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Polymorphic Copper Iodide Anions: Luminescence Thermochromism and Mechanochromism of (PPh4)2[Cu2I4]
journal contribution
posted on 2020-04-01, 17:05 authored by Yaouen Thefioux, Marie Cordier, Florian Massuyeau, Camille Latouche, Charlotte Martineau-Corcos, Sandrine PerruchasThe 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.