Multiscale Modeling of Electronic Spectra Including Nuclear Quantum Effects.

Area of Science:

• Computational Chemistry
• Spectroscopy
• Theoretical Physics

Background:

• Accurate theoretical prediction of electronic absorption spectra is challenging due to factors influencing line shapes.
• Existing methodologies often have limitations in fully addressing these spectral line-broadening factors.
• Experimental input is typically required, limiting purely theoretical predictions.

Purpose of the Study:

• To develop and present a multiscale protocol for the theoretical prediction of electronic absorption spectra.
• To incorporate temperature, solvent, nuclear quantum effects, and anharmonicity into spectral predictions.
• To validate the protocol by calculating UV-vis spectra of an acridine photocatalyst.

Main Methods:

• Quantum mechanics/molecular mechanics (QM/MM) molecular dynamics for generating solute-solvent configurations.
• Generalized smoothed trajectory analysis (GSTA) for generating quantum-corrected ensembles.
• Time-dependent density-functional theory (TDDFT) with implicit solvation for calculating vertical transitions.
• Convolution of TDDFT stick spectra with Gaussian kernels for final spectral shape reconstruction.

Main Results:

• The multiscale protocol successfully reproduced experimental UV-vis spectra of an acridine photocatalyst.
• Nuclear quantization was found to be crucial for accurately reproducing peak intensities and widths.
• Using only optimized geometries with empirical broadening provided surprisingly good results for rigid chromophores.

Conclusions:

• The presented multiscale protocol offers a robust method for theoretical prediction of electronic absorption spectra.
• Accounting for nuclear quantum effects is essential for accurate spectral reproduction.
• The protocol's effectiveness is demonstrated for acridine photocatalysts, with potential applicability to similar systems.