Photoinduced hydrogen atom transfer in salicylic acid derivatives used as matrix-assisted laser desorption/ionization (MALDI) matrices

The triplet states of unsubstituted and 5-substituted derivatives of salicy lic acid and methyl salicylate, some of which are used as matrices in matri x-assisted laser desorption/ionization (MALDI) mass spectrometry of macromo lecules, were studied in acetonitrile and cyclohexane by transient absorpti on and time-resolved luminescence spectroscopy. The results suggest a tauto meric structure of the triplet states of salicylic acid as well as that of its methyl ester and its 5-hydroxy- and 5-methoxy-substituted derivatives. In this tautomeric structure, the ortho-hydroxy hydrogen has been transferr ed to the carbonyl oxygen. No differences were observed between the triplet -triplet absorption spectra of the acids and the corresponding methyl ester s. For the 5-hydroxy- and 5-methoxy-compounds, evidence for long-lived phot otautomers was found. The P-type delayed fluorescence of methyl salicylate is consistent with the known tautomer fluorescence at 440 nm, implying a ta utomeric structure of the triplet state. Similarly, for the 5-methoxy-subst ituted compound a unique delayed fluorescence spectrum, red-shifted relativ e to the prompt fluorescence spectrum, was observed and attributed to excim er or tautomer fluorescence, The results presented here contrast with previ ous reports on the absence of intramolecular hydrogen atom transfer on the singlet surface of the MALDI matrices 5-methoxy-and 5-hydroxysalicylic acid and their respective methyl esters, The resulting tautomers and their reac tion products may be relevant for analyte ionization in MALDI.