Roberto Ballardini

ANTENNA EFFECT IN LUMINESCENT LANTHANIDE CRYPTATES: A PHOTOPHYSICAL STUDY

Excited state emission and absorption decay measurements have been made on the cage‐type cryptate complexes [M bpy.bpy.bpy]n+, where Mn+= Na+, La3+, Eu3+, Gd3+ or Tb3+ and [bpy.bpy.bpy] is a tris‐bipyridine macrobicyclic cryptand. Excitation has been performed in the high intensity 1π‐π* cryptand band with maximum at about 300 nm. Experiments have been carried out in H2O or D2O solutions and at 300 and 77 K to evaluate the rate constants of radiative and nonradiative decay processes. For Mn+= Na+, La3+ and Gd3+ the lowest excited state of the cryptate is a 3ππ* level of the cryptand which decays in the microsecond time scale at room temperature in H2O solution and in the second‐millisecond time scale at 77 K in MeOH‐EtOH. For Mn+= Eu3+, the lowest excited state is the luminescent 5D0 Eu3+ level which in H2O solution is populated with 10% efficiency and decays to the ground state with rate constants 2.9 × 103 s_1 at room temperature and 1.2 × 103 s−′ at 77 K. The relatively low efficiency of 5D0 population upon 1ππ* excitation is attributed to the presence of a ligand‐to‐metal charge transfer level through which 1ππ* decays directly to the ground state. For Mn+= Tb3+ the lowest excited state is the luminescent 5D4 Tb3+ level. The process of 5D4 population upon 1ππ* excitation is ˜100% efficient, but at room temperature it is followed by a high‐efficiency, activated back energy transfer from the 5D4 Tb3+ level to the 3ππ* ligand level because of the relatively small energy gap between the two levels (1200 cm_1) and the intrinsically long lifetime of 5D4. At 77 K back energy transfer cannot take place and the 5D4 Tb3* level deactivates to the ground state with rate constant 5.9 × 102 s‐′ (H2O solution). The relevance of these results toward the optimization of Eu3+ and Tb3+ cryptates as luminescent probes is discussed.

RuII-Polypyridine Complexes Covalently Linked to Electron Acceptors as Wires for Light-Driven Pseudorotaxane-Type Molecular Machines

The photophysical properties of wire-type compounds based on a photosensitizing RuII complex linked to an electron-accepting bipyridinium or diazapyrenium group have been studied with the aim of designing light-driven molecular machines based on pseudorotaxanes (shown schematically). As an outcome, the photoinduced dethreading of a pseudorotaxane formed by a crown ether ring and one of the investigated wire-type compounds has been realized.

Micelle effect on the ‘write–lock–read–unlock–erase’ cycle of 4′-hydroxyflavylium ion

In aqueous solution the 4′-hydroxyflavylium ion (AH + ) can be interconverted into several different neutral forms by light excitation and/or pH changes. All the observed processes are fully reversible and accompanied by strong changes in absorption and emission spectra. This system exhibits properties required by optical memory devices with multiple storage in two different memory levels and non-destructive readout capacity through a write-lock-read-unlock-erase cycle. The effect of micelles on the pH and light induced interconversion of AH + and its neutral forms has been investigated. Negatively charged sodium dodecyl sulfate micelles stabilize AH + , whereas the positively charged cetyltrimethylammonium bromide and neutral polyoxyethylene(10) isooctyl phenyl ether (Triton X-100) micelles stabilize the uncharged (basic) forms. Besides affecting the molar fraction distribution of the various species, the presence of micelles also influences their interconversion rates. Addition of micelles can therefore be considered as a third external stimulus (besides light excitation and pH jump) capable of changing the state of this multistate/multifunctional molecular-level system. Particularly interesting is the possibility to change the autolock pH of the photochromic reaction by addition of micelles.

Aggregation of self-assembling branched [n]rotaxanes

The so-called slippage methodology has been employed to self-assemble novel [2]-, [3]-, and [4]-rotaxanes incorporating, respectively, one, two, and three bis-p-phenylene-34-crown-10 macrocyclic components and a branched ‘dumbbell’ component, consisting of three arms containing bipyridinium units attached covalently to a 1,3,5-trisubstituted benzene central core and each bearing at its other end a substituted tetraarylmethane stopper. The absorption spectra, luminescence properties, and electrochemical behaviour of the branched component and its [2]-, [3]-, and [4]-rotaxanes have been investigated and discussed on the basis of the properties of their chromophoric and electroactive units. Charge- and energy-transfer processes between specific chromophoric units and the correlations between the unusual redox patterns of the various compounds have been evidenced and interpreted. The 1H-NMR spectroscopic investigation of the ‘free’ triply-branched hexacationic core, containing three bipyridinium units, one in each arm and terminated by bulky hydrophobic tetraarylmethane-based stoppers revealed, in chloroform solution, the formation of aggregates—a phenomenon which has been modeled using force field calculations. In addition, the formation of a gel was observed after the slow liquid–liquid diffusion of hexane into a chloroform solution of the triply-branched compound. Field-emission scanning electron microscopic investigation of this gel revealed the presence of domains of regular size. Surface-pressure–area measurements demonstrated the formation of stable monolayers by the ‘free’ backbone and the rotaxanes at an air–water interface: two distinct aggregates are formed by each compound. Interestingly, for the rotaxanes, the measured limiting area per molecule of both aggregates increases with the number of macrocyclic components which are incorporated within the rotaxane molecule. Atomic force microscopic analyses of the monolayers transferred onto mica revealed significant differences in their shapes when the two distinct aggregates formed by the same compound at different pressures were compared. In particular, the section analyses of the monolayers showed nanosized domains possessing diameters ranging from approximately 10 to 56 nm.

Photophysical properties of Eu(SiW11O39)213− and Eu(BW11O39)215−

Abstract The photophysical properties (absorption, emission, and excitation spectra; luminescence quantum yields; luminescence decay lifetimes ) of K 13 [Eu(SiW 11 O 39 ) 2 ] and K 15 [Eu(BW 11 O 39 ) 2 ] in aqueous solution and in the solid state are reported. Both complexes exhibit broad and very intense O → W charge transfer bands in the U.V. region and weak and narrow f → f Eu 3+ bands in the visible. At 77 K the luminescence emission of both complexes, which consists of 5 D O → 7 F J bands split by the local crystal field, can be pumped very efficiently via both the O → W CT and the f → f Eu 3+ levels, whereas at 298 K only pumping via the f → f Eu 3+ is efficient. The values of the luminescence decay lifetimes in H 2 O and D 2 O solution are quite similar, showing that no water molecule is coordinated to the central Eu 3+ ion. The high resolution emission spectra are discussed in an attempt to define the coordination symmetry of Eu 3+ .

NEW TRENDS IN THE DESIGN OF LUMINESCENT METAL COMPLEXES

The requirements needed for an ideal luminophore are discussed with reference to the luminescent properties of organic molecules and metal complexes. Recent strategies for the design of luminescent metal complexes are discussed, with particular emphasis on cage‐type complexes and oligonuclear homo‐ and hetero‐metallic complexes.

Macrocycles, pseudorotaxanes and catenanes containing a pyrrolo-tetrathiafulvalene unit: absorption spectra, luminescence properties and redox behavior

The photophysical properties of (i) three macrocycles (1–3) of different size, each one incorporating a bis(2,5-dimethylpyrrolo[3,4-d])tetrathiafulvalene (TTFP) and a 1,4-dimethoxybenzene (DMB) electron-donating unit, (ii) the six pseudorotaxanes obtained by threading 1–3 with the electron acceptors dimethyldiazapyrenium (DMDAP2+) and dibenzyldiazapyrenium (DBDAP2+) and (iii) the three catenanes obtained by interlocking 1–3 with the electron-accepting cyclophane cyclobis(paraquat-p-phenylene) (CBPQT4+) have been investigated. The monooxidized and dioxidized species obtained by oxidation with Fe(III) of the TTFP unit contained in the above compounds have also been studied. The redox-driven dethreading/rethreading process has been investigated in the case of the pseudorotaxane based on the macrocycle 2 and the DMDAP2+ dication. In the catenanes, oxidation of the TTFP unit causes strong spectral changes, but does not promote disruption of the interlocked structures.

The self assembly of controllable [2]catenanes

The dynamic and electrochemical properties of two new [2]catenanes, in which bisparaphenylene-34-crown-10 is encircled by a cyclophane incorporating either (i) one bipyridinium and one bis(pyridinium)ethylene unit or (ii) two bis(pyridinium)ethylene units, are investigated in solution.

Artificial molecular-level machines with [Ru(bpy)3]2

A molecular-level machine is an assembly of a discrete number of molecular components (that is, a supramolecular structure) designed to perform mechanical-like movements (output) as a consequence of appropriate external stimuli (input). Like macroscopic machines, molecular-level machines are characterized by (i) the kind of energy input supplied to make them work, (ii) the kind of movement performed by their components, (iii) the way in which their operation can be controlled and monitored, (iv) the possibility to repeat the operation at will and establish a cyclic process, (v) the time scale needed to complete a cycle of operation, and (vi) the function performed. The most convenient way to supply energy to an artificial molecular-level machine is through a photochemical reaction. [Ru(bpy)3]2

Photochromic Properties of 3-Methyl-Substituted Flavylium Salts

The photochromic properties of some 3-methyl-substituted synthetic flavylium compounds have been investigated. The main structural feature in these compounds is the existence of a steric effect, caused by the methyl substituent, that forces the phenyl ring to move out of coplanarity with the benzopyrylium moiety. The X-ray structure of 3-methylflavylium tetrafluoroborate shows a torsion angle of 40.4° between the benzopyrylium and the benzene ring. In 3-methyl-substituted synthetic flavylium compounds, the steric effect impedes the formation of trans-chalcone. In the case of the 4′-hydroxy-3-methylflavylium ion, for example, the trans-chalcone form could only be obtained (up to a maximum yield of 60%) by protonation of the trans-chalcone anion, obtained in turn by irradiation of anionic cis-chalcone in basic solution. The multistate/multifunctional properties of the 4′-hydroxy-3-methylflavylium compound are also discussed in detail. For the 4′,7-dihydroxy-3-methylflavylium ion, a detailed investigation of the kinetic and thermodynamic properties of the various forms was performed by means of pH jumps and photochemical experiments. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)