SINGLET AND TRIPLET BIRADICAL INTERMEDIATES IN THE VALENCE ISOMERIZATION OF 2,7-DIHYDRO-2,2,7,7-TETRAMETHYLPYRENE

The photochemical and thermal reaction paths in the valence isomerization 2,7-dihydro-2,2,7,7-tetramethylpyrene (1) reversible 2,2,7,7-tetramethyldicyclopropa[a,g]pyracene (2) were traced by flash photolysis, optical and ESR spectroscopy, thermal reaction kinetics, and oxygen trapping experiments. The heat evolved in the reaction 2 --> 1 was determined by differential scanning and photoacoustic calorimetry. All reactions connecting 1 and the two geometric isomers of 2 proceed through a common perinaphthadiyl biradical intermediate (b, Scheme I). State-symmetry correlation proved to be a valuable guideline to excited-state reactivity. Singlet biradical 1b, which forms with unit quantum yield upon irradiation of either 1 or 2, was observed by picosecond pump-probe flash photolysis (tau = 18 ps). The predominant decay channel of 1b is cyclization yielding 2. Hence, the quantum yield of the photochemical valence isomerization 1 reversible 2 is near unity in the forward direction and only 5% in the backward direction. The ground state of the biradical is a triplet state, 3b, but intersystem crossing (ISC) is negligible in thermal and photochemical isomerization reactions at or above room temperature. Entry to the triplet manifold is provided by triplet sensitization, by oxygen-catalyzed ISC, or by direct irradiation of 2 at low temperature. At room temperature, 3b is long-lived, tau = 660 mus, and readily trapped by molecular oxygen, k = 1.3 x 10(9) M-1 s-1. At 77 K, 3b is persistent and photoisomerization requires two photons, the second for excitation of intermediate 3b. The adiabatic ring opening (3)2 --> 3b occurs exclusively from an upper triplet state of 2, as predicted by state-symmetry analysis.