Intra-molecular quenching

As mentioned in Section 1.3.2 the chemical constitution of an organic molecule determines whether it fluoresces or not [88,89,90]. In the vapour phase and in solvents, intra-molecular quenching effects determine whether emission will take place in addition to radiationless transitions. Furthermore in solvents the interaction with non-excited molecules (mostly solvent molecules) can cause an intermolecular deactivation process, which also reduces the probability of fluorescence. In principle, high fluorescence yield can be expected, if... 

Figure 1.8 Three possible intra-molecular energy transition mechanisms [9]. (Reprinted from Coordination Chemistry Reviews, 99, G.E. Buono-core, H. Li, and B. Marciniak, Quenching of excited states by lanthanide ions and chelates in solution, 55-87, 1990, with permission from Elsevier.)...

Laser flash photolysis studies of nitrospiropyran units which are linked intra-molecularly with triplet quenching or triplet sensitizing side groups show that in these compounds the triplet state does not contribute to the generation of the photochromically active photomerocyanine. 

This was shown by preparing the Z-olefin 63 independently and submitting it to the conditions. No isomerization to 50 was observed. We suspected that the rearrangement proceeded via an allylic anion as an intermediate which could possibly be trapped with deuterium. However, to our surprise, no H-D-incorporation was observed, when the reaction mixture was quenched with D20. Similarly, no deuterium was exchanged on performing the reaction in d7-DMF. This means that the concentration of the anionic species, if present at all, must be very low throughout the reaction. We next turned to the question whether the reaction proceeds inter- or intra-molecularly. To this purpose the monodeuteriated alcohols 68 and 69 were prepared as shown in Scheme 11 and submitted to the rearrangement. 

A porphinatoaluminum alkoxide is reported to be a superior initiator of c-caprolactone polymerization (44,45). A living polymer with a narrow molecular weight distribution (M /Mjj = 1.08) is ob-tmned under conditions of high conversion, in part because steric hindrance at the catalyst site reduces intra- and intermolecular transesterification. Treatment with alcohols does not quench the catalytic activity although methanol serves as a coinitiator in the presence of the aluminum species. The immortal nature of the system has been demonstrated by preparation of an AB block copolymer with ethylene oxide. The order of reactivity is e-lactone > p-lactone. 

---