Excited-State Intramolecular Proton Transfer in 2-(3‘-Hydroxy-2‘-pyridyl)benzoxazole. Evidence of Coupled Proton and Charge Transfer in the Excited State of Some o-Hydroxyarylbenzazoles

Abstract

The influence of solvent, temperature, and viscosity on the phototautomerization processes of a series of o-hydroxyarylbenzazoles was studied by means of ultraviolet-visible (UV-vis) absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. The compounds studied were 2-(2'-hydroxyphenyl)benzimidazole (HBI), 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2-(2'-hydroxyphenyl)benzothiazole (HBT), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (HPyBI), and the new derivative 2-(3'-hydroxy-2'-pyridyl)benzoxazole (HPyBO), this one studied in neutral and acid media. All of these compounds undergo an excited-state intramolecular proton transfer (ESIPT) from the hydroxyl group to the benzazole N3 to yield an excited tautomer in syn conformation. A temperature- and viscosity-dependent radiationless deactivation of the tautomer has been detected for all compounds except HBI and HPyBI. We show that this radiationless decay also takes place for 2-(3-methyl-1,3-benzothiazol-3-ium-2-yl)benzenolate (NMeOBT), the N-methylated analog of the tautomer, whose ground-state structure has anti conformation. In ethanol, the radiationless decay shows intrinsic activation energy for HPyBO and HBO; however, it is barrierless for HBT and NMeOBT and controlled instead by the solvent dynamics. The relative efficiency of the radiationless decay in the series of molecules studied supports the hypothesis that this transition is connected with a charge-transfer process taking place in the tautomer, its efficiency being related to the strength of the electron donor (dissociated phenol or pyridinol moiety) and electron acceptor (protonated benzazole). We propose that the charge transfer is associated with a large-amplitude conformational change of the tautomer, the process leading to a nonfluorescent charge-transfer intermediate. The previous ESIPT step generates the structure with the suitable redox pair to undergo the charge-transfer process; therefore, an excited-state intramolecular coupled proton and charge transfer takes place for these compounds.

Synthesis, characterization and optical properties of aryl and diaryl substituted phenanthroimidazoles

“…1. Upon excitation of N syn an ESIPT process takes place to give T syn * fluorescent species, after which the excited tautomer undergoes a large-amplitude conformational change associated with a charge migration from deprotonated phenol or sulfonamide moiety to the protonated imidazole moiety, yielding the non-fluorescent chargetransfer intermediate T ICT *, which probably deactivates very fast [10]. The electronic and geometric structure of T ICT * in Fig.…”

“…For an efficient ESIPT, the acidity and/or basicity of these groups must increase upon photoexcitation [7]. Among such molecules, hydroxyphenylbenzazoles (HBA) such as 2-(2 -hydroxyphenyl)benzimidazole (HBI) [8][9][10], 2-(2 -hydroxyphenyl)benzoxazole (HBO) [10][11][12], 2-(2 -hydroxyphenyl)benzthiazole (HBT) [10,13], 2-(2 -arylsulfonamidophenyl)benzimidazole [14,15] and 2-(2 -arylsulfonamidophenyl)benzoxazole [16] have attracted particular attention due to their potential for fluorescent probes [17] (for the labeling of proteins and DNA) and model base pairs [18] (for studying the environments within the major and minor grooves of DNA duplexes).…”

Theoretical Study of Structure, Energetic, Hydrogen Bonds Strength and Intramolecular Proton Transfer of 2-(2-R (ROH, NH2, SH) Phenyl (or Pyridyl)) Benzoxazoles (or Benzothiazoles)

“…The excited-state intramolecular proton transfer processes are consistent with experimental results. 16 The relationship between the IMHB strength and the barrier ∆E ≠ can be described with the 2D-QSPR model Eqs. 3, 4 and 5.…”

“…[14][15][16] In this work, they are used as models to study the relationship between proton transfer and hydrogen bond strength. The aim of this work is to clarify the following aspects: (i) in theory, how to determine the quasi-keto-form structure; (ii) whether there is some correlation between the IMHB strength and tautomers energies; (iii) whether there is correlation between the IMHB strength and the barrier.…”

Electronic and photophysical properties of 2-(2′-hydroxyphenyl)benzoxazole and its derivatives enhancing in the excited-state intramolecular proton transfer processes: A TD-DFT study on substitution effect

“…After decaying radiatively to the ground keto form (S 0 keto), the reverse PT takes place to give its initial enol form (Scheme 1). Because of their unique optical property given by ESIPT, HBO and its derivatives have been employed as luminescent material [11,12], chemical sensors for zinc(II) [13] and anions [14], light-emitting diode devices [15], optical switching [16,17], and fluorescent probe [18,19]. In general, fluorescence spectra of these molecules exhibit both enol and keto emission.…”