Benz(2-heteroaryl)cyanoximes and their Tl(i) complexes: new room temperature blue emitters

Abstract

A series of five 2-heteroarylcyanoximes such as: α-oximino-(2-benzimidazolyl)acetonitrile (HBIHCO), α-oximino-(N-methy-l-2-benzimidazolyl)acetonitrile (HBIMCO), α-oximino-(2-benzoxazolyl)acetonitrile (HBOCO), α-oximino-(2-benzothiazolyl)acetonitrile (HBTCO) and α-oximino-(2-quinolyl)acetonitrile (HQCO) and their monovalent thallium(I) complexes were synthesized and characterized using spectroscopic methods (¹H, ¹³C NMR, IR, UV-visible, mass-spectrometry) and X-ray analysis. The HBIMCO (as monohydrate) adopts planar trans-anti configuration in the solid state. The crystal structure of “HBOCO” revealed the presence of nitroso anion a, BOCO⁻, and protonated oxime cation b, H₂BOCO⁺, that form a H-bonded dimer in the unit cell. Both molecules adopt planar structures, but different configurations: cis-anti in the molecule a, and trans-anti for b. This is the first reported case of a zwitterionic pair in oximes and the coexistence of the two geometrical cis/trans isomers in the same crystal. All 2-heteroarylcyanoximes form yellow anions upon deprotonation, which exhibit significant negative solvatochromism in solution. Heterogeneous reactions between hot aqueous solutions of Tl₂CO₃ and solid protonated 2-heteroarylcyanoximes HL afford yellow TlL. The crystal structure of Tl(BTCO) shows the formation of centrosymmetrical dimers, which connect with each other to form a double-stranded one-dimensional coordination polymer. The oxygen atom of the oxime group acts as a bridge between the three different Tl(I) centers. The anion is non-planar and adopts a trans-anti configuration in the complex. The polymeric motif in the complex represents a ladder-type structure. Staggered π–π interactions between benzothiazolyl groups provide additional stabilization of the structure. Both organic ligands and their Tl(I) complexes exhibit strong room temperature blue emission in the solid state.