Excimer Emission Research Articles

Antagonistic Effects of Distance and Overlap toward Anomalous Pressure-Induced Blueshift of π-π Excimer Fluorescence in 9-(2,2-Diphenylvinyl)anthracene Crystals.

Piezochromic materials usually exhibit a gradual redshift of emission as pressure increases due to the formation of a low-energy "dark" state, e.g., excimer. However, our study presents an anomalous excimer-based pressure-induced emission blueshift. A crystal was investigated here with a discrete π-π anthracene dimer stacking and excimer emission, and the dimer is characterized by an overshifted off-center stacking pattern. Intriguingly, under isotropic hydrostatic pressure, this crystal exhibits negative linear compressibility almost along the c-axis of the unit cell. Furthermore, an antagonistic effect between overlap ratio (Sπ-π) and interplanar distance (Dπ-π) within the dimer on emission was identified: reduced Dπ-π typically dominates the emission redshift, while decreasing Sπ-π can cause emission blueshift. When the pressure reaches around 5.00 GPa, the π-π excimer fluorescence exhibits an unexpected blueshift, indicating the reign of decreasing Sπ-π. This study not only sheds light on the modulation of fluorescence properties by noncovalent interactions but also introduces an innovative approach to anomalous piezochromism.

Heteroarene-Fused Benzo[b]arsoles: Structure, Photophysical Properties, and Effects of the Bridging Element.

Heteroarene-fused heteroles have attracted considerable attention owing to their unique electronic and photophysical properties. The bridging element plays a crucial role in determining the electronic characteristics of the resulting π-conjugated molecules. In this study, we synthesized a series of heteroarene-fused benzo[b]arsoles and investigated their structures and photophysical properties. All the synthesized arsoles exhibited phosphorescence at 77 K, whereas arsole oxide did not. The phosphorescence intensities of the pnictogen-containing heteroles (phosphorus and arsenic) were significantly higher than those of the tetrel-containing heteroles (silicon and germanium). This trend was analyzed through theoretical calculations, focusing on the energy levels of the excited singlet and triplet states as well as the spin-orbit coupling matrix elements. Furthermore, the indole-fused benzoarsole oxide exhibited a mechanically induced emission color change. The removal of crystalline water triggered a change in the molecular packing that reduced the excimer emission.

Synthesis of Pyrene Diimide Isomers with Tunable Excimer Emission.

The optoelectronic properties of pyrene diimides (PyDIs) are of strategic interest given the successful application of similar rylene diimides as n-type organic semiconductors in a variety of organic electronics. We present an improved synthesis for 1,5,6,10-pyrene diimide and the first report of its isomer, 1,8,9,10-pyrene diimide. We demonstrate the effect of imide placement on the core structure, optical properties, and electron affinities. Notably, we have discovered that both isomers exhibit tunable aggregation-induced excimer emission and that 1,8,9,10-PyDI exhibits anti-Kasha emission.

Xanthine Nucleosides with Pyrazolo[3,4-d]pyrimidine Skeleton: Functionalization with Halogen Atoms, Clickable Side Chains, Pyrene, and iEDDA Cycloadducts, and Impact of Ionic Forms on Photophysical Properties.

Xanthine nucleosides play a significant role in the expansion of the four-letter genetic code. Herein, 7-functionalized 8-aza-7-deazaxanthine ribo- and 2'-deoxyribonucleosides are described. 2-Amino-6-alkoxy nucleosides were converted to halogenated 8-aza-7-deazaxanthine nucleosides by deamination followed by hydroxy/alkoxy substitution. 8-Aza-7-deaza-7-iodo-2'-deoxyxanthosine served as an intermediate in Suzuki-Miyaura, Sonogashira, and Stille reactions. Alkynyl and vinyl side chains as well as fluorescent tags were introduced. Pyrene conjugates were derived by copper(I)-catalyzed cycloaddition. Inverse-electron-demand Diels-Alder reaction of 8-aza-7-deaza-7-vinyl-2'-deoxyxanthosine with 3,6-dipyridyl-tetrazine proceeded with a second-order rate constant of 0.042 L M-1 s-1. X-ray analysis of 8-aza-7-deaza-7-vinyl-2'-deoxyxanthosine displayed two conformers with a syn conformation. Crystal packing is stabilized by xanthine base pairs. UV spectroscopy confirmed the sensitivity of 7-functionalized 8-aza-7-deazaxanthine nucleosides to pH changes. Halogen and alkynyl substituents decrease pK values, and vinyl, pyrene, or benzofuran leads to an increase. Fluorescence measurements of 8-aza-7-deaza-7-benzofuran-2'-deoxyxanthosine disclosed solvatochromism and enhanced fluorescence when the pH or the viscosity of the solvent is increased. Nucleoside pyrene conjugates connected by a linear linker displayed monomer emission, and two pyrene residues connected by a dendritic linker exhibited excimer emission. According to their fluorescence properties and sensitivity to pH changes, the functionalized 8-aza-7-deazaxanthine nucleosides expand the class of nucleosides applicable to fluorescence detection for diagnostic and therapeutic purposes.

Effects of Excimer Fluorescent UV Lamps on Mold and Fruit Quality in Strawberries

Excimer fluorescent ultraviolet (UV) lamps (UV–228) function as mercury–free sources that use excimer emissions as excitation light sources. First, we evaluated the effects of UV–228–based irradiation (λmax = 228 nm) on the viability of fungi, using Penicillium digitatum, and compared these effects with those of other light sources, such as OEL–222 (λmax = 222 nm) and GL–6 (λmax = 254 nm), to evaluate the effectiveness of the excimer fluorescent lamp. Next, we investigated the effects of UV–228–based irradiation on strawberry storage. Although UV–228 affected weight loss, Brix, ascorbic acid, polyphenol, and DPPH, the effects of UV irradiation for 5 min on strawberry fruit quality were minimal; i.e., only weight loss and ascorbic acid content were higher than those in the non–irradiated treatment. We found that 5 min of UV–C irradiation using UV–228 prevented mold emergence for up to 11 days of storage in strawberries These results indicate that UV–228 contributes to long–term strawberry storage.

Upconversion on the Micrometer Scale: Impact of Local Heterogeneity.

The properties of perovskite/naphtho[2,3-a]pyrene (NaPy) upconversion devices are investigated by a combination of atomic force microscopy and photoluminescence mapping to understand the role of microscopic heterogeneity in the ensemble device properties. The results emphasize strong microscopic inhomogeneity across the perovskite/NaPy upconversion device due to local formation of NaPy microcrystals. NaPy shows emission from three distinct states in the solid state: S1' emission at 520 nm, excimer emission at 560 nm, and S1″ emission at 620 nm. Clear spatial differences in the emission spectrum under 405 nm excitation are found, highlighting that there is a strong microcrystal-to-microcrystal variation in the optical properties─emphasizing a need for multimodal measurements. Our results indicate that microcrystals with strong emission from the strongly coupled low-energy state S1″ (J-dimer) show much higher upconversion intensity than those with dominant emission from the high-energy S1' state (I-aggregate). Hence, our results suggest that microcrystals with strong emission from the low-energy state S1″ act as isolated hotspots for upconversion.

5]Helicene Based π-Conjugated Macrocycles with Persistent Figure-Eight and Möbius Shapes: Efficient Synthesis, Chiral Resolution and Bright Circularly Polarized Luminescence.

π-Conjugated chiral shape-persistent molecular nanocarbons hold great potential as chiroptical materials, though their synthesis remains a considerable challenge. Here, we present a simple approach using Suzuki coupling of a [5]helicene building block with various aromatic units, enabling the one-pot synthesis of a series of chiral macrocycles with persistent figure-eight and Möbius shapes. Single-crystal structures of 7 compounds were solved, and 22 enantiomers were separated by preparative chiral HPLC. A notable pyrene-bridged figure-eight macrocycle, with its rigid, fully π-conjugated and overcrowded structure, exhibited pure excimer emission and outstanding circularly polarized luminescence (CPL) properties, including a large dissymmetric factor (|glum|=3.8×10-2) and significant CPL brightness (BCPL=710.5 M-1cm-1). This method provides a versatile synthetic platform for producing various chiral D2-symmetric figure-eight macrocycles and singly or triply twisted Möbius macrocycles with C2 and D3 symmetry, offering tunable chiroptical properties for CPL applications.

5]Helicene Based π‐Conjugated Macrocycles with Persistent Figure‐Eight and Möbius Shapes: Efficient Synthesis, Chiral Resolution and Bright Circularly Polarized Luminescence

Abstractπ‐Conjugated chiral shape‐persistent molecular nanocarbons hold great potential as chiroptical materials, though their synthesis remains a considerable challenge. Here, we present a simple approach using Suzuki coupling of a [5]helicene building block with various aromatic units, enabling the one‐pot synthesis of a series of chiral macrocycles with persistent figure‐eight and Möbius shapes. Single‐crystal structures of 7 compounds were solved, and 22 enantiomers were separated by preparative chiral HPLC. A notable pyrene‐bridged figure‐eight macrocycle, with its rigid, fully π‐conjugated and overcrowded structure, exhibited pure excimer emission and outstanding circularly polarized luminescence (CPL) properties, including a large dissymmetric factor (|glum|=3.8×10−2) and significant CPL brightness (BCPL=710.5 M−1cm−1). This method provides a versatile synthetic platform for producing various chiral D2‐symmetric figure‐eight macrocycles and singly or triply twisted Möbius macrocycles with C2 and D3 symmetry, offering tunable chiroptical properties for CPL applications.

Exciplex Emission in the POSS Possessing Two Kinds of Luminophores

AbstractHere we report the synthesis and luminescent properties of the modified polyhedral oligomeric silsesquioxane (POSS) that possesses two types of luminophores, cyanobenzene and N,N‐diethylaniline for inducing significant emission not only from each dye but also from molecular interactions on the POSS core. From the optical measurements in the diluted solution, we observed both locally‐excited emission and excimer emission originating from intramolecular interactions assisted by POSS. Moreover, highly‐efficient exciplex emission was observed from the thin film, indicating that the interaction between luminophores easily occurs in the solid state by accumulating two kinds of molecules in the single POSS molecule. Finally, we also found that the modified POSS showed a unique acid responsiveness which can be caused by protonation at two types of amino moieties. In this manuscript, it is shown that POSS is able to play various roles at the same time: recruiting excited molecules to form exciplexes, suppressing concentration quenching in the solid state, and imparting stimulus responsiveness to exciplexes.

Amphiphilic pyrene-functionalized triazoliums for ATP recognition

Four pyrene functionalized triazolium salts with different lengths of alkyl tail from C4 to C16, i.e., PYTAZ-C4, PYTAZ-C8, PYTAZ-C12 and PYTAZ-C16, have been designed and synthesized for ATP recognition. It was revealed that the longer alkyl tail anchored receptors PYTAZ-C12 and PYTAZ-C16 exhibit low CMC values, and self-assemble nanoaggregation at low concentration in aqueous solution, which show a pyrene-based excimer emission at 496 nm. However, the shorter alkyl anchored receptors PYTAZ-C4 and PYTAZ-C8 give only the pyrene-based monomer emission at 380 nm at the test concentration of 10 μM in aqueous solution. Importantly, PYTAZ-C12 and PYTAZ-C16 exhibit a good fluorescence turn-on response toward polyphosphate anions, especially ATP, with the detection limits of 2.5 μM and 0.77 μM, respectively. Furthermore, probe PYTAZ-C16 was successfully used for fluorescence imaging of intracellular ATP in living cells.

Lighting Up Bispyrene‐Functionalized Chiral Molecular Muscles with Switchable Circularly Polarized Excimer Emissions

AbstractAiming at the further extension of the application scope of traditional molecular muscles, a novel bispyrene‐functionalized chiral molecular [c2]daisy chain was designed and synthesized. Taking advantage of the unique dimeric interlocked structure of molecular [c2]daisy chain, the resultant chiral molecular muscle emits strong circularly polarized luminescence (CPL) attributed to the pyrene excimer with a high dissymmetry factor (glum) value of 0.010. More importantly, along with the solvent‐ or anion‐ induced motions of the chiral molecular muscle, the precise regulation of the pyrene stacking within its skeleton results in the switching towards either “inversed” state with sign inversion and larger glum values or “down” state with maintained handedness and smaller glum values, making it a novel multistate CPL switch. As the first example of chiral molecular muscle‐based CPL switch, this proof‐of‐concept study not only successfully widens the application scopes of molecular muscles, but also provides a promising platform for the construction of novel smart chiral luminescent materials for practical applications.

Polymer organic framework-based ratiometric fluorescent probe for non-enzymatic glucose detection

Fluorescent pyrene-based polymer organic frameworks (POFs) PY-POFs were designed and synthesized by a facile solvothermal route for glucose detection, employing melamine, aromatic dialdehydes, 1-pyrenecarboxaldehyde, and phenylboronic acid as building blocks. The structure and photophysical properties of as-prepared POFs were systematically characterized via Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, N2 adsorption isotherms and fluorescence spectroscopy. The PY-POFs possessed intrinsic microporosity derived from extensively interlinked aminal structures, boasting specific surface areas reaching 192 m²/g, pore volumes extending to 0.8079 cm³/g, pore diameters of up to 17 nm, and an average particle dimension of ∼ 31 nm. In addition, the PY-POFs exhibited a dual-wavelength emission of pyrene excimer (λem = 472 nm) and monomer (λem = 382, 400, and 422 nm). When exposed to glucose, the former emission decreased while the latter increased, corresponding to the conformational transition of pyrene molecule from excimer to monomer. The non-enzymatic ratiometric fluorescent sensor displayed remarkable sensitivity towards glucose within 10−200 μM range, with a limit of detection (LOD) of 5.6 µM. Moreover, the glucose detection ability of PY-POFs was also in vivo demonstrated within zebrafish. Application of this sensor to real samples such as human saliva and DMEM biological matrix presented satisfactory results and good reproducibility. These results indicate that the fluorescent glucose sensor PY-POFs have broader prospects for utilization within biological studies.

Narrowband-UVB-emitting gadolinium-doped yttrium oxide nanofilm on xenon excimer lamp for phototherapy

The continuous utilization of ultraviolet (UV) light sources within both academic and industrial contexts necessitates the imperative advancement of alternative UV sources to replace hazardous mercury lamps. Herein, we proposed a mercury-free narrowband UVB (NB-UVB)-emitting device from Y2O3:Gd3+ nanofilm on a tubular quartz substrate. The nanofilm is fabricated utilizing the spin-coating sol-gel precursor technique and is subsequently synthesized via a high-annealing temperature solid-state reaction. This synthesis resulted in the formation of a single-phase cubical structure of Y2O3 characterized by a crack-free morphology. The NB-UVB radiative output with a peak at 315 nm is generated through vacuum UV-induced photoluminescence originating from the emission of Xe excimer, which subsequently excites the Gd3+ luminescent centers incorporated within the Y2O3 host matrix. Moreover, in a high-voltage bipolar power system operating at 19 kV and 19 kHz, the device exhibited an NB-UVB radiance output of 1.34 mW, accompanied by a power efficiency of 0.02 %, whilst preserving exceptional temporal performance. Thus, this investigation introduces an innovative perspective on a mercury-free NB-UVB-emitting device, thereby promoting further exploration into the application of UV radiation sources derived from excimer lamp technology.

A pyrene-based hydrogen-bonded framework with excimer induced ratiometric emission for discriminative luminescence detection of metal ions

Discriminatory fluorescence detection of various metal ions is of significant importance in environmental and health-related applications. A luminescent hydrogen-bonded organic framework (HOF) material, PFC-1 was synthesized using 1,3,6,8-tetra(4-carboxylbenzene) pyrene (H4TBAPy) as the organic building block. The fluorescence behavior of PFC-1 is influenced by the concentration of the suspension, demonstrating different emissions characteristic of monomer and excimer fluorescence, which are highly sensitive to the surrounding environment. This allows for the potential differentiation and sensing of different target analytes. PFC-1 showed discriminatory fluorescence sensing performance towards metal ions such as Al3+, Sc3+, Cr3+, and Cu2+, with changes in fluorescence intensity, emission peak shifts, and intensity ratio changes between monomer and excimer emissions. Furthermore, a smartphone-based detection strategy was proposed, leveraging color recognition capabilities of smartphones for onsite and real-time sensing. The work demonstrate that PFC-1 is a promising material for the development of portable, cost-effective fluorescent sensors for onsite and real-time detection of metal ions. The integration of PFC-1 with smartphone technology paves the way for practical applications in environmental monitoring, industrial processes, and healthcare diagnostics.

Stacking Transformation-Triggered Circularly Polarized Luminescence Reversion in γ-Cyclodextrins-Pyrene Co-Assembly.

Considerable attention has been directed towards cyclodextrins (CDs) in the creation of co-assembled CPL-active materials, owing to their intrinsic chiral host cavities and synergistic host-guest interactions. However, achieving reversed CPL emission regulation with single-handedness CDs moiety poses a significant challenge. In this study, we have devised a series of γ-CD-based host-guest complexes comprising dual pyrene imidazolium derivatives with multiple linkers, which exhibit reversed circularly polarized emission. We have uncovered that the transformation of excimer stacking within γ-CD/pyrene complexes contributes to the inverted CPL emissions originating from a single-handed chiral host. This research elucidates the phenomenon of (+)- and (-)-circularly polarized excimer emission (CPEE) within γ-CD, arising from right- and left-handed stacking conformations, respectively.

Photoresponsive lanthanide-dianthracene framework: Introduction of photoactive anthracene pairs by controlling the synthesis temperature

Metal-organic frameworks (MOFs) containing face-to-face π-π interacting anthracene groups are promising photoresponsive materials because of their rich photophysical properties and their ability to undergo reversible [4+4] photocycloaddition reaction, but it is extremely challenging to obtain such materials. Herein, we propose a generalized method to accomplish photoresponsive MOFs by introducing anthracene pairs into the framework of the dianthracene-phosphonate-based MOFs by controlling the synthesis temperature. Compounds Dy2(ampH)2x(amp2H2)3-x(H2O)6∙4H2O [x = 0.01, Dy-70; x = 0.02, Dy-80; x = 0.037, Dy-90; amp2H4 = pre-photodimerized 9-anthracenemethylphosphonic acid (ampH2)] were obtained by the reaction of DyCl3 and amp2H4 in water at 70, 80, and 90 °C, respectively. They all show excimer emission of paired anthracenes at ca. 555 nm. Detailed studies of Dy-90 have shown that it undergoes a reversible photodimerization reaction under 365 nm and then 280 nm illumination, accompanied by luminescence changes. This property further enables Dy-90 to be used for optical anti-counterfeiting.

Multiple-stimuli fluorescent responsive metallo-organic helicated cage arising from monomer and excimer emission

Effectively regulating monomer and excimer emission in a singular supramolecular luminous platform is challenging due to high difficulty of precise control over its aggregation and dispersion behavior when subjected to external stimuli. Here, we show a metallo-cage (MTH) featuring a triple helical motif that displays a unique dual emission. It arises from both intramolecular monomer and intermolecular excimer, respectively. The distorted molecular conformation and the staggered stacking mode of MTH excimer are verified through single crystal X-ray diffraction analysis. These structural features facilitate the switch between monomer and excimer emission, which are induced by changes in concentration and temperature. Significantly, adjusting the equilibrium between these two states in MTH enables the production of vibrant white light emission in both solution and solid state. Moreover, when combined with a PMMA (polymethyl methacrylate) substrate, the resulting thin films can serve as straightforward fluorescence thermometer and thermally activated information encryption materials.

Ring Rearrangement Reactions of 4-Alkenylisocoumarins and Photophysical Evaluation of Multi-Substituted Anthracene Products.

Herein we report that readily available 4-alkenylisocoumarins can be regarded as potent dienolate equivalents. For example, lactol silyl ethers derived from 4-alkenylisocoumarins were selectively converted to the corresponding benzo-homophthalates through a fluoride-induced ring opening step that was followed by a ring closure through a vinylogous intramolecular aldol condensation. Likewise, nucleophilic activation of 4-alkenylisocoumarins directly yields diversely poly-substituted naphthalenes and anthracenes without formation of any regioisomer. Photophysical evaluation of a set of thus obtained 1,3-di- and 1,3,4-trisubstituted anthracenes reveals their distinct intramolecular charge transfer (ICT) character during light absorption in polar solutions and excimer emission from the solid state when a face-to-face π-stacked molecular assembly is present in the crystal packing.

Magnetic fields reveal signatures of triplet-pair multi-exciton photoluminescence in singlet fission

The photophysical processes of singlet fission and triplet fusion have numerous emerging applications. They involve the separation of a photo-generated singlet exciton into two dark triplet excitons and the fusion of two dark triplet excitons into an emissive singlet exciton, respectively. The role of the excimer state and the nature of the triplet-pair state in these processes have been a matter of contention. Here we analyse the room temperature time-resolved emission of a neat liquid singlet fission chromophore and show that it exhibits three spectral components: two that correspond to the bright singlet and excimer states and a third component that becomes more prominent during triplet fusion. This spectrum is enhanced by magnetic fields, confirming its origins in the recombination of weakly coupled triplet pairs. It is thus attributed to a strongly coupled triplet pair state. These observations unite the view that there is an emissive intermediate in singlet fission and triplet fusion, distinct from the broad, unstructured excimer emission.

Promotion of Fast and Efficient Singlet Fission Process of PDI Dimers by Selenium Substitution.

Singlet fission (SF) is a triplet generation mechanism capable of turning a singlet exciton into two triplet excitons. It has the potential to enhance the power conversion efficiency of single-junction solar cells. Perylene diimides (PDIs) are a class of dye molecules with photovoltaic properties and are beginning to receive more and more attention due to their potential for SF. Here, we report a selenium-substituted PDI dimer, Se-PDI-II, and we studied its SF mechanism by using steady-state, transient absorption, and time-resolved photoluminescence spectroscopy. Compared with the unsubstituted dimer PDI-II, we found that the introduction of selenium atoms can suppress excimer emission during the SF process, showing much higher SF efficiency and triplet yield.