Excited state decay electron transfer

Intramolecular electron transfer in cytochrome c has been investigated by attaching photoactive Ru complexes to the protein surface. Ru(bpy)2(C03) (bpy = 2,2 -bipyridine) has been shown to react with surface His residues to yield, after addition of excess imidazole (im), Ru(bpy)2(im)(His) +. The protein-bound Ru complexes are luminescent, but the excited states ( Ru ) are rather short lived (r 100 ns). When direct electron transfer from Ru to the heme cannot compete with excited-state decay, electron-transfer quenchers (e.g., Ru(NH3)6 + ) are added to the solution to intercept a small fraction (1-10%) of the excited molecules, yielding (with oxidative quenchers) Ru ". If, before laser excitation of the Ru site, the heme is reduced, then the Fe to Ru + reaction (ket) can be monitored by transient absorption spectroscopy. The ket values for five different modified cytochromes have been reported (Ru(His-33), 2.6(3) xlO Ru(His-39), 3.2(4) xlO Ru(His-62), 1.0(2) x 10 Ru(His-... 

FIGURE 12.20 The chemical structure of two Rh-Ru dyads anchored to Ti02 for interfacial electron transfer studies. The Rh(dcb) groups oxidatively quenched the ruthenium MLCT excited state by electron transfer. About 40% of the reduced rhodium units injected electrons to form long-lived charge-separated states, Ti02(e )/Rh(III)-Ru(III), that decayed by back electron transfer to ground-state products on a microsecond to millisecond timescale. 

In the presence of redox couples confined to the hydrophobic liquid phase, photoinduced heterogeneous electron transfer can be effectively monitored by photoelectrochemical techniques under potentiostatic conditions. The photocurrent responses are uniquely related to specifically adsorbed porphyrins, as demonstrated by the photocurrent anisotropy to the angle of polarisation of the incident illumination (Section 4.3). Systematic studies of the photocurrent intensity as a function of the formal potential of the redox couple and the Galvani potential difference revealed that the dynamics of electron transfer are determined by the distance separating the redox species at the interface. Other processes including decay of the electronically excited state, back electron transfer, porphyrin regeneration and coupled ion transfer play important role on the dynamics of the photocurrent responses. 

The energy of an electronically excited state may be lost in a variety of ways. A radiative decay is a process in which a molecule discards its excitation energy as a photon. A more common fate is non-radiative decay, in which the excess energy is transferred into the vibration, rotation, and translation of the surrounding molecules. This thermal degradation converts the excitation energy into thermal motion of the environment (i.e., to heat). Two radiative processes are possible spontaneous emission, just like radioactivity, which is a completely random process where the excited state decays ... 

There are, however, other ways in which excited-state decay can be accelerated by other species, which cannot be classified as reversible chemical reactions. Such processes can be represented generally by (1.12), where a star denotes electronic excitation. The excited state of A is said to be quenched by B. If B is converted into an electronically excited state (B ) during the process, an overall transfer of electronic energy takes place between the excited and unexcited partners of the interaction. 

The theoretical results obtained for outer-sphere electron transfer based on self-exchange reactions provide the essential background for discussing the interplay between theory and experiment in a variety of electron transfer processes. The next topic considered is outer-sphere electron transfer for net reactions where AG O and application of the Marcus cross reaction equation for correlating experimental data. A consideration of reactions for which AG is highly favorable leads to some peculiar features and the concept of electron transfer in the inverted region and, also, excited state decay. 

The use of the terms adiabatic and non-adiabatic in this way leads to a source of confusion. Normally, in describing surface-crossing processes, a process which remains on the same potential curve is called adiabatic and in that sense every net electron transfer reaction is an adiabatic process. Processes which involve a transition between different states as between the two different potential curves in Figure lb are usually called non-adiabatic. Such processes have some special features and will be returned to in a later section dealing with the inverted region and excited state decay. 

As for electron transfer in the normal region, based on the results of time dependent perturbation theory, electron transfer in the inverted or excited state decay region is also determined by the... 

As for normal electron transfer, the vibrational overlap integral for excited state decay contains contributions from those normal modes for which AQe A 0, but the changes in bond distances are now between the excited and ground states. 

Nuclear electromagnetic decay occurs in two ways, y decay and internal conversion (IC). In y-ray decay a nucleus in an excited state decays by the emission of a photon. In internal conversion the same excited nucleus transfers its energy radia-tionlessly to an orbital electron that is ejected from the atom. In both types of decay, only the excitation energy of the nucleus is reduced with no change in the number of any of the nucleons. 

The fact that excited states of these complexes are not generally quenched by water or acid in solution suggests that barriers for the electron-transfer process render it too slow to compete with excited state decay. The redox chemistry of water is in fact complicated because transformations between stable forms (H20 - H2 + 1/2 02) involve multielectron steps. On the other hand, the one-electron reduction of H+ to H and the one-electron oxidation of H20 to H+ + OH require much more energy than that available to the excited complex103). In order to circumvent these difficulties, one can devise cycles in which the excited state undergoes a simple electron transfer reaction to yield a reduced or oxidized quencher which is able to react with water. For example, one could take into con-... 

Elaborate mechanistic schemes have been suggested for the principal rearrangements of cyclohexenone, 2,5-cyclohexadienone, and bicyclo-hexenone systems induced by w - tt excitation which are compatible with the experimental data outlined above. In essence, these mechanisms are based on the common concept that the complicated structural changes are initiated in an electronically excited state. For the appreciably complex ketones considered, reaction initiation in a vibrationally excited ground state produced by adiabatic ir n demotion is expected to be readily suppressed in solution by collisional deactivation. It has been pointed out that by this general concept the rearrangements provide a decay path for electronically excited states which allows transfer of minimal amounts of enei to the environment in each step. 

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