FLUORESCENCE QUANTUM YIELDS AND LIFETIMES OF SUBSTITUTED STILBENES IN N-ALKANES - A REEXAMINATION OF THE RELATIONSHIP BETWEEN SOLUTE SIZE AND MEDIUM EFFECT ON TORSIONAL RELAXATION

This work presents new data and a new analysis on the photoisomerization of trans-stilbenes in n-alkanes. Fluorescence quantum yields of trans-4,4'-di-tert-butylstilbene in n-hexane and n-tetradecane, measured as a function of temperature, are combined with experimental fluorescence lifetimes to define the index of refraction dependence of the radiative rate constant, k(f) = 3.9n(1.8) x 10(8) s(-1). This relationship is used in calculations of torsional relaxation rate constants, k(tp), for trans-4,4'-dimethyl- and trans-4,4'-di-tert-butylstilbene in the n-alkane solvent series. For trans-4,4'-dimethoxystilbene k(f) = 3.45n(x) x 10(8) s(-1) is used with x, confined in the 1.55 less than or equal to x less than or equal to 2.30 range, selected separately for each alkane. In addition, new lifetime data are employed to extend the k(tp) values to lower temperatures. It is found that activation parameters for k(tp), based on Eyring's transition state theory, adhere to the medium-enhanced barrier model relationship, Delta H-t(double dagger) = Delta H-t(double dagger) + aE(eta s), and to the isokinetic relationship. The a value, a measure of the effect of medium friction on solute torsional motion, increases from 0.41 in stilbene to 0.43 in dimethoxystilbene and to 0.47 in the two dialkylstilbenes. A significant increase (similar to 1 kcal/mol) in the activation enthalpy, Delta H-t(double dagger) , of the intrinsic torsional relaxation is found for the dimethoxy derivative. The previously predicted large increase in Delta H-t(double dagger) for di-tert-butylstilbene is not borne out by this analysis. The isokinetic relationship between the activation parameters for the parent trans-stilbene leds to an isokinetic temperature of beta = 600 +/- 11 K and brings it into excellent agreement with beta(eta mu), the isokinetic temperature for activation parameters based on estimated microviscosities, eta(mu), experienced by stilbene in its torsional motion. The conclusion that only when microviscosities rather than shear viscosities, eta(s), are employed in the power law expression k(tp) = k(t)B eta(-a) can a = a be expected is supported by this result.