FLUORINATED ARENE, IMIDE AND LACTAM-FUNCTIONALIZED DONOR ACCEPTOR CONJUGATED POLYMERS: SYNTHESIS, STRUCTURE-PROPERTY AND DEVICE STUDIES After the discovery of doped polyacetylene, organic semiconductor materials are widely studied as high impending active components in consumer electronics. They have received substantial consideration due to their potential for structural tailoring, low cost, large area and mechanically flexible alternatives to common inorganic semiconductors. To acquire maximum use of these materials, it is essential to get a strong idea about their chemical and physical nature. Material chemist has an enormous role to play in this novel area, including development of efficient synthetic methodologies and control the molecular self-assembly and (opto)-electronic properties. The body of this thesis mainly focuses on the substituent effects: how different substituent’s affect the (opto)-electronic properties of the donor-acceptor (D-A) conjugated polymers. The main priority goes to understand, how different alkyl substituent effect to the polymer solubility, crystallinity, thermal properties (eg: glass transition temperature) and morphological order. Three classes of D-A systems were extensively studied in this work. The second chapter mainly focuses on the synthesis and structure-property study of fluorinated arene (TFB) base polymers. Here we used commercially available 1,4-dibromo-2,3,5,6-tetrafluorobenzene (TFB) as the acceptor material and prepare several polymers using 3,3’-dialkyl(3,3’-R2T2) or 3,3’-dialkoxy bithiophene (3,3’-RO2T2) units as electron donors. A detail study was done using 3,3’-bithiophene donor units incorporating branched alkoxy-functionalities by systematic variation of branching position and chain length. The study allowed disentangling the branching effects on (i) aggregation tendency, intermolecular arrangement, (iii) solid state optical energy gaps, and (iv) electronic properties in an overall consistent picture, which might guide future polymer synthesis towards optimized materials for opto-electronic applications. The third chapter mainly focused on the structure-property study of imide functionalized D-A polymers. Here we used thiophene-imide (TPD) as the acceptor moiety and prepare several D-A polymers by varying the donor units. When selecting the donor units, more priority goes to the fused ring systems. One main reason to use imide functionality is due to the, open position of the imide nitrogen, which provides an attaching position to alkyl substituent. Through this we can easily manipulate solubility and solid state packing arrangement. Also these imide acceptors have low-lying LUMOs due to their electron deficient nature and this will allow tuning the optical energy gap by careful choice of donor materials with different electron donating ability. The fourth chapter mainly contribute to the synthesis and structure property study of a completely novel electron acceptor moiety consist of a unsaturated pyrrolidinone unit known as Pechmann dye (PD) core. Pechmann dyes are closely related to the Indigo family. This can refer as 3-butenolide dimer connected via an alkene bridge, containing a benzene ring at the 5 and 5’ positions of the lactone rings. We have prepared several D-A polymers using this PD system with benzodithiophene (BDT) as the donor unit. Different to common D-A polymers the HOMO and LUMO of the PD acceptor moiety are energetically located within the gap of the BDT, so that the electronic and optical properties (HOMO-LUMO transition) are dictated by the PD properties. The promising electronic properties, band gaps, high absorption coefficients and broad absorption suggest this new D-A polymers as an interesting donor material for organic solar cell (OSC) applications.

Due to the drastic required thermodynamical requirements, a photoelectrode material that can function as both a photocathode and a photoanode remains elusive. In this work, we demonstrate for the first time that, under simulated solar light and without co-catalysts, donor-acceptor conjugated acetylenic polymers (CAPs) exhibit both impressive oxygen evolution (OER) and hydrogen evolution (HER) photocurrents in alkaline and neutral medium, respectively. In particular, poly(2,4,6-tris(4-ethynylphenyl)-1,3,5-triazine) (pTET) provides a benchmark OER photocurrent density of ~200 μA cm−2 at 1.23 V vs. reversible hydrogen electrode (RHE) at pH 13 and a remarkable HER photocurrent density of ~190 μA cm−2 at 0.3 V vs. RHE at pH 6.8. By combining theoretical investigations and electrochemical-operando Resonance Raman spectroscopy, we show that the OER proceeds with two different mechanisms, with the electron-depleted triple bonds acting as single-site OER in combination with the C4-C5 atoms of the phenyl rings as dual sites. The HER, instead, occurs via an electron transfer from the tri-acetylenic linkages to the triazine rings, which act as the HER active sites. This work represents a novel application of organic-based materials and contributes to the development of high-performance photoelectrochemical catalysts for the solar fuels’ generation.

Donor-acceptor conjugated polymers opened a new era for conjugated polymer research due to the abundant selection and combination of different conjugated units. This class of polymers function as semiconductor materials with potential application in plastic consumer electronics. The frontier molecular orbital energies of the polymers are generally determined by the selection of donor and acceptor units in the backbone structure, and their substituents. The side chains attached to the backbone not only affect the solubility of the materials, but also their self-assembly and morphological characteristics, which indirectly govern optoelectronic properties. It is important therefore to consider backbone architectures and the side chains together, to control (opto)-electronic properties for specific applications, while also maintaining solution processability without disrupting solid-state packing. The research presented in this dissertation focuses largely on the side chains: how the bulk and position of side chains affect the (opto)-electronic properties of select donor-acceptor (D-A) conjugated polymers. More precisely the intent is to vary the size and position of branches in the alkyl side chains of donor-acceptor polymers, in the attempt to solubilize poorly soluble polymers, without disrupting self-assembly of the polymer backbones into close p-stacks. After an introductory chapter 1, chapter 2 mainly focuses on the synthesis and structure-property study of polymers with 2,3,5,6-tetrafluorobenzene (TFB) as the acceptor motif and benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor units carrying solubilizing substituents. TFB units were chosen based on previous observations that this acceptor unit imparts particularly poor solubility to various donor-acceptor copolymers. The current study indicates that bulky branches placed close to the polymer backbone could solubilize the PBDTTFB copolymers without altering the absorption profile and oxidation potentials. Optical, wide-angle x-ray diffraction (WAXD) and solubility studies shows that solubility is closely related to branching size and position. As the branch size in increased, the solubility of these polymers undergoes a step-change. The third chapter mainly focuses on the structure-property study of D-A polymers with thienopyrroledione (TPD) as acceptor. Unlike TFB, this acceptor can carry additional side chains that can compete with the space-filling demands of the donor unit side chains. As donor, the rigid BDT unit was compared with 3,3’-dialkoxy-2,2’bithiophene (RO2T2) units which have a similar size, but contain a “swiveling” central σ-bond. Bulkiness of side chains attached to the T2 units should be expected to have a more severe impact, possibly causing the two thiophene units of the T2 units to twist out of plane. It was demonstrated that alkoxy side chains with bulky branches in close proximity to the polymer backbones does not disrupt conjugation in these polymers. The UV-Vis absorption spectra of RO2T2-TPD polymers were red-shifted (more than 120 nm) in comparison to PBDTTPD polymers due to the smaller Eg (energy gap), which might be attributed to the expected higher energy HOMO imparted by the donor unit. The π-π stacking of polymers with BDT units was little affected by the bulky side chains. However, the π-π stacking of polymers with RO2T2 units was much more sensitive to side-chain bulk, with high degree of order and close π-π stacking only if proper local free spacing exists for side-chain interdigitation. Chapter 4 reports efforts to study polymers from the same set of RO2T2 monomers studied in Chapter 3, but without acceptor units that might otherwise drive self-assembly. RO2T2 homopolymers were synthesized via the Grignard metathesis (GRIM) method. Further, copolymers were prepared with RO2T2 units alternating with thiophene, thieno[3,2-b]thiophene or bithiophene. The spectroscopic studies suggest these polymers with bulky side chains exhibit some varying level of backbone conjugation. Somewhat surprisingly, despite an expected decrease in the strength of intermolecular donor-acceptor interactions, the solubilities were in some cases low, but varied with volume fraction of side chains. Further, even for polymers that appear to easily dissolve, aggregation in solution is so extensive as to give ensembles “too large” for characterization by GPC and or solution NMR. Oxidation potentials seem essentially insensitive to any of the structural variables (governed mostly by the backbone RO2T2 units).

The present thesis is focused on the rational design of Diketopyrrolopyrrole based π- conjugated polymers for organic electronics. The thesis is organized into six different chapters and a brief description of the individual chapters is provided below. Chapter 1 briefly describes the physics governing the electronic processes occurring in organic photovoltaics (OPVs) and organic field-effect transistors (OFETs) followed by design rules for the synthesis of conjugated polymers for organic electronics. Diketopyrrolopyrrole (DPP) based π-conjugated materials and their development in OPVs and OFETs have been highlighted. Chapter 2 discusses the synthesis and characterization of a series of small molecules of DPP derivatives attached with different alkyl chains. Influence of side chains on the photophysical properties of these DPP derivatives have been studied by UV-visible spectroscopy and DFT calculations. Crystal structure studies revealed the effect of alkyl chains on the torsional angle, crystal packing, and intermolecular interactions such as π-π stacking. Chapter 3 reports the synthesis of novel diketopyrrolopyrrole-diketopyrrolopyrrole (DPPDPP) based conjugated copolymers and their application in high mobility organic field-effect transistors. Effect of insulating alkyl chains on polymer thin film morphology, lamellar packing and π-π stacking interactions have been studied in detail. Investigation of OFET performance of these DPP-DPP copolymers with branched alkyl chains (N-CS2DPP-ODEH) resulted in low charge carrier mobilities as compared to the polymers (N-CS2DPP-ODHE) with linear alkyl chains. Polymer with triethylene glycol side chains (N-CS2DPP-ODTEG) exhibited a high field-effect electron mobility value of ~3 cm2V-1s-1 with a very low threshold voltage of ~2 V. Chapter 4 investigates the effect of torsional angle on the intermolecular interactions and charge transport properties of diketopyrrolopyrrole (DPP) based polymers (PPDPP-OD-HE and PPDPP-OD-TEG). Grazing incidence x-ray diffraction studies shows the different orientation of the polymer crystallites and lamellar packing involved in polymer thin films. Investigation of OFETs evidenced the effect of torsional angle on the charge transport properties where the polymer with higher torsional angle PPDPP-OD-TEG resulted in high threshold voltage with less charge carrier mobility compared to the polymer with lower torsional angle (N-CS2DPP-OD-TEG). Chapter 5 investigates the effect of photoactive material morphology on the solar cell device performance, and charge transfer kinetics by adding high boiling point processing additives. DPP based donor-acceptor (D-A) type low band gap polymers (PTDPPQ and PPDPPQ) have been synthesized and employed in bulk-heterojunction (BHJ) solar cells with the acceptor PC71BM. Addition of processing additive 1,8-diiodooctane (DIO) resulted in three order improvements in power conversion efficiency (PCE). Chapter 6 describes the design and synthesis of two diketopyrrolopyrrole based copolymers (PPDPP-BBT and PTDPP-BBT) for their application in organic devices such as field-effect transistors and bulk-heterojunction solar cells. Investigation of OFET performance of these DPP based copolymers displayed hole mobilities in the order of 10-3 cm2V-1s-1. The semiconductor-dielectric interface has been characterized by capacitance-voltage, and Raman scattering methods. In summary, the work presented in this thesis describes the synthesis and characterization of diketopyrrolopyrrole based new polymeric semiconductors. Effects of insulating side chains and torsional angle on the charge transport properties of these polymers in OFETs have been investigated. This work also describes the effect of solvent additives on the active layer morphology and BHJ solar cell device performance. The results described here show that these materials have potential application as active components in plastic electronics.