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The new heptanuclear ruthenium(ii) dendron, [Cl2Ru{(m-2,3- dpp)Ru[(m-2,3-dpp)Ru(bpy)2]2}2](PF6)12 (1; 2,3-dpp=2,3-bis(2'-pyridyl) pyrazine; bpy=2,2'-bipyridine), was prepared by means of the "complexes as ligands/complexes as metals" synthetic strategy, and its absorption spectrum, redox behavior, and luminescence properties were investigated. Compound 1 is a multicomponent species, which contains three different types of chromophores (namely, the {Cl2Ru(m-2,3-dpp)2} core, the {Ru(m-2,3- dpp)3}2+ intermediate, and the {(bpy)2Ru(m-2,3-dpp)}2+ peripheral subunits) and several redox-active sites. The new species exhibits very intense absorption bands in the UV region (e value in the 105-106 m1cm1 range) as a result of spin-allowed ligand-centered (LC) transitions, and intense bands in the visible region (e value in the 104-105 m1cm1 range) as a result of the various spin-allowed metal-to-ligand charge-transfer (MLCT) transitions. The redox investigation (accomplished by cyclic and differential pulse voltammetry) indicates that 1 undergoes a series of reversible metal-centered oxidation and ligand-centered reduction processes within the potential window investigated (+1.90/1.40 V vs. the standard calomel electrode, SCE). The assignment of each absorption band and redox process to specific subunits of 1 was achieved by comparison with the properties of smaller multinuclear species of the same family, namely [Cl2Ru{(m-2,3-dpp)Ru(bpy) 2}2]4+ (2), [(bpy)2Ru(m-2,3-dpp)Ru(bpy)2]4+ (4), and [Ru{(m-2,3- dpp)Ru(bpy)2}3]4+ (5). The title compound exhibits luminescence both at room temperature in acetonitrile fluid solution and at 77 K in butyronitrile rigid matrix. The emission is attributed to the triplet MLCT (3MLCT) state involving the core {Cl2Ru(m-2,3-dpp)2} subunit. Interestingly, the 3MLCT levels involving the peripheral {(bpy)2Ru(m-2,3-dpp)}2+ subunits are deactivated by energy transfer to the emitting level, in spite of the presence of interposed high-energy {Ru(m-2,3-dpp)3}2+ components, which, in other dendrimers, acted as "isolating" subunits toward energy-transfer processes. Ultrafast experiments on 1 and on the parent species 2 and 5 allowed us to rationalize this behavior and highlight that a sequential two-step electron-transfer process can be held responsible for the efficient overall energy transfer, which offers a way to overcome a limitation in antenna metal dendrimers.

This chapter presents a Glossary that was prepared by the Photochemistry Commission of the Organic Chemistry Division of the International Union of Pure and Applied Chemistry during the period 1978–1985. The purpose of the glossary is to provide definitions of terms and symbols commonly used in the field in order to achieve consensus on the adoption of some definitions and on the abandonment of inadequate terms.

Within the general subject of photoinduced electron and energy transfer, the study of unimolecular processes in covalently linked supramolecular systems constitutes a recent but very active research field. In the present report, the status of this field is briefly outlined. Particular attention is given to the evolution taking place from studies of fundamental interest on simple two-component systems (dyads) to studies of more oriented character involving increasingly complex systems (triads, tetrads, pentads, etc.). The development of this tendency towards the design of photochemical molecular devices is finally pointed out.

The properties of excited states of transition metal complexes are briefly reviewed. The outer-sphere electron transfer reactions of the excited states of complexes containing 2,2'-bipyridine or 1,10-phenanthroline ligands are discussed in detail, with particular reference to electron vs energy transfer quenching and conversion of light into chemical energy. The kinetic aspects of outer–sphere electron transfer reactions involving excited states are examined with particular reference to the so-called “Marcus inverted behavior” which is not observed in most of the systems studied. The extension of the kinetic formalism used for electron transfer reactions to energy transfer processes yields a unified view of “classical” and “ non-classical” energy transfer behavior.

Donor-acceptor interactions at the periphery of a coordination compound can have important consequences on its photochemical and photophysical behavior. Effects of this type have been explored using Ru(I1) bipyridine complexes containing cyanide ligands. The cyanide ligands of these compkxes behave a6 eLectroR donors in interactions with acceptor solvents, protonation, methylation, and bridging to other metals with formation of polynuclear complexes. A variety of effects have been observed, ranging from (i) weak perturbations of the properties of the original complex, to (ii) major changes in the photophysical behavior of the complex via changes in redox potentials and excited state energy ordering, to (iii) induction of completely new properties characteristic of the complex-acceptor "supermolecule". Examples of such effects are discussed in some detail. The use of second-sphere donor-acceptor interactions to tune excited-state properties is stressed.

The photooxidation of ferrocene in CCl 4 ethanol and CHCl 3 ethanol solutions has been investigated at several wavelengths of irradiation in the range 313-472 nm. No photoreaction was detected when the irradiation was carried out in the intramolecular ferrocene band with maximum at 440 nm. Irradiation in the spectral region where the ferrocene-to-halocarbon solvent charge transfer bands overlap the intramolecular ferrocene bands caused the oxidation of ferrocene to ferricenium cation. In this spectral region, only the light intensity absorbed by the charge-transfer to solvent band was active. The photooxidation reaction was found to involve a primary photochemical process was the dissociation of the charge-transfer-to-solvent excited state into a ferricenium cation, a chloride ion, and a radical (CCl 3 or CHCl 2 in the case of CCl 4 and CHCl 3 , respectively). In both acceptor solvents the primary photoprocess occurred with unitary efficiency with respect to the light absorbed by the charge-transfer-tp-solvent band. The secondary thermal oxidation of ferrocene was initiated by the radical species produced in the primary photoprocess. The efficiency of the secondary oxidation depended on concentrations of ferrocene, dissolved oxygen and radical scavengers.

The photochemical behaviour of some cyanide complexes has been investigated at various wavelengths, with particular reference to irradiation in the ligand field spectral region. [Co(CN)6]3− in acid or in aqueous solutions undergoes a photoaquation reaction with quantum yield 0·31 independently of the wavelength of irradiation (254, 313 and 365 mμ), of the concentration of the complex (2 × 10−2−3 × 10−3 F) and of the pH of the solution (2–7·5). The complicated processes that follow the irradiation at 254, 313 and 404 mμ of [Fe(CN)6]3− both in aqueous and in acid solutions have been investigated in detail and a reaction mechanism involving a primary photoaquation reaction is proposed. [Cr(CN)6]3− undergoes a photoaquation reaction when irradiated at 313 and 365 mμ. Irradiation of [Ni(CN)4]2− (254, 313 and 365 mμ), [Pd(CN)4]2− (254 mμ) and [Pt(CN)4]2− (254 mμ) does not cause any reaction. The results obtained in this investigation and in the previous ones concerning the photochemical behaviour of [Mo(CN)8]3−, [W(CN)8]3−, [Mo(CN)8]4−, [W(CN)8]4−, [Mn(CN)6]3− and [Fe(CN)6]4− have been correlated as far as d-d irradiation is concerned; the primary step in the photoreaction appears to be always related to the nature of the electronic state obtained by irradiation even when the overall photochemical reaction is conditioned by non-spectrocopic factors.

Supramolecular photochemistry is an emerging and rapidly growing research area, most attractive from a fundamental point of view and very promising for a variety of applications. An appropriate assembly of molecular components capable of performing light-induced functions can be called a photochemical molecular device (PMD). The most important functions that can be performed by PMDs are (i) generation and migration of electronic energy, (ii) photoinduced vectorial transport of electric charge, (iii) photoinduced conformational changes, and (iv) control and tuning of photochemical and photophysical properties. The aim of this paper is to describe PMDs, to review their possible applications, to single out their molecular components, and to examine the requirements needed for these components and for the entire device.

The excitation of azide-to-platinum charge-transfer transitions in the azidodiethylenetriamine-platinum(II) complex ion causes the formation of N3 radicals.

Treatment of the octahedral Ru(II) complex [trans,cis,cis-RuCl(2)(DMSO-O)(2)(CO)(2)] with an equimolar amount of 5,10-bis(3'-pyridyl)-15,20-diphenylporphyrin (3'-cis-DPyP) yielded, upon selective replacement of the DMSO ligands, the neutral 2 + 2 metallacycle 2. NMR spectroscopy provided unambiguous evidence that only one highly symmetrical species, in which the two chromophores are held in a slipped cofacial arrangement by the external Ru(II) metal fragments, exists in solution. The unprecedented geometry of 2, and of the fully zincated analogue 2a, were confirmed in the solid state by X-ray structural investigations. The spatial arrangement of the two parallel chromophores in 2, with an interplanar distance of 4.18 A and a lateral offset (center-to-center distance) of 9.82 A, is reminiscent of those of the special pair of bacteriophylls in the reaction centers and of adjacent B850 units in the LH2 light-harvesting antenna systems of photosynthetic bacteria. For comparison, the X-ray structure of the corresponding metallacycle with 4'-cis-DPyP, 1a, is also reported. In 1a, the two porphyrins have an almost perfect coplanar arrangement. The semi-zincated metallacycles 1b and 2b, in which only one of the two chromophores bears an inner zinc atom, were prepared from 1 and 2, respectively, and isolated in pure form. Detailed photophysical investigations of the above porphyrin assemblies were performed. In particular, very fast photoinduced intercomponent energy transfer processes from the zinc porphyrin to the free-base unit were detected in the semi-metalated derivatives 1b and 2b (time constants: 14 and 12 ps, respectively).