Excited-state proton-electron simultaneous transfer

5 Excited-state proton-electron simultaneous transfer 

We here report a nonadiabatic electron wavepacket study on the excited state reaction in phenol that is hydrogen-bonded with ammonia clusters Ph OH (NH3) PhO [H (NHs) ] (7.42) 

Phenol has been interesting many chemists as a model system to study how photo-excited bases in DNA can deactivate to the ground state without resulting in mutation. Not only in such biology-oriented studies, this molecule shows many other interesting features when put in surrounding molecules, clusters, and solvents. Small ammonia clusters are frequently used in place of water-molecule clusters, because ammonia is more proton-attractive than water, and very extensive studies have been performed 

The studies so far made for this ESHAT employed the ab initio configuration interaction (Cl) and/or complete active space SCF (CAS-SCF) methods and examined only the two lowest coupled potential energy surfaces (and nuclear wavepacket dynamics simplified [228]). However, because of the fact that the relevant excited states are highly quasi-degenerate in reality, and in view of the complicated nature of electron dynamics associated with proton transfer as described above, it is worthwhile to re-examine the mechanism from the nonadiabatic electron wavepacket dynamics. 

It turns out that the found mechanism is not as schematically simple as the SDDJ model, and it is more appropriate to be referred to as coupled proton-electron transfer in excited state , rather than hydrogen-atom transfer . This mechanism further requires the study of recombination dynamics between the separated charges in clusters and solvents. 

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One may consider the relaxation process to proceed in a similar manner to other reactions in electronic excited states (proton transfer, formation of exciplexes), and it may be described as a reaction between two discrete species initial and relaxed.1-7 90 1 In this case two processes proceeding simultaneously should be considered fluorescence emission with the rate constant kF= l/xF, and transition into the relaxed state with the rate constant kR=l/xR (Figure 2.5). The spectrum of the unrelaxed form can be recorded from solid solutions using steady-state methods, but it may be also observed in the presence of the relaxed form if time-resolved spectra are recorded at very short times. The spectrum of the relaxed form can be recorded using steady-state methods in liquid media (where the relaxation is complete) or using time-resolved methods at very long observation times, even as the relaxation proceeds. 

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