Exploring electronically polarized pentacenes

Article abstract of DOI:10.1021/ol801464k

(Chemical Equation Presented) Unsymmetrically functionalized pentacenes with electron-rich and/or -poor substituents at the 6- and 13-positions were synthesized. The electronic influence was evaluated by solution-state UV-vis absorption and emission spect

Full text of DOI:10.1021/ol801464k

ORGANIC
LETTERS
2008
Vol. 10, No. 19
4163-4166
Exploring Electronically Polarized
Pentacenes
Dan Lehnherr, Robert McDonald,† and Rik R. Tykwinski*
Department of Chemistry, UniVersity of Alberta, Edmonton, AB T6G 2G2, Canada
rik.tykwinski@ualberta.ca
Received June 27, 2008
ABSTRACT
Unsymmetrically functionalized pentacenes with electron-rich and/or -poor substituents at the 6- and 13-positions were synthesized. The
electronic influence was evaluated by solution-state UV-vis absorption and emission spectroscopies. These materials exhibit good solubility
in common organic solvents and are stable in the presence of air and water.
To date, there has been a limited number of examples of
unsymmetrically substituted pentacenes that give rise to a
dipolar acene framework, and there are even fewer such
derivatives that can be isolated or handled in the presence
of air.1-3 In some cases, unsymmetrical pentacenes have
been prepared and characterized as a mixture of
regioisomers,2i while in others, the acene is in equilibrium
with its unconjugated, tautomeric counterpart at high
temperatures.2a,b Other unsymmetrical pentacenes have been
arrived at as part of a specific synthetic sequence that
ultimately presents a rather limited scope.2j,n Desymmetrized
pentacene building blocks have also been used for the
construction of pentacene oligomers, as well as several
polymers.3 Thus, in the vast majority of pentacene systems
studied to date as semiconductors, the functionalized pen-
tacene molecules used as the active component have had a
symmetric or nearly symmetric electronic make up.4 To
provide an opportunity to explore the effects of a pentacene
chromophore electronically polarized due to the presence of
electronically varied substituents, i.e., donors and acceptors,
we targeted a synthetic pathway that would afford ready
† X-Ray Crystallography Laboratory, Department of Chemistry, Uni-
versity of Alberta.
(1) For recent reviews on acenes, see: (a) Bendikov, M.; Wudl, F.;
Perepichka, D. F. Chem. ReV. 2004, 104, 4891–4945. (b) Anthony, J. E.
Chem. ReV. 2006, 106, 5028–5048. (c) Anthony, J. E. Angew. Chem., Int.
Ed. 2008, 47, 452–483
.
(2) For examples of unsymmetrical pentacenes, see: (a) Clar, E.; Wright,
J. W. Nature 1949, 163, 921–922. (b) Clar, E. Chem. Ber. 1949, 82, 495–
514. (c) Takahashi, T.; Kitamura, M.; Shen, B.; Nakajima, K. J. Am. Chem.
Soc. 2000, 122, 12876–12877. (d) Reichwagen, J.; Hopf, H.; Desvergne,
J.-P.; Guerzo, A. D.; Bouas-Laurent, H. Synthesis 2005, 3505–3507. (e)
Swartz, C. R.; Parkin, S. R.; Bullock, J. E.; Anthony, J. E.; Mayer, A. C.;
Malliaras, G. G. Org. Lett. 2005, 7, 3163–3166. (f) Vets, N.; Smet, M.;
Dehaen, W. Synlett 2005, 217–222. (g) Takahashi, T.; Li, S.; Huang, W.;
Kong, F.; Nakajima, K.; Shen, B.; Ohe, T.; Kanno, K.-i. J. Org. Chem.
2006, 71, 7967–7977. (h) Jang, B.-B.; Lee, S. H.; Kafafi, Z. H. Chem. Mater.
2006, 18, 449–457. (i) Okamoto, T.; Senatore, M. L.; Ling, M.-M.; Mallik,
A. B.; Tang, M. L.; Bao, Z. AdV. Mater. 2007, 19, 3381–3384. (j) Wang,
Y.-M.; Fu, N.-Y.; Chan, S.-H.; Lee, H.-K.; Wong, H. N. C. Tetrahedron
2007, 63, 8586–8597. (k) Palayangoda, S. S.; Mondal, R.; Shah, B. K.;
Neckers, D. C. J. Org. Chem. 2007, 72, 6584–6587. (l) Be´nard, C. P.; Geng,
Z.; Heuft, M. A.; VanCrey, K.; Fallis, A. G. J. Org. Chem. 2007, 72, 7229–
7236. (m) Li, S.; Zhou, L.; Song, Z.; Bao, F.; Kanno, K.-i.; Takahashi, T.
Heterocycles 2007, 73, 519–536. (n) Takahashi, T.; Li, Y.; Hu, J.; Kong,
F.; Nakajima, K.; Zhou, L.; Kanno, K. -i. Tetrahedron Lett. 2007, 48, 6726–
6730. (o) Zhao, Y.; Mondal, R.; Neckers, D. C. J. Org. Chem. 2008, 73,
5506–5513. (p) For a recent report of unsymmetrical tetracenes, see: Lin,
Y.-C.; Lin, C.-H. Org. Lett. 2007, 9, 2075–2078
.
(3) Lehnherr, D.; Tykwinski, R. R. Org. Lett. 2007, 9, 4583–4586.
(4) (a) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.;
Silbey, R.; Bre´das, J.-L. Chem. ReV. 2007, 107, 926–952. (b) Bre´das, J.-
L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem. ReV. 2004, 104, 4971–
5003.
(5) A related approach has been used by Dehaen and co-workers to form
an unsymmetrical, 6,13-diarylpentacene (ref 2f).
10.1021/ol801464k CCC: $40.75
Published on Web 09/03/2008
2008 American Chemical Society

access to such molecules.5 The results of this study are
presented here.
however, all attempts to effect a process of selective in situ
desilylation and Sonogashira coupling with substrate 2b have
been unsuccessful. Thus, a more direct method to access
unsymmetrical pentacenes was developed in which the
frustratingly unstable intermediate 2a could be avoided
entirely.
Unsymmetrically substituted pentacenes were synthesized
from a common intermediate, ketone 1 (Scheme 1).6 Briefly,
Scheme 1
.
Attempts toward a Divergent Unsymmetrical
Pentacene Building Block
Scheme 2. Synthesis of Unsymmetrical Pentacenes 2c-j
As outlined in Scheme 2, the formation of an acetylide
nucleophile from the appropriate terminal alkyne was
achieved with HexLi, LDA, or i-PrMgCl, dependent on the
particular nature of the aryl substituent R. When an excess
of this acetylide (typically ca. 3 equiv) was added to ketone
1, unsymmetrical pentacene diols 3c-j were obtained in
excellent yields (84-100%), with the exception of diol 3i
that was isolated in only a moderate yield. Several of these
transformations deserve particular comment. For the forma-
tion of diol 3j, the use of HexLi was not a problem for the
acetylide formation, despite the presence of the fluorine atom
on the aromatic ring. On the other hand, synthesis of diol
3g required formation of the acetylide with LDA rather than
HexLi to circumvent competition from lithium-halogen
exchange at the aromatic bromide in 1-bromo-4-ethynylben-
zene. In the presence of the CF3 group, the synthesis of diol
3i required the use of i-PrMgCl solution to minimize
formation of side products, as previously shown by Ong and
co-workers.8 Unfortunately, the electron-deficient nature of
this substrate also rendered it less nucleophilic, and as a
result, the formation of diol 3i was sluggish. Even after 7 h,
only 54% of 3i was obtained, along with the recovery of
unconsumed starting material 1.
the reaction of one equivalent of TIPS-Ct C-Li with a
slight excess of 6,13-pentacenequinone in THF gives 1 in
88% yield. This reaction can be performed easily on a
multigram scale as a result of a facile purification procedure
which employed orthogonal solubility between 1 and un-
desired materials.
Initially, derivative 2a was envisioned as a general
precursor to unsymmetrical pentacenes via the incorporation
of the desired donor/acceptor moiety via the Sonogashira or
related cross-coupling reactions.7 To this end, the addition
of excess ethynyl Grignard to 1 gave diol 3a in 89% yield,
following workup and purification by column chromato-
graphy. Aromatization through the addition of SnCl2·2H2O
to 3a in THF provided the deep-blue colored solution
expected for pentacene 2a, when the reaction was carried
out under a nitrogen atmosphere. Unfortunately, when the
reaction mixture was exposed to air, the characteristic blue
color quickly disappeared, and none of the desired product,
2a, could be isolated. Since 2a proved to be elusive/unstable
in the presence of oxygen, an alternative route based on the
differentially protected 6,13-bis(silylethynyl)pentacene 2b
was explored. Unsymmetrical diol 3b was obtained in 90%
yield through the reaction of 1 with TMS-Ct C-Li, and
subsequent aromatization gave pentacene 2b as an air-stable
product in excellent yield over the two steps. Unfortunately,
Diols 2c-j were then reduced using excess SnCl2·2H2O
in THF to obtain unsymmetrically substituted pentacenes
2c-j in excellent yield (81-94%). Unlike the conditions
typically reported for the tin(II)-mediated reduction of
diethynylated pentacene diols,9 the reductions here were
carried out without the presence of an acid, such as 10%
HCl. In our hands, these milder conditions have the
(6) (a) Boudebous, A.; Constable, E. C.; Housecroft, C. E.; Neuburger,
M.; Schaffner, S. Acta Crystallogr. Sect. C 2006, 62, o243–o245. (b) See
also: Ried, W.; Dankert, G. Chem. Ber. 1959, 92, 1223–1236.
(7) (a) Tykwinski, R. R. Angew. Chem., Int. Ed. 2003, 42, 1566–1568.
(b) Chinchilla, R.; Na´jera, C. Chem. ReV. 2007, 107, 874–922.
(8) Li, Y.; Wu, Y.; Liu, P.; Prostran, Z.; Gardner, S.; Ong, B. S. Chem.
Mater. 2007, 19, 418–423.
(9) (a) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am.
Chem. Soc. 2001, 123, 9482–9483. (b) Anthony, J. E.; Eaton, D. L.; Parkin,
S. R. Org. Lett. 2002, 4, 15–18.
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Org. Lett., Vol. 10, No. 19, 2008

Figure 1. (a) UV-vis absorption spectra for selected pentacenes (250-725 nm). Inset: expansion of the low-energy region of the UV-vis
absorption spectra (475-725 nm). (b) Fluorescence of selected pentacenes (600-850 nm) using λexc ) 551 nm. (c) Table of photophysical
properties (all spectra and data as measured in CH2Cl2).
advantage of improved functional group tolerance while still
providing excellent yields.
identical to symmetrical analogue 6,13-bis(triisopropylsilyl-
ethynyl)pentacene 7 (λmax ) 643 nm) (Figure 2).11 The effect
Given the success of the stepwise formation of pentacenes
2b-j, the final synthetic target was one with donor-acceptor
functionalization across the pentacene skeleton, compound
4 (Scheme 3).10 As a donor, the para-octyloxy group was
Scheme 3. Synthesis of Polarized Pentacene 4
Figure 2. Related symmetrical pentacenes 7 and 8a-d.
of the various auxochromes at the para position of the
benzene ring resulted mainly in a change of energy for λmax
values, with little or no change in the vibronic fine structure.
The λmax value of 652 nm for 6-triisopropylsilylethynyl-13-
phenylethynyl pentacene 2f represents a red shift of 10 nm
compared to that of 2b, a result of extending the conjugation
length through the incorporation of the additional phenyl ring.
Electron-donating substituents such as alkoxy groups in 2c
and 2d produced slightly red-shifted λmax values of 656 and
655 nm, respectively, in comparison to 2f. A similar red shift
was found for derivatives bearing a CH3, CF3, or Br group,
with λmax ) 654 nm in all three cases. Meanwhile, the fluoro-
incorporated to ensure solubility of the product, given the
formal removal of the TIPS group which provided solubility
to 2b-j. Thus, the reaction of the lithium acetylide formed
from 1-n-octyloxy-4-ethynylbenzene with 6,13-pentacene-
quinone afforded ketone 5 in 60% yield. In the second step,
the addition of excess lithiated 1-fluoro-4-ethynylbenzene
afforded diol 6 in 61% yield. Finally, aromatization of 6 gave
pentacene 4 in 82% yield.
substituted pentacene 2j had a slightly blue-shifted λmax
,
found at 651 nm. The effect of extending the π-system
through the additional TIPS-ethynyl group on the aromatic
ring of 2h (λmax ) 659 nm) red-shifted λmax by 7 nm in
comparison to 2f, in addition to a more intense high-energy
band at 383 nm. Push-pull pentacene 4 had the lowest
HOMO-LUMO gap with a λmax of 664 nm. Compound 4
in toluene has a λmax of 661 nm which is slightly red-shifted
in comparison to that of 8a (λmax ) 655 nm in benzene)12
UV-vis spectroscopy (in CH2Cl2) was used to explore
the electronic outcome of varying the constitution of the
pendent substituents on the pentacene framework. Figure 1a
shows the absorption spectra for the range of 250-725 nm
for selected pentacene derivatives, and the inset highlights
the low-energy absorption region (475-725 nm). The
spectrum of unsymmetrical 2b (λmax ) 642 nm) is nearly
but similar to 8b (λmax ) 660 nm in toluene),8 8c (λmax
662 nm in toluene),8 and bis-donor pentacene 8d (λmax
)
)
(10) To date, attempts to incorporate an aryl moiety with a stronger
acceptor group (e.g., -NO2 or -CN) have not been successful.
(11) λmax of ca. 643 nm is typical for 6,13-bis(trialkylsilylethynyl)
pentacenes. See: (a) ref 9a and (b) Chen, J.; Subramanian, S.; Parkin, S. R.;
Siegler, M.; Gallup, K.; Haughn, C.; Martin, D. C.; Anthony, J. E. J. Mater.
Chem. 2008, 18, 1961–1969.
(12) Maulding, D. R.; Roberts, B. G. J. Org. Chem. 1969, 34, 1734–
1736.
Org. Lett., Vol. 10, No. 19, 2008
4165

664 nm in toluene).8 Thus, there seems to be little added
affect on the optical HOMO-LUMO gap from the incor-
poration of the fluorine atom as a weak acceptor versus an
additional alkoxy donor group. It is worth noting, however,
that overall the effect of placing ethynyl and arylethynyl
groups at the 6- and 13-positions of pentacene provides for
a substantial red shift for λmax compared to that of pristine
pentacene (λmax ) 576 nm in benzene).12
Fluorescence spectra for 2b-j were measured in CH2Cl2
(λexc ) 651 nm), and quantum yields of fluorescence (ΦF)
were determined relative to cresyl violet perchlorate in
methanol (ΦF ) 0.67 at 5.9 × 10-7 M).13 The spectrum of
2b was analogous to symmetrical analogue 7, while the
spectra of 2c-j varied slightly in terms of emission
wavelength, Stokes shift, quantum yield, and the shape of
the emission trace (Figure 1b). The table provided in Figure
1 summarizes the salient emission properties. Overall, λmax
of emission for 2c-j occurred in a narrow range (660-668
nm) irrespective of the nature of the substituent. Stokes shifts
were small, 7-13 nm, as expected for a rigid chromophore.
The ΦF values ranged from 0.05 to 0.13, and pentacenes
bearing an electron-donating alkoxy group (2c and 2d) or a
CF3 group (2i) showed the lowest ΦF values, accompanied
by a broader emission trace and the largest Stokes shifts.
Donor-acceptor 4 has the largest Stokes shift of 15 nm
which was accompanied by the lowest emission efficiency
(ΦF ) 0.03).
Thermal stability of 2b-j and 4 was assessed by thermal
gravimetric analyses (TGA) and differential scanning calo-
rimetry (DSC). TGA showed no significant weight loss
(<5%) below 410 °C for pentacenes 2c-j and 4, which is
slightly higher than that for 2b (385 °C) and 7 (375 °C). In
the DSC traces, however, an exotherm peak was observed
at much lower temperatures and assigned to decomposition,
likely through a Diels-Alder reaction.14 More specifically,
DSC analysis showed melting points for compounds 2c and
2d at 134 and 65 °C, respectively, followed by decomposition
at 136 and 149 °C, respectively. For pentacenes 2e-j and
4, no melting points were found by DSC, and decomposition
occurred in the range of 175-215 °C.15
Figure 3. X-ray structure of 2e illustrating its solid-state geometry
(20% probability level). Selected bond angles (deg): C(1)-C(23)-
C(24) 174.4(2), C(23)-C(24)-Si 174.03(18), C(12)-C(34)-C(35)
177.2(2), C(34)-C(35)-C(36) 179.2(2).
rings of its dimeric neighbor. This packing motif places the
phenyl moiety approximately on top of a TIPS group and
results in a bowing of the conjugated backbone of the
molecule. This is most noticeable in the pendent acety-
lene groups where bond angles range from 174-179°.
Despite this fact, the two pentacene moieties are still able
to achieve close contact in which the interplanar distance
between the acene cores is on average only 3.45 Å.17 This
is comparable to, for example, that of 7 at 3.47 Å, which
adopts a 2-D slipped-stacked arrangement in which the TIPS
groups are staggered while still maintaining overlap of about
2.5 rings from each pentacene moiety.9a
In conclusion, we have demonstrated that a variety of
unsymmetrically substituted pentacenes can be synthesized
in two steps from a common intermediate. This synthetic
method should be broadly applicable as a gateway to new
pentacene derivatives with both varied electronic structure
and extended conjugated skeletons. Evaluation of charge-
transport properties of these new materials is underway
toward fostering a better understanding of the electronic
effects that result from the electronic polarization of the
pentacene core, and these results will be reported in due
course.
A crystal of unsymmetrical tolyl pentacene 2e suitable for
X-ray crystallography was grown from a CH2Cl2 solution
which had been layered with acetone and allowed to slowly
evaporate at 4 °C.16 Pentacene 2e crystallizes as a cen-
trosymmetric dimeric pair (Figure 3) in which four aromatic
rings of one pentacene molecule overlap with four aromatic
Acknowledgment. This work has been generously sup-
ported by the University of Alberta and the Natural Sciences
and Engineering Research Council of Canada (NSERC)
through the Discovery Grant program. D.L. thanks NSERC
(PGS-D), the Alberta Ingenuity Fund, the University of
Alberta, and the Killam Trusts for scholarship support.
(13) Isak, S. J.; Eyring, E. M. J. Phys. Chem. 1992, 96, 1738–1742.
(14) This is analogous to the behavior of related pentacenes. See, for
example, refs 8 and 11b.
Supporting Information Available: Experimental pro-
1
cedures, spectroscopic data, H and 13C NMR spectra for
(15) See Supporting Information for details.
compounds 1-6, and CIF file (2e). This material is available
free of charge via the Internet at http://pubs.acs.org.
(16) X-ray crystallographic data for 2e: C42H40Si, M ) 572.83; triclinic
space group P1 (No. 2); Fcalcd ) 1.153 g cm-3; a ) 12.0801 (14) Å, b )
j
13.1806 (15) Å, c ) 13.6258 (16) Å; R ) 95.3146 (18)°, ꢀ ) 115.2970
(16)°, γ ) 116.2079 (16)°; V ) 1650.6 (3) Å3; Z ) 2; µ ) 0.099 mm-1
.
OL801464K
2
2
Final R1(F) ) 0.0553 (4807 observations [Fo g 2σ(Fo )]); wR2(F2) )
2
2
0.1529 for 386 variables and 6264 data with [Fo g -3σ(Fo )]; CCDC
692481. X-ray data have been deposited at the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+44)-
122-333-6033.
(17) The interplanar distance was measured by calculating the distance
between the least-squares plane of each pentacene moiety as defined by
the 22 carbon atoms of the acene framework.
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Org. Lett., Vol. 10, No. 19, 2008