Showing papers by "Andreas Hauser published in 2006"

Density-Functional Theory Investigation of the Geometric, Energetic, and Optical Properties of the Cobalt(II)tris(2,2'-bipyridine) Complex in the High-Spin and the Jahn-Teller Active Low-Spin States.

Abstract: State-of-the-art generalized gradient approximation (GGA) (PBE, OPBE, RPBE, OLYP, and HCTH), meta-GGA (VSXC and TPSS), and hybrid (B3LYP, B3LYP*, O3LYP, and PBE0) functionals are compared for the determination of the structure and the energetics of the D3 [Co(bpy)3](2+) complex in the (4)A2 and (4)E trigonal components of the high-spin (4)T1g([Formula: see text] [Formula: see text] ) state and in the low-spin (2)E state of octahedral (2)Eg([Formula: see text] [Formula: see text] ) parentage. Their comparison extends also to the investigation of the Jahn-Teller instability of the (2)E state through the characterization of the extrema of C2 symmetry of this spin state's potential energy surface. The results obtained for [Co(bpy)3](2+) in either spin manifold are very consistent among the functionals used and are in good agreement with available experimental data. The functionals, however, perform very differently with respect to the spin-state energetics because the calculated values of the high-spin/low-spin energy difference Δ[Formula: see text] vary between -3212 and 3919 cm(-)(1). Semilocal functionals tend to give too large Δ[Formula: see text] values and thus fail to correctly predict the high-spin state as the ground state of the isolated complex, while hybrid functionals tend to overestimate the stability of the high-spin state with respect to the low-spin state. Reliable results are, however, obtained with the OLYP, HCTH, B3LYP*, and O3LYP functionals which perform best for the description of the isolated complex. The optical properties of [Co(bpy)3](2+) in the two spin states are also analyzed on the basis of electronic excitation calculations performed within time-dependent density functional response theory. The calculated absorption and circular dichroism spectra agree well with experimental results.

Photoexcitation and relaxation dynamics of catecholato-iron(III) spin-crossover complexes.

Abstract: The photophysical properties of the ferric catecholate spin-crossover compounds [(TPA)Fe(R-Cat)]X (TPA=tris(2-pyridylmethyl)amine; X=PF6-, BPh4-; R-Cat=catecholate dianion substituted by R=NO2, Cl, or H) are investigated in the solid state. The catecholate-to-iron(III) charge-transfer bands are sensitive both to the spin state of the metal ion and the charge-transfer interactions associated with the different catecholate substituents. Vibronic progressions are identified in the near-infrared (NIR) absorption of the low-spin species. Evidence for a low-temperature photoexcitation process is provided. The relaxation dynamics between 10 and 100 K indicate a pure tunneling process below 40 K, and a thermally activated region at higher temperatures. The relaxation rate constants in the tunneling regime at low temperature, kHL(T0), vary in the range from 0.58 to 8.84 s-1. These values are in qualitative agreement with the inverse energy-gap law and with structural parameters. A comparison with ferrous spin-crossover complexes shows that the high-spin to low-spin relaxation is generally faster for ferric complexes, owing to the smaller bond length changes for the latter. However, in the present case the corresponding rate constants are smaller than expected based on the single configurational coordinate model. This is attributed to the combined influence of the electronic configuration and the molecular geometry.

Inter- and Intramolecular Interactions in Some Supramolecular Photochemical Systems

Abstract: In supramolecular systems, the interaction between different units modulates their photophysical properties. a) For platinum(II) complexes with ligands that have extended π systems, π-stacking and direct metal–metal interactions result in the formation of excimers with characteristically red-shifted luminescence. Time-resolved emission spectra show clear evidence of dual luminescence. b) In phthalocyanines to which electron-donating tetrathiafulvalene (TTF) groups have been fused, the luminescence is strongly quenched by intramolecular electron transfer. The luminescence can be switched on by oxidation of the TTF groups. c) The luminescence of ruthenium tris-bipyridyl derivatives is strongly influenced by the environment. Linked to biotin, the luminescence quantum yield of such a complex is enhanced by 30 % upon binding to avidin. Furthermore, the binding to avidin induces a circular-dichroism signal from the π–π* transition of the initially racemic ruthenium tris-bipyridyl derivative.

Electronic structure, spectroscopic properties, and reactivity of molybdenum and tungsten nitrido and imido complexes with diphosphine coligands: influence of the trans ligand.

Abstract: A series of molybdenum and tungsten nitrido, [M(N)(X)(diphos)2], and imido complexes, [M(NH)(X)(diphos)2)]Y, (M = Mo, W) with diphosphine coligands (diphos = dppe/depe), various trans ligands (X = N3-, Cl-, NCCH3) and different counterions (Y-= Cl-, BPh4-) is investigated. These compounds are studied by infrared and Raman spectroscopies; they are also studied with isotope-substitution and optical-absorption, as well as emission, spectroscopies. In the nitrido complexes with trans-azido and -chloro coligands, the metal-N stretch is found at about 980 cm(-1); upon protonation, it is lowered to about 920 cm(-1). The 1A1 --> 1E (n --> pi) electronic transition is observed for [Mo(N)(N3)(depe)2] at 398 nm and shows a progression in the metal-N stretch of 810 cm(-1). The corresponding 3E --> 1A (pi --> n) emission band is observed at 542 nm, exhibiting a progression in the metal-N stretch of 980 cm(-1). In the imido system [Mo(NH)(N3)(depe)2]BPh4, the n --> pi transition is shifted to lower energy (518 nm) and markedly decreases in intensity. In the trans-nitrile complex [Mo(N)(NCCH3)(dppe)2]BPh4, the metal-N(nitrido) stretching frequency increases to 1016 cm(-1). The n --> pi transition now is found at 450 nm, shifting to 525 nm upon protonation. Most importantly, the reduction of this nitrido trans-nitrile complex is drastically facilitated compared to its counterparts with anionic trans-ligands (Epred = -1.5 V vs Fc+/Fc). On the other hand, the basicity of the nitrido group is decreased (pKa{[Mo(NH)(NCCH3)(dppe)2](BPh4)2} = 5). The implications of these findings with respect to the Chatt cycle are discussed.

A Redox‐Active Tri‐Star Molecule: Merging of TTF and HAT Chemistry.

Abstract: A planar π-conjugated heteroaromatic molecule 1 has been synthesized and fully characterized; it combines two characteristics, a charge-transfer transition originating from its inherent donor–acceptor nature in its neutral state and an intervalence charge-transfer transition in its 12+ mixed-valence state.