Lead(II) N-oxide-4,4'-bipyridine Coordination Polymers : highly Phosphorescent Materials with Mechanochromic Luminescence Properties

In the Pb(II)X2 / bp4mo (N-oxide-4,4'-bipyridine) system, four Coordination Polymers (CPs) with the formulation of [PbX2(bp4mo)] are obtained. Compounds 1 and 2, in which X= Cl and Br, are polymorphs whose acentric structures are based on 2D CPs. On the contrary, the compound 3 (X= I) shows a close but centrosymmetrical structure compared to the ones of 1 and 2. Finally the structure of compound 4, in which X= NO3-, is described by two-interpenetrated 3D networks. They all exhibit phosphorescence properties characterized by a broad emission band at around 600 nm, with lifetime longer than tens of ?s and quite high quantum yields which increase in the halide series from 6% (1) to 27% (3), and up to 34 % for 4. They also exhibit Mechanochromic Luminescence (MCL) properties: the grinding (g) of samples 1-4 involves a nearly extinction of luminescence as well as a crystal to amorphous transition of the corresponding samples 1-g to 4-g. This phenomenon which can be also considered as a crystallization-enhanced emission process is fully reversible either by heating, exposure to vapor (H2O, acetone) or by recrystallization in a few drops of acetone, however with a loss of emission intensity. These compounds represent the first examples of MCL materials based on Pb2+.