Uniaxial Alignment of a Monolayer of Flat-on Free-Base Porphyrins on an Exfoliable Insulating Substrate

Abstract

Porphyrins are an extremely valuable class of molecules engaged in a variety of roles spanning from biology to optoelectronics. Manipulation of the chemical and physical properties of the inner cavity of porphyrins has been recognized as crucial for the exploitation of these systems in organic devices, particularly when porphyrins self-organize at the interface with a flat-on orientation of the macrocycle. Such an orientation has been mostly observed on metallic surfaces. Unfortunately, the physical-chemical properties of the molecules result in being largely perturbed due to the molecule–metal interaction. In addition, conducting substrates are unsuited to exploit electrically driven devices based on organic layers. To overcome these issues, we performed a topology-based analysis of insulating organic single crystal structures to identify a surface which (i) ensures easy exfoliation through mechanical methods, (ii) ensures epitaxial match with an overlayer of close-packed flat-on porphyrin molecules, and (iii) displays chirality. The outcome of this work is represented by a unique crystal of mixed 2,5-diketopiperazine and fumaric acid in a 1:1 ratio. We demonstrate that the (110) surface of this crystal fulfills the aforementioned requirements and, thanks to its peculiar subnanometric corrugations, allows one to grow uniaxially aligned monolayers of flat-on porphyrin molecules assembled through van der Waals interactions.

Close-Packed Arrangements of Flat-On Free-Base Porphyrins Driven by van der Waals Epitaxy

“…At this thickness, we demonstrated that H 2 TPP (inset Figure A) reaches maximum coverage of its wetting layer, constituted by flat-on molecules with a frozen configuration of their tautomeric form and arranged in a square lattice (Figure S1). ,, The height of the steps on the surface is within the range 0.3–0.5 nm, consistent with the spacing of elemental steps of the substrates ( d 110 = 3.10 Å for DKP:FA and d 0001 = 3.35 Å for HOPG) or the thickness of the wetting layer (4.5–5.5 Å). For a nominal thickness higher that 2.0 Å, 3D islands start nucleating and growing up to a height of 5–10 nm.…”

“…Flat-on arrangements of porphyrins are routinely obtained on metal substrates such as Au, − Ag, − Cu, − and HOPG, but hardly on low interacting surfaces. Recently, we demonstrated the growth of a stable wetting layer of free-base tetraphenyl porphyrin (H 2 TPP; see inset of Figure ) in a flat-on configuration on an insulating substrate, constituted by the triclinic mixed crystal formed by 2,5-diketopiperazine (DKP) and fumaric acid (FA) in a 1:1 mole ratio (DKP:FA). , The difficulty of accomplishing this result lies in the absence in H 2 TPP of functional groups promoting strong interactions with the substrate and/or particular in-plane assemblies. , Under these conditions, the epitaxial match between substrate and overlayer was demonstrated to be the fundamental driving force enabling the controlled growth of the molecular monolayer. Here, we show by high-resolution atomic force microscopy (AFM) investigations that the flat-on arrangement is retained in crystalline domains of subsequent 3D structures and we disclose the presence of two other epitaxial molecular arrangements.…”

“…Cross-sectional profiles as taken along the horizontal scan line indicated by the dashed white segments are reported below each image. Panels A and D report the film morphology for nominal thicknesses corresponding to the formation of a wetting layer of H 2 TPP, , where elemental steps of the cleavage surface of DKP:FA ( d 110 = 3.10 Å) and HOPG ( d 0001 = 3.35 Å) can still be observed. For higher thickness, 3D islands with a height of 5–10 nm decorate the substrate surface.…”

Porphycene Protonation: A Fast and Reversible Reaction Enabling Optical Transduction for Acid Sensing

“…These are reasonably due to a 2D phase of the organic film, which represents a sort of wetting layer for the evolution of the crystals. [16,31] After a single exposure to hydrochloric vapors, the sample morphology is significantly different (see panel b): as expected, the 3D structures are transformed in organic molecules agglomerates. Considering that the intensity of the film optical spectrum is almost recovered after the first acid exposure (see Figure 5) while it is not the case for the morphology (see Figure 6), we deduce that SDR arises mainly from the 2D phase, in close agreement with the results acquired on porphyrin films.…”

“…It is also possible to observe false color changes on the substrate terraces. These are reasonably due to a 2D phase of the organic film, which represents a sort of wetting layer for the evolution of the crystals [16,31] . After a single exposure to hydrochloric vapors, the sample morphology is significantly different (see panel b ): as expected, the 3D structures are transformed in organic molecules agglomerates.…”