Dendrimer analogues of linear molecules to evaluate energy and charge-transfer properties

Article abstract of DOI:10.1021/ol0608956

We have designed and synthesized difunctionalized dendrimers containing two donors in the periphery and an acceptor at the core to serve as scaffolds for comparison with linear analogues to investigate the advantage of dendritic scaffolds for energy and c

Full text of DOI:10.1021/ol0608956

ORGANIC
LETTERS
2006
Vol. 8, No. 14
2981-2984
Dendrimer Analogues of Linear
Molecules to Evaluate Energy and
Charge-Transfer Properties
Arpornrat Nantalaksakul,† Raghunath Reddy Dasari,† Tai-Sang Ahn,‡
Rabih Al-Kaysi,‡ Christopher J. Bardeen,*,‡ and S. Thayumanavan*,†
Department of Chemistry, UniVersity of Massachusetts, Amherst, Massachusetts 01003,
and Department of Chemistry, UniVersity of California, RiVerside, California 92521
thai@chem.umass.edu; christob@ucr.edu
Received April 12, 2006
ABSTRACT
We have designed and synthesized difunctionalized dendrimers containing two donors in the periphery and an acceptor at the core to serve
as scaffolds for comparison with linear analogues to investigate the advantage of dendritic scaffolds for energy and charge transfer. Comparison
of these dendrimers with the fully decorated dendrimers provides information on the advantage of chromophore density in energy/charge
transfer from periphery to the core.
Multichromophore dendrimers provide the ability to surround
a single energy acceptor with a dense periphery of energy
donors. These types of structures are of interest as artificial
light-harvesting systems for use in solar energy conversion
devices.1 Both conjugated2 and nonconjugated3 dendrimers
have been extensively studied for this purpose. Light
harvesting represents only the first step in the photosynthetic
process, which also involves the creation of a long-lived
charge-separated state to convert photon energy into an
electrochemical potential. There have been relatively few
reports on charge transfer within dendritic architectures in
both conjugated4 and nonconjugated5 systems. To the best
of our knowledge, there has been no study that compares
dendrimers with the corresponding linear analogues in the
context of intramolecular electronic energy transfer (EET)
and charge transfer (CT). Such a study has the potential to
provide fundamental insights into the possible advantages
that dendritic architectures provide for these photoinduced
processes. We utilize the combination of donors and accep-
tors that we have recently shown to exhibit a sequential EET
and CT events in dendrimers,6 since these provide a unique
opportunity to investigate the advantages that dendrimers
† University of Massachusetts.
‡ University of California.
(1) (a) Adranov, A.; Fre´chet, J. M. J. Chem. Commun. 2000, 1701-
1710. (b) Balzani, V.; Ceroni, P.; Maestri, M.; Saudan, C.; Vicinelli, V.
Top. Curr. Chem. 2003, 228, 159-191. (c) Jiang, D. L.; Aida, T. Prog.
Polym. Sci. 2005, 30, 403-422. (d) Ceroni, P.; Bergamini, G.; Marchioni,
F.; Balzani, V. Prog. Polym. Sci. 2005, 30, 453-473.
(2) (a) Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996,
118, 9635-9644. (b) Kwon, T. W.; Alam, M. M.; Jenekhe, S. A. Chem.
Mater. 2004, 16, 4657-4666. (c) Weil, T.; Reuther, E.; Beer, C.; Mu¨llen,
K. Chem. Eur. J. 2004, 10, 1398-1414. (d) Mellinger, J. S.; Pan, Y.;
Kleiman, V. D.; Peng, Z.; Davis, B. L.; McMorrow, D.; Lu, M. J. Am.
Chem. Soc. 2002, 124, 12002-12012. (e) Cotlet, M.; Vosch, T.; Lor, M.;
Habuchi, S.; Weil, T.; Mu¨llen, K.; Johan, H.; De Schryver, F. C. J. Am.
Chem. Soc. 2005, 127, 9760-9768.
(3) (a) Jiang, D. L.; Aida, T. J. Am. Chem. Soc. 1998, 120, 10895-
10901. (b) Adronov, A.; Gilat, S. L.; Fre´chet, J. M. J.; Ohta, K.; Neuwahl,
F. V. R.; Fleming, G. R. J. Am. Chem. Soc. 2000, 122, 1175-1185. (c)
Balzani, V.; Ceroni, P.; Giansante, C.; Vicinelli, V.; Klurner, F.-G.;
Verhaelen, C.; Vo¨gtle, F. Angew. Chem., Int. Ed. 2005, 44, 4574-4578.
(d) Wang, S.; Hong, J. W.; Bazan, G. C. Org. Lett. 2005, 7, 1907-1910.
(4) (a) Lor, M.; Thielemans, J.; Viaene, L.; Cotlet, M.; Hofkens, J.; Weil,
T.; Hampel, C.; Mu¨llen, K.; Verhoeven, J. W.; van der Auweraer, M.; De
Schryver, F. C. J. Am. Chem. Soc. 2002, 124, 9918-9925. (b) Qu, J.;
Pschirer, N. G.; Liu, D.; Stefan, A.; De Schryver, F. C.; Mu¨llen, K. Chem.
Eur. J. 2004, 10, 528-537.
10.1021/ol0608956 CCC: $33.50
© 2006 American Chemical Society
Published on Web 06/07/2006

provide in both EET and CT processes. This paper outlines
the design and syntheses of the linear molecules and the
corresponding dendrimer analogues for a straightforward
comparison.
density variable and directly addresses the advantages of
dendritic scaffold in light harvesting. On the other hand,
comparison of F and D provides information on the
advantages of chromophore density in dendrimers.
Comparison of dendrimers with linear analogues has been
previously done in the context of their physical properties.7
However, similar comparison in light-harvesting dendrimers
is more complicated. Classical dendrimers with the periphery
fully decorated by donor moieties and a single acceptor unit
at the core are represented by F (see the Abstract for a
schematic representation; see Figure 1 for the structures of
We have synthesized dendrimers that contain diarylami-
nopyrene-based units as the donor functionality and a
benzthiadiazole-based core as the acceptor. Structures of the
fully functionalized dendrimers 1F-3F, difunctionalized
dendrimers 1D-3D, and the corresponding linear analogues
1L-3L are shown in Figure 1. The key steps in the syntheses
of dendrimers were the repetitive benzylation followed by
the bromination of the hydroxymethyl group by using
methanesulfonyl chloride, triethylamine, and lithium bromide
as reagents. Note that in the syntheses of classical benzyl
ether dendrimers, the conversion of the hydroxymethyl
moiety to the bromomethyl functionality is achieved using
triphenylphosphine and carbontetrabromide (or N-bromo-
succinimide).8 However, when this methodology was re-
peated in this work, we noticed small peaks in the mass
spectra that corespond to M + 80 and M + 160. This was
attributed to the possible aromatic ring bromination, since
the reagents above could generate bromonium ions. The
presence of bromine atoms could affect our photophysical
results, due to the possible heavy atom effect.9 By using the
synthetic pathways outlined here, the formation of electro-
philic Br+ functionality is avoided, thus obviating the
possiblity of ring bromination.
Synthesis of difunctionalized dendrimers D is nontrivial
because only two of the positions in the otherwise identical
peripheral moieties contain the diarylaminopyrene function-
ality. We used the synthetic methodologies developed in our
group to achieve these molecules.10 To obtain the partially
functionalized G-1 dendron 12, we started with the reaction
of 3,5-dihydroxybenzyl alcohol (5) with 1 equiv of benzyl
bromide (6) (Scheme 1). This reaction afforded a mixture
Scheme 1. Synthesis of G1-D Dendrons
Figure 1. Structures of the L, D, and F molecules.
the molecules). Comparison of F with the linear analogue L
accounts for the donor-acceptor distance that dendrimers
and the linear oligomers provide but fails to provide the
equivalent chromophore densities (number of EET/CT donors
vs. acceptor). However, comparison of the dendrimer D with
a difunctionalized periphery with the linear analogue L
accounts for both of these factors. In other words, for this
study, comparison of D and L eliminates the chromophore
of the disubstituted dendron 7 and the monosubstituted
dendron 8. Compound 8 was used to synthesize the G-1
dendron 11 by reaction with the bromomethyl-functionalized
(5) (a) Ghaddar, T. H.; Wishard, J. F.; Thompson, D. W.; Whitesell, J.
K.; Fox, M. A. J. Am. Chem. Soc. 2002, 124, 8285-8289. (b) Braun, M.;
Atalick, S.; Guldi, D. M.; Lanig, H.; Brettreich, M.; Burghardt, S.;
Hatzimarinaki, M.; Ravanelli, E.; Prato, M.; van Eldik, R.; Hirsch, A. Chem.
Eur. J. 2003, 9, 3867-3875. (c) Gutierrez-Nava, M.; Accorsi, G.; Masson,
P.; Armaroli, N.; Nierengarten, J.-F. Chem. Eur. J. 2004, 10, 5076-5086.
(6) Thomas, K. R. J.; Thompson, A. L.; Sivakumar, A. V.; Bardeen, C.
J.; Thayumanavan, S. J. Am. Chem. Soc. 2005, 127, 373-383.
(7) (a) Hawker, C. J.; Malmstrom, E. E.; Frank, C. W.; Kampf, J. P. J.
Am. Chem. Soc. 1997, 119, 9903-9904. (b) Harth, E. M.; Hecht, S.; Helms,
B.; Malmstrom, E. E.; Fre´chet, J. M. J.; Hawker, C. J. J. Am. Chem. Soc.
2002, 124, 3926-3938.
(8) Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638-
7647.
(9) (a) Khudyakov, I. V.; Serebrennikov, Y. A.; Turro, N. J. Chem. ReV.
1993, 93, 537-570. (b) Plummer, B. F.; Steffen, L. K.; Braley, T. L.; Reese,
W. G.; Zych, K.; Dyke, G. V.; Tulley, B. J. Am. Chem. Soc. 1993, 115,
11542-11551. (c) Mac, M.; Danel, A.; Kizior, K.; Nowak, P.; Karocki,
A.; Tokarczyk, B. Phys. Chem. Chem. Phys. 2003, 5, 988-997.
2982
Org. Lett., Vol. 8, No. 14, 2006

diarylaminopyrene compound 10, as shown in Scheme 1.
Compound 11 was converted to the dendron 12 using the
MsCl/Et3N/LiBr combination.
Scheme 3. Synthesis of Linear Analogues
On the other hand, the byproduct of the previous reaction,
7, was used to obtain the next generation of the monofunc-
tionalized dendron. First, the hydroxymethyl compound 7
was converted to the corresponding bromomethyl compound
9, which was then treated with 5 to obtain a mixture of the
monosubstituted 13 and disubstituted 14 dendrons. Dendron
13 was used to synthesize the monofunctionalized G-2
dendron 17 using a pathway similar to that adapted for 12,
as shown in Scheme 2. Similarly, dendron 14 was converted
reactions also do not involve the possibility of any heavy atom
incorporation into the dendrimers. When we attempted the
Mitsunobu reaction for the syntheses of the dendrimers, the
yields of reactions were poor, especially at higher generation.
Finally, the targeted dendrimers 1D-3D were accom-
plished by the benzylation reaction of the benzthiadiazole
core chromophore 4 with the bromomethyl functionalized
dendrons 12, 17, and 20, respectively (Scheme 4). Similarly,
Scheme 2. Synthesis of G2-D and G3-D Dendrons
Scheme 4. Synthesis of D and L Molecules
the linear analogues 1L-3L were obtained by a Mitsunobu
reaction between 4 and the hydroxymethyl compounds 23,
25, and 26, respectively, as shown in Scheme 4. The
dendrons, dendrimers, linear analogues, and key compounds
1
that lead to these molecules were characterized by H and
to the corresponding monofunctionalized G-3 dendron 20
(Scheme 2). Detailed experimental procedures and charac-
terizations for all the compounds are outlined in the Sup-
porting Information.
13C NMR and mass spectrometry. Compounds 1D-3D and
1L-3L were also analyzed by elemental analysis. For
consistency, we also synthesized our previously reported
1F-3F6 using the same synthetic pathway. The purity of
these molecules was also analyzed by elemental analysis.
While recording MALDI spectra of these compounds
particular attention was paid to the presence of even a small
peak containing bromine at M + 80 or M + 160. No such
peaks were observed even in small amounts. Additionally,
the purity of the samples was also further confirmed using
gel permeation chromatography.
The three families of molecules were compared using
fluorescence spectroscopy. For energy transfer study, we
found that, in all these cases, when the donor component of
the molecules is excited at 395 nm, the emission arises
mainly from the acceptor (Figure 2a,b). This indicates a high
degree of energy transfer from the periphery to the core in
all families. The differences in energy transfer among
generations 1-3 were very similar for all three families
within experimental error. We have previously estimated that
the Fo¨rster radius for this combination of donor and acceptor
The linear analogues were synthesized using the Mitsun-
obu11 reaction as the key step in the synthesis. The hydroxy-
methyl-functionalized diarylaminopyrene compound 21 was
treated with 3-hydroxybenzaldehyde (22) under Mitsunobu
reaction conditions to give corresponding aldehyde. The
aldehyde was then reduced to the alcohol 23, linear analogue
for the G-1 dendron (Scheme 3). Reaction of 21 with the
aldehyde 24, followed by reduction afforded the G-2 analog-
ue 25, whereas a similar reaction sequence with the com-
pound 23 afforded the G-3 analogue 26. Note that these
(10) (a) Sivanandan, K.; Vutukuri, D.; Thayumanavan, S. Org. Lett. 2002,
4, 3751-3753. (b) Sivanandan, K.; Britto, S. S.; Thayumanavan, S. J. Org.
Chem. 2004, 69, 2937-2944. For related references, see: (c) Newkome,
G. R.; Kim, H. J.; Moorefield, C. N.; Maddi, H.; Yoo, K. S. Macromolecules
2003, 36, 4345-4354. (d) Zhang, W.; Nowlan, D. T., III; Thomson, L.
M.; Lackowski, W. M.; Simanek, E. E. J. Am. Chem. Soc. 2001, 123, 8914-
8922.
(11) Mitsunobu, O. Synthesis 1981, 1-28.
Org. Lett., Vol. 8, No. 14, 2006
2983

functionality at 490 nm in solvents with different dielectric
constants. The fluorescence of the acceptor was quenched
relative to a control chromophore that does not contain any
diarylaminopyrene moieties (Figure 1). Moreover, the degree
of fluoresence quenching increases with increasing solvent
polarities indicating the presence of CT. An interesting
divergence in generation dependence emerges when the trend
in the extent of fluorescence quenching in the three families
of molecules are compared. As could be seen in Figure 2c,
with increasing generation, the fluorescence quenching de-
creases in the case of 1L-3L and 1D-3D. This trend is
understandable because the distance between the donor and
the acceptor increases with increasing generation. Therefore,
the efficiency of photoinduced electron transfer is expected
to decrease with generation, as observed. However, there is
a discernible difference between L and D families in the
third generation where linear analogues seem to provide
better CT efficiency than their difunctionalized dendrimer
counterparts. This might be due to the steric hindrance gener-
ated by the dendritic arms in D, which could result in in-
creased donor-acceptor distance and therefore decreased CT
efficiency. The generation-dependent trend is opposite in the
case of 1F-3F, where the fluorescence quenching increases
with increasing generation (Figure 2d). In the case of the F
dendrimers, even though the donor-acceptor distance in-
creases with increasing generation, the number of donors also
doubles with each generation, unlike the L and D molecules.
The observed results indicate that doubling the number of
the electron donors around the core chromophore more than
compensates for the loss in charge-transfer efficiency due
to increased distance in these dendrimers.
In summary, we have designed and synthesized linear
versions of dendrimers that could exhibit electronic energy
transfer and electron transfer upon excitation of the donor.
We have synthesized dendrimer analogues of these linear
molecules (difunctionalized dendrimers D) to investigate
whether there are any architectural advantages to dendrimers
in either of these photoinduced processes. Comparison of
these two classes of molecules with the more classical fully
functionalized dendrimers F suggests that dendrimers are
indeed advantageous for these processes. The advantage
primarily emanates from the number density of the energy
or electron donors that one could decorate equidistant from
the acceptor core, a feature that is not available in linear
molecules. A more quantitative understanding of this struc-
ture-property relationship is likely to emerge from time-
resolved spectroscopy and molecular modeling studies, which
are the focus of current work in our laboratories.
Figure 2. Emission spectra in toluene, λex ) 395 nm, of (a) 1D-
3D, 1L-3L and (b) 1F-3F. Spectra of 1.0-3.0 µM solutions are
normalized at the λmax of the acceptor. Emission spectra in DMF
based on relative quantum yields using ethidium bromide as a
reference, λex ) 490 nm of (c) 1D-3D, 1L-3L and (d) 1F-3F.
is approximately 48 Å. Since all these three families have
the donors and acceptors within this distance, the energy
transfer seems to be efficient in all cases. These results
suggest that our dendrimers do not provide an inherent
advantage, when one compares the D and L family of
molecules. However, when one compares these molecules
with the F family of molecules, an obvious advantage of
dendritic architectures can be seen when the absorption
spectra of these molecules were compared (see Figure 3, for
Figure 3. Absorption spectra of 3D, 3L, and 3F (1.0-3.0 µM
solutions in toluene normalized at 500 nm).
Acknowledgment. This work was supported by the
Department of Energy, Basic Energy Sciences program,
Grant Nos. DEFG-01ER15270 (C.J.B.) and DEFG-02ER15503
(S.T.). C.J.B. is a Sloan Fellow.
a comparison of donor vs acceptor in the G-3 molecules).
Although the inherent energy transfer efficiencies are similar
for these two types of molecules, note that the absorption
crosssection of the donor component of the molecule
increases with increasing generation in the case of F
dendrimers, a feature that is obviously not available in linear
molecules.
Supporting Information Available: Experimental details
and characterization data for all of the compounds reported.
This material is available free of charge via the Internet at
http://pubs.acs.org.
To understand the differences in CT efficiencies in these
families, we directly excited the benzthiadiazole-based core
OL0608956
2984
Org. Lett., Vol. 8, No. 14, 2006