Photophysics of meta- and para-nitrophenolate. II. Electronic spectra and quantum dynamics

The vibronic structure and nuclear dynamics underlying the first absorption bands of meta-nitrophenolate (m-NP) and para-nitrophenolate (p-NP) in the coupled manifold of S1-S2-S3-S4-S5 electronic states are investigated here. Attempts are specifically made to understand the broadening of the experimentally recorded gas phase electronic absorption spectra of both the molecules. Justification is also provided for the shorter lifetime of the S1 state of p-NP as compared to m-NP. First principles nuclear dynamics calculations are performed by both time-independent and time-dependent quantum mechanical methods. The model vibronic coupling Hamiltonians developed via extensive ab initio quantum chemical calculations in Paper I [Mondal et al., J. Chem. Phys. 162, 144314 (2025)] are employed for this purpose. The theoretical results presented in this paper are in good agreement with the available experimental data. While the resolved vibrational peaks are observed in the first absorption band of m-NP, the same for p-NP is highly broad and structureless. Occurrence of S1-S3 low-energy conical intersections relative to S1 equilibrium minimum energy in p-NP leads to a nonradiative internal conversion of the S1 state in ∼1.9 ps, whereas the high energy conical intersections of S1 state with other states in m-NP trapped the wave packet prepared on it and led to a much longer decay rate of ∼30.8 ps of the S1 state of m-NP.