Synthesis and Optical Properties of Donor-Acceptor Tetrakis(phenyl-ethynyl) benzenes

Article abstract of DOI:10.1055/s-2003-43372

A simple and efficient synthesis of a series of donor-acceptor substituted 1,2,4,5-tetrakis(phenylethynyl)benzenes is described. The molecules constructed exhibit interesting optical properties. Preliminary studies show that the tetraynes hold promise as

Full text of DOI:10.1055/s-2003-43372

CLUSTER
165
Synthesis and Optical Properties of Donor-Acceptor Tetrakis(phenyl-
ethynyl)benzenes
D
onor-A
c
e
ceptor
T
etr
a
kis(phe
e
nylethynyl)
m
benzenes ie J. Miller, Jeremiah A. Marsden, Michael M. Haley*
Department of Chemistry and the Materials Science Institute, University of Oregon, Eugene, Oregon 97403-1253, USA
Fax +1(541)3460487; E-mail: haley@oregon.uoregon.edu
Received 3 September 2003
Dedicated to Prof. Reg Mitchell on occasion of his 60th birthday.
behavior.6 In a similar manner, 4–10 should show promis-
ing NLO susceptibilities arising from the extensively po-
larized, conjugated pathways between the electron
donating and accepting groups. Comparison of their opti-
cal and nonlinear optical properties with counterparts 1
and 2 should effectively illustrate the net result on elec-
tronic polarization, if any, of locking the p-system into
planarity. Toward that goal, we report herein the synthesis
of models 4–10, their linear optical behavior, and prelim-
inary data to support their potential use as NLO materials.
In addition to known parent 37 and tetradonor system 4,
we prepared a series of ‘tuned’ regioisomeric donor-ac-
ceptor chromophores 5–7 by strategic placement of the
donating N,N-dibutylamino group and the strongly ac-
cepting nitro group. For further ‘tunability’, cyano groups
were also incorporated as weaker electron acceptors on 8–
10. In addition, the cyano functionality was chosen for its
metal-binding ability.8 For example, chelation of 8–10
with silver ions could form macromolecular networks
which are of interest due to their large conductive surfaces
and potential as molecular wires.9
Abstract: A simple and efficient synthesis of a series of donor-ac-
ceptor substituted 1,2,4,5-tetrakis(phenylethynyl)benzenes is de-
scribed. The molecules constructed exhibit interesting optical
properties. Preliminary studies show that the tetraynes hold promise
as nonlinear optical (NLO) materials.
Key words: alkynes, arenes, chromophores, cross-coupling, palla-
dium
In recent years interest in highly conjugated systems
based on phenylacetylene scaffolding has increased dra-
matically.1 The elongated electronic pathways in these
molecules allow chemists to design structures with tech-
nologically significant applications in mind, such as liq-
uid crystal displays, nonlinear optical (NLO) devices,
molecular wires and sensors, and self-assembly of nano-
structures.2 In our lab we have focused on the construction
of phenylacetylene macrocycles based on dehydroben-
zoannulenes,3 and have shown these systems to exhibit
promising optical and NLO properties.4
R
R
R
R
Like 3, the synthesis of tetra-donor 4 requires a four-fold
cross-coupling to a tetraiodobenzene core (Scheme 1).
The requisite alkyne 11 is easily prepared in two steps
from N,N-dibutyl-4-iodoaniline (12).4a Sonogashira
cross-coupling of trimethylsilylacetylene (TMSA) with
12 afforded 13 in high yield, which was readily desilated
with K2CO3 in MeOH–Et2O. Coupling 11 to 1,2,4,5-tet-
raiodobenzene furnished 410 as a bright yellow solid in
79% yield.
R
R
R
R
2
1
For phenylacetylenes 5–10, we needed a simple, rational
synthesis, taking advantage of selective cross-couplings
and selective removal of silane protective groups. By ex-
ploiting the difference in reactivities of aryl iodides vs
aryl bromides for cross-coupling,11 specific positioning of
the alkyne groups could be achieved utilizing all three
known dibromodiiodobenzenes as the central arene core.
The stepwise synthesis of ortho-substituted product 5 is
outlined in Scheme 2. Starting with 1,2-dibromo-4,5-di-
iodobenzene (14),12 Pd-mediated cross-coupling to the
iodo positions with TMSA at room temperature afforded
diyne 15. Use of THF as a co-solvent in the reaction in-
creased product solubility and helped solubilize the cata-
lyst.13 Triisopropylsilylacetylene (TIPSA) then replaced
the bromine, yielding intermediate 16 (79%). In our case,
a temperature of only 40 °C was required for the reaction,
Figure 1 Bisannulenes 1 and 2
We have recently been studying a variety of donor-accep-
tor substituted bisannulenes such as 1 and 2 (Figure 1),5
and were interested in comparing the physical properties
of these molecules with a series of corresponding acyclic
model compounds. Unlike 1 and 2, phenylacetylenes 3–
10 (Figure 2) are not locked into planarity. The molecules
can rotate freely about their C–C single bonds, decreasing
p-orbital overlap and thus reducing their effective conju-
gation lengths. Molecule 3 already has demonstrated NLO
SYNLETT 2004, No. 1, pp 0165–0168
Advanced online publication: 4.12.2003
DOI: 10.1055/s-2003-43372; Art ID: C00203ST
© Georg Thieme Verlag Stuttgart · New York
0
5.
0
1.
2
0
0
4

166
J. J. Miller et al.
CLUSTER
H
H
Bu2N
NBu2
3
5
7
9
4
H
H
Bu2N
Bu2N
NBu2
NBu2
O2N
NBu2
6
O2N
O2N
NBu2
NBu2
O2N
NC
NO2
NBu2
8
Bu2N
Bu2N
NO2
NC
NC
NBu2
NBu2
NBu2
10
Bu2N
CN
NC
CN
Figure 2 Tetrakis(phenylethynyl)benzene target molecules
whereas Sonogashira cross-couplings to bromoarenes of- TBAF to remove the TIPS groups, and cross-coupling of
ten require temperatures in excess of 80 ºC. Selective pro- commercially available 1-iodo-4-nitrobenzene furnished
tiodesilylation of 16 by K2CO3/MeOH removed the more arene 5 as a dark red-orange solid. Table 1 reports the
labile TMS groups. Sonogashira cross-coupling of io- yields for each donor-acceptor substituted tetrakis(phe-
doarene 12 to the resultant terminal alkyne afforded 17. nylethynyl)benzene following analogous synthetic routes.
Another sequence of silane deprotection, this time using
The phenylacetylene systems display interesting linear
optical characteristics attributed to p→p* and other transi-
tions. Alteration of the electronic pathways in 3–7 gives
TMS
rise to notable changes in their UV-Vis spectra (Figure 3).
I
a
b
92%
>95%
Bu2N
Table 1 Reaction Yields for the Synthesis of 5–10
Bu2N
12
13
Haloarene
Step a Step b Step c Step d Final
product
Bu2N
NBu2
95%
82%
90%
79%
99%
74%
44%
35%
42%
45%
62%
5
8
Br
I
c
Br
Br
I
79%
67%
96%
6
9
Bu2N
Br
I
I
I
11
4
Bu2N
NBu2
78%
98%
7
10
Br
Scheme 1 Reagents and conditions: (a) TMSA, PdCl2(PPh3)2, CuI,
DIPA, THF; (b) K2CO3, MeOH, Et2O; (c) 1,2,4,5-tetraiodobenzene,
PdCl2(PPh3)2 CuI, DIPA, THF
I
Br
Synlett 2004, No. 1, 165–168 © Thieme Stuttgart · New York

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Donor-Acceptor Tetrakis(phenylethynyl)benzenes
167
6 are nearly identical, while 7 displays a lesser intensity in
its lower energy band. This difference in 7 may be attrib-
utable to the lack of the 1,4-bis(phenylethynyl)benzene
chromophore between donor and acceptor groups. Exten-
sion of this linear conjugated pathway in similar phenyl-
acetylene systems has been shown to red-shift absorption
wavelengths and even increase NLO susceptibility.6
TMS
TMS
Br
I
I
Br
Br
a
b
Br
15
14
NBu2m
TIPS
TIPS
TMS
TMS
TIPS
Preliminary optical and NLO measurements have been
accomplished for systems 5–7. The two photon absorption
studies14 have provided data showing s(2) values compa-
rable to those of a common reference standard, AF50, as
seen in Table 2.15 The data were taken at 796 nm in a ben-
zene solution with a 160 fs pulse duration. The measure-
ments have yet to be optimized and may show even
greater values which will be disclosed in future publica-
tions along with data from their planar, annulenic counter-
parts.
c
TIPS
16
17
NBu2m
O2N
NBu2
d
Table 2 Preliminary NLO Data for Models 5–7
Compound
s
(2) (cm4 s)
5
O2N
NBu2
3 × 10–49
Scheme 2 Reagents and conditions: (a) TMSA, PdCl2(PPh3)2, CuI,
DIPA, THF; (b) TIPSA, PdCl2(PPh3)2, CuI, DIPA, THF, 40 °C; (c) i:
K2CO3, MeOH, Et2O; ii: 11, Pd(PPh3)4, CuI, DIPA, THF; (d) i: TB-
AF, THF, MeOH; ii: 1-iodo-4-nitrobenzene, Pd(PPh3)4, CuI, DIPA,
THF, 40 °C
Ph2N
Dec
Dec
5
6
7
3 × 10–49
2 × 10–49
2 × 10–49
The parent species 3 and tetradonor 4 display absorption
bands with similar patterns. However, due the increased
electron density of tetradonor 4, a bathochromic shift of
the lower energy bands of about 75 nm is observed. While
only a small red-shift in the higher energy band of 4 is
seen, the absorption intensity is nearly tripled.
Phenylacetylenes 3–10 demonstrate promise as highly ac-
tive optical and nonlinear optical materials. These mole-
cules continue to pave the way to more sophisticated and
elaborate systems that should exhibit higher optical activ-
ity and the ability to be tuned on the basis of certain struc-
tural features. The synthesis and study of these advanced
structures will be the subject of future reports.
Upon addition of the donor and acceptor substituents, a
much more dramatic change in the electronic absorption
spectra was seen. The increased polarization of the conju-
gated backbones gives rise to a large broadening and fur-
ther bathochromic shifts of the UV-Vis signals. The
absorption cut-offs for 5–7 reach into the visible range at
around 550 nm. It is notable that the peak patterns of 5 and
Acknowledgment
This work was supported by the National Science Foundation
(CHE-0104854) and the UO Ronald E. McNair Scholars Program
(summer research fellowship to J. J. M). J. A. M acknowledges the
US Department of Education for a GAANN fellowship.
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Figure 3 UV-Vis spectra of tetraynes 3–7
Synlett 2004, No. 1, 165–168 © Thieme Stuttgart · New York

168
J. J. Miller et al.
CLUSTER
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7: 1H NMR (CDCl3): d = 0.97 (t, J = 7.4 Hz, 8 H), 1.37 (sext,
J = 7.5 Hz, 8 H), 1.57 (quin, J = 7.8 Hz, 8 H), 3.31 (t, J = 7.7
Hz, 12 H), 6.59 (d, J = 9.0 Hz, 4 H), 7.35 (d, J = 9.0 Hz, 4
H), 7.71 (d, J = 9.0 Hz, 4 H) 7.72 (s, 2 H), 8.23 (d, J = 9.0
Hz, 4 H). 13C NMR (CDCl3): d = 13.95, 20.25, 29.30, 50.60,
85.49, 92.92, 93.27, 98.10, 107.84, 111.10, 123.58, 124.09,
125.74, 130.05, 132.32, 132.99, 134.55, 146.95, 148.37.
8: 1H NMR (CDCl3): d = 0.98 (t, J = 7.2 Hz, 8 H), 1.38 (sext,
J = 7.5 Hz, 8 H), 1.59 (quin, J = 8.2 Hz, 8 H), 3.31 (t, J = 7.7
Hz, 12 H), 6.59 (d, J = 9.0 Hz, 4 H), 7.39 (d, J = 9.0 Hz, 4
H), 7.60 (br s, 4 H), 7.66 (d, J = 1.8 Hz, 2 H). 13C NMR
(CDCl3): d = 13.98, 20.28, 29.31, 50.65, 85.95, 91.93, 92.73,
98.35, 108.11, 111.10, 111.67, 118.40, 122.74, 126.76,
127.88, 131.97, 132.10, 133.2, 134.63, 148.30.
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(10) General Alkyne Coupling Procedure. A solution of
haloarene (1 equiv) in i-Pr2NH–THF (3:1, 0.05 M) was
degassed by bubbling Ar for 20 min or by three freeze-
pump-thaw cycles. Pd(PPh3)2Cl2 (for iodoarene) or
Pd(PPh3)4 (for bromoarene) (0.05 equiv) and CuI (0.12
equiv) were added and the flask was sealed under Ar. The
terminal acetylene (1.1 equiv per halide) was then added via
syringe, and the reaction mixture stirred at r.t. (for
iodoarenes) or 40 °C (for bromoarenes) for 12 h. Upon
completion, the reaction mixture was concentrated in vacuo,
diluted with CH2Cl2, and filtered through a bed of silica gel.
The filtrate was concentrated, and the crude material was
purified by column chromatography on silica gel (hexanes–
CH2Cl2).
9: 1H NMR (CDCl3): d = 0.97 (t, J = 7.4 Hz, 12 H), 1.37
(sext, J = 7.5 Hz, 8 H), 1.59 (quin, J = 7.5 Hz, 8 H), 3.30 (t,
J = 7.7 Hz, 8 H), 6.58 (d, J = 8.7 Hz, 4 H), 7.36 (d, J = 8.7
Hz, 4 H), 7.65 (s, 4 H), 7.69 (s, 1 H) 7.71 (s, 1 H). 13C NMR
(CDCl3): d = 14.00, 20.31, 29.33, 50.66, 85.63, 92.36, 92.72,
98.65, 107.78, 111.15, 111.55, 118.56, 122.24, 127.67,
128.17, 132.11, 133.12, 134.04, 135.08, 148.45.
10: 1H NMR (CDCl3): d = 0.97 (t, J = 7.4 Hz, 12 H), 1.38
(sext, J = 7.8 Hz, 4 H), 1.58 (quin, J = 7.7 Hz, 4 H), 3.31 (t,
J = 7.7 Hz, 4 H), 6.58 (d, J = 9.0 Hz, 4 H), 7.35 (d, J = 9.0
Hz, 4 H), 7.65 (s, 8 H), 7.70 (s, 2 H). 13C NMR (CDCl3): d =
13.86, 18.84, 20.17, 29.21, 47.06, 50.54, 54.62, 85.29,
92.20, 92.99, 97.89, 107.74, 111.06, 111.55, 118.40, 124.07,
125.63, 127.96, 131.97, 132.04, 132.89, 134.44, 148.29.
15: 1H NMR (CDCl3): d = 0.26 (s, 18 H), 7.69 (s, 2 H). 13
C
NMR (CDCl3): d = –0.16, 100.84, 100.05, 124.46, 126.05,
136.54.
16: 1H NMR (CDCl3): d = 0.27 (s, 18 H), 1.13 (s, 63 H), 7.53
(s, 2 H). 13C NMR (CDCl3): d = –0.04, 11.29, 18.75, 97.46,
100.60, 102.11, 104.22, 125.10, 125.16, 137.02.
17: 1H NMR (CDCl3): d = 0.96 (t, J = 7.4 Hz, 8 H), 1.14 (s,
42 H), 1.36 (sext, J = 7.4 Hz, 8 H), 1.58 (quin, J = 7.6 Hz, 8
H), 3.29 (t, J = 7.5 Hz, 12 H), 6.57 (d, J = 9.0 Hz, 4 H), 7.41
(d, J = 9.0 Hz, 4 H), 7.57 (s, 2 H). 13C NMR (CDCl3): d =
11.32, 13.98, 18.77, 20.31, 29.36, 50.66, 85.97, 96.06,
97.11, 104.98, 108.56, 111.14, 123.76, 125.72, 133.16,
135.95, 148.17.
4:1H NMR (CDCl3): d = 0.98 (t, J = 7.4 Hz, 24 H), 1.37 (sext,
J = 7.5 Hz, 16 H), 1.59 (quin, J = 7.7 Hz, 16 H), 3.30 (t, J =
7.7 Hz, 16 H), 6.59 (d, J = 9.2 Hz, 8 H), 7.43 (d, J = 9.2 Hz,
8 H), 7.63 (s, 2 H). 13C NMR (CDCl3): d = 13.96, 20.29,
29.36, 50.66, 86.31, 96.23, 108.97, 111.14, 124.60, 133.05,
133.88, 147.98.
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5: 1H NMR (CDCl3): d = 0.97 (t, J = 7.4 Hz, 8 H), 1.37 (sext,
J = 7.4 Hz, 8 H), 1.59 (quin, J = 8.4 Hz, 8 H), 3.31 (t, J = 7.7
Hz, 12 H), 6.59 (d, J = 9.3 Hz, 4 H), 7.42 (d, J = 9.0 Hz, 4
H), 7.67 (d, J = 8.9 Hz, 4 H) 7.73 (s, 2 H), 8.24 (d, J = 8.9
Hz, 4 H). 13C NMR (CDCl3): d = 13.98, 20.30, 29.32, 50.65,
86.12, 92.65, 92.80, 98.49, 108.06, 111.07, 122.66, 123.70,
126.91, 129.90, 132.20, 133.18, 134.65, 146.99, 148.37.
6: 1H NMR (CDCl3): d = 0.98 (t, J = 7.4 Hz, 8 H), 1.38 (sext,
J = 7.5 Hz, 8 H), 1.58 (quin, J = 7.9 Hz, 8 H), 3.30 (t, J = 7.7
Hz, 12 H), 6.57 (d, J = 9.0 Hz, 4 H), 7.35 (d, J = 9.0 Hz, 4
H), 7.66 (d, J = 9.0 Hz, 4 H) 7.66 (s, 1 H), 7.68 (s, 1 H), 8.18
(d, J = 9.0 Hz, 4 H). 13C NMR (CDCl3): d = 13.98, 20.30,
29.32, 50.65, 85.76, 92.55, 93.35, 98.87, 107.77, 111.14,
122.06, 123.64, 127.85, 130.18, 132.25, 133.12, 134.00,
135.20, 146.93, 148.50.
Synlett 2004, No. 1, 165–168 © Thieme Stuttgart · New York