No abstract

Unusual strength and directionality for the charge-transfer motif (established in solution) are shown to carry over into the solid state by the facile synthesis of a series of robust crystals of the [1:1] donor/acceptor complexes of carbon tetrabromide with the electron-rich halide anions (chloride, bromide, and iodide). X-ray crystallographic analyses identify the consistent formation of diamondoid networks, the dimensionality of which is dictated by the size of the tetraalkylammonium counterion. For the tetraethylammonium bromide/carbon tetrabromide dyad, the three-dimensional (diamondoid) network consists of donor (bromide) and acceptor (CBr4) nodes alternately populated to result in the effective annihilation of centers of symmetry in agreement with the sphaleroid structural subclass. Such inherently acentric networks exhibit intensive nonlinear optical properties in which the second harmonics generation in the extended charge-transfer system is augmented by the effective electronic (HOMO−LUMO) coupling between contiguous CBr4/halide centers.

The various aromatic hydrocarbons (Chart ) constitute a sharply graded series of sterically encumbered (unhindered, partially hindered, and heavily hindered) donors in electron transfer (ET) to quinones (Chart ). As such, steric effects provide the quantitative basis to modulate (and differentiate) outer-sphere and inner-sphere pathways provided by matched pairs of hindered and unhindered donors with otherwise identical electron-transfer properties. Thus the hindered donors are characterized by (a) bimolecular rate constants (k 2) that are temperature dependent and well correlated by Marcus theory, (b) no evidence for the formation of (discrete) encounter complexes, (c) high dependency on solvent polarity, and (d) enhanced sensitivity to kinetic salt effectsall diagnostic of outer-sphere electron-transfer mechanisms. Contrastingly, the analogous unhindered donors are characterized by (a) temperature-independent rate constants (k 2) that are 102 times faster and rather poorly correlated by Marcus theory, (b) weak dependency on solvent polarity, and (c) low sensitivity to kinetic salt effectsall symptomatic of inner-sphere ET mechanisms arising from the preequilibrium formation of encounter complexes with charge-transfer (inner-sphere) character. Steric encumbrances which inhibit strong electronic (charge-transfer) coupling between the benzenoid and quinonoid π systems are critical for the mechanistic changeover. Thus, the classical outer-sphere/inner-sphere distinction (historically based on coordination complexes) is retained in a modified form to provide a common terminology for inorganic as well as organic (and biochemical) redox systems.

Time-resolved (fs) spectroscopy allows the direct observation of charge-transfer ion pairs resulting from the photoexcitation of the electron donor−acceptor (EDA) complexes of tetracyanoethylene with various olefin donors, i.e., [olefin, TCNE], in dichloromethane solutions. Measurement of the spectral decays yields first-order rate constants for electron transfer (k ET) in the collapse of the charge-transfer ion pairs [olefin•+ , TCNE•-] by very rapid return to the ground-state EDA complex at 25 oC. [These ultrafast ET rates necessitated the design/construction of a new tunable, high-power pump−probe spectrometer based on a Ti:sapphire laser with 250-fs resolution.] The value of k ET = 5 × 1011 s-1 is strikingly nonvariant for the different TCNE complexes despite large differences in the driving force for electron transfer (ΔG 0), as evaluated from the varying ionization potentials of the olefins. Such a unique nonvariant trend for the free energy relationship (log k ET versus ΔG 0) is analyzed in terms of a dominant inner-sphere component to electron transfer. In a more general context, the inner-sphere (adiabatic) electron transfer in [olefin•+, TCNE•-] relates to a similar, but less pronounced, inner-sphere behavior noted in the analogous [arene•+, TCNE•-] radical-ion pairs. As such, these electron-transfer processes represent an extremum in the continuum of ET transition states based on the inner-sphere/outer-sphere dichotomy.

The encounter complex between photoexcited quinones Q* and various aromatic donors (ArH) is observed directly by time-resolved ps spectroscopy immediately before it undergoes electron transfer to the ion-radical pair [Q•-, ArH•+]. The encounter complex (EC) is spectrally characterized by distinctive (near IR) absorption bands, and its temporal evolution is established by quantitative kinetics analysis. The structural characterization of the 1:1 encounter complex [Q*, ArH] identifies the cofacial juxtaposition of the donor and acceptor moieties for optimal overlap of their π-orbitals. Further comparisons of the (excited-state) encounter complex with the corresponding (ground-state) EDA complex of aromatic donors and quinones establish its charge-transfer character, which directly relates to electron transfer within the encounter complex. The mechanistic significance of the encounter complex to bimolecular electron transfer is discussed (Scheme ).