Ambipolar conjugated ladder polymers by room-temperature Knoevenagel polymerization

Abstract

Two soluble conjugated ladder polymers (cLPs), decorated with multiple electron-poor species (i.e., cyano groups, fused pentagons, and N-heterocyclic rings), have been synthesized from the newly developed tetraketo-functionalized double aza[5]helicene building blocks using a single-step Knoevenagel polycondensation strategy. This facile approach features mild conditions (e.g., room temperature) and high efficiency, allowing us to quickly access a nonalternant ladder-like conjugated system with the in situ formation of multicyano substituents in the backbone. Analysis by ¹H NMR, FT-Raman, and FT-IR spectra confirms the successful synthesis of the resulting cLPs. The combination of theoretical calculations and experimental characterizations reveals that the slightly contorted geometry coupled with a random assignment of trans- and cis-isomeric repeating units in each main chain contributes to improving the solubility of such rigid, multicyano nanoribbon systems. Apart from outstanding thermal stability, the resulting cLPs exhibit attractive red fluorescence, excellent redox properties, and strong π-π interactions coupled with orderly face-on packing in their thin-film states. They are proven to be the first example of ambipolar cLPs that show satisfactory hole and electron mobilities of up to 0.01 and 0.01 cm² V^-1 s^-1, respectively. As we demonstrate, the Knoevenagel polycondensation chemistries open a new window to create complex and unique ladder-like nanoribbon systems under mild reaction conditions that are otherwise challenging to achieve.

Fig. 1. The well-known BBL synthesized from a single-step high-temperature polycondensation route. Room-temperature Knoevenagel condensation strategies toward generation of the known n-type small molecules (TCDADIs) and the two novel conjugated ladder polymers (P1 and P2) studied in this work.

Scheme 1. Synthetic strategies to contorted double aza[5]helicene model compounds (N1 and N2) and their conjugated ladder polymers (P1 and P2).

Fig. 2. Ground-state structures of small-molecule models, N1 (a) and N2 (b), and polymeric dimer models, trans-/cis-P1 (c and d) and trans-/cis-P2 (e and f), calculated at the DFT//B3LYP/def2-SVP level. (g) X-ray crystallographic structures of enantiomers (P,P)-N2 and (M,M)-N2, and geometric details for twisted dihedral angles (9.0°) and H⋯N interactions (2.21 Å). (h) Molecular packing arrangement of racemic N2. Hydrogen atoms and 2-decyltetradecyl side-chains are omitted for clarity.

Fig. 3. ¹H NMR spectra of model small molecules (N1 and N2) and their polymers (P1 and P2).

Fig. 4. Normalized absorption spectra of P1 (a) and P2 (b) in both chlorobenzene solution (purple lines) and as thin films (red lines). Normalized PL spectra (blue lines) in chlorobenzene solution.

Fig. 5. (a) CV curves of N1 and N2 measured in chloroform solution; (b) CV curves of P1 and P2 measured in acetonitrile solution.

Fig. 6. (a and b) 2D-GIWAXS diffraction patterns and (c and d) AFM images of P1 and P2 films; (e) transfer and (f) output curves of P1-based OFETs; (g) transfer and (h) output curves of P2-based OFETs.