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,13