Heavy-atom effect external

Interestingly, it was possible to probe the spin-forbidden component of the tunneling reaction with internal and external heavy atom effects. Such effects are well known to enhance the rates of intersystem crossing of electronically excited triplets to ground singlet states, where the presence of heavier nuclei increases spin-orbit coupling. Relative rates for the low-temperature rearrangements of 12 to 13 were... 

Frosch(84,133) have explained the external heavy-atom effect in intersystem crossing by postulating that the singlet and triplet states of the solute, which cannot interact directly, couple with the solvent singlet and triplet states, which themselves are strongly coupled through spin-orbit interaction. Thus the transition integral becomes<134)... 

Table 5.10. External Heavy-Atom Effect on Naphthalene 32b),a...

Alternatively, rather than to substitute a heavy atom onto a molecule directly, a solvent containing heavy atoms can be used to promote inter-combinational transitions (external heavy-atom effect). Robinson and... 

The external heavy atom effect shows itself when a heavy atom is incorporated in a solvent molecule. For example, 1-chloronaph-thalene has a much stronger S0 —> Ti absorption in iodoethane solution than in ethanol. 

Of particular interest in the application of cyclodextrins is the enhancement of luminescence from molecules when they are present in a cyclodextrin cavity. Polynuclear aromatic hydrocarbons show virtually no phosphorescence in solution. If, however, these compounds in solution are encapsulated with 1,2-dibromoethane (enhances intersystem crossing by increasing spin-orbit coupling external heavy atom effect) in the cavities of P-cyclodextrin and nitrogen gas passed, intense phosphorescence emission occurs at room temperature. Cyclodextrins form complexes with guest molecules, which fit into the cavity so that the microenvironment around the guest molecule is different from that in... 

Intersystem crossing (i.e. crossing from the first singlet excited state Si to the first triplet state Tj) is possible thanks to spin-orbit coupling. The efficiency of this coupling varies with the fourth power of the atomic number, which explains why intersystem crossing is favored by the presence of a heavy atom. Fluorescence quenching by internal heavy atom effect (see Chapter 3) or external heavy atom effect (see Chapter 4) can be explained in this way. 

An interesting example of an external heavy atom effect was reported recently by Fischer.186 Stilbene was irradiated in methyl iodide at the wavelength (436 nm) corresponding to its triplet energy. Although the extinction... 

Figure 3.8 Heavy atom effect in the T - S0 transitions in halonaphthalenes. A. External heavy atom effect 1-chloronaphthalene with (a) ethyl iodide, (b) xenon (143 atml, (c) oxygen (30 atm) and (d) pure 1-chloronaphthalene B. Internal heavy atom effect (ej 1-chloronaphthalene, (f) 2-iodonaphthalene, (g) 1-iodonaphthalene.

Some solvents containing heavy atoms can induce enhancement of phosphorescence at the expense of fluorescence, e.g. ethyl iodide, nitro-methane, CS2 (external heavy atom effect). Irreversible conversion to ionic or radical products is often observed. Hence the system changes with time and the process should be classed a photochemical reaction distinct from the reversible quenching reactions discussed above. For example for anthracene and carbon tetrachloride ... 

After exclusion of a predissociation process responsible for the lifetime shortening in complexes of benzene with noble gases, we consider the external heavy-atom effect on the intersystems crossing rate as the origin of the lifetime shortening [42]. The strong decrease of the lifetime in the... 

In conclusion, we have found that the intramolecular dynamics in the benzene molecule at low excess energy is not strongly influenced by the additional three vibrational degrees of freedom of the benzene-Ar complex. The coupling of the excited intramolecular modes to the low-frequency inter-molecular modes is weak. The observed 40% decrease of the lifetime of the 61 state does not depend on the individual excited rotation and points to an external heavy-atom effect as the source of the lifetime shortening observed for the same selectively excited rovibronic state. 

Now, in aromatic hydrocarbons intramolecular skeletal vibrations, rather than C—H vibrations, dominate the vibronic coupling contribution to the term J m = — . Furthermore, intermolecular vibrations will have negligible effect on the coupling of the electronic states of interest. Thus, in the case of internal conversion, where the (relatively large) matrix elements are solely determined by intramolecular vibronic coupling, no appreciable medium effect on the nonradiative lifetime is to be expected. On the other hand, intersystem crossing processes are enhanced by the external heavy atom effect, which leads to a contribution to the electronic coupling term. 

In this review we shall not be interested in external heavy-atom effects. 

The heavy atom effect can rely on the presence of an atom of high atomic number either within the molecule itself (the internal heavy atom effect) or in the solvent (external heavy atom effect). In both cases the fluorescence... 

The question then arises what is a heavy atom Br and I can be considered as heavy atoms in organic molecules, but Cl is a borderline case. Most metals qualify as heavy atoms and the photophysical properties of metal complexes are related to this increased spin-orbit coupling. Some noble gas heavy atoms like Xe show important external heavy atom effects. 

Figure 3.41 provides an example of a Stern-Volmer plot. In this case the triplet excited state Tj of an aromatic molecule is quenched by the external heavy atom effect. The Stern-Volmer plot must by definition have an... 

In this subsection we describe a more specific type of application that now is possible within the framework of the response methodology, namely the computation and interpretation of heavy atoms effects on S-T absorption spectra. In ref. [150] a new interpretation for the cause of the external heavy atom effect on S-T transitions was thus proposed basing on response theory results for X + C2H4, X = F, Cl, Br, HCl, Ar as model systems. It was found that this effect should be interpreted as an increased SOC due to back-charge-transfer from the heavy atom, which contrasted the more conventional interpretations in terms of hydrocarbon charge penetration into the heavy atom or in terms of an exchange induced effect. 

The internal and external heavy atom effects, IHA and EHA, have attracted a considerable attention in the community of molecular spectroscopists. This is part of an old problem of understanding environmental effects from solvents or solid matrices on S-T absorption or on phosphorescence of solute molecules. For higher temperature studies the triplet decay is quenched either by collision or by vibrational interaction with the matrix or the solvent. The molecules subject to studies in this respect have mostly been aromatic molecules perturbed by molecular oxygen, nitric oxide or other paramagnetic molecules, molecules either with heavy atoms and/or forming charge transfer complexes. 

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