Excimer Formation In Solution Research Articles

BN-Substitution in Dithienylpyrenes Prevents Excimer Formation in Solution and in the Solid State.

Boron–nitrogen substitutions in polycyclic aromatic hydrocarbons (PAHs) have a strong impact on the optical properties of the molecules due to a significantly more heterogeneous electron distribution. However, besides these single-molecule properties, the observed optical properties of PAHs critically depend on the degree of intermolecular interactions such as π–π-stacking, dipolar interactions, or the formation of dimers in the excited state. Pyrene is the most prominent example showing the latter as it exhibits a broadened and strongly bathochromically shifted emission band at high concentrations in solution compared to the respective monomers. In the solid state, the impact of intermolecular interactions is even higher as it determines the crystal packing crucially. In this work, a thiophene-flanked BN-pyrene (BNP) was synthesized and compared with its all-carbon analogue (CCP) in solution and in the solid state by means of crystallography, NMR spectroscopy, UV–vis spectroscopy, and photoluminescence (PL) spectroscopy. In solution, PL spectroscopy revealed the solvent-dependent presence of excimers of CCP at high concentrations. In contrast, no excimers were found in BNP. Clear differences were also observed in the single-crystal packing motifs. While CCP revealed overlapped pyrene planes with centroid distances in the range of classical π-stacking interactions, the BNP scaffolds were displaced and significantly more spatially separated.

Synthesis and photophysical properties of pyrene-functionalized nano-SiO2 hybrids in solutions and doped-PMMA thin films

Luminescent pyrene-functionalized nano-SiO2 (nano-SiO2Pyr) hybrids were synthesized and characterized using thermogravimetry, infrared, UV–vis absorption and, X-ray photoelectron spectroscopy, as well as field emission transmission electron microscopy (FETEM). The organic substituents immobilized on the nano-SiO2Pyr hybrids accounted for approximately 10% of the total weight. Polyethylene glycol 200 (PEG200) was found to be the most suitable solvent to suspend the nano-SiO2Pyr hybrids compared to other commonly used organic solvents. FETEM images indicated an average SiO2 nanoparticle diameter of approximately 12 nm and a 1- to 2-nm thick organic species functionalization layer. Several emission peaks were recorded at wavelengths of 380–580 nm and were designated as emissions arising from either the monomer or excimer of the pyrene substituents. Excimer formation was concentration and solvent polarity dependent, with higher concentrations and a stronger solvent polarity benefiting excimer formation. Further, nano-SiO2Pyr hybrids were doped in poly(methyl methacrylate) (PMMA) thin films; fluorescence spectra indicated that the excimer could be formed almost exclusively from neighboring nano-SiO2Pyr hybrids. Time-resolved fluorescence decays revealed that the emission lifetimes of nano-SiO2Pyr monomers and excimers were approximately 190 ns and 65–100 ns in the PEG200 solution, respectively, which was shortened to 0.45 ns to tens of ns in doped PMMA thin films, depending on the nano-hybrid concentration. Thus, the present study not only provides a method to prepare luminescent nano-materials but also a route to investigate excimer formation in solutions and thin films.

Excimer formation in 9,10-dichloroanthracene – Solutions and crystals

Excimer formation and dissociation in 9,10-dichloroanthracene is investigated in solution phase (low/high concentration) and monocrystalline state (α/β structure) at room temperature by ultrafast absorption spectroscopy combined with streak-camera fluorescence experiments. The transition from the monomeric to the excimeric state is accompanied by a red-shift of the fluorescence and a blue-shift of an excited state absorption band. Excimer formation in solution occurs in the 1ns time regime. In the pre-arranged crystalline state of the β-form a strong acceleration of excimer formation to about 1ps is observed. The slower formation time in solution is attributed to a diffusion process before the two molecules arrange in an ideal excimer geometry. The crystalline structure of the β-form does not allow to reach the optimal geometry, leading to significantly smaller spectral shifts than in solution. For the α-form spectral dynamics typical for the excimer formation are not observed.

Appropriate excitation wavelength removes obfuscations from pyrene excimer kinetics and mechanism studies

The experimental procedures of time-resolved laser photolysis studies of pyrene excimer formation in solution have been scrutinized and the appropriate modifications have been implemented. Contrary to the experimental methods applied in all related previous work, the selection of a suitable excitation wavelength (such that the corresponding pyrene absorbance is less than 0.5 absorbance units) utilized in our study results in simple homogeneous kinetics. Consequently, the rate parameters obtained and the mechanism proposed differ significantly from those published previously. The rate constant values of the unimolecular decay of the pyrene monomer, the unimolecular decay of the pyrene excimer, and the excimer formation in decane solution (η = 0.860 mPa s) at 25 °C are (2.38 ± 0.01) × 10(6) s(-1), (2.78 ± 0.02) × 10(7) s(-1), and (3.11 ± 0.06) × 10(9) M(-1) s(-1), respectively. The dissociation of the excimer to form a singlet excited state pyrene and a ground state pyrene was shown to be negligible. The energies of activation corresponding to the monomer and excimer unimolecular decays were found to be 2.51 ± 0.07 and 25.7 ± 0.7 kJ mol(-1), respectively. Also, our temperature resolved laser photolysis data revealed that the excimer formation has a negative energy of activation equal to -11.2 ± 0.5 kJ mol(-1). This unique phenomenon may be attributed to steric effects in the collision of the reactants. The current findings are important for the correct data analysis and interpretation in many applications of the pyrene excimer.

Fluorescence sensing of Cu 2+ and Hg 2+ by a dipyrene ligand involving an excimer-switch off mechanism

A novel dipyrene ligand 1 has been designed which shows intramolecular excimer formation in solution. Its specific interaction with Cu 2+ or Hg 2+ leads to the disruption of the excimer and results in a fluorescence-mediated sensing of these ions in a mixed organic-aqueous solution. Apart from steady-state studies, time-resolved fluorescence measurements also reveal that excimer-switch off caused by metal ion coordination leads to the selective detection of these ions.

Effect of sequence distribution of PES/PEES random, block, and alternative copolymers on excimer formation in solution

AbstractThree copolymers of poly(ether sulfone) and poly(ether ether sulfone) with the same composition but different sequence distribution were synthesized by three kinds of methods. Their molecular aggregation in dichloromethane was studied by fluorescence spectrophotometer and electron microscope. The experimental results revealed that the formation of intermolecular excimers in alternative copolymer (A50) dichloromethane solution were observed at a A50 concentration about 1.6 × 10−2 g/mL by the fluorescence analysis, but the formation of intermolecular excimers in dichloromethane were not found for random copolymer (R50) and block copolymer (B50). The electron micrograph of three copolymer films, heat‐treated at 200°C for 7 days, presented a diffraction micrograph, which suggest that three copolymer molecular aggregation is changed from a randomly coiled amorphous phase to an ordered one, and the order structure of alternative copolymer (A50) was the most distinct in three copolymers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Photophysical and Electroluminescent Properties of Hyperbranched Polyfluorenes

AbstractSummary: Properties of a series of hyperbranched polyfluorenes were investigated. Increasing the benzene “crosspoint” density blue‐shifts the absorption and PL since the crosspoints interrupt π‐electron conjugation, decreasing conjugation length, and thus increasing chromophore band gap. The relative intensity of S10 → S00 transition in PL increases with crosspoint density increase, indicating that the energy and the energy distribution of vibronic transitions were controlled by hyperbranch structure. When the polymer solution is cooled down, the lattice vibration is reduced and the effective conjugation length (coherence length for electron wavefunction) is increased, thus causing bathochromic shift in absorption and PL. These polymers do not favor excimer formation in solution due to their remarkable rigidity and extra‐large molecular size. With increase in crosspoint density, LED color changed from green to violet. In double‐layer LEDs, PPV not only improves hole injection/transport but also contributes to device emission through its bulk emission and interfacial emission as a result of interfacial energy transfer. Higher electrical field favors interfacial energy transfer probably by facilitating hole–electron recombination and by encouraging the excitons formed in polyfluorene layer to migrate to the interface and to further diffuse into PPV layer. The polymer with more crosspoints shows higher EL efficiency. The crosspoints interrupt π‐electron conjugation, isolate excitons and inhibit intrachain exciton annihilation. They can also increase rigidity of the macromolecular backbone, and the large spatial hindrance of these macromolecules makes the molecular packing very difficult, thus decreasing interchain exciton annihilation. All these structural features help to increase exciton lifetime and improve LED performance.Synthesis of the hyperbranched polyfluorenes.magnified imageSynthesis of the hyperbranched polyfluorenes.

Monomer-excimer kinetics in solution. III. Generalized Smoluchowski approach

A generalized Smoluchowski approach developed in A. Molski, Chem. Phys. 182, 203 (1994) is employed to study reversible excimer formation in solution. For contact excimer formation, relations among the rate coefficients are analyzed for three modes of excitation; initially pulsed, steady-state, and periodic. A new kinetic Laplace transform relation for the frequency domain is demonstrated in the linear harmonic regime. The Laplace transform relations between the time domain and steady states, derived in W. Naumann and A. Molski, J. Chem. Phys. 100, 1511, 1520 (1994) for interaction-free models, are shown to be also valid when the interaction forces are included.

Monomer–excimer kinetics in solution. II. Statistical nonequilibrium thermodynamic approach

The statistical nonequilibrium thermodynamic theory of diffusion-influenced reactions is employed to study reversible excimer formation in solution. Three types of rate coefficients for bimolecular excimer formation are discussed: (i) molecular rate coefficients defined by one-way reactive fluxes, (ii) phenomenological rate constants based on form of the macroscopic rate equations, and (iii) modified, time-dependent phenomenological rate coefficients. Relations are derived linking: (i) the molecular rate coefficients for reversible and irreversible excimer formation rates, and (ii) the steady-state molecular rate constant and the Laplace transform of the time resolved irreversible rate coefficient. The relationship between the present approach and the microscopic–stochastic theory of excimer formation is discussed.

Monomer–excimer kinetics in solution. I. Stochastic many-particle approach

A stochastic many-particle approach is applied to study the kinetics of reversible excimer formation in solution. Coupled dynamic equations for the macroscopic concentrations and for the radial distribution function are derived, and applied to analyze (i) time resolved kinetics after a short pulse, and (ii) steady-state kinetics. Renormalization of the phenomenological excimer dissociation rate coefficient due to nonequilibrium effects is discussed. A relation is demonstrated between steady-state, reversible monomer–excimer kinetics and irreversible fluorescence kinetics. Explicit results are given for the excimer fluorescence yield, assuming the Smoluchowski–Collins–Kimball reactivity model.

Ultrafast Dynamics in Macromolecular Systems

Abstract Picosecond time-resolved fluorescence spectroscopy and computer analysis procedures have been applied to study electronic energy transport in synthetic macromolecules. Second-harmonic generation and sum frequency mixing of actively mode-locked lasers provides tunable ultraviolet radiation for excitation of a wide range of vinyl aromatic polymers. Time-correlated single photon counting detection techniques enable the recording of fluorescence decay data with a time resolution of less than 50 ps and when combined with curve-resolution techniques based on principal factor analysis can provide insight into the heterogeneous fluorescence from polymers. The application of these techniques is described to examine energy transfer and excimer formation in solutions of poly(acenaphthalene), poly(2-naphthylmethacrylate) and films of p-methyl styrene/acrylonitrile co-polymers.

Excimer formation in solution and molecular aggregation of polyethersulfone

AbstractFormation of intermolecular excimers in polyethersulfone (PES) dichloromethane solutions were observed at a PES concentration about 2.7 × 10−2 mol/l by the fluorescence analysis. External light energy irradiation enabled the formation of intramolecular excimers in the PES dichloromethane solution. These chains can gain an access to each other in parallel though the molecular chains are considerably rigid. The electron micrograph of the PES film, heat treated at 200°C for 72 hours, presented a diffraction micrograph, which suggests that the PES molecular aggregation is changed from a randomly coiled amorphous phase to an ordered one.

Excimer emission from glassy films of some aromatic hydrocarbons

The fluorescence emission spectra and fluorescence decay times of amorphous films prepared by evaporation of a number of aromatic hydrocarbons on to a substrate cooled with liquid nitrogen were measured. The fluorescence of such films appears to originate from sites where the interaction between neighbouring molecules is of the same kind as that present in the well-known excimer state. The density of such sites is found to be so high as effectively to prevent energy migration. Some interesting exceptions to this behaviour are noted and comparisons drawn with experimental data reported for excimer formation in solution and in the crystalline phase.