Revealing ultrafast vibronic dynamics of tetracene molecules with sub-8 fs UV impulsive Raman spectroscopy.

Ultrafast dynamics of tetracene molecules in THF solution were investigated using sub-8 fs ultraviolet pulse lasers and calculations. The time trace of absorbance changes exhibited ultrafast decay with a time constant of 165 ± 10 fs because of the relaxation from a vibronically hot excited state to the potential minimum in the S state. From the signals of absorbance changes in the negative time region, we obtained the electronic dephasing time of 31.27 ± 1.63 fs. Inverse Fourier transform of stationary absorption spectra exhibited rapid decay with 2.1 ± 0.08 fs. From these data, we estimated the ratio of total dephasing time to homogenous and inhomogeneous broadening as 6.7% and 93.3%, respectively. Impulsive Raman spectra reflect the wave packet dynamics of vibrational modes. Although inhomogeneous broadening blurred the phase jump across the resonance peak in the spectral range, 1156 and 1680 cm vibrational modes exhibited a phase jump from -π to ∼π and -0.5π to ∼0.5π, respectively. The amplitude profiles of these vibrational modes agree with simulated vibronic progressions of combination bands. Time-frequency analysis revealed coupling dynamics between low- and high-frequency modes, where high-frequency modes are in-plane motions and low-frequency modes are out-of-plane motions. Therefore, these coupling dynamics induce symmetry-breaking of the molecular framework, which fastens the singlet fission process.