Inorg. Chem. 2007, 46, 8051−8058
Transition-Metal-Induced Fluorescence Resonance Energy Transfer in a Cryptand Derivatized with Two Different Fluorophores Kalyan K. Sadhu, Bamaprasad Bag, and Parimal K. Bharadwaj* Chemistry Department, Indian Institute of Technology Kanpur, Kanpur 208016 India Received March 15, 2007
A laterally nonsymmetric aza cryptand has been derivatized with one 7-nitrobenz-2-oxa-1,3-diazole (fluorophore1) and one/two anthracenes (fluorophore2) to obtain 1 and 2. Their emission characteristics are probed in the presence of a number of transition metals and proton. In the case of 1, Cu(II), Zn(II), Cd(II), and proton afford a large enhancement of fluorescence, whereas Fe(II) and Ag(I) exhibit one order of magnitude less enhancement. In contrast, 2 gives a large enhancement with Cu(II), Ag(I), and proton. The enhancement is observed in the diazole moiety even when the anthracene fluorophore is excited because of substantial fluorescence resonance energy transfer from anthracene to the diazole moiety. Compounds 1 and 2 can be termed as the second-generation fluorescence signaling systems.
Introduction Fluoroionophores have been built1-5 that report the binding of a guest to the host with an increase or decrease in fluorescence quantum yields. Two design principles3,6 are mainly adopted for this: (i) both the signaling (fluorophore(s)) and the guest binding (receptor) moieties are integrated into a single unit, or (ii) fluorophore(s) and the receptor are connected through a spacer that acts as a conduit for electronic communication between the two. Because the later design is modular in nature, both the fluorophore and the receptor can be varied easily. Here, the receptor can be modified to induce selectivity in the binding of a guest and to allow for probing of new strategies of fluorescence detection. We have used a cryptand as a receptor, where a * To whom correspondence should be addressed. E-mail:
[email protected]. (1) (a) Lehn, J.-M. Supramolecular Chemistry s Concepts and perspectiVes; VCH Publishers: Weinheim, Germany, 1995. (b) Valeur, B. In Topics in Fluorescence Spectroscopy; Lakowicz, J. R., Ed.; Plenum Press: New York, 1994; Vol. IV. (c) Czarnik, A. W. Acc. Chem. Res. 1994, 27, 302. (2) (a) Valeur, B.; Leray, I. Coord. Chem. ReV. 2000, 205, 3. (b) de Silva, A. P.; Tecilla, P. J. Mater. Chem. 2005, 15. (3) Callan, J. F.; de Silva, A. P.; Magri, D. C. Tetrahedron 2005, 61, 8551. (4) Rurack, K. Spectrochim. Acta, Part A 2001, 57, 2161. (5) Fabbrizzi, L.; Poggi, A. Chem. Soc. ReV. 1995, 25, 197. (6) (a) Bharadwaj, P. K. Prog. Inorg. Chem. 2003, 51, 251. (b) Canary, J. W.; Gibb, B. C. Prog. Inorg. Chem. 1997, 45, 1. (c) Bissell, R. A.; de Silva, A. P.; Gunaratne, H. Q. N.; Lynch, P. L. M.; Maguire, G. E. M.; McCoy, C. P.; Sundanayake, K. R. A. S. Top. Curr. Chem. 1993, 168, 223. (d) Bissell, R. A.; de Silva, A. P.; Gunaratne, H. Q. N.; Lynch, P. L. M.; Maguire, G. E. M.; McCoy, C. P.; Sundanayake, K. R. A. S. Chem. Soc. ReV. 1992, 137.
10.1021/ic700506r CCC: $37.00 Published on Web 08/15/2007
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metal ion forms a highly stable inclusion complex due to the cryptate effect.6a In the absence of a metal ion, the fluorophore, when excited, does not show any emission as a result of intramolecular photoinduced electron transfer (PET)7 from the lone pair of electrons on the nitrogen atom to the fluorophore, and the excited fluorophore comes down to the ground state through a nonradiative pathway. In the presence of a suitable metal ion as an input to the system, the PET is blocked because of the engagement of the nitrogen lone pair of electrons to the metal and fluorescence can be observed. Because transition metals quench fluorescence effectively, different design strategies have been pursued8 to surmount this problem. We had shown earlier that efficient fluorescence signaling by transition-metal ions could be built with cryptands as receptors.9 In systems containing more than one type of fluorophore in which the emission spectrum of one overlaps with the absorption spectrum of the other, substantial fluorescence resonance energy transfer10 can take place. This design (7) Kavarnos, G. J. Fundamentals of Photoinduced Electron Transfer; VCH Publishers: Weinheim, Germany, 1993. (8) (a) Rurack, K.; Resch-Genger, U. Chem. Soc. ReV. 2002, 31, 116. (b) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515. (9) (a) Ghosh, P.; Bharadwaj, P. K.; Roy, J.; Ghosh, S. J. Am. Chem. Soc. 1997, 119, 11903. (b) Bag, B.; Bharadwaj, P. K. Inorg. Chem. 2004, 43, 4626. (c) Bag, B.; Bharadwaj, P. K. Org. Lett. 2005, 7, 1573. (10) (a) Fo¨rster, T. Z. Naturforsch. 1949, 49, 321. (b) Cheung, H. C. In Topics in Fluorescence Spectroscopy; Lakowicz, J. R., Ed.; Plenum Publishing: New York, 1991; Vol. 2, p 128.
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Sadhu et al. ratiometric11
strategy allows for fluorescence signaling and offers a new generation of fluorescence sensors. Because the fluorescence resonance energy transfer also depends on the distance and orientation of the two fluorophores, it becomes an important biological tool to monitor12 enzyme activity and conformational analysis13 of protein structures. Besides, this is potentially important14 in pharmacology, dye lasers, frequency conversion of light, and so on. A cryptand is attractive for studying fluorescence resonance energy transfer because it can act as a perfect skeleton to sequentially add different fluorophores. Besides, it can undergo conformational changes9c to enable partial movement of one of the side arms with respect to another, which changes the distance and the orientation of the fluorophores with respect to one another, with implications on the fluorescence resonance energy transfer efficiency. Herein, we report the synthesis of an aza-oxa cryptand derivatized with one 7-nitrobenz2-oxa-1,3-diazole (hereafter, diazole) and one/two anthracene groups through methylene spacers. The emission band of anthracene overlaps with the absorption band of diazole, making it a fluorescence resonance energy transfer system. When the cavity is empty, no significant interaction between the fluorophores takes place. However, when a metal ion or proton is added, the PET is blocked, and the fluorescence resonance energy transfer is switched on. Both solid-state and solution fluorescence measurements are reported along with absorption data. Experimental Section Materials. All of the solvents and thionyl chloride were purified prior to use. The purified solvents were found to be free from impurities and moisture and were transparent in the region of interest. All of the other reagent-grade chemicals including metal salts were acquired from Aldrich and were used as received. The metal salts were hydrated as mentioned in the Aldrich catalog. For chromatographic separation, 100-200 mesh silica gels (Acme Synthetic Chemicals) were used. The reactions were carried out under a N2 atmosphere, unless otherwise mentioned. Synthesis. The synthetic route for the cryptand derivatives is illustrated in Scheme 1. Cryptand L0 was synthesized and recrystallized from acetonitrile as colorless rectangular crystals following our previously reported15 procedure. Synthesis of L1. Compound L1 was synthesized following the procedure reported previously9b from our laboratory. Synthesis of 1 and 2. To a solution of L1 (0.36 g, 0.5 mmol) in dry acetonitrile (20 mL) was added anhydrous K2CO3 (1.5 g, excess), and the solution was stirred for 15 min. Freshly recrystal(11) (a) Banthia, S.; Samanta, A. J. Phys. Chem. B 2006, 110, 6437. (b) van Dongen, E. M. W. M.; Dekkers, L. M.; Spijker, K.; Meijer, E. W.; Klomp, L. W. J.; Merkx, M. J. Am. Chem. Soc. 2006, 128, 10754. (12) (a) Stryer, L.; Haugland, R. P. Proc. Natl. Acad. Sci. U.S.A. 1967, 58, 719. (b) Wu, P.; Brand, L. Anal. Biochem. 1994, 218, 1. (13) Ramanoudjame, G.; Du, M.; Mankiewicz, K. A.; Jayaraman, V. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10473. (14) (a) Thiery, C. Mol. Photochem. 1970, 2, 1. (b) Scha¨fer, F. P.; Bor, Zs.; Lu¨ttke, W.; Liphardt, B. Chem. Phys. Lett. 1978, 56, 455. (c) Kopainsky, B.; Kaiser, W.; Scha¨fer, F. P. Chem. Phys. Lett. 1978, 56, 458. (d) Ketskemety, I.; Farkas, E.; Toth, Zs.; Gati, L. Acta Phys. Chem. 1982, 28, 3. (e) Bourson, J.; Mugnier, J.; Valeur, B. Chem. Phys. Lett. 1982, 92, 430. (15) Ragunathan, K. G.; Bharadwaj, P. K. Tetrahedron Lett. 1992, 33, 7581.
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Scheme 1. Synthetic Route to the Investigated Compounds 1 and 2a
a Reagent and conditions: (i) 4-chloro-7-nitrobenz-2-oxa-1,3-diazole, toluene, refluxed for 6 h; (ii) PPh3, Br2, MeCN, 25 °C; (iii) K2CO3, MeCN, KI, refluxed for 48 h.
lized 9-bromomethyl anthracene16 (0.21 g, 0.75 mmol) was added to it along with a crystal of KI, and the reaction mixture was allowed to reflux for 72 h. After cooling to room temperature, K2CO3 was removed by filtration. The yellow filtrate was evaporated to dryness under reduced pressure, washed several times with water, and then extracted with CHCl3. The organic layer, after drying over anhydrous Na2SO4, evaporated to dryness to obtain a yellow solid that was a mixture of 1 and 2. Compound 2. This product was eluted out with a mixture of chloroform and methanol (98:2 (v/v)) as the eluent. The dark-red solid obtained upon evaporation of the solvent was recrystallized from acetone. Yield: 30%. Mp: 256 °C. 1H NMR (400 MHz, CDCl3, 25 °C, TMS, δ): 0.81 (t, 4H, 2 × CH2), 1.10 (s, 4H, 2 × CH2), 1.18 (s, 2H, CH2), 1.61 (m, 2H, CH2), 1.93 (m, 4H, 2 × CH2), 2.17 (m, 2H, CH2), 2.31 (m, 2H, CH2), 2.42 (m, 4H, 2 × CH2), 3.29 (m, 2H, CH2), 4.13 (m, 4H, 2 × CH2), 5.29 (s, 4H, 2 × An-CH2), 6.45 (d, 1H, diazole-H1), 6.72 (d, 4H, 2 × AnH2,7), 6.80 (m, 3H, 3 × Ar-H2), 7.15 (d, 4H, 2 × An-H1,8), 7.23 (m, 3H, 3 × Ar-H3), 7.39 (d, 3H, 3 × Ar-H1), 7.48 (d, 4H, 2 × An-H3,6), 7.60 (m, 3H, 3 × Ar-H4), 7.74 (d, 4H, 2 × An-H4,5), 7.92 (d, 2H, 2 × An-H10), 8.06 (d, 1H, diazole-H2). 13C NMR (100 MHz, CDCl3, 25 °C, TMS, δ): 156.98, 132.31, 130.87, 129.69, 128.34, 126.67, 125.58, 120.50, 111.21, 67.61, 55.04, 54.63, 49.55, 47.55, 29.67. FAB-MS (m/z): 1103 (100%) [2]+. Anal. Calcd for C69H66N8O6: C, 75.11; H, 6.03; N, 10.16. Found: C, 74.98; H, 6.18; N, 10.08. Compound 1. This was subsequently eluted out with chloroform/ methanol (96:4 (v/v)) as the eluent. The dark-red solid obtained was recrystallized from acetone. Yield: 35%. Mp: 236 °C. 1H NMR (400 MHz, CDCl3, 25 °C, TMS, δ): 0.81 (m, 2H, CH2), 1.18 (s, 6H, 3 × CH2), 2.25 (br s, 1H, NH), 2.44 (d, 2H, CH2), (16) Bullpit, M.; Kitching, W.; Doddrell, D.; Adcock, W. J. Org. Chem. 1976, 41, 760.
Fluorescence Resonance Energy Transfer
Figure 2. Emission spectra of 1 and 2 (concentration of 1 ) 1.0 × 10-6 M and concentration of 2 ) 2.5 × 10-6 M) (λex are shown in parentheses) in MeCN.
Figure 1. (a) UV-vis spectra of 1, 1 + Cu(II), 1 + H+, and 1 + Cu(II) + H+ (concentration of 1 ) 1.6 × 10-5 M) in MeCN. (b) UV-vis spectra of 2, 2 + Cu(II), 2 + Ag(I), and 2 + H+ (concentration of 2 ) 1.0 × 10-5 M) in MeCN. Inset (above): UV-vis spectra of 2 (solid line), 2 + Ag(I) (dotted line), and 2 + H+ (dashed line). Inset (bottom): UV-vis spectra of 2; and 2 + Cu(II) (dotted line).
2.65 (t, 2H, CH2), 2.75 (m, 2H, CH2), 2.88 (m, 2H, CH2), 3.13 (m, 2H, CH2), 3.24 (m, 4H, 2 × CH2), 3.60 (d, 2H, CH2), 3.95 (m, 2H, CH2), 4.13 (m, 2H, CH2), 4.26 (m, 2H, CH2), 5.23 (s, 2H, AnCH2), 6.05 (d, 1H, diazole-H1), 6.73 (d, 2H, An-H2,7), 6.83 (m, 3H, 3 × Ar-H2), 6.89 (d, 2H, An-H1,8), 6.95 (m, 3H, 3 × ArH3), 7.11 (d, 3H, 3 × Ar-H1), 7.15 (d, 2H, An-H3,6), 7.18 (m, 3H, 3 × Ar-H4), 7.25 (d, 2H, An-H4,5), 7.26 (d, 1H, An-H10), 8.26 (d, 1H, diazole-H2). 13C NMR (100 MHz, CDCl3, 25 °C, TMS, δ): 156.78, 144.70, 135.28, 132.62, 130.14, 122.77, 121.31, 111.99, 102.27, 67.54, 66.36, 55.54, 54.43, 53.48, 52.94, 47.25, 45.71, 29.63. FAB-MS (m/z): 913 (25%) [1]+. Anal. Calcd for C54H56N8O6: C, 71.03; H, 6.18; N, 12.27. Found: C, 71.20; H, 6.09; N, 12.18. Synthesis of Protonated Complexes of 1 and 2. To a stirring solution of either ligand (0.1 mmol) in acetonitrile was added a dilute perchloric acid solution, and the solution was stirred for 2 h at room temperature and then filtered. The filtrate upon evaporation at room temperature for 3-4 days afforded a deep-yellow crystalline solid that was collected by filtration, washed with ether, and dried under a vacuum for solid-state emission measurements. For [1‚H]+ complex. Yield: 75%; ESI-MS (m/z): 914 (20%) [1‚H]+. Anal. Calcd for C54H57ClN8O10: C, 63.99; H, 5.67; N, 11.06. Found: C, 64.20; H, 5.81; N, 10.89. For the [2‚H]+ complex. Yield: 60%. ESI-MS (m/z): 1104 (100%) [2‚H]+ and 1204 (35%) [(2‚2H)ClO4]+. Anal. Calcd for C69H68Cl2N8O14: C, 63.54; H, 5.25; N, 8.59. Found: C, 63.34; H, 5.29; N, 8.68. Synthesis of the Cd(II) Complex of 1. To a stirring solution of ligand 1 (0.1 mmol) in dry acetonitrile was added an acetonitrile solution of Cd(NO3)2‚4H2O (0.1 mmol) followed by NaCl (0.1 mmol), and the solution was stirred for 2 h at room temperature.
After filtration, the filtrate left at room temperature overnight afforded an orange crystalline solid as the desired product. The solid was collected by filtration, washed several times with cold acetonitrile, and dried under a vacuum for use in emission measurements. Yield: 65%. ESI-MS (m/z): 1104 (25%) [Cd(1)NO3‚H2O]+. Anal. Calcd for C54H58N10O13Cd: C, 55.55; H, 5.01; N, 12.00. Found: C, 55.43; H, 5.09; N, 12.08. Synthesis of the Cu(II) Complex of 2. To a stirring solution of 2 (0.1 mmol) in dry ethanol (20 mL) was added an ethanolic solution (5 mL) of Cu(ClO4)2‚6H2O (0.1 mmol) followed by NaCl (0.1 mmol), and the solution was stirred for 2 h at room temperature. After filtration, the filtrate was allowed to evaporate at room temperature, whereupon a greenish-yellow crystalline solid separated. The solid was collected by filtration, washed with diethyl ether, and dried under a vacuum for use in emission measurements. Yield: 72%. ESI-MS (m/z): 1268 (45%) [Cu(2)ClO4]+. Anal. Calcd for C69H66N8O10ClCu: C, 65.45; H, 5.25; N, 8.85. Found: C, 65.36; H, 5.30; N, 8.81. Physical Measurements. Compounds 1 and 2 were characterized by elemental analyses, 1H NMR, 13C NMR, and mass (positive ion) spectroscopy. 1H NMR and 13C NMR spectra were recorded on a JEOL JNM-LA400 FT (400 and 100 MHz, respectively) instrument in CDCl3. FAB mass (positive ion) data were obtained from a JEOL SX 102 /DA-6000 mass spectrometer using argon as the FAB gas at 6 kV and 10 mA, with an accelerating voltage of 10 kV, and the spectra were recorded at 298 K. ESI mass spectra were recorded on a Micromass Quattro II triple-quadrupole mass spectrometer. The ESI capillary was set at 3.5 kV, and the cone voltage was 40 V. Melting points were determined with an electrical melting point apparatus by PERFIT, India, and were uncorrected. UV-vis spectra were recorded on a Jasco V-570 spectrophotometer at 293 K, and the average of three measurements was taken. The deviations in molar absorption coefficients were in the last digit only. Steady-state fluorescence spectra were obtained using a PerkinElmer LS 50B luminescence spectrometer at 293 K with an excitation and emission band pass of 5 nm. Steady-state fluorescence anisotropy data were obtained using a PerkinElmer LS 55 fluorimeter at 293 K with a slit width of 5 nm and an integration time of 40 s. Fluorescence quantum yields of all of the compounds were determined by comparing the corrected spectrum with that of anthracene (φ ) 0.297) in ethanol,17 taking the area under the total (17) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970.
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Sadhu et al. Table 1. Fluorescence Output of 1 and 2 with Different Ionic
Inputsa Quantum yield φ (enhancement factor)
φdiazole (λex ) 344 nm) 1 1 + Mn(II) 1 + Fe(II) 1 + Co(II) 1 + Ni(II) 1 + Cu(II) 1 + Zn(II) 1 + Cd(II) 1 + Ag(I) 1 + Tl(I) 1 + H+ 1 + Cu(II) + H+ 2 2 + Mn(II) 2 + Fe(II) 2 + Co(II) 2 + Ni(II) 2 + Cu(II) 2 + Zn(II) 2 + Cd(II) 2 + Ag(I) 2 + Tl(I) 2 + H+
0.0008 0.0015 (2) 0.0662 (83) 0.0008 (1) 0.0005 (
