Electrochromic polyiminocarbazolylenes with latent hydrogen bonding

Abstract

Polyiminoarylenes with carbazole (cbz) units in the main chain, or main and side chain and thermally cleavable t-butyloxycarbonyl (boc) groups were prepared upon palladium-catalyzed coupling reactions. Polymers were prepared from 2,7-dibromo-9-(2-ethylhexyl)-carbazole M1 and 9-(4-aminophenyl)carbazole hydrochloride M3 (P1-EH), 2,7-dibromo-9-t-butyloxycarbonyl-carbazole M2 and M3 (P1-BOC), M1 and 4-t-butylaniline M4 (P2-EH), or M2 and M3 (P2-BOC). Thermal treatment of boc-substituted polymers P1-BOC and P2-BOC at 180 °C for ≥4 h led to removal of the boc groups and formation of insoluble polymers P1-H and P2-H with strong adhesion to the substrate due to hydrogen bonding (‘latent hydrogen bonding’). For P1-BOC the rate constant k of deprotection could be determined to be about 0.050 min⁻¹. Size exclusion chromatography indicated that the molecular weights of the polymers were between 14 100 and 28 100 g mol⁻¹. Polymers P1-EH, P1-BOC, P2-EH and P2-BOC were readily soluble in toluene, THF, dichloromethane, chloroform, and benzene, and insoluble in acetone and methanol. After removal of the boc groups the resulting polymer films of P1-H and P2-H were insoluble in common organic solvents. Optical and electrochemical properties of the polymers were investigated. Due to the electron-rich nitrogen atoms in the main chain, the polymers can be easily oxidized. Oxidation starts at about 0.31 V vs. SCE with formation of cation radicals. Further oxidation occurs at 0.55 V with formation of dications, and at 0.71 V with oxidation of main chain cbz units. At about 1 V the side chain cbz units are oxidized. The polymers are electrochromic and exhibit color changes from colorless via orange to green and blue. Cbz oxidation at potentials ≥0.71 V leads to the formation of dimers in a side reaction. Thermal cleavage of the boc groups leads to well-adhering films of P1-H and P2-H due to hydrogen bonding of the carbazole N–H-groups between polymer chains, and polymer and ITO-substrate. The best performance with regard to reversible oxidation and adhesion was found for P1-H. For this polymer a reversible electrochromism was found. A contrast ratio of 12% at 900 nm, and a switching time of 400 ms were determined.