Monomer annihilation

The fluorescence spectra measured just upon ablation are given in Figure 2A as a function of laser fluence. The contribution below 370 nm was suppressed, as a Hoya L37 filter was used in order to cut off the laser pulse. Fluorescence spectra of this polymer film consist of sandwich (max. 420 nm, lifetime 35 ns) and partial overlap (max. 370 nm, lifetime 16 ns) excimers (20). The latter excimer is produced from the initially excited monomer state, while the sandwich excimer from the partial overlap excimer and the monomer excited states. Since these processes compete with efficient interactions between identical and different excimers (Si - Si annihilation) (12), the sandwich excimer is quenched to a greater extent compared to the partial overlap one under a high excitation. Actually the fluence-dependent spectral change around the threshold can be interpreted in terms of Si - Si annihilation. 

It is rare for a polymerization reaction to proceed to completion because the viscosity of a polymer increases during chain growth - this is why monomers are usually liquid, while the polymers produced are hard solids. The last step during polymer formation is a termination reaction known as radical annihilation , in which two radicals meet and then spin-pair to form a covalent bond. 

In this report the TTA process has been studied taking into account new experimental data indicating that the ADF emitted from metalloporphyrin upper electronic states is of dimeric nature. Moreover low values of P2 obtained in (6) can be explained by the lesser probability of dimer singlet staTes emitting in comparison with the monomer. But true probabilities of Sp state formation from TTA processes may be quite high. We have been able to establish simultaneous relations between diffusion parameters and annihilation characteristics obtained from spectral kinetic measurements. 

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

The cation-anion annihilation provides the main mode for termination in extraordinarily dry systems. Traces of water terminate propagation in a most efficient way, and then the termination is first order with respect to the growing polymers. It is interesting to digress here and discuss this writers ideas about the role of water in such processes. Under normally achieved states of dryness the concentration of water in a hydrocarbon monomer is probably sufficiently high to maintain the dissolved water in its dimeric form, (H20)2 or, at least, to allow a second molecule of water to collide with carbonium ion associated with one H20 prior to the decomposition of such an associate. Thus, the termination involves processes such as... 

The sequence of monomer additions is terminated by the mutual annihilation of two radicals. Such termination reactions can occur if the radicals combine to form a paired electron bond as in... 

When a free-radical polymerization is first started, the number of radicals in the system will increase from zero as the initiator begins to decompose according to reaction (6-6). The frequency of termination reactions will also increase from zero in the early stages of the polymerization because the rates of these reactions, (6-16) and (6-18), are proportional to the square of the total concentration of radicals in the system. Eventually the rate of radical generation will be balanced by the rate at which radicals undergo mutual annihilation, and the concentration of radicals in the system will reach a steady value. It can be shown that this steady state is reached very early in the reaction with the usual concentrations of initiator and monomer. 

The above investigations clearly show that deqiite their relative stabilities, electro-chemically prepared radical-cations give rise to fairly complicated phenomenologies when they are used as initiators for cationic polymerisations. While the initiation rate constants reported are probably correct, the chemistry of these processes both with reject to the initial step and to the ensuing reactions of the monomer radical cations, is not fully understood. As for the nature and relative concentration of the chain carriers in these systems, more work would need to be done before any firm conclusion can be attained. All these problems stem at least in part from the fact that the radical cations described are only relatively stable and do suffer s)me annihilation reactions. 

The Sj-Sj annihilation rate constant was determined in the following manner. First, the concentration of the excimer was obtained by dividing the observed absorbance by its extinction coefficient and by the effective cell length where the Sj<-Sp absorbance at 266nm was 1. In addition, in pure liquid benzene as well as in solution, there exists rapid equilibrium between monomer and excimer, of which time constants of association and dissociation are in the order of a few ps °. Hence, the sum of the monomer and excimer concentrations obtained by the equilibrium constant at each temperature was used as the concentration of the excited singlet species for the analysis. Although this assumption may affect to some extent the accuracy of the obtained rate constant, the error of this estimation would not depend upon the temperature. 

Electrochemical excitation, photochemistry without light, exhibits many phenomena that are unique to ECL as compared to photochemistry. The efficient production of emission from excimers or exciplexes as compared to excited monomers, efficient generation of excited triplet states, and intense delayed fluorescence caused by triplet-triplet annihilation are the most typical examples. On the other hand, the method offers a chance to populate the excited states that are inaccessible by the processes following photoexcitation. 

The chain growth is terminated at some stage by annihilation of the reactive center by one or more convenient and appropriate mechanism which depends largely on the type of reactive center (radical, cation or anion), nature of the monomer M, and the overall chemical environment and condition of reaction. 

Here n = (n, a) labels the molecules in the crystal, where n is the lattice vector and a enumerates the molecules in a unit cell and / are the creation and annihilation operators of an exciton on molecule n, obeying Pauli commutation relations. Eq is the renormalized excitation energy in the monomer (see Section 3.2), and Mnm is the matrix element of the excitation energy transfer from the molecule m to the molecule n. 

Interaction of two excited triplet states of the same molecule in solution is known as triplet-triplet annihilation (TTA) because it provides an important triplet decay pathway. It often gives rise to monomer and excimer delayed fluorescence (189,190). Rate constants for TTA are usually obtained indirectly from the analysis of the second-order component of the decay of triplet-triplet absorption following flash excitation (191-196). Such analyses are generally based on... 

The propagation step would theoretically have continued until the consumption of all available monomers but for the tendency of pairs of free radicals to react and annihilate their mutual activities. The termination steps can occur by either of two mechanisms combination (coupling) or disproportionation. 

Recall that the rate of consumption of monomers by an active center (disappearance of monomers) is, by definition, the rate of propagation, Rp. Now, in termination by combination, two growing chains undergo mutual annihilation to produce a single inactive polymer molecule, whereas for termination by disproportionation, a biomolecular annihilation of active polymer chains results in two polymers. 

In emulsion polymerization, the rate of generation of free radicals is about lO Vm-s while the number of monomer-polymer particles for typical recipes, N, is in the range 10 to 10 particles/ml of the aqueous phase. Consequently, if all the initiator radicals are captured by the monomer-qxilymer particles, each particle will acquire, at the most, a radical every 1 to 100 s. It can be shown that if a particle contains two radicals, mutual annihilation of radical activity will occur within a time span of the order... 

Here, aj and a,(, represent the creation and annihilation operators for the ik-th state (1 < 4 < 3) of monomer k, N is the number of monomers, and represents the dipole-dipole coupling between transition dipoles nd for... 

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