Avoiding the Center-Symmetry Trap: Programmed Assembly of Dipolar Precursors into Porous, Crystalline Molecular Thin Films

Liquid-phase, quasi-epitaxial growth is used to stack asymmetric, dipolar organic compounds on inorganic substrates, permitting porous, crystalline molecular materials that lack inversion symmetry. This allows material fabrication with built-in electric fields. A new programmed assembly strategy based on metal-organic frameworks (MOFs) is described that facilitates crystalline, noncentrosymmetric space groups for achiral compounds. Electric fields are integrated into crystalline, porous thin films with an orientation normal to the substrate. Changes in electrostatic potential are detected via core-level shifts of marker atoms on the MOF thin films and agree with theoretical results. The integration of built-in electric fields into organic, crystalline, and porous materials creates possibilities for band structure engineering to control the alignment of electronic levels in organic molecules. Built-in electric fields may also be used to tune the transfer of charges from donors loaded via programmed assembly into MOF pores. Applications include organic electronics, photonics, and nonlinear optics, since the absence of inversion symmetry results in a clear second-harmonic generation signal.

Automated summary

The liquid-phase, quasi-epitaxial growth method is used to stack asymmetric organic compounds on inorganic substrates to create porous crystalline molecular materials without inversion symmetry, allowing for the integration of built-in electric fields. A new programmed assembly strategy based on metal-organic frameworks (MOFs) is described to facilitate the formation of crystalline, noncentrosymmetric space groups for achiral compounds. Integration of built-in electric fields into organic, crystalline, and porous materials offers possibilities for band structure engineering and tuning charge transfer for applications in organic electronics, photonics, and nonlinear optics.