Solvatochromic Shift of Brooker's Merocyanine: Hartree-Fock Exchange in Time Dependent Density Functional Calculation and Hydrogen Bonding Effect

The Brooker's merocyanine exhibits a large hypsochromic shift from an apolar aprotic solvent to a polar protic solvent. Quantum chemical calculations have been performed to study the solvatochrornism, but there remained a discrepancy between the calculated and experimental solvatochromic shifts. In this paper we evaluate quantum mechanically the excitation energies of the Brooker's merocyanine in water, methanol, acetonitrile, and dichloromethane to investigate what are important factors to accurately model the solvatochromism of the dye by using TDDFT in combination with implicit and explicit solvation models including the PCM, PCMSMD, RISM-SCF-SEDD, and mean-field QM/MM. The results severely depend on the density functional, especially on the amount of Hartree-Fock exchange included in the functional. Furthermore, an explicit description of the solute solvent hydrogen bonds makes a non-negligible contribution to the shift. The experimental large solvatochromic shift can be accurately reproduced by the TDDFT/RISM-SCF-SEDD and mean-field QM/MM calculations with the LC-BOP functional, although the excitation energies in solutions are considerably overestimated. We also estimated the excitation energies and the solvatochromic shift at the SAC-CI/RISM-SCF-SEDD and mean-field QM/MM level, which are in very good agreement with the experimental values. These results indicate that if an explicit solvent model is used, TDDFT calculations using such a long-range corrected functional can accurately model the solvatochromism. However, an ab initio quantum chemical method including sufficient electron correlation effects is required to reproduce not only the solvatochromism but also the excitation energies in solutions.