Cyclometalated luminescent platinum(II) complexes of dissymmetrical 2,2':4',2''-terpyridine and its self-assembled dimer presenting Pt-Ag dative bonds.

A series of cyclometalated (N^C^N) Pt(II)-platinum complexes featuring a terpyridine ligand with a non-coordinating nitrogen atom and a Pt-C bond was synthesized. In the presence of Ag+, the bis(isonitrile)Pt(II) complex formed a remarkable self-assembled helicoidal dimer stabilized by coordination of Ag(I) and metallophilic Pt-Ag interactions. Its assembly was observed in the solid state and maintained in solution. All complexes show strong luminescence and multiple emitting states, which could be rationalized based on solid state X-ray structures and coordinating environment.

Dual switchable molecular tweezers incorporating anisotropic MnIII-salphen complexes.

An alternative strategy for the synthesis of terpyridine based switchable molecular tweezers has been developed to incorporate anisotropic Mn(iii)-salphen complexes. The free ligand was synthesized using a building block strategy based on Sonogashira coupling reactions and was then selectively metalated with manganese in a last step. The conformation of the tweezers was switched from an open 'W' shaped form to a closed 'U' form by Zn(ii) coordination to the terpyridine unit bringing the two Mn-salphen moieties in close spatial proximity as confirmed by X-ray crystallography. An alternate switching mechanism was observed by the intercalation of a bridging cyanide ligand between the two Mn-salphen moieties that resulted in the closing of the tweezers. These dual stimuli are attractive for achieving multiple controls of the mechanical motion of the tweezers. A crystallographic structure of unexpected partially oxidized closed tweezers was also obtained. One of the two Mn-salphen moieties underwent a ligand-centered oxidation of an imino to an amido group allowing an intramolecular Mn-Oamide-Mn linkage. The magnetic properties of the manganese(iii) dimers were investigated to evaluate the magnetic exchange interaction and analyze the single molecule magnet behavior.

Isolation and Structural Characterization of Eightfold Protonated Octacyanometalates [M(CNH)8 ]4+ (M=MoIV , WIV ) from Superacids.

Octacyanometalates K4 [Mo(CN)8 ] and K4 [W(CN)8 ] are completely protonated in superacidic mixtures of anhydrous hydrogen fluoride and antimony pentafluoride. The resulting hydrogen isocyanide complexes [Mo(CNH)8 ]4+ [SbF6 ]- 4 and [W(CNH)8 ]4+ [SbF6 ]- 4 are the first examples of eight-coordinate homoleptic metal complexes containing hydrogen isocyanide (CNH) ligands. The complexes were crystallographically characterized, revealing hydrogen-bonded networks with short N⋅⋅⋅H⋅⋅⋅F contacts. Low-temperature NMR measurements in HF confirmed rapid proton exchange even at -40 °C. Upon protonation, ν(C≡N) increases of about 50 cm-1 which is in agreement with DFT calculations.

Photoinduced Mo-CN Bond Breakage in Octacyanomolybdate Leading to Spin Triplet Trapping.

The photoinduced properties of the octacoordinated complex K4 MoIV (CN)8 ⋅2 H2 O were studied by theoretical calculations, crystallography, and optical and magnetic measurements. The crystal structure recorded at 10 K after blue light irradiation reveals an heptacoordinated Mo(CN)7 species originating from the light-induced cleavage of one Mo-CN bond, concomitant with the photoinduced formation of a paramagnetic signal. When this complex is heated to 70 K, it returns to its original diamagnetic ground state, demonstrating full reversibility. The photomagnetic properties show a partial conversion into a triplet state possessing significant magnetic anisotropy, which is in agreement with theoretical studies. Inspired by these results, we isolated the new compound [K(crypt-222)]3 [MoIV (CN)7 ]⋅3 CH3 CN using a photochemical pathway, confirming that photodissociation leads to a stable heptacyanomolybdate(IV) species in solution.

Photocatalytic Hydrogen Evolution Driven by a Heteroleptic Ruthenium(II) Bis(terpyridine) Complex.

Since the initial report by Lehn et al. in 1979, ruthenium tris(bipyridine) ([Ru(bpy)3]2+) and its numerous derivatives were applied as photosensitizers (PSs) in a large panel of photocatalytic conditions while the bis(terpyridine) analogues were disregarded because of their low quantum yields and short excited-state lifetimes. In this study, we prepared a new terpyridine ligand, 4'-(4-bromophenyl)-4,4‴:4″,4‴'-dipyridinyl- 2,2':6',2″-terpyridine (Bipytpy) and used it to prepare the heteroleptic complex [Ru(Tolyltpy)(Bipytpy)](PF6)2 (1; Tolyltpy = 4'-tolyl-2,2':6',2'-terpyridine). Complex 1 exhibits enhanced photophysical properties with a higher quantum yield (7.4 × 10-4) and a longer excited-state lifetime (3.8 ns) compared to those of [Ru(Tolyltpy)2](PF6)2 (3 × 10-5 and 0.74 ns, respectively). These enhanced photophysical characteristics and the potential for PS-catalyst interaction through the peripheral pyridines led us to apply the complex for visible-light-driven hydrogen evolution. The photocatalytic system based on 1 as the PS, triethanolamine as a sacrificial donor, and cobaloxime as a catalyst exhibits sustained activity over more than 10 days under blue-light irradiation (light-emitting diode centered at 450 nm). A maximum turnover number of 764 was obtained after 12 days.

Facile One-Pot Synthesis of Ruthenium(II) Quaterpyridine-Based Photosensitizers for Photocatalyzed Hydrogen Production.

We present here the efficient microwave-assisted synthesis and photophysical study of a family of ruthenium(II) complexes of the general formula [Ru(bpy)x(qpy)3-x]2+ (where bpy = 2,2'-bipyridine, qpy = 4,4':2',2″:4″,4‴-quaterpyridine, and x = 0, 1, 2 giving compounds 1 = [Ru(bpy)2(qpy)1]2+, 2 = [Ru(bpy)1(qpy)2]2+, and 3 = [Ru(qpy)3]2+). Compared to the standard reference, [Ru(bpy)3]2+ (τ = 870 ns, Φ = 9.5%), the complexes display longer-lived excited state lifetimes at room temperature (τ: 1 = 1440 ns, 2 = 1640 ns, 3 = 1780 ns) and improved quantum yields (Φ: 1 = 14%, 2 = 19%, 3 = 23%). Theoretical calculations were performed to support the interpretation of these photophysical properties. These complexes are excellent photosensitizers as they absorb light throughout the visible spectrum, have excellent excited state lifetimes at room temperature, and have high quantum yields. In combination with a cobalt dimethylglyoxime catalyst, they exhibit remarkable hydrogen evolution with blue light, and they are far more efficient than the reference in the field, [Ru(bpy)3]2+.

Six States Switching of Redox-Active Molecular Tweezers by Three Orthogonal Stimuli.

A six level molecular switch based on terpyridine(Ni-salphen)2 tweezers and addressable by three orthogonal stimuli (metal coordination, redox reaction, and guest binding) is reported. By a metal coordination stimulus, the tweezers can be mechanically switched from an open "W"-shaped conformation to a closed "U"-shaped form. Theses two states can each be reversibly oxidized by the redox stimulus and bind to a pyrazine guest resulting in four additional states. All six states are stable and accessible by the right combination of stimuli and were studied by NMR, XRD, EPR spectroscopy, and DFT calculations. The combination of the supramolecular concepts of mechanical motion and guest binding with the redox noninnocent and valence tautomerism properties of Ni-salphen complexes added two new dimensions to a mechanical switch.

Mechanical switching of magnetic interaction by tweezers-type complex.

A control of the interaction between two spin centers was achieved by using a mechanical motion in a terpy(Cu-salphen)2 complex. Upon coordination a conformation change and switching from a paramagnetic to an antiferromagnetically coupled system was observed by EPR and SQUID measurements.

Terpy(Pt-salphen)2 switchable luminescent molecular tweezers.

The design and synthesis of switchable molecular tweezers based on a luminescent terpy(Pt-salphen)2 (1; terpy=terpyridine) complex is reported. Upon metal coordination, the tweezers can switch from an open "W"-shaped conformation to a closed "U"-shaped form that is adapted for selective recognition of cations. Closing of the tweezers by metal coordination (M=Zn(2+), Cu(2+), Pb(2+), Fe(2+), Hg(2+)) was monitored by (1)H NMR and/or UV/Vis titrations. During the titration, exclusive formation of the 1:1 complex [M(1)] was observed, without appearance of an intermediate 1:2 complex [M(1)2]. The crystallographic structure of the 1:1 complex was obtained with Pb(2+) and showed a distorted helical structure. Selective intercalation of Hg(2+) cations by the closed "U" form was observed. The tweezers were reopened by selective metal decoordination of the terpyridine ligand by using tris(2-aminoethyl)amine (tren) as a competitive ligand without modification of the Pt-salphen complex. Detailed photophysical studies were performed on the open and closed tweezers. Structured emission was observed in the open form from the Pt-salphen moieties, with a high quantum yield and a long lifetime. The emission is slightly modified upon closing with 1 equivalent of Zn(2+) or Hg(2+), whereas a dramatic quenching was obtained upon intercalation of additional Hg(2+).

Cyclodextrin polyrotaxanes as a highly modular platform for the development of imaging agents.

Selectively functionalized cyclodextrins with a bodipy fluorescent tag or Gd(3+) complex were synthetized and threaded onto a polyammonium chain to form polyrotaxanes. This modular supramolecular assembly makes an ideal platform for bimodal (fluorescent and MRI) imaging applications.

Molecular dyads of ruthenium(II)- or osmium(II)-bis(terpyridine) chromophores and expanded pyridinium acceptors: equilibration between MLCT and charge-separated excited states.

The synthesis, characterization, redox behavior, and photophysical properties (both at room temperature in fluid solution and at 77 K in rigid matrix) of a series of four new molecular dyads (2-5) containing Ru(II)- or Os(II)-bis(terpyridine) subunits as chromophores and various expanded pyridinium subunits as electron acceptors are reported, along with the reference properties of a formerly reported dyad, 1. The molecular dyads 2-4 have been designed to have their (potentially emissive) triplet metal-to-ligand charge-transfer (MLCT) and charge-separated (CS) states close in energy, so that excited-state equilibration between these levels can take place. Such a situation is not shared by limit cases 1 and 5. For dyad 1, forward photoinduced electron transfer (time constant, 7 ps) and subsequent charge recombination (time constant, 45 ps) are evidenced, while for dyad 5, photoinduced electron transfer is thermodynamically forbidden so that MLCT decays are the only active deactivation processes. As regards 2-4, CS states are formed from MLCT states with time constants of a few dozens of picoseconds. However, for these latter species, such experimental time constants are not due to photoinduced charge separation but are related to the excited-state equilibration times. Comparative analysis of time constants for charge recombination from the CS states based on proper thermodynamic and kinetic models highlighted that, in spite of their apparently affiliated structures, dyads 1-4 do not constitute a homologous series of compounds as far as intercomponent electron transfer processes are concerned.

A new versatile class of hetero-tetra-metallic assemblies: highlighting single-molecule magnet behaviour.

Hetero-tetra-metallic species based on hexanuclear assemblies [((valen)M1)Ln(OH2)2(µM2(CN)8)]2(2-) (Ln = Gd(III), Tb(III); M1 = Cu(II), Ni(II) and M2 = Mo(IV), W(IV)) and co-crystallized mononuclear complexes [M3(tpy)2](2+) (M3 = Ni(II), Ru(II), Os(II)) were identified, fully characterized, and shown to constitute a new class of single-molecule magnets.

Single-step versus stepwise two-electron reduction of polyarylpyridiniums: insights from the steric switching of redox potential compression.

Contrary to 4,4'-dipyridinium (i.e., archetypal methyl viologen), which is reduced by two single-electron transfers (stepwise reduction), the 4,1'-dipyridinium isomer (so-called "head-to-tail" isomer) undergoes two electron transfers at apparently the same potential (single-step reduction). A combined theoretical and experimental study has been undertaken to establish that the latter electrochemical behavior, also observed for other polyarylpyridinium electrophores, is due to potential compression originating in a large structural rearrangement. Three series of branched expanded pyridiniums (EPs) were prepared: N-aryl-2,4,6-triphenylpyridiniums (Ar-TP), N-aryl-2,3,4,5,6-pentaphenylpyridiniums (Ar-XP), and N-aryl-3,5-dimethyl-2,4,6-triphenylpyridinium (Ar-DMTP). The intramolecular steric strain was tuned via N-pyridinio aryl group (Ar) phenyl (Ph), 4-pyridyl (Py), and 4-pyridylium (qPy) and their bulky 3,5-dimethyl counterparts, xylyl (Xy), lutidyl (Lu), and lutidylium (qLu), respectively. Ferrocenyl subunits as internal redox references were covalently appended to representative electrophores in order to count the electrons involved in EP-centered reduction processes. Depending on the steric constraint around the N-pyridinio site, the two-electron reduction is single-step (Ar = Ph, Py, qPy) or stepwise (Ar = Xy, Lu, qLu). This steric switching of the potential compression is accurately accounted for by ab initio modeling (Density Functional Theory, DFT) that proposes a mechanism for pyramidalization of the N(pyridinio) atom coupled with reduction. When the hybridization change of this atom is hindered (Ar = Xy, Lu, qLu), the first reduction is a one-electron process. Theory also reveals that the single-step two-electron reduction involves couples of redox isomers (electromers) displaying both the axial geometry of native EPs and the pyramidalized geometry of doubly reduced EPs. This picture is confirmed by a combined UV-vis-NIR spectroelectrochemical and time-dependent DFT study: comparison of in situ spectroelectrochemical data with the calculated electronic transitions makes it possible to both evidence the distortion and identify the predicted electromers, which play decisive roles in the electron-transfer mechanism. Last, this mechanism is further supported by in-depth analysis of the electronic structures of electrophores in their various reduction states (including electromeric forms).

Heterotrimetallic 3d-4d-4f decanuclear metal-capped square showing single-molecule magnet behavior.

The synthesis, the structural characterization and the magnetic properties of a heterotrimetallic decanuclear square, [(Mo(CN)(8))(2)(CuLTb)(4))](4+), noted MoCuTb, are described. This supramolecular compound, viewed as the first example of a metal-capped square, behaves as a single molecule magnet with a magnetic anisotropic barrier energy of U(eff) = 13.40 cm(-1) (19.25 K).

Photomagnetism in clusters and extended molecule-based magnets.

Photomagnetism in molecular systems is a new development in molecular magnetism. It traces back to 1982 and 1984 when a transient effect and then the light-induced excited-spin-state-trapping effect was discovered in spin-crossover complexes. The present contribution gives a definition of the phenomenon, a process that changes the magnetism of a (molecular) system after absorption of a photon. It is limited to the discussion of photomagnetism based on metal-metal electron transfer in clusters and extended molecule-based magnets. The paper is organized around the main pairs of spin bearers, which allowed us to evidence and to study the phenomenon: Cu-Mo, Co-Fe, and Co-W. For each metallic pair, we report and discuss the conditions of appearance of the effect and its characteristics, both in extended structures and in molecular units: structure, spectroscopy, magnetism, thermodynamics and kinetics, and applications. We conclude with some brief prospects. The field is in rapid expansion. We are convinced that the interaction of photons with magnetized matter, to provide original magnetic properties, will meet more and more interest in the future.

Photoswitchable heterotrimetallic chain based on octacyanomolybdate, copper, and nickel: synthesis, characterization, and photomagnetic properties.

Taking into account the general interest in photoswitchable nanomagnets, we present an original synthetic route to well-defined photoswitchable heterotrimetallic complexes. Our strategy demonstrates the feasibility of using a "molybdenum-copper" switchable complex with terminal cyanide ligands as relevant synthons for the rational design and synthesis of heterotrimetallic photoswitchable molecular architectures. Starting from a pentanuclear compound based on an octacyanometalated precursor, [MoIV(CN)4(CNCuII(Me2en)2)4]4+, and a [NiII(cyclam)]2+ assembling building block, we succeeded in synthesizing a photoswitchable heterotrimetallic chain, denoted as {NiMo2Cu7}n. The compound has been characterized by single-crystal X-ray diffraction, UV-vis and IR spectroscopies, and powder magnetic and photomagnetic susceptibility measurements. Before irradiation, the compound behaves as a paramagnet with eight independent spin carriers (CuII d9, S = 1/2, and NiII d8, S = 1). After irradiation, a long-lived metastable state persists until relatively high temperature (120 K) and is thermally reversible.

A kinetic model for photoswitching of magnetism in the high spin molecule [Mo(IV)(CN)2(CN-Cu(II)(tren))6](ClO4)8.

The heptanuclear complex [Mo(IV)(CN)2(CN-CuL)6]8+ switches from a paramagnetic dark state corresponding to six spin-1/2 Cu(II) ions to a predominantly high spin S = 3 state, on prolonged irradiation with 406 nm laser radiation at low temperature. The system returns to a paramagnetic state on warming to room temperature. The temperature dependence of the chiMT vs. T curve depends upon duration of irradiation. An earlier microscopic model showed that the excitation cross sections in different spin manifolds are similar in magnitude and that photomagnetism is not due to preferential excitation to the S = 3 state. In this paper, we attribute photomagnetism to a long lived S = 3 charge transfer excited state for which there appears to be sufficient experimental evidence. Based on this postulate, we model the photomagnetism by employing a kinetic model which includes internal conversions and intersystem crossings. The key feature of the model is the assumption of the existence of two kinds of S = 3 states: one of which has no direct pathway for internal conversion and the other characterized by slow kinetics for internal conversion to the low-energy states. The trapped S = 3 state can decay via a thermally activated barrier to the other S = 3 state. The experimental chiMT vs. T for two different irradiation times are fitted using Arrhenius dependence of the rate constants in the model.

Experimental estimates of dephasing time in molecular magnets.

Muon spin relaxation measurements in isotropic molecular magnets (MM) with a spin value S ranging from 7/2 to 27/2 are used to determine the magnitude and origin of dephasing time tau(phi) of molecular magnets. It is found that tau(phi) approximately 10 nsec with no S or ligand dependence. This indicates a nuclear origin for the stochastic field. Since tau(phi) is a property of the environment, we argue that it is a number common to similar types of MM. Therefore, tau(phi) is shorter than the Zener and tunneling times of anisotropic MM such as Fe(8) or Mn(4) for standard laboratory sweep rates. Our findings call for a stochastic Landau-Zener theory in this particular case.

Intramolecular spin alignment in photomagnetic molecular devices: a theoretical study.

Ground- and excited-state magnetic properties of recently characterized pi-conjugated photomagnetic organic molecules are analyzed by the means of density functional theory (DFT). The systems under investigation are made up of an anthracene (An) unit primarily acting as a photosensitizer (P), one or two iminonitroxyl (IN) or oxoverdazyl (OV) stable organic radical(s) as the dangling spin carrier(s) (SC), and intervening phenylene connector(s) (B). The magnetic behavior of these multicomponent systems, represented here by the Heisenberg-Dirac magnetic exchange coupling (J), as well as the EPR observables (g tensors and isotropic A values), are accurately modeled and rationalized by using our DFT approach. As the capability to quantitatively assess intramolecular exchange coupling J in the excited state makes it possible to undertake rational optimization of photomagnetic systems, DFT was subsequently used to model new compounds exhibiting different connection schemes for their functional components (P, B, SC). We show in the present work that it is worthwhile considering the triplet state of anthracene, that is, P when promoted in its lowest photoexcited state, as a full magnetic site in the same capacity as the remote SCs. This framework allows us to accurately account for the interplay between transient ((3)An) and persistent (IN, OV) spin carriers, which magnetically couple according to a sole polarization mechanism essentially supported by phenyl connector(s). From our theoretical investigations of photoinduced spin alignment, some general rules are proposed and validated. Relying on the analysis of spin-density maps, they allow us to predict the magnetic behavior of purely organic magnets in both the ground and the excited states. Finally, the notion of photomagnetic molecular devices (PMMDs) is derived and potential application towards molecular spintronics disclosed.