Vibronic coupling of competing internal conversion and intersystem crossing in xanthone

Area of Science:

• * Theoretical chemistry
• * Photochemistry
• * Quantum mechanics

Background:

• * Nonradiative decay pathways, including internal conversion (IC) and intersystem crossing (ISC), are crucial for understanding the photophysics of aromatic ketones.
• * The dominance of IC versus ISC in xanthone has been a subject of debate due to rapid ISC in these molecules.
• * El-Sayed's rule provides a framework for predicting these transitions, suggesting a specific pathway for xanthone.

Purpose of the Study:

• * To theoretically investigate the vibronic coupling mechanisms driving IC and ISC in xanthone.
• * To elucidate the factors governing the relative rates of IC and ISC in aromatic ketones.
• * To provide insights for designing molecules with controlled nonradiative decay pathways.

Main Methods:

• * Employed Fermi's golden rule to calculate nonradiative rate constants.
• * Utilized the mixed-spin crude adiabatic approximation to treat IC and ISC as vibronically induced transitions.
• * Analyzed vibronic structures and coupling strengths within the xanthone molecule.

Main Results:

• * Calculated that internal conversion (IC) from the 1ππ* to 1nπ* state is faster than intersystem crossing (ISC) channels.
• * Identified stronger vibronic coupling and less favorable spin-orbit (SO) coupling to nearby triplets as reasons for faster IC.
• * Determined that ISC from 1nπ* to 3ππ* is more efficient than from 1nπ* to 3nπ* due to larger structural changes and smaller energy gaps.

Conclusions:

• * The balance between vibronic coupling, spin-orbit (SO) coupling, and energy gaps dictates whether IC or ISC dominates.
• * Xanthone's nonradiative decay follows a pathway consistent with El-Sayed's rule, with IC being the initial faster process.
• * Molecular design strategies, such as tuning singlet-triplet energy gaps and SO coupling, can control the relative rates of IC and ISC in xanthone derivatives.