Abstract
Theoretical prediction of electronic absorption spectra without input from experiments is no easy feat, as it requires addressing all of the factors that affect line shapes. In practice, however, the methodologies are limited to treat these ingredients only to a certain extent. Here, we present a multiscale protocol that addresses the temperature, solvent, and nuclear quantum effects as well as anharmonicity and the reconstruction of the final spectra from individual transitions. First, quantum mechanics/molecular mechanics (QM/MM) molecular dynamics is conducted to obtain trajectories of solute–solvent configurations, from which the corresponding quantum-corrected ensembles are generated through the generalized smoothed trajectory analysis (GSTA). The optical spectra of the ensembles are then produced by calculating vertical transitions using time-dependent density-functional theory (TDDFT) with implicit solvation. To obtain the final spectral shapes, the stick spectra from TDDFT are convoluted with Gaussian kernels where the half-widths are determined by a statistically motivated strategy. We have tested our method by calculating the UV–vis spectra of a recently discovered acridine photocatalyst in two redox states. Vibronic progressions and broadenings due to the finite lifetime of the excited states are not included in the methodology yet. Nuclear quantization affects the relative peak intensities and widths, which is necessary to reproduce the experimental spectrum. We have also found that using only the optimized geometry of each molecule works surprisingly well if a proper empirical broadening factor is applied. This is explained by the rigidity of the conjugated chromophore moieties of the selected molecules, which are mainly responsible for the excitations in the spectra. In contrast, we have also shown that other parts of the molecules are flexible enough to feature anharmonicities that impair the use of other techniques such as Wigner sampling.
Highlights
Photocatalysts employed under homogeneous catalytic conditions are molecules that harvest the energy of visible light to facilitate new transformations and new synthetic routes that may yield otherwise inaccessible scaffolds and molecules.[1]
The combination of molecular dynamics (MD) and generalized smoothed trajectory analysis (GSTA) methods yields a distribution very similar to that of the quantum harmonic model at room temperature. This similarity indicates that most vibrations are harmonic and the thermal sampling with GSTA works sufficiently, there are a number of low-frequency modes that impairs the application of Wigner sampling
We present a multiscale approach to obtain reliable absorption spectra that includes nuclear quantum effects for selected photocatalyst molecules
Summary
Photocatalysts employed under homogeneous catalytic conditions are molecules that harvest the energy of visible light to facilitate new transformations and new synthetic routes that may yield otherwise inaccessible scaffolds and molecules.[1]. The molecules typically employed in photocatalysis absorb visible light, a feature that has a significant advantage: they can be selectively excited in the visible region, whereas typical organic substrates and solvents absorb in the UV region. This selectivity can be achieved by inserting and tuning chromophore groups, such as delocalized π-systems. The electronic properties of photocatalysts can be explored by measuring or calculating their electronic (UV−vis) spectra. In this regard, calculations can be very useful because they provide a large amount of information not directly available from experiments, such as the assignation of bands to transitions between electronic states or the identification of dark states. Calculations can help to understand how structural and electronic modifications introduced to photocatalysts affect the excitations, facilitating the design of new photocatalysts
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