A computational study of a reduced dye and its O2 reduction: Implication on H2O2 production with dye-sensitized photocathodes
Graphical abstract
Introduction
Coumarin 343 (alternative name: coumarin 519, Fig. S1a) is a very attractive organic dye with versatile applications, including in molecular probes [1], laser dyes [2], and recently, as sensitizers in high-efficiency p-type dye-sensitized solar cells (DSSCs) [3]. Perylenemonoimide (PMI) dyes, such as Perylene dicarboximide–oligothiophene–triphenylamine (Fig. S1b), are other excellent organic sensitizers for p-type DSSCs [4]. Extending the concept of DSSC technology to chemical bond formation, H2-evolving dye-sensitized photocathodes for dye-sensitized photoelectrochemical cells (DSPECs) have been developed over the last few decades as promising platforms for solar fuel generation. Coumarin 343 and PMI-sensitized NiO photocathodes were reported to produce H2 over proton reduction catalysts, with Faradaic efficiencies of ~ 50% [5], [6].
In addition to H2, hydrogen peroxide (H2O2), which is a versatile and clean redox agent for selective organic conversions, environmental purification, bleaching, sterilization, cleaning [7], and energy generation in H2O2 fuel cells [8], is produced from O2, water, and light using DSPECs at a low electrochemical overpotential. The coumarin 343-sensitized NiO photocathode exhibits the largest photocurrent (400 μA cm−2), indicative of its highest H2O2 production amount among the tested dyes, which may have been unprecedented at the time of observing the photoelectrocatalysis on the sensitized NiO semiconductor [9]. The PMI-sensitized NiO photocathode employed for H2O2 production by direct O2 reduction exhibited a current density of 600 μA cm−2 and Faradaic efficiency of 50%–60% [10].
After hole injection into the NiO valence band, effective electron transfer from the reduced dye (dye−•) to O2, followed by superoxide (O2−•) formation, has been suggested to occur based on the results of electron paramagnetic resonance (EPR) [9] and transient absorption spectroscopy measurements [10]. However, not only the dye−• structures but also the intermediate forms with O2, which facilitate the electron transfer, have not been adequately investigated. Notably, the design of dye sensitizers has a significant effect on the energy conversion efficiency and photoelectrode stability [11]. Consequently, to achieve a better understanding and improve the H2O2 production amount with DSPECs, the structures of the dye−• and reaction intermediate derived from the dye−• and O2, as well as the mechanism of the subsequent O2−• formation, must be elucidated.
Herein, we focus on the reactions of reduced coumarin 343 and PMI dyes with the O2 molecule using quantum chemical calculations at the density functional theory (DFT) level. This study is divided into three parts. First, the monomer structures of the neutral and reduced dyes are studied. Second, the dye−•–O2 complexations are investigated. Third, the subsequent reactions of the dye−•–O2 intermediates that afford to the final H2O2 product are examined. Our computational results will facilitate further investigation into the O2 reduction process of p-type dye electrodes.
Section snippets
Computational details
DFT calculations were performed using the Gaussian 16 software at the Research Center for Computational Science, Okazaki, Japan, and Gaussian 16 W on personal computers [12]. The ground state geometry was optimized at the hybrid DFT level using the B3LYP functional, which combines Becke’s three-parameter exchange function (B3) [13] with the correlation function of Lee, Yang, and Parr (LYP) [14]. The Pople-type split-valence basis set 6–31+G(d,p) [15] was adopted in all systems; it provides
Molecular structure of the dyes
Based on previous studies [16], [17], [23], [24], [25], [26], we consider four conformational isomers (conformers) of coumarin 343 differing in the structure of the julolidyl moiety: syn A, whose C18 and C20 atoms bend up; syn B, whose C18 and C20 bend down; anti C, whose C18 bends up and C20 bends down; and anti D, whose C18 bends down and C20 bends up (Fig. 1). Upon excitation, holes are injected into the valence band of the p-type semiconductor, after which the coumarin 343 conformers
Conclusions
We computationally investigated dye−• monomers and their complexes with O2 molecules in water with a full-geometry optimization and a subsequent population analysis. For the coumarin 343 dye, the differences in energy and the distribution of atomic charge, as well as the spin density, were very small among the two syn (A− and B−) and anti (C−, and D−) conformers. For the PMI dye, the perylenemonoimide structure with one π methylthiophene substituent (E−) was extracted, where the spin was
CRediT authorship contribution statement
Hitoshi Kusama: Conceptualization, Methodology, Formal analysis, Investigation, Visualization.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The computations were partially performed using Research Center for Computational Science, Okazaki, Japan.
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