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Molecular bridges that efficiently move charge between remote donor and acceptor sites can be thought of as molecular wires. Insight into the properties of molecular wires can be obtained by studying photoinduced electron transfer in covalently linked donor-bridge-acceptor systems. This article summarizes some of the recent progress in the study of such systems involving transition metal complexes as donor and acceptor units. Specific classes of molecular bridges are considered, namely, polyphenylene, and polyquinoxaline bridges. Basic questions are discussed, such as the transfer mechanisms, the associated distance and bridge structure dependence, and the interplay between energy and electron transfer.

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.

The supramolecular systems [Ru(Pyr(n)bpy)(CN)(4)](2-) (n = 1, 2), where one and two pyrenyl units are linked via two-methylene bridges to the [Ru(bpy)(CN)(4)](2-) chromophore, have been synthesized. The photophysical properties of these systems, which contain a highly solvatochromic metal complex moiety, have been investigated in water, methanol, and acetonitrile. In all solvents, prompt and efficient singlet-singlet energy transfer takes places from the pyrene to the inorganic moiety. Energy transfer at the triplet level, on the other hand, is dramatically solvent dependent. In water, the metal-to-ligand charge transfer (MLCT) emission of the Ru-based chromophore is completely quenched, and rapid (200 ps for n = 1) irreversible triplet energy transfer to the pyrene units is detected in ultrafast spectroscopy. In acetonitrile, the MLCT emission is practically unaffected by the presence of the pyrenyl chromophore, implying the absence of any intercomponent triplet energy transfer. In methanol, triplet energy transfer leads to an equilibrium between the excited chromophores, with considerable elongation of the MLCT lifetime. The investigation of the [Ru(Pyr(n)bpy)(CN)(4)](2-) systems in methanol provided a very detailed and self-consistent picture: (i) The initially formed MLCT state relaxes toward equilibrium in 0.5-1.3 ns (n = 1, 2), as monitored both by ultrafast transient absorption and by time-correlated single photon counting. (ii) The two excited chromophores decay with a common lifetime of 260-450 ns (n = 1, 2), as determined from the decay of MLCT emission (slow component) and of the pyrene triplet absorption. (iii) These equilibrium lifetimes are fully consistent with the excited-state partition of 12-6% MLCT (n = 1-2), independently measured from preexponential factors of the emission decay. Altogether, the results demonstrate how site-specific solvent effects can be used to control the direction of intercomponent energy flow in bichromophoric systems.

Photoinduced electron transfer activation of dioxygen by redoxactive charge-transfer ion pairs of the type {A2+[Pt(mnt)2]2-} (A2+ = 2,2‘-,4,4‘-bipyridinium or cycloalkylated biimidazolium dication; mnt2- = maleonitriledithiolate) occurs through an optical electron transfer within an ion pair. This affords the primary redox products A•+ and [Pt(mnt)2]- as indicated by laser flash photolysis. Under argon the transients recombine by fast second-order kinetics. Under dioxygen a different behavior is observed. In the case of acceptors with a first reduction potential more positive than −0.6 V back electron transfer prevails. When the potential is more negative however, A•+ reduces O2 by pseudo-first-order kinetics to generate O2•-, while [Pt(mnt)2]- accumulates in the solution. Quantum yields increase with decreasing excitation wavelength. This suggests that internal conversion of the initially populated excited state to the photoreactive ion pair charge-transfer state is more efficient upon excitation to the ...

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.

A new hydrogen evolving cobalt catalyst 1 based on a pentapyridine ligand has been synthesized and characterized. Its photocatalytic activity in the presence of a Ru(bpy)32+ sensitizer and ascorbic acid as sacrificial electron donor has been screened in purely buffered aqueous solutions showing TONs and TOFs strongly dependent on both catalyst concentration and pH with the best results obtained at 50 µM 1 and at pH 4 (TON = 87, TOF = 8.1 mi -1). The photochemical mechanism, as revealed by flash photolysis, involves reaction of the excited sensitizer with ascorbic acid to yield Ru(bpy)3+ as primary photo-generated reductant, capable of electron transfer to 1 with a remarkable rate (bimolecular rate constant k = 5.7 (±0.7) × 10e9 M-1s -1). For hydrogen generation, two one-electron photochemical reduction steps of 1 are needed, along with hydride formation and protonation. Under the experimental conditions used, hydrogen evolution is mainly limited by partial decomposition of both sensitizer and catalyst. Moreover, accumulation of the oxidation product of the ascorbic acid donor, dehydroascorbic acid, is observed to strongly decrease the hydrogen production yield. As shown by flash photolysis, this species is capable of quenching the reduced ruthenium species (k = 4.4 (±0.5) × 10e7 M-1s-1) thus competing with electron transfer to the catalyst.

Efficient photocatalytic hydrogen evolution is obtained from 1 M phosphate buffer at pH 7 in the presence of a Ru(bpy)3(2+) sensitizer, an ascorbic acid sacrificial donor, and a water-soluble Co(II) porphyrin catalyst. Spectroscopic investigation of the system by stationary and time-resolved techniques enables a complete characterization of the photoinduced dynamics.

According to NMR evidence the metal-mediated sandwich assemblies of porphyrins 2 and 3 undergo in solution a thermally driven motion that resembles that of a stepper: the spontaneous rotational motion of the pyridyl rings is converted into a reciprocating linear motion of the porphyrins.

Four porphyrin-Re(I) conjugates, in which a pyridylporphyrin chromophore is directly coordinated to the electron-acceptor fragment [ fac-Re(CO) 3(bipy)] (+), were prepared: the dimeric and pentameric compounds [ fac-Re(CO) 3(bipy)(4'MPyP)](CF 3SO 3) ( 1) (4'MPyP = 4'-monopyridylporphyrin) and [ fac-{Re(CO) 3(bipy)} 4(mu-4'TPyP)](CF 3SO 3) 4 ( 2) (4'TPyP = 4'-tetrapyridylporphyrin), and the corresponding compounds with 3' rather than 4' porphyrins, [ fac-Re(CO) 3(bipy)(3'MPyP)](CF 3SO 3) ( 3) and [ fac-{Re(CO) 3(bipy)} 4(mu-3'TPyP)](CF 3SO 3) 4 ( 4). These adducts proved to be very stable in solution and were also structurally characterized in the solid state by X-ray crystallography. A detailed photophysical study was performed on the zincated adducts of the conjugates 1- 3, labeled 5, 6, and 7, respectively. In all adducts the typical fluorescence of the zinc-porphyrin unit was reduced in intensity and lifetime by the presence of the peripheral rhenium-bipy fragment(s) (heavy-atom effect). For the dyads 5 and 7 the photoinduced charge transfer process from the zinc-porphyrin to the Re(I)-bipy unit is only slightly exoergonic. Ultrafast spectroscopy experiments showed no evidence for electron transfer quenching in the dyads as such, whereas the addition of pyridine (that binds axially to zinc and thus affects the porphyrin redox potential) led to a moderately efficient photoinduced electron transfer process. In perspective, an appropriate functionalization of the bipy ligand and/or of the porphyrin chromophore might improve the thermodynamics and, thus the efficiency, of the photoinduced electron transfer process.

A 1,2-bis(2-methylbenzothiophene-3-yl)maleimide model ( DAE) and two dyads in which this photochromic unit is coupled, via a direct nitrogen-carbon bond ( Ru-DAE) or through an intervening methylene group ( Ru-CH 2-DAE ), to a ruthenium polypyridine chromophore have been synthesized. The photochemistry and photophysics of these systems have been thoroughly characterized in acetonitrile by a combination of stationary and time-resolved (nano- and femtosecond) spectroscopic methods. The diarylethene model DAE undergoes photocyclization by excitation at 448 nm, with 35% photoconversion at stationary state. The quantum yield increases from 0.22 to 0.33 upon deaeration. Photochemical cycloreversion (quantum yield, 0.51) can be carried out to completion upon excitation at lambda500 nm. Photocyclization takes place both from the excited singlet state (S 1), as an ultrafast (ca. 0.5 ps) process, and from the triplet state (T 1) in the microsecond time scale. In Ru-DAE and Ru-CH 2-DAE dyads, efficient photocyclization following light absorption by the ruthenium chromophore occurs with oxygen-sensitive quantum yield (0.44 and 0.22, in deaerated and aerated solution, respectively). The photoconversion efficiency is almost unitary (90%), much higher than for the photochromic DAE alone. Efficient quenching of both Ru-based MLCT phosphorescence and DAE fluorescence is observed. A complete kinetic characterization has been obtained by ps-ns time-resolved spectroscopy. Besides prompt photocyclization (0.5 ps), fast singlet energy transfer takes place from the excited diarylethene to the Ru(II) chromophore (30 ps in Ru-DAE, 150 ps in Ru-CH 2-DAE ). In the Ru(II) chromophore, prompt intersystem crossing to the MLCT triplet state is followed by triplet energy transfer to the diarylethene (1.5 ns in Ru-DAE, 40 ns in Ru-CH 2-DAE ). The triplet state of the diarylethene moiety undergoes cyclization in a microsecond time scale. The experimental results are complemented with a combined ab initio and DFT computational study whereby the potential energy surfaces (PES) for ground state (S 0) and lowest triplet state (T 1) of the diarylethene are investigated along the reaction coordinate for photocyclization/cycloreversion. At the DFT level of theory, the transition-state structures on S 0 and T 1 are similar and lean, along the reaction coordinate, toward the closed-ring form. At the transition-state geometry, the S 0 and T 1 PES are almost degenerate. Whereas on S 0 a large barrier (ca. 45 kcal mol (-1)) separates the open- and closed-ring minima, on T 1 the barriers to isomerization are modest, cyclization barrier (ca. 8 kcal mol (-1)) being smaller than cycloreversion barrier (ca. 14 kcal mol (-1)). These features account for the efficient sensitized photocyclization and inefficient sensitized cycloreversion observed with Ru-DAE. Triplet cyclization is viewed as a nonadiabatic process originating on T 1 at open-ring geometry, proceeding via intersystem crossing at transition-state geometry, and completing on S 0 at closed-ring geometry. A computational study of the prototypical model 1,2-bis(3-thienyl)ethene is used to benchmark DFT results against ab initio CASSCF//CASPT2 results and to demonstrate the generality of the main topological features of the S 0 and T 1 PES obtained for DAE. Altogether, the results provide strong experimental evidence and theoretical rationale for the triplet pathway in the photocyclization of photochromic diarylethenes.

Dimeric and pentameric adducts between a meso-4′ pyridylporphyrin core and either one or four rhenium(I) bipyridine tricarbonyl units, fac-[Re(CO)3(bipy)(4′MPyP)][CF3SO3] and fac-[{Re(CO)3(bipy)}4(4′TPyP)][CF3SO3]4(4′MPyP = 4′-monopyridylporphyrin, 4′TPyP = 4′-tetrapyridylporphyrin), respectively, were synthesized and their photophysical behaviors were investigated by emission and absorption time resolved experiments. The adducts exhibit distinctive supramolecular features, different from those of the molecular components. Upon excitation of the core, the typical porphyrin fluorescence is quenched. This effect is attributed to enhanced intersystem crossing in the porphyrin unit, owing to the heavy atom effect provided by the attached rhenium unit(s). Following excitation of rhenium fragments the typical rhenium MLCT emission is not observed while partial sensitization of the porphyrin fluorescence occurs, indicating that fast intercomponent energy and/or electron transfer processes take place in competition with the intersystem crossing in the rhenium unit.

The metal-driven construction of multi-porphyrin assemblies, which exploits the formation of coordination bonds between peripheral basic site(s) on the porphyrins and metal centers, has recently allowed the design and preparation of sophisticated supramolecular architectures whose complexity and function begin to approach the properties of naturally occurring systems. Within this framework, meso -pyridyl/phenyl porphyrins (PyPs), or strictly related chromophores, can provide geometrically well-defined connections to as many as four metal centers by coordination of the pyridyl peripheral groups. Several discrete assemblies of various nuclearities, in which the pyridylporphyrins are linkers binding metalloporphyrins and/or coordination compounds, have been constructed in recent years. In this review, we summarize recent work from our laboratories on metal-mediated multi-porphyrin assemblies and their photophysical properties. The review is organized as follows: after a brief summary of relevant concepts on energy and electron transfer processes and their implications in charge separation and the antenna effect (Section 2), the main assembling strategies leading to side-to-face and metal-mediated assemblies are synthetically reviewed (Section 3). The photophysical studies leading to the identification and kinetic characterization of the photoinduced energy and/or electron transfer processes within the supramolecular structures are surveyed in Section 4.

The metal-driven construction of multi-porphyrin assemblies, which exploits the formation of coordination bonds between exocyclic donor site(s) on the porphyrins and metal centers, has recently allowed the design and preparation of sophisticated supramolecular architectures whose complexity and function begin to approach the properties of naturally occurring systems. Within this framework, meso-pyridyl/phenyl porphyrins (PyPs), or strictly related chromophores, can provide geometrically well-defined connections to as many as four metal centers by coordination of the pyridyl peripheral groups. Several discrete assemblies of various nuclearities, in which the pyridylporphyrins are connected through external coordination compounds, have been constructed in recent years. In this review, we summarize recent work in this field from our and other laboratories. The photophysical properties of some ruthenium-mediated assemblies of porphyrins prepared by our group are also described.

The molecular box 1 comprises of two zinc-porphyrin metallacycles connected by two free-base 4'-trans-dipyridylporphyrins, axially coordinated to the zinc centers. The photophysics of 1 were studied in chloroform by emission and ultrafast absorption spectroscopy. In the molecular box, fast singlet energy transfer (main component, tau=32 ps) is observed to occur from the zinc-porphyrin metallacycles to the free-base chromophores. From wavelength-dependent spectrofluorimetric data, the efficiency of the energy-transfer (ET) process is estimated as 0.5. The lower-than-unity value is tentatively attributed to the possibility of a competing electron-transfer quenching pathway. Molecular box 1 can be considered to be a simple, self-assembling, six-chromophore antenna system. It has an inner cavity, 11.4 Angstrom wide, that could be used, in principle, to host a variety of guest molecules and obtain higher-order assemblies.

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).

The new heptanuclear ruthenium(II) dendron, [Cl(2)Ru{(micro-2,3dpp)Ru[(micro-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 {Cl(2)Ru(micro-2,3-dpp)2} core, the {Ru(micro-2,3dpp)3}2+ intermediate, and the {(bpy)2Ru(micro-2,3-dpp)}2+ peripheral subunits) and several redox-active sites. The new species exhibits very intense absorption bands in the UV region (epsilon value in the 10(5)-10(6) M(-1) cm(-1) range) as a result of spin-allowed ligand-centered (LC) transitions, and intense bands in the visible region (epsilon value in the 10(4)-10(5) M(-1) cm(-1) 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 bond and redox process to specific subunits of 1 was achieved by comparison with the properties of smaller multinuclear species of the same family, namely [Cl(2)Ru{(micro-2,3-dpp)Ru(bpy)2}2]4+ (2), [(bpy)2Ru(u-2,3-dpp)Ru(bpy)2]4+ (4), and [Ru{(micro-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 {Cl(2)Ru(micro-2,3-dpp)2} subunit. Interestingly, the 3MLCT levels involving the peripheral {(bpy)2Ru(micro-2,3-dpp)}2+ subunits are deactivated by energy transfer to the emitting level, in spite of the presence of interposed high-energy (Ru(micro-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.