A novel trigeminal zinc porphyrin and corresponding porphyrin monomers for dye-sensitized solar cells

Abstract

A novel trigeminal zinc porphyrin sensitizer (T) and two zinc porphyrin monomers (M1 and M2) were successfully designed and synthesized. The spectral, electrochemical, and photovoltaic properties of the porphyrin dyes were investigated. Compared with M1, the molecule of M2 has an additional aliphatic n-hexyloxyl chain at the meso-position of the porphyrin framework, and such a structure is favorable for the formation of a compact hydrophobic layer at the TiO₂ surface and the retardation of the diffusion of I₃⁻ ions into the nanoporous TiO₂ electrode, resulting in more effective suppression of the charge recombination process and a higher V_oc. Meanwhile, M2 has larger IPCE values than those of M1, leading to the higher J_sc value. Thus, the DSSC devices based on M2 demonstrated a relatively high power conversion efficiency of 5.77%, with the J_sc, V_oc and ff values of 13.93 mA cm⁻², 732 mV, and 0.566, respectively. Even though dye T has the highest molar absorption coefficients and multiple binding moieties, the corresponding power conversion efficiency is 2.30%, which is lower than those for M1 and M2. These observations may be ascribed to the low efficiency of the electron injection process caused by the isolation of the LUMOs from the anchoring carboxyl groups in addition to the lowest adsorption amount.

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Introduction

Increased energy demands and concerns have encouraged scientists to develop cheap and renewable energy sources in recent years. Solar cells have been developed as one of the most efficient ways to utilize solar energy as a clean and renewable energy source. In comparison with the traditional silicon-based devices, dye-sensitized solar cells (DSSCs) are promising, due to their versatile, energy-saving, low cost, and environmentally friendly nature. Thus, DSSCs have attracted much attention since the breakthrough made by Grätzel et al. in 1991.^1–4

Inspired by the involvement of the porphyrin framework in the photosynthesis of plants in the nature, porphyrin-based chromophores have been widely used in photovoltaic devices, showing the advantages of high molar absorption coefficients, good photostability, and multiple reactive sites for easy structural modification.^5–17 Especially, in 2011, Grätzel, Eric Diau, and Yeh et al. reported the high power conversion efficiency of 12.3% by the utilization of a porphyrin dye YD2-o-C8, which contains a typical donor–π bridge–acceptor (D–π–A) framework, with diarylamino and ethynylbenzoic acid moieties as the donor and the acceptor, respectively.¹⁴ In spite of these excellent examples, the relationship between the cell performance and the porphyrin structures still remains to be further explored.

In an attempt to investigate the cell performance–structure correlations, we have prepared a series of porphyrins with triphenylamine (TPA) as the electron donor and ethynylbenzoic acid as the electron acceptor, and the cell performance can be optimized through modulating the molecular orbital energy levels by the variation of the donors.¹⁸ Whereas, the large π frameworks are rather planar, associated with poor solubility and serious dye aggregation on the TiO₂ film. To overcome these problems,¹⁹ herein we introduced one or two p-(n-hexyloxy)phenyl groups to prepare two porphyrins M1 and M2 (Scheme 1) and investigated the influence of the long chains on the DSSC behavior.

Scheme 1 Molecular structures of the target porphyrins.

On the other hand, it has been demonstrated that the employment of branched structures may increase the dye adsorption amount on TiO₂ due to the presence of multiple binding moieties and thus elevate the DSSC efficiencies.²⁰ In this respect, a few diporphyrin dyes have been reported,²¹ but none of them has the branched structures. Thus, we designed the porphyrin dimer (D) and trimer (T) (Scheme 1) to investigate the influence of the branched structures on the DSSC efficiencies.

Experimental section

Materials and reagents

All reagents and solvents were obtained from commercial sources and used without further purification unless otherwise noted. Compounds AA and BB were prepared by reported procedures.²² The transparent FTO conducting glass (fluorine-doped SnO₂, transmission >90% in the visible range, sheet resistance 15 Ω per square) and the TiO₂ paste was purchased from Geao Science and Educational Co. Ltd. The FTO conducting glass was washed with a detergent solution, deionized water, acetone, and ethanol successively under ultrasonication for 20 min before use, respectively.

Equipments and apparatus

¹H NMR spectra and ¹³C NMR spectra were measured on a Bruker AM 400 spectrometer. HRMS were performed using a Waters LCT Premier XE spectrometer. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was measured using AB Sciex 4800 Plus MALDI TOF/TOF Analyzer with dithranol as the matrix. UV-Vis absorption spectra were recorded on a Varian Cary 100 spectrophotometer and fluorescence spectra were recorded on a Varian Cray Eclipse fluorescence spectrophotometer. The cyclic voltammograms of the dyes were obtained in CH₂Cl₂ with a Versastat II electrochemical workstation (Princeton Applied Research) using 0.1 M TBAPF₆ (Aldrich) as the supporting electrolyte, the sensitizer attached to a nanocrystalline TiO₂ film deposited on the conducting FTO glass as the working electrode, a platinum wire as the counter electrode, and a regular calomel electrode in saturated KCl solution as the reference electrode. The scan rate was 100 mV s⁻¹.

Photovoltaic measurements employed an AM 1.5 solar simulator equipped with a 300 W xenon lamp (model no. 91160, Oriel). The power of the simulated light was calibrated to 100 mW cm⁻² using a Newport Oriel PV reference cell system (model 91150 V). I–V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a model 2400 source meter (Keithley Instruments, Inc. USA). The voltage step and delay time of the photocurrent were 10 mV and 40 ms, respectively. Action spectra of the incident monochromatic photon-to-electron conversion efficiency (IPCE) for the solar cells were obtained with a Newport-74125 system (Newport Instruments). The intensity of monochromatic light was measured with a Si detector (Newport-71640). The electrochemical impedance spectroscopy (EIS) measurements of all the DSSCs were performed using a Zahner IM6e Impedance Analyzer (ZAHNER-Elektrik GmbH & CoKG, Kronach, Germany). The frequency range is 0.1 Hz to 100 kHz. The magnitude of the alternative signal is 10 mV.

Fabrication of the solar cells

The procedures for preparation of TiO₂ electrodes and fabrication of the sealed cells for photovoltaic measurements were adapted from that reported by Grätzel and co-workers.²³ A screen-printed double layer of TiO₂ particles was used as the photoelectrode. The detailed procedure was reported in our previous work.¹⁸ Then the photoelectrode was dipped into a 0.2 mM porphyrin/CHCl₃–CH₃OH (v/v, 2/1) for 12 h at room temperature. The counter electrode was also prepared according to our previously reported procedure.¹⁸ Finally, the DSSCs were assembled, with the electrolyte solution containing 0.1 M LiI, 0.05 M I₂, 0.6 M 1-butyl-3-methyl imidazolium iodide (BMII), and 0.5 M 4-tert-butylpyridine (TBP) in a mixture of acetonitrile and valeronitrile (volume ratio of 1 : 1).

Computational methods

We employed density functional theory (DFT) calculations to study these sensitizers. The PBE functional²⁴ with dispersion correction²⁵ was adopted to optimize molecular geometries of the compounds in their ground state (S₀), and the hybrid B3LYP functional²⁶ was employed to calculate the frontier molecular orbitals at the optimized geometries. The Ahlrichs split valence SVP basis set²⁷ was used together with corresponding auxiliary basis sets,²⁸ and the “resolution of identity”²⁹ was used to speed up the DFT calculations, as implemented in the ORCA program package.³⁰ Subsequent time-dependent DFT (TDDFT) calculations were performed to gain information of the five lowest singlet excited states of the sensitizer dyes, using the Coulomb-attenuated CAM-B3LYP³¹ functional and the SVP basis set implemented in Gaussian 09 program package.³²

Adsorption amount measurements

Three photoelectrodes (area S: 2.5 cm × 1.5 cm) with the adsorbed dye were dipped into 0.1 M NaOH/THF–H₂O (v/v, 1/1, 15 mL) for 24 h at room temperature. Then, the mixed solution was dissolved in CHCl₃ (50 mL) and washed with phosphoric acid (0.05 M, 15 mL) and water (50 mL × 3), respectively, dried over anhydrous sodium sulfate, and vacuum evaporated. Then the desorbed dye was dissolved in the mixture of CHCl₃–CH₃OH (v/v, 2/1, 10 mL), and the absorbance (A) was recorded at the maximum absorption wavelength. The concentration of the dye can be obtained from the equation of A/(εL). Thus, the adsorption amount can be calculated as A × V/(3SεL), where the symbols of ε and L represent the molar extinction coefficient of the dye at the maximum absorption wavelength, and the length of the light path, respectively.

Synthesis of the dyes

Synthesis of compound PM. To a solution of 4-(bis(4-tert-butylphenyl)amino)benzaldehyde (1.7 g, 4.4 mmol), methyl 4-formylbenzoate (0.72 g, 4.4 mmol), dipyrromethane (1.29 g, 8.8 mmol) in CH₂Cl₂ (1.9 L) was added trifluoroacetic acid (0.55 mL). The solution was stirred at room temperature for 3 h. Then, DDQ (2,3-dichloro-5,6-dicyanobenzoquinone) (6 g, 26.45 mmol) was added and stirred at room temperature for 15 min. After the reaction was quenched by the addition of triethylamine (30 mL), the solvent was removed in vacuo. The residue was purified by column chromatography and recrystallization from CH₂Cl₂–CH₃OH gave PM (1.4 g, 39%). ¹H NMR (CDCl₃, Bruker 400 MHz), δ: −3.08 (s, 2H, inner NH), 1.39 (s, 18H, –CH₃), 4.12 (s, 3H, –COOCH₃), 7.36 (d, J = 8.8 Hz, 4H, –Ph), 7.44 (d, J = 8.8 Hz, 4H, –Ph), 7.48 (d, J = 8.4 Hz, 2H, –Ph), 8.08 (d, J = 8.4 Hz, 2H, –Ph), 8.33 (d, J = 8.0 Hz, 2H, –Ph), 8.47 (d, J = 8.4 Hz, 2H, –Ph), 8.98 (d, J = 4.8 Hz, 2H, pyrr), 9.23 (d, J = 4.8 Hz, 2H, pyrr), 9.35 (d, J = 4.8 Hz, 2H, pyrr), 9.38 (d, J = 4.4 Hz, 2H, pyrr), 10.28 (s, 2H, meso-H). ¹³C NMR (CDCl₃, Bruker 100 MHz, 298 K): 31.55, 34.46, 52.49, 105.46, 117.20, 120.03, 120.80, 121.61, 124.65, 125.01, 125.69, 126.36, 126.62, 128.20, 129.47, 130.47, 131.42, 131.59, 131.97, 132.42, 134.00, 134.90, 135.82, 135.92, 145.11, 145.25, 145.26, 146.35, 146.65, 147.56, 148.03, 167.45. HRMS (ESI, m/z): [M + H]⁺ calcd for C₅₄H₅₀N₅O₂, 800.3965; found, 800.3969. IR (KBr pellet, cm⁻¹): 3425 (br), 3039 (w), 2958 (m), 2902 (w), 2857 (w), 2519 (m), 2137 (w), 1797 (m), 1724 (s), 1600 (s), 1507 (s), 1435 (w), 1363 (w), 1320 (m), 1274 (s), 1110 (s), 956 (s), 843 (m), 781 (m), 736 (w), 691 (w), 548 (w).

Synthesis of compound BrPM and Br2PM. To a solution of PM (1.4 g, 1.75 mmol) in CH₂Cl₂ (1.5 L) was added NBS (340 mg, 1.91 mmol) in batches at room temperature. After 1 h, the reaction was quenched with acetone (10 mL). The solvent was removed under reduced pressure. The product was isolated by silica gel columns and recrystallized from CH₂Cl₂–CH₃OH gave BrPM (532 mg, 34.6%) and Br2PM (398 mg, 23.8%).

BrPM: ¹H NMR (CDCl₃, Bruker 400 MHz), δ: −2.97 (s, 2H, inner NH), 1.40 (s, 18H, –CH₃), 4.13 (s, 3H, –COOCH₃), 7.36 (d, J = 8.8 Hz, 4H, –Ph), 7.44 (d, J = 8.8 Hz, 4H, –Ph), 7.46 (d, J = 8.4 Hz, 2H, –Ph), 8.03 (d, J = 8.4 Hz, 2H, –Ph), 8.29 (d, J = 8.4 Hz, 2H, –Ph), 8.47 (d, J = 8.0 Hz, 2H, –Ph), 8.87 (t, J = 4.0 Hz, 2H, pyrr), 9.12 (t, J = 4.6 Hz, 2H, pyrr), 9.27 (d, J = 4.8 Hz, 1H, pyrr), 9.29 (d, J = 4.8 Hz, 1H, pyrr), 9.73 (d, J = 4.8 Hz, 1H, pyrr), 9.75 (d, J = 4.8 Hz, 1H, pyrr), 10.16 (s, 1H, meso-H). HRMS (ESI, m/z): [M + H]⁺ calcd for C₅₄H₄₉N₅O₂Br, 800.3965; found, 800.3969. IR (KBr pellet, cm⁻¹): 3309 (w), 3026 (w), 2943 (m), 2866 (w), 2521 (m), 2131 (w), 1795 (m), 1729 (s), 1596 (s), 1509 (s), 1435 (m), 1399 (m), 1319 (m), 1271 (s), 1191 (m), 1108 (m), 959 (s), 828 (m), 792 (m), 584 (w), 554 (m).

Br2PM: ¹H NMR (CDCl₃, Bruker 400 MHz), δ: −2.71 (s, 2H, inner NH), 1.40 (s, 18H, –CH₃), 4.13 (s, 3H, –COOCH₃), 7.35 (d, J = 8.8 Hz, 4H, –Ph), 7.43 (d, J = 8.0 Hz, 4H, –Ph), 7.44 (d, J = 8.4 Hz, 2H, –Ph), 7.97 (d, J = 8.4 Hz, 2H, –Ph), 8.24 (d, J = 8.0 Hz, 2H, –Ph), 8.45 (d, J = 8.0 Hz, 2H, –Ph), 8.75 (t, J = 4.0 Hz, 2H, pyrr), 9.00 (t, J = 4.0 Hz, 2H, pyrr), 9.62 (t, J = 6.0 Hz, 4H, pyrr). HRMS (ESI, m/z): [M + H]⁺ calcd for C₅₄H₄₈N₅O₂Br₂, 956.2175; found, 956.2172. IR (KBr pellet, cm⁻¹): 3315 (w), 2955 (m), 2860 (w), 2515 (m), 2131 (m), 1795 (m), 1718 (s), 1601 (s), 1500 (s), 1429 (w), 1360 (w), 1319 (m), 1277 (s), 1188 (m), 1108 (m), 959 (s), 870 (m), 792 (s), 700 (m), 626 (m), 554 (m).

Synthesis of compound PM1. To a solution of 1-bromo-4-hexyloxy-benzene (935 mg, 3.65 mmol) in dry THF (20 mL) at −78 °C, n-butyl lithium (2.1 mL, 5.0 mmol, 2.4 M in hexane) was added dropwise. The mixture was stirred at −78 °C for 1 h. Then, trimethyl borate (0.9 mL) was added rapidly to the above solution and the mixture was stirred for another 2 h. The resultant mixture was warmed up to room temperature and stirred overnight and the solution was then injected into a mixture of BrPM (188 mg, 0.21 mmol), cesium carbonate (210 mg, 0.64 mmol), Pd(PPh₃)₄ (26 mg, 0.022 mmol), toluene (25 mL) and DMF (25 mL) in a Schlenk flask (250 mL), which was charged with nitrogen, and stirred at 85 °C for 24 h. Then, the mixture was dissolved in CH₂Cl₂ and washed with water, dried over anhydrous sodium sulfate, evaporated, and purified by a silica gel column to afford the red solid (140 mg, 67%). ¹H NMR (CDCl₃, Bruker 400 MHz), δ: −2.96 (s, 2H, inner NH), 0.99 (t, J = 6.6 Hz, 3H, –CH₃), 1.39 (s, 18H, –CH₃), 1.46 (br, 4H, –CH₂–), 1.63 (m, 2H, –CH₂–), 1.99 (m, 2H, –CH₂–), 4.12 (s, 3H, –COOCH₃), 4.25 (t, J = 6.6 Hz, 2H, –OCH₂–), 7.27 (d, J = 8.4 Hz, 2H, –Ph), 7.35 (d, J = 8.4 Hz, 4H, –Ph), 7.43 (d, J = 8.4 Hz, 4H, –Ph), 7.45 (d, J = 9.2 Hz, 2H, –Ph), 8.07 (d, J = 8.4 Hz, 2H, –Ph), 8.10 (d, J = 8.4 Hz, 2H, –Ph), 8.32 (d, J = 7.6 Hz, 2H, –Ph), 8.40 (d, J = 8.0 Hz, 2H, –Ph), 8.81 (d, J = 4.8 Hz, 1H, pyrr), 8.93 (t, J = 5.2 Hz, 3H, pyrr), 9.05 (d, J = 4.4 Hz, 1H, pyrr), 9.18 (d, J = 4.4 Hz, 1H, pyrr), 9.32 (d, J = 4.8 Hz, 1H, pyrr), 9.34 (d, J = 4.4 Hz, 1H, pyrr), 10.19 (s, 1H, meso-H). HRMS (ESI, m/z): [M + H]⁺ calcd for C₆₆H₆₆N₅O₃, 976.5166; found, 976.5166. IR (KBr pellet, cm⁻¹): 3321 (w), 3033 (w), 2952 (m), 2860 (w), 2521 (w), 1792 (m), 1723 (s), 1604 (s), 1506 (s), 1465 (m), 1319 (m), 1277 (m), 1244 (w), 1176 (m), 1108 (s), 1018 (m), 968 (s), 798 (s), 724 (m), 584 (w), 551 (w).

Synthesis of compound PM2. To a solution of 4-hexyloxy bromobenzene (545 mg, 2.13 mmol) in dry THF (10 mL) at −78 °C, n-butyl lithium (1.1 mL, 2.64 mmol, 2.4 M in hexane) was added dropwise. The mixture was stirred at −78 °C for 1 h. Then, trimethyl borate (0.4 mL) was added rapidly to the above solution and the mixture was stirred for another 2 h. The resultant mixture was warmed up to room temperature and stirred overnight and the solution was then injected into a mixture of Br2PM (192 mg, 0.2 mmol), cesium carbonate (400 mg, 1.23 mmol), Pd(PPh₃)₄ (46 mg, 0.04 mmol), toluene (15 mL) and DMF (15 mL) in a Schlenk flask (100 mL), which was charged with nitrogen, and stirred at 85 °C for 24 h. Then, the mixture was dissolved in CH₂Cl₂ and washed with water, dried over anhydrous sodium sulfate, evaporated, and purified by a silica gel column to afford the red solid (176 mg, 76%). ¹H NMR (CDCl₃, Bruker 400 MHz), δ: −2.73 (s, 2H, inner NH), 0.99 (t, J = 6.8 Hz, 6H, –CH₃), 1.38 (s, 18H, –CH₃), 1.46 (br, 8H, –CH₂–), 1.63 (m, 4H, –CH₂–), 1.98 (m, 4H, –CH₂–), 4.11 (s, 3H, –COOCH₃), 4.25 (t, J = 6.4 Hz, 2H, –OCH₂–), 7.28 (d, J = 8.4 Hz, 4H, –Ph), 7.33 (d, J = 8.8 Hz, 4H, –Ph), 7.42 (t, J = 6.6 Hz, 6H, –Ph), 8.04 (d, J = 8.4 Hz, 2H, –Ph), 8.11 (d, J = 8.4 Hz, 4H, –Ph), 8.30 (d, J = 8.0 Hz, 2H, –Ph), 8.43 (d, J = 8.0 Hz, 2H, –Ph), 8.75 (d, J = 4.4 Hz, 2H, pyrr), 8.89 (t, J = 4.8 Hz, 4H, pyrr), 9.00 (d, J = 4.4 Hz, 2H, pyrr). HRMS (ESI, m/z): [M + H]⁺ calcd for C₇₈H₈₂N₅O₄, 1152.6367; found, 1152.6367. IR (KBr pellet, cm⁻¹): 3318 (w), 3030 (w), 2958 (m), 2920 (m), 2860 (m), 2512 (w), 2134 (w), 1801 (m), 1723 (s), 1604 (s), 1506 (s), 1468 (s), 1378 (w), 1319 (w), 1268 (s), 1241 (s), 1170 (s), 1111 (m), 962 (m), 840 (w), 807 (s), 730 (m), 629 (w), 551 (w).

Synthesis of compound BrPM1. To a solution of PM1 (140 mg, 0.143 mmol) in CH₂Cl₂ (200 mL) was added NBS (26 mg, 0.146 mmol) in batches at room temperature. After 1 h, the reaction was quenched with acetone (1 mL). The solvent was removed under reduced pressure. The product was isolated by silica gel columns and recrystallized from CH₂Cl₂–CH₃OH gave BrPM1 (140 mg, 92%). ¹H NMR (CDCl₃, Bruker 400 MHz), δ: −2.72 (s, 2H, inner NH), 0.98 (t, J = 7.0 Hz, 3H, –CH₃), 1.39 (s, 18H, –CH₃), 1.46 (br, 4H, –CH₂–), 1.63 (m, 2H, –CH₂–), 1.98 (m, 2H, –CH₂–), 4.12 (s, 3H, –COOCH₃), 4.23 (t, J = 6.4 Hz, 2H, –OCH₂–), 7.26 (d, J = 9.2 Hz, 2H, –Ph), 7.34 (d, J = 8.4 Hz, 4H, –Ph), 7.43 (d, J = 8.8 Hz, 6H, –Ph), 8.00 (d, J = 8.4 Hz, 2H, –Ph), 8.06 (d, J = 8.4 Hz, 2H, –Ph), 8.26 (d, J = 8.4 Hz, 2H, –Ph), 8.44 (d, J = 8.0 Hz, 2H, –Ph), 8.70 (d, J = 4.8 Hz, 1H, pyrr), 8.80 (d, J = 4.8 Hz, 1H, pyrr), 8.85 (t, J = 4.8 Hz, 2H, pyrr), 8.95 (d, J = 4.4 Hz, 1H, pyrr), 9.06 (d, J = 4.4 Hz, 1H, pyrr), 9.65 (d, J = 4.8 Hz, 1H, pyrr), 9.67 (d, J = 4.8 Hz, 1H, pyrr). HRMS (ESI, m/z): [M + H]⁺ calcd for C₆₆H₆₆N₅O₃Br, 1054.4271; found, 1054.4268. IR (KBr pellet, cm⁻¹): 3416 (w), 2952 (w), 2925 (w), 2857 (w), 2509 (w), 2128 (w), 1798 (m), 1720 (s), 1604 (s), 1509 (s), 1465 (m), 1381 (m), 1319 (m), 1274 (s), 1241 (m), 1170 (m), 1111 (m), 992 (w), 804 (s), 730 (m), 545 (w), 486 (w).

Synthesis of compound PT. BrPM1 (308 mg, 292 μmol), BB (23 mg, 50 μmol), potassium carbonate (415 mg, 3 mmol), Pd(PPh₃)₄ (20 mg, 17 μmol), toluene (15 mL) and DMF (15 mL) were placed in a Schlenk flask (100 mL), which was charged with nitrogen. The mixture was stirred at 85 °C for 24 h. Then, the mixture was dissolved in CH₂Cl₂ and washed with water, dried over anhydrous sodium sulfate, evaporated, and purified by silica gel column and GPC to afford a red brown solid (50 mg, 33%). ¹H NMR (CDCl₃, Bruker 400 MHz), δ: −2.64 (s, 6H, inner NH), 0.97 (d, J = 6.8 Hz, 9H, –CH₃), 1.39 (s, 54H, –CH₃), 1.43 (br, 12H, –CH₂–), 1.62 (m, 6H, –CH₂–), 1.97 (t, J = 7.0 Hz, 6H, –CH₂–), 4.14 (s, 9H, –COOCH₃), 4.23 (t, J = 6.4 Hz, 6H, –OCH₂–), 7.25 (d, J = 9.2 Hz, 6H, –Ph), 7.35 (d, J = 3.6 Hz, 12H, –Ph), 7.43 (d, J = 8.0 Hz, 18H, –Ph), 8.04 (d, J = 7.2 Hz, 6H, –Ph), 8.08 (d, J = 8.4 Hz, 6H, –Ph), 8.32 (m, 6H, –Ph), 8.45 (m, 6H, –Ph), 8.76 (s, 3H, pyrr), 8.88 (s, 6H, pyrr), 9.02 (d, J = 9.8 Hz, 3H, pyrr), 9.08 (t, J = 7.8 Hz, 3H, pyrr), 9.30 (m, 3H, pyrr), 9.55 (s, 3H, central-Ph), 9.78 (m, 6H, pyrr). MALDI-TOF-MS (dithranol): [M + H]⁺ calcd for C₂₀₄H₁₉₆N₁₅O₉, 2999.53; found, 2999.46. IR (KBr pellet, cm⁻¹): 3416 (br), 2958 (w), 2922 (w), 2851 (w), 2524 (w), 2354 (w), 1792 (w), 1723 (m), 1604 (m), 1509 (s), 1474 (m), 1393 (w), 1319 (w), 1274 (m), 1241 (w), 1182 (m), 1102 (m), 804 (s), 733 (m), 551 (w).

General procedure for hydrolysis of carboxylic ester group and zinc coordination. A mixture of the porphyrin with a carboxylate group (0.03 mmol) and LiOH·H₂O (3 equiv. for each carboxylic ester group) in THF (30 mL) and H₂O (4 mL) was refluxed for 12 h under nitrogen. Then, the resultant mixture was added a CH₂Cl₂ (50 mL) and CH₃OH (5 mL) solution of Zn(OAc)₂·2H₂O (30 equiv.). After the mixture was stirred at room temperature overnight, the solvent was removed under reduced pressure, and the residue was purified on a silica gel column, followed by recrystallization from CH₂Cl₂–CH₃OH.

M1: yield, 60%. ¹H NMR (d⁶-DMSO + CDCl₃, Bruker 400 MHz), δ: 0.97 (t, J = 7.0 Hz, 3H, –CH₃), 1.36 (s, 18H, –CH₃), 1.44 (br, 4H, –CH₂–), 1.57 (m, 2H, –CH₂–), 1.60 (m, 2H, –CH₂–), 4.22 (t, J = 6.4 Hz, 2H, –OCH₂–), 7.29 (t, J = 9.6 Hz, 6H, –Ph), 7.36 (d, J = 8.0 Hz, 2H, –Ph), 7.44 (d, J = 8.4 Hz, 4H, –Ph), 8.04 (d, J = 8.4 Hz, 2H, –Ph), 8.05 (d, J = 8.4 Hz, 2H, –Ph), 8.30 (d, J = 8.0 Hz, 2H, –Ph), 8.39 (d, J = 8.0 Hz, 2H, –Ph), 8.80 (d, J = 4.8 Hz, 1H, pyrr), 8.88 (m, 3H, pyrr), 9.00 (t, J = 4.4 Hz, 1H, pyrr), 9.09 (d, J = 4.4 Hz, 1H, pyrr), 9.42 (t, J = 4.4 Hz, 2H, pyrr), 10.24 (s, 1H, meso-H). HRMS (ESI, m/z): [M + H]⁺ calcd for C₆₅H₆₂N₅O₃Zn, 1024.4144; found, 1024.4149. IR (KBr pellet, cm⁻¹): 3422 (br), 2922 (m), 2851 (m), 2512 (m), 2131 (m), 1798 (m), 1604 (w), 1500 (m), 1319 (w), 1280 (w), 1173 (w), 1063 (w), 986 (m), 792 (w), 721 (w), 548 (w).

M2: yield, 65%. ¹H NMR (d⁶-DMSO + CDCl₃, Bruker 400 MHz), δ: 0.97 (t, J = 7.0 Hz, 6H, –CH₃), 1.34 (s, 18H, –CH₃), 1.44 (br, 8H, –CH₂–), 1.59 (m, 4H, –CH₂–), 1.92 (m, 4H, –CH₂–), 4.24 (t, J = 6.2 Hz, 2H, –OCH₂–), 7.29 (m, 10H, –Ph), 7.44 (d, J = 8.8 Hz, 4H, –Ph), 8.02 (d, J = 8.4 Hz, 2H, –Ph), 8.05 (d, J = 8.4 Hz, 4H, –Ph), 8.28 (d, J = 6.0 Hz, 2H, –Ph), 8.36 (d, J = 8.0 Hz, 2H, –Ph), 8.74 (d, J = 4.4 Hz, 2H, pyrr), 8.82 (d, J = 4.8 Hz, 2H, pyrr) 8.84 (d, J = 4.8 Hz, 2H, pyrr), 8.93 (d, J = 4.8 Hz, 2H, pyrr), 13.15 (s, 1H, –COOH). HRMS (ESI, m/z): [M + H]⁺ calcd for C₇₇H₇₈N₅O₄Zn, 1200.5354; found, 1200.5354. IR (KBr pellet, cm⁻¹): 3428 (br), 2955 (w), 2925 (m), 2848 (m), 2515 (w), 1786 (w), 1685 (w), 1604 (w), 1506 (m), 1462 (w), 1274 (m), 1236 (m), 1170 (m), 1130 (w), 989 (s), 801 (w), 762 (w), 712 (w).

T: yield, 45%. MALDI-TOF-MS (dithranol): [M + H]⁺ calcd for C₂₀₁H₁₈₄N₁₅O₉Zn₃, 3143.22; found, 3143.22. IR (KBr pellet, cm⁻¹): 3419 (br), 2964 (w), 2925 (m), 2839 (m), 2506 (w), 2131 (w), 1792 (m), 1655 (w), 1599 (w), 1503 (m), 1459 (w), 1399 (w), 1262 (w), 1173 (w), 1072 (w), 992 (w), 798 (m), 715 (w), 551 (w).

Result and discussion

Molecular design and syntheses

Generally, efficient DSSC dyes have a push–pull structure,³³ and TPA moieties have been widely used as the electron donors due to their strong electron-donating ability.³⁴ On the other hand, alkoxyl groups may be introduced to suppress the dye aggregation, improve the dye solubility and act as moderate electron donors.^10,35,36 Thus, four target compounds (M1, M2, D, T, Scheme 1) were designed.

The symmetrical porphyrin PM was prepared by TFA-promoted condensation of the corresponding dipyrromethane and aldehyde in CH₂Cl₂, with a high yield of 39%. Bromination of PM with NBS in CH₂Cl₂ at room temperature afforded the monobromoporphyrin (BrPM) and dibromoporphyrin (Br2PM). p-(n-Hexyloxy)phenyl groups were introduced to the meso-positions of BrPM and Br2PM by classical Suzuki coupling reactions to afford PM1 and PM2, which were further hydrolyzed and coordinated with Zn(CH₃COO)₂·2H₂O to yield M1 and M2, respectively (Scheme 2).

Scheme 2 The synthetic routes for M1 and M2. Reaction conditions: (i) TFA, CH₂Cl₂; (ii) NBS, CH₂Cl₂; (iii) 1-bromo-4-hexyloxy-benzene, n-BuLi, trimethylborate, THF; Pd(PPh₃)₄, Cs₂CO₃, DMF, toluene; (iv) LiOH·H₂O, THF, H₂O; and (v) Zn(CH₃COO)₂·2H₂O, CHCl₃, CH₃OH.

We continued to synthesize multiporphyrins PD and PT by Suzuki coupling reactions (Scheme 3). Unfortunately, PD could not be isolated as a pure compound due to the presence of the by-product formed by the self coupling of BrPM1, which has very similar polarity and molecular weight with PD. Hence, only PT could be obtained, followed by hydrolysis and zinc coordination to afford T. The compounds were fully characterized by ¹H NMR and mass spectra (Fig. S1–S20†).

Scheme 3 The synthetic routes for PD and T. Reaction conditions: (i) NBS, CH₂Cl₂; (ii) Pd(PPh₃)₄, K₂CO₃, DMF, toluene; (iii) LiOH·H₂O, THF, H₂O; and (iv) Zn(CH₃COO)₂·2H₂O, CHCl₃, CH₃OH.

Absorption properties in solutions and on TiO₂ films

The UV-visible absorption spectra of M1, M2, and T in CHCl₃–CH₃OH (v/v, 2/1) are shown in Fig. 1a, with the corresponding data summarized in Table 1. Each dye shows an intensive Soret band in the range of 421–436 nm, and two Q bands within 553–604 nm. As expected, the absorption bands are sensitive to the substituents. With increasing numbers of the electron-donors, the Soret and Q bands are prominently broadened and red-shifted. Thus, M2 and T exhibit slightly broader absorption bands at longer wavelengths, compared with those of M1. The molar absorption coefficients of T were dramatically enhanced due to the presence of three porphyrin units in the molecule, which may be favorable for the absorption of the sunlight. The fluorescence spectra of them were also checked in CHCl₃–CH₃OH (v/v, 2/1) (Fig. 1b), and the emission wavelengths varied in a trend similar to that observed for the absorption bands.³⁷

Fig. 1 (a) Absorption spectra and (b) emission spectra of the porphyrins (10 μM) measured in CHCl₃–CH₃OH (v/v, 2/1) at 298 K.

Table 1 Absorption and emission data for the porphyrin dyes in CHCl₃–CH₃OH (v/v, 2/1)^a

Entry	Absorption λmax/nm (ε/10^5 M^−1 cm^−1)	Emission λmax/nm
a Excitation wavelengths/(nm): M1, 421, M2, 428, T, 436.
M1	421(3.32), 553(0.28), 597(0.13)	612
M2	428(3.31), 560(0.25), 602(0.18)	622
T	436(8.36), 562(0.91), 604(0.66)	621

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To clearly elucidate the light harvesting abilities of the dyes in the DSSC devices, we continued to investigate the absorption spectra of the dyes on TiO₂ films (Fig. 2). Compared with the corresponding solution spectra, the absorption bands of the dyes loaded films are broadened, which will result in increased light-harvesting ability. The Soret bands are red-shifted 7 nm, 10 nm, and 5 nm for M1, M2, and T, respectively. The red-shift of the Soret band for T is smaller than those for M1 and M2, indicating that the intermolecular interactions on TiO₂ are less severe for T, compared with M1 and M2. In addition, blue shoulders are observed for the Soret bands. These observations can be ascribed to the J-type aggregation of the porphyrins,^38a,38b as well as the deprotonation of the carboxylic acid on the TiO₂ surfaces^38c,38d and/or H-type aggregation.^13d,17d

Fig. 2 Normalized UV-visible spectra of the porphyrins in CHCl₃–CH₃OH (v/v, 2/1) and on TiO₂ films (5 μm).

Electrochemical properties and energy levels

For an effective DSSC sensitizer, efficient dye regeneration and electron injection processes require suitably aligned energy levels. The highest occupied molecular orbital (HOMO) needs to be lower than the corresponding energy level of the electrolyte, and the lowest unoccupied molecular orbital (LUMO) needs to be higher than the conduction band (CB) of the TiO₂ electrode, thus enabling dye regeneration and electron injection processes, respectively.³⁹ To evaluate this possibility, the HOMO levels of these dyes were determined by cyclic voltammetry, and the LUMO energies were calculated from E_HOMO − E_0–0.

The LUMO orbitals of the porphyrins (Fig. 3 and Table 2) have higher energy levels than the conduction edge of TiO₂, and the HOMO orbitals have lower energy levels than the oxidation potential for the I⁻/I₃⁻ redox couple, indicating that the processes of electron injection from the excited states of the dyes to the conduction band (CB) of TiO₂, and the dye regeneration processes by transferring the electrons from the I₃⁻ ions to the oxidized dyes are both feasible. Thus, these porphyrins may be used as DSSC sensitizers. The HOMO and the LUMO energy levels for M1 and T are almost identical, indicating that the three porphyrin moieties in T are electronically isolated, which may be related to the large interplane angles between the central phenyl unit and the porphyrin frameworks. M2 has slightly higher HOMO and LUMO energy levels than those of M1 and T, which may be caused by the introduction of an additional electron-donating alkoxyl group. And all the dyes have almost the same HOMO–LUMO gaps around 1.95 V. These results are in agreement with our previous observations.¹⁸

Fig. 3 (a) Cyclic voltammetry plots of the dyes adsorbed to nanocrystalline TiO₂ films deposited on conducting FTO glass. (b) Energy-level diagram of the porphyrin dyes. LUMO is estimated by subtracting E_0–0 from HOMO.

Table 2 Electrochemical data for the porphyrin dyes

Entry	HOMO^a/V (vs. NHE)	E0–0^b/V	LUMO^c/V (vs. NHE)
a HOMO levels were measured in CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the electrolyte (working electrode: FTO/TiO2/dye; reference electrode: SCE; calibrated with ferrocene/ferrocenium (Fc/Fc^+) as an external reference. Counter electrode: Pt).b E0–0 was estimated from the absorption threshold of the dyes adsorbed on the TiO2 film.c The LUMO was calculated with the equation of LUMO = HOMO − E0–0.
M1	0.88	−1.96	−1.08
M2	0.78	−1.95	−1.17
T	0.87	−1.94	−1.07

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Theoretical calculations

As we know, suitable electron distribution for the HOMO and LUMO orbitals is essential for the realization of the charge-separated states and subsequent electron injection.⁴⁰ Thus, to gain further insights into the electronic properties of the porphyrins, density functional theory (DFT) calculations and time-dependent density functional theory (TDDFT) calculations were employed.

Fig. 4 shows the corresponding molecular orbitals for dyes M1 and M2. The HOMOs are delocalized over the donors and the porphyrin frameworks while the LUMOs are delocalized over the porphyrin frameworks, the phenyl linkers and the anchoring carboxyl groups, which indicated that intramolecular charge separation states may be possible, accompanied with electron transfer from the donors to the anchoring moieties, thus enabling the electron injection processes.

Fig. 4 Frontier molecular orbital profiles of dyes M1 and M2, obtained from the single point calculations at the CAM-B3LYP/SVP level.

The HOMOs for dye T are located on the donors and the porphyrin frameworks (Fig. 5), but the LUMOs were located only on two of the porphyrin frameworks, which is in sharp contrast to those observed for M1 and M2. These results indicate that the excitation of the electrons from the HOMOs to the LUMOs of T can not efficiently transfer the electrons to the CB of TiO₂ because the LUMOs are not delocalized over the anchoring carboxyl groups, and thus low J_sc value and poor power conversion efficiency are expected for T, which is in accordance with the experimental results (vide infra).

Fig. 5 Frontier molecular orbital contours of dye T, obtained from single point calculations at the CAM-B3LYP/SVP level.

IPCE measurements

From above mentioned results, it can be anticipated that M1, M2 and T may be employed as DSSC dyes. Thus, DSSC devices were fabricated based on these three dyes and the photovoltaic performance was investigated. Fig. 6 shows the incident photon-to-current conversion efficiency (IPCE) action spectra of the corresponding devices and the corresponding current–voltage characteristics.

Fig. 6 (a) IPCE spectra of DSSCs based on the porphyrin dyes. (b) Current–voltage characteristics of DSSCs based on the porphyrin dyes.

M1 and M2 have relatively high IPCE values in a large wavelength range of 350–650 nm. The IPCE values of M2 exceed 60% in the spectral ranges of 390–470 nm, 553–578 nm, and 600–620 nm, and reach a maximum of 77.5% at 440 nm. The IPCE values of M1 reach a maximum of 67.7% at 409 nm, and the value is lower than that of M2 (Fig. 6a). Thus, a lower J_sc value is expected for M1. In sharp contrast to the results observed for M1 and M2, the device based on T exhibits rather poor IPCE performance (Fig. 6a), which can be related to the low efficiency of the electron injection processes induced by the isolation of the LUMOs from the anchoring carboxyl groups, as revealed by the theoretical calculations.

Electrochemical impedance spectroscopy

The electrochemical impedance spectra (EIS) have been used for studying interfacial charge-transfer processes in DSSCs.⁴¹ To further clarify the interrelation between the charge-transfer processes of the dyes and the photoelectric properties of the DSSCs, EIS analyses were performed in the dark at an applied bias of −0.65 V.

A typical Nyquist plot generally contains two semicircles. The first one on the left is associated with the Pt/electrolyte interface charge-transfer processes, and the second one on the right corresponds to the electron transport process at the dye-sensitized TiO₂/electrolyte interface.⁴² In Fig. 7a, the first semicircles are too small to be clearly observed. And the radii of the second ones lie in the order of T < M1 < M2, indicating that the DSSC device based on M2 demonstrates the most effectively suppressed charge recombination process, which may be ascribed to the retardation of the diffusion of I₃⁻ ions into the nanoporous TiO₂ electrode by two long n-hexyloxy chains.³⁶ These results are also further supported by measuring the I–V characteristics of the DSSCs in the dark (Fig. 8). The dark current for M1, M2, and T lie in the order of M2 < M1 < T, indicating that M2 has the largest charge recombination resistance at the dye-sensitized TiO₂/electrolyte interface.

Fig. 7 Electrochemical impedance spectra. (a) Nyquist plots and (b) Bode phase plots for the DSSCs prepared from the porphyrin dyes.

Fig. 8 Dark-current density–potential curves of the DSSCs based on M1, M2 and T, respectively.

In the Bode plots, the peaks at lower frequencies are related to the charge recombination processes, and the electron lifetimes of the devices based on T, M1 and M2 are calculated to be 27.0, 37.9 and 54.1 ms, respectively (Fig. 7b).⁴³ These data revealed that the devices based on dye T demonstrated the fastest charge recombination process and the shortest electron lifetime, which may result in the lowest V_oc. On the other hand, dye M2 has a longer electron lifetime than M1, which may be ascribed to the more effective suppression of the charge recombination between the injected electrons and the electrolyte due to the formation of a more compact hydrophobic layer at the TiO₂ surface and the retardation of the diffusion of I₃⁻ ions into the nanoporous TiO₂ electrode³⁶ by the additional long hexyloxyl chain, and thus the highest V_oc is anticipated for the devices based on M2.

Photovoltaic performance of DSSCs

As demonstrated in Fig. 6b and Table 3, the overall power conversion efficiencies (η) lie in the range of 2.30–5.77%, in the order of T < M1 < M2, which has the same sequence as that for the IPCE curves. As we know, the efficiency of the devices is closely related to the dye absorption capacity and the dye adsorption amount on the TiO₂ film.^39b,44 Among these three dyes, T has the highest molar absorption coefficient and the lowest adsorption amount (173, 154 and 61 μmol m⁻² for M1, M2, and T, respectively). Consequently, the absorption intensities of these dyes on TiO₂ are comparable to each other (Fig. 9).

Table 3 Photovoltaic parameters of the porphyrin-sensitized solar cells under AM 1.5 illumination (power 100 mW cm⁻²) with an active area of 0.25 cm²

Entry	Jsc/mA cm^−2	Voc/mV	ff/%	η (%)
M1	10.36	671	52.3	3.64
M2	13.93	732	56.6	5.77
T	6.47	592	60.1	2.30

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Fig. 9 UV-visible spectra of the porphyrins adsorbed on TiO₂ films (5 μm).

On the other hand, according to our previous report,¹⁸ the cell performance is also closely related to the molecular orbital energy levels. As mentioned above, the energy levels of the HOMO and the LUMO orbitals of M2 and T are almost identical to those of M1 (Fig. 3). Thus, it can be concluded that the absorption and the molecular orbital energy levels are not the main factors for the large differences in the cell performance.

Thus, the lowest efficiency observed for the devices based on T may be ascribed to the low efficiency of electron injection to the CB of TiO₂ and severe charge recombination process as described above. Compared with M1, M2 has a relatively high J_sc, which is consistent with relatively larger IPCE values. Furthermore, the devices based on M2 have higher V_oc than those based on M1, which is consistent with the electrochemical impedance results. Therefore, the combination of the highest V_oc and J_sc values afford the best photovoltaic performance of the solar cell based on dye M2 with the power conversion efficiency of 5.77%.

Conclusions

In this work, we have successfully designed and synthesized a novel trigeminal zinc porphyrin dye T, and two zinc porphyrin monomers M1 and M2. The spectral, electrochemical, and photovoltaic properties of the dyes were investigated. Compared with M1, the molecule of M2 contains an additional aliphatic n-hexyloxyl chain, which favors the formation of a compact hydrophobic layer at the TiO₂ surface and the retardation of the diffusion of I₃⁻ ions into the nanoporous TiO₂ electrode, resulting in better suppression of charge recombination and a higher V_oc. Meanwhile, M2 has larger IPCE values than those of M1, leading to a higher J_sc. Therefore, dye M2 exhibits a power conversion efficiency of 5.77%, which is higher than that of 3.64% for M1.

Compared with M1 and M2, the trigeminal zinc porphyrin dye T shows the largest molar absorption coefficient and the minimum adsorption amount. Thus, its absorption on the TiO₂ film is similar to those for M1 and M2. On the other hand, theoretical calculations on T revealed the isolation of the LUMOs from the anchoring carboxyl groups, which resulted in the low efficiency of electron injection, and thus dramatically reduced J_sc. Moreover, EIS and the dark current indicated that the device based on T have severe charge recombination process, leading to the lowest V_oc. Thus, the device based on T shows the lowest power conversion efficiency of 2.30%.

These results indicated that efficient electron injection process associated with prominent overlapping between the LUMO and the anchoring groups is an important factor for designing high performance sensitizers. In this respect, the trigeminal structure is unfavorable. On the other hand, introducing aliphatic n-hexyloxyl chains to the porphyrin unit can be effective for the suppression of the charge recombination process, prolonging the electron lifetime and improving the V_oc value. The results are useful for the further development of highly efficient porphyrin sensitizers.

Acknowledgements

This work was financially supported by NSFC (21072060, 91227201), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, NCET-11-0638, the Fundamental Research Funds for the Central Universities (WK1013002), and SRFDP (20100074110015).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Characterization data. See DOI: 10.1039/c3ra45791e

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