Competing Excited State Intramolecular Proton Transfer and Photoinduced Electron Transfer in 2-(2 -Arylsulfonamidophenyl)benzimidazoles
Chaudhry, Aneese F.
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Cation-induced inhibition of excited-state intramolecular proton transfer (ESIPT) can be effectively utilized to design fluorescent probes for ratiometric Zn(II)-sensing. The 2-(2'-sulfonamidophenyl)benzimidazole family of fluorophores undergoes efficient ESIPT in protic solvents to yield a highly Stokes-shifted emission of the phototautomer. Upon coordination of Zn(II), ESIPT is disrupted and results in a strongly blue-shifted fluorescence emission. Because Zn(II)-binding competes with protonation of the sulfonamide nitrogen, the coordination equilibrium can principally be tuned by adjusting the sulfonamide pKa. In this study we specifically addressed the question to what extent tuning of the pKa influences the photophysical properties of this class of fluorophores. For this purpose, a series of compounds with varying substituents attached to the sulfonamide aryl-ring were characterized in terms of their pKa, their absorption and emission energies, as well as their quantum yields. Although the pKa varied over almost two orders of magnitude following closely Hammett s free energy relationship, the peak absorption and emission energies remained largely unaltered. Interestingly, the quantum yields of derivatives with strongly electron withdrawing substituents were significantly lower compared to all other fluorophores, suggesting a photoinduced electron transfer process as a possible non-radiative deactivation pathway. Electrochemical analysis revealed indeed an additional reduction wave at a less negative potential for the quenched derivatives. Estimates of the electron transfer driving force based on the Rehm-Weller formalism supported the PET pathway. Furthermore, the quenching mechanism was confirmed through quantum chemical calculations. The findings of this study are expected to aid in the rational design of ESIPT fluorophores for zinc-sensing applications.