Benzo[1,2-d:4,5-d′]bisimidazoles as a Convenient Platform Towards Dyes that are Capable of Excited-State Intramolecular Proton Transfer and of Two-Photon Absorption

New, strongly fluorescent benzo[1,2-d:4,5-d']bisimidazoles have been prepared by the reaction of Bandrowski's base with various aldehydes. Their structures were carefully designed to achieve efficient excited-state intramolecular proton transfer and good two-photon-absorption (2PA) cross-sections. Functional dyes that possessed both high fluorescence quantum yields and large Stokes shifts were prepared. A p-expanded D-A-D derivative that possessed Ffl=50?% and s2=230 GM in the spectroscopic area of interest for biological imaging is an excellent candidate as a fluorescent probe. Thanks to the presence of two reactive amino groups, such compounds can be easily transformed into probes for bioconjugation. All of these benzo[1,2-d:4,5-d']bisimidazoles were also strongly fluorescent in the solid state.