Induced dipole force emission

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

Generally, the room temperature emission spectra of Ln species show incompletely resolved stmcture within the peaks. However, an advantageous attribute of luminescent Ln complexes is the dependence of this emission spectral form on the specific coordination environment of the ion. This sensitivity arises from the selection rules associated with intraconfigurational (4f-4f) electronic transitions the selection rules for forced electric dipole transitions are relaxed due to 5d and 4/orbital mixing. In reality the majority of the complexes included for discussion here are non-centrosymmetric, low symmetry species and the relative intensities of the 4/-4/transitions are generally determined by the induced electric dipole transition selection rules. It should also be noted that visibly emissive Eu also possesses a magnetic dipole transition, F, whose intensity is relatively independent of the coordination environment [1,9]. 

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