Nonadiabatic Dynamics Simulation Predict Intersystem Crossing in Nitroaromatic Molecules on a Picosecond Time Scale.

Area of Science:

• Physical Chemistry
• Quantum Mechanics
• Molecular Dynamics

Background:

• Previous studies attributed ultrafast intersystem crossing (ISC) in nitronaphthalene derivatives to timescales as short as 100 femtoseconds (fs).
• Static quantum-chemical calculations and time-resolved spectroscopic experiments supported these rapid ISC rates.

Purpose of the Study:

• To investigate the actual timescales of intersystem crossing (ISC) and internal conversion (IC) in nitronaphthalene derivatives.
• To challenge the prevailing understanding of ultrafast spin transitions in these organic molecules.

Main Methods:

• Nonadiabatic molecular dynamics simulations were employed to study three nitronaphthalene derivatives.
• Analysis focused on distinguishing between internal conversion (IC) and intersystem crossing (ISC) pathways.

Main Results:

• The simulations demonstrated that the experimentally observed ~100 fs process is internal conversion (IC) within singlet manifolds, not ISC.
• Intersystem crossing (ISC) was found to occur on a significantly longer timescale of approximately 1 picosecond (ps).
• Spin transitions proceed via two distinct pathways, influenced by electronic factors and nitro group torsion.

Conclusions:

• The study questions the existence of sub-picosecond intersystem crossing in nitronaphthalene derivatives.
• This finding suggests that the rapid ISC previously attributed to these molecules needs re-evaluation.
• The results imply that ultrafast ISC may not be as prevalent in nitroaromatic compounds as previously thought.