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Cation Recognition with Fluorophore Crown Ethers1 C ¸ akıl Erk Science Faculty, Chemistry Department, Istanbul Technical University, Maslak, 80626 I˙ stanbul, Turkey
The fluoroionophore behavior of various macrocycles with oxygen and nitrogen donors was reviewed in the present paper, and the cationic recognition of alkali and alkaline-earth metals involving different photophysical effects depending on the fluoroionophore structures was summarized. Spectrofluorometry is a very sensitive technique measuring both emission and excitation intensities of a fluorescent molecule that is influenced by the environment. The cationic recognition of fluoroionophore crown ethers which possess at least two molecular components with sites that interact with photons as well as ions is studied. Cations mostly induce the changes in triplet energy relative to the excited singlet state, S1 f T1, and the ground state, T1 f S0. In the presence of metal cations, the increased phosphorescence lifetime of luminescent macrocycles, in general, gave complexation-enhanced quenching fluorescence spectra that reduce the fluorescence lifetime. However, if the phosphorescence lifetime is reduced, the fluorescence life is increased and the complexation-enhanced fluorescence spectra would be observed. Optical responses originating from the different photophysical mechanisms of photoinduced charge transfer, electronic energy transfer, monomer/excimer equilibrium, and internal charge transfer were represented. The recent studies on the cation recognition of fluorophore macrocyclic ethers are exemplified and discussed in the present paper. 1. Introduction a. Fluorophore Macrocycles. Macrocycles that possess ionophore ability have been subject to wide investigations not only for their synthesis but also for their numerous technological and analytical applications since their discovery by Pedersen.2 A large number of organic syntheses and applications have been reported including solvent extraction and membrane transport procedures, ion-selective electrodes and related analytical utilities, and recently chromogenic functionality of macrocycles via optical spectroscopy. In this paper, among the several reports, we are only dealing with the representative results of fluorophore macrocycles along with the new concepts.3-10 Cationic recognition studies on fluorophore macrocycles have increased because of interest in inorganic cationic chemistry. The interaction mechanism of electropositive ions with the macrocycle dipoles in a solvent is the main concept. The collected reports in this work were selected as evidence for photocationic recognition as we discussed below.11-17 Namely, the macrocycles of photoionic response possess at least two molecular components with sites that interact with photons as well as ions.18-38 b. Introduction to Spectrofluorometry. There are numerous techniques to study on the cation-binding phenomenon. However, spectroscopy has the advantages of fast and relatively simple handling and of calculation procedures as well as attractive performances to analyze the complex mixtures. Spectrofluorometry is a very sensitive and selective technique that measures both emission and excitation intensities of a fluorescent molecule which is usually influenced by the characteristics of the environment and small changes in the positions of the energetic levels ruling the quantum yield and fluorescence decay.11-17 The origin of fluorescence is a quite well-known phenomenon involving relaxation of radiative and nonradiative decays of excited states. The photophysical interactions depend on the nature of the fluorescing
structures of acting partners, although the radiative fluorescence has time to occur when the nonradiative relaxation rate is short enough. The prime practical difficulty of the self-quenching effect of the increased lumophore concentration is decided with quantum yield estimation which may limit the quantitative work. The electronic states transferred are assigned in different ways because we use the enumerative system of S0, S1, S2, ..., Sx or T1, T2, etc., where Sx is the singlet state and Tx is the triplet state. The fluorescence is allowed via radiative S1 f S0 transitions, while the T1 f S0 transition is the radiative phosphorescence decay.11 c. Fluorescence Pathways of Fluoroionophores. The interactions between the cation and macrocycle and the role of the fluorescence structures involved in cationic recognition are considered in this paper.12-38 Interaction of the oxygen dipoles of the macrocycle with electropositive ions may form thermodynamically stable complexes. Plenty of macrocycles have been synthesized to study molecular and cationic recognition using different physical methods.7-10 Fluorescence spectroscopy, although, recently contributed to analytical methods, which estimates the quality and quantity of cationic recognition.11-17 However, the fluorescence strength depends on the structure and substituent which plays the primary role in excited singlet and triplet states. Strong fluorophores, like anthracene, involve the intersystem crossing process in depleting the lowest excited singlet state.18-26 The cation complexes of macrocycles with fluorescent moieties have received more attention recently, in particular focusing the excited-state decays.27-38 The pioneering reports of Sousa et al. on different types of naphthalene crown ethers have outlined the photophysical interactions related to metal complex formations.18-20 However, the cationic effect and its
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quantitation could be estimated by the fluorescence spectroscopy because the cations mostly induce the changes in the triplet state energy relative to the excited singlet state energy, S1 f T1, and the ground state energy, T1 f S0, although they sometimes cause a heavy atom effect. The changes on fluorescence emission properties caused by metal cations which are decided by fluorescence, φf, and phosphorescence, φp, quantum yield measurements are essential. However, the computations among such photophysical interactions may give different results. No difference may sometimes be observed between fluorescence lifetimes, τf, of free and complexed macrocycles. However, in the presence of metal cations, an increased phosphorescence quantum yield, φp, of a luminescent macrocycle gives complex formation enhanced quenching fluorescence spectra (CEQFS), while φf is reduced. However, if φp is reduced while φf is increased, the complexation-enhanced fluorescence spectra (CEFS) is observed. This dichotomous role of naphthalene-erived macrocycles was reported by Sousa et al.18,19 The cationic benzocrowns have shown CEFS in alcohol as reported by Morita et al.20 and by Wolfbeis.21 Because of their extraordinary fluorescent properties, the 9,10-anthracene-substituted macrocycles have been widely investigated for cationic interactions which depend on the temperature as well as the type of cation and the solvent.22-26 Introduction of a nitrogen atom into the fluoroionophore macrocycle would cause electron-transfer properties.27-29 Such molecules were perfectly used for cation detection. The electron transfer between the receptor part and the fluorophore which is quenched by a bound ion is called the photoinduced charge transfer (PET) effect. This interesting area of cation sensing via PET has been, in particular, widely preserved by de Silva et al.22-26 The enhanced internal charge transfer (ICT) types of excited systems have also been applied for cation-ligand titrations.29 It has been reported by Morita et al. that the fluorescence intensity of dibenzo-18-crown-6 was enhanced by alkali cations in alcohol.21 The excitation spectra were quite similar to the absorption spectra of the origin of the 1(π f π*), T1, state, and the quantum yields increase with reduced temperature as well as with cation radii in the complexes. The fluorescence measurements at different temperatures along with viscosity work in cationic solutions have been explained in terms of increases in fluorescence lifetimes due to decreased rates of the nonradiative decays of internal conversion. However, the rotational spin diffusion at the S1 state may also be involved. On the other hand, the conformation of the complex structure could sometimes be defined depending on the photophysical mechanisms. Comprehensive work of Sousa et al. with the different naphthalene crowns showed the complicated computations among the rate of radiative fluorescence, S1 f S0, and phosphorescence, T1 f S0, transitions and the nonradiative loop of intersystem crossing, S1 f T1, and internal conversion, S1 f S0.18-20 2. Results Several types of macrocycles exhibited interesting cation recognition depending on the structure and fluorophore moieties observed from the cation-sensitive
Figure 1. N-(9-Anthracenylmethyl)aza-18-crown-6.
Figure 2. Bis(9,10-anthracenophane) macrocycle.
Figure 3. Excitation spectra of benzo-15-crown-5, in the presence of various NaSCN concentrations from top to bottom at 240 nm. Emission λmax: 308 nm.
fluorescence spectra.21-41 The most typical structures displaying cation-effective fluorescence spectra are discussed as follows: a. Anthracene Crowns. The anthracene-derived macrocycles like 9-anthracene-linked azamacrocycles were reported by Czarnik et al. with water-stable fluorescence spectra and cationic color changes.15,16 Several anthracene derivatives involving the PET mechanism were inhibited by a complex cation, even in the presence of CH2 spacer groups, as was reported by de Silva et al. (Figure 1).22,23 The cyclopolyoxa-9,10-anthracenophanes of the double pseudocavity, as reported by Desvergne et al., have showed interesting guest-induced excimer formation of
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Figure 4. Intensity changes of fluorescence emission of dibenzo-20-crown-6 (n ) 0 and m ) 2) on the addition of Na+ and K+ in AN (excitation λmax: 335 nm).
Figure 5. Ratiometric titration on emission spectra of 2,3-naphtho-18-crown-6 with KSCN and NaClO4 versus intensity changes.
Na+-sensitive fluorescence in acetonitrile (AN) which were less sensitive in methanol (Figure 2).25 b. Benzo and Naphthalene Crowns. The different naphthalene-macrocycle structures have been shown to display alternative photophysical pathways because of a bound cation including the heavy metal role. The earliest examples of ion binding via low-temperature fluorescence spectroscopy have been reported by Sausa et al., who showed the dichotomous effect of altered T1 levels depending on the structure and the cation.18 Morita et al. have presented the nonradiative and radiative deactivation rates of dibenzo-18-crown-6 complexes at various temperatures, evidencing that the K+/ benzo-18-crown-6 system is the strongest complex.20 Desvergne et al. have studied paracyclophane macro-
cycles that exhibited the excimer formation depending on the macrocycle conformation and Na+ complex.26 Erk et al. have studied the synthesis and the steadystate fluorescence spectra of different aromatic crown ethers and their cation complexes in acetonitrile.40-53 At room temperature benzocrowns exhibited typical ionophore behaviors on the cationic fluorescence spectra in acetonitrile as reported by Erk et al. (Figure 3).44 The cation-binding studies exhibited good agreements with the cation radii and the type of counterion as well as the macrocycle size.43-46 The work of Tuncer and Erk on the synthesis and cation-binding effect of dibenzo-(3n + 2)-crown-n macrocycles exhibited interesting cationic fluorescence spectral results (Figure 4).50
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Figure 6. Cationic effect on the fluorescence emission spectra (CEQFS) of 6,7-coumarino-15-crown-5 (n ) 2, R ) CH3) in AN.
Figure 7. Fluorescence emission of NaSCN complexation of 3-(3-cumaril)benzo-15-crown-5 in AN (n ) 2) (excitation λmax: 337 nm).
Such studies in acetonitrile showed, in particular, the validity of the fluorescence method to estimate selective cationic recognition even in the case of weaker cationic interactions.41-53 The study of Yapar and Erk on 2,3-dioxanaphthalene crowns has shown their good sensor role for alkali metals. Ion binding is presented throughout CEQFS, where the counterions have less effect and the cation selectivities observed were mostly in accordance with the macrocycle size (Figure 5).51 However, large dibenzo3n-crown-n types of molecules were shown to properly display such a role.54 c. Coumarin Macrocycles. The coumarin (benzo1,2-pyrone) groups of quite stable fluorogenic molecules which possess too short lifetimes, τf, gave the most interesting results. The observed broad spectral bands and short lifetimes indicate the role of nonradiative transitions from the 1(π f π*) state and intersystem crossing. ISC accounts for only part of the nonradiative deactivation of the excitation energy state by the nearby 1(n f π*) state. The nonradiative deactivation competes with the fluorescence emission of dyes, controlling their laser properties. However, the interactions which restrict the electron flow of the π system would decrease the nonradiative
deactivation probability. The effective lowest triplet state of coumarin can be assigned to a 3(π,π*) type. Therefore, the observed perturbations on the above state, the vibronic interactions between 3(π,π*) and 3(n,π*) states, and the spin-orbit coupling between (n,π*) and (π,π*) states govern the relative dispositions of the excitation energy and lifetimes of the excited states.32,36-38 The preparation and molecular recognition of the coumarin-diazamacrocycles were reported by Valeur et al.,27-29 who reported the ratiometric analyses of the binding role of alkali and alkaline-earth cations. Ionophore properties are attractive for biochemical interests such as the intercellular metal probes. Accordingly, Sammes et al. have reported the synthesis of some coumarin-based macrocycles which displayed very good potassium selectivity in water.30,31 However, the strong solvation may sometimes effect the nonradiative decays that change the interactions to lose the cation selectivity. Erk et al. have reported the serial work on the synthesis and the cationic recognition of different types of coumarin-crowns using steady-state fluorescence in AN (Figure 6).44-46,48-53 Cation-binding investigations were quite satisfactorily observed in AN. However, cations that are well-solvated
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Figure 8. Effect of NaClO4 and LiClO4 on the emission spectra of 3-phenyl-6,7-coumarin-15-crown-5 at λmax ) 443 nm (excitation λmax ) 385 nm).
Figure 9. UV-vis spectra of 9,10-anthraquinone-1,2-15-crown-5 on addition of different amounts of NaClO4 in AN.
by AN, like Li+, may not be completely free to be complexed. The computation between Na+ and K+ observed via fluorescence is the question in AN because six-oxygen-membered macrocycles sometimes show better Na+ binding than K+ binding.43,48 The ion-binding results reported by Erk et al. on the novel crown ethers showed the specific chromophore effect of the coumarins.46 3-Coumarin-attached benzocrowns synthesized very recently displayed an excellent cation sensor role, as shown on Figure 7. The recent studies of Erk and Bulut on the synthesis and photoresponsive binding effect of 3-phenylcoumarin derivatives of macrocycles have shown the role of substituent on the electron efficiency on the coumarin moiety regarding the cation selectivity (Figure 8).52 Cox et al. have reported the synthesis and cationbinding role of 1,8-dioxy- and 4,5-dioxyxanthone crown ethers with spectrofluorometry.38 Recently, Erk et al. prepared the same crowns of 2,3-dioxyxanthone derivatives which showed relatively lower fluorescence quantum yields.53 On the other hand, the UV-vis spectra of 1,2-dioxa9,10-anthraquinone (Alizarin) crown ethers exhibited cation-binding effects. In the presence of alkali and alkaline perchlorate salts, the absorption of macrocyclic ethers at 373 nm is shifted to 373 nm (Figure 9).47 On
the other hand, 2,3-dioxa-9,10-anthraquinone crowns did not display marked cationic fluorescence spectra because of strong quenching.53 However, the binding role of such neutral chromoionophore macrocyclic ethers has been recognized since Pedersen,2 because it was discussed by Lo¨hr and Vo¨gtle.39 3. Conclusions The appearance of multicomponent molecular systems would suggest the different molecular photoionic devices which employ the ions. In principle, the fluoroionophore macrocycle select the ions in matching receptor and incident photons to the fluorophore. Certain stoichiometries of binding constants of cationic complexes estimated from ratiometric analyses displayed the selective sensor role.45,46 The results just gave binding effects relative to quantum yields whatever the photophysical mechanisms involved. However, the photophysical mechanisms might also be investigated via lifetime measurements near the quantitative measurements in preconditioned systems.26,27 Therefore, the results are more versatile from the point of view of practical and engineering use if long-lasting cationinduced light-detecting systems which also respond in a large concentration range are developed.
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Acknowledgment In this paper, we are pleased to give the leading references of the potential laboratories. C¸ .E. appreciates the support of Istanbul Technical University to initiate the topic on Chromo- and Fluorophore Macrocycles. TUBITAK and Turkish National Planning Directory, DPT (Project-126), are acknowledged for their generous support of the continuing projects of C¸ .E. Literature Cited (1) Erk, C¸ . Presented orally in part at the 7th European Symposium on Organic Reactivity, ESOR7, Ulm, FRG, 1999. (2) Pedersen, C. J. New Macrocyclic Ethers. J. Am. Chem. Soc. 1970, 92, 391. (3) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Nielsen, S. A.; Lamb. J. D.; Christensen, J. J.; Sen, D. Thermodynamic and Kinetic Data for Cation-Macrocycle Interaction. Chem. Rev. 1985, 85, 271. (4) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Thermodynamic and Kinetic Data for Cation-Macrocycle Interaction with Cations and Anions. Chem. Rev. 1991, 91, 1721. (5) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Thermodynamic and Kinetic Data for Macrocycle Interactions with Cations, Anions and Neutral Molecules. Chem. Rev. 1995, 95, 2529. (6) Bradshaw, J. S.; Izatt, R. M. Crown Ethers, The Search for Selective Ion Ligating Agents. Acc. Chem. Res. 1997, 30, 338. (7) Inoue, Y.; Gokel, G. W. Cation Binding by Macrocycles; Marcel Dekker: New York, 1990. (8) Vogtle, F. Supramolecular Chemistry; Wiley: Chichester, U.K., 1991. (9) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, Germany, 1995. (10) Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D. Inclusion Commpounds; Academic Press: London, 1984; Vols. 1-3, 1991; Vols. 4 and 5. (11) Kopecky, J. Organic Photochemistry; VCH: Weinheim, Germany, 1991. (12) Schneider, H.-J.; Du¨rr, H. Frontiers in Supramolecular Organic Chemistry and Photochemistry; VCH: Weinheim, Germany, 1991. (13) Valuer, B. In Molecular Luminescence Spectroscopy; Schulman, S. G., Ed.; Wiley: New York, 1993; part III, pp 25-84. (14) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Horwood: Chichester, U.K., 1991. (15) Czarnik, A. W. Fluorescent Chemosensors for Ion and Molecule Recognition; ACS Books: Washington, DC, 1993. (16) Czarnik, A. W. Chemical Communication in Water Using Fluorescent Chemosensors. Acc. Chem. Res. 1994, 27, 302. (17) Desvergne, J.-P.; Czarnik, A. W. Chemosensors of Ion and Molecule Recognition, NATO ASIC; Kluwer: Dordrecht, The Netherlands, 1997; Vol. 492. (18) Sausa, L. R.; Larson, J. M. Crown Ether Model Systems for the study of Photoexcited State Response to Geometrically Oriented Perturbers. The Effect of Alkali Metal Ions on Emission form Naphthalene Derivatives. J. Am. Chem. Soc. 1977, 99, 307. (19) Ghosh, S.; Petrin, M.; Maki, A. H.; Sousa, L. R. Effect of Metal Ion Perturbers on the triplet state of Naphthalene in Naphthalene Crown Ether-Metal Ion Complexes. II. Stark Effect. J. Phys. Chem. 1988, 88, 2913. (20) Shizuka, H.; Takada, K.; Morita, T. Fluorescence Enhancement of Dibenzo-18-crown-6 by Alkali Metal Cations. J. Phys. Chem. 1980, 84, 994. (21) Wolfbeis, O. S.; Offenbacher, H. The Effect of Alkali Cation Complexation on the Fluorescence Properties of Crown Ethers. Monatsh. Chem. 1984, 115, 647. (22) De Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugson, T.; Lynch, P. L. M. Molecular Photoionic Switches with an Internal Reference Channel for Fluorescent pH Sensing Applications. New J. Chem. 1996, 20, 871. (23) de Silva, A. P.; Gunnlaugsson, T.; McCoy, C. P. Photoionic Supramolecules: Mobilizing the Charge and Light Brigades. J. Chem. Educ. 1997, 74, 53. (24) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E.
Signaling Recognition Events with Florescent Sensors and Switches. Chem. Rev. 1997, 97, 1515 and references cited therein. (25) Hinschberger, J.; Desvergne, J.-P.; Bouas-Laurent, H.; Marsau, P. Synthesis and Photophysical Properties of Fluorescent Anthracenophanes Incorporating Two Polyoxadioxoalkane Chains. J. Chem. Soc., Perkin Trans 2 1990, 993. (26) Marquis, D.; Desvergne, J.-P.; Bouas-Laurent, H. Photoresponsive Supramolecular Systems; Synthesis and Photophysical Study of Bis(9,10-anthracenediyl)coronands AAOnOn. J. Org. Chem. 1995, 60, 7984. (27) Fery-Forgues, S.; Le Bris, M.-T.; Guette`, J.-P.; Valeur, B. Ion-Responsive Fluorescent Compounds. 2. Effect of Cation Binding on Photophysical Properties of a Benzoxazin Derivative Linked to Monoaza-15-crown-5. J. Phys. Chem. 1988, 92, 6233. (28) Bourson, J.; Valeur, B. Ion-Responsive Fluorescent Compounds. 2. Cation Steered Intramolecular Charge Transfer in a Crowned Merocyanine. J. Phys. Chem. 1989, 93, 3871. (29) Bourson, J.; Pouget, J.; Valeur, B. Ion-Responsive Fluorescent Compounds. 4. Effect of Cation Binding on the Photophysical Properties of Coumarin Linked to Monoaza- and DiazaCrown Compounds. J. Phys. Chem. 1993, 97, 4552. (30) Crossley, R.; Goolamali, Z.; Gosper, J. J.; Sammes, P. G. Synthesis and Spectral Properties of New Fluorescent Probes for Potassium. J. Chem. Soc., Perkin Trans. 2 1994, 513. (31) Crossley, R.; Goolamali, Z.; Sammes, P. G. Synthesis and Properties of a Potential Extracellular Fluorescent Probe for Potassium. J. Chem. Soc., Perkin Trans. 2 1994, 1615. (32) Alonso, M. T.; Brunet, E.; Hernandez, C.; Rodriguez-Ubis, J. C. Synthesis and Complexation Properties of 3-Aroylcoumarin Crown Ethers. A New Class of Photoactive Macrocycles. Tetrahedron Lett. 1993, 34, 7465. (33) Masilamani, D.; Golchini, K.; Lucas, M. E.; Hammond, G. S. PCT Int. Appl. WO 89 00, 997; Chem. Abstr. 1990, 112, 51578. (34) Nishizawa, S.; Watanabe, M.; Uchida, T.; Teramae, N. Fluorescent Ratio Sensing of Alkali Metal Ions Based on the Intermolecular Exciplex Formation. J. Chem. Soc., Perkin Trans. 2 1999, 141. (35) Jin, T.; Ichikawa, K.; Koyama, T. A fluorescent Calix[4]arene as an Intermolecular Excimer-forming Na+ Sensor in Nonaqueous Solution. J. Chem. Soc., Chem. Commun. 1992, 499. (36) Minta, A.; Tsien, R. Y. Fluorescent Indicators for Cytosolic Sodium. J. Biol. Chem. 1989, 264, 19449. (37) Mantulin, W. M.; Song, P.-S. Excited States of Skin Sensitizing Coumarins and Psoralens Spectroscopic Studies. J. Am. Chem. Soc. 1973, 95, 5122. (38) Cox, B. G.; Hurwood, T. V.; Prodi, L.; Montalti, M.; Bolleta, F.; Watt, C. I. F. Synthesis, Characterisation and Metal Ion Binding Properties of Crown Ether Incorporating 4,5-dioxyxanthones. J. Chem. Soc., Perkin Trans. 2 1999, 289. (39) Lo¨hr, H.-G.; Vo¨gtle, F. Chromo- and Fluoroionophores, A New Class of Dye Reagents. Acc. Chem. Res. 1985, 18, 65. (40) Go¨c¸ men, A.; Bulut, M.; Erk, C ¸ . Synthesis and Characterisation of Coumarin Crown Ethers. Pure Appl. Chem. 1993, 65, 447. (41) Akyu¨z, S.; Go¨c¸ men, A.; Bulut, M.; Erk, C¸ . Fluorescent Spectra of Coumarin Salt Solutions of Coumarin-Crown Ethers. Spectrosc. Lett. 1995, 28, 603. (42) Karsli, N.; Erk, C¸ . Bis-Coumarin Podands and their Complexes with Na+, K+, and Pb2+ Cations. Dyes Pigm. 1996, 32, 85. (43) Go¨c¸ men, A.; Erk, C¸ . Cation Binding of Benzo Crown Ethers in Acetonitril Using Fluorescence Spectroscopy. J. Inclusion Phenom. 1996, 26, 67. (44) Erk, C¸ .; Go¨c¸ men, A.; Bulut, M. The Synthesis of Novel Crown Ethers. IV. Coumarin derivatives of [18]crown-6 and Cation Binding from Fluorescence Spectra. J. Inclusion Phenom. 1998, 31, 319. (45) Erk, C¸ .; Go¨c¸ men, A.; Bulut, M. The Synthesis of Novel Macrocycles. V. The Coumarin Crown Ethers and Cation Binding with Fluorescence Spectra. Supramol. Chem. 1999, 11, 49. (46) Erk, C¸ .; Go¨c¸ men, A.; Bulut, M. The Synthesis of Novel Crown Ethers. VII. Coumarin Derivatives of Benzocrowns and Cation Binding from Fluorescence Spectra. J. Inclusion Phenom. 2000, 37, 441. (47) Erk, C¸ .; Erbay, E. Novel Macrocycles. VI. Synthesis, Structures and Cation Binding from Optical Spectroscopy of 9,10-Anthraquinone-crown Ethers. J. Inclusion Phenom. 2000, 30, 229.
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(48) Go¨c¸ men, A.; Erk, C¸ . Cation Complexing of Crown Ethers using Fluorescence Spectroscopy. II. Talanta 2000, in press. (49) Tuncer, H.; Erk, C¸ . Synthesis and Fluorescence Spectroscopy of bis(ortho- and para- carbonyl)phenyl Glycols. Dyes Pigm. 2000, 44, 81. (50) Tuncer, H.; Erk, C¸ . The New Macrocycle Synthesis. VIII. The Synthesis and Cation Recognition of Dibenzo[3k+2]crown-k with Fluorescence Spectroscopy. Supramol. Chem. 2000, submitted for publication. (51) Yapar, G.; Erk, C¸ . The Naphthalene-2,3-crowns and Metal Complexing with Fluorescence Spectroscopy in Actonitrile. Part III. Dye Pigm. 2000, accepted for publication.
(52) Bulut, M.; Erk, C¸ . Technical University of I˙ stanbul, unpublished results. (53) Yapıcı, M.; Erk, C¸ . Technical University of I˙ stanbul, unpublished results. (54) Yapar, G.; Erk, C¸ . Technical University of I˙ stanbul, unpublished results.
Received for review January 19, 2000 Revised manuscript received July 11, 2000 Accepted July 19, 2000 IE000059E
