Full Paper
was used as the working and auxiliary electrodes; Ag/AgCl (sat.)/
NaCl (3m) was purchased from Bionalytical Systems, Inc. (BASi) and
used as a reference electrode (+0.209 V vs. NHE).
stituted bipyridines (5 and 6) that were end-capped with cya-
noacrylic acid or cyanoacrylic acid ester moieties; these com-
pounds are suitable as ligands for the generation of metal-
based photosensitizers and for the chemical functionalization
of TiO2. Their adsorption behavior on TiO2 was studied by
using spectroscopic ellipsometry and a first approximation of
their LUMO energies was obtained by using voltammetric
methods. These measurements provided solid evidence for the
efficient anchoring of compound 6 on TiO2 and showed a suffi-
cient LUMO energy for efficient electron transfer into the TiO2
conduction band.
4-(1,3-Dioxalan-2-yl)2,5-dimethyloxybenzakdehyde (2)
2,5-Bis-methyloxy-1,4-dibenzaldehyd (500 mg, 2.58 mmol), ethylene
glycol (187 mL, 3.35 mmol), and para-toluenesulfonic acid (9.80 mg,
2 mol%) were dissolved in dry toluene (10 mL) and stirred for 21 h
at reflux. After cooling to RT, the solution was washed with distilled
water (3ꢁ20 mL), dried over MgSO4, and the solvent was removed
by rotary evaporation. Subsequently, the crude product was puri-
fied by column chromatography on silica gel (petroleum ether/
EtOAc, 4:1 v/v), and compound 2 was obtained as the main prod-
uct in the second fraction after removal of the eluent as a white
solid (307 mg, 1.29 mmol, 50% yield). 1H NMR (300 MHz, CDCl3):
d=10.41 (s, 1H), 7.30 (s, 1H), 7.20 (s, 1H), 6.06 (s, 1H), 4.15–4.00
(m, 4H), 3.88 (s, 3H), 3.83 ppm (s, 3H); 13C NMR (100 MHz, CDCl3):
d=189.5, 156.6, 151.9, 133.7, 125.3, 110.8, 109.3, 98.7, 65.4, 56.2,
56.2 ppm; MS (MALDI): m/z: 238 [M+H]+.
Photophysical studies demonstrated the feasibility of these
compounds as potential photosensitizer dyes. Particularly ap-
pealing was the combination of matching the LUMO energies
relative to the TiO2 conduction band with a sufficient long-
lived singlet first excited state (>1 ns), which we anticipate
would give rise to efficient electron transfer from the disubsti-
tuted bipyridine derivative into the TiO2 conduction band; this
property is expected to be confirmed in a subsequent study.
The synthesis of the corresponding Ru, Mn, and Fe complexes
of our new organic photosensitizers (5 and 6; for promising re-
sults from a preliminary complexation experiment, see the
Supporting Information, Figure S17) and their covalent binding
to the surface of TiO2 nanoparticles and/or nanotubes for ap-
plications in visible-light photocatalytic water oxidation and/or
DSSCs is currently underway in our laboratory.
4,4’-Bis((E)-4-(1,3-dioxolan-2-yl)-2,5-dimethoxystyryl)-2,2’-bi-
pyridine (3)
A suspension of potassium tert-butoxide (65 mg) in anhydrous THF
(6 mL) was added dropwise under a nitrogen atmosphere to a solu-
tion of compound 1 (89 mg, 0.195 mmol) and 4-(1,3-dioxolan-2-yl)-
2,5-dimethoxybenz-aldehyde (2; 102 mg, 0.428 mmol) in anhy-
drous THF (2 mL) and the yellow-brownish reaction mixture was
heated at reflux for 39 h. The reaction was quenched by the addi-
tion of water (HPLC grade, 30 mL) and, after filtration, the residue
was taken up in CH2Cl2. The filtered matter was extracted with
CH2Cl2 and the combined organic phases were dried with MgSO4.
After removal of the solvent, the product was dried under vacuum.
The title compound (3) was obtained as a yellowish solid (119 mg,
Experimental Section
Materials and General Procedures
4,4’-Bis(diethylphosphonatomethyl)-2,2’-bipyridine (1)[15] was pre-
pared according to a literature procedure. Chemicals were pur-
chased from commercial sources and used without further purifica-
tion. All solvents were purified by distillation, dried according to
standard procedures, or of HPLC grade. All of the reactions were
performed in flame-dried glassware under a nitrogen atmosphere
by using standard Schlenk techniques. Preparative (flash) column
chromatography was performed on Acros Silica gel 60 (0.035–
0.070) as the stationary phase. All products were dried under high
vacuum (ꢁ10ꢀ3 bar). Thin-layer chromatography (TLC) was per-
formed on precoated aluminum silica gel SIL G/UV254 plates (Ma-
1
0.190 mmol, 97% yield). H NMR (300 MHz, CDCl3): d=8.65 (d, J=
5.1 Hz, 2H), 8.53 (s, 2H), 7.77 (d, J=16.5 Hz, 2H), 7.45 (dd, J1 =
5.2 Hz, J2 =1.6 Hz, 2H), 7.17 (d, J=16.2, 2H), 7.14 (s, 2H), 7.13 (s,
2H), 6.12 (s, 2H), 4.18–4.02 (m, 8H), 3.90 (s, 6H), 3.89 ppm (s, 6H);
13C NMR (100 MHz, CDCl3): d=156.7, 151.9, 151.8, 149.5, 146.2,
128.2, 127.3, 127.0, 126.6, 120.8, 118.8, 110.0, 109.7, 99.0, 65.3, 56.3,
56.1 ppm; MS (MALDI): m/z: 625 [M+H]+.
4,4’-((1E,1’E)-[2,2’-Bipyridine]-4,4’-diylbis(ethene-2,1-diyl))-
bis(2,5-di-methoxybenzaldehyde) (4)
1
cherey–Nagel & Co.). H and 13C NMR spectra were recorded at RT
on Bruker Avance 300 and JEOL JNM GX 400 spectrometers at
300 MHz and 400 MHz, respectively. All chemical shifts (ppm) are
related to undeuterated solvent. NMR data were processed by
using the MestReNova program. MS (MALDI) analysis was per-
formed on a Shimadzu Biotech AXIMA Confidence; MS (ESI) analy-
sis was performed on Bruker Daltonik maXis 4G or Bruker Daltonik
micrOTOF II focus spectrometers. IR spectra were recorded on
a Varian IR-660 apparatus; absorptions are reported in wavenum-
bers (cmꢀ1).
2m HCl (6 mL) was gradually added to a solution of compound 3
(119 mg, 0.190 mmol) in CHCl3 (18 mL) and the red mixture was
stirred for 14.5 h. Then, the organic layer was separated, washed
with a saturated aqueous solution of NaHCO3 and brine, and finally
dried with MgSO4. The solvent was evaporated and, to complete
the deprotection step, the crude product was treated gradually
with 2m HCl (6 mL) in CHCl3 (18 mL). The organic layer was sepa-
rated, washed with a saturated aqueous solution of NaHCO3 and
brine, and finally dried over MgSO4. The solvent was evaporated
and the title compound was obtained as a yellow solid (100 mg,
0.186 mmol, 98% yield). 1H NMR (300 MHz, CDCl3): d=10.44 (s,
2H), 8.69 (d, J=5.2 Hz, 2H), 8.61 (s, 2H), 7.79 (d, J=16.5 Hz, 2H),
7.49 (dd, J1 =5.2 Hz, J2 =1.5 Hz, 2H), 7.36 (s, 2H), 7.30 (d, J=
16.5 Hz, 2H), 7.21 (s, 2H), 3.97 (s, 6H), 3.91 ppm (s, 6H); 13C NMR
(75 MHz, CDCl3): d=189.0, 156.4, 156.1, 151.7, 149.4, 132.6, 129.9,
127.9, 124.9, 121.1, 119.1, 117.8, 110.4, 109.4, 56.2, 56.1 ppm; MS
(MALDI): m/z: 537 [M+H]+.
Sodium tetrafluoroborate (NaBF4) and tetrabutylammonium tetra-
fluoroborate (tBu4NBF4) were purchased from Sigma–Aldrich and
used as supporting salts. CH2Cl2 and DMSO were purchased from
commercial suppliers and used as received. The atomic layer depo-
sition (ALD) precursor Ti(OiPr)4 was purchased from Alfa Aesar, and
water was purified immediately before use in a Millipore Direct-Q
system. Boron-doped [100] CZ silicon wafers with 200 nm thermal
oxide were purchased from Silicon Materials. Pt wire (Alfa Aesar)
&
&
Chem. Asian J. 2016, 00, 0 – 0
6
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!