Isocyanate-, Isothiocyanate-, Urea-, and Thiourea-Substituted Boron Dipyrromethene Dyes as Fluorescent Probes

Boron dipyrromethene dyes (Bodipy) bearing a meso-phenyl substituent carrying a variety of functional
groups can be prepared under mild conditions. A single-crystal X-ray structure determination for the
3,5-dinitrophenyl compound shows the phenyl ring to be almost orthogonal (dihedral angle 84°) to the
plane of the Bodipy core, with one nitro group almost coplanar with the ring and the other tilted by
∼21°. Nitro substituents at the 3-, 4-, and 5- positions of the phenyl group are readily reduced to the
corresponding amino groups and then converted to isocyanato, isothiocyanato, urea, thiourea, and some
polyimine derivatives, the last providing additional functionality (phenazine and pyridylindole units)
suitable for chelation of metal ions. All compounds are redox active, the electron-transfer processes
being assigned on the basis of comparisons with model compounds. Their fluorescence properties are
sensitive to the phenyl group substituents. The Bodipy unit excited state appears to be a strong reductant
(E° ∼ −1.4 V) and a modest oxidant (E° ∼ +1.0 V). Quenching processes in the nitro and phenazine
derivatives appear to involve intramolecular photoinduced electron transfer.