Singlet Fission in Solid 1,6-Diphenyl-1,3,5-hexatriene Dicarboxylic Acids and Esters: Effects of Meta and Para Substitution

As an exciton multiplication process, singlet fission (SF) can potentially increase the energy conversion efficiency of solar cells and other optoelectronic devices. (E,E,E)-1,6-Diphenyl-1,3,5-hexatriene (DPH) is an air-stable SF-active chromophore having a triplet state of higher energy and longer lifetime compared with those of tetracenes and pentacenes. However, functionalization of DPH molecule by the introduction of substituents has not been fully studied yet. In this work, DPH dicarboxylic acids and esters substituted in the meta and para positions of the phenyl rings were prepared to investigate the effects of position isomerism on energetics, molecular arrangements, and solid-state SF properties. We discovered that all four trienes were SF materials. For both the acid and ester, the meta isomers exhibit faster SF and more efficient triplet diffusion than the para isomers as shown by steady-state and time-resolved fluorescence measurements with and without a magnetic field. Crystallographic analysis reveals that molecules of meta ester are arranged to form slipped-parallel stacks, whereas those of the para ester are stacked in a herringbone fashion. The better SF properties, namely the faster SF and more efficient triplet diffusion, brought about by meta substitution can be attributed to more suitable energetics and more favorable molecular arrangements for SF to occur. We also found that propionic acid DPH (PA-DPH), a commercial fluorescence probe for biomembrane studies, underwent SF in the solid state.