Dipole moment merocyanine

Fohlisch99 reported a remarkable dependence of the electron spectra of quino-cyclopropenes on their structure. As shown in Table 9, the merocyanine-like quino-cyclopropenes show positive solvatochromy when they contain an anthraquinonoid chromophore (198), but negative solvatochromy when they contain a benzoquino-noid system (199). This can be interpreted in terms of a markedly increased participation of dipolar resonance forms in the ground state of the benzoquinonoid 199 compared to the anthraquinonoid 198. From the dipole moment of 198 (9.4 D99 ) the dipolar contribution was estimated to be in the range of 23%. 

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. 

Merocyanine dyes 50-55 (Fig. 5.18) possess an electronic structure at the meso-meric center between neutral and zwitterionic electron distribution and have high polarizabilities and dipole moments and exhibit absorption spectra with sharp bands that give rise to exceptionally brilliant magenta hues [60],... 

Thns, 4-[(4-dimethylamino)phenyl)inimo]-2,5-cyclohexadien-l-one (DIA), also known as Phenol Bine, is a merocyanine dye that exists in two extreme resonance hybrids of a keto and a phenolate form (49, 50). Hybrid 49 is expected to contribnte more in the solid state, whereas hybrid 50, owing to its larger dipole moment, is believed to contribute more in polar solvents . 

Net total and tt-electron charges on the atoms of the conjugated path as obtained for merocyanine Ml by CS INDO, INDO and AMI MO SCF calculations. The calculated dipole moments and their components are also reported (last four rows). 

Solvatochromism of merocyanine dyes is accompanied also with the solvent dependence of their dipole moment [Pa 80], So dipole moment measurements have also contributed to the explanation of solvatochrom behaviour of these systems. In such systems the electronic ground and excited states of a molecule are described by two resonance structures non-polar (quinoid) and polar (benzenoid). 

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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