Mauro Ciano

Spectroscopic and photophysical properties of the Europium (III) Cryptate [Eu3+ ⊂ 2.2.1]

Abstract The spectroscope and photophysical properties of [Eu 3+ ⊂ 2.2.1] have been investigated. High-resolution emission spectra in aqueous solution are compatible with the presence of only one Eu-containing species with C 2v symmetry. The emission quantum yields indicate that the conversion of the charge-transfer levels to the 5 D 0 emitting state is relatively inefficient. Luminescence decay measurements show that encapsulation of Eu 3+ in the cryptand cage does not completely shield the metal ion from interaction with the solvent, since three water molecules are still coordinated to Eu 3+ through the cryptand holes.

Polypyridine transition metal complexes as light emission sensitizers in the electrochemical reduction of the persulfate ion

Abstract Electrochemical reduction of polypyridine-type complexes of Cr(III), Ru(II), and Os(II) in the presence of S2O82− ions is accompanied by light emission. The electrochemiluminescence (ecl) spectrum is identical to the photoluminescence spectrum of the complex which undergoes reduction. The net reaction in the electrochemical cell is the reduction of S2O82− to SO2−4 with the polypyridine complex acting as both an electron transfer mediator and as a light emission sensitizer. The experiments were carried out in aqueous solutions for the Cr(III) complexes and in dimethylformamide or acetonitrile for the Ru(II) and Os(II) complexes. The dependence of the ecl emission on S2O2−8 concentration was examined. In preliminary experiments no light emission was obtained with Rh(III) and Ir(III) polypyridine complexes. The interconversion between chemical (and/or electrical) energy and light mediated by suitable sensitizers is briefly discussed.

Absorption and emission properties of a europium(II) cryptate in aqueous solution

Abstract The absorption spectrum of the complex between Eu 2+ and the [2.2.2] cryptand shows narrow, weak transitions within 4f 7 superimposed on broad, strong transitions from 4f 7 to 4f 6 5d. Emission from 4f 6 5d to 4f 7 can be observed at 77 K λ max = 420 nm, τ = 0.55 μs) and at 293 K (λ max = 460 nm, τ = 3 ns) in aqueous solution.

Hexanuclear polypyridine complexes containing different metals, bridging ligands and/or terminal ligands. Absorption spectra, electrochemical oxidation, luminescence properties and intercomponent energy transfer

Abstract Four novel hexanuclear complexes of general formula [(L) 2 M(μ-BL)] 2 M(μ-BL)M[(μ-BL)M(L) 2 ] 2 12+ , where the metal ions M are Ru 2+ and/or Os 2+ , the bridging ligands BL are 2,3-dpp and/or 2,5-dpp, and the terminal ligands L are bpy and/or biq, have been investigated (dpp  bis(2-pyridyl)pyrazine; bpy  2,2′-bipyridine; biq 2,2′-biquinoline). These polymetallic complexes can be considered as supramolecular species made of six distinct metal-containing units. They display very intense ligand centered absorption bands in the UV region (ϵ max in the order of 2.5×10 5 M −1 cm −1 ) and broad and intense bands in the visible region (ϵ max in the order of 5×10 4 M −1 cm −1 ). On electrochemical oxidation, the metal centers are oxidized at the same or different potentials depending on the nature of the metal ions (Ru 2+ or Os 2+ ) and on their positions (inner or outer) in the supramolecular structure. For all the novel compounds, luminescence can be observed in the red or near-IR spectral region. The luminescence properties, which are characteristic of specific metal-containing units, show that exoergonic electronic energy transfer between adjacent units is 100% efficient, whereas it is much lower when higher energy units are interposed. Various energy migration patterns can be obtained by placing different units in suitable sites of the supramolecular array.

A decanuclear ruthenium(II)–polypyridine complex: synthesis, absorption spectrum, luminescence and electrochemical behaviour

A novel oligonuclear Ru(II)–polypyridine complex, Ru{(µ-2,3-dpp)Ru[(µ-2,3-dpp)Ru(bpy)2]2}3(PF6)201, where dpp = bis(2-pyridyl)pyrazine and bpy = 2,2′ bipyridine, has been prepared from the reaction of a Ru(2,3-dpp)32+ core 2 with three Ru[(µ-2,3-dpp)Ru(bpy)2]2Cl24+ units; 1 shows a very intense absorption band at 541 nm (Iµ= 1.25 × 105 dm3 mol–1 cm–1), room temperature luminescence (λmax= 809 nm, τ= 55 ns, Φ= 10–3), and independent one-electron oxidation of the six peripheral Ru2+ ions at +1.43 V vs. saturated calomel electrode (SCE).

Absorption spectra, luminescence properties and electrochemical behaviour of ruthenium(II) complexes containing bis(pyridyl)triazole ligands

Abstract Four novel complexes of general formula Ru(L) 2 (T) 2+ ( 1 , Lbpy, Ttrz; 2 , Lbpy, Ttrz-Q; 3 , Lbiq, Ttrz; 4 , Lbiq, Ttrz-Q; bpy=2,2′-bipyridine, biq=2,2′-biquinoline; trz=4-amino-3,5-di-2-pyridyl-4 H -1,2,4-triazole; trz-Q=4(4′- N,N -dimethylamino-phenyl)imino-3,5-di-2-pyridyl-4 H -1,2,4-triazole) have been synthesized, and their absorption spectra, luminescence properties (both in fluid solution at room temperature and in rigid matrix at 77 K), and electrochemical behaviour have been investigated. The absorption spectra of the complexes show intense absorption bands in the UV region (ϵ in the range 10 4 –10 5 M −1 cm −1 ) that are assigned to ligand- centred transitions and moderately intense absorption bands in the visible (ϵ in the range 10 3 –10 4 M −1 cm −1 ) that are attributed to metal-to-ligand charge transfer (MLCT) transitions. The absorption bands in the visible of the biq-containing complexes are at lower energies than those of the bpy-containing ones. The four complexes emit from a MLCT excited state both at 77 K and at room temperature, with lifetimes in the range 10 −5 – 10 −6 s and 10 −7 –10 −8 s, respectively. The luminescence lifetimes and quantum yields are practically the same for 1 and 3 and for 2 and 4 , respectively, indicating that the presence of the N,N -dimethylamino unit on the triazole ligand does not affect the radiative and radiationless rate constants of the chromophores and does not cause an electron-transfer quenching process. On electrochemical oxidation, 1 and 3 exhibit a reversible one-electron wave at +1.22 and +1.37 V versus SCE, respectively, that are assigned to metal-centred oxidations, while 2 and 4 undergo two successive one-electron oxidations at +1.30 and +1.56 V ( 2 ) and +1.30 and +1.71 V ( 4 ). By comparison with the redox behaviour of the free ligands, in both 2 and 4 the first process is attributed to oxidation of the N,N -dimethylamino moiety, and the second one to metal-centred oxidation. Two reversible reduction processes occur in all the complexes at about −1.15 and −1.40 V ( 1 and 2 ) and at about −0.60 and −0.85 V ( 3 and 4 ). Such processes are assigned as bpy- and biq-centred reductions, respectively. The positive shift of the metal-centred oxidation on passing from 1 and 3 to 2 and 4 is attributed to electronic ‘communication’ between the chromophoric metal unit and the electron-donor N,N -dimethylamino group across the triazole ligand and the conjugate NCHC 6 H 4 bridge, and to an electrostatic term.

Towards an Artificial Photosynthesis. Di-, Tri-, Tetra-, and Heptanuclear Luminescent and Redox-Reactive Metal Complexes

We have synthesized more than 30 mono-, di-, tri-, tetra-, and heptanuclear Ru(II) and/or Os(II) complexes containing 2,3-dpp and 2,5-dpp as bridging ligands and bpy and biq as peripheral ligands. These oligonuclear complexes are quite interesting because they accumulate many chromophoric and redox centers in the same “supermolecule” and exhibit luminescence from relatively long-lived excited states.

Spectroscopic and electrochemical properties of Rh(I) complexes containing diene (norbornadiene or cyclooctadiene) and diimine (bipyrimidine) type ligands

Abstract Some spectroscopic and electrochemical properties of the complexes Rh(nbd)(bpym)+, Rh(cod)(bpym)+, [Rh(nbd)]2(bpym)2+ and [Rh(cod)]2(bpym)2+ are reported; nbd = bicyclo[2,2,1]heptadiene (norbornadiene), cod = 1,5-cyclooctadiene, and bpym = 2,2′- bipyrimidine. In all cases the lowest excited states are of metal-to-ligand charge transfer (Rh → bpym) nature and first reduction involves the bpym ligand. Comparison of spectroscopic and electrochemical data for related mononuclear and dinuclear complexes indicates that the energy levels of the metal centered and ligand centered orbitals involved in the lowest energy metal-to-ligand transition and in the first oxidation and reduction processes are very similar for the two cases. However, while the mono- nuclear complexes are found to luminesce at 77 K, the dinuclear complexes axe not luminescent. The interaction between the two metal centers is regarded as a possible cause of this behavior.

A new hetero-tetrametallic complex of ruthenium and osmium: absorption spectrum, luminescence properties, and electrochemical behaviour

A new hetero-tetrametallic complex, Os[(µ-2,3-dpp)Ru(bpy)2]38+(1), where 2,3-dpp = bis(2-pyridyl)pyrazine and bpy = 2,2′-bipyridine, has been prepared from the reaction of Os(2,3-dpp)32+ with Ru(bpy)2Cl2: luminescence of (1) takes place from the central Os-containing core, which collects the energy absorbed by the peripheral Ru-containing chromophores (antenna effect).

Photophysical and Photochemical Properties of Europium Cryptates

Abstract Several diazapolyoxabicyclic ligands (‘cryptands’) [1, 2] are able to encapsulate metal ions to form coordination compounds (‘cryptates’ which have been the object extensive thermodynamic, kinetic, structural, electrochemical, and analytical investigations [2–12]. By contrast, only a few studies [12–14] have been reported on the electronic absorption and emission spectra of these compounds because both the cryptands and, most cases, the encapsulated metal ions are spectroscopically ‘mute’ species. The spectroscopic behavior of rare earth complexes is a topic of great interest, both theoretical [15–17] and applicative [18–20]. We have thus begun spectroscopic studies on cryptates containing europium or the rare earth ions. The spectroscopic and photophysical properties of type complexes between En2+ and the 2.2.1 and 2.2.2 cryptands are reported and compared to those of the Eu2+ aquo ion. Both complexes show broad relatively intense absorption bands in the near u.v. region due to 4f7 → 4F65d transitions. Some weak narrow bands due to transitions within the 4f7 configuration also appear in the 310–320 region. Both complexes exhibit a strong blue luminescence from 4f6. At 77 K the emission quantum yield is unity, and some vibrational structure can be observed in the broad emission band. Luminescence is also maintained in aqueous solution at room temperature with τ of the order of a few nanoseconds, and o: of the order of 10−3, in contrast with the behavior of the Eu2+aq ion which does not exhibit any luminescence emission under such conditions. The results obtained are discussed in the light of the interaction between Eu2+ and water molecules and of the size and symmetry of the cryptand cage. The spectroscopic and photophysical properties of the Eu3+ complex of the 2.2.1 cryptand are investigated in aqueous solution. The absorption spectrum of the complex, besides the f → f transition of the Eu3+ ion, shows two broad bands 298 and 248 nm (ϵ, 111 and 93, respectively) which are assigned as charge transfer transition from N and, respectively, O atoms of the ligands to Eu3+. High resolution emission spectra show that in aqueous solution there is only one Eu-containing species with C2v symmetry. The emission quantum yield is 3 × 10−2 upon excitation at 393 nm in the 5L6 metal centered band and 3 × 10−3 and 1 × 10−3 upon excitation in the charge transfer bands at 350 nm and 260 nm, showing that the conversion of the charge transfer levels to the 5Do emitting state is relatively inefficient. Luminescence decay measurements in H2O and D2O solutions and comparison with the data obtained for Eu3+aq show that encapsulation of Eu3+ in the cryptand cage does not shield the metal ion towards interaction with solvent since three water molecules are still coordinated to Eu3+ through the cryptand holes. The emission of [Eu+ C 2.2.1] is quenched by Fe(CN)4−6, Ru(CN4−6, Mo(CN)4−8 with Kq = 6.4 × 108, 1.9 × 108, 12 × 109 l mol−1 sec−1, respectively. The quenching takes place via charge-transfer interaction, as shown by the appearance of a new absorption band in the visible region for solutions containing the cryptate and the quencher. Spectrophotometric and electrochemical analysis show that 1:1 complexes are formed with stability constants of the order od 102 1 mol−1.