Synthesis and Fluorescent Properties of Multi-Functionalized C(70) Derivatives of C(70)(OCH(3))(10)[C(COOEt)(2)] and C(70)(OCH(3))(10)[C(COOEt)(2)](2)

Due to the partially reduced π-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)(OCH(3))(10)[C(COOEt)(2)] and C(70)(OCH(3))(10)[C(COOEt)(2)](2), with excellent fluorescence properties, were designed and synthesized. Compared with C(70)(OCH(3))(10) containing a single kind of functional group, both the C(70)(OCH(3))(10)[C(COOEt)(2)] and C(70)(OCH(3))(10)[C(COOEt)(2)](2) exhibited enhanced fluorescence properties with blue fluorescence emission. The fluorescence quantum yields of the C(70)(OCH(3))(10)[C(COOEt)(2)] and C(70)(OCH(3))(10)[C(COOEt)(2)](2) were 1.94% and 2.30%, respectively, which were about ten times higher than that of C(70)(OCH(3))(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.