Picosecond spectroscopy laser-induced electron transfer

The current detailed understanding of photo-induced electron transfer processes has been advanced dramatically by the development of modern spectroscopic methods. For example, the application of time-resolved optical spectroscopy has developed from modest beginnings (flash-phyotolysis with millisecond resolution) [108,109] to the current state of the art, where laser spectroscopy with nanosecond resolution [110-113] must be considered routine, and where picosecond [114-116] or even femtosecond resolution [117] is no longer uncommon. Other spectroscopic techniques that have been applied to the study of electron transfer processes include time-resolved Raman spectroscopy [118], (time resolved) electron spin... 

Intermolecular Photo-Induced Electron Transfer. Picosecond absorption spectroscopy has also been applied recently to studies of intermolecular electron transfer on the picosecond time scale.(34) As in the previously described study of rhodopsin, the photo-induced intermolecular electron transfer between chloranil (CHL) and the arenes, naphthalene (NAP), 9,10-dihydrophenanthrene (DHP), and indene (IN) was studied by means of picosecond absorption spectroscopy which utilizes an OMCD. Difference absorption spectra of samples of CHL and one of these particular arenes in acetonitrile were measured at selected delay times after excitation at 355-nm with 25-ps FWHM laser pulses. These picosecond spectroscopic studies revealed information about the mechanism of intermolecular electron transfer and subsequent radical ion formation that was not possible in previous spectroscopic studies performed on the nanosecond (35-37) and microsecond (38,39) time scales. 

Picosecond spectroscopy has been used to study the photodissociation of Ij-aromatic complex and the formation of iodine atom-aromatic complexes. Shizuka, Nakamura, and Morita have studied the anion-induced fluorescence of aromatic molecules. Electron transfer from the anion to the excited aromatic seems to be the key step (Table 20). Triplet formation has also been studied by nanosecond laser spectroscopy. The rate constants are shown in Table 21. 

Deactivation of 2-naphthylamine singlet state by pyridines in enhanced by dipole moment and the ability to form hydrogen bonds. Picosecond laser spectroscopy shows charge transfer from the excited amine. The fluorescence of 2-iV,A -dimethylaminopyridine induced by p-nitroaniline is also caused by exciplex formation. The latter enhances triplet population of p-nitroaniline. The quenching of the fluoresence of carbazole and some derivatives by trichloroacetic acid and related compounds in fluid solutions has been studied by Johnson.A charge-transfer interaction is involved and the basicity of carbazole and derivatives determined. Charge transfer is also involved by quenching of carbazole by halocarbons. The A -isopropylcarbazole-dimethylterephthalate exciplex has been observed in PMMA films.Photoinduced electron-transfer in the p-phenylenediamine-paraquat complex yields the paraquat cation. ... 

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... 

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