Trapping fullerenes with jellyfish-like subphthalocyanines

Article abstract of DOI:10.1039/c3sc21956a

Six electronically different concave-shaped subphthalocyanines (SubPcs) have been prepared for testing the structural factors governing fullerenes encapsulation. Thus, the supramolecular interaction of SubPcs with C ₆₀ and C₇₀ fuller

Full text of DOI:10.1039/c3sc21956a

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Trapping fullerenes with jellyfish-like
subphthalocyanines†
Cite this: Chem. Sci., 2013, 4, 1338
a
a
b
b
ac
*
*
*
´
I. Sanchez-Molina, C. G. Claessens, B. Grimm, D. M. Guldi and T. Torres
Six electronically different concave-shaped subphthalocyanines (SubPcs) have been prepared for testing
the structural factors governing fullerenes encapsulation. Thus, the supramolecular interaction of SubPcs
with C60 and C70 fullerenes in solution has been studied by Job's plot and titration experiments, which
yielded quantitative information on both the stoichiometry and strength of the complexes in toluene
solution. The importance of the electronic nature of the SubPc was demonstrated, as it influences not
only the stability of the complex, but also its stoichiometry. Alkyl chains incorporated in hexaalkylthio-
substituted SubPcs seem to in some way cooperate in the binding process and influence its kinetics. In
the resulting 2 : 1 complexes, the large absorption cross section of SubPcs throughout the visible part of
the spectrum is the beginning of an unidirectional energy transfer to funnel the excited state energy
from the periphery (i.e., two SubPcs) to the core (i.e., C60 and C70).
Received 10th November 2012
Accepted 14th January 2013
DOI: 10.1039/c3sc21956a
www.rsc.org/chemicalscience
chromophores, including porphyrins, etc., have been shown to
interact favourably with fullerenes. Moreover, remarkable
Introduction
Fullerenes and fullerene derivatives are being widely used as
active components in optoelectronic devices such as photovol-
taic cells due to their outstanding photophysical and electro-
chemical properties.1 Much effort has been devoted to the
understanding of the interactions between fullerenes and other
chromophores, which coexist within these devices in order to
improve their performances.2 One possible approach towards a
better understanding of the morphology of fullerene-based
devices is to study their interactions with supramolecular
receptors which, moreover, are also chromophoric species. The
design of new receptors for fullerenes is based on three prin-
ciples: (i) shape complementarity or optimization of the inter-
acting surface, (ii) electrostatic complementarity with the
electron decient fullerene, and (iii) entropic factors such as the
hydrophobic effect.3 Past experiments have documented that
maximization of p–p stacking interactions is a desirable feature
in the design of fullerene receptors, especially to ne-tune
recognition. In principle, at aromatic surfaces might be
unsuitable as receptors for convex fullerenes. Still, many planar
results have been realized in the area of porphyrin-based
receptors,3g owing to the preorganization of the host. The
strategy of preorganization is very powerful, especially in the
context of enhancing fullerene encapsulation. A convenient
alternative to the aforementioned is the use of aromatic recep-
tors, like, for example, SubPcs, which feature adequate and
complementary shapes to fullerenes.
Subphthalocyanines (SubPcs) are 14 p-electron macrocycles
composed of three 1,3-diiminoisoindole units N-fused around a
central boron atom (Fig. 1). Their synthetic versatility and their
attractive physical properties have given rise to interesting
applications in the elds of nonlinear optics and organic
photovoltaics.4 Their C3 symmetric bowl-shaped structure is
apparently perfectly adapted, at least from a geometric point of
view, to the convex surface of fullerenes. Formation of a 1 : 1
complex of a SubPc with C60 fullerene in solution has been
previously mentioned by Ziessel et al.5 even if no quantitative
data were reported. In the solid state, Kobayashi et al.6 described
the 1 : 1 complex between fullerene and a p-extended sub-
phthalocyanine. Incorporation of C60 fullerene within a homo-
dimeric metallosupramolecular subphthalocyanine capsule was
also demonstrated. Nevertheless, in this case, no quantitative
data were reported since C60 was extracted from acetone, a
solvent in which it is barely soluble.7 Interestingly, there are also
many reports dealing with the photoinduced electron and energy
transfer between C60 and SubPc units within covalent dyads.
These studies have shown that the communication between the
components is highly dependent on the distance, which sepa-
rates them and the nature of the peripheral substituents on the
SubPc moiety.8 However, and to the extent of our knowledge, this
aDepartamento de Quımica Organica (C-1) Universidad Autonoma de Madrid,
´
´
´
Cantoblanco, 28049 Madrid, Spain. E-mail: tomas.torres@uam.es; Fax: +34 913
973 966; Tel: +34 914 0974 151
b
¨
¨
Friedrich-Alexander-Universitat Erlangen-Nurnberg, Institute for Physical Chemistry,
Egerlandstr. 3, D-91058 Erlangen, Germany. Fax: +49 9131-852-8307
cIMDEA-Nanociencia, Ciudad Universitaria de Cantoblanco, c/ Faraday, 9, 28049
Madrid, Spain
† Electronic supplementary information (ESI) available: Experimental procedures
and spectroscopic data. NMR, MS and IR spectra for all novel compounds.
Fluorescence spectra, steady state and transient absorption spectra. See DOI:
10.1039/c3sc21956a
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is the rst systematic and quantitative study of supramolecular
interaction of SubPcs with fullerenes.
Another motivation to address a good understanding of the
interaction of these molecules, was the thought that it might
help in the elucidation of their interaction at the interface
between two layers, which is also crucial for device optimiza-
tion. Evaporated organic solar cells in which the two active
layers were subphthalocyanine and C60 were shown to display
high efficiencies, up to 3.6%.9
Thus, from these preliminary studies it is clear that there is a
strong need for clarifying and quantifying the nature and scope
of the supramolecular interactions between SubPcs and fuller-
enes. In order to do so we describe in this article the synthesis of
a series of acceptor, neutral and donor subphthalocyanines and
the careful study of their complexing ability towards C60 and C70
fullerenes in solution. In the rst instance we expect to control
the intensity of the interaction with fullerene through the
modication of the electronic nature of the SubPc macrocycle.
Thus, we chose dodecauorosubphthalocyanine 1b as
acceptor, unsubstituted SubPc 2b as neutral, and hexathioether
SubPcs 3–6b as donor components.
Furthermore, the donor SubPcs possess alkyl chains which
may strengthen the complexation of C60 and C70 fullerenes
through additional van der Waals interactions. One motivation
for designing new fullerene receptors was built around entropic
factors that would ne tune hydrophobic effects. To this end,
the alkyl chains of the hexaalkylthio-substituted SubPcs are
expected to mutually interact and, thereby, to facilitate/
augment the encapsulation of fullerenes. As a matter of fact,
ne-tuning/optimizing these factors is supposed to impact
the kinetics of SubPc–fullerene excited state interactions. To
Fig. 2 Absorption spectra of 1b (grey), 2b (black) and 6b (red).
Results and discussion
Synthesis
Subphthalocyanines 1b–6b were synthesized following
a
two-step one-pot reaction previously developed in our research
group.10 Accordingly, the corresponding phthalonitriles (tetra-
uorophthalonitrile for 1b, phthalonitrile for 2b and 4,5-dia-
lkylthiophthlontrile for 3b–6b) were cyclotrimerized in the
presence of boron trichloride to yield SubPcs 1a–6a which, aer
evaporation of p-xylene, were further treated in the same reaction
vessel with tert-butyl phenol in toluene at reux temperature.
Final compounds 3b–6b were puried by column chromatog-
raphy on silica gel and were obtained in moderate to good yields
(from 40% to 70%). It is worth mentioning that all the reactions
have been optimized and, more relevantly, scaled up to gram
quantities without appreciable loss of efficiency. All new
compounds were characterized by 1H-NMR, 13C-NMR spectros-
copy, UV-vis spectrophotometry as well as mass spectrometry.
MALDI-TOF mass spectra of compounds 1b–6b show peaks
at 760.14, 544.24, 904.26, 1072.42, 1408.64, 2082.36, respec-
tively, corresponding to [M]+. In the case of thioether-
substituted SubPcs 3b–6b prominent peaks corresponding to
the loss of the axial tert-butylphenoxy group were observed.
These observations conrm the higher liability of the axial
corroborate the aforementioned hypotheses,
a series of
alkylthio-substituted SubPcs was synthesized and investigated
with particular emphasis on the encapsulation of fullerenes by
means of photophysical assays. Thus, four different SubPcs 3–6
with increasing chain-length (C2, C4, C8 and C16, respectively)
were synthesized with the aim of determining the role and
importance of these moieties in the formation of the complexes.
Fig. 1 Synthesis of subphthalocyanines 1–6.
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Fig. 3 Representation of the Job's plots for the complex formation of 1–6b with C60 (from left to right and top to bottom).
ligands observed experimentally when the macrocycle is means of ESI-mass experiments. In both cases, the formation
peripherally substituted with donor groups. Actually, the axial of SubPc–fullerene complexes evolved with 1 : 1, 2 : 1, and in
substitution reaction is complete for thioether SubPcs within 2 some cases with 3 : 1 stoichiometries. Peruorinated sub-
hours, in contrast with dodecauoro and dodecahydro SubPcs, phthalocyanine 1b and C60 or C70 were not expected to interact
which require 18–24 h. Hence, mass spectrometry results are in because their electronic nature is not complementary (both
good agreement with the reactivity observed in the macrocycles. compounds are electronically decient); Job's plot experiments
UV-vis spectra of SubPcs 1b–6b show the expected B and Q carried out in toluene conrmed our hypothesis, as illustrated in
bands at ca. 320 and ca. 580 nm, respectively. In the case of Fig. 3 and S1.† These results indicate that shape complemen-
donor subphthalocyanines 3b–6b, the Q band is bathochromi- tarity alone is not enough for SubPcs to interact appreciably with
cally shied from 570 to 605 nm (Fig. 2). It is worth mentioning fullerenes. In the case of unsubstituted subphthalocyanine 2b, a
the presence of a weak absorption band at ca. 400 nm in 1 : 1 complex with either C60 or C70 was shown to form in toluene
compounds 3b–6b which is most probably due to an n–p* with a clear maximum at 0.5 mole fraction (Fig. 3 and S1†).
transition between the lone pair of the sulphur atoms and the
adjacent aromatic ring.
More strikingly, the introduction of six donor thioalkyl chains
in the periphery of the macrocycles (3b–6b) gave rise to the
formation of 2 : 1 complexes. Here, two SubPcs encapsulate one
fullerene. The length of the alkyl chain inuences dramatically
the time required for the binding process to reach equilibrium.
For example, the latter occurred immediately for lipophilic
SubPcs 5b and 6b, but required some time for SubPc derivatives
bearing short alkyl chains, that is, 15 minutes for 4b, and around
45 minutes for 3b with C60 fullerene. When studying the inter-
actions with C70, the time delay only evolved for SubPc 3b (20
minutes). One can observe that the Job's plots corresponding to
the formation of the 2 : 1 complex between SubPc 3b and C60 are
far from perfect even aer several attempts. From these
Host–guest behaviour
The complexing ability of SubPcs 1b–6b towards C60 and C70
fullerenes was demonstrated through an extensive series of Job's
plots and titration experiments; all of them performed in toluene
and employing the UV-visible spectrophotometry technique.
Notably, extensive elaborations about the impact of the solvent
on the complex formation have not been carried out. However, in
the initial phase of our experiments we checked the interactions
between SubPcs and fullerenes in acetone and chloroform by
Fig. 4 Molecular modelling of the 2 : 1 adducts of SubPcs (from left to right) 3b, 4b and 5b and C60
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Table 1 Binding constants of SubPcs 1–6b and fullerenes C60 and C70
SubPc
with C60 (Mꢀ1
)
with C70 (Mꢀ1
)
1b
2b
3b
4b
5b
6b
—
—
K ¼ 2.1 ꢁ 104 ꢂ 1.1 ꢁ 103
K ¼ 6.6 ꢁ 102 ꢂ 46
K1 ¼ K2 ¼ 2.1 ꢁ 104 ꢂ 2.6 ꢁ 103
K1 ¼ K2 ¼ 1.9 ꢁ 105 ꢂ 5.2 ꢁ 104
K1 ¼ K2 ¼ 2.9 ꢁ 104 ꢂ 1.1 ꢁ 103
K1 ¼ K2 ¼ 4.4 ꢁ 104 ꢂ 2.6 ꢁ 103
K1 ¼ K2 ¼ 1.9 ꢁ 104 ꢂ 1.9 ꢁ 103
K1 ¼ K2 ¼ 1.1 ꢁ 104 ꢂ 2.2 ꢁ 103
K1 ¼ K2 ¼ 1.3 ꢁ 104 ꢂ 3.6 ꢁ 103
K1 ¼ 3.6 ꢁ 105 ꢂ 2.7 ꢁ 104, K2 ¼ 1.8 ꢁ 105 ꢂ 3.8 ꢁ 104
Fig. 6 Upper part – differential absorption spectra (visible and near-infrared)
obtained upon femtosecond pump probe experiments (540 nm) of 5b (2.5 ꢁ
10ꢀ5 M) and C70 in toluene with several time delays between 0.1 and 2800 ps at
room temperature – see figure legend for details. Lower part – time absorption
profiles of the spectra at 675 (black spectrum) and 1250 nm (red spectrum),
monitoring the excited state dynamics.
Fig. 5 Upper part – differential absorption spectra (visible and near-infrared)
obtained upon femtosecond pump probe experiments (540 nm) of 5b (2.5 ꢁ
10ꢀ5 M) in toluene with several time delays between 0 and 2800 ps at room
temperature – see figure legend for details. Lower part – time absorption profiles
of the spectra at 675 (black spectrum), 860 (orange spectrum) and 1250 nm (red
spectrum), monitoring the excited state dynamics.
notable exception is 2b, whose binding constants with C60 and
experimental observations we infer that the alkyl chains have a C70 were obtained from the Job's plots (Fig. 3 and S1†).
structuring role in the formation of the complexes with fuller- At rst glance, no changes appear in the spectra during the
enes. In Fig. 4, representation of the molecular structure of the titration. However, the spectra differ from the sum of the indi-
2 : 1 complexes between SubPcs 3b, 4b and 5b and C60 conrms vidual spectra, that is, SubPc and C60. This difference of absor-
the necessary interactions between the alkyl chains when the bance around 360 nm was plotted versus increasing
number of carbon atoms of the alkyl chains exceeds 4.
concentrations of SubPcs, and curve tting afforded the values of
To obtain a quantitative understanding of the interactions the binding constants (Table 1). It is worth mentioning that,
between SubPcs and fullerenes, binding constants for all of the despite evidence for a 2 : 1 stoichiometry in the Job's plots of 3b–
previously described complexes were derived from titration 6b with C60 and C70, the shape of the titration curves corresponds
experiments monitored by UV-visible spectroscopy. The only to the typical absorbance hyperbola for 1 : 1 complexes. On one
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pump probeexperiments. SubPcs4, 5 and6b wereselectedfor the
study and all of them showed a very similar behaviour. Common
to all of the SubPcs is that photoexcitation at, for example, 540 nm
generates a singlet excited state, whose characteristics involve
minima at around 600 nm accompanied by maxima at 485, 680,
and 870 nm. An example is shown in Fig. 5. With a lifetime of
1250 ꢂ 50 ps these singlet excited states transform into the cor-
responding triplet excited states. The latter feature minima at
600 nm as well as maxima at 490, 645, and 865 nm.
It is important to note that Fig. 6 and 7 attest that the SubPcs
singlet excited state characteristics evolve despite the presence
of C60 and/or C70. Nevertheless, in the presence of C70 a number
of changes are discernable, especially in the near-infrared
region – Fig. 6, S4 and S5.† Firstly, instead of the 1250 ꢂ 50 ps
intersystem crossing dynamics, a 1240 nm maximum evolves in
the case of C70 with a lifetime of 5.5 ꢂ 0.5 ps.
This and the feature at 460 nm resemble those seen in
reference experiments with just C70 and correlate with its
singlet excited state. Secondly, no particular evidence that
would corroborate the involvement of the one electron reduced
form of C70 is gathered. Of importance is the transient
maximum at 1370 nm as registered in spectroelectrochemical
or pulse radiolytical experiments.11 As a matter of fact, we
conclude that energy transfer rather than electron transfer
dominates the electronic interactions between SubPcs and C70
in the excited state. In line with this hypothesis is the obser-
vation that the triplet excited state of C70 with its characteristic
945 nm ngerprint evolves on the longer time scale, that is, up
to 3000 ps. Moreover, this 945 nm growth – C70 triplet excited
state – and the 1240 nm decay – C70 singlet excited state – mirror
each other and feature the same lifetime of 1000 ꢂ 50 ps.
Finally, the SubPc triplet–triplet maxima and the triplet–triplet
minimum are noted at 490/645 and 600 nm, respectively. Their
formation with 1250 ꢂ 50 ps matches that seen in the absence
of C70 and, as such, is rationalized on the basis of free SubPcs in
solution, whose singlet excited state intersystem crosses to the
corresponding triplet excited state. Variation of the C70
concentration, that is, either an increase or decrease, is key in
terms of yielding different C70 triplet to SubPc triplet ratios,
without, however, affecting the underlying energy transfer
dynamics of 5.5 ꢂ 0.5 ps.
Fig. 7 Upper part – differential absorption spectra (visible and near-infrared)
obtained upon femtosecond pump probe experiments (540 nm) of 5b (2.5 ꢁ
10ꢀ5 M) and C60 in toluene with several time delays between 0.1 and 2800 ps at
room temperature – see figure legend for details. Lower part – time absorption
profiles of the spectra at 675 (black spectrum) and 1250 nm (red spectrum),
monitoring the excited state dynamics.
hand, this fact may be rationalized on the basis of a 1 : 1
complex, if the charge transfer occurs only from one of the two
SubPcs. On the other hand, the stepwise formation of a 2 : 1
complex, where both binding constants have the same value,
cannot be ruled out. The former interpretation was, however,
discarded, since, if true, should be reected in the Job's plot
experiments. As a matter of fact, the latter interpretation was
used for the curve tting. The titration of C70 with SubPc 6b (C16
alkyl chains) gave rise to a typical sigmoidal curve, from which
different K1 and K2 values were derived – see ESI.†
Overall, the binding constants are quite high, with values
ranging from 104 to 105 Mꢀ1 and, as such, attest the strong
interactions between SubPcs and fullerenes. In spite of the
different kinetic behaviour in terms of complex formation – vide
supra – no general trend in binding constants is noted. In fact, the
differences between the binding constants are insignicant and,
in turn, hamper the drawing of any meaningful conclusion.
Less unambiguous is the situation when C60 is present. The
corresponding maxima of the C60 singlet excited state, espe-
cially the 960 nm ngerprint are hardly discernable – Fig. 7.
Largely responsible for this trend is the fact that the corre-
sponding SubPc transitions dominate the differential absorp-
tion changes throughout the recorded region in the visible and
the near-infrared. Nevertheless, a closer analysis sheds light
onto the presence of the 960 nm feature. It was identied in the
form of a shoulder. The presence of C60, like that of C70, evokes
a rapid decay, namely 4.5 ꢂ 0.5 ps, of the SubPc singlet excited
state. Taken the aforementioned into concert, an energy trans-
fer scenario is operative upon SubPc–C60 complex formation.
The one electron reduced form of C60, on the other hand, is not
seen at 1080 nm even on longer time scales of up to 3000 ps.
Photophysics
Further insights into the interactions between SubPcs and C70 as Instead, the formation of the C60 triplet excited state at 750 nm
well as between SubPcs and C60 came from femtosecond resolved is noted as a consequence of an efficient intersystem crossing.
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the Comunidad de Madrid (MADRISOLAR-2, S2009/PPQ/1533),
the EU (FP7- ICT-2011.3.6, Nr: 287818), Fonds der Chemischen
Industrie (FCI) and Deutsche Forschungsgemeinscha (SFB
583), and Free State of Bavaria (Solar Technologies go Hybrid).
Conclusions
A systematic and quantitative study of the supramolecular
interaction of SubPcs withfullerenes insolution hasbeen carried
out. Systematic variations of the peripheral substitution of
SubPcs assist in the control over the non-covalent forces at work.
To this end, hydrophobic interactions complemented by p–p
interactions are the major driving forces behind a 2 : 1
complexation between two hexaalkylthioSubPcs and one C60
(or C70), with binding constants as large as 105 Mꢀ1. In the
resulting 2 : 1 inclusion complexes, an unidirectional energy
transfer from the periphery (i.e., two SubPcs) to the core (i.e., C60
and C70) has been determined to occur upon photoexcitation. At
the current stage of our research, it is hard to make serious
predictions about the applicability of our binding motif towards
improving organic photovoltaics. Despite the lack of electron
transfer in our solution based experiments, such events are,
nevertheless, expected in organic photovoltaic devices. On this
connection, we like to draw attention to previous results with
acceptor peruorinated SubPcs.9b Moreover, deaggregation of
fullerenes by the chromophore might be a useful approach for
achieving better architectures in bulk-heterojunction solar cells.
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Experimental
Synthesis
General method for the synthesis of 4,5-dialkylthiophthaloni-
triles:12 4,5-dichlorophthalonitrile (1 g, 5.08 mmol), K2CO3
(2.1 g, 15.24 mmol) and degassed N,N-dimethylacetamide
(13 ml) were placed in a round bottomed ask under argon
atmosphere. The corresponding thiol (12 mmol) was added and
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ꢃ
the mixture was stirred at 90 C for 12 h. Characteristics of 2c
and d are identical to those previously described.
´
´
references therein; (e) E. M. Perez, L. Sanchez,
Generalmethodforthesynthesisofsubphthalocyanines1–6b:
in a 25 ml round bottomed ask, BCl3 (0.3 mmol, 1 M solution in
p-xylene) was added to the corresponding phthalonitrile (0.6
mmol) under an argon atmosphere. The reaction mixture was
stirred under reux for 2 h and then ushed with argon to remove
volatiles. Subsequent axial substitution was carried out without
isolation of chlorosubphthalocyanines intermediate. 4-tert-
buthylphenol (1.8 mmol) and toluene (1 ml) were added and the
mixture was reuxed till reaction was completed (check by TLC;
see eluent below). Reaction was cooled down to room tempera-
ture, the solvent was evaporated under reduced pressure and the
resulting purple-green solid was washed with a 4 : 1 mixture of
methanol and water. Finally, compounds 1–6b were puried by
column chromatography in silica gel using as eluent: hexane–
toluene (1 : 2.5) for 3 and 4b, hexane–CH2Cl2 (1 : 1) for 5b,
hexane–ethyl acetate (20 : 1) for 6b, toluene–THF (20 : 1) for 2b
and hexane–CHCl3 (2 : 1) for 1b. Characteristics of compounds
1c–f are identical to those previously described.12b,13
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´
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Acknowledgements
Support is acknowledged from the Spanish MEC (CTQ2011-
24187/BQU and CONSOLIDER INGENIO 2010, CSD2007-00010
on Molecular Nanoscience), MICINN (PRI-PIBUS-2011-1128),
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D. Gonzalez-Rodrıguez, E. Carbonell, D. M. Guldi and
T. Torres, Angew. Chem., Int. Ed., 2009, 48, 8032–8036.
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