Torsional influences on cross-conjugated thieno[3,4-b]thiophene photochromes

Photoresponsive conjugated polymers are a promising target for modern organic electronics. Numerous photoswitchable repeat units have been included covalently within polymeric structures to enable responsive chromic materials, most commonly through side-chain appendages or through formal conjugation along a pi-conjugated backbone. We recently disclosed a new design whereby the photoswitch elements are cross conjugated to a conjugated polymer main chain. In this case, we found that the extent of photoconversion was dictated in part by competitive main chain light absorption, which could be suppressed by using a photoswitching motif that carried most of the frontier molecular orbital densities. Here, we report the modeling and synthesis of a series of thieno[3,4-b]thiophene (TT)-based photochromes with various aromatic flankers imparting varying degrees of steric bulk and pi-conjugation in order to elucidate the balancing act between steric and electronic factors to promote photochromism. These model systems provide a better understanding of the behavior of photochromic units within extended oligomeric and polymeric pi-conjugated materials. Thieno[3,4-b]thiophene-based diarylethene photochromes present an opportunity to engineer orthogonal conjugation pathways through photoswitchable conjugated polymers. We previously showed the importance of balancing main-chain polymer conjugation with that of the pendant photochrome arms. Herein, we examine structural challenges to this main-chain conjugation, using computational tools to predict photochromism based on conformational considerations. Steric biases along the main-chain polymer backbone led to differing degrees of frontier molecular orbital localization at the switch arms, which subsequently influenced the extent of observed photochromism.image