Spacers charge transfer systems

In contrast to the above-described systems, there are only few systems in which the bound cation can interact with the acceptor part of charge-transfer probes. The case of coumarins linked to crowns (Figure 10.24) is of special interest because the cation interacts directly with the electron-withdrawing group, i.e. the carbonyl group, in spite of the spacer between the fluorophore and the crown. An important consequence is the increase in stability constant of the complexes with respect to the same crown without external complexing atoms. 

Lumophore-receptor systems where the two components are sterically held orthogonal can be considered to have a virtual Co spacer between the lumophore and receptor. Thus these systems can be approximately considered as a PET system. A deeper analysis would take twisted intramolecular charge transfer (TICT) states [116] into account whose thermodynamics is, unsurprisingly, very similar to that of PET systems. 

One might imagine many fascinating applications for supracyclopenta-dienyl metal systems with functional groups. However, this will most likely require a spacer unit to prevent charge transfer between the C5 and functional moiety, if the Cs-metal coordination is to be preserved. Such systems exist or have recently been developed, for example, in connection with the search for a liquid crystalline organometallic complex (see Section IV,A,2,b). 

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