Exciplex formation effects

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

The effects of photophysical intermolecular processes on fluorescence emission are described in Chapter 4, which starts with an overview of the de-excitation processes leading to fluorescence quenching of excited molecules. The main excited-state processes are then presented electron transfer, excimer formation or exciplex formation, proton transfer and energy transfer. 

The basic principle of this method of recognition is a cation-induced conformational change bringing closer together (or moving away) two moieties able to interact and induce photophysical effects excimer or exciplex formation (or disappearance), electronic energy transfer and quenching. 

An exciplex can exert a favorable effect on cycloaddition reactions in two ways. First, the prior association favors the bimolecular process. Second, exciplex formation will bias the system toward bond-formation of the donor-acceptor pair, and away from competing chemical processes, such as dimerization of the donor. 

In nonpolar solvents, exciplex formation is usually favored because of a small AG, and a favorable Coulombic term. The ions are likely to remain in intimate contact for a longer time, i.e., ion pairing is effective because of favorable Coulombic and solvent effects. That dissociation into solvent-separated is not likely for exciplexes formed in nonpolar solvents has been shown by extensive studies dealing with the photochemical additions of donor and acceptors. Reactions via exciplexes or CIP s frequently yield cycloadducts, whereas in polar solvents, coupling via substitution of radical ion pairs and other chemical reactions involving solvated radical ions may predominate [12]. 

One of the problems facing spin chemists performing these measurements is that the observed field effects can be rather small. Thus the method of detection should be as sensitive as possible. In some systems, it is possible to use the inherent fluorescence of one of the species involved in the reaction as a probe of RP activity. The most common of these approaches is the situation with the formation of RlPs that can often lead to spin-selective exciplex formation via the singlet RIP. Systems involving conjugated aromatic molecules, for example, anthracene and pyrene as electron donors/acceptors, amines as electron donors, and substituted benzenes (e.g., dicyanobenzenes) as electron acceptors, have been commonly employed and are now extremely well... 

Steric effects have also been shown to be important in intramolecular exciplex formation (Pragst et al., 1978). It was found that the intramolecular exciplex formed by 3-(N-methyl-N-p-tolyl) amino-l-(9-anthraceno)propane had a dipole moment of 12.2 D whereas 3-(N-p-tolyl)amino-l-(9-anthraceno)-propane gave an exciplex having a dipole moment of 15 D. It was suggested that in the former compound the exciplex deviates from the sandwich conformation and consequently has less charge-transfer character. 

Fig. 8 Effect of inversion at nitrogen upon exciplex formation. (From Van der Auweraer et al., 1980)...

When primary and secondary co-arylalkylamines are excited, exciplex fluorescence is not observed, but fluorescence quenching does take place (Shizuka et al., 1979). Presumably chemical reaction competes effectively with exciplex formation. However, by means of picosecond laser flash photolysis, the time-resolved absorption spectra of the species produced on reaction of pyrene with diphenylamine have been obtained (Okada et al., 1980b). It was shown that the reaction leads to neutral radicals via an exciplex. Although the formation of neutral radicals in such systems had been previously identified (Okada et al, 1976a) the role of exciplexes had been purely speculative. 

Discussion. Copper in Krypton. The absorption spectra of copper atoms Isolated in rare gas matrices have been extensively studied (15-25) and the triplet of bands at 310nm attributed to a number of different causes. These include (1) spin orbit splitting and static axial site distortion (17), (2) multiple matrix sites (18), (3) exciplex formation between the metal and a single matrix atom (19), (4) long range metal-metal interactions (2 ), and (5) Jahn-Teller (JT) effect resulting from matrix cage atom vibrations on the excited metal (21,22,23). The MCD of Cu atoms in the rare gas matrices has recently been reported (24,25) and the results interpreted as consistent with either the distorted site or JT hypotheses (39). 

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