Communications
DOI: 10.1002/anie.201004829
NIR Dyes
Selective NIR chromophores: Bis(Pyrrolopyrrole) Cyanines**
Georg M. Fischer, Ewald Daltrozzo, and Andreas Zumbusch*
New applications have recently led to increased interest in
near-infrared (NIR) dyes. Both strong NIR absorption and
fluorescence emission are important phenomena. While
fluorescence is exploited especially for labeling purposes in
microscopy,[1] NIR absorption has a broad range of applica-
tions mainly in material science.[2] Examples of growing fields
of interest are NIR light emitting diodes[3] and the use of NIR
photosensitizers in dye-sensitized solar cells.[4] In the latter,
the aim is to utilize the NIR part of the solar spectrum to
generate photocurrents. Many applications require NIR
absorbers that absorb strongly in the NIR while exhibiting
negligible absorption in the visible spectral range.[2] Dyes with
these properties find use as NIR absorbers in paints and
windows to block off heat, in laser-protecting glasses, as dyes
for laser welding of transparent polymers,[5] and as antiforgery
markers.[6] In all of these examples it is required that the dye
does not influence the spectral properties of the components
in the visible range. Thus, photostable dyes with strong,
narrow-bandwidth NIR absorptions and negligible absorption
in the visible spectral range are needed.
Common strategies to achieve bathochromic shifts of the
absorption maximum of a dye are the extension of the
chromophoric system and the introduction of donor and
acceptor groups into the chromophore.[2,7] Cyanine dyes[8] and
the rylene dyes[9] are the most popular classes showing
significant bathochromic shifts upon extension of their
p system. In cyanine dyes, however, the chemical stability
decreases with increasing extension. By contrast, rylene dyes
are chemically very stable, but, as a result of the relatively
large changes in bond length accompanying the S0!S1
transition, they show rather intense 01 and 02 vibronic
bands (Franck–Condon principle). Thus, their S0!S1 absorp-
tion is extended into the visible range. Other important
classes of NIR dyes showing strong and narrowband NIR
absorption are squarine dyes,[10] bodipys,[11] and some
aza[18]annulene derivatives such as porphyrins,[12] phthalo-
cyanines,[13] and especially naphthalocyanines.[7,14]
lopyrrole (DPP) 1 and heteroaromatic acetonitriles (HAA) 3.
Complexation with either BF2 or BPh2 yields fluorophores
that exhibit narrowband absorption between 650 and 900 nm
and fluorescence emission with high quantum yields. By
isolation of the monoactivated DPP 2, one of the carbonyl
groups of the DPP can be reacted selectively with HAA 3 to
give the half-converted products 4. Further reaction with
another acetonitrile derivative 3 leads to PPCys with an
asymmetric substitution pattern. This stepwise reaction
scheme makes it possible to introduce just one functional
group.[16]
Herein we describe the synthesis of a new class of NIR
chromophores with unprecedented spectroscopic properties.
They have very strong and narrowband NIR absorption while
featuring negligible visible absorption. The absorption coef-
ficients of these dyes in the NIR are among the strongest ever
reported for organic fluorophores and are twice as high as
those of the recently reported rylene dyes.[9] Our previous
approaches to shifting the absorption of the PPCys bath-
ochromically concerned the modification of the heteroaro-
matic endgroup (A) and/or the complexation agent.[15,16] The
selective synthesis of the half-converted DPPs 4 opens up a
strategy to extend the chromophore of the PPCys by reacting
two equivalents of 4 with a bifunctional bridging heteroar-
omatic acetonitrile such as 5 (Scheme 1.) The resulting
condensation products contain two pyrrolopyrrole moieties
and can therefore be considered bis(pyrrolopyrrole) cyanine
dyes.
In the design of an extended chromophore with preferably
high symmetry, the bifunctional bridging moiety must be
symmetric. As a first appropriate example, we synthesized the
dicyanomethylbisbenzothiazole 5 by a condensation of two
equivalents of malononitrile with 2,5-diaminobenzene-1,4-
dithiol in 86% yield. In most PPCy syntheses described so far,
a para-octyloxy-substituted DPP served to increase the
solubility and facilitate both the reaction and purification
procedures.[15,16] As bis(PPCy)s composed of this DPP are
insoluble in all common solvents, 3,4,5-tridodecyloxy-DPP 1
was synthesized to increase the product solubility. The bis(H-
PPCy)s 6 generated from 1 are soluble in solvents such as
chloroform and toluene and were isolated in yields of 53%
(6a), 86% (6b), and 32% (6c) after purification by column
chromatography. The H-chelates 6 can be stiffened by
exchanging H for BF2 (7) or BPh2 (8). The optical data of
the synthesized bis(PPCy)s are summarized in Table 1. The
long-wavelength absorptions (S0!S1) of 6a, 7a, and 8a are
depicted in Figure 1 (for the absorption spectra of all
derivatives see the Supporting Information).
We recently described pyrrolopyrrole cyanine (PPCy)
dyes as a new class of NIR dyes and fluorophores.[15] PPCys
are synthesized by the condensation reaction of diketopyrro-
[*] Dr. G. M. Fischer, Prof. Dr. E. Daltrozzo, Prof. Dr. A. Zumbusch
Fachbereich Chemie
Universitꢀt Konstanz
78457 Konstanz (Germany)
Fax: (+49)7531-883-870
E-mail: andreas.zumbusch@uni-konstanz.de
The bis(H-PPCy)s
6 show intense long-wavelength
[**] Financial support by the SFB 767 is gratefully acknowledged.
absorption with maxima between 827 and 834 nm. For all
derivatives the spectral shape, intensity, and position of the
first electronic transition are very similar. As is the case for
NIR=near infrared.
Supporting information for this article is available on the WWW
1406
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 1406 –1409
Fischer, Georg M.
