Tuning octopolar NLO chromophores: Synthesis and spectroscopic characterization of persubstituted 1,3,5-tris(ethynylphenyl)benzenes

Article abstract of DOI:10.1021/jo049279i

The synthesis of a series of octopolar 1,3,5-tris(ethynylphenyl)benzenes via Sonogashira coupling is described, varying the substituents on both the central benzene core as well as the acetylenic periphery. In particular, systems bearing an electron-rich

Full text of DOI:10.1021/jo049279i

Tu n in g Octop ola r NLO Ch r om op h or es: Syn th esis a n d
Sp ectr oscop ic Ch a r a cter iza tion of P er su bstitu ted
1,3,5-Tr is(eth yn ylp h en yl)ben zen es
Gunther Hennrich,*,† Inge Asselberghs,‡ Koen Clays,‡ and Andre´ Persoons‡
Departamento de Qu´ımica Orga´nica, Universidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid,
Spain, and Department of Chemistry, University of Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
gunther.hennrich@uam.es
Received April 29, 2004
The synthesis of a series of octopolar 1,3,5-tris(ethynylphenyl)benzenes via Sonogashira coupling
is described, varying the substituents on both the central benzene core as well as the acetylenic
periphery. In particular, systems bearing an electron-rich core and an electron-poor periphery are
obtained that display advanced optical properties. The linear (by UV-vis and fluorescence
spectroscopy) and second-order nonlinear optical properties are studied, the latter by hyper-Rayleigh
scattering (HRS) (Hendrickx, E.; Clays, K.; Persoons, A. Acc. Chem. Res. 1998, 31, 675-683). The
influence of different core and periphery substitution is revealed by the optical properties and
confirms the possibility of fine-tuning those. Because of the presence of (one-photon) fluorescence
for all compounds, femtosecond hyper-Rayleigh scattering has been applied (Olbrechts, G.; Strobbe,
R.; Clays, K.; Persoons, A. Rev. Sci. Instrum. 1998, 69, 2233-2241). The implementation of the
deconvolution in the frequency domain allows for a demodulation and a phase shift between
immediate (nonlinear) scattering and time-delayed (multiphoton) fluorescence for high modulation
frequencies (Wostyn, K.; Binnemans, K.; Clays, K.; Persoons, A. Rev. Sci. Instrum. 2001, 72, 3215-
3220). In accordance with the linear optical properties, the second-order NLO properties can also
be tuned by varying the core and peripheric substitutents.
In tr od u ction
active molecules, most commonly conjugated dipolar
systems, to the supramolecular arrangement in the bulk
material led to problems, namely the extinction of the
macroscopic susceptibility due to antiparallel alignment
of the molecules. This directed increasing attention to
molecular octopoles. Octopolar molecules, possessing a
zero overall dipole moment, can ideally be oriented in the
bulk phase in a way that their molecular susceptibility
is additive.7
A common way to design second-order NLO-active
octopolar molecules is to synthesize noncentrosymmetri-
cally substituted trigonal or tetrahedral π-conjugated
systems that display efficient charge transfer (CT) from
the periphery to the center of the molecule.8 Acetylenic
systems constitute among octopolar NLO chromophores
with the highest second-order polarizabilities known so
far. A small number of alternatingly persubstituted
trisethynylbenzenes has been described in the literature
The growing field of molecular photonics and elec-
trooptics has created a great demand for nonlinear optic
(NLO) materials that can be individually adapted to the
requirements set by a particular application.4 In this
context, organic molecules appear as promising candi-
dates since their molecular properties can be tuned to
an almost infinite extend via organic synthesis.5 On the
macroscopic level, crystal engineering and supramolecu-
lar chemistry can give access to molecular materials and
devices based on suitably prefunctionalized organic mol-
ecules.6 This transition from single second-order NLO-
† Universidad Auto´noma de Madrid.
‡ University of Leuven.
(1) Hendrickx, E.; Clays, K.; Persoons, A. Acc. Chem. Res. 1998, 31,
675-683.
(2) Olbrechts, G.; Strobbe, R.; Clays, K.; Persoons, A. Rev. Sci.
Instrum. 1998, 69, 2233-2241.
(3) Wostyn, K.; Binnemans, K.; Clays, K.; Persoons, A. Rev. Sci.
Instrum. 2001, 72, 3215-3220.
(7) (a) Le Bozec, H.; Le Boulder, T.; Maury, O.; Bondon, A.; Ledoux,
I.; Deveau, S.; Zyss, J . Adv. Mater. 2001, 13, 1677-1681. (b) Thalladi,
V. R.; Brasselet, S.; Weiss, H.-C.; Bla¨ser, D.; Katz, A. K.; Carrel, H.
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1998, 120, 2563-2577.
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F.; Nonell, S.; Mart`ı, C.; Brasselet, S.; Ledoux, I.; Zyss, J . J . Am. Chem.
Soc. 1998, 120, 12808-12817. (b) Cho, B. R.; Park, S. B.; Lee, S. J .;
Son, K. H.; Lee, S. H.; Lee, M.-J .; Yoo, J .; Lee, Y. K.; Lee, G. J .; Kang,
T. I.; Cho, M.; J eon, S.-J . J . Am. Chem. Soc. 2001, 123, 6421-6422.
(c) Wortmann, R.; Glania, C.; Kra¨mer, P.; Martschiner, R.; Wolff, J .
J .; Kraft, S.; Treptow, B.; Barbu, E.; La¨ngle, D.; Go¨rlitz, G. Chem. Eur.
J . 1997, 3, 1765-1773. (d) Lambert, C.; No¨ll, G.; Scha¨mlzlin, E.;
Meerholz, K.; Bra¨uchle, C. Chem. Eur. J . 1998, 4, 2129-2135.
(4) (a) Burland, D. M., Ed. Chem. Rev. 1994, 94, 1-278, Special
issue: Optical Nonlinearities in Chemistry. (b) Nicoud, J . F.; Twieg,
R. J . In Nonlinear Optical Properties of Organic Molecules and Crystals;
Chemla, D. S., Zyss, J ., Eds.; Academic Press: Orlando, 1987; pp 227-
296.
(5) (a) Meyers, F.; Marder, S. R.; Perry, J . W. In Chemistry of
Advanced Materials: An Overview; Interrante, L. V., Hampden-Smith,
M. J ., Eds.; Wiley-VCH: Weinheim, 1998; pp 207-269. (b) Verbiest,
T.; Houbrechts, S.; Kauranen, M.; Clays, K.; Persoons, A. J . Mater.
Chem. 1997, 7, 2175-2189.
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and Crystals; Chemla, D. S., Zyss, J ., Eds.; Academis Press: Orlando,
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10.1021/jo049279i CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/30/2004
J . Org. Chem. 2004, 69, 5077-5081
5077

Hennrich et al.
Due to synthetic-mechanistic considerations, it is in
general strongly disadvantageous to use sterically hin-
dered electron-rich aryl halides in palladium-catalyzed
cross-coupling reactions to obtain such systems.14 Nev-
ertheless, choosing from the plethora of reported meth-
ods,15 the 1,3,5-trisethynylbenzenes 2-4 could be ob-
tained in modest to good yields from the 1,3,5-triiodo-
benzenes16 1a -c and commercially available ethynyl-
benzene derivatives using standard Sonogashira coupling
conditions (Scheme 1).17
For the sake of generality, the reaction conditions,
which have been found to be most appropriate for the
least reactive triiodobenzene 1a , have been applied to all
other coupling reactions without further individual op-
timization. The ethynyl starting material had only little
influence on the reaction time and yield. However, it
should be noted that especially the strongly activated
triiodo compound 1c could be reacted much faster and
at significantly lower temperature.13
Lin ea r Sp ectr oscop y. Regarding their spectroscopic
behavior, the trimethyl-, trimethoxy-, and trifluoroben-
zenes show in general the same following tendencies: An
absorption band of lower intensity around 234 nm
remains relatively unaffected by varying the substituents
in the molecule center and its acetylenic periphery. A
main absorption band can be observed between 290 and
303 nm with a shoulder of lesser intensity between 310
and 318 nm, corresponding to the phenylacetylene chor-
mogenic unit,18 which both shift bathochromically with
increasing electron density in the central benzene core
(F < CH3 < OCH3). Since meta-substitution disrupts
conjugation in simple phenylacetylene systems,19 this
effect can be directly related to an increase in the CT-
character as a result of the substitution pattern and not
of increasing conjugation length. It is also obvious that
the peripheric aryl substituents are less influencial on
the spectroscopic properties compared to the core-
substitution as can be deduced by comparing benzenes
having the same core substitution with those carrying
the same peripheric substituents (Table 1, Figure 2).
The same trend can be observed even more pronounced
by looking at the compounds’ fluorescence properties. The
modestly broadened, unstructured emission band, located
between 358 and 380 nm for all compounds, is signifi-
cantly red-shifted for the trismethoxybenzenes 2a , 3a ,
and 4a , compared to their trimethyl and to an even
stronger extent to their trifluoro analogues. Figure 2
shows a difference in the emission maxima of 4a and 4c
of 22 nm. Comparing the fluorescence quantum yields,
the trifluorobenzenes display the smallest values (0.14-
0.18) while those of the trimethyl- and trimethoxy-
benzenes are in the same order of magnitude, between
0.21 and 0.29. In all cases, the observed Stokes shifts are
large and reach a maximum of 80 nm for compound 4a .
F IGURE 1. Donor-acceptor-substituted 1,3,5-tris(arylethyn-
yl)benzenes.
with donor substitution on the periphery and acceptor
substituents on the center (type I, Figure 1).9 However,
to the best of our knowledge, there are no such systems
reported which possess an electron-rich benzene core and
an electron-poor periphery (type II, Figure 1).
Yet, it is highly desirable to be able to control and vary
both the peripheric as well as the central aryl substitu-
ents in order to allow for a versatile electronic or
structural tuning of the target compounds. In the design
of NLO materials, secondary interaction sites can be used
to direct the alignement of the respective molecules, as
has been recently demonstrated for liquid crystal materi-
als.10 Furthermore, in the context of dendritic systems,11
where most commonly energy transfer takes place in a
directed manner from the outer rim to a central unit or
in an undirected fashion,12 covalent tuning of the respec-
tive building blocks in a way suggested here would put
a handle on the problem of directing energy transfer (CT,
respectively) in the desired way.
Resu lts a n d Discu ssion
Syn th esis. In light of sometimes cumbersome product
isolation from Stille couplings10a and low yields due to
statistical mixtures in cyclotrimerizations,8a,9b the Sono-
gashira coupling figures as the most obvious choice for
the synthesis of 1,3,5-trisalkynylbenzenes. In a recent
communication, we reported a synthetic procedure to
obtain C3-symmetric trisalkynylbenzenes via Sonogashira
coupling. However, in this case an “optimal” substrate,
the electron-poor aryliodide 1c, was employed in the
coupling step.13 Here, we describe the synthesis of 2,4,6-
substituted 1,3,5-trisethynylbenzene derivatives with
different substituents on the central benzene core as well
as on the phenylethynyl periphery, in particular yielding
systems with electron-rich benzene cores.
(9) (a) Wolff, J . J .; Siegler, F.; Matschiner, R.; Wortmann, R. Angew.
Chem., Int. Ed. 2000, 39, 1436-1439. (b) Traber, B.; Wolff, J . J .;
Rominger, F.; Oeser, T. Gleiter, R.; Goebel, M.; Wortmann, R. Chem.
Eur. J . 2004, 10, 1227-1238. (c) Cho, B. R.; Lee, S. J .; Son, K. S.; Kim,
Y. H.; Doo, J .-Y.; Lee, G. J .; Kang, T. I.; Lee, Y. K.; Cho, M.; J eon,
S.-J . Chem. Mater. 2001, 13, 1438-1440. (d) Kobayashi, K.; Kobayashi,
N. J . Org. Chem. 2004, 69, 2487-2497.
(10) Bushey, M. L.; Nguyen, T.-Q., Nuckolls, C. J . Am. Chem. Soc.
2003, 125, 8264-8269. (b) Omenat, A.; Barbera, J .; Serrano, J . L.;
Houbrechts, S.; Persoons, A. Adv. Mater. 1999, 11, 1292-1295.
(11) (a) Adronov, A.; Fre´chet, J . M. J . Chem. Commun. 2000, 1701-
1710. (b) Ma, H.; Liu, S.; Luo, J .; Liu, L.; Kang, S. H.; Haller, M.; Sassa,
T.; Dalton, L. R.; J en, A. L.-Y. Adv. Funct. Mater. 2002, 12, 565-574.
(12) (a) Devadoss, C.; Bharathi, P.; Moore, J . S. J . Am. Chem. Soc.
1996, 118, 9635-9644. (b) Wang, J .; Lu, Pan, Y.; Peng, Z. J . Org. Chem.
2002, 67, 7781-7786. (c) Pan, Y.; Peng, Z.; Melinger, J . S. Tetrahedron
2003, 59, 5495-5506.
(14) Sonogashira, K. In Comprehensive Organic Synthesis; Trost,
B. M., Fleming, I., Eds.; Pergamon Press: New York, 1991; Vol. 3.
(15) Tykwinsky, R. R. Angew. Chem., Int. Ed. 2003, 42, 1566-1568.
(16) For 1b, see: Oshiro, N.; Takei, F.; Onitsuka, K.; Takahashi, S.
J . Organomet. Chem. 1998, 569, 195-202. For 1c: Deacon, G. B.;
Smith, R. M. N. Aust. J . Chem. 1982, 35, 1587-1597.
(17) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N.
Synthesis 1980, 620-630.
(18) Halper, S. R.; Cohen, S. M. Chem. Eur. J . 2003, 9, 4661-4669.
(19) Kopelman, R.; Shortreed, M.; Shi, Z.-Y.; Tan, W.; Xu, Z.; Moore,
J . S.; Bar-Haim, A.; Klafter, J . Phys. Rev. Lett. 1997, 78, 1239-1242.
(13) Hennrich, G.; Echavarren, A. M. Tetrahedron Lett. 2004, 45,
1147-1149.
5078 J . Org. Chem., Vol. 69, No. 15, 2004

Tuning Octopolar NLO Chromophores
SCHEME 1. Syn th esis of 1,3,5-Tr is(eth yn ylp h en yl)ben zen es 2-4
TABLE 1. Lin ea r a n d Non lin ea r Op tica l P r op er ties of Com p ou n d s 2-4
1,3,5-tris[(4-pentylphenyl)-
1,3,5-tris(phenyl-
1,3,5-tris[(4-trifluoromethyl-
ethynyl]- (2)
ethynyl)- (3)
phenyl)ethynyl]- (4)
a
-2,4,6-trimethoxybenzene (a )
-2,4,6-trimethylbenzene (b)
-2,4,6-trifluorobenzene (c)
âHRS,800nm
âHRS,o
λmax,abs
λmax,em
Φb
âHRS,800nm
âHRS,o
λmax,abs
λmax,em
Φb
âHRS,800nm
âHRS,o
λmax,abs
λmax,em
Φb
37 ( 2
11.5 ( 1
318
376
0.27
32 ( 2
10 ( 1
317
365
0.29
25 ( 3
8.5 ( 1
310
369
0.18
34 ( 3
11 ( 1
317
375
0.21
18 ( 2
6 ( 1
313
363
0.28
19 ( 2
6.2 + 1
313
365
0.17
39 ( 5
13 ( 2
317
380
0.25
45 ( 3
14 ( 1
317
365
0.25
28 ( 3
10 ( 1
305
358
0.14
a
a
a
In units of 10-30 esu; absorption (c ) 10-6 mol L-1) and emission (c ) 10-8 mol L-1) maxima in nm, spectra recorded in chloroform.
Samples excited at 300 nm, quantum yields Φ determined using quinine sulfate (c ) 0.1 mol L-1 in H2SO4) as standard,20 estimated
error ( 10%.
b
modulation frequency toward the fluorescence-free hy-
perpolarizability, the multiphoton fluorescence contribu-
tion and the fluorescence lifetime, an accurate and precise
value for the first hyperpolarizability could be obtained.
As reference value, 338 × 10-30 esu was used for the â333
) -â311 ) -â131 ) -â113 for crystal violet (3-axis along
the ethynyl bridge, 1-axis in the plane of the core benzene
ring and perpendicular to the 3-axis, 2-axis perpendicular
to the 13-plane) in methanol. Since the reference com-
pound has the same octopolar symmetry as the un-
knowns, no correction factors for the difference between
dipolar and octopolar contributions need to be taken into
account. The reported hyperpolarizability values are the
dynamic values obtained at 800 nm. For charge-transfer
bands between 305 and 318 nm, this results in a
resonance enhancement factor between 2.80 and 3.23, in
the simple two-level model, that is also applicable to
octopoles.22 The resulting static hyperpolarizability val-
ues are also given in the table. When considering that
the peripheric aryl substitution has only a secondary
effect, the degree of fine-tuning that can be realized by
different core substitution can be demonstrated by the
average static hyperpolarizability value for the tri-
methoxy- (a ), the trimethyl- (b), and the trifluoroben-
zenes (c), respectively, as 11.8, 10.0, and 8.2 × 10-30 esu.
Despite this overall tendency, one also has to consider
F IGURE 2. Normalized absorption and emission spectra of
4a -c in chloroform.
Non lin ea r Sp ectr oscop y. For all nine compounds,
multiphoton fluorescence was also observed. This is not
surprising, since the octopolar molecular motif has also
been shown to be beneficial for this third-order nonlinear
optical property.21 However, by simultaneous fitting of
the demodulation and the phase data as a function of
(20) Velapoldi, R. A. In Advances in Standards and Methodology in
Spectrophotometry; Burgess, C., Mielenz, K. D., Eds.; Elsevier Science
Publishers: Amsterdam, 1987; pp 175-193.
(21) . Beljonne, D.; Wenseleers, W.; Zojer, E.; Shuai, Z. G.; Vogel,
H.; Pond, S. J . K.; Perry, J . W.; Marder, S. R.; Bre´das, J .-L. Adv. Func.
Mater. 2002, 12, 631-641.
(22) Zyss, J .; Dhenaut, C.; Van, T. C.; Ledoux, I. Chem. Phys. Lett.
1993, 206, 409-414.)
(23) Karpitskaya, L. G.; Vasiljeva, V. P.; Merkushev, E. B. J . Org.
Chem. USSR 1991, 1732-1736.
J . Org. Chem, Vol. 69, No. 15, 2004 5079

Hennrich et al.
the crude product. After purification via column chromatog-
raphy or/and recrystallization, the pure products were obtained
in yields between 22 and 71%.
the direct comparison of structurally more closely related
compounds by varying only the central or the peripheric
substitution at a time in order to gain a more accurate
idea of how the substitution pattern influences the NLO
response. For example, the â-value in the trimethyl series
(2b-4b) is the subject of a significant change, varying
from 18 to 45 × 10-30 esu.
1,3,5-Tr is[(4-p en tylp h en yl)eth yn yl]-2,4,6-tr im eth oxy-
ben zen e (2a ). Pure 2a was obtained upon column chroma-
tography (1:1 Hex-cyclohexane) as an orange oil. Yield: 213
1
mg; 58%. Rf ) 0.44. H NMR: δ ) 7.53 (AB, J ) 8 Hz, 6 H),
7.22 (AB, J ) 8 Hz; 6 H), 4.22 (s, 9 H), 2.66 (t, J ) 7.8 Hz, 6
H), 1.66 (m, 6 H), 1.38 (m, 12 H) 0.94 (t, J ) 6.3 Hz, 9 H). 13C
NMR: δ ) 163.0, 143.4, 131.3, 128.4, 120.6, 107.9, 97.6, 80.2,
61.2, 35. 8, 31.3, 30.8, 22.4, 13.9. EI+-MS m/z: 678 (100, [M+]).
Anal. Calcd for C48H44O3: C, 84.96; H, 7.96. Found: C, 84.78;
H, 8.21.
Con clu sion
Straightforward Sonogashira coupling proved to be an
efficient and versatile synthetic method to obtain per-
substituted 1,3,5-trisethynylbenzenes with octopolar sym-
metry. It permits the introduction of different substitu-
ents on both the acetylenic periphery as well as the
central benzene core. The usefulness of chemically tuning
the target compounds in such a way has been demon-
strated first of all by linear UV-vis and fluorescence
spectroscopy where the absorption and emission features
can be tailored according to the respective substitution
patterns. More interestingly, also from the HRS data it
is clear that the nonlinear optical properties can be fine-
tuned by the proper functionalization at the periphery
and at the core. There is good correspondence between
the linear and the nonlinear properties: the stronger
effect of the core substitution and the weaker influence
of the peripheric aryl substituents are confirmed. The
hyperpolarizability values are in a comparable order of
magnitude with respect to known octopolar systems8b,9
and can possibly be further amplified by choosing the
appropriate substituents. Rather than targeting maxi-
mum second hyperpolarizability values, we would like
to emphasize the usefulness and general applicability of
the synthetic strategy. Also, a remarkable degree of
control of the linear and nonlinear optical response can
be obtained by changing the position (periphery/core) and,
more importantly, the electronic nature of the phenyl
substituents.
1,3,5-Tr is[(4-pen tylph en yl)eth yn yl]-2,4,6-tr im eth ylben -
zen e (2b). Pure 2b was obtained upon column chromatogra-
phy (20:1 Hex-EtOAc) and final washing of the solid with
EtOH as colorless crystals. Yield: 346 mg; 55%. Mp: 56 °C.
1H NMR: δ ) 7.52 (AB, J ) 7.8 Hz, 6 H), 7.22 (AB, J ) 7.8
Hz; 6 H), 2.79 (s, 9 H), 2.68 (t, J ) 7.8 Hz, 6 H), 1.68 (m, 6 H),
1.38 (m, 12 H) 0.96 (t, J ) 6.3 Hz, 9 H). 13C NMR: δ ) 143.4,
141.7, 131.3, 128.5, 121.4, 120.8, 97.6, 86.4, 35.9, 31.4, 31.0,
22.5, 20.3, 14.0. EI+-MS m/z: 630 (100, [M+]). Anal. Calcd for
C
48H54: C, 91.43; H, 8.57. Found: C, 91.38; H, 8.55.
1,3,5-Tr is[(4-p en tylp h en yl)eth yn yl]-2,4,6-tr iflu or oben -
zen e (2c). Pure 2c was obtained upon column chromatography
(Hex) and final recrystallization from i-PrOH as colorless
1
crystals. Yield: 449 mg; 70%. Mp: 72 °C. H NMR: δ ) 7.50
(AB, J ) 7.8 Hz, 6 H), 7.20 (AB, J ) 7.8 Hz; 6 H), 2.64 (t, J )
7.8 Hz, 6 H), 1.63 (m, 6 H), 1.36 (m, 12 H) 0.91 (t, J ) 6.3 Hz,
9 H). 13C NMR: δ ) 161.8 (d, J CF ) 261 Hz), 144.5, 131.8,
128.5, 119.3, 100.0, 73.8, 35.9, 31.4, 30.8, 22.5, 14.0. FAB+-
MS: m/z 642 (100, [M+]). Anal. Calcd for C45H45F3: C, 84.11;
H, 7.01. Found: C, 84.55; H, 6.97.
1,3,5-Tr is(p h en yleth yn yl)-2,4,6-m eth oxyben zen e (3a ).
Pure 3a was obtained upon column chromatography (10:1
Hex-EtOAc) and final recrystallization from cyclohexane as
a pale orange solid. Yield: 103 mg; 22%. Mp: 110-111 °C. 1H
NMR: δ ) 7.57-7.55 (m, 6 H), 7.38-7.36 (m, 9 H), 4.18 (s, 9
H). 13C NMR: δ ) 163.4, 131.4, 128.4 123.5 107.8, 97.5 80.9,
61.5. EI+-MS m/z: 468 (100, [M+]). Anal. Calcd for C33H24O3:
C, 84.62; H, 5.13. Found: C, 84.50; H, 5.35.
1,3,5-Tr is(p h en yleth yn yl)-2,4,6-m eth ylben zen e (3b).23
Pure 3b was obtained upon column chromatography (2:1 Hex-
CH2Cl2) and final recrystallization from i-PrOH as an off-white
solid. Yield: 267 mg; 64%. Mp: 248-252 °C. 1H NMR: δ )
7.60-7.55 (m, 6 H), 7.41-7.35 (m, 9 H), 2.76 (s, 9 H). 13C
NMR: δ ) 142.1, 131.4, 128.4, 128.3 123.6, 121.4, 97.4, 86.9,
20.3. EI+-MS m/z: 420 (100, [M+]). Anal. Calcd for C33H24: C,
94.29; H, 5.71. Found: C, 94.27; H, 5.89.
1,3,5-Tr is(p h en yleth yn yl)-2,4,6-tr iflu or oben zen e (3c).
Pure 3c was obtained upon column chromatography (2:1 Hex-
CH2Cl2) and final recrystallization from i-PrOH as colorless
needles. Yield: 309 mg; 71%. Mp: 184-185 °C. 1H NMR: δ )
7.63-7.58 (m, 6 H), 7.42-7.39 (m, 9 H). 13C NMR: δ ) 161.95
(d, J CF ) 261 Hz), 131.8, 129.2, 128.4, 122.1, 99.8 74.3. EI+-
MS m/z: 432 (100, [M+]). Anal. Calcd for C30H15F3: C, 83.33;
H, 3.47. Found: C, 82.88; H, 3.53.
Exp er im en ta l Section
1,3,5-Tr iiod o-2,4,6-tr im eth oxyben zen e (1a ). To a sus-
pension of NaH (0.014 mol, 348 mg of a 60% dispension in
mineral oil) in Et2O (2 mL) was added MeOH (0.014 mol, 0.425
mL) in Et2O (1 mL) dropwise and the mixture stirred for 0.5
h at room temperature. The remaining solvent was blown off
in a stream of argon, and DMI (2 mL) and then, portionwise,
1,3,5-trifluro-2,4,6-triiodobenzene were added subsequently.
The remaining reaction mixture was stirred under argon at
room temperature for 18 h before being poured into hydro-
chloric acid (0.5 N, 30 mL). The resulting solid was filtered,
washed several times with water, and finally recrystallized
from MeOH to give pure 1a as a colorless solid. Yield: 770
mg; 72%. Mp: 163-165 °C. 1H NMR: δ ) 3.86 (s, 9 H). 13C
NMR: δ ) 161.4, 82.6, 60.8. EI+-MS m/z: 546 (100, [M+]).
HRMS: found 545.7677, calcd for C9H9O3I3 545.7686. Rf (TLC)
) 0.52 (Hex-EtOAc 20:1).
Gen er a l P r oced u r e for th e P r ep a r a tion of th e 1,3,5-
Tr is(a r yleth yn yl)ben zen es 2a -4c. The 1,3,5-triiodobenzene
derivatives 1a -c (1.0 mmol) was stirred in dry, degassed i-Pr2-
NH (10 mL) together with CuI (0.15 mmol) and Pd(PPh3)2Cl2
(0.15 mmol) under argon at room temperature for 0.5 h before
the respective arylethynyl compound was added. The mixture
was heated at 80 °C for 48 h, the solvent was removed in vacuo,
and to the remaining residue was added water (50 mL). After
extraction with EtOAc (3 × 30 mL), the combined organic
layers were dried (MgSO4) and concentrated in vacuo to give
1,3,5-Tr is](4-tr iflu or om eth ylp h en yl)eth yn yl]-2,4,6-tr i-
m eth oxyben zen e (4a ). Pure 4a was obtained upon column
chromatography (5:1 Hex-EtOAc) and final recrystallization
from cyclohexane as bright yellow crystals. Yield: 255 mg;
1
38%. Mp: 108-110 °C. H NMR: δ ) 7.65 (s, 12 H), 4.19 (s,
9 H). 13C NMR: δ ) 164.2, 131.6, 130.5 125.4, 122.1, 118.5,
107.2, 96.3, 83.0 61.7. EI+-MS m/z: 672 (100, [M+]). Anal.
Calcd for C36H21O3F9: C, 64.29; H, 3.13. Found: C, 64.30; H,
3.21.
1,3,5-Tr is[(4-tr iflu or om eth ylp h en yl)eth yn yl]-2,4,6-tr i-
m eth ylben zen e (4b). The remaining oily residue was tritu-
rated with MeOH. The resulting solid was filtered and
recrystallized from CH3NO2 to yield pure 4b as colorless
needles. Yield: 387 mg; 62%. Mp: 268-269 °C. 1H NMR: δ )
7.60 (s, 12 H), 2.73 (s, 9 H). 13C NMR: δ ) 143.0, 131.6, 129.9,
5080 J . Org. Chem., Vol. 69, No. 15, 2004

Tuning Octopolar NLO Chromophores
127.1, 125.3, 122.1, 121.0, 118.5, 96.3, 88.9, 20.3. EI+-MS
m/z: 624 (100, [M+]). Anal. Calcd for C36H21F9: C, 69.23; H,
3.37. Found: C, 69.09; H, 3.46.
1,3,5-Tr is[(4-tr iflu or om eth ylp h en yl)eth yn yl]-2,4,6-tr i-
flu or oben zen e (4c). Pure 4c was obtained upon column
chromatography (Hex) and final recrystallization from cyclo-
hexane as colorless crystals. Yield: 364 mg; 57%. Mp: 100-
104 °C. 1H NMR: δ ) 7.71-7.58 (m, 12 H). 13C NMR: δ )
162. 5 (d, J CF ) 263.1 Hz), 132.1, 131.8, 129.2, 125.5, 121.9,
118.3, 98.5, 76.1. EI+-MS m/z: 636 (100, [M+]). Anal. Calcd
for C33H12F12: C, 62.26; H, 1.89. Found: C, 62.74; H, 2.08.
Dan G. Pantos for technical assistance with the fluoro-
metric measurements, and to Profs. Pilar Prados and
J avier de Mendoza. The present work was financially
supported by the MCyT (Project No. PB97-0002-C2 and
Ramo´n y Cajal contract to G.H.).
Su p p or tin g In for m a tion Ava ila ble: Linear spectroscopic
data and general methods. This material is available free of
charge via the Internet at http://pubs.acs.org.
Ack n ow led gm en t. G.H. is grateful to Prof. Antonio
M. Echavarren for support and scientific input, to Dr.
J O049279I
J . Org. Chem, Vol. 69, No. 15, 2004 5081