AZO PIGMENTS AND THEIR INTERMEDIATES - ELECTRONIC-STRUCTURE OF BISAZOPIGMENTS BY CROSS-POLARIZATION MAGIC ANGLE SPINNING C-13 NMR-SPECTROSCOPY

The electronic structures of a number of photoconductive bisazo pigmen ts II-VIII, synthesized by coupling 1,5-diaminonaphthalene, 2,7-diamin onaphthalene, 2,7-diaminofluorenone, 2,7-diaminodibenzothiophene sulfo ne, 4,4'-diamino-3,3'-dichlorobiphenyl, 4,4'-diaminoterphenyl and 4,4' -diaminoazobenzene with 2-hydroxy-3-naphthanilide couplers, respective ly, have been studied by cross-polarization magic angle spinning (CP M AS) C-13 NMR spectroscopy. These bisazo pigments are amorphous (or mic rocrystalline), pigmentary powders in the solid state. Their electroni c structures are neither accessible by solution spectroscopic nor X-ra y powder diffraction studies. C-13 chemical shift data of is(1'-azo-2' -hydroxy-3'-naphthanilide)-naphthalene (IIa) reveal that the hydroxyli c aromatic carbons present in the coupler moieties of IIa are absent. This observation, along with the appearance of a new carbonyl resonanc e, leads us to conclude that IIa exist as a keto-hydrazone in the soli d state. This conclusion is complemented by IR spectral data, where a change in the intramolecular H-bonding structure of the coupler moieti es upon pigment formation, is indicated. Studies of the temperature an d substituent effects on the C-13 NMR spectra of IIa indicate that the keto-hydrazone structure is stable and is unaffected by electronic an d steric effects exerted by substituents in the coupler moieties. The dominance of the keto-hydrazone tautomers is attributable to both intr a- and intermolecular H-bonding effects. CP MAS C-13 NMR data of bisaz o pigments III-VIII clearly reveal that these pigments are keto-hydraz ones also. The impact of the realization of the electronic structure o n the molecular design and synthesis of near-IR-sensitive azo pigments is discussed.