Chromophores intrinsically symmetric

This helicity rule was criticized in 1968 by Kuriyama (36). He demonstrated that for small torsion angles in enones the chirality contribution can be neglected by an electronic interaction of the type, observed in intrinsically symmetric chromophores, which are asymmetrically perturbed. This reasoning is particularly valid when a heteroatom is homo-conjugated with the unsaturated chromophore. 

Chemical enhancement is typically explained by the CT mechanism. When a molecule is adsorbed on a metal surface, new electronic states are formed due to chemisorption. The new electronic states may serve as resonant intermediate states in Raman scattering. If the Fermi level of the metal is located between the Highest Occupied Molecular Orbital (HOMO) Lowest Unoccupied Molecular Orbital (LUMO) in energy, CT excitations may likely occur at lower energy than intrinsic intramolecular excitations of the adsorbate [55-58]. According to Albrechts notation [55], in the CT mechanism via Albrechts A term Franck-Condon term) only the totally symmetric modes are resonantly enhanced when the laser excitation is close to an allowed electronic transition, and only one excited state is involved. The resonance Raman effects for vibrational modes that are non-totally symmetric, are usually observed when these modes couple two excited states of the chromophore. The product of the symmetry of both excited states should be equal or contain the non-totally symmetry. This mechanism is known as the Herzberg-Teller mechanism or B mechanism in Albrechts notation. 

The linear electro-optical (EO) behavior, i.e., the Pockels effect, constitutes a manifestation of nonlinear optical features of anisotropic and non-centro-symmetric media. Functional architectures based on host polymer matrixes and guest SiC nanoparticles (nc-SiC) as active chromophores were realized. The intrinsic dipole moments of the chromophore combined with the eventual polarization at the interfaces with the host matrix constitute the physical origin of the electro-optical responses. The experiments were carried out in hybrid materials based on SiC nanocrystals and matrixes such as PVK, PMMA, or PC. 

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