Mechanochromic Luminescence of N, N'-Dioxide-4,4'-bipyridine Bismuth Coordination Polymers

The association of Bi3+ ions and N,N'-dioxide-4,4?-bipyridine (bp4do) ligands has led to three related compounds [Bi2Br6(bp4do)(dmso)4] (1), [BiBr3(bp4do)(dmso)] (2), and [Bi2Br6(bp4do)3] (3). 2 can be described from 1, and 3 from 2, by the substitution of two dmso molecules by one bp4do molecule, the octahedral geometry of bismuth ions being kept as BiBr3O(bp4do)O2(dmso) in 1, BiBr3O2(bp4do)O(dmso) in 2, and BiBr3O3(bp4do) in 3. In the absorption spectra of 1-3, as well as Bi/bp4do complexes in solution, a metal-to-ligand charge transfer band is observed as a result of the formation of Bi-O(N-oxide) bonds. All 1-3 materials exhibit luminescence properties of phosphorescence type with quantum yields up to 54% for 1. In contrast with 1 (isolated complexes in the solid state), 1D coordination polymer type compounds 2 and 3 exhibit mechanochromic luminescence (MCL) properties: upon grinding - a few seconds for 2 - the broad emission bands of the pristine compounds centered at 575 nm (2, yellow emission) and 600 nm (3, orange emission) are shifted by 75 nm (2) or 50 nm (3) (bands centered at 650 nm), leading to a strong red emission, whereas the process is reversible by heating, fuming, or recrystallization in a few drops of solvent. These MCL properties of 2 and 3 may be related to conformational changes of polymeric chains upon grinding.