Cascade energy transfer in a conformationally mobile multichromophoric dendrimer

Article abstract of DOI:10.1039/b207905d

A conformationally flexible, generation-2,3 poly(aryl ether) dendrimer favors quantitative cascade fluorescence resonance energy transfer without the appearance of undesired chromophore self-quenching interactions such as excimer formation.

Full text of DOI:10.1039/b207905d

Cascade energy transfer in a conformationally mobile
multichromophoric dendrimer
Jason M. Serin, Darryl W. Brousmiche and Jean M. J. Fréchet*
Department of Chemistry, University of California, Berkeley, CA 94720-1460, USA.
E-mail: frechet@cchem.berkeley.edu; Fax: (510) 643-3079; Tel: (510) 643-3077
Received (in Cambridge, UK) 13th August 2002, Accepted 4th October 2002
First published as an Advance Article on the web 25th October 2002
A conformationally flexible, generation-2,3 poly(aryl ether)
dendrimer favors quantitative cascade fluorescence reso-
nance energy transfer without the appearance of undesired
chromophore self-quenching interactions such as excimer
formation.
ether conditions, followed by removal of the TBDPS group
using CsF in DMF afforded 7. Finally, the mixed bichromo-
phoric G-2, G-3 dendron 8 was obtained from 7 by reaction with
succinic anhydride using a catalytic amount of DMAP in
acetone.
The perylenebis(dicarboximide) acceptor core (11) was
synthesized as illustrated in Scheme 2. Aromatic substitution of
Br2 onto 915 followed by imidation using propylamine16
afforded 10 in 78% overall yield. This compound was then
coupled to 4-(hydroxymethyl)phenylboronic acid under Suzuki
conditions and treated with CBr4 in the presence of triphenyl-
phosphine to afford 11. Compound 1 (Scheme 3) was then
obtained by coupling 8 and 11 using K2CO3 in DMF.
In order to establish which of the two possible ET pathways
illustrated in Fig. 2 apply to 1, two key model compounds were
required in addition to 7. Dendrimer 12 (Scheme 3), containing
D2 and Ac, was synthesized by coupling 11 with 13 using
K2CO3 in DMF. Dendrimer 14, containing D1 and Ac, was
synthesized by coupling 11 with 15.17 Several preparative TLC
purifications were performed on all model compounds to ensure
their purity prior to performing absorption and emission
measurements.
Fig. 3 shows the absorption and emission spectra (lex = 342,
418 and 555 nm) of 1 in chloroform. While the absorption
spectrum is a composite of those of the three chromophores, the
emission spectra show a near complete quenching of the D1 and
D2 emissions when 1 is excited at 342 nm (lmax of D1). This
results in a 6.9-fold increase in Ac emission, relative to the
direct excitation of Ac at 555 nm. This very significant ‘antenna
effect’ (calculated using the ratio of integrated emission of Ac
Natural photosynthetic systems employ several light-harvesting
complexes containing many chromophores that efficiently
transfer absorbed radiation unidirectionally over nanometer
distances.1–5 In the past, our group has designed and prepared
dendritic materials inspired by this energy transfer component
of the photosynthetic pathway,2,3 utilizing fluorescence reso-
nance energy transfer (FRET) to concentrate absorbed energy at
a single center.4,5 Systems capable of directional FRET between
several chromophores6 have been the object of much atten-
tion.7–10 Recently, cascade FRET was reported11 using a
conformationally rigid dendrimer that was thought to curtail
energy losses resulting from interactions between chromo-
phores. However, in the absence of a measurement of the energy
transfer (ET) efficiency from the initial donor to the final
acceptor chromophore it is not possible to ascertain whether ET
occurs via sequential transfer from one donor to the other donor
and then to the acceptor, or includes a component of direct ET
from each donor to the acceptor. We now report the design and
synthesis of a cascade light-harvesting system based on a
flexible dendrimer scaffold. This system demonstrates that,
with proper chromophore selection, FRET is favorable in a
vectorial cascade process, and that shape-persistence is not
needed to avoid self-quenching through chromophore aggrega-
tion and excimer formation.
The design of the cascade ET system (1, Fig. 1) places
coumarin 2 (D1) at the third branch point of a poly(aryl ether)
dendrimer and fluorol 7GA (D2) at the second branch point—
closer to the core—representing spatially the desired direction
of FRET. The final energy acceptor (Ac) consists of a
perylenebis(dicarboximide) derivative at the core of the den-
drimer. Earlier studies have demonstrated that the dendritic
backbone selected does not participate in the ET process5 and
energy is transferred through space, as in natural photosynthetic
systems, rather than through the dendritic scaffold. The three
laser dyes were chosen for their spectral properties as well as
their thermal and chemical stability.5,12 A requirement for
FRET is that there be an overlap of the donor chromophore
fluorescence with the absorption of the acceptor chromo-
phore.13,14 Consistent with this requirement, ET is favored from
coumarin 2 to fluorol 7GA, and from fluorol 7GA to the
perylene core. In addition, ET is disfavored between coumarin
2 and the perylene core due to a much smaller spectral overlap
of coumarin 2 fluorescence with the core absorption, and the
increased interchromophoric distance between the two dyes.
Preparation of the target compound starts by coupling fluorol
7GA to the protected AB2 dendritic monomer3 2 followed by
removal of the hexadecanesulfonyl protecting group using
NaOH in ethanol (Scheme 1). The resulting generation 1 (G-1)
dendron 3 was attached to a tert-butyldiphenylsilyl (TBDPS)
protected AB2 dendritic monomer3 (4) using a 20+1 ratio of 4+3
to form the mono-coupled product (5). Coupling the coumarin
2 labeled G-2 dendron3 6 with 5 under standard Williamson
Fig. 1 Structure of the directional cascade ET system (1) and the component
donor and acceptor chromophores.
This journal is © The Royal Society of Chemistry 2002
CHEM. COMMUN., 2002, 2605–2607
2605

View Article Online
when excited at 342 nm, over the integrated emission when
excited at 555 nm) demonstrates how light energy can be
concentrated at a single center using a large, multi-chromo-
phoric antenna. A similar effect is seen when 1 is excited at D2
(lex = 418).
Steady-state photophysical measurements of 7, 12 and 14 in
chloroform enabled the determination of approximate FRET
efficiencies from D1 to D2, D2 to Ac, and D1 to Ac,
respectively. The donor to acceptor FRET efficiencies were
calculated by comparing the integrated donor emission when in
the presence of the acceptor relative to the donor emission in the
absence of the acceptor.5 This is demonstrated using model
dendrimer 14, whose absorption and emission properties, along
with those of 6, are shown in Fig. 4 (6: lex = 342 nm; 14: lex
= 342 and 555 nm). The coumarin 2 absorption is well
separated from that of the perylene core. Coumarin 2 emission
from 14 is quenched by 79% (FRET efficiency) relative to its
emission in 6. Considering that the average distance from D1 to
Ac in 14 is less than in 1, this value can be taken as the upper
limit of FRET efficiency between these chromophores in 1. In
addition, the spectral overlap between D1 to D2 in 1 is larger
than the spectral overlap between D1 and Ac, further increasing
the Förster rate constant for ET in the desired direction.14
Similar studies using 3, 6, 7 and 12 resulted in calculated FRET
efficiencies of 99 % from D1 to D2, and 96 % from D2 to Ac
(Fig. 5).
Scheme 1
Fig. 2 The two possible pathways of FRET. A = Directional cascade FRET.
Energy is transferred from D1 to D2 to Ac. B = Bi-directional FRET.
Energy is transferred from both donor chromophores directly to the
acceptor.
Scheme 2
Fig. 3 Absorption spectrum (top) and emission spectra (bottom, lex = 342,
418 and 555 nm) of 1 in CHCl3 at rt. Calculated FRET efficiencies and
corresponding core emission increases were 98% and 6.9-fold for lex = 342
nm, and 97% and 3.6-fold for lex = 418 nm, respectively.
Scheme 3
2606
CHEM. COMMUN., 2002, 2605–2607

View Article Online
Excitation of 1 at 342 nm results in an overall calculated
FRET efficiency of more than 95%. More specifically, 98% of
the energy D1 absorbs is transferred to D2, and 97% of this
energy is transferred to Ac, resulting in a near 7-fold increase in
core emission (vide supra). The large increase in Ac emission
when excited at either donor indicates that there are no major
pathways competing with the Ac emission. That is, such an
increase in core emission would not be expected if donor
chromophore aggregation and self-quenching were competing
with ET. This has been demonstrated in similar systems,5,11 and
will be further explored using time-resolved studies. Evidence
of core excimer formation was not found in any of the
experiments conducted.
steady-state photophysical analysis of model dendrimer 14,
which places 79% as an upper limit to the amount of FRET that
may occur in the undesired direction, i.e. directly from D1 to
Ac. Time-resolved experiments currently underway should
provide a more detailed view of the dynamics occurring within
this system.
Professor Alex Adronov (McMaster University) is gratefully
acknowledged for the synthesis of several starting materials.
Financial support of this research comes from the AFOSR (No.
FDF-49620-01-1-0167). D. W. B. acknowledges NSERC
(Canada) for a post-doctoral fellowship.
Notes and references
Dendrimer 1 represents a novel approach to examine cascade
unidirectional FRET in a flexible system. It was shown that ET
within this system favors a cascade route, moving from an
initial chromophore through an intermediate chromophore and
onto a final acceptor chromophore. This is evidenced in the
1 (a) G. McDermott, S. M. Prince, A. A. Freer, A. M. Hawthornthwaite-
Lawless, M. Z. Papiz, R. J. Cogdell and N. W. Isaacs, Nature, 1995, 374,
517; (b) W. Kühlbrandt and D. N. Wang, Nature, 1991, 350, 130; (c) W.
Kühlbrandt, D. N. Wang and Y. Fujiyoshi, Nature, 1994, 614; (d) W.
Kühlbrandt, Nature, 1995, 374, 497; (e) A. N. Glazer, Annu. Rev.
Biophys. Biophys. Chem., 1995, 14, 47.
2 S. L. Gilat, A. Adronov and J. M. J. Fréchet, Angew. Chem., Int. Ed.,
1999, 38, 1422.
3 S. L. Gilat, A. Adronov and J. M. J. Fréchet, J. Org. Chem., 1999, 64,
7474.
4 (a) A. Adronov, P. R. L. Malenfant and J. M. J. Fréchet, Chem. Mater.,
2000, 12, 1463; (b) F. V. R. Neuwahl, R. Righini, A. Adronov, P. R.
Malenfant and J. M. J. Fréchet, J. Phys. Chem., 2001, 105, 1307.
5 A. Adronov, S. L. Gilat, J. M. J. Fréchet, K. Ohta, F. V. R. Neuwahl and
G. R. Fleming, J. Am. Chem. Soc., 2000, 122, 1175.
6 (a) V. Vicinelli, P. Ceroni, M. Maestri, V. Balzani, M. Gorka and F.
Vögtle, J. Am. Chem. Soc., 2002, 124, 6461; (b) H. B. Baudin, J.
Davidsson, S. Serroni, A. Juris, V. Balzani, S. Campagna and L.
Hammasström, J. Phys. Chem. A., 2002, 106, 4312; (c) F. Vögtle, M.
Plevoets, M. Nieger, G. C. Azzellini, A. Credi, L. De Cola, V. De
Marchis, M. Venturi and V. Balzani, J. Am. Chem. Soc., 1999, 121,
6290.
Fig. 4 Emission spectra of 6 (lex = 342 nm) and 14 (lex = 342 and 555 nm)
in CHCl3 at rt. Inset: Absorption spectra of 6 and 14 in CHCl3 at rt.
7 J. Kim, T. McQuade, A. Rose, Z. Zhu and T. M. Swager, J. Am. Chem.
Soc., 2001, 123, 11488.
8 M. Berggren, A. Dodabalapur, R. E. Slusher and Z. Bao, Nature, 1997,
389, 466.
9 (a) S. Kawahara, T. Uchimaru and S. Murata, Chem. Commun., 1999,
563; (b) A. K. Tong, S. Jockusch, Z. Li, H. R. Zhu, D. L. Akins, N. J.
Turro and J. Ju, J. Am. Chem. Soc., 2001, 123, 12923.
10 (a) K. Y. Peng, S. A. Chen and W. S. Fann, J. Am. Chem. Soc., 2001,
123, 11388; (b) M. S. Shchepinov and V. A. Korshun, Nucleos.,
Nucleot. Nucl. Acids, 2001, 20, 369.
11 T. Weil, E. Reuther and K. Müllen, Angew. Chem., Int. Ed., 2002, 41,
1900.
12 Y. Geerts, H. Quante, H. Platz, R. Mahrt, M. Hopmeier, A. Böhm and
K. Müllen, J. Mater. Chem., 1998, 8, 2357.
13 (a) T. Förster, Ann. Phys., 1948, 2, 55; (b) T. Förster, Z. Naturforsch.,
Teil A, 1949, 4, 321.
14 B. Weib Van Der Meer, G. Coker III and S. Y. Simon Chen, Resonance
Energy Transfer, Theory and Data, VCH, Weinheim, 1994.
15 A. Böhm, H. Arms, G. Henning and P. Blaschka, US Pat., 6,184,378,
2001.
Fig. 5 Diagram illustrating the FRET efficiencies between chromophores in
1. These calculations were made using model dendrimers 7, 12 and 14, and
the acceptor-lacking counterparts 3 and 6. *The 79% FRET from D1 to Ac
is an upper limit.
16 H. Quante, P. Schlichting, R. Ulrike, Y. Geerts and K. Müllen,
Macromol. Chem. Phys., 1996, 197, 4029.
17 L. A. J. Chrisstoffels, A. Adronov and J. M. J. Fréchet, Angew. Chem.,
Int. Ed., 2000, 39, 2163.
CHEM. COMMUN., 2002, 2605–2607
2607