Manipulation of Precise Molecular Arrangements and Their Photochemical Properties on Inorganic Surfaces via Multiple Electrostatic Interactions

Abstract

Multiple (or multivalent) interaction is the key in many biological systems. One of the most important (photo-)chemical reactions, photosynthesis, is driven by regularly aligned molecules by multiple interactions between proteins and molecules. A grand challenge of modern chemistry therefore includes the construction of supramolecular assemblies and control of their functions for mimicking nature and beyond. While most synthetic systems depend on covalent, coordination and hydrogen bonds between molecules, my approach focuses on multiple electrostatic interactions with two-dimensional clay mineral nanosheets. I here summarize my recent work on manipulation of precise molecular arrangements and photochemical properties via multiple electrostatic interactions. This Account mainly consists of the three parts; 1: manipulation of photochemical properties of molecules and new emission enhancement phenomenon (chapters 2–7), 2: efficient photochemical reactions and artificial photosynthesis model (chapters 8–14), and 3: molecular-scale understanding by means of electron microscopy (chapters 15–17).

Highly Emissive Organic Cage in Single-Molecule and Aggregate States by Anchoring Multiple Aggregation-Caused Quenching Dyes

“…Thus far, there are two pathways to construct this kind of molecule: one strategy is to lock AIE chromophore units within cages or networks, resulting in emissive materials by reducing phenyl rotation, but such a kind of AIE molecule is limited. The other strategy is using ACQ molecules as building blocks by incorporating bulky substituents into their scaffolds to avoid strong π–π stacking − or arranging the ACQ molecules with the help of templates, such as organic–inorganic hybrid materials, , dendritic polymers, − micelles, − organogels, − and biomaterials, − to avoid the self-quenching. However, it remains difficult to satisfy both the high local density and symmetrical distribution of chromophores with a high fluorescence quantum yield and high stability as well as good biocompatibility.…”

Mixed-Metal-Atom Markers Enable Simultaneous Imaging of Spatial Distribution in Two-Dimensional Heterogeneous Molecular Assembly by Scanning Transmission Electron Microscopy

“…The assembly of various molecules is a decisive idea in designing intermolecular chemical reactions, on-surface organic synthesis, and photochemical energy or electron transfers. 7,8,10,11,14 Therefore, identifying the relative arrangement of heterogeneous molecules in an assembly is crucial for a detailed understanding of the functions and chemical reactions occurring in the assembly (see the common distribution models in Figure S1 in the Supporting Information). 7,15,16 Herein, we demonstrate the first direct observation of a wellmixed spatial molecular distribution of two types of porphyrin complexes with Pt and Pd atom markers on relatively thick monolayer clay mineral nanosheet strongly anchored via multiple electrostatic interactions.…”

“…Note that the maximum absorption wavelength shifted to a longer wavelength upon adsorption on the clay mineral nanosheet (Figures S2 and S3) because of the planarization of the methylpyridinium substituent with respect to the porphyrin core, as reported elsewhere. 8 Before the ADF-STEM measurements, a multislice ADF-STEM image simulation was performed to evaluate the difference in ADF-STEM contrast between Pt and Pd markers on clay mineral nanosheets. An atomic model of Pt +PdTMPyP-clay (Figure 3a) was constructed to ensure that the nitrogen atoms of the methylpyridinium substitutes of the molecule and the oxygen atoms on the surface of the clay mineral nanosheets were in the closest proximity considering their van der Waals radii.…”

“…Atomic-scale observation by aberration-corrected (scanning) transmission electron microscopy ((S)­TEM) is essential for the characterization of nanomaterials with nonperiodic structures, such as supramolecular assemblies comprising organic molecules and inorganic substrates. − Understanding the behavior of molecules in nonperiodic supramolecular assemblies at the molecular scale, such as intermolecular distance, orientation, and molecular structural changes, is essential for a deeper understanding and control of the functions of materials. Beyond the averaged structural information generally obtained by spectroscopic, crystallographic, and nuclear magnetic resonance (NMR) techniques, − the direct imaging of individual molecules in supramolecular assemblies provides important guidelines for designing nanomaterials.…”

“…Recently, we successfully demonstrated the first annular dark-field (ADF) STEM observation of an assembly of organic molecules on monolayer montmorillonite-type clay mineral nanosheets anchored via multiple electrostatic interactions using Pt-coordinated porphyrin complexes, where the Pt atom worked as a marker . Montmorillonite, a typical clay mineral, is layered aluminosilicate with atomically flat surfaces and high surface charge densities in which Mg 2+ is isomorphically substituted with Al 3+ in the octahedral layer (Figure , right panel), which generates a Coulombic field to form stable complexes with cationic molecules. − The key factor for the stable observation of individual organic molecules in a supramolecular assembly is the strong fixation of molecules via multiple electrostatic interactions, which allows for the determination of the relative spatial arrangement of the molecules. Because the ADF-STEM contrast is proportional to the 1.4th–2nd power of the atomic number ( Z -contrast), various molecules should be simultaneously identifiable using a series of markers with different atomic numbers.…”