Garry S. Hanan

Luminescent polynuclear assemblies

This tutorial review consists of five main sections. The first gives a general introduction and then a discussion about the need for luminescent assemblies. The next four sections present the various assemblies based on the metal ions used to assemble the final structures. Each of these sections gives a brief overview of the design principles, synthesis, and ground and excited-state properties of the ligands and complexes in question. The review concludes with some suggestions for future avenues of research.

Discrete Covalent Organic–Inorganic Hybrids: Terpyridine Functionalized Polyoxometalates Obtained by a Modular Strategy and Their Metal Complexation

The rational design and synthesis of organic-inorganic hybrids as functional molecular materials relies on both the careful conception of building-blocks and the strategy for their assembly. Three families of trialkoxo polyoxometalates (Lindqvist 2, Anderson 3, Dawson 4) grafted with remote terpyridine coordination sites have been synthesized to extend the available building-blocks. These new units can be combined with metal complexes that play a role as (i) chromophores toward charge-separated systems in light-harvesting devices and (ii) coordination motifs for metal-directed self-assembly toward multifunctional molecular hybrid materials. The X-ray crystal structures of polyoxometalate-terpyridine hybrids indicate distances of 21 Å and 19 Å between the two terpyridyl coordination sites in 2 and 3, respectively, with angles between the coordination vectors of 180° and 177.4°, respectively. Lindqvist 2 displays a reduction at -0.52 V vs SCE while Anderson 3 exhibits one reversible oxidation attributed to Mn(III)/Mn(IV) (+0.75 V vs SCE) and a broad wave at -1.28 V vs SCE assigned to the Mn(III)/Mn(II) reduction. Dawson 4 displays several processes on a wide range of potentials (+0.5 to -2.0 V vs SCE) centered on V(V), W(VI) and the organic ligand in order of decreasing potentials. The grafted terpyridine ligands in Anderson 3 and Dawson 4 were successfully coordinated to {PdCl}(+) and {RuCl(3)} moieties, respectively. The polyoxometalates and transition metal complexes retain their intrinsic properties in the final assemblies.

Near infra-red emission from a mer-Ru(II) complex: consequences of strong σ-donation from a neutral, flexible ligand with dual binding modes

A rare example of dual coordination modes by a novel tridentate ligand gives rise to unique fac-and mer-Ru((II/III)) complexes. The mer-Ru(II)-complex displays the farthest red-shift of a triplet metal-to-ligand charge transfer ((3)MLCT) emission with a tridentate ligand for a mononuclear complex. This observation is a consequence of large bite angle and strong σ-donation by the ligand, the combined effect of which helps to separate the energy of the (3)MLCT and (3)MC states.

Synthesis, Crystal Structure and Photophysical Properties of Pyrene–Helicene Hybrids

Synthesis of helically chiral aromatics resulting from fusion of pyrene and [4]- or [5]helicene has been accomplished using photoredox catalysis employing a Cu-based sensitizer as the key step. Photocyclisation experiments for the synthesis of the target compounds were carried out in batch and using continuous flow strategies. The solid-state structures, UV/Vis absorption spectra and fluorescence spectra of the pyrene-helicene hybrids were investigated and compared to that of the parent [5]helicene to discern the effects of merging a pyrene moiety within a helicene skeleton. The studies demonstrated that pyrene-helicene hybrids adopt co-planar or stacked arrangements in the solid state, in contrast to the solid-state structure of the parent [5]helicene. The UV/Vis and fluorescence spectra of the pyrene-helicene hybrids exhibited strong red-shifts when compared to the parent [5]helicene. DFT calculations suggest that the strategy of extending the π surface in the y axis of the helicenes increased their HOMO levels while also decreasing their LUMO levels, resulting in significantly reduced band gaps.

Azadipyrromethene dye derivatives in coordination chemistry: the structure-property relationship in homoleptic metal(II) complexes.

As a chromophore closely related to dipyrromethene (DPM), the azadipyrromethene (ADPM) family has attracted much interest in the life sciences and optoelectronic fields. A high-yielding microwave-assisted synthesis is reported for new homoleptic complexes of cobalt(II), nickel(II), copper(II) and zinc(II) based on the tetrakis(p-methoxyphenyl)azadipyrromethene ligand 1b. These complexes are compared with other homoleptic complexes of the same metal(II) series based on the tetraphenylazadipyrromethene 1a and also with related BF2(+) chelates (Aza-BODIPYs 6a and 6b) for a better understanding of trends arising from substitution of the chelate and/or the electron-donating effect of the p-methoxy substituents. The electrochemical behavior of the new compounds 2b, 3b, and 5b in dichloromethane revealed two pseudoreversible reductions (2b, -1.09 and -1.25 V vs SCE; 3b, -1.05 and -1.29 V; 5b, -1.13 and -1.25 V) followed by a third irreversible process (2b, -1.78 V; 3b, -1.80 V; 5b, -1.77 V) along with two pseudoreversible oxidations (2b, 0.55 and 0.80 V; 3b, 0.56 and 0.80 V; 5b, 0.55 and 0.80 V) followed by two closely spaced irreversible processes (2b, 1.21 and 1.27 V; 3b, 1.21 and 1.28 V; 5b, 1.22 and 1.25 V). On its side, copper(II) homoleptic complex 4b revealed only one pseudoreversible reduction at -0.59 V followed by three irreversible processes at -0.95, -1.54, and -1.74 V, respectively. The oxidation behavior of this complex exhibited two pseudoreversible processes (0.55 and 0.82 V) and two irreversible processes (1.19 and 1.25 V). The redox processes are assigned and discussed in relation to their photophysical properties. X-ray structures for 1b and related copper(II) complex 2b are also discussed.

Absorption and Emission Properties of Di‐ and Trinuclear Ruthenium(II) Rack‐Type Complexes

The absorption spectra and the luminescence properties of three dinuclear RuII complexes and one trinuclear RuII complex have been investigated. All the complexes have rack-type structures. The dinuclear complexes 1, 2, and 3 incorporate a bis-tridentate bridging ligand made up of a pyrimidine and four pyridine moieties, as well as two 2,2′:6′,2′-terpyridyl (tpy) ligands. The trinuclear complex 4 incorporates a tris-tridentate bridging ligand made up of two pyrimidine and five pyridine moieties, as well as three tpy ligands. The absorption spectra of the complexes show a large number of ligand-centered (LC) and metal-to-ligand charge-transfer (MLCT) bands. All the complexes exhibit emission from a triplet MLCT state, with maxima in the spectral range 840–950 nm (lifetimes between 40 and 80 ns) at 298 K in fluid solution, and in the spectral range 760–810 nm (lifetimes between 2 and 3 μs) at 77 K in rigid matrices. A fine tuning of the absorption and luminescence properties of complexes 1–3can be achieved by changing the substituents on the pyrimidine ring of the bridging ligand. Efficient energy transfer within the rack structure 4 occurs from the (upper-lying) central metal-based chromophore to the (lower-lying) peripheral ones.

CONTROLLING THE DIRECTION OF PHOTOINDUCED ENERGY TRANSFER IN MULTICOMPONENT SPECIES

The direction of the efficient photoinduced energy transfer which takes place in two new multicomponent species that contain RuII-polypyridine chromophores and anthracene subunits is controlled by the temperature. The switch in the direction of the energy-transfer process arises from the quite different intrinsic lifetimes of the luminophores and the relatively small energy gap between the lowest energy excited states located in each subunit. Novel molecular-level devices based on this particular effect are foreseen.

Molecular helicity: a general approach for helicity induction in a polyheterocyclic molecular strand

The structural and conformational features of the non-chiral polyheterocyclic molecule 1 induce a helical shape in the molecular strand both in solution and in the solid state, as shown by the crystal structure.

Symmetric and Asymmetric Coupling of Pyridylpyrimidine for the Synthesis of Polynucleating Ligands

A convenient synthetic approach to build up new polynucleating ligands is presented. Symmetric and asymmetric pyridylpyrimidine dimers are produced by radical anion coupling and nucleophilic addition, respectively. A diruthenium complex of the asymmetric ligand was synthesised and characterised by cyclic voltammetry, luminescence and 99Ru NMR spectroscopy. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Stereoselective formation of a meso-diruthenium(II,II) complex and tuning the properties of its monoruthenium analogues

A novel bis(bidentate) ligand dgpm (dgpm = diguanidylpyrimidine) was synthesized by a catalyst-free C-N bond forming reaction in high yield (90%) by microwave-assisted heating. The ligand was coordinated to two [Ru(bpy)2](2+) cores to afford a meso-di-Ru(II,II) complex (1-meso) with high diastereoselectivity over its homochiral form. Three mononuclear ether-functionalized Ru(II) complexes (2: ethoxyether; 3: butoxyether; 4: 2-hydroxy-1-ethoxyether) were also isolated. The ligand and complexes were fully characterized by a variety of techniques including X-ray crystallography. In cyclic voltammetric studies, the complexes exhibit a Ru(III/II) couple, which is ~500 mV less positive than the Ru(III/II) couple in Ru(bpy)3(2+). The (1)MLCT absorption maxima of all the complexes (510-550 nm) are considerably red-shifted as compared to that of Ru(bpy)3(2+) (450 nm). The (3)MLCT emission maxima of complexes 1-meso and 3 are also red-shifted by about 120 nm compared to that of Ru(bpy)3(2+) (620 nm), whereas the corresponding maxima for complexes 2 and 4 are shifted by 75 nm and 25 nm, respectively. These relative trends in redox potentials and (1)MLCT maxima are in good agreement with DFT and TD-DFT calculations, performed for all complexes. Complexes 1-meso and 3 display emission from a Ru(II)-to-bpy (3)MLCT state, which is rarely the emitting state at λ > 700 nm in [Ru(bpy)2(N-N)](2+) complexes when the ancillary ligand is neutral.