Synthesis of a new type of dibenzopyrromethene-boron complex with near-infrared absorption property

Article abstract of DOI:10.1016/j.tetlet.2010.01.070

A new type of π-extended 4-bora-3a,4a-diaza-s-indacene (BODIPY dye), bis(isoindole)-derived benzo[1,3,2]oxazaborinine 1, has been synthesized by seven-step procedure from 2-methoxybenzoic acid methyl ester. The benzannulation of the pyrrole ring and the formation of a structurally strained intramolecular B-O ring enable the dye to absorb near-infrared light at ca. 750 nm with a molecular extinction coefficient (ε) of ca 8.3 × 10⁴ M^-1 cm^-1 in THF. The absorption properties are discussed on the basis of DFT calculations. Interestingly, a film of 5,5-dihexyloxy derivative 1b, which was fabricated by a spin-coating procedure on a glass plate, exhibited a dramatic bathochromic shift of absorbance as compared to the solution, with λ_max of 922 nm.

Full text of DOI:10.1016/j.tetlet.2010.01.070

Tetrahedron Letters 51 (2010) 1600–1602
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of a new type of dibenzopyrromethene–boron complex
with near-infrared absorption property
a
b
b
Yuji Kubo a, , Yu Minowa , Takayuki Shoda , Kimiya Takeshita
*
a Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan
b Mitsubishi Chemical Group Science and Technology Research Center, 1000 Kamoshida, Aoba, Yokohama, Kanagawa 227-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new type of
p-extended 4-bora-3a,4a-diaza-s-indacene (BODIPY dye), bis(isoindole)-derived benzo-
Received 18 December 2009
Revised 20 January 2010
Accepted 21 January 2010
Available online 28 January 2010
[1,3,2]oxazaborinine 1, has been synthesized by seven-step procedure from 2-methoxybenzoic acid
methyl ester. The benzannulation of the pyrrole ring and the formation of a structurally strained intra-
molecular B–O ring enable the dye to absorb near-infrared light at ca. 750 nm with a molecular extinction
coefficient (
e
) of ca 8.3 Â 104 MÀ1 cmÀ1 in THF. The absorption properties are discussed on the basis of
DFT calculations. Interestingly, a film of 5,5-dihexyloxy derivative 1b, which was fabricated by a spin-
coating procedure on a glass plate, exhibited a dramatic bathochromic shift of absorbance as compared
to the solution, with kmax of 922 nm.
Ó 2010 Elsevier Ltd. All rights reserved.
Functional dyes capable of absorbing near-infrared light beyond
700 nm1 have attracted increasing attention in recent years, and
such dyes are expected to find applications in optical recording
media, laser filters, thermal writing displays, photodynamic ther-
apy, solar cells, and so on. In particular, the demand for such dyes
for applications in solar cells has increased considerably because
approximately 50% of the radiation intensity of sunlight is in the
near-infrared region from 700 to 2000 nm.2 Although many near-
infrared absorbing dyes such as azo dyes,3 cyanine dyes,4 quinone
dyes,5 and phthalocyanine dyes6 have been developed so far, the
synthetic exploration of related dyes with high molecular extinc-
tion coefficients in the near-infrared region as well as photostabil-
ity remains as an area of concern. It has occurred to us that the use
of BODIPY might be an advantageous approach toward this end be-
cause it serves as a stable dye which has strong absorption bands
in the UV/vis spectrum as well as a sharp fluorescence emission
with high quantum yield.7 Despite the highly synthetic versatility,
the development of near-infrared absorbing BODIPY remains in its
infancy.8
(4) and (5), the combination of which would contribute to the
extension of -conjugation in the BODIPY scaffold, thus allowing
p
us to target the synthesis of 1. In addition, our proposed synthetic
path to obtain the target enables us to introduce an alkoxy group at
the 5-position of the isoindole ring (Chart 1); this is feasible for
tuning the solubility as well as the stability. In this Letter, we re-
port the synthesis and absorption properties of 1, and discuss its
properties on the basis of DFT calculations. Interestingly, as de-
scribed below, a film of 5,5-dihexyloxy derivative 1b was found
to exhibit a significant absorption band at 922 nm.
The synthetic path of 1 is shown in Scheme 1; methyl 2-
methoxybenzoate 2 was allowed to react with hydrazine hydrate13
to yield 3, followed by condensation with 1-(2-hydroxy-4-
methoxyphenyl)ethanone. The obtained product 4 was oxidized
with lead tetraacetate14 and then condensed with ammonia to
afford tetramethoxy-substituted and benzo-fused dipyrrin 6. Sub-
sequently BF2-chelation with 6 using BF3ÁEt2O was carried out to
afford F-BODIPY with a bis(isoindole)methene core 7.15 Then,
demethylation of 7 with BBr3 allows spontaneous cyclization to
In this study, in order to synthesize the desired BODIPY dyes, we
considered several approaches to realize shifts to longer wave-
lengths in the absorption spectra: (1) the introduction of styryl
units at 3,5-positions (see Chart 1);9 (2) the replacement of
methine with N at the meso-position;8b,c (3) rigid system of aryl-
substituted BODIPY;8a,10 (4) benzannulation of pyrrole;8b,11 and
(5) the formation of a benzo[1,3,2]oxazaborinine based on intra-
molecular B,O-chelation.8c,12 Among these approaches, we chose
5
5
RO
OR
1
8
7
N
O
N
O
2
6
B
N
F
N
F
B
3
5
1a : R = Me
1b : R = C6H13
BODIPY core
* Corresponding author. Tel./fax: +81 42 677 3134.
E-mail address: yujik@tmu.ac.jp (Y. Kubo).
Chart 1.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.01.070

Y. Kubo et al. / Tetrahedron Letters 51 (2010) 1600–1602
1601
O
OMe
O
MeO
MeO
OMe
OMe
O
O
OMe
N
N
(iii)
(ii)
(iv)
X
HO
NH N
H
OMe
OMe
OMe
2 : X = OMe
(i)
5
6
4
3 : X = NHNH2
(vii)
MeO
(v)
RO
OR
OMe
OMe
1a : R = Me
(vi)
N
B
N
N
N
O
MeO
B
(viii)
F
F
O
1b : R = C6H13
8 : R = H
7
Scheme 1. Synthesis of 1. Reagents and conditions: (i) N2H4ÁH2O, dry EtOH, reflux, 4 h, (94%); (ii) 1-(2-hydroxy-4-methoxyphenyl)ethanone, dry EtOH, reflux, 44 h, (86%); (iii)
Pb(OAc)4, THF, rt, 2 h, (87%); (iv) NH4OH, AcOH, MeOH, rt, 2 days, (24%); (v) BF3ÁEt2O, NEt3, dry toluene, 80 °C, 1.5 h, (91%); (vi) BBr3, CH2Cl2, 0 °C to rt, 3 days, (66%); (vii) MeI,
K2CO3, dry DMF, 50 °C, 36 h, (59%); (viii) C6H13I, NaH, dry DMF, rt, 2 h, (39%).
afford a constrained benzo[1,3,2]oxazaborinine 8. Because the dye
was found to be relatively unstable in a solution, we then con-
verted it to the corresponding dialkoxy derivatives 1a16 and 1b17
using Williamson synthesis with MeI and C6H13I, respectively.
Anisole-annulated F-BODIPY 7 absorbs visible light at 654 nm
(
e
= 8.34 Â 104 MÀ1 cmÀ1). In this way, we have succeeded in syn-
thesizing near-infrared absorbing BODIPY analogues.
For better understanding of the spectra properties, the molecu-
lar orbitals were analyzed on the basis of DFT calculations. A geom-
etry optimization was carried out using B3LYP hybrid function
with 6-31G (d,p) basis set (Fig. 2). Although the first absorption
band is characterized as a mixture of several configurations, it
can mainly be ascribed to the HOMO–LUMO transition. The ener-
gies of the HOMO and LUMO of scaffold 10 are calculated to be
À5.072 and À2.429 eV, respectively. The anisole-annulation at
the pyrrole ring leads to significant unstabilization of the HOMO
(e
= 8.67 Â 104 MÀ1 cmÀ1) in THF (Fig. 1). When compared to the
corresponding F-BODIPY 912, because compound 10 has not been
reported so far (Chart 2), a significant red-shift by 104 nm was ob-
served.18 Although anisole-annulation of the BODIPY scaffold is
advantageous in that it affords a dye capable of absorbing longer
wavelength light, the absorption band of 7 does not reach the
near-infrared region. However, further modification based on the
intramolecular B–O ring formation led to a significant bathochro-
mic shift in the absorption spectra. As a result, 1a absorbs near-
level, possibly due to the
p-extended electron-donor nature of
the segment. Alternatively, it is interesting to note that intramolec-
ular B–O ring formation results in the stabilization of both HOMO
and LUMO whereby the LUMO level is relatively more stabilized
than the HOMO level. Taken together, the calculation not only sup-
ports our design principle in this study but also contributes to the
synthesis of related dyes based on the BODIPY scaffold.
infrared light having
a
wavelength of 746 nm in THF
(e
= 8.29 Â 104 MÀ1 cmÀ1). In addition, the hexyloxy derivative 1b
exhibits
a
similar absorption property at 748 nm in THF
As the next stage, dye films were fabricated by spin-coating a
THF solution of 1 (5.8 mg mLÀ1) onto a glass plate. Each film is com-
posed by 100% dye. Although film 1a absorbs Vis–NIR light with
kmax of 664 nm, we could obtain a greenish film of 1b. Surprisingly,
the 1b film exhibited an intense absorption band up to 1060 nm
with kmax of 922 nm (Fig. 3). A large bathochromic shift by
174 nm was obtained as compared to the solution (vide supra). This
spectral behavior might be attributable to dye aggregation in the
film. Studies aimed at the structural analysis are thus underway.
In conclusion, we have developed new near-infrared absorbing
BODIPY analogues. It is noteworthy that film 1b exhibits a signifi-
cant absorption band up to 1060 nm; an investigation of the
0.5
0.4
0.3
0.2
0.1
0
7
1a
1b
300
400
800
900
500
600
700
Wavelength / nm
Figure 1. Absorption spectra of 7 (5.0
25 °C.
lM), 1a (5.0
l
M), and 1b (5.0
lM) in THF at
Y
N
F
N
F
MeO
OMe
B
9 : Y = p-Iodophenyl
10
: Y = H
Chart 2.
Figure 2. Diagrams showing HOMO and LUMO levels of 10, 7, and 1a.

1602
Y. Kubo et al. / Tetrahedron Letters 51 (2010) 1600–1602
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Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47, 1184–1201.
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0.4
0.3
0.2
0.1
0
1a
1b
9. Saki, N.; Dinc, T.; Akkaya, E. U. Tetrahedron 2006, 62, 2721–2725.
10. Chen, J.; Burghart, A.; Derecskei-Kovacs, A.; Burgess, K. J. Org. Chem. 2000, 65,
2900–2906.
11. (a) Kang, H. C.; Haugland, R. P. U. S. Patent 5,433,896, July 18, 1995.; (b) Wu, Y.;
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12. Kim, H.; Burghart, A.; Welch, M. B.; Reibenspies, J.; Burgess, K. Chem. Commun.
1999, 1889–1890. Arylation of the meso-position has no spectral effect on the
absorption wavelength.7a
.
500
700
900
1100
1300
1500
13. Khan, K. M.; Rasheed, M.; Ullah, Z.; Hayat, S.; Kaukab, F.; Choudhary, M. I.; ur-
Rahman, A.; Perveen, S. Bioorg. Med. Chem. 2003, 11, 1381–1387.
Wavelength / nm
14. Kotali, A.; Lafazanis, I. S.; Harris, P. A. Tetrahedron Lett. 2007, 48, 7181–7183.
15. Difluoro[5-methoxy-1-[[5-methoxy-3-(2-methoxyphenyl)-2H-isoindole-1-
yl]methylene]-3-(2-methoxyphenyl)-1H-isoindolate-N1,N2]boron 7; 1H NMR
(270 MHz, DMSO-d6) d 3.69–3.75 (12H, m), 6.56 (2H, d, J = 1.7 Hz), 6.99 (1H, t,
J = 7.4 Hz), 7.06 (1H, t, J = 7.4 Hz), 7.15–7.21 (4H, m), 7.37 (1H, d, J = 7.4 Hz),
7.43–7.50 (3H, m), 8.02 (2H, dd, J = 8.8, 1.1 Hz), 8.44 (1H, s); 13C NMR (68 MHz,
DMSO-d6) d 55.25, 55.43, 101.61, 111.41, 115.64, 115.79, 119.22, 119.45,
119.72, 119.87, 120.68, 121.14, 127.00, 127.12, 128.15, 128.27, 130.95, 131.11,
131.18, 131.86, 147.34, 147.60, 157.14, 157.21; FAB MS (m/z) 564 [M+]. Anal.
Calcd for C33H27BF2N2O4Á0.2CH2Cl2: C, 68.18; H, 4.75; N, 4.82; B, 1.86. Found: C,
68.00; H, 4.89; N, 4.92; B, 1.8.
Figure 3. Absorption spectra of films 1a and 1b on a glass plate.
electronic and photophysical properties of the film should be of
interest in the field of materials science. Also, the advantage of
the proposed synthetic path enables us to introduce functional
groups at the 5-position in the isoindole ring. We have been syn-
thesizing derivatives in line with the strategy in our laboratory.
References and notes
16. Boron
isoindole-1-yl]methylene]- 3-(2-hydroxyphenyl)-1H-isoindole 1a; 1H NMR
(270 MHz, DMSO-d6) 3.98 (6H, s), 6.90 (2H, d, J = 8.2 Hz), 7.23 (2H, t,
chelated
[5-methoxy-1-[[5-methoxy-3-(2-hydroxyphenyl)-2H-
1. (a) Fabian, J.; Zahradník, R. Angew. Chem., Int. Ed. Engl. 1989, 28, 677–694;
(b) Infrared Absorbing Dyes; Matuyoka, M., Ed.; Plenum Press: New York, 1990;
(c) Fabian, J.; Nakazumi, H.; Matsuoka, M. Chem. Rev. 1992, 92, 1197–1226;
(d) Near-Infrared Dyes For High Technology Applications (NATO Science
Partnership Sub-Series: 3); Daehne, S., Rensch-Genger, U., Wolfbeis, O. S., Eds.;
Kluwer Academic Publishers: Dortrecht, 1998.
2. Yanagida, M.; Onozawa-komatsuzaki, N.; Kurashige, M.; Sayama, K.; Sugihara,
H. Sol. Energy, Mater. Sol. Cells 2010, 94, 297–302.
3. (a) Li, Y.; Patrick, B. O.; Dolphin, D. J. Org. Chem. 2009, 74, 5237–5243; (b)
Salvador, M. A.; Almeida, P.; Reis, L. V.; Santos, P. F. Dyes Pigments 2009, 82,
118–123.
4. (a) Yagi, S.; Ohta, T.; Akagi, N.; Nakazumi, H. Dyes Pigments 2008, 77, 525–536;
(b) Descalzo, A. B.; Rurack, K. Chem. Eur. J. 2009, 15, 3173–3185.
5. (a) Kubo, Y.; Yoshida, K.; Adachi, M.; Nakamura, S.; Maeda, S. J. Am. Chem. Soc.
1991, 113, 2868–2873; (b) Barthram, A. M.; Cleary, R. L.; Kowallick, R.; Ward,
M. D. Chem. Commun. 1998, 2695–2696.
6. (a) Arslanog˘lu, Y.; Sevim, A. M.; Hamuryudan, E.; Gül, A. Dyes Pigments 2006,
68, 129–132; (b) Modibane, D. K.; Nyokong, T. Polyhedron 2009, 28, 479–484;
(c) Li, H.; Nguyen, N.; Fronczek, F. R.; Vicente, M. G. H. Tetrahedron 2009, 65,
3357–3363.
d
J = 7.6 Hz), 7.30 (2H, dd, J = 8.8, 1.8 Hz), 7.42 (2H, t, J = 7.6 Hz), 7.75 (2H, d,
J = 1.9 Hz), 8.13 (2H, d, J = 8.9 Hz), 8.38 (1H, s), 8.46 (2H, d, J = 7.7 Hz); 13C NMR
(68 MHz, DMSO-d6) d 55.69, 102.90, 119.03, 119.83, 120.44, 120.96, 121.46,
126.17, 127.29, 128.25, 128.45, 131.93, 140.68, 152.45, 158.10; FAB MS (m/z)
496 [M+]. Anal. Calcd for C31H21BN2O4Á0.3H2O: C, 74.21; H, 4.34; N, 5.58; B,
2.15. Found: C, 74.08; H, 4.23; N, 5.53; B, 2.1.
17. Boron
chelated
[5-hexyloxy-1-[[5-hexyloxy-3-(2-hydroxyphenyl)-2H-
isoindole-1-yl]methylene]-3-(2-hydroxyphenyl)-1H-isoindole 1b; 1H NMR
(500 MHz, CDCl3) d 0.930–0.958 (6H, m), 1.37–1.42 (8H, m), 1.52–1.57 (4H,
m), 1.88 (4H, quint, J = 7.2 Hz), 4.08 (2H, dt, J = 8.7, 6.6 Hz), 4.13 (2H, dt, J = 8.7,
6.6 Hz), 6.99 (2H, d, J = 8.0 Hz), 7.15 (2H, dd, J = 8.8, 2.0 Hz), 7.19 (2H, t,
J = 7.6 Hz), 7.37 (2H, app. td, J = 7.8, 1.4 Hz), 7.51 (2H, d, J = 1.8 Hz), 7.53 (1H, s),
7.79 (2H, d, J = 8.9 Hz), 8.15 (2H, dd, J = 7.8, 1.3 Hz); 13C NMR (125.7 MHz,
CDCl3) d 14.23, 22.81, 25.98, 29.51, 31.85, 68.57, 103.69, 109.64, 119.95,
120.17, 120.74, 120.93, 125.87, 127.99, 128.81, 129.52, 131.71, 141.50, 153.56,
157.80; FAB MS (m/z) 636 [M+]. Anal. Calcd for C41H41BN2O4: C, 77.36; H, 6.49;
N, 4.40; B, 1.7. Found: C, 77.29; H, 6.48; N, 4.26; B, 1.8.
18. The absorption spectrum of dye 9 was measured in CHCl3.12