Abstract
Molecular switches based on the N-alkylated indanylidene-pyrroline (NAIP) framework mimic some of the outstanding double bond photoisomerization properties of retinal Schiff bases in rhodopsin, most notably, the occurrence of vibrational coherences in the excited and photoproduct ground states. Focusing on the zwitterionic NAIP switch and using broadband transient absorption spectroscopy, our previous investigation of the Z to E photoisomerization dynamics is now extended to the study of the backward E to Z photoisomerization and to the role of the solvent on the vibrational coherence accompanying the photoreaction. Despite very similar signatures of excited-state vibrational coherence and similar isomerization times, the backward reaction has a significantly smaller isomerization yield than the forward reaction, and most interestingly, does not display ground state coherences. This indicates that both the quantum yield and vibrational dephasing depend critically on the photochemical reaction path followed to reach the ground potential energy surface. In addition, investigation of the effect of the solvent viscosity shows that vibrational dephasing is mainly an intramolecular process.
Highlights
The excited-state absorption (ESA) shifts to shorter wavelengths, and is maximum at a delay of t = 0.20 ps in the range 320–335 nm (figure 2(B)). This dynamic blue-shift is evidence for an excited-state vibrational wavepacket, which evolves along the isomerization reaction coordinate on a barrierless excited state potential energy toward the conical intersection (CI)
The IP photoswitches appear as a good model system to investigate the mechanisms that control the photoisomerization dynamics, yield and the accompanying vibrational coherence
The forward and backward reaction dynamics of the zwitterionic ZW-N-alkylated indanylidene-pyrroline (NAIP) switch are studied by broadband transient absorption (TA) spectroscopy with 80 fs resolution
Summary
It is even possible to pursue the trajectory computation after the non-adiabatic decay to the ground state through a conical intersection (CI) [18, 20, 26] where the branching ratio is controlled This opens the way toward accurate comparison with the observed photoproduct dynamics and possibly quantum yield prediction. Studying the forward reaction in a few selected alcohols, among which the higher viscosity decanol, reveals hardly any change in the reaction dynamics and vibrational coherence This suggests that the vibrational coherence is observed in a so-called volume-conserving deformation in line with the ring inversion modes predicted by QM/MM semi-classical trajectory computations [20], and that vibrational dephasing is in essence an intramolecular process in this compound
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