Thermal and Photochemical Generation of Electronically Excited Organic Molecules: Tetramethyl-1,2-dioxetane and Naphthvalene

Turro, Nicholas J.; Lechtken, Peter

In this paper some examples of reactions which yield electronically excited products are presented. In particular, the 1,2-dioxetanes are discussed. These molecules cleave cleanly into two carbonyl fragments when heated or irradiated. It will be shown that these simple, high energy, four atom arrays can efficiently generate electronically excited carbonyl fragments when they decompose. Surprisingly, tetramethyl-1,2-dioxetane (V) yields acetone triplet selectively upon thermolysis or photolysis. A study of the kinetics of thermal decomposition of V as a function of solvent, and the mechanistic implications of these observations will be discussed. Based on summation of available evidence, we propose that the thermolyses of 1,2-dioxetanes require specific vibrational motions which enhance spin—orbit coupling as the molecule fragments, and allow efficient decomposition into triplet states. In the photochemistry of V an exceptional 'anti-Stokes' sensitization is demonstrated, which suggests the possibility of efficient execution of 'blue light' photochemistry with 'red light', and a 'quantum chain' reaction. It is demonstrated that, at 77°K, naphthvalene undergoes efficient photochemical conversion to naphthalene triplets. Finally, the relationship of radiationless electronic relaxation and primary photochemical processes is discussed in the framework of our results.


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Pure and Applied Chemistry

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August 2, 2010