Phosphorus(V) Porphyrin-Manganese(II) Terpyridine Conjugates

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Phosphorus(V) Porphyrin-Manganese(II) Terpyridine Conjugates: Synthesis, Spectroscopy, and Photo-Oxidation Studies on a SnO2 Surface Prashanth K. Poddutoori,*,† Gary N. Lim,‡ Melanie Pilkington,† Francis D’Souza,*,‡ and Art van der Est*,† †

Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario L2S 3A1, Canada Department of Chemistry, University of North Texas, 11555 Union Circle, 305070, Denton, Texas 76203-5017, United States

‡

S Supporting Information *

ABSTRACT: A major challenge in designing artificial photosynthetic systems is to find a suitable mimic of the highly oxidizing photoactive species P680 in photosystem II. Highpotential phosphorus(V) porphyrins have many attractive properties for such a mimic but have not been widely studied. Here, we report the synthesis and photophysical characterization of a novel phosphorus(V) octaethylporphyrin−oxyphenyl−terpyridine conjugate (PPor-OPh-tpy, 1) and its corresponding manganese(II) complex (PPor-OPh-Mn(tpy)Cl2, 2). The X-ray structure of 2 shows that the Mn(II) and P(V) centers are 11.783 Å apart and that the phenoxy linker is not fully conjugated with the terpyridine ligand. The porphyrin fluorescence in 1 and 2 is strongly quenched and has a shorter lifetime compared to a reference compound without the terpyridine ligand. This suggests that electron transfer from tpy or Mn(tpy) to the excited singlet state of the PPor may be occurring. However, femtosecond transient absorbance data show that the rate of relaxation to the ground state in 1 and 2 is comparable to the fluorescence lifetimes. Thus, if charge separation is occurring, its lifetime is short. Because both 1 and 2 are positively charged, they can be electrostatically deposited onto the surface of negatively charged SnO2 nanoparticles. Freeze-trapping EPR studies of 2 electrostatically bound to SnO2 suggest that excitation of the porphyrin results in electron injection from 1PPor* into the conduction band of SnO2 and that the resulting PPor•+ species acquires enough potential to photo-oxidize the axially bound Mn(II) (tpy) moiety to Mn(III) (tpy).

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work.6−8 One crucial problem in designing any such system is to find a suitable mimic for P680. This species must act as a photosensitizer and have an oxidation midpoint potential that is high enough to remove electrons from water. In PS II, the oxidation potential of P680 (∼1.1−1.3 V vs NHE)9 is shifted by hundreds of millivolts compared to chlorophyll in solution through the influence of the surrounding protein matrix. However, in artificial complexes there is no surrounding protein; therefore, other ways of tuning the redox potential of the photosensitizer must be sought. In this regard, numerous examples have been reported in which mainly ruthenium compounds have been selected. This is because they not only generate high oxidation potentials for water oxidation, but they also act as efficient photosensitizers.10−22 Porphyrins are structurally similar to the chlorophylls found in the natural photosystems, and their chemical, physical, and redox properties can be easily tuned by inserting different central elements into the porphyrin cavity or substituting functional groups on the peripheral positions. One of the most

INTRODUCTION The electron transport chain of oxygenic photosynthesis is a multicomponent system consisting of a series of membrane protein complexes that use light to drive the oxidation of water, reduction of NADP, and synthesis of ATP. Two of the most important components in this system are the chlorophyll dimer P680, which acts as the primary donor in photosystem II (PS II) and the Mn4CaO5 cluster referred to as the oxygen-evolving complex (OEC).1−5 Excitation of PS II leads to photooxidation of P680 and subsequent removal of an electron from the OEC via proton-coupled electron transfer involving Tyr161 and His190. After four turnovers of PS II and removal of four electrons from the Mn4CaO5 cluster, the OEC oxidizes two water molecules to molecular oxygen and four protons. The photo-oxidation of P680 also leads to the transfer of electrons through PS II and the reduction and protonation of plastoquinone. Thus, the overall reaction of PS II is the oxidation of water and reduction of plastoquinone. Ultimately, the oxidizing and reducing equivalents produced in this process are used in carbon fixation and respiration, respectively. The construction of artificial mimics of PS II presents a major challenge and has been the subject of a large body of © XXXX American Chemical Society

Received: August 8, 2016

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DOI: 10.1021/acs.inorgchem.6b01924 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis of the Dyad PPor-OPh-tpy and Its Corresponding Mn(II) Complexesa

a

Reaction conditions: (i) CH3PCl3, 2,6-lutidine, dichloromethane, reflux under N2 for 2 days; (ii) 4-(4-hydroxyphenyl)-2,2′,6′,2″-terpyridine (OHPh-tpy), dry pyridine, 65°C, stirred for 6 h under N2; saturated solution of NH4PF6 in water; (iii) dry methanol, stirred for 2 h under N2; saturated solution of NH4PF6 in water; (iv) MnCl2, dry methanol, stirred for 2 h under N2.

employ a phosphorus porphyrin linked to tpy to construct mimics of the OEC and P680. In such a complex, the tpy ligand would be used to bind a metal catalyst capable of water oxidation, and the porphyrin would act as the photosensitizer. The complex would then be bound to the surface of a semiconductor to create a high potential anode for water oxidation. However, P(V)TTP suffers two major drawbacks. First, its excited singlet state is lower in energy than the conduction band of most semiconductors, which makes it unsuitable for use in photoelectrochemical cells. Second, it has a strong tendency to bind two identical axial substituents making it synthetically challenging to construct a multicomponent system with different axial ligands. More recently, we and others reported that these disadvantages can be overcome by using phosphorus(V) octaethylporphyrin (P(V)OEP).40−42 P(V)OEP is slightly less oxidizing than P(V)TTP, and as a result, its first excited singlet state is very wellpositioned for electron transfer to a range of acceptors and electron injection into the conduction band of semiconductors such as SnO2.42 In addition, it can also form axial covalent bonds with two different ligands, which is ideal for construction of multicomponent systems. To study the ability of P(V)OEP to act as a photosensitizer, we have previously synthesized and studied the photophysical properties of two constructs in which the two axial ligands are a methyl group and an ether-linked donor or acceptor.41,42 Here, we refer to the porphyrin unit in

commonly used strategies to raise their oxidation potentials has been the introduction of electron withdrawing groups on the periphery of the porphyrin ring.23−27 Alternatively, a highoxidation-state element can be inserted in the porphyrin cavity. In particular, phosphorus(V) porphyrins have significantly higher oxidation potentials and lower reduction potentials compared to their free-base counterparts or other low-valent metalloporphyrins. For example, the first oxidation midpoint potential of 5,10,15,20-tetratolylporphyrin (TTP) is 1.13 V versus SCE in dichloromethane,28 and upon insertion of phosphorus(V) into the cavity of TTP, this potential shifts positively to >1.80 V.29,30 Moreover, phosphorus(V) porphyrins such as P(V)TTP can form two stable axial covalent bonds with oxygen donor ligands,31−38 which allows multicomponent donor−acceptor systems to be assembled. This unique combination of redox and structural properties makes the phosphorus(V) porphyrins attractive candidates as sensitizers for artificial photosynthesis and in particular as mimics of the donor side of PS II. Despite these attractive properties, they have not been widely used in artificial water oxidation complexes. To address this challenge, we recently prepared a P(V)TTP based conjugate with two terpyridine (tpy) ligands attached axially to the phosphorus center via phenoxy linkers.29,30 Remarkably, excitation of the porphyrin resulted in photooxidation of the tpy.39 This suggests that it might be possible to B


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Inorganic Chemistry these compounds as PPor. In the first study we appended the electron rich tetrathiafulvalene (TTF) donor to PPor. As anticipated, upon excitation of PPor the axially attached TTF unit undergoes oxidation by transferring an electron to 1 PPor*.41 More recently, we constructed a photoanode by anchoring PPor and an IrCp* water oxidation precatalyst to the surface of a thin film of SnO2 nanoparticles on FTO-coated glass.42 Excitation of PPor resulted in ultrafast electron injection from 1PPor* into the conduction band of SnO2. This process generated PPor•+ which was then able to oxidize the water-splitting precatalyst from Ir(III)Cp* to Ir(IV)Cp*. Here we explore the feasibility of binding the catalyst directly to the porphyrin and attaching the resulting dyad to the SnO2 surface. We report the preparation and study of a novel dyad PPor-OPh-tpy (1) and its Mn(II) complex, PPor-OPhMn(tpy)Cl2 (2) (see Scheme 1), where tpy is terpyridine and OPh is a phenyl ether linker. Although the Mn(II) center does not act as a photocatalyst, it is a simple model for a transition metal based catalytic center bound to PPor. The cationic nature of these compounds is particularly useful for electrostatically binding them to the surface of negatively charged SnO2 nanoparticles for proof of concept studies. These studies provide evidence that excitation of PPor in 1 and 2 deposited onto a SnO2 surface first generates the highly oxidative species PPor•+, which has sufficient potential to then oxidize the Mn(II) center in the construct to Mn(III).

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13.5 Hz). 13C (75 Hz): 155.69 (C-7/C-8), 155.39 (C-7/C-8), 149.36 (C-4/C-5), 149.11 (C-12), 148.30 (C-4/C-5), 144.86 (C-14), 138.11 (C-15), 137.07 (C-11), 131.69 (d, J = 4.5 Hz, C-1), 125.39 (d, J = 3 Hz, C-3), 124.14 (C-10), 120.81 (C-9), 117.28 (C-6), 116.02 (d, J = 9.75 Hz, C-2), 95.31 (C-13), 31.00 (d, J = 195 Hz, C-18), 18.94 (C16), 17.40 (C-17). 31P (121 MHz): −194.98 (s), −144.65 (sept, J = 704 Hz). Preparation of 2a and 2b. A solution of 1a (27 mg, 0.026 mmol) in dry methanol (5 mL) and a solution of MnCl2 (3.2 mg, 0.026 mmol) in dry methanol (1 mL) were mixed. The resulting solution was stirred for 2 h under nitrogen at room temperature. The solvent was then evaporated under vacuum. The obtained crude product was redissolved in dichloromethane and gravity filtered. The filtrate was concentrated, and X-ray quality purple needle like crystals were obtained via the slow diffusion of hexane. Yield: 27 mg (90%). FAB MS: m/z 1027 [M − PF6]+, 992 [M − PF6 − Cl]+, 902 [M − PF6 − 2Cl − Mn]+. The corresponding chloride salt 2b was prepared in an analogous manner by the reaction of 1b with MnCl2. Physical Methods. Crystallography. Single crystals of 2a were mounted on a cryoloop with Paratone oil and examined on a Bruker Apex II CCD system equipped with a CCD area detector and an Oxford Cryoflex low-temperature device. Data were measured at 150 K with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) using the APEX II software.43 Cell refinement and data reduction were carried out by SAINT.44 An absorption correction was performed by the multiscan method implanted in SADABS.44 The structures were solved by direct methods using SHELXS-97 and refined using SHELXL-97 in the Bruker SHELXTL suite.45 Hydrogen atoms were included in calculated positions and treated as riding atoms using default parameters. The non-H atoms were refined anisotropically using weighted full-matrix least-squares on F2. Crystallographic parameters and bond lengths and angles for the dyad 2a can be found in Tables S1 and S2 of the SI. NMR Spectroscopy and Mass Spectroscopy. NMR spectra were recorded with Bruker Avance 300 and 600 MHz Digital NMR spectrometers using CD3CN as the solvent. FAB mass spectra were recorded on a Kratos Concept 1S High-Resolution E/B mass spectrometer. UV−Vis Absorption Spectroscopy. The UV−vis spectra were recorded with a ThermoSpectronic/Unicam UV-4 UV−vis spectrometer. The concentration of the samples ranged from 1 × 10−6 M (porphyrin Soret I band) to 8 × 10−5 M (Q-bands and Soret II bands) solutions. Voltammetry. Cyclic and differential pulse voltammetric experiments (CH3CN, 0.1 M tetrabutylammonium hexafluorophosphate, (TBA)PF6) were performed on a BAS Epsilon electrochemical analyzer (working electrode, glassy carbon; auxiliary electrode, Pt; reference electrode, Ag). The Fc+/Fc (Fc = ferrocene) couple was used to calibrate the redox potential values, which are reported in V versus SCE (E1/2 (Fc+/Fc) = 0.40 V versus SCE in CH3CN with 0.1 M (TBA)PF641 under our experimental conditions). The spectroelectrochemical study was performed using a cell assembly (SEC-C) supplied by ALS Co., Ltd. (Tokyo, Japan). This assembly is composed of a Pt counter electrode, a 6 mm Pt gauze working electrode, and a Ag/AgCl reference electrode in a 1.0 mm path length quartz cell. The optical transmission was limited to 6 mm covering the Pt gauze working electrode. Steady-State and Time-Resolved Fluorescence Spectroscopy. Steady-state fluorescence spectra were recorded using a Photon Technologies International (London, Ontario) Quanta Master Model QM-2001 L-format, equipped with double-grating monochromators and a 150 W xenon lamp and running Felix 32 software. A timecorrelated single-photon-counting apparatus utilizing a nanoLED diode laser was used to measure the porphyrin fluorescence decay. Excitation pulses were delivered at 406 nm. Absorption and Fluorescence Titrations. Absorption titrations were carried out in a mixture of methanol and water (1:1, v/v) at concentration of 8 × 10−5 M (appropriate for measuring the porphyrin Q bands). A solution containing the dyad 1 or 2 was placed in a cuvette and titrated by adding aliquots (each 5 μL) of a SnO2 solution

EXPERIMENTAL SECTION

Synthesis. All chemicals and solvents used in this study were purchased from Sigma-Aldrich or Alfa-Aesar. The OEP and chromatography material (Al2O3 neutral AW 3) were obtained from Sigma-Aldrich. The SnO2 (15% in water colloidal dispersion) was obtained from Alfa-Aesar. The precursor porphyrin (PPor-Cl) and reference porphyrin (PPor-OMe, 3) were synthesized from OEP by established methods.41,42 The 4-(4-hydroxyphenyl)-2,2′,6′,2″-terpyridine (OH-Ph-tpy) was prepared according to the literature method.30 Preparation of 1a and 1b. A solution of OEP (300 mg, 0.56 mmol), CH3PCl2 (300 μL, 3.4 mmol), and 2,6-lutidine (300 μL, 2.6 mmol) in dry dichloromethane (20 mL) was refluxed for 36 h under nitrogen. The solvent was evaporated, and the solute was dried under vacuum. The obtained crude PPor-Cl was added to a solution of OHPh-tpy (300 mg, 0.92 mmol) in dry pyridine (20 mL) at 65 °C. The resulting solution was stirred at 65 °C under nitrogen for 2 h. The pyridine was evaporated under reduced pressure, and the crude product was dissolved in dichloromethane and filtered to remove the excess, unreacted OH-tpy. The filtrate was collected and evaporated to dryness, and the product was purified by neutral alumina column chromatography. The compound was loaded onto the column using a benzene:acetonitrile (95:5) solvent mixture and eluted with benzene:methanol (95:5) to remove the low-polarity impurities. The polarity was then increased by eluting with benzene:methanol (90:10) to collect the desired compound as the chloride salt, 1b. To exchange the counterion, the chloride salt was dissolved in 10 mL of ethanol and precipitated with a saturated aqueous solution of NH4PF6. The precipitate was collected by filtration and washed with water and airdried. At this point, the minor OH-Ph-tpy, PPor-OMe, or PPor-OH impurities found in the crude product were separated by silica gel column chromatography using dichloromethane:ethyl acetate (95:5) as the eluent. The polarity was then increased to dichloromethane:methanol (98:2) to elute the desired product, 1a, in pure form. Yield: 550 mg (93%). FAB MS: m/z 902.4675 for [M − PF6]+ calculated 902.4670 for C58H61N7OP; 578 [M − PF6 − C21H14N3O]. NMR (CD3CN, δ ppm), 1H (300 MHz): 9.91 (s, 4H-13), 8.60 (d, 2H-12, J = 4.5 Hz), 8.51 (d, 2H-9, J = 8.1 Hz), 7.98 (s, 2H-6), 7.87 (t, 2H-11), 7.39 (t, 2H-10), 6.14 (d, 2H-3, J = 7.2 Hz), 4.09 (m, 16H-16), 1.79 (t, 24H-17), 1.58 (dd, 2H-2, J = 11.7 and 5.7 Hz), −5.71 (d, 3H-18, J = C


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Inorganic Chemistry (4 × 10−2 M in methanol and water mixture, 1:1, v/v). The SnO2 solution also contained the porphyrin at its initial concentration so that the porphyrin concentration remained constant throughout the titration. In an analogous manner, steady-state fluorescence titrations were carried out in a mixture of methanol and water (1:1, v/v) at constant concentration of porphyrin and varying concentration of SnO2. The solutions were excited at the isosbestic point wavelength, which was obtained from the corresponding absorption titrations. Femtosecond Transient Absorption Spectroscopy. Experiments were performed using an Ultrafast Femtosecond Laser Source (Libra) by Coherent incorporating diode-pumped, mode locked Ti:sapphire laser (Vitesse) and diode-pumped intracavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of 1.45 W. For optical detection, a Helios transient absorption spectrometer coupled with a femtosecond harmonics generator both provided by Ultrafast Systems LLC was used. The source for the pump and probe pulses were derived from the fundamental output of Libra (Compressed output 1.45 W, pulse width 100 fs) at a repetition rate of 1 kHz. 95% of the fundamental output of the laser was introduced into the harmonic generator which produces second and third harmonics of 400 and 267 nm besides the fundamental 800 nm for excitation, while the rest of the output was used for generation of a white light continuum. In the present study, the second harmonic 400 nm excitation pump was used in all the experiments. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. Data analysis was performed using Surface Xplorer software supplied by Ultrafast Systems. All measurements were conducted in degassed solutions at 298 K. EPR Spectroscopy. Freeze-trapping EPR spectra were recorded using a Bruker Elexsys E580 pulse spectrometer operating in CW mode. The temperature was controlled using a Bruker VT 1000 temperature control system. EPR samples were prepared by transferring 100 μL of 2.5 mM porphyrin complex (in methanol) to a Suprasil EPR sample tube (4 mm o.d.) and adding 100 μL of SnO2 (15% colloidal dispersion in water). The obtained suspension was mixed well and formed a transparent glass. Recent studies of the solidification of SnO2 colloidal dispersions suggest that a Wigner glass is formed as a result of the electrostatic repulsion between the negatively charged nanoparticles.46 The tube was sealed and placed in the EPR resonator, which was kept at 80 K, and the EPR spectrum was measured. The sample was then taken out of resonator and warmed to room temperature and irradiated using a 150 W lamp producing white light and a 420 nm long pass cutoff filter for 3 min in a finger Dewar. The sample was then frozen under irradiation by pouring liquid nitrogen into the Dewar. The frozen sample was then transferred to the resonator, and the EPR spectrum was collected. We refer to the EPR spectrum collected prior to illumination as the “dark” spectrum and after freezing during illumination as the “light” spectrum.

ascribable to the mass (m/z) of [M − PF6]+. The 1H and C NMR spectra of dyad 1a in CD3CN are shown in Figures S3−S5. The proton chemical shifts were assigned by employing a range of NMR techniques (1H, 1H−1H COSY, 13C, 1H−13C COSY). Shielding effects are apparent for the protons of the axially linked phenyl terpyridine unit. For example, resonances assigned to the phenyl protons H-2 and H-3 (see Scheme 1 for the numbering of the nuclei), at 6.69, 7.32 ppm in the reference compound OH-Ph-tpy are strongly shifted upfield to 1.58 and 6.14 ppm, respectively, for dyad 1 (see Figure S3) due to the ring current effect of the porphyrin macrocycle. Similarly, the resonance due to the proton H-6 is shifted upfield to 7.98 ppm compared to the corresponding resonance at 8.53 ppm in free OH-Ph-tpy. In contrast, protons H-9, H-10, H-11, and H-12 of the dyad are not much affected by the PPor ring current because they are further away from the porphyrin ring. Hence, the Δδ values (i.e., δOH‑Ph‑tpy − δdyad1) are a function of their proximity to the porphyrin ring. The observed coupling between the phosphorus nucleus of PPor and nearby proton and carbon nuclei provides further support for the formation of the dyad. This coupling is resolved for nuclei up to four consecutive covalent bonds away from the phosphorus nucleus. For example, in the 1H NMR spectrum, the H-2 proton appears as a doublet of doublets due to coupling to proton H-3 and to the central phosphorus nucleus, even though it is four covalent bonds away from the central phosphorus atom. The observed coupling between phosphorus and H-2 shows that the Ph-tpy unit is linked to the PPor unit through the C-1 carbon. The chemical shift of the 31P NMR signal due to the central phosphorus atom in dyad 1 (−194.98 ppm, Figure S3) is slightly higher than that of reference compound 3 (−186.93 ppm) and is consistent with a hexacoordinate phosphorus structure.32,33,35 In the 13P NMR spectra of both compounds, a quintet from the PF6− counterion is also observed. In the 13C NMR spectrum of the dyad (Figure S4), peaks from the C-1, C-2 and C-3 carbon atoms of the axial phenyl ring appear as doublets since they are coupled to the central phosphorus nucleus. Similar results have been reported for other systems in which an organic moiety was appended axially to a PPor macrocycle.41 The NMR spectra of dyad 2 showed broad peaks as expected due to the presence of the paramagnetic Mn(II) center (data not shown). Crystal Structure of 2a·3CH2Cl2. Single crystals of dyad 2a suitable for X-ray diffraction studies were grown via the slow diffusion of hexane. The complex crystallizes in the triclinic space group P1̅ with one independent [PPor-OPh-(tpy)MnCl2]+ cation, one PF6− counterion, and three dichloromethane solvent molecules in the asymmetric unit. The molecular structure of dyad 2a is presented in Figure 1. Supplementary crystallographic data for this compound can be obtained from the Cambridge Crystallographic Data Centre (CCDC 1496883). The structure (Figure 1) confirms that the (tpy)MnCl2 unit is axially appended to the PPor core via the phenol linker. Detailed analysis of the coordinates reveals that the Mn1−P2 distance is 11.783 Å and that the pendant OPh-Mn(tpy) substituent is tilted with respect to the porphyrin macrocyle such that the P−O−C angle is 128.01°. Furthermore, the phenol linker is not fully conjugated with the terpyridine ligand, but twisted such that the angle between their two best planes is 33.85°. The Mn(II) ion adopts a 5-coordinate geometry that is closest to square pyramidal where the basal plane is defined by Cl-2, N-5, N-6, and N-7 and the corresponding τ value is 0.10, 13

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RESULTS AND DISCUSSION Synthesis. The syntheses of dyad 1 and its corresponding Mn complex 2 are summarized in Scheme 1. Briefly, preparation of the construct was achieved in high yield by exploiting the reactivity of the axial P−Cl bond of PPor toward alcohols, in this case OH-Ph-tpy. The counterion exchange was performed after the initial preparation of the chloride salt 1b. This was achieved by dissolving 1b in ethanol and then precipitating the PF6 salt, 1a, by the addition of a saturated aqueous solution of NH4PF6. The Mn2+ complexes, 2a and 2b, were prepared by reacting the corresponding dyads 1a and 1b with MnCl2. The compounds PPor-OMe (3) and HO-Ph-tpy were used as reference compounds in all spectroscopic and redox studies. Characterization. Preliminary characterization of the compounds was carried out by FAB mass spectrometry. The mass spectra of the dyads 1a (Figure S1) and 2a (Figure S2) showed peaks at 991 and 1027, respectively, which are D


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Inorganic Chemistry

with a Soret I and two Q-bands in the ranges 415−432 and 500−630 nm, respectively. It also shows the relatively weak Soret II band at 358 nm, which is typical for phosphorus(V) octaethylporphyrins.40−42,48 The observed absorption at 283 nm for dyad 1a is ascribed to the Ph-tpy moiety. As shown in Figure 3a and Table 2, the porphyrin bands in the dyad (red spectrum) are essentially the same as those of 3 (blue spectrum). For the corresponding Mn(II) complex dyad 2a (green spectrum) the Soret I and Q-bands of the porphyrin remain essentially unchanged. However, in the ultraviolet region a new broad band centered at ∼334 nm reveals the presence of the Mn(tpy) moiety, and it masks the Soret II band of PPor. Overall, the absorption maxima and ε values of PPor in the dyad and its Mn(II) complex are in the same range as those of reference compound 3a suggesting that linking Ph-tpy or PhMn(tpy) axially to PPor does not cause any appreciable changes in the electronic structure of the porphyrin. Thus, we conclude that the Ph-tpy or Ph-Mn(tpy) and PPor units are well-separated in the space so that they do not interact strongly with each other. Electrochemistry. Figure 4 shows the cyclic and differential pulse voltammograms of the investigated compounds in CH3CN with 0.1 M TBAPF6 as a supporting electrolyte. The anodic and cathodic scans of reference compound 3a (trace a) show a one-electron oxidation process at 1.41 V and two oneelectron reduction processes at −0.74 and −1.19 V corresponding to the first oxidation and first and second reduction of PPor, respectively. The reference compound HOPh-tpy shows irreversible oxidation at 1.10 V as reported previously.29,30 The voltammograms of dyad 1a (trace b) are very similar to those of reference compound 3a (trace a). During the anodic scan, the dyad shows only one quasireversible process at 1.40 V, and no features are observed near 1.10 V. Thus, we postulate that the oxidation potential of tpy is raised by ∼0.30 V in the dyad and the process at 1.40 V is a combination of the first oxidations of the PPor and tpy units. Such a shift is expected as a result of the strong electron withdrawing effect of PPor. In the cathodic scan, the dyad shows two one-electron reversible reduction processes that are ascribed to the successive first and second reductions of the PPor entity. In the case of dyad 2 (trace c), oxidation processes are observed at 0.99 and 1.40 V. Comparison with the reference compound Mn(tpy)Cl2 (trace d), which also shows oxidation processes at these voltages, suggests that they are due to oxidation of Mn(II) to Mn(III) and a ligand centered oxidation process, respectively. In dyad 2a, the latter overlaps with the first oxidation of the porphyrin. Spectroelectrochemistry. To aid in the interpretation of the transient absorbance data, which will be discussed below, spectroelectrochemical studies of the oxidation and reduction of reference compound 3a in acetonitrile were also performed. As shown in Figure 5a, one-electron reduction leads to loss of

Figure 1. Molecular structure of dyad 2a. H atoms, PF6− counterions, and solvent molecules are omitted for clarity.

where Cl-1 is the pivot atom that best describes the Berry pseudorotation.47 The axial P−O bond length of 1.686(3) Å is consistent with a fairly long single P−O bond.40 The monoalkylated phosphorus atom in the octaethylporphyrin cavity is in the +5 oxidation state and adopts a distorted octahedral geometry.40 The phosphorus atom lies essentially within the plane of the 4 N atoms of the porphyrin ring such that the deviation Δ4N is only 0.044 Å. The porphyrin core adopts a ruffled conformation with average P−N distances of 1.878 Å. In this respect, the degree of ruffling of the core Δr (calculated as the square root of the sum of the square of the deviation of each atom from the mean plane) is calculated to be 0.460 Å, Table 1, which is consistent with previously reported systems.40 A view of the crystal packing for the dyad 2a·3CH2Cl2 reveals that the PPor molecules are stacked in a head-to-head arrangement which best accommodates the bulky apical OPh(tpy)MnCl 2 substituents, Figure 2. Channels between neighboring molecules accommodate CH2Cl2 molecules and PF6− counterions. Intermolecular H-bonding interactions between H atoms on the dyad PPor-OPh-(tpy)MnCl2 cation and F or Cl atoms of the PF6− counterions and CH2Cl2 solvent molecules are within the range of 2.26 Å for C(2)−H(2)···F(3) to 2.79 Å for C(60)−H(60A)···Cl(1), see Figure 2. UV−Vis Absorption Spectroscopy. The UV−vis absorption spectra of dyads 1a and 2a and reference compound 3a are shown in Figure 3a. The absorption band positions and extinction coefficients are summarized in Table 2. Dyad 1a exhibits a normal-type UV−vis porphyrin absorption spectrum

Table 1. Selected Bond Distances and the Degree of Ruffling for 2a·3CH2Cl2. bond distance

P−O axial (Å)

P−C axial (Å)

P−N in-plane (Å)

av P−N bond distance (Å)

Δra (Å)

Δ4Nb (Å)

1.686(3)

1.845(4)

1.873(4) 1.845(4) 1.871(3) 1.884(3)

1.87

0.460

0.044

a Δr: square root of the sum of square of deviation of each atom from the mean plane of entire 24 core atoms. bΔ4N: distance of the P atom from the plane of the four nitrogen atom.

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Inorganic Chemistry

Figure 2. (Left) Crystal packing of 2a·3CH2Cl2 viewed down the b-axis of the unit cell. H atoms are omitted for clarity. (Right) Hydrogen bonds (H···F) between PF6 anion and different hydrogen atoms of porphyrin and tpy units. Distances are given in Ångstroms.

Figure 3. (a) UV−vis absorption spectra, (b) fluorescence spectra, and (c) fluorescence decay profiles (each channel equals to 4.39 × 10−10 s) of reference compound 3a (blue), dyad 1a (red), and dyad 2a (green) in acetonitrile.

Table 2. UV−Vis Absorption and Redox Data of 1a, 2a, and Their Reference Compounds in CH3CN absorption λ, nm (log ε) compd HO-Ph-tpya 3a 1a 2a MV(PF6)2 a

tpy/Mn(tpy)

B-band

potential (V) vs SCE 0.1 M TBAPF6 Q-band

oxidation

548 (4.08), 590 (4.10) 549 (4.10), 591 (4.07) 549 (4.02), 591 (4.01)

1.00 1.41 1.44 0.99, 1.40

290 (4.57) 283 (4.65) 286 (4.49), 334 (4.55)

357 (4.39), 416 (5.39) 358 (4.40), 418 (5.33) 418 (5.28)

reduction −0.74, −0.70, −0.76, −0.38,

−1.19 −1.14 −1.20 −0.80

Data was collected in DMSO. In redox measurements, 0.1 M TBAClO4 was used as a supporting electrolyte.

the porphyrin Q-bands at 548 and 590 nm and the appearance of a broad peak at 750 nm. In addition, the Soret band at 416 nm is red-shifted by 31 to 447 nm with a large drop in the absorbance. In contrast, the one-electron oxidation (Figure 5b) results in a roughly 10 nm red shift of the Soret and Q-bands to 426, 560, and 601 nm and a loss of intensity of all absorption bands. The isosbestic points in the titrations show that both the oxidation and reduction processes are fully reversible. Photoinduced Electron Pooling. As a test of the electron accepting and donating properties of PPor, electron pooling studies were performed in the presence of the sacrificial electron acceptor methylviologen (MV2+) and the irreversible electron donor 1-benzyl-1,4-dihydronicotinamide (BNAH).49,50 It is anticipated that, with MV2+ and BNAH present, continuous excitation of 3a should result in oxidation of BNAH and reduction of MV2+. Absorption spectroscopy was used to monitor these processes, and the results are shown in Figure 6a. The spectra were collected in acetonitrile after

irradiation with 532 nm laser light (20 mW for 3 min), which excites predominantly PPor. The spectrum of 3a in the presence of MV2+ and BNAH (red spectrum) reveals a new broad absorption band centered at roughly 600 nm with a shoulder at 625 and peaks at 675 and 750 nm typical of the radical monocation MV•+51 in addition to the bands from compound 3a at 550 and 590 nm. To confirm that MV•+ is fphoto-excitation of 3a and oxidation of BNAH, the spectra of various control samples are also shown in Figure 6a. The absorption spectrum of compound 3a irradiated in the presence of MV2+ but without BNAH (blue spectrum) is identical to the spectrum of compound 3a alone (cyan spectrum). Thus, either electron transfer from 1PPor* to MV2+ is not feasible, even though the free-energy change for the process was found to be −0.30 eV, or the oxidized porphyrin and reduced MV2+ undergo rapid back reaction. The steady-state fluorescence and fluorescence lifetime of PPor are unaffected by the presence of MV2+ (data not shown) indicating that the excited F


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Figure 6. (a) Absorption spectral changes of MV2+ + 3a + BNAH (red), MV2+ + 3a (blue), 3a + BNAH (magenta), MV2+ + BNAH (green), and 3a (cyan) after light irradiation (532 nm laser, ca. 20 mW) for 3 min in deaerated acetonitrile. (b) Scheme represents the proposed intermolecular electron transfer processes between MV2+, PPor, and BNAH units.

extent. On the basis of the control experiments, the absorption spectrum of the solution containing MV2+, PPor, and BNAH can be explained by the scheme shown in Figure 6b. Under illumination, electron transfer takes place from BNAH to 1 PPor* to generate PPor•− and BNAH•+ which converts irreversibly to BNA+ via a second oxidation and loss of a proton.52 As a result, PPor•− is sufficiently long-lived to reduce MV2+ giving a solution containing MV•+, PPor, and BNA+. Electron Transfer Energetics. The redox potentials can be used in combination with optical data to estimate the energetics of the energy and electron transfer reactions in the dyads. Figure 7 summarizes the midpoint potentials of the components of dyads 1a and 2a. The energies are given relative to NHE (NHE = SCE + 0.240 V). The excited singletstate energy of the porphyrin and the conduction band energy of the SnO2 semiconductor are also shown.42 As can be seen, the midpoint potential of PPor is ∼420 mV more positive than that of Mn(II) so that photo-oxidation of the metal is feasible. The driving force for electron injection into the conduction band of SnO2 is also about 400 mV. The energy of the charge-separated state ECS (relative to the ground state) and the free-energy changes for charge-separation (ΔGCS) can be calculated using the Rehm and Weller equations.53,54

Figure 4. Cyclic and differential voltammograms of (a) reference compound 3a, (b) dyad 1a, (c) dyad 2a, and (d) Mn(tpy)Cl2 with 0.1 M TBAPF6 in CH3CN.

singlet state does not react. When compound 3a is irradiated in the presence of BNAH (magenta spectrum), a broad absorption band in the near IR with a maximum at ∼730 nm appears. On the basis of the spectroelechemical data (Figure 5), this band can be attributed to PPor•− formed by electron transfer from BNAH to 1PPor*. Since the oxidation of BNAH is irreversible, the transferred electron becomes trapped on PPor, resulting in long-lived PPor•−. The possibility of a direct photochemical reaction between MV2+ and BNAH was also tested by measuring a sample containing only MV2+ and BNAH (green spectrum). The spectrum does show features due to MV•+, but the intensities are much weaker than when compound 3a is present (red spectrum); thus, direct electron transfer between MV2+ and BNAH only occurs to a small

ΔGCS = ECS − E0 − 0

(1)

Figure 5. Spectral changes during (a) the first reduction at −0.74 V and (b) the first oxidation at 1.41 V of compound 3a in acetonitrile with 0.2 M TBA·ClO4. G


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observed fluorescence quenching is consistent with a decrease in the excited singlet-state lifetime as a result of the postulated hole transfer from 1PPor* to tpy (or Mn(tpy)). Singlet−singlet energy transfer can be safely ruled out as a quenching mechanism because the spectral overlap between PPor emission and tpy (or Mn(tpy)) absorption is essentially zero. However, steady-state fluorescence data are only an indirect measure of the excited singlet-state lifetime. Time-resolved fluorescence measurements allow the quenching to be related directly to the excited singlet-state lifetime. Figure 3c shows typical fluorescence decays obtained by singlephoton counting with excitation at 406 nm and detection at 600 nm, where the porphyrin moiety emits. The decay lifetimes (τF), quenching rates (kSq), and quenching quantum yields (ΦSq) are given in Table 3. As illustrated by the fits in Figure 8 and values in Table 3, the fluorescence of reference compound 3a decays with a lifetime of 5.25 ns, whereas that of dyad 1a and its Mn(II) complex 2a decays with much shorter lifetimes of 1.56 and 1.61 ns, respectively. The quenching quantum yields obtained from the fluorescence lifetimes are in good agreement with quenching efficiencies obtained from the steady-state fluorescence spectra showing that the quenching is the result of a decrease in the excited singlet-state lifetime. The fact that the fluorescence lifetimes are the same in 1a and 2a suggests that direct oxidation of Mn(II) by the excited state of the porphyrin does not occur and that the quenching is probably due to oxidation of tpy. Femtosecond Transient Absorption Spectroscopy. To further characterize the postulated hole transfer, femtosecond transient absorbance measurements of the dyads and reference compound were carried out in acetonitrile with excitation at 400 nm in the PPor Soret band. The transient spectral features of compound 3a are typical of those of porphyrins (see Figure S6a). Immediately after excitation, spectral features corresponding to the excited singlet minus ground state spectrum are observed with positive peaks at 461, 572, 623, and 1190 nm due to the excited singlet-state absorbance and negative peaks at 548, 590, and 655 nm due to the ground state bleaching and stimulated emission. The 548 nm peak is mainly due to ground state bleaching; the 655 nm peak is stimulated emission, and the 590 nm peak is due to a combination of bleaching and stimulated emission. The excited singlet-state absorption peak in the near-IR at 1190 nm is similar to that reported earlier for free-base and metalloporphyrins.55,56 The decay of the positive peaks and recovery of the negative peaks (see black traces in Figure 8b,c) are consistent with the 5.25 ns lifetime compound 3a (see Table 3). The corresponding spectra for the dyads, 1a and 2a, are very similar to those of reference 3a; however, there are some subtle differences (see Figure 8a and Figure S6b,c for spectral data). Figure 8b,c shows time profiles for the recovery of the 590 nm peak and decay of 1195 nm peak for both dyads in comparison with reference 3a. In agreement with the shorter fluorescence lifetime, the decay of the positive peak at 1195 nm due to excited singlet-state absorption (Figure 8c) is faster in the dyads than in reference 3a. The recovery of the negative peak at 590 nm is also faster in the dyads than in reference 3a, but the recovery is slightly slower than the decay of the excited singlet state. This implies that the excited singlet state may decay via a short-lived intermediate possibly as a result of electron transfer from the ligand to the porphyrin. The spectroelectrochemical data in Figure 5a show that one-electron reduction of reference 3a leads to increased absorbance at 447 and 750 nm. However,

Figure 7. Midpoint potential diagram of the components of the electron transfer construct 2-SnO2 in acetonitrile.

ECS = E1/2OX (D) − E1/2 RED(A) + GS

(2)

The first excited singlet-state energy of PPor, E0−0, is estimated to be 2.09 eV using the crossing point of the absorption and fluorescence spectra of PPor.42 ECS is the energy of the chargeseparated state and is the difference between the first oxidation potential of the donor and first reduction potential of the acceptor. GS is ion-pair stabilization and incorporates the free energy of solvation of the ions GS = −

e2 4πε0RD − AεS

(3)

where RD−A is center-to-center distance between donor and acceptor. εS is the dielectric constant of solvent used for the photophysical and redox potential measurements, in this case acetonitrile. Using these equations, the energy of the chargeseparated state PPor•−-tpy•+ is also found to be 2.09 V; i.e., it is isoenergetic with the excited singlet state of PPor. However, the states PPor•−-Mn(III) (1.70 eV) and SnO2−PPor•+ (1.60 eV) are both predicted to lie well below the excited singlet-state energy showing that PPor is energetically well-suited to act as a photosensitizer for multistep electron transfer from Mn(II) to SnO2. Fluorescence Spectroscopy. The steady-state fluorescence spectra of dyads 1a and 2a and the reference compound 3a in acetonitrile are shown in Figure 3b. The spectra have been measured with excitation at 555 nm, where the absorption is exclusively due to PPor (Figure 3a). The spectral shapes and the emission maxima (λem) of dyads are essentially the same as those of its reference compound 3a. However, as is apparent in Figure 3b, the fluorescence of the dyads is strongly quenched compared to that of the reference compound. The quenching efficiency (%Q) and λem values are summarized in Table 3. The Table 3. Steady-State and Time-Resolved Fluorescence Data and Estimated Rate Constants for Charge Separation of the Investigated Compounds in CH3CN sample

λem, nm (%Q)

τF/nsa

kSq/s‑1,b (ΦSq)

3a 1a 2a

597, 653 599, 657 (76%) 597, 657 (64%)

5.25 1.56 1.61

4.5 × 108 (0.70) 4.3 × 108 (0.69)

a From time correlated single-photon counting spectroscopy. bkSq = 1/ τdyad − 1/τPPor; ΦSq = 1 − τdyad/τPPor

H


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Figure 8. (a) Femtosecond transient differential spectra of compound 3a (blue) and dyad 2a (red) at a delay time of 100 ps in Ar-saturated acetonitrile solution. The samples were excited using a 400 nm (100 fs pulse width) laser. Panels b and c show the time profiles of compound 3a (black), dyad 1a (red), and dyad 2a (green) with the 590 nm peak representing ground state recovery and decay of the excited singlet-state absorption of phosphorus porphyrin at 1195 nm, respectively.

the peak at 447 nm overlaps strongly with the Soret band of the excited singlet state of the porphyrin, and the peak at 750 nm is weak and broad. Thus, the charge-separated state generated by electron transfer to PPor is difficult to observe. However, the fact that the lifetimes of the ground state recovery and excited singlet-state decay are similar (compare Figure 8b,c) shows that if electron transfer is occurring, the lifetime of the chargeseparated state must be short. Binding of PPor on SnO2 Surface. Because PPor is positively charged it can be bound to a negatively charged SnO2 surface as depicted in Figure 9.57 The negative charge of the SnO2 surface is the result of ionization of Sn−OH groups to Sn−O−.58 The electrostatic binding can be monitored by absorption and fluorescence titrations. Initially, dyad 1a (the PF6− salt) was titrated with a solution of SnO2 nanaoparticles in a 1:1 water/methanol mixture to a maximum concentration of 3.6 mM SnO2. At this concentration no aggregation/solidification of the SnO2 occurs. Despite the opposite charges of the nanoparticles (negative) and the porphyrin (positive), no changes were detected in the absorption and fluorescence spectra suggesting that the dyad does not bind to the SnO2 surface. However, when similar titrations were carried out with the Cl− salt of the same dyad (1b), spectral changes indicative of electrostatic binding interactions between PPor and the SnO2 surface were observed (Figure 9). We speculate that this difference between 1a and 1b is due to hydrogen bonding

Figure 9. Anticipated electrostatic binding of dyad 2b on the negatively charged SnO2 surface.

I


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Figure 10. Titrations of dyad 1b with SnO2 in a 1:1 volume ratio mixture of water and methanol. SnO2 was added up to 3.6 × 10−3 M in 5 μL (4.0 × 10−2 M) increments to a 1 mL (8.0 × 10−5 M) solution of 1b. (a) Absorption titrations. (b) Fluorescence titrations, the excitation wavelength was chosen at the isosbestic point, 541 nm, obtained from UV−vis titrations.

Figure 11. (Left) CW EPR of spectra of 2b-SnO2 (red, dark condition), 2b-SnO2 (green, light condition), and 1b-SnO2 (blue, light minus dark) at 80 K. (Right) Light minus dark CW EPR spectra of 3b (orange), 1b (blue), and 3b-SnO2 (purple) in methanol and water mixture at 100 K. White light was used to irradiate samples with 420 nm long-pass filter.

between PF6− and PPor in 1a. Indeed, the crystal structure of 2a shows multiple hydrogen bonds between PF6− and the PPor and tpy units in 2a (see Figure 1b). Such bonding reduces the effective positive charge available for electrostatic interaction with the SnO2 nanoparticles and may also sterically impede the dyad from contacting the surface. Figure 10a shows the absorption titrations of 1b versus SnO2 in a 1:1 water/methanol mixture. Upon addition of the SnO2, the Q bands at 549 and 590 nm of PPor decrease, and slight broadening occurs. Isosbestic points are observed at 538, 555, and 570 nm, indicating the formation of the anticipated electrostatic complex between PPor and SnO2, see Figure 9. Figure 10b shows the fluorescence spectra of 1b with increasing

amounts of SnO2. The excitation wavelength was set to the isosbestic point at 538 nm in the absorption spectrum (Figure 10a). Upon addition of SnO2, the fluorescence bands of PPor are slightly quenched and blue-shifted, again suggesting that binding of 1b to the SnO2 nanoparticles occurs. Moreover, the observed quenching may be an indication that interfacial electron transfer (IET) from 1PPor* to the conduction band of SnO2 takes place as observed in previous studies in which the same PPor was linked covalently to the SnO2 surface.42 However, the degree of quenching is much weaker when PPor is electrostatically bound compared to when it is covalently linked to the SnO2 surface. This implies poorer electron injection efficiency in the electrostatic binding case. NonetheJ


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less, the titration in Figure 10b provides some indication that electron transfer from 1PPor* to the conduction band of SnO2 may take place. Photo-Oxidation of Mn(II) Complex. Freeze-quenching EPR studies provide more direct evidence for such electron transfer. Figure 11 shows a comparison of the EPR spectra of 2b-SnO2 frozen under illumination and frozen in the dark along with the spectra for several control experiments. The dark EPR spectrum (red spectrum, top left) shows a broad line typical of S = 5/2 Mn(II) in a glassy environment.59 When the sample is frozen in the light (green spectrum), the intensity of the Mn(II) signal decreases, and a narrow peak at the center of the spectrum appears. The narrow peak indicates the formation of an S = 1/2 species, most likely an organic radical; the decrease of the Mn(II) signal is consistent with oxidation of Mn(II) to S = 2 Mn(III) since the latter is not observed by perpendicular mode EPR. When the same experiment is performed with 1bSnO2 or 3b-SnO2, the narrow peak is again observed (blue spectrum in Figure 11, bottom left, and purple spectrum in bottom right). In the absence of SnO2 only a weak narrow peak is accumulated when dyad 1b and reference 3b are irradiated (orange and blue spectra, Figure 11, top right). Together, the data suggest that when PPor is bound to the surface of SnO2, excitation of the porphyrin leads to electron injection into the conduction band of the metal oxide. Accumulation of PPor•+ occurs when the transferred electron are trapped when the sample is frozen.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Sciences and Engineering Research Council Canada (Discovery grants to A.v.d.E. and M.P.) and the Canada Foundation for Innovation (New Opportunities grant to MP and Innovation Fund grant to A.v.d.E.). Support by the National Science Foundation (Grant 1401188 to F.D.) is acknowledged.

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CONCLUSION In summary, we have shown that axially appended phosphorus(V) porphyrin−terpyridine conjugates can be constructed. This provides a successful strategy for attaching metal catalysts to a photosensitizer that can electrostatically bind to a SnO2 surface and inject electrons into the conduction band of the metal oxide. EPR studies suggest that some oxidation of the Mn(II) centers in 2b by the oxidized porphyrin also occurs. However, fluorescence and femtosecond transient absorbance measurements reveal that, in the PPor-OPh-tpy dyad 2a, the chargeseparated state is very short-lived so that the decay of the excited singlet state and recovery of the ground state have essentially the same lifetime. This work represents an important first step toward developing a synthetic P(V)-porphyrin sensitizer that can undergo photo-oxidation. However, further work is needed to tune the system and increase the lifetime of the charge separation in the dyads, which should also increase the overall yield for Mn(II) oxidation. Ultimately, the Mn(II) center will be replaced by a water-splitting catalyst.

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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.6b01924. NMR spectra, crystal data, and transient absorption spectra (PDF)

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REFERENCES

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Phosphorus(V) Porphyrin-Manganese(II) Terpyridine Conjugates

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