Distortion effect, molecular absorption

An apparent decrease in the emission quantum yield and/or distortion of the band shape due to the reabsorption of emitted radiation. If such an effect is not corrected or compensated for, results of an emission experiment may prove to be incorrect. This is especially true in fluorescence quenching experiments conducted to evaluate the stoichiometry and affinity of ligand binding. 2. In a light irradiation experiment, the absorption of incident radiation by a species or molecular entity other than the intended primary absorber. See Fluorescence... 

Stizza et al. (73,274) have investigated amorphous vanadium phosphates, which are also of interest in relation to a XAS study of the butane-maleic anhydride (V, P)0 catalysts (99a). From the V K edge useful information is obtained about the distortions in the vanadium coordination sphere [molecular cage effect on the pre-edge intensity (312)] and on the vanadium oxidation state. Notably, V4+ is silent to most spectroscopic methods. A mixed V4+-V5+ valence state can be measured from the energy shift of the sharp core exciton at the absorption threshold of the Is level of vanadium due to Is -f 3d derived molecular orbitals localized within the first coordination shell of vanadium ions. 

Other symmetry perturbations of the 7t-electron cloud increasing the allowedness of the bands (higher absorptivities) result from substitution in the benzene ring. The bands akin to unsubstituted benzene at 185,210 and 260 nm are usually referred to as the (two) primary bands (or B and LJ and the secondary band (or L ) respectively (Murrell, 1963). Steric repulsions among the substituents and fusion of the benzene system with small rings cause static molecular distortions, and so enhance absorptivity. In some cases this hyperchromic effect is significant, even dramatic. For... 

In addition to the central atom, substituents in the porphyrin ring also affect the absorption spectrum. The number of bromine atoms in the pyrrole position in CoTPP was correlated with the red shift of the Soret band [9], while an effect of distortion of the porphyrin ring should be considered [10] (Fig. 5). As anticipated from the molecular orbital coefficient (see Fig. 3) the aiu orbital would be more dependent on the substitution on the pyrrole position than the a2u orbital. A strong electron-donating substituent was reported to induce an inversion from a2u > aiu to a2 < ai , i.e., the ai of 2-aminoporphyrin was reported to be more destabilized by 0.71 eV than that of 2-nitroporphyrin, while a2 suffers destabili-... 

For several pigments, the origins of crystallochromy could be derived from accurate structure data in combination with spectroscopic investigations. Primarily, the distortion of the molecule due to packing effects may change the absorption spectrum of the solid. Molecular distortions have been identified to be the reason for the solid state absorption properties in various phthalocyanines, especially in titanyl phthalocyanine ... 

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