Versatile bisethynyl[60]fulleropyrrolidine scaffolds for mimicking artificial light-harvesting photoreaction centers

Article abstract of DOI:10.1002/chem.201404372

Fullerene-based tetrads, triads, and dyads are presented in which [60]fulleropyrrolidine synthons are linked to an oligo(p-phenyleneethynylene) antenna at the nitrogen atom and to electron-donor phenothiazine (PTZ) and/or ferrocene (Fc) moieties at the ?±

Full text of DOI:10.1002/chem.201404372

DOI: 10.1002/chem.201404372
Full Paper
&
Fullerenes
Versatile Bisethynyl[60]fulleropyrrolidine Scaffolds for Mimicking
Artificial Light-Harvesting Photoreaction Centers
Adrian Kremer,[a] Emerance Bietlot,[a] Alberto Zanelli,[b] Joanna M. Malicka,[b]
Nicola Armaroli,*[b] and Davide Bonifazi*[a, c]
Abstract: Fullerene-based tetrads, triads, and dyads are
presented in which [60]fulleropyrrolidine synthons are linked
to an oligo(p-phenyleneethynylene) antenna at the nitrogen
atom and to electron-donor phenothiazine (PTZ) and/or fer-
rocene (Fc) moieties at the a carbon of the pyrrolidine cycle
through an acetylene spacer. Cyclic voltammetry and UV/ Vis
absorption spectra evidence negligible ground-state
electronic interactions among the subunits. By contrast,
strong excited-state interactions are detected upon selective
light irradiation of the antenna (UV) or of the fullerene
scaffold (Vis). When only PTZ is present as electron donor,
photoinduced electron transfer to the fullerene unit is unam-
biguously detected in benzonitrile, but this is not the case
when Fc is part of the multicomponent system. These results
suggest that Fc is a formidable energy transfer quencher
and caution should be used in choosing it as electron donor
to promote efficient charge separation in multicomponent
arrays.
Introduction
because of the exceptional electron-accepting properties, the
low-lying level of their excited states, their small reorganization
energy, and their remarkable stability under light irradiation.[6,7]
In particular, their use in donor–acceptor systems for photovol-
taic devices has impressively expanded.[8,9] To this end, a variety
of [60]fullerene derivatives linked to electron-donating units
such as, among others, porphyrins,[10–14] tetrathiafulvalenes
(TTFs),[15] ferrocene,[16] phthalocyanines,[17] oligophenylene-
vinylenes,[18] and transition-metal complexes[19–21] have been
prepared. The mechanisms of photoinduced electron and
energy transfer have been elucidated by means of time-
resolved spectroscopic methods, thus showing a strong intra-
molecular quenching of the excited singlet and triplet states of
C60 through the formation of charge-separated (CS) radical ion
pairs.
In natural photosynthesis, sunlight is converted into chemical
energy by a complex sequence of reactions initiated by a
cascade of energy- and electron-transfer processes.[1,2] Initially,
sunlight is absorbed by light-harvesting pigments, that is, the
antenna moieties, which funnel the excitation energy to the
so-called reaction center through a sequence of energy-trans-
fer steps.[3] At the reaction center, the energy is utilized to trig-
ger an electron-transfer reaction that constitutes the primary
step of energy harvesting.[3] Understanding these fundamental
processes has prompted chemists to engineer artificial
chemical assemblies that mimic them and might provide the
basis for artificial solar-energy conversion devices.[4,5]
In this context, C60 and its derivatives are optimal
components to be incorporated into artificial reaction centers
One of the main challenges in this field of research is the ra-
tional assembly of different molecular components in a spatially
organized fashion to enable stepwise light harvesting and con-
version of light into chemical energy.[22] In this context, we
have now designed and synthesized a novel fullerene scaffold
that features geometrical constraints that afford a hierarchical
and spatial organization of the chromophores (Figure 1). In
particular, by exploiting the 1,3-dipolar cycloaddition on C60,[23]
we have prepared tetrads that bear a chromophoric antenna
(an oligo(p-phenyleneethynylene) derivative, OPE) directly
attached the nitrogen atom of [60]fulleropyrrolidine and two
photoactive electron-donor moieties (i.e., ferrocene (Fc) and
phenothiazine (PTZ)) rigidly attached to the a carbon of the
same pyrrolidine ring (Figure 1) through an acetylene spacer. A
full account of the electrochemical and photophysical proper-
ties of conjugates 1–4 along with those of the corresponding
reference compounds 7–10 (Figure 2) is reported, which shows
the occurrence of a charge-separated state that lives for 250 ns
[a] Dr. A. Kremer, Dr. E. Bietlot, Prof. Dr. D. Bonifazi
Namur Research College (NARC) and Department of Chemistry
University of Namur (UNamur)
Rue de Bruxelles 61, Namur, 5000 (Belgium)
E-mail: davide.bonifazi@unamur.be
[b] Dr. A. Zanelli, Dr. J. M. Malicka, Dr. N. Armaroli
Molecular Photoscience Group
Istituto per la Sintesi Organica e la Fotoreattivitꢀ (ISOF)
Consiglio Nazionale delle Ricerche (CNR)
Via P. Gobetti 101, 40129, Bologna (Italy)
E-mail: nicola.armaroli@isof.cnr.it
[c] Prof. Dr. D. Bonifazi
Department of Pharmaceutical and Chemical Sciences and
INSTM UdR Trieste, University of Trieste
Piazzale Europa 1, Trieste, 34127 (Italy)
Supporting information for this article, including experimental details of
the syntheses and characterization, electrochemical measurements, and
photophysical studies, is available on the WWW under http://dx.doi.org/
10.1002/chem.201404372.
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could be the precursors of choice to form tetrads. Neverthe-
less, only a limited number of reports describe the 1,3-dipolar
cycloaddition of azomethine ylides using ketones,[37–47] and in
most cases with modest yields (<40%). This has prompted us
to further investigate the feasibility of the 1,3-dipolar cyclo-
addition of azomethine ylides generated from the thermal con-
densation of a-amino acids and various ketones.
1,3-Dipolar cycloaddition of azomethine ylides generated in
situ using ketones
Effects of the hydrocarbon substituents: As a starting point
we studied the 1,3-dipolar cycloaddition of azomethine ylides
generated by the condensation reaction of three different sym-
metric ketones (Table 1): an aliphatic (11),[43] an aromatic (12),
and an ethynyl silylated derivative (13 and 14)[48,49], with
sarcosine (15) or other (16 and 17) amino acids under reflux
conditions in PhMe or o-dichlorobenzene (ODCB).
Figure 1. Chromophore hierarchical organization of the fullerene-based pho-
toreaction centers proposed in this work (eT =energy transfer, eꢀT =electron
transfer, EDG1/2 represents the electron donor groups).
for molecule 4. Notably, for the derivatives that contain the fer-
rocenyl moiety, the spectroscopic fingerprints of the electronic
excited states of the individual components are completely
quenched in both toluene (PhMe) and in benzonitrile (PhCN),
thus suggesting ultrafast quenching by energy transfer to the
Fc units.
By using undecan-6-one (11), the corresponding [60]fullero-
pyrrolidine 18a was obtained in modest yields (Table 1,
entry a) after 16 h under reflux conditions in ODCB. Surprising-
ly, when using benzophenone (12), no conversion of the initial
ketone was observed (Table 1, entry b). However, when using
Results and Discussion
Synthesis
Among the synthetic methods to functionalize
C60,[24–27] the 1,3-dipolar cycloaddition of azomethine
ylides[23,28,29] to yield [60]fulleropyrrolidines that are
formed across the 6–6 junction of the fullerene core
is certainly one of the most attractive route for pre-
paring covalent conjugates for materials science. This
because [60]fulleropyrrolidines are known to be very
stable under both oxidative and reductive condi-
tions, and the retrocycloaddition reaction[30,31] is es-
sentially observed under harsh conditions (heating
to reflux in o-dichlorobenzene (ODCB) in the pres-
ence of a strong dipolarophile and a Lewis acid such
as Cu(OTf)2). Although azomethine ylides are unsta-
ble and must be prepared in situ, several methods
have been developed for their synthesis, such as
proton abstraction from imine derivatives of a-amino
acids, thermolysis or photolysis of aziridines, and de-
hydrohalogenation and decarboxylation of immoni-
um salts derived from thermal condensation of a-
amino acids and aldehydes or ketones. In particular,
depending on the chemical nature of the carbonyl
(aldehyde or ketone) and amino acid derivatives, dif-
ferent functional groups can be introduced on the
three different positions of the pyrrolidine ring, and
a large variety of [60]fullerene dyads and triads can
be synthesized a priori.[7,11,17,32–36] By taking advant-
age of the versatile 1,3-dipolar cycloaddition of azo-
methine ylides to C60,[23] we found that ketones
Figure 2. Chemical structures of the targeted [60]fulleropyrrolidine-based chromophores
(Hex=ꢀC6H13).
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tion, the desired ketone (21;
Scheme 1).
Table 1. 3-Dipolar cycloaddition of azomethine ylides to C60 generated in situ through thermal decarboxylation
of immonium salts derived from the condensation of ketones 11–14 and amino acids 15–17.
Whereas from ketone 19 and
sarcosine 15, the corresponding
addition product 18 f was ob-
tained with good yield (48%)
after 48 h under reflux condi-
tions in PhMe (Table 2, entry a),
no product formation was de-
tected when the reaction was
performed in ODCB (Table 2,
entry b) despite the full conver-
sion of the ketone. Notably,
when electron-rich ketone 20
was employed, no conversion
of the latter was observed in
PhMe (Table 2, entry c), whereas
Entry C60 (x equiv) Ketone (R)
y equiv. Amino acid (R’)
z equiv Solvent Product [%]
a
b
c
1
1
1.8
1.8
11 (nPent)
12 (Ph)
5
5
1
1
15 (Me)
1
1
5
5
ODCB
ODCB
PhMe
ODCB
18a (21)
– (0)
18b (61)
18c (72)
15 (Me)
15 (Me)
15 (Me)
13 (ꢀꢁꢀTMS)
d
14 (ꢀꢁꢀTIPS)
e
f
1.8
1.5
14 (ꢀꢁꢀTIPS)
14 (ꢀꢁꢀTIPS)
1.5
1
1
3
PhMe
PhMe
18d (51)
18e (92)
[60]pyrrolidinofullerene
18g
was isolated with modest yield
when ODCB was used as a sol-
vent (Table 2, entry d). Finally,
when using electron-deficient
tetramethylsilane (TMS)- or triisopropylsilyl (TIPS)-protected
ketones 13 and 14, the corresponding [60]fulleropyrrolidines,
18b and 18c, were obtained with high yields (61 and 72%,
respectively; Table 1, entries c and d). Notably, under reflux
conditions in ODCB, TMS-protected ketone 13 led to lower
yields (42%), probably due to thermal decomposition. Interest-
ingly, when sarcosine was replaced by more soluble amino
acids (i.e., derivatives 16 and 17), an unprecedentedly high
yield of 92% was obtained with tert-butyloxycarbonyl
(Boc)-protected amino acid 17 (Table 1).
ketone 21, the corresponding addition product 18h was ob-
tained with good yields (59%) after 48 h (Table 2, entry e).
Again, no product was observed under reflux conditions in
ODCB (Table 2, entry f).
Synthesis of the chromophoric dyad, triads, and tetrads
The synthesis of the dyad (2), triads (3–5), and tetrads (1 and
6) shown in Figure 2 are reported in Scheme 2 and in the
Supporting Information. The same synthetic strategy as that
described in the previous sections has been used for all
chromophores, therefore in this section we describe only the
preparative route for tetrad 1. The reader can find all the
detailed experimental conditions in the Supporting Informa-
tion (all known intermediates have been synthesized following
Electronic effects: We then investigated the formation and
reactivity of azomethine ylides using ketones that bore
different electron-donating (ED) or electron-withdrawing (EW)
groups (19–21; Scheme 1 and Table 2). 1,5-Bis(phenyl)penta-
1,4-diyn-3-one (19) and 1,5-bis[p-
N,N-di(n-hexyl)anilino]penta-1,4-
diyn-3-one (20) were prepared
according to the reported proce-
dures,[50,51] whereas 1,5-bistri-
fluormethylpenta-1,4-diyn-3-one
(21) was synthesized by follow-
ing the route displayed in
Scheme 2. Starting from com-
mercially available prop-2-yn-1-
ol, Sonogashira-type cross-cou-
pling with 1-bromo-4-(trifluoro-
methyl)benzene (22) followed by
oxidation of alcohol intermedi-
ate 25 with BaMnO4 gave 3-[4-
(trifluoromethyl)phenyl]prop-2-
ynal (26) in very good yields
(Scheme 1). Addition of Li-ethy-
nylide analogue of derivative 23
Scheme 1. Synthesis of CF3-terminating ketone 21. Ketones 19–20 were prepared by following the protocols de-
to aldehyde 26 gave, after oxida- scribed in the literature.[50, 51]
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Scheme 2. Synthetic route toward tetrad 1 (Hex=ꢀC6H13). For the other chromophoric systems, see the Supporting Information.
the literature protocols).[23,48–58] By exploiting the ketone strat-
to C60 of the azomethine ylide generated by the condensation
egy, molecule 1 could be obtained by 1,3-dipolar cycloaddition
of OPE-bearing amino acid 37 with ketone 42 (Scheme 2).
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Electrochemical studies
Table 2. 3-Dipolar cycloaddition on C60 using ketones 19–21.
Cyclic voltammetry: The voltammograms of reference molecu-
lar subunits 7–10 and of the multichromophoric systems 1–4
in CH2Cl2 with 0.1 molLꢀ1 nBu4NClO4 (TBAP) are depicted in
Figure 3. Within the electrochemical stability window of the
solvent, fulleropyrrolidine reference 7 exhibits three reversible
reduction waves at ꢀ0.62, ꢀ1.02, and ꢀ1.53 V, which are in
good agreement with the literature data.[62,63] Notably, the re-
duction behavior of the fullerene moiety is virtually unaffected
over the whole series of multicomponent systems (Table 3).
Molecular reference compounds 8, 9, and 10 do not exhibit
any detectable reduction processes. However, reference ferro-
cene 9 and phenothiazine 10 show a quasi-reversible oxidation
wave at 0.63 and 0.78 V, respectively. These potentials are little
or not affected in the two triads but get closer in the tetrad, in
which the wave of ferrocene 9 slightly shifts anodically (0.66 V)
and overlaps that of phenothiazine 10, which in turn moves to
less positive potentials (0.72 V). This behavior likely reflects
a non-negligible electronic interaction between the two gemi-
nal electron donors linked at the 2-position of the pyrrolidine
ring and therefore in close proximity.
Entry
Ketone
Solvent
PhMe
t [h]
48
Product [%]
a
b
c
d
e
f
19
19
20
20
21
21
18 f (48)
18 f (0)
18g (0)
18g (21)
18h (59)
18h (0)
ODCB
PhMe
ODCB
PhMe
ODCB
48
48
16
48
48
The synthesis of amino acid 37 started with the regio-
selective iodination of 27 using ICl in MeOH to give 1-bromo-
4-iodoaryl derivative 28 in good yield. The latter was then
coupled with trimethylsilylacetylene (TMSA) under Sono-
gashira-type cross-coupling conditions to yield bromoaryl 29,
which was subsequently subjected to a second cross-coupling
reaction with 2-ethynylaryl 30 (prepared according to reported
procedures;[59–61] see the Supporting Information) to yield, after
deprotection of the TMS protecting group, bisphenylethynyl
derivative 32 in 63% over three steps. Molecule 32 was then
coupled with another equivalent of 1-bromo-4-iodoaryl deriva-
tive 28 to give bromo-terminating OPE derivative 33. A final
cross-coupling reaction between 33 and 4-pentyn-1-ol gave
alcohol 34, which was converted into aldehyde 35 under
Swern oxidation conditions. Reductive amination of the latter
gave, after saponification, targeted OPE-bearing amino acid 37.
The synthesis of ketone 42 started with the synthesis of
iodophenyl-phenothiazine 38, which was prepared by
following the protocols reported in the literature,[54,55] except
for the Pd-catalyzed cross-coupling reaction, which was per-
formed under microwave conditions (see the Supporting Infor-
mation). Molecule 38 was coupled to TMSA (39) and converted
into aldehyde 40 by successive deprotection (10) and lithiation
exchange reaction with nBuLi followed by addition of DMF.
Subsequent addition of Li-ethynylferrocene to a solution of
aldehyde 40 gave alcohol 41, which, after oxidation with
BaMnO4, was transformed into targeted ketone 42 with an
overall yield of 76% over two steps.
Reference OPE 8 exhibits an irreversible oxidation process
with two partially overlapping waves; the half-wave potential
related to the first shoulder is at 1.17 V (E1/2, Table 3). The oxi-
dation behavior of fullerene derivatives 2, 3, and 4 relative to
fulleropyrrolidine reference 8 is significantly different, with
a new irreversible peak appearing at about 1.07 V (marked
with an asterisk in Figure 3). The related process is likely the
In situ condensation of an equimolar mixture of a-amino
acid 37 and ketone 42 followed by thermal decarboxylation in
the presence of two equivalents of C60 in PhMe under reflux
conditions for five days gave, after purification, the expected
tetrad 1 in a yield of 21% (Scheme 2).
Figure 3. Voltammograms of reference molecular subunits 7–10 and of
multicomponent systems 1–4 obtained in CH2Cl2 with 0.1 molLꢀ1 nBu4NClO4
(TBAP) as supporting electrolyte. Scan rate: 0.2 Vsꢀ1
.
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Photophysical studies
Table 3. Standard redox potentials in CH2Cl2 with 0.1 molLꢀ1 TBAP versus SCE
(ꢀ0.47 V versus Fc/Fc+). Working electrode: Pt, counter electrode: Pt. Scan rate:
0.1 Vsꢀ1. E8=EpaꢀDEpp/2, in which Epc and Epa are the cathodic and anodic peak
potentials, respectively; DEpp =EpaꢀEpc. For the irreversible process, the half-wave
potential has been reported as the best approximation of E8.
Ground-state absorption and luminescence
properties: The absorption spectra of the reference
molecules N-methylfulleropyrrolidine (7), OPE anten-
na module 8, and of the multicomponent systems 2
and 4 in toluene (PhMe) solutions are depicted in
Figure 5. Owing to the negligible absorption of the
ferrocene moiety (see below), the spectra of refer-
ence dyad 1 and triad 3 are virtually identical to
those of fulleropyrrolidine derivatives 2 and 4, re-
spectively, and are not reported in Figure 5. The ab-
sorption spectra of the reference electron-donor
modules 10 and 9 in PhMe are characterized by
moderate to low extinction coefficients, which is in
line with the literature data[66] (Figure S1 in the Sup-
Eored3 [V]
Eored2 [V]
Eored1 [V]
Eoox1 [V]
Eoox1 [V]
E1/2(ox3) [V]
9
10
8
7
2
3
4
1
–
–
–
–
–
–
–
–
–
0.63
–
–
–
–
0.61
–
0.66
–
0.78
–
–
–
–
0.78
0.72
–
–
1.17(irr)
–
1.02
1.06
1.06
1.15
ꢀ1.53
ꢀ1.55
ꢀ1.52
ꢀ1.53
ꢀ1.53
ꢀ1.02
ꢀ1.01
ꢀ1.00
ꢀ1.00
ꢀ1.01
ꢀ0.62
ꢀ0.62
ꢀ0.62
ꢀ0.62
ꢀ0.63
result of an interaction between the OPE antenna and the full-
erene moiety. Notably, the peak at +1.07 V is not detected in
the tetrad, in which the two previously accumulated charges
could anodically shift the related process toward potentials
close to that of the reference OPE module 8.
Spectroelectrochemistry of 10: A 0.5 mmolLꢀ1 solution of
reference phenothiazine 10 in PhCN with 0.1 molLꢀ1 TBAP
exhibits an electronic absorption spectrum with maximum at
314 nm (Figure 4, black dashed line). By increasing the poten-
Figure 5. Absorption spectra in PhMe solution: 7 (black), 8 (gray), 2 (dashed
line), and 4 (dotted line). The inset shows the enlarged spectra in the region
from 430 to 700 nm.
porting Information). Molecule 10 weakly absorbs in the UV
spectral window with
a maximum at 325 nm (e=6ꢁ
103 mꢀ1 cmꢀ1). In agreement with the literature reports, ferro-
cene 9 exhibits a faint band at lmax =450 nm (e=102 mꢀ1 cmꢀ1;
Figure S1 in the Supporting Information), which arises from
spin-allowed d–d electronic transitions.[66] The reference anten-
na unit 8 exhibits a strong absorption band with a maximum
at 385 nm (e=4.6ꢁ104 mꢀ1 cmꢀ1), in line with similar OPEs.[67]
The substantial intensity difference of the absorption spectrum
of 8 with respect to electron donors 9 and 10 allows for the
selective excitation of the antenna moiety in the multicompo-
nent systems. Reference N-methylfulleropyrrolidine (7) exhibits
broad absorption across the UV and visible regions, which is
also in accordance with the literature reports.[7] The absorption
spectra of dyad 2, triads 3 and 4, as well as tetrad 1 are essen-
tially the sum of the spectra of their molecular fragments, thus
indicating negligible intramolecular interactions in the ground
state.
Figure 4. Spectroelectrochemistry of reference phenothiazine 10 in PhCN
with 0.1 molLꢀ1 TBAP as supporting electrolyte. The black profile is obtained
at open circuit voltage (OCV), whereas the other traces are the spectra re-
corded at increasing potentials under steady-state currents (Fc/Fc+ =0.08 V
versus Ag/Ag+). Inset: Voltammogram of 10 at 0.1 Vsꢀ1 on Pt grid in the
spectroelectrochemical cell. The potentials at which the spectra have been
recorded are marked on the oxidation wave.
tial up to the half-wave oxidation value (0.35 V versus Ag/Ag+;
Figure 4, inset) the absorption peak of 10 decreased by about
10% and two envelopes of absorption bands appeared in the
ranges 400–600 and 650–900 nm, which are attributable to the
PTZ radical cation. The extinction coefficient of this species at
the absorption maximum (520 nm) was estimated to be 1.7ꢁ
104 cmꢀ1 molꢀ1, assuming that the 9% absorption decrease
centered at 314 nm and 0.35 V leads to the formation of a cor-
responding amount of the radical cation of 10.[64] This value is
comparable to that reported for phenothiazine derivatives
such as promazine and chlorophenazine.[65]
Reference phenothiazine donor 10 is weakly luminescent
(lmax =450 nm, Fem =1% in PhMe; Figure S2 in the Supporting
Information). As expected, reference ferrocenyl 9 deactivates
nonradiatively from its lowest-lying triplet level and does not
exhibit any luminescence.[68] Reference OPE 8 shows a strong
fluorescence band with lmax =420 nm (Fem =64% in PhMe and
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42% in PhCN, with a t of 1.1 ns in both solvents), which is
attributable to the deactivation from the lowest-lying singlet
excited state (Figure S2 in the Supporting Information). Similar
quantum efficiencies and spectral features were reported earli-
er for analogous oligo(p-phenyleneethynylene) compounds.[67]
Fullerene derivative 7 exhibits a weak fluorescence band
typical of fulleropyrrolidines with lmax =710 nm, t=1.4 ns,
Fem ꢂ0.04% in PhMe and PhCN (Figure S2 in the Supporting
Information). These values are in accordance with the literature
data.[7]
The strong luminescence of the OPE fragment is dramatically
quenched when attached to the fullerene chromophore. In
PhMe, the fluorescence quantum yield of N-OPE fulleropyrroli-
dine 2 (lexc =385 nm) drops from 64 to 0.14%, whereas in
PhCN it drops from 42 to 0.49% (Table 4), thus indicating
Figure 6. Emission spectra of the fulleropyrrolidine moiety upon its selective
excitation at lex =500 nm in PhMe (left) and in PhCN (right) in 2 (black line),
4 (gray line), 3 (dotted line), and 1 (dashed line).
opyrrolidine moiety in dyad 2 upon excitation of the antenna
is not due to quenching by photoinduced electron transfer
but rather to a less efficient energy transfer in polar solvent,
which was also previously observed in similar cases.[70]
Table 4. Fluorescence quantum yields (F) and related excited-state life-
times (t).
OPE moiety
F [%][a] t [ns][b]
PhMe PhCN PhMe
C60 moiety
Photophysical properties of triad 4 in PhMe are similar to
those of dyad 2. Similar quenching of luminescence and life-
time of the antenna moiety is observed as well as slightly less
efficient sensitization of the fullerene fluorescence. For
derivative 4 in PhCN, upon excitation of the OPE moiety, the
sensitized emission from the singlet excited state of the fuller-
opyrrolidine unit is around 75% lower than that of reference
molecules 7 and 2. Likewise, direct excitation of the fulleropyr-
rolidine moiety results in approximately 60% decreased emis-
sion (Figure 6). This trend was confirmed by measurements of
the lifetime of the fulleropyrrolidine singlet excited state of 4,
which was revealed to be reduced to about 750 ps for both
direct (lexc =465 nm) and sensitized excitation (lexc =373 nm)
in PhCN, thus indicating the presence of another quenching
pathway, presumably electron transfer. In molecules 3 and
1 (Figure 6), the observed quenching of the fullerene singlet
emission is virtually complete (ꢃ95%), regardless of the sol-
vent and excitation wavelength. However, the steady-state and
time-resolved data on the quenching of the antenna moiety in
3 and 1 are essentially the same as molecules 2 and 4
(Table 4), which suggests the occurrence of an identical an-
tenna!fulleropyrrolidine energy-transfer process (see above).
In the absence of excited-state deactivation processes, the
lowest singlet level of the fullerene derivatives (1C60*) under-
goes extensive intersystem crossing to yield the corresponding
triplet (3C60*).[71,72] This species can be conveniently monitored
in an indirect way by recording the emission of sensitized
singlet oxygen in the near-infrared (NIR) region,[20,73] which
originates according to the following reaction scheme
[Eq. (1)]:[7]
F [%][c]
t [ns][b]
t [ns][d]
PhMe
PhCN
PhMe PhCN PhMe PhCN
8
7
2
4
3
1
64
–
43
–
1.1
–
–
–
–
–
–
–
0.04
0.04
0.03
0.04
0.03
0.01
1.4
1.4
1.5
1.3
1.4
0.7
1.4
1.4
1.6
1.4
1.4
0.8
0.14 0.49 ꢂ0.025
0.20 0.42 ꢂ0.025
[e]
[e]
[e]
[e]
0.17 0.49 ꢂ0.025 <0.01 <0.01
0.18 0.48 ꢂ0.025 <0.001 <0.001
–
–
–
–
–
–
–
–
[e]
[e]
[e]
[e]
[a] lexc =385 nm. [b] lexc =373 nm. [c] lexc =500 nm. [d] lexc =465 nm.
[e] Too weak to be detected.
strong excited-state intramolecular interactions between the
fullerene and OPE moieties. Although the quenched lifetime of
the antenna is at the limit of the resolution of our instrumenta-
tion, lifetime values of about 25 ps were determined in both
PhMe and PhCN upon deconvolution of the decay profile of
the laser excitation source, which set a lower limit for the rates
of the quenching processes at about 4ꢁ1010 sꢀ1
.
Upon selective excitation of the OPE fragment, efficient
sensitization of the N-methylfulleropyrrolidine was observed
for dyad 2 in both PhMe and PhCN (Figure 6), thus indicating
the expected antenna behavior. In PhMe, when excited at
lexc =385 nm, the luminescence quantum yield (0.04%) and
singlet lifetime (1.4 ns) of the fullerene core of 2 is exactly the
same as that of reference 7.[69] These same values were ob-
tained by the direct excitation of the fullerene moiety of 2 at
500 nm, thus confirming the quantitative energy transfer from
the OPE moiety to the carbon sphere (Table 4).
In PhCN, upon excitation of the antenna, the luminescence
of the fulleropyrrolidine moiety in 2 is about 40% lower
(Fem =0.03%) than (i) the reference 7 in the same solvent or
(ii) 2 in PhMe, which can indicate the presence of a quenching
process for the OPE unit in the more polar solvent. However,
the singlet lifetime value of the fulleropyrrolidine moiety was
determined to be 1.4 ns, regardless of the excitation wave-
length. This suggests that the lower Fem in PhCN of the fuller-
1
3
1
hvUV=Vis
isc
O2
*
*
*
C60
C60
C60
C60 þ O2
!
ƒƒƒ!
ƒ!
ƒ!
ð1Þ
C60 þ O2 þ heat þ hn1270 nm
These measurements are reported in Figure 7 and provide
a clear picture of the presence of the fullerene triplet in all of
Chem. Eur. J. 2014, 20, 1 – 11
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resolution of 30 ps. However, ferrocene is known to have no
detectable fingerprints to be exploited for tracing photoin-
duced electron transfer and this further hampers unequivocal
rationalization.[63] In principle, the strong quenching of the
fullerene excited states in all of the systems containing a ferro-
cenyl moiety can be attributed to the 1) ferrocenyl!fullero-
pyrrolidine electron transfer with ultrafast charge separation
and slower charge recombination,[75] or 2) fulleropyrrolidine!
ferrocenyl singlet to triplet Dexter energy transfer, as was de-
termined for porphyrin–ferrocenyl dyads, for instance.[76] In
PhMe, the latter mechanism is likely to be operative as recently
discussed for ferrocenyl–fullerene dyads[63] and in line with
bimolecular quenching studies that involve pristine C60 and
a number of ferrocenyl derivatives.[68]
Figure 7. Sensitized singlet O2 luminescence spectra in PhMe (left) and in
PhCN (right) upon selective excitation of the fullerene moiety at
lex =500 nm for 2 (black), 4 (gray line), 3 (dotted line), and 1 (dashed line).
The transient absorption spectrum of triad 4 in degassed
PhCN is radically different from those obtained for reference
molecules 10 and 7 (Figures S3 and S4 in the Supporting Infor-
mation), which exhibit maxima at 450 (t=4 ms) and 690 (t=
16.5 ms) nm, respectively. Triad 4 shows a characteristic feature
at 510 nm that can be attributed to the radical cation of the
ED phenothiazine moiety (Figure 8), in accord with the absorp-
tion profiles obtained through spectroelectrochemical meas-
urements for reference compound 10 (Figure 4). Moreover, in
the NIR region, the radical anion of the fullerene moiety was
detected at around 1000 nm,[77] thereby confirming the occur-
rence of photoinduced electron transfer. Notably, the kinetics
determined at 1000 nm are identical to that monitored at
3
our multicomponent arrays. Quantitative production of C60*
was determined only for dyad 2 in both solvents and for triad
4 in PhMe when compared to reference 7 under identical ex-
perimental conditions. These findings, along with the observed
quenching of the OPE antenna by fluorescence measurements
(see above), further confirm the light-harvesting character of
this moiety to funnel the excitation energy to the lowest
available excited state (3C60*), thus validating our molecular
design approach.
3
On the contrary, triad 4 exhibits virtually no traces of C60* in
PhCN. This suggests that, in addition to the OPE!fulleropyrro-
lidine energy transfer, the presence of a phenothiazine donor
unit in a polar medium promotes an additional excited-state
quenching process, presumably an electron transfer. This hy-
pothesis is supported by electrochemical data, from which it is
possible to place the energy of the charge-separated state that
corresponds to the oxidation of the phenothiazine moiety and
the reduction of the C60 moiety in 4 (Table 3) at 1.40 eV in
CH2Cl2.[74] This value sets an upper limit on the energy of the
charge-separated (CS) state, which is expected to be even
lower-lying in PhCN.[7] The energy of the CS level clearly shows
that an electron-transfer process is thermodynamically possible
upon excitation of both the higher-lying antenna and fullerene
singlet levels, placed at about 3.2 (estimated from the
maximum of the fluorescence band, 385 nm) and 1.72 eV,[7]
respectively.
Finally, all architectures that incorporate a ferrocenyl moiety
3
do not exhibit any measurable population of C60*, regardless
of the solvent polarity. This behavior might also in principle
indicate the occurrence of a photoinduced electron transfer,
since the ferrocenyl unit exhibits an even lower oxidation
potential than that of phenothiazine.
Transient absorption spectroscopy: To prove photoinduced
electron transfer through the observation of radical cationic
and anionic species, transient absorption spectra were record-
ed in O2-free PhCN. Formation of charge-separated states for
the systems bearing the ferrocenyl unit (3, 1) in both solvents
was never evidenced. No fullerene radical anion bands in the
NIR spectral region (900–1600 nm) were found down to a time
Figure 8. Top: Transient absorption spectrum of 4 in O2-free PhCN. Bottom:
Absorbance decays of the radical cation (520 nm) and radical (at 1000) anion
species; lexc =355 nm, 1 mJ per pulse.
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510 nm within the experimental error. Therefore, triad 4, which
entails an antenna and a charge-separation module, undergoes
photoinduced electron transfer with a charge-separated state
that lives (250ꢄ10) ns.
FNRS (FRFC contract no. 2.4.550.09). A.K. thanks the Fund for
Scientific Research (FNRS) for his postdoctoral position. We
also thank the “TINTIN” ARC project (09/14-023), the Science
Policy Office of the Belgian Federal Government (BELSPO-IAP
7/05 project), the Italian Ministry of Research (FIRB Futuro in
Ricerca SUPRACARBON, contract no. RBFR10DAK6; PRIN 2010
Conclusion
INFOCHEM
- contract no. CX2TLM), and the Consiglio
Nazionale delle Ricerche (SOLARFUELTANDEM project of the
10-EuroSolarFuels-FP-006 EUROCORES Programme of the Euro-
pean Science Foundation; Progetto Bandiera N-CHEM). We
thank Dr. Filippo Monti for the elaboration of the photophysi-
cal data and the preparation of the related figures.
We have designed and synthesized a family of modular multi-
component systems (one dyad, one tetrad, two triads) that
entail a fulleropyrrolidine scaffold to which were connected
1) at the nitrogen atom an OPE fragment that absorbs in the
UV/Vis region up to 450 nm, and 2) at the a-carbon atom of
the pyrrolidine cycle through acetylene spacers a phenothiazine
and/or ferrocene electron donors that exhibit weak absorption
relative to the other subunits. The syntheses of the triads and
tetrads were accomplished by means of 1,3-dipolar
cycloaddition of azomethine ylides generated in situ by the
condensation reaction of an OPE-bearing amino acid and 1,5-
disubstituted penta-1,4-diyn-3-one ketones. Yields as high as
90% were achieved when TMS- or TIPS-protected ketones
were employed.
Keywords: energy transfer · fullerenes · light harvesting ·
spectroelectrochemistry · synthetic methods
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Acknowledgements
This work was supported by the European Commission
(MC-RTN “PRAIRIES” MRTN-CT-2006-035810 and MC-ITN
“FINELUMEN” PITN-GA-2008-215399), the European Union
through the ERC starting grant “COLORLANDS”, and the FRS-
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Published online on && &&, 0000
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FULL PAPER
A fuller(ene) picture: Fullerene-based
arrays are reported in which [60]fullero-
pyrrolidine synthons are linked to an
oligo(p-phenyleneethynylene) antenna
and to phenothiazine (PTZ) and/or fer-
rocene (Fc) electron donors. When only
PTZ is present photoinduced electron
transfer occurs, whereas energy transfer
is observed whenever Fc is inserted into
the multicomponent array. (see figure;
eT =energy transfer, eꢀT =electron
transfer, EDG1/2 represents the electron
donor groups).
&
Fullerenes
A. Kremer, E. Bietlot, A. Zanelli,
J. M. Malicka, N. Armaroli,* D. Bonifazi*
&& – &&
Versatile
Bisethynyl[60]fulleropyrrolidine
Scaffolds for Mimicking Artificial
Light-Harvesting Photoreaction
Centers
Chem. Eur. J. 2014, 20, 1 – 11
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