Comprehensive Review on Synthesis, Properties, and Applications of Phosphorus (P^III, P^IV, P^V) Substituted Acenes with More Than Two Fused Benzene Rings

Abstract

This comprehensive review, covering the years 1968–2022, is not only a retrospective investigation of a certain group of linearly fused aromatics, called acenes, but also a presentation of the current state of the knowledge on the synthesis, reactions, and applications of these compounds. Their characteristic feature is substitution of the aromatic system by one, two, or three organophosphorus groups, which determine their properties and applications. The (P^III, P^IV, P^V) phosphorus atom in organophosphorus groups is linked to the acene directly by a P-C_sp2 bond or indirectly through an oxygen atom by a P-O-C_sp2 bond.

1. Introduction

Organophosphorus-substituted acenes are an increasingly important group of aromatic hydrocarbons due to the unique properties of the phosphorus atom, which can form tri-, tetra-, and pentacoordinated compounds. This creates the possibility of tuning the electronic properties of the aromatic system of acenes by substituting with organophosphorus groups with different electron characters, from electron-donating phosphine groups to strongly electron-accepting phosphonium groups. The acenes of this type, especially anthracenes, have usually been synthesized with the intention of applying them to organic light-emitting diodes (OLEDs) [1,2]. Other uses of these acenes include the synthesis of ligands for metal catalysts in the hydroformylation reaction [3] or in the Diels–Alder reaction as dienes [4]. Anthrylphosphonic acids and their derivatives have also been employed in the synthesis of self-assembled monolayers [5,6] and anthrylbisphosphonates were used to synthesize fluorescent organic nanoparticles as apoptosis inducers of cancer cells [7].

Scope: Acenes, as defined, are a group of aromatic compounds containing linearly fused benzene rings. In the literature, compounds with angularly fused benzene rings, such as phenanthrene, and compounds with fused heteroaromatic five- and six-membered rings are sometimes included in this group, and they are specifically called angularly fused acenes and heteroacenes, respectively. Both groups, due to the large number of combinations of ring types and the ring positioning in the fused acene, in addition to derivatives of the simplest linearly fused acene, naphthalene, are not the subject of this review. By virtue of their chemical structure, organophosphorus groups have not been restricted and include all groups containing three-, four-, and five-coordinate phosphorus atoms. In the compounds reviewed, organophosphorus groups contain a phosphorus atom, which is linked to the acene directly by a P-C_sp2 bond or indirectly through an oxygen atom by a P-O-C_sp2 bond, such as in phosphates.

This review of the literature covers the period of 1968–2021 and reveals a lack of aromatics that contain more than three fused benzene rings (tetracenes, pentacenes, etc.). Therefore, the subject of this review is anthracenes substituted with any organophosphorus groups.

In addition to the synthesis, this review discusses the reactions that these compounds undergo, and therefore, due to the difficulty of separating the two categories, “synthesis and reactions” are combined together in sections and subsections. Moreover, this review discusses the properties and applications of the synthesized derivatives as well.

This manuscript is divided into five main sections covering phosphines, P^III acids derivatives, diphosphenes, phosphonates and phosphonic acids, phosphates, and the hetero analogs of the mentioned compounds. In the subsections, groups of compounds that can be obtained directly from the precursor included in the main section are identified, e.g., phosphine oxides, phosphonium salts, and phosphoranes can be obtained from phosphines by oxidation, alkylation, and halogenation, respectively, and therefore they are included in the section devoted to the synthesis and reactions of phosphines and derivatives.

Finally, a comment on Section 3 is necessary. Phosphines (λ³-phosphanes), by definition, are P^III compounds containing a combination of three P-Z bonds (Z = C,H). Halophosphines AnthPX₂ and Anthr₂PX (X = F, Cl), which contain at least one P-C bond and one or two halogen atoms, are here classified as halides of the corresponding lower P^III acids [8]. Additionally included in this group are other representatives of the lower P^III acids, i.e., phosphonous acid diamides AnthP(NR₂)₂ (diaminophosphines) and phosphorous acid esters (phosphites).

As a consequence of this division, phosphonous acid P^IV tautomers (H-phosphinic acids) and phosphinous acid P^IV tautomers (H-phosphine oxides) are also reviewed in this section.

Diphosphenes, as the only compounds with a functional group containing more than one phosphorus atom, are discussed separately in Section 6.

2. Synthesis and Reactions of Phosphines (AnthPR₂) (Anth = Anthryl) and Derivatives

This section discusses phosphines, which contain a three-coordinated phosphorus atom linked to three carbon atoms. The subsections of this chapter include groups of compounds that can easily be obtained from phosphine precursors by direct transformation to give compounds with tetra- and pentacoordinated phosphorus atoms. Phosphine oxides, sulfides, and selenides are placed together in one subsection because most papers simultaneously describe the synthesis and reactions of two or three groups of these derivatives. Secondary phosphine oxides (H-phosphine oxides) are discussed in Section 3.5 as phosphinous acid P^IV tautomers.

9-Bromoanthracene 1 is the most frequently used starting material for the syntheses of anthryl phosphines and other derivatives (cf. other subsections throughout this review).

A number of sterically shielded phosphorus ligands for metal catalysts were synthesized by Straub and co-workers via a selective stepwise nucleophilic substitution reaction at the phosphorus atom in triphenyl phosphite (Scheme 1). Thus, first, diphenyl 9-anthrylphosphonite 2 was obtained in a 70% yield by substitution of one phenoxy group by anthryllithium, generated from 9-bromoanthracene 1 and n-butyllithium. Next, the second phenoxy group in triphenyl phosphite was replaced by 1-naphthyllithium to afford phenyl 9-anthryl(1-naphthyl)phosphinite 3 in an overall yield of 46%. Finally, the third least reactive phenoxy group was replaced by phenyllithium to give (anthryl)(naphthyl)(phenyl)phosphine 4 in a 66% isolated yield over three steps. In another synthetic sequence, [di(9-anthryl)](2-methoxyphenyl)phosphine 5 was obtained from triphenyl phosphite via phenyl phosphinite 6 by reacting triphenyl phosphite first with 9-anthryllitium, followed by 2-methoxyphenyllitium at −85 °C. Since phosphorus ligands, employed in homogeneous catalysis, often contain at least one sterically demanding substituent, this method delivers a strategy for the rapid and cost-efficient synthesis of such ligands [9].

Schmutzler et al. reported the synthesis of (anthryl)(diphenyl)phosphine 7 and trianthrylphosphine 8 starting from 9-bromoanthracene 1, which was lithiated to give the intermediate 9-lithioanthracene 9. The latter was reacted with 1 equiv. of chlorodiphenylphosphine or 1/3 equiv. of PCl₃ in diethyl ether at reflux to give 7 or 8 in 71% and 7% yields, respectively. The irradiation of 7 in the presence of W(CO)₆ with a mercury lamp for 34 h formed the pentacarbonyltungsten complex 10 in an 18% yield (Scheme 2) [10].

Chan and co-workers reported a simple monophosphinylation reaction of 1,8-dichloro-anthracene 11, leading to 8-chloro-1-(diphenylphosphino)anthracenes 12 in a 33% yield (Scheme 3) [11]. This reaction was carried out in DMF at 160 °C for 60 h and catalyzed by 10 mol% of palladium supported on charcoal in the presence of 5 equiv. of triphenylphosphine. The addition of 5 equiv. of sodium iodide improved both the rate and yield of the reaction. The second chlorine atom in 11 was not substituted in this reaction and only the mono-derivative 12 was observed, despite the presence of an excess of triphenylphosphine. The authors claimed that steric hindrance of the diphenylphosphino group in position 1 of anthracene protected against the reaction of the second chlorine atom.

Misochko and co-workers [12] obtained 9-(1-phosphirano)anthracene 13 from 9-phosphinoanthracene 14 and ethylene glycol ditosylate as substrates by adapting the procedure of Robinson et al. [13] (Scheme 4). In the next step, 9-(1-phosphirano)anthracene 13 was subjected to UV photolysis to receive a stable triplet anthrylphosphinidene 15, which could be characterized by electron paramagnetic resonance (EPR).

Che and co-authors reported the rhodium(I)-catalyzed C−H arylation of 9-(diphenylphosphino)anthracene 7, as an example of functionalization of phosphines, to give 1-aryl-substituted derivatives 16a and 16b (Scheme 5) [14]. The presented strategy provided access to peri-substituted (naphth-1-yl)phosphines as well.

The synthesis of optically active 1,2-ethylene bis(phosphine) (S,S)-17, presented by Maienza and co-workers, is the only example of a molecule containing two anthrylphosphino moieties linked via an alkyl linker [15]. First, 9-(dichlorophosphino)anthracene 18 was utilized in the reaction with methyl magnesium bromide and BH₃·SMe₂ to obtain 9-anthryldimethylphosphine borane 19. Then, 19 was enantioselectively deprotonated in the presence of (−)-sparteine with s-BuLi and then oxidatively coupled with Cu (II) to give a mixture of enantiomers of 1,2-ethylene bis(phosphine) diboranes (S,S)-20 and (R,S)-20 in a 6:1 ratio and 70% total yield (Scheme 6). Diastereomerically pure (S,S)-20 was received by crystallization from a CH₂Cl₂/Et₂O mixture in a 39% yield and 18% ee. Finally, the diphosphine borane (S,S)-20 was deprotected by stirring in morpholine at room temperature for 12 h. In this reaction, the 1,2-ethylene bis(phosphine) (S,S)-17 was obtained and its enantiomeric excess was determined based on the corresponding phosphine oxide, which was prepared by oxidation of 17 with an excess of H₂O₂.

In the reviewed literature, the synthesis, transformations, and utilization of anthracenes with two phosphino groups on the aromatic moiety were found and are presented below.

1,8-Bis(diphenylphosphino)anthracene 21 was synthesized in a three-step reaction in a 51% overall yield starting from 1,8-dichloro-9,10-anthraquinone 22 by Haenel and co-workers (Scheme 7). The anthraquinone 22 was converted to 1,8-difluoroanthracene 23 by chlorine-fluorine exchange to give 22a followed by reduction with zinc, from which 21 was obtained by reacting it with potassium diphenylphosphide [16].

Gelman and co-workers presented the quantitative Diels–Alder cycloaddition of 1,8-bis-(diphenylphosphino)anthracene 21 to diethyl fumarate 24a. The adduct 25a was used for the synthesis of bifunctional PCsp³P pincer catalyst for the acceptorless dehydrogenation (CAD) of the primary and secondary alcohols to give carbonylic and carboxylic compounds (Scheme 8) [17].

The same research group reported a synthetic scheme that relied on the carbo-Diels–Alder reaction cycloaddition of 1,8-bis-(diphenylphosphino)anthracene 21 to enantiomerically pure bis-(methyl-(S)-lactyl) fumarate 24b, leading to the formation of chromatographically resolvable diastereomers 25b that could be converted into a pair of enantiomerically pure antipodes (Scheme 8) [18].

Jiang and co-workers [19] showed a practical utilization of 9,10-bis(diphenylphosphino)anthracene 26. They obtained a red-light-controllable soft actuator, which was driven by the low-power excited triplet−triplet annihilation-based upconversion luminescence. This system consisted of 9,10-bis(diphenylphosphino)anthracene 26 and the Pt(II) tetraphenyltetrabenzoporphyrin complex 27 (Scheme 9). It was then incorporated into a rubbery polyurethane film and assembled with an azotolane-containing film to study its possible utilization as a highly effective phototrigger of photodeformable cross-linked liquid-crystal polymers. In this system, the Pt(II) complex 27 acted as a sensitizer, whereas 26 was an annihilator, which induced trans-cis photoisomerization of azotolane 28 and 29. The authors achieved a highly effective red-to-blue triplet-triplet annihilation-based upconversion layout with a low-energy excitation light source, large anti-Stokes shift (165 nm), and high absolute quantum yield (9.3%).

In the literature reviewed, the chemistry of anthracenes with two or three phosphino groups and their mixed tetracoordinated derivatives was also found and is presented below.

Thus, 9-bromo-1,8-bis(diisopropylphosphino)anthracene 30, repulsively interacting with 1,8,9-tris(phosphino)anthracene 31 (Scheme 10), single donor stabilized 8-diisopropylphosphino-1-thiophospinoyl-9-metathio/metaselenophosphono)anthracenes 32 and 33 (Scheme 11), and doubly phosphine donor stabilized phosphenium salt 34 (Scheme 12), were synthesized by Kilian and co-workers [20]. The attempted introduction of the third phosphorus atom at the position 9 via Br/Li exchange followed by the reaction with chloro-bis(dimethylamino)phosphine resulted in formation of 1,8,9-tris- and 1,8-bis(phosphino)anthracenes 31 and 35, respectively. The derivative 31 had two relatively inert and bulky diisopropylphosphino groups at positions 1 and 8 whilst the third reactive phosphino group with two P–N bonds on the middle ring opened up the possibility of various transformations on this phosphorus atom, situated in a very crowded surrounding.

Alcoholysis of 31, leading to the intermediate 36, followed by oxidation with sulfur or selenium, afforded phosphine donor stabilized anthracenes 32 or 33 with metaphosphono groups, respectively.

Further reaction of 31 with diphosphorus tetraiodide in 1,2-dichloromethane gave the chlorophosphenium cation 34 stabilized by two phosphino donors at positions 1 and 8, forming a linear P–P–P arrangement. In the first step, the phosphino-phosphonium cation 37 was formed as a transient species. In the next step, the dimethylaminophosphino group was substituted by the chloride anion, which was available from the I/Cl halogen exchange reaction in chlorinated solvent (DCM). Iodide and iodine (I₂) and triiodide originated from disproportionation reaction of P₂I₄.

2.1. Phosphine Oxides (AnthP(=O)R₂), Phosphine Sulfides (AnthP(=S)R₂), Phosphine Selenides (AnthP(=Se)R₂)

Phosphine oxides, sulfides, and selenides are placed together in one subsection because most papers simultaneously describe the synthesis and reactions of two or three groups of these derivatives. All of them were obtained directly from the corresponding phosphines.

A new method for the synthesis of phosphine oxides was published by Yang et al. [21]. They employed a cross-coupling reaction and showed that aryl, vinyl, and benzyl-ammonium triflates reacted with the corresponding phosphorus-based nucleophiles in the presence of the nickel catalyst NiCl₂/dppf, (dppf = 1,1’-bis(diphenylphosphino)ferrocene) (Scheme 13).

The counterion played a minor role in this reaction, so it could be replaced with chloride, bromide, iodide, mesylate, or tosylate anions without a significant loss of yield. A considerable advantage of this synthesis was that ammonium salts are cheap and readily viable.

Another method, which was developed by Zhang et al. [22], involved a direct transformation of aromatic acids into the corresponding phosphine oxides in the presence of palladium(II) salts. Several substrates were shown to react in this manner, including 2-anthroic acid 40, which was transformed to 2-(diphenylphosphinoyl)anthracene 39 (Scheme 14), providing an alternative to the synthesis of the latter from anthryl ammonium triflate 38 (Scheme 13), although in significantly lower yields.

The optimal temperature for the reaction was 115 °C. Changing the temperature reduced the yield, as did changing the solvent to a highly polar one (DMF, N,N-dimethylformamide).

Another study carried out by Zhao and coworkers [1] showed that 9,10-bis(diphenylphosphinoyl)anthracene 41 could readily be synthesized from 9,10-dibromoanthracene 42 by substitution of chlorine in chlorodiphenylphosphine with 9,10-dilithioanthracene obtained from a double Br/Li exchange in 42 followed by oxidation of the resulting bis(diphenylphosphino)anthracene 26 with hydrogen peroxide to yield 41 in a 47% yield. 9,10-Dibromoanthracene 42 was obtained by bromination of anthracene in chloroform (Scheme 15).

The product 41 has been synthesized with the intention of applying it in organic light-emitting diodes (OLEDs). Zhao et al. found that 41 was a yellow-green solid with a melting point that reached 265 °C. A similar study by Tao and co-workers [23] confirmed the fluorescent properties of 41, which could be used in the construction of OLEDs. The authors also claimed that the conversion from 26 to 41 was the first reported triplet-triplet annihilation system activated by hydrogen peroxide.

Furthermore, Xu and co-workers [24] linked the increase in fluorescence properties within the 26/H₂O₂ system to the degree of photooxidation. This system could be used as an indicator of the reaction time, oxygen exposure, and light irradiation or a time-oxygen and light indicator (TOLI). The compound 41 was also the subject of interest in another study by Xu et. al. [25], who used it to investigate the properties of a samarium complex Sm(hfac)₃(41)₃ (hfac = hexafluoroacetylacetonato).

Wu and co-workers [2] showed the synthesis and properties of (9,10-diphenyl-2-phosphinoyl)anthracene 43 with the intention of using it as a true-blue OLED material. The synthesis was similar to the one proposed by Zhao and coworkers [1]: it included treatment of 2-bromo-9,10-diphenylanthracene 44 with n-BuLi, followed by the addition of chlorodiphenylphosphine and subsequent oxidation (Scheme 16).

The compound 43, which readily crystallized as a light-yellow solid, was obtained in a 30% yield. It exhibited red-shift fluorescence compared to 9,10-diphenylanthracene. This study showed that 43 was not suitable for the true-blue OLED material due to the fact that the compound exhibited a red shift.

Yamaguchi et al. [26] reported a synthesis and photochemical characterization of tri(9-anthryl)phosphine 8 and tri(9-anthryl)phosphine oxide 45 (Scheme 17).

Tri(9-anthryl)phosphine 8 was synthesized from 9-bromoanthracene 1 by treatment with n-butyllithium, followed by the addition of phosphorus trichloride. Tri(9-anthryl)phosphine oxide 45 was obtained by oxidation of 8 with hydrogen peroxide (Scheme 17). The tri-coordinated 8 and tetra-coordinated derivative 45 exhibited a weak fluorescence.

The synthesis of phosphine oxides 46a–h containing carboxylic acid esters and 9,10-dihydro-9,10-ethanoanthracene moiety, described by Okada and co-workers, is an example of utilization in the synthesis of 2-(dipenylphospinoyl)anthracene 39. The bulky compounds 46a–h were synthesized in the reaction of 39 with dimethyl methylene malonate, dimethyl fumarate, or methyl acrylate and paraformaldehyde in 21–95% yields. (Scheme 18) [27].

Katagiri et al. [28] synthesized 9-(diphenylphosphinoyl)anthracene 47 and 9-(anthrylphenylphosphinoyl)anthracene 48 from diphenyl chlorophosphine and phenyl dichlorophospine, respectively (Scheme 19). The synthesis was analogous to the previous method by Schwab and co-workers [29] and first required halogen/lithium exchange and then oxidation with H₂O₂. The authors revealed that phosphine oxides 47 and 48 did not lead to the formation of a photodimer in the solid state, whereas in chloroform or acetonitrile under an N₂ atmosphere, at the 365-nm-wavelength irradiation, the [4π + 4π] photodimerization of 47 occurred to give 49. In addition, the absorption and emission spectra of the compound 47 in acetonitrile showed characteristic absorption and emission bands of anthryl moieties while photodimerization of the anthryl groups led to the disappearance of these bands. The authors reversibly returned 49 to 47 by heating the probe at 80 °C. In contrast, analogous irradiation under an O₂ atmosphere resulted in the formation of anthraquinone 50.

During attempts to obtain a “masked” version 51 of the phosphaalkene Mes-P=CH₂, Gates and co-workers [30] synthesized 9-[(methyl)(mesityl)phosphino)]anthracene 52 and the corresponding 9-[(methyl)(mesityl)phosphinoyl)]anthracene 53 in the reaction between (chloro) (chlorometyl)(mesityl)phosphine 54 and the anthracene magnesium (MgAnth•3THF) in THF. The expected adduct 51, which was initially formed, then decomposed to give the anthracene derivative 52 in an 83% yield. The latter was oxidized to 53 in a 9% yield only (Scheme 20).

Wang and Zhu reported the palladium-catalyzed decarbonylation of 9-[(1-keto diphenylphosphinoyl)]anthracene 55 in the presence of 1 mol% of Pd₂(dba)₃ and 8 mol% of the phosphine ligand (PCy₃) to give 9-(diphenylphosphinoyl)anthracene 47 in an 80% yield [31] (Scheme 21).

Drabowicz and co-workers synthesized optically active 9-[(t-butyl)(phenyl)phosphinoyl)]anthracene 56 in a 71% yield in the Hirao reaction of palladium-catalyzed cross-coupling reaction of 9-bromoanthracene 1 with optically active t-butylphenylphosphine oxide 57 (Scheme 22) [32]. The formation of carbon–phosphorus bonds took place with retention of the configuration, and the stereoretention of this reaction was confirmed by X-ray analysis.

Stalke and co-workers synthesized three positional isomers of 1-, 2-, and 9- (diphenylthiophosphinoyl)anthracenes 58, 59, and 60 that revealed a solid-state fluorescence in three different colors with differences in emission wavelengths of over 100 nm. Analysis of the solid-state structure of 59 and photophysical properties allowed the unusual yellow emission to be attributed to the formation of excimer in the solid state. Therefore, substitution at position 1 of the anthracene fluorophore with suitable substituents may be a promising strategy to obtain long wavelength emission in the solid state using structurally easy to modify compounds (Figure 1) [33].

Schillmöller and co-workers [34] synthesized four 9-(diphenylthiophosphinoyl)anthracenes 58 and 61–63 with alkyl and phenyl substituents at the position 10 via sulfurization of 9-(diphenylphosphino)anthracenes 7 and 64–66. The latter were obtained from the corresponding bromoanthracenes 1 and 67–69 (Scheme 23) [35,36].

The compounds 58 and 61–63 were crystallized and their X-ray structures were then determined. These studies revealed that oxidation of the phosphorus atom with sulfur significantly changed the molecular structural parameters and the crystal packing. This caused a strong bathochromic shift, which resulted in a green solid-state fluorescence. Moreover, the authors prepared four host-guest complexes, with 62 as a host molecule and benzene, pyridine, toluene, and quinoline as guest molecules. This resulted in enhanced emission and up to a five times higher quantum yield in comparison to the pure compound 62.

Walensky et al. characterized 9-(diphenylthio- and diphenylselenophosphinoyl)anthracenes 58 and 70, respectively, by NMR and optical spectroscopy (Scheme 24) [37]. The authors demonstrated that ³¹P NMR shifts for 58 and 70 were shifted upfield when compared to the unoxidized analog. This was due to the loss of planarity and relatively greater σ- than π-bonding between the phosphorus atom and the anthracene carbon.

When excited at 310 nm, compounds 58 and 70 showed emission similar to that of unsubstituted anthracene, displaying peaks at 380, 402, 430, and 450 nm. When the excitation wavelength was shifted to 410 nm, the observed emission became structureless and was red-shifted by around 50 nm relative to the typical emission of the unsubstituted anthracene. The authors did not observe excimer formation for these compounds. Moreover, a small deviation from planarity in the anthracene ring was observed for 58 and 70 and the angle of deflection was 3° and 5°, respectively.

Schwab and co-workers [38] obtained 9-bromo-10-(diphenylphosphino)anthracene 71 and its thio- 72 and seleno- 73 derivatives (Scheme 25). In the first stage, 9,10-dibromoanthracene 42 was treated with n-BuLi followed by the addition of chlorodiphenylphosphine to give 9-bromo-10-(diphenylphosphino)anthracene 74. Then, 74 was oxidized to the corresponding oxo-, thio-, and seleno derivatives 71–73 in high yields according to the procedure described by Stalke et al. [39]. Their spectral and structural properties were investigated and shown to be largely consistent with those of the 9,10-diphosphino derivatives 80–82 mentioned below (Scheme 27).

In an analogous manner, the same authors [29] synthesized bulky 9-(diisopropylphosphino)anthracene 75, 9-bromo-10-(diisopropylphosphino)anthracene 76, and 9,10-symmetrically-substituted anthracene 77, which were then oxidized to the corresponding derivatives 78a–f and 79 (Scheme 26).

9,10-Bis(diphenylphosphinoyl)anthracene 80, 9,10-bis(diphenylthiophosphinoyl)anthracene 81, and 9,10-bis(diphenylselenophosphinoyl)anthracene 82 were obtained by oxidation (E = O, S, Se) of 9,10-bis(diphenylphosphino)anthracene 26 again using H₂O₂•(NH₂)₂C=O (urea) (dichloromethane, 0 °C), elemental sulfur (toluene, reflux), and selenium (toluene, reflux), respectively (Scheme 27). The compounds obtained were significantly more soluble in organic solvents than the starting material 26. The absorption and emission spectra of 80–82 were recorded in solution and in the solid state. In solution, only 80 exhibited a detectable emission whereas 81 did not emit. The latter showed strong fluorescence in the solid state at λ = 508 nm. This molecule formed single crystals possessing a groove suitable for binding toluene reversibly to the anthracene chromophore by means of C-H⋯π-ring center interactions. Hence, the crystalline 81 was the first solid-state excimer that could serve as a chemosensor to detect toluene selectively. Single crystals of 80 emitted at λ = 482 nm, whereas the selenium derivative 82 did not emit in the solid state. In addition, the crystal structures of compounds 80–82 were analyzed using the single-crystal X-ray diffraction technique (Scheme 27) [39]. The substrate 26 was obtained based on the procedure described by Prabhavathy and co-workers [40].

2.2. Phosphine Boranes (AnthPR₂•BH₃)

In this subsection, the presented syntheses and reactions of phosphine molecules with P-B coordinate (semipolar, dative) bonds are presented.

The ring opening of enantiomerically pure oxazaphospholidineborane 83 with bulky anthryllithium to give phosphineboranes 84 was studied by Stephan and co-workers. The authors proposed an explanation for the low 4% yield of 84. They reported that in the case when the attack on the phosphorus atom was hindered, deprotonation of the benzylic proton occurred, yielding trans-(N-methylamino)(phenyl)(1-phenyl-1-propenyloxy)phosphineborane 85 instead of 84 (84/85 = 5/95) (Scheme 28) [41].

1,8-Bis-(diisopropylphosphino)-9-methoxyanthracene 86, as a starting material for the synthesis of 9-boron-substituted 1,8-bis-(diisopropylphosphino)anthracene 87a/87b, was prepared by Akiba and co-workers by treatment of 1,8-dibromo-9-methoxy-anthracene 88, first with n-BuLi and then with diisopropylchlorophosphine. Diphenylchlorophosphine was used as well; however, only the diisopropylphosphine derivative 86 could be successfully transformed into 1,8-bis(diisopropylphosphino)-9-bromoanthracene 30 with LDBB (lithium di-tert-butylbiphenylide) followed by treatment with BrCF₂CF₂Br, as a brominating agent, in a 51% yield. The introduction of a boron substituent at the position 9 in 30 via Br/Li exchange followed by reaction with chloroborane 89 led to the formation of 1,8-bis(diisopropylphosphino)-9-borylanthracene 87a/87b. The ¹H and ³¹P NMR spectra of 86 showed a symmetrical anthracene pattern at room temperature. This meant that a very rapid bond switching process between 87a and 87b occurred in solution (Scheme 29) [42].

2.3. Phosphine–Metal Complexes (AnthPR₂-Metal)

In this subsection, complexes of phosphines possessing at least one anthryl substituent with metals, such as Au, Ag, Au/Ag/Sb, Fe, Pd, Pt, Ir, Lu, Eu, Ru, and Ni, are reviewed.

Other metals (W, Os, Co), i.e., the pentacarbonyltungsten complex of 9-diphenylphospino)anthracene, are reported in Section 2 while the triosmiumdodecacarbonyl cluster and dinuclear cobalt complex are discussed in Section 3, respectively.

Gold and platinum(II) complexes of the phosphine ligands PAnth_nPh_3-n (Anth = anthryl) were synthesized by Mingos et al. [43]. The authors recorded ³¹P{¹H} NMR chemical shifts for (anthryl)(diphenyl)phosphine, (dianthry)(phenyl)phosphine and trianthrylphosphine, their oxo derivatives, and gold (I) halide and gold (I) nitrate complexes. Moreover, a crystal structure of the [AuCl(PAnth₂Ph)]•CH₃Cl complex was determined by X-ray analysis. An example of the preparation of the gold (I) complex [Au(NO₃)(PAnthPh₂)] 90 obtained from 9-(diphenylphosphino)anthracene 7 is shown in Scheme 30.

Several other Au and Pt complexes were also synthesized: trans-[PtCl₂(PAnthPh₂)₂] (64%); trans-[Pt (CH₃CN)₂(PAnthPh₂)₂](BF₄)₃ (74%); trans-[Pt (CH₃CN)₂(PAnth₂Ph)₂](BF₄)₃ (68%); [Au(NO₃)(PAnthPh₂)] (95%); [Au(NO₃)(PAnth₂Ph)] (79%); [Au(NO₃)(PAnth₃)] (53%); [AuCl(PAnthPh₂)] (91%); [AuCl(PAnth₂Ph)] (93%); and [AuCl(PAnth₃)] (72%).

A luminescent molecular metalla(Au)cyclophane 91, which was synthesized from the self-assembly of the molecular “clip” 92 and bipyridine, showed a large rectangular cavity of 7.921(3) × 16.76(3) Å (Scheme 31). The electronic absorption/emission spectroscopy and electrochemistry of 91 were studied. The 2⁴⁺ ions were self-assembled into a 2D mosaic in the solid state via complementary edge-to-face interactions between phenyl groups. ¹H NMR titrations ratified the 1:1 complexation of the cations 91 and various aromatic molecules. Comparison of the structures of the inclusion complexes indicated an induced-fit mechanism operating in the binding. The luminescence emission of 91⁴⁺ could be quenched upon the guest binding. The binding constants were determined by both ¹H NMR and fluorescence titrations. Solvophobic and ion-dipole effects were shown to be important in stabilizing the inclusion complexes [44].

Complexes 92 (X = OTf⁻, ClO₄⁻, PF₆⁻, BF₄⁻), as discrete binuclear, trinuclear, and tetranuclear metallacycles, were isolated and characterized, showing novel puckered-ring and saddles-like structures in the tri- and tetranuclear metallacycles (Scheme 31) [45].

The trinuclear Au(I) complex [Au₃(PAnthP)₃][ClO₄]₃ 93 was synthesized by Yip and co-workers in the reaction of 9,10-bis-(diphenylphino)anthracene (PAnthP) 26 and 1 equiv. of Me₂S AuCl in methanol at reflux. The authors observed a stable gold ring in the solution and no NMR signals arising from the free ligand (Scheme 32) [40].

The UV/Vis absorption spectra of 26 and its complex 93 showed intense bands at 396 and 424 nm assigned to ¹π-π* transitions in the anthryl ring. Excitation of CH₃CN solution of 93 at 400 nm gave an emission at 475 nm with a quantum yield of Θ = 0.05.

The reaction of 26 (PAnthP) with 2 equiv. of Me₂SAuX in CH₂Cl₂ led to the new binuclear complexes (µ-PAnthP)(AuCl)₂ 94a and (µ-PAnthP)(AuBr)₂ 94b with Au(I)−X−Ag(I) halonium cation (Scheme 33) [46]. The reaction of 94a and 94b with 2 equiv. of AgSbF₆ led to spontaneous formation of the [(µ-PAnthP)-Au₂]²⁺ ion, and then, after the addition of AgSbF₆ (0.5 equiv.) in a THF solution, gave crystals of {[(µ-PAnthP)(AuCl)₂]2Ag}⁺SbF6⁻ 95a and {[(µ-PAnthP)(AuCl)₂]2Ag}⁺SbF₆⁻ 95b as products.

9,10-Bis(diphenylphosphino)anthracene (PAnthP) 26 with two donor phosphorus atoms has been used as a P-ligand unit for metals. Thus, the double-helicate dinuclear silver(I) complex, [Ag₂(4’-Ph-therpy)₂](SO₃CF₃)₂, was reacted with 26 to give the corresponding dinuclear complex 96 (4’-Ph-therpy = 4’-phenyl-terpyridine). The latter showed a strong fluorescence in the solid state with an excitation band at 383.5 nm, emission band at 535.5 nm, and lifetime of 4.20 ns, but the derived complexes did not show fluorescent properties (Figure 2) [47].

An iron(0)tetracarbonyl complex 97 was synthesized from di(9-anthryl)fluorophosphine 98, which was stable to redox disproportionation (Scheme 34) [48].

Pincer iridium complexes 99 derived from 1,8-bis(diphenylphosphino)anthracene 35 turned out to be suitable platforms for the C−H activation of methyl tert-butyl ether (MTBE) (Figure 3) [49].

A series of thermally stable Ir, Ni, and Pd complexes were obtained from 1,8-bis(dialkyl and diphenylphosphino)anthracenes 100, 35, and 21. The anthracenes 100 and 35 were prepared similarly to 21 by direct nucleophilic substitution of fluorine atoms in 1,8-difluoroanthracene by potassium di-tert-butylphosphide or potassium di-iso-propylphosphide. The reaction of 100 with IrCl₃•3H₂O in 2-propanol/water afforded the complex 101 as a red crystalline powder in an 86% yield (Scheme 35). The reduction of 101 under a hydrogen atmosphere gave mixtures of the yellow-colored iridium tetrahydride 102 and the red-colored iridium dihydride 103. By saturating solutions of such mixtures with hydrogen, the equilibrium was shifted towards 102. Evaporation of the solvent under vacuum resulted in the formation of the analytically pure complex 103 in a >95% yield. The thermally stable complexes 103 and 104 were ideal for homogeneous catalysts in the alkane dehydrogenation above 200 °C. The complex 103 in alkane solution was stable at 250 °C and catalyzed the dehydrogenation reactions at this temperature [50].

Osawa et al. [51] synthesized bis[(9-diisopropylophosphino)anthracene]-tris(hexafluoroacetylacetonato)europium(III) 108 (Scheme 36). First, the authors prepared 9-(diisopropylphosphino)anthracene 78a (Scheme 26) according to the Schwab et al. protocol [29], which was next reacted with tris(hexafluoroacetylacetonato)europium(III) 108a for 8 h in refluxing methanol solution to obtain 108 in a 55% yield after recrystallization.

Osawa and co-workers determined the crystal structure of the Eu(III) complex 108 and studied its intra-complex energy transfer. The studies revealed that laser irradiation of this compound in n-hexane gave blue emission, which was ascribed only to the 9-(diisopropylphospino)anthracene moiety, not to the central Eu(III) ion (Scheme 36).

Kitagawa and co-workers [52] obtained a novel coordination polymer 109 based on 9,10-diphenyl-2,6-bis(diphenylphosphinoyl)anthracene 110 as a core and two molecules of Lu(hexafuoroacetylacetonate)₃ that interacted with the core (Scheme 37).

First, the anthracene 110 was synthesized from 2,6-dibromo-9,10-diphenylanthracene 111 and diphenylphosphine. The first step of the synthesis was performed in the presence of potassium acetate and palladium acetate, and next the resulting bisphosphine intermediate was oxidized to 110 in a 28% yield. The polymer 109 was prepared in a microtube by the liquid-liquid diffusion-assisted crystallization method. The authors studied the photophysical properties and thermal stability of 109 and its oxide 110. The luminescence quantum yield was enhanced from 18% up to 25% (λ_ex = 380 nm) due to the introduction of Lu(hexafuoroacetylacetonate)₃ molecules into the phosphine oxide system, as a result of which bright, pure sky-blue emission was observed. In addition, the compound 109 showed a higher temperature of decomposition (340 °C) than 110.

The diphosphine-bridged dimer of the oxo-centered triruthenium–acetate cluster unit [{Ru₃O(OAc)₆(py)₂}₂(dppan)](PF₆) 112 was synthesized by Chen and his co-workers (Scheme 38). The reaction of [Ru₃O(OAc)₆(py)₂(CH₃OH)](PF₆) with 9,10-bis(diphenylphosphino)anthracene (dppan) 26 resulted in the formation of 112 in a 67% yield. The redox studies of the complex 112 revealed the presence of electronic communication between two triruthenium units mediated through bridging dppan [53].

A number of tri-, tetra-, and penta-ruthenium clusters 113–115 were synthesized by Deeming and co-workers. When a suspension of [Ru₁₃(CO)₁₂] and a slight excess of 9-(diphenylphosphino)anthracene 7 in octane were heated to reflux at 125 °C for 4 h, several products were obtained, including the yellow trinuclear cluster [Ru₃(µ-H)₂(CO)₈(µ₃- C₁₄H₇PPh₂)] 113 and the purple tetraruthenium butterfly complex [Ru₄(CO)₁₁(µ₄-C₁₄H₇PPh²)] 114. Both anthracyne complexes and also the dark purple pentaruthenium bow-tie cluster, [Ru₅(CO)₁₃(µ₅-η¹:η²:η³: η³-C₁₄H₈- η¹- PPh₂)] 115 were obtained via the double metallation from one of the unsubstituted rings (Scheme 39). Furthermore, treatment of the trinuclear species 113 with 1 equivalent of [Ru₃(CO)₁₂] in refluxing octane resulted in a cluster build-up, with the formation of the tetra- and penta-ruthenium species 114 and 115. Likewise, the thermolysis reaction of 114 with [Ru₃(CO)₁₂] also led to 115. The crystal structure of 3 revealed a unique µ₅-interaction of the ligand with the ruthenium cluster [54].

The bulky phosphine ligand 116 was prepared by Claverie et al. and used to generate the phosphine palladium complex 117. The complex catalyzed ethene polymerization to yield linear polyethene; however, its catalytic activity was smaller compared to complexes with phenyl, naphthyl or phenanthryl substituents, which corresponded to increasing cone angles and decreasing basicity (Scheme 40) [55].

Yamamoto and Shimizu synthesized 9-(diphenylphosphino)anthracene-based palladacycles 118a and 118b that catalyzed conjugate addition of arylboronic acids to electron-deficient alkenes, such as α,β-unsaturated ketones, esters, nitriles, and nitroalkenes. The monomeric catalysts, which were synthesized from K₂PdCl₄, 9-(diphenylphosphino)anthracene, and trialkyl phosphites, exhibited turnover numbers of up to 700 (Figure 4) [56].

Mingos and co-workers reported the synthesis and structural characterization of the Pd complex [Pd(dba)L₂] 119 (where L = 120 and dba = dibenzylideneacetone) obtained from [Pd₂(dba)₃] and the corresponding 1-(diphenylphosphino)anthracene 120 (L) (Scheme 41). The single-crystal X-ray structural analyses confirmed that these complexes adopted a trigonal planar structure, with the dba ligand coordinated by a double bond [57].

Dibenzobarrelene-based C(sp³)-metallated pincer complexes 121a, 121b, and 121c were synthesized by the Diels–Alder [4 + 2] cycloaddition reaction of organometallic anthracene dienes 122a, 122b, and 122c with dimethyl alkyne dicarboxylate as a dienophile (Scheme 42). This straightforward approach has an advantage over traditional synthetic routes, such as either C–H activation or oxidative insertion of a coordinated transition metal into the C–X bond of the halogenated spacer [58].

A number of metal complexes 122a, 122b, and 123 have been synthesized using 1,8-bis(diphenylphosphino)anthracene 21 as a ligand (Scheme 43). The latter was synthesized from dipotassium 1,8-anthracenedisulfonate 124 and potassium diphenylphosphide (Ph₂PK). The reaction of 21 with nickel(II) chloride or bis-(benzonitrile)palladium(II) chloride led to cyclometallation of the anthracene C-H bond at 9-position and resulted in the formation of square-planar chelate complexes 122b or 122a, respectively. Treating the complex 122b with aqueous potassium cyanide did not remove nickel from 122b but converted 122b into 123 by substituting chloride with cyanide, confirming the high stability of these cyclometallated chelate complexes. The strong metal bonding in 122b made it an ideal ligand for the development of new catalysts. Like the anthracene unit in 21, other polycyclic acenes or heteroarenes might also be useful as a rigid backbone for bidentate phosphines [59].

The platinum (II) complex 125 and photochemically dimerized product 126 were synthesized from 9-(difluorophosphino)anthracene 127 (Scheme 44), obtained in the reaction of anthryllithium with chlorodifluorophosphine, with the former being synthesized in the reaction of n-butyllithium with 9-bromoanthracene 1. The dimer 126 constituted one of the six possible rotational isomers. A rotation of the PF₂ group was hindered by strong F-H interactions at temperatures up to at least 105 °C [60].

Hu et al. [61] described the cycloplatination reaction of 9,10-bis(diphenylphosphino)anthracene 26 with Pt(bis(diphenylphosphino)methane)(OTf)₂ 128 to give [Pt(bis(diphenylphosphino)methane)(9-(diphenylphosphino)anthracene)PO-H)]OTf 129. The uncoordinated P atom in the complex was oxidized when exposed to air (Scheme 45).

The same authors also studied the influence of the reaction conditions on the regioselectivity of the double cyclometallation process (Scheme 46) [62].

Other dicyclometalated complexes syn- and anti-[Pt₂(L)₂(PAnthP-H₂)](OTf)₂(Pt₂) (Anth = anthrylene) 130–132 have been synthesized in reactions of 26 (PAnthP) with Pt(L)(OTf)₂ (L = diphosphine, OTf) (Scheme 46). To understand the effect of the number of Pt ions on the extent of perturbation, a mononuclear analog 133 was also prepared. The UV–vis absorption spectra of 133 and PAnth displayed moderately intense vibronic bands at around 320–440 nm. The spectra of the binuclear complexes 130–132 were different from that of 133. The spectra of the syn-isomers 130a, 131a, and 132a displayed two intense overlapping absorption bands at 320–520 nm. The anti-isomers 130b and 132b also displayed two intense bands in a similar spectral range (300–500 nm). The emission energies in degassed DCM at room temperature followed the order 133 > 131b, 132b > 130a, 131a, and 132a [62].

9-(Dihydrophosphino)anthracene 134 was prepared in two steps starting from 9-bromoanthracene 1, which was next was converted to 9-(dihalophosphino)anthracenes 135 (X = Cl, Br) via the Grignard reagent 136, which reacted with PCl₃ or PBr₃, respectively. Next, reduction of the latter with 2 equiv. of LiAlH₄ in diethyl ether at −78 °C and then reflux for 1 h delivered 134. In the reaction of the dilithium derivative of 134 with 1,2-dichloroethane, Kubiak and co-workers obtained 9-(1-phosphirano)anthracene 13. Then, the reaction of 13 with 0.5 equiv. of PtCl₂(1,5-cyclooctadiene) gave the platinum complex, cis-dichlorobis[1-(9-anthracene)phosphirano]platinum(II) 137 (Scheme 47) [63]. The complex 137 displayed novel intramolecular π-stacking interactions between the anthracene ring systems.

The same research group synthesized other platinum complexes, such as bis[1-(9-anthracene)phosphirano]dithiolateplatinum complexes 138a–d, in the reaction of 137 with appropriate potassium-ethylene-2,2-dithiolates 139a–d containing two electron-withdrawing groups (EWGs) in positions 1, such as methoxycarbonyl, ethoxycarbonyl and cyano groups. The final products 138a–d were obtained in CH₂Cl₂/MeOH mixture after 18 h at room temperature in high yields. X-ray studies of the complexes displayed the intra- or intermolecular anthracene ring of the cis-bis{1-(9-anthracene)phosphirane} stacked structures (Scheme 48) [64].

All of the platinum complexes 138a–d reported emitted light at low temperatures in the solid state. Complexes 138a–d exhibited a strong green fluorescence at 530 nm at low temperatures in the solid state. Moreover, the complex 138d strongly emitted blue light in the THF or benzene solution at 450 nm after excitation at 420 nm. The blue emission of the complex 138d with two cyano groups and a very small Stokes shift was similar to that observed for free 9-(1-phosphirano)anthracene and anthracene rings.

The pincer complexes 140, 141, and 142 were synthesized by irradiating the cyclometalated complex 122c in the presence of O₂, which led to oxidations of the anthryl ring (Scheme 49). The first photoproduct, a Pt(II)-9,10-endoperoxide complex 143, was converted photochemically to the Pt(II)-9-hydroxyanthrone complex 140, which was further oxygenated to the Pt(II)-hemiketal 141. The oxidation of 140, which could be accelerated by light irradiation, probably involved a Pt(II)-anthraquinone intermediate. The Pt(II)-hemiketal 141 underwent acid-catalyzed ketalization to form a binuclear Pt(II) 2-diketal 142. The structures were characterized by NMR and single-crystal X-ray diffraction. All complexes possessed similar absorption spectra, showing a moderately intense vibronic band at 390–480 nm (λ_max = 454 nm, ε_max = 7.6–9.2 × 10³ M⁻¹ cm⁻¹) and a very intense band at 280 nm (ε_max = 5.2–5.9 × 10⁴ M⁻¹ cm⁻¹). The Pt complexes were also luminescent in solution and in the solid state at room temperature. Irradiating degassed CH₂Cl₂ solutions of the complexes at 390 nm resulted in an emission band at λ_max = 474 nm with a vibronic shoulder at 520 nm [65].

2.4. Phosphoranes (AnthPR₂X₂) (X = F)

Yamaguchi et al. [26] reported the synthesis and photochemical characterization of tri(9-anthryl)difluorophosphorane 144 obtained from the reaction of xenon difluoride with tri(9-anthryl)phosphine 8. The synthesis of 144 is presented in Scheme 50.

The authors proved that the fluorescence intensity is attributed to the coordination number of phosphorus. The tri-coordinated 8 exhibited a weak fluorescence while pentacoordinated 18 showed a significant fluorescence.

2.5. Phospinimines (AnthR₂P = N-R¹) and Phosphiniminium Derivatives (AnthR₂P = NH₂⁺)

Jurisson and co-workers synthesized the N-protected phosphinimine 145 and its phosphiniminium ion pairs 146a and 146b with [ReO^4−] and [TcO^4–] anions. The phosphinimine 145 was fluorescent but the addition of [TcO₄⁻] or [ReO₄⁻] anions to 145 did not change the original spectrum in terms of the overall spectral features or intensity. In addition, the anthracene molecule scintillated in the presence of [⁹⁹TcO⁴⁻], making it a possible reporter group for a scintillation sensor using this molecule (Scheme 51) [66].

2.6. Phosphonium Salts (AnthPR₃⁺)

Usually, phosphonium salts are obtained from phosphines by quaternization of a tricoordinated phosphorus atom with a free electron pair.

The D–π–A type of phosphonium salts, in which electron acceptor (A = ⁺PR₃) and donor (D = NPh₂) groups were linked by polarizable π-conjugated spacers, showed an intense fluorescence classically ascribed to the excited state intramolecular charge transfer (ICT). Therefore, a series of such phosphonium salts with different lengths of spacers and counterions were synthesized and characterized. The salt 147 was synthesized by the two-step approach involving the preparation of tertiary aryl phosphine from 158 followed by methylation with methyl iodide. The peak wavelengths (λ_abs) were gradually red-shifted along with the extension of the π-spacer: π = phenylene (333 nm) < π = biphenylene (387 nm) < π = naphthylene (407 nm) < π = anthrylene (147, 519 nm). The extension of the π–system from phenylene to the polycyclic naphthalene and anthracene motifs in 147 caused a gradual growth of λ_em to 560 and 679 nm for 147 in DCM, which was, however, accompanied by a drop in the quantum yield (Scheme 52) [67].

A metal-free synthesis of 2-anthryl phosphonium bromide 149 by the reaction of triphenylphosphine with 2-bromoanthracene 150 in refluxing phenol was developed by Huang et al. Examination of other solvents with a boiling point of around 200 °C showed that tetralin, PhCN, ethoxybenzene, or 2-chlorophenol could also produce phosphonium salts, although in lower yields (5–44%). A two-step addition-elimination mechanism was proposed, in which the second step of the bromide elimination was fast, as indicated by the deuterium experiment. The authors suggested that phenol could form a hydrogen bond with bromide, facilitating the addition of triphenylphosphine and elimination of bromide by polarizing the carbon–bromide bond, and making phenol the optimal solvent among the solvents tested (Scheme 53) [68].

Nikitin and co-workers [69] synthesized 9-(methylphenylphosphinoyl)anthracene 151 by the oxidation of 9-(methylphenylphosphino)anthracene 152 with hydrogen peroxide in acetonitrile solution (Scheme 54). The compound 151 was then converted into the corresponding phosphonium chloride 153a and bromide 153b using oxalyl chloride and bromide, respectively. The authors measured the exchange barriers of self- and cross-exchange of halides in phosphonium salts using the 2D EXSY NMR technique to visualize the processes.

Tri(9-anthryl)(methyl)phosphonium iodide 154 was synthesized by Yamaguchi et al. in the quaternization reaction of the phosphine 8 with methyl iodide. The tri-coordinated 8 and tetra-coordinated derivatives 154 exhibited a weak fluorescence (Scheme 55) [26].

Bałczewski et al. [70,71] recently presented a novel, one-pot phospho-Friedel–Crafts–Bradsher cyclization, which led to higher-substituted phosphonium salts 155. In the new reaction, (o-diacetaloaryl)arylmethanols 156, as the starting materials, in the presence of triphenylphosphine and acids HA, spontaneously cyclized directly to 155 under very mild reaction conditions (Scheme 56).

3. Synthesis and Reactions of P^III Acids, Their P^IV Tautomers, and Derivatives

This section covers P^III acids derivatives containing at least one anthracene moiety linked either directly to the phosphorus atom via the P-Csp² (Anth) bond (Section 3.1 and Section 3.2) or indirectly via the P-O-Csp² (Anth) bond (Section 3.3). The phosphonous RP(OH)₂, phosphinous R₂POH, and phosphorous P(OH)₃ free acids are the organophosphorus members of the group of substances known as the lower acids of phosphorus. These P^III trivalent species exist as minor tautomers in equilibrium with major P^IV tetravalent forms, which exhibit one less acidic function than might be expected [8]. Derivatives of P^III acids, such as halides, amides, and esters, may exist in stable, trivalent forms and they will be described separately in Section 3.1, Section 3.2 and Section 3.3. The P^IV tautomers are discussed in Section 3.4 and Section 3.5.

3.1. Phosphonous Acid Dihalides and Phosphinous Acid Halides (Halophosphines) (AnthPX₂) and (Anth₂PX) (X = F, Cl, Br)

Halo- and dihaloanthracenes of the formula AnthPX₂ and Anth₂PX (X = F, Cl, Br), which contain at least one P-C bond and one or two halogen atoms, are classified as halides of the corresponding lower P^III acids. Syntheses of 9-(difluoro, dichloro, dibromo)anthracenes are also described in Section 2.3.

9-Difluoroanthracene 127 was synthesized by Schmutzler et al. starting from 9-bromoanthracene 1 and chlorodifluorophosphine in a 96% yield (Scheme 57) [10].

9-(Difluorophosphino)anthracene 127 was also employed by the group of Schmutzler in further investigations. They used 9-(dihydrophosphino)anthracene 14 and irradiated it with a mercury lamp in toluene for 22.5 h to obtain two isomeric dimers 157a and 157b in a 2:1 ratio, which could be observed in ³¹P NMR. Irradiation of 9-(difluorophosphino)anthracene 127 under the same conditions gave only one dimeric isomer 126. Hydrogenation reaction of the latter with 10 equiv. of LiAlH₄ in diethyl ether at reflux for 24 h delivered a single isomer 157a in a 93% yield (Scheme 58) [10].

Kirst and et al. [72] synthesized 9-(dichlorophosphino)anthracene 18 by the reaction of PCl₃ with the organozinc compound 158. The latter was obtained from 9-bromoanthracene 1, which was first lithiated with n-butyllithium to obtain 159 and then submitted to the Li/Zn transmetallation with dry ZnCl₂ (Scheme 59).

9-(Dichlorophosphino)anthracene 18 was further utilized by the group of Schmutzler in the preparation of cyclic (P-anthrylphosphino)phosphonium chloride remaining in equilibrium 160a/160b in the reaction of 9-(dichlorophosphino)anthracene 18 and N-[tert-butyl(phenyl)phosphino]-N,N-dimethyl-N-(trimethylsilyl)urea 161 in CH₂Cl₂ at room temperature in a 46% yield. The existence in solution of the equilibrium between the ionic structure 160a and the covalent form 160b was observed. Next, the chloride 160a/160b was converted into the corresponding phosphonium tetraphenylborate 162 by treatment of NaBPh₄ in CH₂Cl₂/CH₃CN in a 50% yield (Scheme 60) [73].

Schmutzler and co-workers presented a synthesis of 9-(dichlorophosphino)anthracene 18 and 9-(anthrylchlorophosphino)anthracene 163 from 9-bromoanthracene 1 using a large excess of PCl₃ (19 equiv.) in a 48% yield. The resulting mixture of 18 and 163 was reduced with 5.4 equiv. of LiAlH₄ in diethyl ether at reflux to obtain 9-(dihydrophosphino)anthracene 14 and (anthrylhydrophosphino)anthracene 163. Next, pure 14 was reacted with 1 equiv. of Os₃(CO)₁₁(CH₃CN) in CH₂Cl₂ at room temperature to give triosmiumdodecacarbonyl cluster 164 quantitatively (Scheme 61) [10].

3.2. Phosphonous Acid Diamides (AnthP(NR₂)₂)

9-[Bis(diethylamino)phosphino]anthracene 166 and 9,10-bis[bis(diethylamino)phosphino]anthracene 167 were prepared by Tokitoh and co-workers starting from 9-bromoanthracenes 1 and 42, which were first lithiated with n-butyllithium and then reacted with bis(diethylamino)chlorophosphine. The resulting anthracenes 166 and 167 were transformed to 9-(dichlorophosphino)anthracene 18 and 9,10- bis(dichlorophosphino)anthracene 168 using hydrogen chloride in diethyl ether (Scheme 62) [74].

3.3. Phosphorous Acid Esters (Phosphites) (AnthOP(OR)₂)

This subsection covers P^III acids derivatives containing one anthracene moiety linked indirectly to the phosphorus atom via the P-O-Csp² (Anth) bond.

Kloß et al. conducted a study of numerous phosphite-based ligands for rhodium catalysts, which were used in hydroformylation reactions [3]. The authors revealed that anthryl phosphites were susceptible to hydrolysis, which limis their use for the synthesis of catalysts. Therefore, they synthesized relatively stable phosphites, one of which was the phosphite 169.

The latter, as a solid, was synthesized by a procedure involving the treatment of benzopinacol 170 with phosphorus trichloride to give chlorophosphite 171, followed by the addition of lithium anthr-9-olate 172 (Scheme 63).

Implemented in a rhodium catalyst, it exhibited high activity towards hydroformylation. The ligand turned out to be relatively stable under hydrolysis conditions.

3.4. Phosphonous Acid P^IV Tautomers (H-phosphinic Acids) (AnthP(O)H(OH))

The synthesis of the ester of the P^III tautomeric form of phosphonous acid, i.e., diphenyl 9-anthrylphosphonite 2, is mentioned in Section 2.

Schmutzler and co-workers presented the hydrolysis of 9-(dichlorophosphino)anthracene 18 in CH₂Cl₂ with water at room temperature, which gave anthryl-H-phosphinic acid 173 in a 77% yield (Scheme 64) [10].

Yakhvarov et al. reported the synthesis of the first example of dinuclear nickel complex 20 with the bridging anthr-9-yl-P(H)O₂ ligands. Anthr-9-yl-phosphinic acid 173 in the reaction with NiBr₂(bpy)₂ in dimethylformamide after 8 days at room temperature gave the nickel complex 174 in a 46% yield (Scheme 65) [75].

The same authors reported the formation of the first example of a neutral dinuclear cobalt complex 175 formed in the reaction of cobalt dibromide with 2,2’-bipyridine (bpy) and 9-anthrylphosphinic acid 173 (Scheme 66) [76].

Stawinski and co-workers reported a microwave-assisted (MW) synthesis of a series monoaryl-H-phosphinic acids, including anthr-9-yl-H-phosphinic acid 173 [77]. The microwave-assisted cross-coupling of 9-bromoanthracene 1 and anilinium H-phosphinate 176 was catalyzed by 3 mol% Pd₂(dba)₃ CHCl₃/Xantphos^® as a supporting ligand and was carried out in the presence of 2.5 equiv. of triethylamine. Irradiation of the mixture with a microwave (MW) for 5 min at 120 °C produced H-phosphinic acid 173 in an 82% yield (Scheme 67).

Trofimov and co-workers reported another synthesis of anthr-9-yl-phosphinic acid 173 in the reaction of 9-bromoanthracene 1 with elemental phosphorus in a superbasic medium [78]. The authors treated 1 with 3.3 equiv. of phosphorus red in DMSO and the mixture of 4.8 equiv. of KOH and H₂O as a superbase at 60 °C for 3 h (Scheme 68). In this case, the yield of 173 was only 10%.

3.5. Phosphinous Acid P^IV Tautomers (H-phosphine Oxides) (Anth₂P(O)H)

The synthesis of esters of the P^III tautomeric form of phosphinous acid, i.e., phenyl 9-anthryl(1-naphthyl)phosphinite 3 and phenyl dianthrylphosphinite 6, is mentioned in Section 2.

The 1-(phosphino)-1,4-diphenyl-1,3-butadiene moiety, incorporated with a dibenzobarrelene skeleton in 177, was synthesized by Ishii and co-workers [79]. They started the synthesis from lithiation of the starting reagent 178 with n-butyllithium to obtain the organolithium intermediate 179 followed by treatment of the latter with 9-dichlorophosphinoanthracene 18 to obtain the key precursor (Z)-1-(9-anthrylchlorophosphino)butenyne 180. Then, the dibenzobarrelene structure 181 was obtained by an intramolecular [4+2] cycloaddition reaction of 180 (Scheme 69). Further hydrolysis of 181 gave the secondary phosphine oxide 177, which exhibited a long-wavelength absorption (λ_abs = 355 nm) and emission (λ_em = 442 nm).

4. Synthesis and Reactions of Phosphonic Acids (AnthP(O)(OH)₂) and Phosphonates (AnthP(O)(OR)₂) (Anth = Anthryl)

In this section, P^IV organophosphorus-substituted acenes with one P-Csp² (Anth) bond, two P-O, and one P=O bonds are reviewed. Hence, this section includes phosphonic acids and their esters. Interestingly, no thio-and seleno phosphonic acids AnthP(X)(YH)₂ and the corresponding hetero-phosphonates AnthP(X)(YR)₂ (Anth = anthryl), (X, Y = S, Se) were reported in the review period.

The synthesis of a series of anthracenes substituted in position 2 with diethoxyphosphoryl groups was described by French and coworkers (Scheme 70) [80]. The Arbuzov reaction of 2-bromo-anthracene 150 with triethylphosphite, catalyzed by nickel bromide, proceeded in refluxing mesitylene for 20 h and led to 2-(diethoxyphosphoryl)anthracene 182. Then, the latter was transformed into the disilyl diester by treatment with bromotrimethylsilane in dichloromethane and next hydrolyzed to the free acid 183 with methanol. Dissolving 183 in an aqueous solution of a stoichiometric amount of sodium hydroxide gave the sodium salt 184 quantitatively.

Then, the authors investigated the spectroscopic properties of the obtained compounds, including the absorbance, fluorescence, and quantum yields Φ [80]. A slight blue shift was observed after the conversion of phosphonate ester into phosphonic acid and also when the phosphonic acid was converted to the corresponding sodium salt. Shifts of 53, 53, and 49 nm were observed for 182, 183, and 184, respectively. Compounds 182, 183, and 184 showed quantum yields of fluorescence Φ = 33%, 40%, and 0%, respectively. Additionally, compounds 182, 183, and 184 formed micelles in water.

Nagode and co-workers [81] synthesized 1-hydroxy-4-phenyl-2-(dimethoxyphosphoryl)anthracene 185 using α-diazophosphonate 186, phenylacetylene 187, Hantzsch ester, and tetrabutylammonium bromide (TBAB) in dichloroethane under blue LED irradiation. This reaction was conducted at room temperature for 6–8 h and the product 185 was obtained in a 70% yield (Scheme 71).

Shu et al. [82] described a new synthetic method for the preparation of 2-hydroxy-1-(dimethoxyphosphoryl)anthracene 188 in the reaction of an imine derivative of azomethine 189 and dimethyl diazophosphonate 190 in the presence of inorganic additives (Scheme 72). The reaction was carried out at 100 °C and the compound 188 was obtained in a 29% yield.

Nakamura and co-workers [83] studied the photolysis reactions of di(anthr-9-yl)metylphosphonate 191. The authors demonstrated that upon irradiation with monochromatic light at 365 nm, 191 underwent cyclization to 192 (Scheme 73), similarly to the compound 230 (see below Scheme 87).

Bessmertnykh and co-authors presented a direct synthesis of 9-(diethoxyphosphoryl)anthracene 193 bearing an amino group on the aromatic ring at the position 2 in a Hirao reaction. The authors carried out a reaction of 2-amino-9-bromoanthracene 194 with diethyl phosphite in the presence of N,N-dicyclohexylmethylamine in refluxing ethanol for 48 h and using catalytic amounts of palladium acetate (5 mol %) and triphenylphosphine (15 mol %). The outcome of this reaction depended on the stoichiometry used (Scheme 74) [84].

Leenstra and co-workers synthesized zirconium bis-(2-anthrylphosphonate) [Zr(Anth)₂] 196 by mixing zirconyl chloride with hydrofluoric acid, sodium hydroxide, and 2-naphthylphosphonic acid 183 in water for 5 days at reflux (Scheme 75) [85].

The same authors showed the readily excimer formation of zirconium bis-(2-anthrylphosphonate) [Zr(Anth)₂] 196 as a powdered solid in glycerol and its precursor, 2-anthrylphosphonic acid 183, in methanol solution (Scheme 75) [86]. The fluorescence spectrum of 196 [Zr(Anth)₂] had a broad emission band with a maximum at 448 nm in the solid. Additionally, the authors observed that [Zr(Anth)₂] 196 did not display a time-dependent quenching of the fluorescence emission, suggesting that the photodimerization reaction of the anthracene group to bianthryl did not exist in the solid state.

Zhou and coworkers [6] investigated five anthracene-based bis(phosphonic acids), of which three 197a, 197b, and 197c possessed the direct P-C_sp² bonding. They were prepared according to the literature procedures cited there (Scheme 76). Thus, 4,4-(anthracene-9,10-diyl)bis(4,1-phenylene) bis(phosphonic acid) 197a was synthesized by treatment of 1,4-dibromobenzene 198 with n-butyllithium, followed by the addition of the resulting 4-bromo-1-lithiobenzene to anthraquinone and subsequent reduction of the resulting anthryl dialcohol to give 199. Then, the Arbuzov-type reaction followed by hydrolysis with trimethylsilyl bromide (TMS-Br) of the obtained bis(phosphonate) 200 gave the bis(phosphonic acid) 197a in a 36 % yield (Scheme 76) [87]. Using this procedure and 1,3-dibromobenzene as the starting material, the authors obtained the corresponding regioisomeric 4,4′-(anthracene-9,10-diyl)bis(3,1-phenylene)bis(phosphonic acid) 197b [88]. Anthracene-9,10-bis(phosphonic acid) 197c was prepared analogously, based on the procedure by Pramanik et al. of the synthesis of the corresponding tetraethyl ester 201 (Scheme 77) [7], which was next hydrolyzed in this work with TMS-Br to give 197c. Compounds 197a and 197c exhibited red-shift fluorescence. They were deposited on a zirconium dioxide layer as triplet-triplet annihilation acceptors.

M. Pramanik et al. [7] synthesized 201 from 9,10-dibromoanthracene 42 in a 37% yield, at high temperature, using an excess of triethyl phosphite and NiBr₂ in 1,3-diisopropylbenzene (Scheme 77). The compound 201, in the form of fluorescent organic nanoparticles, was explored as a selective anticancer candidate by apoptosis-mediated cancer therapy towards U937 cells.

In another study, Yazji et al. [89] obtained 9,10-diphenyl-2,6-bis(phosphonic acids) 202a and 202b (Scheme 78) from 9,10-diaryl-2,6-dibromoanthracenes 203a and 203b which next were transformed to the corresponding 2,6-dilitio derivatives with t-butyllithium, and then reacted with diethyl phosphorochloridate to give bis(phosphonates) 204a and 204b, which were finally converted, by the hydrolysis of the diester, to the corresponding bis(phosphonic acids) 202a and 202b (Scheme 78).

The authors investigated the use of these compounds in thin films deposited on a silicon dioxide surface, acting as nucleation sites for pentacene crystallization. This study suggested that high optical anisotropy of pentacene, crystallized on such films, indicated that compounds 202a and 202b changed the silicon dioxide surface into a lattice, which induced the nucleation of pentacene. A similar study involving 202a and 202b was also conducted by Cattani-Scholz and co-workers [90], who synthesized these compounds in the same manner as the group of Yazji et al.

Kabachnik and coworkers demonstrated a synthesis of tetramethyl bis(phosphonate) 205 starting from 9,10-dibromo-anthracene 42 and dimethyl phosphite (Scheme 79) [91]. This reaction was catalyzed by palladium acetate/triphenylphosphine and was carried out under bi-phasic conditions for 30 h at 70–80 °C in acetonitrile in the presence of K₂CO₃ as a base and benzyltriethylammonium chloride (BTEAC) as a phase-transfer catalyst (PTC) to give the desired product 205 in a 70% yield.

Next, the authors transformed both phosphonate ester groups in 205 to the corresponding bis(phosphonic acid) 206 by refluxing it in an aqueous solution of hydrochloric acid for 2 h in an 80% yield (Scheme 79) [91].

Organic thin-film transistors based on pentacene as a semiconductor were fabricated on silicon. A self-assembled monolayer derived from the phosphonate (SAMP) 207a showed an improvement over monolayers using octadecylsilane and other phosphonates (Figure 5). These devices had substantially reduced trap states, on/off ratios of 10⁸, subthreshold slopes of 0.2 V/decade, and substantially uniform threshold voltages of −4.5 V across a large number of devices [92].

Good device characteristics were also measured for the monolayer 207b, in which the calculated molecular spacings were about 0.7 nm. This created channels that were on the order of the “thickness” of an aromatic π system, and which could allow intercalation of pentacene units, favoring a π-stacking motif for this first pentacene layer [93].

Tornow and co-workers synthesized SAMPs 208c–e and self-assembled organophosphonate duplexes 209c,d ensemble on nanometer-thick SiO₂-coated, highly doped silicon electrodes (Figure 5) [5].

Most of the reviewed papers in the previous sections discussed organophosphorus-substituted anthracenes that did not contain other substituents or anthracenes with a very low degree of substitution. This problem also concerns a group of phosphonates and phosphonic acids. Bałczewski et al. [70,71] recently presented a new phospho-Friedel–Crafts–Bradsher cyclization, which enabled the synthesis of highly substituted anthracenes 210. In this new reaction, (o-diacetaloaryl)arylmethylphosphonates 211 were cyclized under very mild conditions at room temperature to give 10-(dialkoxyphosphorylanthracenes 212 in a 70–98% yield. The latter were hydrolyzed to the corresponding phosphonic acids 210 in a 67–85% yield (Scheme 80).

5. Synthesis and Reactions of Phosphoric Acids and Phosphates (AnthOP(=O)(OR)₂) (R = H, alkyl, aryl)

Unlike previous sections (except Section 3.3), which reviewed the synthesis and reactions of compounds containing a phosphorus atom linked to the anthracene moiety directly by the P-Csp² (Anth) bond, this section and Section 3.3 cover anthracenes that are bonded to the phosphorus atom indirectly via an oxygen atom by the P -O-Csp² (Anth) bond. Interestingly, no hetero-analogs of phosphates were reported in the reviewed period.

Buckland and Davidson [94] investigated the photo-oxidation of 10-diethoxyphosphoryloxyanthracene 213, which represents a group of acenyl phosphates. The authors showed that in the presence of oxygen, the photochemically labile phosphate 213, dissolved in acetonitrile, could be oxidized to anthraquinone 50 and diethyl hydrogen phosphate 214 upon irradiation with a 365 nm monochromatic light (Scheme 81).

Yamashita et al. [95] conducted a study in which 1,8-dimethoxyanthr-9-ol 215 was used for the synthesis of 9-bromo-1,8-dimethoxyanthracene 216. The former was treated with diethyl phosphorochloridate in the presence of sodium hydride, yielding 9-diethoxyphosphoryloxy-1,8-dimethoxy-anthracene 217. Then, the phosphate/lithium exchange with Li_(DTBB) (DTBB = 4,4′-di-tert-butylbiphenyl) followed by bromination with 1,2-dibromo-1,1,2,2-tetrafluoroethane gave 216 as a pale-yellow solid in a 30 % yield (Scheme 82).

Another study, carried out by the group of Yamashita and co-workers [96], showed further application of the Li_(DTBB) system towards the substrate 217. When 2.5 equiv. of Li_(DTBB) was used, the reaction followed the path from Scheme 82. However, when an excess (10 equiv.) of the Li_(DTBB) reagent was used, the phosphate 217 was transformed to 1,8,9-trilithioanthracene 218, which then underwent bromination with 1,2-dibromo-1,1,2,2-tetrafluoroethane to give 1,8,9-tribromoanthracene 219 (Scheme 83).

Takizawa et. al. [4] discovered an enantioselective, oxidative-coupling reaction of anthr-2-ol 220 with the chiral vanadium complexes 221a and 221b to deliver bianthrol 222, which was then esterified with phosphoric acid to give 4,4’-bianthryl phosphoric acid 223 (Scheme 84).

The authors investigated the catalytic properties of 223 in the Diels–Alder reaction of 2-cyclohexenone with aldimines; however, it gave only racemic mixtures.

Another application of anthryl phosphates in the Diels–Alder reaction was performed by Meek and Koh [97], who described the synthesis of 10-bromo-9-(dimethoxyphosphoryloxy)anthracene 224 obtained from 10,10-dibromoanthrone 225 and trimethyl phosphite. Next, they applied 224 as a diene in the Diels–Alder reaction with acrylic acid and maleic anhydride, which delivered adducts 226 and 227, respectively (Scheme 85).

Then, the authors [97] described the reaction of anthraquinone anil 228 with trimethyl phosphite, yielding 9-dimethoxyphosphoryloxy-10-(phenylamino)anthracene 229 as yellow needles (Scheme 86).

Nakamura and co-workers [83] studied the photolysis reactions of tri(anthr-9-yl)phosphate 230. The authors demonstrated that upon irradiation with a 365 nm monochromatic light, 230 underwent cyclization to 231 (Scheme 87). Interestingly, the product 231 could be transformed back to 230 upon irradiation with a 254 nm monochromatic light.

6. Synthesis and Reactions of Diphosphenes (Anth(P=PR)) and Derivatives

In addition to anthracenes substituted by one, two, or three organophosphorus groups with one phosphorus atom in each group, which have been described in previous sections, this section reviews anthracenes substituted by organophosphorus groups containing two or three phosphorus atoms.

9-(Diphospheno)anthracenes 232 and 9,10-bis(diphospheno)anthracenes 233, presumably the first stable (diphospheno)anthracenes, were synthesized by Tokitoh and co-workers [74,98]. The 2,4,6- tris[bis(trimethylsilyl)methyl]phenyl (Tbt) and 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl (Bbt) groups, reported by Yoshifuji and co-workers, were employed for stabilization of these molecules [99]. First, 9-dichlorophosphinoanthracene 18 and 9,10-bis(dichlorophosphino)anthracene 234 were readily prepared in moderate yields from 9-bromoanthracenes 1 and 42. Next, the condensation reaction of TbtPH₂ 235a and BbtPH₂ 235b with 9-dichlorophosphinoanthracene 18 in the presence of DBU (DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene) as a base afforded 9-diphosphenoanthracenes 232a and 232b as stable red crystals in 71 and 77% yields, respectively (Scheme 88). 9,10-Bis(diphospheno)anthracenes 233a and 233b were synthesized in a ca. 20% yield in a manner similar to the synthesis of 9-diphosphenoanthracenes 232a and 232b using 9,10-bis(dichlorophosphino)anthracene 234 instead of 9-(dichlorophosphino)anthracene 18. The UV-vis spectra of 232 and 233 revealed the electronic communication between the anthryl and P=P units, which was also supported by the TD-DFT calculations. The monodiphosphene derivative 232a exhibited a weak fluorescence in hexane solution, whereas the bis(diphosphene) derivatives 233 displayed no appreciable luminescence under the same conditions. The compounds 232a,b and 233a,b showed absorption maxima at 380–400, 380–400, 403–426, and 404–427 nm, accordingly.

The same research group also examined the specific reactions of the 9-(diphospheno)anthracene 232a, including sulfonation, selenation, and attempted telluration with tributylphosphine telluride (Scheme 89) [74,100].

Thus, sulfonation of 232a gave thiadiphosphirane 236 while selenation delivered selenadiphosphirane 237 in good yields.

Surprisingly, treatment of 232a with tributylphosphine telluride did not result in telluration via the tellurium transfer. Instead, the reaction yielded triphosphirane 238 as yellow crystals and the diphosphene derivative 239 as red crystals.

7. Conclusions

In this review, covering the period 1968–2022, the synthetic methods, reactions, and applications of acenes were discussed. This review revealed that phosphorus-substituted acenes with a number of benzene rings greater than three remain unknown. This opens the way for the development of new syntheses of longer acenes and insights into the properties and novel applications of such materials. Based on the current knowledge of the properties of multi-ring fused aromatics, it can be predicted that such acenes, especially electron-tunable (P^III, P^IV, P^V) phosphorus-substituted tetracenes and pentacenes, will find more effective applications than lower analogs, mainly in optoelectronics. In particular, solid-anchored phosphonic acids that form monolayers may provide an example of such an application [5,92,93].

The second characteristic of the reviewed compounds was the low degree of substitution of aromatic rings by other substituents than organophosphorus groups and, in particular, most of anthracene moieties were unsubstituted. Apart from our preliminary work [70,71], this review showed, practically, a lack of works devoted to the highly substituted acene systems. It is noteworthy that highly substituted acenes containing thioorganic substituents showed extremely high thermal and photochemical stabilities, properties that would be interesting to verify for acenes with organophosphorus substituents [101,102]. Furthermore, the absence of hetero(S, Se)-analogs of phosphonates and phosphates was recorded during the reviewed period, which additionally opens the way for further research in this area.