Vectorial Multistep Electron Transfer at the Gold Electrodes Modified with Self-Assembled Monolayers of Ferrocene−Porphyrin−Fullerene Triads

Abstract

Self-assembled monolayers of ferrocene−porphyrin−C60 triads on gold electrodes were prepared to mimic photosynthetic electron transfer events where efficient conversion of light to chemical energy takes place via the long-lived, charge-separated state with a high quantum yield. Adsorbed amounts of the triads on the gold electrodes, estimated from the charge of the anodic peak of the ferrocene, are comparable to those of the well-ordered porphyrin−alkanethiols and C60−alkanethiols on gold electrodes. The results, together with blocking experiments using a redox probe, indicate that the triad molecules are well-packed with an almost perpendicular orientation on the gold surface. The monolayer thickness obtained using X-ray reflectivity analysis is consistent with the structural model of the monolayer. Photoelectrochemical studies were carried out in a standard three-electrode system using the gold electrodes modified with the self-assembled monolayers of the triads. Stable cathodic photocurrents were observed in the presence of electron carriers such as oxygen and/or methyl viologen in the electrolyte when the modified gold electrodes were illuminated with a monochromic light. A photoinduced multistep electron-transfer mechanism is proposed for the photoelectrochemical cells. Thus, vectorial electron transfer or partial charge transfer occurs from the excited singlet state of the porphyrin to the C60, followed by the successive charge shift from the ferrocene to the porphyrin cation radical, to produce the ferrocene cation radical and the C60 anion radical. The C60 anion radical gives an electron to the counter electrode via the electron carriers in the electrolyte solution, whereas electron transfer takes place from the gold electrode to the ferrocene cation radical, resulting in the recovery of the initial state and the generation of the overall electron flow. The artificial photosynthetic cells show the highest quantum efficiency (20−25%) ever reported for photoinduced multistep electron transfer at monolayer-modified metal electrodes and across artificial membranes using donor−acceptor linked molecules. The result indicates clearly that C60 acts as an excellent electron acceptor as well as an electron mediator in artificial photosynthetic membranes. The molecule-based methodology will provide a new direction for the development of solar energy conversion and molecular devices.

Porphyrin‐ and Fullerene‐Based Molecular Photovoltaic Devices

“…2]. [41,42] The energy gradient of each redox state in the triad is designed to produce the final charge-separated state (Fc + ± P ± C 60 .± ) efficiently with a high quantum yield: Fc± 1 P*±C 60 > Fc±P .+ ±C 60 .± > Fc + ±P±C 60…”

“…These results show that the triad molecules are packed with a nearly perpendicular orientation to the gold surface. [41,42] The photoelectrochemical experiments using the triad-modified gold electrode, a platinum wire, and an Ag/AgCl electrode were performed in the presence of electron carriers (i.e., oxygen or methyl viologen and hereafter represented by Au/3 or 4/MV 2+ /Pt, where / denotes an interface). [41,42] A cathodic photocurrent was observed upon irradiation of the modified gold electrode with monochromatic light of 438.5 nm.…”

“…The internal quantum yields of the photocurrent generation in the Au/3/MV 2+ /Pt and Au/4/MV 2+ /Pt devices under the optimal conditions using an oxygen-saturated solution in the presence of methyl viologen were found to be 20 % and 25 %, respectively. [41,42] These values are some of the highest among the donor±accep-tor linked molecules at monolayer-modified metal electrodes and in artificial membranes. [46±48] The high quantum yield may be attained by photoinduced electron transfer from the porphyrin singlet excited state to the C 60 moiety, followed by a charge shift from the ferrocene moiety to the resulting porphyrin radical cation to produce the charge-separated state Fc + ±P±C 60 .± in the monolayer (Fig.…”

Metal-chelating capacities attached to fullerenes

“…Thio functionalities were introduced in fullereneporphyrin dyads 62 [72] and triad 63 [73,74] to enable the formation of self-assembled monolayer (SAM) on a gold electrode. Cathodic photocurrents under illumination were observed in the presence of electron carriers in solution, such as oxygen or methyl viologen, with high quantum efficiencies of 20-25% [75].…”

Helical Porphyrinoids: Incorporation of Ferrocene Subunits into Macrocyclic Structures

“…To date, metallocene subunits have chiefly been used as electroactive substituents or linkers in single-and multiporphyrin structures, [35][36][37][38][39][40] but have seldom constituted an integral part of the macrocycle. In 1998, Sessler and coworkers reported ansa-bridged ferrocene macrocycles 4 which contain a ferrocene-1,1Ј-diyl unit and two pyrrole rings strapped together with an amide-linked oligoether or alkane chain.…”