Morphology and spectral properties of thin films of benzoxazole derivatives of biphenyl

Abstract

We have studied the spectral properties and morphology of thin films (TVD films) formed by thermal vacuum deposition of 4,4′-bis[(E)-1-(1,3-benzoxazol-2-yl)-2-ethenyl]-1,1′-biphenyl and its substituted derivatives. We have shown that introducing bulky 2,2′-oxyhexyl substituents into the biphenyl units leads to a shift of the fluorescence maximum for the TVD films toward shorter wavelengths, a decrease in their photostability, and aggregation of the films during storage.Introduction. Polyphenylenevinylene (PPV) and its derivatives have been widely used in designing electroluminescent thin-film materials used as components of electronic devices for various purposes [1,2]. However, the low solubility of PPV limits its use, since in making electroluminescent cells the polymer layer is usually formed from solutions by centrifuging. Generally introducing bulky side substituents into the PPV backbone increases its solubility and reduces the tendency toward crystallization in the thin-film state [3,4]. In order to study the effect of substituents on the optical properties of PPV and also the use of PPV as emission layers in light-emitting diodes, its low-molecular weight analogs can certainly be of interest: polyconjugated derivatives of biphenyl [5]. Depending on the electrondonor or electron-acceptor character and steric factors, substituents in these model derivatives affect their electrophysical and optical characteristics [6]. A very important property of biphenyls is also their ability to form thin films by both the liquid method and the thermal vacuum deposition (TVD) method.The aim of this work was to establish the effect of the substituents in benzoxazole derivatives of biphenyl on the spectral characteristics and morphology of their TVD films. Benzoxazole derivatives have, along with electroluminescent properties, good electronic conductivity and can be used as one of the layers in making photoluminescent and electroluminescent devices [7].The Experiment. Substituted biphenyls having trans-trans stereochemistry of the double bonds were synthesized by reaction of the corresponding biphenyl dicarbaldehydes with 2-methylbenzoxazoles [8]: