Polymorphic Copper Iodide Anions: Luminescence Thermochromism and Mechanochromism of (PPh₄)₂[Cu₂I₄]

The photoluminescent stimuli-responsive properties of two crystalline polymorphs with the formula (PPh₄)₂[Cu₂I₄] are reported. Distinct luminescence properties are exhibited by these ionic copper iodide compounds with blue or yellow emission, and original luminescence thermochromism and mechanochromism are demonstrated. While one polymorph displays contrasted temperature-dependent emission properties, the other shows great modification of its emission upon mechanical solicitation. The establishment of structure-properties relationships, supported by a theoretical approach, permits us to get insights into the origin of the photoluminescence properties and the mechanisms at play. According to DFT calculations, the different emission bands originate either from the (PPh₄)⁺ organic cation or from the [Cu₂I₄]^2- anion. Activation of these two emissive centers appears to be dependent on the crystalline packing of the polymorph. The thermochromism displayed by one polymorph can be attributed to a variation in temperature of the relative intensities of two emission bands of two different excited states. The origin is different for the other polymorph, with emission bands coming from two independent emissive centers: namely, (PPh₄)⁺ and [Cu₂I₄]^2-. The luminescence mechanochromism is attributed to a polymorphic transition. The mechanical solicitation induces a partial transformation of one polymorph into the other within a disordered phase. The mechanochromic mechanism can be related to mechanical modifications of intermolecular interactions between the (PPh₄)⁺ cations. By displaying luminescence properties that depend on crystalline structure, excitation wavelength, temperature, and mechanical solicitation, the studied copper iodides offer a great possibility of emissive color control and switching, a clear demonstration of the great potentialities of this family of compounds for the development of photoactive materials.