A series of fluorene-fused benzoquinones (Q1-Q5) were prepared by thermolysis of 4-fluorenyl-4-hydroxycyclobutenones. Red fluorescence observed for Q2 is switched by reduction to blue fluorescence by formation of the hydroquinone. Reaction with hydrogen peroxide restores the original fluorescence colour. The potential use of compound Q2 as a reactive oxygen species detector is discussed.