Theoretical and experimental study of the electronic spectrum and photophysics of Michler's hydrol blue

Abstract

The properties of the lowest electronic states of Michler's hydrol blue, the prototypic diarylmethane dye, were investigated by photophysical measurements and CS-INDO CI calculations. MHB solutions showed very weak fluorescence emissions (Φ F≈2.0–3.0×10 −4) at room temperature and, from the sensitized T 1-T n absorption spectrum of MHB in acetic acid, the T 1 formation quantum yield (Φ T) was estimated to be below 3.5 × 10 −2. Starting from these observations, which point to a cyanine-like nature of MHB, a detailed theoretical study was carried out taking BMNC (bisdimethylaminononamethine cyanine) as the suitable reference chromophore. Calculations included the ground state conformation, the electronic S 0-S n and T 1-T n absorption spectra and the potential energy curves for twisting about one central CC bond in the S 0, T 1 and S 1 states. On the whole, the reported results support the existence of a close relationship between MHB and BMNC and indicate that the fast deactivation of the lowest excited singlet state occurs via internal conversion from twisted configurations characterized by charge localization.