New low-bandgap polymetallaynes of platinum functionalized with a triphenylamine-benzothiadiazole donor–acceptor unit for solar cell applications

Abstract

Two new solution-processable platinum-containing polymetallayne polymers functionalized with both triphenylamine and 2,1,3-benzothiadiazole groups were synthesized and characterized by thermal, spectroscopic and electrochemical methods. Their corresponding diplatinum model complexes were also prepared for comparison. The organometallic polymers exhibit good thermal stability and intense low-energy absorption bands in the visible region and both of them are fluorescent red-emitting materials. The effect of thiophene addition along the polymer chain on the optical, luminescent and photovoltaic properties of these metallated materials was also examined. Bulk heterojunction solar cells based on these polymers were studied and the low-bandgap polymers with internal donor–acceptor–donor π-conjugated fragment can serve as a good electron donor for fabricating photovoltaic devices by blending the polymer with a methanofullerene electron acceptor. At the same donor : acceptor blend ratio of 1 : 4, the light-harvesting ability and solar cell efficiency notably increase with the incorporation of additional thiophene rings in the polymer. The best power conversion efficiency of 1.61% was achieved with the open-circuit voltage of 0.77 V, short-circuit current density of 4.94 mA cm⁻² and fill factor of 0.39 under illumination of an AM 1.5 solar cell simulator.

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1. Introduction

The area of sustainable energy is a hot topic in the 21st century. As the most abundant, renewable and clean energy source on the Earth, solar energy is one of the best candidates for solving the worldwide energy crisis.¹ Novel materials with lower energy gaps (E_g) need to be developed to improve the coverage of the solar spectrum and consequently improve the efficiency. Organic polymer solar cells (PSCs) offer a promising route for low-cost, environmentally friendly energy generation, provided their efficiencies can be improved to a workable level.² The bulk heterojunction (BHJ) concept has improved the power conversion efficiency (PCE) of the PSCs significantly by forming a donor–acceptor (D–A) bicontinuous interpenetrated network, which creates large interfacial areas between the polymers and electron acceptors (e.g.fullerene derivatives), thus leading to efficient photoinduced charge separation in a device.³ In this context, low-bandgap conjugated polymers have shown great promise for photoinduced charge generation in BHJ cells to provide solution-processed fabrication for low-cost photovoltaic devices to capture a larger portion of solar energy. A key characteristic of conjugated polymers is the easy modification of their molecular structure and properties by side chain substitution or copolymerization. Different electron donor (D) and electron acceptor (A) moieties with distinct electronic properties have been selected to generate low-bandgap polymers.² Recently, triphenylamine- and 2,1,3-benzothiadiazole-containing molecules and polymers have attracted much scientific attention because of their intriguing electronic and optical properties and they are widely used in light-emitting diodes (LEDs),⁴ solar cells⁵ and two-photon absorption studies.⁶Aromatic amines such as triphenylamine (TPA) are good hole-transporting chromophores and are commonly used as an electron donor to afford promising photosensitizing materials with improved hole mobility.⁷2,1,3-Benzothiadiazole and its derivatives are known to be strongly fluorescent dyes, which typically exhibit large Stokes shifts.^{4c, 8} Moreover, an electron-withdrawing 2,1,3-benzothiadiazole unit was used as a useful spacer unit in producing semiconducting organic materials.⁹

Although recent advances in organic photovoltaics are largely based on conjugated organic polymers, a possible alternative approach for harvesting sunlight to generate electrical power involves putting metals into organic-based polymers.¹⁰ Although organometallic donor materials are commonly used in small-molecule solar cells,¹¹ soluble π-conjugated metallopolymers used in high-performance polymer solar cells are still rare to date. Metallopolyynes and their derivatives have recently emerged as one of the most promising photovoltaic polymers due to their high solubility, good thermal stability and diverse structural variability.¹² Towards our goal in designing metallopolyyne polymers with low-bandgap π-conjugation for solar cell applications, two new platinum-acetylide polymers consisting of a donor-π-bridge-acceptor-π-bridge-donor (D-π-A-π-D) motif were designed and prepared on the basis of a combination of the 2,1,3-benzothiadiazole unit and two terminal electron-donating TPA groups by π-conjugated spacers to enhance the absorption wavelength and intramolecular charge transfer (ICT) character. BHJ solar cells fabricated from these polymers gave the best power conversion efficiency (PCE) of 1.61% under illumination of an AM 1.5 solar cell simulator. We believe that the present study provides valuable information for the production of new polymer light-harvesting materials for power generation.

2. Experimental

2.1. Materials

Iodine, periodic acid, Pd(OAc)₂, CuI, PPh₃, K₂CO₃ and trimethylsilylacetylene were purchased from commercial sources and used as received unless otherwise specified. [6,6]-Phenyl C₆₁-butyric acid methyl ester (PCBM) was purchased from American Dyes. Compounds L1-H,^{4b, 4c}L2-H,^{4b, 4c, 4d}trans-[Pt(PEt₃)₂PhCl]^13a and trans-[Pt(PBu₃)₂Cl₂]^13b were prepared using the methods reported in the literature. Analytical grade solvents were purified by distillation over appropriate drying agents under an inert nitrogen atmosphere prior to use.

2.2. Characterization

The positive-ion fast atom bombardment (FAB) mass spectra were recorded in m-nitrobenzyl alcohol matrices on a Finngin-MAT SSQ710 mass spectrometer. Infrared spectra were recorded on the Nicolet Magna 550 Series II FTIR spectrometer, using KBr pellets for solid state spectroscopy. NMR spectra were measured in deuterated solvents as the lock and reference on a JEOL JNM-EX270 FT NMR system or a Bruker AV 400 MHz FT-NMR spectrometer, with ¹H, and ¹³C NMR chemical shifts quoted relative to Me₄Si standard and ³¹P chemical shifts relative to an 85% H₃PO₄ external reference. Electronic absorption spectra were obtained with a Hewlett Packard 8453 spectrometer. Optical bandgaps were calculated from the onset of the absorption band. The CV measurements were carried out at a scan rate of 100 mV s⁻¹ using a eDAQ EA161 potentiostat electrochemical interface equipped with a thin film coated ITO covered glass working electrode, a platinum counter electrode and a Ag/AgCl (in 3 M KCl) reference electrode. The solvent in all measurements was deoxygenated MeCN, and the supporting electrolyte was 0.1 M [ⁿBu₄N][PF₆]. The oxidation and reduction potentials were used to determine the HOMO and LUMO energy levels using the equations E_HOMO = [−(Eox(vs._Ag/AgCl) − E(N.H.E.vs._Ag/AgCl)] − 4.50 eV and E_LUMO = [–(Ered(vs._Ag/AgCl) − E(N.H.E.vs._Ag/AgCl)] − 4.50 eV, where the potentials for N.H.E. versus vacuum and N.H.E. versusAg/AgCl are 4.50 and −0.22 V, respectively.¹⁴ Thin polymer films were deposited on the working electrode by dip coating in chlorobenzene solution (6 mg cm⁻³). The molecular weights of the polymers were determined by GPC (HP 1050 series HPLC with visible wavelength and fluorescent detectors) using polystyrene standards and thermal analysis was performed with a Perkin-Elmer TGA6 thermal analyzer.

2.3. Synthesis

2.3.1. L1-I . A mixture of L1-H (312 mg, 0.48 mmol), iodine (106 mg, 0.41 mmol) and periodic acid (39 mg, 0.17 mmol) were dissolved in 95% ethanol (25 mL) and CCl₄ (25 mL). The reaction mixture was heated to reflux for 10 h. After cooling to room temperature, the solvent was removed and saturated NaHSO₃ was added. The precipitate was collected and washed with cold ethanol and dried to provide crude L1-I (390 mg, 0.43 mmol, 90%) as an orange solid. FAB-MS (m/z): 901.9 [M]⁺. The crude product was difficult to be purified and was used for the Sonogashira reaction without further purification.

2.3.2. L2-I . The compound was prepared in a similar way as for L1-I except that L2-H was used instead. Purple solid. Eluent: hexane/CH₂Cl₂ (1 : 2, v/v). Yield: 34%. ¹H NMR (400 MHz, CDCl₃): δ = 8.11 (d, J = 4.0 Hz, 2H, Ar), 7.86 (s, 2H, Ar), 7.57–7.54 (m, 4H, Ar), 7.53–7.51 (m, 4H, Ar), 7.33 (m, 2H, Ar), 7.13–7.11 (m, 4H, Ar), 7.08–7.02 (m, 8H, Ar), 6.87–6.85 (m, 4H, Ar), 2.34 (s, 6H, CH₃) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 152.59, 147.43, 147.10, 145.32, 144.22, 138.11, 133.94, 130.23, 128.69, 128.38, 126.67, 125.65, 125.46, 125.34, 125.19, 123.51, 123.34, 85.00 (Ar), 20.90 (CH₃) ppm. FAB-MS (m/z): 1066.0 [M]⁺.

2.3.3. L1-TMS . To an ice-cooled mixture of L1-I (174 mg, 0.19 mmol) in freshly distilled triethylamine (10 mL) and CH₂Cl₂ (10 mL) solution under nitrogen was added Pd(OAc)₂ (10 mg), PPh₃ (30 mg) and CuI (10 mg). After the solution was stirred for 30 min, trimethylsilylacetylene (0.16 mL, 1.14 mmol) was then added and the suspension was stirred for another 30 min in the ice bath before being warmed to room temperature. After reacting for 30 min at room temperature, the mixture was refluxed for 4 h. The solvents were removed on a rotary evaporatorin vacuo. The crude product was purified by column chromatography on silica gel eluting with hexane–CH₂Cl₂ (2 : 1, v/v) to provide L1-TMS (140 mg, 0.17 mmol, 86%) as an orange solid. ¹H NMR (400 MHz, CDCl₃): δ = 7.88 (d, J = 8.7 Hz, 4H, Ar), 7.74 (s, 2H, Ar), 7.34 (d, J = 8.7 Hz, 4H, Ar), 7.20 (d, J = 8.7 Hz, 4H, Ar), 7.14 (m, 4H, Ar), 7.10–7.03 (m, 8H, Ar), 2.35 (s, 6H, CH₃), 0.24 (s, 18H, Si(CH₃)₃) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 154.12, 147.85, 147.42, 144.26, 134.12, 132.99, 132.17, 131.56, 130.24, 129.98, 127.53, 125.98, 123.48, 122.48, 116.22 (Ar), 105.39, 93.20 (C≡C), 20.93 (CH₃), 0.08 (Si(CH₃)₃) ppm. FAB-MS (m/z): 842.5 [M]⁺. Anal. Calcd for C₅₄H₅₀N₄SSi₂: C, 76.92; H, 5.98; N, 6.64. Found: C, 77.08; H, 6.15; N, 6.88.

2.3.4. L2-TMS . This compound was made using the same approach as for L1-TMS but L2-I was used instead. Purple solid. Eluent: hexane/CH₂Cl₂ (4 : 1, v/v). Yield: 77%. ¹H NMR (400 MHz, CDCl₃): δ = 8.11 (m, 2H, Ar), 7.86 (s, 2H, Ar), 7.57–7.55 (m, 4H, Ar), 7.34–7.32 (m, 6H, Ar), 7.13–7.07 (m, 8H, Ar), 7.04–6.98 (m, 8H, Ar), 2.34 (s, 6H, CH₃), 0.24 (s, 18H, Si(CH₃)₃) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 152.60, 147.75, 147.00, 145.32, 144.17, 138.00, 134.06, 132.99, 130.22, 128.69, 128.59, 126.66, 125.72, 125.67, 125.20, 123.97, 123.38, 122.31, 116.18 (Ar), 105.35, 93.23 (C≡C), 20.92 (CH₃), 0.08 (Si(CH₃)₃) ppm. FAB-MS (m/z): 1006.2 [M]⁺. Anal. Calcd for C₆₂H₅₄N₄S₃Si₂: C, 73.91; H, 5.40; N, 5.56. Found: C, 74.12; H, 5.32; N, 5.68.

2.3.5. L1 . A mixture of L1-TMS (94 mg, 0.11 mmol) and K₂CO₃ (138 mg, 1.00 mmol) in a solution mixture of methanol (6 mL) and CH₂Cl₂ (30 mL), under a nitrogen atmosphere, was stirred at room temperature overnight. The mixture was added to CH₂Cl₂ (30 mL), washed with water (20 mL) three times and dried over anhydrous Na₂SO₄. The solvents were removed on a rotary evaporatorin vacuo. The crude product was purified by column chromatography on silica gel eluting with hexane–CH₂Cl₂ (2 : 1, v/v) to provide L1 (57 mg, 0.08 mmol, 72%) as an orange solid. ¹H NMR (400 MHz, CDCl₃): δ = 7.89 (d, J = 8.7 Hz, 4H, Ar), 7.75 (s, 2H, Ar), 7.37–7.35 (m, 4H, Ar), 7.23–7.21 (m, 4H, Ar), 7.16–7.09 (m, 8H, Ar), 7.07–7.05 (m, 4H, Ar), 3.04 (s, 2H, C≡CH), 2.36 (s, 6H, CH₃) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 154.10, 148.13, 147.36, 144.19, 134.23, 133.11, 132.16, 131.66, 130.28, 130.01, 127.53, 126.07, 123.57, 122.36, 115.01 (Ar), 83.92, 76.30 (C≡C), 20.94 (CH₃) ppm. FAB-MS (m/z): 698.4 [M]⁺. IR (KBr) (cm⁻¹): ν_(C≡C–H): 3259; ν_(C≡C): 2103. Anal. Calcd for C₄₈H₃₄N₄S: C, 82.49; H, 4.90; N, 8.02. Found: C, 82.60; H, 5.07; N, 7.88.

2.3.6. L2 . The same method as for L1 was employed to prepare L2 using L2-TMS as the starting material. Purple solid. Eluent: hexane/CH₂Cl₂ (1 : 1, v/v). Yield: 96%. ¹H NMR (400 MHz, CDCl₃): δ = 8.11 (d, J = 4.0 Hz, 2H, Ar), 7.88 (s, 2H, Ar), 7.59–7.57 (m, 4H, Ar), 7.35 (m, 6H, Ar), 7.12–7.09 (m, 8H, Ar), 7.06–7.00 (m, 8H, Ar), 3.04 (s, 2H, C≡CH), 2.35 (s, 6H, CH₃) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 152.61, 148.05, 146.95, 145.31, 144.12, 138.05, 134.18, 133.12, 130.27, 128.71, 126.70, 125.82, 125.69, 124.08, 123.43, 122.19, 114.97 (Ar), 83.88, 76.30 (C≡C), 20.93 (CH₃) ppm. FAB-MS (m/z): 861.9 [M]⁺. IR (KBr) (cm⁻¹): ν_(C≡C–H): 3266; ν_(C≡C): 2098. Anal. Calcd for C₅₆H₃₈N₄S₃: C, 77.93; H, 4.44; N, 6.49. Found: C, 78.12; H, 4.40; N, 6.69.

2.3.7. P1 . To a stirred mixture of L1 (50.1 mg, 0.07 mmol) and trans-[Pt(PBu₃)₂Cl₂] (48.1 mg, 0.07 mmol) in freshly distilled triethylamine (20 mL) and CH₂Cl₂ (20 mL) solution was added CuI (5 mg). The solution was stirred at room temperature for 24 h under a nitrogen atmosphere. The solvents were removed on a rotary evaporatorin vacuo. The residue was redissolved in CH₂Cl₂ and filtered through a short aluminium oxide column using the same eluent to remove ionic impurities and catalyst residue. After removal of the solvents, the crude product was washed with hexane three times followed by methanol three times and then repeated precipitation from CH₂Cl₂–hexane (or CH₂Cl₂–methanol) and drying in vacuo to afford polymer P1 (72 mg, 79%) as a red solid. ¹H NMR (400 MHz, CDCl₃): δ = 7.85 (d, J = 8.7 Hz, 4H, Ar), 7.72 (s, 2H, Ar), 7.19–7.16 (m, 8H, Ar), 7.10 (m, 8H, Ar), 7.01 (d, J = 8.4 Hz, 4H, Ar), 2.34 (s, 6H, CH₃), 2.17–2.14 (m, 12H, PC₄H₉), 1.62–1.58 (m, 12H, PC₄H₉), 1.48–1.43 (m, 12H, PC₄H₉), 0.95–0.91 (m, 18H, PC₄H₉) ppm. ³¹P NMR (161 MHz, CDCl₃): δ = 2.78 (¹J_P–Pt = 2350 Hz) ppm. IR (KBr) (cm⁻¹): ν_(C≡C): 2098. Anal. Calcd for C₇₂H₈₆N₄P₂SPt: C, 66.70; H, 6.69; N, 4.32. Found: C, 66.98; H, 6.86; N, 4.20. GPC (THF): M_w = 9550, M_n = 6840, PDI = 1.40.

2.3.8. P2 . Using the above procedure for P1, P2 can be prepared similarly from L2. Purple solid. Yield: 60%. ¹H NMR (400 MHz, CDCl₃): δ = 8.11 (d, J = 3.6 Hz, 2H, Ar), 7.86 (s, 2H, Ar), 7.54 (d, J = 8.5 Hz, 4H, Ar), 7.31 (d, J = 3.7 Hz, 2H, Ar), 7.17 (d, J = 8.4 Hz, 4H, Ar), 7.11–7.03 (m, 12H, Ar), 6.96 (d, J = 8.4 Hz, 4H, Ar), 2.33 (s, 6H, CH₃), 2.17–2.13 (m, 12H, PC₄H₉), 1.62–1.59 (m, 12H, PC₄H₉), 1.48–1.43 (m, 12H, PC₄H₉), 0.95–0.91 (m, 18H, PC₄H₉) ppm. ³¹P NMR (161 MHz, CDCl₃): δ = 2.79 (¹J_P–Pt = 2350 Hz) ppm. IR (KBr) (cm⁻¹): ν_(C≡C): 2096. Anal. Calcd for C₈₀H₉₀N₄P₂S₃Pt: C, 65.78; H, 6.21; N, 3.84. Found: C, 65.99; H, 6.12; N, 4.04. GPC (THF): M_w = 10690, M_n = 6540, PDI = 1.63.

2.3.9. M1 . To a stirred mixture of L1 (12.8 mg, 0.018 mmol) and trans-[Pt(PEt₃)₂PhCl] (23.8 mg, 0.042 mmol) in freshly distilled triethylamine (12 mL) and CH₂Cl₂ (12 mL) was added CuI (2.0 mg). The solution was stirred at room temperature under nitrogen over a period of 24 h. After removal of the solvent, the crude product was purified by column chromatography on silica gel eluting with hexane–CH₂Cl₂ (1 : 1, v/v) to give M1 (18.0 mg, 0.010 mmol, 59%) as a red solid. ¹H NMR (400 MHz, CDCl₃): δ = 7.84 (d, J = 8.8 Hz, 4H, Ar), 7.72 (s, 2H, Ar), 7.34–7.32 (m, 4H, Ar), 7.22–7.16 (m, 8H, Ar), 7.10 (s, 8H, Ar), 7.03–7.00 (m, 4H, Ar), 6.98–6.94 (m 4H, Ar), 6.82–6.78 (m, 2H, Ar), 2.34 (s, 6H, CH₃), 1.80–1.73 (m, 24H, PC₂H₅), 1.14–1.06 (m, 36H, PC₂H₅) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 154.18, 148.10, 144.81, 144.16, 139.21, 135.21, 133.03, 132.09, 131.78, 130.18, 129.94, 129.72, 127.27, 125.25, 125.01, 124.47, 124.34, 121.95, 121.16 (Ar), 112.37, 109.73 (C≡C), 20.88 (CH₃), 15.23, 15.06, 14.89, 8.05 (PC₂H₅) ppm. ³¹P NMR (161 MHz, CDCl₃): δ = 9.86 (¹J_Pt–P = 2626 Hz) ppm. FAB-MS (m/z): 1713.7 [M]⁺. IR (KBr) (cm⁻¹): ν_(C≡C): 2094. Anal. Calcd for C₈₄H₁₀₂N₄P₄SPt₂: C, 58.87; H, 6.00; N, 3.27. Found: C, 59.04; H, 5.78; N, 3.21.

2.3.10. M2 . Using the above procedure for M1, M2 can be prepared similarly from M2. Purple solid. Eluent: hexane/CH₂Cl₂ (1 : 1, v/v). Yield: 82%. ¹H NMR (400 MHz, CDCl₃): δ = 8.11 (d, J = 4.0 Hz, 2H, Ar), 7.86 (s, 2H, Ar), 7.54–7.52 (m, 4H, Ar), 7.34–7.30 (m, 6H, Ar), 7.22–7.20 (m, 4H, Ar), 7.08–7.04 (m, 12H, Ar), 6.98–6.94 (m, 8H, Ar), 6.82–6.78 (m, 2H, Ar), 2.33 (s, 6H, CH₃), 1.80–1.74 (m, 24H, PC₂H₅), 1.38–1.06 (m, 36H, PC₂H₅) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 156.50, 152.61, 147.80, 145.69, 144.73, 144.07, 139.21, 137.58, 135.21, 133.00, 128.68, 127.26, 127.72, 126.46, 125.62, 125.14, 125.01, 124.29, 122.96, 122.44, 121.17 (Ar), 112.48, 109.69 (C≡C), 20.87 (CH₃), 15.23, 15.06, 14.89, 8.05 (PC₂H₅) ppm. ³¹P NMR (161 MHz, CDCl₃): δ = 9.87 (¹J_Pt–P = 2626 Hz) ppm. FAB-MS (m/z): 1879.7 [M + 1]⁺. IR (KBr) (cm⁻¹): ν_(C≡C): 2093. Anal. Calcd for C₉₂H₁₀₆N₄P₄S₃Pt₂: C, 58.84; H, 5.69; N, 2.98. Found: C, 58.98; H, 5.84; N, 3.23.

2.4. X-Ray crystallography

Crystals of L2-H suitable for X-ray diffraction studies were grown by slow evaporation of its solution in a CH₂Cl₂–hexane mixture at room temperature. X-Ray diffraction data were collected at 173 K using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) on a Bruker Axs SMART 1000 CCD diffractometer. The collected frames were processed with the software SAINT+¹⁵ and an absorption correction (SADABS)¹⁶ was applied to the collected reflections. The space group was determined from the systematic absences and Laue symmetry check and confirmed by successful refinement of the structure. The structure was solved by the direct method (SHELXTL)¹⁷ in conjunction with standard difference Fourier techniques and subsequently refined by full-matrix least-squares analyses on F². Hydrogen atoms were generated in their idealized positions and all non-hydrogen atoms were refined anisotropically. Crystal data for L2-H: C₅₂H₃₈N₄S₃, M_w = 815.04, monoclinic, space groupP2₁/c, a = 11.2608(7), b = 17.823(1), c = 20.189(1) Å, β = 98.705(1)°, V = 4005.4(4) ų, Z = 4, ρ_c = 1.352 Mg m⁻³, μ(Mo-Kα) = 0.229 mm⁻¹, F(000) = 1704, T = 173 K. 24702 reflections measured, of which 9673 were unique (R_int = 0.0372. Final R₁ = 0.0575 and wR₂ = 0.1505 for 6788 observed reflections with I > 2σ(I). CCDC number 801106.‡

2.5. Fabrication and characterization of bulk heterojunction solar cells

The device structure was ITO/poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/polymer:PCBM blend/Al. Indium tin oxide (ITO) coated glass substrates (10 Ω per square) were cleaned by sonication in toluene, acetone, ethanol, and deionized water, dried in an oven, and then cleaned with UV ozone for 300 s. As-received PEDOT:PSS solution was passed through the 0.45 μm filter and spin-coated on patterned ITO substrates at 5000 rpm for 3 min, followed by baking in N₂ at 150 °C for 15 min. P1–P2 : PCBM (1 : 4 by weight) active layer was prepared by spin-coating the toluene solution (4 mg mL⁻¹ of metallopolyyne, 16 mg mL⁻¹ of PCBM) at 1000 rpm for 2 min. The substrates were dried at room temperature under low vacuum (vacuum oven) for 1 h and then stored under high vacuum (10⁻⁵ to 10⁻⁶ Torr) overnight. An Al electrode (100 nm) was evaporated through a shadow mask to define the active area of the devices (2 mm² circle). All the fabrication procedures (except drying, PEDOT:PSS annealing, and Al deposition) and cell characterization were performed in air. PCE was determined from J–V curve measurement (using a Keithley 2400 sourcemeter) under white light illumination (at 100 mW cm⁻¹). For white light efficiency measurements, an Oriel 66002 solar light simulator with an AM1.5 filter was used. The light intensity was measured by a Molectron Power Max 500D laser power meter.

3. Results and discussion

3.1. Synthesis and characterization

The synthetic routes of the monomers, Pt model compounds and polymers are outlined in Scheme 1. The compounds L1-H and L2-H were synthesized by the Stille coupling of 4,7-dibromo-2,1,3-benzothiadiazole and the appropriate tributylstannyl derivatives with high synthetic yields. Conversion of L1-I and L2-I to their corresponding diethynyl congeners L1 and L2 can be achieved following the typical organic synthetic routes for alkynylation of aromatic halides.¹⁸ The two new metallopolyyne polymers (P1 and P2) and their discrete model compound (M1 and M2) were synthesized via the Sonogashira-type dehydrohalogenation reaction between L1 or L2 and a suitable platinum chloride precursor (Scheme 1).¹⁹ The feed mole ratio of the platinum precursors and the diethynyl ligands were 1 : 1 and 2 : 1 for the polymer and dimer syntheses, respectively. P1 and P2 are thermally and air stable and soluble in common chlorinated hydrocarbons and toluene. P1 and P2 were purified by alumina column chromatography and repeated precipitation, leading to red to purple powders of the polymers in good yield and high purity.

Scheme 1 Synthetic routes to the platinum-containing polymers and dinuclear model compounds.

The chemical structures of the Pt complexes and polymers were verified by NMR (¹H and ³¹P) and IR spectroscopy. Fig. 1 shows the ¹H and ³¹P NMR spectra of polymers P1 and P2. The signals at 8.11–6.95 ppm are assigned to the resonances of protons on the aryl rings. The single sharp peak at 2.34 ppm belongs to the protons of CH₃ groups and the characteristic peaks at 2.17–0.92 ppm are attributed by the PBu units. The single ³¹P–{¹H} NMR signal flanked by platinum satellites for each of the trans-platinum(II) complexes is consistent with a trans geometry of the Pt(PBu₃)₂ group in a square-planar geometry. The ¹J_P–Pt values of 2350 Hz for the PBu₃ moieties are typical of those for related trans-PtP₂ systems.²⁰ All ligands and Pt compounds were subjected to IR measurements in which vibrational frequencies of the acetylenic functional groups were detected. In the IR spectra of the diethynyl ligands, L1 and L2 display IRν_(C≡C) absorption at around 2100 cm⁻¹. The corresponding terminal acetylenic C≡C–H stretching vibrations occur at 3259 and 3266 cm⁻¹. The C≡C stretching frequencies for P1 and P2 are located at 2098 and 2096 cm⁻¹, which are lower than those for the free alkynyl precursors, in line with a higher degree of conjugation in the former.²¹ The molecular structure of L2-H was also confirmed by X-ray crystallography and Fig. 2 depicts a perspective drawing of the molecule.

Fig. 1 ¹H NMR spectra of polymers P1 and P2 in CH₂Cl₂, The inset shows their ³¹P NMR spectra.

Fig. 2 Molecular structure of L2-H, with the thermal ellipsoids shown at the 25% probability level.

The molecular weights of polymers were determined by gel-permeation chromatography (GPC) (see Experimental). The weight-average molecular weights (M_w) of P1 and P2 calibrated against polystyrene standards are 9550 and 10690, respectively. The relatively small polydispersity index (PDI ≈ 1.40–1.63) in molecular weights is consistent with the proposed linear structure from the condensation polymerization. P1 and P2 can possess up to ∼25–28 heterocyclic rings in total along the polymer chain based on number-average molecular weight (M_n). The thermal properties of the polymers were also examined by thermal gravimetric analysis (TGA) under nitrogen (Table 1). P1 and P2 exhibit good thermal stability with the decomposition onsets at ∼355 and 346 °C, respectively, which are higher than those for trans-[–Pt(PBu₃)₂C≡CArC≡C–]_n (Ar = C₆H₄, 300 °C,^{19b, 19c}anthrylene, 315 °C,²² oligothienylene, 278–290 °C,²³etc.).

Table 1 Photophysical data of the new platinum compounds in CH₂Cl₂

	Absorption (293 K)^a	Emission (293 K)				Emission (77 K)
	λ abs/nm	E g/eV^b	λ em/nm	Φ ^c	τ/ns	λ em/nm	τ/ns
a Molar extinction coefficients εmax (10^4 dm^3 mol^−1 cm^−1) are shown in parentheses. b Optical bandgaps determined from the onset of absorption in solution phases. c Quantum yields determined in CH2Cl2 solution with a CH3CN solution of [Ru(bipy)3](PF6)2 as a standard (Φ = 0.062, λex = 450 nm).^29 d Not determined.
M1	348 (9.7), 484 (2.3)	2.09	651	^d	0.69	613	8.11
M2	342 (7.8), 372 (7.2), 542 (4.2)	1.86	704	^d	0.76	677	2.60
P1	363, 484	2.06	665	0.012	0.66	617	8.92
P2	380, 539	1.85	716	0.018	2.84	708	5.16

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3.2. Optical properties

The photophysical properties of all new diethynyl ligands and their metal complexes were investigated by UV/Vis and photoluminescence (PL) spectroscopy in CH₂Cl₂ solution (Table 1). Fig. 3 shows the absorption and PL spectra of the polymers in CH₂Cl₂ solution. All of the compounds were characterized by two major bands in the absorption spectra. The first one peaking at around 319 nm for L1 and its metal congeners is due to the absorption feature of conjugated arylamine-benzothiadiazole segment. The first peak of L2 and its metal compounds, in the range of 334–369 nm, is attributed to both the absorption of conjugated aromatic segment and π–π* transition in the TPA-thiophene core.²⁴ The second broad absorption peak beyond 400 nm can be assigned to the π–π* transition in the conjugated organic system. As compared to the free alkynes, there is a slight red shift in the absorption wavelength for their corresponding platinum compounds. To enhance π-conjugation relative to L1, M1 and P1, the derivatives L2, M2 and P2 having a thiophene moiety as an additional spacer between the benzothiadiazole ring and the N,N-disubstituted amino group were prepared. The low-bandgap property is attributed to the electron-accepting nature of the benzothiadiazole unit, which causes significant ICT even in the ground state by the combination with the electron-releasing amino groups connected with or without the π-conjugated thiophene spacer. The absorption band of P2 can cover a wide range of 300–700 nm with a correspondingly lower HOMO–LUMO gap of 1.85 eV (versus 2.06 eV for P1).

Fig. 3 Normalized absorption and emission spectra of (a) P1 and (b) P2 in CH₂Cl₂ at 293 K.

All of the platinum complexes show weak red to near-infrared emission bands in CH₂Cl₂ solution at room temperature by direct one-photon excitation (Table 1 and Fig. 3). In general, red luminescent polymers usually show a low emission efficiency because red chromophores are prone to aggregation in the solid state and are highly susceptible to concentration quenching.²⁵P1 shows an emission peak at 665 nm with the lifetime of 0.66 ns at 293 K in dilute solution. Compared to P1, polymer P2 can show a weak emission peak at 716 nm. Due to their similar emission pattern, we consider that the emission features in the metal-free ligand precursors and the Pt compounds should have the same origin. In both cases, no triplet emission was observed which manifests the fate of energy gap law for low-gap polyplatinynes.^20d In addition, as described in the literature,^12c the fully extended heteroaryl ring in the ligand chromophore greatly reduces the influence of heavy metal ion in P1 and P2 which is mainly responsible for the intersystem crossing and hence the phosphorescence. Therefore, we consider the ligand-dominating singlet excited state instead of the triplet state to contribute to the photoinduced charge separation in the energy conversion for both polymers. All of the Pt compounds undergo a rigidochromic blue shift upon cooling of their solutions to 77 K. The hypsochromic shift observed at low temperature is mainly caused by the solvent reorganization in a fluid solution at 77 K that can stabilize the charge transfer states prior to emission.²⁶ This process is significantly impeded in a rigid matrix at 77 K, and hence luminescence appears at a higher energy. The absence of vibronic progression in the emission profile (both at 293 and at 77 K) suggests mostly a charge-transfer state here but not the ligand-centered π–π* excited state. The observed emission lifetimes (τ) of our Pt compounds at 77 K are shorter than those recorded at room temperature. In these systems, the τ values are generally determined by nonradiative decay rates which usually decreases with lowering temperature. Thus, a lengthening of τ at low temperature would be anticipated.²⁷

The ICT nature is also discussed on the basis of absorption and fluorescent solvatochromism (Fig. 4). The absorption maxima of P1 and P2 exhibit negligible solvatochromic shifts in solvents of increasing polarity, consistent with very small ground-state dipole moments. Contrary to their absorption process, P1 and P2 exhibit pronounced positive solvatochromism in their fluorescence spectra which is diagnostic of the charge-transfer nature of the excited state (Table 2 and Fig. 5), and the finding can be rationalized by the less polar structure of the ground state compared with that of the excited state.²⁸ As a typical example of this phenomenon, with increasing solvent polarity from CCl₄, toluene, THF, CHCl₃ to CH₂Cl₂, the λ_em values of P1 shift significantly to the longer wavelength region by 71 nm (from 594 nm in non-polar CCl₄ to 665 nm in polar CH₂Cl₂). Such solvent-dependent emission in solution is consistent with our expectation for D–A compounds exhibiting significant ICT interaction.

Fig. 4 The structure of polymers P1 and P2, with the arrows indicating the expected direction of electron transfer after excitation.

Table 2 Charge-transfer absorption and emission wavelength of P1 and P2 in different solvents

Solvent	P1		P2
	λ max/nm	λ em/nm	λ max/nm	λ em/nm
CCl4	489	594	547	649
Toluene	486	608	544	664
THF	483	635	544	694
CHCl3	487	651	546	702
CH2Cl2	489	665	542	715

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Fig. 5 Normalized emission spectra of P1 in different solutions at 293 K.

3.3. Electrochemical properties

The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of P1 and P2 were determined using the redox potentials determined from cyclic voltammetry in deoxygenated MeCN, using 0.1 M [ⁿBu₄N]BF₄ as the supporting electrolyte. From the values of oxidation potential (E_ox) and reduction potential (E_red), the HOMO and LUMO levels of P1 and P2 were calculated according to the following equations E_HOMO = −(E_ox + 4.72) eV and E_LUMO = −(E_red + 4.72) eV (where the unit of potential is V versusAg/AgCl).¹⁴ The relevant data are collected in Table 3. Both polymers display one irreversible wave at 1.37 and 1.25 eV for P1 and P2, respectively, which can be attributed to the oxidation of electron-rich triarylamino unit. When the thiophene ring is inserted into the triphenylamine-benzothiadiazole moiety in P2, the polymer shows an additional irreversible thienyl oxidation at 1.06 V, which is absent in P1. The electrooxidation of oligothiophene is often irreversible because the electrogenerated cations readily undergo rapid coupling reactions resulting in higher oligomers or polymers.³⁰ The irreversible cathodic wave is attributed to the reduction of the electron-deficient benzothiadiazole moiety. Polymer P2 shows an elevated HOMO energy level (–5.78 eV) relative to P1 (–6.09 eV), indicating that P2 is more electropositive (or has a lower ionization potential) than P1, and a better hole transport ability in P2 can therefore be expected.

Table 3 Electrochemical data and frontier orbital energy levels for P1 and P2

Polymer	E ox /eV^a	E HOMO/eV^b	E red/eV^c	E LUMO/eV^d
a E ox is the oxidation potential. b E HOMO = −(Eox + 4.72) eV. c E red is the reduction potential. d E LUMO = −(Ered + 4.72) eV.
P1	1.37	–6.09	–1.41	–3.31
P2	1.06, 1.25	–5.78	–1.26	–3.46

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3.4. Photovoltaic properties

Since light-induced intramolecular electron transfer could easily occur from D to A through the π-bridge which favours the photocurrent generation and photoelectronic energy conversion in photovoltaic devices, PSCs were fabricated by using each of P1 and P2 as an electron donor and PCBM as an electron acceptor (Table 4). The hole collection electrode consisted of indium tin oxide (ITO) with a spin-coated PEDOT:PSS, while Al served as the electron collecting electrode. Fig. 6 shows the J–V curves of solar cells with P1–P2 : PCBM (1 : 4, w/w) active layers under simulated AM1.5 solar irradiation. The open circuit voltage (V_oc), short-circuit current density (J_sc) and the electrical fill factor (FF) values of the device with P2 as electron donor are 0.78 V, 4.94 mA cm⁻² and 0.42, respectively, leading to a PCE of 1.61%. A marked increase in the J_sc and PCE can be observed in P2 relative to P1 at the same blend ratio. It is clear that enhancing the absorption coefficient of the band by increasing the polymer conjugation chain length with additional thienyl rings is an effective way to improve the cell performance. At the same blend ratio of 1 : 4, the PCE increases in the order P1 < P2 and the light-harvesting ability of P2 can be increased by ∼1.5 times relative to P1 by adding one thienyl ring on both sides of benzothiadiazole. Generally, the amount of absorbed light depends not only on the cut-off absorption wavelength, but also on how intense the absorption is. Comparing model complexes M1 and M2 with different m value, there is an increase in ε_max from M1 to M2 (from 2.3 × 10⁴ to 4.2 × 10⁴ dm³ mol⁻¹ cm⁻¹ for the lowest-energy peak), which correlates well with the higher PCE observed for P2 than P1. Therefore, the design of low-bandgap polymers for photovoltaic applications should consider not only lowering the E_g but also increasing the absorption coefficient of the polymer, as well as optimizing the morphology of the blend films. In all diodes, FFs are not very impressive partly because all processing (except PEDOT:PSS annealing and electrode deposition) and measurements have been done in ambient atmosphere which likely results in the presence of traps. We expect FF to improve for fabrication and characterization to be performed in an inert gas environment. Comprehensive study of charge transport and the influence of traps is necessary to further improve FF and overall device performance. Nevertheless, these devices are still not fully optimized. Further studies are needed to optimize the devices based on these materials and the efficient alignment of the energy levels of this type of materials with different acceptors is expected to be a promising strategy for the future advance of organic solar cells.

Table 4 Solar cell performance of the best devices fabricated with P1 and P2

PSCs	V oc/V	J sc/mA cm^−2	FF	PCE (%)
P1:PCBM	0.80	4.00	0.34	1.09
P2:PCBM	0.78	4.94	0.42	1.61

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Fig. 6 J–V curves of solar cells with P1–P2:PCBM (1 : 4, w/w) active layers under simulated AM1.5 solar irradiation.

4. Conclusion

In summary, we have synthesized two stable platinum-acetylide polymers with triphenylamine and 2,1,3-benzothiadiazole as the core components and their photophysical and photovoltaic properties were studied. The solvatochromic effect and low bandgap value are ascribed to ICT through the involvement of strong D–A structural motif in the polymer chain. Adding to this, the extended π-conjugated system including the D and A moieties lowers the excited state energy to let the polymers emit red to near-infrared fluorescence. P2 has a lower E_g than P1 which favours harvesting of more solar photon energy. The expansion of π-electron conjugation is crucial for increasing solar absorption cross-section and the highest PCE of 1.61% was achieved for P2 with a V_oc = 0.77 V, J_sc = 4.9 mA cm⁻² and FF = 0.39 under illumination of an AM 1.5 solar cell simulator in a 1 : 4 (P2 : PCBM) blend ratio. Our preliminary results indicate the potential solar cell application of metallopolymers P1 and P2. These deeply coloured absorbing polymers are attractive candidates as new functional materials for organometallic photovoltaic technology. However, we need to make more effort to improve the photon-to-electricity conversion efficiency further for practical applications.

Acknowledgements

This work has been supported by the Areas of Excellence Scheme, University Grants Committee (AoE/P-03/08) and a Faculty Research Grant from the Hong Kong Baptist University (FRG2/09-10/II-091). We also thank Drs A. B. Djurišić and Kai-Yin Cheung for their assistance in photovoltaic measurements.

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Footnotes

† This paper is part of a Polymer Chemistry issue highlighting the work of emerging investigators in the polymer chemistry field. Guest Editors: Rachel O'Reilly and Andrew Dove.

‡ CCDC reference numbers 801106. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c0py00273a

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