C O M M U N I C A T I O N S
characteristics such as a high extinction coefficient could be
accomplished. Compared to sulforhodamine 101 (λabs ) 575 nm,
λflu ) 590 nm, φ ) 0.95, fwhm ) 930 cm-1, ꢀ ) 139 000 M-1
cm-1 in EtOH), which is a commercially available fluorescent
standard dye for laser applications or bioconjugate probes, the
optical properties of KFL-1 are similar or even superior. Therefore,
KFL-1 is a suitable candidate for becoming a fluorescent standard
dye. The photostability of KFL-1 in EtOH solution was measured
by continuous irradiation with a Xe lamp (150 W) at 10 nm slit
width. The result showed that over 98.5% of the initial fluorescence
intensity was retained after 1 h. Moreover, KFL-1 dissolved in EtOH
retained a strong fluorescence emission after a 1-month storage
under ambient conditions with daylight exposure. This remarkable
photostability indicates the usefulness of KFL-1 for various
applications requiring sufficient photostability, which is difficult
to achieve with less photostable dyes.
Owing to the promising results obtained with KFL-1, we aimed
to further extend the emission maximum into the NIR range. Hence,
we focused on one of the basic phenomena of color chemistry:
the wavelength relies on the strength of π-electron donors (D) and
acceptors (A).10 On the basis of this concept, the new Keio Fluors
(KFL-2, A chromophore; KFL-3, D-D chromophore; and KFL-4,
D-A-D chromophore), which have π-electron donors (at R1) or/
and acceptors (at R2), were designed and synthesized. As a
π-electron donor and acceptor, a 4-methoxyphenyl and a trifluo-
romethyl substituent were selected, respectively.
rigidified aza-dipyrromethene NIR dye reported in the literature7b
has been regarded to have the best optical characteristics among
BDP-derivatives (λflu ) 751 nm, ꢀ ) 159 000 M-1 cm-1, φ ) 0.28,
in chloroform). The very recently reported diketopyrrolopyrrole-
based NIR fluorescent dyes are the only ones to show comparable
optical properties (λflu ) 831 nm, ꢀ ) 256 000 M-1 cm-1, φ )
0.53, in chloroform).12
In conclusion, we successfully designed and synthesized a series
of novel fluorescent dyes with particularly useful optical perfor-
mance such as sharper fluorescence spectral bands compared to
quantum dots, vivid absorption and emission colors, high quantum
yields (up to 0.98), high extinction coefficients (up to 288 000 M-1
cm-1), and high photostability. Also, the wavelength is finely
tunable over a wide range including the NIR region, by the
introduction of electron donors and acceptors. These results indicate
that Keio Fluors have the potential to substitute or to complement
existing commercially available fluoresecent dyes and to be used
as new standard dyes in the vis-NIR region.
Acknowledgment. We thank Prof. Naoki Yoshioka for his
support during X-ray single-crystal structure analysis.
Supporting Information Available: Synthetic details, characteriza-
tion data for each compound, measurement details of optical properties,
X-ray single-crystal structure of KFL-4, and X-ray crystallographic file
for KFL-4 (CIF). This material is available free of charge via the
KFL-2, -3, and -4 in chloroform show vivid blue, blue-green,
and green absorption colors and bright red and purplish red
fluorescence emission (Figure 1). All three dyes exhibit fluorescence
emissions in the red or NIR region (620, 683, and 738 nm) with
high fluorescence quantum yields (φ ) 0.98, 0.86, and 0.56, for
KFL-2, -3, and -4 in chloroform, respectively). Overall, the
fluorescence emission of KFL-4 is red-shifted by 221 nm compared
to the TM-BDP chromophore. Therefore, a drastic spectral red shift
and a fine-tuning of the emission wavelength could be achieved
by relatively simple modifications of the KFL chromophore. In
comparison to the CyDye series (one of the most widely used set
of fluorescent dyes), the wavelengths of fluorescence emission of
KFL-1, -3, and -4 are nearly identical to those of Cy3 (570 nm)/
Cy3.5 (596 nm), Cy5 (670 nm)/Cy5.5 (694 nm), and Cy7 (767
nm), respectively. Furthermore, the fluorescence spectral bands of
the Keio Fluors are extremely sharp (fwhm: 574, 516, and 577
cm-1) and almost no spectral overlap between their emission spectra
is observed (Figure 2). Interestingly, these fluorescent spectra are
even much sharper than those of available quantum dots, which
are known to show narrow spectral bands.11 These characteristics
enable easy spectral separation, and therefore, render Keio Fluors
potentially useful for multicolor assays and imaging. The extinction
coefficients of KFL-3 (288 000 M-1 cm-1) and KFL-4 (253 000
M-1 cm-1) were further increased relative to KFL-1. This drastic
increase is quite remarkable, because the increase of extinction
coefficients by introduction of electron-donating moieties is not
usually expected to such an extent, especially not in the case of
aryl-substituted BDP.5a,7c This increase is supposed to be due to
the planarity of the KFL, which has been confirmed by X-ray single-
crystal structure analysis in the case of KFL-4 (refer to Supporting
Information). The photostabilities of KFL-2, -3, and -4 in ethanol
were also measured under the same condition as for KFL-1. The
results showed that 97.3-98.8% of the initial fluorescence intensi-
ties were retained after 1 h.
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As far as we know, NIR fluorescent dyes including other BDP-
based types, which have higher extinction coefficients and quantum
yields than KFL-3 and -4 have not been reported, yet. So far, a
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