Effects of Methyl-Substituted Phenanthrolines on the Performance of Ruthenium(II) Dye-Sensitizers

Tris-heteroleptic ruthenium(II) cis-[Ru(Me4-phen)(dcbH2)(NCS)2], where Me4-phen = 3,4,7,8-tetramethyl-1,10-phenanthroline, dcbH2 = 4,4’-dicarboxylic acid 2,2’-bipyridine, was synthesized, purified and characterized. Its characteristics were compared with those determined for cis-[Ru(Me2-phen)(dcbH2)(NCS)2] and cis-[Ru(phen)(dcbH2)(NCS)2] aiming to evaluate the effect of the number of methyl groups on the properties of the compounds. Changes in the Fourier transform infrared (FTIR) and H nuclear magnetic resonance (NMR) spectra indicated a modification in the electronic distribution of the complex because of the presence of methyl groups at the 3 and 8 positions of 1,10-phenanthroline. These changes also modified the excited state properties and resulted in a blue shift of the absorption and emission spectra. The complex was incorporated onto TiO2 to prepare dye-sensitized solar cells, achieving up to JSC = 11.9 mA cm−2, VOC = 0.627 V, ff = 0.67 and η = 5.0%.


Introduction
The annual world energy consumption reached 5.5 × 10 20 J in 2010, and it is expected to reach 8.7 × 10 20 J in 2040 as result of population and economic growth. 1,2he main sources responsible for the world's energetic needs are liquid fuels, coal and natural gas, which have several disadvantages because they are not renewable and because they emit large quantities of pollutants to the atmosphere. 3For these reasons, the conversion of solar energy is extremely attractive due to its low environmental impact, infinite supply of energy and its straightforward use in remote areas. 4Dye-sensitized solar cells (DSSCs) are capable of directly converting sunlight into electricity, and so they could potentially fulfill the world's energy needs.[7][8][9][10][11][12][13][14][15][16] The dye sensitizer is the component responsible for harvesting the sunlight and initiating the energy conversion process.Currently, ruthenium polypyridyl tris-heteroleptic compounds are being investigated as dye sensitizers, among other compounds, and have been recently reviewed. 4These complexes usually contain a bipyridine ligand having carboxylic acid groups, which are responsible for its chemical attachment to the TiO 2 surface, and isothiocyanate ligands are responsible to tune the t 2g energy levels of the complex. 13,17The two remaining coordination sites can be used to coordinate other ligands, with an aim to improve the light absorption or reduction of electron recombination processes.The main compounds that have been investigated are those having bipyridine derivatives, and only a few papers have reported the use of phenanthroline and their derivatives for this purpose.The similarity of phenanthroline to the bipyridyl ligand, coupled with its extended π-conjugated structure, enabled phenanthroline derivatives to be employed in the preparation of dye-sensitizers. 18The first compound having 1,10-phenanthroline as an ancillary ligand developed for DSSCs demonstrated its potential as a new class of sensitizers. 19Aiming to improve the performance of this class of compounds, the ancillary ligand was changed to 5,6-dimethyl-1,10-phenanthroline, which exhibited a lower conversion efficiency. 20Subsequently, this lower efficiency was ascribed to the compound's interfacial electron Vol. 26, No. 11, 2015   recombination processes. 21The use of 4,7-diphenyl-1,10phenanthroline resulted in a complex capable of performing the solar-to-electricity conversion better than the standard compound, (TBA) 2 cis-[Ru(dcbH) 2 (NCS) 2 ], N719. 10 A possible way to enhance the solar-to-electrical energy conversion is through the use of electron donating groups. 22,23In a previous work, we investigated the effect of electron donating (i.e., methyl) or withdrawing (i.e., phenyl) substituents on the 4 and 7 positions of 1,10-phenanthroline. 22 In this work, the synthesis and characterization of the new dye cis-[Ru(Me 4 -phen)(dcbH 2 )(NCS) 2 ] (Me 4 -phen = 3,4,7,8-tetramethyl-1,10-phenanthroline; dcbH 2 = 4,4'-dicarboxylic acid 2,2'-bipyridine) is described.In addition, the photoelectrochemical performance of this compound as a dye-sensitizer for solar cells is presented and compared with the performance of complexes that possess 4,7-dimethyl-1,10-phenanthroline (Me 2 -phen) and 1,10-phenanthroline (phen), which enables us to evaluate the effect of methyl groups on the 3 and 8 positions of the 1,10-phenanthroline ligand.

Methods
Nuclear magnetic resonance (NMR) spectra were recorded at 25.0 °C on a DRX-500 Bruker Avance spectrometer at 500.13 MHz using DMF-d 7 as the solvent.The residual DMF peaks were used as an internal standard.Fourier transform infrared (FTIR) spectra were recorded at 25.0 °C in KBr pellets on a Bomem MB 100 spectrometer (with 4 cm −1 resolution).Electronic absorption spectra of the complexes fully protonated with HNO 3 were recorded on an Agilent 8453 diode-array spectrophotometer using quartz cuvettes with a 1.000 cm path length.Uncorrected emission spectra were recorded on a Cary Eclipse spectrofluorimeter using quartz cuvettes with a 1.000 cm path length after the samples had been purged with argon.Cyclic voltammetry experiments were performed with a µautolab III potentiostat/galvanostat (Autolab) using a three electrode arrangement comprised of a glassy carbon, platinum and Ag wires as the working, auxiliary and pseudo-reference electrodes, respectively.TBAPF 6 was used as the supporting electrolyte dissolved in acetonitrile.The ferrocene/ferrocenium pair was used as the internal standard.Potentials were reported versus the normal hydrogen electrode (NHE) using E 1/2 (Fc/Fc + ) = 0.67 V vs. NHE. 24brication of the dye-sensitized solar cells A sandwich-type cell was assembled from a transparent photosensitive electrode, a platinum coated electrode and an I − /I 3 − based electrolyte.Titania films (0.6 × 0.6 cm 2 ) were obtained by the screen-printing technique using Ti-nanoxide T/SP ink on FTO conductive glass plates.First, the TiO 2 film was dried at 150 °C for 10 min and then heated under an air atmosphere at 450 °C for 30 min.The thickness of the transparent TiO 2 layer was measured at 8 µm.The film was then immersed overnight in a 0.5 mmol L −1 dye solution and a 5 mmol L −1 chenodeoxycholic acid solution in methanol/ acetonitrile 1:1 (v:v).Then, the electrode was rinsed with solvent before assembly with the counter electrode.A thin Pt layer was used as the counter electrode, which was also prepared by screen-printing Pt ink onto the FTO coated glass substrate and then heated at 450 °C for 30 min under an air atmosphere.The platinum counter electrode and the dye-adsorbed TiO 2 electrode were assembled together by heating at 110 °C a hot-melt film sandwiched between the two electrodes.Then, the Iodolyte AN50-electrolyte was filled between the two electrodes with a syringe.The reproducibility of the DSSC preparation was controlled by preparing three replicates and by performing various tests as described below.

Photoelectrochemical measurements
The spectral response was determined by measuring the wavelength dependence of the incident photon-tocurrent conversion efficiency (IPCE) using light from an Osram 300-W xenon lamp coupled to a Newport monochromator (Oriel Cornerstone 260).Photocurrents were measured under short circuit conditions (in DC mode) using a Newport 1936R instrument.Incident irradiance was measured with a 1 cm 2 calibrated silicon photodiode.Current-voltage characteristics were determined by applying an external potential bias using a Radiometer PGP 201 potentiostat sweeping the potential with a scan rate of 10 mV s −1 .The irradiation source was a Solaronix Luminox A lamp, which matches the AM 1.5 (100 mW cm −2 ) solar spectrum.Incident irradiance was measured with an ISO-Tech ISM410.The cell temperature was kept below 30 °C.The data were averaged for three cells for each configuration.

Synthesis of cis-[Ru
The cis-[Ru(Me 4 -phen)(dcbH 2 )(NCS) 2 ], cis-[Ru(Me 2phen)(dcbH 2 )(NCS) 2 ] and cis-[Ru(phen)(dcbH 2 )(NCS) 2 ] complexes, Figure 1, were prepared by a one-pot procedure, as previously reported. 22Briefly, the ruthenium p-cymene dimer, [Ru(p-cymene)Cl 2 ] 2 , was added to N,N'dimethylformamide (DMF) and 2 equivalents of R n -phen were then added.The mixture was kept at 80 °C for 2 h under an inert atmosphere.After this period, 2 equivalents of dcbH 2 were added to the mixture, and the temperature was increased to 160 °C.The mixture was kept at this temperature for 4 h.Finally, a 10-fold excess of NaNCS was added to the mixture, the temperature was decreased to 140 °C and the reaction was kept under these conditions for 4 h, allowing it to proceed to completion.All reactions were monitored by UV-Vis spectrophotometry.To purify the compounds, the obtained solution was concentrated, washed and filtered with ultrapure H 2 O, diluted in methanol and deprotonated by using a basic methanolic solution, obtained by dilution of tetrabutylammonium hydroxide (1 mol L −1 ), and centrifuged to remove any residual particles.The solution was applied to a liquid column chromatography containing Sephadex LH-20 as the stationary phase and methanol as the eluent.The pure fraction was concentrated, precipitated by the addition of HNO

FTIR spectroscopy
Typical signals of functional groups are observed in the FTIR spectrum for cis-[Ru(Me 4 -phen)(dcbH 2 ) (NCS) 2 ], Figure 2. The CH bond bending of the aromatic rings (1940-1915 cm −1 region), the methyl groups (1462, 1384 and 1367 cm −1 ) and the OH of the carboxylic acids (1425 and 1407 cm −1 ) can be observed.In addition to the signals described previously, it is important to note the ν CO stretching of the carboxylic groups (1720 cm −1 ) and the ν CN of the NCS -ligand (2109 and 768 cm −1 ).
The ν CN(NCS) signals observed with FTIR are also a useful tool to verify the existence of NCS − linkage isomers.The existence of these peaks indicates the coordination of this ligand by the nitrogen atom, as is described for other compounds. 22The typical peak of NCS − coordination through the sulfur atom can be typically observed at 2050 and 700 cm −1 . 14However, they are not observed in the present spectrum.Thus, it is possible to conclude that the isothiocyanate complex is the only isomer obtained after purification.

H NMR spectroscopy
1 H-1 H COSY was used to assign the protons signals of cis-[Ru(Me 4 -phen)(dcbH 2 )(NCS) 2 ] to the respective ligand, dcbH 2 or Me 4 -phen, Figure 3.Because the protons were assigned, NMR spectroscopy allowed an evaluation for the electronic influence of the substituents on the chemical shifts and proton couplings determined by the 1 H NMR spectrum, Table 1.The number of signals observed in the 1 H spectrum is consistent with the cis configuration of the complex, which reduces its symmetry and results in non-equivalent protons.
The signals corresponding to the Me 4 -phen ligand are consistent with the proposed structure (Figure 1), exhibiting only one proton coupling between H 5 and H 6.

Absorption spectrum
The electronic spectrum of cis-[Ru(Me 4 -phen) (dcbH 2 )(NCS) 2 ] in acetonitrile, Figure 5, exhibits high energy absorption bands (up to 350 nm), which are ascribed to intraligand π-π* transitions of the Me 4 -phen and dcbH 2 ligands.Absorption bands with maxima at 420 nm (1.3 × 10 4 L mol −1 cm -1 ) and at 535 nm (0.8 × 10 4 L mol −1 cm −1 ) are observed in the visible region of the spectrum, which overlap the sunlight spectrum.The high molar absorptivities determined for this compound indicate the presence of MLCT transitions, as has been described in the literature for similar compounds. 10,25The existence of these absorption bands increases the sunlight harvesting by this compound.

Photoelectrochemical measurements
The sensitization of TiO 2 films was monitored up to 14 h by absorption changes on the visible region, Figure 7.These absorption changes occur due to the increase in dye loading, which is responsible for the sensitization, following the MLCT band of each dye along the time.It is  Dye sensitized solar cells were assembled using TiO 2 sensitized films and solar cells were characterized by photocurrent action spectra and current-voltage curves.The photocurrent action spectra establish a relationship between the photocurrent generated by the incident light at a determined wavelength and the light intensity, thus it is possible to access information of the efficiency of the dye at each wavelength.Additionally, important photoelectrochemical parameters from solar cells, such as open-circuit potential, V OC , short circuit current density, J SC , and the overall performance of the solar cell, η, can be determined by the current-voltage curves, which are accessed by exposing the DSSC to simulated solar irradiation. 4he photocurrent action spectra of cis-[Ru(Me 4 -phen) 8, resemble the absorption spectra of the respective dye-sensitized TiO 2 and show that these compounds are capable of harvesting the sunlight and converting it into electrical energy in the visible region of the spectrum.
The spectral features of cis-[Ru(Me 4 -phen)(dcbH 2 ) (NCS) 2 ] and cis-[Ru(Me 2 -phen)(dcbH 2 )(NCS) 2 ] are similar, and the spectra of both are different than the standard N719 in the 350-400 nm region.A lower efficiency can also be observed for the cis-[Ru(Me 4 -phen)(dcbH 2 )(NCS) 2 ] dye relative to cis-[Ru(Me 2 -phen)(dcbH 2 )(NCS) 2 ] for the entire spectrum.Dye-sensitized solar cells prepared by using these complexes are able to convert the sunlight into electricity, and their performances were evaluated by JV curves, Figure 9, and their photoelectrochemical parameters are listed in Table 3.

Discussion
The FTIR spectrum of the compound demonstrates the predominance of isothiocyanate because the typical peaks of the thiocyanate isomer are not observed.The presence of the linkage isomer coordinated by the sulfur atom exhibits a ν SCN− at 2050 cm −1 and a resonance at 700 cm −1 . 14  Additionally, the use of 1 H NMR spectroscopy indicates the presence of only one isomer and confirms the existence of the cis-complexes.This is evident because the number of signals observed in the spectrum is consistent with the existence of non-equivalent protons for the cis-isomer.
Changes of the electronic density on the metal center also shift the redox potential of the ruthenium(II/III) process to lower potentials.A more pronounced shift was observed as the number of methyl groups increases, which suggests that the electronic density on the metal center has increased.Changes to the ruthenium(II/III) E 1/2 value as a function of the nature of the phenanthroline substituents are described for several ruthenium compounds [27][28][29] and can be predicted by using ligand electrochemical parameters. 30he oxidation potentials of the excited states of cis-[Ru(Me n -phen)(dcbH 2 )(NCS) 2 ], n = 2 or 4, are more negative than the conduction band edge of TiO 2 anatase (E CB = −0.4V vs. NHE), 10 and their ground state oxidation potentials are more positive than the I 2 −• /I − (+0.93 V vs. NHE) 31 redox pair, Figure 10.The energy of these components is adequate for their use in dye-sensitized solar cells because they permit the electron to move from the excited state of the complexes into the conduction band of TiO 2 .Thermodynamically, their subsequent regeneration by the redox pair is favorable.
The number of methyl groups on 1,10-phenanthroline changes the electronic distribution of the excited state and has an impact on the electronic spectrum of cis-[Ru(Me 4phen)(dcbH 2 )(NCS) 2 ], which resembles those determined for similar complexes. 10,19,22However, its molar absorptivity in the visible light region (450-600 nm) is lower than that observed for similar compounds where the solar spectral irradiance is at a maximum.Its emission spectrum exhibits a maximum at 800 nm, which is blue shifted by 5 nm relative to cis-[Ru(Me 2 -phen)(dcbH 2 )(NCS) 2 ].This result indicates a destabilization of the 3 MLCT excited state and is responsible for the emission detected.
The photoelectrochemical performance of cis-[Ru(Me 4phen)(dcbH 2 )(NCS) 2 ] reflects the changes to the ground     ] can be ascribed to the lower solar light harvesting capacity of the former complex.The dye regeneration dynamics must also be considered to explain the differences observed on the overall cell efficiencies.Since the ground state redox potentials are not positive enough to access the I • /I − (1.23 V vs. NHE), the regeneration mechanism is expected to proceed by I 2 −• /I − pathway, 31,32  indicating that the less favorable regeneration process reduces the cell efficiency.The lower efficiency observed can also be due to a higher recombination rate of the injected electron to the oxidized dye.This process reduces both the J SC and V OC values, 33,34 as was observed in this case.cis-[Ru(Me 4 -phen)(dcbH 2 )(NCS) 2 ] exhibits a V OC value that is 10 mV lower than the same value for cis-[Ru(Me 2 -phen)(dcbH 2 )(NCS) 2 ], indicating that the recombination process can also reduce the efficiency of energy conversion.If this is the case, the methyl groups at the 3 and 8 positions play an important role in the back electron transfer processes for dye-sensitized solar cells.In this respect, further investigations are being performed on the recombination processes.

Conclusions
The new cis-[Ru(Me 4 -phen)(dcbH 2 )(NCS) 2 ] dye-sensitizer was prepared and characterized.Its characteristics were compared to cis-[Ru(Me 2 -phen) (dcbH 2 )(NCS) 2 ] and cis-[Ru(phen)(dcbH 2 )(NCS) 2 ] to evaluate the influence of methyl groups at the 3 and 8 positions of the phenanthroline ligand.This compound exhibits a wide overlap to the visible spectrum, it sensitizes the TiO 2 film and it is able to convert the sunlight into electrical energy in dye-sensitized solar cells.The presence of the four methyl groups on 1,10-phenanthroline leads to changes in both the ground and excited states of the complex.Overall, this changes reduces the solar light harvesting capacity of the complex and destabilizes its 3 MLCT excited state.Consequently, its efficiency as a sensitizer is lower than that reported for cis-[Ru(Me 2 -phen)(dcbH 2 )(NCS) 2 ], which is consistent with the lower IPCE values along the entire spectrum and the JV performance.The lower photocurrent can be ascribed to the lower solar light harvesting capacity of cis-[Ru(Me 4 -phen)(dcbH 2 )(NCS) 2 ], to dye regeneration dynamics or to a higher recombination rate of the injected electron to the oxidized dye, which is ascribed to a reduction of the J SC and V OC values, indicate that the methyl groups at the 3 and 8 positions play an important role in back electron transfer processes for dye-sensitized solar cells.