External electric field effects on absorption and fluorescence spectra of a fullerene derivative and its mixture with zinc-tetraphenylporphyrin doped in a PMMA film

Electroabsorption and electrofluorescence spectra of a fullerene derivative, C60(C18)2, and its mixture with zinc-tetraphenylporphyrin (ZnTPP) have been measured by using electric field modulation spectroscopy. The change in dipole moment is significant in the electroabsorption spectra both of C60(C18)2 and of a complex composed of C60(C18)2 and ZnTPP, indicating that the excited states both of C60(C18)2 and of a complex between C60(C18)2 and ZnTPP have a large charge-transfer character. The fluorescence quantum yield of C60(C18)2 decreases in the presence of an electric field, which probably arises from the field-induced acceleration of the intramolecular nonradiative process of C60(C18)2 in the fluorescent state. In a mixture between ZnTPP and C60(C18)2, electrofluorescence spectra show the field-induced enhancement for the fluorescence of ZnTPP and the field-induced de-enhancement for the fluorescence both of C60(C18)2 and of the complex between ZnTPP and C60(C18)2. A theoretical analysis clearly shows that the field-induced enhancement of the ZnTPP fluorescence in a mixture results from the field-induced deceleration of the rate of the electron transfer from the excited ZnTPP to C60(C18)2. The standard free energy gap for the photoinduced electron-transfer process is estimated based on the theoretical simulation of the field-dependent fluorescence intensity.