Photo- and Redox-active [ZIRotaxanes and [ZICatenanes Andrew C. Benniston Chemistry Department, University of Glasgow, Glasgow, Scotland, UK G12 8QQ 1 General Introduction [ 2 jRotaxanes and [ 2jcatenanes represent a fascinating class of com- pounds in which two components are inseparable not because they are tethered covalently, but as a result of their molecular topology] (Figure 1). The chemistry of these molecules is now well estab- lished and forms a major part of supramolecular science. Recent synthetic improvements2"J7 have not only led to enhanced yields of rotaxanedcatenanes, but also an ability to build-in further function- ality at any stage of the synthesis. As a consequence, there are now numerous examples of such systems which have been termed photoactive or redoxactive since residing within the architecture is a subunit which can be photochemically and/or electrochemically activated. Nevertheless, the question arises should, for example, a rotaxane be defined as redoxactive even when the redox centre incorporated in the superstructure appears to serve no purpose'? Such a point is clearly open to debate, but for this review an assem- bly is deemed photoactive or redoxactive if the active subunit(s) and molecular architecture are mutually important to the operation of the system.beadfi Figure 1 Simple cartoons representing (a) a [ 2lrotaxane (nomenclature commonly used when referring to (2Jrotaxanes is also shown), and (6) a 12lcatenane Andrew C. Benniston was born in Walsall (West Midlands) in 1966.After obtaining his BSc in I987 from the University of Warwick, he undertook a PhD under the supervision of Professor Peter Moore. After a one year Royal Society Fellowship in 1991 at the Universite' Louk PaJteur de Strasbourg with Dr Jean-Pierre Sauvage, he moved to Austin, Texas, to work at the Center for Fast Kinetics Research with Dr Anthony Harriman. After two and a half years he moved back to Britain in November, I994 to take up his current lecturing position in the Inorganic Department at the University of Glasgow. His major research interests concern donor-acceptor com-plexes and their photophysical properties and ways in which to incorporate them into multi- dimensional structures using cation chelation.1.2 RelevanceSupramolecular Devices To undergo and complete a desired task any machine requires not only an external trigger, but also each independent component per- forming in the correct sequence a specific function. Usually, machines are regarded as large items with many moving parts, however more recently the same notions have been applied to chemical systems, whereby individual subunits are made to perform unique operations but now at the molecular level (suprczmolec~4lar devices).3 Examples of such devices are molecular switches in which the relative positioning of molecular subunits is altered by an external agent such as light or through redox change^.^ Because of the unique structural features of catenanes and rotaxanes research into these molecules as switches is growing, especially rotaxanes where control of bead positioning on a thread forms the basis of a simple molecular abacus.I .2.1 Photosynthetic Models Light-harvesting by plants is possibly the single most important natural process occurring on the Earth, and since solving of the X-ray structure of the photosynthetic reaction centre (RC) for the bac- terium Rhodopseudomonus viridis a great deal of research has resulted in an attempt to understand and mimic this natural process. At a simplified level, within the RC photonic excitation of a donor molecule (D) causes an electron to migrate along a series of elec-tron acceptors (A), leading ultimately to generation of a pair of redox ions .s In the natural system unique donor-acceptor separation and orientation play a major role in controlling individual electron transfer steps, and as a consequence chemists have turned to rotax- anes and catenanes in an attempt to create suitable synthetic proto- types.The two strategies depicted in Figure 2 attempt to illustrate how using the structural traits of 12lrotaxanes, spatial separation of charge is possible through either initial photonic excitation of (a)a stopper entity, or (6)bead portion, coupled with concomitant elec- tron transfer to an acceptor-based stopper. Molecular devices and synthetic models for photosynthetic RCs are merely two potential areas of research pertaining to photoactive and redoxactive [ 2 jrotaxanes/[2 lcatenanes and many more applica- tions are being explored.However, it is fair to say that research is still at a very early stage and there is a long way to go before working molecular devices are commonplace. Thus, it is the inten- c e-e Figure 2 Two modes of charge-separation in a 12lrotaxane through an initial photonic excitation of either: (a)a stopper, or (b)bead. Note: in (a) the bead does not participate in the forward electron transfer process, but may play a role in the restoration of the ground-state by return electron transfer. 427 CHEMICAL SOCIETY REVIEWS, 1996 OMe Figure 3 Examples of [2]catenanes incorporating a P-cyclodextrin as one of the interlocked rings tion of this review to cover some of these recent developments, and in particular give illustrations of real chemical systems supporting photoactive and/or redoxactive groups.Where possible the relation- ship between structural features and physicochemical properties will be highlighted, especially where an understanding of both helps explain observed findings. 2 Cyclodextrin-based Superstructures Cyclodextrins (CD) are well known ‘bucket-shaped’ cyclic sugar compounds ideally suited for inclusion of organic-based guests into their hydrophobic cavity. As a result early research, particularly into redoxactive [2]rotaxanes, utilised the inclusion properties of CDs. A review of this early research can be found in ref. 2(6) and there- fore these systems will not be discussed. Up until very recently, examples of analogous [2lcatenanes incorporating CDs have been noticeably lacking.Recently however, Stoddart and coworkers6 have been successful in pro-ducing workable quantities of cyclodextrin-derived interlocked rings. Consequently, examples of catenanes comprising of lumi-nescent bitolyl groups linked via pol yethoxy chains to bislactam subunits are shown in Figure 3. As pictorially depicted in Figure 3, the X-ray structure of 12lcatenane 1(n = 3,m = 3,p = 1) fully con- firms the bitolyl unit exclusively occupying the cyclodextrin and the bislactam residue lying on the outer casing, In contrast, in the solu- tion phase (C6D6) detailed nuclear Overhauser enhancement experi- ments confirm both the bitolyl and terephthaloyl units residing within the CD cavity.In an apparent contradiction 2D rotating Overhauser enhancement spectroscopy (ROESY) data obtained in the same solvent supports the existence of a single translational isomer in which the bitolyl is encapsulated within the CD. This apparent discrepancy however is explained by the less sensitive nature of ROESY experiments. Detailed photophysical data on (1) (n = 3, m = 3,p = 1) in MeCN are also entirely consistent with the interlocked nature of the two molecular components. For simplic- ity, 1 can be viewed as three simple components (A-C) plus the cyclodextrin. Unlike the absorption spectrum of (A), which is simply a super- imposition of the spectra of (B) and (C), the fluorescence spectrum is somewhat different to those of the individual units and contains two weak bands centred around 320 nm (T < 0.5 ns) and 420 ns (T = 2.5 ns).The emission at 420 nm is accredited to formation of an excited state complex (exciplex), resulting from initial photonic excitation of the bitolyl unit. However, in the resulting [2]catenane no exciplex emission is observed, and only luminescence at 320 nm from the bitolyl group is detected. Such photophysical data would be expected if both bitolyl and terephthaloyl units are well separ- ‘q4-f3 ated and accordingly is entirely in keeping with the interlocked nature of the [2]catenane and structural data. 3 n-n Stacked Assemblies Simple mixing in an appropriate polar solvent of the electron- accepting N,N’-dimethyl-4,4’-bipyridiniumdication and electron- rich 1,4-dimethoxybenzene results in creation of an electron donor-acceptor (EDA) complex, and formation of a charge-trans- fer (CT) absorption band.This CT absorption band of the EDA complex arises from an electronic transition within molecular orbitals formed by direct overlap of appropriate HOMO-LUMO g-orbitals of the two interacting aromatics? Although highly coloured, the resultant solution phase EDA complex is loosely held (K < 1 dm3 mol-I) and consequently of little practical use in the construction of larger molecular assemblies such as rotaxanes and catenanes. Added extra complex stability is however readily obtained through cyclisation of either components into their corre- sponding macrocyclic counterparts.Using such an idea Stoddart and coworkers* have been particularly successful, especially regarding the bipyridinium-based cyclophane 2 (Figure 4) whose internal cavity dimension of ca. 6.8 X 10.3 8, is optimal for encapsulation of varying electron-rich aromatics and formation of EDA complexe~.~ Figure 4 An illustrationof the bipyridinium-based cyclophane synthesised by Stoddart et af. Indeed, cyclophane 2 is now commonly used as the bead in a multitude of (2lrotaxane examples, or alternatively the ring in anal-ogous (2lcatenanes. The ability to address such systems photo- chemically,l*JI and electro~hemically~2~~~ has resulted in intense research into such molecular assemblies. 3.1 [2]Rotaxanes Relevant examples of [2lrotaxane assemblies in which the cyclo- phane 2 forms an integral part of the overall structure are listed in Table I.The 12 Irotaxanes 3-5 represent recently developed systems by Kaifer and coworkers synthesised specifically for influencing properties of the systems through electrochemical means. A closer examination of the electrochemistry of 4-5 is particularly note- worthy as subtle differences in electrochemical attributes of the benzidine- and p-phenylenediamine-stationrotaxanes are observed. PHOTO- AND REDOX-ACTIVE [2JROTAXANES AND [2]CATENANES-A. C. BENNISTON Table 1 Examples of [2]rotaxanes %?0 R (I in CH,CN. Charge-transfer band obscured by porphyrin Soret bands Primarily, the anodic electrochemistry of the two [2]rotaxanes can be summarised as follows: S-eeS’ where S represents the bound station.In both cases, because of proximity to the tetracationic bead oxidation of the bound stations is significantly more difficult than in simple uncomplexed threads. This effect, however, is far more pronounced in 5 compared to 4 and is attributed to the larger size of the benzidine moiety as compared to the corresponding p-phenylenediamine which permits extended ,jositive charge delocalisation over the extended 7r-orbital. llb Ilb 1 lc 1 la 1oc 1Oa 475 1Oa 486 1Ob 1Oa (12)(m= 3,n = 3)b 1Id Electrostatic repulsion between bead and oxidised station is espe- cially important when considering two-station [2 Jrotaxanes, as the possibility of ‘driving’ a bead from alternate stations constitutes an electroactive molecular switch.Accordingly, the [2]rotaxane 6 has been developed containing both a benzidine and biphenol station, which under normal conditions (room temperature, CD,CN) dis-plays bead shuttling. At reduced temperatures shuttling rate is sig- nificantly reduced and from 2D NMR experiments confirms the bead predominantly residing on the benzidine (86%)rather than the biphenol (14%) subunit. More significantly, electrochemical one- electron oxidation of the benzidine station is able, due to generation of enough electrostatic repulsion, to ‘switch’ the bead exclusively over to the biphenol site. It should be noted that similar switching is also possible by protonation of the benzidine basic nitrogens.CHEMICAL SOCIETY REVIEWS, 1996 Radical ion par -I Electrostatic repulsion Molecular motion Charge-transferstate Figure 5 Photoprocesses occurring in the ferrocene containing [21rotax anes illustrated in Table I Conformational control of rotaxanes, but this time using Iight- induction instead of electrochemical means, is again the goal behind the research carried out by Harriman and co-workers? loon a series of one- and two-station I2lrotaxanes 7-11 (Table 1) As CT absorp- tion bands of these EDA complexes occur in the visible region selective excitation of these rotaxanes is possible, resulting in a destabilisation of the charge-transfer interaction For simplicity, the cartoon (Figure 5)is intended to explain the general photoprocesses occurring in [2 Jrotaxanes7-11 In general, excitation of the rotaxanes 7-11 using an ultra-short laser pulse generates a radical ion pair (RIP) through electron trans- fer from the bound hydroquinol unit to the proximal 4,4’-bipyri- dinium dication acceptor of the bead The lifetimes of the RIPS in rotaxanes 7-11 are ultrashort ranging from 14 to 30 ps, as a result of rapid charge recombination from back electron transfer (krec) In rotaxanes 10-11 these short lifetimes exclude the possibility of mol- ecular bead motion, and all initial photonic energy is converted to heat in the surrounding solvent However, in 7-9 an additional sec- ondary electron transfer process occurs in which the ferrocene stopper is oxidised (with a rate constant k,,,) by the photo-generated dialkoxybenzene .rr-radical cation, leading ultimately to formation of a spatially remote charge-transfer state (CTS) The CTS quantum yields range from 8% for 9 to a modest 25% for 7, and represent direct competition between the two rate constants k,, and k,,, To fully explain this successful competition, proximity of the ferrocene to the charge-transfer reaction centre has to be invoked Indeed, a solid-state X-ray crystallographic structure of 7 reveals the ferro- cene stoppers .rr-stacking to the cyclophane bipyridinium units and forming a more compact closed conformer in which the competitive electron-transfer processes occur (Figure 6) Interestingly, lifetimes of generated CTSs are relatively long (0 5-1 ps), indicative of an increase in separation between the ferricinium and reduced bipyridinium-based cyclophane groups This increased component separation is accredited to strong intra- molecular electrostatic repulsion inducing bead motion away from the ferricinium unit and towards the opposing stopper (Figure 26) It is also interesting to note that whilst the lowest CTS quantum IU J Figure 6 A cartoon representing 7r stacking of the ferrocene stoppers to the cyclophane bipyrrdinium groups yield is observed for two-station rotaxane (9) the lifetime is the greatest (1 ps),and at a first glance could be attributed to the bead shuttling to the opposing station However, this is not the case as ‘H NMR studies indicate that bead interchange between the two sta- tions occurs on a much longer timescale than IS required for return electron transfer between the ferricinium and reduced cyclophane Finally, 12 represents another example of a recently reported assembly in which the stopper is itself photoactive Although no detailed photophysical measurements on such an assembly have been reported it is interesting to note that ‘H NMR experiments are consistent with the bead residing proximal to the photoactive por- phyrin stopper as well as at the expected hydroquinol station of the thread 3.2 [2]Catenanes Relevant examples of [2 jcatenane systems recently developed by the group of StoddartI3 incorporating both photoactive and electro- active subunits, are listed in Figure 7 Within such assemblies, control of translational isomerism is expected by trans-cis switch-ing of the bis(pyridinium)ethylenes, or preferential electrochemical bipyridinium reduction Rather disappointingly, photoexcitation of catenanes 13-18 via the bis(pyridinium)ethylene subunits results in no changes attribut- able to molecular component switching Even using external sensitizersno photoreactions occur and the catenanes remain unper- turbed The lack of reactivity, however, is explained by fast photo- excited state deactivation, caused by strong electronic coupling to a low-lying charge-transfer state More encouragingly, switching is observed through electrochemical reduction of bipyridinium units in 13-15, and relying on the preferred translational isomer being that with the bis(pyridinium)ethylene unbound For instance, vari- able-temperature ‘H NMR experiments on 13 in (CD,),CO con-firms the catenane predominantly existing as the translational isomer in which the bipyridinium unit is sandwiched ‘inside’ the two hydroquinol groups of the polyether crown (as shown) Complete switching of this translational isomer is performed through removal of the charge-transfer ‘braking’ action by means of preferential reduction of the inside bipyridinium unit Upon reduction of the bipyridinium unit, diminished aromatic binding allows circumrotation of the cationic cyclophane and a subsequent encapsulation of the bis(pyridinium)ethylene unit Re-oxidation of the reduced bipyridinium cation generates the non-preferred isomer, which by ring rotation reforms the starting translational isomer A simple cartoon representing this process is depicted in Figure 8, starting from the preferred translational Isomer in the top left-hand corner Returning to photoactive assemblies, three examples of azoben- zene-based catenanes recently reported by Vogtle and cowork- ers 140hare illustrated in Figure 9 Catenanes 19-21 differ from those described previously by virtue of the fact that the photoiso- merizable (E)-azobenzene is not in direct contact with the charge- transfer centre Photoisomerization of (@-azobenzene subunits in catenanes 19-21 leads to generation of a photostationary state in which a mixture of the ZIE isomers exists Thermal repopulation of PHOTO- AND REDOX-ACTIVE [2]ROTAXANES AND [2]CATENANES -A C BENNISTON 43 1 A @Ii-0-‘ 13 R=R,’e 16 R=Re 17 R=@ R’=+ Figure 7 Illustrations of photo and redox active (2jcatenanes containing bis(pyr1dinium)ethylene units reduction -rotation rotationI I -oxidation Figure 8 A simple cartoon representing electrochemical and physical pro cesses occurring in 12lcatenane 13 19 A 21 Figure 9 Examples of photoswitchable 12jcatenanes synthesised by the group of Vogtle ground state conformation is somewhat dependent on the size of the catenanes, with half-lives of 20 5 h and 12 days reported for 19 and 20, respectively More importantly, in both cases internal space reduction by generation of the (2)-azobenzene moiety is manifested in an increased ‘friction’ between the bipyridinium and hydroquinol subunits, in a simple sense photoisomerization acts a kind of mole- cular brake to the circumrotation process Interestingly, unlike 19-20 no (E-2)-photoisoherizationis observed within 21, which is not surprising considering the highly compressed nature of the cate nane In contrast, it is worth noting that isomerization in the simple cyclophane (I e without the interlocked crown ether) is possible, hence reiterating the significant influence interlocking two rings in catenanes imposes Finally, to conclude this section on catenanes examples of por phyrin-based assemblies developed by the group of Gunter15ci-” are illustrated in Figure 10 As depicted, control of catenane conformation is exerted by way of close porphyrin proximity to the cyclophane 2 Specifically, within catenanes 22-23 a face-to-face arrangement of the cyclo phane bipyridinium units and porphyrin rings is maintained Strong evidence for such a phenomenon is a shift in the porphyrin Soret band, ca 20 nm 22, which is most likely due to electronic perturba- tions from secondary T-7r interactions However, protonation of the CHEMICAL SOCIETY REVIEWS, 1996 M= ZH,p= 1 B= 0 22 M=2H,p=2B= 23 M=2H, p= 2 B= %24 Figure 10 Porphyrin-based 121catenanes containing cyclophane 2 prepared by Gunter et al.basic porphyrin nitrogens leads to heightened electrostatic repul- sion between the components and an increase in their separation.It is interesting to note that whilst increased molecular separation in catenanes23-24 allows circumrotation of the cyclophane 2, no such rotation in the more restricted catenane 22 is observed. As this example of dynamic control is brought about by protonation, the system is consequently ionactive, but it is worthy of a mention as clearly there is scope for even more interesting work using the photoactive properties of the porphyrin groups. 33 Miscellaneous Examples Although 25 (Figure 11) cannot be labelled a true [2jrotaxane, and is best described as a pseudorotaxane, the work reported by Balzani et a1.I2exemplifies a potential alternative method for photochem- ically addressing I2lrotaxanes.Balzani's approach again relies on destabilisation of the CT interaction but this time by use of 9- anthracenecarboxylic acid as an external photochemical reducing reagent. Without the influence of stoppers, 25 is in dynamic equilibrium with its individual components, namely the tetracationic cyclo- phane 2 and naphthalene-based thread. Owing to a favourable thermodynamic driving force, electron transfer from photoexcited 9-anthracenecarboxylic acid results in rapid reduction of a single bipyridinium unit in 25. Under normal conditions restoration of the ground state by back electron transfer is too fast to permit dethread- ing, and as a result is of little practical use. However, through the use of a sacrificial reductant (triethanolamine) photooxidised 9- anthracenecarboxylic acid is immediately removed and recycled.With removal of the fast destructive return electron transfer pathway dethreading readily takes place. Indeed, under employed conditions after some 25 min of irradiation up to 35% of the pseudo- rotaxane becomes dethreaded. Ho-w 25 Figure 11 An illustration of the pseudorotaxane studied by Balzani and coworkers 26 Figure 12 The surface-modified [2]catenane incorporating cyclophane 2 prepared by Kaifer and coworkers The necessity to think of modes in which to externally manipu- late well-organised [2lrotaxane and [2]catenane superstructures is gaining in momentum. To this extent, the use of an electrode as not only a [2lcatenane component but also the electroactive support, has recently been explored by Kaifer and c0workers.~6 In this instance, 12lcatenane 26 (Figure 12) is formed by sulfur surface attachment of a thiol functionalised thread and cyclophane 2, by simply leaving a clean gold electrode exposed to a solution of the individual ingredients. Cyclic voltammetry of the modified elec- trode displays typical reversible bipyridinium reduction centred at ca.-0.46 V (vs. saturated calomel electrode, SCE). Even though the extent of surface bound catenane is small (ca. 8%) such an example is encouraging for future development of macroscopic devices based on surface-modified electrodes. 4 Cation Chelating Supramolecular Systems It is well established that preorganisation of molecular components on metal ions prior to final cyclisation, leads to significant enhance- ment in yields of macrocyclic ligands.The first successful synthetic application of such an approach to the field of rotaxane and cate- nane chemistry came through the work of Sauvage and Dietrich-Buchecker.2a Through fashioning of two bidentate 2,9- diphenyl-1 ,lo-phenanthroline ligands around Cul they were able to prepare a tetrahedral complex ideal for cyclisation to the 121cate- nane assembly 31 (Table 3). Leading on from this work Sauvage and coworkers have now prepared corresponding rotaxane assem- blies, again using the preferential tetrahedral coordinating proper- ties of Cur.In particular, 12lrotaxane systems developed by Sauvage and studied by Harrimani7"" using fast kinetic techniques have shown to be prime models for the light-harvesting photosynthetic reaction centre [Figure 2(a) I.4.1 [2]Rotaxane Assemblies Examples of rotaxanes based upon entwined 2,9-phenyl- 1,lo-phenanthroline moieties and in which the large bulky stoppers inhibit dethreading are illustrated in Table 2. The rotaxanes 27-29 incorporating gold(1rr) and zinc(I1) por- phyrins were primarily developed as models for determining spacer effects on electron transfer between two well-separated porphyrin subunits. Specifically, previous work on analogous non-rotaxane systems supported the view that photoinduced electron transfer pro- ceeded via a superexchange mechanism employing the HOMO/LUMO orbitals of the phenanthroline-based spacer rnoiety.l8"" Modulation in energy of these spacer-based orbitals through cation changes in 27-29 was expected to determine the effectiveness of the superexchange mechanism at promoting photo- induced electron transfer.19 In general, selective excitation of 27-29 by way of the zinc por- phyrins causes rapid singlet excited-state electron transfer to the adjacent gold porphyrin, and generation of a charge-separated state (CST) leqn.(111. *(ZnP)----Spacer----(AuP+*)-(ZnP+)----Spacer----(A@) (1) PHOTO AND REDOX ACTIVE (2JROTAXANES AND 12JCATENANES-A C BENNISTON Table 2 Examples of 12lrotaxanes incorporating phenanthroline metal chelators R R M Ref w l+ cu 27(n= 1) 19+-g+J-($NN no metal 28(n = 1) 19 TIPS TIPS In rotaxanes 28-29 the rates of formation (ca 3 X lolo s I) and decay (ca 2 X 10" s I) of the CTSs are somewhat slower than those measured for 27 (ca 5 X loll s and ca 5 X 1010s I, respec tively) clearly demonstrating the influence on electron transfer the copper(1) ion imposes Alternatively, excitation of 27-29 via the gold(m) porphyrin subunits results in generation of the gold(rrr) por phyrin triplet state, which again rapidly decays to form the CTS leqn (a1 (ZnP) Spacer (AuP+)* -(ZnP'.) Spacer (AuP) (2) As before, influence of the copper(1) results in significant lifetime differences in the gold(r1r) porphyrin triplet states, ranging from ca 60 ps for 28-29 to 17 ps for 27 As surmised, observed differences in electron transfer properties of the individual rotaxanes are to a first approximation explained using a through bond superexchange mechanism More specifically, rate constants (k) for the aforemen tioned electron transfer processes are governed weakly by the fol lowing expression k a( 1/6EA,)2,where SEABrepresents the energy gap between the spacer LUMO or HOMO and the donating or accepting orbitals of the appropriate porphyrin subunits Generally, for rotaxane 27 SEA, has the minimum value and accordingly cor responds to the highest electron transfer rate constant Moving away from porphryin rotaxanes the teaming up of Sauvage and DiederichZo has recently resulted in the preparation of the novel fullerene stoppered rotaxane 30 Electrochemical results indicate a significant anodic shift in the redox potential for the Cul/Cuilcouple attributable to destabilisation of the higher copper oxidation state by the strong electron withdrawing power of the fullerenes Proximity effects of the fullerene stoppers also account for complete quenching of the 3MLCT luminescence of the Cu' complex 4.2 [2]Catenane Assemblies Illustrations of Cui based 12)catenanes (carenates) In which the linker between individual phenanthroline ligands either maintain Zn2 29(n= 1) 19+ structural integrity, or are themselves photo/redox active groups are listed in Table 3 Influence of interlocking two coordinating subunits is particu larly noticeable in the electrochemistry of catenates 31-32, espe cially the kinetic stability of formal low oxidation state species 21r1 Comparison of redox potentials for CU~~/CUIand Cul/Cuo in similar non catenated phenanthroline based copper(1) complexes to 31-32 reveals only minor differences in recorded values However, for catenanes31-32 an additional highly reversible redox couple exists Table 3 Illustrations of copper I [ Zjcatenanes R R Ref !Idonono~ /-)onoAono~ 31 21 c!onono- r-Tonono- 32 21 CHEMICAL SOCIETY REVIEWS, I996 at ca -1 9 V (vs SCE) assigned to the reduction process Cuo/ Cu I 5 Concluding Remarks Complete electrochemical and chemical reversibility of this rcouple is attributed to the catenanes exhibiting an enhanced kiinertness towards decomplexation, as a consequence of entwined nature of the ligands This kinetic inertness also accfor the ‘free’ catenane ligands (catenands) being very effectiproduction of other stable low oxidation state metal species Ni+ ,Zn+),21h which are otherwise inaccessible in other ‘siphenanthroline-based metal complexes In solution (CH2C12) at room temp , relatively long-lived exstate luminescence (7 ca 190-280 ns) at A,,, cu 720 nm observed for catenanes 31-32, consequently making them edox netic the ounts ve (e mple’ cited ideal in g is Clearly at the present, studies into utilising the unique structural fea- tures and physiochemical properties of rotaxanes and catenanes are still in the early development stage Currently, we are still limited to mainly probing the properties of aforementioned molecular systems in the solution phase, and have only just commenced on the long road of rotaxane/catenane surface attachment for ‘outside world’ manipulation On top of this, the most conducive method for external stimulation is still debatable, but clearly issues such as selectivity, speed of switching, compound degradation, overall control, etc will certainly play a major role in the final choice photosensitizers It should be noted however that these emilifetimes are not particularly unusual, as other Cui complexes i ssion ncor- 6 References porating 2,9-aryl substituted phenanthroline ligands exhibit si milar 1 G Schill, Caterianes Rotaxaries and Knots, Academic Press, New York. photophysical properties 22 Luminescence is also observed in other 1971 metal complexes of the catenand corresponding to 31, with esion maxima being able to be tuned from 730 nm (Cu’) to 40 mis-0 nm 2 (a)J P Sauvage, Acc Chem Res , 1990,23,319 (b)D B Amabilino and J F Stoddart, Chern Rev, 1995,95,2725, and references therein (LI+)23 As a continuation, recently the assembly 3324has been syn- 3 For a comprehensive review of all aspects of supramolecular chemistry see J M Lehn, Suprarnolecular Chemutrv, VCH, 1995 and references therein thesised containing the electron-donating tetrathiafulvalene uis hoped that photoactive properties of the Cui site, coupled wi th its nit It 4 L Fabrizzi and A Poggi, Chem Soc Rev, 1995,24, 197 5 M R Wasielewski, Chem Rev, 1992,92,435 strong excited state reducing properties, can be harnessed to etually produce a catenane capable of yielding charge-separcomparable to the counterpart 12lrotaxanes Along the same ven-ation line, 6 D Armspach, P R Ashton, R Ballardini.V Balzani, A Godi, C P Moore,L Prodi,N Spencer,J F Stoddart,M S Tolley T J Wearand D Williams, Chem Eur J , 1995.1,33 insertion of two photoactive porphyrinic moieties in [ 2lcate nane 7 A C Benniston and A Harriman,J Am Chem Soc , 1994,116, I153 1 3425 is expected to allow study of through-space electron tran sfer 8 P L Anelli, P R Ashton, R Ballardini, V Balzani, M Delgado, M T To conclude, an example of a novel 12I~atenane~~ in which ecular switching is induced through changes in oxidation sta mol-te of Gandolfi, T T Goodnow, A E Kaifer.A M Z Slawin, N Spencer, J F Stoddart,C Vicent and D J Williams,J Am Chem Soc . 1992.114, 193 the central copper ion is shown in Figure 13 Again, beccopper(1) prefers a tetrahedral arrangement, 35 exists exclus ause ively 9 A C Benniston,A Harriman,D Philpand J F Stoddart,J Am Chern Soc , 1993,115,5298 with the two phenanthroline ligands of the catenane coordinatthe metal Upon electrochemical oxidation however the five dinate geometry requirement of copper(l1) results in a ‘swi ed to coor-nging 10 (a)A C Benniston, A Harriman and V M Lynch,J Am Chetn Soc , 1995, 117,5279, (6) A C Benniston, A Harriman and V M Lynch. 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