Benzodichalcogenophenes with Perfluoroarene Termini

Article abstract of DOI:10.1021/ol801569m

(Chemical Equation Presented) Benzodichalcogenophenes are functionalized at their termini via SN_Ar reactions of their bismetalates with perfluoroarenes. The identities of X, Y, and W are varied to study the effects on LUMO energy levels and cry

Full text of DOI:10.1021/ol801569m

ORGANIC
LETTERS
2008
Vol. 10, No. 20
4421-4424
Benzodichalcogenophenes with
Perfluoroarene Termini
Yongfeng Wang, Sean R. Parkin, and Mark D Watson*
Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky, 40506-0055
mdwatson@uky.edu
Received July 10, 2008
ABSTRACT
Benzodichalcogenophenes are functionalized at their termini via SNAr reactions of their bismetalates with perfluoroarenes. The identities of X,
Y, and W are varied to study the effects on LUMO energy levels and crystallization motif. X-ray crystallography reveals that nearly all new
derivatives crystallize with coplanar ring systems within slipped 1D or 2D π-stacks.
A number of fused aromatic polycyclics based on chalco-
genophenes (i.e., thiophene and/or selenophene) have given
good performance as organic semiconductors in various
optoelectronic devices.1 Specifically, benzodichalcogenophene
(BDC) units are excellent building blocks for small molecule
and polymeric semiconductors.2 As for other π-systems, one
may assume that their charge transport and device perfor-
mance are largely affected by their crystallization motif, i.e.,
face-to-face or edge-to-face π-stacking. Face-to-face interac-
tions can be favored via substituents around π-systems that
preclude edge-to-face interactions.3 Arene-perfluoroarene
(π-πF) interactions are increasingly employed to induce
face-to-face π-π packing.4 Another consequence is that
n-channel behavior has been observed after introducing
electron-withdrawing groups as terminal groups into p-type
materials.2b,4b,5
A series of BDCs terminated with electron-withdrawing
benzenes and yielding electron mobilites as high as 0.1 cm2/
Vs in OFETs was reported.2b Solid-state order of these
materials was qualitatively assessed by X-ray diffraction of
sublimed films. We provide here crystallographic studies of
9 out of 12 newly prepared BDC derivatives terminated by
πF units to reveal their packing motifs. We hoped that the
size mismatch between electron-rich BDC cores and smaller
electron-poor terminal rings would cause each π-face to
overlap with two other π-faces.6 Attractive interactions
(1) (a) Takimiya, K.; Ebata, H.; Sakamoto, K.; Izawa, T.; Otsubo, T.;
Kunugi, Y. J. Am. Chem. Soc. 2006, 128, 12604–12605. (b) Takimiya, K.;
Kunugi, Y.; Konda, Y.; Ebata, H.; Toyoshima, Y.; Otsubo, T. J. Am. Chem.
Soc. 2006, 128, 3044–3050. (c) Yamamoto, T.; Takimiya, K. J. Am. Chem.
Soc. 2007, 129, 2224–2225. (d) Wex, B.; Kaafarani, B. R.; Schroeder, R.;
Majewski, L. A.; Burckel, P.; Grell, M.; Neckers, D. C. J. Mater. Chem.
2006, 16, 1121–1124. (e) Gao, P.; Beckmann, Tsao, H. N.; Feng, X.;
Enkelmann, V.; Pisula, W.; Muellen, K. Chem. Commun. 2008, 154, 8–50.
(f) Tang, M. L.; Reichardt, A. D.; Miyaki, N.; Stoltenberg, R. M.; Bao, Z.
J. Am. Chem. Soc. 2008, 130, 6064–6065. (g) Subramanian, S.; Park, S. K.;
Parkin, S. R.; Podzorov, V.; Jackson, T. N.; Anthony, J. E. J. Am. Chem.
Soc. 2008, 130, 2706–2707. (h) Yamada, K.; Okamoto, T.; Kudoh, K.;
Wakamiya, A.; Yamaguchi, S.; Takeya, J. Appl. Phys. Lett. 2007, 90,
072102.
(3) (a) Moon, H.; Zeis, R.; Borkent, E. J.; Besnard, C.; Lovinger, A. J.;
Siegrist, T.; Kloc, C.; Bao, Z. N. J. Am. Chem. Soc. 2004, 126, 15322–
15323. (b) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4,
15–18. (c) Kobayashi, K.; Masu, H.; Shuto, A.; Yamaguchi, K. Chem.
Mater. 2005, 17, 6666–6673.
(4) (a) Patrick, C. R.; Prosser, G. S. Nature 1960, 187, 1021. (b) Yoon,
M. H.; Facchetti, A.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 2006,
128, 5792–5801. (c) Dai, C. Y.; Nguyen, P.; Marder, T. B.; Scott, A. J.;
Clegg, W.; Viney, C. Chem. Commun. 1999, 2493–2494.
(5) (a) Ando, S.; Nishida, J.; Fujiwara, E.; Tada, H.; Inoue, Y.; Tokito,
S.; Yamashita, Y. Chem. Mater. 2005, 17, 1261–1264. (b) Yoon, M.-H.;
DiBenedetto, S. A.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2005,
127, 1348–1349. (c) Ando, S.; Nishida, J. I.; Tada, H.; Inoue, Y.; Tokito,
(2) (a) Takimiya, K.; Kunugi, Y.; Konda, Y.; Niihara, N.; Otsubo, T.
J. Am. Chem. Soc. 2004, 126, 5084–5085. (b) Takimiya, K.; Kunugi, Y.;
Ebata, H.; Otsubo, T. Chem. Lett. 2006, 35, 1200–1201. (c) Pan, H.; Li,
Y.; Wu, Y.; Liu, P.; Ong, B. S.; Zhu, S.; Su, G. J. Am. Chem. Soc. 2007,
129, 4112–4113. (d) Hou, J.; Park, M. H.; Zhang, S.; Yao, Y.; Chen, L. M.;
Li, J. H.; Yang, Y. Macromolecules 2008, 41, 6012–6018.
S.; Yamashita, Y. J. Am. Chem. Soc. 2005, 127, 5336–5337
.
(6) Indicated to improve device performance: (a) Takimiya, K.; Kunugi,
Y.; Konda, Y.; Ebata, H.; Toyoshima, Y.; Otsubo, T. J. Am. Chem. Soc.
2006, 128, 3044–3050. (b) Payne, M. M.; Parkin, S. R.; Anthony, J. E.;
Kuo, C. C.; Jackson, T. N. J. Am. Chem. Soc. 2005, 127, 4986–4987.
10.1021/ol801569m CCC: $40.75
Published on Web 09/19/2008
2008 American Chemical Society

Table 1. Yields and Properties of BDC Derivatives 1
a
λmax
absorption
peak/edge
(nm)
Stokes
shift
b
d
λmax PL
Ered
LUMOe
(eV)
HOMOf
(eV)
c
X
Y
W
yield (%)
(nm)
(cm-1
)
Eg (eV)
onset (V)
1a
1b
1c
1d
1e
1f
1g
1h
1i
S
S
Se
Se
S
S
S
S
S
H
H
H
H
CF
CCF3
CF
CCF3
CF
CCF3
CF
CCF3
CF
CCF3
CCF3
N
76
69
65
56
32
66
47
63
60
70
82
85
367/405
391/430
385/418
408/446
397/442
423/480
392/440
422/475
406/440
420/465
419/462
435/485
405
434
g
2557
2534
g
3.1
2.9
3.0
2.8
2.8
2.6
2.8
2.6
2.8
2.7
2.7
2.6
-2.1
-1.8
-2.1
-1.8
-2.1
-1.8
-2.1
-1.8
-1.8
-1.4
-1.5
-1.7
-2.7
-3.0
-2.7
-3.0
-2.7
-3.0
-2.7
-3.0
-3.0
-3.4
-3.3
-3.1
-5.8
-5.9
-5.7
-5.8
-5.5
-5.6
-5.5
-5.6
-5.8
-6.1
-6.0
-5.7
g
g
O-n-C12H25
O-n-C12H25
OCH3
491
540
498
556
437
464
463
558
4822
5122
5430
5711
1748
2258
2268
5078
OCH3
CCTIPS
CCTIPS
CCTMS
OCH3
1j
1k
1l
S
S
S
a Solution absorption spectra (10-5-10-6 M THF). Solution photoluminescence spectra (10-6-10-8 M THF). c Optical energy gap determined from
b
d
the absorption edge. Versus Fc/Fc+. e Estimated from reduction wave onset. (LUMO ) -4.8 - Ered
)
f Estimated from HOMO ) LUMO - Eg. g No
observable PL.
between chalcogens and pendant fluorine atoms7 may
Scheme 1
.
Synthesis of BDC Derivatives 1
contribute to coplanarization of their ring systems. The effects
of substituents and core chalcogen atoms on frontier orbital
energy levels were estimated by solution optical and
electrochemical measurements. Although chalcogenophenes
are typically coupled to fluorinated arenes via transition-
metal-catalyzed coupling, a few examples7c,8 have exploited
SNAr chemistry of perfluorarenes with metalated chalco-
genophenes. This approach bypasses the need to first
functionalize the chalcogenophene prior to coupling. We
report here also the synthesis of a series of BDCs (1) with
perfluoroarene termini, exploiting SNAr chemistry.
BDC building blocks 2 were prepared by three separate
routes. Silyl-functionalized 2a,b were obtained after metal-
halogen exchange of 3 with t-BuLi followed by adding
chalcogen powder (sulfur or selenium).9 Isolated yields were
diminished due to desilylation during flash chromatography.
Benzo[1,2-b:4,5-b′]dithiophene-4,8-dione 4 was prepared
from 3-bromothiophene via a three-step published proce-
dure.10 Reaction of 4 with alkynyl Grignard reagents and
subsequent deoxygenation/aromatization with SnCl2/HCl3b
yielded trimethylsilylacetylene (TMSA) and triisopropylsi-
lylacetylene (TIPSA) functionalized 2e,f. The low yield for
2f relative to 2e is again attributed to protiodesilylation during
flash chromatography. Reduction of 4 and in situ alkylation
led to 2c/2d bearing alkoxy groups.11
aSee Table 1 for identities of W, X, and Y and isolated yields.
(7) (a) Wang, Y.; Parkin, S. R.; Gierschner, J.; Watson, M. D. Org.
Lett. 2008, 10, 3307–3310. (b) Subramanian, S.; Park, S. K.; Parkin, S. R.;
Podzorov, V.; Jackson, T. N.; Anthony, J. E. J. Am. Chem. Soc. 2008, 130,
2706–2707. (c) Crouch, D. J.; Skabara, P. J.; Lohr, J. E.; McDouall, J. J. W.;
Heeney, M.; McCulloch, I.; Sparrowe, D.; Shkunov, M.; Coles, S. J.; Horton,
P. N.; Hursthouse, M. B. Chem. Mater. 2005, 17, 6567–6578.
The aryl termini of previously reported 2,6-diaryl BDCs
were introduced either in an early stage of the synthesis or
in the final step by transitional-metal-catalyzed coupling,
following a prerequisite functionalization step. SNAr chem-
istry of 2,6-bismetalated BDCs with perfluoroarenes allows
rapid access to a library of such derivatives. We employed
two routes. The carbon-silicon bonds of 2a/b were activated
(8) (a) Wang, Y.; Watson, M. D. J. Am. Chem. Soc. 2006, 128, 2536–
2537. (b) Geramita, K.; McBee, J.; Shen, Y.; Radu, N.; Tilley, T. D. Chem.
Mater. 2006, 18, 3261–3269
(9) Takimiya, K.; Konda, Y.; Ebata, H.; Niihara, N.; Otsubo, T. J. Org.
Chem. 2005, 70, 10569–10571.
.
(10) Slocum, D. W.; Gierer, P. L. J. Org. Chem. 1976, 41, 3668–3673.
(11) Beimling, P.; Kobmehl, G. Chem. Ber. 1986, 119, 3198–3203.
4422
Org. Lett., Vol. 10, No. 20, 2008

by a catalytic amount of fluoride and reacted with
perfluoroarenes,8a or the 2,6-positions of 2c-f were lithiated
with n-BuLi/TMEDA at -78 °C and quenched with per-
fluoroarenes. All BDCs 1 were isolated in analytically pure
form after flash chromatography or in several cases after
simple treatment with methanol.
shifts of 1f and 1h can only be possibly explained by greater
aggregation of the latter.
The effects of substituents and core chalcogen atoms on
LUMO energy levels of 1 were estimated from solution
electrochemical measurements (Table 1). The identity of the
chalcogen atom has little effect on LUMO level (1c/1d versus
1a/1b).12 However, the terminal and central substituents can
significantly affect the LUMO. Alkoxy substituents exert
little or no effect on LUMO levels, and therefore their effect
on Eg is indeed due to destabilization of the HOMO.
Replacement of terminal fluorine atoms (W ) CF) with more
electron-withdrawing CF3 groups consistently lowers the
LUMO level by 0.3-0.4 eV, in agreement with electro-
chemical studies of fluorinated heteroacenes.7a LUMO levels
are estimated to be -2.7, -3.0, and -3.1 eV for 1g, 1h,
and 1l, all bearing identical subsituents on their BDC cores.
Therefore, terminal perfluoropyridine rings (W ) N) exert
a stronger effect on the LUMO level than W ) CCF3.
Substitution of the BDC core with trialkylsilyl acetylenes
lowers LUMO levels by 0.3-0.4 eV compared to compounds
with Y ) H (1a versus 1i, 1b versus 1j and 1k). This and
the effect of terminal CF3 groups are additive here.
Room temperature absorption and photoluminescence
spectra of 1a-l are included in Supporting Information (see
Figure SI2), and relevant data are summarized in Table 1.
All absorption spectra are broad and nearly featureless,
indicating substantial freedom of rotation around the BDC-
πF bonds. As noted for the parent diphenyl BDCs, replace-
ment of S with Se causes a minor absorption red shift.12
Most of the remaining differences in absorption λmax and
optical energy gaps (Eg), estimated from the onset of
absorption, can be explained by tuning of the donor-acceptor
type interaction between electron-rich BDC cores and
electron-poor aryl termini. This behavior is exemplified by
the series 1g f 1h f 1l, bearing perfluorophenyl, perfluo-
rotolyl, and perfluoropyridyl termini with λmax values of 392,
422, and 435 nm. In agreement with a previous study of
fluorinated π-systems,7a replacement of terminal fluorines
with CF3 substituents reduces Eg through preferentially
lowering LUMO energy by 0.3 eV. Alkoxy substituents
destabilize the HOMO by resonance donation (mesomeric)
but have little effect on the Eg via induction, thus inducing
a red shift of 20-30 nm. Trialkylsilylacetylene substituents
decrease the Eg by 0.2-0.3 eV (e.g., 1i/1j vs 1a/1b), similar
to observed and calculated effects13 of these groups on
pentacene. Estimation of frontier orbital energy levels via
electrochemical measurements (see below) indicate that Eg
is modulated by preferential stabilizion of LUMO (trialkyl-
silyl acetylene, πF) or destabilizaion of HOMO (alkoxy) in
a predictable fashion.
One goal here is to force the ring systems of BDCs 1 into
coplanarity and enhance π-stacking, perhaps also leading to
greater dimensionality of the π-stacks. Single-crystal X-ray
analyses revealed that the ring systems of nearly all of the
BDC derivatives 1a-1l are coplanarized (Figure 1). The
exception is 1a, perturbed by intercalation of toluene solvent
molecules into the π-stacks, demanding torsion (51°) between
the BDC core and C6F5 rings to allow commensurate stacking
distances. BDCs 1d and 1k crystallized in motifs similar to
the others, but the diffraction data were of relatively low
quality for small molecules and therefore not included.
Suitable crystals could not be grown from 1c.
No detectable photoluminescence (PL) was observed for
selenophene-based 1c and 1d. This contrasts optical studies12
of the parent 2,6-diphenyl BDCs (X ) S, Se, Te) where it
was only the tellurophene derivative that exhibited no PL.
For the remaining BDCs 1, fairly systematic variation in
Stokes shifts and PL fine structure suggest photoinduced
intramolecular charge transfer states, modulated by electron-
richness of the BDC core (donor) and electron-poorness of
the πF termini (acceptor).14 BDCs 1a, i, j, k without electron-
donating alkoxy substituents exhibit PL with moderate fine
structure and Stokes shifts of 1750-2600 cm-1. BDC 1i with
the least electron-withdrawing πF termini (C6F5) and tri-
alkylsilylacetylene groups (inductively electron-withdrawing)
on the donor yields the smallest Stokes shift and the most
distinct fine structure. The Stokes shifts of BDCs with alkoxy
substituents (greater donor-acceptor character) are ap-
proximately doubled in magnitude, and their PL profiles are
nearly structureless. The discrepancy between the Stokes
Otherwise, all of the new BDCs 1 crystallize into face-to-
face π-stacks with variable degrees of “pitch and roll”15
translation. Face-to-face stacking is enforced by π-πF interac-
tions between overlapping perfluorarene termini and BDC cores
in all cases. All BDCs with CF3 termini, regardless of the steric
or electronic nature of Y, e.g., 1b, (Y ) H), 1f (OC12H25), 1h
(OCH3), and 1j (TIPSCC), pack in slipped stacks along only
one dimension. Their CF3 termini do not cause “large” increases
in the π-stacking distance. The average distances between
π-faces are 3.47 (1b), 3.37 (1f), 3.48 (1h), and 3.33 Å (1j).
Compound 1e without CF3 groups also formed 1-dimensional
slip stacks. Compound 1g with Y ) OCH3 and W ) CF is
one of only two new derivatives that forms slip π-stacks along
two dimensions. One π-stacking axis is defined by four close
contacts (C-S/C-C) per molecule and the other by four C-C
close contacts. The average face-to-face (atom-to-plane) distance
along one stacking axis is 3.33 Å and 3.47 Å along the other.
The packing motif of 1i might loosely be called a 2D π-stack.
One stacking axis is defined by 14 C-C close contacts per
molecule. Along the second axis, two carbons of the C6F5 rings
approach those of others by 3.45 Å, which is slightly greater
(12) Casado, J.; Oliva, M. M.; Delgado, M. C. R.; Ortiz, R. P.; Quirante,
J. J.; Lopez-Navarrete, J. T.; Takimiya, K.; Otsubo, T. J. Phys. Chem. A
2006, 110, 7422–7430.
(13) Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist,
T. Chem. Mater. 2004, 16, 4980–4986.
(14) (a) Fahrni, C. J.; Yang, L.; VanDerveer, D. G. J. Am. Chem. Soc.
2003, 125, 3799–3812. (b) Zucchero, A. J.; Wilson, J. M.; Bunz, U. H. F.
J. Am. Chem. Soc. 2006, 128, 11872–11881.
(15) Curtis, M. D.; Cao, J.; Kampf, J. W. J. Am. Chem. Soc. 2004, 126,
4318–4328.
Org. Lett., Vol. 10, No. 20, 2008
4423

All the BDC derivatives described above crystallize with
edge-to-edge, but no edge-to-face, close contacts. The edge-
to-face, herringbone-like packing motif of compound 1l
bearing terminal perfluoropyridine rings is unique among
BDCs 1. The molecular planes within one π-stack are
inclined by 89° relative to the planes in adjacent stacks. Edge-
to-face close contacts (N-F, S-F, and C-F) bridge adjacent
oblique molecular planes. The π-stacking axis is defined by
four C-C close contacts and two C-S close contacts per
molecule, with the closest contact between 3.2 and 3.3 Å.
In summary, BDCs are easily functionalized with per-
fluoroarenes via SNAr reactions as a final synthetic step.
Electrochemical and optical measurements revealed that the
positions and types of a discrete set of substituents affect Eg
and LUMO levels in a consistent manner. Alkynyl substit-
uents on the BDC core or CF3 termini lower the Eg and
LUMO level by approximately 0.2-0.3 eV, while terminal
perfluoropyridines lower the LUMO further by 0.1 eV
(1l vs 1h). Solution optical measurements indicate intramo-
lecular charge-transfer states with increasing bias of the
donor-acceptor nature of this system. While face-to-face
π-stacking of coplanar ring systems was achieved for nearly
all new derivatives, no clear design principle for 2D
π-stacking emerges as yet.
Acknowledgment. We thank the National Science Foun-
dation (CHE 0616759) and the Donors of the American
Chemical Society (ACS-PRF) for financial support. Y.W. is
a Kentucky Opportunity Graduate fellow.
Supporting Information Available: Synthesis and char-
acterization details, crystallographic information file in CIF
format, and absorption spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
Figure 1. Crystal packing diagrams for BDC derivatives 1.
than van der Waals radii. The average face-to-face (atom-to-
plane) distance is 3.34 Å, and the closest C-C contact is only
3.27Å.
OL801569M
4424
Org. Lett., Vol. 10, No. 20, 2008