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From metal complexes to fullerene arrays: exploring the exciting world of supramolecular photochemistry fifteen years after its birth † Nicola Armaroli Istituto per la Sintesi Organica e la Fotoreattività, Laboratorio di Fotochimica, Consiglio Nazionale delle Ricerche, via Gobetti 101, 40129 Bologna, Italy. E-mail: [email protected] Received 29th October 2002, Accepted 11th December 2002 First published as an Advance Article on the web 22nd January 2003 After over 15 years of extensive research work in many laboratories worldwide, supramolecular photochemistry is a well-established and highly recognized branch of science. A brief retrospective view on the birth and infancy of this research area is given and some of the latest developments are discussed. In supramolecular photochemistry Ru(II) and Cu(I) diimmine complexes and C60 fullerenes are some of the most widely investigated chromophores and over the years big efforts have been made to implement and tune their photophysical and excited state properties, which are briefly reviewed. Thanks to a huge amount of synthetic and analytical research work, it has been possible to insert or combine these organic and inorganic subunits in a variety of fascinating supramolecular architectures. Some results concerned with photoinduced processes occurring in dyads, triads, catenanes, rotaxanes, dendrimers, and protonated self-assembled architectures are briefly illustrated. The overall picture stemming form the current † This manuscript is the 2001 Grammaticakis-Neumann International Prize lecture. Nicola Armaroli was born in 1966; he is married with three children. He graduated in chemistry in 1990 (laurea degree) and obtained his PhD at the University of Bologna, Italy, under the guidance of Professor Vincenzo Balzani in 1994. He carried out post-doctoral research activity at the Center for Photochemical Sciences (Bowling Green, Ohio, USA), the Italian National Research Council (CNR), and the University of Bologna. In 1997 he was appointed researcher at the CNR. His scientific interests concern the photochemistry and photophysics of supramolecular systems, mainly focussing on those containing coordination compounds and fullerene subunits. He was awarded the 2001 Grammaticakis-Neumann International Prize in Photochemistry. Nicola Armaroli DOI: 10.1039/b210569a state of the art in supramolecular photochemistry is that of a discipline gaining an increasing degree of multidisciplinarity. Interconnections with biology, physics and information technology are being established at a very fast pace, suggesting a bright future for this still young research field. 1 Introduction In April 1987 a seminal workshop gathering many prominent chemists in different areas (photochemistry, theoretical and preparative chemistry, catalysis) was organized by Vincenzo Balzani in Anacapri, Italy.1 On that occasion, for the first time, a heterogeneous group of scientists met to discuss the perspectives of a new branch of science that was emerging, which was termed Supramolecular Photochemistry. Some months later the Nobel Prize in Chemistry was awarded to Jean-Marie Lehn 2 (who participated in the meeting), Donald Cram 3 and Charles Pedersen 4 for their seminal contribution to the birth of synthetic supramolecular chemistry. The near occurrence of these events show that in 1987, although supramolecular chemistry was already a highly recognized and mature research field, the encounter between supramolecular chemistry and photochemistry was still at a very preliminary stage. From a retrospective viewpoint, the reading of the landmark proceedings 1 of the Anacapri meeting is quite intriguing because some of the developments that had just been envisaged have now been accomplished. For instance, the key concepts for the construction of photonic molecular switches and machines, now very popular,5,6 were outlined by Balzani, Moggi and Scandola.7 Furthermore, it is interesting to compare the early artificial photosynthetic systems then presented by Gust and co-workers,8,9 with those of the last generation in which an impressive degree of sophistication 10,11 and even operativity 12 has been achieved. In early 1991, as documented by the fundamental monograph of Vincenzo Balzani and Franco Scandola,13 supramolecular photochemistry had already developed to a great extent and, since then, the related scientific production has undergone a spectacular development. The successful story of this discipline has been made possible not only because of the joint efforts of photochemists and synthetic chemists belonging to many research groups worldwide, but also thanks to timely progress in other fields of research and technology that occurred independently. For instance, in the last decade, femtosecond laser equipment for various types of pulsed spectroscopy have become readily available.14 Hence the time resolution for photochemical experiments could be lowered from the pico- to the femtosecond time regime thus allowing a deeper insight into photodynamic processes. On the other hand, advances in optical and electronic technology now permit the investigation of spectral regions which, for a long Photochem. Photobiol. Sci., 2003, 2, 73–87 This journal is © The Royal Society of Chemistry and Owner Societies 2003 73 time, were substantially unaccessible to electronic spectroscopists. Nowadays steady state and time resolved near infrared luminescence spectroscopy is a fast-growing research field 15 with exciting perspectives in terms of technological applications.16 Progress in synthetic and analytical chemistry has provided, starting from the 1980’s, fully characterized supramolecular architectures such as catenanes, knots, pseudorotaxanes, rotaxanes and dendrimers 17–20 which exhibit novel photochemical and photophysical properties.20–25 The key principle that governs the design of a photoactive supramolecular array is relatively simple. A certain number of molecular subunits are assembled in a suitable spatial arrangement. Light excitation of a specific molecular fragment may modify electronic interactions among the components and give rise to phenomena such as energy or electron transfer. In these terms, indeed, nothing seems to be particularly original if one considers that intermolecular photoinduced energy and electron transfer have been studied for decades in bimolecular reaction schemes where two chemical species can freely diffuse to each other. However, linking together different molecular moieties in a supramolecular ensemble and promoting intramolecular or, more precisely, intercomponent processes yields important consequences and opens new research perspectives.13 From the fundamental point of view this approach has allowed the study of the dependence of photoinduced energy or electron transfer as a function of the partners distance or of the nature of interposed chemical bonds (e.g. covalent vs. hydrogen).26–28 Very importantly, the control of intercomponent distance allowed the verification of the predictions of the Marcus theory on electron transfer, as far as the “odd” behaviour in the so-called inverted region is concerned.29 Photoinduced processes on supramolecular arrays can also bring about effects that can in principle be exploited for practical purposes. Funnelling of excitation energy to a specific component,30 molecular rearrangements,31 creation of longlived charge separated states,26 and reversible switching on-off of luminescence following an external input (e.g. chemical or electrochemical) 32 are probably the most investigated. Thus it is not surprising that supramolecular photochemistry has progressively enlarged the spectrum of interest towards light-powered molecular machines,33 molecular switches 5 and sensors,34 and molecular electronics,35 thus reaching the crossroad between chemistry, biology, and information technology.36 A sharp definition of supramolecular array is not straightforward, when based on the nature of the chemical bonds between molecular subunits. According to some authors, intermolecular forces gathering a supramolecular entity should not be of covalent nature. Nevertheless it is now widely accepted that, when supramolecular systems are studied from the viewpoint of effects caused by external stimuli such as light, a useful approach can be based on the degree of intercomponent electronic interaction. This concept, developed by Balzani and Scandola,37 is simply illustrated in Fig. 1 and helps us to define the domain of supramolecular photochemistry. A simple two-component ensemble A–B is a “supermolecule” when a light excitation input is able to generate a localized electronic excited state on a specific subunit. Likewise, when light irradiation stimulates intercomponent electron transfer, A–B is a supermolecule if the positive and negative charge are substantially localized on a specific moiety. By contrast, when excitation does not result in localized excited states or charge distributions, A–B has to be considered a “large molecule”. A further relevant rule defining a supramolecular array is that an intercomponent electronic interaction (though not very strong) must be present to some extent, in order to get some new peculiar ensemble properties. The first consequence of the above concepts is that the chemical linkage between A and B can be of any sort as long as 74 Photochem. Photobiol. Sci., 2003, 2, 73–87 Fig. 1 Schematic representation addressing the difference between supermolecules and large molecules according to the photochemical approach. it provides a “well calibrated” electronic relationship between the molecular subunits able to grant, at the same time, the maintenance of the specific molecular properties (e.g. excited state and redox parameters) and the onset of new features, characteristic of the multicomponent array (e.g. photoinduced processes). Hence, although someone may find it difficult to include covalently linked systems in the family of supramolecular arrays, they can be strictly considered as such for the photochemical vocabulary, once they fulfil the above illustrated electronic requirements. The scope of this paper, of course, cannot be that of presenting the state the of the art in supramolecular photochemistry. Rather, some selected hot topics of this now huge research field are briefly illustrated, mainly focussing on those we have devoted our attention to. An updated and extensive reference section will hopefully act as a springboard for further reading along different directions. 2 Ru(II)-polypyridine complexes : a relentless contribution to the development of photochemical sciences Starting from the late 1960’s Ru() complexes of the polypyridine family have attracted a great deal of attention from the photochemical community.38 Most of these complexes combine remarkable features like: (i) ease of preparation; (ii) reversible electrochemical behaviour; (iii) light absorption in the visible spectral region; (iv) long-lived electronically excited states; (v) intense luminescence from the lowest metal-to-ligand-chargetransfer triplet excited state (3MLCT).39 All these characteristics make them attractive for the study of fundamental processes such as photoinduced energy- and electron-transfer under diffusional (bimolecular) conditions or within multicomponent (supramolecular) arrays. In the late 1980’s the field was developed to a great extent and the photochemical and photophysical properties of a huge number of Ru()-polypyridine complexes had been elucidated.40 Further impetus for this research was given by advances in supramolecular synthetic chemistry, which allowed insertion of [Ru(bpy)3]2⫹-type and related motifs into fascinating architectures,19,41,42 in which they play the role of chromophores, sensitizers, or electron relays 23,43–47 After over 30 years, studies relating to Ru()-polypyridine complexes are still at the forefront of photochemical research.38 These compounds have been assembled in large light harvesting dendrimers 22 where intramolecular energy transfer processes can be controlled by a thorough assembly of the dendrimer shells.21,23 The use of Ru() complexes as sensitizers for nanocrystalline wide band-gap semiconductors is well documented, with interesting fallout from this research in solar energy conversion technology.48,49 In the very active area of artificial photosynthetic mimics,10 Ru() supramolecular arrays play a prominent role.50,51 The same applies to the hot topic of molecular wires were Ru() complexes may serve as energy donor units for Os() analogues, interconnected by covalently linked spacers of different length and chemical nature.28,52–54 Photoinduced energy transfer mediated by hydrogen bonding and involving Ru() complexes as energy donors has also been discussed.55,56 Interestingly, [Ru(bpy)3]2⫹-type complexes have also been employed as active components in prototypic light powered molecular-level machines,57 showing their potential in this expanding area of research.33 Improvement of the photophysical performances of photoactive Ru() complexes (viz. luminescence quantum yield and excited state lifetime) is another challenging task. A strategy leading to a prolongation of the excited state lifetime of [Ru(bpy)3]2⫹-type complexes was suggested by Ford and Rodgers a decade ago.58 By linking this inorganic chromophore to an organic fragment with long lived and low-lying 3ππ* level, such as pyrene, a thermal equilibration between this level and the 3MLCT emitting state centred on the metal complexed moiety is obtained. If 3ππ* is slightly lower lying than 3MLCT and the energy gap is within 500 cm⫺1 a dramatic increase in the 3 MLCT lifetime of the Ru chromophore can be observed.59,60 The extent of lifetime prolongation is related to the number of the organic “reservoir” units, as shown for the series of compounds in Fig. 2.61 Fig. 2 Excited state lifetime prolongation for a series of [Ru(bpy)3]2⫹type complexes with an increasing number of pyrene units appended in deoxygenated CH3CN solution. The [Ru(bpy)3]2⫹ motif is stereogenic and this implies that polynuclear complexes are a mixture of isomers.45,62 An obvious choice to avoid this problem is the [Ru(tpy)2]2⫹ motif (tpy = 2,2⬘:6⬘,2⬙-terpyridine) which, unfortunately, is characterized by a short-lived and virtually non luminescent 3MLCT excited state.63 An increase in the luminescence yield and lifetime was obtained by attaching electron accepting substituents to the tpy ligand (Fig. 3), or by making heteroleptic compounds with different substituents on the chelating unit.64 Remarkable results have been obtained also in dinuclear complexes where [Ru(tpy)2]2⫹ centers are interconnected by spacers able to delocalize the ligand electronic charge in the excited state; both thiophenediyl 65 and butadyinylene 66 spacers have been used (Fig. 3). More recently two different approaches have been proposed to enhance the luminescence performances of [Ru(tpy)2]2⫹-type compounds. Both are aimed at lowering the 3MLCT excited state energy, in order to limit undesired thermal population of upper lying short-lived metal-centred levels that deactivate via non-radiative paths.63,64 First it has been suggested to replace the central pyridine of one tpy ligand with a triazine (tz) unit, thus obtaining a heteroleptic [Ru(tz)(tpy)]2⫹ complex.67 Then it was proposed to attach a coplanar pyrimidine residue to one Fig. 3 Excited state lifetime prologation of [Ru(tpy)2]2⫹ and some related complexes in oxygen free CH3CN solution. central tpy ligand; in this way a respectable lifetime of 200 ns is measured (Fig. 3).68 Notably the [Ru(tpy)2]2⫹ motif has been used to assemble supramolecular triads containing porphyrin terminals, with the aim of obtaining long distance charge separation along the triad.69 However, it was shown that energy transfer may successfully compete over electron transfer, as illustrated in Fig. 4.70–72 Fig. 4 Stepwise photoinduced energy transfer processes (E1, E2, E3) in a supramolecular triad assembled around a [Ru(tpy)2]2⫹ core upon excitation of the free base porphyrin moiety (butyronitrile solution). The final triple–triplet back energy transfer step (E3) is observed only in a rigid matrix at 77 K. The replacement of the central [Ru(tpy)2]2⫹ core with an [Ir(tpy)2]3⫹ motif, was made possible thanks to recent advances in the synthesis of Ir() complexes.73 [Ir(tpy)2]3⫹ is a stronger oxidant than [Ru(tpy)2]2⫹, and its lowest lying electronic level (long-lived and highly luminescent) is located at about 2.5 eV, i.e. much higher that the 3MLCT state of [Ru(tpy)2]2⫹ (1.9 eV).74 This tends to promote electron-transfer over energy transfer,75 the latter being the prevalent quenching process in porphyrin triads assembled around the Ru() complex.72 Switching between the two photoinduced processes is observed upon excitation of different chromophores within the triad.76 3 Cu(I)-phenanthroline complexes 3.1 Mononuclear Cu(I)-phenanthrolines: recent advances in the optimisation and rationalization of their photophysical properties The ground and excited state electronic properties of Cu()bisphenanthroline complexes ([Cu(NN)2]⫹) have been the object of intense investigations throughout 25 years.24,77,78 A key feature that distinguishes them from the hexacoordinated octahedral Ru()-polypyridines is the lower coordination number (4), which leads to a more or less distorted tetrahedral geometry (vide infra). The less demanding coordination environment of Cu()-phenanthrolines allows extended structural distortions in the ground and excited states, thus affording Photochem. Photobiol. Sci., 2003, 2, 73–87 75 a fine tuning of the photophysical and electrochemical properties.24 Similar to Ru()-polypyridine complexes, [Cu(NN)2]⫹ systems exhibit relatively weak absorption features in the VIS spectal region attributed to metal-to-ligand-charge-transfer (MLCT) electronic transitions.24,77,78 The MLCT absorption bands cover a wide spectral range (380–700 nm), as a result of an envelope of at least three different electronic transitions.79 The corresponding spectral intensities are strictly related to the symmetry of the complex that, in turn, is affected by the distortion from the tetrahedral geometry. Such distortion is largely dictated by the position and the chemical nature of the substituents on the chelating ligand.24 For instance, complexes of 2,9-arylphenanthroline are characterized by π-stacking interactions between the aryl groups of one ligand and the phenanthroline moiety of the other ligand, which brings about a strongly distorted ground state tetrahedral geometry (D2 symmetry). This explains the very different MLCT absorption profile that characterizes complexes of 2,9-arylphenanthroline ligands compared to those of the 2,9-alkylphenanthroline type.24 The parent compound of the [Cu(NN)2]⫹ family i.e. [Cu(phen)2]⫹ (phen = 1,10-phenanthroline) is not luminescent in solution, although it proved to be a weak emitter in the solid state.80 Instead, Cu() complexes of substituted phenanthrolines may exhibit respectable luminescence efficiency in fluid media, which is attributed to the deactivation of two MLCT excited states in thermal equilibrium, i.e. a singlet (1MLCT) and a triplet (3MLCT).79 The energy gap between these states is about 1500–2000 cm⫺1 and, at room temperature, the population of the lower lying 3MLCT level largely exceeds that of 1MLCT, though the minority 1MLCT excited molecules are responsible for most of the observed room temperature luminescence. Interestingly, as for ground state electronic properties (e.g. absorption spectra), excited state characteristics (e.g. emission spectra and excited state lifetimes) also strongly depend on the substitution pattern of the phenanthroline ligands.24 The effect on the luminescence intensity and lifetime are attributed to a flattening distortion occurring in the MLCT excited state, since the metal centre changes its formal oxidation state from Cu() to Cu(), thus assuming a more flattened coordination geometry.81 In this ⬙open⬙ structure a fifth coordination site is made available for the newly formed d9 ion,82 that can be attacked by nucleophilic species such as solvent molecules and counterions, leading to pentacoordinated excited complexes (exciplexes), that deactivate via non-emissive deactivation paths.24,83 The effect of the size of substituents on the excited state lifetime of the Cu() complexes is nicely illustrated for the [Cu(NN)2]⫹ complexes of the ligands displayed in Fig. 5. By increasing the size or ramification of the 2,9-alkyl substituents a respectable prolongation of excited state lifetimes is achieved (from 90 to 400 ns in CH2Cl2). Further additions of substituents on other positions leads to a remarkably long lifetime of 920 ns,84 comparable to that of the very popular [Ru(bpy)3]2⫹ in CH3CN.40 The trend of luminescence intensity as a function of temperature is not conventional for [Cu(NN)2]⫹ complexes since usually weaker and red-shifted luminescence is observed with decreasing temperature.85 This is interpreted with the above mentioned two-level scheme; at lower temperatures the 1MLCT–3MLCT thermal equilibrium is shifted towards the lower lying poorly emitting triplet.85 This issue has been addressed in more detail recently, when it was shown that the “odd” intensity vs. temperature trend is obeyed for all [Cu(NN)2]⫹ complexes in fluid matrices (above 150 K), but not in rigid media (below 120 K).86 In a frozen glass, complexes of ligands with long alkyl chains exhibit an opposite trend, i.e. blue-shifting and an increase in intensity, Fig. 6. Again, the different behaviour of alkyl vs. aryl substituted 76 Photochem. Photobiol. Sci., 2003, 2, 73–87 Fig. 5 Selected 2,9-dialkylphenanthrolines with increasing length or ramification of the alkyl substituents. The lifetimes values (CH2Cl2 oxygen-free solution) correspond to those of the corresponding homoleptic [Cu(NN)2]⫹ complexes. Dramatic lifetime prolongation is obtained by adding further substituents in the 3,4,7,8-phenanthroline positions. phenanthroline complexes was attributed to geometric rather than electronic factors. Most importantly, these studies have defined more accurate criteria for the design of highly luminescent [Cu(NN)2]⫹ complexes both in fluid and in rigid media.86 Finally, it is important to note that some heteroleptic Cu()phenanthroline complexes exhibit extremely enhanced photophysical performances 87,88 with a lifetime in fluid solution as long as 16 µs.88 Also in that case the observed behaviour is mainly ascribed to steric factors that affect the coordination geometry and then the electronic properties. Although the insertion of heteroleptic coordination compounds in a supramolecular array can be a very challenging task, it is conceivable that these results can stimulate new research for taking advantage of these new highly performing chromophores. 3.2 Supramolecular arrays with Cu(I)-phenanthroline subunits The high affinity of Cu() for phenanthroline ligands makes the preparation of mononuclear [Cu(NN)2]⫹ complexes relatively easy. The development of sophisticated synthetic strategies, which take advantage of this affinity, has afforded a number of fascinating molecular architectures like catenanes, rotaxanes, and knots, as originally developed by Sauvage, Dietrich-Buchecker and coworkers.17 Notably, the [Cu(NN)2]⫹ coordination motif proved to be very fruitful not only for supramolecular synthetic chemistry but also for supramolecular photochemistry.24,89,90 The Cu()catenate reported in Fig. 7 exhibits absorption and luminescence properties which are typical for [Cu(NN)2]⫹ complexes (see above).91 It is possible, within the same ligand frame, to substitute Cu() with a variety of different metal ions (Li(), Ag(), Co(), Ni(), Zn(), Pd(), Cd()) thus obtaining a fine tuning of the ground and excited state properties of the corresponding complexes. Remarkably, all the catenates of the metal ions listed above are luminescent and their emission bands are spread over the whole visible spectral region.91 This prompted the study of families of polynuclear catenates and knots containing one or two [M(NN)2]n⫹ centres, where the Mn⫹ ion could be varied (see above).92–96 The different metal complexed moieties exhibit rather different photophysical and redox properties. Thus, once assembled within a supramolecular architecture, they may allow a large variety of inter- Fig. 6 Temperature dependence of the luminescence spectra of two [Cu(NN)2]⫹ complexes in CH2Cl2–MeOH 1 : 1 (v/v); the specific ligands are indicated, λexc is on the maximum of the corresponding MLCT absorption bands. In the fluid domain (up to 170 K) emission intensity decrease and spectral red-shifting is observed by lowering temperature in both cases. By contrast when the solvent matrix becomes rigid (around 120 K), the two compounds behave differently. For the 2,9-dialkylphenanthroline complex (right hand-side) a complete reversal of the previous trend is observed, with intensity recovery and blue shift. At 96 K a very strong luminescence band is recorded. dinuclear coordination compounds RuM which are made of an octahedral ([Ru(tpy)2]2⫹-type) and a tetrahedral ([M(NN)2]n⫹type) moiety. The direction of photoinduced energy and/or electron transfer is reversed by substituting Cu() with Ag(), Zn() or by simply leaving the tetrahedral coordination centre free.96 Photoinduced processes, also able to trigger motions at the molecular level,97,98 have been observed in a number of supramolecular Cu()-phenanthroline arrays. This topic has been covered by a recent review where more details can be found.24 Fig. 7 A [2]-catenate where two 30-membered rings with a phenanthroline unit are interlocked around a Cu() ion. The line connecting the oxygen atoms represents –(CH2)2– groups. component photoinduced processes. Particularly interesting is the possibility of tuning the direction of such processes by means of a thorough choice or combination of the [M(NN)2]n⫹ subunits.24 This concept is illustrated in Fig. 8 for a family of 4 Acid–base supramolecular photochemistry: switches, machines and self-assembled structures As briefly outlined above, 1,10-phenanthroline and its derivatives are very popular among supramolecular chemists and photochemists thanks to their capability of chelating metal ions and generating sophisticated supramolecular architectures by self assembly.17 However a no less important feature of these ligands is their ability to bind protons, which brings about some interesting chemical and photochemical results. 4.1 Fig. 8 [2]-catenates RuM and schematic representation illustrating the control of the direction of photoinduced processes by changing the metal ion in the [M(NN)2]n⫹ moiety. e⫺ Indicates electron transfer, E denotes energy transfer. Tuning of excited state energies: proton molecular switches In CH2Cl2 solution, 1,10-phenanthrolines undergo protonation reactions that can be monitored by UV-VIS absorption and luminescence spectroscopy.99 This is exemplified in Fig. 9 where the changes in the absorption and luminescence properties of a solution of a 2,9-dianisyl-1,10-phenanthroline ligand upon addition of increasing amount of acid are reported.100 The presence of well defined isosbestic and isoemissive points, suggests that a single chemical reaction occurs, i.e. the protonation of the phenanthroline unit. Importantly, upon protonation, the energy of the lowest singlet phenanthroline level is decreased from ≈3.1 to ≈2.2 eV, as shown by the remarkable red-shift of the fluorescence band. This prompted us to design a supramolecular dyad (OPV– Phen) in which a strongly fluorescent moiety (oligophenylenevinylene, OPV), is attached to an anisylphenanthroline unit (Phen).100 The luminescent level of OPV is intermediate in energy relative to the levels of Phen and PhenⴢH⫹. The results obtained with OPV–Phen upon reversible additions of acid and base are illustrated in Fig. 10. Photochem. Photobiol. Sci., 2003, 2, 73–87 77 Fig. 9 Changes in the absorption (top) and fluorescence spectra (bottom, λexc = 359 nm, isosbestic point) of a 2,9-dianisyl-1,10phenanthroline (R = –C12H25) in CH2Cl2 solution at room temperature. Fig. 10 On/off switching of the luminescence in OPV–Phen as a consequence of the inversion of photoinduced energy transfer direction. The direction of photoinduced energy transfer is addressed at wish by means of the reversible protonation–deprotonation reaction of the Phen receptor, because the fluorescent energy levels of Phen and PhenⴢH⫹ are put below/above the luminescent level of OPV, which acts act as an energy acceptor (in OPV–Phen) or donor (in OPV–PhenⴢH⫹). The process is conveniently signalled by the on/off switching of the very intense OPV fluorescence. Thus a simple method of controlling the widely exploited luminescence of OPV’s is also suggested.100 Many examples of molecular switches and sensors that take advantage of changes in proton concentration can be found in current literature.101–104 4.2 Unexpected supramolecular architectures by protonation: an example of molecular machine When two or more phenanthroline subunits are integrated in a single array, fascinating and somehow unexpected supra78 Photochem. Photobiol. Sci., 2003, 2, 73–87 molecular architectures can be generated following acidification. Again, these processes can be conveniently monitored by UV-VIS absorption and luminescence spectroscopy.91,93,105,106 The [2]-catenand in Fig. 11 is obtained upon decomplexation of the Cu() [2]-catenate shown in Fig. 7. Acidification of solutions of the [2]-catenand does not bring about protonation of single phen units but, instead, cooperative protonation of the two phenanthroline ligands occurs. This is demonstrated by 1H NMR and UV-VIS absorption and luminescence spectroscopy.91 Practically, a structure resembling that of the metal complexed catenates discussed above is obtained, as also suggested by strong similarities in the absorption spectra. The driving force for the formation of the catenate-type structure is the favourable π-stacking interactions between the aryl groups of one macrocycle and the phenanthroline moiety sitting on the other subunit. The [2]-catenand exhibits a much stronger basic force relative to that of a single isolated macrocyclic unit, since 10 times less acid is necessary to drive the protonation reaction to the end in the supramolecular ensemble.91 Notably, the [2]-catenand here described is a very special case of a multicomponent array (dyad) in which the two macrocyclic moieties are not kept together by chemical interactions (removed upon decomplexation of the parent Cu() complex) but by a purely physical linkage. The supramolecular nature of this dyad, somehow “hidden” in the basic form, is fully disclosed in the presence of acid, when a cooperative effect (protonation) is evidenced. A decade after the publication of this work, we can emphasize a feature that was not recognized at that time. The reversible protonation–deprotonation reactions of the [2]catenand triggers the closure/opening of the system. Under basic conditions the phenanthroline moieties are far apart for steric reasons (open form) while in acidic environment they are entwined around the proton (closed form). Absorption and luminescence spectroscopy allow easy detection of either species (Fig. 12). This is a typical example of what is now indicated as a molecular machine driven by chemical input, a concept not fully developed by the chemical community in the early 1990’s.6 Other phenanthroline arrays can be assembled together by protonation and form double stranded structures, as observed for the bisphenanthroline ligand reported in Fig. 13.106 The first protonation step leads to the formation of a self-assembled structure (helical or face-to-face). Further protonation destroys the structure since electrostatic repulsive forces prevail. This case interestingly compares to that of Fig. 11, where electrostatic repulsion are not able to open the catenate structure in the second reaction step. All these processes are monitored by 1H NMR and UV-Vis absorption and luminescence spectroscopy.106 Cooperative protonation of two or three phenanthroline units has been also described in cage-type arrays.105 On the contrary, in a large 56-membered macrocycle containing two phenanthroline moieties two independent protonation reactions have been monitored.107 Protonation of phenanthroline-containing polyamine macrocycles may also be exploited for sensing purposes.108 5 Fullerene multicomponent systems During the 1990’s a brand new family of molecules, i.e. fullerene C60 109 and its derivatives,110,111 was made available. The fortuitous contemporary growth of two apparently independent research lines, namely synthetic fullerene chemistry and supramolecular photochemistry, has been reciprocally beneficial and contributed to boost activity in both fields. C60 and its derivatives exhibit a number of very attractive photochemical and photophysical properties 112 such as absorption throughout the UV-Vis region,113–116 fluorescence,114–117 Fig. 11 Cooperative protonation reactions of phenanthroline subunits leading to catenate-type structures in a [2]-catenand made of two 30membered rings, CH2Cl2 solution. The process is conveniently monitored by UV-Vis absorption and luminescence spectroscopy. The line connecting the oxygen atoms represents –(CH2)2– groups. Fig. 12 Interconversion between the open and closed form of a [2]catenane upon reversible addition of acid and base. The two forms can be monitored by absorption and luminescence spectroscopy. The wavelength maximum of the emission band is reported. diagnostic transient absorption features,116 singlet oxygen sensitization capability,118–120 electron 121 and energy 122 accepting character. On the other hand functionalized fullerenes have been successfully integrated in a variety of multicomponent supramolecular arrays.10,123–126 Thus it is not surprising that the study of the photochemical and photophysical properties of supramolecular systems containing C60 fullerenes is a very active area of research 10,122,127–129 with important fallouts in terms of fundamental knowledge 130 and practical applications.131 Some of the most actively investigated classes of fullerene supramolecular arrays will now be briefly illustrated. 5.1 Fullerene hybrids with Ru(II), Re(I), Cu(I) complexes and metal porphyrins The relatively long-lived metal-to-ligand-charge-transfer (MLCT) excited states characterizing complexes of Ru(),40 Re(),132 and Cu() 24 with 2,2⬘-bipyridine or 1,10-phenanthroline ligands have been widely exploited in the design of supramolecular molecular architectures featuring photoinduced energy- and electron-transfer processes (see above).24,46,47,54 The MLCT excited states of these metal complexes have a marked reducing character that, in principle, make them ideal partners for C60 fullerene oxidants in the construction of donor–acceptor arrays for photoinduced electron transfer. The popular [Ru(bpy)3]2⫹ chromophore has been coupled with fullerene subunits in supramolecular architectures and electron transfer has been observed.129 However photophysical investigations on the systems reported in Fig. 14 have recently shown that electron transfer in fullerene–[Ru(bpy)3]2⫹ hybrids is rapidly followed by fast and quantitative charge recombination to the low-lying fullerene triplet; the same applies to a Re() analogue.133 This suggests that these hybrid systems are probably not suited for the generation of long-lived and highly exoergonic charge separated states. In a rotaxane made of a Cu()-bisphenanthroline core ([Cu(NN)2]⫹) and C60 terminal units the typical excited state properties of each moiety are strongly quenched, namely MLCT emission of the core, C60 fluorescence, and the C60 triplet absorption. Also, the singlet oxygen sensitization, typical of both (separated) subunits, is dramatically reduced. All these findings are a consequence of the fact that a low-energy chargeseparated state is made available in the multicomponent rotaxane. Excitation of the central inorganic chromophore C60) electron transfer. Importantly, this causes direct (Cu() [Cu(NN)2]⫹ energy transfer step process is preceded by a C60 when the light input is addressed to the fullerene chromophores (Fig. 15).134 ([Cu(NN)2]⫹ complexes are stronger excited state reductants than [Ru(bpy)3]2⫹ compounds 24 thus, in principle, they can be more promising candidates for the construction of photochemical devices for charge separation, since not suffering from the wasting sink effect of the fullerene triplet mentioned above.133 The first report on fullerene–porphyrin arrays dates back to late 1994, when Gust and co-workers demonstrated that in a zinc porphyrin–fullerene array photoinduced electron transfer from the inorganic to the organic moiety occurs.135 This prompted a huge amount of synthetic work aimed at the construction of increasingly sophisticated arrays containing fullerene and porphyrin moieties. Mainly covalently linked arrays have been prepared,10,136,137 however, arrays relying on weaker interactions are increasingly popular.126,138,139 Many of these systems have been designed in order to get artificial models featuring the fundamental acts of natural photoPhotochem. Photobiol. Sci., 2003, 2, 73–87 79 Fig. 13 Spontaneous assembly of a bisphenanthroline molecule in CH2Cl2 upon addition of trifluoroacetic acid (up to 2 equivalents). The double stranded structure (helicoidal or face-to-face) is destroyed when larger amounts of acid are added, owing to effect of electrostatic repulsive forces. Fig. 14 Hybrid dyads containing a C60 fullerene moiety and a Ru() or Re() complex. synthetic systems, namely light harvesting and charge separation.128 To this end impressive successes have been obtained, as already pointed out.10 An interesting aspect of the chemistry of fullerenes and porphyrins is that they are spontaneously attracted to each other, as a result of electronic donor–acceptor interactions. This can be observed both in the solid state 140 and in solution.141 For instance, regardless of solvent polarity, the triad depicted in Fig. 16 adopts a conformation in which one carbon sphere is tangential to the porphyrin plane, as derived by NMR investigations.142 This spontaneous attraction can also be monitored photochemically since ground state charge transfer absorption bands (CT) are recorded, which is not the case for reference solutions containing the three molecular subunits unlinked. Quite remarkably, the CT states are luminescent in the near infrared region (λmax = 890 nm) and exhibit a lifetime of 720 ps.142 Photoinduced charge separation in the triad of Fig. 16 is also observed upon selective excitation of either chromophore. The photochemistry and photo80 Photochem. Photobiol. Sci., 2003, 2, 73–87 Fig. 15 Stepwise photoinduced energy and electron transfer processes (E and e⫺, respectively) in a rotaxane containing a [Cu(NN)2]⫹ core and two fullerene stoppers, following excitation of the organic moieties. physics of face-to-face porphyrin-fullerene arrays is a still rather unexplored field. An extensive and updated treatment on the photophysics of fullerene arrays with metal complexes and porphyrins can be found in recent review papers.128,129 5.2 Fullerodendrimers: C60 inside and outside Dendrimer chemistry is an extremely active area of research. This is attested by the impressive number of review and highlight articles which have appeared in the last couple of years, in which encouraging perspectives for several practical applications are also clearly outlined.19,20,23,25,143–152 The three-dimensional tree-type structure of dendrimers may allow the isolation of internal parts from the external environment, with substantial changes in their chemical and physical solvents such as acetonitrile or THF is obtained and an enhanced wrapping effect is evidenced relative to apolar toluene.157 Further proof of the protective effect exerted on the fullerene center is the reduction of the yield of singlet oxygen sensitization in solution for the dendrimer of Fig. 17, compared to unsubstituted fulleropyrrolidine or smaller dendrimers.157 As already mentioned, C60 fullerene is not only an excellent electron acceptor but also an outstanding excitation energy acceptor, thanks to its low-lying electronic energy levels.122 This makes it attractive for the design of light harvesting dendritic supramolecular arrays as proposed recently.159 In the oligophenylenevinylene (OPV) fullerene array depicted in Fig. 18 the Fig. 16 Preferred conformation of a fullerene–porphyrin triad, as a consequence of attractive donor–acceptor electronic interactions between the two chromophores. properties.151,152 UV-Vis absorption and emission spectroscopy is a powerful tool for investigating encapsulation effects in solution because specific changes in the microenvironment of chromophoric dendrimer cores can be monitored by observing changes in the intensity, shape, or position of absorption and luminescence bands.23 The same applies to electrochemical techniques when electroactive fragments are located in the interior of a dendrimer.144 Several synthetic strategies have been devised to build up and characterize large dendrimers with C60 fullerene subunits as central, intermediate, or peripheral units.150,153,154 All these species are termed fullerodendrimers.125 Fullerene inside. Despite the fact that C60 is a photoactive molecule, it has only recently been employed as a central core in dendritic structures with polyaryl ether branches.155–158 By increasing the size of the dendrimer external shell a reduction in the rate of bimolecular energy and electron transfer processes between the central, increasingly protected fullerene unit and some external molecules was observed.155 The wrapping of the C60 core by the external dendrons has also been monitored by 1 H NMR and UV-Vis spectroscopy and attributed to electronic donor–acceptor interactions.156 When triethyleneglycol chains are attached as terminal units (Fig. 17) solubilization in polar Fig. 18 Photoinduced energy transfer in an OPV–fullerene array. R indicates –C12H25 groups. OPV–C60 energy transfer process is likely to be followed by charge separation in polar solvents.159 Work is underway to elucidate in detail the pattern of photoinduced processes in this fullerodendrimer. Fullerene outside. Nierengarten et al. have prepared three fascinating dendrimers in which a [Cu(NN)2]⫹-type core is surrounded by 4, 8, or 16 C60 terminal units.115 In Fig. 19 the smaller representative of the series is shown. It has been shown that upon excitation of the [Cu(NN)2]⫹ MLCT absorption bands, no MLCT luminescence is detected, thanks to an energy transfer quenching process to the peripheral C60 subunits (Fig.18).115 In the two largest dendrimers, the [Cu(NN)2]⫹ core is buried in a sort of C60-made “black box” since it is hardly or not accessible to external molecules, electrons, and even photons. The much higher absorption coefficient displayed by the (many) C60 units relative to the (single) [Cu(NN)2]⫹ core, practically prevents excitation of the core, relative to the footballene fragments.115 The study of the photochemical properties of fullerodendrimers is still at an early stage, therefore fast and remarkable developments can be expected shortly. 5.3 Fig. 17 Fullerodendrimer with triethyleneglycol chains allowing solubilization of the carbon sphere in a variety of solvents. The large dendritic structure is able to provide some protection of the core towards the external environment. Fullerene–conjugated oligomer dyads A few years ago it was demonstrated that supramolecular systems in which an oligophenylenevinylene unit is covalently linked to a fulleropyrrolidine moiety (OPV–C60, Fig. 20) can be successfully employed in the construction of photovoltaic Photochem. Photobiol. Sci., 2003, 2, 73–87 81 Fig. 19 A [Cu(NN)2]⫹ complex with four fullerene moieties appended, which represents the smallest representative of a series of dendrimers with up to 16 external carbon spheres. R indicates –C8H17 groups. Fig. 21 Energy-level diagram for the oligophenylenevinylene– fulleropyrrolidine array (OPV–C60) reported on top. The reported localized excited levels (full lines) corresponds to the lowest singlet state of the OPV and the lowest singlet and triplet states centred on the C60 moiety. Dashed lines represent the charge separated state (OPV⫹–C60⫺) in solvents of different polarity (PhMe, toluene; PhCN, benzonitrile). Dotted arrows indicate electron transfer steps, EnT stands for energy transfer, i.s.c. for intersystem crossing. Stepwise photoinduced energy and electron transfer steps, following excitation of the OPV moiety, can occur only in polar PhCN. Fig. 20 Oligophenylenevinylene–fulleropyrrolidine arrays (OPV–C60) used as active materials in photovoltaic devices. devices.160,161 Thus, light irradiation of a thin film of these C60 electron materials is able to trigger photoinduced OPV transfer. This so called “molecular” approach to photovoltaic devices, in which the donor and the acceptor are chemically linked, is a viable alternative to the classical approach 162 in which the photoactive material is a blend of fullerene and poly(p-phenylenevinylene).163 Furthermore the study of the electronic and photophysical properties of the supramolecular array may allow structure–activity relationships to be obtained and devise strategies for the implementation of the device performances. Detailed investigations on the OPV–C60 systems reported in Fig. 20 161 and on other similar arrays 164–167 have shown that C60 Förster-type energy transfer occurs in ultrafast OPV C60 electron transfer may solution. In principle direct OPV also take place but it is highly exergonic and is located in the Marcus inverted region, thus it cannot compete with the energy transfer process.165,167 Therefore electron transfer may only be originated from the lowest singlet excited state of the fullerene moiety and the OPV fragment simply act as an antenna unit.159,167 This might be the intimate pattern of photoinduced processes also in solid-state devices, even though it has been suggested that the photovoltaic effect is likely to be the consequence of “material” rather than “molecular” processes.168 Electron transfer from the fullerene singlet in OPV–C60 arrays suffers from competition of internal deactivation and can be promoted by solvent polarity, which can conveniently lower the energy of the OPV⫹–C60⫺ charge separated state. All the above effects are illustrated in the energy diagram of Fig. 21 which concerns an OPV–C60 array recently investigated.167 More sophisticated OPV–C60 arrays have been recently prepared. In the system depicted in Fig. 22 the fullerene is provided with both an energy (OPV) and an electron donor (pyrazoline) unit.167 Detailed studies of the dependence of 82 Photochem. Photobiol. Sci., 2003, 2, 73–87 Fig. 22 Oligophenylenevinylene–fullerene–pyrazoline triad where the fullerene unit acts as energy or electron acceptor for the OPV and the pyrazoline moiety, respectively. Switching of photoinduced processes can be obtained by operating on different parameters, namely excitation wavelength, proton concentration, solvent polarity, and temperature. photoinduced processes on solvent polarity, addition of acid, and temperature reveal that this compound can be considered a fullerene-based molecular switch. The switchable parameters are photoinduced processes, namely OPV–C60 energy transfer C60 electron transfer.167 and pyrazoline The incorporation of this triad in photovoltaic devices results in very low light to current efficiency since charge separation involving the fullerene moiety and the pyrazoline N atom is not able to contribute to the photocurrent and, instead, the pyrazoline unit can act as an electron trap. Nevertheless the design principle of multicomponent arrays featuring an antenna unit (like OPV) and a charge separation module (like pyrazolineC60) is very appealing for the construction of devices for charge separation and light energy conversion. Further work may be expected along this direction Two new approaches to the construction of organic donor– acceptor arrays containing fullerenes have been recently proposed. The first one is based on the so-called double cable concept.169 Fullerene units are grafted to conjugated polymeric backbones so as to obtain intrinsically bipolar double cable polymers in which the negative charge carriers (fullerenes) are spatially close to each other and covalently linked to the positive charge carrier (polymeric backbone). In this way the effective donor–acceptor interfacial area is maximized and positive effects on device efficiency and duration are expected.169 The second approach is termed “supramolecular” and is aimed at creating morphological organization in the active layer of photovoltaic cells via spontaneous supramolecular organization.170 Recently, photophysical investigation on adducts between a methanofullerene and an OPV molecule, both provided with self-complementary 2-ureido-4[1H]-pyrimidinone units have been carried out. In low polarity solvents, the self-association constant is very high and the supramolecular C60 singlet energy transfer, with no adduct exhibits OPV evidence of electron transfer even in polar solvents.171 Finally, it’s worth pointing out that not only OPV–C60 arrays but also oligophenyleneethynylene–C60 172 and oligothiophene– C60 168,173–175 systems are intensively investigated in order to elucidate their photophysical properties and test them for photovoltaic applications. Photocurrents have been generated in devices including gold electrodes modified with fullerene-linked oligothiophenes.176 6 Conclusions Supramolecular photochemistry has gained a great deal of attention from the scientific community since its very beginning, that can be dated back to second half of the 1980’s. Over the years, research activity has developed along a number of routes. Some of them can be outlined by examining the classes of compounds discussed in this article, i.e. Ru()-polypyridines, Cu()- and proton-phenanthrolines and fullerenes. Advances in synthetic and analytical chemistry have allowed extensive modifications to the pristine motifs [Ru(bpy)3]2⫹, [Ru(tpy)2]2⫹, [Cu(phen)2]⫹, and C60. Accordingly, fine tuning of their photophysical properties may be achieved and, at the same time, insertion of these subunits into a variety of multicomponent arrays, such as dyads, triads, catenanes, rotaxanes, dendrimers, etc., can be accomplished. Such systems, when suitably designed in terms of spatial arrangement and electronic/ photophysical properties, may behave as molecular machines,33 switches,5 wires,54 sensors,34 antennas,30 charge separation modules,10 logic gates 35 and so on. Some examples have been described here. Nearly all these results, which are among the most relevant advancements in chemical sciences in recent years, have been obtained in homogeneous solution and, at the present stage, are essentially of academic interest. This important research activity is certainly expected to yield further important results. At the same time, however, the knowledge acquired at the molecular level in bulk solution needs to be transferred into technological applications. This most likely means shifting to solid and/or heterogeneous environments, although we cannot exclude that “liquid” (wet) devices may find some practical use.177 At present, indeed, the know-how acquired by supramolecular photochemistry in solution has been successfully applied in heterogeneous systems for solar energy conversion schemes.12,49,178 Analogous transfer to the solid state environment is expected to be more difficult. For instance it seems that the patterns of photoinduced processes in oligothiophene–C60 arrays may be rather different in solution or in thin films embedded in photovoltaic devices.168 Perspectives for further developments can be easily envisaged, also taking into account the increasing availability of new molecules such as open cage 179,180 and endohedral fullerenes,181 just to remain within the classes of compounds discussed here. The still rather unexplored field of single molecule manipulation and spectroscopy is also quite promising for photochemical sciences.182,183 Then it is not difficult to imagine a leading role for thoroughly designed supramolecular arrays, addressed by light input, in the rapidly expanding field of nanotechnology. Supramolecular photochemistry has been a fundamental research field for many years but this character is changing very quickly. Fruitful interactions with applied research and technology are frequently established. 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