New terpyridine-based ruthenium complexes for dye sensitized solar cells applications

Article abstract of DOI:10.1016/j.ica.2015.11.031

Three new terpyridine-based ruthenium complexes, named as MC124, MC125 and MC127, were synthesized and employed as sensitizers in dye sensitized solar cells. The MC dyes were characterized with experimental techniques followed by theoretical calculations. The promising optical properties with higher molar extinction coefficients compared to N749 prototypical dye, and suitable positioning of energy levels prompted us to employ these dyes in working devices. When used in conjunction with liquid I^-/I₃^- redox electrolyte, the MC dyes have shown modest performances, with a maximum PCE of 2.3% reached with MC124 in combination of CDCA used as co-adsorbent.

Full text of DOI:10.1016/j.ica.2015.11.031

Accepted Manuscript
New Terpyridine-based Ruthenium Complexes for Dye Sensitized Solar Cells
Applications
Ganesh Koyyada, C.H. Pavan Kumar, Paolo Salvatori, Gabriele Marotta, Maria
Grazia Lobello, Olivia Bizzarri, Filippo De Angelis, Chandrasekharam
Malapaka
PII:
S0020-1693(15)00595-2
http://dx.doi.org/10.1016/j.ica.2015.11.031
ICA 16789
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
28 July 2015
19 October 2015
28 November 2015
Please cite this article as: G. Koyyada, C.H. Pavan Kumar, P. Salvatori, G. Marotta, M.G. Lobello, O. Bizzarri, F.
De Angelis, C. Malapaka, New Terpyridine-based Ruthenium Complexes for Dye Sensitized Solar Cells
Applications, Inorganica Chimica Acta (2015), doi: http://dx.doi.org/10.1016/j.ica.2015.11.031
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New Terpyridine-based Ruthenium Complexes for Dye Sensitized
Solar Cells Applications
Ganesh Koyyada,a,b Pavan Kumar CH,a,b Paolo Salvatori,c,d Gabriele Marotta,c,e Maria Grazia
Lobello,c Olivia Bizzarri,c Filippo De Angelis*,c Chandrasekharam Malapaka*.a,b
aInorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology,
Uppal Road, Tarnaka, Hyderabad 500607, India.
bAcademy of Scientific and Innovative Research, CSIR-IICT, India.
cComputational Laboratory for Hybrid and Organic Photovoltaics, Istituto CNR di Scienze e
Tecnologie Molecolari, via Elce di Sotto 8, 06123, Perugia, Italy.
dD3-Computation, Istituto Italiano di Tecnologia, Via Morego 30, I-16163 Genova, Italy.
eDipartimento di Chimica, Biologia e Biotecnologie, Universitàdegli Studi di Perugia, via Elce di
Sotto 8, I-06123 Perugia, Perugia, Italy.
E-mail: Chandra@iict.res.in Phone Fax : +00914027193186.
Keywords: Dye sensitized solar cell (DSSC), Thiocyanate free, Single Thiocyanate, Efficiency,
DFT/TDDFT
Abstract
Three new Terpyridine-based ruthenium complexes, named as MC124, MC125 and MC127,
were synthesized and employed as sensitizers in Dye Sensitized Solar Cells. The MC dyes were
characterized with experimental techniques followed by theoretical calculations. The promising
optical properties with higher molar extinction coefficients compared to N749 prototypical dye,
and suitable positioning of energy levels prompted us to employ these dyes in working devices.
-
When used in conjunction with liquid I-/I3 redox electrolyte, the MC dyes have shown modest
performances, with a maximum PCE of 2.3% reached with MC124 in combination of CDCA
used as co-adsorbent.

1. Introduction
Dye-sensitized solar cells (DSSCs) are considered to be one of the alternative next generation
photovoltaic devices due to their ease of fabrication, low materials cost and attractive unique
features.1 Typical DSSCs is based on a photoelectrode with mesoporous anatase TiO2 layer,2,3
usually composed by a network of nanoparticles (~ 20/30 nm diameter), sensitized with dye
-
molecules and interpenetrated by a liquid electrolyte (usually I-/I3 redox couple in a volatile
organic solvent). The device architecture is completed by a counter-electrode. After the
photoexcitation of the chemisorbed dye, charge separation occurs at the dye/TiO2 interface with
electron injection into the semiconductor conduction band (CB). The injected electrons flow
across the mesoporous structure of TiO2 layer and through the external circuit is collected at the
counter electrode. The oxidized dye molecules are regenerated by the redox mediator in the
telectrolyte solution and finally, the circuit is closed by the electrolyte regeneration at the
counter-electrode.
In the past decade, number of modifications have been attempted on sensitizers, TiO2 photo
electrodes, counter electrodes and electrolyte composition in order to improve the overall
performance of DSSCs.4-8 Sensitizers are the key element to the success of this technology as the
power conversion efficiency of the device is known to be greatly influenced by light harvesting
and electron injection properties of the sensitizer.. A large variety of sensitizers have been tested
for DSSCs including organic dyes,9-13 zinc porphyrin14-16 and Ru(II)-based organometallic
sensitizers.17-19, 55
Although the high power conversion efficiencies (PCE) have been reached with organic dyes
(approaching 10%)20,21 and with porphyrin sensitizers in combination with Cobalt based redox
couple (13%),22-24 the Ru(II)polypyridyl organometallic sensitizers are continued to be deeply

investigated. Apart from molecular sensitizers, recently perovskite hybrid absorbers
(MAPbI3,where MA = methylammonium or other derivatives) have shown 19% efficiency in
solid state devices.25-27 However, health hazards due to lead perovskites are a critical issue that
may undermine its future employment and hence scientists are encouraged to continuously stay
on the track of traditional DSSCs.
One of the prototypical terpyridyl ruthenium (II) sensitizer [Bu4N]3[Ru(tctpy)(NCS)3],was
reported by Nazeeruddin and coworkers.28 This sensitizer, also known as N749 or black dye, has
shown an impressive efficiency of 10.4%. When used in combination with an organic
cosensitizer, an improved certified record efficiency of 11.4% has been reported.45 Though
ruthenium (II) terpyridyl complexes show better photo response in longer wavelength region
than those of bipyridyl complexes, the disadvantage generally lies with its low incident to photon
conversion efficiency due to poor molar extinction coefficient and insufficient free energy for
electron injection.29 Extensive research has been pursued on functionalization of the N749
structure, with electron rich units, in order to increase light harvesting capacity and the charge
transfer efficiency.30-33 Other possible strategies employed to improve the efficacy in both the
visible and near-IR regions, are the tandem DSSCs 35–36 or the cosensitization.37–42
Concerning the stability in working devices, the N749 dyes have shown similar performances
compared to other Ru-bipyridyl dyes.43 The presence of sulphur group in thiocyanate based
ruthenium (II) sensitizers makes intimate contact with the iodide redox couple of the electrolyte
that helps in the regeneration of dye.44 Despite of these advantages, thiocyanates are weakly
coordinating to the ruthenium (II) center and can be substituted by electrolyte additives, such as
tert-butyl pyridine, or solvent molecules, under Stressing thermal and light soaking conditions.45-
47

Being already engaged in synthesis of various ruthenium and metal free sensitizers,48-55 we have
designed a new class of ruthenium (II) sensitizers by partially or fully replacing the thiocyanate
groups in black dye with bipyridine and terpyridine ligands as shown in Figure1, which is
presumed to improve the absorption properties in the longer wavelength region. The bipyridines
were functionalized with carbazole and fluorene as ancillary groups because of their hole
transport capability, electron-richness and elongated conjugation. Furthermore alkyl chains
introduced on the fluorene, carbazole and terpyridine moieties are expected to inhibit the dye
aggregation and interfacial electron recombination processes. Here taking advantage of the
support from theoretical calculations, we demonstrate the synthesis, the characterization and the
photovoltaic performance of these three new terpyridine-based ruthenium complexes suitable for
applications in DSSCs.
C6H13
N
C10H21
C10H21
C10H21
C10H21
N
Ru
N
N
C6H13
N
N
N
N
N
N
NCS
N
N
N
NCS
N
N
N
HO
OH
Ru
N
Ru
N
O
O
HO
HO
OH
OH
O
O
O
O
OH
O
O
OH
O
OH
MC127
MC124
MC125
Figure 1. Structures of MC124, MC125 and MC127
2. Experimental details
2.1 Materials and instrumentation

The starting materials were purchased from Sigma-Aldrich. The solvents were purified using
standard procedures and purged with nitrogen before use. All other chemicals used in this work
were analytical grade and were used without further purification, and all reactions were
performed under argon atmosphere. Absorption spectra were recorded on a Shimadzu ultraviolet-
visible light (UV-vis) spectrometer. Emission spectra were recorded on a J. Y. Horiba model
fluorolog3 fluorescence spectrometer. The Differential Pulse Voltametry (DPV) curves were
recorded using BAS100 electrochemical analyser where a three electrode cell is used in 0.1 M
Bu4NPF6, N,N-dimethylformamide as solvent at a scan rate of 100 mVs-1, Pt wire as a counter
electrode , Ag/AgCl as reference electrode and calibrated with ferrocene.
2. 2 2-bromo-9,9-didecyl-9H-fluorene (1) (83%). To an ice-cooled suspension of KOH (0.137
g, 2.459 mmol) in DMSO (10 mL), 2-bromo-9H-fluorene (0.200 g, 0.819 mmol) and 1-
bromodecane (0.345 mL, 2.276 mmol) was added. The suspension was stirred at room
temperature for 16 h under N2. The mixture was cooled at room temperature and brine was
added. The organic layer was extracted with ethylacetate and the combined organic extractions
were dried over Na2SO4. After evaporation of solvent, the crude compound was purified over
silica gel column and eluted using ethylacetate:hexane (1:9).
1H NMR (300 MHz, CDCl3, ꢀ ): 7.64-7.68 (m, 1H), 7.53-7.56 (m, 1H), 7.42-7.46 (m, 2H), 7.30-
7.34 (m, 3H), 1.86-1.94 (m, 4H), 1.22-1.28 (m, 4H), 1.16-1.20 (m, 10H) 1.12-1.15 (m, 4H), 1.00-
13
1.10 (m, 14H), 0.9 (t, 6H); C NMR (300MHz, CDCl3, ꢀ): 14.11, 22.65, 23.65, 29.22, 29.52,
29.93, 31.85, 40.24, 55.34, 119.70, 120.92, 120.98, 122.83, 126.08, 128.87, 127.41, 129.82,
130.98, 140.09, 150.26, 152.92. ESI-MS:m/z [M]+: 526.
2.3 2-(9,9-didecyl-9H-fluoren-7-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2) (82%).A
solution of compound (1) (0.500 g, 0.952 mmol), bis(pinacolato)diboron (0.265 g, 1.047 mmol),
were dissolved in dry dimethoxy ethane (8 mL) and KOAc (0.227 g, 2.856 mmol) was added.
The mixture was stirred for 15 min under an argon atmosphere at room temperature. Then
Pd(dppf)2Cl2 (0.041 g, 0.057 mmol) catalyst was added and the reaction mixture was stirred

0
overnight at 80 C. After being cooled to room temperature, the mixture was poured into water
(20 mL) and the organic layer was separated. The aqueous layer was extracted with
dichloromethane (3 X 20 mL) and the combined organic layers were dried over anhydrous
Na2SO4. The organic solvent was evaporated under reduced pressure and the crude product was
pre-adsorbed onto silica gel and chromatographed (8:2 n-hexane/ethyl acetate) to give yellow
viscous liquid 2
1H NMR (300 MHz, CDCl3, ꢀ ): 7.78-7.82 (m, 1H), 7.74-7.76 (m, 1H), 7.68-7.73 (m, 2H), 7.30-
7.36 (m, 3H), 1.92-2.02 (m, 4H), 1.39 (s, 12H), 1.20-1.32 (m, 8H), 1.10-1.20 (m, 12H) 0.98-1.08
(m, 12H), 0.88 (t, 6H); 13C NMR (300MHz, CDCl3, ꢀ): 14.13, 22.69, 23.71, 24.96, 29.29, 29.57,
30.05, 31.88, 40.25, 55.07, 83.69, 118.95, 120.09, 122.94, 126.66, 127.48, 128.82, 133.70,
140.92, 144.12, 149.88, 151.30. ESI-MS:m/z [M]+: 572.
2.4
4-(9,9-didecyl-9H-fluoren-2-yl)-2-(4-(9,9-didecyl-9H-fluoren-7-yl)pyridin-2-yl)pyridine
(L1) (76 %). A 50 mL of Schlenk tube was charged with aryl 4,4'-dibromobipyridine (0.500 g,
1.592 mmol) compound (2) (1.912 g, 3.343 mmol), Pd(PPh3) (0.115 g, 0.1 mmol). Dimethoxy
ethane (8 mL), 2 M aqueous sodium carbonate (2 mL) were added and the tube was purged with
0
argon gas with 5 evacuate /refill cycles. The tube was sealed and heated at 90 C for 18 h. Upon
cooling to ambient temperature, the organics were extracted into dichloromethane (3 × 30 mL)
from 30 mL water. The combined organics were washed with water (1 × 30 mL) and brine (1 ×
30 mL), dried over Na2SO4, filtered and the solvent was removed under reduced pressure. The
crude compound was purified by silica gel column chromatography using hexane/ethyl acetate
(9:1) to give light yellow powder.
1H NMR (300 MHz, CDCl3, ꢀ): 8.80-8.04 (m, 4H), 7.74-7.88 (m, 8H), 7.64-7.68 (m, 2H), 7.32-
7.40 (m, 6H), 2.00-2.32 (m, 8H), 1.24-1.28 (m, 8H), 1.12-1.20 (m, 14H), 0.9 (t, 12H); 13C NMR

(300MHz, CDCl3, ꢀ): 14.08, 22.63, 23.77, 29.69, 30.43, 31.49, 31.62, 31.92, 33.82, 40.35, 55.34,
114.05, 121.33, 121.82, 122.92, 123.45, 126.18, 126.85, 127.52, 136.88, 139.23, 140.33, 142.29,
149.53, 149.98, 151.13, 151.66, 156.73.ESI-MS:m/z [M+H]+:1046
2.5 3-bromo-9-hexyl-9H-carbazole (3) (82%). The synthesis procedure resembles that of
compound (1) with 1-bromohexane (0.093 mL, 0.666 mmol) instead of 1-bomodecane in DMF
solvent. Viscous liquid (3) was obtained. 1H NMR (300 MHz, CDCl3, ꢀ): 8.18 (d, 1H), 8.00-8.04
(d, 1H), 7.49-7.54 (m, 1H), 7.44-7.47 (m, 1H), 7.36-7.40 (m, 1H), 7.20-7.25 (m, 1H), 7.19-7.22
(m,1H), 4.21-4.26 (t, 2H), 1.77-1.87 (m, 2H), 1.23-1.36 (m, 6H), 0.9 (t, 3H); 13C NMR
(300MHz, CDCl3, ꢀ): 13.97, 22.48, 26.85, 28.79, 31.48, 43.05, 108.83, 110.01, 119.06, 120.4,
121.68, 122.93, 124.93, 124.42, 126.22, 128.08, 128.85, 138.92. ESI-MS:m/z [M]+: 330.
2.6 9-hexyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (4) (80%). The
1
synthesis method resembles that of compound (2). Colorless liquid (4) was obtained. H NMR
(300 MHz, CDCl3, ꢀ): 8.60 (s, 1H), 8.06 (d, 1H), 7.86 (d, 1H), 7.42-7.44 (m, 1H), 7.38-7.40 (m,
2H), 7.20-7.23 (m, 1H), 4.22-4.36 (t, 2H), 1.82-1.86 (qt, 2H), 1.40 (s, 12H), 1.22-1.38 (m, 6H),
0.9 (t, 3H); 13C NMR (300MHz, CDCl3, ꢀ): 13.93, 22.45, 24.88, 26.86, 28.82, 31.50, 43.00,
83.47, 108.01, 108.65, 119.11, 120.47, 122.54, 123.06, 125.51, 127.69, 132.09, 140.54, 142.54.
ESI-MS:m/z [M+H]+:338.
2.7 9-hexyl-3-(2-(4-(9-hexyl-9H-carbazol-6-yl)pyridin-2-yl)pyridin-4-yl)-9H-carbazole (L2)
(73%). The synthesis method resembles that of compound (L1). White powder of (L2) was
obtained.1H NMR (300 MHz, CDCl3, ꢀ): 8.84-8.86 (m, 2H), 8.78-8.82 (m, 2H), 8.58-8.60 (m,
2H), 8.20-8.22 (m, 2H), 7.96-7.98 (m, 2H), 7.72-7.76 (m, 2H), 7.50-7.56 (m, 4H), 7.44-7.48 (m,
2H), 7.28-7.32 (m, 2H), 4.36-4.40 (t, 4H), 1.88-1.96 (qt, 4H), 1.56-1.64 (m, 4H), 1.40-1.48 (m,
13
4H), 1.28-1.36 (m, 4H),0.9 (t, 6H); C NMR (300MHz, CDCl3, ꢀ): 13.99, 22.51, 26.94, 28.92,

31.54, 43.21, 108.92, 109.08, 119.05, 119.18, 119.22, 120.59, 121.54, 122.80, 123.41, 124.87,
126.03, 128.80, 140.89, 140.94, 149.50, 150.17, 156.71.ESI-MS:m/z [M+H]+:656.
2.8 4,4',4''-trimethoxycarbonyl-2,2':6',2''-terpyridine (5) and trimethyl [2,2':6',2''-
terpyridine]-4,4',4''-tricarboxylate Ruthenium(III) trichloro (6) were reported in our previous
literature (RSC Adv., 2013, 3, 26035–26046)
2.9 General procedure for the synthesis of ruthenium complex (MC124 and MC125)
A solution of ligand L (0.272 mmol) and ruthenium trichloro complex (0.227 mmol) dissolved in
dry DMF (100 mL) was heated and refluxed for 4 h under nitrogen atmosphere in the dark. After
completion of 4 h, the mixture was cooled to 800C. To the resulting dark green solution was
added aqueous solution of NH4NCS (0.573 g in 2 mL of water) and the reaction mixture was
further refluxed for 2 h. The progress of the reaction was monitored by UV-Vis spectroscopy.
After 2 h, the reaction mixture was cooled to room temperature and triethylamine was added
followed by water (2 ml+2 mL). The mixture was further refluxed for 48 h. Then the reaction
mixture was cooled to room temperature and the solvent was removed under reduced pressure
and water (200 mL) was added to get precipitate. The purple solid was filtered off and washed
with distilled water, ether and dried under vacuum. The crude compound was dissolved in
methanol and Terrabutylammonium hydroxide (TBA), further purified on sephadex LH-20 with
methanol as eluent. The main band was collected and concentrated to give
MC124 (63%).1H NMR (300 MHz, CDCl3+CD3OD, ꢀ): 8.96-9.24 (m, 2H), 8.76-8.84 (m, 2H),
8.68-8.72 (m, 2H), 7.96-8.48 (m, 2H), 7.84-7.92 (m, 6H), 7.72-7.82 (m, 4H), 7.52-7.64 (m, 2H),
7.44-7.48 (m, 2H), 7.32-7.40 (m, 6H) 1.96-2.20 (m, 8H), 1.14-1.36 (m, 64H), 0.9 (t, 12H); FT-IR
(KBr) (cm-1): 3064, 2924, 2852, 2092, 1604, 1450, 1364, 1235, 1019. Anal. cal.:

C96H115N5O6RuS. C: 73.53, H: 7.39, N: 4.47, S: 2.04 found: C: 73.50, H: 7.32, N: 4.41, S:
2.01%.
1
MC125 (60%). H NMR (300 MHz, CDCl3+CD3OD, ꢀ): 8.76-8.84 (m, 4H), 8.56-8.64 (m, 2H),
8.20-8.28 (m, 3H), 7.92-8.00 (m,3H), 7.76-7.82 (m, 3H), 7.44-7.60 (m, 8H), 7.21-7.32 (m, 4H),
7.04-7.08 (m, 1H), 4.36 (t, 4H), 3.56-3.76 (m, 4H), 1.88-2.08 (m, 4H), 1.52-1.64 (m, 2H), 1.24-
1.32 (m, 4H), 0.9 (t, 8H); FT-IR (KBr) (cm-1): 3416, 2921, 2852, 2097, 1594, 1466, 1365, 1237,
1156.Anal. cal.: C66H57N7O6RuS. C: 67.33, H: 4.88, N: 8.33, S: 2.72 found: C: 67.30, H: 4.82,
N: 8.29, S: 2.69%.
MC127 (58%).The synthesis method resembles that of MC124 and MC125, except that
NH4NCS was not added.
1H-NMR (300 MHz, CDCl3, ): 9.23(s, 2H), 8.96 (s, 2H), 8.75 (s, 2H), 8.44 (s, 2H), 7.57-7.56
(d, 2H), 7.17(m, 2H), 712 (m, 2H), 7.09 (m, 2H), 1.72 (s, 9H), 1,27 (s, 18H).FT-IR (KBr) (cm-1):
785.96, 915.56, 1033.08, 1242.02, 1354.83, 1400.58, 1541.55,1610.93, 2957.57, 3399.82.ESI-
+
MS:m/z [M+H] :869; Anal Calcd for C45H46N6O6Ru, C, 62.17; H, 5.34; N, 9.64; found C,
61.87; H, 5.39; N, 9.47.
2.10 Device fabrication
Fluorine doped Tin Oxide (FTO) coated glass (Nippon Sheet Glass,4 mm, 10 ꢁ/cm2), was
cleaned by sonication in an aqueous solution of Deaconex 21 (0.2 % wt) followed by 30 min at
o
500 C. To form a dense blocking layer, the substrates were treated in 40 mM aqueous TiCl4
solution (70 °C, 30 min) and rinsed with deionized water. A 12 ꢂm layer of transparent paste (18
NR-T, Dyesol) was spread on the FTO glass by screen printing. Next a 4ꢂm scattering layer was
deposited over the transparent layer by using the same screen printing process. Different film
thicknesses were obtained by repeating the screen printing step several times. In this case the

films were dried at 125 °C for 5 min between each print. The TiO2-coated electrodes (active area
0.28 cm2) were gradually heated under air flow at 325 °C for 5 min, at 375 °C for 5 min, at 450
°C for 15 min, and 500 °C for 15 min. A post treatment with TiCl4 was applied using 40 mM
aqueous TiCl4 solution at 70 °C for 30 min. The area of the deposited films was 0.28 cm2. After
reheating the photoanodes to 500°C for 30 min, they were cooled down to about 80 °C and then
immersed into the dye solutions (0.3 mM and when required 0.6 mM 3a, 7a-dihydroxy-5b-cholic
acid (CDCA) added) containing 10% DMF in acetonitrile and tert-butyl alcohol (volume ratio
1:1) for 20 h. Following the immersion procedure the sensitized electrodes were shortly rinsed
with acetonitrile and dried in air. Counter electrodes were fabricated by drop casting a H2PtCl6
solution (5 mM in isopropanol) onto TEC7 FTO-coated Glass (Solaronix 2.2mm, 7 ꢁ/cm2) and
fired in air at 400 °C for 5 min. These procedures were replicated two times. Cells were
assembled using a 25 ꢂm thick Surlyn thermoplastic resin (DuPont) and sealed by heating. The
Z960 liquid electrolyte (1.0 M 1,3-dimethylimidazolium iodide (DMII), 50 mM LiI, 30 mM I2,
0.5 M ter-butylpyridine, and 0.1 M guanidiniumthiocyanate (GNCS) in a mixture of
acetonitrile/valeronitrile (v/v, 85/15) was introduced via two holes made previously on the
counter electrode. The holes were sealed with Surlyn and a glass cover slip.
2.11 Photovoltaic characterization
The solar cells were measured using a 450 W xenon light source (Oriel) with an irradiance of
100 mW/cm2 at the surface of the devices. A Schott K113 Tempax filter (PräzisionsGlas & Optik
GmbH) was used to reduce the spectral mismatch between AM1.5G and the simulated
illumination to approximately 4% between 350-750 nm. Different light intensities were obtained
using wavelength neutral wire mesh attenuators (RHB) and calibrated with a Si-photodiode
equipped with an IR-cutoff filter (KG3, Schott). Current-Voltage characteristics of the cells were

obtained by applying an external voltage bias (from Voc to 0 V) to the cell while measuring the
current response with a digital source meter (Keithley 2400). The voltage step and equilibration
times were 10 mV and 80 ms, respectively. The cells were covered with a thin mask (0.16 mm2)
to reduce scattered light. Incident-photon-to-collected-electron efficiency (IPCE) spectra were
measured using a home built system composed of a 300 W xenon lamp (ILC Technology), a
Gemini-180 double monochromator with 1200 grooves/mm grating (JobinYvon Ltd) and a lock-
in amplifier (SR830 DSP, Stanford Research Systems). The incident monochromatic light was
chopped at a frequency of approximately 2 Hz and was superimposed onto a constant white light
bias of 10 mW/cm2 supplied by a LED array in order to provide a sufficient base conductivity of
the mesoporous TiO2 for charge collection.
2.12 Computational details
All the calculations have been performed by the Gaussian 09 program package.56 We optimized
the molecular structures using the B3LYP exchange–correlation functional57 and a 3-21G* basis
set58 in vacuo. TDDFT calculations of the lowest singlet–singlet excitations were performed in
DMF solution, on the optimized geometries using the B3LYP functional and a DGDZVP basis
set.59 The C-PCM60,61 was employed for including the solvent effects in TDDFT calculations, as
implemented in G09. To simulate the optical spectra, the 70 lowest spin-allowed singlet–singlet
transitions were computed on the ground state geometry. Excitation energies and oscillator
strengths were interpolated by a Gaussian convolution with a ꢃ value of 0.14 eV, corresponding
to an FWHM of ∼0.35 eV.
3. Results and discussion
3.1 Synthesis

The synthetic route for MC124, MC125 and MC127 were shown in the Scheme 1. The
commercially available starting materials 2-bromo-9H-fluorene and 3-bromo-9H-carbazole were
subjected to alkylation followed by their conversion from bromo to boronic ester 2-(9,9-didecyl-
9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2) and 9-hexyl-3-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9H-carbazole (4) respectively with good yields of 82% and 83%
respectively. Intermediates 2 and 4 further subjected to coupling with dibromobipyridine under
Suzuki conditions to afford 4,4'-bis(9,9-didecyl-9H-fluoren-2-yl)-2,2'-bipyridine (L1) and
9-hexyl-3-(2-(4-(9-hexyl-9H-carbazol-6-yl)pyridin-2-yl)pyridin-4-yl)-9H-carbazole
(L2)
respectively in yields of 76% and 73% respectively. The final step was performed by typical one
pot synthesis using [RuCl3(tetp)] (tercarbmethoxyterpyridine) with ancillary ligand
L1/L2/L3(commercially available) at 150°C for 4h using DMF as solvent, then chlorine ligand
was substituted by thiocyanate ligand, followed by ester hydrolysis using base catalyzed
conditions to afford the Ru-complexes in good yields. Details on the synthetic procedure and
complete characterization of the intermediates and the products were presented in the
experimental part.

Scheme1. Synthesis of sensitizers MC124, MC125 and MC127. Reagents and conditions: i)
0
KOH, DMSO, 1-bromodecane, RT, 18 h; ii) B2(pin)2, Pd(dppf)2Cl2, KOAc, DME, 90 C, 18 h;
0
iii) Pd(PPh3)4, 2M Na2CO3, DME, 90 C, 18 h; iv) MeOH, conc. H2SO4, reflux, 6 hours; v)
RuCl3.3H2O, EtOH/CHCl3, reflux under dark, 4 hours; vi) L1-L3, DMF reflux, 4 hours then
NH4NCS, reflux, 2 hours then TEA, water, reflux, 48 hours.
3.2 Optical and electronic properties
The UV-Vis absorption spectra of MC124, MC125 and MC127 in DMF solution are shown in
Figure 1 and related optical data are summarized in Table 1.

180000
160000
140000
120000
100000
80000
60000
40000
20000
0
MC124
MC125
MC127
300
400
500
600
700
800
Wave length (nm)
Figure.2: UV-vis absorption spectra of MC124, MC125 and MC127 in DMF solution.
Table 1. Optical and electrochemical data of sensitizers MC124, MC125 and MC127.
Dye
λmax Ԑ×10
(
4M-1 m- Eoxd[mV]b
E
0-0 [eV]c
E*oxd[V]d
1) [nm]a
MC124
MC125
MC127
511(1.137)
505(1.289)
483(3.249),
573(1.848)
1.0919
1.1268
1.1094
2.218
2.167
2.003
-1.1261
-1.0402
-0.8936
[a] absorption spectra were recorded in DMF, [b] oxidation potentials were measured by
oxidative differential pulse voltammetry, [c] the bandgap E0-0, was derived from the intersection
b
c
.
of absorption and emission spectra, [d] E*oxd were calculated by Eox - Eo-o
The MC dyes exhibit good optical properties with an intense absorption band in the range 250-
400 nm and another band at longer wavelenghts in the range 430-600 nm. The absorption
maxima follows the order MC127(483 nm) < MC125(505 nm) < MC124(511 nm) while the

molar extinction coefficient (Ԑ) has an opposite trend (MC124 < MC125 < MC127). Overall,
the thiocyanate free complex (MC127) shows the most promising optical properties, both in
terms of wavelength and extinction coefficient while the fluorene and carbazole functionalized
bipyridine dyes (MC124 and MC125) show only slight differences from one another. As a
reference, we compared the optical data of MC dyes with the optical properties of the black dye
(N749). Concerning the absorption band in the visible range, a blue shift in the ꢄmax for all the
three sensitizers compared to N749 (620 nm) was observed, whereas the molar extinction
coefficients are higher than N749 (0.8930*104M-1cm-1).30
Theoretical calculation has been performed in order to reproduce and rationalize the optical and
electronic properties of MC sensitizers. For MC124 and MC125 complexes a good agreement
between experimental and computed spectra has been observed as can be noted in Table 2, apart
from a slight shift toward higher wavelengths (0.09-0.13 ev respectively). For the MC127 bis-
terpyridine complex we computed higher discrepancies between the experimental and theoretical
spectra. It has to be noticed that we employed fully-deprotonated complex geometries in the
simulation that is expected to provide blue shift absorption maxima.62 In any case we were able
to well reproduce the experimental trend in the wavelength of the absorption maxima.
Table 2. Comparison between experimental and computed ꢄmax
.
Dye
λ
max (Exp.)
eV
λmax (Theo.)
nm
512
506
483
nm
494
480
433
eV
MC124
MC125
MC127
2.42
2.51
2.58
2.86
2.45
2.57

In all the cases, the transitions in the visible region are assigned as MLCT while the band at
lower wavelengths, in the range 250-400 nm, is due to the ligand centred ꢅ-ꢅ* electronic
transition. The set of quasi degenerate HOMO-2/HOMO-1 and HOMO have essentially Ru t2g
character. For MC124 and MC125 the HOMOs are localized both on the Ru center and on the
NCS ligand, Figure 3. The MC127 present a higher band gap, due the significantly stabilized
(0.5 eV) HOMO level, localized on the metal center, compared to MC124 and MC125. Moving
to the analysis of LUMOs, for MC124 and MC125 LUMO is localized on the bipyridine ligand
while for the MC127 it is localized on the terpyridine ligand, Figure 3. This result is in
agreement with a previous publication by van Koten et al.63 on similar terpyridine based Ru(II)
dyes, and suggest an unfavorable localization of the electron density in the excited state. The
calculated energy levels are in good agreement with the experimental data reported in Table 1.

Figure 3. Schematic representation of the energy levels of the MC124, MC125 and MC127
complexes. Isodensity surface plots (isodensity contour: 0.035) of selected molecular orbitals are
also shown.
The HOMO-LUMO levels are essential parameters to assess the ease of electron injection from
the excited sensitizer to the conduction band (Ecb) of TiO2 and the subsequent dye regeneration.
Oxidative differential pulse voltammetry were performed with 0.1 tetrabutylammonium
hexaflurophosphate as supporting electrolyte in DMF solution (Figure 4). All potentials were
internally referenced to the ferrocene/ferrocenium (Fe/Fe+) couple which has the redox potentials
of (EFe/Fe+) 0.510 V vs Ag/AgCl. The first oxidation potentials of the dyes have been determined
from their corresponding peak potentials and incorporated in the Table 1. The oxidation
potentials of the RuIII/II are 1.0919, 1.1268 and 1.1094 V for MC124, MC125 and MC127,
-
respectively, Figure 4 and Table 1. This potential is sufficiently more positive than I-/I3 and
provides enough driving force for effective dye regeneration. The excited-state oxidation
*
potentials (Eox ) of MC124, MC125 and MC127 have been calculated from Eoxd-E0-0
formula,64,65 are -1.1261, -1.0402 and -0.8936 V respectively, and are more negative than the
conduction band edge of TiO2 (-0.5 V ) ensuring an efficient electron injection process from the
excited state of the dyes into the TiO2 electrode. The schematic energy level diagram for MC124,
MC125 and MC127 has been shown in Figure 4b according to their electrochemical data.

a)
MC124
0.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Potential (V)
MC125
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Potential (V)
MC127
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Potential (V)
-1.5
-1.0
-0.5
0.0
b)
1.1261
LUMO
1.0402
0.8936
2.003
2.167
2.218
e-
0.5
1.0
1.0919
1.1094
HOMO
1.1268
1.5

Figure 4. a) Oxidative differential pulse voltammetry of MC124, MC125 and MC127. b) The
schematic energy levels of MC124, MC125 and MC127 based on absorption and
electrochemical data.
3. 3 Photovoltaic performances
After the optoelectronic characterization, several working devices have been realized with the
aim to test the photovoltaic properties of MC ruthenium complexes. Details of devices
fabrication are summarized in the experimental section. The J-V curves and the IPCE spectra are
reported in Figure 5 and the related photovoltaic data are summarized in Table 3.
6
MC124+CDCA
MC124
a)
MC125+CDCA
MC125
MC127+CDCA
4
MC127
2
0
-2
0.1
0.2
0.3
0.4
0.5
0.6
0.7
V(v)

60
50
40
30
20
10
0
b)
MC124+CDCA
MC124
MC125+CDCA
MC125
MC127+CDCA
MC127
400
500
600
700
800
Wavelength(nm)
Figure 5. a) J-V curves and dark current b) IPCE spectra of the devices realized with MC dyes
with and without CDCA added in the sensitization solutions.
Table 3. Photovoltaic data of working devices employing MC dyes and iodine based electrolyte.
The average values of three devices are reported in parenthesis.
a
b
Dye
CDCA
No
Jsc
(mA/cm2)
4.3(4.1)
Voc
(mV)
641(647)
FFc
ηd
(%)
MC124
MC125
MC127
MC124
MC125
MC127
0.78(0.78) 2.1(2.1)
0.75(0.75) 0.7(0.7)
0.80(0.79) 1.3(1.2)
0.78(0.78) 2.3(2.2)
0.76(0.75) 0.8(0.7)
0.79(0.78) 1.3(1.2)
No
1.5(1.4)
2.7(2.6)
4.6(4.4)
1.8(1.6
2.7(2.6)
590(592)
594(596)
641(640)
586(585)
609(605)
No
0.6 mM
0.6 mM
0.6 mM

[a] Current density, [b] open-circuit voltage, [c] fill factor, [d] photoelectric conversion
efficiency.
We started the test of all the MC dyes in standard conditions and, then we tried to improve the
photovoltaic performances of the devices with the addition of the chenodeoxycholic acid co-
adsorbent (CDCA) in the sensitizing solutions (Table 3). The addition of CDCA as co-adsorbent
led only to a slight increase of Jsc and hence of the final efficiency, suggesting that the
aggregation of dyes on the TiO2 surface is not the major issue.
The best performances were obtained with MC124 with a maximum PCE of 2.3%, due to the
highest photocurrent density and photo voltage. The presence of long alkyl chains on the
bipyridine ligand ensure a better packing onto TiO2 surface probably leading to the reduction of
parasitic recombination reactions.
Overall the photocurrent density has strongly limited the performances of devices employing
MC dyes, Table 3. The low photocurrent densities are probably due to electron injection
problems caused by an unfavorable localization of the electron density in the excited state, that is
far from the carboxylic anchoring group (Figure 3). A similar behavior has been observed for
other terpyridine-based ruthenium dyes which have shown low IPCE values.63 Another possible
explanation for the low performances of MC dyes, could be the parasitic recombination reactions
competing with the regeneration of dye molecules. Clifford et al.66 in 2007 have shown that the
regeneration process of ruthenium dyes proceed via the formation of an intermediate species
between Ru(II) oxidized dye and iodide ions in the electrolyte. The most likely mechanism for
the interaction between oxidized dyes and iodide is the binding of iodide to the S atom of the
thiocyanate groups67 as confirmed by theoretical calculations.68 Thus the partial (MC124 and
MC125) or total (MC127) replacement of thiocyanate could make the regeneration of oxidized
dye molecules difficoult. It has to be noted, furthermore, that for the oxidized MC127 dye, the

hole remains confined on the ruthenium metal, where the HOMO orbital is localized. The
terpyridine ligand bearing the bulky tert-butyl substituents thus could inhibit the regeneration
process keeping the redox mediator far from the dye hole.
Concerning the stability, direct long-term stability test have not been performed. However
similar terpyridine-based dyes have shown very good photostability upon prolonged irradiation
in both Ru(II) and oxidized Ru(III) state63 Moreover the presence of three carboxylic anchoring
groups suggests a strong adsorption onto the titania surface, as already reported in our previous
papers.48
4. Conclusions
Summarizing we have reported the design and synthesis of three novel Ru(II) complexes, MC
dyes. The MC dyes employ 2,2':6',2''-terpyridine-4,4',4''-tricarboxylic acid as anchor group, and
alternatively a bipyridine (MC124 and MC125) and terpyidine (MC127) auxiliary ligands,
partially or totally replacing the thiocyanate groups of the reference N749 dye. The dyes were
initially characterized from the optoelectronic side, employing cyclic voltammetry DPV and
recording the UV-vis absorption spectra in solution. All the MC dyes exhibit good optical
properties, with an absorption band in the visible range (430-600 nm) characterized by molar
extinction coefficients higher than the reference N749 dye. The MC dyes also shown good
electronic properties with an adequate positioning of energy levels. The ground state oxidation
potentials are more positive than the I-/I-3 redox couple which should provide enough driving
force for the effective dye regeneration, and the excited state oxidation potentials values are
more negative than the conduction band edge of TiO2 ensuring an efficient electron injection
process. After the optoelectronic characterization, the MC dyes were tested in working devices
with iodine based electrolyte. The overall performances were quite low, mainly due to the poor
generated photocurrent that has been possibly related to an unfavorable LUMO localization far
from the anchoring sites. Further improvements could be achieved by modification of molecular
structure.
Acknowledgements:

GK thanks CSIR, New Delhi and P.CH thanks UGC, for senior research fellowships. MC thanks
DST, New Delhi for funding the project
No. DST/TMC/SERI/FR/92
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New Terpyridine-based Ruthenium Complexes for Dye Sensitized Solar Cells
Applications
Ganesh Koyyada,a,b Pavan Kumar CH, a,b Paolo Salvatori,c,d Gabriele Marotta,c,e Maria Grazia Lobello,c Olivia Bizzarri,c
Filippo De Angelis,*c Chandrasekharam Malapaka*a,b
Three new Terpyridine based Ruthenium sensitizers coded as MC124, MC125 and MC127 have been synthesised and
evaluated for DSSC. All the dyes showed high molar extinction coefficients than that of standard N749.
OH
O
R
180000
160000
140000
120000
100000
80000
60000
40000
20000
0
N
N
O
N
R
Ru
N
MC124
MC125
MC127
HO
NCS
O
OH
OH
O
N
N
N
N
O
N
Ru
N
HO
300
400
500
600
700
800
O
OH
Wave length (nm)
TiO2

Synopsis
Here we report three new ruthenium terpyridine complexes with and without thiocyanate for
the application in dye sensitized solar cells. Here the terpyridinic acid ligand acts as an
anchoring group. Sensitizer MC124 found to be more efficient compared to MC125 and
MC127.