Synthesis and Photophysical Properties of Porphyrin Macrorings

Article pubs.acs.org/IC

Synthesis and Photophysical Properties of Porphyrin Macrorings Composed of Free-Base Porphyrins and Slipped-Cofacial Zinc Porphyrin Dimers Yusuke Kuramochi,†,‡ Yuki Kawakami,‡ and Akiharu Satake*,†,‡ †

Department of Chemistry, Faculty of Science Division II, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ‡ Graduate School of Chemical Sciences and Technology, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan S Supporting Information *

ABSTRACT: The self-assembled macroring N-(Zn-Fb-Zn)3 has been constructed by intermolecular complementary coordination among three trisporphyrin Zn-Fb-Zn molecules, each of which consists of a central free-base porphyrin and two imidazolyl-zinc-porphyrin ends. Thus, N-(Zn-Fb-Zn)3 has three slipped-cofacial zinc porphyrin dimers (“special pair model”) and three free-base porphyrins, alternately. The zinc porphyrin dimers in N-(Zn-Fb-Zn)3 are covalently connected by a ring-closing olefin metathesis reaction between the allyl ether groups substituted on the zinc porphyrin dimers, giving a covalently linked macroring C-(Zn-Fb-Zn)3. The fluorescence spectra of C-(Zn-Fb-Zn)3 in several solvents show that the photoinduced energy transfer from one of the zinc porphyrin dimers to a free-base porphyrin occurs intramolecularly in toluene, whereas the photoinduced electron transfer predominantly occurs intramolecularly in N,N-dimethylformamide. Treatment of C-(Zn-Fb-Zn)3 with copper(II) acetate gives a Cu-containing heteromultinuclear porphyrin macroring C-(Zn-Cu-Zn)3, demonstrating that C-(Zn-Fb-Zn)3 could be a good precursor to construct various heteromultinuclear porphyrin macrorings.

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centers.25 The time-resolved transient absorption spectra of the slipped-cofacial zinc porphyrin dimer appending pyromellitimide groups showed that the charge-separated state caused by the photoinduced electron transfer from the zinc porphyrin dimer to the pyromellitimide has a longer lifetime than the zinc porphyrin monomer appending a pyromellitimide group. It is proposed that the stable charge-separated state results from the delocalization of the radical cation over two porphyrin units, preventing the back electron transfer.26 The prolonged lifetime of the charge-separated state was also observed in a conjugate with fullerene as the electron acceptor instead of pyromellitimide groups.27 The supramolecular methodology of the self-assembled dimer formation was applicable to formation of cyclic pentamers and hexamers consisting of gable-porphyrins in which two imidazolyl-zinc-porphyrins were connected via an mphenylene or a bis(ethynylene)phenylene linker.28,29 A a zinc trisporphyrin, in which two imidazolyl-zinc-porphyrins were substituted at the 5,15-meso positions of one zinc porphyrin through m-phenylene linkers, exclusively formed the selfassembled cyclic trimer.30 The cyclic trimer has been studied

INTRODUCTION Porphyrins are well-known macrocyclic dyes having intense absorption bands in the visible region and can coordinate to various metal ions to form very stable complexes, allowing their use in a wide range of applications.1−6 In recent studies, for photochemical reactions, manganese, iron, cobalt, ruthenium, and rhodium porphyrins have been utilized as photoredox catalysts for olefin epoxidation,7,8 CO2 reduction,9 oxidation of hydrocarbons,10,11 water oxidation,12 syntheses of N-acyl sulfimides and sulfoximines,13 and carbon−carbon σ-bond oxidation.14 Zinc porphyrins are used as the photosensitizer in photocatalytic CO2 reduction,15,16 photoinduced living polymerization,17 photoinduced α-functionalization of aldehydes,18 photooxidation of alcohols,19 light-driven O2 reduction,20 and selective oxidation of aromatic alcohols.21 Zinc porphyrins have been actively studied in photoinduced electron transfer by combining electron donor/acceptor22 and used as a component of multiporphyrins for light-harvesting antenna systems.23 In 1994, Kobuke et al. reported that a zinc porphyrin having imidazolyl groups at the meso positions formed a very stable slipped-cofacial dimeric structure by complementary coordination from the imidazolyl group to the zinc ion intermolecularly,24 and that the structure mimics the arrangement of the “special pair” in the photosynthetic reaction © XXXX American Chemical Society

Received: May 23, 2017

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DOI: 10.1021/acs.inorgchem.7b01317 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

and differential pulse voltammogram (DPV) were measured using an ALS-H/CHI Model 610E electrochemical analyzer in a micro-cell equipped with a glassy carbon working electrode (ϕ 1.6 mm) and a Pt counter-electrode. The micro-cell was connected via a Luggin capillary with a reference electrode of Ag/AgNO3 (10 mM in DMF). Tetrabutylammonium hexafluorophosphate (nBu4NPF6) recrystallized from ethyl acetate was used as the supporting electrolyte. Ferrocene was used as an external standard, and all potentials are referenced to the ferrocene/ferrocenium couple (E1/2 = 0.036 vs Ag/AgNO3). Molecular models of the porphyrin macrorings were obtained with Materials Studio 4.4 programs. The structures were optimized using a semi-empirical molecular orbital package (VAMP geometry optimization, PM6). Macroring N-(Zn-Fb-Zn)3. Synthesis of Trisporphyrin Zn-Fb-Zn. The synthetic procedure of trisporphyrin Zn-Fb-Zn was based on a previous report.32 Boronate-zinc-porphyrin dimer (ZnP)2 (90 mg, 1.1 × 10−4 mol), 5,15-bisbromo-10,20-bis(4′-ethoxycarbonylphenyl)porphyrin FbP (42 mg, 5.4 × 10−5 mol), and Cs2CO3 (130 mg, 4.0 × 10−4 mol) dried at 100 °C in vacuo were placed in a Schlenk flask. After the flask was evacuated and filled with Ar gas, dry degassed toluene (18 mL) and dry degassed DMF (9.0 mL) were added to the flask. The reaction proceeded by adding Pd(PPh3)4 (13 mg, 1.1 × 10−5 mol) to the flask, and the reaction mixture was stirred at 90 °C for 17 h. The mixture was cooled to room temperature, and then water was added. The product was extracted with CHCl3 and washed with water and brine. The organic layer was dried over anhydrous Na2SO4 and evaporated to dryness. The residue was passed through a short flash silica gel column (eluent: CHCl3/CH3OH → pyridine) to remove the palladium catalyst. The collected solution was evaporated to obtain a green solid (110 mg). The crude product was further purified with preparative GPC (columns: TSK G2500HHR and G2000HHR; eluent: pyridine; flow rate: 3.0 mL min−1) to give pure trisporphyrin Zn-FbZn (30 mg, 27% yield based on FbP). Reorganization. The obtained trisporphyrin Zn-Fb-Zn (21 mg, 1.0 × 10−5 mol) was dissolved in 200 mL of a mixture of CHCl3/CH3OH (9:1 v/v). The solution, after standing for 2 days, was washed with water and subsequently brine, to remove methanol. Evaporation of the solvent gave 21 mg of the titled compound N-(Zn-Fb-Zn) 3 (quantitative): NMR spectra were collected as a mixture of C3hsymmetry and asymmetry compounds. Parts of 1H NMR signals were distinguished between the structural isomers. They were described separately. Parts of 13C NMR data were obtained from HSQC and HMBC methods. The data were described as the mixture of the structural isomers. 1H NMR (500 MHz, CDCl3) C3h-Symmetric δ 8.05−9.80 (m, 108H, β, Ph), 5.98−6.17 (m, 12H, −CH), 5.57 (d, J = 1.7 Hz, 3H, Im-5 out), 5.34−5.39 (d, J = 4.5 Hz, 6H, β1 out), 5.15− 5.50 (m, 24H, CH2), 5.04 (d, J = 4.5 Hz, 6H, β1 in), 5.00−5.30 (brm, 24H, −CH2CH2CH2O−), 4.65 (q, J = 7.3 Hz, 12H, −CH2CH3), 4.60 (d, J = 1.8 Hz, 3H, Im-5 in), 4.00−4.30 (m, 24H, −OCH2−), 3.72−3.92 (m, 24H, −CH2CH2CH2O−), 2.70−3.10 (brm, 24H, −CH2CH2CH2O−), 2.23 (d, 3H, Im-4 out), 1.70 (d, Im-4 in), 1.67 (s, 9H, Im-CH3 out), 1.62 (t, J = 7.3 Hz, 18H, −CH2CH3), 0.99 (s, 9H, Im-CH3 in), −2.65 (s, 6H, inner NH). Asymmetric δ 8.05−9.80 (m, 108H, β, Ph), 5.98−6.17 (m, 12H, −CH), 5.60 (d, J = 1.5 Hz, 1H, Im-5 out), 5.55 (d, J = 1.8 Hz, 1H, Im-5 out), 5.53 (d, J = 1.9 Hz, 1H, Im-5 out), 5.34−5.39 (d, J = 4.5 Hz, 6H, β1 out), 5.17 (d, J = 4.5 Hz, 2H, β1 in), 5.15−5.50 (m, 24H,  CH2), 5.08 (d, J = 4.5 Hz, 2H, β1 in), 5.00−5.30 (brm, 24H, −CH2CH2CH2O−), 4.96 (d, J = 4.5 Hz, 2H, β1 in), 4.83 (d, J = 1.8 Hz, 1H, Im-5 in), 4.77 (d, J = 1.8 Hz, 1H, Im-5 in), 4.71 (q, J = 7.3 Hz, 4H, −CH2CH3), 4.65 (q, J = 7.3 Hz, 4H, −CH2CH3), 4.59 (q, J = 7.3 Hz, 4H, −CH2CH3), 4.54 (d, J = 1.8 Hz, 1H, Im-5 in), 4.00−4.30 (m, 24H, −OCH2−), 3.72−3.92 (m, 24H, −CH2CH2CH2O−), 2.70−3.10 (brm, 24H, −CH2CH2CH2O−), 2.27 (d, 1H, Im-4 out), 2.20 (d, 1H, Im-4 out), 2.18 (d, 1H, Im-4 out), 1.88 (d, 1H, Im-4 in), 1.85 (d, 1H, Im-4 in), 1.67 (m, 16H, Im-4 in, Im-CH3 out, −CH2CH3), 1.62 (t, J = 7.3 Hz, 6H, −CH2CH3), 1.55 (t, J = 7.3 Hz, 6H, −CH2CH3), 1.16 (s, 3H, Im-CH3 in), 1.06 (s, 3H, Im-CH3 in), 0.89 (s, 3H, Im-CH3 in), −2.47 (s, 2H, inner NH), −2.65 (s, 2H, inner NH), −2.86 (s, 2H, inner NH); 13C NMR (HMQC and HMBC, CDCl3) δ 167.4 (CO), 147.0

for the composite model of the light-harvesting complex (LH1) and the reaction center (RC),31 or for transmembrane nanopores.32 However, earlier studies have been limited in the self-assembled macrorings consisting of all zinc porphyrins. Herein, we have constructed a novel cyclic trimer of Zn-Fb-Zn, which is composed of two imidazolyl-zinc-porphyrin ends and a central free-base porphyrin connected through m-phenylene groups. Cyclic trimers of Zn-Fb-Zn are expected to be a good precursor for constructing heteromultinuclear porphyrin macrorings by introducing various metal ions into the freebase porphyrin parts. Although the synthesis of the cyclic trimers composed of zinc trisporphyrin Zn-Zn-Zn, which is composed of two imidazolyl-zinc-porphyrin ends and a central zinc porphyrin, has been established,30−32 demetalation from only the central zinc porphyrin of Zn-Zn-Zn is difficult. In addition, it is unknown whether the self-assembled process converges to cyclic trimers from Zn-Fb-Zn successfully because the free-base porphyrin plane could be more flexible than the zinc porphyrin plane.33 This is the first report of a synthesized self-assembled macroring having free-base porphyrin parts by complementary coordination and the introduction of copper(II) ions into the free-base porphyrin parts of the macroring to construct a heteromultinuclear macroring containing copper porphyrins, which can be a redox catalyst.34

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EXPERIMENTAL SECTION

General Procedure. All chemicals and solvents were of commercial reagent quality and used without further purification unless otherwise stated. (ZnP)2 and FbP were prepared according to the literature,32 and their chemical structures are shown in Figure S1. Dry N,N-dimethylformamide (DMF) was prepared by distillation under reduced pressure over molecular sieves 4A. Dry toluene was prepared by passing through an activated alumina column and storing over molecular sieves 4A. CHCl3 (Kanto, extra pure) was stabilized with 0.5−1% ethanol. First-generation Grubbs catalyst (benzylidenebis(tricyclohexylphosphine)dichlororuthenium) was purchased from Aldrich. The silica gels utilized for column chromatography were purchased from Kanto Chemical Co. Inc.: Silica-Gel 60N (Spherical, Neutral) 63−210 μm and 40−50 μm (Flash). 1H NMR, 1 H−1H COSY, 1H−13C heteronuclear single quantum correlation (HSQC), 1H−13C heteronuclear multiple bond correlation (HMBC), homonuclear two-dimensional J-resolved NMR, and nuclear Overhauser enhancement spectroscopy (NOESY) were recorded by using a JEOL JNM-ECS-300 or JNM-ECS-500, and chemical shifts were recorded in parts per million (ppm) relative to tetramethylsilane. High-resolution MALDI-TOF mass spectra were collected on a JEOL JMS S-3000 with dithranol or trans-2-[3-(4-tert-butylphenyl)-2methyl-2-propenylidene]-malononitrile (DCTB) as a matrix with and without sodium iodide (NaI). Analytical high-performance liquid chromatographies (HPLCs) using pyridine as an eluent were performed on PU-2080plus and MD-2018plus (JASCO) systems equipped with two TSK G2500HHR (Tosoh, exclusion limit: 20 000 Da) and one TSK G2000HHR (Tosoh, exclusion limit: 10 000 Da) column. Analytical HPLCs using CHCl3 as an eluent were performed on PU-2089plus and UV-2075plus (JASCO) systems equipped with a JAIGEL-3H (JAI, exclusion limit: 70 000 Da) column. Preparative HPLCs were carried out on an LC-908 (JAI) attached to one TSK G2500HHR and one G2000HHR column eluted with pyridine. UV−vis absorption spectra were collected on a JASCO V-650 spectrometer. Steady-state fluorescence spectra were collected on a Hitachi F-4500 spectrometer and corrected for the response of the detector system. The fluorescence intensities were normalized at the absorption of the excitation wavelength. UV/vis λmax values were reported in nm. Fluorescence quantum yields were determined by integrated ratios of the fluorescence spectra relative to that of free-base tetraphenylporphyrine (H2TPP, Φf = 0.11 in toluene).35 Their values were corrected by refractive indices of the used solvents. Cyclic voltammogram (CV) B


Article

Inorganic Chemistry Scheme 1. Synthetic Route of N-(Zn-Fb-Zn)3

paper (Whatman). The filtrate was concentrated and dried under reduced pressure, giving the pure titled compound C-(Zn-Cu-Zn)3 (1.5 mg, quant.): MALDI-TOF MS found m/z ([M + Na]+) 6203.6767, calcd for [C354H288N48O24Zn6Cu3 + Na]+ 6203.6323 (average); UV−vis (in CHCl3) λmax/nm: 621, 566, 541, 440, 410.

(Ph), 146.2 (Im-2 out), 145.9 (Im-2 in), 128.4 (Ph), 127.8 (Ph), 121.4 (Im-4 out), 121.1 (Im-4 in), 118.0 (Im-5 out), 117.5 (Im-5 in), 135.6 (−CHCH 2 ), 117.0 (−CHCH 2 ), 72.0 (−OCH 2 −), 70.8 (−CH2CH2CH2O−), 39.0 (−CH2CH2CH2O−), 33.0 (Im-CH3 out), 32.4 (−CH2CH2CH2O−), 32.3 (Im-CH3 in); MALDI-TOF MS found m/z ([M + H]+) 2051.74, calcd for [C122H106N16O8Zn2 + H]+ 2051.70 (monoisotopic); UV−vis (in CHCl3) λmax/nm: 645, 620, 566, 518, 440, 410; UV−vis (in pyridine) λmax/nm: 647, 613, 563, 519, 441, 431, 421. Covalently Linked Macroring C-(Zn-Fb-Zn)3. N-(Zn-Fb-Zn)3 (20 mg, 3.3 × 10−6 mol) and CHCl3 (50 mL) were placed into a 100 mL flask, and the air in the flask was replaced with Ar gas. Grubbs catalyst (5.8 mg, 7.0 × 10−6 mol) was added to the porphyrin solution, and the reaction mixture was stirred at room temperature. The reaction progress was monitored with UV−vis absorption spectra in pyridine and MALDI-TOF mass spectra. After being stirred for 4 h, further Grubbs catalyst (2.9 mg, 3.5 × 10−6 mol) was added because the reaction was still incomplete. The reaction mixture was stirred for another 2 h. The reaction mixture was diluted with CHCl3 and washed with water and subsequently brine, and the solvent was evaporated. The crude product was purified by a short silica gel column (eluent: CHCl3/acetone = 20:1 v/v → 10:1 v/v). The fractions eluted with CHCl3/acetone (10:1 v/v) were collected and evaporated to afford a green solid (15 mg). Further purification was carried out with preparative GPC (columns: TSK G2500HHR and G2000HHR; eluent: pyridine; flow rate: 3.0 mL min−1) to give the pure titled compound C(Zn-Fb-Zn)3 (9.0 mg, 46% yield based on N-(Zn-Fb-Zn)3): MALDITOF MS found m/z ([M + Na]+) 6018.90, calcd for [C354H294N48O24Zn6 + Na]+ 6018.89 (average); UV−vis (in CHCl3) λmax/nm (log ε): 645 (4.1), 621 (4.8), 567 (4.9), 517 (4.8), 441 (6.1), 410 (6.1); UV−vis (in pyridine) λmax/nm: 646, 621, 567, 518, 442, 412; UV−vis (in toluene) λmax/nm: 646, 621, 566, 517, 441, 411; Fluorescence (in CHCl3, λex = 571 nm) λmax/nm: 624, 648, 713; Fluorescence (in toluene, λex = 571 nm) λmax/nm: 623, 650, 715. Heteromultinuclear Macroring C-(Zn-Cu-Zn)3. C-(Zn-Fb-Zn)3 (1.1 mg, 1.8 × 10−7 mol), copper(II) acetate monohydrate (1.8 mg, 9.0 × 10−6 mol), and CHCl3 (2 mL) were placed in a 10 mL flask. The reaction mixture was stirred at room temperature for 2 days and then diluted with CHCl3 (ca. 50 mL). The organic layer was washed with water and subsequently brine, and passed through Phase Separator

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RESULTS AND DISCUSSION Construction of Noncovalently Linked Macroring N(Zn-Fb-Zn)3. The synthetic procedure and molecular models for N-(Zn-Fb-Zn)3 are shown in Scheme 1 and Figure 1,

Figure 1. Molecular models of the porphyrin macrorings optimized by a semi-empirical molecular orbital method (PM6). The substituents at the meso-position of the zinc-porphyrin parts and the ester groups of the free-base porphyrin parts were replaced by methyl groups for simplicity.

respectively. Palladium-catalyzed cross-coupling reaction of (ZnP)2 and FbP was carried out in a mixture of toluene and DMF, and the reaction progress was monitored by analytical GPC using pyridine as an eluent. Under the GPC conditions, intermolecular coordination bonds of an imidazole-to-zinc ion between Zn-Fb-Zn molecules were dissociated by competitive coordination of the pyridine eluent. The crude mixture C


Article

Inorganic Chemistry

broad peak with the top at 8.4 min, and the peak of as-prepared Zn-Fb-Zn was significantly broader, compared with the GPC chart of the previous zinc trisporphyrin Zn-Zn-Zn.30 This result suggested that as-prepared Zn-Fb-Zn formed larger acyclic and cyclic assemblies. The NMR spectrum of the as-prepared ZnFb-Zn in CDCl3 showed broad signals, as shown in Figure S3a, suggesting various mixtures of cyclic and acyclic polymers. The Zn-Fb-Zn was subjected to a reorganization process.30−32 ZnFb-Zn was dissolved in a diluted mixture of CHCl3/CH3OH (9:1 v/v), and the mixture was allowed to stand for enough time. Under these dilute conditions, a coordinating solvent, methanol, accelerated the equilibrium process, which converged larger acyclic assemblies into stable cyclic assemblies.28−32 In this case, cyclic trimers of Zn-Fb-Zn are expected. After 2 days, the mixture was washed with water to remove methanol before evaporation because the solvent became methanol-rich during the concentration process. The residue obtained by evaporation was analyzed again with a GPC system using CHCl3 eluent. A sharp peak was observed at 11.8 min, suggesting convergence into N-(Zn-Fb-Zn)3 (solid line in Figure 2). 1 H NMR Spectrum of N-(Zn-Fb-Zn)3. The 1H NMR spectrum in CDCl3 of the converged Zn-Fb-Zn is shown in Figure S3b. Sharp and well-resolved signals were observed, and some of the characteristic signals were assigned with a combination of 2-D NMR, such as 1H−1H COSY, 1H−13C HSQC, 1H−13C HMBC, homonuclear J-resolved NMR, and NOESY techniques (Figures S4−S9). Thus, the 1H NMR spectrum after the reorganization showed that the Im-4, Im-5, and Im-CH3 protons appeared in two different positions caused

contained Zn-Fb-Zn dominantly, accompanied with some byproducts, as shown in Figure S2. The crude product was purified with a preparative GPC system using pyridine as an eluent. The fractions only containing Zn-Fb-Zn were concentrated by evaporation, and the residue was analyzed with another GPC system using CHCl3 as an eluent (dotted line in Figure 2). Under the GPC conditions, intermolecular

Figure 2. GPC-HPLC charts (column: JAIGEL-3H; eluent: CHCl3; flow rate: 1.2 mL min−1, detection: 420 nm) of Zn-Fb-Zn before (blue dotted line) and after (red solid line) the reorganization with CHCl3/ CH3OH = 9:1 and [Zn-Fb-Zn] = 0.05 mM at room temperature.

coordination of imidazole-to-zinc ion among Zn-Fb-Zn molecules was achieved predominantly due to lack of competitive solvent. The chromatogram chart showed a very

Figure 3. 1H NMR spectra at (a) 2.5−6.3 ppm, (b) 0.7−2.4 ppm, and (c) −3.7 to −1.8 ppm of N-(Zn-Fb-Zn)3 in CDCl3. The open (○) and filled (●) circles indicate signals for the zinc porphyrin dimer parts of the C3h-symmetric and asymmetric macrorings, respectively. The open (□) and filled (■) squares exhibit signals for the free-base porphyrin parts of the C3h-symmetric and asymmetric macrorings, respectively. D


Article

Inorganic Chemistry Scheme 2. Ring-Closing Metathesis Reaction To Provide C-(Zn-Fb-Zn)3

asymmetric macrorings. The product was further purified using preparative GPC using pyridine as an eluent, giving a pure sample of C-(Zn-Fb-Zn)3 (Figure S12). The MALDI-TOF mass spectrum in Figure 4 showed isotope peaks corresponding

by the inside and outside of the cyclic structure (Figure 3). The Im-5 protons appeared at 5.5−5.6 ppm for the outside and 4.5− 4.8 ppm for the inside, and the Im-4 protons appeared at 2.2− 2.3 ppm for the outside and 1.7−1.9 ppm for the inside. The outside and inside Im-CH3 protons were observed at 1.7 ppm and 0.9−1.2 ppm, respectively. The inside signals, e.g., Im-CH3 at 0.9−1.2 ppm, were not observed in the linear polymeric/ oligomeric assemblies (Figure S3a). In addition, most of their signals split into four peaks, which consisted of one big and three small peaks with an integration ratio of 2:1 (open and closed circles in Figure 3). The splits were also observed in the β-pyrrole protons (Figures 3 and S10). The split signals could be explained by considering two topological isomers of C3hsymmetric and asymmetric, whose mole ratio was 2:3.30 For the three zinc porphyrin dimer parts in the C3h-symmetric and asymmetric macrorings, one set and three sets of the proton signals were, respectively, observed at different positions. However, the ethyl ester and inner NH proton signals of the free-base porphyrin units showed three peaks consisting of one big and two small peaks with an integration ratio of 3:1 (open and closed squares in Figure 3). It is thought that one of the three free-base porphyrin parts in the asymmetric macroring is in a similar environment to the free-base porphyrin parts in the C3h-symmetric macroring. Covalently Linked Macroring C-(Zn-Fb-Zn)3. To fix the self-assembled macroring with covalent bonds, a ring-closing metathesis reaction was carried out in CHCl3 between the allyl ether groups on the zinc porphyrin dimers using Grubbs catalyst (Scheme 2).28a,36,37 Under these conditions, the transformation between C3h-symmetric and asymmetric rings does not occur. The reaction progress was monitored with the UV−vis absorption spectra in pyridine. In the early stage of the reaction, the self-assembled macroring was dissociated into the Zn-Fb-Zn units by coordination of pyridine to the zinc porphyrin, resulting in the elimination of the large split of the Soret band (410 and 440 nm) caused by the exciton couplings from the slipped-cofacial zinc porphyrin dimers (Figure S11). When the reaction proceeded, the UV−vis absorption spectrum of the reaction mixture in pyridine became almost the same as that in CHCl3, because no more dissociation occurred even in a coordinating solvent, pyridine. This result indicated that the metathesis reaction proceeded completely, and the terminal allyl groups in the coordination dimers were doubly covalentlinked. From monitoring the reaction progress, no difference was observed between the reactivity of the C3h-symmetric and

Figure 4. MALDI-TOF mass (matrix: DCTB) of C-(Zn-Fb-Zn)3. Top: found, bottom: simulated by [C354H294N48O24Zn6Na]+ ([M + Na]+).

to C-(Zn-Fb-Zn)3 (calcd for the average peak of [M + Na]+: m/z: 6018.89; found: 6018.90). The observed peaks were almost coincident with the theoretical peaks and showed no additional peaks, such as [M + 28 + H]+ or [M + 28 + Na]+, corresponding to the macroring with unreacted allyl ether groups. The 1H NMR spectrum of the purified C-(Zn-Fb-Zn)3 was compared with that of the nonmetathesized ring N-(Zn-FbZn)3. (Figure S13). A distinct change was observed at the eliminated exomethylene and methine signals of the allyl ether groups, where the multiplet signals coupled with the neighboring sp3-methylene and terminal exomethylene protons at 6.0−6.2 ppm in N-(Zn-Fb-Zn)3 were simplified to appear as two triplet signals coupled with only the neighboring sp3methylene protons at 6.0 and 6.3 ppm in C-(Zn-Fb-Zn)3 (Figure 5a). A metathesis reaction using the first-generation Grubbs catalyst can provide both E- and Z-olefins, and the Eolefin is preferentially formed.36 Taking into account the relative chemical shifts of their signals,37 the major signal at 6.3 ppm is assigned to the E-isomer, the minor one at 6.0 ppm to E


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Inorganic Chemistry

Figure 5. 1H NMR spectra of (top) N-(Zn-Fb-Zn)3 and (bottom) C-(Zn-Fb-Zn)3 in CDCl3 at (a) 5.7−6.6 ppm and (b) 0.5−2.5 ppm. The open and filled marks indicate signals for the zinc porphyrin dimer parts of the C3h-symmetric and asymmetric macrorings, respectively.

Figure 6. Three isomers for the zinc porphyrin dimer parts in C-(Zn-Fb-Zn)3 caused by the E- and Z-olefins.

Electrochemistry of C-(Zn-Fb-Zn)3. The electrochemical properties of C-(Zn-Fb-Zn)3 were measured in DMF (Figure 7). C-(Zn-Fb-Zn)3 showed three quasi-reversible reduction waves at E1/2 = −1.44, −1.91, and −2.09 V, two quasi-reversible oxidation waves at E1/2 = 0.25 and 0.40 V, and an irreversible oxidation wave at ca. 0.6 V vs Fc/Fc+. Judging from the redox potentials of H2TPP and the model compound of the zinc porphyrin dimer37 (Figure S14), the waves at −1.44 V and ca. 0.6 V corresponded to the first reduction and oxidation of the free-base porphyrin parts. The second-electron reduction of the free-base porphyrin parts would appear at ca. −1.9 V, overlapping with the cathodic wave of the zinc porphyrin dimer parts. The zinc porphyrin dimer parts showed the first reduction wave at ca. −1.9 V, followed by the second one at −2.09 V, and the first oxidation wave at 0.25 V, followed by the second one at 0.40 V. The separated second reduction and oxidation waves observed in the zinc porphyrin dimer are caused by delocalization of the anion or cation radical over the framework of the dimer pair in the first reduction or oxidation state.38 The difference between the cathodic and anodic peak potentials at E1/2 = −1.44 V was estimated to be 73 mV (Figure S15), which was larger than the theoretical value of a reversible one-electron redox reaction (59 mV at 25 °C),39 implying that each of the free-base porphyrin parts is electrically independent. In addition, the molecular radius of C-(Zn-Fb-Zn)3 was estimated to be 2.5 nm from the diffusion coefficient (9.5 ×

the Z-isomer. The mole ratio of the E-isomer to the Z-isomer is 3.5:1. The E- and Z-olefin groups would give three isomers for each zinc porphyrin dimer part (Figure 6). If the metathesis reaction independently proceeded without being affected by the other reaction sites, the mole ratios of the three isomers were estimated to be 1:0.57:0.08, as shown in Figure 6. Although the existence of the three isomers further complicated the 1H NMR spectrum, we could find apparent splittings caused by the isomers at the Im-4 and Im-CH3 signals (Figure 5b). For example, the Im-CH3 signals at 0.9−1.2 ppm split into big (hexagon) and small (diamond) peaks accompanied with other very small peaks. The integration ratio of the big to the small peak was approximately 1:0.5, reflecting the mole ratio of the (E, E) isomer to the sum of the (E, Z) and (Z, E) isomers. As a conclusion in the structural analyses, C-(Zn-Fb-Zn)3 was a mixture of C3h-symmetric and asymmetric rings, and each of the two includes geometrical isomers on the alkenyl parts. Despite this, UV−vis spectra of N-(Zn-Fb-Zn)3 and C-(Zn-Fb-Zn)3 in CHCl3 were almost identical, as shown in Figure S11a,b, indicating that the covalent linkages by the alkenyl parts did not affect the relative geometries of the porphyrin parts in both N(Zn-Fb-Zn)3 and C-(Zn-Fb-Zn)3. In the following electro- and photochemical studies, C-(Zn-Fb-Zn)3 was used as such geometrical mixtures. F


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Inorganic Chemistry

correspond to the free-base porphyrin parts, whereas the peaks at 566 and 621 nm are contributed from the zinc porphyrin dimer parts. Compared with the macroring consisting of nine zinc porphyrins C-(Zn-Zn-Zn)3,30−32 C-(Zn-Fb-Zn)3 can absorb a wide range of visible light, and this feature is beneficial for light-harvesting antenna complexes.43 Figure 9 shows the fluorescence spectra of C-(Zn-Fb-Zn)3 in toluene, CHCl3, CH2Cl2, and DMF, in which the zinc

Figure 7. Cyclic voltammogram (scan rate = 100 mV/s) and differential pulse voltammogram of C-(Zn-Fb-Zn)3 (0.36 mM) in DMF containing nBu4NPF6 (0.1 M). The anodic signals marked by asterisks (∗) were only observed on the reversed scan.

10−7 cm2 s−1) obtained from the slopes of linear plots of ip at the wave of E1/2 = −1.44 V versus the square root of the scan rate by assuming that each porphyrin part is electrically independent.40,41 The estimated radius is comparable to the radius (ca. 2 nm) estimated from a molecular model (Figure 1). The anodic signals marked as asterisks (∗) on the reversed scan in Figure 7 were only observed after sweeping more negative potential than ca. −2 V (Figure S17), indicating that the species such as phlorine formed by the reduction.42 Photophysics of C-(Zn-Fb-Zn)3. The UV−vis absorption spectrum of C-(Zn-Fb-Zn)3 is shown in Figure 8. The shape of the Q-band was close to a simulation curve of the sum of (ZnP)2 and H2TPP in a ratio of 1:1, while the spectral shape of the Soret band was not expressed by the sum of them due to the strong excitonic coupling interactions via the phenylene linkages in Zn-Fb-Zn.31f The peaks at 517 and 646 nm mainly

Figure 9. Fluorescence spectra of C-(Zn-Fb-Zn)3 excited at 571 nm at 298 K in toluene, CHCl3, CH2Cl2, and DMF from the top to the bottom. Inset shows the normalized fluorescence spectrum of C-(ZnZn-Zn)3 (broken line) and the spectrum by subtracting the normalized spectrum of C-(Zn-Zn-Zn)3 from C-(Zn-Fb-Zn)3 (dotted line) in toluene.

porphyrin dimer parts can be selectively excited (λex = 571 nm). According to the molar absorption coefficients of the zinc porphyrin dimers and the free-base porphyrins in C-(Zn-FbZn)3, 94% of the light at 571 nm was absorbed by the zinc porphyrin dimer parts. The fluorescence spectrum in toluene showed peaks at 650 and 714 nm, which were mainly contributed from the free-base porphyrin, indicating that the photoinduced energy transfer occurred from the zinc porphyrin dimer to the free-base porphyrin. By comparing the peak at 623 nm with the fluorescence intensity of C-(Zn-Zn-Zn)3 (Figure S18),32 the energy-transfer efficiency from the zinc porphyrin dimer to the free-base porphyrin was estimated to be ca. 50%. Lindsey et al. reported the efficient energy transfer from the excited zinc porphyrin to the free-base porphyrin in the diphenylethyne-linked cyclic porphyrin hexamer, resulting in emission almost exclusively from the free-base porphyrin.44 The center-to-center distance between adjacent zinc porphyrin and free-base porphyrin through the diphenylethyne linkage was 18 Å. Gossauer et al. reported that the energy-transfer efficiency was estimated to be 91% from the excited zinc porphyrin to the free-base porphyrin, whose center-to-center distance between adjacent porphyrins was 23 Å.45 Judging from the center-tocenter distance of ca. 10 Å between the zinc porphyrin dimer and the free-base porphyrin in C-(Zn-Fb-Zn)3, the energytransfer efficiency of 50% seems to be very low. This behavior could be explained by the fact that the N-coordinated zinc porphyrin has a lower energy level for the singlet excited state than the uncoordinated zinc porphyrin.46 From the UV−vis and fluorescence spectra, the energy gaps between the ground state and the lowest singlet excited state are estimated to be 1.99 and 1.92 eV for the zinc porphyrin dimer and the free-base porphyrin, respectively. Because the energy difference between

Figure 8. UV−vis absorption spectra of C-(Zn-Fb-Zn)3 (red solid line), H2TPP (green dotted line), (ZnP)2 (blue dotted line), and the sum of H2TPP and (ZnP)2 as 1:1 molar ratio (black dotted line) in CHCl3. G


Article

Inorganic Chemistry the two excited states is not so significant, the back-energy transfer from the excited free-base porphyrin to the zinc porphyrin would occur frequently. Actually, the fluorescence spectrum that was excited at 518 nm, at which the free-base parts dominantly absorbed the light, was almost the same as that excited at 571 nm, at which the zinc porphyrin dimer parts dominantly absorbed the light (Figure S19). This observation indicates that energy hopping occurs among the zinc porphyrin dimer parts ((ZnP)2) and the free-base porphyrin parts (FbP) in C-(Zn-Fb-Zn)3 as below in eq 1: FbP‐1(ZnP)2 * ⇄ 1FbP*‐(ZnP)2

(1)

Figure 9 also shows that the fluorescence of C-(Zn-Fb-Zn)3 was quenched by increasing the solvent polarity. The fluorescence quantum yield of 0.058 in toluene (dielectric constant: εs = 2.38) was decreased to 0.026 in CHCl3 (εs = 4.81), 0.003 in CH2Cl2 (εs = 8.93), and 0.002 in DMF (εs = 36.7).47 The efficient fluorescence quenching in the polar solvent suggested that the photoinduced electron transfer occurred among the zinc porphyrin dimer parts and the freebase porphyrin parts in C-(Zn-Fb-Zn)3. From the electrochemical data, the free energies of charge separation (ΔGCS) were calculated using the following eq 240,48

Figure 10. Energy diagram of C-(Zn-Fb-Zn)3 in different solvents.

e2 1 4πε0εs RDA ⎤ 1 ⎞⎛⎜ 1 1⎞ + − ⎟⎟⎥ ⎟⎜ εs ⎠⎥⎦ 2rA− ⎠⎝ εref (2)

the MALDI-TOF mass spectrum (Figure 11), which showed isotope peaks corresponding to the heteromultinuclear macroring C-(Zn-Cu-Zn)3 (calcd for the average peak of [M + Na]+: m/z: calcd 6203.63; found: 6203.68). The MALDITOF mass spectrum showed that the introduction of copper(II) ions into three free-base porphyrin parts proceeded completely. The UV−vis absorption spectrum at the Q-band of C-(Zn-Cu-Zn)3 is shown in Figure 12. The spectral shape was expressed by the sum of (ZnP)2 and CuTPP in a ratio of 1:1, indicating that the copper(II) insertion proceeded quantitatively. In addition, the fluorescence was almost completely quenched in CHCl3 by the introduction of copper(II) ions. Judging from the above data, we confirmed that the heteromultinuclear porphyrin macroring of C-(Zn-Cu-Zn)3 was quantitatively formed.

ΔGCS = e[E 0(D+ /D) − E 0(A /A−)] − E00 − −

⎡ e 2 ⎢⎛ 1 ⎜ 4πε0 ⎢⎣⎝ 2r D+

where e is the elementary charge, E0(D+/D) and E0(A/A−) are the first oxidation and reduction potentials for the electron donor and acceptor, respectively, measured in a solvent with εref (DMF: 36.7), E00 is the zero−zero transition energy of the first singlet excited energy, rD+ and rA− are the radii of the electron donor and electron acceptor, respectively. In the present case, we assumed rD+ = rA− = 5 Å. RDA is the center-to-center donor− acceptor distance (RDA = 10 Å). The values of ΔGCS, caused by the electron transfer to form the radical anion on the free-base porphyrin, were estimated to be +0.30, −0.01, −0.15, and −0.27 eV in toluene, CHCl3, CH2Cl2, and DMF, respectively (Figure 10). The electron transfer to form the radical anion on the zinc porphyrin dimer is highly endothermic, whose ΔGCS is > +0.5 eV even in DMF. The ΔGCS values well explain the observation of the fluorescence quenching in Figure 9. The energy level between the excited state and the charge-separated state in CHCl3 is almost the same, and the fluorescence intensity in CHCl3 is almost half that in toluene. The fluorescence is significantly quenched in CH2Cl2 and DMF, reflecting the significant negative value of ΔGCS. In C-(Zn-FbZn)3, energy hopping occurs among the zinc porphyrin dimer parts and the free-base porphyrin parts. Thus, the electron transfer might occur from both excited states of the zinc porphyrin dimer parts and the free-base porphyrin parts. To clarify these processes in detail, further investigations using time-resolved transient absorption measurements would be required. Introduction of Heterometal Ions into C-(Zn-Fb-Zn)3. The free-base porphyrin parts of C-(Zn-Fb-Zn) 3 can incorporate various metal ions to give heteromultinuclear macrorings. Here, as a demonstration, copper(II) ion was introduced into C-(Zn-Fb-Zn)3 by treatment with copper(II) acetate (Scheme 3). The reaction progress was confirmed by

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CONCLUSIONS We have synthesized the macroring, N-(Zn-Fb-Zn)3, having three slipped-cofacial zinc porphyrin dimers and three free-base porphyrins by self-assembly of Zn-Fb-Zn with the complementary coordination from the imidazole groups to the zinc ions. The self-assembled macrorings were fixed via the covalent linkages using a metathesis reaction. The covalently linked macroring, C-(Zn-Fb-Zn)3, enables us to investigate the electrochemical and photophysical properties in various solvents, including coordinating solvent. The fluorescence spectrum of C-(Zn-Fb-Zn)3 in toluene showed that the photoinduced energy transfer from the excited zinc porphyrin dimer to the free-base porphyrin resulted in emission from the free-base porphyrin part by selective excitation of the zinc porphyrin dimer. The energy diagram using the electrochemical data suggested that the electron transfer from the zinc porphyrin dimer to the excited free-base porphyrin occurred further in a polar solvent such as DMF. Finally, we introduced copper(II) ions into C-(Zn-Fb-Zn) 3 to construct the heteromultinuclear macroring C-(Zn-Cu-Zn)3. A novel photocatalyst would be designable using C-(Zn-Fb-Zn)3 because the kind of metal ions can be freely selected. Because the slippedcofacial porphyrin parts have a great potential to act as a good H


Article

Inorganic Chemistry

Scheme 3. Reactions of C-(Zn-Fb-Zn)3 with Copper(II) Ions To Provide the Heteromultinuclear Macroring C-(Zn-Cu-Zn)3

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01317. Structures; GPC-HPLC charts; 1H NMR spectra, 1H−1H COSY, HSQC, HMBC, NOESY, and homonuclear twodimensional J-resolved NMR data; selected NOESY correlations; UV−vis absorption spectra; cyclic voltammograms; and fluorescence spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

Figure 11. MALDI-TOF mass (matrix: DCTB) of the sample after the metathesis reaction of the self-assembled macroring. Top: found, bottom: simulated by [C354H288N48O24Zn6Cu3Na]+ ([M + Na]+).

*E-mail: [email protected]. ORCID

Yusuke Kuramochi: 0000-0001-5936-5010 Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 12. UV−vis absorption spectra of C-(Zn-Cu-Zn)3 (red solid line), CuTPP (green dotted line), (ZnP)2 (blue dotted line), and the sum of CuTPP and (ZnP)2 as 1:1 molar ratio (black dotted line) in CHCl3.

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