An EPR, electrochemical, and ab initio investigation on the nature of the radical ions formed in the reduction of some photochromic compounds of the spiroindolinic series

The oxidation and reduction peak potentials of 1-methyl-5-nitrospiro[indoline-2,3?-naphthoxazine], 1, 1-methyl-8?-nitrospiro[indoline-2,3?-naphthoxazine], 2, and 1-methyl-5,8?-dinitrospiro[indoline-2,3?-naphthoxazine], 3, have been determined by cyclic voltammetry. The radical anions of the three compounds, generated by reduction with potassium tert-butoxide in DMSO, by photoinduced reduction with tetrabutylammonium borohydride, or by electrochemical reduction have also been investigated by EPR spectroscopy. In the anions from 1 and 2 the unpaired electron is confined on the molecular fragment bearing the nitrogroup, conjugative interactions between the indolinic and naphthoxazinic moieties being prevented by the quaternary carbon. As expected on the basis of the measured reduction potentials, in the radical anion of the dinitro compound 3 the unpaired electron is accommodated in the naphthoxazinic unit, a behavior apparently at odds with that previously observed in the reduction of the structurally related 1-methyl-5,7?-dinitrospiro[indoline-2,3?-benzopyran], 6, and 1-methyl-5,8?-dinitrospiro[indoline-2,3?-naphthopyran], 9. The reduction of the latter two compounds has been reinvestigated, and the observed species have been identified as the corresponding radical trianions. Ab initio calculations carried out on 1-methyl-5-nitrospiro[indoline-2,3?-benzopyran], 4, 1-methyl-7?-nitrospiro[indoline-2,3?-benzopyran], 5, and the dinitro compound 6 as model systems correctly predict the relative stability of the radical anions as well as the spin density distribution; they also indicate that the formation of the trianions from the dinitro derivatives is possible provided that alkali metal counterions are present, as is the case of the reduction with potassium tert-butoxide. © 1995 American Chemical Society.