The influence of structural effects and the solvent properties on spectral, generation characteristics, photostability and lipophilicity of 1,3,5,7-tetramethyl-BODIPY and its alkylated and iodinated derivatives

This paper presents the results of a comparative analysis of the effect of iodination features, meso-substitution of 1,3,5,7-tetramethyl-BODIPY, and the solvent properties on the spectral, generation properties, photostability and lipophilicity of boron(III) dipyrromethenates. Like most alkyl-substituted BODIPYs, the BDP1-BDP3 dipyrromethenates exhibit intense emission properties with fluorescence quantum yields of 40–100 %. However, they are practically not endowed with the ability to generate singlet oxygen (1O2), showing ФΔ values of 0–9 %. Iodinated derivatives BDP4–BDP7 revealed a strong heavy atom effect resulting in the red-shifted (up to 37 nm and 46 nm, respectively) absorption and emission bands, fluorescence quenching with φfl values of about 2–28 %, a decrease (up to 4.38 times) in photostability, but increasing the ability to generate singlet oxygen with excellent ФΔ values of 0.73–0.93 and 0.76–0.98 in 1-octanol and ethanol, respectively. The introduction of a –(CH2)3COOCH3 substituent into the meso-position of the alkyl- and iodo-substituted luminophores BDP3 and BDP7 make it possible to increase the UV stability of the dyes up to 3 times due to the screening of the carbon atom of the methine meso-spacer, which is one of the most photoactive centres of dipyrromethene dyes. In addition, meso-substitution enhances the lipophilicity of BDP3 and BDP7 by almost ∼1.5 times compared to meso-unsubstituted BDP1 and BDP6 analogues. The results of the studies indicate that the introduction of the –(CH2)3COOCH3 substituent into the diiodine-BODIPY meso-position provides a combination of photostability, efficiency of singlet oxygen generation, and lipophilicity that is optimal for a PDT agent.