Supramolecular host-guest interaction of neutral and cationic (protonated) forms of two boron-dipyromethane (BODIPY)-benzimidazole (mono- and di-benzimidazole) conjugate dyes with the macrocyclic host cucurbit[7]uril (CB7) has been investigated using photophysical and density functional theory studies. Expectedly, cationic forms of the dyes show exceptionally stronger binding than that of the neutral forms with CB7, which can be ascribed to the strong ion-dipole interaction between the positive charge of the dye and the highly polarizable carbonyl portals of the host. The formation of dye-host inclusion complexes is supported by the significant changes in the photophysical properties and longer rotational relaxation times of the dye in the presence of CB7. Job's plot studies indicate the formation of a 1:1 inclusion complex for the mono and a 1:2 inclusion complex for the dibenzimidazole BODIPY dyes. Quantum chemical calculations are in good agreement with the inferences outlined from photophysical measurements. Findings from the studied dye-CB7 systems are of direct relevance to applications such as drug delivery, aqueous dye lasers, sensors, and so on.

The fluorescence lifetime and quantum yield are seen to depend in an unexpected manner on the nature of the solvent for a pair of tripartite molecules composed of two identical boron dipyrromethene (BODIPY) residues attached to a 1,10-phenanthroline core. A key feature of these molecular architectures concerns the presence of an amide linkage that connects the BODIPY dye to the heterocyclic platform. The secondary amide derivative is more sensitive to environmental change than is the corresponding tertiary amide. In general, increasing solvent polarity, as measured by the static dielectric constant, above a critical threshold tends to reduce fluorescence but certain hydrogen bond accepting solvents exhibit anomolous behaviour. Fluorescence quenching is believed to arise from light-induced charge transfer between the two BODIPY dyes, but thermodynamic arguments alone do not explain the experimental findings. Molecular modelling is used to argue that the conformation changes in strongly polar media in such a way as to facilitate improved rates of light-induced charge transfer. These solvent-induced changes, however, differ remarkably for the two types of amide.

An efficient and chemoselective route for transforming BF2-BODIPYs to Et2B-BODIPYs (masking) was developed using Et2AlCl. The Et groups can be easily replaced with F atoms using BF3·Et2O in moist CH2Cl2 to regenerate the BF2-BODIPYs (demasking). The masking-demasking strategy is very useful for synthesizing functionalized BODIPYs via nucleophilic and reductive reactions. The masking strategy was used to synthesize a BODIPY dimer by McMurry coupling of a formyl Et2B-BODIPY, while a new BODIPY with an asymmetrically substituted B-center was synthesized using the demasking strategy.

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