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.
University of Minnesota M.S. thesis. February 2010. Major: Chemistry. Advisor: Prof. Paul Kiprof. 1 computer file (PDF); xii, 109 pages, appendix pages 60-109. Ill. (some col.)
Carlson, Jeffrey Collin.
Strategic synthetic color tuning of a family of simple azole based luminescent organoboron compounds..
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