Significantly large Stokes shifts and enhanced solid state emissions were achieved in a novel series of asymmetric core-expanded aza-BODIPY analogues, 4a-4d, synthesized by a facile and scalable two-step reaction in high yields.

A boron-dipyrromethene (BODIPY) dye containing a styryl substituent with a hydroxyl and nitro group, 1a, exhibits a 'turn-on' fluorescence response for hypoxic cells. The presence of the electron-withdrawing nitro group results in a highly efficient nonradiative decay of the S1 state, and hence a recovery of fluorescence intensity when nitro reduction occurs under hypoxic conditions.

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