1.02 g (9.05 mmol) of potassium tert-butoxide and then 2.20 g (8.05 mmol) of 10-methyl-5,5-dioxophenothiazine-3-carbaldehyde (Example 21) were added under nitrogen at room temperature with stirring to a solution of 1.82 g (4.00 mmol) of diethyl[4′-(diethoxyphosphorylmethyl)biphenyl-4-ylmethyl]phosphonate in 41 ml of dimethyl sulfoxide dried over molecular sieve. After stirring at room temperature for four hours, the reaction solution was diluted with 90 ml of methanol and stirred for a further 15 min. The precipitate was filtered off, washed with 220 ml of methanol and dried at 35° C. under reduced pressure. The crude product (2.50 g) was recrystallized in 58 ml of dimethyl sulfoxide. After solvent residues had been removed under high vacuum at 200° C., 1.64 g (59% of theory) of analytically pure yellow solid having a melting point of 360° C. were obtained, whose solution in N-methylpyrrolidone fluoresced at λ=460 nm.
