Synthesis and properties of oligo[n]cruciforms: Nanosized sterically encumbered tetraethynylphenyl-homologated fluorophores

Article abstract of DOI:10.1021/jo802797e

(Figure Presented) A series of nanosized phenyleneethynylenes have been prepared which are sterically insulated from the surrounding environment by multiple functionalization with triisopropylsilyl (TIPS) substituents. The phenyleneethynylenes comprise ol

Full text of DOI:10.1021/jo802797e

Synthesis and Properties of Oligo[n]cruciforms: Nanosized Sterically
Encumbered Tetraethynylphenyl-Homologated Fluorophores
Abdelaziz Al Ouahabi,† Paul N. W. Baxter,*,† Jean-Paul Gisselbrecht,‡ Andre´ De Cian,§
Lydia Brelot,§ and Nathalie Kyritsakas-Gruber§,|
Institut Charles Sadron, CNRS UPR 022, 6 rue du Loess, F-67083 Strasbourg, France, and Laboratoire
d’Electrochimie et de Chimie Physique du Corps Solide, SerVice de Radiocrystallographie,
CNRS UMR 7177, and Laboratoire de Chimie de Coordination Organique, CNRS UMR 7140, Institut Le
Bel, UniVersite´ Louis Pasteur, 4 rue Blaise Pascal, F-67070 Strasbourg, France
baxter@ics.u-strasbg.fr
ReceiVed December 23, 2008
A series of nanosized phenyleneethynylenes have been prepared which are sterically insulated from the
surrounding environment by multiple functionalization with triisopropylsilyl (TIPS) substituents. The
phenyleneethynylenes comprise oligo[n]cruciforms 1-4 (n ) 3-5) and a dehydrotribenzo[12]annulene
5, the former of which possess para-acyclic and the latter ortho-cyclic electronic conjugation pathways.
1
All compounds were characterized by H and 13C NMR, IR, and mass spectroscopic techniques. The
X-ray crystal structure of 1 confirmed the sterically isolating properties of the TIPS substituents. A
comparison of the physical properties of these electronically differing systems revealed that they were
all luminescent upon UV irradiation displayed negligible aggregation in dilute solution and that particular
members of the series studied were electrochemically active, undergoing facile reversible reductions.
The phenyleneethynylenes also exhibited significantly enhanced thermal stability by virtue of the presence
of the TIPS substituents. The properties of 1-5 suggest that they are promising building blocks for the
construction of materials for novel molecular electronics applications.
Introduction
to corrosion, and mechanically robust. Organic polymers can
also be easily processed into thin films of varying thickness
and topology via solvent evaporation, spin coating, etc. and may
be directly functionalized with, e.g., biomaterials or quantum
dots, and used in sensing applications.3a-j In particular,
conjugated organic polymers are now of significant commercial
importance for the construction of organic light emitting diodes
(OLEDs), thin film transistors (TFTs), and field effect transistors
(FETs), which are finding applications in computing hardware
and electronic appliances.4a-f
Since the first report of doping-induced electrical conductivity
in polyacetylene in the 1970s,1a,b the synthesis and study of
conjugated organic polymers has rapidly developed into a field
of major technological and economic importance.2a-d Electronic
components and devices incorporating organic polymers offer
many advantages over those fabricated from traditional inorganic
and metallic materials in that the former are lightweight, resistant
† Institut Charles Sadron.
‡ Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide,
Universite´ Louis Pasteur.
(2) (a) Kiebooms, R.; Menon, R.; Lee, K. In Handbook of AdVanced
Electronic and Photonic Materials and DeVices: Vol. 8 Conducting Polymers;
Nalwa, H. S., Ed.; Academic Press: New York, 2001. (b) Skotheim, T. A.,
Elsenbaumer, R. L., Reynolds, J. R., Ed. Handbook of Conducting Polymers,
2nd ed.; Dekker: New York, 1998. (c) Nalwa, H. S., Ed. Handbook of Organic
ConductiVe Molecules and Polymers; Wiley: New York, 1997; Vols. 1-4. (d)
Skotheim, T. A., Ed. Handbook of Conducting Polymers; Dekker: New York,
1986; Vols. 1 and 2.
§ Service de Radiocrystallographie, Universite´ Louis Pasteur.
| Current address: Laboratoire de Chimie de Coordination Organique,
Universite´ Louis Pasteur.
(1) (a) Etemad, S.; Heeger, A. J.; MacDiarmid, A. G. Annu. ReV. Phys. Chem.
1982, 33, 443–469. (b) MacDiarmid, A. G.; Heeger, A. J. Synth. Met. 1980, 1,
101–118.
10.1021/jo802797e CCC: $40.75  2009 American Chemical Society
Published on Web 06/12/2009
J. Org. Chem. 2009, 74, 4675–4689 4675

Al-Ouahabi et al.
An important physicochemical property displayed by the
majority of conjugated organic oligomers and polymers is that
of luminescence both in solution and the solid state when
irradiated with light of particular wavelengths. For OLEDs, the
luminescence is generated electrically, and it is of special
importance to tune the conjugation of the oligomer/polymer in
a controllable way in order to obtain electroluminescence of a
desired color.
For both electrically and light-stimulated emission, surpris-
ingly little is currently understood about how the relative
alignments between the oligomer/polymer chains and the
detailed nature of their interchain contacts affect their physical
properties such as luminescence color, quantum yield, and
electrochemical behavior.5a-e The overall efficiency of OLEDs
is determined by both the charge transport and emissive
properties of the materials used, however it is clear that short
interchain contacts such as aromatic stacking will favor charge
transport but may significantly decrease the overall luminescence
quantum yield.
In general, short interchain contacts would thus facilitate
luminescence energy transfer between oligomer/polymer chains
and maximize the chance of loss in energy as heat. The excitons
would also be able to hop between polymer chains enabling
the preferred light re-emission from the chromophores of lowest
energy. This effect is observed with blended mixtures of
different chromophores which emit light only from the chro-
mophore of lowest energy, thereby preventing the possibility
of luminescence color tuning by mixing chromophores of
differing emission energies.6
One way of increasing the luminescence emission efficiency
of conjugated organic polymers and oligomers would be to
increase the interchain distances by encapsulation in media
transparent to light,7a-c or alternatively to protect the chains as
rotaxanes or wrapped inside helical polysaccharides.8a-c Both
approaches have had some success, although the former avenue
provides little control of interchain contacts at a molecular level
while the latter may be synthetically demanding. A third
approach is to synthesize conjugated polymers and oligomers
with sterically bulky substituents covalently attached either to
the main backbone or capping the chain ends. Such substituents
are often branched or dendritic and serve to both increase
solubility and minimize interchain contacts.8b,9a,b
In the case of OLEDs, however, if conjugated systems are
too isolated, then the efficiency of interchain charge transport
would be reduced. With long polymers, significant charge
transport would be possible along the polymer chains, and the
negative effects of isolation upon the charge transport would
be less severe. An alternative approach would also be to
construct block copolymers comprising segments which may
engage in aromatic stacking interactions to aid interchain charge
transport and sterically isolated segments to enhance emission
efficiency.
One of the most highly studied and synthetically accessible
classes of conjugated organic polymers is that of the polyphe-
nylethynylenes.10 They have been prepared with a diverse range
of different substituents for the purpose of developing new types
of OLEDs, sensors, redox, antenna, and photoconducting and
photorefractive materials. Recently, a class of molecular phe-
nyleneethynylenes comprising 1,2,4,5-tetraarylethynylbenzenes
and 1,2-diarylethynyl-4,5-diarylethenylbenzenes, known as cru-
ciforms, have been the focus of a surge in interest due to their
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Synthesis and Properties of Oligo[n]cruciforms
SCHEME 1. Synthesis of Sterically Encumbered Oligocruciforms 1-4a
a Key: (i) n-BuLi, Et2O, -78 °C, 1.5-2 h then ICH2CH2I/Et2O, -78 °C, 4 h (76% 7), (80% 9); (ii) TMSA, [PdCl2(PPh3)2], CuI cat., toluene/Et3N, 20
°C, 7 d (77% 8); (iii) K2CO3, MeOH, 20 °C, 4 d, (77% 11);16 (iv) [PdCl2(dppf)]·CH2Cl2, CuI cat., toluene/Et3N, 20 °C, 14-19 d (64% 1, argon), (52% 1,
10% 2, argon/air 80: 20); (v) K2CO3, 4:1 THF/MeOH, 20 °C, 7 d (94% 3); (vi) [PdCl2(dppf)]·CH2Cl2, CuI cat., toluene/Et3N, 20 °C, 4 d then 60 °C, 5 d
(17% 4).
fascinating luminescent and sensory properties and potential use
in nanomechanical machines.11a-u
molecular modeling showed that the TIPS groups would prevent
intermolecular contacts between the alkynyl and benzenoid
portions of each oligomer.
With the aforementioned considerations in mind, we therefore
reasoned that oligomeric strings of conjugated cruciforms
comprising para-connected 1,2,4,5-tetraethynylbenzene units
functionalized with sterically bulky substituents would exhibit
physicochemical properties characteristic of single molecules,
unperturbed by effects resulting from intermolecular aggregation
and associations. They would therefore function as ideal models
for studying the effect of steric isolation at a molecular level
on their electronic and spectroscopic properties12 and in
particular serve as promising candidates for the creation of
OLED polymers and block copolymers with enhanced emission
characteristics.
The fact that the TIPS substituents are lipophilic would also
be expected to significantly assist the solubility of the larger
oligomers and their lack of conjugation and saturation further
reduce the ease of intermolecular exciton hopping in these
systems. Furthermore, the use of 1,2,4,5-tetraethynylbenzene
units extends the chromophoric surface area of the oligomers
and increases the conjugation of the phenylethynylene backbone.
Oligo[n]cruciforms of this type where n g 1 would therefore
be expected to yield chromophores with superior luminescence
emission properties. The TIPS substituents were also anticipated
to increase the stability of the chromophores toward degradation
by heat and light, an expectation that was born out upon
subsequent exploration of their properties. The fulfillment of
this latter requirement is important if real-life applications are
to be found for such materials.13a-e
We decided to prepare the sterically encumbered TIPS-
substituted oligocruciforms 1-4 shown in Scheme 1, as
(12) Reports of oligo[n]cruciforms are rare in the literature, see, for
example: (a) An oligo[2]cruciform comprising two 1,2,4,5-tetraethynylbenzene
subunits with a 1,4-butadiyne bridge, used as a precursor for the syntheses of
acediynes: Gallagher, M. E.; Anthony, J. E. Tetrahedron Lett. 2001, 42, 7533–
7536. For an oligo[2]cruciform incorporating extended TTF units with a 1,4-
butadiyne bridge, see: (b) Vestergaard, M.; Jennum, K.; Sørensen, J. K.; Kilså,
K.; Nielsen, M. B. J. Org. Chem. 2008, 73, 3175–3183.
In order to expand our studies to include sterically encum-
bered phenyleneethynylenes with alternative conjugation path-
ways to that of the para-/cross-conjugation of 1-4, we prepared
the TIPS-functionalized dehydrotribenzo[12]annulene 5 (Scheme
J. Org. Chem. Vol. 74, No. 13, 2009 4677

Al-Ouahabi et al.
SCHEME 2. Synthesis of Sterically Encumbered Dehydrotribenzo[12]annulene 5a
a Key: (i) five steps;22 (ii) n-BuLi, Et2O, -78 °C, 1.5 h then ICH2CH2I/Et2O, -78 °C, 3 h (95% 14); (iii) K2CO3, 0.6:1 THF/MeOH, 20 °C, 14 h (98%
15); (iv) 1: 2 mol ratio of 15: t-BuOCu, pyridine, 35-40 °C, 4 h, then 125 °C-140 °C, 10 h (20% 5); (v) [(n-Bu)4N]F/THF, 20 °C, 48 h (94% 16).
2). Compound 5 offers the opportunity for investigating the
electronic and spectroscopic effects of steric encumbrance on a
cyclically ortho-conjugated analogue of 1 which possesses the
same number of benzene rings.
lated with K2CO3 in MeOH to give 11,16 which served as the
core building-block for the construction of the required oli-
gocruciforms 1-4.
The cross-coupling reaction of g2 equiv of 9 with 11 in 6:1
17
toluene/Et3N solution, using [PdCl2(dppf)]·CH2Cl2 and CuI
catalysts, afforded 64% of oligo[3]cruciform 1 after workup.
The employment of [PdCl2(PPh3)2] catalyst and pure Et3N as
solvent resulted in the formation of significantly reduced yields
of 1. In order to obtain optimal yields of 1, it was also necessary
to ensure complete exclusion of oxygen from the reaction. For
example, one run was conducted under an atmosphere of 80:
20 parts argon/air. In this case, oligo[4]cruciform 2 was isolated
in 10% yield as a yellow powder with significantly reduced
solubility compared to 1, which was also obtained in 52% yield
from the same reaction. An explanation consistent with the
oxygen-assisted formation of 2 may be that a significant
difference exists between the relative rates of palladium-
catalyzed cross-coupling of each equivalent of 9 with the in
situ generated organocuprate of 11. The initial step in the
reaction involves a fast cross-coupling between the organocu-
prate of 11 and the first equivalent of 9 to yield a triarylmo-
noethyne intermediate. However, in the presence of oxygen an
alternative reaction pathway is available to this latter intermedi-
ate which involves the copper-catalyzed oxidative self-coupling
of the remaining alkyne to afford 2. The slower second cross-
coupling reaction with 9 may therefore occur at a rate which is
comparable to the oxidative self-coupling. Thus, in the presence
of air, oxidative dimerization of the intermediate monoalkyne
effectively competes with the second cross-coupling step,
resulting in a mixture of 1 and 2. Alternatively, comparable
homo- and cross-coupling rates may lead to 2 via the initial
coupling of two molecules of 11 followed by end-capping by
9.18a,b
Results and Discussion
Oligomer Synthesis and Characterization. The sterically
isolated oligocruciforms 1-4 were synthesized according to the
reaction sequence shown in Scheme 1. Thus, the halogen-lithium
exchange reaction of 614 in Et2O solution at low temperature
resulted in clean monolithiation when treated with 1 equiv of
n-BuLi. Subsequent quenching of the monolithiophenyl with
diiodoethane at low temperature afforded 7, which was obtained
in 76% yield after workup. The ambient temperature reaction
of 7 with TMSA (trimethylsilylacetylene) in Et3N solution under
Sonogashira conditions, using [PdCl2(PPh3)2] and CuI catalysts,
resulted in exclusive reaction with the iodine substituent of 7
to give 8, which was isolated in 77% yield.15a,b Although 8
was reactive toward a variety of arylethynes at elevated
temperatures in the presence of the above catalysts, we found
that the products of cross-coupling were formed in reduced
yields, rarely clean, and often problematic to purify. We
therefore converted 8 to the iodo analogue 9 via a second low-
temperature lithium-halogen exchange reaction followed by
treatment with diiodoethane. The enhanced reactivity of 9
compared to 8 toward Sonogashira cross-couplings was antici-
pated to result in enhanced yields and cleaner reaction products,
an expectation that was confirmed during subsequent experi-
mentation. Tetraethynylphenyl 10 was then selectively desily-
(13) For oligomeric enediynes and annulenes stabilized and solubilized with
appended TIPS-ethynyl groups, see: (a) Boldi, A. M.; Anthony, J.; Gramlich,
V.; Knobler, C. B.; Boudon, C.; Gisselbrecht, J.-P.; Gross, M.; Diederich, F.
HelV. Chim. Acta 1995, 78, 779–796. (b) Anthony, J.; Boudon, C.; Diederich,
F.; Gisselbrecht, J.-P.; Gramlich, V.; Gross, M.; Hobi, M.; Seiler, P. Angew.
Chem., Int. Ed. Engl. 1994, 33, 763–766. (c) Anthony, J.; Knobler, C. B.;
Diederich, F. Angew. Chem., Int. Ed. Engl. 1993, 32, 406–409. (d) Boldi, A. M.;
Anthony, J.; Knobler, C. B.; Diederich, F. Angew. Chem., Int. Ed. Engl. 1992,
31, 1240–1242. For some dendritically branched phenyleneethynylenes with
terminal TIPS-ethynyl groups, see: (e) Nierengarten, J.-F.; Zhang, S.; Ge´gout,
A.; Urbani, M. J. Org. Chem. 2005, 70, 7550–7557.
(14) Tovar, J. D.; Swager, T. M. J. Organomet. Chem. 2002, 653, 215–222.
(15) (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis
1980, 627–630. (b) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 4467–4470.
(16) Marsden, J. A.; Miller, J. J.; Shirtcliff, L. D.; Haley, M. M. J. Am. Chem.
Soc. 2005, 127, 2464–2476.
The formation of 2 is particularly surprising as it involves a
multistep, one-pot generation of a 35 Å conjugated and insulated
nanowire from simple monomeric starting materials. This
observation further suggests that the reaction may become a
(17) Dierkes, P.; van Leeuwen, P. W. N. M. J. Chem. Soc., Dalton Trans.
1999, 1519–1529.
(18) For the copper catalyzed oxidative self-coupling of ethynes, see, for
example: (a) Batsanov, A. S.; Collings, J. C.; Fairlamb, I. J. S.; Holland, J. P.;
Howard, J. A. K.; Lin, Z.; Marder, T. B.; Parsons, A. C.; Ward, R. M.; Zhu, J.
J. Org. Chem. 2005, 70, 703–706. (b) Anderson, H. L. Inorg. Chem. 1994, 33,
972–981.
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Synthesis and Properties of Oligo[n]cruciforms
synthetically useful method for the direct generation of large
conjugated nanowires if performed in air.
Oligo[3]cruciform 1 may serve as a central core for the further
elaboration to longer [3 + 2m]cruciform oligomers (where m
) integer). Thus, the selective deprotection of the trimethylsilyl
groups of 1 using K2CO3 in 4:1 THF/MeOH afforded diethyne
3 in 94% yield after workup. It may be noted that the
triisopropylalkynyl substituents on 3 and 11 result in an
enhanced stabilization of the unsubstituted alkyne groups toward
heat and light. Aromatic polyethynylbenzenes, in which the
ethynes are unsubstituted, normally thermally polymerize or
slowly degrade at ambient temperature and show a marked
sensitivity to light. Compounds 3 and 11, on the other hand,
can be stored for several years in the dark at ambient temperature
without any visible sign of decomposition and undergo only
slight color darkening on exposure to indirect light for several
months. As in the formation of 1 from 11 and 9, the cross
coupling of 3 with g2 equiv of 9 in toluene/Et3N solution, using
[PdCl2(dppf)]·CH2Cl2 and CuI catalysts, afforded 17% of
oligo[5]cruciform 4 after workup. The low yield of 4 was
principally attributed to its reduced solubility, which hampered
purification. This latter observation is contrary to what was
expected, i.e., that the longer oligomers would remain soluble.
However, a decrease in solubility of longer TIPS-ethynyl
functionalized enediyne oligomers in hexane has been observed
by Diederich et al.13
FIGURE 1. 1H NMR of 1 showing expanded aromatic region (upper
spectrum) and aliphatic region (lower spectrum). (As the aromatic region
has been vertically enlarged relative to the aliphatic region, the
integrations of the upper and lower spectral sections are not proportion-
ate.)
The oligocruciforms 1-4 were characterized on the basis of
1
mass spectroscopic, infrared, H and 13C NMR spectroscopic
measurements, and spectral comparisons with their respective
precursors. Due to the similarity in the chemical and magnetic
environments of many of the protons and carbons in 1-4, the
NMR spectra always displayed some overlapping bands which
hampered assignment. However, for each compound, particular
groups of protons and carbons were numerically correctly
represented, which supported characterization. Uniquely in the
case of 1, the 1H NMR (Figure 1) displayed the expected number
of peaks corresponding to the three TIPS and three aromatic
protons, although situated in close proximity, as well as all nine
peaks in its 13C NMR due to the aromatic carbons. The
dilution studies performed on the longest oligomer 4 and cycle
5 between, respectively, 4 × 10-7 and 5 × 10-6 mol/L in CDCl3
and 1 × 10-6 and 1 × 10-5 mol/L in CD2Cl2 afforded only
negligible changes in the chemical shifts, demonstrating that
aggregation was either absent or unchanging over the concentra-
tion range studied, i.e., the range usual for NMR measurements.
The infrared spectra of 1-4 each exhibited a weak band
within the range 2157-2161 cm-1 assignable to the ethynyl
Ct C stretching vibration. In the case of 3, the weak band at
3307 cm-1 was clearly assignable to the CH stretching mode
of the terminal ethynyl t C-H groups.19
1
assignment of the H NMR of 1 was based on the expectation
that the chemical shifts of the outer facing H3 and 5-TIPS
protons of the terminal phenyl rings would be significantly
different from those of the inner aromatic and TIPS protons.
Accordingly, the magnetically and chemically most distinct
singlets at 7.620 and 1.151 ppm were assigned to the outer
phenyl H3 and 5-TIPS, respectively.
The clearest evidence for the oligocruciform identity of 1-4
was afforded by mass spectroscopic studies. Initial attempts at
recording the ESI and MALDI-TOF mass spectra of 1-4 in
the presence of various matrices and organic acids such as TFA
failed to yield any peak clusters corresponding to those expected
for the [M + H+] molecular ions, presumably due to acid-
mediated degradation of the oligocruciforms.
It is well known, however, that phenyleneethynylenes form
complexes with Ag+ and Cu+ ions,20a-c which should be
detectible by ESI and MALDI-TOF mass spectroscopy. How-
ever, the spectra of 1-4 mixed with AgCF3SO3 exhibited only
very low intensity bands corresponding to the presence of [M
+ Ag+] ions. The majority of peaks were assignable to the loss
of TIPS substituents, and in the MALDI-TOF mass spectra,
direct replacement of the TMS by Ag to afford the ethynylar-
gentate. Clearly, the Ag+ was causing significant desilylation
The assignment of the remaining closely situated peaks to
the inner phenyl and TIPS protons was supported by NOESY
1
and ROESY measurements. The H NMR of the structurally
similar 3 also displayed the three peaks expected for the aromatic
protons as well as the 10 peaks in its 13C NMR originating from
the ethynyl carbons. The 1H NMR spectrum of 2 exhibited four
strong singlets due to the four chemically and magnetically
inequivalent TIPS groups, and its 13C spectrum displayed the
expected 14 peaks corresponding to the ethynyl carbons and
four peaks assignable to the four TIPS secondary carbons. As
anticipated, oligo[5]cruciform 4 possessed the most complex
NMR with the greatest number of overlapping peaks; however,
five peaks corresponding to all five aromatic protons were
clearly visible in its 1H NMR. The characteristic signals
assignable to the terminal TMS and ethynyl-H substituents were
also correctly represented in the NMR spectra of 1-4. 1H NMR
(19) Silverstein, R. M.; Webster, F. X. Spectrometric Identification of Organic
Compounds, 6th ed.; J. Wiley & Sons, Inc.: New York, 1997.
(20) For the study and isolation of silver complexes with ethynes, see, for
example: (a) Hellbach, B.; Rominger, F.; Gleiter, R. J. Organomet. Chem. 2006,
691, 1814–1816. (b) Le´tinois-Halbes, U.; Pale, P.; Berger, S. J. Org. Chem.
2005, 70, 9185–9190. (c) Schulte, P.; Behrens, U. J. Organomet. Chem. 1998,
563, 235–249.
J. Org. Chem. Vol. 74, No. 13, 2009 4679

Al-Ouahabi et al.
under the mass spectral conditions.21 However, the employment
of CuCF3SO3 yielded significantly cleaner ESI and/or MALDI-
TOF mass spectra with relatively intense [M + Cu+] cluster
peaks. Interestingly, the ESI technique only afforded [M + Cu+]
ions for the smaller oligomers 1 and 3, while MALDI-TOF
yielded [M + Cu+] ions for all linear oligomers studied.
As mentioned earlier, we were interested in comparing the
physical properties of the oligocruciforms with other similarly
functionalized and sterically encumbered phenyleneethynylene
nanostructures possessing alternative electronic conjugation
pathways. Toward this goal, we prepared the ethynyl-TIPS-
functionalized dehydrotribenzo[12]annulene 5 as shown in
Scheme 2. Thus, key intermediate 13 was prepared from 12
via a five-step sequence as reported.22 When treated with 1 equiv
of n-BuLi in Et2O at low temperature, 13 underwent a clean
lithium-halogen exchange to the lithiophenyl, which was
subsequently quenched with diiodoethane to afford 14 in 95%
yield. The trimethylsilyl group of 14 could then be selectively
removed using K2CO3 in 0.6:1 THF/MeOH to afford 15 in 98%
yield after workup. Finally, addition of 15 to an excess of
t-BuOCu in pyridine under argon yielded an organocuprate,
which upon heating, underwent the Stephens-Castro cyclization
to 5 which was isolated in 20% yield after purification.23a-c
Upon treatment with g3 equiv of [(n-Bu)4N]F in THF, the TIPS
groups of 5 could be removed to provide the symmetrically
functionalized triethynyldehydrotribenzo[12]annulene 16, which
after purification was obtained as a poorly soluble yellow solid
in 94% yield. The latter macrocycle is a particularly interesting
compound in that it may be anticipated to partake in Sonogash-
ira-type cross-couplings with many readily available function-
alized iodoaromatics, thereby serving as a key synthon for the
creation of a wide variety of symmetrically trifunctionalized
dehydrotribenzo[12]annulenes.24a,b
The dehydrotribenzo[12]annulenes 5 and 16 were character-
1
ized on the basis of mass spectroscopic, infrared, H and 13C
NMR spectroscopic measurements. In contrast to the oligo[n]-
cruciforms, the ESI technique afforded superior mass spectra
for the macrocycles than did MALDI-TOF, with the [M + Cu+]
ions clearly visible in both cases. The 1H NMR spectra, on the
other hand, were particularly second order, comprising only two
sets of multiplets in the aromatic region. The 13C NMR spectra
of 5 and 16, however, displayed the correct number of peaks
expected for such cyclic structures.
X-ray Crystal Structural Characterization of 1. In order
to obtain detailed structural parameters of the oligo[n]cruciforms
at a molecular level and determine the steric influence that the
TIPS substituents exert on intermolecular interactions, we
therefore determined the X-ray crystal structure of 1, which
served as a representative member of the series. Oligomer 1
crystallized with no included solvent in a monoclinic crystal
system with the C2/c space group.
FIGURE 2. X-ray crystal structure of oligo[3]cruciform 1 (top:
wireframe representation; center: spacefilling; bottom: packing motif
as viewed down main axes of 1 molecules).
Despite the relatively large number of conformations available
to 1 through twisting of the phenyl rings about its main axis,
the molecule is essentially planar in the solid state with identical
directional alignment of all phenyl rings with respect to each
other (Figure 2, upper: wireframe; center: space-filling repre-
sentations). The Si(TMS)-Si(TMS) distance is 25.5 Å, which
places the molecule within the nanostructural domain. This
preferred solid-state all-planar conformation has also been
observed in the crystal structure of the related TIPS-function-
alized linear tetraethynylethene trimer oligomers synthesized by
Diederich and co-workers.13b,d
The three TIPS substituents within each row on either side
of the molecule are in very close proximity and interdigitated
with each another. In order to further accommodate this
closeness of approach, the central TIPS-ethyne-phenyl-ethyne-
TIPS moiety is partially bent into a slight S-shaped configura-
tion, and each inner-terminal TIPS-ethyne substituent is slightly
splayed out toward the TMS groups. Furthermore, the main axis
of 1, which passes through the three phenyl rings and TMS
substituents, is not linear as expected but slightly curved into
an S-shape, resulting from the combined effects of the sub-
(21) Yamamoto, Y. Chem. ReV. 2008, 108, 3199–3222.
(22) Tobe, Y.; Takaharu, H. Jpn. Sci. Tech. Agency 2005-09-
22, JP2005255778.
(23) For syntheses of unsubstituted dehydrotribenzo[12]annulene via the
Stephens-Castro reaction, see: (a) Solooki, D.; Ferrara, J. D.; Malaba, D.;
Bradshaw, J. D.; Tessier, C. A.; Youngs, W. J.; John, J. A.; Tour, J. M. Inorg.
Synth. 1997, 31, 122–128. (b) Staab, H. A.; Graf, F. Tetrahedron Lett. 1966,
751–757. (c) Campbell, I. D.; Eglinton, G.; Henderson, W.; Raphael, R. A. Chem.
Commun. 1966, 87–89.
(24) Dehydrobenzoannulenes functionalized with terminal ethynes are rare
in the literature; see, for example: (a) Matzger, A. J.; Shim, M.; Vollhardt, K. P. C.
Chem. Commun 1999, 1871–1872. (b) Eickmeier, C.; Junga, H.; Matzger, A. J.;
Scherhag, F.; Shim, M.; Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1997,
36, 2103–2108.
4680 J. Org. Chem. Vol. 74, No. 13, 2009

Synthesis and Properties of Oligo[n]cruciforms
TABLE 1. Electrochemical Data of 1 and 5 Observed by Cyclic
Voltammetry (CV) in CH2Cl2 + 0.1 mol/L Bu4NPF6 (All Potentials
Are Given versus Ferrocene, Used as Internal Standard)
stituent distortions described above and crystal packing factors
(Figure 2, wireframe representation).
In the case of our series of oligo[n]cruciforms, it was of
particular interest to determine whether the conjugated backbone
of each molecule was effectively sterically isolated from its
neighbors in the solid state due to the steric volume of the TIPS
substituents.
a
oligomer/monomer
scan rate (V s-1
)
E° (V/Fc+/Fc
)
∆Ep (mV)
10
1
0.1
0.1
>1.0
0.2
-2.25
-1.86
-2.04b
-1.84
-2.13
100
80
5
60
100
0.2
The crystal lattice of 1 comprises two-dimensional layers,
each 27 Å in thickness, within which the molecules of 1 are
vertically packed together with the TMS groups forming the
extreme upper and lower surfaces of every layer. The layers of
1 are also interdigitated with each other via the TMS substit-
uents, with the shortest interlayer Si(TMS)-Si(TMS) distance
of 6.376 Å. The steric volume of the trialkylsilyl substituents
lining the upper and lower surface of each layer prevents any
ethyne and phenyl contacts being formed between each layer.
However, the interlayer domains are not tightly close-packed,
as evidenced by some disorder within the terminal TIPS and
TMS substituents which function as the interlocking groups
lining the surface of each layer.
a ∆Ep is the potential difference between the cathodic and anodic peak
potentials. b reversible electron transfer only at scan rates higher than 1
Vs-1
.
Within each layer, as defined by the plane through the b-
and c-axes, the molecules of 1 are all tilted 66° in the same
direction. The direction of tilting is also identical for all layers.
When viewed down their main axes (i.e., 66° to the layer plane),
the molecules of 1 are seen to be packed into a slipped
herringbone motif (Figure 2, bottom: packing motif). The
molecules of 1 therefore laterally slot together in an edge-to-
face type arrangement, where the edge is defined by the row of
three TIPS substituents on each side of 1 and the face is the
axial cavity that lies between the two rows of TIPS groups in
each molecule of 1. In this way, each molecule is completely
sterically isolated from its nearest neighbors by the TIPS
substituents so that the conjugated ethynyl and phenyl parts of
1 are unable to form any intermolecular lateral contacts. Thus,
within each layer, the shortest intermolecular phenyl-phenyl
distance of 8.594 Å was found between C17 and H18 of the
terminal phenyl rings. The shortest phenyl-phenyl interlayer
distance was found to be 7.325 Å between the H21 hydrogens
of the terminal phenyl rings. The X-ray crystal structure of 1
unambiguously demonstrates that the TIPS groups are of
sufficient steric volume to prevent intermolecular π-π contacts.
Electrochemical Properties. Both linear and cyclic conju-
gated oligomeric hydrocarbons functionalized with stabilizing
and solubilizing ethynyl-TIPS groups have been the focus of
electrochemical investigations due to their novel topologies and
interesting electronic delocalization pathways.13b,25a-c We
therefore considered that it would be of interest to perform some
investigations into the electrochemical properties of selected
members of our series of TIPS-ethynyl functionalized oligophe-
nyleneethynylenes. Thus, the cyclic voltammetry of 1, 5, and
10 (Table 1 and Figure 3) was performed in CH2Cl2 in the
presence of 0.1 mol/L of n-Bu4NPF6 and referenced against
ferrocene as an internal standard. The cyclic voltammetry of
cycle 5 at low scan rates exhibited two reversible reductions at
-1.84 and -2.13 V vs Fc+/Fc, with a cathodic/anodic peak
separation ∆Ep of 60 and 100 mV, respectively.
FIGURE 3. Cyclic voltammetry of 1 (1 V s-1 scan rate, upper) and 5
(0.2 V s-1 scan rate, lower voltammogram) in CH2Cl2 solution, with
0.1 mol/L of Bu4NPF6 in the presence of ferrocene.
first and second reductions of -1.12 and -1.52 V vs Fc+/Fc,
respectively. However, cycle 5 is more difficult to reduce than
the TIPS-protected tetraethynylethene-based dehydro-
[18]annulene, reflecting the longer cyclic conjugation pathway
and greater number of π-electrons in the latter system, as well
as the presence of an increased number of ethynyl-TIPS
substituents, which are known to be stabilize the reduction
process.25b
Interestingly,recenttheoreticalcalculationsondehydrotribenzo-
[12]annulene have also revealed that the electronic properties
of this system are dominated by the individual aromaticity of
the benzene rings rather than the antiaromatic contribution of
the 12 π-electron circuit,26 a factor which would also be
expected to contribute to the difference in the electrochemical
reduction propensities of 5 and the reported TIPS-protected
dehydro[18]annulene.25a
The linear oligo[3]cruciform 1 also afforded two one-electron
electron transfers at -1.86 and -2.04 V vs Fc+/Fc; however,
at low scan rates, only the first reduction was reversible, the
second reduction becoming reversible at scan rates higher than
1 V s-1. This latter behavior is characteristic of an electron
transfer mechanism which triggers an irreversible chemical
reaction. Increased scan rates ensure that the dianion is rapidly
(25) (a) Mitzel, F.; Boudon, C.; Gisselbrecht, J.-P.; Gross, M.; Diederich, F.
Chem. Commun. 2002, 2318–2319. (b) Boudon, C.; Gisselbrecht, J.-P.; Gross,
M.; Anthony, J.; Boldi, A. M.; Faust, R.; Lange, T.; Philp, D.; Van Loon, J.-D.;
Diederich, F. J. Electroanal. Chem. 1995, 394, 187–197. (c) Ferrara, J. D.;
Tanaka, A. A.; Fierro, C.; Tessier-Youngs, C. A.; Youngs, W. J. Organometallics
1989, 8, 2089–2098.
The electrochemical properties of 5, which is a Hu¨ckel-
antiaromatic 12π electron system, are similar to those reported
for the parent triangular hydrocarbon dehydrotribenzo[12]-
annulene25c and a structurally related TIPS-protected dehydro-
[18]annulene constructed from tetraethynylethene units,25a which
also exhibited multiple reversible reductions, with E° for the
(26) Tahara, K.; Yoshimura, T.; Sonoda, M.; Tobe, Y.; Williams, R. V. J.
Org. Chem. 2007, 72, 1437–1442.
J. Org. Chem. Vol. 74, No. 13, 2009 4681

Al-Ouahabi et al.
TABLE 2. UV-vis and Luminescence Emission Spectroscopic Data for 1-4 and 10 (Presented in Order of Increasing Oligomer Size) and
Cycles 5 and 16
c
d
monomer/
oligomer
no. of aromatic
rings (length, Å)
λabs
λem solid
-
optical bandgap
quantum
yield Φ
λem solutiond (nm)
Eg (eV)e
opt
(nm)
λ
em solution (nm)
λ
em solid (nm)
370d
10
3
1 (12)a
3 (24)b
3 (26)a
4 (35)a
274
346d
362
24
3.5
3.0
2.9
2.8
0.18
285
286
289
396d
416
434d
445
38
41
45
0.89
0.93
0.55
1
399d
422
440d
453
2
422d
447
467d
483
462sh
417d
441
4
5
5 (39)a
3
287
318
469d
486
516sh
539d
572
52
36
2.8
0.76 (0.78)g
0.06
f
486
-
503d
514sh
537
542
f
16
3
480
504sh
539d
556sh
571sh
-
308
496d
505sh
527
43
533
a Si(TMS)-Si(TMS) distance. b H(alkyne)-H(alkyne) distance. c Absorption maximum of highest intensity. d Highest intensity emission peaks.
e Determined from the lowest energy absorption edge of the thin film absorption spectrum. f Lowest energy absorption edge poorly defined. g Quantum
yield in argon-degassed heptane. Unless otherwise stated, all UV-vis and luminescence solution measurements were performed in air-equilibrated
heptane at 20-25 °C.
reoxidized before it can chemically react. The chemical reaction
probably involves the protonation of the generated dianion by
trace impurities of water. Indeed, dianions generated at such
negative potentials are strong bases, and may react easily with
residual water.
Somewhat surprisingly, the cyclic voltammetry of 2, per-
formed under conditions identical to those of 1 and 5, showed
no reduction processes either at relatively low scan rates or at
scan rates >1 V s-1. A possible contributory factor to this
behavior may be related to the poor solubility of 2 in
dichloromethane.
The first and second electrochemical reduction potentials of
1 and 5 are therefore strikingly similar despite their very
different topologies, suggesting that their electrochemical
properties are determined more by the presence and number of
the benzene rings. It may be noted that both 1 and 5 possess
the same number of benzene rings.
than 1. This behavior is consistent with the conjugation
extension of 1, which is expected to facilitate the first electron
transfer.
From the above studies, it can therefore be seen that both 1
and 5 are able to undergo multiple electrochemical reductions,
suggesting that these systems possess potential for the future
development of electron reservoirs and capacitors within the
field of molecular electronics. Further stabilization of the above
TIPS-ethynyl functionalized phenyleneethynylene oligomers
toward electrochemical reduction should also be possible upon
selective functionalization with electron-withdrawing substitu-
ents and increasing the axial conjugation.
UV-vis Spectroscopic Properties. The spectroscopic prop-
erties of π-electron conjugated oligomers continue to be the
focus of intensive studies in order to gain further insight into
their electronic properties and to assess their potential as a
reservoir of new materials with novel commercial applications.
Within our series of phenyleneethynylene oligomers, it was
important to determine the effect of the ethynyl-TIPS substit-
uents and steric isolation upon the spectroscopic properties of
these compounds. We therefore measured the UV-vis spectra
of 1-5 and 16 in heptane solution over several concentrations
from 6 × 10-5 to 9 × 10-7 mol/L and within the limits of
experimental error showed no significant or progressive devia-
tion from the Beer and Lambert law, demonstrating that
aggregation was not occurring in dilute solution. The results of
the UV-vis spectroscopic investigations are summarized in
Table 2 and Figures 4 and 5.
Diederich and co-workers25b have reported a TIPS-function-
alized linear tetraethynylethene trimer oligomer, structurally
related to 1, for which the E° for the first and second reduction
potentials were -1.23 V (reversible) and -1.47 V (irreversible)
vs Fc+/Fc, respectively. Although the two systems exhibited
the same number of reduction steps, the aromatic 1 was more
difficult to reduce than Diederich’s linear oligomeric tetraethy-
nylethene analogue, following the same trend as in the macro-
cyclic systems discussed above. Interestingly, elongation of the
axial conjugation by end-capping the tetraethynylethene trimer
oligomer with phenyl rings, was found to increase its stability
toward electrochemical reduction, affording three reductions at
E° ) -1.17, -1.42, and -2.00 V vs Fc+/Fc, respectively.13b,25b
For comparison, the electrochemical properties of 10 were
also measured (Table 1). It was found to afford only one well
resolved reduction step at -2.25 V vs Fc+/Fc, which was
situated very close to the electrolyte discharge, and thereby
preventing the observation of further reductions if present.
Monomer 10 is therefore significantly more difficult to reduce
As expected, upon increasing the conjugation length from
cruciform 10 through oligo[3]cruciform 1 to oligo[5]cruciform
4, the majority of the spectroscopic transitions move to lower
energy as a result of the concomitant decrease in the
HOMO-LUMO energy difference. The difference between the
absorbance maxima of 1 and 4 is however considerably smaller
than that between the maxima of 10 and 1, indicating that the
pentameric oligo[5]cruciform 4 is closely approaching the
4682 J. Org. Chem. Vol. 74, No. 13, 2009

Synthesis and Properties of Oligo[n]cruciforms
FIGURE 6. UV-vis spectra of 5 in heptane solution (solid curve, left)
and 1:1 THF/DMSO (dotted curve, left) and magnification of the lower
energy domain of 5 in heptane (solid curve, right).
FIGURE 4. UV-vis spectra of oligo[n]cruciforms 1-4 and monomer
10 in heptane solution.
1-4 they were poorly resolved (Figures 4 and 5). For example,
the second absorption domain in 10 which directly follows the
most intense absorption peak merges into a single region of
peaks or hill in the case of 1 and 4. These findings are consistent
with the degree of flexibility of the compounds investigated.
Thus, the spectra exhibiting the greatest number of vibrational
peaks are those of the rigid oligomers, i.e., 5, 10, 11, and 16,
and those of the conformationally mobile 1-4 accordingly
exhibit comparatively few clearly defined vibrational bands.
Within the series of linear TMS-terminated compounds the
molar absorption coefficients of the absorption maxima of
greatest intensity also increase as expected in the order 10 < 1
< 4 from 1.86 × 105 (10) through 2.38 × 105 (1) to 3.13 × 105
M-1 cm-1 in the case of 4.
The terminal TMS groups exert a small energy lowering effect
upon the absorption maxima of the linear oligomers, as the
absorption maxima within the region 280-420 nm move 1-4
nm to a shorter wavelength upon passing from 1 to the TMS-
deprotected 3. The electron-donating nature of the trialkylsilyl
groups may therefore be raising the energy of the ground state
HOMO, thereby reducing the HOMO-LUMO energy gap.
At wavelengths higher than 280 nm, the oligo[4]cruciform 2
exhibits the most red-shifted absorption spectrum within the
series of linear oligomers, displaying a low energy absorption
which tails well into the visible region at 445 nm. Elongation
of the axial ethynyl bridges therefore results in a reduction of
the π-electron HOMO-LUMO energy gaps of the tetraethy-
nylphenyl-based oligo[n]cruciforms.
The UV-vis spectra of 1-5, 10, 11, and 16 are slightly
solvatochromic. For example, all compounds in THF solution
exhibited small but significant 1 nm shifts in the majority of
absorption maxima to lower energy. In the case of 5, a further
increase in the solvent polarity using 1:1 THF/DMSO resulted
in 1-3 nm peak shifts to lower energy (Figure 6). Confirmation
that the absorption shifts were not resulting from aggregation
in these more polar solvents, was obtained by dilution studies
which showed no significant or progressive deviation from the
Beer and Lambert law. These findings suggest that the absorp-
tion maxima in the UV-vis spectra of the oligomers originate
from π-π* transitions.28
FIGURE 5. Comparison of UV-vis spectra of macrocycles 5 and 16,
oligo[3]cruciform 1, and monomer 10 in heptane solution.
effective conjugation length for this system. This conclusion
opt
was also supported by the values of the optical energy gap (Eg
)
of the linear oligomers (Table 2), in each case determined from
the lowest energy absorption edge of the thin film absorption
spectrum,27a,b which decreases in the order 10 . 1 > 2 = 4.
Although 10 and 11 are structurally much simpler than either
1 or 4, their UV-vis spectra were particularly complex and
comprised three main domains. The first highest energy domain
consisted of five to six transitions from 226 to 274 nm which
increased successively in molar absorption coefficient, the lowest
energy transition in this series being the most intense absorption
in the spectrum. The second domain comprised seven peaks
from 277 to 321 nm of intermediate molar absorption coef-
ficients. The third and lowest energy domain comprised three
to four absorptions from 322 to 350 nm with very low absorption
coefficients characteristic of forbidden transitions.
The absorption peak separations in the UV-vis spectra of
10 and 11 varied between 5 and 15 nm, equating to the energies
of vibrational transitions28 and confirming that the complexity
of the spectra originated from an enhanced degree of vibronic
coupling.
In fact, all of the UV-vis spectra of 1-5 and 16 exhibited
peaks or shoulders due to some vibronic coupling, although in
As mentioned above, the macrocycles 5 and 16 afforded
UV-vis spectra of greater similarity to that of 10 and 11 with
respect to the sharpness and multiplicity of vibronic couplings.
As in 10 and 11, the UV-vis spectra of 5 and 16 each display
a single sharp absorption of much higher intensity compared to
(27) (a) Steckler, T. T.; Abboud, K. A.; Craps, M.; Rinzler, A. G.; Reynolds,
J. R. Chem. Commun. 2007, 4904–4906. (b) Mu¨hlbacher, D.; Neugebauer, H.;
Cravino, A.; Sariciftci, N. S. Synth. Met. 2003, 137, 1361–1362.
(28) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Herndon, VA, 1991.
J. Org. Chem. Vol. 74, No. 13, 2009 4683

Al-Ouahabi et al.
the other peaks. These intense absorption peaks which occur at
318 nm in the UV-vis spectrum of 5 and at 308 nm in that of
16 exhibit very large molar absorption coefficients of 6.36 ×
105 and 4.35 × 105 M-1 cm-1 respectively, compared to those
of the most intense absorptions of the longest linear oligomers
2 and 4 at 289 nm (2.63 × 105 M-1 cm-1) and 287 nm (3.13 ×
105 M-1 cm-1), respectively (Figure 4).
With respect to shape and the number of absorptions, the
UV-vis spectra of 5 and 16 are virtually identical. However,
the presence of the TIPS substituents significantly lower the
energy of the absorption transitions, as is seen in the spectrum
of 5 where all absorptions are red-shifted by 5-10 nm compared
to those in the spectrum of 16. The molar absorption coefficients
of the three most intense absorptions in the UV-vis spectrum
of 5 are also significantly enhanced (by 2.93 × 104-2.01 ×
105 M-1 cm-1) compared to those of 16. Thus the sterically
isolating TIPS substituents may result in an enhancement in
the molar absorptivity of the dehydrotribenzo[12]annulene
chromophore.
FIGURE 7. Normalized luminescence emission spectra of 1-5, 10,
and 16 recorded in heptane solution. *2λ excitation scattering peak
(see the Supporting Information for the respective excitation wave-
lengths and solution concentrations).
Interestingly, the most intensely absorbing peak in each of
the UV-vis spectra of 5 and 16 exhibits considerable tailing
on its lower energy edge which encompasses additional peaks
of very low absorption coefficients. This unusual feature is most
clearly observed in the case of 5, where magnification of the
UV-vis spectrum of a relatively concentrated solution in
heptane (7.13 × 10-5 mol/L) revealed that the absorption tail
comprised a suite of maxima presumably arising from vibronic
coupling (Figure 6) and which finally disappeared into the
baseline at 459 nm.29
Luminescence Spectroscopic Properties. A particularly
characteristic physical property of oligophenyleneethynylenes
is their enhanced luminescence when exposed to UV light. The
color of the emission can be tuned by alterations in conjugation
length and molecular topology, as well as by variation in the
electronic properties and positions of appended substituents and
functional groups. For this reason, oligophenyleneethynylenes
have for example been utilized as emissive substrates in
OLEDs.4a,b
In accord with the above considerations, we were therefore
interested in the emissive properties of the sterically encumbered
TIPS-functionalized oligophenyleneethynylenes 1-5 and the
unhindered 16, and observed that particular members of the
series were indeed strongly luminescent when exposed to UV
light both in solution and the solid state. The spectral emission
data are summarized in Table 2, the luminescence spectra of
1-5 and 16 recorded in heptane solution shown in Figure 7,
and spectra of the same compounds recorded as solvent-
evaporated thin films are shown in Figure 8.
FIGURE 8. Normalized luminescence emission spectra of 1-5, 10,
and 16 recorded as thin films.
corresponding to the absorption maxima of the respective
UV-vis spectra (see the Supporting Information), revealing that
reabsorption effects were not dominant in these systems.
The luminescence emission data (Table 2) reveal a general
trend, i.e., that the highest intensity emission maxima of all
compounds decrease in energy along the series 10 . 3 > 1 > 4
> 2 . 16 > 5 in solution and 10 . 3 > 1 > 2 = 4 . 16 = 5
in the solid state. Thus, within the series of linear oligomers
both in solution and the solid state, the luminescence energy
decreases and becomes more red-shifted with increase in
conjugation length, and the macrocycles are emissive at much
lower energies than the longest linear oligomers. This finding
is in accord with the expected decrease in the gap between the
excited state and ground state energy levels with increasing
conjugation. As in the absorption spectra, the TMS groups of
the linear oligomers and TIPS substituents of the macrocycles
are exerting an electronic effect on the spectroscopic properties
of these oligophenyleneethynylenes. Thus, removal of the
terminal TMS groups results in a small increase in energy of
the emission maxima upon passing from 1 to 3. A similar effect
is also observed in the solution luminescence spectra of the
macrocycles where the emission energy of 16, which lacks the
TIPS substituents, is greater than that of 5. This again reflects
the electron donating nature of the trialkylsilyl groups which
would be expected to raise the energy of the ground state
HOMO and reduce the energy gap.
The solution luminescence spectra for all compounds were
recorded over a range of concentrations from 5 × 10-4 to 9 ×
10-7 mol/L and exhibited either negligible or zero changes in
the shape and position of the emission maxima, showing that
excited state aggregation was minimal in dilute solution. The
emission spectra of each compound were also relatively
unchanged upon excitation at high and low energy wavelengths
(29) The presence of this low energy tailing phenomenon resulted in a poorly
defined absorption edge in the thin film UV-vis spectra of 5 and 16, thereby
opt
introducing ambiguity in the measurement of Eg for these compounds. The
possibility exists that these comparatively weak absorptions may be the fingerprint
of unidentified trace contaminants. However, cumulative evidence suggests
otherwise. See for example the first report on the synthesis of dehydrotribenzo[12]-
annulene, in which the UV-vis spectrum also clearly shows the presence of
some weak absorptions on the lower energy edge of the principal maximum.23c
4684 J. Org. Chem. Vol. 74, No. 13, 2009

Synthesis and Properties of Oligo[n]cruciforms
FIGURE 9. Comparison of the UV-vis spectrum (left) and normalized
luminescence emission spectrum of 10 in heptane solution.
FIGURE 10. Comparison of the UV-vis spectrum (left) and normal-
ized luminescence emission spectrum of oligo[5]cruciform 4 in heptane
solution.
The solid state emissions for 1-5 and 16 are all of lower
energy than the emissions in solution, with the energy difference
between the λem solid and λem soln increasing in the following order,
10 < 1 < 4, i.e., as the molecular size increases within the series
of structurally most closely related linear oligomers. The values
of the λem solid - λem soln of the remaining oligomers 2, 3, 5, and
16 is intermediate between that of 10 and 4, averaging at 41
nm and therefore the same as that for 1. Despite the steric
isolation, an increased degree of exciton hopping is occurring
in the solid state with increased radiationless energy loss prior
to re-emission and a concomitant reduction in the energy of
luminescence. This process would be expected to be augmented
upon increasing the intermolecular contact surface area and
yield lower than those of the linear oligomers as expected, due
to its comparatively small size and reduced degree of conjuga-
tion. However, the longest oligomers 2 and 4 exhibited lower
quantum yields than the shorter oligo[3]cruciforms 1 and 3. This
effect has been previously noted for phenyleneethynylene
oligomers.30
The butadiyne-bridged oligomer 2 was found to possess a
lower quantum yield than 4, suggesting that increasing the
interphenyl ring separation and reducing the steric encumbrance
of the oligomer may be detrimental to the strength of its
luminescence. The possible existence of oxygen-quenching
effects upon the luminescence of the oligomers was investigated
in the case of the longest oligo[n]cruciform 4. Thus, directly
after measurement of the quantum yield of air-equilibrated
solutions of 4, the solutions were degassed with argon and the
quantum yield redetermined. Although small compared to the
inherent experimental errors, a slight increase in quantum yield
from 0.76 to 0.78 was observed after argon degassing suggesting
that some luminescence quenching by oxygen may occur in this
system.
molecular size. The larger λem
- λem
value for 16
solid
soln
compared to 5 is also consistent with increased intermolecular
contacts within the latter in the solid state due to the absence
the TIPS substituents.
This latter conclusion is also supported by the finding that
the thin film emission spectrum of 16 was of poorer resolution
than that of 1, suggesting that an increased amount of
intermolecular interactions exists in the solid state of the former
macrocycle.
As in the solution UV-vis spectra, the solution emission
spectra of 1-5 and 16 all comprise multiple numbers of bands
due to vibronic coupling, which is particularly well resolved in
the case of cycles 5 and 16, a finding that is consistent with
their enhanced rigidity.28
Somewhat surprisingly in view of its comparative rigidity
and relatively large conjugation surface area, cycle 5 afforded
the lowest luminescence quantum yield of all compounds
studied, suggesting that dehydrotribenzo[12]annulenes may be
inferior modules for the construction of strong fluorophores
compared to the noncyclic analogues.
Thermochemical Properties. Polyacetylenic compounds are
high energy materials with characteristic thermochemical prop-
erties that have been the subject of considerable interest over
the past forty years. In particular, their thermally induced
polymerizations, rearrangements and reactions have been widely
exploited as an efficient and direct method of access to high
The unsymmetric relationship between the shape of the
solution absorbance and emission spectra in 1-5 and 16
introduces some ambiguity into the precise determination of the
Stokes shifts of these compounds (see, for example, the spectra
of 10 and 4 in Figures 9 and 10). A clarification of the absorption
and emission bands that involve related transitions will require
theoretical calculation of the spectra. However, comparison of
the spectra does lead to a general conclusion, i.e., that the
absorption and emission bands of the macrocycles 5 and 16
exhibit significantly greater separations than the linear oligomers,
and have considerably broadened emissions. The relatively large
apparent Stokes shifts of 5 and 16 suggest that dehydrotribenzo-
[12]annulenes may have potential future applications as fre-
quency downconversion materials.
(30) Sudeep, P. K.; James, P. V.; Thomas, K. G.; Kamat, P. V. J. Phys.
Chem. A, 2006, 110, 5642–5649.
(31) For topotactic polymerizations, see: (a) Tachibana, H.; Yamanaka, Y.;
Sakai, H.; Abe, M.; Matsumoto, M. Macromolecules 1999, 32, 8306–8309, and
references therein. Carbonization polymerizations: (b) Zhou, N.; Merschrod, E. F.;
Zhao, Y. J. Am. Chem. Soc. 2005, 127, 14154–14155. (c) Kijima, M.; Tanimoto,
H.; Shirakawa, H. Synth. Met. 2001, 119, 353–354. For explosive decompositions,
see, for example: (d) de Meijere, A.; Kozhushkov, S.; Haumann, T.; Boese, R.;
Puls, C.; Cooney, M. J.; Scott, L. T. Chem.sEur. J. 1995, 1, 124–131.
(32) (a) Laskoski, M.; Steffen, W.; Morton, J. G. M.; Smith, M. D.; Bunz,
U. H. F. J. Am. Chem. Soc. 2002, 124, 13814–13818. (b) Glezakou, V.-A.; Boatz,
J. A.; Gordon, M. S. J. Am. Chem. Soc. 2002, 124, 6144–6152. (c) Dosa, P. I.;
Erben, C.; Iyer, V. S.; Vollhardt, K. P. C.; Wasser, I. M. J. Am. Chem. Soc.
1999, 121, 10430–10431. (d) Boese, R.; Matzger, A. J.; Vollhardt, K. P. C.
J. Am. Chem. Soc. 1997, 119, 2052–2053.
Luminescence quantum yield estimations (Table 2) confirmed
that the linear oligomers were strong fluorophores, and in
particular the oligo[3]cruciforms 1 and 3, which exhibited very
high Φ values. Monomer 10 afforded an estimated quantum
J. Org. Chem. Vol. 74, No. 13, 2009 4685

Al-Ouahabi et al.
TABLE 3. DSC Data for 1, 3, 5, 11, and 16a
b
c
compd
Tonset (°C)
Tmax (°C)
∆T1/2 (°C)
∆H (kJ mol-1
)
1
48
271
278
53
52
276
307
60
3
4
26
6
21d
65
-186
2
6d
3
5
11
247
283
169
174
157
258
304
174
181
176
13
21
3
9
27
-295
-316
29
-171
-312
16
a Data obtained under an N2 atmosphere (heating rate 2 °C/minute).
b Maximum temperature of exo/endothermic peak. c Half-width of the
exo/endothermic peak at Tmax.
d Reversible when heating terminated
before passing through higher temperature transitions.
FIGURE 12. DSC heating trace of 1 (solid line), and reheating trace
(dotted line) after passing through the 307 °C exotherm (2 °C/min
heating rate).
FIGURE 11. TGA curves for 1, 4, and 5 (10 °C/min heating rate).
carbon content polymers which would otherwise be difficult to
prepare using conventional organic synthetic techniques. De-
pending upon heating rate and spatial orientations of the
molecules within the crystal, a range of possible outcomes may
be available to such materials upon heating, which include
topotactic and amorphous carbonization polymerizations, or even
explosive decomposition.31a-d A significant recent development
concerning the latter behavior has been the discovery of carbon
nanotubes in the residues remaining from the explosive decom-
position of particular dehydroannulenes and metal-complexed
phenyleneethynylenes.32a-d Phenyleneethynylenes become par-
ticularly unstable when the ethyne substituents are unfunction-
alized. For example, in the case of cruciform-type molecules,
1,2,4,5-tetraethynylbenzene, the completely desilylated analogue
of 10, has been reported to explode upon heating.33
However, thermally unstable polyphenyleneethynylenes also
often possess intrinsically interesting physicochemical properties
in their own right, thus offering potential as new types of
functional materials once their stability toward heat can be
enhanced. In the light of these latter considerations, we
performed differential scanning calorimetric (DSC) and ther-
mogravimetric (TGA) measurements on selected oligo[n]cru-
ciforms and dehydrotribenzo[12]annulenes in order to investigate
their stability toward heat (Table 3 and Figures 11-13).
The DSC measurements showed that the larger TIPS-
protected oligomers 1, 3, and 5 exhibited the greatest stability
to heat, but with the macrocycles 5 and 16 involving exothermic
energy changes of the greatest magnitude. After heating past
the exotherm followed by cooling to ambient temperature, the
FIGURE 13. DSC heating curve of 1 showing reversible low tem-
perature endothermic (Tmax ) 52 °C) and exothermic (Tmax ) 49 °C)
transitions (2 °C/min heating rate).
original enthalpy changes were absent upon reheating in all
compounds studied, demonstrating that the exothermic phase
in each case involved an irreversible chemical reaction. The
TGA measurements on oligomers 1, 4, and 5 (Figure 11)
revealed that the onset of mass loss upon heating occurred at
340 °C, showing that at least in the cases of 1, 4, and 5, the
DSC exotherms and melting profiles were the signature of
intramolecular chemical reactions or polymerizations rather than
decompositions.
The DSC measurements of 3, 5, and 16 displayed a single
exothermic peak. By contrast, the DSC of monomer 11 exhibited
a sharp endothermic transition at 174 °C followed directly by
a broader exothermic phase at 181 °C. This latter behavior is
indicative of a melting transition which then develops into an
irreversible chemical reaction. The DSC trace of 1 (Figure 12)
is particularly interesting in that it displays two sharp endo-
thermic transitions at 52 and 276 °C, respectively. The latter
endotherm is directly followed by a broad exotherm at 307 °C
in parallel to the behavior of 11, signaling the onset of an
irreversible chemical reaction. The endotherm at 52 °C is,
however, reversible, as heating just past the endothermic
transition followed by slow recooling to ambient temperature
results in the development of a “mirror image” exothermic
transition of similar enthalpy change but at a slightly lower
temperature (Figure 13). Similarly, the DSC trace of 2 exhibited
a weak reversible endothermic transition at 60 °C (Table 3). In
(33) Berris, B. C.; Hovakeemian, G. H.; Lai, Y.-H.; Mestdagh, H.; Vollhardt,
K. P. C. J. Am. Chem. Soc. 1985, 107, 5670–5687.
4686 J. Org. Chem. Vol. 74, No. 13, 2009

Synthesis and Properties of Oligo[n]cruciforms
the case of 2, there were no further clearly definable transitions
in the DSC trace up to 350 °C, in accordance with its visual
melting behavior, which showed no changes other than gradual
color-darkening over a wide temperature range (281-350 °C).
The crystals of 1 and 2 exhibited no visible changes upon
heating and cooling over the reversible low temperature DSC
transitions, suggesting that they may originate from unusual
crystal-to-crystal phase changes involving structural reorganiza-
tion within the crystal networks.
As stated above, the DSC and TGA measurements support
the conclusion that the apparent melting of oligomers 1 and
3-5 and monomer 11 (see the Experimental Section) is actually
assignable to the onset of irreversible chemical reactions rather
than simple melting behavior. A more detailed examination of
the products from these high temperature transformations would
be an avenue of future interest to determine if this is a route to
new types of conjugated oligomers and polymers generated in
the solid state. It may be noted that the visual and DSC transition
temperatures appear to be sensitive to the heating rate, and
increase in temperature as the heating rate is increased. This
dependence accounts in part for the fact that the visual transitions
occur at lower temperatures than the DSC transitions, the former
of which being conducted at slower heating rates.
The high temperature thermal behavior of 1, 4, and 5 as
measured by TGA, showed that continued heating from 340
°C resulted in mass reduction, signifying the onset of decom-
position reactions. In the cases studied, the greatest loss in mass
occurred between 340 and 570 °C. After 570 °C, the principal
decomposition reaction had finished, with negligible mass
changes up to 900 °C. The initial decomposition of 5 from 340
to 430 °C was slower than that of 1 and 4 and involved a 33%
mass loss from 340 to 570 °C, whereas the oligo[n]cruciforms
exhibited a 42% loss in mass over the same temperature range.
Thus, the thermochemical measurements revealed that the
oligomer stability increases with increased TIPS and TMS
functionalization. The stabilizing effects of the TIPS substituents
suggests that this is a viable approach to the stabilization of
otherwise thermally sensitive phenyleneethynylenes with po-
tential materials applications.
complete absence of intermolecular contacts between the ethyne
and phenyl subunits in the solid state.
Oligophenyleneethynylenes 1-5 and 16 all functioned as
luminescentchromophores,andinparticulartheoligo[3]cruciforms
1 and 3 which exhibited particularly high quantum yields.
UV-vis and luminescence spectroscopic dilution studies re-
vealed that intermolecular associations between the conjugated
parts of all oligomers were absent in dilute solution. However,
thin film luminescence studies showed a significant red shifting
in the emissions of all oligomers compared to those in solution,
consistent with increased energy loss via intermolecular exciton
hopping across the contacting TIPS substituents. The apparent
Stokes shifts of the cycles 5 and 16 were surprisingly large
compared to the longest oligo[n]cruciforms. Organic materials
with large Stokes Shifts offer considerable potential as energy
downconversion materials and luminescent concentrators in solar
energy technology and as fluorescence labels for biomolecules.35a,b
The unusually detailed vibronic coupling observed in the
UV-vis and room temperature emission spectra of some of the
oligophenyleneethynylenes and particularly in the UV-vis
spectra of 10 and 11, suggests that the spectroscopic properties
of these compounds may be of theoretical interest.
Preliminary electrochemical studies showed that 1 and 5 were
reversibly reducible, suggesting that they may possess future
potential as electron reservoir materials upon further structural
elaboration. The TIPS-protected oligomers also exhibited en-
hanced thermochemical and photochemical stability for poly-
ethynylated-phenylenes, properties of particular importance for
successful applications in molecular electronics.
Finally, with respect to the potential nanotechnological
applications concerning controlled motions on surfaces,11u it may
be of interest to note that oligo[n]cruciforms 1-4 are geared
molecular systems. Rotation of a terminal phenyl ring about
the linear axis of the oligo[n]cruciform will bring its ethynyl-
TIPS substituents in contact with those of the adjacent ring,
which are forced to rotate in the same direction. Propagation
of this motion down the axis of the oligomer would result in
unidirectional motion of all the ethynyl-TIPS substituents,
thereby resulting in a geared molecular motion.
The spectrum of physicochemical properties exhibited by 1-5
and 16 suggests that further synthetic modifications via selective
deprotection and Sonogashira-mediated functionalization will
yield a rich diversity of structurally new polyconjugated
oligo[n]cruciforms and dehydrotribenzo[12]annulene-based struc-
tures with potentially novel functions and molecular electronic/
materials science applications. Work along these lines is
currently underway.
Conclusion
A series of TIPS-substituted sterically encumbered oligo[n]-
cruciform phenyleneethynylenes 1-4 constructed from tetra-
ethynylbenzene units have been synthesized34 and their physical
properties compared to those of similarly functionalized cyclic
analogue 5 and its desilylation product 16. All oligophenyle-
neethynylenes were characterized by NMR, ESI, and/or MALDI-
TOF mass spectroscopic measurements. The characterization
of the oligomers as [M + Cu+] ions was found to be the cleanest
and most effective method of mass spectral analysis of these
systems. This technique may therefore be of general utility for
the mass spectral characterization of nonpolar phenyleneethy-
nylenes in cases when other approaches are unsuccessful. The
steric isolation of the oligo[n]cruciforms was unambiguously
demonstrated by the crystal structure of 1 which showed a
Experimental Section
See the Supporting Information for general experimental meth-
ods, synthetic procedures and characterization data for 7-9, 11,
14, and 15 and X-ray experimental data for 1.
Electrochemistry. Electrochemical measurements were carried
out in CH2Cl2 (with gentle degassing with argon) containing 0.1
mol/L of Bu4NPF6 in a classical three-electrode cell by cyclic
voltammetry (CV). The working electrode was a glassy carbon disk
(3 mm in diameter), the auxiliary electrode a Pt wire, and the
pseudoreference electrode a Pt wire. The cell was connected to an
Autolab PGSTAT20 potentiostat (Eco Chemie, Holland) driven by
a GPSE software running on a personal computer. All potentials
are given vs Fc+/Fc used as internal reference and are uncorrected
from ohmic drop. E° ) (Epa + Epc)/2 where Epa and Epc are the
respective anodic and cathodic peak potentials.
(34) It may be noted that 1-4 do not constitute an exact homologous series
due to the fact that 2 uniquely possesses a butadiene bridge. The “n” in
oligo[n]cruciform is used in this work as a descriptor solely for the number of
tetraethynylbenzene units within each oligomer, independent of the structural
composition of the bridging units.
(35) (a) Peng, X.; Song, F.; Lu, E.; Wang, Y.; Zhou, W.; Fan, J.; Gao, Y.
J. Am. Chem. Soc. 2005, 127, 4170–4171. (b) van Roosmalen, J. A. M.
Semiconductors 2004, 38, 970–975.
J. Org. Chem. Vol. 74, No. 13, 2009 4687

Al-Ouahabi et al.
Luminescence quantum yield estimations. The luminescence
inner 2′,5′-Si(CH(CH3)2)3}, 0.252 ppm (s, 18H; Si(CH3)3). 13C NMR
(CDCl3, 100.61 MHz, 25 °C): δ ) 136.8, 136.3, 136.1, 125.4,
125.29, 125.26, 125.23, 125.14, 125.1, 104.0 (-Ct ), 103.97
(-Ct ), 103.9 (-Ct ), 102.3 (-Ct ), 100.6 (-Ct ), 97.91 (-Ct ),
97.88 (-Ct ), 97.4 (-Ct ), 92.6 (-Ct ), 18.8 (CH(CH3)2), 18.7
(CH(CH3)2), 11.30 (CH(CH3)2), 11.26 (CH(CH3)2), -0.1 ppm
(Si(CH3)3). UV/vis (heptane): λmax (ε) ) 242sh (42925), 260sh
(97590), 276 (188988), 286 (238042), 320 (50598), 350 (57708),
358 (59268), 374 nm (56983 M-1 cm-1). IR (thin film): 2942 (m),
2890 (w), 2864 (s), 2157 (w) (Ct C), 1490 (m), 1462 (m), 1250
(m), 1188 (m), 996 (m), 906 (m), 881 (vs), 840 (s), 768 (vs), 679
(s), 657 cm-1 (s). High res ESI MS m/z: 1551.9825 (5) [M+],
1615.9138 (10) [M + Cu+]. MALDI TOF MS m/z: 1615.82 (100)
[M + Cu+] calcd for C98H150Si8 1551.9910, C98H150CuSi8 1615.92.
Oligo[4]cruciform (2). Following the general procedure for
Sonogashira couplings, a mixture of 11 (0.360 g, 7.39 × 10-4 mol),
9 (1.100 g, 1.66 × 10-3 mol), and [PdCl2(dppf)] ·CH2Cl2 (0.068 g,
8.33 × 10-5 mol) in toluene (25 mL) and CuI (0.068 g, 3.57 ×
10-4 mol) in Et3N (8 mL) under an atmosphere of argon/air
(approximately 80: 20) was stirred for 19 d in the dark. All solvent
was then removed under reduced pressure on a water bath, the
residue extracted with boiling hexane (3 × 50 mL), and the com-
bined hexane extracts left to cool to ambient temperature. The
mixture was gravity filtered in order to separate the suspended
yellow solid which formed upon cooling and the filtrate reduced
in volume to 20 mL under reduced pressure on a water bath. The
filtrate was then purified by chromatography and EtOH treatment
as described above to yield 1 (0.598 g, 52%). The yellow solid
was boiled in hexane (60 mL) and the hot mixture gravity filtered.
The filtrate was concentrated by continued boiling to 20 mL and
allowed to cool to ambient temperature. The solid which formed
was isolated by filtration under vacuum, washed with hexane (2 ×
1 mL), and then recrystallized three times from hexane to afford 2
(0.076 g, 10%) as a bright yellow solid after air drying. (mp:
microcrystals darken to an amber-brown color from 281.0-287.0
°C but remain transparent. Color-darkening continues up to the
heating limit of 350 °C with no observable melting transitions).
1H NMR (CDCl3, 400.13 MHz, 25 °C): δ ) 7.609 (s, 2H; phH),
7.583 (s, 4H; phH), 7.568 (s, 2H; phH), 1.166 (s, 42H;
Si(CH(CH3)2)3), 1.153 (s, 42H; Si(CH(CH3)2)3), 1.112 (s)/1.110 (s);
{84H; Si(CH(CH3)2)3}, 0.251 ppm (s, 18H; Si(CH3)3). 13C NMR
(CDCl3, 100.61 MHz, 25 °C): δ ) 136.9, 136.7, 136.2, 135.8, 126.3,
125.7, 125.46, 125.44, 125.2, 124.8, 124.5, 103.96 (-Ct ), 103.93
(-Ct ), 103.8 (-Ct ), 103.3 (-Ct ), 102.3 (-Ct ), 100.7 (-Ct ),
98.7 (-Ct ), 98.3 (-Ct ), 97.9 (-Ct ), 97.4 (-Ct ), 93.4 (-Ct ),
92.5 (-Ct ), 81.3 (-Ct ), 79.7 (-Ct ), 18.8 (CH(CH3)2), 18.69
(CH(CH3)2), 18.66 (CH(CH3)2), 11.32 (CH(CH3)2), 11.30
(CH(CH3)2), 11.28 (CH(CH3)2), 11.2 (CH(CH3)2), -0.1 ppm
(Si(CH3)3). UV/vis (heptane): λmax (ε) ) 274 (176727), 289
(263185), 300sh (149680), 359 (63832), 387 (89395), 408 nm
(65036 M-1 cm-1). IR (thin film): 2942 (m), 2890 (w), 2864 (m),
2158 (w) (Ct C), 1485 (m), 1462 (m), 1397 (m), 1250 (m), 1187
(m), 996 (m), 904 (m), 874 (vs), 853 (s), 842 (s), 769 (s), 735 (m),
677 (s), 660 cm-1 (vs). MALDI TOF MS m/z: 2101.19 (100) [M
+ Cu+], calcd for C130H194CuSi10 2101.22.
quantum yields36 were calculated using the equation
ΦX ) ΦST(AST/AX)(FX/FST)(ηX/ηST)2
which can be rearranged to
ΦX ) ΦST(FX/AX)/(FST/AST)(ηX/ηST)2
where Φ is the luminescence quantum yield, A is the absorbance
at the excitation wavelength, F is the area under the instrumentally
corrected emission curve, η is the refractive index of the solvent,
and subscripts ST and X refer to the standard and unknown,
respectively. The function (FX/AX) is therefore the gradient of the
plot of FX verses AX, which was determined from a series of dilute
air-equilibrated heptane solutions with absorbances between 0.005
and 0.07 for each oligomer under investigation. Similarly, (FST/
AST) was determined from the plot of FST verses AST, using
anthracene in argon-degassed ethanol (Φ ) 0.27)37 as the lumi-
nescence standard for compounds 1-4 and 10. Quinine hemisulfate
in 0.5 M aqueous H2SO4 (Φ ) 0.546 at 25 °C)37 was used as the
luminescence standard for the calculation of the quantum yield of
5. We also used the quantum yield reported by Melhuish for
deoxygenated ethanolic anthracene to calculate the quantum yield
for air equilibrated ethanolic anthracene at ambient temperature,
and found it to be in exact agreement with that determined by
Venturi et al., i.e., Φ ) 0.21.38 The experimental error associated
with all determined quantum yield measurements (Table 2) was
estimated to be within ( 10%.
Compound Syntheses. General Procedure for Sonogashira
Heterocouplings. To the respective aryl iodide in the presence of
a catalytic amount of PdCl2(PPh3)2 or [PdCl2(dppf)]·CH2Cl2 under
an atmosphere of argon was added by syringe Et3N or toluene as
solvent, followed by the appropriate alkyne reactant. After the
mixture was stirred for 0.25 h, a solution of CuI in Et3N, prepared
in a separate argon-filled Schlenk, was syringed into the reaction,
and stirring continued for 7-19 days at ambient temperature in
the absence of light. A gray, brown, or yellow precipitate slowly
formed in all cases, visually indicating the progress of the reaction.
All solvent was then removed under reduced pressure on a water
bath at 70 °C, and the residue isolated as described separately for
each product.
Oligo[3]cruciform (1). Following the general procedure for
Sonogashira couplings, a mixture of 11 (0.161 g, 3.31 × 10-4 mol),
9 (0.504 g, 7.63 × 10-4 mol), and [PdCl2(dppf)] ·CH2Cl2 (0.030 g,
3.67 × 10-5 mol) in toluene (12 mL) and CuI (0.020 g, 1.05 ×
10-4 mol) in Et3N (2 mL) was stirred at ambient temperature for
14 d in the dark. All solvent was then removed under reduced
pressure on a water bath and the residue extracted with boiling
hexane (5 × 40 mL). The combined hexane extracts were
chromatographed on a column of silica, eluting with hexane. The
product thus obtained was suspended in absolute EtOH (250 mL)
and the volume reduced to 90 mL with boiling and stirring. The
suspension was then allowed to stand overnight, and the solid
isolated by filtration under vacuum, washed with EtOH (3 × 2 mL),
and air-dried to afford 1 (0.331 g, 64%) as an amorphous cream
Oligo[3]cruciform (3). To a solution of 1 (0.598 g, 3.85 × 10-4
mol) in THF (36 mL) and MeOH (9 mL) was added powdered
K2CO3 (0.159 g, 1.15 × 10-3 mol) and the mixture stirred at
ambient temperature in the dark for 7 d. All solvent was then
removed under reduced pressure at ambient temperature to yield a
white solid which was extracted with boiling hexane (4 × 30 mL).
The combined hexane extracts were then added to a column of
silica, which was first washed with some hexane and then eluted
with 2% Et2O/hexane. The product thus obtained was then
recrystallized from boiling hexane (40 mL) to afford 3 (0.511 g,
94%) as a microcrystalline white powder after air drying. (mp:
crystals transform into a partly melted amber-colored glass at 227.0
- 228.0 °C, with no further change other than color-darkening up
to the heating limit of 350 °C). 1H NMR (CDCl3, 400.13 MHz, 25
1
solid (mp ) 252.0-255.0 °C). H NMR (CDCl3, 400.13 MHz,
25 °C): δ ) 7.620 (s, 2H; outer phH3), 7.589 (s, 2H)/7.583
(s, 2H); {outer phH6/inner phH3′,6′}, 1.151 (s, 42H; outer
5-Si(CH(CH3)2)3), 1.098 (s)/1.093 (s); {84H; outer 2-Si(CH(CH3)2)3/
(36) Fery-Forgues, S.; Lavabre, D. J. Chem. Educ. 1999, 76, 1260–1264.
(37) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229–235.
(38) Venturi, M.; Marchioni, F.; Balzani, V.; Opris, D. M.; Henze, O.;
Schlu¨ter, A. D. Eur. J. Org. Chem. 2003, 4227–4233. The quantum yield of
anthracene in air-equilibrated ethanol (Φ ) 0.21) was determined by the group
of Venturi, who found that their value agrees with the quantum yield (Φ )
0.27) of anthracene in de-aerated ethanol reported by Melhuish, as well as the
rate constant for the oxygen quenching published by the latter author. We thank
Professor Margherita Venturi for this information.
4688 J. Org. Chem. Vol. 74, No. 13, 2009

Synthesis and Properties of Oligo[n]cruciforms
°C): δ ) 7.626 (s, 2H; phH), 7.617 (s, 2H; phH), 7.608 (s, 2H;
phH), 3.343 (s, 2H; -Ct CH), 1.145 (s, 42H; Si(CH(CH3)2)3), 1.106
ppm (s, 84H; Si(CH(CH3)2)3). 13C NMR (CDCl3, 100.61 MHz, 25
°C): δ ) 136.4, 136.1, 135.8, 125.9, 125.5, 125.3, 125.2, 124.7,
103.9 (-Ct ), 103.88 (-Ct ), 103.6 (-Ct ), 98.2 (-Ct ), 97.9
(-Ct ), 97.87 (-Ct ), 92.8 (-Ct ), 92.6 (-Ct ), 82.9 (-Ct ),
81.3 (-Ct ), 18.7 (CH(CH3)2), 11.28 (CH(CH3)2), 11.25 ppm
(CH(CH3)2). UV/vis (heptane): λmax (ε) ) 256sh (82714), 269sh
(123070), 274 (132814), 285 (226223), 347 (49597), 356 (51683),
370 nm (51604 M-1cm-1). IR (thin film): 3307 (w) (t C-H), 2941
(m), 2889 (w), 2863 (m), 2159 (w) (Ct C), 1489 (m), 1461 (m),
1402 (m), 1187 (m), 997 (m), 902 (s), 880 (s), 868 (vs), 769 (s),
730 (s), 674 (vs), 662 (vs), 650 cm-1 (vs). High res ESI MS m/z:
1471.8226 (100) [M + Cu+]. MALDI TOF MS m/z: 1471.83 (70)
[M + Cu+], calcd for C92H134CuSi6 1471.84.
columns of silica, eluting with heptane, to afford 5 (0.028 g, 20%)
as a bright yellow solid after air-drying. In some runs, further
purification was necessary, either by repeated chromatography as
above, or by brief ultrasonication in n-propanol, isolation by
filtration under vacuum, washing with n-propanol (3 × 2 mL), and
air drying. (mp ) 270.5-274.5 °C transforming to an amber glass
which slowly darkened in color up to the heating limit of 310 °C).
1H NMR (CD2Cl2, 400.13 MHz, 22 °C): δ ) 7.451 (t, 3H; H3),
7.298 (d, 6H; H5, H6), 1.137 ppm (m, 63H; Si(CH(CH3)2)3). 13C
NMR (CD2Cl2, 100.61 MHz, 22 °C): δ ) 135.9, 132.63, 132.59,
127.1, 126.5, 124.7, 105.9 (-Ct ), 94.7 (-Ct ), 94.1 (-Ct ), 93.4
(-Ct ), 19.0 (CH(CH3)2), 11.8 ppm (CH(CH3)2). UV/vis (heptane):
λmax (ε) ) 227 (33526), 254 (27002), 267 (18988), 280 (34750),
288 (57485), 298 (190345), 307 (182985), 318 (636242), 367 nm
(15168 M-1cm-1). IR (thin film): 2941 (m), 2890 (w), 2864 (m),
2142 (w) (Ct C), 1490 (m), 1462 (m), 995 (m), 940 (m), 887 (s),
881 (s), 823 (s), 793 (s), 705 (vs), 676 (vs), 656 cm-1 (vs). High
res ESI MS m/z: 903.4162 (100) [M + Cu+], 1745.9004 (9) [2M
+ Cu+], calcd for C57H72CuSi3 903.4232.
4,4′,4′′-Triethynyldehydrotribenzo[12]annulene (16). To a
solution of 5 (0.127 g, 1.51 × 10-4 mol) in THF (20 mL) was
added n-Bu4NF (1.0 M solution in THF; 0.60 mL, 6.00 × 10-4
mol) and the solution stirred at ambient temperature in the dark
for 48 h. All solvent was then removed under reduced pressure at
ambient temperature, MeOH (15 mL) added to the residue, and
the suspension briefly ultrasonicated and filtered under vacuum.
The isolated solid was washed with MeOH (4 × 5 mL), air-dried,
dissolved in warm CH2Cl2 (100 mL), and chromatographed on a
column of silica, eluting with CH2Cl2. The product obtained from
the chromatography was finally suspended in CH2Cl2 (5 mL), briefly
ultrasonicated, and filtered under vacuum and the isolated solid
washed with CH2Cl2 (3 × 0.5 mL) and air-dried to yield 16 (0.053
g, 94%) as a fibrous yellow solid. (mp: color-darkening was
observed from 160.0 to 169.0 °C, but no melting phase up to the
heating limit of 200 °C). 1H NMR (CD2Cl2, 500.13 MHz, 25 °C):
δ ) 7.476 (m, 3H; H3), 7.328 (m, 6H; H5, H6), 3.268 ppm (s, 3H;
-Ct CH). 13C NMR (CD2Cl2, 100.61 MHz, 24 °C): δ ) 136.0,
132.9, 132.7, 127.13, 127.08, 123.5, 94.1 (-Ct ), 93.3 (-Ct ),
82.5 (-Ct ), 80.3 ppm (-Ct ). UV/vis (heptane): λmax (ε) ) 219
(50738), 248 (30009), 273 (48527), 280 (63223), 289 (161041),
297 (134079), 308 (435119), 347 (6108), 362 nm (8398 M-1cm-1).
IR (thin film): 3290 (m) (t C-H), 3258 (s) (t C-H), 1488 (s),
902 (s), 890 (s), 836 (m), 826 (vs), 664 (vs), 635 (s), 610 (vs), 604
cm-1 (vs). High res ESI MS m/z: 372.0897 (100) [M+], 435.0196
(30) [M + Cu+], calcd for C30H12Cu 435.0230.
Oligo[5]cruciform (4). Following the general procedure for
Sonogashira couplings, a mixture of 3 (0.050 g, 3.55 × 10-5 mol),
9 (0.063 g, 9.53 × 10-5 mol), and [PdCl2(dppf)] ·CH2Cl2 (0.009 g,
1.10 × 10-5 mol) in toluene (9 mL) and CuI (0.012 g, 6.30 × 10-5
mol) in Et3N (1.5 mL) was stirred at ambient temperature for 4 d
in the dark and then at 60 °C for 5 d. All solvent was then removed
under reduced pressure on a water bath, the residue extracted with
boiling heptane (3 × 10 mL), and the combined heptane extracts
chromatographed on a column of Silica eluting with heptane. When
the product was sufficiently separated from the more slowly moving
contaminants (as visualized with a fluorescent lamp operating at
312 nm), the latter were dug out from the column, and the crude 4
isolated by elution with 2% Et2O/heptane. The product was then
rechromatographed on Silica with 5%CH2Cl2/hexane and after
isolation from the eluate, boiled in absolute EtOH (10 mL). The
mixture was allowed to cool to ambient temperature and the
suspended solid isolated by filtration under vacuum, washed with
absolute EtOH (3 × 1 mL), and then treated in the same way with
n-propanol to afford 4 (0.015 g, 17%) as a bright yellow solid after
air drying. (mp: slow transformation into an amber-colored glass
from 235.1 to 243.5 °C, which color-darkened up to the heating
limit of 330 °C). 1H NMR (CDCl3, 400.13 MHz, 25 °C): δ ) 7.640
(s, 2H; phH), 7.635 (s, 2H; phH), 7.631 (s, 2H; phH), 7.594 (s,
2H; phH), 7.586 (s, 2H; phH), 1.153 (s, 42H; Si(CH(CH3)2)3), 1.105
(s, 126H; Si(CH(CH3)2)3), 1.098 (s, 42H; Si(CH(CH3)2)3), 0.253
ppm (s, 18H; Si(CH3)3). 13C NMR (CDCl3, 100.61 MHz, 25 °C):
δ ) 136.8, 136.3, 136.1, 125.4, 125.3, 125.28, 125.26, 125.23,
125.12, 125.05, 103.98 (-Ct ), 103.96 (-Ct ), 103.91 (-Ct ),
102.3 (-Ct ), 100.6 (-Ct ), 97.9 (-Ct ), 97.3 (-Ct ), 92.76
(-Ct ), 92.74 (-Ct ), 92.65 (-Ct ), 92.63 (-Ct ), 18.8
(CH(CH3)2), 18.7 (CH(CH3)2), 11.28 (CH(CH3)2), 11.26
(CH(CH3)2), -0.1 ppm (Si(CH3)3). UV/vis (heptane): λmax (ε) )
276 (219907), 287 (313112), 377 nm (94174 M-1cm-1). IR (thin
film): 2942 (m), 2890 (w), 2864 (m), 2161 (w) (Ct C), 1493 (m),
1461 (m), 1401 (m), 1250 (m), 1187 (m), 997 (m), 903 (m), 881
(vs), 858 (s), 841 (s), 768 (vs), 677 (s), 659 cm-1 (vs). MALDI
TOF MS m/z: 2537.52 (100) [M + Cu+], calcd for C158H238CuSi12
2537.52.
Acknowledgment. The Centre National de la Recherche
Scientifique and the Institut Charles Sadron are acknowledged
forfinancialsupport(P.N.W.B.)andtheMiniste`redel′Enseignement
Supe´rieur et de la Recherche is acknowledged for a PhD
fellowship (A.A.). Raymonde Baltenweck-Guyot and Romain
Carriere of the Service de Spectrometrie de Masse are thanked
for the MALDI and ESI mass spectrometric measurements. Y.
Guilbert of the ICS is thanked for the TGA measurements,
Professor Mir Wais Hosseini is thanked for the use of the DSC,
and Dr. Sylvie Ferlay is thanked for the DSC measurements.
Dr. Marie-The´re`se Youinou is thanked for constructive comments.
4,4′,4′′-Tris(triisopropylsilylethynyl)dehydrotribenzo[12]annulene
(5). To a dried, argon -filled 25 mL 2-necked flask fitted with a
reflux condenser vacuum/argon inlet adaptor and rubber septum,
and containing CuCl (0.104 g, 1.05 × 10-3 mol) and t-BuOK 0.117
g, 1.04 × 10-3 mol), was added N2-purged pyridine (6 mL) via
syringe. The reaction was stirred at ambient temperature for 0.5 h,
and then 15 (0.200 g, 4.90 × 10-4 mol) added by syringe. With
continued stirring, the resulting orange-red suspension was heated
at 35-40 °C for 4 h and then at 125 °C-140 °C for 10 h during
which time the color changed to brown. After heating, the pyridine
was removed by distillation under reduced pressure on a water bath
at 35 °C and the black residue extracted with heptane (4 × 200
mL). Removal of the heptane by distillation on a water bath yielded
a beige solid which was chromatographed on two successive
Supporting Information Available: General experimental
methods, synthetic procedures, and characterization data for
1
7-9, 11, 14, and 15, UV-vis spectral data for 10, H and 13C
spectra of 1-5, 7-9, 11, 14-16, X-ray experimental data for
1 and full page thermal ellipsoid plot of 1. This material is
available free of charge via the Internet at http://pubs.acs.org.
JO802797E
J. Org. Chem. Vol. 74, No. 13, 2009 4689