Aromatic difluoroboron β-diketonate complexes: effects of π-conjugation and media on optical properties

Aromatic difluoroboron β-diketonate complexes (BF2bdks) are classic fluorescent molecules that have been explored as photochemical reagents, two-photon dyes, and oxygen sensors. To gain a better understanding of their emissive properties in both solution and polymer matrices, BF2bdks with varying aromatic groups were synthesized and their photophysical properties were investigated in both methylene chloride and poly(lactic acid) (PLA). Absorption spectra showed systematic variations that are well correlated with structural features, including the size of the aryl substituent and the presence of a para electron-donating methoxy substituent. Computational modeling of the absorption spectra with the TD-B3LYP/6-311+G(d)//B3LYP/6-31G(d) formulation of density functional theory and a polarizable continuum model of dichloromethane solvent shows that all systems show intense π-π* one-electron excitations, usually from one of the highest occupied molecular orbitals (HOMO - k, k = 0, 1, 2) to the lowest unoccupied molecular orbital (LUMO). Emission properties are sensitive to the dye structure and medium. Based on spectroscopic and lifetime studies, BF2bdks exhibit comparable fluorescence properties in both solutions and polymers when the diketonate group is functionalized with smaller aromatic ring systems such as benzene. For BF2bdks with larger arene ring systems, such as anthracene, emission from a strong intramolecular charge-transfer (ICT) state was also noted in both solution and in PLA. There are differences in relative intensities of peaks arising from π-π* and ICT excitations depending upon dye loading in PLA. Substituent effects were also observed. Electron-donating methoxyl groups on the aromatic rings lead to enhanced fluorescence quantum yields. For certain dyes, phosphorescence is detected at low temperature or under a nitrogen atmosphere in PLA matrices.