Lead(II) 4,4′‐Bipyridine N ‐Oxide Coordination Polymers – Highly Phosphorescent Materials with Mechanochromic Luminescence Properties

In the Pb II X 2 /bp4mo (4,4′‐bipyridine N ‐oxide) system, four coordination polymers (CPs) with the formulation of [PbX 2 (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 with the ones of 1 and 2 . Finally, the structure of compound 4 , in which X = NO 3 – , is described by two‐interpenetrated 3D networks. They all exhibit phosphorescence properties characterized by a broad emission band at around 600 nm, with lifetimes longer than tens of µs and quite high quantum yields that 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 nearly complete extinction of the 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 a crystallization‐enhanced emission process, is reversible either by heating, exposure to vapour (H 2 O, acetone) or by recrystallization in a few drops of acetone; however, it is accompanied by a loss of emission intensity. These compounds represent the first examples of MCL materials based on Pb 2+ .