Photophysical Fundamentals

The results of photoexcitation may be divided into three broad categories  

Re-emission of the absorbed energy as light (fluorescence or phosphorescence). 

Chemical reaction of the excited state (secondary charge separation, isomerisation, disassociation etc.). 

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 . 

In general, in complex ground states, the Ol and Tti ligand orbitals are full. The orbitals (the tjg set in 

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Laser-induced fluorescence has proven to be the key to these pioneering studies of transuranic hexafluoride electronic state photophysics and photochemistry. This is a research area of unique opportunity in which fundamental and technical research interests strongly converge. 

Metal dendrimers containing photoactive units exhibit very interesting properties, both from a fundamental and applicative viewpoint. For example, well-designed photoactive dendrimers can play the role of artificial antennae in supra-molecular structures devoted to solar energy conversion [37,47]. Photophysical... 

From the above discussion, it should be possible to appreciate how extremely subtle differences in guest-host interactions in the ground and excited states of Cu and Ag atoms and dimers in both non-reactive and reactive supports can lead to dramatically distinct chemical reactivity patterns and dynamical processes. Photochemical and photophysical phenomena of this kind should provide chemists of the 21st century with a rich field for fundamental and applied research, offering considerable scope for experimental challenges and intellectual stimulation. 

Based upon the fundamental photophysical transitions outlined in Figure 4, a set of elementary chemical reactions may be written, as shown below. These... 

The U.S. - Australia Symposium on Radiation Effects on Polymeric Materials contained research presentations on fundamental radiation chemistry and physics as well as on technological applications of polymer irradiation. This paper represents a hybrid contribution of these two areas, examining a field of extensive technological importance. Spin casting of radiation sensitive polymer resists for microelectronic fabrication was studied using photophysical techniques that are sensitive to the fundamental radiation response in the ultraviolet range. 

It should be emphasized that best design (each application corresponding to a particular design), proper choice, and correct use of fluorescent probes require a thorough knowledge of the basic phenomena involved in ion recognition medium effect on complexation equilibrium, fundamental photophysical processes, and possible changes from other causes than complexation. 

In spite of the numerous studies on practical applications of luminescence sensors, the understanding of the fundamental primary processes and underlying photophysics is still in its infancy. Only when details of the types of sites occupied by the sensor molecules, the local environment, and the quenching processes are understood and correlated with a variety of sensor molecules and supports can new materials and supports be rationally designed. 

As mentioned in the introduction, the ability to shape femtosecond laser pulses with unprecedented precision is the key to efficient control of photophysical and photochemical processes at the quantum level. In this section, we present the fundamentals of femtosecond pulse shaping and introduce specific pulse shapes that are used in the experiments and simulations presented in the following sections. We start with the electric field of a bandwidth-limited (BWL) femtosecond laser pulse written in terms of its positive frequency analytic signal... 

It should also be recalled that a full electrochemical, as well as spectroscopic and photophysical, characterization of complex systems such as rotaxanes and catenanes requires the comparison with the behavior of the separated molecular components (ring and thread for rotaxanes and constituting rings in the case of catenanes), or suitable model compounds. As it will appear clearly from the examples reported in the following, this comparison is of fundamental importance to evidence how and to which extent the molecular and supramolecular architecture influences the electronic properties of the component units. An appropriate experimental and theoretical approach comprises the use of several techniques that, as far as electrochemistry is concerned, include cyclic voltammetry, steady-state voltammetry, chronoampero-metry, coulometry, impedance spectroscopy, and spectra- and photoelectrochemistry. 

In the first place, we shall take a look at the recent advances in fast reaction photochemical kinetics and spectroscopy, in particular at picosecond laser flash photolysis and femtosecond observations. Next, photophysics and photochemistry in molecular beams will be considered. Here observations are made under single molecule-single photon conditions, and these experiments provide insight into the most fundamental unimolecular gas phase reactions. 

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