On the Role of Electronic Correlation and State-Specific Environment Polarization in Singlet-Triplet Gap Inversion

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Molecules with inverted singlet-triplet gaps (ΔEST < 0) are crucial for optoelectronic applications.
• Electronic correlation and solvent polarization significantly influence these negative ΔEST values.
• A comprehensive understanding of solvation effects on molecular structures and energy gaps is lacking.

Purpose of the Study:

• To evaluate computational strategies for calculating negative singlet-triplet gaps, incorporating electronic correlation and solvent polarization.
• To investigate the impact of solvation on singlet and triplet excited states.
• To develop and validate a protocol for accurate prediction of singlet-triplet gaps and transition rates.

Main Methods:

• Utilized RMS-CASPT2 as a benchmark for computational accuracy.
• Employed double-hybrid density functionals and mixed-reference spin-flip TD-DFT (MRSF-TD-DFT) to account for electronic correlation.
• Investigated solvation effects using continuum solvation models and developed a state-specific solvent polarization approach (VEM).

Main Results:

• Double-hybrid functionals and MRSF-TD-DFT partially recover electronic correlation.
• Linear-response schemes in continuum solvation models show limitations for excited states.
• The developed B2PLYP/VEM(UD) protocol accurately reproduces experimental transition rates for systems with negative ΔEST.

Conclusions:

• The B2PLYP/VEM(UD) protocol effectively combines electronic correlation and state-specific solvent polarization for accurate calculations.
• This approach provides a reliable method for studying molecules with inverted singlet-triplet gaps.
• The findings facilitate the design of novel optoelectronic materials.