Keto–enol tautomerism of two structurally related Schiff bases: Direct and indirect way of creation of the excited keto tautomer

Femtosecond time-resolved absorption spectra of two structurally related, internally H-bonded Schiff bases are reported. The 2-hydroxynaphthylidene-1′-naphthylamine (HNAN) stable as an enol tautomer undergoes an ultrafast excited state intramolecular proton transfer, while the 2-hydroxynaphthylidene-(8′-aminoquinoline) (HNAQ), stable as a keto structure, reveals unusual relaxation routes after electronic excitation. In particular, the rise of the bleaching band with the characteristic time of ∼700 fs was found and attributed to a gradual population of the S1 fluorescent state from a ‘hot’ excited state. The results accompanied by TDDFT calculations are used to construct the diagram of relaxation routes of an excited HNAQ molecule.