Highly fluorinated benzobisbenzothiophenes

Article abstract of DOI:10.1021/ol8003468

(Figure Presented) Expedient, facile syntheses of highly fluorinated benzobisbenzothiophenes (BBBT) are reported. Defined peripheral arrangements of sulfur and fluorine atoms lead to extensive crystalline networks of edge-to-edge S-F close contacts. The e

Full text of DOI:10.1021/ol8003468

ORGANIC
LETTERS
2008
Vol. 10, No. 15
3307-3310
Highly Fluorinated
Benzobisbenzothiophenes
Yongfeng Wang,† Sean R. Parkin,† Johannes Gierschner,‡ and Mark D. Watson*,†
Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506-0055,
Laboratory for Chemistry of NoVel Materials, UniVersity of Mons-Hainaut, Mons,
Belgium, and Instituto Madrilen˜o de Estudios AVanzados IMDEA Nanoscience,
Madrid, Spain
mdwatson@uky.edu
Received May 22, 2008
ABSTRACT
Expedient, facile syntheses of highly fluorinated benzobisbenzothiophenes (BBBT) are reported. Defined peripheral arrangements of sulfur
and fluorine atoms lead to extensive crystalline networks of edge-to-edge S-F close contacts. The effects of various substitution patterns on
self-assembly and electronic properties are described.
One of the many critical factors governing the performance of
organic semiconductors is the mode in which they self-assemble
to ordered arrays having extensive intermolecular orbital over-
lap.1 Some approaches to modify intermolecular orbital overlap
are the following: (a) increase the size of the π-system,2a (b)
decorate the periphery with groups that preclude edge-to-face
interactions,2b (c) enhance face-to-face interaction via attraction
between electron-rich and -poor segments,2c–e and (d) construct
peripheries from heteroatoms.2f
can convert known p- to n-types.3 Most organic semiconduc-
tors are p-types, such as pentacene,4 poly-3-hexylthiophene,
or oligothiophenes,5 while n-types are rarer.6 Lower LUMO
(3) (a) Jones, B. A.; Facchetti, A.; Wasielewski, M. R.; Marks, T. J.
J. Am. Chem. Soc. 2007, 129, 15259–15278. (b) Chesterfield, R. J.; Newman,
C. R.; Pappenfus, T. M.; Ewbank, P. C.; Haukaas, M. H.; Mann, K. R.;
Miller, L. L.; Frisbie, C. D. AdV. Mater. 2003, 15, 1278–1282. (c) Facchetti,
A.; Deng, Y.; Wang, A.; Koide, Y.; Sirringhaus, H.; Marks, T. J.; Friend,
R. H. Angew. Chem., Int. Ed. 2000, 39, 4547–4551. (d) Facchetti, A.;
Mushrush, M.; Yoon, M. H.; Hutchison, G. R.; Ratner, M. A.; Marks, T. J.
J. Am. Chem. Soc. 2004, 126, 13859–13874.
In addition to altering self-assembly, decoration with
electron-withdrawing groups such as imide, cyano, or fluorine
(4) (a) Lin, Y. Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE
Electron DeVice Lett. 1997, 18, 606–608. (b) Kelly, T. W.; Boardman, L. D.;
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† University of Kentucky.
(5) (a) Chabinyc, M. L.; Toney, M. F.; Kline, R. J.; McCulloch, I.;
Heeney, M. J. Am. Chem. Soc. 2007, 129, 3226–3237. (b) Dimitrakopoulos,
C. D.; Purushothaman, S.; Kymissis, J.; Callegari, A.; Shaw, J. M. Science
1999, 283, 822–824. (c) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen,
M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.;
Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999,
401, 685–688.
‡ University of Mons-Hainaut and Instituto Madrilen˜o de Estudios
Avanzados IMDEA Nanoscience.
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10.1021/ol8003468 CCC: $40.75
Published on Web 07/09/2008
2008 American Chemical Society

energy levels can facilitate electron injection and transport,
leading to n-type characteristics.1
Scheme 1. Synthesis of BBBT’s 1
A recent search of the Cambridge Structural Database
reveals 99 structures with close contacts (less than the
sum of van der Waals radii) between organofluorine and
thienyl sulfur atoms. This interaction may be electrostatic
as in the interaction of thienyl sulfur atoms with oxygen
atoms7 or it may resemble that which imparts a rich
supramolecular chemistry to chalcogenodiazoles, where
nitrogen lone pairs donate into nitrogen-chalcogen σ*
orbitals.8 Regardless of the nature of the S-F interaction,
it stands to reason that lath-shaped molecules with
triangular arrangements of thienyl sulfur atoms and two
fluorine atoms could crystallize into infinite sheets re-
sembling urea self-assembly. Indeed, the crystal structure
of perfluorodibenzothiophene reveals close contacts be-
tween every sulfur atom and two fluorine atoms.9
We report here that the benzobisbenzothiophene (BBBT)
scaffold, if fully fluorinated, provides two such antiparallel
triangular F-S-F units, thus leading to extensive edge-
to-edge interactions. This report does not delineate the
nature of these interactions, but rather demonstrates their
use as a supramolecular synthon. We also report a facile
rapid synthesis of BBBT molecules, based largely on
nucleophilic aromatic substitution chemistry (SNAr). These
highly fluorinated BBBT molecules may also be further
functionalized via SNAr chemistry. Crystal packing, solu-
bility, and the positions of frontier orbitals are modified
by altering the nature of substituents.
All BBBTs reported here were prepared via relatively
short synthetic sequences (Scheme 1). Heavily halogenated
building blocks allowed all new bonds forming the
polycyclic framework to be constructed by nucleophilic
aromatic substitution (SNAr) over 3 steps. For comparison,
the recently reported10 parent BBBT required 6-7 steps.
Compounds 3 were prepared via substitution of bromine
or fluorine by alkyl thiolate. The yield of 3a was not
determined as it was obtained as a minor coproduct during
the synthesis of 1,2,4,5-tetrakis(tert-butylthio)benzene for
other purposes. Compounds 3 were converted to para-
bis-metalates via ortho-lithiation or halogen-lithium
exchange and reacted with excess perfluoroarene to
produce terphenylenes 2. The final step in each sequence
is one-pot dealkylation11 of compounds 2 and ring closure
via the intramolecular SNAr reaction.
aYield not determined. See text.
Scheme 2. Functionalization of BBBT 1d. R ) n-C6H13
Highly fluorinated BBBTs 1 can also serve as building
blocks via substitution of selected or all fluorine atoms.
The preliminary examples shown in Scheme 2 are each
the result of a single unoptimized trial. The regioselectivity
of partial subsitution reactions should be a function of
the nucleophile and the relative directing abilities of the
(7) Pomerantz, M. Tetrahedron Lett. 2003, 44, 1563–65.
(8) Cozzolino, A. F.; Vargas-Baca, I.; Mansour, S.; Mahmoudkhani,
A. H. J. Am. Chem. Soc. 2005, 127, 3184–3190.
(9) Jones, J. B.; Brown, D. S.; Massey, A. G. Phosphorus, Sulfur, Silicon
1988, 35, 67–70.
sulfur- and carbon-centered substituents. Compound 6,
which is a potential substrate for alkyne metathesis, was
isolated from the reaction of 1d with butynyllithium. As
expected based on reports12 of other perthiolated π-sys-
(10) Ebata, H.; Miyazaki, E.; Yamamoto, T.; Takimiya, K. Org. Lett.
2007, 9, 4499–4502.
(11) (a) U.S. Patent 6376,716, B1. (b) Reddy, T. J.; Iwama, T.; Halpern,
H. J.; Rawal, V. H. J. Org. Chem. 2002, 67, 4635–4639.
3308
Org. Lett., Vol. 10, No. 15, 2008

tems, the reduction wave onset of 7 is approximately 200
mV more positive than its precursor 1d (Table 1).
orinated compound 1a. The calculated overall shift is 0.32
eV, in reasonable agreement with the experimental value
(0.26 eV). Simulated spectra relating the pure vertical
transitions (S0 f S1,2,3) are included in the Supporting
Information for comparison.
Solution electrochemical measurements of 1a, 1b, and
1c revealed reduction peaks at -1.99, -1.60, and -2.18
V (vs Fc/Fc+). From reduction wave onsets (Table 1),
LUMO levels are estimated to be -2.81, -3.20, and
-2.62 eV, respectively. These values cannot be strictly
compared with that reported from another laboratory for
the parent 5 (LUMO ) -2.50 eV),10 but the effect of
fluorination is clear. Greater fluorine substitution lowers
the LUMO (1a vs 1c) and more electron-withdrawing CF3
groups provide the largest effect.
Single-crystal X-ray analyses revealed that all BBBTs
1a-e form slip stacks with face-to-face orientation. With
the exception of 1d, all assemble with S-F close contacts.
Partial packing diagrams are included in Figure 2 along with
that of the nonfluorinated parent 5 for comparison. BBBT 5
packs in a typical herringbone-like arrangement with edge-
to-face interactions along its long edges.
Table 1. Electrochemical and UV-Vis Data for 1, 5, and 7
b
λmax
a
d
Ered cath./
peak/edge
(nm)
LUMOc
(eV)
Eg
HOMOe
(eV)
onset (V)
(eV)
1a
1b
1c
1d
5f
-2.13/-1.99
-1.72/-1.60
-2.33/-2.18
-2.42/-2.22
372/383
400/416
366/379
383/397
369/380
467/490
-2.81
-3.20
-2.62
-2.58
-2.50
-2.77
3.24
2.98
3.27
3.12
3.26
2.53
-6.05
-6.18
-5.89
-5.70
-5.76
-5.30
7
-2.04/-2.03
a Versus Fc/Fc+ (E1/2 ) 0.60V in this system). b Solution absorption
spectra. c Estimated from reduction wave onset (LUMO ) -Ered - 4.8).
d Estimated from the absorption edge. e Estimated from HOMO ) LUMO
- Eg. f Reference 10.
Energy gaps Eg of the lowest allowed optical transitions
(Table 1) are 3.24 and 3.27 eV for 1a and 1c, determined
from the absorption edges of UV-vis spectra (Figure 1).
They are very similar to the unsubstituted compound 5
(3.26 eV),10 while that for 1b is significantly lower (2.98
eV). Substitution effects can be well understood13 by
quantum-chemical calculations, carried out at the DFT
Derivatives 1a,b crystallize with predicted full edge-
to-edge arrangements with eight S-F close contacts per
molecule. BBBT 1b assembles to sheets with all unsatur-
ated atoms lying in parallel planes while the molecular
planes of 1a tilt slightly against similar planes containing
the molecular centroids. Both fully fluorinated BBBTs
form slip π-stacks along two dimensions, a feature that
has been shown to improve FET performance.15 For 1a,
one π-stacking axis is defined by 10 close contacts per
molecule (6 C-C, 4 C-S), and the other by two S-S
close contacts. The average face-to-face (atom-to-plane)
distance is 3.34 Å. For 1b, π-overlap is minimal and
defined along each of two dimensions by 4 C-S distances
(3.51 Å) which approximate the sum of the vdW radii of
carbon and sulfur. The interplanar distance is exceptionally
small (3.2 Å), similar to that reported16 for perfluoropen-
tacene and likewise attributed to electrostatic attraction.
The large slip along the short molecular axis may also
contribute. Whether recrystallized via vapor sublimation17
or from solution, 1b forms foil-like, “large” flexible plates.
While BBBTs 1d,e lack central fluorine atoms to com-
plete the F-S-F edge-to-edge interactions, side chains
preclude edge-to-face interactions along their long edges. The
side chains of 1d, 1e, and 6 expand the π-stacking distance
(3.40-3.45 Å) relative to the other BBBTs.
Figure 1.
Solution absorption spectra for 1a-c (10-6 M THF).
(density functional theory) level of theory, employing the
B3LYP functional and the 6-311+G* basis set within the
GAUSSIAN03 package.14 According to calculations,
the transition responsible for Eg is described mainly by
the HOMO-LUMO transition. Fluorination at the teminal
positions (“Y”, Scheme 1) induces a hypsochromic shift
that is essentially negated by a bathochromic shift due to
fluorination in the lateral positions, leading to little
difference in Eg for 1a,c relative to 5. Due to terminal
CF3 substiuents, the S0 f S1 band of compound 1b is
significantly red-shifted compared to that of the perflu-
BBBT 1c (X ) H) serves as a control, lacking fluorine
atoms on the central benzene ring to complete the F-S-F
groups. It is the only derivative reported here that exhibits a
herringbone-like packing with edge-to-face interactions (at
molecular termini). Its crystal plates were exceedingly thin,
twinned, and somewhat disordered, and therefore the preci-
sion of the reported structure is relatively low for a small
molecule. However, the molecular structure and packing
motif are unambiguous. Edge-to-edge close contacts (S-F,
H-F) bridge adjacent oblique, rather than (nearly) parallel,
molecular planes. BBBT 1c is highly soluble in common
Org. Lett., Vol. 10, No. 15, 2008
3309

organic solvents at room temperature while alkylated 1c,d
require mild heating and analogues with full F-S-F groups
(1a,b) are soluble only at elevated temperature (e.g., boiling
toluene).
In summary, we have established an efficient route to
prepare fluorinated polycyclics 1. Experimental and
computational estimates of the effects of substituents on
the energy gap and frontier orbital energy levels are in
accord: CF3 substituents (1b) induce a bathochromic shift
of the (optical) energy gap and decrease the LUMO level
relative to the fluorine substituents of 1a. Fluorine atoms
attached to the central benzene ring lower the LUMO in
comparison to protons at the same positions (1a vs 1c).
Although the nature and strength of the fluorine-sulfur
interaction is unknown in this case, the crystal packing
of 1a,b indicates its utility in engineering self-assembly.
Interactions between antiparallel, triangular F-S-F units
along each side of the lath-shaped molecules, combined
with face-to-face electrostatic attraction, preclude edge-
to-face interactions. Two S-F units (1c) are insufficient
to preclude edge-to-face arrangements, except when bulky
alkyl chains are included.
Acknowledgment. We thank the National Science Foun-
dation (CHE 0616759) and the Donors of the American
Chemical Society (ACS-PRF) for financial support. J.G. is
a Ramo´n y Cajal research fellow (Spanish Ministry for
Science and Education). Y.W. is a Kentucky Opportunity
Graduate fellow.
Supporting Information Available: Synthesis and char-
acterization details, crystallographic information file (CIF),
simulated absorption spectra, CV data, and complete ref 14.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL8003468
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Figure 2.
Crystal packing diagrams for (a) 5,10 (b) 1c, (c) 1a, (d)
1b, (e) 1d, (f) 1e, and (g) 6. Red dashed lines: S-F close contacts.
3310
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