Predicting Excited-State Absorption Spectra from Non-Aufbau Configurations

Area of Science:

• Computational Chemistry
• Spectroscopy
• Theoretical Chemistry

Background:

• Accurate prediction of excited-state absorption (ESA) spectra is vital for interpreting transient absorption spectroscopy (TAS) data.
• Existing electronic structure methods often face challenges in balancing accuracy, computational efficiency, and chemical intuition.

Purpose of the Study:

• To develop a novel computational approach for predicting ESA spectra that overcomes limitations of current methods.
• To provide a tool that enhances the interpretation of TAS features by mapping them to specific electronic and geometric species.

Main Methods:

• Introduction of the LR-TDA/ΔSCF method, combining linear-response Tamm-Dancoff approximation (LR-TDA) with Δ self-consistent-field (ΔSCF) and maximum overlap method (MOM).
• This method incorporates excited-state orbital relaxation while maintaining computational efficiency and interpretability.
• Benchmarking against experimental femto- and nanosecond TAS data for azobenzene, a BODIPY derivative, and a zinc porphyrin complex.

Main Results:

• The LR-TDA/ΔSCF method successfully reproduces experimental ESA spectra with good accuracy.
• The approach demonstrates reliability even when neglecting vibronic effects or describing multiconfigurational excited states with single determinants.
• Successful mapping of TAS spectral features to specific species and transitions was achieved.

Conclusions:

• LR-TDA/ΔSCF presents an accurate, cost-effective, and interpretable method for ESA spectral prediction.
• This method significantly aids in assigning TAS spectral features, facilitating the elucidation of photochemical and photophysical mechanisms.