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| - Excited-State Intramolecular Proton Transfer in2-(2‘-Arylsulfonamidophenyl)benzimidazole Derivatives: The Effectof Donor and Acceptor Substituents
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| - 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 beensynthesized and characterized in aqueous solution at 0.1 M ionic strength. The measured pKa's fordeprotonation of the sulfonamide group of monosubstituted derivatives range between 6.75 and 9.33 andfollow closely Hammett's free energy relationship. In neutral aqueous buffer, all compounds undergoefficient excited-state intramolecular proton transfer (ESIPT) to yield a strongly Stokes-shifted fluorescenceemission from the phototautomer. Upon deprotonation of the sulfonamide nitrogen at high pH, ESIPT isinterrupted to yield a new, blue-shifted emission band. The peak absorption and emission energies werestrongly influenced by the nature of the substituents and their attachment positions on the fluorophoreπ-system. The fluorescence quantum yield of the ESIPT tautomers revealed a significant correlationwith the observed Stokes shifts. The study provides valuable information regarding substituent effectson the photophysical properties of this class of ESIPT fluorophores in an aqueous environment and mayoffer guidelines for designing emission ratiometric pH or metal-cation sensors for biological applications.
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