Tuning Nonradiative Lifetimes via Molecular Aggregation.

Area of Science:

• Photochemistry
• Molecular Spectroscopy
• Quantum Chemistry

Background:

• Nonradiative decay (NRD) is a critical process governing the excited-state lifetime of molecules.
• Molecular aggregation can alter photophysical properties compared to isolated monomers.
• Understanding the factors influencing NRD is essential for controlling molecular luminescence.

Purpose of the Study:

• To investigate how molecular aggregation influences the nonradiative decay (NRD) lifetime of electronic excitations.
• To explore the dependence of NRD lifetime on intermolecular interaction strength in a model dimer system.
• To establish design principles for tuning fluorescence quantum yield through aggregation control.

Main Methods:

• Theoretical modeling of a transition-dipole-dipole-interacting dimer.
• Utilized harmonic potential energy surfaces (PESs) for the monomer units.
• Analyzed the dependence of NRD lifetime on the position of the NRD channel (q_nr) relative to monomer dynamics.

Main Results:

• The NRD lifetime (τ_nr^dim) of the dimer exhibits distinct dependencies on interaction strength based on q_nr location.
• When q_nr is near the Franck-Condon region, τ_nr^dim increases with intermolecular interaction strength.
• When q_nr is near the monomer excited PES minimum, interaction strength has minimal effect on τ_nr^dim.
• When q_nr is near the classical turning point, τ_nr^dim decreases with increasing interaction strength.

Conclusions:

• Molecular aggregation profoundly affects NRD lifetimes, with the outcome dependent on the interplay between intermolecular interactions and the NRD pathway.
• The position of the nonradiative decay channel relative to the potential energy surfaces dictates whether aggregation enhances or diminishes the NRD lifetime.
• These findings provide a foundation for designing molecular systems with tailored fluorescence quantum yields by controlling aggregation and excited-state dynamics.