2.1 Materials

All chemicals used in this study were of the highest purity available. 3,4-diaminobenzoic acid was purchased from Arcos Organics, ruthenium trichloride hydrate was purchased from Merck while, 2,2′-bipyridine, 2-hydrazino pyridine, anthranilic acid, dichloro(p-cymene)ruthenium(II) dimer, ethanol, N,N-dimethylformamide (DMF), tetrabutyl ammonium hexafluorophosphate (TBAPF₆) and methanol were purchased from Sigma–Aldrich. The structural formulas of the investigated ligands are given in Scheme 1.

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Scheme 1 Structural formulas of the investigated ligands.

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2.2 Measurements

Infrared measurements of the complexes were taken by using Jasco FT-IR – 460 plus (range 400–4000 cm⁻¹). Mass spectra were taken with a Jeol JMS-AX500 mass spectrometer. Thermal analysis of the complexes was carried out by using a Shimadzu thermo-gravimetric analyzer TGA-50H; under a nitrogen atmosphere with a heating rate of 10 °C/min over a temperature range from room temperature up to 1000 °C. UV–Vis and fluorescence spectra were recorded in a 1 cm path length quartz cell by using an Analytic Jena S 600 UV diode array spectrophotometer and Edinburgh FLS920P fluorescence spectrometers, respectively. Electrochemical data were obtained using a CH Instrument 660 B Model Electrochemical Workstation. Cyclic voltammograms were measured in a cell containing a glassy carbon working electrode, silver wire reference electrode, platinum wire counter electrode and supporting electrolyte consisting of 0.1 M TBAPF₆ in DMF (scan rate 100 mV s⁻¹).

2.3 Synthesis of complexes

2.4 Molecular modeling

An attempt to gain a better insight into the molecular structure of the synthesized ruthenium complexes, geometric optimization and conformation analysis has performed by using a semiempirical parameterized PM3 method as implemented in HyperChem 7.5 (HyperChem version 7.5 Hypercube, Inc., 2003). A gradient of 1 × 10⁻² cal Å⁻¹ mol⁻¹ was set as a convergence criterion in all the quantum calculations.

3.1 IR spectra

The IR spectra of the complexes are compared with the free ligands (hzpy, bipy, Anth and DABA) in order to determine and monitor the change in the vibration frequency of the coordination sites that are involved in chelation. The characteristic peaks of all ligands and its complexes are listed in Table 1. The FT-IR spectra of the diamine ligands showed medium to strong bands at (3390–3305) cm⁻¹ and (3324–3209) cm⁻¹ which are assigned to stretching vibration of the NH₂ (hzpy, Anth and DABA) groups. These anti-symmetric and symmetric stretching bands appeared as a broad band in each spectrum with higher frequencies ranging between 3433 and 3415 cm⁻¹ indicating the coordination of these groups to metal ions. The coordination of the NH₂ or NH groups is also confirmed from the shift of their deformation vibrations found at (1236–1203) cm⁻¹ (ρ_tNH₂), (1154–1031) cm⁻¹ (ρ_wNH₂) and (767–759) cm⁻¹ (ρ_rNH₂) to higher frequencies (Table 1) (Nakamoto, 1986). In addition, complexes I and II showed characteristic vibration peaks for the coordinated SCN (νSCN) at 2125 and 2095 cm⁻¹. The bands appeared at 555–619 cm⁻¹ may be assigned for M—N bonding, while the band appeared at 652 cm⁻¹ in complex IV may be assigned for M—O bonding (Ali et al., 2002).

3.2 Mass spectra of complex

The major fragmentations of the complexes are listed in Table 2. The molecular ion peaks of the complexes appeared as in the following: I, 522 (M—3H); II, 478 (M⁺); III, 584 (M—H); IV, 526 (M—3H); V, 632 (M⁺). In addition, the mass spectra of the complexes showed the m/e peaks of the ligands: 152 (DABA), 156 (bipy), 137 (anth) 58 (NCS) and the stable Ru isotopes (96, 98, 99, 101, 102, 104).

3.3 Thermal analysis

The thermal studies of the ruthenium complexes are carried out to investigate the stability of the complexes. Calculation of the thermodynamic parameters was performed by using the integral method of Coats–Redfern and approximation method of Horowitz–Metzger (Coats and Redfern, 1964; Horowitz and Metzger, 1963). The temperature ranges of decompositions along with the corresponding mass loss of species are given in Table 3. A representative TG and DrTG (derivative TG) plot of complex I is shown in Fig. 1. Complex I is decomposed in two steps. The first decomposition step observed at 347.40–609.48 K is attributed to the loss of bipy and NCS groups with mass losses of 214.22% and 41.31%, respectively. This is followed by the second step with the loss of DABA and NCS groups with mass losses of 210.22% and 40.35%, respectively at 608.48–689.35 K, leaving Ru as the metallic residue (18.34%) (Soliman et al., 2007; Soliman, 2007). Similar trends are observed in the thermal decomposition of complexes II and V, with Ru or RuO₂ as the metallic residues. The temperature ranges of decomposition along with the corresponding mass losses of species are given in Table 3. Thermodynamic parameters of the five complexes are summarized in Table 4. The complexes showed low thermal stability which is reflected from the relatively lower overall activation energy. The entropy change (ΔS^∗) for the formation of the activated complexes from the starting reactants is, negative. The negative sign of the ΔS^∗ suggests that the degree of structural randomness of the activated complex was lower than that of the starting reactants (more disorder, random) and the decomposition reactions are slow reactions (Soliman et al., 2006; Valaev and Gospodinov, 2001).

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Figure 1 TGA and DrTGA curves of complex I.

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3.4 UV/visible absorption and fluorescence spectra

The UV–Vis absorption and emission spectra of ruthenium complexes in DMF solvents are shown in Fig. 2. The maximum absorption wavelengths and the molar extinction coefficients are summarized in Table 5. All of the complexes show visible bands in the 350–580 nm regions due to metal-to-ligand charge transfer (MLCT) (Klein et al., 2004; Nazeeruddin et al., 2004). The lowest energy MLCT absorption bands are attributed to the filled metal t_2g orbital to the empty π^∗ orbitals of the 3,4-diaminobenzoic acid and 2-aminobenzoate for the complexes I, II, III, V and IV, respectively (Fantacci et al., 2003). The lowest-energy MLCT band of ruthenium complex IV is slightly red-shifted to 582 nm when compared with other complexes. The redshift is attributed to ligand-field strengths of the σ-donating 2-aminobenzoate ligand (Lesh et al., 2011). The related classes of ruthenium(II) complexes that contain mixtures of bipyridine and phenanthroline typically show the lowest-energy MLCT band between 520 and 540 nm (Klein et al., 2004; Li et al., 2008) whereas synthesized ruthenium(II) complexes with some dinitrogen ligands; 3,4-diamino benzoic acid, 2-hydazinopyridine and anthranilic acid are redshifted due to the ligand-field strengths of ligands. The emission spectra of the ruthenium complexes were obtained at room temperature by excitation at their lowest energy MLCT absorption maxima band. The emission spectra of the complexes containing NCS group i.e. complexes I and II show bands around 697 nm. The emission maxima of complexes containing two carbonyl groups (III, IV and V) are red-shifted compared to other complexes due to stronger acceptor properties of the carbonyl group lowering the energy of the excited state (Nazeeruddin et al., 2004). The emission quantum yield of ruthenium complexes was calculated according to the Eq. (1), where Φ_f is the fluorescence quantum yield, A is the absorption intensity, S is the integrated emission band area and n is the solvent reflective index, u and s refer the unknown and standard, respectively (Karapire et al., 2003).

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Figure 2 UV–Vis absorption spectrum of 4 × 10⁻⁵ M DMF solution of complex I and normalized photoluminescence spectra of the complexes in DMF.

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The quantum yield measurements for solutions were prepared in DMF and [Ru(bpy)₃](PF₆)₂ was used as the reference (Φ = 9.5% in acetonitrile) (Swanick et al., 2012). The calculated values are in the range of 0.9–2.5% and are in agreement with the reported values for ruthenium complexes containing bipyridine ligands (Lee et al., 2003).

3.5 Electrochemical properties

The redox potentials of complexes are summarized in Table 6 and the cyclic voltammogram of complex I in DMF is given in Fig. 3. The cyclic voltammogram of complexes I and II shows three oxidation and three reduction peaks around 0.71, 1.04, 1.27 and −1.47, −1.79, −2.09 vs. Ag/Ag⁺, respectively. The oxidation peaks are attributed to the amine moiety (N—H) of the ligand, the Ru (II/III) and Ru (III/IV) couples (Lesh et al., 2011; Smith and Masheder, 1976; Sahin et al., 2013; Zourab et al., 2005) whereas the reduction peaks are assigned to the reduction of carboxylic acid protons, carbonyl group and the bipyridine, respectively (Klein et al., 2004; Nazeeruddin et al., 2004). The redox potentials of complexes containing two carbonyl groups, i.e. III, IV and V are slightly shifted to anodic area when compared to the others which is attributed to stronger acceptor properties of the carbonyl groups of the complexes (Nazeeruddin et al., 2004). This results in a decrease in the energy of the LUMO energy of the ligand. The cyclic voltammogram of complexes III and V shows an additional reduction peak at −0.30 eV, which is attributed to the chloride ion in the second coordination sphere of ruthenium complex. The chloride ion interacts with Ag electrode surface and leads to the reduction of the chloride (Arai et al., 1996). The HOMO and LUMO energy levels of the complexes were calculated using the maximum of first oxidation and reduction potentials (Sahin et al., 2013). Ferrocene was used as an internal standard (0.65 V vs. Ag/Ag⁺). The calculated HOMO and LUMO energy levels of the complexes are in the range of −4.94 to −4.85 eV and −2.86 to −2.68 eV, respectively. Consecutive cyclic behavior of the complexes was also investigated in order to determine their electrochemical stability. No significant change in peak currents and potentials of anodic and cathodic areas are observed (Fig. 3b). Although the energy states and electrochemical stability of the complexes are appropriate to be used as photosensitizers in DSSCs, the limited solubility of them hinders their utilization in this application area. Nevertheless, the HOMO and LUMO energy levels and thermal stabilities of all of the complexes show that they may find application in vacuum evaporated organic photonic systems. The low lying HOMO energy levels will allow hole injection from commonly used hole transport layers, e.g. PEDOT:PSS whereas the LUMO energy levels are appropriate to gain electron from metals with high work function and widely used electron transport materials, e.g. Ca, TPBi, BPhen, etc. (Oner et al., 2011, 2012; Saygili et al., 2011, 2012).

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Figure 3 (a) Differential cyclic voltammogram of complex I measured in DMF solution, (b) 5 consecutive cyclic voltammograms of complex I in DMF solution. The couple at 0.65 V vs. Ag/Ag⁺ is due to ferrocenium/ferrocene couple, which was used as an internal standard; scan rate is 100 mV s⁻¹.

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3.6 Molecular modeling

In the absence of a crystal structure and to obtain the molecular conformation of the complexes, energy minimization studies were carried out on the basis of the semi-empirical PM3 level provided by HyperChem 7.5 software. The molecular structure of complexes along with the atom numbering scheme is given in Figs. 4–8.

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Figure 4 The molecular structure of complex I along with the atom numbering scheme.

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Figure 5 The molecular structure of complex II along with the atom numbering scheme.

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Figure 6 The molecular structure of complex III along with the atom numbering scheme.

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Figure 7 The molecular structure of complex IV along with the atom numbering scheme.

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Figure 8 The molecular structure of complex V along with the atom numbering scheme.

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