Excited-State Intramolecular Proton Transfer in 2-(2‘-Arylsulfonamidophenyl)benzimidazole Derivatives:  The Effect of Donor and Acceptor Substituents

A series of water-soluble 2-(2‘-arylsulfonamidophenyl)benzimidazole derivatives containing electron-donating and accepting groups attached to various positions of the fluorophore π-system has been synthesized and characterized in aqueous solution at 0.1 M ionic strength. The measured pK a's for...

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Bibliographic Details
Published in:Journal of organic chemistry 2007-06, Vol.72 (13), p.4784-4797
Main Authors: Henary, Maged M, Wu, Cody, John, Sumalekshmy, S, Li, Jing, Mandal, Subrata, Fahrni, Christoph J
Format: Article
Language:English
Subjects:
Amides - chemistry
Benzimidazoles - chemistry
Chemistry
Exact sciences and technology
Heterocyclic compounds
Heterocyclic compounds with several n hetero atoms in the same ring, in separated rings or in fused rings
Isomerism
Magnetic Resonance Spectroscopy
Molecular Structure
Organic chemistry
Phenols - chemistry
Photochemistry
Preparations and properties
Protons
Solutions
Sulfones - chemistry
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Summary:A series of water-soluble 2-(2‘-arylsulfonamidophenyl)benzimidazole derivatives containing electron-donating and accepting groups attached to various positions of the fluorophore π-system has been synthesized and characterized in aqueous solution at 0.1 M ionic strength. The measured pK a's for deprotonation of the sulfonamide group of monosubstituted derivatives range between 6.75 and 9.33 and follow closely Hammett's free energy relationship. In neutral aqueous buffer, all compounds undergo efficient excited-state intramolecular proton transfer (ESIPT) to yield a strongly Stokes-shifted fluorescence emission from the phototautomer. Upon deprotonation of the sulfonamide nitrogen at high pH, ESIPT is interrupted to yield a new, blue-shifted emission band. The peak absorption and emission energies were strongly influenced by the nature of the substituents and their attachment positions on the fluorophore π-system. The fluorescence quantum yield of the ESIPT tautomers revealed a significant correlation with the observed Stokes shifts. The study provides valuable information regarding substituent effects on the photophysical properties of this class of ESIPT fluorophores in an aqueous environment and may offer guidelines for designing emission ratiometric pH or metal-cation sensors for biological applications.
ISSN:0022-3263
1520-6904
DOI:10.1021/jo070433l