Noncentrosymmetric environments

Optical second harmonic generation (SHG), which stems from the conversion of two photons of frequency to to a single photon of frequency 2(o, is an inherently surface-sensitive technique. Whereas no optical second harmonic wave is generated in the centrosymmetric bulk of a liquid, molecules participating in the asymmetry of the interface between two liquids (noncentrosymmetric environ-ment) contribute to SHG. Since the square root of SHG signal intensity, is proportional to the number N (per unit area), the molecular orientation (I) and the second order nonlinear polarizability of the SHG active species at the interface... 

In (a) the ion is so situated as to be in a noncentrosymmetric field, even when it is not vibrating. In this case electric-dipole emission is allowed. In (b) there is inversion symmetry when the ion is not vibrating, but vibration carried it to some other point Py at which the center of symmetry is lost. It should be self-evident that, even when the ion is in a noncentrosymmetric environment, vibrations may be important. That is, changes in the crystal-field symmetry induced by the vibronic motion will lead to violations of the crystal-field-selection rules. 

Recent NMR measurements of a series of complexes in the solid state indicate that many of the same principles apply to H-bonds of the FH F type. Centrosymmetric H-bonds occur only for short interfluorine distances, about 2.3 A or less. The gas-phase bond length of 2.278 A is therefore consistent with the ab initio finding of a centrosymmetric H-bond in (P H"F) . The crystalline environment can alter the proton transfer potential to a noncentrosymmetric type by just a small H-bond elongation . 

The luminescence of Ln ion from the f-f transitions can be classified as two types of transitions the parity-allowed magnetic dipole transitions and the parity-forbidden electric dipole transitions. When the Ln " ion is inserted into a chemical enviromnent, noncentrosymmetric interactions allow the mixing of electronic states of opposite parity into the 4f wave functions, and electric dipole transitions become partly allowed. The intensity of some of these transitions is particularly sensitive to the nature of the metal ion environment, and these transitions are called hypersensitive transition a typical example is the Dq p2 transition of Eu [106]. Thus, the luminescence of lanthanide ions can provide valuable information about the local enviromnent and make them very suitable for acting as a structural probe deciphering the symmetry of the chemical environment and the coordination sphere. 

Next we refer to Section X, where it was shown that nonlinear optical properties of noncentrosymmetric polyene systems can be modulated by changing the push-pull strength of the end groups and/or by changing CL or the environment. The strength of the acceptor end group of retinal and the environment have been... 

---