Charge-transfer, heavy atoms

The best evidence for a charge-transfer exciplex (hetero excimer) has been provided by Thomaz and Stevens.<148,149) They note a reduction in fluorescence yield of pyrene with increasing heavy-atom concentration and proposed the following set of reactions to explain their results ... 

Although some workers suggest that exchange interactions are dominant in determining the effects of external heavy atoms/1455 experimental evidence at this point tends to favor charge transfer as the mechanism for external... 

Wheland RC (1976) Correlation of electrical conductivity in charge-transfer complexes with redox potentials, steric factors, and heavy atom effects. J Am Chem Soc 98 3926-3930... 

The first (and still the foremost) quantum theory of stopping, attributed to Bethe [19,20], considers the observables energy and momentum transfers as fundamental in the interaction of fast charged particles with atomic electrons. Taking the simplest case of a heavy, fast, yet nonrelativistic incident projectile, the excitation cross-section is developed in the first Born approximation that is, the incident particle is represented as a plane wave and the scattered particle as a slightly perturbed wave. Representing the Coulombic interaction as a Fourier integral over momentum transfer, Bethe derives the differential Born cross-section for excitation to the nth quantum state of the atom as follows. 

The conclusions reached by Costentin and SavOant are in fact quite consistent with our own. The main difference is that, according to these authors, the notion of an imbalanced transition state should be placed within the context of charge localization-delocalization heavy-atom intramolecular reorganization rather than of synchronization (or lack thereof) between charge delocalization and proton transfer. ... 

Relaxation of the rules can occur, especially since the selection rules apply strongly only to atoms that have pure Russell-Saunders (I-S) coupling. In heavy atoms such as lanthanides, the Russell-Saunders coupling is not entirely valid as there is the effect of the spin-orbit interactions, or so called j mixing, which will cause a breakdown of the spin selection rule. In lanthanides, the f-f transitions, which are parity-forbidden, can become weakly allowed as electric dipole transitions by admixture of configurations of opposite parity, for example d states, or charge transfer. These f-f transitions become parity-allowed in two-photon absorptions that are g g and u u. These even-parity transitions are forbidden for one photon but not for two photons, and vice versa for g u transitions [46],... 

Figure 1. Charge transfer Aq/e from the zigzag SWCNTs (p,0) with p = 5-15 to the F atom adsorbed on their surfaces as a function of the tube radius R. The heavy dots and triangles refer to semiconducting and metallic SWCNTs, respectively. The solid lines are intended as a guide to the eye.

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. 

Table 7-2. Main geometrical parameters for the ground state (GS) and theintramolecular charge transfer state (ICT) of pNA in gas phase and in the two solvents. The values in parentheses refer to calculations including diffuse basis functions on the heavy atoms...

---