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 ab initio 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 S₁ 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^-1 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.