Twisted molecular probes

Y-P Sun, G Beimett, KP Johnston, MA Fox. Quantitative resolution of dual fluorescence spectra in molecules forming twisted intramolecular charge-transfer states. Toward establishment of molecular probes for medium effects in supercritical fluids and mixtures. Anal Chem 64 1763, 1992. 

A recent summary of the history and dynamics of the theoretical models of benzene39 cites a view that even though the current molecular orbital (MO) view of benzene seems complete and ultimate while the valence bond (VB) view seems obsolete, the recent findings about zr-distortivity in benzene indicate that the benzene story is likely to take additional twists and turns that will revive the VB viewpoint (see footnote 96 in ref 39). What the present review will show is that the notion of delocalized zr-systems in Scheme 1 is an outcome of both VB and MO theories, and the chemical manifestations are reproduced at all levels. The use of VB theory leads, however, to a more natural appreciation of the zr-distortivity, while the manifestations of this ground state s zr-distortivity in the excited state of delocalized species provides for the first time a physical probe of a Kekule structure .3... 

A basic understanding of the structure and behavior of liquid-crystalline cellulosics has yet to evolve. From a conceptual point of view, the chirality of the cellulosic chain is most sensitively expressed in the super-molecular structure of the cholesteric phase, which may be described by the twisting power or the pitch. At present, no information is available about domains or domain sizes (correlation lengths) of supermo-lecular structures. The chirality in the columnar phases has not been addressed at all. The principal problem, i.e., how does chirality on a molecular or conformational level promote chirality on the supermolecular level, has not been solved. If this correlation were known, it would enable the determination of the conformation of cellulosic chains in the mesomorphic phase and the development of models for the polymer-solvent interactions for lyotropic systems. On the other hand, direct probing of this interaction would provide a big leap towards an understanding of lyotropic phases. 

Molecular mechanics serves to probe the dynamics of macromolecules as well as their structure. From the potential energy barrier that lies between two stable conformers, molecular mechanics can predict the temperature at which the molecule twists easily between the two forms. Specific folding pathways can be tested in principle, but the enormous range of possible conformations often limits these studies to statistical methods, described in TK Chapter 7. 

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