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(L-R)X-2] (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 L-R is 1-(pyridin-2-yl)-3-o-tolylimidazo[1,5-a]pyridine (L-Me) or 2-[1-(pyridin-2-yl) imidazo[1,5-a]pyridin-3-yl]phenol (L-OH), have been prepared and characterized. The X-ray structural analysis performed on complexes 1, 3, and 4 showed that both L-Me and L-OH act as N,N-bidentate ligands and coordinate to the Zn-II metal center through the pyridine ring (N-py) and the pyridine-like nitrogen atom of the imidazo[1,5-a]pyridine group (N-im). 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 (lambda(exc) = 366-410 nm) with lambda(max) 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 L-Me (0.60). On the other hand, the PLQY of L-OH increased after coordination to ZnCl2, and a PLQY of 0.29 was recorded for 4. A possible interpretation of this behavior is reported.