Motivation

The main objective of this dissertation is to construct the donor-acceptor polymer or metallo-polymer architectures by incorporating various donors and acceptors and to study their performance in polymer bulk heterojunction solar cells as electron donors with fullerene derivatives as electron acceptors. Addition of electron-withdrawing imine nitrogen to a conjugated polymer backbone generally enhances its electron-accepting properties and makes it susceptible to n-doping. Benzodiazole units are, in that sense, typical examples of such units containing imine nitrogen,⁵⁶ which have been widely used electron acceptors for the synthesis of D-A polymers. For example, copolymers of benzodiazole with variety of donors such as, fluorene,⁴⁹ silafluorene,⁸⁶ carbazole,⁵⁶ dithienosilole,⁸⁷ dithienocyclopentadiene,²⁶ and dithieno[3,2-b:2‟,3‟-d]pyrroles⁸⁸ were synthesized and applied to PSCs, yielding PCE values in the range of 0.18-5.4%. Recently, many LBG copolymers have been synthesized by sandwiching acceptors in the midst of two thiophene units to alleviate the severe steric hindrance between the electron donors and acceptors, resulting in more planar structures to facilitate inter-chain associations and improve the hole mobilities of the LBG polymers. Among all heterocyclic donors, phenothiazine contains both electron-rich sulfur and nitrogen heteroatoms. The electron-rich nature of phenothiazine contributes for the efficient electron donor and hole transporting materialsin polymers. In order to have better photophysical, electrochemical, and photovoltaic properties in the polymers, we incorporated of phenothiazine donor units with various benzodiazole acceptors (such as benzothiadiazole, benzoselenodiazole, and benzoxadiazole units) sandwiched between two hexyl thiophene units to form alternating conjugated donor-acceptor polymers. These polymers were synthesized by palladium(0)-catalysed Suzuki coupling reactions. The effects of donor-acceptor

strengths on the electronic and optoelectronic properties of the LBG polymers were also investigated. In addition, the PSC devices fabricated by polymer/PC61BM or polymer/PC₇₁BM blends sandwiched between a transparent anode (ITO/PEDOT:PSS) and a cathode (Ca) were explored.

LBG polymers containing electron donating moieties from 2,2‟-bithiophene unit covalently bridged with an atom, such as C, N, S, and Si, at 3,3‟-position have attracted considerable research attentions. The bridging atoms at 3,3‟-position of donor moieties play an important role for LBG polymers in terms of solubility, planarity, band gap, and interchain packing, as well as for the performance of the bulk heterojunction (BHJ) solar cells.⁵⁹ As compared with polythiophene and polyfluorene derivatives, LBG polymers based on these donor moieties showed relatively high conductivities due to more extensive π-conjugation lengths, narrow band gaps, high planarities, and strong intermolecular π-π interactions of donor units.^105-106 Again, To obtain the broad absorption bands with high absorptivities, electron-donating groups and/or electron-withdrawing groups are substituted on the main-chains of the conjugated polymers to raise the HOMO levels and/or to reduce the LUMO levels of the polymers.^107-108 Hence, introduction of electron-withdrawing cyano-vinylene groups to polymer backbones to lower the LUMO levels,¹⁰⁹ tune their electro-optical properties,¹¹⁰ and enhance the electrochemical stabilities of the polymers¹¹¹ are desirable for optoelectronic device applications. Moreover, LBG polymers containing electron-accepting cyano-vinylene groups were proven to possess higher hole mobilities,¹¹² and were applied as photovoltaic materials in BHJ solar cells.^113-116 However, the PCE values of these photovoltaic cells are still low at present. All these research results inspire further development in exploring cyclopentadithiophene- and dithienosilole- derivatives with cyano-vinylene cgroups for the better photophysical,

electrochemical, and photovoltaic properties. Based on this concept, soluble cyclopentadithiophene- and dithienosilole-based LBG D-A polymers (CPDT-CN, DTS-CN) containing -cyano-thiophenevinylene groups are designed and synthesized. The effects of the bridged atoms on the optical, electrochemical, charge transporting and photovoltaic properties of the polymers are compared and reported also in this study.

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. But PCE values of these devices were limited either by the low open-circuit voltage (V_oc) or low short circuit current (J_sc).

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 V_oc 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. 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 intriguing us. So, we design, synthesis, properties, and device applications of Ru^II-containg metallo-polymers 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. 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 and metallo-polymers with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) inserted between a transparent anode (ITO/PEDOT:PSS) and a cathode (Ca) were explored.

Chapter 2.

Synthesis and Applications of Low-Bandgap Conjugated Polymers Containing Phenothiazine Donor and Various Benzodiazole Acceptors for Polymer Solar Cells

2.1 Introduction

In spite of poor long-term stability, polymer solar cell (PSC) devices based on conjugated polymers as electron donors and fullerene derivatives as electron acceptors are of broad interests because of the advantages of low cost, light-weight flexible devices, tunable electronic properties, and ease of processing for the conversion of solar energy to electricity.^9-10,21,69 Although poly(3-hexylthiophene) (P3HT) is proven to be one of the most efficient donor materials ever tested in PSCs for giving the power conversion efficiency (PCE) up to 5%,^37-47 further enhanced PCE values are limited due to both lower photocurrent generation and intrinsic absorption properties.

In order to conquer these problems, low-bandgap (LBG) polymers composed of electron-rich (donor) and electron-deficient (acceptor) units have been utilized recently in PSCs with fullerene derivatives, such as [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) or [6, 6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), yielding a power conversion efficiency (PCE) value up to 7.7%.^61,70-74 Polymer solar cells consisting of such donor-acceptor (D-A) LBG polymers have attracted more attention owing to their tunable optical, electrochemical, electronic, and photovoltaic properties.^23-24 Incorporation of wide ranges of donors and acceptors into LBG polymers can manipulate the electronic structures, i.e., the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels through the partial intramolecular charge transfer (ICT) in the D-A systems.^75-76 By optimizing materials and device structures, photovoltaic parameters, such as the short-circuit

current (J_sc) and open-circuit voltage (V_oc), can be further improved to obtain higher PCE values in the PSCs. In solar cell devices, Jsc is determined by the creation and subsequent dissociation of excitons at the polymer/acceptor interface followed by transport of free charge carriers towards the collecting electrodes,⁷⁷ Voc is primarily determined by the effective band gap of the bulk hetero-junction (BHJ) film.⁷⁸ For this purpose, the electron donor polymer should exhibit a band gap between 1.2 and 1.9 eV, which corresponds to a HOMO energy level between -5.8 and -5.2 eV and a LUMO energy level between -4.0 and -3.8 eV.³¹ Again, if the energy difference between the LUMO levels of polymer and acceptor is less than 0.3 eV,³² the driving force for charge separation will be reduced, and Voc can be reduced by raising the HOMO level. Consequently, it is of great importance to match the energy levels of the polymer and acceptor carefully to develop BHJ solar cells with high efficiencies.

Among all heterocyclic compounds, phenothiazine contains both electron-rich sulfur and nitrogen heteroatoms. The electron-rich nature of phenothiazine contributes for the efficient electron donor and hole transporting materials in polymers and organic molecules for photo-induced charge separation and it has been also proven as a superior electron donor for reductive quenching.⁷⁹ Due to their unique electro-optical properties, these materials are potential candidates for diverse applications for light-emitting diodes,^80-81 solar cells, chemiluminescence devices,^82-83 and organic field effect transistors.⁸⁴ Phenothiazine ring hampers stacking aggregation and intermolecular excimer formation in the main chain of the polymer due to its non-planar structure.⁸⁵ However, till now only a limited number of phenothiazine-based polymers for photovoltaic devices have been explored.^52-55

Addition of electron-withdrawing imine nitrogen to a conjugated polymer backbone generally enhances its electron-accepting properties and makes it susceptible

to n-doping (reduction). Benzodiazole units are, in that sense, typical examples of such units containing imine nitrogen.⁵⁶ 2,1,3-Benzothiadiazole is a widely used electron acceptor for the synthesis of D-A polymers. For example, copolymers of benzothiadiazole with fluorene,⁴⁹ silafluorene,⁸⁶ carbazole,⁵⁶ dithienosilole,⁸⁷ dithienocyclopentadiene,²⁶ and dithieno[3,2-b:2‟,3‟-d]pyrroles⁸⁸ were synthesized and applied to PSCs, yielding PCE values in the range of 0.18-5.4%. Recently, many photovoltaic papers have reported LBG copolymers made of electron donors and acceptors sandwiched between two thiophene units.26,49,56,87-88

Incorporation of acceptor units in the midst of two thiophene units, alleviate the severe steric hindrance between the electron donors and acceptors, resulting in more planar structures to facilitate inter-chain associations and improve the hole mobilities of the LBG polymers. Despite of these advantages, addition of thiophene units could induce solubility problems and yield low molecular weights in polymers.⁸⁹ To utilize the aforementioned merits of thiophene units, structural modifications, such as incorporation of alkyl or alkoxy chains on the 3- and/or 4-position of thienyl units⁹⁰ or addition of supplementary alkylated thiophene units,⁹¹ have been outfitted to acquire higher molecular weights and better solubilities than the original polymers without any soluble side-chains.

In order to have better photophysical, electrochemical, and photovoltaic properties in the resulting LBG polymers, the incorporation of phenothiazine donor units with various acceptor units are very intriguing and thus to motivate this study.

Herein, we report the design, synthesis, properties, and device applications of phenothiazine-based alternating conjugated donor-acceptor polymers, in which the acceptor benzodiazole units include benzothiadiazole, benzoselenodiazole, and benzoxadiazole sandwiched between two hexyl thiophene units. These polymers were

synthesized by palladium(0)-catalysed Suzuki coupling reactions. The effects of donor-acceptor strengths on the electronic and optoelectronic properties of the LBG polymers were also investigated. In addition, the PSC devices fabricated by polymer/PC61BM or polymer/PC71BM blends sandwiched between a transparent anode (ITO/PEDOT:PSS) and a cathode (Ca) were explored.

2.2 Experimental Section 2.2.1 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.

2.2.2 Measurements and Characterization

1H NMR and ¹³C NMR spectra were recorded on a Varian Unity 300 MHz spectrometer using CDCl₃ solvent. 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. The molecular weights of polymers were measured by gel permeation chromatography (GPC) using Waters 1515 separation module (concentration = 1 mg/mL in THF; flow rate = 1 mL/min), and polystyrene was used as a standard with THF as an eluant. UV-visible absorption spectra were recorded in dilute chlorobenzene solutions (10^-5 M) as well as on solid films (spin-coated with a spin rate ca. 1000 rpm for 60 s on glass substrates from chlorobenzene solutions with a

concentration of 10 mg/mL) on a HP G1103A. Cyclic voltammetry (CV) measurements were performed using a BAS 100 electrochemical analyzer with a standard three-electrode electrochemical cell in a 0.1 M solution of tetrabutylammonium hexafluorophosphate ((TBA)PF6) 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 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 onset potentials were determined from the intersections of two tangents drawn at the rising currents and background currents of the cyclic voltammetry (CV) measurements. Surface morphology images of thin films (on glass substrates) were obtained using atomic force microscopy (AFM, Digital instrument NS 3a controller with D3100 stage).

2.2.3 Device Fabrication of Polymer Solar Cells.

The polymer solar cells in this study were composed of an active layer of blended polymers (Polymer: PCBM) in solid films, which was sandwiched between a transparent indium tin oxide (ITO) anode and a metal cathode (Ca). 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. 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 glovebox.

Then, on the top of PEDOT:PSS layer, an active layer was prepared by spin coating from blended solutions of polymers:PC61BM (with 1:1 w/w) and PP6DHTBT:PC71BM (with 1:1, 1:3, and 1:4 w/w) subsequently with a spin rate ca.

1500 rpm for 60 s, and the thickness of the active layer was typically ca. 80 nm.

Initially, the blended solutions were prepared by dissolving both polymers and PCBM in 1,2-dichlorobenzene (20 mg/mL), followed by continuous stirring for 12 h at 50^°C.

In the slow-growth approach, blended polymers in solid films were kept in the liquid phase after spin-coating by using the solvent with a high boiling point. 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. The active area of the device was 0.12 cm². All PSC 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 intensity was calibrated by a mono-silicon photodiode with KG-5 color filter (Hamamatsu, Inc.). The external quantum efficiency (EQE) action spectrum was obtained at short-circuit condition.

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.

2.2.4 Synthesis of Monomers and Polymers General Synthetic Procedures for 4a-4c

In a 100 mL flame-dried two-neck flask fitted with a condenser, 1.00 eq of dibromoarene (3a-3c), 2.2 eq of 2-(4-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-

1,3,2-dioxaborolane (2), and 0.03 eq. of tetrakis(triphenylphosphine)palladium was added. The mixture was degassed and purged nitrogen. Afterward, 40 mL of anhydrous toluene and 2 M aqueous potassium carbonate solution (8 mL) was added.

The reaction mixture was heated to 90 ^°C with vigorous stirring until reaction completion by TLC analyses (~ 24 h). The mixture was poured into water (100 mL) and extracted with methylene chloride. The organic layer was washed thrice with water, once with brine and dried over magnesium sulfate. The solvent was evaporated and the residue was purified by column chromatography on silica gel with hexane/ethyl acetate = 20/1 to give the products. Their chemical characterization analyses are shown as follows:

4,7-bis(4-hexylthiophen-2-yl)-2,1,3-benzothiadiazole (4a) Orange needles (yield: 88%); mp 75-77^°C. ¹H NMR (ppm, CDCl₃):  7.97 (dd, 2H), 7.82 (d, J=1.8 Hz 2H), 7.04 (dd, 2H), 2.66 (t, J=7.5 Hz, 4H), 1.70 (m, 4H), 1.25-1.53 (m, 12H), 0.90 (t, J=6.7 Hz, 6H). ¹³C NMR (ppm, CDCl₃):  153.02, 139.75, 128.42, 127.90, 127.21, 126.38,126.15,. 31.68, 29.70, 29.65, 29.03, 22.67, 14.14.

4,7-bis(4-hexylthiophen-2-yl)-2,1,3-benzoselenadiazole (4b) Purple solid (yield: 87%); mp 82-83^°C. ¹H NMR (ppm, CDCl3):  7.87 (d, J= 1.2 Hz, 2H), 7.71 (s, 2H), 7.04 (d, J=1.5 Hz, 2H), 2.68 (t, J=7.8 Hz, 4H), 1.68 (m, 4H), 1.20-1.43 (m, 12H), 0.90 (t, J=6.9 Hz, 6H). ¹³C NMR (ppm, CDCl3):  158.19, 143.98, 139.29, 128.87, 127.42, 125.75,121.83,. 31.68, 30.56, 30.45, 29.04, 22.62, 14.10.

4,7-bis(4-hexylthiophen-2-yl)-2,1,3-benzoxadiazole (4c) Yellow solid (yield:

92%); mp 78-79^°C. ¹H NMR (ppm, CDCl3):  7.95 (d, J= 1.2 Hz, 2H), 7.57 (s, 2H), 7.02 (d, J=1.2 Hz, 2H), 2.67 (t, J=7.6 Hz, 4H), 1.70 (m, 4H), 1.20-1.43 (m, 12H), 0.89

(t, J=7.2 Hz, 6H). ¹³C NMR (ppm, CDCl3):  148.081, 145.28, 137.77, 30.40, 126.32, 122.31, 121.88, 31.90, 30.83, 30.65, 29.24, 22.85, 14.34.

General Bromination Procedures for 5a-5c

In a 100 mL flask, 1.00 eq of 4,7-di(4-hexyl-2-thienyl)-arene (4a-4c) was added into THF under nitrogen flow. After solids were dissolved completely, 2.10 eq N-bromosuccinimide (NBS) was added in portion wise. The reaction mixtures were

stirred at a room temperature for 5 h. Subsequently, hexane was added into the mixture, and the white precipitate formed was filtered off. In addition, the filtrate was extracted with ethyl acetate, and the organic layer was washed with brine followed by being dried over anhydrous sodium sulfate. After that, the residue was purified by column chromatography on silica gel with hexane/methylene chloride = 1/2 to give the products. Their chemical characterization analyses are shown as follows:

4,7-Bis(5-bromo-4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (5a) Red solid (yield: 94%); mp 101-103 ^°C. ¹H NMR (ppm, CDCl₃):  7.75 (s, 2H), 7.71 (s, 2H), 2.63 (t, J=7.2 Hz, 4H), 1.67 (m, 4H), 1.33-1.40 (m, 12H), 0.89 (t, J=7.1 Hz, 6H). ¹³C NMR (ppm, CDCl₃):  152.19, 143.01, 138.46, 128.03, 125.25, 124.80, 111.59, 31.62, 29.74, 29.67, 28.96, 22.64, 14.11. Element Anal. Calcd for C₂₆H₃₀Br₂N₂S₃: C, 49.84%; H, 4.83%; N, 4.47%; Found: C, 49.62%; H, 5.02%; N, 4.62%. EIMS (m/z):

calcd for C₂₆H₃₀ Br₂N₂S₃, 626.53; found, 626.

4,7-Bis(5-bromo-4-hexyl-2-thienyl)-2,1,3-benzoselenadiazole (5b) Purple solid (yield: 96%); mp 92-94 ^°C. ¹H NMR (ppm, CDCl3):  7.65 (s, 2H), 7.64 (s, 2H), 2.62 (t, J=7.2 Hz, 4H), 1.67 (m, 4H), 1.33-1.42 (m, 12H), 0.90 (t, J=6.9 Hz, 6H). (ppm, CDCl3):  157.71, 142.62, 138.73, 127.63, 126.75, 124.93, 112.19, 31.62, 29.74, 29.67, 28.96, 22.64, 14.11. Element Anal. Calcd for C26H30Br2N2S2Se: C, 46.37%; H,

4.49%; N, 4.16. Found: C, 46.78%; H, 5.14%; N, 4.33%. EIMS (m/z): calcd for zine (7) A solution of 3,7-dibromo-10-hexyl-10H-phenothiazine (4.41 g, 10 mmol) in anhydrous tetrahydrofuran (100 mL) was cooled to -78 ^°C under nitrogen and stirred at this temperature for 5 min in the flame-dried two-neck round-bottom flask. n-Butyl lithium (8.4 mL of 2.5 M solution in hexane, 21 mmol) was added dropwise, using a syringe, and the mixture was stirred at -78 ^°C, warmed to 0 ^°C for 15 min, and cooled again at -78^°C for 15 min. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6.13 mL, 30 mmol) was added rapidly to the solution, and the resulting mixture was warmed to room temperature and stirred overnight. The mixture was poured into water and extracted with ether. The organic extracts were washed with brine and dried over magnesium sulfate. The solvent was removed by rotary evaporation, and the residue was recrystalized from acetone to obtain 3.96 g (74%) of the title product as a slight yellow solid ; mp 212-214 ^°C. ¹H NMR (ppm, CDCl3):  7.54 (m, 4H), 6.80 (d, J = 7.8 Hz, 2H), 3.84 (t, J = 6.9 Hz, 2H), 1.78 (m, 2H), 1.4 (m, 2H), 1.32 (s, 24H), 1.25 (m, 4H), 0.86 (t, J = 7.2 Hz, 3H). ¹³C NMR (ppm, CDCl3): 

133.97, 124.15, 114.90, 83.91, 47.71, 31.62, 26.89, 26.72, 25.06, 22.78, 14.21.

Element Anal. Calcd for C₃₀H₄₃B₂NO₄S: C, 67.31%; H, 8.10%, N, 2.62%. Found: C, 66.47%; H, 7.73% N, 2.93%. EIMS (m/z): calcd for C30H43B2NO4S, 535.35; found, 536.

General Polymerization Procedure

All polymerization steps were carried out through the palladium(0)-catalyzed Suzuki coupling reactions. In a 50 mL flame dried two-neck flask, 1 eq of 10-hexyl-3,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-10H-phenothiazine (7), 1 eq of bis(bromo-4-hexyl-2-thienyl) arene (5a-5c), and Pd(PPh₃)₄ (1.5 mol %) were dissolved in a mixture of toluene ([monomer] = 0.5 M) and aqueous 2 M Na₂CO₃ (2:3). The solution was first put under a nitrogen atmosphere and vigorously stirred at 90-95 ^°C for 4-5 days. After reaction completion, an excess of bromobenzene was added to the reaction then one hour later, excess of phenylboronic acid was added and the reaction refluxed overnight to complete the end-capping reaction. The polymer was purified by precipitation in methanol/water (10:1), filtered through 0.45 μm nylon filter and washed on Soxhlet apparatus using hexane, acetone and chloroform. The chloroform fraction was reduced to 40-50 mL under reduced pressure, precipitated in methanol/water (10:1, 500 mL), filtered through 0.45 μm nylon filter and finally air-dried overnight.

Poly[(10-hexyl-10H-phenothiazine-3,7-ylene)-alt-(4,7-bis(4-hexylthien-2-yl)-2,1,3-benzothiadiazole)2’,2’’-diyl] (PP6DHTBT) Dark orange solid (yield: 71%).

1H NMR (ppm, CDCl3):  8.02 (br, 2H), 7.84 (br, 2H), 7.32 (br, 4H), 6.93 (br, 2H), 3.92 (br, 2H), 2.74 (br, 4H), 1.88 (br, 2H), 1.72 (br, 4H), 1.50 (br, 4H), 1.18-1.40 (br, 14H), 0.81-0.90 (br, 9H). Anal. Calcd C, 70.45%; H, 6.85%, N, 5.60%. Found: C, 69.78%; H, 6.97% N, 5.42%.

Poly[(10-hexyl-10H-phenothiazine-3,7-ylene)-alt-(4,7-bis(4-hexylthien-2-yl)-2,1,3- benzoselenadiazole)2’,2’’-diyl] (PP6DHTBSe) Dark black solid (yield: 76%).

1H NMR (ppm, CDCl3):  7.92 (br, 2H), 7.77 (br, 2H), 7.32 (br, 4H), 6.94 (br, 2H), 3.92 (br, 2H), 2.74 (br, 4H), 1.88 (br, 2H), 1.71 (br, 4H), 1.50 (br, 4H), 1.18-1.40 (br, 14H), 0.81-0.90 (br, 9H). Anal. Calcd C, 66.30%; H, 6.45%, N, 5.27%. Found: C, 64.49%; H, 6.38% N, 4.94%.

Poly[(10-hexyl-10H-phenothiazine-3,7-ylene)-alt-(4,7-bis(4-hexylthien-2-yl)-2,1,3- benzoxadiazole)2’,2’’-diyl] (PP6DHTBX) Dark solid (yield: 69%). ¹H NMR (ppm, CDCl3):  8.00 (br, 2H), 7.53 (br, 2H), 7.30 (br, 4H), 6.91 (br, 2H), 3.89 (br, 2H), 2.69 (br, 4H), 1.88 (br, 2H), 1.69 (br, 4H), 1.48 (br, 4H), 1.18-1.40 (br, 14H), 0.81-0.90 (br, 9H). Anal. Calcd C, 71.99%; H, 7.00%, N, 5.72%. Found: C, 72.00%;

H, 6.84% N, 5.75%.

Figure 2.1 Synthetic Routes of Monomers (5a-5c and 7).

Figure 2.2 Synthetic Routes of Polymers (PP6DHTBT, PP6DHTBSe, and PP6DHTBX).

2.3 Results and Discussions

2.3.1 Synthesis and Structural Characterization

The general synthetic routes of monomers 5a-5c and 7 are shown in Figure 2.1.

Synthesis of 2-(4-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2), 4,7-dibromo-2,1,3-benzothiadiazole (3a), 4,7-dibromo-2,1,3-benzoselenadiazole (3b), and 4,7-dibromo-2,1,3-benzoxadiazole (3c) were prepared by following the literature procedures.^56,92 Hexyl-thiophene units were added to both sides of each acceptor units

through the Suzuki coupling reaction between

2-(4-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2) and dibromo arenes (3a-3c) in presence of a catalyst Pd(PPh3)4. Next, these compounds were brominated with NBS to produce monomers 5a-5c. The diboronic ester monomer (7) was prepared according to the literature method,⁵⁴ i.e., alkylation of phenothiazine with 1-bromohexane, followed by bromination with molecular bromine, then lithiation of dibromo compound with n-Buli and quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane produced monomer 7.

Monomers (5a-5c and 7) were satisfactorily characterized by ¹H NMR, ¹³C NMR, MS spectroscopy, and elemental analyses. As shown in Figure 2.2, three alternating polymers PP6DHTBT, PP6DHTBSe, and PP6DHTBX were prepared with the well-known Suzuki polymerization between the diboronic ester of phenothiazine (7) and the dibromide monomers (5a-5c). The obtained polymers were further purified by washing on Soxhlet apparatus using hexane, acetone, and chloroform. The chloroform

Monomers (5a-5c and 7) were satisfactorily characterized by ¹H NMR, ¹³C NMR, MS spectroscopy, and elemental analyses. As shown in Figure 2.2, three alternating polymers PP6DHTBT, PP6DHTBSe, and PP6DHTBX were prepared with the well-known Suzuki polymerization between the diboronic ester of phenothiazine (7) and the dibromide monomers (5a-5c). The obtained polymers were further purified by washing on Soxhlet apparatus using hexane, acetone, and chloroform. The chloroform