Polymers having one of the three complementary colors (red, green, and blue) in the reduced state and high transmissivity in the oxidized state are key materials towards use in electrochromic devices and displays. Although many neutral state red and blue polymers were reported up to date, neutral state green polymeric materials appear to be limited. For potential application of electrochromic materials in display technologies, one should have to create the entire color spectrum and this can be only achieved by having materials with additive or subtractive primary colors in their neutral states. To obtain a green color there should be at least two simultaneous absorption bands. Although the neutral state color is of great importance, the transmittance in the oxidized state is crucial too. The materials having one of the three primary colors should also possess highly transmissive oxidized states in order to be used in commercial electrochromic device applications. Donor-acceptor molecules lead to lower band gap due to resonances that enable a stronger double bond character between the donor and acceptor units. The materials with low band-gaps produce cathodically coloring polymers due to the lower energy transition in the doped state. Moreover, donor-acceptor type materials commonly show two absorption maxima. Since donor-acceptor approach seems to be the key to the complex nature of producing these materials, novel donor-acceptor type polymers were synthesized, and electrochromic properties were investigated in detail.Additionally a solution-processable donor-acceptor type polymer was realized using method of introducing alkyl side chains in the polymer structures.

Synthesis of new electroactive monomers are highly desired since these compounds can be utilized as active layers in many device applications such as ECDs, LEDs and solar cells. EDOT (3,4 ethylenedioxythiophene) and thiophene bearing polymers were also proven to be excellent candidates as electrochromic materials. Benzotriazole can be coupled to EDOT and thiophene to yield materials that can be polymerized to give donor acceptor type polymers. These materials are promising candidates as components in fast switching polymeric electrochromic devices. Donor acceptor type materials / 2-dodecyl-4-(2,3-dihydrothieno [3,4- b][1,4]dioxin-5-yl)-7- (2,3-dihydrothieno[3,4-b] [1,4] dioxin-7-yl)-2H-benzo [d][1,2,3]triazole and 2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole were synthesized via N-alkylation, bromination, stannylation and Stille coupling reactions. Electrochemical and electrochromic properties of the polymers were examined in detail.

Polymeric electrochromic materials that has as one of the three complementary colors (red, green, and blue) in the neutral form and become transparent via oxidation (or reduction), has a crucial importance towards use of these materials in electrochromic devices and displays. To reflect red or blue color in neutral state, the materials have to absorb at only one dominant wavelength. On the contrary, to have a green color, there should exist at least two simultaneous absorption bands in the red and blue regions of the visible spectrum where these bands should be controlled with the same applied potential. The transmissivity in the oxidized state is significantly important in addition to the neutral state color of the polymer. The optical contrast between the states is the decisive point for use of these materials for many electrochromic applications, especially as smart windows and displays. Hence, the material should possess two absorption bands with definite maximum points, and upon oxidation these bands should simultaneously vanish to have a transmissive state. A donor&ndash / acceptor approach can be utilized to solve this puzzle. It has been shown that insertion of alternating donor&ndash / acceptor units on the polymer backbone leads to a significant decrease in band gap due to the increased double bond character in the structure. In this study novel donor-acceptor type polymers were synthesized, and electrochromic properties were investigated in detail. PBDT is the first green electrochromic material which has a highly transmissive sky blue oxidized state. PDETQ was shown to be one of the few examples of neutral state green polymeric materials in literature. PDEQ has a bluish green color in the neutral state and a highly transmissive light blue oxidized state.

Synthesis of new electroactive monomers are highly desired since these compounds can be utilized as active layers in many device applications such as ECDs, LEDs and solar cells. Pyrrole and selenophene bearing polymers were also proven to be excellent candidates as electrochromic materials. Benzothiodiazole can be coupled to to pyrrole and selenophene yield materials that can be polymerized to give donor acceptor type polymers. These donor-acceptor type materials / Poly(4,7-di(1H-pyrrol-2-yl)benzo[c][1,2,5]thiadiazole P(PYBTPY) and poly(4,7-di(selenophen-2-yl)benzo[c][1,2,5]thiadiazole P(SEBTSE) were synthesized via bromination, stannylation and Stille coupling reactions. Electrochemical and electrochromic properties of the polymers were examined in detail.

In traditional electronics, inorganic materials such as silicon and germanium are used as semiconductors due to their outstanding semiconducting properties. Unfortunately, inorganic materials are rigid due to their high crystalline nature, and processing these materials is complex and expensive. Furthermore, traditional semiconducting materials do not have favorable mechanical properties in applications such as wearable devices and large-area applications with complicated shapes. Conjugated conducting polymers (CCPs) are being explored as alternative materials to conventional semiconductors due to their mechanical properties and high conductivity. CCPs offer properties such as solution and low-temperature processability, flexibility, thermal and optical properties that traditional semiconductors could not provide. These characteristics are essential in Organic Light-Emitting Diodes (OLEDs), Organic Field-Effect Transistors (OFETs), and Photovoltaic (PVs) devices. This dissertation focuses on synthesizing rhodamine- and diketopyrrolopyrrole- containing CCPs. Chapter I focuses on the synthesis, and characterization of polyrhodamine (PRho), a semiconducting conjugated polymer containing the rhodamine core in the polymer’s backbone. PRho was synthesized by the Buchwald-Hartwig polycondensation and characterized for its optical and electrochemical properties. We have discovered that the polymer is electrochemically reversible and stable up to 1000 cycles as recorded by cyclic voltammetry between -0.4 and 1.0 V vs. Ag/AgCl and stable to extreme acidic and basic conditions without noticeable degradation. Remarkably, the polymer has a conductivity in the semiconductor range of 8.38 x 10-2 S cm–1 when treated with 20% HCl. Chapter II focuses on the synthesis and characterization of four different alkenyl flanked diketopyrrolopyrrole (DPP) polymers ( PDPPVTV, PDPPVTT, PDPPV3T, and PDPPV4T) synthesized via Stille polycondensation. Different pi-conjugated segments (alkenyl/ PDPPVTV, thiophene/ PDPPVTT, thienothiophene/ PDPPV3T, and dithienothiophene/ PDPPV4T) were used to tune the optoelectrical properties of the polymers. The effect of the alkenyl groups and different pi-conjugated segments on the optoelectrical and charge mobility properties were determined by UV/visible spectroscopy, cyclic voltammetry, and FET characteristics. Three of the four polymers, except PPP4T, showed good solubility in chloroform. All the polymers showed high thermal stabilities in TGA and semi-crystalline nature in X-Ray diffraction patterns. PDPPVTV and PDPPVTT exhibited hole mobilities of 1.8 x 10-3 cm2 V-1 s-1 and 0.25 cm2 V-1 s-1, respectively. .

Philosophiae Doctor - PhD / This thesis is based on the study of electropolymerization and electrochromism of poly(4,7- dithien-2-yl-2,1,3-benzothiadiazole) (P(DTBT) and its copolymer with 3-methoxythiophene (MOT) in imidazolium ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate (bmimbf4) hydrophilic, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimpf6) and 1- butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimtnf2) hydrophobic ILs. Traditional organic solvents acetonitrile and dichloromethane in the presence of 0.1 M tetrabutylammonium perchlorate (n-Bu4NClO4) as a supportive electrolyte was also used as a medium for comparison. Besides, a more hydrophobic ionic liquid, 1-octyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide (octmimtnf2) was also used for the electrodeposition of poly(3-methoxythiophene) (PMOT). The techniques employed are cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), uv-visible spectroelectrochemistry and scanning electron microscopy (SEM). 4,7-Dithien-2-yl-2,1,3-benzothiadiazole (DTBT) a type of donor –acceptor monomer and 3- methoxythiophene (MOT) has been electropolymerized using ionic liquids as growth and supportive electrolytes.

The present thesis is focused on rational design and synthesis of π-conjugated donor-acceptor (D-A) type oligomers and polymers. It is organized in six different chapters and a brief discussion on the content of the individual chapter is provided below. Chapter 1 briefly describes the charge transport properties of organic semiconductors followed by recent development of different organic semiconducting materials mainly for applications in OFET and solar cells have been highlighted. Chapter 2 explores the synthesis and characterization of two new liquid crystalline, D-A type bithiophene-benzothiazole derivatives. The liquid crystalline properties of the materials have been studied in detail with optical polarizing microscopic images and differential scanning calorimetry and found that these materials possess highly ordered smectic A liquid crystalline phase. Their charge transport properties have also been investigated by fabricating OFET devices. Chapter 3 describes the photophysical properties and OFET performance of quinoxaline based donors-acceptor-donor (D-A-D) type molecules. Depending on the flexibility and rigidity of the conjugated backbone these materials show liquid crystalline behaviour. Investigation of their OFET performance indicated that these molecules exhibit p-type mobility up to 9.7 x 10-4 cm2V-1s-1 and on/ off ratio of 104. Chapter 4 investigates excited state properties and OFET behavior of D-A-D type diketopyrrolopyrrole (DPP) derivatives end-capped with alkoxynaphthalene group. UV-Visible spectroscopy measurement shows strong intramolecular charge transfer (ICT) between donor and acceptor unit. Steady-state and time-resolved fluorescence measurements confirm the formation of excimer. The excited state interactions, the interchromophore separation and geometry of the molecules influence the extent of excimer formation. Finally, the OFET behavior of these DPP based materials has been studied using different dielectric layers. Chapter 5 discusses the synthesis, characterization and properties of two new thieno[3,2-b]thiophene-DPP based donor-acceptor (D-A) type low band gap polymers (PTTDPP-BDT and PTTDPP-BZT). Investigation of OFET performance indicated that polymers exhibited ambipolar behaviour with hole mobility upto 1.0 x 10-3 cm2/Vs and electron mobility upto 8 x 10-5 cm2/Vs. Using polymer PTTDPP-BDT with electron acceptor C70PCBM, power conversion efficiency (PCE) around 3.26% in bulk heterojunction solar cell has been achieved. Chapter 6 describes the approach to tailor the energy levels of conjugated polymers (PTDPP-IDT and PTTDPP-IDT) based on Indacenodithiophene (IDT) coupled with DPP moieties. We have studied the photovoltaic performance of these conjugated polymers by blending with PCBM and P3HT. The importance of these materials in polymer/polymer blend solar cell has been emphasized. The photovoltaic devices with polymer/polymer blend solar cell exhibit high open-circuit voltages (VOC) of ~ 0.8 V. In summary, the work presented in this thesis describes synthesis, characterization and photophysical properties of new organic semiconductors and their importance in optoelectronic devices. This work also describes a general design principle of nonfullerene organic solar cell. The results described here show that these materials have potential application as active components in plastic electronics.