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Chemistry

 

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1     Double-spaced (225 words)

 

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5486-Lab3F.docx
4013-CHEMISTRY116BLPEERREVIEWFORM.docx

 

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Description:

 

Peer review of student lab report. Attached is a lab report and a review sheet. Review the lab report and fill out the reviewers sheet, as discussed.

 

Lab #3 (Working Title)
Name
May 27, 2020
Abstract: This lab focuses on three parts-the synthesis of the ruthenium compound, Ru(bpy)32+, the electron transfer of the ruthenium compound, and oxygen sensing. Analytical techniques utilized in this lab, like luminescence quenching and the use of cyclic voltammetry, are used to monitor oxidation and reduction reactions.

Introduction:
Ruthenium complexes can be monitored using a wide variety of analytical techniques. Ru(bpy)32+ undergoes a metal to ligand charge transfer (MTLC) to Ru(bpy)32+* which can then undergo luminescence quenching. Ruthenium complexes also display redox activity in both the ground state, singlet state, as well as excited state, triplet state.[1] These factors make ruthenium complexes an ideal subject matter.
Methods:
Characterization of Ru(bpy)32+ uses electrochemical techniques; the techniques that are used are cyclic voltammetry and differential pulse polarography. Ru(bpy)32+ is also characterized by spectroscopy using UV-visible absorbance and emission spectroscopy. The cyclic voltammetry method can record and measure redox reactions. Compounds have their own unique absorption and UV-VIS spectrum. For Ru(bpy)32+ there are peaks at ~300 nm and ~650 nm.[2]
When going from an exited state to the ground state the compound must release that excess energy. Normally Ru(bpy)32+ would release its energy in the form of luminescence, but a quencher can also go through the electron transfers instead. Using a quencher allows one to monitor the redox reactions that occur.
Oxygen can also act as a quencher in the presence of Ru(bpy)32+. This means that an oxygen sensor can be used to monitor O2 levels. Oxygen sensing works in the same sense as luminescence quenching. The general apparatus consists of a tube with silicone rubber that has been soaked in ruthenium solution. Nitrogen and oxygen gases are passed over the film and the emission intensity is recorded. The following results must then be calibrated to get accurate results.
Materials and Equipment:
A. Synthesis and Characterization

Compound Amount Note
RuCl23H2O 0.5111g Corrosive
DMSO 1.5 mL Toxic
Acetone 20 mL Flammable
Diethyl Ether 1 mL Flammable
KBr 100-200 mg Irritating
BPY
2,2’-bipyridine 0.2663 g Toxic, Irritating
Ethanol 25 mL Flammable
Toluene Flammable
NH4PF6 0.5542 g Expensive

Equipment Model
Potentiostat PAR Versastat II
Electrodes BASi Glass WE &
AlCl RE
UV-Vis Spectrophotometer JASCO V-730
Spectroflourometer JASCO FP-8200

B. Luminescence Quenching

Compound Hazards
PTZ Toxic, Irritant
TMPD Irritant
Acetonitrile Flammable, Irritant
Ru[(bby)3]2+ Mildly toxic

Equipment Model
YAG 3rd Harmonic Generation INDI
Digital Storage Oscilloscope Wavesurfer 3200
Spectrofluorometer FP-8200

C. Oxygen Sensor

Equipment Model
Lock in Amplifier Stanford Research- SR510
Mass flow meter Alicat Scientific-M model

Experiment:
Experimental procedure was followed using the lab manual, Chemistry 116 Lab Manual.[3]
To synthesize Ru(DMSO)4Cl2, RuCl33H2O is combined with DMSO (dimethyl sulfoxide) and refluxed in anhydrous conditions for five minutes. When the reaction turns brown orange the reaction is complete. The solution is then transferred into a flask and the solution is reduced to 0.5-1.0 mL using nitrogen gas. Then dry acetone is added to the reaction separating it into two layers. The reaction is then cooled in an ice bath and yellow crystals should precipitate out. The solution should then be filtered and washed with acetone and diethyl ether. The compound, Ru(DMSO)4Cl2, should then be tested through IR and melting point to ensure that the correct compound was synthesized.
Ru(bpy)3(PF6)2 is synthesized from Ru(DMSO)4Cl2 and the ligand 2,2’-bipyridine (bpy). The ratio of this reaction is 1 Ru(DMSO)4Cl2:3 bpy. The two compounds are refluxed together overnight. The color change from yellow to red indicates that the reaction has taken place. The solution is then filtered and dried. Then the metathesis with ammonium hexafluorophosphate (NH4PF6) is done to get the final product. There should be five molar excess of NH4PF6. NH4PF6 is added when Ru(bpy)3Cl2 is dissolved in water and warm. A color change of red to orange should occur as well as the formation of a precipitate. After heating for an additional five minutes the solution is cooled and filtered to get the final product.
To prepare the samples for luminescence quenching 100mL sample solutions of ~1mM of the quenchers: phenothiazine (PTZ) , N,N,N’,N’-tetramethyl-pphenylenediamine (TMPD) or parachloronitrobenzene (PCNB) were prepared. Four 50mL volumetric flasks were prepared so that each of the flasks ranged from 0.00-0.1mM. The solutions were then diluted with 15-20mL of acetonitrile. To make sure that the solution is within the absorbance spectrum an aliquot of [Ru(L)3]2+ was added to one of the flasks and tested. If the added amount is acceptable add the same amount the rest of the flasks. The flask containing TMPD should be added under dimmed lights as it is photosensitive.
Then the solutions were tested by filling a cuvette with 2.5mL of stock solution, making sure everything is under the conditions stated earlier. The solutions’ fluorescence is then measured with the spectrofluorometer.
To prepare the oxygen sensor a thin film of silicon needs to be cured for a week prior to testing. The silicon rubber is placed between two sheets of Parafilm and left for about a week. After the silicon has cured it is soaked in a solution of methylene chloride and the ruthenium complex. Allow the solvent to evaporate and put the film into the cuvette. To test the sensor nitrogen or oxygen gas is passed through and the results are observed.
Data:
A. Characterization of Ru(bpy)32+

Fig 1. Cyclic Voltammetry

Fig 2. Differential Pulse Polarography

Fig 3. Absorption Spectroscopy

Fig 4. Emission Spectroscopy,
B. Electron Transfer of Ru(bpy)32+

Fig 5. Marcus Fit

Fig 6. PTZ Lifetime

Fig 7. PTZ Luminescence

Fig 8. TMPD Lifetime

Fig. 9 TMPD Luminescence

Quenching Kq
PTZ Lifetime 3972902.9
PTZ Luminescence 4011794.31344006
TMPD Lifetime 16538371727.6045
TMPD Luminescence 16641689.3100621

C. Oxygen Sensing of Ru(bpy)32+

Discussion:
Looking at the standard curves for both absorption and emission spectroscopy, pictured above, the peak values are at around 290 nm and 650 nm. The data from this lab also had peaks at similar values, 288 nm and 608 nm . By comparing the experimental data with that of the literature value it can be concluded that the product synthesized in lab is indeed [Ru(bpy)3](PF6)2.
The peak point for the cathode reaction in cyclic voltammetry is around -1.3 volts (Epc). This is for the reduction of the 2+ state to the 1+ state. While the peak point for the anode reaction is around -1.2 volts (Epa). This is for the oxidation of the 1+ state to the 2+ state. The E1/2 value is calculated by computing the difference between Epc and Epa then dividing by two. The E1/2 value is about 0.05 volts. Looking at the peaks for DPP the third peak is closest to this value at around -1 volt. The peak values were determined by manually picking it out. This results in inaccurate measurements and is the most likely reason for this discrepancy.
The two techniques used in luminescence quenching were lifetime and steady-state. Lifetime quenching is a measurement of how long the molecule spends in the excited state. On the other hand, steady-state is the intensity of the fluorescence. Looking at the graphs the R2 value is higher for the steady-state graphs. From this the steady-state technique is slightly more accurate than the lifetime method. The time spent in the excited state is affected by the concentration used in lab. Creating a solution that is consistent in concentration gives way to human error. These slight deviations may cause the smaller R2 value, thus making the results less accurate than steady-state analysis.
Looking at literature values for TMPD with ruthenium II complexes, 1.67x109M-1s-1, the values are similar.[4] Looking at literature values for PTZ in acetonitrile also gave similar results to the experimental data. The rate for the literature varied from compound to compound but the general basis was N x106 with ‘N’ being a positive integer.[5] This also implies that the experimental data obtained in this experiment is valid.
The calibration curve obtained in the oxygen sensing experiment is linear; this is a good indication that the data obtained was consistent. Since the percentage of oxygen in air is around 21% the signal value can be determined from the calibration curve. The calculation of this was 0.0911 which corresponds with the data obtained. Looking at the graph it seems that the sensor is more suited for lower levels of oxygen as the points further up the line stray from linear line. This conclusion is also concluded in a similar lab. [6]
Works Cited:
[1] David W. Thompson, Akitaka Ito,Thomas J. Meyer2, ; [Ru(bpy)3]2+* and other remarkable metal-toligand charge transfer (MLCT) excited states*
[2] Polydefkis Diamantis, Jerome Florian Gonthier, Ivano Tavernelli, Ursula Rothlisberger ; Study of the Redox Properties of Singlet and Triplet Tris(2,2′- bipyridine)ruthenium(II) ([Ru(bpy)3] 2+) in Aqueous Solution by Full Quantum and Mixed Quantum/Classical Molecular Dynamics Simulations
[3] L.E. Laverman, Chemistry 116 Lab Manual; University of California Santa Barbara
[4] Nathir A. F. Al-Rawashdeh,* Khaled Shawakfeh; Luminescence quenching of mixed-ligand ruthenium (II) complexes by different quenchers
[5] S. L. Larson, C. Michael Elliott, D. F. Kelley; Electron Transfer in Phenothiazine/Ru(bpy)32+ Donor−Chromophore Complexes; Inorganic Chemistry 1996 35 (7), 2070-2076
[6] Yan Xiong,, Jun Tan, Chengjie Wang, Ying Zhu,, Shenwen Fang, Jiayi Wu, Qing Wang, Ming Duan; A miniaturized oxygen sensor integrated on fiber surface based on evanescent-wave induced fluorescence quenching

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