The Overlooked Photochemistry of Iodine in Aqueous Suspensions of Fullerene Derivatives

Abstract

Fullerene's low water solubility was a serious challenge to researchers aiming to harness their excellent photochemical properties for aqueous applications. Cationic functionalization of the fullerene cage provided the most effective approach to increase water solubility, but common synthesis practices inadvertently complicated the photochemistry of these systems by introducing iodide as a counterion. This problem was overlooked until recent work noted a potentiation effect which occurred when photosensitizers were used to inactivate microorganisms with added potassium iodide. In this work, several photochemical pathways were explored to determine the extent and underlying mechanisms of iodide's interference in the photosensitization of singlet oxygen by cationic fulleropyrrolidinium ions and rose bengal. Triplet excited state sensitizer lifetimes were measured via laser flash photolysis to probe the role of I^- in triplet sensitizer quenching. Singlet oxygen production rates were compared across sensitizers in the presence or absence of I^-, SO₄^2-, and other anions. 3,5-Dimethyl-1H-pyrazole was employed as a chemical probe for iodine radical species, such as I·, but none were observed in the photochemical systems. Molecular iodine and triiodide, however, were found in significant quantities when photosensitizers were irradiated in the presence of I^- and O₂. The formation of I₂ in these photochemical systems calls into question the interpretations of prior studies that used I^- as a counterion for photosensitizer materials. As an example, MS2 bacteriophages were inactivated here by cationic fullerenes with and without I^- present, showing that I^- moderately accelerated the MS2 deactivation, likely by producing I₂. Production of I₂ did not appear to be directly correlated with estimates of ¹O₂ concentration, suggesting that the relevant photochemical pathways are more complex than direct reactions between ¹O₂ and I^- in the bulk solution. On the basis of the results here, iodine photochemistry may be underappreciated and misunderstood in other environmental systems.

Keywords: C60; Iodine; MS2 bacteriophage; cationic fullerene; photochemistry; photosensitizer; singlet oxygen.

Figure 1

Triplet Lifetimes for fullerene in toluene (Ar sparging) and its derivatives in DMSO (5% O₂ sparging). In an organic solvent, like DMSO, the fullerene does not aggregate and is instead simply dissolved.

Figure 2

Triplet lifetimes for fullerene and fullerene derivatives in water (20% O₂ sparging). The solutions contained 5.0 μM of nC₆₀. KI was added in excess (4.76 mM) for solutions containing it.

Figure 3

Estimated steady-state ¹O₂ molar concentrations for 25 μM RB or fullerene aggregates under several conditions: no additional constituents; 75 μM NaCl; 75 μM NaI; 75 μM Na₂SO₄; 75 μM NaBr; and 50 μM l-histidine.

Figure 4

(a) Photochemical production of I₂ over by sensitizers with NaI and 10 mM starch under UV₃₉₅ light; and (b) observed I₂ production yields (I₂ production rate constants normalized by estimated steady-state ¹O₂ concentrations) for RB, nC₆₀-FP-I, and nC₆₀-FP-SO₄.

Figure 5

MS2 Inactivation by 250 nM of nC₆₀-FP-SO₄ or nC₆₀-FP-I under visible light irradiation in DI.

Figure 6

Two-dimensional illustration of the fulleropyrrolidinium ions studied here. Counterions were either I^– or SO₄^2–.