N‐Methyl‐4,4′‐bipyridinium and N‐Methyl‐N′‐oxide‐4,4′‐bipyridinium Bismuth Complexes – Photochromism and Photoluminescence in the Solid State

Three bismuth complexes based on N ‐methyl‐4,4′‐bipyridinium (hMV + ), (hMV)[Bi(hMV)Cl 5 ] ( 1 ), and N ‐methyl‐ N′ ‐oxide‐4,4′‐bipyridinium (MVO + ), [Bi(MVO)X 4 (dmso)] · dmso [X = Cl ( 2 ), Br ( 3 )], are reported. All three compounds show luminescence in the solid state with maxima at 545 nm (yellow for 1 ) and 560 nm (orange for 2 and 3 ) with quantum yields up to 10 %. Upon UV irradiation, 1 undergoes a color change from white to blue accompanied by a reduction of the photoluminescence intensity. The analysis of the crystal structure of the three complexes points to a photoinduced charge‐transfer (PICT) process at the origin of the photochromism in 1 .