Gianluca Accorsi

Photochemistry and Photophysics of Coordination Compounds: Copper

Cu(I) complexes and clusters are the largest class of compounds of relevant photochemical and photophysical interest based on a relatively abundant metal element. Interestingly, Nature has given an essential role to copper compounds in some biological systems, relying on their kinetic lability and versatile coordination environment. Some basic properties of Cu(I) and Cu(II) such as their coordination geometries and electronic levels are compared, pointing out the limited significance of Cu(II) compounds (d 9 configuration) in terms of photophysical properties. Well-established synthetic protocols are available to build up a variety of molecular and supramolecular architectures (e.g. catenanes, rotaxanes, knots, helices, dendrimers, cages, grids, racks, etc.) containing Cu(I)-based centers and exhibiting photo- and electroluminescence as well as light-induced intercomponent processes. By far the largest class of copper complexes investigated to date is that of Cu(I)-bisphenanthrolines ([Cu(NN)2]+) and recent progress in the rationalization of their metal-to-ligand charge-transfer (MLCT) absorption and luminescence properties are critically reviewed, pointing out the criteria by which it is now possible to successfully design highly emissive [Cu(NN)2]+ compounds, a rather elusive goal for a long time. To this end the development of spectroscopic techniques such as light-initiated time-resolved X-ray absorption spectroscopy (LITR-XAS) and femtosecond transient absorption have been rather fruitful since they have allowed us to firmly ground the indirect proofs of the molecular rearrangements following light absorption that had accumulated in the past 20 years. A substantial breakthrough towards highly emissive Cu(I) coordination compounds is constituted by heteroleptic Cu(I) complexes containing both N- and P-coordinating ligands ([Cu(NN)(PP)]+) which may exhibit luminescence quantum yields close to 30% in deaerated CH2Cl2 solution and have been successfully employed as active materials in OLED and LEC optoelectronic devices. Also copper clusters may exhibit luminescence bands of halide-to-metal charge transfer (XMCT) and/or cluster centered (CC) character and they are briefly reviewed along with miscellaneous Cu(I) compounds that recently appeared in the literature, which show luminescence bands ranging from the blue to the red spectral region.

Electrophosphorescent homo- and heteroleptic copper(I) complexes prepared from various bis-phosphine ligands

Homo- and heteroleptic copper(I) complexes obtained from various chelating bis-phosphine ligands and Cu(CH3CN)4BF4 have been used for the preparation of light emitting devices.

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium(III) complexes Ir(2,4-di-X-phenyl-pyridine)(2)(picolinate) (X = H, F, Cl, Br) HIrPic, FIrPic, ClIrPic, and BrIrPic has been synthesized and characterized by NMR, X-ray crystallography, UV-vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with FIrPic and ClIrPic, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of BrIrPic and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin-orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for ClIrPic and BrIrPic, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect.

Blue-emitting dinuclear N-heterocyclic dicarbene gold(I) complex featuring a nearly unit quantum yield.

Dinuclear N-heterocyclic dicarbene gold(I) complexes of general formula [Au(2)(RIm-Y-ImR)(2)](PF(6))(2) (R = Me, Cy; Y = (CH(2))(1-4), o-xylylene, m-xylylene) have been synthesized and screened for their luminescence properties. All the complexes are weakly emissive in solution whereas in the solid state some of them show significant luminescence intensities. In particular, crystals or powders of the complex with R = Me, Y = (CH(2))(3) exhibit an intense blue emission (λ(max) = 450 nm) with a high quantum yield (Φ(em) = 0.96). The X-ray crystal structure of this complex is characterized by a rather short intramolecular Au···Au distance (3.272 Ǻ). Time dependent density functional theory (TDDFT) calculations have been used to calculate the UV/vis properties of the ground state as well as of the first excited state of the complex, the latter featuring a significantly shorter Au···Au distance.

The exceptional near-infrared luminescence properties of cuprorivaite (Egyptian blue)

Cuprorivaite (CaCuSi(4)O(10), also known as Egyptian blue) exhibits an exceptionally high emission quantum efficiency in the near-infrared region (lambda(max) = 910 nm, Phi(EM) = 10.5%) and a long excited state lifetime (107 mus); these properties make it appealing for several applications in the fields of biomedical analysis, telecommunications and lasers.

Photoinduced processes in fullerenopyrrolidine and fullerenopyrazoline derivatives substituted with an oligophenylenevinylene moietyElectronic supplementary information (ESI) available: synthetic procedures and full characterization of all new compounds. See http://www.rsc.org/suppdata/jm/b2/b200432a/

Fullerene derivatives A-3PV and B-3PV in which an oligophenylenevinylene trimeric subunit (3PV) is attached to C60 through, respectively, a pyrrolidine or a pyrazoline ring have been prepared. The electrochemical and excited-state properties of the multicomponent arrays A-3PV and B-3PV have been investigated in solution using the related oligophenylenevinylene derivative 3PV, fullerenopyrrolidine A and fullerenopyrazoline B as reference compounds. In A-3PV quantitative OPV → C60 photoinduced singlet–singlet energy transfer has been observed. Population of the lowest fullerene singlet excited state is followed by nearly quantitative intersystem crossing to the lowest fullerene triplet excited state in CH2Cl2 and toluene, whereas OPV → C60 electron transfer is able to compete significantly in the more polar solvent benzonitrile. In the case of B-3PV, the excited-state properties are more complex due to the electron donating ability of the pyrazoline ring. As observed for A-3PV, quantitative OPV → C60 photoinduced singlet–singlet energy transfer occurs in B-3PV. However, in this case, the population of the lowest fullerene singlet excited state is followed by an efficient electron transfer from the pyrazoline ring in CH2Cl2 and benzonitrile. In B-3PV, studies of the dependence of photoinduced processes on solvent polarity, addition of acid, and temperature also reveal that this compound can be considered as a fullerene-based molecular switch, the switchable parameters being the photoinduced processes. Finally, A-3PV and B-3PV have been tested as active materials in photovoltaic devices and the differences of light to energy conversion efficiencies found for the two compounds have been rationalised on the basis of their photophysical properties.

Photophysical Properties of the ReI and RuII Complexes of a New C60-Substituted Bipyridine Ligand

The rhenium(I) and ruthenium(II) complexes of a fullerene-substituted bipyridine ligand have been prepared. Electrochemical studies indicate that some ground state electronic interaction between the fullerene subunit and the metal-complexed moiety are present in the Re(I) but not the Ru(II) complex. The photophysical properties have been investigated by steady-state and time-resolved UV/Vis-NIR luminescence spectroscopy and nanosecond laser flash photolysis in CH2Cl2 solution, and compared to those of the corresponding model compounds. Excitation of the methanofullerene moiety in the dyads does not lead to excited state intercomponent interactions. Instead, excitation of the metal-complexed unit shows that the lowest triplet metal-to-ligand-charge-transfer excited state ((3)MLCT) centered on the Re(I)- or Ru(II)-type unit is quenched with a rate constant of about 2.5 x 10(8) s(-1). The quenching is attributed to an electron-transfer (ElT) process leading to the reduction of the carbon sphere, as determined by luminescence spectroscopy for the Ru(II) dyad. Experimental detection of electron transfer in the Re(I) dyad is prevented due to the unfavorable absorption of the metal-complexed moiety relative to the fullerene unit. However, it can be postulated on the basis of energetic/kinetic arguments and by comparison with the Ru(II)-type array. The primary ElT process is followed by charge-recombination to give the lowest-lying fullerene triplet excited state (3C60) with quantitative yield, as determined by sensitized singlet oxygen luminescence experiments. Direct (3)MLCT-->3C60 triplet-triplet energy-transfer (EnT) does not successfully compete with ElT since it is highly exoergonic and located in the Marcus inverted region. The quantum yield of singlet oxygen sensitization (Phi(delta)) of the Re(I)-based dyad is found to be lower (0.80) than for the corresponding Ru(II) derivative (1.0). This is likely to be the consequence of different conformational structures for the two dyads, rather than a different yield of 3C60 formation.

A fullerene core to probe dendritic shielding effects

Abstract Dendrimers with a C 60 core have been obtained by cyclization of dendritic bis-malonate derivatives at the carbon sphere. The resulting bis-methanofullerene derivatives have been characterised by electrospray (ES) and/or MALDI-TOF mass spectrometries. UV–VIS absorption spectra, fluorescence spectra, and fullerene singlet excited state lifetimes have been determined in solvents of different polarity (toluene, dichloromethane, acetonitrile). These data suggest a tighter core/periphery contact upon increase of solvent polarity and dendrimer size. In all the investigated solvents, the fullerene triplet lifetimes are steadily increased with the dendrimer volume, reflecting lower diffusion rates of O 2 inside the dendrimers along the series. Measurements of quantum yields of singlet oxygen sensitization indicate that longer lived triplet states generate lower amounts of singlet oxygen ( 1 O 2 ∗ ) in dichloromethane but not in apolar toluene suggesting a tighter contact between the dendritic branches and the fullerene core in CH 2 Cl 2 . In acetonitrile, the trend in singlet oxygen production is peculiar. Effectively, enhanced singlet oxygen production is monitored for the largest dendrimer. This reflects specific interactions of excited 1 O 2 ∗ molecules with the dendritic wedges, as probed by singlet oxygen lifetime measurements, possibly as a consequence of trapping effects.

Fullerodendrimers with peripheral triethyleneglycol chains: synthesis, mass spectrometric characterization, and photophysical properties

Poly(aryl ether) dendritic branches terminated with peripheral triethyleneglycol chains have been attached to C60. The resulting fullerodendrimers have been characterised by electrospray mass spectrometry (ESMS), which appears to be a particularly interesting analytical tool for the unambiguous structural assignment of such high molecular weight fullerene derivatives. Their photophysical properties have been systematically investigated in three solvents, namely toluene, dichloromethane, and acetonitrile. The changes observed in the photophysical properties along the series suggest an increasing interaction between the poly(aryl ether) dendritic wedges and the fullerene core, which brings about an increasing isolation of the central chromophore from the exterior. Finally, thanks to their high solubility, fullerodendrimers 1–4 have been easily incorporated in mesoporous silica glasses and preliminary measurements on the resulting doped samples have revealed efficient optical limiting properties.

Syntheses and structures of mononuclear {Re(CO)3Cl(NN)} 'complex ligands' with a pendant imino /pyridine binding site, and preparation of some heterodinuclear Re(I) /lanthanide(III) complexes

Abstract A series of potentially dinucleating ligands, each with two bidentate imino–pyridine compartments, has been used to prepare the mononuclear complexes [Re(CO)3Cl(Ln)] (n=1–4) which each have a vacant binding site and so constitute ‘complex ligands’ for the stepwise synthesis of heterodinuclear complexes. Three of the complexes (with L1, L2 and L4) have been structurally characterised. The mononuclear complex [Re(CO)3Cl(L5)], arising from decomposition of [Re(CO)3Cl(L3)] by hydrolysis of the non-coordinated imine unit, was also isolated and structurally characterised. Luminescence studies show that these compounds are only very weak emitters. The ‘complex ligand’ [Re(CO)3Cl(L1)] has been reacted with [Ln(hfac)3]·2H2O (Ln=Yb, Er) to prepare the heterodinuclear complexes [Re(CO)3Cl(μ-L1)Ln(hfac)3] in which the {Ln(hfac)}3 unit is attached to the second bidentate imino–pyridine site of [Re(CO)3Cl(L1)]. The crystal structures of these show that the bridging ligand has to twist to accommodate the two bulky metal fragments, and that the lanthanide units have an 8-coordinate square-antiprismatic geometry.