Radical Ions from Spiro Compounds

Spirofindoline-benzopyrans], spiro[indoline-naphthopyrans], and spiro[indo-line-naphthoxazines] can rather readily undergo electron-transfer processes in either direction i.e they can be readily oxidized and reduced. Because the presence of nitro groups in their molecular framework normally enhances the photochromaticity of these compounds, several combined electrochemical and EPR studies have been carried out on a variety of mono- and dinitro-substituted spiroindolinic photochromes.52 56-60 

The peak reduction potential for nitrospiro[indoline-benzopyrans] 57-62 

The radical cations of the various spiroindolinic compounds could not be studied by EPR spectroscopy, being too short-lived to be observed. The oxidation and reduction potentials of nitro derivatives vary with the position and number of 

As mentioned before, the radical cations were very short-lived (totally irreversible cyclic voltammograms), while all the radical anions were rather persistent,57-59 with the notable exception of that from the dinitrospironaphthox-azinic derivative 69.60 Actually, while the dinitro derivatives 65 and 67 exhibited a reduction voltammogram with two well-defined reduction peaks at potentials close to the reduction potentials of the corresponding mononitro derivatives, the voltammogram of 69 indicated an irreversible first reduction wave and a small, broad second wave attributed to reduction of degradation products of the radical anion formed in the first reduction step. 

Hgu re- S. EPR sjteclra plained after reduction Of (he dinilro compounds 65 fa. experimental c. Simulated) urtd W (t , experimental d simulated I with excess potassium terf-bultutidcindimcdtylsuir- 

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This chapter, far from being exhaustive, is aimed at outlining the properties of the radical ions and of the free radicals derived from photochromic compounds independently of whether or not these paramagnetic species are involved in the photochromic process itself. In particular, it will deal with the EPR of dihydro-and dialkyldipyridyl radical cations (viologens) and of the radical anions of a variety of nitro-substituted spirofmdoline-benzopyrans], spiro[indoline-naphtho-pyrans], and spiro[indoline-naphthoxazines] as well as their triphenylgermyloxy nitroxides and with the triplet spectra observed upon irradiation of 2,2,4,4-tetrachloro-1 -keto-1,4-dihydronaphthalene. 

The analogous primary amine had been earlier studied by Wubbels and gave only the photo-Smiles reaction, but this process was not observed for the present system and the reaction is suggested to proceed by a photo-electron transfer mechanism to yield the radical ion pair. Photoaddition of N-H bonds to aromatic compounds has been known for some years, and the intramolecular process which leads to cyclization products has been reported for the reaction of 9-(aj-anilinolalkyl)phenanthrenes (220). The formation of the spiro-compounds (221) by addition of the N-H bond across the phenan-threne 9,10-positions is suggested to arise from the intramolecular exciplex in... 

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