Molecular Tetris by sequence-specific stacking of hydrogen bonding molecular clips

Click To Stack: Shape-Assisted Self-Assembly of Unflattened Macrocycles into Waveguiding Elastic Crystals

Designing molecular crystals for practical applications requires precise control over intermolecular interactions. We report a click-to-stack strategy to enable facile construction and systematic structural modification of tetraarene-fused cyclooctatetraenes. These saddle-shaped molecules self-assemble into one-dimensional columns with exceptional precision, driven solely by weak van der Waals interactions. Single-crystal X-ray crystallographic and computational studies revealed fully eclipsed π-π stacking, reinforced by the shape complementarity of the negatively curved molecular contour. With alkoxy groups tethered, the antiparallel alignment of coaxial cable-like assemblies produces centimeter-long voidless crystals that exhibit remarkable structural durability under mechanical stress. These glow stick-like elastic crystalline rods display optical waveguiding properties, demonstrating the practical utility of highly anisotropic molecular assemblies with structural uniformity and resilience.

Mutually antagonistic molecular clips: symmetry-breaking non-covalent bonds at the chiral-nonchiral interface

The homochirality of life remains an unresolved scientific question. Prevailing models postulate that homochirality arose through mutual antagonism. In this mechanism, molecules of opposite handedness deactivate each other, amplifying even a small enantiomeric excess into a larger proportion. In this paper, we present chiral molecular clips that replicate this process. Through π-π stacking and complementary hydrogen bonds, shape-persistent clips of opposite chirality bind to each other more strongly than those of the same chirality, resulting in chiral amplification. This process was studied quantitatively, revealing a remarkably high degree (180-fold) of stereoselection, unmatched by any non-covalent assemblies reported to date. We demonstrate how this symmetry-breaking, in conjunction with the chiral composition of the host, impacts the binding of nonchiral molecules. Our findings illustrate how chirality transfer and amplification occur non-covalently from hosts to guests, offering insights into the evolutionary origins of homochirality in life's molecular building blocks.

Self-Assembled Bis-Acridinium Tweezer Equilibria Controlled by Multi-Responsive Properties

Protonated and methylated bis-acridinium tweezers built around a 2,6-diphenylpyridyl and an electron enriched 2,6-di(p-anisyl)pyridyl spacer have been synthesized. These tweezers can self-assemble in their corresponding homodimers and the associated thermodynamic parameters have been probed in organic solvents. The switching properties of the tweezers have been exploited in biphasic transfer experiments showing the shift of the equilibria towards the homodimers. Moreover, the thermodynamic parameters of the formation of the reduced methylated homodimers investigated by electrochemical experiments revealed the dissociation of the dimers. Thus, in addition to solvent and temperature, the pH and redox responsiveness of the acridinium units of the tweezers make it possible to modulate to a larger extent the monomer-dimer equilibria.

Silapillar[n]arenes: Their Enhanced Electronic Conjugation and Conformational Versatility

During recent decades, methylene-bridged macrocyclic arenes have been widely used in supramolecular chemistry. However, their π-conjugations are very weak, as the methylene bridges disrupt the electronic communication between π orbitals of the aromatic units. Herein, we successfully synthesized a series of silapillar[n]arenes (n = 4, 6, and 8) using silylene bridging. These showed enhanced electronic conjugation compared with the parent pillar[n]arenes because of σ*-π* conjugation between σ* (Si-C) orbitals and π* orbitals of the benzenes. Owing to the longer Si-C bond compared with the C-C bond, silylene-bridging provides additional structural flexibility into the pillar[n]arene scaffolds; a strained silapillar[4]arene was formed, which is unavailable in the parent pillar[n]arenes because of the steric requirements. Furthermore, silapillar[n]arenes displayed interesting size-dependent structural and optical properties.

Small is Beautiful: Electronic Origin and Synthetic Evolution of Single-Benzene Fluorophores

ConspectusSingle-benzene fluorophores (SBFs) are small molecules that produce visible light by using only one benzene ring as the sole aromatic core. This Account centers around the chemistry of a new class of SBF that we accidentally discovered but rationally developed and refined afterward. In a failed experiment that took an unintended reaction pathway, we encountered the bright green fluorescence of ortho-diacetylphenylenediamine (o-DAPA). Despite its uninspiring look, reminiscent of textbook examples of simple benzene derivatives, this molecule had neither been synthesized nor isolated before. This discovery led to our studies on the larger DAPA family, including isomeric m-DAPA and p-DAPA. Remarkably, p-DAPA is the lightest red fluorophore, with a molecular weight of only 192. While o- and p-DAPA are emissive, m-DAPA rapidly undergoes internal conversion, facilitated by sequential proton transfer reactions in the excited state.Leveraging the synthetic utility of the amine group, we carried out straightforward single-step modifications to create a full-color SBF library from p-DAPA as the common precursor. During the course of the investigation, we made another fortuitous discovery. With increasing acidity of the N-H group, the excited-state intramolecular proton transfer reaction is promoted, opening up additional pathways for emission to occur at even longer wavelengths. Tipping the balance between the two excited-state tautomers enabled the first example of a single-benzene white-light emitter. We demonstrated the practical utility of these molecules in white light-emitting devices and live cell imaging.According to the particle-in-a-box model, it is difficult to expect a molecule with only one small aromatic ring to produce long-wavelength emission. SBFs rise to this challenge by exploiting electron donor-acceptor pairs around the benzene core, which lowers the energy of light absorption. However, this answers only half of the question. Where do the exceptionally large spectral shifts in the light emission of SBFs originate from? Chemists have long been curious about the molecular mechanisms underlying the dramatic spectral shifts observed in SBFs. Prevailing paradigms invoke the charge transfer (CT) between electron donor and acceptor groups in the excited state. However, without a large π-skeleton for effective charge separation, how could benzene support a CT-type excited state? Our experimental and theoretical studies have revealed that large excited-state antiaromaticity (ESAA) of the benzene core itself is responsible for this remarkable phenomenon. The core matters, not the periphery. With appropriate molecular design, large and extended π-conjugation is no longer a prerequisite for long-wavelength light emission.

Assembling Molecular Clips To Build π-Stacks

Nature utilizes an intimate stacking of aromatic motifs to construct functional structures, as demonstrated in protein folding and polynucleotide assembly. However, organized π-stacks of artificial molecules are difficult to build, primarily due to the weak, non-directional, and context-sensitive nature of van der Waals forces. To overcome these challenges, chemists have invented ingenious architectural designs to construct π-stacked supramolecular assemblies using clip-like molecules. This Concept article focuses on molecular clips that enable precise spatial control over assembly patterns, beyond the scope of simple host-guest chemistry. Different design strategies are analyzed and compared that leverage non-covalent interactions to create multi-layer π-stacks. Particular emphasis is placed on the choice of spine units as they play a crucial role in controlling the (i) spacing, (ii) orientation, and (iii) conformational pre-organization of linked aromatics to achieve long-range spatial ordering.