From metal complexes to fullerene arrays: exploring the excitingworld of supramolecular photochemistry fifteen years after itsbirth†

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: armaroli@isof.cnr.it

Received 29th October 2002, Accepted 11th December 2002First published as an Advance Article on the web 22nd January 2003

After over 15 years of extensive research work in manylaboratories worldwide, supramolecular photochemistry isa well-established and highly recognized branch of science.A brief retrospective view on the birth and infancy of thisresearch area is given and some of the latest developmentsare discussed. In supramolecular photochemistry Ru(II)and Cu(I) diimmine complexes and C60 fullerenes are someof the most widely investigated chromophores and overthe years big efforts have been made to implement andtune their photophysical and excited state properties, whichare briefly reviewed. Thanks to a huge amount of syntheticand analytical research work, it has been possible to insertor combine these organic and inorganic subunits in avariety of fascinating supramolecular architectures. Someresults concerned with photoinduced processes occurringin dyads, triads, catenanes, rotaxanes, dendrimers, andprotonated self-assembled architectures are brieflyillustrated. The overall picture stemming form the current

† This manuscript is the 2001 Grammaticakis-Neumann InternationalPrize lecture.

Nicola Armaroli was born in 1966; he is married with threechildren. He graduated in chemistry in 1990 (laurea degree) andobtained his PhD at the University of Bologna, Italy, under theguidance of Professor Vincenzo Balzani in 1994. He carried outpost-doctoral research activity at the Center for PhotochemicalSciences (Bowling Green, Ohio, USA), the Italian NationalResearch Council (CNR), and the University of Bologna. In1997 he was appointed researcher at the CNR. His scientificinterests concern the photochemistry and photophysics ofsupramolecular systems, mainly focussing on those containingcoordination compounds and fullerene subunits. He was awardedthe 2001 Grammaticakis-Neumann International Prize inPhotochemistry.

Nicola Armaroli

state of the art in supramolecular photochemistry is thatof a discipline gaining an increasing degree of multi-disciplinarity. Interconnections with biology, physics andinformation technology are being established at a very fastpace, suggesting a bright future for this still young researchfield.

1 IntroductionIn April 1987 a seminal workshop gathering many prominentchemists in different areas (photochemistry, theoretical andpreparative chemistry, catalysis) was organized by VincenzoBalzani in Anacapri, Italy.1 On that occasion, for the first time,a heterogeneous group of scientists met to discuss the perspec-tives of a new branch of science that was emerging, which wastermed Supramolecular Photochemistry. Some months later theNobel Prize in Chemistry was awarded to Jean-Marie Lehn 2

(who participated in the meeting), Donald Cram 3 and CharlesPedersen 4 for their seminal contribution to the birth ofsynthetic supramolecular chemistry. The near occurrenceof these events show that in 1987, although supramolecularchemistry was already a highly recognized and mature researchfield, the encounter between supramolecular chemistry andphotochemistry was still at a very preliminary stage.

From a retrospective viewpoint, the reading of the landmarkproceedings 1 of the Anacapri meeting is quite intriguingbecause some of the developments that had just been envisagedhave now been accomplished. For instance, the key concepts forthe construction of photonic molecular switches and machines,now very popular,5,6 were outlined by Balzani, Moggi andScandola.7 Furthermore, it is interesting to compare the earlyartificial photosynthetic systems then presented by Gust andco-workers,8,9 with those of the last generation in which animpressive degree of sophistication 10,11 and even operativity 12

has been achieved. In early 1991, as documented by thefundamental monograph of Vincenzo Balzani and FrancoScandola,13 supramolecular photochemistry had alreadydeveloped to a great extent and, since then, the related scientificproduction has undergone a spectacular development.

The successful story of this discipline has been made possiblenot only because of the joint efforts of photochemists and syn-thetic chemists belonging to many research groups worldwide,but also thanks to timely progress in other fields of research andtechnology that occurred independently. For instance, in thelast decade, femtosecond laser equipment for various types ofpulsed spectroscopy have become readily available.14 Hence thetime resolution for photochemical experiments could belowered from the pico- to the femtosecond time regime thusallowing a deeper insight into photodynamic processes. On theother hand, advances in optical and electronic technology nowpermit the investigation of spectral regions which, for a long

DOI: 10.1039/b210569a Photochem. Photobiol. Sci., 2003, 2, 73–87 73

This journal is © The Royal Society of Chemistry and Owner Societies 2003

time, were substantially unaccessible to electronic spectro-scopists. Nowadays steady state and time resolved nearinfrared luminescence spectroscopy is a fast-growing researchfield 15 with exciting perspectives in terms of technologicalapplications.16

Progress in synthetic and analytical chemistry has provided,starting from the 1980’s, fully characterized supramoleculararchitectures such as catenanes, knots, pseudorotaxanes,rotaxanes and dendrimers 17–20 which exhibit novel photo-chemical and photophysical properties.20–25 The key principlethat governs the design of a photoactive supramolecular arrayis relatively simple. A certain number of molecular subunits areassembled in a suitable spatial arrangement. Light excitationof a specific molecular fragment may modify electronic inter-actions among the components and give rise to phenomenasuch as energy or electron transfer.

In these terms, indeed, nothing seems to be particularlyoriginal if one considers that intermolecular photoinducedenergy and electron transfer have been studied for decadesin bimolecular reaction schemes where two chemical speciescan freely diffuse to each other. However, linking togetherdifferent molecular moieties in a supramolecular ensembleand promoting intramolecular or, more precisely, intercom-ponent processes yields important consequences and opens newresearch perspectives.13

From the fundamental point of view this approach hasallowed the study of the dependence of photoinducedenergy or electron transfer as a function of the partners dis-tance or of the nature of interposed chemical bonds (e.g.covalent vs. hydrogen).26–28 Very importantly, the control ofintercomponent distance allowed the verification of the predic-tions 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 alsobring about effects that can in principle be exploited forpractical purposes. Funnelling of excitation energy to a specificcomponent,30 molecular rearrangements,31 creation of long-lived charge separated states,26 and reversible switching on-offof luminescence following an external input (e.g. chemical orelectrochemical) 32 are probably the most investigated. Thusit is not surprising that supramolecular photochemistry hasprogressively enlarged the spectrum of interest towardslight-powered molecular machines,33 molecular switches 5 andsensors,34 and molecular electronics,35 thus reaching the cross-road between chemistry, biology, and information technology.36

A sharp definition of supramolecular array is not straight-forward, when based on the nature of the chemical bondsbetween molecular subunits. According to some authors,intermolecular forces gathering a supramolecular entity shouldnot be of covalent nature. Nevertheless it is now widelyaccepted that, when supramolecular systems are studied fromthe viewpoint of effects caused by external stimuli such aslight, a useful approach can be based on the degree of inter-component electronic interaction. This concept, developed byBalzani and Scandola,37 is simply illustrated in Fig. 1 and helpsus to define the domain of supramolecular photochemistry.

A simple two-component ensemble A–B is a “supermole-cule” when a light excitation input is able to generate a localizedelectronic excited state on a specific subunit. Likewise, whenlight irradiation stimulates intercomponent electron transfer,A–B is a supermolecule if the positive and negative charge aresubstantially localized on a specific moiety. By contrast, whenexcitation does not result in localized excited states or chargedistributions, A–B has to be considered a “large molecule”. Afurther relevant rule defining a supramolecular array is that anintercomponent electronic interaction (though not very strong)must be present to some extent, in order to get some newpeculiar ensemble properties.

The first consequence of the above concepts is that thechemical linkage between A and B can be of any sort as long as

it provides a “well calibrated” electronic relationship betweenthe molecular subunits able to grant, at the same time, themaintenance of the specific molecular properties (e.g. excitedstate and redox parameters) and the onset of new features,characteristic of the multicomponent array (e.g. photoinducedprocesses). Hence, although someone may find it difficultto include covalently linked systems in the family of supra-molecular arrays, they can be strictly considered as such for thephotochemical vocabulary, once they fulfil the above illustratedelectronic requirements.

The scope of this paper, of course, cannot be that of present-ing the state the of the art in supramolecular photochemistry.Rather, some selected hot topics of this now huge research fieldare briefly illustrated, mainly focussing on those we havedevoted our attention to. An updated and extensive referencesection will hopefully act as a springboard for further readingalong different directions.

2 Ru(II)-polypyridine complexes : a relentlesscontribution to the development of photochemicalsciencesStarting from the late 1960’s Ru() complexes of the polypyr-idine family have attracted a great deal of attention from thephotochemical community.38 Most of these complexes combineremarkable features like: (i) ease of preparation; (ii) reversibleelectrochemical behaviour; (iii) light absorption in the visiblespectral region; (iv) long-lived electronically excited states; (v)intense luminescence from the lowest metal-to-ligand-charge-transfer triplet excited state (3MLCT).39 All these characteristicsmake them attractive for the study of fundamental processessuch as photoinduced energy- and electron-transfer under dif-fusional (bimolecular) conditions or within multicomponent(supramolecular) arrays.

In the late 1980’s the field was developed to a great extent andthe photochemical and photophysical properties of a hugenumber of Ru()-polypyridine complexes had been elucidated.40

Further impetus for this research was given by advances insupramolecular synthetic chemistry, which allowed insertionof [Ru(bpy)3]

2�-type and related motifs into fascinatingarchitectures,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()-polypyridinecomplexes are still at the forefront of photochemical research.38

These compounds have been assembled in large light harvestingdendrimers 22 where intramolecular energy transfer processescan be controlled by a thorough assembly of the dendrimershells.21,23 The use of Ru() complexes as sensitizers for nano-crystalline wide band-gap semiconductors is well documented,with interesting fallout from this research in solar energyconversion technology.48,49 In the very active area of artificialphotosynthetic mimics,10 Ru() supramolecular arrays play a

Fig. 1 Schematic representation addressing the difference betweensupermolecules and large molecules according to the photochemicalapproach.

74 Photochem. Photobiol. Sci., 2003, 2, 73–87

prominent role.50,51 The same applies to the hot topic of molec-ular wires were Ru() complexes may serve as energy donorunits for Os() analogues, interconnected by covalently linkedspacers of different length and chemical nature.28,52–54 Photo-induced energy transfer mediated by hydrogen bonding andinvolving Ru() complexes as energy donors has also beendiscussed.55,56 Interestingly, [Ru(bpy)3]

2�-type complexes havealso been employed as active components in prototypic lightpowered molecular-level machines,57 showing their potential inthis expanding area of research.33

Improvement of the photophysical performances ofphotoactive Ru() complexes (viz. luminescence quantumyield and excited state lifetime) is another challenging task.A strategy leading to a prolongation of the excited state lifetimeof [Ru(bpy)3]

2�-type complexes was suggested by Ford andRodgers a decade ago.58 By linking this inorganic chromophoreto an organic fragment with long lived and low-lying 3ππ* level,such as pyrene, a thermal equilibration between this level andthe 3MLCT emitting state centred on the metal complexedmoiety is obtained. If 3ππ* is slightly lower lying than 3MLCTand the energy gap is within 500 cm�1 a dramatic increase in the3MLCT lifetime of the Ru chromophore can be observed.59,60

The extent of lifetime prolongation is related to the numberof the organic “reservoir” units, as shown for the series ofcompounds in Fig. 2.61

The [Ru(bpy)3]2� motif is stereogenic and this implies that

polynuclear complexes are a mixture of isomers.45,62 An obviouschoice to avoid this problem is the [Ru(tpy)2]

2� motif (tpy =2,2�:6�,2�-terpyridine) which, unfortunately, is characterized bya short-lived and virtually non luminescent 3MLCT excitedstate.63 An increase in the luminescence yield and lifetime wasobtained by attaching electron accepting substituents to thetpy ligand (Fig. 3), or by making heteroleptic compounds withdifferent substituents on the chelating unit.64 Remarkableresults have been obtained also in dinuclear complexes where[Ru(tpy)2]

2� centers are interconnected by spacers able todelocalize the ligand electronic charge in the excited state; boththiophenediyl 65 and butadyinylene 66 spacers have been used(Fig. 3).

More recently two different approaches have been proposedto enhance the luminescence performances of [Ru(tpy)2]

2�-typecompounds. Both are aimed at lowering the 3MLCT excitedstate energy, in order to limit undesired thermal population ofupper lying short-lived metal-centred levels that deactivate vianon-radiative paths.63,64 First it has been suggested to replacethe central pyridine of one tpy ligand with a triazine (tz) unit,thus obtaining a heteroleptic [Ru(tz)(tpy)]2� complex.67 Then itwas proposed to attach a coplanar pyrimidine residue to one

Fig. 2 Excited state lifetime prolongation for a series of [Ru(bpy)3]2�-

type complexes with an increasing number of pyrene units appended indeoxygenated CH3CN solution.

central tpy ligand; in this way a respectable lifetime of 200 ns ismeasured (Fig. 3).68

Notably the [Ru(tpy)2]2� motif has been used to assemble

supramolecular triads containing porphyrin terminals, with theaim of obtaining long distance charge separation along thetriad.69 However, it was shown that energy transfer may success-fully compete over electron transfer, as illustrated in Fig. 4.70–72

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 porphyrintriads assembled around the Ru() complex.72 Switchingbetween the two photoinduced processes is observed uponexcitation of different chromophores within the triad.76

3 Cu(I)-phenanthroline complexes

3.1 Mononuclear Cu(I)-phenanthrolines: recent advances in theoptimisation and rationalization of their photophysical properties

The ground and excited state electronic properties of Cu()-bisphenanthroline complexes ([Cu(NN)2]

�) have been theobject of intense investigations throughout 25 years.24,77,78

A key feature that distinguishes them from the hexacoordin-ated octahedral Ru()-polypyridines is the lower coordinationnumber (4), which leads to a more or less distorted tetrahedralgeometry (vide infra). The less demanding coordinationenvironment of Cu()-phenanthrolines allows extended struc-tural distortions in the ground and excited states, thus affording

Fig. 3 Excited state lifetime prologation of [Ru(tpy)2]2� and some

related complexes in oxygen free CH3CN solution.

Fig. 4 Stepwise photoinduced energy transfer processes (E1, E2, E3) ina supramolecular triad assembled around a [Ru(tpy)2]

2� core uponexcitation of the free base porphyrin moiety (butyronitrile solution).The final triple–triplet back energy transfer step (E3) is observed only ina rigid matrix at 77 K.

Photochem. Photobiol. Sci., 2003, 2, 73–87 75

a fine tuning of the photophysical and electrochemicalproperties.24

Similar to Ru()-polypyridine complexes, [Cu(NN)2]�

systems exhibit relatively weak absorption features in the VISspectal region attributed to metal-to-ligand-charge-transfer(MLCT) electronic transitions.24,77,78 The MLCT absorptionbands cover a wide spectral range (380–700 nm), as a result ofan envelope of at least three different electronic transitions.79

The corresponding spectral intensities are strictly related to thesymmetry of the complex that, in turn, is affected by the distor-tion from the tetrahedral geometry. Such distortion is largelydictated by the position and the chemical nature of thesubstituents on the chelating ligand.24 For instance, complexesof 2,9-arylphenanthroline are characterized by π-stackinginteractions between the aryl groups of one ligand and thephenanthroline moiety of the other ligand, which brings abouta strongly distorted ground state tetrahedral geometry (D2

symmetry). This explains the very different MLCT absorptionprofile that characterizes complexes of 2,9-arylphenanthrolineligands compared to those of the 2,9-alkylphenanthrolinetype.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 solidstate.80 Instead, Cu() complexes of substituted phenanthrolinesmay exhibit respectable luminescence efficiency in fluid media,which is attributed to the deactivation of two MLCT excitedstates in thermal equilibrium, i.e. a singlet (1MLCT) and atriplet (3MLCT).79 The energy gap between these states is about1500–2000 cm�1 and, at room temperature, the population ofthe lower lying 3MLCT level largely exceeds that of 1MLCT,though the minority 1MLCT excited molecules are responsiblefor most of the observed room temperature luminescence.

Interestingly, as for ground state electronic properties (e.g.absorption spectra), excited state characteristics (e.g. emissionspectra and excited state lifetimes) also strongly depend on thesubstitution pattern of the phenanthroline ligands.24 The effecton the luminescence intensity and lifetime are attributed to aflattening distortion occurring in the MLCT excited state, sincethe metal centre changes its formal oxidation state from Cu()to Cu(), thus assuming a more flattened coordinationgeometry.81 In this �open� structure a fifth coordination site ismade available for the newly formed d9 ion,82 that can beattacked by nucleophilic species such as solvent molecules andcounterions, leading to pentacoordinated excited complexes(exciplexes), that deactivate via non-emissive deactivationpaths.24,83

The effect of the size of substituents on the excited statelifetime 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 lifetimesis achieved (from 90 to 400 ns in CH2Cl2). Further additionsof substituents on other positions leads to a remarkably longlifetime 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 oftemperature is not conventional for [Cu(NN)2]

� complexessince usually weaker and red-shifted luminescence is observedwith decreasing temperature.85 This is interpreted with theabove mentioned two-level scheme; at lower temperaturesthe 1MLCT–3MLCT thermal equilibrium is shifted towards thelower lying poorly emitting triplet.85 This issue has beenaddressed 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 notin rigid media (below 120 K).86 In a frozen glass, complexesof 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

phenanthroline complexes was attributed to geometric ratherthan electronic factors. Most importantly, these studies havedefined more accurate criteria for the design of highly lumin-escent [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 photo-physical performances 87,88 with a lifetime in fluid solution aslong as 16 µs.88 Also in that case the observed behaviour ismainly ascribed to steric factors that affect the coordinationgeometry and then the electronic properties. Although theinsertion of heteroleptic coordination compounds in a supra-molecular array can be a very challenging task, it is conceivablethat these results can stimulate new research for takingadvantage of these new highly performing chromophores.

3.2 Supramolecular arrays with Cu(I)-phenanthroline subunits

The high affinity of Cu() for phenanthroline ligands makesthe preparation of mononuclear [Cu(NN)2]

� complexesrelatively easy. The development of sophisticated syntheticstrategies, which take advantage of this affinity, has afforded anumber 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 chemistrybut also for supramolecular photochemistry.24,89,90 The Cu()-catenate reported in Fig. 7 exhibits absorption and luminescenceproperties which are typical for [Cu(NN)2]

� complexes (seeabove).91

It is possible, within the same ligand frame, to substituteCu() with a variety of different metal ions (Li(), Ag(), Co(),Ni(), Zn(), Pd(), Cd()) thus obtaining a fine tuning of theground and excited state properties of the correspondingcomplexes. Remarkably, all the catenates of the metal ionslisted above are luminescent and their emission bands arespread over the whole visible spectral region.91

This prompted the study of families of polynuclear catenatesand knots containing one or two [M(NN)2]

n� centres, where theMn� ion could be varied (see above).92–96 The different metalcomplexed moieties exhibit rather different photophysicaland redox properties. Thus, once assembled within a supra-molecular architecture, they may allow a large variety of inter-

Fig. 5 Selected 2,9-dialkylphenanthrolines with increasing length orramification of the alkyl substituents. The lifetimes values (CH2Cl2

oxygen-free solution) correspond to those of the correspondinghomoleptic [Cu(NN)2]

� complexes. Dramatic lifetime prolongation isobtained by adding further substituents in the 3,4,7,8-phenanthrolinepositions.

76 Photochem. Photobiol. Sci., 2003, 2, 73–87

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 andspectral red-shifting is observed by lowering temperature in both cases. By contrast when the solvent matrix becomes rigid (around 120 K), the twocompounds 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.

component photoinduced processes. Particularly interesting isthe possibility of tuning the direction of such processes bymeans 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

Fig. 7 A [2]-catenate where two 30-membered rings with a phen-anthroline unit are interlocked around a Cu() ion. The line connectingthe oxygen atoms represents –(CH2)2– groups.

Fig. 8 [2]-catenates RuM and schematic representation illustrating thecontrol of the direction of photoinduced processes by changing themetal ion in the [M(NN)2]

n� moiety. e� Indicates electron transfer, Edenotes energy transfer.

dinuclear coordination compounds RuM which are made of anoctahedral ([Ru(tpy)2]

2�-type) and a tetrahedral ([M(NN)2]n�-

type) moiety. The direction of photoinduced energy and/orelectron transfer is reversed by substituting Cu() with Ag(),Zn() or by simply leaving the tetrahedral coordination centrefree.96

Photoinduced processes, also able to trigger motions atthe molecular level,97,98 have been observed in a number ofsupramolecular Cu()-phenanthroline arrays. This topic hasbeen covered by a recent review where more details can befound.24

4 Acid–base supramolecular photochemistry:switches, machines and self-assembled structuresAs briefly outlined above, 1,10-phenanthroline and its deriv-atives are very popular among supramolecular chemists andphotochemists thanks to their capability of chelating metal ionsand generating sophisticated supramolecular architectures byself assembly.17 However a no less important feature of theseligands is their ability to bind protons, which brings about someinteresting chemical and photochemical results.

4.1 Tuning of excited state energies: proton molecular switches

In CH2Cl2 solution, 1,10-phenanthrolines undergo protonationreactions that can be monitored by UV-VIS absorption andluminescence spectroscopy.99 This is exemplified in Fig. 9 wherethe changes in the absorption and luminescence properties of asolution of a 2,9-dianisyl-1,10-phenanthroline ligand uponaddition of increasing amount of acid are reported.100

The presence of well defined isosbestic and isoemissivepoints, suggests that a single chemical reaction occurs, i.e. theprotonation of the phenanthroline unit. Importantly, uponprotonation, the energy of the lowest singlet phenanthrolinelevel is decreased from ≈3.1 to ≈2.2 eV, as shown by the remark-able red-shift of the fluorescence band.

This prompted us to design a supramolecular dyad (OPV–Phen) in which a strongly fluorescent moiety (oligophenyl-enevinylene, OPV), is attached to an anisylphenanthroline unit(Phen).100 The luminescent level of OPV is intermediate inenergy relative to the levels of Phen and Phen�H�. The resultsobtained with OPV–Phen upon reversible additions of acid andbase are illustrated in Fig. 10.

Photochem. Photobiol. Sci., 2003, 2, 73–87 77

The direction of photoinduced energy transfer is addressedat wish by means of the reversible protonation–deprotonationreaction of the Phen receptor, because the fluorescent energylevels of Phen and Phen�H� are put below/above the lumin-escent level of OPV, which acts act as an energy acceptor(in OPV–Phen) or donor (in OPV–Phen�H�). The process isconveniently signalled by the on/off switching of the veryintense OPV fluorescence. Thus a simple method of controllingthe widely exploited luminescence of OPV’s is also suggested.100

Many examples of molecular switches and sensors that takeadvantage of changes in proton concentration can be found incurrent literature.101–104

4.2 Unexpected supramolecular architectures by protonation:an example of molecular machine

When two or more phenanthroline subunits are integrated in asingle array, fascinating and somehow unexpected supra-

Fig. 9 Changes in the absorption (top) and fluorescence spectra(bottom, λexc = 359 nm, isosbestic point) of a 2,9-dianisyl-1,10-phenanthroline (R = –C12H25) in CH2Cl2 solution at room temperature.

Fig. 10 On/off switching of the luminescence in OPV–Phen as aconsequence of the inversion of photoinduced energy transferdirection.

molecular architectures can be generated following acidific-ation. Again, these processes can be conveniently monitored byUV-VIS absorption and luminescence spectroscopy.91,93,105,106

The [2]-catenand in Fig. 11 is obtained upon decomplexationof the Cu() [2]-catenate shown in Fig. 7. Acidification ofsolutions of the [2]-catenand does not bring about protonationof single phen units but, instead, cooperative protonationof the two phenanthroline ligands occurs. This is demonstratedby 1H NMR and UV-VIS absorption and luminescencespectroscopy.91

Practically, a structure resembling that of the metal com-plexed catenates discussed above is obtained, as also suggestedby strong similarities in the absorption spectra. The drivingforce for the formation of the catenate-type structure is thefavourable π-stacking interactions between the aryl groups ofone macrocycle and the phenanthroline moiety sitting on theother subunit. The [2]-catenand exhibits a much stronger basicforce relative to that of a single isolated macrocyclic unit, since10 times less acid is necessary to drive the protonation reactionto the end in the supramolecular ensemble.91

Notably, the [2]-catenand here described is a very special caseof a multicomponent array (dyad) in which the two macrocyclicmoieties are not kept together by chemical interactions(removed upon decomplexation of the parent Cu() complex)but by a purely physical linkage. The supramolecular natureof this dyad, somehow “hidden” in the basic form, is fullydisclosed in the presence of acid, when a cooperative effect(protonation) is evidenced.

A decade after the publication of this work, we canemphasize a feature that was not recognized at that time. Thereversible protonation–deprotonation reactions of the [2]-catenand triggers the closure/opening of the system. Underbasic conditions the phenanthroline moieties are far apart forsteric reasons (open form) while in acidic environment they areentwined around the proton (closed form). Absorption andluminescence spectroscopy allow easy detection of eitherspecies (Fig. 12). This is a typical example of what is nowindicated as a molecular machine driven by chemical input,a concept not fully developed by the chemical community inthe early 1990’s.6

Other phenanthroline arrays can be assembled together byprotonation and form double stranded structures, as observedfor the bisphenanthroline ligand reported in Fig. 13.106 The firstprotonation step leads to the formation of a self-assembledstructure (helical or face-to-face). Further protonation destroysthe structure since electrostatic repulsive forces prevail. Thiscase interestingly compares to that of Fig. 11, where electro-static repulsion are not able to open the catenate structure inthe second reaction step. All these processes are monitoredby 1H NMR and UV-Vis absorption and luminescencespectroscopy.106

Cooperative protonation of two or three phenanthroline unitshas been also described in cage-type arrays.105 On the contrary,in a large 56-membered macrocycle containing two phenan-throline moieties two independent protonation reactions havebeen monitored.107 Protonation of phenanthroline-containingpolyamine macrocycles may also be exploited for sensingpurposes.108

5 Fullerene multicomponent systemsDuring the 1990’s a brand new family of molecules, i.e. fuller-ene C60

109 and its derivatives,110,111 was made available. Thefortuitous contemporary growth of two apparently independ-ent research lines, namely synthetic fullerene chemistryand supramolecular photochemistry, has been reciprocallybeneficial and contributed to boost activity in both fields. C60

and its derivatives exhibit a number of very attractive photo-chemical and photophysical properties 112 such as absorptionthroughout the UV-Vis region,113–116 fluorescence,114–117

78 Photochem. Photobiol. Sci., 2003, 2, 73–87

Fig. 11 Cooperative protonation reactions of phenanthroline subunits leading to catenate-type structures in a [2]-catenand made of two 30-membered rings, CH2Cl2 solution. The process is conveniently monitored by UV-Vis absorption and luminescence spectroscopy. The line connectingthe oxygen atoms represents –(CH2)2– groups.

diagnostic transient absorption features,116 singlet oxygensensitization capability,118–120 electron 121 and energy 122 accept-ing character. On the other hand functionalized fullerenes havebeen successfully integrated in a variety of multicomponentsupramolecular arrays.10,123–126 Thus it is not surprising that thestudy of the photochemical and photophysical properties ofsupramolecular systems containing C60 fullerenes is a veryactive area of research 10,122,127–129 with important fallouts interms of fundamental knowledge 130 and practical applic-ations.131 Some of the most actively investigated classes offullerene supramolecular arrays will now be briefly illustrated.

5.1 Fullerene hybrids with Ru(II), Re(I), Cu(I) complexes andmetal 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-phenan-throline ligands have been widely exploited in the design ofsupramolecular molecular architectures featuring photo-induced energy- and electron-transfer processes (seeabove).24,46,47,54 The MLCT excited states of these metalcomplexes have a marked reducing character that, in principle,make them ideal partners for C60 fullerene oxidants in the

Fig. 12 Interconversion between the open and closed form of a [2]-catenane upon reversible addition of acid and base. The two forms canbe monitored by absorption and luminescence spectroscopy. Thewavelength maximum of the emission band is reported.

construction of donor–acceptor arrays for photoinducedelectron transfer.

The popular [Ru(bpy)3]2� chromophore has been coupled

with fullerene subunits in supramolecular architectures andelectron transfer has been observed.129 However photophysicalinvestigations on the systems reported in Fig. 14 have recentlyshown that electron transfer in fullerene–[Ru(bpy)3]

2� hybrids israpidly followed by fast and quantitative charge recombinationto the low-lying fullerene triplet; the same applies to a Re()analogue.133 This suggests that these hybrid systems areprobably not suited for the generation of long-lived and highlyexoergonic charge separated states.

In a rotaxane made of a Cu()-bisphenanthroline core([Cu(NN)2]

�) and C60 terminal units the typical excited stateproperties of each moiety are strongly quenched, namelyMLCT emission of the core, C60 fluorescence, and the C60

triplet absorption. Also, the singlet oxygen sensitization, typicalof both (separated) subunits, is dramatically reduced. All thesefindings are a consequence of the fact that a low-energy charge-separated state is made available in the multicomponentrotaxane. Excitation of the central inorganic chromophorecauses direct (Cu() C60) electron transfer. Importantly, thisprocess is preceded by a C60 [Cu(NN)2]

� energy transfer stepwhen 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 photo-chemical devices for charge separation, since not sufferingfrom the wasting sink effect of the fullerene triplet mentionedabove.133

The first report on fullerene–porphyrin arrays dates back tolate 1994, when Gust and co-workers demonstrated that in azinc porphyrin–fullerene array photoinduced electron transferfrom the inorganic to the organic moiety occurs.135 Thisprompted a huge amount of synthetic work aimed at theconstruction of increasingly sophisticated arrays containingfullerene and porphyrin moieties. Mainly covalently linkedarrays have been prepared,10,136,137 however, arrays relying onweaker interactions are increasingly popular.126,138,139 Many ofthese systems have been designed in order to get artificialmodels featuring the fundamental acts of natural photo-

Photochem. 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 doublestranded structure (helicoidal or face-to-face) is destroyed when larger amounts of acid are added, owing to effect of electrostatic repulsive forces.

synthetic systems, namely light harvesting and chargeseparation.128 To this end impressive successes have beenobtained, as already pointed out.10

An interesting aspect of the chemistry of fullerenes andporphyrins is that they are spontaneously attracted to eachother, as a result of electronic donor–acceptor interactions.This can be observed both in the solid state 140 and insolution.141 For instance, regardless of solvent polarity, thetriad depicted in Fig. 16 adopts a conformation in which onecarbon sphere is tangential to the porphyrin plane, as derivedby NMR investigations.142 This spontaneous attraction can alsobe monitored photochemically since ground state chargetransfer absorption bands (CT) are recorded, which is not thecase for reference solutions containing the three molecularsubunits unlinked. Quite remarkably, the CT states areluminescent in the near infrared region (λmax = 890 nm) andexhibit a lifetime of 720 ps.142 Photoinduced charge separationin the triad of Fig. 16 is also observed upon selective excitationof either chromophore. The photochemistry and photo-

Fig. 14 Hybrid dyads containing a C60 fullerene moiety and a Ru() orRe() complex.

physics of face-to-face porphyrin-fullerene arrays is a stillrather unexplored field.

An extensive and updated treatment on the photophysics offullerene arrays with metal complexes and porphyrins can befound 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 andhighlight articles which have appeared in the last couple ofyears, in which encouraging perspectives for several practicalapplications are also clearly outlined.19,20,23,25,143–152

The three-dimensional tree-type structure of dendrimers mayallow the isolation of internal parts from the external environ-ment, with substantial changes in their chemical and physical

Fig. 15 Stepwise photoinduced energy and electron transfer processes(E and e�, respectively) in a rotaxane containing a [Cu(NN)2]

� core andtwo fullerene stoppers, following excitation of the organic moieties.

80 Photochem. Photobiol. Sci., 2003, 2, 73–87

properties.151,152 UV-Vis absorption and emission spectroscopyis a powerful tool for investigating encapsulation effects insolution because specific changes in the microenvironment ofchromophoric dendrimer cores can be monitored by observingchanges in the intensity, shape, or position of absorption andluminescence bands.23 The same applies to electrochemicaltechniques when electroactive fragments are located in theinterior of a dendrimer.144 Several synthetic strategies have beendevised to build up and characterize large dendrimers withC60 fullerene subunits as central, intermediate, or peripheralunits.150,153,154 All these species are termed fullerodendrimers.125

Fullerene inside. Despite the fact that C60 is a photoactivemolecule, it has only recently been employed as a central core indendritic structures with polyaryl ether branches.155–158 Byincreasing the size of the dendrimer external shell a reduction inthe rate of bimolecular energy and electron transfer processesbetween the central, increasingly protected fullerene unit andsome external molecules was observed.155 The wrapping of theC60 core by the external dendrons has also been monitored by1H NMR and UV-Vis spectroscopy and attributed to electronicdonor–acceptor interactions.156 When triethyleneglycol chainsare attached as terminal units (Fig. 17) solubilization in polar

Fig. 16 Preferred conformation of a fullerene–porphyrin triad, as aconsequence of attractive donor–acceptor electronic interactionsbetween the two chromophores.

Fig. 17 Fullerodendrimer with triethyleneglycol chains allowingsolubilization of the carbon sphere in a variety of solvents. The largedendritic structure is able to provide some protection of the coretowards the external environment.

solvents such as acetonitrile or THF is obtained and anenhanced wrapping effect is evidenced relative to apolartoluene.157 Further proof of the protective effect exerted on thefullerene center is the reduction of the yield of singlet oxygensensitization in solution for the dendrimer of Fig. 17, comparedto unsubstituted fulleropyrrolidine or smaller dendrimers.157

As already mentioned, C60 fullerene is not only an excellentelectron acceptor but also an outstanding excitation energyacceptor, thanks to its low-lying electronic energy levels.122 Thismakes it attractive for the design of light harvesting dendriticsupramolecular arrays as proposed recently.159 In the oligo-phenylenevinylene (OPV) fullerene array depicted in Fig. 18 the

OPV–C60 energy transfer process is likely to be followed bycharge separation in polar solvents.159 Work is underway toelucidate in detail the pattern of photoinduced processes in thisfullerodendrimer.

Fullerene outside. Nierengarten et al. have prepared threefascinating dendrimers in which a [Cu(NN)2]

�-type core issurrounded by 4, 8, or 16 C60 terminal units.115 In Fig. 19 thesmaller 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 per-ipheral 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 coef-ficient 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 fulleroden-drimers is still at an early stage, therefore fast and remarkabledevelopments can be expected shortly.

5.3 Fullerene–conjugated oligomer dyads

A few years ago it was demonstrated that supramolecularsystems in which an oligophenylenevinylene unit is covalentlylinked to a fulleropyrrolidine moiety (OPV–C60, Fig. 20) can besuccessfully employed in the construction of photovoltaic

Fig. 18 Photoinduced energy transfer in an OPV–fullerene array.R indicates –C12H25 groups.

Photochem. Photobiol. Sci., 2003, 2, 73–87 81

devices.160,161 Thus, light irradiation of a thin film of thesematerials is able to trigger photoinduced OPV C60 electrontransfer.

This so called “molecular” approach to photovoltaic devices,in which the donor and the acceptor are chemically linked, isa viable alternative to the classical approach 162 in which thephotoactive material is a blend of fullerene and poly(p-phenyl-enevinylene).163 Furthermore the study of the electronic andphotophysical properties of the supramolecular array mayallow structure–activity relationships to be obtained and devisestrategies for the implementation of the device performances.

Detailed investigations on the OPV–C60 systems reported inFig. 20 161 and on other similar arrays 164–167 have shown thatultrafast OPV C60 Förster-type energy transfer occurs insolution. In principle direct OPV C60 electron transfer mayalso take place but it is highly exergonic and is located in theMarcus inverted region, thus it cannot compete with the energytransfer process.165,167 Therefore electron transfer may only beoriginated from the lowest singlet excited state of the fullerenemoiety and the OPV fragment simply act as an antennaunit.159,167 This might be the intimate pattern of photoinducedprocesses also in solid-state devices, even though it has beensuggested that the photovoltaic effect is likely to be the con-sequence of “material” rather than “molecular” processes.168

Electron transfer from the fullerene singlet in OPV–C60 arrayssuffers from competition of internal deactivation and can bepromoted by solvent polarity, which can conveniently lower theenergy of the OPV�–C60

� charge separated state. All the aboveeffects are illustrated in the energy diagram of Fig. 21 whichconcerns an OPV–C60 array recently investigated.167

More sophisticated OPV–C60 arrays have been recentlyprepared. In the system depicted in Fig. 22 the fullerene isprovided with both an energy (OPV) and an electron donor(pyrazoline) unit.167 Detailed studies of the dependence of

Fig. 19 A [Cu(NN)2]� complex with four fullerene moieties appended,

which represents the smallest representative of a series of dendrimerswith up to 16 external carbon spheres. R indicates –C8H17 groups.

Fig. 20 Oligophenylenevinylene–fulleropyrrolidine arrays (OPV–C60)used as active materials in photovoltaic devices.

photoinduced processes on solvent polarity, addition of acid,and temperature reveal that this compound can be considered afullerene-based molecular switch. The switchable parametersare photoinduced processes, namely OPV–C60 energy transferand pyrazoline C60 electron transfer.167

The incorporation of this triad in photovoltaic devices resultsin very low light to current efficiency since charge separationinvolving the fullerene moiety and the pyrazoline N atom is notable to contribute to the photocurrent and, instead, the pyrazo-line unit can act as an electron trap. Nevertheless the designprinciple of multicomponent arrays featuring an antenna unit(like OPV) and a charge separation module (like pyrazoline-C60) is very appealing for the construction of devices for chargeseparation and light energy conversion. Further work may beexpected along this direction

Two new approaches to the construction of organic donor–acceptor arrays containing fullerenes have been recentlyproposed. The first one is based on the so-called double cableconcept.169 Fullerene units are grafted to conjugated polymericbackbones so as to obtain intrinsically bipolar double cablepolymers in which the negative charge carriers (fullerenes)are spatially close to each other and covalently linked to thepositive charge carrier (polymeric backbone). In this way theeffective donor–acceptor interfacial area is maximized andpositive effects on device efficiency and duration are expected.169

Fig. 21 Energy-level diagram for the oligophenylenevinylene–fulleropyrrolidine array (OPV–C60) reported on top. The reportedlocalized excited levels (full lines) corresponds to the lowest singlet stateof 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 energytransfer, i.s.c. for intersystem crossing. Stepwise photoinduced energyand electron transfer steps, following excitation of the OPV moiety, canoccur only in polar PhCN.

Fig. 22 Oligophenylenevinylene–fullerene–pyrazoline triad where thefullerene unit acts as energy or electron acceptor for the OPV and thepyrazoline moiety, respectively. Switching of photoinduced processescan be obtained by operating on different parameters, namelyexcitation wavelength, proton concentration, solvent polarity, andtemperature.

82 Photochem. Photobiol. Sci., 2003, 2, 73–87

The second approach is termed “supramolecular” and isaimed at creating morphological organization in the active layerof photovoltaic cells via spontaneous supramolecular organiz-ation.170 Recently, photophysical investigation on adductsbetween a methanofullerene and an OPV molecule, both pro-vided with self-complementary 2-ureido-4[1H]-pyrimidinoneunits have been carried out. In low polarity solvents, theself-association constant is very high and the supramolecularadduct exhibits OPV C60 singlet energy transfer, with noevidence of electron transfer even in polar solvents.171

Finally, it’s worth pointing out that not only OPV–C60 arraysbut also oligophenyleneethynylene–C60

172 and oligothiophene–C60

168,173–175 systems are intensively investigated in order to elu-cidate their photophysical properties and test them for photo-voltaic applications. Photocurrents have been generated indevices including gold electrodes modified with fullerene-linkedoligothiophenes.176

6 ConclusionsSupramolecular photochemistry has gained a great deal ofattention from the scientific community since its very begin-ning, that can be dated back to second half of the 1980’s. Overthe years, research activity has developed along a number ofroutes. Some of them can be outlined by examining the classesof compounds discussed in this article, i.e. Ru()-polypyridines,Cu()- and proton-phenanthrolines and fullerenes. Advances insynthetic and analytical chemistry have allowed extensivemodifications to the pristine motifs [Ru(bpy)3]

2�, [Ru(tpy)2]2�,

[Cu(phen)2]�, and C60. Accordingly, fine tuning of their photo-

physical properties may be achieved and, at the same time,insertion of these subunits into a variety of multicomponentarrays, such as dyads, triads, catenanes, rotaxanes, dendrimers,etc., can be accomplished. Such systems, when suitablydesigned in terms of spatial arrangement and electronic/photophysical properties, may behave as molecular machines,33

switches,5 wires,54 sensors,34 antennas,30 charge separationmodules,10 logic gates 35 and so on. Some examples have beendescribed here. Nearly all these results, which are among themost relevant advancements in chemical sciences in recentyears, have been obtained in homogeneous solution and, at thepresent stage, are essentially of academic interest. This import-ant research activity is certainly expected to yield furtherimportant results. At the same time, however, the knowledgeacquired at the molecular level in bulk solution needs to betransferred into technological applications. This most likelymeans shifting to solid and/or heterogeneous environments,although we cannot exclude that “liquid” (wet) devices may findsome practical use.177 At present, indeed, the know-howacquired by supramolecular photochemistry in solution hasbeen successfully applied in heterogeneous systems for solarenergy conversion schemes.12,49,178 Analogous transfer to thesolid state environment is expected to be more difficult. Forinstance it seems that the patterns of photoinduced processes inoligothiophene–C60 arrays may be rather different in solutionor in thin films embedded in photovoltaic devices.168

Perspectives for further developments can be easily envis-aged, also taking into account the increasing availability of newmolecules such as open cage 179,180 and endohedral fullerenes,181

just to remain within the classes of compounds discussedhere. The still rather unexplored field of single moleculemanipulation and spectroscopy is also quite promising forphotochemical sciences.182,183 Then it is not difficult to imaginea leading role for thoroughly designed supramolecular arrays,addressed by light input, in the rapidly expanding field ofnanotechnology.

Supramolecular photochemistry has been a fundamentalresearch field for many years but this character is changingvery quickly. Fruitful interactions with applied research andtechnology are frequently established. Thus we may anticipate

that supramolecular photochemistry is doomed to acquirethe character of a truly interdisciplinary research enterprise ina not too distant future.

Acknowledgements

I want to express my gratitude to my colleagues of the photo-chemistry group at ISOF/CNR Institute: Francesco Barigel-letti, Lucia Flamigni and Sandra Monti for their support andmany stimulating discussions. Special thanks are due toJean-Francois Nierengarten, Jean-Pierre Sauvage, FrancoisDiederich, Mike Ward, Ed Constable, Fernando Langa andmany people from their research teams for our fruitfulcollaborations. This paper is dedicated to Professor VincenzoBalzani, the father of supramolecular photochemistry.

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