The observed Cf 2+ 5f→6d transitions are further supported by photoluminescence measurements of single crystals at 93.15 K. As shown in Figure , excitation of these crystals with 546 nm (18,315 cm -1 ) light yields broadband emission at 14,087 cm -1 (710 nm) which corresponds to emission of the first group of 5f→6d transitions (16,303 cm -1 ). Compared to the photoluminescence observed from the isomorphous Sm(18-crown-6)I2 under the same conditions, the emission exhibited by Cf(18-crown-6)I2 is far less intense. The solution-phase absorption spectrum for this system corroborates the presence of Cf 2+ prior to crystallization. As shown in Figure , the absorption spectrum of [Cf (18-crown-6)x] 2+ in acetonitrile exhibits several broad features between 32,500 and 15,000 cm -1 . Most of these features can be attributed to the transitions identified in the solid-state absorption spectra of Cf(18-crown-6)I2. However, there are notable differences in the spectra obtained from these two phases that are likely the result of competing coordination environments of the [Cf(18-crown-6)x] 2+ ion. An example of this is an observed color change that is dependent on the concentration of [Cf(18crown-6)x] 2+ in solution. The more concentrated solution from which Cf(18-crown-6)I2 crystallizes is a dark peach-brown color, but the color of this solution shifts dramatically when diluted with acetonitrile resulting in a blue-green solution (Figure ). In the presence of excess acetonitrile, it is probable that the more labile iodide ions would be displaced by solvent molecules. Given the possibility of different coordination environments in solution, the allowance of 5f→5f transitions cannot be overlooked. However, the pseudo-axial symmetry imposed by the coordination of the 18-crown-6 molecule restricts the overall intensity of these transitions compared to the dipoleallowed 5f→6d transitions (Extended Data Figure ).