Emission mechanisms for OLEDs and their characteristics.

No abstract

Substituent effects on the photophysical and electrochemical properties of 1,7-diaryl-substituted perylene diimides (1,7-Ar(2)PDIs) have been carefully explored. Progressive red-shifts of the absorption and emission maxima were observed when the electron-donating ability of these substituents was increased. Linear Hammett correlations of 1/lambda(max) versus sigma(+) were observed in both spectral analyses. The positive slopes of the Hammett plots suggested that the electronic transitions carry certain amounts of photoinduced intramolecular charge-transfer (PICT) character from the aryl substituents to the perylene diimide core which leads to the reduction of the electron density on the substituents. The substituent electronic effects originated mainly from the perturbation of the core PDI HOMO energy level by the substituents. This conclusion was supported by PM3 analyses and confirmed by cyclic voltammetry experiments. More interestingly, the Ph(2)NC(6)H(4)-substituted PDI, 4i, showed an unusual dual-band absorption that spans from 450 to 750 nm. We tentatively assigned these two bands as the charge-transfer band and the PDI core absorption, respectively.

Starburst triarylamines 2 and 3 were electropolymerized to form electrochromic thin films. Film from 2 showed intense absorption at 372 nm before voltage was applied. There were two absorption bands at 496 nm and 1000-2000 nm at 0.3 V, and a broad band, which peaked at 807 nm at 0.6 V. Film from 3 showed intense absorption at 372 nm before voltage was applied. There were two absorption bands at 498 nm and 1000-2000 nm at 0.3 V, and a broad band, which peaked at 890 nm at 0.7 V. The switching time studies revealed that thin film from 2 would require 3 s at 0.46 V for switching absorbance at 1600 nm and 1 s for bleaching. It would also require 4 s at 0.66 V for coloration at 800 nm and 2 s for bleaching. On the other hand, thin film from 3 would require 3 s at 0.46 V for switching absorbance at 1500 nm and 1 s for bleaching. It would also require 3.3 s at 0.76 V for switching absorbance at 900 nm and 1.5 s for bleaching. Electropolymerized thin films of 2 and 3 were also used as the surface modification layers to modify the surface of ITO for polymeric light emitting diode (PLED). To evaluate the performance of the modification layer, we selected a device of ITO/modification layer/PVK-Alq 3 -coumarin 6/metal electrode as the standard for comparison. We discovered that co-electropolymerization of the triarylamines 2 or 3 with poly(vinylcarbazole) (PVK) on ITO greatly reduces the turn-on voltage of the PLED devices and their performance is comparable to that of the commercially available PEDOT.