Synthesis and Fluorescent Properties of Multi-Functionalized C-70 Derivatives of C-70(OCH3)(10)[C(COOEt)(2)] and C-70(OCH3)(10)[C(COOEt)(2)](2)

Due to the partially reduced pi-conjugation of the fullerene cage, multi-functionalized fullerene derivatives exhibit remarkable fluorescent properties compared to pristine fullerenes, which have high potential for application in organic light-emitting diodes (OLEDs). In this study two multi-functionalized C-70 derivatives, C-70(OCH3)(10)[C(COOEt)(2)] and C-70(OCH3)(10)[C(COOEt)(2)](2), with excellent fluorescence properties, were designed and synthesized. Compared with C-70(OCH3)(10) containing a single kind of functional group, both the C-70(OCH3)(10)[C(COOEt)(2)] and C-70(OCH3)(10)[C(COOEt)(2)](2) exhibited enhanced fluorescence properties with blue fluorescence emission. The fluorescence quantum yields of the C-70(OCH3)(10)[C(COOEt)(2)] and C-70(OCH3)(10)[C(COOEt)(2)](2) were 1.94% and 2.30%, respectively, which were about ten times higher than that of C-70(OCH3)(10). The theoretical calculations revealed that the multi-functionalization of the C-70 increased the S-1-T-1 energy gap, reducing the intersystem crossing efficiency, resulting in the higher fluorescence quantum yield of the C-70 derivatives. The results indicate that multi-functionalization is a viable strategy to improve the fluorescence of fullerene derivatives.