Lan-Ying Yang

Mechanisms of Photoisomerization of Polyenes in Confined Media: From Organic Glasses to Protein Binding Cavities†

Photochemical reactivities of model organic systems (stilbene and diphenylbutadiene) in organic glasses were first examined and compared with those in solution and in organized media. These observations were in turn compared with reactivities of polyene chromophores in protein binding cavities (specifically PYP, rhodopsin and bacteriorhodopsin). The obvious conclusion is that the preference for the most volume‐conserving Hula‐twist mechanism isomerization in organic glasses is because of the close interaction between the guest and the host molecules. In organized media (zeolites, crystals and protein binding cavities), the residual empty space coupled with any specific guest–host interactions that are characteristic of a given system, could lead to involvement of the more volume‐demanding one‐bond‐flip (i.e. torsional relaxation) or bicycle‐pedal or an extended HT process in photoisomerization.

Mechanism of photoisomerization of 1-naphthyl-2-phenylethylenes in organic glasses.

Photoisomerization reactions of cis isomers of 1‐β‐naphthyl‐2‐phenylethylene, an o‐methylated homolog and 1‐α‐naphthyl‐2‐phenylethylene in organic glasses at liquid nitrogen temperature were studied. Reactions were followed by changes in UV‐absorption spectra of the irradiated samples. Formation of an unstable trans‐photoproduct was detected only with the o‐methylated homolog. The results are consistent with high regioselective Hula‐twist photoisomerization at the benzylic sites of the three compounds examined. Calculated data on relative energies of the conformers of both the cis and the trans isomers are in agreement with the suspected conformational population of the starting materials and the photoproducts.

Stilbene analogs in Hula-twist photoisomerization.

Photoisomerization of several cis- or Z-stilbene analogs and two E-analogs in low temperature organic glasses was examined. From a mechanistic view-point, the compounds can be divided into three types: (i) those giving identical Hula-twist (HT) and one-bond-flip (OBF) products, (ii) those giving a single HT product that is different (hence distinguishable) from the OBF product and (iii) those showing two distinct HT processes but only one OBF process. Examples for all three types of analogs are provided emphasizing the most informative Type-II (stilbene analogs with identical but unsymmetrically substituted phenyl rings), including linear as well as conformationally constrained compounds. Conditions necessary for establishing HT and OBF processes are defined. Proper choice and design of model systems are essential for establishing or eliminating HT mechanism(s) of isomerization.

Designing systems for one-way trans to cisphotoisomerization for solar reactions

We have examined triplet photosensitized isomerization of sterically crowded derivatives of stilbene, diene, triene and styrene. Under selective triplet sensitization, several such compounds proceeded in the synthetically desirable manner of complete one-way trans to cis conversion or in a highly stereoselective manner. In the process, we have identified several systems suitable for solar irradiation as a “green” procedure for preparation of hindered olefinic isomers. A simple procedure for determining the relative efficiency of a solar reaction has also been introduced.