BODIPY parent compound: Fluorescence, singlet oxygen formation and properties revealed by DFT calculations

Abstract

The BODIPY (boron-dipyrromethene) parent compound is synthesized to measure its fluorescence properties and singlet oxygen generation ability. The ground state, excited singlet state S1 and triplet state T1 geometry and properties in gas, solid state and organic solvents are calculated by DFT method. This unsubstituted BODIPY (BDP) has good solubility in all solvents ranging from non polar n-hexane to highly polar water. BDP exhibits bright green fluorescence with high fluorescence quantum yield (Φf) and long fluorescence lifetime (τf) values in all solvents, Φf of BDP varies from the lowest 0.79 in toluene to the highest 0.98 in water, while τf varies from the shortest 6.02 ns in toluene to the longest 7.36 ns in water. τf and Φf show good linear correlation with the refractive index (n) of a solvent: τf = 17.13 – 7.36·n and Φf = 2.51–1.16·n (for air-saturated solution). The fluorescence lifetime in each solvent, however, becomes longer in deoxygenated solutions, suggesting that the presence of molecular oxygen leads to the formation of BDP T1 and singlet oxygen: BDP(S1)+O2 → BDP(T1)+1O2. Singlet oxygen formation quantum yield was measured to be up to 0.083. The absorption and emission maximum (λabs and λem), however, are only slightly changed by solvent nature: λabs= 500 ± 4 nm and λem= 510 ± 4 nm. The spectral shape and peak maximum for its absorption and emission are little affected by solvent nature. DFT calculation reveals that the geometry difference between S0 and S1 state of BDP is very small, so that the dipole moment change is also small. The calculated charges of carbon atoms, however, are very different and can well explain their different reactivity to nucleophiles. The calculated absorption spectra (peak maximum λcal) by TD-DFT method shows a good linear relation with experimental value λexp: λexp = 1.19 λcal. These results provide the basis for elucidating the mechanism or photophysical processes involving BODIPY as the functional units.