Exploring the influence of polymorphism and chromophore co-ligands on linkage isomer photoswitching in [Pd(bpy4dca)(NO2)(2)]

The polymorphic Pd(II)-nitrite complex [Pd(bpy4dca)(NO2)(2)] (1) (bpy4dca = 2,2'-bipyridine-4,4'-dicarboxylic acid methyl ester) is shown to undergo photoinduced nitro -> nitrito linkage isomer switching in two crystal forms, to varying excited state population levels. Detailed photocrystallographic kinetic studies, structural analyses of the ground and photoexcited states and density functional theory calculations all combine to explain the unusually high maximum excited state population of 80% in 1, where other linkage isomer complexes containing strong chromophore co-ligands have traditionally been challenging to excite. Comparison of the photo-response in crystals for forms I and II reveals that, while the local crystal packing environment has a role in controlling the maximum photostationary population that can be achieved, the rate of isomerisation is comparable across different nitrite ligand environments. Our results reinforce the hypothesis that a complex combination of steric, electronic and kinetic factors govern the progress of linkage isomer switching in the solid-state and highlight the need for better understanding of the structural dynamics involved in isomer switching at the molecular level.