Intermolecular annihilation

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

Clear-cut evidence on the intermolecular nature of the excitation step and the need for in-cage radical annihilation has been gained from a study on the solvent-cage effect in the peroxyoxalate reaction. The viscosity effect on the singlet quantum yields was verified using the binary solvent system toluene-diphenyhnethane. A rise in solvent viscosity from 0.50 cP (toluene) to 2.67 cP (diphenylmethane) leads to an up to tenfold increase in the quantum yields, demonstrating the intermolecular nature of the excitation step. 

In the gas phase, the asymmetric CO stretch lifetime is 1.28 0.1 ns. The solvent can provide an alternative relaxation pathway that requires single phonon excitation (or phonon annihilation) (102) at 150 cm-1. Some support for this picture is provided by the results shown in Fig. 8. When Ar is the solvent at 3 mol/L, a single exponential decay is observed with a lifetime that is the same as the zero density lifetime, within experimental error. While Ar is effective at relaxing the low-frequency modes of W(CO)6, as discussed in conjunction with Fig. 8, it has no affect on the asymmetric CO stretch lifetime. The DOS of Ar cuts off at "-60 cm-1 (108). If the role of the solvent is to open a relaxation pathway involving intermolecular interactions that require the deposition of 150 cm-1 into the solvent, then in Ar the process would require the excitation of three phonons. A three-phonon process would be much less probable than single phonon processes that may occur in the polyatomic solvents. In this picture, the differences in the actual lifetimes measured in ethane, fluoroform, and CO2 (see Fig. 3) are attributed to differences in the phonon DOS at 150 cm-1 or to the magnitude of the coupling matrix elements. 

The mobility of excited states, imposed by intermolecular interactions (see Sec. 2.4), can lead to their collision with each other and/or with other types of excited states as well as trapped or free carriers generated in an organic solid. Such collision processes, realizing various excitonic interactions, may result in annihilation of the excitons and/or their transformation into another set of particles and quasi-particles. As different types of excitonic interactions show up in different optical and electrical phenomena, we divide them into two categories corresponding to the interaction between quasiparticles (exciton-exciton interactions) and to the interaction between quasi-particles and particles (exciton-charge carrier interactions). 

Figure 23. Proposal intermolecular energy migration between surface bound sensitizers. Excited states proximate to each other may annihilate and ultimately yield ground state products and heat.

Fig. 7. A three-input INHIBIT gate exemplified by the tetraanion 54 and /1-cyclodextrin (j8-CD). a With neither protons nor /1-CD present in the solution, phosphorescent output is low, because of both PET from the tertiary amine, and through intermolecular triplet-triplet collisions of the bromonaphthalene phosphor, b Addition of calcium ions leads to a reduction in the PET-based quenching of the phosphorescence - however, intermolecular collisions still lead to a low emission, c Shielding of the phosphor with /l-CD reduces intermolecular triplet annihilations, but quenching still occurs via PET. d Only with both Ca2+ and /1-CD present does the solution phosphoresce, e In any combination of Ca2+ and /1-CD, the solution will yield a low output in the presence of molecular oxygen (the INHIBIT stimulus), as a consequence of triplet-triplet collisions...

FIGURE 12.6 Schematic of how lateral intermolecular energy transfer across the semiconductor surface can lead to a second-order excited-state annihilation reaction. First-order excited-state relaxation was observed for sensitizers with short excited-state lifetimes, t < 50 ns, and was predominant at low irradiances and surface coverages for all sensitizers. 

Here, ancj are the creation and annihilation operators, respectively, for phonons in mode q = (q, r), where q denotes the vector of the phonon and r is the branch label. The energy of these phonon modes is given by u>q. Furthermore, the single-molecule Hamiltonian as well as the intermolecular transfer interaction are still considered to be operators in phonon space. 

The specific fluorescent probe technique used in the present research is excimer formation between pyrene groups terminally attached to both ends of poly(ethylene glycol) (PEG) chains. Excimer formation in such a labeled polymer may result from both intramolecular and intermolecular interactions. A large body of literature over the past decade deals with intramolecular cyclization as detected by fluorescence techniques. This work has come largely from three groups. Cuniberti and Perico (24, 25) were the first to explore the possibility that excimer fluorescence could be used as a probe of end-to-end cyclization dynamics. Later, Horie and co-workers (26, 27) used triplet annihilation as a probe of cyclization in long chains. By... 

The fluorescence signal from fluorophores of complex organic compound (COC) under powerful laser excitation is represented as the nonlinear function of the number of detected fluorescence photons Nh (or fluorescence intensity In) on the photon fluxes F of pumping radiation (Filipova et al., 2001). The dependence Nn(F) is called fluorescence saturation curve, its typical view is represented in the Fig. 1(a). There are several reasons for that nonlinear dependence the non-zero lifetime of orj nic molecules in excited state intercombination conversion intermolecular interactions including singlet-singlet annihilation, etc. 

These models have been quite useful as a means of explaining some of the phenomena associated with the rate of positron annihilation. Other experiments, however, seemed to indicate that the "free volume" model includes far too few properties apart from the factor of density as to satisfactorily explain variations in the positron lifetimes which occur as a result of phase transitions. It would appear that in this case an important part in the positron annihilation process is played by the nature of the intermolecular Interaction and by the internal order of the structures of the molecular substance. 

These findings were further confirmed in a series of investigations on liquid crystal systems carried out in our own laboratory (32) and it was concluded that the mechanism of positron annihilation is extremely sensitive not only to variations in the free volume but also to slight changes in the arrangements of molecules in a condensed matter and thus to changes in the nature and magnitude of intermolecular interactions. 

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