PHOSPHORESCENCE AND TRANSIENT ABSORPTION OF AZOALKANE TRIPLET-STATES

Fused derivatives of 2,3-diazabicyclo[2,2.1]hept-2-ene (DBH) were stud ied by transient absorption, optical emission, and photoelectron spect roscopy, and their reaction quantum yields were measured. For the firs t time, long-lived azoalkane triplet states were observed by phosphore scence and transient absorption spectroscopy. The assignment of the tr ansient absorptions to the triplet states of the azoalkanes was proven by quenching and energy transfer experiments in solution. Quantitativ e agreement was found between the real-time kinetic data of triplet qu enching determined by flash photolysis and Stem-Volmer analyses of the effect of quenchers on the product distributions and quantum yields. Triplet energies of 62.5 +/- 1 kcal mol(-1) were determined from the 0 -0 bands of the phosphorescence spectra. The triplet energies of azoal kanes that have been previously evaluated by techniques other than pho sphorescence are critically evaluated. The radiative rate constant of azoalkane phosphorescence was determined as ca. 3 s(-1). The transient absorption spectra had maxima at ca. 315 and 500 nm, and the triplet lifetimes were as long as 0.63 mu s. Intersystem crossing (ISC) was fo und to be an efficient process for most of the azoalkanes examined; fo r azoalkane la, the triplet quantum yield was estimated to be 0.5 +/- 0.2 at ambient temperature. No internal or external heavy-atom effects were observed on phosphorescence and ISC. In some cases, formation of 1,3-cyclopentanediyl biradicals from the azoalkane triplet states was observed by transient absorption spectroscopy. The yield of triplet b iradicals, generated by direct photolysis of azoalkanes in solution, s erved as a qualitative probe for the involvement of ISC and azoalkane triplet states. Possible factors which govern the triplet lifetimes an d ISC of azoalkanes are discussed; molecular rigidity appears to favor both high triplet yields and long triplet lifetimes. Assignments of t he two T-T absorption bands are proposed. All experimental results are consistent with the expected n,pi configuration for the lowest excit ed singlet and triplet states of aliphatic azoalkanes.