Abstract
A modified form of the steady state treatment for the determination of excited state proton transfer rate constants was successfully applied to a variety of oxygen and nitrogen center aromatic excited acids. These compounds shared the characteristic of being more acidic in the lowest excited singlet state (S1) than in the ground electronic state (S0) and of requiring a concentrated mineral acid medium for protonation. This treatment was extended to phenanthridone, the lactam of 6-hydroxyphenanthridine, which is a weak enough base in the ground and the excited states to require moderately concentrated mineral acids for protonation, and becomes a stronger base in the excited state than it is in the ground state. Phenanthridone exists as an α-lactam and is a weaker base in the ground and excited states than the α-lactams derived from 2-hydroxypyridine, 2-hydroxyquinoline, and 1-hydroxyisoquinoline. It is also a much weaker base than the vinylogous γ-lactams. The reasons for this are discussed here.
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