Soft-landing experiments of Cr(benzene)₂ sandwich complexes onto a carboxyl-terminated self-assembled monolayer matrix

Gas-phase synthesized Cr(benzene)₂ sandwich complexes were soft-landed onto a carboxyl-terminated selfassembled monolayer (C ₁₅COOH-SAM) on a gold substrate at hyperthermal collision energy (5-20 eV). The deposition process and adsorption properties of the soft-landed complexes were investigated by means of infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) measurements. With the soft-landing, the Cr(benzene)₂ complexes are adsorbed at the SAM surface while keeping their native sandwich structure intact even after the hyperthermal collision event. Moreover, the complexes tend to orient their molecular axes reasonably or comparatively close to the surface normal direction to the SAM surface. This orientation might arise from chemical interactions between the benzene ring in the sandwich complexes and carboxyl groups on the SAM surface. An Arrhenius analysis of the TPD spectra reveals the desorption activation energy of soft-landed complexes as 133 ± 10 kJ/mol, a value which is comparable to typical adsorption heat of chemisorptions. Chemical interactions between the Cr(benzene)₂ complexes and carboxyl groups would provide firm trapping of the complexes at the SAM surface, preventing the supported complexes from being desorbed until near room temperature.