Tetrahedron Letters 50 (2009) 1738–1740
Tetrahedron Letters
Boradiazaindacene (Bodipy)-based building blocks for the construction
of energy transfer cassettes
Gokhan Barin a, M. Deniz Yilmaz b, Engin U. Akkaya a,
*
a UNAM-Institute of Materials Science and Nanotechnology and Department of Chemistry, Bilkent University, Ankara, TR-06800, Turkey
b Department of Chemistry, Middle East Technical University, Ankara, TR-06531, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Energy transfer cassettes composed entirely of boradiazaindacene (Bodipy) units were designed and syn-
thesized to capture photonic energy and convert it to longer wavelength fluorescence emission. The new
energy transfer systems obtained by simple condensation reactions are capable of elaborating efficient
energy transfer from donor Bodipy units to the distyryl-Bodipy acceptor.
Received 9 December 2008
Revised 16 January 2009
Accepted 28 January 2009
Available online 1 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
In the past two decades, much attention has been focused on
the design and synthesis of molecular and supramolecular systems
which can eventually function as light harvesting antennae in arti-
ficial systems for the photochemical conversion of solar energy.1 In
multichromophoric energy transfer cassettes, one of the chro-
mophores is excited by the absorption of a photon of light, and en-
ergy transfer can occur when the donor returns to its ground state
simultaneously with raising of the acceptor to its excited state.2
Excitation energy transfer (EET) can take place essentially via
two pathways: (i) through bond (Dexter mechanism)3 and (ii)
through space (Förster mechanism).4 The former is a non-radiative
energy transfer from a donor to an acceptor, which are connected
by a conjugated bond. The latter, on the other hand, requires strong
overlap between the emission spectrum of the donor and absorp-
tion of the acceptor. Experimentally, the most convenient method
for the preparation of such systems is when the donor and acceptor
components are brought together as a single unit in an energy
transfer cassette.5
aldehyde of 3 was reacted with 3-ethyl-2,4-dimethylpyrrole to
yield cassette 4. Similar condensation reactions were pursued in
order to obtain cassettes 7 and 9 (Scheme 1). The structures of
all new compounds including 4, 7 and 9 were confirmed by 1H
NMR, 13C NMR and HRMS (Supplementary data). The absorption
spectra of the three bichromophoric species (4, 7 and 9) are pre-
sented in Figure 1, which are normalized to 650 nm, the peak
absorption wavelength (kmax) of the distyryl-Bodipy core. As ex-
pected, the absorption spectra of compounds 4, 7 and 9 are com-
posites of donor and acceptor spectra, with the band at 520 nm
arising from the donor chromophore and the band at 650 nm orig-
inating from the acceptor chromophore. At equal concentrations,
the absorbance at 520 nm, the peak absorption wavelength (kmax
)
of the Bodipy donor, increases with increasing number of donor
chromophores.
The emission spectra of compounds 4, 7 and 9 are shown in Fig-
ure 2. Excitation of the donor chromophore at 525 nm results in a
strong emission from the acceptor at 670 nm. The spectral overlap
between the donor emission peak and the absorption peak of the
distyryl-Bodipy chromophore is apparent when Figures 1 and 2
are inspected carefully. For comparison, an overlay of these two
normalized peaks is given in the supporting information. Excita-
tion spectra (Figure 3) provide further evidence for the energy
transfer. The spectra obtained by collecting emission data at
670 nm show two peaks, one for the shorter wavelength chromo-
phore and other for the distyryl chromophore. As the number of
light harvesting Bodipy chromophores around the distyryl-substi-
tuted Bodipy increases, the excitation peak around 525 nm be-
Boradiazaindacenes (Bodipy dyes)6 have been studied exten-
sively as building blocks of energy transfer cassettes,7 artificial
light harvesting complexes8 and as sensitizers for dye sensitized
solar cells.9 In this report, we describe the design, synthesis and
spectroscopic properties of new energy transfer cassettes based
entirely on differentially functionalized boradiazaindacene (Bodi-
py) dyes.
The structures of compounds 4, 7 and 9 are shown in Scheme 1.
In order to access the cassettes synthetically, singly protected
terephthaldehyde 1 was prepared and condensed with 3-ethyl-
2,4-dimethylpyrrole and was then treated with boron trifluoride-
diethyl etherate to give compound 2. Condensation of 2 with
p-tolualdehyde, followed by acidic deprotection of the crude
compound gave distyryl-Bodipy10 derivative 3. The terminal
comes more prominent,
a clear indication of efficient light
harvesting and energy transfer. The energy transfer should be
mostly through space because individual peaks clearly correspond
to those in the model Bodipy and distyryl-Bodipy compounds, and
the 1,7-dimethyl groups on the Bodipy core place the peripheral
Bodipy chromophore orthogonal to the core, thus essentially
breaking the conjugation between the bodipy units. The photo-
* Corresponding author. Tel.: +90 312 290 2450; fax: +90 312 266 4068.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.01.141