Synthesis And Self-Assembly Properties of Chiral Diketopyrrolopyrrole Based Copolymers
Applications of conjugated polymer (CPs) in optoelectronic devices are critically depend on nature of thin film morphology. In thin film of CPs, the distribution of conjugation length is highly heterogeneous because of conformational defects, distortions of polymer chain and aggregates. A greater understanding of the self-assembly properties of polymer in solution, in particular control over aggregation leads to richer description of electronic properties and hence reproducible fabrication of thin film devices. Recently, chiral CPs have attracted profound interest because of their promising chiroptical properties in thin films and easy control over the selective agglomeration process. In this thesis, we have investigated the role of chiral side-chains on a series of thiophene diketopyrrolopyrrole (TDPP)-benzodithiophene (BDT) based copolymers. Chiral 3,7-dimethyloctyl chain was introduced as an asymmetric chain to incorporate chirality on one of the repeating unit (TDPP) of copolymers. Two polymers with side-chains of identical chirality (S),(S)-PTDPP-BDT; (R),(R)-PTDPP-BDT and a third polymer with similar side-chains of opposite chirality (R),(S)-PTDPP-BDT were synthesized. The chiroptical properties were investigated by UV-visible and circular dichroism (CD) spectroscopy. Figure 1: The structure of the TDPP-BDT copolymers. The copolymers dissolved in a good solvent (e.g. chloroform, chlorobenzene) in which polymers adopts random coil conformation, no chiral response has been observed. However, a critical addition of non-solvent (methanol), the copolymers stack in a chiral fashion and leads to typical bisignate Cotton effects. It is noteworthy that the two polymers, (S),(S)-PTDPP-BDT and (R),(R)-PTDPP-BDT exhibiting a nearly ideal mirror-image relationship in CD spectra (Figure 2a) whereas the (R),(S)-PTDPP-BDT lacks chiropticity even with the addition of methanol. The aggregation induced CD phenomena are dependent on the temperature of solution and do not exhibit reversibility in a heating-cooling cycle. Figure 2: (a) The mirror image Cotton effects of (S),(S)-PTDPP-BDT and (R),(R)-PTDPP-BDT (b) No CD signal was observed for the (R),(S)-PTDPP-BDT polymer. Figure 3: The variation of (a) UV-vis and (b) CD spectra of (R),(R)-PTDPP-BDT polymer with thickness of the solid film. To investigate the role of thickness and annealing temperature on optical and chiroptical properties of polymer films, thin films were prepared using drop-casting method from a solution of chlorobenzene. Both the polymer showed gradual enhancement of CD signal with the increase of film thickness but we did not see any such order with temperature (Figure 3). Figure 4: The morphology observed for the film by (a) AFM; (b); (c) FESEM. The thin film morphology of polymers is characterized by atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) (Figure 4). AFM studies show the polymer molecules self-assembled and formed interconnected nanofibers. Whereas FESEM images clearly revealed that, the nanofibers of polymers are predominantly stack in a chiral fashion and mimic a one-handed helix which leads to bisignate Cotton effects. The (S),(S)-PTDPP-BDT and (R),(R)-PTDPP-BDT form fibers with opposite handedness whereas (R),(S)-PTDPP-BDT do not have such preferred handedness. The research described in this thesis aims to explore the role of chiral side-chains to impose chiral stacking and hence resulting chiral expression. Chirality in this class of polymers may endows them promising optoelectronic properties. (For figures pl see the abstract pdf file)
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