Theoretical and experimental study of the electronic spectrumand photophysics of Michler’s hydrol blue

The propertles of the lowest electronic states of Michler’s hydrol blue, the prototypic dlarylmethane dye, were investigated byphotophysical measurements and CS-INDO CI calculations. MHB solutions showed very weak fluorescence emissions (&x 2.0-3.0~ 10m4) at room temperature and, from the sensitized T,-T. absorption spectrum of MHB in acetic acid, the T, formatlonquantum yield (&) was estimated to be below 3.5 x 1 O-‘. Starting from these observations, which point to a cyanme-like natureof MHB, a detailed theoretical study was carried out taking BMNC (bisdlmethylammononamethine cyanine) as the suitablereference chromophore. Calculations included the ground state conformation, the electronic So-S,, and T,-T, absorption spectraand the potential energy curves for twisting about one central C-C bond in the SO. T, and S, states. On the whole, the reportedresults support the existence of a close relationship between MHB and BMNC and indicate that the fast deactivation of the lowestexcited singlet state occurs via internal conversion from twisted configurations characterized by charge localization