Synthesis of furan-substituted aza-BODIPYs having near-infrared emission

Article abstract of DOI:10.1016/j.tetlet.2017.06.054

Development of near-infrared-emissive aza-boron dipyrromethene (aza-BODIPY) derivatives having furanyl groups is reported. From the optical measurements, it was clearly indicated that the emission bands were presented in the longer wavelength region than

Full text of DOI:10.1016/j.tetlet.2017.06.054

Accepted Manuscript
Synthesis of Furan-Substituted Aza-BODIPYs Having Near-Infrared Emission
Honami Yamane, Shunsuke Ohtani, Kazuo Tanaka, Yoshiki Chujo
PII:
S0040-4039(17)30786-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.06.054
TETL 49046
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
6 May 2017
15 June 2017
19 June 2017
Please cite this article as: Yamane, H., Ohtani, S., Tanaka, K., Chujo, Y., Synthesis of Furan-Substituted Aza-
BODIPYs Having Near-Infrared Emission, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.
2017.06.054
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Tetrahedron Letters
Synthesis of Furan-Substituted Aza-BODIPYs Having Near-Infrared Emission
Honami Yamane, Shunsuke Ohtani, Kazuo Tanaka* and Yoshiki Chujo*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Development of near-infrared-emissive aza-boron dipyrromethene (aza-BODIPY) derivatives
having furanyl groups is reported. From the optical measurements, it was clearly indicated that
the emission bands were presented in the longer wavelength region than those of the
conventional aza-BODIPYs. The emission bands with the peaks at 730 nm and 758 nm were
observed from the bis- and tetra-substituted furanyl aza-BODIPYs with similar extents of
emission efficiencies, respectively. According to the computer calculations, it was proposed
that molecular planarity could be enhanced in the case of the furan groups. As a result, band-
gap energy could be lowered comparing to those of the conventional benzene and thiophene-
substituted aza-BODIPYs.
Available online
Keywords:
Boron dipyrromethene
Near-infrared emission
Fluorescence
2009 Elsevier Ltd. All rights reserved.
Because of high permeability on the NIR light through
biological samples and even vital bodies, NIR-emissive dyes can
be used as a versatile signal transducer in a luminescent probe for
bioimaging.1 By the combination of the NIR-emissive dyes with
the stimuli-responsive units toward target reactions or
environmental changes, smart bioprobes for surveying biological
functions at the deep spot inside vital organs can be
constructed.2,3 Moreover, since the NIR light can efficiently
penetrate quartz and aqueous media, NIR-emissive dyes are a
building block for fabricating advanced optical devices such as
telecommunication devices, laser amplifiers and organic light-
emitting diodes.4,5 In these applications in material science and
Scheme 1. Chemical structures of aza-BODIPYs
atom on the meso-position instead of the methylene bridge in
BODIPYs showed emission in the longer wavelength region than
common BODIPYs because of lower-lying LUMO energy
levels.41−43 As a typical example, it was reported that tetraphenyl
aza-BODIPY (TPAB) exhibited emission in the red region with a
high quantum yield (Scheme 1).44 Based on TPAB, the NIR-
emissive polymeric materials with large electron-carrier ability
were also obtained.45,46 More recently, it was reported that
thiophene-substituted aza-BODIPYs including the tetra-
substituted derivative (TTAB) presented emission in far longer
wavelength regions than TPAB.47,48 These results could be
derived from less steric hindrance and electron-donating
properties of the substituted thiophenes. However, as is often the
case with the development of NIR-emissive dyes, thiophene-
substituted aza-BODIPYs also suffered from decreases in
emission efficiencies. Therefore, it is still required to demonstrate
the design strategy for providing red-shifted emission in the NIR
region without critical losses of emission efficiencies.
biotechnology, the NIR-emissive dye presenting
a sharp
luminescent spectrum is especially advantageous. The intense
emission within small wavelength width is valid for improving
the signal to noise ratios in the detection as well as for fabricating
multi-channel telecommunication system. To meet this demand,
conjugated organic molecules are regarded as a suitable platform,
and indeed several NIR-emissive dyes have been reported so
far.6,7
Boron-containing “element-blocks”, which are defined as a
minimum functional unit composed of heteroatoms, are a
promising building block for developing luminescent materials.8
Particularly, in the boron-containing luminescent “element-
blocks”, boron dipyromethenes (BODIPYs) are well known to be
one of beneficial light-emitting dyes because they have various
advantages such as large absorption and luminescent abilities in
the narrow wavelength regions, high photo-stability, and
environment-resistant emissive properties.9−17 In addition, optical
properties of BODIPYs can be easily tuned. By the introduction
of substituents to its core structure, various photochemical
properties such as emission color, stimuli-responsive intensity
changes, phosphorescence characteristics and energy-transfer
efficiencies were drastically varied.18−32 The NIR emissive
materials were also prepared with the BODIPY derivatives.33−40 It
was reported that the class of aza-BODIPYs having the nitrogen
Herein, the new aza-BODIPYs were designed to receive the
NIR emission with high quantum yields (Scheme 1). To improve
molecular planarity, furan was chosen as the substituents instead
of the thiophene groups.49,50 Furan also works as a donor unit and
is a smaller five-membered ring compound than thiophene.
Therefore, it was expected that introducing of furan should be the
effective tactic to realize strong NIR emission without loss of
emission quantum yields. To evaluate validity of this idea, two
kinds of furanyl aza-BODIPYs with various numbers of furan

2
substituents were synthesized, and the emission bands were
found in the longer wavelength regions than those of the
conventional aza-BODIPYs.
Scheme 2. Synthesis of 5a
The synthetic routes of furan-substituted aza-BODIPYs are
outlined in Scheme 2.51 In the previous reports on the syntheses
of TPAB and TTAB, the ligands were directly transformed from
the nitro compounds via coupling reactions.44,47 In the case of
furanyl-aza-BODIPYs, these synthetic methods were not
applicable. Instead, 3a and 3b which were proposed as an
intermediate in the conventional methods were once isolated,
followed by preparing each ligand in the coupling reaction in the
presence of sodium nitrite which was a nitrogen source at the
meso position in the aza-BODIPY skeleton. The products 5a and
aReagents and conditions: (a) CH3NO2, K2CO3, EtOH, reflux, 12 h; (b) i)
NaOMe, MeOH, THF, r.t., 1.5 h; ii) H2SO4, MeOH, 0 °C to r.t., then stirred
for 1.5 h; iii) NH4OAc, AcOH, 100 °C, 1 h; (c) AcOH, Ac2O, NaNO2, r.t., 1 h;
(d) BF3∙Et2O, Et3N, CH2Cl2, r.t., 12 h for 5a, 1 d for 5b.
1
5b had good solubility enough for obtaining H NMR in the
common organic solvents such as chloroform, THF, and ethanol
(ca. 1 mg/mL in chloroform). The structures of all compounds
were confirmed by NMR spectroscopies and ionization mass
measurements (Charts S1−S6). 5a and 5b hardly showed critical
degradation in the air and during optical measurements. From
these results, we concluded that the desired boron complexes
were obtained. TPAB and TTAB were also prepared according to
the previous studies for comparing optical properties under the
same conditions.44,47
Table 1. Optical properties of aza-BODIPYsa
abs
(nm)a
648
741
721
em
(nm)b
669
750
730
FWHM
(cm−1)d
802
584
543

c
ФPL
(104 M−1cm−1)
82,400
TPAB
TTAB
5a
0.12
0.14
0.22
0.08
99,500
133,900
130,700
751
758
644
5b
a1.0×10‒5 M. b1.0×10‒6 M. cDetermined as an absolute value with the
integration sphere method. dCalculated from the data in Figure 1b.
The optical properties of aza-BODIPYs were evaluated, and
the results are presented in Table 1. Figure 1a shows the
absorption spectra of aza-BODIPYs in the dichloromethane
solution. 5a and 5b showed sharp absorption bands with the
peaks at 721 nm and 751 nm with larger molar extinction
coefficients than those of TPAB and TTAB, respectively. The
absorption maxima of 5 and TTAB were obtained in the NIR
region, while the absorption band of TPAB was observed in the
shorter wavelength region. In particular, 5b and TTAB with
tetra-heterole substitutions had the peak in the longer wavelength
region. These data indicate that more effective expansion of
conjugation can occur in these heteroles-substituted aza-
BODIPYs. It is implied that 5-memebered rings could have less
steric hindrance compared with the phenyl ring. Therefore, the
bandgap energy of 5 and TTAB decreased, resulting in red-
shifted absorption bands. In particular, 5b presented the emission
band in the longer wavelength region than TTAB having
thiophene rings, indicating that furan should be favorable for
expansion of π-conjugation though the ligand moiety. From
electrochemical measurements, the reversible processes in the
reduced scan were detected (Figure S1 and Table 2). From the
onsets of the first reduction waves, the energy levels of lowest-
unoccupied molecular orbital (LUMO) were determined, and
similar values were obtained. The band gap energy should be
determined by the energy levels of highest-occupied molecular
orbital (HOMO) of aza-BODIPYs.
Table 2. Results of the absorption spectroscopy and the cyclic
voltammetry
opt
CV
λonset
(nm)a
691
783
759
Eg
Ered
EHOMO
(eV)e
−2.23
−2.48
−2.39
−2.48
ELUMO
(eV)f
−4.02
−4.06
−4.02
−4.04
(eV)b
1.79
1.58
1.63
1.56
(V)c,d
−0.78
−0.74
−0.78
−0.76
TPAB
TTAB
5a
794
5b
aOnset value of the UV–vis spectra measured in dichloromethane (1.0 ×10–5
M). bOptical band gap estimated from the onset wavelength of the UV–vis
spectra in dichloromethane. cCyclic voltammetry was carried out in
dichloromethane with 0.1 M tetrabutylammonium hexafluorophosphate as a
supporting electrolyte and the Fc/Fc+ redox couple as an external standard.
dEred is the onset potential of first reduction wave. Calculated from LUMO
e
and optical band gap (Egopt) of the synthesized compounds, EHOMO / eV =
ELUMO / eV – Egopt / eV. fCalculated from the empirical formula, ELUMO / eV =
– Ered / eV – 4.80.
a
b
1.6
1.2
0.8
0.4
0
TPAB
TTAB
5a
TPAB
TTAB
5a
5b
5b
300
500
700
900
600 650 700 750 800 850
Wavelength (nm)
Figure 1b shows the photoluminescence (PL) spectra of aza-
BODIPYs in dichloromethane solution. 5a presented the NIR
emission with the largest emission efficiency of the aza-
BODIPYs used in this study. 5b showed red-shifted emission
bands compared with those of TTAB as well as TPAB with the
similar extent of emission efficiency to these aza-BODIPYs.
Similarly to the absorption spectra, less steric hindrances of 5-
membered rings should play a significant role in extension of π-
conjugation though the ligand moiety. Typical sharp emission
bands were observed from 5. Even in the spectrum of 5b, the
smaller full width at half maximum (FWHM) of the emission
band was obtained than that of TPAB (Table 1). Because of
intrinsic rigidity of the aza-BODIPY ligand, sharp shapes of
emission bands should be obtained. From the emission decay
measurements, it was shown that all components were within ns
orders, indicating aza-BODIPYs showed fluorescence (Table
Wavelength (nm)
Figure 1. UV–vis–NIR absorption (1×10–5 M) and PL (1×10–6 M)
spectra of aza-BODIPYs in dichloromethane.
S1). Solvent dependencies on optical properties were investigated
in various solvents, and significant changes were hardly obtained
(Tables S2−S5). This fact means that emission should be derived
not from intramolecular charge transfer states but from locally-
excited states in the ligand moiety. Therefore, optical properties
were sensitive in the degree of π-conjugation.
To elucidate optical properties originated from electronic
structures of aza-BODIPYs, computer calculations were
performed. By the density functional theory (DFT), the optimized
geometries of aza-BODIPYs in the ground state were estimated
(Figure 2). Obviously, the 5-membered rings had higher planar

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In conclusion, this manuscript demonstrates that introduction
of furan should be an effective strategy to obtain strong emission
in the NIR region by employing aza-BODIPY. Molecular
planarity can be significantly improved by introducing furan,
leading to bathochromic shifts not only in the absorption band
with a larger extinct coefficient but also in the emission without
critical losses of sharp spectrum shapes and emission
efficiencies. Taking versatility of BODIPY dyes into account, our
findings could be widely applicable in the design of NIR-
emissive materials in a variety of fields.
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This work was partially supported by the Fukuoka Naohiko
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on Element-Blocks (No.2401)” (JSPS KAKENHI Grant Number
JP24102013).

4
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Highlights
-Novel BODIPY derivatives having furan rings
were synthesized.
-The synthesized BODIPYs showed bright
emission in the NIR region.
-We clarified that introduction of furan rings should
enhance molecular planarity.
Graphical Abstract