Visible Light Photo-switching in a Conformationally-strained Electron Acceptor <i>via</i> a Dual Singlet-triplet Mechanism

Molecular photo-switches have significant potential for use in smart materials that can be controlled by light. Photo-switches function due to lowering or complete removal of the barrier to switching in the excited state. This constraint on the ground and excited state potential energy surfaces means that relatively few photo-switching molecules are known, and fewer that only utilise visible photons. Here, we report the unanticipated visible-light photo switching behavior of a conformationally strained electron acceptor molecule, NIDCS-A, which consists of an anthracene core linked via vinylic double bonds to two thiophene-naphthalamide substituents. Using a combination of spectroscopic techniques, we find that NIDCS-A exhibits an unusual dual photo-switching mechanism via both singlet and triplet excited states. In the singlet state, photoisomerisation precedes geometric relaxation, which enables the molecule to overcome a surprisingly large energy barrier. At later times, photoisomerisation is mediated by long-lived triplet states. We draw several lessons for future work. First, photo-switching of vibrationally excited molecules is able to outcompete geometry relaxation. Second, our work reveals intrinsic molecular ISC as a complementary strategy to intramolecular triplet energy transfer for triplet-mediated photo-switching. Finally, we suggest that NIDCS-A may provide a template for the construction of all-optical three state molecular photo-switches.