Photochemistry and Photophysics of Coordination Compounds

94 N. Armaroli et al.
tionalized derivative. The combined effect of these two factors (kinetic and
thermodynamic) can explain the different and unexpected trend in photoprocesses
of multicomponent arrays containing Cu(I)-phenanthrolines linked to
methanofullerenes vs. bismethanofullerenes, which has been found in a variety
of molecular architectures such as dendrimers [38], rotaxanes [98] and
sandwich-type dyads [110].
Exhaustive review articles presenting photophysical investigations on
fullerene- and porphyrin-type arrays built-up around [Cu(NN)2] + centers
have been published recently and we suggest the reader refers to these papers
for a comprehensive and updated overview on this topic [15, 25, 111, 112].
2.6
Bimolecular Quenching Processes
Excited state electrochemical potentials can be obtained from the ground
state monoelectronic electrochemical potentials and the spectroscopic energy
(E ◦◦ in eV units, to be considered divided by a unitary charge) related to the
involved transition, according to Eqs 2 and 3 [6]:
E(A + / ∗ A)=E(A + /A)–E ◦◦
E( ∗ A/A – )=E(A/A – )+E ◦◦
Hence the variation of the electron-donating or accepting capability of a given
molecule A, upon light excitation, can be easily assessed. In Eqs 2 and 3: ∗ A
denotes the lowest-lying electronically excited state of A and its spectroscopic
energy (E ◦◦ ) can be estimated from the onset of emission spectra [6].
Oxidation from Cu(I) to Cu(II) is easily accomplished and the MLCT
excited states of Cu(I)-bisphenanthrolines are, therefore, potent reductants.
For example [Cu(3)2] + is a more powerful reductant than the very popular
photosensitizer [Ru(bpy)3] 2+ (A + /A = – 1.11 and – 0.85 V, respectively)
owing to its more favorable ground state 2+/+ potential (+ 0.69 vs. + 1.27 V),
that largely compensates the lower content of excited state energy (1.80 vs.
2.12 eV) [15]. By contrast reduction of Cu(I)-bisphenanthrolines is strongly
disfavored and they are mild excited state oxidants; accordingly, only a few examples
of reductive quenching of [Cu(NN)2] + complexes are reported in the
literature, with ferrocenes as donors [113, 114].
Oxidative quenching of [Cu(NN)2] + ’s by Co(III) and Cr(III) complexes as
well as nitroaromatic compounds and viologens has been reported and comprehensively
reviewed [115]. Some attempts to sensitize wide band-gap semiconductors
with Cu(I) complexes were also carried out [115] but so far they
do not seem to be competitive in terms of stability and efficiency with those
based on Ru(II) complexes [12]. Energy transfer quenching to molecules
possessing low-lying triplets such as anthracene has been demonstrated via
transient absorption spectroscopy [116, 117], whereas oxygen quenching,
(2)
(3)