Steric effects in the phototransposition reactions of dialkylbenzenes

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The photochemistry, photophysical properties, and temperature dependence (–25 to +65 °C) of fluorescence by quantum yields and excited singlet state lifetimes in acetonitrile have been examined for three sets of dialkylbenzene derivatives: Set 1 — ortho-xylene (10), tetralin (11), and indan (12); Set 2 — 2,3-dimethylbenzonitrile (9-23), 5-cyanotetralin (T-23), and 4-cyanoindan (I-23); and Set 3 — 3,4-dimethylbenzonitrile (9-34), 6-cyanotetralin (T-34), and 5-cyanoindan (I-34). Phototransposition reactions occur for 10, 9-23, 9-34, and T-34. Fitting of the temperature-dependent fluorescence data to an Arrhenius expression gave A and Ea values for all substrates studied except I-23 and I-34. The fluorescence intensity of these two compounds was essentially independent of temperature. For the other compounds, the data revealed that the activation barrier separating the excited singlet state (S1) from the reactive intermediate, a prefulvene biradical, was the important one in determining the reaction efficiency. The dominant mode of decay of the reactive intermediate was internal return to the starting material. Moreover, the general observation was made that nitrile substitution ortho to one of the alkyl groups in these dialkylbenzene derivatives reduced the rate at which they were converted to the reactive intermediate and, therefore, also the efficiency of the phototransposition reactions.Key words: phototranspositions, substituted benzenes, temperature-dependent fluorescence, activation parameters.