PICOSECOND EMISSION-SPECTROSCOPY OF 2 PREDISSOCIATIVE ISOMERIC METHYLENEPYRAZOLINES AND PRODUCT TRIMETHYLENEMETHANE
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The kinetics of the predissociation of two isomeric methylenepyrazolines and the resulting singlet trimethylenemethane (TMM) derivative have been studied by picosecond emission spectroscopy in room-temperature solution and 4.2 K rare gas matrices. The transient species are produced by a ~0.5-mJ, 266-nm pulse having a duration of 25 ps. The data indicate that excited diazenes 1* and 2* have respective lifetimes of 38 and 25 ps in room-temperature hexane solution and 5.0 and ~3 ns in 4.2 K argon matrices, and dissociate to singlet TMM 3S. Highly excited 2** 1* isomerization occuss rapidly in xenon matrices and the lifetime of 1* is 6.5 ns. Emission occurs from an excited singlet TMM biradical which is produced by a two-photon process. Two production mechanisms are discussed. The TMM has a lifetime of 300 ps in room-temperature hexane solution and ~30 ns in argon matrices. This lifetime is greatly reduced in xenon matrices, and it is concluded that, in the absence of chemical reactions, intersystem crossing to the triplet manifold facilitated by an external heavy atom effect is the dominant singlet biradical quenching mechanism. These results and the fluorescence spectra are discussed in terms of known chemistry and theoretical calculations. 1983, American Chemical Society. All rights reserved.