Photochemically-assisted synthesis and photophysical properties of difluoroboronated β-diketones with fused four-benzene-ring chromophores, chrysene and pyrene

We investigated the photophysical properties of difluoroboronated β-diketones (BF₂DK) with chrysene and pyrene skeletons (ChB and PyB, respectively) in solution and in the solid state. Acetylchrysenes, as the key precursors to ChBs, were photochemically prepared from the corresponding (acetylphenyl)naphthylethenes by means of a modified photocyclization method. The absorption and emission spectra of the BF₂DKs were obtained in chloroform and acetonitrile, and the quantum yields and lifetimes of the fluorescence were determined. Excimeric fluorescence from PyB was absent even in highly concentrated solution. Based on the Lippert-Mataga analysis of the absorption and fluorescence features, the photophysical properties of the ChBs were discussed in comparison with those of PyB. The fluorescence states of the studied BF₂DKs are shown to be of a charge-transfer character. The fluorescence quantum yields decrease with increasing the solvent polarity due to the enhanced internal conversion process. The fluorescence quantum yields in the solid state of the studied BF₂DKs were determined, and it was found that PyB is fluorescent, whereas the fluorescence quantum yields of the ChBs depend on the substituted position of the chrysene moiety.