2,7-substituted hexafluoroheterofluorenes as potential building blocks for electron transporting materials

Article abstract of DOI:10.1021/jo802171t

A series of 2,7-substituted hexafluoro-9-heterofluorenes was synthesized via nucleophilic aromatic substitution (S_NAr_N) reactions of phenyllithium, thienyllithium, and lithium phenylacetylide with various octafluoroheterofluorenes and 2,2'-dibromooctafluorobiphenyl. These compounds are of interest as possible building blocks for materials with useful electron transport properties, since they possess relatively low LUMO energy levels. The HOMO-LUMO energy gaps, as determined by UV-vis spectroscopy, range between 3.0 and 3.9 eV, while photoluminescence emission spectra reveal λ _ems values in the range of 365 to 420 nm (corresponding to ultraviolet to violet/blue emission). Dilute solution state quantum yields vary significantly with the nature of the heteroatom and the 2,7-substituents, and approach unity for a number of the di(phenylethynyl) derivatives. The experimentally determined LUMO energy levels (- 2.7 to - 3.3 eV as determined by differential pulse voltammetry) suggest that these compounds may be good candidates for electron transport applications. Single-crystal X-ray analyses of a number of compounds revealed cofacial packing in all cases, with intermolecular distances as short as 3.4 A.

Full text of DOI:10.1021/jo802171t

2,7-Substituted Hexafluoroheterofluorenes as Potential Building
Blocks for Electron Transporting Materials
Katharine Geramita, Jennifer McBee, and T. Don Tilley*
Department of Chemistry, UniVersity of California at Berkeley, Berkeley, California 94720
tdtilley@berkeley.edu
ReceiVed September 28, 2008
A series of 2,7-substituted hexafluoro-9-heterofluorenes was synthesized via nucleophilic aromatic
substitution (SNArF) reactions of phenyllithium, thienyllithium, and lithium phenylacetylide with various
octafluoroheterofluorenes and 2,2′-dibromooctafluorobiphenyl. These compounds are of interest as possible
building blocks for materials with useful electron transport properties, since they possess relatively low
LUMO energy levels. The HOMO-LUMO energy gaps, as determined by UV-vis spectroscopy, range
between 3.0 and 3.9 eV, while photoluminescence emission spectra reveal λems values in the range of
365 to 420 nm (corresponding to ultraviolet to violet/blue emission). Dilute solution state quantum yields
vary significantly with the nature of the heteroatom and the 2,7-substituents, and approach unity for a
number of the di(phenylethynyl) derivatives. The experimentally determined LUMO energy levels (-2.7
to -3.3 eV as determined by differential pulse voltammetry) suggest that these compounds may be good
candidates for electron transport applications. Single-crystal X-ray analyses of a number of compounds
revealed cofacial packing in all cases, with intermolecular distances as short as 3.4 Å.
Introduction
Useful n-type organic materials possess a low barrier to
electron injection and effectively transport a negative charge.
While it is generally accepted that a low-energy LUMO should
facilitate charge injection, the attributes that contribute to high
electron mobility are not well understood.7 Electron-deficient
π-systems are thought to be associated with high electron
mobility, and pyridines, oxadiazoles, perylene diimides, met-
alloles, and fluorinated aromatics have been identified as
potential n-type materials.8-13 Additionally, close cofacial
intermolecular packing has been associated with increased
charge transport in a number of organic systems.14 Therefore,
molecular organic building blocks that contain an electron-
deficient, extended π-system, a stabilized LUMO, and close
cofacial stacking in the solid state would be of great interest to
the development of this field.
The use of organic semiconducting materials for applications
in “plastic electronics”, such as field effect transistors (FETs),
organic light emitting diodes (OLEDs), and photovoltaics (PVs),
continues to attract considerable interest.1 Organic systems offer
the possibility for cheap raw materials and low processing costs,
and are readily tailored to access a wide range of physical,
optical, and electrical properties in the final device.1-4 In
general, conjugated organic π-systems are effective hole-
transporting (p-type) materials;5 however, the development of
improved devices (e.g., FET and PV) should be facilitated by
the introduction of efficient electron-conducting (n-type)
materials.4,6
(1) (a) Forrest, S. R. Chem. ReV. 1997, 97, 1793. (b) Jenekhe, S. Chem. Mater.
2004, 16 (23), 4382 (special issue on organic electronics). (c) Forrest, S. R.;
Thompson, M. E. Chem. ReV. 2007, 107 (4), 923 (special issue on organic
electronics).
(2) Hwang, D. H.; Kim, S. K.; Park, M. J.; Koo, B. W.; Kang, I. N.; Kim,
S. H.; Zyung, T. Chem. Mater. 2004, 16, 1298.
(4) Zhu, Y.; Alam, M. M.; Jenekhe, S. A. Macromolecules 2003, 36, 8958.
(5) Renak, M. L.; Bartholomew, G. P.; Wang, S. J.; Ricatto, P. J.; Lachicotte,
R. J.; Bazan, G. C. J. Am. Chem. Soc. 1999, 121, 7787.
(6) Tonzola, C. J.; Alam, M. M.; Kaminsky, W.; Jenekhe, S. A. J. Am. Chem.
Soc. 2003, 125, 13548.
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H.; Friend, R. Nature 2005, 434, 194. (b) Zaumseil, J.; Sirringhaus, H. Chem.
ReV. 2007, 107, 1296.
(3) Schwartz, M.; Srinivas, G.; Yeates, A.; Berry, R.; Dudia, D. Synth. Met.
2004, 143, 229.
820 J. Org. Chem. 2009, 74, 820–829
10.1021/jo802171t CCC: $40.75 2009 American Chemical Society
Published on Web 12/09/2008

Potential Building Blocks for Electron Transporting Materials
CHART 1
Metalloles, which are five-membered heterocycles containing
a heteroatom and a diene moiety, are associated with low-energy
LUMO levels, resulting from the stabilization of the diene
π-system via interaction with the heteroatom-based σ* oribtals.15,16
The LUMO energy level is generally unaffected by minor
variations in the R groups on the heteroatom (vida infra), which
can be changed to modify solubility and packing properties.
Siloles in particular have exhibited exceptionally good perfor-
mance in OLED devices but often exhibit poor intermolecular
cofacial interactions due to the orientation of the aryl groups
that usually occupy the 3- and 4-positions.10a,17,18 The analogous
sulfur (thiophene) and phosphorus (phosphole)-based systems
have shown great promise in a number of electronic applica-
tions.16 Often thiophene- and phosphole-based systems function
as hole transporting materials; however, it is possible that their
incorporation into the highly electron-deficient perfluorofluorene
framework might afford interesting n-type compounds. Ad-
ditionally, the related phosphole oxide derivatives possess a
more electron-deficient π-system and should afford compounds
with high electron affinities.
SCHEME 1
Compounds containing the dibenzometallole (or metallofluorene)
fragment are interesting candidates for n-type materials, since they
possess a planar aromatic structure while maintaining the LUMO-
lowering ability of the metallole fragment. In general fluorene-
based systems are electron rich and tend to pack in a herringbone
structure.19-21 However, fluorine atom substituents in the aromatic
system may improve both electron transport and cofacial
packing.12,13,22,23 Previous work in our laboratories on the synthesis
and characterization on 2,7-bis(pentafluorophenylethynyl)hexa-
fluoro-9-heterofluorenes has demonstrated that these systems pack
with parallel heterofluorene fragments and possess stabilized
LUMO energy levels.24 Additional work in our laboratories has
demonstrated that nucleophilic aromatic substitution (SNArF) is a
(8) Kulkarni, A.; Tonzola, C.; Babel, A.; Jenekhe, S. Chem. Mater. 2004,
16, 4556.
(9) Babudri, F.; Farinola, G.; Naso, F.; Ragni, R. Chem. Commun. 2007, 10,
1003.
(10) (a) Geramita, K.; McBee, J.; Shen, Y.; Radu, N.; Tilley, T. D. Chem.
Mater. 2006, 18, 3261. (b) Tannaci, J. F.; Noji, M.; McBee, J.; Tilley, T. D. J.
Org. Chem. 2007, 72, 5567.
(11) Tamao, K.; Uchinda, M.; Izumizawa, T.; Furukawa, K.; Yamaguchi, S.
J. Am. Chem. Soc. 1996, 118, 11974.
(12) Facchetti, A.; Yoon, M. H.; Stern, C. L.; Katz, H. E.; Marks, T. J. Angew.
Chem., Int. Ed. 2003, 42, 3900.
(13) Heidenhain, S. B.; Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa, H.;
Mori, T.; Tokito, S.; Taga, T. J. Am. Chem. Soc. 2000, 122, 10240.
(14) Hutchison, G.; Ratner, M.; Marks, T. J. Am. Chem. Soc. 2005, 127,
16866.
(15) (a) Tamao, K.; Yamaguchi, S.; Shiro, M. J. Am. Chem. Soc. 1994, 116,
11715. (b) Yamaguchi, S.; Tamao, K. J. Chem. Soc., Dalton Trans. 1998, 3693.
(16) (a) Hissler, M.; Dyer, P. W.; Reau, R. Coord. Chem. ReV. 2003, 244, 1.
(b) Bumgartner, T.; Reau, R. Chem. ReV. 2006, 106, 4681.
(17) Murata, H.; Malliaras, G. G.; Uchida, M.; Shen, Y.; Kafafi, Z. H. Chem.
Phys. Lett. 2001, 339, 161.
(18) (a) Luo, J.; Xie, Z.; Lam, J.; Cheng, L.; Chen., H.; Qui, C.; Kwok, H.;
Zhan, X.; Lui, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740. (b)
Boydston, A. J.; Yin, Y.; Pagenkopf, B. L. J. Am. Chem. Soc. 2004, 126, 3724.
(19) (a) Rathnayake, H.; Cirpan, A.; Lahti, P.; Karasz, F. J. Am. Chem. Soc.
2006, 18, 560. (b) Heeney, M.; Bailey, C.; Giles, M.; Shkunov, M.; Sparrowe,
D.; Tierney, S.; Zhang, W.; McCulloch, I. Macromolecules 2004, 37, 5250.
(20) Chen, C.-H.; Shen, W.-J.; Jakka, K.; Shu, C.-F. Synth. Met. 2004, 143,
215.
highly effective method for carbon-carbon bond formation
involving perfluoroaromatic compounds, and high conversion
to a single product was observed for a variety of substrates.10a,24,25
In this report we describe the synthesis and physical, optical,
and electrochemical characterization of a variety of 2,7-
substituted hexafluoroheterofluorenes (Chart 1), prepared via
nucleophilic aromatic substitution (SNArF). The synthesis and
characterization of these electron-deficient compounds greatly
expands this class of 2,7-substituted hexafluoroheterofluorenes
beyond the more highly fluorinated 2,7-bis(pentafluorophenyl-
ethynyl)hexafluoroheterofluorenes previously communicated.24
(21) Coropceanu, V.; Nakano, T.; Gruhn, N.; Kwon, O.; Yade, T.; Katsukawa,
K.; Bredas, J.-L. J. Phys. Chem. B 2006, 110, 9482.
Results and Discussion
(22) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Hutchison, G. R.; Ratner, M. A.;
Marks, T. J. J. Am. Chem. Soc. 2004, 126, 13859.
(23) (a) Gierschner, J.; Ehni, M.; Egelhaaf, H.-J.; Medina, B. M.; Beljonne,
D.; Benmansour, H.; Bazan, G. J. Chem. Phys. 2005, 123, 144914. (b) Cho, D.;
Parkin, S.; Watson, M. Org. Lett. 2005, 7, 1067. (c) Sakamoto, Y.; Komatsu,
S.; Suzuki, T. J. Am. Chem. Soc. 2001, 123, 4643.
Synthesis of Fluorinated Heterofluorenes. The synthesis of
octafluoroheterofluorenes (Scheme 1) from 1,2-dibromotetrafluo-
robenzene followed procedures slightly modified from those of
(24) Geramita, K.; McBee, J.; Tao, Y.; Segalman, R. A.; Tilley, T. D. Chem.
Commun. 2008, 5107.
(25) Nitschke, J.; Tilley, T. D. J. Am. Chem. Soc. 2001, 123, 10183.
J. Org. Chem. Vol. 74, No. 2, 2009 821

Geramita et al.
SCHEME 2
SCHEME 3
SCHEME 4
SCHEME 5
method, which takes advantage of the electron-deficient nature
of the aromatic system and the electronegativity of the fluorine
atom.10a,24,29 Control over the position of substitution can present
a challenge for synthetic applications of SNArF. It is well-
established that the position of substitution is dictated by the
stability of the anionic intermediate, whose dominant resonance
structure is that with the negative charge localized in the position
para to substitution, as illustrated for octafluoroheterofluorenes
in eq 1.30-32 While the relative stabilities of possible intermedi-
ates are easy to predict for simple perfluorinated arenes, such
as decafluorobiphenyl or octafluoronaphthlanene, the situation
is more complex for substituted fluoroarenes.31-33
In these highly fluorinated systems 19F NMR spectroscopy
is a useful diagnostic tool, and valuable information is obtained
from the fluorine chemical shifts and the F-F coupling
constants. The chemical shift for a given fluorine atom (relative
to C6F6) appears further downfield as the number of neighboring
fluorine atoms is reduced. Additionally, very large through-bond
F-F coupling (ranging from 75-175 Hz) can be observed
between inequivalent fluorine atoms located near one another
in space, even if they are 4 or 5 bonds apart.34-39 In the work
described here, this type of large coupling (100-160 Hz) is
observed between fluorine atoms in the 4 and 5 positions of
the monosubstituted heptafluoroheterofluorene intermediates that
lead to the final disubstituted products (readily observed by
monitoring the reactions by 19F NMR spectroscopy). Observa-
tion of this F-F coupling for all monosubstituted products
indicates that substitution never occurs in the 4 position. This
is consistent with the fact that previously reported nucleophilic
aromatic substitutions on P′-F and S-F, with oxygen and
nitrogen-based nucleophiles, occur exclusively in the 2-posi-
tion.33 The disubstitutions reported here primarily occur in the
2- and 7-positions, as indicated by characterizations of four
examples by X-ray crystallography (vide infra). Indeed, sub-
stitutions in the 2- and 3-positions are expected to be favored,
since the intermediates generated by attack at the 1- or
Cohen et al. (Si-F, Si′-F, Ge-F, Sn-F)26 and Chambers et al. (P-
F).27 Their syntheses were previously reported to give low to
moderate yields, but the methods have now been optimized to give
the products in 50-90% yield. Additionally, it was possible to
use this method to synthesize octafluorothiafluorene (S-F), which
had previously been synthesized by a copper-mediated coupling
of S(2-Br-C6F4)2.28 X-ray crystal structures (vide infra) were
determined for octafluoroheterofluorenes containing silicon (Si-F,
Si′-F), germanium (Ge-F), and tin (Sn-F).
(29) (a) Burdon, J. Tetrahedron 1965, 21, 3373. (b) Wang, Y.; Watson, M. D.
J. Am. Chem. Soc. 2006, 128, 2536. (c) Ding, J.; Day, M. Macromolecules 2006,
39, 6054.
(30) Zimmerman, H. Tetrahedron 1961, 16, 169.
All 2,7-substituted hexafluoroheterofluorenes were synthe-
sized via nucleophilic aromatic substitution (SNArF) under
relatively mild reaction conditions (Schemes 2-5). Fluorinated
arenes are good substrates for this carbon-carbon bond-forming
(31) Burdon, J. Tetrahedron 1965, 21, 3373.
(32) Burdon, J.; Parsons, I. J. Am. Chem. Soc. 1977, 99, 7445.
(33) (a) Chambers, R. D.; Spring, D. J. Tetrahedron 1971, 1, 669. (b) Burdon,
J.; Kane, B. L.; Tatlow, J. C. J. Fluorine Chem. 1971/72, 1, 185.
(34) Burdon, J.; Childs, A. C.; Parsons, I. W.; Tatlow, J. C. J. Chem. Soc.,
Chem. Commun. 1982, 10, 534.
(26) (a) Cohen, S.; Fenton, D.; Tomlinson, A.; Massey, A. J. Organomet.
Chem. 1966, 6, 301. (b) Cohen, S.; Massey, A. J. Organomet. Chem. 1967, 10,
471.
(27) Chambers, R. D.; Spring, D. J. J. Fluorine Chem. 1971/72, 1, 309.
(28) Chambers, R.; Cunningham, J.; Spring, D. J. Tetraherdron 1968, 24,
3997.
(35) Bolton, R.; Sandall, J. J. Chem. Soc., Perkin 2 1978, 746.
(36) Burdon, J.; Parsons, I.; Gill, H. J. Chem. Soc., Perkin 1 1979, 1351.
(37) Burdon, J.; Gill, H.; Parsons, I.; Tatlow, J. J. Chem. Soc., Perkin 1 1980,
1726.
(38) Matthews, R. Org. Magn. Reson. 1982, 18, 266.
(39) Servis, K.; Fang, K.-N. J. Chem. Soc. 1968, 6712.
822 J. Org. Chem. Vol. 74, No. 2, 2009

Potential Building Blocks for Electron Transporting Materials
4-positions would result in high-energy pπ-pπ repulsive interac-
tions at the fluorine-substituted carbanion.32,35-37 Additional
structural information is derived from the 19F chemical shifts,
since a given fluorine atom experiences a downfield shift of
between 15 and 30 ppm upon substitution of an adjacent fluorine
atom by a carbon atom. Thus, the NMR data allow for
assignment of the chemical shift for the fluorine atom in the
1-position (resulting from substitution at the neighboring
2-position) or the 4-position (resulting from substitution at the
neighboring 3-position).37
Nucleophilic substitution on octafluorosilafluorenes Si-F and
Si′-F, and octafluorostannafluorene Sn-F, occurred less readily
and required more forcing conditions (Scheme 4). The reaction
of Sn-F with a variety of nucleophiles predominantly resulted
in decomposition, as indicated by the observation of 2,2′-
dihydrooctafluorobiphenyl in the 19F NMR spectra of crude
reaction mixtures, suggesting nucleophilic attack at tin rather
than on the aromatic ring. For the octafluorosilafluorenes the
reaction yields and the position of substitution were found to
be highly dependent on reaction conditions (solvent, nature of
the nucleophile, etc.). It is unclear why these silicon and tin
heterocycles are less well-behaved than the germanium ana-
logue, but the more electropositive nature of these elements may
render the fluorene derivative less electron deficient, and
therefore less susceptible to nucleophilic attack. Also, silyl and
stannyl groups are known to stabilize a negative charge on the
adjacent carbon,40 and this may lead to a reduction in 2,7-
selectivity for nucleophilic attack. Overall, conversions to the
disubstituted product and the 2,7-selectivities were much lower
in polar solvents, as indicated by 19F NMR spectra. The highest
product yields were achieved with toluene or hexanes as the
main solvent and a minimum amount of Et2O required to
solubilize the lithium reagent. Generally, the conversions and
the selectivities for 2,7-substitution were increased when the
silicon R′ substitutent was phenyl (vs methyl). While moderate
yields of ∼50% were obtained for the phenyl (Si-Ph, Si′-Ph)
and thienyl (Si-Th) derivatives, the di(phenylethynyl) analogues
(Si-PE, Si′-PE) were not obtained by this method.
A superior synthetic route to 2,7-substituted di(phenylethy-
nyl)silafluorene and di(phenylethynyl)stannafluorene compounds
is based on 2,2′-dibromo-4,4′-di(phenylethynyl)-3,3′,5,5′,6,6′-
hexafluorobiphenyl (1) as a starting material. This compound
was synthesized via addition of 2.3 equiv of lithium phenyl
acetylide to 2,2′-dibromooctafluorobiphenyl in THF (Scheme
5). Lithium-bromine exchange between 1 and BuLi in Et2O/
THF at -78 °C followed by addition of the appropriate
dichlorosilane produced the desired silafluorene product in good
yield (65% Si′-PE, 56% Si-PE). With this approach, it was
possible to synthesize the corresponding 2,7-di(phenylethynyl)-
substituted stannafluorene (Sn-PE, 71%), germafluorene (Ge-
PE, 40%), thiafluorene (S-PE, 40%), and phosphafluorene oxide
(P-PE, 55%) compounds. This strategy was not effective for
preparing the phenyl- or thienyl-derivatives of the stannafluorene
because lithium-bromine exchange between 1 and the nucleo-
phile was favored over SNArF. Single-crystal X-ray analyses for
Sn-PE and Si-Ph (vide infra) confirmed the connectivity of 1
and the 2,7-substitution pattern for Si-Ph, respectively.
The synthesis of phenyl-, thienyl-, and phenylethynyl-
substituted hexafluorophosphafluorene oxide compounds is
illustrated in Scheme 2. These procedures involved generation
of the parent hexafluorophosphafluorenes as synthetic intermedi-
ates. Following aqueous workup, these species were oxidized
by addition of aqueous H2O2, and the products were obtained
in moderate to good yields after column chromatography.
Compound P′-F was readily converted to the 2,7-disubstituted
products by addition of the appropriate carbon nucleophile at
-78 °C followed by slow warming to room temperature. Even
the relatively less reactive lithium phenylacetylide was com-
pletely consumed over 10 h at room temperature (by 19F NMR
spectroscopy). Monitoring the formation of P′-PE allowed
assignments for the mono- and disubstituted compounds (see
the Supporting Information for details). The absence of strong
F-F coupling for the most downfield shifted resonance suggests
substitution in the 2-position of the monosubstituted product.
This assignment is further substantiated by independent synthesis
of P′-PE from 2,2′-dibromo-4,4′-di(phenylethynyl)-3,3′,5,5′,6,6′-
hexafluorobiphenyl (vide infra), for which the substitution
pattern of a derivative (Sn-PE) was established by X-ray
crystallographic studies. For the thienyl derivative P-Th, the
2,7-substitution pattern was also confirmed by X-ray crystal-
lography (vide infra).
Octafluorothiafluorene S-F and octafluorogermafluorene Ge-F
were readily converted to the corresponding diphenyl- and
dithienyl-substituted products via addition of the appropriate
lithium reagent at -78 °C followed by slow warming to room
temperature (Scheme 3). The analogous reactions with lithium
phenylacetylide, to give S-PE and Ge-PE, required heating at
120 °C for at least 2 h. Isolated yields of the disubstituted phenyl
and thienyl derivatives ranged from 50% to 80% but were only
∼30% for the phenylethynyl derivatives. The lower isolated
yields for S-PE and Ge-PE are likely due to product loss during
purification, resulting from inefficient separations from phenyl-
ethynyl-containing byproducts. Higher yields for both S-PE and
Ge-PE were achieved when the heterofluorene was formed from
2,2′-dibromo-4,4′-di(phenylethynyl)-3,3′,5,5′,6,6′-hexafluorobi-
phenyl, as described in the following section. Monitoring the
formation of S-Ph and Ge-Ph allowed assignments of 19F NMR
spectra for the mono- and disubstituted compounds (see the
Supporting Information). The absence of strong F-F coupling
for the most downfield shifted resonances suggests substitution
in the 2-position of the monosubstituted product. The 2,7-
substitution pattern for Ge-Ph was confirmed by a single-crystal
X-ray analysis (vide infra). The 2,7 substitution patterns for S-F
and Ge-F derivatives are further substantiated by independent
synthesis of S-PE and Ge-PE from 2,2′-dibromo-4,4′-di(phe-
nylethynyl)-3,3′,5,5′,6,6′-hexafluorobiphenyl (vide infra), for
which the substitution pattern of a derivative (Sn-PE) was
established by X-ray crystallographic studies.
Spectroscopic Characterization of Fluorinated Heterof-
luorenes. The solution state UV-vis data collected for all
compounds are summarized in Table 1. The corresponding
HOMO-LUMO energy gaps (Egopt) were calculated from the
absorption onset wavelength, which was taken to be the
intersection of the leading edge tangent with the x-axis.
UV-visible absorption behavior for the parent octafluorohet-
erofluorenes exhibit little to no dependence on the heteroatom,
and HOMO-LUMO energy gaps are between 3.75 and 3.90
eV. Derivatizations of the heterofluorenes in the 2- and
7-positions lead to a red-shift in the absorption maximum, as
illustrated in Figure 1 for phosphafluorene oxide derivatives P-F,
P-Ph, P-Th, and P-PE. Compared to the parent compounds,
the 2,7-diphenylhexafluoroheterofluorenes exhibit a slight red-
(40) Alt, H.; Franke, E. R.; Bock, H. Angew. Chem., Int. Ed. 1969, 8, 525.
J. Org. Chem. Vol. 74, No. 2, 2009 823

Geramita et al.
TABLE 1. Selected Optical Characterization Data for
Heterofluorenes
λ
abs/nm
(log ε)
Egopt/eV,
soln
λ
ems/nm,
soln
ΦPL
,
λ
ems/nm,
solid
soln
P-F
P-Ph
P-Th
325(3.6)
338(3.8)
385
369
380(3.7)
3.7
3.4
3.0
365
395
424
443
414
432
364
369
380
397
392
404
331
365
395
415
399
411
337
369
389
402
365
376
400
420
392
414
0.18a
0.60a
0.54b
378
428
P-PE
3.1
0.98b
472
447
473
S-F
S-Ph
S-Th
322(3.6)
344(3.5)
336
352
359(3.7)
3.75
3.8
3.3
<0.01a
0.03a
0.27b
S-PE
3.3
0.08b
Ge-F
Ge-Ph
Ge-Th
308(3.5)
315(4.1)
367(3.5)
3.9
3.7
3.2
0.03a
0.25a
0.80b
406
467-484
Ge-PE
368(4.6)
3.2
1.0b
467
Si′-F
Si′-Ph
Si′-PE
307
321(4.3)
367(4.3)
3.9
3.6
3.2
0.20a
0.70a
1.0a
336
430449
473
Si-F
Si-Ph
Si-Th
315(3.6)
325(4.8)
358(4.5)
3.75
3.7
3.2
0.26a
0.70a
0.80b
378
436457
450-477
Si-PE
354
3.2
1.0b
Sn-F
Sn-PE
309
363
350
3.95
3.2
389
410
0.65b
485
FIGURE 1. Solution state absorption (a) and emission (b) in THF for
phosphafluorene oxide compounds P-F, P-Ph, P-Th, and P-PE.
a Referenced to 1,4-bis-(5-phenyloxaole-2-yl)benzene (POPOP) in
THF (ΦP ) 0.97). b Referenced to 9,10-diphenylanthracene in THF
L
(ΦP L ) 0.9).
300 nm excitation wavelength light for parent octafluoroheter-
ofluorene and phenyl-derivatized compounds, and with 350 nm
excitation wavelength light for the thienyl and phenylethynyl
derivatives. Derivatization of the heterofluorenes in the 2- and
7-positions leads to a red-shift in the emission energies
consistent with the change in absorption energies described
above, as illustrated in Figure 1 for phosphafluorene oxide
derivatives P-F, P-Ph, P-Th, and P-PE. The energies of the
emitted light (λems) for these compounds exhibit a constant stokes
shift of 25 to 30 nm from the absorption maximum, producing
ultraviolet to violet light, and display little heteroatom dependence.
Dilute solution state (optical density 0.1) photoluminescence
quantum yields (ΦPL, Table 1) were determined for all com-
pounds and it was found that the intensity of the emitted light
(ΦPL) varied widely across the series. These observations suggest
that although the heteroatom does not seem to influence
absorption or emission energies, there is significant HOMO and/
or LUMO electron density located on the heteroatom, which
can influence the intensities of these transitions. The observed
decrease in ΦPL values for the Group 14 derivatives, Si′-F, Si-F
> Ge-F > Sn-F, may be explained by the heavy atom effect
and the increase in intersystem crossing and nonradiative decay
pathways with increasing principal quantum number.43 The
lower ΦPL value for thiafluorene S-F versus phosphafluorene
oxide P-F or silafluorenes Si′-F and Si-F may be explained by
a more accessible triplet state or more nonradiative decay
shift, ca. 20 nm, in the absorption energies suggesting a minor
extension of the conjugation length. Exchange of the phenyl
groups for thienyl groups results in a significant red-shift in
the absorption energies, and the HOMO-LUMO energy gaps
drop by ca. 0.4 eV as compared to the parent and phenyl-
substituted compounds. The phenylethynyl derivatives display
HOMO-LUMO energy gaps that are similar to those of the
thienyl analogues despite their potentially longer conjugation
length, further illustrating that the nature of the substitutents is
important in determining the electronic structure.
The effect of fluorination, which can stabilize both the HOMO
and LUMO energy levels, on the absorption and emission
behavior of organic compounds is often difficult to predict.41
Examplesofincreases,decreases,andnochangeinHOMO-LUMO
energy gaps for fluorinated (vs nonfluorinated) versions of the
same compounds have been reported.9,42 In this case, λabs values
for the nonderivatized octafluoroheterofluorenes were similar
to those of the analogous nonfluorinated heterofluorenes, sug-
gesting that fluorination leads to comparable stabilization of both
the HOMO and LUMO energy levels. Additionally, the 2,7-
opt
bis(phenylethynyl) derivatives possess Eg values that are
similar to those of the 2,7-bis(pentafluorophenylethynyl) deriva-
tives, further illustrating a comparable stabilization in both the
HOMO and LUMO energy levels.24
Solution state photoluminescence data for all compounds are
summarized in Table 1. Emission energies were measured with
(43) (a) Prodi, A.; Kleverlaan, C. J.; Indelli, M.; Scandola, F.; Alessio, E.;
Iengo, E. Inorg. Chem. 2001, 40, 34–98. (b) Gutierrez, A.; Whitten. D. J. Am.
Chem. Soc. 1976, 98, 6233. (c) Beljonne, D.; Shuai, Z.; Pourtois, G.; Bredas,
J. L. J. Phys. Chem. A 2001, 105, 3899.
(41) Matsuda, T.; Kadowaki, S.; Goya, T. Org. Lett. 2007, 9, 133–136.
(42) Medina, B.; Beljonne, D.; Egehaaf, H.-J.; Gierschner, J. J. Chem. Phys.
2007, 126, 111101.
824 J. Org. Chem. Vol. 74, No. 2, 2009

Potential Building Blocks for Electron Transporting Materials
TABLE 2. Selected Electrochemical Characterization Data for
Perfluoroheterofluorenes
pathways due to the lone pairs of electrons on the sulfur atom.
Indeed, it was observed that the unoxidized phosphafluorene
P-F′, with one lone pair of electrons, exhibits a much lower
LUMOa/eV HOMOa/eV EgCV/eV, soln Egopt/eV, soln
P-F
-3.0b
-3.0b
-3.1b
-3.3b,d
3.7
3.4
3.0
3.1
Φ
PL value (not shown) than the oxidized phosphafluorene oxide.
P-Ph
P-Th
P-PE
-6.7
-6.2
-6.5
3.7
3.1
3.2
Additionally, the nature of the 2,7-substituents has a signifi-
cant effect on the emission intensities. The 2,7-diphenyl-
hexafluoroheterofluorenes exhibit a significant increase in the
Φ
PL value (3 times or more) versus the corresponding octafluo-
S-F
-2.5
-6.7
4.2
3.75
3.8
3.3
3.3
S-Ph
S-Th
S-PE
-2.7
roheterofluorenes. Exchanging the phenyl groups for thienyl
groups results in a moderate increase in ΦPL for all compounds
except the phosphafluorene oxide derivative (which demon-
strates a slight decrease in emission intensity). All 2,7-
di(phenylethynyl)hexafluoroheterofluorenes, except for thiaflu-
orene S-PE, exhibit high ΦPL values (>0.65), even approaching
unity with the silafluorene- and phosphafluorene oxide-based
systems. Similarly high ΦPL values (∼1) were also observed
for the 2,7-bis(pentafluorophenylethynyl) derivatives.24
-2.8c
-3.0c,d
-6.1
-6.4
3.3
3.4
Ge-F
-2.6
3.9
3.7
3.2
3.2
Ge-Ph
Ge-Th
Ge-PE
-2.7b
-2.8b
-3.0b,d
-6.3
-6.1
-6.4
3.6
3.3
3.4
Si′-F
Si′-Ph
Si′-PE
-2.7
3.9
3.6
3.2
-2.6c
-3.0c,d
-6.5
-6.4
3.9
3.4
In the literature there are examples of both increases and
decreases of ΦPL values upon fluorination.9,42 In this work a
significant increase in emission intensity is observed for many
of the octafluoroheterofluorenes when compared to their non-
fluorinated counterparts.16,41,42 Although optical properties for
the nonfluorinated analogues of the 2,7- derivatized heterof-
luorenes have not been reported, polymers and oligomers
containing silafluorene units are highly emissive; thus high ΦPL
values (>0.7) can be expected for these types of molecules.41
Si-F
-2.6
3.75
3.7
3.2
Si-Ph
Si-Th
Si-PE
-2.7b
-2.9c
-3.0d
-6.5
-6.1
3.8
3.2
3.2
Sn-F
Sn-PE
-2.6
3.95
3.2
-2.8d
a Referenced to Fc/Fc+ (HOMO energy level -4.8 eV below
vacuum). b Reversible reduction. c Quasireversible reduction. d Three
drops of toluene was added to improve solubility in CH3CN.
Despite the poor film-forming properties of these materials,
which result in nonuniform layers by solution casting, it was
possible to collect solid state emission data for a number of
compounds (Table 1). The solid state emission energy (λems,solid
)
for the parent compounds exhibits almost no change from those
observed in solution. However, the λems,solid energies for the 2,7-
derivatized compounds are red-shifted between 30 and 80 nm
from the corresponding values in solution, which strongly
suggests the existence of intermolecular electronic communica-
tion for these extended structures.
Electrochemical Studies on Fluorinated Heterofluorenes.
The solution state redox behavior of all compounds was
investigated via differential pulse voltammetry (reduction and
oxidation potentials) and cyclic voltammetry (reversibility). The
relevant results are summarized in Table 2. All measurements
were performed in a CH3CN electrolyte solution (0.1 M solution
of tetrabutylammonium hexafluorophosphate in CH3CN) with
a Ag/AgNO3 nonaqueous reference electrode and HOMO/
LUMO energy levels were calculated with respect to the
appropriate ferrocene-ferrocenium (Fc/Fc+) oxidation or reduc-
tion (Fc HOMO energy of -4.8 eV relative to vacuum).44 The
LUMO energies range from -2.6 to -3.3 eV. The reversibility
of the reduction varies throughout the entire series and
representative cyclic voltammetry (CV) curves are shown in
Figure 2. Oxidation behavior, when observed, is always ir-
reversible and HOMO-LUMO energy gaps calculated from the
electrochemical analysis are in good agreement with those
calculated from the UV-vis data. In many cases, multiple
reduction and oxidation events are observed and the details of
these transitions are available in the Supporting Information.
FIGURE 2. Representative cyclic voltammetry curves demonstrating
the differences in reduction behavior: (a) irreversible reduction (Ge-
F), (b) reversible reduction (P-PE), and (c) quasireversible reduction
(Si′-Ph).
The LUMO energy values of -2.6 to -3.3 eV are signifi-
cantly lower than those of the nonfluorinated heterofluorene
compounds previously reported in the literature,45-47 and
slightly higher than those reported for the 2,7-bis(pentafluo-
rophenylethynyl) derivatives (∼0.2 eV).24 This clearly illustrates
the effect of fluorination in reducing the LUMO energy level
and puts these small molecules in the range of a number of
(44) Koepp, H.; Wendt, H.; Strehlow, H. Z. Elektrochem. 1960, 64, 483.
(45) Huang, T.-H.; Whang, W.-T.; Shen, J. Y.; Wen, Y.-S.; Lin, J. T.; Ke,
T.-H.; Chen, L.-Y.; Wu, C.-C. AdV. Funct. Mater. 2006, 16, 1449–1456.
(46) Chan, K. L.; McKiernan, M.; Towns, C.; Holmes, A. B. J. Am. Chem.
Soc. 2005, 127, 7662–3.
(47) Mo, Y.; Tian, R.; Shi, W.; Cao, Y. Chem. Commun. 2005, 4925.
J. Org. Chem. Vol. 74, No. 2, 2009 825

Geramita et al.
FIGURE 3. Crystal structure for silafluorene Si′-F: (a) view perpendicular to the stacking direction and (b) view parallel to the stacking direction
illustrating the hexagonal packing of individual π-stacked columns. Hydrogen atoms have been removed for clarity.
useful n-type conducting materials.4,12,48 The LUMO levels for
the parent octafluoroheterofluorenes are all around -2.7 eV,
except for phosphafluorene oxide P-F which possesses a slightly
lower LUMO level at -3.0 eV. Additionally, P-F was the only
nonderivatized heterofluorene to exhibit reversible reduction
behavior. Phenyl substitution in the 2- and 7-positions does little
to alter the LUMO energy level, although it does increase the
reversibility of the radical anion formation for the silafluorene
and germafluorene compounds. The thienyl and phenylethynyl
derivatives exhibit significantly lower LUMO energy levels, a
decrease of about 0.2-0.3 eV versus those of the parent
compounds, which is to be expected due to their slightly longer
conjugation length. The thienyl derivatives maintain some of
the reversibility observed for the phenyl derivatives, although
small amounts of electropolymerization at the Pt working
electrode was observed. This electropolymerization could likely
be prevented by capping the 5-position on the terminal
thiophenes and may improve the reversibility of the electro-
chemical transitions. The thienyl derivatives exhibit higher
HOMO energy levels than the phenylethynyl derivatives, which
would explain the similarities in their Egopt values despite their
differences in conjugation length. Of the phenylethynyl deriva-
tives, only the phosphafluorene oxide P-PE exhibits completely
reversible reduction behavior. All other bis(phenylethynyl)
compounds exhibit quasireversible or irreversible reductions.
The reason for the poor reversibility of these compounds is
unclear but could possibly be improved by the use of a more
electron deficient aryl moiety, or by the use of a better solvent
in the supporting electrolyte solution.
Solid State Structures of Fluorinated Heterofluorenes.
While it would be preferable to demonstrate electron transport
in these materials via the construction of a functioning device
(e.g., FET, OLED, etc.), numerous attempts to solution cast these
compounds failed to produce uniform films. However, single-
crystal X-ray analysis of representative samples provides a
means for investigating intermolecular interactions in the solid
state. Since close cofacial intermolecular packing is desirable
for many electronic applications, it is important to assess how
molecular-level modifications influence the solid state assembly
in these systems. Singly-crystal X-ray analyses of 2,7-bis(pen-
tafluorophenylethynyl)hexafluorogermafluorene and -phosphaflu-
orene oxide derivatives have shown that these compounds
exhibit very planar π systems with parallel heterofluorene
fragments;24 however, the current study allows for a more
extensive investigation into the influence of 2,7 functional
groups and the heteroatom on packing in the solids state.
Examination of the solid state structures of the octafluoro-
heterofluorenes and 2,7-derivatized hexafluoroheterofluorenes
illustrates the tendency for heterofluorenes to pack via the
formation of cofacial intermolecular interactions, and the
significant influence of small changes in molecular structure on
the long-range intermolecular order. Additionally, the crystal
structures of Si-Ph, Ge-Ph, P-Th, and Sn-PE confirm the 2,7-
substitution positions inferred from the 19F NMR spectroscopic
data. Full crystallographic information on all X-ray structures
is available in the Supporting Information.
All of the crystal structures involve cofacial intermolecular
π-π interactions with heterofluorene-heterofluorene distances
ranging from 3.2 to 3.7 Å. This cofacial packing is typical of
highly fluorinated aromatic systems,23 whereas the less desired
herringbone structure is observed for many of the nonfluorinated
fluorene compounds.19-21 The specific interfluorene spacing,
as well as the extent of the π-system overlap, is influenced by
both the heteroatom substituent and the 2,7-substituents. The
influence of the heteroatom substituent can be illustrated by a
comparison of the structures of octafluorosilafluorenes Si′-F and
Si-F (Figures 3 and 4). While both structures exhibit cofacial
stacking, the silafluorene with methyl groups on the silicon, Si′-
F, stacks in hexagonally arranged columns (Figure 3) and
exhibits appreciable π-stacking throughout the entire crystal.
The silafluorene with phenyl substituents on the silicon, Si-F,
pack in two nearly perpendicular directions and significant
fluorene-fluorene π-interaction (3.4 Å) in a given direction
occurs only between pairs of molecules which are offset from
an optimal π-stacking direction and are separated by >3.6 Å.
Comparison of the structures of octafluorostannafluorene, Sn-
F, and 2,7-di(phenylethynyl)hexafluorostannafluorene, Sn-PE,
clearly illustrates the effect of 2,7-derivatizations on the solid
state packing (Figures 5 and 6). As shown in Figure 5, Sn-F
adopts a packing motif similar to that observed for Si-F, with
the molecules aligned in two nearly perpendicular directions,
and cofacial fluorene-fluorene π-overlap is much less signifi-
cant between pairs of molecules. A very different packing
arrangement is exhibited by 2,7-di(phenylethynyl)hexafluo-
rostannafluorene, Sn-PE, which exhibits unidirectional π-stack-
ing throughout the crystal (Figure 6).
(48) Lee, B. L.; Yamamoto, T. Macromolecules 1999, 32, 1375–1382.
826 J. Org. Chem. Vol. 74, No. 2, 2009

Potential Building Blocks for Electron Transporting Materials
FIGURE 4. Crystal structure for silafluorene Si-F: (a) view illustrating π-interactions between molecules and (b) view illustrating the lateral offset
between pairs of molecules. Hydrogen atoms and Si-phenyl groups have been removed for clarity.
FIGURE 5. Crystal structure for stannafluorene Sn-F: (a) view illustrating cofacial interactions and (b) view illustrating the offset between the
pairs of π-stacked molecules. Hydrogen atoms and Sn-phenyl groups have been removed for clarity.
FIGURE 6. Crystal structure for stannafluorene Sn-PE. (a) view perpendicular to the stacking direction and (b) view parallel to the stacking
direction, illustrating the continuous π-overlap. Hydrogen atoms and Sn-phenyl groups have been removed for clarity.
The presence of thienyl groups (P-Th) instead of phenyl
groups (Ge-Ph), as illustrated in Figures 7 and 8 results in a
much more planar aryl-fluorene-aryl ring system. For P-Th
(Figure 7) the torsion angles between the fluorene core and the
thiophenes rings are 7° and 25° versus torsion angles of ∼50°
between the 2,7-phenyl rings and the fluorene core for Ge-Ph
(Figure 8). Even though the molecules in P-Th pack in one
direction throughout the crystal and the molecules in Ge-Ph
pack in two nearly perpendicular directions, both systems exhibit
a similar “slip” away from a π-stacked column, with close π-π
interactions occurring only for pairs of molecules. It is possible
that slightly less bulky groups on the heteroatoms would result
in a more continuous π-system overlap, especially for the more
planar thienyl derivatives.
J. Org. Chem. Vol. 74, No. 2, 2009 827

Geramita et al.
FIGURE 7. Crystal structure for 2,7-dithienylperfluorophosphafluorene oxide P-Th. (a) view illustrating π-interactions between molecules and (b)
view illustrating the lateral offset between pairs of molecules. Hydrogen atoms have been removed for clarity.
FIGURE 8. Crystal structure for 2,7-diphenylgermafluorene Ge-Ph: (a) view illustrating cofacial interactions between molecules and (b) view
illustrating the offset between pairs of molecules. Hydrogen atoms and Ge-phenyl groups have been removed for clarity.
and characterization of P-F′, P-Th, S-Th, S-PE, Si-Ph, Si-Th, Si-
PE, Si′-Ph, Si′-PE, and Sn-PE is described in the Supporting
Information.
Concluding Remarks
A series of 2,7-substituted hexafluoroheterofluorenes has been
prepared via nucleophilic aromatic substitution, and their optical,
electrochemical, and solid state packing properties have been
investigated. The synthesis and characterization of these com-
pounds builds upon the previous work in our laboratories on
the evaluation of such materials as n-type components with
potential electronic applications (n-type FETs, OLED and PV
electron transport layers, etc.).24 The silafluorene and phos-
phafluorene oxide derivatives exhibit very intense, high-energy
emissions, making them interesting candidates for devices based
on electro- and photoluminescence. The low LUMO energies
and cofacial solid state packing observed for many of these
compounds suggest that they may function well as building
blocks for electron transporting materials. In this respect, it is
worth noting that changes in the heteroatom substituents
influence the nature of crystal packing, without significantly
altering electronic properties. Thus, these systems may be
synthetically tunable such that electronic and solid state proper-
ties may be independently varied. Unfortunately the poor film-
forming and adhesion properties of the compounds in this study
prevented studies on electron transport in the solid state. Current
work is focused on addressing these problems through modi-
fications of the structures of these fluorinated heterofluorene
molecules via the incorporation of various substituents.
2,7,9-Triphenylhexafluorophosphafluorene Oxide (P-Ph). To
a solution of P-F′ (0.100 g, 0.25 mmol) in toluene (2 mL) at -78
°C was added a solution of phenyllithium (0.052 g, 0.62 mmol) in
Et2O (1.5 mL). The mixture was allowed to warm to room
temperature over 12 h with stirring. The resulting light yellow
solution was quenched with water (10 mL) and the organic
components were extracted into toluene (2 × 5 mL). The combined
extracts were dried over MgSO4 and concentrated via rotary
evaporation. The resulting solid was dissolved in CH2Cl2 (30 mL)
and then H2O2 (0.1 mL, 30 wt % solution in H2O) was added and
the resulting mixture was stirred for 4 h. The reaction mixture was
washed with water (2 × 15 mL) and the organic components were
dried over MgSO4, filtered through Celite, and concentrated via
rotary evaporation. The disubstituted product was isolated via slow
evaporation of hot hexanes/CH2Cl2 to give 0.034 g (0.07 mmol,
25% yield) of P-Ph as a tan powder. 1H (CDCl3, 400 MHz, 25 °C)
δ 7.48 (12H, m), 7.61 (1H, m), 7.79 (2H, m); 19F NMR (CDCl3,
376 MHz, 25 °C) δ -112.21 (1F, m), -126.71 (1F, m), -132.93
(1F, m); 31P (CDCl3, 161.9 MHz, 25 °C) δ 28.12 (s). MS 520 (M+)
443. Anal. Calcd for C30H15F6OP: C, 67.17; H, 2.82. Found: C,
67.11; H, 3.18. Mp 265-267 °C dec.
2,7-Bis(phenylethynyl)-9-phenylhexafluorophosphafluorene Ox-
ide (P-PE). Method 1: To a solution of P-F′ (0.050 g, 0.12 mmol)
in toluene (2 mL) at -78 °C was added a solution of lithium
phenylacetylene (0.040 g, 0.36 mmol) in THF (2 mL). The mixture
was allowed to warm to room temperature over 12 h with stirring.
The resulting yellow solution was quenched with water (30 mL)
and the organic components were extracted into toluene (2 × 10
mL). The extracts were dried over MgSO4, filtered through celite,
and concentrated via rotary evaporation. The resulting solid was
dissolved in CH2Cl2 (15 mL) and then H2O2 (0.1 mL, 30 wt %
solution in H2O) was added and the resulting solution was stirred
over 4 h. The reaction mixture was quenched with water (2 × 15
Experimental Section
There are literature procedures for the synthesis of 9-phenyloc-
tafluorophosphafluorene and the underivatized octofluoroheterof-
luorenes (Si′-F, Si-F, P-F, S-F, Ge-F, Sn-F) and the changes made
to the synthesis and purification to improve yields are described in
the Supporting Information. Additionally the synthesis, isolation,
828 J. Org. Chem. Vol. 74, No. 2, 2009

Potential Building Blocks for Electron Transporting Materials
mL) and the organic components were dried over MgSO4, filtered
through Celite, and concentrated via rotary evaporation. The
disubstituted product was isolated via slow evaporation of CH2Cl2:
hex to give 0.035 g (0.06 mmol, 50% yield) of P-PE as a bright
yellow powder.
MgSO4, filtered through Celite, and concentrated via rotary
evaporation. The resulting solid was dissolved in CH2Cl2, absorbed
onto alumina, and passed through an alumina plug (hexanes:CH2Cl2
10:1). The disubstituted product was isolated via slow evaporation
of CH2Cl2 from a hexanes/CH2Cl2 mixture to give 0.035 g (0.7
mmol, 24% yield) of S-PE as a yellow powder.
Method 2: 2,2′-Dibromo-4,4′-bis(phenylethynyl)hexafluorobi-
phenyl (0.030 g, 0.05 mmol) was dissolved in toluene/THF (10
mL/ 2 mL) and the resulting solution was cooled to - 78 °C. To
this was added butyllithium (0.06 mL, 1.6 M in hexanes) and the
solution was mixed for 2 h at -78 °C. In a separate Schlenk flask
dichlorophenylphosphine (0.05 mL) was added to dry THF (1 mL)
and this mixture was deoxygenated via two series of freeze/pump/
thaw cycles. The dichlorophenylphosphine solution was added to
the cooled dilithio solution and the resulting mixture was warmed
to room temperature over 8 h with stirring. The resulting yellow
solution was quenched with water (10 mL) and the organic
components were extracted into CH2Cl2 (2 × 10 mL). The
combined extracts were dried over MgSO4, filtered through Celite,
and concentrated via rotary evaporation. The resulting solid was
dissolved in CH2Cl2 (10 mL) and then H2O2 (0.1 mL, 30 wt %
solution in H2O) was added and the resulting solution was stirred
for 4 h. The reaction mixture was quenched with water (2 × 15
mL) and the organic components were dried over MgSO4, filtered
through Celite, and concentrated via rotary evaporation. The
disubstituted product was isolated via washing with acetone to give
0.015 g (0.03 mmol, 55% yield) of P-PE as a bright yellow powder.
1H (CDCl3, 400 MHz, 25 °C) δ 7.40 (6H, m), 7.53 (1H, m),
7.55 (4H, m), 7.64 (2H, m), 7.78 (2H, m); 19F NMR (CDCl3, 376
MHz, 25 °C) δ -105.94 (1F, m), -120.04 (1F, m), -132.89 (1F,
m); 31P (CDCl3, 161.9 MHz, 25 °C) δ 28.47 (s). Anal. Calcd for
C34H15F6OP: C, 69.87; H, 2.59. Found: C, 69.97; H, 2.58. Mp
285-290 °C dec.
2,7-Diphenylhexafluorothiafluorene (S-Ph). To a solution of
S-F (0.022 g, 0.07 mmol) in toluene (2 mL) at -78 °C was added
a solution of phenyllithium (0.028 g, 0.33 mmol) in Et2O (1.5 mL).
The resulting mixture was allowed to warm to room temperature
over 12 h with stirring. The resulting light yellow solution was
quenched with water (15 mL) and the organic components were
extracted into toluene (2 × 10 mL). The combined extracts were
dried over MgSO4, filtered through Celite, and concentrated via
rotary evaporation. The disubstituted product was isolated via
recrystallization from hot hexanes to give 0.024 g (0.06 mmol, 80%
yield) of S-Ph as a white powder. 1H (CDCl3, 400 MHz, 25 °C) δ
7.50-7.60 (m); 19F NMR (CDCl3, 376 MHz, 25 °C) δ -120.92
(1F, m), -136.78 (1F, m), -141.47 (1F, m). MS 444 (M+). Anal.
Calcd for C24H10F6S: C, 64.87; H, 2.27; S, 7.22. Found: C, 64.81;
H, 1.97; S, 6.98. Mp 235-237 °C dec.
Method 2: 2,2′-Dibromo-4,4′-bis(phenylethynyl)hexafluorobi-
phenyl (0.050 g, 0.08 mmol) was dissolved in Et2O/THF (7 mL/2
mL) and the resulting solution was cooled to -78 °C. To this was
added butyllithium (0.1 mL, 1.6 M in hexanes) and the resulting
solution was stirred for 2 h at -78 °C. In a separate Schlenk flask
sulfur monochloride (0.5 mL) was added to dry THF (2 mL) and
this mixture was deoxygenated via two series of freeze/pump/thaw
cycles. The sulfur monochloride solution was added to the cooled
dilithio solution and the resulting mixture was warmed to room
temperature over 8 h with stirring. The resulting yellow solution
was quenched with water (20 mL) and the organic components
were extracted into CH2Cl2 (2 × 20 mL). The combined extracts
were dried over MgSO4 and concentrated via rotary evaporation.
The disubstituted product was isolated via slow evaporation of
CH2Cl2 from a hexanes/CH2Cl2 mixture to give 0.016 g (0.03 mmol,
40% yield) of S-PE as a yellow powder.
1H (CDCl3, 400 MHz, 25 °C) δ 7.43 (3H, m), 7.65 (2H, m); 19
F
NMR (CDCl3, 376 MHz, 25 °C) δ -114.06 (1F, m), -135.01 (1F,
m), -136.51 (1F, m). Anal. Calcd for C28H10F6S: C, 68.29; H, 2.05.
Found: C, 68.17; H, 1.92. Mp 245 °C dec.
2,2′-Dibromo-4,4′-bis(phenylethynyl)hexafluorobiphenyl (1).
To a solution of 2,2′-dibromooctafluorobiphenyl (0.535 g, 1.2
mmol) in THF (75 mL) was added a solution of lithium pheny-
lacetylene (0.290 g, 2.8 mmol) in THF (10 mL) and the resulting
solution was stirred for 16 h. The resulting yellow solution was
quenched with water (20 mL) and the organic components were
extracted into CH2Cl2 (2 × 20 mL). The combined extracts were
dried over MgSO4, filtered through celite, and concentrated via
rotary evaporation. The disubstituted product was isolated via
column chromatography (hexanes on alumina) to give 0.400 g (55%,
0.66 mmol) of 1 as a white powder. 1H (CDCl3, 400 MHz, 25 °C)
δ 7.42 (3H, m), 7.62 (2H, m); 19F NMR (CDCl3, 376 MHz, 25 °C)
δ -103.28 (1F, m), -131.24 (1F, m), -137.80 (1F, m). MS 620
(M+), 460, 310. Anal. Calcd for C28H10Br2F6: C, 54.23; H, 1.63.
Found: C, 53.95; H, 1.60. Mp 213-215 °C.
Acknowledgment. This work was supported by the National
Science Foundation research grant CHE0314709 and the DuPont
Center for Collaborative Research and Education.
Supporting Information Available: Full experimental de-
tails, NMR data for determination of substitution patterns, and
crystallographic data for Si′-F, Si-F, Si-Ph, Ge-F, Ge-Ph, P-Th,
and Sn-PE. This material is available free of charge via the
Internet at http://pubs.acs.org.
2,7-Bis(phenylethynyl)hexafluorothiafluorene (S-PE). Method
1: Octafluorothiafluorene (0.100 g, 0.3 mmol) and lithium pheny-
lacetylene (0.145 g, 1.4 mmol) were dissolved in toluene (2 mL)
and THF (0.5 mL). The resulting solution was heated, with stirring,
at 90 °C for 8 h. The resulting brown solution was quenched with
water (15 mL) and the organic components were extracted into
toluene (2 × 15 mL). The combined extracts were dried over
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