Spin-Vibronic Coupling Enhanced Intersystem Crossing beyond El-Sayed Restrictions.

Area of Science:

• Photochemistry
• Quantum Chemistry
• Materials Science

Background:

• Intersystem crossing (ISC) is vital in photochemistry, typically driven by spin-orbit coupling.
• Designing molecules to control ISC rates is a key research area.
• Heavy atoms are conventionally used to enhance spin-orbit coupling for efficient ISC.

Purpose of the Study:

• To investigate the mechanisms behind unusually rapid ISC in organic molecules lacking heavy atoms.
• To explore the role of symmetry-perturbation in enabling ISC.
• To understand ISC in dibutylaniline thiosquaraine's lowest excited states (S(nπ*) and T(ππ*)).

Main Methods:

• Computational analysis of intersystem crossing rates.
• Investigation of spin-vibronic mechanisms.
• Focus on symmetry-perturbation effects.

Main Results:

• Identified symmetry-perturbation as the primary driver for ISC in the studied system.
• Demonstrated that ISC occurs via a symmetry-perturbing spin-vibronic mechanism.
• Showed that El-Sayed restrictions are not the dominant factor in this case.

Conclusions:

• Symmetry considerations are crucial for designing efficient heavy-atom-free molecular systems for ISC.
• This work provides new insights for chemists and engineers developing novel photochemical materials.
• The findings enable the rational design of molecules with tunable ISC properties.