Photophysical Properties of Nucleic Acid Components—ii. Effect of N, N'dimethyl Substitution on Intersystem Crossing in Pyrimidine Bases

The intersystem crossing quantum yields to the lowest triplet states of N, N'dimethyluracil and N, N'dimethylthymine are 0.02 and ≥0.004, respectively. The very low yields are interpreted in terms of a 1 ( n , π*) to 1 (π.π*) state order change compared with the unsubstituted parents.