- Email: [email protected]
Dual anchored Ruthenium(II) sensitizer containing 4-Nitro-phenylenediamine Schiff base ligand for dye sensitized solar cell application
Inorganic Chemistry Communications 104 (2019) 88–92
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Dual anchored Ruthenium(II) sensitizer containing 4-Nitrophenylenediamine Schiff base ligand for dye sensitized solar cell application
T
Subramaniam Kamalesu, Athanas Anish Babu, Kalaiyar Swarnalatha
⁎
Photochemistry Research Laboratory, Department of Chemistry, Manonmaniam Sundaranar University, Abishekapatti, Tirunelveli 627012, Tamil Nadu, India
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Dual anchor 4-Nitro-phenylenediamine Crystal structure Ruthenium complex FTIR DSSC
New heteroleptic dual anchored Ruthenium(II) sensitizer (RNPDA) was synthesized using 4-Nitro-phenylenediamine Schiff base as ligand (NPD-PC) and the complex was characterized by diverse spectroscopic techniques. The structure of NPD-PC was resolved by single crystal X-ray diffraction method. FT-IR spectra showed that the 4-Nitro-phenylenediamine ligand (NPD-PC) behaves as a bidentate N and N donors coordinate to ruthenium via the azomethine nitrogen and the amine nitrogen. Their optical and electrochemical properties were also investigated. The dye containing electron withdrawing group of pyridine and nitro group act as an anchoring unit and they evince sensitization behavior as well as fascinating interfacial phenomena on TiO2 substrates. The new ruthenium dye was used as photosensitizer for the DSSC applications which expressed overall photoconversion efficiency (η) of 3.42%.
Global warming, effect of mining fossil fuels, resource depletion and power deficiency worldwide are the most pressing environmental issues that require to be addressed, additionally to the challenges to find renewable energy solutions for the longer term. Solar energy is the most copious natural resource on earth. Solar energy is converted into electrical energy engineered by the application of dye-sensitized solar cells (DSSCs) [1,2]. DSSCs are auspicious alternatives for the conventional silicon based solar cells because of their simple fabrication technique ⁎
and low-cost material. Inorganic dyes have high thermal and chemical stability than the organic dye [3–6]. Ruthenium(II) complexes are utilized with achievable applications of light activated switches, catalysis and solar energy conversion devices. Ruthenium dyes are still studied by the highest certified sunlight conversion efficiency (11.9%) [7–10]. Anchoring groups have shown to be important for enabling light harvesting, intramolecular charge transfer, and interfacial charge transfer of a DSSC [11,12]. In recent years, some innovative anchoring groups
Corresponding author. E-mail address: [email protected] (S. Kalaiyar).
https://doi.org/10.1016/j.inoche.2019.03.043 Received 28 January 2019; Received in revised form 28 March 2019; Accepted 30 March 2019 Available online 01 April 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 104 (2019) 88–92
K. Subramaniam, et al.
Scheme 1. a) Synthetic route of NPD-PC and RNPDA, b) energy level diagram of TiO2/RNPDA/I−/I3−.
Fig. 1. a) ORTEP view of NPD-PC, b) FT-IR spectra of RNPDA and RNPDA adsorbed on TiO2, c) absorption spectrum of NPD-PC, RNPDA, d) emission spectrum of RNPDA.
have emerged as potential substitutes, such as sulfonate, salicylate, hydroxylquinoline, amino, pyridine and hydroxamate groups [13,14]. The nitro group is a well-known electron accepting chromophore and it was exposed to act as a new anchoring group for DSSCs [15]. The dual anchoring to the surface of TiO2 strengthens the binding in agreement with increased time required for desorption bleached dyes from the TiO2 films [16]. Phenylenediamine derivatives and their metal complexes are used in photovoltaic cells and the efficiency are 1.09% to
5.29%. Azomethine (RC=N-) based compounds are very ingenious because of their innocuous, good electrical conductivity and efficient [17–19]. In this regard, we have synthesized and characterized dual anchoring 4-Nitro-phenylenediamine Schiff base ligand and its ruthenium sensitizer. Moreover, electrochemical and photovoltaic properties of the heteroleptic sensitizer are investigated in this paper. The ligand, (E)-5-nitro-N1-(pyridin-4-ylmethylene) benzene-1,2diamine (NPD-PC) is synthesized by the condensation of 4-Nitro89
Inorganic Chemistry Communications 104 (2019) 88–92
K. Subramaniam, et al.
Fig. 2. Frontier molecular orbitals for NPD-PC and the RNPDA: (a) HOMO of complex NPD-PC, (b) LUMO of complex NPD-PC, (c) HOMO of complex RNPDA, (d) LUMO of complex RNPDA.
Fig. 3. a) TGA curve of the RNPDA, b) cyclic voltammogram of RNPDA.
phenylenediamine with 4-pyridine carboxaldehyde in 1:1 molar ratio in methanol (10 mL) (Supplementary material). The complex [Ru (bpy)2(NPD-PC)] synthesized by mixing NPD-PC ligand with a solution of Ru(bpy)2Cl2 (1:1) in 50 mL of methanol and refluxed in an inert atmosphere for 7 h. The solution turned from purple to reddish brown. Brown precipitate (RNPDA) formed was filtered, washed with diethyl ether/acetone and dried at room temperature (Scheme 1a). The structure of NPD-PC is shown in Fig. 1a. The single crystal XRD
data of NPD-PC crystal denotes that the grown crystal belongs to monoclinic crystal system with I2/a space group (Table S1) (CCDC 1011703). FT-IR spectra of the ligand display a band at 3422 cm−1 and 3299 cm−1 corresponding to the symmetric and asymmetric stretching vibration of NeH group. IR spectra of the new complex shows the symmetric and asymmetric stretching vibration of NeH group in the region of 3392 cm−1 and 3268 cm−1 respectively, stretching bands of amine groups are shifted to lower frequency for the complex, which 90
Inorganic Chemistry Communications 104 (2019) 88–92
K. Subramaniam, et al.
Fig. 4. a) J-V characteristics of DSSC, b) photostability of DSSC, c) Nyquist plot of DSSC, d) Bode plot of DSSC.
reveals the formation of a linkage between the metal ion and nitrogen atom (Fig. S1). The band around 1578 cm−1 in the spectra of ligand can be attributed to ν(C=N) which is shifted to 1558 cm−1 in complex indicating the coordination of azomethine nitrogen with central metal ion. The new bands observed in the region 507 cm−1 and 612 cm−1 are assigned to the ν(M-Nimin) and ν(M-Namine) band respectively, it denotes NPD-PC acts as a bidentate ligand towards the central ruthenium ion via azomethine nitrogen atom and amine nitrogen atom. A new band appeared at around 1615 cm−1, which is assigned to the pyridine coordinated to the TiO2 surface (Fig. 1b). The symmetric and asymmetric stretching vibrations of NO2 group in RNPDA are located at 1521 cm−1 and 1346 cm−1. These two peaks are absent in FTIR spectra of the RNPDA adsorbed on TiO2. It indicates RNPDA adsorbed through dual anchoring group on the surface of TiO2 [14–16]. The 1H NMR spectrum of the complex (Fig. S2) shows a multiplet in the region 8.80–6.23 ppm due to aromatic protons of bipyridine and NPD-PC. A sharp singlet appears at 9.14 ppm assigned to the azomethine proton and the NH2 protons observed as a singlet at 4.23 ppm of the complex. The positions of the azomethine and -NH2 signal in the complex are slightly downfield in comparison with the protons of NPDPC (Fig. S3), suggesting deshielding of the azomethine and -NH2 due to its coordination to ruthenium. Fig. 1c. shows the absorption in the visible region at 482 nm is due to MLCT transition and the bands observed in the region 370–380 nm are due to n–π* transition of nonbonding electrons present on nitro group in the RNPDA. The band observed around 224–293 nm is assigned to π-π* transitions of the
ligand. Fig. 1d shows the broad red emission band of complex is 609 nm on excitation at their absorption maximum. Fig. 2 shows the frontier molecular orbitals of the NPD-PC and the RNPDA. The HOMO (highest occupied molecular orbital) is localized on the metal (Ru), carbon atoms of pyridine ring, azomethine carbon and nitrogen atoms of phenylenediamine with a weak contribution, on the carbons atoms of two bipyridine rings with strong contribution. The LUMO (lowest unoccupied molecular orbital) is localized on the metal (Ru), carbons atoms of one bipyridine, carbon atoms of pyridine ring, azomethine nitrogen and amine Nitrogen with a great contribution and on the carbon atoms of the one bipyridine ring and phenylenediamine with weak contribution. According to calculation, HOMO energy = −4.4339 eV, LUMO energy = −3.5698 eV, HOMO-LUMO energy gap = 0.8641 eV. This small energy gap confirms the compounds with high chemical reactivity as well as high polarizability. The energetic properties of NPD-PC and RNPDA are listed in Table S2 [20]. The Thermogravimetric curve illustrates the absence of any detectable weight loss upto 281 °C and the material decomposed immediately after melting (Fig. 3a). The differential thermal analysis shows the sharp endothermic dip at 281 °C indicates the melting point, high-quality degree of crystallinity and purity of the material. Fig. 3b shows the onset oxidation potential (Eox) and the onset reduction potential (Ered) of RNPDA were measured to be Ered = −0.119 and Eox = 0.913 V respectively (Table S3), and the energy bandgap was 1.036 eV, the energy value of the HOMO was −5.323 eV and for LUMO was −4.281 eV calculated according to the equations, 91
Inorganic Chemistry Communications 104 (2019) 88–92
K. Subramaniam, et al.
HOMO =
(Eox + 4.40) (eV)
Appendix A. Supplementary data
LUMO =
(Ered + 4.40) (eV)
Materials and methods and experimental procedures associated with this article can be found in the supplementary data. Supplementary data to this article can be found online at doi: https:// doi.org/10.1016/j.inoche.2019.03.043.
The results indicate that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the RNPDA is suitable for efficient electron injection from the excited dye into the conduction band of TiO2 and the regeneration of the oxidized dye by I−/I3− redox couple (Scheme 1b). The photovoltaic performance of DSSC fabricated with RNPDA under standard AM 1.5G irradiation is given in Fig. 4a. The power conversion efficiency (PCE) under irradiation can be measured using the equation,
=
References [1] N.A. Ludin, N.I. Mustafa, M.M. Hanafiah, M.A. Ibrahim, M.A.M. Teridi, S. Sepeai, A. Zaharim, K. Sopian, Prospects of life cycle assessment of renewable energy from solar photovoltaic technologies: a review, Renew. Sust. Energ. Rev. 96 (2018) 11–28. [2] R. Su, S. Ashraf, A. El-Shafei, Structure-property relationships: “double-tail versus double-flap” ruthenium complex structures for high efficiency dye-sensitized solar cells, Sol. Energy 177 (2019) 724–736. [3] M. Gratzel, Dye-sensitized solar cells, J Photochem Photobiol C: Photochem Rev 4 (2003) 145–153. [4] A. Baysal, M. Aydemir, F. Durap, S. Ozkar, L.T. Yildirim, Y.S. Ocak, A ruthenium(II) bipyridine complex containing a 4,5-diazafluorene moiety: synthesis, characterization and its applications in transfer hydrogenation of ketones and dye sensitized solar cells, Polyhedron 89 (2015) 55–61. [5] T. Chakrabarti, A. Dey, S.K. Sarkar, Comparative analysis of physical organic and inorganic dye-sensitized solar cell, Opt. Mater. 82 (2018) 141–146. [6] J.Y. Shao, Y.W. Zhong, Stabilization of a cyclometalated ruthenium sensitizer on nanocrystalline TiO2 by an electrodeposited covalent layer, Inorg. Chem. 58 (2019) 3509–3517. [7] L. Wang, P. Chen, Y.C. Wang, G.S. Liu, C. Liu, X. Xie, J.Z. Li, B.R. Yang, Tape-based photodetector: transfer process and persistent photoconductivity, ACS Appl. Mater. Interfaces 10 (2018) (16596-16604). [8] J. Yum, E. Baranoff, S.M. Wenger, K. Nazeeruddin, M. Gratzel, Panchromatic engineering for dye-sensitized solar cells, Energy Environ. Sci. 4 (2011) 842–857. [9] R.N. Prabhu, R. Ramesh, Synthesis, structural characterization, electrochemistry and catalytic transfer hydrogenation of ruthenium(II) carbonyl complexes containing tridentate benzoylhydrazone ligands, J. Organomet. Chem. 718 (2012) 43–51. [10] M. Gierszewski, I. Gradzka, A. Glinka, M. Ziot, Insights into the limitations of solar cells sensitized with ruthenium dyes revealed in time-resolved spectroscopy studies, Phys. Chem. Chem. Phys. 19 (2017) 20463–20473. [11] W.C. Chen, F.T. Kong, R. Ghadari, Z.Q. Li, X.P. Liu, T. Yu, Y. Huang, Y. Huang, T. Hayat, S.Y. Dai, Insight into electron-donating ancillary ligands in ruthenium terpyridyl complexes configuration on performances of dye- sensitized solar cells, J. Phys. Chem. C 121 (2017) 8752–8759. [12] M. Urbani, M. Gratzel, M.K. Nazeeruddin, T. Torres, Meso-substituted porphyrins for dye-sensitized solar cells, Chem. Rev. 114 (2014) 12330–12396. [13] L. Zhang, J.M. Cole, Anchoring groups for dye-sensitized solar cells, ACS Appl. Mater. Interfaces 7 (2015) 3427–3455. [14] A.A. Babu, T. Shankar, K. Swarnalatha, Co-sensitization of ruthenium(II) dye sensitized solar cells by coumarin based dyes, Chem. Phys. Lett. 699 (2018) 32–39. [15] L. Zhang, J.M. Cole, Can nitro groups really anchor onto TiO2? Case study of dye-to TiO2 adsorption using azo dyes with NO2 substituents, Phys. Chem. Chem. Phys. 18 (2016) 19062–19069. [16] J. Cong, X. Yang, J. Liu, j. Zhao, Y. Hao, Y. Wang, Nitro group as a new anchoring group for organic dyes in dye-sensitized solar cells, Chem. Commun. 48 (2012) 6663–6665. [17] A.G. Imer, R.H.B. Syan, M. Gulcan, Y.S. Ocak, A. Tombak, The novel pyridine based symmetrical Schiff base ligand and its transition metal complexes: synthesis, spectral definitions and application in dye sensitized solar cells, J. Mater. Sci. Mater. Electron. 29 (2018) 898. [18] M.A. Shakour, W.A. El-Said, I.M. Abdellah, R. Su, A. El-Shafei, Low-cost Schiff bases chromophores as efficient co-sensitizers for MH-13 in dye-sensitized solar cells, J. Mater. Sci. Mater. Electron. 30 (2019) 5081–5091. [19] A.W. Jeevadason, K.K. Murugavel, M.A. Neelakantan, Review on Schiff bases and their metal complexes as organic photovoltaic materials, Renew. Sust. Energ. Rev. 36 (2014) 220–227. [20] A.V. Medved'ko, V.K. Ivanov, M.A. Kiskin, A.A. Sadovnikov, E.S. Apostolova, V.A. Grinberg, V.V. Emets, A.O. Chizhov, O.M. Nikitin, T.V. Magdesieva, S.A. Kozyukhin, The design and synthesis of thiophene-based ruthenium(II) complexes as promising sensitizers for dye- sensitized solar cells, Dyes Pigments 140 (2017) 169–178. [21] C.Y. Li, C. Su, H.H. Wang, P. Kumaresan, C.H. Hsu, I.T. Lee, Design and development of cyclometalated ruthenium complexes containing thiophenylpyridine ligand for dye-sensitized solar cells, Dyes Pigments 100 (2014) 57–65.
JSC VOC FF × 100 Pin
where, Jsc is the short-circuit current, Voc is the open-circuit voltage, FF is the fill factor and Pin is the incident light intensity (100 mW cm−2), RNPDA sensitized DSSC afforded Jsc value of 7.12 mA/cm2, Voc value of 0.79 V, FF of 0.61 and efficiency (η) of 3.42% (Table S4). The maximum efficiency (3.42%) is achieved by the presence of dual anchoring units in the dye which implies that increasing the number of anchoring units leads to increase the binding properties of dye on TiO2 surface compared to mono anchoring unit of the dye. The long-term photostability of DSSC is a vital factor for the commercialization. RNPDA obtained good reproducible efficiency (Fig. 4b) up to 80 h [14,20,21]. Fig. 4c shows the Nyquist plot of electrochemical impedance spectrum of DSSC, which was measured with a frequency range of 0.1 Hz to 100 kHz. The smaller semicircle at higher frequency was attributed to the charge transfer at the counter electrode/electrolyte interface and the incomplete arc at low frequency corresponds to charge transfer at TiO2-dye/electrolyte interface. The experimental data are fitted in to equivalent circuit Rs[(R1Q1)(R2Q2)] is shown in insert of Fig. 4c. R and Q denote resistance and constant phase element (CPE) respectively (Table S5). The resistance R1(1.453 × 10−5 Ω) and R2(187.1 Ω) are smaller and results in fast electron transfer and higher efficiency (η = 3.42%). In Bode's plot (Fig. 4d), the middle frequency of peak is directly related to the electron lifetime in conduction band (CB) of TiO2 and this was estimated using τCB=(2πf)−1. It is the essential factor for enhance the efficiency. The high electron life time (τCB = 4.4 ms) indicates the electron recombination is delayed at the TiO2/electrolyte interface. We reported the synthesis and characterization of new monometallic Ruthenium(II) complex containing NPD-PC ligand possessing dual anchoring units. The RNPDA is found to have great photophysical and redox properties. The sensitizer has good thermal and photostability. NPD-PC has been designed to allow RNPDA to bind on TiO2 surface through the simultaneous use of two anchoring groups for enhancing the efficiency. Dual anchoring effect of the fabricated DSSC could improve the spectral response, diminish the recombination resistance, extend the electron life time and enhances the efficiency. The DSSC fabricated using the RNPDA sensitizer and the device exhibited PCE of 3.42% with Voc of 0.79 V, and a JSC of 7.12 mA/cm2. Acknowledgement Dr. S. Kamalesu thanks to Council for Scientific and Industrial Research (CSIR), New Delhi, India for Research Associate Fellowship [09/652(0029)/2018-EMR-I].
92
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Dual anchored Ruthenium(II) sensitizer containing 4-Nitrophenylenediamine Schiff base ligand for dye sensitized solar cell application
T
Subramaniam Kamalesu, Athanas Anish Babu, Kalaiyar Swarnalatha
⁎
Photochemistry Research Laboratory, Department of Chemistry, Manonmaniam Sundaranar University, Abishekapatti, Tirunelveli 627012, Tamil Nadu, India
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Dual anchor 4-Nitro-phenylenediamine Crystal structure Ruthenium complex FTIR DSSC
New heteroleptic dual anchored Ruthenium(II) sensitizer (RNPDA) was synthesized using 4-Nitro-phenylenediamine Schiff base as ligand (NPD-PC) and the complex was characterized by diverse spectroscopic techniques. The structure of NPD-PC was resolved by single crystal X-ray diffraction method. FT-IR spectra showed that the 4-Nitro-phenylenediamine ligand (NPD-PC) behaves as a bidentate N and N donors coordinate to ruthenium via the azomethine nitrogen and the amine nitrogen. Their optical and electrochemical properties were also investigated. The dye containing electron withdrawing group of pyridine and nitro group act as an anchoring unit and they evince sensitization behavior as well as fascinating interfacial phenomena on TiO2 substrates. The new ruthenium dye was used as photosensitizer for the DSSC applications which expressed overall photoconversion efficiency (η) of 3.42%.
Global warming, effect of mining fossil fuels, resource depletion and power deficiency worldwide are the most pressing environmental issues that require to be addressed, additionally to the challenges to find renewable energy solutions for the longer term. Solar energy is the most copious natural resource on earth. Solar energy is converted into electrical energy engineered by the application of dye-sensitized solar cells (DSSCs) [1,2]. DSSCs are auspicious alternatives for the conventional silicon based solar cells because of their simple fabrication technique ⁎
and low-cost material. Inorganic dyes have high thermal and chemical stability than the organic dye [3–6]. Ruthenium(II) complexes are utilized with achievable applications of light activated switches, catalysis and solar energy conversion devices. Ruthenium dyes are still studied by the highest certified sunlight conversion efficiency (11.9%) [7–10]. Anchoring groups have shown to be important for enabling light harvesting, intramolecular charge transfer, and interfacial charge transfer of a DSSC [11,12]. In recent years, some innovative anchoring groups
Corresponding author. E-mail address: [email protected] (S. Kalaiyar).
https://doi.org/10.1016/j.inoche.2019.03.043 Received 28 January 2019; Received in revised form 28 March 2019; Accepted 30 March 2019 Available online 01 April 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 104 (2019) 88–92
K. Subramaniam, et al.
Scheme 1. a) Synthetic route of NPD-PC and RNPDA, b) energy level diagram of TiO2/RNPDA/I−/I3−.
Fig. 1. a) ORTEP view of NPD-PC, b) FT-IR spectra of RNPDA and RNPDA adsorbed on TiO2, c) absorption spectrum of NPD-PC, RNPDA, d) emission spectrum of RNPDA.
have emerged as potential substitutes, such as sulfonate, salicylate, hydroxylquinoline, amino, pyridine and hydroxamate groups [13,14]. The nitro group is a well-known electron accepting chromophore and it was exposed to act as a new anchoring group for DSSCs [15]. The dual anchoring to the surface of TiO2 strengthens the binding in agreement with increased time required for desorption bleached dyes from the TiO2 films [16]. Phenylenediamine derivatives and their metal complexes are used in photovoltaic cells and the efficiency are 1.09% to
5.29%. Azomethine (RC=N-) based compounds are very ingenious because of their innocuous, good electrical conductivity and efficient [17–19]. In this regard, we have synthesized and characterized dual anchoring 4-Nitro-phenylenediamine Schiff base ligand and its ruthenium sensitizer. Moreover, electrochemical and photovoltaic properties of the heteroleptic sensitizer are investigated in this paper. The ligand, (E)-5-nitro-N1-(pyridin-4-ylmethylene) benzene-1,2diamine (NPD-PC) is synthesized by the condensation of 4-Nitro89
Inorganic Chemistry Communications 104 (2019) 88–92
K. Subramaniam, et al.
Fig. 2. Frontier molecular orbitals for NPD-PC and the RNPDA: (a) HOMO of complex NPD-PC, (b) LUMO of complex NPD-PC, (c) HOMO of complex RNPDA, (d) LUMO of complex RNPDA.
Fig. 3. a) TGA curve of the RNPDA, b) cyclic voltammogram of RNPDA.
phenylenediamine with 4-pyridine carboxaldehyde in 1:1 molar ratio in methanol (10 mL) (Supplementary material). The complex [Ru (bpy)2(NPD-PC)] synthesized by mixing NPD-PC ligand with a solution of Ru(bpy)2Cl2 (1:1) in 50 mL of methanol and refluxed in an inert atmosphere for 7 h. The solution turned from purple to reddish brown. Brown precipitate (RNPDA) formed was filtered, washed with diethyl ether/acetone and dried at room temperature (Scheme 1a). The structure of NPD-PC is shown in Fig. 1a. The single crystal XRD
data of NPD-PC crystal denotes that the grown crystal belongs to monoclinic crystal system with I2/a space group (Table S1) (CCDC 1011703). FT-IR spectra of the ligand display a band at 3422 cm−1 and 3299 cm−1 corresponding to the symmetric and asymmetric stretching vibration of NeH group. IR spectra of the new complex shows the symmetric and asymmetric stretching vibration of NeH group in the region of 3392 cm−1 and 3268 cm−1 respectively, stretching bands of amine groups are shifted to lower frequency for the complex, which 90
Inorganic Chemistry Communications 104 (2019) 88–92
K. Subramaniam, et al.
Fig. 4. a) J-V characteristics of DSSC, b) photostability of DSSC, c) Nyquist plot of DSSC, d) Bode plot of DSSC.
reveals the formation of a linkage between the metal ion and nitrogen atom (Fig. S1). The band around 1578 cm−1 in the spectra of ligand can be attributed to ν(C=N) which is shifted to 1558 cm−1 in complex indicating the coordination of azomethine nitrogen with central metal ion. The new bands observed in the region 507 cm−1 and 612 cm−1 are assigned to the ν(M-Nimin) and ν(M-Namine) band respectively, it denotes NPD-PC acts as a bidentate ligand towards the central ruthenium ion via azomethine nitrogen atom and amine nitrogen atom. A new band appeared at around 1615 cm−1, which is assigned to the pyridine coordinated to the TiO2 surface (Fig. 1b). The symmetric and asymmetric stretching vibrations of NO2 group in RNPDA are located at 1521 cm−1 and 1346 cm−1. These two peaks are absent in FTIR spectra of the RNPDA adsorbed on TiO2. It indicates RNPDA adsorbed through dual anchoring group on the surface of TiO2 [14–16]. The 1H NMR spectrum of the complex (Fig. S2) shows a multiplet in the region 8.80–6.23 ppm due to aromatic protons of bipyridine and NPD-PC. A sharp singlet appears at 9.14 ppm assigned to the azomethine proton and the NH2 protons observed as a singlet at 4.23 ppm of the complex. The positions of the azomethine and -NH2 signal in the complex are slightly downfield in comparison with the protons of NPDPC (Fig. S3), suggesting deshielding of the azomethine and -NH2 due to its coordination to ruthenium. Fig. 1c. shows the absorption in the visible region at 482 nm is due to MLCT transition and the bands observed in the region 370–380 nm are due to n–π* transition of nonbonding electrons present on nitro group in the RNPDA. The band observed around 224–293 nm is assigned to π-π* transitions of the
ligand. Fig. 1d shows the broad red emission band of complex is 609 nm on excitation at their absorption maximum. Fig. 2 shows the frontier molecular orbitals of the NPD-PC and the RNPDA. The HOMO (highest occupied molecular orbital) is localized on the metal (Ru), carbon atoms of pyridine ring, azomethine carbon and nitrogen atoms of phenylenediamine with a weak contribution, on the carbons atoms of two bipyridine rings with strong contribution. The LUMO (lowest unoccupied molecular orbital) is localized on the metal (Ru), carbons atoms of one bipyridine, carbon atoms of pyridine ring, azomethine nitrogen and amine Nitrogen with a great contribution and on the carbon atoms of the one bipyridine ring and phenylenediamine with weak contribution. According to calculation, HOMO energy = −4.4339 eV, LUMO energy = −3.5698 eV, HOMO-LUMO energy gap = 0.8641 eV. This small energy gap confirms the compounds with high chemical reactivity as well as high polarizability. The energetic properties of NPD-PC and RNPDA are listed in Table S2 [20]. The Thermogravimetric curve illustrates the absence of any detectable weight loss upto 281 °C and the material decomposed immediately after melting (Fig. 3a). The differential thermal analysis shows the sharp endothermic dip at 281 °C indicates the melting point, high-quality degree of crystallinity and purity of the material. Fig. 3b shows the onset oxidation potential (Eox) and the onset reduction potential (Ered) of RNPDA were measured to be Ered = −0.119 and Eox = 0.913 V respectively (Table S3), and the energy bandgap was 1.036 eV, the energy value of the HOMO was −5.323 eV and for LUMO was −4.281 eV calculated according to the equations, 91
Inorganic Chemistry Communications 104 (2019) 88–92
K. Subramaniam, et al.
HOMO =
(Eox + 4.40) (eV)
Appendix A. Supplementary data
LUMO =
(Ered + 4.40) (eV)
Materials and methods and experimental procedures associated with this article can be found in the supplementary data. Supplementary data to this article can be found online at doi: https:// doi.org/10.1016/j.inoche.2019.03.043.
The results indicate that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the RNPDA is suitable for efficient electron injection from the excited dye into the conduction band of TiO2 and the regeneration of the oxidized dye by I−/I3− redox couple (Scheme 1b). The photovoltaic performance of DSSC fabricated with RNPDA under standard AM 1.5G irradiation is given in Fig. 4a. The power conversion efficiency (PCE) under irradiation can be measured using the equation,
=
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JSC VOC FF × 100 Pin
where, Jsc is the short-circuit current, Voc is the open-circuit voltage, FF is the fill factor and Pin is the incident light intensity (100 mW cm−2), RNPDA sensitized DSSC afforded Jsc value of 7.12 mA/cm2, Voc value of 0.79 V, FF of 0.61 and efficiency (η) of 3.42% (Table S4). The maximum efficiency (3.42%) is achieved by the presence of dual anchoring units in the dye which implies that increasing the number of anchoring units leads to increase the binding properties of dye on TiO2 surface compared to mono anchoring unit of the dye. The long-term photostability of DSSC is a vital factor for the commercialization. RNPDA obtained good reproducible efficiency (Fig. 4b) up to 80 h [14,20,21]. Fig. 4c shows the Nyquist plot of electrochemical impedance spectrum of DSSC, which was measured with a frequency range of 0.1 Hz to 100 kHz. The smaller semicircle at higher frequency was attributed to the charge transfer at the counter electrode/electrolyte interface and the incomplete arc at low frequency corresponds to charge transfer at TiO2-dye/electrolyte interface. The experimental data are fitted in to equivalent circuit Rs[(R1Q1)(R2Q2)] is shown in insert of Fig. 4c. R and Q denote resistance and constant phase element (CPE) respectively (Table S5). The resistance R1(1.453 × 10−5 Ω) and R2(187.1 Ω) are smaller and results in fast electron transfer and higher efficiency (η = 3.42%). In Bode's plot (Fig. 4d), the middle frequency of peak is directly related to the electron lifetime in conduction band (CB) of TiO2 and this was estimated using τCB=(2πf)−1. It is the essential factor for enhance the efficiency. The high electron life time (τCB = 4.4 ms) indicates the electron recombination is delayed at the TiO2/electrolyte interface. We reported the synthesis and characterization of new monometallic Ruthenium(II) complex containing NPD-PC ligand possessing dual anchoring units. The RNPDA is found to have great photophysical and redox properties. The sensitizer has good thermal and photostability. NPD-PC has been designed to allow RNPDA to bind on TiO2 surface through the simultaneous use of two anchoring groups for enhancing the efficiency. Dual anchoring effect of the fabricated DSSC could improve the spectral response, diminish the recombination resistance, extend the electron life time and enhances the efficiency. The DSSC fabricated using the RNPDA sensitizer and the device exhibited PCE of 3.42% with Voc of 0.79 V, and a JSC of 7.12 mA/cm2. Acknowledgement Dr. S. Kamalesu thanks to Council for Scientific and Industrial Research (CSIR), New Delhi, India for Research Associate Fellowship [09/652(0029)/2018-EMR-I].
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