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Abstract
A model luminescent [(PhCO2)4Cu4] (Cu4) complex in the crystalline state was investigated via combined crystallographic and spectroscopic techniques contributed substantially by theoretical modelling. The complex appeared to exhibit luminescence thermochromism, i.e., red phosphorescence at room temperature which changes to green when lowering the temperature to 90 K. The low-energy emissive state was assigned as a cluster-centred triplet, 3CC. The emission from this state predicted in TDDFT (~635 nm) matches the experimental red band observed at 660–715 nm. In contrast, the nature of the high-energy “green” band was less straightforward. The next reached cluster-centred triplet excited state occurred to be energetically close to the experimental value of ~545 nm. The two excited states also exhibit significant metal-to-ligand and ligand-to-metal charge transfer characteristics, especially for solid-state distorted geometries. In both cases the cluster core was expected to become notably contracted when compared to the ground state. Time-resolved photocrystallographic results supported the computationally predicted core contraction upon excitation. Additionally, the differences between the spectroscopic behaviour of the related tetra- and hexanuclear copper(I) complexes, Cu4 and Cu6 (i.e., [(PhCO2)6Cu6]) in the crystalline state were discussed and examined. It appeared that crystal packing may constitute an important factor as far as the lack of luminescence thermochromism in the latter case is concerned. Synopsis: Structure–property relationships characterising a model luminescent [(PhCO2)4Cu4] (Cu4) complex in the crystalline state were investigated via combined crystallographic and spectroscopic techniques contributed by theoretical modelling, and compared with the properties of the related [(PhCO2)6Cu6] (Cu6) complex.
Highlights
One of the most important current areas in chemistry is developing new functional and controllable materials for technological applications
That upon distortion of the D2 symmetry, which is observed in crystals, as well as in the quantum-mechanics/molecular mechanics (QM/MM)-optimized structures, the two types of states gain significantly more MLCT and LMCT contributions
Temperature-dependent phosphorescence observed and characterised by us for crystals of the studied tetranuclear copper complex characterising a model luminescent [(PhCO2)4Cu4] (Cu4) is similar to the luminescence thermochromism reported for the Cu4 I4 -core-containing compounds [52,53,54,55], its nature differs substantially between the two systems due to cluster cores chemical and geometrical features
Summary
One of the most important current areas in chemistry is developing new functional and controllable materials for technological applications. Among interesting ones due to several reasons, two of which are: (i) the presence of metal centres induces spectroscopic properties related to metal-ligand interactions not achievable for purely organic in solar-energy conversion [1] and other fields, ranging from molecular electronics [2], light-emitting molecules, and (ii) the flexibility in designing new molecules by changing metal centres and organic diodes (LEDs) [3], through photocatalysts [4], to biolabels [5]. Polynuclear d copper complexes may create properties related to metal-ligand interactions not achievable for purely organic molecules, and (ii) the diverse molecular assemblies and are often unusually strongly luminescent Their photochemical flexibility in designing new molecules by changing metal centres and organic ligands, which allows properties can be modified by the choice of ligands and by variation of a number of metal centres
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