Impact of Intermolecular Non‐Covalent Interactions in a Cu^I₈Pd^II₁ Discrete Assembly: Conformers’ Geometries and Stimuli‐Sensitive Luminescence Properties

Abstract

A new highly solid-state luminescent phase of a previously reported weakly luminescent Cu I 8 Pd II 1 dicationic assembly is reported revealing the high geometrical versatility of this moiety that importantly alters its luminescent properties. This very minor new species B c is based on a different conformer scaffold than the one encountered in the previously reported B o form and, essentially differs from B o by displaying shorter Cu I -Cu I intermetallic distances. DFT calculations allow concluding that the predominance in the solidstate of the weakly luminescent and less stable B o phase is due to the extra stability induced by a larger number of intermolecular non-covalent π-CH interactions in its crystalline packing and not by the intrinsic stability of the Cu I 8 Pd II 1 dicationic moiety. Calculations also revealed that a more stable conformation B calc is expected in vacuum, which bears a different distribution of Cu I -Cu I intermetallic distances than the dications in B o and B c phases. Taking into account that the geometrical alterations are associated to drastic changes of luminescence properties, this confer to the Cu I 8 Pd II 1 assembly high potentiality as stimuli-sensitive luminescent materials. Indeed, by applying mechanical or thermal stress to samples of B o phase, new phases B g and B m , respectively, were obtained. Alterations of the solid-state photophysical properties of these new species compared to those recorded for B o are reported together with a combined experimental and computed study of the structures/properties relationships observed in these phases.

Luminescence and Palladium: The Odd Couple

“…Costuas, Yam, and Lescop have reported a different approximation to the building of highly luminescent supramolecular moieties containing Pd(II) centers [ 109 , 110 ]. In these cases, the tetranuclear Cu(I) derivative {[Cu 2 (μ-dppm) 2 ] 2 (μ-CN) 2 }(PF 6 ) 2 120 (dppm = Ph 2 PCH 2 PPh 2 ), which shows an impressive photophysical performance in solid state due to thermally activated delayed fluorescence (TADF), is used as a basic building block for the construction of higher luminescent nuclearities.…”

“…Further work showed that [Cu 8 Pd(CN) 8 (dppm) 8 ](PF 6 ) 2 121 can be obtained as different polymorphs arising from different sources [ 110 ]: (i) as minor components during the crystallization of the main component shown in Figure 33 , for which the only difference is the μ 3 -bonding mode of the central cyanide ligands; (ii) mechanical treatment of the major component; (iii) thermal heating of the main component giving an irreversible melting transition; and (iv) even from DFT calculations. Not surprisingly, the polymorphs only differ in subtle details of the molecular structure, but these small are translated into huge differences in photophysical properties, and strong enhancements of the luminescence are observed [ 110 ]. As detailed previously, the quantum yield of the main component is 2%.…”

“…As detailed previously, the quantum yield of the main component is 2%. This quantum yield increases in the minor component up to 42%, in the case of the mechanotreated sample, the increment is lower but a remarkable 27% is obtained, while for the melt sample, a quantum yield of 7% is measured [ 110 ]. The flexible coordination chemistry of Cu(I) further favours this type of remarkable behaviour.…”

Phase dependence and mechanical and thermal ductility of the luminescence properties of tetranuclear Cu(i) metallacycle assemblies stabilized by ditopic organo-pnictogen (P,As) ligands

“…Indeed, previous investigations conducted on related luminescent ‘Cu 6 ’ and ‘Cu 8 Pd 1 ’ assemblies have revealed that the crystal packing greatly influences the solid-state molecular conformations observed in single crystals and therefore the metric data and the solid-state photophysical properties of these derivatives. 58,59 Taking these aspects into account, indirect simulations of these packings were performed by constraining the copper atomic positions in the metallacycles to those of the X-ray structures. These partially constrained geometry optimizations lead to A opt-const (S 0 ) and B opt-const (S 0 ).…”

“…Regarding the fascinating structural flexibility and luminescence properties that can be associated with Cu( i )-based molecular assemblies, these derivatives exhibit appealing potential for designing such stimuli-sensitive materials, 1 and indeed attractive mechanochromic luminescent mononuclear or mainly polynuclear Cu( i ) complexes have been reported in recent years. 1,28,38,43,44,58…”