Modelling pressure and solvation effects in intermolecular charge-transfer excitations

The excitation energy of intermolecular charge-transfer (ICT) states depends asymptotically on the distance between the donor and acceptor partners, and is strongly influenced by the electrostatic environment exposed by solvent molecules. Conversely, the transition dipole moment (TDM) of ICT states increases when donor and acceptor molecules are brought close together by applying hydrostatic pressure. In this work, we explore these effects with low-scaling excited-state electronic-structure methods based on density functional theory (DFT). In the first part of the study, we benchmark time-dependent (TD-)DFT and the ALMO-SGM orbital-optimized DFT method against experimental data. Results obtained with ALMO-SGM are systematically improved by more accurately modeling the electrostatic environment, whereas the performance of TD-DFT is deteriorated. ALMO-SGM results show little system dependence, whereas TD-DFT results vary significantly across systems. In the second part of our study, we investigate the effects of donor-acceptor separation on the TDM of ICT states with low-scaling methods. ALMO-SGM reproduces systematic changes in the TDM obtained by moving the donor and acceptor molecules far apart, whereas TD-DFT failed at times. Finally, we model the effects of hydrostatic pressure on a flavin-indole system, which was previously investigated experimentally. TD-DFT results depend critically on the quality of the exchange-correlation functional (XCF), whereas ALMO-SGM results remain consistent for different XCF parametrizations. Applying higher hydrostatic pressure pushes the donor and acceptor partners closer together, leading to a redshift of the excitation energy and an increase in the TDM of the ICT state. This result is consistent across three conformations of the flavin-indole dimer and is in qualitative agreement with experimental findings, albeit requiring a higher applied empirical pressure to observe significant absorbance changes.