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Article

Copper-Catalyzed Sonogashira-Type Coupling Reaction of Vinylacetylene ortho-Carborane with Boronic Acid in the Synthesis of Luminophores with Phosphorescent Emission

by
Timofey D. Moseev
1,*,
Tair A. Idrisov
1,
Mikhail V. Varaksin
1,2,*,
Anton N. Tsmokaluk
1,
Valery N. Charushin
1,2 and
Oleg N. Chupakhin
1,2
1
Department of Organic and Biomolecular Chemistry, Ural Federal University, 19 Mira Street, 620062 Ekaterinburg, Russia
2
Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, 22 S. Kovalevskaya Street, 620990 Ekaterinburg, Russia
*
Authors to whom correspondence should be addressed.
Reactions 2024, 5(4), 868-882; https://doi.org/10.3390/reactions5040046
Submission received: 6 October 2024 / Revised: 29 October 2024 / Accepted: 5 November 2024 / Published: 7 November 2024
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Abstract

A synthetic approach to prepare boron-enriched π-conjugated photoactive molecular systems based on ortho-carborane using the Cu(I)-catalyzed Sonogashira-type coupling reaction has been developed. The obtained luminophores have been found to possess absorption in the range of 300 to 400 nm, emission of up to 700 nm, and photoluminescence quantum yields of up to 99% in non-polar solvents. TD-DFT calculations have demonstrated that the luminophores are characterized by phosphorescent emission behavior with a lifetime of about 7 μs. In addition, the rigidochromism for the synthesized compounds has been revealed; particularly, the transition electronic state and bathochromic shift have been elucidated in the emission spectra. The exhibited luminescent characteristics indicate that the elaborated vinylcarborane fluorophores could be considered as promising building blocks in the design of advanced photofunctional materials for molecular electronics.

Graphical Abstract

1. Introduction

Carboranes are known to be cluster-type compounds containing several carbon and boron atoms located at the vertices of convex polyhedra with triangular faces. The most numerous are closo-carboranes, which are characterized by their unique thermal and chemical stability, spatial, structural, and electronic organization [1,2,3]. Due to their physical and chemical properties, a number of ortho-carborane-based compounds have been explored for applications as light-emitting diodes (OLEDs, PhOLEDs), electrochemical cells (LECs) [4,5,6,7,8,9,10,11,12], organic transistors (OFETs and OTFTs) [13,14,15,16], photoelectric converters (PV cells) [17,18,19,20,21,22], chemosensors [23,24,25,26,27], photosensitizers [21,28,29,30,31], catalysts [32,33,34], and other molecular electronics materials (Figure 1).
The majority of organic luminophores that are commonly utilized in molecular electronics devices are reported to be composed of conjugated π-systems or transition metal complexes. Additionally, highly efficient ones are considered to be structures containing both electron-donating (EDG) and electron-withdrawing (EWG) groups connected to each other by π-linkers. Examples of such systems are D-π-A, A-π-D-π-A, D-π-A-π-D, etc., where D is EDG, π is a π-linker, and A is EWG [35,36,37,38,39,40,41,42,43,44,45,46,47]. It should be noted that one can design target compounds with specific properties, such as emission, absorption, or other useful properties, that are tailored to the particular material or device by modifying the donor/withdrawing strength or π-conjugation [48,49,50,51,52]. According to the recent literature sources, these systems are widely known as push–pull fluorophores and are most often characterized by the intramolecular charge transfer (ICT) effect and other ones, such as PET (photoinduced electron transfer), TICT (twisted intramolecular charge transfer), AIE (aggregation-induced emission), AIEE (aggregation-induced emission enhancement), and TADF (thermally activated delayed fluorescence). The described opportunities inspire the further development of numerous advanced materials based on the push–pull molecular systems [53,54,55,56,57,58,59].
In the design of organic luminophores for practical applications, the C-substituted o-carborane derivatives with extended π-conjugation systems are becoming increasingly attractive. It is worth noting that the negative inductive effect of the o-carboranyl moiety acquires the EWG properties of the whole photoactive systems. Moreover, the boron cluster provides conformational and steric hindrance to organic molecules, which sometimes determine their specific and useful properties, as well as can impart them with a thermal, electrochemical, and photolytical stability [60].
In our previous work [61], the CuAAC (Cu-Catalyzed Alkyne-Azide Cycloaddition) reaction was used in the synthesis of 1H-1,2,3-triazole-derived vinylcarborane fluorophores characterized by AIEE and ICT effects. The emission was in the blue region (407 nm) and quantum yields were not determined. In this work, a synthetic strategy of the Cu(I)-catalyzed and Ag(I)-promoted Sonogashira-type cross-coupling reaction was applied to the vinylacetylene o-carborane with boronic acids to produce novel push–pull fluorophores systems, with comprehensive photophysical properties being studied. (Figure 2).

2. Materials and Methods

Nuclear Magnetic Resonance (NMR) spectra were recorded on a Bruker Avance II (400 MHz). All 1H NMR experiments were reported in δ units, parts per million (ppm), and were measured relative to a residual chloroform CDCl3 (7.26 ppm) signal in the deuterated solvent. All 11B NMR spectra were reported in ppm relative to BF3·Et2O (0 ppm). All 13C NMR spectra were reported in ppm relative to CDCl3 (77.16 ppm) and all spectra were obtained with 1H decoupling. All coupling constants J were reported in Hertz (Hz). The following abbreviations were used to describe peak splitting patterns (s = singlet, d = doublet, t = triplet, q = quartet, dd = doublet of doublet, m = multiplet, and br s = broad singlet). The mass spectra were recorded on a SHIMADZU GCMS-QP2010 Ultra mass spectrometer (SHIMADZU, Kyoto, Japan) with sample ionization by electron impact (EI). The IR spectra were recorded using a Fourier-transform infrared spectrometer (Bruker Corporation, Billerica, MA, USA)equipped with a diffuse reflection attachment. The elemental analysis was carried out on a Perkin Elmer Instrument equipped with a PE 2400 II CHN-analyzer. The course of the reactions was monitored by TCL on 0.25 mm silica gel plates (60F 254). UV-vis absorption spectra were recorded on a Shimadzu UV-1800 spectrophotometer (SHIMADZU, Kyoto, Japan). The molar absorption coefficients maxima were determined using the Bouguer–Lambert–Beer law. UV-vis emission spectra and phosphorescence decay kinetic curves were recorded on a Horiba FluoroMax-4 spectrofluorimeter (HORIBA Scientific, Piscataway, NJ, USA). The absolute quantum yield was determined using an integrating sphere.
1-(But-1-en-3-ynyl)-1,2-dicarba-closo-dodecaborane (1a) was synthesized according to the published procedure [62]. Copper iodide, cesium carbonate, argentum oxide, triethylamine, potassium carbonate, argentum carbonate, 1,3,5-trimethoxybenzene, copper chloride, 1,10-phenanthroline, dichloroethane (DCE), (4-(diphenylamino)phenyl)boronic acid (2a), (9-ethyl-9H-carbazol-3-yl)boronic acid (2b), (3,4,5-trimethoxyphenyl)boronic acid (2c), pyren-1-yl-boronic acid (2d), hexane, and ethyl acetate were purchased from commercial sources.
Solvents for optical studies include the following: cyclohexane (CyH), toluene (Tol), dichloromethane (DCM), acetonitrile (MeCN), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), dimethyl sulfoxide (DMSO), and methanol (MeOH) were purchased at pure or extra pure quality and used without additional purification.
The general procedure for the synthesis of push–pull luminophores 3a–3d is as follows.
(But-1-en-3-ynyl)-o-carborane (100 mg, 0.5 mmol, 1 equiv), boronic acid (1.1 equiv), cesium carbonate (335 mg, 1 mmol, 2 equiv), copper (I) iodide (15 mg, 0.08 mmol, 0.15 equiv), silver (I) oxide (238 mg, 1 mmol, 2 equiv), and DCE (5 mL) were added to a round-bottom flask (10 ml), equipped with reflux condenser, heated to 85 °C in an oil bath with magnetic stirring, and refluxed for 12 h. The cooled to room temperature reaction mixture was directly subjected to silica gel (0.04–0.063 mm). The desired compound was obtained by silica gel column chromatography using hexane/EtOAc (from 9.5/0.5 to 8/2) as an eluent.
(E)-(4-(4-(Diphenylamino)phenyl)but-1-en-3-yn-1-yl)-o-carborane (3a)
The general procedure was applied using 4-(diphenylamino)phenyl boronic acid (164 mg, 0.55 mmol, 1.1 equiv).
Yellow powder. Yield 121 mg (0.275 mmol, 55%). mp = 231–233 °C. Rf 0.60 (hexane/EtOAc 19/1). 1H NMR (400 MHz, CDCl3): δ 7.35–7.20 (m, 6H), 7.15–7.05 (m, 6H), 7.00–6.95 (m, 2H), 6.25–6.21 (d, 1H, HC=, 3J = 15.50 Hz), 6.21–6.15 (d, 1H, =CH, 3J = 15.54 Hz), 3.67 (s, 1H, –CH); 3.00–1.60 (m, 10H, B10H10) ppm. 13C {1H} NMR (100 MHz, CDCl3): δ 148.9, 148.0, 147.0, 133.3, 132.8, 129.6, 129.3, 125.5, 124.3, 124.1, 123.5, 122.8, 121.6, 118.9, 114.3, 95.9 (≡C–), 84.5 (–C≡), 73.3 (–C–), 60.7 (–CH) ppm. NMR 11B (128 MHz, CDCl3): δ 0–(−6.5) (2B), −6.5–(−21.0) (8B) ppm. Calculated m/z for [M]+: 438. Found m/z for [M]+ (EI-qMS): 438. Calculated (%) for C24H27B10N: C, 65.87; H, 6.22; N, 3.20. Found (%): C, 65.97; H, 6.25; N, 3.36.
(E)-(4-(9-Ethyl-9H-carbazole-3-yl) but-1-en-3-yn-1-yl)-o-carborane (3b)
The general procedure was applied using 9-ethyl-9H-carbazole-3-boronic acid (135 mg, 0.55 mmol, 1.1 equiv).
Yellow powder. Yield 82 mg (0.21 mmol, 42%). mp = 96–98 °C. Rf 0.39 (hexane/EtOAc 8/2). 1H NMR (400 MHz, CDCl3): δ 8.49 (s, 1H), 8.36 (d, 1H, 3J = 7.90 Hz), 7.85–7.80 (m, 1H), 7.80–7.77 (m, 1H), 7.75–7.68 (m, 1H), 7.67–7.63 (m, 1H), 7.60–7.52 (m, 1H), 6.60–6.54 (d, 1H, HC=, 3J = 15.58 Hz), 6.54–6.48 (d, 1H, =CH, 3J = 15.56 Hz), 4.67 (q, 2H, CH2, 3J = 7.27 Hz), 3.96 (s, 1H, –CH), 3.50–1.60 (m, 10H, B10H10), 1.74 (t, 3H, CH3, 3J = 7.22 Hz) ppm. 13C {1H} NMR (100 MHz, CDCl3): δ 140.5, 140.2, 133.0, 129.4, 129.5, 126.5, 125.7, 123.1, 122.5, 120.7, 119.8, 119.1, 112.0, 108.8, 97.2 (≡C–), 83.7 (–C≡), 73.4 (–C–), 60.8 (–CH), 37.8 (CH2), 13.9 (CH3) ppm. NMR 11B (128 MHz, CDCl3): δ 0–(−6.5) (2B), −6.5–(−21.0) (8B) ppm. Calculated m/z for [M]+: 387. Found m/z for [M]+ (EI-qMS): 387. Calculated (%) for C20H25B10N: C, 61.99; H, 6.50; N, 3.61. Found (%): C, 62.24; H, 6.68; N, 3.60.
(E)-(4-(3,4,5-Trimethoxyphenyl)but-1-en-3-yn-1-yl)-o-carborane (3c)
The general procedure was applied using 3,4,5-trimethoxyphenylboronic acid (120 mg, 0.55 mmol, 1.1 equiv).
Brown powder. Yield 42 mg (0.115 mmol, 23%). mp = 169–173 °C. Rf 0.46 (hexane/EtOAc 9/1). 1H NMR(400 MHz, CDCl3): δ 6.66 (s, 2H), 6.25–6.21 (d, 1H, HC=, 3J = 15.75 Hz), 6.21–6.16 (d, 1H, =CH, 3J = 15.76 Hz), 3.85 (s, 9H, OCH3), 3.65 (s, 1H, –CH), 3.10–1.50 (m, 10H, B10H10) ppm. 13C {1H} NMR (100 MHz, CDCl3): δ 153.3, 139.8, 134.3 (HC=), 118.5 (=CH), 117.0, 109.1, 95.3 (≡C–), 84.3 (–C≡), 73.0 (–C–), 61.1 (–CH), 56.3 (OCH3) ppm. NMR 11B (128 MHz, CDCl3): δ 0–(−6.5) (2B), −6.5–(−21.0) (8B) ppm. Calculated m/z for [M]+: 361. Found m/z for [M]+ (EI-qMS): 361. Calculated (%) for C15H24B10O3: C, 49.98; H, 6.71; O, 13.32. Found (%): C, 50.17; H, 6.87.
(E)-(4-(Pyrene-1-yl)but-1-en-3-yn-1-yl)-o-carborane (3d)
The general procedure was applied using pyrene-1-boronic acid (139 mg, 0.55 mmol, 1.1 equiv).
Yellow powder. Yield 85 mg (0.215 mmol, 43%). mp = 215–218 °C. Rf 0.15 (hexane/EtOAc 19/1). 1H NMR (400 MHz, CDCl3): δ 8.47 (d, 1H, 3J = 9.09 Hz), 8.22–8.00 (m, 8H), 6.48–6.42 (d, 1H, HC=, 3J = 15.55 Hz), 6.42–6.37 (d, 1H, =CH, 3J = 15.53 Hz), 3.72 (s, 1H, –CH), 3.00–1.60 (m, 10H, B10H10) ppm. 13C {1H} NMR (100 MHz, CDCl3): δ 134.3, 132.3, 132.1, 131.3, 131.1, 129.9, 128.9, 127.5, 127.3, 126.6, 126.1, 125.2, 125.1, 124.7, 124.5, 124.3, 118.8, 116.4, 94.5 (≡C–), 90.7 (–C≡), 73.2 (–C–), 60.7 (–CH) ppm. NMR 11B (128 MHz, CDCl3): δ 0–(−6.5) (2B), −6.5–(−21.0) (8B) ppm. Calculated m/z for [M]+: 395. Found m/z for [M]+ (EI-qMS): 395. Calculated (%) for C22H22B10: C, 66.98, H, 5.62. Found (%): C, 66.06; H, 6.91.

3. Results and Discussion

For the synthesis of novel π-conjugated systems based on the vinylacetylene o-carborane, a Cu (I)-catalyzed coupling of terminal alkynes with arylboronic acids was chosen [63]. It was found that the reaction of vinylacetylene o-carborane 1a with arylboronic acids 2a–2d led to the formation of the corresponding coupling products 3a–3d. The synthesized compounds are trans isomers confirmed by the double bond vicinal protons spin–spin interaction constants (3J ≈ 16 Hz). The reaction mechanism comprises an oxidative addition, transmetalation, and reduction eliminative stage; the latter has been extensively investigated with regard to this type of coupling [64,65,66,67,68]. Thus, four novel compounds have been obtained containing (hetero)aromatic building blocks as electron-donating groups (Figure 3).
First, we explored the optimal conditions for the Cu (I)-catalyzed arylation reaction. The coupling between 1a, p-(diphenylamino)phenylboronic acid 2a was chosen as a model reaction (Table 1). Initially, the desired product was obtained in a 55% yield under the following conditions: CuI (15 mol %), Cs2CO3 (2 equiv), Ag2O (2 equiv), DCE (0.1 M), and reflux for 12 h. The utilization of other bases, oxidizing agents, higher boiling solvents, and ligands resulted in a lower yield of 3a (20–48%). It is noteworthy that in some instances, deborylation processes were observed, which could potentially contribute to the reduced yield of the desired product 3c. On account of the C-substituted o-carboranyl fragment, its alkenyl derivatives could be considered as an electron-deficient system and the traditional Sonogashira cross-coupling conditions, namely Pd and Cu dual catalysis, are not eligible for obtaining the desired products. However, the very similar Sonogashira-type reaction conditions with copper catalysis and argentum oxide as an oxidant turned out to be effective to afford the novel photoactive conjugated system based on o-carborane.
All novel compounds were fully characterized by 1H, 1H {11B}, 13C {1H}, 11B, 11B {1H} NMR, mass spectrometry, IR, and elemental analysis; the proposed structures are fully matched with analysis results.
To assess the photophysical properties, the absorption and emission spectra in solvents of different polarities (cyclohexane, toluene, dichloromethane, 2-methyltetrahydrofuran, tetrahydrofuran, acetonitrile, dimethylsulfoxide, methanol), the rigidochromism phenomenon, quantum yields, and lifetimes have been investigated. The absorption spectra of compound 3a exhibit two broad absorption bands in the regions 290–300 nm and 350–380 nm with intensities up to 0.2 and 0.4 a.u., respectively (Figure 4, Table 2). Luminophore 3b has three absorption maxima in the same region as 3a (ESI, Figure S22). For compound 3c, one intense band was observed in the range of 300–325 nm (ESI, Figure S23). The most intricate spectral pattern was discovered for compound 3d, which has two groups of bands in the region of 275–300 nm and 360–400 nm, respectively (Figure 4).
The molar absorption coefficients for all studied compounds significantly exceed 10,000 cm−1·M−1, which is typical for π→π* transitions in π-conjugated systems. For compounds 3a and 3c, a blue shift of the absorption maxima was detected with increasing solvent polarity from 374 to 361 and from 325 to 302 nm in cyclohexane and methanol for 3a and 3c, respectively.
The emission spectra in the solution of 3a–3d were also studied in different solvents. Under the photoexcitation by λex = 280–400 nm (UV-A and UV-B), several bands have been discovered; in particular, narrow bands in the NUV region, wide intense bands in the violet and blue region (λem = 350–450 nm), narrow bands in the red region (λem = 630–700 nm), and broad intensity bands in the NIR region (Figure 5 and Figures S29–S39) were discovered. In particular, one broadened emission band was observed for 3a in polar solvents at 380–450 nm, whereas in non-polar solvents, it was localized at 460–660 nm and 540–740 nm in the cases of cyclohexane and toluene, respectively (Figure 5, top left). At the same time, several emission bands were detected in the region 370–410 nm for 3b; it most likely might be associated with singlet and triplet transition states. Furthermore, broadened bands appeared in the region of 500–650 nm in solutions of cyclohexane and toluene, similar to 3a (Figure 5, top right). In the spectra of compound 3c, a few narrow and broad absorption bands were revealed in the region from 400 to 700 nm, while an additional broadened emission band with a maximum at 540 nm was found in cyclohexane (Figure 5, bottom left). Lastly, only one broad emission band from 400 to 750 nm was revealed in the emission spectra of 3d in all solvents. It is worth noting that in polar solvents (MeCN, DMSO, MeOH) the band was localized at 420–480 nm, whereas in low-polar solvents (cyclohexane, toluene) one was at 520–600 nm. The significant red shift was observed in solvents of medium polarity (DCM, THF), in which one was located from 700 to 750 nm (Figure 5, bottom right).
To evaluate the luminescence efficiency, the absolute quantum yield has been measured using the integrating sphere (Table 3). Compounds 3a and 3d have the highest quantum yields of 66% and 99%, respectively, in cyclohexane, whereas the highest quantum yields of 20 and 23% for 3b and 3c, respectively, are revealed in acetonitrile. It is also worth noting that AIE, AIEE, and chemosensory properties have not been found.
To reveal the phenomenon of rigidochromism, emission spectra of synthesized substance solutions in 2-MeTHF have been measured at 298 and 77 K (Table 4 and Figure 6). A significant number of fluorescent dyes that exhibit one [69,70,71,72,73] operate through a mechanism known as the restriction of intramolecular motions (RIM). This means that when the local environment becomes more rigid, the molecule is unable to adopt conformations that allow it to reach the ground state through non-radiative pathways. The RIM effect most likely occurred; namely, the splitting of peaks into narrower ones was observed in all cases. Moreover, 3a, 3c, and 3d exhibit slightly bathochromic shifts, and new bands at 570 nm were detected for 3b and 3d at 77 K. The highest observed peak was in the case of 3a and 3c at 400–450 nm, which may be explained by the dominance of the ICT over the LE state. The main emission band of 3b was specified at 77 K; this indicated several singlet states (S0-S1, S0-S2, etc). Further, a novel widened band at 570 nm has appeared. Lastly, a bathochromic shift of 50 nm and a novel widened band at 550 nm have been observed in the case of 3d, which may be related to excimer emission.
It should be noted that there are several examples of similar photoactive systems in the literature. In the study [74], authors have synthesized compounds similar to 3a–3b and 3d, in which p-phenylene acts as a π-linker. Multiple emissions were observed in non-polar solvents, where the short-wave emission (350–500 nm) was due to the LE states of the donor units and the long-wave emission (500–700 nm) was ascribed to the CT emissions. Molecules 3a–3b and 3d exhibit a red shift in the absorption spectra at approximately 25–50 nm, whereas the emission spectra exhibit the same pattern. In the other study [75], authors obtained a series of tolane-o-carborane dyads, several of which demonstrated the LE/TICT dual emission and thermochromic luminescence effects. The absorption and emission spectra of 3c and the reference compounds bearing a 1,2,3-trimethoxybenzene fragment are very similar. Both compounds exhibit dual emission properties in 450–700 nm region, but 3c has a phosphorescent emission, while another one exhibits the TICT effect. The absorption spectra of o-carboranyl pyrene compounds [26] with ethynyl and vinylacetylene linkers are similar, but the signals of 3d are red-shifted by ~20 nm. Dual emission with a characteristic band in the region of 500–700 nm in the reference compound was observed only under rigid conditions (2-MeTHF solutions, 77 K); in our case, rigidochromism was observed for all compounds. A quantum yield of 1-(o-carboranylethynyl)pyrene is <1%, as opposed to 3d, for which it is 99% in cyclohexane, 9% in toluene, and 1–2% in THF, DMSO, and MeCN. This indicates that the molecular configuration of the compound bearing the vinylacetylene spacer facilitates the occurrence of a distinctive broadband emission in the visible region to a greater extent than that with the ethynyl spacer.
As mentioned above, the C-substituted o-carboranyl moiety is engaged by EWG properties of the whole molecular system [60]. To take these data into account, as well as the presence of a π-linker and an EDG, we assumed that the designed compounds could be considered as push–pull fluorophores with the intramolecular charge transfer (ICT) effect. However, the solvatochromic studies did not yield the anticipated results for such photoactive systems; namely, the polar environment led to a longwave emission. In particular, the appearance of broadened emission bands, which could be associated with the ICT state, was observed exclusively in non-polar solvents and was not detected in others.
To understand the nature of the excited state, we have employed the DFT and time dependent DFT (TD-DFT) calculations for molecules 3a and the visualization of molecular orbitals (HOMO-2, HOMO-1, HOMO, LUMO, and LUMO+1) for the other (Figures S41–S56). The HOMO (highest occupied molecular orbital) mainly resides on the triphenylamine moiety and partly on the vinylacetylene moiety. Its LUMO (lowest unoccupied molecular orbital) was distributed over the o-carboranyl cluster and also partly on the vinylacetylene moiety (Figure 7). Thus, the HOMO-LUMO gap is around ΔE = −0.25365 eV (for detailed molecular orbital energy, see Table S6). Moreover, an additional calculation of the molecule, in which o-carborane is replaced by hydrogen, has been carried out. It turned out that the main contribution to the HOMO is formed on the triphenylamine fragment, while the main contribution to the LUMO is distributed between one phenyl ring and the conjugated system (ESI, Figures S57–S58). In our case, since o-carborane exhibits EWG properties, there is a small contribution to LUMO.
The TD-DFT/wB97XB/6-311++g(d,p) method in toluene media has been used to understand the nature of the absorption spectra of 3a, on the basis of the ground state (S0) optimized geometries shown in Table 5. One can observe that the calculated absorbance band for 372 nm is in excellent agreement with the experimentally observed value of 373 nm (ESI, Figure S40). The S1 state is mainly composed on the HOMO → LUMO transition. Subsequently, to understand the nature of the emission spectra, the TD-DFT approach was used to calculate the fluorescence and the phosphorescent spectra using the optimized structure of S1 and T1, respectively. The obtained results have revealed that the calculated triplet excited state energies of 3a are closer to the experimental results. The lowest-lying triplet states (T1) of 3a originate from the HOMO → LUMO (69%) and HOMO → LUMO (26%) transitions, respectively (for the full calculation see ESI, Table S7). The calculated emission wavelength of 3a is 625 nm, which is in close agreement with an experimental wavelength of 670 nm, whereas the lowest S0 → S1 transition 3a is located at 325 nm. Thus, the absence of typical emission behavior for push–pull fluorophores might be explained by the fact that the compound has a phosphorescent emission, which has additionally been supported by calculation.
To determine phosphorescence lifetimes, we have studied the kinetics of the most intense long-wave visible emission: for 3a—λem = 569 nm at λex = 296 nm, for 3b—λem = 536 nm at λex = 351 nm, for 3c—λem = 532 nm at λex = 313 nm in cyclohexane, and for 3d—λem = 613 nm at λex = 299 nm in toluene. The decay of phosphorescence of compounds 3a–3d could be described by a single-exponential dependence with lifetime τ ~ 7 μs (Table 6). It should be noted that the fluorescence lifetime for the obtained compounds were found to be from 4 to 7 ns (ESI, Table S5).

4. Conclusions

In summary, the Cu (I)-catalyzed coupling strategy of vinylacetylene ortho-carborane with boronic acids has successfully been applied to afford a series of novel photoactive boron-enriched π-conjugated molecular systems. The elaborated photoluminescent compounds are characterized by an absorption in the range from 300 to 400 nm with molar absorption coefficient maxima over 103 cm−1·M−1 and emissions of up to 700 nm with various complexity band patterns. The performed solvatochromism studies have indicated that these compounds do not exhibit the characteristics typically associated with push–pull fluorophores. TD-DFT calculations have shown that the compounds have typical phosphorescent emission behavior; namely, the phosphorescence lifetime is around 7 μs, and the quantum yields are up to 99%. In addition, the rigidochromism phenomenon has been revealed; in particular, both the transition electronic state and bathochromic shift are observed in the emission spectra. Thus, the elucidated luminescent characteristics indicate that the elaborated compounds have the potential to be used in the design of advanced photofunctional materials for molecular electronics.

Supplementary Materials

The following supporting information can be download at: https://www.mdpi.com/article/10.3390/reactions5040046/s1, Figures S1–S20: Copies of NMR spectra; Figures S21–S28: Copies of absorbance spectra; Figures S29–S39: Copies of emission spectra; Figure S40: Simulated UV-Vis Spectrum of 3a; Figures S41–S58: Visualization of molecular orbitals; Tables S1–S4: Emission maxima of compounds 3a–3d; Table S5: Results of short-lived emission decay kinetic curves approximation for solutions of compounds 3a–3d; Table S6: Energies of molecular orbitals of 3a; Table S7: Results of the TD-DFT calculations.

Author Contributions

Conceptualization, T.D.M. and M.V.V.; methodology, T.D.M. and T.A.I.; software, A.N.T.; validation, O.N.C.; formal analysis, A.N.T.; investigation, T.A.I.; writing—original draft preparation, T.D.M. and T.A.I.; writing—review and editing, M.V.V. and V.N.C.; supervision, V.N.C. and O.N.C.; funding acquisition, T.D.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Russian Science Foundation, Project № 24-13-20023, https://rscf.ru/en/project/24-13-20023/ (accessed on 29 October 2024).

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Ortho-carborane-based functional materials.
Figure 1. Ortho-carborane-based functional materials.
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Figure 2. Previous and current work.
Figure 2. Previous and current work.
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Figure 3. Synthesis of conjugated push–pull fluorophores based on ortho-carboranes via copper-catalyzed Sonogashira-type coupling.
Figure 3. Synthesis of conjugated push–pull fluorophores based on ortho-carboranes via copper-catalyzed Sonogashira-type coupling.
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Figure 4. Absorbance spectra of 3a (left) and 3d (right). Sample preparation: C = 1 × 10−5 M in corresponding solvent at room temperature.
Figure 4. Absorbance spectra of 3a (left) and 3d (right). Sample preparation: C = 1 × 10−5 M in corresponding solvent at room temperature.
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Figure 5. Emission spectra of 3a–3d. Rayleigh scattering peaks are removed. Excitation (λex) wavelength was at the absorption maximum.
Figure 5. Emission spectra of 3a–3d. Rayleigh scattering peaks are removed. Excitation (λex) wavelength was at the absorption maximum.
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Figure 6. Emission spectra at different temperatures: 3a (λex = 300 nm) (top left), 3b (λex = 305 nm) (top right), 3c (λex = 308 nm) (bottom left), 3d (λex = 296 nm) (bottom right). Rayleigh scattering peaks are removed.
Figure 6. Emission spectra at different temperatures: 3a (λex = 300 nm) (top left), 3b (λex = 305 nm) (top right), 3c (λex = 308 nm) (bottom left), 3d (λex = 296 nm) (bottom right). Rayleigh scattering peaks are removed.
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Figure 7. Visualization of HOMO and LUMO for molecule 3a with isovalue = 0.04.
Figure 7. Visualization of HOMO and LUMO for molecule 3a with isovalue = 0.04.
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Table 1. Optimization of the reaction conditions for the synthesis of 3a.
Table 1. Optimization of the reaction conditions for the synthesis of 3a.
EntryDifference from Standard Conditions 1Yield 2 (%)
1None60 (55 3)
2Reflux 5 h instead of 12 h29
3NEt3 instead of Cs2CO3<1
4K2CO3 instead of Cs2CO320
5Ag2CO3 instead of Ag2O36
6CuCl instead of CuI45
7Toluene instead of DCE42
8Addition of Pd(OAc)2 (15 mol%)20
9Addition of 1,10-phenanthroline (15 mol%)34
10CuI (0.05 mol% instead of 0.15 mol%)31
112a (2 mmol instead of 1.1 mmol)58
12Inert atmosphere (Argon) instead of air51
134-(Diphenylamino)phenylboronic acid pinacol ester instead of 2a57
1 Standard conditions: 1a (1 mmol), 2a (1.1 mmol), CuI (15 mol %), Ag2O (2 mmol), Cs2CO3 (2 mmol), DCE (0.1 M, reflux 12 h, air). 2 1H NMR yield using 1,3,5-trimethoxybenzene as internal standard. 3 Isolated yield.
Table 2. Absorption maxima (λabs (nm), ε (cm−1·M−1)) 3a–3d.
Table 2. Absorption maxima (λabs (nm), ε (cm−1·M−1)) 3a–3d.
Entry 1CyHTolueneDCM2-MeTHFTHFDMSOMeCNMeOH
3a296 (16,799), 374 (26,151)296 (16,000), 373 (29,700)293 (8200), 372 (13,400)294 (21,800), 369 (29,400)294 (20,800), 367 (31,400)296 (21,000), 367 (30,400)293 (18,500), 363 (35,000)294 (11,800), 361 (19,000)
3b278 (23,845), 304 (29,112), 333 (26,120), 351 (30,267)306 (30,700), 336 (26,400), 351 (27,600)270 (21,800), 279 (25,500), 305 (32,300), 336 (27,200), 348 (27,800)304 (45,800), 335 (36,800), 346 (37,100)278 (24,900), 304 (31,900), 334 (26,300), 346 (26,500)280 (25,700), 305 (30,900), 337 (24,600), 347 (24,500)278 (28,800), 302 (38,200), 334 (30,200), 343 (29,600)262 (27,500), 269 (27,000), 277 (25,100), 302 (33,900), 332 (26,600), 343 (25,700)
3c313 (18,886), 325 (16,445)312 (24,600)310 (17,300)308 (29,200)311 (19,000)309 (17,700)306 (23,900)302 (26,700)
3d273 (20,000), 284 (28,700), 297 (35,700), 367 (35,500), 387 (28,600), 397 (36,100)286 (33,912), 299 (37,931), 369 (40,058), 387 (32,969), 398 (34,580)275 (29,100), 285 (40,700), 297 (50,600), 367 (48,500), 387 (42,600), 396 (38,800)296 (45,800), 366 (45,400), 386 (40,300), 395 (33,000)284 (27,300), 296 (34,800), 366 (34,700), 386 (31,100), 395 (25,700)276 (21,400), 286 (29,800), 298 (37,100), 369 (34,900), 388 (30,500), 396 (28,400)283 (29,200), 294 (38,000), 364 (35,500), 384 (33,700), 393 (22,800)262 (15,800), 269 (18,300), 282 (22,700), 294 (28,300), 364 (28,000), 384 (26,000), 393 (17,200)
1 Sample preparation: C = 1 × 10−5 M in corresponding solvent, at room temperature.
Table 3. Emission maxima (nm) and absolute quantum yield (%) of 3a–3d.
Table 3. Emission maxima (nm) and absolute quantum yield (%) of 3a–3d.
№ 1CyHTolueneDCMTHFDMSOMeCNMeOH
3a405, 567, 797412, 667, 800403, 810397, 410, 802412, 806408, 795403, 811
ϕ, %6310<0.1<0.1141213
3b333, 370, 387, 404, 530, 667, 691, 777337, 383, 615, 674, 772336, 373, 390, 783334, 391, 406, 633, 669, 785336, 392, 673, 786332, 373, 391, 408, 633, 651, 665, 787333, 388, 665, 781
ϕ, %2<0.1137206
3c345, 373, 532, 651, 690, 755365, 382, 643, 767342, 405, 687, 799342, 367, 384, 514, 647, 686, 775341, 405, 633, 682, 798382, 781332, 388, 666, 790
ϕ, %<0.114<0.1623<0.1
3d424, 507, 803430, 607416, 449, 728, 817420, 444, 713419, 556417, 428464
ϕ, %999<0.1221<0.1
1 Sample preparation: C = 1 × 10−5 M, room temperature. Excitation (λex) wavelength was at the absorption maximum.
Table 4. Emission maxima in 2-MeTHF at different temperatures.
Table 4. Emission maxima in 2-MeTHF at different temperatures.
EntryTemperature
298 K77 K
3a 1397, 413, 798406, 650, 819
3b 2355, 372, 715, 780342, 346, 365, 368, 384, 395, 405, 418, 431, 578, 741, 769, 790
3c 3350, 712, 770, 784353, 371, 405, 418, 432, 756
3d 4423, 446, 689424, 577
1–4 Excitation wavelength λex: 1 369, 2 305, 3 308, 4 395 nm.
Table 5. TD-DFT calculations for 3a.
Table 5. TD-DFT calculations for 3a.
StateTypeTransitionContribution
S0 → S1AbsHOMO-1 → LUMO3%E = 3.3241 eV
λ = 373 nm
f = 1.4384
HOMO-1 → LUMO + 42%
HOMO → LUMO85%
HOMO → LUMO + 45%
S0 → S2AbsHOMO-1 → LUMO + 13%E = 4.1238 eV
λ = 300 nm
f = 0.0170
HOMO → LUMO + 175%
HOMO → LUMO + 24%
HOMO → LUMO + 54%
S0 → S3AbsHOMO-1 → LUMO + 39%E = 4.4154 eV
λ = 280 nm
f = 0.2145
HOMO → LUMO + 33%
HOMO → LUMO + 479%
S0 → T1EmHOMO-1 → LUMO26%E = 1.9824 eV
λ = 625 nm
HOMO → LUMO69%
S0 → T2EmHOMO-1 → LUMO20%E = 3.2471 eV
λ = 381 nm
HOMO → LUMO14%
HOMO → LUMO + 338%
S0 → T3EmHOMO-1 → LUMO + 217%E = 3.7894 eV
λ = 327 nm
HOMO → LUMO + 129%
HOMO → LUMO + 222%
S0 → S1EmHOMO-1 → LUMO4%E = 3.8067 eV
λ = 325 nm
HOMO → LUMO86%
HOMO → LUMO + 44%
S0 → S2EmHOMO → LUMO + 176%E = 4.1189 eV
λ = 301 nm
HOMO → LUMO + 23%
S0 → S3EmHOMO → LUMO + 251%E = 4.3674 eV
λ = 283 nm
HOMO → LUMO + 331%
Table 6. Long-lived emission kinetic decay curves approximation of compounds 3a–3d (10−5 M, room temperature).
Table 6. Long-lived emission kinetic decay curves approximation of compounds 3a–3d (10−5 M, room temperature).
EntryCompounds 3a–3dτ, μsR2 1
13a 27.080.99976
23b 37.530.99959
33c 47.920.99935
43d 57.150.99983
1 R2—coefficient of determination (R squared). 2 Cyclohexane as solvent, λex = 296 nm, λem = 569 nm. 3 Cyclohexane as solvent, λex = 351 nm, λem = 536 nm. 4 Cyclohexane as solvent, λex = 313 nm, λem = 532 nm. 5 Cyclohexane as solvent, λex = 299 nm, λem = 613 nm.
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Moseev, T.D.; Idrisov, T.A.; Varaksin, M.V.; Tsmokaluk, A.N.; Charushin, V.N.; Chupakhin, O.N. Copper-Catalyzed Sonogashira-Type Coupling Reaction of Vinylacetylene ortho-Carborane with Boronic Acid in the Synthesis of Luminophores with Phosphorescent Emission. Reactions 2024, 5, 868-882. https://doi.org/10.3390/reactions5040046

AMA Style

Moseev TD, Idrisov TA, Varaksin MV, Tsmokaluk AN, Charushin VN, Chupakhin ON. Copper-Catalyzed Sonogashira-Type Coupling Reaction of Vinylacetylene ortho-Carborane with Boronic Acid in the Synthesis of Luminophores with Phosphorescent Emission. Reactions. 2024; 5(4):868-882. https://doi.org/10.3390/reactions5040046

Chicago/Turabian Style

Moseev, Timofey D., Tair A. Idrisov, Mikhail V. Varaksin, Anton N. Tsmokaluk, Valery N. Charushin, and Oleg N. Chupakhin. 2024. "Copper-Catalyzed Sonogashira-Type Coupling Reaction of Vinylacetylene ortho-Carborane with Boronic Acid in the Synthesis of Luminophores with Phosphorescent Emission" Reactions 5, no. 4: 868-882. https://doi.org/10.3390/reactions5040046

APA Style

Moseev, T. D., Idrisov, T. A., Varaksin, M. V., Tsmokaluk, A. N., Charushin, V. N., & Chupakhin, O. N. (2024). Copper-Catalyzed Sonogashira-Type Coupling Reaction of Vinylacetylene ortho-Carborane with Boronic Acid in the Synthesis of Luminophores with Phosphorescent Emission. Reactions, 5(4), 868-882. https://doi.org/10.3390/reactions5040046

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