Heavy atom effect intermolecular

A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). 

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. 

Such intermolecular heavy atom effects on S <-> T transitions are... 

The photochemistry of polysilane derivatives may occur also via the triplet state 13, 30), on the basis of the observation of a weak-structured phosphorescence characteristic of a localized excited state for a number of polysilane derivatives. In principle, the halogenated additives could promote intersystem crossing via an intermolecular heavy-atom effect (39). Again, however, why the two structurally similar polysilanes should respond so differently to the presence of the additive is unclear. 

The exact nature of the reaction (oxidative vs. reductive) will depend on the redox properties of I ) and Q. The electron transfer process is a special case of exciplex formation favored in the strongly polar solvents, such as water. The involvement of an exciplex in a photochemical reaction is generally established by studying the effects of known exciplex quenchers such as amines on the exciplex fluorescence and the product formation. The heavy atom effect, due to the presence of substituents such as bromine or iodine intra- or intermolecularly, causes an exciplex to move to the triplet state preferentially, with a quenching of fluorescence. 

Z. S. Romanova, K. Deshayes, P. Piotrowiak, Remote intermolecular heavy-atom effect spin-orbit coupling across the wall of a hemicarcerand, J. Am. Chem. Soc., 2001, 123, 2444-2445. 

The role of the triplet state in the cis-trans isomerization of stilbenes effected by photosensitizers, such as acetophenone, benzophenone, or anthraquinone, which have large So Ti excitation energies, was first revealed in Ref. [65]. Theoretical considerations and experimental data on intermolecular triplet-triplet energy transfer leading to the sensitized stilbene photoisomerization are described in Section 4.2.2. It was shown that data on positional dependence of the heavy-atom effect on the cis-trans photoisomerization of bromostilbenes were consistent with the fact that, in contrast to the para position, the meta position is near a node in the highest occupied and the lowest unoccupied MO of stilbene [66]. According to [67], internal and external heavy-atom effects induce phosphorescence in frans-stilbene... 

The effect is observed when the heavy atom is substituted in the molecule (intramolecular effect) as also when the molecule collides with a heavy atom containing perturber (intermolecular or external effect). The dramatic enhancement of S —T transition due to intramolecular and intermolecular heavy atom perturbations are respectively shown in Figure 3.8 for chloronaphthalenes in ethyl iodide and other perturbants. Molecular oxygen has a perturbing effect on S — T absorption spectra of organic molecules in solution. Under a pressure of 100 atm of Os, well defined... 

In the case of aromatic molecular liquids, the intermolecular vibrational dynamics in the lower frequency region, which is typically less than 50 cm"i, includes the interaction-induced motion, which is translation-like, coupled with the librational motion, which is rotation-like, whereas that in the higher frequency region is predominantly due to the librational motion (Ryu Stratt, 2004 Elola Ladanyi, 2006). If the molecular motions and timescale of the interionic vibrational dynamics in the ILs are similar to those of simple aromatic molecular liquids, it would be permissible to consider the origins of the heavy atom substitution effects on the interionic vibrational dynamics in the ILs by taking into account the analogy of simple aromatic liquid dynamics. 

On the other hand, the reduced mass is another parameter for the peak frequency of the harmonic oscillator model. The liquid density is substantially increased by the heavy atom substitution of As for P in [XFe], but becomes slightly lower by the substitution of P for N in a cation (Table 1). By an analogy of the better correlation between the first moment of the intermolecular vibrational spectrum and the square root of the value of surface tension divided by liquid density than by formula weight in aprotic molecular liquids (Shirota et al., 2009), we find that the relatively large substitution effect on the low-frequency spectrum in the [XFe]" ILs could be attributed to the difference in liquid density. 

Shirota, H. Funston, A. M. Wishart, J. F. Castner, E. W., Jr., (2005). Ultrafast dynamics of pyrrolidinium cation ionic liquids. Journal of Chemical Physics, 122,184512/1-12 Shirota, H. Wishart, J. F. Castner, E. W., Jr. (2007). Intermolecular interactions and dynamics of room temperature ionic liquids that have silyl and siloxy-substituted imidazolium cations. Journal of Physical Chemistry B, 111, 4819-4829 Shirota, H. Nishikawa, K. Ishida, T. (2009). Atom substitution effects of [XFg]- in ionic liquids. 1. Experimental study. Journal of Physical Chemistry B, 113, 9831-9839 Shirota, H. Fujisawa, T. Fukazawa, H. Nishikawa, K. (2009). Ultrafast dynamics in aprotic molecular liquids a femtosecond Raman-induced Kerr effect spectroscopic study. Bulletin of the Chemical Society of Japan, 82,1347-1366 Shirota, H. Fukazawa, H. Fujisawa, T. Wishart, J. F. (2010). Heavy atom substitution effects in non-aromatic ionic liquids Ultrafast dynamics and physical properties. Journal of Physical Chemistry B, 114, 9400-9412... 

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