DEVELOPMENT OF NEW DYEING PHOTOINITIATORS BASED ON AZOMETHINE DYES

Several dyes containing the azomethine moiety have been synthesized an d evaluated as photoinitiators for free radical polymerization induced with the argon-ion or He-Ne lasers. Two methods of dye modification a re applied in the study. The first method is a change in the type of t he substituent in the pyrazolone skeleton: methyl group (a weak electr on donor) and ester group (an electron acceptor). The replacement of t he substituent causes a significant red shift in the electronic absorp tion spectra but does not affect photoinitiation ability. Elimination or limitation of the rotation of the phenyl group (PBAD-1 and PBAD-2) causes an additional red shift and a change of the shape of the electr onic absorption curve. The coplanarization of the azomethine part (ChA D) shifts to the red an absorption maximum of about 200 nm and also de creases the molar absorption coefficient. A change in the dye structur e caused by the opening of the pyrazolone skeleton causes a blue shift in the absorption spectra but does not affect the photoinitiation abi lity. One observes a dramatic increase in the photoinitiation efficien cy and an increase in the quantum yield of the bleaching process when the twisting motion of the C=N bond is severely hindered. Elimination of the motion of C=N bound by coplanarization of the azomethine residu e with other parts of the dye decreases the degree of branching. This decrease in branching stabilizes the molecule in its excited state. Th e red shift of the absorption spectra maximum of ChAD allows the initi ation of polymerization using both argon-ion and He-Ne lasers.