Showing papers on "Excimer published in 2015"

Electronic communication between fluorescent pyrene excimers and spin crossover complexes in nanocomposite particles

Abstract: Bistable [Fe(H-trz)2(trz)]BF4 core and silica shell hybrid nanoparticles with different sizes between 50–150 nm were grafted with a pyrene-derivative fluorophore. The fluorescent composites exhibited both monomer and excimer luminescence (with a controllable ratio), whilst their spin transition properties remained unaltered. When switching the particles from the diamagnetic to the paramagnetic state the excimer lifetime and luminescence intensity increased and the emission spectra were redshifted, while the monomer emission exhibited negligible spin-state dependence. The strong coupling of the pyrene excimers with the spin-state of the ferrous ions was shown to occur via a non-radiative mechanism, brought about, to a large extent, by the spin transition induced mechanical strain.

Theoretical investigations of the perylene electronic structure: Monomer, dimers, and excimers

Abstract: The structural and electronic properties of perylene molecule, dimers, and excimers have been computationally studied. The present work represents the first systematic study of perylene molecule and dimer forms by means of long-range corrected time-dependent density functional theory (TDDFT) approaches. Initially, the study explores the photophysical properties of the molecular species. Vertical transitions to many excited singlet states have been computed and rationalized with different exchange-correlation functionals. Differences between excitation energies are discussed and compared to the absorption spectrum of perylene in gas phase and diluted solution. De-excitation energy from the relaxed geometry of the lowest excited singlet is in good agreement with the experimental fluorescence emission. Optimization of several coplanar forms of the perylene pair prove that, contrary to generalized gradient approximation (GGA) and hybrid exchange-correlation functionals, corrected TDDFT is able to bind the perylene dimer in the ground state. Excitation energies from different dimer conformers point to dimer formation prior to photoexcitation. The fully relaxed excimer geometry belongs to the perfectly eclipsed conformation with D2h symmetry. The excimer equilibrium intermolecular distance is shorter than the separation found for the ground state, which is an indication of stronger interchromophore interaction in the excimer state. Excimer de-excitation energy is in rather good agreement with the excimer band of perylene in concentrated solution. The study also scans the energy profiles of the ground and lowest excited states along several geometrical distortions. The nature of the interactions responsible for the excimer stabilization is explored in terms of excitonic and charge resonance contributions. © 2015 Wiley Periodicals, Inc.

Photo‐Ionization of 3d‐Ions in Fluoride‐Phosphate Glasses

Abstract: Interaction of radiation with glasses often modifies their properties (e.g., the refractive index). Most noticeable is a loss in transmission. Such phenomena can be exploited in photo-sensitive materials, but have to be avoided in other applications. To improve the understanding of defect generation processes, a systematic comparison of defect formation in (fluoride-)phosphate glasses doped with low concentrations of 3d ions was attempted. Samples doped with 10–5000 ppm of Ti, V, Cr, Mn, Fe, Co, or Ni were irradiated in the UV range by excimer lasers. Defects, which generally form in the levels of several ppm, were characterized by optical and by electron spin resonance spectroscopy. V4+ was photo-oxidized to the empty valence shell d0 ion. Co2+, Mn2+, and Fe2+ were all photo-oxidized to the trivalent state while Ni2+ was photo-reduced. The fully oxidized Ti4+ was also photo-reduced. Cr3+ showed photo-disproportionation into Cr2+ and Cr6+. Qualitative and quantitative changes in defect formation rates depend not only on the ion, but also on the radiation parameters, for example, the wavelength of the excimer lasers used (193, 248, 351 nm) or the initial transmission of the glass samples at the chosen laser wavelength. Defect recovery was followed up to 10 years after the irradiation experiments.

Ensemble density functional theory method correctly describes bond dissociation, excited state electron transfer, and double excitations

Abstract: State-averaged (SA) variants of the spin-restricted ensemble-referenced Kohn-Sham (REKS) method, SA-REKS and state-interaction (SI)-SA-REKS, implement ensemble density functional theory for variationally obtaining excitation energies of molecular systems. In this work, the currently existing version of the SA-REKS method, which included only one excited state into the ensemble averaging, is extended by adding more excited states to the averaged energy functional. A general strategy for extension of the REKS-type methods to larger ensembles of ground and excited states is outlined and implemented in extended versions of the SA-REKS and SI-SA-REKS methods. The newly developed methods are tested in the calculation of several excited states of ground-state multi-reference systems, such as dissociating hydrogen molecule, and excited states of donor–acceptor molecular systems. For hydrogen molecule, the new method correctly reproduces the distance dependence of the lowest excited state energies and describes an avoided crossing between the doubly excited and singly excited states. For bithiophene–perylenediimide stacked complex, the SI-SA-REKS method correctly describes crossing between the locally excited state and the charge transfer excited state and yields vertical excitation energies in good agreement with the ab initio wavefunction methods.

Excited state evolution of DNA stacked adenines resolved at the CASPT2//CASSCF/Amber level: from the bright to the excimer state and back

Abstract: Deactivation routes of bright ππ* (La) and excimer charge transfer (CT) states have been mapped for two stacked quantum mechanical (CASPT2//CASSCF) adenines inside a solvated DNA double strand decamer (poly(dA)·poly(dT)) described at the molecular mechanics level. Calculations show that one carbon (C2) puckering is a common relaxation coordinate for both the La and CT paths. By mapping the lowest crossing regions between La and CT states, together with the paths connecting the two states, we conclude that at least one CT state can be easily accessible. The lowest-lying conical intersections between ground state (GS) and CT states have been fully characterized in a realistic DNA environment for the first time. We show that the path to reach this crossing region from the CT minima involves high barriers that are not consistent with experimental data lifetimes. Instead, the multiexponential decay recorded in DNA, including the longest (ca. 100 picoseconds) lifetime component detected in oligomeric single- and double-stranded systems, is compatible with both intra-monomer relaxation processes along the La deactivation path (involving small barriers) and the population of the excimer (CT) state that behaves as a trap. In the latter case, deactivation is feasible only going back to the La state by following its preferred decay coordinate.

Theoretical study on the dehydrogenation reaction of dihydrogen bonded phenol-borane-trimethylamine in the excited state.

Abstract: Time dependent density functional theory (TDDFT) and transition state theory (TST) have been performed to study the dehydrogenation process of dihydrogen bonded phenol–borane-trimethylamine (phenol–BTMA) in the excited state. The potential curve of phenol–BTMA in the ground state confirms that the dehydrogenation process does not occur in the ground state. The analysis of the geometric structure and infrared spectra demonstrate that the dihydrogen bond O–H⋯H1–B of phenol–BTMA is considerably strengthened with the cleavage of O–H when excited to the first excited state. Based on the geometric structure in the first excited state, a transition state is found with the only imaginary frequency pointing to the formation of the hydrogen molecule. This finding implies the occurrence of the dehydrogenation process of phenol–BTMA in the excited state. The dehydrogenation reaction is fully completed in the reaction product and the new formed hydrogen molecule moves away from the plane of the benzene ring. This work provides a theoretical model for the dehydrogenation process of phenol–BTMA in the excited state.

Photoinduced electron transfer (PET) versus excimer formation in supramolecular p/n-heterojunctions of perylene bisimide dyes and implications for organic photovoltaics.

Abstract: Foldamer systems comprised of two perylene bisimide (PBI) dyes attached to the conjugated backbones of 1,2-bis(phenylethynyl)benzene and phenylethynyl-bis(phenylene)indane, respectively, were synthesized and investigated with regard to their solvent-dependent properties. UV/Vis absorption and steady-state fluorescence spectra show that both foldamers exist predominantly in a folded H-aggregated state consisting of π-π-stacked PBIs in THF and in more random conformations with weaker excitonic coupling between the PBIs in chloroform. Time-resolved fluorescence spectroscopy and transient absorption spectroscopy reveal entirely different relaxation pathways for the photoexcited molecules in the given solvents, i.e. photoinduced electron transfer leading to charge separated states for the open conformations (in chloroform) and relaxation into excimer states with red-shifted emission for the stacked conformations (in THF). Supported by redox data from cyclic voltammetry and Rehm-Weller analysis we could relate the processes occurring in these solution-phase model systems to the elementary processes in organic solar cells. Accordingly, only if relaxation pathways such as excimer formation are strictly avoided in molecular semiconductor materials, excitons may diffuse over larger distances to the heterojunction interface and produce photocurrent via the formation of electron/hole pairs by photoinduced electron transfer.

Controllable multicolor switching of oligopeptide-based mechanochromic molecules: from gel phase to solid powder

Abstract: A series of molecules with pyrene and rhodamine B as color-producing mechanophores linked by different spacers were synthesized and their mechanochromic properties were studied. Interestingly, we found that the molecule with diphenylalanine as a linker (PHE-2) showed a sequential multicolored switch from deep blue to bluish green and further to a reddish color, which was associated with the phase change from gel to xerogel as the solvent evaporated, and to a solid powder triggered by grinding. However, the gel and xerogel of the molecule with the pentaphenylalanine linker (PHE-5) exhibited the same deep blue color that switched to bluish green and further to a reddish powder by virtue of continuously grinding the xerogel sample in situ. The multicolored switching of PHE-2 and PHE-5 was realized by the variation of the self-assembled structures, which induced the transition of the pyrene excimers from excimer 1 (deep blue) to excimer 2 (bluish green), and by the chemical reaction of rhodamine B from a spirolactam to a ring-opened amide (red). From the experimental results, we may conclude that the crucial point for controlling and tuning the tricolored fluorescent switch of this system is to constrict the pyrene excimer in an overlapped packing mode, which can be achieved (1) by controlling the molecular structure; and (2) by confining the excimers of pyrene in a restricted environment.

Energy transfer and spectroscopic characterization of a perylenetetracarboxylic diimide (PDI) hexamer

Abstract: We report a comprehensive study on a newly synthesized perylenetetracarboxylic diimide (PDI) hexamer together with its corresponding monomer and dimer by means of steady-state absorption and fluorescence as well as femtosecond broadband transient absorption measurements. The structure of the PDI hexamer is nearly arranged in a 3-fold symmetry by three identical and separated dimers. This unique structure makes the excited state energy relaxation processes more complex due to the existence of two different intramolecular interactions: a strong interaction between face-to-face PDIs in dimers and a relatively weak interaction between the three separated PDI dimers. The steady-state spectra and the ground state structural optimization show that the steric effect plays a dominant role in keeping the formation of the face-to-face stacked PDI-dimer within the PDI-hexamer, indicating that some level of a pre-associated excimer had formed already in the ground state for the dimer in the hexamer. Femtosecond transient absorption experiments on the PDI hexamer reveal a fast (∼200 fs) localization process and a sequential relaxation to a pre-associated excimer trap state from the delocalized exciton state with about 1.2 ps after the initially delocalized excitation. Meanwhile, excitation energy transfer among the three separated dimers within the PDI-hexamer is also revealed by the anisotropic femtosecond pump–probe transient experiments, where the hopping time is about 2.8 ps. A relaxed excimer state is further formed in 7.9 ps after energy hopping and conformational relaxation.

A pyrene-bridged macrocage showing no excimer fluorescence.

Abstract: Pyrene is a common organic luminescent material. To improve the fluorescence properties of pyrene, we have designed a pyrene-2,7-diyl bridged macrocage in which the pyrene moiety is sterically protected by the outside alkyl chains. The macrocage shows intense fluorescence from a monomeric excited state without excimer fluorescence even in saturated solutions, although the parent pyrene shows excimer fluorescence in highly concentrated solutions. These results indicate that the steric shielding by the cage prevents the formation of the excimer. Intensities of florescence in the presence of nitrobenzene were investigated to clarify the cage effects on fluorescence quenching. Lower efficiency of the fluorescence quenching caused by intermolecular collision between the caged pyrene (fluorophore) and nitrobenzene (quencher) was revealed by the analysis of the bimolecular quenching constants kq.

Smart excimer fluorescence probe for visual detection, cell imaging and extraction of Hg2+

Abstract: Smart pyrene-based simple fluorescent probes 2 and 4 were designed, synthesized and characterized by different spectroscopic methods. The photophysical properties of the probes and their affinity towards different metal ions in phosphate buffer were investigated. Upon selective interaction with Hg2+, the molecular probes showed enhanced static excimer emission at 506 nm along with a naked-eye detectable chromo- and fluoro-genic response. Probe 2 sensitively showed a high limit of detection (3.4 pM) for Hg2+ in the solution. The pyrene silicate derivative, 4, was utilized to detect and extract Hg2+ in solution as well as in the solid state. The data obtained from NMR and ESI-MS spectroscopy supported the postulate that the mode of interaction of the probe with Hg2+ involves the N and O atoms of the –CN and –OH functional groups to complex Hg2+ in 2:1 stoichiometry. Moreover, probe 2 exhibited excellent selectivity for Hg2+ in protein medium (BSA/HSA) and was used to detect Hg2+ in live HeLa cells, on test paper strips and in real contaminated water samples.

Sn(II) induced concentration dependent dynamic to static excimer conversion of a conjugated naphthalene derivative

Abstract: The X-ray structurally characterized naphthalene appended diformyl-p-cresol derivative (L) selectively detects Sn2+ by both colorimetric and fluorescence methods. In the presence of Sn2+, L exhibits a monomer emission at 420 nm along with a strong red excimer emission at 582 nm in acetonitrile. The excimer formation highly depends on Sn2+ concentration. The dynamic excimer, observed with up to 2.5 equivalents of Sn2+, gradually converts to a static form above 2.5 equivalents of Sn2+. Moreover, in a different solvent media, viz. in aqueous methanol, L can also detect Al3+ through the generation of intense green fluorescence. The photophysical interactions are rationalized by 1H NMR, mass spectra, steady state and lifetime fluorescence measurements. DFT studies support the experimental findings.

Topographical changes in polyester after chemical, physical and enzymatic hydrolysis

Abstract: Polyester was treated with alkali, VUV excimer, and a hydrolytic enzyme. The effect of each treatment on the physicochemical properties of polyester (PET) was studied and compared by qualitative and quantitative methods. Scanning electron microscopy and atomic force microscopy analysis show that alkali treatment causes severe fiber degradation with formation of pits on the fabric surface. In contrast, excimer treatment results in uniform pitting at nanometer level, which is restricted to the surface. In case of enzyme-treated samples, non-uniform roughness was observed due to deposition of residual protein formed due to enzyme hydrolysis. K/S value and moisture regain value of excimer-treated samples were found to be higher than alkali- or enzyme-treated PET. Out of the three methods, excimer treatment appears to be the best method for PET modification with maximum number of polar groups being created on hydrolysis with marginal loss in strength and weight.

Solvent dependent intramolecular excimer emission of di(1-pyrenyl)silane and di(1-pyrenyl)methane derivatives

Abstract: Di(1-pyrenyl)silane and di(1-pyrenyl)methane derivatives showed unprecedented intramolecular excimer emission in polar organic solvents such as DMSO and the ratio of excimer/monomer emissions strongly depends on the dielectric constants of the solvents.

Electron-beam-ignited, high-frequency-driven vacuum ultraviolet excimer light source

Abstract: Transformation of a table-top electron beam sustained 2.45 GHz RF discharge in rare gases into a self burning discharge has been observed for increasing RF-amplitude. Thereby, the emission spectrum undergoes significant changes in a wide spectral range from the vacuum ultraviolet (VUV) to the near infrared. A strong increase of VUV excimer emission is observed for the self burning discharge. The so called first excimer continuum, in particular, shows a drastic increase in intensity. For argon this effect results in a brilliant light source emitting near the 105 nm short wavelength cutoff of LiF windows. The appearance of a broad-band continuum in the UV and visible range as well as effects of RF excitation on the atomic line radiation and the so called third excimer continuum are briefly described.

Intramolecular Excimer Formation Dynamics of 1,3-Bis-(1-pyrenyl)propane within 1-Butyl-3-methylimidazolium Hexafluorophosphate and Its Polyethylene Glycol Mixtures.

Abstract: Mixtures of ionic liquid with polyethylene glycol (PEG) have shown interesting features as solubilizing media. Intramolecular excimer formation dynamics of 1,3-bis-(1-pyrenyl)propane [1Py(3)1Py] is investigated within mixtures of a common and popular ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) with PEGs of average molecular weight (MW) 200 (PEG200), average MW 400 (PEG400), number-average MW Mn 570–630 (PEG600), and number-average MW Mn 950–1050 (PEG1000) over the complete composition range at a 10° interval in the temperature range 10–90 °C. Irrespective of the composition of the medium and the temperature, excited-state intensity decay of the excimer fluorescence best fits to a three-exponential decay function, suggesting the presence of one excited-state monomer and two kinetically distinguishable excimers where both excimers are populated simultaneously by the excited monomer with no interconversion between the two excimers. In neat PEGs for temperatures ≤ 50 °C, intensi...

Mid-infrared ultrafast laser pulses induced third harmonic generation in nitrogen molecules on an excited state

Abstract: We report on generation of third harmonic from nitrogen molecules on the excited state with a weak driver laser pulse at a mid-infrared wavelength. The excited nitrogen molecules are generated using a circularly polarized intense femtosecond pulse which produces energetic electrons by photoionization to realize collisional excitation of nitrogen molecules. Furthermore, since the third harmonic is generated using a pump-probe scheme, it enables investigation of the excited-state dynamics of nitrogen molecules produced under different conditions. We also perform a comparative investigation in excited argon atoms, revealing different decay dynamics of the molecules and atoms from the excited states in femtosecond laser induced filaments.

State-selective all-optical detection of Rydberg atoms

Abstract: We present an all-optical protocol for detecting population in a selected Rydberg state of alkali-metal atoms. The detection scheme is based on the interaction of an ensemble of ultracold atoms with two laser pulses: one weak probe pulse which is resonant with the transition between the ground state and the first excited state, and a pulse with high intensity which couples the first excited state to the selected Rydberg state. We show that by monitoring the absorption signal of the probe laser over time, one can deduce the initial population of the Rydberg state. Furthermore, it is shown that---for suitable experimental conditions---the dynamical absorption curve contains information on the initial coherence between the ground state and the selected Rydberg state. We present the results of a proof-of-principle measurement performed on a cold gas of $^{87}\mathrm{Rb}$ atoms. The method is expected to find application in quantum computing protocols based on Rydberg atoms.

Excited-State Deactivation of Branched Phthalocyanine Compounds.

Abstract: The excited-state relaxation dynamics and chromophore interactions in two phthalocyanine compounds (bis- and trisphthalocyanines) are studied by using steady-state and femtosecond transient absorption spectral measurements, where the excited-state energy-transfer mechanism is explored. By exciting phthalocyanine compounds to their second electronically excited states and probing the subsequent relaxation dynamics, a multitude of deactivation pathways are identified. The transient absorption spectra show the relaxation pathway from the exciton state to excimer state and then back to the ground state in bisphthalocyanine (bis-Pc). In trisphthalocyanine (tris-Pc), the monomeric and dimeric subunits are excited and the excitation energy transfers from the monomeric vibrationally hot S1 state to the exciton state of a pre-associated dimer, with subsequent relaxation to the ground state through the excimer state. The theoretical calculations and steady-state spectra also show a face-to-face conformation in bis-Pc, whereas in tris-Pc, two of the three phthalocyanine branches form a pre-associated face-to-face dimeric conformation with the third one acting as a monomeric unit; this is consistent with the results of the transient absorption experiments from the perspective of molecular structure. The detailed structure-property relationships in phthalocyanine compounds is useful for exploring the function of molecular aggregates in energy migration of natural photosynthesis systems.

Laser damage of calcium fluoride by ArF excimer laser irradiation

Abstract: Artificially grown excimer grade calcium fluoride is one of key optical materials used in microlithography applications. Such calcium fluoride is required for optical components requiring high laser durability and laser induced bulk damage threshold (LIDT). The mechanical properties of calcium fluoride can vary depending on the crystal axis, , and . For example, material hardness is highest in the {100} crystal orientation. Furthermore, it is also known to cleave in the {111} plane. Therefore there is a possibility of a property that originates in such a crystal structure that influences LIDT. In this study, we investigated the relationship between crystal structure, laser durability and LIDT. The influence in the relation between the polarization plane of the ArF excimer laser and the crystal orientation of calcium fluoride in regards to LIDT was examined. The samples were all prepared from the same CaF2 crystal with optical axis's of , and . The azimuth of the samples was measured by the reflection Laue method. For the experiment, the samples were rotated to the polarization plane of the ArF excimer laser, and the change in the number of irradiation pulses that damage was observed and measured. As a result, we determined the position of the crystal orientation of the calcium fluoride relative to the polarization plane of the ArF excimer laser that produced the highest LIDT.

Cystine-derived bis-naphthalimides as stimuli-responsive fluorescent gelators

Abstract: Two cystine-derived bis-naphthalimide gelators (L1, L2) were synthesised and characterised. Both L1 and L2 exhibited similar absorptions and emission spectra in solvents such as acetonitrile and DMF. The fluorescence spectra of both the compounds featured a distinct monomer and long-wavelength excimer emissions in the aforementioned solvents. It was found that the excimer emissions for the two compounds could be preferentially quenched by triethylamine, and subsequently restored with hydrofluoric acid. The stimuli-responsive nature of the excimer emissions was demonstrated using anion stimuli in solution and in the gel phase. Thus, the excimer emission for L1 (or L2) could be switched ‘off’ using fluoride anions, and subsequently re-activated using tetrafluoroborate anions as the chemical stimulus.