Time-dependent density functional theory (TD-DFT) with linear response polarizable continuum model (LR-PCM) is widely applied to study the excited states of solvated organic fluorescence materials and provide theoretical predictions for emission energies. Despite various benchmark studies, the choice of theoretical level remains problematic partly due to the system dependence of optimal functional and the divergent method recommendation based on existing datasets. As one underlying reason, we describe here a surprisingly significant influence of solvent polarity on the functional choice in TD-DFT/LR-PCM computations. More than 200 experimentally measured emission wavelengths for a set of 22 typical organic dyes in up to 33 common solvents spanning a wide spectrum of low-to-high polarity were gathered. Assuming vertical emissions, statistics over TD-DFT/LR-PCM predictions in non-polar solvents shows the best agreement with experiments using LC-ωPBE(20) (0.12 eV by mean absolute error (MAE)), PBE38 (0.14 eV), M06-2X (0.15 eV) and CAM-B3LYP (0.16 eV). However, these methods lead to notably higher MAEs of at least 0.38 eV in polar solvents. Instead, the B3LYP hybrid (0.16 eV) outperforms all other tested functionals in the high-polarity regime. PBE0 and M06 exhibit balanced accuracy in polar and non-polar solvents, the MAEs being ca. 0.2 eV. Besides unifying some discrepancies in literature, the studies emphasize the importance of considering solvent polarity in the evaluation and selection of TD-DFT/LR-PCM methods.

Organic fluorescence emission from TD-DFT/LR-PCM computations: The importance of solvent polarity

ACS Spring 2022

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