Spiropyrans dipole moment

Photo-aggregation of the nitro-BIPS compounds continues to fascinate researchers and articles are being produced still regarding this phenomenon [24,38-45,60-62]. It is, of course, noteworthy before we continue with this discussion that spiropyran merocyanines are zwitterions possessing a significant dipole moment and that their stability in a nonpolar solvent will therefore be minimal. Spontaneous precipitation upon formation may be expected as the final outcome of the photo-formation of the merocyanine. 

Two permanent merocyanines have been reported for the spiro-oxazines [85]. These were NOSH heteroanellated by imidazo [l,2-a]pyridine and imidazo [l,2-a]pyrimadine. Several tests have been conducted to determine the nature of these species. H -NMR data show that the indoline nitrogen is not highly charged and the crystal structure indicates that the ground state is essentially the quinoidal form. The most stable form was found to be the TTC isomeric form by x-ray analysis. The dipole moment of these permanent spiro-oxazine merocyanines was around 3.84 D, which is much lower than the values reported for spiropyran merocyanines. 

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... 

A. Le Beuze, A. Botrel, A. Samat, and R. Guglielmetti, Structural study of benzothiazolinic spiropyrans by the theoretical and experimental determination of dipole moments,/. Mol. Struct. 40, 77-87 (1977). 

A change in dipole moment caused by isomerization from spiropyran to the merocyanine form would be expected to alter intramolecular interaction of polymer chains when the chromophores are incorporated into the polymer pendant groups or backbone groups. The change of intramolecular interac-... 

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