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

Abstract

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 intersystem 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 S2,FC(ππ*) to S0 or T1,min via various CIs and ISCs were proposed.