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J. Phys. Chem. 1996, 100, 5420-5425
Aggregation of Tetraaryl-Substituted Porphyrins in Homogeneous Solution Daniel L. Akins,* Han-Ru Zhu, and Chu Guo Center for Analysis of Structures and Interfaces (CASI), Department of Chemistry, The City College of The City UniVersity of New York, New York, New York 10031 ReceiVed: May 25, 1995; In Final Form: NoVember 27, 1995X
Electronic absorption, fluorescence and fluorescence excitation spectra, and vibrational Raman spectra of selected homogeneous solution phase, free-base tetraaryl-substituted porphyrinssspecifically, tetraphenylporphine (TPPH2), tetra(p-pyridyl)porphine (TPyPH2), tetra(p-N-methylpyridyl)porphine (TMPyPH2), tetra(pcarboxylphenyl)porphine (TCPPH2), and tetra(p-sulfonatophenyl)porphine (TSPPH2)sas well as their protonated diacids are acquired and analyzed. N-protonation induces a structural change in which the freebase porphyrin is converted from a configuration in which the aryl moiety is twisted relative to the macrocycle plane to one in which it is nearly coplanar, thus promoting aggregation. However, spectroscopic investigations of the tetraaryl-substituted porphyrins reveal that coplanarity, although necessary, is insufficient for aggregation to occur. Also required is that the doubly protonated porphyrin have a net zero charge, i.e., be zwitterionic. The structural alignments of monomers in tetraaryl-substituted porphine aggregates are suggested to be similar to those of J-aggregates of cyanine dyes.
I. Introduction The structure and dynamical behavior of molecular aggregates are subjects of intense interest. Two fundamental reasons are the opportunity to study intermolecular interactions with reduced degrees of freedom and the fact that important processes such as light-harvesting and the primary charge-separation steps in photosynthesis are facilitated by aggregated species, i.e., chlorophylls.1-4 Theoretical efforts regarding aggregate systems have focused on excitonic coupling, often with the hope of clarifying biological interactions and processes. Applied research efforts have endeavored to develop and exploit artificial molecular aggregate systems for device applications, since closestacked molecular structures may possess properties suitable for superconductivity, optical frequency conversion, and information processing, transmission, and storage.5-9 Recently, there has been renewed interest in the study of Hand J-type aggregates. Molecules that are widely known to form such aggregate structures include dyes such as the cyanines10-22 and the xanthenes,23-25 as well as some polycyclic aromatics.26-31 These molecules form H- and/or J-aggregates, generally, through self-assembly in homogeneous solution, on surfaces (such as on an electrode) as well as on interfaces of artificial bilayers and native membranes. In addition, aggregate formation has been observed in various environments for chlorophylls,32 chlorins,33 and several synthetic porphyrins.34-41 Investigations of systems, such as those mentioned above, have verified, in most cases, the relationship among the dipoledipole interaction energy, V, the transition moment, M, the center-to-center separation distance between the dipoles, rcc, and geometrical factors relating to the mutual inclination, R, of the aligned monomers,26,42-48 such as that represented by the following:26
V)-
M2 (1 - 3 cos2 R) 3 rcc
(1)
In addition, Raman studies of aggregated cyanine dyes on electrode surfaces have led to a theoretical formulation of X
Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-5420$12.00/0
enhanced Raman scattering by molecular aggregates.14,16 We have developed in the literature a quantum theory derived analytical intensity expressionswhich is based on a different enhancement mechanism than that used for surface-enhanced Raman scattering (SERS)sand explained the enhancement of vibrational Raman band intensities for aggregated molecules (viz., “aggregation-enhancement Raman scattering”) in terms of an increased-size effect and near resonance terms in the polarizability.14,16 The former effect is associated with the polarizability of the aggregate deriving from additivity of single molecule polarizabilities, while, for all but a small number of “broad” low frequency bands, the latter is attributable to small energy separations between the molecular aggregate electronic state (viz., the molecular excitonic state)49-51 and other electronic states.14,52 Moreover, the theory allowed us, in the case of 1,1′-diethyl2,2′-cyanine with iodide or chloride counterion (hereinafter referred to as 2,2′-cyanine, without reference to the halide), to assign the origins of two “broad” low-frequency bands at 232 and 278 cm-1. These bands, which appear when excitation resonant with the exciton state is used, have half-widths nearly twice that of the intramolecular modes and are attributed to absorption resonances and the existence of nonvanishing overlap integral products involving intramolecular (single molecule) wavefunctions and intermolecular wavefunctions (e.g., excitonphonon modes).52,53 Evidence in support of the theoretical formulation concerning enhanced Raman scattering by molecular aggregates has recently been provided through a unique aggregating system. Specifically, we have shown that free-base tetrakis(p-sulfonatophenyl)porphine (TSPPH2) aggregates in homogeneous aqueous acidic solution.54 Also, in addition to the observation of a red-shifted, sharp absorption band at 490 nm, a dramatic resonance enhancement of Raman bands at 242 and 316 cm-1 is found (see below and ref 54). The fact that aggregation occurs in a homogeneous system obviates concern of whether SERS is active. Moreover, utilization of Raman spectra of porphyrins has the important benefit that vibrational mode assignments have been extensive, providing an opportunity, unlike in the case of the cyanine dyes, for assessment of which motions in the molecule are most active in the enhancement process. © 1996 American Chemical Society
Aggregation of Tetraaryl-Substituted Porphyrins
J. Phys. Chem., Vol. 100, No. 13, 1996 5421 substituents of vicinal porphines, thus, further facilitating aggregation. Regarding this last point, as will be discussed in detail below, we have not detected aggregate formation for N-protonated porphines that have zero or positively charged aryl substituents, e.g., TPPH2, TPyPH2 (zero charge), or TMPyPH2 (positive charge), respectively. II. Experimental Section
Figure 1. Structures of tetraarylporphines.
Despite what is known about excitonic interactions in molecular aggregates, as a result of studies such as those mentioned above concerning both electronic and vibrational spectroscopic theory as well as experiments, the mechanisms of aggregate formation and their geometrical structures are of continuing interest. In order to shed more light on mechanistic and structural details of the aggregation phenomenon, we have chosen to investigate aggregation of tetraaryl-substituted porphyrins (Figure 1). In general, compounds of this nature have been well characterized, since their electronic structures have been measured using UV-vis absorption and fluorescence spectroscopies,55 their vibrational structures have been determined through Raman spectroscopic investigations,56,57 and information concerning the molecular geometry of typical representatives, such as tetraphenylporphine (TPPH2) and tetra(p-N-methylpyridyl)porphine (TMPyPH2), is available from crystallographic measurements.58 In addition, since aggregates of porphyrin derivatives are known to play central roles in biological events, such as photosynthetic light energy conversion, oxygen transport, and biocatalysis, the tetraaryl-substituted porphyrins represent model systems for gaining insight on the design of molecular devices. Furthermore, a recent study from this laboratory regarding aggregate formation of TSPPH2 54 suggests that tetraarylporphine derivatives are suitable model systems for elucidating mechanistic as well as structural aspects of molecular aggregation in homogenous solution. In the present paper we show that the primary step in the transformation of free-base tetraarylporphines to aggregates is the protonation of the imino nitrogens of the two pyrroleninelike rings. This protonation induces twisting from the near perpendicular orientation of the meso-aryl substituents to an essentially coplanar conformation relative to the mean plane of the porphinato macrocycle, as indicated by reported crystallographic data for TPPH42+ and TMPyPH42+.58 This reorientation of the meso-aryl substituent is expected to significantly reduce steric hindrance, which might limit close contact between porphinato macrocycles. Moreover, N-protonation creates positively charged sites in the protonated porphinato macrocycle, which promotes electrostatic attraction with negatively charged
Tetraphenylporphine (TPPH2), tetra(p-pyridyl)porphine (TPyPH2), tetra(p-N-methylpyridyl)porphine (TMPyPH2), tetra(p-carboxylphenyl)porphine (TCPPH2), and tetra(p-sulfonatophenyl)porphine (TSPPH2) (see Figure 1) were purchased from Porphyrin Product Inc. (Logan, Utah) and used without further purification. Solutions were prepared with spectrograde dichloromethane for TPPH2 and TPyPH2 and with deionized water for TMPyPH2, TCPPH2, and TSPPH2. Protonation of these freebase porphyrins was accomplished by dropwise addition of trifluoroacetic acid (CF3COOH) and monitored by visual color change as well as by the appearance of an absorption band, which was red-shifted relative to that of the parent free-base porphine. Electronic absorption and fluorescence excitation spectra were acquired using a Perkin-Elmer Lambda 3B UV-vis spectrophotometer. Fluorescence spectra were acquired using a SPEX Fluorolog-τ2 spectrofluorometer. Raman spectra were acquired with a 0.6 m SPEX 1877 spectrometer coupled to a SPEX Spectrum-1 CCD detector cooled to 140 K with liquid nitrogen. Vibrational Raman spectra were acquired by placing the sample solution in a Pyrex capillary and exciting with radiation from a Coherent 899 dye laser pumped by a Coherent Innova 200 Ar+ laser, using stilbene as the laser dye. Raman spectra, in general, were acquired in one accumulation of 5 s duration, and reported spectra have been refined by background subtraction, using IGOR from Wavematrics. The spectral resolution was set at ca. 2 cm-1. III. Results and Discussion A. Absorption Spectra and Their Changes Induced by N-Protonation. Absorption spectra of solutions of the five freebase tetraaryl-substituted porphines used in the present study are shown in Figure 2, together with spectra of the same solution containing trifluoroacetic acid. From these spectra, we note that upon addition of trifluoroacetic acid (10 vol. %), the spectral pattern changes from the four Q-band spectrum, indicating D2h symmetry for free-base porphine, to a two Q-band spectrum, indicating D4h symmetry, characteristic of porphyrin coordinated to a metal ion through the four N-heteronuclei. In addition, in all cases, the intense Soret band is red-shifted (to an extent dependent on the particular meso-substituents). Moreover, it can be discerned that an additional intense absorption band, shifted even farther to the red, is found for the negatively charged tetraarylporphines (i.e., bands appear at 467 nm for TCPPH2 and at 490 nm for TSPPH2). The change in spectra upon addition of trifluoroacetic acid can generally be attributed to the attachment of protons to the two imino nitrogen atoms of the pyrrolenine-like ring in the free-base.55 Indeed, the N-protonation-induced red-shifts are consistent with frontier molecular orbital calculations for protonated porphyrins.59,60 However, the occurrence of the second, more red-shifted, intense band for TCPPH2 and TSPPH2 is not easily rationalized in term of protonation of the monomeric species alone. We, in fact, attribute these bands, as discussed later, to the aggregate formed upon protonation of the porphine monomers.
5422 J. Phys. Chem., Vol. 100, No. 13, 1996
Figure 2. Absorption spectra: (A) TPPH2 and (B) TPyPH2 in dichloromethane and (C) TMPyPH2, (D) TCPPH2, and (E) TSPPH2 in water. Solid line and dashed line spectra relate to the absence and presence, respectively, of trifluoroacetic acid (10 vol. %).
B. Emission Spectra and Their Changes Induced by N-protonation. Fluorescence spectra excited at Soret frequencies for the five tetraarylporphines in solutions, with and without trifluoroacetic acid, are shown in Figure 3. In the absence of trifluoroacetic acid, the five tetraarylporphines exhibit emission spectral patterns typical of free-base porphines, i.e., the presence, in general, of two red-shifted emission peaks, Q(0, 0) and Q(0, 1), which are the mirror images of their respective absorption bands at Q(0, 0) and Q(1, 0); see Figure 2. In the case of TMPyPH2 (part C of Figure 3), the two emission bands strongly overlap and appear broader than those of the other porphyrins. Also, their relative intensity is found to be strongly pH dependent. Furthermore, for all the porphyrins, the excitation spectra (not shown) of both emission bands are the same and coincide with the respective absorption spectrum. Upon addition of trifluoroacetic acid (10 vol. %) and excitation at the peak wavelength of the red-shifted absorption bands of TPPH2, TPyPH2, and TMPyPH2 (Figure 2), broad emission bands (“dashed line” spectra in Figure 3) are found with maxima located between the two emission bands originally observed before addition of trifluoroacetic acid. The asymmetrical shapes of these emission bands suggest that there is more than one emitting species. However, this prospect can be discounted, since excitation spectra of these bands, using various detection wavelengths that cover the same spectral region as that of the emission bands, coincide with the redshifted absorption band of the corresponding protonated porphine; see Table 1. In the case of TCPPH2 and TSPPH2, a different picture emerges in regard to the emission spectra when trifluoroacetic acid is present. We find, for example, that excitation of an
Akins et al.
Figure 3. Fluorescence spectra: (A) TPPH2 and (B) TPyPH2 in dichloromethane and (C) TMPyPH2, (D) TCPPH2, and (E) TSPPH2 in water. In parts A, B, and C, solid line spectra (for excitation wavelengths of 414, 413 and 421 nm, respectively) and dashed line spectra (for excitation wavelengths of 433, 400, and 441 nm, respectively) relate to the absence and presence, respectively, of trifluoroacetic acid (10 vol. %). In parts D and E, the solid line spectra result when no trifluoroacetic acid is present. In part D, the open circles (a) and solid circles (b) correspond to fluorescence resulting from excitation at 439 and 467 nm, respectively, when trifluoroacetic acid is present (10 vol. %). In part E, the open circles (a) and solid circles (b) correspond to fluorescence resulting from excitation at 432 and 490 nm, respectively, when trifluoroacetic acid is present (10 vol. %).
TABLE 1: Soret Absorption Bands of Free-Base Tetraarylporphines (TxPPH2) and Their Protonated Diacids (TxPPH42+)
porphines
free-base TxPPH2 λ (nm)
PH2 TPPH2 TPyPH2 TMPyPH2 TCPPH2
389 414 413 421 412
TSPPH2
421
a
protonated diacid TxPPH42+ λ(nm) ∆λ (nm) 394 (sh) 433 440 441 414 469a 432 490a
5 19 27 20 2 57 11 69
Assigned to aggregated diacid.
aqueous solution of TCPPH2 containing (10 vol. %) trifluoroacetic acid, at the peak of the red-shifted absorption at 414 nm, gives rise to an emission band quite similar to that which occurs for the nonacidic solution, although there does exist a distinguishable long wavelength “shoulder”. Moreover, this long wavelength “shoulder” is found to increase in relative intensity as the exciting frequency is shifted bathochromically toward the second, more “red-shifted” absorption band at ca. 467 nm (see part D of Figure 2).
Aggregation of Tetraaryl-Substituted Porphyrins
J. Phys. Chem., Vol. 100, No. 13, 1996 5423
TABLE 2: Fluorescence Wavelengths (λfl) and Corresponding Peaks in Excitation Spectra (λex) for Free-Base Porphines (TxPPH2) and Their Protonated Diacids (TxPPH42+) free-base TxPPH2
protonated diacid TxPPH42+
porphines
λfl (nm)
λex (nm)
λfl (nm)
λex (nm)
TPPH2 TPyPH2 TMPyPH2 TCPPH2 TSPPH2
647 714 646 711 659 705 642 701 642 702
417 414 421 412 413
681 691 (?) 661 661 700 680 693a 665 716a
436 437 441 437 478 432 489
a
Assigned to aggregated diacid.
The fluorescence excitation spectra at different detection wavelengths (not shown) of the broad-band emission shown in part D of Figure 3 (whose intensities are represented by “open” and “solid” circles) also suggest the presence of two emitting species. For example, the emission on the long wavelength shoulder leads to an excitation spectrum, which associates that emission with aggregated TCPPH42+ diacid (attributed as giving rise to the more red-shifted absorption in the Soret region), while the excitation spectrum of the emission near the maximum correlates with the less red-shifted peak and is attributed to monomeric TCPPH42+ diacid. For TSPPH2 (part E of Figure 3), the picture is much clearer. The emission spectrum of TSPPH2 in water with (10 vol. %) trifluoroacetic acid reveals an emission whose dominant peak’s wavelength depends on the excitation wavelength. Specifically, when the excitation wavelength is 432 nm, a dominant peak near 660 nm appears. When the excitation wavelength is near 490 nm, a very symmetrical band with a peak at ca. 700 nm appears, and at intermediate excitation frequencies, between 432 and 490 nm, both bands at ca. 660 and ca. 700 nm appear, with a regular change in the relative intensities of the two fluorescence bands. The excitation spectra (not shown) of the two emission bands (detecting emissions at ca. 660 and ca. 700 nm; see Table 2) possess peaks at 432 and 490 nm, respectively. The emission spectra, combined with the excitation spectra, for all of the tetraarylporphines suggest that upon addition of trifluoroacetic acid the tetraarylporphines are converted to their protonated forms, with the diacid having an emission band located between the wavelengths of the two emission bands of the corresponding parent free-base. However, the protonated diacids of TCPPH2 and TSPPH2, distinguished from the others by the presence of negatively charged sites at the peripheral aryl substituents, are additionally capable of self-aggregating, which results in emissions even farther to the red than emissions from the parent free-base. C. Raman Spectral Characterization of ProtonationInduced Change in the Structure of the Porphinato Macrocycle. Porphyrin aggregation has been reported to result in only weak changes in the Raman spectrum.37,40 However, we have found striking changes in the intensity pattern of Raman spectra for negatively charged tetraarylporphyrins once they are induced to aggregate. In Figure 4, Raman spectra of the freebase and the corresponding diacid tetraarylporphines for TPPH2, TCPPH2, and TSPPH2 are shown in parts A, B, and C, respectively. The spectral pattern for the various tetraarylporphines are similar, although a closer examination reveals slight frequency shifts between the unprotonated and protonated systems. For modes composed of bond vibrations as designated in Figure 1, in general, the band shifts upon protonation are as follows: (i) downshifts for bands associated with vibrational modes containing a major contribution from the Cb-Cb stretch and (ii) upshifts for bands containing a major contribution from the Ca-Cb stretch.
Figure 4. Raman spectra of the free-base and the corresponding diacid tetraarylporphines for (A) TPPH2, (B) TCPPH2 and (C) TSPPH2. The excitation wavelengths used to excite Raman spectra were 422, 432, and 488 nm in parts A, B, and C, respectively. The pH for the nonacidified solution (labeled b) was 13.4, while that of the acidified solution (labeled a) was 1.6.
However, addition of trifluoroacetic acid (10 vol. %), for vibrational modes with sizable contributions from Ca-Cm and Ca-N stretching (see Figure 1), is found to result in either upshifts or downshifts, depending on the particular band. Of course, differences in the extent of the shifts for corresponding bands are observed for porphine with different substituents, since these differences derive from differences in electronic properties of the substituentssthe general pattern of the various Raman band shifts, for a given tetraaryl substituted porphine, is consistent between the various tetraarylporphines. Moreover, the observed shifts are consistent with those found for unsubstituted porphine, PH2, for which the relevant band shifts have been rationalized as a protonation-induced structural change in the porphinato macrocycle at the two imino nitrogen atoms of the pyrrolenine-like ring. In particular, for protonated porphine, PH42+, an alternate shortening of single bonds and lengthening of double bonds of Ca-Cm occur relative to the bonds in the free-base form. Also, the Ca-N and Ca-Cb bonds are lengthened while the Cb-Cb bond is contracted.60 An additional indicator that a coplanar precursor might be required for aggregation of protonated tetraarylporphines to occur is provided by the finding of an upshift in frequency upon protonation for the Raman band near 1080 cm-1, which has been assigned to a vibrational mode associated with Cb-H bending. This upshift suggests increased steric hindrance associated with conversion of the macrocycle to a coplanar arrangement when protonation occurs. Indeed, X-ray crystallographic data of tetraphenylporphine and tetra(p-4-methylpyridyl)porphine diacids (TPPH42+ and TMPyPH42+, respectively) have indicated that these protonated tetraarylporphines possess
5424 J. Phys. Chem., Vol. 100, No. 13, 1996 essentially a planar conformation, despite the slight out-of plane tilting of their pyrrolic rings.59 Although, as mentioned above, the spectral pattern of Raman bands among the free-base forms and among the protonated forms of the tetraarylporphines are similar, a striking difference in spectral pattern exists in the low-frequency region for protonated tetra(p-carboxylphenyl)porphine and tetra(p-sulfonatophenyl)porphine diacids (TCPPH42+ and TSPPH42+, respectively) compared to the other protonated tetraarylporphines. Specifically, a substantial enhancement in intensity (relative to other bands of the same molecule) is found for the Raman bands at 236 and 314 cm-1 of TCPPH42+ and, particularly, for the 242 and 316 cm-1 bands of protonated TSPPH42+. No such enhancement is observed for the other protonated tetraarylporphines, which possess either neutral substituents, such as phenyl and pyridyl groups, or a positively charged substituent, such as in the case of tetra(p-N-methylpyridyl)porphine. This enhancement effect is rationalized as being due to a resonance with the excitonic transition created when molecules aggregate. For TCPPH42+ and TSPPH42+ the excitonic absorptions correspond to the red-shifted absorption bands at 467 and 490 nm, respectively, created when the solution is made highly acidic. An identical picture for what we term “aggregation-enhanced Raman scattering”, as applied to cyanine dyes, has been advanced from this laboratory.15-20 IV. Conclusion The conflation of UV-vis absorption, fluorescence, fluorescence excitation, and Raman studies leads us to the conclusion that free-base tetraarylporphines, regardless of the charge of the aryl substituent, are readily protonated at the two pyrroleninelike rings of the porphinato macrocycles when dissolved in strongly acidic solution containing, for example, trifluoroacetic acid (10 vol. %). In fact, N-protonation of tetraphenylporphines has also been reported by others and discussed in terms of their resultant absorption properties.55 We have found, however, that some important differences in spectral properties exist between tetraarylporphines with negatively charged, as opposed to neutral or positively charged, aryl substituents. Spectroscopic characteristics are interpreted as signaling the formation of aggregates when the negatively charged tetraarylporphines are protonated, despite the fact that no aggregation occurs for the free-base species. We are led to conclude that both a coplanar conformation of the tetraarylporphines, as a result of protonation of the inner N atoms, and the presence of negatively charged sites in the porphyrin are requirements for formation of aggregates in homogeneous solution. Both of these requirements are, in fact, intuitively obvious, since a planar conformation undoubtedly creates a favorable condition for close contact of vicinal molecular species, while negative sites can allow for strong energy-lowering interactions with positive sites. In terms of other reported studies that might support our view, it is to be noted that coplanar alignments have been shown to promote aggregation for the cyanine dyes10,11 as well as for tetrakis(amidophenyl)porphyrin.38 In the latter case, J-aggregation occurs under premicellar concentration for a porphyrin tropisomer of the “picket fence” type, with four long-chain substituents that assume an all-cis conformation due to weak steric hindrance as a result of the cofacial interaction. Additionally, our recent Raman studies on the structure of 2,2′cyanine aggregates reveal that the Raman bands attributable to C-H bending are upshifted compared to the monomer,61 which suggests that aggregation necessitates some distortion of the monomeric species to a more coplanar conformation. On the basis of the above observations for porphyrins as well as cyanine dyes, we are drawn to the conclusion that a planar
Akins et al.
Figure 5. Two possible linear J-aggregates resulting from the protonated macrocycle. These structures would be stabilized by electrostatic interaction between the negative aryl group and the positive macrocycle and by the resulting coplanar alignment of the mesosubstituted aryl moiety and the macrocycle.
conformationswhich may be inherent for the interacting species or induced through intermolecular forces, intramolecular charge transfer, or protonationsand intermolecular alignments that lead to electrostatic energy lowering are general prerequisites for aggregation to occur in homogeneous solution. As a result, one would anticipate that the molecular subunits in aggregates are arranged in a cofacial fashion with a displacement between next nearest neighbors such that oppositely charged sites might be positioned close to one another. Figure 5 indicates what typical molecular arrangements for a protonated tetraarylporphyrin such as TCPPH42+ and TSPPH42+ might be, based on our findings above. Such arrangements as in Figure 5 correspond to the head-to-tail alignment of the transition dipole moments that define a J-aggregate structure and that leads to a red-shifted, exciton absorption band. Acknowledgment. Support for this research by the National Science Foundation (NSF) under Grant HRD-9353488 is gratefully acknowledged. References and Notes (1) Pearlstein, R. M. In Photosynthesis; Amesz, J., Ed.; Elsevier: Amsterdam, 1987; pp 299-317. (2) Warshel, A.; Parson, W. W. J. Am. Chem. Soc. 1987, 109, 6143, 6152. (3) Creighton, S.; Hwang, J.-K.; Warshel, A.; Parson, W. W.; Norris, J. Biochemistry 1988, 27, 774. (4) Michel-Beyerle, M. E.; Plato, M.; Deisenhofer, J.; Michel, H.; Bixon, M.; Jorter, L. Biochem. Biophys. Acta 1988, 932, 52. (5) Hanack, M. Mol. Cryst. Liq. Cryst. 1984, 105, 133. (6) Collman, J. P.; McDeevitt, J. T.; Yee, G. T.; Leidner, C. R.; McCullough, L. G.; Little, W. A.; Torrance, J. B. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 4581. (7) Seta, P.; Bienvenue, E.; Mailland, P.; Momenteau, M. Photochem. Photobiol. 1989, 49, 537. (8) Lamrabte, A.; Janot, J. M.; Bienvenue, E.; Momenteau, M.; Seta, P. Photochem. Photobiol. 1991, 54, 123. (9) Schouten, P. G.; Warman, J. M.; de Haas, M. P.; Fox, M. A.; Pan, H.-L. Nature (London) 1991, 353, 736. (10) Sturmer, D. M.; Heseltine, D. W. In The Theory of the Photographic Process, 4th ed.; James, T. H., Ed.; MacMillian Publishing Company: New York, 1977; Chapter 8. (11) Herz, A. H. AdV. Colloid Interface Sci. 1977, 8, 237. (12) Mobius, D.; Kuhn, H. Isr. J. Chem. 1979, 18, 375. (13) Mobius, D. Acc. Chem. Res. 1981, 14, 63. (14) Akins, D. L. J. Phys. Chem. 1986, 90, 1530. (15) Akins, D. L.; Lombardi, J. R. Chem. Phys. Lett. 1987, 136, 495. (16) Akins, D. L.; Akpabli, C. K.; Li, X. J. Phys. Chem. 1989, 93, 1977. (17) Akins, D. L.; Macklin, J. W. J. Phys. Chem. 1989, 93, 5999.
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