Ultrafast intersystem crossing for nitrophenols: ab initio nonadiabatic molecular dynamics simulation

Area of Science:

• * Theoretical Chemistry
• * Photochemistry
• * Molecular Dynamics

Background:

• * Understanding excited-state dynamics is crucial for photochemistry.
• * Intersystem crossing (ISC) is a key process in molecular relaxation.
• * Nitrophenols are model systems for studying photophysical phenomena.

Purpose of the Study:

• * To investigate the ultrafast intersystem crossing mechanisms in p-nitrophenol (PNP) and m-nitrophenol (MNP).
• * To elucidate the role of specific relaxation pathways and their time constants.
• * To correlate electronic structures and relaxation dynamics with molecular motions.

Main Methods:

• * Ab initio nonadiabatic molecular dynamics simulations.
• * 6SA-CASSCF level of theory for electronic structure.
• * Trajectory surface hopping within a defined intersystem crossing network (S0, S1, T1, T2).

Main Results:

• * The dominant S1 → T2 pathway accounts for 65.4% (PNP) and 85.0% (MNP) of quantum yield with ultrafast time constants (13.4 fs for PNP, 22 fs for MNP).
• * Secondary pathways like S1 → T2 → S0 and S1 → T2 → T1 → S0 were quantified with longer time constants.
• * Relaxation mechanisms are closely linked to the torsional motions of the nitro and hydroxyl groups.

Conclusions:

• * The study reveals distinct ultrafast intersystem crossing dynamics for PNP and MNP.
• * The dominant S1 → T2 pathway plays a critical role in the rapid relaxation of these molecules.
• * Findings provide new physical insights into the mechanisms governing ultrafast photochemical intersystem crossing.