3 Photochemistry and photophysics of fullerenyl molecular micelle and chromophore-fullerene conjugates

Photoexcitation of C₆₀ and fullerene derivatives induces a singlet fullerenyl excited state that is transformed to the corresponding triplet excited state, via intersystem energy crossing, with nearly quantitative efficiency [73]. Subsequent energy transfer from the triplet fullerene derivatives to molecular oxygen produces singlet molecular oxygen in aerobic media. This photocatalytic effect becomes one of key mechanisms in photodynamic treatments using fullerene derivatives as photosensitizers. However, a high degree of functionalizaton on C₆₀ for the enhancement of solubility and compatibility in biomedia resulted in a progressive decrease of the singlet oxygen production quantum yield [Φ (¹O₂)]. Examples were given by Bingel-type malonic acid, C₆₀[C(COOH)₂]_n, and malonic ester, C₆₀[C(COOE_t)₂]_n, [60]fullerene adducts [74], showing a decreasing trend of Φ (¹O₂) as the number of addends (n) increases. When the number n reached 6 for a hexaadduct, its Φ (¹O₂) value declined to only 13% or less of that for C₆₀ [75]. However, it was not the case for molecular micellar FC₄S, a relatively high singlet oxygen production quantum yield for FC₄S may indicate its unique electronic features in difference with Bingel-type malonic hexaadducts of C₆₀ [71]. The efficiency was substantiated by direct detection of ¹O₂ emission at 1270 nm upon photoirradiation of self-assembled FC₄S nanospheres at 500–600 nm.