Anal. Chem. 2005, 77, 1999-2006
Photoresponsive Liquid Membrane Transport of Alkali Metal Ions Using Crowned Spirobenzopyrans Hidefumi Sakamoto, Hideyuki Takagaki, Makoto Nakamura, and Keiichi Kimura*
Department of Applied Chemistry, Faculty of Systems Engineering, Wakayama University, 930 Sakae-dani, Wakayama, Wakayama 640-8510, Japan
Several azacrown ether derivatives, which are monoaza12-crown-4, -15-crown-5, and -18-crown-6 and diaza-12crown-4 and -18-crown-6, bearing one or two spirobenzopyran(s), which we call crowned spirobenzopyran or crowned bis(spirobenzopyran), were synthesized and were used as carriers for liquid membrane transport of alkali metal ions. The passive alkali metal transports through liquid membranes containing crowned spirobenzopyrans were carried out under dark, and UV- and visible-light irradiation conditions. The metal ion transport was accelerated and retarded by UV- and visible-light irradiation, respectively. On the other hand, the photoresponse of the metal ion selectivity in membrane transport by crowned spirobenzopyrans was different, depending on the kind of crown ether units. Especially, diaza-12-crown-4-bis(spirobenzopyran) exhibited an excellently selective and effective transporting ability for Li+. The uphill transports of Li+ through a liquid membrane containing monoaza12-crown-4-spirobenzopyran or diaza-12-crown-4-bis(spirobenzopyran) were realized under the conditions where the same aqueous solution was used as the source and receiving phases with UV and visible lights being irradiated onto the boundary phases between the source and membrane phases and between the receiving and membrane phases, respectively. The uphill transport of Li+ from the source to receiving phases through a liquid membrane containing a crowned spirobenzopyran was also attained by the proton-concentration gradient between the source and receiving phases under dark conditions, and the transporting ability was remarkably increased by photoirradiation. For the separation of a particular alkali metal ion, membrane transport using a crown ether derivative is a convenient, effective, and low-energy-consuming method. Thus, there are several papers published for the liquid and solid membranes containing crown ether derivatives, which have functional groups, such as proton dissociable, oxidative or reductive, and photoinduced isomerizable groups.1-9 The extractability of alkali metal ion from an aqueous * To whom correspondence should be addressed. E-mail: kkimura@ sys.wakayama-u.ac.jp. (1) Fyles, T. M. In Cation Binding by Maclocycles; Inoue, Y., Gokel, G. W., Eds.; Marcel-Dekker: New York, 1990; pp 203-310. (2) Shinkai, S. In Cation Binding by Maclocycles; Inoue, Y., Gokel, G. W., Eds.; Marcel-Dekker: New York, 1990; pp 397-428. 10.1021/ac048642i CCC: $30.25 Published on Web 02/19/2005
© 2005 American Chemical Society
solution into an organic solution containing a crown compound bearing a proton-dissociable group, which forms an anion by proton dissociation, can be controlled by pH in the aqueous solution. This phenomenon of crown compounds bearing proton dissociable group(s) is useful for the liquid membrane transport of alkali metal ion, because the transport can be accelerated by pH difference between the source and receiving phases. Therefore, an uphill transport of alkali metal ions can be attained by pH control.10-19 On the other hand, the metal ion complexing ability and selectivity of crown ether derivatives bearing a photochromic moiety such as an azobenzene group, which is isomerized from its trans to cis forms by UV-light irradiation and vice versa by visible-light irradiation, were controlled by photoirradiation. For example, a bis(crown ether) bearing an azobenzene moiety as the bridging unit of two crown ethers was used as a photoresponsive ionophore for alkali metal ions, and the metal ion transporting ability and selectivity of the bis(crown ether) derivative were controlled by photoirradiation.2,7 The transporting ability and selectivity, however, were not very easy to control by photoirradiation, because the complexing ability of the crown ether (3) Izatt, R. M.; LindH, G. C.; Bruening, R. L.; Huszthy, P.; McDaniel, C. W.; Bradshaw, J. S.; Cheristensen, J. J. Anal. Chem. 1988, 60. 1694-1699. (4) Matsuno, S.; Ohki, A.; Takagi, M.; Ueno, K. Chem. Lett. 1981, 1543-1546. (5) Ohki, A.; Takeda, T.; Takagi, M.; Ueno, K. Chem. Lett. 1982, 1529-1532. (6) Ozeki, E.; Kimura, S.; Imanishi, Y. J. Chem. Soc., Chem. Commun. 1988, 1353-1356. (7) Shinkai, S.; Manabe, O. In Host-Guest Complex Chemistry III; Vo ¨gtle, F., Weber, E., Eds.; Topics in Current Chemistry 121; Springer-Verlag: Tokyo, 1984; pp 67-104. (8) Jong, F. D.; Visser, H. C In. Supramolecular Technology; Atwood, J. L., Davies, J. E. D., Macnicol, D. D., Vo¨gtle, F., Eds.; Comprehensive Supramolecular Chemistry 10; Elsevier Science Ltd.: Oxford, 1996; pp 13-51. (9) Khairutdinov, R. F.; Hurst, J. K. Langmuir 2004, 20, 1781-1785. (10) Fyles, T. M.; Mcgavin C. A.; Thompson. D. E. J. Chem. Soc., Chem. Commun. 1982, 924-925. (11) Dulyea, L. M.; Fyles, T. M.; Whitfield, D. M. Can. J. Chem. 1984, 62, 498506. (12) Fyles, T. M.; Hansen, S. P. Can. J. Chem. 1988, 66, 1445-1453. (13) Matsushima, K.; Kobayashi, H.; Nakatsuji, Y.; Okahara, M. Chem. Lett. 1983, 701-704. (14) Casa, M. D.; Fabbrizzi, L.; Perotti, A.; Poggi, A.; Tundo, P. Inorg. Chem. 1984, 24, 1610-1611. Grimaldi, J. J.; Lehn, J.-M. J. Am. Chem. Soc. 1979, 101, 133-1334. (15) Sakamoto, H.; Kimura, K.; Shono, T. Eur. Polym. J. 1986, 22, 97-101. (16) Sakamoto, H.; Kimura, K.; Koseki, M.; Shono, T. J. Chem. Soc., Perkin Trans. 2 1987, 1181-1185. (17) Sakamoto, H.; Kimura, K.; Shono, T. Anal. Cham. 1987, 59, 1513-1517. (18) Ishikawa, J.; Sakamoto, H.; Otomo, M. Analyst 1997, 122, 1383-1386. (19) Shinkai, S.; Manabe, O. In Host-Guest Complex Chemistry III; Vo ¨gtle, F., Weber, E., Eds.; Topics in Current Chemistry 121; Springer-Verlag: Tokyo, 1984; pp 67-104.
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Chart 1. Crowned Spirobenzopyran Derivatives Used in This Study
derivative for a particular metal ion could not be changed dramatically by the photoinduced conformational change of their azobenzene. It is well known that spirobenzopyran derivatives are typical photochromic compounds, which are isomerized from their electrically neutral spiropyran form to the corresponding zwitterionic merocyanine form by UV-light irradiation and vice versa by visible-light irradiation.20-22 We have synthesized crown ether derivatives bearing a spirobenzopyran moiety 1-3 (Chart 1), which are called crowned spirobenzopyrans, and have studied their complexing ability for alkali metal ions under dark and UVand visible-light irradiation conditions.23-33 It was found in the liquid-liquid extraction and electrospray ionization mass spectroscopy that the complexing ability of crowned spirobenzopyran for a particular metal ion was increased and decreased remarkably by UV- and visible-light irradiation, respectively, as compared with that under dark conditions.34-36 It is because the phenolate ion of merocyanine, which is formed by complexation of the crown ether moiety with metal ion or UV-light irradiation, interacts strongly with a metal ion bound to the crown ether moiety to enhance the (20) Sunamoto, J.; Iwamoto, K.; Akutagawa, M.; Nagase, M.; Kondo, H. J. Am. Chem. Soc. 1982, 104, 4904-4907. (21) Sunamoto, J.; Iwamoto, K.; Mohri, Y.; Kominato, T. J. Am. Chem. Soc. 1982, 104, 5502-5504. (22) Winkler, J. D.; Deshayes, K.; Shao, B. J. Am. Chem. Soc. 1989, 111, 769770. (23) Kimura, K.; Sakamoto, H.; Nakamura, M. Bull. Chem. Soc. Jpn. 2003, 76, 225-245. (24) Kimura, K.; Yamashita, T.; Yokoyama, M. J. Chem. Soc., Chem. Commun. 1991, 147-148. (25) Kimura, K.; Yamashita, T.; Yokoyama, M. J. Chem. Soc., Perkin Trans.2 1992, 613-619. (26) Kimura, K.; Sumida, M.; Yokoyama, M. Chem. Commun. 1997, 1417-1418. (27) Tanaka, M.; Kamada, K.; Ando, H.; Shibutani, Y.; Kimura, K. J. Org. Chem. 2000, 65, 4342-4347. (28) Tanaka, M.; Nakamura, M.; Salhin, M. A. A.; Ikeda, T.; Kamada, K.; Ando, H.; Shibutani, Y.; Kimura, K. J. Org. Chem. 2001, 66, 1533-1537. (29) Kimura, K.; Yamashita, M.; Yokoyama, M. Chem. Lett. 1991, 965-968. (30) Kimura, K.; Yamashita, T. Yokoyama, M. J. Phys. Chem. 1992, 96, 56145617. (31) Kimura, K.; Utsumi, T.; Teranishi, T.: Yokoyama, M.; Sakamoto, H.; Okamoto, M.; Arakawa, T.; Moriguchi, H.; Miyaji, Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 2452-2454. (32) Kimura, K.; Sakamoto, H.; Kado, S.; Yokoyama, M. Analyst 2000, 125, 1091-1095. (33) Kimura, K.; Teranishi, T.; Yokoyama, M.; Miyake, S.; Sakamoto, H.; Tanaka, M. J. Chem. Soc., Perkin. Trans. 2 1999, 199-204. (34) Sakamoto, H.; Yokohata, T.; Yamakura, T.; Kimura, K. Anal. Chem. 2002, 74, 2522-2528. (35) Nakamura, M.; Fujioka, T.; Sakamoto, H.; Kimura, K. New, J. Chem. 2002, 3, 554-559. (36) Nakamura, M.; Takahashi, K.; Fujioka, T.; Kado, S.; Sakamoto, H.; Kimura, K. J. Am. Soc. Mass Spectrom. 2003, 14, 1110-1115.
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complexing ability. The higher the charge density of the metal ion, the stronger the interaction between phenolate ion of the merocyanine form and the metal ion.37-39 On the other hand, under visible-light irradiation conditions, the spiropyran form is retained and the metal ion is complexed only with the crown ether moiety. In this case, the metal ion complexing ability and selectivity of the crowned spirobenzopyran derivative depend primarily on those of the crown ether moiety. The cation selectivity of crown ethers is mainly based on the size fit between the crown ether cavity and cation diameter.40 Therefore, the cation complexing ability and selectivity of the crowned spirobenzopyran derivative are remarkably influenced by photoirradiation. In the liquid-liquid extraction of alkali metal ions, 12-crown-4 spirobenzopyran 1, which has an octadecyl group as the lipophilic moiety, exhibited a high Li+ selectivity, and the extractability was increased and decreased by UV- and visible-light irradiation, respectively.34 Moreover, the diaza-12-crown-4 derivative bearing two spirobenzopyran moieties, 4, which is 12-crown-4-bis(spirobenzopyran), showed an excellent selectivity for Li+.41 Here, we describe the alkali metal ion transport through liquid membranes of 12-crown-4-, 15-crown-5-, and 18-crown-6-spirobenzopyrans, 1-3, and 12-crown-4- and 18-crown-6-bis(spirobenzopyran)s, 4 and 5, under dark and UV- and visible-light irradiation conditions. The uphill transport behavior of alkali metal ions through the liquid membranes of the crowned spirobenzopyrans was also examined under the conditions where UV and visible light were irradiated on the boundary phases between the source and membrane phases and between the membrane and receiving phases, respectively. An attempt to accelerate the transport rate of the uphill transport was further carried out by the combination of a proton concentration difference between the source and receiving phases with photoirradiation. EXPERIMENTAL SECTION Reagents. Crowned spirobenzopyrans 1-5 were synthesized in the same manner as mentioned elsewhere.33-35 1,2-Dichloroethane (Sigma Aldrich Japan, Ltd.) was of analytical grade and was shaken with deionized water before being used for membrane transport experiments. Lithium hydroxide monohydrate (Nakarai Tesque, Inc.), lithium chloride (Nakarai Tesque, Inc.), 1.0 mol dm-3 sodium hydroxide aqueous solution (Nakarai Tesque, Inc.), sodium chloride (Katayama Chemical Industry Co., Ltd.), 1.0 mol dm-3 potassium hydroxide aqueous solution (Katayama Chemical Industry Co., Ltd.), potassium chloride (Nakarai Tesque, Inc.), 10% tetramethylammonium hydroxide aqueous solution (TMAOH) (Nakarai Tesque, Inc.), 84% picric acid (Katayama Chemical Industry Co., Ltd.), and 1.0 mol dm-3 hydrochloric acid (Sigma Aldrich Japan, Ltd.) were of analytical grade. Good’s buffer, (3-morpholino)propanesulfonic acid (MOPS), was purchased as analytical grade from Dojindo Laboratories. (37) Takagi, M. In Cation Binding by Maclocycles; Inoue, Y., Gokel, G. W., Eds.; Marcel Dekker: New York, 1990; pp 465-495. (38) Takagi, M.; Ueno, K. In Host-Guest Complex Chemistry III; Vo ¨gtle, F., Weber, E., Eds.; Topics in Current Chemistry 121; Springer-Verlag: Tokyo, 1984; pp 39-65. (39) Gokel, G. W.; Trafton, J. E. In Cation Binding by Maclocycles; Inoue, Y., Gokel, G. W., Eds.; Marcel Dekker: New York, 1990; pp 253-310. (40) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S. Chem. Rev. 1991, 91, 1721-2085. (41) Nakamura, M.; Fujioka, T.; Sakamoto, H.; Kimura, K. Bunseki Kagaku 2003, 52, 419-424.
Figure 1. Apparatus for liquid membrane transport under photoirradiation conditions.
Liquid Membrane Transport. A U-shaped glass cell was used for the liquid membrane transport (Figure 1). The source and receiving phases were separated by a liquid membrane of 1,2dichloroethane. UV light (300-400 nm) and visible light (>500 nm) were obtained by passing a light from a Xe lamp equipped with a quartz waveguide through a Toshiba UV-D36B and a Toshiba Y-50 color filter, respectively, and were irradiated from above the aqueous phase to the boundary phase between the aqueous and membrane phases through the aqueous phase. The aqueous solutions of the source and receiving phases were stirred by mechanical stirrers equipped with glass rods, and the liquid membrane solution was stirred by a magnetic stir bar. Aliquots (100 µL) of the source and receiving solutions were taken at regular time intervals to be subjected to flame spectrophotometry, whose measurement errors observed were within (3%. For passive transport of alkali metal ion, a 1,2-dichloroethane solution (25 mL) containing crowned spirobenzopyran in 1.0 × 10-3 mol dm-3 was used as the membrane phase. An aqueous solution (15 mL) containing LiOH, NaOH, and KOH each in 2.0 × 10-2 and 1.0 × 10-2 mol dm-3 picric acid and an aqueous solution (15 mL) of 5.0 × 10-2 mol dm-3 TMAOH was used as the source and receiving phases, respectively. The selectivity ratio of the alkali metal ion transported was calculated from the concentrations of metal ions transported into the receiving phase after 10 h. On the other hand, the light-driven uphill transports of Li+ were carried out under conditions where the membrane phase used was the same as mentioned above, and an aqueous solution containing 3.0 × 10-2 mol dm-3 LiOH and 1.0 × 10-2 mol dm-3 picric acid was used as both the source and receiving phases. UV and visible lights were irradiated onto the source and receiving phases, respectively. For uphill transport of Li+ under different pH conditions between the source and receiving phases, an aqueous solution containing 3.0 × 10-2 mol dm-3 LiCl, 1.0 × 10-2 mol dm-3 picric acid, and 0.10 mol dm-3 TMAOH (pH 12.7) was used as the source phase, while an aqueous solution containing 3.0 × 10-2 mol dm-3 LiCl and 1.0 × 10-2 mol dm-3 picric acid, adjusted to pH 3.7 with 1.0 × 10-2 mol dm-3 MOPS, was used as receiving phase. RESULTS AND DISCUSSION Alkali Metal Ion Transport under Dark Condition. Passive competitive transport of alkali metal ions through liquid membranes containing crowned spirobenzopyrans was carried under dark condition. For the competitive transport of alkali metal ions,
Figure 2. Competitive transport of alkali metal ions through liquid membrane of 1 under dark conditions.
an aqueous solution containing alkali metal hydroxide and picric acid was used as the source phase, and an aqueous solution of tetramethylammonium hydroxide was used as receiving phase. It is because the metal ion complexing ability of the crown ether unit of the crowned spirobenzopyran is influenced by the protonation on the nitrogen atom on the crown ether unit under neutral pH and acidic conditions.42 The concentrations of the alkali metal ions transported through a liquid membrane of 1 to the receiving phase are shown in Figure 2. Lithium ion was selectively transported through a membrane of 1; only Na+ was slightly transported among alkali metal ions. The metal ion selectivities in the membrane transport are summarized in Table 1. The Li+ selectivities against Na+ and K+ for the membrane of 1 were 10 and 88, respectively. The transport mechanism of Li+ under dark conditions can be postulated as shown in Scheme 1. The crown ether unit of 1 binds Li+ at the boundary phase between the source and membrane phases, and the spirobenzopyran moiety is simultaneously isomerized to the corresponding (open) merocyanine form. The phenolate anion of the merocyanine form interacts with Li+ bound to the crown ether unit to form the more stable complex.37,38 The ternary complex with a picrate ion as a counterion permeates into the membrane phase, and then the complex is transported to the boundary phase between the membrane and receiving phases. At the boundary phase, Li+ and picrate ion are released into the receiving phase based on the concentration gradient of Li+ between the membrane phase and the receiving phase, and the merocyanine form is simultaneously isomerized back to the (closed) spiropyran form thermally. The metal ion transporting selectivity for the membrane under dark conditions is dependent on the metal ion extractability of the crowned spirobenzopyran into the membrane phase at the boundary phase between the source and membrane phases. The high Li+ selectivity is, therefore, derived from the extractability of 1 for Li+, which is based on the combination of the complexing ability of monoaza-12-crown-4 unit for Li+ and the powerful interaction of phenolate ion of merocyanine form with Li+. It is known that the p-nitrophenolate ion interacts more strongly with metal ions of a higher charge density.17 Thus, the interaction of the phenolate ion of the merocyanine form of 1 with Li+ should be much stronger than with Na+. In the alkali metal ion transport, the membranes of 2 and 3 exhibited Na+ and K+ selectivities, respectively, which were in accord with the metal ion selectivities (42) Sakamoto, H.; Kimura, K.; Koseki, Y.; Matsuo, M.; Shono, T. J. Org. Chem. 1986, 51, 4974-4979.
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Table 1. Metal Ion Selectivity Ratios for Competitive Transport through Liquid Membranes Containing Crowned Spirobenzopyrans under Different Photoirradiation Conditions photoirradiation condition compd
source
phasea
receiving
selectivity ratioc
phaseb Li+/Na+
1
2
3
4
5
UV light dark visible light UV light
UV light dark visible light UV light
UV light dark visible light UV light
UV light dark visible light UV light
UV light dark visible light UV light
dark dark dark visible light
dark dark dark visible light
dark dark dark visible light
dark dark dark visible light
dark dark dark visible light
15 10 7 25
Li+/K+ 119 88 37 121
Na+/Li+
Na+/K+
2 3 3 2
6 5 5 5
K+/Li+
K+/Na+
34 34 32 33
5 4 5 5
Li+/Na+
Li+/K+
50 24 17 53
ndc nd nd nd
K+/Li+
K+/Na+
20 31 33 16
6 10 9 6
a Photoirradiation was made at the boundary phase adjacent to the membrane phase. b Values were calculated from concentration ratio of metal ions extracted into the receiving phase after 10 h. c Could not be determined.
Scheme 1. Plausible Mechanisms for Transport of Li+ through Liquid Membrane of 1 under Dark Conditions
of the crown ether units of crowned spirobenzopyrans. The selectivity ratio of 2 for Na+ against Li+, however, is very low as compared with that of the Na+ selectivity of monoaza-15-crown-5. It is because the complexing selectivity of 2 for Na+ depends on 2002 Analytical Chemistry, Vol. 77, No. 7, April 1, 2005
Figure 3. Competitive transport of alkali metal ions through liquid membrane of 4 under dark conditions.
Figure 4. Lithium ion transport behavior through liquid membranes of 4 under different photoirradiation conditions: (b) when the source and receiving phases are kept under dark conditions; (4) when visible light is irradiated onto source phase with the receiving phase being kept under dark conditions; (0) when UV light is irradiated onto source phase with the receiving phase being kept under dark conditions; (9) when UV and visible lights were irradiated onto the source and receiving phases, respectively.
the balance between the complexing selectivity of monoaza-15crown-5 for Na+ and the powerful interaction of phenolate ion of merocyanine form with Li+, as mentioned above. A similar phenomenon was also found in the K+ selectivity against Na+ for the membrane of 3. An excellent Li+ selectivity, a Li+/Na+ ratio of 24 after 10-h transport, was attained using the membrane of diaza-12-crown-4-bis(spirobenzopyran) 4, which has a diaza-12crown-4 unit and two spirobenzopyran moieties (Figure 3). When, in the Li+ complex of crowned bis(spirobenzopyran) 4, the crown ether moiety binds Li+, its spirobenzopyran moieties are simultaneously isomerized to their merocyanine forms whose phenolate ions interact strongly with Li+ bound to the crown ether unit.35,41 The complexing ability of 4 with Li+ is much stronger than that of the diaza-12-crown-4 unit itself, because of the additional interaction of two phenolate ions. On the other hand, the membrane of diaza-18-crown-6-bis(spirobenzopyran) 5 did not show very high selectivity for K+, because two phenolate ions of its merocyanine form hardly interact with K+ due to the lower charge density of K+ than those of Na+ and Li+.32,33 Alkali Metal Ion Transport under Photoirradiation Condition. Alkali metal ion transports through liquid membranes containing a crowned spirobenzopyran derivative were also carried out under the photoirradiation conditions. At first, UV or visible lights were irradiated onto the boundary phase between the source and membrane phases through the source phase, while the
Scheme 2. Plausible Mechanisms at Boundary Phase between Source and Membrane Phases for Transport of Li+ through Liquid Membrane of 4
boundary phase between the receiving and membrane phases was kept under dark conditions. The Li+ transporting behavior through a liquid membrane of 4 under photoirradiation conditions is shown in Figure 4. The concentration changes of only Li+ are shown in this figure for the simplification of the figure, although an aqueous solution containing equal amounts of Li+, Na+, and K+ was used as the source phase in the transport experiment. It is recognized from Figure 4 that the transport of Li+ was accelerated as compared with that under dark conditions, when UV light was irradiated onto the source phase while the receiving phase was kept in dark conditions. UV-light irradiation on the source phase isomerizes the spirobenzopyran moiety to its corresponding merocyanine form, and thereby the phenolate ion of the merocyanine moiety interacts strongly with Li+ bound to the crown ether unit to form the more stable complex. To the contrary, the transport of Li+ was retarded by visible-light irradiation on the source phase. When the visible-light irradiation on the source phase stabilizes the spirobenzopyran moieties, the complex stability of the 4 depends predominantly on the complexing ability of the crown ether unit itself, being much less than the combined system of the interaction of two phenolate ions of the merocyanine moieties to Li+ and the complexation of crown ether unit with Li+. For the membranes containing the other crowned spirobenzopyrans, similar phenomena were observed for the most transported metal ions through the corresponding liquid membrane under photoirradiation conditions. The extent of change in the transport rate of the most transported ion under photoirradiation was the highest in the membrane transport using 4, which carries two spirobenzopyran moieties isomerizing to the corresponding merocyanine forms to interact with Li+ strongly by UV-light irradiation. In addition, when visible light was also irradiated on the receiving phase for a liquid membrane transport system of 4, with UV light being irradiated on the source phase, the metal ion transport was much more accelerated as compared with that under the conditions where UV light was irradiated on the source phase
with the receiving phase being kept under dark conditions, as shown in Figure 4. It is because the complex stability of 4 with Li+ is remarkably decreased by visible-light irradiation at the boundary phase between the membrane and receiving phases. At that time, Li+ is released into the receiving phase more easily, due to the disappearance of the interaction between Li+ bound to the crown ether unit and the phenolate ion of the merocyanine form of the spiropyran moiety of 4 (Scheme 2).41 Similar phenomena were also observed with the membranes containing the other crowned spirobenzopyrans. The metal ion ratios for membrane transport under photoirradiation conditions as well as those under dark conditions are summarized in Table 1. The most remarkable change in the selectivity by photoirradiation was observed in the Li+ transport through the membrane of 4, while the membrane of 5 bearing a diaza-18-crown-6 moiety exhibited the less remarkable change for the K+ transport. This is because the complexing ability of 5 for K+ is predominantly governed by the cation complexing ability of the diaza-18-crown-6 unit itself of 5, but not by the interaction between the phenolate ion of the merocyanine form and K+ bound to the crown unit due to the lesser charge density of K+.37 The extent in the photoinduced enhancement of the transporting ability for the most transported metal ions is decreased in the following order: 4 > 1 > 2 > 5 ≈ 3. This result is attributed to the difference in the charge densities among the metal ions; i.e., the higher charge density the metal ion has, the stronger is the interaction between the phenolate ion of the merocyanine form and the metal ion bound to the crown ether unit. Effect of Photoirradiation on Alkali Metal Ion Selectivity. The change in metal ion selectivity for the liquid membrane transport was examined under photoirradiation conditions as mentioned above. The metal ion selectivity ratio was calculated from the concentrations of the metal ions transported into the receiving phase after 10-h transport, as summarized in Table 1. The concentrations of Na+ and K+ transported to the receiving phase through the liquid membrane of 4 in 10 h were hardly Analytical Chemistry, Vol. 77, No. 7, April 1, 2005
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determined, while the selective transport for Li+ was observed, using an aqueous solution containing equimolar amounts of Li+, Na+, and K+ as the source phase. Thus, for the determination of the Li+/Na+ transport ratio, an aqueous solution containing Na+ at 10 times higher concentrations than that of Li+ and K+ was used as the source phase. The Li+ selectivities against Na+ for the membrane of 4 under different photoirradiation conditions was decreased in the following order: when the UV and visible lights were irradiated onto the source and receiving phases, respectively > when UV light was irradiated onto the source phase with the receiving phase being kept under dark conditions > when both the source and receiving phases were kept under dark conditions > when visible light was irradiated onto the source phase with the receiving phase being kept under dark conditions. UV-light irradiation on the source phase induces isomerization from the two spirobenzopyran moieties of 4 to their corresponding merocyanine form, and thereby, the phenolate ions of the merocyanine moieties interact more strongly with Li+ than Na+ due to the higher charge density of Li+ than Na+.37 On the other hand, the irradiation of visible light on the receiving phase isomerizes the merocyanine form of the spirobenzopyran moieties back to the original spiropyran form, decreasing the stability constant for the complex of 4 with Li+, due to the disappearance of the strong interaction between the phenolate ion of the merocyanine form and Li+ bound to the crown ether unit (Scheme 2). Therefore, the extraction of Li+ into the membrane phase is much more accelerated than that of Na+ by UV-light irradiation onto the source phase, and the release of Li+ from the membrane into the receiving phases is easier than that of Na+ by visiblelight irradiation to the receiving phase. Thus, the Li+ selectivity against Na+ was the highest under the conditions where UV and visible lights were irradiated to the source and receiving phases, respectively, being 53, which is the greatest selectivity ratio of Li+/Na+ in the liquid membrane transport as far as we know. A similar result was found using the liquid membrane of 1. To the contrary, the K+ selectivity against Li+ for the alkali metal ion transport through a liquid membrane of 5 under different photoirradiation conditions was decreased in the following order: when visible light was irradiated onto the source phase with the receiving phase being kept under dark conditions > when both the source and receiving phases were kept under dark conditions > when UV light was irradiated onto the source phase with the receiving phase being kept under dark conditions > when UV and visible lights were irradiated onto the source and receiving phases, respectively. This selectivity change of the K+ against Li+ for the system of 5 is the opposite to the selectivity change of the Li+ against Na+ for the system of 4. These results show that a most important factor governing the metal ion selectivity is the intensity of the interaction between the phenolate ion of the merocyanine form and a metal ion bound to the crown ether unit. Light-Driven Uphill Transport of Li+. Compounds 1 and 4, whose Li+-transporting abilities were remarkably changed by UVor visible-light irradiation, were used as ionophores for Li+ uphill membrane transports by photoirradiation. The same aqueous solutions containing LiOH and picric acid were used as the source and receiving phases. UV and visible lights were irradiated onto the source and receiving phases, respectively. A typical Li+ transport profile through a liquid membrane of 1 is shown in 2004 Analytical Chemistry, Vol. 77, No. 7, April 1, 2005
Figure 5. Behavior of light-driven uphill transport of Li+ through liquid membrane of 1.
Figure 6. Behavior of light-driven uphill transport of Li+ through liquid membrane of 4.
Figure 7. Behavior of proton-driven (b), and proton- and light-driven (9) uphill transport of Li+ through liquid membrane of 1.
Figure 5. The vertical axis denotes the Li+ concentration changes in the source and receiving phases with the transporting time. The figure shows that the Li+ concentration in the receiving phase increased with transport time, which means that Li+ is enriched in the receiving phase by photoirradiation. This phenomenon is similar to that of the proton-driven uphill transport using a crown ether bearing proton dissociable group.43,44 As mentioned above, (43) Fyles, T. M.; Malik-Diemer, V. A.; Whitfield, D. M. Can. J. Chem. 1981, 59, 1734-1744. (44) Sakamoto, H.; Kimura, K.; Shono, T. Anal. Chem. 1987, 59, 1513-1517.
Scheme 3. Plausible Mechanism for Proton-Driven Uphill Transport of Li+ through Liquid Membrane of 1
the Li+ complexing ability of crowned spirobenzopyran 1 is enhanced by UV-light irradiation and vice versa by visible-light irradiation. The enhancement of Li+ complexing ability of 1 by photoirradiation is reflected in this light-driven transport. It is also found from the figure that the the Li+ concentration decrease in the source phase is much more than the increase in the receiving phase.45,46 And the Li+ concentration change of the receiving phase was scarcely observed in the early stage of transport, from 0 to 5 h, and then the Li+ concentration in the receiving phase was increased. The Li picrate in both of the source and receiving phases was transferred into the membrane phase until the formation of Li+ complex reached an equilibrium in the membrane phase, since the crowned spirobenzopyran can also form complexes with Li+ at the boundary phase between the membrane and receiving phases even under visible-light irradiation. It is also because the membrane phase is larger in volume than those of the source and receiving phases and contains a large amount of ligand corresponding to about 5.6% of the Li+ concentration in an aqueous phase. That is to say, the transport of Li+ complex, which is extracted from the source phase into the membrane phase and then is transferred to the boundary phase between the membrane and receiving phases by its diffusion in the liquid membrane, is a time-consuming process. Thus, the time lag between the decrease of the Li+ concentration in the source phase and the increase in the receiving phase depends on the diffusion of Li+ complex in the liquid membrane. With a liquid membrane of 4, the enrichment of Li+ in the receiving phase is much more effective than with that of 1 (Figure 6). It is because the photoinduced change in the Li+ complexing ability of 4 is much more significant than in that of 1. Proton- and Light-Driven Uphill Transport of Li+. The cation complexing ability of monoazacrown ether derivatives is decreased by the protonation of the nitrogen atom in the crown ether unit, as mentioned above. We had already reported protondriven uphill transport of alkali metal ions through membranes (45) Frederick, L. A.; Fyles, T. M.; Malik-Diemer, V. A.; Whitfield, D. M. J. Chem. Soc., Chem. Commun. 1980, 1211-1212. (46) Ishikawa, J.; Sakamoto, H.; Otomo, M. Analyst 1997, 122, 1383-1386.
containing monoazacrown derivatives.47 The result persuaded us to apply the difference in the proton concentration between the source and receiving phases to the Li+ transport system using the present crown spirobenzopyrans, which turns out to be a proton-driven transport system. Li+ transport through a liquid membrane of 1 was carried out under dark conditions, using an aqueous solution containing LiCl, picric acid, and 0.1 mol dm-3 TMAOH as the source phase and an aqueous solution containing 1.0 × 10-2 mol dm-3 MOPS and LiCl and picric acid in the same concentration as that of the source phase as the receiving phase. The Li+ transport profile using a liquid membrane of 1 is shown in Figure 7. The uphill transport of Li+ from the source to receiving phases was also attained by the proton concentration gradient under dark conditions. The transport mechanism is as follows. The crowned spirobenzopyran and Li+ forms a complex, and the spirobenzopyran moiety is simultaneously isomerized to its merocyanine form at the boundary phase, and then the complex is extracted into the membrane phase. After diffusion of the complex to the boundary phase between the membrane and receiving phases, the crown-ether-ring nitrogen atom of the compound was protonated. Finally, due to the decreased complexing ability, the Li+ at the boundary phase was released into the receiving phase (Scheme 3). Furthermore, the photoirradiation system was combined with the proton-driven metal ion transport. The transport of Li+ through a liquid membrane of 1 was carried out by UV- and visible-light irradiation to the source and receiving phases, respectively, under different pH conditions between the source and receiving phases. Thus, the uphill membrane transport of Li+ was remarkably accelerated by the photoirradiation, as compared with the system using only different pH conditions between the source and receiving phases (Figure 7). CONCLUSION In the passive transport of alkali metal ions, the liquid membranes containing crowned spirobenzopyrans 1-3, bearing monoaza-12-crown-4, -15-crown-5, and -18-crown-6, exhibited the ion selectivities for Li+, Na+, and K+, respectively. The ion (47) Sakamoto, H.; Kimura, K.; Koseki, Y.; Matsuo, M.; Shono, T. J. Org. Chem. 1986, 51, 4974-4979.
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selectivities are consistent with the cation complexing abilities of the crown ether units of the crowned spirobenzopyrans. The liquid membrane of monoaza-12-crown-4-spirobenzopyran 1 exhibited high Li+ selectivity, and its transport was accelerated and retarded by the UV- and visible-light irradiations, respectively, as compared with that under dark conditions. The liquid membrane of diaza12-crown-4-bis(spirobenzopyran) 4 showed the excellent Li+ selectivity in the competitive transport of alkali metal ions and the largest change in Li+ transporting ability and selectivity by photoirradiation. The uphill transports of metal ions were also attained using the liquid membranes containing the crowned spirobenzopyrans and bis(spirobenzopyran)s under the conditions where UV and visible lights were irradiated on the source and receiving phases, respectively, with the same solutions containing
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LiOH and picric acid being used as the source and receiving phases. Especially, the most effective transport and enrichment of Li+ into the receiving phase were observed using the liquid membrane of 4, which should be a candidate for Li+ selective enrichment. In addition, the combination of photoirradiation and pH gradient conditions between the source and receiving phases resulted in the enhancement of the uphill transporting ability of Li+ through the liquid membrane of 1.
Received for review September 13, 2004. Accepted January 6, 2005. AC048642I
