Enhanced Photostability through Rapid Exciton Decay in Desymmetrized Cyclopentannulated Acenes with Strong Face-to-Face pi Stacking.

The photophysics of organic semiconductors impacts their efficiency in optoelectronic devices where exciton transitions, including singlet fission, intersystem crossing and the formation of charge transfer states influence the ability to convert between bright and dark states and to dissociate into free charges. Unfortunately, photodegradation and spurious signals often confound the results of optical studies, especially of important triplet states. Here four asymmetric cyclopentannulated acenes are synthesized and studied. This system represents an extreme in photophysics achieved via molecular design to fully quench the photoluminescence and bypass triplet formation allowing for comparative studies with other highly absorbing acenes. Rapid molecular exciton decay that is unaffected by strong electronic coupling induced by the crystal packing is found. The quick return to the ground state inhibits the formation of triplets and leads to heating in the solid state. These aceacenes are photostable both in solution and as single crystals, likely because the short excited-state lifetime diminishes the chances for deleterious photoreactions. Density functional theory calculations highlight excited state twisting in the five-membered ring, indicating a key driver of rapid internal conversion.