628
J. Phys. Chem. 1983, 87,628-634
highly dominant for Ura (-100%) and for Thy (5methyluracil, 96-9970) in any solvent. Substitution of a carboxyl group at the 6-position of Ura, ORO, does not significantly alter the situation in an aprotic polar solvent but the radiationless pathways are significantly decreased in an H-bonding solvent. Substitution of a carboxyl group at the 5-position, iso-ORO, decreases the radiationless processes in both aprotic and protic polar solvents, although phosphorescence is never observed. The results for both OR0 and iso-OR0 indicate the importance toward decreasing the dominance of radiationless processes of (1)the presence of the carboxyl group and (2) either or both of intra- and intermolecular H bonding. This most likely is the result of a '(T,T*) state being the lowest excited state (rather than l(n,a*)) and the diminution of vibrational modes which lead to radiationless deactivation. Finally, we shall consider all of the existing data on the triplet yield $T and the dimerization yield $D. These are related by $D = fD&, where f D is the fraction of triplets which react to form stable dimers. In water," the $T of Ura is -2 X and of Thy is -6 X In the case of Ura, a relatively small f D of 0.2' appears to account for the low value of $D (5 X For Thy, it would appear that f D would also have to be relatively small, -0.1, in order (11) Salet, C.; Bensasson, R. Photochem. Photobiol. 1975,22,231-5.
to account for the value of
C#Q
(5 x 10-4)7compared with
@T*
In acetonitrile,2the $T of DMUra is -2 X and of Since there is generally a 10-fold DMThy is
