Photophysical processes, molecular

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. 

Our motivation for offering a further consideration of excimer fluorescence is that it is a significant feature of the luminescence behavior of virtually all aryl vinyl polymers. Although early research was almost entirely devoted to understanding the intrinsic properties of the excimer complex, more recent efforts have been directed at application of the phenomenon to solution of problems in polymer physics and chemistry. Thus, it seems an appropriate time to evaluate existing information about the photophysical processes and structural considerations which may influence excimer formation and stability. This should help clarify both the power and limitations of the excimer as a molecular probe of polymer structure and dynamics. 

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-... 

Because of the presence of a well-defined energy gap between the conduction and the valence band, semiconductors are ideally suited for investigation of the interfacial interactions between immobilized molecular components and solid substrates. In this chapter, interfacial assemblies based on nanocrystalline TiOz modified with metal polypyridyl complexes will be specifically considered. It will be shown that efficient interaction can be obtained between a molecular component and the semiconductor substrate by a matching of their electronic and electrochemical properties. The nature of the interfacial interaction between the two components will be discussed in detail. The application of such assemblies as solar cells will also be considered. The photophysical processes observed for interfacial triads, consisting of nanocrystalline TiO 2 surfaces modified with molecular dyads, will be discussed. Of particular interest in this discussion is how the interaction between the semiconductor surface and the immobilized molecular components modifies the photophysical pathways normally observed for these compounds in solution. 

In a more general sense, these observations show that upon immobilization of photoactive compounds onto a solid substrate a substantial difference is detected between the photophysical processes observed for the heterotriad and the dyad in solution. More importantly, direct injection from those moieties not directly bound to the oxide surface can be efficient - this is always fully realized and such an observation is important for the further development of real devices. As a result of this through-space interaction, no osmium-based emission is observed and injection from both the ruthenium and the osmium centers is faster than the laser pulse. An interesting observation is also that upon irradiation of the heterotriad Ti02-Ru-0s, only one final product, i.e. Ti02(e)-Ru(ll)0s(lll), is obtained. In view of the potential of these modified surfaces as potential molecular devices, this is an important feature. The presence of a rigid structure rather than a flexible one, as observed in the Ru-Rh case, clearly leads to a more uniform behavior. 

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 term energy transfer is used to describe a photophysical process in which an excited state of one molecular entity (donor, e.g. AB ) is deactivated to a... 

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. 

Photophysical processes include radiative transition in which excited molecule emits light in the form fluorescence or phosphorescence and returns to the ground state, intra-molecular non-radiative transitions, in which some of the energy of the absorbed photon ultimately gets converted to heat. 

Absorption of UV/VIS radiation in the solid state is different from UV/VIS absorption in the liquid or gaseous phase with respect to photophysical processes taking place in the crystal lattice and to the metallic, semiconductor (SC) or insulator properties of the absorbing solid (Bottcher, 1991). In crystals, multiple atomic or molecular orbitals are combined to form broad energy bands, i.e. a valence band (vb) fully occupied by electrons and a conduction band (cb) unoccupied or only partly occupied by electrons. Conduction bands and valence bands have different energetic positions relative to one another depending on the specific substrate. In a SC cluster, electronic transitions between the valence band and the conduction... 

Internal conversion A photophysical process. Isoenergetic radiationless transition between two electronic states of the same multiplicity. When the transition results in a vibrationally excited molecular entity in the lower electronic state, this usually undergoes deactivation to its lowest vibrational level, provided the final state is not unstable to dissociation. 

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. 

It has become apparent over the last few years that photophysical research has been largely concerned with the very detailed analysis of particular systems. The economics of research funding are reflected in the style and extent to which groups in di+ferent parts of the world are able to investigate specific problems. The application of photophysical processes in areas such as molecular electronics is still in an embryonic state and much more work needs to be done before substantial progress towards useful devices will be achieved. 

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