Micellar Fibers of Octopus Porphyrin. Photoinduced Electron Transfer

Nov 15, 1996 - Symmetrical tetraresorcinolporphyrin with eight alkylphosphocholine side chains (octopus porphyrin) and its zinc(II) complex were dispe...
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Langmuir 1996, 12, 6242-6249

Micellar Fibers of Octopus Porphyrin. Photoinduced Electron Transfer Reactions in Aqueous Media Teruyuki Komatsu,†,‡ Kazuhiro Yamada,‡ Eishun Tsuchida,‡ Ulrich Siggel,§ Christoph Bo¨ttcher,† and Ju¨rgen-Hinrich Fuhrhop*,† Institut fu¨ r Organische Chemie der Freien Universita¨ t Berlin, Takustrasse 3, 14195 Berlin, Germany, Department of Polymer Chemistry, Waseda University, Tokyo 169, Japan, and Max Volmer Institut fu¨ r Physikalische Chemie der Technischen Universita¨ t Berlin, Strasse des 17 Juni 135, 10623 Berlin, Germany Received April 26, 1996. In Final Form: September 2, 1996X Symmetrical tetraresorcinolporphyrin with eight alkylphosphocholine side chains (octopus porphyrin) and its zinc(II) complex were dispersed in aqueous media to give a stable colloidal solution. Electron microscopy showed either planar monolayers or micellar fibers with a uniform thickness of 7 nm. A unique spherical arrangement of eight methyl groups on each side of the porphyrin center provides fat droplet like centers which align in a string of pearls in an extremely open fiber structure. Exciton calculations on the basis of a long wavelength shift of the Soret bands (7-8 nm), line broadening, and electron micrographs indicated a tilt stacking arrangement with a porphyrin separation of 11 Å. The octopus porphyrin fibers fluoresced strongly, and laser flash photolysis lead to electron transfer from the porphyrin to hydrophobic phenyl-p-benzoquinone as well as to hydrophilic 1,2-naphtoquinone-4-sulfonate and dimethylviologen acceptors. The fibers were also active as photocatalysts in the reduction of dimethylviologen to the corresponding radical by amines.

Introduction There is much current interest in the design and study of vectorial photoinduced electron transfer systems using micelles and vesicle membranes in aqueous media.1-4 In these systems, the photoactive centers, namely synthetic porphyrin derivatives, play a crucial role.2,5 It has been clarified that the intermolecular electron transfer in phospholipid bilayer membranes is possible only when the porphyrin plane is fixed within a distance of less than 12 Å from the acceptor.6 Furthermore it has been found that some porphyrin amphiphiles self-organize in aqueous media to produce long-lived membrane structures with distinct photoreactivity.7-13 14,16-Diaminoporphyrin fibers held together by hydrogen bond chains, for example, showed no fluorescence but light-induced charge separation.12 Ultrathin tin(IV) or zinc porphyrinate fibers also did not fluoresce and were photochemically inactive.13 Here †

Institut fu¨r Organische Chemie der Freien Universita¨t Berlin. Waseda University. § Max Volmer Institut fu ¨ r Physikalische Chemie der Technischen Universita¨t Berlin X Abstract published in Advance ACS Abstracts, November 15, 1996. ‡

(1) Fendler, J. H. Membrane Mimetic Chemistry; John Wiley and Sons: New York, 1982; Chapter 13. (2) Baral, S.; Fendler, J. H. In Photoinduced Electron Transfer, Part B; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; p 541. (3) Fox, M. A. Top. Curr. Chem. 1991, 159, 68. (4) Armitage, B.; O’Brien, D. F. J. Am. Chem. Soc. 1992, 114, 7396. (5) Hurst, J. K.; Lee, L. Y. C.; Gra¨tzel, M. J. Am. Chem. Soc. 1983, 105, 7048. (6) Tsuchida, E.; Kaneko, M.; Nishide, H.; Hoshino, M. J. Phys. Chem. 1986, 90, 2283. (7) Barber, D. C.; Freitag-Beeston, R. A.; Whitten, D. G. J. Phys. Chem. 1991, 95, 4074. (8) Fuhrhop, J.-H.; Demoulin, C.; Bo¨ttcher, C.; Ko¨ning, J.; Siggel, U. J. Am. Chem. Soc. 1992, 114, 4159. (9) Endisch, C.; Bo¨ttcher, C.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1995, 117, 8273. (10) Tsuchida, E.; Komatsu, T.; Arai, K.; Yamada, K.; Nishide, H.; Fuhrhop, J.-H. Langmuir 1995, 11, 1877. (11) Tsuchida, E.; Komatsu, T.; Kumamoto, S.; Toyano, N.; Nishide, H. J. Chem. Soc., Chem. Commun. 1993, 1731. (12) Fuhrhop, J.-H.; Bindig, U.; Siggel, U. J. Am. Chem. Soc. 1993, 115, 11036. (13) Fuhrhop, J.-H.; Bindig, U.; Siggel, U. J. Chem. Soc., Chem. Commun. 1994, 1583.

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we report on bolaamphiphilic meso-tetraresorcinolporphyrins with eight octadecyl-ω-phosphocholine side chains (octopus porphyrin)14 which formed either monolayers in water showing no curvature or extremely open fiber structures with maximum curvature. The fibers have a “micellar” character because they have a polar surface and a hydrophobic core made of alkyl chains and porphyrins. Furthermore, the electron microscopic diameter of the fiber corresponds to a molecular monolayer micelle of the bolaamphiphilic porphyrin. No critical micellar concentration (cmc) could be determined. Long-lived triplet states of the free base and zinc complex fibers will be described as well as their photoinduced electron transfer reactions. Experimental Section Methods and Materials. Chemicals were commercial highpurity grades and not further purified. The water used was deionized using a Millipore-Q system. 18-(Benzyloxy)-2,2-dimethyleicosanoic acid, 18-hydroxy-2,2-dimethyleicosanoic acid, 5,10,15,20-tetrakis(2,6-dihydroxyphenyl)porphine, and 2-chloro2-oxo-1,3,2-dioxophospholane were prepared according to the previously reported procedure.14-16 Infrared spectra were recorded with a JASCO FT/IR-5300 spectrometer. The 1H-NMR and COSY (1H-1H) spectra were measured with samples dissolved in CDCl3 or CD3OD at room temperature using a JEOL GSX-400 spectrometer. Chemical shifts are expressed in ppm downfield from Me4Si as an internal standard. UV-vis absorption spectra were obtained on a Shimadzu UV-2200 or a Perkin Elmer Lambda 16 spectrophotometer. Elemental analysis was performed on a Yanagimoto MT3 CHN corder. Thin-layer chromatography (TLC) was carried out on 0.2 mm precoated plates of silica gel 60 F-254 (Merck). Purification was performed by silica gel 60 (Merck) flash column chromatography. Transmission Electron Microscopy. The negatively stained specimens for transmission electron microscopy were prepared as follows. Droplets of the aqueous solution of the octopus porphyrin were placed onto carbon-coated copper grids (400 mesh), and after 1 min excess fluid was blotted off. The remaining (14) Komatsu, T.; Nakao, K.; Nishide, H.; Tsuchida, E. J. Chem. Soc., Chem. Commun. 1993, 728. (15) Tsuchida, E.; Komatsu, T.; Hasegawa, E.; Nishide, H. J. Chem. Soc., Dalton Trans. 1990, 2713. (16) Matsushita, Y.; Hasegawa, E.; Eshima, K.; Tsuchida, E. Chem. Lett. 1983, 1387.

© 1996 American Chemical Society

Micellar Fibers of Octopus Porphyrin thin film of the sample was stained with 1% uranyl acetate. After removal of the excess fluid, the grids were air-dried. The obtained grids were observed in a Philips CM12 electron microscope at an accelerating voltage of 100 kV. The vitrified specimen for cryomicroscopy was prepared and observed as described elsewhere.17 The vitrified sample was observed in a Phillips CM12 using a Gatan cold stage model 626 at an accelerating voltage of 100 kV. Gel Chromatography. Gel chromatography of an aqueous octopus porphyrin solution was performed as follows. The sample solution (1 mL) was applied onto a column (1.5 cm × 20 cm) of Sepharose CL-4B (Pharmacia Co.) and developed with deionized water. As molecular mass standards, several polymers in the molecular weight range from 6.6 × 104 Da (albumin) to 2.0 × 106 Da (blue dextran) were used. The contents of the porphyrin fractions were detected by UV-vis spectroscopy. Fluorescence Measurement. Fluorescence emission spectra were recorded on a Perkin-Elmer MPF-44B. Excitation was 510 and 540 nm for 1a and 1b, respectively, and emission spectra

were generally recorded within the range 550-800 nm. Methanol solutions of phenyl-p-benzoquinone (PBQ) and aqueous solutions of 1,2-naphtoquinone-4-sulfonate (NQS-) and 4,4′-dimethylviologen dichloride (MV2+) were prepared as stock solutions of quencher. Several microliters of the quencher solutions were injected with a microsyringe to 3 mL of the porphyrin solutions. The quenching of the fluorescence emission of octopus porphyrin was then observed immediately after addition of the quencher. The added volume of the quencher solution was always less than 4% of the total volume. Excited singlet lifetimes were measured (17) Ko¨ning, J.; Bo¨ttcher, C.; Winkler, H.; Zeitler, E.; Talmon, Y.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1993, 115, 693.

Langmuir, Vol. 12, No. 26, 1996 6243 using a Horiba NAES-500 single-photon counter equipped with a hydrogen lamp. Laser Flash Photolysis. Laser flash photolysis studies were carried out by using a Q-switched Nd-YAG laser (Spectrum Co.). This laser generated a second-harmonic (532 nm) pulse of 2 ns duration with an energy of 25 mJ per pulse; a repetition rate of 3 Hz was used for excitation of the sample solutions. A 150 W Xenon arc lamp was used as the monitor light source, and light detection was realized by a silicon avalanche photodiode. The output signal was recorded on a Nicolet 1170 averager. The octopus porphyrin solutions for flash photolysis were held in a 1 cm cuvette and purged of oxygen by nitrogen bubbling. Used porphyrin concentrations were normally 25 µM for 1a and 10 µM for 1b. Most experiments were carried out at 20 °C. Injection of quenchers to the porphyrin solution was carried out as in the fluorescence quenching experiment. 5,10,15,20-Tetrakis[2,6-bis((2,2-dimethyl-20-(benzyloxy)eicosanoyl)oxy) phenyl]porphine (2a). 2,2-Dimethyl-20-

(benzyloxy)eicosanoic acid (18.5 g, 41.2 mmol) was suspended in oxalyl chloride (12.7 g, 0.1 mol). The mixture was stirred for 3 h, and excess oxalyl chloride was removed in vacuo to yield a white solid. A dry THF solution (100 mL) of 5,10,15,20-tetrakis(2,6-dihydroxy)phenylporphine (3a)15 (1.0 g, 1.3 mmol) was added dropwise to the crude acid chloride at room temperature for 1 h, and DMAP (0.3 g, 2.5 mmol) was added. After reaction for 12 h at 60 °C, the solution was brought to dryness on a rotary evaporator and extracted with CHCl3. The organic layer was washed, first with water and then with aqueous NaHCO3. The organic phase dried over anhydrous Na2SO4 was concentrated, and the residue was chromatographed on a silica gel column using CH2Cl2/acetone, 100:1 (v/v) as the eluent. The main band was collected, and the solvent was reduced to a small volume on a rotary evaporator. The residue was filtered off and dried at room temperature for several hours in vacuo, to give a purple powder (2a) (3.0 g, 55%). Rf ) 0.2 (CH2Cl2/acetone, 100:1 v/v). 1H-NMR (CDCl , 400 MHz): δ -2.8 (2H, s, inner H), -0.7 (48H, 3 s, dimethyl), 0.8-1.6 (272H, m, (CH2)17), 3.4 (16H, t, CH2O), 4.5 (16H, s, OCH2Ph), 7.4-7.9 (52H, m, phenyl), 8.8 (8H, s, pyrrole β-H). IR (KBr): ν 1760 (CdO) cm-1. UV-vis (CHCl3): λmax ) 651, 584, 540, 510, 416 nm. 5,10,15,20-Tetrakis[2,6-bis((2,2-dimethyl-20-hydroxyeicosanoyl)oxy) phenyl]porphine (4a). 2a (3.0 g, 0.7 mmol) was dissolved in dry CH2Cl2 (50 mL) at 5 °C, followed by the addition of boron tribromide (2.1 mL, 1.68 mmol). The resulting green solution was stirred for 6 h at room temperature and then cautiously dropped into ice-water. Diethyl ether (200 mL) was added to the suspension, and the mixture was neutralized with

6244 Langmuir, Vol. 12, No. 26, 1996 NaHCO3. The separated organic layer was washed with water and dried over anhydrous Na2SO4. The solution was reduced to a small volume on a rotary evaporator, and the residue was chromatographed on a silica gel column using CHCl3/CH3OH, 20:1 (v/v) as the eluent, giving 4a (1.9 g, 80%). Rf ) 0.3 (CHCl3/ CH3OH, 20:1 v/v). 1H-NMR (CDCl3, 400 MHz): δ -2.8 (2H, s, inner H), -0.7 (48H, s, dimethyl), 0.8-1.6 (272H, m, (CH2)17), 3.5 (16H, t, CH2OH), 7.5, 7.9 (8H, 4H, d, t, phenyl), 8.9 (8H, s, pyrrole β-H). IR (KBr): ν 3350 (OH), 1760 (CdO) cm-1. UV-vis (CHCl3): λmax ) 651, 584, 540, 510, 416 nm. 5,10,15,20-Tetrakis[2,6-bis[[2,2-dimethyl-20-[[2-((trimethylammonio)ethoxy)phosphonat]oxy]eicosanoyl]oxy]phenyl]porphine (1a). 4a (0.3 g, 88 µmol) was dissolved in a CH2Cl2 solution (50 mL) of triethylamine (0.3 mL, 2.1 mmol) under argon. 2-Chloro-2-oxo-1,3,2-dioxaphospholane (0.2 mL, 2.1 mmol) was added to the mixture, and the resulting solution was further stirred for 4 h at room temperature under argon. The solution was brought to dryness on a rotary evaporator (Rf ) 0.6 (CHCl3/CH3OH, 10:1 v/v)) and redissolved in dry DMF (20 mL). The DMF solution and trimethylamine (25 mL) were sealed in a pressure bottle and allowed to react for 18 h at 60 °C. The solvents were removed, and the residue was recrystallized (CH3OH/acetone). The obtained purple solid was gel chromatographed on a Sephadex LH-60 using CH3OH as the eluent to give a purple product (1a) (0.37 g, 80%). 1H-NMR (CD3OD, 400 MHz): δ -2.8 (2H, s, inner H), -0.6 (48H, s, dimethyl), 0.7-1.7 (272H, m, (CH2)17), 3.2 (72H, s, choline-CH3), 3.6 (16H, s, OCH2CH2N+(CH3)3), 3.9 (16H, t, -CH2O-), 4.3 (16H, s, OCH2CH2N+(CH3)3), 7.5, 7.9 (8H, 4H, d, t, phenyl), 8.9 (8H, s, pyrrole β-H). IR (KBr): ν 1760 (CdO), 1251 (PdO), 1096 (POC) cm-1. UV-vis (CH3OH): λmax ) 654, 585, 538, 508, 413 nm. Anal. Calcd for C260 H462N12O48P8: N, 3.52. Found: N, 3.22. (((2,2,2-Trichloroethoxy)carbonyl)oxy)-2,2-dimethyleicosanoic Acid (5). (2,2,2-Trichloroethoxy)carbonyl chloride (1.37 mL, 10.2 mmol) was added dropwise at room temperature to a dry THF/CH2Cl2 (4:1, v/v) solution (250 mL) of 18-hydroxy2,2-dimethyleicosanoic acid (3.3 g, 9.27 mmol) and pyridine (0.9 mL, 11.12 mmol) and reacted for 12 h. After 100 mL of a 5% HCl aqueous solution was added, the mixture was further stirred for 30 min. CH2Cl2 (200 mL) was added to the suspension, and the organic layer was separated, washed first with dilute aqueous HCl and then twice with water, and dried over anhydrous Na2SO4. The solution was reduced to a small volume on a rotary evaporator, and the residue was chromatographed on a silica gel column using CHCl3/ethyl acetate, 10:1 (v/v) as the eluent, giving 5 (4.28 g, 87%). Rf ) 0.54 (CHCl3/ethyl acetate, 10:1 v/v). 1HNMR (CDCl3, 400 MHz): δ 1.2 (6H, s, dimethyl), 1.2-1.4 (32H, m, CH2), 1.7 (2H, t, CH2C(CH3)2COOH), 4.2 (2H, t, C(dO)OCH2), 4.8 (2H, s, Cl3CCH2). IR (KBr): ν1757 (CdO (carbonate)), 1705 (CdO (carboxyl)) cm-1. Anal. Calcd for C24H45O5Cl3: C, 56.44; H, 8.53. Found: C, 56.78; H, 8.89. 5,10,15,20-Tetrakis[2,6-bis[[2,2-dimethyl-20-(((2,2,2-trichloroethoxy)carbonyl)oxy)eicosanoyl]oxy]phenyl]porphinatozinc(II) (6b). Zinc insertion to 3a was carried out by a general procedure using ZnCl2 and 2,6-lutidine to give zinc(II) complex 3b (98%). Thionyl chloride (1.8 mL, 24.8 mmol) was added to a dry benzene solution of 5 (1.59 g, 2.98 mmol), and the mixture was stirred for 2 h. Excess thionyl chloride and benzene were removed in vacuo to yield a white solid. A dry THF solution (15 mL) of 3b (0.1 g, 0.12 mmol) and DMAP (0.37 g, 2.98 mmol) was then added dropwise to the crude acid chloride at room temperature. The mixture was reacted for 12 h, and the solution was brought to dryness on a rotary evaporator. The residue was extracted with CHCl3, and the organic layer was washed with water and aqueous NaHCO3. After it was dried over anhydrous Na2SO4, the organic layer was evaporated, and the residue was chromatographed on a silica gel flash column using CHCl3/ acetone, 100:1 (v/v) as the eluent. The major band was collected, and the solvent was reduced to a small volume on a rotary evaporator. The residue was filtered off and dried at room temperature for several hours in vacuo to give a purple powder (6b) (90 mg, 15%). Rf ) 0.52 (CHCl3/acetone, 100:1 v/v). 1HNMR (CDCl3, 400 MHz): δ -0.7 (48H, s, dimethyl), 0.8-1.6 (272H, m, (CH2)17), 4.2 (16H, t, C(dO)OCH2), 4.8 (16H, s, Cl3CCH2), 7.4, 7.8 (8H, 4H, d, t, phenyl), 8.9 (8H, s, pyrrole β-H). IR (KBr): ν 1759 (CdO (ester, carbonate)) cm-1. UV-vis (CHCl3): λmax ) 570, 539, 411 nm.

Komatsu et al. 5,10,15,20-Tetrakis[2,6-bis((2,2-dimethyl-20-hydroxyeicosanoyl)oxy)phenyl]porphinatozinc(II) (4b). Removal of the trichloroethoxycarbonyl protecting groups of 6b was carried out according to the previously reported procedure.10 The mixture was chromatographed on a silica gel flash column using CHCl3/ CH3OH, 10:1 (v/v) as the eluent. 4b was isolated as a red-purple powder (30 mg, 48%). Rf )0.23 (CHCl3/CH3OH, 50:1 v/v). 1HNMR (CDCl3, 400 MHz): δ -0.8 (48H, s, dimethyl), 0.8-1.6 (272H, m, (CH2)17), 3.6 (16H, t, HOCH2), 7.4, 7.8 (8H, 4H, d, t, phenyl), 8.9 (8H, s, pyrrole β-H). IR (KBr): ν 3335 (OH), 1755 (CdO (ester)) cm-1. UV-vis (CHCl3): λmax ) 573, 537, 411 nm. Zinc(II) Complex (1b). The synthetic procedure for 1b was similar to that used for 1a, except for using 4b. Finally, a redpurple powder (1b) was obtained (35 mg, 85%). 1H-NMR (CD3OD, 400 MHz): δ -0.6 (48H, s, dimethyl), 0.7-1.7 (272H, m, (CH2)17), 3.2 (72H, s, choline-CH3), 3.6 (16H, s, OCH2CH2N+(CH3)3), 3.9 (16H, t, CH2O), 4.3 (16H, s, OCH2CH2N+(CH3)3), 7.5, 7.9 (8H, 4H, d, t, phenyl), 8.9 (8H, s, pyrrole β-H). IR (KBr): ν 1759 (CdO), 1253 (PdO), 1099 (POC) cm-1. UV-vis (CH3OH): λmax ) 573, 541, 412 nm. Preparation of Aqueous Octopus Porphyrin Solution. (a) Fibers. A methanol solution of 1a or 1b was evaporated in a round flask to give a thin film on the bottom. It was dried in vacuo at 50 °C for 2 h, and then deionized water was added. The mixture was heated up to 80 °C in a hot water bath and fully homogenized by vortex mixing. A clear, red solution of octopus porphyrin (1 µM to 1 mM) was obtained. (b) Platelet. The mixture of water and the porphyrin thin film was incubated at room temperature. The octopus porphyrin was slowly hydrated and then a homogeneous, red solution was obtained after 3 days. Infrared Spectra of Octopus Porphyrin Fibers and Platelets. The aqueous solutions of the octopus porphyrin assemblies (0.5 mM) in a round flask were plunged into a liquid nitrogen bath and lyophilized in vacuo. After lyophilization, KBr disks for IR measurement were prepared with the obtained porphyrin powder. The spectra at 4 cm-1 resolution were recorded with a Nicolet 800 FT/IR spectrometer.

Results and Discussion Synthesis and NMR Spectra. The porphyrin synthesis and condensation reactions to add the long chains were in general following published procedures.10,14-16 Octopus porphyrin could, however, not be metalated with zinc acetate, which is very unusual. The central methyl groups on the side chains obviously prevented the approach of zinc ions to the porphyrin nitrogen atoms. Zinc insertion was therefore carried out before the condensation reactions. In 1H-NMR spectra of both 1a and 1b, the signal for the 2,2-dimethyl groups at the bottom of the chains was strongly upfield shifted to -0.6 ppm. This indicates a folded conformation with the eight dimethyl ester groups fixed at a distance 2.5 Å above the ring plane in CD3OD solution. Morphologies of Octopus Porphyrin Aggregate. 1a and 1b were easily dispersed in deionized water by heating and vortex mixing to give a transparent red solution. These homogeneous solutions did not change within one year at room temperature. No precipitation was observable. Transmission electron microscopy of the evaporated and negatively stained colloids of 1a and 1b showed long fibers with a width of 7 nm corresponding to a porphyrin monolayer and several hundred nanometers in length (Figure 1a). These fibers were stable for more than 1 year. Cryomicroscopy gave the same images (Figure 1b). Since the fiber is so thin, it can only consist of a monolayered porphyrin stack (Figure 1d). This is a surprising arrangement for this particular molecule because it implies that there are only four alkyl chains within a circle with a diameter of 7 nm (Figure 1c and d). On the periphery there is a very low density of head groups, and one wonders why such a structure exists. Why does it not form the much more tightly packed planar monolayer described below? The answer lies probably in the unique

Micellar Fibers of Octopus Porphyrin

Langmuir, Vol. 12, No. 26, 1996 6245

(d)

(c)

Figure 1. Transmission electron micrographs of octopus porphyrin fibers: (a) negatively stained sample of 1b fibers with uranyl acetate; (b) cryo preparation in vitreous ice without stain. (c) Full space model of octopus porphyrin molecule. (d) Proposed model of octopus porphyrin fiber. The spherical arrangement of eight methyl groups above and below the porphyrin center forms hydrophobic fat globules.

hydrophobic porphyrin center. Eight methyl groups are forced together here, and they tend to spread on a disk or

sphere rather than in a more ordered crystal plane. The sphere enforces a statistical arrangement of the alkyl

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Komatsu et al.

Figure 3. Absorption spectra of octopus porphyrin fibers (a) 1a and (b) 1b.

Figure 2. (a) Transmission electron micrograph of monolayered platelets of 1b negatively stained with uranyl acetate. (b) Model of the porphyrin platelet corresponding to conformation 1.

chains in the neighborhood of the methyl groups, and they become independent of each other. In water the fat forms globules above and below the porphyrin center which then align as a string of pearls. The hydrophobic porphyrin centers rather than the alkyl chains lead to fiber formation. The flexible chains do, however, prevent side-on aggregation. The molecular weight of the 1a fibers was determined by comparative chromatography on agarose gel with some molecular mass standards. More than 90% of the 1a fibers appear at >2.2 × 106 Da, suggesting that aggregation of at least 500 porphyrin molecules has taken place, which is consistent with the observed fiber lengths of >500 nm. If the dispersion of the octopus porphyrins in water was carried out more gently, namely without heating and vortexing, large monolayer platelets instead of fibers were consistently obtained (Figure 2a). Colloidal solutions of these platelets were also stable for many days, before precipitation started. Similar platelets were sometimes obtained by injection of methanol solutions of the porphyrin into water. In these platelets the expected parallel

arrangement of the side chains (Figure 2b) was probably realized. A comparison of the infrared spectra of lyophilized fibers and platelets revealed more disturbed alkyl chains in the fibers, as one would expect, but the differences were not large. In the 1400-1200 cm-1 wagging band region, we cannot assign the bands because of overlaps with a strong band around 1251 cm-1 due to the antisymmetric stretching vibration of the phosphate groups. On the other hand, the bandwidths of the symmetric and antisymmetric CH2 stretching bands of the 1a fibers at 2852 and 2924 cm-1 were broadened by 5-10 cm-1 as compared to those of the platelets and by 17 cm-1 when compared to those of corresponding crystalline alkyl chains. Furthermore CH2 bending of the 1a fibers produced a broad singlet band at 1465 cm-1 whereas in platelets it was split (1469 and 1456 cm-1), which is often found in crystalline alkanes.18,19 Absorption Spectra and Exciton Coupling Model. The visible absorption spectra of the octopus porphyrin fibers are shown in Figure 3. The 1a fibers showed a Soret band at 420 nm ( ) 3.11 × 105) which was predominantly red-shifted relative to that observed in a methanol homogeneous solution of 1a (λmax ) 413 nm). The bandwidth at half-height (λ1/2 ) 19 nm) was broader than that of monomer (λ1/2) 15 nm). In contrast, the Q bands remained essentially unaltered. The absorption spectra of the octopus porphyrin fibers did not change with time (6 months) and or temperature (5-85 °C). octopus porphyrin fibers formed from zwitterionic phosphocholine chains were barely sensitive to the presence of various ionic solutes. They survived, for example, ∼0.8 M NaCl. The zinc complex 1b fibers showed very similar spectra (λmax ) 412 nm f 420 nm ( ) 4.05 × 105), λ1/2 ) 11 nm f 14 nm). (18) Cameron, D. G.; Casal, H. L.; Mantsch, H. H. Biochemistry 1980, 19, 3665. (19) Allara, D. L.; Arte, S. V.; Elliger, C. A.; Snyder, R. G. J. Am. Chem. Soc. 1991, 113, 1852.

Micellar Fibers of Octopus Porphyrin

Since local inhomogeneities in the aggregate would lead to a broadening of all absorption spectra,20,21 we suggest that the observed narrow line widths and the shifts in Soret bands indicate exciton interaction. The small observed red-shift can be explained by the interaction of stacked and tilted porphyrin rings (tilt angle θ) at a relatively large distance. For a calculation of distance and tilt angle, we need at least one more experimental quantity. A close inspection of the aggregate Soret bands shows that they are asymmetrical in contrast to the monomer bands. A blue-shifted band of low intensity seems to be hidden under the short wavelength slopes. Under the condition that the red-shifted bands are symmetrical, we have localized the side bands at 407 and 412 nm for the 1a and 1b fibers, respectively. For the exciton calculation, we have introduced the additional assumption that the porphyrin rings are rotated by the angle R ) 45° around the fiber axis with respect to the nearest neighbors. Such arrangements, which minimize phenyl-phenyl interactions, are also found in crystal structures.22 Both the red-shifted and the blue-shifted exciton bands are allowed for the Sx and Sy transitions, and four absorption bands are expected. The real spectra show only two bands, which leads to the conclusion that two bands appear at the same wavelength. This would indicate a tilt angle close to θ ) 35°.23 The distance between the transition dipole, i.e. the distance of the porphyrin ring centers along the fiber axis, has been calculated by the nearest neighbors approximation using the total energy splitting between red-shifted and blueshifted bands24 to be 11.2 and 12.5 Å for the 1a and 1b fibers, respectively. Since exciton splitting is always symmetrical whereas the experimental splitting is not, an additional red-shift due to van der Waals interaction is apparent. The van der Waals shift amounts to 50% of the experimental red-shift (230 cm-1) for zinc porphyrinate and is negligible (23 cm-1) for free base fibers. The use of extended dipoles25 (dipole length 4.7 Å) yields a smaller dipole interaction, whereas the consideration of second and third neighbors leads to additional terms. Since both effects cancel approximately, the results of the two model calculations are essentially the same. Protonation of Octopus Porphyrin Fibers. If we assume a 7 nm diameter of the porphyrin fiber as deduced from electron micrographs and a head group diameter of 9 Å, then less than 20% of the fiber’s surface is covered by phosphocholine groups. The hydrophobic porphyrin center is therefore in direct contact with water, and water soluble reagents can approach the porphyrin periphery. They may, however, not react with the porphyrin center covered by sixteen methyl groups. This was indeed found: hydrochloric acid down to pH 1 did not react with the free base (no protonation) or the zinc complex (no demetalation). The methyl groups efficiently shield the (20) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 3, Chapter 1. (21) Kim, D.; Holten, D.; Gouterman, M. J. Am. Chem. Soc. 1984, 106, 2793. (22) Scheidt, W. R.; Lee, Y. J. In Structure and Bonding; Buchler, J. W., Ed.; Springer Velarg: Berlin, 1987; p 1. (23) The condition is that the exciton energies for the Sx and Sy transitions, V12x ) 2h-1M2r-3(1 - 3 cos2 θ) and V12y ) 2h-1M2r-3(cos R), are equal. V12 is the spectral shift from monomer absorption, h is Planck’s constant, r is the distance between the centers of the interacting transition dipoles, and M is the monomer transition dipole moment, which was calculated by integrating plots of the exciton coefficient divided by wavenumber, (ν)/ν, versus wavenumber ν and applying the equation M2 ) (9.19 × 10-3∫[(ν)/ν] dν) (M ) 8.68 and 8.05 D for 1a and 1b, respectively). (24) Hochstrasser, R. M.; Kasha, M. Photochem. Photobiol. 1964, 3, 317. (25) Czikklely, V.; Fo¨sterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207.

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Figure 4. Stern-Volmer plots of fluorescence emission quenching for octopus porphyrin fibers in water corresponding to the singlet state quenching. [Por]: 1 µM.

porphyrin center. The same holds true even for porphyrin monomers in methanol solution. In a sense, these phenomena are in agreement with our previous findings of reversible oxygen adduct formation of octopus hemes.26 Fluorescence Quenching. The octopus porphyrin fibers fluoresce strongly. The fluorescence emission intensities for the 1a and 1b fibers were slightly broadened relative to those in monomeric methanol solutions, although they showed almost the same intensity. The fluorescence excitation spectra of the fibers corresponded to each absorption spectrum. This clearly indicates that the obtained fluorescence emission originates from the aggregates and not the trace amount of monomers dissociated from the fibers. The excited singlet state lifetimes of the 1a and 1b fibers were determined to be 9.3 and 1.9 ns, which are also identical to those of the monomers and other water-soluble porphyrins.27-29 The octopus porphyrin fibers are the first porphyrin fibers which show fluorescence. This difference from other stacked and lateral assemblies is presumably related to the exceptionally large distance between the neighboring porphyrins. Fibers with a porphyrin-porphyrin distance of 5-7 Å do not fluoresce at all, and leaflets with a 7.5 Å distance9 and fibers with a 11 Å distance fluoresce strongly. We have not found any such correlation in the literature and at the present time do not know the mechanism of intermolecular fluorescence quenching in our fibers and platelets. The fluorescence intensity quenching of the octopus porphyrin fibers by added electron acceptors was then studied in aqueous solution. The corresponding SternVolmer plots (Figure 4) show a linear correlation in the concentration range from 0 to 1 mM for all the quenchers used. The hydrophobic phenyl-p-benzoquinone (PBQ) is much more efficient than the hydrophilic quenchers, 1,2naphthoquinone-4-sulfonate (NQS-) and dimethylviologen (MV2+). The sensitivities of the fluorescence toward PBQ are equal for the 1a and 1b fibers and approximately equal for the reaction with NQS-. The effect of MV2+ on the (26) Tsuchida, E.; Komatsu, T.; Arai, K.; Nishide, H. J. Chem. Soc., Dalton Trans. 1993, 2465. (27) Kalyanasundaram, K.; Neumann-Spallart, M. J. Phys. Chem. 1982, 86, 5163. (28) Harriman, A.; Porter, G.; Richoux, M.-C. J. Chem. Soc., Faraday Trans. 2 1981, 77, 833. (29) Richoux, M.-C.; Harriman, A. J. Chem. Soc., Faraday Trans. 1 1982, 78, 1873.

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free base fibers is smaller, since oxidative quenching of the porphyrin singlet state to give the radical cation is the least exergonic with MV2+. Products could not be identified because the back reactions were fast. The slopes of the Stern-Volmer plots lie between 1.1 × 104 and 2.1 × 102 M-1. They cannot be interpreted in terms of dynamic and static quenching as in homogeneous solutions.28,30,31 None of the quencher molecules caused a spectroscopic shift of the porphyrin absorption bands. There is no π-overlap between the electron accepting molecules and the donating porphyrins. Static quenching is thus excluded. The methyl group sphere cannot be penetrated by other molecules, and its radius of 4-5 Å represents the minimum distance to which quencher molecules can approach the excited porphyrin. Quenching of the fluorescence only occurs by tunneling of the excited electron through this barrier to the acceptor molecule. This process must, however, be as fast as the lifetime of the singlet state, since the quenching is not of the dynamic type. The corresponding quantitative evaluation leads to unreasonably high quenching constants (5.9 and 1.1 × 1012 M-1 s-1 for the 1b-PBQ and 1a-PBQ systems). The interpretation of the Stern-Volmer slope m must therefore take into account that the quenchers at first bind to the surface of the hydrophobic core of the fiber. The m values then represent a heterogeneous binding constant multiplied with the probability of electron transfer. The large difference between hydrophobic and hydrophilic acceptors is thus explained, and the linearity of the Stern-Volmer plots (Figure 4) indicates nonsaturated binding sites. It is tempting to compare the micellar fibers with micelles. Fluorescence quenching of probes solubilized in micelles has been thoroughly investigated mainly for low occupancy.32 The evaluation of fluorescence decay kinetics33 is widely used to characterize the micelles. Our porphyrin fibers are, however, to some extent similar to functional micelles. Mostly the photoreactive group is localized on the surface,34 in contrast to our fibers, where the porphyrin ring is somewhat remotely localized in the center. Fluorescence lifetime measurements will give a more detailed picture of the mechanisms involved. Electron Transfer Reaction from the Triplet State. Flash photolysis experiments for the octopus porphyrin fibers were carried out using nanosecond laser excitation (λex ) 532 nm). A typical oscilloscope trace of a photoexcited fibrous solution of 1a under nitrogen is shown in Figure 5a. The dark decay of the absorption change obeys first-order kinetics (τ0 ) 612 µs) and is strongly accelerated by oxygen. In the case of the 1b fibers, the decay consists of two phases but shows predominantly first-order kinetics (τ0 ) 532 µs). The transient absorption difference spectra of the 1a and 1b fibers essentially display the triplet state difference spectra of the monomeric tetraphenylporphyrin and its zinc derivative (Figure 5).35 Moreover the lifetimes are comparable to those of water soluble monomeric and dimeric porphyrins.27-29,36 The octopus porphyrin therefore provides the first example of a reasonably long-lived (30) Harriman, A.; Porter, G.; Wilowska, A. J. Chem. Soc., Faraday Trans. 2 1984, 80, 191. (31) Kalyanasundaram, K.; Gra¨tzel, M. Helv. Chim. Acta 1980, 63, 478. (32) Almgren, M. In Kinetics and Catalysis in Microheterogeneous Systems; Kalyanasundaram, K., Gra¨tzel, M., Eds.; Marcel Dekker: New York, 1991; p 63. (33) Infelta, P. P.; Gra¨tzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 70, 190. (34) Humphry-Baker, R.; Moroi, Y.; Gra¨tzel, M.; Pelirretti, E.; Tundo, P. J. Am. Chem. Soc. 1980, 102, 3689. (35) Pekkarinen, L.; Linschitz, H. J. Am. Chem. Soc. 1960, 82, 2407. (36) Harriman, A.; Porter, G.; Walters, P. J. Photochem. 1982, 19, 183.

Komatsu et al.

Figure 5. Difference spectra of octopus porphyrin fibers in water recorded at the end of the laser flash excitation (λ ) 532 nm): (a) 1a; (b) 1b. The inset in part a shows the triplet decay.

triplet state in porphyrin fibers which allows us to study the triplet photochemistry of these assemblies. The triplet quenching by externally added electron acceptors was investigated for the same quenchers as used for singlet quenching. With the exception of the 1a-MV2+ pair, an acceleration of the triplet decay was observed in all cases. The mechanism of quenching is again electron transfer from the excited porphyrin to the quencher. Products are formed here in contrast to the singlet quenching reaction. In the case of the nonreactive couple, the charge separation would be endergonic and consequently does not occur. If measured at a wavelength larger than 900 nm, the triplet state decays back to the base line. A very short lifetime of 13 µs was measured with the fiber of 1a in the presence of 0.7 mM PBQ. At 550 nm, there is a residual absorbance which decays much slower, indicating the formation of the product. At 579 nm where the ground and triplet states are isobestic, it can in fact be shown that the formation kinetics of the product is identical to the decay kinetics of the triplet state (firstorder reaction with a lifetime of 13 µs). The product can be identified as the porphyrin radical cation (P•+), whereas the second product of the charge separation, the quinone radical anion, could not be seen in the wavelength region studied. The reduced quencher is easy to detect if one uses MV2+. With 1.5 mM MV2+, the triplet state has a lifetime of 22 µs. At 558 nm, the formation of the products can be seen predominantly with the same rise time. The spectrum after total decay of the triplet state (300 µs) is characterized by three peaks at 675, 610, and 555 nm. It represents a mixture of ZnP•+ and MV•+ (Figure 6).30,37,38 We assign the 675 nm peak to the porphyrin cation radical, which is not consistent with published broad and structureless (37) Wolberg, A.; Manassen, J. J. Am. Chem. Soc. 1970, 92, 2982. (38) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J. Am. Chem. Soc. 1970, 92, 3451.

Micellar Fibers of Octopus Porphyrin

Figure 6. Difference spectrum of octopus porphinatozinc(II) (1b) fibers in water containing MV2+ (1.5 mM) recorded 300 µs after the laser flash excitation (λ ) 532 nm).

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constant can be calculated. The highest value is kqT ) 1.4 × 108 M-1 s-1 for the 1b-PBQ couple, and the lowest is 3.1 × 107 M-1 s-1 for the 1b-MV2+ couple. For the interpretation of these values, it is helpful to compare the results of the quenching reaction of a similar system in homogeneous solution. Then direct contact of the reactants is possible, and the electron transfer is very fast. The quenching constants are diffusion controlled. They vary between 1.8 × 107 and 1.4 × 1010 M-1 s-1, corresponding to diffusion of ions with opposite and like charges, e.g. ZnTMPyP4+ and ZnTSPP4-.28-30,36 In our heterogeneous system the quenching constants might be interpreted as being determined by the diffusion reactions within the fiber. The quenching reaction becomes faster, when the quencher molecule is more hydrophobic (PBQ > NQS- > MV2+). The probability of quenching the porphyrin triplet state by the quenchers (Φq) is expressed as kqT[Q]/(τ0-1 + kqT[Q]). Even in the cases of MV2+ (1 mM), Φq is nearly 1.0; the photoexcited porphyrin triplet state was almost completely quenched by MV2+. When the 1b fiber solution containing MV2+ and triethanolamine (TEA) as electron donor was irradiated, sacrificial photoreduction of the MV2+ was observed. The obtained MV•+ absorption disappeared immediately after exposure to oxygen, and the spectrum returned to the original shape completely. We presumed therefore that oxidative quenching by MV2+ is predominant under these experimental conditions. These fibers will act as an effective photosensitizer for photoreduction of H+ to hydrogen in the presence of platinum. Conclusion

Figure 7. Stern-Volmer plots of triplet state quenching for octopus porphyrin fibers in water.

spectra.37,38 The measured species is, however, not monomeric. In an experiment with sodium persulfate as electron acceptor, which does not produce any absorption, the 675 nm peak was the only one occurring in this region and has to be interpreted as absorption of ZnP•+. Furthermore, Batteas et al. have published a difference spectrum for the photoreaction between the free base tetraphenylporphyrin and MV2+ with peaks at 560, 630, and 680 nm, similar to our peaks, and interpreted it in the same way.39 Stern-Volmer plots of the relative triplet lifetime (τ0/τ) versus quencher concentration are shown in Figure 7. Straight lines are obtained in all cases. As in the case of singlet quenching, PBQ is the most efficient quencher, and MV2+ is the least effective, but the differences are small. And again the 1b fibers tend to be more sensitive than the free base 1a fibers. From the slope, the quenching (39) Batteas, J. D.; Harriman, A.; Kanda, Y.; Mataga, N.; Nowak, A. K. J. Am. Chem. Soc. 1990, 112, 126.

The string of porphyrin fat droplets held in solution by a few separated phosphocholine C18 side chains reported here provides some unique properties: (i) the porphyrin chromophores are 11 Å apart, and bimolecular radiationless decay is thus prevented; (ii) the bulky methyl groups in the center provide a good adsorbent but not a solvent for hydrophobic quencher molecules, and they prevent the attack of ions on the porphyrin center; (iii) water soluble quencher molecules can also approach the porphyrin periphery because the fiber is loosely packed, which then (iv) allows sacrificial electron transport from amines to dimethylviologen. The fiber is thus photoreactive with both hydrophobic and hydrophilic systems. Work on vectorial photoreactions from the hydrophobic center to hydrophilic systems in the periphery is currently being undertaken. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research (No. 05403028, 05236103) from the Ministry of Education, Science, Sport and Culture, Japan, and by the Deutsche Forschungsgemeinschaft (SFB312, Vectorial Membrane Processes), the Fonds der Chemischen Industrie, and the Fo¨rderungskommission of the Free University Berlin. T.K. thanks the JSPS Postdoctoral Fellowships for Research Abroad. LA960409V