A new method for measuring intramolecular charge transfer.

The direct measurement of intramolecular electron transfer through detection of the electromagnetic (EM) waveform that is emitted during this process is reviewed. The waveform is detected in the time-domain via free-space electrooptic sampling and then related to the dynamics of the charge transfer event. Electronmagnetic generation from two systems, Betaine-30 in chloroform and DMANS in toluene, are studied to illustrate this technique. A finite-difference time-domain calculation with a time-dependent polarization is used to model the EM generation and propagation through the solution. This method is very general since the movement of charge itself generates the EM waveform, and is sensitive to charge transfer occurring on a 0.1-10ps timescale. The potential for studying the primary steps of charge transfer in photosynthetic bacteria is also discussed. Keywords: measuring intramolecular change transfer Introduction Charge transfer is a ubiquitous phenomenon. Examples include photosynthesis, redox chemistry, corrosion, photography, xerography, respiration, and many other fundamental processes. Before reviewing research aspects of charge transfer, it is worth briefly reminding ourselves of the scope and magnitude of these processes that involve charge transfer. Photosynthesis represents the most important form of solar energy imaginable. It allows plants to generate their mass from building blocks of primarily C[O.sub.2] and [H.sub.2]O, and over several hundred million years has led to immense, yet finite, stores of energy as fossil fuels. In addition, [O.sub.2] is generated as a by product which is also quite beneficial. Redox chemistry and electrochemistry have tremendous industrial importance, from electroplating, to batteries, to extraction of metallic aluminum from aluminum oxide. For example, it is estimated that about 1.8% of the USA's industrial energy supply is directed toward production of metallic aluminum. (1) Corrosion represents an important example of redox chemistry. The corrosion and subsequent repair or replacement of structures that are underground andlor underwater is a costly enterprise, in terms of both money and manpower. In fact, it is estimated that the annual costs of metallic corrosion are about 4% of the gross domestic product (GDP) of industrialized nations (which amounted to roughly $300 billion in the USA in 1995). (2,3) Photography and xerography each permeate our everyday lives, and even if they do not seem as significant as photosynthesis and corrosion, they deserve continued attention. Finally, respiration, which in some sense is the inverse of photosynthesis, is of obvious importance for all plants and animals. Clearly, further understanding of charge transfer, especially at a very fundamental level, is a worthwhile endeavor. Background Broader issues aside, the remainder of this review will focus on basic research. There has been a particularly strong interest in photo-induced charge transfer, because spectroscopic investigations can monitor changes that occur due to electron transfer (ET), and the process can be initiated at a well defined instant. Also, extremely high temporal resolution can be obtained with modem lasers, on the order of 10 fs (1 fs = [10.sup.-15] S), which allows the dynamics accompanying ET to be monitored in addition to kinetics. That is, conformational changes of molecules or proteins in response to ET can be observed. A generic depiction of photoinduced ET is: DA[[right arrow].sup.hv] [D.sup.*] A[[right arrow].sup.E][[D.sup.+] [A.sup.-]][[right arrow].sup.separation] [D.sup.+] + [A.sup.-] (1) where D is the donor, A is the acceptor, and [D.sup.*] is the photoexcited donor, and the hope is that the charge will remain separated after excitation. The donor and acceptor can be individual molecules or ions, leading to intermolecular ET, or they can be separate chromophores within a large molecule, leading to intramolecular ET. …