Synthesis of trans-A(2)B(2)- and trans-A(2)BC-Porphyrins with Polar 4 '-(Dimethylamino)tolan-4-yl Substituents, and a Screening Protocol for Vapor-Phase Deposition on Metal Surfaces

The role of polar 4-[p-(dimethylamino) phenylethynyl] phenyl substituents, with a calculated dipole moment of 3.35 Debye, in the self-assembly of trans-A(2)B(2)- and A(2)BC-substituted porphyrins was explored in the solid state by X-ray crystallography, and on an Au(111) surface by scanning tunneling microscopy (STM). Our results demonstrate that the dipolar character of these substituents blocks the 2D self-assembly of porphyrins into larger ordered domains on Au(111) at low coverage, whereas antiparallel dipole-dipole interactions govern the molecular ordering in the crystal. The STM analysis revealed an adaptation of the conformation of the prochiral building blocks and a site-selectivity of the adsorption. We present a general protocol for testing the suitability of higher-molecular-weight compounds, such as porphyrins, to be deposited on surface by sublimation in ultra-high vacuum (UHV). This protocol combines classical methods of chemical analysis with typical surface science techniques.