New two- and three-cationic polymethine dyes. Synthesis, properties and application

• Benzothiazolestyrylium dyes and borate salts as new dyeing photoinitiating systems.
• Spectroscopic and electrochemical properties of new donor-π-acceptor based photosensitizers.
• Kinetics of visible light induced photopolymerization.

Three new two- and three-cationic monochromophoric polymethine dyes based on the 2-methylbenzothiazole were synthesized and applied as visible light initiators for acrylic monomer polymerization. In the photoinitiating system, the polymethine dye cation acts as an electron acceptor whereas borate anion is an electron donor.The actual photoinitiators can only operate in a restricted part of the visible light region; as a consequence, a several photoinitiators are usually necessary to harvest all the emitted visible photons. In the present paper, new dyes based on a donor-π-acceptor structure (2-methylbenzothiazole derivatives) are incorporated into visible light sensitive photoinitiating systems of triacrylate polymerization. They exhibit an unusual and remarkable broad absorption lying from the blue to the red. When employed in the presence of organoborate salts, these dyes can efficiently initiate the radical photopolymerization of acrylates under exposure at 514 nm.

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