Synthesis, characterization, and photophysical properties of luminescent gallium and indium complexes constructed using tridentate 6-azolyl-2,2′- bipyridine chelates

Three systematically functionalized 6-azolyl-2,2′-bipyridine ligands were prepared from reactions initiated by 6-cyano-2,2′-bipyridine. These ligands readily reacted with the metal alkyl reagents GaMe₃ and InMe₃ to afford the pentacoordinate complexes [(fpzbpy)MMe ₂] (1a, M = Ga; 1b, M = In), [(ftzbpy)MMe₂] (2a, M = Ga; 2b, M = In), and [(N4bpy)MMe₂] (3a, M = Ga; 3b, M = In), in which (fpzbpy)H, (ftzbpy)H, and (N4bpy)H denote 6-pyrazolyl-, 6-triazolyl-, and 6-tetrazolyl-substituted 2,2′-bipyridine, respectively. These complexes exhibited moderate blue-green emission ranging from 412 to 493 nm. On the other hand, treatment of the bidentate 2-pyridyl tetrazole ligand (pyN4)H with InMe₃ afforded the bridged dimer [Me₂In(pyN4)]₂ (4). Calculation based on time-dependent density function theory (TDDFT) showed that the S₁ state of complexes 1-3 is mainly attributed to an allowed intraligand π → π* electronic transition located at tridentate chelating moieties, together with a small contribution (< 10%) of gallium (or indium) → π* (ligand) charge transfer transition. Accordingly, the corresponding emission properties of 1a-3a (or 1b-3b) can be rationalized using the correlation between the substituent effect of azolyl groups and the relative HOMO/LUMO energy level.