Short-time dynamics and decay mechanism of E,E-2,4-hexadienal in the first light-absorbing S₂(ππ^*) state

The initial nonadiabatic decay dynamics of E,E-2,4-hexadienal (HAL) in the light absorbing S₂(ππ^*) state were studied using resonance Raman spectroscopy and complete-active space self-consistent field (CASSCF) calculations. The UV and vibrational spectra were assigned on the basis of the UV absorption, Fourier transform (FT)-Raman and FT-infrared measurements, the density-functional theory computations, and the normal mode analysis. The A-band resonance Raman spectra in cyclohexane and acetonitrile were obtained at 282.4, 273.9, 266.0, 252.7, and 245.9 nm excitation wavelengths, respectively, to probe the corresponding structural dynamics of HAL. The A-band absorption cross section and the corresponding absolute resonance Raman cross sections were simulated using a simple model based on the time-dependent wave-packet theory in a Brownian oscillator model. The geometric structures of the singlet electronic excited states and their curve-crossing points were optimized at the CASSCF level of theory. The obtained short-time structural dynamics in easy-to-visualize internal coordinates were then compared with the CASSCF-predicted structural-parameter changes of S₂(ππ^*)S₁(nπ^*)-n (n = 1-4). Our results indicate that the initial population of HAL in the S₂ state ramifies in or nearby the Franck-Condon (FC) region, leading to five S₂(ππ^*) → S₁(nπ^*) internal conversion pathways due to the flexibility of the molecular chain and the different electronic resonant structures formed nearby FC of the S₂ state. Then, the formed S₁ transient species, which have different geometric structures and different energy partitions, undergo different photophysical processes, such as S₁ → S₀ internal conversion, S₁ → T₁ intersystem crossing, and the S₁ → S'₁ photoisomerization reaction. The substitution effect on the S₂(ππ^*) → S₁(nπ^*) internal conversion dynamics and the trans-cis photoisomerization reaction is proposed in terms of the p-π conjugation interaction or the p-σ superconjugation interaction.