Controlling the Charge State and Redox Properties of Supported Polyoxometalates via Soft Landing of Mass-Selected Ions

We investigate the controlled deposition of Keggin polyoxometalate (POM) anions, PMo12O403 and PMo12O402, onto different self-assembled monolayer (SAM) surfaces via soft landing of mass-selected ions. Utilizing in situ infrared reflection absorption spectroscopy (IRRAS), ex situ cyclic voltammetry (CV), and electronic structure calculations, we examine the structure and charge retention of supported multiply charged POM anions and characterize the redox properties of the modified surfaces. SAMs of alkylthiol (HSAM), perfluorinated alkylthiol (FSAM), and alkylthiol terminated with NH3+ functional groups (NH(3)(+)SAM) are chosen as model substrates for soft landing to examine the factors that influence the immobilization and charge retention of multiply charged anionic molecules. The distribution of charge states of POMs on different SAM surfaces is determined by comparing the IRRAS spectra with vibrational spectra calculated using density functional theory. In contrast with the results obtained previously for multiply charged cations, soft-landed anions are found to retain charge on all three SAM surfaces. This charge retention is attributed to the substantial electron binding energy of the POM anions. Investigation of redox properties by CV reveals that while surfaces prepared by soft landing exhibit similar features to those prepared by adsorption of POM from solution, the soft-landed POM2 has a pronounced shift in oxidation potential compared with POM3 for one of the redox couples. These results demonstrate that ion soft landing is uniquely suited for precisely controlled preparation of substrates with specific electronic and chemical properties that cannot be achieved using conventional deposition techniques