Electron-transfer mechanism transient lifetimes

We have obtained additional evidence supporting the electron transfer mechanism of fluorescence quenching in 2 and 2 from picosecond transient absorption and fluorescence measurements. The fluorescence lifetimes of 1-2 in butyronitrile are reported in Table II. These lifetimes are proportional to the observed fluorescence quantum yields of these compounds and therefore indicate that the observed fluorescence quenching is not due simply to a change in the radiative rate for emission. 

The use of short (fs) laser pulses allows even highly transient ion-radical pairs with lifetimes of t 10 12 s to be detected, and their subsequent (dark) decay to products is temporally monitored through the sequential spectral changes. As such, time-resolved (ps) spectroscopy provides the technique of choice for establishing the viability of the electron-transfer paradigm. This photochemical (ET) mechanism has been demonstrated for a variety of donor-acceptor interactions, as presented in the foregoing section. 

The lack of well-resolved kinetic data in support of the formation of [P -Ba -BOa] presented a dilemma, since the monomeric BChl is in van der Waals contact with the primary donor, whereas BO is further removed at a distance of 17 A. Rudolf Marcus examined the available kinetic data and reviewed two alternative mechanisms proposed for the reduction of the intermediate electron carrier, BChl. One was the super-exchange mechanism of electron transfer that implicates the existence of a virtually populated [P BA ]-state in the mediation of electron transfer between P and BOa and the other a two-step electron transfer to BO that can be kinetically resolved by an intermediary [P BA"]-state. In view of the lack of resolution of such a state in the data obtained by Martin and Breton , Marcus estimated that the putative [P Ba -BOa] state would have an upper lifetime limit of -1 ps. Of course, undertaking measurements of such a brief kinetic event in the neighborhood of a 3-ps event would demand substantially improved measuring techniques and procedures. With this in mind, Holzapfel et al. extended their femtosecond measurements to look for the intermediary [P -BA"]-state. Their measurements entailed the use of short excitation pulses (60/v), appropriate wavelengths for selective excitation of the primary electron donor P, high time resolution (-100 fs), and sufficiently low excitation intensity to avoid double photon excitation and consequently nonlinear processes. As the results summarized below show, these measurements provided new evidence for the existence of a kinetically resolvable, though extremely transient, intermediary [P -Ba -BCJa] state. 

The direct characterization of an eT mechanism requires a much more complicated technique time-resolved spectroscopy. The solution containing the system under investigation is irradiated by a laser pulse, and the absorption spectra of the solution are consecutively recorded at chosen and very short time intervals (e.g. every 10 ns). If, in the envisaged two-component system F1 M, an M-to-Fl eT process takes place upon illumination, one should be able to measure the absorption spectra of Fl and M" ", as well as their decay, which allows the determination of the lifetime of the transient species F1 M. It goes without saying that very sophisticated and expensive instrumentation is required to carry out this type of experiment. Moreover, the smaller the fluorophore lifetime and the faster the back-electron transfer process, the more rapid and expensive the data acquisition equipment required. In particular, narrow laser pulses and especially fast data collections are needed for systems such as 1, where a short-living polyaromatic fluorophore (anthracene, r = 5 ns) is linked to the electron donor (or acceptor) group by a rather short carbon chain. 

The photophysical and electron transfer properties of bacteriochlorophylls (Bchl) and bacteriopheophytins (Bpheo) found in the reaction centers of photosynthetic bacteria have been directly associated with the mechanism of charge separation which underlies photosynthesis [1]. The appearance of the Bpheo anion (Bpheo ) within 3-5 ps after excitation of the special pair of Bchl (P) is well documented from transient absorption spectroscopy [2-4]. The 200 ps lifetime of Bpheo which is primarily determined by the electron transfer process to a quinone also has been established by picosecond changes in absorption [5,6], Thus, the general kinetic time scale for the primary processes in bacterial photosynthesis has been determined by the transient differences in electronic state properties. 

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