Systematic electronic control in ambipolar compounds optimizes their photoluminescence properties: Synthesis, characterization, and device fabrication of four-coordinate boron compounds containing an N,O-chelating oxazolylphenolate ligand

Abstract

A series of four-coordinate boron compounds of the type N,O-(OPh OxZArX)BPh2 or N,O-(OPhOxZNPh 2)BPh2 (5a-g) has been prepared by treating triphenylboron (TPB) with 2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-4-X-arylphenols (HOPh OxZArX; 4a-f) or 2-(4,4-dimethyl-4,5-dihydrooxazol-2- yl)-4-(4-diphenylamino)phenol (HOPhOxZNPh2; 4g). A systematic change in the electronic structures is achieved in these compounds by incorporating electron-withdrawing (EW) and -donating (ED) groups [ArX (X = 4-cyano-, 2,4-difluoro-, 4-chloro-, phenyl, 4-methoxy-, and 4- dimethylaminophenyl) or NPh2] at the 4-position of the phenoxide. The absorption and emission maxima of the ED groups show a significant red-shift compared to those observed in the EW groups. This red-shift suggests that φ-conjugation is effectively extended over the arylphenoxides and oxazoline moiety once the boron center has been plugged into the corresponding ligands. The gradual decrease observed in the bandgaps on going from EW to ED groups is found to be in agreement with the increase in oxidation potentials determined by the cyclic voltammetry (CV) experiments. In particular, a Hammet plot between the EW and ED substituents and oxidation potentials confirms the ground state electronic perturbation. Such alterations are attributed mainly to an elevation of the energy levels of the HOMOs), as further confirmed by a series of theoretical calculations on the frontier orbitals of each system. The ED group (-NPh2) substituted compound 5g shows a bipolar character due to intramolecular charge transfer in the complex, with the highest photoluminescence quantum yield being obtained in toluene. This new boron complex was found to function as an emitter in electroluminescence (EL) devices, with a maximum brightness of 2905 cd/m2 at 13 V and a current efficiency of 1.63 cd/A at 6 mA/cm2, with a turn-on voltage of 4.3 V. (© Wiley-VCH Verlag GmbH and Co. KGaA, 69451 Weinheim, Germany, 2009).