Synthesis and characterization of 2,7-bis(pentafluorophenylethynyl) hexafluoroheterofluorenes: New materials with high electron affinities

Article abstract of DOI:10.1039/b813440e

The synthesis, optical, electrochemical and physical characterization of 2,7-bis(pentafluorophenylethynyl)hexafluoroheterofluorenes is described along with a preliminary evaluation of their performance in photovoltaic applications. The Royal Society of Chemistry.

Full text of DOI:10.1039/b813440e

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COMMUNICATION
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Synthesis and characterization of 2,7-bis(pentafluorophenylethynyl)-
hexafluoroheterofluorenes: new materials with high electron affinitiesw
Katharine Geramita,a Jennifer McBee,a Yuefei Tao,b Rachel A. Segalmanb and
T. Don Tilley*a
Received (in Cambridge, UK) 4th August 2008, Accepted 9th September 2008
First published as an Advance Article on the web 30th September 2008
DOI: 10.1039/b813440e
The synthesis, optical, electrochemical and physical character-
ization of 2,7-bis(pentafluorophenylethynyl)hexafluorohetero-
fluorenes is described along with a preliminary evaluation of
their performance in photovoltaic applications.
reactions. The availability of the key 4,40-bisethynyl-2,
20-dibromo-3,30,5,50,6,60-hexafluorobiphenyl
intermediate
(Scheme 1), which is readily coupled to aryl halides (vide infra),
greatly facilitates incorporation of the hexafluorohetero-
fluorene fragment into a variety of extended conjugated
organic systems (e.g. compounds 1–4). Here we describe use
of this method to obtain several molecular species with high
electron affinities, and preliminary results on their behavior as
n-type components in photovoltaic devices.
The use of organic semiconducting materials for applications in
‘‘plastic electronics’’ continues to attract considerable interest, as
organic systems offer the possibility for economical raw materi-
als and low processing costs, and are readily modified to access a
wide range of physical, optical and electrical properties.1–4 In
contrast to the development of improved conjugated organic
p-systems that demonstrate effective hole-transporting (p-type)
properties,5 the advancement of efficient electron-conducting (n-
type) organic compounds has been slow. A number of devices
are expected to exhibit improved performance with incorpora-
tion of n-type organic materials that exhibit a low barrier to
electron injection and effective charge transport.4,6 To achieve
this, compounds with electron-deficient p-systems and close
cofacial intermolecular packing in the solid state are desired.7–13
In general, many of the electron-deficient building blocks
(i.e. pyridines, oxadiazoles, perylene diimides, metalloles,
fluorinated aromatics) used to construct n-type materials are
difficult to synthetically manipulate, as required for the tuning
of important physical and electronic properties.7–12 A new
class of compounds, the perfluoroheterofluorenes (structures
2–4, Scheme 1), includes a number of interesting structural
units that are expected to exhibit good electron-accepting
properties and cofacial p-stacking in the solid state (vide infra).
While synthetic variation of the heteroatom (and its substitu-
ents) is relatively straightforward, substitutions on the fluori-
nated biaryl framework are more challenging since the
typically employed nucleophilic aromatic substitution reac-
tions are very sensitive to minor variations in substrate.9a,14 In
this communication, we describe a facile, general synthetic
The synthesis of 2,7-bis(pentafluorophenylethynyl)hexafluoro-
heterofluorenes is described in Scheme 1. The alkynyl functionality
was installed via a nucleophilic aromatic substitution reaction
of triisoproylsilyl lithium acetylide with 2,20-dibromo-
octafluorobiphenyl. Use of a 4 : 1 toluene : THF solvent
mixture at B2 ꢀ 10ꢁ2 M concentration gave the 4,40-bis-
(triisopropylsilylethynyl)-2,20-dibromo-3,30,5,50,6,60-hexafluorobi-
phenyl product in moderate conversion (B65% as
determined by 19F NMR) after 3 days at room temperature.
Longer reaction times increased the conversion to the
disubstituted product; however, elevated temperatures led to
a mixture of mono-, di- and trisubstituted material. The
triisopropylsilyl groups were removed by the addition of
Bu4NF to a dilute (0.0001 M) toluene–THF solution (B2%
THF) of 4,40-bis(triisopropylsilylethynyl)-2,20-dibromo-3,30,
5,50,6,60-hexafluo robiphenyl. Aqueous work-up and
filtration through a silica plug gave a crude reaction mixture
predominantly containing the key 4,40-bisethynyl-2,20-
dibromo-3,30,5,50,6,60-hexafluorobiphenyl intermediate that
was then capped with C6F5 groups by a Pd catalyzed
Sonogashira coupling reaction with C6F5I at 45 1C to give 1.
No conversion to 1 was observed when C6F5Br was used as the
aryl halide. Compound 1 was isolated via column chromato-
graphy in 55% yield based on the starting 2,20-dibromoocta-
fluorobiphenyl. Lithiation of 1 with BuLi in Et2O–THF
at ꢁ78 1C, followed by addition of the appropriate
route to
fluorenes, which utilizes
a
variety of 2,7-substituted hexafluorohetero-
combination of nucleophilic
a
aromatic substitution (SNArF) and Pd-catalyzed coupling
a Department of Chemistry, University of California at Berkeley,
Berkeley, California 94720, USA. E-mail: tdtilley@berkeley.edu
b Department of Chemical Engineering, University of California at
Berkeley, Berkeley, California 94720, USA
w Electronic supplementary information (ESI) available: Experimental
details, absorption, emission and electrochemical data, crystallo-
graphic data (3a, 4a) and current/voltage data for the PV devices.
CCDC reference numbers 697514, 697515. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
b813440e
Scheme 1
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Table 1 Select optical and electrochemical characterization data
LUMO
energy/eVd
Egopt/eVa
l
ems/nmb
FPL
c
Compound
2a
2b
3a
3b
4a
4b
a
3.2
3.3
3.3
3.3
3.1
3.2
382
378
378
376
385
391
B1
B1
B1
B1
B1
B1
ꢁ3.2e
ꢁ3.2e
ꢁ3.2f
ꢁ3.2e
ꢁ3.5g
ꢁ3.4g
HOMO–LUMO energy gaps (Egopt) were calculated from the absorp-
tion onset wavelength, determined by the intersection of the leading edge
b
tangent with the X-axis. Emission energies were measured with 350 nm
c
excitation. Dilute solution state (optical density 0.1) photoluminescence
quantum yields calculated with respect to 9,10-diphenylanthracene in
d
THF = 0.9. In CH3CN electrolyte solution (0.1 M solution of Bu4NPF6
in CH3CN) and LUMO energy levels calculated with respect to ferroce-
nium–ferrocene (Fc+/Fc) reduction (Fc HOMO energy of ꢁ4.8 eV
f
g
relative to vacuum).20 e Irreversible. Quasireversible. Reversible.
dichloroheteroatom reagent, produced the desired hetero-
fluorene product in quantitative yield (as determined by 19F
NMR), and isolated yields are good in all cases except with
dichlorodiphenylsilane. With the latter reagent, the reaction pro-
ceeds to 65% conversion resulting in low isolated yields (o10%).
The solution state UV-vis data collected for all compounds
are summarized in Table 1. UV-visible absorption behavior of
the disubstituted hexafluoroheterofluorenes exhibit very little
dependence on the heteroatom, and possess HOMO–LUMO
energy gaps of 3.1–3.3 eV. Furthermore, the energy gaps are
rather insensitive to changes of substituents at the heteroatom
(Ph vs. alkyl); thus modifications of these groups to tune
physical properties should not greatly perturb the electronic
properties of interest. Strong violet emission, with a Stokes shift
of 25 to 30 nm, was observed for all compounds (Table 1). It
has been observed that the photoluminescent quantum yield
(FPL) may be increased or decreased upon fluorination, depend-
ing on both the molecular framework and the degree of
fluorination.8,9 The vibronic coupling observed in both the
absorption and emission spectra (see ESI for detailsw) was
independent of heteroatom. The specific origin of this coupling
has not been explored but it is quite typical for many organic
systems. Although optical properties for the non-fluorinated
analogues of compounds 2–4 have not been reported, polymers
and oligomers containing fluorene and heterofluorene units are
highly emissive;15,16 thus the near unity emission intensity of the
reported 2,7-bis(pentafluorophenylethynyl)hexafluorohetero-
fluorenes indicates that fluorination of the terminal rings does
not diminish the intensity of the emitted light.
Fig. 1 Crystal structures for germafluorene 3a (Fig. 1a) and phos-
phafluorene 4a (Fig. 1b). Hydrogen atoms have been removed for
clarity. View (a) illustrates the unidirectional p-stacking of 3a and view
(b) illustrates the bidirectional interactions of 4a.
CH3CN). It was observed that the reduction potentials of the
disubstituted hexafluoroheterofluorenes exhibit a slight depen-
dence on the heteroatom itself (Si, Ge vs. P), but as was
opt
observed for the Eg
values, they are rather insensitive to
the nature of the substituents at the heteroatom (aryl vs. alkyl).
Single crystal X-ray analyses of 3a and 4a (Fig. 1) show that
both molecules possess planar p systems that pack to give
stacked columns with parallel heterofluorene fragments.21
Germafluorene 3a packs in hexagonally arranged columns
with close p-stacking interactions (3.2–3.4 A; Fig. 1a) and
significant interaction between adjacent heterofluorene cores.
This motif is thought to be ideal for many electron transporting
applications.7–13 The packing of phosphafluorene 4a (Fig. 1b and
Fig. S11w) illustrates how changes in the heteroatom substituent
can lead to dramatically different packing characteristics.
Although 4a maintains a heterofluorene-layered structure, the
molecules stack in two nearly perpendicular directions throughout
the crystal, unlike the unidirectional assembly of 3a, with p-systems
that do not exhibit direct interactions. The latter structural feature
may be described by an interplane distance between the hetero-
fluorenes of ca. 6 A, and a tilting of the molecular stacks such that
the molecules are offset laterally from one another to avoid a
cofacial arrangement. It is also worth noting short O1ꢃ ꢃ ꢃC24 and
O1ꢃ ꢃ ꢃC25 contacts (B2.8 A) which suggest that an interaction
between the electron-rich oxygen atom and the electron-deficient
p-system influence the solid state assembly.
The solution state redox behavior of all compounds was
investigated via differential pulse voltammetry (reduction po-
tentials) and cyclic voltammetry (reversibility) and the LUMO
energy levels are reported in Table 1. The LUMO energies
range from ꢁ3.2 eV to ꢁ3.5 eV and the reversibility of these
reductions is highly dependent on heteroatom. These LUMO
energy levels are significantly lower than those of related
heterofluorene compounds previously reported in the litera-
ture (ꢁ2.1 to ꢁ2.6 eV),16–18 and are in the range established
for useful n-type conducting materials.4,11,17–19 The oxidation
behavior of these compounds was not observed within the
solvent/electrolyte window (0.1 M solution of Bu4NPF6 in
As a preliminary proof of concept that the compounds may
work well as electron transporting materials, device studies were
performed to evaluate germafluorene 3b and phosphafluorene
4b as electron transport materials in a basic photovoltaic (PV)
configuration. Unoptimized PV cells were made from 1 : 1 wt%
blends of 3b–regioregular poly-3-hexylthiophene (P3HT) and
4b–P3HT that were spin-coated onto an ITO surface from
chloroform solution (10 mg mLꢁ1, B80 nm thick films) with
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Notes and references
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Fig. 2 Photovoltaic performance of spin-coated devices with P3HT as the
hole transport material and 3b (blue) or 4b (red) as the electron transport
material (performance for 3b: Voc = 0.905 V, Jsc = 0.211 mA cmꢁ2, fill
factor = 0.184, Z = 0.035%; 4b: Voc = 0.743 V, Jsc = 0.064 mA cmꢁ2, fill
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factor = 0.255, Z = 0.012%; P3HT-only: Voc = 0.04 V, Jsc
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ꢁ0.044 mA cmꢁ2, fill factor = 0.253, Z = 0.0004%.)
a vapor deposited aluminium cathode. The devices were tested
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an AM 1.5G solar filter, and the resulting current density–
voltage curves are illustrated in Fig. 2 (additional current–vol-
tage data are available in the ESIw).
As illustrated in Fig. 2, the PV devices made with blends of
3b–P3HT and 4b–P3HT clearly exhibit improved performance
over the P3HT-only devices. This can be attributed to a much
higher open circuit voltages (Voc) in the blend systems (0.905 V
and 0.743 V respectively for the devices containing 3b and 4b,
vs. 0.040 V for pure P3HT), which result in greater charge
separation than in the pure P3HT system. In fact, these Voc
values are greater than the typical values observed for the
highest efficiency P3HT–PCBM devices (B0.65 V, PCBM =
[6,6]-phenyl-C61-butyric acid methyl ester).22 Additionally, the
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significantly larger than that of the pure P3HT system, which
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charge transport in the devices. The efficiencies of these devices
are rather low (0.035% for 3b and 0.012% for 4b) compared to
those obtained for optimized devices based on P3HT–PCBM
(4–5%)21 and P3HT–perylene diimide (0.5–1%).23 Further
modifications to the structures of such fluorinated, hetero-
fluorene-based compounds should provide increased light
absorption and more efficient charge transport.24 Also, opti-
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conditions, etc.) and device assembly (e.g. cathode material,
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21 Crystal data for 3a: C40H10F16Ge, M = 867.07, triclinic, a =
7.922(2), b = 15.049(3), c = 15.823(3) A, U = 1626.1(8) A3, T =
ꢀ
118 K, space group P1, Z = 2, 9156 reflections measured, 3384
unique (Rint = 0.1023) were used in all calculations. The final
wR(F2) = 5.45. CCDC 697514. 4a: C34H5F16OP, M = 764.35,
monoclinic, a = 13.798(13), b = 5.938(5), c = 35.27(3) A, U =
2840(5) A3, T = 140 K, space group P21/n, Z = 4, 9224 reflections
measured, 2955 unique (Rint = 0.0857) which were used all
calculations. The final wR(F2) = 4.20. CCDC 697515.
In summary, 2,7-bis(pentafluorophenylethynyl)hexafluoro-
heterofluorenes show great potential as electron transporting
organic materials, in applications such as organic PV and LED
devices. The availability of the 4,40-bisethynyl-2,20-dibromo-
3,30,5,50,6,60-hexafluorobiphenyl synthetic intermediate, and
its ready conversion to heterofluorene derivatives, demon-
strates the versatility of these compounds and enables the
convenient incorporation of this fragment into a wide variety
of extended organic systems.
22 S. A. Backer, K. Sivula, D. F. Kavulak and J. F. J. Fre
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24 Target difference between donor LUMO energy (LUMOD) and
acceptor LUMO energy (LUMOA) is B0.5 eV. For 3b–P3HT
D(LUMOD ꢁ LUMOA) is 0.1 eV and for 4b–P3HT D(LUMOD
ꢁ
This work was supported by the National Science Foundation
research grant CHE0314709, the DuPont Center for Collaborative
Research and Education and the DOE-BES Plastic Electronics
Program at Lawrence Berkeley National Laboratories.
LUMOA) is 0.2 eV; J. G. S. Ramon and E. R. Bittner, J. Phys.
Chem. B, 2006, 110, 15.
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´
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ꢂc
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