Enhancing Excited State Intramolecular Proton Transfer in 2‑(2′- Hydroxyphenyl)benzimidazole and Its Nitrogen-Substituted Analogues by β‑Cyclodextrin: The Effect of Nitrogen Substitution # Francis A. S. Chipem, Santosh Kumar Behera, and G. Krishnamoorthy* Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India * S Supporting Information ABSTRACT: Excited state intramolecular proton transfer (ESIPT) in nitrogen- substituted analogues of 2-(2′-hydroxyphenyl)benzimidazole (HPBI), 2-(2′-hydrox- yphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b), and 2-(2′-hydroxyphenyl)-3H-imidazo- [4,5-c]pyridine (HPIP-c) have been investigated in a β-cyclodextrin (β-CD) nanocavity and compared with that of HPBI. The stoichiometry and the binding constants of the complexes were determined by tautomer emissions. Both pK a and NMR experiments were employed to determine the orientation of the molecules inside of the β-CD cavity. Huge enhancement in the tautomer emission of HPIP-b and HPIP-c compared to that of HPBI in β-CD suggests that not only is the ESIPT favored inside of the cavity, but also, the environment reduces the nonradiative decay through the formation of an intramolecular charge-transfer (ICT) state. Unlike HPBI, the tautomer emission to normal emission ratio of HPIP-b increases from 0.9 to 2.6, and that of HPIP-c increases from 4.9 to 7.4 in 15 mM β-CD. The effect of dimethylsulfoxide (DMSO) on complexation was also investigated for all three guest molecules. In DMSO, HPBI is present in neutral form, but the nitrogen-substituted analogues are present in both neutral and monoanionic forms. However, in DMSO upon encapsulation by β-CD, all three molecules are present in both neutral and monoanionic forms in the nanocavity. The monoanion is stabilized more inside of the β-CD cavity. The studies revealed that the ESIPT of nitrogen-substituted analogues is more susceptible to the environment than HPBI, and therefore, they are more promising probes. 1. INTRODUCTION Studies on organized microheterogeneous assemblies have been growing rapidly during the past few decades as these serve as good miniature models for studying and mimicking important phenomena in biosystems. 1-3 Among the studies on micro- heterogeneous media, inclusion complexes are one of the interesting subjects for many researchers. Inclusion complexes provide valuable information about noncovalent intermolecular interactions between the host and guest molecules where the guest component is lying within the cavity of the host molecule without forming any covalent bond. 1 Apart from biomimick- ing, 2,4 inclusion complexes find applications in drug delivery, 3 nanosized electronic devices, 5 and energy storage devices. 6 Cyclodextrins (CDs) 1-3,5 are the most sought systems for studying such inclusion complexes. CDs are cyclic oligosac- charides composed of glucopyranose units linked by α-(1,4) bonds with hydrophilic external walls and an interior hydrophobic nanosized cavity of different size and shape. 1-3 The hydrophobicity of the cavity enhances the solubility 7 and the fluorescence of the encapsulated guest molecule. 8 There- fore, CDs have been used as microenvironments to study excited-state processes such as proton transfer, 8-10 charge transfer, 11-13 and energy transfer. 14,15 Douhal et al. reviewed the dynamics and structural aspects of the host-guest interaction in CD. 16,17 The inclusion complexes of fluorophore in aqueous host molecules including CDs were recently reviewed by Nau et al. 18 Wagner more recently reviewed the hydrogen bonding of excited states in supramolecular inclusion complexes including CDs. 19 Douhal et al. investigated the effect of β-CD on the dynamics of an excited-state intramolecular proton transfer (ESIPT) molecule. 16 Warner et al. studied the dual fluorescence of 10- hydroxybenzo[h]quinone and also 2-(2′-hydroxyphenyl)- benzimidazole (HPBI), its corresponding benzoxazole, and benzothiazole in the presence of β-CD. 20,21 They showed the existence of weak intramolecular hydrogen bonding in HPBI and formation of strong intermolecular hydrogen bonds with the hydroxyl groups of CD. They further showed that the phototautomers exist as zwitterions. More recently, Guchhait et al. studied the ESIPT process of 1-hydroxy-2-napthaldehyde in CDs. 22 HPBI and its analogues form a class of fluorophores that are extensively studied due to the ESIPT exhibited by them. 21,23-40 Hence, this class of compounds finds applications as lasers, probes, sensors, and devices. 25,34,41-43 The ESIPT reaction is greatly affected by the hydrogen bonding capability of the solvent. 44-49 In protic solvents, the intramolecular hydrogen bonded ring in the fluorophore molecule might break to form Received: November 19, 2012 Revised: April 26, 2013 Published: April 26, 2013 Article pubs.acs.org/JPCA © 2013 American Chemical Society 4084 dx.doi.org/10.1021/jp311438s | J. Phys. Chem. A 2013, 117, 4084-4095