8-Nitro spiro indoline-benzopyran

Distribution of spin density in the framework of trianion radicals allows determining whether or not there is some conjugation between substituents at different sites. For example, reduction of potassium tert-butoxide in dimethysulfoxide (DMSO) proceeds according to Scheme 1-62 (Alberti et al. 1995). 

As shown by means of electrochemical ESR spectroscopic methods and confirmed by ab initio calculations (Alberti et al. 1995), the first electron is accommodated in the more accessible nitrobenzopyran moiety. Next, the second electron fits into the nitroindoline moiety in virtually the same orbital that in the anion radical of mon<9-nitro-spiro(indoline-benzopyran), is occupied by the unpaired electron. The mono-nitro anion radical is also formed by the aforementioned reaction with terf-BuOK in DMSO, Scheme 1-63. 

As to the third step of the step-wise one-electron reduction of t//-nilro-spiro(indoline-benzopyran), the third extra electron is again accommodated into the nitrobenzopyran unit without affecting the spin density distribution on the opposite fragment, which therefore remains essentially identical to that of the anion radical of mono-nitro-spiro(indoline-ben-zopyran). Hence, the spiro-nodal carbon atom prevents conjugate interactions between the indoline and benzopyran moieties. 

Smets and coworkers worked with stretched spirobenzopyran rubber networks obtained by copolymerization of ethyl aaylate with variaWe amounts of a bis-photochrome dimethacrylate as cross-linking agent, namely l,l -(a,a -p-xylyl)-bis-[3, 3 -dimethyl-8-methacryloyloxymethyl-6-nitro-spiro(2H-l-benzopyran-2,2 -indoline)]. The chemical structure of these DIPS-rubbers is given in Fig. 11. 

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. 

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... 

Hovery, R.J., Chu, N.Y., Piusz, P.G., and Eudsmann, C.H., Photochromic Compounds, U. S. Patent 4342, 668, 1982 Kawanishi, Y, Seki, K., Tamaki, T, Sakuragi, M., and Suzuki, Y, Tuning reverse ring closure in the photochromic and thermochromic transformation of l, 3, 3 -trimethyl-6-nitro-spiro[2H-l-benzopyran-2,2 -indoline] analogues by ionic moieties, /. Photochem. Photobiol., A Chem., 109, 237, 1997. 

Another class of merocyanines consists of ring-opened spiropyrans, such as spiro[2H-l-benzopyran-2,2 -indolines] (BIPS), especially of 6-nitro-substituted derivatives (NOjBIPS). The photochromism of spiro... 

---