Highly selective initial excitation

The synthesis of luminescent organoboron quinolate polymers (21) (Fig. 15) via a three-step procedure starting from a silylated polystyrene has been communicated. The synthesis was initiated by the highly selective borylation of poly (4-trimethylsilylstyrene) (PS-Si), followed by the replacement of the bromine substituents in poly(4-dibromoborylstyrene) (PS-BBr) with substituted thienyl groups (R = H, 3-hexyl, 5-hexyl). In the final step, the 8-hydroxyquinolato moiety was introduced. The hexyl-substituted polymers efficiently emitted light at 513-514nm upon excitation at 395 nm.40... 

In connection with the total synthesis of grandisol, an asymmetric addition of ethylene on chiral heterocyclic aminals and ketals was examined (Scheme 23). The selectivity can be high, with a preferred approach of ethylene from the less hindered side, especially when chiral pyrrolidone 97 or furanones 100 were used in place of cyclic enones [70]. The diastereoisomeric excess of 101 or 102 remains modest with 5-menthyloxy furanone, even if the dark addition of nucleophiles or radicals on 100 occurs with a total facial selectivity. From a detailed analysis of the dependence of the product ratio with temperature and substituents, it was proposed that a pyramidalization of the 3-carbon in the relaxed -ir,- of the excited furanones and a homoanomeric effect were responsible for the observed selectivity [71]. Excited cyclopentenones also possess a biradical character in their relaxed state. However, no regio and no stereoselectivity could be detected when cyclopentenone was selectively excited in the presence of 5-men-thyloxyfuranone. An initial energy transfer, followed by a cycloaddition of the triplet excited furanone with cyclopentenone, explains these poor results. With more flexible or more electron deficient cyclenones the facial selectivity increases, but mixtures of regioisomers and syn/anti stereoisomers are obtained [72]. 

We may expect that the electronic predissociation of the excited complex will be also highly selective. If the initially excited 5> state is the vibrationless level of the j electronic manifold, the coupling to lower lying vibronic levels of the / manifold will induce the transition ... 

The intensity patterns of the vibrational substructure of the fluorescence spectra indicate that the van der Waals stretch and bend modes are extensively mixed. High resolution experiments performed with a single frequency laser show that the mixing of the pumped level C (in the present study 6a ) with near-resonant levels (in this case 16a ) depends on the rotational state of the complex. The quantum yields of IVR and photodissociation depend on which rotational states of the cluster are initially excited. Almost all the selectivity with respect to the rotational degrees of freedom is in the first step (IVR) and not in the last step (fragmentation). 

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