Dipyrene-Terminated Oligosilanes Enable Ratiometric Fluorescence Response in Polymers toward Mechano- and Thermo-Stimuli

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 Si_n-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 Si_n-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 (T_g) 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.

Excimer Formation in the Non-Van-Der-Waals 2D Semiconductor Bi2 O2 Se

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.

Structural Phase Transitions in Anthracene Crystals

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.

Structure and Photophysical Properties of 1,1,2,2-Tetra(1-anthryl)ethane: A C(sp3 )-C(sp3 ) Bond Substituted with Four Anthracene Units

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.

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.

Gated Photoreactivity of Pyrene Copolymers in Multiresponsive Cross-Linked starPEG-Hydrogels

The synthesis and manufacturing of multiresponsive polymer hydrogels using simple components is a notable challenge. Pyrene is an excimer-forming fluorophore mostly used as microenvironmental probe and for the localization of molecules in close proximity in artificial and biomaterials. Here we make use of the solvophobic preaggregation and photolysis properties of pyrene to construct multiresponsive hydrogels. We synthesize poly(ethylene glycol) (PEG) hydrogels from well-defined pyrene-substituted macro-cross-linkers and elucidate their intricate intra- and intermolecular excimer formation pathways. We find that controlling the water content of the hydrogels through the degree of swelling acts as a gating stimulus governing the photoinduced solvolysis of pyrenylmethyl esters from their poly(methacrylate) backbone. This allows the implementation of a simple transient photolithography process. We thus demonstrate that multiresponsive soft materials with complex optical and mechanical responses can be obtained with comparatively little synthetic effort.

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.

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.

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.

Tuning the Photonic Behavior of Symmetrical bis-BODIPY Architectures: The Key Role of the Spacer Moiety

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.

Unravelling Why and to What Extent the Topology of Similar Ce-Based MOFs Conditions their Photodynamic: Relevance to Photocatalysis and Photonics

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.

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.

Coupled Excited-State Dynamics in N-Substituted 2-Methoxy-9-Acridones

Fluorophores of the acridone family have been widely employed in many applications, such as DNA sequencing, the detection of biomolecules, and the monitoring of enzymatic systems, as well as being the bases of intracellular sensors and even antitumoral agents. They have been widely used in fluorescence imaging due to their excellent photophysical properties, in terms of quantum yield and stability. However, frequently, the fluorescence emission data from acridones are not easily interpretable due to complex excited-state dynamics. The formation of π-stacking aggregates and excimers and excited-state proton transfer (ESPT) reactions usually result in emission features that are dependent on the experimental conditions. Therefore, an in-depth understanding of the dynamics involved in the excited-state transients of these dyes is mandatory for their appropriate application. Herein, we synthesized and fully characterized different 2-methoxy-9-acridone dyes. Their transient fluorescence emission spectra exhibited a complex dynamic behavior that can be linked to several excited-state reactions. We performed a thorough study of the excited-state dynamics of these dyes by means of time-resolved fluorimetry supported by computational calculations. All this allowed us to establish a multistate kinetic scheme, involving an ESPT reaction coupled to an excimer formation process. We have unraveled the rich dynamics behind this complex behavior, which provides a better understanding of the excited states of these dyes.

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.

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.

Reversible Mechanochromic Delayed Fluorescence in 2D Metal-Organic Micro/Nanosheets: Switching Singlet-Triplet States through Transformation between Exciplex and Excimer

Mechanochromic luminescent materials have attracted much attention and present a variety of applications in information security, data recording, and storage devices. However, most of these smart luminescent systems are based on typical fluorescence and/or phosphorescence mechanisms; the mechanochromic delayed fluorescence (MCDF) materials involving switching singlet and triplet states are rarely studied to date. Herein, new 2D layered metal-organic micro/nanosheets, [Cd(9-AC)2(BIM)2] (named as MCDF-1; 9-AC = anthracene-9-carboxylate and BIM = benzimidazole) and its solvate form containing interlayer CH3CN (named as MCDF-2), which exhibit reversible mechanochromic delayed fluorescence characteristics, are presented. With applying the mechanical force, the luminescent center of MCDF-1 can be converted from 9-AC/BIM exciplex to 9-AC/9-AC excimer, resulting in alternations of delayed fluorescence. Such luminescent change can be further recovered by CH3CN fumigation, accompanied by the structural transformation from MCDF-1 to MCDF-2. Furthermore, the force-responsive process also refers to the energy redistribution between singlet and triplet states as inferred by both temperature-dependent photophysics and theoretical calculations. Therefore, this work not only develops new 2D micro/nanosheets as MCDF materials, but also supplies a singlet-triplet energy switching mechanism on their reversible mechanochromic process.

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.

Dual-Emissive Waterborne Polyurethanes Prepared from Naphthalimide Derivative

Fluorescent and room-temperature phosphorescent (RTP) materials are widely used in bioimaging, chemical sensing, optoelectronics and encryption. Here, a series of single-component dual-emissive waterborne polyurethanes (WPUs) with both fluorescence and room-temperature phosphorescence were synthesized. Dye without halogen atom incorporated into WPUs can only exhibit fluorescence due to poor spin-orbit coupling. When bromine atom is introduced into dye, we found that WPUs can emit both fluorescence and room-temperature phosphorescence with lifetimes up to milliseconds because of enhanced spin-orbit coupling. Moreover, with an increase in dye concentrations in WPUs, excimers are formed due to the aggregation effect, and may promote communication between singlet and triplet states. At different dye concentrations, structural, thermal, and luminescent properties serve as the main focus.

Metal-Cluster-Based Colloidal Excimer Superstructures

Luminescent network: Colloidal excimer superstructures with unique optical and electronic properties have recently been described. Ground-state gold cluster cores were held together by the hydrogen-bonding network formed by their capping ligands, which enabled excimer formation.

Crystal structure of 2-(thio-phen-3-yl)ethyl pyrene-1-carboxyl-ate

In the title compound, C23H16O2S, the thio-phene group is rotationally disordered into two fractions almost parallel to each other, with occupation factors of 0.523 (7) and 0.477 (7), and subtending dihedral angles of 10.5 (5) and 9.3 (5)°, respectively, to the thio-phene group. The mol-ecules are held together by weak C-H⋯O and C-H⋯π hydrogen bonds, producing a laminar arrangement, which are further connected in a perpendicular fashion by S⋯π contacts [S⋯centroid = 3.539 (8) and 3.497 (8) Å]. In spite of the presence of the entended pyrene group, the structure does not present any parallel π-π stacking inter-actions. The structure was refined as an inversion twin.

Mechanofluorochromism of 1-Alkanoylaminopyrenes

To create a new series of mechanofluorochromic materials and to elucidate the mechanism of the phenomenon of mechanofluorochromism, 1-alkanoylaminopyrenes including 1-acetylaminopyrene (AAPy), 1-octanoylaminopyrene (OAPy), and 1-stearoylaminopyren (SAPy) were prepared. It was found that these materials exhibited mechanofluorochromism with emission colors in the crystalline samples changing reversibly from bluish purple to yellowish green, which could be induced by mechanical grinding. X-ray crystal structure analysis, electronic absorption, and fluorescence spectroscopies, as well as fluorescence lifetime analysis and powder X-ray diffraction analysis of AAPy suggested that the present mechanofluorochromism was caused by developing crystal defects through grinding. Intermolecular hydrogen bonds were suggested to play an important role in the occurrence of mechanofluorochromism, suppressing the face-to-face overlapping of pyrene moieties to form excimers in the pristine crystal.

Excimer formation in a confined space: photophysics of ladderphanes with tetraarylethylene linkers

Communication between chromophores is vital for both natural and non-natural photophysical processes. Spatial confinements offer unique conditions to scrutinize such interactions. Polynorbornene- and polycyclobutene-based ladderphanes are ideal model compounds in which all tetraarylethylene (TAE) linkers are aligned coherently. The spans for each of the monomeric units in these ladderphanes are 4.5-5.5 Å. Monomers do not exhibit emission, because bond rotation in TAE can quench the excited-state energy. However, polymers emit at 493 nm (Φ=0.015) with large Stokes shift under ambient conditions and exhibit dual emission at 450 and 493 nm at 150 K. When the temperature is lowered, the emission intensity at 450 nm increases, whereas that at 493 nm decreases. At 100 K, both monomers and polymers emit only at 450 nm. This shorter-wavelength emission arises from the intrinsic emission of TAE chromophore, and the emission at 493 nm could be attributed to the excimer emission in the confined space of ladderphanes. The fast kinetics suggest diffusion-controlled formation of the excimer.

Supramolecular clippers for controlling photophysical processes through preorganized chromophores

A novel supramolecular clipping design for influencing the photophysical properties of functional molecular assemblies, by the preorganization (clipping) of chromophores, is described. Several chromophores end functionalized with molecular recognition units were designed. These molecular recognition units serve as handles to appropriately position these systems upon noncovalent interactions with multivalent guest molecules (supramolecular clippers). Towards this goal, we have synthesized 1,5-dialkoxynaphthalene (DAN) and naphthalenediimide (NDI) functionalized with dipicolylethylenediamine (DPA) motifs. These molecules could preorganize upon noncovalent clipping with adenosine di- or triphosphates, which resulted in preassociated excimers and mixed (cofacial) charge-transfer (CT) assemblies. Chiral guest binding could also induce supramolecular chirality, not only into the individual chromophoric assembly but also into the heteromeric CT organization, as seen from the strong circular dichroism (CD) signal of the CT transition. The unique ability of this design to influence the intermolecular interactions by changing the binding strength of the clippers furthermore makes it very attractive for controlling the bimolecular photophysical processes.

A Review of Luminescent Anionic Nano System: d10 Metallocyanide Excimers and Exciplexes in Alkali Halide Hosts

Dicyanoaurate, dicyanoargentate, and dicyanocuprate ions in solution and doped in different alkali halide hosts exhibit interesting photophysical and photochemical behavior, such as multiple emission bands, exciplex tuning, optical memory, and thermochromism. This is attributed to the formation of different sizes of nanoclusters in solution and in doped hosts. A series of spectroscopic methods (luminescence, UV-reflectance, IR, and Raman) as well as theoretical calculations have confirmed the existence of excimers and exciplexes. This leads to the tunability of these nano systems over a wide wavelength interval. The population of these nanoclusters varies with temperature and external laser irradiation, which explains the thermochromism and optical memory. DFT calculations indicate an MLCT transition for each nanocluster and the emission energy decreases with increasing cluster size. This is in agreement with the relatively long life-time for the emission peaks and the multiple emission peaks dependence upon cluster concentration.

Fluorescence spectroscopy and molecular dynamics simulations in studies on the mechanism of membrane destabilization by antimicrobial peptides

Since their initial discovery, 30 years ago, antimicrobial peptides (AMPs) have been intensely investigated as a possible solution to the increasing problem of drug-resistant bacteria. The interaction of antimicrobial peptides with the cellular membrane of bacteria is the key step of their mechanism of action. Fluorescence spectroscopy can provide several structural details on peptide-membrane systems, such as partition free energy, aggregation state, peptide position and orientation in the bilayer, and the effects of the peptides on the membrane order. However, these "low-resolution" structural data are hardly sufficient to define the structural requirements for the pore formation process. Molecular dynamics simulations, on the other hand, provide atomic-level information on the structure and dynamics of the peptide-membrane system, but they need to be validated experimentally. In this review we summarize the information that can be obtained by both approaches, highlighting their versatility and complementarity, suggesting that their synergistic application could lead to a new level of insight into the mechanism of membrane destabilization by AMPs.