Benzimidazole-based receptor for Zn 2þ recognition in a biological system: a chemosensor operated by retarding the excited state proton transfer Min Joung Kim a , Kamalpreet Kaur b , Narinder Singh b , Doo Ok Jang a, * a Department of Chemistry, Yonsei University, Wonju 220-710, Republic of Korea b Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India article info Article history: Received 20 March 2012 Received in revised form 23 April 2012 Accepted 24 April 2012 Available online 2 May 2012 Keywords: Chemsensor Benzimidazole Zinc Espt Tautomerism Dipodal abstract A new benzimidazole-based receptor was developed with potential functional groups for excited state proton transfer (ESPT) through ketoeenol tautomerism. The enol form of the receptor selectively rec- ognizes Zn 2þ , allowing it to be used as a ratiometric fluorescence sensor in DMSO/CH 3 CN (1:9, v/v). The binding event triggers a blue-shifted band through the modulation of charge transfer transitions. The sensor is applicable for recognizing Zn 2þ in the cytoplasm of Saccharomyces cerevisiae. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Zn 2þ is an essential trace metal element in many cellular pro- cesses, and many enzymatic reactions are under its direct control. 1 Zinc is also a contributory factor in neurological disorders, such as epilepsy and Alzheimer’s disease. 2 Depletion of the biological zinc level would lead to a decrease in wound healing strength as a result of impaired collagen synthesis. 3 Contrary to these structural and functional applications, a high level of zinc is cytotoxic and may lead to skin disease, diabetes, and prostatic adenocarcinoma. 4 The unregulated amount of zinc in the diets of children in many parts of Africa and Asia is believed to cause slow growth, frequent infections and loss of appetite. 5 Considerable research activity has focused on Zn 2þ estimation; however, there is still room for improvement through detailed mechanistic studies of sensors and applications in biological systems. 6 A number of mechanisms control the activities of a fluorescence sensor. Previous reports typically use photoinduced proton transfer (PET), modulation of charge transfer (CT) transitions, excited state proton transfer (ESPT), and eximer formation. 7 A PET sensor oper- ates through ‘oneoff’/‘offeon’ of the fluorescence intensity at a single wavelength. Measurements at a single wavelength may lead to an error in estimation due to phototransformation, receptor concentrations and environmental effects. 8 Alternatively, sensors based on the ESPT mechanism, sensors operating through modu- lations of the CT band, and excimer formation lead to a system in which the estimation is free from these drawbacks as they dem- onstrate dual channel fluorescence emissions. 9 2. Results and discussion In the continuation of our on-going research in the development of benzimidazole-based chemosensors, 10 we report a new type of dipodal receptor 1 , which was prepared by the condensation of dialdehyde 2 with amine 3 (Scheme 1). Receptor 1 has the potential to operate through either one mechanism (PET, CT, ESPT, and mo- lecular rigidity) or through a combination of mechanisms. The re- ceptor bears an eOH groups, from which the PET may originate and that can quench the fluorescence intensity of the receptor. The metal binding in a coordination sphere of the receptor using an eOH group can eliminate the PET and restore the fluorescence emission. Metal binding using the nitrogen donors of benzimid- azole may affect the CT band of the receptor. The receptor consists of imine linkages, and the fluorescence is quenched through cis/ trans isomerism at eCH]N. Metal binding with such a receptor causes rigidity in the receptor structure and also freezes the eCH] N in one isomer; consequently, the fluorescence will again occur. 11 The receptor may undergo ketoeenol tautomerism through the * Corresponding author. E-mail address: dojang@yonsei.ac.kr (D.O. Jang). Contents lists available at SciVerse ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2012.04.097 Tetrahedron 68 (2012) 5429e5433