Synthesis and Electrochemical and Photophysical Characterization of New 4,4′-π-Conjugated 2,2′-Bipyridines that are End-Capped with Cyanoacrylic Acid/Ester Groups

Article abstract of DOI:10.1002/asia.201501324

Two new functionalized 4,4′-disubstituted 2,2′-bipyridines that were end-capped with cyanoacrylic acid or cyanoacrylic acid ester anchoring groups, which might allow their efficient functionalization on TiO₂ or other metal-oxide semiconductor surfaces, have been synthesized and characterized by electrochemical, photophysical, and spectroscopic measurements. The electrochemical and photophysical properties of these 4,4′-disubstituted 2,2′-bipyridines with extended π systems, in particular their LUMO energies, make them promising candidates to build up inorganic-organic hybrid photosensitizers for the sensitization of metal-oxide semiconductors (e.g., TiO₂ nanoparticles and/or nanotubes). The fantastic 4,4′: The electrochemical and photophysical properties of new 4,4′-disubstituted 2,2′-bipyridines with extended π systems and cyanoacrylic acid or cyanoacrylic acid ester anchoring groups make them promising candidates to build up inorganic-organic hybrid photosensitizers for the sensitization of metal-oxide semiconductors (e.g., TiO₂ nanoparticles and/or nanotubes).

Full text of DOI:10.1002/asia.201501324

DOI: 10.1002/asia.201501324
Full Paper
Bipyridines
Synthesis and Electrochemical and Photophysical Characterization
of New 4,4’-p-Conjugated 2,2’-Bipyridines that are End-Capped
with Cyanoacrylic Acid/Ester Groups
Anja Fingerhut,[a] Yanlin Wu,[b] Axel Kahnt,*[c] Julien Bachmann,*[b] and
Svetlana B. Tsogoeva*[a]
Abstract: Two new functionalized 4,4’-disubstituted 2,2’-bi-
pyridines that were end-capped with cyanoacrylic acid or cy-
anoacrylic acid ester anchoring groups, which might allow
their efficient functionalization on TiO2 or other metal-oxide
semiconductor surfaces, have been synthesized and charac-
terized by electrochemical, photophysical, and spectroscopic
measurements. The electrochemical and photophysical prop-
erties of these 4,4’-disubstituted 2,2’-bipyridines with ex-
tended p systems, in particular their LUMO energies, make
them promising candidates to build up inorganic–organic
hybrid photosensitizers for the sensitization of metal-oxide
semiconductors (e.g., TiO2 nanoparticles and/or nanotubes).
Introduction
of clean and renewable energy. For this to be the case, solar
energy in the form of light needs to be efficiently converted
into storable forms, such as electrical power, or transformed
into “solar fuel”, such as hydrogen, through photocatalytic
water splitting.[1]
Owing to the finite supply of fossil fuels and the negative
impact of fossil-fuel usage on the climate, it is foreseeable that
harvesting solar energy will obviously become the main source
Such light-harvesting systems typically comprise a photosen-
sitizer dye and a metal-oxide or electron acceptor molecule,
which show strong absorption cross-sections in a wide range
of the visible spectrum, as well as sufficiently high LUMO ener-
gies to allow efficient electron-transfer reactions to the accept-
or (the metal-oxide semiconductor conduction band or the
HOMO of the molecular electron acceptor). Strongly fluores-
cent dyes based on nitrogen-containing heterocycles (e.g., pyr-
idines) are commonly employed as photosensitizer building
blocks,[2,3] because of their ease of synthesis; moreover, deriva-
tization of the pyridine ring allows fine-tuning of the optical
and electronic properties of the building blocks, as well as at-
taching anchoring groups for linking to metal-oxide surfa-
ces.[2b,4]
[a] A. Fingerhut, Prof. Dr. S. B. Tsogoeva
Department of Chemistry and Pharmacy
Organic Chemistry Chair I and Interdisciplinary Center for Molecular Materi-
als (ICMM)
Friedrich-Alexander-Universitꢀt Erlangen-Nꢁrnberg (FAU)
Henkestrasse 42
91054 Erlangen (Germany)
E-mail: svetlana.tsogoeva@fau.de
[b] Y. Wu, Prof. Dr. J. Bachmann
Department of Chemistry and Pharmacy
Inorganic Chemistry Chair II
Friedrich-Alexander-Universitꢀt Erlangen-Nꢁrnberg (FAU)
Egerlandstrasse 1
91058 Erlangen (Germany)
E-mail: julien.bachmann@fau.de
[c] Dr. A. Kahnt
Organic bipyridine-based p-extended compounds have
been employed as photoluminescent materials in, for example,
nonlinear optics (NLO). These systems absorb light through in-
tramolecular charge transfer (ICT) and emit from the corre-
sponding photoexcited state.[5] Moreover, bipyridine-, terpyri-
dine-, or phenanthroline-based p-extended compounds are
widely used as ligands in metal complexes (Figure 1),[6] which
are used in electrochemical applications,[7] in particular in light-
emitting diodes[8] or as dyes for dye-sensitized solar cells
(DSSCs).[2b,4] These ligands enable the fine-tuning of the photo-
physical and electrochemical properties of the complexes by
influencing the metal-to-ligand charge transfer (MLCT) pro-
cess.[9,10]
Department of Chemistry and Pharmacy
Physical Chemistry Chair I
Friedrich-Alexander-Universitꢀt Erlangen-Nꢁrnberg (FAU)
Egerlandstrasse 3
91058 Erlangen (Germany)
E-mail: axel.kahnt@fau.de
Supporting information and the ORCID identification numbers for the au-
thors of this article can be found under http://dx.doi.org/10.1002/
asia.201501324.
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This is an open access article under the terms of Creative Commons Attri-
bution NonCommercial License, which permits use, distribution and repro-
duction in any medium, provided the original work is properly cited and is
not used for commercial purposes.
This manuscript is part of a special issue on energy conversion and storage.
A link to the Table of Contents of the special issue will appear here when
the complete issue is published.
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Figure 1. Known pyridine-based p-extended ligands and their metal complexes.[6b, c,f,g]
Such fine-tuning can either be performed by an p-acceptor
Interestingly, p-conjugated 2,2’-bipyridines with cyanoacrylic
ligand, which has a low-lying p* molecular orbital, or a strong
donor ligand, which destabilizes the metal t2g orbitals.[6g,11]
A further possible method for modifying the spectroscopic
and redox properties of the complexes is through extension of
the coordinating pyridine-based moiety with a large p-extend-
ed system, the so-called “antenna function”, to suppress
charge recombination by spatial separation.[12] Application of
this method to p-conjugated 2,2’-bipyridines[6] has been domi-
nated by using donor end-capped antennae.
acids as anchoring acceptor moieties have never been report-
ed before. Herein, we report the synthesis and electrochemical
and photophysical properties of new fluorescent 4,4’-p-conju-
gated 2,2’-bipyridines (5 and 6) that are end-capped with cya-
noacrylic acid or cyanoacrylic acid ester moieties, and investi-
gate their potential applications in photocatalytic processes,
for example, as ligands in inorganic–organic hybrid hole-trans-
port materials and DSSCs.
To the best of our knowledge, examples of 2,2’-bipyridine
4,4’-based p-extended systems that are end-capped with elec-
tron-withdrawing groups (which can also be used as anchoring Results and Discussion
acceptor moieties on metal-oxide surfaces) are scarce;[6b,13]
Synthesis
therefore, the development of new derivatives is in high
demand. Notably, only carboxylic acid (COOH) and ester
(COOR) groups that were directly attached to the correspond-
ing phenyl framework have been used as anchoring groups in
these systems so far. However, cyanoacrylic acid groups are
commonly employed as anchoring moieties of photosensitizers
in DSSCs,[14] because excited electrons can be effectively inject-
ed into the conduction band of TiO2 through the carboxy an-
choring group adjacent to the electron-accepting cyano group
of the organic photosensitizer.[15]
Our design of new 4,4’-p-conjugated 2,2’-bipyridines 5 and 6
was inspired by known neutral organic dyes, which contain cy-
anoacrylic acid as an acceptor moiety.[16] We assumed that the
introduction of an additional C=C double bond between the
2,5-dimethoxyphenyl framework and the cyanoacrylic acid/
ester anchoring groups might increase the p conjugation and
can result in a broader absorption in the visible-light region of
the electromagnetic spectrum. In addition, such molecules are
synthetically more accessible in comparison to known litera-
ture compounds 4,4’-p-conjugated 2,2’-bipyridines.[6b,13]
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Figure 2. Reaction conditions: a) tBuOK, THF, 708C, 24 h (97% yield); b) 2m HCl, CHCl3, RT, 10 h (98% yield); c) cyanoacetic acid (40 equiv), piperidine, MeCN,
908C, 24 h (46% yield); d) cyanoacetic acid butyl ester, (40 equiv), piperidine, MeCN, 908C, 24 h (63% yield).
For the preparation of new bipyridine dyads, initially, a four-
step literature procedure of 4,4’-bis(diethylphosphonatometh-
yl)-2,2’-bipyridine (1; Figure 2) was performed, starting from
commercially available 4,4’-dimethyl-2,2’-bipyridine.[13]
To avoid the formation of side products in the subsequent
Horner–Wadsworth–Emmons (HWE) reaction (Figure 2), an ad-
ditional dialdehyde-protection step was performed (see the
Supporting Information). Thus, monoprotected dibenzaldehyde
4-(1,3-dioxalan-2-yl)2,5-dimethyloxybenzalde-hyde (2) was syn-
thesized under stoichiometric control by using ethylene glycol
and para-toluenesulfonic acid as a catalyst. After purification
by column chromatography on silica gel, monoacetal 2 was
obtained in moderate yield. Then, 4-(1,3-dioxalan-2-yl)2,5-dime-
thyloxybenzaldehyde (2) and 4,4’-bis(diethylphosphonato-
methyl)-2,2’-bipyridine (1) in a 2:1 molar ratio were applied in
a trans-selective C=C bond-forming HWE reaction (Figure 2). p-
Conjugated two-armed bipyridine compound 3 was obtained
in excellent yield without further purification. After the suc-
cessful HWE coupling reaction, acid hydrolysis of the acetals re-
generated the free aldehyde moieties in bipyridine compound
4 in almost quantitative yield.
A typical Knoevenagel condensation reaction between free-
aldehyde-containing bipyridine 4 and an excess of cyanoacetic
acid (as a CH-acidic compound) was performed with piperidine
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as a base to obtain compound 5 as an orange, sparingly solu-
ble solid after trituration with THF and acetone. Owing to its
insolubility, we could not characterize this compound by
13C NMR spectroscopy.
Both compounds 5 and 6 adsorbed onto amorphous TiO2
(Figure 3a), even from the polar solvent DMSO (5). The spec-
troscopic ellipsometry curves displayed marked shifts upon
soaking the planar substrates in the corresponding solutions,
and the curves did not revert to their initial states, even after
extensive rinsing in pure solvent. The results were different on
crystalline TiO2, which provided fewer reactive surface
sites.[17,18] Compound 5 (end-capped with cyanoacrylic acid
groups) did not yield a stable chemisorbed layer, whereas
compound 6 did (Figure 3b). This difference was not owing to
solvent effects, because it was also observed if both com-
pounds were dissolved in a mixture of CH2Cl2 and DMSO (1:1
v/v). The difference could be owing to hydrogen-bonding in-
teractions between the carboxylic acid moieties and the bipyri-
dine units, which might significantly decrease the reactivity of
the linker.
To improve the solubility of the bipyridine dyads, a less-
polar ester function, rather than the carboxylic acid function
derived from cyanoacetic acid, was desirable. Thus, a Knoevena-
gel condensation reaction was performed between aldehyde-
containing two-armed bipyridine 4 and an excess of cyanoace-
tic acid butyl ester (as a CH-acidic compound), which yielded
target compound 6 with good solubility in CH2Cl2.
Adsorption Behavior
The utilization of molecular dyes in dye-sensitized solar cells
necessitates that the bipyridine ligands bind to the surface of
TiO2 colloids through an ester linkage. For this purpose, crystal-
line colloids are typically coated with a thin layer of amor-
LUMO Energies
phous TiO2, which provides
sites.[17,18]
a high number of reactive
The next requirement of bipyridine ligands for use in dye-sen-
sitized solar cells or other light-harvesting devices is that an
electron that is photoexcited into the LUMO of the bipyridine
must have sufficient energy to be injected into the TiO2 con-
duction band.
A first approximation of the LUMO energy was obtained by
using voltammetric methods. Figure 4 shows differential pulse
Figure 4. Differential pulse voltammetry of compounds 5 (blue) and 6
(green) as dilute (5 mm) solutions in CH2Cl2/DMSO (1:1 v/v). Arrows denote
the reduction peak for each compound.
voltammetry (DPV) curves for dilute solutions of compounds 5
and 6. Reduction was observed at ꢀ0.22 V and ꢀ0.06 V (vs.
NHE) for compounds 5 and 6, respectively. The difference be-
tween these values was most likely owing to electrostatic (pro-
tonation) effects. Importantly, both reduction potentials were
close the energy of the TiO2 valence band (ꢀ4.21 eV on the ab-
solute scale, ꢀ0.31 V vs. NHE).[19]
Figure 3. Adsorption behavior of compounds 5 (blue) and 6 (green) on TiO2
studied by spectroscopic ellipsometry of Si/SiO2/TiO2 wafers: a) with an
amorphous TiO2 layer (before annealing) and pure DMSO (5) or CH2Cl2 (6);
b) with a crystalline TiO2 layer (after annealing) and DMSO/CH2Cl2 (1:1, v/v)
mixtures for both compounds. In each case, three curves are shown: before
soaking (light blue/green), after soaking (blue/green), and after an additional
overnight rinse in pure solvent (dark blue/green).
Photophysical Properties
The ground-state absorption spectra of compounds 6 and 5 in
THF exhibited almost-identical characteristic absorptions, with
bands in the UV and blue-light regions of the optical spectrum
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have fluorescence quantum yields of 0.11 and 0.03 in toluene,
respectively.[20] Fluorescence lifetimes of 1.1 ns (6) and 1.3 ns
(5) were obtained upon photoexcitation at 403 nm (Figure 6
and the Supporting Information, Figure S15).
Transient absorption measurements based on femtosecond
laser photolysis provided insight into the formation and fate of
the excited states, in particular the S1!SN transitions. Solutions
of compounds 6 and 5 in THF were photoexcited with femto-
second laser pulses with a wavelength of 387 nm. Both sam-
ples showed essentially identical transient absorption spectra,
with two dominating maxima at 580 and 810 nm, which were
owing to S1!SN transitions, accompanied by a minor mini-
mum around 510 nm, which originated from spontaneous
emission (see the fluorescence spectra), and poorly resolved
transient bleaching about 430 nm, which mirrored the ground-
state absorption (see Figure 7 and the Supporting Information,
Figure S16). For both samples, transient absorption decays to
the ground state were recorded, with lifetimes of 1.0 ns (6)
and 1.2 ns (5), in line with the measured fluorescence lifetimes.
Figure 5. Absorption spectrum of compound 6 in THF.
Figure 6. a) Fluorescence spectrum of compound 6 in THF upon photoexci-
tation at 420 nm. b) Fluorescence–time profile for compound 6 in THF at
500 nm, upon photoexcitation at 403 nm (red); the Instrument Response
Function (IRF) is shown in black.
Figure 7. a) Femtosecond transient absorption spectra of compound 6 in Ar-
saturated THF at different times after excitation with a femtosecond laser
pulse (387 nm, 200 nJpulseꢀ1, <150 fs FWHM): 1 ps (black), 10 ps (red),
100 ps (green), 1000 ps (blue), and 5500 ps (cyan). b) Corresponding absorp-
tion–time profiles at 580 nm (black) and 810 nm (red).
and maxima at 350 and 436 nm (6) and 346 and 430 nm (5;
see Figure 5 and the Supporting Information, Figure S14).
Upon photoexcitation at 420 nm, solutions of compounds 6
and 5 in THF exhibited strong fluorescence between 450 and
750 nm, with maxima at 513 nm (6) and 496 nm (5)— see
Figure 6 and the Supporting Information, Figure S15. Fluores-
cence quantum yields of 0.075 (6) and 0.085 (5) in THF were
calculated by using tetraphenylporphyrin (H2TPP) and zinc tet-
raphenylporphyrin (ZnTPP) as reference compounds, which
Conclusion
Herein, we have reported the synthesis and electrochemical
and photochemical properties of two new symmetrically disub-
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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)
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(2E,2’E)-3,3’-(((1E,1’E)-[2,2’-Bipyridine]-4,4’-diylbis(ethene-2,1-
diyl))bis (2,5-dimethoxy-4,1-phenylene))bis(2-cyanoacrylic
acid) (5)
Adsorption Tests
Si wafers were coated with TiO2 to a thickness of several nanome-
ters by using. The substrates before annealing (amorphous TiO2) or
after annealing (crystalline TiO2) were soaked in solutions of com-
pound 5 or 6 (0.01 mm in DMSO, CH2Cl2, or a 1:1 v/v mixture) for
20 min. The samples were well-rinsed with pure solvent (or the sol-
vent mixture) before performing the ellipsometry measurements.
As an additional check, the samples were subsequently soaked in
the solvent overnight and rinsed before the ellipsometry measure-
ments.
Cyanoacetic acid (970 mg, 11.4 mmol) and piperidine (1.13 mL,
11.4 mmol) were added to a suspension of compound 4 (153 mg,
0.285 mmol) in dry MeCN (12 mL). The yellow suspension was
stirred for 24 h at 908C. Then, CH2Cl2 and a 2m aqueous solution
of H3PO4 were added and the mixture turned orange. The mixture
was filtered and washed with THF and acetone. The filtrate was
dried under vacuum and the title compound was obtained as an
orange, sparingly soluble solid (87.2 mg, 0.13 mmol, 46% yield).
1
M.p. 2808C (dec.); H NMR (400 MHz, [D6]DMSO; NMR analysis was
Electrochemical Measurements
facilitated by the addition of a small amount of hot DMF): d=8.72
(d, J=4.8 Hz, 2H), 8.60 (s, 2H), 8.24 (s, 2H), 7.84 (s, 2H), 7.78 (d, J=
16.3 Hz, 2H), 7.70 (dd, J1 =4.9 Hz, J2 =1.5 Hz, 2H), 7.66 (d, J=
16.3 Hz, 2H), 7.54 (s, 2H), 3.95 (s, 6H), 3.90 ppm (s, 6H); FTIR (ATR):
n˜ =3599 (w), 2948 (w), 2836 (w), 2363 (m), 2215 (w), 2115 (w), 1921
(w), 1711 (m), 1588 (s), 1498 (m), 1464 (m), 1415 (m), 1352 (m),
1251 (s), 1215 (s), 1031 (s), 960 (s), 864 (m), 810 (m), 731 (m), 660
(m), 567 (s), 484 (m), 430 cmꢀ1 (s); MS (MALDI): m/z: 671 [M+H]+;
HRMS (MALDI): m/z calcd for C38H30N4O8: 671.2136 [M+H]+; found:
see the Supporting Information, Figure S9.
Differential pulse voltammetry (DPV) was performed in 5 mm
NaBF4 solution in a mixture of CH2Cl2 and DMSO (1:1 v/v). Com-
pounds 5 and 6 were added to a concentration of 50 mm. The pa-
rameters for the DPV experiments were as follows: voltage range:
ꢀ0.791 V to +1.209 V (vs. NHE), step size: 2 mV, sample period:
0.25 s, pulse duration: 0.05 s, pulse size: 15 mV.
Photochemical Measurements
Steady-state UV/Vis absorption spectra were recorded on Perki-
nElmer Lambda 2 two-beam spectrophotometers. Fully corrected
steady-state fluorescence spectra were recorded on a FluoroMax3
spectrometer (Horiba Jobin Yvon).
(2E,2’E)-Dibutyl-3,3’-(((1E,1’E)-[2,2’-bipyridine]-4,4’-diylbis
(ethene-2,1-diyl))bis(2,5-dimethoxy-4,1-phenylene))bis(2-cya-
noacrylate) (6)
Fluorescence lifetimes were determined by using the time-correlat-
ed single-photon-counting (TCSPC) technique on a Fluorolog 3
(Horiba Jobin Yvon). The samples were excited with a NanoLED-
405 (403 nm), and the signal was detected by a Hamamatsu MCP
photomultiplier (type R3809U-50). The time profiles were recorded
at 500 nm.
Butyl cyanoacetate (1.24 mL, 14.5 mmol) and piperidine (862 mL,
14.5 mmol) were added to a suspension of 4,4’-((1E,1’E)-[2,2’-bipyri-
dine]-4,4’-diylbis(ethane-2,1-diyl))bis(2,5-dimethoxybenzaldehyde)
(4; 117 mg, 0.218 mmol) in dry MeCN (9 mL). The yellow suspen-
sion was stirred for 24 h at 908C. After cooling to RT, the mixture
was filtered and the residue was taken up with CH2Cl2. After evapo-
ration of the solvent, the product was dried under vacuum. The
title compound was obtained by recrystallization from CH2Cl2/Et2O
(107 mg, 0.137 mmol, 63% yield). M.p. 2558C; 1H NMR (300 MHz,
CDCl3): d=8.73 (s, 2H), 8.69 (d, J=5.0 Hz, 2H), 8.57 (s, 2H), 7.98 (s,
2H), 7.77 (d, J=16.5 Hz, 2H), 7.48 (d, J=5.1 Hz, 2H), 7.31 (d, J=
16.4 Hz, 2H), 7.15 (s, 2H), 4.31 (t, J=6.6 Hz, 4H), 3.95 (s, 6H), 3.94
(s, 6H), 1.73 (m, 4H), 1.46 (dq, J1 =14.4 Hz, J2 =7.3 Hz, 4H),
0.97 ppm (t, J=7.4 Hz, 6H); 13C NMR (75 MHz, CDCl3): d=162.9,
156.5, 154.0, 151.4, 149.6, 149.6, 148.4, 145.5, 132.0, 129.9, 127.4,
120.9, 116.4, 110.8, 109.3, 109.2, 101.3, 66.3, 56.2, 56.2, 30.5, 19.1,
13.5 ppm; MS (MALDI): m/z: 783 [M+H]+; HRMS (ESI): m/z calcd for
C46H47N4O8: 783.3388 [M+H]+; found: 783.3378; FTIR (ATR): n˜ =
3604 (w), 3057 (w), 2963 (m), 2362 (m), 2213 (m), 2017 (w), 1913
(w), 1804 (w), 1717 (s), 1635 (w), 1576 (s), 1497 (s), 1457 (s), 1404
(s), 1357 (m), 1296 (m), 1209 (s), 1150 (m), 1084 (m), 1024 (s), 951
(s), 906 (m), 849 (m), 737 (m), 677 (m), 641 (m), 602 (m), 550 cmꢀ1
(m).
Transient absorption measurements based on femtosecond laser
photolysis were performed by using the output of a Ti/sapphire
laser system (CPA2110, Clark-MXR Inc.): 775 nm, 1 kHz, and 150 fs
full width at half maximum (FWHM) pulses. The excitation wave-
length was generated by second harmonic generation (387 nm),
with pulse widths of <150 fs and energies of 200 nJpulse-1. Transi-
ent absorption detection was performed by using a transient ab-
sorption pump/probe system (TAPPS, Ultrafast Systems).
Acknowledgements
S.B.T. is thankful for funding from the Interdisciplinary Center
for Molecular Materials (ICMM) and the Solar Technologies go
Hybrid (Research Network SolTech) initiative of the Bavarian
State Ministry for Science, Research and Art. J.B. is grateful for
generous financial support from the European Research Coun-
cil under the European Union’s Horizon 2020 Research and In-
novation Program (Grant No. 647281, “Solacylin”). A.K. grateful-
ly acknowledges funding from the Deutsche Forschungsge-
meinschaft (KA 3491/2-1). The authors kindly thank Mr. Jacob
Hitzenberger (Univ. Erlangen-Nꢂrnberg, Germany) for perform-
ing mass spectrometry (MALDI) analysis of compound 5. Finan-
cial support from the Erika Giehrl-Stiftung (doctoral research
fellowship) and the Friedrich-Alexander-Universitꢃt Erlangen-
Nꢂrnberg, Germany (Fçrderung der Chancengleichheit fꢂr
Frauen in Forschung und Lehre (FFL) scholarship) to A.F. is
gratefully acknowledged.
Instrumental Methods
Spectroscopic ellipsometry data were collected from 400–1000 nm
at a 708 incidence angle on an EL X-02 P Spec apparatus (DRE Dr
Riss Ellipsometerbau GmbH). Fits were performed by using the da-
tabase of material files provided with the instrument. Electrochem-
istry data were collected on a Gamry interface 1000. ALD of TiO2
was performed at 1208C from Ti(OiPr)4 and water in a homemade
hot-walled reactor.
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Keywords: adsorption
fluorescence · photosensitizers
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Manuscript received: November 29, 2015
Accepted Article published: January 14, 2016
Final Article published: && &&, 0000
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Full Paper
FULL PAPER
Bipyridines
The fantastic 4,4’: The electrochemical
and photophysical properties of new
4,4’-disubstituted 2,2’-bipyridines with
extended p systems and cyanoacrylic
acid or cyanoacrylic acid ester anchor-
ing groups make them promising candi-
dates to build up inorganic–organic
hybrid photosensitizers for the sensitiza-
tion of metal-oxide semiconductors
(e.g., TiO2 nanoparticles and/or nano-
tubes).
Anja Fingerhut, Yanlin Wu, Axel Kahnt,*
Julien Bachmann,* Svetlana B. Tsogoeva*
&& – &&
Synthesis and Electrochemical and
Photophysical Characterization of New
4,4’-p-Conjugated 2,2’-Bipyridines that
are End-Capped with Cyanoacrylic
Acid/Ester Groups
Chem. Asian J. 2016, 00, 0 – 0
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ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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