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Tetraphenylmethane appended with four pyridylpyridinium units works as a scaffold to self-assemble four ruthenium porphyrins in a tetrahedral shape-persistent giant architecture. The resulting supramolecular structure has been characterised in the solid state by X-ray single crystal analysis and in solution by various techniques. Multinuclear NMR spectroscopy confirms the 1 : 4 stoichiometry with the formation of a highly symmetric structure. The self-assembly process can be monitored by changes of the redox potentials, as well as by modifications in the visible absorption spectrum of the ruthenium porphyrin and by a complete quenching of both the bright fluorescence of the tetracationic scaffold and the weak phosphorescence of the ruthenium porphyrin. An ultrafast photoinduced electron transfer is responsible for this quenching process. The lifetime of the resulting charge separated state (800 ps) is about four times longer in the giant supramolecular structure compared to the model 1 : 1 complex formed by the ruthenium porphyrin and a single pyridylpyridinium unit. Electron delocalization over the tetrameric pyridinium structure is likely to be responsible for this effect.

We present a stepwise synthetic strategy for the preparation of the unprecedented heteroleptic 2+2 neutral metallacycle [{t,c,c-RuCl2(CO)2}2(4′cisDPyP)(3′cisDPyP)] (5), in which two different 5,10-meso-dipyridylporphyrins, 4′cisDPyP [i.e., 5,10-bis(4′-pyridyl)-15,20-diphenylporphyrin] and 3′cisDPyP [i.e., 5,10-bis(3′-pyridyl)-15,20-diphenylporphyrin], are joined through equal 90°-angular Ru(II) connectors. The synthesis of 5 was accomplished through the preparation of a reactive ditopic intermediate in which one of the two pyridylporphyrins is linked to two neutral ruthenium fragments, each having one residual readily available coordination site (a dmso-O). Thus, compound 5 was obtained under mild conditions through two complementary routes: either by treatment of [{t,c,c-RuCl2(CO)2(dmso-O)}2(4′cisDPyP)] (3) with 1 equiv of 3′cisDPyP or, alternatively, by treatment of [{t,c,c-RuCl2(CO)2(dmso-O)}2(3′cisDPyP)] (4) with 1 equiv of 4′cisDPyP. Heteroleptic metallacycle 5 was isolated in pure form in acceptable yield and fully characterized. Spectroscopic data and a molecular model show that 5 has an L-shaped geometry, with the two porphyrins almost orthogonal to one another. The modular approach that we established is highly flexible and opens the way to several possible exciting developments., The unprecedented heteroleptic 2+2 neutral metallacycle [{t,c,c-RuCl2(CO)2}2(4′cisDPyP)(3′cisDPyP)] (5), which features two different 5,10-meso-dipyridylporphyrins and has an L-shaped geometry, was isolated in pure form through a flexible stepwise synthetic approach.

Aromatic amino acids such as l-tyrosine and l-tryptophan are deployed in natural systems to mediate electron transfer (ET) reactions. While tyrosine oxidation is always coupled to deprotonation (proton-coupled electron-transfer, PCET), both ET-only and PCET pathways can occur in the case of the tryptophan residue. In the present work, two novel conjugates 1 and 2, based on a Sn-IV tetraphenylporphyrin and Sn-IV octaethylporphyrin, respectively, as the chromophore/electron acceptor and l-tryptophan as electron/proton donor, have been prepared and thoroughly characterized by a combination of different techniques including single crystal X-ray analysis. The photophysical investigation of 1 and 2 in CH2Cl2 in the presence of pyrrolidine as a base shows that different quenching mechanisms are operating upon visible-light excitation of the porphyrin component, namely photoinduced electron transfer and concerted proton electron transfer (CPET), depending on the chromophore identity and spin multiplicity of the excited state. The results are compared with those previously described for metal-mediated analogues featuring Sn-IV porphyrin chromophores and l-tyrosine as the redox active amino acid and well illustrate the peculiar role of l-tryptophan with respect to PCET.

Model dyads, in which the Ru(bda) water oxidation catalyst (WOC) is connected to a porphyrin, were prepared following two different modular strategies: i) the direct linkage approach, in which porphyrins bearing peripheral pyridyl rings are bound to the {Ru(bda)} fragment as axial ligands, and ii) the metal-containing ligand approach, in which symmetrical trans-[Ru(bda)(L)2] or unsymmetrical trans-[Ru(bda)(L)(pic)] compounds (L = bifunctional pyridyl linker), are axially bound to metalloporphyrins. Both synthetic strategies are modular and thus rather flexible in nature. As proofs of principle, the following Ru(bda) – porphyrin dyads are described: trans-[Ru(bda)(4′-MPyP)2] (2, 4′-MPyP = 5-(4′-pyridyl)-10,15,20-phenylporphyrin), and the sandwich compounds [trans-Ru(bda)(4,4′-bpy)2{Ru(CO)(TPP)}2] (5) and [trans-Ru(bda)(4,4′-bpy)2(6Zn)]2 (7), where 6Zn is the porphyrin metallacycle [trans,cis,cis-RuCl2(CO)2(Zn·4′-cisDPyP)]2 (4′-cisDPyP = 5,10-(4′-pyridyl)-15,20-(phenyl)-porphyrin.

Aromatic amino acids such as l‐tyrosine and l‐tryptophan are deployed in natural systems to mediate electron transfer (ET) reactions. While tyrosine oxidation is always coupled to deprotonation (proton‐coupled electron‐transfer, PCET), both ET‐only and PCET pathways can occur in the case of the tryptophan residue. In the present work, two novel conjugates 1 and 2, based on a SnIV tetraphenylporphyrin and SnIV octaethylporphyrin, respectively, as the chromophore/electron acceptor and l‐tryptophan as electron/proton donor, have been prepared and thoroughly characterized by a combination of different techniques including single crystal X‐ray analysis. The photophysical investigation of 1 and 2 in CH2Cl2 in the presence of pyrrolidine as a base shows that different quenching mechanisms are operating upon visible‐light excitation of the porphyrin component, namely photoinduced electron transfer and concerted proton electron transfer (CPET), depending on the chromophore identity and spin multiplicity of the excited state. The results are compared with those previously described for metal‐mediated analogues featuring SnIV porphyrin chromophores and l‐tyrosine as the redox active amino acid and well illustrate the peculiar role of l‐tryptophan with respect to PCET., Photoinduced electron transfer (ET) or concerted proton‐electron transfer (CPET) occur in tin(IV) porphyrin‐tryptophan conjugates in dichloromethane in the presence of an organic base depending on the chromophore identity and spin multiplicity of the excited state which point out how peculiar the l‐tryptophan amino acid is with respect to charge transfer reactions.

Proton-coupled electron transfer (PCET) plays a key role in many biological processes, and a thorough comprehension of its subtle mechanistic complexity requires the synthesis and characterization of suitable artificial systems capable of mimicking this fundamental, elementary step. Herein, we report on a detailed photophysical investigation of conjugate 1, based on a tin(IV) tetraphenylporphyrin (SnTPP) chromophore bound to two l-tyrosinato amino acids, in CH2Cl2 in combination with organic bases of different strength and the preparation of a novel conjugate 3, based on a tin(IV) octaethylporphyrin (SnOEP) in place of the tetraphenyl analogue, and its photophysical characterization in CH2Cl2 in the presence of pyrrolidine. In the case of compound 1 with all bases examined, quenching of both the singlet and triplet excited states is observed and attributed to the occurrence of concerted proton-electron transfer (CPET). Rates and quenching yields decrease with the strength of the base used, consistent with the decrease of the driving force for the CPET process. Conjugate 3 with pyrrolidine is quenched only at the triplet level by CPET, albeit with slower rates than its parent compound 1, ascribable to the smaller driving force as a result of SnOEP being more difficult to reduce than SnTPP. For both systems, the quenching mechanism is confirmed by suitable blank experiments, specific kinetic treatments, and the observation of kinetic isotope effects (KIEs). Differently from what has been previously proposed, a detailed reinvestigation of the triplet quenching of 1 with pyrrolidine shows that no long-lived radical pair state is formed, as diradical recombination is always faster than formation. This is true for both 1 and 3 and for all bases examined. The kinetics of the CPET pathways can be well described according to Marcus theory and point toward the involvement of substantial reorganization energy as typically observed for PCET processes of concerted nature.

A side-to-face array DPy-gPBI[Ru(4-tBuTPP)(CO)]

In this paper, we describe three new stereoisomers of the already known 2 + 2 metallacycle of porphyrins [trans,cis,cis-RuCl2(CO)2(4′cisDPyP)]2 (2, 4′cisDPyP = 5,10-bis(4′-pyridyl)-15,20-diphenylporphyrin), namely [{trans,cis,cis-RuCl2(CO)2}(4′cisDPyP)2{cis,cis,cis-RuCl2(CO)2}] (14) and [cis,cis,cis-RuCl2(CO)2(4′cisDPyP)]2 (15), in which the chiral {cis,cis,cis-RuCl2(CO)2} fragment has either a C or A handedness. The least abundant 15 exists as a mixture of two stereoisomers defined as alternate (15alt, both porphyrins are trans to a Cl and a CO) and pairwise (15pw, one porphyrin is trans to two chlorides and the other to two carbonyls), each one as a statistical mixture of meso (AC) and racemic (AA and CC) diastereomers. Remarkably, both 14 and 15 are—to the best of our knowledge—unprecedented examples of 2D metallacycles with octahedral chiral-at-metal connectors, and 14 is the first example of a 2 + 2 molecular square with stereoisomeric Ru(II) corners. Whereas 2 is selectively obtained by treatment of trans,cis,cis-RuCl2(CO)2(dmso-O)2 (1) with 4′cisDPyP, 14 and 15 were obtained, together with 2 (major product), using stereoisomers of 1, either cis,cis,trans-RuCl2(CO)2(dmso-S)2 (5) or cis,cis,cis-RuCl2(CO)2(dmso)2 (6), as precursors. From a general point of view, this work demonstrates that—even for the smallest 2 + 2 metallacycle and using a symmetric organic linker—several stereoisomers can be generated when using octahedral metal connectors of the type {MA2B2} that are not stereochemically rigid. As a proof-of-concept, it also opens the way to new—even though challenging—opportunities: unprecedented and yet unexplored chiral metallosupramolecular assemblies can be obtained and eventually exploited (e.g., for supramolecular catalysis) by using stereogenic octahedral metal connectors amenable to become chiral centers.

A series of four arrays made of a central Sn(IV) porphyrin as scaffold axially connected, via carboxylate functions, to two free-base porphyrins has been prepared and fully characterized. Three arrays in the series feature the same free-base unit and alternative tin-porphyrin macrocycles, and one consists of a second type of free-base and one chosen metallo-porphyrin. A thorough photophysical investigation has been performed on all arrays by means of time-resolved emission and absorption techniques. Specific focus has been given at identifying how structural modifications of the free-base and tin-porphyrin partners and/or variation of the solvent polarity can effectively translate into distinct photophysical behaviors. In particular, for systems SnTPP(Fb) 2 (1) and SnOEP(Fb) 2 (2), an ultrafast energy transfer process from the excited Sn(IV) porphyrin to the free-base unit occurs with unitary efficiency. For derivative SnTPP(FbR) 2 (3), the change of solvent from dichloromethane to toluene is accompanied by a neat change in the intercomponent quenching mechanism, from photoinduced electron transfer to energy transfer, upon excitation of the Sn(IV) porphyrin unit. Finally, for array SnTpFP(Fb) 2 (4), an ultrafast electron transfer quenching of both chromophores is detected in all solvents. This work provides a general outline, accompanied by clear experimental support, on possible ways to achieve a systematic fine-tuning of the quenching mechanism (from energy to electron transfer) of Sn(IV) multiporphyrin arrays.

This contribution, that readdresses the insertion of the Ru(II)–CO fragment into model porphyrins (i.e. ruthenation), has a Janus character, with one speculative and one practical side. As a proof of concept we demonstrate that ruthenation of a porphyrin can be performed under relatively mild conditions using the Ru(II) monocarbonyl complex [Ru(CO)(dmso)5][PF6]2 that – besides CO – features exclusively labile dmso ligands. Even though this finding might seem trivial, it is only the second example that uses a Ru(II) carbonyl for porphyrin ruthenation, the first one having been reported almost 50 years ago and then neglected. From a practical point of view, we show the spectacular effect of propionic acid as solvent for performing the ruthenation of neutral and anionic model porphyrins with Ru3(CO)12 (1). This process turned out to be extremely efficient and advantageous in terms of both reaction rates and yields (e.g. 100% ruthenation of TPP in 30 min at 140°C) compared to the procedures described in the literature.

Preparation and characterization of a small library of symmetric trans-di(4-pyridyl)porphyrin dimers, obtained by either Glaser–Hay or Sonogashira coupling reactions from appropriately prepared trans-di-4-pyridylporphyrin precursors, is presented. The porphyrin dimers are differentiated by a phenyl-alkynyl bridge of increasing length at one meso-position, while for all the derivatives the two remaining opposite meso-positions are tailored with a phenyl moiety bearing a short polyether chain. Coordination of the four pyridyl groups with appropriate metal fragments may be exploited to construct tubular hollow structures, with varied internal sizes, depending on the choice of the porphyrin dimer component.

We report here the synthesis, solution and solid state (single crystal X-ray structure) of a novel robust conjugate, obtained via axial coordination of two Re(diimine) fragments to one tin(IV) porphyrin center. The photophysical behavior of the conjugate was investigated by emission and absorption time resolved experiments. The conjugate exhibits distinctive supramolecular features, different from those of the molecular components. Upon excitation of the Re(I)(diimine) units, the typical rhenium MLCT emission is completely quenched and efficient sensitization of the Sn-porphyrin triplet state occurs. The synthetic method proposed here may be easily extended to the facile and efficient obtainment of a large number of tin(IV) porphyrin–Re(diimine) conjugates, with tuned photophysical features.

While its main current use is that of a feedstock in the chemical and petrochemical industry, molecular hydrogen can also be considered, in perspective, as an interesting energy carrier to be used in place of conventional fuels, e.g., in devices such as fuel cells. From this viewpoint, of particular interest would be the possibility to generate hydrogen from water splitting using a source of renewable energy such as solar light. This could represent, in principle, an inexhaustible, environmentally friendly energy source. In this review we summarize recent work on photochemical hydrogen evolution carried out with systems based on porphyrins or metalloporphyrins, either as photosensitizers or as catalysts. The systems discussed are varied including different levels of complexity, performance, and mechanistic insight. In general porphyrin molecules appear as a very promising class of photosensitizers for photocatalytic hydrogen production.

The novel conjugate 1, featuring two l-tyrosinato residues axially coordinated to the tin centre of a Sn(iv)- tetraphenylporphyrin, is reported as the first example of a supramolecular dyad for photochemical PCET. It is noteworthy that the excitation of 1 in the presence of a suitable base is followed by photoinduced PCET leading to a radical pair state with a surprisingly long lifetime.

This work demonstrates how a careful reexamination of well-trodden fields can fill conceptual gaps that previously escaped full understanding. The coordination of 2,2′-bipyridine (bpy) to the known RuII–chlorido–carbonyl precursors – the dinuclear [RuCl2(CO)3]2 (P1) and the polymeric [RuCl2(CO)2]n (P2) – has been investigated by several groups in the past, and a remarkably large number of ruthenium–mono(bpy)–carbonyl compounds were identified and fully characterized. Many were investigated as catalysts or key intermediates for the photochemical, electrochemical, and photo-electrochemical reduction of CO2, and for the water–gas shift reaction. Nevertheless, even though most – if not all – of the reaction products are known already, a careful examination of the literature led us to believe that a convincing general scheme interconnecting them all was still missing and important questions remained unanswered. For this reason, we investigated the reactivity of two mononuclear RuII–carbonyl–dmso precursors, trans,cis,cis-[RuCl2(CO)2(dmso-O)2] (P3) and fac-[RuCl2(CO)3(dmso-O)] (P4) – which can be considered as “activated forms” of P2 and P1, respectively – towards the coordination of bpy. Compounds P3 and P4 allowed us to gain new mechanistic insight and a deeper level of understanding. In particular, we found that coordination of bpy to P4 (or P1) generates first the tricarbonyl cation fac-[Ru(bpy)Cl(CO)3]+. This key intermediate undergoes the facile and selective nucleophilic attack on the CO trans to Cl (by RO– in alcoholic solvents or OH– from adventitious water in other solvents), leading to all other species. We also demonstrated that Cl– – even when in large excess – is unable to replace a carbonyl on fac-[Ru(bpy)Cl(CO)3]+. However, the chloride set free from the precursor competes efficiently with bpy for the coordination to RuII (chloride rebound mechanism).

A new triad system featuring one zinc porphyrin and one fullerene moieties attached to a central redox-active Re(I) connector was obtained in remarkable yield by cleverly exploiting a facile two-step synthesis. Detailed description and discussion on the characterization of this multicomponent system and of its parent free-base analogue are presented, along with a kinetic study of the stepwise electron-transfer processes occurring upon visible excitation.

A very efficient metal-mediated strategy led, in a single step, to a quantitative construction of a new three-component multichromophoric system containing one fullerene monoadduct, one aluminium(III) monopyridylporphyrin, and one ruthenium(II) tetraphenylporphyrin. The Al(III) monopyridylporphyrin component plays the pivotal role in directing the correct self-assembly process and behaves as the antenna unit for the photoinduced processes of interest. A detailed study of the photophysical behavior of the triad was carried out in different solvents (CH2Cl2, THF, and toluene) by stationary and timeresolved emission and absorption spectroscopy in the pico- and nanosecond time domains. Following excitation of the Alporphyrin, the strong fluorescence typical of this unit was strongly quenched. The time-resolved absorption experiments provided evidence for the occurrence of stepwise photoinduced electron and hole transfer processes, leading to a chargeseparated state with reduced fullerene acceptor and oxidized ruthenium porphyrin donor. The time constant values measured in CH2Cl2 for the formation of charge-separated state Ru-Al+-C60 - (10 ps), the charge shift process (Ru-Al+-C60 - → Ru+-Al-C60 -), where a hole is transferred from Al-based to Ru-based unit (75 ps), and the charge recombination process to ground state (>5 ns), can be rationalized within the Marcus theory. Although the charge-separating performance of this triad is not outstanding, this study demonstrates that, using the self-assembling strategy, improvements can be obtained by appropriate chemical modifications of the individual molecular components.

Three porphyrin-cobaloxime dyads, suitable for application in photoinduced hydrogen generation with sacrificial donors, are characterized by ultrafast spectroscopy in order to clarify the primary photochemical events.

The monodentate phosphane ligand 1,3,5-triaza-7-phosphaadamantane (PTA) imparts excellent water solubility to its complexes. We aimed to prepare precursors with one or more PTA coligands for solubility and one or more labile ligands for facile replacement by a linker. For this purpose, we investigated the reactivity of the neutral isomers trans- and cis-RuCl2(PTA)4 (1 and 2) towards 2,2′-bipyridine (bpy), as a model chelating diimine linker. The new derivatives mer-[Ru(bpy)Cl(PTA)3]Cl (9) and fac-[Ru(bpy)Cl(PTA)3]Cl (10) were prepared and characterized. We also found that PTA reacts rapidly with cis,fac-RuCl2(dmso-O)(dmso-S)3 (11) and trans-RuCl2(dmso-S)4 (13) under mild conditions through the replacement of pairs of mutually trans dmso ligands with high selectivity, even when in stoichiometric defect. Thus, 11 affords cis,cis,trans-RuCl2(dmso-S)2(PTA)2 (12), whereas 13 gives 1. The two dmso ligands of 12 can be replaced selectively by chelating diimines such as bpy to afford the less symmetrical all-cis product cis,cis-Ru(bpy)Cl2(PTA)2 (16).

The preparation and the full characterization, including the X-ray structure determination, of a polar trans -dipyridylporphyrin functionalized with two short polyoxyethylene chains is reported. Reaction of the porphyrin with a Re(I) complex yielded a 4+4 metallacycle showing an improved solubility and a lower tendency to aggregate with respect to analogous porphyrin cyclic derivatives. These properties allowed a full NMR characterization of the metallacycle including VT 1 H DOSY-NMR experiments and, for the first time, the recording of a 13 C-NMR spectrum giving further insight into the structural definition of these type of metallacycles. Spray deposition of the metallacycle on a heated mica substrate shows the formation of regular ring-like nano-structures which are not formed by the parent porphyrin.

The combination of cis-protected metal fragments with linear linkers is expected to yield molecular squares. We found instead that treatment of the 90 degrees angular precursor trans-[RuCl2(dmso-S)4] (1) with an equivalent amount of the linear and rigid pyrazine (pyz) linker unexpectedly yields, in a number of different experimental conditions, the molecular triangle [{trans,cis-RuCl2(dmso-S)2(mu-pyz)}3] (3), together with polymeric material. Very similar results were also obtained from the reaction between 1 and the preformed corner fragment trans,cis,cis-[RuCl2(dmso-S)2(pyz)2] (6). In both cases, the expected molecular square [{trans,cis-RuCl2(dmso-S)2(mu-pyz)}4] (4) was observed only as a transient species. These results suggest that 3, which is the first example of a neutral molecular triangle with octahedral metal corners and pyrazine edges, is both the thermodynamic and the kinetic product of the reactions described above. The X-ray structure of 3 shows that the main distortions from ideal coordination geometry concern the N-Ru-N angles, which are narrower than 90 degrees , and the coordination bonds of pyz. The pyrazine molecules, which are basically planar, are significantly tilted from linearity. Calculations performed on 6 indicated that the N-Ru-N angle is ca. six times more rigid than the tilt angle of pyrazine. The structural and theoretical findings on 3 and 6, together with the previous examples of molecular triangles and squares with cis-protected metal corners and linear pyz edges, suggest that the entropically favored molecular triangles might be preferred over the expected molecular squares with metal corner fragments that spontaneously favor Npyz-M-Npyz angles narrower than 90 degrees because of the presence of ancillary ligands with significant steric demand on the coordination plane. The rather-flexible coordination geometry of pyrazine can accommodate the moderate distortions from linearity required to close the small metallacycle with modest additional strain.

We investigated the reactivity of three Ru(ii) precursors -trans,cis,cis-[RuCl2(CO)2(dmso-O)2], cis,fac-[RuCl2(dmso-O)(dmso-S)3], and trans-[RuCl2(dmso-S)4] - towards the diimine linker 2-(2'-pyridyl)pyrimidine-4-carboxylic acid (cppH) or its parent compound 4-methyl-2-(2'-pyridyl)pyrimidine ligand (mpp), in which a methyl group replaces the carboxylic group on the pyrimidine ring. In principle, both cppH and mpp can originate linkage isomers, depending on how the pyrimidine ring binds to ruthenium through the nitrogen atom ortho (N(o)) or para (N(p)) to the group in position 4. The principal aim of this work was to establish a spectroscopic fingerprint for distinguishing the coordination mode of cppH/mpp also in the absence of an X-ray structural characterization. By virtue of the new complexes described here, together with the others previously reported by us, we successfully recorded {(1)H,(15)N}-HMBC NMR spectra at natural abundance of the (15)N isotope on a consistent number of fully characterized Ru(ii)-cppH/mpp compounds, most of them being stereoisomers and/or linkage isomers. Thus, we found that (15)N NMR chemical shifts unambiguously establish the binding mode of cppH and mpp - either through N(o) or N(p)- and can be conveniently applied also in the absence of the X-ray structure. In fact, coordination of cppH to Ru(ii) induces a marked upfield shift for the resonance of the N atoms directly bound to the metal, with coordination induced shifts (CIS) ranging from ca.-45 to -75 ppm, depending on the complex, whereas the unbound N atom resonates at a frequency similar to that of the free ligand. Similar results were found for the complexes of mpp. This work confirmed our previous finding that cppH has no binding preference, whereas mpp binds exclusively through N(p). Interestingly, the two cppH linkage isomers trans,cis-[RuCl2(CO)2(cppH-κN(p))] (5) and trans,cis-[RuCl2(CO)2(cppH-κN(o))] (6) were easily obtained in pure form by exploiting their different solubility properties.

The aim of this work is the preparation of new, easy-to-make, octahedral Ru I I precursors with facial geometry to be used in self-assembly reactions. We chose dmso as a leaving group and 1,4,7-trithiacyclononane ([9]aneS 3 ), tris(pyrazolyl)methane [CH(pz) 3 , pz = pyrazol-1-yl], or triphos [CH 3 C(CH 2 PPh 2 ) 3 ] as the face-capping ligand (fcl). We describe here the preparation and structural characterization of three new dicationic Ru I I complexes (as triflate salts) of the type fac[Ru(fcl)(solv) 3 ] 2 + , namely fac-[Ru([9]aneS 3 )(dmso-O)(dmso-S) 2 ] 2 + (3b), which equilibrates in solution with the linkage isomer fac-[Ru([9]aneS 3 )(dmso-O) 2 (dmso-S)] 2 + (3a), fac[Ru{CH(pz) 3 }(dmso-O)(dmso-S) 2 ] 2 + (7), and fac-[Ru(triphos)-(dmso-O) 2 (H 2 O)] 2 + (9). In all cases the preferred synthetic procedure involved coordination of the face-capping ligand to cis-[RuCl 2 (dmso) 4 ] first, followed by replacement of the chlorides with dmso (or water) molecules assisted by addition of AgCF 3 SO 3 (AgOTf). NMR investigations showed that hydrolysis of the dmso ligands occurs very easily for 3 in water and for 9 in chloroform, while for complex 7 only the O-bound dmso ligand is replaced in water. In the course of this work, the following complexes were also isolated and structurally characterized: fac-[Ru([9]aneS 3 )Cl(dmso-S) 2 ][OTf] (2), trans,cis,cis-[RuCl 2 (dmso-S) 2 (CH(pz) 2 (pz))] (4), trans,cis,cis-[RuCl 2 (dmso-S) 2 (pzH) 2 ] (5), fac-[Ru(CH(pz) 3 }Cl 2 (dmso-S)] (6), fac-[Ru([9]aneS 3 )(py) 3 ][OTf] 2 (11), fac-[Ru([9]aneS 3 )(py) 2 -(CH 3 CN)][OTf] 2 (13), and fac-[Ru([9]aneS 3 O)(dmso-O) 2 -(dmso-S)][OTf] 2 ([9]aneS 3 O = 1,4,7-trithiacyclononane 1-oxide); in 4 the tripodal tris(pyrazolyl)methane acts as a bidentate ligand.

The bis-porphyrin system ZnP 2 , in which two zinc porphyrins are connected by a phenanthroline linker in an oblique fashion, acts as a bifunctional receptor towards the complexation of free-base meso-5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin (4'-cisDPyP). In solution, NMR spectroscopy evidenced quantitative formation of the tris-porphyrin macrocyclic assembly ZnP 2 (4'-cisDPyP), in which the two fragments are held together by two axial 4'-N(pyridyl)-Zn interactions. The remarkable stability of the edifice (an association constant of about 6 x 10 8 M - 1 was determined by UV/Vis absorption and emission titration experiments in toluene) is due to the almost perfect geometrical match between the two interacting units. The macrocycle was crystallized and studied by X-ray diffraction, which confirmed the excellent complementarity of the two components. Photoinduced energy transfer from the singlet excited state of the zinc porphyrin chromophores to the free-base porphyrin occurs with an efficiency of 98% (k e n = 2 x 10 1 0 s - 1 in toluene, ambient temperature) with a mechanism consistent with a dipole - dipole process with a low orientation factor.

The monocarbonyl Ru(II)–dmso precursor cis,cis,trans-RuCl2(dmso-S)2(dmso-O)(CO) (1) equilibrates slowly in light protected chloroform solution with two new geometrical isomers of formula cis,cis,cis-RuCl2(dmso-S)2(dmso-O)(CO) (2) and cis,mer-RuCl2(dmso-S)3(CO) (3). The three isomers have similar stabilities in solution. In 2 and 3 the three sulfoxides are in a meridional geometry, with the CO ligand coordinated trans to one of the cis chlorides. The new complexes 2 and 3 differ only in the coordination mode of one of the two trans sulfoxides: they are both S-bonded in 3, while one is S-bonded and the other O-bonded in 2. Thus 2 and 3 are linkage isomers of each other. The three isomers 1–3 are also obtained as a mixture by treatment of cis-RuCl2(dmso)4 with CO. The spectroscopic characterization of the new compounds 2 and 3 is reported, together with the X-ray structure of 2. Isomer 2 is the first example of a Ru(II)–CO complex in which coordination through the oxygen atom is found for a dmso that is not trans to CO; isomer 3 is the first example of a Ru(II)–dmso complex featuring three S-bonded dmso ligands with a meridional geometry. Considerations relating the stability of the three isomers to their geometry and to the binding mode of the dmso ligands are reported.

There is substantial recent interest worldwide in the construction of multiporphyrin assemblies which can either mimic naturally occurring multichromophore aggregates, such as the photosynthetic reaction center and the light harvesting complex of purple bacteria, or which can be used as electron- and/or energy-transfer molecular devices for advanced technological tasks. The metal-mediated self-assembly approach, 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 groups. Several discrete assemblies of various nuclearities, in which the pyridylp...

Treatment of the octahedral Ru(II)-dimethyl sulfoxide complexes trans-RuCl(2)(dmso-S)(4) (1), trans-RuCl(2)(dmso-O)(2)(CO)(2) (2), and trans-RuCl(2)(dmso)(3)(CO) (3) with a stoichiometric amount of 5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin (4'-cis-DPyP) yields, after chromatographic purification, the novel 2+2 molecular squares of formula [trans,cis,cis-RuCl(2)(dmso-S)(2)(4'-cis-DPyP)](2) (4), [trans,cis,cis-RuCl(2)(CO)(2)(4'-cis-DPyP)](2) (5), and [trans,cis,cis-RuCl(2)(dmso-S)(CO)(4'-cis-DPyP)](2) (6), respectively. Compound 6 exists as an equimolar mixture of the isomeric metallacycles 6a and 6b, depending on whether the 4'-N(py) rings of 4'-cis-DPyP's are trans to CO or to dmso-S. Compounds 4-6 were fully characterized by NMR and IR spectroscopy and by FAB mass spectrometry. Treatment of 5 with excess zinc acetate in chloroform/methanol mixtures led to the isolation of the corresponding zinc adduct [trans,cis,cis-RuCl(2)(CO)(2)(Zn x 4'-cis-DPyP)](2) (5Zn). Treatment of 5Zn with 1 equiv of a trans ditopic N-donor ligand L (L = 4,4'-bipy, 5,15-bis(4'-pyridyl)-2,8,12,18-tetra-n-propyl-3,7,13,17-tetramethylporphyrin (4'-trans-DPyP-npm), or 5,15-bis(4'-pyridyl)-10,20-diphenylporphyrin (4'-trans-DPyP)) leads readily and selectively, according to (1)H NMR spectroscopy, to the quantitative assembling of 2:2 supramolecular adducts of stacked metallacycles of formula [(5Zn)(2)(mu-L)(2)] (7-9), which were thoroughly characterized in solution by NMR spectroscopy. NMR features indicate that, at ambient temperature, the equilibrium between 5Zn and L to yield [(5Zn)(2)(mu-L)(2)] has an intermediate to slow rate on the NMR time scale (relatively broad signals for L) and is totally shifted toward the 2:2 product (all or nothing process). Single-crystal X-ray investigations showed that, depending on the nature of the bridging ligand, in the solid state these sandwich structures can either be maintained or originate polymeric chains formulated as [(5Zn)(mu-L)](infinity). When L = 4'-trans-DPyP, both solution- and solid-state data indicate that [(5Zn)(2)(mu-4'-trans-DPyP)(2)] (9) is a discrete supramolecular assembly of two molecular squares of metalloporphyrins axially connected through other porphyrins. In this molecular box, the two bridging porphyrins are coplanar at a distance of about 11.4 A. When L = 4,4'-bipy, the corresponding adduct 7 has the anticipated sandwich-like discrete architecture [(5Zn)(2)(mu-4,4'-bipy)(2)] in solution, but it assumes a stair-like polymeric wire structure in the solid state. The polymer [(5Zn)(mu-4,4'-bipy)](infinity) is made by 5Zn squares bridged by 4,4'-bipy ligands which are axially coordinated alternatively on the two opposite sides of each square. Our work clearly established that relatively simple supramolecular adducts of porphyrins, such as molecular squares, are suitable building blocks for the construction of more elaborate assemblies of higher order by axial coordination of bridging ligands.

Over the last two decades the metal-mediated directional-bonding has established as an efficient tool for the construction of complex and functional supramolecular architectures. The success of this approach has strongly benefited from design principles based on the known and fixed ligand and metal coordination geometry, thus ensuring a high level of control on the topology and energetics of the supramolecular adducts. Among them, self-assembled pyridylporphyrin metallacycles have attracted considerable attention due to their symmetric structure, to their peculiar optical and redox properties, to their large inner cavity, that can be easily addressed by axial ligation to metal ions inserted in the porphyrin ring. This review is focused on this particular subset of metal–organic architectures and in particular on discrete metallacyclic system with a 4 + 4 and 2 + 2 metal/ligand stoichiometry formed by coordination of 4-pyridylporphyrins with different metal ions (Pd(II), Pt(II), Ru(II), Re(I)). Their synthesis, characterization and applications in the field of catalysis, molecular recognition, sensing and ionophoric behavior will be reviewed with the aim to stimulate further interest on these versatile supramolecular systems.

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.

The [Pd(dppp)(OTf)2] complex acts as an efficient transporter of halide anions, in particular the biologically relevant chloride anion, across a phospholipid bilayer.

For the fi rst time the two linkage isomers of a Ru( II ) complex with 2-(2 ’ -pyridyl)pyrimidine-4-carboxylic acid (cppH) – that form in comparable amounts – have been fully characterized individually. The X-ray structure of each isomer is related to its NMR spectrum in solution.

We report on the efficient use of different coordinative affin- ities to quantitatively construct a three-component multi- chromophore system containing one free-base porphyrin, one aluminium(III) monopyridylporphyrin, and one ruth- enium(II) tetraphenylporphyrin in a single step. The Al III monopyridylporphyrin plays the pivotal role in directing the correct self-assembly process, and it behaves as the antenna unit for the photoinduced processes of interest. A two-com- ponent system, containing the same free-base porphyrin and one aluminium(III) tetraphenylporphyrin, was prepared for comparative studies. Both systems are remarkably stable in chlorinated solutions. Their photophysical behaviour was studied in detail by stationary and time-resolved emission techniques. In the two-component system, fast and efficient singlet energy transfer from the Al–porphyrin to the free- base porphyrin is observed; in the three-component system, upon excitation of the Al–porphyrin, a fast (minor of 250 ps) electron transfer process from the Ru–porphyrin to the aluminium–porphyrin is observed.

Over the last two decades the metal-mediated directional-bonding has established as an efficient tool for the construction of complex and functional supramolecular architectures. The success of this approach has strongly benefited from design principles based on the known and fixed ligand and metal coordination geometry, thus ensuring a high level of control on the topology and energetics of the supramolecular adducts. Among them, self-assembled pyridylporphyrin metallacycles have attracted considerable attention due to their symmetric structure, to their peculiar optical and redox properties, to their large inner cavity, that can be easily addressed by axial ligation to metal ions inserted in the porphyrin ring. This review is focused on this particular subset of metal–organic architectures and in particular on discrete metallacyclic system with a 4 + 4 and 2 + 2 metal/ligand stoichiometry formed by coordination of 4-pyridylporphyrins with different metal ions (Pd(II), Pt(II), Ru(II), Re(I)). Their synthesis, characterization and applications in the field of catalysis, molecular recognition, sensing and ionophoric behavior will be reviewed with the aim to stimulate further interest on these versatile supramolecular systems.

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.

A novel class of dianionic Ru(III) dimers of formula Na2[[trans-RuCl4(Me2SO)]2(mu-L)], with L = pyrazine (pyz, 1), pyrimidine (pym, 2), 4,4'-bipyridine (bipy, 3), and 1,2-bis(4-pyridine) ethane (etbipy, 4), was developed by us with the specific aim of assessing their antitumor properties. The dimers are in fact structurally related to the antimetastatic mononuclear compound (ImH) [trans-RuCl4(Me2SO)(Im)] (NAMI-A, Im = imidazole). Preliminary results concerning the antineoplastic activity of 1-4 against the murine MCa carcinoma model, a tumor which spontaneously metastasizes in the lungs, are reported. Similarly to what is normally observed with NAMI-A, the treatment with the dimeric complexes was scarcely effective against the growth of the primary tumor. However, dimers 1, 2, and 4 reduced very effectively the number and, in particular, the weight of lung metastases (to about 5% with respect to controls); in particular, Na2[[trans-RuCl4(Me2SO)]2(mu-etbipy)] (4) was as effective as NAMI-A in reducing the spontaneous metastases at a dosage which, in terms of moles of ruthenium, is about 3.5 times lower compared to that normally used for NAMI-A. Furthermore, in vitro tests showed that dimers 1-4 are capable of forming interstrand cross-links with linearized plasmidic DNA in a time-dependent manner. All the dimeric species are more active in inducing cross-links compared to NAMI-A, and the dimer bridged by the etbipy ligand (4) is the most effective among those tested.

The reaction of Na[transRuCl4Me2SO(Im)] (NAMI; where Im is imidazole), a novel anti-neoplastic ruthenium(III) complex, with BSA, was studied in detail by various physico-chemical techniques. It is shown that NAMI, following chloride hydrolysis, binds bovine serum albumin tightly; spectrophotometric and atomic absorption data point out that up to five ruthenium ions are bound per albumin molecule when BSA is incubated for 24 h with an eightfold excess of NAMI. CD and electronic absorption results show that the various ruthenium centers bound to albumin exhibit well distinct spectroscopic features. The first ruthenium equivalent produces a characteristic positive CD band at 415 nm whereas the following NAMI equivalents produce less specific and less marked spectral effects. At high NAMI/BSA molar ratios a broad negative CD band develops at 590 nm. Evidence is provided that the bound ruthenium centers remain in the oxidation state +3. By analogy with the case of transferrins it is proposed that the BSA-bound ruthenium ions are ligated to surface histidines of the protein; results from chemical modification experiments with diethylpyrocarbonate seem to favor this view. Spectral patterns similar to those shown by NAMI are observed when BSA is reacted with two strictly related ruthenium(III) complexes Na[transRuCl4(Me2SO)2] and H(Im)[transRuCl4(Im)2] (ICR), implying a similar mechanism of interaction in all cases. It is suggested that the described NAMI-BSA adducts may form in vivo and may be relevant for the biological properties of this complex; alternatively NAMI/BSA adducts may be tested as specific carriers of the ruthenium complex to cancer cells. Implications of these findings for the mechanism of action of NAMI and of related ruthenium(III) complexes are discussed.

Factors influencing the orientation and dynamic motions of planar N-donor heterocyclic ligands (L) are of interest since such features have broad relevance in metallobiochemistry [Marzilli, L. G.; Marzilli, P. A.; Alessio, E. Pure Appl. Chem. 1998, 70, 961-968]. We found that mu-oxorhenium(V) dinuclear complexes [ReOCl2LsLt]-O-[ReOCl2LsLt] bearing either symmetrical (L = py = pyridine; 3,5-lut = 3,5-lutidine) or lopsided (L = Me3-Bzm = 1,5,6-trimethylbenzimidazole) cis L ligands are particularly useful for studying these factors. NMR data showed that terminal (Lt) and stacked (Ls) ligands were exchanged by approximately 180 degrees rotation about the Re-O-Re bond system. Such exchange occurred, however, between degenerate chiral conformers. Here we report a combined X-ray structural and solution NMR investigation of the AA + CC (racemic) and AC (meso) forms of two mixed-ligand mu-oxorhenium dimers that bear one lopsided and one symmetrical ligand on each Re atom, namely, Re2O3-Cl4(py)2(Me3Bzm)2 (1rac and 1meso) and Re2O3Cl4(3,5-lut)2(Me3Bzm)2 (2rac and 2meso). The presence of two different cis L ligands in 1 and 2 breaks the local symmetry at each Re atom, so that, in the racemic dimers, the exchange of terminal and stacked ligands leads to nondegenerate conformers. Overall, NMR data showed that the unsymmetrical dimers 1 and 2 undergo two dynamic processes contemporaneously, namely, 180 degrees rotation about the Re-N(py or 3,5-lut) bond and coupled rotation about the Re-O-Re/Re-N bonds. Both processes reach the slow exchange limit below -80 degrees C. Rotation of py in 1 occurs faster than that of 3,5-lut in 2; this difference is attributed to the higher steric demands of 3,5-lut compared to py. For both dimers NMR data provided compelling evidence of the preferred conformers in solution, including ligand orientations. The low-T solution structure of 1meso and 2meso is chiral, the same as that found in the solid state for 2meso, where the Me3Bzm on one Re atom is stacked with the 3,5-lut on the other Re atom. The remaining Me3Bzm and 3,5-lut, one on each Re atom, are both terminal. In solution the coupled Re-O-Re/Re-N rotations interconvert the two halves of each meso dimer to yield the same overall stable chiral conformation. For the racemic dimers, however, this process does not interconvert one enantiomer into the other, but instead interconverts two rotamers, R1 and R2, each of which is chiral. We found that, in the case of both 1rac and 2rac, the conformer with stacking symmetrical ligands (R1) is roughly 1 order of magnitude more stable than that with stacking Me3Bzm ligands (R2). Moreover, the solution conformation of R1 is the same as that found in the solid state of 1rac. Solution- and solid-state data indicate that the key interaction favoring the observed conformations is very likely the electrostatic attraction between the delta+ H2 atoms on the Me3Bzm ligands and the negative O and Cl groups in the core of the dimers. Finally, for both meso and racemic dimers we were also able to elucidate the preferred pathways of the coupled dynamic motions and establish that, very likely, the two halves of the dimers swing back and forth by approximately 130 degrees through the anti eclipsed form.

We demonstrate here that, whereas the rhenium(I) − zinc porphyrin dyad fac-[Re(CO) 3 (bpy)(Zn · 4 ′ MPyP)](CF 3 SO 3 ) [1; 4 ′ MPyP = 5-(4 ′ -pyridyl)-10,15,20-triphenylporphyrin] shows no evidence for photoinduced electron transfer upon excitation in the visible region because the charge-separated state ZnP + − Re − is almost isoenergetic with the singlet excited state of the zinc porphyrin ( Δ G = − 0.05 eV), the introduction of electron-withdrawing ethyl ester groups on the bpy ligand signi fi cantly improves the thermodynamics of the process ( Δ G = − 0.42 eV). As a consequence, in the new dyad fac-[Re(CO) 3 (4,4 ′ -DEC-bpy)(Zn · 4 ′ MPyP)](CF 3 SO 3 ) (4; 4,4 ′ - DEC-bpy = 4,4 ′ -diethoxycarbonyl-2,2 ′ -bipyridine), an e ffi cient and ultrafast intramolecular electron-transfer process occurs from the excited zinc porphyrin to the rhenium unit upon excitation with visible light. Conversely, the introduction of electron-donor tert-butyl groups on the meso-phenyl moieties of the zinc porphyrin has a negligible e ff ect on the photophysics of the system. For dyad 4, the time constants for the charge-separation and charge- recombination processes in solvents of di ff erent polarity (PrCN, DCM, and toluene) were measured by an ultrafast time-resolved absorption technique ( λ exc = 560 nm).

Three new molecular cages-a trigonal prism (2) and two cubic-shaped boxes of increasing size (3, 4)-featuring as many as 10 porphyrins were obtained in quantitative yields by self-assembly of a single bis-zincporphyrin metallacycle (1) with appropriate polytopic N-linkers: the trigonal planar tpt or the cruciform porphyrins 4'-TPyP and 4'-TPhPyP.

A stable tetraporphyrin metallacycle with Re(I) corners (1) is capable of forming nanopores in a liposomial membrane, provided that the porphyrin units are properly functionalized with peripheral carboxylic acid residues that, by establishing an hydrogen bond network, allow the formation of dimers that span the depth of the membrane.

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.

A very efficient and successful metal-mediated strategy towards the formation of a non-symmetric triad is described: appropriate Lewis acid and/or base functions on the molecular components (a naphthalenediimide, an aluminium(III) porphyrin, and a ruthenium(II) porphyrin) lead to the desired product uniquely. The photophysics of the triad was investigated in detail using time-resolved spectroscopy in the pico- and nanosecond time domains. The strategy is of great potential interest as, while confining the synthetic effort to the single components, it can give access to a wide range of photoactive systems.

The conjugation of porphyrins to metal fragments is a strategy for making new compounds that are expected to combine the phototoxicity and the tumour-localization properties of the porphyrin chromophore with the cytotoxicity of the metal fragment for additive antitumour effect. We report here the preparation of new classes of porphyrin-ruthenium conjugates with potential bio-medical applications. Ruthenium was chosen because several Ru compounds have shown promising anticancer activity. The conjugation with the porphyrin moiety was accomplished either through peripheral pyridyl rings (e.g.meso-4'-tetrapyridylporphyrin, 4'TPyP) or through bpy units (e.g.meso-(p-bpy-phenyl)porphyrins, bpy(n)-PPs, n = 1-4). The number of Ru fragments attached to the porphyrins ranges from 1 to 4 and the total charge of the conjugates from -4 to +8. Different types of peripheral fragments, both Ru(III) and Ru(II), have been used: in some cases they are structurally similar to established anticancer compounds. Examples are [Na](4)[4'TPyP{trans-RuCl(4)(dmso-S)}(4)] (2), that bears four NAMI-type Ru(III) fragments, or [4'TPyP{Ru([9]aneS3)(en)}(4)][CF(3)SO(3)](8) (3) and [bpy(4)-PP{Ru([9]aneS3)(dmso-S)}(4)][CF(3)SO(3)](8) (9) (en = ethane-1,2-diamine, [9]aneS3 = 1,4,7-trithiacyclononane) that have four half-sandwich Ru(II) compounds. The Ru fragments may either contain one or more labile ligands, such as in 2 or in 9, or be coordinatively saturated and substitutionally inert, such as in 3 or in [bpy(4)-PP{Ru([12]aneS4)}(4)][CF(3)SO(3)](8) (11) ([12]aneS4 = 1,4,7,10-tetrathiacyclododecane). Most of the ruthenium-porphyrin conjugates described in this work are soluble--at least moderately--in aqueous solution and are thus suitable for biological investigations, in particular for cytotoxicity and photo-cytotoxicity tests.

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.

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 supra- molecular 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 connec- tions 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 laborato- ries. The photophysical properties of some ruthenium-mediated assemblies of porphyrins prepared by our group are also described.

Porphyrins play a major role as active chromophores in artificial systems mimicking the natural photoinduced processes. The formation of coordination bonds between peripheral donor sites on the porphyrins and external metal fragments has proven to be an efficient alternative to covalent synthesis for the construction of multiporphyrin assemblies, whose complexity and beauty gradually approach those of the multichromophore systems found in nature. In a modular approach, relatively simple metal-mediated porphyrin adducts, owing to their thermodynamic and kinetic stability, can be exploited as building blocks in the construction of higher order architectures. Thus, multichromophore systems become accessible on demand, with a limited synthetic effort. The collection of solid-state structures reported here demonstrates that the flexibility of the porphyrins and the metal junctions, combined with the conformational freedom of the coordination bonds, may lead to assemblies with hardly predictable architectures. Examples in which X-ray structural determination was essential for establishing the real composition and geometry of the multiporphyrin assemblies are highlighted.

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.