Core structure dependence of cycloreversion dynamics in diarylethene analogs.

Diarylperfluorocyclopentenes are a well-characterized class of molecular photoswitches that undergo reversible photocyclization. The efficiency of cycloreversion (<∼30%), in particular, is known to be limited by a competition with excited-state deactivation by internal conversion that is strongly impacted by the electron-withdrawing/donating character of pendant aryl groups. Here we present a first study to determine how varied structural motifs for the core bridge group impact excited-state dynamics that control cycloreversion quantum yields. Specifically, we compare photophysical behaviors of 3,3'-(perfluorocyclopent-1-ene-1,2-diyl)bis(2-methylbenzo[b]thiophene) with diarylethene derivatives possessing the same benzo[b]thiophene pendant group but with a rigid 1-methyl-1H-pyrrole-2,5-dione and a rigid/aromatic thieno[3,4-b]thiophene bridge (TT) core bridge group. We find that the flexible perfluorocyclopentene core undergoes cycloreversion 3-4× slower than the rigid core photoswitches (9 vs. 2-3 ps in acetonitrile, 25 vs. 5-6 ps in cyclohexane) despite comparable cycloreversion quantum yields. To distinguish effects induced by bridge vs. pendant groups, we also studied a series of photoswitches with the same thieno[3,4-b]thiophene bridging group, but with varied pendant groups including 2,5-dimethylthiophene and 2-(3,5-bis(trifluoromethyl)phenyl)-5-methylthiophene. Analysis of temperature-dependent excited-state lifetimes and cycloreversion quantum yields reveals that both the rates of nonreactive internal conversion and reactive cycloreversion increase with greater structural rigidity of the core. This difference is attributed to smaller energy barriers on the excited-state potential energy surface for both reactive and non-reactive deactivation from the 21A electronic state relative to the flexible perfluorocyclopentene switch, implying that a rigid core results in a net shallower excited-state potential energy surface.

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 < 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 (∼4 ps) and slow (∼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 (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.