Tunable optoelectronic performances of novel 7H-benzo[c]fluoren-7-one acceptor unit-based conjugated D-A polymers

In this paper, two novel D-A-D type molecules (EBZFO1 and TBZFO2), which consisting of 7H-benzo[c]fluoren-7-one as electron acceptor (A) unit, 3,4-ethylenedioxythiophene (EDOT) and thiophene (Th) as the terminal donor (D) units, were designed and synthesized. Afterwards, the corresponding D-A polymers of P(EBZFO1) and P(TBZFO2) were prepared by electrochemical deposition method. The electrochemical behavior of the monomers, electrochemical, and electrochromic properties of the D-A polymers were comparatively studied. Although both of the monomers have lower onset oxidation potentials (1.0V/1.2V) and good adherence on the electrode, the redox stability of their polymers was very different. P(EBZFO1) displayed much better redox stability (remaining 78.4% electroactivity after 1000 cycles), good optical stability (2% reduction in optical contrast after 1500s), and stable optical memory, while P(TBZFO2) remained inferior redox stability with only 8.82% of its original electroactivity was retained after 1000 cycles. As a result, higher optical band gap of P(TBZFO2) (1.65eV) displayed much worse electrochromic performances with the normal optical contrast (11.4%~46%) and relatively slow switching time (2.5~3.8s). By contrast, P(EBZFO1) (band gap: 1.55eV) showed favorable optical contrast (15.8%) at 1100nm with a very fast response time of 0.24s, and decent coloration efficiency (228.8 cm2 C-1 at 433nm). From these results, P(EBZFO1) can further render an appropriate electrochromic material for the smart windows or displays.