Efficient panchromatic sensitization of nanocrystalline TiO2 films by a black
dye based on a trithiocyanato–ruthenium complex
Md. K. Nazeeruddin, P. Pe´chy and M. Gra¨tzel*
Institute of Photonics and Interfaces, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland
A black trithiocyanato–ruthenium(II) terpyridyl complex
where the terpyridyl ligand is substituted by three carboxyl
groups in 4,4A,4B-positions achieves very efficient panchro-
matic sensitization of nanocrystalline TiO2 solar cells over
the whole visible range extending into the near-IR region up
to 920 nm.
decrease in the p* level of the 4,4A,4B-tricarboxy-2,2A:6A,2B-
terpyridine ligand and an increase in the energy of the t2g metal
orbital.
The MLCT and the p–p* bands of 1 were displaced to higher
energy (590 nm) upon deprotonation of the carboxyl groups,
which could be due to increase in the energy of the p* orbital of
the 4,4A,4B-tricarboxy-2,2A :6A,2B-terpyridine ligand. When ex-
cited within the MLCT absorption band, 1 in ethanol solution at
298 K, exhibits a luminescence (measured as described in ref. 4)
consisting of a single band with a maximum at 950 nm
[Fig. 1(b)] and a lifetime of 18 ± 5 ns.
The cyclic voltammogram of 1 measured in dmf containing
0.1 m [NBu4][CF3SO3] shows one quasireversible wave at 0.6 V
(vs. Ag/AgCl), which is attributed to the [RuII/IIIL(NCS)3] redox
couple. In the negative potential range two irreversible waves at
21.1 and 21.2 V (vs. Ag/AgCl) are observed which are
assigned to ligand reduction. A monolayer of complex 1 was
adsorbed from a 2 3 1024 m solution in ethanol, containing 40
mm sodium taurodeoxycholate, as a co-adsorbent, onto a 12 nm
thick nanocrystalline TiO2 (anatase) film prepared on conduct-
ing glass as previously described.4 The black film exhibited
striking performance when tested in a photovoltaic cell in
Dye-sensitized nanocrystalline TiO2 solar cells are presently
under intensive investigation.1–3 So far, the best performing
charge transfer sensitizer employed for such an application is
a cis-dithiocyanato bis(4,4A-dicarboxy-2,2A-bipyridine)ruthe-
nium(ii) complex,4 yielding solar to electric power conversion
efficiency of 10% under standard AM 1.5 condition. To
improve further the efficiency of such systems, an enhanced
spectral response of the sensitizer in the red and near-IR region
is required. The present study reports the synthesis and
characterization of a new class of trithiocyanato–ruthenium(ii)
terpyridyl complex. Substitution of the terpyridyl ligand by
three carboxyl groups in 4,4A,4B-positions yields a black dye
displaying very efficient panchromatic sensitization over the
whole visible range extending into the near-IR region up to 920
nm.
The oxidation of 4,4A,4B-trimethyl-2,2A:6A,2B-terpyridine5,6
(0.32 g, 1.16 mmol) was carried out in sulfuric acid (4 ml), with
chromium trioxide (1.05 g, 10.5 mmol) at 75 °C, yielding
4,4A,4B-tricarboxy-2,2A:6A,2B-terpyridine (0.15 g, 41%).†
4,4A,4B-Tricarboxy-2,2A:6A,2B-terpyridine (127 mg) was es-
terified by refluxing in methanol (15 ml) and sulfuric acid (70
mg) for 3 days. The white crystals were filtered, washed with
methanol and diethyl ether successively, yielding 4,4A,4B-
trimethoxycarbonyl-2,2A:6A,2B-terpyridine L (102 mg, 72%).‡
The [RuLCl3] complex was synthesized by adding a solution
of 4,4A,4B-trimethoxycarbonyl-2,2A:6A,2B-terpyridine (40 mg),
in dichloromethane (20 ml), to a solution of hydrated ruthenium
trichloride (26 mg) in ethanol (30 ml). The reaction mixture was
refluxed for 2 h and the solution was concentrated to 20 ml. The
precipitated complex was collected and washed thoroughly with
ethanol to remove any unreacted ruthenium trichloride (yield 48
mg, 79%).§
The complex [NHEt3][RuL(NCS)3] 1 was obtained by
refluxing [RuLCl3] (48 mg) and ammonium thiocyanate (100
mg) in dmf–water for 3.5 h at 120 °C, in the dark under an argon
atmosphere. The ester groups of the terpyridine ligand were
hydrolysed by further refluxing for 15 h in the presence of
triethylamine (6 ml). The isolated solid was recrystallised from
methanol–diethyl ether (yield 39 mg, 69%).¶
The UV–VIS absorption spectrum of 1 in ethanol shows an
intense metal-to-ligand charge transfer band at 620 nm
[Fig. 1(a)], with a molar absorption coefficient of 6500 dm3
mol21 cm21. Bands at 291 and 329 nm with a distinct shoulder
at 340 nm are assigned to the intraligand p–p* transition of the
4,4A,4B-tricarboxy-2,2A:6A,2B-terpyridine ligand. The interest-
ing feature of complex 1 is the red-shifted MLCT absorption
band in the visible region compared to the [Ru(tpy)2]2+ (tpy
= 2,2A:6A,2B-terpyridine) family.7 The three thiocyanato an-
ionic ligands stabilize the excited states by electron donation to
Ru causing a red shift in the MLCT bands. The electrochemical
data of 1 suggest that the red shift in the MLCT is due to the
(a)
1.8
1.2
6
0
200
400
600
λ / nm
800
1000
(b)
400
300
200
100
0
800
1000
λ / nm
1200
1400
Fig. 1 (a) Absorption spectrum of complex 1 in ethanol solution and (b)
emission spectrum in ethanol solution at room temperature. The smoother
curve corresponds to a Gaussian fit (instrument limit is 1000 nm).
Chem. Commun., 1997
1705
Nazeeruddin
