Energy- and conformer-dependent excited-state relaxation of an E/Z photoswitchable thienyl-ethene Electronic supplementary information (ESI) available: Additional transient absorption spectra, their spectral analyses, and computed absorption spectra. See DOI: 10.1039/c9cp01226e

摘要

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 < 5%) and occurs via the lowest-lying triplet. We present excitation-wavelength dependent (565-420 nm) transient absorption (TA) studies to probe the photophysics of cis-to-trans isomerization to identify sources of switching inefficiency. TA data reveals contributions from more than one switch conformer and relaxation cascades between multiple states. Fast (similar to 4 ps) and slow (similar to 40 ps) components of spectral dynamics observed at low excitation energies (>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 (P-A and P-B) characterized by roughly parallel dipoles for the thiophenes on opposite sides of the ethene bond. Only the P-B conformer is found to contribute to triplet population and the switching of cis-4TCE: high-energy excitation (<470 nm) of P-B involves direct excitation to S-2, relaxation from which prepares an ISC-active S-1 geometry (ISC QY 0.4-0.67, k(ISC) similar to 1.6-2.6 x 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 P-A conformer (75-85% loss) as well as loses along the P-B S-2 -> S-1 -> T-1 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.