Theoretical study of the excitation of proflavine H-dimers in an aqueous solution: the effect of functionals and dispersion corrections

Abstract

Analysis of the ground and excited states of the H-dimer of the proflavine dye cation (PF) in an aqueous solution was performed using the DFT/TD-DFT method with an implicit specification of the aqueous environment and using various hybrid functionals (APFD, B2PLYP, B3LYP, B3PW91, BMK, CAM-B3LYP, M05, M052X, M06, M062X, M06HF, mPW2PLYP, PBE0, PW6B95, and ωB97XD), Grimme dispersion corrections, and Becke–Johnson damping. To our knowledge, this is the first theoretical study on the dimerization of charged monomers. The use of the B2PLYP, B3LYP, B3PW91, BMK, CAM-B3LYP, PBE0, M05, mPW2LYP, and PW6B95 functionals without additional dispersion corrections led to dimer dissociation due to Coulombic repulsion between PF cations. At the same time, the M052X, M06, M062X, and M06HF functionals without dispersion corrections showed reliable stabilization of PF₂cations. APFD and ωB97XD with built-in dispersion corrections also performed well. This photoinduced attraction is explained by the different overlaps of the respective molecular orbitals. In this case, there is no significant intermolecular charge transfer in PF₂. The electron density shifts in dye molecules during their dimerization significantly exceeded those upon excitation of both the free monomer and the dimer. The transition moments M showed that the CAM-B3LYP, M052X, M062X, M06HF, and ωB97XD functionals gave M(monomer) < M(H-dimer), while all other functionals used in this work showed M(monomer) > M(H-dimer). The former pattern was described using the strong coupling theory for H-aggregates. The interaction between PF molecules in the H-dimer is indeed strong and, therefore, is correctly described precisely using the functionals with the long-range correction or large exact Hartree–Fock exchange. Only these five functionals were found to satisfy the condition λ_max(H-dimer) < λ_max(monomer), which is observed experimentally for PF and is explained using the Kasha exciton theory. They alone give rise to very strong intramolecular vibrations in the spectrum of the excited dimer. The vibronic absorption spectra could only be calculated using the M062X, M06HF, and ωB97XD functionals, which had the lowest RMSD between the ground and excited states. Based on the results obtained, we recommend the CAM-B3LYP (with dispersion correction), M052X, M062X, M06HF, and ωB97XD functionals for the theoretical description of aromatic cation dimers in both the ground and excited states.