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Coordination Chemistry Reviews 252 2008 251272527 Contents lists available at ScienceDirect Coordination Chemistry Reviews ELLEVIER journal homepage wwwelseviercomlocateccr Review Recent developments in the eld of supramolecular lanthanide luminescent sensors and self assemblies Cidalia MG dos Santos Andrew J Harte Susan J Quinn Thor nnur Gunnlaugsson School ofChemistIy Centrefor Synthesis and ChemicalBiologl Trinity College Dublin Dublin 2 Ireland Contents 1 lntrnrlnrtinn 2512 2 Some of the physical properties oflanthanirle 2513 3 Recent examples of cationeinduced luminescent rhan e 2514 4 Recent examples of metal directed synthesis of lanthanide luminescent self hquot 2517 5 Recent examples of anioneinduced lanthanide luminescent rhan e 2520 6 Recent examples of anion directed lanthanide luminescent selfeassemblies 2523 7 39 2524 8 rnmIn inn 2525 2525 Rp arpnrp 2525 ARTICLEINFO ABSTRACT Am39de MSW The lanthanide ions possess long lived excited states which can be populated by sensitizing antennae RECEiVEd 26 MarChmOg and emit at long wavelengths in the visible and the near infrared NIR regions These are particularly Accept 28 13 2008 desirable features for a sensing as it 4 L L 1 ht 39 Available online 22 August 2008 associated with short wavelength emitting sensors and b for probing metal directed synthesis of large J t m n o med between frf or fed metal ions This review article focuses on some Synlama l ofthe recent work published in the areas of quot 39 in ofiou auu illUlELulE and the 53452quot Simmescence 39 eassembly structures 39 NIR or coordination ligands which complement either as a single ligands or through metaledirected self n a L n 1 qunlp L Euro ium J Termini of ligands that can either in a stepwise manner or simultaneously bind lanthanide and transition metal Ytterbium ions will also be discussed Neodymium 2008 Elsevier BV All rights reserved Erbium Sensors Chemosensors Anions Cations Selfeassemblies fifeAssemblies fedeAssemblies Metal directed synthesis 1 Introduction nescence output generated from excited state species can be modulated by external perturbation or inputs be it chemical development of luminescent signalling systems is highly ions andor molecules4e6 or physical eg by using light 78 topical within the eld of supramolecular chemistry 13 Lumie heat 9 etc Such modulation has been used to develop and study many elegant supramolecular systems such as switches sensors etc The attractiveness of using luminescence to study ConespondingauthonTeL35318963459fax35316712826 supramolecular function lies in the fact that emission can be gene Email address gunnlautgjtcdje I Gunnlaugsson erated through energy transfer or electron transfer processes from 0010785453 7 see front matter 2008 Elsevier BV All rights reserved doi101016jccr200807018 CMG dos Santos et al Coordination Chemistry Reviews 252 2008 2512 2527 2513 either purely organic ie aromatic chromophores or conjugated systems or from inorganic coordination complexes and conju gates 10 The use of lanthanide f ions such as EuIII TbIII SmIII YbIII NdIII etc in such luminescent supramolecular sys tems has become a very active area of research ll 13 From the view of developing luminescent chemical sensors changes in var ious photophysical properties such as wavelength lifetimes and quantum yield the outputs can all be modulated as a result of external perturbation This phenomenon can also be employed to investigate the formation and physical properties of complexed supramolecular structures and self assemblies 14 These prop erties of the lanthanides have been the scope of many reviews 15 and we would like to draw the readers attention to the work of Tsukube and co workers 16 Parker and co workers 17 Bunzli 1218 de Sa et al 19 Piguet and co workers 20 Ward 21 and Faulkner and Matthews 22 to name just a few The scope of this review is however to discuss some of the more recent work in the area of luminescence supramolecu lar chemistry This review begins with a short introduction to some of the physical and photophysical properties of the lan thanide Next examples of lanthanide based luminescent switches and sensors where the emission is modulated by the presence of cations and anions is considered Finally supramolecular sys tems where changes in the lanthanide emission are used to probe the structural nature of metal directed self assemblies will be discussed 2 Some of the physical properties of lanthanides The 15 elements from lanthanum to lutetium with access to their 4f shell are known as the lanthanides They are all hard elec tropositive metals and the nature of their 4f shells imparts some unique properties to the lanthanides 23 The electronic shielding of the f electrons is quite weak and hence as one proceeds from La to Lu the increasing atomic number or nuclear charge causes a decrease in the radii of the atoms which is known as the lan thanide contraction The lanthanides are easily oxidised and favour the 111 oxidation state with few exceptions Since the forbitals are buried within the atom they interact only very weakly with lig and orbitals As a result the f gt f transitions which occur in the visible region of the spectrum give rise to very narrow bands and the lifetimes of their excited states are quite long lived us ms 24 In this review article the luminescent properties of the lan thanide ions such as EuIII TbIII NdIII and YbIII will be given particular attention As stated systems which utilise lanthanide A Energy Transfer Vite luminescence as reporters or luminescent signalling moieties have some signi cant advantages over analogous uorescent systems allowing them to overcome the auto uorescence and light scat tering of body tissue 2425 Their long lived excited states ms for EuIII and TbIII allows the short lived background uores cence to disperse before the lanthanide emission occurs Moreover the long wavelengths at which lanthanides emit 500 750nm for EuIII occur beyond the absorbance of body tissue which is important for signal quality Furthermore ions such as YbIII and NdIII emit at even longer wavelengths in the near infrared part of the spectrum which is particularly attractive for diagnostic appli cations 26 The lanthanides line like emission bands 10nm bandwidth can also give a rise to better signal to noise ratio com pared to uorescent bands several hundred nm Furthermore some of these are considered as in the case of EuIII hypersensitive to changes in either the coordination or the magnetic properties of the lanthanide environment coordination sphere This gives rise to signi cant changes in the intensity or the appearances of the lan thanide emission all of which can be used to quantify the number or the nature of the input 2728 In order for lanthanide emission to occur the lanthanide ion must rst be elevated into an excited state Direct excitation of a lan thanide ion is not easily achieved as it requires a Laporte forbidden f f transition low extinction coef cient 8 5 10 M 1 cm 1 As a result lanthanides are usually photophysically silent and have low molar absorbancies However this problem can be circumvented via indirect excitation by a sensitizing chromophore or antenna Fig 1A 29 Excitation of the antennae can pass this energy onto the lanthanide ion by an energy transfer process The indirect exci tation of a lanthanide ion is best illustrated by aJablonski diagram Fig TB The sensitizing antenna denoted Ar absorbs a photon of energy hv and is excited to its singlet excited state 1Ar From here the energy can be passed into the antennas triplet excited state 3Ar via intersystem crossing ISC The lanthanide s excited state Ln can now be populated from the antennas triplet state by means of an intramolecular energy transfer ET provided that this triplet state is higher in energy than the lanthanide excited state 30 The excited energy now resides within the lanthanide ion and can be lost by emission of light resulting in the characteristic lanthanide emission spectra with each transition corresponding to an emission band Though singlet and triplet excited states share a common geometry where their potential energy curves inter sect intersystem crossing from a S1 to T1 state is a spin forbidden process However the presence of the lanthanide ions improves the ef ciency of this process due to the heavy atom affect of the lanthanide which gives rise to a signi cant amount of spin orbit A lAr ISC S Energy Transfer 1 33 gtkLl l 5 D0 gtn 5 Q m GU HI ogm lh l H b Fig 1 A Indirect lanthanide excitation or sensitization involving the excitation of the antenna followed by energy transfer to the lanthanide ion resulting in a lanthanide emission BJablonski diagram illustrating sensitization of a lanthanide ion Other deactivation energy loss pathways have been omitted for clarity 2514 CMG dos Santos et d Coordination Chemistry Reviews 252 2008 251272527 coupling which provides a mechanism for intersystem crossing from the S1 to T1 5 ate Recent examples of antenna sensitized lanthanide luminesi cence developed for a range of applications include systems 174 Complex 1 was developed by Pope who used a diethylenetrii aminepentaacetic acid DTPA core as the lanthanide chelating ligand that was appended with two pyrenyl chromophores by amide bonds 3 Excitation of these antennae resulted in ef 7 cient energy transfer to the excited states of NIR7emitting ions such as ErIII NdIII and YbIII in ethanol A related system is 2 recently developed by Bunzli and coworkers and involved the use of an 87hydrozxyquinolinate based tripodal platform 32 Here the antennae sensitized the NdIII ErIII and YbIII ions in HBS buffered aqueous solution at pH 74 Such tripodal lige ands have recently been explored for the use in the construction of anion sensors using EuIII based emission and chiral quinoi line ligands Examples include that of Tsukube and coworkers who developed 3 for the recognition of Cl after the formation of a EuIII complex between 3 and EuN033 through coordina tion of the pyridyl nitrogen atoms 33 Another recent example is compound 4 that was developed by Parker and coiworkers and employed for intracellular imaging upon excitation at 355 nm 34 1 Ln Eran Ndlll and Yblll The mechanism for energy transfer from the 3ArgtLn state is somewhat debated One possible mechanism is the Forster energy transfer mechanism which involves overlap of energy lev7 els 35 In the Forster mechanism energy transfer occurs through space and has 1 distance dependence where r is the dis tance between the metal ion and the antennae The alternative mechanism for the energy transfer is the Dexter mechanism and involves an electron exchange between the excited chromophore and the metal ion in a through bond interaction and should display a e r dependency 36 For both of these mechanisms the energy transfer process is made more ef cient by minimise ing the distance between the lanthanide ion and the antennae 37 The pathways discussed above are not the only means by which the excited energy can be lost This pathway must compete with other mechanisms through which the energy can be discarded Energy can be lost from the singlet excited state 1Ar through radiative decay ie fluorescence or noniradiative decay by collie sions and vibration interactions with the surrounding molecules Triplet states may also be quenched by interactions with molecular oxygen The lanthanide excited state Ln can also lose its energy through noniradiative decay by vibrational interactions with other molecules such as water In order for the preferred pathway to operate ie population of the lanthanide excited state followed by a L 394 39 39 t e 39 mnt quot 39 lu mother pathways By maximising the ef ciency of the intersystem crossing and 111y are 2 it39 quot quot 39 oftime that the excited energy spends in the 1 Ar and 3Ar states and 1a 1 1 Hr therefore minimise the loss to deactivation pathways from these states As stated before ISC is made possible by spinrorbit coupling which is increased by the presence ofa heavy atom so the very pres ence of a lanthanide ion itselfwill elegantly increase the ef ciency ofrhi nrnr p 12 39 39 391 w I L 39 what isolated from its environment However back energy transfer is possible but can be minimised if the 3Ar state is of suf ciently higher energy than that of the Ln state 38 Vibrational quenching by metal bound water molecules is one of the main competing deactivation pathways 39 The use of a 1 In 39 quot 39 1i dllu the ufmetalbound water molecules and thus minimises noniradiative quenching by 07H but NrH oscillators are often as effective in such quenching In fact as demonstrated by Parker and coworkers such quenching is possible by CrH oscillators though to a much lesser extend 40 The number of such deactivating water molecules associated with lanthanide ion structures can be determined spectroscopically as established by Horrocks and Sudnick by measuring the luminesi cent lifetimes I of the lanthanideiexcited states in H20 and in D20 where it is not quenched by 07D vibrations 41 The method provides a convenient way to monitor the coordination environ ment of the lanthanide ion This quanti cation of the hydration O O O soaH O 1me N E MSW N 0quot x N N N x l N OH N o 2 i I N 3 4 state is referred to as the qevaluequot and for EuIII was determined by Horrocks and Sudnick 41 and later modi ed by Parker 38 by Eq I unltIIIJ12 L 7 702570075x 1 IH20 ID20 Other modi cations of this equation have also recently been published 42 o summarise in order to produce a lanthanide emission an antenna with triplet energy above that of the lanthanide excited state must be positioned in close proximity to the lanthanide ion Furthermore the design requires the lanthanide ion to be tightly bound by a ligand to minimise the number of metal bound water molecules and maximise stability and solubility The next section shall detail some examples where sensors have utilised lanthanide luminescence as a signalling moiety 3 Recent examples of cationinduced luminescent changes 14 111 en nr and 1 1 1 examples of 1 L L switches have b quot cations such as H groups I and II as well as transition metal ions The rst examples ofsuch systems were developed by de Silva et al 4344 and Parker et al 2745 respectively for protons These and many other recent examples 28 have been extensively reviewed 111324 CMG dos Santos et al Coordination Chemistry Reviews 252 2008 251272527 2515 0 2F 0 2 N 2 I N Ol39NN l O In the last a fewyears many new examples have been published New members of this family of sensors have included the Znll sensors 5 and 6 and the pH sensor 7 the latter having also been incorporated into soft materials The charge neutral complex 5 was developed by Pope and Laye 46 who showed that the modulation of Eulllemission could be achieved using the picolyl unit as a Znll receptor This elegant example was designed in such a way that the Eulll coordination sphere involved one of the pyridyl nitroe gen moieties which was directly bound to the cyclen based Eulll ion This not onlyjust ful lled the high coordination requirement of the ion but also enabled the pyridyl antenna to populate the excited state of the lanthanide ion effectively due to its close proxi imity to the ion This coordination also prevented water molecules binding to the Eulll ion Hence quenching by 07H oscillators was also removed However upon addition of Znll the pyridyl moieties coordinates to the Znll opening up a vacant side a the Eulll ions O O I 0 0e 39 o 5 I39 S S O N TN lo R o Lnquot N 39y N l 8 Ln Eull or Tbll pm l SI l l 0 which becomes lled with water and hence the Eulll emission is quenched In a similar manner Nagano and coworkers 47 devele oped a sensor for Znll using a diethylenetriaminepentaacetic acid DTPA based chelate conjugated to a quinoline antenna to prepare the Eulll and Tblll complexes of 6 This sensor was shown to bind Znll through the tetrakis27pyridylmethylethylenediamine TPEN part of the structure which also function as the antenna A pH dependence study showed the ability of this receptor to bind to group I II and transition metal ions The results showed that the sensor was pH independent between pH ca 479 and that the emission was signi cantly modulated in the presences of Znll Unlike the above two examples com ound 7 was only found to be sensitive to pH 48 While the solution behaviour of this molecule and other related pH sensors is established the sensing behaviour within soft materials such as hydrophilic polymers had not been explored 49 With this in mind lt III 0 O 0 2kg N N l N N O EEugo N o N O I Eu NJ we incorporated this molecule into three different hydrogel matrices comprised ofcrosslinked polymethylmethacrylateecoel hydroxyiethylmethacrylate The control hydrogel matrices were prepared using methylmethacrylate MMA and 27hydroxyethyli methacrylate HEMA in three different ratios 1 MMAHEMA ww 13 MMA HEMA ww and 100 HEMA All the hydrogels demonstrated Eulll emission upon excitation of the antenna and 39 be ith l quot fpasafunci than that imnnrtanth tionoan Nu 39 39 quot u 0 observed in solution Recently Accorsi and coworkers 50 have incorporated several lanthanide complexes into silica single layers by doping the SiOZ layers with compounds 8 Here the emission was not modulated by external source such as cations but served as an extension ofthe hydrogel approach and a further example of the versatile role the lanthanides play in current material design 9 Ln Eulll or Tbll NMez n12 Another example of the use of cations to modulate the lane thanide emission is 9 which was developed to sense Na or K ions depending of the size of the crown ether ring 5152 The resulting Eulll or Tblll complexes could yield an overall charge neutral or cationic species Only the Tblll complexes gave rise to signi cant changes in the emission spectra The Eulll systems were found to be only weakly emissive due to effective electron transfer quench ing of the Eulll excited state by the crown ether antenna Both the nl or 2 Tbicomplexes of 9 were investigated in buffered aque ous solutions The luminescence of both exhibited interesting pH dependence where the emission was switched on in either highly acidic or basic media while being quenched within the physiologi ical pH range Consequently the complexes could be employed as a sensor for Na or K within the physiological pH range The ditopic macrocyle 10 developed by Pope and Rice is an extension of this idea 53 It consists of a crowneether fused pyridylicryptane ring the latter able to complex lanthanide ions 2516 such as Ndlll In CH3CN complexation of Ball by the crown ether moiety modulated the NIR emission of Ndlll Here the chelation of the Ball perturbed the sensitization ability of the bipyridine ligand resulting in signi cant emission quenching Sig ni cant changes were also observed in the absorption spectra of 10 upon binding to Ball Wong and coworkers developed the Tblll complexes of 11 and related analogues which showed that the emission was modulated in the presence of ions such as Na but particularly in the presence of K in aqueous solution 54 This design principle was also used by the same authors to develop sensors for anions 55 DEC EOJO N O H 27 H N gig NW 0 twin g VKOv f 01Li O o 11 O 12 0 N H N I We NJ I N W 039 Eum O N N NJV o Previously we discussed the Znll sensors 5 and 6 Related to these systems is compound 12 56 This example was originally developed for group 11 metal ions in collaboration with Parker and coworkers 57 However the sensors gave excellent selectivity for Znll where the Tblll emission of 12 was signi cantly mode ulated The corresponding Eulll complex suffered however from poor quantum yield due to quenching of the Eulll excited sate by the antenna Compound 12 was also one of the rst examples of fed metal ion assemblies Compound 13 was developed in our laboratory as a potene tial sensor for demetal ions 58 This compound based on the 5N5 can Eulll Emission Intensity Wavelength n m CMG dos Santos et ed Coordination Chemistry Reviews 252 2008 251272527 phenanthroline antenna which has also been elegantly employed by Armaroli and coworkers 59 for the formation of lanthanide luminescent complexes also displayed pHemodulated emission in a similar manner to that seen for 9 However here the emission was switched on within the physiological pH range 58 This system wasusedfor L 39 39 Culll but 39 L I and Collalso modulated the Eulll emission of13 albeit at signif icantly higher concentration than Cull 58 We had anticipated that Cull would be capable of coordinating two equivalents of13 to form a new selfeassembled system Indeed this was found to be COZH COZH N N EEUIH COZH LN N O N the case but unexpectedly we also observed the formation of 1 3 complex between 13 and Cull Thiswas one ofthe rst examples of the formation of fed metal ion assembly through the use of struce turally de ned ligands Previously the charge neutral compound 14 had been prepared for the use in MRI and while no luminescent properties were reported the same approach to constructing mixed fed metal ion frameworks was explored 60 To overcome the formation of mixed frdrf and f3 rd assemblies in solution we undertook the synthesis of15 where the terpyridine chromophore acted as both antenna and demetal coordinating Hg and 61 The Eulll emission from 15 was clearly visible at pH 74 u 1 700 500 550 Wavelength nm Fig 2 Left The changes in EuIII emission upon coordination of FeII to 14 Right The formation of the MLCT band in the ground state spectmm of 14 upon addition of FeII CMG dos Santos et al Coordination Chemistry Reviews 252 2008 251272527 2517 in buffered aqueous solution The addition of transition metal ions such as Fe II resulted in ca 95 quenching of5D0 7F This clearly demonstrated the sensitivity of the Eulll emission to coordination status of the chelating antenna Fig 2 left This could also be fol lowed by UVrvis absorption measurements the appearance of a long wavelength absorption band signifying the presence of the MLCT state In this system the Eulll luminescence quenching is most likely due to energy transfer occurring from the lanthanide excited state to the 3MLCT state of the Fell terpy complex Fig 2 right The y i39uility ft 39 wa also 39 39 and as had previously been seen for 13 the addition of ion scavengers such as EDTA resulted in the reeestablishment of the lanthanide emission 4 Recent examples of metal directed swthesis of lanthanide luminescent selfassemblies As can be seen in the above discussion the use of lanthanide ions as reporters for sensing of cations is particularly attractive Furthermore the interaction between 13 14 and 15 with dimetal ions all results in the formation of mixed and ordered fed metal assemblies Building from this rationale several other exam ples of lanthanideibased metaledirected synthesis of functional assemblies have recently been reported These lanthanideebased selfiassemblies have the potential to yield novel photochemical properties Compound 16 capable of binding two lanthanide ions was developed byTremblay and Sames 62 This elegant structure is based on two welliknown lanthanide coordinating ligands DOTA and DTPA both ofwhich featured in the above discussion In 16 the presence of these two binding domains gives rise to mixed lane thanide luminescence upon excitation of the tryptophan antenna This frf metal assembly is one of the rst examples of this type where two different lanthanide ions could be incorporated into one ligand Faulkner and Pope 63 also attempted to developed such systems using DTPA as a central linker and two coordinatively unsaturated Tblll based cyclen moieties the heterotrimetallic lanthanide complex 17 was formed This frfrf system gives rise to Yblll based NIR emission at 980nm upon sensitization from the cyclen based Tblll centres The heterotrimetallic luminescent Ndlll complex 18 was developed by Pikramenou and coworkers for the sensing of DNA 64 Again this system was based on the DTPA ligand as a central linker which is flanked with two diethylene etriaminepentaacetic acid derivatives which provide two binding sides for the Ptll This gives rise to the formation ofa difid sys tem r Ptll r L39 J to DNA through intercalation The two dimetal moieties were also used as sensitize ers for Ndlll which gave rise to NIR emission upon excitation of the Ptll centres E o 2 6 0 As can be seen in the above discussion the use of fif and fed mixed cations and simple ligands has led to the formation of novel selfiassemblies Other examples ofsuch designsinclude that ofBeer et al who developed the Rull bipyridyl complexes 19 contain ing one two or six lower rim acidiamide modi ed calix4arene macrocycles covalently linked to a bipyridine group 65 The 2518 CMG dos Santos et d Coordination Chemistry Reviews 252 2008 251272527 calix4arene moieties were able to coordinate several lanthanide ions such as Ndlll Eulll and Tblll in a 21 Lncomplex This coordination gave rise to signi cant changes in both the absorp tion and emission spectra of the resulting assemblies The authors conducted inedepth luminescence studies of these systems upon excitation ofthe trisbipyridine7ruthenium II complex at 452 nm minescent nature of the donating state the 3MLCT state allowed a detailed study of the energy transfer process to the lane thanide centres Here the ruthenium luminescence was quenched by lanthanide ions such as Ndlll while for ions such as Tblll and Eulll it was enhanced A related system is 20 66 As in the case of19 the Rull bipyridyl complexes sensitized the excited states of both Ndlll and Yblll by energy transfer from the triplet excited state of the rutheniumitrisbipyridine complex upon excitation in the visible region Other examples of NIR emitting selfeassemblies based on 13 and developed in our group is compound 21 In collaboration with Faulkner we demonstrated that the rutheniumrbisbipyridine capped complex of 13 gave 21 which emitted in the NIR for ions such as Ndlll and Yblll upon excitation of the MLCT band of the complex in water 67 While the Yblll emission was signi cantly quenched due to back energy transfer the Ndlll emission was found to be particularly strong for this system The Rull cene tred emission being signi cantly less intense for the latter system indicating ef cient sensitization of the Ndlll excited state The interaction of this complexwith calfethymus DNA was also investie gated under buffered conditions However only about 20 changes were seen in the NIR emission for these systems while the Rull based MLCT and the Rull centred emission at ca 900nm was reduced in intensity upon binding of 21 to DNA The porphyrin based cyclen complex 22 is another example of the use of mixed frd complexes 68 This system developed by Parker et al has a Pdll conjugated porphyrin moiety that was covalently linked to a chiral cyclen Yblll complex In this mixed metal ion system the excited triplet state of the porphyrinate was quenched by 02 upon binding ofthe complex to DNA This resulted in an enhancement of the NIR lanthanide luminescence Me 22 CF3 N o 5 3 OH N N 0 CF NI OH N LNlII N N 3 3 L J o o 25 24 Another example of such mixed fed systems for DNA targeting is that of Pikramenou and coworkers discussed above 64 While all of these systems employ excitation of the demetal complex Bruce and coworkers has recently extended such DNA targeting toward the use of organic chromophores eg 23 69 This com pound was found to bind to DNA with concomitant changes in the lanthanide emission upon excitation in the visible region The use of organic chromophores as antennae in such NIR emitting systems has become quite popular in the last a few years as exam ples 173 demonstrate Additionally Faulkner and coworkers 70 h that39 r 39 Lasthe anthracene based system 24 can also be used as sensitizers for NIR lanthanide ions This NIR emitting work has been further devele oped by Faulkner an example is that developed in collaboration with RodriguezeBlas based on the use of simple aryl imine based azacrown ethers eg 25 71 It is clear that the examples discussed above highlight the scope afforded by mixed frf or fed metal sys tems and in particular of current developments in NIR emitting organic supramolecular lanthanides as well as being is a highly attractive area of research CMG dos Santos et al Coordination Chemistry Reviews 252 2008 251272527 2519 N CV N E1N NEt N N N HOOC 26 N COOH I N N H Njyf l O N 7 2 I 1 HO H H OH N N O O O O 23 OOON C a NOOO 2N N N N HOOC 29 COOH WE JNC N N O OH OH O 31 N 39 N N gN 32 N N N 0 O N o 7 NK 0 3 N O 0 0 N i 39l H H 34a RR 34b 38 Furthermore mixed fif metal systems also offer the ability to r ff 39 a sime ilar manner to the transition metal based helicates developed by many others A recent elegant example is that of Faulkner and Ward which involved the use of polynuclear lanthanide complexes that were bridged by two tridentate NOidonors 72 This gave rise to the formation of helical structures that were analysed using both solution and solid states methods A number of other such lanthanide directed structures have been achieved using acyclic ligands such as 26732 Compound 26 developed by Bunzli and co workers was designed to facilitate the assembly of a new class of lumine tem quot quot 1 73 TL39 Jwas used in the formation of the neutral tripleistranded dimetallic helie cates where all the lanthanide ions were investigated where the three equivalents of 26 wrapped around the two nineicoordinate LnIII ions in pseudoiD3 symmetry Ln2263 The formation of this helicate was monitored by changes in the ground and the excited states of the ligand and by monitoring the appearance of the lane thanide emission at long wavelengths Compound 27 is another example of such a selfiassembly that gives rise to the formation of tripleistranded helicate an273 6 for a 23 Ln27 ratio 74 Many other examples were developed in collaboration between the Swiss based groups ofBunzli and Piguet in this area 12 Com ound 28 is another example ofa ligand able to form a lanthanide directed helie cal system 75 Developed by Horrocks et al ligand 28 consists of the use of tridentate chelating units dpa 267pyridinedicarboxylic acid linked by an organic diamine In these each ligand can coordii 439 metalinn 39 39 rise to the formation of dinuclear trigonal prismatic D3 geometry However the authors reported that these assemblies were formed as mixtures in solution A number of related systems have been developed by the Bunzli group for application in lanthanide luminescenceibased intraceli lular imaging which has only recently been achieved successfully The addition of two polyethoxy chains onto the structure of 26 afforded 29 76 Again lanthanide emission changes yield infor7 mation on both the stability and the ionitoeligand stoichiometry of the resulting assembled complex Importantly the ethoxy chain conferred water solubility to the complexes allowing their incor7 poration into cells such as HeLa jurkat and SD10 facilitated most likely through edocytosis The complexes were imaged by excitai 2520 CMG dos Santos et d Coordination Chemistry Reviews 252 2008 251272527 tion of the ligand based antenna and observing the Eu III emission This design is very attractive as it also opens the door to the use ofmixed luminescent and magnetic detection MRI of tissues and cells and possiblylarger organs Other recent example oflanthanide complexes for cellular imaging is 4 previously discussed above and developed by Parker and coworkers 34 This area ofresearch has recently brought new dimension to the eld oflanthanide chem istry 14l7a Other helical structures developed for lanthanide directed syn thesis include examples such as 30 77a The structure obtained from 30 was analysed by various spectroscopic techniques but the authors were unable to obtain Xeray crystallographic structure of their resulting helicates Several other examples of the use ofboth 39 and 39 If mh which results in the formation of selfiassemblies containing large nume ber of metal ions and detailed analyses of their optical properties including studies oftheirvisible and NIR emissions have also been developed by several researchers such as Bunzli and coiworkers 77b Bunzli and coworkers 77c Mazzanti coeworkers 77d and Houetal77e Lquot L 394 L of this review we direct interesting readers to these publications and the references therein Compound 31 recentlyyielded another example ofan NIR emit ting helicate constructed using either two Yblll or a mixture of Yblll and Allll ions 78 Interestingly for these structures KI was also found to coordinate the center of the helicate possibly functioning in the selfiassembly process as a template as it coordi7 nates both the hydroxy groups on each wing of 31 giving rise to a six coordinate KI species Compound 32 was designed and synthei sised by Piguet and coworkers and is the rst example ofa triple stranded helicate with more than three metal ions 74 This elegant structure was designed by employing a novel strategy developed by the Geneva group While this example is a major step in the nf predictable 7 quot usin lanthanide directed synthesis it does suffer from lengthy ligand synthesis In the case of33 developed byMamula et al 79 a large chiral complex was formed which gave rise to both intense lane thanide luminescence that could be resolved by circularly polarised luminescence CPL The use of metal directed synthesis has become a very popular area of research and ligand 34a and 34b consisting of a sime ple pyridy17267diamide unit flanked by two chiral naphthalene moieties was developed in our laboratorywith the aim ofusing lane thanide ions to direct the formation of novel selfiassemblies 80 Inourca 2 L 39 theRR 39 39 eithe visible or NIR emitting lanthanide ions resulted in the formation ofa single product for each ligand per lanthanide ion In all cases a l 3 Lnligand stoichiometry was obtained The presence of the chi ral moieties in 34 aided in the formation of highly ordered chiral selfiassembly The chirality of these selfiassemblies was resolved using Xiray crystallography These result showed that this rather simple g and gave rise to helical structure and that the 55 ligand induces A chirality about the metal ions whereas the RR ligand induces A chirality as demonstrated in Fig 3A These chiral systems also showed each of the threeipyridyl moieties intercalated between two naphthalene moieties one from each of the remaining three ligands This gave rise to an extended lTi lT network that was also observed in the packing diagram of these structures which con sisted of honeycomb networks ofinterpenetrating Lnecomplexes of 34 This was characterized by a variety of spectroscopic techniques including Circular Dichroism and CPL The complexes gave rise to CPL upon excitation of the antennae with enantiomers giving rise to equal and opposite signs in the CPL spectra as shown in Fig 3B for the Smlll complexes The steadyistate Smlll luminescence is also demonstrated in Fig 3B These examples demonstrate the use oflanthanide directed synthesis in the formation of chiral self assemblies from structurallysimple but de ned ligands Here both the steadyestate as well as the CPL can be employed to elucidate the structural features of the selfeassembly of such systems These ligi ands are similar to those developed by Piguet and coworkers that gave only a mixture of products 81 and more recently of that of Muller and coworkers who also elegantly demonstrate the use of CPL in the analysis of his systems 82a Similarly the use of CPL has recently been demonstrated by Muller and coworkers and Raymond and coiworkers for probing the luminescence of Eulll complexes derived from octadentate ligands based on leh droxyi 27pyridinones 82b and chiral 27hydroxyisophthalamide ligands 82c This section has dealt with the modulation of lanthanide lumii he cenceeiLhe 39 39 39 ex rna pe 39 39e group I II and d7 or even fimetal ions modulated the lanthanide luminescence within a coordination complex This was followed by the discussion of some of the more recent examples of other supramolecular architectures which are formed by the use oflani thanide ions and the way their luminescence can be employed to elucidate their structural features The next section details some of the recent examples of lanthanide complexes that have been developed where the lanthanide emission is modulated through the coordination of anions 5 Recent examples of anioninduced lanthanide luminescent changes Monitoring the concentration of ions and molecules in vivo is of critical importance as man of these ions and molecules are involved in crucial biological and chemical processes 45 Anions in particular 5 are essential to life as many biological processes depend on the presence or transport of these negatively charged species or use them to carry out chemical transformations They TL and CPL3910 a 2 700 750 600 650 Wavelength nm Fig 3 AThe Xiray crystalsn39ucture ofthe selfiassembl of 39 rm L 394 39 39 the A chiralin39es of34 as measured using CPL Published with permission from Journal of the American Chemical Society CMG dos Santos et al Coordination Chemistry Reviews 252 2008 251272527 have also become progressively more used in industrial processes as well as in agriculture which consequently puts them in the class of pollutants It has thus become evident that there is a signi cant need for the development of synthetic receptors able to report the presence ofanions This has been the driving force for the formation of anion sensors in recent time The high coordination numbers of the lanthanides is often dif cult to achieve Hence coordinative unsaturated complexes often ful l this requirement by coordinating additional solvent molecules or ligands such as anions Such binding interactions may perturb the excited state properties of the complex directly due to the direct binding to the lanthanide ion or through interac7 tions with other coordinating ligands such as antennae Relatively hard anions such as carboxylates or phosphates can displace metal bound water molecules with subsequent modulation in the lumii nescence properties of the lanthanide ions Examples of such architectures that exploit this are compounds 35738 2521 mined by excited state measurements in H20 and D20 However it was observed that in aqueous solution both water molecules could be displaced from 36 by anions such as HZPO4 acetate HCOg and C0321 to yield a 1 adduct The formation of the resulting ternary com lex was signalled by large changes in the lifetime of the EuIII and TbIII excited states In contrast the lifetimes remained unchanged for ions such as Br and Cl while in the case of CH3COO F and HZPO4 1 was determined to be ca 1 imply ing the loss ofa single bound water molecule upon coordination of these anions to the lanthanide ions Compounds 37 and 38 also exploit coordinatively unsaturated lanthanide complexes all of which have 12 for sensing Come pounds 373 and 37b were developed in our laboratory 86 and lack the antennae shown for 36 In these systems displacement of the metal bound water molecules by coordinating aromatic su s N quot 39 39 39 39 39 aci and salii cylic acid results in the population of the TbIII excited states 02 ff R HSC N 20 l H OH N NI 2 ofan R N N OH2 O 39l L Z CH3 0 5 0 Eu 0 p Me Oquot 35 0 N CH3 Me o rMe NH R Ph 38 Me 373 Ln Tb R CH3 35 37b LnTbIRH Ln Tbll Eul Compound 35 was developed by Parker and Williams as a sen sor for CI 83 The system is based on the use of Nealkylated phenanthridinium antenna which is known to be quenched by halide anions in aqueous media Indeed the addition of halide ions to 35 leads to a decrease in the intensity of the phenani thridinium fluorescence 405nm and an associated decrease in the EuIII luminescence intensity by a factor of four The halide quenching effect was independent of added acetate HZPO4 cite rate and HCOg and followed the quenching order I gtBr gtCl the mechanism of which was proposed to involve charge trans fer from the halide ion to the excited singlet of the chromophore This phenomenon has been further employed by the Parker group in the development of assays for CI ion in biological media 84 Developed by Parker and coworkers 85 complex 36 is coordii natively unsaturated as the cylen ligand only satis es seven of the coordination sites for lanthanide ions such as EuIII and TbIII This complex possesses three chiral phenyl antennae which ope ulated the excited state of these ions through sensitization The luminescence of36 is quenched by effective 07H harmonics in the presence of two metal bound water molecules 36 129 q 2 as deter 87 A similar design was reported by Faulkner et al who used the charge neutral IR7emitting YbIII complex 38 for the sense ing of tetrathiafulvalene carboxylate 88 In methanolic solution the carboxylate coordinate 38 through displacement of the two bound water molecules in a similar manner to that of 36 and 37 upon which the tetrathiafulvalene antenna is then able to transfer energy to YbIII with concomitant emission from the lanthanide ion in the NIR More recently this research group demonstrated the sensitization of the NdIII analogue of 38 using pyrene care boxylate as the sensitizing antenna 89 Upon coordination of the pyrene carboxylate in water intense characteristic NdIII emis sion bands at 1055 and 1350 nm were observed These results are of signi cant importance for the development of lanthanide based sensors and probes for biological applications in the NIR as well as it demonstrated the use of organic chromophores with longer absorption wavelengths gt400 nm as antennae for such systems 2522 OH 3 o 2 R O 04 1 40 R COZEL R39 Me 41 R 00239 R39 Me CMG dos Santos et d Coordination Chemistry Reviews 252 2008 251272527 R39HNrR ZNFTNNN O HN PhCHzYCOQEt o NTN O oEE 1M EKOZC N N s N PhCH2 H 0 NH PhCHZJ COZEt 43 42 R cog R39 CH2200239 o U 0 I 0 V0 0H 0H2 44 45 In a similar manner to that shown above Li and Wong have reported that a cyclen based Tblll complex 39 capable of sigi nalling the presence of anions such as acetate and salicylate under physiological conditions 90 As in the case for the previous exam ples 39 lacks the presence ofa sensitizing chromophore and hence is noniluminescent However the luminescence lifetime and inten7 sity are observed to vary greatly upon addition of lactate and salicylate Other derivatives of this design have also been reported by this research group 5455 A series of cationic zwitterionic and anionic Eulll complexes such as 40742 have recently been developed by Parker and coworkers for the detection of HCOg 9192 Here the acridone chromophore served as an antenna the advantages of using such an antenna as a sensitizer had been pre viously noted by Faulkner et al which allowed sensitization of the Eulll emission following excitation at 3907410nm Both 40 and 41 were found to selectively bind bicarbonate at physiological pH 91 The binding process was signalled by changes of the lumii nescence lifetimes and emission intensities of the Eulll where q changed from 1 a 0 This supported the displacement of the metal bound inner sphere water molecule By monitoring the ratio of the emission intensity of Eulll ofthe 618588 nm or 618702 nm emis sion bands the solution concentration of HCOg could be assessed in a background of competing anions such as acetate citrate and 1421704 Compound 42 is a particularly interesting extension to this design idea as it could be employed for the sensing of phosphates 92 The binding of selected phosphates as well as phosphorylated amino acids such as Ser70P and Tyr70P was investigated by observe inc L I I t L Obnlnl I J the formation of 11 adducts which displayed subtly different emission spectra with Tyr70P showing the lowest Aj2Ajl intensity ratio In each case the data obtained was consistent with formation of monoiaqua species with phosphate anion act ing as a monodentate ligand The Parker group has recently further extended this design principle by employing coordinatively unsati urated lanthanide complexes for sensing larger polyianions such as DNA as well as developing novel lanthanide based markers for imaging of living cells an example of the latter is compound 43 93 This was found to selectively stain nucleoli of NIH 3T3 HeLa and HDF cells and could be used in xed cell imaging 17a Come pound 44 was developed for targeting DNA where the antenna was able to intercalate into DNA with concomitant modulation in the lanthanide emission 94 The Parker group has more recently further extended this design to target uric acid in serum 95 as well as other biological species 96 Compound 45 was developed in our laboratory 97 This dinucleariEulllibismacrocyclic conjugate was designe as a 1 ed 39 39 39 394 forquot 39 39 sen sor was shown to bind small dicarboxylic acids such as aspartic malonic succinic and glutaric acid in pH 65 solutions Neverthe7 less only malonic acid gave rise to selective Eulll luminescence enhancements as the emission intensitywas reduced for the other acids Compound 45 gave rise to ternary complex formation with anions which was formed upon displacement ofmetal bound water molecules In a similar manner we have also developed complexes of the Eulll complex of36 using the antenna 46 which can bind to the metal ion by displacing the aforementioned metal bound water molecules 98 This gave rise to a very stronglyluminescent assem7 bly where the red emission from the Eulll ion was visible to the naked eye However upon titrating this complex with anions such carboxylates and phosphates the antenna was displaced and the emission 39 L offdue to t This work has recently been extended by incorporating these come fthp ntpnn CMG dos Santos et al Coordination Chemistry Reviews 252 2008 251272527 2523 plexes onto gold nanoparticles via an alkylthiol linkage This gave rise to the formation of highly saturated lanthanide complexes that were employed for the sensing of large biological relevant anions through the use ofdisplacement assays 99 This part of this review has discussed some of the recent devele opment of lanthanide complexes that can interact with anions which results in modulation of the lanthanide luminescence While the focus has been mostly on anion sensing then the idea ofbeing able to replace either metal bound water molecules or coordinate ing antennae can also be employed to give rise to the formation of higher order selfeassembly structures This will be the topic of next section where a few recent examples will be discussed 6 Recent examples of anion directed lanthanide luminescent selfassemblies In many of the examples discussed in previous section anion recognition and sensing was achieved by the coordination of the anion directly to the lanthanide centre yielding the formation of selfeassembly ternary complexes Thus anions offer the potential to direrthioherordera emhlie Nextthen enf quot 39 offers the ability to extend this rationale to the assembly of multi lanthanide centred systems is considered The diinuclear come plexes 47 100 and 48 101 are examples of lanthanide complexes capable of this Faulkner et al reported the bridged bisecyclen sys tem 47 which was able to accommodate two lanthanide ions with each of the cyclen moieties giving rise to the formation of overall charge neutral complex Excitation of the bridging antenna of the various lanthanide complexes gave rise to the typical lanthanide emission In contrast the cationic xylyl based complex 48 was prepared in our laboratorywith the aim ofa developing luminesi cent sensors for diicarboxylates where the anion would bridge the two metal ion centres or b forming novel selfiassemblies where intermolecular manner binding of a suitable anion would give rise to 22 stoichiometry This complex was able to detect the pres ence of mono or bisiaromatic carboxylates such as terephthalic acid in buffered aqueous solution at physiological pH which was found to occur through the binding of the carboxylates to the metal centre via the displacement of the metal bound water molecules This gave rise to signi cant enhancements in the Tblll emission However in the presence of tartaric acid the recognition process gave rise to quenching of the Tblll emission intensity Compound 49 is an example of an pH dependent offron switch where the Eulll emission of49 was greatly enhanced upon deprotonation of the sulphonamide which lead to displacement of one of the metal bound water molecules of49 102 O O N N N ogbn Hzo O ETb H o N O N 2 OH AV NLN o 2 o 4 0 N o o 47 48 49 f lt0 O 2524 Reinhoudt and coworkers have recently described the Eulll and Tblll complexes of the EDTAibased Bicyclodextrin dimer 50 which operate in H20 at pH 70 as luminescent sensors for aromatic carboxylates 103 As the ligand does not provide any sensitizing antenna no luminescence was observed Nevertheless upon the addition of 51 the luminescence intensities of both the Eulll and Tblll complexes increased by a factor of 350 and 310 respectively upon excitation of the aromatic antenna at 250nm These large enhancements were ascribed to the hydrophobic bind ing contribution of the Bicyclodextrin units which can bind the aromatic unit thereby positioning the Lnlll coordinating carboxyl group correctly to function as a sensitizing antenna and allow ef 7 cient energy transfer to the lanthanide ion The stoichiometry was reported to be 12 with a stability constant of K 50 x 104 M4 As expected the addition of the benzyl alcohol 52 led to much smaller enhancements of ca twofold The authors explained this luwei 39 a nwin tn thelack of 39 39 E I 39 ity through which the chromophore can bind to the lanthanide ion T 39 39 llHrnmnlex nf503nrlquot 39 biphenyl sensitizers 53 and 54 was also investigated Once again upon excitation of the sensitizers at 285nm large increases in the Eulll luminescence was observed ca 3007 and 3607fold for 53 and 54 respectively in D20 A 11 binding stoichiometry was observed for both selfiassemblies with very high association con stants which suggest that both of the Bicyclodextrin cavities of50 were involved in the binding of the bisadamantyl biphenyl sensii tizers 53 and 54 CMG dos Santos et ed Coordination Chemistry Reviews 252 2008 251272527 for the population ofNIR emitting lanthanide ions within the visii ble region An example of one of these designs is 56 105 but this work has recently been reviewed in depth byWard 21 and in other publications by Faulkner and coworkers 106 and consequently will not be discussed further herein Compound 57 is an example of a selfiassembly developed by DeCola and coworkers consisting of a heterotrimetallic drfrd in which the sensitization of the Eulll excited state occurred energy transfer from the two Irlllibased moieties 107 Importantly the complex is achieved though anionidirected coori dination Unlike many of the above examples here the emission from the selfiassembly was almostiwhite light originating from the combination of the red Eu III emission combined with residual blue emission occurring from the iridium complexes Such white emitting molecules are of great current interests and the present example demonstrates the role that lanthanide ions have in the 39 of such systems 7 Miscellaneous Unlike that of dimetal ions the use of lanthanide ions in the metal directed synthesis of more traditional supramoleci ular structures comprising either rotaxanes and catenanes has not been much explored Loeb and coworkers have recently N F F N 55 N 1 57 N N quotN N Ir F F I I I I N CN CNN N Ell0N NC I I F F IN CNN CNNI Tb N Pk 56 Compound 55 developed by Faulkner and BurtoniPye is an extension to the displacement ofthe metal bound water displace ment discussed above 104 This mixed frd selfiassembly was formed by the direct coordination of carboxylate functionalised Rull or the Osll M in structure 55 coordination complexes directly to the lanthanide ion LnYbIII Ndlll or Erlll via the displacement ofwater molecules form the cyclenrlanthanide com plex This enabled the population of the lanthanide ions by the MLCT band of the transition metal complexes in a similar man ner to that discussed above for 22725 This idea has been further extended by the Faulkner group in collaboration with Ward et al who has used varieties of such coordination complexes as antennae addressed this de cit by developing threeidimensional polyrotaxi ane from lanthanideiion nodes based on pyridinum Nioxides axles and crown ethers ie 58 108 The use of 58 in conjunction with various lanthanide ions gave rise to the formation of coordination networks where each of the lanthanide ions was coordinated to six equivalents of the ligand as shown in Fig 4A This gave rise to a three dimensional network of interpenetrating polyrotaxane self assemblies Fig 4B This is to the best of our knowledge the only i p L 4 J i the potential that the fimetal ions can play in the construction of such higher order selfiassemblies However the luminescent prop erties of this selfiassembly were not reported CMG dos Santos et al Coordination Chemistry Reviews 252 2008 251272527 2525 Fig 4 The crystal structure of 58 after reaction with SmIII giving rise to the formation of 61 coordination environment around the lanthanide ion B The packing diagram ofthe resulting polyrotaxane Published with permission fromAngewcmdte Chemie The last example of this short review is 59 developed in collabe oration between Faulkner and Beer which upon interacting with 60 gives rise to the formation of 2 pseudorotaxanes between the two molecules where 59 LnGd Nd Yb is complexed within the cavity of 60 109 The resulting structure was described to be formed through anion templating effects and is to the best of out knowledge the only example of such structures which results in I I I I IHIAY nP I tion of the Reecoordination complex ef cient energy transfer was observed to the excited state of the NIR emitting ions YbIII and NdIII and concomitant emission in the NIR of these ions 8 Conclusion This short review has focused on the development of a few and selective examples from the recent literature on the modula tion of lanthanide luminescence either in the visible or the NIR by both cations and anions and the formation of selfeassembly structures which are formed either in such a way that the lane thanide ions directs the formation ofthe selfeassembly or partakes in it through mediating as an anion acceptor The examples herein 1 ml 1 39 39 392 J quot JiiiLiii eld ofresearch in the past few years and that there still exist is an enore p E n 1 function as recently pointed our in an excellent review by Bunzli et al 110 entitled New Opponunitiesfor Lantham39de Luminescence We look forward to follow and participate in the steady grow of this highly topical research area in the years to come Acknowledgements The authors would like to thank the University of Dublin Trinity College Ireland Kinerton Ltd now lpsen Ltd Enterprise Ireland comm N I O H N 04 7O O 60 EI Science Foundation Ireland SFI Irish Research Council for Science Engineering and TechnologyIRCSET CSCB and the Well come Trust for nancial assistance References 1 See special volumes on Photochemistry and Photophysics of Coordination Compounds V Balzani S Campagna Eds Top Curr Chem 281 2007 2 Special issue on colorimetric and luminescent sensing J Mater Chem 1 2005 2617 3 a MartinezeMa ez F Sancenon Chem Rev 103 2003 4419 b C Suksai T Tuntulani Chem Soc Rev 32 2003 192 a AP de Silva B McCaughan BOF McKinney M Querol Dalton Trans 2003 4 1902 bF Callan AP de Silva DC Magri Tetrahedron 36 2005 8551 c V Balzani Photochem Photobiol Sci 2 2003 459 d V Balzani A Credi M Venturi Pure Appl Chem 75 2003 541 e K Rurack Spectrochem Acta A 57 2001 2161 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