Photoinduced charge accumulation by metal ion-coupled electron transfer.

Annabell G Bonn,Oliver S Wenger

doi:10.1039/c5cp04718h

Abstract

An oligotriarylamine (OTA) unit, a Ru(bpy)3(2+) photosensitizer moiety (Ru), and an anthraquinone (AQ) entity were combined to a molecular dyad (Ru-OTA) and a molecular triad (AQ-Ru-OTA). Pulsed laser excitation at 532 nm led to the formation of charge-separated states of the type Ru(-)-OTA(+) and AQ(-)-Ru-OTA(+) with lifetimes of ≤10 ns and 2.4 μs, respectively, in de-aerated CH3CN at 25 °C. Upon addition of Sc(OTf)3, very long-lived photoproducts were observed. Under steady-state irradiation conditions using a flux of (6.74 ± 0.21) × 10(15) photons per second at 450 nm, the formation of twofold oxidized oligotriarylamine (OTA(2+)) was detected in aerated CH3CN containing 0.02 M Sc(3+), as demonstrated unambiguously by comparison with UV-Vis absorption spectra obtained in the course of chemical oxidation with Cu(2+). Photodriven charge accumulation on the OTA unit of Ru-OTA and AQ-Ru-OTA is possible due to the lowering of the O2 reduction potential caused by the interaction of superoxide with the strong Lewis acid Sc(3+). The presence of the anthraquinone unit in AQ-Ru-OTA accelerates the rate-determining reaction step for charge accumulation by a factor of 10 compared to the Ru-OTA dyad. This is attributed to the formation of Sc(3+)-stabilized anthraquinone radical anion intermediates in the triad. Possible mechanistic pathways leading to charge accumulation are discussed. Photodriven charge accumulation is of key importance for solar fuels because their production will have to rely on multi-electron chemistry rather than single-electron reaction steps. Our study is the first to demonstrate that metal ion-coupled electron transfer (MCET) can be exploited to accumulate charges on a given molecular unit using visible light as an energy input. The approach of using a combination of intra- and intermolecular electron transfer reactions which are enabled by MCET is conceptually novel, and the fundamental insights gained from our study are relevant in the greater context of solar energy conversion.

Highlights

The primary events of natural photosynthesis are light absorption, energy transfer, and electron transfer.[1]
In the vast majority of cases explored to date, sacrificial redox reagents were used,[10] and there are only very few exceptions.6g,8a,b,11 Using sacrificial reagents, photoinduced charge accumulation in purely molecular systems has been achieved for example in quinone-based triads,6d–f in various coordination compounds containing precious metals such as Rh, Ir, Pd or Pt,6b,c,12 and in many cobaloximes in which the accumulation of two negative charges was employed for the formation of H2.13 Many other examples from the realms of
The key ligand of the Ru(bpy)32+ photosensitizer moiety (Ru)-OTA dyad was prepared as illustrated in the uppermost line of Scheme 2. 5-Bromo-2,20-bipyridine (1)[18] was reacted with (2,5-dimethyl-4-(trimethylsilyl)phenyl)boronic acid (2)[19] under standard Suzuki cross-coupling conditions, and the trimethylsilyl protection group of the coupling product (3) was substituted by an iodine atom using ICl.[19]

Summary

Introduction

The primary events of natural photosynthesis are light absorption, energy transfer, and electron transfer.[1]. We report here on the use of metal ion-coupled electron transfer (MCET) for photoinduced accumulation of two positive charges on the oligotriarylamine (OTA) units of the two compounds from Scheme 1.

Results

Conclusion