Structural dynamics of 4-cyanobenzaldehyde in S2(ππ*) state

Abstract

The excited state structural dynamics of 4-cyanobenzaldehyde (p-CNB) were studied by using the resonance Raman spectroscopy and the quantum mechanical calculations. The experimental A- and B-band absorptions were, respectively, assigned to the major nO [RIGHTWARDS ARROW] π3* and π2 [RIGHTWARDS ARROW] π3* transitions according to the B3LYP-TD/6-31G(d) and CIS/6-31G(d) computations, and the resonance Raman spectra. It was determined that the actual S2(π2π3) state was in energy lower than S3(π1π3), which was just opposite to the B3LYP-TD/6-31G(d) calculated order of the S2(π1π3) and S3(π2π3). The vibrational assignments were carried out for the A- and B-band resonance Raman spectra. The B-band resonance Raman intensities of p-CNB were dominated by the C2–C3/C5–C6 symmetric stretch mode ν8, the overtones nν8 and their combination bands with the ring C–H bend mode ν17, the C9–N10 stretch mode ν6, the C7–O8 stretch mode ν7 and the remaining modes. The conical intersection between S1(nOπ3) and S2(π2π3) states of p-CNB was determined at complete active space self-consistent field (CASSCF)(8,7)/6-311G(d,p) level of theory. The B-band short-time structural dynamics and the corresponding decay dynamics of p-CNB were obtained by analysis of the resonance Raman intensity pattern and CASSCF computations. The resonance Raman spectra indicated that CI[S1(nOπ3)/S2(π1π2π3π4)] located nearby the Franck–Condon region. The excited state decay dynamics evolving from the S2, FC(π2π3) to the S1(nOπ3) state was proposed.