Electronic conductivity of solid-state, mixed-valent, monolayer-protected Au clusters

Electronic conductivity, sigma (EL), in solid-state films of alkanethiolate monolayer protected Au clusters (Au MPCs) occurs by a bimolecular, electron self-exchange reaction, whose rate constant is controlled by (a) the core-to-core tunneling of electronic charge along alkanethiolate chains and (b) the mixed valency of the MPC cores (e.g., a mixture of cores with different electronic charges). The tunneling mechanism is demonstrated by an exponential relation between the electronic conductivity of Au-309(C-n)(92) MPCs (average composition) and n, the alkanethiolate chainlength, which varies from 4 to 16. The electron tunneling coefficient beta (n) = 1.2/CH2 or, after accounting for alkanethiolate chain interdigitation, beta (dis) = 0.8 Angstrom (-1). Quantized electrochemical double layer charging of low polydispersity Au-140(C6)(53) MPCs was used to prepare solutions containing well-defined mixtures of MPC core electronic charges (such as MPC0 mixed with MPC1+). Electronic conductivities of mixed-valent, solid-state Au-140(C6)(53) MPC films cast from such solutions are proportional to the concentration product [MPC0][MPC1+], and give a MPC0/1+ electron self-exchange rate constant of ca. 10(10) M-1 s(-1).