Synthesis and characterization of perylene diimide based molecular multilayers using CuAAC: towards panchromatic assemblies

Abstract

Three perylene diimides (PDI) were synthesized with propargyl groups at the diimide positions to enable the fabrication of molecular multilayer thin films via sequential copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) coupling reactions in a layer-by-layer (LbL) fashion. The new PDIs with phenoxy (PhO-PDI), dodecylthiol (Thiol-PDI), and pyrrolidine (Pyrr-PDI) substituents in bay positions have optical absorption spectra that span a large portion of the visible region. Multilayer growth on 11-azidoundecylsiloxane coated silica glass and indium tin oxide (ITO) surfaces was demonstrated up to 10 bilayers of each PDI with 1,3,5-tris(azidomethyl)-benzene (N₃Mest) via optical and infrared spectroscopy. Polarized UV-visible absorption measurements showed a strong dependence on the orientation of the film in the light path and was consistent with a preferential orientation of the PDI molecules in the film at 40–42° with respect to the substrate surface. Grazing-angle attenuated total reflectance (GATR) infrared spectra of the films show an increase in content of unreacted azides during multilayer growth from N₃Mest as a result of the smaller molecular footprint as compared to the PDI components. Electrochemical scans of the multilayer films grown on ITO exhibited reversible two-electron reduction waves for the PDIs with linear increases in charge with bilayer growth. The electrochemically derived surface coverages of PDIs averaged 1.2–1.3 × 10¹⁴ molecules per cm² for each layer added to the multilayer films. The flexibility of the CuAAC based LbL assembly methodology is highlighted by the assembly of a mixed multilayer film containing five layers of each PDI, resulting in a panchromatic film that is a summation of individual chromophores.