J. Phys. Chem. B 2008, 112, 16517–16524
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Substituent Effects of Porphyrins on Structures and Photophysical Properties of Amphiphilic Porphyrin Aggregates Kohei Hosomizu,† Masaaki Oodoi,† Tomokazu Umeyama,† Yoshihiro Matano,† Kaname Yoshida,‡ Seiji Isoda,‡ Marja Isosomppi,§ Nikolai V. Tkachenko,*,§ Helge Lemmetyinen,§ and Hiroshi Imahori*,†,|,⊥ Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, Institute for Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan, Institute of Materials Chemistry, Tampere UniVersity of Technology, P.O. Box 541, FIN-33101 Tampere, Finland, Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, and Fukui Institute for Fundamental Chemistry, Kyoto UniVersity, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan ReceiVed: September 9, 2008; ReVised Manuscript ReceiVed: October 6, 2008
Substituent effects of porphyrin on the structures and photophysical properties of the J-aggregates of protonated 5-(4-alkoxyphenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin have been examined for the first time. Selective formation of the porphyrin J-aggregate was attained when suitable length of the alkoxy group was employed for the amphiphilic porphyrin. Namely, a regular leaflike structure was observed for the J-aggregates of protonated 5-(4-octyloxyphenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin, which was consistent with the results obtained by using the UV-visible absorption and dynamic light-scattering measurements. A bilayer structure in which the hydrophobic alkoxyl groups facing inside the bilayer are interdigitated to each other, whereas the hydrophilic porphyrin moieties are exposed outside, was proposed to explain the unique porphyrin J-aggregate. Fast energy migration and efficient quenching by defect site in the J-aggregates were suggested to rationalize the short lifetimes of the excited J-aggregates. Introduction Self-assembly of photoactive molecules has attracted considerable attention in materials as well as biological sciences in the past years.1 Various photoactive molecules including porphyrins,2 phthalocyanines,3 hexabenzocoronenes,4 and perylene bisimides5 have been intensively investigated aiming to their application as optoelectronic and photovoltaic devices. In particular, self-assembled synthetic porphyrins6-8 and chlorophylls9 have been frequently prepared in connection with natural light-harvesting complexes in photosynthesis.10-12 In purple bacteria, bacteriochlorophylls (BChl) are organized in highly symmetric ringlike structures with the help of protein.10 The transition dipoles of BChls are circularly arranged so as to point nearly in the direction of the ring tangent, located on the circumference, enhancing the interaction for excitation energy transfer (EN) between neighboring BChls as much as possible. On the other hand, in green photosynthetic bacteria the supramolecular self-assembled aggregates of BChl form the major light-harvesting antenna complex, known as chlorosomes.11 They include several thousands of BChls, arranged into rodlike aggregates without the help of protein, which are tightly packed into an organelle surrounded by a lipid monolayer. Owing to a regular organization of BChls and strong excitonic interactions, the dipole strength is concentrated in a few allowed excitonic transitions, which is suitable for the highly * E-mail:
[email protected] (H.I.); nikolai.tkachenko@ tut.fi (N.V.T.). † Graduate School of Engineering, Kyoto University. ‡ Institute for Chemical Research, Kyoto University. § Tampere University of Technology. | Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University. ⊥ Fukui Institute for Fundamental Chemistry, Kyoto University.
efficient light-harvesting and EN properties of these aggregates. However, lack of information on the atomic-resolution structures of chlorosomes has precluded the detailed elucidation on the relationship between the structure and function. Natural chlorophyll aggregates in purple bacteria and chlorosomes have strong transition dipole moments stemming from the alignment of the “head-to-tail” direction. Thus, J-aggregates of synthetic porphyrins are highly promising as light-harvesting models to examine the structure-function relationship. However, the porphyrin J-aggregates have been rather limited to tetrakis(4-sulfonatophenyl)porphyrin (H2TSPP4-) and its derivatives in acidic solutions,7 protonated tetraphenylporphyrins at the liquid-liquid8a,f or gas-liquid interface,8b,f cationic tetraphenylporphyrins,6j dendritic porphyrins,8c and amphiphilic porphyrins.8j As such, substituent effects of H2TSPP4- on the structures and photophysical properties of J-aggregates have yet to be investigated in detail. We report herein the first systematic comparison of H2TSPP4- derivatives with different lengths of hydrophobic alkoxy groups. The electrostatic interaction between the positively charged protonated core and negatively charged sulfonato groups is the driving force for the association of the protonated porphyrin monomers to J-aggregates under acidic conditions. Thus, the minimum requirement is the presence of two sulfonato groups at the para-positions of the meso-phenyl groups in 5,15 positions of the porphyrin ring to attain the slipped head-to-tail structure. To stabilize the J-aggregates, one sulfonate group is replaced by hydrophobic group (i.e., alkoxy group), whereas the three sulfonate groups remain intact (Figure 1). The length of alkyl moiety in the alkoxy group (H2C13-, H2C83-, H2C183-) would affect the interaction between the porphyrins under acidic
10.1021/jp807991k CCC: $40.75 2008 American Chemical Society Published on Web 11/19/2008
16518 J. Phys. Chem. B, Vol. 112, No. 51, 2008
Figure 1. Molecular structures of amphiphilic porphyrins and reference used in this study.
conditions, making it possible to control the structures and photophysical properties of the porphyrin J-aggregates. Results and Discussion Preparation of Amphiphilic Porphyrins. The synthetic route to H2Cn3- (n ) 1, 8, 18) is shown in Scheme 1. Trimethylsilyl-substituted porphyrin 3 was synthesized by Lindsey’s method.13 Cross-condensation of benzaldehydes 1 and 214,15 and pyrrole in the presence of BF3 · Et2O as catalyst in CHCl3 followed by oxidation with DDQ gave asymmetrically substituted porphyrin 3. Sulfonated porphyrin H2Cn3- (n ) 1, 8, 18) was obtained by sulfonation of 3 with ClSO3SiMe3 in refluxing CCl4 for 5 h followed by hydrolysis of the intermediate with aqueous NaOH.16 H2TSPP4- was also synthesized following the same procedures as described previously.17 The products were characterized on the basis of their 1H NMR and MALDI-TOF mass spectra and elemental analysis (see Experimental Section). Absorption Spectra under Acidic Conditions. H2Cn3- (n ) 1, 8, 18) and H2TSPP4- are soluble in water, and at low pH (
