Structures, Electronic States, and Electroluminescent Properties of a Zinc(II) 2-(2-Hydroxyphenyl)benzothiazolate Complex

Bis(2-(2-hydroxyphenyl)benzothiazolate)zinc (Zn(BTZ)2) is one of the best white electroluminescent materials used in organic light-emitting diodes (LEDs). Despite a large number of studies devoted
to this complex, very little is known about its basic molecular and electronic structures and electron transport
properties in LEDs. Therefore, we investigate the structures and electroluminescent properties. The
unsolvated single crystal of Zn(BTZ)2 was grown and its crystalline structure was determined from X-ray
diffraction data. The crystal is triclinic, space group P-1, a = 9.4890(19) Å, b = 9.5687(19) Å, c = 11.685(2)
Å, α = 84.38(3)°, β = 78.94(3)°, γ = 83.32(3)°. The structure of the chelate is dimeric [Zn(BTZ)2]2 with two
isotropic Zn2+ ion centers having five-coordinate geometry. The present study provides direct evidence for
the sole existence of dimeric structure in the powder and the thin film. The dimer is energetically more
stable than the monomer. Analysis of the electronic structure of [Zn(BTZ)2]2 calculated by density functional
theory reveals a localization of orbital and the distribution of four orbital “tetrads”. The structural stabilities
of both anion and cation and the distribution of the hole in the cation and that of the excess electron in the
anion are discussed in terms of theoretical calculations. Strong intermolecular interaction may be expected
to enable good electron transport properties as compared with tris(8-hydroxyquinolinato)aluminum.