Marcus Theory, chemiluminescence

Kolasinski KW, Hartline JD, Kelly BT, Yadlovskiy J (2010) Dynamics of porous silicon formation by etching in HF + V2O5 solutions. Mol Phys 108 1033-1043 Kolasinski KW, Gogola JW, Barclay WB (2012) A test of Marcus theory predictions for electroless etching of silicon. J Phys Chem C 116 21472-21481 Kooij ES, Butter K, Kelly JJ (1998) Hole injection at the silicon/aqueous electrolyte interface a possible mechanism for chemiluminescence from porous silicon. J Electrochem Soc 145 1232-1238... 

The experiment is conducted by pulsing the potential applied to an electrode between the oxidation and reduction potentials of Ru(bpy)3 The efficiency of the electrogenerated chemiluminescence from Ru(bpy)3 is close to 100%, with the reaction being so exergonic that it falls in the inverted region predicted by Marcus theory. 

In a series of papers between 1956 and 1965, Marcus solved much of the mystery by outlining a description of the probability of fluctuations in the geometry of reactants and their solvents. These fluctuations lead to changes in the energy barriers that the reactants must surmount before an electron can be transferred from one molecule to another. Marcus extended the theory to other systems, such as electrochemical rate constants at electrodes, and to chemiluminescent electron transfer reactions. The by-now famous inverted effect is a consequence of his theory after a certain point, adding more energy to an electron transfer reaction actually slows the process. Scientists believe photosynthesis can occur because of the inverted effect. 

As mentioned above, the formation of excited states in chemical reactions may be understood in the context of an electron transfer model for chemiluminescence, first proposed by Marcus [2]. According to this model the formation of excited states is competitive with the formation of the ground state, even though the latter is strongly favored thermodynamically. Thus, understanding the factors that determine the electron transfer rate is of considerable importance. The theory of electron transfer reactions in solution has been summarized and reviewed in many reviews (e.g., [30-36]). Therefore, in this chapter the relevant ideas and equations are only briefly summarized, to serve as a basis for description of the ECL experiments. 

An indirect consequence of the considerable interest in Marcus s theory is that, as brilliantly suggested by Marcus himself in 1965, the inverted region is responsible for most chemiluminescent effects. Indeed, as shown in Fig. 36.30, when the Vr and Vp curves intersect with a high activation barrier AG because of the inverted region effect, there may be an electron transfer to a more easily accessible Vp curve. In this case, one of the products is electronically excited and intersects the Vr curve in the normal region with a low activation barrier. 

The title paper was enormously important by itself, but in addition it was the first step (and the cornerstone) in a long series of papers on electron-transfer reactions which were published by Marcus from 1956 to 1965. During those years he extended [3, 4] the theory to include, for instance, intramolecular vibrational effects, numerically calculated rates of self-exchange and cross reactions, electrochemical electron-transfer reactions (i.e. including electrodes), chemiluminescent electron transfers, the relation between nonequilibrium and... 

The strongly exothermic transfer of electrons between fluorescent organic molecules represents one of the most general mechanisms in chemiluminescence [50, 51]. It can be found in electroluminescence, radical ion annihilation and peroxide decomposition. The basic concept was introduced by Hercules [39] following the general theory of Marcus [52]. The reaction co-ordinate can be roughly indicated by the potential energy curves shown. 

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