Synthesis and Characterization of Fully Conjugated Ladder Naphthalene Bisimide Copolymers

Fully conjugated ladder copolymers have attracted considerable attention due to their unique fused-ring structure and optoelectronic properties. In this study, two fully conjugated ladder naphthalene diimide (NDI) copolymers, P(NDI-CZL) and P(NDI-TTL) with imine-bridged structures are presented in high yields. Both of the two copolymers have good solubility and high thermal stability. The corresponding compounds with the same structure as the copolymers were synthesized as model system. The yields for each step of the synthesis of the model compounds are higher than 95%. These results suggest that P(NDI-CZL) and P(NDI-TTL) can be synthesized successfully with fewer structural defects. The structures and optoelectronic properties of compounds and copolymers are investigated by NMR, fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-vis), and cyclic voltammetry (CV). Both in solution and as a thin film, the two copolymers show two UV-vis absorption bands (around 300–400 nm and 400–750 nm) and a very weak fluorescence. The collective results suggest that the two fully conjugated ladder copolymers can be used as potential acceptor materials.


Compound NDI-CZA
A mixture of NDI-CZN (1.50 g, 0.71 mmol), Pd/C (70 mg) and THF (10 mL) was heated at reflux and stirred under H2 for overnight. After cooling, Pd/C was removed by filtration, the solution was collected and removed to get the crude black product NDI-CZA (1.43 g, 97%). The compound NDI-CZA was directe to the next step without further purified.

Polymer P(NDI-CZN)
A mixture of P(NDI-CZ) (1.10 g), concentrated nitric acid/acetic acid (2 mL, v/v = 1/3) and chloroform (10 mL) was heated at 40 °C for 24 h. The reaction was then cooled to room temperature, precipitated in methanol, and filtered. The solids were washed three times with methanol, water, methanol and petroleum ether, respectively. Then dried to yield the polymer product P(NDI-CZN) (1.13 g, 96%).

Polymer P(NDI-CZA)
A mixture of P(NDI-CZN) (1.10 g), Pd/C (70 mg) and THF (10 mL) was heated at reflux and stirred under H2 for overnight. After cooling, Pd/C was removed by filtration, the solution was collected and removed to get the black product P(NDI-CZA) (1.03 g).

Polymer P(NDI-CZL)
A mixture of P(NDI-CZA), acetic acid (2 mL) and THF (15 mL) was heated at reflux and stirred under nitrogen for overnight. The reaction was then cooled to room temperature, precipitated in methanol, and filtered. The solids were washed three times with methanol, water, methanol and petroleum ether, respectively. Then dried to yield the polymer product P(NDI-CZL) (0.97 g, the total yield of two steps was 93%). Scheme S3. The synthetic routes to compound NDI-TTL.

Polymer P(NDI-TTN)
A mixture of P(NDI-TT) (1.60 g), fuming nitric acid (2 mL) and chloroform (10 mL) was heated at 70 °C for 10 h. The reaction was then cooled to room temperature, precipitated in methanol, and filtered. The solids were washed three times with methanol, water, methanol and petroleum ether, respectively. Then dried to yield the polymer product P(NDI-TTN) (1.66 g, 95%).

Polymer P(NDI-TTA)
A mixture of P(NDI-TTN) (1.60 g), Pd/C (70 mg) and THF (10 mL) was heated at reflux and stirred under H2 for overnight. After cooling, Pd/C was removed by filtration, the solution was collected and removed to get the black product P(NDI-TTA) (1.45 g).

Polymer P(NDI-TTL)
A mixture of P(NDI-TTA), acetic acid (2 mL) and THF (15 mL) was heated at reflux and stirred under nitrogen for overnight. The reaction was then cooled to room temperature, precipitated in methanol, and filtered. The solids were washed three times with methanol, water, methanol and petroleum ether, respectively. Then dried to yield the polymer product P(NDI-TTL) (1.31 g, the total yield of two steps was 93%).