Excimer

About: Excimer is a research topic. Over the lifetime, 3725 publications have been published within this topic receiving 75104 citations.

Intramolecular excimer formation in macromolecules. II. Energy migration and excimer formation in acenaphthylene-methyl methacrylate copolymers

Abstract: Excimer formation has been studied in a series of ancenaphthylene—methyl methacrylate copolymers of content varying over the total composition range. In accord with a previous model [Reid and Soutar, J. Polym. Sci. Polym. Lett. Ed., 15, 153 (1977)], the excimer formation has been described by a composite function representative of energy migration and excimer site concentration. In tramolecular energy migration in glassy solutions of the polymers was characterized by use of fluorescence depolarization measurements. The extent of energy migration in the acenaphthylene copolymers is determined by the mole fraction of aromatic chromophore fa. A function Σ descriptive of the excimer site concentration has been generated on the assumption that excimer formation results predominantly from next- to nearest-neighbor interactions. Σ is based upon the pentad distribution of aromatic chromophores in the chain and contains pentad fractions appropriately weighted according to the frequency and relative importance of their potential excimer sites. Reactivity ratios of acenaphthylene and methyl methacrylate in copolymerization at 70°C were ra = 0.80 and rm = 0.57.

Exciplex formation in blended spin-cast films of fluorene-linked dyes and bisphthalimide quenchers.

Abstract: Spin-cast films of dyes (donor−π–donor, donor−π–acceptor, and acceptor−π–acceptor type, where the donor is Ph2N–, the acceptor is 2-benzothiazoyl, and the π-linker is 9,9-diethylfluorene) blended with nonconjugated bisphthalimides were prepared. Upon visible-light excitation of the dyes, quenching of the excited state occurs by exciplex formation between dye and bisphthalimide molecules or, in some cases, by excimer formation or aggregation-induced emission between two dye molecules. The extent of exciplex formation is dependent on the driving force, which can be calculated using the energy difference between the lowest unoccupied molecular orbitals (LUMOs) of the dyes and bisphthalimides. The results show that complete exciplex formation occurs when this driving force is greater than 0.57 eV whereas partial exciplex formation occurs when the driving force is between 0.28 and 0.57 eV. The exciplex emission energies can also be predicted by calculating the difference between the LUMO level of the bisphthal...

Mobility of pyrene on chemically modified silica surfaces

Abstract: Pyrene was adsorbed on three different chemically modified silicas, RP-2, RP-8, and RP-18, commonly used in liquid chromatography separations. The mobility of pyrene on the surfaces and its dependence of temperature was studied by measuring the time dependence of pyrene excimer formation with a fluorescence method. The rate constant for excimer formation is 1.2 x 10/sup 8/ s/sup -1/ M/sup -1/ at room temperature on RP-18. The apparent activation energy for diffusion E/sub d/, of pyrene on RP-18 is approx. 20 kJ/mol in the temperature range 290-325 K. For lower temperatures E/sub d/ increases rapidly, which indicates a phase transition in the temperature range 273-290 K. The values for k/sub DM/ and E/sub d/ at room temperature are consistent and show that the surface of RP-18 has liquid-like properties. The authors found that pyrene excimer formation is a dynamic process on RP-2 and RP-8 as well. In these cases the evaluation of the different rate constants is more complex, mainly due to differences in the microenvironment for different pyrene molecules.

Energetics and Dynamics of Naphthalene Polyaminic Derivatives. Influence of Structural Design in the Balance Static vs Dynamic Excimer Formation

Abstract: Two new fluorescent macrocyclic structures bearing two naphthalene (Np) units at both ends of a cyclic polyaminic chain were investigated with potentiometric, fluorescence (steady-state and time-resolved) and laser flash photolysis techniques. The fluorescence emission studies show the presence of an excimer species whose formation depends on the protonation state of the polyamine chains implying the existence of a bending movement (occurring in both the ground and in the first singlet excited state), which allows the two naphthalene units to approach and interact. For comparison purposes, one bis-chromophoric compound containing a rigid chain (piperazine unit) was also investigated. Its emission spectra shows a unique band decaying single exponentially thus showing that no excimer is formed. With the two new ligands, excimer formation occurs in all situations even at very acidic pH values when the protonation of the polyamine bridges is extensive. Coexistence of ground-state dimers with dynamic excimers was established based on steady-state and timeresolved fluorescence data. The energetics of excimer formation and dissociation were determined in ethanol and water. Different methods of decay analysis (independent decay deconvolution, global analysis and excimer deconvolution with monomer) were used to extract the kinetic (rate constants for excimer formation, dissociation, and decay) and thermodynamic parameters. In ethanol and acidified ethanol:water mixtures, an additional short decay time was found to exist and assigned to a dimer, whose presence is assumed to be responsible by the decrease in activation energy for excimer formation in this solvent. The results are globally discussed in terms of the small architectural differences that can induce significant changes in the photophysical behavior of the three studied compounds.