Modulating the Shape of Short Metal-Mediated Heteroleptic Tapes of Porphyrins.

In view of developing artificial light-responsive complex systems, the preparation of discrete and robust heteroleptic assemblies of different chromophores in precisely defined positions is of great value since they would allow to investigate directional processes unavailable in symmetrical architectures. Here we describe the preparation, through a modular stepwise approach, and characterization of four novel and robust metal-mediated heteroleptic 4+3 porphyrin tapes, labeled D4 -T4 -D4 , D3 -T4 -D3 , D4 -T3 -D4 , and D3 -T3 -D3 , where a central meso-tetrapyridylporphyrin (either 3'-TPyP=T3 or 4'-TPyP=T4 ) is connected to two equal cis-dipyridylporphyrins (either 3'cisDPyMP=D3 or 4'cisDPyMP=D4 ) through four {t,c,c-RuCl2 (CO)2 } fragments. Whereas D4 -T4 -D4 is flat, the tapes containing at least one 3'PyP, i. e. D3 -T4 -D3 , D4 -T3 -D4 , and D3 -T3 -D3 , have unprecedented - and well defined - 3D geometries, and each exists in solution as a pair of stereoisomers in slow conformational equilibrium. The X-ray molecular structures of two such conformers, the C-shaped (D3 -T4 -D3 )C and the z-shaped (D4 -T3 -D4 )z , were determined and are fully consistent with the solution NMR findings.

Giant Shape-Persistent Tetrahedral Porphyrin System: Light-Induced Charge Separation.

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.

A Flexible Synthetic Strategy for the Preparation of Heteroleptic Metallacycles of Porphyrins.

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.

Photoinduced Electron vs. Concerted Proton Electron Transfer Pathways in SnIV (l-Tryptophanato)2 Porphyrin Conjugates.

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 CH2 Cl2 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.

Orthogonal Coordination Chemistry of PTA toward Ru(II) and Zn(II) (PTA = 1,3,5-Triaza-7-phosphaadamantane) for the Construction of 1D and 2D Metal-Mediated Porphyrin Networks.

This work demonstrates that PTA (1,3,5-triaza-7-phosphaadamantane) behaves as an orthogonal ligand between Ru(II) and Zn(II), since it selectively binds through the P atom to ruthenium and through one or more of the N atoms to zinc. This property of PTA was exploited for preparing the two monomeric porphyrin adducts with axially bound PTA, [Ru(TPP)(PTA-κP)2] (1, TPP = meso-tetraphenylporphyrin) and [Zn(TPP)(PTA-κN)] (3). Next, we prepared a number of heterobimetallic Ru/Zn porphyrin polymeric networks-and two discrete molecular systems-mediated by P,N-bridging PTA in which either both metals reside inside a porphyrin core, or one metal belongs to a porphyrin, either Ru(TPP) or Zn(TPP), and the other to a complex or salt of the complementary metal (i.e., cis,cis,trans-[RuCl2(CO)2(PTA-κP)2] (5), trans-[RuCl2(PTA-κP)4] (7), Zn(CH3COO)2, and ZnCl2). The molecular compounds 1, 3, trans-[{RuCl2(PTA-κ2P,N)4}{Zn(TPP)}4] (8), and [{Ru(TPP)(PTA-κP)(PTA-κ2P,N)}{ZnCl2(OH2)}] (11), as well as the polymeric species [{Ru(TPP)(PTA-κ2P,N)2}{Zn(TPP)}]∞ (4), cis,cis,trans-[{RuCl2(CO)2(PTA-κ2P,N)2}{Zn(TPP)}]∞ (6), trans-[{RuCl2(PTA-κ2P,N)4}{Zn(TPP)}2]∞ (9), and [{Ru(TPP)(PTA-κ3P,2N)2}{Zn9(CH3COO)16(CH3OH)2(OH)2}]∞ (10), were structurally characterized by single crystal X-ray diffraction. Compounds 4, 6, 9, and 10 are the first examples of solid-state porphyrin networks mediated by PTA. In 4, 6, 8, 9, and 11 the bridging PTA has the κ2P,N binding mode, whereas in the 2D polymeric layers of 10 it has the triple-bridging mode κ3P,2N. The large number of compounds with the six-coordinate Zn(TPP) (the three polymeric networks of 4, 6 and 9, out of five compounds) strongly suggests that the stereoelectronic features of PTA are particularly well-suited for this relatively rare type of coordination. Interestingly, the similar 1D polymeric chains 4 and 6 have different shapes (zigzag in 4 vs "Greek frame" in 6) because the {trans-Ru(PTA-κ2P,N)2} fragment bridges two Zn(TPP) units with anti geometry in 4 and with syn geometry in 6. Orthogonal "Greek frame" 1D chains make the polymeric network of 9. Having firmly established the binding preferences of PTA toward Ru(II) and Zn(II), we are confident that in the future a variety of Ru/Zn solid-state networks can be produced by changing the nature of the partners. In particular, there are several inert Ru(II) compounds that feature two or more P-bonded PTA ligands that might be exploited as connectors of well-defined geometry for the rational design of solid-state networks with Zn-porphyrins (or other Zn compounds).

Photoinduced Energy- and Electron-Transfer Processes in a Side-to-Face RuII -Porphyrin/Perylene-bisimide Array.

A side-to-face array DPy-gPBI[Ru(4-tBuTPP)(CO)]2 , based on a "green" perylene bisimide chromophore sandwiched between two RuII -porphyrins, has been prepared by self-assembly. Its photophysical properties have been characterized in detail by a combination of steady-state and time-resolved techniques upon selective excitation of the two different components. Different photoinduced processes are observed as a function of the excitation wavelength. Electron transfer quenching is attained upon "red light" excitation of the perylene unit, whilst an energy transfer pathway is followed upon "green light" excitation of the metallo-porphyrin moiety. Regardless of the excitation wavelength efficient population of the triplet excited state of the perylene chromophore is achieved. The photophysical results are discussed within the framework of classical electron transfer theory and compared with those of a previously reported system.

Rare Example of Stereoisomeric 2 + 2 Metallacycles of Porphyrins Featuring Chiral-at-Metal Octahedral Ruthenium Corners.

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.

Sn(IV) Multiporphyrin Arrays as Tunable Photoactive Systems.

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.

Formation of a long-lived radical pair in a Sn(iv) porphyrin-di(l-tyrosinato) conjugate driven by proton-coupled electron-transfer.

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.

Self-Assembled Ruthenium(II)Porphyrin-Aluminium(III)Porphyrin-Fullerene Triad for Long-Lived Photoinduced Charge Separation.

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 time-resolved emission and absorption spectroscopy in the pico- and nanosecond time domains. Following excitation of the Al-porphyrin, 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 charge-separated 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.

(15)N NMR spectroscopy unambiguously establishes the coordination mode of the diimine linker 2-(2'-pyridyl)pyrimidine-4-carboxylic acid (cppH) in Ru(ii) complexes.

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.

Zinc porphyrin-Re(I) bipyridyl-fullerene triad: synthesis, characterization, and kinetics of the stepwise electron-transfer processes initiated by visible excitation.

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.

Photoinduced hydrogen evolution by a pentapyridine cobalt complex: elucidating some mechanistic aspects.

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)3(2+) sensitizer and ascorbic acid as a 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 = 187, TOF = 8.1 min(-1)). The photochemical mechanism, as revealed by flash photolysis, involves reaction of the excited sensitizer with ascorbic acid to yield Ru(bpy)3(+) as a primary photo-generated reductant, capable of electron transfer to 1 with a remarkable rate (bimolecular rate constant k = 5.7 (±0.7) × 10(9) M(-1) s(-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 the sensitizer and the 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) × 10(7) M(-1) s(-1)) thus competing with electron transfer to the catalyst.

Anion transport across phospholipid membranes mediated by a diphosphine-Pd(II) complex.

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.

Efficient photocatalytic hydrogen generation from water by a cationic cobalt(II) porphyrin.

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.

Porphyrin-cobaloxime dyads for photoinduced hydrogen production: investigation of the primary photochemical process.

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.

Photocatalytic hydrogen evolution with a self-assembling reductant-sensitizer-catalyst system.

A noble-metal-free system for photochemical hydrogen production is described, based on ascorbic acid as sacrificial donor, aluminium pyridyl porphyrin as photosensitizer, and cobaloxime as catalyst. Although the aluminium porphyrin platform has docking sites for both the sacrificial donor and the catalyst, the resulting associated species are essentially inactive because of fast unimolecular reversible electron-transfer quenching. Rather, the photochemically active species is the fraction of sensitizer present, in the aqueous/organic solvent used for hydrogen evolution, as free species. As shown by nanosecond laser flash photolysis experiments, its long-lived triplet state reacts bimolecularly with the ascorbate donor, and the reduced sensitizer thus formed, subsequently reacts with the cobaloxime catalyst, thereby triggering the hydrogen evolution process. The performance is good, particularly in terms of turnover frequencies (TOF=10.8 or 3.6 min(-1), relative to the sensitizer or the catalyst, respectively) and the quantum yield (Φ=4.6%, that is, 9.2% of maximum possible value). At high sacrificial donor concentration, the maximum turnover number (TON=352 or 117, relative to the sensitizer or the catalyst, respectively) is eventually limited by hydrogenation of both sensitizer (chlorin formation) and catalyst.

New meso-substituted trans-A(2)B(2) di(4-pyridyl)porphyrins as building blocks for metal-mediated self-assembling of 4 + 4 Re(I)-porphyrin metallacycles.

The reaction between 5-(4-pyridyl)dipyrrylmethane and aromatic aldehydes affords meso-arylsubstituted trans-A2B2 di(4-pyridyl)porphyrins which are key building blocks in the metal-mediated self-assembling of supramolecular structures. A careful optimization of the reaction conditions allowed us to obtain 5,15-diphenyl-10,20-di(4-pyridyl)porphyrin (P1), and two analogues bearing on the meso-phenyl substituents two dipropyl- (P4) or dihexyl-alkyl chains (P5), with yields ranging from 53 to 63%. Porphyrin P1 reacts with Re(CO5)Br to give the expected 4 + 4 Re(I)-porphyrin metallacycle which has been fully characterized by means of infrared, NMR and UV-Vis (absorption and emission) spectroscopies and by guest inclusion studies. Unexpectedly the addition of alkyl chains to the porphyrin fragment, which increase the solubility of the porphyrin in organic solvents, has the opposite effect on the adduct with Re(I). Indeed, the reaction between Re(CO5)Br and porphyrins P4,5 gives very insoluble materials, hampering their complete characterization.

Improving the efficiency of the photoinduced charge-separation process in a rhenium(I)-zinc porphyrin dyad by simple chemical functionalization.

We demonstrate here that, whereas the rhenium(I)-zinc porphyrin dyad fac-[Re(CO)3(bpy)(Zn·4'MPyP)](CF3SO3) [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 (ΔG = -0.05 eV), the introduction of electron-withdrawing ethyl ester groups on the bpy ligand significantly improves the thermodynamics of the process (ΔG = -0.42 eV). As a consequence, in the new dyad fac-[Re(CO)3(4,4'-DEC-bpy)(Zn·4'MPyP)](CF3SO3) (4; 4,4'-DEC-bpy = 4,4'-diethoxycarbonyl-2,2'-bipyridine), an efficient 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 effect on the photophysics of the system. For dyad 4, the time constants for the charge-separation and charge-recombination processes in solvents of different polarity (PrCN, DCM, and toluene) were measured by an ultrafast time-resolved absorption technique (λ(exc) = 560 nm).

Metal-organic transmembrane nanopores.

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.