Femtosecond coherent nuclear dynamics of excited tetraphenylethylene: Ultrafast transient absorption and ultrafast Raman loss spectroscopic studies.

Ultrafast torsional dynamics plays an important role in the photoinduced excited state dynamics. Tetraphenylethylene (TPE), a model system for the molecular motor, executes interesting torsional dynamics upon photoexcitation. The photoreaction of TPE involves ultrafast internal conversion via a nearly planar intermediate state (relaxed state) that further leads to a twisted zwitterionic state. Here, we report the photoinduced structural dynamics of excited TPE during the course of photoisomerization in the condensed phase by ultrafast Raman loss (URLS) and femtosecond transient absorption (TA) spectroscopy. TA measurements on the S1 state reveal step-wise population relaxation from the Franck-Condon (FC) state → relaxed state → twisted state, while the URLS study provides insights on the vibrational dynamics during the course of the reaction. The TA spectral dynamics and vibrational Raman amplitudes within 1 ps reveal vibrational wave packet propagating from the FC state to the relaxed state. Fourier transformation of this oscillation leads to a ∼130 cm-1 low-frequency phenyl torsional mode. Two vibrational marker bands, Cet =Cet stretching (∼1512 cm-1 ) and Cph =Cph stretching (∼1584 cm-1 ) modes, appear immediately after photoexcitation in the URLS spectra. The initial red-shift of the Cph =Cph stretching mode with a time constant of ∼400 fs (in butyronitrile) is assigned to the rate of planarization of excited TPE. In addition, the Cet =Cet stretching mode shows initial blue-shift within 1 ps followed by frequency red-shift, suggesting that on the sub-picosecond time scale, structural relaxation is dominated by phenyl torsion rather than the central Cet =Cet twist. Furthermore, the effect of the solvent on the structural dynamics is discussed in the context of ultrafast nuclear dynamics and solute-solvent coupling.