Photodimerization liquid crystals

Nagamatsu et al. have shown that liquid crystal and various other media can greatly influence the stereochemistry of uracil photodimerization. Pirrung and Nunn have shown that irradiation of a series of quinone monoacetals (5) in acetic acid provides a flexible high-yield route to substituted cyclopentenones. 

Similar experiments performed at higher CTAB concentrations near the phase transition from isotropic solution to lyotropic liquid crystals show that the phase transition temperature is affected by the presence of rheologically active compounds (155,161). Figure 13 demonstrates that the phase transition temperature increases when small amounts of 9-anthracene carboxylic acid are solubilized. Irradiation at X = 366 nm, i.e. photodimerization, removes the effect, and reirradiation at X = 254 nm (splitting of the dimers) causes a reincrease of the phase transition temperature. 

The photodimerization of the anthracene (176) having a 9-dendritic substituent has been studied within the vaterite crystal of CaCOs (R = H),"" and in its liquid crystalline smectic phase (R = Me). The crystal phase of the CaCOs is... 

General Considerations. In the well-known photodimerization of anthracenes in liquid solution 9, lO /lO, 9 -dimers (head-to-tail [4 + 4]) are formed in most cases. However, there have also been instances where head-to-head-photodimers (9,9 /10,10 ) are produced [19], and these were overseen previously. The solid phase photoreactions of anthracenes charged the topochemical postulate [7] for decades with hitherto unsolvable difficulties. All examples that contradict this assumption were eliminated without hesitation from the scope of topochemistry and termed to be crystal defect reactions, because the topochemically allowed processes were taken as support for topochemistry without further proof. The later provision, that the dimerizations occur within reaction cavities in the bulk of the crystal [20], did not help in this respect. A summary of the various arguments is given in Ref. 8. From examples 7 to 8 only b and perhaps c formally meet... 

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