Spectral signature of 2-[4-(dimethylamino)styryl]-1-methylquinolinium iodide: a case of negative solvatochromism in water.

Photophysics of the 2-[4-(dimethylamino) styryl]-1-methylquinolinium iodide (DASQMI) molecule has been studied in different solvents by steady-state and time-resolved emission spectroscopy and also with quantum chemical calculations. The probe molecule exhibits a strong solvent-polarity-dependent characteristic. The low-energy fluorescence band of DASQMI shows an anomalous 40 nm blue shift in water from that in dimethyl sulfoxide (DMSO); though in deuterium oxide the normal trend of red shift was observed. A marked increase in intensity of this band at 77 K and an increase in lifetime in viscous solvent point clearly to the intramolecular charge-transfer (ICT) character of the low-energy band. From the temperature-dependent emission and emission spectra in mixed solvents, the negative solvatochromism of DASQMI has been established, which means that the ICT state moves toward ground state with polarity and hydrogen-bond ability and beyond a critical dielectric constant coupled with protic nature of the solvent ground state gets further stabilized to show anomalous blue shift. In ethanol, below a critical temperature, 253 K, a blue shift starts due to greater solvent molecular polarization. A third long-lifetime component with dominant 75% amplitude was observed only in aqueous solution and may be due to the cis-isomer of hydrophobic DASQMI, a stable form in the excited state predicted from polarizable continuum model (PCM) calculations in water with 6-31G+(d,p) as basis set.