Phenazine-Substituted Poly(benzimidazobenzophenanthrolinedione): Electronic Structure, Thin Film Morphology, Electron Transport, and Mechanical Properties of an n-Type Semiconducting Ladder Polymer

Abstract

Unlike naphthalene diimides, perylene diimides, and other classes of n-type conjugated polymers with numerous derivatives that enable understanding of structure–property relationships, the electronic structure and properties have not been reported for any derivative of ladder poly­(benzimidazobenzophenanthroline) (BBL). Herein, we report the synthesis and properties of BBL-P, a phenazine derivative of BBL. In acid solution, BBL-P has a broad absorption spectrum with a lowest energy absorption peak at 840 nm due to protonation-enhanced intramolecular charge transfer. Compared to BBL, BBL-P thin films have decreased crystallinity with face-on molecular orientations on substrates, resulting in a substantially decreased field-effect electron mobility of 1.2 × 10–4 cm2/V s. BBL-P films have excellent mechanical properties exemplified by a Young modulus of 11 GPa. The results demonstrate that BBL-P is a promising n-type semiconducting polymer and provide new insights into the effects of backbone structure on electronic structure, thin film microstructure, and charge transport properties of conjugated ladder polymers.

p-Type Semiconducting Ladder Poly(pyrrolobenzothiazine)s: Effects of N-Alkyl Side Chains on the Chain Conformation, Electronic Structure, and Charge Transport Properties

“…In MSA solution (Figure a), LPBT has two well-resolved high-energy bands at 354 nm (ε max = 1.05 × 10 4 M –1 cm –1 ) and 502 nm (ε max = 1.06 × 10 4 M –1 cm –1 ), of which the latter can be assigned to the π–π* transition. The intense lowest-energy absorption band centered at 933 nm originates from protonation-enhanced intramolecular charge transfer. , This phenomenon is seen in many imine nitrogen-containing π-conjugated polymers in protic acid solutions where the ICT band is significantly red-shifted relative to the π–π* band. ,− The effects of protonation on the ICT band are corroborated with DFT calculations shown in Figure S6a, which show protonation (+1 and +6 protonation states) induces spatial localization of the HOMO and LUMO molecular orbitals that correspond to the ICT character seen in the optical absorption spectra. ,, …”

“…Thus, we can conclude that while the double-stranded ladder architecture explored here has not eliminated structural fluctuations, it has suppressed them compared to the single-stranded polymer architecture. It is likely that the complete electron delocalization and lack of ICT character in BBL previously shown by DFT calculations enhances its rigidity in protic acids. Indeed, the protonation-enhanced flexibility of the ladder backbone does impact the solid-state morphology and charge transport properties, which are discussed below.…”

“…Solutions of both polymers were prepared in deuterated nitromethane containing GaCl 3 according to the known procedures . However, we point out the use of the Lewis-acid solutions as NMR solvents is known to produce impurities in the NMR spectra; , these impurities appear at ∼5.9 ppm, and ∼6.3–6.5 ppm for both polymers (Figure S2). The NMR spectra show two main peaks from the benzene protons at 7.2 and 8.1 ppm for LPBT, and a cluster of peaks ∼7.7–8.6 ppm for LPBT-Me.…”

“…It is well known that the backbone conformation of π-conjugated polymers directly impacts their electronic structure and properties including molecular orbital distribution, reorganizational energy during charge transfer process, intermolecular transfer integrals, , and the degree of electronic delocalization. In contrast to semiflexible polymers that show conformational torsions and rotations along the single bonds that link the polymer repeat units, π-conjugated ladder polymers feature a double-stranded architecture that is thought to limit conformational distortions and promote backbone rigidity. ,− The most notable member of the π-conjugated ladder polymers, poly­(benzimidazobenzophenanthroline) (BBL) (Chart ), first synthesized in 1966 for aerospace applications due to its extremely high thermal and chemical stability, has been successfully used in a variety of organic electronic and optoelectronic applications over the past 3 decades. ,− BBL and its derivatives are the only known examples of π-conjugated ladder polymers that have rigid-rod backbones in solution and solid state; − the scarcity is primarily because of the difficulty in the synthesis of π-conjugated ladder polymers in general. ,,− For example, in polymers such as BBL and ladder-type polyaniline derivatives, installing basic sites such as imine nitrogens in the backbone is a key design feature that enables either processing in protic acids or offers unique properties such as acid-doped conductivity. Beyond BBL, there is one other conjugated ladder polymer, called ladder poly­( p -phenylene) (LPPP), that has been subjected to rigorous studies of its backbone conformation and persistence length .…”

Observation of super-Nernstian proton-coupled electron transfer and elucidation of nature of charge carriers in a multiredox conjugated polymer

“…Furthermore, a high doping level could be achieved, which will aid in distinguishing whether polaron pairs or bipolarons are the multiply charged species in heavily doped BBL-P. Furthermore, the more disordered nature of BBL-P 28 will help to stabilize the formation of multiply charged species, 29,30 which otherwise would be harder in the more crystalline parent polymer, BBL (Chart 1a). 31,32…”