Conjugation-length dependence of regioregular oligo 3-alkyl(thienylene-vinylene)s demonstrates polyene-like behaviour with weak electron-electron correlations

Poly(3-alkyl(thienylene-vinylene)) (P3TV) and its longer oligomers have negligibly low photoluminescence quantum yields, however, the reason for their low yields is currently debated. Here, we prepare a series of regioregular (3-dodecyl)thienylene-vinylene oligomers with n = 2-8 repeat units by iterative Horner-Wadsworth-Emmons reactions, and report their steady-state, transient absorption, and emission spectroscopy. The results presented here demonstrate that 3-alkyl(thienylene-vinylene) oligomers form part of the polyene family. The shortest (n = 2) oligomer emits from the bright 1B_u state, while fluorescence in oligomers with n = 3, 4 is from the formally dark 2A_g state, allowed via Herzberg-Teller vibronic coupling to the nearby bright 1B_u state as described for diphenyl-polyenes. Longer oligomers and the polymer are essentially non-emissive as the 2A_g state can no longer intensity-borrow from the 1B_u state. We demonstrate that the spectral shapes, photoluminescence quantum yield, and transient spectral behaviour can all be explained using a polyene model with weak electronic correlations.