S. Goeb, R. Ziessel / Tetrahedron Letters 49 (2008) 2569–2574
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Fig. 1. Absorption spectra measured for 20 in dichloromethane at rt. Inset
Fig. 2. Absorption spectra measured for 21 in dichloromethane at rt. Inset
emission spectra measured by excitation at 475, 419, or 308 nm.
emission spectra measured by excitation at 708, 419, 308, and 280 nm.
sized using a rational protocol. The key aspect of these
syntheses is the introduction of iodophenyl units via substi-
tution at boron, enabling the stepwise construction, via
Pd(0) promoted coupling reactions, of multi-chromophoric
dyes. The introduction of the polar propargyl alcohol unit
as a protected ethyne entity enables easy purification of the
dyes. In all cases, the excitonic energy is channeled effi-
ciently from the peripheral modules into the central Bodipy
units. Work is currently in progress in order to study the
kinetics of the photo-induced energy transfer in these large
multi donor–acceptor systems. These series of fluorescent
Bodipys’ dyes nicely complement the library of exciting
dyes, which has been recently reviewed.21,22
absorption. The single absorption at 516 nm (e =
78,500 Mꢁ1 cmꢁ1) is due to the S0?S1 transition of the yel-
low Bodipy dye.19 The S0?S2 transition usually found for
these dyes around 370–390 nm is hidden by the intense
absorption of the perylene moiety. Compared to the
respective mono-, di-, and tri-substituted dyes 9a, 14, and
18a, no significant red-shift of the relative absorption of
the different residues is observed. This indicates that there
is little conjugation between each module which maintains
its electronic individuality, as a consequence of the mole-
cular design. Interestingly, when excited at each wave-
length of the absorption spectra (Fig. 1) including the
one at 516 nm, a single emission is observed at 534 nm.
Only very weak residual emission of the pyrene or perylene
fragments was detected, indicating that the excitonic
energy is very efficiently channeled to the Bodipy dye as
the final energy acceptor. The emission quantum yield of
the Bodipy unit was estimated as 95% using Rhodamine
6G as the fluorescent reference.20 The efficiency of the
energy transfer processes, expressed by the ratio of the
quantum yield measured by excitation of the concerned
module to the quantum yield measured for excitation at
516 nm, lies in the 75–100% range.
Acknowledgments
`
This work was supported by the CNRS and Ministere
de la Recherche. We are also indebted to Professor J.
Harrowfield for his comments on the manuscript and
Alexandre Haefele for the optical measurements and
Figure drawings.
References and notes
Concerning the green series of dyes and the final com-
pound 21, the same trends are observed (Fig. 2). The major
difference arises from the absorption of the bis-isoindole
dye which has a strong absorption at 708 nm (e about
78,000 Mꢁ1 cmꢁ1). As previously observed for the yellow
series of dyes, very efficient energy transfer to the green
acceptor is effective by irradiation in each isolated residues.
Little or no residual emission from the pyrene or perylene
subunits attests to the ideal design. The effective emission
of compound 21 is 46% using cresyl violet as the refer-
ence.20 Also noteworthy are the excitation spectra which
in both cases (excitation measured at 534 nm for 20 and
at 750 nm for 21) perfectly match the absorption spectra,
showing that each module contributes to the emission of
the yellow or green Bodipy acceptors.
1. Blankenship, R. E. Molecular Mechanism of Photosynthesis; Blackwell
Science: Oxford, UK, 2002.
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1996, 118, 11166; (b) Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am.
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2871.
In summary, two series of photo-sensitive Bodipy dyes
bearing an increasing number of chromophoric units (pyr-
ene, perylene, and fluorene) have been successfully synthe-
10. (a) Kollmannsberger, M.; Rurack, K.; Resch-Genger, U.; Daub, J. J.
Phys. Chem. A 1998, 102, 10211; (b) Rurack, K.; Kollmannsberger,
M.; Resch-Genger, U.; Daub, J. J. Am. Chem. Soc. 2000, 122, 968;