Applications In Materials Science

The formation of liquid crystals and their light-induced phase transitions as imaging techniques were already discussed in the Sections 5 and 8. The most important application of stilbenoid dendrimers is in the field of light-emitting diodes (LEDs). A strong photoluminescence together with the high film-forming quality in spin-coating procedures seems to be very promising. The (E)-stilbene chromophore has a large HOMO-LUMO gap (bandgap); therefore, (E,E)-1,4-distyrylbenzene units, present in 2-5, are a better choice. We could observe, for example, in a single-layer system (thickness 17.1 nm) of 2b (R = C12H25) a photocur-rent on irradiation with a 10 ns laser pulse of 7 mJ • cm-1. Raising the temperature from 22 to 80 °C led to an increase of the current by a factor of 100 at 200 V An experiment performed under the same conditions with the related compound 1b (R1 = R2 = OC12H25) did not lead to a charge-carrier generation.

More detailed studies have been made for the series 4 [42] and 5 [43]. Poly(3,4-ethylenedioxythiophene) was used as a hole-injecting layer for 100 nm thick dendrimer films of 4a-d [42]. The ITO-Al device furnished the emission spectra shown in Figure 9. For the lower generations, the electroluminescence bands are much broader than the photoluminescence band, which was measured in solution. The tailing on the low-energy side is attributed to intermolecular interactions.

At a bias of 4 V, electroluminescence could be observed for all four generations. The external quantum efficiency (EQE) of the device increased exponentially by a factor of 25 on going from 4a to 4d. The charge-carrier mobility is reduced with growing generation; consequently, the probability of polaron recombination and electroluminescence

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