Modeling absorption spectra of molecules in solution

Abstract

The presence of solvent tunes many properties of a molecule, such as its ground and excited state geometry, dipole moment, excitation energy, and absorption spectrum. Because the energy of the system will vary depending on the solvent configuration, explicit solute-solvent interactions are key to understanding solution-phase reactivity and spectroscopy, simulating accurate inhomogeneous broadening, and predicting absorption spectra. In this tutorial review, we give an overview of factors to consider when modeling excited states of molecules interacting with explicit solvent. We provide practical guidelines for sampling solute-solvent configurations, choosing a solvent model, performing the excited state electronic structure calculations, and computing spectral lineshapes. We also present our recent results combining the vertical excitation energies computed from an ensemble of solute-solvent configurations with the vibronic spectra obtained from a small number of frozen solvent configurations, resulting in improved simulation of absorption spectra for molecules in solution.excited states, TDDFT, solvation, inhomogeneous broadening, absorption spectra | INTRODUCTIONThe presence of solvent around a molecule affects its energy, properties, dynamics, and reactivity, in both the ground and excited state. Because many excited state phenomena of chemical interest happen in solution and in complex interfacial environments, including photosynthesis, photocatalysis, photoinduced charge and proton transfer, and fluorescence in biological systems, it is important to be able to model these excited state reactions while accurately simulating the solvent environment. In addition, both static and time-resolved absorption and fluorescence spectroscopies serve as useful tools to elucidate chemical reactions and pathways, and simulation of these solution-phase spectra can help to achieve understanding of the electronic rearrangements governing chemical reactions.If theoretical models are to provide reliable simulations of solution-phase chemistry, accurate models of the solvent environment are required.The solvent environment can have a strong influence on an absorption spectrum due to polarization and direct solute-solvent interactions such as hydrogen bonding. Therefore, the modeling of absorption spectra of molecules in solution is often considered a vital step in developing and benchmarking methods to account for the complex solvent environment.Although continuum solvent approaches are computationally attractive because they use the bulk properties of the solvent to polarize a solute, and thus, usually do not sample solute-solvent configurations, in reality it is specific solvent configurations rather than a solvent continuum that determine excited state properties. Also, the simulation of spectral lineshapes, such as those shown in Figure 1 that include both the highenergy tail due to vibronic transitions and the inhomogeneous broadening due to solute-solvent interactions, poses a significant challenge for continuum methods. T...

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