Synthetic Semiconductor Photoelectrochemistry

Abstract

In the field of synthetic organic chemistry, photochemical and electrochemical approaches are often considered to be competing technologies that induce single electron transfer (SET). Recently, their fusion, i. e., the “photoelectrochemical” approach, has become the focus of attention. In this approach, both solar and electrical energy are used in creative combinations. Historically, the term “photoelectrochemistry” has been used in more inorganic fields, where a photovoltaic effect exhibited by semiconducting materials is employed. Semiconductors have also been studied intensively as photocatalysts; however, they recently have taken a back seat to molecular photocatalysts. In this account, we would like to revisit semiconductor photocatalysts in the field of synthetic organic chemistry to demonstrate that semiconductor “photoelectrochemical” approaches are more than mere alternatives to molecular photochemical and/or electrochemical approaches.

Photochemical Radical Cation Cycloadditions of Aryl Vinyl Ethers

“…Since not only the formation of vinylcyclobutane ( 6) but also that of the cycloadduct ( 5) were observed even in the very early stage, it seems that two pathways, direct and indirect, are possible to construct the six-membered ring. Namely, the radical cation (1 * + ) is first trapped by the diene (3) to form the initial carboncarbon bond and formally generate the distonic radical cation (7). Radical cation (7) can then be cyclized via single electron reduction to form both six-and four-membered rings.…”

“…Namely, the radical cation (1 * + ) is first trapped by the diene (3) to form the initial carboncarbon bond and formally generate the distonic radical cation (7). Radical cation (7) can then be cyclized via single electron reduction to form both six-and four-membered rings. Although mechanistic aspects remain unclear, single electron oxidation may trigger the rearrangement of the vinylcyclobutane (6) into the cycloadduct (5), which accords well with our previous observations.…”

“…Taken together, we regard semiconductor nanoparticles as photochemically driven electrodes. [7] The use of lithium perchlorate (LiClO 4 )/ nitromethane (CH 3 NO 2 ) solution is essential in most cases, in which carbon-carbon bond formations using radical cations as reactive intermediates are facilitated. [8] The radical cation precursors are generally electron-rich alkenes, with styrene derivatives being representative examples, where the double bond is conjugated to an arene (Figure 1).…”

“…It should be noted that semiconductor nanoparticles are heterogeneous platforms and, in this sense, they are more similar to electrodes than molecular sensitizers. Taken together, we regard semiconductor nanoparticles as photochemically driven electrodes [7] . The use of lithium perchlorate (LiClO 4 )/nitromethane (CH 3 NO 2 ) solution is essential in most cases, in which carbon‐carbon bond formations using radical cations as reactive intermediates are facilitated [8] .…”

Radical cation Diels–Alder reactions of arylidene cycloalkanes

“…We have developed radical cation cycloadditions using (photo)electrochemical single-electron transfer in lithium per- chlorate (LiClO 4 )/nitromethane (CH 3 NO 2 ) solution [36][37][38][39][40][41][42][43][44]. During the course of our studies, we found that the TiO 2 photoelectrochemical approach was more beneficial than simple electrochemistry in most cases, probably because both single-electron oxidation and reduction are made possible at the same surface [45]. This is especially true for the radical cation Diels-Alder reaction, since non-substituted β-methylstyrene, which was previously reported as an unsuccessful dienophile, was found to participate under TiO 2 photoelectrochemical conditions (Scheme 1) [46,47].…”