Cationic 3H-indolium dyes by ring-opening of benzo[1,3]oxazine

The reaction of 2-methylbenzo[1,3]oxazine with (hetero)aromatic aldehydes under acid catalysis afforded directly polymethine dyes, formed by a thermal ring-opening of the oxazine cycle followed by condensation with the aldehydes. These dyes have the structure of a cationic thermally stable coloured open form of a photochromic benzo[1,3]oxazine and do not undergo ring-closure to afford the closed form again, even under basic treatment. All of the dyes showed reversible acidochromic properties.

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► Condensation of 2-methylbenzo[1,3]oxazine with aldehydes afforded 3H-indolium cationic dyes.
► Thermal ring-opening of benzo[1,3]oxazines.
► In basic media the opened oxazine dyes do not undergo ring-closure.
► In basic media the indole derived dye has a zwitterionic structure containing two chromophores.
► All dyes exhibit reversible acidochromic behaviour.