covalently linked multichromophoric system with short
separation between the chromophores and many flexible
linkers and/or side chains compared to simple reference
chromophore modules. It has been recognized for some
time that these values for energy transfer efficiency re-
present an upper limit and in most cases a significant
overestimation.
We have been interested in the synthesis and character-
ization of light-harvesting systems for some time.3 In many
other examples, BODIPY chromophores have been uti-
lized as either energy donors or energy acceptors. In recent
years, brightly emitting BODIPY dyes are in the focus of
renewed interest with an eye toward practical applications
and newderivatization opportunities.4 Oneparticularderi-
vatization which already proved to be highly fruitful is
Knoevenagel condensation of acidic methyl groups found
in methyl-substituted BODIPYs with selected aromatic
aldehydes. Mono- and distyryl-BODIPY compounds were
reported in 20015 and 2006,6 respectively, and in 2009, our
research group reported7 both tri- and tetrastyryl-BODI-
PY derivatives. Each styryl unit increases π-conjugation
further, shifting the main absorption band (S0fS1 transition)
to lower energy (longer wavelength region). By these simple
sequences of reactions, the absorption band of BODIPY dyes
can be easily tuned in the range of 500À900 nm. Relative ease
of this modification alone puts BODIPY derivatives in a very
privileged place among organic chromophores.
In order to investigate light-harvesting properties of
near-IR-emitting tetrastyryl BODIPY dyes, we targeted
modular harvesters 4 and 5 (Figure 1). The synthesis plan
makes use of common intermediate 3. Compound 3 is a
tetrastyryl dye obtained by quadruple Knoevenagel con-
densation of a previously reported 2,6-disubstituted inter-
mediate with 4-propynyloxybenzaldeyhde.
Key intermediate compound 3 carries four clickable
terminal alkyne groups. This design conveniently allows
(3) (a) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2006, 128,
14474–14475. (b) Bozdemir, O. A.; Cakmak, Y.; Sozmen, F.; Ozdemir,
T.; Siemiarczuk, A.; Akkaya, E. U. Chem.;Eur. J. 2010, 16, 6346–6351.
(c) Kolemen, S.; Bozdemir, O. A.; Cakmak, Y.; Barin, G.; Erten-Ela, S.;
Marszalek, M.; Yum, J.-H.; Zakeeruddin, S. M.; Nazeeruddin, M. K.;
€
Gratzel, M.; Akkaya, E. U. Chem. Sci. 2011, 2, 949–954. (d) Atilgan, S.;
Ozdemir, T.; Akkaya, E. U. Org. Lett. 2010, 12, 4792–4795. (e) Bozdemir,
O. A.; Yilmaz, M. D.; Buyukcakir, O.; Siemiarczuk, A.; Tutas, M.;
Akkaya, E. U. New J. Chem. 2010, 34, 151–155. (f) Guliyev, R.; Coskun,
A.; Akkaya, E. U. J. Am. Chem. Soc. 2009, 131, 9007–9013. (g) Barin, G.;
Yilmaz, M. D.; Akkaya, E. U. Tetrahedron Lett. 2009, 50, 1738–1740. (h)
Yilmaz, M. D.; Bozdemir, O. A.; Akkaya, E. U. Org. Lett. 2006, 8, 2871–
2873. (i) Saki, N.; Dinc, T.; Akkaya, E. U. Tetrahedron 2006, 62, 2721–
1740.
Figure 1. Structures of the modules used in the construction of
tetrastyryl-BODIPY-based light harvester and the target com-
pounds 4 and 5. R denotes decyl.
(4) (a) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891–4932. (b)
Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47,
1184–1201. (c) Burghart, A.; Thoresen, L. H.; Chen, J.; Burgess, K.;
Bergstrom, F.; Johansson, L. B.-A. Chem. Commun. 2000, 2203–2204.
modular attachment of essentially any azide-functiona-
lized chromophore. In the synthesis of compound 4, we
reacted intermediates 3 and 1. For compound 5, modular
units 2 and 3 were joined together by click chemistry.
Our light-harvesting design involves attachment of four
shorter wavelength antenna units and one central core.
Absorbance spectra of compound 1, 2, 3, 4, and 5 are
shown in Figure S1 (Supporting Information). As ex-
pected, both light harvesters show two major absorption
bands corresponding to two different chromophores. In
compound 4, absorption bands with peaks at 545 and 730 nm
€
(d) Wan, C.-W.; Burghart, A.; Chen, J.; Bergstrom, F.; Johansson,
L. B.-A.; Wolford, M. F.; Kim, T. G.; Topp, M. R.; Hochstrasser, R. M.;
Burgess, K. Chem.;Eur. J. 2003, 9, 4430–4441. (e) Ulrich, G.; Goze, C.;
Guardigli, M.; Roda, A.; Ziessel, R. Angew. Chem., Int. Ed. 2005, 44,
3694–3698. (f) Harriman, A.; Mallon, L.; Ziessel, R. Chem.;Eur. J.
2008, 14, 11461–11473.
(5) Coskun, A.; Akkaya, E. U. Tetrahedron Lett. 2004, 45, 4947–
4949.
(6) Dost, Z.; Atilgan, S.; Akkaya, E. U. Tetrahedron 2006, 62, 8484–
8488.
(7) Buyukcakir, O.; Bozdemir, O. A.; Kolemen, S.; Erbas, S.;
Akkaya, E. U. Org. Lett. 2009, 11, 4644–4647.
Org. Lett., Vol. 14, No. 14, 2012
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