Fine tuning of a solvatochromic fluorophore for selective determination of Fe 3+ : a new type of benzimidazole-based anthracene-coupled receptor Doo Youn Lee a , 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 140 001, Punjab, India article info Article history: Received 2 December 2010 Revised 30 December 2010 Accepted 17 January 2011 Available online 20 January 2011 Keywords: Solvatochromic Sensor Fluorescent Ratiometric Fe 3+ abstract We synthesized a novel benzimidazole-based, anthracene-coupled fluorescent receptor capable of recog- nizing and estimating the concentrations of Fe 3+ in semi-aqueous solution by ratiometric estimation. Our sensor can be made highly selective for Fe 3+ over other metal ions by changing the solvent composition. Ó 2011 Elsevier Ltd. All rights reserved. Fluorescence-based techniques are important tools for chemical and biochemical research because of their non-invasive nature and high intrinsic sensitivity. 1 They rely on several fundamental photo- physical mechanisms including photo-induced electron transfer (PET), internal charge transfer (ICT), excimer and exciplex forma- tions, electronic energy transfer (EET), or Förster resonance energy transfer (FRET). 2 Receptors attached to fluorescent dyes commonly employ these mechanisms for bioimaging, characterization of bio- logical micro-structures, and quantitative estimation of analytes. 3 The methodology has rapidly progressed to meet new demands, such as analytes associated with new disease states, the need for simultaneous estimation of multiple analytes, and molecular logic gates and switches. 4 Solvents are known to influence the physico-chemical nature of receptor binding subunits as well as the fluorescence output. Quantitative measurement of the influence of solvents on the rec- ognition of metal ions would be greatly aided by the investigation of the solvatochromic behavior of donor–receptor units fabricated with more than one fluorophore. 5 This approach is very helpful for developing fluorescent sensor materials that exhibit changes in their fluorescent signals when exposed to different solvents and/ or cations. Under present investigation, we wish to develop selec- tive sensors for Fe 3+ because of the role of Fe 3+ as an important nutrient for human health and in numerous physiological functions. We previously reported the development of benzimidazole- based receptors. 6 Here we present the synthesis of a receptor unit comprised of two fluorophores, benzimidazole, and anthracene. The solvatochromic behavior of the receptor was fine tuned to be highly selective for Fe 3+ . The two fluorophores were chosen in such a way that both could be excited at the same wavelength, but the fluorophore output had to be different; the anthracene moiety gen- erally exhibits ‘on–off’ behavior, 7 while the benzimidazole moiety may undergo both ‘on–off’ behavior and a charge transfer (CT) band shift. 6 This design is important for the improvement of ratio- metric fluorescent recognition that measures the ratio of fluores- cence intensity at two wavelengths. The ratiometric approach eliminates errors associated with receptor concentration, photo- bleaching, and environmental effects. 8 Receptor 1 was synthesized as shown in Scheme 1. Compound 2 was synthesized as described in the literature. 9 Compound 4 was prepared with a 43% yield by the reaction of compound 2 with sal- icylaldehyde (3) in the presence of KOH. 10 Receptor 1 was prepared by a condensation reaction of dialdehyde 4 with 2-(2-aminophe- nyl)-1H-benzimidazole (5) in the presence of a catalytic amount of Zn(ClO 4 ) 2 , followed by a reduction with NaBH 4 , with a 46% yield. 11 Receptor 1 displayed a well defined maximum at 414 nm in the fluorescence spectrum recorded at a concentration of 10 lM in pure acetonitrile, when excited at 367 nm. There were no other emission bands at any wavelengths, indicating the absence of duel channel emission due to two different fluorophores in the pure host. Sensors used in biological and environmentally important samples must be capable of monitoring analytes in aqueous media. 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.01.077 ⇑ Corresponding author. Tel.: +82 337602261; fax: +82 337602182. E-mail address: dojang@yonsei.ac.kr (D.O. Jang). Tetrahedron Letters 52 (2011) 1368–1371 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet