A first principles examination of phosphorescence.

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Photophysics

Background:

• Phosphorescence, a spin-forbidden radiative decay process, is crucial for understanding molecular luminescence.
• Calculating the T1 → S0 transition dipole moment is computationally challenging.
• Accurate phosphorescence lifetime predictions require a deep understanding of electronic state couplings.

Purpose of the Study:

• To simplify the computation of spin-forbidden phosphorescence.
• To investigate the role of specific electronic states in phosphorescence.
• To provide a first-principles computational framework for phosphorescent spectra.

Main Methods:

• First-principles calculations of phosphorescence.
• Coupling of electronic states within Franck-Condon and Herzberg-Teller regimes.
• Analysis of T1 → S0 transition dipole moment contributions.

Main Results:

• The T1 → S0 transition dipole moment calculation is less complex than previously thought.
• A second-order corrected rate constant of 0.402 s^-1 was obtained for gas-phase acridine, aligning with experimental data.
• Only specific electronic states with strong spin-orbit coupling significantly contribute to the transition dipole moment.

Conclusions:

• Phosphorescence calculations can be streamlined by focusing on essential electronic states.
• The nπ* state's role in phosphorescence is primarily through fluorescence quenching and enhancing non-radiative decay, not direct transition dipole moment contribution.
• Accurate phosphorescence modeling relies on identifying and including states with significant spin-orbit coupling.