Effects of charge transfer on the ESIPT process in methyl 5-R-salicylates.

Abstract

The fluorescent behavior of the methyl-5-R-salicylates is analyzed in media of negligible acidity and basicity so that the methyl-5-R-salicylates may undergo solvent dipolarity changes or not in a controlled manner based on the following guidelines: (i) The molecular forms of these methyl-5-R-salicylates possessing an intramolecular hydrogen bond (IMHB) between their hydroxyl group and ether type oxygen (rotated tautomer) undergo no excited-state intramolecular proton transfer (ESIPT) in their first excited electronic state; (ii) on the other hand, the molecular species with an IMHB between its hydroxyl group and carbonyl oxygen (normal tautomer) exhibits both ESIPT and normal emission when charge transfer (CT) from the R-substituent to the phenol group is slight to moderate, but only normal emission is monitored when CT is strong. The special insensitivity of the first UV absorption band for the normal tautomer of methylsalicylate (MS, with R = H) to the polarity of the solvent is not echoed by the normal forms of methyl-5-R-salicylates containing substituents R with a substantial effect of CT in the IMHB of the compound. These solvatochromic features of MS are shared by the emissions of its derivatives. The photophysical evidence found for the methyl-5-R-salicylates confirms the photophysical model recently reported (Phys. Chem. Chem. Phys. 2012, 14, 8903-8909), which assigns three fluorescent emissions to the methyl-5-R-salicylates: two of them coming from the IMHB normal tautomer, which undergoes ESIPT, and another from the IMHB rotated tautomer, which cannot undergo ESIPT.