Characterization and optical properties of oligoazomethines with triphenylamine moieties exhibiting blue, blue-green and green light

Abstract

New photoluminescence oligoazomethines possessing both hole and electron-transporting units in the main chain were synthesized. Triphenylamine (TPA) was used as the electron-donating group, while 4,4′-diaminooctafluorobiphenyl, 4,4′-(hexafluoro-isopropylidene)dianiline, 4-aminophenylsulfone, 4,4′-(4,4′-isopropylidenediphenyl-1,1′diyldioxy)dianiline and 2,5-bis(4-aminophenyl)-1,3,5-oxadiazole were used as the electron-acceptor or as the electron-donating group. The bifunctional oligomers (D-π-A and D-π-D) were soluble in some organic solvents such as chloroform, DMA, HMPA, NMP and formed transparent films on glass support. All oligomers exhibit high glass transition temperature in the range of 188–227 °C as determined by differential scanning calorimetry (DSC). The photoluminescence (PL) emission maximum peaks of the oligomers in solution are in the range of 459–552 nm (2.70–2.25 eV) corresponding to blue, blue-green or green light. The solvatochromic and protonation behavior of the oligomers in two solvents (DMA, chloroform) were detected. Oligomers protonated with methanesulfonic acid (MSA) are hypsochromically shifted with respect to the PL spectra of the pristine oligomers measured in solution. Relative PL intensity of the oligomers investigated in chloroform solution was found in the range of 0.10–0.50%, while for protonated ones it was detected in the range of 33–50% in relation to 9,10-diphenylanthracene. Blends of the oligoazomethines with poly(methylmethacrylate) (PMMA) (0.1%, w/w) emitted blue light. Thin films were also doped with iodine. Calculated energy gap for the undoped films following the Tauc relation was in the range of 2.46–2.69 eV while for the iodine doped oligomers in the range of 1.76–2.38 eV was found.