Deactivation of excited states

The 3 Pi/2, 3 P2/2 excited states involved in the sodium D lines are the lowest energy excited states of the atom. Consequently, in a discharge in the vapour at a pressure that is sufficiently high for collisional deactivation of excited states to occur readily, a majority of atoms find themselves in these states before emission of radiation has taken place. Therefore... 

PCSs are systems of chromophores bound into a single macromolecule. Therefore, the study of processes of electronic excitation and energy transfer, as well as the investigation of the ways of deactivation of excited states, should lay a foundation for the understanding of such properties of PCSs as reactivity in photochemical transformations, photosensitizing and photoelectric activity, photoinitiated paramagnetism, etc. 

As with other first-row transition metals, copper complexes are not expected to be satisfactory singlet oxygen photogenerators, because of the rapid deactivation of excited states in the presence of partially filled d-orbitals. The exceptional case of the copper(II) benzochlorin iminium salt ((18), M = Cu) has already been referred to (Section 9.22.5.6) this showed bioactivity, although the nickel(II) complex ((18), M = Nin) was inactive.195... 

Figure 3.1 Physical deactivation of excited states of organic molecules...

Explain the deactivation of excited states by other molecules in terms of quenching processes, excimer/exciplex formation, energy transfer and electron transfer. 

The intramolecular processes responsible for radiative and radiationless deactivation of excited states we have considered so far have been uni-molecular processes that is, the processes involve only one molecule and hence follow first-order kinetics. 

The occurrence and deactivation of excited states of the first type are schematically shown in Fig. 35. Let the minority carriers (holes) be injected into the semiconductor in the course of an electrode reaction (reduction of substance A). The holes recombine with the majority carriers (electrons). The energy, which is released in the direct band-to-band recombination, is equal to the energy gap, so that we have the relation ha> = Eg for the emitted light quantum (case I). More probable, however, is recombination through surface or bulk levels, lying in the forbidden band, which successively trap the electrons and holes. In this case the excess energy of recombined carriers is released in smaller amounts, so that hco < Eg (case II in Fig. 35). Both these types of recombination are revealed in luminescence spectra recorded with n-type semiconductor electrodes under electrochemical generation of holes (Fig. 

Although the existence of the M.I.R. may have appeared counter-intuitive to many chemists, photophysicists had a different point of view, since an inverted" relationship of the rate constant of nonradiative transitions and the energy difference between the states is well established [91]. This energy gap law results from the decreasing vibrational overlap of electronic states, the so-called Franck-Condon factor. It predicts an exponential relationship of the rate constant of nonradiative deactivation of excited states with the energy gap, of the form ... 

In competition to electron transfer processes is internal conversion (IC), in which deactivations of excited states occur via a nonradiative transition to the electronic ground state. Visualizing IC rates is practically impossible because of the lack of a direct probe mechanism for nonradiative transitions. A notable approach to overcome this lack of detectability implies fluorescence quantum yield measurements, which is, however, only indirect. 

Crespo-Hernandez et al. have recently reported femtosecond pump-probe measurements in the liquid phase, pointing to the role of stacked structures in the rapid deactivation of excited states [24], This points to the complexity of the excited state dynamics, requiring further detailed experiments to determine the contributions of possibly competing pathways. 

The rates of diffusion of solutes and surfactants in and out of micelles have been measured using photophysical techniques. The most commonly used method is to measure the deactivation of excited states of the probe by added quenchers, which are only soluble in the aqueous phase. The measurement of either the decrease in emission intensity or a shortening of the emission lifetime of the probe can be employed to determine exit and entrance rates out of and into micelles 7d). The ability of an added quencher to deactivate an excited state is determined by the relative locations and rates of diffusion of the quenchers and excited states. Incorporation of either the quencher or excited state into a surfactant allows one to determine the rates of diffusion of surfactants. Because of the large dynamic range available with fluorescent and phosphorescent probes (Fig. 3), rates as fast as... 

Triplet excimers of aromatic hydrocarbons have proved very difficult to detect and hence their role in deactivation of excited states is largely speculative. However, on the basis of emission experiments (Subudhi and Lim, 1976 Okajima et al, 1977 Chandra and Lim, 1977 Webster et ai, 1981), it has been suggested that some di(l-naphthyl)alkanes form such species. It is suggested that the favoured conformation of the triplet excimer does not have the two naphthalene rings lying parallel to each other. 

The three-mode expression is most useful when discussing the rates of non-radiative deactivation of excited states in the inverted region. In this region, where —AG° Xy + Xc + As, a much simpler expression can be used since the product is created with a high vibrational quantum number in the high-frequency mode. This expression is Eq. 65 provided that 5Ac and lOAs are each < AG°1. au2 r., -3 lV2... 

Excimers are excited state complexes which consist of two identical species, one of which is in the excited state prior to complexation (See Scheme I). The subject has been thoroughly reviewed for polymers in a recent article by Semerak and Frank ( ). Briefly (Scheme I), an excited monomer species M combines with an identical ground state molecule M to produce an excimer E. Both excited species M and E may undergo the normal processes for deactivation of excited states, i.e., non-radiative decay, radiative decay, or product formation. 

Thus, the rate of deactivation of excited states of all investigated HAC - stabilizers is much larger, than the rate of intramolecular transfer of proton. Large difference is observed between values of A for HAC - XLIX and HAC - L in spite of small difference in their chemical structmes. Computer calculation of electronic structure of HAC - XLIX and HAC -L (Figure 2.10-2.12) shows that HAC - L structure is more planar than that of HAC - XLIX. 

The radiative deactivation of excited states in polymers has found widespread application in the field of plastic scintillators. Most of the knowledge on light emission during irradiation results from research performed in the years 1950—1965 to achieve the optimum scintillation efficiency of solid polymer systems. The ionizing radiation is absorbed in a polymer, usually polystyrene, and produces excited states which are transferred with a high yield to a fluorescent solute. The theory of excitation transfer will be developed in Chapter 3. If the energy donor is P and the fluorescent solute S, competition between energy transfer to S... 

Deactivation of excited states by transfer of energy to a suitable acceptor can efficiently inhibit secondary photochemical processes. The reaction can be represented schematically as... 

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