Conjugated " B-entacenes": Polycyclic aromatics containing two borepin rings

Article abstract of DOI:10.1021/ol2010159

The synthesis and characterization of functionalized bora-acenes (B-entacenes) where Stille and Sonogashira cross-couplings were used to attach a series of electron-donating and -withdrawing substituents is reported. Photophysical, electrochemical, and co

Full text of DOI:10.1021/ol2010159

ORGANIC
LETTERS
2011
Vol. 13, No. 12
3106–3109
Conjugated “B-Entacenes”: Polycyclic
Aromatics Containing Two Borepin Rings
Anthony Caruso Jr.† and John D. Tovar*,†,‡
Department of Chemistry and Department of Materials Science and Engineering,
Johns Hopkins University, Baltimore, Maryland 21218, United States
tovar@jhu.edu
Received April 16, 2011
ABSTRACT
The synthesis and characterization of functionalized bora-acenes (B-entacenes) where Stille and Sonogashira cross-couplings were used to
attach a series of electron-donating and -withdrawing substituents is reported. Photophysical, electrochemical, and computational analyses
revealed that the LUMO level can be tuned by changing the para-conjugated substituent. Furthermore, the dimethylamino-functionalized molecule
exhibited intense solvatochromism due to the intramolecular charge-transfer interaction.
Inthe searchfor new organic electronicmaterials, acenes
and heteroacenes have become leading candidates for
replacing amorphous silicon and other inorganics in sys-
tems that require low cost, highly tunable and easily
processable materials for applications such as thin film
transistors (TFTs),1 sensors2 and photovoltaics.3 Infact, in
some areas pentacene already rivals the performance of
amorphous silicon.4 To date, the majority of acenes and
heteroacenes synthesized and studied have been p-channel
(hole-transporting) semiconductors. Progress has been
made in preparing n-channel (electron-transporting)
materials based on acene-like platforms. Anthony5 and
Bao6 have independently reported the functionalization of
thienoacenes, pentacenes, and perylene diimides with
electron-deficient groups as a method of increasing the
electron-transporting ability of these materials. Bunz and
co-workers systematically replaced carbons in the acene
backbone with heteroatoms (specifically nitrogen) result-
ing in hole- and electron-transporting materials.7 There
remains a need for new electron-transporting materials
because of the difficulty in synthesizing air-stable n-chan-
nel semiconductors in their doped forms.
† Department of Chemistry.
(5) (a) Anthony, J. E.; Facchetti, A.; Heeney, M.; Marder., S. R.;
Zhan, X. Adv. Mater. 2010, 22, 3876–3892. (b) Swartz, C. R.; Parkin,
S. R.; Bullock, J. E.; Anthony, J. E.; Mayer, A. C.; Malliaras, G. G. Org.
Lett. 2005, 7, 3163–3166.
(6) (a) Okamoto, T.; Nakahara, K.; Saeki, A.; Seki, S.; Oh, J. H.;
Akkerman, H. B.; Bao, Z.; Matsuo, Y. Chem. Mater. 2011, 23, 1946–
1953. (b) Tang, M. L.; Oh, J. H.; Reichardt, A. D.; Bao, Z. J. Am. Chem.
Soc. 2009, 131, 3733–3740. (c) Tang, M. L.; Okamoto, T.; Bao, Z. J. Am.
Chem. Soc. 2006, 128, 16002–16003.
(7) (a) Wu, J. I.; Wannere, C. S.; Mo, Y.; Schleyer, P. v. R.; Bunz,
U. H. F. J. Org. Chem. 2009, 74, 4343–4349. (b) Miao, S.; Appleton, A.;
Berger, N.; Barlow, S.; Marder, S. R.; Hardcastle, K. I.; Bunz, U. H. F.
Chem.;Eur. J. 2009, 15, 4990–4993.
‡ Department of Materials Science and Engineering.
(1) (a) Pisula, W.; Feng, X.; Mullen, K. Chem. Mater. 2011, 23, 554–
567. (b) Jung, B. J.; Lee, K.; Sun, J.; Andreou, A. G.; Katz, H. E. Adv.
Funct. Mater. 2010, 20, 2930–2944.
(2) Khan, H. U.; Roberts, M. E.; Knoll, W.; Bao, Z. Chem. Mater.
2011, 23, 1946–1953.
(3) Facchetti, A. Chem. Mater. 2011, 3, 733–758. (b) Brunetti, F. G.;
Gong, X.; Tong, M.; Heeger, A. J.; Wudl, F. Angew. Chem., Int. Ed.
2010, 49, 532–536. (c) Gorodetsky, A. A.; Cox, M.; Tremblay, N. J.;
Kymissis, I.; Nuckolls, C. Chem. Mater. 2009, 21, 4090–4092. (d) Ling,
M. M.; Erk, P.; Gomez, M.; Koenemann, M.; Locklin, J.; Bao, Z. Adv.
Mater. 2007, 19, 1123–1127.
(4) (a) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm,
C.; Schutz, M.; Maisch, S.; Effenerger, F.; Brunnbauer, M.; Stellacci, F.
Nature 2004, 431, 963–966. (b) Dimitrakopoulos, C. D.; Malenfant,
P. R. L. Adv. Mater. 2002, 14, 99–117. (c) Lin, Y. Y.; Gundlach, D. J.;
Nelson, S. F.; Jackson, T. N. IEEE Trans. Electron Devices 1997, 44,
1325.
(8) (a) Wakamiya, A.; Mori, K.; Araki, T.; Yamaguchi, S. J. Am.
Chem. Soc. 2009, 131, 10850–10851. (b) Wakamiya, A.; Mishima, K.;
Ekawa, K.; Yamaguchi, S. Chem. Commun. 2008, 579–581. (c) Yama-
guchi, S.; Wakamiya, A. Pure Appl. Chem. 2006, 78, 1413–1424. (d)
Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. J. Am. Chem.
Soc. 2002, 124, 8816–8817.
r
10.1021/ol2010159
Published on Web 05/23/2011
2011 American Chemical Society

The insertion of neutral, planar boron centers,
which are inherently electron deficient, into polycyclic
aromatic frameworks has been less developed. For
example, Yamaguchi,8 Perepichka,9 and we recently
inserted boron centers into π-conjugated systems such
as diborins, azaborineꢀthiophene heteroacenes, and
borepins, respectively. The borepins displayed well-
behaved cathodic electrochemistry at the boron cen-
ters, which is useful for n-channel and NLO materials.
Herein, we report the synthesis of extended fused
polycyclic aromatics containing two formally aromatic
borepin rings and the study of a series of functionalized
systems with electron-donating, -withdrawing, and
-neutral substituents. We refer to these systems as
“B-entacenes” because they are boron-containing pen-
tacyclic acenes.
used as a common core from which double Wittig olefina-
tions with bromochlorobenzaldehyde 3b resulted in the
meta-chlorinated tetrabromide 4b. Chemoselective lithiumꢀ
halogen exchange with the bromides followed by in situ
treatment of the tetralithio species with 2 equiv of
dimethyltin dichloride yielded fused stannepin 5b.
Tinꢀboron exchanges between boron trichloride and
the dimethyltin centers of 5b followed by treatment
with Mes*Li (Mes* = 2,4,6-tri-tert-butylphenyl) pro-
vided 1b in 40% yield.
Table 1 shows the functionalized borepins accessible
with more active palladium-catalyzed cross-coupling
conditions. Stille coupling between 1a and 2-tributyl-
stannylthiophene using Fu’s Pd(P-t-Bu3)2 catalyst14
resulted in p-thienyl B-entacene 6a in 34% yield. The
1
transformation was quantitative by crude H NMR;
however, purification of these compounds often resulted in
lower isolated yields because of poor solubility. Sonoga-
shira couplings under Buchwald conditions resulted in a
series of para-substituted phenylethynyl B-entacenes 6bꢀf.
The series included electron-withdrawing 4-ethyl ester
phenylacetylene and 4-fluorophenylacetylene, neutral phe-
nylacetylene, and electron-donating anisole and N,N-di-
methylaminophenylacetylene B-entacenes. The molecules
were thermally robust up to their melting points in DSC
(see Figures S26ꢀ28, Supporting Information).
Scheme 1. Synthesis of Para- and Meta-Chlorinated
B-Entacenes
Unexpectedly, the meta-chlorinated B-entacene 1b would
not react under several variations of active palladium
cross-coupling conditions, resulting in the isolation
of starting material in each case. As a result, 1b was
subjected to Kumada coupling conditions using Ni-
(dppp)Cl2, which is known to be more reactive toward
chlorides; however, to our surprise no reaction oc-
curred. A lack of reactivity of the meta chloride B-
entacene 1b was unforeseen since we previously re-
ported the functionalization of meta bromide DBBs
by common cross-coupling procedures.15 For this rea-
son, electronic factors arising from the inductive effects
of the more electron-deficient p-diphenylene diborane
are thought to be the barrier preventing successful
cross-couplings and not steric encumbrance.
Figure 1a shows representative electronic spectra for the
p-phenylethynyl B-entacenes (data for all measurements
are compiled numerically in Tables 2 and S2, Supporting
Information). Borepins 6a,cꢀf showed low energy bands
of low oscillator strength at ca. 445 nm attributed to
charge-transfer between the appendedelectron-richhandle
and the electron-deficient boron center. Intense πꢀπ*
transitions were observed, some with structured vibronic
features at higher energy. Not surprisingly, the strong-
er dimethylamino phenylacetylene donor substituent ap-
pendedtothe B-entacene (6b) had a muchmore intensebut
ill-defined low energy absorption. The photoluminescence
We previously reported chlorinated B-entacene 1a10 but
found that only nickel-catalyzed cross-couplings of aryl
Grignards were able to functionalize the polycyclic core.
Subsequently, Buchwald conditions using the Pd(Me-
CN)2Cl2/XPhos catalyst11 were used by Piers to enable
palladium-catalyzed reactions with chlorinated dibenzo-
[b,f]silepins.12 Thus, we revisited the reactivity of 1a and
sought to extend this to the meta isomer 1b, prepared as
shown in Scheme 1. The bis(phosphonium salt) 213 was
(9) Lepeltier, M.; Lukoyanova, O.; Jacobson, A.; Perepichka, D. F.
Chem. Commun. 2010, 46, 7007–7009.
(10) Caruso, A.; Siegler, M. A.; Tovar, J. D. Angew. Chem., Int. Ed.
2010, 49, 4213–4217.
(11) Gelman, D.; Buchwald, S. L. Angew. Chem., Int. Ed. 2003, 42,
5993–5996.
(12) Mercier, L. G.; Furukawa, S.; Piers, W. G.; Wakamiya, A.;
Yamaguchi, S.; Parvez, M.; Harrington, R. W.; Clegg, W. Organome-
tallics 2011, 30, 1719–1729.
(14) Littke, A. F.; Schwarz, L.; Fu, G. C. J. Am. Chem. Soc. 2002,
124, 6343–6348.
(15) Caruso, A., Jr.; Tovar, J. D. J. Org. Chem. 2011, 76, 2227–
2239.
(16) Swager, T. M.; Gil, C. J.; Wrighton, M. S. J. Phys. Chem. 1995,
99, 4886–4893.
(13) Bonifacio, M. C.; Robertson, C. R.; Jung, J. Y.; King, B. T. J.
Org. Chem. 2005, 70, 8522–8526.
Org. Lett., Vol. 13, No. 12, 2011
3107

Figure 1b shows a representative cyclic voltammagram
(CV) for the reductions of 6b. The first reduction half-wave
potential (E1/2) was ꢀ1.93 V, while the second process
was ca. 600 mV more negative. All of the functionalized
B-entacenes exhibited similar cathodic electrochemistry
with two one-electron reductions; furthermore, the elec-
tron-rich B-entacenes 6aꢀc were more difficult to reduce
than the neutral and electron-deficient B-entacenes 6dꢀf.
In addition to the well-behaved nature of these cathodic
processes, the ability to tune the LUMO level by changing
the functional handle para to the boron center (e.g., ꢀ2.76
to ꢀ3.08 eV) could enable the fashioning of B-entacene
into electron-accepting materials via substituent fine-tuning.
Table 1. Palladium-Catalyzed Cross-Couplings of Para-
Chlorinated B-Entacene 1a a
a Isolated yields. b Quantitative by 1H NMR. a Conditions: (a) Pd2-
(dba)3/ P(tBu)3, dioxane, CsF, 100 °C; (b) Pd(MeCN)2Cl2/XPhos,
Cs2CO3, MeCN, THF, heat.
Figure 2. PL spectra for 6b in various solvents (a) (acquired at
room temperature, with excitation at 392 nm), image of solu-
tions without (left) and with (right) irradiation with hand-held
UV (365 nm) (b) and DFT (B3LYP/6-31G*) calculations of the
LUMO (top) and HOMO (bottom) surfaces (c).
The PL spectra of the B-entacenes with electron-donat-
ing groups 6bꢀc were also acquired in solvents with
different ET30 values in order to see if there were any
solvatochromic effects due to the “push-pull” electronic
character. The methoxy donor 6c showed no more than a 6
nm shift in going from nonpolar to polar solvents (see
Figure S15, Supporting Information); however, the di-
methylamino donor 6b showed a shift of nearly 100 nm
(see Figure 2a,b). DFT calculations (B3LYP/6-31G*)
rationalized this behavior by revealing a large shift in
orbital density between the HOMO and LUMO surface
for 6b (Figure 2c), while the HOMO and LUMO wave
functions are predominantly localized on the B-entacene
core for 6c (see Figure S20, Supporting Information).
The ability to perform Sonogashira couplings on 1a
allowed for the synthesis of p-TIPS-ethynyl substituted
B-entacenes that could be deprotected by treatment with
Figure 1. UVꢀvis and PL spectra (a) for 6b,e (acquired in CHCl3
at room temperature with excitation at 392 nm) and CV of 6b (b)
(2 mM acquired in 0.1 M n-Bu4NPF6/THF and reported relative
to Ag/Agþ.
(PL) spectra of the B-entacenes 6aꢀf were bimodal with a
more intense higher energy λmax at ca. 464 nm and a less
intense low energy shoulder. Time-resolved photolumines-
cence on the B-entacenes revealed single exponential de-
cays with lifetimes between 6.8 and 9.6 ns with no
noticeable trends. We did not observe significant spectral
differences regardless of substituent electronics. We attri-
bute this to the node present in the wave function at the site
of attachment to the B-entacene core preventing delocali-
zation of the frontier molecular orbital surfaces (see the
Supporting Information).
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Org. Lett., Vol. 13, No. 12, 2011

alkynylated monomer with localized electronic structure
rather than an extended system with any sort of long-range
communication between the oligomeric B-entacenes as
witnessed by the localized HOMO and LUMO surfaces
of the B-entacenes (see the Supporting Information) with
the exception of 6b. The inherent electronic structure of
this B-entacene pathway is thus restricted relative to other
π-conjugated polymers.
Scheme 2. Sonogashira Polymerization Leading to 7
Table 2. Summary of Photophysical and Electrochemical
Data
compd abs λonset (nm) PL λmax (nm)
Φ
τa (ns)
E1/2b (V)
1c11
6a
6b
6c
6d
6e
6f
458
459
462
463
466
471
467
459
456
460
465
463
465
464
465
466
0.71 9.3 ꢀ1.89, ꢀ2.46
0.36 7.3 ꢀ1.99, ꢀ2.52
0.69 6.8 ꢀ1.93, ꢀ2.49
0.52 9.2 ꢀ1.85, ꢀ2.62
0.45 8.9 ꢀ1.89, ꢀ2.38
0.29 9.1 ꢀ1.84, ꢀ2.34
0.49 9.6 ꢀ1.83, ꢀ2.22
0.45 8.6 ꢀ1.86, ꢀ2.37
tetrabutylammonium fluoride to provide the terminal
alkyne 6f in 34% yield over two steps as shown in
Scheme 2. This alkyne was then polymerized under stan-
dard Sonogashira conditions with bis(decyloxy)-2,5-diio-
dobenzene16 to form polymer 7 in 70% yield (Mn 4400,
PDI 1.60). This oligomeric material was readily dissolved
in common organic solvents such as chloroform and
toluene, which would allow for solution processability.
7
a All lifetimes are single exponential. E1/2’s are reported
relative to Ag/Agþ with which the Fc/Fcþ couple was found to be
205 mV.
b
This report describes the synthesis of para- and meta-
chlorinated B-entacenes and the subsequent functiona-
lization of the para system by Stille and Sonogashira
cross-coupling reactions. With an expanded cross-cou-
pling repertoire, we were able to synthesize a series of
electron-donating, -withdrawing, and -neutral groups
appended to the electron-accepting B-entacenes cores.
Optical and electrochemical characterization revealed
that the band gap remains quantitatively the same, as
seen by the similar λonsets of absorption for 6aꢀf, while
the LUMO was tuned by changing the electronic char-
acter of the conjugated substituent. Electron-deficient
acceptor materials in which the energy levels can be
tuned to optimize interactions with electron-rich donor
materials are of great interest for future work with
heterostructures or blends for use in organic device
fabrication.
Figure 3. UVꢀvis, PL (a) and CV (b) data for 7. UVꢀvis and PL
spectra were acquired in CHCl3 at room temperature with
excitation at 399 nm, CV acquired in 0.1 M n-Bu4NPF6/THF
and reported relative to Ag/Agþ.
Polymer 7 had an onset of absorption at 459 nm and a
more intense higher energy band at 396 nm. The typical
lower energy, low oscillator strength bands associated with
a charge-transfer interaction were more intense, but over-
lapping with the πꢀπ* transition. PL spectra for 7 showed
a peak at 466 nm that had a low energy shoulder at 490 nm
and a quantum yield of 45% (Figure 3a). The CV spectrum
of 7 showed two reductions (E1/2 = ꢀ1.86, ꢀ2.37 V)
that were more broad than those seen for the mono-
mers (Figure 3b). The photophysical and electrochemical
data for 7 were quite similar to those for related dibenzo-
borepin polymers15 and suggested that 7 behaved like an
Acknowledgment. We thank Johns Hopkins University
for generous support. A.C. was supported by a JHU
Chemistry Alumni Graduate Fellowship.
Supporting Information Available. Experimental de-
tails, characterization data, and molecular orbital sur-
faces. This material is available free of charge via the
Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 12, 2011
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