Photoisomerization of Stilbene and 1,4-Diphenylbutadiene in Compressed Gases and Liquids — Density-Dependent Solvent Shift and Transport Contributions

Abstract

Reactions in the liquid phase are strongly influenced by reactant-solvent interactions, which one usually investigates by simply changing the solvent. This approach entails the simultaneous variation of a whole set of parameters that may affect the reaction, so that it appears desirable to employ methods, where only a single parameter is varied continuously over a wide range [1]. One such parameter is the bath gas or solvent pressure, which we have chosen to study the density dependence of halogen photolysis and atom recombination from the low-pressure gas right through the gas-liquid transition region into the dense liquid under several kilobars of pressure [2–6]. We wish to report here on similar experiments concerning the photoisomerization of trans-stilbene and 1,4-diphenylbutadiene (DPB), which have been studied in detail with picosecond time-resolution under collision-free conditions, in dense gases, and a variety of liquid solvents, see, e. g. refs. [7–16], and references therein. The gas phase reaction can be represented very well by RRKM-theory for both stilbene [17] and DPB [18]. If one calculates the limiting high-pressure rate-constant k_∞ from the specific rate-constants k(E) of the isolated molecule reaction, one obtains a value that is more than one order of magnitude below the rate coefficients measured in low viscosity liquid solvents for the case of stilbene, while for DBP it is only slightly above the experimental liquid phase value. Recently, we have suggested a simple model involving a density-dependent lowering of the isomerization barrier in the first excited singlet state, which could be used to describe the observed behaviour [19]. Another question arises as a result of the approximate m^−0.5-dependence of the rate coefficient in high viscosity liquid solvents [15,16,20–22], which is not in accord with the predictions of Kramers theory [23]. A solvent -induced barrier shift could also serve as an explanation for this phenomenon [19], as an alternative to interpretations considering frequency-dependent friction coefficients or non-Markovian effects (see discussion in [19] and references therein). The aim of our experiments was to gain more insight into the way the solvent density and viscosity influence the photoisomerization rate by changing them continuously from the gas to the compressed liquid phase.