A dynamical model of excimer instability in molecular crystals

Catastrophe theory is applied to the analysis of complex bandshapes observed in the excimer emission of molecular crystals. Excited state potential surfaces in one- and two-well regimes are computed by means of a simple anharmonic quartic potential, modulated by the macroscopic parameters. Model calculations are reported for the pressure dependence of the excimer emission of 9-cyano- and of β-9,10-dichloroanthracene, and a satisfactory fit is found.     

Theoretical study of the benzene excimer using time-dependent density functional theory

A theoretical characterization of the potential energy surfaces of the singlet benzene excimer states derived from the B2u monomer excited state has been performed using time-dependent density functional theory. The excited-state potential energy surfaces were initially characterized by computations along the parallel and perpendicular intermolecular translational coordinates. These calculations predict that the lowest excited state for parallel translation is bound with a minimum at 3.15 angstroms and with a binding energy of 0.46 eV, while the perpendicular translational coordinate was essentially found to be a repulsive state. At the calculated minimum distance, the effect of in-plane rotation, out-of-plane rotation, and slipped-parallel translation were examined. The rotational calculations predict that deviations from the D6h geometry lead to a destabilization of the excimer state; however, small angular variations in the range of 0 degrees -10 degrees are predicted to be energetically feasible. The slipped-parallel translational calculations also predict a destabilizing effect on the excimer state and were found to possess barriers to this type of dissociation in the range of 0.50-0.61 eV. When compared to experimentally determined values for the benzene excimer energetics, the calculated values were found to be in semiquantitative agreement. Overall, this study suggests that the time-dependent density functional theory method can be used to characterize the potential energy surfaces and the energetics of aromatic excimers with reasonable accuracy.     

The absorption and fluorescence properties of pyrene crystal: a theoretical approach. II. relaxation processes

It is shown that by takin into account anhormonicity it is possible to explain why, in ground-to-excimer absorption in pyrene, the non-phonon peak can be resolved whereas the other peaks are overlapped. The same treatment applied to emission gives an explanation of the lack of structure in excimer emission. A possible model for vibrational relaxation of the excimer in pyrene crystal is considered and the corresponding time for relaxation is estimated.     

Emission studies of 9-cyanoanthracene excimers generated in glassy and crystalline matrices

Fluorescence spectra of 9-cyanoanthracene polycrystalline films and glasses of different viscosities and polarities are reported. The 490 nm emitting species is identified as the trans excimer. The blue-shift of 9-CNA crystal emission upon increasing temperature is explained in terms of a temperature-dependent competition between two crystal traps giving rise to cis or trans excimer position.     

Fluorescence Emission from 2,6‐Naphthylene‐Bridged Mesoporous Organosilicas with an Amorphous or Crystal‐Like Framework

We report that 2,6‐naphthylene‐bridged periodic mesoporous organosilicas exhibit unique fluorescence behavior that reflects molecular‐scale periodicities in the framework. Periodic mesoporous organosilicas consisting of naphthalene–silica hybrid frameworks were synthesized by hydrolysis and condensation of a naphthalene‐derived organosilane precursor in the presence of a template surfactant. The morphologies and meso‐ and molecular‐scale periodicities of the organosilica materials strongly depend on the synthetic conditions. The naphthalene moieties embedded within the molecularly ordered framework exhibited a monomer‐band emission, whereas those embedded within the amorphous framework showed a broad emission attributed to an excimer band. These results suggest that the naphthalene moieties fixed within the crystal‐like framework are isolated in spite of their densely packed structure, different from conventional organosilica frameworks in which only excimer emission was observed for both the crystal‐like and amorphous frameworks at room temperature. This key finding suggests a potential to control interactions between organic groups and thus the optical properties of inorganic/organic hybrids.     

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.     

Effect of pressure on the luminescence of 9,10-dichloroanthracene crystals

Luminescence spectra of β-9,10-dichloroanthracene have been studied at room temperature in the pressure range ≈ 0–10 GPa. The initial shift to lower energies of the excimer emission is linear (≈500 cm⁻¹/GPa) up to ≈3 GPa. Above this pressure a sharper emission grows at the high-energy side at the expense of the excimer luminescence. The suppression of the excimer state at pressures above ≈ 7 GPa is attributed to the strong repulsive interactions between the molecular pair. The new emission is thought to arise from a more loosely bound configuration in the excited state which could originate from a crystal phase change around 3 GPa.     

The spectroscopy and photochemistry of the excimer-emitting 9-cyanoanthracene crystal; The importance of multiphonon processes

A study of the spectroscopy and photochemistry of crystalline 9-cyanoanthracene is reported. This crystal gives rise to excimer emission; at 77 K the excimer has a lifetime of 160 ±5 ns and the emission spectrum is of gaussian shape. There is, prior to reaction, transfer of excitation energy by a process occurring at the excimer—exciton level and involving an activation energy of about 420 ^cm?1. Radiationless deactivation of the excimer involves an activation energy of 660 cm^?1. These experimental observations are in good agreement with the predictions of theories based on the model of strong exciton—phonon coupling. The photoreaction occurs at specific sites which are believed to be head-to-tail defects.     

"标记"芘的激基缔合物荧光在水溶性高分子研究中的应用

首先介绍了芘的荧光发射光谱的特点及其激基缔合物荧光、然后给出了用芘标记高分子的各种方式和方法,讨论了水溶性高分子的”标记”芘形成激基缔合物的原理,评述了芘标记高分子箕缔合物荧光在水溶性分子疏水相互作用,静电相互作用、氢键、络合等研究中的应用,也介绍了芘标记激基缔的荧光在高分子凝胶体积相变、分子链运动等领域的研究进展。     

Theoretical investigation of perylene dimers and excimers and their signatures in X-ray diffraction

The structures of the ground and excimer states of perylene pairs are calculated [using density functional theory (DFT) and time-dependent DFT techniques] in a free as well as a crystal environment, and their spectroscopic properties are studied for the most stable configurations. The vertical transition energies for the absorption and emission bands are obtained, and they are in good agreement with experimental data. In these calculations, up to six excited states are considered. With the calculated structures of the ground and excimer states, the scattering factors are analyzed as a function of the concentration of excimers in a crystal. The intensity of the 110, 005, and 0 10 0 reflections are found to be fairly sensitive to the presence of excimers in the crystal. The finite (nanosecond) lifetime of the excimer may make it possible to observe this state using time-resolved X-ray diffraction techniques.     

Immobilization of pyrene on quartz plate surface via a flexible long spacer and its sensing properties to dicarboxylic acids

It is known that polycyclic aromatic hydrocar-bons (PAHs) form excimer easily, and show both monomer and excimer emission in polar medium[1]. This character comes from their hydrophobic nature and plane structures. The ratio of the intensity of ex-cimer emission to that of monomer emission is a func-tion of distance between neighbouring PAHs in a sys-tem. It is not difficult to understand that many factors, such as the concentration of PAHs, temperature, pres-ence and concentration of othe…     

On the relationship between fluorescnece and molecular geometry. The effect of intermolecular shift on the excimer emission of crystalline dianthronylidene ethane.

The existence of two luminescent excimer geometries in polymorphic dianthronylidene ethane has been verified by X-ray structure determinations of three crystal modifications. The different excimer emission energies are interpreted in terms of variations in transannular π-orbital interaction resulting from an intermolecular shift of adjacent molecules across the log axis of the anthronylidene moiety.     

Resolution of ultrafast pyrene excimer emission rise times in zeolites X and Y

Pyrene has been a favorite photophysical probe molecule for zeolite research because of its ability to exhibit both monomer and excimer emission upon excitation. This study combines the use of ultrafast time-resolved fluorescence spectroscopy with steady-state fluorescence spectroscopy to study the excimer emission of pyrene incorporated within zeolites LiY, NaY, KY and NaX. The effects of sealing technique and coincorporated solvents are also explored. Pyrene excimer emission is resolvable with the use of an ultrafast streak camera under all conditions examined in this study with a rise-time range of 6.8 to 16.0 picoseconds. For each zeolite sample the addition of cosolvents decreases the rise time, with a greater decrease for polar solvents than for a nonpolar solvent. The presence of a detectable rise time for excimer emission indicates that pyrene excimer formation is a dynamic process when pyrene is embedded within the cavities of zeolite host materials.     

Excimer emission in 9-aminoacridine hydrochloride

Excimer emission in concentrated aqueous solutions of 9-aminoacridine hydrochloride has been observed at 555 nm. A study of monomer, dimer and excimer emission, absorption and excitation spectra shows that the excited dimer emission is transformed into excimer emission at higher temperatures. Unusually large hypochromism is observed on dimer formation.     

On low temperature fluorescence of perylene crystals

At low temperatures, the broad excimer fluorescence band of α-perylene crystals is replaced by a weakly structured emission at higher energy. This emission originates from a new crystal state (Y-state) which is populated independently of the high temperature excimer (E-state). Due to the temperature dependence of its first order decay rate and due to the thermally activated formation of the E-state, the Y-emission grows rapidly at temperatures below 90 K. The Y-emission differs from the fluorescence of the monomeric β-perylene at 5.5 K by its Stokes shift of 1300 cm^?1, the lack of vibronic structure, the long first order decay time of 40 ns and the absence of bimolecular annihilation indicating a localized state. The Y-state is attributed to a less relaxed pair state formed upon contraction of the dimeric crystal lattice.     

THE 1-CYANONAPHTHALENE EXCIMER IN HOMOGENEOUS AND MICELLAR SOLUTIONS

Abstract— An excimer of 1-cyanonaphthalene is produced in homogeneous organic solvents and in micelle containing detergent solutions. From solvent effects in homogeneous solution it is concluded that the excimer is relatively polar (dipole moment ∼ 4D). From a comparison with emission from that observed in homogeneous solvents, it is concluded that 1-cyanonaphthalene is solubilized mainly in the Stern Layer of sodium dodecyl sulfate (SDS) and hexadecyltrimethyl ammonium bromide (HDTBr) micelles. The influence of variation of detergent concentration on the excimer to monomer emission ratio and the influence of NaCl concentration on the excimer to monomer emission ratio have been determined. Excimer formation is shown to be a convenient method for determination of the Kraft point of SDS solutions.     

EXCITED STATES OF DNA AND ITS COMPONENTS AT ROOM TEMPERATURE—IV. SPECTRAL, POLARIZATION AND QUANTUM YIELD STUDIES OF EMISSIONS FROM CpC AND POLY rC

Abstract— Corrected normalized emission spectra are reported for poly rC, CpC and CMP in neutral aqueous solution at room temperature. The emissions from CpC and poly rC show progressive and extensive depolarization across the emission band compared with CMP. Self-consistent analysis of the polarization and spectral data allows the resolution of red-shifted component emissions from the overall spectra and the evaluation of component polarizations. By comparison with experimental reference spectra three long wavelength components are identified in the poly rC emission and assigned as excimer fluorescence, monomer phosphorescence and excimer phosphorescence, whereas for CpC only two components are observed. The quantum yields are interpreted quantitatively in terms of the known stacking behavior of CpC and poly rC. with excimer emissions originating from the stacked fraction. Quantitative differences between CpC and poly rC are due to their different hypochromism and degree of stacking. Differences of polarization of excimer fluorescence are a consequence of different stacking geometries, and the directional properties of the transition moment suggest the excimer fluorescence is largely charge-resonance in nature. The behavior of the CpC/poly rC system is compared with ApA/ poly rA.     

Protease Probes that Enable Excimer Signaling upon Scission

Peptide‐based probes that fluoresce upon proteolytic cleavage are invaluable tools for monitoring protease activity. The read‐out of protease activity through pyrene excimer signaling would be a valuable asset because the large Stokes shift and the long lifetime of the excimer emission facilitate measurements in autofluorescent media such as blood serum. However, proteolytic cleavage abolishes rather than installs the proximity relationships required for excimer signaling. Herein, we introduce a new probe architecture to enable the switching on of pyrene excimer emission upon proteolytic scission. The method relies on hairpin‐structured peptide nucleic acid (PNA)/peptide hybrids with pyrene units and anthraquinone‐based quencher residues positioned in a zipper‐like arrangement within the PNA stem. The excimer hairpin peptide beacons afforded up to a 50‐fold enhancement of the pyrene excimer emission. Time‐resolved measurements allowed the detection of matrix metalloprotease 7 in human blood serum.     

Examination of intramolecular potential surfaces of flexible conjugated molecules by calculation of crystal structures. equilibrium geometries of chalc

A method for refinement of intramolecular potential surfaces of flexible molecules is proposed based on the knowledge of the equilibrium molecular geometry in the crystal and on a good first approximation to the intra- and intermolecular surfaces. The equilibrium conformation in the crystal is calculated by minimization of the total potential with respect to all Cartesian coordinates of the asymmetric unit as well as the components of the unit cell parameters. The initial approximations to the potential function are further refined with the purpose of obtaining better agreement between the calculated crystal equilibrium geometry and the corresponding X-ray results. The previously proposed QCFF/PI potential surfaces for conjugated hydrocarbons are re-examined by calculation of the crystal structures of chalcone, p-methylchalcone, p,p'-dimethylchalcone and α,ω-diphenyloctatetraene. The resulting potentials of the present refinement are somewhat shallower than the previous QCFF/PI surfaces. It is suggested that the application of the method as a model calculation may be considered a reliable “quasi-experimental” technique for the determination of gas phase equilibrium conformations.     

Excimer fluorescence in β-9,10-dichloroanthracene crystals

The fluorescence of single crystals of β-9,10-dichloroanthracene at 4.2 K consists solely of excimer emission (τ = 95 ± 5 ns). The absence of monomenc emission shows that excimer formation in this crystal is not a thermally activated process. This result is confirmed by excimer-excimer annihilation studies (γ(4.2 K) = 6 × 10⁻¹³, γ(298K) = 3 × 10⁻¹² cm³ s⁻¹).