Conformational changes in regioregular polythiophenes due to crosslinking

Regioregular polythiophene copolymers 10a-d containing hexyl and 11-hydroxy undecyl side chains have been synthesised, by nickel-catalysed cross-coupli ng of well-defined Grignard intermediates. The hydroxy groups were protecte d during the polymerisation as tetrahydropyranyl ethers, and subsequently t ransformed into azide groups. These azide-functionalised copolymers 11a-d w ere heated under vacuum, leading to azide decomposition, nitrene formation and crosslinking. The resultant polymer films showed decreased solubility ( or insolubility) and a shift in the absorption spectrum to shorter waveleng ths dependent on the azide content of the polymer. This colour change is ra tionalised in terms of the conformational change in the polythiophene backb one, associated with thermochromism, which has been partially fixed by the crosslinking at high temperature. This is supported by the modified thermoc hromism and photoluminescence behaviour of the crosslinked polymer films.