Excited state mechanisms

Experiments were carried out to determine if during the ac electrolysis the ligand isomerization requires the formation of the reduced and oxidized form (59). This would indicate an excited state mechanism. If the intermediate formation of the reduced or oxidized complex is sufficient to induce the isomerization, excited states are not required. First support in favor of a true epc was obtained by the results of the ac electrolysis of Re(trans-SP)2(CO)jCl in the presence of redox buffers. Tetramethyl-p-phenylenediamine (TMPD) was used as reductant and the paraquat cation (PQ +) served as oxidant. 

Additional evidence in support of an excited state mechanism was obtained by continuous potential step chronocoulometric experiments (59). When the electrode potential was stepped only over the oxidation potential of the complex at a frequency of 10 Hz a slow net oxidation took place. Potential steps involving only the reduction wave led to rapid net reduction but no ligand isomerization. The isomerization occurred only when the potential steps included both reduction and oxidation of the complex. Since the voltammograms of Re(trans-SP O Cl and Re(cis-SP (C0) C1 are virtually indistinguishable, the ligand isomerization was not accompanied by a potential change. 

As a further possibility the ac electrolysis may lead to other products than those of the photolysis. In this case an excited state mechanism is, of course, excluded. Although there is a certain similarity between the electronic structure of an excited state and the reduced or oxidized form of a molecule, they are not identical. Consequently, it is not surprising when photolysis and electrolysis do not yield the same product. Another reason for such an observation may be the different lifetimes. An excited state can be extremely short-lived. Non-reactive deactivation could then compete successfully with a photoreaction. The compound is not light-sensitive. On the contrary, the reduced and oxidized intermediates generated by ac electrolysis should have comparably long life times which may permit a reaction. The ac electrolysis of Ni(II)(BABA)(MNT) (BABA = biacetyl-bis(anil) and MNT - = disulfidomaleonitrile) is an example of this reaction type (63). 

Pyrolysis of acetylene to a mixture of aromatic hydrocarbons has been the subject of many studies, commencing with the work of Berthelot in 1866 (1866a, 1866b). The proposed mechanisms have ranged from formation of CH fragments by fission of acetylene (Bone and Coward, 1908) to free-radical chain reactions initiated by excitation of acetylene to its lowest-lying triplet state (Palmer and Dormisch, 1964 Palmer et al., 1966) and polymerization of monomeric or dimeric acetylene biradicals (Minkoff, 1959 see also Cullis et al., 1962). Photosensitized polymerization of acetylene and acetylene-d2 and isotopic analysis of the benzene produced indicated involvement of both free-radical and excited state mechanisms (Tsukuda and Shida, 1966). 

The rate of decomposition was directly proportional to the rate of light absorption and was directly proportional to the pressure. They found no effect of nitrogen on the reaction rate and they proposed a primary split to N and O atoms rather than the excited state mechanism,... 

The case of Pt(tpy)2 is particularly interesting since this complex is able to emit a relatively strong luminescence in fluid solution at room temperature, that is, under the experimental conditions used for photochemical studies. This rather unusual property for a Pt(II) complex has offered the opportunity for an attempt to elucidate the excited state mechanism of the photochemical reaction via comparative photochemical and luminescence quenching experiments [117]. 

The proposed excited state mechanism is schematized in Figure 12. Light excitation at 313 or 430 nm is followed by an almost complete internal deactivation to the luminescent 3MLCT excited state. However, small fractions of the LC and 3MLCT excited states are converted to a charge... 

Figure 12. Pictorial representation of the excited-state mechanism of the photoreaction of Pt(tpy)2 with CH2C12. From Ref, 117 with permission of American Chemical Society.

Irradiation of phenyl substituted diacetylenes yields both protonated products and reduction products. The photoreduction products seem to originate from the triplet excited state, since oxygen quenches the photoreduction almost completely. However, oxygen does not quench the polar addition reaction, supporting a singlet excited state mechanism for the formation of type B photoproducts (Cl protonation product). 

In Fig. 11, we characterize the excited-state mechanism in more detail, depicting the geometries associated with nonadiabatic transitions. For each such transition leading to population transfer to the ground electronic state, we... 

The cyclopropylcarbinyl-allylcarbinyl rearrangement of 2-arylcyclopropylcarbinyl acetates is well known thermally, but the first authentic photochemical case has only now been reported. Irradiation of the trans-isomer (655 R = Me or Bu X = H, OMe, or Cl) gives the cis-isomer (665b) and the ester (666), possibly by an ionic excited-state mechanism. 

Stabilization interrupts this sequence. Mechanisms include inhibition of sequence initiation by incorporating additives to screen UV energy, to preferentially absorb it, or to quench the excited state. Mechanisms also include inhibition of the propagation process by incorporating additives that will react chemically with the free radicals and hydroperoxides as soon as they are formed to render them harmless. 

FIGURE 27.4 The Fu6 hot excited state mechanism for cZc-previtamin D photochemistry in Sj (2A) (reprinted from Reference 27, with permission from the copyright owner, Elsevier Science, Oxford, UK). 

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