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

A series of five 2-heteroarylcyanoximes such as: alpha-oximino-(2-benzimidazolyl)acetonitrile (HBIHCO), alpha-oximino-(N-methy-l-2-benzimidazolyl)acetonitrile (HBIMCO), alpha-oximino-(2-benzoxazolyl)acetonitrile (HBOCO), alpha-oximino-(2-benzothiazolyl)acetonitrile (HBTCO) and alpha-oximino-(2-quinolyl)acetonitrile (HQCO) and their monovalent thallium(i) complexes were synthesized and characterized using spectroscopic methods ((1)H, (13)C 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, H(2)BOCO(+), 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 Tl(2)CO(3) 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 pi-pi 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.