Photophysical relaxation process

Photophysics of Rhenium(I) Diimine Complexes A. Electronic Structure of Rhenium(I) Diimine Complexes Photophysical Relaxation Process of Rhenium(I)... 

After Section I, the photophysical processes of the rhenium complexes are briefly siunmarized the electronic structures, the photophysical relaxation processes, and the effects of intramolecular weak interaction between ligands on the photophysical properties. 

B. Photophysical Relaxation Process of Rhenium(I) Diimine Complexes... 

The typical results reported in this chapter, clearly demonstrate how the lifetime of excited states and the low-spin/high-spin character of such states can be tuned by pressure. Furthermore, photochemical bond formation and cleavage processes are accelerated or decelerated by pressure, respectively, in a similar way as found for the corresponding thermal reactions. As a result of this, the associative or dissociative nature of such substitution reactions can be characterized. A further characterization of the intimate nature of the reaction mechanism can also be obtained for photochemical isomerization and electron-transfer reactions as reported in Sections V and VI, respectively. The same applies to photoinduced thermal reactions, where the interpretation of the pressure dependence is not complicated by photophysical relaxation processes. The results for the subsequent thermal reactions can be compared with a wealth of information available for such processes [1-6]. Especially the construction of reaction volume profiles has turned out to be a powerful tool in the elucidation of such reaction mechanisms. 

Early studies of the photophysical radiationless processes of molecular systems were carried out on molecules in condensed media, liquids, rigid matrices, and high-pressure gases. This experimental situation introduces the complication associated with the presence of the possible occurrence of a number of different competing photophysical relaxation processes in the same molecular system in a fashion that mimics the complexity of a full photochemical reaction scheme. In order to study the primary photophysical radiationless transitions, it is optimal to consider experiments in which only the elementary individual processes of interest appear. Such investigations often involve the experimental determination of radiationless transition rates in isolated collision-free molecules. " For instance, collision-free experiments enable the consideration of the important phenomena of electronic relaxation and intramolecular vibrational redistribution. Studies on isolated molecules have greatly contributed to our... 

As in the case of thermal reactions, the reaction scheme introduced in Section 2.1.1.1 can be used to set up the differential equations. However, the degrees of advancement are primed, since the number of steps can be reduced as will be demonstrated by use of the Bodenstein hypothesis. In the last column of this scheme, the number of moles of light quanta are written for a photochemical step, which are absorbed by the reactant starting this photochemical step. According to this assumption and the different photophysical relaxation processes discussed in Section 1.3 the primary exited molecule A completely deactivates into the lowest level of vibrational energy of the first exited singlet state. Three further steps are possible ... 

The most convenient direct spectroscopic means of obtaining the rates of photophysical relaxation processes involves measuring the temporal decays and quantum yields of fluorescence and phosphorescence of radiative excited states. Kasha summarized the relevant empirical data that existed in the field of luminescence spectroscopy abont a half century ago ... 

It should be noted that no emission from the zwitterionic form of the proton-transferred tautomer was observed from any of the benzotriazoles studied in the present work. This implies that non-radiative relaxation processes from the excited state of this species are very efficient in all of the solvent and polymer environments studied. Thus no information is available on the effect of the medium polarity on the room-temperature photophysics of the zwitterionic form using fluorescence techniques. 

As we have seen above, a photochemical primary step can itself be followed only by transformations of other than the original molecules. It follows, therefore, that a photochemical primary step can only be part of a primary process if it is the last step in the sequence. Such a primary process, ending on a photochemical primary step, is a photochemical primary process. A primary process which does not end with a photochemical primary step (and thus does not contain any such step in the sequence) is a photophysical primary process. Those which end with a final photophysical primary step are final photophysical primary processes. In Figure 1, sequences 02-24, 02-23-15, and 02-23-14 represent photochemical primary processes, while sequences 02 (absorption alone or absorption followed by vibrational relaxation as appropriate to the medium),... 

Following photo excitation a solution sample returns to thermal equilibrium by a variety of photochemical and photophysical processes. The faster processes, e.g. vibrational relaxation and solvent relaxation, have only recently begun to be studied by direct kinetic methods (1-5). Picosecond emission spectroscopy has been especially useful in elucidating these ultrafast processes (1,/3, 5). As electronically excited molecules relax, their fluorescence spectrum shows time dependence that reflects the relaxation processes. 

We adopt the nomenclature of molecular spectroscopy to describe the excitation and relaxation processes in PPV derivatives, since the polymer photophysics is similar to the photophysics of large organic molecules [146]. Figure 22.11 shows schematically the configuration coordinate diagram of all... 

Figure 1 Simplified energy level diagram of an organic luminescent molecule showing basic photophysical processes. /< vibrational relaxation processes, Kp, rate of phosphorescence, Kq, rate of quenching.

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 photophysical properties of 9,10-dioxabimanes (l,5-diazabicyclo[3.3.0j-octadienediones) were extensively investigated by Kosower and his coworkers [llj. The most striking feature of their results is the stereoselectivity of the relaxation processes from 81. The 5yn-dioxabimanes fluoresce so well that they can... 

Volume 7 summarizes new trends on liquid crystals, display, and laser materials. The topics include liquid crystals for electro-optic applications, switchable holographic polymer-dispersed liquid crystals, electrochromism and electrochromic materials for displays, materials for solid-state dye lasers, photophysical properties of laser orientational relaxation processes in luminescence, and lasing of dyes and photosensitive materials for holographic recording. 

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