Coordination polymers containing dimeric Cu2X2 and polymeric (CuI)n clusters linked by unsymmetrical isomeric pyridine-benzimidazole linkers: modulating photophysical properties by mechanical stimuli†

Two rigid unsymmetrical linkers, 4-PBI and 3-PBI, have been successfully employed to synthesize three CuX cluster-based coordination polymers (CPs). The complexation reaction of 4-PBI with CuI and CuBr resulted in isostructural crystalline CP1 and CP2, respectively, containing Cu₂X₂ clusters. On the other hand, the isomeric 3-PBI with CuI resulted in crystalline CP3 containing a one-dimensional polymeric (CuI)_n cluster. Furthermore, the mechanochromism of the CPs has been examined using DRS, solid state PL, FT-IR and Raman spectroscopy. CP1 and CP2 exhibited mechanochromism as their colours changed from reddish brown to orangish yellow upon grinding. The pressure induced local distortion around the flexible dimeric cores of CP1 and CP2 was responsible for the resultant mechanochromism. The presence of a polymeric CuI cluster with μ₃-bridging I atoms within the 2D layer of CP3 renders it rigid, thereby hampering its ability to display mechanochromism.