Intramolecular Proton or Hydrogen-Atom Transfer in the Ground and Excited States of 2-Hydroxybenzoyl Compounds † J. Catala ´ n,* J. Palomar, and J. L. G. de Paz Departamento de Quı ´mica Fı ´sica Aplicada, UniVersidad Auto ´ noma de Madrid, Cantoblanco, E-28049 Madrid, Spain ReceiVed: May 14, 1997; In Final Form: July 24, 1997 X Potential energy surfaces for the intramolecular proton transfer of ground (GSIPT) and excited (ESIPT) states of 2-hydroxybenzoyl compounds were obtained. Based on the results, intramolecular proton transfer in this type of compound is strongly dependent on the distances between the oxygen atoms that bear the intramolecular hydrogen bond (IMHB). Also, the GSIPT curves for these compounds contain a single minimum that is located in the zone for the normal (enol) form. The ESIPT curves also contain a single minimum but lie in the zone for the keto form. There is no correlation between the strength of the IMHB and the proton transfer barrier through it. The energy for the excited singlet 1(n,π * ) for these compounds is strongly dependent on the resonance effect of the substituent, -R, so this state is the first excited singlet only in derivatives with nearly nonresonating R. The ESIPT processes are of the proton transfer type, even though the final form possesses no zwitterionic connotations. Finally, these theoretical features are quite consistent with photophysical experimental evidence for this type of compounds. 1. Introduction More than 40 years ago, Albert Weller, 1 using a 2-hydroxy- benzoyl compound [Ph(OH)COR] such as methyl salicylate, laid the foundation for the subsequently called “excited-state in- tramolecular proton transfer” (ESIPT) mechanism (Scheme 1). This mechanism is currently being employed to understand the behavior of some compounds that exhibit such interesting properties as ultraviolet stabilization, 2-7 stimulated radiation production, 8,9 and information storage, 10 as well as environ- mental probes in biomolecules. 11 The signature of an ESIPT process is the emission of strongly Stokes-shifted fluorescence following absorption of UV photons. This spectral feature is the result of both the exothermal behavior of the excited singlet state potential curve that governs the ESIPT process while the proton transfer develops, and the endothermal behavior of the potential curve for the process in the ground state (GSIPT). These combined effects bring the two electronic states involved in the emission dramatically nearer. While the curve for the excited electronic state is that which dictates whether the proton phototransfer is to take place, the role played by the curve for the ground electronic state is spectroscopically as relevant because it contributes to the Stokes shift and is responsible for the spectral envelope with no vibronic structure that is observed in the fluorescence of compounds undergoing an ESIPT process. 2-Hydroxybenzoyl compounds possess a strong intramolecu- lar hydrogen bond (IMHB) as a result of their bearing a hydroxyl group and a carbonyl group that act as a proton donor (acid) and acceptor (base), respectively, in adjacent positions. It is widely accepted that the presence of this strong IMHB endows the structure with increased photostability, which in turn is partly responsible for the interesting, characteristic properties of these compounds. According to Weller, 1,12 the acid-base properties of the hydroxyl and carbonyl groups on an aromatic ring can change to such an extent by the effect of electronic excitation that the hydrogen atom in the hydroxyl group (structure I in Scheme 1) may shift to the vicinity of the oxygen atom in the carbonyl group (structure II in Scheme 1), thereby giving rise to a proton phototransfer and the consequent formation of a zwitterionic structure. The proton transfer takes place via the IMHB; the ease with which it does increases with increasing bond strength, which in turn is dictated at first by the acidity and basicity of the two groups involved in the IMHB. The energy curves that describe the displacement of the hydrogen atom in the ground (GSIPT) and excited states (ESIPT) play central roles in the photophysics of 2-hydroxy- benzoyl compounds and as such have aroused much attention in both the experimental and theoretical domains. There is currently accepted evidence 13,14 that the GSIPT curves for 2-hydroxybenzaldehyde (-H), 2-hydroxyacetophe- none (-Me), methyl salicylate (-OMe), and salicylamide (-NH 2 ) (see Scheme 2) exhibit a single minimum that is located in the zone for the enol formsthe normal form (I in Scheme 1)swhereas their ESIPT curves, which control the fluorescent † This paper is dedicated to the memory of Albert Weller, who passed away in September 1996, in appreciation of his pioneering research into such an interesting topic as proton phototransfer (ESIPT) processes. X Abstract published in AdVance ACS Abstracts, September 15, 1997. SCHEME 1 7914 J. Phys. Chem. A 1997, 101, 7914-7921 S1089-5639(97)01582-X CCC: $14.00 © 1997 American Chemical Society