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J. Phys. Chem. B 2008, 112, 8134–8138
J-Aggregates of Diprotonated Tetrakis(4-sulfonatophenyl)porphyrin Induced by Ionic Liquid 1-Butyl-3-Methylimidazolium Tetrafluoroborate Jian-Jun Wu, Na Li, Ke-An Li, and Feng Liu* Beijing National Laboratory for Molecular Sciences, The Key Laboratory of Bioorganic Chemistry and Molecular Engineering, Ministry of Education, College of Chemistry, Peking UniVersity, Beijing 100871, China ReceiVed: March 21, 2008; ReVised Manuscript ReceiVed: April 30, 2008
The J-aggregation behavior of diprotonated tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS42-) in aqueous solution in the presence of the hydrophilic ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) was investigated in detail using UV-vis absorption spectroscopy, fluorescence spectroscopy, resonance light scattering (RLS) spectroscopy, Raman spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. With the addition of bmimBF4, increasing peaks appeared at a wavelength of 490 nm in the absorption spectra to account for the formation of H2TPPS42- J-aggregates. In addition, the experimental results also showed decreased fluorescence emission, enhanced RLS signals, intensified Raman scattering peaks, and the disappearance of NMR signals to further indicate that porphyrin J-aggregates exist in the studied system. NMR shifts of bmimBF4 toward high field occurred corresponding to H2, H4, and H5 in the cationic imidazolium ring (bmim+), suggesting that bmim+ enters the magnetic shielding domain of the anionic phenyl sulfonate ion owing to the association process between the “large” cation and anion. Additionally, the fact that the absorption spectral shifts occurred in the nonprotonated porphyrin TPPS44- further indicates the existence of the ion association effect of bmim+, which functions as an important factor in porphyrin aggregation. Introduction The size of supramolecular assemblies lies between the dimensions of bulk solid materials and molecules, and the assembly process is driven by noncovalent interactions including hydrogen bonding, metallic coordination, electrostatic forces, and π-π stacking. Because the assemblies have some special characteristics unlike those of the component molecules, studies on the assembly behaviors are of both theoretical and practical importance to further understand and apply supramolecular chemistry. Porphyrins are planar macrocyclic dyes that abound in nature, and porphyrin aggregation has shown great value in photodynamic cancer therapies, artificial light-harvesting devices, and nonlinear optical materials.1–3 J-aggregates consist of “side-by-side” porphyrin arrays with characteristic red-shifted absorption wavelengths, whereas H-aggregates are composed of “face-to-face” porphyrin monomers that show blue-shifted absorption.4 It is widely reported that the tuning factors of porphyrin aggregation in aqueous solution vary depending on the porphyrin structure5 and concentration,6 as well as the acidity,4 ionic strength, and counterions of inorganic salts in the media.7 In addition, some organized media including normal surfactant micelles,8 reverse micelles,9 nucleic acids,10 polypeptides,11 proteins,12 carbon nanotubes,13 spermine,14 and cyclodextrins15 are also found to be capable of influencing porphyrin aggregation behaviors. Additionally, porphyrin aggregates are reported to form in some organic solvents, which opens up more information for aggregation processes and mechanisms.16 Regardless of the factors used to obtain organized porphyrin arrays, the purpose is to change the porphyrin structures or the characteristics of their microenvironment and thus the final porphyrin forms. In supramolecular systems, it is always * Corresponding author. E-mail:
[email protected]. Tel.: 86-1062761187. Fax: 86-10-62751708.
desirable to develop more interactions between porphyrins and new media in order to shed new light on porphyrin assemblies for further potential applications in biological or material fields. As new and environmentally friendly solvents, ionic liquids are a current hot topic. Ionic liquids are composed of organic cations and inorganic/organic anions and are liquid at room temperature. Because of their unique physicochemical properties such as negligible vapor pressure, wide liquid temperature range, excellent thermodynamic stability, wide electrochemical window, tunable polarity and amphiphilicity, good solvent behavior for various materials, excellent designable space through the changing of components, and so on, they have found many applications in the fields of synthesis, catalysis, separation, and extraction to function as new media and liquid materials.17 Moreover, ionic liquids are also used in supramolecular assembly, being mixed with surfactant micelles or cyclodextrins to expand their research range, as well as potential applications.18,19 However, to the best of our knowledge, no report has focused on porphyrin aggregation in ionic liquid aqueous solutions, and it is quite promising to explore the relevant aggregation mechanism and potential technological applications of porphyrin assembly in such a green solvent. Herein, the hydrophilic ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) was used as a new medium to induce diprotonated tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS42-) to form J-aggregates in aqueous solution, as verified by light absorption, emission, scattering, and NMR measurements. In addition, the aggregation mechanism is elucidated by the ion association process between imidazolium and phenyl sulfonate ions to shed more light on the role of ionic strength during porphyrin aggregation.
10.1021/jp802482f CCC: $40.75 2008 American Chemical Society Published on Web 06/18/2008
J-Aggregates of H2TPPS42- Induced by Ionic Liquid bmimBF4
Figure 1. Chemical structures of (a) H2TPPS42- and (b) bmimBF4.
J. Phys. Chem. B, Vol. 112, No. 27, 2008 8135
Figure 2. Absorption spectra of porphyrin at different pH values: (a) 5.82, (b) 2.24, (c) 0.86 ([TPPS44-] ) 2 µM).
Experimental Methods Chemicals. Diprotonated tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS42-, Figure 1a) was obtained by adding appropriate amounts of hydrochloric acid to the nonprotonated form TPPS44-, which was purchased from Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan) with a purity of over 99.5% and prepared as a stock aqueous solution of 0.1 mM. The hydrophilic ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4, Figure 1b) was purchased from He’nan Lihua Pharmaceutical Co. Ltd. (Henan, China) with a purity of over 99.0% and prepared as a stock aqueous solution of 2 M. Sodium tetrafluoroborate (NaBF4) was purchased from Beijing Chemical Reagent Company (Beijing, China), and its purity was >99.0%. D2O was purchased from Cambridge Isotope Laboratories (Andover, MA), and the deuterium ratio was above 99.9%. All other reagents were of analytical grade and wereused without further purification. Deionized water was used for all experiments. Spectroscopic Measurements. UV-vis absorption spectra are obtained using a U-3010 UV-vis spectrophotometer (Hitachi, Tokyo, Japan). Both the fluorescence and resonance light scattering (RLS) spectra were recorded on an F-4500 fluorescence spectrophotometer (Hitachi, Tokyo, Japan), with excitation and emission slit widths of 10 nm and a photomultiplier tube voltage of 700 V. Fluorescence spectra at an emission wavelength of 664 nm were monitored at an excitation wavelength of 435 nm, and RLS spectra were recorded by scanning simultaneously the excitation and emission monochromators (∆λ ) 0) of the F-4500 spectrophotmeter. Raman spectra were measured on a LabRAM HR-UV Spectrometer (Horiba Jobin Yvon, Villeneuve d’Ascq, France). The Raman scattering signals were recorded with an excitation wavelength of 488 nm for the Ar+-laser line, and all samples were verified by measuring the absorption spectra before and after the fluorescence, RLS, and Raman spectra had been obtained to ensure the consistent state of the solutions. NMR spectra were monitored on a Varian Mercury Plus 300 MHz apparatus (Varian, Palo Alto, CA) with the internal standard of D2O (δ ) 4.67). Stock solutions for NMR measurements were also prepared to control the pH conditions using hydrochloric acid, according to procedures similar to those used in ref 20. The acidity was recorded with an 868 pH meter, which is produced by Thermo Orion (Waltham, MA). Results and Discussion Porphyrin J-Aggregation in Strong Acidic Media. Porphyrins tend to self-assemble through π-π stacking interactions because of their hydrophobic porphine rings, whereas charged substitutent groups are repulsive and hinder aggregation in the
Figure 3. Influence of bmimBF4 concentration on the absorption spectrum of H2TPPS42-: [bmimBF4] ) 0, 0.04, 0.08, 0.12, 0.16, 0.20, 0.24, 0.28, 0.32, 0.36, 0.40 M ([H2TPPS42-] ) 2 µM, pH 2.24).
case of ionic species; therefore, the final assembly depends on the two competitive equilibria. The protonation process makes the number of protons increase from 0 to 2, and the inner charge increases from 0 to 2+ as the symmetry changes from D2h to D4h. Both the protonated porphine core and the peripheral sulfonate groups have the characteristics of “inner positive and outer negative”, which makes it possible for the porphyrin to stack in a J-aggregation manner by electrostatic forces and hydrophobic interactions. Figure 2 shows that the absorption spectrum of the porphyrin changes with pH, and the inset plot gives an enlarged scale of the spectra in the range of 500-750 nm. At pH 5.82, curve a corresponds to the nonprotonated porphyrin TPPS44- with a Soret band at 413 nm and Q bands at 516, 550, 581, and 635 nm. At pH 2.24, the protonation process leads to the formation of H2TPPS42-, for which the Soret band exhibits a red shift to 435 nm and the four Q bands merge into two bands at 593 and 646 nm, as shown in curve b. For curve c at pH 0.86, two new and red-shifted bands at 490 and 708 nm are characteristic bands corresponding to porphyrin J-aggregates.4 bmimBF4-Dependent Light Absorption and Emission of J-Aggregates. The concentration of H2TPPS42- at pH 2.24 was fixed at 2 µM, and the absorption and fluorescence spectra were recorded by varying the concentration of bmimBF4. Figure 3 shows that the Soret band at 434 nm and the Q band at 646 nm gradually transformed into two new bands at 490 and 708 nm, indicating that J-aggregates formed owing to the presence of ionic liquid. The corresponding inset plot shows that the bands for the porphyrin monomers initially decreased and then increased, whereas the bands for the J-aggregates exhibited the opposite variation tendency, suggesting that an equilibrium between porphyrin monomers and J-aggregates exists. The
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Figure 4. Influence of bmimBF4 concentration on resonance light scattering spectrum of H2TPPS42-: [bmimBF4] ) 0, 0.04, 0.08, 0.12, 0.16, 0.20, 0.24, 0.28, 0.32, 0.36, 0.40 M ([H2TPPS42-] ) 2 µM, pH 2.24).
Wu et al.
Figure 6. Comparison of 1H NMR spectra of different porphyrin forms and bmimBF4 in D2O solution: (a) TPPS44- (pH 5.82), (b) H2TPPS42(pH 2.24), (c) bmimBF4, (d) TPPS44- (pH 5.82) and bmimBF4 ([TPPS44-] ) 0.5 mM, [H2TPPS42-] ) 0.5 mM, and [bmimBF4] ) 2 M).
TABLE 1: Variation of 1H Chemical Shifts of bmimBF4 and TPPS44- in the bmimBF4-Dependent Porphyrin J-Aggregation System δ0a (ppm) H2 H4 H5 Ho Hm
Figure 5. Comparison of Raman spectra of different porphyrin forms and bmimBF4: (a) TPPS44- (pH 5.82), (b) H2TPPS42- (pH 2.24), (c) bmimBF4, (d) TPPS44- (pH 5.82) and bmimBF4, (e) H2TPPS42- (pH 2.24) and bmimBF4 ([TPPS44-] ) 2 µM, [H2TPPS42-] ) 2 µM, and [bmimBF4] ) 0.2 M).
interaction between bmimBF4 and H2TPPS42- can indded change the porphyrin from monomers into J-aggregates. However, excess ionic liquid breaks the aggregates, just as too much H+ from HNO3 behaves.15 Because the light absorbed by porphyrin aggregates experiences nonluminescent decay of self-absorption, the porphyrin aggregation system induced by bmimBF4 also definitely exhibits decreased fluorescence emission. However, it could not be completely quenched because of the presence of porphyrin monomer,15 which also first decreased and then increased in the present study (spectra not shown here). bmimBF4-Dependent Scattering Signals of J-Aggregates. The resonance light scattering technique is a very useful tool for verfiying the existence of porphyrin aggregates, as the scattering intensity correspondingly increases when electrons delocalize over many components in porphyrin aggregates.20 Figure 4 shows that the RLS signals at a wavelength of 494 nm first increase and then decrease, suggesting that the number of aggregates undergoes the same variation. An apparent enhancement in the lower-frequency Raman region occurs when porphyrin aggregates form under appropriate conditions.21,22 Curves a-e in Figure 5 are Raman spectra that correspond to the five combination conditions of nonprotonated and diprotonated porphyrin and ionic liquid. Negligible Raman signals were observed in the cases of nonprotonated porphyrin monomer, diprotonated porphyrin monomer, and ionic liquid, as shown in curves a-c, respectively. However, with the addition of bmimBF4 to the porphyrin system, sharp peaks at 247 and 318 cm-1 indicate the formation of porphyrin Jaggregates in curves d and e. Compared to the strong Raman signals in curve e, the two corresponding peaks in curve d are
8.56 7.36 7.31 7.67 8.08, 8.11
δ1b (ppm) 8.43 7.21 7.17
-
∆δc (ppm) -0.13 -0.15 -0.14 -
a δ0 represents the 1H chemical shifts of bmimBF4 (H2, H4, and H5) or TPPS44- (Ho and Hm), respectively. b δ1 represents the corresponding chemical shifts in the complex system of bmimBF4 and TPPS44-. c ∆δ ) δ1 - δ0.
much weaker, suggesting that excess bmimBF4 also induces nonprotonated porphyrin to form J-aggregates. Moreover, some increasing Raman signals appear in the higher-frequency region to further confirm the occurrence of porphyrin J-aggregation. NMR Shifts in bmimBF4-Dependent Porphyrin J-Aggregate System. NMR spectra can provide valuable information on molecular interactions. Although the concentration needed in NMR measurements is so high that it is beyond the UV-vis absorption limit and cannot ensure that the same species form, NMR spectroscopy can still provide some clues to explain aggregation behaviors.23 Curves a-d in Figure 6 correspond to 1H NMR spectra of the four combination conditions for nonprotonated and diprotonated porphyrin and ionic liquid, and the given chemical shift range is from 7.0 to 10.0 ppm. Table 1 lists the original chemical shifts of TPPS44- or bmimBF4, as well as the corresponding difference to summarize the NMR behaviors of the bmimBF4induced porphyrin J-aggregation system. The single peak at 7.67 ppm is ascribed to the ortho-H (Ho) in the sulfonated phenyl group, and the doublet peak at 8.08 and 8.11 ppm is ascribed to the meta-H (Hm) as shown in curve a.24 When excess hydrochloric acid is added to the porphyrin solution, both the Ho and Hm peaks disappear, and no new peak appears in curve b, indicating the formation of porphyrin J-aggregates. In curve d, the doublet peaks at 7.31 and 7.36 ppm are from H5 and H4 in the imidazolium ring of bmimBF4, and the single peak at 8.56 ppm is from H2 in the ring. When bmimBF4 is added to TPPS44-, curve c shows that no porphyrin peak is obtained, and the peaks corresponding to H5, H4, and H2 shift to the higher magnetic field of the chemical shifts at 7.17, 7.21.,and 8.43 ppm, respectively. The lessening chemical shifts indicate that the imidazolium ring and sulfonated phenyl groups interact with each other, and the hydrogens atoms of the former ring enter
J-Aggregates of H2TPPS42- Induced by Ionic Liquid bmimBF4
Figure 7. Influence of bmim+ and Na+ concentrations on the absorbance of H2TPPS42- at a wavelength of 490 nm: [bmim+] ) [Na+] ) 0, 0.04, 0.08, 0.12, 0.16, 0.20, 0.24, 0.28, 0.32, 0.36, 0.40 M ([H2TPPS42-] ) 2 µM, pH 2.24).
into the shielding region of the latter group. Moreover, just as shown in curve b, there are no analyzable signals for H2TPPS42of sufficiently high concentration for NMR detection, and it is not possible to determine the dominant interaction factor in the binary system of bmimBF4 and H2TPPS42-. Therefore, it is of great value to investigate the interaction mode between bmimBF4 and TPPS44- by using NMR spectra to obtain more clues about the bmimBF4 induction effect on porphyrin aggregation. Moreover, as for similar inorganic-salt-induced porphyrin J-aggregation systems such as NaCl-dependent porphyrin, it is impossible to measure 1H NMR signals for sodium ion in order to obtain useful information on their interaction behaviors. In addition, the present study makes the role of ionic strength better understood, by providing the NMR shifts of the hydrogen atoms in the imidazolium ring of bmimBF4 during its induction of porphyrin aggregation. Aggregation Mechanism. The presence of an inorganic salt can effectively induce H2TPPS42- to form J-aggregates.7 Ionic liquids consist solely of cations and anions, and they form aqueous organic salt solutions when dissolved in water; therefore, it is expected that the presence of ionic liquids could result in high ionic strengths to favor the formation of porphyrin J-aggregates. Additionally, both the hydrophobic interaction of π-π stacking and the electrostatic attractive force exist between the imidazolium ring and sulfonated phenyl group,25 so bmim+ can much more effectively intercalate into porphyrin J-aggregates than Na+ to reduce the repulsive forces among porphyrins, meaning that J-aggregates could exist more stably in aqueous solution. As for the carboxylphenyl porphyrin aggregates induced by HNO3, the distance between molecular dipole centers is about 0.83 nm, and it is reasonable that ions can intercalate into aggregates to act as balancing ions.23 With the same anion of BF4-, Figure 7 shows a comparison of Na+ and bmim+ in terms of their promotion abilities to form porphyrin aggregates, and the curves indicate that bmim+ is more favorable for promoting J-aggregate formation characterized by a higher slope of Beer’s law at 490 nm. Then, the curve corresponding to bmim+ decreases, whereas that for Na+ constantly increases. It is proposed that bmim+ is much larger than Na+, to provide a more appropriate microenvironment to facilitate porphyrin J-aggregation, and excess bmim+ tends to make aggregates dissociate, as discussed above. It is well-documented that, when the title porphyrin is diprotonated by using HCl, HNO3, H2SO4, CH3COOH, and CX3COOH, porphyrin J-aggregates subsequently come into existence in the bulk solution if there are enough protons in the sensing system.15,16 The present experimental results show that bmimBF4 also effectively induces diprotonated porphyrin to
J. Phys. Chem. B, Vol. 112, No. 27, 2008 8137
Figure 8. Influence of bmimBF4 concentration on the absorption spectrum of TPPS44-: [bmimBF4] ) 0, 0.04, 0.08, 0.12, 0.16, 0.20, 0.24, 0.28, 0.32, 0.36, 0.40 M ([TPPS44-] ) 2 µM, pH 5.82).
form J-aggregates, and it is necessary to investigate how bmimBF4 affects nondiprotonated porphyrin to obtain more information on the aggregation mechanism. With the addition of bmimBF4, Figure 8 shows that the absorption Soret band at 413 nm of nonprotonated TPPS44- at pH 5.82 first decreases and then increases, and the splitting Soret bands at 434 and 490 nm first increases and then decreases. The equilibria among porphyrin forms of TPPS44-, H2TPPS42-, and the J-aggregates depend on the concentration of bmimBF4, and the spectral changes are 413 nm (tetraanion), 434 nm (dianion), and 490 nm (J-aggregates). It is widely accepted that the interaction between Na+ and TPPS44- is due to the “ion association” mechanism, where spectral shifts appear at 413, 434, and 490 nm similarly to those shown in Figure 8. Between bmim+ and TPPS44-, there certainly exist such interactions as dipole-dipole interactions and electrostatic attractive forces to favor their assembly. Therefore, it is reasonable that the spectral shifts in Figure 8 are caused by a similar association process, which provides more convincing evidence for the induction effect of bmim+ on the aggregation process of H2TPPS42- indicated in Figures 3–6. In other words, the ion association mechanism is fundamental to understanding the spectral changes concerning the interaction modes of such systems as Na+-TPPS44-, Na+-H2TPPS42-, bmim+-TPPS44-, and bmim+-H2TPPS42-. As for dyes with large molar extinction coefficients, strong interactions among dye monomers exist, and thus the excitation energy actually delocalizes over the whole assembly of aggregated molecules. Kasha et al. proposed the following equation to explain the dipole-dipole coupling interaction among dye molecules26
V)-
M2 (1 - 3 cos2 θ) r3
(1)
where M is the transition dipole moment, r is the distance between the centers of two dipoles, and θ represents the angle between the line connecting the two dipole centers and the dipole orientation. As for J-aggregates, the dipoles are aligned in a parallel fashion, and the θ value falls into an acute angle range (0 < θ < π/2). Figure 9 illustrates the ion association process between bmimBF4 and sulfonated porphyrins from a side view. bmimBF4 induces porphyrin J-aggregation through the ion association process by entering the space of porphyrins to function as electrostatic balancing ions. Conclusions For the first time, it is reported that H2TPPS42- J-aggregates are induced by the hydrophilic ionic liquid bmimBF4, and these
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Figure 9. Side-view schematic structure of porphyrin J-aggregates of H2TPPS42- in the presence of bmim+ cations.
aggregates were characterized using UV-vis absorption spectroscopy, fluorescence spectroscopy, RLS spectroscopy, Raman spectroscopy, and NMR spectroscopy. The NMR chemical shifts of bmimBF4 move upward to higher magnetic field, indicating that the imidazolium ring of bmimBF4 might intercalate into the porphyrin aggregates to act as a balancing cation, much stronger than Na+. In addition, the ion association process should be taken into account as a major driving force for J-aggregation behavior. The present study is a novel attempt to study porphyrin J-aggregates in bmimBF4, to find more potential applications of aggregation phenomena. Therefore, it is meaningful to enrich the viewpoint of the self-assembly process for aggregates in ionic liquids, which are both structurally designable and characteristically versatile. Acknowledgment. The authors are very grateful for the generous help and instructive discussion on recording Raman spectra from Jie Shen and Wei Feng in Prof. Chun-Hua Yan’s group, Peking University. This work was financially supported by the National Natural Science Foundation of China (20675003, 90713013 and 20275003) and the Finance Bureau of Beijing of China (PXM2007_178305_048917). References and Notes (1) Bonnett, R. Chem. Soc. ReV. 1995, 24, 19–33. (2) Okamura, M. Y.; Feher, G.; Nelson, N. In Photosynthesis 1982, Govindjee, H. J., Ed.; Academic Press: New York, 1982; pp 195-272. (3) Calvete, M.; Yang, G. Y.; Hanack, M. Synth. Met. 2004, 141, 231– 243.
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JP802482F
