Thieno[1,3,2]oxazaborinine-containing aza-BODIPYs with near infrared absorption bands: Synthesis, photophysical properties, and device applications

Article abstract of DOI:10.1039/c9nj04612g

As part of an ongoing study of near infrared (NIR) absorbing dyes applicable to optoelectronics, thieno[1,3,2]oxazaborinine-containing aza-BODIPYs were synthesized for the first time. 3,5-Di(thiophene)-substituted N₂O₂-type 1 showed a NIR absorption band centered at 780 nm, with a molar extinction coefficient (?_max) of 5.51 × 10⁴ M^-1 cm^-1 in CH₂Cl₂; however, it was easily hydrolyzed in solution. In comparison, 3-thiophene-containing N₂O-type analogues 2 and 3 had greater stability, and NIR absorption bands at 768 nm (?_max = 8.62 × 10⁴ M^-1 cm^-1) and 779 nm (?_max = 6.32 × 10⁴ M^-1 cm^-1), respectively, in CH₂Cl₂. Notably, these dyes had longer absorption bands than those of all of the structurally constraining aza-BODIPYs reported thus far. CV measurements and theoretical calculations indicated that this shift was the result of a higher-lying HOMO energy level generated by incorporation of thiophene into the aza-BODIPY core. As a potential device application, the 3-loaded film was prepared on indium tin oxide (ITO). The film showed a NIR absorption band at 827 nm, with a wide spectral range, and a λ_onset value of 897 nm; thus, its applicability as a NIR photodetector was evaluated through fabrication of a single-component device. The ITO/3 (85 nm)/Al (100 nm) device produced a photocurrent of 9.57 × 10^-7 A cm^-2 at a bias potential of 1 V upon illumination at 850 nm and a fluence of 130 μW cm^-2. This demonstrated the potential of thieno[1,3,2]oxazaborinine-containing aza-BODIPY materials in optoelectronic applications such as NIR photodetectors.

Full text of DOI:10.1039/c9nj04612g

Volume 44
Number 1
7 January 2020
Pages 1–284
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New Journal of Chemistry
A journal for new directions in chemistry
rsc.li/njc
ISSN 1144-0546
PAPER
Yuji Kubo et al.
Thieno[1,3,2]oxazaborinine-containing aza-BODIPYs with
near infrared absorption bands: synthesis, photophysical
properties, and device applications

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PAPER
Thieno[1,3,2]oxazaborinine-containing
aza-BODIPYs with near infrared absorption bands:
synthesis, photophysical properties, and device
applications†
Cite this: New J. Chem., 2020,
44, 29
Yuji Kubo, *a Takuma Shimada,a Kentaro Maedab and Yuta Hashimotob
As part of an ongoing study of near infrared (NIR) absorbing dyes applicable to optoelectronics,
thieno[1,3,2]oxazaborinine-containing aza-BODIPYs were synthesized for the first time. 3,5-Di(thiophene)-
substituted N2O2-type 1 showed a NIR absorption band centered at 780 nm, with a molar extinction
coefficient (emax) of 5.51 ꢀ 104 Mꢁ1 cmꢁ1 in CH2Cl2; however, it was easily hydrolyzed in solution. In
comparison, 3-thiophene-containing N2O-type analogues 2 and 3 had greater stability, and NIR absorption
bands at 768 nm (emax = 8.62 ꢀ 104 Mꢁ1 cmꢁ1) and 779 nm (emax = 6.32 ꢀ 104 Mꢁ1 cmꢁ1), respectively, in
CH2Cl2. Notably, these dyes had longer absorption bands than those of all of the structurally constraining
aza-BODIPYs reported thus far. CV measurements and theoretical calculations indicated that this shift was
the result of a higher-lying HOMO energy level generated by incorporation of thiophene into the aza-
BODIPY core. As a potential device application, the 3-loaded film was prepared on indium tin oxide (ITO).
The film showed a NIR absorption band at 827 nm, with a wide spectral range, and a lonset value of
897 nm; thus, its applicability as a NIR photodetector was evaluated through fabrication of a single-
component device. The ITO/3 (85 nm)/Al (100 nm) device produced a photocurrent of 9.57 ꢀ 10ꢁ7 A cmꢁ2
at a bias potential of 1 V upon illumination at 850 nm and a fluence of 130 mW cmꢁ2. This demonstrated
the potential of thieno[1,3,2]oxazaborinine-containing aza-BODIPY materials in optoelectronic applications
such as NIR photodetectors.
Received 7th September 2019,
Accepted 29th October 2019
DOI: 10.1039/c9nj04612g
rsc.li/njc
of NIR dyes have been produced through the introduction of
electron donor or acceptor groups at the 1,7 and/or 3,5-positions,21–23
Introduction
Aza-boron-dipyrromethene complexes (aza-BODIPYs)1 have been with a few examples of dyes synthesized from electron-donating
recognized as promising candidates for near infrared (NIR) groups introduced at the 2,6-positions.24 Specifically, substitution of
absorbing dyes.2–5 Replacement of the C atom at the meso thiophene at the 3,5-positions leads to effective spectral perturbation
position of BODIPY6 with a N atom leads to a significant, in the form of a redshift,25–27 which is the result of a high-lying
E100 nm, bathochromic shift in the absorbance and fluorescence HOMO energy level.27 Notably, a conjugated polymer composed of
spectra of the dye. This feature is advantageous for numerous 1,3,5,7-tetra-thiophene-substituted aza-BODIPY showed absorption
applications in optoelectronics,7 photovoltaics,8–10 probes in in the deep near infrared region.28 With this knowledge, our research
biology11 such as in vivo imaging,12 photodynamic therapy13 and has focused on the structural constraint of 3,5-thiophene rings
chemosensing.14,15 Various synthetic strategies for dye core modifi- through intramolecular B–O chelation. Past reports involving cyclic
cation have enabled extensions of p-conjugation to produce fixation of 3,5-aryl moieties through the formation of bonds
[a]-benzene-fused,16,17 [b]-thiophene-fused18 and phenanthrene- with b-pyrrole carbons29–33 or chelation with boron centers34–36
fused19,20 aza-BODIPYs with a NIR region. Additionally, a number have been proposed to produce redshifts in the absorption
bands (Fig. 1). However, to the best of our knowledge, aza-
a Department of Applied Chemistry, Graduate School of Urban Environmental
Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo,
192-0397, Japan. E-mail: yujik@tmu.ac.jp
b Nippon Kayaku Co., Ltd., 31-12, Shimo 3-Chome, Kita-ku, Tokyo, 115-8588, Japan
† Electronic supplementary information (ESI) available: Synthesis and character-
ization data of compounds, additional information, detailed single crystal X-ray
diffraction study and calculation results. CCDC 1948374. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c9nj04612g
BODIPYs substituted with thieno[1,3,2]oxazaborinine are yet to
be reported.
Our continuous efforts to prepare NIR dyes for application in
optoelectronics37 have motivated us to investigate intramolecular
B–O chelation of 3,5-dithiophene-substituted aza-BODIPYs.
Currently, organic NIR dyes applicable to solar cells, photodetectors,
and various electronic devices have been extensively studied.38
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Fig. 1 Synthesis strategy for structural constraint of aza-BODIPYs.
Notably, photodetectors that can translate incident NIR photons Experimental section
to an electrical signal have attracted considerable attention
due to their tremendous applications involving image sensing,
General
NMR spectra were measured on a Bruker Avance 500 spectrometer
(1H: 500 MHz, 13C: 126 MHz, 11B: 160 MHz, and 19F: 471 MHz).
In 1H and 13C NMR measurements, chemical shifts (d) are
reported downfield from the internal standard Me4Si. BF3ꢂOEt2
and C6F6 were used as the external standard for 11B NMR and
internal standard for 19F NMR, respectively. Mass spectrometry
data were obtained by using a JEOL JMS-700 fast atom bombard-
ment (FAB) spectrometer where m-nitrobenzylalcohol was used
as a matrix or a Bruker micrOTOF II-SDT1 spectrometer with
atmospheric pressure chemical ionization (APCI) method. The
absorption and fluorescence were measured using a Shimadzu
UV-3600 UV/vis/NIR spectroscope and a JASCO FP-6500 spectro-
fluorometer, respectively. Elemental analyses were performed on
an Exeter Analytical, Inc. CE-440F elemental analyzer. Ionization
potential data were measured using a Riken photoelectron
spectroscopy instrument, AC-3. An AFM-5400 (Hitachi High-
Technologies Corporations) atomic force microscope (AFM)
was used in dynamic force mode using silicon probes with a
optical communication, night surveillance, etc.39,40 To this end,
development of sensitive photodetectors capable of capturing
very weak NIR signals are necessary. Narrow-band gap
p-conjugated polymers that serve as organic semiconductors
make it possible to achieve high-performance photodetectors
with spectral responses.41–46 Given the advantages of small
molecules, including their high purities and well-defined mole-
cular structures, it is worthwhile to investigate NIR dyes for such
applications. In this work, the synthesis and characterization of
thieno[1,3,2]oxazaborinine-containing aza-BODIPYs 1–3 are
reported (Fig. 2). These new dyes have lmax values in the NIR
range of 768–780 nm in CH2Cl2, with significant redshifts
relative to their 3,5-benzo[1,3,2]oxazaborinine type counterpart,
BO1.34 Among these, 3-thiophene-containing N2O–boron chelate
analogue 3 was isolated as a stable NIR dye and investigated
for possible applicability as an organic photodetector.38 The
3-containing film absorbed NIR light at 827 nm, had a long
spectral width, and a lonset value of 897 nm. As a preliminary
result, the related indium tin oxide (ITO)/3/Al device allowed
photocurrent detection under light irradiation at 850 nm
(130 mW cmꢁ2), which demonstrated for the first time that an
aza-BODIPY-based NIR dye could serve as an organic NIR
photodetector for application in optoelectronics. In this way,
a new family of B–O constrained aza-BODIPYs were introduced
for use as NIR materials.
resonant frequency of 70 kHz and a force constant of 2 N mꢁ1
.
Materials
Reagents used for the synthesis were commercially available
and used as supplied. Dry CH2Cl2 and dry toluene were prepared
according to standard procedures. 3,5-Bis(o-methylthiophene)-
substituted aza-BODIPY 4 was synthesized in line with the reaction
Fig. 2 Chemical structures of thieno[1,3,2]oxazaborinine-containing aza-BODIPYs and related dyes, BO1 and BO2.
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path in Scheme S1 (ESI†). 3,5-Benzo[1,3,2]oxazaborinine type 4.06 (3H, s). 13C NMR (126 MHz, CDCl3): d (ppm) 161.5, 159.9,
counterpart BO1 was synthesized by reference to the method 146.0, 145.8, 145.0, 143.0, 142.7, 139.5, 133.3, 132.4, 132.2,
reported previously.34
129.4, 129.3, 128.7, 128.7, 128.4, 128.4, 128.2, 120.8, 119.1,
115.2, 111.7, 111.6, 110.5, 59.0. 11B NMR (CDCl3, 160 MHz):
d (ppm) 0.674 (d, J = 46.5 Hz). 19F NMR (CDCl3, 471 MHz): d (ppm)
Synthesis
Boron chelated [5-(3-hydroxythiophen-2-yl)-3-phenyl-1H-pyrrol- 26.4 (q, J = 46.5 Hz). APCI-MS: m/z = 536 [M + 1]+. Elemental
2-yl]-[5-(3-hydroxythiophen-2-yl)-3-phenyl-2H-pyrrol-2-ylidene]- analysis: calcd for C29H19BFN3O2S2ꢂ0.3CH2Cl2: C, 62.74; H, 3.52;
amine 1. To a solution of 4 (296 mg, 0.52 mmol) in dry CH2Cl2 N, 7.49. Found: C, 62.79; H, 3.56; N, 7.52.
(75 mL) was added 1 M BBr3 (2.5 mL, 2.5 mmol) under ice
[5-(3-Hydroxythiophen-2-yl)-3-phenyl-1H-pyrrol-2-yl]-[5-(3-meth-
cooling conditions. The mixture was stirred for 1 h under ice oxythiophen-2-yl)-3-phenyl-2H-pyrrol-2-ylidene]amine 6. To a
cooling conditions and was then stirred at room temperature. solution of 4 (0.102 g, 0.19 mmol) in dry CH2Cl2 (25 mL) under
After that the solution was poured into a saturated aqueous a N2 atmosphere was added 1 M BBr3 solution (0.88 mL,
NaHCO3 solution and the organic layer was washed with water 0.88 mmol). The resulting mixture was stirred for 10 min under
and evaporated. The residue was washed with water, CH2Cl2 ice cooling conditions. After quenching by adding saturated
and hexane in a step-by-step manner. 235 mg of the resultant aqueous NaHCO3 solution, the resulting organic layer was
solid was obtained as a dark green solid, being assignable to washed with water. After evaporation, the residue was chromato-
crude aza-dipyrrin 5 (FAB-MS: m/z = 493 [M]+). To a solution of graphed on silica gel (Wakogel C-300) using CH2Cl2 as an eluent to
the solid (235 mg) in dry toluene (50 mL) was added B(OMe)3 give 6 as a red-purple solid as well as 2 as a brown solid with yields
1
(0.53 mL, 4.74 mmol). The mixture was stirred at 100 1C. After of 74% and 9%, respectively. For 6, H NMR (500 MHz, CDCl3):
that the solution was poured into hexane, and the precipitated d (ppm) 10.30 (1H, br. s), 8.03–8.01 (2H, m), 7.93 (2H, d, J = 7.57 Hz),
solid was dissolved in CH2Cl2. The solution was filtrated with 7.68 (1H, d, J = 5.44 Hz), 7.42–7.41 (3H, m), 7.37 (2H, t, J = 7.68 Hz),
silica gel, evaporated, and dispersed in hexane. In this way, 7.29 (1H, t, J = 6.93 Hz), 7.20 (1H, d, J = 5.36 Hz), 7.08 (1H, s),
21.6 mg of 1 was obtained with a 8% yield from 4. 1H NMR 6.96 (1H, d, J = 5.44 Hz), 6.77 (1H, s), 6.72 (1H, d, J = 5.52 Hz,), 4.15
(500 MHz, CDCl3): d (ppm) 8.14 (4H, d, J = 7.25 Hz), 7.49 (2H, d, (3H, s). 13C NMR (126 MHz, CDCl3): d (ppm) 200.0, 156.9, 149.2,
J = 5.12 Hz), 7.47 (4H, t, J = 7.57 Hz), 7.40 (2H, tt, J = 7.33 and 147.7, 146.3, 137.0, 134.4, 134.3, 132.4, 132.0, 131.6, 129.6, 129.0,
1.49 Hz), 7.01 (2H, s), 6.85 (2H, d, J = 5.28 Hz). 13C NMR 128.8, 128.3, 128.2, 127.1, 123.6, 122.2, 121.6, 116.4, 111.5, 107.8,
(126 MHz, CDCl3): d (ppm) 159.8, 146.0, 144.4, 141.0, 132.5, 107.7, 59.6. APCI-MS: m/z = 508 [M + 1]+.
130.7, 129.1, 128.9, 128.7, 120.8, 112.1, 77.9. 11B NMR (CDCl3,
Phenylboron chelated [5-(3-hydroxythiophen-2-yl)-3-phenyl-
160 MHz): d (ppm) ꢁ0.104 (s). APCI-MS: m/z = 502 [M + H]+; 1H-pyrrol-2-yl]-[5-(3-methoxythiophen-2-yl)-3-phenyl-2H-pyrrol-
elemental analysis: calcd for C28H16BN3O2S2ꢂ0.25H2O: C, 66.48; 2-ylidene]amine 3. Aza-dipyrrin 6 (101 mg, 0.20 mmol) and
H, 3.29; N, 8.31, found: C, 66.37; H, 3.22; N, 8.33.
phenylboronic acid (51.3 mg, 0.42 mmol) were dissolved in
Fluoroboron chelated [5-(3-hydroxythiophen-2-yl)-3-phenyl- CH2Cl2 (30 mL). The resulting mixture was refluxed for 2 days.
1H-pyrrol-2-yl]-[5-(3-methoxythiophen-2-yl)-3-phenyl-2H-pyrrol- After silica gel filtration, the solvent was removed in vacuo. The
2-ylidene]amine 2. To a solution of 4 (1.22 g, 2.15 mmol) in dry resultant residue was treated with hexane to give 3 (90.4 mg)
CH2Cl2 (300 mL) under a N2 atmosphere was added 1 M BBr3 with a yield of 76%. 1H NMR (500 MHz, CDCl3): d (ppm)
solution (10.0 mL, 10.0 mmol). The resulting mixture was 8.17–8.15 (2H, m), 8.13–8.11 (2H, m), 7.59 (1H, d, J = 5.20 Hz),
stirred for 10 min under ice cooling conditions. After quenching 7.56 (1H, s), 7.49–7.43 (4H, m), 7.41 (1H, d, J = 5.60 Hz), 7.40–
by adding saturated aqueous NaHCO3 solution, the resulting 7.37 (2H, m), 7.14 (2H, dd, J = 7.56 and J = 1.82 Hz), 7.04 (1H, d,
organic layer was washed with water and was almost evaporated. J = 5.20 Hz), 7.03–6.99 (3H, m), 6.86 (1H, d, J = 5.44 Hz), 6.82
The residue was then dispersed in hexane. The collected solid (1H, s), 3.87(3H, s); 13C NMR (126 MHz, CDCl3): d (ppm) 162.6,
was dissolved in dry toluene (200 mL). To the solution was 160.4, 146.7, 145.9, 145.3, 143.3, 142.2, 139.0, 133.6, 133.1,
added DIPEA (3.8 mL, 22.4 mmol). The solution was stirred for 132.4, 131.4, 129.3, 129.1, 128.7, 128.6, 128.3, 128.2, 128.1,
30 min at room temperature under a N2 atmosphere. After 127.0, 126.8, 120.5, 119.6, 115.3, 112.6, 111.9, 110.4, 59.0. 11B NMR
adding BF3ꢂOEt2 (3.8 mL, 30.79 mmol) dropwise to the solution, (CDCl3, 160 MHz): d (ppm) 2.77 (s). APCI-MS: m/z = 594 [M + H]+.
the resulting mixture was refluxed overnight. The reaction was Elemental analysis: calcd for C35H24BN3O2S2ꢂ1H2O: C, 68.74; H, 4.29;
quenched by saturated aqueous NaHCO3 solution, extracted N, 6.87. Found: C, 68.96; H, 4.12; N, 6.89.
with toluene and chloroform, washed with water, and evaporated.
Determination of fluorescence quantum yield
After washing with MeOH, the residue was chromatographed on
silica gel (Wakogel C-300) using CHCl3 as an eluent and repreci-
pitated with CH2Cl2 and hexane to give 2 (0.672 g, 62%) as a brown
solid. Further purification was thus achieved by reprecipitation
with CH2Cl2 and hexane because a little contamination was
present. 1H NMR (500 MHz, CDCl3): d (ppm) 8.17–8.14 (2H,
m), 8.07–8.05 (2H, m), 7.69 (1H, s), 7.58 (1H, d, J = 5.28 Hz), 7.49
The fluorescence quantum yield (Fexp) was calculated using
eqn (1).3
Ð
1FðlÞdl
AR
A
n2
2
0
Ð
Fexp ¼ FR
ꢀ
ꢀ
ꢀ
(1)
1FRðlÞdl
nR
0
(1H, d, J = 5.52 Hz’), 7.48–7.43 (4H, m), 7.41–7.37 (2H, m), 6.98 where F(l) and FR(l) describe the measured fluorescence inten-
(1H, d, J = 5.60 Hz), 6.97 (1H, d, J = 5.28 Hz’), 6.90 (1H, s), sities of the dye and the reference, respectively, and A and AR
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describe the corresponding absorbances at the excitation wave- Results and discussion
length. The reference used was indocyanine green (FR = 0.23 in
Synthesis
DMSO)47 for 1–3 when excited at 730 nm and zinc phthalo-
The key synthon, 3,5-bis(o-methoxythiophene)-substituted aza-
BODIPY 4, was prepared by a straightforward reaction of 3-methoxy-
thiophene involving a chalcone intermediate (Scheme S1, ESI†).
An initial reaction, involving intramolecular B–O chelation
of 4 with BBr3 in dry CH2Cl2 at room temperature, was
attempted to obtain 1 (Scheme 1). After quenching with saturated
aqueous NaHCO3 solution, work-up of the reaction never pro-
duced the target dye. Instead, various products involving in situ
generated aza-dipyrrin 5 (m/z = 493 [M]+) were isolated. This may
have been due to the instability of 1 as the result of hydrolysis
under the acidic conditions generated by HBr produced during
the reaction. To avoid this, a two-step one-pot reaction involving
demethylation with BBr3, followed by boron coordination using
B(OMe)3, was conducted. Assuming that 1 would be easily
hydrolyzed, a work-up of the reaction was carried out without
washing or column purification. Instead, reprecipitation of the
resultant sample with CH2Cl2 and hexane enabled isolation of
pure 1 from 4 in a yield of 8%. To gain insight into the water-
sensitivity of 1, time-course measurements of absorption spectra
in water-saturated CH2Cl2 were carried out (Fig. S1, ESI†). The
absorption band with a lmax of 780 nm decreased as time passed,
whereas new bands at 664 nm and 730 nm appeared with
cyanine (FR = 0.25 in THF)48 for 4 and BO1 when excited at
650 nm. The refractive indexes are n = 1.407 for THF, n = 1.424
for CH2Cl2 and n = 1.479 for DMSO, respectively.
Theoretical calculation
All geometries of the dyes at the ground state were fully
optimized by means of the B3LYP/6-31G(d,p) level method,
and density functional theory (DFT) calculations at the B3LYP/
6-31G(d,p) level were performed in the Gaussian 09 package49
(Gaussian 09. Rev C.01). The molecular orbitals in Fig. 4 were
visualized using the Gauss view 6.0.16 program.
Device fabrication
The dye films with a thickness of 85 nm were formed on the
surface of the indium tin oxide (ITO) glass substrate using a
resistance heating type vacuum deposition method. Subsequently,
the devices were completed by evaporating a 100 nm thick
aluminum film as an electrode. Current–voltage ( J–V) measurements
of the dye-loaded devices were performed using a Keithley 4200-SCS
source meter under irradiation at 850 nm (130 mW cmꢁ2) by means
of a PVL-3300 (Asahi Spectra Co., Ltd) for the device with 3.
Scheme 1 Synthesis of target dyes (1–3). Reaction conditions: (i) 1 M BBr3 in dry CH2Cl2, and 1 h ice bath followed by room temperature for 14 h;
(ii) B(OMe)3 in dry toluene, 100 1C, and overnight; (iii) 1 M BBr3, and 10 min ice bath; (iv) BF3ꢂOEt2 and DIPEA in dry toluene; and (v) phenylboronic
acid in CH2Cl2.
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isosbestic points at 747 nm, suggesting that the transformation tetrahedral geometry. The N1–B–N3 and O–B–N1 bite angles
of 1 to the corresponding aza-dipyrrin 5 occurred via hydrolysis.50 are 104.06 and 108.191, respectively, and the B–O distance is
Once the water-sensitivity of 1 was determined to hamper the 1.470 Å. These data are similar to those of the 3,5-
efficiency of the work-up of the synthetic reaction, an alteration of bis(benzo[1,3,2]oxazaborinine) aza-BODIPY derivative BO2, pre-
target dyes was made. As a result, demethylation of 4 with BBr3 viously reported by Burgess, et al.34 The thiophene ring was found
was conducted for only 10 min in an ice bath, followed by boron to be tilted with respect to the aza-BODIPY plane by 151 (Fig. 3b).
coordination by BF3ꢂOEt2 in the presence of diisopropylethyla- This value is half as long as that of BO2. This flattening of the
mine (DIPEA) to give monothiophene-involved B–O constrained 2 structure may be advantageous not only to its stability against
following chromatography, with 2 identified analytically. An hydrolysis but also its increase in e relative to 1, as described in
assessment of BBr3-induced demethylation for 10 min in an ice the discussion of photophysical properties in Fig. 4 (vide infra).
bath enabled the detection of 2 and 1-(o-hydroxythiophene)-9-(o- Fig. 3c shows the packing structure of 2, where two dyes are
methoxythiophene)-substituted aza-dipyrrin 6, in 9% and 74% present as an enantiomer pair in the I2/a space group to form a
yields, respectively. The facile isolation of 6 allowed the prepara- dimer in which a fluorine atom at the boron center formed
tion of a corresponding 3 analogue bearing a phenyl on boron by intermolecular hydrogen bonds with phenyl-H at the 1,7 posi-
reacting 6 with phenylboronic acid (76% yield). Unlike 1, tions (C–Hꢂ ꢂ ꢂF distances of 2.622
Å and 2.624 Å) and
thiophene-containing N2O-type aza-BODIPYs 2 and 3 were stable methoxythiophene-H (C–Hꢂ ꢂ ꢂF distance = 2.437 Å) of an adjacent
in solution. Characterization of these dyes was conducted by 1H dye with the same enantiomeric structure.
and 13C NMR spectroscopies, mass spectrometry, and elemental
Photophysical properties
analysis. A single crystal of 2 was obtained by the solvent
evaporation technique of CHCl3/hexane, which had a monoclinic Fig. 4 and Table 1 summarize the series of dyes prepared in this
crystal system in the I2/a space group. The structure was analyzed work, with 3,5-benzo[1,3,2]oxazaborinine type dye, BO1, as a
by X-ray crystallography (CCDC No. 1948374†). The Oak Ridge reference. Target 1 absorbed NIR light at 780 nm, with a molar
thermal ellipsoid plot (ORTEP) diagram is shown in Fig. 3a. extinction coefficient (emax) of 5.51 ꢀ 104 Mꢁ1 cmꢁ1, and a
Boron binds to the two nitrogen atoms of the pyrrole rings, the bathochromic shift of 32 nm relative to 4 as a result of
thiophene-linked oxygen, and a fluorine atom, to form a intramolecular B–O chelation. Comparatively, the absorption
Fig. 3 ORTEP structures of 2, where thermal ellipsoids are drawn at the 50% probably levels. Front view (a), side view (b), and packing structure (c).
Fig. 4 Absorption (a) and fluorescence (b) spectra of dyes in CH2Cl2. lex = 650 nm for 4 and BO1. lex = 730 nm for 1–3.
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Table 1 UV-vis absorption properties and emission properties of the dyes
(1 mM) in CH2Cl2
Table 2 Electrochemical properties of the dyes in CH2Cl2
II
E
/V
Ered(1/2)/V
lonseta/eV
ELUMOb/eV
EHOMOc/eV
red(1/2)
Dye
lmax/nm
e/104 Mꢁ1 cmꢁ1
labs onset/nm
lema/nm
Ff
d
1
2
3
4
—
ꢁ1.01
ꢁ0.99
ꢁ1.08
ꢁ0.97
ꢁ0.91
1.51
1.54
1.51
1.59
1.65
ꢁ3.79
ꢁ3.81
ꢁ3.72
ꢁ3.83
ꢁ3.89
ꢁ5.30
ꢁ5.35
ꢁ5.24
ꢁ5.42
ꢁ5.54
1
2
3
4
780
768
779
748
721
5.51
8.62
6.32
11.8
8.29
822
805
820
780
751
808
791
808
767
740
0.04
0.07
0.02
0.17 BO1
0.25
ꢁ1.69
ꢁ1.78
ꢁ1.71
ꢁ1.65
BO1
a
Optical band gap (Eg) was estimated from the onset wavelength in the
a
b
lex = 650 nm for 4 and BO1. lex = 730 nm for 1–3.
absorption spectra measured in CH2Cl2. ELUMO [eV] = ꢁ(4.8 + Ered(1/2)
[V vs. Fc/Fc+]). HOMO = LUMO ꢁ Eg(eV). The peak was too broad to
c
d
determine the value.
band of 1 was redshifted by 59 nm relative to that of BO1, due to
replacement of the benzo[1,3,2]oxazaborinine unit with its corres-
ponding thiophene counterpart. This structural perturbation led
to a change in the shape of the absorption band, with 1 having a
longer spectral width ranging from 670 to 820 nm. The shoulder
peak observed in the range of 700–750 nm is related to a
vibrational mode S0–S1 transition. The mono(thiophene)-
containing N2O–boron chelated dye, 2, was found to absorb
NIR light at 768 nm (emax = 8.62 ꢀ 104 Mꢁ1 cmꢁ1), suggesting
that a significant bathochromic shift in the absorption band was
attained even by introducing one thiophene B–O chelate in the
aza-BODIPY core. It should be noted that 3, containing a phenyl
unit at the boron, showed a relatively longer absorption band,
centered at 779 nm with a lonset value of 820 nm, being close to
the lmax value of 1. The emax value of the absorbance (6.32 ꢀ
104 Mꢁ1 cmꢁ1) was somewhat larger than that of 1. Unexpectedly,
replacement of the fluorine with a phenyl unit led to a redshift of
11 nm in the absorption band, which contradicted the trend for
diallyl aza-BODIPYs, reported previously.51 This interpretation
was made based on the electrochemical measurements discussed
later (electrochemistry section). We found that the absorption
band of 3 was insensitive to solvent polarity; almost no shifts in
the absorption or fluorescence spectra were observed for various
solvents (Fig. S2a, ESI†). Use of a Lippert–Mataga plot52 allowed
determination of a transition moment (D) of 2.07 Debye (Fig. S2b,
ESI†), suggesting that 3 has a typical p–p* transition.
Fluorescence spectra (Fig. 4b) showed a typical mirror-image
relationship53 for 2 and 4. The fluorescence maxima of the
thiophene B–O chelated dyes were redshifted with reference to
that of BO1 by 51–68 nm. Subsequently, 1 and 3 emitted NIR light
at 808 nm. However, their fluorescence quantum yields were
lower (Ff o 0.05) than that of BO1 (Ff = 0.25), possibly due to
increased nonradiative deactivation as the result of a decreased
energy gap.2 Interestingly, the spectral shape of 3 was consistent
with that of 1, supporting the idea that the photophysical proper-
ties observed are mainly due to the presence of at least one
thieno[1,3,2]oxazaborinine in the dye structure.
As a result of the electron-donor properties of thiophene,
incorporation of thiophene units in the dye cores led to
reduced oxidation and first reduction potentials compared to
those of BO1. Additionally, by varying the substituent groups at the
3,5-positions, oxidation potentials for the thiophene-containing
dyes (1–4) were more sensitive than those of reduction. Further
analysis of the CV and absorption data enabled determination of
the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) energy levels (Table 2).54 It
was found that the HOMO levels were energetically higher in the
following order: 3 4 1 4 2 4 4. Higher-lying HOMO energy levels
led to smaller HOMO–LUMO band gaps (DE), resulting in batho-
chromic shifts in the absorption bands (Table 1). Unexpectedly,
the LUMO energy level of 3 had the highest value; this can be
interpreted as the replacement of the fluorine with an aryl
substitution group on the boron creating instability in the LUMO
level.51 However, the significant high-lying of the corresponding
HOMO energy level resulted in a smaller DE value relative to 2.
This insight rationalized the replacement of the fluorine with a
phenyl substituent on the boron which led to a redshift in the
absorption band (Table 1). It is interesting to note that the
substitution effect on the boron was opposite to that of conven-
tional 1,3,5,7-tetraphenyl aza-BODIPYs.51 It is thought that
steric hindrance between the thieno[1,3,2]oxazaborinine unit
and the phenyl substituent on the boron leads to significant
perturbation of the chromophore system.
Theoretical analysis
Time-dependent density functional theory (TD-DFT) and DFT
analyses at the B3LYP/6-31G(d,p) level49 (Gaussian 09 Rev. C.01)
were conducted to better understand the optical properties
observed. Table 3 summarizes the absorption and theoretical
data of 3,5-dithiophene-containing N2O2-type aza-BODIPY 1,
the related N2O-type dye having a phenyl unit on the boron 3
and 3,5-bis(o-methoxythiophene) aza-BODIPY 4 along with
transition energies (lcalcd) and oscillator strengths ( f ). Based
on the f values listed, the major absorption bands were repre-
sentative of S0–S1 transitions. Corresponding electron-density
Electrochemistry
The electrochemical properties of the dyes were investigated distributions are illustrated in Fig. 5.
+
using cyclic voltammetry (CV). The formal potential of Fc/Fc
(E1/2
The electron-density distributions of both the HOMO and
Fc/Fc+
) was 0.191 V versus Ag/Ag+. Two reversible reduction LUMO levels spread over the aza-BODIPY core, with substituents
waves were observed with half-wave potentials (Er1e/2d) of Eꢁ1.7 at the 3,5 positions being mainly responsible for p–p* transitions,
and ꢁ1.0 V, with an irreversible oxidation wave also detected as supported by the small Stokes shifts of the dyes (Table 1). For
(Fig. S3, ESI†). A summary of the data is presented in Table 2. aza-BODIPY 4, a decrease in the electron-density distribution of
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Table 3 Theoretical data for the dyes
Dye State
l (calcd.)/nm f/a.u.
MO transition assignmenta
1
S0 - S1 643
0.4007 HOMO - LUMO (51%)
0.1868 HOMO–1 - LUMO (47%)
0.0357 HOMO–2 - LUMO (49%)
0.0734 HOMO–3 - LUMO (48%)
S0 - S2 469
S0 - S3 452
S0 - S4 424
3
S0 - S1 654
S0 - S2 515
S0 - S3 477
S0 - S4 458
0.4053 HOMO - LUMO (50%)
0.0259 HOMO–1 - LUMO (49%)
0.0560 HOMO–3 - LUMO (48%)
0.2091 HOMO–4 - LUMO (5%)
HOMO–2 - LUMO (42%)
4
S0 - S1 627
S0 - S2 529
S0 - S3 505
S0 - S4 465
0.4078 HOMO - LUMO (46%)
0.1348 HOMO–1 - LUMO (49%)
0.0768 HOMO–2 - LUMO (46%)
0.1556 HOMO–3 - LUMO (49%)
a
Data given in parentheses are (CI coefficient)2 ꢀ 100%.55
the methoxythiophene units at the 3,5 positions through the
HOMO - LUMO transition was observed (Fig. 5). However,
based on the surface plots of the HOMO and LUMO energy
levels, intramolecular charge transfer within the chromophore
upon photoexcitation was not detected. This was supported by
the transition moment (D) of 4 being 2.04 Debye, calculated
based on the solvent-dependency of its absorption and fluores-
cence properties (Fig. S4, ESI†) being similar to those of 3 (Fig. S2,
ESI†). This suggests that 4 still has p–p* transition characteristics
in its photophysical properties. Conversely, formation of thieno-
[1,3,2]oxazaborinine through intramolecular B–O chelation,
mainly contributed to the expansion of the p–p* transition of
the dye chromophore. In this context, the broad absorption band
at E500 nm may include plural transitions of the HOMO–1 -
LUMO, HOMO–2 - LUMO, and HOMO–3 - LUMO. Based on
these findings, the optical properties of 3 are similar to those of 1,
strongly suggesting that 3 would be the most advantageous for
device application from a structural stability point of view.
Fig. 5 The frontier molecular orbitals of the optimized structures of the
dyes.
Photodetector fabrication
An investigation of 3 was performed to determine its applic-
ability as a NIR photodetector. Prior to device fabrication, the
thermal properties of 3 were investigated by differential scan- under ambient conditions,56 which was lower than that of the
ning calorimetry (DSC) and thermal gravimetric analysis (TGA) ITO electrode (ꢁ5.0 eV). Taking into account the energy level of
(Fig. S5, ESI†); a melting point of 273 1C was clearly observed. the work function of Al (ꢁ4.3 eV), it was expected that the
With consideration to this thermal property, resistance-heating electron and hole injection would be energetically favorable
type vacuum deposition was employed to prepare the dye film enough to provide photodetector functionality. Fig. 6b shows
on an ITO glass substrate (Experimental section). Subsequently, the current–voltage (J–V) measurements of the dye-loaded device
an amorphous film was successfully formed on the glass with a under irradiation at 850 nm (130 mW cmꢁ2). The device with 3
sublimation yield of 40%. An atomic force microscopy (AFM) produced photocurrents of 3.19 ꢀ 10ꢁ7 A cmꢁ2 and 9.57 ꢀ
image of a 1 ꢀ 1 mm area of the dye-coated ITO substrate 10ꢁ7 A cmꢁ2 at bias potentials of 0.5 and 1 V, respectively. On-to-
suggested a flat amorphous film with a root mean square (RMS) off current ratios of 2.65 and 2.08 were calculated at bias
thickness of 0.6 nm (Fig. S6, ESI†). A film on quartz substrate potentials of 0.5 and 1.0 V, respectively, relative to dark current
was used to measure the absorption spectrum of the material, values at the same bias potentials. The increase in the forward
having a near infrared absorption band at 827 nm and a lonset injection of current under NIR light irradiation was ascribed to
value of 897 nm (Fig. 6a). Compared to its absorption spectrum direct dissociation of photoexcited electron–hole pairs, although
in CH2Cl2, a bathochromic shift of 48 nm was observed. the on-to-off current ratio was not very large. To support
The HOMO energy level of the thin film was determined to be this phenomenon, the wavelength-dependent photocurrent den-
ꢁ5.42 eV, based upon photoelectron spectroscopy measurements sities of the ITO/3 (85 nm)/Al (100 nm) device were measured,
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Fig. 6 (a) Absorption spectra of film 3 on a quartz substrate. (b) J–V curves of a 3-loaded device (ITO/3 (85 nm)/Al (100 nm)) measured in the dark (blue)
and under illumination (red) at 850 nm (130 mW cmꢁ2). (c) Photocurrent densities of the 3-loaded device at a 1 V applied potential as a function of time.
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