Theoretical investigation on proton-induced intramolecular charge transfer of a D-π-A dye for a pH molecular switch

The ground and excited state properties of the neutral and protonated dye molecule, 3-(dicyanomethylene)-5,5-dimethyl-1(4-[(2-hydroxy-ethyl)-methyl-amino]-styryl) cyclohexene (DCDHC), for a pH fluorescent switch have been investigated using density functional theory (DFT) and time-dependent DFT (TDDFT) in combination with polarizable continuum models (PCM) and compared with experimental absorption and fluorescence spectra. The global-hybrid B3LYP and global-hybrid meta-generalized gradient approximations functional M06 have been used. The calculations have reproduced both the UV–Vis absorption and fluorescent emission spectra of the experimental observations. It has been found that the N atom of the amino group should be the major site to accept a proton. Upon the addition of protons, proton-induced intramolecular charge transfer occurred from the electronic donor to acceptor resulting in the fluorescence quenching. The mechanism on the operation of the pH molecular switch has been given.