255549 Synthesis, Optical and Electrochemical Properties of Novel Luminescent Compounds Containing Triphenylamine Units
Organic semiconductors have attracted much interest due to the application in organic light-emitting devices (OLEDs), in particular for next generation flat-panel display. Layered devices consisting of charge-transporting and charge-emitting layers can achieve higher charge injection efficiency and better charge balance than those of single layer devices using emitting materials alone. The hole-transporting layer in the layered OLEDs can impact the efficiency of injection and formation of exciton, thus influencing the luminance, efficiency and working life of the device directly. In this paper, two novel luminescent hole-transporting materials (HTMs) containing triphenylamine and olefinic linkers (Fig. 1) were synthesized via wittig reaction and characterized by 1H NMR, FT-IR, and MS. The compounds show good solubility in common organic solvents such as chloroform, tetrahydrofuran and so on. Optical, electrochemical and thermal properties have been investigated. Quantum chemical calculations were carried out through the Gaussian 03 program at the B3LYP/6-31G* level. The results show the molecules of compounds TM1 and TM2 present linear structures and proper highest occupied molecular orbital (HOMO) levels. The UV-Vis absorption and fluorescence emission spectra of the two compounds in dilute tetrahydrofuran were measured. The compounds exhibit two absorption peaks at 302 nm & 403 nm (TM 1) and 303 nm & 407 nm (TM 2), respectively. The maxima emission peaks are located at 519 nm (TM 1) and 456 nm (TM 2), corresponding to blue-green light emission. The results reveals that along with the increase of the дл-conjugation of the orbitals, compounds have a much stronger absorption in the visible area, thereby the fluorescence peaks of the two compounds red-shifted by 63 nm. Glass-transition temperature (Tg) of the sythesized compounds was determined by differential scanning calorimetry (DSC). The two compounds possess higher Tg than that of widely used HTM such as NPB, suggestting that the two compounds have a good morphological stability. Cyclic voltammetry (CV) was used to assess the the ionization potentials and electrochemical properties of the compounds. Results show that these compounds have proper HOMO levels for hole injection consistented with the calculated data. The HOMO levels of two compounds are higher than that of NPB (-5.40 eV). The experimental results indicate that these compounds have good solubility, high thermal stability, proper HOMO energy level for hole injection, thus these compounds can be candidates for the HTMs in OLEDs.
Fig.1 Molecular sructures of compounds, TM1 and TM2
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