Photocatalytic CO₂ reduction sensitized by a special-pair mimic porphyrin connected with a rhenium(I) tricarbonyl complex

Abstract

Zn porphyrins with an imidazolyl group at the meso position generate a highly stable porphyrin dimer by complementary coordination from the imidazolyl to the Zn ion in noncoordinating solvents such as chloroform, which mimics the natural special pair in photosynthesis. In this work, we have synthesized an imidazolyl-substituted Zn porphyrin connected with a Re 2,2-bipyridine tricarbonyl complex as a CO₂ reduction catalyst via a p-phenylene linker, affording a homodimer with two Re complexes on both sides (ReDRe). The dimeric structure is easily dissociated into the corresponding monomers in coordinating solvents. Therefore, we prepared a mixture containing a heterodimer with the Re carbonyl complex on one side (ReD) by simple mixing with an imidazolyl Zn porphyrin and evaporating the solvent. Using the Grubbs catalyst, the subsequent olefin metathesis reaction of the mixture gave covalently linked porphyrin dimers through the allyloxy side chains, enabling the isolation of the stable hetero- (ReD′) and homo-dimers (ReD′Re) with gel permeation chromatography. The Zn porphyrin dimers have intense absorption bands in the visible light region and acted as good photosensitizers in photocatalytic CO₂ reduction in a mixture of N,N-dimethylacetamide and triethanolamine (5 : 1 v/v) containing 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the electron donor, giving CO with high selectivity and durability. Under irradiation with strong light intensity, the reaction rate in ReD′ exceeded that of the previous porphyrin=Re complex dyad, ZnP-phen=Re. For instance, after irradiation at 560 nm for 18 h, the turnover number (TON_CO) of ReD′ reached 2800, whereas the TON_CO of ZnP-phen=Re was 170. The high activity in the system using the porphyrin dimer originates from no accumulation of the one-electron reduced species of the porphyrin that inhibit light absorption due to the inner-filter effect.

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Introduction

Undoubtedly, CO₂ emissions caused by human activity have drastically changed the global climate. Various systems have been developed to reduce CO₂ in the atmosphere, and systems converting CO₂ into energy-rich compounds with the aid of light (solar energy) have much potential to solve not only the problem of global warming, but simultaneously also that of a shortage of fossil-fuel resources.¹ Sunlight consists mainly of visible light with low photon density. The low photon density can be disadvantageous, in particular in multielectron reactions, and therefore photosensitizers capturing visible light with high efficiency are essential.² In natural photosynthesis, dilute sunlight is collected by networks of pigments—the so-called light-harvesting complexes.³ The collected energy is transferred to the chlorophyll dimer in the reaction center, the so-called “special pair”, that initiates photoinduced electron transfer to give a charge-separated (CS) state in the reaction center.⁴

In 1994, Kobuke et al. reported that Zn porphyrins having an imidazolyl group at the meso position form a slipped-cofacial dimeric structure by complementary coordination from the imidazolyl to the Zn ion of the porphyrin center in noncoordinating solvents, and that the dimeric structure mimics the special pair in photosynthetic systems.⁵ When the dimer is one-electron oxidized, the produced radical cation delocalizes over two porphyrin units. The CS rate is accelerated in the photoinduced electron transfer, while the charge-recombination rate decelerates to give the ground state.⁶ The dimeric structure is sufficiently stable to be treated as a photosensitizer in noncoordinating solvents such as chloroform and toluene, but it easily dissociates into monomeric units in coordinating solvents such as pyridine and dimethyl sulfoxide (DMSO). These properties are somewhat advantageous for preparing various combinations of dimers.⁷ The porphyrin units can be freely exchanged by simply mixing two porphyrin components in a coordinating solvent followed by evaporating the solvent, giving a hetero-porphyrin dimer. For instance, a triad system consisting of a porphyrin dimer with ferrocene as an electron donor on the one side and fullerene as an electron acceptor on the other side was easily formed by the simple mixing of two kinds of Zn porphyrin having either ferrocene or fullerene. The supramolecular triad gave long-lived CS species after efficient photoinduced electron transfer and charge shift.⁸ In addition, this dimer-based supramolecular methodology is applicable in the construction of various self-assembled porphyrin arrays having cyclic, chain, and sheet structures for photosynthetic antenna models,^9–11 two-photon absorption materials,¹² solar cell materials,¹³ solvation/desolvation indicators,¹⁴ and so on.

Zn porphyrin has been used as the photosensitizer in homogeneous photoredox reactions.^15,16 For example, dyads combining Zn porphyrin and the Re diimine tricarbonyl complex catalyst have been developed for photocatalytic CO₂ reduction.^16–18 Among them, we have recently reported on a Zn porphyrin=Re complex dyad, ZnP-phen=Re (Fig. 1), in which fac-Re(phen)(CO)₃Br (where phen = 1,10-phenanthroline) is connected with a Zn porphyrin, and affords CO with high selectivity (>99.9%) and efficiency (Φ_CO = 8% and turnover number (TON_CO) > 1300) in photocatalytic CO₂ reduction using 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as an electron donor, phenol as a proton source, and N,N-dimethylacetamide (DMA) as a solvent.¹⁸

Fig. 1 Structures of the porphyrin dimers having the Re complex(es) (ReD′Re and ReD′) and the porphyrin=Re complex dyad (ZnP-phen=Re). The asterisks in the IS group indicate asymmetric carbons.

In this work, Kobuke's special-pair mimic porphyrin was first used as the photosensitizer for photocatalytic CO₂ reduction. It is expected that the slipped-cofacial dimer has the advantage that the dimer can utilize light of a wider wavelength because the intense Soret band appearing at ∼420 nm of monomeric porphyrin is split by strong excitonic coupling between two Zn porphyrins in close proximity.¹⁹ In addition, because the dimeric structure can be easily dissociated in coordinating solvents, unsymmetric heterodimers can be constructed by simply mixing two kinds of porphyrin in a coordinating solvent and then evaporating the solvent. Furthermore, a multisite ring-closing metathesis reaction using the Grubbs catalyst on olefin moieties at the meso positions of the porphyrin fixes the dimeric structure through covalent bonds, giving a stable dimer that does not dissociate even in coordinating solvents such as pyridine or DMSO.²⁰ We have synthesized a Re bipyridine tricarbonyl complex connected to the opposite meso position of the imidazolyl Zn porphyrin through a p-phenylene linker. The porphyrin forms a stable dimer having the Re complex on both sides (ReDRe). The reorganizing procedure by mixing ReDRe and a dimer without the Re complex (D) in pyridine and evaporating the solvent gave a mixture of ReDRe, D, and a hetero-dimer having the Re complex on one side (ReD). The ring-closing metathesis reaction followed by separation of the mixture with gel permeation chromatography (GPC) produced covalently linked dimers, ReD′ and ReD′Re (Fig. 1). Here, a bulky isostearyl group (IS group) was introduced on the imidazolyl group in ReDRe to be soluble in practical organic solvents. The photocatalytic reaction in CO₂-saturated DMA containing BIH showed that the triethanolamine (TEOA) presence dramatically improved the catalytic activity. In addition, we performed photocatalytic CO₂ reduction using ReDRe and ReD′Re in DMSO, and ReD′Re had higher durability than ReDRe, which was dissociated to the monomers in DMSO, demonstrating that the dimeric structure plays an important role for the high activity in photocatalytic reactions.

Results and discussion

Synthesis of ReDRe

First, we attempted to synthesize the porphyrin dimer having 1-methyl-imidazolyl groups at the meso positions of the porphyrins (see the ESI†). However, when the Re tricarbonyl complexes were introduced into the bpy groups of [Im^Me-ZnP-Ph-5Bpy]₂, the product became insoluble in preferred solvents (chloroform, dichloromethane, toluene, and acetonitrile) for the ring-closing metathesis reaction using the Grubbs catalyst as the dimer. [Im^Me-ZnP-Ph-5Bpy=Re]₂ was soluble in N,N-dimethylformamide (DMF) and the ¹H NMR spectrum recorded under high-concentration conditions showed an almost dimeric structure in DMF-d₇ (Fig. S14†). However, [Im^Me-ZnP-Ph-5Bpy=Re]₂ was partially dissociated intothe monomer in the concentration range for the intramolecular metathesis reaction of the dimer. We also tried the ring-closing metathesis reaction with the dimer, [Im^Me-ZnP-Ph-5Bpy]₂, before the introduction of the Re complex, but the reaction did not proceed, probably due to the deactivation of the Grubbs catalyst (1^st generation) by coordination of the bpy groups.²¹ Therefore, we changed the substituent group on the imidazolyl moiety from the methyl group to a bulky IS group (Fig. 1) to improve organic solvent solubility. Scheme 1 shows the synthetic route for the porphyrin dimer having the IS groups at the imidazolyl groups and Re tricarbonyl complexes at the bpy groups ([Im^IS-ZnP-Ph-5Bpy=Re]₂, ReDRe). The free-base porphyrin Im^IS-H₂P-PhI was synthesized with 19% yield by condensation of 4-iodobenzaldehyde and Im^IS-CHO with meso-(3-allyloxypropyl)dipyrromethane at room temperature (rt). The three asymmetric C atoms of the IS groups provide 2³ = 8 stereoisomers and their conformational isomers, resulting in broadened and complicated signals in their ¹H and ¹³C NMR spectra. Successful syntheses of the porphyrins were confirmed by not only using 2-dimensional (2D) NMR but also by comparing with the spectra of porphyrins substituted by 1-methyl-imidazolyl. The complicated signals appearing from 1.38 to −0.80 ppm in the ¹H NMR spectrum of Im^IS-H₂P-PhI were assigned to the methine, methylene, and methyl signals of the IS groups (Fig. S36†). The Zn(II) ion was introduced into Im^IS-H₂P-PhI by treatment with a methanol solution of Zn(OAc)₂ in CHCl₃. The ¹H NMR spectrum showed that the Im-4 and Im-5 protons that appeared at 7.74–7.72 ppm and 7.52–7.50 ppm in Im^IS-H₂P-PhI were shifted to a higher field by the Zn insertion to show the peaks at 2.06 ppm for the Im-4 proton and at 5.56 ppm for the Im-5 proton. The spectral change was similar to that of [Im^Me-ZnP-PhI]₂, indicating that all the stereoisomers quantitatively construct the slipped-cofacial dimer, [Im^IS-ZnP-PhI]₂ (Fig. S42†). The dimeric structure was also clarified by the characteristic split Soret band in the ultraviolet visible (UV-vis) absorption spectrum (Fig. S46†).¹⁹

Scheme 1 Synthetic route of the noncovalently linked porphyrin dimer, [Im^IS-ZnP-Ph-5Bpy=Re]₂ (ReDRe). Reagents and conditions: (a) (i) trifluoroacetic acid/CHCl₃, rt, and 3.5 h, (ii) Et₃N, p-chloranil, rt, overnight, and 19%; (b) Zn(OAc)₂ (8 equivalent (equiv.)), CHCl₃/CH₃OH, rt, overnight, and 96%; (c) 5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,2′-bipyridine (5Bpy-B(OR)₂, 1.5 equiv.), Pd(PPh₃)₄ (0.2 equiv.), CsCO₃ (2.9 equiv.), toluene/DMF/water, 90 °C, 4 h, and 89%; (d) Re(CO)₅Br (0.97 equiv.), toluene, 90 °C, 2 d, and 72%.

The porphyrin dimer with the bpy groups, [Im^IS-ZnP-Ph-5Bpy]₂, was synthesized by the Suzuki–Miyaura coupling reaction of [Im^IS-ZnP-PhI]₂ and 5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,2′-bipyridine (5Bpy-B(OR)₂) in toluene and DMF. The reaction was first performed under anhydrous conditions, but the coupling reaction did not proceed. The coupling reaction proceeded with water addition, giving [Im^IS-ZnP-Ph-5Bpy]₂ at 89% yield. The product, [Im^IS-ZnP-Ph-5Bpy]₂, also showed the split Soret bands in the UV-vis absorption spectrum and the upper-field shifts of the β-pyrrole and imidazolyl protons compared with the free-base porphyrin in the ¹H NMR spectrum (Fig. S48†), indicating that the bpy group does not interrupt the formation of the dimeric structure. Here, 5Bpy-B(OR)₂ was difficult to purify with column-chromatographic techniques due to the decomposition of boron derivatives on the silica gel column.²² Thus, 5Bpy-B(OR)₂ was purified by partitioning between the aqueous and organic layers. The lithiation of 5-bromo-2,2′-bipyridine followed by addition of triisopropyl borate gave a boronic acid derivative, which was extracted at pH = 11 to the aqueous layer to remove the starting material and by-products existing the organic layer. Next, the aqueous layer was evaporated to dryness, and the residue was reacted with pinacol. Pure 5Bpy-B(OR)₂ was extracted with toluene and other impurities remained as insoluble materials (see the ESI†).

The introduction of the Re(I) tricarbonyl complex into the bpy groups of [Im^IS-ZnP-Ph-5Bpy]₂ was performed by refluxing with Re(CO)₅Br in toluene. The use of excess amounts (1.5 equiv.) of Re(CO)₅Br against the bpy group made the characteristic split Soret band of the dimeric structure change into a single Soret band (Fig. S55†). This observation indicates that the coordination of the excess Re(I) ion to the imidazolyl group of the porphyrin dissociates the dimer to give a monomeric species. The reflux of the mixture in pyridine recovered the split Soret band and gave the target compound, [Im^IS-ZnP-Ph-5Bpy=Re]₂ (ReDRe, Fig. S55 and 56†), indicating that the monodentate imidazole Re complex dissociated in pyridine, whereas the bidentate bpy complex was stable in pyridine even under reflux conditions. The use of 0.97 equiv. of Re(CO)₅Br selectively gave the target compound without forming the monomeric species, (Fig. S55†), and the purification on a silica gel column afforded ReDRe at 72% yield. Three CO peaks at 197.17, 196.88, and 189.20 ppm in the ¹³C NMR spectrum (Fig. S59†) and three CO stretching peaks at 1901, 1923, and 2020 cm⁻¹ in the infrared (IR) spectrum (Fig. S63†) indicate that the Re tricarbonyl complex has an fac-Re(bpy)(CO)₃Br-type structure.²³ The bulky IS substituent on the imidazolyl group significantly improved the solubility for organic solvents, and ReDRe was easily soluble in chloroform, dichloromethane, toluene, and acetonitrile over a 0.1 mM concentration.

Fixation of the dimers via covalent bonds in the side chains

To prevent the dissociation of the coordination bond during the catalytic reaction in polar solvents, the dimeric structure was fixed by the ring-closing olefin metathesis reaction on the allyloxy groups at the meso positions of the porphyrin.^20,21 First, the metathesis reaction for ReDRe was performed using the Grubbs catalyst (1^st generation) in dichloromethane (Scheme 2). The reaction progress was monitored with the UV-vis absorption spectrum in pyridine. The split Soret band was observed even in pyridine after the overnight reaction, indicating that all four allyloxy side chains were covalently linked (Fig. S64†). GPC using a pyridine eluent showed a single peak at 21.2 min, suggesting that the reaction proceeded almost perfectly (Fig. S65†). The matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry using trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]-malononitrile (DCTB) as a matrix gave signals corresponding to the covalently linked dimer, C-[Im^IS-ZnP-Ph-5Bpy=Re]₂ (ReD′Re) in Scheme 2 (Fig. S66†). The ¹H NMR spectrum of the purified ReD′Re was compared with that of the nonmetathesized compound ReDRe (Fig. S68†). The elimination of the exomethylene signals and a change in the methine signals of the allyl ether groups were observed. The broad signals coupled with the neighboring sp³-methylene and terminal exomethylene protons were split into two signals at 6.46 and 6.11 ppm, which were assigned to the E- and Z-olefin isomers, respectively. The molar ratio of the E- and Z-isomers was 1.9 : 1, suggesting that three isomers exist in a 0.96 : 1:0.27 ratio (Fig. S69†). Therefore, ReD′Re is a diastereo-mixture formed from the two IS groups and the two olefin parts. The diastereomers are difficult to separate in chromatography and are used as a mixture for a catalytic reaction. In addition, three CO peaks for ReDRe in the ¹³C NMR spectrum were split into six peaks for ReD′Re, which would result from the E- and Z-olefin isomers (Fig. S70†).

Scheme 2 Ring-closing metathesis reaction of ReDRe.

We confirmed that the ring-closing metathesis reaction of the homodimer ReDRe quantitatively proceeds to give the covalently linked dimer, ReD′Re. Next, a covalently linked heterodimer in which two types of imidazolyl Zn porphyrins are paired was prepared. Here, to investigate whether a heterodimer can be formed using the porphyrin with the Re complex, a simple p-tolyl-substituted Zn porphyrin dimer was used (D in Scheme 3). Two types of dimer, ReDRe and D (1 : 1 molar ratio), were dissolved in pyridine and then the solvent was evaporated. In pyridine, the dimer is dissociated into a pyridine-coordinated monomer. The evaporation of pyridine gives the reorganized dimers. In addition, the ring-closing metathesis reaction was applied to the resulting product in chloroform, giving the heterodimer ReD′Re, accompanied by the homodimers ReD′Re and D′. In the analytical GPC traces of the crude product (Fig. 2), the peaks at 21.2 and 23.8 min were respectively assigned to be ReD′Re and D′ by comparing with the corresponding homodimers. The central peak at 22.1 min was expected as the target ReD′. Larger components (oligomers), which were probably formed by the intermolecular metathesis reaction of the dimers, were also observed before 21 min. The MALDI-TOF mass spectrum of each isolated component proved that the species at 22.1 min is ReD′. The peak area ratio of ReD′Re, ReD′, and D′ in Fig. 2 is a statistical ratio of 0.5 : 1:0.5. When ReDRe and D were used in a 1 : 4 molar ratio as the starting dimers, the ratio of ReD′Re, ReD′, and D′ was 0.04 : 0.26 : 0.70, which was consistent with the ratio (0.04 : 0.32 : 0.64) assumed for the covalently linked dimers to be statistically obtained (Fig. S75†). Thus, if the proportion of D, which can be easily synthesized, is increased, ReD′ can be efficiently obtained from ReDRe. The isolated peaks of ReD′Re and ReD′ have a single Gaussian shape, indicating that the diastereomers originating from the IS and alkene groups hardly affect the molecular size or the hydrodynamic volume.

Scheme 3 Synthesis of the heterodimer, ReD′. Reagents and conditions: (a) (i) mixing in pyridine at rt, (ii) evaporating the solvent in vacuo; (b) Grubbs catalyst (0.5 equiv.), CHCl₃, and rt.

Fig. 2 GPC-high-performance liquid chromatography (HPLC) charts (column: TSK gel G2500H_HR × 2 + G2000H_HR × 1, eluent: pyridine, flow rate: 1.0 mL min⁻¹, and detection: 440 nm) of the crude product obtained after the metathesis reaction using ReDRe and D (molar ratio 1 : 1), and the isolated samples by preparative GPC-HPLC. The inset shows the logarithm of molecular weight against the retention time.^9e,24

Photochemical properties

To investigate the substituent effect on the imidazolyl group on the photochemical properties, the UV-vis absorption spectrum of [Im^IS-ZnP-Ph-5Bpy]₂ was compared with that of [Im^Me-ZnP-Ph-5Bpy]₂ in chloroform (Fig. S83†). Here, [Im^Me-ZnP-Ph-5Bpy=Re]₂ is insoluble in chloroform. The two spectra overlap almost completely, suggesting that there is no strong electronic interaction between the IS group and the porphyrin unit and that differences in the photochemical properties due to the presence of isomers of the IS groups would be negligible. Previous studies have reported that the E- and Z-olefin isomers at the meso positions of the porphyrins do not affect the electrochemical and photochemical properties of porphyrins.²⁰ In fact, the absorption and fluorescence spectra between ReDRe and ReD′Re in chloroform are almost the same, indicating that the olefin side chains do not affect the electronic state of the porphyrin ring (Fig. S84†).

Fig. 3a shows the UV-vis absorption spectra of D′, ReD′Re, ReD′, and fac-Re(bpy)(CO)₃Br in DMA, a solvent used for photocatalytic CO₂ reduction.²⁵ Because DMA weakly coordinates to Zn porphyrin,^18a the noncovalently linked dimer, D, was mostly dissociated into the monomer in the micromolar concentration range. The inset shows a spectral comparison between the Zn porphyrin dimer and fac-Re(bpy)(CO)₃Br, indicating that the Zn porphyrin unit has a much larger absorption coefficient than the Re(bpy)(CO)₃Br unit. The Soret bands split into two peaks (at 414 and 437 nm; ΔE = 1270 cm⁻¹ for ReD′) due to strong exciton coupling between the two porphyrins in the dimer.¹⁹ The excitonic coupling in the Q bands was negligibly small due to the smaller oscillator strengths, and the spectral shape of the Q bands in the dimer was similar to that of the monomeric porphyrin.^9e,19 The introduction of the Re(bpy)(CO)₃Br unit caused a slight broadening of the Soret bands and a slight red-shift of the peaks throughout the spectrum (Table 1). The dimer shows a much broader Soret band (caused by the exciton coupling) than that of monomeric Zn porphyrin, enabling the use of a very wide wavelength range of visible light for the photocatalytic reactions.

Fig. 3 (a) UV-vis absorption spectra of D′, ReD′, ReD′, and fac-Re(bpy)(CO)₃Br in DMA. The inset presents the magnification of the near-UV region. (b) Fluorescence spectra of D′, ReD′Re, and ReD′ in DMA at 298 K. The inset shows the spectra in toluene. The fluorescence spectra were normalized with the absorbance at the excitation wavelength (λ_ex = 568 nm).

Table 1 Absorption data in DMA and fluorescence data in toluene and DMA^a

	λ abs/nm (ε/10^4 M^−1 cm^−1)	λ em/nm^b	Φ f
			Toluene	DMA^c
a Excited at 568 nm. b In DMA. c The values in parentheses are the ratio to the fluorescence quantum yield of D.
D	413 (23), 436 (27)	622	7.9%	4.2% (1)
	567 (2.3), 620 (1.6)
ReD′	414 (25), 437 (29)	625	8.1%	0.50% (0.12)
	568 (2.7), 622 (2.0)
ReD′Re	414 (24), 439 (30)	629	8.0%	0.31% (0.073)
	568 (2.9), 625 (2.6)

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The fluorescence spectra of D′, ReD′Re, and ReD′ are shown in Fig. 3b and their fluorescence quantum yields (Φ_f) in DMA and toluene are summarized in Table 1. The fluorescence intensity decreases in polar DMA (dielectric constant: ε_s = 38.3) as the Re complex is introduced, whereas the fluorescence quenching by the introduction of the Re complex is not observed in nonpolar toluene (ε_s = 2.43) (inset of Fig. 3b). The behavior suggests that the intramolecular electron transfer occurs from the lowest excited singlet state (S₁) of the porphyrin dimer unit to the Re complex unit in ReD′Re and ReD′.^9a,26 According to the energy levels of the CS state (1.9 eV) estimated from the electrochemical data of D′ and fac-Re(bpy)(CO)₃Br and of the S₁ (2.0 eV) of D′,²⁶ the intramolecular electron transfer, the so-called “oxidative quenching”,²⁸ is thermodynamically possible (Fig. S85†). In DMA, the quenching efficiencies of ReD′ and ReD′Re compared to D′ were 88% and 93%, respectively (Table 1). Assuming that the value of the CS rate constant (k_CS) in ReD′Re is twice the value in ReD′, the ratio of the fluorescence quantum yields of ReD′Re to D was calculated to be 0.064 (see p. S68 in the ESI†). The value is consistent with the experimental value (0.073), indicating that each Re complex acts independently to quench the S₁ of the porphyrin unit in ReD′Re. The quenching efficiencies show that most of the excited energy on the porphyrin mainly produces the CS state between the porphyrin and the Re complex both in ReD′Re and ReD′. The fluorescence quenching experiments of D′, ReD′, and ReD′Re by BIH gave the Stern–Volmer constants K_SV = 3.5 M⁻¹, 0.65 M⁻¹, and 0.32 M⁻¹, respectively (Fig. S86†). The K_SV values of ReD′ and ReD′Re were smaller compared with that of D′ and reflect that the lifetime of the S₁ of the porphyrin is shortened by the formation of the CS state. The lifetimes of ReD′ and ReD′Re were estimated to be 0.37 and 0.18 ns from the Stern–Volmer plots by assuming that the fluorescence quenching process is diffusion-controlled,^18b which was well consistent with the lifetimes (0.24 and 0.13 ns for ReD′ and ReD′Re, respectively) estimated from the fluorescence quantum yields (see p. S68 in the ESI†).

In a previous report, ZnP-phen=Re showed phosphorescence from the Zn porphyrin in Ar-saturated DMA even at rt by strong spin–orbit coupling imposed by the large Re atom in the vicinity.^18b The phosphorescence was efficiently quenched by an electron donor used in photocatalytic CO₂ reduction, i.e., BIH, indicating that quantitative photoinduced electron transfer occurred from BIH to the long-lived excited triplet state (T₁) of ZnP-phen=Re. In the present systems, no phosphorescence was observed in ReD′Re and ReD′. It is considered that the distance between the Re atom and the porphyrin unit is too long to induce rt phosphorescence from the Zn porphyrin. In fact, the fluorescence quantum yields of ReD′Re and ReD′ are almost the same as that of D′ in toluene (Table 1), indicating that the heavy atom effect of the Re atom on porphyrin was negligible. This also supports that the S₁ state of Zn porphyrin was quenched to predominantly produce the CS states in ReD′Re and ReD′. According to the energy diagram using the phosphorescence spectrum of D′ at 77 K,²⁷ the charge-recombination process from the CS states to generate the corresponding T₁ state is thermodynamically possible, as is the generation of ground states (Fig. S85†).

Photocatalytic CO₂ reduction in DMA

Photocatalytic CO₂ reduction was conducted in a DMA solution containing ReD′ (0.025 mM) and BIH (0.1 M) as the electron donor^18,28 under photoirradiation at 560 nm using light-emitting diode (LED) lamps. A trace amount of CO was produced and its turnover number against the Re atom was only TON_CO = 15 after 18 h. By contrast, the addition of either phenol (PhOH, 0.1 M) or TEOA (17 vol%) dramatically increased CO production, and its TON_CO reached 780 and 2800 after 18 h, respectively (Fig. 4). PhOH has been reported to act as a proton source to promote the reaction with CO₂ on the Re complex,²⁹ and TEOA assists trapping CO₂ by forming the Re(bpy)(CO)₃(CO₂-TEOA) species.³⁰ Herein, CO was selectively produced without forming detectable amounts of H₂, CH₄, and HCOOH. Because TEOA can work as an electron donor,^16c,28 irradiation of a CO₂-saturated DMA-TEOA solution containing ReD′ was carried out in the absence of BIH as a control experiment. The reaction produced a trace amount of CO (TON_CO = 14 after 48 h (Table S1 entry 5†)), indicating that BIH acts as the major electron donor.

Fig. 4 CO formation with time during irradiation at 560 nm from LED lamps with a merry-go-round apparatus for CO₂-saturated DMA solutions (total 1.0 mL) containing ReD′ (0.025 mM) and BIH (0.1 M) in the absence (green) and presence of additives ([PhOH] = 0.1 M (blue) and DMA : TEOA = 5 : 1 v/v (red)).

The formation of CO using ReD′, ReD′Re, mixed systems of D′ and fac-Re(bpy)(CO)₃Br (1 and 2 equiv. against D′), and ZnP-phen=Re in CO₂-saturated DMA-TEOA (5 : 1 v/v) solutions containing BIH (0.1 M) is illustrated in Fig. 5. All systems photocatalyzed CO₂ reduction, selectively giving CO. The time profiles of the CO production between ReD′ and ReD′Re were similar, indicating that the absorbed light energy can be used by one Re complex site for the CO₂ reduction reaction and that the catalytic activity of the Re complex is unaffected by another Re complex. From the fluorescence quenching experiment (Fig. S86†), the quenching efficiencies (η_q) of the S₁ states in the presence of [BIH] = 0.1 M can be estimated to be 6% and 3% for ReD′ and ReD′Re, respectively.²⁸ The quenching efficiency of ReD′ having one Re complex was higher than that of ReD′Re by a factor of 2. If the initial photoinduced electron transfer from BIH occurred mainly through the S₁ state of the porphyrin dimer (reductive quenching),²⁸ the difference in photocatalytic activities between ReD′ and ReD′Re would be larger. The results indicate that the reductive quenching mechanism can be excluded. Thus, the electron transfer from BIH is expected to occur mainly through the subsequent state after the intramolecular electron transfer (Fig. S85†) (oxidative quenching). The mixed systems of D′ and fac-Re(bpy)(CO)₃Br (Re) show a mainly relatively lower activity than the connected systems, ReD′ and ReD′Re, but the mixed system using only two equivalents of Re shows a moderate activity. In the mixed system, the intermolecular electron transfer from the S₁ state of D′ to Re (oxidative quenching) is unlikely to occur due to the low concentration of Re. Considering that the reaction through the T₁ state of D′ is thermodynamically difficult (vide infra), the reductive quenching from BIH to the S₁ state of D′ would start under the condition of high BIH concentration (0.1 M). We believe that the S₁ state of D′ has a sufficient lifetime to be quenched by the concentrated BIH (η_q = 26% at 0.1 M from Fig. S86†).

Fig. 5 CO formation with time under irradiation from LED lamps at 560 nm with a merry-go-round apparatus for CO₂-saturated DMA-TEOA (5 : 1 v/v, total 1.0 mL) solutions containing BIH (0.1 M) in the presence of ReD′ (0.025 mM), ReD′Re (0.025 mM), ZnP-phen=Re (0.050 mM), and a mixed system of D′ (0.025 mM) and fac-Re(bpy)(CO)₃Br (Re: 0.025 mM or 0.05 mM).

The CO production by ReD′ is much larger than that of ZnP-phen=Re (Fig. 5). The photocatalytic CO₂ reduction upon irradiation at 560 nm of DMA solutions (1.0 mL) containing ZnP-phen=Re (0.05 mM) and BIH (0.1 M) gave a linear increase in CO production in both the absence and presence of PhOH, whereas the addition of TEOA enhanced the CO production at the initial stage but decreased the durability (Fig. S87†). The TON_CO of ZnP-phen=Re reached 172 after 18 h when PhOH (0.1 M) was added but was 1/16 compared with that of ReD′ in DMA-TEOA (Table S1†).

The UV-vis absorption spectral changes of the reaction solutions containing either ReD′ or ZnP-phen=Re during the photocatalytic reactions under CO₂ and Ar atmospheres are shown in Fig. 6. Under the CO₂ atmosphere, the CO formation catalytically proceeded even under micromolar-order concentration conditions of the photocatalysts. The spectrum of ReD′ was almost unchanged (Fig. 6a), whereas the original bands of ZnP-phen=Re significantly decreased, accompanied by the appearance of a band at 620 nm and a featureless broad band at about 700 nm (Fig. 6c), which are assignable to Zn chlorins^16c,d,31 and the one-electron reduced species (OERS) of Zn porphyrin,³² respectively. Even under an Ar atmosphere, ReD′ showed no spectral change (Fig. 6b), suggesting that there is a rapid back-electron transfer process returning to the ground state in ReD′. In addition, in ZnP-phen=Re, TEOA promotes the formation of chlorins and tends to decrease the OERS of porphyrin (Fig. 6c and d). Visible-light irradiation of tetraphenylporphyrin in the presence of aliphatic amine produces chlorins via an adduct of porphyrin and amine.³³ Thus, the low catalytic durability of ZnP-phen=Re in the presence of TEOA (Fig. S87a†) would come from chlorin formation. In the absence of TEOA, ZnP-phen=Re shows high durability (Fig. S87a†), indicating that the OERS of porphyrin is not a species that directly decomposes the photocatalyst. In contrast to ZnP-phen=Re, TEOA is essential for ReD′ and ReD′Re to obtain high catalytic activity and would not react with the porphyrin dimer to form chlorin-producing adducts.

Fig. 6 UV-vis absorption spectral changes of (a, c and d) CO₂– and (b) Ar-saturated DMA solutions in the (a, b and c) presence and (d) absence of 17 vol% TEOA containing the photocatalysts; ReD′(a and b; 2.5 μM), ZnP-phen=Re (c and d; 5.0 μM), and 0.01 M BIH during irradiation at 560 nm (3.5 × 10⁻⁸ einstein s⁻¹) from a Xe lamp, 25 °C. The peak marked with an asterisk (*) in Fig. 6c is assigned to chlorins. Red dotted lines show the spectra of resulting solutions after standing for 2 d in the dark.

Table 2 summarizes the reaction quantum yields (Φ_CO) of CO and the TON_CO using ReD′ and ReD′Re in DMA-TEOA (5 : 1 v/v). The Φ_CO upon excitations to the upper excited singlet states (S₂) by λ_ex = 420 and 450 nm and to S₁ by λ_ex = 560 nm has similar values of 2%, indicating that there is no deactivation process from S₂ to S₁. In addition, the Φ_CO values are similar between ReD′Re and ReD′, indicating that one Re complex site is sufficient for the CO₂ reduction reaction and that the electron transfer from BIH to the S₁ of the porphyrin dimer is a minor process in the photocatalytic CO₂ reduction described above. Considering that the Φ_CO of ZnP-phen=Re is 8% in DMA containing PhOH,^18b the Φ_CO of ReD′ does not seem to reflect a TON_CO of ReD′ much larger than that of ZnP-phen=Re using the merry-go-round apparatus (Fig. 5 and Table S1†). Focusing on the fact that the most different factor between the Φ_CO and TON_CO measurements is light irradiation intensity,³⁴ we investigated the dependence of Φ_CO on the light intensity and found that the dependence between ReD′ and ZnP-phen=Re was significantly different.

Table 2 Quantum yield and products for ReD′ and ReD′Re as a function of excitation wavelength

	Quantum yield (ΦCO)^a			Product/μmol (TON(dimer)^b)
	420 nm	450 nm	560 nm	CO	H2	HCOOH
a Dependence of quantum yield on the excitation wavelength. Quantum yield as a ratio of number of CO and absorbed photons (light intensity: 2.6 × 10^−8, 2.6 × 10^−8, and 3.6 × 10^−8 einstein s^−1 for 420, 450, and 560 nm, respectively (Fig. S89)). A Xe lamp with band path filters was used. b A merry-go-round apparatus equipped with LED lamps was used (λex = 560 nm). [ReD′] = [ReD′Re] = 0.025 mM and [BIH] = 0.1 M in CO2-saturated DMA-TEOA (5 : 1 v/v). The turnover number is based on the porphyrin dimer unit. c n.d., not detected.
ReD′	0.023	0.032	0.024	70.5 (2820)	n.d.^c	n.d.^c
ReD′Re	0.020	0.019	0.016	62.5 (2500)	n.d.^c	n.d.^c

---

The plots of Φ_CO under favorable conditions for ReD′ and ZnP-phen=Re with respect to the light irradiation intensity are shown in Fig. 7. We controlled the light intensity using neutral density filters of 560 nm monochromic light from a Xe lamp with a band path filter. The Φ_CO of ReD′ was independent of the light intensity (red circles in Fig. 7 and S90†), whereas the Φ_CO of ZnP-phen=Re significantly decreases as the light intensity increases. The decrease in the reaction quantum yield with increasing light intensity is rather commonly observed in photocatalytic systems, because OERS accumulation reduces the catalytic activity due to the inner-filter effect, which is caused by OERS absorption.^35,36 By contrast, the OERS of ReD′, which absorbs 560 nm light, did not form during the irradiation (Fig. 6a), resulting in the independence of Φ_CO on the light intensity. The strong light from LED lamps with the merry-go-round apparatus lowered the photocatalytic activity of ZnP-phen=Re by the inner-filter effect of the OERS, whereas the photocatalytic activity of ReD′ was not lowered due to no OERS accumulation (Fig. 6a). The independence of ReD′ on light intensity was also observed on the TON_CO using the merry-go-round apparatus (Fig S91†).

Fig. 7 Φ _CO as a function of light irradiation intensity (Xe lamp, λ_ex = 560 nm) for ReD′ (0.1 mM) in DMA-TEOA (5 : 1 v/v) solutions containing BIH (0.1 M) (red circles) and ZnP-phen=Re (0.3 mM) in DMA solutions containing BIH (0.05 M) and PhOH (0.1 M) (blue squares).

Another interesting difference between ReD′ and ZnP-phen=Re is the dependence of the catalytic activity on the BIH concentration. The catalytic activity of ZnP-phen=Re does not depend on the concentration of BIH because the photoinduced electron transfer from BIH occurs via the long-lived T₁ (K_SV = 180 000 M⁻¹) with reductive quenching from the T₁ state.^18b Upon excitation at 560 nm, the overlaid plots of CO production against the absorbed photon using 0.01 M and 0.05 M BIH show the independence of the BIH concentration in ZnP-phen=Re (Fig. S92†). By contrast, the Φ_CO and TON_CO strongly depend on the BIH concentration in ReD′ (Fig. 8 and S93†). The simulation of the quenching efficiencies on the state that mediates the electron transfer from BIH gave the same order values of K_SV = 8 M⁻¹ and 15 M⁻¹ from the plots of Φ_CO and TON_CO, respectively. The obtained value was much smaller than that of ZnP-phen=Re, suggesting that a relatively short-lived state such as the CS state (ZnP˙⁺−Re˙⁻ in Fig. S85†) contributes to the electron transfer from BIH. In fact, the coordination of the imidazolyl group to the Zn porphyrin lowers the energy level of T₁ (ref. 27) and shifts the reduction potential of the porphyrin to the negative side,^26,37 so that the electron transfer from BIH to the T₁ of the porphyrin dimer forming the OERS of the porphyrin is thermodynamically unfavorable (Fig. S94†).

Fig. 8 (a) CO formation during irradiation at 560 nm (Xe lamp, 3.6 × 10⁻⁸ einstein s⁻¹) using BIH (red circle, 0.1 M; green diamond. 0.026 M; blue triangle, 0.01 M) and ReD′ (0.1 mM) in CO₂-saturated DMA-TEOA (5 : 1 v/v). (b) The reaction quantum yield as a function of BIH concentration. The quenching efficiency relative to η_q at [BIH] = 0.1 M was calculated using η_q = [BIH] K_SV/(1 + [BIH] K_SV) assuming K_SV = 8 M⁻¹.

Photocatalytic CO₂ reduction in DMSO

The above results indicate that ReD′ (ReD′Re), which has the porphyrin dimer as the photosensitizer, proceeds by a different mechanism from that of ZnP-phen=Re, and shows an extremely high activity under irradiation with strong light. However, the bond distance and direction between the porphyrin and the Re complex in ReD′ (ReD′Re) were different from those of ZnP-phen=Re. The differences could affect the catalytic activity. Therefore, to clarify whether the dimeric structure of the porphyrin contributes to the high activity, photocatalytic reactions were performed under the same conditions using ReDRe and ReD′Re (Scheme 2) in a coordination solvent of DMSO. Here, DMSO is a suitable solvent for photocatalytic CO₂ reduction using a Re diimine tricarbonyl complex in the presence of BIH and TEOA.³⁸ Under micromolar-order concentration conditions, ReDRe dissociated into the corresponding monomer (Fig. 9). The first oxidation and reduction potentials of the porphyrin dimer D′ and monomer D (Scheme 3) were almost the same in DMSO in differential pulse voltammetry (DPV) (Fig. S95†) and the potentials of ReD′Re were similar to those in DMA. In addition, the fluorescence of the monomer prepared from ReDRe in DMSO was quenched by the Re complex unit, indicating that the intramolecular electron transfer from the S₁ of the porphyrin unit to the Re complex unit would occur in the monomer as well as in the dimer (Fig. S96†).

Fig. 9 (Top) dissociation of the porphyrin dimer (ReDRe) into porphyrin monomers in DMSO. (Bottom) CO formation with time during irradiation at 560 nm from LED lamps with a merry-go-round apparatus for CO₂-saturated DMSO-TEOA (5 : 1 v/v) solutions containing ReD′Re (2.5 μM) and ReDRe (2.5 μM) in the presence of BIH (0.01 M). The TON_CO is calculated against the Re atom.

The CO production with time using dilute ReDRe and ReD′Re of the micromolar range (2.5 μM) is shown in Fig. 9. From the UV-vis absorption spectrum (Fig. 10a inset), ReDRe completely dissociates into the monomeric structure under these reaction conditions (2.5 μM ReDRe). The Q band is inert to whether the porphyrin is the monomer or dimer, and the initial absorbance of ReDRe and ReD′Re at 560 nm is almost the same even in DMSO. Thus, 560 nm was selected as the excitation wavelength of the photocatalytic reaction. In the system using the porphyrin dimer (ReD′Re), the TON_CO was much larger than in the system using the porphyrin monomer (ReDRe). The UV-vis spectral changes during irradiation show that the spectrum of ReD′Re hardly changed (Fig. 10), but the monomer spectrum shows significant reductions in both the Soret and Q bands (Fig. 10a inset), indicating that the porphyrin monomer decomposed into colorless species. To investigate the degradation process, we carried out the irradiation of a CO₂-saturated DMSO-TEOA (5 : 1 v/v) containing a higher concentration of ReDRe (10 μM) in the presence of BIH (0.05 M). The UV-vis absorption spectrum after the irradiation for 7 h (Fig. S97†), when the rate of CO formation became slow, showed the appearance of distinct absorption peaks at 500 and 760 nm for bacteriochlorins³⁹ and 610 nm for chlorins.^16c,40 Thus, as shown in the literature,^16c it is though that the monomeric porphyrin is degraded to colorless species via chlorins and bacteriochlorins, whose process is initiated by two electron reduction of Zn porphyrin (Fig. S98a†). The system using ReD′Re reaches TON_CO ∼ 1800 (based on the Re atom). Considering that the equivalent of added BIH against the Re atom was 2000, BIH donated two electrons and an almost quantitative amount of added BIH was consumed.^28,41

Fig. 10 (a) UV-vis absorption spectral change of a CO₂-saturated DMSO-TEOA (5 : 1 v/v) solution containing BIH (0.01 M) and ReD′Re (2.5 μM) during irradiation at 560 nm (Xe lamp, 3.5 × 10⁻⁸ einstein s⁻¹) and 25 °C. Inset shows change using ReDRe instead of ReD′Re. Red dotted lines show the spectra after the irradiation for 60 min. (b) Relative absorbance with time at 430 nm of the reaction solutions during the irradiation.

Reaction mechanisms in systems using the porphyrin dimer

Based on the above results, a plausible reaction mechanism is shown in Fig. 11. The porphyrin dimer in ReD′ and ReD′Re can absorb a wide wavelength range of visible light to give the S₁ state, which would efficiently produce the CS state by intramolecular electron transfer. Although the CS state can be stabilized by the dimeric structure in ReD′ and ReD′Re,⁷ a rapid back-electron transfer process generally competes with the catalytic reaction. The nonspectral change in ReD′ under an Ar atmosphere (Fig. 6b) is explained by the rapid back-electron transfer process. The proton promotes the formation of an adduct between the reduced Re complex and CO₂,⁴² and TEOA coordinates to the Re complex in the ground state to form an adduct with CO₂.^30,38 Thus, with the assistance of PhOH or TEOA, the transient reduced Re complex would rapidly react with CO₂ to afford a more stable reaction intermediate that suppresses the back-electron transfer. Considering the fact that the formation of Re–COOH should be rather slower as reported in the literature,⁴³ in the presence of PhOH the hole scavenging process by BIH more probably occurs before the Re–C bond formation. In this mechanism, BIH is used to neutralize the cation radical of the porphyrin to give the ground state porphyrin, and no long-lived OERS of the porphyrin was formed.

Fig. 11 (a) A plausible reaction mechanism of ReD′ (ReD′Re) in the presence of a proton source such as PhOH.^42,43 (b) The CO₂-capturing reaction with the Re(I) complex with the assistance of TEOA.^30,38

In ZnP-phen=Re, it is thought that the adjacent Re atom enhances the intersystem crossing from the S₁ to the T₁ of porphyrin and the initial electron transfer mainly occurs via the long-lived T₁.^18b Thus, the efficient electron transfer from BIH to T₁ allows the catalytic reaction to proceed with the same activity even at a low concentration of the electron donor. However, although the accumulation of the porphyrin's OERS would not directly lead to a decrease in the catalytic durability, the inner-filter effect by the OERS under strong light irradiation lowers the reaction efficiency. By contrast, the formation of the CS state in ReD′ and ReD′Re causes independence of the CO₂ reduction reaction rate on light intensity because no species that inhibits light absorption is accumulated.

The porphyrin dimer delocalizes the radical cation over the two porphyrin units, giving the stable CS state that can promote the subsequent reaction before the back-electron transfer. The dimeric structure has been reported to accelerate the CS rate and decelerate the charge-recombination rate.⁷ReD′ had a higher quenching efficiency in DMA against D′ (88% in Fig. 3b and Table 1) than ReDRe against D in DMSO (73% in Fig. S96†), which might support the acceleration of the CS rate. The dimer is superior to the monomer in durability (Fig. 9). Judging from the UV-vis spectrum (Fig. S97†), the irradiation of the DMSO-TEOA solution containing ReDRe (monomer) in the presence of BIH gave chlorins and bacteriochlorins. Because BIH produces a very strong reductant BI˙,^28,41 the two-electron reduced species on the porphyrin unit can be generated. Thus, it is thought that the monomeric structure is easily decomposed by two electron reduction, whereas the dimeric structure can suppress degradation by sharing electrons between the two porphyrins of the dimer (Fig. S98b†), even when a two-electron reduced species is produced on the porphyrin unit.⁴⁴ In the case of ZnP-phen=Re, the high durability might be due to the proximity between Re and the porphyrin, causing rapid electron transfer to the Re complex unit and preventing the accumulation of two electrons on the porphyrin. The detailed reaction mechanisms of ReD′ and ReD′Re in addition to ZnP-phen=Re are currently under investigation using time-resolved measurements.

Conclusion

The special-pair mimic porphyrin dimer was used here as a photosensitizer in a photocatalytic reaction for the first time. Using the fact that the coordination bond of the dimer can be easily rearranged and that the coordination structure can be fixed by a covalent bond by the ring-closing metathesis reaction, dimers having Re complex(es) on either one side (ReD′) or two sides (ReD′Re) have been synthesized. The photocatalytic CO₂ reductions using ReD′ and ReD′Re selectively gave CO, the reaction quantum yield of which was estimated to be 2%, independent of the excitation wavelength, light intensity, and number of Re complex units. A high TON_CO reaching >2800 after 18 h was observed in ReD′ under irradiation of relatively strong light, which is an order of magnitude higher than the value (172 after 18 h) of ZnP-phen=Re under the optimal conditions. The high catalytic activity stems from the fact that the absorption spectrum of the dimer remains almost unchanged during the irradiation and there is no deactivation caused by the internal filter effect that the OERS absorbs light as observed in general photosensitizers. It is thought that the catalytic reaction proceeds via the CS state formed by the intramolecular electron transfer from the S₁ of the porphyrin unit to the Re complex unit, and that BIH neutralizes the cation radical of the porphyrin. Thus, the excessive OERS does not form during the irradiation. In addition, it was shown that the dimeric structure contributes to the improvement of the catalytic durability of CO₂ reduction by sharing electrons between the two porphyrins of the dimer. In this work, we prepared a heterodimer of ReD′ using a simple porphyrin D having a tolyl group. The simple porphyrin can be substituted with an imidazolyl Zn porphyrin having more functional units. As a result, it may be possible to create functional heterodimers that can bind to semiconductors and supramolecular systems such as chain structures and cyclic structures with a higher absorption ability.

Experimental section

General procedure

All chemicals and solvents were of commercial reagent quality and were used without further purification unless otherwise stated. Dry diethyl ether was prepared by distillation over benzophenone/Na. DMA was dried over molecular sieves of size 4 Å and distilled under reduced pressure. The Grubbs catalyst (1^st generation), benzylidene-bis(tricyclohexylphosphine)dichlororuthenium, was purchased from Aldrich. 1-Methylimidazole-2-carbaldehyde, meso-(3-allyloxypropyl)dipyrromethane, and BIH were synthesized according to the literature.^9,45 FINEOXOCOL180 was provided by Nissan Chemical Corporation. Super dehydrated N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) were purchased from Fujifilm Wako Pure Chemical Corporation. TEOA was distilled under reduced pressure. The reaction was monitored on silica-gel 60F₂₅₄ TLC plates (Merck). 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 and ¹³C NMR, distortionless enhancement by polarization transfer 135 (DEPT 135), ¹H–¹H correlation spectroscopy (COSY), ¹H–¹³C heteronuclear single quantum correlation (HSQC), and ¹H–¹³C heteronuclear multiple bond correlation (HMBC) spectra were recorded by using a JEOL JNMECS-300, JNMECZ-400, and JNMECS-500. Chemical shifts were recorded in parts per million (ppm) relative to tetramethylsilane. UV-vis absorption spectra were collected using square cells (path length = 1.0 cm) on JASCO V-650 and V-660 spectrometers, and a Asahi Spectra PRA-201. Steady-state emission 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. Fluorescence quantum yields were determined from the integrated ratios of the fluorescence spectra relative to that of ZnTPP (Φ_f = 3.3% in toluene).⁴⁶ Their values were corrected with the refractive indices of the used solvents. Hi-resolution MALDI–TOF mass spectra were collected on a JEOL JMS S-3000 with dithranol or trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]-malononitrile (DCTB) as a matrix. FTIR spectra were recorded in KBr using a JASCO FT/IR-4600. Analytical GPC-HPLCs using pyridine as the eluent were performed on a PU-2080plus and MD-2018plus (JASCO) system equipped with two TSK G2500H_HR (Tosoh, exclusion limit: 20 000 Da) and one TSK G2000H_HR (Tosoh, exclusion limit: 10 000 Da) columns. Preparative HPLCs were carried out on an LC-908 (JAI) attached to one TSK G2500H_HR and one G2000H_HR columns eluted with pyridine. The differential pulse voltammogram (DPV) was recorded using an ALS-H/CHI Model 612E 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 DMSO). Tetrabutylammonium hexafluorophosphate (ⁿBu₄NPF₆) recrystallized from ethyl acetate was used as a supporting electrolyte. Ferrocene was used as an external standard, and all potentials were referenced to the ferrocene/ferrocenium couple (E_1/2 = +0.191 V vs. Ag/AgNO₃).

5-(4-Iodophenyl)-15-(1-isostearylimidazole-2-yl)-10,20-bis(3-allyloxypropyl)porphyrin (Im^IS-H₂P-PhI). In a three-necked 500 mL flask, Im^IS-CHO (420 mg, 1.2 mmol), 4-iodobenzaldehyde (270 mg, 1.2 mmol), and meso-(3-allyloxypropyl)dipyrromethane (740 mg, 3.0 mmol), and CHCl₃ (300 mL) were placed. The mixture was degassed by bubbling with Ar gas for 15 min and then trifluoroacetic acid (TFA, 0.39 mL, 4.8 mmol) in CHCl₃ (ca. 2 mL) was slowly added. After being stirred for 3.5 h at rt, the mixture was added by triethylamine (0.68 mL, 4.9 mmol) and p-chloranil (1.4 g, 5.2 mmol) and stirred at rt overnight. The resulting mixture was evaporated to dryness and the residue was purified with an alumina column (eluent: CHCl₃/acetone = 10/1) and silica gel columns (eluent: CHCl₃ → CHCl₃/acetone = 10/1). The collected solution was evaporated to dryness, giving a purple solid (230 mg, 19% yield). TLC (silica gel, CHCl₃/acetone = 10/1) R_f = 0.5; MALDI-TOF-mass (dithranol) m/z [M + H]⁺ 1027.5002, calcd for [C₅₉H₇₆IN₆O₂]⁺ 1027.5068; ¹H NMR (400 MHz, CDCl₃) δ/ppm = 9.54 (m, 2H, β-pyrrole), 9.49 (m, 2H, β-pyrrole), 8.96–8.76 (m, 4H, β-pyrrole), 8.12 (d, J = 8.7 Hz, 1H, phenylene), 8.12 (d, J = 8.7 Hz, 1H, phenylene), 7.98 (m, 1H, phenylene), 7.86 (m, 1H, phenylene), 7.74 (s, 0.4H, Im), 7.72 (s, 0.6H, Im), 7.52 (s, 0.4H, Im), 7.50 (s, 0.6H, Im), 6.09 (m, 2H, –CH=), 5.44 (d, J = 17 Hz, 2H, =CH₂), 5.28 (d, J = 10 Hz, 2H, =CH₂), 5.10 (m, 4H, –CH₂CH₂CH₂–), 4.08 (d, J = 5.5 Hz, 4H, –OCH₂–), 3.85–8.40 (m, 2H, CH₂), 3.66 (m, 4H, –CH₂CH₂CH₂–), 2.78 (m, 4H, –CH₂CH₂CH₂–), 1.38–8.40 (m, 2H, CH₂), 1.38–(–0.80) (m, 35H), −2.64 (s, 0.6H, NH), −2.66 (s, 0.4H, NH);¹³C NMR (100 MHz, CDCl₃) δ/ppm = 150–145 (C), 148.95 (C), 148.66 (C), 142.30 (C), 142.27 (C), 136.10 (CH), 135.85 (CH), 135.17 (CH), 131.99–131.50 (CH), 131.22–130.74 (CH), 129.66–128.90 (CH), 128.57 (CH), 128.51 (CH), 128.12 (CH), 128.10 (CH), 128.02–127.55 (CH), 121.41 (CH), 121.29 (CH), 120.33 (CH), 120.27 (CH), 119.61 (C), 119.56 (C), 119.48 (C), 119.09 (C), 119.02 (C), 117.00 (CH₂), 105.31 (C), 105.26 (C), 105.11 (C), 94.21 (C), 72.14 (CH₂), 69.29 (CH₂), 51.20 (CH₂), 50.75 (CH₂), 50.68 (CH₂), 50.46 (CH₂), 49.39 (CH₂), 49.29 (CH₂), 49.20 (CH₂), 49.12 (CH₂), 48.93 (CH₂), 47.70 (CH or CH₃), 47.42 (CH₂), 47.21 (CH₂), 46.72 (CH₂), 46.32 (CH₂), 37.96 (CH₂), 37.47 (CH₂), 37.23 (CH₂), 36.90 (CH₂), 36.59 (CH₂), 31.44 (CH₂), 31.18 (C), 30.93 (C), 30.75 (C), 30.61 (C), 30.55 (C), 30.23 (C), 30.03 (CH or CH₃), 29.89 (CH or CH₃), 29.86 (CH or CH₃), 29.64 (CH or CH₃), 29.41 (CH or CH₃), 29.18 (CH or CH₃), 29.10 (CH or CH₃), 29.03 (CH or CH₃), 28.61 (CH or CH₃), 28.38 (CH or CH₃), 28.16 (CH or CH₃), 28.00 (CH or CH₃), 27.77 (CH or CH₃), 26.85 (CH₂), 26.72 (CH₂), 24.61 (CH₂), 24.55 (CH₂), 22.51 (CH or CH₃), 22.15 (CH or CH₃), 22.08 (CH or CH₃), 22.01 (CH or CH₃), 16.61 (CH or CH₃), 16.51 (CH or CH₃), 16.40 (CH or CH₃), 16.22 (CH or CH₃).

Zinc(II) 5-(4-iodophenyl)-15-(1-isostearylimidazole-2-yl)-10,20-bis(3-allyloxypropyl)porphyrin ([Im^IS-ZnP-PhI]₂). Zinc(II) acetate (330 mg, 1.8 mmol) in methanol (ca. 3 mL) was added to a solution of Im^IS-H₂P-PhI (230 mg, 0.22 mmol) in CHCl₃ (230 mL). The mixture was stirred overnight at rt. The resulting solution was washed with water. The organic layer was dried over anhydrous Na₂SO₄ and evaporated to dryness, affording the titular compound as a purple solid (230 mg, 96% yield). TLC (silica gel, CHCl₃/acetone = 10/1) R_f = 0.9; MALDI-TOF-mass (dithranol) m/z [M + H]⁺ 1089.4229, calcd for [C₅₉H₇₄IN₆O₂Zn]⁺ 1089.4204; ¹H NMR (400 MHz, CDCl₃) δ/ppm = 9.63 (m, 2H, β-pyrrole), 9.00 (m, 2H, β-pyrrole), 8.91 (m, 2H, β-pyrrole), 8.43 (m, 1H, phenylene), 8.27 (d, J = 7.9 Hz, 1H, phenylene), 8.11 (d, J = 7.9 Hz, 1H, phenylene), 7.89 (m, 1H, phenylene), 6.22 (m, 2H, –CH= ), 5.57 (m, 2H, =CH₂), 5.56 (s, 1H, Im), 5.42 (m, 2H, β-pyrrole), 5.36 (d, J = 10 Hz, 2H, =CH₂), 5.22 (m, 4H, –CH₂CH₂CH₂–), 4.24 (m, 4H, –OCH₂–), 3.92 (m, 4H, –CH₂CH₂CH₂–), 3.02 (m, 4H, –CH₂CH₂CH₂–), 2.06 (m, 1H, Im), 1.84–1.55 (m, 2H, CH₂), 0.30–(–1.77) (m, 35H); ¹³C NMR (100 MHz, CDCl₃) δ/ppm = 151.14 (C), 150.05 (C), 148.65 (C), 148.38 (C), 146.02 (C), 144.10 (C), 136.47 (CH), 136.41 (CH), 135.59 (CH), 135.53 (CH), 135.46 (CH), 135.32 (CH), 131.63 (CH), 131.50 (CH), 129.45 (CH), 129.31 (CH), 128.48 (CH), 127.84 (CH), 121.20 (CH), 119.60 (C), 119.33 (C), 117.66 (CH), 116.88 (CH₂), 96.93 (C), 93.41 (C), 72.18 (CH₂), 70.20 (CH₂), 50.53 (CH₂), 50.22 (CH₂), 50.01 (CH₂), 49.91 (CH₂), 47.75 (CH₂), 47.58 (CH₂), 47.38 (CH₂), 47.31 (CH₂), 46.50 (CH₂), 45.81 (CH or CH₃), 45.40 (CH or CH₃), 45.21 (CH or CH₃), 44.98 (CH or CH₃), 44.800 (CH or CH₃), 38.74 (CH₂), 35.60 (CH₂), 35.44 (CH₂), 32.16 (CH₂), 30.26 (C), 30.21 (C), 30.16 (C), 30.05 (C), 29.80 (CH₂), 29.60 (CH or CH₃), 29.44 (CH or CH₃), 29.39 (CH or CH₃), 29.31 (CH or CH₃), 28.65 (CH or CH₃), 28.48 (CH or CH₃), 29.21 (CH or CH₃), 27.01 (CH or CH₃), 26.86 (CH or CH₃), 26.63 (CH or CH₃), 21.92 (CH or CH₃), 21.56 (CH or CH₃), 21.27 (CH or CH₃), 15.74 (CH or CH₃).

Zinc(II) 5-{4-(2,2′-bipyrid-5-yl)phenyl}-15-(1-isostearylimidazole-2-yl)-10,20-bis(3-allyloxypropyl)porphyrin ([Im^IS-ZnP-Ph-5Bpy]₂). In a 100 mL three-necked flask, [Im^IS-ZnP-PhI]₂ (310 mg, 0.28 mmol), 5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,2′-bipyridine (120 mg, 0.42 mmol), Cs₂CO₃ (26.0 mg, 0.80 mmol), toluene (30 mL), DMF (15 mL), and water (2.3 mL) were placed. The mixture was degassed by freeze–thaw cycles and replaced with Ar gas and Pd(PPh₃)₄ (68 mg, 0.059 mmol) was added and stirred at 90 °C for 4 h. CHCl₃ (100 mL) and water (100 mL) were added to the resulting solution, and the organic layer was washed with water (100 mL) and brine (100 mL). The organic layer was passed through Phase Separator paper (Whatman). The solvent was evaporated to dryness and the residue was purified with a flush silica gel column (eluent: CHCl₃ → CHCl₃/acetone = 10/1). Further purification by precipitation with CHCl₃, methanol and water afforded 280 mg (89% yield) of the titular compound as a dark purple solid. TLC (silica gel, CHCl₃/acetone = 10/1) R_f = 0.1; MALDI-TOF-mass (dithranol) m/z [M + H]⁺ 1117.5768, calcd for [C₆₉H₈₁N₈O₂Zn]⁺ 1117.5768; ¹H NMR (500 MHz, CDCl₃) δ/ppm = 9.67 (m, 2H, β-pyrrole), 9.40 (s, 1H, bpy 6), 9.11 (m, 2H, β-pyrrole), 8.92 (m, 2H, β-pyrrole), 8.82 (m, 2H, phenylene and bpy 6′), 8.71 (d, J = 7.9 Hz, 1H, bpy 3), 8.61 (d, J = 7.7 Hz, 1H, bpy 3′), 8.49 (d, J = 7.9 Hz, 1H, bpy 4), 8.31 (brs, 1H, phenylene), 8.26 (d, J = 7.6 Hz, 1H, phenylene), 8.10 (brs, 1H, phenylene), 7.94 (t, J = 7.7 Hz, 1H, bpy 4′), 7.41 (t, J = 7.7 Hz, 1H, bpy 5′), 6.21 (m, 2H, -CH= ), 5.56 (s, 1H, Im), 5.54 (m, 2H, =CH₂), 5.44 (m, 2H, β-pyrrole), 5.37 (m, 2H, =CH₂), 5.26 (m, 4H, –CH₂CH₂CH₂–), 4.26 (m, 4H, –OCH₂–), 3.94 (m, 4H, –CH₂CH₂CH₂–), 3.05 (m, 4H, –CH₂CH₂CH₂–), 2.16 (m, 1H, Im), 2.09–(–1.72) (m, 35H); ¹³C NMR (125 MHz, CDCl₃) δ/ppm = 156.20 (C), 155.23 (C), 151.15 (C), 150.05 (C), 149.50 (CH), 148.92 (C), 148.87 (C), 148.34 (C), 148.13 (CH), 146.28 (C), 146.09 (C), 144.68 (C), 137.20 (CH), 136.79 (C), 136.29 (C), 135.65 (CH), 135.57 (CH), 135.48 (CH), 131.74 (CH), 129.60 (CH), 129.43 (CH), 129.29 (CH), 128.46 (CH), 127.84 (CH), 125.12 (CH), 124.90 (CH), 123.92 (CH), 121.41 (CH), 121.33 (CH), 120.37 (C), 119.32 (C), 119.24 (C), 117.59 (CH), 116.87 (CH₂), 116.84 (CH₂), 96.86 (C), 72.17 (CH₂), 70.22 (CH₂), 67.87 (CH₂), 50.58 (CH₂), 50.25 (CH₂), 50.08 (CH₂), 49.91 (CH₂), 47.82 (CH₂), 47.39 (CH₂), 46.53 (CH₂), 45.40 (CH or CH₃), 45.25 (CH or CH₃), 45.01 (CH or CH₃), 44.83 (CH or CH₃), 39.01 (CH or CH₃), 38.75 (CH₂), 36.46 (CH₂), 36.24 (CH₂), 35.64 (CH₂), 35.50 (CH₂), 32.19 (CH or CH₃), 30.67 (C), 30.49 (C), 30.42 (C), 30.29 (C), 30.21 (C), 29.81 (C), 29.63 (CH or CH₃), 29.53 (CH or CH₃), 29.48 (CH or CH₃), 29.45 (CH or CH₃), 29.41 (CH or CH₃), 29.34 (CH or CH₃), 29.09 (CH₂), 28.63 (CH or CH₃), 28.50 (CH or CH₃), 28.25 (CH or CH₃), 28.06 (CH or CH₃), 27.03 (CH or CH₃), 26.91 (CH or CH₃), 26.67 (CH or CH₃), 25.49 (CH₂), 24.09 (CH₂), 23.95 (CH₂), 23.81 (CH₂), 23.75 (CH₂), 23.07 (CH₂), 21.95 (CH or CH₃), 21.91 (CH or CH₃), 21.60 (CH or CH₃), 21.34 (CH or CH₃), 21.28 (CH or CH₃), 16.11 (CH or CH₃), 15.78 (CH or CH₃).

Dimer of Zn porphyrin=Re complex dyad ([Im^IS-ZnP-Ph-5Bpy=Re]₂, ReDRe). In a 200 mL flask, [Im^IS-ZnP-Ph-5Bpy]₂ (130 mg, 0.12 mmol), Re(CO)₅Br (46 mg, 0.11 mmol), and toluene (135 mL) were placed. The mixture was heated to 90 °C and stirred for 2 days. Pyridine (2 mL) was added to the resulting solution and the solvent was evaporated to dryness. The residue was purified with a flush silica gel column (eluent: CHCl₃ → CHCl₃/acetone = 20/1). The collected fractions were evaporated to dryness, giving the titular compound as a purple solid (120 mg, 72% yield). TLC (silica gel, CHCl₃/acetone = 10/1) R_f = 0.5; HPLC (GPC, pyridine, 1.0 mL min⁻¹) RT = 23.4 min; MALDI-TOF-mass (dithranol) m/z [M + H]⁺ 1465.4329, calcd for [C₇₂H₈₁BrN₈O₅ReZn]⁺ 1465.4329; ¹H NMR (500 MHz, CDCl₃) δ/ppm = 9.69 (m, 3H, β-pyrrole and bpy 6), 9.17 (d, J = 5.6 Hz, 1H, bpy 6′), 9.07 (m, 2H, β-pyrrole), 8.96 (m, 2H, β-pyrrole), 8.90 (m, 1H, phenylene), 8.53 (q, J = 7.7 Hz, 1H, bpy 3), 8.35 (m, 1H, bpy 3′), 8.27 (m, 2H, phenylene and bpy 4), 8.18 (m, 1H, phenylene), 8.06 (m, 2H, phenylene and bpy 4′), 7.57 (t, J = 6.6 Hz, 1H, bpy 5′), 6.21 (m, 2H, –CH=), 5.58 (s, 1H, Im), 5.54 (m, 2H, =CH₂), 5.44 (m, 2H, β-pyrrole), 5.36 (m, 2H, =CH₂), 5.25 (m, 4H, –CH₂CH₂CH₂–), 4.25 (m, 4H, –OCH₂–), 3.94 (m, 4H, –CH₂CH₂CH₂–), 3.05 (m, 4H, –CH₂CH₂CH₂–), 2.13 (m, 1H, Im), 2.09–(–1.77) (m, 35H); ¹³C NMR (125 MHz, CDCl₃) δ/ppm = 197.17 (CO), 196.88 (CO), 189.20 (CO), 155.80 (C), 153.91 (C), 153.52 (CH), 151.86 (CH), 151.20 (C), 150.25 (C), 148.65 (C), 148.34 (C), 146.61 (C), 145.97 (C), 140.79 (C), 139.00 (CH), 137.04 (CH), 135.99 (CH), 135.88 (CH), 135.43 (CH), 133.18 (C), 131.64 (CH), 131.51 (CH), 129.60 (C), 129.59 (CH), 129.40 (CH), 128.75 (CH), 127.91 (CH), 127.00 (CH), 125.40 (CH), 125.15 (CH), 123.42 (CH), 123.19 (CH), 121.26 (CH), 119.47 (C), 116.94 (CH₂), 97.08 (C), 72.19 (CH₂), 70.17 (CH₂), 67.85 (CH₂), 50.58 (CH₂), 50.24 (CH₂), 50.25 (CH₂), 49.88 (CH₂), 47.84 (CH₂), 47.39 (CH₂), 46.53 (CH₂), 45.24 (CH₃), 45.04 (CH₃), 44.84 (CH₃), 38.78 (CH₂), 36.49 (CH₂), 36.23 (CH₂), 35.62 (CH₂), 35.49 (CH₂), 32.20 (CH₂), 30.65 (C), 30.49 (C), 30.43 (C), 30.23 (C), 29.80 (C), 29.63 (CH₂), 29.45 (CH or CH₃), 29.40 (CH or CH₃), 29.34 (CH or CH₃), 29.08 (C), 28.61 (CH or CH₃), 28.48 (CH or CH₃), 28.22 (CH or CH₃), 28.04 (CH or CH₃), 26.84 (CH or CH₃), 26.61 (CH or CH₃), 25.48 (CH₂), 24.06 (CH₂), 23.94 (CH₂), 23.80 (CH₂), 23.07 (CH₂), 21.94 (CH or CH₃), 21.61 (CH or CH₃), 21.29 (CH or CH₃), 16.09 (CH or CH₃), 15.75 (CH or CH₃); FT-IR (KBr) ν_CO = 1900, 1920, 2022 cm⁻¹.

Covalently linked dimers, C-[Im^IS-ZnP-Ph-5Bpy=Re]₂ (ReD′Re) and C-[Im^Me-ZnP-PhCH₃][Im^IS-ZnP-Ph-5Bpy=Re] (ReD′). In a test tube, two kinds of noncovalently linked dimers, [Im^IS-ZnP-Ph-5Bpy=Re]₂ (70 mg, 0.048 mmol) and [Im^Me-ZnP-PhCH₃]₂ (35 mg, 0.048 mmol), were placed. Pyridine (6 mL) was added to it and the dimers were dissociated to the corresponding monomers. The solvent was evaporated to dryness under reduced pressure and the residue was dried in vacuo at 40 °C for 2 h. The solid was dissolved in CHCl₃ (25 mL) and the solution was degassed by freeze–thaw cycles and purged with Ar gas. The Grubbs catalyst (1^st generation, 1.3 mg, and 0.0016 mmol) was added to it, and the reaction mixture was stirred at rt under an Ar atmosphere. The reaction progress was monitored with UV−vis absorption spectra in pyridine. The reaction was completed by gradually adding the Grubbs catalyst (total 18 mg, 0.022 mmol). The resulting solution was evaporated to dryness, and the residue was analyzed with gel permeation chromatography (GPC; two TSK G2500H_HR and one TSK G2000H_HR columns) using pyridine as the eluent. The products of ReD′Re and ReD′ were separated with preparative recycle GPC (one preparative TSK G2500H_HR and one preparative TSK G2000H_HR columns) using pyridine as the eluent. The second fraction was collected and the solvent was evaporated to dryness, giving a purple solid (16 mg, 12% yield) as ReD′Re: HPLC (GPC, pyridine, 1.0 mL min⁻¹) RT = 21.2 min; MALDI-TOF-mass (DCTB) m/z [M + H]⁺ 2880.7907, calcd for [C₁₄₀H₁₅₃Br₂N₁₆O₁₀Re₂Zn₂]⁺ 2880.7910 (peak); ¹H NMR (500 MHz, CDCl₃) δ/ppm = 9.67 (m, 2H, bpy 6), 9.60 (m, 4H, β-pyrrole), 9.16 (m, 2H,bpy 6′), 9.05 (m, 8H, β-pyrrole), 8.92 (m, 2H, phenylene), 8.54 (m, 2H, bpy 4), 8.31 (m, 4H, bpy 3 and bpy 3′), 8.27 (m, 2H, phenylene), 8.20 (m, 2H, phenylene), 8.06 (m, 4H, phenylene and bpy 4′), 7.58 (m, 2H, bpy 5′), 6.46 (s, 2.6H, –CH= (E form)), 6.11 (s, 1.4H, –CH= (Z form)), 5.56 (s, 2H, Im), 5.42 (m, 8H, β-pyrrole and –CH₂CH₂CH₂–), 5.15 (m, 4H, –CH₂CH₂CH₂–), 4.69 (m, 2.8H, –OCH₂-(Z form)), 4.44 (m, 5.2H, –OCH₂-(E form)), 4.21 (m, 7H, –CH₂CH₂CH₂–), 3.95 (m, 1H, –CH₂CH₂CH₂–), 3.29 (m, 1H, –CH₂CH₂CH₂–), 3.13 (m, 3H, –CH₂CH₂CH₂–), 3.00 (m, 4H, –CH₂CH₂CH₂–), 2.12 (m, 2H, Im), 2.07–(–1.33) (m, 70H); ¹³C NMR (125 MHz, CDCl₃) δ/ppm = 197.53 (CO), 197.23 (CO), 197.14 (CO), 196.86 (CO), 189.79 (CO), 189.19 (CO), 155.82 (C), 153.93 (C), 153.51 (CH), 153.40 (CH), 151.85 (C), 150.91 (C), 150.13 (C), 148.38 (C), 146.54 (C), 140.84 (C), 140.78 (C), 139.10 (CH), 138.98 (CH), 137.10 (CH), 137.05 (CH), 135.93 (CH), 135.84 (CH), 133.23 (C), 131.58 (CH), 129.60 (CH), 128.32 (CH), 127.88 (CH), 127.05 (CH), 125.38 (CH), 125.16 (CH), 123.43 (CH), 123.20 (CH), 119.70 (C), 116.91 (CH₂), 70.80 (CH₂), 70.58 (CH₂), 67.24 (CH₂), 49.93 (CH₂), 47.75 (CH₂), 40.06 (CH₂), 39.01 (CH₂), 32.82 (CH₂), 30.50 (C), 30.30 (C), 29.70 (CH or CH₃), 29.40 (CH or CH₃), 28.25 (CH or CH₃), 27.04 (CH or CH₃), 21.58, (CH or CH₃) 15.97 (CH or CH₃); FT-IR (KBr) ν_CO = 1901, 1919, 2021 cm⁻¹. The third fraction of the preparative GPC was collected and the solvent was evaporated to dryness, giving a purple solid (36 mg, 35% yield) as ReD′: HPLC (GPC, pyridine, 1.0 mL min⁻¹) RT = 22.1 min; MALDI-TOF-mass (DCTB) m/z [M]⁺ 2146.6340, calcd for [C₁₁₁H₁₁₄BrN₁₄O₇ReZn₂]⁺ 2146.6292; ¹H NMR (500 MHz, CDCl₃) δ/ppm = 9.70–9.62 (m, 2H, β-pyrrole), 9.56 (m, 2H, β-pyrrole), 9.20 (m, 1H, bpy 6), 9.15 (m, 2H, β-pyrrole), 9.08 (m, 4H, β-pyrrole), 8.90 (m, 3H, β-pyrrole and bpy 6′), 8.64 (m, 1H, bpy 3), 8.50 (brs, 2H, phenylene and bpy 3′), 8.18 (m, 1H, phenylene), 8.03 (m, 1H, phenylene), 7.80 (m, 1H, bpy 4), 7.76–7.60 (m, 5H, phenylene and bpy 4′), 7.36 (m, 2H, bpy 5′), 7.25 (brs, 1H, phenylene), 6.49 (s, 2.75H, –CH= (E form)), 6.14 (s, 1.25H, –CH= (Z form)), 5.57 (m, 3H, β-pyrrole and Im), 5.51 (m, 2H, β-pyrrole), 5.41 (m, 1H, Im), 5.15 (m, 8H, –CH₂CH₂CH₂–), 4.70 (m, 2.5H, –OCH₂–(Z form)), 4.45 (m, 5.5H, –OCH₂–(E form)), 4.28–4.21 (m, 8H, –CH₂CH₂CH₂–), 3.33–3.034 (m, 8H, –CH₂CH₂CH₂–), 2.85 (s, 3H, CH₃), 2.16 (m, 1H, Im), 1.94 (m, 1H, Im), 1.64 (m, 3H, CH₃), 2.09–(–1.77) (m, 35H); ¹³C NMR (125 MHz, CDCl₃) δ/ppm = 197.36 (CO), 197.16 (CO), 197.00 (CO), 196.80 (CO), 189.60 (CO), 188.98 (CO), 155.25 (C), 153.04 (CH), 151.05 (CH), 150.88 (CH), 150.21 (C), 150.10 (C), 149.81 (CH), 149.27 (C), 149.25 (C), 148.42 (C), 148.16 (C), 146.14 (C), 146.06 (C), 141.16 (C), 138.94 (CH), 138.85 (CH), 137.07 (C), 136.08 (CH), 135.90 (CH), 135.46 (CH), 134.82 (CH), 134.61 (CH), 132.84 (C), 132.50 (CH), 132.40 (CH), 131.47 (CH), 129.97 (CH), 129.62 (CH), 128.29 (CH), 127.87 (CH), 127.64 (CH), 127.35 (CH), 127.26 (CH), 126.54 (CH), 125.06 (CH), 123.82 (CH), 122.77 (CH), 121.47 (CH), 119.81 (C), 118.14 (CH), 116.91 (CH₂), 96.94 (C), 96.47 (C), 72.21 (CH₂), 70.83 (CH₂), 70.60 (CH₂), 67.30 (CH₂), 50.58 (CH₂), 49.90 (CH₂), 47.85 (CH₂), 47.34 (CH₂), 45.02 (CH₃), 40.06 (CH₂), 39.01 (CH₂), 35.63 (CH₂), 32.91 (CH₂), 32.03 (C), 30.50 (C), 30.26 (C), 30.19 (C), 29.81 (CH₂), 29.66 (CH or CH₃), 29.61 (CH or CH₃), 29.50 (CH or CH₃), 29.44 (CH or CH₃), 29.40 (CH or CH₃), 29.35 (CH or CH₃), 28.51 (CH or CH₃), 28.37 (CH or CH₃), 28.28 (CH or CH₃), 28.13 (CH or CH₃), 26.93 (CH or CH₃), 21.94 (CH or CH₃), 21.79 (CH or CH₃), 21.60 (CH or CH₃), 21.29 (CH or CH₃), 15.80 (CH or CH₃); FT-IR (KBr) ν_CO = 1900, 1920, 2022 cm⁻¹.

Covalently linked dimer, C-[Im^IS-ZnP-Ph-5Bpy=Re]₂ (ReD′Re). The noncovalently linked dimer, [Im^IS-ZnP-Ph-5Bpy=Re]₂, (4.8 mg, 0.0033 mmol) and CH₂Cl₂ (5.0 mL) were placed in a 20 mL Schlenk tube and the solution was degassed by freeze–thaw cycles. Grubbs catalyst (1^st generation, 0.22 mg, and 0.00027 mmol) was added to it, and the reaction mixture was stirred at rt under an Ar atmosphere. The reaction progress was monitored with UV-vis absorption spectra in pyridine. After being stirred overnight, the resulting solution was evaporated to dryness. The residue was purified with a flush silica gel column (eluent: CHCl₃ → CHCl₃/acetone = 10/1). The collected solution was evaporated to dryness, giving a purple solid (3.8 mg, 87% yield).

Quenching experiments. The Stern–Volmer relationship (eqn (1)) was obtained from the plots of the relative emission peak area (Φ₀/Φ) versus the concentration of BIH:

Φ_o/Φ = 1 + K_sv[BIH] = 1 + k_qτ[BIH] (1)

where Φ₀ and F represent the emission peak area in the absence and presence of BIH, respectively, and K_sv, k_q, and τ are the Stern–Volmer constant, the quenching rate constant, and the emission lifetime, respectively. The DMA solutions containing the porphyrins in square quartz cells (l = 1.0 cm) were used.

Photocatalytic CO₂ reduction. In glass tubes (8.0 mL, i.d. = 10 mm), 1.0 mL of CO₂-saturated DMA solutions containing BIH was added to 1.0 mL of Ar-saturated DMA solutions containing the photocatalysts, and the reaction solutions were bubbled through septum caps with CO₂ gas for 20 min. Photo-irradiations to determine the TON were carried out using a merry-go-round irradiation apparatus (Iris-MG, Cell Systems) equipped with LED lamps at 25 °C. Reaction quantum yields (Φ) were determined with an Asahi Spectra PRA-201 apparatus equipped with a Xe lamp (Asahi Spectra, MAX-350) and calculated according to eqn (2):

The solutions in 7 mL quartz cubic cells (optical path length: 1.0 cm) were irradiated with light at 420, 450 or 560 nm using bandpass filters. The concentrations of the catalysts were 0.025 mM (420 and 450 nm) and 0.1 mM (560 nm). The gaseous reaction products (CO and H₂) were quantified with a gas chromatography system (GC-2014, Shimadzu Science) equipped with a Shincarbon column (i.d. 3.0 mm × 3.0 m) and a thermal conductivity detector (TCD). The product (formate) in the solutions was analyzed with a capillary electrophoresis system (Otuka Electronics Co. CAPI-3300I).

Author contributions

Y. K. conceived the idea, designed and performed some of the photocatalytic experiments, and wrote the manuscript. R. S. synthesized and characterized the isostearyl-substituted porphyrins, and performed most of the photocatalytic experiments. H. S. synthesized the methyl-substituted porphyrins. A. S. supervised the project, and provided suggestions on the experiments and writing the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Numbers JP19K05677 and JP22H02186. We thank Prof. Akihiko Kudo (Tokyo University of Science) for help with capillary electrophoresis analyses.

References

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, NMR, MALDI-TOF mass, IR, UV-vis and fluorescence spectra, GPC-HPLC charts, calculations for fluorescence quenching experiments, CV, DPV, energy diagrams, and photocatalytic CO₂ reduction. See https://doi.org/10.1039/d2sc03251a

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