Thermochromic Luminescence of Copper Iodide Clusters: The Case of Phosphine Ligands

Abstract

Three copper(I) iodide clusters coordinated by different phosphine ligands formulated [Cu(4)I(4)(PPh(3))(4)] (1), [Cu(4)I(4)(Pcpent(3))(4)] (2), and [Cu(4)I(4)(PPh(2)Pr)(4)] (3) (PPh(3) = triphenylphosphine, Pcpent(3) = tricyclopentylphosphine, and PPh(2)Pr = diphenylpropylphosphine) have been synthesized and characterized by (1)H and (31)P NMR, elemental analysis and single crystal X-ray diffraction analysis. They crystallize in different space groups, namely, monoclinic P21/c, cubic Pa ̅3, and tetragonal I ̅42m for 1, 2, and 3, respectively. The photoluminescence properties of clusters 1 and 3 show reversible luminescence thermochromism with two highly intense emission bands whose intensities are temperature dependent. In accordance to Density Functional Theory (DFT) calculations, these two emission bands have been attributed to two different transitions, a cluster centered (CC) one and a mixed XMCT/XLCT one. Cluster 2 does not exhibit luminescence variation in temperature because of the lack of the latter transition. The absorption spectra of the three clusters have been also rationalized by time dependent DFT (TDDFT) calculations. A simplified model is suggested to represent the luminescence thermochromism attributed to the two different excited states in thermal equilibrium. In contrast with the pyridine derivatives, similar excitation profiles and low activation energy for these phosphine-based clusters reflect high coupling of the two emissive states. The effect of the Cu-Cu interactions on the emission properties of these clusters is also discussed. Especially, cluster 3 with long Cu-Cu contacts exhibits a controlled thermochromic luminescence which is to our knowledge, unknown for this family of copper iodide clusters. These phosphine-based clusters appear particularly interesting for the synthesis of original emissive materials.

Cu(I) hybrid inorganic–organic materials with intriguing stimuli responsive and optoelectronic properties

“…When 39, which displays a bright green-yellow emission at 544 nm (of 3 CC character), was exposed to vapors of toluene it rapidly converted into 42. The "classical" thermochromic behavior of the two polymorphs was here reinvestigated with results analogous to those previously observed [47]. Hong et al have exploited cuprophilic interactions as a multi-responsive chromophore to prepare a simultaneously tri-stimuli-responsive luminescent material with reversible and independent thermo, mechano and vapochromism [88].…”

“…The population of the triplet states was suggested to arise from an ISC process at low temperature. [46,47]. In particular, clusters 39-41 with L = PPh 3 (triphenylphosphine, 39), Pcpent 3 (tricyclopentylphosphine, 40) and PPh 2 Pr (diphenylpropylphosphine, 41) were synthetized by reaction of CuI with the corresponding phosphine.…”

Stimuli-responsive Luminescent Copper(I) Complexes for Intelligent Emissive Devices

“…26 X-ray analysis clearly revealed similar tetranuclear cubane-type {Cu 4 I 4 } core structures of these three complexes. However, the Cu£Cu distances in the cores were quite different, increasing in the order of 2 (2.9012(6) ¡) < 4 (3.14(8) ¡) < 3 (3.2758(8) ¡).…”

Geometry Flexibility of Copper Iodide Clusters: Variability in Luminescence Thermochromism

“…A second lowest triplet state (T 2 ) was also optimized in the case of compounds 1 and 3. 16 Relevant computed data for all these states (S 0 , T 1 , and T 2 ) are reported in Table 4 along with the experimental metrical data (X-ray analysis data in brackets). The computed values are in a satisfying agreement with the experimental ones.…”