Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Copper 73
Cu(I) cluster compounds are characterized by a variety of emitting electronic
levels whereas Cu(I) cationic complexes show only luminescence originating
from metal-to-ligand charge-transfer (MLCT) states, as long as empty
π orbitals are easily accessible in the ligands. Such MLCT transitions, which
clearly take advantage of the low oxidation potential of Cu(I), arealsocommonly
observed in other classes of coordination compounds, for example
those of d 6 metals like Ru(II)-bipyridines [6] and Ir(III)-phenylpyridine complexes
[7].
MLCT electronic transitions in coordination compounds are normally
more intense when compared to MC (metal-centered) ones since they do not
undergo the same prohibitions by orbital symmetry; accordingly MLCT absorption
bands exhibit relatively high molar extinction coefficients. As far as
emission is concerned, when MLCT excited states are the lowest-lying, they
are generally characterized by long lifetimes, and potentially intense luminescence,
even though exceptions are possible (vide infra). Complexes exhibiting
long-lived MLCT excited states have been extensively investigated in the last
decades both for a better comprehension of fundamental phenomena [8, 9]
and for potential applications related to solar light harvesting and conversion
[10–12]. Among them the highest attention was probably devoted to
Ru(II) [13], Os(II) [14] and, more recently, Ir(III) [7] complexes, however,
economical and environmental considerations make Cu(I) compounds interesting
alternatives [15].
As extensively discussed in the literature, long-lived luminescent MLCT
excited states of d 6 metal complexes, in particular those of Ru(II), can be
strongly affected by the presence of upper lying MC levels. The latter can
be partially populated through thermal activation from the MLCT states and
prompt non-radiative deactivation pathways and photochemical degradation
[6, 16]. Closed shell d 10 copper(I) complexes cannot suffer these kinds of
problems, but undesired non-radiative deactivation channels of their MLCT
levels can be favored by other factors, as will be discussed in detail further
on in this review. An orbital diagram illustrating the electronic transitions of
Ru(II) and Cu(I) complexes is reported in Fig. 2.
1.3
Copper in Biology
Copper, even if present in traces, is an essential metal for the growth and development
of biological systems. Copper plays a fundamental role in cerebral
activity, nervous and cardiovascular systems, oxygen transport and cell protection
against oxidation. Copper is important to strengthen the bones and to
guarantee the performances of the immune system [17].
Metals are commonly found as natural constituents of proteins and, in
thecourseofevolution,Naturehaslearnedhowtousethespecialproperties
of metal ions to perform a wide variety of specific functions associated