Chiral binaphthyl-linked BODIPY analogues: synthesis and spectroscopic properties

Abstract

A chiral binaphthyl linked at the periphery and boron atom to form chiral BODIPY analogues has been prepared and characterized by various spectroscopies including single-crystal X-ray diffraction, ¹H NMR, electronic absorption and CD, as well as fluorescence. Their chiroptical properties are dependent on the position of the chiral substituent. Attachment of a chiral binaphthyl group at the periphery to form (R)/(S)-2 displays positive and negative CD curves in the 260–400 nm region, and a chiral binaphthyl linked at the boron atom to form (R)/(S)-4 exhibits a negative/positive and positive/negative sign pattern in ascending energy terms in the main absorption spectra, respectively. Very low fluorescence quantum yields and solvent-dependent fluorescent bands for 2 are observed due to the excited-state intramolecular charge transfer. In the case of 4, the fluorescence is totally quenched, which can be attributed to the complete charge localization and negligible orbital overlap of the frontier MOs. TDDFT calculations reproduce experimentally observed spectroscopic data, and provide further insight into the nature of the electronic transitions.