Effuects of long-chain alkyl substituents on the protolytic reactions of naphthols

Kinetics and mechanism of excited-state proton-transfer reactions of 1-naphthol (1N) and 2-octadecyl-1-naphthol (2O1N) were studied in aqueous acetonitrile and absolute ethanol. Dissociation of 1N and 2O1N in the singlet excited state was characterized by similar rate and equilibrium constants (k(1) similar to 0.5 ns(-1), pK similar to 1) in contrast to the ground state, where 2O1N is a weaker acid than IN (ApK=0.6 in an acetonitrile-water mixture 2: 1, v/v). Main features of the ground and excited-state pK and pK* and excited-state protolytic dissociation rate constants k(1) and k(R) were rationalized in terms of solvent effects on the energetics of equilibrium between hydrogen bonded complex and ion pair and in terms of electron donating effects of alkoxy group in the ground state and aromatic system in the excited singlet state. An efficient deactivation process competing with the excited-state proton transfer (ESPT) was observed both for 1N and 2O1N. This needs more complicated kinetic analysis, yet provided a deeper insight into mechanisms of the excited-state proton-transfer reactions. A simple kinetic scheme including transient formation of excited hydrogen-bonded complex and geminate ion-pair and fast deactivation of both transients provided a good description of the protolytic photodissociation for the compounds studied. The rate constants for proton transfer and induced deactivation were determined for photodissociation of excited IN and 20 IN in aqueous acetonitrile and their reactions with acetate anion in absolute ethanol. A remarkable decrease of ESPT rate constant and a substantial increase of the radiationless decay rate constant was observed in aqueous acetonitrile as compared to water. The origin of dissimilar solvent effects on these rate constants was discussed. (c) 2005 Elsevier B.V. All rights reserved.