Synthesis and characterization of highly photoresponsive fullerenyl dyads with a close chromophore antenna-C₆₀ contact and effective photodynamic potential

We report the synthesis of a new class of photoresponsive C ₆₀-DCE-diphenylaminofluorene nanostructures and their intramolecular photoinduced energy and electron transfer phenomena. Structural modification was made by chemical conversion of the keto group in C₆₀(>DPAF-C _n) to a stronger electron-withdrawing 1,1-dicyanoethylenyl (DCE) unit leading to C₆₀(>CPAF-C_n) with an increased electronic polarization of the molecule. The modification also led to a large bathochromic shift of the major band in visible spectrum giving measureable absorption up to 600 nm and extended the photoresponsive capability of C₆₀-DCE-DPAF nanostructures to longer red wavelengths than C₆₀(>DPAF-C _n). Accordingly, C₆₀(>CPAF-C_n) may allow 2γ-PDT using a light wavelength of 1000-1200 nm for enhanced tissue penetration depth. Production efficiency of singlet oxygen by closely related C₆₀(>DPAF-C_2M) was found to be comparable with that of tetraphenylporphyrin photosensitizer. Remarkably, the ¹O₂ quantum yield of C₆₀(>CPAF-C_2M) was found to be nearly 6-fold higher than that of C₆₀(>DPAF-C_2M), demonstrating the large light-harvesting enhancement of the CPAF-C_2M moiety and leading to more efficient triplet state generation of the C ₆₀> cage moiety. This led to highly effective killing of HeLa cells by C₆₀(>CPAF-C_2M) via photodynamic therapy (200 J cm^-2 white light). We interpret the phenomena in terms of the contributions by the extended π-conjugation and stronger electron-withdrawing capability associated with the 1,1-dicyanoethylenyl group compared to that of the keto group.