First-Order Interacting Space Approach to Excited-State Molecular Interaction: Solvatochromic Shift of <i>p</i>‑Coumaric Acid and Retinal Schiff Base

A triple-layer QM/sQM/MM method was developed for accurately describing the excited-state molecular interactions between chromophore and the molecular environment (Hasegawa, J.; Yanai, K.; Ishimura, K. <i>ChemPhysChem</i> <b>2015</b>, <i>16</i>, 305). A first-order-interaction space (FOIS) was defined for the interactions between QM and secondary QM (sQM) regions. Moreover, configuration interaction singles (CIS) and its second-order perturbation theory (PT2) calculations were performed within this space. In this study, numerical implementation of this FOISPT2 method significantly reduced the computing time, which realized application to solvatochromic systems, <i>p</i>-coumaric acid in neutral (<i>p</i>-CA) and anionic forms in aqueous solution, retinal Schiff base in methanol (MeOH) solution, and bacteriorhodopsin (bR). The results were consistent with the experimentally observed absorption spectra of the applied systems. The QM/sQM/MM result for the opsin shift was in better agreement to the experimental result than that of the ordinary QM/MM. A decomposition analysis was performed for the excited-state molecular interactions. Among the electronic interactions, charge-transfer (CT) effect, excitonic interaction, and dispersion interaction showed significant large contributions, while the electronic polarization effect presented only minor contribution. Furthermore, the result was analyzed to determine the contributions from each environmental molecule and was interpreted based on the distance of the molecules from the π system in the chromophores

Electronic Polarization Effect of the Water Environment in Charge-Separated Donor–Acceptor Systems: An Effective Fragment Potential Model Study

The electronic polarization (POL) of the surrounding environment plays a crucial role in the energetics of charge-separated systems. Here, the mechanism of POL in charge-separated systems is studied using a combined quantum mechanical and effective fragment potential (QM/EFP) method. In particular, the POL effect caused by charge separation (CS) is investigated at the atomic level by decomposition into the POL at each polarizability point. The relevance of the electric field generated by the CS is analyzed in detail. The model systems investigated are Na⁺–Cl^– and guanine–thymine solvated in water. The dominant part of the POL arises from solvent molecules close to the donor (D) and acceptor (A) units. At short D–A distances, the electric field shows both positive and negative interferences. The former case enhances the POL energy. At longer distances, the interference is weakened, and the local electric field determines the POL energy