DOI: 10.1002/chem.201406625
Communication
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Donor–Acceptor Systems
Effective Tuning of Ketocyanine Derivatives through Acceptor
Substitution
Ambata Poe, Andrea Della Pelle, Sean Byrnes, and S. Thayumanavan*[a]
It is reasonable to ascertain that the electronic delocalization
Abstract: A series of ketocyanine derivatives possessing
in ketocyanines will depend on the donor functional group’s
bis(diarylamino)fluorenyl donors and variable acceptors in-
ability to stabilize the positive charge (Figure 2a).[12–14] Howev-
stalled at the bridging carbon atom were synthesized to
er, as the name itself indicates, the acceptor moiety in these
investigate how the electronic structure of the dye can be
dye molecules is often based on ketone functional group. It is
systemically tuned through stabilization of the cyanine-
reasonable to hypothesize that if the acceptor is more capable
like character of the donor by increasing the acceptor
of stabilizing the negative charge, the extent of the cyanine
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strength. Analysis of the H NMR spectra indicates that the
character in the molecule will increase. To test this hypothesis,
“charge-separated” species dominates in these dyes, given
we have designed a series of bis(diarylamino)fluorenone deriv-
that carbons possessing a positive or negative charge in
atives with systematic variations in the acceptor moiety at the
the resonance structures of this state purposefully shift
bridgehead benzlyic position (Figure 2b). We found that when
downfield or upfield, respectively, depending on the
the strength of the acceptor functional group increases, the
strength of the acceptor moiety. In DAA-Fl-PI, the acceptor
ketocyanine-type molecules have a stronger tendency for the
strength and the gain of acceptor aromaticity indicates
symmetrical charge distribution as with cyanines. However,
a predisposition of the separated state, indicated by asym-
when these moieties are coupled using the strong phenyl isox-
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metry in the H NMR spectrum, as well as uneven distribu-
azolone acceptor, the electron density distribution is akin to
tion of the HOMO on the fluorenyl donor.
a donor–acceptor molecule, that is, merocyanines.
Cyanines and merocyanines are
both dye molecules that are
known for their intense absorp-
tion features, but differ in their
delocalization of electron density
in that it is symmetrical in the
former structures, while it is
asymmetrical because of the
donor–acceptor-like features in
the latter (Figure 1a and b).[1–6]A
Figure 1. Generic structures of: a) cyanines, b) merocyanines, and c) ketocyanines.
third class of molecules, ketocya-
nines (Figure 1c), has features
that resemble both the cyanines and the merocyanines.[7–11]
The charge transfer resonance forms of the ketocyanines clear-
ly resemble the symmetrical electronic delocalization of cya-
nines, while the donor–acceptor functional group feature is
merocyanine-like. We were interested in identifying whether it
is possible to predictably bias the ketocyanine-type molecules
to exhibit either of these characteristics. We present our initial
findings here.
Among the molecules shown in Figure 2b, the acceptor
strength increases with the following trend: ketone<malono-
nitrile (CN)<1,3-indandione (In)<1,3-dimethylbarbituric acid
(6O)<1,3-diethyl-2-thiobarbituric acid (6S)<3-phenyl-5-isox-
azolone (PI).[15–20] DAA-Fl was synthesized according to a previ-
ously published procedure.[21] Briefly, 3,6-dibromo-9H-fluoren-9-
one was synthesized from the bromination and subsequent
ring contraction of phenanthrene-9,10-dione. 3,6-Dibromo-9H-
fluoren-9-one was then coupled to diphenylamine under Hart-
wig–Buchwald coupling conditions to obtain DAA-Fl. DAA-Fl-
CN was synthesized by a piperidine-catalyzed Knoevenagel
condensation of DAA-Fl with malononitrile. DAA-Fl-In, DAA-Fl-
6O, DAA-Fl-6S, and DAA-Fl-PI were synthesized through HOAc/
Ac2O-mediated condensation with 1,3-indandione, barbituric
acid, thiobarbituric acid, and 3-phenyl-5-isoxazolone, respec-
tively.
[a] A. Poe,+ A. Della Pelle,+ S. Byrnes, Prof. S. Thayumanavan
Department of Chemistry, University of Massachusetts Amherst
710 North Pleasant St., Amherst, MA 01003 (USA)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406625.
Chem. Eur. J. 2015, 21, 1 – 6
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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