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| - Effect of Substitution on the Photoinduced Intramolecular Proton Transfer in Salicylic Acid
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| - The influence of methyl and methoxy substitution in thepara position of the phenolic OH functionalgroupon the intramolecular proton-transfer properties of electronicallyexcited salicylic acid (ESIPT) has beeninvestigated both in solution and in the isolated gas-phase conditionsprovided by supersonic cooling. Thedual fluorescence observed for 5-methylsalicylic acid (5-MeSA) inalkane solutions has been attributed forits blue part to the excited tautomer resulting from the intramolecularproton-transfer process and for its UVcomponent to the dimer. A single fluorescence emission peaking at400 nm is observed in alkane solutionsof 5-methoxysalicylic acid (5-MeOSA). In the presence of protonacceptors such as diethyl ether, the 5-MeSAsolution emits only in the blue region while 5-MeOSA exhibits twofluorescence bands at 400 and 475 nm.This behavior shows that the ESIPT process is promoted bycomplexation with proton-accepting molecules.In the supersonic expansion, the excitation and dispersed emissionspectra of 5-MeSA are very similar tothose previously observed for unsubstituted salicylic acid and showthat the ESIPT mechanism takes placewithout barrier, in agreement with the model of a distorted potentialsurface in the excited state. In contrast,the 5-MeOSA excitation and dispersed fluorescence spectra present amirror-image relationship that indicatesthat the molecule keeps a similar geometry in the ground and excitedstate. In this case the ESIPT reactionis prevented. Complexation with diethyl ether and acetone does notgive rise to a dual fluorescence as insolutions but results in a broad emission extending toward the visible.This result may be explained by amodification of the excited potential energy surface along thetautomerization coordinate without introducingan energy barrier in the proton-transfer reaction.
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