Synthesis and optical properties of isomeric branched π-conjugated systems

Article abstract of DOI:10.1021/jo0506581

Branched conjugated systems with a terminal alkyne function have been prepared starting from 4-(triisopropylsilylethynyl) phenylacetylene by applying the following iterative reaction sequence: (i) metal-catalyzed cross-coupling reaction of the terminal al

Full text of DOI:10.1021/jo0506581

Synthesis and Optical Properties of Isomeric Branched
π-Conjugated Systems
Jean-Franc¸ois Nierengarten,* Sheng Zhang, Aline Ge´gout, and Maxence Urbani
Groupe de Chimie des Fullere`nes et des Syste`mes Conjugue´s, Ecole Europe´enne de Chimie,
Polyme`res et Mate´riaux (ECPM), Universite´ Louis Pasteur et CNRS, 25 rue Becquerel,
67087 Strasbourg Cedex 2, France
Nicola Armaroli,* Giancarlo Marconi, and Yannick Rio
Istituto per la Sintesi Organica e la Fotoreattivita`, Molecular Photoscience Group,
Consiglio Nazionale delle Ricerche, via Gobetti 101, 40129, Bologna, Italy
jfnierengarten@chimie.u-strasbg.fr; armaroli@isof.cnr.it
Received April 4, 2005
Branched conjugated systems with a terminal alkyne function have been prepared starting from
4-(triisopropylsilylethynyl) phenylacetylene by applying the following iterative reaction sequence:
(i) metal-catalyzed cross-coupling reaction of the terminal alkyne with 3,4-dibromobenzaldehyde
or 2,5- dibromobenzaldehyde; (ii) Corey-Fuchs dibromoolefination and treatment with an excess
of LDA. The building blocks thus prepared have been subjected to a Pd-catalyzed cross-coupling
reaction with 1,4-diiodobenzene to yield isomeric branched π-conjugated systems containing 7 (first
generation) or 15 (second generation) phenyl units connected by ethynyl spacers. The different
π-conjugation patterns in those isomeric derivatives have a dramatic effect on their electronic
properties, as attested by the differences observed in their absorption and emission spectra. Finally,
theoretical calculations have been performed to rationalize the optical properties of these compounds.
Introduction
has generated some spectacular molecular architectures
with high potential for optoelectronic applications.1-3 This
has given impetus to make this research a truly inter-
disciplinary branch of science located at the interfaces
between chemistry, physics, and materials science. It
must also be added here that the bottom-up approach
for the construction of electronic circuitry starting from
nanometer-sized π-conjugated molecular nanowires is
The past several years have seen a considerable
interest in the synthesis of new linear monodisperse
π-conjugated oligomers.1,2 Whereas the initial driving
force for the synthesis and the study of well-defined
oligomers was the development of structure-property
relationships to rationalize the properties of the corre-
sponding π-conjugated polymers, this research has evolved
into an independent field of its own.1,2 Its development
(3) (a) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int.
Ed. 1998, 37, 403. (b) Bernius, M. T.; Inbasekaran, M.; O’Brian, J.;
Wu, W. Adv. Mater. 2000, 12, 1737. (c) Mitshke, U.; Ba¨uerle, P. J.
Mater. Chem. 2000, 10, 1471. (d) Sano, T.; Nishio, Y.; Hamada, Y.;
Takahashi, H.; Usuki, T.; Shibata, K. J. Mater. Chem. 2000, 10, 157.
(e) Shirota, Y. J. Mater. Chem. 2000, 10, 1. (f) Nalva, H. S. Adv. Mater.
1993, 5, 341. (g) Marks, T. J.; Ratner, M. A. Angew. Chem., Int. Ed.
1995, 34, 155. (h) Nierengarten, J.-F. Sol. Energy Mater. Sol. Cells
2004, 83, 187.
(1) Mu¨llen, K.; Wegner, G. Electronic Materials: The Oligomer
Approach; Wiley-VCH: Weinheim, 1998.
(2) (a) Tour, J. M. Chem. Rev. 1996, 96, 537. (b) Martin R. E.,
Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350. (c) Roncali, J.
Acc. Chem. Res. 2000, 33, 147. (d) Tour, J. M. Acc. Chem. Res. 2000,
33, 791. (e) Diederich, F. Chem. Commun. 2001, 219. (f) Segura, J. L.;
Martin, N. J. Mater. Chem. 2000, 10, 2403.
10.1021/jo0506581 CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/13/2005
7550
J. Org. Chem. 2005, 70, 7550-7557

Isomeric Branched π-Conjugated Systems
CHART 1
particularly appealing and gives rise to tremendous
research efforts nowadays.4
of these dendritic molecules is, however, rather limited.
More recently, Peng and co-workers have reported the
preparation of new conjugated dendrimers based on 1,2,4-
trisubstituted phenyl units in which the different branches
exhibit different conjugation lengths.9 Indeed, the un-
symmetric substitution on the aromatic ring produces
para-linked phenylacetylene units allowing an increase
of the conjugation length when the dendritic generation
is increased. By following a similar approach, we now
report the synthesis of the isomeric branched π-conju-
gated systems H1-2 and X1-2 (Chart 1). Whereas X1
and X2 are similar to the dendritic systems reported by
Peng and co-workers, compounds H1 and H2 have
unprecedented structural features. To understand the
influence of the conjugation pathways within these
isomeric systems, the excited-state properties of H1-2
and X1-2 have been investigated and compared to those
of the corresponding model compound HX0.
The rapid progress achieved in the synthesis of mono-
disperse π-conjugated oligomers has also stimulated the
development of other fascinating families of compounds.
These novel molecules include macrocyclic π-systems,5
substructures of new graphite-like two-dimensional net-
works,5 fullerene-like three-dimensional cages,5 and hy-
perbranched structures.6-8 As far as conjugated dendrim-
ers are concerned, almost all of the systems described so
far have been constructed from symmetrical building
blocks based on 1,3,5-trisubstituted phenyl rings.7,8 As a
result of the meta-branching scheme, the π-conjugation
(4) (a) Kovtyukhova, N. I.; Mallouk, T. E. Chem. Eur. J. 2002, 8,
4355. (b) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002,
41, 4379. (c) Balzani, V.; Credi, A.; Venturi, B. Chem. Eur. J. 2002, 8,
5525.
(5) (a) Diederich, F.; Rubin, Y. Angew. Chem., Int. Ed. Engl. 1992,
31, 1101. (b) Diederich, F. Nature 1994, 369, 199. (c) Haley, M. M.
Synlett 1998, 557. (d) Bunz, U. H. F.; Rubin, Y.; Tobe, Y. Chem. Soc.
Rev. 1999, 28, 107. (e) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K.
Chem. Rev. 2001, 101, 1267.
Results and Discussion
(6) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendrimers and
Dendrons: Concepts, Syntheses, Applications; VCH: Weinheim, 2001.
(7) (a) Moore, J. S.; Xu, Z. Polym. Prep. 1991, 32, 629. (b) Moore, J.
S.; Xu, Z. Macromolecules 1991, 24, 5893. (c) Moore, J. S.; Xu, Z. J.
Am. Chem. Soc. 1994, 116, 4537. (d) Pesak, D. J.; Moore, J. S. Angew.
Chem., Int. Ed. 1997, 36, 1636. (e) Moore, J. S.; Xu, Z. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 246. (e) Moore, J. S. Acc. Chem. Res. 1997, 30,
402. (f) Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc.
1996, 118, 9635.
(8) (a) Deb, S. K.; Maddux, T. M.; Yu, L. J. Am. Chem. Soc. 1997,
119, 9079. (b) Meier, H.; Lehmann, M. Angew. Chem., Int. Ed. 1998,
37, 643. (c) Lehmann, M.; Schartel, B.; Hennecke, M.; Meier, H.
Tetrahedron 1999, 55, 13377. (d) Meier, H.; Lehmann, M.; Kolb, U.
Chem. Eur. J. 2000, 6, 2462. (e) Diez-Barra, E.; Garcia-Martinez, J.
C.; Rodriguez-Lopez, J. Tetrahedron Lett. 1999, 40, 8181. (f) Diez-
Barra, Garcia-Martinez, J. C.; Rodriguez-Lopez, J.; Gomez, R.; Segura,
J. L.; Martin, N. Org. Lett. 2000, 2, 3651. (g) Accorsi, G.; Armaroli, N.;
Eckert, J.-F.; Nierengarten, J.-F. Tetrahedron Lett. 2002, 43, 65. (h)
Segura, J. L.; Gomez, R.; Martin, N.; Luo, C. P.; Swartz, A.; Guldi, D.
M. Chem. Commun. 2001, 707. (i) Guldi, D. M.; Swartz, A.; Luo, C.;
Gomez, R.; Segura, J. L.; Martin, N. J. Am. Chem. Soc. 2002, 124,
1087. (j) Langa, F.; Gomez-Escalonilla, M. J.; Diez-Barra, E.; Garcia-
Martinez, J. C.; de la Hoz, A.; Rodriguez-Lopez, J.; Gonzalez-Cortes,
A.; Lopez-Arza, V. Tetrahedron Lett. 2001, 42, 3435.
Synthesis. The preparation of HX0 is depicted in
Scheme 1. 4-Bromobenzaldehyde (1) was subjected to a
Pd-catalyzed cross-coupling reaction10 with triisopropyl-
silylacetylene to give 2 in 98% yield. Subsequent treat-
ment with CBr4/PPh3/Zn under the conditions described
by Corey-Fuchs11 yielded dibromoolefine 3 in 99% yield.
Elimination of HBr and halogen-metal exchange was
best achieved with an excess of LDA12 in THF at -78
°C, and the resulting anion was quenched with NH4Cl
to give monoprotected bisalkyne 4 in 90% yield. The OPE
trimer HX0 was then obtained in 84% yield by Pd-
(9) (a) Peng, Z.; Pan, Y.; Xu, B.; Zhang, J. J. Am. Chem. Soc. 2000,
122, 6619. (b) Pan, Y.; Lu, M.; Peng, Z.; Melinger, J. S. J. Org. Chem.
2003, 68, 6952.
(10) (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Nagihara,
N. Synthesis 1980, 627. (b) Metal-Catalyzed Cross-Coupling Reactions;
Diederich, F., Stang, P. J., Eds.; Wiley-VCH: Weinheim, 1998.
(11) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 13, 3769.
J. Org. Chem, Vol. 70, No. 19, 2005 7551

Nierengarten et al.
SCHEME 1a
a Reagents and conditions: (i) triisopropylsilylacetylene, PdCl2-
(PPh3)2, CuI, PPh3, Et3N, THF (98%); (ii) CBr4, PPh3, Zn dust,
CH2Cl2 (99%); (iii) LDA, THF then NH4Cl, H2O (90%); (iv) 1,4-
diiodobenzene, PdCl2(PPh3)2, CuI, PPh3, Et3N, THF (84%).
SCHEME 2a
FIGURE 1. 1H NMR spectra (300 MHz, CDCl3) of compounds
6 (bottom) and 9 (top).
dendron 9. Treatment with CBr4/PPh3/Zn and subsequent
reaction of the resulting intermediate 10 with an excess
of LDA yielded terminal alkyne 11. Owing to the presence
of the triisopropylsilyl (TIPS) substituents, dendrons
6-11 are highly soluble in common organic solvents
1
(CH2Cl2, CHCl3, THF), and H and 13C NMR character-
ization was easily achieved. As typical examples, the 1H
NMR spectra of compounds 6 and 9 recorded in CDCl3
are shown in Figure 1. For both 6 and 9, the resonance
arising from the aldehydic proton is observed at ca. 10
ppm. In the aromatic region, the spectrum of 6 shows
three sets of signals in a typical pattern for a 3,4-
disubstituted benzaldehyde moiety as well as a pseudo-
singlet at 7.50 ppm for the protons of the two para-
a Reagents and conditions: (i) 4, PdCl2(PPh3)2, CuI, PPh3, Et3N,
THF (75%); (ii) CBr4, PPh3, Zn dust, CH2Cl2 (95%); (iii) LDA, THF
then NH4Cl, H2O (92%); (iv) 1,4-diiodobenzene, PdCl2(PPh3)2, CuI,
PPh3, Et3N, THF (81%); (v) 5, PdCl2(PPh3)2, CuI, PPh3, Et3N, THF
(78%); (vi) CBr4, PPh3, Zn dust, CH2Cl2 (94%); (vii) LDA, THF then
NH4Cl, H2O (94%); (viii) 1,4-diiodobenzene, PdCl2(PPh3)2, CuI,
PPh3, Et3N, THF (88%).
1
disubstituted phenyl groups. The H NMR of 9 is also
consistent with the proposed structure. In addition to the
signals corresponding to the protons of the central 3,4-
diethynylbenzaldehyde moiety, the resonances arising
from the two 1,3,4-triethynylbenzene units appear as a
single set of three signals. These two aromatic rings are
in principle nonequivalent; however, both benzene rings
are substituted by three alkyne groups and must be in a
similar chemical environment and are therefore pseudo-
equivalent. Finally, the peaks of the peripheral p-
diethynyl phenyl moieties appear between 7.44 and 7.47
ppm.
catalyzed cross-coupling between 4 and 1,4-diiodoben-
1
zene. Compound HX0 was characterized by H and 13C
NMR, UV-vis, and IR spectroscopies. In addition, the
structure of HX0 was confirmed by FAB mass spectrom-
etry.
The synthesis of X1-2 is shown in Scheme 2. Reaction
of 3,4-dibromobenzaldehyde (5) with monoprotected bis-
alkyne 4 under Sonogashira conditions gave the fist
generation dendron 6 in 75% yield. Dibromoolefination
according to Corey-Fuchs then provided 7, which after
treatment with an excess of LDA in THF at -78 °C and
quenching with NH4Cl afforded terminal alkyne 8.
Compound 8 was subjected to a Pd-catalyzed cross-
coupling reaction with 5 to yield the second generation
Compounds 6-11 were further characterized by IR
spectroscopy. For all the derivatives, the characteristic
CtC stretching band is observed at 2151-2152 cm-1. In
the IR spectrum of 6 and 9, the diagnostic aldehyde band
is observed at ca. 1700 cm-1. In the case of 8 and 11, the
C-H stretching band characteristic of the terminal
alkyne function is seen at ca. 3300 cm-1
.
Compounds 15 and 18, the key building blocks for the
synthesis of H1 and H2, respectively, were obtained from
2,5-dibromobenzaldehyde (12) by following a similar
synthetic approach as the one described above for the
preparation of the corresponding isomers 8 and 11
(Scheme 3). Pd-catalyzed cross-coupling reaction of ter-
minal alkyne 4 with 2,5-dibromobenzaldehyde (12),
(12) (a) Nierengarten, J.-F.; Schreiber, M.; Diederich, F.; Gramlich,
V. New J. Chem. 1996, 20, 1273-1284. (b) Solladie´, N.; Gross, M.
Tetrahedron Lett. 1999, 40, 3359. (c) Nierengarten, J.-F.; Gu, T.;
Hadziioannou, G.; Tsamouras, D.; Krasnikov, V. Helv. Chim. Acta
2004, 87, 2948.
7552 J. Org. Chem., Vol. 70, No. 19, 2005

Isomeric Branched π-Conjugated Systems
SCHEME 3a
FIGURE 2. 1H NMR spectra (300 MHz, CDCl3) of compounds
H1 (bottom) and H2 (top).
CHART 2
a Reagents and conditions: (i) 4, PdCl2(PPh3)2, CuI, PPh3, Et3N,
THF (76%); (ii) CBr4, PPh3, Zn dust, CH2Cl2 (94%); (iii) LDA, THF
then NH4Cl, H2O (96%); (iv) 1,4-diiodobenzene, PdCl2(PPh3)2, CuI,
PPh3, Et3N, THF (85%); (v) 12, PdCl2(PPh3)2, CuI, Et3N (81%);
(vi) CBr4, PPh3, Zn dust, CH2Cl2 (86%); (vii) LDA, THF then
NH4Cl, H2O (87%); (viii) 1,4-diiodobenzene, PdCl2(PPh3)2, CuI,
PPh3, Et3N, THF (76%).
Corey-Fuchs dibromoolefination, and treatment with an
excess of LDA gave 15 in an overall 69% yield. Subse-
quent Sonogashira coupling with 12 and treatment of the
resulting 16 with CBr4/PPh3/Zn followed by reaction with
an excess of LDA yielded terminal alkyne 18.
The branched π-conjugated systems H1-2 and X1-2
were finally obtained by reaction of 1,4-diiodobenzene
with an excess of the appropriate terminal alkyne
precursor in the presence of PdCl2(PPh3)2, CuI, and PPh3
in Et3N/THF (Schemes 2 and 3). In addition to the
desired compounds, byproducts resulting from the ho-
mocoupling of the alkyne precursors were also formed
under these conditions. However, the desired products
were easily purified by column chromatography on silica
gel, and H1-2 and X1-2 were isolated in good yields.
Compounds H1-2 and X1-2 are well-soluble in common
organic solvents such as CH2Cl2, CHCl3, or THF, and
complete spectroscopic characterization was easily
achieved. Both 1H and 13C NMR spectra are in full
agreement with the centrosymmetric structures of H1-2
and X1-2. The 1H NMR spectra of H1 and H2 recorded
in CDCl3 are shown in Figure 2.
aromatic unit (Ha) is observed at 7.51 ppm. When
compared to H1, the latter signal is shifted slightly
upfield.
In addition, benzylic alcohols 19 and 20 (Chart 2) used
as reference compounds in the optical studies were
prepared by diisobutylaluminum hydrid (DIBAL-H) re-
duction of benzaldehydes 6 and 9, respectively.
Optical Properties. H1 and H2. The electronic
absorption spectra of HX0, H1, and H2 in cyclohexane
solution are depicted in Figure 3 (top left). The spectrum
of the reference phenyeneacetylene (PA) HX0 is very
similar to an analogous trimer system lacking the
peripheral triple bonds,13 suggesting negligible increase
of π-conjugation length. By connecting two HX0 units via
an ortho PA bridge, little band broadening is observed,
with an absorption onset red-shift of only 17 nm (Table
1); this indicates negligible criss-cross electronic delocal-
ization over the whole structure in H1. In other words,
the conjugation is effective for the para-linked subunits
but the ortho- and meta-linked ethynylbenzene moieties
do not really increase the conjugation length of the
system.
1
In the aromatic region, the H NMR spectrum of H1
is characterized by a singlet for the aromatic protons of
the central phenyl ring (Ha), three sets of signals for the
1,3,4-trisubstituted phenyl ring (Hb, Hc, and Hd), and a
pseudo-singlet for the protons of the four para-disubsti-
1
tuted phenyl groups. The H NMR of compound H2 is
also in full agreement with the proposed structure. The
resonances arising from the three different pairs of
equivalent 1,3,4-triethynylbenzene moieties are observed;
in particular, Hb, He, and Hh give rise to three distin-
guishable signals in the expected 2:2:2 ratio. Three sets
of pseudo-singlets are seen at 7.49, 7.40, and 7.39 for the
peripheral para-disubstituted phenyl groups. Finally, the
singlet corresponding to the protons of the central
The fluorescence spectra of HX0 and H1 have practi-
cally the same shape and vibronic structure (Figure 3,
top right), but the emission maximum of H1 is red-shifted
by 20 nm, in line with the absorption trend. The singlet
(13) Melinger, J. S.; Pan, Y. C.; Kleiman, V. D.; Peng, Z. H.; Davis,
B. L.; McMorrow, D.; Lu, M. J. Am. Chem. Soc. 2002, 124, 12002.
J. Org. Chem, Vol. 70, No. 19, 2005 7553

Nierengarten et al.
FIGURE 3. Top left: Absorption spectra of HX0 (dotted line), H1 (dashed line), and H2 (full line) in cyclohexane. Top right:
Emission spectra of isoabsorbing solutions (A ) 0.18) of HX0 (dotted line), H1 (dashed line), and H2 (full line) in cyclohexane. λex
) 320 nm. Bottom left: Absorption spectra of X1 (dashed black line), X2 (full black line), 19 (dashed gray line), and 20 (full gray
line) in cyclohexane. Bottom right: Emission spectra of isoabsorbing solutions (A ) 0.18) of X1 (dashed black line), X2 (full black
line), 19 (dashed gray line), and 20 (full gray line) in cyclohexane. λex ) 320 nm.
TABLE 1. Optical Properties in Cyclohexane
aggregation (e.g., excimer emission) is detected within
the range of concentrations used for optical measure-
ments (10-5/10-7 M).
ab
ab
edge
em
max
λ
λ
ꢀmax
(nm) a (nm) b (M-1 cm-1
λ
τ
max
c
e
)
(nm) d
φfl
(ns) f
X1 and X2. The electronic absorption spectra of X1
and X2 in cyclohexane solution are depicted in Figure 3
(bottom left). Such spectra are broad and poorly resolved
(especially X1) and do not bear much resemblance to
those of 19 and 20, which may be considered the end
units of X1 and X2. This indicates that the electronic
π-delocalization mainly occurs through the central mo-
lecular backbone made of five and seven p-phenylacety-
lene units for X1 and X2, respectively; this pattern
mainly dictates the absorption profile. The larger delo-
calization of the central molecular wire in X2 leads to a
36 nm red-shift of the absorption onset relative to X1,
and a 14 nm shift of the first feature of the fluorescence
spectra (Figure 3, Table 1). The fluorescence quantum
yield and the singlet excited-state lifetime of X1 are
identical to that of the reference system HX0. The
excitation spectra of X1 and X2 in cyclohexane match the
absorption profile at any emission wavelength. It is
noteworthy to mention here that the lowest energy
absorption peak observed in freshly prepared solutions
of X2 tends to decrease over time (minutes), this is
attributed to some molecular aggregation of this large
extended π-conjugated system.
HX0
H1
H2
X1
342
332
334
360
309
377
394
408
406
442
7 850
188 200
370 000
124 500
368 500
372
392
410
402
416
1.00
0.97
0.89
1.00
0.61
0.6
1.2
2.4
0.6
0.7
X2
a Maximum absorption wavelength. b Absorption onset corre-
sponding to the wavelength at which ꢀ is 5% of ꢀmax.
c Molar
extinction coefficient of the absorption maximum. d Fluorescence
emission values obtained from uncorrected spectra. e Fluorescence
quantum yield. f Singlet lifetimes from fluorescence decays.
excited state of HX0 deactivates only via the radiative
path (Φfl ) 1.0), whereas for H1 some little nonradiative
deactivation occurs (Φfl ) 0.97), probably as a conse-
quence of the larger structure that favors vibrational
deactivation. The radiative deactivation rate constant (kr
) Φ/τ) amounts to 1.7 × 109 and 8.3 × 108 s-1 for HX0
and H1, respectively. H2 is made of two H1 units linked
through a phenylacetylene spacer, but the electronic
absorption spectrum is practically identical and twice as
intense relative to that of H1 for a large part of the
spectrum (190-385 nm). Notably, the spectral onset of
H2 is slightly red-shifted compared to H1, and a weak
absorption tail is observed down to 420 nm. These results
suggest that, in the ground state, the two H1-type
fragments of H2 are substantially decoupled, but this
does not prevent the occurrence of a lower-lying singlet
state, as signaled by a red-shifted and longer lived
fluorescence (λmax ) 408 nm, τ ) 2.4 ns, Table 1, Figure
3) relative to H1. The excitation spectra of HX0, H1, and
H2 in cyclohexane perfectly match the absorption profile
at any emission wavelength and no evidence of molecular
It must be emphasized that H1/X1 and H2/X2 are
isomer couples whose different electronic π-conjugation
pattern is reflected in the differences of the absorption
spectra (Figure 4). The X-type molecules exhibit a lower
lying absorption onset and a wider spectral profile. The
fluorescence yield is virtually unitary for H1 and X1,
whereas for X2 a substantially lower value is found,
compared to H2 (Table 1). X2 does not exhibit any
detectable phosphorescence at 77 K.
7554 J. Org. Chem., Vol. 70, No. 19, 2005

Isomeric Branched π-Conjugated Systems
FIGURE 4. Left: absorption spectra of H1 (full line) and X1
(dashed line) isomers in cyclohexane. Right: absorption spectra
of H2 (full line) and X2 (dashed line) isomers in cyclohexane.
FIGURE 6. Calculated HOMO and LUMO Orbital Distribu-
tions for H1 (dark gray negative, light-blue positive) and H1
MM+ minimized structure.
lated energies of the simpler compounds H1 and X1,
where the large absorption system detected in the region
of 280-400 can be accounted for by two main transitions
to S1 (calculated at 406 and 399 nm, respectively) and S2
(calculated at 355 and 346 nm). The calculated red shift
of the X1 bands with respect to those of H1 can be fairly
well accounted for by a larger conjugation shown by the
former compound, with a wire of five phenylacetylene
units versus three in the case of H1. Also the different
shape of the absorption bands shown by the two classes
of compounds (H1, H2 versus X1, X2) can be explained
by the different amount of π electron delocalization. In
fact, the oscillator strength of the most intense band of
H1 (S2), which is calculated to be about twice the amount
of X1 (S1), can be explained by the sum of two separated
wires made of three phenylacetylene units, due to the
poor electronic conjugation brought about by the trans-
versal bridge. This feature appears evident when inspect-
ing the nature of the molecular orbitals involved in the
transitions to these states and displayed in Figures 6 and
7. In H1, the very intense band calculated at 355 nm is
largely due to the transition (HOMO - 1) f (LUMO +
1), showing a break of electron density in correspondence
of the central phenyl of the bridge. On the other hand,
the most intense calculated band of X1 (S1) arises
essentially from the HOMO f LUMO transition, local-
ized on the central wire made of five phenylacetylene
units, with minor participation of the side branches.
FIGURE 5. Measured (gray full line) and simulated absorp-
tion (black full line) spectra of H1 (top left), H2 (top right),
X1 (bottom left), and X2 (bottom right). The calculated energies
have been shifted in order to let the calculated and experi-
mental spectra maxima coincide.
Analysis of Absorption Spectra. The absorption
spectra of the main compounds examined in this work
have been simulated using the oscillator strengths cal-
culated with the ZINDO/S program14 included in the
Hyperchem package,15 having previously optimized the
ground-state geometry within the MM+ force field. The
simulation was obtained using a Gaussian shape of the
bands centered on the calculated wavelength and a
semibandwidth of 30 nm, having shifted the computed
energies so that the wavelengths of the experimental and
calculated maxima coincide. The method used, despite
its simplicity, provides a satisfactory description of the
main features of the experimental spectra, as already
underlined by comparison with more sophisticated meth-
ods employed in the study of analogous compounds.16,17
In particular, the spectra displayed in Figure 5 show
a reasonable agreement for the band shape and calcu-
Conclusion
(14) Ridley, J.; Zerner, M. C. Theor. Chim. Acta 1973, 32, 111.
(15) Hyperchem, version 6.02; Hypercube Inc.: Gainesville, FL, 2000.
(16) Magyar, R. J.; Tretiak, S.; Gao, Y.; Wang, H.-L.; Shreve, A. P.
Chem. Phys. Lett. 2005, 401, 149.
(17) Houk, K. N., Lee, P. S., Nendel, M. J. Org. Chem. 2001, 66,
5517.
We have developed an iterative approach for the
preparation of new branched conjugated systems with a
terminal alkyne function starting from 4-(triisopropyl-
silylethynyl) phenylacetylene (4) by applying the follow-
J. Org. Chem, Vol. 70, No. 19, 2005 7555

Nierengarten et al.
General Procedure for Dibromoolefination Reactions.
A mixture of CBr4, PPh3, and Zn dust in dry CH2Cl2 was
stirred at room temperature for 24 h. The suspension was then
cooled to 0 °C, and the appropriate aldehyde dissolved in CH2-
Cl2 was added at once. The resulting mixture was slowly
warmed to room temperature and stirred overnight. The
resulting thick suspension was filtered and evaporated. The
residue was dissolved in a minimum of CH2Cl2, and then
hexane was added to precipitate the remaining P-containing
byproducts. The resulting mixture was filtered and evaporated.
The product was then purified as outlined in the following text.
Compound 3. This compound was prepared from 2 (6.77
g, 23.7 mmol), CBr4 (39.9 g, 118.6 mmol), PPh3 (31.1 g, 118.6
mmol), and Zn dust (7.75 g, 118.6 mmol) in CH2Cl2 (450 mL),
and column chromatography (SiO2, hexane) yielded 3 (10.48
g, 99%). Colorless oil. IR (CH2Cl2): 2288 (CtC). 1H NMR
(CDCl3, 200 MHz): δ 7.48 (s, 4H), 7.46 (s, 1H), 1.15 (s, 21 H).
13C NMR (CDCl3, 50 MHz): δ 136.0, 134.9, 131.9, 128.1, 123.6,
106.7, 92.1, 90.4, 18.7, 11.3. Anal. Calcd for C19H26Br2Si: C
51.59, H 5.92. Found: C 51.51, H 6.01.
General Procedure for Preparation of Alkynes from
Dibromoolefines. A solution of LDA in THF was slowly
added to a solution of the appropriate dibromoolefine in THF
at -78 °C. After 3 h, a saturated aqueous NH4Cl solution was
added. The reaction mixture was diluted with hexane, washed
with water, dried with MgSO4, and evaporated. The product
was then purified as outlined in the following text.
Compound 4. This compound was prepared from 3 (14.09
g, 32.8 mmol) and LDA (134 mmol) in THF (450 mL), and
column chromatography (SiO2, hexane) yielded 4 (8.34 g, 90%).
Colorless oil. IR (CH2Cl2): 3315 (tC-H), 2152 (CtC). 1H NMR
(CDCl3, 200 MHz): δ 7.42 (s, 4H), 3.17 (s, 1H), 1.14 (s, 21 H).
13C NMR (CDCl3, 50 MHz): δ 131.8, 131.7, 123.9, 122.1, 106.5,
92.8, 83.2, 18.6, 11.3. Anal. Calcd for C19H26Si: C 80.78, H
9.28. Found: C 80.80, H 9.01.
FIGURE 7. Calculated HOMO and LUMO Orbital Distribu-
tions for X1 (light-blue negative, dark-grey positive) and X1
MM+ minimized structure.
General Procedure for Sonogashira Cross-Coupling
Reactions. To an oven-dried glass screw capped tube were
added all solids including the aryl halide (bromide or iodide),
alkyne, CuI, PPh3, and palladium catalyst. The atmosphere
was removed via vacuum and replaced with dry argon (3×).
THF and triethylamine were added by syringe, and the
reaction was conducted at room temperature (for aryl iodides)
or heated at 65 °C in an oil bath with stirring (for aryl
bromides). Upon cooling the reaction mixture was filtered via
gravity filtration to remove solids and diluted with dichlo-
romethane. The reaction mixture was extracted with an
aqueous NH4Cl solution. The organic layer was dried with
MgSO4 and filtered through a plug of SiO2 (CH2Cl2). The
solvent was evaporated, and the product was purified as
outlined in the following text.
Compound 2. This compound was prepared from triiso-
propylsilylacetylene (15 mL, 67 mmol), 1 (10.0 g, 54 mmol),
Pd(PPh3)2Cl2 (248 mg, 0.36 mmol), CuI (72 mg, 0.36 mmol),
and PPh3 (0.23 g, 0.88 mmol) in THF/Et3N (100 mL). Column
chromatography (SiO2, hexane/CH2Cl2 2:1) yielded 2 (15.16 g,
98%). Colorless oil. IR (CH2Cl2): 2286 (CtC), 1704 (CdO). 1H
NMR (CDCl3, 200 MHz): δ 10.01 (s, 1H), 7.83 (d, J ) 7 Hz,
2H), 7.62 (d, J ) 7 Hz, 2H), 1.15 (s, 21H). Anal. Calcd for
C18H26SiO: C 75.46, H 9.15. Found: C 75.62, H 9.26.
General Procedure for Reduction. A 1 M solution of
DIBAL-H in hexane was slowly added to a solution of the
appropriate benzaldehyde in THF at 0 °C under argon. After
3 h, methanol and then water were added. The reaction
mixture was filtered through a pad of Celite, dried (MgSO4),
filtered, and evaporated. The product was then purified as
outlined in the following text.
ing reaction sequence: (i) metal-catalyzed cross-coupling
reaction of the terminal alkyne with a dibromobenzal-
dehyde derivative (5 or 12); (ii) Corey-Fuchs dibromoole-
fination and treatment with an excess of LDA.12c The
building blocks thus prepared have been subjected to a
Pd-catalyzed cross-coupling reaction with 1,4-diiodoben-
zene to yield the isomeric branched π-conjugated systems
containing 7 (H1 and X1) or 15 (H2 and X2) phenyl units
connected by ethynyl spacers. The different π-conjugation
patterns in those isomeric derivatives have a dramatic
effect on their electronic properties as attested by the
differences observed in their absorption spectra. Finally,
it can be added that all of these extended conjugated
derivatives are highly luminescent with nearly quantita-
tive fluorescence quantum yields. The results described
in this paper provide a hint for the design of novel
branched conjugated systems with tailored optical prop-
erties, and the characteristic features of these compounds
make them attractive photoactive components for the
preparation of photochemical molecular devices or new
sensors. Work in this direction is now underway in our
laboratories.
Experimental Section
General. Reagents and solvents were purchased as reagent
grade and used without further purification. THF was distilled
over sodium benzophenone ketyl. Compounds 518 and 1219 were
prepared as previously reported.
Compound 19. This compound was prepared from 6 (80
mg, 0.12 mmol) and DIBAL-H (0.5 mL, 0.5 mmol), in THF (5
mL). Column chromatography (SiO2, hexane/CH2Cl2 1:4) yielded
19 (59 mg, 73%). Pale yellow solid (mp 124 °C). IR (KBr): 3300
(O-H), 2152 (CtC). 1H NMR (200 MHz, CDCl3): δ 7.58 (broad
s, 1H), 7.(d, J ) 7 Hz, 1H), 7.48 (s, 8H), 7.33 (broad d, J ) 7
(18) Swoboda, P.; Saf, R.; Hummel, K.; Hofer, F.; Czaputa, R.
Macromolecules 1995, 28, 4255.
(19) Schweikart, K.-H.; Hanack, M.; Lu¨er, L.; Oelkrug, D. Eur. J.
Org. Chem. 2001, 293.
7556 J. Org. Chem., Vol. 70, No. 19, 2005

Isomeric Branched π-Conjugated Systems
Hz, 1H), 4.73 (s, 2H), 1.15 (s, 42H). Anal. Calcd for C45H56-
OSi2: C 80.78, H 8.44. Found: C 80.80, H 8.50.
Recherche of the French Ministry of Research, and A.G.
thanks the ADEME-Re´gion Alsace for their fellowships.
We further thank L. Oswald for technical help and
J.-M. Strub for MS measurements.
Acknowledgment. This research was supported by
CNRS, CNR (commessa PM-P04-ISTM-C1/PM-P04-
ISOF-M5, Componenti Molecolari e Supramolecolari o
Macromolecolari con Proprieta` Fotoniche ed Optoelettron-
iche) the French Ministry of Research (ACI Jeunes
Chercheurs), the Italian MIUR (contract FIRB RBNE-
019H9K, Molecular Manipulation for Nanometric
Machines) and the EU (RTN Contract “FAMOUS”,
HPRN-CT-2002-00171). S.Z. thanks the Direction de la
Supporting Information Available: 1H and 13C NMR
spectra of compounds 6-11, 13-18, X1, X2, H1 and H2; the
computational data for X1 and H1; and the experimental
details for the preparation of compounds 6-11, 13-18, HX0,
X1, X2, H1, H2, and 20. This material is available free of
charge via the Internet at http://pubs.acs.org.
JO0506581
J. Org. Chem, Vol. 70, No. 19, 2005 7557