Molecular photophysical/chemical processes

Within the context of supramolecular devices, re-emission of the radiation by luminescence is of interest in sensing and signalling applications, while chemical reactions are of interest in applications such as molecular switches and photocatalysis. Strictly speaking absorption and re-emission type processes are termed molecular photophysics while light-induced chemical reactions or chemical processes are termed photochemistry . 

It should be stressed that the wave-packet picture of photophysical relaxation and photochemical reaction dynamics described in this chapter is substantially different from the traditional concepts in this area. In contrast to the established picture of radiationless transitions in terms of interacting tiers of zero-order molecular eigenstates, the dynamics is rationalized in terms of local properties of PE surfaces such as slopes, barriers and surface intersections, a view which now becomes widely accepted in photochemistry. This picture is firmly based on ah initio electronic-structure theory, and the molecular relaxation d3mamics is described on the basis of quantum mechanics, replacing previously prevaUing kinetic models of electronic decay processes. Such a more detailed and rigorous description of elementary photochemical processes appears timely in view of the rich and specific information on ultrafast chemical processes which is provided by modern time-resolved spectroscopy. " ... 

The term photophysics designates all the physical processes that occur upon interaction of electromagnetic radiation with matter. In the case of molecular photophysics this means all those phenomena which are connected and follow the absorption of photons from a molecular system. Closely related to molecular photophysics is also photochemistry, which is the branch of chemistry that studies photo-induced chemical processes [62]. 

The study of molecular systems using quantum mechanics is based on the Born-Oppenheimer approximation. This approximation relies on the fact that the electrons, because of their smaller mass, move much faster than the heavier nuclei, so they follow the motion of the nuclei adiabatically, whereas the latter move on the average potential of the former. The Born-Oppenheimer approximation is sufficient to describe most chemical processes. In fact, our notion of molecular structure is based on the Born-Oppenheimer approximation, because the molecular structure is formed by nuclei being placed in fixed positions. There are, however, essential nonadiabatic processes in nature that cannot be described within this approximation. Nonadiabatic processes are ubiquitous in photophysics and photochemistry, and they govern such important phenomena as photosynthesis, vision, and charge-transfer reactions. 

A photoexcited process that, while exhibiting no chemical change, leads to different states of a molecular entity by way of radiation and radiationless transitions. Light absorption is a photophysical process, as is fluorescence. 

The photochemical and photophysical processes discussed above provide illustrations and incentives for further studies of photoeffects brought about by the formation of supramolecular species. Such investigations may lead to the development of photoactive molecular and supramolecular devices, based on photoinduced energy migration, electron transfer, substrate release, or chemical transformation. Coupling to recognition processes may allow the transduction of molecular infor-... 

Review of literature concerning the photochemistry of inorganic compounds shows us that a substantial progress was achieved during the past two decades in understanding the photophysical and photochemical properties of nanometer semiconductor particles [1-4] and structurally organized semiconductor materials, including the nanostructured semiconductor films, mesoporous molecular sieves [5 - 10] etc., and, also in the elaboration of physical and chemical techniques for their synthesis and examination of their photocatalytic activity in various chemical and electrochemical redox-processes. 

Photoexcitation by photon absorption and subsequent events that lead from one to another state of a molecular entity through radiation and radiationless transitions without any chemical change are called photophysical processes. The processes are classified as radiative and radiationless ones, depending on the photon emission (or absorption) and energy loss without any photon emission according to the kinetic aspects the monomolecular (spontaneous) and bimolecular (quenched) processes are distinguished (see Figure 4.1). 

The deactivation of an excited molecular entity through a non-radiative process can also occur as a result of an external environmental influence. A molecular entity that deactivates (quenches) an excited state of another molecular entity, by energy transfer, electron transfer, or a chemical mechanism is called a quencher [29]. Photophysical processes (energy or electron transfer) were described in Chapter 4 the present discussion is confined to the chemical consequences of quenching. 

The excited-state species can also reduce its energy by transfer to another molecule (quencher), which may diffuse it or change it into chemical energy. The process by which a photochemical or photophysical alteration occurs in one molecular entity, as a result of initial absorption of radiation by another molecular entity, is called photosensitization. 

Photophysical processes Photoexcitation and subsequent events which lead from one to another state of a molecular entity through radiation and radiationless transitions. No chemical change results. 

This composite satellite image displays areas on the surface of the Earth where chlorophyll-bearing plants are located. Chlorophyll, which is one of nature s most important biomolecules, is a member of a class of compounds called porphyrins. This Glass also includes hemoglobin and cytochrome c, which is discussed in Feature 19-1. Many analytical techniques have been used to measure the chemical and physical properties of chlorophyll to explore Its role in photosynthesis. The redox titration of chlorophyll with other standard redox couples reveals the oxidation/ reduction properties of the molecule that help explain the photophysics of the complex process that green plants use to oxidize water to molecular oxygen. 

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