Photophysical properties of 2-phenylanthracene and its conformationally-stabilized derivatives

Abstract

A synthetic route to a series of anthracene derivatives consisting of 2-phenylanthracene and its conformationally-stabilized planar derivatives, containing carbon, oxygen, sulphur and nitrogen atoms, is reported. Thermal, optical and electrical properties of the derivatives featuring an extended π-conjugated electron system were also investigated. Experimental results were supplemented by the energy spectra and molecular orbital calculations performed in the framework of density functional theory. Some of the compounds were found to be molecular glasses with glass transition temperatures from 28 to 55 °C as revealed by differential scanning calorimetry. The anthracene compounds in dilute solutions were found to emit in the 400–500 nm wavelength range with the fluorescence quantum yields varying from 0.22 up to 0.46. The solid films of the anthracene derivatives exhibited slightly narrower emission wavelength-tunability with clearly-expressed excimer emission bands (for the benzothiophene- and indole-anthracene compounds) causing a significant lowering of the fluorescence quantum efficiency down to 0.005. Rigidity of the studied anthracene molecules was assessed by evaluating the Huang-Rhys factor (indicating strength of electron-vibrational coupling) from the vibronic profile of the fluorescence spectra, which indicated that the cyclized indole- and benzofuran-anthracenes possess the most rigid molecular structures as opposed to more flexible structures of 2-phenylanthracene and indeno-anthracene. Wet-casted films of the anthracene compounds demonstrated moderate charge carrier drift mobilities, in the order of 10−5–10−4 cm2/V, which are considered as sufficient for light-emitter applications. Moreover, introduction of additional C, O, S and N atoms into 2-phanylanthracene molecule enabled a tuning of the HOMO level from 4.9 up to 5.74 eV, which is crucial for the optimization of hole injection into the active layers, and thus for balancing electron and hole currents in light-emitting devices.