Effect of substitution on the photoinduced intramolecular proton transfer in salicylic acid

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 accepters 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.