This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. http://pubs.acs.org/journal/acsodf Article Novel Heteroleptic Ruthenium(II) Complexes with 2,2′- Bipyridines Containing a Series of Electron-Donor and Electron-Acceptor Substituents in 4,4′-Positions: Syntheses, Characterization, and Application as Sensitizers for ZnO Nanowire-Based Solar Cells Fernando F. Salomoń , Nadia C. Vega, Teodor Parella, Faustino Eduardo Morań Vieyra, Claudio D. Borsarelli, Claudia Longo, Mauricio Cattaneo, and Neś tor E. Katz* Cite This: ACS Omega 2020, 5, 8097−8107 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: A novel series of complexes of the formula [Ru(4,4′-X2-bpy)2(Mebpy-CN)](PF6)2 (X = −CH3, −OCH3, −N(CH3)2; Mebpy-CN = 4-methyl-2,2′-bipyridine-4′-carbonitrile) have been synthesized and characterized by spectroscopic, electrochemical, and photophysical techniques. Inclusion of the electron-withdrawing substituent −CN at one bpy ligand and different electron- donor groups −X at the 4,4′-positions of the other two bpy ligands produce a fine tuning of physicochemical properties. Redox potentials, electronic absorption maxima, and emission maxima correlate well with Hammett’s σp parameters of X. Quantum mechanical calculations are consistent with experimental data. All the complexes can be anchored through the nitrile moiety of Mebpy-CN over ZnO nanowires in dye-sensitized solar cells that exhibit an improvement of light to electrical energy conversion efficiency as the electronic asymmetry increases in the series. ■ INTRODUCTION Developing new materials for conversion of solar into electrical in the bipyridine rings can be accomplished. It is well known that addition of electron-donor or electron-acceptor groups to or chemical energy is a relevant area of research for the the 4- and 4′- positions of bpy (bpy = 2,2′-bipyridine) alter the physicochemical properties of trisbipyridineruthenium com- substitution of fossil fuels and mitigation of climate change. plexes.2 Bpys with electron-donor substituent groups have π Dye-sensitized solar cells (DSCs) using n-type semiconductor orbitals suitable to mix with d orbitals of Ru(II), the net effect materials such as TiO2 and sensitizers such as ruthenium being the destabilization of the highest occupied molecular complexes are promising low-cost devices based on the orbital (HOMO). On the other hand, the lowest unoccupied discovery of the Grätzel cell.1 Polypyridylruthenium(II) molecular orbital (LUMO) is a π* orbital centered in the bpys complexes have been extensively studied since their photo- and it can be stabilized by introducing electron-acceptor physical and redox properties are suitable for using them as substituents in the bpy rings. The usual procedure to obtain sensitizers in optoelectronic devices and DSCs.2−4 The red-emitting Ru(II) complexes is the addition of better π- commercial dye N7195 and the black dye6 are examples of efficient sensitizers that absorb a wide range of wavenumbers of visible light. Although they are very efficient converters, Received: January 17, 2020 chemical stability problems arise when considering the Accepted: March 24, 2020 possibility of decomposition of the −NCS group, as suggested Published: April 2, 2020 before.7 Tris-substituted bipyridineruthenium(II) complexes are highly stable and a large variety of chemical transformations © 2020 American Chemical Society https://dx.doi.org/10.1021/acsomega.0c00243 8097 ACS Omega 2020, 5, 8097−8107 ACS Omega ■ http://pubs.acs.org/journal/acsodf Article acceptor ligands than bpy in order to decrease the energy of RESULTS AND DISCUSSION the LUMO and better σ-donor ligands than bpy in order to Syntheses, IR, and NMR Spectra. The precursors increase the energy of the HOMO, such as those complexes Ru(4,4′-X2-bpy)2Cl2 were prepared using procedures similar described by Hanan et al.8 The overall result of combining to those previously reported.26,27 As described by Pannwitz et both trends is a decrease of the HOMO−LUMO gap and a al.,27 [Ru(4,4′-{N(CH3)2}2-bpy)2Cl2]Cl was obtained first as a shift of light absorption and emission maxima to the near- Ru(III) precursor due to the stabilization of higher oxidation infrared (NIR) spectrum region, among other physicochemical states induced by a strong electron-donor group in the bpy ring property changes. such as dimethylamine. For this particular complex, trimethyl- With these modifications, the sensitization of a wide band amine was added in the next preparative step in order to gap semiconductor at longer wavelengths of the solar spectrum reduce Ru(III) to Ru(II). The new complexes 1−3 were led to the creation of new electrodes in DSC devices, as already prepared by protocols already known.26 reported for TiO2-nanostructured electrodes.9,10 In recent Fourier transform infrared (FTIR) spectra of complexes 1− years, ZnO have become very common in semiconductor 3 exhibit a very weak band at 2235 cm−1, which is devices11 due to its unique optical, chemical, electrical, and characteristic of νCN, the stretching vibration of the CN piezoelectrical properties,11−16 being also a material easy to group of Mebpy-CN. Typical bands corresponding to the obtain in different morphologies (films, nanoparticles, and vibrational modes of the polypyridyl moieties (4,4′-(X)2-bpy nanowires)17−19 and having the added possibility of making and Mebpy-CN) are observed between 1600 and 1400 cm−1. core−shell structures with other materials to change their Figure S2 shows the FTIR spectrum of complex 3 as a optical and electrical properties.20,21 In the broad spectrum of representative example. The values of νCN obtained using the challenges related to this issue, we address in this work the Raman spectra of complexes 1−3 are identical with those possibility of improving electron injection efficiency in measured by FTIR and almost equal to that of the free ligand, ruthenium complex DSCs by a careful selection of coligands indicating that Mebpy-CN is not coordinated to the metal and using ZnO as a semiconductor, since its sensitization with through the nitrile group. Ru complexes has received much less attention. ZnO is Complexes 1−3 possess C1 symmetry and all the signals of currently considered as one of the most promising materials to the atoms of the substituted bpy ligands in the NMR spectra replace the TiO2 semiconductor22 in solar energy conversion are therefore inequivalent. For this reason, signals of 1H and technology. In particular, ZnO nanowires (NWs) in a DSC 13 C for the 4,4′-X2-bpy ligands are reported as broad bands. create direct electron pathways, which lead to an increase of On the other hand, signals for the Mebpy-CN ligand can be electron diffusion length and lifetime.22,23 In some of our assigned straightforwardly by comparison with data previously previous studies, very efficient charge transfer between Ru reported for similar complexes with this ligand.25 Detailed complexes and ZnO nanowires24,25 was evidenced, indicating assignments of 1H-NMR, 13C-NMR, and 15N-NMR signals of the possibility of exploring new Ru complex/ZnO nanowire complexes 1−3 as well as mass spectroscopy (MS) data can be structures to develop better DSCs. found in the Syntheses Section. Figures S3−S9 show all the In this paper, three new ruthenium(II) polypyridyl NMR spectra of complex 3 as a representative example and the complexes of the formula [Ru(4,4′-X2-bpy)2(Mebpy-CN)]- complete assignments are shown in Figure S10. NMR and MS (PF6)2, where X = −CH3, −OCH3, −N(CH3)2 and Mebpy- results confirm the proposed structures depicted in Scheme 1. CN = 4-methyl-2,2′-bipyridine-4′-carbonitrile, have been Electrochemistry. Redox potentials of complexes 1−3 in synthesized (Scheme 1). Electrochemical, spectroscopic, and CH3CN with 0.1 M TBAH are shown in Table 1. The first two Scheme 1. Molecular Structures of the New Trisbipyridine Table 1. Electrochemical Data for Complexes 1−3 in Ruthenium(II) Complexes CH3CN vs SCE a complex Eox red1 red2 red3 1/2(V) E1/2 (V) E1/2 (V) E1/2 (V) ΔE1/2(V) [Ru(bpy)2(Mebpy- 1.28 −1.23 −1.51 −1.77 2.51 CN)] (PF6)2 1 1.24 −1.25 −1.61 −1.86 2.49 2 1.06 −1.26 −1.61 −1.76 2.32 3 0.64 −1.38 −1.62 −1.93 2.03 a ΔE1/2 = Eox 1/2 − E1/2 red1 reduction potentials were measured by CV, while the following ones were measured by DPV. The values for the reference complex of formula [Ru(bpy)2(Mebpy-CN)](PF6)225 were photophysical studies have been carried out and the electronic measured again under the same conditions. Cyclic and properties can be rationalized by density functional theory differential pulse voltammograms of complex 3 as a (DFT) and time-dependent density functional theory (TD- representative example are shown in Figures S11 and S12, DFT) calculations. Finally, we demonstrate that ZnO nano- respectively. wires, grown in fluoride tin oxide (FTO) substrates using a When the electron-donor strength of the substituent X of hydrothermal technique, can be sensitized by these complexes 4,4′-(X)2-bpy was increased, a cathodic shift of redox potential anchored to the semiconductor surface through a nitrile group of the RuIII/II couple, Eox1/2,is observed. Less pronounced and used as working electrodes in DSCs that perform at low cathodic shifts are also observed for the redox potentials of efficiencies but could be improved by increasing the number of the ligand reductions E 1r/e2d 1 , E 1r/e2d 2 , and E 1r/e2d 3 . In anchoring nitrile groups. polypyridylruthenium(II) complexes, when the metal center 8098 https://dx.doi.org/10.1021/acsomega.0c00243 ACS Omega 2020, 5, 8097−8107 ACS Omega http://pubs.acs.org/journal/acsodf Article is oxidized to Ru(III), electron-donor groups in the bpy rings maxima and molar extinction data are summarized in Table S1. stabilize this latter state by sharing more electron density with For comparative purposes, data for [Ru(bpy)3](PF6)2 and the metal. The one-electron reduction process is expected to [Ru(bpy)2(Mebpy-CN)](PF6)2 have also been included in the be centered in the best electron-accepting bpy, Mebpy-CN in table. this case. Increasing the electron-donor strength of the All the complexes show intense absorption bands in the substituent X in the bpy rings changes the amount of metal- visible region, which can be assigned to metal-to-ligand charge to-ligand π-backbonding, making their reduction more difficult. transfer (MLCT) transitions.2 As shown in Table S1, The one-electron oxidation process involves removal of an coordination of a 4,4′-X2-bpy ligand diminishes the wave- electron from the HOMO, which is a dπ(Ru) orbital. On the number of the lowest-energy MLCT band in the order of other hand, the one-electron reduction process involves increasing donor strength of −X: addition of an electron to the LUMO, which is centered in Mebpy-CN, as discussed in the Calculations Section. A plot of −H < −CH3 < −O(CH 2)2 < −N(CH3)2 the difference of potentials of the oxidative and the first Besides, the asymmetry within the coordination sphere reductive processes, ΔE1/2,as a function of Hammett’s sigma increases in the same order, resulting in splitting of all the parameters σp,28 shown in Figure 1, is linear and evidences that visible absorption bands, as compared to [Ru(bpy)3](PF6)2 and [Ru(bpy)2(Mebpy-CN)](PF6)2. The splitting goes from 0.20 eV in complex 1 (λmax = 478 and 444 nm) to 0.24 eV in complex 2 (λmax = 488 and 446 nm) and to 0.44 eV in complex 3 (λmax = 532 and 448 nm). As explained in the Calculations section, the least energetic band can be assigned to a dπ(Ru) → π*(Mebpy-CN) MLCT transition, while the following band can be assigned to a dπ(Ru) → π*(4,4′-X2-bpy) MLCT transition. Absorption of visible light is thus extended to a broader range when increasing the donor strength of −X, making complex 3with X = −N(CH3)2almost black in color. The area below the two split bands is almost the same as that of the MLCT bands of [Ru(bpy)3](PF6)2. The maximum frequency of the lowest-lying MLCT band, νMLCT, and the electrochemical energy gap, ΔE1/2 (see Table 1), are correlated Figure 1. Difference of potentials, ΔE1/2,of the oxidative and the first linearly, as expected since both values depend on the HOMO− reductive processes in [Ru(4,4′-X2-bpy)2(Mebpy-CN)](PF6)2 com- LUMO gap.30 plexes as a function of Hammett’s σp parameter of X. Spectroelectrochemistry. UV−vis−NIR spectroelectro- chemical measurements for complexes 1−3 are shown in increasing the donor strength of the substituent group −X Figure 3. On one-electron oxidation, MLCT bands for all these destabilizes the HOMO more than the LUMO. This was complexes decrease in intensity, as expected upon changing the already reported by Maestri et al. for a series of electron-donor- metal configuration from d6 to d5. Also, new bands arise at λmax substituted terpyridines.29 The net effect is a reduction of the between 300 and 400 nm, which can be assigned to intraligand HOMO−LUMO energy gap with the increase of the electron- charge transfer (ILCT) transitions in the oxidized species. donor strength of −X , as discussed below. Complex 3 exhibits a wide band at λmax from 600 to 1000 nm, UV−Vis Spectra. The electronic absorption spectra of which is not present in the other complexes. TD-DFT complexes 1−3 are displayed in Figure 2. The absorption calculations of 33+ show a set of ligand-to-metal charge transfer (LMCT) transitions from a π(4,4′-DMA-bpy) to a dπ-Ru(III) orbital, as explained below in the Calculations Section. On the other hand, upon one-electron reduction, two new bands appear at λmax around 350 to 370 and 530 to 550 nm. Both are typical of bpy radical formation.31 Photophysical Properties. Table 2 shows the photo- physical properties of complexes 1−3 in CH3CN at room temperature, together with the previously studied complex with X = −H.25 The room-temperature emission spectra of complexes 1−3 are depicted in Figure S13. Time-resolved differential absorption spectra of complexes 1−3, obtained by LFP, are shown in Figure 4. In all cases, bleaching of the MLCT bands and appearance of new bands at λmax = 370 nm are detected. This latter band is associated with ILCT transitions of the Mebpy-CN− radical and is unaffected by the electron-donor ability of −X in the substituted bpys. The spectroelectrochemical measurements described above show that upon one-electron oxidation and reduction of complexes 1−3, similar bands are detected; therefore, the lowest-lying MLCT excited state can be formulated as Figure 2. UV−vis spectra of complexes 1 (orange), 2 (blue), and 3 [RuIII(4,4′-X2-bpy)2(Mebpy-CN·)−]2+. As the electron-donor (red) in CH3CN at rt. ability of X increases, the excited state is stabilized and the 8099 https://dx.doi.org/10.1021/acsomega.0c00243 ACS Omega 2020, 5, 8097−8107 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 3. (a) Oxidative difference spectra acquired at an applied potential 100 mV higher than the oxidative potential of the metallic center of complexes 1−3, respectively, in CH3CN solutions. (b) Reductive difference spectra acquired at an applied potential 100 mV lower than the first reductive potential of complexes 1−3, respectively, in CH3CN solutions. Table 2. Photophysical Properties of Complexes 1−3 and [Ru(Bpy)2(Mebpy-CN)](PF6)2 in CH3CN at rt complex λem [nm] Φem τ [μs] kobs × 10−5 [s−1] kr × 10−5 [s−1] knr × 10−5 [s−1] [Ru(bpy)2(Mebpy-CN)](PF6)2 623 0.120 0.76 13.2 1.58 11.6 1 666 0.024 0.67 14.9 0.36 14.5 2 709 9.3 × 10−3 0.32 31.3 0.29 31.0 3 798 2.0 × 10−4 0.02 465 0.093 465 emission band shifts to longer wavelengths reaching the near- an increase in the nonradiative kinetic constant, knr, and infrared region (see Table 2 and Figure S13). The emission consequently a decrease of the excited state lifetime. Calculations. DFT and TD-DFT calculations allowed us to wavenumbers correlate with the MLCT absorption wave- explain the experimental results that depend on electronic numbers. structures. Geometries were assumed to be the same as those As predicted by the energy gap law,32 lowering of the energy of a similar complex determined by crystal structure studies.25 gap between the excited and ground state is accompanied by The molecular orbital energy levels of complexes 1−3 and 8100 https://dx.doi.org/10.1021/acsomega.0c00243 ACS Omega 2020, 5, 8097−8107 ACS Omega http://pubs.acs.org/journal/acsodf Article complexes are delocalized over the Mebpy-CN ligand. A small destabilization of the LUMO with an increase of the electron-donor strength of X is observed due to some mixing with π(4,4′-(X)2-bpy) orbitals. The overall effect of increasing the electron-donor strength of X is a decrease in the HOMO− LUMO gap (see Figure 5). Orbital pictures of HOMOs and LUMOs for complexes 1−3 are displayed in Figure 6. For Figure 6. Orbital representations of LUMOs (top) and HOMOs (bottom) of complexes 1−3. example, the HOMO of complex 3 has a larger contribution of orbitals localized at the bpy ligand with dimethylamino substituents, indicating higher delocalization between elec- Figure 4. Time-resolved differential absorption spectra of 1(top), 2 tron-donor-substituted bpys and metal orbitals than that (middle), and 3 (bottom) in CH3CN solutions at different times (λexc observed for the other two complexes. This is also reflected = 370 nm) (left) and the absorbance decay at λ = 370 nm and mono- in Table S2, where participation of X2bpy in the HOMO exponential fitting (right). increases from 10% in [Ru(bpy)2(Mebpy-CN)]2+ to 12% in 1, to 15% in 2, and to 40% in 3. [Ru(bpy)2(Mebpy-CN)](PF6)2, calculated by DFT, are shown TD-DFT calculations of complexes 1−3 were done to in Figure 5. support assignments of electronic transitions. Calculated For all complexes (see Figure 5 and Table S2), the HOMOs transitions and experimental UV−vis spectra of complexes are dπ-Ru(II)-type orbitals close in energy with some 1−3 are depicted in Figure 7. As stated before, a higher contribution of π orbitals of 4,4′-(X)2-bpy that increases with amount of splitting of both visible bands is observed when the increasing electron-donor strength of X. LUMOs for all electron-donor strength of X grows. Since complex 3 has the larger asymmetry, it will be taken as the representative example to explain general features resulting from these calculations. The band at λmax = 532 nm in the spectrum of 3 is mainly assigned to HOMO-2 → LUMO, supporting the MLCT assignment of that band. The broad band at λmax = 448 nm in the experimental spectrum is assigned to a set of transitions from dπ-Ru(II) orbitals (HOMO, HOMO-1, and HOMO-2) to π* ligand orbitals (LUMO, LUMO+1, LUMO+2, and LUMO+3) (see Table S3). As a considerable mixing of π(4,4′- {N(CH3)2}2-bpy) and dπ-Ru(II) orbitals is predicted (see above), these transitions can be assigned as MLCT/ LLCT. Moreover, the band at λmax = 304 nm in the experimental spectrum can be assigned to a transition from HOMO to a 4,4′-(N(CH3)2-bpy-centered orbital (LUMO+8), being mostly MLCT in nature. Five strong transitions (f > 0.1) in the UV region (λmax ≈ 260 nm) confirm the ILCT π → π* assignment of the band at λmax = 261 nm in the experimental spectrum. Figure 5. Molecular orbital (MO) diagrams calculated by DFT for The calculated extinction molar coefficients are overestimated complexes 1−3 and [Ru(bpy)2(Mebpy-CN)](PF6)2. Different by a factor of 3, as previously reported for similar complexes.25 contributions of the groups are marked in different colors: ruthenium Table S4 and Figure S14 show TD-DFT calculations for the in black, 4,4′-(X)2-bpy in red, and Mebpy-CN in blue. The HOMO− oxidized species 3+ that confirms the assignments discussed in LUMO energy gaps are also depicted. the Spectroelectrochemistry Section. 8101 https://dx.doi.org/10.1021/acsomega.0c00243 ACS Omega 2020, 5, 8097−8107 ACS Omega http://pubs.acs.org/journal/acsodf Article Raman spectra of FTO|ZnO NW electrodes sensitized by complexes 1−3 are shown in Figure S17. In all cases, two bands are detected in the nitrile stretching frequency region, νCN: one at 2240 cm−1, which is identical with those measured in the FTIR spectra of the solid complexes, as described before, corresponding to free nitriles, and one at 2333 cm−1, which can be assigned to nitrile groups bound to the ZnO surface, indicating chemical adsorption of the complex on the semiconductor. This shift has been observed before when the complex with X = H was adhered to TiO2.33 The appearance of an unbound dye can be attributed to the presence of only one anchoring nitrile group in all the complexes studied here. Ongoing work on the series [Ru(4,4′- X2-bpy)(Mebpy-CN)2](PF6)2 containing two anchoring nitrile groups will hopefully improve the quantity of sensitizers adsorbed on the surface and consequently the solar cell efficiency. It must be noted that the highest efficiency was detected in our previously studied Ru systems with two nitrile groups adsorbed on TiO2.33 The QE spectra of the solar cells, Figure 9a, were acquired under ambient temperature (25 ± 1 °C). The higher efficiencies were observed for incident radiation with the wavelength ranging from 450 to 600 nm, consistent with the UV−visible spectra of sensitized FTO |ZnO NW electrodes (Figure S16). A red shift in the QE band of the electrode is detected when going from 1 to 3, showing that the sample sensitized with complex 3 presents a higher value of efficiency than the other complexes for identical times of immersion and a better response to lower wavelengths. In Figure 9b, I−V curves for all the solar cells are presented: they were obtained under irradiation with a solar simulator (linear sweep voltammetry, 5 mV s−1) at 28 ± 1 °C. Table 3 shows the parameters obtained for these DSCs: under polychromatic irradiation, they all show an overall efficiency, η, of 0.03−0.017 %, open circuit voltages (Voc) in a 0.38−0.26 V range and short circuit current values between 0.17 and 0.14 mA/cm2. Figure 7. TD-DFT calculated (straight line) and experimental Considering these data, the best photoresponse was that of (dashed line) UV−vis spectra of complexes 1−3. the cell sensitized by complex 3, according to its higher absorption in the NIR region. When comparing these data with the reference N719, the efficiencies were found to be lower by Application as Sensitizers for ZnO Nanowire-Based a factor of 10 for our complexes in the same conditions, as Solar Cells. Figure 8a,b shows the side and front SEM images, expected because of lower adsorption of the dyes, as discussed respectively, corresponding to a FTO|ZnO NW sample grown before. Although the efficiencies obtained are low, our data by the hydrothermal technique; both images prove that support the fact that panchromatic shifts in absorption lead to nanowires are formed. Besides, the morphology of the ZnO increasing photocurrent generation by modulating the NWs shows the expected hexagonal wurtzite structure in electronic asymmetry by small structural changes in the agreement with the XRD results, shown in Figure 8c, with a sensitizer. We hope to improve the cell performances using mean diameter of 100 nm. complexes with two anchoring nitrile groups. Electrodes of FTO|ZnO NWs were characterized by EDAX, It has already been reported that very low efficiencies are UV−vis, and Raman spectra. The EDAX spectrum, shown in obtained when loading ruthenium polypyridyl complexes onto Figure S15, indicates only the presence of ZnO, evidencing the ZnO surfaces with anchoring carboxylate groups.34 We purity of this semiconductor. As shown in Figure 8c, the XRD demonstrate in this work that anchoring similar complexes patterns of ZnO NWs, as compared with those of a standard with nitrile groups onto ZnO nanowires is a promising ZnO polycrystalline powder sample, indicate that the NWs approach when combined with the occurrence of panchro- grew with a preferential orientation normal to the (002) plane, matic shifting induced by small structural variations in the bipyridine rings as described in this work. ■ corresponding to the c axis of the wurtzite structure. Secondary peaks, corresponding to (101) and (102) planes, are not present in the NW structure. The normalized UV−vis spectra CONCLUSIONS of ZnO NWs sensitized with complexes 1−3 are displayed in Changing the electron-donor ability of substituent −X in Figure S16; they present bands in the visible region, which heteroleptic complexes of the formula [Ru(4,4′-(X) 2 - correspond to those previously measured in CH3CN solutions bpy)2(Mebpy-CN)]2+, with X = −CH3, −OCH3, and −N- of complexes 1−3, as shown in Figure 2, which are somewhat (CH3)2, allows us to obtain a fine tuning of the electronic masked by bands in the UV region due to the ZnO NWs. properties of this series of new complexes. Enhancing 8102 https://dx.doi.org/10.1021/acsomega.0c00243 ACS Omega 2020, 5, 8097−8107 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 8. (a) Side and (b) front view SEM images of ZnO NWs grown using the hydrothermal technique. (c) XRD data of FTO|ZnO NWs compared with those of a bulk sample of the ZnO (commercial powder) spectrum. Figure 9. Performance of solar cells assembled with the sensitized FTO|ZnO NWs, Pt counter electrodes, and a liquid electrolyte: (a) quantum efficiency spectra and (b) current−potential (I−V) curves under polychromatic irradiation. Table 3. Parameters Obtained from I−V Curves for the rationalized using Hammett σp parameters of X, consistent Dye-Sensitized ZnO Solar Cells under Polychromatic with the important contribution of −X in modulating electron Irradiation densities and orbital energies in these complexes, which can be used as dyes in DSCs over Zn NWs by anchoring the nitrile Voc Isc Ptheoa Pmaxb dye (V) (mA/cm2) (mW/cm2) (mW/cm2) ffc η (%)d group of Mebpy-CN to the semiconductor surface. Enlarging the electron density asymmetry in these molecules by (1) 0.26 0.14 3.7 2.1 0.58 0.016 increasing the donor strength of −X gives more pronounced (2) 0.22 0.15 3.4 2.2 0.64 0.018 panchromatic shifting and higher photocurrent generation, a (3) 0.38 0.17 6.6 3.9 0.59 0.030 concept that may be useful for the development of more a Ptheo = Voc × Isc. bPmax = Imax × Vmax. cff = (Pmax/Ptheo). dη = overall efficient DSCs. ■ efficiency. EXPERIMENTAL SECTION asymmetry induces absorption over all visible wavelengths and Materials and Instrumentation. All the chemicals used emission with appreciable quantum yields in the NIR region. in this work were of analytical reagent grade and were used as The observed changes in redox potentials, MLCT absorption purchased. CH3CN was freshly distilled over KMnO4. Infrared and emission maxima, and excited state dynamics can be spectra (4000−400 cm−1) of the solids were obtained using 8103 https://dx.doi.org/10.1021/acsomega.0c00243 ACS Omega 2020, 5, 8097−8107 ACS Omega http://pubs.acs.org/journal/acsodf Article KBr pellets with a Perkin-Elmer FTIR RX-I spectrometer. effect of the solvent (CH3CN) was included using the Characterization of the new complexes were carried out in conductor-like polarizable continuum model (CPCM).43 CH3CN solutions by spectroscopic, electrochemical, and Frequency calculations were performed to ensure that the photophysical techniques. Construction and instruments used optimized structure is a real minimum; no imaginary for characterizing the dye-sensitized ZnO nanowire-based solar frequencies were found. No symmetry restrictions were cells are described below. NMR spectra were obtained in imposed. From optimized structures, molecular orbital CD3CN with a Bruker 500 MHz equipment operating at a energies were determined and TD-DFT calculations were frequency of 500.13 MHz for 1H NMR, 125.75 MHz for 13C made. The contribution of the different groups of molecules to NMR, and 50.67 MHz for 15N NMR. Electrospray ionization orbitals and electronic transitions were obtained using version (ESI) mass spectra were recorded on a Bruker Esquire 6000 3.0 of the GaussSum software package.44 MS. Chemical analyses were carried out at INQUIMAE, Syntheses. The precursors [Ru(4,4′-X2-bpy)2Cl2] were University of Buenos Aires, Buenos Aires, Argentina, with an prepared by following previously published methods.26,27 estimated error of ±0.5%. Electrochemical measurements were Mebpy-CN was prepared as reported previously.25 4,4′- performed with BAS Epsilon EC equipment, using 0.1 M tetra- (CH3)2-bpy and 4,4′-(OCH3)2-bpy were purchased from n-butylammonium hexafluorophosphate (TBAH) as the Sigma-Aldrich and 4,4′-{N(CH3)2}2-bpy from Biosynth. supporting electrolyte in solutions that were thoroughly (i) [Ru(4,4′-(CH3)2-bpy)2(Mebpy-CN)](PF6)2 (1). A mixture degassed with Ar prior to each measurement. A standard of 26 mg of Mebpy-CN (0.13 mmol) and 72 mg of [Ru(4,4′- three-electrode arrangement was used with Pt as the working (CH3)2-bpy)2Cl2] (0.13 mmol) was added to 30 mL of and auxiliary electrode and nonaqueous Ag/AgNO3 (0.01 M deaerated methanol and heated at reflux under Ar for 4 h. The AgNO3 in CH3CN with 0.1 M of TBAH) as the reference obtained orange solution was rotaevaporated to dryness, electrode. For reversible electrochemical processes, reported dissolved in a minimum amount of water, and passed through E1/2 values were calculated from cyclic voltammetry (CV) a CM Sephadex C-25 column. The complex was eluted with 1 measurements as the averages between the peak values M HCl, and the collected fractions were rotaevaporated to corresponding to the cathodic (Ec) and anodic (Ea) waves: dryness, dissolved in a minimum amount of water, and E1/2 = (Ec + Ea)/2. In cases where redox processes were quasi- precipitated with a concentrated aqueous solution of KPF6 (1 g reversible, potentials were obtained by differential pulse in 5 mL of water). The mixture was kept in the fridge voltammetry (DPV). UV−vis absorption spectra were overnight and filtered; the obtained orange powder was stored recorded on a Varian Cary 50 spectrophotometer using 1 cm in vacuum over P4O10 for 1 day. Yield: 68 mg (54%). Chemical quartz cells. Absorption spectra of solid films were measured in analyses and NMR were coherent with the formula [Ru(4,4′- the same instrument using a solid sample holder for Cary 50/ (CH3)2-bpy)2(Mebpy-CN)](PF6)2.(KPF6)0.5. Anal. calcd 60. Raman spectra were obtained with a DXR spectrometer (found) for C36H33N7P2.5F15K0.5Ru: C, 41.3 (41.9); H, from Thermo Scientific equipped with a binocular Olympus 3.2(2.5); N, 9.4 (8.1). 1H-NMR (500.13 MHz, CD3CN) microscope. Steady-state emission measurements were made at 2.56 (s, 12H), 2.58(s, 3H), 7.26 (br, 4H), 7.33 (d, 6.1 MHz, room temperature on a Shimadzu RF-5301 PC spectrofluor- 1H), 7.53 (br, 4H), 7.59 (d, 6.1, 1H), 7.61 (d, 6.0 1H), 7.98 ometer. Relative radiative quantum yields were calculated by (d, 6.0 Hz, 1H), 8.38 (br, 4H), 8.42 (s, 1H), 8.76 (s, 1H). 13C- eq 1 NMR (125.75 MHz, CD3CN): 20.5, 20.1, 115.8, 124.9, 125.4, 2 125.8, 128.2, 128.3, 129.1, 150.4, 150.7, 150.9, 152.3, 152.9, r (I x / A x )/(Ir / A r )](ηx / ηr ) Φx = Φ[ (1) 155.3, 156.5, 158.9. 15N-NMR (50.67 MHz, CD3CN): 241.7, where Φr is the quantum yield of a reference ([Ru(bpy)3]- 242.8, 269.0. Positive ESI-MS ion clusters at m/z: 809.8 {M − (PF6)2 in acetonitrile (Φr = 0.095).35 UV−vis spectroelec- PF6}+, 332.5 {M − 2PF6}2+. trochemical experiments were performed in CH3CN (0.1 M (ii) [Ru(4,4′-(OCH3)2-bpy)2(Mebpy-CN)](PF6)2 (2). A mixture TBAH) using a Honeycomb spectroelectrochemical cell (Pine of 20 mg of Mebpy-CN (0.10 mmol) and 60 mg of [Ru(4,4′- Research) with Pt as the working and counter electrode and (OCH3)2-bpy)2Cl2] (0.10 mmol) was added to 30 mL of Ag/AgCl as the reference electrode. Lifetime and transient deaerated methanol and heated at reflux under Ar for 4 h. The spectra of the lowest-lying3MLCT excited states were obtained obtained reddish-orange solution was rotaevaporated to with the laser flash photolysis (LFP) setup described dryness, dissolved in a minimum amount of water, and passed previously.36,37 Luminescence lifetimes were also determined through a CM Sephadex C-25 column. The complex was by the time-correlated single photon counting (TCSPC) eluted with 1 M HCl, and the collected fractions were technique with a Tempro-01 apparatus from HORIBA Jobin rotaevaporated to dryness, dissolved in a minimum amount of Yvon (Glasgow, U.K.) using as the excitation pulse source an water, and precipitated with a concentrated aqueous solution ultrafast 450(±15) nm Nanoled operating at 250 kHz. of KPF6 (1 g in 5 mL of water). The mixture was kept in the Fluorescence intensity decay was fitted with the fluorescence refrigerator overnight and filtered; the obtained dark orange decay analysis software DAS6 of HORIBA Jobin Yvon by powder was stored in vacuum over P4O10 for 1 day. Yield: 46 deconvolution of the pulse function using a single-exponential mg (45%). Chemical analyses and NMR were coherent with model function. All measurements were performed at room the formula [Ru(4,4′-(OCH3)2-bpy)2(Mebpy-CN)](PF6)2. temperature in CH3CN solutions deoxygenated by bubbling (KPF6). Anal. calcd (found) for C36H33N7O4P3F18KRu: C, high-purity Ar (>99.98, Indura SRL, Argentina) for 20 min. 36.0 (35.9); H, 2.8(2.9); N, 8.2 (10.4). 1H-NMR (500.13 Computational calculations were carried out with Gaussian MHz, CD3CN) 2.58 (s, 3H), 4.01 (s, 12H), 6.93 (br, 2H), 03.38 Molecular geometries were optimized using Becke’s 7.01 (br, 2H), 7.32 (br, 1H), 7.44 (br, 4H), 7.66 (d, 5.9 Hz, three-parameter hybrid functional39 with local terms of Lee, 1H), 7.77 (d, 5.9 Hz, 1H), 7.98 (br, 4H), 8.49 (s, 1H), 8.76 (s, Yang, and Parr.40 For light atoms, a 6-311G(d,p) basis set41 1H). 13C-NMR (125.75 MHz, CD3CN): 20.1, 56.7, 111.0, was chosen, while for the ruthenium atom, LANL2DZ effective 113.7, 121.2, 125.5, 128.6, 149.8, 150.8, 152.2, 152.5, 158.1, core potential with its corresponding basis set was used.42 The 159.0, 167.1, 167.3. 15N-NMR (50.67 MHz, CD3CN): 225.2, 8104 https://dx.doi.org/10.1021/acsomega.0c00243 ACS Omega 2020, 5, 8097−8107 ACS Omega http://pubs.acs.org/journal/acsodf Article 226.6, 244.1, 269.4. Positive ESI-MS ion clusters at m/z: 873.9 data for DSCs are reported in this media,3 and no experimental {M − PF6}+. evidence of adsorption of this solvent on the ZnO surface has (iii) [Ru(4,4′-{N(CH3)2}2-bpy)2(Mebpy-CN)](PF6)2 (3). A been obtained. Double-sided tape (thickness of 60 μm) was mixture of 47 mg of [Ru(4,4′-{N(CH3)2}2-bpy)2Cl2]Cl used as an electrode separator. Solar cells were characterized (0.067 mmol), 14 mg of Mebpy-CN (0.071 mmol), and 0.2 using I−V (current−potential) curves and quantum efficiency mL of triethylamine was added to 10 mL of deaereated ethanol (QE) measurements as described previously.33 and heated to reflux under Ar for 4 h. The obtained dark red The performance of each DSC for solar energy conversion solution was rotaevaporated to a minimum volume, recrystal- was evaluated using the quantum efficiency/incident photon to lized with ether, and filtered. The solid was dissolved in 5 mL charge carrier efficiency (QE/IPCE) Newport measurement of water and precipitated with a concentrated aqueous solution kit, consisting of a 300 W ozone-free Xe lamp, chopper, of NH4PF6 (1 g in 5 mL of water). The mixture was kept in a monochromator, detector, and other accessories. The cells refrigerator overnight and filtered; the obtained dark red were irradiated using a “homemade” solar simulator, which was powder was stored in vacuum over P4O10 for 1 day. Yield: 26 assembled with a metallic vapor lamp (Metalarc Sylvania HIS- mg (36%). Chemical analyses and NMR were coherent with YHX 400 W). Considering the distance from the lamp and the formula [Ru(4,4′-{N(CH 3 ) 2 } 2 -bpy) 2 (Mebpy-CN)]- solar cell (15 cm), the polychromatic irradiance was estimated (PF6)2.H2O. Anal. calc (found) for C40H47N11OP2F12Ru: C, to be 130 ± 5 mW cm−2. 44.1 (44.1); H, 4.3(4.2); N, 13.4 (14.1). 1H-NMR (500.13 MHz, CD3CN): 2.56 (s, 3H), 3.13 (s, 12H), 3.17 (s, 12H), 6.49 (br, 2H), 6.63 (br, 2H), 7.03 (br, 2H), 7.23 (br, 2H), 7.29 ■ ASSOCIATED CONTENT * Supporting Information sı (d, 1H), 7.51 (br, 2H), 7.53 (br, 2H), 7.63 (d, 1H), 7.76 (d, The Supporting Information is available free of charge at 1H), 8.10 (d, 1H), 8.47 (s, 1H), 8.71 (s, 1H). 13C-NMR https://pubs.acs.org/doi/10.1021/acsomega.0c00243. (125.75 MHz, CD3CN): 20.1, 39.1, 105.8, 109.3, 121.1, 124.0, 124.9, 128.0, 148.2, 148.9, 149.5, 150.6, 152.3, 154.2, 156.6, Construction of DSCs, FTIR spectrum of complex 3, 1D 157.0, 159.4. 15N-NMR (50.67 MHz, CD3CN): 68.0, 204.1, and 2D NMR spectra and assignments of complex 3, 205.9, 252.0, 272.2. Positive ESI-MS ion clusters at m/z: 925.9 UV−vis absorption data of complexes 1−3, CV and {M − PF6}+, 390.3 {M − 2PF6}2+. DPV curves of complex 3, emission spectra of complexes FTO |ZnO NWs. For ZnO NW growth, glass-FTO 1−3, TD-DFT calculations and assignments of com- substrates (Sigma-Aldrich) of 2 × 1.5 cm were used. They plexes 1−3 and 3+, EDAX of ZnO NWs, UV−vis were initially cleaned in ultrasonic baths of ethanol and absorption spectra of bare and sensitized FTO |ZnO acetone for 3 min. Then, seeds of ZnO were made on glass/ NWs, and Raman spectra of sensitized FTO |ZnO NWs FTO using a spin-coating deposit of an ethanolic solution of (PDF). 20 mM zinc acetate dihydrate (spinning at 1500 rpm during 1 min) followed by drying at 125 °C for 1 min. This procedure was repeated 5 times, and finally the substrates were heated at ■ AUTHOR INFORMATION Corresponding Author 125 °C for 4 h to improve the seed adherence. In a second Néstor E. Katz − INQUINOA (UNT- CONICET), Instituto de step, ZnO NWs were synthesized by a solvothermal method: ́ Quimica ́ Fisica, ́ Facultad de Bioquimica, ́ Quimica y Farmacia, ZnO-seeded substrates were put in 25 mL polytetrafluoro- Universidad Nacional de Tucumán, T4000INI San Miguel de ethylene (PTFE) vessels, which were then filled with a solution Tucumán, Argentina; orcid.org/0000-0001-5576-2692; of distilled water (DW), hexamethylenetetramine (HMTA), Phone: (+54)(381)(4200960); Email: nkatz@ and zinc nitrate dihydrate (0.5 M, aqueous solutions). The fbqf.unt.edu.ar; Fax: (+54)(381)(4248169) vessels were placed inside a stainless steel autoclave and heated at 125 °C for 4 h. The autoclaves were then cooled down to Authors room temperature, and after an hour, each sample was taken Fernando F. Salomón − INQUINOA (UNT- CONICET), out of the vessel, rinsed with DW, and dried in air. Instituto de Quimicá ́ Fisica, ́ Facultad de Bioquimica, ́ Quimica y Characterization of the FTO|ZnO NWs was carried out Farmacia, Universidad Nacional de Tucumán, T4000INI San using a Zeiss Supra 55VP scanning electron microscope Miguel de Tucumán, Argentina (SEM). X-ray powder diffraction analyses between 30 and 40° Nadia C. Vega − INQUINOA (UNT- CONICET), Instituto de were carried out in an X-ray diffraction (XRD) RIGAKU Mini ́ Quimica ́ Fisica, ́ Facultad de Bioquimica, ́ Quimica y Farmacia, Flex 600 instrument using Cu Kα1 radiation. UV−visible, and Universidad Nacional de Tucumán, T4000INI San Miguel de Raman measurements of films were made in the equipment Tucumán, Argentina already described. Teodor Parella − Servei de RMN, Universitat Autonóma de Ru Complexes/ZnO Solar Cells. In order to assemble the Barcelona, Bellaterra E-08193, Spain; orcid.org/0000- DSCs, the FTO/ZnO NW samples were heated at 100 °C for 0002-1914-2709 60 min to remove traces of water and then immersed in Faustino Eduardo Morán Vieyra − INBIONATEC (UNSE- solutions of complexes 1−3 in CH3CN (C = 1.2 mM) for 24 h. CONICET), Universidad Nacional de Santiago del Estero, Samples were rinsed with DW and dried in air to obtain the 4206 Santiago del Estero, Argentina different sensitized electrodes. They were characterized by Claudio D. Borsarelli − INBIONATEC (UNSE-CONICET), Raman and UV−vis absorption techniques. Universidad Nacional de Santiago del Estero, 4206 Santiago del Solar cells were subsequently assembled with an area of 1 Estero, Argentina cm2 as described in Figure S1: a glass-FTO with a layer of Pt Claudia Longo − Institute of Chemistry, University of Campinas, (deposited by sputtering) was used as a counter electrode, and UNICAMP, 13083-970 Campinas, Sao Paulo, Brazil a solution 0.5 M KI/0.005 M I2 in acetonitrile was used as the Mauricio Cattaneo − INQUINOA (UNT- CONICET), electrolyte. Acetonitrile was chosen as the solvent since most ́ Instituto de Quimica ́ Fisica, ́ Facultad de Bioquimica, ́ Quimica y 8105 https://dx.doi.org/10.1021/acsomega.0c00243 ACS Omega 2020, 5, 8097−8107 ACS Omega http://pubs.acs.org/journal/acsodf Article Farmacia, Universidad Nacional de Tucumán, T4000INI San (14) Wisz, G.; Virt, I.; Sagan, P.; Potera, P.; Yavorskyi, R. Structural, Miguel de Tucumán, Argentina Optical and Electrical Properties of Zinc Oxide Layers Produced by Pulsed Laser Deposition Method. Nanoscale Res. Lett. 2017, 12, 253. Complete contact information is available at: (15) He, J. H.; Hsin, C. L.; Liu, J.; Chen, L. J.; Wang, Z. L. https://pubs.acs.org/10.1021/acsomega.0c00243 Piezoelectric Gated Diode of a Single ZnO Nanowire. Adv. Mater. 2007, 19, 781−784. Notes (16) Emanetoglu, N. W.; Gorla, C.; Liu, Y.; Liang, S.; Lu, Y. Epitaxial The authors declare no competing financial interest. ZnO Piezoelectric Thin Films for Saw Filters. Mater. Sci. Semicond. ■ ACKNOWLEDGMENTS We thank CONICET (Grant PIP-2015-098), FONCyT Process. 1999, 2, 247−252. (17) Vega, N. C.; Wallar, R.; Caram, J.; Grinblat, G.; Tirado, M.; LaPierre, R. R.; Comedi, D. ZnO Nanowire Co-Growth on SiO2 and C by Carbothermal Reduction and Vapour Advection. Nanotechnology (Grant PICT-2016-0553), and UNT (Grant PIUNT 26D/ 2012, 23, 275602. 620) for financial support. F.F.S. and N.C.V. thank CONICET (18) Grinblat, G.; Capeluto, M. G.; Tirado, M.; Bragas, A. V.; for fellowships. M.C., F.E.M.V., C.D.B., and N.E.K. are Comedi, D. Hierarchical ZnO Nanostructures : Growth Mechanisms members of the Research Career (CONICET). T.P. acknowl- and Surface Correlated Photoluminescence. Appl. Phys. Lett. 2012, edges financial support for this research provided by Spanish 100, 233116. MINECO (Project PGC2018-095808-B-I00). We thank Dr. (19) Vega, N. C.; Tirado, M.; Comedi, D.; Rodriguez, A.; Rodriguez, Miriam Pérez (UAB) for MS measurements, Dr. Mónica T.; Hughes, G. M.; Grovenor, C. R. M.; Audebert, F. Electrical , Tirado (FACET, UNT) for letting us use their facilities, and Photoelectrical and Morphological Properties of ZnO Nanofiber Dr. Carolina Navarro (FQByF, UNT) for XRD measurements. Networks Grown on SiO2 and on Si Nanowires. Mater. Res. 2013, 16, We also thank Bioq. Luciano Martinez and Ing. Hernán 597−602. Esquivel (CIME-CONICET-UNT) for SEM and EDS (20) Vega, N. C.; Marin, O.; Tosi, E.; Grinblat, G.; Mosquera, E.; Moreno, M. S.; Tirado, M.; Comedi, D. The Shell Effect on the measurements. ■ Room-Temperature Photoluminescence from ZnO/MgO Core/Shell Nanowires: Exciton-Phonon Coupling and Strain. Nanotechnology REFERENCES 2017, 28, 275702. (1) Grätzel, M. Dye-Sensitized Solar Cells. J. Photochem. Photobiol. C (21) Grinblat, G.; Bern, F.; Barzola-quiquia, J.; Tirado, M.; Comedi, 2003, 4, 145−153. D.; Esquinazi, P. Luminescence and Electrical Properties of Single (2) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; ZnO/MgO Core/Shell Nanowires. Appl. Phys. Lett. 2014, 104, von Zelewsky, A. Ru(II) Polypyridine Complexes: Photophysics, 103113. Photochemistry, Eletrochemistry, and Chemiluminescence. Coord. (22) Anta, J. A.; Guillén, E.; Tena-Zaera, R. ZnO-Based Dye- Chem. Rev. 1988, 84, 85−277. Sensitized Solar Cells. J. Phys. Chem. C 2012, 116, 11413−11425. (3) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye- (23) Zhang, Q.; Dandeneau, C. S.; Zhou, X.; Cao, G. ZnO Sensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663. Nanostructures for Dye-Sensitized Solar Cells. Adv. Mater. 2009, 21, (4) Sharma, K.; Sharma, V.; Sharma, S. S. Dye-Sensitized Solar Cells 4087−4108. : Fundamentals and Current Status. Nanoscale Res. Lett. 2018, 13, 381. (24) Vega, N. C.; Mecchia Ortiz, J. H.; Tirado, M.; Katz, N. E.; (5) Nazeeruddin, M. K.; Humphry-Baker, R.; Liska, P.; Grätzel, M. Comedi, D. ZnO Nanowire Sensitization with Ru Polypyridyl Investigation of Sensitizer Adsorption and the Influence of Protons on Complexes : Charge-Transfer Probed by Spectral and Relaxation Current and Voltage of a Dye-Sensitized Nanocrystalline TiO2 Solar Photocurrent Measurements. Mater. Res. Express 2018, 5, No. 075020. Cell. J. Phys. Chem. B 2003, 107, 8981−8987. (25) Mecchia Ortiz, J. H.; Vega, N.; Comedi, D.; Tirado, M.; (6) Nazeeruddin, M. K.; Péchy, P.; Renouard, T.; Zakeeruddin, S. Romero, I.; Fontrodona, X.; Parella, T.; Morán Vieyra, F. E.; M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Borsarelli, C. D.; Katz, N. E. Improving the Photosensitizing Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Grätzel, M. Properties of Ruthenium Polypyridyl Complexes Using 4-Methyl- Engineering of Efficient Panchromatic Sensitizers for Nanocrystalline 2,2′-bipyridine-4′-carbonitrile as an Auxiliary Ligand. Inorg. Chem. TiO2-Based Solar Cells. J. Am. Chem. Soc. 2001, 123, 1613−1624. 2013, 52, 4950−4962. (7) Bessho, T.; Yoneda, E.; Yum, J.-H.; Guglielmi, M.; Tavernelli, I.; (26) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Mixed Phosphine Imai, H.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. New 2,2′-Bipyridine Complexes of Ruthenium. Inorg. Chem. 2002, 17, Paradigm in Molecular Engineering of Sensitizers for Solar Cell Applications. J. Am. Chem. Soc. 2009, 131, 5930−5934. 3334−3341. (8) Pal, A. K.; Nag, S.; Ferreira, J. G.; Brochery, V.; La Ganga, G.; (27) Pannwitz, A.; Prescimone, A.; Wenger, O. S. Ruthenium(II)- Santoro, A.; Serroni, S.; Campagna, S.; Hanan, G. S. Red-Emitting Pyridylimidazole Complexes as Photoreductants and PCET Reagents. [Ru(bpy)2(N-N)]2+ Photosensitizers:Emission from a Ruthenium(II) Eur. J. Inorg. Chem. 2017, 609−615. to 2,2′-Bipyridine 3MLCT State in the Presence of Neutral Ancillary (28) Hansch, C.; Leo, A.; Taft, R. W. A Survey of Hammett “Super Donor” Ligands. Inorg. Chem. 2014, 53, 1679−1689. Substituent Constants and Resonance and Field Parameters. Chem. (9) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Rev. 1991, 91, 165−195. Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, (29) Maestri, M.; Armaroli, N.; Balzani, V.; Constable, E. C.; Cargill 737−740. Thompson, A. M. W. Complexes of the Ruthenium(II)-2,2’:6’,2’’- (10) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, terpyridine Family. Effect of Electron-Accepting and -Donating 338−344. Substituents on the Photophysical and Electrochemical Properties. (11) Ö zgür, Ü .; Hofstetter, D.; Morkoç, H. ZnO Devices and Inorg. Chem. 1995, 34, 2759−2767. Applications : A Review of Current Status and Future Prospects. Proc. (30) Dodsworth, E. S.; Lever, A. B. P. Correlation of Electronic IEEE 2010, 98, 1255−1268. Charge Transfer Transitions and Electrochemical Potentials. The (12) Rodnyi, P. A.; Khodyuk, I. V. Optical and Luminescence Bispyrazine(tetracarbonyl)molybdenum(0) System in Various Sol- Properties of Zinc Oxide ( Review ). Opt. Spectrosc. 2011, 111, 776− vents. Chem. Phys Lett. 1984, 112, 567−570. 785. (31) Wallin, S.; Davidsson, J.; Modin, J.; Hammarströ m, L. (13) Lee, J.-H.; Ko, K.-H.; Park, B.-O. Electrical and Optical Femtosecond Transient Absorption Anisotropy Study on [Ru- Properties of ZnO Transparent Conducting Films by the Sol − Gel (bpy)3]2+ and [Ru(bpy)(py)4]2+. Ultrafast Interligand Randomization Method. J. Cryst. Growth 2003, 247, 119−125. of the MLCT State. J. Phys Chem. A 2005, 109, 4697−4704. 8106 https://dx.doi.org/10.1021/acsomega.0c00243 ACS Omega 2020, 5, 8097−8107 ACS Omega http://pubs.acs.org/journal/acsodf Article (32) Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J. Application of the Energy Gap Law to the Decay of Charge-transfer Excited States. J. Am. Chem. Soc. 1982, 104, 630−632. (33) Mecchia Ortiz, J. H.; Longo, C.; Katz, N. E. Polypyridyl Ruthenium Complexes Containing Anchoring Nitrile Groups as TiO2 Sensitizers for Application in Solar Cells. Inorg. Chem. 2015, 55, 69− 72. (34) Keis, K.; Lindgren, J.; Lindquist, S.-E.; Hagfeldt, A. Studies of the Adsorption Process of Ru Complexes in Nanoporous ZnO Electrodes. Langmuir 2000, 16, 4688−4694. (35) Ishida, H.; Tobita, S.; Hasegawa, Y.; Katoh, R.; Nozaki, K. Recent Advances in Instrumentation for Absolute Emission Quantum Yield Measurements. Coord. Chem. Rev. 2010, 254, 2449−2458. (36) Cattaneo, M.; Fagalde, F.; Katz, N. E.; Borsarelli, C. D.; Parella, T. pH-Induced Luminescence Changes of Chromophore-Quencher Tricarbonylpolypyridylrhenium(I) Complexes with 4-Pyridinealda- zine. Eur. J. Inorg. Chem. 2007, 3, 5323−5332. (37) Cattaneo, M.; Fagalde, F.; Borsarelli, C. D.; Katz, N. E. Improvement of the Dynamic Range of pH Sensing by Using a Luminescent Tricarbonylpolypyridylrhenium(I) Complex with Three Different Protonation Sites. Inorg. Chem. 2009, 48, 3012−3017. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02. Gaussian, Inc.: Wallingford, CT, 2004. (39) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (40) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle- Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (41) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self- Consistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wave Functions. J. Chem. Phys. 1980, 72, 650−654. (42) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270−283. (43) Barone, V.; Cossi, M. Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model. J. Phys. Chem. A 1998, 102, 1995−2001. (44) O’Boyle, N. M.; Tenderholt, A. L.; Langner, K. M. Cclib: A Library for Package-Independent Computational Chemistry Algo- rithms. J. Comput. Chem. 2008, 29, 839−845. 8107 https://dx.doi.org/10.1021/acsomega.0c00243 ACS Omega 2020, 5, 8097−8107