A collection of fullerenes for synthetic access toward oriented charge-transfer cascades in triple-channel photosystems

Article abstract of DOI:10.1002/anie.201402042

The development of synthetic methods to build complex functional systems is a central and current challenge in organic chemistry. This goal is important because supramolecular architectures of highest sophistication account for function in nature, and synthetic organic chemistry, contrary to high standards with small molecules, fails to deliver functional systems of similar complexity. In this report, we introduce a collection of fullerenes that is compatible with the construction of multicomponent charge-transfer cascades and can be placed in triple-channel architectures next to stacks of oligothiophenes and naphthalenediimides. For the creation of this collection, modern fullerene chemistry - methanofullerenes and 1,4-diarylfullerenes - is combined with classical Nierengarten-Diederich-Bingel approaches.

Full text of DOI:10.1002/anie.201402042

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Angewandte
Communications
DOI: 10.1002/anie.201402042
Supramolecular Chemistry
A Collection of Fullerenes for Synthetic Access Toward Oriented
Charge-Transfer Cascades in Triple-Channel Photosystems**
Altan Bolag, Javier Lꢀpez-Andarias, Santiago Lascano, Saeideh Soleimanpour, Carmen Atienza,
Naomi Sakai, Nazario Martꢁn,* and Stefan Matile*
Abstract: The development of synthetic methods to build
complex functional systems is a central and current challenge
in organic chemistry. This goal is important because supra-
molecular architectures of highest sophistication account for
function in nature, and synthetic organic chemistry, contrary to
high standards with small molecules, fails to deliver functional
systems of similar complexity. In this report, we introduce
a collection of fullerenes that is compatible with the construc-
tion of multicomponent charge-transfer cascades and can be
placed in triple-channel architectures next to stacks of oligo-
thiophenes and naphthalenediimides. For the creation of this
collection, modern fullerene chemistry—methanofullerenes
and 1,4-diarylfullerenes—is combined with classical Nieren-
garten–Diederich–Bingel approaches.
important to ensure directionality, which is a critical issue to,
for example, construct oriented multicomponent antiparallel
redox gradients (OMARGs) which drive holes and electrons
in opposite directions after their generation with light,[14] thus
resembling biological photosystems.[1]
Toward this general objective, zipper assembly, a sticky-
end-layer-by-layer method, has been introduced.[10] The
discovery of surface-initiated ring-opening disulfide-
exchange polymerization as a most convenient approach to
complex architectures followed.[11,12] Based on this robust,
somewhat bioinspired method, self-organizing surface-initi-
ated polymerization (SOSIP) was conceived to grow charge-
transporting p stacks on indium tin oxide (ITO).[11] To adapt
SOSIP to the synthesis of multicomponent architectures with
two[14–16] or three[17] coaxial charge-transporting channels,
templated self-sorting (TSS) during co-SOSIP[13] and tem-
plated stack exchange (TSE, Figure 1) after SOSIP[14–17] were
introduced next. Sometimes referred to as supramolecular
n/p heterojunctions (SHJs),[3] surface architectures with
coaxial channels to transport holes and electrons are of
interest for maximizing photoinduced charge separation
without losses in charge mobility.
Initial efforts to use the SOSIP-TSE methodology for the
synthesis of the first OMARG-SHJ architectures focused
exclusively on naphthalenediimides (NDIs).[14] However, in
todayꢀs functional supramolecular materials, fullerenes[4–8]
and oligothiophenes[9] are considered as most powerful
components for the construction of hole- and electron-
transporting channels. By combining classics with recent
progress in fullerene chemistry, that is, Nierengarten–Die-
derich–Bingel fullerenes,[5] methanofullerenes,[6] and 1,4-dia-
rylfullerenes,[7] we herein report the design, synthesis, and
evaluation of a collection of fullerenes which 1) are compat-
ible with the construction of charge-transfer cascades, that is,
multicomponent OMARGs, and 2) can be placed, with
molecular-level precision, next to oligothiophene stacks in
operational triple-channel architectures.
B
iological function originates from supramolecular archi-
tectures of highest sophistication.[1] No one knows what could
be found if functional materials of similar sophistication could
be synthesized with similar precision because the ability to do
so remains rudimentary, despite enormous efforts world-
wide.[1–9] To help improve on this situation, we became
interested in developing synthetic methods to grow complex
architectures directly on solid surfaces.[10–17] This approach is
[*] Dr. A. Bolag,[+] S. Lascano,[+] S. Soleimanpour, Dr. N. Sakai,
Prof. S. Matile
Department of Organic Chemistry, University of Geneva
Geneva (Switzerland)
E-mail: stefan.matile@unige.ch
Homepage: http://www.unige.ch/sciences/chiorg/matile/
J. Lꢀpez-Andarias,[+] Dr. C. Atienza, Prof. N. Martꢁn
Departamento de Quꢁmica Orgꢂnica, Universidad Complutenese
Madrid (Spain)
and
IMDEA Nanoscience, Madrid (Spain)
E-mail: nazmar@quim.ucm.es
[+] These authors contributed equally to this work.
[**] We thank J.-F. Nierengarten for advice, A. Sobczuk, H. Hayashi, and
L. Cervini for contributions to synthesis, the NMR and Mass
Spectrometry platforms for services, and the University of Geneva,
the European Research Council (ERC Advanced Investigator), the
National Centre of Competence in Research (NCCR) Chemical
Biology and the Swiss NSF for financial support (S.M.). We also
thank the European Research Council ERC-2012-ADG (Chirallcar-
bon), Ministerio de Economꢁa y Competitividad (MINECO) of Spain
(project CTQ2011-24652; Ramꢀn y Cajal granted to C.A. and FPU
granted to J.L.-A.) and the CAM (MADRISOLAR-2 project S2009/
PPQ-1533; N.M.). N.M. is indebted to the Alexander von Humboldt
Foundation.
For compatibility with SOSIP-TSE, the fullerenes F have
to offer an aldehyde and good solubility in aprotic polar
solvents (Figure 1). The double Bingel fullerene DBF has
been shown previously, in a different context, to fulfill these
prerequisites.[15] The SBF (single Bingel fullerene), with only
one cyclopropane ring, was expected to have about a 0.06 eV
lower HOMO and LUMO energy level compared to those of
DBF (Figure 2).[5] In SBF, the central benzaldehyde and both
triethyleneglycol (TEG) solubilizers are preserved (solubility
with only one TEG was insufficient). To raise the HOMO/
LUMO levels from those of DBF, the recently disclosed
methanofullerenes were considered.[6] DBMF and SBMF,
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402042.
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Figure 1. Structure of the fullerenes F and their incorporation into triple-channel architectures, for example, ITO-NDI-T4-DBF. SBMF and DBMF
are mixtures of regioisomers. [a,b] Templated stack exchange (TSE); for yields, see Table 1. [c] For synthesis of T4-F, see Scheme 1.
accessible as regioisomeric mixtures from Bingel reaction of
methanofullerenes with the same malonates, should repro-
duce the energy difference between DBF and SBF at a higher
level. To more profoundly modulate the HOMO/LUMO
levels of fullerenes, the recently introduced direct coupling
with phenyl rings appeared most promising.[7] The compar-
ison between the electron-richer TAF, having three alkoxy
substituents, and the electron-poorer SAF, having only one
alkoxy substituent, was expected to clarify the potential of
this alternative approach.
The oligothiophene-fullerene dyad T4-SBF was synthe-
sized as follows (Scheme 1).[18] The diol 1,[5] obtained from the
triester 2 by a more user-friendly pathway,[19] was converted
Figure 2. Energy levels of HOMO (bold) and LUMO (dashed) of the
into the malonate 3 by consecutive esterification with the
fullerenes F and T4[15] (in eV against vacuum, assuming À5.1 eV for
Fc+/Fc [compare Figures 1 (structures) and 3 (DPVs)]. [a] Gradually
decreasing LUMO levels designed to build oriented gradients into the
electron (eÀ) transporting fullerene channels in, for example, triple-
channel architectures of ITO-NDI-T4-DBF (Figure 1). Charge separa-
tion between the co-axial T4-F stacks can occur by [b] eÀ transfer after
excitation of T4 (hnT4) and [c] h+ transfer after excitation of F (hnF).
[d] Measured for the methyloxime derivatives.[18] [e] The HOMO levels
of F, estimated from the gradual onset of their absorption, are affected
by non-negligible uncertainty [LUMO levels, from DPV (Figure 3), are
accurate].
TEG-glycolate 4 and TEG-malonate 5. Cyclopropanation of
the fullerene 6 with 3 gave the Bingel fullerene 7, which was
deprotected to yield the desired SBF. The quaterthiophene 8
was prepared from the bithiophene 9 and bromothiophene 10
as described.[16]
The diacid 8 was first coupled with the amine 11—
equipped with an essential leucine spacer to assure good
solubility—and then with amine 12 (Scheme 1). Chemo-
selective Alloc removal liberated the reactive alkoxyamine in
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Scheme 1. Synthesis of the fullerene collection. a) 1. LiBH4, THF, 60%; 2. PCC, 99%; 3. 2,2-dimethyl-1,3-propanediol, p-TsOH, 86%; 4. LiAlH4,
THF, 57%;[19] b) 1. 1, 4, EDC, DMAP, CH2Cl2, 67%; 2. same with 5, 66%; c) 3, I2, DBU, 64%; d) TFA, RT, 4 h, 52%; e) 1a. 9, nBuLi; 1b. Bu3SnCl;
2. 10, [Pd(PPh3)4], 82%; 3. TFA, RT, quant;[16] f) 1. 8, 11, HBTU, TEA, 46%; 2. same with 12, 42%; 3. [Pd(PPh3)2Cl2], Bu3SnH; g) AcOH, RT,
15 min, quant; h) TFA, quant; i) as for T4-SBF (g, h) with DBF;[15] j) 1. DMF, ODCB, 89%; 2a. tBuOK, ODCB; 2b. CuCl2, 1008C, 12 h, 79%;[6] k) as
for T4-SBF with 14 (c, d, g, h); l) as for T4-DBF with 14 (i); m) 17, NaNO2, HCl, O2, 15%; n) 18 (from phenol, rac 2-ethylhexylbromide, tBuOK,
98%), p-TsOH, 808C; o) 1. HgO, BF3, 10% (2 steps); 2. T4, AcOH, RT, 15 min, 47%; p) TFA, 72%; q) 1. 1,2-ethanedithiol, BF3·Et2O, 96%; 2. tBu
carbazate, CuI, phenanthroline, Cs2CO3, 67%; 3. HCl, quant; r) 1. 22 (from pyrogallol, n-hexylbromide, K2CO3, 76%), p-TsOH, 808C; 2. HgO, BF3,
28% (2 steps); 3. T4, AcOH, RT, 15 min, 52%; 4. TFA, 93%. DBU=,8-diazabicyclo[5.4.0]undec-7-ene, DMAP=4-(N,N-dimethyl)pyridine,
DMF=N,N-dimethylformamide, EDC=1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, HBTU=O-benzotriazole-N,N,N’,N’-tetramethyluronium-
hexafluoro phosphate, ODCB=o-dichlorobenzene, PCC=pyridnium chlorochromate, TFA=trifluoroacetic acid, THF=tetrahydrofuran, Ts=4-
toluenesulfonyl.
T4. Spontaneous oxime formation between T4 and SBF then
gave the dyad 13 essentially in situ. Aldehyde deprotection
afforded the desired dyad T4-SBF under mild reaction
conditions, and made ready for TSE with the activated ITO-
NDI architecture (Figure 1). T4-DBF was prepared following
the exceptionally mild procedure developed for T4-SBF.
The fullerene 14, having a very small 1,2-dihydromethano
group, was synthesized using the original procedure.[6]
Namely, addition of the Grignard reagent 15 and subsequent
deprotonation of the obtained silylfullerene and oxidative
cyclopropane formation in the presence of CuCl2 at 1008C.
The fullerene 14 was converted into T4-SBMFand T4-DBMF
by using the procedure described above for the complemen-
tary T4-SBF and T4-DBF.
The introduction of dithiane-protected benzaldehydes
into the fullerenol 16 turned out to be feasible (Scheme 1).
The Wang reaction with the phenyl hydrazine 17 in the
presence of NaNO2 gave the desired product 16 in, at least in
this context, very reasonable 15% yield. The 1,4-addition was
completed with the alkoxybenzene 18 in the presence of para-
toluenesulfonic acid (p-TsOH), and HgO-mediated hydrol-
ysis of the non-isolable intermediate 19 gave the key product
SAF (Figure 1). Incubation of SAF with T4 gave the dyad 20
in situ, and mild acidic deprotection afforded T4-SAF, the
dyad needed for stack exchange with ITO-NDI. The hydra-
zine 17 was readily accessible from p-iodobenzaldehyde (21)
in a three-step synthetic procedure.[18] The alkoxybenzene 18
was prepared by simple alkylation of the corresponding
phenol. T4-TAF, the electron-rich homologue of T4-SAF, was
prepared analogously from 16 using the alkylated pyrogallol
22 instead of 18.
The redox potential values for all fullerenes were
determined by cyclic voltammetry (CV) and differential
pulse voltammetry (DPV). LUMO energy levels against
vacuum were estimated assuming À5.1 eV for Fc+/Fc
(Figure 2).[20] For the Bingel quartet DBMF, SBMF, DBF,
and SBF, a marvelous stepwise decrease of the LUMO
energies by 60 meVeach could be detected unambiguously by
DPV against Fc+/Fc as the internal standard (Figure 3). This
finding provided experimental support that the Bingel
methanofullerene series is suitable for the synthesis of four-
component electron-transfer cascades in complex architec-
tures.
For DPV measurements, SAF and TAF were converted
into their respective methyloximes. This transformation of the
aldehyde acceptors into formal N-alkoxyimine donors raised
the LUMO levels, but only slightly. Consistent with the
presence of fewer alkoxy donors, the LUMO level of SAF was
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Figure 3. DPVs of DBMF, SBMF, DBF, and SBF (left to right),
measured in CH2Cl2 against internal Fc+/Fc standards at 0 mV.
À60 meV below that of TAF (Figure 2). The HOMO energies
of none of the fullerenes could be directly measured by DPV,
and an accurate estimation from the HOMO/LUMO gap in
the absorption spectra is difficult because of the weak and
gradually decreasing tail reaching up to about 650 nm. A
constant HOMO/LUMO gap of 2.3 eV was assumed based on
the literature.[21] For SAF and TAF, the HOMO/LUMO gap
was arbitrarily reduced to 2.2 eV considering the previously
reported[7a] red shift caused by their lowered symmetry
(Figure 2). The direct determination of the LUMO energy
levels of the fullerenes, of central importance for this study, is
most accurate (Figure 3 and Figure S2 in the Supporting
Information), while the intrinsic uncertainty of the absolute
values of the HOMO energy levels is inconsequential.
Figure 4. Action spectra of a) ITO-NDI-T4-SBF and b) ITO-NDI-T4-
SBMF (dashed lines) compared to their absorption spectra (solid
lines) and those of c) T4-SBF in DMSO and d) ITO-NDI. The grey
dashed lines in a) and b) mark the area where photocurrent is mostly
generated by T4.
Table 1: Characteristics of triple-channel photosystems.
Photosystem[a]
TSE
JSC
DEHOMO
DELUMO
[%][b]
[mAcmÀ2 [c]
]
[eV][d]
[eV][e]
To explore the compatibility of the new fullerenes with
triple-channel architectures, original NDI stacks were grown
by the established disulfide-exchange SOSIP on ITO in the
presence of NDI templates on the surface and benzaldehyde
hydrazone templates along the stacks.[14] The absorption of
the obtained ITO-NDI-B was kept constant around A380
ꢀ 0.17 at the low-energy maximum of the NDI stack at l =
380 nm (Figure 4d). This constant A380 ꢀ 0.17 was important
to maximize comparability of the results. In an ideal
architecture, A380 ꢀ 0.17 corresponds to about 142 NDIs
stacked on top of each other, or a formal thickness of about
47 nm. The orthogonal dynamic covalent chemistry[22] of
TSE[14] is the key to cleaving the hydrazones along the NDI
stacks in ITO-NDI-B with excess hydroxylamine (Figure 1).
The large pores left behind in ITO-NDI with reactive
hydrazides along their walls were then filled with the T4-F
and control T4-C with an oxime from 4-tert-butyl benzalde-
hyde in place of the fullerene (see Scheme S3 in the
Supporting Information). In the absorption spectrum of
ITO-NDI-T4-SBF, the presence of all components was clearly
observable (Figures 4a,c,d). From the absorption spectra,
a TSE yield of 67% was estimated (Table 1, entry 1). The
formation of all other triple-channel architectures occurred
with similar satisfactory yield (Table 1). Only the control
architecture ITO-NDI-TAF (compound lacking the T4
moiety) was obtained with a low 33% yield, presumably
because the accommodation of the bulky fullerenes in close
proximity to the NDI stack was hindered.
1
2
3
4
5
6
7
ITO-NDI-T4-SBF
ITO-NDI-T4-DBF
ITO-NDI-T4-SBMF
ITO-NDI-T4-DBMF
ITO-NDI-T4-SAF
ITO-NDI-T4-TAF
ITO-NDI-T4-C
67
89
78
62
82
70
90
12.0
8.8
2.1
2.2
3.9
2.8
2.4
+0.5
+0.4
+0.3
+0.3
+0.4
+0.3
–
À0.85
À0.79
À0.73
À0.67
À0.85
À0.79
–
[a] See Figure 1 for structures.[18] [b] Yield for templated stack exchange,
estimated from absorption spectra, see Figure 4. [c] Short-circuit
photocurrent density generated by irradiation with white light (solar
simulator, W=100 mW, 50 mm TEOA, 0.1m Na2SO4, Pt counter
electrode). [d] Energy difference to the HOMO of T4 (from DPV, compare
Figure 2 and 3). [e] Energy difference from the LUMO of T4.
counter electrode, and triethanolamine (TEOA) as sacrificial
mobile hole transporter. Other, also reversible, hole carriers
were applicable but gave lower currents. While results
obtained under these conditions are not comparable with
those from optimized optoelectronic devices, they are suitable
to evaluate and compare different multicomponent architec-
tures. The highest currents were observed for ITO-NDI-T4-
SBF (Figure 5a, Table 1, entry 1). This is the most active
triple-channel photosystem prepared so far.[17] With a distinct
shoulder around l = 430 nm, the action spectrum demon-
strated that the oligothiophenes contribute to photocurrent
generation (Figure 4a, dashed line). With absorption and
action spectra of fullerenes and NDIs being very weak above
l = 400 nm, the action spectrum of ITO-NDI-T4-SBF dem-
onstrated that these contributions of the oligothiophenes are
highly significant (Figure 4a, dashed gray lines). High activity
below l = 400 nm indicated that fullerenes and NDIs also
Photocurrent generation of the triple-channel architec-
tures was measured under routine conditions with the photo-
systems as the working electrode, a platinum wire as the
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T4-SBF and ITO-NDI-T4-DBF with identical LUMO but
lower HOMO could be explained by reduced hole transfer
(Table 1). The decisive importance of hole transfer from
excited fullerenes has been suggested for other systems.[8]
Taken together, the origin of the observed trends are
presumably quite complex, and electron transfer and hole
transfer, as well as charge mobility and charge recombination
might all contribute to the final photocurrent, although
electron transfer from excited oligothiophenes to fullerenes
is probably most relevant (Figure 4a versus b).
In summary, this study introduces a collection of ful-
lerenes which is compatible with the construction of triple-
channel photosystems having oriented strings of different
fullerenes for directional electron transfer along multicom-
ponent gradients, similar to biological photosystems. The
redox potentials of the fullerenes could be adjusted without
losses in reactivity and solubility by combining cutting-edge
fullerene chemistry with classical Nierengarten–Diederich–
Bingel strategies. Confirmed compatibility with SOSIP-TSE
methodology provided exceptionally mild, fast, and easy
conditions for the directional positioning of the new ful-
lerenes in triple-channel architectures with molecular-level
precision. The obtained multicomponent surface architec-
tures are as sophisticated as it gets today, and the reported
activities include the best observed so far with triple-channel
photosystems. Charge recombination efficiencies larger than
50% are important because they demonstrate that future
suppression of charge recombination with antiparallel redox
gradients could further increase activity and would be easily
detectable.[14] To achieve this general objective, the redox
engineering of oligothiophene stacks will be required next,
preliminary results are promising and will be reported in due
course.
Figure 5. a) Photocurrent generated by ITO-NDI-T4-SBF (left) and ITO-
NDI-T4-SBMF (right, W=100 mW, 50 mm TEOA, 0.1m Na2SO4, Pt).
b) Dependence of the short-circuit current density JSC on the light
*
*
intensity I for ITO-NDI-T4-SBF ( ) and ITO-NDI-T4-SBMF ( ), with
curve fit and bimolecular charge recombination efficiencies hBR deter-
mined from the slope.[18]
contribute to photocurrent generation (Figure 4a). For com-
parison, the fullerene-free double-channel control ITO-NDI-
T4-C was prepared with a record TSE yield of 90% (probably
as a result of reduced steric demand; Table 1, entry 7).
Photocurrent generation by ITO-NDI-T4-C never exceeded
J
SC = 2.4 mAcmÀ2. The presence of an electron-transporting
SBF channel next to the hole-transporting T4 channel in ITO-
NDI-T4-SBF thus caused a greater than fivefold increase in
activity, although the TSE yield and thus the relative T4
content was with 67% clearly lower than in the fullerene-free
control ITO-NDI-T4-C (Table 1, entry 1 versus 7). This result
confirmed the significance of operational triple-channel
architectures.
High photocurrents were also found for ITO-NDI-T4-
DBF, whereas the 1,4-diarylfullerenes containing ITO-NDI-
T4-SAF and ITO-NDI-T4-TAF were less active, and ITO-
NDI-T4-SBMF and ITO-NDI-T4-DBMF with methanoful-
lerenes were worst (Figure 5a; Table 1, column 2). Bimolec-
ular charge recombination efficiencies were determined from
the dependence of the photocurrent generation on the
intensity of irradiation as described.[14] The most active
Received: February 3, 2014
Published online: April 1, 2014
Keywords: charge transfer · fullerenes · hydrazones ·
.
polymerization · supramolecular chemistry
*
ITO-NDI-T4-SBF gave hBR = 58% (Figure 5b; ), whereas
in the least active ITO-NDI-T4-SBMF, recombination was
ˇ
[1] R. Bhosale, J. Mꢁsek, N. Sakai, S. Matile, Chem. Soc. Rev. 2010,
*
very high with hBR = 92% (Figure 5b, ). This difference
39, 138 – 149.
suggested that when compared to Bingel fullerenes, the poor
activity observed for methanofullerenes could stem from poor
charge mobility. The organization of strings of methanoful-
lerenes and, although less pronounced, also of 1,4-adducts in
triple-channel architectures could be hindered by the random
location of the third cylcopropane ring and the clumsy diaryl
substituents, respectively. Moreover, poor electron transfer
from excited oligothiophenes to fullerenes with high LUMO
could also reduce activity (Table 1, column 4). The weak
photocurrent generation by the oligothiophenes in the action
spectrum of ITO-NDI-T4-SBMF, compared to that of ITO-
NDI-T4-SBF, was in support of this interpretation (Figure 4a
versus b: dashed lines). The same could be said for poor hole
transfer from excited fullerenes with high HOMO to oligo-
thiophenes (Table 1, column 3). The weaker activities of ITO-
NDI-T4-SAF and ITO-NDI-T4-TAF compared to ITO-NDI-
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