The volume-conserving hula-twist cis/trans isomerization process has been incorporated in a general scheme for photoisomerization of polyenes, applicable to small organic molecules as well as to protein-bound polyene chromophores. The main theme is that in solution the conventional one-bond-flip mechanism dominates, while in frozen media (or under other forms of supramolecular constraint) the hula-twist mechanism takes over. Literature examples of photoisomerization obtained under confined conditions have been critically reviewed, the applicability of HT has been examined, and new systems unambiguously testing this volume-conserving process are proposed.

A model for the primary process of vision is proposed, which involves a novel concerted-twist motion. Application of such motions to rhodopsin and bathorhodopsin successfully accounts for the properties of bathorhodopsin and related intermediates, including specific assignment of molecular structures to bathorhodopsin, to lumirhodopsin, and, less specifically, to hypsorhodopsin.In this paper we propose a mechanism for cis-trans isomerization of polyenes. This mechanism is a special case applicable only to compounds in a constrained medium. Arguments are presented to show that it is the preferred pathway for the primary process of vision.Background on the primary process of vision

This paper critically reviews examples in the literature of photochemical cis-trans isomerization paying particular attention to the medium effect and accompanied conformational changes. A case is made that the Hula-Twist mechanism, postulated in 1985 as a photochemical reaction pathway for a polyene chromophore imbedded in a protein binding cavity such as those of rhodopsin and bacteriorhodopsin, is also a dominant reaction pathway for a diene, or a longer polyene confined in a rigid (relative to isomerization rate) medium. The conventional one-bond-flip process is the preferred reaction pathway in a fluid medium. While defining experiments are proposed, this dual mechanistic approach successfully accounts for all examples in the literature on photoisomerization reactions whether involving conformational changes or not.

Factors that govern inclusion of organic molecules within octa acid (OA), a synthetic deep cavity cavitand, have been delineated by examining the complexation behavior of a number of organic molecules with varying dimensions and functionalities with OA. The formation of two types of complexes has been noted: the one which we call cavitandplex is a partially open complex in which a part of the guest molecule remains exposed to water, and the other termed capsuleplex is formed through assembly of two OA molecules. In capsuleplex, the guest is protected from water. Generally, guest molecules that possess ionic head groups form cavitandplex, and all others form capsuleplex. Capsuleplex may contain one or two guest molecules within the capsule. Small organic molecules (<10 A in length) may form both 2:1 and 2:2 capsuleplex, while longer ones (>12 A) preferentially form 2:1 capsuleplex. Extensive 1H NMR experiments have been carried out to characterize host-guest complexes. In the absence of the guest, OA tends to aggregate in water. The extent of aggregation depends on the concentration of OA and the presence of salts in solution. We expect the information obtained from this study to be of great value in predicting the nature of complexes with a given guest and facilitating appropriate guest chosen by researchers.

New azulene derivatives containing resonantly electron withdrawing formyl and resonantly electron donating fluorine substituents on the seven-as well as the five-member ring have been prepared. Systematic color changes as a result of perturbation of HOMO, LUMO, and LUMO + 1 energies were discussed. Corresponding excited-state properties related to S 2 -fluorescence were determined.