FLUORESCENCE QUANTUM YIELDS AND LIFETIMES OF SUBSTITUTED STILBENES INN-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 photoisomerizati on of trans-stilbenes in n-alkanes. Fluorescence quantum yields of tra ns-4,4'-di-tert-butylstilbene in n-hexane and n-tetradecane, measured as a function of temperature, are combined with experimental fluoresce nce lifetimes to define the index of refraction dependence of the radi ative 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) = 3.45 n(x) x 10(8) s(-1) is used with x, confined in the 1.55 less than or e qual to x less than or equal to 2.30 range, selected separately for ea ch alkane. In addition, new lifetime data are employed to extend the k (tp) values to lower temperatures. It is found that activation paramet ers for k(tp), based on Eyring's transition state theory, adhere to th e medium-enhanced barrier model relationship, Delta H-t(double dagger) = Delta H-t(double dagger) + aE(eta s), and to the isokinetic relatio nship. The a value, a measure of the effect of medium friction on solu te torsional motion, increases from 0.41 in stilbene to 0.43 in dimeth oxystilbene and to 0.47 in the two dialkylstilbenes. A significant inc rease (similar to 1 kcal/mol) in the activation enthalpy, Delta H-t(do uble 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 paramete rs for the parent trans-stilbene leds to an isokinetic temperature of beta = 600 +/- 11 K and brings it into excellent agreement with beta(e ta mu), the isokinetic temperature for activation parameters based on estimated microviscosities, eta(mu), experienced by stilbene in its to rsional motion. The conclusion that only when microviscosities rather than shear viscosities, eta(s), are employed in the power law expressi on k(tp) = k(t)B eta(-a) can a = a be expected is supported by this re sult.