Proton transfer from excited electronic states

The future is surely for direct dynamical examination of the individual elementary steps in such reactions. For proton transfer from excited electronic states this is already a reality, and we look forward to additional work along the same lines of inquiry for other archetypal reactions. 

Fig. 46. Scheme of optical transitions, explaining the dual fluorescence resulting from proton transfer in excited electronic state. 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

The photochemistry of imides, especially of the N-substituted phthalimides, has been studied intensively by several research groups during the last two decades [233-235]. It has been shown that the determining step in inter- and intramolecular photoreactions of phthalimides with various electron donors is the electron transfer process. In terms of a rapid proton transfer from the intermediate radical cation to the phthalimide moieties the photocyclization can also be rationalized via a charge transfer complex in the excited state. 

Irradiation of aromatic hydrocarbons such as phenanthrene, anthracene, naphthalene, and certain substituted naphthalenes in the presence of NaBH4 and m- or p-(NC)2C6H4 promotes a Birch-type photoreduction.The reaction seems to occur by electron transfer from the excited singlet state of the arene to the electron acceptor giving the arene radical cations, which are then reduced by the borohydride. Other reducing agents such as NaBH, NaBHjCN, and NaBH(OMe)3 have been found to be effective and all lead to different isomer ratios. In a mechanistically related reaction, both fluoren-9-ol and the corresponding acetate are reported to be photoreduced to the parent hydrocarbon in the presence of aliphatic amines. The products arise by photoinduced electron transfer followed by proton transfer from the amine. The yield depends on the structure of the amine and increases in the order primary < secondary < tertiary amine. In... 

Proton transfer is a fundamental process in both chemistry [1-3] and biology [4]. In particular, proton dissociation namely, proton transfer to solvent, from aromatic dye molecules in their excited electronic state [5] can be easily studied by virtue of their strong fluorescence signal [6]. The older fluorescence measurements did not possess time resolution It was only possible to obtain steady-state quantum yields under conditions of constant illumination [6]. The conventional interpretation of the experimental data assumed a chemical kinetic scheme, such as [3]... 

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