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.

Chiroptical properties of dithia[3.3]cyclophanes composed of anthracene and pyridine/pyridinium moieties: A combined experimental and theoretical study

“…The excitation spectrum for the low‐energy transition was essentially superimposable with the UV‐vis spectrum of 1‐H + (Figure S6 in the Supporting Information), and the band was assigned to the emission from the CT state. An exceptionally large Stokes shift of ≈150 nm was observed and is explained by the possible excited‐state proton transfer . Such characteristic CT emission (and absorption) have also been observed in the anthracene‐pyridinium dyad tethered with an o ‐methylenephenylene linker, where the CT absorption and emission were observed at 410 and 563 nm, respectively …”

Comment on ‘On Saltiel's isopolarizability approach and its applicability to diphenylpolyenes’ by J. Catalán, Chem. Phys. Lett. 635 (2015) 56–59

“…Ignoring a large body of experimental and theoretical evidence [1,2,16], Catalán rejected the well-established Hudson-Kohler 1 1 B u /2 1 A g model for diphenylpolyene fluorescence [17,18]. He contends that "The spectral emissions of α,ωdiphenylpolyenes can be ascribed to the presence of several different molecular structures in their excited electronic state.…”

Recent progress on the excited-state multiple proton transfer process in organic molecules

“…Excited-state proton transfer (ESPT) is a basic reaction in biology and chemistry [1] and has been widely used in fluorescent probes [2][3][4][5], organic solid-state lasers [6,7], fluorescent sensors [8]. Since the first observation of excitedstate intramolecular proton transfer (ESIPT) in salicylic acid in 1955 by Weller [9], several types of ESPT have been discovered and investigated (Figure 1) [10,11]. In 1969, Kasha et al [12,13] demonstrated for the first time that 7azaindole (7-AI) underwent an excited-state double proton transfer (ESDPT) in both its self-formed dimer and its ethanol solvate (Figure 1).…”