Photochromism paramagnetism

Electron paramagnetic resonance (EPR) studies on pyridine solutions of bianthrone by Wasserman (41) suggest that the thermochromic form may have one unpaired electron localized on each half of the molecule. The structure proposed by Woodward and Wasserman (42) is VIII which they consider to be identical to the colored photochromic form. Klochkov (43) has warned that free radicals may be involved in decomposition processes occuring at high temperatures in studies of the bian-... 

Few photochromes exhibit paramagnetism as evidenced by magnetic susceptibility and electron spin resonance measurements. In spite of this selectivity, these are probably the most powerful techniques to aid in the elucidation of mechanisms for certain photochromic systems. 

Chapter 4 (EPR and Radical Processes). The radical species sometimes resulting from photochromic systems can be investigated by electron paramagnetic resonance (EPR) spectroscopy. This technique can also be employed to detect radical species formed during photodegradation. 

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. 

More recently, the new photochromic spin probe 74 was synthesized.67 As the paramagnetic unit is provided by a 4-(2,2,6,6,-tetramethylpiperidinyl-l-oxyl) (TEMPO) moiety, it is not unexpected that the spectral parameters of this radical, i.e., aN= 15.5 G and g = 2.0055, did not differ from those of TEMPO itself. 

M. Campredon, R. Guglielmetti, A. Samat, G. Gronchi, and A. Alberti, Radical anions from some photochromic nitro compounds. An electron paramagnetic resonance and electrochemical study, Res. Chem. Interned. 19, 307-318 (1993). 

It is relevant to cite here two more examples of the application of molecular calculations to spiropyran derivatives. First, in a study of the relationship between photochromism and second-order nonlinear optical (NLO) properties in spiropyran- and spirooxazine-doped polymer films, dipole moments, polarizabilities, and hyperpolarizabilities were estimated by the MNDO method.46 Second, the relative stabilities of the radical anions as well as their spin density distributions for a spiroindolinic series were calculated by an ab initio method in a combined electron paramagnetic resonance (EPR)/electrochemistry study.47... 

The second volume of this new treatise is focused on the physicochemical properties and photochromic behavior of the best known systems. We have included chapters on the most appropriate physicochemical methods by which photochromic substances can be studied (spectrokinetic studies on photostationary states, Raman spectroscopy, electron paramagnetic resonance, chemical computations and molecular modeling, and X-ray diffraction analysis). In addition, special topics such as interactions between photochromic compounds and polymer matrices, photodegradation mechanisms, and potential biological applications have been treated. A final chapter on thermochromic materials is included to emphasize the chemical similarities between photochromic and thermochromic materials. In general, the literature cited within the chapters covers publications through 1995. However, in several cases, publications from as late as 1997 are included. 

The triarylimidazolyls of this invention are relatively 70 long-lived free (paramagnetic) radicals, which can exist for long periods of lime under the influence of the light stimulus required to dissociate the photochromic dimers thereof. They have finite lifetimes in the absence of such stimulus and are characterized by their color, light ab- 7B... 

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