Tuning J-Aggregates of Tetra(p-hydroxyphenyl)porphyrin by the

Apr 5, 2007 - J-aggregates of the diacid form of tetra(p-hydroxyphenyl)porphyrin (THPP) were found to be stable in nonionic micellar solution in the p...
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4342

J. Phys. Chem. B 2007, 111, 4342-4348

Tuning J-Aggregates of Tetra(p-hydroxyphenyl)porphyrin by the Headgroups of Ionic Surfactants in Acidic Nonionic Micellar Solution Xiwen Li,† Zhongliang Zheng,† Mingyue Han,† Zhangping Chen,*,‡ and Guolin Zou*,† State Key Laboratory of Virology, College of Life Sciences, and College of Chemistry and Molecular Sciences, Wuhan UniVersity, Wuhan 430072, P.R. China ReceiVed: October 30, 2006; In Final Form: February 3, 2007

J-aggregates of the diacid form of tetra(p-hydroxyphenyl)porphyrin (THPP) were found to be stable in nonionic micellar solution in the presence of trace ionic surfactant with an oxyacid headgroup. The excitation energy of exciton coupling depends systematically on the headgroups of the ionic surfactant, by which strong and weak coupling can be accomplished in the J-aggregates. The J-aggregates have two strong exciton bands corresponding to the B- and Q-bands of the protonated monomers. The total fluorescence of THPP is quenched through aggregate formation. A strong and sharply peaked resonance light-scattering signal that suggests a delocalized excitonic state was observed just slightly to the red of the absorption maximum of the J-aggregates. The overall resonance Raman intensities appeared to be stronger in the aggregates than in the monomers. In the kinetics of aggregation induced by sodium dodecyl sulfate (SDS), no characteristics of autocatalyzed reactions were observed, and there was only a logarithmic phase that lasted only several seconds.

Introduction In recent years, many efforts have been made to investigate the aggregation process and the aggregate structure of porphyrins and cyanines because of their unique properties. It is well-known that molecular dye aggregates play an important role in many technological applications and have been employed as markers for biological and artificial membrane systems.1-8 In particular, binding of porphyrins and metalloporphyrins to the simplest models for membranes (ionic surfactants) has also attracted much interest because of the possibility of understanding many of their biological and photochemical processes, such as photosynthesis, oxygen transport, oxidation-reduction, and electron transport. The interaction of water-soluble porphyrins and metalloporphyrins with ionic surfactants has been widely demonstrated.9-17 The kinetics of formation and structures of porphyrin-surfactant complexes (aggregates) are sensitive to the type of surfactant (anionic, cationic, or neutral). As natural chlorophyll aggregates in the light-harvesting proteins or chlorosomes have a strong transition dipole moment aligned in the “head-to-tail” direction,18 porphyrin J-aggregates are important for the study of the excited-state model of the organisms.19 Recently, most of the attention has focused on water-soluble tetra(4-sulfonatophenyl)porphyrin (TSPP). The appearance of TSPP J-aggregates has been reported not to depend on the nature of the positively charged surfactant or inorganic counterions (H+, K+, Ca2+, etc.). The role of these counterions is mainly to screen the negatively charge repulsion. Additionally, the TSPP aggregates observed in ionic and nonionic surfactant solutions should be qualified as premicellar porphyrin-surfactant complexes.20 Formation of colloidal particles of TSPP J-aggregates in acidic solutions under a variety of experimental conditions has been reported in several * Corresponding authors. E-mail: [email protected] (Z.C.), [email protected] (G.Z.). Tel.: +86 27 87645674. Fax: + 86 27 68752560. † College of Life Sciences. ‡ College of Chemistry and Molecular Sciences.

studies,21-26 as indicated by the appearance of a sharp intense absorption band that is shifted to the red with respect to the monomeric B-band. With respect to water-insoluble porphyrin derivative J-aggregates, almost only “interfacial aggregation” of protonated tetraphenylporphyrin and premicellar atroplsomerspecific porphyrin J-aggregates has been reported.27,28 In the present work, Triton X-100 (TX-100) micelles were used to dissolve water-insoluble tetra(p-hydroxyphenyl)porphyrin (THPP). We report evidence for the aggregation of THPP induced by trace ionic surfactant in acidic TX-100 solution and the resulting excitonic spectra, which depend on oxyacid headgroups of surfactants. An attempt is made to provide more insight into the influence of the nature of the headgroups on the tendency of THPP to aggregate. Experimental Section Sodium dodecyl sulfate (SDS, Serva, Ultra Pure, 99%), tertoctylphenoxypolyethoxyethanol (Triton X-100), and Tween-20 (Amresco, Purity > 99%) were used as received. N-Lauroyl sarcosine (SKL, Amresco, Purity > 97%) was recrystallized twice from 95% ethanol. THPP was purchased from Aldrich. Figure 1 shows their structures. All other reagents were pure analytical grade and were used without further purification. Milli-Q-quality water was used for all experiments. Stock solutions (20 mM surfactant) were prepared using Milli-Q water. Water was mixed with appropriate aliquots of nonionic surfactant stock solutions to attain the final surfactant concentrations, and the pHs were adjusted with 3 M HCl. Solubilization of THPP was achieved by injection of a small volume of THPP dissolved in 0.01 M NaOH solution followed by membrane filtration (0.2 µm, nylon-66). The concentration of THPP for all of the following measurements is 5 µM. SDS and SKL were added at the end. The conversion of the porphyrin into the various forms was directly monitored by UV-vis spectroscopy in 1-cm quartz cells at room temperature. All experiments were repeated at least three times to check for reproducibility. UVvis absorption spectra were obtained using a Cary-100 Conc

10.1021/jp067148a CCC: $37.00 © 2007 American Chemical Society Published on Web 04/05/2007

Headgroup-Deciding Porphyrin J-Aggregates

J. Phys. Chem. B, Vol. 111, No. 17, 2007 4343 (RM-1000, Renishaw, U.K.), in which the Raman scattering of the sample was excited with the 514.5-nm laser line from a Spex 2000 Ar+ laser. Samples were circulated through a glass capillary cell. The stability of each sample was verified by measuring the absorption spectrum before and after gathering the Raman and fluorescence spectra. Results and Discussions

Figure 1. Molecular structures of Tween-20, Triton X-100, THPP, and SKL.

Figure 2. Absorption spectra in aqueous TX-100 of H2THPP2+ at pH 2.2 (solid line), free base at pH 6.4 (dashed line), and THPP aggregates induced by SDS at pH 2.2 (dotted line). [SDS] ) 20 µM, [THPP] ) 4 µM, [TX-100] ) 0.4 mM.

UV-vis spectrophotometer (Varian, American), and the kinetics were measured with an SFA-20 Rapid Kinetics Accessory (Hitachi, Japan). Fluorescence and resonance light-scattering (RLS) measurements were obtained on a Hitachi F-4500 fluorescence spectrophotometer. RLS spectra were recorded by scanning simultaneously the excitation and emission monochromators (∆λ ) 0 nm) of the F-4500 spectrofluorometer from 300 to 700 nm with a slit width 5.0 nm both for the excitation and emission (PMT voltage ) 400 V). The background light scattering (less than 20 counts per second) of a solution containing only surfactant was subtracted from the lightscattering spectra of the aggregated samples. Raman spectra were measured using a confocal Raman microspectroscope

Comparison of Absorption Spectra of Three Independent THPP Samples. Figure 2 compares the visible absorption bands of THPP in different systems. THPP is water-insoluble in neutral aqueous solution, so it exists as an irregular aggregate that shows only a very broad and faint absorption peak (centered at 426 nm, not shown) in porphyrin stock solution diluted with pure water. However, in aqueous TX-100 solution, at concentrations above the critical micelle concentration (cmc, 0.3 mM), the overall absorption spectra of THPP are similar to those of THPP monomers in organic solvent such as ethanol: a strong B-band centered at 424 nm and four absorption peaks in the Q-band region (Table 1). The commonly known interaction of surfactants and organic dye molecules in aqueous solution is the process of micellization of the hydrophobic molecules in the hydrophobic region of the micelles at surfactant concentrations above the cmc.29,30 The chemical structure of TX-100 is shown in Figure 1. It consists of a long hydrophilic polyoxyethylene chain and a short hydrophobic alkyl chain. The nonuniform distribution of the hydrophilic terminations along the aggregate interface leads to a hydration layer of the micelles with a hydrophobic inner core aggregated by alkyl chains. THPP, with four hydrophilic hydroxyl groups, inserts mostly into the hydration layer and is stabilized by hydrogen-bonding and hydrophobic interactions between the porphyrin and the polyoxyethylene. However, we cannot exclude the possibility that a few porpyrin molecules insert into the hydrophobic inner core. In summary, THPP dissolves as a free base monomer in aqueous 0.4 mM TX-100 solution, because of the dissolving effects of micelles. As the pH in the above solution is adjusted to 2.2 with HCl, the outer protons will cross into the hydration layer as a result of the concentration gradient so that the porphyrin is protonated. Porphyrin inner-core protonation leads to a small red shift of the B-band at 444 nm and the Q(I) band (red shift from 654 to 689 nm). As the porphyrin is not fully protonated, the solid line spectrum of Figure 2 seems to present a shoulder peak at 424 nm. A spectrum of the completely protonated species was obtained in ethanol/HCl. Additional light-scattering experiments suggest that there is only a quite faint resonance light-scattering (RLS) signal at pH 2.2 in 0.4 mM TX-100 and ethanol/HCl, which shows two peaks at 444 and 449 nm (Table 1) representing the H2THPP2+ monomer. As 20 µM SDS is added to the above TX-100 solution, the B- and Q-bands red shift from 444 to 477 nm and from 689 to 734 nm, respectively. In the description of the aggregates formed by the free base TPPS4 with CTAB, Maiti et al.30 discussed in detail the formation of H- and J-aggregates, which are characterized by face-to-face and side-by-side arrangements of the porphyrins, respectively. In the case of J-aggregates, a red shift of the B-band

TABLE 1: Absorption and RLS Features of THPP in Various Systems at 298 K system

species

B-band

Q-band(s) (λ, nm)

RLS (λ, nm)

ethanol ethanol/0.01 M H+ water/0.4 mM TX-100, pH 6.4 water/0.4 mM TX-100, pH 2.2 water/0.4 mM TX-100/20 M SDS, pH 2.2 water/0.2 mM TX-100/30 M SKL, pH 2.2

THPP H2THPP2+ THPP H2THPP2+ H2THPP2+J-agg H2THPP2+J-agg

419 449 424 444 477 466

516, 554, 592, 651 693 522, 557, 599,654 689 734 719

492 492

4344 J. Phys. Chem. B, Vol. 111, No. 17, 2007

Figure 3. Absorption (thin line) and fluorescence (thick line) spectra of THPP in 0.4 mM TX-100 at pH 3.0 as a function of SDS concentration.

is expected, and in fact, a B-band centered at 489 nm was observed together with a Q-band at around 700 nm for TSPP in aqueous CTAB or CTAC solution. We tentatively assign the red-shifted B-band components of H2THPP2+ to J-aggregate transitions. The putative J-band and red-shifted Q-band absorption peaks signify that the assembly consists of head-to-tail aggregates (J-aggregates).27,31 Evidently, in this case, the J-aggregates are induced by added trace SDS, which is able to insert into the TX-100 micelles because of its alkyl chains. It is very possible that SDS (negatively charged) attracts protonated porphyrin and that aggregation then happens in the hydration layer of the TX-100 micelles. SDS-Dependent Absorption and Fluorescence Spectra. The SDS-dependent absorption spectra at pH 3.0 in 0.4 mM TX-100 exhibit apparent isosbestic points at about 439 nm (see Figure 3). This suggests that two forms of THPP mainly contribute to the absorption spectra. Additional experiments show that the pKa of THPP central-ring nitrogens is 2.32 in the presence of 0.4 mM TX-100. Furthermore, at a TX-100 concentration of 0.2 mM, pKa ) 3.22, and at a TX-100 concentration of 20 mM, pKa < 2.0. Therefore, the pKa decreases with increasing TX-100 concentration. However, when 20 µM SDS is added to 0.4 mM TX-100 solution, the pKa increases to 3.62. Evidently, as SDS that inserts into the TX100 micelles attracts H+, the porphyrin molecules are more prone to be protonated, and subsequent aggregation occurs. Therefore, at pH 3.0 (pH > pKa) in 0.4 mM TX-100 in the absence of SDS, a majority of the THPP monomers are not protonated. However, if 5 µM SDS is added to the above solution, the absorption spectra of THPP change a great deal: the J-band and red-shifted Q-band absorption peaks increase sharply. When the SDS concentration is increased to 10 µM, both red-shifted absorption peaks show only a slight increase, but the characteristic spectra of the free base monomers disappear completely (the B-band centered at 424 nm and the four absorption peaks in the Q-band region). Here, the minimum SDS/THPP ratio for maximum aggregate formation is close to 2:1. The SDS-dependent fluorescence spectra in the Q-band regions shown in Figure 3 also support this conclusion. As the concentration of SDS increases, the emission at 655 nm, which is identical to that of the free base monomer in organic solvents, decreases sharply, and above 10 µM, only a quite faint emission peak is observed. The quite weak fluorescence observed is from residual free base monomers. This suggests that quenching of the Q-band fluorescence is evidence that the spectral perturbations are a result of aggregation.31 Concentration-Dependent Aggregation. Figure 4 illustrates the effect of the TX-100 concentration on THPP J-aggregates

Li et al.

Figure 4. Change in maximum absorption (at 477 nm) of THPP J-aggregates induced by SDS as a function TX-100 concentration at pH 2.4. [THPP] ) 6 µM.

Figure 5. Absorption of THPP in 20 mM TX-100 at pH 2.2 as a function of SDS concentration. The THPP concentration in the solid line spectrum is 5 µM, and the other SDS and THPP concentrations are shown in the figure.

induced by SDS at pH 2.4. When the TX-100 concentration is lower than the cmc, the absorption of the J-band increases sharply with increasing concentration, which suggests that TX100 is a prerequisite to aggregation. When the TX-100 concentration increases to 0.4 mM, the J-band absorption reaches a maximum. As the concentration of TX-100 increases from 0.4 to 0.8 mM, the J-band absorption decreases slightly. At concentrations higher than 0.8 mM, the J-band absorption decreases dramatically. However, once the SDS/TX-100 ratio is readjusted to around 3:40 with SDS stock solution, all of the absorptions of J-band in the following samples increase to a maximum. Therfore, the minimum SDS/TX-100 ratio for maximum aggregate formation is around 3:40. Additional estimation of the cmc of TX-100 (based on the fluorescence mutation of cyanine dye) shows that the cmc of TX-100 changes from 0.301 to 0.309 as the SDS/TX-100 ratio is adjusted to around 3:40. Evidently, a small amount of SDS leads to only a slight change of the cmc of nonionic TX-100, so that the effect of trace SDS on the cmc of TX-100 can be ignored. Figure 5 presents the absorption spectra of THPP in 20 mM TX-100 (far higher than the cmc) at pH 2.2 as a function of the concentration of SDS. When the concentration of THPP is 5 µM and the SDS concentration is lower than 1.0 mM, THPP is partially protonated (solid line). At higher TX-100 concentrations, THPP is more prone to be dissolved in the micelles, and the number of porphyrin molecules per micelle decreases, so it is more difficult for the dissolved porphyrin to become protonated, and then the pKa of THPP decreases. Thus, in Figure 5, the peaks corresponding to H2THPP2+ appear only in the presence of SDS because SDS attracts H+ and induces protonation. When the concentration of THPP is increased to 8 µM, the result is the same (dashed line), which further supports the

Headgroup-Deciding Porphyrin J-Aggregates

Figure 6. (a) Overlay of absorption spectra of THPP in 0.4 mM Tween20 at pH 2.4 in the presence of 20 µM SDS at different times (∆t ) 30 s). Arrows indicate the time evolution of the two main contributions: monomers and aggregates. (b) Inset: Kinetic profile for the aggregation of THPP in 0.4 mM TX-100 at pH 2.4 in the presence of 20 µM SDS.

foregoing conclusion that THPP is protonated because of the inductive effect of SDS. As the SDS concentration increases to 1.5 mM (here, the SDS/TX-100 ratio also reaches 3:40), the two red-shifted peaks appear, which shows only the partial THPP form of the J-aggregates (dotted line). As the concentration of THPP increases further to 12 µM, a majority of mononers form J-aggregates (dotted-dashed line). The broad B-band results from the free protonated porphyrins. On account of dilution effect of TX-100 micelles on THPP, the increasing TX-100 concentration leads to a decrease in the number of porphyrin molecules in each TX-100 micelle. Therefore, both a minimum SDS/TX-100 ratio and a high enough relative concentration of porphyrin in the TX-100 micelles are essential for aggregate formation. Kinetics of Assembly of THPP. Figure 6b shows a kinetic profile for the aggregation of 5 µM THPP in 0.4 mM TX-100 at pH 2.4 in the presence of 20 µM SDS. Kinetic runs were conducted at 476 nm to monitor the appearance of the J-aggregates. In the kinetics of aggregate formation, the absorption intensities of J-aggregates usually show a sigmoidal time profile, suggesting a slow initial reaction (lag phase) followed by a rapid aggregation (logarithmic phase) in aqueous solution.32-34 In this process, the rate-determining step for aggregation is the formation of a “critical-size” assembly that catalyzes further growth. However, in this micelle system, no characteristics of autocatalyzed reactions are observed, and there is only a logarithmic phase that takes less than 4 s. We also measured aggregation kinetics (see Figure 6a) of THPP in 0.4 mM Tween20 at the same pH value in the presence of 20 µM SDS, where the equilibration requires several minutes. It is possible that the short and branched polyoxyethylene chains of Tween-20 in some measure result in slower occurrence of aggregation as compared to the long unbranched polyoxyethylene chains of TX-100. Figure 6 also indicates apparent isosbestic points at about 440 nm. The two red-shifted absorption peaks increase markedly at the expense of the peak of the monomeric free base, which indicates that the monomers translate into J-aggregates to a great extent after full equilibration. Headgroup-Controlled Exciton in THPP J-Aggregates. Amphoteric surfactant (SKL) can also induce formation of THPP J-aggregates. Evidently, at pH 2.4 in 0.2 mM TX-100, the headgroup of SKL, whose molecular structure is shown in Figure 1, is fully protonated, so here, SKL functions as a cationic surfactant. However, SKL can induce formation of THPP J-aggregates only at low TX-100 concentration (under its cmc). Perhaps because of the positive-charge repulsion between the

J. Phys. Chem. B, Vol. 111, No. 17, 2007 4345

Figure 7. Comparison of (a,b) RLS and (c,d) absorption spectra of THPP aggregates in 0.2 mM TX-100 in the presence of (a,c) 30 µM SKL and (b,d) 30 µM SDS at pH 2.4.

headgroups of SKL and H2THPP2+, SKL cannot induce the formation of THPP J-aggregates in TX-100 micelles. However, in TX-100 premicellar solution, Cl- is prone to contact H2THPP2+ and the headgroup of SKL, which inserts polyoxyethylene chains into TX-100 premicellar aggregates, so that the positive-charge repulsion can be effectively screened. Figure 7 displays absorption and RLS spectra of THPP aggregates induced by SDS and SKL at pH 2.4 in 0.2 mM TX-100. As shown by Pasternack and co-workers, RLS is a powerful tool for detecting the presence of electronically coupled porphyrin aggregates.22 Although any chromophore can exhibit RLS when excited within its absorption band, in the absence of aggregation, the effect is generally offset by absorption. In both cases, a strong RLS signal at 492 nm, just slightly to the red of the J-band maximum, is very similar to that already reported for TSPP J-aggregates, which suggests that the aggregates indicate very good quality of the optical resonance. The pair of redshifted absorption bands and the intense RLS signal imply a delocalized excitonic state in which the chromophores are strongly coupled electronically.21 Notably, the J-band of the J-aggregates induced by SDS shows increasing transition dipole moments and bathochromic shifts as compared to that of the J-aggregates induced by SKL (see Table 1 and Figure 7), which suggests that the coupling is stronger in the former because of the closer association of the monomer units. This comparison is in accord with the more exchanged narrow RLS peak in the former. We propose that the strong RLS signal in the latter is superimposed on a broad background resulting from a distribution of physical aggregate sizes in which the electronic coupling is relatively weak as compared to that in the former. The spectra of THPP J-aggregates are qualitatively similar to those of TSPP J-aggregates in acidic solution, but the J-band shows more exchange narrowing in the latter, which suggests that the exciton state is delocalized over more molecules and the association of porphyrin units is closer in TSPP aggregates.22 However, THPP aggregates show a more distinctive enhancement of Q-band absorption than TSPP aggregates. The striking changes in the absorption spectrum, particularly a distinctive enhancement of Q-band absorption, must be attributed to intensity transfer from the B-band region to the Q-band region that is mediated by an excitonic coupling between B and Q transition dipoles.35 The spectral characteristics of the Jaggregates in this study are also very similar to those of the J-aggregates of protonated water-insoluble tetraphenylporphyrin derivatives at the liquid-liquid or gas-liquid interface,27 both

4346 J. Phys. Chem. B, Vol. 111, No. 17, 2007 of which show two strong exciton bands corresponding to the B- and Q-bands of the protonated porphyrin. Mechanism and Structure of Aggregation. It was verified that J-aggregates were not obtained when cationic surfactant (cetyltrimethyl ammonium bromide, CTAB) was used in this system. Notably, SKL functions as a cationic surfactant because the carboxylic acid groups of SKL are uncharged at pH 2.4. Therefore, it is the oxyacid headgroups of surfactants that seem to act not only as counterions but also as important groups interacting with porphyrin J-aggregates. Additionally, when H2SO4, HNO3, or CH3COOH is used in place of HCl to adjust the pH in the system, ionic surfactants are still required for the formation of J-aggregates. Thus, only the headgroups of the ionic surfactant determine the exciton in J-aggregates of THPP, and inorganic acid is only used to protonate the porphyrin. Taking into account the distortion of the porphine plane and the rotation of the aryl groups, the protonation of the imino nitrogens of the two pyrrolenine-like rings induces a more planar structure than in the parent free base. This reorientation of the meso-aryl substituent is expected to significantly reduce steric hindrance, which might limit close contact between porphinato macrocycles. However, spectroscopic investigations of the tetraaryl-substituted porphyrins reveal that coplanarity, although necessary, is insufficient for aggregation to occur. As discussed above, nonionic micelles are a prerequisite to aggregation, and the kinetics of aggregation are different for Tween-20 and TX-100 micelles. Unlike aggregation in aqueous solution, no characteristics of autocatalyzed reactions were observed in the kinetics of aggregate formation in TX-100 micellar solution. Furthermore, both a minimum SDS/TX-100 ratio and a high enough relative concentration of porphyrin in the TX-100 micelles are essential for aggregate formation. Therefore, we consider that the assembly of THPP-SDS a complex occurs in nonionic micelles. A hypothetical model for the J-type structure in the hydration layer of a TX-100 micelle is presented in Figure 8a. In analogy with the proposed structural arrangement of zwitterionic TSPP in J-aggregates, we are inclined to think that hydrogen-bonding interactions between the peripheral hydroxyl groups and the protonated central N-H residues are bridged by the oxyacid headgroups of surfactants in THPP J-aggregates. As a result, a stable hydrogen-bonding network forms. This pronounced tendency to self-aggregate is not surprising considering the hydrophobic nature and symmetry of the porphyrin skeleton. The peripheral rigid phenyl groups can also stabilize the slipped face-to-face structure. Because porphyrin diacids and the headgroups of SDS are electriferous, the J-aggregates are prone to form and stabilize in the hydration layer of the TX-100 micelles. Transition moment coupling of strongly absorbing dyes often results in the delocalization of the excitation energy over a number of the aggregated molecules as a result of the intermolecular interactions within aggregated domains. The exciton theory developed by Kasha36 predicts that, for interacting chromophores, the dipole-dipole interaction energy V is given by

V)-

M2 (1 - 3 cos2 θ) 3 r

where M is the transition moment, r is the distance between the centers of the two dipoles, and θ is a geometric factor accounting for the mutual orientation of the aligned monomers. In the case of J-type aggregates, the dipole moments are parallel, assuming an angle θ (0 < θ < π/2) with respect to the line

Li et al.

Figure 8. (a) Schematic representation of exciton coupled J-aggregates in the hydration layer of TX-100 micelles and (b,c) sketch of a side view for the proposed ion-pair and hydrogen-bonding model for THPP J-aggregate formation through the intermediacy of the headgroups of (b) SDS and (c) SKL. Cl- and H+ are represented by - and +, respectively.

connecting the centers of the dipoles. In the TCPP J-aggregates induced by HNO3, the r value is considered to be about 8.3 Å, so that the derived interplanar distance allows for the possibility that anions are intercalated between neighboring porphyrin molecules.31 Similarly, oxyacid headgroups of surfactants can insert between neighboring THPP molecules. Furthermore, because of the nonplanar structures with mainly saddle-type distortions of the porphyrin core, direct hydrogen bonding between O-H and N-H cannot form in THPP J-aggregates. Therefore, the above molecular mechanics model for the proposed hydrogen bonding in THPP J-aggregates through the intermediacy of oxyacid headgroups is very possible. For the J-aggregates induced by SDS, the anionic headgroups (-SO4-) not only establish a hydrogen-bonding network but also contribute to charge neutralization (Figure 8b). All of these noncovalent interactions are expected to be largely enhanced in the relatively low-polarity hydration layer. In TX-100 premicellar solution, the J-aggregates can also form among polyoxyethylene chains of TX-100 premicellar aggregation in like manner. As indicated in Figure 3, the minimum SDS/THPP ratio for maximum aggregate formation seems to be 2:1 in 0.4 mM TX-100. Therefore, the model in which the SDS/THPP ratio is also close to 2:1 seems to be very reasonable. Because of the positive-charge repulsion between the headgroups of SKL and H2THPP2+, SKL cannot induce formation of THPP Jaggregates in the hydration layer. However, in TX-100 premicellar solution, the positive-charge repulsion can be effectively screened by Cl- (Figure 8c), and the proposed hydrogen bonding in THPP J-aggregates through the intermediacy of -COOH is still possible, so aggregation can also occur. Additionally, dispersive interactions should play an important role in stabilizing the overall structure.

Headgroup-Deciding Porphyrin J-Aggregates

J. Phys. Chem. B, Vol. 111, No. 17, 2007 4347 Conclusions

Figure 9. Resonance Raman spectra of 10 µM THPP solutions, excited at 514.5 nm: (a) THPP free base monomer in 0.4 mM TX-100 at pH 6.4, (b) H2THPP2+ in 0.2 mM TX-100 at pH 2.4 in the presence of 30 µM SKL, and (c) H2THPP2+ in 0.4 mM TX-100 at pH 2.4 in the presence of 30 µM SDS.

It is possible that the additional electrostatic interactions in the J-aggregates induced by SDS cause the shorter slipping distance and closer association of monomer units in the headto-tail direction as compared to those induced by SKL. As a result, the former show a higher-energy absorption and a greater red shift of the B- and Q-bands than the latter. This can be explained by the above equation in which the larger θ and smaller r values correspond to the higher V. Therefore, the strength of the exciton coupling of the transition dipole moment can be systematically changed by the headgroups, by which strong and weak coupling can be accomplished in the Jaggregates. Aggregation-Enhanced Raman Effect. Previous studies have shown that low-frequency features are strongly enhanced in Raman spectra of aggregates relative to those of monomers. Various explanations for this enhancement have been offered in the literature. Akin et al. reported that the two bands at 241 and 317 cm-1 can be attributed to out-of-plane ruffling and doming modes, respectively; they also found that these two bands are dramatically enhanced by a factor of more than 30 with respect to the corresponding two bands of diacid monomer at 233 and 310 cm-1.37,38 Kano et al.39 observed coherent oscillations in the