The X-ray crystallographic, optical spectroscopic, and electrochemical properties of a newly synthesized class of boron-diindomethene (BDI) dyes and their tetrahydrobicyclo precursors (bc-BDP) are presented. The BDI chromophore was designed to show intensive absorption and strong fluorescence in an applicationary advantageous spectral range. Its modular architecture permits fusion of a second subunit, for example, a receptor moiety to the dye's core to yield directly linked yet perpendicularly prearranged composite systems. The synthesis was developed to allow facile tuning of the chromophore platform and to thus adjust its redox properties. X-ray analysis revealed a pronounced planarity of the chromophore in the case of the BDIs, which led to a remarkable close packing in the crystal of the simplest derivative. On the other hand, deviation from planarity was found for the diester-substituted bc-BDP benzocrown that exhibits a "butterfly"-like conformation in the crystal. Both families of dyes show charge- or electron-transfer-type fluorescence-quenching characteristics in polar solvents when equipped with a strong donor in the meso-position of the core. These processes can be utilized for signaling purposes if an appropriate receptor is introduced. Further modification of the chromophore can invoke such a guest-responsive intramolecular quenching process, also for receptor groups of low electron density, for example, benzocrowns. In addition to the design of various prototype molecules, a promising fluoroionophore for Na+ was obtained that absorbs and emits in the 650 nm region and shows a strong fluorescence enhancement upon analyte binding. Furthermore, investigation of the remarkable solvatokinetic fluorescence properties of the "butterfly"-like bc-BDP derivatives suggested that a second intrinsic nonradiative deactivation channel can play a role in the photophysics of boron-dipyrromethene dyes.

Four novel borondipyrromethene (BDP) and -diindomethene (BDI) dyes with one or two (dimethylamino)styryl extensions at the chromophore were synthesized and spectroscopically investigated. An X-ray crystal structure shows that the extended auxochrome is virtually planar. All dyes thus display intense red/near infrared (NIR) absorption and emission. The (dimethylamino)styryl group induces a charge-transfer character that entails bright solvatochromic fluorescence, which is only quenched with increasing solvent polarity according to the energy-gap law. The dye with an additional dimethylanilino group at the meso position of BDP shows a remarkable switching of lipophilicity by protonation. Two dyes with an 8-hydroxyquinoline ligand at the meso position display quenched emission in the presence of Hg2+ or Al3+ owing to electron transfer from the excited BDP to the complexed receptor. The BDI dye presents a pH indicator with bright fluorescence and extremely low fluorescence anisotropy.

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