Highly Sensitive and Selective Rhodamine-Based “Off-On” Reversible Chemosensor for Tin (Sn 4+ ) and Imaging in Living Cells Ajit Kumar Mahapatra,* ,† Saikat Kumar Manna, † Debasish Mandal, § and Chitrangada Das Mukhopadhyay ‡ † Department of Chemistry and ‡ Centre for Healthcare Science & Technology, Bengal Engineering and Science University, Shibpur, Howrah711103, India § Department of Spectroscopy, Indian Association for The Cultivation of Science, Jadavpur, Kolkata700032, India * S Supporting Information ABSTRACT: A structurally characterized new oxo-chromene function- alized rhodamine derivative L1 exhibits high selectivity toward Sn 4+ by forming a 1:1 complex, among other biologically important metal ions, as studied by fluorescence, absorption, and HRMS spectroscopy. Complexing with Sn 4+ triggers the formation of a highly fluorescent ring-open form which is pink in color. The sensor shows extremely high fluorescence enhancement upon complexation with Sn 4+ , and it can be used as a “naked- eye” sensor. DFT computational studies carried out in mimicking the formation of a 1:1 complex between L1 and Sn 4+ resulted in a nearly planar pentacoordinate Sn(IV) complex. Studies reveal that the in situ prepared L1-Sn complex is selectively and fully reversible in presence of sulfide anions. Further, confocal microscopic studies confirmed that the receptor shows in vitro detection of Sn 4+ ions in RAW cells. ■ INTRODUCTION The design and development of optical sensors for selective recognition and sensing of environmentally and biologically important metal ions is an important and contemporary research area. In this regard, metal ions that are known to have harmful effects on living organisms or the environment are generally more common as target metal ions for such studies. 1 Among the different metal ions, tetravalent tin, Sn 4+ , is an important trace mineral for human and animal biology as it is involved in several biochemical processes at the cellular level. 2a Tin deficiency can increase the risk factors associated with poor growth, hearing loss, and cancer prevention. Some severe immunotoxic and neurotoxic effects of tin have been reported, with symptoms mostly associated to gastrointestinal complaints such as nausea, abdominal pain, and vomiting. 2b In addition, tin compound is a known environmental pollutant due to its widespread use as chemical reagent, agricultural, chemical weapon, and industrial activities. 3 Therefore, great importance is attached to developing selective chemosensors for tin. In this aspect, fluorogenic methods with suitable probes are preferable approaches for the measurement of these analytes because fluorimetry is rapidly performed, is nondestructive, is highly sensitive, is suitable for high-throughput screening applications, and can afford real information on the localization and quantity of the targets of interest. In recent years, significant emphasis has been placed on the development of new, highly selective fluorescent chemosensors of different architectures for metal cations because of their potential applications in biochemistry and environmental research. 4 It is familiar that rhodamine B is a classic fluorescent dye and its derivatives (RhB) are extensively used as a fluorescent chemosensor because of their excellent photophysical proper- ties, such as high extinction coefficients (>75 000 cm -1 M -1 ), excellent quantum yields, 5 great photostability, 6 and relatively long emission wavelengths in the visible region. 7 In addition, the equilibrium between the nonfluorescent spirocyclic form and the highly fluorescent ring-open form provides a better model for the development of turn-on sensors. So, rhodamine B is widely used as a fluorescent probe for the detection of cysteine 8 and metal ions, 9 including Cu 2+ , 10 Cr 3+ , 11 Fe 3+ , 12 Hg 2+ , 13 Pb 2+ , 14 Zn 2+ , 15 Au 3+ , 16 Ba 2+ , 17 and Al 3+ , 18 due to the ring-opening reaction of rhodamine. With the above-mentioned criteria in mind, herein we introduced oxo-benzopyran or oxo-chromene moiety to rhodamine, and we synthesized a chemosensor, L1 (Scheme 1), 19 which can give highly selective and rapid responses to Sn 4+ in living cells. As far as we are aware, L1 is the first oxo- chromene based Sn 4+ sensor on rhodamine derivative. The structure of chemosensor L1 was verified by 1 H NMR, 13 C NMR, mass spectra, and elemental analyses (Figures S1-S3, Supporting Information). Our sensor shows extremely high sensitivity compared to the recently developed 9d Sn 4+ sensors attributed to the very high association constants for the binding of Sn 4+ . Received: March 21, 2013 Published: September 25, 2013 Article pubs.acs.org/IC © 2013 American Chemical Society 10825 dx.doi.org/10.1021/ic4007026 | Inorg. Chem. 2013, 52, 10825-10834