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D2-symmetric (R)-binaphthyl-bridged pyrenophanes containing thioether bonds were synthesized. The pyrenophanes exhibited the temperature-induced sign inversion of circularly polarized luminescence (CPL) while maintaining the emission wavelength and reversibility. The Δglum value reached 0.02, and the FL quenching by heat was negligible. The sign inversion of CPL originates from the inversion of intramolecular excimer chirality associated with excitation dynamics. The two pyrenes form a kinetically trapped left-handed twist excimer at low temperatures, while they form a thermodynamically favored right-handed twist excimer at high temperatures. The thioether linkers can impart flexibility suitable for the inversion of chirality of the excimers.

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Time-dependent double hybrids with spin-component or spin-opposite scaling to their second-order perturbative correlation correction have demonstrated competitive robustness in the computation of electronic excitation energies. Some of the most robust are those recently published by our group (M. Casanova-Páez, L. Goerigk, J. Chem. Theory Comput. 2021, 20, 5165). So far, the implementation of these functionals has not allowed correctly calculating their ground-state total energies. Herein, we define their correct spin-scaled ground-state energy expressions which enables us to test our methods on the noncovalent excited-state interaction energies of four aromatic excimers. A range of 22 double hybrids with and without spin scaling are compared to the reasonably accurate wavefunction reference from our previous work (A. C. Hancock, L. Goerigk, RSC Adv. 2023, 13, 35964). The impact of spin scaling is highly dependent on the underlying functional expression, however, the smallest overall errors belong to spin-scaled functionals with range separation: SCS- and SOS- ω PBEPP86, and SCS-RSX-QIDH. We additionally determine parameters for DFT-D3(BJ)/D4 ground-state dispersion corrections of these functionals, which reduce errors in most cases. We highlight the necessity of dispersion corrections for even the most robust TD-DFT methods but also point out that ground-state based corrections are insufficient to completely capture dispersion effects for excited-state interaction energies.

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Aggregation-Induced Emission (AIE) luminogens have garnered significant interest due to their distinctive applications in different applications. Among the diverse molecular architectures, those based on triphenylamine and thiophene hold prominence. However, a comprehensive understanding of the deactivation mechanism both in solution and films remains lacking. In this study, we synthesized and characterized spectroscopically two AIE luminogens: 5-(4-(bis(4-methoxyphenyl)amino)phenyl)thiophene-2-carbaldehyde (TTY) and 5'-(4-(bis(4-methoxyphenyl)amino)phenyl)-[2,2'-bithiophene]-5-carbaldehyde (TTO). Photophysical and theoretical analyses were conducted in both solution and PMMA films to understand the deactivation mechanism of TTY and TTO. In diluted solutions, the emission behavior of TTY and TTO is influenced by the solvent, and the deactivation of the excited state can occur via locally excited (LE) or twisted intramolecular charge transfer (TICT) state. In PMMA films, rotational and translational movements are constrained, necessitating emission solely from the LE state. Nevertheless, in the PMMA film, excimers-like structures form, resulting in the emergence of a longer wavelength band and a reduction in emission intensity. The zenith of emission intensity occurs when molecules are dispersed at higher concentrations within PMMA, effectively diminishing the likelihood of excimer-like formations. Luminescent Solar Concentrators (LSC) were fabricated to validate these findings, and the optical efficiency was studied at varying concentrations of luminogen and PMMA.

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Developing fluorescent motifs capable of displaying mechano- and thermo-stimuli reversibly and ratiometrically is appealing for monitoring the deformation or temperature to which polymers are subjected. Here, a series of excimer-type chromophores Sin-Py (n = 1-3) consisting of two pyrenes linked with oligosilanes of one to three silicon atoms is developed as the fluorescent motif incorporated in a polymer. The fluorescence of Sin-Py is steered with the linker length where Si2-Py and Si3-Py with disilane and trisilane linkers display prominent excimer emission accompanied by pyrene monomer emission. Covalent incorporation of Si2-Py and Si3-Py in polyurethane gives fluorescent polymers PU-Si2-Py and PU-Si3-Py, respectively, where intramolecular pyrene excimers and corresponding combined emission of excimer and monomer are obtained. Polymer films of PU-Si2-Py and PU-Si3-Py display instant and reversible ratiometric fluorescence change during the uniaxial tensile test. The mechanochromic response arises from the reversible suppression of excimer formation during the mechanically induced separation of the pyrene moieties and relaxation. Furthermore, PU-Si2-Py and PU-Si3-Py show thermochromic response toward temperature, and the inflection point from the ratiometric emission as a function of temperature gives an indication of the glass transition temperature (Tg) of the polymers. The design of the excimer-based mechanophore with oligosilane provides a generally implementable way to develop mechano- and thermo-dual-responsive polymers.

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Hydrogen sulfide (H2S) is produced endogenously by several enzymatic pathways and modulates physiological functions in mammals. Quantification of H2S in biochemical systems remains challenging because of the presence of interferents with similar reactivity, particularly thiols. Herein, we present a new quantification method based on the formation of pyrene excimers in solution. We synthesized the probe 2-(maleimido)ethyl 4-pyrenylbutanoate (MEPB) and determined that MEPB reacted with H2S in a two-step reaction to yield the thioether-linked dimer (MEPB)2S, which formed excimers upon excitation, with a broad peak of fluorescence emission centered at 480 nm. In contrast, we found that the products formed with thiols showed peaks at 378 and 398 nm. The difference in emission between the products prevented the interference. Furthermore, we showed that the excimer fluorescence signal yielded a linear response to H2S, with a limit of detection of 54 nM in a fluorometer. Our quantification method with MEPB was successfully applied to follow the reaction of H2S with glutathione disulfide and to quantify the production of H2S from cysteine by Escherichia coli. In conclusion, this method represents an addition to the toolkit of biochemists to quantify H2S specifically and sensitively in biochemical systems.

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The layered semiconductor Bi2 O2 Se is a promising new-type 2D material that holds layered structure via electrostatic forces instead of van der Waals (vdW) attractions. Aside from the huge success in device performance, the non-vdW nature in Bi2 O2 Se with a built-in interlayer electric field has also provided an appealing platform for investigating unique photoexcited carrier dynamics. Here, experimental evidence for the observation of excimers in multilayer Bi2 O2 Se nanosheets via transient absorption spectroscopy is presented. It is found that the excimer formation is the primary decay pathway of photoexcited excitons and three-stage excimer dynamics with corresponding time scales are established. Excitation-fluence-dependent excimer dynamics further suggest that the excimer is diffusive and its formation can be simply described as excitons relaxed to an excimer geometry. This work indicates the outstanding promise of unique excitonic processes in Bi2 O2 Se, which may motivate novel device designs.

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Anthracene (C14 H10 ) and its derivatives, π-conjugated molecules in acenes, have been widely researched in terms of their reactions, physical properties, and self-assembly (or crystal engineering). These molecules can be functionalized to tune reactivities, optoelectronic properties, and self-assembling abilities. Structural changes in the molecular assemblies, solid states, and crystals have recently been discovered. Therefore, a systematic discussion of anthracene's molecular structure, packing, and optical properties based on its intermolecular structure and phase transitions is important for future chemical and structural design. In the present review, we discuss anthracene's molecular design, dimer packing, and crystal structure, focusing on the structural phase transitions of its crystals. We also provide examples of the phase transitions of anthracene crystals. Changes to edge-to-face of CH-π interaction and face-to-face packing of π-π interaction affect the thermodynamic stabilities of various crystal structures. These structures can inform the prediction of structural and physical properties.

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A series of D2 -symmetric macrocycles composed of alternately linked pyrene and binaphthyl moieties (binaphthyl-bridged pyrenophanes) have been synthesized. Among them, a pyrenophane possessing ether linkers at the 2,7-positions of the pyrenes exhibited intense circularly polarized luminescence (CPL) with a |glum | value of 0.053. This value is by far the highest for excimers and was not sensitive to temperature, solvent, or concentration. The CPL originated from a twisting pyrene excimer, with the (R)-binaphthyl moieties producing a left-handed twist excimer, which exhibited (-)-CPL. The electric and magnetic transition dipole moments are perfectly parallel, which is the best relationship for strong CPL.

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The title aromatic compound comprising four anthracene units was synthesized by the McMurry coupling of di(1-anthryl) ketone as a hydrogenated product in 65 % yield. The molecule forms a C2 symmetric structure with the ap conformation about the C(sp3 )-C(sp3 ) single bond, as revealed by X-ray analysis and DFT calculations. The UV/vis and fluorescence spectra of this compound were compared with those of anthracene, di(1-anthryl)methane, and 1,2-di(1-anthryl)ethane. The fluorescence spectrum showed a broad emission band at 450 nm having a long lifetime at 21 ns, which was assignable to an excimer-type emission, in contrast to the other reference compounds. The characteristic photophysical property is discussed in terms of the molecular structure with the aid of the noncovalent interaction plots and the conformational analysis.

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Chirality at different levels is widely observed in nature, but the clue to connect it all together, and the way chirality transfers among different levels are still obscure. Herein, a l-/d-lysine-based self-assembly system was constructed, in which two-step chirality transfer among three different levels was observed in aqueous solution. The chirality originated from the point chirality of amino acid derivatives l-/d-PyLys hydrochloride, and was transferred to the planar conformational chirality of water-soluble pillar[5]arene pR-/pS-WP5. Then, with the aid of pR-/pS-WP5, nanoparticles were formed that exhibited L-/R-handed circularly polarized luminescence with a dissymmetry factor of up to ±0.001, arising from pyrene chiral excimers. This multilevel chirality transfer not only provides a perspective to trace potential clues, and to pursue certain ways by which the chirality transfers, but also offers a strategy to create controllable CPL emission in aqueous media.

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A supramolecular mechanophore that can be integrated into polymers and indicates deformation by a fluorescence color change is reported. Two perylene diimides (PDIs) were connected by a short spacer and equipped with peripheral atom transfer polymerization initiators. In the idle state, the motif folds into a loop and its emission is excimer dominated. Poly(methyl acrylate) (PMA) chains were grown from the motif and the mechanophore-containing polymer was blended with unmodified PMA to afford materials that display a visually discernible fluorescence color change upon deformation, which causes the loops to unfold. The response is instant, and correlates linearly with the applied strain. Experiments with a reference polymer containing only one PDI moiety show that looped mechanophores that display intramolecular excimer formation offer considerable advantages over intermolecular dye aggregates, including a concentration-independent response, direct signaling of mechanical processes, and a more pronounced optical change.

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Variation of DNA conformation is important in regulating gene expression and mediating drug-DNA interactions. However, directly probing transient DNA conformation changes is challenging owing to the dynamic nature of this process. We show a label-free fluorescence method to monitor transient DNA conformation changes in DNA structures with various lengths and shapes using a DNA intercalator, K21. K21 can form transient excimers on the surface of DNA; the ratiometric emission of monomer and excimer correlate to DNA transient conformation stability in numerous DNA structures, including i-motifs, G-quadruplex structures, and single nucleotide mutation at random position. We analyzed the conformation dynamics of a single plasmid before and after enzyme digestion with confocal fluorescence microscopy. This method provides a label-free fluorescence strategy to probe transient conformation changes of DNA structures and has potential in uncovering transient genomic processes in living cells.

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A [c2]daisy chain rotaxane with two diarylacetylene cores was efficiently synthesized in 53 % yield by capping a C2 -symmetric pseudo[2]rotaxane composed of two diarylacetylene-substituted permethylated α-cyclodextrins (PM α-CDs) with aniline stoppers. The maximum absorption wavelength of the [c2]daisy chain rotaxane remained almost unchanged in various solvents, unlike that of the stoppered monomer, indicating that the two independent diarylacetylene cores were insulated from the external environment by the PM α-CDs. Furthermore, the [c2]daisy chain rotaxane exhibited fluorescence emission derived from both diarylacetylene monomers and the excimer, which implies that the [c2]daisy chain structure can undergo contraction and extension. This is the first demonstration of a system in which excimer formation between two π-conjugated molecules within an isolated space can be controlled by the unique motion of a [c2]daisy chain rotaxane.

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Dual-emissive tetraphenylethene (TPE) and pyrene-containing amphiphilic molecules are of great interest because they can be integrated to form stimuli responsive materials with various biological applications. Herein, we report the study of mechanically interlocked molecules (MIMs) with aggregation-induced static excimer emission (AISEE) property through a series of TPE and pyrene-based amphiphilic [2]rotaxanes, where t-butylcalix[4]arene with hydrophobic nature was used as the macrocycle. Evidently, by adorning TPE and pyrene units in [2]rotaxanes P1, P2, P1-b, and P2-b, they display remarkable emission bands in 70% of water fraction (fw) in tetrahydrofuran (THF)/water mixture, which could be attributed to the restricted intramolecular rotation of phenyl groups, whereas prominent blue-shifted excimer emission of pyrene started to appear as fw reached 80% for P1 and 90% for P1-b, P2, and P2-b, which was ascribed to the favorable π-π stacking and hydrophobic interactions of the pyrene rings that enabled their static excimer formation. The well-defined distinct amphiphilic nanostructures of [2]rotaxanes including hollowspheres, mesoporous nanostructures, spheres, and network linkages can be driven smoothly depending on the molecular structures and their aggregated states in THF/water mixture. These fascinating diversiform nanostructures were mainly controlled by the skillful manner of reversible molecular shuttling of t-butylcalix[4]arene macrocycle and also the interplay of multinoncovalent interactions. To further understand the aggregation capabilities of [2]rotaxanes, the human lung fibroblasts (MRC-5) living cell incubated with either P1, P2, P1-b, or P2-b was studied and monitored by confocal laser scanning microscopy. The AISEE property was achieved at an astonishing level by integrating TPE and pyrene to MIM-based reversible molecular switching [2]rotaxanes; furthermore, distinct nanostructures, especially hollowspheres and mesoporous nanostructures, were observed, which are rarely reported in the literature but are highly desirable for future applications.

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Excimers and exciplexes are defined as assemblies of atoms or molecules A/A' where interatomic/intermolecular bonding appears only in excited states such as [A2 ]* (for excimers) and [AA']* (for exciplexes). Their formation has become widely known because of their role in gas-phase laser technologies, but their significance in general chemistry terms has been given little attention. Recent investigations in gold chemistry have opened up a new field of excimer and exciplex chemistry that relies largely on the preorganization of gold(I) compounds (electronic configuration AuI (5d10 )) through aurophilic contacts. In the corresponding excimers, a new type of Au⋅⋅⋅Au bonding arises, with bond energies and lengths approaching those of ground-state Au-Au bonds between metal atoms in the Au0 (5d10 6s1 ) and AuII (5d9 ) configurations. Excimer formation gives rise to a broad range of photophysical effects, for which some of the relaxation dynamics have recently been clarified. Excimers have also been shown to play an important role in photoredox binuclear gold catalysis.

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The self-association of fluoroquinolones (FQ) in water would play a relevant role in their translocations across lipid membranes. Triplet excited states of these drugs have been shown as reporters of FQ self-association using laser flash photolysis technique. A study using low-temperature phosphorescence technique was performed with quinolone derivatives such as enoxacin (ENX), norfloxacin (NFX), pefloxacin (PFX), ciprofloxacin (CPX, ofloxacin (OFX), nalidixic acid (NLA), pipemidic acid (PPA) and piromidic acid (PRA) to explore emission changes associated with self-associations and to shed some light on the triplet excited state energy (ET) discrepancies described in the literature for most of these drugs. The emissions obtained at 77 K in buffered aqueous medium revealed that the amphoteric nature of the quinolones CPX, NFX, PFX, ENX, OFX and PPA must generate their self-associations because a redshift of their phosphorescence maxima is produced by FQ concentrations increases. Hence, this effect was not observed for NLA and PRA or when all quinolones were analysed using ethanol or ethylene glycol aqueous mixtures as glassed solvents. Interestingly, the presence of these organic mixtures produced a blue-shift in the phosphorescence emission maximum of each FQ. Additionally, laser flash photolysis experiments with PRA and the amphoteric quinolone PPA, compounds with the same skeleton but different peripheral substituent, confirm the expected correlations between the amphoteric nature of compounds and their self-associations in aqueous media because the excimer generation was only detected for PPA. Now, the discrepancies described in the literature for the ET of FQs can be understood considering that changes of medium polarity or proticity as well as the temperature can considerably modify their ET values. Thereby, low-temperature phosphorescence technique, is an effective way to detect molecular self-associations and surrounding changes in quinolones that opens the possibility to evaluate these effects in other drug families.

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Herein we describe the synthesis, computationally assisted spectroscopy, and lasing properties of a new library of symmetric bridged bis-BODIPYs that differ in the nature of the spacer. Access to a series of BODIPY dimers is straightforward through synthetic modifications of the pending ortho-hydroxymethyl group of readily available C-8 (meso) ortho-hydroxymethyl phenyl BODIPYs. In this way, we have carried out the first systematic study of the photonic behavior of symmetric bridged bis-BODIPYs, which is effectively modulated by the length and/or stereoelectronic properties of the spacer unit. The designed bis-BODIPYs display bright fluorescence and laser emission in non-polar media. The fluorescence response is governed by the induction of a non-emissive intramolecular charge transfer (ICT) process, which is significantly enhanced in polar media. The effectiveness of the fluorescence quenching and also the prevailing charge transfer mechanism (from the spacer itself or between the BODIPY units) rely directly on the electron-releasing ability of the spacer. Moreover, the linker moiety can also promote intramolecular excitonic interactions, leading to excimer-like emission characterized by new spectral bands and the lengthening of lifetimes. The substantial influence of the bridging moiety on the emission behavior of these BODIPY dyads and their solvent-sensitivity highlight the intricate molecular dynamics upon excitation in multichromophoric systems. In this regard, the present work represents a breakthrough in the complex relationship between the molecular structure of the chromophores and their photophysical signatures, thus providing key guidelines for rationalizing the design of tailored bis-BODIPYs with potential advanced applications.

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Metal-organic frameworks (MOFs) are emerging materials for luminescent and photochemical applications. Armed with femto to millisecond spectroscopies, and fluorescence microscopy, the photobehaviors of two Ce-based MOFs are unravelled: Ce-NU-1000 and Ce-CAU-24-TBAPy. It is observed that both MOFs show ligand-to-cluster charge transfer reactions in ≈100 and ≈70 fs for Ce-NU-1000 and Ce-CAU-24-TBAPy, respectively. The formed charge separated states, resulting in electron and hole generation, recombine in different times for each MOF, being longer in Ce-CAU-24-TBAPy: 1.59 and 13.43 µs than in Ce-NU-1000: 0.64 and 4.91 µs. The linkers in both MOFs also undergo a very fast intramolecular charge transfer reaction in ≈160 fs. Furthermore, the Ce-NU-1000 MOF reveals excimer formation in 50 ps, and lifetime of ≈14 ns. The lack of this interlinkers event in Ce-CAU-24-TBAPy arises from topological restriction and demonstrates the structural differences between the two frameworks. Single-crystal fluorescence microscopy of Ce-CAU-24-TBAPy shows the presence of a random distribution of defects along the whole crystal, and their impact on the observed photobehavior. These findings reflect the effect of linkers topology and metal clusters orientations on the outcome of electronic excitation of reticular structure, key to their applicability in different fields of science and technology, such as photocatalysis and photonics.

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Two novel pyrene-based isocyanide gold(I) complexes have been designed and synthesized. The different structures lead to distinct and diverse photophysical properties both in solution and in the aggregate state. Multiple photoluminescence, involving monomer emission, locally excited emission and excimer emission, are observed. Notably, an excimer is formed by aggregation in solution and external mechanical stimulation in the solid state, showing aggregation- and mechano-induced excimer emission.

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Phenothiazine derivatives based on the 10-phenyl-10H-phenothiazine (NAS) chromophore, namely 7-phenyl-7H-benzo[c]phenothiazine (NAS-1) and 12-phenyl-12H-benzo[a]phenothiazine (NAS-2), were designed and synthesized. NAS-1 and NAS-2 are constitutional isomers with different steric strains imposed on the phenothiazine core moiety. In solution, the more-strained NAS-2 possesses a bent structure and undergoes photoinduced structural planarization (PISP). In the crystal, despite the absence of PISP, bent NAS-2 exhibits prominent excimer emission as well as emission mechanochromism, which is not observed in the planar-like NAS and NAS-1. This unconventional observation results from the bent core structure facilitating π-π stacking of the peripheral naphthalene moieties. Two-photon-coupled depth-dependent emission shows spectral differences between the surface and kernel of the NAS-2 crystal, and is believed to be a general phenomenon, at least in part, for materials exhibiting emission mechanochromism.