Diphenilamino-substituted carbazoles were used as guest compounds for the preparation of highly efficient blue organic light-emitting diodes based on the phenomenon of delayed fluorescence. It was shown that the spectra of the delayed fluorescence of host−guest systems are identical to those of the prompt fluorescence and in general coincide with the photoluminescence spectra of the guest films. The congruence of the prompt and delayed fluorescence spectra is explained by the effective intermolecular triplet−singlet (T → S) energy transfer from the excited T states of the host to the S states of the guest molecules. High external electroluminescence efficiency of the fabricated electroluminescent devices, reaching 17%, is comparable to that achieved in phosphorescence-based organic light-emitting diodes.

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The synthesis and properties of carbazole and diphenylamine derivatives with different numbers and positions of methoxy groups in the diphenylamino moiety are reported. A comparative study on their thermal, optical, and photoelectrical properties is presented. All the synthesized compounds are found to form glasses with the glass transition temperatures in the range of 39-69 °C, as characterized by differential scanning calorimetry. The ionization potentials of the synthesized compounds were estimated both theoretically by quantum chemical calculations and experimentally by an electron photoemission in air technique. The trends observed by theoretical calculations are confirmed by experimental results. The experimental values of ionization potentials range from 5.10 to 5.56 eV. Compounds containing monomethoxy-substituted phenyl rings exhibited lower ionization potentials than compounds containing di-and trimethoxy-substituted phenyl moieties. The highest ionization potential was observed for the compound containing trimethoxysubstituted phenyl rings. The best charge transport properties were observed for the compounds containing one methoxy group in para and ortho positions of phenyl rings of the diphenylamino moiety. Room-temperature hole drift mobility in the amorphous film of 3-[N,N-(bis-4-methoxyphenyl)]amino-9-ethylcarbazole established by a xerographic time-of-flight technique was found to be 1.2 Â 10 -4 V/cm at an electric field of 6.4 Â 10 5 V/cm.

The single-layer and bilayer devices showed blue monomer, electromer or interface exciplex emission.

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