In rigid media

The termination of radical polymerization cannot be prevented under normal conditions. This would be possible only in a polymerization initiated in rigid media, assuming that no chain transfer occurs, or if the radicals are trapped, for instance, by precipitation of the polymer during the process of its formation. Both methods have been used, and indeed the termination was considerably slowed down or even prevented permanently. However, such systems are of little value for synthesizing polymers according to a preconceived pattern. 

Interest in carbenes in rigid media can be dated to the 1960s when Murray et al. reported diphenylcarbene as the first organic species with a triplet ground state. ... 

Light intensity at the usual levels seldom has an effect on the primary photochemical step if all other variables are kept constant, although it may affect overall results considerably since it may control the concentrations of reactive intermediates. However, it will affect the outcome of a competition between primary one-photon and two-photon processes. The latter are still somewhat of a rarity but may be more important than is commonly realized, namely in rigid media where triplets have long lifetimes and quite a few of them are likely to absorb a second photon. The additional available energy may permit motion to new minima in Ti and thus give new products. 

Related ion-radicals have been prepared photochemically in rigid media by Lewis.146 When tetramethylbenzidine is exposed to ultraviolet light in a rigid medium, an electron is ejected as in the production of color centers in glass. 

Perrin s model has been used in particular for the interpretation of non-radiative energy transfer in rigid media (see Chapter 9). 

Because of rapid intersystem crossing, the lifetime of the n,n singlet is usually extremely short. The n,n triplet in rigid media, where the lifetime is at a maximum, exists for less than 10"2 sec.12 However, in fluid solution, where nonradiative processes are more important, the... 

Second, the excited quartet states convert to the doublet state which then acts as a common chemical intermediate. This attractive suggestion, owing partly to to Plane and partly to Schlafer, is plausible because this conversion is known to occur in rigid media. 

To these three types may possibly be added a fourth, namely, Recombination Delayed Fluorescence which has been reported by other workers in rigid media. It requires ejection of an electron as a first step and would therefore be expected to occur preferentially by excitation with high-energy quanta. A fifth source of delayed fluorescence— Triplet Excitation —might also conceivably operate in fluid solution. Since... 

Does the structure or fluorescence of an excimer in rigid media differ from that in fluid solution ... 

The intensity of P-type delayed fluorescence should decrease with viscosity and should be observed, if at all, with very low efficiency in rigid media. 

C) Recombination luminescence. Still another kind of delayed emission observed in dye solutions in low temperature glasses is by cation-electron recombination mechanism. Under high intensity irradiation and in rigid media, a dye molecule can eject an electron which is trapped in suitable sites. When this electron recombines with the dye cation, an Sj state is generated and a photon is emitted as fluorescence. 

The quenching of donor phosphorescence is a function of the acceptor concentration in rigid media and follows the expression ... 

Aromatic solutes can photosensitize the homolysis of C—H bonds in hydrocarbon solvents at low temperatures in rigid media.222,241,242 The reaction is biphotonic in rigid media where triplet lifetimes are sufficiently long, intense irradiation can populate second excited triplets by T-T absorption. The second excited triplets of several aromatic compounds apparently can transfer their energy to solvent molecules and cause bond cleavage. 

These authors have developed a theory for radiationless transitions in rigid media. Their theory has been reviewed recently by several authors.11,17 Nevertheless, for the sake of completeness we shall discuss it so that we can place emphasis on those parts which are of particular interest to us. 

No difference in the absorption spectrum is observed. The lifetimes of the two fluorescence emissions are both of the order of 10-8 sec. Another effect has been observed with hydrocarbons both as vapor and in solution, namely, a "delayed fluorescence emission with a life of a few milliseconds (56,63). This was at first interpreted as process 9, and the term excimer applied to the hypothetical excited dimer. However, C. A. Parker and C. G. Hatchard have shown that the intensity of the delayed fluorescence, which has the same spectrum as that from the excited singlet molecule, depends upon the square of the intensity of the exciting light (49). The mean life of the delayed fluorescence of anthracene solutions is (about) one-half that of the triplet state, and the effect is not observed in rigid media. These facts show that the delayed emission must be caused by an interaction between two triplet-state molecules ... 

Of comparable importance was the subsequent discovery that Ams = + 2 transitions could be detected at low fields for triplet molecules in random orientations in rigid media (van der Waals and de Groot, 1959, 1960). These are best observed with the apparatus modified so that the static and oscillating magnetic fields are parallel, but if weak spectra can be detected with the normal arrangement of static and oscillating fields perpendicular to each other then the combined results are almost as informative as those from single crystal studies. 

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