Molecular Driving Forces for Z/E Isomerization Mediated by Heteroatoms: The Example Hemithioindigo

Abstract

A combined experimental and theoretical investigation of photoinduced Z/E isomerizations is presented. Unsubstituted Hemithioindigo is selected as a representative minimal model to unravel the reaction mechanism in the presence of heteroatoms on an atomic level. Time-resolved spectroscopy reveals multiexponential reaction dynamics on the few picoseconds time scale, which are interpreted by quantum chemical calculations at the CASSCF/CASPT2 level of theory. Detailed insight into the processes governing the ultrafast decay from the first excited state, mediated by a number of conical intersections, is provided. Charge separation and charge balance recovery on the reaction pathway play the leading role and are controlled by the electron-donating or -withdrawing character of the heteroatoms. The electronic and geometric structures of the individual minimum energy conical intersections governing the reaction are rationalized, and an extended energetically low lying conical intersection seam is extracted. Comparison to the experimental results permits linking the observed time constants to molecular intermediates and pathways. An explanation is provided for the pronounced differences of Z → E and the E → Z photoreactions upon excitation to the first excited singlet state.

Making Fast Photoswitches Faster—Using Hammett Analysis to Understand the Limit of Donor–Acceptor Approaches for Faster Hemithioindigo Photoswitches

“…The measured lifetimes of the excited state of the Z isomers describe directly the Z / E photoisomerization kinetics, because this is the main pathway for de‐excitation. The corresponding lifetimes of the E isomers however, are a composite of two processes leading to de‐excitation (see Figure 5 in the Supporting Information) 12. The faster process is an internal conversion (IC) leading exclusively to E isomer without a significant barrier.…”

“…The photoisomerization of unsubstituted HTIs takes place at the picosecond timescale12—a convenient starting point to create ultrafast photoswitching. In general, increasing the photoisomerization rate of a photoswitch can be done by external influences, such as solvent viscosity14 or polarity,15 or internally by substitution.…”

Hemithioindigo—an emerging photoswitch

“…After excitation of the E isomer a similar photoisomerization mechanism is followed with similar polarizations [46] along the way, but with one important difference. In the S 1 excited state of the E isomer, a virtually barrier-less transition from the excited state to the ground state is found in the theoretical description, [42] which takes place before isomerization of the double bond has occured. This nonradiative "loss channel" is responsible for the low quantum yield of the E/Z photoisomerization as well as the faster deexcitation and very small fluorescence that is observed for the E isomer.…”

“…Seminal studies of the ultrafast behavior of HTIs upon photoexcitation were carried out by the groups of Zinth and Rück-Braun [39,41] and a thorough theoretical description of the excited state of unsubstituted HTI 1 by de Vivie-Riedle and coworkers. [42] Earlier theoretical descriptions have been given by the groups of Ganguly [43] and Plötner. [44] In a simplified picture (Figure 2a), the excited state potential energy surface of photoexcited HTI is a combination of two (S 1 and S 2 ) excited states, which mix at a certain geometry point.…”

Photoisomerization around a Fulvene Double Bond: Coherent Population Transfer to the Electronic Ground State?

“…In c-HEX the characteristic cooling times are 12 and 10 ps. [31,[41][42][43] An induced spectrum remains even after ground-state recovery and cooling are complete, as shown by the thick black line in Figure 3. It amounts to 3-4 % of the initial bleach for excitation with excess of energy in c-HEX and ACN.…”