Excimer Emission Research Articles

Blending or Bonding? Mechanochromism of an Aggregachromic Mechanophore in a Thermoplastic Elastomer

A straightforward way for the preparation of mechanochromic polymers consists of incorporating a suitable content of a mechanophore in the polymeric matrix either by physical dispersion or via covalent functionalization. Although covalent incorporation may require demanding chemical efforts, this approach can offer significant advantages over physical dispersion. In this work, a common thermoplastic elastomer, styrene-b-(ethylene-co-butylene)-b-styrene triblock copolymer grafted with maleic anhydride (SEBS-MAH), was covalently functionalized with 1-aminomethylpyrene (AMP). MAH functional groups are covalently linked to the ethylene-co-butylene blocks, thus allowing a precise and selective confinement of the chromogenic AMP units in the soft block. Flat, fully conjugated pyrene units undergo the reversible formation of π–π aggregates, readily distinguishable by their red-shifted emission. These aggregates were heavily affected by the application of mechanical stimuli. Despite the low degree of mechanophore functionalization (about 1 wt %), uniaxial deformation of the polymer was reliably monitored via fluorescence and a clear drop in the excimer to monomer emission ratio (IE/IM) was observed starting from 50% of strain. The marked mechanochromism was confirmed by emission lifetime measurements and also by near-field investigations. In addition, the mechanoresponse showed good reversibility after repeated stress–relaxation cycles. Control experiments performed on formulations comprising a physical dispersion of pyrene in unfunctionalized SEBS showed faint excimer emission and a negligible mechanochromic response up to 5 wt % of doping, in substantial agreement with the scanning near-field optical microscopy analysis. An evident drop of the IE/IM ratio occurred for 10 wt % of pyrene, albeit the excimer emission remained predominant even at the highest deformation, being a smaller fraction of pyrene moieties involved. Overall, the covalent approach appeared as an elegant procedure to confine the chromogenic unit in the soft phase of block copolymers and thus to provide an elastomeric film showing a detectable and reversible mechanochromic response with a modest (i.e., ∼1 wt %) amount of pyrene molecules, i.e., 10 times smaller compared to the dispersed system.

Luminescence Sensing of Biomacromolecules Heparin and Protamine in 100% Human Serum and Plasma by Supramolecular Polymeric Assemblies.

Heparin, an anionic biomacromolecule, is routinely used as an anticoagulant during medical surgery to prevent blood clot formation and in the treatment of several heart, lung, and circulatory disorders having a higher risk of blood clotting. We herein report supramolecular polymeric nanoassemblies of cationic pyrene-tagged bis-imidazolium amphiphiles for heparin detection with high sensitivity and selectivity in aqueous buffer, plasma, and serum media. The nano-assemblies exhibited cyan-green excimeric emission in aqueous media, and their multivalent array of positive surface charges allowed them to form co-assemblies with heparin, resulting in significantly enhanced emission. This provided a convenient method for heparin detection in buffer at nanomolar concentrations, and most notably, a ratiometric fluorescence response was obtained even in highly competitive 100% human serum and 100% human plasma in a clinically relevant concentration range. Moreover, using the heparin-based luminescent co-assemblies, protamine sulfate, a clinically administered antidote to heparin, was also detected in 100% human serum and 100% human plasma at sub-micromolar concentrations.

Circularly polarised luminescence from excimer emission of anthracene derivatives complexed with γ-cyclodextrin in the solid state.

In this study, we report circularly polarised luminescence (CPL)-active molecules that exhibit high fluorescence quantum yields in the solid state. We developed anthracene derivatives with substituents at the 9 and 10 positions, such as ethyl(anthracene-9-carbonyl)glycinate (9AnGlyEt), N-butylanthracene-9-carboxamide (9AnB), N-benzylanthracene-9-carboxamide (9AnPh), and N 9,N 10-dibutylanthracene-9,10-dicarboxamide (9,10AnB). These compounds were complexed with γ-cyclodextrin (γ-CD) in the solid state by grinding, and the fluorescence properties of the resulting γ-CD complexes were investigated. The fluorescence quantum yields were enhanced after γ-CD complexation. Among the prepared γ-CD complexes, 9AnGlyEt/γ-CD had the highest fluorescence quantum yield (Φ f = 0.35), which was enhanced up to 5.8 times after γ-CD complexation. This was probably due to the interaction between the two anthracene molecules in the γ-CD cavity, which prevented fluorescence quenching caused by aggregation of the compounds. Positive CPL of g CPL = 1.3 × 10-3 was observed for 9AnGlyEt/γ-CD based on its excimer emission.

A catalytic system based on π-π stacking interactions between a pyrene substituted gold NHC catalyst and amphiphilic polymers for alkyne hydration reactions.

Pyrene-substituted amphiphilic ATRP polymers (P1) were used to stabilize the pyrene-substituted gold(I) NHC complex (Au-1) within the hydrophobic compartment of micellar structures in a methanol/water mixture through non-covalent π-π stacking interactions. The stacking interactions between pyrene groups of the polymer and the catalyst were investigated by means of fluorescence spectroscopy by comparing excimer and monomer emission signals of the pyrene moiety. The supported catalyst (Au-1@P1) formed spherical micellar structures in a water/methanol mixture with an average size of 55.6 (±13.1) nm as confirmed from TEM analysis. The performance of the catalytic system was tested on alkyne hydration reactions of alkynes. When compared to the unsupported analog, the supported catalyst showed an improved performance as a result of stabilization through π-stacking interactions within the hydrophobic compartment in the micellar structure.

An ultra-sensitive ratiometric fluorescent thermometer based on monomer and excimer dual emission.

By leveraging natural saturated fatty acids with distinct melting points and swift reversible phase transitions, we correlated external thermal cues to monomer and excimer emissions of difluoroboron β-diketonate fluorophores. This integration yielded a ratiometric fluorescent thermometer showcasing unparalleled sensitivity and thermochromism in the physiological temperature range.

A ratiometric radio-photoluminescence dosimeter based on a radical excimer for X-ray detection.

A novel radio-photoluminescence material featuring fluorochromic responses toward UV or X-ray irradiation has been obtained. Such a unique monomer- to excimer-based luminescence transition allows for dosimetry of ionizing radiation in a ratiometric manner. Rather than quenching the luminescence, the radiation-induced radical species of Th-105 boost the excimer emission, rendering it as a rare material possessing radical-excimers.

Strong circularly polarized luminescence via intramolecular excited-state symmetry-breaking charge separation.

Trans-1,2-di(1-pyrenylamino)cycloxexane was found to display circularly polarized excimer emission (glum = 0.016) both in polar and non-polar solvents that is assigned to charge separation symmetry breaking on the basis of its large transition state dipole moment (12.1 D).

Multi-resonance TADF in optical cavities: suppressing excimer emission through efficient energy transfer to the lower polariton states

Strong light-matter coupling is exploited as a way to reduce aggregation-induced excimer losses in multi-resonance thermally activated delayed fluorescence emitters.

Titelbild: Alternately π‐Stacked Systems Assisted by o‐Carborane: Dual Excimer Emission and Color Modulation by Bcage‐Methylation (Angew. Chem. 2/2023)

Titelbild: Alternately π‐Stacked Systems Assisted by <i>o</i>‐Carborane: Dual Excimer Emission and Color Modulation by B_cage‐Methylation (Angew. Chem. 2/2023)

Cover Picture: Alternately π‐Stacked Systems Assisted by o‐Carborane: Dual Excimer Emission and Color Modulation by Bcage‐Methylation (Angew. Chem. Int. Ed. 2/2023)

Cover Picture: Alternately π‐Stacked Systems Assisted by <i>o</i>‐Carborane: Dual Excimer Emission and Color Modulation by B_cage‐Methylation (Angew. Chem. Int. Ed. 2/2023)

Julolidine based red emitting ESIPT/AIE active material showing luminescence beyond excimer emission: An “on-off” emission response to Cu2+

A new julolidine-fluorene based excited state intramolecular proton transfer (ESIPT)/aggregate induced emission (AIE) active Schiff-base (JDF) has been synthesized and evaluated for its photophysical properties in solution and aggregated/solid states. The correlation between the emission behavior and the solid state crystal packing structure revealed the interplay of ESIPT coupled excimer reaction occurring in the solid state, which is one of the rare examples reported so far. For a comprehensive comparison, we synthesized a non-ESIPT methyl derivative (JDF-Me) of JDF capable of showing excimer emission only in the solid state. Further, JDF exhibits normal as well as keto emission in solution, upon addition of water, its poor solvent, that promotes aggregation, the fluorescence emission shows the preponderance of the excimer band in the low energy region. It was also interesting to note that in the solid state (thin films), JDF shows emission beyond the excimer emission, which is wavelength dependent. This is attributed to the formation of diverse clusters leading to the extended delocalization beyond excimers, and represents a clustering-triggered emission ascribing bright red color to the solid JDF. Such mélange of emission characteristics of JDF are responsible for the multicolor emission covering a broad range of electromagnetic spectrum, which is demonstrated by the confocal microscopy images of the JDF recorded in different states. Further, in its aggregated state, JDF recognized Cu2+ ions, selectively, manifested in the form of emission quenching via the interaction of Cu2+ ions with the oxygen and nitrogen atoms of JDF inhibiting the excimer formation.

Monomer and Excimer Emission in a Conformational and Stacking‐Adaptable Molecular System

AbstractA design strategy that combines molecular conformation, alkyl chain length, and charge‐transfer effects has been developed to obtain conformational and stacking‐adaptable donor‐acceptor‐π type molecules for precisely regulating the monomer and excimer emission in a single luminous platform under different environments. These fluorophores can exhibit bright monomer emissions when they are in the dispersed state based on their planar conformation. However, when the luminous molecules with short alkyl side chains are in the crystalline state, their molecular conformation can become distorted, further inducing strong intermolecular interactions and staggered π‐π stacking for bright excimer emission. More importantly, their dispersed and aggregated states can be reversibly regulated in a phase‐change fatty acid matrix, to achieve temperature‐responsive fluorescence for temperature monitoring and advanced information encryption.

Monomer and Excimer Emission in a Conformational and Stacking-Adaptable Molecular System.

A design strategy that combines molecular conformation, alkyl chain length, and charge-transfer effects has been developed to obtain conformational and stacking-adaptable donor-acceptor-π type molecules for precisely regulating the monomer and excimer emission in a single luminous platform under different environments. These fluorophores can exhibit bright monomer emissions when they are in the dispersed state based on their planar conformation. However, when the luminous molecules with short alkyl side chains are in the crystalline state, their molecular conformation can become distorted, further inducing strong intermolecular interactions and staggered π-π stacking for bright excimer emission. More importantly, their dispersed and aggregated states can be reversibly regulated in a phase-change fatty acid matrix, to achieve temperature-responsive fluorescence for temperature monitoring and advanced information encryption.

Emissive Substoichiometric Covalent Organic Frameworks for Water Sensing and Harvesting.

Emissive covalent organic frameworks (COFs) have recently emerged as next-generation porous materials with attractive properties such as tunable topology, porosity, and inherent photoluminescence. Among the different types of COFs,substoichiometric frameworks (so-called Type III COFs) are especially attractive due to the possibility of not only generating unusual topology and complex pore architectures but also facilitating the introduction of well-defined functional groups at precise locations for desired functions. Herein, the first example of a highly emissive (PLQY 6.8%) substoichiometric 2D-COF (COF-SMU-1) featuring free uncondensed aldehyde groups is reported. In particular, COF-SMU-1 features a dual-pore architecture with an overall bex net topology, tunable emission in various organic solvents, and distinct colorimetric changes in the presence of water. To gain further insights into its photoluminescence properties, the charge transfer, excimer emission, and excited state exciton dynamics of COF-SMU-1 are investigated using femtosecond transient absorption spectroscopy in different organic solvents. Additionally, highly enhanced atmospheric water-harvesting properties of COF-SMU-1 are revealed using FT-IR and water sorption studies.The findings will not only lead to in-depth understanding of structure-property relationships in emissive COFs but also open new opportunities for designing COFs for potential applications in solid-state lighting and water harvesting.

Bola-type PEG-linked polyaromatic hydrocarbon-based chemosensors for the “turn-off” excimer fluorescence detection of nitro-analytes/explosives in aqueous solutions

Diesters of 1-pyrenecarboxylic acid with tri- and tetra - ethylene glycols were prepared and characterized for the first time and their photophysical and sensory properties were investigated. In aqueous solutions these compounds exhibited an intensive excimeric emission at 521 nm, which is attributed to the aggregation-induced fluorescence of the dimers, pre-associated in the ground state. In aqueous solutions an intensive “turn-off” fluorescence response towards common nitro-explosive components, such as 2,4-dinitrotoluene (2,4-DNT), 2,4,6-trinitrotoluene (TNT), as well as aliphatic nitro compounds, such as pentaerythritol tetranitrate (PETN), 1,3,5-trinitro-1,3,5-triazinane (RDX) and 2,3-dimethyl-2,3-dinitrobutane (DMDNB) was observed via the excimeric emission quenching. For the most representative chemosensor 4a the highest Stern-Volmer fluorescence quenching constant of 8.75 × 106 M−1 was observed in the presence of TNT.

The Journey to Fluorescent Bullvalenes: Pitfalls and Prospects

AbstractA dinaphthoylmethyl‐substituted bullvalene derivative is presented that exhibits excimer fluorescence. It is demonstrated that the fluxional bullvalene core supports the excimer formation of the aroyl substituents, thus offering the opportunity to monitor externally induced structural changes by fluorescence spectroscopy, as exemplarily shown by fluorimetric analysis of host‐guest interactions of the diaroylbullvalene with metal cations, which lead to significant changes of the excimer emission. During the synthesis of the bullvalenes, it was discovered coincidentally that the corresponding triplet‐sensitized photoreaction also yielded three unknown lumibullvalene isomers. The NMR‐spectroscopic analysis of this reaction revealed a sequence and combination of different di‐ and tri‐π‐methane rearrangements and sigmatropic rearrangements. More importantly, the static lumibullvalenes proved useful as reference compounds for the evaluation of the emission properties of the fluxional bullvalenes.

Alternately π-Stacked Systems Assisted by o-Carborane: Dual Excimer Emission and Color Modulation by Bcage -Methylation.

Herein, we report the unique solid-state excimer emission of three types of acridine-tethered o-carboranes with variable degrees of methylation at the o-carborane unit. They all showed columnar packing structures based on dimer formation, and two types of π-overlapping motifs were alternately stacked. From the photoluminescence (PL) measurements on the crystalline samples, it was found that three types of luminescence bands can simultaneously appear: monomer emission, excimer emission from the moderately π-stacked intra-dimer unit, and excimer emission from the widely π-stacked inter-dimer unit. Consequently, the PL colors were drastically changed by the steric effect of the methyl groups, with a strong correlation found between the π-overlapping and excimer character. In addition, variable-temperature PL measurements revealed that these PL species should be in thermal equilibrium at room temperature, with the intensity ratios sensitive toward temperature changes.

Alternately π‐Stacked Systems Assisted by o‐Carborane: Dual Excimer Emission and Color Modulation by Bcage‐Methylation

AbstractHerein, we report the unique solid‐state excimer emission of three types of acridine‐tethered o‐carboranes with variable degrees of methylation at the o‐carborane unit. They all showed columnar packing structures based on dimer formation, and two types of π‐overlapping motifs were alternately stacked. From the photoluminescence (PL) measurements on the crystalline samples, it was found that three types of luminescence bands can simultaneously appear: monomer emission, excimer emission from the moderately π‐stacked intra‐dimer unit, and excimer emission from the widely π‐stacked inter‐dimer unit. Consequently, the PL colors were drastically changed by the steric effect of the methyl groups, with a strong correlation found between the π‐overlapping and excimer character. In addition, variable‐temperature PL measurements revealed that these PL species should be in thermal equilibrium at room temperature, with the intensity ratios sensitive toward temperature changes.

Polydimethylsiloxanes with Grafted Dibenzoylmethanatoboron Difluoride: Synthesis and Properties.

A method for the preparation of polydimethylsiloxanes with grafted methoxy-substituted dibenzoylmethanatoboron difluoride has been described. The structures of prepared polymers were confirmed using NMR, IR spectroscopy and gel permeation chromatography methods. Their thermal properties were investigated using thermal gravimetric analysis, differential scanning calorimetry and thermomechanical analysis. The prepared polymers had good thermal (Td5% up to 393 °C) and thermo-oxidative (Td5% = 413 °C) stability. The polymers started to transit in a viscous flow state at about 40 °C (for 3 a) and at about 20 °C (for 3 b). The viscoelastic characteristics of prepared polymers were determined in the sinusoidal oscillating vibrations mode. It was shown that the studied polymers at low frequencies at room temperature are viscoelastic fluids (G′ < G″). Increasing the frequency led to inversion (crossover) of dependences G′ and G″, which indicated the transition of polymers from viscous to elastomeric behavior characteristics, and the beginning of the formation of a physical network. Optical properties were studied using electron absorption, steady-state and time-resolved fluorescence spectroscopy. It was shown that intramolecular H-dimers exist in the ground state. The polymers studied had a bright fluorescence in the solution and in the solid state, consisting of bands of monomer and excimer emission. Thermally-activated delayed fluorescence was observed in the solution and the solid state. The prepared polymers possess intriguing properties that make them useful as optical materials, sensors or imaging agents.

Turn-off near-infrared fluorescent probe for free bilirubin detection constructed by enhanced excimer emission

In this study, an amphiphilic near-infrared fluorescent molecule (denoted BCPB) was employed as a fluorescent probe to detect free bilirubin. In an aqueous solution, the micellar assemblies of BCPB possess a strong excimer emission at 660 nm, which was dramatically quenched upon the addition of bilirubin. It has been proven that fluorescence quenching is mainly attributed to photoinduced electron transfer (PET) from BCPB to bilirubin. As a fluorescent probe of bilirubin, BCPB showed advantages, such as fast response (<1 min), good anti-interference ability, and low limit of detection (0.33 μmol L−1, S/N = 3). BCPB was successfully applied to detect free bilirubin in human serum and urine, and the detection showed very high accuracy.