Effect of Substitution on the Photoinduced Intramolecular Proton Transfer in Salicylic Acid

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 101; no. 35; pp. 6141 - 6147
• Main Authors: Lahmani, F, Zehnacker-Rentien, A
• Format: Journal Article
• Language: English
• Published: American Chemical Society 28.08.1997
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp9712516

The influence of methyl and methoxy substitution in the para position of the phenolic OH functional group on the intramolecular proton-transfer properties of electronically excited salicylic acid (ESIPT) has been investigated both in solution and in the isolated gas-phase conditions provided by supersonic cooling. The dual fluorescence observed for 5-methylsalicylic acid (5-MeSA) in alkane solutions has been attributed for its blue part to the excited tautomer resulting from the intramolecular proton-transfer process and for its UV component to the dimer. A single fluorescence emission peaking at 400 nm is observed in alkane solutions of 5-methoxysalicylic acid (5-MeOSA). In the presence of proton acceptors such as diethyl ether, the 5-MeSA solution emits only in the blue region while 5-MeOSA exhibits two fluorescence bands at 400 and 475 nm. This behavior shows that the ESIPT process is promoted by complexation with proton-accepting molecules. In the supersonic expansion, the excitation and dispersed emission spectra of 5-MeSA are very similar to those previously observed for unsubstituted salicylic acid and show that the ESIPT mechanism takes place without barrier, in agreement with the model of a distorted potential surface in the excited state. In contrast, the 5-MeOSA excitation and dispersed fluorescence spectra present a mirror-image relationship that indicates that the molecule keeps a similar geometry in the ground and excited state. In this case the ESIPT reaction is prevented. Complexation with diethyl ether and acetone does not give rise to a dual fluorescence as in solutions but results in a broad emission extending toward the visible. This result may be explained by a modification of the excited potential energy surface along the tautomerization coordinate without introducing an energy barrier in the proton-transfer reaction.