Alkyl chain dynamics in monolayer-protected clusters (MPCs): a quasielastic neutron-scattering investigation

Abstract

Dynamics of alkyl chains in an isolated monolayer protected cluster (MPC), AuC₁₈ (C_n = n carbon n-alkanethiolate), and a superlattice of AgC₈ MPC have been investigated by quasielastic neutron scattering (QENS) technique in the temperature range of 300-400 K. The results have been compared with a layered silver thiolate, AgC₁₂, chosen to represent a planar two-dimensional monolayer. In the cluster superlattice, QENS broadening was observed even at room temperature and below the chain melting temperature (T_cm) whereas for the isolated cluster it was seen only above T_cm. In the layered silver thiolate, it was observed above the melting point. Data pertain to the rotational motions of the alkyl chains and can be best described by jump diffusion among N equivalent sites or continuous rotation about the molecular axis. At room temperature, the observed dynamics in AgC₈ correspond only to the noninterdigitized molecular chains which amount to about 50% of the total. Above 340 K, contribution from the interdigitized chains manifest and at 380 K, close to the superlattice melting, all the chains become dynamic. It is clearly established that the temperature required for the dynamics to manifest increases from the isolated cluster to the superlattice to the thiolate, suggesting a gradual increase in intermolecular interactions. Reorientation times for all these systems have been obtained at each temperature.