Photophysical Characterization of Benzylidene Malononitriles as Probes of Solvent Friction

Abstract

Steady-state absorption and emission and femtosecond time-resolved emission spectroscopy of two benzylidene malononitriles, 2-[4-(dimethylamino)benzylidene])malononitrile (DMN) and julolidinemalononitrile (JDMN), are reported in a variety of room-temperature solvents. Solvatochromic shifts of these molecules are consistent with dielectric continuum descriptions and an S(1)-S(0) dipole moment change of 8.5 D. Time-resolved spectra show modest dynamic Stokes shifts of approximately 1000 cm(-1) occurring independently of fluorescence decay, which takes place in 0.5-5 ps in most room-temperature solvents. Absorption transition moments and fluorescence decay times are used to determine radiative rate constants: k(rad) = 0.32 +/- 0.02 ns(-1) in DMN and 0.28 +/- 0.02 ns(-1) in JDMN, assumed to be independent of solvent. Quantum yield data together with these radiative rates provide the reaction rate constants k(rxn) associated with the internal conversion process of these molecules in 33 representative solvents at 298 K and in several solvents as functions of temperature. Reaction rates of JDMN are systematically lower than those of DMN by a factor of 2.0. Values of k(rxn) in series of homologous solvents or in a single solvent at different temperatures are correlated to solvent viscosity eta and temperature T in the manner k(rxn)/T proportional to eta(-p) with exponents 0.2 < or = p < or = 0.8. Solvent polarity appears to influence these reactions such that for a given viscosity reaction in high polarity solvents is significantly slower than in nonpolar solvents. However, this conclusion is predicated on the assumption that reactive friction is identical in solvents of the same viscosity, which is unlikely to be quantitatively correct. The observed reaction rates and their solvent dependence are discussed in terms of isomerization about the C=C bond occurring on a shelf-like potential.