Structural Dynamics of 3-Dimethylamino-2-methyl-propenal in S₂(ππ^*)State (cited: 3)

The photophysics of 3-dimethylamino-2-methyl-propenal (DMAMP) after excitation to the S₂(ππ^*) electronic state was studied using the resonance Raman spectroscopy and complete active space self-consistent field method calculations. The transition barriers of the ground state tautomerization reactions between DMAMP and its three isomers were determined at B3LYP/6-311++G(d,p) level of theory. The vibrational spectra were assigned. The A-band resonance Raman spectra were obtained in acetonitrile with excitation wavelengths in resonance with the first intense absorption band to probe the structural dynamics of DMAMP. The B3LYP-TD computation was carried out to determine the relative A-band resonance Raman intensities of the fundamental modes, and the result indicated that the vibronic-coupling existed in Franck-Condon region. Complete active space self-consistent field (CASSCF) calculations were carried out to determine the excitation energies of the lower-lying singlet and triplet excited states, the conical intersection points and the inter-system crossing points. The A-band short-time structural dynamics and the corresponding decay dynamics of DMAMP were obtained by analysis of the resonance Raman intensity pattern and CASSCF computations. It was found that a sudden de-conjugation between C1=O6 and C2=C3 occurred at the Franck-Condon region of the S₂(ππ^*) state, while the enhancement of the conjugation interaction between C3 and N(CH₃)₂, and between C1 and C2 evolutions shortly after the wavepacket leaves away the Franck-Condon region via the excited state charge redistribution. The de-conjugation interaction between C1=O6 and C2=C3 made the rotation of C3=N(CH₃)₂ group around the C2-C3 bond much easier, while the enhanced conjugation between C1 and C2, and between C3 and N(CH₃)₂ made the rotation around the C1-C2 bond and C3-N5 more difficult. It was revealed that the initial structural dynamics of DMAMP was predominantly towards the CI-1(S₂/S₀) point, while the opportunities towards either CI-2(S₂/S₀) or CI-3(S₂/S₀) point were negligible. Two decay channels of DMAMP from S_2,FC(ππ^*) to S₀ or T_1,min via various CIs and ISCs were proposed.