Substituent Effects of Porphyrin Monolayers on the Structure and Photoelectrochemical Properties of Self-Assembled Monolayers of Porphyrin on Indium−Tin Oxide Electrode

Abstract

Self-assembled monolayers (SAMs) of porphyrins have been prepared to examine the substituent effects of porphyrin monolayers on the structure and photoelectrochemical properties of the SAMs on an ITO electrode. The ultraviolet (UV)-visible absorption, steady-state fluorescence, and cyclic voltammetry measurements for the porphyrin SAMs revealed that the interaction between the porphyrins without bulky tert-butyl groups is much larger than that of the porphyrins with bulky tert-butyl groups. Photoelectrochemical measurements were performed in nitrogen-saturated Na 2 SO 4 aqueous solution containing triethanolamine as an electron sacrificer using the modified ITO working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode. The internal quantum yields of photocurrent generation together with the porphyrin fluorescence lifetimes remain virtually the same irrespective of the steric hindrance, which is in sharp contrast with severe self-quenching of the porphyrin excited singlet state in conventional molecular assemblies such as Langmuir-Blodgett films. The two-dimensional, densely packed structure of the porphyrins in SAM is responsible for the long-lived excited singlet state, which is similar to the antenna function of photosystem I in cyanobacteria. These results will provide valuable information on the construction of artificial lightharvesting systems.

First Cellulose Langmuir‐Blodgett Films towards Photocurrent Generation Systems

“…The photocurrent quantum yield (F) of the LB monolayer film increased on applying more positive potentials, and increased at around 3.8-4.6% at 0.2-0.3 V versus SCE ( Figure 6). These F values are comparable to the values of 3.4 AE 0.6% obtained for a porphyrin SAM on an ITO electrode (applied potential ¼ 0.4 V, light intensity ¼ 0.83 mW Á cm À2 , l ¼ 423 AE 5 nm) reported by Imahori et al [20] . They pointed out that the self-quenching of the porphyrin excited state cannot be reduced in conventional molecular assemblies such as LB films.…”

“…They pointed out that the self-quenching of the porphyrin excited state cannot be reduced in conventional molecular assemblies such as LB films. [20] However, we achieved the same quantum yields of photocurrent generation as those of SAMs. In other words, the porphyrins in the LB film have a densely packed structure, analogous to SAMs.…”

“…[20] We observed the quantum efficiency (F) of the LB monolayer film to be 1.6% under the experimental conditions (applied potential ¼ 0 V versus SCE, light intensity ¼ 2.0 mW Á cm…”

Porphyrins as Active Components for Electrochemical and Photoelectrochemical Devices

“…A big effort has been made to elucidate relationships between structure and function, but the antenna complexes show a great variety in their structures, making it difficult to achieve this goal. Attempting to understand the effect of surface packing on photocurrent efficiency, Imahori et al 390 synthesized two porphyrins with different substituents in order to analyze the substituent effect on the film morphology and thus the photocurrent efficiency ( Figure 136). They immobilized on an ITO surface a classical tetraphenylporphyrin such as TPP-amide, forming a SAM, and another film of a porphyrin containing 3,5-di-tert-butylphenyl groups at the meso positions (TBPPamide).…”

Effect of Fullerene on Photocurrent Performance of 6‐O‐Porphyrin‐2,3‐di‐O‐stearoylcellulose Langmuir‐Blodgett Films

“…The photocurrent quantum yield (F) based on the number of photons adsorbed by the porphyrin moiety in the LB film was calculated according to the following equation: [34] F ¼ 100ði=eÞ=½Ið1 À 10 ÀA Þ; I ¼ ðWlÞ=ðhcÞ (1) where i is the observed photocurrent density, e is the charge of the electron, I is the number of photons per unit area and unit time, A is the absorbance of the monolayer at l nm, l is the wavelength of light illumination, W is light power irradiation at l nm, h is the Planck constant, and c is the light velocity. At the same time, the incident photon-to-current conversion efficiency (IPCE) value was also determined as the following equation: [15] IPCEð%Þ ¼ 100 Â 1240 Â i=ðWlÞ (2) Results and Discussion…”