Understanding the AIE phenomenon of nonconjugated rhodamine derivatives via aggregation-induced molecular conformation change

The bottom-up molecular science research paradigm has greatly propelled the advancement of materials science. However, some organic molecules can exhibit markedly different properties upon aggregation. Understanding the emergence of these properties and structure-property relationship has become a new research hotspot. In this work, by taking the unique closed-form rhodamines-based aggregation-induced emission (AIE) system as model compounds, we investigated their luminescent properties and the underlying mechanism deeply from a top-down viewpoint. Interestingly, the closed-form rhodamine-based AIE system did not display the expected emission behavior under high-viscosity or low-temperature conditions. Alternatively, we finally found that the molecular conformation change upon aggregation induced intramolecular charge transfer emission and played a significant role for the AIE phenomenon of these closed-form rhodamine derivatives. The application of these closed-form rhodamine-based AIE probe in food spoilage detection was also explored.

Tailor-Made Pdn L2n Metal-Organic Cages through Covalent Post-Synthetic Modification

Post-synthetic modification (PSM) is an effective approach for the tailored functionalization of metal-organic architectures, but its generalizability remains challenging. Herein we report a general covalent PSM strategy to functionalize Pd_n L_2n metal-organic cages (MOCs, n=2, 12) through an efficient Diels-Alder cycloaddition between peripheral anthracene substituents and various functional motifs bearing a maleimide group. As expected, the solubility of functionalized Pd₁₂ L₂₄ in common solvents can be greatly improved. Interestingly, concentration-dependent circular dichroism and aggregation-induced emission are achieved with chiral binaphthol (BINOL)- and tetraphenylethylene-modified Pd₁₂ L₂₄ , respectively. Furthermore, Pd₁₂ L₂₄ can be introduced with two different functional groups (e.g., chiral BINOL and achiral pyrene) through a step-by-step PSM route to obtain chirality-induced circularly polarized luminescence. Moreover, similar results are readily observed with a smaller Pd₂ L₄ system.

Synergism between LDLB and true CD to achieve angle-dependent chiroptical inversion and switchable polarized luminescence emission in nonreciprocal nanofibrous films

Chiroptical nanomaterials including chiralized advanced functional nanofibers have attracted ever-increasing interest in multi-disciplinary fields. Nowadays, electronic circular dichroism (CD) is the most widely used technique to evaluate chiral properties. Numerous studies have shown that the occurrence of linear dichroism and linear birefringence (LDLB) phenomenon in chiral micro-/nano-architectures makes it challenging to distinguish their inherent chiral information. However, tactfully combining LDLB with true circular dichroism (true CD) may lead to a new class of chiroptical materials. In this study, we demonstrate that transparent nanofibrous films infiltrated with poor solvents can show unique LDLB properties, which allows achiral nanofibrous films to exhibit these features regularly opposite Cotton signals by simply flipping the two faces of the same film or even rotating the testing angles of the sample around its optical axis in a single face. More importantly, we can quantitatively distinguish the contribution of LDLB from true CD in the chiral composite nanofibrous system and even effectively combine them into a single unity. This kind of adjustable flexible film material shows outstanding value in the field of optoelectronics. Its large intrinsic LDLB feature and angle-dependent, non-reciprocal transmission for polarized lights are exploited to fabricate programmable polarized light-emitting devices through the resin encapsulation process. This study may provide an easy and powerful way for the construction and implementation of novel polarization optical materials towards future intelligent optical encryption and advanced anti-counterfeiting devices.

A curcumin-based AIEE-active fluorescent probe for Cu2+ detection in aqueous solution

Curcuminoids have been extensively investigated as metal ion probes, but the intrinsic aggregation-caused-quenching (ACQ) characteristic of curcumin would hinder their applications in aqueous solution. Fortunately, tetraphenylethylene (TPE) could endow the compounds with aggregation-induced emission (AIE)/aggregation-induced enhanced emission (AIEE) characteristics to eliminate the ACQ effect. According to this strategy, a series of TPE-modified curcumin derivatives L1–4 were prepared and studied for their AIEE properties. Among the four TPE-curcumin analogues, only L1 particles have been successfully used as an on-off fluorescence probe for detecting Cu²⁺ in aqueous solution. The fluorescence titration experiment determined its detection limit of 1.49 × 10⁻⁷ mol L⁻¹, and the binding ratio between L1 and Cu²⁺ was estimated as 2 : 1, which was in agreement with the results of high resolution mass spectrum and Job's plot. In addition, the binding constant was evaluated as 6.77 × 10² M⁻¹ using a Benesi–Hildebrand plot. Finally, the obtained L1-based indicator paper showed significant fluorescence response to Cu²⁺ aqueous solution. This TPE-modified strategy improves the detection capability of curcumin probe in aqueous solution and provides a feasible way to obtain other probes with ACQ characteristics.

A curcumin-based AIEE-active L1 was synthesized and used to prepare an on-off fluorescent probe for Cu²⁺ detection in aqueous solution.

Spontaneous and induced chiral symmetry breaking of stereolabile pillar[5]arene derivatives upon crystallisation

Stereolabile pillar[5]arene (P[5]) derivatives, which are dynamic racemic mixtures in solution on account of their low inversion barriers, were employed as platforms to study chiral symmetry breaking during crystallisation. In the solid state, we showed that crystal enantiomeric excess of a conglomerate-forming P[5] derivative can be obtained by handpicking and Viedma ripening without the intervention of external chiral entities. On the other hand, in the presence of ethyl d/l-lactate as both optically-active solvents and chiral guests, the handedness of P[5] derivative crystals, either forming racemic compounds or conglomerates upon condensation, can be directed and subsequently inverted in a highly controllable manner.

Stereolabile pillar[5]arene derivatives, which are dynamic racemic mixtures in solution on account of their low inversion barriers, were employed as platforms to study chiral symmetry breaking during crystallisation.

Truncated Face-Rotating Polyhedra Constructed from Pentagonal Pentaphenylpyrrole through Graph Theory

Discovering novel families of molecular polyhedra through graph theory has attracted increasing interest. Nevertheless, the design principles of molecular polyhedra based on graph theory remain elusive, especially for those containing five-node units. Herein, we construct a series of chiral truncated face-rotating polyhedra (T-FRP) from pentagonal pentaphenylpyrrole (PPP) derivatives and chiral diamines. Graph theory is used to elucidate the geometry of these novel T-FRP, which represent a new family of molecular polyhedra. The phenyl flipping of PPP faces in these T-FRP is significantly restricted, thus making T-FRP chiral and strongly emissive in solution. In addition, T-FRP also generate circularly polarized luminescence. This study provides new insights into the rational design of novel molecular polyhedra through graph theory.

Aggregate Science: From Structures to Properties

Molecular science entails the study of structures and properties of materials at the level of single molecules or small interacting complexes of molecules. Moving beyond single molecules and well-defined complexes, aggregates (i.e., irregular clusters of many molecules) serve as a particularly useful form of materials that often display modified or wholly new properties compared to their molecular components. Some unique structures and phenomena such as polymorphic aggregates, aggregation-induced symmetry breaking, and cluster excitons are only identified in aggregates, as a few examples of their exotic features. Here, by virtue of the flourishing research on aggregation-induced emission, the concept of "aggregate science" is put forward to fill the gaps between molecules and aggregates. Structures and properties on the aggregate scale are also systematically summarized. The structure-property relationships established for aggregates are expected to contribute to new materials and technological development. Ultimately, aggregate science may become an interdisciplinary research field and serves as a general platform for academic research.

Aggregation-Induced Emission: New Vistas at the Aggregate Level

Aggregation-induced emission (AIE) describes a photophysical phenomenon in which molecular aggregates exhibit stronger emission than the single molecules. Over the course of the last 20 years, AIE research has made great strides in material development, mechanistic study and high-tech applications. The achievements of AIE research demonstrate that molecular aggregates show many properties and functions that are absent in molecular species. In this review, we summarize the advances in the field of AIE and its related areas. We specifically focus on the new properties of materials attained by molecular aggregates beyond the microscopic molecular level. We hope this review will inspire more research into molecular ensembles at and beyond the meso level and lead to the significant progress in material and biological science.

Chiral molecular face-rotating sandwich structures constructed through restricting the phenyl flipping of tetraphenylethylene

Chiral tetraphenylethylene (TPE) derivatives have great potential in chiral recognition and circularly polarized luminescence. However, they were mainly constructed through introducing chiral substituents at the periphery of the TPE moiety, which required additional chemical modifications and limited the variety of chiralities of products. Herein, we constructed a series of chiral face-rotating sandwich structures (FRSs) through restricting the phenyl flipping of TPE without introducing any chiral substituents. In FRSs, the complex arrangements of TPE motifs resulted in a variety of chiralities. We also found that non-covalent repulsive interactions in vertices caused the facial hetero-directionality of FRSs, and the hydrogen bonds between imine bonds and hydroxy groups induced excited-state intramolecular proton transfer (ESIPT) emission of FRSs. In addition, the fluorescence intensity of FRSs decreases with the addition of trifluoroacetic acid. This study provides new insights into the rational design of chiral assemblies from aggregation-induced emission (AIE) active building blocks through restriction of intramolecular rotation (RIR).

Brønsted acid-catalyzed aromatic annulation of alkoxyallenes with naphthols: a reaction sequence to larger π-conjugated naphthopyrans with aggregation-induced emission characters

A practical and readily scalable reaction sequence was developed for the straightforward synthesis of a new family of larger π-conjugated naphthopyrans by a Brønsted acid-catalyzed aromatic annulation of alkoxyallenes with inert naphthols. The cascade pathway involves allylation/cyclization/debenzyloxylation/isomerization/dehydration. The new class of solid state diphenylmethylene substituted naphthopyrans are fluorescent emissive and proved to have aggregation-induced emission (AIE) behavior.

A pair of Schiff base enantiomers studied by absorption, fluorescence, electronic and vibrational circular dichroism spectroscopies and density functional theory calculation

A pair of enantiomeric Schiff bases were synthesized and characterized, in particular their absolute configurations were determined by vibrational circular dichroism spectroscopy.

Strong intermolecular exciton couplings in solid-state circular dichroism of aryl benzyl sulfoxides

A series of 13 enantiopure aryl benzyl sulfoxides () with different substituents on the two aromatic rings has been previously analyzed by means of electronic circular dichroism (CD) spectroscopy. Most of these compounds are crystalline and their X-ray structure is established. For almost one-half of the series, CD spectra measured in the solid state were quite different from those in acetonitrile solution. We demonstrate that the difference is due to strong exciton couplings between molecules packed closely together in the crystal. The computational approach consists of time-dependent density functional theory (TDDFT) calculations run on "dimers" composed of nearest neighbors found in the lattice. Solid-state CD spectra are well reproduced by the average of all possible pairwise terms. The relation between the crystal space group and conformation, and the appearance of solid-state CD spectra, is also discussed.