2.28 g (19.9 mmol) of potassium tert-butoxide and then 5.90 g (17.6 mmol) of 10-phenyl-5,5-dioxophenothiazine-3-carbaldehyde (Example 21) were added under nitrogen at room temperature with stirring to a solution of 4.00 g (8.80 mmol) of diethyl[4′-(diethoxyphosphorylmethyl)biphenyl-4-ylmethyl]phosphonate in 45 ml of dimethyl sulfoxide dried over molecular sieve. After stirring at room temperature for 72 h, the reaction solution was diluted with 200 ml of methanol and stirred for a further 1 h. The precipitate was filtered off, washed with 500 ml of methanol and dried at 80° C. under reduced pressure. The crude product (5.44 g) was recrystallized in 250 ml of o-dichlorobenzene. The crystals were filtered off with suction, washed successively with o-dichlorobenzene and ethanol, filtered off with suction and dried at 80° C. under reduced pressure. After solvent residues had been removed in high vacuum (2×10−5 mbar) at 250° C., 4.35 g (60% of theory) of yellow microcrystals having a melting point of 392° C. were obtained, whose solution in methylene chloride fluoresced at λ=456 nm.
