Synthesis, structure, and photophysical properties of blue-emitting Zinc(II) Complexes with 3-aryl-substituted 1-pyridylimidazo[1,5-a]pyridine ligands

Six zinc(II) complexes of the general formula [Zn(LR)X2] (1:R = Me, X = Cl; 2: R = Me, X = Br; 3: R = Me, X = I; 4: R = OH, X = Cl; 5: R = OH, X = Br; 6: R = OH, X = I), in which LR is 1-(pyridin-2-yl)-3-o-tolylimidazo[1,5-a]pyridine (LMe) or 2-[1-(pyridin-2-yl)imidazo[1,5-]pyridin-3-yl]phenol (LOH), have been prepared and characterized. The X-ray structural analysis performed on complexes 1, 3, and 4 showed that both LMe and LOH act as N,N-bidentate ligands and coordinate to the ZnII metal center through the pyridine ring (Npy) and the pyridine-like nitrogen atom of the imidazo[1,5-a]pyridine group (Nim). The photophysical properties of these compounds have been investigated, both in solution and in the solid state. The complexes showed intense blue luminescence when excited with UV light (λexc = 366–410 nm) with λmax of emission between 440–460 nm in the solid state. Complexes 1–3 showed moderate-to-good absolute photoluminescence quantum yields (PLQYs) in the solid state with a maximum PLQY of 0.40 for 1. These values were slightly lower than the PLQY of free LMe (0.60). On the other hand, the PLQY of LOH increased after coordination to ZnCl2, and a PLQY of 0.29 was recorded for 4. A possible interpretation of this behavior is reported.