(714d) 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 ¦Ð-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 (T_g) of the
sythesized compounds was determined by differential scanning calorimetry (DSC).
The two compounds possess higher T_g 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