O,S-Acetals in a New Modification of oxo-Friedel–Crafts–Bradsher Cyclization—Synthesis of Fluorescent (Hetero)acenes and Mechanistic Considerations

This paper presents the use of O,S-acetals in a new modification of the oxo-Friedel–Crafts–Bradsher cyclization. In this reaction, under mild reaction conditions (25 °C), three- and four-ring fused RO-acenes (major) and/or HO(CH2)2S-acenes (minor) are formed, the latter products having never been observed before in this type of cyclization. In this way, two electronically different fluorophores could be obtained in a single cyclization reaction, one of them having strong electron donor properties (+M effect of alkoxy groups) and the other having donor-acceptor properties (+M and −I effects of the HO(CH2)2S-group, Hammett’s constants). Further increasing the reaction temperature, HCl concentration or prolonging reaction time, surprisingly, yielded a 2:1 mixture of cis and trans dimeric isomers, as the only products of this cyclization. The DFT calculations confirmed a greater stability of the cis isomer compared to the trans isomer. The formation of unexpected dimeric products and HO(CH2)2S-acenes sheds light on the mechanism of oxo-Friedel–Crafts–Bradsher cyclization, involving competitive O/S atom protonation in strained O,S-acetals and in strain-free side groups of intermediate species.


Introduction
Organic electronics and optoelectronics are relatively new fields of basic knowledge and technology, which have become a subject of interest to chemists, physicists and process engineers [1]. Therefore, a search for organic fluorescent and semiconducting materials for the construction of new-generation electronic devices, such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic solar cells (OPVs), organic solar concentrators (OSCs), organic lasers, etc., has drawn the attention of numerous multidisciplinary joint laboratories [2]. Among aromatic hydrocarbons, linearly fused acenes are being considered as key organic compounds for achieving these goals. Anthracene and its derivatives are particularly attractive due to high thermal stability [3], relatively good solubility, low price, blue photoluminescent [4] and electroluminescent properties [5]. Many blue-light-emitting materials with an anthracene core structure [6][7][8][9][10][11][12][13][14][15][16][17][18] have been developed; however, deep blue is still in demand due to the lack of electrically and photochemically stable light-emitting materials [19,20].
In the literature, examples of intramolecular cyclizations of o-formyl [21], o-acyl [22] and o-carboxy [23] diarylmethanes as well as o-carboxy [24,25] diarylketones, leading to the required fused aromatic systems, have been described. The first two types of reactions and our present modification of the oxo-Friedel-Crafts-Bradsher cyclization, utilizing O,Sacetals, lead directly to fused aromatic hydrocarbons, while the remaining transformations require additional steps involving reductions in intermediate products, i.e., anthrones or anthraquinones, followed by aromatization of the obtained cyclic system. In addition, these reactions require harsh reaction conditions, such as high concentrations of Brønsted acids and high temperatures up to 180 • C or more, which preclude the presence of most substituents on the aromatic system [26][27][28]. Only a few examples of reactions, carried out under milder, non-aqueous reaction conditions, are known [29,30]. Our approach employs a dilute, aqueous methanolic solution (2:1) of hydrochloric acid as a strong carbocation solvating medium and room temperature, being the mildest reaction conditions ever used in these types of intramolecular, electrophilic and aromatic cyclizations [31][32][33]. These mild conditions allow for the installation of thermally and chemically sensitive functional groups on aromatic systems and, thus, the oxo-Friedel-Crafts-Bradsher cyclization gives rise to highly substituted, fused aromatics, as we demonstrated in this study.
Earlier, we obtained hetero (XR = OR, SR)-substituted acenes I via cyclization of diarylmethanol derivatives, i.e., ortho-O,O-acetals III (path a, Scheme 1) via oxo-Friedel-Crafts-Bradsher cyclization [32][33][34] or S,S-dithioacetals IV (path b, Scheme 1) via the thio-Friedel-Crafts-Bradsher cyclization [3]. In both hetero-Friedel-Crafts-Bradsher cyclizations, a new benzene ring, fused to two other (hetero)aromatic moieties, ArI and ArII, is formed in the acene I. It is worth noting that the cyclization of O,O-acetals III took place only in the Brønsted acid aqueous solutions and did not occur under anhydrous conditions in the presence of Lewis acid (FeCl 3 /KI). On the other hand, the cyclization of S,S-dithioacetals IV proceeded exclusively in the presence of FeCl 3 /KI in an organic solvent solution.
Molecules 2023, 28, x FOR PEER REVIEW 2 of 24 and our present modification of the oxo-Friedel-Crafts-Bradsher cyclization, utilizing O,S-acetals, lead directly to fused aromatic hydrocarbons, while the remaining transformations require additional steps involving reductions in intermediate products, i.e., anthrones or anthraquinones, followed by aromatization of the obtained cyclic system. In addition, these reactions require harsh reaction conditions, such as high concentrations of Brønsted acids and high temperatures up to 180 °C or more, which preclude the presence of most substituents on the aromatic system [26][27][28]. Only a few examples of reactions, carried out under milder, non-aqueous reaction conditions, are known [29,30]. Our approach employs a dilute, aqueous methanolic solution (2:1) of hydrochloric acid as a strong carbocation solvating medium and room temperature, being the mildest reaction conditions ever used in these types of intramolecular, electrophilic and aromatic cyclizations [31][32][33]. These mild conditions allow for the installation of thermally and chemically sensitive functional groups on aromatic systems and, thus, the oxo-Friedel-Crafts-Bradsher cyclization gives rise to highly substituted, fused aromatics, as we demonstrated in this study. Earlier, we obtained hetero (XR = OR, SR)-substituted acenes I via cyclization of diarylmethanol derivatives, i.e., ortho-O,O-acetals III (path a, Scheme 1) via oxo-Friedel-Crafts-Bradsher cyclization [32][33][34] or S,S-dithioacetals IV (path b, Scheme 1) via the thio-Friedel-Crafts-Bradsher cyclization [3]. In both hetero-Friedel-Crafts-Bradsher cyclizations, a new benzene ring, fused to two other (hetero)aromatic moieties, ArI and ArII, is formed in the acene I. It is worth noting that the cyclization of O,O-acetals III took place only in the Brønsted acid aqueous solutions and did not occur under anhydrous conditions in the presence of Lewis acid (FeCl3/KI). On the other hand, the cyclization of S,Sdithioacetals IV proceeded exclusively in the presence of FeCl3/KI in an organic solvent solution. This differentiated behavior of O,O-acetals and S,S-dithioacetals is due to the greater hydrolytic susceptibility of acetal C-O bonds than dithioacetal C-S bonds towards relatively dilute Brønsted acids and a lack of reactivity of the FeCl3/KI system towards C-O bonds. In this way, suitable reaction conditions can be selected for the preservation of sensitive substituents on the aromatic system. This differentiated behavior of O,O-acetals and S,S-dithioacetals is due to the greater hydrolytic susceptibility of acetal C-O bonds than dithioacetal C-S bonds towards relatively dilute Brønsted acids and a lack of reactivity of the FeCl 3 /KI system towards C-O bonds. In this way, suitable reaction conditions can be selected for the preservation of sensitive substituents on the aromatic system.
In the present study, we employed O,S-acetals (1,3-oxathiolanes) II, which possess C-O and C-S bonds, as precursors of three-and four-ring fused aromatics I. Both bonds are cleaved under different reaction conditions, yielding carbocation intermediates that are active in the new oxo-Friedel-Crafts-Bradsher cyclization modification (Scheme 1). The importance of O,S-acetals, as carbocation-equivalent reagents for carbon-carbon bond Cyclization reaction. As mentioned, in the case of O,O-acetals, especially five-and sixmembered ones, the cyclization to acenes proceeded only with Brønsted acids in an aqueous media (path a, Scheme 1) [31][32][33] with the cleavage of C-O acetal bonds, while with six-membered S,S-acetals, the cyclization occurred only with Lewis acids in anhydrous media with the cleavage of the C-S dithioacetal bonds (path b, Scheme 1). One of the reasons for this differentiated behavior is the lower electron density on the bigger sulfur atom  (Table 1). The cyclization was performed with aqueous solution of hydrochloric acid in methanol (r.t., 72 h, Method C, Scheme 2) and in the presence of the FeCl 3 /KI in methanol under anhydrous conditions (65 • C, 12 h, Method D, Scheme 2). The latter conditions gave better yields (up to 78%) of fused RO-acenes 7 while the former ones delivered up to 53% yields and required longer reaction times. Interestingly, the yield of the four-ring acene 13 was 62% when the aldehyde 10 was used while the yield of another four-ring acene 7d was only 15% when the aldehyde 3d was employed, both aldehydes derived from benzothiophene. The structure of 7d was unambiguously confirmed via X-ray analysis ( Figure 1). Table 1. A comparison of the cyclization results of ethers 5 and 12.

Substrate
Reaction Conditions RO-Acene (Yield) 1 HO(CH 2 ) 2 S-Acene (Yield) 1 in which both C-O and C-S bonds can be cleaved in the presence of mineral acids (HCl), especially when the reaction conditions were intensified (higher HCl concentration, higher temperature, longer reaction time). Therefore, in this study, we installed ortho-O,Sacetal moiety on one of the aryl groups in 5 and 12 to benefit from the ability to cleave both C-O and C-S bonds under different reaction conditions (Method C and D, Scheme 2) and to study the mechanism of this electrophilic modification. Thus, having in hand o-(O,S-acetalaryl)arylmethyl methyl ethers 5 and 12, we started the investigation of the oxo-Friedel-Crafts-Bradsher cyclization with these substrates (Table 1). The cyclization was performed with aqueous solution of hydrochloric acid in methanol (r.t., 72 h, Method C, Scheme 2) and in the presence of the FeCl3/KI in methanol under anhydrous conditions (65 °C , 12 h, Method D, Scheme 2). The latter conditions gave better yields (up to 78%) of fused RO-acenes 7 while the former ones delivered up to 53% yields and required longer reaction times. Interestingly, the yield of the four-ring acene 13 was 62% when the aldehyde 10 was used while the yield of another four-ring acene 7d was only 15% when the aldehyde 3d was employed, both aldehydes derived from benzothiophene. The structure of 7d was unambiguously confirmed via X-ray analysis ( Figure 1). in which both C-O and C-S bonds can be cleaved in the presence of mineral acids (HCl especially when the reaction conditions were intensified (higher HCl concentratio higher temperature, longer reaction time). Therefore, in this study, we installed ortho-O,S acetal moiety on one of the aryl groups in 5 and 12 to benefit from the ability to cleav both C-O and C-S bonds under different reaction conditions (Method C and D, Scheme and to study the mechanism of this electrophilic modification. Thus, having in hand o-(O,S-acetalaryl)arylmethyl methyl ethers 5 and 12, we starte the investigation of the oxo-Friedel-Crafts-Bradsher cyclization with these substrates (T ble 1). The cyclization was performed with aqueous solution of hydrochloric acid in meth anol (r.t., 72 h, Method C, Scheme 2) and in the presence of the FeCl3/KI in methanol unde anhydrous conditions (65 °C , 12 h, Method D, Scheme 2). The latter conditions gave bette yields (up to 78%) of fused RO-acenes 7 while the former ones delivered up to 53% yield and required longer reaction times. Interestingly, the yield of the four-ring acene 13 wa 62% when the aldehyde 10 was used while the yield of another four-ring acene 7d wa only 15% when the aldehyde 3d was employed, both aldehydes derived from benzothio phene. The structure of 7d was unambiguously confirmed via X-ray analysis ( Figure 1). than on the oxygen atom and the lower electronegativity of the former, which means that the 1,3-dithiane sulfur atoms in moderately concentrated mineral acid aqueous solutions at room temperature do not undergo an effective protonation, with the consequence that they also do not undergo apparent hydrolysis through the intermediate benzyl-type carbocations that are required for the thio-Friedel-Crafts-Bradsher cyclization to occur. We discovered that a different situation exists in strained five-membered O,S-acetals, in which both C-O and C-S bonds can be cleaved in the presence of mineral acids (HCl), especially when the reaction conditions were intensified (higher HCl concentration, higher temperature, longer reaction time). Therefore, in this study, we installed ortho-O,Sacetal moiety on one of the aryl groups in 5 and 12 to benefit from the ability to cleave both C-O and C-S bonds under different reaction conditions (Method C and D, Scheme 2) and to study the mechanism of this electrophilic modification.
Thus, having in hand o-(O,S-acetalaryl)arylmethyl methyl ethers 5 and 12, we started the investigation of the oxo-Friedel-Crafts-Bradsher cyclization with these substrates (Table 1). The cyclization was performed with aqueous solution of hydrochloric acid in methanol (r.t., 72 h, Method C, Scheme 2) and in the presence of the FeCl3/KI in methanol under anhydrous conditions (65 °C , 12 h, Method D, Scheme 2). The latter conditions gave better yields (up to 78%) of fused RO-acenes 7 while the former ones delivered up to 53% yields and required longer reaction times. Interestingly, the yield of the four-ring acene 13 was 62% when the aldehyde 10 was used while the yield of another four-ring acene 7d was only 15% when the aldehyde 3d was employed, both aldehydes derived from benzothiophene. The structure of 7d was unambiguously confirmed via X-ray analysis ( Figure 1). in which both C-O and C-S bonds can be cleaved in the presence of mineral acids (HCl especially when the reaction conditions were intensified (higher HCl concentratio higher temperature, longer reaction time). Therefore, in this study, we installed ortho-O,S acetal moiety on one of the aryl groups in 5 and 12 to benefit from the ability to cleav both C-O and C-S bonds under different reaction conditions (Method C and D, Scheme and to study the mechanism of this electrophilic modification. Thus, having in hand o-(O,S-acetalaryl)arylmethyl methyl ethers 5 and 12, we starte the investigation of the oxo-Friedel-Crafts-Bradsher cyclization with these substrates (T ble 1). The cyclization was performed with aqueous solution of hydrochloric acid in meth anol (r.t., 72 h, Method C, Scheme 2) and in the presence of the FeCl3/KI in methanol unde anhydrous conditions (65 °C , 12 h, Method D, Scheme 2). The latter conditions gave bette yields (up to 78%) of fused RO-acenes 7 while the former ones delivered up to 53% yield and required longer reaction times. Interestingly, the yield of the four-ring acene 13 wa 62% when the aldehyde 10 was used while the yield of another four-ring acene 7d wa only 15% when the aldehyde 3d was employed, both aldehydes derived from benzothio phene. The structure of 7d was unambiguously confirmed via X-ray analysis ( Figure 1). in which both C-O and C-S bonds can be cleaved in the presence of mineral acids (HCl), especially when the reaction conditions were intensified (higher HCl concentration, higher temperature, longer reaction time). Therefore, in this study, we installed ortho-O,Sacetal moiety on one of the aryl groups in 5 and 12 to benefit from the ability to cleave both C-O and C-S bonds under different reaction conditions (Method C and D, Scheme 2) and to study the mechanism of this electrophilic modification. Thus, having in hand o-(O,S-acetalaryl)arylmethyl methyl ethers 5 and 12, we started the investigation of the oxo-Friedel-Crafts-Bradsher cyclization with these substrates (Table 1). The cyclization was performed with aqueous solution of hydrochloric acid in methanol (r.t., 72 h, Method C, Scheme 2) and in the presence of the FeCl3/KI in methanol under anhydrous conditions (65 °C , 12 h, Method D, Scheme 2). The latter conditions gave better yields (up to 78%) of fused RO-acenes 7 while the former ones delivered up to 53% yields and required longer reaction times. Interestingly, the yield of the four-ring acene 13 was 62% when the aldehyde 10 was used while the yield of another four-ring acene 7d was only 15% when the aldehyde 3d was employed, both aldehydes derived from benzothiophene. The structure of 7d was unambiguously confirmed via X-ray analysis ( Figure 1). in which both C-O and C-S bonds can be cleaved in the presence of mineral acids (HC especially when the reaction conditions were intensified (higher HCl concentratio higher temperature, longer reaction time). Therefore, in this study, we installed ortho-O,S acetal moiety on one of the aryl groups in 5 and 12 to benefit from the ability to cleav both C-O and C-S bonds under different reaction conditions (Method C and D, Scheme and to study the mechanism of this electrophilic modification. Thus, having in hand o-(O,S-acetalaryl)arylmethyl methyl ethers 5 and 12, we starte the investigation of the oxo-Friedel-Crafts-Bradsher cyclization with these substrates (T ble 1). The cyclization was performed with aqueous solution of hydrochloric acid in meth anol (r.t., 72 h, Method C, Scheme 2) and in the presence of the FeCl3/KI in methanol und anhydrous conditions (65 °C , 12 h, Method D, Scheme 2). The latter conditions gave bett yields (up to 78%) of fused RO-acenes 7 while the former ones delivered up to 53% yield and required longer reaction times. Interestingly, the yield of the four-ring acene 13 wa 62% when the aldehyde 10 was used while the yield of another four-ring acene 7d wa only 15% when the aldehyde 3d was employed, both aldehydes derived from benzothi phene. The structure of 7d was unambiguously confirmed via X-ray analysis ( Figure 1).
in which both C-O and C-S bonds can be cleaved in the presence of mineral acids (HCl), especially when the reaction conditions were intensified (higher HCl concentration, higher temperature, longer reaction time). Therefore, in this study, we installed ortho-O,Sacetal moiety on one of the aryl groups in 5 and 12 to benefit from the ability to cleave both C-O and C-S bonds under different reaction conditions (Method C and D, Scheme 2) and to study the mechanism of this electrophilic modification. Thus, having in hand o-(O,S-acetalaryl)arylmethyl methyl ethers 5 and 12, we started the investigation of the oxo-Friedel-Crafts-Bradsher cyclization with these substrates (Table 1). The cyclization was performed with aqueous solution of hydrochloric acid in methanol (r.t., 72 h, Method C, Scheme 2) and in the presence of the FeCl3/KI in methanol under anhydrous conditions (65 °C , 12 h, Method D, Scheme 2). The latter conditions gave better yields (up to 78%) of fused RO-acenes 7 while the former ones delivered up to 53% yields and required longer reaction times. Interestingly, the yield of the four-ring acene 13 was 62% when the aldehyde 10 was used while the yield of another four-ring acene 7d was only 15% when the aldehyde 3d was employed, both aldehydes derived from benzothiophene. The structure of 7d was unambiguously confirmed via X-ray analysis ( Figure 1).  Increasing the reaction temperature, HCl concentration or prolonging reaction time resulted in the formation of unexpected HO(CH2)2S-acenes 8 in yields up to 27%, which had never been observed in this type of reaction before.
Surprisingly, neither the acene 7 nor the acene 8 was formed in this case. TLC and HPLC analysis confirmed the presence of the two products (Scheme 3). In both 1 H and 13 C NMR spectra, doubling signals were observed in a 2:1 ratio, which corresponded to the formation of the two isomers cis-9b/trans-9b. In the 13 C NMR spectrum, characteristic signals due to cis-9b/trans-9b carbonyl groups were observed at 195.09 and 194.34 ppm. The presence of the latter was further confirmed by observation of the band at 1697 cm −1 in the IR spectrum and by the DFT calculation.  Increasing the reaction temperature, HCl concentration or prolonging reaction time resulted in the formation of unexpected HO(CH 2 ) 2 S-acenes 8 in yields up to 27%, which had never been observed in this type of reaction before.
Surprisingly, neither the acene 7 nor the acene 8 was formed in this case. TLC and HPLC analysis confirmed the presence of the two products (Scheme 3). In both 1 H and 13 C NMR spectra, doubling signals were observed in a 2:1 ratio, which corresponded to the formation of the two isomers cis-9b/trans-9b. In the 13 C NMR spectrum, characteristic signals due to cis-9b/trans-9b carbonyl groups were observed at 195.09 and 194.34 ppm. The presence of the latter was further confirmed by observation of the band at 1697 cm −1 in the IR spectrum and by the DFT calculation.  Increasing the reaction temperature, HCl concentration or prolonging reaction time resulted in the formation of unexpected HO(CH2)2S-acenes 8 in yields up to 27%, which had never been observed in this type of reaction before.
Surprisingly, neither the acene 7 nor the acene 8 was formed in this case. TLC and HPLC analysis confirmed the presence of the two products (Scheme 3). In both 1 H and 13 C NMR spectra, doubling signals were observed in a 2:1 ratio, which corresponded to the formation of the two isomers cis-9b/trans-9b. In the 13 C NMR spectrum, characteristic signals due to cis-9b/trans-9b carbonyl groups were observed at 195.09 and 194.34 ppm. The presence of the latter was further confirmed by observation of the band at 1697 cm −1 in the IR spectrum and by the DFT calculation.

DFT Calculations for 9a and 9b
The optimized geometries and electronic structures of both the cisand trans-isomers of 9b in the gas phase, at the ground state obtained from DFT calculations using the gradient-corrected three-parameter hybrid functional (B3LYP) with the 6-31++G(d,p) basis set, are presented in Figure 2 (see also Tables S1 and S2 in Supplementary Materials). According to the DFT calculations, the cis-9b is more thermodynamically stable than the trans-9b by 4.58 kcal/mol (Figure 2a). It is seen from Figure 2b that cis-9b is also chemically more stable with HOMO-LUMO energy gap (E G ) of 3.871 eV compared to the trans isomer with E G = 3.742 eV. The higher thermodynamic and chemical stability of the cis isomer compared to the trans isomer may be due to the presence of an intramolecular non-covalent interaction between the S atom of the substituent attached to the cyclohexadiene ring of the cis isomer and the H atom of the cyclohexanone ring (dashed pink lines in Figures 2a and 3), as revealed by the non-covalent interaction (NCI) analysis (see Supplementary Materials for details). The distance between non-covalently bonded S and H atoms (2.746 Å) is smaller by 0.25 Å than the sum of their van der Waals radii (3.00 Å). The geometrical parameters  (Figure 3), indicated that it could be treated as a weak unconventional hydrogen bond of the C-H···S type. This interaction forms a closed seven-membered ring S(7) (Figure 2a). The distance between the corresponding S and H atoms in trans-9b is 4.534 Å, so this kind of non-covalent interaction has no possibility to occur in this molecule. It is worth noting that in the case of the trans isomer, the S atom is involved in the formation of short non-covalent contacts with the hydrogen atoms attached to aromatic carbons (Figure 3). Different non-covalent interactions involving the sulfur atom, i.e., C-H···S (in cis-9b) and two C-H···S (in trans-9b) interactions (Figures 2a and 3), led to differences in the molecular configuration of the two isomers. For example, in cis-9b, the distance between the sulfur atom and the cyclohexan-1-one 4-C sp3 atom is only 3.692 Å, while in the trans isomer, it is about 1.5 Å greater. In the cis isomer, the C-H···S interaction forms an intramolecular ring S(7) that prevents free rotation around the C sp3 -C sp3 bond connecting the two-ring systems.

Mechanistic Considerations and DFT Calculations
The formation of two unexpected types of products 8 and 9 made it possible to explain not only a pathway for obtaining these products but also to propose the overall mechanism of the oxo-Friedel-Crafts-Bradsher cyclization reaction using O,S-acetals (Scheme 4). To make the proposed mechanism credible, we performed DFT calculations (B3LYP 6-311++G(d,p)) in the gas phase in the ground state and the quantitative analysis of molecular surfaces [42,43] for 5b, 16a and 16b. As a result of this analysis, the largest minima of electrostatic potential (ESP) on the van der Waals surfaces of the compounds and more precise ESP values on the local surfaces (surface corresponding to a given atom) were calculated for oxygen and sulfur atoms in the O,S-acetal 5b as well as for the MeO oxygen and X atom (X = O, S) in the XCH2CH2YH side chains in 16a and 16b (Scheme 4, Figure 4). ESP values reflect the electron density in these atoms. As the electron density in a given atom decreases, its affinity for the proton also decreases, making the atom less basic, and vice versa. The atom is red, indicating that it is rich in electrons, and if the color of the atom gradually changes toward yellow and green, then the atom becomes steadily less rich in electrons.

Mechanistic Considerations and DFT Calculations
The formation of two unexpected types of products 8 and 9 made it possible to explain not only a pathway for obtaining these products but also to propose the overall mechanism of the oxo-Friedel-Crafts-Bradsher cyclization reaction using O,S-acetals (Scheme 4). To make the proposed mechanism credible, we performed DFT calculations (B3LYP 6-311++G(d,p)) in the gas phase in the ground state and the quantitative analysis of molecular surfaces [42,43] for 5b, 16a and 16b. As a result of this analysis, the largest minima of electrostatic potential (ESP) on the van der Waals surfaces of the compounds and more precise ESP values on the local surfaces (surface corresponding to a given atom) were calculated for oxygen and sulfur atoms in the O,S-acetal 5b as well as for the MeO oxygen and X atom (X = O, S) in the XCH 2 CH 2 YH side chains in 16a and 16b (Scheme 4, Figure 4). ESP values reflect the electron density in these atoms. As the electron density in a given atom decreases, its affinity for the proton also decreases, making the atom less basic, and vice versa. The atom is red, indicating that it is rich in electrons, and if the color of the atom gradually changes toward yellow and green, then the atom becomes steadily less rich in electrons.
Thus, on the basis of the obtained experimental and calculation data, we assumed that both oxygen and sulfur atoms can be protonated in strained five-membered O,S-acetals systems, with an obvious preference for the O,S-acetal oxygen atom because the difference in the ESP values between oxygen (−22.836 kcal/mol) and sulfur (−16.859 kcal/mol) atoms in 5b is only −5.977 kcal/mol. It means that the difference in electron density in sulfur and oxygen in the cyclic O,S-acetal, and consequently affinity of the latter for the proton, may be regarded as comparable (cf. 16b, vide infra). We also assumed that protonation of the sulfide sulfur atom in the strain-free side groups of intermediates 15-18 (X = S) and also in the final products 9b would be more difficult than in the strained O,S-acetals because the difference in the ESP values between oxygen (−31.478 kcal/mol) and sulfur (−21.292 kcal/mol) atoms in 16b is twice as high as in 5b and equals 10.186 kcal/mol. However, the protonation in 16b to give 17 (X = S) can be possible, to some extent, under harsh reaction conditions, SUCH AS higher HCl concentration, higher temperature and/or longer reaction times. This minor pathway, as in the case of major pathway for 16a, also leads to carbocation 19 and then to anthracene 7b.
Thus, the possible protonation of both heteroatoms leads to O,S-acetal cleavage and formation of the reactive benzylic carbocation 14 followed by the intramolecular S E Ar cyclization to give 15 and next aromatization one of the benzene rings to form 16a,b. Preferential protonation of oxygen in 16a or sulfur atoms in 16b (the latter under harsh con-ditions), as mentioned above, produces 17, which next undergoes aromatization through the intermediate dibenzylic carbocation 19 to give the major cyclization product 7. Protonation of the MeO oxygen atom in 16b gives 18 and, after aromatization through 19, delivers the minor aromatic product 8b of the oxo-Friedel-Crafts-Bradsher reaction. Finally, the obtained product 7b couples with the intermediate dibenzylic carbocation 20 to give the dimeric products cis-9b and trans-9b. This reaction predominates over pathways leading to 7b and 8b at higher temperature (65 • C) and higher HCl concentration (c = 0.34 mol/dm 3 ).

Electron Character of RO-Acenes 7, 13 and HO(CH 2 ) 2 S-Acenes 8
The electron nature of the obtained highly substituted acenes 7 and 8 and, in consequence, their photophysical properties are related to the character of the substituents at-tached to the acene system. The measure of the electron effect exerted by these substituents is the Hammett constants, which were calculated using the ACD/Percepta program [44]. The methoxy and methylene-1,3-dioxa groups with negative σ p values of −0.27 and −0.13, respectively, have a strong electron donor character and increase electron density in acene systems 7a-d and 13. On the other hand, the small but positive values of σ p = 0.07 constants confirm a weak electron-acceptor character of the HO(CH 2 ) 2 Sgroup when attached to electron-rich acenes 8b and 8c substituted by electron-donating alkoxy groups [3,45].
The electron effects operating in the discussed functional groups (MeO, methylene-1,3-dioxa-and HO(CH 2 ) 2 S-) were further analyzed with the σ ind and σ res Hammett's components. They show that the electron-donating properties of the methoxy group with σ ind /σ res = 0.30/−0.58 and methylene-1,3-dioxa group with σ ind /σ res = 0.35/−0.48 are connected with a dominance of the positive resonance (+M) effect over the inductive (−I) effect of both alkoxy-type substituents.
On the other hand, in the HO(CH 2 ) 2 Sgroup with σ ind /σ res = 0.26/−0.21, the predominantly negative inductive effect (−I) dominates over the resonance effect. It accounts for the electron-withdrawing character of the RS group in electron-rich aromatics 8b and 8c.

Photopysical Properties
Thus, RO-acenes 7 and 13 belong to a group of highly substituted donor chromophores absorbing UV light in a typical range of 270-395 nm and emitting blue light at 380-445 nm [32]. Sulfur-substituted products, represented here by HO(CH 2 ) 2 S-acenes 8b and 8c, belong to a group of donor-acceptor chromophores that normally absorb light in a range of 270-425 nm and emit blue light at longer wavelengths of 404-457 nm [3]. UV/Vis absorption and emission spectra of the obtained substituted acene derivatives 7b, 7c, 8c and 13 are shown in Figure 5 and Table 2.  In particular, in a range of 240-300 nm electron-donor anthracene 7b, 7c and a weak electron donor-acceptor anthracene 8c due to the presence of the thio group with the -I effect, revealed almost identical absorption maxima at c.a. 269 nm and the same absorp- In particular, in a range of 240-300 nm electron-donor anthracene 7b, 7c and a weak electron donor-acceptor anthracene 8c due to the presence of the thio group with the −I effect, revealed almost identical absorption maxima at c.a. 269 nm and the same absorption profile. A further comparison of absorption spectra of electron-rich three-ring acenes (7b, 7c), with the four-ring acene 13 of the same electron character, showed a redshift by 11 nm in a range of 240-300 nm, which indicated the effect of a larger aromatic conjugation in 13 (Figure 5a). In a range of 300-420 nm, all investigated compounds exhibited absorption bands of lower intensity in the long-wavelength part of the spectrum. The TD-DFT calculations in the gas phase, made for 7b, revealed, in this part of the spectrum, two strong transitions, i.e., HOMO → LUMO (0.97), corresponding to a band at 380.79 nm and HOMO-1 → LUMO (0.73), HOMO → LUMO+1 (0.26) at 346.56 nm ( Figure 6). Table 2. Absorption and emission maxima (λ max ), Stokes shifts in dichloromethane solution (10 −5 mol/dm 3 , 25 • C) for 7b,c, 8c and 13 (underlined values are highest absorption maxima in the lower part of the spectrum in the 300-420 nm range).

Compound
Absorption λ max (nm)  The fluorescence spectra of the obtained acenes exhibited blue emission and covered a region from 385 to 438 nm (Figure 5b). A redshift of 19-31 nm was observed for emission maxima of 7c, 8c and 13 relative to 7b.

Materials and Methods
Organic solvents were purchased from commercial sources (ChemPur, Poland) and used as received or dried using standard procedures. Tetrahydrofuran (THF) was purchased from J.T. Baker and purified on Solvent Purification System (MBraun SPS-800). All reagents were from commercial suppliers (Sigma-Aldrich, Merck-USA, China) and used without further purification. The 1 H NMR and 13 C NMR spectra were measured with a Bruker AV 200 or AV 500 spectrometer (Billerica, MA, USA), with chemical shifts given in ppm relative to TMS as an internal standard. High-resolution mass spectrometry (HRMS) measurements were performed using SQ Detector 2 mass spectrometer (Waters, Milford, MA, USA). Melting points were measured using Boetius apparatus. Thin-layer chromatography (TLC) was performed on precoated Merck 60 (F254 60, Darmstad, Germany) silica gel plates with fluorescent indicator, with detection by means of UV light at 254 and 360 nm. Column chromatography was performed on Merck silica gel (Kieselgel 60, Darmstad, Germany, 230-400 mesh) or using Pure FlashPrep 850 Chromatography System (Büchi). The UV-Vis absorption spectra were recorded in 1 cm cuvettes on a Shimadzu UV-2700 spectrophotometer (Kioto, Japan) using two types of light source: a D2 64604 deuterium lamp and a Wl L6380 halogen lamp (Kioto, Japan, 220-600 nm). The emission spectra were obtained with the Horiba Jobin Yvon, FluoroMax-4 spectrofluo- The fluorescence spectra of the obtained acenes exhibited blue emission and covered a region from 385 to 438 nm (Figure 5b). A redshift of 19-31 nm was observed for emission maxima of 7c, 8c and 13 relative to 7b.

Materials and Methods
Organic solvents were purchased from commercial sources (ChemPur, PiekaryŚląskie, Poland) and used as received or dried using standard procedures. Tetrahydrofuran (THF) was purchased from J.T. Baker and purified on Solvent Purification System (MBraun SPS-800). All reagents were from commercial suppliers (Sigma-Aldrich, Merck-USA, Beijing, China) and used without further purification. The 1 H NMR and 13 C NMR spectra were measured with a Bruker AV 200 or AV 500 spectrometer (Billerica, MA, USA), with chemical shifts given in ppm relative to TMS as an internal standard. High-resolution mass spectrometry (HRMS) measurements were performed using SQ Detector 2 mass spectrometer (Waters, Milford, MA, USA). Melting points were measured using Boetius apparatus. Thin-layer chromatography (TLC) was performed on precoated Merck 60 (F254 60, Darmstad, Germany) silica gel plates with fluorescent indicator, with detection by means of UV light at 254 and 360 nm. Column chromatography was performed on Merck silica gel (Kieselgel 60, Darmstad, Germany, 230-400 mesh) or using Pure FlashPrep 850 Chromatography System (Büchi, Flawil, Switzerland). The UV-Vis absorption spectra were recorded in 1 cm cuvettes on a Shimadzu UV-2700 spectrophotometer (Kioto, Japan) using two types of light source: a D2 64604 deuterium lamp and a Wl L6380 halogen lamp (Kioto, Japan, 220-600 nm). The emission spectra were obtained with the Horiba Jobin Yvon, FluoroMax-4 spectrofluorometer (Glasgow, UK), using a xenon lamp as a light source. The IR absorption spectra were recorded on Nicolet 6700 FT-IR spectrometer (Thermo Scientific, Waltham, MA, USA).  13   The reaction mixture was warmed to room temperature. The saturated aqueous NH 4 Cl solution was added, and organic layer was concentrated. The residue was diluted with ethyl acetate (3 × 10 mL), washed with water (15 mL) and dried over anhydrous MgSO 4 . After filtration, ethyl acetate was removed in vacuum and the crude product 5 was purified by column chromatography over silica gel with a mixture of toluene/ethyl acetate (1:1 v/v) as an eluent. o-(O,S-Acetalaryl)arylmethanol 4a,b or 4d (1.0 mmol) and KI (5 mol%) were placed in the round-bottom flask (50 mL) and dissolved in dry THF (8 mL) at room temperature; then, NaH (1.1 mmol) was added and stirred for 30 min under argon atmosphere. Then, the resulting mixture was treated with MeI (1.5 mmol) and was left at room temperature overnight. After 12 h, the residue was diluted with ethyl acetate (3 × 10 mL), washed with water (15 mL) and dried over anhydrous MgSO 4 . After filtration, ethyl acetate was removed in vacuum and the crude product 5 was purified by column chromatography over silica gel with a mixture of toluene/ethyl acetate (1:1 v/v) as an eluent. To a solution of o-(O,S-acetalaryl)arylmethyl methyl ether 5 or 11 (0.8 mmol), dissolved in MeOH (20 mL), aqueous solution of 1N or 2N HCl (4 mL) was added and the resulting mixture was stirred at the relevant temperature (see Table 1) until disappearance of the starting material (monitoring by TLC). The reaction mixture was extracted with ethyl acetate (50 mL) and the organic layer was washed with water (30 mL), saturated solution of NaHCO 3 (30 mL) and again with water (30 mL). After drying over anhydrous MgSO 4 and filtration, the solvent was removed in vacuum and the crude products were purified by column chromatography over silica gel with a mixture of n-hexane/ethyl acetate 10:1 (v/v) to afford corresponding acenes 7 and 13. A mixture of n-hexane/ethyl acetate 3:1 (v/v) was used to purify anthracene 8.  2-Mercaptoethanol (2.62 mmol, 205 mg, 184 µL) and p-TsOH·H 2 O (50 mg, 0.2 mmol, 10 mol%) were added to a solution of 3-bromobenzo[b]thiophene-2-carbaldehyde 10 (2.62 mmol, 0.633 g) in benzene (6 mL), and the resulting mixture was refluxed for 24 h using the Dean-Stark trap to remove water. The mixture was concentrated and purified with column chromatography using toluene as an eluent. Evaporation of the solvent gave a colorless oil of 11 (0.580 g, 74%) [47]. 1

Synthesis of o-(O,S-acetalaryl)arylmethyl Methyl Ether 12
o-Bromothioacetal 11 (500 mg, 1.66 mmol) was placed in the round-bottom flask (50 mL) and dissolved in dry THF (6 mL) at −78 • C under argon atmosphere. Next, n-BuLi (0.7 mL, 2.6 M in hexanes, 1.83 mmol) was added. The resulting mixture was stirred for 15 min under argon and then solution of the aldehyde 3a (326 mg, 1.66 mmol) in dry THF (4 mL) was added. After 2 h, MeI (1.13 g, 8.0 mmol) was added. The reaction mixture was warmed to room temperature and stirred for 12 h. The saturated aqueous NH 4 Cl solution was added, and organic layer was concentrated. The residue was diluted with ethyl acetate (3 × 10 mL), washed with water (15 mL) and dried over anhydrous MgSO 4 . After filtration, ethyl acetate was removed in vacuum and the crude product was purified by column chromatography over silica gel with a mixture of toluene/ethyl acetate (10:1 v/v). Fraction with R f = 0.65 yielded 617 mg of product 12.  13

Synthesis of Acene 13 Using HCl aq
To a solution of o-(O,S-acetalaryl)arylmethyl methyl ether 12 (0.073 g, 0.17 mmol), dissolved in MeOH (4 mL), the aqueous solution of 2 N HCl (0.8 mL) was added and the resulting mixture was stirred at room temperature for 12 h. The reaction mixture was extracted with ethyl acetate (20 mL) and the organic layer was washed with water (10 mL), saturated solution of NaHCO 3 (15 mL) and again with water (10 mL), then dried over anhydrous MgSO 4 . After filtration, ethyl acetate was removed in vacuum and the crude product was purified with PLC plate with a mixture of hexane/acetone (3:1 v/v). Fraction with R f = 0.45 yielded 9 mg of product 13.