NIR-Absorbing Dye Based on BF2-Bridged Azafulvene Dimer as a Strong Electron-Accepting Unit

Article abstract of DOI:10.1021/acs.orglett.8b02056

BF₂-bridged azafulvene dimers designed to be strong electron-accepting units were selectively synthesized using a bulky base. Single-crystal X-ray diffraction analysis revealed that the high electron-accepting ability of this structure stems from the contribution of the ?-conjugation mode of the azafulvene dimer upon formation of B-N coordination bonds. As a result of the low-lying LUMO energy of this electron-accepting unit, the corresponding D-A-D dye exhibits an intense NIR absorption band at 922 nm, which tails up to 1150 nm, while significant absorption bands in the visible region are absent. As a NIR dye this molecule exhibits moreover exceptional photostability and resistance to oxidation by atmospheric oxygen, even in dilute solution.

Full text of DOI:10.1021/acs.orglett.8b02056

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
NIR-Absorbing Dye Based on BF2‑Bridged Azafulvene Dimer as a
Strong Electron-Accepting Unit
Hiroyuki Shimogawa, Yasujiro Murata, and Atsushi Wakamiya*
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
S
* Supporting Information
ABSTRACT: BF2-bridged azafulvene dimers designed to be
strong electron-accepting units were selectively synthesized
using a bulky base. Single-crystal X-ray diffraction analysis
revealed that the high electron-accepting ability of this
structure stems from the contribution of the π-conjugation
mode of the azafulvene dimer upon formation of B−N
coordination bonds. As a result of the low-lying LUMO
energy of this electron-accepting unit, the corresponding D−
A−D dye exhibits an intense NIR absorption band at 922 nm,
which tails up to 1150 nm, while significant absorption bands
in the visible region are absent. As a NIR dye this molecule
exhibits moreover exceptional photostability and resistance to
oxidation by atmospheric oxygen, even in dilute solution.
yes that absorb near-infrared (NIR) light have been
investigated intensively in the context of a variety of
intramolecular B−N bond. Based on this observation, we
designed a 2-fold BF2-bridged azafulvene dimer as a strongly
electron-accepting building block (Figure 1). We synthesized 1
D
applications, including photovoltaics, photodetectors, heat
absorbers, and medical applications.1 NIR dyes can be small
molecules2 and polymers3 with donor−acceptor (D−A) units,
such as rylenes,4 cyanines,5 and the corresponding heteroatom-
bridged analogues.6,7 In many applications, the photostability
and resistance to oxidation of these NIR dyes are very
important.1 NIR dyes usually have narrow HOMO−LUMO
gaps on account of the destabilized HOMO level and are thus
easily oxidized by atmospheric oxygen. A strong electron-
accepting unit having low-lying LUMO levels should therefore
be expected to lead to NIR dyes with improved air- and
photostablility. The combination of an electron-accepting
building block and electron-donating units can generate narrow
HOMO−LUMO gaps while maintaining moderate HOMO
levels.8
Intramolecular coordination bonds are effective at inducing
strong electron-accepting properties. Wakamiya and Yamaguchi
reported that boryl-substituted thienylthiazoles show much
stronger electron-accepting properties than the parent skeleton
due to the formation of the intramolecular B−N coordination
bond.9 The methodology can be applied to a variety of CN-
containing molecular structures,6a,c,10,11 including stable boron-
bridged dyes, for example, BODIPY derivatives with N−B−N
bridges7 as well as boron diketonato complexes with O−B−O
bridges.12 Recently, numerous boron-bridged cyanine analogues
have been developed as new building blocks for NIR
dyes.6a,c,10c,k,12c Examining the structure and properties of
these building blocks, we noticed that the origin of their strong
electron-accepting ability appears to originate from the large
contribution of the azafulvene resonance structure, a contribu-
tion that is further enhanced by the formation of the
Figure 1. Molecular design concept for a strongly electron-accepting
BF2-bridged azafulvene dimer and structures of model compounds 1−3.
and 2 as model compounds and 3 as a model D−A−D dye
composed of the electron-acceptor unit and triarylamines as
electron-donor units. The structural, electrochemical, and
photophysical properties of 1−3 were examined in detail. The
stability of 3 was compared to the commercially available
naphthalocyanine (Napht) NIR dye.5a,6c
Following a typical protocol for the synthesis of BODIPY,7b
dithienyl derivative 1 was synthesized from the borylation of
dipyrrolylethanedione derivative 4 using i-Pr2EtN in THF at
room temperature to afford 1 in only 29% yield together with
Received: July 1, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02056
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
large amounts of a complex mixture of unidentified products
(Scheme 1). To try to improve the yield, various bases were
Scheme 1. Synthesis of BF2-Bridged Azafulvene Dimers 1 and
1-5-mem
Figure 2. (a) ORTEP representation of the molecular structure of 1
(thermal ellipsoids set at 50% probability; only selected atoms are
labeled); (b) possible resonance structures A and B.
bond alternations: C1−C2 [1.377(3) Å], C3−C4 [1.356(3) Å].
Meanwhile the C5−N1 [1.351(3) Å] bonds are much shorter
than the C1−C1* [1.459(4) Å], C2−C3 [1.431(3) Å], C4−C5
[1.439(3) Å], and C2−N1 [1.396(3) Å] bonds. From this we
can conclude that the electronic structure of 1 has a significant
contribution from the π-conjugation mode B having azafulvene
character. The crystal structures of 1-5-mem and 2 also show high
planarity and a significant contribution from the π-conjugation
mode of the azafulvene dimer (Figures S3−S4 and Table S1).
The N1−B1−O1 bond angle around the coordinated
tetrahedral boron center in 1-5-mem [100.0(2)°] is however
much smaller than a typical tetrahedral bond angle (109.5°) as
well as that in 1 [109.79(16)°]. The lower degrees of distortion
in the BF2-bridged 6-membered ring of 1 should endow 1 with
higher stability than 1-5-mem.
In order to gain a deeper understanding of the effects of the
BF2-bridges on the electronic structure in the central building
block, DFTcalculations (B3LYP/6-31G(d)) were conducted on
model compounds 1′ and dipyrrolylethanedione 4′, in which the
thienyl moieties were replaced with hydrogen atoms. Dipyrro-
lylethanedione 4′ can be represented by its tautomeric structures
(Figure 3): One is well described as pyrrole and oxalyl groups in
keto form (4′-keto), while the other exhibits an azafulvene dimer
with enol form (4′-enol). Although 4′-enol is the energetically
unfavorable tautomer, its electron-deficient azafulvene character
lowers the LUMO level by 0.66 eV relative to that of 4′-keto. In
BF2-bridged 1′, the formation of two B−N coordination bonds
not only stabilizes the enol form but also induces a further
decrease of the LUMO level by 0.93 eV, suggesting a substantial
electron-accepting ability of the BF2-bridged azafulvene dimer
structure.
a
b
Reaction conditions: base (4 equiv), BF3·OEt2 (6 equiv). Reaction
conditions: base (46 equiv), BF3·OEt2 (60 equiv). Reduced amounts
resulted in poor yield. A mixture of 1 and 1-5-mem was obtained;
ratio determined by H NMR.
c
1
screened. Typical organic bases, such as DBU, triethylamine, and
pyridine, furnished merely complex mixtures, whereas in the
presence of inorganic bases, such as K2CO3 and K3PO4, no
reactions were observed even in excess amounts. We
subsequently focused on a bulky base, 2,6-di-tert-butylpyridine,
which led to a significant increase of the yield (52%) of 1 having
6-membered-ring moieties. Another compound, the 5-mem-
bered-ring product 1-5-mem, was formed in 26% yield. Both
structures 1 and 1-5-mem were determined by single-crystal X-
ray diffraction analysis (vide infra). A selective synthetic route for
1 or 1-5-mem was found after careful examination of the reaction
conditions (Scheme 1): When the borylation was performed in
dry CH2Cl2 instead of THF, 1-5-mem was obtained as a sole
product in 71% yield. According to 1H NMR and TLC, 1-5-mem
was found to gradually deborylate back to 4 in wet CH2Cl2 or
even in dry THF. Conversely, 1 was found to be stable under
such conditions. DFT calculations at the B3LYP/6-31G(d) level
of theory suggested that 1-5-mem is less stable by +8.2 kcal/mol
than 1 (Figure S1). 1 and 1-5-mem therefore represent the
thermodynamically and kinetically controlled product, respec-
tively. Thus, we expect that in THF solvent the formed 5-mem
will be selectively deborylated to increase the yield of 1, and
indeed, the borylation of4 in refluxingTHF afforded 1as the sole
product in 74% yield. In a similar manner, dimesityl derivative 2
and D−A−D dye 3 with 6-membered rings were synthesized
from the corresponding dipyrrolrylethanedione derivatives in
62% and 83% yields, respectively (the details are shown in the
Supporting Information (SI)).
To experimentally confirm the electronic features of the BF2-
bridged azafulvene dimer building unit, we measured the cyclic
voltammogram for 1−3 in CH2Cl2. The electrochemical data are
summarized in Table 1. Dithienyl derivative 1 showed two
reversible reduction waves at E1/2 = −0.48 and −0.89 V (vs Fc/
Fc+), which appear at much less negative potential than that of 4
(Epc = −1.60 V) (Table 1). The first reduction potential of 1
appears at a substantially more positive potential than those of
typical n-type materials such as fullerene C60 (E1/2 = −0.98 V)13a
or perylene diimide (E1/2 = −0.97 V).13b Dimesityl derivative 2
also showed two reversible reduction waves at E1/2 = −0.50 and
−1.05 V, which is also much less negative compared to that of
Single-crystal X-ray diffraction analysis revealed a highly
planar structure for 1, evident from a torsion angle of 0.0°
between the pyrrole rings, and the fact that the terminal thienyl
rings adopt torsion angles of 2.5° relative to the pyrrole rings
(Figure 2a). For compound 1, two resonance structures are
possible: Dipyrrolylethanedione A with B−O coordination and
azafulvene dimer B with B−N coordination as shown in Figure
2b. In the X-ray structure, the central structure of 1 exhibits large
B
DOI: 10.1021/acs.orglett.8b02056
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 4. (a) UV/vis/NIR absorption spectra of 1−3 in CH2Cl2 and
films of 3. Photographic images of their CH2Cl2 solutions and the films
of 3 are included. (b) Change of the absorption maxima after irradiation
of NIR light in dilute degassed or nondegassed toluene.
Figure 3. Effect of introducing BF2-bridges into dipyrrolylethanedione;
energy diagramfor 1′ and 4′, calculated at the B3LYP/6-31G(d) level of
theory.
The observed intense NIR absorption profile can be
rationalized by DFT and TD-DFT calculations. DFT calcu-
lations (B3LYP/6-31G(d)) on model compound 3′ confirmed
that the HOMO and LUMO are delocalized over the entire π-
conjugated skeleton (Figure S15). TD-DFT calculations on this
optimized structure (CAM-B3LYP/6-31G(d)//B3LYP/6-
31G(d)) suggest that the longest absorption in 3 is attributed
to the HOMO → LUMO transition and exhibits a large
oscillator strength (f = 2.313). No solvatochromism was
observed in the solutions of 3 (λabs = 907 nm in toluene and
902 nm in DMF, Figure S12). This was also confirmed by TD-
DFT calculations using the polarizable continuum model
(PCM) at the CAM-B3LYP/6-31G(d) as shown in Table S3.
In order to examine the photophysical properties in the solid
state, the absorption spectra of spin-coated thin films of 3 were
recorded (Figure 4a). Whereas the poly(methyl methacrylate)
a
Table 1. Electrochemical Properties of 1−4
b
b
d
d
dye
Eox (V)
Ered (V)
HOMO (eV) LUMO (eV)
1
2
3
4
−0.48, − 0.89
−0.50, − 1.05
−0.78, − 0.98
−4.68
−4.66
−4.40
−3.63
+0.21, + 0.54
+1.16
−5.26
−6.00
c
c
−1.60
a
Measured in CH2Cl2 (1 mM); supporting electrolyte: [(n-Bu)4N]-
b
[PF6] (0.1 M); scan rate: 100 mV s−1. Half-wave potentials (vs Fc/
c
d
Fc+). Irreversible wave; Epc or Epa is shown. Estimated by − (Eonset
+
5.1 eV).14
bis(mesityl)-substituted BODIPY derivative (E1/2 = −1.40 V).8a
These results demonstrate the significant electron-accepting
ability of the BF2-bridged azafulvene dimer building unit.
Whereas 1 and 2 show no oxidation waves in the potential
window of CH2Cl2, D−A−D dye 3 with diarylaminothienyl
groups exhibits two reversible oxidation waves (E1/2 = +0.21 and
+0.54 V). In addition, 3 showed two reversible reduction waves
at E1/2 = −0.78 and −0.98 V, whichare slightlyshifted tonegative
from 1 and 2 due to the effect of the electron-donating
diarylaminothienyl groups. Nevertheless, the first reduction
potential of 3 is still less negative compared to those of typical n-
type materials (vide supra).13 The HOMO and LUMO levels
estimated from these potentials are given in Table 1.14 Owing to
the high electron-accepting ability of the BF2-bridged azafulvene
dimer building unit, 3 exhibits significantly narrower HOMO−
LUMO gap (0.86 eV) with a moderate HOMO level (−5.26
eV).
In the UV/vis/NIR absorption spectra in CH2Cl2 (Figure 4a),
the absorption band of 1 and 2 appears at λabs = 625 nm (log ε =
4.76) and 457 nm (log ε = 4.57), respectively. D−A−D dye 3
exhibits an intense NIR absorption at λabs = 922 nm (log ε =
5.06) tailing up to λedge = 1150 nm, reflecting the narrow
HOMO−LUMO gap, while relatively little absorption in the
visible region (400−700 nm) was observed. This is the ideal
photophysical property for a selective NIR absorber.
(PMMA) film doped with 1 wt % of 3 shows a spectrum (λabs
=
914 nm) similar to that in solution, the neat film of 3 shows
additional red-shifted absorption bands (λabs = 1066 and 1174
nm) due to intermolecular interactions.
To assess the effectiveness of the BF2-bridged azafulvene
dimer unit with significant electron-accepting ability as a NIR
dye, the photostability of 3 was investigated under irradiation
with NIR light (Xe lamp, 690−1100 nm, 1.3 × 104 lux) in
degassed or nondegassed toluene (4 × 10−6 M) and compared to
that of a commercially available naphthalocyanine (Napht) NIR
dye5a,6c (Figure 4b). After irradiation for 1 h, the absorbance of
the Napht dye (λabs = 865 nm) diminished to 27% in degassed
toluene and to 6% in nondegassed toluene. In sharp contrast, the
absorbance of 3 remained unchanged (97%), even after
irradiation for 50 h in nondegassed toluene, clearly demonstrat-
ing the high photostability of 3 and its resistance to oxidation by
atmospheric oxygen.
We have designed and developed a 2-fold BF2-bridged
azafulvene dimer and demonstrated that the formation of
intramolecular B−N coordination bonds to dipyrrolylethane-
dione endows this structure with extraordinary electron-
accepting properties. On the basis of this concept, a D−A−D
dye was synthesized which exhibits a narrow HOMO−LUMO
gap while maintaining a moderate HOMO level. This leads to
C
DOI: 10.1021/acs.orglett.8b02056
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02056.
Experimental details, spectra of new compounds, and
Cartesian coordinates of optimized structures (PDF)
Accession Codes
CCDC 1547424−1547426 and 1547497 contain the supple-
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obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wakamiya@scl.kyoto-u.ac.jp.
ORCID
Yasujiro Murata: 0000-0003-0287-0299
Atsushi Wakamiya: 0000-0003-1430-0947
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by JST ALCA (Grant No.
JPMJAL1603), JST COI, and JST ERATO (Grant No.
JPMJER1302) and a Grant-in-Aid for Scientific Research (B)
(JP26288093) and JSPS Core-to-Core Program. H.S. thanks the
JSPS for a Research Fellowship for Young Scientists.
Synchrotron single-crystal X-ray diffraction analyses were
carried out at the SPring-8 beamline BL38B1 with the approval
of JASRI (2015B1074). We thank Dr. K. Suzuki (Hamamatsu
Photonics K. K.) and T. Handa and Prof. Y. Kanemitsu (Kyoto
University) for the photophysical measurements. We appreciate
Prof. L. T. Scott (Boston College and ERATO) for fruitful
discussions.
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DOI: 10.1021/acs.orglett.8b02056
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