Nonlinear dependence of solvent polarity effects on twisted intramolecular charge-transfer states and linear relation for electronic spectra in a stilbazolium-like dye

The photophysical properties of a stilbazolium-like dye, trans-1-(N-methylpyrrol-2-yl)-2-(N-methyl-4-pyridinium)-ethene iodide, in 20 solvents and its photochemistry were investigated. A division into two groups in the linear relation between the solvatochromic shifts (absorption, fluorescence emission and corresponding Stokes shifts) and solvent polarity parameter EN was observed. The dipole moments of fluorescence emission and twisted intramolecular charge-transfer state were estimated. The quantum yields of fluorescence and trans cis photoisomerization change by almost two orders of magnitude. The rate constant of radiative decay is in the order of 10(7)-10(8) s(-1), which is about one or two orders of magnitude less than that of the photochemistry decay, whereas the rate constant of the radiationless decay is in the order of 10(8)-10(10) s(-1). The nonlinear dependence and division into two groups which differs very much from that in the above linear relation were also observed for the solvent polarity influences on the properties of the twisted intramolecular charge-transfer state. The decay scheme from the excited singlet state was assigned as controlled by the equilibrium constant for interconversion between the twisted and planar conformations, which differs from that of reported organic molecules, such as ordinary stilbazolium dyes, stilbene derivatives, or rhodamine B, etc. (C) 2002 Elsevier Science B.V All rights reserved.