Ultrafast electronic-excitation energy transfer

A microscopic theory for describing ultrafast radiationless transitions in particular for, photo-induced ultrafast radiationless transitions is presented. For this purpose, one example system that well represents the ultrafast radiationless transaction problem is considered. More specifically, bacterial photosynthetic reaction centers (RCs) are investigated for their ultrafast electronic-excitation energy transfer (EET) processes and ultrafast electron transfer (ET) processes. Several applications of the density matrix method are presented for emphasizing that the density matrix method can not only treat the dynamics due to the radiationless transitions but also deal with the population and coherence dynamics. Several rate constants of the radiationless transitions and the analytic estimation methods of those rate... 

The events taking place in the RCs within the timescale of ps and sub-ps ranges usually involve vibrational relaxation, internal conversion, and photo-induced electron and energy transfers. It is important to note that in order to observe such ultrafast processes, ultrashort pulse laser spectroscopic techniques are often employed. In such cases, from the uncertainty principle AEAt Ti/2, one can see that a number of states can be coherently (or simultaneously) excited. In this case, the observed time-resolved spectra contain the information of the dynamics of both populations and coherences (or phases) of the system. Due to the dynamical contribution of coherences, the quantum beat is often observed in the fs time-resolved experiments. 

A powerful application of TRIR has been to use the z/(CO) and n(CN) vibrations of supramo-lecular coordination compounds to monitor electron- and energy-transfer processes in solution. For example, a combination of fast and ultrafast TRIR was used to show that excitation into the Re-MLCT transition of [(phen)(CO)3Re NC)Ru CN)(bpy)2] (phen= 1,10-phenanthroline) produces the Re-MLCT state, followed by energy transfer to the Ru-based MLCT state." For the related complex, (CO)5W(4,4 -bpy)W(CO)5 ns-TRIR was used to confirm that, on the IR... 

Abstract Photoinduced processes in extended molecular systems are often ultrafast and involve strong electron-vibration (vibronic) coupling effects which necessitate a non-perturbative treatment. In the approach presented here, high-dimensional vibrational subspaces are expressed in terms of effective modes, and hierarchical chains of such modes which sequentially resolve the dynamics as a function of time. This permits introducing systematic reduction procedures, both for discretized vibrational distributions and for continuous distributions characterized by spectral densities. In the latter case, a sequence of spectral densities is obtained from a Mori/Rubin-type continued fraction representation. The approach is suitable to describe nonadiabatic processes at conical intersections, excitation energy transfer in molecular aggregates, and related transport phenomena that can be described by generalized spin-boson models. 

The heterometallic system [(bpy)2Ru(234)] exhibits several intramolecular energy-transfer processes (i) ultrafast singlet-to-singlet transfer, (ii) fast triplet-to-singlet transfer and (iii) singlet-to-triplet transfer. Excitation into the Ru(bpy)3 " " domain is followed by rapid energy transfer to the triplet state of the Zn(porphyrin) fragment. There is no evidence for intramolecular electron transfer between the Ru(bpy)3 and Zn(porphyrin) units. 

With site-directed mutation and femtosecond-resolved fluorescence methods, we have used tryptophan as an excellent local molecular reporter for studies of a series of ultrafast protein dynamics, which include intraprotein electron transfer [64-68] and energy transfer [61, 69], as well as protein hydration dynamics [70-74]. As an optical probe, all these ultrafast measurements require no potential quenching of excited-state tryptophan by neighboring protein residues or peptide bonds on the picosecond time scale. However, it is known that tryptophan fluorescence is readily quenched by various amino acid residues [75] and peptide bonds [76-78]. Intraprotein electron transfer from excited indole moiety to nearby electrophilic residue(s) was proposed to be the quenching... 

A state-of-the-art description of broadband ultrafast infrared pulse generation and multichannel CCD and IR focal plane detection methods has been given in this chapter. A few poignant examples of how these techniques can be used to extract molecular vibrational energy transfer rates, photochemical reaction and electron transfer mechanisms, and to control vibrational excitation in complex systems were also described. The author hopes that more advanced measurements of chemical, material, and biochemical systems will be made with higher time and spectral resolution using multichannel infrared detectors as they become available to the scientific research community. 

Is Ultrafast Electron Injection Useful for Solar Energy Conversion While ultrafast injection is necessary for sensitizers with inherently short excited-state lifetimes, such as those based on iron polypyridyl compounds, it remains unclear whether ultrafast injection is necessary, or even desirable, for solar energy conversion. A recent example of trapping hot carriers provides some clues as to how ultrafast electron transfer might be exploited for enhanced energy conversion efficiency.123,124... 

The ultrafast PT, which occurs typically on time scales of 10 13-10 14 s will not be considered. Such transfers are observed in molecular systems in which the potential energy surface (PES) governing the proton motion is essentially barrierless but has different minima positions in different electronic states, so that the proton finds itself in an off-equilibrium position after electronic excitation and relaxes to the new equilibrium position. The contribution of tunneling may be disregarded and the rate of these processes does not depend very strongly on temperature. These reactions, which are of great current interest, are intensely studied by ultrafast laser spectroscopy and are reviewed elsewhere [16,17],... 

---