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A synthetic route to new heterocyclic 1,1-donor-acceptor-substituted alkenes starting with N -methyl-acridone, xanthone, and thioxanthone was investigated, leading to the acridone- and xanthone-derived products methyl 2-methoxy-2-(10-methylacridin-9 (10H)-ylidene)acetate ( 7 ) and methyl 2-methoxy-2-(9H-xanthen-9-ylidene)acetate ( 10 ) in low yields with the de-methoxylated product methyl 2-(10-methylacridin-9 (10H)-ylidene)acetate ( 8 ) and the reduced compound methyl 2-methoxy-2-(9H-xanthen-9-yl)acetate ( 11 ) as the major products from N -methyl acridone and xanthone. From thioxanthone, only the rearrangement and reduction products ( 14 ) and ( 15 ) resulted. The photophysical properties of compounds ( 7 ), ( 8 ), and ( 10 ) were investigated in the presence and absence of the Brønsted acid TFA by NMR, UV-VIS absorption, and fluorescence spectroscopy. Protonation of the acridone-derived alkenes ( 7 ) and ( 8 ) led to strong bathochromic and hyperchromic fluorescence shifts and a substantial increase in Stokes shift. The photooxygenation experiments with these substrates showed an unusual reactivity pattern in the singlet oxygen processes: whereas the electron-rich enolether ( 7 ) was chemically unreactive, ( 8 ) and ( 10 ) were oxidatively cleaved, presumably via intermediate 1,2-dioxetanes.

New Acridone- and (Thio)Xanthone-Derived 1,1-Donor–Acceptor-Substituted Alkenes: pH-Dependent Fluorescence and Unusual Photooxygenation Properties