Energy- and conformer-dependent excited-state relaxation of an E/Z photoswitchable thienyl-ethene

Bis(bithienyl)-1,2-dicyanoethene (4TCE) is a photoswitch that operates via reversible E/Z photoisomerization following absorption of visible light. cis-to-trans photoisomerization of 4TCE requires excitation below 470 nm, is relatively inefficient (quantum yield 470 nm) are readily attributed to deactivation of two conformers; this assignment is supported by computed thermal populations and absorption strengths of two molecular geometries (PA and PB) characterized by roughly parallel dipoles for the thiophenes on opposite sides of the ethene bond. Only the PB conformer is found to contribute to triplet population and the switching of cis-4TCE: high-energy excitation (<470 nm) of PB involves direct excitation to S2, relaxation from which prepares an ISC-active S1 geometry (ISC QY 0.4–0.67, kISC ∼ 1.6–2.6 × 10−9 s−1) that is the gateway to triplet population and isomerization. We ascribe low cis-to-trans isomerization yield to excitation of the nonreactive PA conformer (75–85% loss) as well as loses along the PB S2 → S1 → T1 cascade (10–20% loss). In contrast, electrocyclization is inhibited by the electronic character of the excited states, as well as a non-existent thermal population of a reactive “antiparallel” ring conformation.