Report on Synthesis, Characterization and Photochemistry of Tris | CHEM 116BL, Study notes of Chemistry

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 Synthesis, Characterization, and Photochemistry of Tris(4,7-diphenyl-1,10- phenanthroline)ruthenium(II) Bis(hexafluorophosphate) for the Preparation of a Thin Film Oxygen Sensor Barahona, Jenny* and Cena, Nico Department of Chemistry and Biochemistry, University of California Santa Barbara, Santa Barbara, CA, 93106-3025 ABSTRACT: [Ru(4,7-diphenyl-1,10-phenanthroline)3](PF6)2 was synthesized and characterized by structural, spectroscopic, and electrochemical methods. Bimolecular rate constants of 4.95×109 M-1 s-1 and 1.34×108 M-1 s-1 for the quenching of Ru(dpphen)32+* (dpphen is 4,7-diphenyl-1,10-phenanthroline) by PTZ and TMPD, respectively, have been determined. An efficient oxygen sensor was constructed by fixing the luminescent [Ru(dpphen)3](PF6) on silicone gel. The sensor was able to detect O2 even at low concentrations. INTRODUCTION: Photo-induced electron transfer reactions of Ruthenium polypyridyl complexes are particularly important in inorganic chemistry because they allow for the quantitation of compounds that interact energetically and quench the MLCT emission. One such compound is molecular oxygen. Absorption of light by Ruthenium polypyridyl complexes initiates the intramolecular electron transfer reaction that results in an electronically excited state, which gives a strong emission in the visible spectrum.1 The emission originates from a metal-to-ligand charge transfer (MLCT) state, which is essentially a triplet state (90%) and has a lifetime around 1µs at room temperature.1,2 4,7- diphenyl-1,10-phenanthroline (dpphen) ligands coordinate to Ruthenium and exhibit strong absorption bands in the visible light region of the spectrum; making [Ru(dpphen)3](PF6)2 an inorganic metal complex with an MLCT absorption that can be characterized by UV-Vis, fluorescence spectroscopy, and voltammetry. Long lifetimes of the excited states contribute to efficient excited-state reactivity pathways that are kinetically competitive.3 The radiative pathway takes the complex from its triplet MLCT orbital and results in the emission of light (luminescence). The non-radiative results in the conversion of energy to heat via vibrational cooling. The reactive pathway demands interaction with another species in solution via energy or electron transfer (bimolecular quenching) in order to deactivate the excited state back to the ground state of the Ruthenium complex. The reactive pathway was analyzed using Stern- Volmer kinetics to find the bimolecular quenching rate constant (kq) corresponding to Ru(dpphen) quenching in the presence of quenchers and oxygen. The results are of value in demonstrating how quencher rate data can be used to estimate excited-state potentials, which have important implications for the lifetime and electron-transfer properties of the excited state.4 The results are also important in the determination of the Barahona
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 Gibbs free energy (ΔG), which determines whether quenching of the MLCT emission of the Ruthenium complex will occur spontaneously. The highly luminescent Ruthenium complexes containing dpphen ligands are frequently used as a class of oxygen sensors.5 The following paper examines the synthesis, characterization, and photophysics of [Ru(dpphen)3](PF6)2 in preparation of an O2 sensor in order to detect the concentration of O2 in various systems. The different methods of characterization served to establish the structure, MLCT, lifetime of the excited state, determine whether the kq is competitive with fluorescence, and determine whether it is thermodynamically favorable. The results were then used to determine whether [Ru(dpphen)3](PF6)2 could be used to construct an O2 sensor that could detect O2 at low concentrations. EXPERIMENTAL METHODS: Instrumentation Infrared spectroscopy was performed using a JASCO FT/IR-4100 spectrophotometer and air as the background signal. Absorption spectra were recorded using a JASCO V-530 UV/Vis spectrophotometer. Luminescence measurements were determined using a JASCO FP-6300 spectrofluorometer. Cyclic voltammograms were obtained on a VersaStat II electrochemical workstation. Time resolved luminescence studies were performed on a Jarrell Ash model 82-140 spectrometer. Synthesis and Structural Characterization of Ru(DMSO)4Cl2 A 0.5076 g (1eq., 1.94 mmol) sample of ruthenium trichloride trihydrate (RuCl3•3H2O) was weighed and placed in a 10mL round-bottomed flask containing a magnetic stir bar. Argon gas was bubbled through 1.8mL (25.35 mmol) of dimethyl sulfoxide (DMSO) because O2 can act as a quencher. The degassed DMSO was added to the 10 mL round-bottomed flask to create a dark brown solution upon mixture with RuCl3•3H2O. The flask containing the solution was attached to a reflux condenser and allowed to reflux at 95°C for 5 minutes over a hot plate. The reaction resulted in a brown-orange solution. The solution was cooled and transferred to a 25mL Erlenmeyer flask using a Pasteur pipette. Argon gas was passed over the solution to reduce the volume to 1.0mL. 20mL of dry reagent grade acetone was added to the flask and the solution was allowed to cool in an ice bath for 25 minutes. Cooling the solution lead to the precipitation of pale yellow dichlorotetrakis (dimethyl sulfoxide) ruthenium (II) (Ru(DMSO)4Cl2) crystals (191.6 mg, 20.37% yield). The crystals were collected using a Hirsch funnel and washed with 1mL acetone and 1mL diethyl ether. The melting point was determined using a Melt-Temp hot-stage microscope (205°C m.p.) and an infrared spectrum was obtained using a KBr pellet. The pellet was prepared by grinding 0.1882 g dry IR grade KBr and 0.0052 g (3% by weight) of Ru(DMSO)4Cl2 crystals in a mortar. The powder was loaded into a press to form a 1mm deep layer and placed in the . (IR(KBr) (cm-1): ν(S=O) 1097 and1123 S-bonded; ν(S=O) 922 O- bonded as determined by Evans et al.6). Barahona
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 by placing a drop of silicone between the pieces of weighing paper and allowed to cure for 2 two nights. A small silicone rectangle of each thickness was cut out and soaked in a vial that contained 2mL dichloromethane (DCM) and 1mg [Ru(dpphen)3](PF6)2. After 10 mins, each film was removed, allowed to dry, and placed in separate pipettes. A stream of air, and varying amounts of N2 and O2 were passed through the pipette using a photodiode and SR 510 lock-in amplifier to measure to measure the emission intensity. RESULTS: Synthesis of Ru(DMSO)4(Cl)2 The melting point of the synthesized Ru(DMSO)4Cl2 was observed at 205°C, which is higher than the literature value of 193°C.6 IR analysis of the synthesized Ru(DMSO)4Cl2 revealed two strong bands with frequencies of 1097 cm-1 and 1123 cm-1 which can be attributed to the SO stretch of S-bonded DMSO6 and another peak at 922 cm-1 which can be attributed to the SO stretch of O-bonded DMSO6 (Figure 1). Figure 1. IR spectrum of Ru(DMSO)4Cl2 with key SO stretch peaks at 1123 cm-1, 1097 cm-1 and 922 cm-1. Spectroscopic Characterization of [Ru(phen)3](PF6)2 
 
 The absorption spectra of three concentrations of [Ru(phen)3](PF6)2 in CH3CN is reported in Figure 2. The blue absorption spectrum is for 0.034 mM, the red is for 0.017 mM, and the green is for 0.01133 mM [Ru(phen)3](PF6)2. The broad bands at 445 nm were assigned to a metal-to-ligand charge-transfer (MLCT) transition7 and used to determine the extinction coefficient. A graph of the absorbance vs. concentration of [Ru(phen)3](PF6)2 yielded an extinction coefficient of the MLCT band equal to 8749 M-1cm-1 at a λmax of 445nm as shown in Figure 2 inset. Figure 2. Absorption spectrum of [Ru(phen)3](PF6)2 in CH3CN at three different concentrations: 0.034mM (blue), 0.017mM (green), and 0.01133mM (red). Top-right inset is a graph of the absorbance vs. concentration of [Ru(phen)3](PF6)2. A graph of the emission spectrum of 0.017 mM [Ru(phen)3](PF6)2 was obtained and shown in Figure 3 by exciting at λmax. The emission spectrum exhibited a strong luminescence band at 595 nm. The E00 energy was estimated using the “10% rule”8 and determined to be 543nm, which was converted to 2.277 eV. Barahona
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 Figure 3. Normalized emission spectrum of 0.017 mM [Ru(phen)3](PF6)2 in CH3CN at λexc=445nm. Arrow denotes wavelength of 10% intensity of max (E00). Electrochemical Characterization: Cyclic Voltammetry and Differential Pulse Voltammetry of [Ru(phen)3](PF6)2 The cyclic voltammogram of Ru(phen)3](PF6)2 showed that it is reversible but only undergoes two redox reactions with reduction peaks at -1.36, and -1.485V over the range of +1.75 to - 2.25 V (Figure 4). The E1/2 potential value was determined to be -1.35 V. The differential pulse polarogram (Figure 4 inset) showed three reductions at -1.73 V, -1.45 V, and -1.3 V and an oxygen peak at -0.79 V. Figure 4. Cyclic voltammogram of [Ru(phen)3](PF6)2 in CH3CN with 0.1 M TBAH supporting electrolyte. Conditions: glassy carbon electrode; scan rate 200 mV/sec. Inset: Differential pulse polarogram of [Ru(phen)3](PF6)2 in CH3CN with 0.1 M TBAH supporting electrolyte. Arrow denotes the reduction potential (E1/2). Spectroscopic Characterization of [Ru(dpphen)3](PF6)2 The absorption spectra of three concentrations of [Ru(dpphen)3](PF6)2 in CH3CN is reported in Figure 5. The blue spectrum is for 0.010 mM, the red is for 0.0067 mM, and the green is for 0.00504 mM [Ru(dpphen)3](PF6)2. The broad bands at 436 nm were assigned to a metal-to-ligand charge-transfer (MLCT) transition7 and used to determine the extinction coefficient. A graph of the absorbance vs. concentration of [Ru(dpphen)3](PF6)2 yielded an extinction coefficient of the MLCT band equal to 38,872 M-1cm-1 at a λmax of 436nm as shown in Figure 5 inset.

 
 Figure 5. Absorption spectrum of [Ru(dpphen)3](PF6)2 in CH3CN at three different concentrations: 0.010mM (blue), 0.0067mM (green), and 0.00504mM (re d). Top-right inset is a graph of the absorbance vs. concentration of [Ru(dpphen)3](PF6)2. Electrochemical Characterization: Cyclic Voltammetry and Differential Pulse Voltammetry of [Ru(dpphen)3]
 (PF6)2 and Quenchers The cyclic voltammogram of Ru(dpphen)3](PF6)2 showed that it is reversible and undergoes several redox reactions with reduction peaks at -1.325, -1.41V, and -1.7V over the range of Barahona
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 +1.75 to -2.25 V (Figure 6). The E1/2 potential value was determined to be - 1.1975 V. The differential pulse polarogram (Figure 6 inset) showed three reductions at -1.24 V, -1.36 V, and -1.66 V. Figure 6. Cyclic voltammogram of [Ru(dpphen)3](PF6)2 in CH3CN with 0.1 M TBAH supporting electrolyte. Conditions: glassy carbon electrode; scan rate 200 mV/sec. Inset: Differential pulse polarogram of [Ru(dpphen)3](PF6)2 in CH3CN with 0.1 M TBAH supporting electrolyte. Arrow denotes the reduction potential (E1/2). The cyclic voltammograms of PTZ, TMPD, and PCNB (Figure 7) were all reversible with E1/2 potentials of 0.665V, 0.1625V, and -1.0675 for PTZ, TMPD, and PCNB, respectively, over the range of -0.250 to +1.
0 V. Figure 7. Cyclic voltammogram of PTZ, TMPD, and PCNB in CH3CN with 0.1 M TBAH supporting electrolyte. Conditions: glassy carbon electrode; scan rate 100 mV/sec. Quenching of [Ru(dpphen)3](PF6)2* by PTZ and TMPD Figures 8-9 show the emission spectra and linear Stern-Volmer plots of [Ru(dpphen)3](PF6)2 with added PTZ and TMPD quencher concentrations, respectively. The integrated peaks areas (I0) for Ru(dpphen)3](PF6)2 in the absence of PTZ and TMPD were 69430 and 83447, respectively. The bimolecular quenching constants (kq) were 4.95×109 M-1 s-1 for PTZ and 1.34×108 M-1 s-1 for TMPD. Figure 8. Emission spectra of [Ru(dpphen)3](PF6)2 in CH3CH in the absence and presence of PTZ quencher. Inset: Stern- Volmer plot for the emission quenching by PTZ. Figure 9. Emission spectra of [Ru(dpphen)3](PF6)2 in CH3CH in the absence and presence of TMPD quencher. Inset: Stern- Volmer plot for the emission quenching by TMPD. Figures 10-11 show the time- resolved fluorescence spectra and Stern- Barahona
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 Bis(hexafluorophosphate) was not synthesized. Photoluminescence helped to calculate the triplet energy (E00) of the excited state ([Ru(phen)3](PF6)2) complex. A graph of the emission spectrum of 0.017 mM [Ru(phen)3](PF6)2* (Figure 3) exhibited a strong emission band at 595 nm and an E00 energy of 543 nm, which converts to 2.277 eV. The single emission band indicated the purity9 of the [Ru(phen)3](PF6)2. This observation falls within range of previously reported E00 values for Ruthenium polypyridyl complexes.7 The E00 value was used to determine the reduction potential at the cathode and ultimately the Gibbs free energy. Electrochemical Characterization: Cyclic Voltammetry and Differential Pulse Voltammetry of [Ru(phen)3](PF6)2 The electrochemistry of the [Ru(phen)3](PF6)2 complex was studied in CH3CN by cyclic voltammetry and differential pulse voltammetry (Figure 4) to determine the half-wave potentials for the ruthenium complex. The diagnostic criteria usually employed to establish reversibility in cyclic voltammetry are that the separation of the anodic and cathodic peaks, ΔE (Epa- Epc) is equal to or less than 60 mV for a one-electron process and that the ratio of anodic to cathodic current, Ipa/Ipc is unity.10 While the obtained voltammogram is reversible and similar to analogues11, only two reduction (-1.36, and -1.485V) peaks were observed over the range of +1.75 to -2.25 V. This pattern is not common to the three reduction peaks observed for most Ruthenium polypyridyl complexes, where the redox orbitals are localized on the individual ligands12. Furthermore, the current observed at the cathode (26.4 µA) was not equal to current observed at the anode (24.8 µA), which is key10 to the voltammogram of a reversible process. The E1/2 potential of [Ru(phen)3](PF6)2 was calculated from cyclic voltammetry to be -1.35 V, which corresponds to reduction of ruthenium(II) to ruthenium(III).13 The differential pulse polarogram showed three reductions (-1.73 V, -1.45 V, and - 1.3 V) and an oxygen peak (-0.79 V). The electrochemical data obtained indicates that the bis form of [Ru(phen)3](PF6)2 was obtained instead of the tris form and therefore a different Ruthenium polypyridyl complex ([Ru(dpphen)3](PF6)2) was synthesized for purposes of ultimately making an O2 sensor. Synthesis of [Ru(dpphen)3](PF6)2 Orange colored Tris(4,7- diphenyl-1,10-phenanthroline) ruthenium(II) Bis(hexafluorophosphate) ([Ru(dpphen)3](PF6)2) crystals were synthesized as presented in Scheme 2. Scheme 2. Synthetic routes for the preparation of [Ru(dpphen)3](PF6)2 (i) RuCl3•3H2O (ii) Ru(dpphen)3Cl2 (iii) 5 NH4PF6 Spectroscopic Characterization of [Ru(dpphen)3](PF6)2 The absorbance of three different concentrations of [Ru(dpphen)3](PF6)2 was obtained via UV-Vis spectrophotometry (Figure 5). The sharp bands at 300 nm were assigned to the Barahona
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 internal π-π* transition of the dpphen ligands3 while the broad bands at 436 nm were assigned to a metal-to-ligand charge-transfer (MLCT) transition.7 A plot of the absorbance vs. the concentration of [Ru(dpphen)3](PF6)2 (Figure 5 inset) yielded an extinction coefficient of the MLCT band equal to 38,872 and a λmax of 436nm. The large extinction coefficient indicates how readily the compound absorbs light. The experimental extinction coefficient was far above the literary value of 29,500 M- 1cm-1 and a λmax of 460nm2 in a different solvent. Photoluminescence helped to calculate the triplet energy (E00) of the excited state ([Ru(dpphen)3](PF6)2) complex. The E00 energy was determined to be 562nm, which was converted to 2.20 eV. This observation falls within range of previously reported E00 values for Ruthenium polypyridyl complexes.7 The E00 value was used to determine the reduction potential at the cathode and ultimately the Gibbs free energy to determine whether quenching of the MLCT emission would be an exergonic reaction. Electrochemical Characterization: Cyclic Voltammetry and Differential Pulse Voltammetry of [Ru(dpphen)3](PF6)2 and Quenchers The electrochemistry of the [Ru(dpphen)3](PF6)2 complex was studied in CH3CN by cyclic voltammetry and differential pulse voltammetry (Figure 6) to determine the half-wave potentials for the Ruthenium complex. The diagnostic criteria usually employed to establish reversibility in cyclic voltammetry are that the separation of the anodic and cathodic peaks, ΔE (Epa- Epc) is equal to or less than 60 mV for a one-electron process and that the ratio of anodic to cathodic current, Ipa/Ipc is unity.10 The obtained voltammogram is reversible and similar to analogues11, with three reduction (- 1.325, -1.41, and -1.7V) peaks over the range of +1.75 to -2.25 V. This pattern is in accordance to the three reduction peaks observed for most Ruthenium polypyridyl complexes, where the redox orbitals are localized on the individual ligands.12 The E1/2 potential of [Ru(dpphen)3](PF6)2 was calculated from cyclic voltammetry to be -1.1975 V, which corresponds to reduction of ruthenium(II) to ruthenium(III).13 The differential pulse polarogram showed three reductions at -1.24 V, -1.36 V, and -1.66 V. The electrochemical data obtained indicates that the tris form of [Ru(dpphen)3](PF6)2 was obtained. The cyclic voltammograms of PTZ, TMPD, and PCNB were also recorded (Figure 7) to determine their E1/2 values, and ultimately the change in free energy for the redox reaction. All three quencher voltammograms proved to be reversible with E1/2 potentials at 0.665V, 0.1625V, and -1.0675 for PTZ, TMPD, and PCNB, respectively, over the range of -0.250 to +1.0 V. These E1/2 values correspond to the reduction potential at the cathode. Quenching of [Ru(dpphen)3](PF6)2 by PTZ and TMPD The quenching of the luminescence of the MLCT excited state of [Ru(dpphen)3](PF6)2 was achieved with added PTZ and TMPD quencher concentrations. Emission spectroscopy and time resolved photoluminescence helped to calculate the quenching rate constant (kq) for the deactivation of the MLCT excited state of Barahona
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 [Ru(dpphen)3](PF6)2. The kq values are a good indicator of the compound’s susceptibility to quenching. Figures 8-9 show the emission spectra and linear Stern-Volmer plots of [Ru(dpphen)3](PF6)2 with added PTZ and TMPD quencher concentrations, respectively. Addition of PTZ and TMPD both dramatically affected the emission intensity of and effectively quenched the excited-state Ru(dpphen)3](PF6)2 in CH3CN as shown by the reduction of the emission intensity spectra with increasing quencher concentration. The integrated peaks areas (I0) for Ru(dpphen)3](PF6)2 in the absence of PTZ and TMPD were 69430 and 83447, respectively.
 The data was plotted according to the Stern-Volmer equation, I0/I = 1 + kqτo[Q] where kq is the experimental quenching rate constant, τo is the excited-state lifetime for Ru(dpphen)32+, I0 is the intensity of light emitted at a fixed wavelength in the absence of quencher, and I is the emitted intensity in solutions with added quencher.4 Stern-Volmer quenching constants, KSV, of 31.96 mM for PTZ and 1966 mM for TMPD were obtained. The bimolecular quenching constants (kq) were 4.95×109 M-1 s-1 for PTZ and 1.34×108 M-1 s-1 for TMPD which are of the same order as literature kq values4 obtained for this complex in different quenchers of the order ×109 M-1 s-1. The large experimental kq values imply that the MLCT emission of [Ru(dpphen)3](PF6)2 can be readily quenched by PTZ, TMPD, and most important O2. Time-resolved fluorescence spectroscopy also helped calculate the quenching rate constant for the deactivation of the MLCT excited state of [Ru(dpphen)3](PF6)2. Figures 10-11 show the time-resolved fluorescence spectra and Stern-Volmer plots of [Ru(dpphen)3](PF6)2 with added PTZ and TMPD quencher concentrations, respectively. Addition of PTZ and TMPD both dramatically affected the lifetime intensity of the excited-state Ru(dpphen)3](PF6)2* in CH3CN as show by the reduction of the lifetime intensity with increasing quencher concentration. This again proves that the compound readily be quenched, making it a great compound for the construction of an O2 sensor. The radiative lifetimes for Ru(dpphen)3](PF6)2 in the absence of PTZ and TMPD were 6733 ns and 6651 ns, respectively; suggesting that it is a good complex to undergo quenching because it can remain in the excited state long enough to collide with a quenching molecule. The excited state is sufficiently long lived to be in thermal equilibrium with its surroundings and should have its own distinctive thermodynamic properties including oxidation and reduction potentials.4 The data was plotted according to the Stern-Volmer equation and gave KSV values of 33.30 mM for PTZ and 88.90 mM for TMPD were obtained. The calculated kq values were 4.75×109 M-1 s-1 for PTZ and 2.96×107 M-1 s-1 for TMPD. While the kq value obtained from PTZ quenching is of the same order as literature kq values11, the kq value obtained from TMPD quenching was far below literature values. Because the quenching rate constant itself does not give much insight into the mechanism of quenching,14 the photochemical studies performed helped determine the Gibbs free energy of the quenching reaction between the Ruthenium complex and the quencher (ΔG). According to the Marcus Theory of electron transfer, the rates of