Excimer Formation Research Articles

Acid Sensors Based on Luminochromism of Solid‐State Excimer Emission of Pyrene‐Modified Polyhedral Oligomeric Silsesquioxane

To obtain solid‐state fluorescence sensors, it is essential to simultaneously obtain solid‐state emission and stimuli‐responsiveness. In this research, the modified polyhedral oligomeric silsesquioxane (POSS) tethered with pyrenes was synthesized and we confirmed that both demands for developing solid‐state fluorescence sensors can be satisfied. The POSS derivative exhibited strong emission even in the solid state. In particular, we found that exposure of the POSS films to trifluoroacetic acid vapor resulted in a significant red‐shift of the peak wavelength of the emission band. Excitation spectroscopy and comparison with model compounds suggest that this significant red‐shift should be attributable to the formation of static excimer, meaning a preformed dimer in the ground state induced by protonation of the secondary amino groups. These results indicate that POSS is a promising scaffold for a solid‐state fluorescence sensor.

Pyrene Based Schiff Base for Cascade Fluorescent "On-Off" Detection of Aluminium(III) and Fluoride ions and its Applications in Live Cells Imaging.

An easy-to-prepare pyrene-based Schiff base PNZ was synthesized by condensing equimolar amount of 1-pyrenebutyric hydrazide with 2-hydroxy-naphthaldehyde, and employed as a fluorescent chemosensor for in-situ cascade detection of aluminium (Al3+) and fluoride (F-)ions. In DMSO:H2O (1:1, v/v), the weakly emissive PNZ showed a significant fluorescence enhancement at 455 nm selectively upon the addition of Al3+ due to the complexation-induced formation of a pyrene excimer. Schiff base PNZ and Al3+ formed a complex in 2:1 binding ratio with the estimated binding constants of K1:1 = 9826.01 M-1 and K2:1 = 3188.49 M-1. The sensing mechanism was explored by performing quantum mechanical calculations and 1H NMR titration of PNZ with Al3+. The in-situ formed PNZ-Al3+ complex species enabled the fluorescent turn-off detection of F-. Using PNZ and PNZ-Al3+, the concentrations of Al3+ and F- ions can be detected down to 2.89×10-7 M and 1.88×10-7 M, respectively. The cytotoxicity of the PNZ and its ability to bioimage Al3+ and F- ions was examined in the human cervical cancer cell line. Finally, the receptor PNZ was applied for the quantification of Al3+ and F- ions in various real samples, such as tap water, river water, rainwater, mouthwash, and toothpaste.

Nanosecond-Lived Excimer Observation in a Crystal of a Rhodium(I) Complex via Time-Resolved X-ray Laue Diffraction.

The rare observation of transient Rh···Rh excimer formation in a single crystal is reported. The estimated excited-state lifetime at 100 K is 2 ns, which makes it the shortest-lived small-molecule species caught experimentally using the laser-pump/X-ray-probe time-resolved Laue method. Upon excitation with 390 nm laser light, the intermolecular Rh···Rh distance decreases from 3.379(4) to 3.19(1) Å, and the metal-metal contact gains more bonding character. On the basis of the experimental results and theoretical modeling, the structural changes determined with 100 ps time resolution reflect principally the S0 → S1 electronic transition.

Illumination-Induced Solute Uptake in Liquid Crystal Droplets: The Role of 5CB Excimers.

This study investigates the phenomenon of solute uptake into liquid crystal (LC) droplets, illuminated under UV light, focusing on the role of 4-cyano-4'-pentylbiphenyl (5CB) excimer formation in this process. Our experiments reveal that upon UV irradiation solute molecules, including surfactants and dyes, are actively drawn into the LC phase, forming distinctive assemblies within the droplets. Contrary to previous assumptions that the uptake was driven by the direct photoreactivity of the solutes, we found that the 5CB excimer state plays a critical role for this phenomenon. This state, identifiable through its photoluminescence yet invisible to conventional UV/vis absorption spectroscopy, correlates strongly with the molecular assembly process inside of the droplets. Notably, this mechanism operates across a broad spectrum of solute molecules, from simple surfactants like sodium dodecyl sulfate (SDS) to complex dyes, demonstrating the generality of the excimer-induced solute uptake. The excimer's role is pivotal, facilitating solute incorporation without reliance on their inherent light-absorbing properties. This insight not only advances our understanding of LC behavior under light irradiation but also opens new avenues for designing light-responsive materials.

Ultrafast and Coherent Dynamics in a Solvent Switchable "Pink Box" Perylene Diimide Dimer.

Perylene diimide (PDI) dimers and higher aggregates are key components in organic molecular photonics and photovoltaic devices, supporting singlet fission and symmetry breaking charge separation. Detailed understanding of their excited states is thus important. This has proven challenging because interchromophoric coupling is a strong function of dimer architecture. Recently, a macrocyclic PDI dimer was reported in which excitonic coupling could be turned on and off simply by changing the solvent. This presents a useful case where coupling is modified without synthetic changes to tune supramolecular structure. Here we present a detailed study of solvent dependent excited state dynamics in this dimer by means of coherent multidimensional spectroscopy. Spectral analysis resolves the different coupling strengths, which are consistent with solvent dependent changes in dimer conformation. The strongly coupled conformer forms an excimer within 300 fs. The low-frequency Raman active modes recovered from two-dimensional electronic spectra reveal frequencies characteristic of exciton coupling. These are assigned to modes modulating the coupling from the corresponding DFT calculations. Further analysis reveals a time dependent frequency during excimer formation. Analysis of two-dimensional "beatmaps" reveals features in the coupled dimer which are not predicted by the displaced harmonic oscillator model and are assigned to vibronic coupling.

Boosting Stimulated Emission via a Hydrogen‐Bonded Co‐Crystallization Approach in Molecular Crystals

AbstractStimulated emission (SE) of organic π‐conjugated molecule in solid state remains a significant challenge, mainly involving the mode of molecular stacking that invariably alters the photo‐physical processes. Herein, we successfully realized stimulated emission in molecular crystals using a hydrogen‐bonded co‐crystallization strategy. Two hydrogen‐bonded co‐crystals, obtained from 1,4‐bis‐p‐cyanostyrylbenzene (CNDSB) and two types of co‐formers, can boost stimulated emission and show decent amplified spontaneous emission (ASE), whereas the parent CNDSB crystal is not SE‐active. Crystal structural analysis demonstrated that the co‐crystallization eliminated excimer formation. The resulting higher kr and shorter excited lifetime led to a larger stimulated emission cross‐section, which benefited to the occurrence of ASE. Simultaneously, the uniaxial arrangements along the long axis of co‐crystal together contributed to highly polarized emission. This system presents very rare evidence of boosting stimulated emission by binary co‐crystallization, which enriches our insights into organic solid‐state lasers.

Valence Electronic Structure of Interfacial Phenol in Water Droplets.

Biochemistry and a large part of atmospheric chemistry occur in aqueous environments or at aqueous interfaces, where (photo)chemical reaction rates can be increased by up to several orders of magnitude. The key to understanding the chemistry and photoresponse of molecules in and "on" water lies in their valence electronic structure, with a sensitive probe being photoelectron spectroscopy. This work reports velocity-map photoelectron imaging of submicrometer-sized aqueous phenol droplets in the valence region after nonresonant (288 nm) and resonance-enhanced (274 nm) two-photon ionization with femtosecond ultraviolet light, complementing previous liquid microjet studies. For nonresonant photoionization, our concentration-dependent study reveals a systematic decrease in the vertical binding energy (VBE) of aqueous phenol from 8.0 ± 0.1 eV at low concentration (0.01 M) to 7.6 ± 0.1 eV at high concentration (0.8 M). We attribute this shift to a systematic lowering of the energy of the lowest cationic state with increasing concentration caused by the phenol dimer and aggregate formation at the droplet surface. Contrary to nonresonant photoionization, no significant concentration dependence of the VBE was observed for resonance-enhanced photoionization. We explain the concentration-independent VBE of ∼8.1 eV observed upon resonant ionization by ultrafast intermediate state relaxation and changes in the accessible Franck-Condon region as a consequence of the lowering of the intermediate state potential energy due to the formation of phenol excimers and excited phenol aggregates. Correcting for the influence of electron transport scattering in the droplets reduced the measured VBEs by 0.1-0.2 eV.

Advantages of Pyrene Excimer Formation (PEF) over Fluorescence Resonance Energy Transfer (FRET) for Probing the Conformation of Macromolecules in Solution

Advantages of Pyrene Excimer Formation (PEF) over Fluorescence Resonance Energy Transfer (FRET) for Probing the Conformation of Macromolecules in Solution

Gold Nanoparticles Modulate Excimer and Exciplex Dynamics of PDDCP-Conjugated Polymers.

How plasmonic nanostructures modulate the behavior of exciplexes and excimers within materials remains unclear. Thus, advanced knowledge is essential to bridge this gap for the development of optoelectronic devices that leverage the interplay between plasmonic and conjugated polymer hybrid materials. Herein, this work aims to explore the role of gold nanoparticles (AuNPs) in modulating exciplex and excimer states within the conjugated polymer poly(2,5-di(3,7-dimethyloctyloxy) cyanoterephthalylidene) (PDDCP), known for its photoluminescent and semi-conductive properties, aiming to create innovative composite materials with tailored optical features. The spectral analysis was conducted to investigate the effects of AuNPs on the PDDCP, varying AuNP volume percentages to measure changes in the absorption profile, molar extinction coefficient (ε), absorption cross-section (σa), and optical bandgap (Eg). Fluorescence spectra of the mixture at different volume ratios were also examined to assess exciplex formation, while amplified spontaneous emission (ASE) profiles were analyzed to study the behavior and photochemical stability of the polymer-NP hybrid material. Increasing AuNP volume induced both blue and red shifts in the absorption profile of the PDDCP. Higher AuNPs concentrations correlated with decreased ε and σa, inversely impacting Eg. The emission spectra obtained at varied AuNP volume ratios indicated significantly enhanced exciplex and excimer formations. The ASE profiles remained consistent but showed reduced intensity with increasing AuNPs concentrations, indicating their influence on hybrid material behavior and stability. The findings suggest that AuNPs affect PDDCP's optical characteristics, altering the absorption profile, bandgap, and fluorescence spectra. Furthermore, the observed reduction in ASE intensity highlights their influence on the behavior and photochemical stability of the hybrid material. These results contribute to a better understanding of the versatile applications of plasmonic-conjugated hybrid polymers.

Ultrafast and Coherent Dynamics in a Solvent Switchable “Pink Box” Perylene Diimide Dimer

AbstractPerylene diimide (PDI) dimers and higher aggregates are key components in organic molecular photonics and photovoltaic devices, supporting singlet fission and symmetry breaking charge separation. Detailed understanding of their excited states is thus important. This has proven challenging because interchromophoric coupling is a strong function of dimer architecture. Recently, a macrocyclic PDI dimer was reported in which excitonic coupling could be turned on and off simply by changing the solvent. This presents a useful case where coupling is modified without synthetic changes to tune supramolecular structure. Here we present a detailed study of solvent dependent excited state dynamics in this dimer by means of coherent multidimensional spectroscopy. Spectral analysis resolves the different coupling strengths, which are consistent with solvent dependent changes in dimer conformation. The strongly coupled conformer forms an excimer within 300 fs. The low‐frequency Raman active modes recovered from two‐dimensional electronic spectra reveal frequencies characteristic of exciton coupling. These are assigned to modes modulating the coupling from the corresponding DFT calculations. Further analysis reveals a time dependent frequency during excimer formation. Analysis of two‐dimensional “beatmaps” reveals features in the coupled dimer which are not predicted by the displaced harmonic oscillator model and are assigned to vibronic coupling.

Distinct vibrational motions promote disparate excited-state decay pathways in cofacial perylenediimide dimers.

A complex interplay of structural, electronic, and vibrational degrees of freedom underpins the fate of molecular excited states. Organic assemblies exhibit a myriad of excited-state decay processes, such as symmetry-breaking charge separation (SB-CS), excimer (EX) formation, singlet fission, and energy transfer. Recent studies of cofacial and slip-stacked perylene-3,4:9,10-bis(dicarboximide) (PDI) multimers demonstrate that slight variations in core substituents and H- or J-type aggregation can determine whether the system follows an SB-CS pathway or an EX one. However, questions regarding the relative importance of structural properties and molecular vibrations in driving the excited-state dynamics remain. Here, we use a combination of two-dimensional electronic spectroscopy, femtosecond stimulated Raman spectroscopy, and quantum chemistry computations to compare the photophysics of two PDI dimers. The dimer with 1,7-bis(pyrrolidin-1'-yl) substituents (5PDI2) undergoes ultrafast SB-CS from a photoexcited mixed state, while the dimer with bis-1,7-(3',5'-di-t-butylphenoxy) substituents (PPDI2) rapidly forms an EX state. Examination of their quantum beating features reveals that SB-CS in 5PDI2 is driven by the collective vibronic coupling of two or more excited-state vibrations. In contrast, we observe signatures of low-frequency vibrational coherence transfer during EX formation by PPDI2, which aligns with several previous studies. We conclude that key electronic and structural differences between 5PDI2 and PPDI2 determine their markedly different photophysics.

Blue thermally activated delayed fluorescence excimers with high external quantum efficiency and accelerated reverse intersystem crossing

This study reports for the first time the development of an efficient blue excimer with thermally activated delayed fluorescence (TADF) properties using a multi-resonance TADF-type fused indolo[3,2,1-jk]carbazole-derived compound with a planar structure for facile excimer formation. The blue excimer was efficiently generated by doping the indolo[3,2,1-jk]carbazole-derived emitter at a high doping concentration. The photophysical analysis demonstrated that the photoluminescence quantum yield (PLQY), rate coefficients of reverse intersystem crossing (RISC), and horizontal emitting dipole orientation values were improved by excimer formation in the solid film compared with the monomer-dominant solid film by suppressing the concentration-quenching effect. Specifically, the delayed PLQY increased because of the efficient RISC process at high doping concentrations. Consequently, an enhanced maximum external quantum efficiency of 26.3% was obtained in the excimer device (15 wt%) compared with 15.2% in the monomer prevalent device (1 wt%). Moreover, the excimer device showed a peak wavelength of 473 nm and color coordinate of (0.13, 0.19). This demonstration is the first of excimer TADF organic light-emitting diodes with a blue emission and a high external quantum efficiency of 26.3%.

Surface Loading Dictates Triplet Production via Singlet Fission in Anthradithiophene Sensitized TiO2 Films.

Singlet fission, the process of transforming a singlet excited state into two lower energy triplet excited states, is a promising strategy for improving the efficiency of dye-sensitized solar cells. The difficulty in utilizing singlet fission molecules in this architecture is understanding and controlling the orientation of dyes on mesoporous metal oxide surfaces to maximize triplet production and minimize detrimental deactivation pathways, such as electron injection from the singlet or excimer formation. Here, we varied the concentration of loading solutions of two anthradithiophene dyes derivatized with either one or two carboxylic acid groups for binding to a metal oxide surface and studied their photophysics using ultrafast transient absorption spectroscopy. For the single carboxylic acid case, an increase in dye surface coverage led to an increase in apparent triplet excited-state growth via singlet fission, while the same increase in coverage with two carboxylic acids did not. This study represents a step toward controlling the interactions between molecules at mesoporous interfaces.

(Invited) A General Approach to Induce Ultralong Room Temperature Phosphorescence in Organic Small Molecules

Interest in organic small molecules that exhibit second-scale phosphorescence at room temperature has grown immensely in recent years due to their potential applications in sensing, anticounterfeiting, and bioimaging. However, such material systems are rare—requiring second-scale triplet lifetimes, efficient intersystem crossing, and slow rates of nonradiative recombination. This third requirement has been met by isolating phosphors in a rigid matrix, or by aggregating them into densely packed crystals or powders to suppress the molecular vibrations that lead to recombination. While these techniques work well for a small subset of molecules with specific properties, most isolated molecules in a rigid matrix do not phosphoresce, and most macroscopic aggregates experience significant triplet quenching. In this work, we find a middle ground between these extremes by forming microscopic [approximately submicron sized] phosphor aggregates in rigid polymer matrices using a simple drop casting and thermal annealing process. Using this technique, we activate second-scale phosphorescence at room temperature in 20 molecules that do not otherwise phosphoresce using conventional matrix-isolation or crystallization approaches. We find that increased chromophore loading increases aggregate sizes. Excitons are thus able to diffuse further and interact more, and triplet-triplet annihilation dominates. Furthermore, we determine that excimer formation in some aggregates leads to increased rates of triplet generation—complementing the effect of nonradiative recombination suppression and further enhancing phosphorescence. In sum, the simplicity and robustness of this blending approach significantly loosens the design constraints to access second-scale emission with organic phosphors, allowing researchers to choose from a broader catalog of organic materials to match the desired properties for a given application.

Optical Monitoring of Supramolecular Interactions in Polymers.

Many stimuli-responsive materials harness the reversible association of supramolecular binding motifs to enable advanced functionalities such as self-healing, switchable adhesion, or mechanical adaptation. Despite extensive research into the structure-property relationships of these materials, direct correlations between molecular-level changes in supramolecular binding and macroscopic material behaviors have mostly remained elusive. Here, we show that this challenge can be overcome with supramolecular binding motifs featuring integrated binding indicators. We demonstrate this using a novel motif that combines a hydrogen-bonding ureido-4-pyrimidinone (UPy) with two strategically placed pyrene fluorophores. Dimerization of this motif promotes pyrene excimer formation, facilitating the straightforward optical quantification of supramolecular assembly under various conditions. We exploit the new motif as a supramolecular cross-linker in poly(methyl acrylate)s to probe the extent of (dis)assembly as a function of cross-linker content, processing history, and applied stimuli. We demonstrate that the stimuli-induced dissociation of hydrogen-bonding linkages strongly depends on the initial cross-link density, which also dictates whether the force-induced dissociation in polymer films correlates with the applied stress or strain. Thus, beyond introducing a robust tool for the in situ study of dynamic (dis)assembly mechanisms in supramolecular systems, our findings provide new insights into the mechanoresponsive behavior of such materials.

Optical Monitoring of Supramolecular Interactions in Polymers

AbstractMany stimuli‐responsive materials harness the reversible association of supramolecular binding motifs to enable advanced functionalities such as self‐healing, switchable adhesion, or mechanical adaptation. Despite extensive research into the structure–property relationships of these materials, direct correlations between molecular‐level changes in supramolecular binding and macroscopic material behaviors have mostly remained elusive. Here, we show that this challenge can be overcome with supramolecular binding motifs featuring integrated binding indicators. We demonstrate this using a novel motif that combines a hydrogen‐bonding ureido‐4‐pyrimidinone (UPy) with two strategically placed pyrene fluorophores. Dimerization of this motif promotes pyrene excimer formation, facilitating the straightforward optical quantification of supramolecular assembly under various conditions. We exploit the new motif as a supramolecular cross‐linker in poly(methyl acrylate)s to probe the extent of (dis)assembly as a function of cross‐linker content, processing history, and applied stimuli. We demonstrate that the stimuli‐induced dissociation of hydrogen‐bonding linkages strongly depends on the initial cross‐link density, which also dictates whether the force‐induced dissociation in polymer films correlates with the applied stress or strain. Thus, beyond introducing a robust tool for the in situ study of dynamic (dis)assembly mechanisms in supramolecular systems, our findings provide new insights into the mechanoresponsive behavior of such materials.

Flexibility of Poly(alkyl methacrylate)s Characterized by Their Persistence Length Determined through Pyrene Excimer Formation.

A series of poly(alkyl methacrylate)s and poly(oligo(ethylene glycol) methyl ether methacrylate)s labeled with 1-pyrenebutanol were referred to as the PyC4-PCnMA samples with n = 1, 4, 6, 8, 12, and 18 and the PyC4-PEGnMA samples with n = 0-5, 9, 16, and 19, respectively. Pyrene excimer formation (PEF) upon the encounter between an excited and a ground-state pyrenyl labels was employed to determine their persistence length (lp) in o-xylene. The fluorescence decays of the PyC4-PCnMA and PyC4-PEGnMA samples were acquired and analyzed with the fluorescence blob model to yield the number (Nblob) of structural units in the volume probed by an excited pyrenyl label. Nblob was found to decrease with an increasing number (NS) of non-hydrogen atoms in the side chain, reaching a plateau for the PyC4-PEGnMA samples with a longer side chain (n = 16 and 19). The Nblob values were used to determine lp. The lp values for the PyC4-PCnMA and PyC4-PEGnMA samples increased linearly with increasing NS2 as predicted theoretically, which agreed with the lp values obtained by viscometry for a series of PCnMA samples. The good agreement between the lp values retrieved by PEF and viscometry served to validate the PEF-based methodology for determining lp for linear polymers.

Crystal structure, theoretical calculations, mechanoluminescence feature and antimicrobial activity based on a double-armed Salamo-type ligand and its trinuclear Cu(Ⅱ) complex

A double-armed salamo-type ligand H4L and its Cu(Ⅱ) complex have been designed and synthesized. The stoichiometric ratios of the prepared H4L and its Cu(Ⅱ) complex were structurally characterized by a variety of analytical and spectroscopic techniques, including elemental analysis, FT-IR, UV–visible, fluorescence spectroscopy, and X-ray crystal diffraction. The results showed that H4L is a linear molecule, while the Cu(Ⅱ) complex is a trinuclear structure with three copper atoms exhibiting different coordination modes due to varying coordination cavities and bridging atoms. The quantum chemical parameters and molecular structures of the H4L and the Cu(Ⅱ) complex have been theoretically calculated and studied. Using molecular electrostatic potential (MEP) and density functional theory (DFT) calculations to understand the selectivity of the H4L and the Cu(Ⅱ) complex chemistry and related reactions. Detailed energy level transitions of the H4L and the Cu(Ⅱ) complex can be determined through time-dependent density functional theory (TD-DFT) calculations. In studying the fluorescence properties of liquid and solid states, it was observed that increasing water content leads to the formation of excimer in both the H4L and its Cu(Ⅱ) complex, the ligand H4L shown good mechanoluminescence, but the Cu(Ⅱ) complex did not. We studied the antimicrobial effectiveness of the H4L and the Cu(Ⅱ) complex against E. coli., the Cu(Ⅱ) complex showed higher inhibition of the pathogenic microorganisms.

Synthesis and optoelectronic properties of bis-BODIPY-meso-phenyleneethynylene dimers

Three different bis-BODIPY-meso-phenyleneethynylene dimers were prepared in a straightforward manner following a sequence Liebeskind-Srogl- Stille cross-couplings, which allowed us to control the architecture of the final products. The phenyleneethynylene (PE) bridged bis-meso-BODIPYs, indistinctly of the ortho, meta or para substitution exhibit in dichloromethane, a main excitonic (S0–S1 electronic transition) peak at 503 ± 1 nm, similar to that of the monomer meso-phenyl BODIPY, evidencing the poor electronic communication between the two BODIPYs. This is supported by theoretical study where modest energy splitting of the S0–S1 electronic transition was found and by voltammetry where voltammograms of para and meta dimers show similar redox process to that of the monomer. The fluorescence spectra are also quite similar, but the ortho dimer presents two emission bands. The fluorescence quantum yield (φ) is much lower for para (0.6 %) and meta (2.9 %) dimers with respect to the monomer (4.3 %) and to the ortho homologous (15.1 %). This is likely due to non-radiative losses derived from the free rotation of the two BODIPYs in meso that is restricted in the ortho substitution. Intramolecular charge transfer (ICT) through photoinduced symmetry breaking can also occur in this latter dimer according to the Weller equation using the voltammetric oxidation/reduction potentials, and to the fluorescence quenching in polar solvents. Noncovalent interactions studies indicate significant π-π interactions in the excited state that could promote excimer formation. For which, the two emission bands in this compound could be also due to a monomer-like ICT state and excimers.

Enhancement of Excimer Formation Ability by Modulating the Length of Side-Chains in Polyhedral Oligomeric Silsesquioxane (POSS) and Application for Fluorescence Sensors for Metal Cations

Abstract Polyhedral oligomeric silsesquioxane (POSS) is a molecule with an inorganic cubic structure and organic side-chains and has attracted great attention for its application to luminescent materials by modifying luminophores. In this study, pyrenes-integrated POSSs with various lengths of side-chains were synthesized and the effect of the length on luminescent properties was evaluated. In optical measurements, highly efficient excimer emission was observed under dilute solution conditions. The higher value of the intensity ratio of the excimer emission to the monomer one was detected in the shortest side-chains. It is likely that the shorter side-chains of POSS lead to more efficient intramolecular interaction. Interestingly, we also found that the luminescence color was changed in response to metal cations in the dilute solutions. From the mechanistic study, metal cations such as Cu2+ can accelerate hydrolysis at the linker moiety. As a result, highly-sensitive luminescent sensors were obtained. These data represent that POSS can work as a reaction field where chemical reactions are accelerated through the accumulation of reactive species.