Phosphonate-Derivatized Porphyrins for Photoelectrochemical

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Phosphonate-Derivatized Porphyrins for Photoelectrochemical Applications Animesh Nayak, Subhangi Roy, Benjamin D. Sherman, Leila Alibabaei, Alexander M Lapides, M. Kyle Brennaman, Kyung-Ryang Wee, and Thomas J. Meyer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10587 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on January 25, 2016

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ACS Applied Materials & Interfaces

Phosphonate-Derivatized Porphyrins for Photoelectrochemical Applications. Animesh Nayak, Subhangi Roy, Benjamin D. Sherman, Leila Alibabaei, Alexander M. Lapides, M. Kyle Brennaman, Kyung-Ryang Wee and Thomas J. Meyer* Department of Chemistry, University of North Carolina at Chapel Hill, 121 South Road, Chapel Hill, NC 275993290 U.S.A.

Keywords: Porphyrin, Chromophore, photocurrent, electron injection, excited state potential Abstract A series of phosphonate-derivatized, high redox potential porphyrins with mesityl, pentafluorophenyl, and heptafluoropropyl meso-substituents were synthesized by acid-catalyzed condensation reactions. Ground and excited state redox potentials in the series were varied systematically with the electron donating or accepting nature of the meso substitutents. The extent of excitation and injection by porphyrin singlet excited states surface-bound to SnO2/TiO2 core/shell metal oxide nanoparticle films varies with the excited state reduction potential, Eo’(P+/P*). With the mesityl-substituted porphyrin, high current density, sustained photocurrents are observed at pH 7 with the addition of the electron transfer donor hydroquinone. Introduction The dye-sensitized photoelectrosynthesis cell (DSPEC) provides a systematic basis for integrating the light absorption and catalytic properties of molecular assemblies with the stability and bandgap properties of semiconductor oxides. The targets are solar fuels, based on water splitting to give H2 or reduction of CO2 to carbon-based fuels.1–4 At the heart of a DSPEC photoanode lies a chromophore that absorbs light, injects electrons into a metal oxide semiconductor and transfers oxidative equivalents to a catalyst for water oxidation to O2. Organic chromophores are desirable in these applications because they are relatively inexpensive, easily available and readily tunable by systematic synthetic modifications. In a DSPEC application, there are significant thermodynamic requirements: 1) The excited state or states following excitation must be sufficiently reducing to undergo efficient electron injection into the conduction band of the metal oxide; 2) The oxidized chromophore following injection must be sufficiently oxidizing to drive the water oxidation cycle at an accessible catalyst. 3) The chromophore and its oxidized forms must be capable of recycling through an indefinite number of excitation/injection/electron transfer cycles. Porphyrins have been used in dye-sensitized solar cells (DSSC) and DSPEC applications, mainly with carboxylate anchors that, due to hydrolysis, can be problematic for long term use under aqueous

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conditions.5–14 Here we report the synthesis and characterization of a family of phosphonate-derivatized, high-redox potential porphyrins including values for their ground and excited state redox potentials. We also demonstrate surface binding to mesoscopic SnO2/TiO2 core/shell nanoparticle films and provide evidence for injection under conditions appropriate for applications in DSPEC photoanodes. Experimental Materials All commercially available reagents and solvents were obtained from Sigma-Aldrich or Fisher Scientific and used without purification. An inert atmosphere was maintained in every reaction unless otherwise stated. Chromatographic purification (silica gel 60, 230-400 mesh, Silicycle Inc.; Sephadex LH-20, GE Healthcare) of all newly synthesized compounds was accomplished inside a fume hood under gravity unless otherwise mentioned. All NMR solvents were used as received. Chemical shifts for 1H NMR spectra are relative to solvent residual protium (CDCl3 δ= 7.24 ppm, (CD3)2SO δ= 2.50 ppm). The number of attached protons and coupling constants are found in parentheses following the chemical shift values. Instrumentation Mass spectra were obtained by a high resolution FT-ICR (Thermo LTQ-FT-ICR-MS – 7T) instrument equipped with electrospray ionization (ESI). Electronic absorption spectra were recorded on an Agilent 8453 UV/Visible photo diode array spectrophotometer. Steady-state emission spectra were recorded of the porphyrin solutions (solvent: CH3OH/CHCl3 (1:1)) at room temperature with an Edinburgh FLS920 spectrometer with emitted light first passing through a 515 nm long-pass filter, then a single grating (1800 1/mm, 500 nm blaze) Czerny-Turner monochromator (5 nm bandwidth) and finally detected by a peltiercooled Hamamatsu R2658P photomultiplier tube. The samples were excited with the light output from a housed 450 W Xe lamp/single grating (1800 1/mm, 250 nm blaze) Czerny-Turner monochromator combination with 5 nm bandwidth. Cyclic voltammetric measurements of the porphyrin phosphonate esters in CH2Cl2 with 0.1 M tetra n-butylammonium hexafluorophosphate were carried out on a CHI 660D potentiostat. Preparation of core/shell metal oxide electrodes and loading of porphyrins Core/shell metal oxide electrodes were made in two steps following a literature procedure.15First, a transparent thin film electrode of nanocrystalline mesoporous SnO2 (core) on FTO was made; then layer of TiO2 (shell) was deposited by atomic layer deposition (ALD). The SnO2 colloidal paste used to prepare electrodes in this study was prepared by using a protocol similar to that in the literature.15,16 In brief, acetic acid (1 mL) was added to SnO2 colloidal dispersion (30 mL of 15 wt %) in water and the mixture was stirred overnight at room tempera ture. Hydrothermal treatment was carried out by using a Parr


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Instruments pressure vessel at 240 °C for 60 h. The resulting solution was then sonicated and 2.5 wt % of both polyethylene oxide (mol wt 100,000) and polyethylene glycol (mol wt 12,000) were added. Stirring for 12 h yielded a homogeneous colloidal paste. Transparent thin-film electrodes were prepared by depositing the sol–gel paste onto conductive FTO glass substrates 4 cm × 2.2 cm using the doctor blade method with tape casting followed by sintering at 450 °C for 30 min under air. ALD was performed in a commercial reactor (Savannah S200, Cambridge Nanotech). Titanium dioxide (TiO2) was deposited by using tetrakis (dimethylamido) titanium, Ti(NMe2)4 (TDMAT, 99.999%, Sigma-Aldrich), and water. The reactor temperature was 130 °C. The TDMAT reservoir was kept at 75 °C. ALD coating conditions were 130 °C and 20 torr of N2 carrier gas with a sequence of 0.3 s metal precursor dose, 10 s hold, 20 s N2 purge, 0.02 s H2O dose, 10 s hold, 20 s N2 purge. Porphyrins were surface-bound to the metal oxide electrodes by dipping the core-shell slides in a solution of porphyrin (1 mM in CHCl3/MeOH (1:1)) for 16 h. At the end of the soaking period, the slides were washed withCHCl3 and MeOH and dried with a stream of N2 gas to give fully loaded surfaces with Γ ~ 2.6 x 10-8 mol/cm2 as determined from optical absorption of porphyrin bound electrodes. Electrochemical and Photoelectrochemical measurements The electrochemical cell used for electrochemical measurements utilized a glassy carbon working electrode, a platinum wire counter electrode, and a Ag/AgCl reference electrode (E = 0.197 V vs NHE). The photoelectrochemical measurements were performed with a 3-electrode configuration with a white light source (Thorlabs, model HPLS-30-4) outfitted with a 5 mm liquid light guide and 400 nm long pass filter (Figure S1, ESI). The phosphate buffer consisted of NaH2PO4 and Na2HPO4 salts dissolved in deionized water. KNO3 (0.4 M) was added as a supporting electrolyte. Hydroquinone (20 mM) was added as a sacrificial electron transfer donor in the photoelectrochemical experiments. Solutions were degassed by Ar bubbling for 30 min. Synthesis of porphyrins Synthesis of meso-susbtituted porphyrins was carried out by acid catalyzed condensation reactions following procedures developed by Lindsey et. al.17,18 The synthetic route to the porphyrins is shown in Scheme 1. Compounds 719, 820, 921, 1022 and 1122 (Scheme 1; Scheme S1, ESI) were synthesized by literature procedures. Compound 12 was reported earlier. Syntheses of porphyrins 1E, 2E, 1 and 2 were published earlier.23 5-(Heptafluoropropyl)dipyrrylmethane (12): 2,2,3,3,4,4,4-Heptafluorobutanal (4g, 20.2 mmol) was dissolved in THF (60 mL) and the solution was degassed by argon bubbling for 20 min. To this solution was added pyrrole (1.4 mL, 40.4 mmol)



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followed by conc. HCl (1 mL, 12 mmol). The mixture was refluxed for 2.5 h, cooled to 25 oC and then CH2Cl2 (100 mL) was added. The mixture was washed with NaHCO3 solution (100 mL × 3) and water

Scheme 1. Synthetic scheme for porphyrins 1-6

(100 mL × 3). After drying the organic phase over anhydrous Na2SO4, the solvent was removed. The desired product was purified from the crude mixture by flash column chromatography on silica gel using CH2Cl2 as eluant. The first band (which fluoresced blue upon exposure with long UV light) was collected and the solvent evaporated to give a light yellow oil which crystallized on standing. (3.25 g, 51% yield based on 2,2,3,3,4,4,4-heptafluorobutanal). 1H NMR (500 MHz, CDCl3): 8.10 (s, 2H), 6.75 (d, 2H, J = 5.0 Hz), 6.23 (d, 2H, J = 5.0 Hz), 6.19 (m, 2H), 4.89 (t, 4H, J = 13.1 Hz) 5-[4-(Diethoxyphosphoryl)phenyl]-15-[4-(bromomethyl)phenyl]-10,20bis(pentafluoropehenyl)porphyrin (3E): In a round bottom flask were added 5(pentafluorophenyl)dipyrrylmethane (1.21 g, 3.88 mmol), 4-(diethoxyphosphoryl) benzaldehyde (471.5 mg, 1.95 mmol) and 4-bromomethyl benzaldehyde (387.5 mg, 1.95 mmol) in CHCl3 (2 L). The solution was degassed by passing N2 through it for 15 min and then BF3.OEt2 (0.1mL, 0.8 mmol) was added to the solution. The mixture was stirred under N2 for 16h during which the solution color turned reddish yellow. 2,3-dichloro-5,6-dicyano-1,4-benzoquinone was added to the solution and stirred for 2h. The crude material was then passed through a bed of silica gel under vacuum and eluted with CHCl3/MeOH (98:2) until all reddish purple materials were removed. After removing the solvent from the filtrate the crude material was further purified by column chromatography on silica gel (using CHCl3/MeOH (98:2) as


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eluant) and onSephadex LH-20 dextran beads (using CHCl3/MeOH (50:50) as eluant). The major reddish purple band was collected and characterized as the desired compound 3E (455mg, yield = 22%). 1H NMR (400 MHz, CDCl3): 8.93 (d, 2H, J = 4.8 Hz), 8.88 (d, 2H, J = 4.8 Hz), 8.81-8.79 (m, 4H), 8.33-8.30 (m, 2H), 8.25-8.16 (m, 4H), 7.80 (d, 2H, J = 6.3 Hz), 4.85 (s, 2H), 4.43-4.34(m, 4H), 1.51 (t, 6H, J = 6.6 Hz), -2.88 (s, 2H). 19F NMR (376 MHz, CDCl3): -136.83 (dd, 4F, J = 28 Hz, J = 9 Hz), -152.06 (t, 2F, J = 22 Hz), -161.83 (td, 4F, J = 22 Hz, J = 9 Hz). 31P NMR (162 MHz, CDCl3): 18.62. ESI FT- ICR: 1023.113 (calcd M+H 1023.116). 5,15-bis[3,5-bis(Diethoxyphosphorylmethyl)phenyl]-10,20-bis(pentafluoropehenyl)porphyrin (4E): A solution of 5-(pentafluorophenyl)dipyrrylmethane (300 mg, 0.96 mmol), 3,5bis(diethoxyphosphorylmethyl) benzaldehyde (471.5 mg, 1.95 mmol) in CHCl3 (200 mL) was degassed by passing N2 through it for 30 min. BF3.OEt2 (0.05mL, 0.4 mmol) was added to the solution. The mixture was stirred under N2 for 18h. After 18 h 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (350 mg, 1.54 mmol) was added to the solution and stirred for 2h. The crude material was purified by silica gel column chromatogrpahy eluted with CH2Cl2/MeOH (96:4) with a major reddish purple band collected. (174 mg, yield = 26%). 1H NMR (400 MHz, CDCl3): 9.01 (d, 4H, J = 4.8 Hz), 8.79 (d, 4H, J = 4.8 Hz), 8.07-8.05 (m, 4H), 7.68-7.67 (m, 2H), 4.20-4.13 (m, 16H), 3.41 (d, 8H, J = 21.8 Hz), 1.29 (t, 24H, J = 7.2 Hz), -2.88 (s, 2H). 19F NMR (376 MHz, CDCl3): -136.82 (dd, 4F, J = 28 Hz, J = 9 Hz), -152.35 (t, 2F, J = 22 Hz), -162.07 (td, 4F, J = 22 Hz, J = 9 Hz). 31P NMR (162 MHz, CDCl3): 26.37. ESI FT-ICR: 1395.335 (calcd M+H 1395.339). 5,15-bis[4-(diethoxyphosphoryl)phenyl]-10,20-bis[(2,4,6-trimethyl)phenyl] porphyrin (5E): A solution of 5-(2,4,6-trimethylphenyl)dipyrrylmethane (660 mg, 2.5 mmol), 4-(diethoxyphosphoryl) benzaldehyde (605 mg, 2.5 mmol) in CHCl3 (250 mL) was degassed by passing N2 through it for 30min and BF3.OEt2 (0.3mL, 2.4 mmol) was added. The mixture was stirred under N2 for 2.5 h. After 2.5 h 2,3dichloro-5,6-dicyano-1,4-benzoquinone (850 mg, 3.74 mmol) was added to the solution and stirred for 2h. The crude material was purified by silica gel column chromatography with CH2Cl2/MeOH (98:2) as eluant followed by column chromatography on Sephadex LH-20 beads with CHCl3/MeOH (1:1) as eluant. A major reddish purple band was collected. (328 mg, yield = 27%). 1H NMR (400 MHz, CDCl3): 8.72 (dd, 8H, J = 11.9 Hz, J = 4.8 Hz), 8.07-8.05 (m, 4H), 8.21-8.16 (m, 4H), 7.27 (s, 4H), 4.41-4.31 (m, 8H), 2.62 (s, 6H), 1.82 (s, 12H), 1.50 (t, 12H, J = 7.0 Hz), -2.67 (s, 2H). ESI FT- ICR: 971.404 (calcd M+H 971.407). 5,15-bis[4-(diethoxyphosphoryl)phenyl]-10,20-bis (2,2,3,3,4,4,4-heptafluoropropyl)porphyrin (6E): In a degassed solution of 5-(heptafluoropropyl)dipyrrylmethane (325 mg, 1.03 mmol), 4(diethoxyphosphoryl) benzaldehyde (250 mg, 1.03 mmol) in CHCl3 (250 mL) was sdded BF3.OEt2 (0.05mL, 0.4 mmol). The mixture was stirred under N2 for 22h. After 22 h 2,3-dichloro-5,6-dicyano-1,4-



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benzoquinone (400 mg, 1.76 mmol) was added to the solution and stirred for 2h. The crude material was purified by silica gel column chromatography eluted with CH2Cl2/MeOH (98:2) followed by column chromatography on Sephadex LH-20 beads eluted with CHCl3/MeOH (1:1). A major reddish purple band was collected. (116 mg, yield = 21%). 1H NMR (400 MHz, CDCl3): 9.50-9.48 (m, 4H), 8.86 (d, 4H, J = 5.1 Hz), 8.29-8.22 (m, 8H), 4.45-4.39 (m, 8H), 1.55 (d, 12H, J = 7.1 Hz), -2.55 (s, 2H). 19F NMR (376 MHz, CDCl3): -79.14 (t, 6F, J = 11.4 Hz), -81.96 (m, 4F), -119.98 (s, 4F). 31P NMR (162 MHz, CDCl3): 18.28. ESI FT- ICR: 1071.210 (calcd M+H 1071.212). Procedure for hydrolyzing porphyrin phosphonate esters To the porphyrin phosphonate ester in anhydrous CH2Cl2 was added trimethylsilyl bromide (excess, 10 equivalents) under N2. The color of the solution turned green after addition of trimethylsilyl bromide. The mixture was heated at reflux under N2 for 2 days. The solvent was evaporated and a mixture of water and methanol was added. The color of the solution slowly changed to red. After stirring for 2h, the solvent was evaporated to obtain the hydrolyzed product. The material was purified by column chromatography on Sephadex LH-20 withCHCl3/MeOH (50:50) as eluant. A major reddish purple band was collected and the solvent was evaporated to obtain hydrolyzed product in 85-90% yield. 5-[4-(Dihydroxyphosphoryl)phenyl]-15-[4-(bromomethyl)phenyl]10,20bis(pentafluoropehenyl)porphyrin (3): 1H NMR (400 MHz, (CD3)2SO): 9.27 (d, 4H, J = 4.8 Hz), 8.93 (t, 4H, J = 5.8 Hz), 8.38 (dd, 2H, J = 7.9 Hz, J = 3.2 Hz), 8.28 (d, 2H, J = 7.9 Hz), 8.17-8.12 (m, 2H), 7.93 (d, 2H, J = 7.9 Hz), 5.10 (s, 2H), -3.05 (s, 2H). 19F NMR (376 MHz, (CD3)2SO): -139.47 (dd, 4F, J = 27 Hz, J = 8 Hz), -153.88 (t, 2F, J = 24 Hz), -162.07 (td, 4F, J = 24 Hz, J = 8 Hz). 31P NMR (162 MHz, (CD3)2SO): 12.46. ESI FT-ICR: 887.125 (calcd M-Br 887.127). 5,15-bis[4-(dihydroxyphosphoryl)phenyl]-10,20-bis[(2,4,6-trimethyl)phenyl] porphyrin (5): 1H NMR (400 MHz, (CD3)2SO): 8.77 (d, 4H, J = 4.9 Hz), 8.64 (d, 4H, J = 4.9 Hz), 8.34-8.31 (m, 4H), 8.13-8.08 (m, 4H), 7.34 (s, 4H), 2.58 (s, 6H), 1.74 (s, 12H), -2.77 (s, 2H). 31P NMR (162 MHz, (CD3)2SO): 12.43. ESI FT- ICR: 859.280 (calcd M+H 859.281). 5,15-bis[4-(dihydroxyphosphoryl)phenyl]-10,20-bis (2,2,3,3,4,4,4-heptafluoropropyl)porphyrin (6): 1

H NMR (400 MHz, (CD3)2SO): 9.44 (s, broad, 4H), 8.89 (d, 4H, J = 5.0 Hz), 8.34-8.32 (m, 4H), 8.18-

8.13 (m, 4H), -2.79 (s, 2H). 19F NMR (376 MHz, (CD3)2SO): -78.58 (t, 6F, J = 11.4 Hz), -81.66 (s, 4F), 119.82 (s, 4F). 31P NMR (162 MHz, (CD3)2SO): 12.23. ESI FT- ICR: 959.086 (calcd M+H 959.087). Results and Discussion Photochemical Stability on Metal Oxide Surfaces Porphyrins have been widely used in light-to-energy conversion applications,5–12 typically with carboxylic acid binding to semiconductor oxides in non-aqueous solvents.5–12 Although carboxylate-to-oxide surface


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binding is stable in organic solvents, surface hydrolysis limits stability in aqueous media.24–26 Relative stabilities for phosphonate and carboxylate surface binding under photolytic conditions were investigated on mesoporous, nanoparticle TiO2 films for the carboxylate- and phosphonate-substituted

Scheme 2. Structures of carboxylate- and phosphonatederivatized porphyrins compared for photostability on TiO2.

porphyrins, 5-(4-(carboxylic)phenyl)-10,15,20-tris(mesityl) porphyrin and 5-(4(dihydroxyphosphoryl)phenyl)-10,15,20-tris(mesityl) porphyrin(Scheme 2 and Figure S2, ESI) by an established protocol.27 In this protocol, porphyrin-loaded TiO2 slides, (~4 µm thick) with single-site carboxylate and phosphonate surface binding were immersed in aqueous 0.1 M HClO4 solutions and irradiated with an intense LED light source (455 nm, fwhm ~30 nm, 475 mW/cm2). Absorption spectra of the slide during the illumination periods were acquired every 15 min for 16 h. Absorbance-time plots at 515 nm for both porphyrins are shown in Figure S2, ESI. From the data, the contrast in stability between carboxylate and phosphonate-bound derivatives under these conditions is significant. At the end of the 16 h photolysis period, only ~20% of the carboxylate derivative remained on the surface while > 90% of the phosphonate derivative remained. The loss of absorbance was due to surface desorption with no spectral shifts observed to indicate decomposition of the porphyrin. (Figure S2, ESI). Porphyrin design and synthesis Inspired by the results of the surface photostability measurements, we synthesized the phosphonatederivatized porphyrin chromophores in Scheme 1. The rationale behind the synthetic strategy was to utilize variations in the meso substitution from mesityl to pentafluorophenyl to heptafluoropropyl to modify electronic properties systematically. All except structure 3 inScheme 1 are structurally symmetric with phenyl phosphonate substitutuents on the opposite meso position (10-,20-) of the porphyrin macrocyle. The symmetry of the structures is advantageous in the preparation of chromophore-catalyst assemblies on metal oxide surfaces by allowing for both co-loading of chromophore and catalyst12,28 or by utilizing a layer-by-layer approach with a metal ion as bridge.23,29–31 Porphyrin 3 incorporates a 4bromomethyl phenyl substituent which provides a site for synthetic covalent attachment of catalysts.



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Absorption and Emission Spectra

Figure 1. Absorption spectrum of 5E in CHCl3/MeOH (1:1). The inset shows absorption and normalized emission spectra.

Absorption and emission spectra of porphyrins 1E-6E were recorded in CHCl3/MeOH mixtures. Representative absorption and emission spectra for porphyrin 5E are shown in Figure 1. Absorption and emission maxima for porphyrin phosphonate esters 1E-6E are listed in Table 1. The visible absorptions arise from π→π* transitions and fall into two groups: 1) Q bands, a group of quasi allowed transitions from 500-650 nm (molar absorbance, ε ~ 20,000 M-1cm-1) arising from ground singlet state (S0) to first singlet excited state (S1) transitions, and 2) B band (Soret), which appears as a narrow, intense absorption at ~410-420 nm with ε ~ 500,000 M-1cm-1) arising from excitation of the ground state (S0) to second singlet excited state (S2). The structure in the Q band S0 → S1 transition is vibronic in nature32 as is the structure in the S1 to S0 fluorescence at 650-750 nm. Excited and Ground State Redox Potentials Ground state reduction potentials for the 1st oxidation (Eo’(P+/P)) and 1st reduction (Eo’ (P/P−)) were obtained by cyclic voltammetric (CV) measurements on the corresponding phosphonate esters of porphyrins 1E-6E in CH2Cl2 (Table 1). Excited state 0-0 energies, E0-0, were estimated from the cross sections of normalized absorption and emission spectra for the porphyrin phosphonate esters in solution (Figure 1 and Figure S3-S6, ESI). Excited state reduction potentials for the emitting porphyrin singlet states as electron donors (Eo’(P+/P*)) and electron acceptors (Eo’(P*/P−)) were estimated by subtracting or adding E0-0 values to the ground state reduction potentials for the 1st oxidation or reduction as shown in eqs 1-2 with E0-0 in eV and e the unit electron charge in eV/V. Eo’(P+/P*) = Eo’(P+/P) – E0-0/e (1) Eo’(P*/P−) = Eo’ (P/P−) + E0-0/e (2)


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Calculated formal potentials for the singlet excited states as electron donors, Eo’(P+/P*), are listed in Table 1 and illustrated in Figure 2 relative to the conduction band edges for the n-type oxides TiO2 and Table 1. Electrochemical and spectroscopic properties of the porphyrins in solution.

Compou

E1/2

E1/2

nd

(P+/P)

( P/P−)

E0-0

E1/2

E1/2

Absorption

Emission

(P+/P*)

( P*/P−)

λmax/nm (ε×10-5/M-

λmax/nm

1

1E

1.56

-0.72

2.00

-0.44

1.28

cm-1)

413(2.92), 509(0.19),

644, 712

542(0.04), 586(0.05), 640(0.01) 2E

1.31

-0.94

2.13

-0.82

1.19

422(4.74), 554(0.23),

603, 654

594(0.05) 3E

1.54

-0.73

1.98

-0.44

1.25

415(3.05), 510(0.21),

645, 712

544(0.05), 587(0.07), 641(0.03) 4E

1.45

-0.78

1.99

-0.54

1.21

415(3.28), 510(0.20),

647, 713

543(0.05), 587(0.06), 643(0.03) 5E

1.29

-1.02

1.95

-0.66

0.93

418(5.20), 515(0.26),

649, 716

549(0.11), 589(0.08), 645(0.05) 6E

1.67

-0.57

1.92

-0.25

1.35

410(1.91), 510(0.11),

647, 715

546(0.12), 590(0.05), 643(0.11) Electrochemical potentials are reported in V vs NHE. E0-0 values are reported in eV.

SnO2 at pH 7. Similarly, the potentials for the excited states as electron acceptors, Eo’(P*/P−), are listed in Figure 2 and Table 1 and compared to the valence band edge for the p-type oxide, NiO at pH 7. Inspection of the data in Table 1 shows that Eo’(P+/P) values for the oxidized porphyrins are all more positive than the thermodynamic potential for water oxidation at 1.23 V vs NHE at pH 0. Eo’(P+/P) increases systematically with the introduction of progressively stronger electron withdrawing substituents in the meso-position of the porphyrin with: 6E (C3F7)2P (1.67 V) > 1E (PhF5)2P (1.56 V) > 5E (mes)2P (1.29 V). The formal potentials for the singlet excited states as electron donors, Eo’(P+/P*), provide a measure of the thermodynamic ability of the singlet excited states to undergo electron injection into semiconductor



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oxides with injection favored for Eo’(P+/P*) < Ecb. Ecb is the conduction band potential of the metal oxide. Similarly, the Eo’(P*/P−) values provide insight into the ability of the excited states to accept electrons from the valence band of p-type oxides with hole transfer favored for Eo’(P*/P− ) > Evb with Evb the valence band potential of the metal oxide. As shown in Figure 2 and Table 1, injection into the conduction band of SnO2, with Ecb = 0.0 V vs. NHE at pH 7, is favorable for the series of porphyrin singlet excited states with Eo’(P+/P*) < Ecb. Similarly, all of the singlet excited states are thermodynamically capable of hole injection into NiO with Eo’(P*/P− ) > Evb and Evb = 0.6 V vs NHE for NiO. These results are relevant only to the emitting porphyrin singlet S1 states.32 The corresponding triplet states of free base and Zn

Figure 2. Excited (singlet) and ground state reduction potentials vs. NHE shown relative to the conduction band edge potentials for TiO2 (-0.55 V33 vs NHE at pH 7), SnO2 (0.00 V33 vs NHE at pH 7) and valence band edge potential of NiO (0.60 V34 vs NHE).

porphyrin are ~0.4-0.5 eV less energetic35,36 with a corresponding loss in both oxidizing and reducing strength. Excited State Injection on core/shell SnO2/TiO2 The ability of the phosphonate-derivatized free base porphyrins to undergo excited state injection into TiO2 shells on SnO2/TiO2 core/shell electrodes was investigated by photocurrent measurements.37 The results of a preliminary transient absorption study had shown that porphyrins 1 and 2 underwent singlet injection into mesoporous SnO2 to give porphyrin cation radicals 1+ and 2+.23 These experiments also provided evidence that intersystem crossing to the lower-lying triplet excited state was competitive with


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electron injection from the singlet. A related observation was made for a porphyrin-Ru(II) polypyridyl chromophore-catalyst assembly on SnO2 for which rapid intra-assembly transfer of the oxidative equivalent occurs from the oxidized porphyrin to the catalyst following porphyrin excitation and injection.23 The photoelectrochemical experiments were conducted on 3.5 µm thick, SnO2/TiO2 core/shell films. The core-shell electrodes were prepared from films of ~20 nm nanoparticle SnO2 on fluorine doped SnO2 (FTO) coated glass slides. The outer shell of TiO2 was deposited by atomic layer-deposition (ALD), as described previously with shell thicknesses of 4.5 nm.38 Core/shell electrodes were used to exploit their impact on interfacial electron transfer kinetics with significantly enhanced photocurrent efficiencies compared to TiO2. For the SnO2/TiO2 core/shells used here, injection and rapid electron transfer through the thin TiO2 shell to the SnO2 core creates a ~0.4 V barrier to back electron transfer due to the more positive conduction band potential for SnO2. 15,38,39

Figure 3. Light on/off (A) and 15 min (B) current-time traces for porphyrins 1, 5, and 6 with white light excitation at pH 7 in 0.1 M



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H2PO4-/HPO42- buffers 0.4 M in KNO3 with 20 mM added hydroquinone (H2Q).

The photocurrent response of porphyrin-derivatized SnO2/TiO2 core/shell slides, Г = 2.6 x 10-8 mol/cm2, was measured in a 3 electrode photoelectrochemical H-cell (Figure S1, ESI). The measurements were carried out on solutions containing 20 mM hydroquinone (H2Q) in a 0.1 M phosphate buffer (H2PO4/HPO42-) at pH ~ 7 with 0.4 M added KNO3 as the electrolyte. Under these conditions the porphyrins exist in their free base forms as shown by UV-visible measurements. The working electrode was held under a 0.2 V bias vs SCE with a Thor lab white light (100 mW/cm2) as the light source. Photocurrent-time traces are shown in Figure 3. In the photoelectrochemical experiments, H2Q was added as a reductive scavenger for the porphyrin cations formed following excitation and porphyrin excited state injection, eq 3-6. Under these conditions, with a relatively high concentration of H2Q (20 mM), the observed photocurrents are a measure of the efficiency of injection by the singlet excited states. hν

SnO2/TiO2|-P → SnO2/TiO2|-1P*

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SnO2/TiO2|-1P* → SnO2/TiO2(e-)|-P+ → SnO2(e-)/TiO2|-P+ SnO2(e-)/TiO2|-P+ → SnO2/TiO2|-P

(4)

(5)

SnO2(e-)/TiO2|-P+ + ½ H2Q → SnO2(e-)/TiO2|-P + ½ Q + H+

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Light on/light off photoelectrochemical cycles with added H2Q over 30 sec intervals are shown in Figure 3A and over 900 sec periods in Figure 3B. Based on these data, the magnitudes of the observed photocurrents were 500, 94, and 49 µA/cm2 for 5, 1, and 6 after 40 seconds of photolysis and 443, 82, and 45 µA/cm2 after 880 seconds. As shown by the results of nanosecond transient absorption experiments, the photocurrent do not arise from a photogalvanic mechanism with excited state reduction of the porphyrin excited state or states by H2Q followed by electron injection by the reduced porphyrin. Transient absorption measurements were conducted on 1 loaded on a nanoparticle film of the inert oxide ZrO2 whose conduction band potential is inaccessible to the porphyrin excited states. Experiments with and without 20 mM H2Q in the external solution exhibited the same excited state decay kinetics with no evidence for reductive quenching. The magnitudes of the photocurrents track the thermodynamic driving force for injection with o’

E (P*/P-) = -0.66 V for 5, -0.44 V for 1, and -0.25 V for 6. Steady state photocurrents diminish from the mesityl substituted porphyrin (~500 µA/cm2 for 5) to the pentafluorophenyl substituted porphyrin (~100 µA/cm2 for 1) to the heptafluoropropyl substituted porphyrin (~50 µA/cm2 for 6). For the latter two, electron injection into the TiO2 shell is thermodynamically disfavored based on the excited state reduction potentials. For these excited states, there may be contributions to injection into trap state levels below the


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conduction band or, perhaps, by direct electron tunneling through the shell to the SnO2 core. Under the same conditions, the photocurrent response with the reference Ru(II) polypyridyl complex, [Ru(bpy)(4,4’PO3H2bpy)2]2+, was ~300 µA/cm2 (Figure S7, ESI). For 5, relatively high, sustained photocurrent levels of ~0.5 mA/cm2 are observed over an extended period consistent with relatively efficient, sustained injection. UV-visible measurements at the end of the photolysis periods show that the porphyrin-derivatized surfaces are stable on this timescale, both toward porphyrin decomposition and surface loss (Figure S8, ESI). Conclusion We describe here the synthesis and characterization of the family of phosphonate-derivatized porphyrins, 1-6, with changes in substituents on the porphyrin core used to vary electronic structure and with it, excited and ground state redox potentials. The potentials for the P+/P couples for all 6 members of the series are sufficient to drive water oxidation to oxygen over an extended pH range and the excited states sufficiently reducing for electron injection into the conduction band of SnO2. The singlet excited states are also sufficiently oxidizing to undergo hole injection into the valence band of NiO. For the most highly reducing excited state, 5, significant, sustained photocurrents are obtained in a photoanode configuration on SnO2/TiO2 core/shell electrodes with the added sacrificial electron transfer donor hydroquinone. Supporting information NMR, absorption, emission of porphyrins and synthetic scheme of precursors are supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Author information Corresponding Author * [email protected] Acknowledgement This research was supported primarily by the UNC EFRC: Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0001011. A.N. acknowledges support from National Science Foundation under grant CHE-1362481. A.M.L. acknowledges a graduate fellowship supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. References (1)

Swierk, J. R.; Mallouk, T. E. Design and Development of Photoanodes for Water-Splitting Dye-



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Sensitized Photoelectrochemical Cells. Chem. Soc. Rev. 2013, 42 (6), 2357–2387. (2)

Concepcion, Javier J; Houser, Ralph L; Papanikolas, John M; Meyer, T. J. Chemical Approaches to Artificial Photosynthesis. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (39), 15560–15564.

(3)

Gust, D.; Moore, T. A.; Moore, A. L. Solar Fuels via Artificial Photosynthesis. Acc. Chem. Res. 2009, 42 (12), 1890–1898.

(4)

Alstrum-Acevedo, J. H.; Brennaman, M. K.; Meyer, T. J. Chemical Approaches to Artificial Photosynthesis. Inorg. Chem. 2005, 44 (20), 6802–6827.

(5)

Urbani, M.; Grätzel, M.; Nazeeruddin, M. K.; Torres, T. Meso-Substituted Porphyrins for DyeSensitized Solar Cells. Chem. Rev. 2014, 114 (24), 12330–12396.

(6)

Griffith, M. J.; Sunahara, K.; Wagner, P.; Wagner, K.; Wallace, G. G.; Officer, D. L.; Furube, A.; Katoh, R.; Mori, S.; Mozer, A. J. Porphyrins for Dye-Sensitised Solar Cells: New Insights into Efficiency-Determining Electron Transfer Steps. Chem. Commun. 2012, 48 (35), 4145-4162.

(7)

Milot, R. L.; Moore, G. F.; Crabtree, R. H.; Brudvig, G. W.; Schmuttenmaer, C. A. Electron Injection Dynamics from Photoexcited Porphyrin Dyes into SnO 2 and TiO2 Nanoparticles. J. Phys. Chem. C 2013, 117 (42), 21662–21670.

(8)

Walter, M. G.; Rudine, A. B.; Wamser, C. C. Porphyrins and Phthalocyanines in Solar Photovoltaic Cells. J. Porphyrins Phthalocyanines 2010, 14 (09), 759–792.

(9)

Moore, G. F.; Konezny, S. J.; Song, H. E.; Milot, R. L.; Blakemore, J. D.; Lee, M. L.; Batista, V. S.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W. Bioinspired High-Potential Porphyrin Photoanodes. J. Phys. Chem. C 2012, 116 (7), 4892–4902.

(10)

Swierk, J. R.; Méndez-Hernández, D. D.; McCool, N. S.; Liddell, P.; Terazono, Y.; Pahk, I.; Tomlin, J. J.; Oster, N. V.; Moore, T. A.; Moore, A. L.; Gust, Devens; Mallouk T. E. Metal-Free Organic Sensitizers for Use in Water-Splitting Dye-Sensitized Photoelectrochemical Cells. Proc. Natl. Acad. Sci. U. S. A. 2015, 112 (6), 1681–1686.

(11)

Li, L.-L.; Diau, E. W.-G. Porphyrin-Sensitized Solar Cells. Chem. Soc. Rev. 2013, 42 (1), 291– 304.

(12)

Moore, G. F.; Blakemore, J. D.; Milot, R. L.; Hull, J. F.; Song, H.; Cai, L.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W. A Visible Light Water-Splitting Cell with a Photoanode Formed by Codeposition of a High-Potential Porphyrin and an Iridium Water-Oxidation Catalyst. Energy Environ. Sci. 2011, 4 (7), 2389-2392.

(13)

Wang, Y.; Chen, B.; Wu, W.; Li, X.; Zhu, W.; Tian, H.; Xie, Y. Efficient Solar Cells Sensitized by Porphyrins with an Extended Conjugation Framework and a Carbazole Donor: From Molecular Design to Cosensitization. Angew. Chem. Int. Ed. 2014, 53 (40), 10779–10783.

(14)

Xie, Y.; Tang, Y.; Wu, W.; Wang, Y.; Liu, J.; Li, X.; Tian, H.; Zhu, W.-H. Porphyrin Cosensitization for a Photovoltaic Efficiency of 11.5%: A Record for Non-Ruthenium Solar Cells Based on Iodine Electrolyte. J. Am. Chem. Soc. 2015, 137 (44), 14055–14058.

(15)

Alibabaei, L.; Sherman, B. D.; Norris, M. R.; Brennaman, M. K.; Meyer, T. J. Visible Photoelectrochemical Water Splitting into H2 and O2 in a Dye-Sensitized Photoelectrosynthesis Cell. Proc. Natl. Acad. Sci. U. S. A. 2015, 112 (19), 5899–5902.

(16)

Chappel, S.; Zaban, A. Nanoporous SnO2 Electrodes for Dye-Sensitized Solar Cells: Improved Cell Performance by the Synthesis of 18nm SnO2 Colloids. Sol. Energy Mater. Sol. Cells, 2002, 71, 141–152.

(17)

Littler, B. J.; Ciringh, Y.; Lindsey, J. S. Investigation of Conditions Giving Minimal Scrambling in the Synthesis of Trans -Porphyrins from Dipyrromethanes and Aldehydes. J. Org. Chem. 1999, 64


Page 14 of 17

Page 15 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8), 2864–2872. (18)

Lee, C.-H.; S. Lindsey, J. One-Flask Synthesis of Meso-Substituted Dipyrromethanes and Their Application in the Synthesis of Trans-Substituted Porphyrin Building Blocks. Tetrahedron 1994, 50 (39), 11427–11440.

(19)

Muthukumaran, K.; Loewe, R. S.; Ambroise, A.; Tamaru, S.; Li, Q.; Mathur, G.; Bocian, D. F.; Misra, V.; Lindsey, J. S. Porphyrins Bearing Arylphosphonic Acid Tethers for Attachment to Oxide Surfaces. J. Org. Chem. 2004, 69 (5), 1444–1452.

(20)

Wen, L.; Li, M.; Schlenoff, J. B. Polyporphyrin Thin Films from the Interfacial Polymerization of Mercaptoporphyrins. J. Am. Chem. Soc. 1997, 119 (33), 7726–7733.

(21)

Bhosale, S. V.; Kalyankar, M. B.; Langford, S. J.; Bhosale, S. V.; Oliver, R. F. Synthesis and Supramolecular Properties of a Novel Octaphosphonate Porphyrin. Eur. J. Org. Chem. 2009, 2009 (24), 4128–4134.

(22)

Laha, J. K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.; Lindsey, J. S. A Scalable Synthesis of Meso-Substituted Dipyrromethanes. Org. Process Res. Dev. 2003, 7 (6), 799–812.

(23)

Nayak, A.; Knauf, R. R.; Hanson, K.; Alibabaei, L.; Concepcion, J. J.; Ashford, D. L.; Dempsey, J. L.; Meyer, T. J. Synthesis and Photophysical Characterization of Porphyrin and porphyrin– Ru(II) Polypyridyl Chromophore–catalyst Assemblies on Mesoporous Metal Oxides. Chem. Sci. 2014, 5 (8), 3115–3119.

(24)

Bauer, T.; Schmaltz, T.; Lenz, T.; Halik, M.; Meyer, B.; Clark, T. Phosphonate- and CarboxylateBased Self-Assembled Monolayers for Organic Devices: A Theoretical Study of Surface Binding on Aluminum Oxide with Experimental Support. ACS Appl. Mater. Interfaces 2013, 5 (13), 6073– 6080.

(25)

Bae, E.; Choi, W.; Park, J.; Shin, H. S.; Kim, S. Bin; Lee, J. S. Effects of Surface Anchoring Groups (Carboxylate vs Phosphonate) in Ruthenium-Complex-Sensitized TiO 2 on Visible Light Reactivity in Aqueous Suspensions. J. Phys. Chem. B 2004, 108 (37), 14093–14101.

(26)

Bae, E.; Choi, W. Effect of the Anchoring Group (Carboxylate vs Phosphonate) in Ru-ComplexSensitized TiO 2 on Hydrogen Production under Visible Light. J. Phys. Chem. B 2006, 110 (30), 14792–14799.

(27)

Hanson, K.; Brennaman, M. K.; Luo, H.; Glasson, C. R. K.; Concepcion, J. J.; Song, W.; Meyer, T. J. Photostability of Phosphonate-Derivatized, Ru(II) Polypyridyl Complexes on Metal Oxide Surfaces. ACS Appl. Mater. Interfaces 2012, 4 (3), 1462–1469.

(28)

Sherman, B. D.; Pillai, S.; Kodis, G.; Bergkamp, J.; Mallouk, T. E.; Gust, D.; Moore, T. A.; Moore, A. L. A Porphyrin-Stabilized Iridium Oxide Water Oxidation Catalyst. Can. J. Chem. 2011, 89 (2), 152–157.

(29)

Feldheim, D. L.; Mallouk, T. E. Layer-by-Layer Assembly and Intercalation Reactions of iron(III) and iron(II) Alkanebisphosphonates on Gold Surfaces. Chem. Commun. 1996, No. 22, 2591–2592.

(30)

Hanson, K.; Torelli, D. A.; Vannucci, A. K.; Brennaman, M. K.; Luo, H.; Alibabaei, L.; Song, W.; Ashford, D. L.; Norris, M. R.; Glasson, C. R. K.; Concepcion J. J.; Meyer, T. J. Self-Assembled Bilayer Films of ruthenium(II)/polypyridyl Complexes through Layer-by-Layer Deposition on Nanostructured Metal Oxides. Angew. Chem. Int. Ed. 2012, 51 (51), 12782–12785.

(31)

Terada, K.; Kobayashi, K.; Hikita, J.; Haga, M. Electric Conduction Properties of Self-Assembled Monolayer Films of Ru Complexes with Disulfide/Phosphonate Anchors in a Au–(Molecular Ensemble)–(Au Nanoparticle) Junction. Chem. Lett. 2009, 38 (5), 416–417.

(32)

Gouterman, M. The Porphyrins: Vol 3; Dolphin, D., Ed.; Academic Press: New York, 1978.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(33)

Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414 (6861), 338–344.

(34)

Stevanović, V.; Lany, S.; Ginley, D. S.; Tumas, W.; Zunger, A. Assessing Capability of Semiconductors to Split Water Using Ionization Potentials and Electron Affinities Only. Phys. Chem. Chem. Phys. 2014, 16 (8), 3706–3714.

(35)

Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Diederich, F. N.; Whetten, R. L.; Rubin, Y.; Alvarez, M. M.; Anz, S. J. Photophysical Properties of Sixty Atom Carbon Molecule (C60). J. Phys. Chem. 1991, 95 (1), 11–12.

(36)

Spellane, P. J.; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. C. Porphyrins. 40. Electronic Spectra and Four-Orbital Energies of Free-Base, Zinc, Copper, and Palladium Tetrakis(perfluorophenyl)porphyrins. Inorg. Chem. 1980, 19 (2), 386–391.

(37)

Hambourger, M.; Liddell, P. A.; Gust, D.; Moore, A. L.; Moore, T. A. Parameters Affecting the Chemical Work Output of a Hybrid Photoelectrochemical Biofuel Cell. Photochem. Photobiol. Sci. 2007, 6 (4), 431–437.

(38)

Alibabaei, L.; Farnum, B. H.; Kalanyan, B.; Brennaman, M. K.; Losego, M. D.; Parsons, G. N.; Meyer, T. J. Atomic Layer Deposition of TiO2 on Mesoporous nanoITO: Conductive Core-Shell Photoanodes for Dye-Sensitized Solar Cells. Nano Lett. 2014, 14 (6), 3255–3261.

(39)

Alibabaei, L.; Brennaman, M. K.; Norris, M. R.; Kalanyan, B.; Song, W.; Losego, M. D.; Concepcion, J. J.; Binstead, R. a; Parsons, G. N.; Meyer, T. J. Solar Water Splitting in a Molecular Photoelectrochemical Cell. Proc. Natl. Acad. Sci. U. S. A. 2013, 110 (50), 20008–20013.

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