Ultrahigh electron-deficient pyrrolo-acenaphtho-pyridazine-dione based donor–acceptor conjugated polymers for electrochromic applications

Novel electron acceptors 2-alkyl-6,9-di(thiophen-2-yl)-1H-pyridazino[4′,5′:2,3]indeno[6,7,1-def]isoquinoline-1,3(2H)-dione and 2-alkyl-6,9-di(furan-2-yl)-1H-pyridazino[4′,5′:2,3]indeno[6,7,1-def]isoquinoline-1,3(2H)-dione derived from pyrrolo-acenaphtho-pyridazine-diones (PAPD) with a very low-lying lowest unoccupied molecular orbital (LUMO) level have been synthesized via a regioselective inverse electron demand Diels–Alder reaction between thiophene- and furan-substituted 1,2,4,5-tetrazine and an electron-deficient compound 2-(2-alkyl)-1H-indeno[6,7,1-def]isoquinoline-1,3(2H)-dione. The chemical structures of two PAPD monomers were confirmed by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry as well as single-crystal X-ray structural analysis. The time-dependent density functional theory (TD-DFT) calculations were performed to show that PAPD series monomers have a LUMO energy level down to −3.42 eV, much lower than the popular electron acceptors such as benzotriazole, benzothiadiazole and its fluorinated derivatives (−2.19–−2.98 eV). The PAPD based monomer was incorporated into a series of donor–acceptor type conjugated polymers comprising 3,3-bis((dodecyloxy)methyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine and thieno[3,2-b]thiophene as co-monomers through Stille coupling polymerization to give electrochromic conjugated polymers P1–P5 with high number-average molecular weights in the range of 42 000–67 000 (g mol−1). The polymers showed optical bandgaps between 1.90–1.99 eV. Electrochromic devices displayed reversible color changes between purple/red in the neutral state and greyish blue/grey in the oxidized state with an outstanding redox stability of less than 1% decrease in contrast after 800 cycles using polymer P3 as an example, and high optical contrasts of up to 80% at 1500 nm in the near infrared region.