Photovoltaic properties

To investigate the potential use of these polymers in polymer solar cell (PSC), the bulk heterojunction PSC devices with a configuration of ITO/PEDOT:PSS/CPDT-CN or DTS-CN:PCBM/Ca/Al were fabricated from an active layer where polymers were blended with PC61BM in a weight ratio of 1:1 w/w initially. Later on, the active layer compositions were modified with various weight ratios for the optimum polymer DTS-CN with PC₇₁BM. Figure 3.5 (a) and 3.5 (b) illustrate the J-V curves (current density J versus voltage V) and external quantum efficiency (EQE) curves as a function of wavelengths, respectively. The photovoltaic properties, i.e., the values of open circuit voltage (V_oc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE) of BHJ solar cell devices are listed in Table 3.3. Due to negligible differences in HOMO levels of both polymers, they exhibited similar Voc values (0.74-0.75 V) in the BHJ solar cell devices containing CPDT-CN or DTS-CN:PC61BM in 1:1 weight ratio. With the similar Voc values and fill factors (33.9-35.7 %), the PCE values of polymers CPDT-CN and DTS-CN were dependent on their Jsc values of 5.46 and 6.05 mA/cm², respectively. The PSC device based on the polymer blend of DTS-CN:PC61BM (1:1 wt%) reached a higher PCE value of 1.60% with Voc = 0.74 V, Jsc = 6.05 mA/cm², and FF = 35.7%. As reported, the more balanced hole and electron transportig properties in the films and the higher hole mobilities are favorable factors for LBG polymers utilized in the BHJ solar cell devices,¹²² so the PCE and Jsc values in the PSC device based on DTS-CN were higher than that based on CPDT-CN. This may be due to the higher hole mobility (9.82  10^-4 cm²/V s) and more balanced charge transport (μe/μh = 1.6) in the polymer blend of DTS-CN compared with those proerties (5.99  10^-4 cm²/V and 3.4, respectively)

of the polymer blend of CPDT-CN. Furthermore, from the absoption spectra and EQE curves (Figure 3.5 (b)), it is evident that DTS-CN exhibited broader absorption bands and higher EQE values between 350 to 800 nm than those of CPDT-CN, which may be another reason for the higher Jsc value.

Table 3.3 Photovoltaic properties of PSC devices with the configuration of

Since the best performance of PSC device was observed in the previous optimum polymer blend of DTS-CN:PC61BM (1:1 wt%) as an active layer, the PSC devices as a function of polymer blends DTS-CN:PC₇₁BM in various weight compositions were fabricated. Another electron acceptor PC71BM was used to optimize the device properties due to its stronger light absorption in the visible region than that of PC61BM.⁹ The Voc values observed in DTS-CN:PC71BM solar cells were fairly stable in all polymer blend compositions (1:1 to 1:3 w/w) with PC₇₁BM, but the J_sc, FF, and PCE values are strongly dependent on the donor-to-acceptor weight ratios in the active layers. The PSC device based on DTS-CN:PC71BM (1:2,w/w) exhibited the highest PCE= 2.25% with Voc= 0.74 V, Jsc=8.39 mA cm^-2, and FF=36.1%. Though these polymers exhibited a higher J_sc compared with the other CPDT and DTS- based

polymers, their efficiencies were limited by their low FF and EQE values. Therefore, if the EQE values of the PSC devices can be enhanced by increasing the thickness of the active layer without hampering charge separation and transporting properties, the PSC device performance can be improved significantly. Low EQE values were also observed in some other LBG polymer systems, but this problem can be solved by developing new electron acceptor materials to replace PCBM.¹²³

Figure 3.6 AFM images of (a) CPDT-CN: PC61BM 1:1 (w/w) and (b) DTS-CN:

PC61BM 1:1 (w/w).

3.4 Conclusion

The concept of incorporation of electron deficient -cyano-vinylene groups with donor-acceptor polymer architectures was utilized to improve the efficiencies of polymer solar cells. Cyano-vinylene groups were introduced via palladium(0)-catalyzed Stille coupling reactions into electron-rich building blocks, such as cyclopentadithiophene and dithienosilole to yield LBG polymers (CPDT-CN and DTS-CN). These polymers showed excellent charge-transporting properties with high hole mobilities in the range of (5.99-9.82) × 10^-4 cm²V^-1s^-1 and good processabilities for PSC applications. Due to the lowest band gap and the

highest hole mobility with more balanced charge transport of DTS-CN (with Si atom), an optimum PSC device based on the blended polymer DTS-CN:PC71BM = 1:2 (w/w) achieved the maximum power conversion efficiency (PCE) value up to 2.25 %, with Voc = 0.74 V, Jsc = 8.39 mA/cm², and FF = 36% (under AM 1.5 G 100 mW/cm²). Regardless of the high open-circuit voltages and the large short-circuit currents of all PSC devices, the low fill factor values indicated the possibility of further device performance improvements by the optimization of film morphology of the polymer/PCBM blends and/or device architectures.

Chapter 4.

Synthesis and Applications of Main-Chain RuII Metallo-Polymers Containing Bis-terpyridyl Ligands with Various Benzodiazole Cores for Solar Cells

4.1 Introduction

In the extensive search of new materials for optoelectronics applications, supramolecular metallo-polymers have gained much interest in last decades.^124-127 The self-recognition and self-assembly of functional structures into supramolecular metallo-architectures through the interactions of transition metal ions and appropriate chelating ligands has afforded many intriguing architectures that attains highly attention in modern supramolecular chemistry.^128-129 Polypyridyl ligands, especially 2,2‟:6‟,2‟‟-terpyridine (terpy) derivatives, which have high binding affinities towards transition-metal ions due to dπ-pπ*

back-bonding of metal ions to pyridine rings have been utilized recently for multinuclear supramolecular interactions.^130-132 Terpy derivatives bearing π-conjugated substituents at the 4‟-position possess fascinating photophysical, electrochemical, catalytic, and magnetic properties with promising applic ations in the field of self-assembled molecular devices and photoactive molecular wires,¹³³ and their properties can be tuned by varying substituents at 4‟-position of terpy moieties.^134-135 Furthermore, chemically and thermally stable ditopic bis-terpyridine derivatives, where two terpy units are linked back-to-back with a covalent bond through a spacer, are able to form stable complexes with a large variety of transition-metal ions for the design of functional materials.¹³⁶ Three chelating pyridyl units in terpy ligands offer a higher binding constants and the formation of distorted octahedral 2:1 ligand-metal complexes. In particular, by

proper selections of metal-ligand combinations, it is possible to realize the formation of ligand-metal complexes from kinetically labile to inert but nevertheless thermodynamically stable bonds.

Constable et al. have developed the concept concerning the utilization of ditopic bis-terpyridyl ligands as building blocks for coordination oligomers and polymers.¹³⁷ By coordinating to suitable metal ions, the electronic properties of these materials can be tuned and that highlights their potential in the applications of new functional materials.^138-140 In the past years, a large number of terpy complexes containing heavy transition metal ions, such as ruthenium(II), iridium(III), or osmium(II), as photoactive species, as well as coordination polymers built up from terpy ligands have been introduced by several research groups.¹³⁶ Zinc(II) ions have also recently attracted much interest as novel templates for the fabrication of structurally well-defined photoluminescent and electroluminescent supramolecular metallo-cycles and metallo-polymers.^141-146 In our previous reports,^144-146 a series of terpyridyl Zn(II) metallo-polymers containing polyfluorene backbones as emitters were fabricated into PLED devices with multilayer structures.

Nowadays, some terpyridyl Ru(II) complexes have attracted researchers to use in the applications of photovoltaic cells (PVC).62-65,147-150

The insertion of ruthenium metals into conjugated backbones has several advantages, such as to facilitate the charge generation by extending its absorption range due to its characteristic long-lived metal to ligand charge transfer (MLCT) transition¹³⁶ and to exhibit a reversible Ru^II,III redox process along with some ligand-centered redox processes. Motivations for examining the potential incorporation of such conjugated polyelectrolytes into solar cell development include the easy

processability, layer-by-layer (LBL) processing capability, and also due to efficiently quenched by electron acceptors. Recently, terpyridyl Ru(II) complexes have been utilized in bulk heterojunction (BHJ) PVC devices by using LBL self-assembled techniques. Chan et. al. reported that the use of a terpyridyl Ru(II) complex containing conjugated polymer (Ru-PPV) and sulfonated polyaniline (SPAN) for the fabrication of different multilayer PVCs by LBL deposition method. The short-circuit currents in the PVC devices were measured ca. 7.5-32.9 μA/cm² and the PCE values were in the range of 0.95-4.4×10^-3 %.⁶⁵ They also synthesized conjugated polymers with pendant Ru^II terpyridine trithiocyanato complexes and applied in bulk heterojunction photovoltaic cells with high PCE values ca. 0.12%.⁶² Similarly, Mikroyannidis et al used metallo-polymers in bulk heterojunction photovoltaic cells as a buffer layer along with an active layer of polymer blend P3HT:PCBM (1:1 w/w) and found the maximum power conversion efficiency value of 0.71% among four metallo-polymers.⁶³ In all cases, the PCE values were limited either by the low open-circuit voltage (V_oc) or low short circuit current (Jsc). Due to relatively high HOMO levels and less sensitization ranges in all reported polymers, there were inefficient photocurrents generated which probably affected their PCE values.

One of the feasible solutions to conquer these problems, i.e. to get a higher Voc value and a more favorable overlap of the absorption spectra from both active

layer and solar emission, is to introduce electron donor-acceptor structures to the cores of bis-terpyridyl ligands. By choosing different electron donors and acceptors, their absorption edges can be extended due to the intramolecular charge transfer (ICT) between them and also the energy band structures, i.e., highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital

(LUMO) levels, of these materials can be tuned. The incorporation of the thiophene donor units with benzodiazole acceptor units at the cores of bis-terpyridyl ligands in Ru^II-containing metallo-polymers to have better photophysical, electrochemical, and photovoltaic properties are very intrigui ng and thus to motivate this study. Here, we report the design, synthesis, properties, and device applications of Ru^II-containg metallo-polymers (P1-P3) containing donor-acceptor (D-A) bis-terpyridyl ligands bearing different benzodiazole acceptors, including benzothiadiazole, benzoselenodiazole, and benzoxadiazole cores sandwiched between symmetrical thiophene and terpyridyl units. By introducing such D-A structures, broad absorption ranges ca. 300-750 nm and ideal HOMO/LUMO levels of the metallo-polymers were obtained. The effects of their donor-acceptor strengths on the electronic and optoelectronic properties were also investigated. In addition, the PVC devices fabricated by these bis-terpyridyl ligands (M1-M3) and metallo-polymers (P1-P3) with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) inserted between a transparent anode (ITO/PEDOT:PSS) and a cathode (Ca) were explored, and all photovoltaic parameters obtained are also comparable with the BHJ solar cells fabricated from ionic polythiophene and C60.^66-67

4.2 Experimental Section 4.2.1 Materials

All chemicals and solvents were reagent grades and purchased from Aldrich, ACROS, Fluka, TCI, TEDIA, and Lancaster Chemical Co. Toluene, tetrahydrofuran, and diethyl ether were distilled over sodium/benzophenone to keep anhydrous before use. Chloroform (CHCl3) was purified by refluxing with

calcium hydride and then distilled. If not otherwise specified, the other solvents were degassed by nitrogen 1 h prior to use.

4.2.2 Measurements and Characterization

1H NMR spectra were recorded on a Varian Unity 300 MHz spectrometer using CDCl3 and DMSO-d6 solvents. Elemental analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Thermogravimetric analyses (TGA) were conducted with a TA Instruments Q500 at a heating rate of 10°C/min under nitrogen. Viscosity measurements were proceeded by 10% weight of metallo-polymer solutions (in NMP) in contrast to that proceeded by bis-terpyridyl ligand solutions under the same condition (with viscosity η = 6 cp) on a BROOKFILEL DV-III+ RHEOMETER system at 25°C (100 RPM, Spindle number 4). UV-visible absorption were recorded in dilute chloroform (for M1-M3) and DMF (for P1-P3) solutions (10^-6 M) on a HP G1103A spectrophotometer.

Solid films of UV-vis measurements were spin-coated on quartz substrates from chloroform and DMF solutions with a concentration of 10 mg/mL for bis-terpyridyl ligands (M1-M3) and polymers (P1-P3), respectively. UV-vis titrations were performed by taking 10^-5 M of bis-terpyridyl ligands (M1-M3) in the co-solvent of chlorofom: acetonitrile (8:2 v/v), and titrated with 50 µ l aliquots of 3.9×10^-4 M solutions containing metal salts Zn(OAc)2 in the EtOH. Cyclic voltammetry (CV) measurements were performed using a BAS 100 electrochemical analyzer with a standard three-electrode electrochemical cell in a 0.1 M tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) solution (in acetonitrile) at room temperature with a scanning rate of 100 mV/s. During the CV measurements, the solutions were purged with nitrogen for 30 s. In each case, a carbon working electrode coated with a thin layer of monomers or polymers, a

platinum wire as the counter electrode, and a silver wire as the quasi -reference electrode were used, and Ag/AgCl (3 M KCl) electrode was served as a reference electrode for all potentials quoted herein. The redox couple of ferrocene/ferrocenium ion (Fc/Fc⁺) was used as an external standard. The corresponding HOMO and LUMO levels were calculated using E_ox/onset and Ered/onset for experiments in solid films, which were performed by drop-casting films with the similar thicknesses from THF or DMF solutions (ca. 5 mg/mL).

4.2.3 Device fabrication of polymer solar cells

The photovoltaic (PV) cells in this study were composed of an active layer of blended bis-terpyridyl ligands (M1-M3) or metallo-polymers (P1-P3) with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in solid films, which was sandwiched between a transparent indium tin oxide (ITO) anode and a metal cathode. Prior to the device fabrication, ITO-coated glass substrates (1.5×1.5 cm²) were ultrasonically cleaned in detergent, deionized water, acetone, and isopropyl alcohol sequentially. After routine solvent cleaning, the substrates were treated with UV ozone for 15 min. Then, a modified ITO surface was obtained by spin-coating a layer of poly(ethylene dioxythiophene): polystyrenesulfonate (PEDOT:PSS) (~30 nm). After baking at 130°C for one hour, the substrates were transferred to a nitrogen-filled glove box. The PVC devices were fabricated by spin-coating solutions of blended bis-terpyridyl ligands (M1-M3) or metallo-polymers (P1-P3):PCBM (1:1 w/w) onto the PEDOT:PSS modified substrates at 1500 rpm for 60 s (ca. 100 nm), and placed in a covered glass Petri dish. Initially, the blended solutions were prepared by dissolving both bis-terpyridyl ligands (M1-M3) and PCBM (1:1 w/w) in chloroform (20 mg/1 mL) and both metallo-polymers (P1-P3) and PCBM (1:1 w/w) in DMF (20 mg/1 mL),

and followed by continuous stirring for 12 h at 50°C. Finally, a calcium layer (30 nm) and a subsequent aluminum layer (100 nm) were thermally evaporated through a shadow mask at a pressure below 6×10^-6 Torr, where the active area of the device was 0.12 cm². All PVC devices were prepared and measured under ambient conditions. The solar cell testing was done inside a glove box under simulated AM 1.5G irradiation (100 mW/cm²) using a Xenon lamp based solar simulator (Thermal Oriel 1000W). The light source was a 450 W Xe lamp (Oriel Instrument, model 6266) equipped with a water-based IR filter (Oriel Instrument, model 6123NS). The light output from the monochromator (Oriel Instrument, model 74100) was focused onto the photovoltaic cell under test.

4.2.4 Synthesis of Monomers and Polymers

4′-(2-Bromo-5-thienyl)-2,2′,6′,2″-terpyridine (1). Aqueous potassium hydroxide (8.4 g, 150 mmol in 40 mL water) was added to a solution of 2-Acetylpyridine in 400 mL of methanol. The reaction mixture was stirred for 30 min and then 9.55 g (50 mmol) of 2-bromothiophene carboxaldehyde in 50 mL of methanol was added dropwise. The solution was stirred overnight at room temperature. The solvent was then evaporated off under vacuum and extracted with dichloromethane. Then the crude was taken for the next step without further purifications. To the above crude an excess amount (50 gm) ammonium acetate in 200 mL of ethanol/acetic acid (2/1) was added. The mixture was heated to reflux for 6 h.

The reaction mixture was cooled to room temperature, poured onto ice and water (1 liter) to give a pale yellow precipitate that was recrystallized with ethanol to yield the title compound as a white solid. (9.25 g, 47%). ¹H NMR (CDCl3, 300 MHz, δ): 8.72 (d, J = 3.2 Hz, 2H), 8.61 (d, J = 7.8 Hz, 2H), 8.58 (s, 2H), 7.88 (ddd, J = 1.8 Hz, J = 7.5 Hz, J = 7.5 Hz, 2H), 7.51 (d, J = 4.2 Hz, 1H), 7.35 (ddd, J = 1 Hz, J = 4.8 Hz, J

=7.8 Hz, 2H), 7.11 (d, J = 3.9 Hz, 1H). ¹³C NMR (CDCl₃, 75 MHz, δ): 156.5, 156.1, 149.5, 143.6, 142.8, 137.3, 131.6, 126.3, 124.4, 121.7, 116.6, 114.6.

4'-(5-Tributylstannanyl-thiophen-2-yl)-[2,2';6',2'']terpyridine (2). To a solution of 4′-(2-Bromo-5-thienyl)-2,2′,6′,2″-terpyridine (4.00 g, 10.15 mmol) in toluene (20 mL), bis(tributyltin) (13 mL, 25 mmol, 2.5 equiv) was added in one portion, and the mixture was degassed with argon. (PPh3)4Pd(0) (400 mg, 0.34 mmol) was added, and the mixture was refluxed overnight. The reaction mixture was cooled to room temperature, filtered and solvents were removed by reduced pressure. The residue was purified by column chromatography on alumina with 3:1 hexane/ ethyl acetate to give product as slightly yellowish oil, (3.2 g, 52%). ¹H NMR (CDCl3, 300 MHz, δ): 8.74 (d, J = 3.2 Hz, 2H), 8.69 (s, 2H), 8.63 (d, J = 7.8 Hz, 2H), 7.88 (ddd, J

= 1.8 Hz, J = 7.5 Hz, J = 7.5 Hz, 2H), 7.81 (d, J = 4.2 Hz, 1H), 7.36 (ddd, J = 1 Hz, J

= 4.8 Hz, J = 7.8 Hz, 2H), 7.21 (d, J = 3.9 Hz, 1H), 1.55−1.63 (m, 6H), 1.30−1.44 (m, 6H), 1.09−1.15 (m, 6H),. 0.92 (t, J = 7.3 Hz, 9H). ¹³C NMR (CDCl3, 75 MHz, δ):

157.5, 155.1, 149.5, 147.6, 141.8, 137.3, 131.6, 127.3, 124.4, 121.7, 117.6, 114.6, 29.8, 28.0, 14.4, 10.4.

General Synthetic Procedure for Bis-terpyridyl Ligands (M1-M3)

M1-M3 were prepared via Stille coupling reaction using tetrakis(triphenylphosphine)palladium as a catalyst. In a flame dried two-neck flask, 1.00 eq of dibromo compounds (5a-5c) and 2.50 eq of compound 2 in toluene were degassed with Argon. Then, 0.03 eq Pd(PPh3)4 were added and refluxed for 2 days. The reaction mixtures were cooled to room temperature, and solvents were removed by reduced pressure. After removal of the solvents, the product was precipitated from methanol. Further purification was achieved by

column chromatography on alumina with chloroform as an eluant to give the products.

M1. According to the above-mentioned general procedure, M1 was obtained as a purple solid (yield: 68%). ¹H NMR (CDCl3, 300 MHz, δ): 8.77 (d, J=4.2 Hz,

M2. According to the above-mentioned general procedure, M2 was obtained as a black solid (yield: 63%). ¹H NMR (CDCl3, 300 MHz, δ): 8.74 (d, J = 4.2 Hz, 121.67, 117.84, 116.88, 31.99, 30.63, 29.98, 29.61, 22.93, 14.39. MS (FAB): m/z [M+] 1143; calcd m/z [M+] 1142.39. Element Anal. Calcd for C₆₄H₅₄N₈S₄Se: C, 67.29; H, 4.76; N, 9.81. Found: C, 66.77; H, 5.30; N, 9.63.

M3. According to the above-mentioned general procedure, M3 was obtained as a dark purple solid (yield: 76%). ¹H NMR (CDCl3, 300 MHz, δ): 8.75 (d, J =4.2 Hz, 4H), 8.69 (m, 8H), 7.96 (s, 2H), 7.87 (dd, J = 7.2 Hz, J = 1.8Hz, 4H), 7.73 (d,

J = 3.6 Hz, 2H), 7.50 (S, 2H), 7.37 (dd, J = 6.9 Hz, J = 1.2 Hz, 4H), 7.25 (d, J =3.9

Hz, 2H), 2.88(t, J =7.2 Hz, 4H), 1.78 (m, 4H), 1.40-1.29 (m, 12H), 0.93 (t, J =6.9 Hz, 6H), ¹³C NMR (CDCl₃, 75 MHz, δ): 155.72, 148.96, 148.35, 143.10, 141.16, 138.10, 137.36, 133.05, 132.09, 130.68, 129.41, 128.63, 126.85, 125.28, 122.94, 121.61, 117.69, 116.97, 31.98, 30.60, 30.03, 29.60, 22.94, 14.38. MS (FAB): m/z [M+] 1078; calcd m/z [M+] 1078.33. Element Anal. Calcd for C64H54N8S4O: C, evaporated to dryness. The remaining solid was redissolved in n-butanol (15 mL), and to this solution, bis-terpyridyl ligand M1, M2, or M3 (0.1 mmol) was added, and the resulting solution is refluxed for 5 days. As soon as the precipitation of the formed polymer was observed, a small portion of DMA was added to the mixture (Σ ≈ 20 mL) to redissolve the product. Finally, an excess of NH4PF6 (50 mg in 20 mL DMA) was added to the hot solution and stirring was continued for 1 h. The resulting solution is dropwise poured into methanol (200 mL). The precipitated metallo-polymer product was filtered off, washed with methanol (200 mL). Further purification was achieved by repetitively dissolving the metallo-polymer in NMP (2 mL) and precipitating from diethyl ether. Finally, the products were dried under vacuum at 40 ^°C for 24 h.

P1. According to the above-mentioned procedure, metallo-polymer P1 was obtained as a dark solid (yield: 66%). ¹H NMR (DMSO-d6, 300 MHz, δ): 9.36 (br, 4H), 9.12 (br, 4H), 8.52 (br, 2H), 8.24 (br, 8H), 7.67 (br, 6H), 7.31 (br, 4H), 3.05

(br, 4H), 1.86 (br, 4H), 1.25-1.40 (br, 12H), 0.92 (br, 6H).

P2. According to the above-mentioned procedure, metallo-polymer P2 was obtained as a dark solid (yield: 58%). ¹H NMR (DMSO-d₆, 300 MHz, δ): 9.36 (br, 4H), 9.12 (br, 4H), 8.48 (br, 2H), 8.14 (br, 8H), 7.68 (br, 6H), 7.30 (br, 4H), 3.08 (br, 4H), 1.83 (br, 4H), 1.20-1.53 (br, 12H), 0.91 (br, 6H).

P3. According to the above-mentioned procedure, metallo-polymer P3 was obtained as a dark solid (yield: 77%). ¹H NMR (DMSO-d₆, 300 MHz, δ): 9.39 (br, 4H), 9.13 (br, 4H), 8.54 (br, 2H), 8.10 (br, 8H), 7.65 (br, 6H), 7.33 (br, 4H), 2.94 (br, 4H), 1.83 (br, 4H), 1.20-1.53 (br, 12H), 0.89 (br, 6H).

Figure 4.1 Synthetic Route for Bis-terpyridyl Ligands (M1-M3) and Ru^II-containing Metallo-Polymers (P1-P3)

4.3 Results and Discussion

4.3.1 Synthesis and Structural Characterization

The general synthetic routes of ditopic bis-terpyridyl ligands (M1-M3) and metallo-polymers (P1-P3) are shown in Figure 4.1. The ditopic bis-terpyridyl monomers in which, acceptor spacer units sandwiched in between two hexyl thiophene units were synthesized in multistep procedures.

4′-(2-Bromo-5-thienyl)-2,2′,6′,2″-terpyridine (1) was prepared by the modified method described in the literature,¹⁵¹ then tributyltin group was introduced by a palladium(0) catalyzed reaction of compound 1 with excess bis-(tributyltin).

Synthesis of 5a-5c were described in Chapter 2.¹⁵² The aromatic dibromides 5a-5c

were reacted with two equivalents of

4'-(5-Tributylstannanyl-thiophen-2-yl)-[2,2';6',2'']terpyridine (2) under Pd⁰-catalyzed Stille cross-coupling conditions to form ditopic bis-terpyridyl ligands (M1-M3). After precipitation from methanol and column chromatographic purification, the bis-terpyridyl ligands (M1-M3) were obtained in moderate to good yields and fully characterized by ¹H NMR, ¹³C NMR, MS spectroscopy, and elemental analysis.

The synthesis of Ru^II-based metallo-polymers (P1-P3) is also depicted in Figure 4.1. The metallo-polymerization by self-assembly was carried out according to the methods described in the literature.¹⁵³ In a typical polymerization process, an appropriate quantity of ruthenium trichloride was activated by de-chlorinating with AgBF4 in acetone. The resulting hexa-acetone Ru^III complex was reacted with exactly 1 equiv of bis-terpyridyl ligands M1-M3 in n-butanol/DMA for 5 days, which involved a reduction of Ru^III to Ru^II with the chain growth process using n-butanol solvent itself as a reducing agent. The

resulting metallo-polymers (P1-P3) were purified by repetitive precipitation from NMP in diethyl ether and dried in vacuum, leading to homopolymers P1−P3 (yield:

58-78%). The resulting highly linear-rigid polymer containing charged metal ions exhibited less solubility in common organic solvents as compared with terpyridyl ligands M1-M3, but were soluble in highly polar aprotic solvents, e.g., DMSO, DMF, NMP, or DMA.

Figure 4.2 ¹H NMR spectra (aromatic region) of bis-terpyridyl ligands M1-M3 (in CDCl3) and metallo-polymers P1-P3 (in DMSO-d6).

Figure 4.2 shows ¹H NMR spectra in aromatic regions of ligands and

Figure 4.2 shows ¹H NMR spectra in aromatic regions of ligands and