New ruthenium complexes (Ru[3+2+1]) bearing π-extended 4-methylstyryl terpyridine and unsymmetrical bipyridine ligands for DSSC applications

Inorganica Chimica Acta 435 (2015) 46–52

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New ruthenium complexes (Ru[3+2+1]) bearing p-extended 4-methylstyryl terpyridine and unsymmetrical bipyridine ligands for DSSC applications CH. Pavan Kumar a,b, V. Anusha a, K. Narayanaswamy a,b, K. Bhanuprakash a, A. Islam c, L. Han c, Surya Prakash Singh a,b,⇑, M. Chandrasekharam a,b,⇑ a

CSIR – Indian Institute of Chemical Technology, I&PC Division, Uppal Road, Tarnaka, Hyderabad 500 607, India Academy of Scientific and Innovative Research, CSIR-IICT, India c Photovoltaic Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b

a r t i c l e

i n f o

Article history: Received 23 September 2014 Received in revised form 27 March 2015 Accepted 10 April 2015 Available online 19 June 2015 Keywords: Dye sensitized solar cell (DSSC) Ru[3+2+1] complex Triisopropyl phenyl Ancillary bipyridine Efficiency DFT/TDDFT

a b s t r a c t Two novel heteroleptic Ru[3+2+1] sensitizers, 1 and 2, with unsymmetrical bipyridine as ancillary ligand and electron donating 4-methylstyryl group in the anchoring p-extended terpyridyl ligand were synthesized and characterized. DFT studies reveal that the lowest unoccupied molecular orbital (LUMO) is distributed over the terpyridine. The two new sensitizers showed an improvement in the molar extinction coefficient compared to reference standard N749 dye. Among the two new sensitizers, 1 exhibited maximum solar to electric conversion efficiency (g) of 5.19% with short circuit current density of 14.032 mA cm2, open circuit voltage of 0.520 V and fill factor of 0.712, under Air Mass (AM) 1.5 sunlight, while the reference N749 sensitized solar cell exhibited g-value of 6.44%. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction In recent times research on dye sensitized solar cells (DSSCs) attracted attention as an inexpensive alternative to conventional solar cells, owing to their low material cost, easy manufacturing processes, flexibility and efficient solar energy conversion [1–6]. The electrochemical, photophysical, ground-and excited-state properties of sensitizers play major roles in achieving high power conversion efficiency. Despite high efficiencies achieved by ruthenium sensitizers [7–10], the DSSCs warrant further improvements in order to make these devices competitive with conventional silicon solar cells. Therefore, efforts are in progress to develop an optimal photosensitizer with an absorption band extending up to red region of the visible spectrum and high molar extinction coefficient. An ideal sensitizer should harvest all photons to electrical current below a threshold wavelength of about 920 nm [5]. Therefore, various sensitizers have been designed and employed in the DSSC devices to harvest a broader spectrum in sunlight. ⇑ Corresponding authors at: CSIR – Indian Institute of Chemical Technology, I&PC Division, Uppal Road, Tarnaka, Hyderabad 500 607, India. E-mail address: [email protected] (S.P. Singh). http://dx.doi.org/10.1016/j.ica.2015.04.038 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

The spectral properties, HOMO and LUMO energy levels of Ru complexes can be tuned by substitution at different positions of the ligands. However, to the best of our knowledge, only a few examples of extended p-conjugation on the acceptor terpyridine (tpy) ligand of the Ru–tpy complex with high conversion efficiencies have been reported [11–14]. Recently, p-extended black dye derivatives as well as Ru-bipyridine (bpy) sensitizers have been reported with conversion efficiencies up to 10% [15–18]. Substitution on tpy ligand with electron donating groups is expected to impart increased electron density on the pyridyl ring, which may decrease the electron accepting character. Theoretical calculations also indicate that the LUMO levels of the Ru–tpy complexes with highly electron rich p-spacer are mainly localized on the carboxyl bipyridine part and not on the pyridyl ring substituted with the electron donating groups [14,15]. On the other hand the electron withdrawing groups on Ru–tpy sensitizers decrease the LUMO level, resulting in more difficult electron injection [15,16]. Recently, we have achieved a reasonable conversion efficiency (11.1%) using a new p-extended dicarboxylterpyridyl ligand with an electron donating 4-methyl styryl group (HIS-2) [19]. As a part of our continued program we have been engaged in the design and synthesis of new and efficient ruthenium

CH. Pavan Kumar et al. / Inorganica Chimica Acta 435 (2015) 46–52

complexes [20,21] and metal free organic molecules [22] for DSSC applications. A simple coadsorbent also improved the efficiency of the black dye based DSSC to 11.4% [23]. We also improved the efficiencies of DSSCs by employing novel unsymmetrical bipyridines as ancillary ligands in the Ru-complex [24,25]. In this paper we present the design and synthesis of ruthenium(II) [3+2+1] mixed ligand complexes 1 and 2, where a new p-extended dicarboxyterpyridyl ligand is tethered with a electron donating 4-methylstyryl moiety. In the complex 1, unsymmetrically substituted ancillary bipyridine (L1) is ligated as shown in the Scheme 1 [24,25], Alkyl (hexyl) thiophene and mesityl are tethered on each pyridine ring of the ligand L1. The advantages of this ligand are (i) the thiophene moiety serves as the source of two double bonds in conjugation for increasing the light harvesting property and shifts the low energy metal to ligand charge transfer (MLCT) band towards red region; (ii) hydrophobic hexyl chain increases the solubility, and reduces the recombination of electrons in the vicinity of TiO2 with redox electrolyte; (iii) the mesityl moiety facilitates electron pumping to oxidized dye and red-shifts the absorption. In the complex 2, Ru is ligated wih diethyl 2,20 -bipyridine-4,40 -dicarboxylate (L2) while the anchoring terpyridyl ligand is same as in 1. The new sensitizers showed improvement in the molar extinction coefficient compared to the reference dye (N749). The chemical structures of new Ru-sensitizers 1 and 2 are shown in Scheme. 1. 2. Experimental 2.1. Materials and instrumentation The starting materials dibromobipyridine, diethyl 2,20 -bipyridine-4,40 -dicarboxylate, trimethyl terpyridine, RuCl33H2O, tetrabutylammoniumhydroxide, ammonium thiocyanate, tetrabutyl ammoniumhydoxide were purchased from Sigma–Aldrich. Silica gel was purchased from Fluka. The solvents were purified by standard procedures and purged with nitrogen before use. All other chemicals used were analytical grade and were used without further purification. All reactions were performed under argon atmosphere unless otherwise mentioned. UV–Vis and fluorescence spectra were recorded in a 1 cm path length quartz cell on a Shimadzu UV–Vis to near IR 3600 spectrometer and a Fluorolog 3, J.Y. Horiba fluorescence spectrometer, respectively. Electrochemical data were obtained by Cyclic Voltammetry using a conventional three-electrode cell and a BAS100 electrochemical analyzer. The working electrode was a Pt rod, the auxiliary (counter) electrode was a Pt wire, the reference electrode was SCE, saturated KCl and the supporting electrolyte was 0.1 M tetrabutylammoniumhexafluorophosphate in DMF. 1H NMR, 13C NMR spectra were measured with Avance ACP-300 or AMX2-500 spectrometers at 300 and 500 MHZ respectively, using TMS as internal standard. Mass spectra were recorded on Shimadzu LCMS-2010EV model with ESI probe. C, H, N, S data was recorded on Elementar (variomicrotube) instrument. 2.2. Device fabrication and photo-electrochemical measurements Fluorine-doped SnO2 (FTO) coated glasses were sequentially washed with glass-cleaning detergent, water, acetone and ethanol. The FTO glass plates were dipped into a 40 mM aq.TiCl4 solution at 70 °C for 30 min and washed with ethanol and water. The photoanodes composed of TiO2 were prepared using literature procedures [26]. The films were further treated with 0.1 M HCl solution before use [27]. Photovoltaic measurements were performed in a twoelectrode sandwich cell configuration. A colloidal TiO2 electrode (0.25 cm2) was stained by immersing into a 0.3 mM dye solution

47

containing 1 or 2 sensitizers in a mixture of acetonitrile and tertbutanol (volume ratio 1/1) overnight. After drying (air flow), the sensitized titania electrode was assembled with a thermally platinized counter electrode. The two electrodes were separated by a surlyn spacer (40 lm thick) and sealed up by heating the polymer frame. The electrolyte was composed of 0.5 M dimethylpropyl-imidazolium iodide (DMPII), 0.05 M I2, and 0.1 M LiI in acetonitrile and is introduced into the device by the vacuum filling method through the predrilled hole on the counter electrode. Incident photon-to-current conversion efficiency (IPCE) spectra were measured by a CEP-2000 spectrometer (Bunkoh-Keiki Co. Ltd). The current–voltage characteristics of sensitizers 1 and 2 were obtained from a WXS-155S-10 solar simulator (Wacom Denso Co., Japan). Measurements were performed with 0.25 cm2 active surface area devices defined by a metal mask under AM 1.5 G irradiation (100 mW cm2) [28]. 2.3. Synthesis and characterization 2.3.1. Synthesis of ruthenium complex (1) A solution of ligand L1 (78 mg, 0.147 mmol) and ruthenium trichloro complex (a) (80 mg, 0.122 mmol) [25] dissolved in dry DMF (60 mL) was refluxed for 4 h under nitrogen atmosphere in dark. The mixture was cooled to 80 °C and to the resulting dark green solution, aqueous solution of NH4NCS (320 mg in 1 mL of water) was added and the reaction mixture was further refluxed for 2 h. The progress of the reaction was monitored by UV–Vis spectroscopy. The reaction mixture was hydrolyzed by the addition of triethylamine (4 mL) and water (4 mL) at RT followed by reflux for 48 h. After cooling to room temperature, the solvent was removed under reduced pressure. The purple solid obtained on the addition of water (200 mL) was filtered off, washed with distilled water, ether and dried under vacuum. The crude compound was dissolved in methanol and purified on Sephadex LH-20, using methanol: chloroform (2:3) as eluent. The main fraction was collected and concentrated to give 1 (60%, 79.8 mg, 0.071 mmol). 1 H NMR (500 MHz, CDCl3+CD3OD, d): 9.87–9.71 (m, 1H), 9.29–9.08 (m, 3H), 8.56 (s, 1H),8.40 (m, 1H), 8.12–8.00 (m, 2H), 7.80 (t, 3H), 7.60–7.44 (br s, 3H), 7.32–7.20 (m, 4H), 7.14–6.96 (m, 5H), 6.80 (d, 1H), 3.04–2.84 (m, 3H), 2.80 (br m, 2H), 2.40 (s, 3H), 2.16–2.00 (m, 2H),1.80–1.60 (m, 2H), 1.48–1.21 (br m, 22H), 1.00 (t, 3H). FT-IR (KBr) (cm1): 3404, 3063, 2958, 2925, 2861, 2095, 1607, 1539, 1464, 1409, 1356, 1237, 1021. ESI-HRMS calcd for C62H63N6O4RuS2 1121.34, found 1121.34161. Anal. Calc. C62H63N6O4RuS2: C, 66.40; H, 5.66; N, 7.49; S, 5.72. Found: C, 66.78; H, 5.68; N, 7.37; S, 4.98%. 2.3.2. Synthesis of ruthenium complex (2) The complex 2 was prepared following the procedure employed for the synthesis of 1 using L2 (50 mg, 0.166 mmol) in place of L1. The crude compound was dissolved in MeOH and methanolic TBA (0.1 mL) and purified on Sephadex LH-20, using methanol as eluent. The main fraction was collected and concentrated to give 2 in 50% yield (107 mg, 0.059 mmol). 1H NMR (500 MHz, CDCl3+CD3OD, d): 9.72 (b, 1H), 9.12 (m, 2H), 8.88 (m, 2H), 8.48–8.40 (m, 2H), 8.24–8.20 (m, 1H), 7.80 (t, 1H), 7.60 (d, 1H), 7.52–7.44 (m, 5H), 7.14 (d, 1H), 7.20 (d, 2H), 7.14 (m, 2H), 3.20 (t, 32H), 2.40 (s, 3H), 1.60 (q, 32H), 1.40 (q, 32H), 1.00 (t, 48H). FT-IR (KBr) (cm1): 3413, 2960, 2872, 2100, 1972, 1609, 1539, 1467, 1359, 1235, 1022. Anal. Calc. C103H167N10O8RuS: C, 68.48; H, 9.32; N, 7.75; S, 1.77. Found: C, 68.90; H, 9.38; N, 7.59; S, 1.16%. 2.4. Computational details Density Functional Theory (DFT) and Time Dependent DFT calculations were performed using Gaussian09 software [29].

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C6H13 S

HOOC

HOOC

N

N

N

Ru

N

NCS

HOOC

N

N

N

N

Ru

N

NCS

HOOC

N S

C6H13

1B

1A

COOHOOC

COO N(C4H9)4 N N COO N(C4H9) 4

N Ru NCS

N

COO N(C4H9)4

N

N

NCS Ru

N COO N(C4H9) 4

(n-Bu4N+)3

NCS

N NCS

-

OOC

N749

2

C6H13 S COOMe

MeOOC N

MeOOC

N N

N

N

N

Cl

L1 N

Ru

(i)

N MeOOC

Cl

(ii)

Cl

1

a O

O O

O

(ii) N

L2

N

2 Scheme 1. Top: Structure of the 1, 2 and N749 Sensitizers. Bottom: synthesis of sensitizers 1 and 2. Reagents and conditions: (i) RuCl33H2O, EtOH/CHCl3, reflux under dark, 4 h; (ii) DMF reflux, 4 h then NH4NCS, reflux, 2 h then TEA, water, reflux, 48 h.

The atomic positions in closed shell configurations of the ruthenium(II) [3+2+1] mixed ligand sensitizers with a charge of +1 were fully optimized in gas phase. The bulky –CO2[N(C4H9)4]

groups on anchoring ligands and ancillary ligands, are replaced with –COOH, to reduce the computational complexity. Becke’s LYP (B3LYP) exchange correlation functional [30–32] with Hay

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and Wadt’s double zeta quality Los Alamos electron effective core potential basis set (LANL2DZ ECP) [33,34] was adopted on all atoms. The minimized geometries were analyzed by vibrational frequencies and the calculations resulted in all real frequencies. Thus, the optimized structures correspond to real minima on the potential energy surface. At the optimized ground state geometry, TDDFT calculations were performed to estimate the singlet excitation energies at M06 [35]/LANL2DZ level of theory in DMF solvent by means of the Polarizable Continuum Model [36,37] (PCM), based on the default linear response theory, as implemented in Gaussian09. The software GaussSum 2.2.5 [38] was used to interpret the nature of transitions. The molecular orbital surfaces were visualized with Gaussview [39] and the percentage contributions of the Ru(II) and the ligands to the respective molecular orbitals were calculated using GaussSum. 3. Results and discussion 3.1. Synthesis of Ru complexes 1 and 2 Fig. 1a. Absorption Spectra of 1 and 2.

Scheme 1 shows the synthetic approach to obtain Ru-complexes 1 and 2. An ethanolic solution of RuCl3.3H2O and 4,40 -bis(methoxycarbonyl)-400 -(4-methylstyryl)-2,20 :60 ,200 -terpyridine (LMe) was refluxed to produce compound (a). The next step was performed by heating [RuCl3(LMe)] with ancillary ligands (L1) [24,25] or diethyl 2,20 -bipyridine-4,40 -dicarboxylate (L2) at 150 °C for 4 h in DMF solvent. Finally, the chloro complex was subjected to substitution by thiocyanate using ammonium thiocyanate, followed by hydrolysis to afford the desired Ru-complexes. Due to the asymmetry of the L1 ligand, the Ru(II)-complex (1) is obtained as a mixture of two isomers, 1A and 1B (Scheme 1), where the hexyl thiophene moiety lying either trans or cis to NCS ligand, respectively (ESI.). This can be inferred from the 1H NMR in the aliphatic region showing two distinct signals (ESI.) and DFT calculation shows that the isomer B is more stable by 0.58 kcal mol1 than A. 3.2. Geometries and orbitals of complexes 1 and 2 The optimized geometries of 1 (1A and 1B) computed at B3LYP/LANL2DZ level of theory are given in Table S1 ESI. The carboxylic acid groups and styryl substituents on anchoring ligands of both the complexes are in plane with that of pyridines. The calculated HOMO, LUMO energies (eV), the isodensity surface plots of the molecular orbitals of 1A, 1B and 2, along with percentage contribution of groups to each molecular orbital at B3LYP/LANL2DZ level in DMF solvent are given in Table S2 ESI. The plots depict that the HOMO, HOMO-1 of 1A, 1B is distributed on Ru atom, NCS ligand and the LUMO, LUMO+1 are centered on terpy ligand. The HOMO, HOMO-1 of 2 is present on Ru atom, NCS ligand. Whereas LUMO and LUMO+2 of 2 is on terpy ligand and LUMO+1 on bipy ligand (the values are given in Table S2 ESI.). The HOMO–LUMO gaps calculated at B3LYP/LANL2DZ level of theory, in DMF solvent are 2.32 eV, 2.40 eV, 2.41 eV for 1A, 1B and 2 respectively. 3.3. Photophysical characterization The UV–Vis absorption spectra measured in DMF are shown in Fig 1(a), and selected parameters are summarized in Table 1. The two new sensitizers showed a broad absorption spectrum covering a wide spectral range in UV–Vis region from 400 to 700 nm with higher molar absorption coefficients. The higher molar extinction coefficient (kmax) corresponding to the ancillary ligand in the

340 nm region for sensitizer 1 is the result of extended conjugation arising from thienyl moiety. These Ru-complexes showed intense absorption band at around 300 nm, referred to the ligand centered p–p⁄ transition of the 4, 40 -bis(methoxycarboxyl)-400 -(4-methylstyryl)-2,20 :60 ,20 -terpyridine ligand. The low energy MLCT absorption band of 1 at 506 nm with molar extinction coefficient 1.8  104 M1 Cm1, which is higher than that of 2 (473 nm, 1.5  104 M1 Cm1) is resulting from the presence of thienyl group in 1. The absorption spectra of two sensitizers adsorbed onto a transparent thin TiO2 film were recorded and depicted in Fig. 1(b). The broad absorption spectra obtained are observed to be similar to that in solutions, with the absorption peaks at 516, 504 nm respectively red shifted by 10–30 nm. The luminescence spectra of 1 and 2 in DMF at excitation of the low energy MLCT transition produces a broad emission band with a maximum at 722 and 709 nm, as shown in Fig. 1(c). The excitation spectrum of each complex overlays well with the corresponding absorption spectrum as shown in Table. S3 (ESI.). The excited state energies (E0-0) were estimated from the intersection point of absorption and emission spectra. TDDFT calculations are carried out at M06/LANL2DZ level of theory, in the framework of PCM in DMF solvent to understand the photophysical behavior of the complexes 1A, 1B and 2. The simulated UV–Vis spectra with the data of important allowed transitions within the range of 450–650 nm in comparison with experimental kmax of 1A, 1B, and 2 are given in Table S3 ESI. The most intense peak in 1A is at 558 nm with oscillator strength f = 0.1179 and in 1B it is at 516 nm with oscillator strength f = 0.1785 showing charge transfer from HOMO1 to LUMO+1. In complex 2, it is mainly from HOMO to LUMO+2 at 525 nm with f = 0.1284. The main allowed charge transfer transitions in the molecules are of Metal–Ligand (MLCT) and Ligand–Ligand (LLCT) type. The differential pulse voltammetry (DPV) of the sensitizers 1 and 2 were examined in the potential range +1.4–0 V using a SCE electrode with 0.1 M tetrabutylammonium hexaflurophosphate as supporting electrolyte in DMF solvent, in order to assess the possibility of electron transfer at the interface of TiO2/dye/redox electrolyte. As shown in the Fig. 2(a), the oxidation potential of the sensitizers 1 and 2 was determined from the peak potentials of the DPV. The ground state oxidation potentials (Eox) corresponds to the HOMO level of the sensitizers while the LUMO could be calculated from EoxE0-0. E0-0 [39] is the zero–zero transition value obtained from the intersection of the corresponding normalized

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Table 1 Optical, redox, and DSSC performance parameters of sensitizers 1 and 2.

a b

c d e f

Dye

kmax (e  104 M1 cm1) (nm)a

kem (nm)a

Eoxd/(HOMO) (V)b

E0-0 (eV)c

LUMO (V)b

Jscd (mA/cm2)

Voce (V)

f.ff

Eff. (%)

1 2 N749

286(5.1), 344(5.6), 506(1.8) 302(4.8), 327(4.1), 473(1.5)

722 709

0.486 0.593

2.006 2.091

1.520 1.498

14.032 13.326 14.150

0.52 0.49 0.67

0.712 0.698 0.680

5.19 4.63 6.44

Absorption and emission spectra were recorded in DMF solutions at 298 K. Redox potentials were measured by DPV in DMF vs ferrocene/ferrocenium (Fc/Fc+) with 0.1 M n-Bu4NPF6 as an supporting electrolyte (scanning rate: 100 mV s1, working electrode and counter electrode: Pt rod and Pt wire, and reference electrode: SCE). The HOMO and LUMO were calculated with the following formulas: HOMO = Eox  EFc/Fc+ V; LUMO = HOMO  E0-0. The band gap, E0-0, was derived from the intersection of the absorption and emission spectra. Current density. Open-circuit voltage. Fill factor.

Fig. 2a. DPV of 1 and 2 sensitizers in DMF.

Fig. 1b. Absorption Spectra of 1 and 2 on TiO2.

Fig. 1c. Emission Spectra of 1 and 2. Fig. 2b. The schematic energy levels of 1 and 2 based on absorption and electrochemical data.

absorption and emission spectra. [40] The values are shown in the Fig. 2(b), and tabulated in Table 1. The redox potential of Fc+/Fc was found at (EFc/Fc+) 0.478 V which is relatively similar to reported potential (0.47 V versus SCE). [41] The HOMO level of the sensitizers 1 and 2 are 0.486 and 0.593 V (versus ferrocene/ferrocenium

(Fc+/Fc)) and are positive enough compared to iodine/iodide redox potential (0.42 V versus SCE) [42], indicating that the oxidized sensitizers formed after electron injection into the conduction band of TiO2 could be regenerated. To effectively inject the electron into

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the conduction band (CB) of TiO2, the LUMO levels of the sensitizers 1 and 2 should be sufficiently more negative than the CB of TiO2 (0.5 V versus SCE) [43]. The LUMO of the new sensitizers are 1.520 and 1.498 V (versus Fc+/Fc) respectively for 1 and 2, which are satisfying the above condition.

3.4. Photovoltaic characterization The photovoltaic performance of DSSC with an active area of 0.25 cm2 constructed from sensitizers 1 and 2 were studied employing an electrolyte composed with 0.6 M dimethylpropylimidazolium iodide (DMPII), 0.05 M I2, and 0.1 M LiI in acetonitrile. Fig. 3(a) shows the incident photon-to-current conversion efficiency (IPCE) spectra as a function of excitation wavelength for DSSCs based on 1 and 2 and compared with the device constructed from N749. It is notable that the DSSCs based on new sensitizers 1 and 2 exhibit efficient panchromatic sensitization of nanocrystalline TiO2 over the entire visible wavelength range, extending into the near IR region; the steep rise in the IPCE of the new sensitizers started at 860 nm, matching well with their TiO2 thin films absorption spectra (Fig. 1(b)). The IPCE value of sensitizer 1 is above 65% over the visible range of 460–560 nm, reaching up to a maximum of 75% at 515 nm. The sensitizer 2 achieved a maximum of 55% over the visible range 440–560 nm, reaching up to a maximum of 64% at 510 nm. The dip in the IPCE spectra at 350– 400 nm region of the black dye was due to competitive light absorption between the iodide present in the electrolyte and Ru sensitizer as seen in Fig. 3(a). More intense absorptions of 1 and 2 observed in the spectral range 350–400 nm compared with N749 is attributed to the 4-methylstyrylgroup. As a result, the devices of new sensitizers show higher IPCE values than those of N749 at 300–650 nm (for 1) and 300–600 nm (for 2). The sensitizer 1 shows a larger photocurrent than 2. Inspection of Fig. 3(a) shows that the IPCE of 2 is broader (it reaches to 850 nm) than that of 1 (it reaches to 800 nm). Fig. 3(b) shows the characteristic photocurrent density–voltage (J–V) curves of the DSSC devices recorded under an illumination of AM 1.5 G full sunlight (100 mW cm2). The short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (FF), and overall cell efficiencies (g) of the devices constructed from 1, 2 and N749 are summarized in Table 1. The highest efficiency value of 5.19% is obtained for the sensitizer 1 with corresponding short circuit current density (Jsc, mA cm2) of 14.032, open circuit

Fig. 3b. J–V Curves of 1 and 2 sensitizers.

voltage (Voc, V) of 0.52 and fill factor (FF) of 0.712, whereas the sensitizer 2 shows slightly lower efficiency 4.63% with the corresponding Jsc (13.326, mA cm2), Voc (0.49, V) and FF (0.698) parameters. The increase in the Jsc of 1 is consistent with increased value of IPCE. These results indicate that the introduction of hydrophobic long alkyl chain and triisopropyl groups on ancillary bipyridine ligand lowers the unwanted back electron transfer from the TiO2 conduction band to oxidized charge carriers in the electrolyte solution and thus 1 represents better sensitizer compared to 2 for DSSC application. Despite of having alkyl chains, these sensitizers showed lower Voc values compared to N749, this might be attributed to the more charge recombination. Under similar fabrication conditions, the standard N749 exhibited Jsc, Voc and ff of 14.150 mA cm2, 0.67 V and 0.680 respectively corresponding to overall light to electricity conversion efficiency 6.44%.

4. Conclusion In summary, the two new Ru[3+2+1] sensitizers 1 and 2 were designed, synthesized and characterized using spectroscopic and electrochemical methods. These sensitizers showed intense absorption over the whole spectral region. The presence of p-conjugated 4-methylstyryl group is presumed to be responsible to overcome the competitive absorption from redox couple. Unsymmetrical ancillary bipyridine ligand increased the molar extinction coefficient of Ru-complex as well as decreases the direct recombination with I 3 . The DSSC based on 1 exhibited the high overall light-to-electrical conversion efficiency of 5.19%. Acknowledgements Ch.P.K. thanks UGC, New Delhi for a senior research fellowship. SPS thanks the DST for Fast Track Young Scientist Project (CS-83/2012). MC thanks DST, New Delhi for funding the project No. DST/TMC/SERI/FR/92.

Appendix A. Supplementary material

Fig. 3a. IPCE Curves of 1 and 2 sensitizers.

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2015.04.038.

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References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737. [2] M.K. Nazeeruddin, S.M. Zakeeruddin, R. Humphry-Baker, M. Jirousek, P. Liska, N. Vlachopoulos, V. Shklover, C.H. Fischer, M. Grätzel, Inorg. Chem. 38 (1999) 6298. [3] K. Hara, H. Sugihara, Y. Tachibana, A. Islam, M. Yanagida, K. Sayama, H. Arakawa, G. Fujihashi, T. Horiguchi, T. Kinoshita, Langmuir 17 (2001) 5992. [4] H. Lindstrm, A. Holmberg, E. Magnusson, S.E. Lindquist, L. Malmqvist, A. Hagfeldt, Nano Lett. 1 (2001) 97. [5] M.K. Nazeeruddin, P. Péchy, T. Renouard, S.M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G.B. Deacon, C.A. Bignozzi, M. Grätzel, J. Am. Chem. Soc. 123 (2001) 1613. [6] P. Wang, S.M. Zakeeruddin, J.E. Moser, M.K. Nazeeruddin, T. Sekiguchi, M. Grätzel, Nat. Mater. 2 (2003) 402. [7] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humpbry-Baker, E. Müller, P. Liska, N. Vlachopoulos, M. Grätzel, J. Am. Chem. Soc. 115 (1993) 6382. [8] T. Bessho, E. Yoneda, J.-H. Yum, M. Guglielmi, I. Tavernelli, H. Imai, U. Rothlisberger, M.K. Nazeeruddin, M. Grätzel, J. Am. Chem. Soc. 131 (2009) 5930. [9] Q. Yu, Y. Wang, Z. Yi, N. Zu, J. Zhang, M. Zhang, P. Wang, ACS Nano 4 (2011) 6032. [10] T. Funaki, H. Funakoshi, O. Kitao, N.O. Komatsuzaki, K. Kasuga, K. Sayama, H. Sugihara, Angew. Chem., Int. Ed. 51 (2012) 7528. [11] Ch. Ramesh Kumar, K.S.V. Gupta, M. Chandrasekhram, F. Reza, A. Islam, L. Han, K. Bhanuprakash, S.P. Singh, Phys. Chem. Chem. Phys. 16 (2014) 2630. [12] S.-H. Yang, K.-L. Wu, Y. Chi, Y.-M. Cheng, P.-T. Chou, Angew. Chem., Int. Ed. 50 (2011) 8270. [13] H. Ozawa, Y. Yamamoto, K. Fukushima, S. Yamashita, H. Arakawa, Chem. Lett. 42 (2013) 897. [14] D. Jérémy, H. Jérôme, G. Laurent, O. Frédéric, M. David, Renewable Energy 66 (2014) 588. [15] K.-L. Wu, C.-H. Li, Y. Chi, J.N. Clifford, L. Cabau, E. Palomares, Y.-M. Cheng, H.-A. Pan, P.-T. Chou, J. Am. Chem. Soc. 134 (2012) 7488. [16] H.-W. Lin, Y.-S. Wang, Z.-Y. Huang, Y.-M. Lin, C.-W. Chen, S.-H. Yang, K.-L. Wu, Y. Chi, S.-H. Liu, P.-T. Chou, Phys. Chem. Chem. Phys. 14 (2012) 14190. [17] N. Onozawa-Komatsuzaki, M. Yanagida, T. Funaki, K. Kasuga, K. Sayama, H. Sugihara, Sol. Energy Mater. Sol. Cells 95 (2011) 310. [18] A. Islam, H. Sugihara, H. Arakawa, J. Photochem. Photobiol., A 158 (2003) 131.

[19] Y. Numata, S.P. Singh, A. Islam, M. Iwamura, A. Imai, K. Nozaki, L. Han, Adv. Funct. Mater. 23 (2013) 1817. [20] M. Chandrasekharam, T. Suresh, S.P. Singh, B. Priyanka, K. Bhanuprakash, A. Islam, L. Han, M. Lakshmi, Dalton Trans. 41 (2012) 8770. [21] T. Suresh, K. Ganesh, S.P. Singh, A. Islam, L. Han, M. Chandrasekharam, Dyes Pigm. 99 (2013) 850. [22] G. Marotta, M. Anil Reddy, S.P. Singh, A. Islam, L. Han, Filippo De Angelis, M. Pastore, M. Chandrasekharam, ACS Appl. Mater. Interfaces 5 (2013) 9635. [23] L. Han, A. Islam, H. Chen, M. Chandrasekharam, B. Chiranjeevi, S. Zhang, X. Yang, M. Yanagida, Energy Environ. Sci. 5 (2012) 6057. [24] M. Chandrasekharam, M.A. Reddy, S.P. Singh, B. Priyanka, K. Bhanuprakash, M. Lakshmi, J. Mater. Chem. 22 (2012) 18757. [25] M. Chandrasekharam, CH. Pavan Kumar, S.P. Singh, V. Anusha, K. Bhanuprakash, A. Islam, L. Han, RSC Adv. 3 (2013) 26035. [26] S. Ito, T.N. Murakami, P. Comte, P. Liska, C. Grätzel, M.K. Nazeeruddin, M. Grätzel, Thin Solid Films 516 (2008) 4613. [27] Z. Wang, T. Yamaguchi, H. Sugihara, H. Arakawa, Langmuir 21 (2005) 4272. [28] X. Yang, M. Yanagida, L. Han, Energy Environ. Sci. 6 (2013) 54. [29] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford, CT, 2010. [30] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [31] A.D. Becke, J. Chem. Phys. 104 (1996) 1040. [32] C.T. Lee, W.T. Yang, R.G. Parr, Phys. Rev. B. 37 (1988) 785. [33] T.H. Dunning, P.J. Hay, Modern Theoretical Chemistry, Vol. 3, Plenum, New York, 1976. 1. [34] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270. [35] Y. Zhao, D. Truhlar, Theor. Chem. Acc. 120 (2008) 215. [36] S. Miertuš, E. Scrocco, J. Tomasi, Chem. Phys. 55 (1981) 117. [37] M. Cossi, V. Barone, R. Cammi, J. Tomasi, Chem. Phys. Lett. 255 (1996) 327. [38] N.M. O’Boyle, A.L. Tenderholt, K.M. Langner, J. Comp. Chem. 29 (2008) 839. [39] S.H. Yang, K.L. Wu, Y. Chi, Y.M. Cheng, P.T. Chou, Angew. Chem., Int. Ed. 50 (2011) 8270. [40] R. Dennington, T. Keith, J. Millam, Shawnee Mission KS, GaussView, Version 5, 2009. [41] J.R. Aranzaes, M.C. Daniel, D. Astruc, Can. J. Chem. 84 (2006) 288. [42] K. Chen, Y.H. Hong, Y. Chi, W.H. Liu, B.S. Chen, P.T. Chou, J. Mater. Chem. 19 (2009) 5329. [43] A. Hagfeldt, M. Grätzel, Chem. Rev. 95 (1995) 49.