The predominance
of one of the bands over the other at different temperatures (i.e., the luminescence thermochromism effect) is ultimately due to the variation in size of the inner Cu4 core, originated by the closer crystal packing of the pyridine ligands: having partial ionic bonding character, the inner Cu4I4 framework can deform flexibly according to the “positive” or “negative” pressure exerted by the ligand packing at different temperatures, thereby favoring more one or the other excited states geometries, as it has recently been demonstrated.9c