Synthesis and characterization of highly photoresponsive fullerenyl dyads with a close chromophore antenna-C-60 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 C-60(>DPAF-C-n) to a stronger electron-withdrawing 1,1-dicyanoethylenyl (DCE) unit leading to C-60(>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 C-60-DCE-DPAF nanostructures to longer red wavelengths than C-60(>DPAF-C-n). Accordingly, C-60(>CPAF-C-n) may allow 2 gamma-PDT using a light wavelength of 1000-1200 nm for enhanced tissue penetration depth. Production efficiency of singlet oxygen by closely related C-60(>DPAF-C-2M) was found to be comparable with that of tetraphenylporphyrin photosensitizer. Remarkably, the O-1(2) quantum yield of C-60(>CPAF-C-2M) was found to be nearly 6-fold higher than that of C-60(>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 C-60(>CPAF-C-2M) via photodynamic therapy (200 J cm(-2) white light). We interpret the phenomena in terms of the contributions by the extended p-conjugation and stronger electron-withdrawing capability associated with the 1,1-dicyanoethylenyl group compared to that of the keto group.