Synthesis of pentamethyldisilanyl-substituted starlike molecule with triazine core and its application to dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jorganchem.2016.10.022

Two-dimensional starlike molecule 7 consisting of benzothiadiazole-thiophenylphenyltriazine groups and pentamethyldisilanyl substituted thienylene units that extend to three directions was synthesized by the reactions of 2,4,6-tris[4-(4-hexyl-5-(7-(3-hexy

Full text of DOI:10.1016/j.jorganchem.2016.10.022

Journal of Organometallic Chemistry 825-826 (2016) 63e68
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis of pentamethyldisilanyl-substituted starlike molecule with
triazine core and its application to dye-sensitized solar cells
,
Akinobu Naka a *, Kiyoshi Fujishima a, Erika Okada a, Mao Noguchi a, Joji Ohshita b,
Yohei Adachi b, Yousuke Ooyama b, Mitsuo Ishikawa a
a Department of Life Science, Kurashiki University of Science and the Arts, 2640 Nishinoura, Tsurajima-cho, Kurashiki, Okayama 712-8505, Japan
b Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Two-dimensional starlike molecule 7 consisting of benzothiadiazole-thiophenylphenyltriazine groups
and pentamethyldisilanyl substituted thienylene units that extend to three directions was synthesized by
the reactions of 2,4,6-tris[4-(4-hexyl-5-(7-(3-hexyl-5-iodothiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)-
thiophen-2-yl)phenyl]-1,3,5-triazine (5) with the pentamethyldisilanylthienyl substituted stannyl de-
rivative (6). UV-visible absorption and fluorescence properties of starlike molecules 4 and 7 have been
investigated in dioxane and in the solid state. The application of TiO2-electrode coated on the surface
with the use of 7 to dye-sensitized solar cells has been discussed.
Received 23 July 2016
Received in revised form
24 September 2016
Accepted 15 October 2016
Available online 15 October 2016
Keywords:
Starlike molecule
Triazine
© 2016 Elsevier B.V. All rights reserved.
Pentamethyldisilanyl
UV absorption
Photoluminescence
Dye-sensitized solar cell
1. Introduction
rise to mutual interaction between HOMO of the donor and LUMO
of the acceptor, and reduction of the band gap in these systems.
Three-dimensional organosilicon dendrimers containing
p
-
Recently, Ohshita et al. prepared disilanylene polymers with con-
jugated donoreacceptor systems and used them as sensitizing dyes
[20e24]. In this paper we report synthesis of a new donor-acceptor
type of the disilanyl substituted starlike molecule bearing a triazine
core, and its application to dye-sensitized solar cells (DSSC)s.
conjugated units have attracted a great deal of attention to their
potential application in organic photo- and electroluminescent
materials [1e13]. We have reported synthesis and optical proper-
ties of various types of three-dimensional starlike silicon com-
pounds [14,15], and also two-dimensional ones, whose arms
extended to three or four directions [16,17] (see Chart 1). They show
interesting properties, depending on their unique molecular
structure, and may be used as functionality materials. For example,
the compounds bearing the arms consisting of a regular alternating
arrangement of a silicon-silicon bond and bithienylene unit, which
extend to three directions reveal high fluorescence quantum yields
and long lifetimes of the excited state [15e17].
2. Results and discussion
The synthesis of the arms having benzothiadiazole and pen-
tamethyldisilanyl substituted thienylene units were carried out as
shown in Schemes 1e3. Treatment of the lithio derivative prepared
by the reaction of 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (1) and
n-butyllithium,
with
2-isopropyl-4,4,5,5-tetramethyl-1,3,2-
Recently, a considerable attention has been focused on low-
band gap materials as active layer in photovoltaic cells for light
absorption [18,19]. The alternating arrangements of electron-
donating and electron-accepting moieties in the materials give
dioxaborolane afforded compound 2 in 18% yield (Scheme 1) [25].
Suzuki-Miyaura cross coupling reaction of 2 with 4-(5-bromo-3-
hexylthiophen-2-yl)-7-(3-hexylthiophen-2-yl)benzo[c][1,2,5]thia-
diazol (3) in THF afforded 2,4,6-tris(4-(4-hexyl-5-(7-(3-
hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)
phenyl)-1,3,5-triazine (4) in 24% yield.
Compound 5 was prepared by treating 4 with 3 quiv. of N-
iodosuccinimide (NIS) in chloroform in 47% yield. The starlike
* Corresponding author.
E-mail address: anaka@chem.kusa.ac.jp (A. Naka).
http://dx.doi.org/10.1016/j.jorganchem.2016.10.022
0022-328X/© 2016 Elsevier B.V. All rights reserved.

64
A. Naka et al. / Journal of Organometallic Chemistry 825-826 (2016) 63e68
Chart 1. Various types of three-dimensional starlike silicon compounds.
Scheme 1. Synthesis of compound 2.
molecule,
2,4,6-tris(4-(4-hexyl-5-(7-(4-hexyl-5’-(1,1,2,2,2-
to the methylsilyl carbons, and twenty three signals due to the
aromatic carbons, as well as the signals due to the hexyl carbons.
The 29Si NMR spectrum reveals three signals at ꢀ23.5
and ꢀ18.8 ppm, due to two different kinds of the silicon atoms.
These results are wholly consistent with the structure proposed for
7.
pentamethyldisilyl)-2,2’-bithiophen-5-yl)benzo[c][1,2,5]thiadia-
zol-4-yl)thiophen-2-yl)phenyl-1,3,5-triazine (7) was prepared by
Stille coupling reaction of 1,1,2,2,2-pentamethyl-2-(5-(tributyl-
stannyl)thiophen-2-yl)disilane (6) with 5. The product 7 was iso-
lated from the resulting reaction mixture, using a silica gel column
in 24% yield.
The structure of 7 was verified by mass and 1H, 13C, and 29Si
NMR spectrometric analysis (see Experimental Section).
In fact, mass spectrometric analysis for 7 indicates the presence
of a parent ion at m/z 2343, which is consistent with the calculated
value for C126H153N9Si6S12. The 1H NMR spectrum of 7 shows two
signals at 0.13, 0.40 ppm due to the methylsilyl protons, and the
signals at 7.10, 7.21, 7.31, 7.52 ppm attributed to thienyl ring protons,
along with hexyl protons and phenylene protons. The 13C NMR
spectrum of 7 reveals two signals at ꢀ2.9 and ꢀ2.4 ppm attributed
2.1. Optical properties of starlike molecules 4 and 7
The UVevisible absorption spectra and fluorescence spectra of
compounds 4 and 7 are shown in Figs. 1 and 2, respectively, and
their spectral data are summarized in Table 1. Compound 4 in
dioxane exhibits two bands at 373 nm (ε ¼ 74000 Mꢀ1 cmꢀ1) and
414 nm (ε ¼ 56000 Mꢀ1 cmꢀ1). The fluorescence spectrum of 4
excited at 373 nm displays the emission maximum at 560 nm. In
the case of excitation at 373 nm, the fluorescence quantum yield of

4 in dioxane is found to be fF ¼ 17%. While compound 7 with the
pentamethyldisilanyl unit also reveals two absorption maxima at
lmax
¼
361 nm (ε
¼
81000 Mꢀ1 cmꢀ1
) and 441 nm
(ε ¼ 57000 Mꢀ1 cmꢀ1). The PL spectrum of 7 exhibits a broad band
with emission maximum of 596 nm. The fluorescence quantum
yield of 7 is 43%, which is much higher than that of 4, having no
disilanylene groups. The
s-p conjugation consisting of disilanyl
units and oligothiophenes would be an important factor for a great
enhancement in the fluorescence for the starlike molecule [26].
Emission spectra of 4 and 7 in the solid state were recorded at
room temperature. Considerable changes are observed in the
fluorescence maxima (605 nm for 4, 643 nm for 7) and the quantum
yields (8% for 4, 11% for 7) compared to those in dioxane. The lmax of
4 and 7 in the solid state is red-shifted from that in solution. The
red-shift in the solid spectra may be attributed to the formation of
aggregate sites derived from the strong intermolecular interaction
[27,28]. The fluorescence quantum yields of 4 and 7 in the solid
state become smaller than those in solution [29]. The aggregation-
induced quenching would be caused by
p-p stacking contacts of
fluorophore in the solid state.
2.2. Applications of 7 to dye sensitizer of DSSC
The starlike molecule 7 was coated on the surface of the TiO2-
electrode, according to the following method. Namely, the TiO2-
electrodes prepared on FTO plates was immersed in chloroform
containing compound 7 (2 g/L) in argon for 40 min under irradia-
tion at >400 nm (photochemical condition) or in the dark for 24 h
(thermal condition). The resulting orange-colored TiO2-electrode
Scheme 2. Synthesis of compound 4.
Scheme 3. Synthesis of compound 7.

66
A. Naka et al. / Journal of Organometallic Chemistry 825-826 (2016) 63e68
Fig. 1. (a) UVevis of compound 4 and 7 in dioxane. (b) FL spectra of compound 4 and 7
in dioxane. 4 and 7.
Fig. 3. (a) IPCE spectra of TiO2 electrodes modified compound 7 (7-P: photochemical
condition, 7-T: thermal condition). (b) I-V characteristics of DSSCs based on TiO2
electrodes modified compound
7 (7-P: photochemical condition, 7-T: thermal
condition).
Compound 7 exhibited clear sensitizing effects and the PCEs (po-
wer conversion efficiencies) were 0.14% (7-P; photochemical con-
dition) and 0.27% (7-T; thermal condition) for the DSSCs. Thermally
modified cell showed higher performance. Though the photo-
chemical condition might form SieOeTi bonds, in which direct
reaction of Ti-OH surface with the photo-excited SieSi bonds would
be involved, the longer immersing time in the thermal condition
seems to affect the performance due to the higher degree of com-
pound 7 attached.
In conclusion, we prepared the starlike molecule consisting of a
benzothiadiazole- thiophenylphenyltriazine group and pentam-
ethyldisilanyl substituted thienylene units. The compound was
attached to TiO2 electrodes under photochemical and thermal
Fig. 2. FL spectra of compound 4 and 7 in powder.
excited at 361 nm.
4 excited at 373 nm and
7
was rinsed thoroughly with chloroform. Thus, the starlike com-
pound 7 with a triazine unit could readily coat on the surface of the
TiO2 electrode under both photochemical and thermal conditions
[30,31]. DSSCs (FTO/7-attached TiO2/Iꢀ$I3ꢀ/Pt) were fabricated, and
then the performance was evaluated. The IPCE (incident photon to
current efficiency) spectra and the I-V curves of the cells are shown
in Fig. 3 and the DSSC parameters are summarized in Table 2.
Table 1
Absorption and fluorescence data for compounds 4 and 7.
Compound
lmax, Abs (in dioxane) (nm)
lmax, F (in dioxane) (nm)
lmax, F (powder) (nm)
FF (in dioxane)
FF (powder)
4
7
373, 414
361, 441
560
596
605
643
0.17
0.43
0.08
0.11

A. Naka et al. / Journal of Organometallic Chemistry 825-826 (2016) 63e68
67
Table 2
hydrolyzed with dilute hydrochloric acid. The organic layer was
separated, washed with water, and dried over anhydrous magne-
sium sulfate. After the solvent was evaporated, compound 5
(0.1157 g, 47% yield) was isolated by a silica gel column eluting with
hexane-chloroform (2:1) as a red solid: M.p. 92e93 ꢁC. HRMS (ESI)
exact mass calcd for C99H103N9S9I3 (Mþ þ H): 2086.29514, found
DSSC performance for compound 7.
Compound
VOC/mV
JSC/mAcmꢀ2
FF
PCE (h)%
7-P
7-T
315
363
0.70
1.21
0.63
0.61
0.14
0.27
2086.29744. MS m/z 2085 (Mþ); 1H NMR (
d CDCl3) 0.83 (t, 9H, CH3,
J ¼ 7.4 Hz), 0.84 (t, 9H, CH3, J ¼ 7.4 Hz), 1.16e1.34 (m, 36H, CH2), 1.62
(quint, 6H, CH2, J ¼ 7.4 Hz), 1.71 (quint, 6H, CH2, J ¼ 7.4 Hz), 2.65 (t,
6H, CH2, J ¼ 7.4 Hz), 2.72 (t, 6H, CH2, J ¼ 7.4 Hz), 7.26 (s 3H, thie-
nylene protons), 7.50 (s, 3H, thienylene protons), 7.63 (d, 3H, phe-
nylene protons, J ¼ 7.0 Hz), 7.70 (d, 3H, phenylene protons,
J ¼ 7.0 Hz), 7.87 (d, 6H, phenylene protons, J ¼ 8.6 Hz), 8.80 (d, 6H,
condition and the resulting TiO2 electrodes were applied to DSSCs.
Studies to explore starlike molecules other than DSSC applications
are in progress.
3. Experimental
phenylene protons, J ¼ 8.6 Hz); 13C NMR (
d CDCl3) 14.02, 14.04
3.1. General procedures
(CH3), 22.47, 22.53, 29.0, 29.1, 29.2, 29.7, 30.5, 30.6, 31.47, 31.54
(CH2), 125.4, 126.1, 126.4, 127.3, 129.5 (3C), 129.6, 132.8, 135.1, 137.8,
138.2, 139.0, 143.1, 143.5, 143.7, 153.8, 153.9, 170.7 (thienylene and
phenylene carbons).
All reactions were performed under an atmosphere of dry ni-
trogen. NMR spectra were recorded on JNMeECS400 spectrometer
and JNMeLA500 spectrometer. Loweresolution mass spectra were
measured on a JEOL Model JMSe700 instrument. High-resolution
mass spectra were measured using LTQ Orbitrap XL at the Natural
Science Center for Basic Research and Development (N-BARD),
Hiroshima University. UVevisible absorption spectra were
measured with a JASCO Ve560 spectrometer in dioxane. Fluores-
cence spectra were measured with a JASCO FPe8200 spectrometer
and JASCO FPe8300 spectrometer. Melting point was measured
with a YanacoeMPeS3 apparatus. Column chromatography was
performed by using Wakogel Ce300 (WAKO). THF used as a solvent
was distilled from sodium/benzophenone ketyl, just before use.
Compounds 2 and 3 were synthesized following a procedure
described in literature [25,32]. Fabrication and evaluation of DSSCs
were performed as reported in the literature [22,23].
3.1.3. Synthesis of compound 6
In a 50 mL threeenecked flask was placed 9.2435 g (31.5 mmol)
of 2-bromo-5-pentamethyldisilanylthiophene in 150 mL of THF. To
this was added 20.0 mL (32.0 mmol) of a 1.6 M n-butylli-
thiumehexane solution at e80ꢁC. After the mixture was stirred for
2 h at e80ꢁC, the resulting mixture was slowly added to 10.8589 g
(33.4 mmol) of tributyltin chloride at e80ꢁC. After the mixture was
stirred for 1 h at e80ꢁC, it was warmed to room temperature and
stirred over night. After the mixture was hydrolyzed with dilute
hydrochloric acid. The organic layer was separated, washed with
water, and dried over anhydrous magnesium sulfate. After the
solvent was evaporated, compound 6 (10.4707 g, 66% yield) was
isolated by a silica gel column eluting with hexane: MS m/z 504
(Mþ) 1H NMR (
d CDCl3) 0.09 (s, 9H, SiMe3), 0.37 (s, 6H, SiMe2), 0.89
3.1.1. Synthesis of compound 4
(t, 9H, CH3, J ¼ 7.2 Hz), 1.08e1.12 (m, 6H, CH2), 1.34 (sext, 6H, CH2,
In
a 200 mL twoenecked flask was placed 0.6270 g
J ¼ 7.2 Hz), 1.53e1.64 (m, 6H, CH2), 7.25 (d 1H, thienylene proton,
J ¼ 3.6 Hz), 7.33 (d, 1H, thienylene proton, J ¼ 3.6 Hz); 13C NMR (
d
(0.912 mmol) of 2 and 2.4078 g (4.40 mmol) of 4-(5-bromo-3-
hexylthiophen-2-yl)-7-(3-hexylthiophen-2-yl)benzo[c][1,2,5]thia-
diazol (3) and 0.8927 g (2.74 mmol) of cesium carbonate and
0.0676 g (0.0963 mmol) of bis(triphenylphosphine)dichloropalla-
dium in 75 mL of THF and 15 mL of H2O. After the mixture was
heated to reflux for 3 days, it was hydrolyzed with dilute hydro-
chloric acid. The organic layer was separated, washed with water,
and dried over anhydrous magnesium sulfate. After the solvent was
evaporated, compound 4 (0.3743 g, 24% yield) was isolated by a
silica gel column eluting with hexane-ethyl acetate (50:1) as a red
solid: M.p. 75e76 ꢁC. HRMS (ESI) exact mass calcd for C99H106N9S9
(Mþ þ H): 1708.60521, found 1708.60650. MS m/z 1707 (Mþ); 1H
CDCl3) ꢀ2.5 (Me2Si), ꢀ2.4 (Me3Si), 10.9, 13.7, 27.2, 29.0 (Bu), 134.7,
136.2, 142.1, 144.5 (thienyl ring carbons).
3.1.4. Synthesis of compound 7
In
a 100 mL twoenecked flask was placed 0.1031 g
(0.0493 mmol) of 5 and 0.1668 g (0.331 mmol) of 6 and 0.0041 g
(0.0058 mmol) of bis(triphenylphosphine)dichloropalladium in
23 mL of THF. After the mixture was heated to reflux for 4 days, it
was hydrolyzed with dilute hydrochloric acid. The organic layer
was separated, washed with water, and dried over anhydrous
magnesium sulfate. After the solvent was evaporated, compound 7
(0.0274 g, 24% yield) was isolated by a silica gel column eluting with
hexane-chloroform (2:1) as a dark red solid: M.p. 86e87 ꢁC. HRMS
(ESI) exact mass calcd for C126H154N9Si6S12 (Mþ þ H): 2344.75858,
NMR (
d
CDCl3) 0.83 (t, 9H, CH3, J ¼ 7.4 Hz), 0.85 (t, 9H, CH3,
J ¼ 7.4 Hz), 1.21e1.32 (m, 36H, CH2), 1.65 (quint, 6H, CH2, J ¼ 7.4 Hz),
1.72 (quint, 6H, CH2, J ¼ 7.4 Hz), 2.69 (t, 6H, CH2, J ¼ 7.4 Hz), 2.74 (t,
6H, CH2, J ¼ 7.4 Hz), 7.12 (d 3H, thienylene protons, J ¼ 5.2 Hz), 7.46
(d, 3H, thienylene protons, J ¼ 5.2 Hz), 7.52 (s, 3H, thienylene
protons), 7.69 (d, 3H, phenylene protons, J ¼ 7.6 Hz), 7.73 (d, 3H,
phenylene protons, J ¼ 7.6 Hz), 7.90 (d, 6H, phenylene protons,
found 2344.75747. MS m/z 2343 (Mþ); 1H NMR (
d CDCl3) 0.13 (s,
27H, SiMe3), 0.40 (s, 18H, SiMe2), 0.84 (t, 9H, CH3, J ¼ 7.0 Hz), 0.85 (t,
9H, CH3, J ¼ 7.0 Hz), 1.20e1.34 (m, 36H, CH2), 1.69 (quint, 6H, CH2,
J ¼ 7.6 Hz), 1.73 (quint, 6H, CH2, J ¼ 7.6 Hz), 2.69 (t, 6H, CH2,
J ¼ 7.6 Hz), 2.74 (t, 6H, CH2, J ¼ 7.6 Hz), 7.10 (d, 3H, thienylene
protons, J ¼ 3.2 Hz), 7.21 (s 3H, thienylene protons), 7.31 (d, 3H,
thienylene protons, J ¼ 3.2 Hz), 7.52 (s, 3H, thienylene protons), 7.70
(d, 3H, phenylene protons, J ¼ 7.6 Hz), 7.74 (d, 3H, phenylene pro-
tons, J ¼ 7.6 Hz), 7.90 (d, 6H, phenylene protons, J ¼ 8.4 Hz), 8.83 (d,
J ¼ 8.8 Hz), 8.83 (d, 6H, phenylene protons, J ¼ 8.8 Hz); 13C NMR (
d
CDCl3) 13.9 (2C) (CH3), 22.4, 22.5, 29.0, 29.1, 29.3, 29.6, 30.5 (2C),
31.4 (2C) (CH2), 125.2, 125.8, 126.3, 126.9, 127.4, 129.1, 129.3, 129.5,
129.7, 132.1, 132.9, 135.0, 137.6, 141.5, 142.9, 143.5, 153.8, 154.1, 170.4
(thienylene and phenylene carbons).
6H, phenylene protons, J ¼ 8.4 Hz); 13C NMR (
d CDCl3) e2.9 (Me2Si),
3.1.2. Synthesis of compound 5
e2.4 (Me3Si), 14.1 (2C) (CH3), 22.5 (2C), 29.2 (2C), 29.7 (2C), 30.6,
30.7, 31.64 (2C) (CH2), 125.2, 125.6, 126.0, 126.6, 126.9, 127.2, 129.5,
129.6, 129.8, 131.0, 133.1, 134.8, 135.2, 137.9, 138.1, 139.1, 142.2, 142.7,
143.1, 143.6, 154.0 (2C), 171.0 (thienylene and phenylene carbons);
In a 100 mL twoenecked flask was placed 0.2020 g (0.118 mmol)
of 4 in 18 mL of chloroform and 3 mL of acetic acid. To this was
added 0.0997 g (0.443 mmol) of N-iodosuccinimide with ice cool-
ing and the mixture was stirred over night. The mixture was
29Si NMR (
d
CDCl3) e23.5, ꢀ18.8.

68
A. Naka et al. / Journal of Organometallic Chemistry 825-826 (2016) 63e68
T. Takeuchi, Organometallics 19 (2000) 2406.
Acknowledgment
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A. Kunai, T. Takeuchi, M. Ishikawa, J. Organomet. Chem. 694 (2009) 346.
[17] A. Naka, R. Fukuda, R. Kishimoto, Y. Yamashita, Y. Ooyama, J. Ohshita,
M. Ishikawa, J. Organomet. Chem. 702 (2012) 67.
This work was supported by JSPS KAKENHI Grant Number
26410061 and Wesco Science Promotion Foundation.
€
[18] B. O'Regan, M. Gratzel, Nature 353 (1991) 737.
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[19] M. Gratzel, Inorg. Chem. 44 (2005) 6841.
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