"click-chemistry" approach in the design of 1,2,3-triazolyl- pyridine ligands and their Ru(II)-complexes for dye-sensitized solar cells

Article abstract of DOI:10.1039/c0jm03750h

The synthesis of new 1,2,3-triazolyl-pyridine ligands via "click-chemistry" and their corresponding Ru(L)(2,2′-bipyridyl- 4,4′-dicarboxylic acid)(NCS)₂ complexes (L = 1,2,3-triazolyl-pyridine) are presented. The complexes have been photophysica

Full text of DOI:10.1039/c0jm03750h

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PAPER
‘‘Click-chemistry’’ approach in the design of 1,2,3-triazolyl-pyridine ligands
and their Ru(II)-complexes for dye-sensitized solar cells
a
a
b
b
Ilona Stengel, Amaresh Mishra, Nuttapol Pootrakulchote, Soo-Jin Moon, Shaik M. Zakeeruddin,
b
*
b
Michael Gratzel and Peter Bauerle
a
*
*
€
€
Received 2nd November 2010, Accepted 13th December 2010
DOI: 10.1039/c0jm03750h
The synthesis of new 1,2,3-triazolyl-pyridine ligands via ‘‘click-chemistry’’ and their corresponding
Ru(L)(2,20-bipyridyl-4,40-dicarboxylic acid)(NCS)2 complexes (L ¼ 1,2,3-triazolyl-pyridine) are
presented. The complexes have been photophysically and electrochemically characterized and have
been used as sensitizers in dye-sensitized solar cells (DSSC). In DSSCs with an acetonitrile-based
electrolyte the cells comprising of Ru(2-(1-(4-hexylphenyl)-1H-1,2,3-triazol-4-yl)pyridine)(2,20-
bipyridyl-4,40-dicarboxylic acid)(NCS)2 TBA salt 1 showed an overall power conversion efficiency of
7.8% under full sunlight intensity, and Ru(2-(4-(4-hexylphenyl)-1H-1,2,3-triazol-1-yl)pyridine)(2,20-
bipyridyl-4,40-dicarboxylic acid)(NCS)2 TBA salt 2 an efficiency of 4.7%. Transient photovoltage and
photocurrent decay measurements showed an enhanced performance for dye 1 due to faster electron
transport into the TiO2 film and lower recombination rate in comparison to dye 2 sensitized devices.
Additionally, solid-state devices were prepared with 2 mm thick TiO2 films using spiro-OMeTAD as
a hole-transport material. The solid-state dye-sensitized solar cells showed power conversion
efficiencies of 1.92% and 0.38% for sensitizer 1 and 2, respectively.
In general, bi- and terpyridines (bpy and terpy) are the most
frequently investigated ligands for ruthenium(II) complexes.4
Relatively little is known about Ru(II) complexes in which 1,2,4-
triazoles5 or 1,2,3-triazoles are used as chelating ligands.
However, growing interest in 1,2,3-triazolyl-derivatives as
chelates is evolving since 2001, when the ‘‘click-chemistry’’
approach was presented.6a In particular, the 1,3-dipolar Cu(I)-
catalyzed Huisgen cycloaddition of azides and terminal acety-
lenes is widely used in various areas in order to build up a diverse
series of 1,2,3-triazoles under mild reaction conditions and in
high yields.6–8
Introduction
Dye-sensitized solar cells (DSSC) have attracted great interest in
materials science in the past years. First results were reported in
1991,1 and ever since chemists are in search of new dyes which
appear as suitable sensitizers in titanium dioxide-based organic
photovoltaic cells.2 DSSCs with power conversion efficiencies
over 10% were initially demonstrated using prototype cis-
di(thiocyanato)-bis[2,20-bipyridyl-4,40-dicarboxylic acid] Ru(II)
(N3), its bis-tetrabutylammonium (TBA) salt counterpart N719
or black dye [tri(thiocyanato)-(4,40,400-[2,20:60,200-terpyridine]
tricarboxylic acid] Ru(II) as sensitizers. The dyes are the key-
components in DSSCs as they absorb sunlight and induce
intramolecular charge transfer from the ancillary ligand to the
anchoring ligand with subsequent electron injection into the
TiO2 semiconductor. In this respect, numerous reports about
bipyridine-Ru(II) dyes, which, for example were functionalized
with either hydrophobic chains to improve the device stability or
with elongated p-systems to enhance absorption have been
published.3
Very recently, several bi- and terdentate ligands such as (1,2,3-
triazol-4-yl)pyridines,7a,c,d,8a,b
2,6-bis(1,2,3-triazol-4-yl)pyr-
idines7b,8c and 4,40-bis(1,2,3-triazoles)7c–e and their corresponding
Ru(II) complexes were synthesized using click-chemistry. Crys-
tallographic analysis has shown that upon replacing one pyridyl
unit with one triazolyl unit in the Ru(II) coordination sphere, the
geometry of the complex is only slightly affected.8b,c Complexes
with asymmetrical triazolyl-pyridine ligands were generally iso-
lated as isomeric mixtures. Concerning the photophysical prop-
erties of such compounds, it was observed that with an increasing
number of triazole rings the HOMO/LUMO energy gap of the
octahedral Ru(II) complex increases in comparison to their bi-
and terpyridine counterparts.8a,c As a result, the absorption and
emission spectra are blue-shifted. Triazolyl-pyridine has a higher
acceptor strength compared to bipyridine, therefore, better back-
bonding from the Ru(II) metal centre to the ligand-centered p*
orbital is possible, which lowers the metal-centered highest
aInstitute of Organic Chemistry II and Advanced Materials, University of
Ulm, Albert-Einstein-Allee 11, D-89081 Ulm, Germany. E-mail: peter.
baeuerle@uni-ulm.de; Fax: +49 (0)731 50 22840
bLaboratory for Photonics and Interfaces, Institute of Chemical Sciences
and Engineering, School of Basic Sciences, Swiss Federal Institute of
Technology, CH-1015 Lausanne, Switzerland. E-mail: shaik.zakeer@
epfl.ch; michael.graetzel@epfl.ch; Fax: +41 (0)21 693 61 00
3726 | J. Mater. Chem., 2011, 21, 3726–3734
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occupied molecular orbital (HOMO) and increases the p* orbital
energy of the triazolyl ligand. In heteroleptic complexes, with one
bpy ligand and a triazolyl-pyridine ligand, the lowest unoccupied
molecular orbital (LUMO) is mostly located on the bpy ligand,
because the p* orbital of the triazole ligand is more destabilized.
In general, one can say that the LUMO is shifted to higher
energies with an increasing number of triazole units in the Ru(II)
complex, but it also strongly depends on the nature of the
substituents which are attached to the triazole or the pyridine.
Acceptor substituents are able to lower the LUMO energy and
they show the biggest influence if they are linked to the bpy
chelator or to the pyridine moiety in the triazolyl-pyridine
ligand.8
In search of new ligands for Ru(II) dyes in DSSCs it is shown
that pyridines can successfully be replaced by other N-containing
ligands, for example, pyrazoles.9 In a recent report, the click-
chemistry approach was used to functionalize the periphery of an
ancillary bipyridine-ligand in a Ru(II) dye,10 while there are no
examples reported up to now where 1,2,3-triazoles have been
used as chelating ligands in Ru(II) sensitizers for solar cell
applications. In order to gain more insight into structure–prop-
erty-device performance relationships, we report herein the
synthesis
of
new
dithiocyanato-(1H-1,2,3-triazolyl-
pyridine)(2,20-bipyridyl-4,40-carboxylic acid) ruthenium(II) dyes
1 and 2. The photophysical and electrochemical investigations
provide insight into the electronic structure and potential
applicability of the novel dyes. The complexes were incorporated
and characterized in DSSCs using a volatile solvent-based elec-
trolyte on one hand and a solid-state organic hole conductor
(spiro-MeOTAD) as redox mediator on the other hand.
Scheme 1 Synthesis of bidentate 1,2,3-triazolyl-pyridine ligands. a)
NaN3, CuI, Na ascorbate, DMEDA, EtOH–water (7 : 3), 100 ꢁC, 1 h,
92% (4) and 68% (8). b) KF, Cu(CH3CN)4PF6, Cu0, DIPEA, DCM–
MeOH (4 : 1), rt, 3 d, 89%.c) Cu(CH3CN)4PF6, Cu0, DIPEA, DCM–
MeOH (4 : 1), reflux, 5 d, 73%.
separated via column chromatography. It has been previously
reported that the isomers are generally difficult to separate when
bidentate triazolyl-pyridine were used as ligands.8a
Results and discussion
Synthesis
Azides 4 and 8 were synthesized from their corresponding halides
via a Cu(I)-catalyzed substitution reaction with sodium azide,11
(Scheme 1). The click-reaction was performed either by in situ
deprotection of the trimethylsilyl-protected ethynyl group with
potassium fluoride or directly with the deprotected ethynyl
compound. Ligand 6 was obtained in 89% yield by the Cu(I)-
catalyzed click-reaction of azide 4 and trimethylsilyl-protected
ethynylpyridine 5 at room temperature. However, in the case of
ligand 10 the reaction is much slower at room temperature.
Therefore, click-reaction of azide 8 and ethynyl compound 9
were performed under reflux conditions for five days and ligand
10 was obtained in 73% yield.
Photophysical properties
Absorption and emission spectra of dyes 1 and 2 are depicted in
Fig. 1. Both dyes show broad absorption from the UV region up
to 650 nm. The high energy absorption band at 250–300 nm can
be assigned to intraligand p–p* transitions. Both dyes show two
additional absorption bands at lower energies which can be
attributed to metal-to-ligand charge transfer transitions
(MLCT). Dye 1 showed the lowest energy MLCT absorption
band at 493 nm with a molar extinction coefficient of 7600 L
molꢀ1 cmꢀ1, which is lower than the higher energy MLCT band
(lmax ¼ 369 nm, 3 ¼ 12000 L molꢀ1 cmꢀ1). In contrast, dye 2
showed the lowest energy MLCT band at 465 nm with a higher
extinction coefficient of 9200 L molꢀ1 cmꢀ1 than the higher
energy MLCT band at 366 nm (3 ¼ 8000 L molꢀ1 cmꢀ1). The
lowest energy MLCT band of 2 is blue-shifted by 30 nm
compared to 1. In comparison to bipyridine-based Ru(II) dyes
the new dyes exhibit blue-shifted absorption bands with lower
molar extinction coefficients.3
The Ru(II) dyes 1 and 2 were obtained from a one-pot synthesis
in which di-m-chloro(p-cymene)ruthenium(II) dimer was used as
a precursor (Scheme 2). In a first step, the Ru(II) precursor was
reacted with ligands 6 and 10, respectively, in the second step,
2,20-bipyridyl-4,40-dicarboxylic acid (dcbpy) was introduced and
in the last reaction step thiocyanate was added. Both complexes
were purified via Sephadex LH-20 column chromatography with
methanol as eluent. After precipitation, the dyes were isolated as
mono-TBA salt. The structures were confirmed by 1H-NMR and
ESI high resolution mass spectroscopy. The proton NMR
spectrum showed two isomers for each compound (fac- and mer-
isomer in a ratio of 2 : 1 for 1 and 3 : 1 for 2), which could not be
Emission maxima of dyes 1 and 2 are found at 738 and 765 nm,
respectively, when excited at 480 nm, in the region of the lowest
energy MLCT absorption. A large Stoke’s shift was observed for
both dyes 1 (6734 cmꢀ1) and 2 (8433 cmꢀ1), which is an indication
of significant structural changes between the ground and excited
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Scheme 2 Synthesis of Ru(II) dyes 1 and 2.
Fig. 1 UV-VIS and normalized emission spectra of dye 1 and 2,
measured in acetonitrile. Emission spectra were measured in aerated
acetonitrile solution with c ꢂ 10ꢀ5 mol Lꢀ1 (lex ¼ 480 nm).
Fig. 2 Cyclic voltammogram (CV) of dye 1 and 2, in DMF (0.1 M
TBAPF6), c ¼ 10ꢀ3 mol Lꢀ1. The differential pulse voltammogram (DPV)
plots are presented as black dashed lines.
NCS ligand. Both dyes 1 and 2, display one reversible reduction
wave at ꢀ2.15 V, which can be assigned to the one electron
reduction of the dcbpy ligand. The HOMO/LUMO energy levels
were calculated from the onset of the first oxidation and reduc-
tion waves, respectively. The smaller band gap obtained for 1
(Eg ¼ 2.12 eV) compared to 2 (Eg ¼ 2.19 eV) is in agreement with
the red-shift of the absorption spectrum. For an efficient sensi-
tizer in DSSCs the LUMO energy level has to be higher in energy
than the quasi-Fermi level of the TiO2 photoanode, ensuring
efficient electron transfer from the excited dye into the titanium
dioxide semiconductor. The HOMO level has to be sufficiently
low in energy to accept electrons from the I3ꢀ/Iꢀ-based redox
electrolyte (Fig. 3). Thus, the HOMO/LUMO energy values of 1
and 2 fit well with the energy levels of TiO2 (ꢀ4.0 eV vs. vacuum)
and I3ꢀ/Iꢀ (ꢀ4.83 eV vs. vacuum).
state. Similar to the absorption spectra, the emission bands are
also blue-shifted compared to bipyridine-based Ru(II)-
complexes.
Electrochemical properties
The redox behaviour of the dyes was measured by cyclic vol-
tammetry (CV) and differential pulse voltammetry (DPV) in
DMF solutions containing 0.1 M tetrabutylammonium hexa-
fluorophosphate (TBAPF6) as the supporting electrolyte (Fig. 2).
The data is presented in Table 1. The redox properties of both
dyes are similar. They show two irreversible oxidation waves, one
around 0.3 V and the second at about 0.4 V vs. Fc/Fc+. The first
oxidation wave can be attributed to the RuII/RuIII oxidation,
whereas the second wave can be assigned to the oxidation of the
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Table 1 Photophysical and electrochemical data of complexes 1 and 2
Dye
labs [nm] a (3 [L molꢀ1 cmꢀ1])
lem [nm]a
E0ox1 [V] b
E0 [V] b
E0red [V] b
ꢀ2.14
HOMO [eV] c
LUMO [eV] c
Eg [eV] d
ox2
1
284 (33 000)
369 (12 000)
493 (7600)
302 (32500)
366 (8 000)
465 (9200)
738
0.29
0.37
0.42
ꢀ5.23
ꢀ3.11
ꢀ3.09
2.12
2
765
0.32
ꢀ2.15
ꢀ5.28
2.19
a
Measured in acetonitrile solution. b Measured vs. Fc/Fc+ in DMF (0.1 M TBAPF6), [c] ¼ ꢂ10ꢀ3 mol Lꢀ1, 295 K, scan rate ¼ 100 mV sꢀ1. Potentials
c
d
were determined using DPV. Determined from the onsets of oxidation and reduction waves, set Fc+/Fc EHOMO ¼ ꢀ5.1 eV.12 Calculated by the
onset
onset
difference of the values of E red1
and E
.
ox1
Fig. 3 Representation of the HOMO/LUMO energies of sensitizers 1
and 2 together with TiO2 and I3ꢀ/Iꢀ.
In order to get a better insight into the location of the
HOMO and LUMO energy levels we performed semi-
empirical calculations using the ZINDO/1 method to opti-
mize the geometry of both dyes. Fig. 4 depicts molecular
orbitals of 1 and 2 at HOMO ꢀ 1 to LUMO + 1 levels. In
both complexes the electron density in the HOMO and
HOMO ꢀ 1 is distributed homogeneously on the Ru(II) and
the thiocyanate ligands. The LUMO is located on the
anchoring ligand in both dyes 1 and 2. However, the LUMO
+ 1 in 2 is located on the dicarboxybipyridine and on the
triazol-1-yl-pyridine ligand, whereas in 1 the LUMO + 1 is
situated only on the dicarboxybipyridine anchoring ligand.
Therefore, the strength of the MLCT transition from the
Ru(NCS)2 moiety to the dcbpy ligand is reduced in 2 and
therefore electron injection from the excited dye to the TiO2
semiconductor in a DSSC might be better for sensitizer 1.
Photovoltaic performance
Both dyes 1 and 2 were tested in liquid and solid-state DSSCs and
the data is presented in Table 2. The photocurrent density–
voltage (J–V) curves and incident photon-to-current conversion
efficiency (IPCE) spectra of the devices prepared with an aceto-
nitrile-based electrolyte (Z959) are depicted in Fig. 5, together
with their corresponding dark-current curves. The devices were
prepared using a double layer TiO2 film which consisted of
a transparent 8 mm thick layer and a 5 mm scattering layer, and
DINHOP (dineohexyl bis(3,3-dimethylbutyl)phosphinic acid)
was added as co-adsorbent to the dye solution. The IPCE
Fig. 4 Graphical representation of the frontier orbitals of 1 and 2
optimized using the ZINDO/1 semi-empirical method. Atoms: C black,
N blue, S yellow, O red, Ru brown.
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Table 2 Summarized data of the liquid and solid-state DSSC performance
Dye
Device
VOC [mV]
JSC [mA cmꢀ2
]
FF
h [%]
IPCEmax [%] (l [nm])
1
2
1
2
liquida
liquida
solidb
solidb
761
673
692
569
13.2
8.9
4.05
0.98
0.77
0.77
0.68
0.69
7.8
4.7
1.92
0.38
77(500)
58 (480)
33 (500)
8 (460)
a
b
Liquid device: Electrolyte: Z959; co-adsorbent: DINHOP (4 : 1); film thickness: 8 + 5 mm. Solid-state device: Spiro-OMeTAD as hole conductor,
tert-Butyl pyridine and Li(CF3SO2)2N.
a power conversion efficiency of 7.8% is quite impressive. The
higher photocurrent density in the case of the dye 1-based device
can be explained by an increase in the light absorbing capability
of the dye 1-sensitized TiO2 film. The higher Voc in case of the dye
1-based device is due to either the reduction of charge recombi-
nation or the upward shift of the conduction-band edge position.
In order to elaborate more on this assumption, transient
absorption spectroscopy (TAS) measurements were performed
on these devices.
As shown in Fig. 6a, the structural differences of the two
sensitizers have a significant effect on the photocurrent decay
rate across the TiO2 film and on the slope of decay rate vs. charge
density measured under open circuit conditions. At identical
charge density, the photoinduced electrons in the case of the dye
1-based device diffuse faster through the TiO2 film than those of
Fig. 5 (a) Photocurrent density–Voltage (J–V) characteristic and (b)
incident photon-to-current conversion efficiency (IPCE) spectrum of
acetonitrile-based devices using 1 and 2 sensitizers. Electrolyte compo-
sition (Z959): 1.0 M 1,3-dimethylimidazolium iodide, 0.03 M iodine, 0.1
M guanidinium thiocyanate, 0.5 M tert-butylpyridine in acetonitrile/
valeronitrile (85 : 15).
spectrum of 1 covers a broad range from 380 to 700 nm, reaching
its maximum of 77% at 500 nm, whereas the IPCE spectrum of 2
is slightly blue-shifted with a maximum of 58% at 480 nm.
Under full sunlight (AM 1.5 G, 100 mW cmꢀ2) the dye 1-
sensitized device exhibited an open-circuit photovoltage (VOC),
a short-circuit photocurrent density (JSC), fill-factor (FF) and
power conversion efficiency (h) of 761 mV, 13.2 mA cmꢀ2, 0.77
and 7.8%, respectively. On the other hand, a dye 2-sensitized
device gave a VOC of 673 mV, JSC of 8.9 mA cmꢀ2, and FF of
0.77, yielding a lower h of 4.7%. The power conversion efficiency
of dye 1 bearing the 1,2,3-triazol-4-yl ligand is superior to the
1,2,3-triazol-1-yl counterpart dye 2 under similar conditions.
Fig. 6 (a) Open circuit transient photocurrent decay rate for DSSC
Considering the lower molar extinction coefficient and the blue-
shift in the absorption spectrum of 1 compared to other bipyr-
idine-based Ru(II) sensitizers, such as Z-907,3c the device with
devices sensitized with complexes 1 and 2 as a function of photoinduced
charge density. (b) Charge recombination lifetime vs. photoinduced
charge density of the two devices measured at open circuit conditions.
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DSSCs. Two new substituted 1,2,3-triazol ligands were synthe-
sized via the 1,3-dipolar Cu(I)- catalyzed Huisgen cycloaddition
(click-reaction). Their corresponding heteroleptic Ru(II)
complexes 1 and 2 were prepared and used as sensitizers in liquid
and solid-state DSSCs. They have shown overall power conver-
sion efficiencies up to 7.8% which is a remarkable value consid-
ering that in comparison to other bipyridine-based Ru(II)
sensitizers the absorption spectra are considerably blue-shifted
and the molar extinction coefficients are significantly lower. The
transient absorption spectral studies revealed that the higher
photovoltaic performance of devices with dye 1 is due to the
faster electron transport into the TiO2 film and a lower recom-
bination rate in comparison to the dye 2-sensitized devices. The
large difference in device performance correlates with minor
structural changes in the isomeric ligands 4 and 8, and in the
corresponding Ru(II) complexes 1 and 2, which is the different
substitution pattern of the 1,2,3-triazole unit. Taking into
account that it is possible to red-shift and to increase the
absorption of these dyes by, e.g., extending the p conjugation,
a new class of highly efficient 1,2,3-triazolyl-pyridine Ru(II)
sensitizers should be accessible via the convenient and versatile
click-approach.
Fig. 7 Open-circuit voltage of the DSSC based on sensitizer 1 and 2,
plotted versus the light intensity, with illumination from a white LED
light source.
the dye 2-based device, resulting from a larger number of occu-
pied trap states for the given Fermi level in the TiO2 film.
Fig. 6b shows the dependence of the charge recombination
lifetime on the photoinduced charge density. The recombination
lifetime is determined by the reciprocal of the photovoltage decay
rate obtained from a small perturbation voltage decay technique.
The value of the charge density is obtained from the experimental
measurements by collecting electrons when switching the device
from open circuit to short circuit conditions.13 It is noted that the
dye 1-sensitized device shows a longer charge recombination
lifetime than that of the dye 2-sensitized device at an identical
photoinduced charge density, clearly suggesting an advantage of
1 in retarding charge recombination. However, this influence
becomes smaller under incident low light intensity where fewer
charges are accumulated in the TiO2 film.
Experimental
Synthesis
1-Iodo-4-n-hexylbenzene was purchased from Alfa Aesar,
sodium azide, copper iodide and Cu(CH3CN)4PF6 were
purchased from Aldrich, sodium ascorbate, 2,20-bipyridine 4,40-
dicarboxylic acid (dcbpy) and di-m-chlorobis[(p-cymene)chlor-
oruthenium(II)] ([RuCl2(p-cymene)]2) were purchased from
ABCR,
N,N0-dimethylethylenediamine
(DMEDA)
was
The VOC as a function of incident light intensity is displayed in
Fig. 7. The VOC of both devices changes linearly according to the
logarithm of the light intensity with the slope of 99 mV per
decade and 102 mV per decade for devices sensitized with dyes 1
and 2, respectively. The almost identical slopes indicate that the
trap states on the surface of TiO2 are identical when both dyes
are sensitized on TiO2. The slope of 59 mV per decade is reported
for an ideal diode.14a The difference in the slope between the ideal
diode and the real devices is attributed to a non-linear recom-
bination occurring in the DSSCs.14b
purchased from Acros and 2-bromopyridine, diisopropylethyl-
amine (DIPEA) and potassium fluoride were purchased from
Merck. All chemicals were used as received. 2-(TMS-ethy-
nyl)pyridine 5 was synthesized from 2-bromopyridine and tri-
methylsilylacetylene via
a Sonogashira coupling reaction
according to the literature,15 and 1-ethynyl-4-n-hexyl-benzene 9
was synthesized from 1-iodo-4-n-hexylbenzene and trimethylsi-
lylacetylene with subsequent deprotection with potassium fluo-
ride (KF) in dichloromethane (DCM) and methanol (MeOH) in
the same way.
The 1 and 2 dyes were also used as sensitizers for solid-state
devices using spiro-OMeTAD (2,207,70-tetrakis(N,N-di-p-
methoxyphenyl-amine)-9,90-spirobifluorene) as a hole transport
material. The power conversion efficiencies of 1.92% and 0.38%
were obtained for 1- and 2-containing devices, respectively. The
corresponding photovoltaic parameters are tabulated in Table 2.
Similar to the liquid-devices, in the solid-state cells dye 1
outperforms dye 2 sensitized cells. In comparison to liquid elec-
trolyte devices, the JSC of the solid-state devices are lower. This is
mainly due to thin TiO2 films (2 mm) used in the solid-state
devices which limits the photocurrent density.
NMR spectra were recorded on an Avance 400 spectrometer
(1H NMR: 400 MHz), at 25 ꢁC. Chemical shift values (d) are
expressed in parts per million using residual solvent protons
(CDCl3: H d ¼ 7.26 ppm and 13C d ¼ 77.0 ppm) as internal
1
standard. Chemical shifts of the isomer are given in square
brackets. Melting points were determined using a Buchi B-545
apparatus. Elemental analyses were performed on an Elementar
Vario EL (University of Ulm). CI mass spectra were recorded
on a Finnigan MAT SSQ-7000. High resolution mass spectra
(HRMS) were measured at the University of Stuttgart with
a micrOTOF-Q43 apparatus and ESI source. Optical measure-
ments were carried out in 1 cm cuvettes with Merck spectro-
scopic grade solvents, absorption spectra were recorded on
a Perkin Elmer Lambda 19 spectrometer and fluorescence
spectra on a Perkin Elmer LS 55 spectrometer. The emission
spectra are fully corrected for the photodetector response. IR
Conclusion
We have shown that click-chemistry is a beneficial approach in
the design of new chelating ligands for Ru(II) sensitizers in
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spectroscopy was measured on a Perkin Elmer FT-IR Spectrum
2000. Cyclic voltammetry experiments were performed with
a computer-controlled Autolab PGSTAT30 potentiostat in
a three-electrode single-compartment cell with a platinum
working electrode, a platinum wire counter electrode, and an
Ag/AgCl reference electrode. All potentials were internally
referenced to the ferrocene/ferrocenium (Fc/Fc+) couple.
(40%), 2ꢁ07 (14%). RF-value: 0.33 (basic alumina, DCM/PE 1 : 1).
Mp. 93 C. IR (KBr): [cmꢀ1] ¼ 3129 (m), 1602 (s, C]N), 1520
(s). Elemental analysis (%): calc. C: 74.48, H: 7.24, N: 18.29;
found C: 74.57, H: 7.24, N: 18.36.
2-Azidopyridine (8)
1.23 ml (12.66 mmol, 2.0 g) 2-Bromopyridine was dissolved in 42
ml ethanol and 18 ml water and the solution was degassed. 1.65 g
(25.32 mmol) Sodium azide, 0.24 g (1.27 mmol) CuI, 0.13 g (0.63
mmol) sodium ascorbate and 0.2 ml (1.90 mmol, 0.17 g)
DMEDA were added. The suspension was heated under reflux
for 1 h. After cooling to rt 100 ml water and 50 ml DCM were
added. The layers were separated and the aqueous layer was
extracted with 100 ml DCM. The combined organic layers were
washed with 100 ml water and were dried over Na2SO4. The
solvent was removed in vacuo and it was purified via column
chromatography with basic alumina and DCM. 1.04 g (8.66
mmol, 68%) of a white solid was obtained. 1H-NMR (CD3Cl, 400
MHz): d ¼ 7.24 (td, 1H, 3J ¼ 6.9 Hz, 4J ¼ 1.0 Hz, H-5), 7.68 (ddd,
1H, 3J ¼ 9.0 Hz, 3J ¼ 7.0 Hz, 4J ¼ 1.1 Hz, H-4), 8.05 (dt, 1H, 3J ¼
9.0 Hz, 4J ¼ 1.1 Hz, H-3), 8.84 (dt, 1H, 3J ¼ 7.0 Hz, 4J ¼ 1.1 Hz,
H-6). 13C-NMR (CD3Cl, 100 MHz): d ¼ 116.0 (C-5), 116.2 (C-3),
125.4 (C-4), 131.8 (C-6), 148.6 (C-2). RF-value: 0.66 (basic
alumina, DCM).
1-Azido-4-n-hexyl-benzene (4)
2.0 g (6.94 mmol) 1-Iodo-4-n-hexylbenzene was dissolved in 28
ml ethanol and 12 ml water and the solution was degassed. 0.902
g (13.88 mmol) Sodium azide, 0.131 g (0.69 mmol) CuI, 0.069 g
(0.35 mmol) sodium ascorbate and 0.11 ml (1.04 mmol, 0.092 g)
DMEDA were added. The suspension was heated under reflux
for 45 min. After cooling to room temperature (rt) the reaction
mixture was diluted with 100 ml water and extracted with 70 ml
diethyl ether three times. The combined organic layers were dried
over Na2SO4 and the solvent was removed in vacuo. It was
purified via column chromatography with silica and petrol ether
(PE). The yellow liquid was dried in vacuo and 1.29 g (6.36
mmol, 92%) of the product was obtained. 1H-NMR (CDCl3, 400
MHz): d ¼ 0.89 (m, 3H, H-60), 1.30 (m, 6H, H-30, H-40, H-50),
1.59 (qui, 2H, 3J ¼ 7.6 Hz, H-20), 2.59 (t, 2H, 3J ¼ 7.7 Hz, H-10),
6.95 (d, 2H, 3J ¼ 8.4 Hz, H-2), 7.16 (d, 2H, 3J ¼ 8.4 Hz, H-3). 13C-
NMR (CDCl3, 100 MHz): d ¼ 14.1 (C-60), 22.6 (C-50), 28.9, 31.4,
31.7 (C-20, C-30, C-40), 35.3 (C-10), 118.8 (C-2), 129.7 (C-3), 137.3
(C-1), 139.8 (C-4). MS (CI, 100 eV): m/z ¼ 204 (43%, M + H+),
176 (100%), 132 (9%). RF-value: 0.69 (silica, PE). IR (NaCl):
[cmꢀ1] ¼ 2106 (s, azide). Elemental analysis (%): calc. C: 70.90, H:
8.43, N: 20.67; found C: 70.79, H: 8.47, N: 20.77.
2-(4-(4-Hexylphenyl)-1H-1,2,3-triazol-1-yl)pyridine (10)
136 mg (0.733 mmol) 1-Ethynyl-4-n-hexyl-benzene and 88 mg
(0.733 mmol) 2-azidopyridine 8 were dissolved in 40 ml DCM
and 10 ml methanol and the solution was degassed. 55 mg (0.147
mmol) Cu(CH3CN)4PF6, 9 mg (0.147 mmol) Cu0 and 0.25 ml
(1.47 mmol, 0.189 g) DIPEA were added and it was heated under
reflux for five days. After cooling to rt, 20 ml 25% ammonia in 80
ml water and 100 ml DCM were added. After stirring the solu-
tion vigorously for some minutes the layers were separated and
the aqueous layer was extracted with 50 ml DCM. The combined
organic layers were washed with 90 ml 0.1 M Na2-EDTA solu-
tion/10 ml 25% ammonia and dried over MgSO4. The solvent
was removed in vacuo and it was purified via column chroma-
tography on basic alumina and DCM and petrol ether 2 : 3 as
eluent. 163 mg (0.532 mmol, 73%) Product was obtained as white
2-(1-(4-Hexylphenyl)-1H-1,2,3-triazol-4-yl)pyridine (6)
1.0 g (4.92 mmol) 1-Azido-4-n-hexylbenzene 4 and 0.862 g (4.92
mmol) 2-(TMS-ethynyl)pyridine were dissolved in 30 ml DCM
and 6 ml methanol. 0.429 g (7.38 mmol) KF were added and the
solution was degassed. Subsequently, 0.365 g (0.98 mmol)
Cu(CH3CN)4PF6, 0.062 g (0.98 mmol) Cu(0) and 1.7 ml (9.84
mmol, 1.27 g) DIPEA were added, and stirring was continued at
rt for 3 days. The mixture was then diluted with 100 ml DCM and
30 ml water/10 ml 25% ammonia. The layers were separated and
the organic layer was washed with 30 ml 0.1 M disodium dihy-
drogen ethylenediaminetetraacetate (Na2-EDTA) solution/10 ml
25% ammonia. It was dried over Na2SO4 and the solvent was
removed in vacuo. Purification via column chromatography on
basic alumina with PE/DCM 1 : 2, then 100% DCM as eluent,
afforded 1.35 g (4.40 mmol, 89%) of the product as a white solid.
1H-NMR (CDCl3, 400 MHz): d ¼ 0.88 (m, 3H, H-600), 1.31 (m,
6H, H-300, H-400, H-500), 1.64 (qui, 2H, 3J ¼ 7.7 Hz, H-200), 2.67 (t,
2H, 3J ¼ 7.6 Hz, H-100), 7.26 (m, 1H, H-5), 7.33 (d, 2H, 3J ¼ 8.4
Hz, H-30), 7.70 (d, 2H, 3J ¼ 8.4 Hz, H-20), 7.82 (td, 1H, 3J ¼ 7.8
Hz, 4J ¼ 1.7 Hz, H-4), 8.26 (dd, 1H, 3J ¼ 7.9 Hz, 4J ¼ 0.7 Hz, H-
3), 8.59 (m, 1H, triazole-H), 8.62 (m, 1H, H-6). 13C-NMR
(CDCl3, 100 MHz): d ¼ 14.1 (C-600), 22.6 (C-500), 28.8, 31.3, 31.6
(C-400, C-300, C-200), 35.5 (C-100), 120.0 (C-5), 120.3 (C-30), 120.5
(C-3), 123.0 (C]CH), 129.7 (C-20), 134.7 (C-10), 137.2 (C]CH),
144.0 (C-4), 148.5 (C-40), 149.2 (C-6), 149.9 (C-2). MS (CI, 100
eV): m/z ¼ 308 (22%), 307 (100%), 306 (7%), 279 (30%), 278
1
crystals. H-NMR (CD3Cl, 400 MHz): d ¼ 0.89 (m, 3H, H-600),
3
1.32 (m, 6H, H-500, H-400, H-300), 1.66 (qui, 2H, J ¼ 7.6 Hz, H-
200), 2.64 (t, 2H, 3J ¼ 7.7 Hz, H-100), 7.28 (d, 2H, 3J ¼ 8.2 Hz, H-
3
3
4
20), 7.36 (ddd, 1H, J ¼ 6.5 Hz, J ¼ 4.9 Hz, J ¼ 0.9 Hz, H-4),
7.85 (d, 2H, 3J ¼ 8.2 Hz, H-30), 7.94 (td, 1H, 3J ¼ 7.6 Hz, 3J ¼ 1.8
Hz, H-5), 8.26 (d, 1H, 3J ¼ 8.2 Hz, H-3), 8.53 (m, 1H, H-6), 8.78
(s, 1H, triazole-H). 13C-NMR (CD3Cl, 100 MHz): d ¼ 14.1 (C-
600), 22.6 (C-500), 29.0 (C-400), 31.4 (C-300), 31.7 (C-200), 35.8 (C-
100), 113.8 (]CH-triazole), 116.4 (C-5), 123.5 (C-4), 125.8 (C-20),
127.6 (C-10), 128.9 (C-30), 139.1 (C-3), 143.4 (C-40), 148.2 (Cq-
triazole), 148.5 (C-6), 149.2(C-2). CI-MS (100 eV): m/z ¼ 307
(100%, M + H+), 279 (35%). RF-value: 0.19 (basic alumina,
DCM/petrol ether 2 : 3). Mp. 60 ꢁC. IR (KBr): [cmꢀ1] ¼ 3146
(m), 1611 (m), 1595 (s, C]N), 1577 (s), 1471 (s), 1461 (s).
Elemental analysis (%): calc. C: 74.48, H: 7.24, N: 18.29; found
C: 74.34, H: 7.22, N: 18.21.
3732 | J. Mater. Chem., 2011, 21, 3726–3734
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Ru(2-(1-(4-hexylphenyl)-1H-1,2,3-triazol-4-yl)pyridine)(2,20-
nm sized TiO2 particles was first printed on the fluorine doped
SnO2 (FTO) conducting glass electrode and subsequently coated
with a 5 mm thick second layer of 400 nm light-scattering anatase
particles (CCIC, Japan). The porosity was determined from BET
measurements to be 68% for the 20 nm TiO2 transparent layer
and 42% for the 400 nm TiO2 scattering layer. The detailed
methods for TiO2 film preparation, device fabrication, and the
photocurrent-voltage measurements can be found in an earlier
report.16 After sintering at 500 ꢁC and cooling to 80 ꢁC, the
sintered TiO2 electrodes were sensitized by dipping 16 h into the
dye solution (300 mM) in 10% DMSO and acetonitrile and tert-
butyl alcohol (volume ratio 1 : 1) mixture, with 75 mM DINHOP
(4 : 1) as a co-adsorbent. For the solid-state devices, a compact
TiO2 layer was first deposited onto the FTO substrate by spray
pyrolysis, onto which 23 nm sized TiO2 particles were deposited
by doctor-blading to obtain a 2 mm thick mesoporous film. After
sintering the TiO2 layers at 500 ꢁC, the film was cooled to room
temperature and immersed overnight in 0.02 M aqueous TiCl4.
The film was then rinsed with deionized water, annealed in air at
450 ꢁC for 20 min, and cooled to 80 ꢁC before immersing it in the
dye solution (300 mM) for sensitization. The hole transporting
material solution containing 0.17 M spiro-MeOTAD, 0.11 mM
tert-butylpyridine, and 0.21 mM LiN(CF3SO2)2 in chloroben-
zene was used. We deposited this solution onto the dye-coated
TiO2 film, leaving it to penetrate into the pores of the TiO2 layer
for 1 min prior to spin-coating. Finally, a 50 nm-thick gold
contact was deposited onto the organic semiconductor to close
the cell.
bipyridine-4,40-dicarboxylic acid)(NCS)2 TBA salt (1)
100 mg (0.326 mmol) Triazolyl-pyridine ligand 6 was dissolved in
50 ml dry DMF and the solution was degassed. 100 mg (0.163
mmol) [RuCl2(p-cymene)]2 was added and the solution was
ꢁ
heated with stirring to 75 C in the dark for 6 h. Then, 80 mg
(0.326 mmol) dcbpy was added and it was heated to 140 ꢁC for 4
h. Subsequently, 1.0 g (excess) NH4NCS was added and it was
heated to 145 ꢁC for additional 4 h with constant stirring. It was
cooled to rt over night and then the solvent was removed in vacuo
until about 5 ml of DMF remained. 50 ml water was added and
the suspension was left in the ultrasonic bath for 30 min. It was
cooled to 4 ꢁC in the refrigerator overnight. The brown solid was
filtered off and it was washed with large amounts of water and
then diethyl ether. The crude material was dried in vacuo. The
product was purified via column chromatography on Sephadex
LH20, by dissolving the complex in methanolic tetra-n-butyl-
ammonium (TBA) hydroxide solution and methanol was used as
eluent. The volume of the main fraction was reduced in vacuo and
the brown product was precipitated with diethyl ether, collected
by filtration and dried in high vacuum. 1H NMR (d4-MeOH, 400
MHz): d ¼ 0.89 [0.95] (t, 3H, H-100), 1.02 (t, 12H, TBA), 1.31 (m,
6H, H-70, H-80, H-90), 1.43 (sext, 8H, TBA), 1.69 (m, 10H, TBA +
H-60), 2.65 [2.80] (t, 2H, H-50), 3.26 (m, 8H, TBA), 7.11 [7.80] (td,
1H, H-5), 7.31 [7.47] (d, 2H, H-30), 7.50 [7.53] (d, 2H, H-20), 7.63
(dd, 1H, H-500), 7.75 (td, 1H, H-4), 7.87 (d, 1H, H-500), 7.99–8.17
(m, 2H, H-6, H-600), 8.24 (m, 1H, H-600), 8.83 [8.87] (s, 1H, H-300),
8.95 [9.03] (s, 1H, H-300), 9.33 [9.61] (s, 1H, triazole-H), 9.41 [9.57]
(d, 1H, H-3). HR-MS (ESI): found m/z ¼ 1008.36 (M +
1TBA – H+; C49H64N9O4RuS2); calc. m/z ¼ 1008.36 for
C49H64N9O4RuS2. IR (KBr): [cmꢀ1] ¼ 2107 (s, NCS). Elemental
analysis (%) (C33H29N8O4RuS2 ꢃ 1.3TBA ꢃ 3H2O): calc.
C: 57.04, H: 7.31, N: 11.43; found C: 56.85, H: 7.22, N: 11.30.
Transient absorption spectroscopy
The photovoltage transient decay was recorded using an exciting
pulse generated by a ring of red LEDs (Lumiled) controlled by
a fast solid-state switch. The pulse widths were 100 ms. The pulse
was incident on the photoanode side of the device. An array of
InGaN diodes (Lumiled) supplied the white bias light incident
from the same direction. Transients were measured at different
white light intensities via tuning the voltage applied to the bias
diodes. Before the LEDs switched to the next light intensity,
a charge-extraction routine was executed to measure the electron
density in the film. In the charge-extraction technique, the LED
illumination source was turned off in <1 ms, while the cell was
simultaneously switched from open to short circuit. The resulting
current, as the cell returned to V ¼ 0 and J ¼ 0, was integrated to
give a direct measurement of the excess charge in the film at
Ru(2-(4-(4-hexylphenyl)-1H-1,2,3-triazol-1-yl)pyridine)(2,20-
bipyridine-4,40-dicarboxylic acid)(NCS)2 TBA salt (2)
The complex was synthesized in an identical manner to complex
1 using 92 mg (0.30 mmol) triazolyl-pyridine ligand 10, 92 mg
(0.15 mmol) [RuCl2(p-cymene)]2, 73 mg (0.30 mmol) dcbpy
1
ligand and 1.0 g (excess) NH4NCS. H NMR (d4-MeOH, 400
MHz) d ¼ 0.87 [0.92] (t, 3H, H-100), 1.01 (t, 12H, TBA), 1.29 (m,
6H, H-70, H-80, H-90), 1.40 (sext, 8H, TBA), 1.58 (qui, 2H, H-60),
1.66 (m, 8H, TBA), 2.58 [2.72] (t, 2H, H-50), 3.23 (m, 8H, TBA),
7.18 [7.41] (d, 2H, H-30), 7.26 [7.90] (t, 1H, H-5), 7.56 (d, 2H, H-
20), 7.61 (m, 1H, H-4), 7.62 (m, 1H, H-500), 7.89 (m, 1H, H-500),
8.09 [8.25] (d, 1H, H-6), 8.20 (m, 1H, H-600), 8.34 (m, 1H, H-600),
8.85 [8.87] (s, 1H, H-300), 8.98 [9.03] (s, 1H, H-300), 9.37 [9.50] (d,
1H, H-3), 9.49 [9.71] (s, 1H, triazole-H). HR-MS (ESI): m/z ¼
1008.36 (M + 1TBA – H+; C49H64N9O4RuS2); calc. m/z ¼
1008.36 for C49H64N9O4RuS2. IR (KBr): [cmꢀ1] ¼ 2107 (s, NCS).
Elemental analysis (%) (C33H29N8O4RuS2 ꢃ 1.3TBA ꢃ 3H2O):
calc. C: 57.04, H: 7.31, N: 11.43; found C: 57.23, H: 7.62, N:
10.91.
that VOC
.
Acknowledgements
The authors thank the German Ministry of Education and
Research (BMBF) in the frame of project OPEG 2010 and the
Swiss National Science Foundation for the financial support.
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