Development of 4,4′-bibenzo[c]thiophene fluorophores with substituents on the thiophene rings†
4,4′-Bibenzo[c]thiophene fluorophores (1,1′-Si-4,4′-BBT, 1,1′-Sn-4,4′-BBT, 1,1′-CHO-4,4′-BBT and 1,1′-PhtBu-4,4′-BBT) with various substituents (–Si(CH3)2C(CH3)3, –Sn(CH3)3, –CHO and –PhtBu) at the 1,1′-positions on the thiophene rings have been developed. The photoabsorption and fluorescence maxima (λabsmax and λflmax) of the 1,1′-disubstituted 4,4′-bibenzo[c]thiophene derivatives in toluene exhibit bathochromic shifts in the order of 1,1′-Si-4,4′-BBT (366 and 420 nm) ≈ 1,1′-Sn-4,4′-BBT (367 and 422 nm) < 1,1′-PhtBu-4,4′-BBT (386 and 469 nm) ≤ 1,1′-CHO-4,4′-BBT (395 and 456 nm), compared to those (359 and 410 nm) of the unsubstituted 4,4′-bibenzo[c]thiophene (4,4′-BBT). The fluorescence quantum yields (Φfl) of the 1,1′-disubstituted 4,4′-bibenzo[c]thiophene derivatives except 1,1′-CHO-4,4′-BBT (Φfl = 0.03) are ca. 0.4 in toluene. The HOMO energy levels rise in the order of 4,4′-BBT (−5.55 eV) < 1,1′-Si-4,4′-BBT (−5.45 eV) ≈ 1,1′-Sn-4,4′-BBT (−5.43 eV) < 1,1′-PhtBu-4,4′-BBT (−5.22 eV), but the LUMO energy levels (ca. −2.35 eV) of the four derivatives are similar to each other, leading to the bathochromic shift of the photoabsorption band from 4,4′-BBT to 1,1′-PhtBu-4,4′-BBT. On the other hand, the HOMO and LUMO energy levels (−5.79 eV and −2.90 eV) of 1,1′-CHO-4,4′-BBT are significantly lower than those of 4,4′-BBT, but the lowering in the LUMO energy level is larger than that in the HOMO energy level, resulting in the bathochromic shift of the photoabsorption. It is worth mentioning that in the solid state, 1,1′-Si-4,4′-BBT and 1,1′-Sn-4,4′-BBT show relatively high Φfl-solid values of 0.22 and 0.40, respectively, whereas 4,4′-BBT, 1,1′-CHO-4,4′-BBT and 1,1′-PhtBu-4,4′-BBT exhibit feeble solid-state fluorescence properties (Φfl-solid ≤ 0.04). This work provides the synthetic strategy for 1,1′-disubstituted 4,4′-bibenzo[c]thiophene derivatives and reveals their optical properties in the solution and the solid state and electrochemical properties.