Nanoscale Organization of Chlorophyll a in Mesoporous Silica: Efficient Energy Transfer and Stabilized Charge Separation as in Natural Photosynthesis

Abstract

Chlorophyll a molecules were adsorbed to mesoporous silica (FSM: folded-sheet mesoporous material) to form a chlorophyll−FSM conjugate capable of nanometer-scale interaction between chlorophyll a molecules as seen in a living plant leaf. The chlorophyll a molecules introduced into nanopores of FSM-22 (with a pore diameter of 4 nm) exhibited a red shift in the absorption peak and showed efficient excitation energy transfer from the shorter to the longer wavelength forms as characterized by the fast transient phase of 38 ps detected in the decay-associated spectrum (DAS) analysis of the fluorescence decay. Illumination of the chlorophyll−FSM-22 conjugate with visible light generated a stable chlorophyll a cation radical in the electron spin resonance (ESR) measurement, although the illumination of free chlorophyll a in solution produced almost no cation signal. The physiological function of the chlorophyll−FSM conjugate intercalated with ruthenium oxide (RuO2) was explored as chlorophyll−FSM exhibited the photoinduced ability to catalyze the reduction of DCIP (an electron carrier). The mesoporous silica−chlorophyll conjugate in an appropriate system thus produced a solar energy conversion system enabling fast energy transfer and stable charge separation, presumably by producing the appropriate interaction of pigments as occurs in living photosynthetic apparatus.