On the Nature of Luminescence Thermochromism of Multinuclear Copper(I) Benzoate Complexes in the Crystalline State

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.

Exploring Photoswitchable Properties of Two Nitro Nickel(II) Complexes with (N,N,O)-Donor Ligands and Their Copper(II) Analogues

“…To gain some more insights into the examined processes and support the experimental observations, theoretical computations were conducted. Therefore, geometry optimizations of the nitro and endo -nitrito isomers of Ni-2a and Ni-2b were performed both for the isolated molecules and using the QM/MM approach, which reflects crystal confining effects well. ,,,− The latter optimizations were based on the crystal structures after irradiation (collected at 140 K for Ni-2a and at 160 K for Ni-2b ). A comparison of the experimental and optimized molecular geometries is presented in Figure , whereas the molecular energy values computed for each geometry complemented by the hypothetical exo -nitrito isomer are gathered in Table .…”

Time-Resolved Single-Crystal X-Ray Crystallography

“…Most recently, the luminescent properties of a tetranuclear Cu(I) benzoate complex has been investigated by a combination of time-resolved spectroscopy and crystallography. The complex [Cu4(PhCO2)4] displays luminescence thermochromism, with red phosphorescence at room temperature and turns green on lowering the temperature to 90 K [135]. The low-energy triplet state has been assigned to a cluster-centred triplet state and the emission from this state matches the experimental red band observes at 660-715 nm.…”

Impact of Cis–Trans Isomerism on Crystal Packing and Photo-Activity of a Cobalt(III) Room-Temperature Photoswitch

“…In order to justify the formation of any other PLI, theoretical computations were conducted. For that purpose, geometry optimizations for the most common NO 2 linkage isomers ,, (i.e., nitro, exo - and endo -nitrito) of cis -Co and trans -Co were performed for the isolated molecules and by exploiting the QM/MM approach, which relatively well mimics the crystal environment. ,− All optimizations were done starting from the crystal structures measured at 100 K. Relative molecular energies computed for all optimized molecular geometries are gathered in Table . As expected, the nitro isomer turns out to be the most energetically stable form of the NO 2 group for both the trans -Co and cis -Co compounds.…”