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Predicting Excitation Energies of Twisted Intramolecular Charge-Transfer States with Time-Dependent Density Functional Theory: Comparison with Experimental Measurements in the Gas-Phase and Solvents Ranging from Hexanes to Acetonitrile
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submitted on 21.06.2020, 04:13 and posted on 23.06.2020, 12:47by James Shee, Martin Head-Gordon
Electronically-excited states characterized by intramolecular charge-transfer play
an essential role in many biological processes and optical devices. The ability to make
quantitative ab initio predictions of the relative energetics involved is a challenging
yet desirable goal, especially for large molecules in solution. In this work we present a
data set of 61 experimental measurements of absorption and emission processes, both
in the gas phase and solvents representing a broad range of polarities, which involve intramolecular charge-transfer mediated by a non-zero, “twisted” dihedral angle between one or more donor and acceptor subunits. Among a variety of density functionals investigated within the framework of linear-response theory, the “optimally tuned”
LRC-ωPBE functional, which utilizes a system-specific yet non-empirical procedure
to specify the range-separation parameter, emerges as the preferred choice. For the
entire set of excitation energies, involving changes in dipole moment ranging from 4
to >20 Debye, the mean signed and absolute errors are 0.02 and 0.18 eV, respectively
(compared, e.g., to -0.30 and 0.30 for PBE0, 0.44 and 0.47 for LRC-ωPBEh, 0.83 and
0.83 for ωB97X-V). The performance of polarizable continuum solvation models for
these charge-transfer excited states is closely examined, and clear trends emerge when
measurements corresponding to the four small DMABN-like molecules and a charged
species are excluded. We make the case that the large errors found only for small
molecules in the gas phase and weak solvents cannot be expected to improve via the
optimal tuning procedure, which enforces a condition that is exact only in the wellseparated donor-acceptor limit, and present empirical evidence implicating the outsized
importance for small donor-acceptor systems of relaxation effects that cannot be accounted for by linear-response TDDFT within the adiabatic approximation. Finally, we
demonstrate the utility of the optimally tuned density functional approach by targeting the charge-transfer states of a large biomimetic model system for light-harvesting
structures in Photosystem II.