Substituted 1,3-bis(imino)isoindole diols: a new class of proton transfer dyes.

A new class of excited-state intramolecular proton transfer (ESIPT) dyes based on a 1,3-bis(imino)isoindole diol motif has been prepared. These molecules exhibit orange emission (∼600 nm) with a large apparent Stokes shift (>6000 cm(-1)) and quantum efficiencies up to 45%. Selective modification of the substituents can be used to shift the equilibrium between the enol and keto forms of the molecule in both the ground and excited states.