Heteroleptic copper(I) complexes coupled with methano[60]fullerene: Synthesis, electrochemistry, and photophysics

Article abstract of DOI:10.1021/ic800315e

Heteroleptic copper(I) complexes CuPOP-F and CuFc-F have been prepared from a fullerene-substituted phenanthroline ligand and bis[2-(diphenylphosphino) phenyl] ether (POP) and 1,1′-bis(diphenylphosphino)ferrocene (dppFc), respectively. Electrochemical studies indicate that some ground-state electronic interaction between the fullerene subunit and the metal-complexed moiety are present in both CuPOP-F and CuFc-F. Their photophysical properties have been investigated by steady state and time-resolved UV-vis-NIR luminescence spectroscopy and nanosecond laser flash photolysis in a CH₂Cl ₂ solution and compared to those of the corresponding model copper(I) complexes CuPOP and CuFc and of the fullerene model compound F. Selective excitation of the methanofullerene moiety in CuPOP-F results in regular deactivation of the lowest singlet and triplet states, indicating no intercomponent interactions. Conversely, excitation of the copper(I)-complexed unit (405 nm, 40% selectivity) shows that the strongly luminescent triplet metal-to-ligand charge-transfer (³MLCT) excited state located at 2.40 eV is quenched by the carbon sphere with a rate constant of 1.6 × 10 ⁸ s^-1. Details on the mechanism of photodynamic processes in CuPOP-F via transient absorption are hampered by the rather unfavorable partition of light excitation between the two chromophores. By determination of the yield of formation of the lowest fullerene triplet level through sensitized singlet oxygen luminescence in the NIR region, it is shown that the final sink of photoinduced processes is always the fullerene triplet. This can be populated via a two-step charge-separation charge-recombination process and a less favored ³MLCT→³C₆₀ triplet-triplet energy-transfer pathway. In CuFc-F, both of the photoexcited copper(I)-complexed and fullerene moieties are quenched by the presence of the ferrocene unit, most likely via ultrafast energy transfer.

Full text of DOI:10.1021/ic800315e

Inorg. Chem. 2008, 47, 6254-6261
Heteroleptic Copper(I) Complexes Coupled with Methano[60]fullerene:
Synthesis, Electrochemistry, and Photophysics
Andrea Listorti,† Gianluca Accorsi,† Yannick Rio,† Nicola Armaroli,*,† Omar Moudam,‡ Aline Ge´gout,‡
Be´atrice Delavaux-Nicot,‡ Michel Holler,§ and Jean-Franc¸ois Nierengarten*,§
Molecular Photoscience Group, Istituto per la Sintesi Organica e la FotoreattiVita`, Consiglio
Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy, Laboratoire de Chimie de
Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 4, France, and
Laboratoire de Chimie des Mate´riaux Mole´culaires, Ecole Europe´enne de Chimie, Polyme`res et
Mate´riaux (ECPM), UniVersite´ Louis Pasteur (ULP) et CNRS, 25 rue Becquerel,
67087 Strasbourg Cedex 2, France
Received February 19, 2008
Heteroleptic copper(I) complexes CuPOP-F and CuFc-F have been prepared from a fullerene-substituted
phenanthroline ligand and bis[2-(diphenylphosphino)phenyl] ether (POP) and 1,1′-bis(diphenylphosphino)ferrocene
(dppFc), respectively. Electrochemical studies indicate that some ground-state electronic interaction between the
fullerene subunit and the metal-complexed moiety are present in both CuPOP-F and CuFc-F. Their photophysical
properties have been investigated by steady state and time-resolved UV-vis-NIR luminescence spectroscopy
and nanosecond laser flash photolysis in a CH2Cl2 solution and compared to those of the corresponding model
copper(I) complexes CuPOP and CuFc and of the fullerene model compound F. Selective excitation of the
methanofullerene moiety in CuPOP-F results in regular deactivation of the lowest singlet and triplet states, indicating
no intercomponent interactions. Conversely, excitation of the copper(I)-complexed unit (405 nm, 40% selectivity)
shows that the strongly luminescent triplet metal-to-ligand charge-transfer (3MLCT) excited state located at 2.40 eV
is quenched by the carbon sphere with a rate constant of 1.6 × 108 s-1. Details on the mechanism of photodynamic
processes in CuPOP-F via transient absorption are hampered by the rather unfavorable partition of light excitation
between the two chromophores. By determination of the yield of formation of the lowest fullerene triplet level
through sensitized singlet oxygen luminescence in the NIR region, it is shown that the final sink of photoinduced
processes is always the fullerene triplet. This can be populated via a two-step charge-separation charge-recombination
process and a less favored 3MLCT f 3C60 triplet-triplet energy-transfer pathway. In CuFc-F, both of the photoexcited
copper(I)-complexed and fullerene moieties are quenched by the presence of the ferrocene unit, most likely via
ultrafast energy transfer.
Introduction
appropriate electron/energy acceptors to be used in parallel
with reducing coordination compounds, several criteria are
usually met, which include reasonably long-lived excited-
state lifetimes, good absorption in the UV-vis spectral
region, photostability, and possibly photoluminescence. Ac-
cordingly, a large number of dyads in which fullerene is
coupled with carefully chosen chromophores have been
investigated over the years: porphyrin3–8 and conjugated
d6 and d10 coordination compounds may possess low-lying
metal-to-ligand charge-transfer (MLCT) excited states with
marked reducing character; thus, they can be excellent
partners for C60 fullerene electron acceptors in photoactive
multicomponent hybrid systems.1,2 In the search for the most
* To whom correspondence should be addressed. E-mail: armaroli@
isof.cnr.it (N.A.), nierengarten@chimie.u-strasbg.fr (J.-F.N.).
†
Consiglio Nazionale delle Ricerche.
Laboratoire de Chimie de Coordination du CNRS.
Universite´ Louis Pasteur (ULP) et CNRS.
(2) Clifford, J. N.; Accorsi, G.; Cardinali, F.; Nierengarten, J. F.; Armaroli,
N. C. R. Chim. 2006, 9, 1005.
‡
§
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(4) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J. Photochem.
Photobiol. C 2004, 5, 79.
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ReV. 2002, 230, 141.
6254 Inorganic Chemistry, Vol. 47, No. 14, 2008
10.1021/ic800315e CCC: $40.75  2008 American Chemical Society
Published on Web 06/26/2008

Heteroleptic Copper(I) Complexes
organic oligomer systems9–15 are probably the most popular,
whereas relatively less attention has been paid to hybrid
systems with photoactive coordination compounds of ruthe-
nium(II),16–19 rhenium(I),16 and copper(I).20–24 In a ruthe-
nium(II) system that we investigated previously, the energy
of the lowest 3MLCT level (2.4 eV) is higher lying than that
of the fullerene singlet (1.8 eV) and triplet (1.5 eV), whereas
the charge-separated state is intermediate (2.0 eV).16 Upon
excitation of the metal-complexed moiety, charge separation
followed by charge recombination to the fullerene triplet is
observed. Practically, because direct excitation of the fullerene
moiety results in regular deactivation without intercomponent
interactions, the fullerene triplet level is the final energy sink
of the dyad, whatever the excitation wavelength.16 The
situation can be rather different in the case of copper(I)
phenanthroline-fullerene hybrids. Copper(I) complexes are
indeed stronger reducing agents than ruthenium(II) systems;
thus, the charge-separated state can be the lowest in the
energy diagram, originating a different pattern of photoin-
duced processes also finely depending on the fullerene cage
functionalization [e.g., mono- vs bis(methanofullerenes)].20
In the past few years, there has been growing interest in
heteroleptic copper(I) complexes with a phenanthroline and
a phosphine-type ligand.25 Some of them are strong green
Chart 1
emitters from their lowest 3MLCT* state26–29 and are rather
promising for the development of electroluminescent devices
based on materials cheaper than traditional iridium(III)
compounds.30–33 Interestingly, the electronic and photo-
physical properties of these copper(I) complexes are more
similar to ruthenium(II) polypyridine than to copper(I)
bis(phenanthroline) analogues.33 For instance, the energy of
the excited state may exceed 2.40 eV, whereas a relatively
high oxidation potential can render the charge-separated state
in fullerene dyads almost isoenergetic to that obtainable with
analogous ruthenium(II) polypyridyl/fullerene arrays.
Herein we describe the synthesis and electrochemical and
photophysical properties of a new hybrid system, CuPOP-
F, made of a methanofullerene unit (F) linked to a green
emitting and weakly reducing heteroleptic copper(I) complex
(CuPOP; Chart 1). The pattern of photoinduced processes
in the dyad is discussed in comparison to previously
investigated hybrids with ruthenium(II) and copper(I) com-
plexes and with an analogous complex (CuFc-F) bearing
an electrochemically and photochemically active ferrocene
center in place of the POP ligand.
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Inorganic Chemistry, Vol. 47, No. 14, 2008 6255

Listorti et al.
Scheme 1. Preparation of Ligand 5
Table 1. Electrochemical Properties of CuPOP-F, CuFc-F, CuPOP,
CuFc, and F Determined by CV on a Pt Working Electrode in CH2Cl2
+ 0.1 M n-Bu4NBF4 Solutions at Room Temperaturea
Synthesis
The preparation of fullerene-substituted phenanthroline
ligand 5 is depicted in Scheme 1. Compound 2 was obtained
from 2,9-dimethyl-1,10-phenanthroline by deprotonation of
one of the methyl groups with lithium diisopropylamide
(LDA) to generate the corresponding carbanionic species,
followed by reaction with 4-(bromopropyl)tetrahydropyran-
2-yl ether (1). Compound 2 was not isolated in a pure form
and was used as received in the next step. The cleavage of
the 3,4,5,6-tetrahydro-2H-pyranyl (THP) protecting group
was achieved by treatment with p-toluenesulfonic acid (p-
TsOH) in refluxing ethanol. Compound 3 was thus obtained
in 50% yield (from 2,9-dimethyl-1,10-phenanthroline). Fi-
nally, the treatment of alcohol 3 with carboxylic acid 4 under
esterification conditions [N,N′-dicyclohexylcarbodiimide
(DCC), 4-(dimethylamino)pyridine (DMAP), and 1-hydroxy-
benzotriazole (HOBt)] yielded ligand 5. The corresponding
complexes CuPOP-F and CuFc-F were obtained by the
reaction of bis[2-(diphenylphosphino)phenyl] ether (POP)
and 1,1′-bis(diphenylphosphino)ferrocene (dppFc), respec-
tively, with stoichiometric quantities of 5 and [Cu(CH3-
CN)4]BF4 in dichloromethane at 25 °C. All of the spectro-
scopic studies and elemental analysis results were consistent
with the proposed molecular structures.
reduction
oxidation
E2
E1
E1
E2
CuPOP
CuFc
F
CuPOP-F
CuFc-F
-1.75
-1.75
-0.51
-0.45
-0.46
+1.36
+0.87
+1.66b
+1.53
+0.98
+1.68b
-0.89c
-0.86c
-0.86c
+1.68b
a Values for (Epa + Epc)/2 in V vs SCE at a scan rate of 0.1 V s-1
.
b Irreversible process, Epa value reported. Reversible for V > 2 V s-1
.
c
attributed to the oxidation of the dialkyloxyphenyl unit.34
In the cathodic region, compound F revealed the typical
electrochemical response of methanofullerene derivatives,35
and several reduction steps are seen. Whereas the first one
is reversible, the second is irreversible at low scan rates, in
accordance with observations on other C60 derivatives;36 it
becomes partially reversible upon an increase in the scan
rate. The third wave gradually disappears when the second
one becomes reversible, so that it probably implies a
reduction of the product formed after the second reduction.
For this reason, only the two first reduction potentials are
listed in Table 1. Model compound CuPOP exhibits a one-
electron-transfer process both in the cathodic and in the
anodic region. As reported earlier,37 the oxidation process
can be assigned to the CuII/CuI redox couple and the
reduction is centered on the phenanthroline ligand. Com-
pound CuFc shows a one-electron-transfer process in the
cathodic region at a potential value identical with that of
CuPOP (-1.75 V vs SCE) and corresponding to the
reduction of the phenanthroline ligand. In the anodic region,
Electrochemical Properties
The electrochemical properties of CuPOP-F and CuFc-F
were investigated by cyclic voltammetry (CV). For the sake
of comparison, electrochemical measurements have also been
carried out with model compounds CuPOP, CuFc, and F.
All of the experiments were performed at room temperature
in CH2Cl2 solutions containing tetra-n-butylammonium tet-
rafluoroborate (0.1 M) as the supporting electrolyte, with a
platinum wire as the working electrode and a saturated
calomel electrode (SCE) as the reference electrode. Potential
data for all of the compounds are collected in Table 1.
In the anodic region, model compound F presents an
irreversible peak at ca. +1.7 V vs SCE, which can likely be
(34) Armaroli, N.; Barigelletti, F.; Ceroni, P.; Eckert, J. F.; Nicoud, J. F.;
Nierengarten, J. F. Chem. Commun. 2000, 599.
(35) Felder, D.; Nierengarten, H.; Gisselbrecht, J. P.; Boudon, C.; Leize,
E.; Nicoud, J. F.; Gross, M.; Van Dorsselaer, A.; Nierengarten, J. F.
New J. Chem. 2000, 24, 687.
(36) Hahn, U.; Maisonhaute, E.; Amatore, C.; Nierengarten, J. F. Angew.
Chem., Int. Ed. 2007, 46, 951.
(37) Armaroli, N.; Accorsi, G.; Bergamini, G.; Ceroni, P.; Holler, M.;
Moudam, O.; Duhayon, C.; Delavaux-Nicot, B.; Nierengarten, J. F.
Inorg. Chim. Acta 2007, 360, 1032.
6256 Inorganic Chemistry, Vol. 47, No. 14, 2008

Heteroleptic Copper(I) Complexes
two processes are observed. The first is completely reversible,
while the second one is irreversible whatever the scan rate
is. The first oxidation process involves the ferrocene moiety
and the second one the CuII/CuI redox couple.37 The
irreversibility of the second electron-transfer process reflects
the instability of a copper(II) complex with an oxidized
dppFc ligand, which is likely to undergo fast dissociation.37
The cyclic voltammogram recorded for the hybrid compound
CuPOP-F shows the characteristic electrochemical features
of both CuPOP and F. The comparison of the E1/2 potentials
of CuPOP-F and F clearly shows that the two first reduction
waves correspond to fullerene-centered reductions and the
first oxidation process can be assigned to the CuII/CuI redox
couple. Oxidation of the copper(I) center is more difficult
in the case of CuPOP-F when compared to CuPOP (∆E )
170 mV). This positive shift is explained, at least partially,
by the presence of a longer alkyl substituent on the
phenanthroline ligand, thus hindering the formation of a more
flattened structure appropriate for the copper(II) oxidation
state.37 Similarly, the cyclic voltammogram of hybrid
compound CuFc-F shows the typical electrochemical sig-
natures of both CuFc and F. Clearly, the two first reductions
are centered on the fullerene moiety and the first oxidation
on the ferrocene subunit. Interestingly, in both CuPOP-F
and CuFc-F, the two first fullerene-centered reduction
potentials are shifted to more positive values by about 30-60
mV with respect to the corresponding fullerene model
compound F. On the other hand, the ferrocene-centered
oxidation potential is found to be more positive (∆E ) 110
mV) when compared to CuFc. The latter observations
indicate a small electronic interaction between the copper
complex and the methanofullerene unit in both CuPOP-F
and CuFc-F, suggesting that these compounds may adopt a
folded conformation in which the two moieties are close to
each other. This is in agreement with previous reports in
which derivatives with a metal center and a fullerene unit,
forced to be spatially close, behave similarly (more facile
fullerene reduction, harder metal-centered oxidation).21
Figure 1. Absorption spectra of CuPOP-F (red), CuPOP (green), and F
(black) in CH2Cl2 along with the profile obtained by the sum CuPOP + F
(blue). For the sake of clarity, after 360 nm the spectra are multiplied by a
factor of 10. Inset a: Emission spectra of CuPOP (green) and CuPOP-F
(red; magnified by a factor of 160) in oxygen-free CH2Cl2 at 298 K; λexc
405 nm. Inset b: Emission decays of CuPOP-F (red) and CuPOP (green)
at 550 nm; λexc ) 407 nm, 298 K, oxygen-free CH2Cl2.
)
systems encompassing fullerenes and copper(I) complexes21
but also in fullerodendrimers38–40 and host-guest adducts.41
Both molecular subunits of CuPOP-F are luminescent.
At 298 K, CuPOP exhibits strong green luminescence26 in
an oxygen-free CH2Cl2 solution (λmax ) 562 nm, τ ) 4.6
µs, and Φem ) 0.09),37 which originates from deactivation
29
3
of the lowest MLCT state and is blue-shifted in a 77 K
CH2Cl2 matrix due to rigidochromism (λmax ) 514 nm and
τ ) 33 µs).
The photophysical properties of methanofullerenes have
been extensively investigated in the past.23,42 F exhibits a
weak and structured fluorescence band in CH2Cl2 at 298 K
with the highest-energy feature peaked at 700 nm (τ ) 1.5
ns, Φem ) 4.5 × 10-4, CH2Cl2), also observable at 77 K
(λmax ) 706 nm).16
In CuPOP-F, selective excitation of the fullerene moiety
can be achieved above 520 nm, while the copper-complexed
moiety can be excited at most with 40% selectivity at 405
nm. The fullerene-centered emission properties are substan-
tially unaffected in CuPOP-F relative to F: the fluorescence
quantum yields and lifetimes at λexc > 520 nm are identical
for the two compounds (Figure 2, top panel). The fullerene
triplet properties investigated at λexc ) 532 nm are marginally
affected in CuPOP-F compared to F: (i) the intense transient
absorption features are peaked at 700 nm in both cases; (ii)
the triplet lifetime of CuPOP-F is 18 µs, to be compared to
27 µs for F (oxygen-free CH2Cl2); (iii) CuPOP-F is a slightly
weaker singlet-oxygen (1O2*) sensitizer than F, as is observ-
able from the intensity of the senzitized 1O2* luminescence
in the NIR region (Figure 2, top panel).16
Photophysical Properties
CuPOP-F. The electronic absorption spectra of CuPOP-
F, CuPOP, and F in CH2Cl2 are depicted in Figure 1. In
the UV region above 300 nm, there is substantial superim-
position of the bands of the two individual chromophores;
furthermore, the MLCT absorption envelope of CuPOP,
peaked at 380 nm, is also substantially overlapped to the
profile of the carbon cage, hampering detailed investigations
of photoinduced processes when the two moieties are
combined (vide infra). The spectrum of the hybrid dyad
matches well the profile obtained by summing the spectra
of the model compounds CuPOP and F, except around 430
nm. Enhanced absorption in this region is often observed in
fullerene multicomponent systems where the carbon sphere
is in tight vicinity with other chromophore(s)20 and ground-
state donor-acceptor interactions at relatively high energy
can be established. This is observed not only in hybrid
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Listorti et al.
Figure 3. Schematic energy level diagram for CuPOP-F. The energy
positions of the lowest excited states centered on each moiety are estimated
from experimental (emission and electrochemical) and literature data. Cu+F-
represents the charge-separated state obtained upon electron transfer from
the metal-complexed moiety to the carbon sphere.
Figure 2. Vis-NIR emission spectra of F (full line) and CuPOP-F (dashed
line) in CH2Cl2 at 298 K: (a) λexc ) 520 nm (100% excitation on the F
moiety); (b) λexc ) 405 nm (light partitioning: 40% copper-complexed
moiety; 60% fullerene moiety).
(i) can be ruled out because excitation of CuPOP-F at 405
nm, where 40% of the light is addressed to the copper-
complexed chromophore, leads to a 40% decrease of the
fullerene fluorescence intensity compared to what was
recorded upon excitation above 520 nm, where only the
fullerene chromophore absorbs (Figure 2, bottom panel). This
clearly shows that transfer of the excitation energy from
3CuMLCT*-F to Cu-1F* does not take place. Finding out if
hypotheses (ii) and (iii) are the active mechanisms is very
difficult because of the unfavorable distribution of the light
absorption between the two chromophores. Nd:YAG laser
lines at 266, 355, and 532 nm cannot be used to excite
significantly the copper(I) moiety and carry out transient
absorption studies. On the other hand, excitation at more
favorable wavelengths, e.g., 400-410 nm (Ti:sapphire or
355-nm-fed OPO), does not give meaningful results because
of the wide and intense singlet and triplet transient absorption
signals of the directly excited fullerene moiety, which
undergoes regular deactivation (see above).
These observations indicate that no photoinduced processes
occur upon excitation of the carbon sphere in CuPOP-F,
and this is fully rationalized based on the energy level
diagram (vide infra). The minor variations of the triplet
properties of the dyad can be related to the specific chemical
environment of the C60 moiety, in close vicinity to the
copper(I) chromophore. It is well-known that, upon aggrega-
tion at the nano- and microscopic level, fullerenes may
undergo variations of intersystem crossing efficiencies and
triplet lifetimes.43,44
Upon excitation of CuPOP-F at 405 nm in oxygen-free
CH2Cl2 (40% selectivity on the copper(I)-complexed moiety),
a substantial decrease of the intense MLCT luminescence is
detected when compared to CuPOP (Figure 1, inset). The
measured emission quantum yields and lifetimes (λexc ) 407
nm, laser diode) are 0.0003 and 6.1 ns, to be compared to
0.09 and 4600 ns for the reference system CuPOP under
the same conditions. From lifetime data, it is possible to get
the quenching rate constant for the MLCT excited state
centered on the copper moiety of CuPOP-F (3CuMLCT*), k
) 1.6 × 108 s-1. An energy level diagram for CuPOP-F
can be drawn where the energies of the copper-type moiety
and of the methanofullerene fragments are estimated from
77 K emission spectra (λmax ) 514 nm; CuPOP) and
literature data,16 respectively (Figure 3). The charge-
separated state energy (Cu+F-, 1.98 eV) is estimated from
the first one-electron-oxidation and -reduction potentials of
CuPOP-F (Table 1).45
Because of the difficulty of studying in detail the photo-
induced excited-state dynamics of CuPOP-F by means of
transient absorption spectroscopy, a different approach was
attempted aimed at determining the yield of formation of
the lowest available excited level, i.e., the fullerene triplet
(Figure 3). This value may provide some hints about excited-
state intercomponent processes involving upper-lying levels,
as was also pointed out previously in an analogous case of
two dyads made of a methanofullerene chromophore coupled
to a ruthenium(II) or rhenium(I) complex.16 The approach
is based on the fact that fullerene triplets are potent singlet-
oxygen sensitizers46 and the yields of triplet formation (ΦT)
3
With reference to Figure 3, quenching of the MLCT
excited state can occur, in principle, via (i) spin-forbidden
(triplet-singlet) energy transfer to the fullerene lowest singlet
level (Cu-1F*); (ii) triplet-triplet energy transfer to Cu-3F*;
(iii) electron transfer to Cu+F-, followed by charge recom-
bination back to the ground state or to Cu-3F*. Hypothesis
1
are found to be identical with that of sensitization of O2*
(Φ∆) for C60,47 its close42 and open cage48 derivatives, as
well as for higher fullerenes like C7049 and C76.50 Therefore,
the measure of Φ∆ can be taken as an indirect evaluation of
(43) Quaranta, A.; McGarvey, D. J.; Land, E. J.; Brettreich, M.; Burghardt,
S.; Schonberger, H.; Hirsch, A.; Gharbi, N.; Moussa, F.; Leach, S.;
Gottinger, H.; Bensasson, R. V. Phys. Chem. Chem. Phys. 2003, 5,
843.
(46) Armaroli, N. In Fullerenes: from synthesis to optoelectronic properties;
Guldi, D. M., Martin, N., Eds.; Kluwer Academic Publishers:
Dordrecht, The Netherlands, 2002; p 137.
(47) Redmond, R. W.; Gamlin, J. N. Photochem. Photobiol. 1999, 70, 391.
(48) Stackow, R.; Schick, G.; Jarrosson, T.; Rubin, Y.; Foote, C. S. J. Phys.
Chem. B 2000, 104, 7914.
(44) Hosomizu, K.; Imahori, H.; Hahn, U.; Nierengarten, J. F.; Listorti,
A.; Armaroli, N.; Nemoto, T.; Isoda, S. J. Phys. Chem. C 2007, 111,
2777.
(49) Bensasson, R. V.; Schwell, M.; Fanti, M.; Wachter, N. K.; Lopez,
J. O.; Janot, J.-M.; Birkett, P. R.; Land, E. J.; Leach, S.; Seta, P.;
Taylor, R.; Zerbetto, F. ChemPhysChem 2001, 109.
(45) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis
Horwood: Chichester, U.K., 1991; p 44.
6258 Inorganic Chemistry, Vol. 47, No. 14, 2008

Heteroleptic Copper(I) Complexes
ΦT for all fullerenes.51 Φ∆, in its turn, is determined via the
comparative method by measuring the intensity of the
sensitized singlet-oxygen luminescence in the NIR region
(λmax ) 1268 nm).40,52,53
Using as a reference C60 (Φ∆ ) 1), we have determined
Φ∆ of the reference fullerene compound F in CH2Cl2 as 0.8,
in line with those of methanofullerenes investigated ear-
lier.16,42 Then, upon selective excitation of the carbon sphere
(λexc ) 520 nm; Figure 2a), we obtained Φ∆ ) 0.6 for
CuPOP-F, a slightly lower value that might reflect specific
interaction of the two chromophores as commented on above.
Then, the same experiment was repeated under 405 nm
excitation (Figure 2b), where a fraction of the incoming
photons (40%) are absorbed by the copper(I) moiety of
CuPOP-F.54 Notably, despite the net loss of 40% of fullerene
singlets (see the fluorescence signal around 700 nm), the
Figure 4. Emission spectra of F (full line) and CuFc-F (dashed line) at
λexc ) 325 nm. Inset: Fullerene fluorescence decay at 730 nm of F (full
black circles) and CuFc-F (empty black circles). In the full gray circles is
reported the decay profile of the excitation source, λexc ) 331 nm. x axis:
1 channel ) 51 ps; 298 K, CH2Cl2.
1
intensity of the 1268 nm band of O2* is slightly higher
compared to the experiment made at λexc ) 520 nm. Clearly,
we have to rule out quantitative deactivation of Cu+F- to
expected to increase in a rigid matrix, where the effective
solvent reorganization energy increases dramatically because
the solvent molecules cannot reorient to stabilize the newly
formed ions.56 Consequently, for multicomponent arrays
where the occurrence of photoinduced processes is conve-
niently signaled by luminescence quenching in solution, the
lack of such quenching in a rigid matrix can be taken as
evidence to support the electron-transfer mechanism in the
fluid medium. Unlike the case of a methanofullerene dyad
coupled with a ruthenium(II) complex, in which we observed
recovery of the MLCT emission band of the latter at 77 K,16
here the intense emission band of the copper(I) unit remains
quenched in the low-temperature rigid matrix. However, this
observation cannot rule out electron transfer in the fluid
solution because a 0.45 eV difference between 3CuMLCT* and
the charge-separated state (Figure 3) is likely to be too large
to make the electron-transfer reaction endoergonic at 77 K,
similarly to a methanofullerene dyad with a rhenium(I)
complex, characterized by an energy gap between the MLCT
and charge-separated states of 0.42 eV.16 Accordingly, the
3CuMLCT* in CuPOP-F might undergo electron transfer also
in a 77 K rigid matrix.
3
the ground state following CuMLCT*-F f Cu+F- electron
transfer. In fact, under these conditions, a much lower 1268
nm emission signal should be found from the 60% excited
fullerene moiety characterized by intrinsic Φ∆ ) 0.60, i.e.,
approximately 45% and not the measured (dashed line in
Figure 2, bottom panel) 80% of the intensity of F. The
measured singlet-oxygen luminescence intensity of Cu-
POP-F excited at λexc ) 405 nm is compatible, within the
experimental error, with quantitative population of the lowest
fullerene triplet following excitation of the copper(I)-
complexed moiety. Both direct (3CuMLCT*-F f Cu-3F*)
and stepwise (3CuMLCT*-F f Cu+F- f Cu-3F*) deactiva-
tions may be responsible for the observed trend (Figure 3).
The first step in the latter process is substantially less
exoergonic (∆GCS° ) -0.42 eV) than direct triplet energy
transfer (∆GEnT° ) -0.90 eV), and its related free energy
change is comparable to the value found at the maximum of
the Marcus parabola in zinc(II) porphyrins/C60 molecular
dyads.55 Thus, the two-step route 3CuMLCT*-F f Cu+F- f
Cu-3F* is likely to be kinetically more favored compared
3
to the direct CuMLCT*-F f Cu-3F* one, as suggested in
CuFc-F. The electronic absorption spectrum of CuFc-F
in CH2Cl2 is practically identical to that of CuPOP-F because
the ferrocene and POP fragments exhibit negligible absorp-
tion intensities compared to the other moieties of their dyad
systems. In the reference complex CuFc, the potentially
luminescent MLCT state(s) is (are) totally quenched by
energy transfer to the ferrocene unit, as was already
discussed.37 Therefore, it is not surprising that the copper(I)-
complexed moiety does not luminesce also in the dyad CuFc-
F. Contrary to what is observed for CuPOP-F, in CuFc-F
only partial quenching of the fullerene singlet state is
observed (Φ ) 0.00018 and τ ) 0.9 ns; Figure 4) relative
to F. A much stronger triplet quenching occurs: a vanishingly
weak transient absorption signal is detected, with a lifetime
comparable to that of F, which suggests the presence of trace
amounts of free methanofullerene. Virtually no sensitized
the case of ruthenium(II) and rhenium(I) fullerene dyads
having similar energy diagrams.16
When an electron-transfer process is slightly exoergonic
at room temperature, it may become endoergonic (blocked)
at 77 K.16 In fact, the energy of a charge-separated state is
(50) Bensasson, R. V.; Bienvenue, E.; Janot, J. M.; Land, E. J.; Leach, S.;
Seta, P. Chem. Phys. Lett. 1998, 283, 221.
(51) Kordatos, K.; Da Ros, T.; Prato, M.; Leach, S.; Land, E. J.; Bensasson,
R. V. Chem. Phys. Lett. 2001, 334, 221.
(52) Darmanyan, A. P.; Arbogast, J. W.; Foote, C. S. J. Phys. Chem. 1991,
95, 7308.
(53) Wilkinson, F.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data
1993, 22, 113.
(54) The quantum yield of singlet-oxygen sensitization of CuPOP has also
been measured and turns out to be 30%, i.e., substantially lower than
that of the fullerene moiety. At any rate, this value has no consequences
in the interpretation of our data because the copper-centered excited
state responsible for photosensitization (3MLCT) is completely
quenched in CuPOP-F.
(55) Imahori, H.; Tkachenko, N. V.; Vehmanen, V.; Tamaki, K.; Lem-
metyinen, H.; Sakata, Y.; Fukuzumi, S. J. Phys. Chem. A 2001, 105,
1750.
(56) Chen, P. Y.; Meyer, T. J. Inorg. Chem. 1996, 35, 5520.
Inorganic Chemistry, Vol. 47, No. 14, 2008 6259

Listorti et al.
singlet-oxygen luminescence is observed, confirming that a
fullerene triplet is not generated (Figure 4).
energy acceptor, often promoting energy transfer rather than
the most wanted electron transfer.
Quenching of fullerene excited states in CuFc-F is clearly
due to the presence of the ferrocene unit and, in principle,
can occur via (i) singlet57 and triplet58 energy transfer to
the lowest-lying ferrocene triplet electronic level (1.16 eV)58
or (ii) ferrocene f fullerene electron transfer, generating the
charge-separated state located at 1.45 eV according to the
electrochemical data (Table 1). The latter process could be
evidenced via transient absorption spectroscopy, through
the detection of the fullerene radical anion in the NIR region.
We found no evidence for this fingerprint down to a
resolution of 10 ns, even via bimolecular quenching studies
with ferrocene and F.57 We have already discussed the
elusive nature of the intimate mechanism of the quenching
processes between ferrocene and fullerenes,58 pointing out
that the two mechanisms are also solvent- and distance-
dependent.57 Often, unambiguous rationalization of this
process is hardly obtainable because of the lack of reliable
spectroscopic fingerprints from the lowest ferrocene triplet,59
and this becomes particularly challenging in the present case
because of the tight vicinity of the donor-acceptor partners,
which prompts ultrafast processes.
Experimental Section
General Procedures. All reagents were used as purchased from
commercial sources without further purification. Compounds 4,60
F,60 CuPOP,37 and CuFc37 were prepared according to previously
reported procedures. All reactions were performed in standard
glassware. Evaporation was done using a water aspirator and drying
in vacuo at 10-2 Torr. Column chromatography: Merck silica gel
60, 40-63 µm (230-400 mesh). TLC: precoated glass sheets with
silica gel 60 F254 (Merck), visualization by UV light. Melting points
were determined on a Electrothermal Digital Melting Point ap-
paratus and are uncorrected. IR spectra (cm-1) were determined
on an ATI Mattson Genesis Series FTIR instrument. NMR spectra
were recorded on a Bruker ARX 300 spectrometer. Elemental
analyses were performed by the analytical service at the Laboratoire
de Chimie de Coordination (Toulouse, France).
Compound 2. A 2 M solution of LDA in tetrahydrofuran (THF;
5.4 mL) was added slowly to a solution of 2,9-dimethyl-1,10-
phenanthroline (3.73 g, 17.92 mmol) in anhydrous THF (70 mL)
at 0 °C under argon. After 1 h, a solution of 1 (4.0 g, 17.92 mmol)
in THF (25 mL) was added dropwise. The resulting mixture was
stirred 2 h at 0 °C and then 15 h at room temperature. The solution
was then poured into ice water (150 mL). The mixture was extracted
with CH2Cl2 (3 × 100 mL), and the combined organic layers were
dried (MgSO4), filtered, and evaporated to yield 2 (6.0 g) as a pale-
orange oil, which was used in the next step without purification.
1H NMR (300 MHz, CDCl3): δ 1.51 (m, 4 H), 1.81 (m, 4 H), 1.99
(m, 2 H), 2.91 (s, 3 H), 3.23 (m, 2 H), 3.48 (m, 2 H), 3.83 (m, 2
H), 4.57 (br s, 1 H), 7.47 (m, 2 H), 7.66 (s, 2 H), 8.09 (m, 2 H).
Compound 3. A solution of 2 (6.0 g, 17.12 mmol) and p-TsOH
(2.50 g, 17.92 mmol) in EtOH (400 mL) was refluxed for 4 h. The
solvent was then evaporated. Column chromatography (Al2O3,
CH2Cl2 containing 5% MeOH) yielded 3 (360 mg, 50% from 2,9-
Conclusions
Two novel hybrid heteroleptic copper(I) complexes Cu-
POP-F and CuFc-F were prepared from a fullerene-
substituted phenanthroline ligand and two bisphosphine
ligands, namely, POP and dppFc, respectively. Ground-
state electronic interaction between the closely spaced
fullerene subunit and the metal-complexed moiety is evi-
denced with electrochemical studies in both cases. Photo-
physical studies indicate that no intercomponent interactions
were observed upon excitation of the methanofullerene
moiety in CuPOP-F, while excitation of the copper(I)-
complexed unit (40% selectivity only) shows that the strongly
1
dimethyl-1,10-phenanthroline) as a colorless glassy product. H
NMR (300 MHz, CDCl3): δ 1.77 (m, 2 H), 2.19 (m, 2 H), 2.92 (s,
3 H), 3.23 (t, J ) 6 Hz, 2 H), 3.67 (t, J ) 5 Hz, 2 H), 7.46 (d, J
) 8 Hz, 1 H), 7.47 (d, J ) 8 Hz, 1 H), 7.68 (s, 2 H), 8.09 (d, J )
8 Hz, 1 H), 8.12 (d, J ) 8 Hz, 1 H). Anal. Calcd for C17H18N2O:
C, 76.66; H, 6.81; N, 10.52. Found: C, 76.45; H, 6.82; N, 10.88.
Compound 5. DCC (46 mg, 0.226 mmol) was added to a stirred
solution of 4 (277 mg, 0.207 mmol), 3 (50 mg, 0.188 mmol), DMAP
(9 mg, 0.075 mmol), and BtOH (catalytic amount) in CH2Cl2 (20
mL) at 0 °C. After 1 h, the mixture was allowed to slowly warm
to room temperature (within 1 h) and then stirred for 12 h, filtered,
and evaporated. Column chromatography (SiO2, CH2Cl2 containing
0.1% MeOH) yielded 1 (198 mg, 67%) as a dark-red glassy product.
IR (KBr): 1746 (CdO) cm-1. 1H NMR (300 MHz, CDCl3): δ 0.88
(t, J ) 6 Hz, 6 H), 1.30 (m, 36 H), 1.72 (m, 4 H), 1.88 (m, 2 H),
2.04 (m, 2 H), 2.94 (s, 3 H), 3.24 (t, J ) 6 Hz, 2 H), 3.87 (t, J )
6 Hz, 4 H), 4.33 (t, J ) 6 Hz, 2 H), 4.97 (s, 2 H), 5.45 (s, 2 H),
6.38 (t, J ) 2 Hz, 1 H), 6.59 (d, J ) 2 Hz, 2 H), 7.47 (d, J ) 8 Hz,
1 H), 7,53 (d, J ) 8 Hz, 1 H), 7.69 (AB, J ) 9 Hz, 2 H), 8.10 (d,
J ) 8 Hz, 1 H), 8.13 (d, J ) 8 Hz, 1 H). 13C{1H} NMR (75 MHz,
CDCl3): δ 14.1, 22.7, 26.1, 29.3, 29.4, 29.5, 29.6, 29.7, 31.9, 39.0,
62.7, 65.8, 68.1, 69.1, 71.2, 101.7, 107.3, 122.3, 123.5, 125.5, 125.6,
126.9, 127.2, 136.2, 136.4, 136.6, 138.4, 139.8, 140.8, 140.9, 141.8,
141.9, 142.1, 142.2, 142.85, 142.9, 142.95, 143.8, 143.85, 144.4,
144.5, 144.6, 144.9, 145.0, 145.1, 145.2, 145.4, 145.5, 159.4, 160.5,
3
luminescent MLCT excited state at 2.40 eV is quenched
by the fullerene moiety. The determination of the detailed
mechanism(s) of photodynamic processes in CuPOP-F via
transient absorption was hampered by the rather unfavorable
partition of light excitation between the metal complex and
the fullerene moiety. Nevertheless, by measurement of the
yield of formation of the lowest fullerene triplet level through
sensitized singlet-oxygen luminescence in the NIR region,
it was unambiguously demonstrated that the final sink of the
photoinduced processes is the fullerene triplet. In contrast,
in CuFc-F, both the photoexcited copper(I)-complexed and
fullerene moieties are quenched by the presence of the
ferrocene unit, most likely via ultrafast energy transfer. This
is in line with recent reports57,58 having also shown that
ferrocene, which, in principle, is an excellent electron donor
in multicomponent arrays, is also indeed an outstanding
(57) Figueira-Duarte, T. M.; Rio, Y.; Listorti, A.; Delavaux-Nicot, B.;
Holler, M.; Marchioni, F.; Ceroni, P.; Armaroli, N.; Nierengarten, J.-
F. New J. Chem. 2008, 32, 52.
(58) Araki, Y.; Yasumura, Y.; Ito, O. J. Phys. Chem. B 2005, 109, 9843.
(59) Faraggi, M.; Weinraub, D.; Broitman, F.; Defelippis, M. R.; Klapper,
M. H. Radiat. Phys. Chem. 1988, 32, 293.
(60) Gegout, A.; Figueira-Duarte, T. M.; Nierengarten, J. F.; Listorti, A.;
Armaroli, N. Synlett 2006, 3095.
6260 Inorganic Chemistry, Vol. 47, No. 14, 2008

Heteroleptic Copper(I) Complexes
162.2, 163.0, 163.05, 166.5. Anal. Calcd for C113H74N2O8: C, 85.48;
H, 4.70; N, 1.76. Found: C, 85.19; H, 4.85; N, 1.70.
Electrochemistry. The CV measurements were carried out with
a potentiostat Autolab PGSTAT100. Experiments were performed
at room temperature in an homemade airtight three-electrode cell
connected to a vacuum/argon line. The reference electrode consisted
of a SCE separated from the solution by a bridge compartment.
The counter electrode was a platinum wire of ca. 1 cm2 apparent
surface. The working electrode was a platinum microdisk (0.5 mm
diameter). The supporting electrolyte [n-Bu4N][BF4] (Fluka, 99%
electrochemical grade) was used as received and simply degassed
under argon. Dichloromethane was freshly distilled over CaH2 prior
to use. The solutions used during the electrochemical studies were
typically 10-3 M in compound and 0.1 M in supporting electrolyte.
Before each measurement, the solutions were degassed by bubbling
argon and the working electrode was polished with a polishing
machine (Presi P230). Under these experimental conditions, Fc+/
Fc is observed at +0.54 ( 0.01 V vs SCE.
Photophysics. Spectroscopic investigations were carried out in
CH2Cl2 (Carlo Erba, spectrofluorimetric grade). Absorption spectra
were recorded with a Perkin-Elmer λ40 spectrophotometer. Emis-
sion spectra were obtained with an Edinburgh FLS920 spectrometer
(continuous 450 W xenon lamp), equipped with a Peltier-cooled
Hamamatsu R928 photomultiplier tube (185-850 nm) or a
Hamamatsu R5509-72 supercooled photomultiplier tube (193 K,
800-1700 nm range). Emission quantum yields were determined
according to the approach described by Demas and Crosby61 using
[Ru(bpy)3Cl2] (Φem ) 0.028 in an air-equilibrated water solution)62
as the standard. Emission lifetimes (10-10-10-5 s time scale) were
determined with the time-correlated single-photon-counting tech-
nique using (a) an Edinburgh FLS920 spectrometer equipped with
a laser diode head as the excitation source (1 MHz repetition rate,
λexc ) 407 nm, 200 ps time resolution upon deconvolution) and a
Hamamatsu R928 PMT as the detector (setup details are reported
elsewhere)7 and (b) an IBH spectrometer equipped with a nanoLed
as the excitation source (50 kHz repetition rate, λexc ) 331 or 560
nm, 200 ps time resolution upon deconvolution) and an Horiba
TBX05 PMT as the detector; analysis of the luminescence decay
profiles against time was accomplished by using the DAS6 software
provided by the manufacturer. Transient absorption spectra and
lifetimes were detected upon excitation at 532 nm (Nd:YAG laser);
details on the apparatus have been already reported.44 Experimental
uncertainties are estimated to be 8% for lifetime determinations,
20% for emission quantum yields, 10% for relative emission
intensities in the NIR, and 1 and 5 nm for absorption and emission
peaks, respectively.
Compound CuPOP-F. Cu(CH3CN)4BF4 (20 mg, 0.07 mmol)
was added to a stirred solution of POP (40 mg, 0.07 mmol) in
CH2Cl2 (10 mL). After 1 h, a solution of 5 (110 mg, 0.07 mmol)
in CH2Cl2 (5 mL) was added. The resulting mixture was stirred
for 2 h and evaporated. Gel permeation chromatography (Biorad,
Biobeads SX-1, CH2Cl2) yielded CuPOP-F (113 mg, 71%) as a
black glassy product. IR (KBr): 1746 (CdO) cm-1. 1H NMR (300
MHz, CDCl3): δ 0.87 (t, J ) 6 Hz, 6 H), 1.06 (m, 2 H), 1.24 (s, 36
H), 1.37 (m, 2 H), 1.72 (m, 4 H), 2.46 (s, 3 H), 2.86 (t, J ) 6 Hz,
2 H), 3.88 (m, 6 H), 4.94 (s, 2 H), 5.48 (s, 2 H), 6.39 (t, J ) 2 Hz,
1 H), 6.60 (d, J ) 2 Hz, 2 H), 6.97 (m, 18 H), 7.20 (m, 8 H), 7.33
(m, 2 H), 7.62 (d, J ) 8 Hz, 1 H), 7.72 (d, J ) 8 Hz, 1 H), 7.87
(AB, J ) 8 Hz, 2 H), 8.42 (d, J ) 8 Hz, 1 H), 8.43 (d, J ) 8 Hz,
1 H). 13C{1H}{31P} NMR (75 MHz, CDCl3): δ 14.1, 22.7, 24.5,
26.1, 27.3, 28.2, 29.3, 29.4, 29.5, 29.64, 29.65, 29.7, 31.9, 51.4,
62.6, 65.0, 68.2, 69.2, 71.3, 101.7, 107.5, 120.3, 123.5, 125.4, 125.8,
126.1, 126.4, 127.8, 128.2, 128.6, 128.65, 129.9, 130.1, 131.6,
132.3, 132.6, 133.1, 133.7, 136.6, 138.2, 138.4, 139.8, 140.9, 141.7,
141.9, 142.1, 142.2, 142.9, 142.95, 143.0, 143.8, 144.5, 144.6,
144.9, 145.2, 145.25, 158.2, 159.0, 160.5, 161.8, 163.0, 163.1,
166.4. FAB-MS: 2189.7 ([M - BF4]; calcd for C149H102N2O9P2Cu,
2189.95). Anal. Calcd for C149H102N2O9P2CuBF4: C, 78.60; H, 4.52;
N, 1.23. Found: C, 78.32; H, 4.84; N, 1.19.
Compound CuFc-F. Cu(CH3CN)4BF4 (40 mg, 0.13 mmol) was
added to a stirred solution of dppFc (70 mg, 0.13 mmol) in CH2Cl2
(10 mL). After 1 h, a solution of 5 (200 mg, 0.13 mmol) in CH2Cl2
(10 mL) was added. The resulting mixture was stirred for 2 h and
evaporated. Gel permeation chromatography (Biorad, Biobeads SX-
1, CH2Cl2) yielded CuFc-F (244 mg, 82%) as a black glassy
product. IR (KBr): 1747 (CdO) cm-1. 1H NMR (300 MHz, CDCl3):
δ 0.87 (t, J ) 6 Hz, 6 H), 1.25 (s, 36 H), 1.40 (m, 4 H), 1.73 (m,
4 H), 2.27 (s, 3 H), 2.68 (m, 2 H), 3.88 (t, J ) 6 Hz, 4 H), 4.01 (t,
J ) 6 Hz, 2 H), 4.65 (s large, 4 H), 4.77 (s large, 4 H), 4.97 (s, 2
H), 5.49 (s, 2 H), 6.40 (t, J ) 2 Hz, 1 H), 6.60 (d, J ) 2 Hz, 2 H),
7.11 (m, 16), 7.31 (m, 4 H), 7.58 (d, J ) 8 Hz, 1 H), 7.65 (d, J )
8 Hz, 1 H), 8.09 (AB, J ) 8 Hz, 2 H), 8.55 (d, J ) 8 Hz, 1 H),
8.58 (d, J ) 8 Hz, 1 H). 13C{1H}{31P} NMR (75 MHz, CDCl3): δ
14.1, 22.7, 23.85, 23.9, 26.1, 27.5, 28.3, 29.3, 29.4, 29.5, 29.6, 29.7,
31.9, 40.4, 62.7, 64.9, 68.2, 69.3, 71.3, 72.9, 74.2, 75.0, 101.7,
107.5, 123.5, 125.8, 126.7, 126.9, 128.3, 128.6, 128.8, 130.1, 132.1,
133.8, 136.6, 138.5, 138.7, 138.9, 139.8, 140.9, 141.7, 141.9, 142.1,
142.2, 142.7, 142.75, 142.9, 142.95, 143.0, 143.05, 143.8, 144.5,
144.55, 144.6, 144.7, 144.9, 145.2, 159.4, 160.5, 162.1, 163.0,
IC800315E
163.1, 166.4. FAB-MS: 2205.6 ([M - BF4]; calcd for C147H102
-
N2O8P2CuFe, 2205.7). Anal. Calcd for C147H102N2O8P2CuFeBF4:
C, 77.01; H, 4.48; N, 1.22. Found: C, 76.92; H, 4.60; N, 1.10.
(61) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(62) Nakamaru, K. Bull. Soc. Chem. Jpn. 1982, 55, 2697.
Inorganic Chemistry, Vol. 47, No. 14, 2008 6261