Electron ejection, photochemical

Photochemical electron ejection from the anion is also observed by ESR spectroscopy (5). 

Following the classical observation of Lewis and his school (29, 30, 34) of the photoionization of aromatic molecules in rigid solvents, Land, Porter and Strachan (25) proved such processes in the flash photolysis of aqueous solutions of phenols. It was suggested (7, 15, 25, 38) that the primary photochemical act involves electron ejection. 

Photochemically induced electron ejection from anions has been used in the preparation of radical anions in the cavity of an ESR spectrometer. Subtle structural differences between the radical anions prepared by photoirradiation and by other ways were outlined [50, 51], Two modes of radical anion photoinitiated formation were used. An electron is photoejected from the excited fluorene anion and adds to a neutral fluorene molecule in a subsequent step (A) or a dianion obtained from the potassium mirror reduction of tetrabenzocyctloocatetraene is directly photooxidized to the corresponding radical anion (B). This last method was also used to generate the otherwise inaccessible radical anion of pentalene, starting from the corresponding dianion irradiated in an ESR probe [52],... 

Thus with the cyclopropyl anions 331- and 340 - it has been established that the disrotatory mode, as predicted by Woodward and Hoffmann , is the preferred one. It is however not clear whether a photochemical cyclopropyl-allyl anion or a thermal cyclopropyl radical ring-opening (the latter caused by photochemical electron ejection) takes place. It has also been realized that systems with X = vinyl or Br (342- and 343-, respectively) do not open photochemically . 

It is not clear, however, whether a photochemical anion or a thermal cyclopropyl radical ring-opening — the latter caused by photochemical electron ejection — took place. 

Photoionization provides an alternative photochemical method for generating radical cations in laser experiments. Laser excitation of a wide range of easily oxidized substrates (usually aromatic compounds) has been demonstrated to result in direct electron ejection to give a radical cation and the solvated elec-tron. - In most cases photoionization occurs from an upper excited state produced either directly by absorption of a single... 

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. 

The photosensitized electron transfer reaction forms the reduced lipophilic electron acceptor BNA which is ejected into the continuous organic phase and thus separated from the oxidized product. In order to monitor the entire phase transfer of the reduced acceptor, BNA, a secondary electron acceptor, p-dlmethyl-amlnoazobenzene (dye),was solubilized in the continuous oil phase. The photochemically induced electron transfer reaction in this system results in the reduction of the dye (0 = 1.3 x 10 3). Exclusion of the sensitizer or EDTA or the primary electron acceptor, BNA, from the system resulted in no detectable reaction. Substitution of the primary acceptor with a water soluble derivative, N-propylsulfonate nicotinamide, similarly results in no reduction of the dye. These results indicate that to accomplish the cycle formulated in Figure 6A the amphiphilic nature of the primary electron acceptor and its phase transfer ability in the reduced form are necessary requirements. 

It is probable that the course of reactions discussed here is photochemical CO ejection followed by oxidative addition of Si—H to the 16-electron metal center this is reflected in the nature of the products (Table 1). Few reactions of mononuclear metal carbonyls with silanes are induced thermally in preference, and these often lead to different products. For example, M(CO)2C5H5-t (M = Co, Rh) and Cl3SiH give M(SiCl3)2(CO)CjH5t at 100"C and RT respectively whereas FefCO), and the same silane provide Fe(SiCl3)2(CO)4 at 140°C and Fe3(CO),2(/<-SiCl2)3 at 160°C . 

A special instance of photoinitiation was already di us in Sect. VI-B, within the context of bare cation formation. In those sterns in fact, h -energy photons were used to produce the ejection of an electron from the moiK>mer molecule. In this chapter we will briefly review other photochemical tediniques involving the framation of electronically excited intermediates, which in turn generate suitable species for the initiation of cationic polymerisation. Crivello has recently reviewed this topic and other authors have published more specialised monographs on some specific aspects of cationic photo-initiation. We will therefore reduce our coverage to the basic premises on whidi the various methods are founded and to a few comments on the more recent contributions and medianistic interpretations. 

The reverse process can also be achieved by a photochemical process which can eject an electron from a dianion and form a radical-anion 1819>, (Eq. 3) ... 

Phase diagram. A diagram showing the conditions at which a substance exists as a solid, liquid, or vapor. (11.9) Photochemical smog. Formation of smog by the reactions of automobile exhaust in the presence of sunlight. (17.6) Photoelectric effect. A phenomenon in which electrons are ejected from the surface of certain metals exposed to light of at least a certain minimum frequency. (7.2)... 

On irradiation of DOM, the one primary photochemical event is photo-ionization. This process is less sensitive to environmental factors than triplet formation. All of the remaining photochemistry depends on the subsequent fate of the three primary products of photoionization triplets, solvated electrons, and cation radicals [13]. Laser flash photolysis studies supplied early evidence for hydrated electron (e q) production on irradiation of DOM, with significant primary quantum yields [9,12,84,85]. The hydrated electron was identified by its characteristic broad absorption spectrum from 700 nm to 750 nm [86, 87]. The formation of e q is thought to result from the photo-ejection of an electron from excited-state humic substances ... 

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