Intramolecular Electron Transfer Reactions of Ion Pairs: Thermal, Optical, and Photochemical Pathways

KINETIC SALT EFFECTS IN THE PHOTOCHEMICAL REACTION BETWEEN* Ru(bpy)2/3+ Fe3+

“…Table 6 shows k,,, for the different sodium nitrate solutions. These activation-controlled rate constants are of the same order than those estimated for the process *Ru(bpy):+ + Fe(CN);-(1.5 10" mol-I s-' dm3) (Haim, 1985). This circumstance supports the validity of our procedure of estimating k,,,.…”

Spectroscopic and Electrochemical Probes of Electronic Coupling in Some Cyanide-Bridged Transition Metal Donor/Acceptor Complexes

“…Consequently, the electrochemical half-wave potentials are expected to vary systematically with ε s as in eq 21, , where is the half-wave potential (e.g., for the A/A - couple of a D/A pair) when there is no CT coupling and the sign of ε s is determined by which component of the particular couple (e.g., A or A - ) is stabilized by the CT coupling. In the limit that D/A coupling is dominated by “transfer” (or local dipole) terms (i.e., if there is significant direct overlap of the donor and acceptor orbitals of the ground state), a simple model for the transition dipole in an electronic absorption can be used to give the electronic matrix element as in eq 22, where is the distance (in Å) between the effective centers of charge of states e and g; ,,,,, ν max ε max and Δν 1/2 are the frequency and molar absorptivity of the absorption maximum and the full width at half-maximum of the corresponding CT absorption band. If significant charge is delocalized in the various CT mixings, as discussed above, then should differ from the equilibrium distance between the geometrical centers of donor and acceptor, r DA .…”

“…Such systems have been extensively studied experimentally − and theoretically, − ,,,− but a number of fundamental issues remain unresolved. Among these are various aspects of the electronic coupling between donors and acceptors in electron-transfer systems. ,− ,− ,− , We have recently been examining the effects of strong D/A electronic coupling on the properties of covalently linked transition metal (D/A) complexes. , The results of our observations have led us to consider the applicability of “vibronic” models in which the electronic coupling matrix element, H DA , is a function of some of the nuclear coordinates involved in the electron-transfer process. ,− ,...…”

“…Electron-transfer kinetics of D/A systems are usually analyzed in terms of the product of functions of an electronic factor (e.g., the electronic matrix element H DA ) and a nuclear reorganizational factor (λ r ); for example in the semiclassical limit the electron-transfer rate constant can be represented as in eq 1, in which κ i ( i = el for electronic; i = nu for nuclear) are transmission coefficients, and ν nu is the correlated nuclear frequency at the transition state. Very clearly, related parameters govern a D/A charge transfer (DACT) absorption: the absorption band maximum can be correlated with the transition-state energy, the oscillator strength is proportional to H DA 2 , , and the square of the bandwidth is correlated with λ r . , While the contributions of nuclear reorganizational factors to electron-transfer rates and to CT absorption bandwidths have been well documented, ,− ,− ,, the magnitudes and the origins of the contributions of electronic factors have often been difficult to define and difficult to directly investigate experimentally and the experimental observations have sometimes been subject to conflicting interpretations. − ,− ,,,− More specifically, it has been difficult to establish how (and, sometimes, whether) the components of the spatial region between donor and acceptor contribute to the magnitude of the electronic factor. − …”

Optical and Thermal Outer-Sphere Electron Self-Exchange Reaction of the Hexacyanoferrate(II/III) Couple: Comparative Analysis of Band-Shape and Activation Parameters and Large Solvent Kinetic Isotope Effect

“…Electron transfer theory establishes physical interrelations between thermal and optical electron transfer steps occurring under the participation of weakly coupled electron trapping centers. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The detection of both types of electron transfer processes in a given system under comparable experimental conditions promises further insight into the details of charge transfer mechanisms. Despite the success in separate investigations of thermal and optical electron transfer processes in various systems, involving inorganic or organometallic polynuclear complexes and ion pairs, the experimental documentation of both kinds of electron transitions in a given system is restricted to only a few cases due to experimental limitations.14•21 •22 The thermal processes corresponding to the best characterized optical transitions10-12•16-18•23 are in most cases either extremely rapid or thermodynamically unfavorable.…”