DETERMINATION OF THE TRIPLET LIFETIMES OF 1,3-CYCLOPENTADIYL BIRADICALS DERIVED FROM THE PHOTODENITROGENATION OF AZOALKANES WITH TIME-RESOLVED PHOTOACOUSTIC CALORIMETRY

The triplet lifetime of the parent 1,3-cyclopentanediyl (2a) and the 4-methylene and the 1-phenyl derivatives 2b and 2c were determined by time-resolved photoacoustic calorimetry (PAC). For the biradical 2b, PAC was the only suitable method to date for determining its hitherto inaccessible triplet lifetime. A comparison with results for biradicals 2a,c obtained in earlier studies shows that PAC is a reliable method for measuring triplet lifetimes of biradicals. The effect on the triplet lifetimes of allyl and benzyl conjugation at one radical center in 2b,c proved to be insignificant. In view of the large enhancement of the biradical lifetime as the result of benzylic conjugation at both radical sites, as in the diphenyl derivative 2d, the present results imply that the biradicals 2b,c are still flexible enough to enable effective intersystem crossing through pyramidalization. The much larger (ca. 200-fold) triplet lifetime of biradical 2b compared to its gem-dimethyl derivative 2e suggests that geminal substitution is a general phenomenon for the effective reduction of the lifetimes of triplet biradicals through changes in the singlet-triplet energy gap.