Fluorescence of methylated derivatives of hydroxyphenylimidazopyridine. Resolution of strongly overlapping spectra and a new ESIPT dye showing very efficient radiationless deactivation

Abstract

The ground- and excited-state behaviour of the isomeric species 2-(2'-methoxyphenyl)imidazo[4,5-b]pyridine (1-OMe) and 2-(2'-hydroxyphenyl)-4-methylimidazo[4,5-b]pyridine (1-NMe) in neutral and acid media has been studied by UV-vis absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy. The new dye 1-NMe is non-fluorescent in neutral media except in trifluoroethanol, where it shows a very weak fluorescence. 1-NMe also exhibits highly solvent-dependent fluorescence intensity in acidic media. We propose that the neutral species experiences a fast excited-state intramolecular proton transfer (ESIPT), relaxing afterwards by intramolecular twisting associated with internal charge transfer (TICT) and subsequent very fast internal conversion of the proton-transferred TICT structure. The behaviour of 1-NMe in acidic media is explained by the existence of a ground-state tautomeric equilibrium between species with intramolecular hydrogen bonds N-HOH and NHO. The first type of tautomers dissociates at the hydroxyl group in water and ethanol, but fluoresces in acetonitrile and trifluoroethanol due to the inability of these solvents to accept the proton. The second type of tautomers is non-emissive due to fast radiationless deactivation through an ESIPT-TICT process. The fluorescence of 1-OMe was investigated in neutral and acidic media, demonstrating the photobasic character of the pyridine nitrogen. A ground-state equilibrium between pyridinium and imidazolium cations was found for this species, showing overlapping absorption and fluorescence spectra. We devised a method to resolve the spectra by applying principal component global analysis to a series of excitation spectra taken at different emission wavelengths, which allowed estimation of the equilibrium constant between the cations.

Intramolecular Proton Transfer in 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐b]pyridine: Evidence for Tautomer in the Ground State

“…Rodríguez et al . elaborated that when the electron withdrawing ability of the imidazopyridine ring increases the quantum yield of the tautomer emission of HPIP‐b decreases . Same way it was shown that the substitution of electron donating diethylamino group on the phenolic ring reduces the fluorescence of HPBI .…”

“…More recently, Rodríguez et al . showed that the deprotonation of the hydroxyl group (increasing the charge on the donor moiety) or protonation/methylation on the pyridyl nitrogen of HPIP‐b (increasing the electron withdrawing ability of the acceptor group) enhances the radiationless deactivation of HPIP‐b .…”

Towards Ratiometric Sensing of Amyloid Fibrils In Vitro

“…A similar TICT process has been reported for the tautomer of various HPIP-related hydroxyarylbenzazoles. [25,29,33,34] As shown in Figure 2, the value of the ratio SR for HPIP is highly sensitive to the environment of the dye. In aqueous medium, the spectral ratio SR has a value of approximately 0.3, which drops abruptly with the addition of a small percentage of ethanol.…”

Triphenylamine‐Based Bis‐ and Tris‐ESIPT Compounds and Their Boron Complexes: Synthesis, Photophysical Properties and DFT Study of ICT and ESIPT Emissions

“…The red shifted keto emission in ESIPT process is mainly attributed to the four level cyclic proton transfer reaction (E →E*→K*→K→E) which completes after the reverse proton transfer from the keto form (K) to the original enol form (E) in the ground state. Hence variety of ESIPT chromophores e. g. 2‐(benzo[d]thiazol‐2‐yl)phenol (HBT) 2‐(benzo[d]oxazol‐2‐yl)phenol (HBO), 2‐(1H‐benzo[d]imidazol‐2‐yl)phenol (HBI), oxadiazole, 3‐hydroxyflavone(3HF), 3‐hydroxychromones, hydroxyl phenyl imidazole (HPI), imidazopyridine methyl salicylate have been extensively studied for their…”