J. Phys. Chem. B 1998, 102, 6759-6765
6759
Monolayer Assemblies Made of Octopusporphyrins with Pyridinium Headgroups: Electron-Transfer Reactions in Noncovalent Porphyrin-Quinone Platelets in Aqueous Media Teruyuki Komatsu,† Tetsuya Yanagimoto,† Eishun Tsuchida,†,‡,* Ulrich Siggel,§ and Ju1 rgen-Hinrich Fuhrhop¶ Department of Polymer Chemistry, AdVanced Research Institute for Science and Engineering, Waseda UniVersity, Tokyo 169-8555, Japan, Max Volmer Institut fu¨ r Physikalische Chemie der Technischen UniVersita¨ t Berlin, Strasse des 17, Juni 135, D-10623 Berlin, Germany, and Institut fu¨ r Organische Chemie der Freien UniVersita¨ t Berlin, Takustrasse 3, D-14195 Berlin, Germany ReceiVed: December 16, 1997; In Final Form: June 3, 1998
Water-soluble tetraresorcinolporphyrins with eight ω-pyridinium alkyl chains (octopusporphyrins) formed fluorescent and nonfluorescent monolayer assemblies by anion exchange of the headgroups. Electron microscopy of the evaporated solution of the octopusporphyrin having 11-pyridinium-undecanoyl side chains (1) with sodium perchlorate showed planar sheets. The uniform thickness of the layer was 4.0 ( 0.5 nm, corresponding to monomolecular platelets. An exciton calculation on the basis of the red shift of the Soret band (6 nm) is consistent with a two-dimensional arrangement with porphyrin separations of 25.6 and 17.4 Å in the x and y directions, respectively. Organic dianions such as anthraquinone-2,6-disulfonate (AQDS2-) were more effective than perchlorates or iodides for aggregate formation. Arrays of a 1:3 complex of 1/AQDS2produced a curvature to yield nonfluorescent vesicles. The introduction of dimethyl groups at the bottom of the alkyl chains (octopusporphyrin 2) did not lead to enhanced aggregation, while the octopusporphyrin with long 2,2-dimethyl-C20-pyridinium chains (3) formed fluorescent fibers without assistance of special anions. The electron-transfer reactions of the 1 platelets with perchlorates, in which the relative fluorescence intensity was 30% of the monomer, were investigated. External addition of negatively charged electron acceptors, 1,2-naphthoquinone-4-sulfonate (NQS-) and anthraquinone-2-sulfonate (AQS-), led to partial quenching of the fluorescence of the central porphyrin layer. In both cases, the corresponding Stern-Volmer plots showed plateaus at sufficiently high concentration of the quinones. The results have been evaluated using equations derived for this special case of dynamic quenching by an electrostatically bound quencher. Binding constants of 3.4 × 104 and 1.7 × 105 M-1 and electron-transfer constants of 5 × 109 and 1.3 × 109 s-1 have been calculated for NQS- and AQS-, respectively.
Introduction Vectorial photoinduced electron- and energy-transfer reactions in micelles and phospholipid membranes have been a topic of great interest during the last 2 decades.1-6 In these synthetic systems, sensitizers of the cited reactions, e.g., amphiphilic porphyrins, were incorporated into the host structures with a low molecular guest-to-host ratio. It has been recently found that some porphyrin amphiphiles are self-organized in aqueous media to form supramolecular assemblies (fibers,7-10 vesicles,11,12 and tubules8), in which the highly ordered porphyrin arrays can act as redox centers for light energy conversion and as oxygen binding sites for O2 transport. Amphiphilic porphyrins are now attractive building blocks to form functional molecular architecture in bulk aqueous solution. Monomeric porphyrins may exhibit high quantum efficiency in their photochemistry. The photoexcited states of the porphyrin aggregates, however, are †
Waseda University. CREST investigator, Japan Science and Technology Corporation. * To whom correspondence should be addressed. FAX, +81 3-32054740; E-mail,
[email protected] § Technischen Universita ¨ t Berlin. ¶ Freien Universita ¨ t Berlin. ‡
mostly quenched by additional nonradiative pathways. Thus fluorescence and the triplet state are practically totally quenched in stacked and lateral fibers made of proto- and mesoporphyrins.8,10 The fibers of tetraresorcinolporphyrin with eight ω-phosphocholine alkyl chains, on the other hand, are strongly fluorescent and have the triplet state populated.7 Bilayer vesicle membranes formed by tetraanilinoporphyrin with four doublealkyl chains11 and a J-aggregate of picket-fence porphyrins represent an intermediate case.13 In this paper, we report on a new monolayered porphyrin assembly with similar optical properties. It is the rare case of platelets made of tetraresorcinolporphyrins with eight ω-pyridinium alkyl chains (octopusporphyrins). Formation of aggregates is caused by special anions which are necessary to couple the positive headgroups. The regular structure makes exciton calculations feasible. The saturation behavior of fluorescence quenching by electrostatically bound quinones yields rate constants for the electron transfer between the central porphyrin layer to the quinones incorporated into the surface (porphyrin-quinone platelets). Results and Discussion Synthesis. The condensation reaction of ω-bromoalkylated tetraresorcinolporphyrins with pyridine gave ω-pyrdinium alkyl-
S1089-5647(98)00529-X CCC: $15.00 © 1998 American Chemical Society Published on Web 08/11/1998
6760 J. Phys. Chem. B, Vol. 102, No. 35, 1998
Komatsu et al.
SCHEME 1
ated porphyrins quantitatively. Zinc ions, for example, were easily introduced to 5,10,15,20-tetrakis{2,6-bis(11-bromoundecanoyloxy)phenyl}porphyrin, which is a precursor of octopusporphyrin 1, by stirring with methanolic Zn(OAc)2. This result is in contrast to the fact that octopusporphyrin 2 could not be metalated under the same conditions. The 2,2-dimethyl groups at the bottom of the chains prevented the contact of zinc ion with the porphyrin center, as we reported previously for octopusporphyrins with longer chains.7 Morphology of Octopusporphyrin Assemblies. Octopusporphyrin 1 was readily dispersed in water, when the counteranions were bromides. The UV-vis absorption spectrum of the aqueous solution (e1 × 10-4 M) was almost equal to that of the monomer solution in methanol and showed neither enhanced light scattering nor definite structures in electron micrographs. Addition of an excess amount (200-fold in terms of moles) of sodium perchlorate, a chaotropic anion, to this solution produced the expected turbidity. Electron micrographs of the negatively stained colloids showed planar sheets (Figure 1a). The membrane thickness was estimated to be 4.0 ( 0.5 nm corresponding to a monomolecular length of 1. In these platelets, the parallel arrangement of the side chains was presumably realized. The turbidity increased very slowly and small precipitates were observed after a week at room temperature. Water soluble R,ω-bis(paraquat)-C40-bolaamphiphiles also formed vesicular membranes and molecular crystals by stepwise anion exchange.14 Self-assembly of octopusporphyrin 1, on the other hand, needed a relatively high concentration of ClO4-, because of its hydrophilic property. Iodides also should cause a similar aggregation but were actually less effective.
Figure 1. Electron micrographs of negatively stained samples of the octopusporphyrin (1) assemblies: (a) planar sheets with NaClO4 (200fold moles), (b) monolayered crystals with NaClO4 (2 × 103-fold moles), (c) nonfluorescing spherical vesicles with AQDS2- (3-fold moles), (d) platelets with AQDS2- (4-fold moles).
Addition of a still larger excess amount of sodium perchlorate (2 × 103-fold) to the monomeric solution of 1 led to turbidity immediately, and electron micrographs showed highly organized planar porphyrin crystals (Figure 1b). The thickness was again 4.0 ( 0.5 nm, indicating monolayer crystals.
Octopusporphyrins Organic dianions such as anthraquinonedisulfonates (AQDS2-) were more effective reagents for aggregation of octopusporphyrins compared to perchlorates or iodides. When a 3-fold excess of AQDS2- was added to an aqueous solution of 1, we obtained nonfluorescent spherical vesicles (Figure 1c). AQDS2was obviously effective on one side of the porphyrin planes, thus producing a curvature. Accordingly, the addition of one more molecule of AQDS2- per porphyrin caused the transformation of the vesicles into planar sheets (Figure 1d). The thickness of the layer was now 8.6-14 nm, which is thicker than that of the sheet with perchlorate anions. This hybrid membrane presumably consists of double or triple layers of porphyrins held together by intercalated quinone molecules. The necessity of a bifunctional molecule also follows from the fact that anthraquinone-2-sulfonate (AQS-) as an organic monoanion did not lead to any formation of a precise structure. Porphyrin tetraanions, e.g., tetrakis-4-sulfonatephenylporphinatocopper(II) (CuTPPS4-), are also interesting candidates for constructing a supramolecular architecture with cationic octopusporphyrins. Addition of CuTPPS4- to 1 in an equivalent amount gave relatively stable colloids, but only small micelles (diameter, ca. 15 nm) were observed by electron microscopy. We had expected that an octopusporphyrin with 2,2-dimethyl groups at the bottom of the chains (2) would give uniform micellar fibers in water by self-organization. This expectation originates from our results with octa(ω-phosphocholineeicosanoyl)resorcinolporphyrin, an octopusporphyrin with longer chains, which formed beautiful monomolecular fibers of 0.5 µm length.7 Eight methyl groups are forced together on the porphyrin plane and form a hydrophobic sphere around the porphyrin center. We have advanced the hypothesis that the interaction of these hydrophobic porphyrin centers is mainly responsible for the formation of the fibers, in which the porphyrins are stacked with some inclination with respect to the fiber axis and with the possibility of attracting van der Waals forces between the methyl groups. In contrast to our expectation, octopusporphyrin 2 did not show any effects on aggregation. No structures were observed under the electron microscopy, nor did the absorption spectrum show an exciton shift of the Soret band, not even at high concentration (10-4 M) and high temperature (80 °C). Obviously the hydrophobic interaction of the porphyrin centers is not sufficient to promote aggregation. We should conclude that the hydrophobic effect of the eight side chains is decisive. In compound 2, the relatively short C10 chains do not yield a sufficiently large term of negative entropic free energy in order to realize fiber formation. This is in line with our most recent finding that an octopusporphyrin with long C20-2,2-dimethylpyridinium chains (3) forms fluorescent monomolecular fibers (EM picture is not shown). Excitonic Interactions in Absorption Spectra of the Platelets. In the UV-vis. absorption spectrum of the octopusporphyrin 1 platelets with perchlorate anions, the Soret band appeared at 422 nm (max 1.5 × 105 M-1cm-1), which was redshifted relative to that observed with bromide anions (λmax 416 nm) (Figure 2). The bandwidth at half-height (∆λ1/2 ) 22 nm) was larger than that of the monomer (∆λ1/2 ) 16 nm). The Q-bands showed even smaller red shifts of 3 and 2 nm for Qy(1,0) and Qx(1,0), respectively. The red shift of the Q-bands (58-115 cm-1) is a van der Waals shift, caused by the replacement of solvent by porphyrin molecules. The larger red shift of the Soret band (342 cm-1) should include some exciton interaction, because the positions of the porphyrin molecules should be regular in the platelets. Gouterman has interpreted
J. Phys. Chem. B, Vol. 102, No. 35, 1998 6761
Figure 2. Visible absorption spectra of octopusporphyrin (1) in aqueous solution: (a) with counteranions Br- (monomer) and (b) with 200fold moles of NaClO4 (planar sheet).
Figure 3. Planar lattice of the porphyrin molecules as a model for the monomolecular sheets of 1. The represented porphyrin molecules are tetrakis{2,6-bis(acetoxy)phenyl}porphyrin as simplified model.
the red shifts in the Soret band of covalent free base porphyrin dimers as a solvent shift.15 He pointed out that the existence of tautomers with respect to the position of the protons at the pyrrole rings would lead to a variety of different exciton interactions and consequently to no exciton shift. We have, however, evidence for an alignment of the protons in free base porphyrin fibers, so as to give maximal exciton effects.16 We think that such an alignment also takes place in the platelets of the free base octopusporphyrin. We have based our exciton calculation on a space-filling molecular model. The tightest packing of the octopusporphyrins is realized by a rhomboidal lattice with diagonal distances of 25.6 Å for 2a and 17.4 Å for 2b (Figure 3). Alternatively, it may be interpreted as a rectangular lattice with 2a and 2b as sides of the unit cell and an additional molecule in the center of the unit cell. This arrangement is actually observed in the pure triclinic crystal of the tetraphenylporphyrin.17 The exciton interaction between any two porphyrins is given by
V1n ) M2(1 - 3 cos2 θ)/r1n3
(1)
6762 J. Phys. Chem. B, Vol. 102, No. 35, 1998 where r1n is the distance between porphyrins 1 and n, and θ is the angle between transition moment 1 and the connecting vector to moment n. In principle, we used the method of Hochstrasser and Kasha,18 summing all the interactions in the plane, but we stopped the summation at a distance of some 100 Å of the molecule considered. Thus we took into account the interaction with 48 neighbors. In a rectangular lattice, the exciton interaction V is different for the two Soret transitions Sx and Sy. In our special case, the difference turned out to be very small. With M2 ) 61.4 D2 for the dipolar strength of the monomer, V was calculated to be -416 and -334 cm-1 for Sx and Sy, respectively. The wavelength difference between the corresponding transitions is much smaller than the halfwidth of the Soret band of the monomer. Consequently, the splitting of the Soret band will not be seen. The result is one exciton band, red-shifted by 375 cm-1 with respect to the monomer and slightly broadened (by 2 nm). This calculated band shift is almost the same as the experimental value (larger by only 33 cm-1). A calculation using extended transition dipoles instead of point dipoles would probably lead to slightly lower exciton interaction.19,20 In conclusion, the aggregate spectrum seems to be describable by exciton interaction. The real lattice of the platelets might be more rectangular to account for the greater broadening of the band (6 nm compared to 2 nm in our calculation). Fluorescence Quenching. The fluorescence intensity of the octopusporphyrin 1 platelets (after 6 h at room temperature from the addition of 200-fold moles of NaClO4) was ca. 30% of the aqueous monomer solution. The fluorescence excitation spectra of the platelets corresponded to the absorption spectrum, clearly indicating that the fluorescence emission obtained at 645 nm came from the platelets and not from any trace of monomers dissociated from the aggregates. The decrease in fluorescence intensity suggests an acceleration of the nonradiative decay from the first excited singlet state probably due to vibrational interaction in the assembly. We also studied the quenching of fluorescence intensity by external electron acceptors. If we used a negatively charged quencher, such as 1,2-naphthoquinone-4-sulfonate (NQS-), we expected binding to the positively charged pyridinium groups of the porphyrin. The quenching of the monomer fluorescence was characterized by a nonlinear Stern-Volmer plot with upward curvature (Figure 4). This indicates static quenching, as expected. The initial slope m0 ) 3.3 × 103 M-1 should be equal to the binding constant of the quinone and indicates relatively weak binding. In contrast to this result, the fluorescence quenching of the colloidal solutions of 1 platelets by NQS- was more effective. The Stern-Volmer plot was now linear up to 0.7 mM and saturated at higher quencher concentration. The slope was 2.4 × 104 M-1, which is seven times larger than that for a homogeneous solution of the monomer. The quencher anthraquinone-2-sulfonate (AQS-) yielded an even greater slope of 5.2 × 104 M-1 but showed constant fluorescence intensity from 0.1 mM concentration (quenching level 5.2 compared to 17.7 for NQS-). For the quantitative evaluation of these effects, we cannot use the formula for static or dynamic quenching in homogeneous solutions, in which saturation does not occur. The qualitative interpretation has to consider the binding of quencher molecules in the presence of a surface potential of the platelets, caused by the positive charges on the planes. The surface concentration of negatively charged ions will be enlarged, giving rise to apparently enhanced binding. The higher apparent binding constant of AQS- compared to NQS- is probably a result of
Komatsu et al.
Figure 4. Stern-Volmer plots of fluorescence intensity quenching for octopusporphyrin (1) in water: excitation at 519 nm and emission at 644 nm. The solid lines are connecting the measured points and have been drawn to evaluate the initial slopes. Equation 2 describes the initial slope and saturation level only. Saturation can be calculated to occur at much higher quencher concentration than measured. For NQS-, F0/F would be 10 for cq ) 0.8 mM.
deeper insertion of the molecule into the platelet surface because of its more hydrophobic nature. The attainment of a maximal quenching level indicates, of course, that there is a limited number of binding sites on the sheet surfaces. If all of them are occupied by quencher molecules, no further quenching is possible. The persistence of some fluorescence intensity indicates that the quenching is not of static nature. The binding of the quencher molecules to the pyridinium surface takes place at a distance of some 10 Å from the porphyrin planes. The electron transfer from the excited state of the porphyrins occurs by tunneling through the layer of alkyl chains to the quinone acceptors in the surface layer. This process takes time and there is competition with fluorescence, internal conversion and intersystem crossing, similar to that in homogeneous dynamic quenching. The ratio of rate constants for electron transfer and competing processes determines the level of attainable quenching. Thus, because of this competition and despite binding of the quencher, the quenching process is dynamic in nature and the corresponding rate constant is of first order as in covalent sensitizer-quencher systems. In the quantitative formulation of the phenomena, we described the binding of the quencher by an adsorption of the Langmuir type and did not explicitly include the surface potential. In this approximation, we obtained:
F0 /F ) 1 + cq(1 - a)K/(1 + Kcqa)
(2)
with 1/a ) 1 + ke/Σki. The ke is the rate constant for the electron transfer, and Σki represents the sum of the rate constants for all the competing processes. K is the apparent binding constant and equal to the product of the true binding constant and a Boltzmann term depending on the surface potential, which may change in a quenching experiment; cq is the concentration of free quencher. The equation describes linear quenching with saturation. If formulated as a function of total quencher concentration, it would become nonlinear in the case of strong binding as for homogeneous static quenching. This improvement will be necessary for high binding constants when the concentration of free quencher is no longer equal to the total
Octopusporphyrins
J. Phys. Chem. B, Vol. 102, No. 35, 1998 6763
TABLE 1: Rate Constants of Electron Transfer and Binding Constants of Quencher Molecules to the Octopusporphyrin (1) Platelets with Perchlorate Anions in Aqueous Media quencher
10-9ke (s-1)
10-4K (M-1)
10-4Kcor (M-1)
NQS-
5 1.3
2.5 6.3
3.4 17
AQS-
concentration. For the equation given above, the slope of the Stern-Volmer curve is:
m ) K(1 - a) ) Kke /(ke +
∑ki)
(3)
and the maximum quenching level is
(F0/F)max ) 1 + ke /
∑ki
(4)
Thus this level depends on the ratio of the rate constants of the electron transfer and the sum of the competing processes. The slope is proportional to the binding constant as in homogeneous quenching but is multiplied by the probability of electron transfer to occur. The binding constant K and the rate constant for electron transfer ke can be calculated from the experimental values of m and (F0/F)max:
ke ) [(F0/F)max - 1](1/τ0)
(5)
K ) m(F0/F)max/[(F0/F)max - 1]
(6)
The results of the calculation are given in Table 1. The electron-transfer constants were calculated under the assumption that the rate constant for the sum of all competing processes corresponding to the reciprocal singlet lifetime 1/τ0 of the aggregate is 3 × 108 s-1. This is derived from the value 1/τ0 ) 109 s-1 for the monomeric porphyrin and the reduction of the fluorescence intensity to 30% on aggregation. The rate constant for AQS- comes out to be about four times smaller than that for NQS-. A comparison of the rate constants with electron-transfer theories is only possible with assumptions for the values of reorganization energy λ and the exponential coefficient β of the electronic coupling. From the normal potential of the quinones at pH 7, E0(NQS-) ) 0.05 V and E0(AQS-) ) -0.38 V,21 and the porphyrin excited-state potential of -0.67 V for oxidative quenching, we can calculate the free energy changes to be ∆G0(NQS-) ) -0.72 eV and ∆G0(AQS-) ) -0.29 eV. If we assume a reorganization energy of 0.9 eV,22 the system would be in the normal region of the Marcus parabola and the ratio of Franck-Condon factors FC(NQS-)/FC(AQS-) would be 39, that is, much larger than the experimental ratio of the rate constants. This difference is compensated by the ratio of electron coupling which is larger for AQS-, because its van der Waals (edge-to-edge) distance to the porphyrin is some 3 Å smaller than for NQS-. With an exponential coefficient β ) 0.8,22 the ratio of electron coupling would be V2(NQS-)/V2(AQS-) ) 1/11. The calculated ratio of the rate constants ke(NQS-)/ke(AQS-) would be therefore 3.6, which is very near to the experimental value of 3.8. Thus the observed electron transfer seems to be describable by the Marcus equation. It is not possible to calculate the absolute value of the rate constants, as the maximum electronic coupling is not known. If the dependence on the distance is given by ke ) 1013e-βR, the experimental value for ke(NQS-) can be verified taking the edge-to-edge distance as R ) 9.5 Å. This is not far from the 10 Å, which is a guess from the space-filling molecular
Figure 5. Transient absorption spectra of octopusporphyrin (1) in water recorded after laser flash excitation (λex 532 nm). Aqueous momomeric 1 (X-: Br-) solution displayed typical triplet-triplet absorption pattern of tetraphenylporphyrin, but the platelet solution with sodium perchlorate showed only 4% of the monomer signals.
model, assuming that the charges of the sulfonate and the pyridinium group have the same distance from the porphyrin ring. The binding constants calculated with eq 6 are so high that an appreciable part of the quencher would be bound to the platelets. So actually a nonlinear Stern-Volmer plot is expected. We rationalize the experimental linearity by assuming a compensation of the upward curvature due to the strong binding with a downward curvature due to the decrease of the surface potential with progressive binding of the charged quencher. We obtained corrected values for the binding constants using the eq 12 of the Appendix for the initial slope of a nonlinear quenching function, which are included in Table 1. Triplet States of the Platelets. Flash photolysis experiments were also carried out with laser excitation (λex 532 nm). The transient absorption spectra of the aqueous monomeric 1 solution essentially displayed the triplet-triplet absorption pattern of the tetraphenylporphyrin (Figure 5),23 in which the dark decay obeys first-order kinetics (τT ) 1.0 ms) and is strongly accelerated by oxygen. Excitation of the octopusporphyrin platelets, however, yielded only 4% of the monomer signals in this wavelength range. If the internal conversion would be the only process to be affected by aggregation, the triplet state would be quenched to the same extent as the fluorescence, which obviously is not realized. There are two possibilities for the reduction of the triplet yield: (i) decrease in kisc, the rate constant of intersystem crossing, or (ii) increase in k0, the rate constant of triplet deactivation. Because of the very small amplitude of the triplet absorption changes, we have not been able to determine the triplet lifetime. Whitten has reported a large decrease in the triplet lifetime for 5,10,15,20-tetrakis(R4hexanamidophenyl)porphyrin J-aggregates in dilute aqueous surfactant solution (from 1 ms to
