Electron transfer pathways in mixed-valence paracyclophane-bridged bis-triarylamine radical cations.

A series of paracyclophane (PC) bridged mixed-valence (MV) bis-triarylamine radical cations with different ([2.2], [3.3], [4.4]) linkers, with and without additional ethynyl spacers, have been studied by quantum-chemical calculations (BLYP35-D3/TZVP/COSMO) of ground-state structures, thermal electron-transfer barriers, hyperfine couplings, and lowest-lying excited states. Such PC-bridged MV systems are important intra-molecular model systems for inter-molecular electron transfer (ET) via π-stacked aromatics, since they allow enforcement of a more or less well-defined geometrical arrangement. Closely comparable ET barriers and electronic couplings for all [2.2] and [3.3] bridges are found for these class-II MV systems, irrespective of the use of pseudo-para and pseudo-meta connections. While the latter observation contradicts notions of quantum interference for off-resonant conduction through molecular wires, it agrees with the less intricate nodal structures of the highest occupied molecular orbitals. The ET in such MV systems may be more closely connected with hole conduction in the resonant regime. Computations on model cations, in which the [2.2] linkers have been truncated, confirm predominant through-space π-π electronic coupling. Systems with [4.4] PC bridges exhibit far more structural flexibility and concomitantly weaker electronic interactions between the redox centers.

Mixed-valence ruthenium complexes rotating through a conformational Robin-Day continuum.

The conformational energy landscape and the associated electronic structure and spectroscopic properties (UV/Vis/near-infrared (NIR) and IR) of three formally d(5)/d(6) mixed-valence diruthenium complex cations, [{Ru(dppe)Cp*}2(µ-C≡CC6H4C≡C)](+), [1](+), [trans-{RuCl(dppe)2}2(µ-C≡CC6H4C≡C)](+), [2](+), and the Creutz-Taube ion, [{Ru(NH3)5}2(µ-pz)](5+), [3](5+) (Cp = cyclopentadienyl; dppe = 1,2-bis(diphenylphosphino)ethane; pz = pyrazine), have been studied using a nonstandard hybrid density functional BLYP35 with 35 % exact exchange and continuum solvent models. For the closely related monocations [1](+) and [2](+), the calculations indicated that the lowest-energy conformers exhibited delocalized electronic structures (or class III mixed-valence character). However, these minima alone explained neither the presence of shoulder(s) in the NIR absorption envelope nor the presence of features in the observed vibrational spectra characteristic of both delocalized and valence-trapped electronic structures. A series of computational models have been used to demonstrate that the mutual conformation of the metal fragments--and even more importantly the orientation of the bridging ligand relative to those metal centers--influences the electronic coupling sufficiently to afford valence-trapped conformations, which are of sufficiently low energy to be thermally populated. Areas in the conformational phase space with variable degrees of symmetry breaking of structures and spin-density distributions are shown to be responsible for the characteristic spectroscopic features of these two complexes. The Creutz-Taube ion [3](5+) also exhibits low-lying valence-trapped conformational areas, but the electronic transitions that characterize these conformations with valence-localized electronic structures have low intensities and do not influence the observed spectroscopic characteristics to any notable extent.

A combined computational and spectroelectrochemical study of platinum-bridged bis-triarylamine systems.

The character of the electronic transitions in the ultraviolet-visible-near infrared (UV-vis-NIR) spectra of platinum-bis(alkynyl) bridged, bis-triarylamine mixed-valence systems trans-[Pt(C≡CC6H4NAr2)2 (PR3)2](n+) (R = ethyl, Ar = C6H4CH3-4 (1) or C6H4OCH3-4 (2); R = Ph, Ar = C6H4CH3-4 (3) or C6H4OCH3-4 (4), n = 0, 1, 2) has been determined from a combination of spectroscopic measurement and density functional theory calculations. The hybrid functional BLYP35 in combination with a suitable solvent model (i.e., conductor-like screening model (COSMO)) has been used to model the UV-vis-NIR and IR spectroscopic properties of [1-4](+), to confirm the description of [1-4](+) as examples of metal-bridged organic mixed-valence compounds, and to assign the principal features of the electronic spectra, including the triarylamine-based intervalence charge transfer transition located in the NIR region. The successful modeling of the charge distribution within the system demonstrates the utility of the BLYP35-COSMO protocol as a tool for use in the study of intramolecular charge transfer properties in mixed-valence complexes.

Refining the interpretation of near-infrared band shapes in a polyynediyl molecular wire.

Spinning to improve (band) shape: A blend of theoretical and experimental work demonstrates that the rotational conformation of mixed-valence complexes influences the low-energy (NIR) transitions in such molecules. Interpretations of the NIR band shapes are presented.

The preparation, characterisation and electronic structures of 2,4-pentadiynylnitrile (cyanobutadiynyl) complexes.

Convenient preparative routes to mononuclear ruthenium complexes containing the 2,4-pentadiynylnitrile, or cyanobutadiynyl, ligand are described. The electronic properties of the [C(5)N](-) ligand are closely related to those of not only the cyanide ([CN](-)) and 2-propynylnitrile or cyanoacetylide ([C≡CC≡N](-)) ligands, but also those of the isoelectronic polyynyl ([{C≡C}(n)R](-)) ligands.

Computational and spectroscopic studies of organic mixed-valence compounds: where is the charge?

This article discusses recent progress by a combination of spectroscopy and quantum-chemical calculations in classifying and characterizing organic mixed-valence systems in terms of their localized vs. delocalized character. A recently developed quantum-chemical protocol based on non-standard hybrid functionals and continuum solvent models is evaluated for an extended set of mixed-valence bis-triarylamine radical cations, augmented by unsymmetrical neutral triarylamine-perchlorotriphenylmethyl radicals. It turns out that the protocol is able to provide a successful assignment to class II or class III Robin-Day behavior and gives quite accurate ground- and excited-state properties for the radical cations. The limits of the protocol are probed by the anthracene-bridged system 8, where it is suspected that specific solute-solvent interactions are important and not covered by the continuum solvent model. Intervalence charge-transfer excitation energies for the neutral unsymmetrical radicals are systematically overestimated, but dipole moments and a number of other properties are obtained accurately by the protocol.