Odd-Even Effect of Thiophene Chain Lengths on Excited State Properties in Oligo(thienyl ethynylene)-Cored Chromophores

In a self-assembly material system, odd-even effects are manifested from long-range periodic packing motifs. However, in an amorphous material system, due to long-range disorder, such phenomena are less prone to appear. Here, we report the discovery of a remarkable odd-even effect on the excited state properties of a series of conjugated thienyl ethynylene (TE) oligomers with truxene as end-capping units, Tr(TE)_nTr (n = 2-6), in solution. Using steady-state and time-resolved spectral measurements, we found the fluorescence quantum yield and excited state dynamics, both showing odd-even alternation with increasing thiophene-ethynylene chain lengths in apolar cyclohexane (CHX). It is found that the symmetry properties with different torsional modes dominate the excited state processes. In polar tetrahydrofuran (THF), solvation lowers the twisting barriers, leading to symmetry breaking without special odd-even alternation over structures. The results presented here will be helpful for understanding odd-even effects of conjugated polymers and designing novel photoelectric materials. (Graph Presented).