Field-Induced Fluorescence Quenching and Enhancement of Porphyrin Sensitizers on TiO2 Films and in PMMA Films

Three highly efficient porphyrin sensitizersYD2, YD2-oC8, and YD30, either sensitized on TiO2 films or embedded in PMMA filmswere investigated using electrophotoluminescence (E-PL) spectra. Under both thin-film conditions, on application of an external electric field we observed the quenching of fluorescence of push−pull porphyrins (YD2 and YD2-oC8) and a slightly enhanced fluorescence of the reference porphyrin without an electron donor group (YD30). A nonfluorescent state with charge separation (CS) is proposed to be involved in both YD2 and YD2-oC8 systems so that the electron injection becomes accelerated in the presence of a strong electric field. In contrast, the retardation of the nonradiative process not involving a CS state was the reason for the field-induced enhancement of fluorescence of YD30. The extent of fluorescence quenching of YD2-oC8 was greater than that of YD2 on TiO2 films, indicating that the ortho-substituted long alkoxyl chains play a key role to accelerate the consecutive electron injection involving the CS state. Our E-PL results indicate that a field-induced variation of fluorescent intensity is related to the efficiency of conversion of solar energy and that further improvement of the performance of devices containing push−pull porphyrin dyes is achievable under an applied electric field.