The Preparation and Characterization of [Ru(bpy)3](PF6)2
Introduction
Photo-induced electron transfer reactions in metal complexes play an important role in inorganic chemistry. These electron transfer reactions are typically initiated by light absorption into charge transfer bands.1 A metal to ligand charge transfer (MLCT) is one type of reactive state that has been extensively studied. [Ru(bpy)3](PF6)2, where bpy is 2,2´-bipyridine, is an inorganic metal complex with an MLCT absorption in the visible region of the spectrum. These excited state complexes are of interest for their insight into the electron transfer process.
The synthesis of [Ru(bpy)3](PF6)2 utilizes Ru(DMSO)4Cl2 as a precursor, since it binds bidentate ligands more efficiently than readily available starting materials, such as RuCl3·3H2O. Once purified, the Ru(DMSO)4Cl2 is refluxed with 2,2´-bipyridine in the proper molar ratio to generate [Ru(bpy)3]Cl2.
Following its synthesis, [Ru(bpy)3](PF6)2
will be characterized by UV-vis and fluorescence spectroscopies,
as well as cyclic voltammetry. A Stern-Volmer analysis will be
performed using methyl viologen dichloride (PQ2+Cl2)
or phenothiazine (PTZ) as an electron transfer acceptor or donor,
respectively. This type of analysis involves a luminescence
experiment,2 in which a reaction mechanism is proposed. The
reaction mechanism involves competition between a unimolecular decay of A* - the excited
state of a luminescent compound, in this case Ru(bpy)32+ -
and a bimolecular quenching of A* by Q (phenothiazine):
k1
unimolecular decay of
A*:
A* --------->
product
[1]
kq
quenching of A* by Q: A* + Q
---------> quenching of
A*
[2]
It can be shown that:
[ F0f] / [Ff]
= 1 + kqtA[Q]
[3]
where: F0f is the initial
quantum yield of fluorescence in the absence of quencher, Ff
is the fluorescence following addition of some amount of quencher Q, [Q] is the
concentration of quencher, kq is the rate of quenching and tA
is the lifetime of species A.
A plot of the relative efficiencies of fluorescence
of A, in the presence (Ff) or absence (F0f) of
quencher Q, versus [Q]
will therefore result, according to equation [3], in a straight line, with a slope equal
to kqtA
and a y intercept equal to 1. The units of kq are
M-1 sec-1 for a bimolecular reaction. A plot with a straight line
and y intercept
equal to 1 would confirm the proposed mechanism. Stern-Volmer analysis will
therefore provide the quenching rate constant
for the electron transfer reaction. Finally, the CAChe molecular
modeling software will be used for theoretical analysis of the complex.
Experimental
Synthesis of [Ru(bpy)3](PF6)2
Method A. [Ru(bpy)3](PF6)2 from the DMSO Complex (1 lab period)
The DMSO complex of ruthenium that was synthesized in a previous laboratory will be used as a source reagent in this experiment. [Ru(bpy)3](PF6)2 is synthesized from Ru(DMSO)4Cl2 by refluxing with the ligand bipyridine, in a stoichiometric ratio slightly less than 1:3. A round-bottom flask is equipped with a magnetic stirring bar, into which 1.28 g of Ru(DMSO)4Cl2 and 1.5 g of 2,2´-bipyridine are placed. Purge 25 mL of reagent quality ethanol with nitrogen for 15 minutes, then add to the flask. The mixture is heated to reflux, with continuous stirring, for two hours. The solution changes in color from yellow to dark red.
The final step in the synthesis is the conversion of [Ru(bpy)3]Cl2 to its hexafluorophosphate (PF6-) salt by metathesis. Make a saturated solution of [Ru(bpy)3]Cl2 in distilled water. In a separate beaker, a five-fold molar excess of ammonium hexafluorophosphate is heated in 20 mL of distilled water. When the solution iswarm (not boiling), the latter solution should slowly be added to the former [Ru(bpy)3]Cl2 solution. The color of the solution changes from red to orange. Continue heating the resulting solution for 5 minutes (be careful not to bring the solution to a boil). The solution is then cooled in an ice bath. The crystals are collected by suction filtration and dried under vacuum.
Method B. [Ru(bpy)3](PF6)2 from RuCl3·3H2O (1 lab period)
An alternate method of preparing the complex is to start
from a suspension of RuCl3·3H2O (1.0 g) and 2,2´-bipyridine
(bipy, 3.2 g) in ethylene glycol (15 mL, purged with nitrogen first for 15 mintues).
Divide all three masses by one-half if there is not much ruthenium trichloride.
Reflux the mixture for 3 hours and allow to cool to room temperature.
Add 10 mL of a saturated aqueous solution of NH4PF6, and
an orange precipitate should form. Filter and leave to dry under vacuum.
In some cases, column chromatography may be necessary to purify the product.
Characterization and Write-up
1. Obtain a UV-Vis spectrum of Ru(bpy)32+ in acetonitrile. This will allow you to determine the lmax in the MLCT region of the spectrum (455-465 nm). You will also be able to determine the appropriate concentration for an acetonitrile stock solution of Ru(bpy)32+. The stock solution will be diluted to make five different solutions (25 mL each). Each solution will contain the same concentration of Ru(bpy)32+, with varying concentrations of quencher. Keep in mind that standard volumetric pipette sizes are 3 or 5 mL. When the absorbance is ~1 in the 200 nm region, the absorbance in the MLCT region will be 0.1. The lmax in the MLCT region will be the excitation wavelength in the fluorescence experiment.
2. Using equation [3], the ratio [F0f]/[Ff] can be set at 1.00, 1.25, 1.67, 2.50, 5.00 and 10.0, so that by back-calculation, you can obtain the concentration of quencher to be added to each solution. An approximate value of kq is 1 × 1010 M-1 sec-1, where kq is the rate of diffusion in acetonitrile. The lifetime, tA, of Ru(bpy)32+ may be obtained from the literature; it is 8.70 × 10-7 sec in acetonitrile.3
3. An acetonitrile stock solution (~1 × 10-3 M) of phenothiazine is prepared, based upon these calculations. This solution will be used to prepare your final samples for analysis. Your final solutions will consist of acetonitrile, a constant concentration of Ru(bpy)32+, and a variable concentration of quencher. Into each of five 25 mL volumetric flasks, you will pipet a convenient amount of quencher, such as 0, 5, 10, 15 or 20 mL.
4. The fluorescence experiment is performed using the MLCT lmax from the UV-Vis as the excitation wavelength and 500-900 nm is the emission wavelength range. A fluorescence spectrum is acquired for each of the five solutions. All five spectra may be printed on the same graph, with the individual concentrations designated. The area of each of the five peaks is calculated. These values are used for the ratio [F0f]/[Ff], with the sample in the absence of quencher as the numerator.
5. The [F0f]/[Ff] ratios are plotted against [Q] and the line of best fit is obtained. The y intercept is obtained and the slope is determined. Using the literature value for tA, the experimental value of kq may be calculated.
Iterative Component
Using the results obtained in the fluorescence experiment, discuss how you could modify the ruthenium complex, or the quencher, in such a way as to influence kq. Would your proposed modifications result in a larger or smaller rate of emission quenching? Be sure to justify your predictions concerning your modified molecule, based upon a comparison with [Ru(bpy)3](PF6)2.
Finally, a CAChe modeling project may be used to provide insight into the electronic structure of Ru(bpy)32+, including the molecular orbitals involved in the HOMO and LUMO, and the location of the electron density in the ground versus the excited state. Are these states singlet or triplet states? Since ZINDO will only optimize molecules containing 60 atoms or less, Ru(bpy)2(CN)2 may be used as a model compound. What are the effects of, for instance, additional substituents on the bipyridine ligand?
References
1. Kalyanasundaram, K., Photochemistry of Polypridine and Porphyrin Complexes, Academic Press Inc.: San Diego, 1992.
2. Turro, N. J., Modern Molecular Photochemistry, University Science Books: Mill Valley, California, 1991.
3. Jones, W. E., Jr.; Fox, M. A. J. Phys. Chem.
1994, 98, 5095.
Additional References
MLCT
1. Shriver, D. F.; Atkins, P.; Langford, C. H.,
Inorganic Chemistry, W.H. Freeman and Company: New York,
1994.