Ethynyl thiophene-appended unsymmetrical zinc porphyrin sensitizers for dye-sensitized solar cells

Abstract

Four unsymmetrical porphyrins of A2B donor–π–acceptor type have been designed, synthesized, characterized and their photovoltaic properties explored. Polycyclic aromatic hydrocarbons (PAH), such as pyrene or fluorene, act as a donor, the porphyrin is the π-spacer, appended with an ethynyl thiophene linker, and either cyanoacrylic acid or malonic acid acts as the acceptor. All of the compounds were characterized by ¹H NMR and mass spectrometry. UV-Vis absorption spectra and B or Soret (λ_ex at 440 nm for the four sensitizers reported) band-excited fluorescence emission spectra were also obtained. The electrochemical properties suggest that the first oxidation is ring-centred, which is supported by in situ spectro-electrochemical and DFT computational studies. The synthesized porphyrins were applied in dye-sensitized solar cells (DSSCs). A conversion efficiency of up to 3.14%was realized for PYR–Por–MA under our experimental conditions.

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Introduction

The increasing global energy crisis has demanded the search for new alternative energy conversion materials as competitors of silicon photovoltaic devices.¹ In this regard, dye-sensitized solar cells (DSSCs) are found to display low-cost, ease of fabrication, a short-energy pay-back time, low sensitivity to temperature changes and environmental friendliness compared with the conventional solid-state p–n photovoltaic devices.^2–8 Typical DSSCs consist of a dye-sensitized nanocrystalline semiconductor sandwiched between the platinum-coated counter electrode, within which is the redox electrolyte of the type I⁻/I₃⁻. The sensitizer is one of the key components in achieving the high efficiency and durability of the device. The most widely studied sensitizers employed so far are Ru(II) polypyridyl complexes (N719, N3, N945, Z-907), which produced solar energy-to-electricity conversion efficiencies (η) of more than 11%.^9–12 In spite of this high conversion efficiency, the main drawbacks of Ru(II) polypyridyl complexes are their expense due to the rarity of the metal in the earth's crust, and the lack of absorption in the near-IR region of the visible spectrum, where the solar flux of photons is still significant, thus limiting the realization and usability of highly-efficient devices. For this reason, dyes with large π-conjugated systems such as porphyrins and phthalocyanines have received considerable attention as sensitizers for DSSC applications.^13,14

A great variety of porphyrin sensitizers^15–26 used for DSSC applications comprise the anchoring group either at the pyrrole-β (η up to 7.1%)¹⁹ or -meso positions (η up to 12.3%).²⁶ Consistent with recent reports, unsymmetrical porphyrin sensitizers designed with unique directionality have proven to be promising alternatives and are competitive with conventional Ru(II) polypyridyl complexes. Enormous effort has been put forth in recent years to advance the existing highest power conversion efficiency by various structural modifications at the -meso positions of porphyrin macrocycles by adopting the donor–π–acceptor approach (D–π–A).^22–27 Recently, Yella et al. reported a porphyrin sensitizer (YD–o–C8) with an N,N′-diphenyl amine as the donor, a porphyrin macrocycle as the π-spacer and an aryl benzoxy group as the acceptor, with a highest η value of 12.3% using a Co(II)/Co(III) based redox electrolyte.²⁶ This has stimulated further investigations of porphyrin-based sensitizers in order to enhance the photovoltaic performance of DSSC devices.

The performance of porphyrin sensitizers can be significantly improved by grafting the molecule with a precisely chosen donor, π-linker and acceptor/anchoring groups (D–π–A approach).^22–27 According to previous reports, the combination of a wide range of donor moieties, such as functionalized arylamines, polyaromatic or heterocyclic donors, with an ethynyl benzoic acid anchoring group at the -meso position of the porphyrin, has revealed significantly-improved cell performance.^28,29 In contrast to many available porphyrin sensitizers reported with the ethynyl benzoic acid anchoring group, in the present scheme we have attempted to introduce an ethynyl thiophene π-conjugated linker with either cyanoacrylic or malonic acid anchoring groups and study its influence on the solar cell performance. Herein, we report four new unsymmetrical zinc metallated porphyrin sensitizers (PYR–Por–CA, PYR–Por–MA, FLU–Por–CA and FLU–Por–MA), as shown in Fig. 1, appended with an ethynyl thiophene linker between the porphyrin and anchoring group to facilitate a shift in the absorption towards the red region, and either a cyanoacrylic or malonic acid anchoring group and a polycyclic aromatic hydrocarbon (PAH) such as pyrene or fluorene as the donor moiety.^22–27,30

Fig. 1 Molecular structures of the porphyrin sensitizers.

All four sensitizers have been characterized by elemental analysis, MALDI-MS, IR, UV-Visible and fluorescence spectroscopy (both steady-state and time resolved) as well as electrochemical methods.

Experimental

General

Chemical reagents and catalysts used in the reactions were of analytical reagent grade (AR), purchased from Sigma-Aldrich (India) and were used without further purification. All the reactions were performed using dried and distilled solvents of laboratory reagent (LR) grade. The solvents CHCl₃, CH₂Cl₂ and triethylamine (TEA) were dried over CaH₂. Toluene was dried over sodium chunks overnight and distilled under a nitrogen atmosphere. The purification of compounds by column chromatography was performed on ACME Silica Gel (100–200 mesh).

Synthesis

Dipyrromethane, 4,4,5,5-tetramethyl-2-(pyren-1-yl)-1,3,2-dioxaborolane and 2-(9,9-dihexyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane were prepared according to the literature-reported procedures.^28,29,31

5-Pyrenyl-10,20-bis[4-(hexyloxy)-3,5-dimethoxyphenyl]porphyrin (5a). Monobromo porphyrin (4) (190 mg, 0.24 mmol) was dissolved in 40 mL of dry toluene, to which CsCO₃ (393.6 mg, 1.2 mmol), Pd(PPh₃)₄ (0.25 equivalents) and 1-pyrenylborane (218 mg, 0.713 mmol) were added and the reaction mixture was refluxed under a nitrogen atmosphere for 12 h. After cooling to room temperature (RT, 25 °C), the crude mixture was purified using a silica gel column with ethyl acetate–hexane (1 : 4 v/v) to afford the desired product (90% yield). Elemental analysis of anal. calcd for C₆₄H₆₂N₄O₆ % (983.20): C, 78.18; H, 6.36; N, 5.70. Found: C, 78.20; H, 6.33; N, 5.68. ESI-MS: m/z C₆₄H₆₂N₄O₆: calculated: 983.20, found: 984 [M + 1H]⁺ (100%). ¹H NMR (CDCl₃, 300 MHz, δ ppm): 10.25 (s, 1H), 9.37 (d, 2H), 9.14 (d, 2H), 8.90 (dd, 2H), 8.70 (s, 1H), 8.46 (m, 6H), 8.05 (m, 2H), 7.45 (m, 6H), 4.28 (m, 4H), 3.95 (s, 12H), 1.99 (m, 4H), 1.55 (m, 12H), 0.97 (m, 6H), −2.78 (b, 2H). UV-Vis (CH₂Cl₂) λ_max (nm) (log ε M⁻¹ cm⁻¹): 419 (5.81), 511 (4.44), 547 (3.85), 584 (3.81), 640 (3.33).

5-Fluorenyl-10,20-bis[4-(hexyloxy)-3,5-dimethoxyphenyl]porphyrin (5b). This compound was synthesized by adopting a similar procedure that was used to prepare 5a, in 87% yield. ¹H NMR (CDCl₃, 300 MHz, δ ppm): 2.95 (s, 2H), 0.78 (m, 12H), 0.98 (m, 8H), 1.25 (m, 8H), 1.45 (m, 10H), 1.60 (m, 4H), 2.10 (m, 8H), 3.95 (s, 12H), 4.31 (t, 4H), 7.50 (m, 7H), 7.91–8.29 (m, 4H), 8.94 (d, 4H), 9.13 (d, 2H), 9.35 (s, 2H), 10.21 (s, 1H). ESI-MS: m/z C₇₃H₈₆N₄O₆: calculated: 1115.49, found: 1115 [M⁺]. UV-Vis (CH₂Cl₂) λ_max (nm) (log ε M⁻¹ cm⁻¹): 272 (1.5), 306 (4.4), 418 (5.9), 512 (4.3), 547 (3.9), 587 (3.8), 641 (3.5).

5-Pyrenyl-15-ethynyl-10,20-bis[4-(hexyloxy)-3,5-dimethoxyphenyl]porphyrin zinc(II) (9a). Porphyrin (8a) (65 mg, 0.057 mmol) and K₂CO₃ (0.5 g) were dissolved in 30 mL of CH₂Cl₂–MeOH and allowed to stir at RT for 5 h. The crude mixture was filtered to remove K₂CO₃, washed with water and extracted with CHCl₃. Purification by a silica gel column using CHCl₃–hexane (4 : 1 v/v) afforded a more polar green product (90% yield). Elemental analysis of anal. calcd for C₆₆H₆₀N₄O₆Zn % (1068.4): C, 74.04; H, 5.65; N, 5.23. Found: C, 74.00; H, 5.68; N, 5.20. ESI-MS: m/z C₆₆H₆₀N₄O₆Zn: found: 1093 [M + Na]⁺ (100%). ¹H NMR (CDCl₃, 300 MHz, δ ppm): 9.82 (d, 2H), 9.12 (d, 2H), 8.80 (dd, 2H), 8.70 (s, 1H), 8.46 (m, 6H), 8.05 (m, 2H), 7.45 (m, 6H), 4.23 (m, 4H), 3.91 (s, 12H), 1.93 (m, 4H), 1.44 (m, 12H), 0.97 (m, 6H). UV-Vis (CH₂Cl₂) λ_max (nm) (log ε M⁻¹ cm⁻¹): 431 (5.85), 557 (4.54), 599 (4.13).

5-Fluorenyl-15-ethynyl-10,20-bis[4-(hexyloxy)-3,5-dimethoxyphenyl]porphyrin zinc(II) (9b). This compound was synthesized by adopting a similar procedure that was used to prepare 9a, in 91% yield. ¹H NMR (CDCl₃, 300 MHz, δ ppm): 0.86 (m, 12H), 0.99 (m, 8H), 1.44 (m, 8H), 1.55 (m, 10H), 1.65 (m, 4H), 1.99 (m, 4H), 2.11 (m, 4H), 3.67 (s, 1H), 3.93 (s, 12H), 4.33 (t, 4H), 7.46 (m, 7H), 7.90–8.25 (m, 4H), 8.98 (d, 4H), 9.12 (d, 2H), 9.45 (s, 2H). ESI-MS: m/z C₇₅H₈₄N₄O₆Zn: calculated: 1202.90, found: 1204 [M + 2H]⁺. UV-Vis (CH₂Cl₂) λ_max (nm) (log ε M⁻¹ cm⁻¹): 267 (4.5), 308 (4.4), 430 (5.7), 557 (4.3), 600 (5.0).

5-Pyrenyl-15-(ethynyl (5-formyl thiophene-2-yl)-10,20-bis[4-(hexyloxy)-3,5-dimethoxyphenyl]porphyrinato zinc(II) (10a). Porphyrin 9a (50 mg, 0.047 mmol) and Pd(PPh₃)₂Cl₂ (5.75 mg, 0.005 mmol) were dissolved in 10 mL of dry triethylamine, to which 5-bromothiophene-2-carboxaldehyde (44.16 mg, 0.025 ml, 0.23 mmol) and CuI (0.95 mg, 0.005 mmol) were added and the solution was heated to 50 °C for 8 h. After cooling to RT, the crude mixture was washed with water and extracted with CHCl₃. The green product was purified using a silica gel column with CHCl₃–hexane (3 : 1 v/v) as the eluant to afford the desired product (85% yield). Elemental analysis of anal. calcd for C₇₁H₆₂N₄O₇SZn % (1180.72): C, 72.22; H, 5.29; N, 4.75. Found: C, 72.20; H, 5.30; N, 4.70. ESI-MS: m/z C₇₁H₆₂N₄O₇SZn: calculated: 1180.72, found: 1182 [M + 2H]⁺ (100%). ¹H NMR (CDCl₃, 300 MHz, δ ppm): 10.11 (s, 1H), 9.90 (d, 1H), 9.77 (s, 1H), 9.20 (d, 2H), 8.77 (m, 3H), 8.50 (m, 6H), 8.21 (m, 2H), 7.83 (m, 4H), 7.45 (m, 7H), 6.90 (s, 1H), 4.22 (m, 4H), 3.93 (s, 12H), 1.93 (m, 4H), 1.44 (m, 12H), 0.97 (m, 6H). UV-Vis (CH₂Cl₂) λ_max (nm) (log ε M⁻¹ cm⁻¹): 426 (5.83), 454 (4.83), 566 (3.82), 623 (4.00).

5-fluorenyl-15-(ethynyl (5-formyl thiophene-2-yl)-10,20-bis[4-(hexyloxy)-3,5-dimethoxyphenyl]porphyrinato zinc(II) (10b). This compound was synthesized by adopting a similar procedure that was used to prepare 10a in 83% yield. ¹H NMR (CDCl₃, 300 MHz, δ ppm): 0.87 (m, 12H), 0.99 (m, 8H), 1.43 (m, 8H), 1.56 (m, 10H), 1.63 (m, 4H), 2.00 (m, 4H), 2.12 (m, 4H), 3.92 (s, 12H), 4.28 (t, 4H), 7.15 (d, 1H), 7.35 (d, 1H), 7.44 (m, 7H), 7.92–8.23 (m, 4H), 9.03 (m, 4H), 9.48 (d, 2H), 9.54 (s, 2H), 9.64 (s, 1H). ESI-MS: m/z C₈₀H₈₆N₄O₇SZn: calculated: 1313.04, found: 1315 [M + 2H]⁺. UV-Vis (CH₂Cl₂) λ_max (nm) (log ε M⁻¹ cm⁻¹): 267 (4.5), 308 (4.5), 423 (5.6), 453 (sh, 5.1), 551 (4.2), 621 (4.0).

5-pyrenyl-15-[3-(5-(ethynyl thiophen-2-yl))-10,20-Bis(4-(hexyloxy)-3,5-dimethoxyphenylphenyl)porphyrinatozinc(II)]-acrylic acid PYR–Por–CA. Porphyrin 10a (50 mg, 0.042 mmol) was dissolved in 30 mL of CH₃CN–CHCl₃ (3 : 1), to which piperidine and cyanoacetic acid (0.21 mmol) were added. The reaction mixture was refluxed for 8 h. After cooling to RT, the reaction mixture was washed with water and 0.1 M HCl and extracted with CH₂Cl₂. The product was purified with a silica gel column using CH₂Cl₂–MeOH (3 : 1 v/v) as the eluant to afford the desired product (85% yield). Elemental analysis of anal. calcd for C₇₄H₆₃N₅O₈SZn % (1247.77): C, 71.23; H, 5.09; N, 5.61. Found: C, 71.25; H, 5.10; N, 5.65. MALDI-MS: m/z C₇₄H₆₃N₅O₈SZn: calculated: 1247.77, found: 1246 [M − H]⁺ (100%). ¹H NMR (CDCl₃, 300 MHz, δ ppm): 9.67 (s, 2H), 9.08 (s, 2H), 8.73 (m, 4H), 8.29 (m, 6H), 8.11 (m, 2H), 7.36 (m, 8H), 4.17 (m, 4H), 3.85 (s, 12H), 1.93 (m, 4H), 1.44 (m, 12H), 0.97 (m, 6H). UV-Vis (CH₂Cl₂) λ_max (nm) (log ε M⁻¹ cm⁻¹): 447 (5.32), 575 (4.04), 639 (4.36).

5-pyrenyl-15-[3-(5-(ethynyl thiophen-2-yl)methylene)-10,20-Bis(4-(hexyloxy)-3,5-dimethoxyphenylphenyl)porphyrinatozinc(II)]-malonic acid (PYR–Por–MA). This compound was synthesized by an analogous procedure to the previous compound. The only difference is that here malonic acid was used instead of cyanoacrylic acid. Elemental analysis of anal. calcd for C₇₄H₆₄N₄O₁₀SZn % (1266.77): C, 70.16; H, 4.49; N, 5.61. Found: C, 70.15; H, 5.10; N, 4.51. MALDI-MS: m/z C₇₄H₆₄N₄O₁₀SZn: calculated: 1266.77, found: 1264 [M − 2H]⁺ (60%). ¹H NMR (CDCl₃, 300 MHz, δ ppm): 9.67 (s, 2H), 9.08 (s, 2H), 8.73 (m, 4H), 8.29 (m, 6H), 8.11 (m, 2H), 7.36 (m, 7H), 4.17 (m, 4H), 3.85 (s, 12H), 1.93 (m, 4H), 1.44 (m, 12H), 0.97 (m, 6H). UV-Vis (CH₂Cl₂) λ_max (nm) (log ε M⁻¹ cm⁻¹): 468 (5.12), 580 (4.01), 654 (4.46).

5-fluorenyl-15-[3-(5-(ethynyl thiophen-2-yl))-10,20-Bis(4-(hexyloxy)-3,5-dimethoxyphenylphenyl)porphyrinatozinc(II)]-acrylic acid (FLU–Por–CA). This compound was synthesized by adopting a similar procedure as used to prepare 11a. ¹H NMR (CDCl₃, 300 MHz, δ ppm): 0.65 (m, 8H), 0.82 (m, 12H), 1.23 (m, 18H), 1.63 (m, 4H), 2.02 (m, 8H), 3.61 (s, 12H), 4.23 (m, 4H), 6.85 (d, 2H), 7.42 (m, 8H), 7.92–8.05 (m, 4H), 9.03 (m, 4H), 9.61 (m, 4H). MALDI-TOF MS: m/z C₈₃H₈₇N₅O₈SZn: calculated: 1380.08, found: 1381 [M + H]⁺. UV-Vis (CH₂Cl₂) λ_max (nm) (log ε M⁻¹ cm⁻¹): 267 (4.6), 308 (4.5), 444 (5.2), 575 (4.2), 638 (4.5).

5-fluorenyl-15-[3-(5-(ethynyl thiophen-2-yl)methylene)-10,20-Bis(4-(hexyloxy)-3,5-dimethoxyphenylphenyl)porphyrinatozinc(II)]-malonic acid (FLU–Por–MA). This compound was synthesized by adopting a similar procedure as used to prepare 12a. ¹H NMR (CDCl₃, 300 MHz, δ ppm): 0.87 (m, 12H), 0.99 (m, 8H), 1.22 (m, 17H), 1.63 (m, 4H), 1.98 (m, 8H), 3.92 (s, 12H), 4.23 (m, 4H), 7.03 (d, 2H), 7.36 (m, 8H), 7.92–8.05 (m, 4H), 8.85 (m, 4H), 9.02 (d, 2H), 9.62 (s, 2H). MALDI-TOF MS: m/z C₈₃H₈₈N₄O₁₀SZn: calculated: 1399.08, found: 1400 [M + H]⁺. UV-Vis (CH₂Cl₂) λ_max (nm) (log ε M⁻¹ cm⁻¹): 266 (4.6), 309 (4.6), 448 (5.3), 572 (4.2), 632 (4.4).

Methods

The optical absorption spectra were recorded on a Shimadzu (Model UV-3600) spectrophotometer. Concentrations of the solutions were ca. 1 × 10⁻⁶ M for Soret band and 1 × 10⁻⁵ M for Q band absorption. Steady state fluorescence spectra were recorded (Spex model Fluorlog-3) for solutions showing optical density at the wavelength of excitation (λ_ex) ≈ 0.11. Time-resolved fluorescence measurements were carried out using a HORIBA Jobin–Yvon spectrofluorometer. Briefly, the samples were excited at 650 nm and the emission was monitored at 780 nm. The count rates employed were typically 10³–10⁴ s⁻¹. Deconvolution of the data was carried out by iterative reconvolution of the instrument response function and the assumed decay function using DAS-6 software. The accuracy of the fit of the experimental data to the assumed decay function was judged by the standard statistical tests (i.e., random distribution of weighted residuals, the autocorrelation function and the values of reduced χ²).

Cyclic voltammetric measurements were performed on a PC-controlled CH instrument (model CHI 620C electrochemical analyser) using 1 mM unsymmetrical porphyrin solution in dichloromethane (DCM) solvent at a scan rate of 100 mV s⁻¹ with 0.1 M tetrabutylammonium perchlorate (TBAP) as a supporting electrolyte. Glassy carbon was the working electrode, standard calomel electrode (SCE) was the reference electrode and platinum wire was an auxiliary electrode. After a cyclic voltammogram (CV) had been recorded, ferrocene was added, and a second voltammogram was measured. Spectroelectrochemical measurements were performed at a fixed temperature between 233 and 270 K by the optically transparent thin layer electrochemistry (OTTLE) technique in dry CH₂Cl₂ containing 0.3 M [TBA][BF₄] supporting electrolyte using a 2 mm thick quartz cuvette. The conventional three-electrode electrochemical cell consisted of a platinum gauze working electrode, platinum wire counter electrode and a Ag/AgCl reference electrode (ferrocene E_1/2 = + 0.63 V). The UV-Vis-NIR spectra were recorded with a Jasco V-670 spectrophotometer. The TG curves of the samples were performed on a Mettler Tolledo TGA/SDTA 851c thermogravimetric analyzer under a nitrogen atmosphere (99.999%) from 25 to 600 °C, in Al₂O₃ crucibles. The heating rates were 10 °C min⁻¹ and the flow rate of nitrogen was 80 mL min⁻¹.

Cell fabrication

A commercially available TiO₂ paste (Dyesol), of 18 nm diameter, was used to prepare the nanocrystalline TiO₂ electrodes. DSSCs were prepared on 3 mm thick float glass substrates coated with FTO (TEC-8 from Pilkington Group Limited). These FTO thin films have a sheet resistance of ∼8 Ω sq⁻¹ and an average transmission of ∼80% in the visible and NIR spectral region. The TiO₂ electrodes were made by the screen printing method to prepare transparent layers. Subsequently, a second scattering layer was made from a paste containing 400 nm anatase TiO₂ nanoparticles. As a post deposition treatment the TiO₂ electrodes were annealed at 450 °C for 30 min on a hotplate. The dye molecules were dissolved in ethanol at a concentration of 0.1 × 10⁻³ M. The TiO₂ thin films were soaked in the dye solution and then kept at room temperature for 16 h so that the dye was adsorbed onto the TiO₂ film. The TiO₂ electrodes were soaked overnight in dye solution, sandwiched with a platinised conducting counter electrode using a Surlyn frame (Solaronix SA), filled with the electrolyte through a hole in the counter electrode, and sealed. The iodide/tri-iodide electrolyte comprising 0.4 M LiI, 0.4 M tetrabutylammonium iodide (TBAI), and 0.04 M I₂ dissolved in 0.3 M N-methylbenzimidazole (NMB) in acetonitrile (ACN) and 3-methoxypropionitrile (MPN) solvent mixture at a volume ratio of 1 : 1, was used. The area of the cells was 0.25 cm².

Results and discussions

The synthetic scheme of the porphyrin sensitizers is illustrated in Fig. 2 (for a detailed synthetic scheme of each compound, experimental procedures and analytical data see the ESI†). We have adopted the Lindsey method for the synthesis of the unsymmetrical porphyrin (3).³¹ The donors, polycyclic aromatic hydrocarbons such as pyrene or fluorene, were introduced at the -meso position via Suzuki–Miyaura cross-coupling with the corresponding donor boronic acid pinacol ester and monobrominated porphyrin (3). A thiophene moiety with a rigid ethynyl linker was introduced via Sonogashira cross-coupling reaction of the corresponding TMS (trimethylsilylacetylene)-deprotected porphyrin (either 9a or 9b) with 5-bromothiophene carboxaldehyde. Finally, the presence of hexyloxy substituents on 10- and 20-meso phenyl rings of the porphyrin macrocycle is to increase the solubility of the porphyrin, and was also expected to minimise charge recombination.²⁶ The presence of the thiophene group is to enhance the molar absorption coefficient, bathochromically shift the absorption and increase the excited state lifetime.³² Moreover, some of the thiophene-based organic D–π–A sensitizers have been shown to improve the open circuit voltage and enhance the efficiency to up to 7%.^33,34 All of the new unsymmetrical porphyrins were characterized by various spectroscopic techniques. The MALDI-TOF MS mass spectrum of each compound displays peaks at PYR–Por–CA: 1246 [(M − H)⁺], PYR–Por–MA: 1264 [(M − 2H)⁺], FLU–Por–CA: 1381 [(M + H)⁺] and FLU–Por–MA: 1400 [(M + H)⁺], which are ascribed to the presence of the molecular ion peak (see ESI†).

Fig. 2 Synthetic scheme of the porphyrin sensitizers.

The electronic absorption spectra of typical metalloporphyrins exhibit an intense Soret band at around 420 nm, which is an a_1u(π)/e_g(π*) electronic transition, assigned to the second excited state (S₂), and two less intense Q bands (500–700 nm) originating from the a_2u(π)/e_g(π*) electronic transition, attributed to the first excited state (S₁). The absorption spectra of all four sensitizers have been measured in dichloromethane solvent, and the representative absorption spectrum of PYR–Por–CA is depicted in Fig. 3 and the corresponding absorption maxima and molar extinction coefficients are given in Table 1. Fig. 3 and Table 1 suggests that the absorption peaks in the ultraviolet region, i.e. between 230 and 380 nm, belong to the absorption of the donor PAH group in all four sensitizers, and it is not much altered when compared to its isolated donor molecules. In contrast, a split in the Soret band was observed in both fluorene derivatives i.e., FLU–Por–CA and FLU–Por–MA (see ESI†). Both the Soret and Q bands of all four sensitizers are broadened and red-shifted in comparison with ZnTTP, which can be attributed to the reduced molecular symmetry, extended-π conjugation via the ethynyl linker, and also due to electronic communication between the aromatic hydrocarbon donors/thiophene with the porphyrin macrocycle. The broadening of the absorption bands is more pronounced in the sensitizers with the malonic acid anchoring group (PYR–Por–MA & FLU–Por–MA) than the corresponding cyanoacrylic acid derivatives (PYR–Por–CA & FLU–Por–CA) (see ESI†). Similarly, the sensitizers with pyrene derivatives are more bathochromically shifted when compared to their fluorene derivatives, which may be due to the more electron releasing nature of the pyrene moiety.^25,35 Fig. 3 also displays the absorption spectrum of PYR–Por–CA adsorbed onto a 2 μm thick TiO₂ electrode. It is similar to that of the solution spectrum but exhibits a small red shift. This may be due to the anchoring of the carboxylic protons of porphyrin on TiO₂ which releases the proton upon binding to Ti⁴⁺.³⁶

Fig. 3 UV-Vis absorption spectra of PYR–Por–CA in CH₂Cl₂ ([thick line, graph caption]) and adsorbed onto a 2 μm thick TiO₂ film ([dash dash, graph caption]).

Table 1 UV-Visible absorption and electrochemical data

Compound	Absorption,^a λmax, nm (log ε, M^−1 cm^−1)							Potential^c (V, vs. SCE)
	Porphyrin bands			Donor PAH bands^b				Reduction	Oxidation
a Solvent CH2Cl2, error limits: λmax, ±1 nm, log ε, ±10%.b D = pyrene or fluorene.c CH2Cl2, 0.1 M TBAP; glassy carbon is the working electrode, standard calomel electrode is the reference electrode, Pt electrode is the auxillary electrode. Error limits, E1/2 ± 0.03 V.
PYR–Por–CA	447 (5.60)	575 (4.13)	639 (4.40)	339 (4.60)	326 (4.50)	275 (4.60)	234 (4.70)	−1.05, −1.34, −1.62	0.76, 1.12, 1.50
PYR–Por–MA	468(5.11)	580 (4.02)	654 (4.41)	339 (4.50)	326 (4.50)	275 (4.50)	234 (4.70)	−1.26, −1.50, −1.60	0.64, 1.06, 1.29
FLU–Por–CA	444 (5.21)	569 (4.17)	638 (4.51)	267 (4.61)	308 (4.50)			−1.14, −1.47, −1.70	0.79, 1.09, 1.47
FLU–Por–MA	448 (5.33)	572 (4.20)	632 (4.41)	266 (4.62)	309 (4.61)			−0.98, −1.35, −1.51	0.66, 1.27

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The emission spectra of all four porphyrin sensitizers were measured at room temperature in dichloromethane solvent, and the representative spectrum of PYR–Por–CA is illustrated in Fig. 4, with the corresponding emission maxima with quantum yields reported in Table 2. From Fig. 4 and Table 2, it is clear that the quantum yields of all four sensitizers is enhanced in comparison with the reference compound 5,10,15,20-tetraphenyl zinc porphyrin (ZnTPP). The singlet state energies (E_0–0) of all four sensitizers, estimated from the excitation and emission spectra, are presented in Table 2. No emission spectra are observed for the porphyrin sensitizers adsorbed onto the 6 μm thick TiO₂ layer as a consequence of electron injection from the excited singlet state of the porphyrin into the conduction band of TiO₂. The singlet excited-state lifetimes of all four unsymmetrical porphyrins were measured in DCM solvent (λ_ex = 440 nm & λ_em = 650 nm) and were found to be 0.82, 0.97, 0.71 and 0.95 ns for PYR–Por–CA, PYR–Por–MA, FLU–Por–CA and FLU–Por–MA, respectively (see ESI†). In all four cases, the excited state lifetime was quenched when adsorbed onto the 2 μm thick TiO₂ layer.

Fig. 4 Fluorescence spectra of PYR–Por–CA in CH₂Cl₂ ([thick line, graph caption]) and adsorbed onto a 2 μm thick TiO₂ film ([dash dash, graph caption]). Excitation wavelength λ_ex = 440 nm.

Table 2 Fluorescence data^a and fluorescence decay parameters

Compound	λem,^a nm (ϕ)	τ,^b ns (A%)	E0–0^c (eV)	Eox^*^d (eV)
a Error limits: λex, ±2 nm; ϕ, ±10%.b All lifetimes are in nanoseconds (ns). Error limits of τ ∼ 5%.c Error limits: ±0.05 eV.d Excited state oxidation potentials are calculated by using E* = E1/2(OX) − E0–0.
PYR–Por–CA	651	0.82	1.92	−1.16
	(0.069)	(80.03)
		0.09
		(19.97)
PYR–Por–MA	688	0.97	1.85	−1.21
	(0.060)	(63.62)
		1.94
		(36.38)
FLU–Por–CA	662	0.71	1.91	−1.12
	(0.070)	(78.15)
		1.84
		(21.85)
FLU–Por–MA	639	0.95	1.95	−1.29
	(0.078)	(43.50)
		2.00
		(56.42)

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With a view to evaluating the HOMO–LUMO levels of the porphyrin sensitizers, we performed electrochemistry by using cyclic and differential pulse voltammetric techniques in dichloromethane solvent. The redox potentials were determined from the half-wave potentials (E_1/2) (E_OX − (E_red)/2) by cyclic voltammetry (CV) or peak potentials (E_p) by differential pulse voltammetry (DPV). The redox potential data are presented in Table 1. Each new porphyrin sensitizer undergoes three reductions and either two or three oxidations under the experimental conditions employed. Wave analysis suggested that both the oxidation and reduction reactions are either quasireversible or totally irreversible. The first two oxidation processes belong to the porphyrin macrocycle, generating the π-cation radical and the dication, and the third oxidation belongs to the polycyclic aromatic hydrocarbon moiety in all four investigated sensitizers. The data presented in Table 1 suggest that the sensitizers with the cyanoacrylic acid group are more difficult to oxidize than the corresponding sensitizers with the malonic acid group. This is due to the more electron withdrawing nature of the cyanoacrylic acid group. The excited state oxidation potential of all four porphyrin sensitizers (Table 1) was found to be more negative than the energy level of the conduction band edge of TiO₂ (−0.8 V vs. SCE),³⁷ and the E_OX energy level is more positive than the redox potential of the iodine/iodide system (0.2 V vs. SCE)³⁸ for these dyes.

In DSSCs, the sensitizer promotes an electron to the excited state by absorbing a photon, and then injects the excited electron to the semiconductor on an ultra-rapid time scale. To gain insight into the electronic properties of the oxidized species of the porphyrin sensitizers, we carried out a spectro-electrochemical study.³⁹ Fig. 5 shows the spectral changes of PYR–Por–CA under an applied potential. During the controlled potential oxidation of PYR–Por–CA at 1.00 V, the absorption of both the Soret and Q bands decrease in intensity without any shift, while a new band appears at 11 000 cm⁻¹. Prolonged oxidation at 1.0 V results in a further decrease in the intensity of the Soret band and the disappearance of the Q bands. On the other hand, the bands in the spectral region corresponding to the donor pyrene moiety show an increase in intensity. During this process, isosbestic points were initially observed, indicating that oxidation gives a single product, however at longer times these were lost. These characteristic changes indicate the formation of the porphyrin cation radical.⁴⁰ The electrochemical oxidation is not fully reversible under these conditions as the porphyrin cation generated at +1.00 V cannot be fully recovered to its neutral form when the applied potential is changed to +0.2 V. Thus, although the oxidation was observed to be reversible during cyclic voltammetry, during the longer timescale of the spectroelectrochemical study some degradation was observed. The species after spectroelectrochemistry still displays features characteristic of a porphyrin, however it is presumed that some chemical change to the substituent groups has occurred. We note that during the operation of the solar cell, the sensitizer remains in the oxidised form for a much shorter time and indeed, PYR–Por–CA shows greater oxidative stability during spectroelectrochemistry than the well-known Ru-sensitizer N719 under similar conditions.^40,41 Similar spectral changes are observed in the PYR–Por–MA, FLU–Por–CA and FLU–Por–MA sensitizers (see ESI†).

Fig. 5 In situ UV-Vis spectral changes of PYR–Por–CA at an applied potential of 1.0 V.

Quantum mechanical calculations

In order to obtain insights into the effect of differing donor (pyrene or fluorene) and acceptor (CA or MA) groups on the electrochemical, optical and geometrical properties of these new porphyrin sensitizers, we have performed TD-DFT calculations using the B3LYP/6-31G(d) level in the dichloromethane polarisable continuum model (PCM) solvent phase. The molecular orbital analysis of the frontier orbitals of PYR–Por–CA is illustrated in Fig. 6, while their energy levels are summarized in Table 3. The electron distribution pattern in Fig. 6 suggests that the first two HOMOs are essentially porphyrin ring-centred with slight electron delocalization onto the donor pyrene and on the thiophene part of the anchoring group, whereas in the HOMO−2, the delocalization was mainly on the donor pyrene along with a contribution from the macrocyclic ring. In contrast, the electron density of the LUMO level was mainly localized on the thiophene–cyanoacrylic acid moiety, anchoring group and on the ethynyl linker, with a considerable contribution from the porphyrin ring as well. In contrast, for LUMO+1, the electron density was exclusively distributed on the porphyrin ring, and for LUMO+2 there was delocalization of the electron density on both the porphyrin and the ethynyl thiophene linker. The above discussion suggests that the porphyrin unit is mainly responsible for the first oxidation process, although with a notable contribution from the thiophene adjacent to the anchoring group. Similarly, the first reduction process in each of the reported porphyrins was contributed by charge delocalization on both the ethynyl thiophene acrylic acid groups and the porphyrin macrocyclic ring. The pyrene unit plays a negligible role in the frontier orbitals, suggesting that its potential role as an electron donating unit is not realised in practice, which is consistent with the small change in redox potential between the fluorene and pyrene analogues. This may be detrimental in terms of charge separation following electron transfer to TiO₂, since a noticeable component of the positive charge density on the dye will be distributed close to the TiO₂, which can promote charge recombination. Fig. 7 depicts the energy level diagram of the D–π–A porphyrins with the conduction band (CB) of TiO₂, as well as the redox energy of I⁻/I₃⁻. As suggested in Fig. 7, all four new porphyrin sensitizers should be capable of injecting electrons to the CB of TiO₂ upon excitation. More importantly, electron injection from the sensitizers to TiO₂ should be more favourable for PYR–Por–MA than the other sensitizers owing to the higher LUMO levels.

Fig. 6 Frontier orbitals of PYR–Por–MA calculated using B3LYP/6-31G(d) within dichloromethane PCM.

Table 3 Percentage contributions from the components of PYR–Por–CA to selected molecular orbitals. Also quoted are the calculated energies for these molecular orbitals. (Ar-based = trimethoxyaryl unit, S-based = thiophene–cyanoacetic acid unit)

MO	MO energy (eV)	% Contribution from
		Zn-based	Porphyrin-based	Pyrene-based	S-based	Ar-based
HOMO−2	−5.52	0.01	15.66	82.80	0.16	1.37
HOMO−1	−5.47	0	79.93	12.91	0.04	7.12
HOMO	−5.22	0.84	62.12	3.17	23.46	10.41
LUMO	−3.09	0.16	32.48	1.86	63.91	1.59
LUMO+1	−2.44	0.23	88.60	1.42	0	9.75
LUMO+2	−2.37	0.14	53.27	3.21	41.23	2.15

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Fig. 7 Energy level diagram of the D–π–A dyes.

In Fig. 8 we report the calculated absorption spectrum of PYR–Por–CA in dichloromethane solvent. All of the calculated electronic transition energies, along with their oscillator strengths and molecular orbital compositions of the dyes, are summarized in the ESI.† In general, the calculations show excellent agreement with the experimental spectra, although the Q-band, assigned to the HOMO–LUMO transition (>80%), shows a higher than expected intensity.

Fig. 8 The red dashed curve is the calculated spectrum of PYR–Por–CA in dichloromethane and the solid columns are the calculated electronic transitions.

Photovoltaic measurements

Fig. 9 shows the performance of the DSSCs of different sensitizers on the basis of their steady-state current–voltage characteristics. Table 4 summarizes the key cell parameters for DSSCs as a function of the different porphyrin sensitizers. The DSSC parameters are significantly influenced by the porphyrin sensitizers. The maximum conversion efficiency was achieved for the cells sensitized with PYR–Por–MA. It shows increased J_sc and V_oc effects over the other sensitizers as the donor PAH plays no significant role in the frontier orbitals. However, the overall conversion efficiency is less than the controlled cells made with the standard N719 sensitizer. The reason for the decreased efficiency compared to the previously reported acene-modified dyes can be deduced as follows. The flaccidity of the acrylic bond causes the dye molecule anchored on TiO₂ to be inclined with respect to the adjacent dye molecule, consequently reducing the extent of dye adsorption on TiO₂ (FF), and henceforth results in decreased IPCE and current density values.⁴² Moreover there is also a chance of an increased recombination rate at the TiO₂/electrolyte interface, due to the possibility of halogen bonding between iodine in the electrolyte and some electron-rich segments like the sulfur atom in the thiophene moiety.⁴³ The presence of long side chains may be an added advantage to minimize dye aggregation but still they may lead to surface blocking, resulting in low dye uptake.^44,45 The reduced V_oc values of certain zinc porphyrin dyes may be due to a faster recombination rate between TiO₂ electrons and the acceptor species in the I⁻/I₃⁻ redox electrolyte and also by means of other deactivation pathways which compete and lower the electron injection efficiency.⁴⁵ The introduction of a twisted spacer as an alternative to the rigid benzoic acid anchoring group may interrupt the overall conjugation in the dye molecule and hence weaken the ICT interaction, and also replacing a phenyl spacer with a thiophene moiety may also lead to a decrease in the total absorption cross-section by half, which will consequently lead to poor photovoltaic performance.^43,46,47

Fig. 9 J–V characteristics of the DSSCs constructed with different porphyrin sensitizers under 1 sun illumination.

Table 4 Cell parameters of the DSSCs made with different sensitizers (1 sun illumination)

Compound	Voc^a (mV)	Jsc^a (mA cm^−2)	FF^a (%)	η (%)
a Error limits: Jsc: ±0.20 mA cm^−2, Voc = ±30 mV, FF = ±0.03.
PYR–Por–CA	439	4.59	67.15	1.35
PYR–Por–MA	525	8.19	73.21	3.14
FLU–Por–CA	443	3.04	68.92	0.92
FLU–Por–MA	424	4.21	71.88	1.28
N719^48	622	18.1	73.79	8.35

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Thermal stability

Finally, we studied the thermal stability of these porphyrin sensitizers by using thermogravimetric analysis. This thermal stability is essential for rooftop applications of DSSC devices. It is well known in the literature that tetraphenylporphyrin and its metallo-derivatives are thermally stable up to 400 °C. Fig. 10 shows the thermal behaviour of PYR–Por–CA. It is clear that the sensitizer PYR–Por–CA is stable up to 250 °C. The initial weight loss (∼2%) observed between 200 and 250 °C is attributed to the removal of the carboxyl group. A similar trend in thermal stability was also obtained in the other sensitizers of this series (see ESI†). It is clear from the thermal data that these dyads are highly durable for longstanding outdoor applications.

Fig. 10 TG/DTG curves of PYR–Por–CA with a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere.

Conclusions

In conclusion, we have designed four unsymmetrical zinc porphyrins based on the D–π–A approach for DSSC applications. Both the Soret and Q band absorption of all four sensitizers are broadened and red-shifted. The emission maxima and excited state lifetimes were quenched when adsorbed onto nanocrystalline TiO₂. The electrochemical and spectroelectrochemical properties suggest that the first oxidation is localized on the porphyrin centre. Upon the photosensitization of nanocrystalline TiO₂, the pyrene-substituted sensitizers have shown efficiencies of up to 3.14%. The reason may be due to the more electron releasing nature of the pyrene derivatives than the corresponding fluorene derivatives. This results in the Soret band absorption being more red-shifted in the case of the pyrene derivatives as they can harvest more sunlight and have more favorable conditions for the injection of electrons from the excited state of the sensitizer to the TiO₂ conduction band. However, for the exact reasons, one has to perform the dynamic studies of these sensitizers in detail and also impedance spectroscopy. Such studies are currently in progress.

Acknowledgements

The authors are thankful to DST-EPSRC (UK) (‘APEX’) programme and the EPSRC supergen programme for financial support of this work. This work has made use of the resources provided by the EaStChem Research Computing Facility (http://www.eastchem.ac.uk/rcf). This facility is partially supported by the eDIKT initiative (http://www.edikt.org).

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Footnote

† Electronic supplementary information (ESI) available: detailed experimental procedure, absorption and emission spectra, fluorescence decay curves, spectro-electrochemical data and HOMO–LUMO energy level information available. See DOI: 10.1039/c3ra47948j

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