ANALYSIS OF THE NONEXPONENTIAL TRIPLET TRIPLET FLUORESCENCE DECAY IN METHYL-SUBSTITUTED META-XYLENE BIRADICALS

The triplet-triplet fluorescence decay observed earlier in a series of m-xylylene biradicals is reanalyzed on the basis of the assumption that intersystem crossing is the only process leading to spin alignment in these systems. To analyze the three triplet sublevel decay rate constants derived in this manner, semiempirical LCAO-SCF-CI calculations are carried out to locate the singlet states involved in the intersystem crossings. It is found that the crossings from all three sublevels are to the same singlet state, namely, S3 separated from the emitting triplet by the smallest energy gap. The rate constants indicate that some m-xylylenes, especially those substituted in position 2, show out of plane deformation leading to triplet sublevel mixing which affects mostly the smallest of the three rate constants. The effect of methyl substitution in other ring positions can be explained satisfactorily in terms of energy gaps and Franck-Condon factors. For planar m-xylenes the ratio k(x):k(y):K(z) congruent-to 1:5:10 is similar to that for triplet decay in planar aromatic hydrocarbons.