Electron Transfer in Films of Atomically Precise Gold Nanoclusters

In applications of monolayer-protected gold clusters, assessing the dynamic behavior of the capping monolayer is crucial, as this determines the cluster’s effective size and electron-transfer (ET) properties. Here, we describe a systematic study on the effect of the monolayer thickness on the ET between molecular Au25(SR)180 nanoclusters in films. The length of the ligands protecting the Au core was varied by using a series of linear-chain thiolate (SCnH2n+1) protected Au25 clusters, where n = 3, 4, 5, 6, 7, 8, and 10. The effect of branching was assessed by using the 2-methyl-1-butanethiolate-protected Au25 cluster. Conductivity measurements were carried out on dry films obtained by drop-casting Au25(SR)180 solutions onto interdigitated gold electrodes. Changing the alkyl chain from C3 to C10 induces a smooth decrease in the film conductivity by 3.5 orders of magnitude. By using electrochemically determined Stokes radii, the conductivity results were transformed into the corresponding ET rate constants (kETs) between neighboring clusters. kET exponentially depends on the distance, and the data show that the average Au center-to-center distance in the film is not larger than the sum of two Stokes radii. This indicates that linear alkyl chains hold a detectable degree of fluidity in films, although not as much as in solution, as shown by the trend of the corresponding heterogeneous standard rate-constant values. For both physical states, these conclusions are precisely confirmed by the much slower ET rate determined for the Au25 cluster protected by the hindered thiolate. ET in films was also studied as a function of temperature, and the results were analyzed in the framework of the ET theory. We found that the outer and inner reorganization energies increase with n. For these ETs, the electronic coupling resonance between the reactant and product states, HRP, ranges from 12 (n = 3) to 0.7 cm–1 (n = 10) and therefore ETs are nonadiabatic. The data also show that 3/4 of the effect of distance on kET is related to HRP, while the rest is associated with the intrinsic barrier and thus the distance dependence of the outer and inner reorganization energies.