How do spin-scaled double hybrids designed for excitation energies perform for noncovalent excited-state interactions? An investigation on aromatic excimer models.

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.

Photophysical Analysis of Aggregation-Induced Emission (AIE) Luminogens Based on Triphenylamine and Thiophene: Insights into Emission Behavior in Solution and PMMA Films.

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.

Temperature-Induced Sign Inversion of Circularly Polarized Luminescence of Binaphthyl-Bridged Tetrathiapyrenophanes.

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.

Fluorescent detection of hydrogen sulfide (H2S) through the formation of pyrene excimers enhances H2S quantification in biochemical systems.

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.

Binaphthyl-Bridged Pyrenophanes: Intense Circularly Polarized Luminescence Based on a D2 Symmetry Strategy.

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.

Cyclodextrin-Based [c2]Daisy Chain Rotaxane Insulating Two Diarylacetylene Cores.

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.

Multilevel Chirality Transfer from Amino Acid Derivatives to Circularly Polarized Luminescence-Active Nanoparticles in Aqueous Medium.

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.

Folded Perylene Diimide Loops as Mechanoresponsive Motifs.

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.

Probing Transient DNA Conformation Changes with an Intercalative Fluorescent Excimer.

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.

Excimer and Exciplex Formation in Gold(I) Complexes Preconditioned by Aurophilic Interactions.

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.

Mechanoresponsive, Luminescent Polymer Blends Based on an Excimer-Forming Telechelic Macromolecule.

A well-known approach toward mechanochromic polymers relies on the incorporation of excimer-forming fluorophores into a matrix polymer and the disruption of aggregated chromophores when such materials undergo macroscopic mechanical deformation. However, the required aggregates and stress-transfer processes have so far only been realized with select dye/polymer combinations. As demonstrated here, the utility of this approach can be extended by tethering an excimer-forming cyano-substituted oligo(p-phenylene vinylene) fluorophore to the two ends of a telechelic poly(ethylene-co-butylene) and blending small amounts (0.1-2 wt%) of the resulting aggregachromic macromolecule into polymer matrices such as poly(ε-caprolactone), poly(isoprene), or poly(styrene-b-butadiene-b-styrene). All blends display mechanofluorochromic responses, and the ratio between the monomer and excimer emission intensities can be used to correlate the luminescence signal to the extent of deformation and to follow subsequent relaxation processes. The developed approach significantly expands the scope of blend-based mechanoresponsive luminescent materials.

Excitation Wavelength Dependent Fluorescence of an ESIPT Triazole Derivative for Amine Sensing and Anti-Counterfeiting Applications.

Excitation wavelength dependent (Ex-De) emission materials have potential applications in anti-counterfeiting labels and bioimaging. Nevertheless, few purely organic chromophores are used in these areas. In this study, multiple excited states were incorporated into a molecule that was excited state intramolecular proton transfer (ESIPT) active, with the goal of manipulating the relaxation pathways of the excited states. The triazole derivative exhibits Ex-De photoluminescence (PL), and the maximum PL wavelength is located at 526 nm and 593 nm under a series of excitation wavelengths. Spectral identification indicates that the excimer and ESIPT processes are responsible for the green (526 nm) and orange (593 nm) fluorescence, respectively. Importantly, the quick response code and test strip prepared with this triazole derivative can be used for anti-counterfeiting and food spoilage detection applications, respectively. This research opens the door for developing novel Ex-De materials for anti-counterfeiting purposes.

Phenothiazine Scope: Steric Strain Induced Planarization and Excimer Formation.

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.

Luminescent Radical-Excimer: Excited-State Dynamics of Luminescent Radicals in Doped Host Crystals.

The excited-state dynamics of the photostable luminescent organic radical (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl (PyBTM) doped in a host crystal was investigated by using optically detected magnetic resonance (ODMR) and time-resolved emission spectroscopies. In the radical system, the unpaired electron can be used as the probe for studying the electronic state and its dynamics. The mixed crystal with a high concentration of the radical showed excimer emission, together with the monomer emission. The ODMR signals were observed with opposite signs for monitoring the monomer and the excimer emissions. Based on their temperature and concentration dependencies, the excited-state dynamics on the doped crystal and the mechanism of the excimer formation and the ODMR signal generation are discussed with the help of the quantum mechanical simulation of the excited-state spin dynamics. The initial process of excimer formation has been clarified for the first time from the viewpoint of the spin-dynamics.

Paintable Room-Temperature Phosphorescent Liquid Formulations of Alkylated Bromonaphthalimide.

Organic phosphors have been widely explored with an understanding that crystalline molecular ordering is a requisite for enhanced intersystem crossing. In this context, we explored the room-temperature phosphorescence features of a solvent-free organic liquid phosphor in air. While alkyl chain substitution varied the physical states of the bromonaphthalimides, the phosphorescence remained unaltered for the solvent-free liquid in air. As the first report, a solvent-free liquid of a long swallow-tailed bromonaphthalimide exhibits room-temperature phosphorescence in air. Doping of the phosphor with carbonyl guests resulted in enhanced phosphorescence, and hence a large-area paintable phosphorescent liquid composite with improved lifetime and quantum yield was developed.

Programmable and Sequence-Selective Supramolecular Assembly of Two Different Chromophores along DNA Templates.

The templated self-assembly of two different chromophores attached to the same binding motif gives oligomers of statistical mixtures without any control on the resulting chromophore sequence. The first self-assembled bichromophore system is reported, in which the resulting chromophore sequence is simply governed by the DNA template. A new ethynylpyrene-diaminopurine 2'-deoxynucleoside was combined with the ethynyl nile red-modified 2'-deoxyuridine to achieve this completely programmable sequence selective self-assembly. The helical ordering of the two different chromophores by DNA templates with different mixtures of A-T sequences yields precise control over the optical properties of the formed assemblies, which was proven by methods of optical spectroscopy.

Enhanced Fluorescence Properties of Stilbene-Containing Alternating Copolymers.

In recent years, nonconjugated, fluorophore-free organic polymers have emerged as potentially useful light-emitting materials. The fluorescence properties of a novel class of nonconjugated, tert-butyl carboxylate functionalized stilbene-containing alternating copolymers are investigated in this work. These sterically crowded, semi-rigid copolymers exhibit very strong blue fluorescence in organic solvents upon irradiation. The origin of the fluorescent band with high quantum yield is attributed to the "through space" π-π interactions between the phenyl rings from the stilbene and CO groups from the anhydride groups. To the best of our knowledge, the di-tert-butyl group-containing stilbene and maleic anhydride alternating copolymer showed one of the highest fluorescent intensities among all fluorophore-free polymers. The excellent linearity of the luminescence property of this copolymer is an important attribute for future potential quantitative applications. The fluorescence is maintained when the tert-butyl groups are removed and the resulting carboxylic acid-functionalized copolymer is dissolved in water at neutral pH, which can render these copolymers as attractive candidates for diagnostic and therapeutic applications.

Carborane-Induced Excimer Emission of Severely Twisted Bis-o-Carboranyl Chrysene.

The synthesis of a highly twisted chrysene derivative incorporating two electron deficient o-carboranyl groups is reported. The molecule exhibits a complex, excitation-dependent photoluminescence, including aggregation-induced emission (AIE) with good quantum efficiency and an exceptionally long singlet excited state lifetime. Through a combination of detailed optical studies and theoretical calculations, the excited state species are identified, including an unusual excimer induced by the presence of o-carborane. This is the first time that o-carborane has been shown to induce excimer formation ab initio, as well as the first observation of excimer emission by a chrysene-based small molecule in solution. Bis-o-carboranyl chrysene is thus an initial member of a new family of o-carboranyl phenacenes exhibiting a novel architecture for highly-efficient multi-luminescent fluorophores.

Bottom-up Synthesis and Self-Assembly of Copper Clusters into Permanent Excimer Supramolecular Nanostructures.

Metal clusters with appropriate molecular ligands have been shown to be suitable subnanometer building blocks for supramolecular architectures with controlled secondary interactions, providing access to physical regimes not achievable with conventional intermolecular motifs. An example is the excimer photophysics exhibited by individual cluster-based superstructures produced by top-down etching of gold nanoparticles. Now, a supramolecular architecture of copper clusters is presented with controlled optical properties and efficient non-resonant luminescence produced via a novel bottom-up synthesis using mild green reductants followed by a ligand exchange reaction and spontaneous supramolecular assembly. Spectroscopic experiments confirm the formation of the intercluster network and reveal the permanent nature of their excimer-like behavior, thus extending the potential impact and applicability of metal cluster superstructures as efficient and stable non-resonant single-particle emitters.

Janusarene: A Homoditopic Molecular Host.

A homoditopic molecular host, janusarene, is presented that has two back-to-back compactly arranged nanocavities for guest complexation. The unique two-face structural feature of janusarene allows it to bind and align various guest compounds concurrently, which include spherical pristine fullerene C60 and planar polycyclic aromatic hydrocarbons (PAHs), such as pyrene, perylene, and 9,10-dimethylanthracene. The host-guest interactions were characterized by single-crystal X-ray diffraction. A pairwise encapsulation of the PAH guests by janusarene enables PAH dimers to be obtained that deliver spectroscopic properties distinct from those of PAHs dissolved in solution, or in the bulk state. A monotopic control host was also synthesized and used to characterize the host-guest complexing behavior in solution.