Excited-State Odd-Even Effect in Through-Space Interactions

Clusteroluminescence from Random Aggregation of Micrometers to Ordered Assembly of Sub-10 Nanometers by Space-Confined Template Synthesis

Clusteroluminescence (CL) clusters in the absence of aromatic or π-conjugated structures have emerged as a new family of luminescent materials due to their abundant sources and inherent biocompatibility. However, there is an inborn challenge in controlling the size of CL clusters from random aggregation of micrometers to ordered assembly of sub-10 nanometers. Such an inherent drawback significantly restricts their progress from theoretical research to practical applications. To address this obstacle, a novel approach using space-confined templates is proposed to synthesize CL clusters with tunable sizes. Space-confined templates with sizes of ≈2-6 nm are constructed to effectively confine non-aromatic molecules, yielding size-controllable and luminescence-tunable CL clusters. The versatility of this synthetic strategy is further proved by using non-conjugated molecules, such as L-valine and L-isoleucine. Finally, the potential applications of the synthesized CL clusters have been implemented in cell nucleus imaging owing to their sub-10 nm size and efficient luminescence. The success of this work not only offers a versatile space-confined approach to synthesize size-tunable CL clusters within sub-10 nm, but also opens new avenues for the deployment of CL clusters in advanced applications.

Emergent clusteroluminescence from nonemissive molecules

Once considered the exclusive property of conjugated molecules, efficient and visible-light luminescence from non-conjugated and nonemissive molecules in the clustered state, known as clusteroluminescence (CL), has attracted much attention recently due to its special photophysical behaviors and behind electronic interactions. This perspective discusses the development of the CL phenomenon, followed by the typical photophysical features, examples, mechanisms, and potential applications of CL materials, to provide a comprehensive picture of this emerging field. Starting with organic clusters, inorganic, metallic, and hybrid clusters with CL properties are also introduced, and the perspective shift from covalent interactions at the molecular level to non-covalent interactions at the aggregate level is invoked.

Flexible Regulation of Optical Properties Based on Structure Size-Driven Intermolecular Interactions for Phototherapy

The precise control of optical properties in molecular systems remains a challenge for phototherapy. Herein, the strategic combination of aggregation-caused quenching (ACQ) and aggregation-induced emission (AIE) molecule creates ACQ@AIE bimolecular systems with tunable optical properties, which are almost unattainable by single-component materials. Through systematic investigation of three ACQ@AIE bimolecular systems, it is established that molecule structure size differentials dictate their intermolecular interactions and consequent optical behaviors. Crucially, AIE molecule with a smaller structure size promotes ACQ molecule clustering to enhance the photothermal effect, while when the size becomes larger, particularly approaching that of ACQ molecule, facilitating π-π stacking and boosting the photodynamic effect. These distinct assembly modes revealed through combined experimental and theoretical analyses, enable precise regulation of photothermal versus photodynamic effects by simply regulating the structure size and ratio of ACQ and AIE molecules. Building on these mechanistic insights, the optimal molecule combination of ACQ@AIE bimolecular system is engineered into nanoparticles that exhibit mild photothermal effect, strong photodynamic effect, and excellent tumor accumulation and retention, achieving near-complete tumor eradication with minimal treatment cycles while maintaining good biosafety. This work not only elucidates the fundamental structure size-interaction-property relationships in ACQ@AIE bimolecular systems but also provides generalizable strategies for developing intelligent photo theranostic materials through controlled intermolecular interaction.

Fluorene-containing binuclear gold(I) complexes: High-contrast mechanochromic fluorescence quenching and information encryption application

A series of fluorene-based binuclear gold(I) complexes I-VI have been successfully synthesized. Their structures were characterized by nuclear magnetic resonance, high-resolution mass spectrometry, and single crystal X-ray diffraction techniques. The fluorescence switching characteristics of complexes I-VI in the solid state were studied by photoluminescence spectroscopy. Luminogens I-VI showed different solid-state fluorescence involving blue-green, yellow-green and yellow colors before mechanical stimulation, which suggests that the solid fluorescent properties of I-VI can be effectively manipulated by positional isomerism and increasing conjugation strategies. Interestingly, their solid fluorescence intensities of all luminogens weakened significantly upon grinding. Impressively, luminogen IV barely displayed macroscopic fluorescence after grinding, indicative of its remarkable high-contrast mechanochromic fluorescence quenching feature. The unique mechanofluorochromic phenomena of I-VI were fully elucidated by analyzing X-ray diffraction patterns changes of I-VI before and after grinding and crystal packing structure of III. Furthermore, an effective dual information encryption system was constructed based on complex IV.

Pressure-Engineered Through-Space Conjugation for Precise Control of Clusteroluminescence

Clusteroluminogens (CLgens) represent an innovative class of nonconjugated luminophores that address the limitations of conventional π-conjugated molecules. Different from the through-bond conjugation mechanism in π-conjugated luminophores, through-space conjugation (TSC) plays dominant roles in CLgens. However, precisely controlling TSC to customize the optical properties of CLgens remains a significant challenge. This work proposes a novel strategy of high pressure to engineer TSC within tetraphenylalkanes (TPAs)-based CLgens at molecular level. High-pressure exploration enables accurate manipulation of clusteroluminescence and elucidates the intrinsic structure-property relationships involved. Upon initial compression, the predominant molecular distortions marked by increased interfacial angles between benzene rings diminish TSC, resulting in anomalous hypochromatic shift in emission. Subsequently, considerable structural contraction enhances TSC and suppresses molecular motion, resulting in a pronouncedly enhanced and bathochromic-shifted emission. Notably, a series of TPAs-based CLgens exhibit intense white-light emission upon pressure release, attributed to irreversible structural distortion and destruction. This study not only advances the understanding of CLgens, but also underscores the crucial structural factors for effective TSC control, paving the way for establishing new photophysical theories for aggregate science.

Calcium Carbonate as an Ionic Molecular Lock for Ultrastrong Fluorescence of Single Organic Molecules

Locking molecular conformation are widely applied in molecular engineering for improved performance. However, locking via organic functional groups often changes the original molecular properties. Following the rigidity and stability of ionic interaction in ionic compounds, we suggested the use of a molecular-scale ionic compound, calcium carbonate oligomer, as a robust molecular segment to functionalize organic molecules. The rigid structure of the ionic molecular segments locked the organic molecules, which could remarkably limit the intramolecular motion and intermolecular interactions. This ensured a stable and ultrastrong fluorescence of the single organic molecule while preserving its original maximum emission wavelength. The locking strategy was general and extendable to multiple organic molecules. Additionally, the ultrastrong single-molecular fluorescence can be maintained in inorganic solids with even higher quantum yields and almost unchanged maximum emission wavelength. The highest quantum yield of the investigated molecules reached 99.9 %, superior to all reported organic-inorganic fluorescent composite under air conditions. This work demonstrates a general strategy to restrict intramolecular motion and intermolecular interactions by using ionic oligomers as molecular locks, providing an alternative method for realizing ultraemissive molecules. This further demonstrates a fascinating example of molecular engineering in the presence of inorganic ionic molecules.

Two-Photon Clusteroluminescence Enabled by Through-Space Conjugation for In Vivo Bioimaging

Clusteroluminescence (CL) materials without largely conjugated structures have gained significant attention due to their unique photophysical properties and potential in bioimaging. However, low luminescence efficiency and short emission wavelength limit their development. This work designs three luminogens with CL properties (CLgens) by introducing n-electron-involved through-space conjugation (TSC) into diarylmethane. Apart from single-photon excited long-wavelength (686 nm) and high-efficiency (29 %) CL, two-photon clusteroluminescence (TPCL) is successfully achieved in such small luminogens with only two isolated heteroatomic units. TSC stabilized in the aggregate state has been proven to realize efficient spatial electron delocalization similar to conventionally conjugated compounds. Encouraged by the excellent TPCL properties, two-photon imaging of blood vessels in vivo and biocompatibility verification utilizing CLgens are also achieved. This work illustrates the essential role of TSC in promoting nonlinear optical properties of CLgens and may facilitate further design and development of the next generation of bioprobes with excellent biocompatibility.

Self-aggregation of zinc bacteriochlorophyll-d analogs with an acylhydrazone moiety as the 13-keto-carbonyl alternative

Zinc methyl 3-hydroxymethyl-pyropheophorbides-a possessing an acylhydrazinylidene group at the 131-position were prepared by chemically modifying chlorophyll-a, which were models of bacteriochlorophyll-d as one of the light-harvesting pigments in photosynthetic green bacteria. Similar to the self-aggregation of natural bacteriochlorophyll-d in the antenna systems called chlorosomes, some of the synthetic models self-aggregated in an aqueous Triton X-100 solution to give red-shifted and broadened visible absorption bands. The newly appeared oligomeric bands were ascribable to the exciton coupling of the chlorin π-systems along the molecular y-axis, leading to intense circular dichroism bands in the red-shifted Qy and Soret regions. The self-aggregation in the aqueous micelle was dependent on the steric size of the terminal substituent at the 13-acylhydrazone moiety. An increase in the length of the oligomethylene chain as the terminal moved the red-shifted Qy maxima to shorter wavelengths, and branched alkyl and benzyl substitutes afforded no more self-aggregates to leave monomeric species in the hydrophobic environment inside the micelle. These results indicated that the acyl groups on the 13-hydrazone as the alternative of the natural 13-ketone regulated the chlorosome-like self-aggregation.

Thiol-containing hyperbranched polysiloxane for scavenging reactive oxygen species

Unconventional luminescent polymers have attracted considerable attention in the biological field due to their intrinsic fluorescence properties and excellent biocompatibility. However, exploring the luminescent properties of thiol-containing polymers and the mechanism of their scavenging of ROS remains unclear. In this work, we synthesised three kinds of hyperbranched polysiloxanes (HE, HP, and HB) with terminal thiol groups containing different chain lengths by the polycondensation reaction. Surprisingly, HP exhibits longer-wavelength emission at 480 nm with a quantum yield of 12.23% compared to HE and HB. Experiments and density functional theory (DFT) calculations have revealed that the rigidity of the conformation is enhanced by substantial hydrogen bonds and through-space O⋯O interactions in the polymer structure. These interactions, combined with the rigid carbon chains, balance the flexibility of the Si-O-C chain segments, which emerges as a critical factor contributing to the superior fluorescence performance of HP. In addition, HP exhibits excellent biocompatibility and ROS scavenging ability with a scavenging capacity of up to 35.095%. This work provides a new fluorescent polymer for scavenging ROS for the treatment of ROS-related diseases.

Azaylide-based gemini amphiphiles displaying unique self-assembling behavior via an even-odd effect of alkyl linker chain length

Herein, we report a straightforward synthesis of azaylide-based gemini amphiphiles using bis(diphenylphosphino)alkanes via the Staudinger reaction. The prepared gemini amphiphiles exhibited an even-odd effect in their self-assembly behavior depending on the length of the alkyl linkers. Furthermore, the assembled micelles had high host capability toward hydrophobic guests in water.

Narrowband clusteroluminescence with 100% quantum yield enabled by through-space conjugation of asymmetric conformation

Different from traditional organic luminescent materials based on covalent delocalization, clusteroluminescence from nonconjugated luminogens relies on noncovalent through-space conjugation of electrons. However, such spatial electron delocalization is usually weak, resulting in low luminescent efficiency and board emission peak due to multiple vibrational energy levels. Herein, several nonconjugated luminogens are constructed by employing biphenyl as the building unit to reveal the structure-property relationship and solve current challenges. The intramolecular through-space conjugation can be gradually strengthened by introducing building units and stabilized by rigid molecular skeleton and multiple intermolecular interactions. Surprisingly, narrowband clusteroluminescence with full width at half-maximum of 40 nm and 100% efficiency is successfully achieved via an asymmetric conformation, exhibiting comparable performance to the traditional conjugated luminogens. This work realizes highly efficient and narrowband clusteroluminescence from nonconjugated luminogens and highlights the essential role of structural conformation in manipulating the photophysical properties of unconventional luminescent materials.

Near-Infrared Emission Beyond 900 nm from Stable Radicals in Nonconjugated Poly(diphenylmethane)

Organic radicals with narrow energy gaps are highly sought-after for the production of near-infrared (NIR) fluorophores. However, the current repertoire of developed organic radicals is notably limited, facing challenges related to stability and low fluorescence efficiency. This study addresses these limitations by achieving stable radicals in nonconjugated poly(diphenylmethane) (PDPM). Notably, PDPM exhibits a well-balanced structural flexibility and rigidity, resulting in a robust intra-/inter-chain through-space conjugation (TSC). The stable radicals within PDPM, coupled with strong TSC, yield a remarkable full-spectrum emission spanning from blue to NIR beyond 900 nm. This extensive tunability is achieved through careful adjustments of concentration and excitation wavelength. The findings highlight the efficacy of polymerization in stabilizing radicals and introduce a novel approach for developing nonconjugated NIR emitters based on triphenylmethane subunits.

Regulation of Photophysical Behaviors in Hyperbranched Aggregation-Induced Emission Polymers for Reactive Oxygen Species Scavenging

Developing nonconjugated materials with large Stokes shifts is highly desired. In this work, three kinds of hyperbranched aggregation-induced emission (AIE) polymers with tunable n/π electronic effects were synthesized. HBPSi-CBD contains alkenyl groups in the backbone and possesses a promoted n-π* transition and red-shifted emission wavelength with a large Stokes shift of 186 nm. Experiments and theoretical simulations confirmed that the planar π electrons in the backbone are responsible for the red-shifted emission due to the strong through-space n···π interactions and restricted backbone motions. Additionally, the designed HBPSi-CBD could be utilized as an ROS scavenger after coupling with l-methionine. The HBPSi-Met exhibits remarkable ROS scavenging properties with a scavenging capacity of 77%. This work not only gains further insight into the structure-property relationship of nonconjugated hyperbranched AIE polymers but also provides a promising ROS-scavenging biomaterial for the treatment of ROS-related diseases.

How the Length of Through-Space Conjugation Influences the Clusteroluminescence of Oligo(Phenylene Methylene)s

The length and mode of conjugation directly affect the molecular electronic structure, which has been extensively studied in through-bond conjugation (TBC) systems. Corresponding research greatly promotes the development of TBC-based luminophores. However, how the length and mode of through-space conjugation (TSC), one kind of weak interaction, influence the photophysical properties of non-conjugated luminophores remains a relatively unexplored field. Here, we unveil a non-linear relationship between TSC length and emission characteristics in non-conjugated systems, in contrast to the reported proportional correlation in TBC systems. More specifically, oligo(phenylene methylene)s (OPM[4]-OPM[7]) exhibit stronger TSC and prominent blue clusteroluminescence (CL) (≈440 nm) compared to shorter counterparts (OPM[2] and OPM[3]). OPM[6] demonstrates the highest solid-state quantum yield (40 %), emphasizing the importance of balancing flexibility and rigidity. Further theoretical calculations confirmed that CL of these oligo(phenylene methylene)s was determined by stable TSC derived from the inner rigid Diphenylmethane (DPM) segments within the oligomers instead of the outer ones. This discovery challenges previous assumptions and adds a new dimension to the understanding of TSC-based luminophores in non-conjugated systems.