Studies of Semiconducting Ladder Polymers for Organic Electronics

Abstract

Organic semiconducting polymers have garnered much interest over the last several decades due to their potential to be fabricated on a large scale for use in low-cost, flexible electronic devices. A key feature of semiconducting polymers is their synthetic tunability though molecular design that allows fine-tuning of their properties and enables the study of their structure-property relationships. The goal of the research presented in this thesis is to synthesize and study the structure-property relationships of semiconducting polymers that feature a double-stranded architecture, called π-conjugated ladder polymers. π-Conjugated ladder polymers by design have properties beneficial for organic electronics including planar and rigid backbones, low conformational disorder, large persistence lengths, and tight intermolecular contacts. However, these polymers are rarely studied due to their difficult synthesis and processing. My first approach to studying the structure-property relationships of semiconducting ladder polymers was to synthesize functionalized and derivatized poly(benzimidazobenzophenanthroline) (BBL) polymers and then compare their properties to those of BBL. Towards this goal, I report the synthesis and properties of BBL-P, a phenazine derivative of BBL (Chapter 2), and two series of electron-deficient random copolymers, BBL-xCN and BBL-xTCN (x = 0, 0.2, 0.35, 0.5) featuring known and novel electron-deficient, cyanated monomers (Chapter 3). Compared to BBL, I found that while phenazine substitution in BBL-P does not impact the lowest unoccupied molecular orbital (LUMO) energy level, the thin films have decreased crystallinity with preferential face-on molecular orientations on substrates, which contributed to a decreased field-effect electron mobility of 1.2 x 10-4 cm2/V s. The random copolymer series BBL-x2CN and BBL-xTCN discussed in Sections 3.1 and 3.2, respectively, demonstrate that as little as 20 mol% of the electron-deficient monomers increased the speed of the polycondensations compared to BBL. Additionally, I found that while 20-50 mol% of the cyanated monomers had little impact on the optical bandgaps, 50 mol% reduced the LUMO levels by ~ 0.2 eV and facilitated the reduction processes. The n-doped conductivity of the BBL-x2CN series was found to decrease with increasing 2CN content. These studies provide important insights into the synthesis and structure-property relationships of electron-deficient BBL derivatives. My second approach focused on N-alkyl side chain engineering of p-type ladder poly(pyrrolobenzothiazine)s (Chapter 4). Here, I found that polymer with N-methyl (LPBT-Me) and N-H (LPBT) groups both have strong intramolecular charge transfer (ICT) character when protonated, and N-methylation does not effect the narrow optical bandgaps of ~1.5 eV. Moreover, temperature-dependent absorption spectroscopy and theoretical calculations revealed that both polymers show planar/non-planar conformations that vary with the degree of protonation; this conformational variation was enhanced by N-methylation, which contributed to a decreased field-effect hole mobility in LPBT-Me. My third approach involved the synthesis and studies of a new p-type thiophene-containing ladder polymer (LTBT) that I discussed in Chapter 5. Although the solubility in strong acids is limited, the polymer demonstrated excellent film-forming properties evidenced by the high quality freestanding and thin films that I used to characterize the molecular structure using infrared and Raman spectroscopies. I found that this polymer has a narrow optical bandgap of 1.28 eV, enhanced protonation-enhanced ICT character extending out into the infrared region, and a modest average electrical conductivity of (3.31 ± 0.31) x 10-1 S/cm when p-doped with FeCl3.