What interaction makes Hoechst fluorescence blue?

Hoechst stains are excited by ultraviolet light at about 350 nm. Upon interaction with UV light from a mercury arc lamp, xenon lamp or ultraviolet laser, the dyes emit a broad spectrum of blue fluorescence in the region of 460 to 490 nm. On binding with DNA, their fluorescence increases as much as 30 times, ensuring a clear signal-to-noise ratio.