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Optical Control of Divalent Metal Ion Binding to a Photochromic Catechol: Photoreversal of Tightly Bound Zn

2+

Mark T. Stauffer, and Stephen G. Weber Anal. Chem., 1999, 71 (6), 1146-1151 • DOI: 10.1021/ac980582r Downloaded from http://pubs.acs.org on January 19, 2009

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Anal. Chem. 1999, 71, 1146-1151

Optical Control of Divalent Metal Ion Binding to a Photochromic Catechol: Photoreversal of Tightly Bound Zn2+ Mark T. Stauffer and Stephen G. Weber*

Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

Photochromic molecules with functional groups that bind metal ions strongly make possible the development of photoreversible sensors and preconcentrators for detection of metal ions at trace concentrations. While several photochromic metal chelators have been reported, none have been shown to bind strongly to divalent metal ions. A novel photochromic benzopyran 2, which is a photochromic catechol, binds Zn2+ strongly and reversibly. The thermal equilibrium between open and closed 2 is slow. Thus, binding of Zn2+ to 2 is slow in the dark. Under illumination with 306-416-nm light, Zn2+ binds rapidly to open 2. The stable complex dissociates upon illumination with visible light, showing that both formation and dissociation of the complex are controllable with light. A binding constant of 1.1 × 109 is estimated from a Job plot, which also indicates formation of a 1:1 Zn2+/open 2 complex. Photochromic molecules, e.g., spiropyrans, spirooxazines, and benzopyrans (chromenes) (Scheme 1),1 that possess metal ioncoordinating functional groups are potentially interesting ligands for remote metal ion sensors.2 Photochromic molecules undergo structural changes that are induced by light. Thus, with such ligands, light can control the binding and dissociation of target metal ions. Reports of investigations into the use of photochromes as metal ion-complexing agents have focused on the effect of the metal ion on the photochromism of the ligands, mainly spiropyrans and spirooxazines.3 Preigh et al.4 first demonstrated the ability to detect Zn2+ at submicromolar concentrations using a photochromic spiroquinoxazine. Zn2+ binds to the photoisomerized spiroquinoxazine, yielding a fluorescent signal that is linear with [Zn2+]. More recently, Winkler et al. demonstrated that a photochromic, nitro-substituted 8-hydroxyquinoline was also capable of generat* Corresponding author: (phone) 412/624-8520; (fax) 412/624-8511; (fax) [email protected]. (1) (a) Guglielmetti, R. 4n+2 Systems: Spiropyrans. In Photochromism: Molecules and Systems; Du ¨ rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; pp 314-455. (b) Chu, N. Y. C. 4n+2 Systems: Spirooxazines. Ibid., pp 493-508. (c) Bertelson, R. C. Photochromic Processes Involving Heterolytic Cleavage. In Techniques of Chemistry, Volume III: Photochromism; Brown, G. H., Ed.; Wiley-Interscience: New York, 1971. (d) Livingston, R. Behavior of Photochromic Systems. Ibid. (2) (a) Chapman, T. P.; Johnson, K. S., Coale; K. H. Anal. Chim. Acta 1991, 249, 469. (b) Sakamoto-Arnold, C. M.; Johnson, K. S. Anal. Chem. 1987, 59, 1789. (c) Ward, N. I. Trace Elements. In Environmental Analytical Chemistry; Fifield, F. W., Haines, P. J., Eds.; Blackie Academic and Professional: Glasgow, 1995; pp 337-338, 341-342, 381.

1146 Analytical Chemistry, Vol. 71, No. 6, March 15, 1999

Scheme 1

ing a Zn2+-dependent metal ion signal.5 The first published account of metal ion binding to a chromene was by Stauffer et al.,6 in which the authors demonstrated photoreversible binding of Pb2+ to a naphthochromene containing a benzo-15-crown-5 moiety. The authors found that Pb2+ binds to the crown in the dark, with dissociation of the Pb2+-crown complex upon irradiation with near-ultraviolet light and recomplexation of the Pb2+ following (3) (a) Inouye, M.; Akamatsu, K.; Nakazumi, H. J. Am. Chem. Soc. 1997, 119, 9160. (b) Atabekyan, L. S.; Lilikin, A. I.; Zakharova, G. V.; Chibisov, A. K. High Energy Chem. 1996, 30, 409. (c) Inouye, M. Coord. Chem. Rev. 1996, 148, 265. (d) Kobayashi, N.; Sato, S.; Takazawa, K.; Ikeda, K.; Hirohashi, R. Electrochim. Acta 1995, 40, 2309. (e) Zhou, J.; Zhao, F.; Li, Y.; Zhang, F.; Song, X. J. Photochem. Photobiol. A: Chem. 1995, 92, 193. (f) Zhou, J.; Li, Y.; Song, X. J. Photochem. Photobiol. A: Chem. 1995, 87, 37. (g) Willner, I.; Willner, B. Chemistry of Photobiological Switches. In Biological Applications of Photochemical Switches; Morrison, H., Ed.; John Wiley and Sons: New York, 1993; pp 1-110. (h) Winkler, J. D.; Deshayes, K.; Shao, B. Photochemical Binding, Release, and Transport of Metal Ions. Ibid., pp 171-196. (i) Kimura, K.; Yamashita, T.; Yokoyama, M. J. Phys. Chem. 1992, 96, 5614. (j) Kimura, K.; Yamashita, T.; Kaneshiga, M.; Yokoyama, M. J. Chem. Soc., Chem. Commun. 1992, 969. (k) Atabekyan, L. S.; Chibisov, A. K. J. Photochem. 1986, 34, 323. (4) Preigh, M. J.; Lin, F.-T.; Ismail, K. Z.; Weber, S. G. J. Chem. Soc., Chem. Commun. 1995, 2091. (5) Winkler, Jeffrey D.; Bowen, Corrine M.; Michelet, Veronique J. Am. Chem. Soc. 1998, 120, 3237. (6) Stauffer, M. T.; Knowles, D. B.; Brennan, C.; Funderburk, L.; Lin, F.-T.; Weber, S. G. Chem. Commun. 1997, 287. 10.1021/ac980582r CCC: $18.00

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illumination by visible light. The authors attributed the effect of light on coordination to withdrawal of electron density from the crown ether moiety upon photoisomerization of the naphthochromene. Photochromic chelators may find application to analytical chemistry by their ability to collect and release metal ions in a reagentless manner. Thus, one can envision sensors or preconcentrators. A critical need, if photochromic ligands are ever to be useful for trace metal ion sensors, is for the photochromic ligand to have a large binding constant with the metal ion of interest. Several investigators3,7 have developed ligands based on placement of a metal ion-coordinating functional group, X, ortho to the pyran/ oxazine oxygen (1/1′) to convert otherwise poorly coordinating

photochromic spiropyrans/spirooxazines (analogous to 1) into merocyanine chelators (1′). One of the more frequently used functional groups used in this capacity is the methoxy group.3f,k,7 Other metal ion-coordinating functional groups used include X ) piperidino,8 CO2Me,3e SO3,3c,7 and azacrown ethers.3i,j,9 Generally, the bidentate chelators listed are weak and, though interesting from a fundamental point of view, are not useful analytically. This work focuses on the development of a strong, divalent metal-chelating molecule and demonstrating the strength of binding from equilibrium measurements. Furthermore, if metal is tightly bound, is it possible to liberate it with light? The approach is to use hydroxynaphthochromene 2, a photochromic catechol.

To our knowledge, this work is the first to describe a photochromic catechol. (7) (a) Oda, H. Dyes Pigm. 1993, 23, 1. (b) Tamaki, T.; Kawanishi, Y.; Seki, T.; Sakuragi, M.; Ichimura, K.; Yamaguchi, T. J. Photopolym. Sci. Technol. 1990, 3, 85. (c) Tamaki, T.; Ichimura, K. J. Chem. Soc., Chem. Commun. 1989, 1477. (d) LeBaccon, M.; Guglielmetti, R. N. J. Chem. 1988, 12, 825. (e) Taylor, L. D.; Nicholson, J. R.; Davis, B. Tetrahedron Lett. 1967, 17, 1585. (8) Winkler, J. D.; Deshayes, K.; Shao, B. J. Am. Chem. Soc. 1989, 111, 769. (9) Kimura, K.; Yamashita, T.; Yokoyama, M. J. Chem. Soc., Perkin Trans. 2 1992, 613.

EXPERIMENTAL SECTION Chemicals. (3-(2-Fluorophenyl)-3-(4-methoxy-3-methylphenyl)5-hydroxynaphtho[2,1-b]pyran) (2) was obtained by base-catalyzed hydrolysis of 3 in 10% (w/v) aqueous ethanol at ambient temperature for 20 min and was purified (98-99%) by RPLC (20% aqueous MeCN mobile phase). (3-(2-Fluorophenyl)-3-(4-methoxy3-methylphenyl)-5-acetoxynaphtho[2,1-b]pyran) (3) was donated by PPG Industries, Inc. Sodium perchlorate (GFS Chemicals) was recrystallized from methanol.10 Zinc (GFS Chemicals), cadmium (GFS), nickel (GFS), cobalt (Johnson Matthey Electronics), ferrous (ICN Pharmaceuticals) and tetraethylammonium (TEAP, GFS) perchlorates, ethanol (EtOH, CAS No. 64-17-5, Pharmco Products), acetonitrile (MeCN, Mallinckrodt AR grade), 2,2,2trifluoroethanol (TFE, Aldrich, 99.5+%, NMR grade), 1,1,1,3,3,3hexafluoro-2-propanol (HFP, Aldrich, 99.8+%), tetramethylammonium hydroxide (TMAOH, 20 wt % in water, Aldrich), and 2,2,6,6tetramethylpiperidine (TMP, 99+%, Aldrich), were used as received. Apparatus and Instruments. UV-visible measurements were made with a Hewlett-Packard HP8452A diode array spectrophotometer in general scanning mode. Spectra were measured in 1-mm (Quaracell and Starna) and 1-cm (Fisher Scientific) Teflon-stoppered quartz cuvettes. A 150-W Xe lamp with power supply (Varian LX150UV) served as the photolysis source. Glass filters (Turner, model 7-60, 306-416-nm bandpass (λmax ) 365 nm), and model 2A, g420 nm transmission) were used for selective transmission of near-ultraviolet and visible light, respectively. Spectroelectrochemical studies of 2 were carried out with a PAR model 173 potentiostat coupled to a digital waveform generator (Stanford Research) and an Allen X-Y recorder for cyclic voltammetry, the HP8452A diode array spectrophotometer operated in general scanning mode, and a three-electrode cell (reticulated vitreous carbon (100 pores/in.) working electrode, Ag wire pseudoreference, stainless steel syringe counter electrode/ sample entry port) in a 1-mm quartz cuvette. Solution Preparation. Stock solutions of 2 (10-4-10-3 M), Zn(ClO4)2 (10-2 M), and TMP (10-2 M) were prepared in EtOH containing 0.1 M NaClO4 to maintain the ionic strength approximately constant. Solutions of 10-4-10-3 M Zn2+ in ethanolic 0.1 M NaClO4 were prepared by dilution of the 10-2 M stock Zn2+ solution with the solvent. Solutions of 2 (10-3 M) were prepared for spectroelectrochemistry in MeCN containing 0.1 M TEAP as supporting electrolyte. TMAOH (0.01 M) was prepared in EtOH and stored under argon. Solutions of the other metal perchlorates listed above were prepared in the 10-2 M range in EtOH. Solutions of 2 were prepared in EtOH due to its insolubility in H2O. UV and Visible Photolysis of 2 and Its Complex with Zn2+. Solutions of 2, and its complex with Zn2+, were pipetted into 1-mm quartz cuvettes. The cuvettes were placed in the spectrophotometer cell compartment at 45° to the photolysis and spectrophotometer source beams, which were orthogonal to each other. The solutions were irradiated with the appropriate wavelength region of light for selected times. The Xe lamp was operated about 9 and 11 A (half- and full-iris aperture) for photolyses of 2 and its complex with Zn2+, respectively. (10) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals, 3rd ed.; Pergamon Press: New York, 1988; p 360.

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Spectroelectrochemistry of 2 in Acetonitrile in the Dark.13 Solutions of 2 in MeCN/0.1 M TEAP were added to the 1-mm quartz cuvette spectroelectrochemical cell. The cell was placed in the spectrophotometer cell compartment perpendicular to the spectrophotometer source beam and then connected to the potentiostat. A triangular waveform (frequency 5 × 10-3 Hz, phase angle 0°, amplitude 1.10 V) was programmed into the digital waveform generator for cyclic voltammetry at a sweep rate of 11 mV s-1. UV-visible spectra (190-820 nm) were recorded at selected potentials during the cyclic voltammetry. Formation of the 2/Zn2+ Complex in the Dark. The effect of 1.67 × 10-4 M HClO4, 1.67 × 10-4 M TMP, 1.67 × 10-4 M TMAOH, and a 1.67 × 10-4 M TMP/1.0 × 10-4 M HClO4 buffer, added to solutions containing 9.0 × 10-5 M 2 in EtOH/0.1 M NaClO4, on binding of Zn2+ to 2 was examined. [Zn2+] was kept constant, at 8.24 × 10-4 M, in all solutions. Formation of the complex of 2 with Zn2+ was followed spectrophotometrically, at 496 nm (vs 0.1 M NaClO4 in EtOH) in the dark, in the absence and presence of added HClO4, HClO4/TMP buffer, TMP, and TMAOH. The formation of the complex was monitored over 1215 h to allow the reaction to reach equilibrium. Determination of the Stoichiometry of the 2/Zn2+ Complex in the Dark. The Job method11 was used to obtain the stoichiometry of the Zn2+/2 complex. Solutions ([Zn2+] + [2] ) 2 × 10-4 M) containing varying mole ratios of Zn2+ and 2, all containing amounts of TMP ranging from 0 to 4 molar equiv TMP per molar equiv 2, were prepared and allowed to equilibrate in the dark overnight. The UV-visible spectrum of each solution was measured from 190 to 820 nm, with absorbance values from 496 nm (λmax of the complex) through 510 nm employed for the Job plots. Calculation of Molar Absorptivities of Closed and Open 2, and the Equilibrium Constant (Ktherm) for the Thermal Equilibrium between Closed and Open 2. Calculation of Molar Absorptivities of Closed and Open 2. For a series of solutions of 2 in the concentration range (0-2.59) × 10-4 M, UV-visible spectra (200-800 nm) were measured (1-mm quartz vs EtOH/0.1 M NaClO4) in the dark13 and after irradiation with 306-406-nm light for 60 s. Baseline-corrected absorbances were measured for both series of spectra at each [2], at 238 (corresponding to closed 2) and 462 nm (corresponding to open 2 which will be called 2′). A238 decreased during near-UV irradiation, with a corresponding increase in A462. The fraction of 2 converted to 2′ by UV photolysis was estimated from 1 - (A238,UV/A238,dark), assuming that 2 is the (11) (a) Beck, M. T.; Nagypa´l, I. Chemistry of Complex Equilibria; Halsted Press: New York, 1990; pp 112-118. (b) Ho¨gfeldt, E. Graphic and Computational Methods in the Evaluation of Stability Constants. In Treatise on Analytical Chemistry, Part I: Theory and Practice, 2nd ed.; Kolthoff, I. M., Elving, P. J., Eds.; John Wiley and Sons: New York, 1979; Vol. 2, pp 100-103. (c) McBryde, W. A. E. Talanta 1974, 21, 979. (d) Likussar, W. D.; Boltz, F. Anal. Chem. 1971, 43, 1265. (e) Woldbye, F. Acta Chem. Scand. 1955, 9, 299. (f) W. C. Vosburgh, G. R. Cooper, J. Am. Chem. Soc. 1941, 63, 437. (12) (a) Flannery, J. B., Jr. J. Am. Chem. Soc. 1968, 90, 5660. (b) Kosower, E. M. J. Am. Chem. Soc. 1958, 80, 3253. (c) Brownstein, S. Can. J. Chem. 1960, 38, 1590. (13) Obviously spectra cannot be taken in the dark. However, for simplicity, we refer to spectroscopic observations of dark-adapted solutions as having been taken in the dark. To ensure against the possibility that spectroscopic observation causes photochemical changes, spectra are recorded repetitively at various cycle times. Solutions susceptible to this error show changing spectra for short cycle times. For such solutions, longer cycle times are used.


Figure 1. (a) Irradiation of 2 × 10-4 M 2 in EtOH, containing 0.1 M NaClO4 background electrolyte, with 306-416-nm light, followed by thermal relaxation of photoisomerized 2 to its closed form. (b) Same conditions for UV photolysis as in (a), except photoisomerized 2 returns to its closed form by irradiation with g420-nm light. Measurements made in 1-mm quartz cuvette vs solvent.

predominant form in the dark and that 2 is converted exclusively to 2′ during UV photolysis. The concentration of 2′ produced by UV photolysis was calculated from the initial concentration of 2 in the dark and the fraction of 2 converted to 2′ by UV photolysis. The molar absorptivities of 2 and 2′ (238 and 462, respectively) were obtained from the slopes of the linear regressions of A238,dark vs [2] and A462,UV vs [2′], respectively. Calculation of the Thermal Equilibrium Constant, Ktherm. For the series of solutions of 2 described above, baseline-corrected absorbances were measured at 238 and 462 nm, using the UVvisible instrument. Beer’s law (i.e., 238 and 462) was used to calculate [2] and [2′] in the dark. Finally, the thermal equilibrium constant Ktherm ) [2′]/[2] was calculated for the thermal equilibrium between closed and open 2. Data Analysis. Plots were generated, and linear regressions performed, with Microsoft Excel (Windows 95, version 7.0). Factor analysis and nonlinear regressions were performed with STATA (Windows 95, version 5). RESULTS AND DISCUSSION Ethanolic solutions of the chromene 2 change from pale yellow to deep orange (λmax 462 nm) when irradiated with 306-416-nm light (Figure 1a). The colored, ring-open, merocyanine isomer will be called 2′. Accompanying the change in the visible region of the UV-visible spectrum of 2 is a shift in λmax from 238 to 230

Figure 3. Photodissociation (g420-nm light) of the complex resulting from addition of 5.4 × 10-4 M Zn2+ to 5.4 × 10-4 M 2, both in EtOH containing 0.1 M NaClO4 as background electrolyte. The spectra correspond to the following irradiation times 0 (maximum absorbance near 500 nm), 5, 10, and 15 min (minimum absorbance near 500 nm). Measured in 1-mm quartz vs solvent.

Figure 2. UV “on”/visible “off” photoswitching of 1.6 × 10-4 M 2 (a, O; b, s) and 2.2 × 10-4 M 3 (+) in EtOH (0.1 M NaClO4), in (a) the absence and (b) presence of 1 mol equiv of TMP. λmax(open 2) ) 462 nm; λmax(open 3) ) 480 nm. Measurements made in 1-mm quartz cuvettes vs the solvent.

nm. The 462-nm peak disappears over a several-minute period when the solution stands in the dark. Irradiation with g420-nm light accelerates the relaxation process (Figure 1b). (Concurrently, the 230-nm peak increases in absorbance and shifts back to 238 nm.) Factor analysis of the thermal and photorelaxation data yields, in each case, two distinct factors corresponding to the UVvisible spectra of 2 and 2′, respectively. The factor analysis rules out a dominating influence of products of photodestruction on the spectra. The control compound, 3 (3′ is a much weaker ligand than 2′), photoisomerizes in EtOH upon illumination with 306-416nm light (λmax 484 nm) and undergoes thermal and photoinduced ring closure also. 2 returns more slowly than 3 (Figure 2a) to the original closed isomer. The thermal return is accelerated by similar amountssa factor of 2-3sfor 2′ and 3′ in the presence of 1 mol equiv of base (TMP). (The thermal relaxation is biexponential, so more quantitative comparisons are not really useful in the absence of mechanistic information.)1,12 There is a small absorbance at 462 nm in the UV-visible spectra of EtOH solutions of 2 in the dark.13 This λmax corresponds to that of 2′ during UV photolysis. Thus, we attribute this peak to 2′ in thermal equilibrium with closed 2. The molar absorptivity of 2 is 1.13 × 104 M-1 cm-1 (r2 ) 0.9998) at 238 nm (λmax) and that of 2′ is 8.97 × 103 M-1 cm-1 (r2 ) 0.9995) at 462 nm. At room temperature, in EtOH, the concentrations of 2 and 2′ lead to a value of Ktherm([2′]/[2]) ) 1.5 × 10-2 (rsd ) 10.6%). For the

thermal equilibrium between the 3′ and 3, Ktherm is 3 × 10-3. This is based on absorbance measurements at 244 (3) and 484 nm (3′). Spectroelectrochemistry of 2 in the Dark. Cyclic voltammetry of a 1 × 10-3 M solution of 2 in MeCN (0.1 M TEAP supporting electrolyte) in the dark yields an ill-defined anodic wave ∼+1.7 V (vs Ag) and two overlapping cathodic peaks about -0.2 and -0.35 V (both vs Ag). Both the anodic and cathodic waves are irreversible. UV-visible spectroscopic monitoring of the cyclic voltammetry gives no evidence of visible absorptions corresponding to those observed during UV photolysis or from addition of Zn2+. We do not expect redox activity from Zn2+, but photoxodiation may occur from ambient oxygen. There may be reactive metal ion impurities that catalyze photooxidation. Fortunately, none of this seems to be occurring in the current instance. Binding of Zn2+ To Open 2 in EtOH in the Dark. Photodissociation of the Complex with Visible Light. Addition of Zn2+ to EtOH solutions of 2 produces a red color (λmax 496 nm) that is distinct from the orange color (λmax 462 nm) typical of 2′. In the dark the solutions require at least 4 h to reach equilibrium, as measured by the absorbance at 496 nm. The slow growth of A496 is due to the slow thermal rate of conversion from 2 to 2′. Irradiation of equilibrated solutions of 2 containing Zn2+ with g420-nm light causes a transient decrease in A496 (Figure 3), with an increase in A496 when the solution is again allowed to stand in the dark. The decrease in A496 results from dissociation of the complex accompanied by the conversion of 2′ f 2. Factor analysis of the UV-visible spectra obtained from the photodissociation experiment yields two factors corresponding to the spectra of closed 2 and the complex. This again rules out photodestruction as being a dominant factor in the spectra. It also supports the contention that there is one metal/ligand complex formed under these conditions. The ring-open form (e.g., 2′) of this sort of compound can exist as several isomers which are expected to have different spectra. Zn2+ can also bind to open 3 in the dark. Addition of excess 2+ Zn to EtOH solutions of 3 during irradiation with 306-416-nm Analytical Chemistry, Vol. 71, No. 6, March 15, 1999

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Table 1. Visible Absorption Maximums Obtained upon Addition of Metal Ions to Millimolar Solutions of 2 in Ethanol metal ion Fe2+ Cu2+ Cd2+

λmax (nm)

metal ion

λmax (nm)

556 524 f 554 484

Co2+

492 404; 484

Ni2+

light produces a small absorption ∼490 nm during thermal relaxation. Effect of Other Metal Ions on 2. Complexes of 2′ are formed by other divalent and trivalent metal ions. Cd2+, Co2+, and Ni2+ yielded results similar to those obtained with Zn2+, producing complexes in the dark with visible λmax values close to that for Zn2+ (Table 1). Cu2+ produced an initially lavender solution (λmax 524 nm) when added to 2 in EtOH; upon standing for at least 1 day, the solution became violet (λmax 554 nm). Such color changes in solutions of metal ion complexes usually represent changes in the number and types of ligands coordinated to the metal ion.14-16,18 Addition of Fe2+ to 2 in EtOH yielded an initially yelloworange solution, which became light violet and finally produced a significant peak at 556 nm (deep violet solution) with time. These results suggest initial air oxidation of Fe2+ to Fe3+, followed by complexation of Fe3+ with 2. The violet color is similar to that observed for coordination of Fe3+ by gallic acid15 catechol,15 pyrogallol,15 protocatechuic acid,15 and Tiron16 in aqueous solutions. Fe2+ is reported to give a wine-red color when complexed with gallic acid in aqueous media;15 the wine-red color is different from that produced by coordination of Fe3+ to gallic acid at the same pH.15 2/2′ are related to phenols and are therefore oxidizable.17 However, spectroelectrochemistry shows no visible absorption bands produced during cyclic voltammetry. Thus, while the oxidation of 2/2′ cannot be ruled out, it is not responsible for the color changes that are observed. Effect of Acid and Base on Formation of the Complex of 2 with Zn2+ in the Dark. 2′ contains a phenolic hydroxyl group with a dissociable proton. Therefore, the progress of the reaction of metal ion with 2/2′ was followed in solutions of different composition. The absorbance (496 nm) vs time curves for solutions containing 9.0 × 10-5 M 2 and 8.24 × 10-4 M Zn2+ in (14) (a) Jameson, R. F.; Wilson, M. F. J. Chem. Soc., Dalton Trans. 1972, 2617. (b) Lee, J. D. Concise Inorganic Chemistry, 4th ed.; Chapman and Hall: New York, 1991; pp 938-971. (c) Nemcova´, I.; Cerma´kova´, L.; Gasparic, J. Spectrophotometric Reactions; Marcel Dekker: New York, 1996; pp 57-134. (15) Powell, H. K. J.; Taylor, M. C. Aust. J. Chem. 1982, 35, 739. (16) W. A. E. McBryde, Can. J. Chem. 1964, 42, 1917. (17) Hydroxychromenes are phenols and are thus susceptible to oxidation by Fe3+. The following references discuss phenol oxidation by Fe3+ via coordination of the metal ion: (a) Mentasti, E.; Pelizzetti, E. J. Chem. Soc., Dalton Trans. 1973, 2605, 2609. (b) Green, S.; Mazur, A.; Shorr, E. J. Biol. Chem. 1956, 220, 237. (c) Baxendale, J. H.; Hardy, H. R. Trans. Faraday Soc. 1954, 50, 808. (d) Baxendale, J. H.; Hardy, H. R.; Sutcliffe, L. H. Trans. Faraday Soc. 1951, 47, 963. (e) Wesp, E. F.; Brode, W. R. J. Am. Chem. Soc. 1934, 56, 1037. (18) (a) Satyanarayana, S.; Venugopal Reddy, K. Indian J. Chem. 1989, 28A, 630. (b) Gergely, A.; Kiss, T.; Dea´k, Gy. Inorg. Chim. Acta 1979, 36, 113. (c) Avdeef, A.; Sofen, S. R.; Bregante, T. L.; Raymond, K. N. J. Am. Chem. Soc. 1978, 100, 5362. (d) Tyson, C. A.; Martell, A. E. J. Am. Chem. Soc. 1968, 90, 3379. (e) Athavale, V. T.; Prabhu, L. H.; Vartak, D. G. J. Inorg. Nucl. Chem. 1966, 28, 1237. (f) Muramaki, Y.; Nakamura, K.; Tokunaga, M. Bull. Chem. Soc. Jpn. 1963, 36, 669.


Figure 4. Effect of addition of acid and base on formation of the complex of 2 with Zn2+ (9.0 × 10-5 M 2, 8.24 × 10-4 M Zn2+) in the dark, in EtOH containing 0.1 M NaClO4 background electrolyte. The acid used was HClO4; the bases, TMAOH and TMP. Measurements made in 1-mm quartz cuvettes vs the solvent: (A) 1.67 × 10-4 M HClO4 added; (B) neither HClO4 or TMP added; (C) 5:3 (1.67 × 10-4 M:1.0 × 10-4 M) TMP/HClO4 nonaqueous buffer added; (D) 1.67 × 10-4 M TMAOH; (E) 1.67 × 10-4 M TMP.

Figure 5. Job plots of 2.0 × 10-4 M 2 and 2.0 × 10-4 M Zn2+ in the dark in EtOH with 0.1 M NaClO4, in the absence and presence of 1 mol equiv of TMP. Measured in 1 mm quartz vs solvent. (]) 0 and (9) 1 mol equiv of TMP.

EtOH/0.1 M NaClO4, shown in Figure 4, illustrate the effect of acid (HClO4), base (TMP and TMAOH), and a nonaqueous buffer (5:3 TMP/HClO4) on binding of Zn2+ to 2′ in the dark. No complex forms in the presence of 1.67 × 10-4 M HClO4. The greatest enhancement of complex formation occurs in the presence of 1.67 × 10-4 M TMP or TMAOH. In this regard, both bases behave identically. The basic TMP/HClO4 buffer enhances formation of the complex over the case in which neither acid nor base is present in solution. Thus, the use of base to control the activity of protons dissociated from open 2 by coordinated Zn2+ is important to formation of the Zn2+/open 2 complex in EtOH, analogous to the use of pH to control metal ion complexation in aqueous solutions.18 Stoichiometry and Binding Constant. Figure 5 illustrates the Job plots for the determination of the stoichiometry of the complex of open 2 with Zn2+, in the absence and presence of 1 mol equiv of TMP. In the absence of TMP, the Job curve is very broad. With 1 mol equiv of TMP present, the Job curve takes on the shape characteristic of 1:1 metal ion/ligand stoichiometry, with a clearly defined maximum at X ) 0.5.

The binding constant, K1, for the 1:1 complex of 2′ with Zn2+ is 1.1 × 109 (log K1 ) 9.04) as determined from the Job plot shown in Figure 5. The molar absorptivity, 496, is 1.45 × 104 M-1 cm-1 for the 1:1 complex of Zn2+ with 2′ from linear regression of the linear portions of Figure 5. The value of K1 compares favorably with literature values (log Kf ) 8.2-10.2)19 of K1 for 1:1 chelates of Zn2+ with various substituted catechols in aqueous media. For comparison of the binding strength of the Zn2+ complex with 2 vs that with 3, we attempted to obtain the stoichiometry and formation constant of the complex of open 3 with Zn2+ via the Job method. Our attempts to generate reliable Job plots for the Zn2+/3 proved unsuccessful. However, we estimated K1 to be 6 × 102 from the UV-visible spectra of thermally relaxed 3 in the presence of excess Zn2+ added during UV photolysis, and the estimated Keq for the thermal equilibrium between closed and open 3 given above. Comparison of the estimated K1 values for the Zn2+ complexes of 3 and 2, respectively, shows that 3 binds Zn2+ weakly and 2 coordinates Zn2+ much more strongly. The photochemistry and photophysics of the photoinduced ring-closing reaction are not understood, and to our knowledge there are no theoretical discussions on the ring-closing reaction from a metal-bound photochromic (Scheme 2). Intuitively, however, one would expect the efficiency of the ring-closing reaction to be lower the more tightly bound it is to metal ion. In practical terms, the success of photoreversal will depend on the binding constant. There are not enough data here to test this quantitatively. However, knowledge of Keq can be used fruitfully. The amount of energy per mole put into Zn2+/2′ is something over 50 kcal/mol (492 nm). As the reaction mechanism is unknown, it is not obvious how much of that energy is available. In more well-studied systems, such as azobenzene/stilbenes,20 geometrical changes (trans f cis) occur as molecules relax from a vibrationally excited state to the ground vibrational level of the first excited electronic state. Unfortunately, we have no information on the amount of (19) Martell, A. E.; Smith, R. M.; Motekaitis, R. J. NIST Critical Stability Constants of Metal Complexes Database; U.S. Department of Commerce: Washington, DC, 1993. (20) Mukherjee, S.; Bera, S. C. J. Photochem. Photobiol. A: Chemistry 1998, 113, 23-26.

Scheme 2

energy available in photoexcited 2′. However, we4 have measured fluorescence from one metal-bound compound, a spirooxazine. The difference in excitation and emission energies (Stokes shift) is ∼16 kcal/mol. The free energy of the Zn2+/2′ binding reaction is 12-13 kcal/mol. Clearly there is reason to be optimistic about the possibility of photoreversing even more tightly bound systems. A more detailed understanding of the excited state of these species would help in understanding the limitations of the ability to photoreverse metal ion binding. ACKNOWLEDGMENT We gratefully acknowledge the financial support of the Office of Naval Research. We also acknowledge Mr. David B. Knowles (PPG Industries, Inc.) for synthesis of the starting compound 3 and helpful discussions. Received for review May 27, 1998. Accepted December 17, 1998. AC980582R

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