Polynuclear metal complexes of nanometre size. A versatile synthetic strategy leading to luminescent and redox-active dendrimers made of an osmium(II )-based core and ruthenium(II )-based units in the branches Scolastica Serroni,*a Alberto Juris,*b Margherita Venturi,*b Sebastiano Campagna,a Immaculada Resino Resino,b† Gianfranco Denti,c Alberto Credib and Vincenzo Balzanib aDipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universita` diMessina—98166 V illaggio S.Agata,Messina, Italy bDipartimento di Chimica ‘G. Ciamician’, Universita` di Bologna—40126 Bologna, Italy cIstituto di Chimica Agraria, Universita` di Pisa—56124 Pisa, Italy A docosanuclear metal complex of nanometric size and dendritic shape made of an osmium(II)-based core and containing 21 ruthenium(II)-based units in the branches has been prepared.The key building blocks are the [Os(2,3-dpp)3]2+ ‘complex ligand’, and the [Ru(2,3-Medpp)2Cl2 ]2+ and [{Ru(bpy)2(m-2,3-dpp)}2RuCl2]4+ ‘complex metals’ {2,3-dpp=2,3-bis(2-pyridyl)pyrazine; 2,3-Medpp+=2-[2-(1-methylpyridiniumyl)]-3-(2-pyridyl)pyrazine; bpy=2,2¾-bipyridine}. The first step of the synthesis is the formation of the tetranuclear [Os{(m-2,3-dpp)Ru(2,3-Medpp)2 }3]14+ species in which the peripheral ligands 2,3-Medpp+ are 2,3- dpp ligands with the second chelating site inactivated (protected) by methylation.This species is obtained from the reaction of the [Os(2,3-dpp)3]2+ ‘complex ligand’ core, which contains three open chelating positions, with three equivalents of the [Ru(2,3- Medpp)2Cl2]2+ ‘complex metal’, where the labile Cl- ligands can be replaced by the chelating units of the core.Successive demethylation (deprotection) of the tetranuclear compound opens the six peripheral chelating sites. At this stage, the divergent synthesis can be iterated {reaction with six equivalents of the [Ru(2,3-Medpp)2Cl2]2+ ‘complex metal’} with formation of the protected decanuclear compound [Os{(m-2,3-dpp)Ru[(m-2,3-dpp)Ru(2,3-Medpp)2]2}3]32+.Alternatively, in a convergent approach, the reaction of the deprotected tetranuclear species with six equivalents of the trinuclear [{Ru(bpy)2(m-2,3- dpp)}2RuCl2 ]4+ ‘complex metal’ leads to the docosanuclear [Os{(m-2,3-dpp)Ru[(m-2,3-dpp)Ru{(m-2,3-dpp)Ru(bpy)2}2]2}3 ]44+ species. The absorption spectra, luminescence properties, and electrochemical behaviour of [Os(2,3-dpp)3]2+, [Os(m-2,3- dpp)3{Ru(2,3-Medpp)2}3]14+, [Os{(m-2,3-dpp)Ru[(m-2,3-dpp)Ru(2,3-Medpp)2]2}3 ]32+, and [Os{(m-2,3-dpp)Ru[(m-2,3- dpp)Ru{(m-2,3-dpp)Ru(bpy)2}2]2}3]44+ have been investigated.complexes as ligand’ syntheticstrategy is that one can introduce Miniaturization of components for the construction of useful devices is currently pursued by a ‘large-downward’ approach.the desired building block at each stage of the synthesis. We have taken advantage of this flexibility to build up a new This approach, however, which leads physicists to deal with progressively smaller molecular aggregates, becomes dicult docosanuclear dendrimer which contains an osmium(II) core and three branches, each one containing 7 ruthenium(II) units. or even impossible when the size of the miniaturized component has to be in the dimension of nanometres.The metals and ligands used to build up our compounds are shown in Fig. 1 together with their graphic symbols and Chemists can construct nanometre-size species by a ‘smallupward’ approach starting with molecular components. This abbreviations.Fig. 2 shows the schematic representations of relevant compounds dealt with in this paper. The abbreviations approach is very appealing since it allows the assembly of functionally integrated molecular building blocks to obtain indicate the type and number of metal atoms contained in the compound, and the nature of the peripheral ligands (d= molecular-level devices.1 Assembly of molecular components into large and functional deprotected 2,3-dpp ligands; p=protected 2,3-dpp ligands; t=terminal monochelating ligands).arrays (supramolecular species) can be based on a variety of intermolecular forces such as hydrogen bonds, donor–acceptor interactions, stacking interactions, or on coordination and covalent bonds.2 When the molecular building blocks contain Results and Discussion transition metals, a strategy called ‘complexes as metal/com- Synthetic strategies plexes as ligand’ can allow the construction of large polynuclear metal complexes via metal–ligand coordination bonds.3,4 In Dendrimers can be constructed by two dierent approaches.7 the last few years we have synthesized a variety of polynuclear One approach, called divergent or ‘inside out’, starts with an transition-metal complexes having dendrimer type structure initial core containing n reactive sites.To this core (zero- and exhibiting interesting photophysical and electrochemical generation of the dendrimer), n units (building blocks) can be properties.5 The largest metal-based dendrimer reported by connected. This process aords the first generation of the our group contains 22 RuII centres.6 The interest in these dendrimer.If the peripheral units of the dendrimer so obtained species is not only related to their size, but also to the presence still contain reactive sites, the process can be iterated, yielding of chemically dierent units since each unit introduces into the a second-generation dendrimer, and so on.For example, if the supramolecular structure its own ‘pieces of information’ (in chosen core and each building block both contain three the form of specific properties such as excited states, redox chelating reactive sites (as is the case of our compounds, see levels, etc.). One of the advantages of the ‘complexes as metal/ later), the peripheral reactive sites will progress as follows: 3 (zero generation); 6 (first generation); 12 (second generation); 24 (third generation), etc.† Present address: Departamento de Quimica Inorganica, Facultad de Quimicas—08028 Barcelona, Spain. An alternative strategy is the convergent (or ‘outside in’) J. Mater. Chem., 1997, 7(7), 1227–1236 1227The divergent iterative approach requires the availability of a bifunctional species.In dealing with coordination compounds, such species have to be complexes capable of behaving both as a ligand and as a metal. A simple example is the compound Ru(2,3-dpp)2Cl2, shown in Fig. 4. Unfortunately, a species like this is unavoidably self-reactive because the free chelating sites of one molecule would substitute the labile ligands of another molecule, leading to a mixture of dispersed and uncontrolled nuclearities.Thus, in order to carry on a Fig. 1 Metals, ligands, and abbreviations used divergent synthesis, potentially bifunctional species, where one of the two functions is temporarily blocked, have to be used. construction of the dendrimer, whereby preformed branched This is the case of the preparation of the complex metal arms are attached to a more or less large core.[Ru(2,3-Medpp)2Cl2]2+,6b obtained by using the ‘protected’ The ‘complexes as metal/complexes as ligand’ synthetic ligand 2,3-Medpp+,4b where one of the two chelating sites is strategy3,4 can be exploited both for the divergent and the methylated. The protection is stable in the reaction conditions convergent approaches. Such a strategy is based on the use, used when [Ru(2,3-Medpp)2Cl2]2+ acts as a complex metal.as building blocks, of metal complexes that possess free chelat- Subsequent demethylation of the product restores the presence ing sites and metal complexes containing labile ligands (Fig. 3). of free chelating sites, i.e. its complex ligand ability. An itera- The complexes with free chelating sites play the role of ligands tive, divergent synthesis is therefore possible, as shown in (‘complex ligands’), and those with labile ligands play the role Fig. 5. of metals (‘complex metals’). The reaction of complex ligands It should also be emphasized that each deprotected comwith complex metals in the appropriate stoichiometric ratio pound of the divergent approach can be used as a ligand core in a convergent process with complex metals to yield dendri- then leads to complexes of higher nuclearity (Fig. 3). Fig. 2 Schematic representation of the compounds used for and/or obtained by our synthetic strategy. The graphic symbols used to represent metals and ligands are explained in Fig. 1. In the label used to designate each compound, the number indicates the nuclearity (number of metal ions), the letter indicates the type of the peripheral ligands (d, deprotected bridging ligand; p, protected bridging ligand; t, terminal monochelating ligand). 1228 J. Mater. Chem., 1997, 7(7), 1227–1236Fig. 3 Schematic representation of the reaction between the mononuclear Os-d complex metal and the mononuclear Cl2Ru-t complex ligand to give the tetranuclear OsRu3-t complex.Arrows indicate free chelating sites of Os-d and labile ligands of Cl2Ru-t. mers of higher generation (Fig. 5).8 Such a strategy has been previously used to obtain homonuclear ruthenium(II) dendrimers containing up to 22 metal centres.6 As mentioned in the introductory section, the interest in highly branched polynuclear metal complexes, and more generally in dendritic species, is related to the ‘small upward’ construction of nanometre-size species.In this field, an important goal is the design and construction of dendrimers containing units of dierent chemical nature. An ordered array of dierent components can in fact generate valuable properties, such as the presence of cavities having dierent sizes, surfaces with specific functions, gradients for photoinduced directional energy and electron transfer, and sites for multi-electron transfer catalysis (Fig. 6). The divergent and convergent synthetic strategies shown in Fig. 5 allow a step-by-step control of the growing process, so that building blocks containing dierent metals and/or ligands can be introduced at each step. In this work, we have used an [Os(2,3-dpp)3]2+ core and [Ru(2,3-Medpp)2Cl2]2+ units in a divergent approach to prepare [Os{(m-2,3-dpp)Ru(2,3- Medpp)2}3]14+ and [Os{(m-2,3-dpp)Ru(2,3-dpp)2 }3]8+ (tetranuclear dendrimers of the first generation) and then [Os{(m- 2,3-dpp)Ru[(m-2,3-dpp)Ru(2,3-Medpp)2]2}3 ]32+ and [Os{(m- 2,3-dpp)Ru[(m-2,3-dpp)Ru(2,3-dpp)2]2}3]20+ (decanuclear dendrimers of the second generation).Moreover the tetranuclear [Os{(m-2,3-dpp)Ru(2,3-dpp)2}3]8+ core and the trinuclear [{Ru(bpy)2(m-2,3-dpp)}2RuCl2]4+ units have been used in a convergent approach to obtain the docosanuclear [Os{(m-2,3- dpp)Ru[(m-2,3-dpp)Ru{(m-2,3-dpp)Ru(bpy)2}2]2}3]44+ (third generation) dendrimer.To our knowledge, the docosanuclear compound is the first mixed-metal third-generation dendrimer Fig. 4 Bifunctional self-reactive Ru(2,3-dpp)2Cl2 complex and its prepared so far.protected [Ru(2,3-Medpp)2Cl2]2+ form A further extension of this strategy towards the synthesis of J. Mater. Chem., 1997, 7(7), 1227–1236 1229Fig. 5 Divergent and convergent synthetic approaches for the preparation of dendrimers based on transition metal complexes. The symbols used are explained in Fig. 1. dendrimers containing units of dierent chemical nature can can exist as dierent isomers, depending on the fac or mer arrangement of the ligands around each metal centre.be based on the use, in the convergent approach, of branches Furthermore, since each metal centre is a stereogenic centre, containing metals and/or ligands dierent from those present the preparation of polymetallic complexes necessarily leads to in the core.Besides [{Ru(bpy)2(m-2,3-dpp)}2RuCl2]4+,4 intermixtures of several diastereoisomeric species. For these reasons esting examples of trinuclear complex metal branches structural investigations on these systems are dicult. are [{Ru(biq)2(m-2,3-dpp)}2RuCl2]4+,8c,f [{Ru(bpy)2(m- Dierences in the electrochemical and spectroscopic properties 2,5-dpp)}2RuCl2]4+,4 [{Os(bpy)2(m-2,3-dpp)}2RuCl2]4+,4b,8f arising from the presence of isomeric species are not expected [{Os(bpy)2(m-2,3-dpp)}2OsCl2 ]4+,9 [{Ru(bpy)2 (m-2,3-dpp)}2 to be large.10 OsCl2]4+.9 The species with high nuclearity exhibit a three-dimensional branching structure.Therefore, endo- and exo-receptor proper- General properties ties7,11 can be expected (Fig. 6). The largest of our dendrimers The compounds reported in this paper are soluble in common contains 22 metal atoms, 21 bridging ligands, 24 peripheral solvents (e.g., water, acetone and acetonitrile) and are stable ligands, and has an approximate size of 5 nm.We would like to stress that our dendrimers dier from most of those prepared both in the dark and under light excitation. In principle, they 1230 J.Mater. Chem., 1997, 7(7), 1227–1236two metal centres, as evidenced by its reduction potential that becomes more positive by about 0.4 V.6 Since the methylation of 2,3-dpp mainly concerns one of the pyridine rings, the electron donor and acceptor properties of the chelating site are only slightly aected; on the other hand a new easily reducible centre (the methylated pyridine ring) becomes present, 22 that is likely to contain the LUMO orbital of the complex.In the dendritic species each metal-based unit will bring its own excited state and redox properties. It should be pointed out, however, that these properties are aected by intercomponent interaction (vide infra). From the above considerations, it follows that the energy order of the lowest excited states for the metal-based units relevant for our discussion (see later) is: [Os(m-2,3-dpp)3]2+<[(m-2,3-dpp)Ru(bpy)2]2+<[Ru(m-2,3- dpp)3]2+<[(m-2,3-dpp)Ru(2,3-Medpp)2]4+. Absorption and emission properties Fig. 7 shows the absorption spectra of OsRu21-t and of the lower nuclearity OsRu9-t and OsRu3-t analogues. The spectra of the corresponding homonuclear Ru22-t, Ru10-t, and Ru4-t complexes6b are shown in Fig. 8. In Fig. 9 the spectra of the protected OsRu3-p and OsRu9-p dendrimers are displayed. Absorption and emission data are collected in Table 1. As one can see, all the compounds exhibit intense ligand-centred (LC) bands in the UV region and moderately intense metal-to- Fig. 6 Schematic representation of a docosanuclear dendrimer.Some ligand charge transfer (MLCT) bands in the visible. To a first properties are indicated. approximation, each metal-based unit carries its own absorption properties in the polynuclear species, so that the molar so far for two fundamental reasons: (i) each metal-containing absorption coecients are proportional to the nuclearity unit exhibits valuable intrinsic properties such as absorption of visible (solar) light, luminescence, and oxidation and reduction levels at accessible potentials;12 (ii) by a suitable choice of the building blocks, they can incorporate many ‘pieces of information’ and therefore they can be used to perform valuable functions such as light harvesting, directional energy transfer, and exchange of a controlled number of electrons at a certain potential.13,‡ Previous investigations carried out on mono- and polynuclear compounds of the ruthenium(II) and osmium(II) polypyridine family6,8,15,16 have shown that: (i) oxidation is metal centred; (ii) OsII is oxidized at potentials considerably less positive than RuII; (iii) the electron donor power decreases in the ligand series bpy>2,3-dpp>2,3-Medpp+m-2,3-dpp; (iv) the interaction between equivalent metal centres is noticeable for metals coordinated to the same bridging ligand, whereas it is negligible for metals that are further apart; (v) the interaction between equivalent ligands is noticeable for ligands Fig. 7 Absorption spectra of (a) OsRu3-t, (b) OsRu9-t, and (c) coordinated to the same metal, whereas it is negligible for OsRu21-t in acetonitrile solution at room temperature.Inset shows ligands that are further apart. their emission spectra under the same experimental conditions. In the ruthenium(II) and osmium(II) polypyridine complexes luminescence originates from the lowest MLCT excited state which is formally spin forbidden.12,17 Deactivation of the upper excited states to the lowest one is a very fast (picosecond timescale)18 and highly ecient (100%)19 process.In the ‘localized molecular orbital’ approach,20 the excited electron and the corresponding hole are considered to be centred on the ligand and on the metal, respectively. The ligands involved in our dendrimers exhibit dierent electron donor and electron acceptor properties. The electrochemical6,8b, 21 and luminescent6,8b behaviour of the complexes of the [Ru(bpy)n(2,3-dpp)3-n]2+ family indicates that bpy is a better electron donor ligand than 2,3-dpp, and that monochelated 2,3-dpp is easier to reduce than coordinated bpy.When 2,3-dpp plays the role of a bis-chelating bridging ligand, its electron donor power toward a single metal decreases because the pyrazine ring is involved in the coordination of Fig. 8 Absorption spectra of (a) Ru4-t, (b) Ru10-t, and (c) Ru22-t in acetonitrile solution at room temperature.Inset shows their emission ‡ Dendrimers made of dierent organic chromophores have been recently reported by Moore and coworkers.14 spectra under the same experimental conditions. J. Mater. Chem., 1997, 7(7), 1227–1236 1231other (at higher energy) due to the peripheral [(m-2,3- dpp)Ru(bpy)2]2+ units.8f This can be rationalized on the basis of the energy level diagram of Fig. 10(a). The lowest energy excited state is localized on the [Os(m-2,3-dpp)3]2+ core, and the energy of the peripheral [(m-2,3-dpp)Ru(bpy)2]2+ units is lower than that of the intermediate [Ru(m-2,3-dpp)3]2+ units. Therefore energy transfer from the peripheral units to the central one is at least in part prevented.For OsRu21-t, a stronger contribution from ruthenium-based units is apparent (Fig. 7, inset). This is consistent with (i) a larger number of ruthenium-based units and (ii) the presence of intermediate ruthenium-based units having higher energy than the peripheral ones as in OsRu9-t. In the homonuclear ruthenium-based t-type dendrimers, luminescence occurs almost at the same wavelength (Fig. 8, inset) and exhibits comparable lifetimes and quantum yields Fig. 9 Absorption spectra of (a) OsRu3-p and (b) OsRu9-p in regardless of nuclearity. This is an expected result because in acetonitrile solution at room temperature. Inset shows their emission all cases the lowest energy excited states are localized on the spectra under the same experimental conditions.peripheral [(m-2,3-dpp)Ru(bpy)2]2+ units and there is a gradient for energy transfer from the centre to the periphery Table 1 Spectroscopic and photophysical dataa [Fig. 10(b)].6,8f The emission properties of the p-type family can be inter- absorptionb luminescence preted on the basis of the discussion above. lmax/nm (e/dm3 mol-1 cm-1 ) lmaxc/nm td/ns Wemd/103 Electrochemical properties Os-de 476 (18 000), 300 (27 800) 765 82 — The electrochemical behaviour has been studied in argon- OsRu3-pf 526(sh) (34 500), 454 (45 000) 832 445g 6.5 purged MeCN solution at room temperature.The polynuclear OsRu3-th 551 (40 000), 283 (144 000) 875 18 1.0 complexes contain a large number of redox-active units, and Ru4-ti 545 (46 000), 285 (149 000) 811 60 1.0 OsRu9-p 459 (118 000), 298 (460 000) 700 380 4.1 it is known from previous investigations on analogous com- OsRu9-tj 550 (117 000), 283 (358 000) 808 65 ca.0.5 plexes6,8,12,21 that each metal centre can undergo a one-electron Ru10-tj 541 (125 000), 282 (329 000) 809 55 1.0 oxidation and each ligand can undergo at least one reduction OsRu21-t 547 (213 000), 284 (801 000) 805 200 0.85 process.The half-wave potential values are gathered in Table 2. Ru22-tk 542 (202 000), 284 (682 000) 786 45 0.30 Oxidation. The polynuclear complexes examined here con- aAcetonitrile solution, 298 K. bLowest energy band in the visible and prominent absorption maximum in theUV region. cCorrected emission tain one osmium-based core and several ruthenium-based maxima.dDeaerated solution, unless otherwise noted. eSee also units. In all cases, the observed oxidation processes are revers- ref. 8(a), 15 and K. Kalyanasundaram and Md. K. Nazeeruddin, Chem. ible and, as expected,6,8,12 oxidation of the osmium-based unit Phys. L ett., 1989, 158, 45. fFor preliminary data, see V. Balzani, preceeds oxidation of the ruthenium-based ones. In the polynu- S.Campagna, G. Denti, A. Juris, S. Serroni and M. Venturi, Coord. clear compounds oxidation of the osmium-based core occurs Chem. Rev., 1994, 132, 1. gAir-equilibrated solution. hSee also ref. 8(a). at more positive potentials than in the mononuclear Os-d iSee also ref. 8(b) and W. R. Murphy Jr., K. J. Brewer, G. Gettlie and J. D. Petersen, Inorg. Chem., 1989, 28, 81. jRef. 8( f ).kRef. 6. compound, according to the weaker electron donor power of m-2,3-dpp compared with 2,3-dpp (see General properties). The overall positive charge of the dendrimer increases strongly (Fig. 8 and 9). Therefore the light harvesting capability (i) with increasing nuclearity and (ii) on replacing the peri- increases with increasing nuclearity. It is well known that the pheral bpy ligands with protected 2,3-Medpp+.The increase transition from the singlet ground state to the lowest triplet of positive charge has only little eect on the oxidation MLCT level is more allowed in the osmium(II) than in the potential of the osmium-based core because (i) the increase in ruthenium(II) complexes. This results in a more intense tail in nuclearity is accompanied by an increase in the dendrimer size the red region of the spectra of the osmium-based complexes.and (ii) the charges of the protected ligands are localized in This dierence can be easily noted comparing the spectra of the periphery of the structure. OsRu3-t (Fig. 7) and Ru4-t (Fig. 8). In the p-type compounds the MLCT bands are displaced to higher energy compared to the analogous t-type compounds because the protected bridging ligand is a worse electron donor than bpy (Fig. 9). All the dendrimers reported in this paper display luminescence in fluid solution at room temperature. The emission spectra of OsRu21-t, OsRu9-t and OsRu3-t (Fig. 7, inset), Ru22-t, Ru10-t and Ru4-t (Fig. 8, inset) and OsRu3-p (Fig. 9, inset) are shown. The most important photophysical parameters are gathered in Table 1.In the heteronuclear t-type dendrimers, the emission band moves to higher energy and broadens with increasing nuclearity (Fig. 7, inset). The emission of OsRu3-t is characteristic of the osmium-based core, showing that energy transfer takes place from the three peripheral ruthenium-based moieties to the core. The corrected excitation spectrum shows that energy transfer occurs with unitary eciency.8a In OsRu9-t, the emis- Fig. 10 Simplified schematic diagram showing the energy of the lowest sion band clearly results from two dierent contributions excited state for the metal-based units along a branch of the OsRu9-t (a) and Ru10-t (b) dendrimers (Fig. 7, inset), one due to the [Os(m-2,3-dpp)3]2+ core, and the 1232 J. Mater. Chem., 1997, 7(7), 1227–1236Table 2 Electrochemical results in argon-purged acetonitrile solution at room temperaturea compound E1/2ox [n]b (site)c E1/2red[n]b (site)c Os-dd +1.21[1](Os) -0.83[1](2,3-dpp), -1.04[1](2,3-dpp), -1.39[1](2,3-dpp) OsRu3-td +1.36[1](Os), +1.61[3](Rup) -0.44[1](2,3-dpp), -0.62[1](2,3-dpp), -1.08[1](2,3-dpp), -1.20[ca.1](2,3-dpp)e OsRu3-p +1.36[ca.1](Os) -0.45[1](2,3-dpp), -0.78irr[6](2,3-Medpp+), -0.99[1](2,3-dpp), -1.18[1] (2,3-dpp), -1.31[1](2,3-dpp)e OsRu9-t +1.35[1](Os), +1.55[6](Rup)f -0.43[1](2,3-dpp), -0.58[2](2,3-dpp), -0.68[6](2,3-dpp), -0.91[1]e OsRu9-p +1.45[1](Os), +1.82[3](Rui) -0.78irr[12](2,3-Medpp+), -1.00[ca.6](2,3-dpp)e OsRu21-t +1.42[1](Os), +1.54[12](Rup) -0.63[ca.12](2,3-dpp)e aPotentials in volts vs.SCE; unless otherwise noted, the waves are reversible; for irreversible reductions (irr), the potential is evaluated from the DPV peaks; for the symbols used to indicate the compounds, see caption to Fig. 2. bNumber of exchanged electrons. cSite(s) involved in the redox processes; the subscripts p and i on the metal stand for peripheral and intermediate positions. dRevised data; for previously reported data, see G.Denti, S. Campagna, L. Sabatino, S. Serroni, M. Ciano and V. Balzani, in Photochemical conversion and storage of solar energy, ed. E. Pelizzetti and M. Schiavello, Kluwer, Dordrecht, 1991, p. 27. eOther broad waves follow at more negative potential. fRevised data; for previously reported data, see ref. 8( f ). Oxidation of the osmium-based core can be followed by (for the mononuclear Ru-d compound, as many as 12 reduction processes have been evidenced21 in the range -0.94 to oxidation processes involving ruthenium-based units which occupy internal or peripheral sites (Fig. 2). In the deca- and -2.83 V). We will only discuss briefly the behaviour of the compounds of lower nuclearity, excluding the d-type com- docosa-nuclear compounds, all the internal ruthenium-based units are equivalent from the viewpoint of the first coordination pounds because of their electrode adsorption.The mononuclear Os-d compound shows three one-electron sphere since each metal is surrounded by three bridging ligands, [Ru(m-2,3-dpp)3]2+. The dendrimers, however, have an onion- processes. They correspond to the successive reduction of the three 2,3-dpp ligands which interact because they are linked shaped structure (Fig. 2) and therefore only the units that belong to the same shell are topologically equivalent and to the same metal. In OsRu3-t the osmium-based core is linked to three exhibit the same properties. Slightly dierent properties can be expected for units belonging to dierent inner shells. [Ru(bpy)2]2+ units. It is well known that the bpy ligands are more dicult to reduce than the 2,3-dpp ligands.21 Therefore The peripheral sites in our dendrimers can be of type t, or p, or d (Fig. 2). Since the d-type dendrimers give rise to even in this compound the first reduction processes should involve 2,3-dpp. Furthermore, it is expected that bridging 2,3- electrode adsorption, we discuss only the t- or p-type compounds.Previous studies showed that the units belonging to dpp is easier to reduce that monocoordinated 2,3-dpp. In agreement with such expectations, OsRu3-t shows three one- the same shell (inner or peripheral) interact only weakly and are therefore oxidized simultaneously in multielectron electron reduction waves at less negative potentials than Osd, assigned to the first reduction of the three bridging ligands.processes. For OsRu3-p no oxidation process is observed after the This series of waves is followed by a single wave, presumably due to the second reduction of a bridging ligand, and other oxidation of the osmium-based core. In OsRu9-p, oxidation of the core is followed by a three-electron oxidation process at high and broad waves assigned to overlapping second reduction of the other two bridging ligands and first reduction +1.82 V (Table 2).The number of electrons involved and the potential value (which is that found for the [Ru(m-2,3-dpp)3]2+ of the six bpy ligands. In OsRu3-p the osmium-based core is linked to three inner units of Ru4-p and Ru10-p)6b suggest that this process is due to the simultaneous oxidation of the three equivalent [Ru(2,3-Medpp)2]4+ units.The methylated site of the 2,3- Medpp+ ligand undergoes an irreversible reduction at ruthenium-based units occupying the intermediate positions in the dendrimer structure. The results obtained for OsRu3-p and -0.76 V.22 Such a reduction process is practically unaected by coordination of a metal on the other site of the ligand.In OsRu9-p indicate that the peripheral p-type units cannot be oxidized in the potential window examined, as previously fact, it occurs practically at the same potential as in the free ligand.22 Therefore in OsRu3-p it is expected that the reduction observed for the protected homonuclear ruthenium dendrimers. 6b In the analogous OsRu3-t and OsRu9-t compounds, pattern of the three inner bridging ligands, observed for OsRu3- t, is perturbed by the reduction processes of the six peripheral oxidation of the osmium core is always followed by the simultaneous oxidation of the peripheral t-type ruthenium- protected ligands.The experimental results show that a small wave at -0.45 V is followed by an approximately six times based units. Accordingly, for OsRu3-t a three-electron process at +1.61 V, and for OsRu9-t a six-electron process at +1.55 V larger wave at -0.78 V, followed by three small waves at -0.99, -1.18, and -1.31 V, and other large waves.are observed. The slightly more positive potential observed for the first complex is probably due to the closeness of the Tentatively, we assign (i) the first wave to the one-electron reduction of an inner bridging ligand, as observed for OsRu3- ruthenium units with the already oxidized osmium core.Even in the case of OsRu21-t, oxidation of the osmium core is t, (ii) the big wave at -0.78 V to the simultaneous reduction of the six peripheral methylated sites, (iii) the two successive followed by the oxidation of the peripheral t-type units, as shown by the twelve-electron process observed at +1.54 V small waves to the first reduction of the two other inner bridging ligands, (iv) the small wave at -1.31 V to the second (Table 2).Previous studies6 have shown that the oxidation of the peripheral t-type ruthenium-based units in the homonuclear reduction of a bridging ligand, and (v) the following broad and large waves to overlapping second reduction of the other ruthenium dendrimers occurs at a potential very similar to that found for the t-type heteronuclear compounds.These two bridging ligands and of the peripheral ligands. For the decanuclear OsRu9-t and OsRu9-p, the reduction results show that the potential at which the oxidation of peripheral units takes place does not substantially depend on patterns are similar to those of the corresponding tetranuclear compounds, albeit much more complicated because of the the nature of the internal part of the dendrimer.presence of an intermediate shell containing six bridging ligands not exactly equivalent to the three inner ones. In Reduction. Because of the presence of a large number of polypyridine ligands, each capable of undergoing several addition, for OsRu9-p, the irreversible reduction of the twelve protected peripheral ligands prevents the observation of one- reduction processes,6b,21 the electrochemical reduction of polynuclear compounds of this type shows very complex patterns electron process(es) that could be present at less negative J.Mater. Chem., 1997, 7(7), 1227–1236 1233potential. For OsRu21-t, the first, broad, strongly asymmetric cyclic voltammetric peaks.For irreversible processes the wave with a maximum at -0.63 V, corresponding to approxi- reported values are those evaluated from the peak potentials mately 12 electrons, can be assigned to the reduction of in the DPV experiments. Both CV and DPV techniques have bridging ligands, and the following huge waves with maxima been used to measure the number of the exchanged electrons at-1.43 and-1.76 V (not reported in Table 2), corresponding in each redox process,25 utilizing [Ru(bpy)3]2+ as a reference to large numbers of electrons, should be due to overlapping compound, for which the oxidation and reduction processes second reduction of the bridging ligands and first and second are reversible and monoelectronic.26 To establish the reversireduction of the peripheral bpy ligands.bility of a process, we used the criteria of (i) separation of 60 mV between cathodic and anodic peaks, (ii) close to unity ratio of the intensities of the cathodic and anodic currents, Conclusions and (iii) constancy of the peak potential on changing sweep rate in the cyclic voltammograms. The divergent and convergent synthetic strategies to the dendri- Experimental errors in the reported data are as follows: mers described in this paper are characterized by a step-by- absorption maxima, 2 nm; emission maxima, 5 nm; emission step control of the growing process.Therefore, building blocks lifetimes, 10%; emission quantum yields, 20%; redox potentials, containing dierent metals and/or ligands can be introduced 20 mV.As far as molar absorption coecients are concerned, at each step. We have used the [Os(2,3-dpp)3]2+ core and the uncertainty in their absolute values is ca. 10% because of [Ru(2,3-Medpp)2Cl2]2+ units in a divergent approach to the highly diluted solutions used (10-5–10-4 M). prepare tetra- and deca-nuclear mixed-metal dendrimers. Moreover, we have used the tetranuclear [Os{(m-2,3- dpp)Ru(2,3-dpp)2}3]8+ core and trinuclear [{Ru(bpy)2(m-2,3- Characterization of the dendrimers dpp)}2RuCl2 ]4+ units in a convergent approach in order to obtain the [Os{(m-2,3-dpp)Ru[(m-2,3-dpp)Ru{(m-2,3- Characterization of large molecules like dendrimers is a dicult dpp)Ru(bpy)2}2]2}3]44+ docosanuclear compound.To our task.7 For neutral compounds, techniques based on colligative knowledge, this compound is the first mixed-metal third- properties can be used to determine the molecular mass.Our generation dendrimer prepared so far. compounds, however, are highly charged ions, and the use of To a first approximation, in the dendrimers each metal- the above techniques is not advisable because of the large based unit carries its own absorption properties.Therefore the number of counter ions. Mass spectrometry has not yet been light harvesting capability increases with increasing nuclearity. developed for compounds carrying such a high electric charge, All the dendrimers reported in this paper display luminescence and light scattering can hardly be used because of the strong in fluid solution at room temperature.Exoergonic inter- absorption in all the UV and visible spectral region. component energy-transfer processes are very ecient. In spite of the above diculties, a reliable characterization The dendrimers contain a large number of redox-active of our dendrimers has been achieved by using a variety of units. On oxidation, a reversible one-electron process of the techniques.osmium-based core occurs first, and is then followed by (1) Each compound (including precursors) was purified until oxidation processes involving ruthenium-based units which TLC showed the presence of only one spot. occupy internal or peripheral sites. On reduction, complex (2) Each building block was checked to be stable under the patterns are observed because of the presence of a large number experimental conditions.It is already known that in such of polypyridine ligands (as many as 45 in the docosanuclear compounds no ligand or metal scrambling occurs, as shown dendrimer), each capable of undergoing several reduction by the synthesis of a number of tetra-,8c hexa-4a and deca- processes. nuclear8d,f species containing dierent metals and/or ligands in predetermined sites of the structure.(3) Each one of the reaction steps (Fig. 5) was accurately Experimental monitored as follows. (i) The reaction of the ‘complex ligand’ Equipment and methods compounds (Os-d and OsRu3-d) with the ‘complex metal’ Cl2Ru-p was carried out under stoichiometric conditions. TLC Absorption spectra were obtained in acetonitrile solution at analysis [Al2O3, CH2Cl2–MeOH (951)] showed that in each room temperature using a Perkin-Elmer l-6 spectrophoto- case at least 90% of Cl2Ru-p had reacted.(ii) For the product meter. Luminescence spectra were obtained with a Perkin- of the p-type compounds obtained in each growing step the Elmer LS-50 spectrofluorimeter. Emission lifetimes were meas- ratio between aromatic and aliphatic protons in the 1H NMR ured with an Edinburgh 199 single-photon counting equip- spectrum (where the strong signals of the methyl protons lie ment.Emission quantum yields were measured at room in a clean spectral window around d 4) was consistent with temperature (20°C) with the optically dilute method23 calibrat- the expected formulations. (iii) IR analysis on the p-type ing the spectrofluorimeter with a standard lamp.[Ru(bpy)3]2+ compounds showed the absence of the 990 cm-1 band of in aerated aqueous solution was used as a quantum yield unbridged 2,3-dpp. standard, assuming a value of 0.028.24 The deprotection steps were carried out with a large excess Electrochemical measurements were carried out in argon- of demethylating agent. The purified products did not show purged acetonitrile solution at room temperature with a PAR any 1H NMR signal due to the presence of methyl groups, so 273 multipurpose equipment interfaced to a PC.The working we can exclude the presence of residual methylated sites (<1%). electrode was a Pt microelectrode or a glassy carbon (8 mm2, The reaction of OsRu3-d with the ‘complex metal’ Cl2Ru3-t Amel) electrode.The counter electrode was a Pt wire, and the (which was fully characterized by several techniques including reference electrode was a SCE separated with a fine glass frit. FAB MS)8f,27 was carried out under stoichiometric conditions The concentration of the complexes was 3×10-4 M and tetrauntil complete (90%) disappearance of Cl2Ru3-t (TLC ethylammonium hexafluorophosphate 0.05 M was used as supanalysis).porting electrolyte. Cyclic voltammograms were obtained at (4) Consistent elemental analyses were obtained. It should sweep rates of 20, 50, 200, 500 and 1000 mV s-1; dierential be noted, however, that the C, H, and N values do not change pulse voltammetry (DPV) experiments were performed with a significantly on increasing the dendrimer size.For this reason, scan rate of 20 mV s-1, a pulse height of 75 mV, and a duration we measured the ruthenium and osmium content of the of 40 ms. For reversible processes the half-wave potential OsRu21-t species with atomic absorption spectrometry using a values are reported; the same values are obtained from the DPV peaks and from an average of the cathodic and anodic method described elsewhere.28 1234 J.Mater. Chem., 1997, 7(7), 1227–1236(5) The luminescence and electrochemical properties are acetonitrile (15 ml) was refluxed for 6 d. After cooling of the solution to room temperature and addition of EtOH, the fully consistent with the reported formulations. reaction mixture was concentrated to induce the precipitation of the crude product which was separated by filtration.This Synthesis was then dissolved in a small amount of MeCN and purified The preparations of the complexes [Os(2,3-dpp)3][PF6]2 (Os- by chromatography on Sephadex G-25 (acetonitrile eluent). d),15 [Ru(2,3-Medpp)2Cl2][PF6]2 (Cl2Ru-p),6b and [Ru{(m- On adding Et2O to the partially evaporated eluate the product 2,3-dpp)Ru(bpy)2}2Cl2][PF6]4 (Cl2Ru3-t)4a have been pre- precipitated as brown powder, which was filtered o, washed viously reported.All the compounds dealt with in this paper with Et2O and dried in vacuo (yield: 90%). Anal. calc. for have been characterized using the same methods previously C294H210F120N84P20OsRu9: C, 39.5; H, 2.4; N, 13.2. Found: C, described.6b 39.4; H, 2.5; N, 13.1%. [Os{(m-2,3-dpp)Ru(2,3-Medpp)2}3][PF6]14 (OsRu3-p).Solid [Os{(m-2,3-dpp)Ru[(m-2,3-dpp)[Ru(m-2,3-dpp)Ru- AgNO3 (0.009 g, 0.053 mmol) was added to a suspension of (bpy)2]2]2}3 ][PF6 ]44 (OsRu21-t). [Ru{(m-2,3- [Ru(2,3-Medpp)2Cl2][PF6]2 (0.026 g, 0.027 mmol) in dpp)Ru(bpy)2}2Cl2][PF6]4 (0.106 g, 0.052 mmol) in 5 ml of H2O–EtOH (152, v/v) (10 ml). After 2 h stirring at room water–ethanol (151, v/v) was treated with silver nitrate (0.014 g, temperature, [Os(2,3-dpp)3][PF6]2 (0.011 g, 0.009 mmol) was 0.082 mmol).After the mixture had been stirred at room added, and the reaction mixture refluxed for 72 h. After cooling temperature for 2 h, a solution of [Os{(m-2,3-dpp)Ru(2,3- to room temperature the AgCl formed was removed by dpp)2}3][PF6]8 (OsRu3-d) (0.032 g, 0.009 mmol) in ethylene repeated centrifugations, then the reaction mixture was rotary- glycol (3 ml) was added.The reaction mixture was refluxed evaporated to a small volume (about 3 ml) and a saturated for 7 d, then cooled to room temperature. AgCl was separated methanolic solution of NH4PF6 (ca. 4 ml) was added. The by repeated centrifugations, the mother-liquor concentrated solid so obtained was separated by filtration, dissolved in a by evaporation in vacuo, and an excess of solid NH4PF6, small volume of MeCN and purified by flash chromatography methanol (2 ml) and diethyl ether (10 ml) were added.The on Sephadex G-10. From the eluate a wine-red solid was crude product was filtered o, dissolved in a small amount of recovered by addition of CH2Cl2 , partial evaporation in vacuo MeCN and purified by chromatography on Sephadex G-75 and filtration. A further recrystallization from MeCN–CH2Cl2 (acetonitrile eluent). The eluate was rotary evaporated in vacuo of the solid provided the product which was washed with small to 2 ml, and the OsRu21-t product was recovered as a purple portions of CH2Cl2, then with Et2O, and dried in vacuo (yield: powder by addition of about 20 ml of diethyl ether (yield: 67%).Anal. calc. for C132H108F84N36P14OsRu3·2H2O: C, 33.3; 76%). Anal. calc. for OsRu21-t: Os, 1.1; Ru, 12.2. Found: Os, H, 2.4; N, 10.6. Found: C, 33.3; H, 2.5; N, 10.5%. 0.9; Ru, 10.5%. [Os{(m-2,3-dpp)Ru(2,3-dpp)2}3][PF6]8 (OsRu3-d). A solu- We thank Mr. Fausto Puntoriero for his valuable help in the tion containing [Os{(m-2,3-dpp)Ru(2,3-Medpp)2}3][PF6 ]14 synthesis of some trinuclear compounds.This work was sup- (0.080 g, 0.17 mmol) and a very large excess of 1,4-diazabicy- ported by Consiglio Nazionale delle Ricerche (Progetto clo[2.2.2]octane (DABCO, 0.370 g, 3.3 mmol) in dry aceto- Strategico Tecnologie Chimiche Innovative), Ministero nitrile (15 ml) was refluxed for 6 d. After cooling of the solution dell’Universita` e della Ricerca Scientifica e Tecnologica, and to room temperature and addition of EtOH, the reaction Universita` di Bologna (Funds for selected research topics).mixture was concentrated to induce the precipitation of the crude product that was separated by filtration. This was then References dissolved in a small amount of MeCN and purified by flash 1 (a)Molecular Electronics Devices, ed.F. L. Carter, R. E. Siatkowski chromatography on Sephadex G-10. On adding Et2O to and H. Wohltjen, North-Holland, Amsterdam, 1988; the partially evaporated eluate the product precipitated as (b) Nanostructure based on MolecularMaterials, ed. W. Go�pel and a brown powder, which was filtered o, washed with Ch. Ziegler, VCH, Weinheim, 1992; (c) A. J. Bard, Integrated Et2O, and dried in vacuo (yield: 75%).Anal. calc. for Chemical Systems, Wiley, New York, 1994; (d) J.-M. Lehn, C126H90F48N36P8OsRu3·3H2O: C, 39.6; H, 2.5; N, 13.2. Found: Supramolecular Chemistry, VCH, Weinheim, 1995; (e) G. Denti, C, 39.4; H, 2.4; N, 13.0%. S. Campagna and V. Balzani, in Mesomolecules: fromMolecules to Materials, ed. D. Mendenhall, A. Greenberg and J. Liebman, Chapman and Hall, New York, 1995, p. 69. [Os{(m-2,3-dpp)Ru[(m-2,3-dpp)Ru(2,3-Medpp)2 ]2}3[PF6]32 2 (a) Supramolecular Chemistry, ed. V. Balzani and L. De Cola, (OsRu9-p). Solid AgNO3 (0.035 g, 0.208 mmol) is added to Kluwer, Dordrecht, 1992; (b) F.Vo� gtle, Supramolecular Chemistry, a suspension of [Ru(2,3-Medpp)2Cl2][PF6]2 (0.100 g, Wiley, Chichester, 1993. 0.104 mmol) in H2O–EtOH (152, v/v) (10 ml).After 2 h stirring 3 G. Denti, S. Serroni, S. Campagna, A. Juris, M. Ciano and V. Balzani, in Perspectives in Coordination Chemistry, ed. at room temperature, [Os{(m-2,3-dpp)Ru(2,3-dpp)2 }3][PF6]8 A. F. Williams, C. Floriani and A. E. Merbach, VCH, Basel, 1992, (0.065 g, 0.017 mmol) was added, and the reaction mixture was p. 153. refluxed for 7 d. After cooling to room temperature the AgCl 4 (a) S.Campagna, G. Denti, S. Serroni, M. Ciano and V. Balzani, formed was removed by repeated centrifugations, then the Inorg. Chem., 1991, 30, 3728; (b) S. Serroni and G. Denti, Inorg. reaction mixture was rotary-evaporated to a small volume (ca. Chem., 1992, 31, 4251 and references therein. 3 ml) and an excess of solid NH4PF6 was added. The solid so 5 S.Serroni, S. Campagna, G. Denti, A. Juris, M. Venturi and V. Balzani, in Advances in Dendritic Macromolecules, ed. obtained was separated by filtration, dissolved in a small G. R. Newkome, JAI Press Inc., Greenwich, CT, 1996, vol. 3, p. 61. volume of MeCN and purified by flash chromatography 6 (a) S. Serroni, G. Denti, S. Campagna, A. Juris, M. Ciano and on Sephadex G-25. From the eluate the brown–red product V.Balzani, Angew. Chem., Int. Ed. Engl., 1992, 31, 1493; was recovered by addition of EtOH, partial evaporation (b) S. Campagna, G. Denti, S. Serroni, A. Juris, M. Venturi, in vacuo and filtration (yield: 25%). Anal. calc. for V. Ricevuto and V. Balzani, Chem. Eur. J., 1995, 1, 211. C306H246F192N84P32OsRu9: C, 33.8; H, 2.3; N, 10.8. Found: C, 7 (a) D.A. Tomalia and H. D. Durst, T op. Curr. Chem., 1993, 165, 193; (b) J.M. J. Fre� chet, Science, 1994, 263, 1710; (c) N. Ardoin and 33.7; H, 2.4; N, 10.6%. D. Astruc, Bull. Soc. Chim. Fr., 1995, 132, 875; (d) G. R. Newkome, C. N. Moorefield and F. Vo� gtle, Dendritic Molecules, VCH, [Os{(m-2,3-dpp)Ru[(m-2,3-dpp)Ru( 2,3-dpp)2]2}3 ][PF6 ]20 Weinheim, 1996. (OsRu9-d ). A solution containing [Os{(m-2,3-dpp)Ru[(2,3- 8 (a) S.Campagna, G. Denti, L. Sabatino, S. Serroni, M. Ciano and dpp)Ru(2,3-Medpp)2]2}3][PF6 ]32 (0.030 g, 0.003 mmol) and V. Balzani, J. Chem. Soc., Chem. Commun., 1989, 1500; (b) G. Denti, S. Campagna, L. Sabatino, S. Serroni, M. Ciano and V. Balzani, a very large excess of DABCO (0.185 g, 1.65 mmol) in dry J. Mater. Chem., 1997, 7(7), 1227–1236 1235Inorg.Chem., 1990, 29, 4750; (c) G. Denti, S. Serroni, S. Campagna, 16 A. Juris, V. Balzani, S. Campagna, G. Denti, S. Serroni, G. Frei and H. U. Gu�del, Inorg. Chem., 1994, 33, 1491. V. Ricevuto and V. Balzani, Coord. Chem. Rev., 1991, 111, 227; (d) S. Serroni, G. Denti, S. Campagna, M. Ciano and V. Balzani, 17 (a) G. A. Crosby, R. J. Watts and D. H. W. Carstens, Science, 1970, 170, 1195; (b) V.Balzani, A. Juris, M. Venturi, S. Campagna and J. Chem. Soc., Chem. Commun., 1991, 944; (e) S. Campagna, G. Denti, S. Serroni, M. Ciano, A. Juris and V. Balzani, Inorg. S. Serroni, Chem. Rev., 1996, 96, 759. 18 (a) P. C. Bradley, N. Kress, B. A. Hornberger, R. F. Dallinger and Chem., 1992, 31, 2982; ( f ) G. Denti, S. Campagna, S. Serroni, M. Ciano and V.Balzani, J. Am. Chem. Soc., 1992, 114, 2944; W. H. Woodru, J. Am. Chem. Soc., 1989, 103, 7441; (b) P. J. Carrol and L. E. Brus, J. Am. Chem. Soc., 1987, 109, 7613; (c) T. Yabe, (g) S. Serroni, A. Juris, S. Campagna, M. Venturi, G. Denti and V. Balzani, J. Am. Chem. Soc., 1994, 116, 9086. D. R. Anderson, L. K. Orman, Y. J. Chang and J. B. Hopkins, J. Phys. Chem., 1989, 93, 2302; (d) L.F. Cooley, P. Bergquist and 9 S. Serroni, A. Juris, M. Venturi, S. Campagna, A. Credi and V. Balzani, unpublished work. D. F. Kelley, J. Am. Chem. Soc., 1990, 112, 2612. 19 (a) J. N. Demas and G. A. Crosby, J. Am. Chem. Soc., 1971, 93, 10 See, for example: R. Hage, A. H. J. Dijkhnis, J. G. Haasnoot, R. Prins, J. Reedijk, B. E. Buchanan and J. G. Vos, Inorg. Chem., 2841; (b) J. N. Demas and D. G. Taylor, Inorg. Chem., 1979, 18, 3177; (c) F. Bolletta, A. Juris, M. Maestri and D. Sandrini, Inorg. 1988, 27, 2185. 11 (a) D. A. Tomalia, A. M. Naylor and W. A. Goddard III, Angew. Chim. Acta, 1980, 44, L175. 20 V. Balzani and V. Carassiti, Photochemistry of Coordination Chem., Int. Ed. Engl., 1990, 29, 138; (b) G. R. Newkome, C. N. Moorefield and G. R. Baker, Aldrichim. Acta, 1992, 25, 31; Compounds, Academic, London, 1970. 21 S. Roa, M. Marcaccio, C. Paradisi, F. Paolucci, V. Balzani, (c) E.M. M. de Brabander-van den Berg and E. W. Meijer, Angew. Chem., Int. Ed. Engl., 1993, 32, 1308; (d) R. Dagani, Chem. Eng. G. Denti, S. Serroni and S. Campagna, Inorg. Chem., 1993, 32, 3003. News, 1996, June 3rd, p. 30. 22 A. Juris, M. Venturi, L. Pontoni, I. R. Resino, V. Balzani, 12 (a) A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and S. Serroni, S. Campagna and G. Denti, Can. J. Chem., 1995, 73, A. von Zelewsky, Coord. Chem. Rev., 1988, 84, 85; (b) T. J. Meyer, 1875. Acc. Chem. Res., 1989, 22, 163; (c) F. Scandola, M. T. Indelli, 23 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991. C. Chiorboli and C. A. Bignozzi, T op. Curr. Chem., 1990, 158, 73; 24 K. Nakamaru, Bull. Chem. Soc. Jpn., 1982, 55, 2697. (d) K. Kalyanasundaram, Photochemistry of Polypyridine and 25 J. B. Flanagan, S. Margel, A. J. Bard and F. C. Anson, J. Am. Porphyrin Complexes, Academic Press, London, 1991. Chem. Soc., 1978, 100, 4248. 13 V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni and 26 M. K. DeArmond and C. M. Carlin, Coord. Chem. Rev.,Venturi, Sol. Energy Mater. Sol. Cells, 1995, 38, 159; G. Denti, 36, 325. S. Campagna, S. Serroni, V. Balzani, A. Juris and M. Venturi, in 27 G. Denti, S. Serroni, G. Sindona and N. Uccella, J. Am. Soc.Mass Polymeric Materials Encyclopedia, ed. J. C. Salamone, CRC Press, Spectrom., 1993, 4, 306. Boca Raton, FL, 1996, vol. 3, p. 1799. 28 M. Taddia, C. Lucano and A. Juris, Euroanalysis IX, September 14 C. Devadoss, P. Bharathi and J. S. Moore, J. Am. Chem. Soc., 1996, 1–7, 1996, Bologna, Italy, Abstracts p.We P166. 118, 9635. 15 G. Denti, S. Serroni, L. Sabatino, M. Ciano, V. Ricevuto and S. Campagna, Gazz. Chim. Ital., 1991, 121, 37. Paper 7/00426E; Received 17th January, 1997 1236 J. Mater. Chem., 1997, 7(7), 1227–1236