Photochemistry and Photophysics of Coordination Compounds

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 CH2Cl2 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.
Keywords Clusters · Copper · Electron transfer · Energy transfer · Luminescence ·
OLED · Phenanthroline
1
An Overview of Copper
1.1
Historical Notes, Current Use, Consumption Trends
Copper was known to some of the oldest civilizations on record, and has a historyofusethatisatleast10
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 cast, and bronze alloys
were extensively found in the Nile valley. The use of bronze was so pervasive
in a certain era of civilization that the period spanning from 2500 to 600
B.C. is named the Bronze Age. In Roman times, copper became known as aes
Cyprium, aes being the generic Latin term for copper alloys such as bronze
or other metals, and Cyprium because so much of it was mined in the island
of Cyprus. From this, the phrase was simplified to cuprum (originating the
current chemical symbol) and then eventually Anglicized into copper.