On the Impact of Linear Siloxanated Side Chains on the Molecular Self-Assembling and Charge Transport Properties of Conjugated Polymers

Herein reported is the impact of the functionalization of four different semiconducting polymer structures by a linear siloxane-terminated side-chains. The latter is tetrasiloxane (Si₄) or trisiloxane (Si₃) chains, substituted at their extremity to a pentylene linker. The polymer structure is based on 5,6-difluorobenzothiadiazole comonomer (PF2), a diketopyrrolopyrrole unit (PDPP-TT), a naphtalediimide unit (PNDI-T₂), and a poly[bis(thiophen-2-yl)thieno[3,2,b]thiophene (PBTTT). The properties of these siloxane-functionalized polymers are scrutinized and compared with the ones of their alkyl-substituted polymer analogues. The impact of the alkyl-to-siloxane chain substitution clearly depends on the molecular section of the side chains. When a branched 2-octyldodecyl chain (C₂₀) is replaced by a Si₄ chain of same molecular section, the greatest impact is the strong increase of the π-stacking overlap of the polymer backbones. This effect leads to a significative enhancement of the charge mobility values of the polymers. As in-plane and out-of-plane mobility are increased simultaneously, this π-overlap enhancement effect happens to be preponderant over the polymer orientation variations. When a linear tetradecyl chain (C₁₄) is replaced by a linear Si₃ chain of twice larger molecular section, the polymer structure is profoundly affected. While PBTTT-C₁₄ is crystalline and purely edge-on, PBTTT-Si₃ is mesomorphic and shows a mixed face-on/edge-on orientation.