B. Hannah Arendt
B.6. Juicio, Urphänomen y la banalidad del mal
The overal l aim of thi s work was to produce chemical compounds that could be used to form conducting polymers with a sensing capabil i ty. A successful chemical sensor m ust possess two attri butes. Fi rstl y it must be able to selectively recogn i ze the desi red analyte, and then it must be capable of producing a measurable signal . I It can also be added that if the functional ities ful fi l l ing these two roles are separated within the molecule, then there must be some form of electronic communication between them. Combi n i ng the well-known ion recognition abil ity of crown ethers with the electrical conducti vity of conducting polymers has the potential to satisfy both these requirements, and could result in the fabrication of a range of metal ion sensors.
The specific objecti ves of this work were to:
i ) Carry out a survey of the l i terature to i nvesti gate the chemi stry of crown ethers, conducting pol y mers, and how these areas have previously been merged in order to produce conducting polymers functional i sed with crown ethers that can be used as metal ion sensors (Chapter I).
i i ) Design and synthesise a range of polyether-substituted monomers that have the potential both to selectively complex metal ions. and to form conductin g poly mers (Section 1 .6, Chapters 2 , 3 and 7).
i i i ) I nvestigate the response of the monomers t o a variety of metal ions (Chapter 4).
i v) I nvestigate the polymerisation of the monomers both chemicall y and electrochemicall y , and i n vesti gate the response of the polymers to metal ions (Chapter 5).
1.2
Crown Ethers
S i nce the serendi pitous di scovery of crown ethers by Charles Pedersen i n 1 967,2, 3 there has been a huge vol ume of work published on their synthesi s and appl ications. The crown ethers ori g i nal ly reported by Pederson consi sted of a cyclic polyether chain, and i ncl uded compounds contai ning either benzo or cyclohexyl ri ngs ( Fig. 1 . 1 ).
12-crown-4 cyclohexyl-15-crown-5 dibenzo-18-crown-6
Figure 1 .1 Examples of crown ethers
The crown nomenclature was appl ied to the compounds due to their abi l ity to crown alkali metal i ons and form stable complexes. Thi s was vi sual ized by Pedersen to be the result of ion-dipole interactions between the cation and the oxygen atoms, with the ion sitti n g i nside the crown ri ng. H i s hypothesis was l ater shown by X-ray crystallographic studies to be correct. The term 'supramolecul ar chemi stry ' was s ubsequently introduced to descri be thi s type of work i nvol v i ng "two or more chemical species held together by i ntermolecular forces":� A typical ion-di pol e interaction has an energy of ca. 1 5 kJ/mol, certainly less than the 250 kJ/mol for an ion-ion interaction, or 370 kJ/mol for a covalent C-C bond.5, 6 The strength of a crow n-cati on com plex comes from having multiple ion-dipole i nteractions, and because of thi s, s i ze i s one of the main factors determi ning the stab i l ity of s uch complexes. When this ratio of the ionic diameter of the metal ion to the ' hole' in the center of the crown is c lose to u ni ty , I: 1 complexes w i l l be formed ( Fi g . 1 .2). Complexes of thi s type are formed between Li' and 1 2-crown-4, Na+ and 1 5-crown-5 ,
and K+ and 1 8-crown-6. I n addition, when the ion i s larger than the c rown cavity, sandwich type 2: I and 3 : 2 complexes can be formed (Fig. 1 .3 ) .7 9
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Figure 1 .2 Crown ether - metal ion complexation
Figure 1 .3 Schematic representation of 2 : 1 and 3 : 2 'sandwich' complexes
Crown compounds bind metal cations m uch more strongly than thei r open-chain equi valents, call ed podands. lo This is due to the level of preorgani sation i nherent in the c rown molecules. While podands can exist i n hundreds of conformations, only a few of these are suitable for ion bi ndi ng. The e nergy gai ned by a podand on complexation must be greater than the l oss i n free energy associated with organi zi ng the chain i nto a suitable conformation. For crown ethers however, thi s rearrangement has already occurred duri ng synthesis, so that the molecule has substantially less freedom of movement and the oxygen atoms are held in a conformation more suitable for ion binding. The more preorganised a compound is for binding, the more stable i ts resulting complexes will be. ID
The i ncorporation of benzene rings i nto the structure of crown ethers reduces both thei r flexi b i l i ty" and the i r sol ubi l ity i n waterl 2, affect i ng the i r com p l exation properties. The benzene rin g also reduces the electron density at two of the oxygen bindi n g sites, further alterin g cation selectivity. 13 Benzo-crown ethers are frequently exploited in synthe s i s, as they are easily functional i sed by elec trophi l i c aromatic
s ubst it ut ion. However, t he int rod uct ion of elect ron-wit hd rawing or elect ron-d onat ing groups ont o t he aromat ic rings affects t he bas icit y of t he oxygen at oms , again mod ifying complexing abi l it y.14 Normal cat ion s elect ivit y can be alt eredl5 16 or even reversed'7 by t his proces s, t he effect bei ng great er for benz o-\S-crown-S t han for benz o-\S- crown-6. Not s urpris i ngly, t he nat ure of t he solvent us ed when meas uring cat ion select ivit y also has a large effect . 13, 18
Since t heir d is covery, crown compounds have been synt hesis ed t hat i ncorporat e nit rogen and s ul phur d onor at oms rat her t han oxygen. This is anot her fact or t hat changes t he crowns complexing abil it ies , allowing complex es t o be formed wit h t rans it ion met al and heavy met al ions .9 I n add it ion, t he field of s upramolecular chemist ry has been extended to lariat crown et hers ,'9 22 crypt andsB and s pherandslO 24. The pioneers of t he field of supramolecular chemist ry, Charles Ped ers en, Donald Cram and l ean-Marie Lehn, were award ed t he N obel Prize i n Chemist ry i n 1 987 for t heir work in t his area.)O
Applicat ions of crown et hers range from t heir archet y pal use as phas e t ransfer cat alysts in s ynt het ic chemist r/5, t o t he st ud y of ion t ransport t hrough membranes, 26 27 met al ion s eparat ion20, 28 or isot ope fract ionat ion29, chiral it y s ens ing of ami no acids30, s ol id phas e microext ract ion31 and ion s elect i ve elect rodes 2u2. They have also been used i n chromat ography as add it ives in t he el uent,3 , ad sorbed on'" or covalent ly att ached t035, 36 various st at ionary phases or as a coat ing on a piez oelect ric quart z cryst al for d et ect ion37 .
1.3
Conducting Polymers
Ten y ears after Pedersen reported the d iscovery of crown ethers, A lan MacDiarmid, A l an Heeger and Hideki Shi rakawa p ubl i shed papers announci n g that they had formed a fil m of polyacetyl ene that was capable of conducting electricity .38-41 Like the di scovery of crown ethers, thi s startli ng news caused a fl urry of research activity i n what was a completely new area, and led to thei r being jointly awarded the Nobel Prize i n Chemi stry i n 2000.42-47 Figure l A shows the structural variety in types of pol ymers that have since been shown to be conducting.
N
nPolyacetylene Poly thiophene
A
nPolypyrrole Polyfuran Polyparaphenylene vinylene
H H
to-�-O-N=O=N-O-�1
n Polyparaphenylene Polyheptadiyne PolyanilineFigure 1 .4 Examples of conjugated polymers
All the polymers i l l ustrated i n Fig. lA share one structural feature, their conj ugated 1(;
system of alternati ng singl e and double bonds. Pol y mers such as these may be transformed from i nsulatin g to semi -conducting by doping.46. 48 The mechanisms of charge transport i n conducting poly mers w i l l be further di scussed i n Secti on 1 .3.2. Figure 1 .5 shows the i nterconversion of poly thiophene between the neutral i nsulating and the conductive oxidi sed form. Oxidation requires the i ncorporation of counter
ions i nto the polymer in order for the polymer to remain electrical l y neutral, and these counter-ions can be exploited to add functionali ty to the polymer e.g. recognition sites.49
�A
+eM
Reduction + AS
nS
n Oxidation Conducting -e InsulatingFigure 1 .5 Redox activity of poly thi ophene
In addition to th e change i n conducti vit y on switchi ng bet ween th e oxi di sed and r educ ed for m of a polymer , th er e may also be ch anges i n ph ysical pr operti es such as colour and mor ph ology. In partic ular , th e electr ochr omic pr operties of poly thioph ene h ave dr awn att enti on due to possi ble applications i n eyewear, ' smart wi ndows' and
optical i nf or mation st or age. 50 5-1