Slow Charge Recombination and Enhanced Photoelectrochemical

Article pubs.acs.org/JPCC

Slow Charge Recombination and Enhanced Photoelectrochemical Properties of Diazaporphyrin-Fullerene Linked Dyad Masanori Yamamoto,† Yuta Takano,‡ Yoshihiro Matano,§ Kati Stranius,∥ Nikolai V. Tkachenko,*,∥ Helge Lemmetyinen,∥ and Hiroshi Imahori*,†,‡ †

Department of Molecular Engineering, Graduate School of Engineering and ‡Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan § Department of Chemistry, Faculty of Science, Niigata University, Nishi-ku, Niigata 950-2181, Japan ∥ Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FIN-33101 Tampere, Finland S Supporting Information *

ABSTRACT: Diazaporphyrin-C60 linked dyad has been prepared to assess intrinsic electron transfer properties of diazaporphyrins for the first time. The dyad exhibited the efficient formation of a chargeseparated state which has a lifetime 4× longer than that of the corresponding porphyrin-C60 linked dyad with the same spacer. In accordance with this elongation, a SnO2 nanostructured electrode modified with the diazaporphyrin-C60 dyad also revealed enhanced photocurrent generation in the visible region in comparison with that with the porphyrin-C60 dyad. These results show that diazaporphyrins are highly promising building blocks for the construction of artificial photosynthesis and solar energy conversion.

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INTRODUCTION Over the last two decades much effort has been devoted to exploring excellent donor−acceptor (D−A) couples for artificial photosynthesis and solar energy conversion.1−16 In this regard, fullerenes have been frequently employed as acceptors to exhibit outstanding electron transfer (ET) properties in D−A systems as well as organic photovoltaics.17−20 Meanwhile, various donors such as porphyrins,21 phthalocyanines,22 and ruthenium bipyridyl complexes23 have been involved in such systems and devices. However, the examples of superior donors are still limited in terms of lightharvesting and ET properties. 5,15-Diazaporphyrins (DAPs) are a class of porphyrins that bear two meso-nitrogen atoms linking two dipyrromethene units. DAPs have received continuing interest in relation to their analogues, porphyrins and phthalocyanines.21,22,24,25 It has been shown that 5,15-aza-substitution significantly changes the intrinsic properties of the porphyrin π systems that would be comparable to or even better than their analogues. Nevertheless, due to synthetic difficulties of the cyclization and peripheral modification of the core in DAPs, the opportunity of using DAPs as materials has poorly been addressed.26−29 As such, DAPs have yet to be used as donors in D−A linked systems to elucidate the intrinsic ET properties. Here we report, for the first time, the synthesis and photophysical and photoelectrochemical properties of zinc 5,15-diazaporphyrin (ZnDAP)-C60 linked dyad (Figure 1). To evaluate the intrinsic photophysical properties of ZnDAP as a © 2014 American Chemical Society

donor accurately, the corresponding zinc porphyrin (ZnP)-C60 linked dyad with the same spacer was also designed (Figure 1). Owing to the electron-withdrawing character of the 5,15-azasubstitution ZnDAP would be more difficult to be oxidized than ZnP.30,31 Accordingly, we expected that once a chargeseparated state was generated via photoinduced ET, charge recombination (CR) from the charge-separated state to the ground state in ZnDAP-C60 would be slowed down as compared with ZnP-C60, because CR in ZnDAP-C60 is shifted more deeply into the Marcus inverted region. To attain sufficient solubility of the dyads, bulky long alkoxy chains were also introduced to the ortho-positions of the meso-phenyl group. In this article, we present the synthesis, optical, electrochemical, photophysical, and photoelectrochemical properties of ZnDAPC60 in comparison with the corresponding ZnP-C60.

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EXPERIMENTAL SECTION General. 1H and 13C NMR spectra were recorded on a JEOL AL300 (300 MHz for 1H and 75.5 MHz for 13C) and a JEOL ECX-400P (400 MHz for 1H and 99.6 MHz for 13C) spectrometer in CDCl3 and chemical shifts were noted in δ ppm with reference to internal tetramethylsilane peak (Si(CH3)4, 0.00 ppm) and internal residual solvent peak (CHCl3, 7.27 ppm). Silica gel column chromatography was performed Received: October 22, 2013 Revised: December 29, 2013 Published: January 21, 2014 1808

dx.doi.org/10.1021/jp410436f | J. Phys. Chem. C 2014, 118, 1808−1820

The Journal of Physical Chemistry C

Article

Figure 1. Structures of ZnP-C60, ZnDAP-C60, and their references.

were studied, that is, 450−770 and 850−1100 nm. The typical response time of the instrument was 150 fs (full width at halfmaximum (fwhm)). A global multiexponential fitting procedure was applied to process the data. The procedure takes into account the instrument time response function and the group velocity dispersion of the white continuum and allows one to calculate the decay time constants and dispersion-compensated transient absorption spectra. The excitation energy was adjusted to low density enough to avoid the photodegradation of samples and the energy-dependent decay processes. All measurements were carried out at room temperature. Emission decays were measured using up-conversion technique as described elsewhere.32 In brief, the samples were excited by the second harmonic (405 nm) of 50 fs pulses produced by the same Ti:sapphire generator. Emission was mixed with the fundamental pulses to achieve frequency up-conversion, and resulting UV photons were detected by a photon counting photomultiplier coupled with a monochromator. Typical time resolution of the instrument was 150−200 fs. Photoelectrochemical Measurement. All photoelectrochemical measurements were carried out in a standard threeelectrode system using an ALS 630A electrochemical analyzer.33 The deposited film as a working electrode was immersed into an electrolyte solution containing 0.5 M LiI and 0.01 M I2 in acetonitrile. A Pt coil was employed as a counter electrode, which was fixed with ∼1 mm distance from the working electrode. A Pt wire covered with a glass Luggin capillary, whose tip was located near the working electrode, was used as a quasi-reference electrode. The potential measured was converted to the saturated calomel electrode (SCE) scale by adding +0.05 V. A 500 W xenon lamp (USHIO, XB50101AAA) was used as a light source. White light (λ > 380 nm; input power, 35 mW cm−2) or monochromatic light through a monochromator (Ritsu, MC-10N) was illuminated on the modified area of the working electrode (0.20 cm2) from the backside. The light intensity was monitored by an optical power meter (Anritsu, ML9002A) and corrected for calculation of the incident photon-to-current efficiency (IPCE) values.

using UltraPure silica gel (230−400 mesh, SiliCycle Inc.). Alumina column chromatography was carried out using activated alumina (300 mesh, Wako). Thin layer chromatography (TLC) was conducted on aluminum plates coated with silica gel 60 F254 (Merck) or aluminum oxide 60 F254 (Merck). Infrared (IR) spectra were recorded in KBr pellet by using FT/IR-470Plus (JASCO). High-resolution mass spectra (HRMS) were measured on a JEOL JMS-HX110A spectrometer. All reactions were performed under argon. Atomic force microscopy (AFM) images were obtained with a SII NanoTechnology SIDF40P. Optical Measurements. Steady-state absorption spectra were measured with a Lambda 900 (Perkin-Elmer) UV/vis/ NIR spectrometer with a data interval of 0.5 nm. These spectra were taken with about 10−5−10−6 M solutions in a quartz cell with a path length of 1 cm. Steady-state fluorescence spectra were measured with a FluoroMax-3 (JOBIN YVON, HORIBA) spectrofluorophotometer with a data interval of 1 nm. These spectra were taken with about 10−6 M solutions in a quartz cell with a path length of 1 cm. Solvents were degassed by bubbling with argon before use. Spectral grade toluene (Wako) and HPLC grade benzonitrile (Aldrich) were used for these measurements. Time-Resolved Spectroscopy. Subpicosecond to nanosecond time-resolved absorption spectra were collected using a pump−probe technique as described elsewhere.32 The fast processes were also monitored by using the fluorescence upconversion techniques with time resolutions of about 100 fs.32 The femtosecond pulses of the Ti:sapphire generator were amplified by using a multipass amplifier (CDP-Avesta) pumped by a second harmonic of the Nd:YAG Q-switched laser (Solar TII, model LF114). The amplified pulses were used to generate a second harmonic (405 nm) for sample excitation (pump beam) and a white continuum for time-resolved spectrum detection (probe beam). The transient spectra were recorded by a charge-coupled device (CCD) detector (Andor, Newton DU920) coupled with a monochromator in the visible and near-infrared ranges. The wavelength range for a single measurement was roughly 300 nm and typically two regions 1809



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Scheme 1. Synthesis of ZnDAP-C60 Dyad

Scheme 2. Synthesis of ZnP-C60 Dyad

Quantum Chemical Calculations. The initial geometries for the optimization procedure of the neutral molecules were based on the structures built on Chem 3D by MM2. The geometry optimization calculations were performed with the Gaussian 03 program34 with an HPC2500 computer at the PM3 level, followed by the B3LYP/6-31G(d) level. The frequency analyses were performed at the same level. The initial geometries of the radical cation states were based on the structures optimized for the neutral molecules and the geometry optimizations and the frequency analyses were performed at the UB3LYP/6-31G(d) level. The energies of the neutral and radical cation states were also calculated by the B3LYP/6-31G(d) and UB3LYP/6-31G(d) levels, respectively. Electrochemical Measurements. Differential pulse voltammetry measurements were performed on an ALS660A electrochemical analyzer in deaerated benzonitrile containing 0.1 M TBAPF6 as a supporting electrolyte. A conventional three-electrode cell was used with a glassy carbon working electrode and a platinum wire as a counter electrode. The measured potentials were recorded with respect to the Ag/ AgNO3 reference electrode. The first oxidation potential of ferrocene used as a standard is 0.37 V versus SCE in CH2Cl2 under the present experimental conditions. Synthesis. The synthetic routes toward ZnDAP-C60, ZnPC60, and their references are illustrated in Schemes 1 and 2. 1,3Didecoxybenzene, 3 5 5-(4-methoxycarbonyphenyl)dipyrromethane,36 and C60-ref37 were synthesized following the previously reported methods. Synthesis of 1. To a solution of 1,3-didecoxybenzene35 (21.68 g, 55.5 mmol) in distilled THF (150 mL) were added

tetramethylethylene diamine (12.8 g, 110 mmol) and nbutyllithium (52.5 mL, 86.1 mmol, 1.64 M in hexanes) at 0 °C. The resulting mixture was stirred at room temperature for 5 h. To this solution was added DMF (20.0 mL, 259 mmol) and stirring continued for an additional 12 h. To this mixture were added water (100 mL) and dichloromethane (100 mL). The aqueous layer was extracted with dichloromethane. The combined organic extracts were washed with brine and dried over MgSO4. The solution was filtrated, concentrated in vacuo, and purified by silica column chromatography (hexane/ dichloromethane = 1/1) to afford 1 as a colorless oil (21.1 g, 5.04 mmol, 91%). 1H NMR (300 MHz, CDCl3): δ 10.54 (s, 1H), 7.37 (t, J = 8.4 Hz, 1H), 6.53 (d, J = 8.4 Hz, 2H), 4.02 (t, J = 6.6 Hz, 4H), 1.82 (m, 4H), 1.50−1.40 (m, 4H), 1.40−1.10 (m, 24H), 0.88 (t, J = 5.9 Hz, 6H). 13C NMR (75 MHz, CDCl3): δ 189.3, 161.7, 135.5, 104.6, 69.0, 31.9, 29.5, 29.3, 29.1, 26.0, 22.7, 14.1. IR (neat): 2919, 2848, 1688, 1583, 1458, 1394, 1251, 1180, 1093, 823, 776, 721, 663, 592 cm−1. HRMS (ESI): m/z calcd for C27H47O3+ ([M + H]+), 419.3520; found, 419.3521. Synthesis of 2. Two-necked, round-bottomed flask was added 100 mL of pyrrole and degassed with argon for 10 min. To the solution were added well grinded 1 (3.80 g, 9.08 mmol) and trifluoroacetic acid (TFA, 0.15 mL). After stirring for 15 min at the temperature in the dark, to the reaction mixture were added triethylamine (30 mL) and dichloromethane (200 mL). The mixture was then washed with water, dried over MgSO4, filtrated, evaporated under reduced pressure, and then purified with silica column chromatography (hexane/ethyl acetate = 10/1) to give 2 as a brown liquid (4.16 g, 7.78 mmol, 1810



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86%). 1H NMR (300 MHz, CDCl3): δ 8.55 (br, 2H), 7.12 (t, J = 8.4 Hz, 1H), 6.61 (dd, J = 4.4, 2.2 Hz, 2H), 6.56 (t, J = 8.4 Hz, 2H), 6.20 (s, 1H), 6.08 (dd, J = 5.5, 2.2 Hz, 2H), 5.88 (m, 2H), 3.86 (t, J = 6.2 Hz, 4H), 1.51 (m, 4H), 1.35−1.05 (m, 28H), 0.87 (t, J = 7.0 Hz, 6H). 13C NMR (75 MHz, CDCl3): δ 133.3, 115.8, 107.8, 105.8, 31.9, 29.6, 29.3, 22.7, 14.1. IR (neat): 3417, 2922, 2852, 1591, 1460, 1239, 1089, 1026, 710, 630, 533, 490 cm−1. HRMS (ESI): m/z calcd for C35H55N2O2+ ([M + H]+), 535.4258; found, 535.4248. Synthesis of 3. A solution of 2 (4.164 g, 7.79 mmol) in THF (105 mL) was cooled to −78 °C. To the reactor Nbromosuccinimide (NBS, 2.717 g, 15.3 mmol) was added at one portion. After stirring for 1 h at −78 °C, 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ, 1.593 g, 7.02 mmol) in THF was added over 10 min, and then the reaction mixture was warmed to room temperature. The solvent was evaporated under reduced pressure. The residue was immediately subjected to alumina short column chromatography with dichloromethane as eluent, followed by purification with silica column chromatography (hexane/dichloromethane = 2/1, then 8/5) to give 3 as a dark liquid (3.54 g, 5.13 mmol, 66%). This compound was unstable at room temperature, yielding the isomers. 1H NMR (300 MHz, CDCl3): δ 7.29 (t, J = 8.4 Hz, 1H), 6.57 (d, 8.4 Hz, 2H), 6.36 (d, J = 4.0 Hz, 2H), 6.22 (d, J = 4.0 Hz, 2H), 3.85 (t, J = 6.2 Hz, 4H), 1.51 (m, 4H), 1.35−1.05 (m, 28H), 0.89 (t, J = 6.2 Hz, 6H). 13C NMR (75 MHz, CDCl3): δ 158.2, 140.9, 130.2, 128.8, 127.9, 119.7, 104.8, 68.7, 31.9, 29.6, 29.5, 29.4, 29.2, 29.0, 25.8, 22.7, 14.1. IR (neat): 2921, 2851, 1579, 1456, 1419, 1361, 1334, 1242, 1100, 1034, 632, 531. HRMS (ESI): m/z calcd for C35H51Br2N2O2+ ([M + H]+), 691.2291; found, 691.2272. Synthesis of 4. A solution of 5-(4-methoxycarbonylphenyl)dipyrromethane (0.703 g, 2.51 mmol) in THF (30 mL) was cooled to −78 °C. To the reactor NBS (0.900 g, 5.06 mmol) was added at one portion. After stirring for 1 h at −78 °C, DDQ (0.583 g, 2.57 mmol) in THF was added over 10 min, and then the reaction mixture was warmed to room temperature. The solvent was evaporated under reduced pressure. The residue was immediately subjected to alumina short column chromatography with dichloromethane as eluent, followed by purification with silica column chromatography (hexane/dichloromethane = 1/1). The product was recrystallized from hexane/dichloromethane to give 4 as an orange solid (0.941 g, 2.16 mmol, 86%). Mp 145−147 °C. 1H NMR (300 MHz, CDCl3): δ 8.13 (dd, J = 6.6, 1.8 Hz, 2H), 7.51 (dd, J = 6.6, 1.8 Hz, 2H), 6.40 (d, J = 4.4 Hz, 2H), 6.34 (d, J = 4.4 Hz, 2H), 3.97 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 166.5, 140.0, 137.8, 131.0, 130.7, 130.1, 129.8, 129.1, 120.8, 52.4. IR (pellet): 3137, 2947, 1715, 1573, 1552, 1501, 1436, 1405, 1355, 1309, 1256, 1244, 1203, 1150, 1104, 1038, 1009, 952, 928, 902, 868, 814, 778, 759, 716, 635, 605 cm−1. HRMS (ESI): m/z calcd for C17H13Br2N2O2+ ([M + H]+), 436.9318; found, 436.9323. Synthesis of 5. A solution of 3 (0.690 g, 0.999 mmol), 4 (0.431 g, 0.987 mmol), bis(acetylacetonato)lead (0.447 g, 1.10 mmol), and sodium azide (0.285 g, 4.38 mmol) in n-propanol (500 mL) was refluxed for 5 h under an argon atmosphere. To the reactor were added water and toluene. The organic phase was washed with water and dried over Na2SO4, and solvents were removed under reduced pressure. The residue was purified by column chromatography on silica gel with hexane/ethyl acetate = 5/1, 2/1, and then 1/1 (v/v) as the eluent to give a mixture of cyclization products where the core is metalated with lead atom. The mixture was again dissolved in

dichloromethane (100 mL) and TFA (10 mL) was then added. After stirring for 1 h, the solution was washed with satd NaHCO3 and brine, dried over Na2SO4, and filtrated. The product was purified by column chromatography on silica gel with hexane/dichloromethane = 1/1 and then dichloromethane/ethyl acetate = 50/1 as the eluents. Reprecipitation from dichloromethane/methanol gave 5 as a purple solid (36.6 mg, 0.0424 mmol, 4.3% yield). 1H NMR (300 MHz, CDCl3): δ 9.31 (d, J = 4.7 Hz, 2H), 9.24 (d, J = 4.7 Hz, 2H), 8.97 (pseudo triplet, J = 5.1 Hz, 4H), 8.50 (d, J = 7.8 Hz, 2H), 8.27 (d, J = 7.8 Hz, 2H), 7.72 (t, J = 8.4 Hz, 1H), 7.00 (d, J = 8.4 Hz, 2H), 4.51 (t, J = 6.6 Hz, 2H), 3.88 (t, J = 6.2 Hz, 4H), 1.97 (m, 2H), 1.30−0.50 (m, 41H), −2.58 (s, 2H). 13C NMR (75 MHz, CDCl3): δ 159.8, 144.3, 134.6, 132.8, 131.0, 130.5, 128.3, 121.4, 105.2, 68.7, 67.0, 31.4, 28.8, 28.7, 28.5, 25.2, 22.4, 22.3, 13.9, 10.7. HRMS (ESI): m/z calcd for C54H67N6O4+ ([M + H]+), 863.5218; found, 863.5202. UV−vis (benzonitrile): λmax (ε) 401 (140000), 509 (9400), 543 (31400), 628 nm (40800 M−1 cm−1). Synthesis of 6.38 To a flask containing 5 (44.1 mg, 0.0511 mmol) and THF (10 mL) was added lithium aluminum hydride (LAH, 5.2 mg, 0.14 mmol). The reaction was monitored every 1 h by TLC (silica gel, hexane/ethyl acetate = 5/1, Rf = 0.28). At the same time, 5 mg of LAH was added to the mixture to complete the reaction. Once TLC indicated the consumption of 5, the reaction mixture was cooled and excess LAH was quenched by cautious addition of several lumps of ice. The resulting suspension was filtered through Celite and the residue was thoroughly washed with dichloromethane and methanol. The filtrate was concentrated and the residue was dried under reduced pressure. Then, the crude product was dissolved in dichloromethane (15 mL) and portions of activated manganese dioxide were added until TLC (silica gel) indicated the conversion to the corresponding aldehyde. The reaction mixture was filtered through Celite and the residue was washed thoroughly with dichloromethane and methanol. The filtrate was evaporated to dryness and the residue was chromatographed on silica gel with chloroform as eluent to give 6 as a purple solid (37.4 mg, 0.0465 mmol, 91%). 1 H NMR (300 MHz, CDCl3): δ 10.42 (s, 1H), 9.32 (d, J = 3.7 Hz, 2H), 9.25 (d, J = 3.7 Hz, 2H), 8.99 (d, J = 3.7 Hz, 2H), 8.95 (d, J = 3.7 Hz, 2H), 8.38 (d, J = 6.0 Hz, 2H), 8.35 (d, J = 6.0 Hz, 2H), 7.72 (t, J = 6.2 Hz, 1H), 7.00 (t, J = 6.2 Hz, 2H), 3.88 (t, J = 6.2 Hz, 4H), 1.00−0.25 (m, 38H), −2.58 (s, 2H). 13 C NMR (75 MHz, CDCl3): δ 192.2, 159.8, 146.1, 136.0, 135.7, 135.2, 132.9, 131.1, 128.4, 120.9, 117.1, 115.9, 105.2, 68.7, 31.5, 28.8, 28.7, 28.5, 25.2, 22.4, 14.0. HRMS (ESI): m/z calcd for C51H61N6O3+ ([M + H]+), 805.4800; found, 805.4793. Synthesis of ZnDAP-C60. A solution of 6 (37.4 mg, 46.5 μmol), C60 (133 mg, 185 μmol), and N-methylglycine (20.0 mg, 244 μmol) in toluene (200 mL) was degassed with argon for 10 min. The reaction mixture was warmed to 110 °C and stirred at that temperature for 20 h. Once cooled, the contents of the flask were concentrated and the residue was chromatographed on silica gel with dichloromethane as the eluent to give the desired product as a purple solid. To a round-bottomed flask equipped with reflux condenser were added the product, Zn(OAc)2·2H2O (238.7 mg, 1.09 mmol), CHCl3 (30 mL), and MeOH (6 mL). The reaction mixture was stirred at 65 °C for 24 h under an argon atmosphere. The mixture was then allowed to cool, washed with satd NaHCO3 and water, dried over Na2SO4, and the solvent was evaporated under reduced 1811



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dichloromethane as eluent to give 8 as a purple solid (107 mg, 0.133 mmol, 88%). Mp 141−142 °C. 1H NMR (300 MHz, CDCl3): δ 10.38 (s, 1H), 10.23 (s, 2H), 9.36 (d, J = 4.8 Hz, 2H), 9.29 (d, J = 4.8 Hz, 2H), 9.01 (d, J = 4.8 Hz, 2H), 8.97 (d, J = 4.8 Hz, 2H), 8.42 (d, J = 7.9 Hz, 2H), 8.29 (d, J = 7.9 Hz, 2H), 7.70 (t, J = 8.2 Hz, 1H), 7.00 (d, J = 8.2 Hz, 2H), 3.83 (t, J = 6.2 Hz, 4H), 1.10−0.95 (m, 4H), 0.90−0.25 (m, 34H), −3.07 (s, 2H). 13C NMR (75 MHz, CDCl3): δ 192.5, 160.0, 148.5, 148.3, 145.7, 145.4, 145.0, 135.4, 131.5, 131.4, 131.1, 130.3, 130.0, 128.2, 119.5, 116.6, 112.6, 105.4, 104.9, 68.7, 31.5, 28.9, 28.5, 25.2, 22.5, 14.0. IR (pellet): 2919, 2850, 1691, 1586, 1453, 1409, 1384, 1239, 1192, 1142, 1094, 1061, 1048, 986, 972, 954, 902, 876, 846, 823, 760, 733, 710, 689, 643 cm−1. HRMS (ESI): m/z calcd for C53H63N4O3+ ([M + H]+), 803.4895; found, 803.4890. Synthesis of ZnP-C60. A solution of 8 (100.8 mg, 0.126 mmol), C60 (366.3 mg, 0.508 mmol), and N-methylglycine (45.1 mg, 0.506 mmol) in toluene (450 mL) was degassed with argon for 10 min. The reaction mixture was warmed to 110 °C and stirred for 20 h. After cooling to room temperature, the contents of the flask were concentrated, and the residue was chromatographed on silica gel with toluene and then dichloromethane as the eluent to give the freebase porphyrinC60 dyad. To a round-bottomed flask were added the product, Zn(OAc)2·2H2O (515.6 mg, 2.35 mmol), CHCl3 (100 mL), and MeOH (5 mL). The reaction mixture was stirred at 70 °C overnight under an argon atmosphere. The mixture was then allowed to cool, washed with satd NaHCO3 and water, dried over Na2SO4, and the solvent was evaporated under reduced pressure. The residue was purified by column chromatography with chloroform as the eluent, and recrystallized from dichloromethane/methanol to afford ZnP-C60 as a purple needle crystal (107.8 mg, 66.8 μmol, 53% yield). Mp > 300 °C. 1 H NMR (400 MHz, CDCl3): δ 10.22 (s, 2H), 9.36 (br, 2H), 9.34 (d, J = 4.4 Hz, 2H), 9.08 (d, J = 4.4 Hz, 2H), 8.95 (br, 2H), 8.28 (br, 4H), 7.70 (t, J = 8.3 Hz, 1H), 7.01 (d, J = 8.3 Hz, 2H), 5.34 (s, 1H), 5.16 (d, J = 9.3 Hz, 1H), 4.47 (d, J = 9.3 Hz, 1H), 3.82 (t, J = 6.3 Hz, 4H), 3.19 (s, 3H), 1.00 (m, 4H), 0.87 (m, 4H), 0.76 (m, 4H), 0.71 (t, J = 7.3 Hz, 6H), 0.58 (m, 4H), 0.42−0.26 (m, 8H), 0.22−0.10 (m, 8H). IR (pellet): 2923, 2851, 2780, 1587, 1521, 1456, 1392, 1246, 1180, 1098, 1059, 997, 851, 781, 721, 526 cm−1. HRMS (ESI): m/z calcd for C115H66N5O2Zn+ ([M + H]+), 1613.4536; found, 1613.4551. Synthesis of ZnDAP-ref. To a round-bottomed flask equipped with reflux condenser were added 5 (10.5 mg, 12.2 μmol), Zn(OAc)2·2H2O (76.7 mg, 0.349 mmol), CHCl3 (9 mL), and MeOH (1 mL). The reaction mixture was stirred at 70 °C for 24 h under an argon atmosphere. The mixture was then allowed to cool, washed with satd NaHCO3 and water, dried over Na2SO4, and the solvent was evaporated under reduced pressure. The residue was purified by column chromatography with chloroform as the eluent, and recrystallized from dichloromethane/hexane to afford ZnDAP-ref as a purple plate crystal (9.2 mg, 9.9 μmol, 81% yield). Mp 140 °C. 1 H NMR (300 MHz, CDCl3): δ 9.02 (d, J = 4.4 Hz, 2H), 8.96 (br, 4H), 8.95 (d, J = 4.4 Hz, 2H), 8.49 (d, J = 8.4 Hz, 2H), 8.31 (d, J = 8.4 Hz, 2H), 7.75 (t, J = 8.4 Hz, 1H), 7.04 (d, J = 8.4 Hz, 2H), 4.52 (t, J = 6.6 Hz, 2H), 3.90 (t, J = 6.6 Hz, 4H), 1.97 (m, 2H), 1.21 (t, J = 7.3 Hz, 3H), 1.02−0.85 (m, 8H), 0.76−0.68 (m, 4H), 0.65 (t, J = 7.0 Hz, 6H), 0.62−0.50 (m, 4H), 0.49 (m, 12H), 0.34−0.19 (m, 4H). 13C NMR (75 MHz, CDCl3): δ 159.9, 157.6, 157.0, 153.2, 149.6, 135.0, 134.7, 134.1, 133.3, 132.7, 128.1, 118.3, 105.3, 68.7, 67.0, 31.4, 28.8, 28.7,

pressure. The residue was purified by column chromatography with chloroform as the eluent to afford ZnDAP-C60 as a purple solid (42.2 mg, 26.3 μmol, 57% yield). Mp > 300 °C. 1H NMR (300 MHz, CDCl3): δ 9.28 (br, 2H), 9.26 (d, J = 4.4 Hz, 2H), 9.02 (d, J = 4.4 Hz, 2H), 8.91 (br, 2H), 8.26 (br, 4H), 7.72 (t, J = 8.4 Hz, 1H), 7.00 (d, J = 8.4 Hz, 2H), 5.33 (s, 1H), 5.16 (d, J = 9.2 Hz, 1H), 4.48 (d, J = 9.2 Hz, 1H), 3.87 (t, J = 6.3 Hz, 4H), 3.17 (s, 3H), 1.40−1.30 (br, 4H), 1.00−0.25 (m, 34H). 13 C NMR (75 MHz, CDCl3): δ 159.9, 156.7, 156.3, 153.6, 153.4, 153.0, 150.0, 147.4, 147.0, 146.5, 146.4, 146.3, 146.2, 146.0, 145.9, 145.6, 145.5, 145.4, 145.2, 144.8, 144.5, 143.3, 143.1, 142.7, 142.2, 142.1, 141.8, 141.7, 140.3, 140.1, 139.6, 137.1, 136.7, 135.8, 135.1, 134.8, 134.1, 132.8, 130.6, 127.8, 123.3, 105.2, 69.3, 68.7, 31.5, 28.9, 28.8, 28.7, 28.5, 25.2, 22.4, 13.9. IR (pellet): 2916, 2847, 1651, 1585, 1514, 1453, 1375, 1300, 1246, 1180, 1092, 1052, 979, 864, 796, 713 cm−1. HRMS (ESI): m/z calcd for C113H64N7O2Zn+ ([M + H]+), 1614.4407; found, 1614.4395. UV−vis (benzonitrile): λmax (ε) 412 (96900), 555 (7900), 586 nm (53700 M−1 cm−1). Synthesis of 7. A solution of dipyrromethane (0.581 g, 3.97 mmol), methyl 4-formylbenzoate (0.332 g, 2.02 mmol), and 1 (0.835 g, 1.99 mmol) in dichloromethane (1500 mL) was degassed with argon for 15 min. To the reactor TFA (0.2 mL) was added in the dark. After stirring for 16 h at room temperature, DDQ (2.736 g, 12.0 mmol) was added, and the solution was further stirred for 1 h. To the reactor was added triethylamine (6 mL). The crude mixture was then subjected to the alumina chromatography with dichloromethane as eluent and then purified with silica chromatography (hexane/ethyl acetate = 20/1) to afford 7 as a purple solid (202 mg, 0.242 mmol, 12% yield). Mp 125−126 °C. 1H NMR (300 MHz, CDCl3): δ 10.24 (s, 2H), 9.37 (d, J = 4.7 Hz, 2H), 9.29 (d, J = 4.7 Hz, 2H), 9.00 (d, J = 4.7 Hz, 2H), 8.97 (d, J = 4.7 Hz, 2H), 8.48 (dd, J = 6.6, 1.8 Hz, 2H), 8.35 (dd, J = 6.6, 1.8 Hz, 2H), 7.71 (t, J = 8.4 Hz, 1H), 7.02 (d, J = 8.4 Hz, 2H), 4.14 (s, 3H), 3.84 (t, J = 6.2 Hz, 4H), 1.10−0.95 (m, 4H), 0.90−0.25 (m, 34H), −3.07 (s, 2H). 13C NMR (75 MHz, CDCl3): δ 160.1, 146.7, 134.8, 131.4, 131.3, 131.0, 130.3, 130.2, 129.4, 128.1, 117.2, 112.4, 105.4, 104.8, 68.7, 52.4, 31.6, 28.9, 28.6, 28.5, 25.2, 22.5, 14.0. IR (pellet): 2919, 2849, 1717, 1582, 1453, 1272, 1246, 1193, 1143, 1095, 1021, 988, 954, 847, 714, 690, 642 cm−1. HRMS (ESI): m/z calcd for C54H65N4O4+ ([M + H]+), 833.5000; found, 833.4988. UV−vis (benzonitrile): λmax (ε) 411 (301000), 503 (15300), 537 (6000), 579 nm (4900 M−1 cm−1). Synthesis of 8. To a flask containing 7 (202 mg, 0.242 mmol) and THF (20 mL) was added LAH (10.4 mg, 0.274 mmol). The reaction was monitored every hour by TLC (silica gel, dichloromethane/ethyl acetate = 100/1, Rf = 0.16). At the same time, 10 mg of LAH was added to the mixture to complete the reaction. After TLC indicated the consumption of 7, the reaction mixture was cooled, and excess LAH was quenched by cautious addition of several lumps of ice. The resulting suspension was filtered through Celite and the residue was thoroughly washed with dichloromethane and methanol. The filtrate was concentrated and the residue was dried under reduced pressure. Then, the crude product was dissolved in dichloromethane (12 mL) and portions of activated manganese dioxide were added until TLC indicated the conversion to the corresponding aldehyde. The reaction mixture was filtered through Celite and the residue was washed thoroughly with dichloromethane and methanol. The filtrate was evaporated to dryness and the residue was chromatographed on silica gel with 1812



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28.5, 25.2, 22.4, 22.3, 13.9, 10.7. IR (pellet): 2920, 2850, 1717, 1589, 1515, 1455, 1386, 1301, 1267, 1248, 1189, 1097, 1050, 1009, 982, 940, 833, 818, 790, 757, 744, 709 cm−1. HRMS (ESI): m/z calcd for C54H65N6O4Zn+ ([M + H]+), 925.4353; found, 925.4348. UV−vis (benzonitrile): λmax (ε) 412 (89300), 553 (7400), 586 nm (50100 M−1 cm−1). Synthesis of ZnP-ref. To a round-bottomed flask were added 7 (19.8 mg, 23.8 μmol), Zn(OAc)2·2H2O (117 mg, 0.533 mmol), CHCl3 (14 mL), and MeOH (2 mL). The reaction mixture was stirred at 70 °C for 24 h under an argon atmosphere. The mixture was then allowed to cool, washed with satd NaHCO3 and water, dried over Na2SO4, and the solvent was evaporated under reduced pressure. The residue was purified by column chromatography with chloroform as the eluent, and recrystallized from dichloromethane/hexane to afford ZnP-ref as a purple needle crystal (18.1 mg, 20.2 μmol, 85% yield). Mp 139 °C. 1H NMR (300 MHz, CDCl3): δ 10.24 (s, 2H), 9.38 (d, J = 4.4 Hz, 2H), 9.37 (d, J = 4.4 Hz, 2H), 9.09 (d, J = 4.4 Hz, 2H), 9.01 (d, J = 4.4 Hz, 2H), 8.45 (d, J = 8.1 Hz, 2H), 8.29 (d, J = 8.1 Hz, 2H), 7.72 (t, J = 8.4 Hz, 1H), 7.02 (d, J = 8.4 Hz, 2H), 4.12 (s, 3H), 3.84 (t, J = 6.2 Hz, 4H), 1.04−0.93 (m, 4H), 0.93−0.82 (m, 4H), 0.80−0.70 (m, 4H), 0.71 (t, J = 7.3 Hz, 6H), 0.62−0.52 (m, 4H), 0.42−0.26 (m, 8H), 0.24−0.10 (m, 8H). 13C NMR (75 MHz, CDCl3): δ 167.5, 160.1, 151.3, 149.6, 149.1, 148.8, 143.2, 134.6, 132.4, 131.9, 131.5, 129.9, 129.2, 127.8, 113.1, 105.8, 105.5, 100.6, 68.7, 52.4, 31.5, 28.8, 28.5, 28.4, 25.1, 22.5, 14.0. IR (pellet): 2919, 2850, 1720, 1584, 1520, 1453, 1392, 1267, 1246, 1172, 1145, 1056, 993, 904, 852, 838, 779, 753, 728, 712, 700, 647 cm−1. HRMS (ESI): m/z calcd for C54H63N4O4Zn+ ([M + H]+), 895.4135; found, 895.4107. UV−vis (benzonitrile): λmax (ε) 420 (277000), 548 (12400), 586 nm (2900 M−1 cm−1).

Figure 2. (a) Absorption spectra of ZnP-C60 (solid line), ZnP-ref (dashed line), and C60-ref (dotted line) in benzonitrile. (b) Absorption spectra of ZnDAP-C60 (solid line), ZnDAP-ref (dashed line), and C60-ref (dotted line) in benzonitrile. The magnified absorptions of Q-bands are shown as an inset.

than that of ZnP (Figure S1). The steady-state fluorescence spectra of ZnP-C60 and ZnDAP-C60 in benzonitrile reveal efficient quenching of the fluorescence in comparison with those of ZnP-ref and ZnDAP-ref (Figure 3). This suggests the occurrence of rapid quenching of the ZnP excited singlet state (1ZnP*) and ZnDAP excited singlet state (1ZnDAP*) by the attached C60. Similar results were obtained in toluene. From the intercept of the absorption and the fluorescence spectra, the zeroth−zeroth energies (E0−0) in benzonitrile were

■

RESULTS AND DISCUSSION Synthesis. The synthetic route of ZnDAP-C60 is illustrated in Scheme 1. Dibromodipyrrin 3 was prepared by the condensation of benzaldehyde 1 and pyrrole, followed by bromination of 2, whereas 4 was synthesized by bromination of the corresponding dipyrromethane. Next important step is asymmetric cyclization from different dibromodipyrrins 3 and 4 using metal-template method.30,31 Namely, treatment of 3 and 4 with NaN3 in the presence of Pb(acac)2 in refluxing propanol and subsequent demetalation afforded a mixture of cyclization products from which asymmetrical 5 was separated by column chromatography. Note that asymmetrical propyl ester 5 was produced by an ester exchange reaction between the methoxycarbonyl group and propanol. After conversion of propyl ester 5 to aldehyde 6, 1,3-dipolar addition of 6 to C60 followed by treatment with Zn(OAc)2 yielded ZnDAP-C60.39 The corresponding dyad ZnP-C60 (Scheme 2), as well as ZnDAP-ref and ZnP-ref, was prepared according to similar procedures. C60-ref was also synthesized following the previously reported method.37 Their structures were verified by spectroscopic analyses including 1H NMR, 13C NMR, highresolution mass, and IR spectra. Optical and Electrochemical Properties. The UV− visible absorption spectra of ZnP-C60 and ZnDAP-C60 in benzonitrile are the superposition of those of each chromophore, implying that there is no significant electronic interaction in their ground states (Figure 2). It is noteworthy that the absorption of ZnDAP-ref is broadened and Q-bands are intensified relative to ZnP-ref, ensuring better matching of ZnDAP with solar energy distribution spectrum on the earth

Figure 3. (a) Fluorescence spectra of ZnP-C60 (solid line) and ZnPref (dashed line) in benzonitrile. (b) Fluorescence spectra of ZnDAPC60 (solid line) and ZnDAP-ref (dashed line) in benzonitrile. The samples were excited at the peaks of the Soret bands where the absorbances were adjusted to be identical for comparison. 1813



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excited singlet state to the acceptor or from the donor to the acceptor excited singlet state were determined by applying eq 2, where e stands for the elementary charge. The Coulombic terms in the dyads are neglected in the evaluation of the driving forces in Table 1 because of the high polarity of benzonitrile.

determined to be 2.12 eV for ZnP-C60 and 2.14 eV for ZnDAPC60. The driving forces (−ΔG0ET) for all the intramolecular ET processes were determined by measuring the redox potentials of ZnP-C60 and ZnDAP-C60 in benzonitrile (Figure 4 and

−ΔG 0CR = e[E 0 ox (D+• /D) − E 0 red(A/A−•)]

(1)

−ΔG 0CS = ΔE0 − 0 + ΔG 0CR

(2) 1

The driving force for charge separation (CS) from ZnDAP* to C60 (−ΔG0CS1 = 0.50 eV) is smaller than the value from 1 ZnP* to C60 (−ΔG0CS1 = 0.77 eV), whereas the driving force for CR in ZnDAP-C60 (−ΔG0CR = 1.64 eV) is larger than the value in ZnP-C60 (−ΔG0CR = 1.35 eV; Table 1). Photophysical Properties of ZnP-C60. Time-resolved transient absorption spectroscopy and fluorescence lifetime measurements were performed for the dyads by exciting at 405 nm. Figure 5a shows the transient absorption component

Figure 4. (a) Cyclic voltammograms (top) and differential pulse voltammograms (bottom) of (a) ZnP-C60 and (b) ZnDAP-C60 obtained in benzonitrile containing 0.1 M nBu4NPF6 as an electrolyte; Ag/Ag+ [AgNO3 (benzonitrile)] as a reference electrode; scan rate 60 mV s−1.

Table 1. Optical and Electrochemical Data and Driving Forces of the Dyads in Benzonitrilea

Figure 5. Transient absorption component spectra of (a) ZnP-C60 and (b) ZnDAP-C60 in benzonitrile obtained from global exponential fit of the data. The fitted time constants are shown on the figures. ΔOD is a change in optical density.

potential

compound ZnP-C60 ZnDAPC60

E0−0 (eV)

E0ox (V vs Fc/ Fc+)

E0red (V vs Fc/ Fc+)

−ΔGCRb (eV)

−ΔGCS1c (eV)

−ΔGCS2d (eV)

2.12 2.14

0.30 0.59

−1.05 −1.05

1.35 1.64

0.77 0.50

0.40 0.11

spectra of ZnP-C60 in benzonitrile obtained from global threeexponential fit of the data. The first state has a lifetime (τ) of 1 ps, which matches well that of the porphyrin fluorescence (1.4 ps) at 640 nm (Figure 6a and Table 2). Thus, the first component can be assigned to the decay of 1ZnP*. The final state with τ = 65 ps exhibits characteristic absorption of ZnP radical cation (ZnP+•) at 600−700 nm39 and C60 radical anion (C60−•) at 1000 nm,39 corroborating the formation of the charge-separated state. The 8 ps component with negative absorption is almost a mirror image of the final component, implying the efficient conversion of the intermediate state to the charge-separated state. We have already reported that an analogous ZnP-S1-C60 with the same spacer reveals the initial formation of an exciplex arising from 1ZnP* (τ = 1 ps), followed by the formation (τ = 12 ps) and decay (τ = 130 ps) of the completely charge-separated state.40,41 Considering the similarity in the lifetime and characteristic absorption feature, the middle component can be attributed to the exciplex state (ZnP-C60)*. The exciplex state is generated quantitatively from 1 ZnP* given the long-lived 1ZnP* state in ZnP-ref (τ = 2700 ps). On the other hand, minor, but efficient, CS also takes place

a

The redox potentials were determined by differential pulse voltammetry in benzonitrile containing 0.1 M n-Bu4NPF6 as an electrolyte with scan rate 60 mV s−1. bCoulombic terms were neglected because of high polarity of benzonitrile. cDriving forces for CS from 1ZnDAP* or 1ZnP* to C60. dDriving forces for CS from ZnDAP or ZnP to C60 excited singlet state (1C60*). The energy level of 1C60* is reported to be 1.75 eV.39

Table 1). As predicted, the first one-electron oxidation potential of ZnDAP (0.59 V vs Fc/Fc+) in ZnDAP-C60 is shifted to a positive direction by 0.29 V relative to that of ZnP (0.30 V vs Fc/Fc+) in ZnP-C60. On the other hand, the first one-electron reduction potentials (−1.05 V vs Fc/Fc+) of C60 in ZnP-C60 and ZnDAP-C60 are identical. The driving forces (−ΔG0CR) for the intramolecular CR processes from the acceptor radical anion (A−•) to the donor radical cation (D+•) were determined by applying eq 1, whereas the driving forces (−ΔG0CS) for the intramolecular CS processes from the donor 1814



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Scheme 3. Photophysical Processes and Their Rate Constants of ZnP-C60 in Benzonitrilea

k1 = 1/(1.4 ps) − 1/(2700 ps) = 7.1 × 1011 s−1, k2 = 1/(8 ps) = 1.3 × 1011 s−1, k3 = 1/(65 ps) = 1.5 × 1010 s−1, k1′ = 1/(61 ps) − 1/(1300 ps) = 1.6 × 1010 s−1, k2′ = 1/(1300 ps) = 7.7 × 108 s−1.37 a

Figure 6. Fluorescence decay of (a) ZnP-C60 and (b) ZnDAP-C60 in benzonitrile measured with the up-conversion method (white squares). The solid line shows a fit of the data and the fitted lifetimes and amplitude ratios are shown on the figures. The samples were excited at 405 nm and the fluorescence intensities were monitored at 640 nm for ZnP-C60 and 590 nm for ZnDAP-C60.

from the C60 excited singlet state (1C60*) based on the shortlived 1C60* (τ = 61 ps at 710 nm, Table 2) in ZnP-C60 relative to C60-ref (1300 ps).42 The overall CS efficiency at 405 nm was estimated to be 99% considering the absorption ratio of ZnP/ C60 = 88:12. The relaxation pathway of ZnP-C60 in benzonitrile is illustrated in Scheme 3. Figure 7a displays the transient absorption component spectra of ZnP-C60 in toluene obtained from global threeexponential fit (1.1 ps, 41 ps, >1 ns) of the data. There was no evidence of the ET in toluene, since no formation of ZnP+• was observed at 600−700 nm. The up-conversion emission measurements show that the porphyrin fluorescence has a lifetime of 1.2 ps, which matches well with the first component obtained by the transient absorption spectroscopy (Figure 8a and Table 3). Therefore, the first component with τ = 1.1 ps in the transient absorption can be identified as the relaxation of 1 ZnP*. The third long-lived component in the transient absorption has a broad structureless spectrum typical for 1 C60*. The formation of the 1C60* state is also confirmed by the emission spectroscopy studies. The steady-state fluorescence spectrum of ZnP-C60 in toluene reveals the fluorescence from the ZnP and C60 moieties. The time-resolved emission

Figure 7. Transient absorption component spectra of (a) ZnP-C60 and (b) ZnDAP-C60 in toluene obtained from global exponential fit of the data. The fitted time constants are shown on the figures.

spectroscopy yields a long-lived component (1.9 ns) at 710 nm (Table 3), which largely agrees with the fluorescence lifetime of C60-ref (1.3 ns) at 710 nm.39,42 Moreover, there is a short-lived component (24 ps) at 640 nm, which can correspond to the middle component (41 ps) obtained by the transient absorption spectroscopy. Taking into account the similarity between ZnP-C60 and the analogous ZnP−S1-C60 dyad with the same phenylene spacer,40,41 the intermediate

Table 2. Rate Constants of the Dyads and Their References in Benzonitrilea fluorescence lifetime up-conversionb

transient absorption compound

τ1 (ps)

τ2 (ps)

τ3 (ps)

ZnP-C60 ZnP-ref ZnDAP-C60 ZnDAP-ref

1

8

65

2

9

120

τ4 (ps)

τ5 (ps)

τ1 (ps)

τ2 (ps)

1.4 280

>1000

2.2 (41%)

15 (59%)

SPCc τ (ps) 61d 2700e 100d 540e

a

Excitation wavelength of 405 nm. bMonitored at 640 nm for ZnP-C60 and 590 nm for ZnDAP-C60. The value in parentheses represents the amplitude ratio. cMeasured by single photon counting (SPC) method. dMonitored at 710 nm where fluorescence from the C60 moiety exhibits the strongest intensity. eMonitored at 640 nm for ZnP-ref and 590 nm for ZnDAP-ref. 1815



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Scheme 4. Photophysical Processes and Their Rate Constants of ZnP-C60 in Toluenea

k1 = 1/(1.2 ps) − 1/(1500 ps) = 8.3 × 1011 s−1, k2 = 1/(41 ps) = 2.4 × 1010 s−1, k2′ = 1/(1300 ps) = 7.7 × 108 s−1.37

a

Figure 8. Fluorescence decay of (a) ZnP-C60 and (b) ZnDAP-C60 in toluene measured with up-conversion method (white squares). The solid line shows a fit of the data and the fitted lifetimes and amplitude ratios are shown on the figures. The samples were excited at 405 nm and the fluorescence intensities were monitored at 640 nm for ZnPC60 and 590 nm for ZnDAP-C60.

Figure 9. Spectral change in the addition of antimony(V) chloride (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, and 1.5 equiv) in deaerated dichloromethane solution containing ZnDAP-ref (1.0 × 10−5 M). Absorptions at 318, 440, and 606 nm increase, while those at 396, 546, and 583 nm decrease as SbCl5 is added to the solution. The spectral change can be ascribed to the decay of neutral ZnDAP and the formation of ZnDAP+•.

component can be assigned to the exciplex state (ZnP-C60)* (Scheme 4). Photophysical Properties of ZnDAP-C60. In contrast, ZnDAP-C60 in benzonitrile has more complex behavior and five exponentials were used to fit the data (Figure 5b). At first, the fourth component with τ = 280 ps can be identified as the charge-separated state with characteristic absorption of ZnDAP radical cation (ZnDAP+•) at 550−650 nm and C60−• at 1000 nm.39 The assignment of ZnDAP+• was performed by comparsion with the absorption spectra produced by the chemical oxidation of ZnDAP-ref with antimony(V) chloride (Figure 9). The spectral change in Figure 9 can be ascribed to the decay of neutral ZnDAP and the formation of ZnDAP+•. Indeed, the transient absorption component spectrum with the time constant of 280 ps reveals bleaching at 580 nm and weak positive absorption at 600−650 nm, which matches well with

the spectral change in Figure 9. The fifth component with lifetime longer than 1 ns is minor and may arise from fit inaccuracy or from minor formation of the excited triplet states of ZnDAP (3ZnDAP*) and C60 (3C60*) with cumulative yield less than 5%. The latter may not have any significant effect on the photochemical and photoelectrochemcial properties of the dyad. On the other hand, the third component with τ = 120 ps has feature typical for the formation of the charge-separated state from the 1C60*. Indeed, the τ value of fluorescence from the 1C60* moiety in ZnDAP-C60 at 710 nm is 100 ps, which is consistent with the τ value of the third component. It is noteworthy that τ = 2 ps for the first component and τ = 9 ps for the second component agree with the respective τ values

Table 3. Rate Constants of the Dyads and Their References in Toluenea fluorescence lifetime up-conversionb

transient absorption compound

τ1 (ps)

τ2 (ps)

τ3 (ps)

ZnP-C60 ZnP-ref ZnDAP-C60 ZnDAP-ref

1.1 >1000 3 440

41

>1000

15

>1000

τ4 (ps)

SPCc

τ1 (ps)

τ2 (ps)

τ (ps)

1.2 (91%) 1500 2.4 (45%) 500

24 (9%)

1900d

13 (55%)

1500d

a Excitation wavelength of 405 nm. bMonitored at 635 nm for ZnP-ref, 640 nm for ZnP-C60, and 590 nm for ZnDAP-ref and ZnDAP-C60. The value in parentheses represents the amplitude ratio. cMeasured by single photon counting (SPC) method. dMonitored at 710 nm where fluorescence from the C60 moiety exhibits the strongest intensity.

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(2.2, 15 ps) of fluorescence from the 1ZnDAP* moiety in ZnDAP-C60 at 590 nm (Figure 6b). Taking into account the strong Q-band bleaching with two different minima at 588 and 598 nm, the two components may be interpreted as two different conformers.43 Accordingly, we can conclude that energy transfer (EN) occurs from each conformer in 1ZnDAP* to generate 1C60* in ZnDAP-C60. Meanwhile, the shape of the transient absorption spectrum at 30 ps shows significant formation of the charge-separated state, ruling out the exclusive EN to C60 and subsequent CS (Figure S2). Therefore, some part of 1ZnDAP* undergo direct CS to yield the charge-separated state. An estimation based on degree of the Q-band bleaching at 0 and 30 ps leads to 25% contribution from ET and 75% from EN (Scheme 5). This is in

linked systems, photoinduced CS is accelerated in polar solvents relative to nonpolar solvents, whereas EN is not affected by solvent polarity.42 Thus, in benzonitrile photoinduced ET from 1ZnDAP* to C60 can compete with EN to C60, triggering the dual deactivation pathways and resulting in accelerated quenching of 1ZnDAP* in benzonitrile relative to toluene. The overall CS efficiency at 405 nm was estimated to be 94% considering the absorption ratio of ZnDAP/C60 = 97:3. It should be emphasized here that the lifetime of chargeseparated state in ZnDAP-C60 is 4× longer than in ZnP-C60. This can be explained largely by the large driving force for CR (−ΔG0CR = 1.64 eV) in ZnDAP-C60 in comparison with that (−ΔG0CR = 1.35 eV) in ZnP-C60. It is noteworthy that the calculated solvent reorganization energies (λs) of ZnDAP-C60 (0.648 eV) and ZnP-C60 (0.656 eV) are comparable because of the same size of the ZnDAP and ZnP and the same separation distances between the donor and acceptor,44,45 whereas the calculated vibrational reorganization energy (λv) of ZnDAP-C60 (0.395 eV) is slightly smaller than that of ZnP-C60 (0.430 eV). −ΔGCR values of ZnP-C60 (1.35 eV) and ZnDAP-C60 (1.64 eV) are larger than the total reorganization energies (λ = λs + λv) of ZnP-C60 (1.09 eV) and ZnDAP-C60 (1.04 eV), respectively, corroborating that the CR processes are located in the Marcus inverted region. Therefore, small total reorganization energy and electronic coupling (vide infra) of ZnDAP-C60 relative to ZnP-C60 may also contribute to the slow CR in ZnDAP-C60 (Tables S1 and S2). We also performed the DFT calculations of ZnP-C60 and ZnDAP-C60 to obtain the optimized molecular structures as well as determine the energy levels and electron distribution of the HOMO and LUMO. The optimized molecular structures are almost identical with the same center-to-center distance (12.5 Å) between the donor and acceptor (Figure S3 and Table S2). However, the 1ZnP* state of ZnP-C60 (2.12 eV) is comparable to that of ZnDAP-C60 (2.14 eV), whereas the charge-separate state of ZnP-C60 (1.35 eV) is lower than that of ZnDAP-C60 (1.64 eV). Moreover, the significant electron distribution on the phenylene spacer is visible in the HOMO of ZnP-C60, but that of ZnDAP-C60 does not possess any electron distribution on the phenylene spacer. Therefore, the differences in photophysics of the dyads may be associated with the energy differences between the excited state and charge-separated state and the different electronic coupling between the ZnP or ZnDAP with different electronic wave function and the C60 through the phenylene spacer (vide supra). At present we have no reasonable explanation for the selective decay of the chargeseparated state of ZnDAP-C60 to the ground state rather than to the triplet states of ZnDAP and C60. The charge-separated state may possess exciplex character in some extent, which accelerates the decay to the ground state (vide infra). Further detailed study would be necessary to elucidate the unique photodynamics. Photoelectrochemical Properties. To shed light on the correlation between the ET and photoelectrochemical properties of ZnP-C60 and ZnDAP-C60, their aggregates were formed in a mixture of toluene (good solvent) and acetonitrile (poor solvent), and then deposited onto nanostructured SnO2 electrodes under high electric field (200 V, 120 s), as previously reported.46−48 Upon subjecting the resultant aggregate suspension in the mixed solvent to the high electric (DC) field, the aggregates were deposited onto a nanostructured SnO2 electrode to give a modified electrode (denoted as FTO/ SnO2/(ZnP-C60)m or FTO/SnO2/(ZnDAP-C60)m. The ab-

Scheme 5. Photophysical Processes and Their Rate Constants of ZnDAP-C60 in Benzonitrilea

k1 = {[1/(2.2 ps) − 1/(540 ps)] + [1/(15 ps) − 1/(540 ps)]} × 0.75 = 3.9 × 1011 s−1, k2 = {[1/(2.2 ps) − 1/(540 ps)] + [1/(15 ps) − 1/ (540 ps)]} × 0.25 = 1.3 × 1011 s−1, k3 = 1/(280 ps) = 3.6 × 109 s−1, k1′ = 1/(100 ps) − 1/(1300 ps) = 9.2 × 109 s−1, k2′ = 1/(1300 ps) = 7.7 × 108 s−1.37 a

good agreement with the 1.5 times accelerated biexponential decay of the excited singlet state (τ = 2, 9 ps) in polar benzonitrile compared with biexponential (τ = 3, 15 ps) in nonpolar toluene. Namely, in toluene the fast and biexponential recovery of the ZnDAP ground state absorption corresponds to exclusive EN occurring from the corresponding two conformers in 1ZnDAP* to C60 (Figures 7b and 8b and Scheme 6), and the final long-lived component (>1 ns) is attributed to the resulting 1 C60* state that shows broad absorption at 900 nm.39 In D−A Scheme 6. Photophysical Processes and Their Rate Constants of ZnDAP-C60 in Toluenea

k1 = [1/(2.4 ps) − 1/(500 ps)] + [1/(13 ps) − 1/(500 ps)] = 3.4 × 1011 s−1, k2′ = 1/(1300 ps) = 7.7 × 108 s−1.37

a

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that (1.3% at 410 nm) of the FTO/SnO2/(ZnP-C60)m device. More importantly, the integrated IPCE value of the FTO/ SnO2/(ZnDAP-C60)m device is about 2× larger than that of the FTO/SnO2/(ZnP-C60)m device. The enhanced photocurrent generation in the visible region matches well with the longer lifetime of the charge-separated state in ZnDAP-C60 than ZnPC60 as well as the better light-harvesting properties of ZnDAPC60. We have already reported the results of time-resolved nanosecond transient absorption measurements for analogous ZnP-S1-C60 dyad that has the same phenylene spacer of ZnPC60 and ZnDAP-C60 dyads.49 The charge-separated state was observed, but the stronger interaction between the porphyrin and C60 resulted in the small quantum yield of CS (18%) that matched the low IPCE value (0.026%). ZnDAP-C60 as well as ZnP-C60 with the same spacer may also show similar photodynamic behavior, leading to the low IPCE values.

sorption feature of the FTO/SnO2/(ZnP-C60)m electrode is largely similar to that of the FTO/SnO2/(ZnDAP-C60)m electrode (Figure S4). The broad absorption of these films and high absorbance in the visible region make these films suitable for harvesting solar energy. AFM observation visualizes similar film morphology on the FTO/SnO2/(ZnP-C60)m and the FTO/SnO2/(ZnDAP-C60)m electrodes (Figure S5). The thicknesses of the deposited films were also found to be comparable for the FTO/SnO2/(ZnP-C60)m and the FTO/ SnO2/(ZnDAP-C60)m electrodes. Finally, photoelectrochemical measurements were conducted in deaerated acetonitrile containing 0.5 M LiI and 0.01 M I2 with the modified SnO2 electrode as a working electrode under standard three-electrode conditions. The current vs potential curves of the FTO/SnO2/(ZnP-C60)m and the FTO/SnO2/ (ZnDAP-C60)m devices under the white light illumination (λ > 380 nm) were measured (Figure S6). With increasing the positive bias up to 0.23 V versus SCE, the photocurrent increased compared with the dark current for the both devices. Increased CS and the facile transport of charge carriers under positive bias are responsible for the enhanced photocurrent generation, as seen in photoelectrochemical devices with analogous porphyrin-fullerene dyads.46 To gain further insights into the photoelectrochemical properties of the deposited films, we evaluated the wavelength-dependent incident photon-tocurrent efficiency (IPCE) spectra under the same applied potential of 0.23 V versus SCE where the material intrinsic properties can be extracted maximally (Figure 10). The

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CONCLUSION In conclusion, we have synthesized ZnDAP-C60 dyad for the first time. In addition to the better matching of ZnDAP with solar energy distribution in the visible region than ZnP, the lifetime of the charge-separated state in ZnDAP-C60 is 4× longer than that in ZnP-C60 due to the high oxidation potential of ZnDAP relative to ZnP. This difference also parallels the enhanced photocurrent generation of the ZnDAP-C60-based device in the visible region. These results unambiguously exemplify the potential utility of DAPs as an excellent donor in artificial photosynthesis and solar energy conversion.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Experimental details, tables, and additional figures. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author

*E-mail: [email protected]; nikolai.tkachenko@tut.fi. Notes

The authors declare no competing financial interest.

■ ■

Figure 10. Photocurrent action spectra of (a) the FTO/SnO2/ (ZnDAP-C60)m device (solid line) and (b) the FTO/SnO2/(ZnPC60) m device (dashed line).

ACKNOWLEDGMENTS This work was supported by Grant-in-Aid (Nos. 25220801 and 25620148 to H.I.) and WPI Initiative, MEXT, Japan.

photocurrent action spectra largely resemble the absorption spectra of the deposited electrode. Taking into account the well-established photocurrent generation mechanism of similar photoelectrochemical devices,46,47 these results lead to the conclusion that the donor excited singlet state is quenched by C60 to generate the charge-separated state, followed by an electron injection from the reduced C60 into the conduction band of SnO2 through electron hopping on the C60 molecules and ET from I− to the resultant donor radical cation, thus generating the photocurrent. The maximum IPCE values of the FTO/SnO2/(ZnDAPC60)m (2.2% at 400 nm) and the FTO/SnO2/(ZnP-C60)m (1.3% at 410 nm) devices are moderate. This may result from the rather short lifetime of the charge-separated states as well as undesirable packing of the dyad molecules in the films due to the bulky substituents at the ortho-positions of the meso-phenyl rings. Nevertheless, the maximum IPCE value (2.2% at 400 nm) of the FTO/SnO2/(ZnDAP-C60)m device is higher than

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