Photochemistry and Photophysics of Coordination Compounds

102 N. Armaroli et al.
Cuprous clusters display many structural formats that are characterized by
largely different emission behavior (vide infra). The variety of structural motifs
and stoichiometries is related to the remarkable flatness of their ground
state potential energy surfaces [143]. The most extensive studies carried
out on these Cu(I) complexes concern cubane-type clusters of general formula
[CuXL]4 [144, 145]. The solid state structural variety observed among
cuprous clusters can be illustrated by examining some crystallographic data
reported by Ford and co-workers, who found that compounds containing
Cu(I), iodine, and pyridine generate different structures depending on the
reaction stoichiometry, Fig. 30.
For a 1:1:1 Cu:I:L ratio (Fig. 30 A), the most common motif is the tetranuclear
“cubane” structure (Cu4I4L4) in which a tetrahedron of copper atoms is
included by a larger I4 tetrahedron where each iodine is placed on a triangular
face of the Cu4 cluster and the fourth coordination site of each copper is occupied
by the ligand (L). For stoichiometry 1:1:2 (Fig. 30 B), the most common
structure is an isolated rhombohedron of Cu2I2 with alternating copper and
halide atoms. Sometimes, clusters with stoichiometry (1:1:1) exist in more
than one crystalline structure. For example (Fig. 30 C) Cu4I4py4 can also give
rise to a polymeric “stair” made of an infinite chain of steps [146].
4.2
Cuprous Iodide Clusters
The interest in the luminescence properties of Cu(I) iodide clusters goes back
to the pioneering work of Hardt and co-workers [144]. They found that the
emission spectra of solid samples of [CuxIy(py)z] are markedly temperaturedependent
and defined the term “luminescent thermochromism”.
In some cases cuprous iodide clusters exhibit two emission bands termed
HE (high energy) and LE (low energy), which sharply change their relative intensities
upon temperature variation. As an example, in Fig. 31 are depicted
the temperature-dependent emission spectra of Cu4I4(4 – phenylpyridine)4
[147]. The LE band dominates at room temperature, while the HE band is by
far the strongest at temperatures below 80 K.
The HE band dominating at low temperature has been attributed, on the
basis of ab initio calculations and experimental work, to ligand-to-ligand
(I – → phenylpyridine) charge transfer states, also indicated as XLCT. The LE
emission dominating at room temperature has been assigned to an excited
state of mixed halide-to-metal charge transfer (XMCT) and d → s,p metalcentered
character which is usually referred to as “cluster-centered” (CC).
This term was introduced to highlight that these transitions are localized on
the Cu4I4 cluster and are essentially independent on ligand L. The Cu–Cu distance
is a fundamental parameter to allow the presence of CC bands and must
be shorter than the orbital interaction radius, estimated to be 2.8 ˚A. Ifthe
distance between the two metal centers exceeds this critical value the metal