The results of AMI molecular orbital calculations on two series of arylmethylsulfonium salt derivatives have indicated that the nature of the lowest unoccupied molecular orbital (LUMO) switches from * when aryl = phenyl, 1-naphthyl, and 9-anthryl to * when aryl = 5-naphthacenyl. Electrochemical, photochemical, fluorescence quantum yield, and singlet-lifetime data were found to support the conclusions from the calculations.
7656 Figure 4. Energy surface (one-electron contributions) of the butadiene-cyclobutene ground configuration as a function of independent rotational angles $ and $'. Dotted lines: $ = -$' conrotation, @ = +$' disrotation. (Note: @' = +90° is electronically equivalent to $' = -90" and the surface repeats periodically for angles outside the given range.) path in this molecular state lies on a smooth energy slope, with no intervening energy maximum between the two paths. (This is true on either side of the peak value!) This in effect represents a highly unstable situation with strong restoring forces (negative derivatives of the energy with respect to the coordinates) directed toward 4' = -4. Within this simplified model a synchronized disrotational motion appears therefore extremely improbable, not only on the grounds of a higher activation energy.The energy difference between con-and disrotation is primarily controlled by the energy of the new u bond. We expect therefore the same type of qualitative behavior in all reacting states with an appreciable value of the bond order: the energy minimum path is close to the predicted preferred rotation; the opposite rotation is energy wise unstable. The model supports the view of the concerted electrocyclic reaction as simultaneous rotations of the two reacting centers; along the minimum energy path, the rotations are, however, not in perfect synchronization. A twofold axis or a symmetry is then not retained during the reaction.This observation detracts in no way from any previous conclusions as con-and disrotation refer to convenient, but not necessarily physically realized reference paths. It points, however, again to the fact that the bond order criterion is by its very nature a qualitative criterion for the prediction of rotations in electronically controlled electrocyclic reactions. Abstract: The influence of cholesteric liquid crystalline mesophase pitch (degree of helicity), temperature, texture, solute concentration, and cholesteric matrix on the intensity and sign of the cholesteric liquid crystal induced circular dichroism (LCICD) in pyrene was investigated. The spectroscopic information (polarizations of electronic transitions, detection of hidden transitions) provided by the LCICD spectra is also discussed. The LCICD intensity is dependent on pitch of the cholesteric mesophase, temperature, and texture. The sign of the LCICD is dependent on the position of Xa of the cholesteric pitch band relative to the wavelength of absorption, cholesteric matrix properties, as well as the chirality of the mesophase. The molecular ellipticity, [e] (deg cm2/dmol), for pyrene in a single cholesteric matrix is independent of solute concentration over a range in which the concentration was altered by two orders of magnitude. The concentration independence of [e] indicates the lack of a mechanism change over the concentration range studied and that solute-solute interactions do not play an important role in the origin of LCICD for dilute solutions of solutes in cholesteric mesophas...
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