Strategic synthetic color tuning of a family of simple azole based luminescent organoboron compounds.

Abstract

Many organic dopants for use in SMOLED displays have been discovered, but few are derived from one family that spans the visible spectrum. Through careful design, a rational approach to synthesizing a small family of simple azole based tetra-coordinate organoboron compounds of the type B(N,O)Ph2 has been developed with emission that spans the visible spectrum from blue to orange. Azole based ligands were strategically designed and utilized for ease of heteroatomic substitution. Significant changes in emission were observed via substitution of the azole based non-chelate heteroatom, and large bathochromic shifts were observed by increasing the number of polycyclic ligand rings. Boron clearly exploits the lone electron pair from nitrogen forming a dative bond with similar length to a carbon-boron single bond. The realm of the coordination sphere is mildly distorted from tetrahedral and each ligand moiety relatively coplanar. The large family of space groups, resulting from conformational changes associated with variation of torsion angles about the coordination sphere, suggest a low steric rotational energy barrier for the adduct phenyl rings. These large hydrophobic phenyl groups serve to enhance compound stability by protecting the coordination sphere from water and inhibiting hydrolysis of the adduct species. The nature of emission was elucidated computationally using B3LYP and PBE1PBE at the 6-311G (d,p)++ level with PCM solvent optimization. The model was validated by TD-DFT comparison to experimental absorption spectra and clearly shows that fluorescence can be characterized by a ligand centered p̟ * to p̟ transition. Analysis of the frontier molecular orbitals reveals a clear reduction in energy with increasing emission wavelength that correlates to several trends; an increase in compound aromaticity as evidenced by proton deshielding, and a decrease in the band gap with extension of the pi conjugate system. Two striking features are evident in this trend. Energy of the LUMO is lowered with sulfur substitution of the ligand heteroatom. Analysis of MO distribution reveals that this may be due to a nearly two-fold increase in the auxochromic orbital participation. The second salient feature is the large increase in the HOMO energy for each naphthyl substituted compound. Evidence of destabilization is apparent in the HOMO with an increase of nodes in the wave function, or an increase in antibonding like character of the 1,2-type substituted compounds, and with concentration of the wave function to the aryl moiety for each 2,3-type substituted compound. Orbital distribution behavior about each ligand, with respect to chelate geometry, also appears to display properties and characteristics consistent with what is described by nodal plane theory.