N,N-di(4-methoxyphenyl)hydrazones of carbazole and phenothiazine carbaldehydes containing 4-methoxyphenyl groups as hole transporting materials

In a quest of exploitation of low-cost chemistry for the preparation of hole-transporting materials, we investigated the synthesis of N,N-di(4-methoxyphenyl)hydrazones and the corresponding dihydrazones of carbazole and phenothiazine carbaldehydes. The electron photoemission spectrometry measurements of the layers of the synthesized compounds revealed low ionization potentials in the range of 5.27-5.41 eV. Charge-transporting properties of the compounds were characterized by xerographic time of flight method. The measurements revealed hole mobilities reaching 10-3 cm2/Vs at high electric fields. DFT calculations of the geometry, electronic structure, absorption and photoluminescence spectra of the compounds were carried out to explain and supplement experimental data. The results of calculations show that the first maxima in absorption spectra of the compounds can be attributed to the pi-pi * local excitation of conjugated system including mainly lone electron pairs of the nitrogen atoms of hydrazone and carbazole or phenothiazine moieties. It is not accompanied by any significant charge transfer. The experimentally observed dramatic effect on the emissive properties of the compounds by attachment of the hydrazone moieties is explained by rotation of para-methoxyphenyl ring into the plane of carbazole moiety upon excitation, which leads to significantly larger charge transfer in the case of S1 -> S0 emission, compared to S0 -> S1 absorption.