Effect of additives on the photophysics of 2-acetyl benzimidazole and 2-benzoyl benzimidazole encapsulated in cyclodextrin cavity

Interesting excited state proton transfer photophysics of 2-acetyl benzimidazole (2ABI) and 2-benzoyl benzimidazole (2BBI) encapsulated in cyclodextrin (CD) (alpha and beta) cavity and its modulation on addition of acid/base and inorganic salt have been delineated in this paper based on steady state and time-resolved photoemission spectroscopy. In the ground state both 2-acetyl benzimidazole and 2-benzoyl benzimidazole form complexation with beta-CD, but not with alpha-CD. Large intensification of anion band and relatively small enhancement of neutral emission of 2BBI in beta-CD points to the complex formation in the excited state with a preferential orientation of >= N-H group in bulk water. Increased lifetime and enhancement of band intensity of 2BBI in beta-CD compared to that of 2ABI indicate a definite inclusion of 2BBI inside the cavity. Base induced protons at the rims of the cavity liberated in the process of formation of anions quench the anionic species of 2BBI in beta-CD solution. Cavity size of cyclodextrin influences the modulation of photodynamics of the guest. Addition of acid leads to the formation of a cationic species. Inorganic salt hinders the deprotonation and lowers the barrier for anion formation. (c) 2005 Elsevier B.V. All rights reserved.