Wide-Range Near-Infrared Sensitizing 1 H -Benzo [c, d] indol-2-ylidene-Based Squaraine Dyes for Dye-Sensitized Solar Cells

Article abstract of DOI:10.1021/acs.joc.8b00070

NIR absorbing squaraine dyes SQ1-SQ7 having 1H-benzo[c,d]indol-2-ylidene as a donor moiety were designed for application in DSSCs. Annulation of the benzene ring to an 3H-indolium-based anchor moiety led to a red-shifted and broadened absorption band on TiO₂ film, which were reflected in the improved short-circuit current density of SQ2 (6.22 mA cm^-2) compared to the nonbenzene fused derivative SQ1 (4.39 mA cm^-2). Although the introduction of a butoxy (SQ4: 806 nm) or dialkylamino group (SQ5-SQ7: 815-820 nm) to the 1H-benzo[c,d]indol-2-ylidene-based donor moiety resulted in red-shifted absorption maxima in ethanol compared to the nonsubstituted derivative SQ2 (784 nm), the HOMO energy level of SQ4-SQ7 gave rise to an undesirable approximation to the redox potential of I^-/I₃^-. Thus, the butoxy (SQ4: 0.56) and dialkylamino (SQ5-SQ7: 0.25-0.30) derivatives had relatively lower conversion efficiencies. Since the 2-ethylhexyl derivative SQ3 exhibited red-shifted absorption (λ_max: 796 nm), suitable HOMO and LUMO energy levels, and relatively efficient restriction of charge recombination, this dye achieved the highest conversion efficiency (1.31%), along with a high IPCE response of over 20% over a wide range from 640 to 860 nm and an onset of IPCE at 1000 nm.

Full text of DOI:10.1021/acs.joc.8b00070

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Wide-Range Near-Infrared Sensitizing 1H-Benzo[c,d]indol-2-
ylidene-Based Squaraine Dyes for Dye-Sensitized Solar Cells
YUKI HAISHIMA, Yasuhiro Kubota, Kazuhiro Manseki, Jiye Jin, Yoshiharu
Sawada, Toshiyasu Inuzuka, Kazumasa Funabiki, and Masaki Matsui
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00070 • Publication Date (Web): 26 Mar 2018
Downloaded from http://pubs.acs.org on March 26, 2018
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Wide-Range Near-Infrared Sensitizing
1H-Benzo[c,d]indol-2-ylidene-Based Squaraine Dyes for
Dye-Sensitized Solar Cells
Yuki Haishima,a Yasuhiro Kubota,a,* Kazuhiro Manseki,a Jiye Jin,b Yoshiharu Sawada,c Toshiyasu Inuzuka,c
Kazumasa Funabiki,a and Masaki Matsui,a,*
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aDepartment of Chemistry and Biomolecular Scienece, Faculty of Engineering, Gifu University, 1ꢀ1 Yanagido, Gifu 501ꢀ1193, Japan.
bDepartment of Chemistry, Faculty of Science, Shinshu University, 3ꢀ1ꢀ1ꢀAsahi, Matsumoto, Nagano 390ꢀ8621, Japan.
cDivision of Instrumental Analysis, Life Science Research Center, Gifu University, 1ꢀ1 Yanagido, Gifu 501ꢀ1193, Japan.
Eꢀmail : kubota@gifuꢀu.ac.jp, matsuim@gifuꢀu.ac.jp
NIR absorbing squaraine dyes SQ1–SQ7 having 1Hꢀbenzo[c,d]indolꢀ2ꢀylidene as a donor moiety
were designed for application in DSSCs. Annulation of the benzene ring to an 3Hꢀindoliumꢀbased anchor
moiety led to a redꢀshifted and broadened absorption band on TiO2 film, which were reflected in the
improved shortꢀcircuit current density of SQ2 (6.22 mA cmꢀ2) compared to the nonꢀbenzene fused derivative
SQ1 (4.39 mA cmꢀ2). Although the introduction of a butoxy (SQ4: 806 nm) or dialkylamino group (SQ5–
SQ7: 815–820 nm) to the 1Hꢀbenzo[c,d]indolꢀ2ꢀylideneꢀbased donor moiety resulted in redꢀshifted
absorption maxima in ethanol compared to the nonꢀsubstituted derivative SQ2 (784 nm), the HOMO energy
−
level of SQ4–SQ7 gave rise to an undesirable approximation to the redox potential of I−/I3 . Thus, the butoxy
(SQ4: 0.56) and dialkylamino (SQ5–SQ7: 0.25–0.30) derivatives had relatively lower conversion
efficiencies. Since the 2ꢀethylhexyl derivative SQ3 exhibited redꢀshifted absorption (λmax: 796 nm), suitable
HOMO and LUMO energy levels, and relatively efficient restriction of charge recombination, this dye
achieved the highest conversion efficiency (1.31%), along with a high IPCE response of over 20% over a
wide range from 640 to 860 nm and an onset of IPCE at 1000 nm.
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Introduction
Since first being reported in 1991 by Grätzel and O’Regan, dyeꢀsensitized solar cells (DSSCs) have
attracted a great deal of attention as next generation solar cells due to advantages such as their facile
fabrication process and low production cost.1 The lightꢀtoꢀenergy conversion efficiency (η) of DSSC cells is
strongly affected by the type of sensitizer that is used as an absorber of sunlight. Therefore, the development
of new sensitizers is being actively studied. Although metal organic sensitizers such as ruthenium
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polypyridine complexes2 (η: 10–12 %) and zinc porphyrin complexes3 (SM315:
η = 13.15 %) are known to
have high conversion efficiencies, the high cost of the metals, relatively low molar extinction coefficients (ε)
and difficulty of the syntheses4 are considered to be drawbacks. On the other hand, metalꢀfree organic
sensitizers have many advantages, including their low cost, high
η values, flexibility of molecular design,
and easily tunable absorption properties.5 Several metalꢀfree organic sensitizers such as
tetrathienoaceneꢀbased dyes (TPA-TTART-A: η = 10.1 %),6 indolineꢀbased dyes (YA422: η = 10.65 %),7
and indenoperyleneꢀbased dyes (C275: η = 12.5%)4 have been reported so far. Since these sensitizers show
high incident photonꢀtoꢀcurrent efficiencies (IPCEs) of greater than 90 % in the visible light region (ca. 350–
600 nm), their utilization of sunlight in the visible region is already sufficient. To further improve their
conversion efficiencies, utilization of sunlight in the nearꢀinfrared (NIR) region is required. Coꢀsensitization8
and tandem systems9 using NIR dyes are promising approaches to achieve higher η values. In addition,
transparent NIR dyes that do not absorb light in the visible region can be applied to transparent solar cells.10
Lead halide perovskite solar cells (PSCs) have also received increased attention recently due to their
excellent conversion efficiency (>20 %).11 As one method to enhance the
systems with NIR DSSCs are a useful approach.12 Thus, the development of efficient NIR sensitizers is very
η values of PSCs, hybridized
important.
Several NIR sensitizers, such as methyinecyanine,13 BODIPY,14 squaraine (SQ),15 porphyrin,16
phthalocyanine,17 and other dyes,18 have been reported. However, the IPCE response of these NIR sensitizers
is still insufficient. The development of efficient NIR dyes with absorption over the entire NIR region (ca.
750–1000 nm) is a challenging task. Among the NIR sensitizers, SQ dyes are considered the most promising
candidates because of their intense absorption, high photostability, and their ability to extend absorption to
longer wavelengths.19 In order to achieve the sensitization of SQ dyes in the longer wavelength NIR region,
oligomerization20
and
the
introduction
of
the
ethylenedioxythiophene
(EDOT)21
and
1Hꢀbenzo[c,d]indolꢀ2ꢀylidene moieties22 have been examined. Notably, in the oligomerization approach, the
SQ oligomer TSQa showed a high IPCE response of over 15 % from 750 nm to 900 nm.20d In the case of
EDOTꢀcontaining SQ dyes, JYL-SQ5 exhibited a high IPCE response in the NIR region: ca. 40 % at 750 nm,
ca. 30 % at 800 nm, ca. 15 % at 850 nm, and ca. 5 % at 900 nm.21a Despite their simple structure,
1Hꢀbenzo[c,d]indolꢀ2ꢀylideneꢀbased SQ dyes also show IPCE response in the NIR region. Among the
reported 1Hꢀbenzo[c,d]indolꢀ2ꢀylideneꢀbased SQ dyes, VG5 had the highest IPCE value (ca. 30 % at 750 nm,
ca. 35 % at 800 nm, ca. 15 % at 850 nm, and ca. 1 % at 900 nm).22c
Recently, we also have reported 1Hꢀbenzo[c,d]indolꢀ2ꢀylideneꢀbased SQ dyes with an anchor group
on the 3Hꢀindolium ring as NIR sensitizers.22e Among the SQ dyes, the highest η and IPCE values were
0.58 % and 14 % at 790 nm, respectively. In this paper, in order to improve the conversion efficiency and
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achieve greater redꢀshift of the sensitization region, we report the molecular design and synthesis of novel
1Hꢀbenzo[c,d]indolꢀ2ꢀylideneꢀbased SQ sensitizers. We focused on the following three points: (1) extension
of the πꢀconjugation by annulation of the benzene ring to an 3Hꢀindoliumꢀbased anchor moiety for further
redꢀshifted absorption, (2) enhancement of the intramolecular charge transfer property by introducing an
electron donating group such as a dialkylamino group into the 1Hꢀbenzo[c,d]indolꢀ2ꢀylideneꢀbased donor
moiety to realize further spectral red shift, and (3) introduction of various substituent groups onto the
1Hꢀbenzo[c,d]indolꢀ2ꢀylideneꢀbased donor moiety to adjust the HOMO and LUMO energy levels.
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Results and discussion
Synthesis
1HꢀBenzo[c,d]indolꢀ2ꢀylideneꢀbased SQ dyes SQ1–SQ7 were synthesized as sensitizers for
dyeꢀsensitized solar cells (DSSCs) (Scheme 1). Semisquaric acid 6,22e 3Hꢀindolium salt 7a,23
1Hꢀbenzo[e]indolium salt 7b,24 and semisquarine 824 are known compounds. These compounds were
synthesized according to the previously reported method. First, in order to efficiently introduce various types
of electron donating groups, bromoꢀsubstituted 1Hꢀbenzo[c,d]indolꢀ2ꢀylidene 3 was synthesized from
commercially available 1Hꢀbenzo[c,d]indolꢀ2ꢀone 1 (Scheme 1a). 1HꢀBenzo[c,d]indolꢀ2ꢀone 1 was
brominated using bromine to obtain the bromoꢀsubstituted derivative 2. NꢀAlkylation of 2 was carried out
with hexyl iodide in the presence of sodium hydride to obtain the Nꢀhexylated derivative 3.
Next, 1Hꢀbenzo[c,d]indolium perchlorates 5a–5e were synthesized from 3 (Scheme 1b). These
reactions were performed according to modified literature procedures.25ꢀ30 To introduce a 2ꢀethylhexyl group
directly onto the bromo derivative 3 under mild conditions, we carried out a Negishi cross coupling reaction
using an organozinc reagent and a Pd catalyst. 2ꢀEthylhexyl zinc bromide was prepared by the reaction of
zinc powder and 2ꢀethylhexyl bromide in the presence of iodine,25 and was then titrated according to
Knochel’s method to determine its concentration26 which was 0.42 M. The Negishi cross coupling reaction
of
3 with 2ꢀethylhexyl zinc bromide in the presence of Pd2(dba)3 and SPhos gave the
2ꢀethylhexylꢀsubstituted derivative 4a.27 The reaction of 3 with sodium butoxide in the presence of CuI gave
butoxyꢀsubstituted derivative 4b.28 Dialkylaminoꢀsubstituted derivatives 4c–4e were obtained by the
Buchwald–Hartwig cross coupling reaction of 3 with secondary amines in the presence of Pd2(dba)3, XPhos,
and LiHMDS.29 The reactions of amides 4a–4e with methylmagnesium chloride, followed by dehydration
using perchloric acid, afforded the corresponding 1Hꢀbenzo[c,d]indolium perchlorates 5a–5e.30
Finally, the synthesis of squaraine dyes SQ1–SQ7 was carried out by the reaction of semisquarine
with the indolium salts (Scheme 1c). Squaraine dyes SQ1 and SQ2 were synthesized from the reactions of
semisquarine 6 with 3Hꢀindolium salt 7a and 1Hꢀbenzo[e]indolium salt 7b, respectively. Semisquarine 8 was
hydrolyzed with aqueous sodium hydroxide, followed by acidification by adding hydrochloric acid to
provide the corresponding semisquarine. The obtained semisquaric acid was reacted without further
purification with 1Hꢀbenzo[c,d]indolium perchlorates 5a–5e to yield the corresponding squaraine dyes SQ3–
SQ7.
(a)
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(c)
Scheme 1. Synthesis of 1Hꢀbenzo[c,d]indolꢀ2ꢀylideneꢀbased SQ dyes SQ1–SQ7.
UV/Vis/NIR absorption properties
The UV/Vis/NIR absorption spectra of the SQ dyes in ethanol are shown in Figure 1a. The
absorption maximum (λmax) and molar extinction coefficient (ε) values are listed in Table 1. All the
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synthesized SQ dyes exhibited intense and weak absorption bands at around 800 nm (λmax: 771–820 nm, ε:
94,100–115,800) and 450 nm (λmax: 412–486 nm, ε: 11,000–15,200), respectively. The λmax value of SQ2
(784 nm) was slightly redꢀshifted compared to that of SQ1 (771 nm) due to the extension of the
πꢀconjugation by the fusion of the benzene ring. The introduction of electron donating groups onto the
1Hꢀbenzo[c,d]indolꢀ2ꢀylidene moiety of SQ2 led to a further red shift in the λmax values (796–820 nm). The
λmax values exhibited greater redꢀshift as the electron donating ability of the substituent groups increased:
SQ3 (2ꢀethylhexyl, 796 nm), SQ4 (butoxy, 806 nm), SQ5 (dibutylamino, 818 nm), SQ6 (dioctylamino, 820
nm), SQ7 (di(2ꢀethylhexyl)amino, 816 nm). Additionally, the introduction of dialkylamino groups not only
caused a significant red shift of λmax, but also broadened the absorption bandwidth. As a result, the absorption
edge of the dialkyaminoꢀsubstituted derivatives reached up to 932 nm (SQ7).
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The UV/Vis/NIR absorption spectra of the SQ dyes adsorbed on a 4 ꢁm TiO2 film are shown in
Figure 1b. Compared to the corresponding solution spectra, the absorption bands on the TiO2 films were
slightly redꢀshifted and broadened, probably due to the formation of aggregates and the interaction of the
anchor group with the TiO2 surface.31 Furthermore, the intensities of the shorter absorption bands at around
450 nm were relatively higher in the absorption spectra on the TiO2 films. As observed in the solution
measurements, the absorption bands of SQ dyes with strong electron donating groups tended to be more
redꢀshifted. The absorption edges were extended to 1000 nm on the TiO2 film.
(a)
(b)
SQ1
SQ2
SQ3
SQ4
SQ5
SQ6
SQ7
SQ1
SQ2
SQ3
SQ4
SQ5
SQ6
SQ7
1
1
0.5
0
0.5
ε
0
400
500
600
700
800
900
1000
400
500
600
700
800
900
1000
Wavelength / nm
Wavelength / nm
Figure 1. UV/Vis/NIR absorption spectra of SQ dyes (a) in ethanol (1.0 × 10ꢀ5 M) and (b) on TiO2 (4 ꢁm)
film in the presence of CDCA.
Table 1. Photophysical properties of SQ1-SQ7a
SQ dyes
λmax (ε)a / nm
λonseta / nm
827
λmaxb / nm
412 (11,700), 718 (93,700), 771 (103,000)
419, 720, 785
SQ1
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413 (11,000), 728 (110,000), 784 (103,000)
838
849
873
919
919
932
422, 726, 800
431, 736, 814
447, 745, 821
479, 761, 827
479, 757, 830
486, 756, 823
SQ2
SQ3
SQ4
SQ5
SQ6
SQ7
424 (11,600), 740 (100,000), 796 (116,000)
438 (10,800), 749 (63,800), 806 (94,100)
474 (12,400), 760 (77,900), 818 (102,000)
474 (13,900), 761 (81,600), 820 (110,000)
486 (15,200), 760 (73,600), 815 (97,700)
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aMeasured in ethanol (1.0 × 10−5 M). bMeasured on a 4 ꢁm transparent TiO2 film which was dipped in 0.12 mM of SQ dye and 6 mM of CDCA in a
solution of acetonitrile and chloroform (v : v = 1 : 1) for 30 min at room temperature.
Theoretical calculation
To obtain deeper insight into the absorption properties, density functional theory (DFT)
calculations were performed using the Gaussian 09 package.32 The geometries were optimized at the
DFT/B3LYP level using the 3ꢀ21G basis set. Singleꢀpoint energy calculations of the optimized structures
were then performed using the B3LYP/6ꢀ31G(d,p) method (Figure 2). Timeꢀdependent DFT (TDDFT)
calculations were also performed using the B3LYP/6ꢀ31G(d) method. The calculated λmax values, the main
orbital transition, the oscillator strength ( f ) values, and the experimentally obtained λmax and
ε values are
shown in Table 2. The calculated λmax values were in good agreement with the experimental values.
For all the SQ dyes, the S0 → S1 transitions were mainly attributed to HOMO to LUMO transitions,
and the λmax and f values were estimated to fall between 653–714 nm and 1.288–1.511, respectively. The
HOMO and LUMO orbitals of SQ1–SQ7 were delocalized from the 1Hꢀbenzo[c,d]indolꢀ2ꢀylidene donor
moiety to a particular part of the indolium acceptor moiety. Therefore, the observed first absorption bands of
SQ1–SQ7 (λmax: 771–820 nm) were attributed to the π → π* transition. In the LUMO orbitals of SQ2–SQ7,
no electron density was observed around the carboxyl group. This indicates that the electron transfer from
the excited S1 state of SQ2–SQ7 by light absorption to the TiO2 conduction band was unfavorable.
Although the λmax values of the S0 → S2 transitions of SQ1–SQ7 were estimated to fall between
558–572 nm, these transitions are forbidden (f ≤ 0.001). Thus, the observed absorption peaks at around 718–
761 nm in ethanol were attributed to the vibrational peaks (S0,0 → S1,1 transitions) or absorption by
aggregated dyes. In order to reveal the effect of aggregates on the absorption properties, absorption spectra
of SQ3 were measured in ethanol–water mixtures of various ratios (Figure S1). When the water content of
the mixture reached 40 %, the absorption intensity at around 690 nm increased obviously. The increase in the
absorption intensity became more pronounced with increasing water content. The formation of aggregates
was confirmed by the Tyndall phenomenon (Figure S2). Therefore, the absorption peak at approximately
690 nm was assigned to the aggregates of SQ3. Therefore, the absorption peak observed at 740 nm for SQ3
in ethanol was probably a vibrational peak. These results suggested that the vibrational absorption peak and
the absorption peak of the aggregates both appeared at approximately 720 nm.
The λmax and f values of the S0 → S3 transitions were estimated to fall between 439–528 nm and
0.011–0.059, respectively. Therefore, the observed weak absorption peaks at 412–486 nm for SQ1–SQ7 in
ethanol were attributed to the S0 → S3 transitions, which were mainly assigned to the HOMO to LUMO+1
transitions. The LUMO+1 orbitals of SQ2–SQ7 were located on the 1Hꢀbenzo[e]indolium acceptor moiety.
Thus, the HOMO to LUMO+1 transitions of SQ2–SQ7 were attributed to the intramolecular charge transfer
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(ICT) transition. The calculated results indicated that electron transfer from the S3 states of SQ2–SQ7 that
originated from the absorption at 412–486 nm to the TiO2 was favorable.
We then focused on the HOMO, LUMO, and LUMO+1 energy levels. DFT calculations suggested
that the fusion of benzene to the 3Hꢀindoliumꢀbased anchor moiety slightly reduced the LUMO energy level
(SQ1: ꢀ2.76 eV, SQ2: ꢀ2.77 eV) and that the HOMO–LUMO gap became slightly smaller (SQ1: 1.84 eV,
SQ2: 1.83 eV). The introduction of substituent groups to the 1Hꢀbenzo[c,d]indolꢀ2ꢀylidene donor moiety led
to an increase in both the HOMO and LUMO energy levels. The changes to the HOMO (SQ2: ꢀ4.60 eV,
SQ3: ꢀ4.54 eV, SQ4: ꢀ4.44 eV, SQ2: ꢀ4.39 eV) and LUMO (SQ2: ꢀ2.77 eV, SQ3: ꢀ2.73 eV, SQ4: ꢀ2.64 eV,
SQ2: ꢀ2.63 eV) energy levels became more pronounced as the electronꢀdonating properties of the substituent
groups increased. The calculated HOMO and LUMO energy levels were in good agreement with the
experimentally obtained EHOMO and ELUMO values, which will be described below (Table 3).
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Figure 2. Molecular orbital energy diagram and isodensity surface plots of the HOMO−1, HOMO, LUMO,
and LUMO+1 of SQ1−SQ7 calculated using the B3LYP/6ꢀ31G(d,p)//B3LYP/3ꢀ21G method.
Table 2. Calculated absorption maxima (λmax), oscillator strengths (f), and main orbital transitions and
experimentally observed λmax and molar extinction coefficient (ε) values
SQ dyes Transition λmaxa / nm
Main orbital transition
f
λmaxb / nm
εb
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718
771
93,700
S0 → S1
S0 → S2
S0 → S3
653
572
439
HOMO → LUMO (0.72)
1.288
SQ1
SQ2
SQ3
SQ4
SQ5
SQ6
SQ7
103,000
c
c
HOMOꢀ1 → LUMO (0.70)
HOMO → LUMO+1 (0.51)
HOMOꢀ2 → LUMO (ꢀ0.47)
0.000
0.059
─
─
412
11,700
728
784
110,000
103,000
9
S0 → S1
S0 → S2
S0 → S3
663
565
503
HOMO → LUMO (0.72)
1.342
0.000
0.012
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c
c
HOMOꢀ1 → LUMO (0.70)
HOMO → LUMO+1 (0.69)
HOMOꢀ2 → LUMO (0.14)
─
─
413
11,000
740
796
100,000
116,000
S0 → S1
S0 → S2
S0 → S3
675
563
508
HOMO → LUMO (0.71)
1.439
0.000
0.011
c
c
HOMOꢀ1 → LUMO (0.70)
HOMO → LUMO+1 (0.69)
HOMOꢀ2 → LUMO (0.14)
─
─
424
11,600
749
806
63,800
94,100
S0 → S1
S0 → S2
S0 → S3
678
558
515
HOMO → LUMO (0.71)
1.440
0.001
0.012
c
c
HOMOꢀ1 → LUMO (0.70)
HOMO → LUMO+1 (0.69)
HOMOꢀ2 → LUMO (ꢀ0.13)
─
─
438
10,800
760
818
77,900
S0 → S1
S0 → S2
S0 → S3
701
559
524
HOMO → LUMO (0.71)
1.481
0.001
0.011
102,000
c
c
HOMOꢀ1 → LUMO (0.70)
HOMO → LUMO+1 (0.67)
HOMOꢀ2 → LUMO (ꢀ0.21)
─
─
474
12,400
761
820
81,600
S0 → S1
S0 → S2
S0 → S3
703
559
524
HOMO → LUMO (0.71)
1.494
0.001
0.011
110,000
c
c
HOMOꢀ1 → LUMO (0.70)
HOMO → LUMO+1 (0.67)
HOMOꢀ2 → LUMO (ꢀ0.21)
─
─
474
13,900
760
815
73,600
97,700
S0 → S1
S0 → S2
S0 → S3
714
559
528
HOMO → LUMO (0.71)
1.511
0.000
0.011
c
c
HOMOꢀ1 → LUMO (0.70)
HOMO → LUMO+1 (0.63)
HOMOꢀ2 → LUMO (ꢀ0.30)
─
─
486
15,200
aCalculated in vacuum. bMeasured in ethanol. cNot observed.
Electrochemical properties
Cyclic voltammetry measurements were performed in DMF solution using 0.1
M
tetrabutylammonium perchlorate as a supporting electrode and ferrocene (Fc) as an external standard (Figure
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3). The first halfꢀwave potential for oxidation (E1/2 (ox)) of SQ1, corresponding to the HOMO energy level
(EHOMO), was observed at 0.07 V vs. Fc/Fc+. In DMF, the E1/2 (vs. Fc/Fc+) value can be converted to the E1/2
(vs. NHE) value as follows: E1/2 (vs. NHE) = E1/2 (vs. Fc/Fc+) + 0.72 V.33 Thus, the EHOMO value of SQ1 was
estimated to be 0.79 V vs NHE. The energy gap between the HOMO and LUMO energy levels (E0ꢀ0) of SQ1
obtained from the onset of the UV/Vis/NIR absorption spectrum was 1.50 eV. Thus, the LUMO energy level
(ELUMO) of SQ1 estimated by subtracting the E0ꢀ0 value from the EHOMO value vs. NHE was ꢀ0.71 V. The
EHOMO and ELUMO values of the other SQ dyes were determined in a similar manner (Figure 4).
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The ELUMO values of all the synthesized SQ dyes (ꢀ0.91 to ꢀ0.71 V vs. NHE) were more negative
than the conduction band level of TiO2 (ꢀ0.5 V vs. NHE), which suggested that the electron transfer from the
excited SQ dyes to the conduction band of TiO2 was thermodynamically allowed.34 However, in order for an
oxidized dye to undergo thermodynamically allowed regeneration by iodide, the EHOMO value of the dye must
be more positive than the redox potential of I−/I3 in the electrolyte (0.4 V vs. NHE).34 Since all the
−
synthesized SQ dyes had positively shifted EHOMO values (0.52 to 0.83 V vs. NHE), regeneration of these SQ
dyes was possible.
SQ1
SQ2
SQ3
SQ4
SQ5
SQ6
SQ7
0.4
0.2
0
ꢀ0.2
ꢀ0.4
ꢀ0.6
Potential / V vs. Fc / Fc+
Figure 3. Cyclic voltamogram of SQ1−SQ7 in DMF.
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ꢀ1.0
ꢀ0.5
ꢀ0.91
ꢀ0.86
ꢀ0.84
ꢀ0.83
ꢀ0.78
ꢀ0.80
ꢀ0.71
TiO2
ꢀ0.5
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SQ4
SQ7
SQ3
SQ6
SQ5
SQ1
SQ2
0.0
0.5
1.0
ꢀ
Iꢀ/I3
0.44
0.4
0.50
0.51
0.56
0.68
0.68
0.79
Figure 4. Energy level diagram for SQ1−SQ7.
Table 3. Electrochemical properties of SQ1–SQ7
SQ dyes
SQ1
E
1/2 (ox) (vs. Fc/Fc+)a / V
EHOMO (vs. NHE)b / V
E0ꢀ0c / eV
ELUMO (vs. NHE)d / V
0.07
ꢀ0.04
ꢀ0.04
ꢀ0.16
ꢀ0.21
ꢀ0.28
ꢀ0.22
0.79
0.68
0.68
0.56
0.51
0.44
0.50
1.50
1.48
1.46
1.42
1.35
1.35
1.33
ꢀ0.71
ꢀ0.80
ꢀ0.78
ꢀ0.86
ꢀ0.84
ꢀ0.91
ꢀ0.83
SQ2
SQ3
SQ4
SQ5
SQ6
SQ7
aThe first halfꢀwave potential for oxidation was measured in DMF solution using 0.1 M tetrabutylammonium perchlorate as a supporting electrode
b
c
d
and ferrocene (Fc) as an external standard. E1/2 (vs. NHE) = E1/2 (vs. Fc/Fc+) + 0.72 V in DMF. E0−0 (eV) = 1240/λonset. ELUMO was calculated as
EHOMO − E0−0
.
Photovoltaic performance
Photovoltaic measurements were carried out on the DSSC cells, which consisted of the TiO2
photoelectrode (transparent layer: 8 ꢁm, scattering layer: 4 ꢁm) with squaraine dyes SQ1–SQ7 absorbed as a
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sensitizer and chenodeoxycholic acid (CDCA, 50 equiv.) as a coꢀadsorbent in acetonitrile/tꢀbutyl alcohol (v :
v = 1 : 1), a Pt counter electrode, and an iodineꢀbased electrolyte. The IPCE spectra and IꢀV curves are
shown in Figure 5, and the performance parameters are summarized in Table 4. For comparison,
UV/Vis/NIR absorption and photovoltaic measurements in the absence of CDCA were also conducted
(Figure S3 and S4). In the UV/Vis/NIR absorption spectra of without CDCA, the absorption intensities at
around 700 nm which correspond to dye aggregates were relatively increased and the spectra were broadened
compared to those of with CDCA. In all synthesized SQ dyes, the shortꢀcircuit current density (JSC),
openꢀcircuit voltage (Voc) and power conversion efficiency (η) values were significantly improved by
addition of CDCA due to the inhibition of unfavorable dye aggregation (Table 4).
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The maximum IPCE value of SQ2 (17% at 800 nm) was slightly redꢀshifted compared to that of
SQ1 (17% at 790 nm) (Figure 5a). Moreover, SQ2 showed a higher IPCE value than SQ1 over the entire
wavelength range from 400 nm to 950 nm. Accordingly, the JSC value of SQ2 (6.22 mA cmꢀ2) was larger than
that of SQ1 (4.39 mA cmꢀ2). The slightly higher IPCE value of SQ2 in the longer wavelength region (600–
950 nm) was consistent with the broadened absorption spectrum of SQ2 on TiO2 (Figure 1a). Although both
SQ1 and SQ2 showed similar absorption spectra on TiO2, SQ2 had a significantly higher IPCE value in the
shorter wavelength region (400–550 nm). Based on the calculation results discussed above, the absorption
peak at around 450 nm was attributed to the HOMO to LUMO+1 transition. The LUMO+1 orbital of SQ1
was delocalized over the whole molecule, while that of SQ2 was located around the anchor group (Figure 2).
This implied the electron transfer from the LUMO+1 to the conduction band of TiO2 was more effective in
SQ2 than in SQ1. The effective electron transfer properties of SQ2 were probably the reason for the higher
IPCE value in the shorter wavelength region. The Voc values of SQ1 (0.300 V) and SQ2 (0.303 V) were
nearly equal. The fill factor (FF) value of SQ2 (0.47) was smaller than that of SQ1 (0.56). Consequently, the
η value of SQ2 (0.89) was larger than that of SQ1 (0.74), due to the improved IPCE response.
In the 2ꢀethylhexyl derivative SQ3, a remarkable enhancement of the IPCE spectrum was observed.
The IPCE value of SQ3 was higher than that of SQ2 over the whole wavelength range, and the IPCE spectral
edge of SQ3 was significantly redꢀshifted, reaching 1000 nm. The IPCE response of SQ3 was as follows:
24 % at 680–840 nm, 22 % at 850 nm, 10 % at 900 nm, and 2 % at 950 nm. Thus, the JSC value of SQ3 (9.20
mA cmꢀ2) was further improved. On the other hand, introduction of a butoxy group caused a decrease in the
IPCE value over a wide wavelength range from 400 to 850 nm, and an increase over a narrow wavelength
range from 850 to 1000 nm. The overall decrease in the IPCE response led to the relatively low JSC value for
SQ4 (3.81 mA cmꢀ2). The dialkylaminoꢀsubstituted derivatives showed even lower JSC values (SQ5: 1.77 mA
cmꢀ2, SQ6: 1.45 mA cmꢀ2, SQ7: 2.05 mA cmꢀ2), which explains the substantially inferior IPCE responses.
Among the dialkylaminoꢀsubstituted derivatives, the di(2ꢀethylhexyl)amino derivative SQ7 had a
significantly higher IPCE value in the range from 650 nm to 1000 nm. The difference in the longer
wavelength region (850–950 nm), which was probably associated with the broadened absorption peak of
SQ7 on TiO2 in this region, was especially remarkable.
In addition to efficient electron transfer, appropriate ELUMO and EHOMO values are also desirable.
Roughly speaking, the EHOMO and ELUMO values must be more positive than 0.7 vs. NHE and more negative
than ꢀ0.7 V vs. NHE, respectively.15e, 35 In the 1Hꢀbenzo[e]indolium derivatives SQ2–SQ7, the EHOMO values
increased in the order SQ6 (0.44 V) < SQ7 (0.50 V) < SQ5 (0.51 V) < SQ4 (0.56 V) < SQ2 (0.68 V), SQ3
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(0.68 V) (Figure 4). This was highly consistent with the trend of the increase in the JSC values: SQ6 (1.45
mA cmꢀ2) < SQ5 (1.77 mA cmꢀ2) < SQ7 (2.05 mA cmꢀ2) < SQ4 (3.81 mA cmꢀ2) < SQ2 (6.22 mA cmꢀ2) < SQ3
(9.20 mA cmꢀ2). The EHOMO values of the butoxyꢀ (SQ4: 0.56 V vs NHE) and dialkylaminoꢀsubstituted
derivatives (SQ5–SQ7: 0.44–0.51 V vs. NHE) were obviously close to the I−/I3− redox potential (0.40 V vs.
NHE). Therefore, the lower JSC value of SQ4–SQ7 could be ascribed to ineffective regeneration of the
oxidized dye by iodide. Due to its suitable ELUMO and EHOMO values, the 2ꢀethylhexyl derivative SQ3 showed
the highest JSC value.
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In order to understand the effect of the dye loading of SQ1−SQ7, the dyes were desorbed by
immersing the TiO2 photoelectrode in the mixture of 2M HCl and ethanol with the volume ratio of 1 : 9. The
UV/Vis/NIR absorption spectra of the desorbed solutions and the dye loading were shown in Figure S6 and
Table S1, respectively. In the case of using CDCA (50 equiv.), the dye loading of SQ1−SQ7 were in the
range of 2.80 × 10ꢀ8 to 5.80 × 10ꢀ8 mol/cm2. SQ2 (4.39 × 10ꢀ8 mol/cm2) exhibited a higher dye loading than
SQ1 (2.80 × 10ꢀ8 mol/cm2). This also contributes to the improved Jsc value by the annulation of a benzene
ring to the anchor moiety. Introduction of 2ꢀethylhexyl group lead to a slightly increase of the dye loading
(SQ3: 5.41 × 10ꢀ8 mol/cm2). This indicates that introduction of the bulky substituent to a separate position
from the anchor moiety is associated with the formation of wellꢀpacked layers rather than decrease of the dye
loading. The dye loading of butoxy (SQ4: 3.16 × 10ꢀ8 mol/cm2) and dialkylamino (SQ5–SQ7: 3.69 × 10ꢀ8 to
5.80 × 10ꢀ8 mol/cm2) derivatives has no significant differences compared to that of SQ3. Thus, the lower Jsc
values of SQ4–SQ7 are not due to the difference of the dye loading but due to their higher HOMO energy
levels.
The Voc value increased compared to the nonꢀsubstituted derivative SQ2 (0.303 V) with the
introduction of the 2ꢀethylhexyl group (SQ3: 0.315 V), while it decreased with the butoxy (SQ4: 0.266 V)
and dialkylamino (SQ5: 0.276 V, SQ6: 0.289 V, SQ7: 0.279 V) groups. On the other hand, no remarkable
trend was observed for the FF values. Accordingly, the η values followed the order: SQ6 (0.25) < SQ5 (0.29)
< SQ7 (0.30) < SQ4 (0.56) < SQ2 (0.89) < SQ3 (1.31).
Table 4. Photovoltaic performance of DSSCs based on SQ1–SQ7a
SQ dye
CDCA (equiv.)
Jsc / mA cmꢀ2
Voc / V
0.258
0.300
0.248
0.300
0.269
0.315
0.197
0.266
0.195
0.276
0.226
0.289
FF
η / %
0.29
0.74
0.36
0.89
0.69
1.31
0.16
0.56
0.09
0.29
0.07
0.25
0
50
0
1.84
0.60
0.56
0.56
0.47
0.50
0.45
0.48
0.56
0.45
0.59
0.55
0.60
SQ1
4.39
2.56
SQ2
SQ3
SQ4
SQ5
SQ6
50
0
6.22
5.11
50
0
9.20
1.72
50
0
3.81
1.00
50
0
1.77
0.58
50
1.45
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1.53
2.05
0.186
0.279
0.45
0.52
0.13
0.30
SQ7
50
aConditions: electrode: 8 + 4 ꢁm (transparent + scattering) layer of TiO2, electrolyte: 0.05 M I2, 0.5 M DMPImI, and 0.5 M LiI in
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acetonitrile/chloroform (v : v = 1 : 1), dye absorption: 0.12 mM of SQ dye in acetonitrile/tꢀbutanol (v : v = 1 : 1), 6.0 mM CDCA (or without CDCA),
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b
dipping time: 15 h, measurement: active area of 0.20 cm2 using a mask under AM1.5G (100 mW cmꢀ2). Dye absorption: 0.12 mM of SQ4 in THF,
CDCA 6.0 mM (or without CDCA).
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Figure 5. (a) IPCE spectra and (b) IꢀV curve of DSSCs based on SQ1–SQ7 with CDCA.
Electrochemical impedance spectroscopy (EIS)
2ꢀEthylhexylꢀsubstituted derivative SQ3 showed the highest performance (Jsc = 9.2 mA cmꢀ2, Voc =
0.315 V, FF = 0.45, η = 1.31 %). In order to investigate the effects on the Voc value, electrochemical
impedance spectroscopy (EIS) was performed on cells based on SQ1, SQ2, and SQ3 with CDCA (50 equiv.)
under an illumination of AM 1.5 G (100 mW cmꢀ2). The measurement results are summarized in Table 5, and
the Nyquist plots are shown in Figure 6a. The large semicircles in the midfrequency range of the Nyquist
plots were assigned to charge transfer at the TiO2/dye/electrolyte interface.36 The charge recombination
resistance (Rrec) at the TiO2 surface can be estimated by fitting Nyquist plots. The Rrec of SQ3 (60 ꢂ) was
obviously larger than that of SQ1 (49 ꢂ) and SQ2 (48 ꢂ), which indicates a slower charge recombination
rate.15e, 37 The Bode phase plots are shown in Figure 6b. The electron lifetime (τ) was calculated from the
peak frequency of the lowꢀfrequency peak in the Bode phase plots using the formula τ = 1/2πf.15e, 15h, 38 The τ
value of SQ3 (15.9 ms) was longer than those of SQ1 (12.6 ms) and SQ2 (12.6 ms). These results strongly
supported the higher Voc value of SQ3 (0.315 V) compared to those of SQ1 (0.300 V) and SQ2 (0.303 V),
which suggested that the introduction of the 2ꢀethylhexyl group could prevent charge recombination from the
TiO2 surface to triiodide and/or the oxidized dye.
Table 5. EIS measurement data for DSSCs based on SQ1, SQ2, and SQ3 with CDCA (50 equiv.)
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SQ dye
SQ1
Rrec / ꢂ
49
f / Hz
12.6
12.6
10.0
τ / ms
12.6
12.6
15.9
48
SQ2
60
SQ3
8
9
(a) 40
40
30
20
10
0
(b)
CE/electrolyte TiO2/Dye/electrolyte
SQ1
SQ2
SQ3
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R
RCE
SQ1
SQ2
SQ3
Rｓ
30
CCE
C
20
10
0
1
10
100
Fequency / Hz
1000
10000
0
20
40
60
80
100
Z' / ꢁ
Figure 6. EIS characteristics of DSSCs based on SQ1–SQ3 with CDCA (50 equiv.) under an illumination of
AM1.5G (100 mW cmꢀ2). (a) Nyquist plot and (b) Bode phase plot.
Conclusions
The novel squaraine dyes SQ1–SQ7, in which a 1Hꢀbenzo[c,d]indolꢀ2ꢀylidene moiety bearing
various substituent groups acts as a donor moiety, were synthesized, and their application to DSSCs was
investigated. Squaraine dyes SQ1–SQ7 exhibited a strong first absorption band (λmax: 771–820 nm, ε:
94,100–116,000) corresponding to the HOMO–LUMO transition, and a relatively weak second absorption
band (λmax: 412–486 nm, ε: 10,800–15,200) corresponding to the HOMO–LUMO+1 transition in ethanol.
When we investigated the use of different anchor moieties, we found that the 1Hꢀbenzo[e]indolium
derivative SQ2 (784 nm) showed a more redꢀshifted λmax value compared to the 3Hꢀindolium derivative SQ1
(771 nm) because of the extension of πꢀconjugation by the annulation of the benzene ring. Due to its
broadened absorption band and suitable intramolecular HOMO to LUMO+1 charge transfer transition
properties, SQ2 (6.22 mA cmꢀ2) exhibited a higher Jsc value than SQ1 (4.39 mA cmꢀ2). When we focused on
the substituent group on the 1Hꢀbenzo[c,d]indolꢀ2ꢀylidene moiety of SQ2, we found that the λmax value
showed a bathochromic shift that became more pronounced as the electronꢀdonating properties of the
substituent group increased: 2ꢀethylhexyl (SQ3: 796 nm) < butoxy (SQ4: 806 nm) < dibutylamino (SQ5:
818 nm) < dioctylamino (SQ6: 820 nm) < di(2ꢀethylhexyl)amino (SQ7: 815 nm). Cyclic voltammetry
measurements and DFT calculations indicated that the introduction of butoxy and dialkylamino groups gave
rise to a significant increase in the HOMO energy levels (SQ4–SQ7: 0.44 to 0.56 V vs. NHE, ꢀ4.44 to ꢀ4.37
eV) compared to the nonꢀsubstituted derivative SQ2 (0.68 V vs. NHE, ꢀ4.60 eV), which resulted in the
undesirable approximation of the HOMO levels to the redox potential of I−/I3 (0.4 V vs. NHE). Due to the
−
ineffective regeneration of the oxidized dye by iodide, SQ4–SQ7 had lower Jsc values (1.45–3.81 mA cmꢀ2).
Introduction of the 2ꢀethylhexyl group led to desirable properties such as a spectral red shift, suitable HOMO
and LUMO energy levels, and prevention of charge recombination. Therefore, SQ3 showed the highest Jsc
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(9.20 mA cmꢀ2), Voc (0.315), and η (1.31) values among the synthesized SQ dyes.
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Experimental section
6
7
General information
NMR spectra were recorded on ECX400P or ECA–600 spectrometers. Chemical shifts are referred to TMS
(1H and 13C) as internal standards. Infrared (IR) spectra were determined on a Simazu IR–Affinity
spectrometer. UV–vis spectra were taken on a Hitachi U4100 spectrophotometer. Mass spectra were
recorded on a JEOL JMS–700 spectrometer. Highꢀresolution mass spectroscopy (HRMS) was obtained using
a doubleꢀfocusing magnetic sector mass spectrometer (JEOL JMS–700). Melting points were measured on a
Yanagimoto MP–S2 micro–melting–point apparatus. Analytical thinꢀlayer chromatography (TLC) was
performed on preꢀcoated plates (Merck, silica gel 60 F254). Silica gel (Wakogel C–200) was used for
column chromatography.
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Synthesis of 2: Benz[c,d]indolꢀ2(1H)ꢀone (1) (5.01 g, 29.1 mmol) was dissolved in chloroform (120 mL).
Bromine (7.10 g, 44.4 mmol) was added to the solution and stirred at room temperature for 60 h. A saturated
sodium thiosulfate aqueous solution (100 mL) was poured into the reaction mixture. The resulting precipitate
was filtered off and washed with water to give 2 as a yellow solid (6.89 g, 94%). 2: mp 219.0−220.0°C; 1H
NMR (400 MHz, DMSOꢀd6)
8.10 (d, J = 7.8 Hz, 1H), 8.24 (d, J = 7.8 Hz, 1H), 10.9 (s, 1H); 13C NMR (100 MHz, DMSOꢀd6)
δ
6.91 (d, J = 7.8 Hz, 1H), 7.76 (d, J = 7.8 Hz, 1H), 7.93 (t, J = 7.8 Hz, 1H),
107.5,
δ
111.9, 124.8, 126.6, 127.3, 128.3, 129.5, 130.4, 131.8, 138.1, 168.2; IR (KBr) 3194, 1705, 1636, 1485, 1462,
1396, 1369, 1258, 1219 cmꢀ1; HRMS (FAB) m/z calcd for C11H7BrNO [M + H]+ 247.9711, found 247.9715.
Synthesis of 3: Sodium hydride (60wt% in oil, 1.09 g, 27.2 mmol) was added to a solution of 1 (2.25 g, 9.08
mmol) in anhydrous DMF (40 mL) at 0°C and stirred at 0°C for 5 min. To the solution, hexyl iodide (2.89 g,
13.6 mmol) was added and stirred at room temperature for 2 h. Water was then poured into the mixture, and
the mixture was extracted with ethyl acetate. The organic extract was dried over anhydrous Na2SO4 and
concentrated in vacuo. The resulting residue was purified by column chromatography (silica gel, Rf = 0.30,
1
dichloromethane : hexane = 5 : 2) to give 3 as a yellow solid (2.23 g, 76%). 3: mp 53.0−54.0°C; H NMR
(400 MHz, CDCl3)
7.3 Hz, 2H), 6.77 (d, J = 7.3 Hz, 1H), 7.66 (d, J = 7.3 Hz, 1H), 7.80 (t, J = 7.7 Hz, 1H), 8.09 (d, J = 7.7 Hz,
1H), 8.15 (d, J = 7.7 Hz, 1H); 13C NMR (100 MHz, CDCl3)
14.2, 22.7, 26.8, 28.8, 31.6 40.5, 106.1, 114.0,
δ 0.87 (t, J = 7.3 Hz, 3H), 1.24ꢀ1.46 (m, 6H), 1.76 (quint, J = 7.3 Hz, 2H), 3.89 (t, J =
δ
125.2, 126.3, 127.2, 129.1, 129.8, 130.4, 131.3, 139.4, 167.5; IR (KBr) 1712, 1628, 1493, 1466, 1373, 1308,
1207 cmꢀ1; HRMS (FAB) m/z calcd for C17H19BrNO [M + H]+ 332.0650, found 332.0647.
Preparation of 2-ethylhexylzinc bromide and synthesis of 4a: Zinc powder (983 mg, 15.0 mmol) was
heated to 70 °C under vacuum for 1 h. After backꢀfilling with argon, iodine (127 mg, 0.50 mmol) was added,
followed by application of a vacuum and filling with argon. Dry DMA (10 mL) was added, and the resulting
mixture was stirred at room temperature until the red color of the iodine disappeared (ca. 2 min). Then,
2ꢀethylhexyl bromide (1.93 g, 10.0 mmol) was added, and the mixture was stirred at 80 °C for 13 h. The
resulting gray solution was cooled to room temperature and then was titrated according to Knochel’s method
to determine concentration25, which was found to be 0.42 M. The prepared 2ꢀethylhexylzinc bromide (0.42
M solution in DMA, 3.6 mL, 1.50 mmol), 3 (332 mg, 1.00 mmol), SPhos (10.3 mg, 0.025 mmol, 2.5 mol%)
and Pd2(dba)3 (9.2 mg, 0.010 mmol, 1.0 mol%) were stirred at 80 ºC for 21 h. After completion of the
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reaction, 2N HCl (1 mL) was added to the reaction mixture, followed by the addition of water (20 mL). The
resulting solution was extracted with ethyl acetate, washed with brine, dried over Na2SO4, and concentrated
in vacuo. The residue was purified by column chromatography (silica gel, Rf = 0.25, hexane : ethyl acetate =
30 : 1) to give 4a as a yellow oil (345 mg, 94%). 4a: oil; 1H NMR (400 MHz, CDCl3)
δ 0.83ꢀ0.95 (m, 9H),
1.21ꢀ1.46 (m, 14H), 1.65ꢀ1.83 (m, 3H), 2.89 (dd, J = 7.2, 7.5 Hz, 1H), 2.93 (dd, J = 7.2, 7.5 Hz, 1H), 3.90 (t,
J = 7.4 Hz, 2H), 6.82 (d, J = 7.2 Hz, 1H), 7.19 (d, J = 7.2 Hz, 1H), 7.71 (dd , J = 7.3, 1.4 Hz, 1H), 8.05 (d, J
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= 7.3 Hz, 1H), 8.11 (d, J = 7.3 Hz, 1H); 13C NMR (100 MHz, CDCl3)
δ 10.9, 14.1, 14.2, 22.6, 23.2, 25.7,
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26.8, 28.9, 31.6, 32.7, 36.4, 40.3, 41.3, 105.0, 123.8, 125.6, 127.4, 128.1, 128.30, 128.34, 129.0, 132.6, 137.9,
168.0; IR (KBr) 1701, 1628, 1501, 1474, 1377, 1312, 1172 cmꢀ1; HRMS (FAB) m/z calcd for C25H36NO [M
+ H]+ 366.2797, found 366.2781.
Synthesis of 4b: Anhydrous nꢀbutanol (1.50 g, 20.2 mmol) was added to a suspension of sodium hydride (60
wt% in oil, 200 mg, 5.00 mmol) in anhydrous DMF (2 mL) at room temperature. After stirring at room
temperature for 1 h, the resulting solution was added to a suspension of copper (I) iodide (381 mg, 2.00
mmol) and 2 (332 mg, 1.00 mmol) in anhydrous DMF (2 mL). The reaction mixture was stirred at 120 °C for
3 h, and the resulting precipitate was then filtered off and washed with THF to remove copper (I) iodide. The
washed THF solution and the filtrate were combined. After concentrating the solution under reduced pressure,
the residue was purified by column chromatography (silica gel, Rf = 0.35, dichloromethane : hexane = 5 : 4)
to give 4b as a yellow oil (320 mg, 98%). 4b: oil; 1H NMR (400 MHz, CD3OD)
δ 0.88 (t, J = 7.3 Hz, 3H),
1.05 (t, J = 7.3 Hz, 3H), 1.24ꢀ1.46 (m, 6H), 1.61 (sext, J = 7.3 Hz, 1H), 1.78 (quint, J = 7.3 Hz, 2H),
1.84ꢀ1.95 (m, 2H), 3.91 (t, J = 7.3 Hz, 2H), 4.15 (t, J = 7.3 Hz, 2H), 6.81 (d, J = 7.8 Hz, 1H), 6.98 (d, J = 7.8
Hz, 1H), 7.74 (t, J = 8.0 Hz, 1H), 8.04 (d, J = 8.0 Hz, 1H), 8.27 (d, J = 8.0 Hz, 1H); 13C NMR (100 MHz,
CD3OD)
δ 14.4, 14.5, 20.6, 23.8, 27.8, 30.0, 32.6, 32.8, 41.4, 48.5, 48.7, 48.9, 49.2, 49.4, 49.6, 49.8, 69.6,
107.3, 108.4, 124.2, 125.8, 127.0, 127.5, 128.0, 129.2, 133.2, 153.6, 169.9; IR (KBr) 1693, 1636, 1474, 1450,
1377, 1315, 1219 cmꢀ1; HRMS (FAB) m/z calcd for C21H28NO2 [M + H]+ 326.2120, found 326.2099.
Synthesis of 4c: A solution of 3 (498 mg, 1.50 mmol), diꢀnꢀbutylamine (0.30 mL, 1.80 mmol), lithium
bis(trimethylsilyl)amide (552 mg, 3.30 mmol), XPhos (17.2 mg, 0.036 mmol, 2.4 mol%) and Pd2(dba)3 (13.7
mg, 0.015 mmol, 1.0 mol%) in anhydrous THF (5 mL) was stirred at 65 ºC for 24 h. Then, to the solution 2N
HCl (3 mL) was added, followed by a saturated aqueous sodium bicarbonate solution (20 mL). The resulting
solution was extracted with ethyl acetate, washed with brine, dried over Na2SO4 and concentrated in vacuo.
Column chromatography of the residue on silica gel (Rf = 0.25, dichloromethane : hexane = 5 : 4) gave 4c as
1
a yellow oil (191 mg, 34%). 4c: oil; H NMR (400 MHz, CDCl3)
δ 0.82ꢀ0.92 (m, 9H), 1.22ꢀ1.55 (m, 8H),
1.36ꢀ1.44 (m, 2H), 1.45ꢀ1.54 (m, 4H), 1.78 (quint, J = 7.8 Hz, 2H), 3.17 (t, J = 7.8 Hz, 4H), 3.90 (t, J = 7.8
Hz, 2H), 6.80 (d, J = 7.8 Hz, 1H), 6.96 (d, J = 7.8 Hz, 1H), 7.66 (t, J = 7.5 Hz, 1H), 8.03 (d, J = 7.5 Hz, 1H),
8.23 (d, J = 7.5 Hz, 1H); 13C NMR (100 MHz, CDCl3)
δ 14.1, 14.2, 20.6, 22.7, 26.9, 29.0, 29.4, 31.7, 40.4,
54.5, 105.6, 118.5, 124.1, 126.5, 127.5, 127.67, 127.74, 128.4, 134.8, 144.3, 168.2; IR (KBr) 1701, 1628,
1605, 1470, 1450, 1373, 1312, 1215 cmꢀ1; HRMS (FAB) m/z calcd for C25H37N2O [M + H]+ 381.2906, found
381.2904.
Synthesis of 4d: A solution of 3 (665 mg, 2.00 mmol), diꢀnꢀoctylamine (0.73 mL, 2.40 mmol), lithium
bis(trimethylsilyl)amide (736 mg, 4.40 mmol), XPhos (23.0 mg, 0.048 mmol, 2.4 mol%) and Pd2(dba)3 (18.3
mg, 0.020 mmol, 1.0 mol%) in anhydrous THF (10 mL) was stirred at 65 ºC for 24 h. Then, to the solution
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2N HCl (6 mL) was added, followed by a saturated aqueous sodium bicarbonate solution (40 mL). The
resulting solution was extracted with ethyl acetate, washed with brine, dried over Na2SO4 and concentrated
in vacuo. Column chromatography of the residue on silica gel (Rf = 0.20, hexane : ethyl acetate = 30 : 1)
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gave 4d as an orange oil (548 mg, 56%). 4d: oil; H NMR (400 MHz, CDCl3)
δ 0.80ꢀ0.92 (m, 9H),
1.15ꢀ1.37 (m, 24H), 1.38ꢀ1.46 (m, 2H), 1.51 (quint, J = 7.3 Hz, 4H), 1.77 (quint, J = 7.3 Hz, 2H), 3.16 (t, J =
7.3 Hz, 4H), 3.88 (t, J = 7.3 Hz, 2H), 6.80 (d, J = 8.0 Hz, 1H), 6.95 (d, J = 8.0 Hz, 1H), 7.66 (t, J = 7.5 Hz,
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1H), 8.03 (d, J = 7.5 Hz, 1H), 8.22 (d, J = 7.5 Hz, 1H); 13C NMR (100 MHz, CDCl3)
δ 14.1 14.2, 22.6, 22.7,
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26.8, 27.1, 27.4, 29.0, 29.4, 29.5, 31.6, 31.9, 40.3, 54.6, 105.5, 118.4, 124.0, 126.4, 127.4, 127.57, 127.64,
128.3, 134.7, 144.2, 168.1; IR (KBr) 1694, 1632, 1605, 1470, 1450, 1377, 1312, 1219 cmꢀ1; HRMS (FAB)
m/z calcd for C33H53N2O [M + H]+ 493.4158, found 493.4173.
Synthesis of 4e: A solution of 3 (997 mg, 3.00 mmol), diꢀnꢀ(2ꢀethylhexyl)amine (1.1 mL, 3.60 mmol),
lithium bis(trimethylsilyl)amide (1.10 g, 6.60 mmol), XPhos (34.3 mg, 0.072 mmol, 2.4 mol%) and
Pd2(dba)3 (27.5 mg, 0.030 mmol, 1.0 mol%) in anhydrous THF (7 mL) was stirred at 65 ºC for 24 h. Then, to
the solution HCl (9 ml) was added, followed by a saturated aqueous sodium bicarbonate solution (60 mL).
The resulting solution was extracted with ethyl acetate, washed with brine, dried over Na2SO4 and
concentrated in vacuo. Column chromatography of the residue on silica gel (Rf = 0.20, hexane : ethyl acetate
= 30 : 1) gave 4e as an orange oil (167 mg, 11%). 4e: oil; 1H NMR (400 MHz, (CD3)2CO)
δ 0.77ꢀ0.92 (m,
15H), 1.12ꢀ1.52 (m, 22H), 1.55ꢀ1.64 (m, 2H), 1.77 (quint, J = 7.3 Hz, 2H), 3.10 (t, J = 7.3 Hz, 4H), 3.90 (t, J
= 7.3 Hz, 2H), 7.04 (d, J = 7.8 Hz, 1H), 7.24 (d, J = 7.8 Hz, 1H), 7.78 (dd, J = 7.9, 1.4 Hz, 1H), 7.98 (d, J =
7.9 Hz, 1H), 8.38 (d, J = 7.9 Hz, 1H); 13C NMR (100 MHz, CDCl3)
δ 10.75, 10.81, 14.17, 14.21, 22.7, 23.3,
24.47, 24.50, 26.8, 28.7, 28.8, 29.0, 31.17, 31.21, 31.7, 37.3, 40.4, 59.4, 105.7, 119.6, 124.1, 126.3, 127.4,
127.6, 127.8, 128.4, 134.4, 134.9, 145.7, 168.2; IR (KBr) 1690, 1628, 1605, 1474, 1443, 1377, 1312, 1219
cmꢀ1; HRMS (FAB) m/z calcd for C33H53N2O [M + H]+ 493.4158, found 493.4130.
Synthesis of 5a: Methylmagnesium chloride (1.0 mL, 3 M solution in THF, 3.08 mmol) was added to a
solution of 4a (280 mg, 0.766 mmol) in anhydrous THF (2.0 mL) at 0°C. The mixture was stirred at 60°C for
2 h. After cooling to 0°C, water (0.5 mL) was added, and then perchloric acid (60%, 0.6 mL) was added to
the reaction mixture. Ice water (90 mL) was added to the obtained yellow solution. The resulting yellow
precipitate was collected by vacuum filtration and washed with water to yield 5a as a yellow solid (350 mg,
1
98%). 5a: mp 110.0−111.0°C; H NMR (400 MHz, CDCl3)
δ 0.81ꢀ0.96 (m, 9H), 1.19ꢀ1.43 (m, 10H),
1.44ꢀ1.52 (m, 2H), 1.70ꢀ1.79 (m, 1H), 2.0 (quint, J = 7.3 Hz, 2H), 3.07ꢀ3.17 (m, 2H), 3.25 (s, 3H), 4.68 (t, J
= 7.3 Hz, 2H), 7.61 (d, J = 7.8 Hz, 1H), 8.01ꢀ8.08 (m, 2H), 8.61 (d, J = 7.8 Hz, 1H), 8.74 (d, J = 7.8 Hz, 1H);
13C NMR (100 MHz, CDCl3)
δ 10.6, 12.8, 13.8, 13.9, 22.2, 22.8, 25.5, 26.3, 28.6, 30.3, 31.1, 32.4, 36.5,
42.7, 47.4, 121.1, 123.0, 128.7, 129.3, 129.9, 130.6, 135.1, 136.1, 136.7, 146.8, 169.2; IR (KBr) 1593, 1512,
1477, 1443, 1342, 1219 cmꢀ1; HRMS (FAB) m/z calcd for C26H39N+ [M + H]+ 365.3078, found 365.3090.
Synthesis of 5b: Methylmagnesium chloride (0.8 mL, 3 M solution in THF, 2.44 mmol) was added to a
solution of 4b (198 mg, 0.608 mmol) in anhydrous THF (2.0 mL) at 0°C. The mixture was stirred at 60°C for
2 h. After cooling to 0°C, water (0.4 mL) was added, and then perchloric acid (60%, 0.8 mL) was added to
the reaction mixture. Ice water (90 mL) was added to the obtained orange solution. The resulting orange
precipitate was collected by vacuum filtration and washed with water to yield 5b as an orange solid (220 mg,
90%). 5b: mp 93.0−94.0°C; 1H NMR (400 MHz, DMSOꢀd6)
δ 0.86 (t, J = 7.3 Hz, 3H), 1.00 (t, J = 7.3 Hz,
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3H), 1.21ꢀ1.36 (m, 4H), 1.37ꢀ1.48 (m, 2H), 1.57 (sext, J = 7.3 Hz, 2H), 1.85ꢀ1.96 (m, 4H), 3.15 (s, 3H), 4.40
(t, J = 7.3 Hz, 2H), 4.63 (t, J = 7.3 Hz, 2H), 7.34 (d, J = 8.2 Hz, 1H), 8.11 (t, J = 7.7 Hz, 1H), 8.47 (d, J = 8.2
Hz, 1H), 8.77 (d, J = 7.7 Hz, 1H), 8.94 (d, J = 7.7 Hz, 1H); 13C NMR (100 MHz, CDCl3)
δ 13.87, 13.93,
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14.0, 19.4, 22.5, 26.6, 30.7, 31.0, 31.4, 47.7, 70.1, 108.5, 122.3, 123.9, 124.9, 128.5, 129.7, 131.3, 134.3,
134.5, 161.7, 165.5; IR (KBr) 1740, 1447, 1366, 1215 cmꢀ1; HRMS (FAB) m/z calcd for C22H31NO+ [M +
H]+ 325.2401, found 325.2381.
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Synthesis of 5c: Methylmagnesium chloride (0.6 mL, 3 M solution in THF, 1.88 mmol) was added to a
solution of 4c (179 mg, 0.470 mmol) in anhydrous THF (1.5 mL) at 0°C. The mixture was stirred at 60°C for
2 h. After cooling to 0°C, water (0.3 mL) was added, and then perchloric acid (60%, 0.6 mL) was added to
the reaction mixture. Ice water (90 mL) was added to the obtained blue purple solution. The resulting blue
purple precipitate was collected by vacuum filtration and washed with water to yield 5c as a blue purple solid
(217 mg, 97%). 5c: mp 82.0−83.0°C; 1H NMR (400 MHz, CDCl3)
δ 0.87 (t, J = 7.3 Hz, 3H), 1.04 (t, J = 7.3
Hz, 3H), 1.41ꢀ1.53 (m, 4H), 1.80ꢀ1.91 (m, 4H), 1.97 (quint, J = 7.3 Hz, 2H), 3.05 (s, 3H), 3.78 (t, J = 7.3 Hz,
4H), 4.60 (t, J = 7.3 Hz, 2H), 6.87 (d, J = 9.2 Hz, 1H), 7.87 (t, J = 8.0 Hz, 1H), 8.00 (d, J = 9.2 Hz, 1H), 8.51
(d, J = 8.0 Hz, 1H), 8.53 (d, J = 8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3)
δ 12.7, 14.0, 14.1, 20.3, 22.5,
26.6, 28.7, 31.1, 31.4, 47.0, 55.1, 111.7, 121.4, 124.0, 124.6, 126.7, 127.4, 128.0, 132.6, 135.8, 153.0, 154.0;
+
IR (KBr) 1709, 1628, 1466, 1369, 1312, 1285, 1207 cmꢀ1; HRMS (FAB) m/z calcd for C26H40N2 [M + H]+
380.3186, found 380.3192.
Synthesis of 5d: Methylmagnesium chloride (0.8 mL, 3 M solution in THF, 2.40 mmol) was added to a
solution of 4d (296 mg, 0.60 mmol) in anhydrous THF (2.0 mL) at 0°C. The mixture was stirred at 60°C for
2 h. After cooling to 0°C, water (0.5 mL) was added, and then perchloric acid (60%, 0.5 mL) was added to
the reaction mixture. Ice water (90 mL) was added to the obtained blue purple solution. The resulting blue
purple precipitate was collected by vacuum filtration and washed with water to yield 5d as a blue purple
solid (325 mg, 92%). 5d: mp 81.0−82.0°C; 1H NMR (400 MHz, CDCl3)
δ 0.80ꢀ0.94 (m, 9H), 1.22ꢀ1.50 (m,
26H), 1.79ꢀ1.90 (m, 4H), 1.96 (quint, J = 7.3 Hz, 2H), 3.04 (s, 3H), 3.76 (t, J = 7.3 Hz, 4H), 4.60 (t, J = 7.3
Hz, 2H), 6.85 (d, J = 8.7 Hz, 1H), 7.87 (t, J = 7.8 Hz, 1H), 7.98 (d, J = 8.7 Hz, 1H), 8.50 (d, J = 7.8 Hz, 1H),
8.55 (d, J = 7.8 Hz, 1H); 13C NMR (100 MHz, CDCl3)
δ 12.6, 14.0, 14.2, 22.5, 22.7, 26.56, 26.62, 26.9, 29.3,
29.4, 31.1, 31.4, 31.8, 46.9, 55.3, 111.5, 121.3, 123.9, 124.5, 126.5, 127.4, 127.9, 132.7, 135.8, 152.8, 153.9;
+
IR (KBr) 1631, 1570, 1466, 1312, 1219 cmꢀ1; HRMS (FAB) m/z calcd for C34H56N2 [M + H]+ 492.4438,
found 492.4437.
Synthesis of 5e: Methylmagnesium chloride (0.52 mL, 3 M solution in THF, 1.56 mmol) was added to a
solution of 4e (190 mg, 0.386 mmol) in anhydrous THF (2.0 mL) at 0°C. The mixture was stirred at 60°C for
2 h. After cooling to 0°C, water (0.2 mL) was added, and then perchloric acid (60%, 0.4 mL) was added to
the reaction mixture. Ice water (90 mL) was added to the obtained blue solution. The resulting blue
precipitate was collected by vacuum filtration and washed with water to yield 5e as a blue solid (210 mg,
1
91%). 5e: mp 52.0−53.0°C; H NMR (400 MHz, CD3OD)
δ 0.73ꢀ0.95 (m, 15H), 1.10ꢀ1.54 (m, 22H),
1.90ꢀ2.05 (m, 4H), 3.06 (s, 3H), 3.85 (d, J = 7.3 Hz, 4H), 4.60 (t, J = 7.3 Hz, 2H), 7.12 (d, J = 8.7 Hz, 1H),
7.96 (t, J = 7.8 Hz, 1H), 8.10 (d, J = 8.7 Hz, 1H), 8.70 (d, J = 7.8 Hz, 1H), 8.90 (d, J = 7.8 Hz, 1H); 13C
NMR (100 MHz, CDCl3)
δ 10.4, 12.7, 13.8, 13.9, 22.3, 22.9, 23.8, 26.3, 28.3, 30.5, 30.8, 31.2, 37.4, 46.8,
58.6, 114.3, 122.8, 123.8, 124.9, 127.6, 127.8, 128.3, 133.2, 135.7, 155.8, 156.3; IR (KBr) 1681, 1601, 1512,
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1477, 1427, 1219 cmꢀ1; HRMS (FAB) m/z calcd for C34H56N2+ [M + H]+ 492.4438, found 492.4460.
Synthesis of SQ1: A solution of semisquaric acid 6 (191 mg, 0.550 mmol) and 7a (253 mg, 0.550 mmol) in
toluene (5 mL) and nꢀbutanol (5 mL) was heated to reflux in a DeanꢀStark apparatus for 16 h. After
completion of the reaction, the mixture was concentrated in vacuo to give a residue. The residue was purified
by column chromatography (silica gel, Rf = 0.35, chloroform : methanol = 15 : 1) to yield SQ1 (67 mg, 18%)
as a brown solid. SQ1: mp 219.0−220.0°C; 1H NMR (400 MHz, CDCl3)
δ 0.80ꢀ0.92 (m, 6 H), 1.16ꢀ1.51 (m,
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16 H), 1.72ꢀ1.95 (m, 10 H), 4.15 (br s, 2H), 4.18 (br s, 2H), 6.10 (s, 1H), 6.37 (s, 1H), 7.03 (d, J = 7.9 Hz,
1H), 7.07 (d, J = 7.9 Hz, 1H), 7.51 (t, J = 7.9 Hz, 1H), 7.58 (d, J = 7.9 Hz, 1H), 7.85 (t, J = 7.9 Hz, 1H), 7.96
(d, J = 8.3 Hz, 1H), 8.08 (s, 1H), 8.13 (dd, J = 8.3, 1.8 Hz, 1H), 9.20 (brs, 1H); 13C NMR (150 MHz, CDCl3)
δ
14.1, 14.2, 22.67, 22.72, 27.0, 27.15, 27.19, 27.4, 28.9, 29.3, 29.5, 31.7, 31.9, 44.2, 49.2, 89.0, 91.3, 107.2,
109.1, 121.8, 124.2, 125.6, 125.8, 128.5, 129.8, 129.9, 130.5, 131.2, 132.0, 141.7, 142.4, 146.6, 152.0, 170.3,
170.54, 177.9, 179.9, 183.2; IR (KBr) 1731, 1693, 1566, 1501, 1354, 1285, 1223 cmꢀ1; HRMS (FAB) m/z
calcd for C42H49N2O4 [M + H]+ 645.3692, found 645.3722.
Synthesis of SQ2: A solution of semisquaric acid 6 (167 mg, 0.480 mmol) and 7b (245 mg, 0.480 mmol) in
toluene (5 mL) and nꢀbutanol (5 mL) was heated to reflux in a DeanꢀStark apparatus for 16 h. After
completion of the reaction, the mixture was concentrated in vacuo to give a residue. The residue was purified
by column chromatography (silica gel, Rf = 0.30, chloroform : methanol = 15 : 1) to yield SQ2 (46 mg, 13%)
as a brown solid. SQ2: mp 261.0−262.0°C; 1H NMR (400 MHz, THFꢀd8)
δ 0.85ꢀ0.94 (m, 6 H), 1.26ꢀ1.46 (m,
16 H),1.48ꢀ1.57 (m, 4 H), 1.86ꢀ1.94 (m, 4 H), 2.10 (s, 6H), 4.22 (t, J = 7.3 Hz, 2H), 4.32 (t, J = 7.3 Hz, 2H),
6.22 (s, 1H), 6.30 (s, 4H), 7.07 (dd, J = 7.6, 2.1 Hz, 1H), 7.42ꢀ7.48 (m, 2H), 7.63 (d, J = 8.0 Hz, 1H), 7.74 (t,
J = 8.60 Hz, 1H), 7.87 (d, J = 8.0 Hz, 1H), 8.10 (d, J = 8.0 Hz, 1H), 8.16 (dd, J = 8.9, 1.4 Hz, 1H), 8.39 (d, J
= 8.9 Hz, 1H), 8.68 (s, 1H), 9.58 (d, J = 8.0 Hz, 1H); 13C NMR (150 MHz, THFꢀd8)
δ 14.06, 14.11, 23.2,
26.7, 27.4, 27.6, 28.2, 29.2, 29.9, 30.0, 32.3, 32.5, 43.9, 44.5, 52.2, 89.4, 92.2, 105.9, 112.1, 120.4, 123.3,
126.8, 127.6, 127.7, 128.6, 128.9, 129.8, 130.9, 131.3, 131.6, 132.0, 133.3, 135.6, 142.4, 143.1, 150.1, 167.3,
173.2, 180.8, 181.9, 183.6, 201.0, 201.2, 201.3; IR (KBr) 1728, 1690, 1555, 1501, 1470, 1431, 1254, 1219,
cmꢀ1; HRMS (FAB) m/z calcd for C42H49N2O4 [M + H]+ 645.3692, found 645.3722.
Synthesis of SQ3: Semisquarine 8 (293 mg, 0.600 mmmol) was dissolved in a mixed solution of 40 wt%
aqueous NaOH solution (0.1 mL) and EtOH (3 mL). The solution was refluxed for 5 min. The reaction
mixture was neutralized with 2N HCl, and the resulting organic layer was concentrated in vacuo. The residue
was then mixed with a solution of 5a (278 mg, 0.600 mmol) in toluene (6 mL) and nꢀbutanol (6 mL). The
mixture was heated to reflux in a DeanꢀStark apparatus for 17 h. After completion of the reaction, the
mixture was concentrated in vacuo to give a residue. The residue was purified by column chromatography
(silica gel, Rf = 0.36, chloroform : methanol = 20 : 1) to yield SQ3 (108 mg, 22%) as a brown solid. SQ3: mp
232.0−233.0°C; 1H NMR (400 MHz, CDCl3)
δ 0.82ꢀ0.90 (m, 12H), 1.18ꢀ1.39 (m, 20 H), 1.40ꢀ1.51 (m, 4H),
1.67ꢀ1.76 (m, 1H), 1.83ꢀ1.94 (m, 4 H), 2.10 (s, 6H), 2.94 (d, J = 6.4 Hz, 1H), 4.04ꢀ4.24 (m, 4H), 6.13 (s, 1H),
6.33 (s, 1H), 6.95 (d, J = 7.6 Hz, 1H), 7.24 (d, J = 7.6 Hz, 1H), 7.36 (d, J = 9.0 Hz, 1H), 7.85 (t, J = 8.1 Hz,
1H), 8.01 (d, J = 9.0 Hz, 1H), 8.05 (d, J = 8.1 Hz, 1H), 8.18ꢀ8.29 (m, 2H), 8.74 (s, 1H), 9.15 (brs, 1H); 13C
NMR (150 MHz, CDCl3)
δ 11.0, 14.16, 14.21, 14.3, 22.7, 23.3, 25.9, 26.9, 27.0, 27.2, 27.7, 29.96, 29.03,
29.3, 29.5, 31.7, 31.9, 32.8, 36.6, 41.8, 44.1, 44.2, 51.3, 88.1, 90.8, 106.8, 111.2, 122.9, 125.5, 126.0, 127.2,
128.7, 129.4, 129.5, 130.4, 131.1, 131.7, 133.7, 134.7, 134.8, 134.8, 140.20, 140.23, 142.0, 151.4, 170.3,
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171.6, 176.3, 177.7, 183.3; IR (KBr) 1728, 1690, 1574, 1493, 1250, 1219 cmꢀ1; HRMS (FAB) m/z calcd for
C54H67N2O4 [M + H]+ 807.5101, found 807.5079.
Synthesis of SQ4: Semisquarine 8 (195 mg, 0.400 mmmol) was dissolved in a mixed solution of 40 wt%
aqueous NaOH solution (0.07 mL) and EtOH (2 mL). The solution was refluxed for 5 min. The reaction
mixture was neutralized with 2N HCl, and the resulting organic layer was concentrated in vacuo. The residue
was then mixed with a solution of 5b (170 mg, 0.400 mmol) in toluene (6 mL) and nꢀbutanol (6 mL). The
mixture was heated to reflux in a DeanꢀStark apparatus for 17 h. After completion of the reaction, the
mixture was concentrated in vacuo to give a residue. The residue was purified by column chromatography
(silica gel, Rf = 0.41, dichloromethane : methanol = 15 : 1) to yield SQ4 (146 mg, 48%) as a brown solid.
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SQ4: mp 226.0−227.0°C; 1H NMR (600 MHz, DMSOꢀd6)
δ 0.82 (t, J = 6.9 Hz, 3H), 0.86 (t, J = 7.6 Hz, 3H),
1.01 (t, J = 7.6 Hz, 3H), 1.20ꢀ1.38 (m, 12H), 1.40ꢀ1.47 (m, 4H), 1.57 (sext, J = 7.6 Hz, 2H), 1.78ꢀ1.89 (m, 6
H), 2.00 (s, 6H), 4.22 (t, J = 6.9 Hz, 2H), 4.25ꢀ4.30 (m, 4H), 5.97 (s, 1H), 6.16 (s, 1H), 7.00 (d, J = 8.3 Hz,
1H), 7.29 (d, J = 8.3 Hz, 1H), 7.73 (d, J = 8.6 Hz, 1H), 7.82 (t, J = 8.7 Hz, 1H), 8.08 (dd, J = 8.6, 2.1 Hz,
1H), 8.15 (d, J = 8.0 Hz, 1H), 8.19 (d, J = 8.7 Hz, 1H), 8.30 (d, J = 8.7 Hz, 1H), 8.65 (d, J = 2.0 Hz, 1H),
9.15 (d, J = 8.0 Hz, 1H); 13C NMR (150 MHz, DMSOꢀd6)
δ 14.1, 14.18, 14.24, 19.4, 22.4, 26.5, 26.67, 26.70,
27.4, 28.7, 29.0, 29.1, 31.3, 31.4, 31.6, 40.0, 44.0, 51.0, 69.2, 88.0, 90.4, 109.5, 112.7, 122.9, 123.0, 125.2,
126.0, 126.8, 127.2, 129.1, 130.4, 130.5, 130.6, 131.3, 132.1, 132.8, 133.9, 135.4, 142.3, 150.0, 152,8, 167.7,
170.9, 176.6, 177.4, 181.9; IR (KBr) 1728, 1682, 1551, 1477, 1443, 1369, 1250, 1207 cmꢀ1; HRMS (FAB)
m/z calcd for C50H59N2O5 [M + H]+ 767.4424, found 767.4393.
Synthesis of SQ5: Semisquarine 8 (98 mg, 0.200 mmmol) was dissolved in a mixture solution of 40wt%
aqueous NaOH solution (0.07 mL) and EtOH (2 mL). The solution was refluxed for 5 min. The reaction
mixture was neutralized with 2N HCl and the resulting organic layer was concentrated in vacuo. The residue
was then mixed with a solution of 5c (96 mg, 0.200 mmol) in toluene (6 mL) and nꢀbutanol (6 mL). The
mixture was heated to reflux in a DeanꢀStark apparatus for 17 h. After completion of the reaction, the
mixture was concentrated in vacuo to give a residue. The residue was purified by column chromatography
(silica gel, Rf = 0.35, chloroform : methanol = 15 : 1) to yield SQ5 (19 mg, 12%) as a dark green solid. SQ5:
1
mp 250.0−251.0°C; H NMR (400 MHz, CDCl3)
δ 0.80ꢀ0.94 (m, 12H), 1.14ꢀ1.39 (m, 16H), 1.40ꢀ1.50 (m,
4H), 1.56 (quint, J = 7.3 Hz, 4H), 1.78ꢀ1.94 (m, 4H), 2.10 (s, 6H), 3.29 (t, J = 7.3 Hz, 4H), 4.04ꢀ4.24 (m, 4H),
6.06 (s, 1H), 6.32 (s, 1H), 6.93 (d, J = 7.8 Hz, 1H), 6.99 (d, J = 7.8 Hz, 1H), 7.32 (d, J = 8.7 Hz, 1H), 7.82 (t,
J = 8.5 Hz, 1H), 7.98 (d, J = 8.7 Hz, 1H), 8.16ꢀ8.20 (m, 2H), 8.24 (d, J = 8.5 Hz, 1H), 8.72 (s, 1H), 9.11 (d, J
= 7.8 Hz, 1H); 13C NMR (150 MHz, CDCl3)
δ 14.1, 14.16, 14.21, 19.4, 20.6, 22.70, 22.73, 26.95, 27.00, 27.2,
27.6, 29.1, 29.3, 29.45, 29.54, 29.9, 31.7, 31.9, 44.3, 50.9, 54.3, 87.68, 87.71, 90.3, 108.6, 111.1, 117.4,
122.8, 125.1, 126.8, 126.9, 127.1, 127.7, 128.6, 130.1, 131.0, 131.2, 131.6, 133.8, 136.0, 142.3, 146.7, 151.4,
169.83, 170.3, 173.5, 177.4, 183.4; IR (KBr) 1709, 1605, 1566, 1493, 1439, 1331, 1250, 1200 cmꢀ1; HRMS
(FAB) m/z calcd for C54H68N3O4 [M + H]+ 822.5210, found 822.5233.
Synthesis of SQ6: Semisquarine 8 (230 mg, 0.470 mmmol) was dissolved in a mixture solution of 40wt%
aqueous NaOH solution (0.07 mL) and EtOH (2 mL). The solution was refluxed for 5 min. The reaction
mixture was neutralized with 2N HCl and the resulting organic layer was concentrated in vacuo. The residue
was then mixed with a solution of 5d (278 mg, 0.470 mmol) in toluene (6 mL) and nꢀbutanol (6 mL). The
mixture was heated to reflux in a DeanꢀStark apparatus for 17 h. After completion of the reaction, the
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(silica gel, Rf = 0.40, chloroform : methanol = 20 : 1) to yield SQ6 (59 mg, 13%) as a dark green solid. SQ6:
1
mp 209.0−210.0°C; H NMR (400 MHz, CDCl3)
δ 0.80ꢀ0.94 (m, 12H), 1.14ꢀ1.39 (m, 32H), 1.40ꢀ1.51 (m,
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4H), 1.57 (quint, J = 7.3 Hz, 4H), 1.79ꢀ1.94 (m, 4H), 2.11 (s, 6H), 3.28 (t, J = 7.3 Hz, 4H), 4.04ꢀ4.24 (m, 4H),
6.07 (s, 1H), 6.33 (s, 1H), 6.92 (d, J = 7.8 Hz, 1H), 6.99 (d, J = 7.8 Hz, 1H), 7.33 (d, J = 9.0 Hz, 1H), 7.82 (t,
J = 8.0 Hz, 1H), 7.98 (d, J = 9.0 Hz, 1H), 8.16ꢀ8.20 (m, 2H), 8.24 (d, J = 8.0 Hz, 1H), 8.72 (s, 1H), 9.12 (d, J
= 7.3 Hz, 1H); 13C NMR (150 MHz, CDCl3)
δ 14.16, 14.23, 22.69, 22.71, 22.8, 26.9, 27.0, 27.2, 27.3, 27.4,
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27.6, 29.1, 29.3, 29.4, 29.5, 29.6, 31.7, 31.86, 31.93, 43.9, 44.3, 50.9, 54.5, 87.7, 90.3, 108.6, 111.0, 117.2,
122.7, 125.3, 126.7, 126.8, 127.1, 127.8, 128.6, 130.1, 130.9, 131.1, 131.6, 132.0, 133.7, 134.3, 135.9, 142.2,
146.7, 151.4, 169.8, 170.4, 173.1, 177.1, 183.7; IR (KBr) 1735, 1690, 1550, 1493, 1443, 1250, 1219 cmꢀ1;
HRMS (FAB) m/z calcd for C62H84N3O4 [M + H]+ 934.6462, found 934.6436.
Synthesis of SQ7: Semisquarine 8 (157 mg, 0.320 mmmol) was dissolved in a mixture solution of 40wt%
aqueous NaOH solution (0.07 mL) and EtOH (2 mL). The solution was refluxed for 5 min. The reaction
mixture was neutralized with 2N HCl and the resulting organic layer was concentrated in vacuo. The residue
was then mixed with a solution of 5e (189 mg, 0.320 mmol) in toluene (6 mL) and nꢀbutanol (6 mL). The
mixture was heated to reflux in a DeanꢀStark apparatus for 17 h. After completion of the reaction, the
mixture was concentrated in vacuo to give a residue. The residue was purified by column chromatography
(silica gel, Rf = 0.40, chloroform : methanol = 20 : 1) to yield SQ7 (34 mg, 11%) as a dark green solid. SQ7:
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mp 234.0−235.0°C; H NMR (400 MHz, CDCl3)
δ 0.75ꢀ0.92 (m, 18H), 1.14ꢀ1.51 (m, 32H), 1.62ꢀ1.73 (m,
2H), 1.80ꢀ1.94 (m, 4H), 2.11 (s, 6H), 3.24 (d, J = 6.9 Hz, 4H), 4.04ꢀ4.24 (m, 4H), 6.05 (s, 1H), 6.31 (s, 1H),
6.95ꢀ7.01 (m, 2H), 7.32 (d, J = 8.8 Hz, 1H), 7.81 (t, J = 7.8 Hz, 1H), 7.98 (d, J = 8.8 Hz, 1H), 8.14ꢀ8.28 (m,
3H), 8.72 (s, 1H), 9.10 (d, J = 6.4 Hz, 1H); 13C NMR (150 MHz, CDCl3)
δ 10.8, 14.2, 22.7, 23.3, 24.4, 27.0,
27.2, 28.7, 28.8, 29.2, 29.3, 29.5, 31.08, 31.11, 31.3, 31.7, 31.9, 37.5, 43.9, 44.3, 50.8, 58.5, 87.7, 90.3, 109.0,
111.1, 117.9, 122.8, 125.1, 126.66, 126.73, 127.1, 128.1, 128.5, 130.1, 131.0, 131.2, 131.6, 133.8, 135.6,
142.4, 148.2, 151.3, 169.5, 170.4, 172.8, 177.2, 183.6; IR (KBr) 1733, 1705, 1570, 1493, 1443, 1250, 1200
cmꢀ1; HRMS (FAB) m/z calcd for C62H84N3O4 [M + H]+ 934.6462, found 934.6479.
Fabrication and measurement of the dye-sensitized solar cell: The dyeꢀsensitized solar cells were
fabricated according to the previously reported method of Ito et al.39 FTO (Fꢀdoped tin oxide; 4 mm
thickness, 13 ꢂ □‒1, Nippon Sheet Glass) glass substrate was washed for 15 min each with an aqueous
detergent solution, distilled water, and ethanol using an ultrasonic bath. The FTO glass substrate was
immersed in a 40 mM aqueous TiCl4 solution at 70 °C for 30 min, and then washed with distilled water and
ethanol. A commercial TiO2 paste (PSTꢀ18NR, JGC Catalysts and Chemicals Ltd.) was deposited onto the
FTO glass substrate by the squeegee method using mending tape (4 µm thickness) twice to obtain a 8 µm
transparent TiO2 film. After drying the FTO glass substrate at 125 °C in air for 10 min, a commercial TiO2
paste (PSTꢀ400C, JGC Catalysts and Chemicals Ltd.) was deposited once to obtain a 4 µm scattering layer.
The resulting substrate was sintered at 500 °C for 15 min. After cooling to room temperature, the sintered
TiO2 film was immersed in a 40 mM aqueous TiCl4 solution at 70 °C for 30 min, and then washed with
distilled water and ethanol. After drying, the TiO2 film was sintered at 500 °C for 30 min to obtain a
mesoporous TiO2 photoelectrode consisting of a transparent layer (8 µm thickness) and a scattering layer (4
µm thickness). The TiO2 electrode was then immersed in a solution of SQ dye (0.12 mM) and
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chenodeoxycholic acid (6 mM) in acetonitrile/tꢀbutyl alcohol (v : v = 1 : 1). The dye adsorption was
performed in the dark at room temperature for 15 h, washed with ethanol to remove any nonꢀadsorbed dye,
and dried. The dyeꢀcoated TiO2 electrode and Ptꢀsputtered FTO glass counter electrode were assembled into
a sandwich type cell (active area: 5 mm × 4 mm). An electrolyte composed of 0.5 M DMPImI
(1,2ꢀdimethylꢀ3ꢀnꢀpropylimidazolium iodide), 0.05 M I2, and 0.5 M LiI in a mixed solution of acetonitrile
and chloroform (v : v = 1 : 1) was added to the cell. After assembling the cell, silver paint was applied to the
edges of the TiO2 photoelectrode and counter electrode of the cells to reduce series resistance. The I–V
curves of the cells were measured under illumination with a simulated sunlight condition at AM1.5 (100 mW
cm−2) generated by a BunkoꢀKeiki CEPꢀ2000 system. A mask was used to regulate the active area to 0.2 cm2.
IPCE spectra were measured on a BunkoꢀKeiki CEPꢀ2000 system under monochromatic light illumination.
Electrochemical impedance spectra were measured using VersaSTAT3 potentiostat at open circuit conditions
under 1 sun light intensity (25°C). The spectra were fitted using Zview software (Scribner).
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Deposition studies of SQ dyes
A TiO2 photoelectrode (transparent layer: 8 ꢁm, cell area: 1.0 cm × 1.0 cm, 1.0 cm2) was immersed in a
solution of SQ dye (0.12 mM) and chenodeoxycholic acid (6 mM) in acetonitrile/tꢀbutyl alcohol (v : v = 1 :
1) (SQ1, SQ2, SQ3, SQ5, SQ6 and SQ7) or in THF (SQ4) for 15 h. The dyeꢀadsorbed TiO2 photoelectrode
was washed with ethanol and dried. Then the adsorbed SQ dye was desorbed by using 4.0 mL (SQ1, SQ4,
SQ5, SQ6 and SQ7) or 8.0 mL (SQ2 and SQ3) of a mixture solution of 2M HCl and ethanol at 1 : 9 ratio by
volume. After that, the UV/Vis/NIR absorption spectra of the desorbed solution were measured (Figure S6).
In order to obtain the
values (SQ1, SQ2, SQ4−SQ7: 1.0 × 10ꢀ5 M, SQ3: 5.0 × 10ꢀ6 M) were measured in the mixture of 2M HCl
and ethanol with the volume ratio of 1 : 9 (Figure S5). The max and values in the mixture of 2M HCl and
ethanol in the ratio of 1 : 9 (v : v) were as follows: SQ1 ( max = 767 nm,
= 123,000), SQ3 (λmax = 792 nm, = 132,000), SQ4 (λmax = 800 nm,
85,000), SQ6 ( max = 728 nm, = 82,000), SQ7 ( max = 727 nm, = 23,000).
ε values, the UV/Vis/NIR absorption spectra of the SQ dyes with reliable concentration
λ
ε
λ
ε
= 109,000), SQ2 (
λmax = 781 nm,
ε
ε
ε
= 90,000), SQ5 ( max = 728 nm,
λ
ε
=
λ
ε
λ
ε
Supporting Information Available: 1H and 13C NMR spectra of new compounds. This material is available
free of charge via the internet at http://pubs.acs.org.
Acknowledgements: This work was partially supported by JSPS KAKENHI Grant Number JP16K05894,
the Koshiyama Research Grant, the Research Foundation for the Electrotechnology of Chubu and the
Strategic Core Technology Advancement Program, “Supporting Industry Program”.
References
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AlꢀAlwani, M. A. M.; Mohamad, A. B.; Ludin, N. A.; Kadhum, A. A. H.; Sopian, K. Renew. Sust.
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