Molecular conformation and electronic properties of Protoporphyrin-IX self assembled monolayers adsorbed on a Pt(111) surface

Monolayers of protoporphyrinIX molecules are prepared on a Pt(111) surface by self-assembly process in order to manufacture organic devices with controlled electronic properties. Scanning Tunneling Microscopy (STM) and Two-Colour Sum-Frequency Generation (2C-SFG) are performed ex situ at ambient air, in order to characterize their molecular conformation and electronic properties at the monolayer level, respectively. STM measurements performed with functionalized gold tips reveal a high covering rate of the metal surface. 2C-SFG measurements highlight CH stretching modes of vinyl substituted groups (R-CH=CH2) in the 2800-3200 cm-1 infrared spectral range and particular electronic features in the visible spectral range, i.e. Soret band red shift and band separation compared to the liquid phase. Moreover, similar measurements are performed on Zn(II)ProtoporphyrinIX and 5-[p-(6-mercaptohexoxy)-phenyl]-10,15,20-triphenylporphin films for comparison. All these results suggest a film conformation with the molecules having different tilt angles with respect to the substrate normal, depending on the ion metal presence or the chain length bonded to the porphyrin moiety.