Beyond the Condon limit: Condensed phase optical spectra from atomistic simulations

Area of Science:

• Computational chemistry
• Spectroscopy
• Materials science

Background:

• Simulating non-Condon effects in condensed phase spectroscopy is challenging.
• Molecular motions significantly influence optoelectronic properties by brightening dark transitions.

Purpose of the Study:

• To develop a Gaussian theory for predicting and analyzing condensed phase optical spectra beyond the Condon limit.
• To provide a framework for understanding how nuclear motions modulate electronic transition properties.

Main Methods:

• Derivation of a Gaussian theory incorporating spectral densities for energy gap and transition dipole modulation.
• Statistical framework using thermal averages and fluctuations to handle anharmonic interactions and strong non-Condon effects.
• Calculation of spectral densities using first-principles simulations with finite-temperature, disorder, and dynamical effects.

Main Results:

• Accurate prediction of spectra for systems with strong non-Condon effects (e.g., phenolate).
• Identification of distinct mechanisms for electronic peak splitting, including timescale separation and spectral interference.
• Development of analysis tools to assess the impact of vibrations, solute-solvent interactions, and polarization on dark transitions.

Conclusions:

• The novel Gaussian theory effectively predicts condensed phase optical spectra beyond the Condon limit.
• The theory elucidates mechanisms of peak splitting and quantifies the influence of environmental factors.
• An upper bound on fluctuation correlations provides a condition for accurate spectral prediction.