No abstract

The intramolecular magnetic exchange coupling constants (J) for a series of tetrathiafulvalene (TTF) and verdazyl diradical cations connected by a range of pi conjugated linkers have been investigated by means of methodology based on unrestricted density functional theory. The magnetic interaction between radicals is transmitted via pi-electron conjugation for all considered compounds. The calculation of J yields strong or medium ferromagnetic coupling interactions (in the range of 56 and 300 K) for diradical cations connected by linkers with an even number of carbon atoms that are able to provide a spin polarization pathway, while antiferromagnetic coupling is predicted when linkers with an odd number of carbon atoms are employed. The topological analysis of spin density distributions have been used to reveal the effects of the spin polarization on both linkers and spin carriers. The absence of heteroatoms that impede the spin polarization pathway, and the existence of a unique spin polarization path instead of several possible competitive routes are factors which contribute to large positive J values favoring ferromagnetic interactions between the two terminal pi-radicals. The magnitude of J depends strongly on the planarity of the molecular structure of the diradical cation since a more effective orbital overlap between the two pi-systems can be achieved. Hence, the dependence of J on the torsion angle (theta) of each spin carrier has been analyzed. In this respect, our findings show that this geometrical distortion reduces largely the calculated J values for ferromagnetic couplings, leading to weak antiferromagnetic interactions for a torsion angle of 90 degrees .

The distance dependence of sequential electron transfer has been studied in six, vertical, linear supramolecular triads, (TTF-Ph(n)-py → AlPor-Ph(m)-C60, n = 0, 1 and m = 1, 2, 3), constructed using tetrathiafulvalene (TTF), aluminum(III) porphyrin (AlPor) and fullerene (C60) entities. The C60 and TTF units are bound to the Al center on opposite faces of the porphyrin; the C60 through a covalent axial bond using a benzoate spacer, and the TTF through a coordination bond via an appended pyridine. Time-resolved optical and EPR spectroscopic methods and computational studies are used to demonstrate that excitation of the porphyrin leads to step-wise, sequential electron transfer (ET) between TTF and C60, and to study the electron transfer rates and exchange coupling between the components of the triads as a function of the bridge lengths. Femtosecond transient absorption studies show that the rates of charge separation, k(CS) are in the range of 10(9)-10(11) s(-1), depending on the length of the bridges. The lifetimes of the charge-separated state TTF˙(+)-C₆₀˙⁻ obtained from transient absorbance experiments and the singlet lifetimes of the radical pairs obtained by time-resolved EPR are in good agreement with each other and range from 60-130 ns in the triads. The time-resolved EPR data also show that population of the triplet sublevels of the charge-separated state in the presence of a magnetic field leads to much longer lifetimes of >1 μs. The data show that a modest stabilization of the charge separation lifetime occurs in the triads. The attenuation factor β = 0.36 Å(-1) obtained from the exchange coupling values between TTF˙(+) and C₆₀˙⁻ is consistent with values reported in the literature for oligophenylene bridged TTF-C60 conjugates. The singlet charge recombination lifetime shows a much weaker dependence on the distance between the donor and acceptor, suggesting that a simple superexchange model is not sufficient to describe the back reaction.

A comprehensive review of tuneable polypyridine complexes as the emissive components of OLED and LEC devices is presented, with a view to bridging the gap between molecular design and commercialization.

The coordination properties of ortho- and meta-substituted [(2-diphenylphosphanylethyl)phenyl]methanol 4a and 4b toward ruthenium(II) have been investigated. To ensure coordination of both the arene and the tethered phosphine, the labile ruthenium arene dimer [RuCl2(EtO2CC6H5)]2 (7) was synthesized and structurally characterized. Both the ortho and meta isomers [Ru(4a)Cl2] (9a) and [Ru(4b)Cl2] (9b) were characterized by X-ray crystallography. The lack of reactivity of the benzylic alcohol functionality in complexes 9a and 9b toward various P and C electrophiles is rationalized with extended Hückel calculations.

No abstract

The synthesis and structural chemistry of a series of new divalent transition metal complexes of the bis-bidentate ligand 3,3Ј-diamino-2,2Ј-bipyridine (L1) and its dication L1H 2 are described. Ligand L1 reacts with salts of divalent transition metals to afford the (1:1) metal-ligand complexes (2a−2d) as well as the tris complexes (3a−3f). All complexes were fully characterised by spectroscopic methods and the following compounds [Cu(L1)Cl 2 ] 2 (2a), [Cu(L1)(OAc) 2 ] (2b), [Zn(L1) 3 ][OTf] 2 (3a), and [Zn(L1) 3 ][ZnCl 4 ] (3e and 3f) were structurally characterised. Results from single crystal X-ray diffraction measurements indicate that formation of an intramolecular hydrogen bond between the two amino groups allows the ligand to coordinate divalent metal ions through their diimine binding sites. Furthermore, the structure of compound 2a reveals that it crystallises as a dimer in which each copper ion is bound to two pyridine nitrogen atoms and [a]

Two mononuclear Dy III crown ether complexes [Dy(15C5)(H 2 O) 4 ](ClO 4 ) 3 Á(15C5)ÁH 2 O (1) and [Dy(12C4)(H 2 O) 5 ](ClO 4 ) 3 ÁH 2 O (2) have been prepared and characterized. X-ray diffraction studies show that both compounds crystallize as half sandwich type structures with muffin and pseudo-capped square antiprismatic geometries respectively. Despite the comparable local environments of the Dy III ions they display remarkably different dynamic magnetic properties with only 1 displaying SMM properties in zero field. The solid state emission spectra for both 1 and 2 display sharp bands associated with f-f transitions.From the fine structure of the 4 F 9/2 -6 H 15/2 band, the Stark splitting of the 6 H 15/2 ground state permitted the energy difference between the ground and first excited state to be determined. For 1 this value (DE = 58.0 AE 3.0 cm À1 ) is in excellent agreement with ab initio calculations and the experimentally observed SMM behaviour. For 2, the photoluminescence data and theoretical calculations support a less well isolated ground state (DE = 30 AE 3.0 cm À1 ) in which a rapid relaxation process affords no SMM behaviour in zero-field.