Singlet Oxygen Generation by an Indotricarbocyanine Dye with Bulky Substituents

A study was carried out on the spectral-luminescence and phosphorescence properties of an indotricarbocyanine dye with an ortho-phenylene bridge in the conjugation chain as well as two 300-Da polyethylene glycol (PEG) substituents (PD1) and its analog without PEG (PD2). The presence of the bulky PEG300 substituents in the dye structures was shown to alter the efficiency of singlet oxygen generation. The yield of singlet oxygen in ethanol for both dyes in the concentration range from 5∙10 –8 to 10 –5 M has a constant value γ Δ = 0.031 ± 0.005 for PD1 and 0.050 ± 0.008 for PD2. An increasing value of γ Δ from 0.022 ± 0.004 when C dye = 2.6·10 –7 M to 0.104 ± 0.016 when C dye = 5.8·10 –5 M was found in the concentration range from 10 –7 to 10 –5 M in low-polarity chloroform for PD2, whereas the quantum yield for PD1 with bulky substituents is invariant in this concentration range (0.032 ± 0.003). The increase in the singlet oxygen formation quantum yield with increasing concentration of PD2 in low-polarity chloroform is attributed to an increase in the fraction of contact ion pairs in solution and a heavy atom effect related to the Br – anion. The presence of two PEG300 chains in the structure of the cationic indotricarbocyanine dye (~770 Da) prevents the counterion from moving away from the cation of dye PD1 in low-polarity chloroform. Furthermore, the dye molecules are in the form of contact ion pairs at any concentration and it is hence difficult for the chromophore to interact with dissolved oxygen due to steric hindrance.