DISTANCE DEPENDENCE OF THE LOW-TEMPERATURE ELECTRON-TRANSFER KINETICSOF (FERROCENYLCARBOXY)-TERMINATED ALKANETHIOL MONOLAYERS

Results are presented for rate constants (k(0)) and reorganizational e nergy barriers (lambda) for interfacial electron transfer at ultralow- temperatures (120-150 K) across mixed CpFeCpCO(2)(CH2)(n)SH/CH3(CH2)n- 1SH monolayers (n = 8, 12, 16). The monolayers are kinetically dispers e, i.e., the ferrocene sites exhibit a range of rate constants. Averag e values of k(0) were measured by cyclic voltammetry with application of Marcus theory corrected for the density of electronic states in the gold electrode. The k(0) and pre-exponential (mu rho) values exhibit exponential dependencies on alkane chain length characterized by expon ential coefficients of 1.06 and 1.44/CH2, respectively. The former val ue agrees with aqueous phase results by others for analogous but more highly ordered monolayers near ambient temperatures; the latter result corresponds to an electronic coupling coefficient of beta(EL) of 1.1 Angstrom(-1). The activation analysis-derived reorganizational barrier energies decrease somewhat with increasing chain length, contrary to theoretical expectations.