Photosensitized Isomerization by Internal Reflection
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An Investigation of Isomerization of 1,3=PentadieneSensitized by Solid Benzophenone Using Internal Reflection Photolysis Josb S. DeGurman and G. R. McMillan* Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44 106 (Received September 14, 1972; Revised Manuscript Received February 7, 1975) Publication costs assisted by the Environmental Protection Agency
Illumination by internal reflection of crystalline benzophenone deposited on a light pipe induces isomerization of cis- or trans-1,3-pentadiene vapor in contact with the solid. Triplet-triplet energy transfer is implicated. The mixture of diene isomers at the photostationary state contains 56.5% trans. This differs from the reported composition (58.5%) of the photostationary state for homogeneous excitation in solution but agrees with the value obtained through surface illumination of the benzophenone crystals. If the energy transfer is treated strictly as a surface reaction, the pressure dependences of the isomerization yields lead to a value of the ratio of rate constants for energy transfer to the two isomers and to a value of the ratio of decay probabilities of diene triplet to the two isomers somewhat different from those obtained by other investigators in studies of energy transfer in homogeneous and heterogeneous systems. I t is demonstrated that internal reflection photolysis applied in studies of gas-solid systems yields results of acceptable reproducibility and that quantum yields can be measured. The quantum yield of isomerization for trans cis is about 2 X a t 3660 8, for a 1,3-pentadiene pressure of 2 Torr. -+
Introduction
The cis-trans isomerization of 1,3-pentadiene vapor induced by energy transfer from a solid sensitizer represents a heterogeneous photochemical reaction relatively uncomplicated by side rea~tions.l-~ In the earlier attempt,l illumination of polycrystalline benzophenone in the presence of cis-1,3-pentadiene vapor led to formation of the trans isomer through a process believed to be triplet-triplet energy transfer on the basis of comparison with sensitized isomerization in s o l ~ t i o n Interpretation .~~~ of the results was somewhat uncertain because of contributions to isomerization from gas-phase photosensitization and from sensitization by pentadiene adsorbed on Pyrex.I Attempts to study quantitatively the isomerization rate yielded data with considerable ~ c a t t e rIt . ~is extremely difficult to measure the absorbed intensity in such experiments. The method used in these experiment^,^^^ wherein the radiation passes through the vapor layer, has limited usefulness in gas-solid photochemical studies if the gas is not transparent or if the solid has an appreciable vapor pressure. Some of these difficulties are circumvented by using the method of internal reflection photolysis (IRP).I The solid is coated on a light pipe, and the light is introduced into the pipe under conditions ensuring total internal reflection. The optical path through the vapor surrounding a thin layer of solid is negligible provided no light is scattered out of the solid. The amount of light absorbed by the solid can he monitored accurately. This study is supposed to demonstrate the applicability of IRP to a heterogeneous photochemical reaction and to compare triplet-triplet energy transfer in the system benzophenone(s)-1,3-pentadiene vapor with other cases of energy transfer from sensitizers to 1,3-pentadiene. Experimental Section
Both cis- and trans-1,3-pentadiene of purity 99% were obtained from Chemical Samples Co. Purities of 99.9%
were realized by preparative gas chromatography, but a slow isomerization was observed under storage conditions. All samples used for photolysis contained
