Understanding the fundamental reactivity of polymetallic complexes is challenging due to the complexity of their structures with many possible bond breaking and forming processes. Here, we apply ion mobility mass spectrometry coupled with density functional theory to investigate the disassembly mechanisms and energetics of a family of heterometallic rings and rotaxanes with the general formula [NH 2 RR'][Cr 7 MF 8 (O 2 C t Bu) 16 ] with
In this tutorial review, we present an introduction to structural characterisation techniques commonly used for non-crystalline supramolecular compounds and discuss their application based on recent case studies.
Molecular aggregates with long-range excitonic couplings have drastically different photophysical properties compared to their monomer counterparts. From Kasha's model for one-dimensional systems, positive or negative excitonic couplings lead to blue or red-shifted optical spectra with respect to the monomers, labeled H-and J-aggregates, respectively. The overall excitonic couplings in higher dimensional systems are much more complicated and cannot be simply classified from their spectral shifts alone. Here, we provide a unified classification for extended 2D aggregates using temperature dependent peak shifts, thermal broadening, and quantum yields. We discuss the examples of six 2D aggregates with J-like absorption spectra but quite drastic changes in quantum yields and superradiance. We find the origin of the differences is, in fact, a different excitonic band structure where the bright state is lower energy than the monomer but still away from the band edge. We call this an “I-aggregate.” Our results provide a description of the complex excitonic behaviors that cannot be explained solely on Kasha's model. Furthermore, such properties can be tuned with the packing geometries within the aggregates providing supramolecular pathways for controlling them. This will allow for precise optimizations of aggregate properties in their applications across the areas of optoelectronics, photonics, excitonic energy transfer, and shortwave infrared technologies.
Specific
molecular arrangements within H-/J-aggregates of cyanine
dyes enable extraordinary photophysical properties, including long-range
exciton delocalization, extreme blue/red shifts, and excitonic superradiance.
Despite extensive literature on cyanine aggregates, design principles
that drive the self-assembly to a preferred H- or J-aggregated state
are unknown. We tune the thermodynamics of self-assembly via independent
control of the solvent/nonsolvent ratio, ionic strength, or dye concentration,
obtaining a broad range of conditions that predictably stabilize the
monomer (H-/J-aggregate). Diffusion-ordered spectroscopy, cryo-electron
microscopy, and atomic force microscopy together reveal a dynamic
equilibrium between monomers, H-aggregated dimers, and extended J-aggregated
2D monolayers. We construct a model that predicts the equilibrium
composition for a range of standard Gibbs free energies, providing
a vast aggregation space which we access using the aforementioned
solvation factors. We demonstrate the universality of this approach
among several sheet-forming cyanine dyes with tunable absorptions
spanning visible, near, and shortwave infrared wavelengths.
Following electrospray ionization, it is common for analytes to enter the gas phase accompanied by a charge-carrying ion, and in most cases, this addition is required to enable detection in the mass spectrometer. These small charge carriers may not be influential in solution but can markedly tune the analyte properties in the gas phase. Therefore, measuring their relative influence on the target molecule can assist our understanding of the structure and stability of the analyte. As the formed adducts are usually distinguishable by their mass, differences in the behavior of the analyte resulting from these added species (e.g., structure, stability, and conformational dynamics) can be easily extracted. Here, we use ion mobility mass spectrometry, supported by density functional theory, to investigate how charge carriers (H + , Na + , K + , and Cs + ) as well as water influence the disassembly, stability, and conformational landscape of the homometallic ring [Cr 8 F 8 (O 2 C t Bu) 16 ] and the heterometallic rotaxanes [NH 2 RR′][Cr 7 MF 8 (O 2 C t Bu) 16 ], where
A series of aromatic
Schiff bases, featuring 7-diethylamino-coumarin
and with five different substituents at an adjacent phenyl ring, were
synthesized and characterized. With the aim of assessing the stability
of these dyes in acidic medium, their hydrolysis reactions were kinetically
studied in the absence and presence of the macrocycle cucurbit[7]uril
(CB[7]). Our results are consistent with a model containing three
different forms of substrates (un-, mono-, and diprotonated) and three
parallel reaction pathways. The p
K
a
values
and the rate constants were estimated and discussed in terms of the
presence of a hydroxyl group at the ortho position and electron-releasing
groups on the phenyl ring of the dyes. The kinetic study in the presence
of CB[7] led to two different behaviors. Promotion of the reaction
by CB[7] was observed for the hydrolysis of the Schiff bases containing
only one coordination site toward the macrocycle. Conversely, an inhibitor
effect was observed for the hydrolysis of a Schiff base with two coordination
sites toward CB[7]. The latter effect could be explained with a model
as a function of a prototropic tautomeric equilibrium and the formation
of a 2:1 host/guest complex, which prevents the attack of water. Therefore,
the kinetic results demonstrated a supramolecular control of the macrocycle
toward the reactivity and stability of 7-diethylaminocoumarin Schiff
bases in acidic medium.
Five new lanthanide(III) sandwich and half‐sandwich complexes with bulky cyclooctatetraenyl ligands have been prepared and fully characterized, including single‐crystal X‐ray structure determination. The treatment of anhydrous lanthanide(III) chlorides, LnCl3 (Ln = Pr, Tb, Yb), with 2 equivalents of Li2COT′′ [COT′′ = 1,4‐bis(trimethylsilyl)cyclooctatetraene dianion] followed by crystallization in the presence of a coordinating solvent afforded the anionic sandwich complexes [Li(THF)4][Pr(COT′′)2] (1), [Li(DME)3][Tb(COT′′)2] (2; DME = 1,2‐dimethoxyethane), and [Li(TMEDA)2][Yb(COT′′)2] (3; TMEDA = N,N,N′,N′‐tetramethylethylenediamine). Attempted oxidation of 2 with silver iodide did not afford the neutral terbium(IV) sandwich complex [Tb(COT′′)2]. Instead, the tri(µ‐iodido)‐bridged dinuclear half‐sandwich complex [Li(DME)2][Tb2(µ‐I)3(COT′′)2] (4) was isolated in 72 % yield. In this complex, the Li(DME)2 fragment is attached to one of the µ‐iodido ligands. A closely related binuclear lutetium complex, [Li(DME)3][Lu2(µ‐Cl)3(COTbig)2]·DME (5), was obtained by using the “superbulky“ COTbig ligand [COTbig = 1,4‐bis(triphenylsilyl)cyclooctatetraenyl dianion]. In addition to the spectroscopic and structural characterization, the magnetic properties of the paramagnetic terbium(III) derivative 2 have been investigated and further complemented by ab initio computational methods. These results have been used to discuss the structural requirements for potential terbium(III) single‐ion magnets.
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