Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis

Abstract

Earth-abundant first row transition metal complexes are important for the development of large-scale photocatalytic and solar energy conversion applications. Coordination compounds based on iron are especially interesting, as iron is the most common transition metal element in the Earth’s crust. Unfortunately, iron-polypyridyl and related traditional iron-based complexes generally suffer from poor excited state properties, including short excited-state lifetimes, that make them unsuitable for most light-driven applications. Iron carbene complexes have emerged in the last decade as a new class of coordination compounds with significantly improved photophysical and photochemical properties, that make them attractive candidates for a range of light-driven applications. Specific aspects of the photophysics and photochemistry of these iron carbenes discussed here include long-lived excited state lifetimes of charge transfer excited states, capabilities to act as photosensitizers in solar energy conversion applications like dye-sensitized solar cells, as well as recent demonstrations of promising progress towards driving photoredox and photocatalytic processes. Complementary advances towards photofunctional systems with both Fe(II) complexes featuring metal-to-ligand charge transfer excited states, and Fe(III) complexes displaying ligand-to-metal charge transfer excited states are discussed. Finally, we outline emerging opportunities to utilize the improved photochemical properties of iron carbenes and related complexes for photovoltaic, photoelectrochemical and photocatalytic applications.

Design and Synthesis of Photoactive Iron N-Heterocyclic Carbene Complexes

“…In the following chapters, we aim to explain in simple terms these fundamental differences and resulting difficulties to generate viable photoactive iron complexes, present strategies to overcome these issues, summarize the progress made so far and give inspiration for new generations of photofunctional iron complexes. For a more detailed description of the photophysical aspects, the reader is referred to the complementary review by Persson and Sundström et al [12] in this issue of Catalysts and a recent concept article by Wenger [13].…”

Towards Iron(II) Complexes with Octahedral Geometry: Synthesis, Structure and Photophysical Properties

“…Other ligands giving rise to more rigid complexes [ 15 ] or different coordination architectures [ 16 ] have achieved impressive MLCT lifetimes beyond hundreds of picoseconds. A comprehensive review of the rapid advances in this area was recently published [ 17 ].…”

Photophysical Investigation of Iron(II) Complexes Bearing Bidentate Annulated Isomeric Pyridine-NHC Ligands

“…[1][2][3][4][5][6] Recently, these ligands have allowed to strongly improve the photophysical properties of iron(II) complexes making them appealing for optically active devices or as cheaper and non-toxic alternative to ruthenium-based sensitizers in solar energy applications. [7][8][9][10][11][12][13][14] However, controlling the photophysics of first-row transition metal complexes is challenging compared with heavier elements. For example, while ruthenium(II) polypyridyl complexes exhibit long-lived metal-to-ligand (MLCT) triplet states, 15 ultrafast relaxation to metal centred (MC) states and subsequent ground-state recovery are observed for iron analogues.…”