Synthesis of penta-and hexa(3,4-thienylene): size-dependent structural properties of cyclic oligothiophenes †

Penta-and hexa(3,4-thienylene)s were synthesized as a potential precursor for thiophene-containing polyarenes, and the structures were determined via X-ray crystallography. The interconversion of thiophene rings is fast in penta(3,4-thienylene), and slow in hexa(3,4-thienylene) reflecting the activation energy for enantio-merization. Size-dependent bathochromic shifts were observed in UV-vis absorption spectra.

Cyclic oligoarylenes have long received considerable attention due to their structural, electronic, and optical properties. 1 The most representative arylenes are phenylenes, in which the phenyl groups can be connected to each other in three ways, i.e., at the o-, m-, or p-positions.Of these, cyclo-o-phenylenes are of little interest and are not very well explored.Even so, biphenylene, triphenylene, tetraphenylene, hexaphenylene, and octaphenylene derivatives have already been synthesized (Fig. 1a), and their reactivity, structural properties, and applications have been investigated. 2Replacing the benzene rings with thiophene rings allows modifying the electronic and structural properties to induce, for example, weaker aromaticity, lower steric hindrance, and intermolecular SÁ Á ÁS interactions in the solid state. 3Structurally, cyclo-o-phenylenes correspond approximately to cyclic oligo(3,4-thienylene)s in as much that the thiophene rings are linked at the b,b 0 -positions (Fig. 1b).To date, cyclic oligo(3,4-thienylene)s with two to four thiophene rings 4 have been explored.Given that all their a-positions are reactive sites, tri-and tetra(3,4-thienylene)s (3T and 4T) can be transformed into a variety of electron-rich p-conjugated molecules, such as heterosumanenes 5 and hetero [8]circulenes, 6 as well as tetra(3,4-thienylene)s can be employed as e.g., 3D building blocks 7 or solvent-responsive molecular assemblies. 8 addition, tri(3,4-thienylene)s have been studied as chargetransfer complexes 9 or planar [6]radialenes. 10However, cyclic oligo(3,4-thienylene)s with five or more thiophene rings have not yet been synthesized. 11To create larger b,b 0 -linked cyclic oligo(3,4-thienylene)s, building blocks that contain functional groups at the b-positions are a prerequisite.In 2017, a practical synthetic method for a b-substituted unit, i.e., a 3,4-diborylated thiophene derivative, was developed by Osuka, Tanaka, and coworkers, which widened the range of synthetic approaches for cyclic compounds that contain 3,4-thienylenes and other heteroarenes. 12erein, we report the synthesis of penta-and hexa(3,4thienylene)s (5T and 6T) via Pd-catalyzed cross-coupling and Ni-mediated homocoupling reactions using 3,4-diborylthiophene 1 or the newly synthesized 4,4 0 -diboryl-3,3 0 -bithiophene 2 (Fig. 1c  and 2a).The structures and electronic properties of 5T and 6T were determined using NMR and X-ray crystallography as well as UV-vis absorption spectroscopy.The structural characteristics and optoelectronic properties of 5T and 6T were compared to This journal is © The Royal Society of Chemistry 2023 those of 4T, and examined using density functional theory (DFT) calculations.
To identify the thus-obtained compounds 5T and 6T, and to investigate their size-dependent structural characteristics, NMR spectra were recorded (Fig. 2b).In the room-temperature 1 H NMR spectrum of 5T in dichloromethane-d 2 , the a-protons of the thiophene rings appear as a singlet at 7.01 ppm.Even at À95 1C, the singlet remains almost unchanged.In the 13 C NMR spectrum in chloroform-d 1 , two signals were observed at 124.6 and 137.6 ppm.These results indicate that the dynamic interconversion of the thiophene rings is fast on the NMR timescale.Unlike that of 5T, the 1 H NMR spectrum of 6T in dichloromethane-d 2 at room temperature displays three signals, i.e., doublets at 6.5 and 6.9 ppm as well as a singlet at 7.3 ppm.Only very slight broadening of the signals was observed even at 140 1C in tetrachloroethane-d 2 , indicating that 6T has three diastereotopic protons and those are not exchanged in the NMR timescale.The 1 H NMR spectra of 4T-6T presented in Fig. 2b clearly demonstrate the size dependence of the symmetry in solution.The NMR assignments were supported by chemical shift calculations using the GIAO method (see Fig. S2 in ESI † for details).
The solid-state structures of 5T and 6T were determined via X-ray diffraction analysis of single crystals obtained from a hexane/chloroform and a pentane/THF solution, respectively (Fig. 3a and b).Compound 5T adopts a C 2 -symmetric structure in which five thiophene rings are fused at the 3,4-positions to form a 10-membered ring.Compound 6T adopts a screw (or helical) structure with D 2 symmetry, similar to hexaphenylene 2b,e-h and hexa(2,3-thienylene)s.11b,f The distance between the centroids of the central thiophene rings (av.crystallography, including the dihedral angles (see Fig. S3 in ESI † for details).Compound 6T has two possible stable conformations, i.e., a screw and a crown conformation.‡ The Gibbs free energy of 6T with the screw conformation is 3.5 kcal mol À1 lower than that of the crown conformation, which was not observed by NMR spectroscopy in this study (see Fig. S4 in ESI † for details).Strain energies of 5T and 6T were calculated to be 19.9 and 1.1 kcal mol À1 , respectively, using hypothetical homodesmotic reactions. 15This result indicates that 5T has a strained structure, and the decrease in energy of 6T might be due to the stabilization by intramolecular p-p interaction (see Fig. S5 in ESI † for details).The enantiomerization between C 2 -symmetric 5T and 5T 0 as well as between D 2 -symmetric 6T and 6T 0 is illustrated in Fig. 4a and b.Our calculations suggest C s symmetry for the transition states (TSs) of 5T and 6T, and activation energies for the interconversion of 5T and 6T of 5.0 and 26.5 kcal mol À1 , respectively.The highlighted thiophene ring resides initially on the C 2 axis in 5T; after interconversion via the enantiomerization pathway, this thiophene ring does not reside anymore on the C 2 axis in 5T 0 .By repeating these enantiomerization steps, all a-protons become magnetically equivalent.This dynamic motion, together with the low activation energy, leads to a rapid exchange of the a-protons even at low temperatures.In the enantiomerization process of 6T, the highlighted thiophene rings on the C 2 axis become the central thiophene rings, and four thiophene rings are symmetrically identical.Although this process leads to the magnetic equivalence of the a-protons, the reaction rate should be slow because of the high activation energy (26.5 kcal mol À1 ).Taken together, the NMR spectrum of 4T shows a singlet because of its highly symmetric structure (D 2d ).Although 5T has low symmetry (C 2 ), the fast dynamic motion of the molecule enables the a-protons of the thiophene rings to reach magnetic equivalence.Compound 6T also has low symmetry (D 2 ), albeit that its unsymmetrical NMR signals indicate a slow exchange of the diastereotopic a-protons on the NMR timescale.Thus, the size-dependent structural characteristics of 4T, 5T, and 6T were determined using a combination of NMR measurements and DFT calculations.
To further examine the size-dependent characteristics of 5T, 6T, and reference compound 4T, their UV-vis absorption spectra were recorded (Fig. 5a).The maximum absorption for all three compounds was observed at B230 nm, and the longestwavelength absorption maxima were observed at 263, 273, and 276 nm for 4T, 5T, and 6T, respectively.Fluorescence of 5T and 6T was negligible in dichloromethane solution.The frontier molecular orbitals related to the longest absorption bands and their energy levels are depicted in Fig. 5b, taking into account the vertical excitation energies, oscillator strengths, and transition contributions obtained from time-dependent (TD) DFT calculations (see Fig. S6 in ESI † for details).Because 4T adopts a highly symmetrical structure, the electronic wavefunctions of the HOMO and LUMO are distributed over the entire molecule, and the HOMO and the HOMOÀ1 are degenerate.In contrast, the HOMO and LUMO of 5T and 6T are relatively localized, presumably because of their lower-symmetry structures.The shoulder peaks in the range of 260-280 nm can be attributed to a combination of the HOMOÀ1 -LUMO and HOMO -LUMO transitions for 4T, and the HOMO -LUMO transitions for 5T and 6T.With increasing number of thiophene rings, the HOMO level rises gradually from À6.09 eV to À5.76 eV, while the LUMO energy decreases from 4T to 5T and is similar for 5T and 6T.Consequently, it can be concluded that the HOMO-LUMO energy gap decreases from 4T to 6T, which is consistent with the bathochromic shift observed for the longestwavelength absorption maxima.
In summary, we have developed synthetic routes to 5T and 6T via Suzuki-Miyaura cross-coupling and a Ni-mediated homocoupling reaction that involved the synthesis of a new b,b 0 -substituted moiety 2. Using an excess of 3 effectively suppressed the formation of polymers.While only a single singlet was observed in the NMR spectra of 4T and 5T, three different signals were observed for 6T.The solid-state structures of 5T and 6T were determined using X-ray crystallography; a corresponding analysis of the metric parameters revealed that the smallest dihedral angle of the 3,3 0 -bithiophene moiety decreases with increasing number of thiophene rings.The size-dependence observed in the NMR spectra can be rationally interpreted in terms of the activation energies for the interconversion of the thiophene rings.The size-dependence manifests in the optoelectronic measurements, in which the longest-wavelength absorption maxima are by 13 nm bathochromically shifted with increasing size of the molecules.The results of our DFT calculations suggest that gradually increasing the number of thiophene rings gradually increases the HOMO energy and decreases the LUMO energy.With synthetic routes to 5T and 6T established, it will now be possible to explore the synthetic chemistry of a variety of unprecedented thiophene-containing arenes.
This work was supported by FOREST program (JPMJFR211R to Y. S.) from JST, JSPS KAKENHI (JP22K19038 and JP22H02068 to Y. S.), UBE Foundation, Mitsubishi Foundation, and Asahi Glass Foundation.We thank Tetsuro Kusamoto, Ryota Matsuoka (IMS) and Takayuki Tanaka (Kyoto Univ.) for their support of experiments and fruitful advices.M. N. is a recipient of JSPS Research Fellowship for Young Scientists (DC2) and IMS SRA fellowship.This work was conducted in IMS supported by ARIM (JPMXP1223MS5012).Calculations were performed using the resources of the Research Center for Computational Science, Okazaki, Japan (23-IMS-C202).

Fig. 3
Fig. 3 (a) and (b) Molecular structures of 5T (a) and 6T (b) with thermal ellipsoids at 50% probability; gray = carbon; yellow = sulfur; white = hydrogen.The C 2 axis of 5T and one of the C 2 axes of 6T are noted with dashed lines.The hydrogen atoms indicated by the colored marks correspond to the 1 H NMR signals shown in Fig. 2b.

Fig. 4 Fig. 5
Fig. 4 Energy diagrams for the enantiomerization of 5T (a) and 6T (b).Selected thiophene rings are highlighted in blue to illustrate the enantiomerization.