Cryptands podands

Podates AcycHc analogues of crown ethers /coronands and cryptands (podands, eg, (11) (30) are also capable of forming inclusion compounds (podates) with cations and uncharged organic molecules, the latter being endowed with a hydrogen bond fiinctionahty. Podates normally are less stable than coronates and cryptates but have favorable kinetics. 

Organic acids, amino acids, 241 aminocarboxylates, (tio) crown ethers, (tio) cryptands, podands, phenols, heterocyclic and aromatic compounds, adenosine derivatives... 

Protons are relatively simple targets for sensor molecules and do not require engineered receptors, however, achievement of selective interactions with other chemical species requires much more elaborate receptors. In the most cases cations are bound via electrostatic or coordinative interactions within the receptors alkali metal cations, which are rather poor central ions and form only very weak coordination bonds, are usually bound within crown ethers, azacrown macrocycles, cryptands, podands, and related types of receptor moieties with oxygen and nitrogen donor atoms [8], Most of the common cation sensors are based on the photoinduced electron transfer (PET) mechanism, so the receptor moiety must have its redox potential (HOMO energy) adjusted to quench luminescence of the fluorophore (Figure 16.3). 

Weber. E. Vogtle, F. Classification and nomenclature of coronands, cryptands, podands. and their complexes. Inorg. Chim. Acta 1980. 45. L65-L67. 

F. Vogtle, Crown ethras, cryptands, podands and spherands, in Supramolecular Chemistry, John Wiley Sons, Ltd., Chichester, 1991, pp. 27-83. 

Fig. 7. Crown type and analogous receptor molecules of different varieties (1) crown ethers (2) cryptands (3) a podand (4) a spherand and (5) the natural...

The terms crown and cryptand have been universally adopted. A number of other terms have enjoyed less widespread recognition as noted above. Recently, Vogtle and Weber have proposed use of the terms crown ether, cryptand and podand according to the following scheme. Their suggested definitions are as follows ... 

Podand Open-chained (acyclic) analogs of either coronands or cryptands. 

Complexes Tlie term cryptate is now accepted to mean the complex formed between a cryptand and a substrate. Tlie corresponding complex with a coronand would be a coronate, a term suggested some years ago by the same authors . Presumably, a complex between a podand and some substrate would be a podate . 

The problem of molecular recognition has attracted biologically oriented chemists since Emil Fischer s lock-and-key theory l0). Within the last two decades, many model compounds have been developed micelle-forming detergents11, modified cyclodextrins 12), many kinds of crown-type compounds13) including podands, coronands, cryptands, and spherands. Very extensive studies using these compounds have, however, not been made from a point of view of whether or not shape similarity affects the discrimination. 

Figure 2.1. Schematic illustration of various structures of fluoroionophores. (a) Chelators and podands (b) coronands (c) cryptands (d) other structures.

As stated above, systematic names of macrocyclic host molecules were absurdly complicated for routine discussions [22]. Therefore Vogtle proposed the name coronand for crown ethers, and that of coronates for their complexes while cryptand complexes were called cryptates . The corresponding noncyclic analogues are podands such as 64 [23] and podates, respectively. The cumbersome name podando-coronands (and correspondingly podando-coronates ) was proposed for lariat ethers [24] having at least one sidearm like 65. Examples of hemispherands 66 [25], cavitands 25 [26] and those of some other hosts are discussed in Chapter 7 in some detail, whilst the exceptional stability of fragile guests 4 [2a] and 67 [27] in the hemicarcerand 5 cavity are discussed in Chapters 1 and Section 7.3. 

Various approaches have been taken to the synthesis of effective luminescent materials, using a variety of large encapsulating antenna-containing ligands, including podands, calixarenes, macrocycles, and macrobicycles (cryptands). These have been divided into acyclic (sub-section A) and cyclic (sub-section B). Representative ligands and complexes will be presented and discussed. 

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