Excited-State Intramolecular Proton Transfer in Salicylidene-α-Hydroxy Carboxylate Derivatives: Direct Detection of the Triplet Excited State of the cis-Keto Tautomer

Excited-state intramolecular hydrogen transfer on the triplet surface of salicylideneaniline derivatives has received much less attention than the corresponding ultrafast process on the singlet surface. To enhance the understanding of this triplet reactivity, the photochemical properties of a series of salicylidene-α-hydroxy acid salts with different substituents on the phenol moiety (1-3) were characterized. UV/vis absorption and phosphorescence measurements in ethanol revealed that 1-3 exist as both enol and keto tautomers, with the enol form being predominant. Irradiation of 1 at 310 nm in ethanol glass (77 K) yielded an absorption band with a λ_max at ∼405 nm, which was assigned to the trans-keto tautomer (trans-1K). In contrast, laser flash photolysis of 1-3 in methanol or acetonitrile resulted in a transient absorption with λ_max at 440-460 nm. This transient, which decayed on the microsecond timescale and was significantly shorter lived in methanol than in acetonitrile, was assigned to the triplet excited state (T₁) of the cis-keto tautomer (cis-1K-3K) and residual absorption of trans-1K-3K by comparison with TD-DFT calculations. The assignment of the T₁ of cis-1K was further supported by quenching studies with anthracene and 2,5-dimethyl-2,4-hexadiene. Laser flash photolysis of 1 in the temperature range of 173-293 K gave an activation barrier of 6.7 kcal/mol for the decay of the T₁ of cis-1K. In contrast, the calculated activation barrier for cis-1K to undergo a 1,5-H atom shift to reform 1 was smaller, indicating that intersystem crossing of the T₁ of cis-1K is the rate-determining step in the regeneration of 1.