Functionally layered dendrimers: A new building block and its application to the synthesis of multichromophoric light-harvesting systems

Article abstract of DOI:10.1021/ol0516824

(Chemical Equation Presented) A divergent synthesis of internally functionalized dendrimers based on a modular functional monomer has been developed. This strategy was applied to the construction of a light-harvesting dendrimer containing one set of napht

Full text of DOI:10.1021/ol0516824

ORGANIC
LETTERS
2005
Vol. 7, No. 20
4451-4454
Functionally Layered Dendrimers: A
New Building Block and Its Application
to the Synthesis of Multichromophoric
Light-Harvesting Systems
William R. Dichtel, Stefan Hecht,† and Jean M. J. Fre´chet*
Department of Chemistry, UniVersity of CaliforniasBerkeley, Berkeley, California
94720, and Materials Science DiVision, Lawrence Berkeley National Laboratory,
Berkeley, California 94720-1460
frechet@berkeley.edu
Received July 17, 2005
ABSTRACT
A divergent synthesis of internally functionalized dendrimers based on a modular functional monomer has been developed. This strategy was
applied to the construction of a light-harvesting dendrimer containing one set of naphthopyranone dyes located at the interior and another
set of coumarin chromophores located in the adjacent outer layer surrounding a porphyrin acceptor. Quantitative energy transfer from both
donor pigments is observed, giving rise to exclusive emission from the porphyrin core over all excitation wavelengths.
In recent years, the special architecture of dendrimers1,2 has
been exploited in the design of functional macromolecules
both capable of mimicking many natural phenomena,3 such
as site isolation,3-5 catalysis,6-13 and light-harvesting,13-18
and useful in a variety of technological19-21 or therapeutic22
applications. Due to available synthetic protocols, functional
groups have typically been incorporated at the core and/or
periphery of the dendrimer, with the backbone acting only
as a passive scaffold separating these two domains. The
difficult functionalization of the interior of the dendrimer23
constitutes a serious obstacle to the full exploitation of the
dendritic scaffold.24-28 For example, internally functionalized
layered structures are desirable to establish complex energy
gradients25,26 between core and periphery or to tailor specific
properties of the dendritic interior.13,25
As a first example of the complex structures readily
accessed via this synthetic approach, the sophisticated light-
harvesting system 1 was synthesized containing two different
types of donor chromophores in a layered arrangement
† Current address: Max-Planck-Institut fu¨r Kohlenforschung 45470
Mu¨lheim an der Ruhr, Germany.
(1) Fre´chet, J. M. J.; Tomalia, D. Dendrimers and Other Dendritic
Polymers; Wiley: Chichester, U.K., 2001.
(2) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Mol-
ecules: Concepts, Synthesis, PerspectiVes; Wiley-VCH: Weinheim, Ger-
many, 2001.
(3) Hecht, S.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74.
(4) Furuta, P.; Brooks, J.; Thompson, M. E.; Fre´chet, J. M. J. J. Am.
Chem. Soc. 2003, 125, 13165.
(5) Furuta, P.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2003, 125, 13173.
(6) Kreiter, R.; Kleij, A. W.; Gebbink, R. J. M. K.; van Koten, G. Top.
Curr. Chem. 2001, 163.
(7) van Heerbeek, R.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Reek,
J. N. H. Chem. ReV. 2002, 102, 3717.
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Commun. 2003, 2524.
(12) Hecht, S.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2001, 123, 6959.
(13) Piotti, M. E.; Rivera, F., Jr.; Bond, R.; Hawker, C. J.; Fre´chet, J.
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(14) Adronov, A.; Fre´chet, J. M. J. Chem. Commun. 2000, 1701.
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10.1021/ol0516824 CCC: $30.25
© 2005 American Chemical Society
Published on Web 09/08/2005

around a porphyrin core (Figure 1). The 25 chromophore
types of donor chromophores have recently been re-
ported,18,26,29 our approach differs significantly as it is
generally applicable to a broad range of functionality and
avoids statistical monofunctionalization reactions in its
synthesis. Donor chromophores were selected based on their
emission characteristics, enabling efficient sensitization of
the porphyrin absorption bands as well as the presence of a
carboxylic acid functionality necessary for incorporation into
the functional monomer synthesis. For the first donor
chromophore, novel naphthopyranone dye 4 was targeted
(Scheme 1) as it was expected that its absorbance and
Scheme 1
Figure 1. Internally functionalized dendrimer containing eight
naphthopyranone and 16 coumarin-3-carboxylate donor chromo-
phores.
emission bands would undergo a bathochromic shift relative
to the coumarin derivative, thereby enhancing the absorption
cross-section of the overall assembly while also leading to
sensitization of the porphyrin Q-bands.30 Indeed, this chromo-
phore has an absorption maximum at 365 nm, to the red of
the coumarin absorbance, and exhibits a broad emission from
425 to 625 nm with a maximum at 472 nm (vide supra).
The dye was synthesized via a Peckmann condensation
between 2,7-dihydroxynaphthalene and ethyl trifluoroaceto-
acetate to exclusively afford the bent isomer upon crystal-
lization from ethanol. A carboxylic acid group necessary for
incorporation into the branched monomer was introduced by
alkylation with tert-butyl bromoacetate followed by acid-
catalyzed deprotection of the tert-butyl ester. The synthesis
of 4 requires no chromatography and can be scaled up to
assembly absorbs light over a broad range of the UV and
visible spectrum and converts harvested photons into a single
emission from the central porphyrin acceptor in the red region
of the spectrum via fluorescence resonance energy transfer
(FRET). Light-harvesting systems containing multiple ac-
cessory pigments mimic natural photosynthesis and offer
potential technological advantages in a variety of photonic
applications.3,14,15 The target molecule 1 was synthesized via
an iterative esterification/deprotection divergent growth
procedure using two different functionalized monomers
carrying the chosen donor chromophore at each branch point.
Though other light-harvesting dendrimers containing multiple
(16) Thomas, K. R. J.; Thompson, A. L.; Sivakumar, A. V.; Bardeen,
C. J.; Thayumanavan, S. J. Am. Chem. Soc. 2005, 127, 373.
(17) Weil, T.; Reuther, E.; Beer, C.; Mullen, K. Chem.sEur. J. 2004,
10, 1398.
Scheme 2
(18) Hahn, U.; Gorka, M.; Vo¨gtle, F.; Vicinelli, V.; Ceroni, P.; Maestri,
M.; Balzani, V. Angew. Chem., Int. Ed. 2002, 41, 3595.
(19) Fre´chet, J. M. J. J. Polym. Sci., Part A: Polym. Chem. 2003, 41,
3713.
(20) Hedrick, J. L.; Magbitang, T.; Connor, E. F.; Glauser, T.; Volksen,
W.; Hawker, C. J.; Lee, V. Y.; Miller, R. D. Chem.sEur. J. 2002, 8, 3308.
(21) Tully, D. C.; Fre´chet, J. M. J. Chem. Commun. 2001, 1229.
(22) Gillies, E. R.; Fre´chet, J. M. J. Drug DiscoVery Today 2005, 10,
35.
(23) Lochmann, L.; Wooley, K. L.; Ivanova, P. T.; Fre´chet, J. M. J.
J. Am. Chem. Soc. 1993, 115, 7043.
(24) Hecht, S. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 1047.
(25) Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996,
118, 9635.
(26) Serin, J.; Brousmiche, D.; Fre´chet, J. M. J. Chem. Commun. 2002,
2605.
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Top. Curr. Chem. 2001, 217, 163.
4452
Org. Lett., Vol. 7, No. 20, 2005

Scheme 3
produce multigram quantities of the dye. Commercially
With these monomers in hand, synthetic conditions for
dendrimer growth and deprotection were developed (Scheme
3). The first generation dendrimer 11 was synthesized from
an octahydroxyporphyrin 10 and 7 using a polystyrene-bound
carbodiimide reagent34 as the coupling agent in the presence
of DMAP and dimethylaminopyridinium p-toluenesulfonate
(DPTS).35 The use of a resin-bound carbodiimide allowed
available coumarin-3-carboxylic acid was chosen as the first
donor chromophore because it had already been shown to
be an excellent sensitizer of the porphyrin Soret band.31,32
Once each donor chromophore had been selected, mono-
mers bearing each dye were prepared (Scheme 2). Depending
on the availability of the carboxylic acid being used, two
different acylation procedures were developed. With readily
available coumarin-3-carboxylate, equimolar amounts of 533
and the carboxylic acid are mixed in the presence of EDC
at 0 °C. In the case of more precious synthetic chromophores,
such as 4, an excess of the acetonide was used and HOBT
was added to the mixture to enhance the selectivity for the
formation of the monoacylated species. Following successful
amide formation, the alcohol in each monomer was reacted
with succinic anhydride to provide a carboxylic acid handle
required for dendrimer growth. While TLC and NMR
analyses of the crude reaction mixtures suggest that the
reactions proceeded in good to excellent yields, the un-
optimized yields reported here are only moderate due to
difficulties in purification arising from the low solubility of
the relatively polar synthetic intermediates.
(28) Oosterom, G. E.; Reek, J. N. H.; Kamer, P. C. J.; Van Leeuwen, P.
W. N. M. Angew. Chem., Int. Ed. 2001, 40, 1828.
(29) Weil, T.; Reuther, E.; Mu¨llen, K. Angew. Chem., Int. Ed. 2002, 41,
1900.
(30) Li, F.; Yang, S. I.; Ciringh, Y.; Seth, J.; Martin, C. H.; Singh, D.
L.; Kim, D.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am.
Chem. Soc. 1998, 120, 10001.
(31) Hecht, S.; Ihre, H.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1999, 121,
9239.
(32) Hecht, S.; Vladimirov, N.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2001,
123, 18.
(33) Forbes, D. C.; Ene, D. G.; Doyle, M. P. Synthesis 1998, 879.
(34) PS-Carbodiimide, Argonaut Technologies.
(35) Moore, J. S.; Stupp, S. I. Macromolecules 1990, 23, 65.
(36) Ates, A.; Gautier, A.; Leroy, B.; Plancher, J. M.; Quesnel, Y.;
Vanherck, J. C.; Marko, I. E. Tetrahedron 2003, 59, 8989.
(37) Marko, I. E.; Ates, A.; Gautier, A.; Leroy, B.; Plancher, J. M.;
Quesnel, Y.; Vanherck, J. C. Angew. Chem., Int. Ed. 1999, 38, 3207.
Figure 2. MALDI-TOF mass spectra of the porphyrin core (10),
the G-1 acetonide protected dendrimer (11), the G-1 deprotected
dendrimer (12), and the final target (1). The observed molecular
weight of each compound in each spectrum corresponds to the
expected value, and the monodispersity of the spectra indicates that
each esterification and deprotection step proceeded to completion.
Org. Lett., Vol. 7, No. 20, 2005
4453

the stoichiometric carbodiimide-derived byproducts to be
removed by filtration, greatly simplifying the chromato-
graphic purification of the dendrimers. Complete deprotection
of all eight acetonide protecting groups of 11 could not be
accomplished using aqueous acid without a small amount
of hydrolysis of the polyester backbone. However, complete
deprotection without degradation was achieved using a
catalytic amount of ammonium nitrate (CAN) in an aceto-
nitrile/borate buffer mixture at pH 7, modified from the
procedure reported by Marko et al.36,37 The deprotection
provides the activated first generation compound 12 contain-
ing 16 hydroxyl groups that can serve as reactive sites for
another iteration of the esterification procedure. The final
target 1 was prepared under similar conditions as 11 by using
coumarin monomer 9 in the coupling procedure. The purity
of all compounds in this synthetic sequence was confirmed
by NMR spectroscopy and by MALDI-TOF mass spectrom-
etry (Figure 2), paying special attention to monitor each
growth and deprotection step and for completion and for
unwanted side reactions.
Figure 4. Fluorescence spectra of 1, 7, and 9 in THF at room
temperature. Fluorescence spectra of 7 and 9 are normalized to the
absorbance of compound 1 at 358 and 335 nm, respectively.
Emission from the coumarin and naphthopyranone moieties in 1 is
quenched relative to that seen from the monomers in the absence
of the porphyrin acceptor. Emission is, instead, observed from the
porphyrin core.
The absorption spectra of each of the multiple chromo-
phore assemblies 1 and 11 (Figure 3) show peak shapes and
of the two donor chromophores. When 1 is excited at
335 nm, virtually no coumarin emission is observed as
compared to the emission of coumarin from an isoabsorbing
solution of 9. Instead, the characteristic porphyrin emission,
with maxima at 651 and 717 nm, is observed. Extremely
low levels of residual naphthopyranone emission are also
observed, representing a slight deviation from quantitative
energy transfer from naphthopyranone excited states gener-
ated by either direct excitation or FRET from the coumarin
donors. When 1 is excited at 358 nm, corresponding to direct
naphthopyranone excitation, the expected naphthopyranone
absorption is quenched by 97%, and predominant emission
from the porphyrin is seen, indicating efficient FRET from
the naphthopyranone to the porphyrin.
With this modular synthetic approach to complex dendritic
architectures allowing for precise control over interior
functionality in hand, ongoing efforts in our laboratories are
focused on improving photon collection of light-harvesting
dendrimers3,31,32 and subsequent conversion of the excited-
state energy concentrated at the dendrimer core into chemical
energy.12 Such light-driven nanoreactors closely imitate the
function of natural photosynthetic machinery.38
Figure 3. Absorbance spectra of the target and model compounds
in THF at room temperature. The spectra of 7 and 9 are multiplied
by appropriate factors to account for their dye composition relative
to 1 and 11. The spectra of 1 and 11 show the expected extinction
coefficients and peak shapes based on the number and type of
chromophores they contain.
molar absorbtivities that represent linear additions of the
absorption spectra of their component chromophores. This
confirms the presence of all of the chromophores in each of
the dendrimers and demonstrates that there is little ground-
state interaction between the dyes.
Acknowledgment. Support of this research by the
AFOSR (FDF-49620-01-1-0167) and the Department of
Energy (DE-AC03-76SF00098) is gratefully acknowledged.
The fluorescence spectra of 1 (Figure 4) taken at two
excitation wavelengths corresponding to donor absorption
are indicative of near quantitative energy transfer from each
Supporting Information Available: Experimental details
and characterization data of all compounds (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
(38) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34,
40.
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