The Development of Molecular Wires PART 11: ROLE OF RUTHENIUM AND OSMIUM POLYPYRIDINE COMPLEXES FOR FAST VECTORIAL ELECTRON TRANSFER By V. Grosshenny, A. Harriman, M. Hissler and R. Ziessel Ecole Europkenne de Hautes Etudes des Industries Chimique de Strasbourg, Universitk Louis Pasteur, Laboratoire de Chimie d’Electronique et de Photonique Molkculaires, Strasbourg, France The concludingpart of thispaper on the use of ruthenium(ZI) and osmium(II) polypyridyl complexes, as molecular sized terminal subunits that are linked together bypolyyne bridges functioning as molecular girders to retain the stereo- chemical rigidity, deals with the process of electron transfer between the subunits and considers the benefits conferred by the use of polyyne bridges. The ruthe- nium and osmium complexes have properties which aid the selective promo- tion of an electron from the metal to the bridging ligand, together with amenable absorption and emission spectral pro+, and facile oxidation-reduction processes. This makes them promising candidates for vectorial electron transfer. Future work to extend the lengths of the linkages, to ensure unidirectional and long- range electron tunnelling, and to anchor the wires to supports is discussed. These are the necessary requirements for the development of molecular wiring made from these materials forfuture use with molecular-scale electronic devices. The first part of this paper introduced the sub- metal centres, must occur between more widely- ject of molecular wires and considered the struc- spaced reactants and it displays a more signifi- ture and chemistry of the complexes that can cant attenuation factor. be used for them, and other materials currently Interestingly, the insertion of a platinum(I1) believed to be the best for this purpose (13). bis-trialkylphosphine complex into the polyyne Here, the topic is further illustrated by reference bridge, see Figure 6 and Panel 6, has the effect to light-induced electron transfer. of inhibiting through-bond electron-transfer The selective population by an electron of the reactions occurring from the terminal ruthe- ethynyl-substituted terpyridyl ligand, followed nium(I1) polypyridine complex (14). This by electron delocalisation over an extended n*- situation arises because the platinun(I1) centre orbital is of extreme importance for the design donates charge to the ethynyl-substituted of effective molecular wires. It gives direction bipyridyl ligand, making its reduction potential to the electron flow, since, upon excitation of more negative than that of the corresponding the ruthenium(I1) complex, an electron is platinum(I1)-free ligand, see Panel 6. pushed along the molecular axis towards the Consequently, excitation of the ruthenium(I1) acceptor. In principle, this electron can approach complex results in selective electron donation close to the cation that is co-ordinated to the from the metal centre to the unsubstituted lig- second terpyridyl ligand, so that the actual elec- and. The electron is localised at this ligand and tron-transfer step might occur over a relatively is unable to migrate to an ethynyl-substituted short distance. However, there is no suggestion ligand because of the unfavourable thermody- that the extended x*-orbital includes the sec- namic position; thus the photophysical proper- ond terpyridyl ligand bound to the bridge. The ties remain virtually unchanged from those of corresponding hole transfer, which involves both the unsubstituted complex. Platinum Metals Rev., 1996,40, (2), 72-77 72 Fig. 6 Molecular structures of the trinudear tmns-Ru"J't"and trans-Ru"Pt"0s"complexes The same situation is observed for the anal- polyyne wire (7), as illustrated in Figure 8. Here, ogous osmium(II) complexes, see Figure 6, but excitation of one of the chromophores results not for the rhenium0 bipyridine complex shown in the formation of the corresponding triplet in Figure 7. In this latter case, excitation of excited state of the metal complex, which is of the rheniumm chromophore results in the trans- metal-to-ligand charge-transfer character. These fer of an electron from the rhenium(1) centre to binuclear complexes luminesce at much longer the co-ordinated bipyridine ligand. wavelength than do the reference compounds, Although the presence of a central platinum@) due to the lower energy of the charge-transfer complex lowers the reduction potential of the state. Furthermore, the triplet lifetimes of the bipyridine ligand because of charge injection, binuclear complexes are significantly longer, rel- the metal-to-ligand charge-transfer transition ative to analogous mononuclear ruthenium(I1) still occurs in the platinum(I1)-contaig rhe- polypyridine complexes of similar energy, at nium(I) complexes, with the result that the pro- both room temperature and 77 K. moted electron resides in an extended n*-orbital. In these polyyne-bridged binuclear complexes This causes a substantial enhancement in the it appears that, under illumination, an triplet lifetimes of the platinum(I1)-containing rhenium(1) complexes (- 80 ns) relative to that of the unsubstituted complex (- 40 ns), as mea- sured in acetonitrile solution at room temper- ature. The stabilised triplet state arises because of decreased vibronic coupling between triplet and ground states, and is a common feature of ethynyl-substituted polypyridine ligands. Symmetrical Binuclear Complexes A special case can be made for binuclear com- Fig. 7 Molecular structure of the trinuclear plexes possessing identical ruthenium(I1) trans-Re'J't" complex polypyridine complexes at opposite ends of a I Platinum Metals Rm., 1996, 40, (2) 73 Panel 6 The o-alkynyl-bipyridine derivatives of was prepared by a catalytic homo-coupling platinum(II) were stereoselectivelyprepared reaction of the ethynyl-substitutec from the ethynyl-substitutedbuilding blocks bipyridine in the presence of CuCl, and nuns-[Ptn(P"Bu3),C1,], using a CuI catal- tetramethylethylenediamine complexes ysed reaction. The platinum(I1)-free ligand see Chart A, below. Br (II) KF, MeOH 3 84% Chart A 98% NQ) The parent &compound of the metallo- the heterotrinuclear complex RunPtnOsn,sel synthon was similarly prepared (85 per Chart B, below. cent yield) from the parent cis- W"(P "B~,>,Cl,l. \ Selective com- plexation of one of the two bipy- ridine domains with complex ~-rRU(bPY),CU. 2H,O (bpy is 2,2'-bipyridine), afforded either the dinuclear PtnRunor the tri- nuclear Pt1'Ru1lZ complexes. Further com- plexation of the free bipyridine subunit within the PtnRuncom- plex with the cis- [OS(bPY)2C121 UUULI afforded, in a RuPtOs '86% clean reaction, Platinum Metals Rev., 1996,40, (2) 74 Fig. 8 Molecular structures of the ethynyl-substituted, diethynyl-substituted, diphenyl acetylene-substituted and diphenyldiaeetylene-substihrted dinuclear ruthenium(I1) complexes electron is promoted from one of the ruthe- erties of the mononuclear reference compounds. niumm centres to a n*-orbital associated with Overall, it is clear that polyynes provide sev- the bridging ligand. As suggested earlier, this eral important benefits when covalently attached n*-orbital is not restricted to the polypyridine to transition metal polypyridine complexes of ligand but, because of conjugation, it extends the type considered here. They can cause a over part of the bridging polyyne chain. marked increase in triplet lifetime, thereby Due to the similarity of the terminal groups, favouring reactions with adventitious quenchers, photon migration can take place in which simul- and can induce a sense of directionality into taneous electron- and hole-transfer steps serve intramolecular energy- or electron-transfer steps. to alternate the photon between the two termini, Polyynes operate as molecular girders to retain see Panel 7. In this manner, the mplet state shut- stereochemical rigidity and, at the same time, tles between the terminal metal complexes, each they provide highly effective electron and hole journey taking about 1 ns, and can react with channels. a quencher molecule at either site. This is a remarkable process, made possible by the strong Future Work on Molecular Wires electronic communication provided by the It is now necessary to fabricate longer polyyne polyyne bridge, and closely resembles the bridges and to ensure that the excellent elec- natural light harvesting complexes found in pho- tronic conductivity is not restricted to short tosynthetic bacteria (1 5). chain lengths. In this respect, it is important Decreasing the extent of electronic coupling to realise that the energy of the bridge is likely between the ruthenium(I1) polypyridine com- to decrease with increasing length and, in terms plexes, by inserting a bridging phenyl ring, see of superexchange theory, the degree of through- Figure 8, severely inhibits triplet migration and bond electronic coupling will increase as the the photophysical properties of such binuclear energy of the bridge approaches that of the complexes resemble the photophysical prop- excited triplet state. Each added ethynyl group, Platinum Metals Rev., 1996, 40, (2) 75 Panel 7 According to superexchange theory, the ate energy gap would be between HOMOS electronic matrix coupling element (V,.,,), a associated with bridge and triplet. The mag- term that quantifies the amount of electronic nitudes of C, and Cpare controlled by the communication between the terminal metal nature of the reactants, especially stereo- complexes, can be expressed in the follow- chemical aspects, and can be estimated by ing form: detailed molecular orbital