Photochemistry and Photophysics of Coordination Compounds: Copper

Abstract

Cu(I) complexes and clusters are the largest class of compounds of relevant photochemical and photophysical interest based on a relatively abundant metal element. Interestingly, Nature has given an essential role to copper compounds in some biological systems, relying on their kinetic lability and versatile coordination environment. Some basic properties of Cu(I) and Cu(II) such as their coordination geometries and electronic levels are compared, pointing out the limited significance of Cu(II) compounds (d 9 configuration) in terms of photophysical properties. Well-established synthetic protocols are available to build up a variety of molecular and supramolecular architectures 70 N. Armaroli et al.(e.g. catenanes, rotaxanes, knots, helices, dendrimers, cages, grids, racks, etc.) containing Cu(I)-based centers and exhibiting photo-and electroluminescence as well as light-induced intercomponent processes. By far the largest class of copper complexes investigated to date is that of Cu(I)-bisphenanthrolines ([Cu(NN) 2 ] + ) and recent progress in the rationalization of their metal-to-ligand charge-transfer (MLCT) absorption and luminescence properties are critically reviewed, pointing out the criteria by which it is now possible to successfully design highly emissive [Cu(NN) 2 ] + compounds, a rather elusive goal for a long time. To this end the development of spectroscopic techniques such as light-initiated time-resolved X-ray absorption spectroscopy (LITR-XAS) and femtosecond transient absorption have been rather fruitful since they have allowed us to firmly ground the indirect proofs of the molecular rearrangements following light absorption that had accumulated in the past 20 years. A substantial breakthrough towards highly emissive Cu(I) coordination compounds is constituted by heteroleptic Cu(I) complexes containing both N-and P-coordinating ligands ([Cu(NN)(PP)] + ) which may exhibit luminescence quantum yields close to 30% in deaerated CH 2 Cl 2 solution and have been successfully employed as active materials in OLED and LEC optoelectronic devices. Also copper clusters may exhibit luminescence bands of halide-to-metal charge transfer (XMCT) and/or cluster centered (CC) character and they are briefly reviewed along with miscellaneous Cu(I) compounds that recently appeared in the literature, which show luminescence bands ranging from the blue to the red spectral region. An Overview of Copper Historical Notes, Current Use, Consumption TrendsCopper was known to some of the oldest civilizations on record, and has a history of use that is at least 10 000 years old. A copper pendant was found in what is now northern Iraq that dates to 8700 B.C. and by 5000 B.C. there are signs of copper smelting from simple copper compounds such as malachite or azurite. This process appears to have been developed independently in several parts of the world since several centuries B.C., including Anatolia, China, Central America and West Africa. The Egyptians found that, upon addition of small amounts of tin, copper becomes easier to...

Highly efficient thermally activated fluorescence of a new rigid Cu(i) complex [Cu(dmp)(phanephos)]+

“…with energetically low-lying π* orbitals, such as aromatic diimines, often display distinct low-energy metal-toligand charge-transfer (MLCT) transitions in the visible part of the spectrum. With this type of electronic excitation, a flattening distortion of the molecular structure takes place.13a, [16][17][18] Such structural rearrangements are usually connected with an increase of non-radiative deactivation or even quenching of the emission due to an increase of the Franck-Condon factors that couple the excited state and the ground state. 19,20 This is especially distinct in non-rigid environments.…”

Dual luminescence in solid CuI(piperazine): hypothesis of an emissive 1-D delocalized excited state

“…Ir or Os) and rare earth metals, which are expensive and environmentally problematic. 12 The luminescence properties of the copper halide clusters arise from a remarkable variety of emissive states, which have been extensively investigated in the last 30 years and can be summarized as follows: (a) a low-energy emission, attributed to a triplet cluster-centered ( 3 CC) excited state, is observed in the case of metal center interactions [13][14][15][16][17][18][19] and is independent of the nature of the ligand engaged in the complex; (b) a highenergy emission, attributed to a triplet halide-to-ligand charge transfer ( 3 XLCT) excited state, may show up in the presence of unsaturated ligands with accessible π-orbitals. 20 Moreover, recent studies have shown that several copper compounds present a thermally activated delayed fluorescence (TADF): Cu(I) halide-bridged compounds with P^N ligands are characterized by (metal + halide)-to-ligand charge transfer ((M + X) LCT) excited states where the low singlet-triplet energy gap allows emission from both the singlet and the triplet excited states, depending on the temperature.…”

Efficient luminescence from easily prepared three-coordinate copper(i) arylamidophosphines

“…4 Indeed, their comparative ease of synthesis has in part driven the widespread success enjoyed by iridium cyclometalates in roles ranging from dopants in OLED displays to sensitizers in photochemical hydrogen production. 5,6 Copper-based luminophores 7 have been investigated as an inexpensive alternative to the more ubiquitous noble metal emitters and recently our group introduced copper(I) amidophosphine complexes as a class of highly luminescent compounds. To date we have focused on binuclear 8 and mononuclear 9 copper(I) complexes that feature tridentate and bidentate arylamidophosphine ligands, respectively.…”

“…While their quantum efficiencies are not in general as high as those complexes highlighted above, 8,9 they are favorable when compared to other classes of copper emitters. 7 Moreover, their ease of structural variation makes this class of complexes a convenient family for further expansion and study.…”

“…phosphinodiimine complexes (F PL r 0.16) that have been the most extensively studied monomeric copper emitters. 7 The electronic structure of 2 was explored using Density Functional Theory (DFT) calculations at the B3LYP/ 6-31+G* level of theory. The computed highest occupied molecular orbital (HOMO) is localized primarily on the diphenylamide ligand with substantial nitrogen pp and C-C p character (Fig.…”