ACTIVATION PARAMETERS IN THE PHOTOCYCLOADDITION OF PHENANTHRENE TO DIMETHYL FUMARATE

The [2 + 2] direct photocycloaddition of phenanthrene to dimethyl fumarate was studied as a function of temperature in the range 21-65°C. From temperature dependencies of the quantum yields for the various photoprocesses, activation parameters were derived for the processes originating from the singlet exciplex 1E. The cycloaddition and internal conversion processes are activated, while intersystem crossing is not. Fluorescence was assumed to be temperature independent. The ben-zophenone-sensitized reaction is not sensitive to temperature in this range. Cycloaddition enthalpies of activation are found to be larger and entropies of activation less negative for 1E when compared to previous results for cyanophenanthrene-styrene exciplexes. The difference is attributed to a combination of increased charge transfer and less favorable remote orbital overlap in 1E. © 1979, American Chemical Society. All rights reserved.