Xanthene dyes, radicals from

For uniformity with the stmctures given in the Colourindex the ammonium radical (9) is used for the amino-substituted xanthenes and the keto form for the hydroxy derivatives. The xanthene dyes may be classified into two main groups diphenylmethane derivatives, called pyronines, and triphenylmethane derivatives (eg, (4)), which are mainly phthaleins made from phthaUc anhydride condensations. A third much smaller group of rosamines (9-phenylxanthenes) is prepared from substituted ben2aldehydes. The phthaleins may be further subdivided into the following fluoresceins (hydroxy-substituted) rhodamines (amino-substituted), eg, (6) and mixed hydroxy/amino-substituted. 

When xanthene is oxidized by certain 1,4-naphthoquinones, radical products are obtained which contain a xanthenyl moiety (e.g., 42) for which splittings are quoted in gauss. The reactions have been investigated by the CIDNP technique. Radical 41 has been involved as a substrate in Feldman and co-workers series of papers on stabilities of trivalent carbon spe-144.185,186 Section II,B,5,e for radicals from xanthene dyes.)... 

Pulse Radiolysis of Aqueous Fluorescein Dyes. The xanthene dyes of the fluorescein type were investigated using l-/xsec. pulses of 30 Mev. electrons (4, 5, 8). The transient spectra obtained with deaerated fluorescein solutions show three characteristic sets of bands. A prominent peak that shifts from 355 m/x in neutral solutions to 395 m/x in alkaline solutions corresponds with the semiquinone monoanion (pKa = 9.5) (13). This band is quenched by e q scavengers, such as oxygen or H202, and was attributed to reduction of the dye by eaq. A band at 415-420 m/x which does not change with pH was identified with the semioxidized radical monoanion, a phenoxyl derivative first observed in flash photolysis also (13). This band is quenched by formate and was attributed to the oxidative attack of OH. The remaining transient consists of a diffuse... 

The dye-sensitized photo-oxidation of phenols has been examined using CIDNP techniques.148 It is concluded from the observed polarizations of nuclear spin that sensitization by xanthene dyes (such as Rose Bengal) is the result of reversible hydrogen abstraction by the triplet dye molecule from the phenolic hydroxyl group. Any resulting photochemical reactions (e.g. with oxygen) arise from irreversible reactions of the phenoxy-radicals produced, rather than from direct reactions of the phenols with 102. Dye-sensitized photo-oxidation of p-hydroxyphenylpyruvic acid (94) in solution at pH 7 (conditions under which... 

Bradley and Davidson pointed out that for any particular aromatic ketone, the efficiency of photoinitiation is determined by the structure of the ketone and by the molecular geometry of the a-aminoalkyl radical produced. On the other hand, Paczkowski et al, suggested that the initial electron-transfer reaction from aromatic amines to the excited state of xanthene dyes, used in their study, is responsible for the variation of the photoinitiation efficiency. 

Other examples include acridine dyes (with absorption peaks aroimd 475 nm), xanthene dyes ( 500-550 nm), fluorone dyes ( 450-550 nm), coiunarin dyes (" 350-450 nm), cyanine dyes ( 400-750 nm), and carbazole dyes ( 400 nm) (12,19-21). The oxidation or reduction of the dye is dependent on the co-initiator for example, methylene blue can be photoreduced by accepting an electron from an amine (22) or photo-oxidized by transferring an electron to benzyltrimethyl-stannane (12). Either mechanism will result in the formation of a free-radical active center capable of initiating a growing polymer chain. For a more detailed discussion of the mechanisms, see Reference 12. 

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. 

---