Ultrafast Dynamics of a Molecular Switch from Resonance Raman Spectroscopy: Comparing Visible and UV Excitation

Abstract

Resonance Raman spectroscopy probes the ultrafast dynamics of a diarylethene (DAE) molecular switch following excitation into the first two optical absorption bands. Mode-specific resonance enhancements for Raman excitation at visible (750–560 nm) and near-UV (420–390 nm) wavelengths compared with the calculated and experimental off-resonance Raman spectrum at 785 nm reveal different Franck–Condon active vibrations for the two electronically excited states. The resonance enhancements at visible wavelengths are consistent with initial motion on the first excited-state that promotes the cycloreversion reaction, whereas the enhancements for excitation at near-UV wavelengths highlight motions involving conjugated backbone and phenyl ring stretching modes that are orthogonal to the reaction coordinate. The results support a mechanism involving rapid internal conversion from the higher-lying state followed by cycloreversion on the first excited state. These observations provide new information about the reactivity of DAE derivatives following excitation in the visible and near-UV.

Two-Dimensional Electronic Spectroscopy Resolves Relative Excited-State Displacements

“…The ability to initiate coherent dynamics and probe the structure of higher-lying excited states is becoming increasingly important in ultrafast photochemistry. Several recent experiments show that the excitation of electronic states higher than S 1 leads to novel photophysical and photochemical processes not accessible by other pathways, encompassing processes such as novel reactive channels in photochromics, charge injection for photovoltaics, and anti-Kasha emission. − The present data suggest that coherence spectroscopies provide new information and potentially suggest routes to control such phenomena.…”

Two-Dimensional Electronic Spectroscopy Resolves Relative Excited State Displacements

“…Several recent observations show that excitation of electronic states higher than S1 leads to novel photophysical and photochemical processes not accessible by other pathways. [31][32][33][34][35] used in the 2DES simulations are reported in Figure 1e, following the colour code of Figure 1a. The spectra in Figure 1 f-h were calculated using the hierarchical equations of motion (HEOM) method that fully accounts for solvent effects through multiple overdamped baths in combination with the equation of motion-phase matching approach (EOM-PMA), explicitly accounting for the electric fields of the pump and probe pulses, as previously described by Green et al 36 Full details of the model are given in the SI.…”