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

Results are presented for rate constants (k(0)) and reorganizational energy 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 disperse, i.e., the ferrocene sites exhibit a range of rate constants. Average 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 exponential coefficients of 1.06 and 1.44/CH2, respectively. The former value 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.