Synthesis, electronic, and photophysical properties of cruciform OPE/OPV hybrid oligomer bridged bisfullerene triads

Article abstract of DOI:10.1016/j.tetlet.2007.03.092

A series of cruciform-shaped phenyleneethynylene and phenylenevinylene hybrid oligomers was prepared via a sequence of Sonogashira reactions. Using an in situ ethynylation protocol, these oligomers were end-functionalized with bis(fullerenyl) groups at th

Full text of DOI:10.1016/j.tetlet.2007.03.092

Tetrahedron Letters 48 (2007) 3563–3567
Synthesis, electronic, and photophysical properties of cruciform
OPE/OPV hybrid oligomer bridged bisfullerene triads
Ningzhang Zhou, Li Wang, David W. Thompson* and Yuming Zhao*
Department of Chemistry, Memorial University of Newfoundland, St. John’s, NL, Canada A1B 3X7
Received 11 January 2007; revised 9 March 2007; accepted 17 March 2007
Available online 21 March 2007
Abstract—A series of cruciform-shaped phenyleneethynylene and phenylenevinylene hybrid oligomers was prepared via a sequence
of Sonogashira reactions. Using an in situ ethynylation protocol, these oligomers were end-functionalized with bis(fullerenyl) groups
at the termini of the oligomers. Spectroscopic and electrochemical properties of these novel bisfullerene triads were studied in detail
by UV–vis, fluorescence, and cyclic voltammetric analyses, and the results of these investigations are reported.
ꢀ 2007 Elsevier Ltd. All rights reserved.
There is a growing interest in fullerene (C60) and p-con-
jugated oligomer hybrids, owing to their versatile
applications in artificial photosynthesis, molecular opto-
electronics, and nano-assemblies.1 Especially, in organic
photovoltaic cells, active components consisting of
blends of organic conjugated polymers and C60 moieties
can constitute ‘bulk heterojunctions’ which lead to con-
siderably improved efficiency of light-to-current conver-
sion.2 Nevertheless, in these devices spontaneous phase
segregation is a major setback that needs to be over-
come. One way to deal with this problem is to covalently
attach C60 groups to various oligomers. Therefore, the
simple dyad motif, that is, C60–oligomer, has been
extensively adopted as a straightforward strategy for de-
sign of C60 based photovoltaic materials at the molecu-
lar level. More recently, sophisticated C60–oligomer
hybrid architectures, for instance, the dumbbell-shaped
C60–oligomer–C60 triad,3 has emerged as an appealing
design motif for functional C60–oligomer hybrids.
Although covalently incorporating multiple C60 cages
into one molecule is synthetically challenging, the C60–
oligomer–C60 type of molecules have been found to
show some advantageous properties and performance
characteristics in comparison to the simple C60–oligo-
mer dyads. For example, enhanced stabilization effects
of charge-separation (CS) state stemming from the sec-
ond C60 group and better solid-state ordering could be
achieved.4
In recent research, a number of electroactive linearly
conjugated oligomers, including oligo(arylenevinyl-
ene)s,5 oligo(phenylene ethynylene)s (OPEs),6 and
oligo(thiophene)s,7 have been employed to serve as p-
bridges for the C60–oligomer–C60 molecular dumbbells.
The nature of the central oligomer p-bridge can signifi-
cantly affect electronic, optical, and photophysical prop-
erties.3,5–7 However, the effects originating from
conjugation pattern and dimensionality of the p-bridge
are yet to be clarified due to lack of compounds whose
p-bridge complexity is more than simple one-dimen-
sional linear conjugation.
We report herein the first synthesis and property study
of two novel C60–oligomer–C60 triads 7 and 11, in which
cruciform-shaped OPE/OPV hybrid oligomers are en-
listed as the bridging units instead of commonly used
linear conjugated oligomers (Scheme 1). Analogous
OPE/OPV cruciform oligomers were first investigated
by Bunz’s group8 and showed intriguing electron delo-
calization characteristics and appealing applications in
metal ion sensing. In our work, it was envisioned that
adding two-dimensionally conjugated oligomer bridges
(chromophores) in between two C60 groups would likely
impart the molecule with interesting molecular behavior.
Moreover, the cruciform bridge may also provide new
access to multiple and versatile functionalization for ful-
lerene-oligomer hybrids.
As outlined in Scheme 1, the synthesis of triads 7 and 11
began with a diiodophenylenevinylene building block 1.
Sonogashira coupling of 1 with 2 equiv of trimethylsilyl-
acetylene (TMSA) afforded compound 2 in 87% yield.
*
Corresponding authors. Tel.: +1 709 737 8046; fax: +1 709 737 3702
(D.W.T.); tel.: +1 709 737 8747; fax: +1 709 737 3702 (Y.Z.); e-mail
addresses: dthompso@mun.ca; yuming@mun.ca
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.03.092

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N. Zhou et al. / Tetrahedron Letters 48 (2007) 3563–3567
Scheme 1. Synthesis of cruciform oligomers and C60–cruciform–C60 triads.
Compound 2 after protiodesilylation with K2CO3 was
cross-coupled with iodoarene 4 to give TMS group end-
capped cruciform oligomer 5. Desilylation of 5 with
K2CO3 yielded terminal alkyne 6, which was immedi-
ately reacted with excess C60 via an in situ ethynylation
reaction to afford triad 7 in a reasonable yield of 41%. In
a similar manner, extended cruciform oligomers 9 and
10 were obtained via Sonogashira coupling and desilyla-
tion reactions. In situ ethynylation of C60 with 10 thus
afforded another C60–p–C60 triad 11 in 21% yield.
Molecular structures of the OPE/OPV cruciform oligo-
mers and respective bisfullerenyl triads were clearly ver-
ified by spectroscopic analyses (detailed IR, NMR, and
MS characterizations are provided in the Supplementary
data).9 The redox properties of triads 7 and 11 were
characterized by cyclic voltammetry (CV) at room tem-
perature. Detailed cyclic voltammograms and redox po-
tential data are given in Figure 1 and Table 1. In spite of
the presence of a cruciform p-bridge (supposedly a do-
nor) in each of the molecules of 7 and 11, their cyclic
voltammograms give no meaningful oxidation features.
In the negative potential region, distinctive reduction
Figure 1. Cyclic voltammograms of triads 7 and 11.
features were observed, however, the results of which
were quite different from that observed in related C60–
OPE–C60 systems.6c,d The characteristic four reversible
or quasi-reversible wave pairs ascribed to sequential

N. Zhou et al. / Tetrahedron Letters 48 (2007) 3563–3567
3565
Table 1. Summary of electrochemicala and spectroscopicb properties of compounds 6, 7, 10 and 11 at room temperature
Entry
Reduction
Epa, V
Absorption
max, nm (e, Mꢀ1 cmꢀ1
324 (5.8 · 104)
360 (7.8 · 104)
390 (5.4 · 104, sh)
Emission
Epc, V
ꢀ1.67
k
)
Energy, cmꢀ1
k
em, nm
Energy, cmꢀ1
Uem
Assignment
6
ꢀ1.57
30,860
27,780
25,320
428
452 (sh)
23,360
22,120
0.96
p-Bridge
7
10
11
ꢀ0.59, ꢀ0.85,
ꢀ1.00, ꢀ1.24,
ꢀ1.52, ꢀ1.73
ꢀ0.38, ꢀ0.84,
ꢀ1.12, ꢀ1.27,
ꢀ1.64
254 (1.8 · 104, sh)
325 (9.4 · 103)
38,910
30,770
27,780
24,815
451
469 (sh)
711
22,175
21,320
14,045
ꢁ10ꢀ4
p-Bridge
360 (8.7 · 103)
3C60 em
403 (6.0 · 103, sh)
ꢀ1.80
n/a
312 (2.1 · 104)
330 (2.4 · 104)
372 (3.3 · 104)
410 (3.2 · 104, br)
32,970
30,300
26,890
24,390
452
484 (sh)
22,125
20,660
0.96
p-Bridge
ꢀ0.29, ꢀ0.60,
ꢀ0.80, ꢀ0.99,
ꢀ1.17, ꢀ1.51,
ꢀ1.97
ꢀ1.03
255 (2.6 · 105)
315 (1.5 · 105, sh)
326 (1.6 · 105)
370 (1.4 · 105)
415 (1.3 · 105)
39,220
31,950
30,680
27,030
24,100
cfc
<0.01
p-Bridge
711
14,045
3C60 em
a Redox properties were determined by cyclic voltammetry. Cyclic voltammograms were recorded in solutions of o-dichlorobenzene–CH3CN (4:1).
Bu4NBF4 (0.1 M) as the supporting electrolyte, glassy carbon as the working electrode, and platinum wire as the counter electrode. Potentials are
given in volts versus a Ag/AgCl reference electrode. Scan rate: 100 mV sꢀ1
.
b UV–vis spectra were obtained in N2 saturated CHCl3. Fluorescence spectra were measured in N2 saturated toluene, uncorrected for instrument
response.
c Obscured by emission from trace amounts of 10.
reduction of functionalized C60 cage were not obvious in
this case. Rather, triad 7 gives three characteristic quasi-
reversible redox waves at E1/2 = ꢀ0.48, ꢀ0.92, and
ꢀ1.20 V (vs Ag/AgCl) accompanied by two irreversible
cathodic peaks (Epc = ꢀ0.85 and ꢀ1.52 V vs Ag/AgCl)
and one irreversible anodic peak (Epa = ꢀ1.27 V vs
Ag/AgCl). In the cyclic voltammogram of 11, only one
quasi-reversible wave appears at E1/2 = ꢀ1.10 V versus
Ag/AgCl along with four irreversible cathodic peaks in
the negative potential region. The origins of these
irreversible reduction processes are not clear. However,
these features are suggestive that the electronic inter-
actions between the cruciform p-oligomer and C60 are
more significant than those in the linear OPE bridged
C60–p–C60 system.6
UV–vis spectral data, extinction coefficients, emission
spectral data, quantum yield, and optical energies are gi-
ven in Table 1. Representative absorption and emission
spectra for 6 and 7 are shown in Figure 2. Absorption
spectra for 6 and 7 have a series of overlapping p!p*
Figure 2. (a) UV–vis spectrum of 6. (b) Fluorescence spectrum of 6
excited at 360 nm. Inset: excitation spectrum of 6. (c) UV–vis spectrum
of 7. (d) Fluorescence spectrum of 7 excited at 320 nm.
transitions localized on bridge in the 300–450 nm re-
gime. These p!p* bands are superimposed transitions
localized on the stilbene fragment, and the connecting
phenylactylene moieties.10 The absorption spectrum
for 7 differs significantly from that observed for 6. There
is increased absorption below 300 nm and broad sloping
shoulder that extends out past 700 nm. These transitions
are associated with the C60 termini.11 The transitions at
325 and 360 nm in 7 are also apparent in 6, albeit the
intensities of these absorption bands in 7 are smaller.
The origin of the diminished intensity in 7 is not known
and awaits a more detailed spectral deconvolution and
theoretical analysis. Absorption spectra for 6 and 10
have a series of overlapping bridge localized p!p* tran-
sitions in the 300–450 nm regime. The emax values for 6
and 10 range from 2.0 · 104 to 7.8 · 104 Mꢀ1 cmꢀ1
respectively, consistent with assignment of highly al-
lowed S0!S1 transitions. Generally, the energies of the
p!p* transitions are found at lower energies for 10 ver-
sus 6, an expected observation based on the increased
conjugation in 10 giving rise to more extended charge
delocalization in 10. Comparative spectra for 10 and

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N. Zhou et al. / Tetrahedron Letters 48 (2007) 3563–3567
11 follow the same trends as those described above for 6
and 7, albeit the absorption spectral band intensities in
11 are higher than 10, a trend that is reversed from
observations made for 6 and 7.
Acknowledgments
The authors thank NSERC Canada, the Canadian
Foundation for Innovation (CFI), Industrial Research
Innovation Fund (IRIF), and Memorial University of
Newfoundland for funding. Professor C. Jablonski is
acknowledged for funding of Li Wang. Dane Sheppard
and Paul Inder are acknowledged for preliminary
investigations.
Emission spectra for 6 and 7 are shown in Figure 2.
Emission from 6 at room temperature in N2 saturated
toluene shows a structured emission band envelope with
a Uem = 0.96 and possesses a lifetime of 2.8 ns (k =
3.8 · 108 sꢀ1) which is reasonably assigned as S1 ! S0
radiative transition. The excitation profile overlaps
extensively with absorption spectra. However, the
absorption band envelope and emission band envelope
are not mirror images to each other, which is not sur-
prising given that the absorption band envelope is com-
posed of a series of p!p* transitions localized on
various light absorbing fragments in the bridge. The
emission for 7 is more complex than that observed in
6; the most intense emission transition of 7 occurs at
451 nm, some 1185 cmꢀ1 lower in energy than 6, with
Uem ꢁ 10ꢀ4 for 7 versus 0.96 for 6, and a new emission
band appears beyond 700 nm. The emission at 711 nm
is assigned to 3C60 based emission based on comparison
with analogous systems described elsewhere.11 The
attenuated bridge based emission in 7 is due to an
additional non-radiative decay pathway(s), presum-
ably energy transfer from the bridge giving rise to a
3C60 emission and/or electron transfer quenching
of the C60 termini based excited state by the
bridge.11,12 A similar behavior is apparent in 10 versus
11. The exact nature of these decay pathways is under
investigation and will be reported in a subsequent
manuscript.
Supplementary data
Supplementary data including synthetic details and
spectroscopic characterizations for new compounds
can be found in the online version, at doi:10.1016/
j.tetlet.2007.03.092.
References and notes
1. For recent reviews of C60/conjugated oligomer hybrids,
´
see: (a) Segura, J. L.; Martın, N.; Guldi, D. M. Chem. Soc.
Rev. 2005, 34, 31–47; (b) Nierengarten, J. F. Sol. Energy
Mater. Sol. Cells 2004, 83, 187–199.
2. (a) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv.
Funct. Mater. 2001, 11, 15–26; (b) Marcos Ramos, A.;
Rispens, M. T.; van Duren, J. K. J.; Hummelen, J. C.;
Janssen, R. A. J. J. Am. Chem. Soc. 2001, 123, 6714–
6715.
´
´
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3. Sanchez, L.; Herranz, M. A.; Martın, N. J. Mater. Chem.
2005, 15, 1409–1421, and references cited therein.
´
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4. Sanchez, L.; Sierra, M.; Martın, N.; Guldi, D. M.; Wienk,
M. W.; Janssen, R. A. J. Org. Lett. 2005, 7, 1691–
1694.
´
5. (a) Segura, J. L.; Martın, N. Tetrahedron Lett. 1999, 40,
At this juncture, some qualitative comments on the
relative photochemical stabilities of 1, 6, and 7 are use-
ful. Light excitation into the absorption manifold of 1
result in facile changes in the emission and absorption
spectra presumably due to trans!cis photo-isomeriza-
tion of the stilbene based moiety.12b The photosensitiv-
ity of 6 is significantly attenuated, however, still
occurs. For 7, the trans!cis photo-isomerization is
not observed to an appreciable extent, indicating rapid
deactivation of the bridge based excited state via non-
radiative decay processes alluded to in the previous
paragraph.
3239–3242; (b) Martineau, C.; Blanchard, P.; Rondeau,
D.; Delaunay, J.; Roncali J. Adv. Mater. 2002, 14, 283–
287; (c) Guldi, D. M.; Luo, C.; Swartz, A.; Gomez, R.;
Segura, J. L.; Martın, N. J. Phys. Chem. A 2004, 108, 455–
´
´
467.
´
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6. (a) Atienza, C. M.; Fernandez, G.; Sanchez, L.; Martın,
N.; Dantas, I. S.; Wienk, M. M.; Janssen, R. A. J.;
Rahman, G. M. A.; Guldi, D. M. Chem. Commun. 2006,
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514–516; (b) Atienza, C.; Insuasty, B.; Seoane, C.; Martın,
N.; Ramey, J.; Aminur Rahman, G. M.; Guldi, D. M. J.
Mater. Chem. 2005, 15, 124–132; (c) Zhao, Y.; Shirai, Y.;
Slepkov, A. D.; Cheng, L.; Alemany, L. B.; Sasaki, T.;
Hegmann, F. A.; Tour, J. M. Chem. Eur. J. 2005, 11,
3643–3658; (d) Shirai, Y.; Zhao, Y.; Cheng, L.; Tour, J.
M. Org. Lett. 2004, 6, 2129–2132.
In conclusion, we have prepared a series of novel cruci-
form shaped OPE/OPV hybrid oligomers and their
bis(fullerenyl) endcapped derivatives, with their redox
and photophysical properties well characterized. Two
points are worth some final remarks here: (1) triads 7
and 11 show quite different electrochemical properties
than other previously reported C60–oligomer–C60 com-
pounds. Likely, the relatively complex p-bridge struc-
ture have played a crucial role by exerting significant
influence on factors such as electronic interactions and
others; (2) the substantially quenched fluorescence of
the bridge units in triads 7 and 11 is indicative of rapid
photoinduced intramolecular energy/electron transfer,
which may render these new fullerene-oligomer species
potential candidates for advanced organic optoelec-
tronic materials.
7. (a) van Hal, P. A.; Knol, J.; Langeveld-Voss, B. M. W.;
Meskers, S. C. J.; Hummelen, J. C.; Janssen, R. A. J. J.
Phys. Chem. A 2000, 104, 5974–5988; (b) Dhanabalan, A.;
Knol, J.; Hummelen, J. C.; Janssen, R. A. J. Synth. Met.
2001, 119, 519–522.
8. (a) Wilson, J. N.; Bunz, U. H. F. J. Am. Chem. Soc. 2005,
127, 4124–4125; (b) Wilson, J. N.; Smith, M. D.; Enkel-
mann, V.; Bunz, U. H. F. Chem. Commun. 2004, 1700–
1701; (c) Wilson, J. N.; Josowicz, M.; Wang, Y.; Bunz, U.
H. F. Chem. Commun. 2003, 2962–2963.
9. The purities of all new compounds were satisfactory
enough to give trustworthy electrochemical, UV–vis and
fluorescence spectroscopic results, except the emission of
11.
10. Spectral data for
1 in toluene: UV–vis: 299 nm
(33,445 cmꢀ1), 310 nm (32,260 cmꢀ1); emission: 338 nm

N. Zhou et al. / Tetrahedron Letters 48 (2007) 3563–3567
3567
(29,590 cmꢀ1), 352 nm (28,410 cmꢀ1), 375 nm (26,679
cmꢀ1). Spectral data for 4 in toluene: UV–vis: 331 nm
(30,210 cmꢀ1), sh 344 nm (29,070 cmꢀ1); emission: 370 nm
(27,030 cmꢀ1).
12. (a) Excitation at 263 nm, a transition localized on the C60
cage does not give rise to a bridge-based emission. (b)
Note that the cis–trans isomerization hypothesis will be
elaborated on in a subsequent manuscript.
11. Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695–
703.