Tuning the bandgaps of polyazomethines containing triphenylamine by different linkage sites of dialdhyde monomers

Two series of conjugated aromatic polyazomethines (PAMs) have been designed and obtained by solution polycondensation of N,N′-bis(4-aminophenyl)-N,N′-di-2-naphthalenyl-4-benzene diamine, N,N′-bis(4-aminophenyl)-N,N′-di-1-naphthalenyl-(1,1′-biphenhyl)-4,4′-diamine with different aromatic dialdehydes, namely terephthalaldehyde, 1,3-isophthalaldehyde, phthaldialdehyde, 4,4′-diformyltriphenylamine, 2,5-thiophenedialdehyde, respectively. The structures of polymers were characterized by means of FTIR, 1H NMR spectroscopy, which showed an agreement with the proposed structure. All the polymers were amorphous determined by XRD technique with good solubility in many organic solvents, such as CHCl3, THF, N-methyl-2-pyrrolidinone (NMP) and N,N′-dimethylacetamide (DMAc), and could be cast into tough and flexible polymer films. The morphologies of PAMs have been detected by atom force microscopy (AFM) which showed more planar surface.All polymers displayed outstanding thermal stability, i.e. 20 wt% loss in excess of 515 °C under nitrogen atmosphere. Hybrid density functional theory (DFT) method was used to calculate the optimized geometry and electronic structure of PAMs. The calculated HOMO and LUMO energy levels of these polymers by theory method are in the range of −4.879 to −5.494 and −2.344 to −2.927 eV vs. the vacuum level, respectively, which are similar to the experiment results that indicated the PAMs would be used in organic layer electroluminescent diode (OLED) and photovoltagic cell as hole transporting materials or functional dyes. All obtained PAMs revealed excellent stability of electrochromic characteristics, changing color from original yellowish to red or blue. And multiple reversible colors states were observed upon I2, HCl, FeCl3, UV irradiation doping and dedoping. The properties prove that the PAMs are multifunctional materials which will be subject of hole-transporting, electrochromic, chemical sensor application in near future.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The energy levels and band gaps of the polymers could be effectively tuned by alternating the dialdehyde unit and biphenyl ring in the main chains. ► The PAMs exhibit good solubility in common organic solvents and excellent thermal stability. ► The obtained PAMs show sensitive multi-colored chromic properties under stimulation of potential, pH and chemical reagents.