116
J . Org. Chem. 2000, 65, 116-123
Syn th esis a n d Ch a r a cter iza tion of Mon od en d r on s Ba sed on
9-P h en ylca r ba zole
Zhengguo Zhu and J effrey S. Moore*
Roger Adams Laboratory, Departments of Chemistry and Materials Science & Engineering, and
the Beckman Institute for Advanced Science and Technology, University of Illinois at
Urbana-Champaign, Urbana, Illinois 61801
Received J uly 21, 1999
A series of 9-phenylcarbazole ethynylene monodenrons have been prepared by palladium-catalyzed
coupling reactions creating well-organized arrays of redox centers. The tert-butyl groups attached
to the 3,6-positions of peripheral 9-phenylcarbazole monomers provide adequate solubility to a
limited degree. Trimer and 7-mer monodendrons were prepared using a monomer with 3,3-
diethyltriazene at its focal point. To facilitate purification, the synthesis of 15-mer monodendron,
however, required a monomer bearing a 3-hydroxy-3-methyl-but-1-ynyl group at its focal point as
a masking group for the terminal acetylene functionality. Although the solubility was limited, high
generation monodendrons were found to be readily soluble in carbon disulfide, a solvent of high
polarizability. Spectroscopic studies showed that there is limited through-bond conjugation over
the monodendrons, but fluorescence studies suggested the presence of long-range through-space
interactions in the higher members of the series.
In tr od u ction
constructions. There have already been many photo-
physical studies involving dendritic macromolecules as
scaffolding for building chromophore arrays.10-12 In
most cases, however, the photoactive unit has been
incorporated either as a single core12-16 or as peripheral
groups10,17-19 rather than as the dendritic repeating unit.
Carbazole has strong absorption in the near-UV region
and a low redox potential. The electrochemical and
spectroscopic properties of carbazole and its derivatives
have been extensively studied.20-22 Chemically, carbazole
can be easily functionalized at its 3-, 6-, or 9-positions
and covalently linked to other molecular moieties.23 As
a result of its special photo, electrical, and chemical
properties, carbazole has been used as a functional
building block in the fabrication of organic photoconduc-
The organization of chromophores plays essential roles
in controlling fundamental photolytic processes such as
light absorption, energy transfer (exciton diffusion),
photoinduced charge generation, and charge transfer.1,2
These effects are relevant in a variety of interesting
applications such as artificial photosynthesis, thin-film
transistors, photovoltaic cells, electroluminescent materi-
als, and photorefractive materials. Both covalent and
noncovalent strategies have been exploited in the con-
struction of spatially well-defined chromophore arrays.2
Examples of noncovalent systems include micelles and
reverse micelles, Langmuir-Blodgett films, liquid crys-
tals, lipid bilayers, and multilayer thin films.3,4 Nonco-
valent systems can involve large numbers of chromo-
phores and be easily extended into macroscopic dimen-
sions. However, these systems afford less precise control
over chromophore organization than covalent systems.5
Through covalent synthesis, chromophores can be ac-
curately positioned over dimensions up to 10 nm within
a single molecule. Studies on these systems will help
elucidate the basic structural parameters controlling the
energy and electron flow in molecular systems.
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J .; Albinsson, B. J . Phys. Chem. A 1997, 101, 2218-2220.
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Chromophore arrays based on linear systems are
simple, usually easy to synthesize, and have been exten-
sively studied.5-9 However, linear chain molecules can
usually afford control in only one dimension. To realize
more compact arrangements characterized by higher
dimensions, chromophores can be incorporated into a
well-defined dendritic structure. Rigid dendritic macro-
molecules can provide a suitable framework for such
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Gross, M. Angew. Chem., Int. Ed. Engl. 1996, 34, 2725-2728.
(16) Pollak, K. W.; Leon, J . W.; Frechet, J . M. J .; Maskus, M.;
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F. J . Am. Chem. Soc. 1998, 120, 12187-12191.
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(19) Gilat, S. L.; Adronov, A.; Frechet, J . M. J . Angew. Chem., Int.
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10.1021/jo991167h CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/10/1999