Fluorescence thermochromism of pyridine copper iodides and copper iodide

Copper(I)iodide reacts with pyridine to form (Cu py)I, and further to form (Cu py 2 )I. A tripyridino compound, although existing in contact with pyridine, is not obtainable at room temperature and normal pressure. The three solvates are different in structure and in their fluorescence colour in ultraviolet light. (Cu py 3 )I crystals are not fluorescent at room temperature but show yellow fluorescence at −180°C. (Cu py 2 )I is fluorescent with a green colour at room temperature as well as at very low temperatures. (Cu py)I shows yellow fluorescence at room temperature which reversibly changes to orange, then to red and finally to violet by cooling in liquid nitrogen („fluorescence thermochromism”︁). The last named compound is spontaneously decomposed by benzene to form highly dispersed copper iodide, which fluoresces red at room temperature, but not at all in liquid nitrogen.