Highly efficient green-light ionization of an aryl radical anion: key step in a catalytic cycle of electron formation

Abstract

A sustainable generation of hydrated electrons with green light would allow solar-driven applications of this potent reductant, such as the detoxification of halogenated organic waste. Using two-color laser flash photolysis, we have studied the photoionizations of the 1,5-naphthalene disulfonate radical anion and triplet with 532 nm as well as 355 nm. The radical anion is prepared by reducing the triplet with the bioavailable ascorbate monoanion under physiological conditions; its photoionization recovers the starting substrate, so turns the reaction sequence into a catalytic cycle. A comparison of the four ionizations suggests that their efficiency is strongly influenced by the electronic configuration of the state ejecting the electron. The quantum yield for ionizing the radical anion with 532 nm (0.27) is at least four times higher than for the very few known examples of such green-light ionizations and comparable to the most efficient UV ionizations known to date, so this system might represent a breakthrough towards the "green" production of hydrated electrons.

Sustainable, inexpensive and easy-to-use access to the super-reductant e˙−aq through 355 nm photoionization of the ascorbate dianion—an alternative to radiolysis or UV-C photochemistry

“…The first laser pulse (355 nm, at 0 μs) acts only on Asc 2− and quasi-instantaneously generates e •− aq , which is then scavenged by Bpc − to give Bpc •2− ; only the latter is present immediately before the second laser pulse (532 nm, at 2 μs). 21 The linear plot of e •− aq formation against Bpc •2− bleaching with a slope of 1 (inset of the figure) establishes that the green-light ionization of Bpc •2− is not accompanied by side reactions, as we and others already found for this and other aryl radical anions. By experiments with the UV-laser blocked or without Bpc − , we verified that the green flash alone has no effect on Bpc − , on Asc 2− , and on Asc •− .…”

“…This second flash bleaches about 70% of Bpc •2− , in the process liberating the same amount of e •− aq . 18,21,49 To determine the quantum yield of the green-light ionization, we performed relative actinometry with the ruthenium-(tris)bipyridine dication as the reference, 17,18 comparing the spectrophotometrically measured amount of e •− aq produced by the green pulse with the 532 nm induced quantitative formation of the metal-to-ligand charge-transfer excited state of the complex, 46 monitored through its emission. We further varied the concentration of Bpc •2− through the intensity of the first pulse (compare Fig.…”

“…Our examples include radical anions that cannot be obtained through electron-transfer quenching of their excited parent compounds because of competing photochemical reactions or because they lie too high in energy, as well as radical anions for which this standard route is viable but gives spectra that are complicated by the absorptions of earlier intermediates or are difficult to calibrate because of incomplete charge separation. 25) but no longer at 308 nm or 355 nm, 21,22 we recently observed that the red shift of the absorption band by about 40 nm (see, the ESI †) and the decrease of the redox potential associated with the second deprotonation step suffice to extend the photoionizability of the dianion Asc 2− not only to 308 nm but also to 355 nm. 8 2 Results and discussion 2.1 The method 2.1.1 Asc 2− as an electron source in the near UV.…”

“…In our quest for a way to transfer this technique to water, where e •− aq is an extremely well-characterised and relatively long-lived unique species, 3 we discovered that the ascorbate dianion Asc 2− , which we had previously utilized as a sacrificial donor in catalytic photoionization cycles, 17,18,21,22 can itself be ionized with a 355 nm laser. Being a bioavailable and completely nontoxic substance, this extremely inexpensive precursor obviously provides sustainable access to e •− aq , and its photoionizability in the UV-A avoids all potential issues associated with radiolysis or UV-C excitation.…”

Photoionization access to cyclodextrin-encapsulated resveratrol phenoxy radicals and their repair by ascorbate across the phase boundary

“…[52] All solutions were purged with argon (5.0, Air Liquide) or N 2 O (5.0, Air Liquide) for 30 minutes before and for the whole duration of the laser flash photolysis experiments. A detailed description has been given elsewhere.…”