Supersized contorted aromatics

Article abstract of DOI:10.1039/c3sc50374g

We describe here the synthesis and electronic device properties of a new type of polycyclic aromatic molecule, the contorted octabenzocircumbiphenyl (c-OBCB). Contorted polycyclic aromatic hydrocarbons (PAHs) are promising small active molecules for organic devices. We present two different methods to synthesize c-OBCB derivatives that allow the smooth incorporation of functional groups. The material has a highly contorted exterior with six 4-helicenes and two 5-helicenes around the exterior of the expanded core of the aromatic. With appropriate sidechains, the material is soluble in common organic solvents and forms thin films. In thin films, the tetradodecyloxy-substituted c-OBCB self-assembles to form the active layer in organic field effect transistors. It is a hole transporting organic semiconductor. In the transistors, the c-OBCB forms good contact with source and drain contacts made from graphene. The c-OBCB self-assembles into a heterojunction from solution with phenyl-C ₇₀-butyric acid methyl ester (PC₇₀BM). We observed power conversion efficiencies of ～2.9 % under 100 mW cm^-2 illumination at a 1:4 weight ratio of the c-OBCB relative to PC₇₀BM. The c-OBCB is shape complementary to the ball shaped PC₇₀BM.

Full text of DOI:10.1039/c3sc50374g

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Supersized contorted aromatics†
Shengxiong Xiao,‡a Seok Ju Kang,‡b Ying Wu,‡b Seokhoon Ahn,be Jong Bok Kim,c
Cite this: DOI: 10.1039/c3sc50374g
c
d
b
a
*
Yueh-Lin Loo, Theo Siegrist, Michael L. Steigerwald, Hexing Li
ab
*
and Colin Nuckolls
We describe here the synthesis and electronic device properties of a new type of polycyclic aromatic
molecule, the contorted octabenzocircumbiphenyl (c-OBCB). Contorted polycyclic aromatic
hydrocarbons (PAHs) are promising small active molecules for organic devices. We present two different
methods to synthesize c-OBCB derivatives that allow the smooth incorporation of functional groups.
The material has a highly contorted exterior with six 4-helicenes and two 5-helicenes around the
exterior of the expanded core of the aromatic. With appropriate sidechains, the material is soluble in
common organic solvents and forms thin films. In thin films, the tetradodecyloxy-substituted c-OBCB
self-assembles to form the active layer in organic field effect transistors. It is a hole transporting organic
semiconductor. In the transistors, the c-OBCB forms good contact with source and drain contacts made
from graphene. The c-OBCB self-assembles into a heterojunction from solution with phenyl-C70-butyric
acid methyl ester (PC70BM). We observed power conversion efficiencies of ꢀ2.9 % under 100 mW cmꢁ2
Received 7th February 2013
Accepted 1st March 2013
DOI: 10.1039/c3sc50374g
illumination at
a 1 : 4 weight ratio of the c-OBCB relative to PC70BM. The c-OBCB is shape
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complementary to the ball shaped PC70BM.
Here we describe the synthesis, structure, and self-assembly adopt unique p-to-p contacts that produce active materials in
in electrical devices for the largest member of the contorted applications that require charge transport.1,4c,d,6 In addition,
aromatics synthesized to date, the contorted octabenzo- these contorted aromatics are shape- and size-complementary
circumbiphenyl (c-OBCB).1 Polycyclic aromatic hydrocarbons to fullerenes in thin lms and co-crystals. The association
(PAHs) are aesthetically beautiful, synthetically challenging, between contorted aromatics and fullerenes produces efficient
and fundamentally interesting.2 More importantly, PAHs have photovoltaic cells.7
an increasing role as an active element in molecular elec-
tronic devices3 such as eld-effect transistors (FETs)4 and OPV's the smallest of this class of materials, the contorted
solar cells.5 hexabenzocoronene (c-HBC).1,4c,d,6a,8 While the c-HBC and its
Until this study, we have only synthesized and tested in
One strategy that we have employed when making electronic derivatives are efficient in solar cells,7 they are limited by the
materials from PAHs is to introduce strain into their molecular poor overlap of their absorption proles with the solar spec-
structure that forces the appended rings to adopt a ruffled trum.8 Here we describe the synthesis of a new type of contorted
conformation. The peripheral groups that are bent out of the PAH that has eight aromatic rings appended around a circum-
ring plane produce several advantageous consequences for the biphenyl core (c-OBCB).
materials properties. In thin lms and crystals, they are able to
Furthermore, we describe their behavior in solar cells and
thin lm transistors. In transistors, the large aromatic face of
the c-OBCB allows it to make efficient contact to single-layer
graphene that we use as the OFET source and drain electrodes.
The c-OBCB, like the c-HBC, forms a complementary structure
with fullerenes and provides solar cells that are more efficient
than the c-HBC.
aDepartment of Chemistry, Optoelectronic Nano Materials and Devices Institute,
Shanghai Normal University, Shanghai, China. E-mail: hexing-li@shnu.edu.cn;
cn37@columbia.edu
bDepartment of Chemistry, Columbia University, New York, NY, USA
cDepartment of Chemical and Biological Engineering, Princeton University, Princeton,
NJ, USA
dDepartment of Chemical and Biomedical Engineering, and National High Magnetic
Design
Field Laboratory, Tallahassee, FL, USA, 32310
eInstitute of Advanced Composite Materials, Korea Institute of Science and Technology,
Eunhari san 101, Bongdong-eup, Wanju-gun, Jeollabukdo, Republic of Korea
† Electronic supplementary information (ESI) available. CCDC reference number
923610. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3sc50374g
Fig. 1 shows the DFT-generated structure9 of the lowest energy
conformer of the c-OBCB. The structure is similar to the c-HBC
in many respects but has some important differences. Like the
c-HBC, the c-OBCB has six benzophenanthrenes arrayed around
the exterior of the circumbiphenyl core. The c-OBCB has two
‡ These authors contributed equally to the work.
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both syntheses is 3 shown in Scheme 1. It consists of three
tetrasubstituted double bonds separated by two six-membered
rings. Given the crowded structure of this tris-olen, we grew
crystals to investigate its conformational preference. Fig. 2
displays the structure obtained crystallographically. The two
pentacene-like units along the short axis of the molecule are
bent into opposite directions due to steric crowding. The three
double bonds along the long axis of the molecule adopt a “zig–
zag” conformation.
Each of the olens 3a–c photocyclizes to the c-OBCB skeleton
under the Katz-modied Mallory photocyclization conditions
(Scheme 1).11 The yield is 83%, 80% and 82% for 1a, 1b, and 1c,
respectively. The derivatization of this type of c-OBCBs is
simple, following our previously reported c-HBC syntheses.1,6a
We easily include functional groups such as the solubilizing
alkoxyl chains.
Scheme 2 displays the two routes we developed to produce
tris-olen (3) and its derivatives. Scheme 2A utilizes the
thioketone (4) and diazoalkane (5) to produce coupling
Fig. 1 The energy minimized structure from DFT calculations for c-OBCB 1a.
Hydrogens have been removed to clarify the view.
partners for
a
Barton–Kellogg olenation.12 Using this
method we were able to demonstrate that these large
aromatics have unusual properties as proton sensors in thin
lm devices.13
additional [5]-helicenes around its exterior. In this conforma-
tion the molecule is chiral. We found two other conformations
by DFT. They are shown in the ESI (Fig. S1).† These two other
geometries are essentially isoenergetic, and each is ꢀ21 kcal
molꢁ1 higher in energy than the one shown in Fig. 1.
There are no reported syntheses of circumbiphenyls except
for the original 1972 Clar synthesis of the parent circum-
biphenyl.10 The harsh conditions employed in the Clar synthesis
prevent using it to make functionalized derivatives. Our strategy
was to synthesize the parent with solubilizing chains and to
study its property in thin lm devices.
The synthesis in Scheme 2A is step-intensive. The alter-
nate synthesis in Scheme 2B obviates the synthesis of either
the thioketone or the diazoalkane by utilizing the dione (6).14
The dione is prepared in two steps from commercially
available reagents.14 Subjecting this dione to Ramirez ole-
nation15,16 creates the versatile tetrabromide (7). 7 is a yellow
solid that has poor solubility in common organic solvents at
room temperature, and thus can be puried simply by
washing with toluene and ltering. Fortunately, compound 7
is soluble enough to allow the subsequent reactions to occur.
The synthesis of the tris-olen (3) can be completed easily
with the Suzuki coupling17,18 of 7 with substituted aryl-
boronic esters.
Synthesis of c-OBCBs
We developed two syntheses for c-OBCB. Full synthetic details
are contained in the ESI.† The key common intermediate in
Fig. 2 Structure of tris-olefin 3a from single-crystal X-ray diffraction. Hydrogen
atoms have been removed to clarify the view. Crystals were grown by slow
diffusion of methanol into a solution of 3a in 1,2,4-trichlorobenzene.
Scheme 1 The photocyclization of the tris-olefins (3a–c) to c-OBCBs (1a–c). (a)
hv, I2, propylene oxide, anhydrous benzene.
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Fig. 3 UV-vis (red) and fluorescence (green curve, excited at 433 nm) spectros-
copy of c-OBCB 1b in o-xylene at ꢀ10ꢁ6 M concentration with a path length of l ¼
1 cm. The inset shows a 5 mM solution of 1b in chloroform.
The absorbance and emission maxima for the c-OBCBs are red-
shied by ꢀ65 nm when compared to the smaller c-HBCs.8
Scheme 2 Synthesis of 3 employing: (A) Barton–Kellogg olefination or (B)
Ramirez olefination/Suzuki coupling sequence. (a) Dichloromethane in the dark
followed by PPh3, xylenes, reflux; (b) CBr4, PPh3, toluene, 80 ꢂC; (c) substituted aryl
boronic esters, Pd(PPh3)4, Na2CO3, DME/H2O, 100 ꢂC.
Characterization in OFETs
We created OFETs from the c-OBCB 1b in a device where the
source, drain, and active layer were transfer printed. We use
graphene source and drain contacts that were pattern transfer
printed using a process we described earlier.19 The c-OBCB
solution in o-xylene solvent was spin cast onto 20 mm lines
patterned in a PDMS mold that was transferred onto the gra-
phene electrodes on a silicon wafer with 300 nm of silicon oxide
on its surface. The silicon serves as a global back gate for the
Molecular and materials characterization
The unsubstituted c-OBCB (1a) is a red powder that has limited
solubility in common organic solvents (ꢀ10 mg mLꢁ1 in 1,1,2,2-
tetrachloroethane). It precipitates from the benzene solution
upon the completion of the photocyclization. We synthesized
derivatives 1b with four sidechains because the corresponding
c-HBC forms a columnar liquid crystalline lm that acts as a
p-type organic semiconductor.1 The c-HBC that corresponds to
1c with eight sidechains forms nanoscale cables that can be
placed in devices.6a
As expected the sidechains drastically increase solubility. 1b
and 1c have a solubility of over 100 mg mLꢁ1 and 200 mg mLꢁ1
in chloroform, respectively. The melting point decreased
dramatically with dodecyloxy substitution on the c-OBCB core.
1a melts above 550 ꢂC. 1b melts below 200 ꢂC. 1b forms rod-like
nanostructure from mixtures of methylene chloride and meth-
anol. 1c is a waxy-oil that melts near room temperature.
Fig.
3 displays the UV-visible (UV-vis) and photo-
luminescence (PL) spectra of 1b in o-xylene at ꢀ10ꢁ6 M. It shows
strong absorbance at 411 nm, 433 nm and 493 nm. The strong
absorptions below 350 nm are due to the pendant phenyl
groups. The peaks at 411 nm and 433 nm are the b-bands of c-
OBCB. The peak at 493 nm is the p-band. The weak absorptions
at 550 nm are most likely due to (radialene p)–(radialene p*)
triplets. The assignment of these transitions to a triplet state is
supported by the PL spectrum shown in Fig. 3. Moreover,
transition moment calculations show that the lowest-energy
excitations are strongly allowed. So, weak absorptions must be
due to otherwise forbidden processes. The uorescence spectra
collected by excitation at 433 nm showed two strong emission
Fig. 4 Transfer (a) and output (b) curve of patterned c-OBCB 1b transistor. Inset
of (a) shows an optical microscope image of c-OBCB 1b channel bridged between
peaks at 549 nm and 593 nm, along with a small peak at 645 nm. CVD-graphene S/D.
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devices. The inset in Fig. 4 shows the active layer located
between a pair of graphene electrodes. The important point is
that both the active layer of the c-OBCB and the graphene
electrodes can be transfer printed in the working devices.
Fig. 4a and b display the transfer and output characteristics
of the OFET. We calculate the eld effect mobility from the
transfer characteristics (Fig. 4a) to be 0.002 cm2 Vꢁ1 sꢁ1
.
20
Fig. 4b shows the representative output characteristics of the
OFET with increasing gate eld (Fig. 4b). The lm displays an
increase of IDS with negative gate bias due to the accumulation
of holes in the molecular lm; thus c-OBCB 1b transports holes
readily. The micropatterned c-OBCB lm reduces the gate
leakage current and operates well with the single layer graphene
electrodes. In addition, the graphene S/D electrodes provide
very low contact resistance with the c-OBCB lm as seen in the
low bias region of Fig. 4a. The low contact resistance is due to
the large PAH making good contact to graphene electrodes and
to the intrinsic work function adjustment of graphene.21
Characterization in OPVs
Our previous studies on the contorted aromatics in solar cells7
have shown that a specic association between the donor and
acceptor is important for efficacious devices spun from solu-
tion.7 We mix the c-OBCB 1b with varying amounts of the n-type
phenyl-C70-butyric acid methyl ester (PC70BM). We suppose that
the curved structure of the c-OBCB will be able to make a ball
and socket interaction with the convex guest. Stern–Volmer
analysis22 of the uorescence quenching reveals an association
constant of ꢀ5 ꢃ 104 Mꢁ1 in chloroform. The data is contained
in the ESI (Fig. S2†).
We next test whether the 1b/PC70BM complex when cast into
lms would form the active layer in a photovoltaic device. We
measure the HOMO and LUMO levels of the c-OBCB using cyclic
voltammetry in Fig. S3† with a ferrocene standard. The c-OBCB
1b:PC70BM mixture solution (solvent: o-xylene) was spin cast on
the top of ꢀ30 nm PEDOT:PSS/ITO substrate. Fig. 5a shows the
energy diagram of each layer in the nal device architecture. A
TiOx layer was employed on the top of the c-OBCB 1b:PC70BM
layer prior to evaporation of ꢀ60 nm aluminum counter elec-
trode.23 We tested several ratios of the donor and acceptor and
found optimal performance at 1 : 4 weight ratio of the donor to
acceptor (see ESI, Fig. S4† for results based on other ratios).
Fig. 5b displays the J–V curves for the blended lms. The open
circuit voltage (Voc) is 0.98 V and very close to what would be
predicted theoretically from the energy level alignment in Fig. 5a.
The short-circuit current (Jsc) is 7.9 mA cmꢁ2 and the ll factor is
0.37. These values result in a 2.88% power conversion efficiency.
The average value is 2.86% from twelve devices. The external
Fig. 5 (a) Energy level diagram of the device. Current density–voltage (J–V)
characteristics (b) and EQE spectrum (c) of c-OBCB 1b:PC70BM solar cell.
Conclusion
quantum efficiency (EQE) spectra in Fig. 5c shows the photo- We describe here the synthesis and electronic device properties of
current response of c-OBCB 1b:PC70BM solar cell devices as a a new type of polycyclic aromatic molecule. We reveal two different
function of wavelengths. We observe a red shied maximum methods to synthesize c-OBCB derivatives that allow the smooth
wavelength in EQE spectra compared with contorted c-HBC and incorporation of functional groups. The material has a highly
its derivatives.7 The red-shi in absorbance is responsible for the contorted exterior with six 4-helicenes and two 5-helicenes around
higher PCE for c-OBCB compared to the smaller c-HBC series and the exterior of the expanded core of the aromatic. With appro-
charts a clear path to improving the properties of these materials priate sidechains, the material is soluble in common organic
in OPVs by further red-shiing the absorbance.
solvents and forms thin lms. The tetradodecyloxy-substituted
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c-OBCB self-assembles to form the active layer in organic eld
effect transistors. The c-OBCB also self-assembles into a hetero-
junction from solution with PC70BM.
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Acknowledgements
Design synthesis, OFET characterization, and initial OPV
properties were performed at Columbia University as part of the
Center for Re-Dening Photovoltaic Efficiency through Molec-
ular Scale Control, an Energy Frontier Research Center funded
by the U.S. Department of Energy, Office of Science, Office of
Basic Energy Science under Award Number DE-SC0001085. OPV
characterization was performed at Princeton University, sup-
ported by the Photovoltaics Program at ONR (N00014-
11010328), and the SOLAR Initiative at the NSF (DMR-1035217),
and an NSF sponsored MRSEC through Princeton Center for
Complex Materials (DMR-0819860). Crystal structure determi-
nation was performed at the National Magnetic Field Labora-
tory which is supported by the National Science Foundation
Cooperative Agreement (DMR-1157490), the State of Florida,
and the U.S. Department of Energy.
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was acquired using a Keithley 2635 source measurement
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