Halide ion inclusion into a dicopper(II) bistren cryptate containing 'active' 2,5-dimethylfuran spacers: The origin of the bright yellow colour

The inclusion of chloride and bromide ions into a dicopper(II) bistren cryptate complex containing 2,5-dimethylfuran spacers is signalled by the development of an intense yellow colour. The drastic colour change results from a halide-to-metal charge transfer transition, which has been shifted to the visible region through the interaction of the encapsulated halide with the oxygen atoms of the furan spacer.