Determination Of Electron Transfer Rate By Fluoresence Quenching Of Ruthenium(Ii) Tris Bipyridine - Laboratorio Quimica Inorganica

Determination of Electron Transfer rate by Fluoresence quenching of ruthenium(II) Tris Bipyridine



ABSTRACT: The rate (kET) of the electron transfer process of Ruthenium (II) tris(2,2’-bypridine) was analyzed and quantified by using an electron transfer quencher Copper (II) sulfate. Solutions contained constant concentrations of Ru(bpy) (10-5M) and varied concentrations of Cu2+ (OM-.2M).Using fluorometer, Intensities(I) of excitation and photoluminescence decreased as Cu2+ concentra-tions increased. By plotting the ratio of the original Intensity (Io,intensity of solution without quencher) and the intensity of solutions with quencher vs. concentration of Cu2+, kET (1.57x108M-1s-1) was calculated. Experimental kET was found to be two orders of magnitude less than the kET(5.68x106M-1s) from literature value .
Introduction
Molecules, when exposed to electromagnetic radiation absorb energy. This energy excites the electrons of a molecule. Then the energy is released when electrons go back to their ground state. Compounds such as Ruthenium(II) Trisipyridine behave differently when exposed to electromagnetism because they have different photo physicalproperties(Figure1). Ru(bpy)32+, a luminiphore, is a heavily studied compound because its ability to undergo in a wide variety of energy transfer process. For example, Ru(bipy)32+ in solution and in precence? of light (hv) will go to an excited state (1) and then go back to its ground state. (2) For it to go back to ground state or “deactivate” it releases energy as both radiative (photoiluminesnce) decay andnon-radiative decay (heat). The radiative decay is known as photoiluminesence and, the photoluminescence intensity is labeled as I.6

RuII(bpy)32+ + hv - - - - - -> RuIII(bpy-)(bpy)2 2+ (1)

RuIII(bpy-)(bpy)2 2+ -------- > RuII(bpy)32+ + hv + (2)

Interestingly, when in the presence of what is known as an electron transfer quencher, the energy or electrontransfer of an excited Ru(bpy)32+* takes a different direction. The excited molecule, instead of releasing the energy as radiative decay, transfers its excited electron to the electron transfer quencher (3). Later it will be discussed the effects of quencher in solution and how it affects the photoluminescence intensity I. In this experiment Cu2+ was used as the quencher. Yet, by measuring howthe intensities change in relation to the concentration of the quencher present, the rate of electron transfer (kET) can be calculated. This method is known as the Stern-Volmer Analysis. This method basically demonstrates that by plotting concentration of quencher vs. ratio of Intensity without quencher (Io) and Intensity of solutions with different concentrations of quenchers, kET can becalculated.1

Figure1 Structure of Ruthenium (II) tris Bipyridine
*Strcutres were done on “SkectchEl molecule draw”, not retrieved in literature


RuII(bpy)32=* + Cu2+ ---> RuII(bpy)33+ + Cu+ K12 (3)

RuII(bpy)32=*+RuII(bpy)32 -- >RuII(bpy)32 +RuII(bpy)32=* k11 (4)

Cu2+ + Cu+ ---- > Cu+ + Cu2+ k22 (5)Marcus Theory is a theory developed by no other than Rudolph A. Marcus. The theory elucidates the rate of electron transfer (ket) from one compound to another by the simple use of the relationship of two parabolas. 5Figure2. The parabolas are the result of plotting Free energy G and reaction coordination. G of activation is defined as the difference between the lowest minima and the intersection ofparabolas and, G is the difference in the two minimas. Marcus also proved that by using the self exchange coefficients (k11k22) from each species (4,5) and the equilibrium constant of the overall reaction (K12) (3). As it can be seen, he discovered a relationship between the electron transfer rate (kET=k12) and the coefficients already mentioned.5

kET=k12=(k11k22K12f)1/2...