Inorganica Chimica Acta 322 (2001) 7–16

www.elsevier.nl/locate/ica

Synthesis and photophysical properties of ruthenium(II) chargetransfer sensitizers containing 4,4�-dicarboxy-2,2�-biquinoline and

5,8-dicarboxy-6,7-dihydro-dibenzo[1,10]-phenanthroline

Ashraful Islam a, Hideki Sugihara a,*, Lok Pratap Singh a, Kohjiro Hara a,Ryuzi Katoh a, Yoshinobu Nagawa b, Masatoshi Yanagida a, Yoshiaki Takahashi a,

Shigeo Murata a, Hironori Arakawa a,*a National Institute of Materials and Chemical Research (NIMC), 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

b National Institute for Ad�anced Interdisciplinary Research, 1-1-4 Higashi, Tsukuba, Ibaraki 305-8562, Japan

Received 17 January 2001; accepted 28 May 2001

Abstract

Ruthenium (II) complexes of the type cis-Ru(H2dcbiq)2X2 and cis-Ru(H2dcdhph)2X2, where H2dcbiq=4,4�-dicarboxy-2,2�-biquinoline, H2dcdhph=5,8-dicarboxy-6,7-dihydro-dibenzo[1,10]-phenanthroline, and X=Cl−, NCS− or CN−, have beensynthesized and spectroscopically characterized. The resulting complexes show a broad and intense metal-to-ligand charge transfer(MLCT) band in the visible region with a peak between 580 and 700 nm and are emissive at room temperature. The ground-statefirst pKa value of cis-Ru(H2dcbiq)2(NCS)2 (2) was determined to be 2.9 by the spectrophotometric method. Photoelectrochemicalmeasurements show that all dyes, when anchored to a nanocrystalline TiO2 film electrode, present low light-harvesting efficienciesdue to inefficient driving force for electron injection into the conduction band of TiO2 from their lower energy MLCT band. Thephotoelectrochemical performance of 2 was also investigated on a number of oxide semiconductor thin films such as Nb2O5, ZnO,SnO2 and In2O3. The results show that a high value of short-circuit photocurrent (Jsc) is observed for the semiconductors havinga low-energy conduction band potential (SnO2 and In2O3). In the dye 2-sensitized TiO2 film, the absorbed photon-to-currentconversion efficiency (APCE) spectrum shows an absorption band selective electron injection yield, while a wavelengthindependent electron injection yield is observed when dye 2 is anchored to SnO2. These results indicate that the lowest excitedMLCT state is energetically favorable for electron injection into the conduction band of SnO2 but not for TiO2. © 2001 ElsevierScience B.V. All rights reserved.

Keywords: Solar cells; Ruthenium(II) sensitizers; 4,4�-Dicarboxy-2,2�-biquinoline; Titanium dioxide; Tin dioxide

1. Introduction

The development of efficient devices for solar energyconversion has been a very active research field. Dye-sensitized nanocrystalline metal oxide solar cells presenta promising strategy for solar energy conversion [1–7].Gratzel and coworkers have developed efficient pho-toelectrochemical cells based on a porous TiO2 semi-conductor electrode sensitized with ruthenium-basedpolypyridyl-type complexes, making practical applica-

tions feasible [1,2]. So far, the highest efficiency re-ported for such systems has been obtained usingnanocrystalline TiO2 coated with the cis-dithiocyanato-bis(4-4�-dicarboxy-2,2�-bypyridine)ruthenium (II) com-plex (cis-Ru(dcbpy)2(NCS)2), yielding an overall solarto electrical energy conversion efficiency of 10% [2].This has led to a high absorption of light and efficientconversion of visible photons into electricity in thedevices. In these systems, after absorption of a photon,the adsorbed dye molecule undergoes a transition fromthe ground state to an excited state located energeticallyabove the conduction band of the semiconductor. Sub-sequently, an efficient and rapid electron transfer occursfrom the dye-excited state to the conduction band of

* Corresponding authors. Tel.: +81-298-61 4581; fax: +81-298-614511.

E-mail address: sughihara@nimc.go.jp (H. Sugihara).

0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 0 -1693 (01 )00548 -5

A. Islam et al. / Inorganica Chimica Acta 322 (2001) 7–168

the semiconductor. The oxidized form of the dye israpidly reduced back to its ground state by redoxmediators such as iodide. Thermodynamic driving force[4,8–12] and electronic coupling [13] between the dyemolecules and the semiconductor are found to dictateto a large extent the kinetics of the electron transferprocesses and thus control the device efficiency.

To improve further the efficiency of such systems, anenhanced spectral response of the sensitizer at lowerenergies is required while maintaining sufficient thermo-dynamic driving forces for both the electron transferand the dye regeneration processes. In the rutheniumpolypyridyl complexes, absorption properties can betuned to a lower energy region by introducing a ligandwith a low-lying �* molecular orbital [8–12,14–17] orby destabilization of the metal t2g orbital with a strongdonor ligand [18]. Recently, using the 4,4�-dicarboxy-2,2�-biquinoline (H2dcbiq) ligand, which has a low-lying�* molecular orbital, several ruthenium complexes ofthe Ru(NNN)(NN)X type, where NNN=2,6-bis(1-methylbenzimidazole-2-yl)pyridine or 2,2�:6�,2�-ter-pyridine, NN=H2dcbiq, X=NCS− or Cl−, have beensynthesized and the visible absorption (MLCT) bandwas tuned up to 600 nm [8,9,12]. Unfortunately, the useof low-lying �*-level ligands was accompanied bystrong reduced injection yields into the conductionband of TiO2.

We report here the synthesis and photoelectrochemi-cal properties of a new family of ruthenium complexesof the cis-RuL2X2 type where L is one of the twodiimine ligands with a low-lying �* molecular orbitalshown in Chart 1: 4,4�-dicarboxy-2,2�-biquinoline(H2dcbiq) and 5,8-dicarboxy-6,7-dihydro-dibenzo[1,10]-phenanthroline (H2dcdhph), and X is CN−, NCS− orCl−. By using these complexes the MLCT transitionscan be tuned up to 700 nm. In order to understand theenergetics at the dye/semiconductor interface, we alsoexamined the photoelectrochemical behavior of cis-Ru(H2dcbiq)2(NCS)2 on a number of oxide semicon-ductor thin films such as TiO2, Nb2O5, ZnO, SnO2 andIn2O3.

2. Experimental

2.1. Materials

All the solvents and chemicals were of reagent gradeand used as received unless otherwise noted. Dichloro-

tetrakis(dimethylsulfoxide)ruthenium (Ru(Me2SO)4Cl2)was synthesized according to the published procedure[19]. Commercial oxide semiconductor powder, Nb2O5,ZnO, SnO2, In2O3 (Wako Pure Chemical Industries),and TiO2 (Nihon Aerosil, P25) were used for the prepa-ration of the colloids.

2.1.1. 5,8-Dicarboxy-6,7-dihydro-dibenzo[1,10]-phena-nthroline (H2dcdbph)

6,7 - Dihydro - 5,8 - dimethyl - dibenzo[1,10] - phenan-throline (3 g, 9.7 mmol) was dissolved in 200 cm3 of1,4-dioxane containing 4% v/v water, then seleniumoxide (7.5 g, 68 mmol) was added. The mixture washeated to reflux for 3 h and then filtered through Celitewhile hot. 5,8-Diformyl-6,7-dihydro-dibenzo[1,10]-phenanthroline was separated from the cold filtrate as abrown powder (1.3 g, 40%) and recrystallized fromtetrahydrofuran. MS (ESI MS); m/z : 339.0 (M+H)2−.1H NMR (CDCl3, 300 MHz): � 11.06 (s), 8.50 (q), 7.75(d), 7.64 (d), 3.45 (s). Anal. Found: C, 77.05; H, 4.12;N, 7.91. Calc. for C22H14N2O8: C, 78.09; H, 4.17; N,8.28%.

5,8 - Diformyl - 6,7 - dihydro - dibenzo[1,10] - phenan-throline (0.5 g, 1.5 mmol) in 10 cm3 of 60% nitric acidwas heated under reflux for 3 h, then poured onto ice,and the precipitated brown solid (0.16 g, 30%) wasrecrystallized from methanol. This brown product wasfurther purified by column chromatography using silicagel as the column support. The solid was dissolved in aminimum amount of DMF and loaded onto thecolumn (2.5 cm×25 cm). Some impurities were elutedfirst using acetonitrile and discarded. The deep yellowproduct, H2dcdbph, was eluted using methanol, and thesolvent was removed by rotary evaporation. MS (ESIMS); m/z : 368.9 (M−H)−, 183.9 (M−2H)2−. 1HNMR (D2O–NaOD, 300 MHz): � 8.26 (d), 7.90 (q),7.74 (t), 3.20 (s). Anal. Found: C, 70.65; H, 3.72; N,7.61. Calc. for C22H14N2O4: C, 71.35; H, 3.81; N,7.56%.

2.1.2. cis-Ru(H2dcbiq)2Cl2 (1)Ru(Me2SO)4Cl2 (600 mg) and ethylene glycol (40 ml)

were heated near reflux until the Ru(Me2SO)4Cl2 wentinto solution. Na2dcbiq (960 mg) was added and heat-ing continued for approximately 5 min until the solu-tion became deep green. Approximately 0.2 g of LiCland an additional 10 ml of ethylene glycol were thenadded with continued heating until the LiCl dissolved.The cis-Ru(H2dcbiq)2Cl2 complex was precipitated bythe addition of 0.1 M HNO3 and the product wasplaced in a refrigerator overnight. Compound 1 wasisolated by filtration, washed well with H2O, and driedunder vacuum. Yield: 960 mg, 90%. MS (ESI MS);m/z : 430.0 (M−2H)2−, 859.2 (M−H)−. 1H NMR(D2O–NaOD, 300 MHz): � 8.68 (s), 8.56 (s), 8.10 (d),7.94 (d), 7.47 (t), 7.37 (d), 7.13 (tt), 6.53 (t), 6.14 (d).Chart 1.

A. Islam et al. / Inorganica Chimica Acta 322 (2001) 7–16 9

Anal. Found: C, 50.38; H, 3.61; N, 6.05. Calc. forC40H34N4O13Cl2Ru (cis-Ru(H2dcbiq)2Cl2(H2O)5): C,50.53; H, 3.60; N, 5.90%.

2.1.3. cis-Ru(H2dcbiq)2(NCS)2 (2)cis-Ru(H2dcbiq)2Cl2 (200 mg) was dissolved in DMF

(50 ml) under reduced light. KNCS (400 mg) wasseparately dissolved in 2 ml of water and subsequentlyadded to the above solution. The reaction mixture wasthen heated to reflux with vigorous stirring for 5 hunder Ar atmosphere. After this time, the reactionmixture was allowed to cool, and the solvent wasremoved on a rotary evaporator. The resulting solidwas dissolved in 0.1 M aqueous NaOH and filteredthrough a sintered glass crucible. The cis-Ru-(H2dcbiq)2(NCS)2 complex was precipitated by the ad-dition of 0.1 M HNO3. The green solid was isolated byfiltration, washed well with H2O, and dried under vac-uum. The product was dissolved in a minimum amountof aqueous 0.1 M tetrabutylammonium hydroxide(TBAOH) and purified by column chromatography us-ing Sephadex LH-20 as a column support and water aseluent. The main band, which was deep green andeluted very slowly, eventually becoming very wide, wascollected. The pH of the solution was lowered to 2.8 byadding dilute HNO3, and the solution was placed in arefrigerator overnight. The resulting precipitate wasfiltered and dried. The isolated solid was recrystallizedfrom methanol–diethyl ether. Yield 40%. MS (ESIMS); m/z : 301.1 (M−3H)3−, 381.2 (M−4H+TBA−

)3−, 451.7 (M−2H)2−, 572.5 (M−3H+TBA−)2−,694.3 (M−4H+2TBA−)2−. 1H NMR (CD3OD, 500MHz, −50 °C): � 9.12 (s), 9.04 (s), 8.53 (d), 8.32 (d),7.83 (d), 7.61 (t), 7.35 (t), 7.29 (t), 6.75 (t), 6.67 (d)(weak signals were omitted). Anal. Found: C, 59.49; H,5.57; N, 8.09. Calc. for C60H67N7O10S2Ru (cis-Ru(Hd-cbiq)2(NCS)2(TBA)(CH3OH)2): C, 60.40; H, 5.54; N,8.07%.

2.1.4. cis-Ru(H2dcbiq)2(CN)2 (3)cis-Ru(H2dcbiq)2Cl2 (500 mg) was dissolved in 30 ml

of DMF under reduced light. KCN (190 mg) wasseparately dissolved in 10 ml of water and subsequentlyadded to the above solution. The reaction mixture wasthen heated to reflux with vigorous stirring for 5 hunder Ar. During this time the solution color changedfrom green to blue. After cooling, the mixture wasfiltered and the solvent was evaporated from the filtrateusing a rotary evaporator. The product was then redis-solved in water and precipitated as a neutral complexby addition of 0.1 M HClO4. cis-Ru(H2dcbiq)2(CN)2

was isolated by vacuum filtration, washed with waterand dried under vacuum. This product was dissolved inaqueous TBAOH solution and further purified bycolumn chromatography on Sephadex LH-20 with wa-ter as eluent. The major purple band was collected and

the eluent was evaporated to a few milliliters using arotary evaporator. HClO4 (0.1 M) was then added toprecipitate cis-Ru(H2dcbiq)2(CN)2 (3). Compound 3was isolated by filtration, washed with water and ether,and dried under vacuum. Yield 45%. MS (ESI MS);m/z : 279.8 (M−3H)3−, 359.9 (M−4H+TBA−)3−,419.7 (M−2H)2−, 540.4 (M−3H+TBA−)2−, 660.9(M−4H+2TBA−)2−. 1H NMR (D2O–NaOD, 300MHz): � 9.88 (d), 8.26 (d), 8.11 (t), 7.91 (d), 7.82 (t),7.62 (s), 7.57 (s), 7.34 (t), 6.76 (t), 6.70 (d). Anal.Found: C, 61.56; H, 5.46; N, 8.80. Calc. forC58H62N7O10Ru (cis-Ru(Hdcbiq)2(CN)2(TBA)(H2O)2):C, 62.30; H, 5.59; N, 8.77%.

2.1.5. cis-Ru(H2dcdhph)2Cl2 (4)The procedure used to prepare cis-Ru(H2dcdhph)2Cl2

is analogous to that used to prepare cis-Ru(H2dcbiq)2-Cl2. MS (ESI MS); m/z : 303.8 (M−3H)3−, 455.8(M−2H)2−. 1H NMR (D2O–NaOD, 300 MHz): �

7.74 (d), 7.42 (d), 7.36 (t), 7.02 (t), 6.98 (t), 6.41 (t), 5.98(d), 3.55 (d). Anal. Found: C, 55.30; H, 3.65; N, 5.75.Calc. for C44H32N4O10Ru (cis-Ru(H2dcdhph)2Cl2-(H2O)2): C, 55.70; H, 3.40; N, 5.91%.

2.1.6. cis-Ru(H2dcdhph)2(NCS)2 (5)The procedure used to prepare cis-Ru(H2dcdhph)2-

(NCS)2 is analogous to that used to prepare cis-Ru(H2dcbiq)2(NCS)2. MS (ESI MS); m/z : 398.6 (M−4H+TBA−)3−, 477.9 (M−2H)2−, 598.5(M−3H+TBA−)2−, 719.2 (M−4H+2TBA−)2−.1H NMR (D2O–NaOD, 300 MHz): � 7.80 (d), 7.56 (d),7.30 (s), 6.60 (d) 3.55 (d). Anal. Found: C, 61.37; H,6.44; N, 7.88; S, 4.48. Calc. for C46H26N6O8S2Ru-(H2O)3(TBA)1.5: C, 61.20; H, 6.31; N, 7.65; S, 4.67%.

2.1.7. cis-Ru(H2dcdhph)2(CN)2 (6)The procedure used to prepare cis-Ru(H2dcdhph)2-

(CN)2 is analogous to that used to prepare cis-Ru(H2dcbiq)2(CN)2. MS (ESI MS); m/z : 296.3 (M−3H)3−, 305.0 (M−4H+Na)3−, 456.8 (M−3H+23)2−. 1H NMR (D2O–NaOD, 300 MHz): � 9.80(d), 8.02 (t), 7.86 (d), 7.81 (t), 7.54 (d), 7.36 (t), 6.81 (t),6.50 (d) 3.55 (d). Anal. Found: C, 54.50; H, 3.50; N,8.25. Calc. for C46H36N7O12RuNa2 (cis-Ru(Hd-cdhph)2(CN)2(H2O)2Na2): C, 54.60; H, 3.59; N, 8.31%.

2.2. Methods

2.2.1. Spectroscopic studiesUV–Vis and fluorescence spectra were recorded in a

Shimadzu UV-3101PC spectrophotometer and HitachiF-4500 spectrophotometer, respectively. Spectrophoto-metric titrations were performed in 10−5 M aqueousdye solution containing 0.1 M LiCl, and the initial pHwas adjusted to 10.5 by adding 0.1 M NaOH solution.The UV–Vis spectrum of each solution was obtained

A. Islam et al. / Inorganica Chimica Acta 322 (2001) 7–1610

Scheme 1. Synthesis of the various Ru complexes examined in thiswork.

2.2.4. Photoelectrochemical measurementsPhotoelectrochemical measurements were performed

in a two-electrode sandwich cell configuration as previ-ously reported [21]. As electrolyte, a mixture containing0.5 M 1,2-dimethyl-3-propylimidazolium iodide(DMPII), 0.02 M I2, 0.5 M 4-tert-butylpyridine (TBP),and 0.04 M LiI in methoxyacetonitrile was used. Theworking electrode was illuminated through a conduct-ing glass and the illuminated surface area was 0.25 or1.0 cm2.

Photocurrent and photovoltage were measured undersimulated solar light (Wacom Co., WXS-80C-3, AM1.5, 100 mW cm−2) using a potentiostat with a nonre-sistance ammeter (Nikko Keisoku Co., NPGS-2501). Inaddition, a 500 W Xe lamp with a �420 nm cut-offfilter and a ND 25 filter (83 mW cm−2) was employedas the light source. Monochromatic illumination wasobtained using a 500 W halogen lamp (Ushio DenkiCo.) in combination with a grating monochromatormodel (JASCO, CT-10), a scanning controller (JASCO,SMD-25C), and a multimeter (Keithley, 2000). Thelight intensities of monochromatic and solar simulatedlight were estimated with an optical power meter (Ad-vantest, TQ8210) and a thermopile (The Eppley Lab.,Inc., Newort, RI), respectively.

Incident photon-to-current conversion efficiencies(IPCE) at each incident wavelength were calculatedfrom Eq. (1)

IPCE(%)=(1240 eV· nm) Iph

�P0

×100 (1)

where Iph is the incident photocurrent density in�A cm−2, � is the wave length of the incident radiationin nm, and P0 is photon flux in W cm−2. The absorbedphoton-to-current conversion efficiency (APCE) wasobtained by dividing the IPCE by the light-harvestingefficiency (LHE, the percentage of light absorbed by theadsorbed chromophores defined as LHE=1−10−A(�)

with A(�) the absorbance at �).

3. Results and discussion

3.1. Synthesis

Scheme 1 shows the synthetic strategy adopted forthe synthesis of Ru complexes of the general formulacis-RuL2X2, with L=H2dcbiq or H2dcdhph and X−=Cl−, NCS− or CN−. Usually, complexes of this typeare prepared in two steps, starting with the reaction of1:2 molar amounts of Ru(III)Cl3 and the bidentateligand in dimethyl formamide, yielding cis-RuL2Cl2 [2].Subsequently, the CN− or NCS− ligand is introducedby reacting a large excess of it with cis-RuL2Cl2 indimethyl formamide–water. We found a very low yield(�10%) of the cis-RuL2Cl2 complexes when prepared

after adding acid and allowing the solution to equili-brate for 5 min. The measured emission spectra werecorrected for the wavelength dependent features. Spec-tra at 77 K were obtained by cooling the samples in aquartz Dewar flask filled with liquid nitrogen. Theemission lifetimes were measured by exciting the samplewith a �7 ns pulse at 500 nm from an optical paramet-ric oscillator (Surelite OPO) pumped at 355 nm by aNd:YAG laser (Continum Surelite II). The emissiondecay was followed in a digitizing signal analyzer,having used a Hamamatsu R928 photomultiplier toconvert the light signal to a voltage signal. Electrosprayionization mass spectra (ESI MS) were obtained withMicromass Quattro II. 1H NMR spectra were recordedin a Varian 300BB NMR spectrometer or a BrukerAVANCE 500 NMR spectrometer.

2.2.2. ElectrochemistryThe redox potentials of the complexes were measured

using a standard three-electrode apparatus. The counterelectrode was a platinum wire, the working electrodewas a carbon or platinum disk and the reference elec-trode was Ag/AgCl (saturated aqueous KCl) in contactwith a KCl salt bridge. Cyclic voltammograms werecollected using a BAS-100 electrochemical analyzer(Bioanalytical System). Methanol was used as a solventand the supporting electrolyte was 0.1 M tetrabutylam-monium perchlorate. Electrode potential values werecorrected to the SCE.

2.2.3. Preparation of thin-layer filmNanoporous oxide semiconductor thin films on ITO

electrodes were prepared using a previously publishedprocedure [20]. The thin films were coated for 15 h in5×10−5 M ethanolic dye solutions at room tempera-ture. The dye solutions contained 10 mM deoxycholicacid (choleic acid) as a co-adsorbate. The amount ofadsorbed dye was determined by desorbing the dyefrom the oxide semiconductor films into a solution of10−4 M NaOH in ethanol and measuring its absorp-tion spectrum.

A. Islam et al. / Inorganica Chimica Acta 322 (2001) 7–16 11

directly from RuCl3·3H2O. The yield was improved upto 90% by using a DMSO coordinating ruthenium(II)precursor complex, i.e. Ru(Me2SO)4Cl2 as a startingmaterial for the dichloro complex in ethylene glycol.

The 1H NMR spectroscopy of the dichloro complexshows a typical cis configuration, and the substitutionof chloride ligands by cyanates gives a similar 1H NMRspectrum [22]. Both Ru(H2dcbiq)2Cl2 and Ru(H2-dcbiq)2(CN)2 showed ten signals of individual chemicalshifts for the protons of the 4,4�-dicarboxy-2,2�-biquino-line moiety (Fig. 1(a) and (b)). On the other hand, thesubstitution of chlorides by thiocyanates gave a differ-ent result (Fig. 1(c)).

To investigate these differences we measured the 1HNMR spectra at different temperatures (Fig. 2). Theobservation indicates that at lower temperatures thestructure of the dithiocyanato complex is quite similarto that of the corresponding dichrolo and dicyanatoanalogs except for the very weak signals, which mightbe attributed to the S-bound thiocyanato isomer(s). Theassignments of the major and minor signals are indi-cated in Fig. 2(e). While the solution is warmed up, the1H NMR signal becomes more complicated, which canbe attributed to a coalescence of the signals of theseN-bound and S-bound isomers.

3.2. Absorption and emission spectra

The absorption and emission spectra of cis-Ru(H2dcbiq)2Cl2 (1), cis-Ru(H2dcbiq)2(NCS)2 (2), and

Fig. 2. Temperature dependence of 1H NMR spectra (500 MHz) ofcis-Ru(H2dcbiq)2(NCS)2 complex in CD3OD: (a) at 50; (b) at 25; (c)at 5; (d) at −15; (e) at −50 °C, ‘M’ denotes the major signals and‘m’ denotes the minor signals (see Fig. 1).

Fig. 1. Aromatic region of the 1H NMR spectra of H2dcbiq com-plexes at 25 °C: (a) cis-Ru(H2dcbiq)2Cl2 in D2O+NaOD; (b) cis-Ru(H2dcbiq)2(CN)2 in D2O+NaOD; (c) cis-Ru(H2dcbiq)2(NCS)2 inCD3OD.

Fig. 3. Absorption and emission spectra of cis-Ru(dcbiqH)2Cl2 (— ),cis-Ru(dcbiqH)2(NCS)2 (– – – ) and cis-Ru(dcbiqH)2(CN)2 (– · · – ):(a) absorption spectrum in methanol solution; (b) emission spectrumin ethanol–methanol glass (4:1, v/v) at 77 K.

cis-Ru(H2dcbiq)2(CN)2 (3) are shown in Fig. 3. Thewavelengths of the absorption bands of all the com-plexes investigated are shown in Table 1. The absorp-

A. Islam et al. / Inorganica Chimica Acta 322 (2001) 7–1612

Table 1Absorbance and luminescence properties of ruthenium complexes

Complex Emission bAbsorption a, �max (nm) (�, 103 M−1 cm−1)

MLCT band �max (nm)�–�* band � (ns)

77 K 298 K 77 K

627 (sh), 701 (6.4) 910cis-Ru(H2dcbiq)2Cl2 (1) 25269 (69.0), 350 (39.6) 210cis-Ru(H2dcbiq)2(NCS)2 (2) 269 (82.0), 349 (47.0) 518 (4.0), 627 (12.0) 873 34 590

480 (5.3), 589 (10.0) 786cis-Ru(H2dcbiq)2(CN)2 (3) 84268 (91.0), 341 (51.0) 600626 (sh), 723 (2.9) 905264 (50.9), 352 (29.1) 35cis-Ru(H2dcdhph)2Cl2 (4) 200

266 (69.0), 353 (38.5)cis-Ru(H2dcdhph)2(NCS)2 (5) 521 (4.3), 625 (12.1) 838 45 620263 (56.5), 344 (33.6)cis-Ru(H2dcdhph)2(CN)2 (6) 482 (4.2), 587 (6.9) 726 100 760

a Measured in methanol with NaOH.b Measured in ethanol–methanol (4:1).

tion spectra of the complexes in methanol are domi-nated by strong ligand-centered �–�* transitions ataround 268 and 350 nm on H2dcbiq or H2dcdhphligands. Intense Ru-to-dcbiq or Ru-to-dcdhph CT ab-sorption bands are observed in the visible region withan extinction coefficient between 12 000 and 6000mol−1 cm−1 and their maximum absorption wave-lengths depend on the strength of donor property of theancillary groups X used. The lowest MLCT band isred-shifted when the ancillary ligands are changed fromCN− to NCS− to Cl−, consistent with an increase inelectron density on the metal resulting in destabilizationof the metal t2g orbitals. All the complexes showed anadditional CT absorption band at higher energy thanthe main CT band.

Deprotonation of the carboxylic groups causes blueshifts of the �–�* and MLCT bands. The blue shift isdue to an increase in the energy of the LUMO of theH2dcbiq or H2dcdhph ligands, causing the �–�* andd�–�* transitions to occur at higher energies. Complex2 at pH 10 shows visible MLCT bands at 605 and 504nm, and the high-energy �–�* bands at 267 and 345nm. Upon lowering the pH from 10 to 2.5, the MLCTand �–�* transition bands shift from 605 to 640 nmand from 345 to 358 nm, respectively. Fig. 4 shows thepH dependent absorbance changes at 345 and 605 nm.The ground-state pKa of complex 2 is estimated to be2.9 with the pH at the inflection point. Nazeeruddin etal. investigated the pKa of the cis-Ru(H2dcbpy)2(NCS)2

complex and the dissociation of protons were found tooccur in two steps (pKa=3 and 1.5). In complex 2, wecould not observed the second inflection point becausethe neutral form of complex 2 was insoluble in waterand precipitation occurred at pH�2. The low pKa

value of 2 compared to cis-Ru(H2dcbpy)2(NCS)2 is dueto the strong �-accepting nature of the H2dcbiq ligandthan the H2dcbpy ligand.

The absorption spectra of cis-Ru(H2dcbiq)2(NCS)2

(2) adsorbed on transparent TiO2 and SnO2 thin filmsare shown in Fig. 5. Upon adsorption on TiO2 or SnO2

the absorption bands become broad and the MLCTabsorption bands at 520 and 640 nm, observed inMeOH solution, are red-shifted to 530 and 700 nm,respectively. The onset of the low-energy part of theabsorption spectrum is shifted at around 850 nm. Thispeak energy shift may be due to the change in theenergy levels of the ground and excited states comparedto that in solution, due to the interaction between thedye and the substrate.

It is well known that the excited state responsible forthe luminescence of the Ru(II)–polypyridine com-pounds is the lowest energy triplet metal-to-ligandcharge-transfer (3MLCT) state. All the complexes (1–6)show an intense emission at 77 K ethanol–methanolglass matrix. On raising the temperature the emissionspectra become weak and broad with a small shift to

Fig. 4. Absorbance as a function of pH for complex 2 at 345 (�) and605 nm (�).

A. Islam et al. / Inorganica Chimica Acta 322 (2001) 7–16 13

Fig. 5. Incident photon-to-current efficiency (– – – ), absorbed pho-ton-to-current efficiency (– · · – ) and absorption spectra (— ) ofcomplex 2 anchored to transparent films of TiO2 (a) and SnO2 (b).Absorption spectra are corrected for absorbance of the correspondingundyed films. The IPCE and APCE are plotted as a function ofwavelength. A sandwich-type cell configuration was used to measurethese spectra.

lower energy. The luminescence data are gathered inTable 1. As expected, the emission maxima were alsored-shifted by about 100 nm with a decrease of thedonor property of the ancillary group X (Fig. 3).According to the energy gap law, the rate constant ofthe radiationless decay of the 3MLCT excited state tothe ground state must increase as the energy gap be-tween the ground and excited states decreases. Theexpected decrease in the excited-state lifetimes withdecreasing excited-state energy is observed (Table 1).

3.3. Electrochemical properties

Fig. 6 shows the cyclic voltammogram of complex 2in acetonitrile. The reversible wave at 0.92 V versusSCE is assigned to the Ru(III)/(II) couple. On thecathodic side, there is a reversible peak at −0.85 Vversus SCE, assigned to the reduction of the H2dcbiqligand. Table 2 displays the ground-state redox poten-tials of all the complexes. The oxidation potential forcis-Ru(H2dcbiq)2Cl2 is 0.68 V (vs. SCE), which is morepositive than the corresponding value of 0.43 V (vs.SCE) obtained for the unsubstituted analog cis-Ru(-biq)2Cl2 [23]. This is attributed to the electron-with-drawing properties of the carboxylate groups.Ru–dcdhph complexes exhibit reduction potentials ataround −0.72 V (vs. SCE), which is more positive thanthe Ru–dcbiq complexes. A similar trend was alsoobserved between Ru(biq)3(PF6)2 and Ru(dhph)3(PF6)2

[24]. Substitution of the chloro ligand by NCS− (2 and5) and CN− (3 and 6) results in a steady increase in thepotential of the Ru(III)/(II) couple from 0.68 to 1.15 Vversus SCE, illustrating the tuning of the redox poten-tials by selecting appropriate ligands. The trend for theground-state oxidation potential was RuL2Cl2�RuL2(NCS)2�RuL2(CN)2, where L=H2dcbiq orH2dcdhph. This trend correlates with the electron do-nating/�-accepting properties of the ligands around themetal center. Ligands with more electron donating

Fig. 6. Cyclic voltammogram of complex 2 measured in 0.1 MTBAClO4 acetonitrile solution.

Table 2Electrochemical and photoelectrochemical a properties of ruthenium complexes

E1/2* d (V)Complex Redox potentials b E0–0 c (eV) � (%)Fill factorVoc (mV)Jsc (mA cm−2)

Red. (mV)Oxid. (mV)

0.116801 −850 1.40 −0.72 0.70 450 0.420.300.725000.80−0.612 1.53−8509200.33−825 1.68 −0.53 0.883 5301145 0.73

750 −720 1.45 −0.714 0.20 460 0.56 0.050.415 −0.701.54−740 0.130.63480830

1.79−70011506 −0.64 0.40 460 0.60 0.11

a Semiconductor thin films: TiO2; light source: a solar simulator AM1.5 (100 mW cm−2); electrolyte: 0.5 MDMPII+0.03 M I2+0.5 MTBP+0.04 M LiI in methoxyacetonitrile; surface area of electrodes: 0.25 cm2.

b mV vs. SCE.c E0–0 estimated from a tangent to the high-energy side of the corrected emission spectrum.d E1/2* estimated by Eq. (2).

A. Islam et al. / Inorganica Chimica Acta 322 (2001) 7–1614

Table 3Photovoltaic performance of cis-Ru(H2dcbiq)2(NCS)2-sensitized oxide semiconductor solar cells a

IPCE at 517 nm b (%) Jsc (mA cm−2) Voc (mV) Fill factor � (%)Semiconductor Particle size (nm)

6.0 1.36TiO2 0.3330–50 0.62 0.34Nb2O5 100 0.1 0.02 0.21 0.35 0.01ZnO 3.8100–400 0.74 0.21 0.29 0.05

6.6 2.12 0.2615–20 0.52SnO2 0.29100–500In2O3 3.2 1.17 0.11 0.28 0.04

a Light source: a 500 W Xe lamp with a �420 nm cut-off filter and a ND 25 filter (83 mW cm−2); electrolyte: 0.3 M Pr4NI+0.03 M I2 inethylene carbonate–acetonitrile (60:40); surface area of electrodes: 1.0 cm2.

b Light source: a 500 W Xe lamp with a band-pass filter (517 nm).

properties (such as Cl−) will confer additional stabilityon the Ru(III) oxidation state relative to the Ru(II)oxidation state, which will produce a more negativeground-state Ru(III)/(II) redox potential. In complexcis-Ru(H2dcbpy)2(NCS)2, the Ru(II)/(III) couple wasreported at 0.85 V versus SCE which is 0.07 V morenegative than complex 2. The difference (0.07 V) in theoxidation potential of complex 2 from that of cis-Ru(H2dcbpy)2(NCS)2 is due to the strong �-acceptingnature of H2dcbiq. Increasing the �-accepting proper-ties of the ligands, however, leads to stabilization of themetal t2g symmetry d-orbitals, which therefore producesa more positive Ru(III)/(II) formal potential. The pKa

decreases with increasing Ru(II)/(III) oxidation poten-tial. There is a good correlation between the oxidationpotential and the pKa values of these two complexes.

The excited-state oxidation potentials are estimatedby Eq. (2), where E1/2(RuIII/II) is the oxidation potentialof the ground state, E0–0 is the minimum energy be-tween the ground state and the excited state. E0–0 wasestimated from a tangent to the high-energy side of thecorrected emission spectrum of the complex. The ex-cited-state oxidation potentials are given in Table 2.The excited-state oxidation potential of the H2dcbiq orH2dcdhph complexes is less negative compared to thecis-Ru(H2dcbpy)2(NCS)2 complex [2].

E1/2(RuIII/II�)=E1/2(RuIII/II)−E0−0 (2)

3.4. Photo�oltaic performance

3.4.1. Sensitization of TiO2 by Ru–dcbiq andRu–dcdhph complexes

The performance of the ruthenium complexes assensitizers on nanocrystalline TiO2 is summarized inTable 2. Though both H2dcbiq and H2dcdhph com-plexes have spectral response in a wide energy range(400–850 nm), but show low values of short-circuitphotocurrent (Jsc) and poor cell efficiencies (�). Theshort-circuit photocurrent density depends upon thespectral property of the dye, the energy gap between theexcited-state oxidation potential of the dye and thepotential of the conduction band of the semiconductor

substrate, rapid charge recombination, sluggish iodideoxidation and so on. It has been reported that theinterfacial electron-transfer processes on TiO2 elec-trodes are in the femtosecond domain and the excited-state oxidation potential of the dye should be above thetail of the conduction band [25–27]. The excited-stateslifetime (25–100 ns) of all the complexes is long enoughfor the process of electron injection into the conductionband of the TiO2 electrode to be efficient. Recently,Tachibana et al. and Meyer et al. independently showedthat charge recombination kinetics are highly nonexpo-nential and independent of sensitizer properties [14,26].A rapid iodide oxidation is expected for all these sensi-tizers (1–6) due to their positive oxidation potentials[28]. Therefore, the low photocurrent efficiency ob-served for the studied complexes may be due to the low�* energy level of the anchoring H2dcbiq or H2dcdhphligand (Table 2). The lowest excited-state oxidationpotential (� −0.7 vs. SCE) of these sensitizers is notenergetically favorable to transfer an electron into theconduction band (−0.82 V vs. SCE, pH 7) [29] of theTiO2 semiconductor.

3.4.2. Dependence of photo�oltaic performance ofcomplex 2 on the potential of the conduction band ofse�eral nanoporous oxide semiconductors

The photovoltaic performances for complex 2 ad-sorbed on Nb2O5, ZnO, SnO2, In2O3, and TiO2

nanoporous oxide semiconductors are summarized inTable 3. The amount of dye molecules adsorbed onboth TiO2 and SnO2 was nearly the same (6×10−8

mol cm−2). But it was decreased ten times for the largeparticle size (100–500 nm) semiconductor films of ZnOand In2O3. Considering both the particle size of thesemiconductors and the amount of adsorbed moleculeson it, it is found that the values of Jsc as well as IPCE(at 517 nm) are higher for SnO2 and In2O3 than forTiO2, Nb2O5, and ZnO. The trend for the conductionband potential of the semiconductors studied here isTiO2 (−0.82 V, pH 7)�Nb2O5 (−0.81 V, pH 7)�ZnO (−0.61 V, pH 4.8)�SnO2 (−0.3.1 V, pH 7)�In2O3 (0.1 V) versus SCE [29–31]. Therefore, thepotentials of the conduction band edge of SnO2 and

A. Islam et al. / Inorganica Chimica Acta 322 (2001) 7–16 15

In2O3 are energetically more favorable for electroninjection than the conduction band edge of TiO2,Nb2O5, and ZnO.

3.4.3. Spectral response characteristics of 2 coatedTiO2 and SnO2 electrodes

The spectral dependence of the photocurrent canprovide useful information about the injection effi-ciency of the dye. Since the extent of dye adsorption isdependent on numerous factors, IPCE data has alsobeen converted to absorbed photon-to-current conver-sion efficiency (APCE) to convert the data from adevice efficiency to a quantum efficiency for complex 2in the device. The APCE spectra can be a good relativeapproximation of injection yields for dye 2-sensitizedTiO2 and SnO2 films if the film properties and elec-trolyte conditions are identical. Fig. 5 shows the pho-tocurrent action spectra for complex 2 adsorbed onTiO2 and SnO2 films where the IPCE and APCE valuesare plotted as a function of wavelength along with theabsorption spectra of the adsorbed dye. It is observedthat in the dye 2-sensitized TiO2 film (Fig. 5(a)), thelower energy MLCT band (700 nm) has a higher ab-sorbance value than the higher energy MLCT band(530 nm) but their relative contributions to the actionspectra are opposite to their absorbance values. Theinjection quantum yield (APCE) for the higher energy(480 nm) transition is nearly 25%, whereas it is onlyabout 4% from the lower energy (700 nm) transition. Arelatively more efficient injection occurs from the higherenergy MLCT and �–�* transitions than from thelower energy MLCT transition. The energy level of thelowest excited MLCT state is estimated to be about 0.1

eV lower than the conduction band edge of TiO2.Hence, the lowest excited MLCT state is not energeti-cally favorable to inject an electron into the conductionband of TiO2.

The IPCE spectrum for complex 2 on SnO2 film,which has a conduction band edge �0.5 V [30] morepositive than TiO2, qualitatively traces the dye’s ab-sorbance feature, that is, the photocurrent contributionfrom the two MLCT bands are equally proportional totheir absorbances (Fig. 5(b)). Between 450 and 800 nm,the APCE spectrum is nearly flat, indicating that anefficient injection occurred in this area at all energies.These results indicate that the lowest excited MLCTstate of 2 is energetically allowed to transfer an electroninto the conduction band of SnO2 but not into TiO2.

Fig. 7 shows the proposed energy level scheme fordye 2-sensitized TiO2 or SnO2. High-energy (h�1) pho-ton absorption forms the (S+/S*)2 state (higher lyingexcited MLCT state) that may inject (kinj) an electroninto the semiconductor (TiO2 or SnO2) or internalconversion (kic) to the (S+/S*)1 state (lowest excitedMLCT state) followed by an electron injection (k �inj) tothe semiconductor SnO2 only or return to the groundstate S+/S. On the other hand, low-energy photon (h�2)absorption forms the (S+/S*)1 state that may inject(k �inj) an electron into the semiconductor of SnO2 ormay inject (k�inj) directly form the hot vibrational levelinto the TiO2 conduction band.

An efficient electron injection into the TiO2 conduc-tion band (APCE�23%, at 500 nm) from the higherlying excited MLCT state ((S+/S*)2) occurs prior to theinternal conversion event between the (S+/S*)1 and(S+/S*)2 excited states. Although the lowest excitedMLCT state is not thermodynamically favorable forelectron injection in the dye 2-sensitized TiO2 film, thesmall APCE value (�3%) observed between 550 and800 nm is due to the hot electron injection as discussedin Fig. 7. Here, both electron injection and vibrationalrelaxation processes are competing with each other.Thus, the dual electron injection phenomenon observedin the dye 2-sensitized TiO2 film indicates an ultrafastrate of electron injection to TiO2, and agrees with thefindings of femtosecond dynamical studies [25–27].

4. Conclusions

A new class of Ru(II) complexes of the cis-RuL2X2

type with bulky dcbiqH2 and dcdbphH2 ligands weresynthesized and used to sensitize TiO2 to solar illumina-tion. Due to the low �* energy level of these ligands thelight absorption profile at longer wavelengths was im-proved. Despite better light harvesting at lower ener-gies, the poor cell efficiency of these complexes wasobserved. The poor efficiency may be ascribed to thelow excited-state oxidation potential, which plays a

Fig. 7. Schematic representation of the excited-state charge injectioninto a conduction band of the semiconductors TiO2 and SnO2 in dye2-sensitized solar cell for excitation at two different wavelengths h�1

and h�2 (h�1�h�2). S+/S0, (S+/S*)1, and (S+/S*)2 are the redoxpotential levels of the ground state, the lowest excited state and thesecond lowest excited state, respectively. kinj and k �inj are excited-stateelectron transfer processes. Electron injection from hot or vibra-tionally unrelaxed excited state is denoted by k�inj. k0 indicates radia-tive and radiationless decay of (S+/S*)1 to S+/S0. kic is the internalconversion rate between (S+/S*)1 and (S+/S*)2.

A. Islam et al. / Inorganica Chimica Acta 322 (2001) 7–1616

crucial role in the electron-transfer process. The bandselective electron injection yield (APCE) observed in thedye cis-Ru(dcbiq)2(NCS)2/TiO2 system suggests a veryfast rate of electron injection to TiO2, before the inter-nal conversion event between the two lowest excitedstates. A wavelength independent electron injectionyield in cis-Ru(dcbiq)2(NCS)2/SnO2 indicates that anefficient electron injection occurs from the lowest ex-cited MLCT state into the low-lying conduction bandof the SnO2.

Acknowledgements

Financial support of this work by Science and Tech-nology Agency, Center of Excellence Development Pro-ject (COE), Japan, is gratefully acknowledged.

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