Study 6.1 Supramolecular chemistry photoresponsive stilbene dendrimers

Case Study 6.1 Supramolecular chemistry - photoresponsive stilbene dendrimers 

Sensitization is often indispensable for inducing reaction from the triplet state and it is practical in the case of aliphatic non-conjugated alkenes, which absorb weakly over 200 nm. The triplet energy (ET) of the sensitizer should be higher than that of the acceptor to afford efficient energy transfer (Section 2.2.2). Nevertheless, triplet 

Asymmetric synthesis introduces one or more new features of chirality in a molecule. Several approaches are possible. In general, preferential formation of an enantiomer or diastereoisomer is achieved as a result of the influence of a chiral element present in the substrate, a reagent, catalyst or the environment. Chirality control is also possible in the electronically excited state,584 as demonstrated in the following examples. 

Excitation with circularly polarized light (CPL) is an interesting direct method of asymmetric synthesis, which is based on preferential excitation of one of the enantiomers thanks to a difference in the molar absorption coefficients towards left or right CPL at a given wavelength.585 Unfortunately, only a small enantiomeric excess (ee) can be generated by this method. Some scientists, nevertheless, believe that 

Encmtiodifferentiating photosensitization involves a prochiral or racemic substrate interacting with an optically active sensitizer via quenching (Section 2.2.2) or exciplex formation (Section 2.2.3),585 rather than in the subsequent thermal process. Asymmetric photosensitization requires only a catalytic amount of the chiral sensitizer. An example is provided in Case Study 6.2 (E Z photoisomerization). 

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