Electronic and optical properties of 4H-cyclopenta[2,1-b:3,4-b']bithiophene derivatives and their 4-heteroatom-substituted analogues: a joint theoretical and experimental comparison.

The electronic and optical properties of 2,6-dialkyl and 2,6-bis(5-alkyl-2-thienyl) derivatives of the fused-ring systems 4H-cyclopenta[2,1-b:3,4-b']bithiophene, 4,4-di-n-hexyl-4H-cyclopenta[2,1-b:3,4-b']bithiophene, 4H-cyclopenta[2,1-b:3,4-b']bithiophene-4-one, 4-alkyl and 4-aryldithieno[3,2-b:2',3'-d]pyrrole, 4-phenyldithieno[3,2-b:2',3'-d]phosphole, 4-phenyldithieno[3,2-b:2',3'-d]phosphole 4-oxide, dithieno[3,2-b:2',3'-d]thiophene, dithieno[3,2-b:2',3'-d]thiophene 4-oxide, and dithieno[3,2-b:2',3'-d]thiophene 4,4-dioxide have been compared to those of the analogous unbridged 5,5'-substituted 2,2'-bithiophene derivatives using electrochemistry, UV-visible absorption and emission spectroscopy, and DFT and TD-DFT calculations. The planarization in the fused-ring compounds means that the methylene-bridged cyclopentabithiophenes are more readily oxidized than their unbridged bithiophene analogues. In each case, the bridging group (X) lies on a nodal plane of the HOMO; accordingly, within each series of fused-ring compounds, electrochemical oxidation potentials and calculated ionization potentials depend primarily on the inductive donor/acceptor strength of the bridging group. On the other hand, significant LUMO coefficients can be found on X groups with π-donor or acceptor properties; accordingly, the electrochemical reduction potentials, calculated electron affinities, and the energies of the HOMO→LUMO optical transitions depend on both inductive and mesomeric donor and acceptor strengths. In particular, within the 2,6-bis(5-alkyl-2-thienyl) series, increasingly electron-withdrawing bridging groups lead to a bathochromic shift and weakening of the low-energy absorption band relative to that of methylene- or π-donor-bridged examples and also to a loss of vibronic structure, with the compound that has the strongest π-accepting bridge of those examined (X = CO) showing a particularly low-energy and weak band. The fluorescence of acceptor-bridged compounds exhibits greater Stokes shifts and a loss of vibronic structure relative to those of methylene- or π-donor-bridged analogues, with the carbonyl-bridged derivative showing no observable fluorescence. These results can be related to increasing localization of the LUMO on the core and toward the bridging group, leading to increased charge-transfer character for the first excited state. The radical cations of some examples have been generated by chemical oxidation and investigated using visible-NIR and ESR spectroscopy and DFT and TD-DFT calculations. The absorption spectra of the radical cations of the 2,6-bis(5-alkyl-2-thienyl) compounds are generally similar to those previously reported for quaterthiophene derivatives, while the hyperfine couplings obtained from ESR spectra are consistent with delocalization of the unpaired electron over both the core and terminal thienyl rings of the π system.