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 ).

Spherical polyoxometalates (POMs) such as M6O19 2and SiM12O40 4-(with M = Mo or W) and planar arene donors (anthracenes and pyrenes) can be cocrystallized (despite their structural incompatibility) by attaching a cationic "anchor" onto the arene which then clings to the POM anion by Coulombic forces. As a result, novel chargetransfer (CT) salts are prepared from arene donors and Lindqvist-type [M6O19] 2and Keggin-type [SiM12O40] 4acceptors with overall 2:1 and 4:1 stoichiometry, respectively. The CT character of the dark-colored (yellow to red) crystalline materials is confirmed by the linear Mulliken correlation between the CT transition energies and the reduction potentials of the POM acceptors, as well as by the transient (diffuse reflectance) absorption spectra (upon picosecond laser excitation) of anthracene or pyrene cation radicals (in monomeric and π-dimeric forms). X-ray crystallographic studies reveal a unique "dimeric" arrangement of the cofacially oriented arene couples which show contact points with the oxygen surface of the POMs that vary with distance, depending on the POM/arene combination. Moreover, the combination of X-ray crystallographic and spectroscopic techniques results in the observation of a logical structure/property relationship the shorter the distance between the POM surface and the arene nucleus, the darker is the color of the CT crystal and the faster is the decay of the laser-excited charge-transfer state (due to back-electron transfer).

Solid-state irradiation of the crystalline inclusion complex of (E)-stilbene in gamma-cyclodextrin (gamma-CD) yields a single isomer of syn-tetraphenylcyclobutane stereoselectively in high yield. In contrast, the photodimerization of stilbene in solution is very inefficient and unselective, and no photodimer is observed even upon prolonged irradiation of pure crystals. The monosubstituted stilbenes form a pair of photodimers stereoselectively, viz. the syn head-to-head and syn head-to-tail isomers, in comparable yields. The photodimer yields of about 70% and the biphasic decay kinetics of the excited stilbene (as established by picosecond time-resolved diffuse-reflectance spectroscopy) indicate that the stilbene guests are located in at least two distinct sites in the gamma-CD crystal lattice, i.e., a dimerization site where excited stilbene is in close reach of another stilbene guest molecule and an isomerization site where excited stilbene does not find a close neighbor for dimerization and thus undergoes trans --> cis isomerization only.

Photoexcitation of chloranil (CA) produces initially the excited singlet state 1 CA*, as demonstrated for the first time by time-resolved spectroscopy on the femtosecond/picosecond time scale. Electron transfer from aromatic donors (D) to singlet chloranil leads to short-lived (ca. 5 ps) singlet radical-ion pairs, 1[D•+, CA •-]. This ultrafast quenching process competes with intersystem crossing (k ISC ≈ 1011 s-1) to generate the triplet excited state, 3 CA*. The follow-up electron transfer from D to 3 CA* yields triplet radical-ion pairs, which are distinguished from their singlet analogues by their long (nanosecond) lifetimes. The competition between electron transfer and intersystem crossing on the early picosecond time scale also pertains to a wide variety of other photoexcited quinones related to chloranil. Electron transfer to singlet quinone as established here adds a new dimension to the generally accepted mechanisms which proceed from the triplet state, and the inclusion of reactions on both the triplet and the singlet manifolds provides a complete picture of photoinduced electron transfer to various quinone acceptors.