Fluorescence Enhancement by Supramolecular Sequestration of a C54-Nanographene Trisimide by Hexabenzocoronene

C 3-symmetric aromatic triimides as useful building blocks for supramolecular chemistry and advanced materials

C 3-symmetric aromatic triimides (ATIs) represent an interesting class of electron-deficient molecular backbones, characterized by π-conjugated aromatic cores symmetrically substituted with three imide groups. Their unique structural and electronic features, combined with exceptional stability and straightforward N-functionalization at three imide sites, make ATIs promising as versatile building blocks for applications in supramolecular chemistry and materials science. Despite their promising attributes, research on ATIs remains underexplored compared to the extensively studied aromatic imides and diimides, largely due to the synthetic challenges involved in constructing three imide groups on the aromatic frameworks. In recent years, advances in synthetic protocols have significantly propelled ATI-related chemistry, expanding their applications in the realm of functional materials. This review compiles advancements in the synthesis, structural and physicochemical properties, and applications of ATIs in supramolecular chemistry and advanced materials. We anticipate that this review will not only draw greater attention to ATIs but also serve as a valuable guide for future research into these molecular building blocks.

Controlled Self-assembly of Nanographdiynes Mediated by Molecular Dipoles Induced by Rotatory Asymmetric Substituents

The discrete π- stacks of specific lengths and orientation is crucial for understanding the impact of intermolecular interactions on optical or electronic properties of nanographdiynes. We designed and synthesized nanographdiynes modified with bulky rotatable asymmetric substituents. The peripheral substituents with different push-pull electronic properties can induce molecular dipoles perpendicular to nanoGDY π surface with different orientation. Mediated by directional dipole-dipole interactions, o-TBGDY substituted with electron-donating tert-butylphenyl groups cofacially dimerized with a twist angle, involving intermolecular interlocking of ortho-substituted tert-butylphenyl groups. In contrast, OTFGDY substituted with electron-withdrawing trifluorophenyl groups formed dimeric H-aggregates with an in-plane shift along one diyne linkage, in which the ortho-substituted trifluorophenyl groups point toward the outside of the interacting π-plane. o-TBGDY can only exist as isolated H-dimer in the solid state, while OTFGDY can form a hybrid tetramer of H- and J-aggregates. Our researches reveal a new method for preparing discrete π-stacked dye assemblies with well-defined optoelectronic properties.

Photofunctional cyclophane host-guest systems

Modulation of optical properties through smart protein matrices is exemplified by a few examples in nature such as rhodopsin (absorption wavelength tuning) and the green fluorescence protein (emission), but in general, the scope found in nature for the matrix-controlled photofunctions remains rather limited. In this review, we present cyclophane-based supramolecular host-guest complexes for which electronic interactions between the cyclophane host and mostly planar aromatic guest molecules can actively modulate excited-state properties in a more advanced way involving both singlet and triplet excited states. We begin by highlighting photofunctional host-guest systems for on-off fluorescence switching and chiroptical functions using bay-functionalized perylene bisimide cyclophanes. Next, we examine the impact of π-extension in perylene bisimide cyclophanes for multiple guest binding, showcasing photofunctional properties including circularly polarized luminescence (CPL). We then focus on triplet-generating cyclophanes, i.e. coronene bisimide cyclophane, with high intersystem crossing (ISC) rates, where we demonstrate modulation of excited state pathways upon guest encapsulation and triplet sensitization through phosphorescence and thermally activated delayed fluorescence (TADF). Furthermore, using supramolecular strategies, we advance non-covalent designs, involving either heavy-atom-based Pt(acac)2 guests or heavy-atom free charge transfer complexes, for triplet harvesting under ambient conditions and demonstrate the role of supramolecular nanoenvironments in stabilizing triplet excitons in aerated solutions. Additionally, we showcase examples for triplet-triplet annihilation (TTA) upconversion in defined cyclophane complexes in aqueous solutions and the application of host-guest chemistry in organic light-emitting diodes (OLEDs).

Tetraphenylethylene-Derived Tetracarboxylate Featuring AIE Properties for Dual Ion Sensing and Mechanochromic Self-Erasable Writing

The integration of multiple functions within a single fluorescent molecule provides a promising platform for developing versatile, efficient, and cost-effective materials with enhanced performance across diverse applications. In this study, we introduce TPEC, an aggregation-induced emission (AIE) molecule derived from tetraphenylethylene-based tetracarboxylate, which demonstrates multifunctional capabilities, including metal ion sensing and self-erasable writing. TPEC exhibits amphiphilicity in water, self-assembling into single-layer nanosheets with robust blue fluorescence. Notably, the aqueous solution of TPEC displays a fluorescence colorimetric response to Al3+ ions and fluorescence quenching in the presence of Fe3+ ions. Additionally, TPEC powders undergo fluorescence colorimetric changes under mechanical stimulation, enabling self-erasable writing on prepared paper. This study presents a straightforward strategy for the development of multifunctional luminescent materials based on the self-assembly of a single-component fluorophore.

Ternary π-π Stacking Complexes by Allosteric Regulation in Multilayer Nanographenes

Construction of π-π stacking supramolecular complexes with more than two different components is challenging due to the weak and directionless nature of dispersion interactions. Here, we report ternary complexes of a ditopic nanographene tetraimide (1), α-substituted phthalocyanine (Pc), and polyaromatic hydrocarbons (PAHs) in solution and the crystalline state via allosteric regulation. Binding of one Pc gives rise to significant distortion and conformational changes in 1 that in turn lead to the inhibition of the second binding of Pc. The conformational changes associated with first binding allowed an allosteric binding of a third component (PAHs) to form ternary complexes in solution. 1H NMR titration revealed moderately high thermodynamic stability for the ternary complexes in CDCl3. Competition between allosterically regulated ternary complexes ([Pc·1·PAH]) and 1:2 stoichiometric binary complexes of 1 with PAHs ([PAH·1·PAH]) was elucidated. Further, the selective formation of ternary complexes in solution led to the generation of ternary cocrystals from a 1:1:1 mixture of three components in solution. Our work shows that large π-conjugated nanographenes designed with allosteric recognition sites allow the construction of multilayer ternary complexes in solution and the solid state even with dispersive π-π interactions.

Nonplanar Nanographene with a Large Conjugated π-Surface

Conjugated π-surfaces are ubiquitous in molecules and materials. However, large π-surfaces up to a few nanometers in size are difficult to construct in an atomically precise manner. They tend to aggregate because of strong π-π interactions, resulting in notorious problems for both purification and spectroscopic investigations. Here, by contrast, we report the design, synthesis, and full characterizations of a nonplanar nanographene 1, which has a large, precise, and nonstacked π-surface. It is soluble in common organic solvents and allows for thorough investigations. The structure of 1, comprising 85 fused rings with an extended π-surface of 3 nm in size, is unambiguously confirmed by single-crystal X-ray diffraction. Unusual electronic structures, record-high near-infrared absorption, pronounced magnetic shielding, and ultrastrong heteromolecular van der Waals complexations are demonstrated, enabling us to establish a clear structure-property relationship, which has been elusive for decades. These results have broad implications for studying and understanding various phenomena and processes relevant to both discrete and interacting π-surfaces.

Room-Temperature Near-Infrared Phosphorescence from C64 Nanographene Tetraimide by π-Stacking Complexation with Platinum Porphyrin

Near-Infrared (NIR) phosphorescence at room temperature is challenging to achieve for organic molecules due to negligible spin-orbit coupling and a low energy gap leading to fast non-radiative transitions. Here, we show a supramolecular host-guest strategy to harvest the energy from the low-lying triplet state of C64 nanographene tetraimide 1. 1H NMR and X-ray analysis confirmed the 1 : 2 stoichiometric binding of a Pt(II) porphyrin on the two π-surfaces of 1. While the free 1 does not show emission in the NIR, the host-guest complex solution shows NIR phosphorescence at 77 K. Further, between 860-1100 nm, room temperature NIR phosphorescence (λmax=900 nm, τavg=142 μs) was observed for a solid-state sample drop-casted from a preformed complex in solution. Theoretical calculations reveal a non-zero spin-orbit coupling between isoenergetic S1 and T3 of π-stacked [1 ⋅ Pt(II) porphyrin] complex. External heavy-atom-induced spin-orbit coupling along with rigidification and protection from oxygen in the solid-state promotes both the intersystem crossing from the first excited singlet state into the triplet manifold and the NIR phosphorescence from the lowest triplet state of 1.

Cooperative Binding and Chirogenesis in an Expanded Perylene Bisimide Cyclophane

The encapsulation of more than one guest molecule into a synthetic cavity is a highly desirable yet a highly challenging task to achieve for neutral supramolecular hosts in organic media. Herein, we report a neutral perylene bisimide cyclophane, which has a tailored chiral cavity with an interchromophoric distance of 11.2 Å, capable of binding two aromatic guests in a π-stacked fashion. Detailed host-guest binding studies with a series of aromatic guests revealed that the encapsulation of the second guest in this cyclophane is notably more favored than the first one. Accordingly, for the encapsulation of the coronene dimer, a cooperativity factor (α) as high as 485 was observed, which is remarkably high for neutral host-guest systems. Furthermore, a successful chirality transfer, from the chiral host to encapsulated coronenes, resulted in a chiral charge-transfer (CT) complex and the rare observation of circularly polarized emission originating from the CT state for a noncovalent donor-acceptor assembly in solution. The involvement of the CT state also afforded an enhancement in the luminescence dissymmetry factor (glum) value due to its relatively large magnetic transition dipole moment. The 1:2 binding pattern and chirality-transfer were unambiguously verified by single-crystal X-ray diffraction analysis of the host-guest superstructures.

The Delayed Box: Biphenyl Bisimide Cyclophane, a Supramolecular Nano-environment for the Efficient Generation of Delayed Fluorescence

Activating delayed fluorescence emission in a dilute solution via a non-covalent approach is a formidable challenge. In this report, we propose a strategy for efficient delayed fluorescence generation in dilute solution using a non-covalent approach via supramolecularly engineered cyclophane-based nanoenvironments that provide sufficient binding strength to π-conjugated guests and that can stabilize triplet excitons by reducing vibrational dissipation and lowering the singlet-triplet energy gap for efficient delayed fluorescence emission. Toward this goal, a novel biphenyl bisimide-derived cyclophane is introduced as an electron-deficient and efficient triplet-generating host. Upon encapsulation of various carbazole-derived guests inside the nanocavity of this cyclophane, emissive charge transfer (CT) states close to the triplet energy level of the biphenyl bisimide are generated. The experimental results of host-guest studies manifest high association constants up to 104 M-1 as the prerequisite for inclusion complex formation, the generation of emissive CT states, and triplet-state stabilization in a diluted solution state. By means of different carbazole guest molecules, we could realize tunable delayed fluorescence emission in this carbazole-encapsulated biphenyl bisimide cyclophane in methylcyclohexane/carbon tetrachloride solutions with a quantum yield (QY) of up to 15.6%. Crystal structure analyses and solid-state photophysical studies validate the conclusions from our solution studies and provide insights into the delayed fluorescence emission mechanism.

Self-Assembly of Wheel-Shaped Nanographdiynes and Self-Template Growth of Graphdiyne

Graphdiyne (GDY) multilayers show stacking-style-dependent physical properties; thus, controlling the stacking style of nanostructures is crucial for utilizing their electrical, optical, and transport properties in electro-optical devices. Herein, we report the assemblies of nanographdiynes decorated with substituents with different steric hindrances to adjust the stacking style. We show that the π-stacked aggregates were influenced by peripheral substituents and the substrate. Steric hexaterphenyl-substituted nanoGDY scaffolds led to dimer structures stacked in the AB-3 configuration with a twist angle of 26.01° or the AB-1 configuration with an in-plane shift along one diyne link. With the interval replacement of steric substituents with long C12 alkyl chains, nanoGDYs were stacked in the AB-2 configuration to decrease the steric congestion, eventually leading to one-dimensional (1D) nanofibrous aggregates. Self-assembly in the presence of substrates can result in ABC-stacked nanoGDYs, which endowed us with the possibility of using nanoGDY as the template for GDY growth in a homogeneous reaction. High-resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and near-infrared-ultraviolet-visible (NIR-UV-vis) absorption spectroscopy indicate that the crystalline GDY prepared in this way is a 1.18 eV bandgap semiconductor.

Frustrated π-stacking

The properties of functional materials based on organic π-conjugated systems are governed extensively by intermolecular interactions between π-molecules. To establish clear relationships between supramolecular structures and functional properties, it is essential to attain structurally well-defined π-stacks, particularly in solution, as this enables the collection of valuable spectroscopic data. However, precise control and fine-tuning of π-stacks pose significant challenges due to the weak and bidirectional nature of π-π stacking interactions. This article introduces the concept of "frustrated π-stacking," strategically balancing attractive (π-π interaction) and repulsive (steric hindrance) forces in self-assembly to exert control over the sizes, sequences of π-stacks, and slip-stacked structures. These research efforts contribute to a deeper understanding of the correlation between π-stacks and their properties, thereby providing useful insights for the development of molecular materials with the desired performance.

Cucurbituril-based supramolecular host-guest complexes: single-crystal structures and dual-state fluorescence enhancement

Two supramolecular complexes were prepared using cucurbiturils [CBs] as mediators and a four-armed p-xylene derivative (M1) as a guest molecule. The single crystals of these two complexes were obtained and successfully analyzed by single-crystal X-ray diffraction (SCXRD). An unexpected and intriguing 1 : 2 self-assembly arrangement between M1 and CB[8] was notably uncovered, marking its first observation. These host-guest complexes exhibit distinctive photophysical properties, especially emission behaviors. Invaluable insights can be derived from these single-crystal structures. The precious single-crystal structures provide both precise structural information regarding the supramolecular complexes and a deeper understanding of the intricate mechanisms governing their photophysical properties.

Naphthalimide-Annulated [n]Helicenes: Red Circularly Polarized Light Emitters

Two [n]heliceno-bis(naphthalimides) 1 and 2 (n = 5 and 6, respectively) where two electron-accepting naphthalimide moieties are attached at both ends of helicene core were synthesized by effective two-step strategy, and their enantiomers could be resolved by chiral stationary-phase high-performance liquid chromatography (HPLC). The single-crystal X-ray diffraction analysis of enantiopure fractions of 1 and 2 confirmed their helical structure, and together with experimental and calculated circular dichroism (CD) spectra, the absolute configuration was unambiguously assigned. Both 1 and 2 exhibit high molar extinction coefficients for the S₀-S₁ transition and high fluorescence quantum yields (73% for 1 and 69% for 2), both being outstanding for helicene derivatives. The red circularly polarized luminescence (CPL) emission up to 615 nm for 2 with CPL brightness (B_CPL) up to 66.5 M^-1 cm^-1 demonstrates its potential for applications in chiral optoelectronics. Time-dependent density functional theory (TD-DFT) calculations unambiguously showed that the large transition magnetic dipole moment |m| of 2 is responsible for its high absorbance dissymmetry (g_abs) and luminescence dissymmetry (g_lum) factor.

Helical Trilayer Nanographenes with Tunable Interlayer Overlaps

The synthesis of well-defined nanocarbon multilayers, beyond the bilayer structure, is still a challenging goal. Herein, two trilayer nanographenes were synthesized by covalently linking nanographene layers through helicene bridges. The structural characterization of the trilayer nanographenes revealed a compact trilayer-stacked architecture. The introduction of a furan ring into the helicene linker modulates the interlayer overlap and π-conjugation of the trilayer nanographenes, enabling the tuning of the interlayer coupling, as demonstrated by optical, electrochemical, and theoretical analyses. Both synthesized trilayer nanographenes are rigid chiral nanocarbons and show a chirality transfer from the helicene moiety to the stacked nanographene layers. These helical trilayer nanographenes reported here represent the covalently linked multilayer nanographenes rather than bilayer ones, showing the tunable multilayer stacking structure.

Molecular Carbon Imides

The creation and development of new forms of nanocarbons have fundamentally transformed the scientific landscape in the past three decades. As new members of the nanocarbon family with accurate size, shape, and edge structure, molecular carbon imides (MCIs) have shown unexpected and unique properties. Particularly, the imide functionalization strategy has endowed these rylene-based molecular carbons with fascinating characteristics involving flexible syntheses, tailor-made structures, diverse properties, excellent processability, and good stability. This Perspective elaborates molecular design evolution to functional landscapes, and illustrative examples are given, including a promising library of multi-size and multi-dimensional MCIs with rigidly conjugated π-architectures, ranging from 1D nanoribbon imides and 2D nanographene imides to cross-dimensional MCIs. Although researchers have achieved substantial progress in using MCIs as functional components for exploration of charge transport, photoelectric conversion, and chiral luminescence performances, they are far from unleashing their full potential. Developing highly efficient and regioselective coupling/ring-closure reactions involving the formation of multiple C-C bonds and the annulation of electron-deficient aromatic units is crucial. Prediction by theory with the help of machine learning and artificial intelligence research along with reliable nanotechnology characterization will give an impetus to the blossom of related fields. Future investigations will also have to advance toward─or even focus on─the emerging potential functions, especially in the fields of chiral electronics and spin electronics, which are expected to open new avenues.