Abstract

The electronic excited states of the iron(II) complex [FeII(tpy)(pyz-NHC)]2+ [tpy = 2,2′:6′,2″-terpyridine; pyz-NHC = 1,1′-bis(2,6-diisopropylphenyl)pyrazinyldiimidazolium-2,2′-diylidene] and their relaxation pathways have been theoretically investigated. To this purpose, trajectory surface-hopping simulations within a linear vibronic coupling model including a 244-dimensional potential energy surface (PES) with 20 singlet and 20 triplet coupled states have been used. The simulations show that, after excitation to the lowest-energy absorption band of predominant metal-to-ligand charge-transfer character involving the tpy ligand, almost 80% of the population undergoes intersystem crossing to the triplet manifold in about 50 fs, while the remaining 20% decays through internal conversion to the electronic ground state in about 300 fs. The population transferred to the triplet states is found to deactivate into two different regions of the PESs, one where the static dipole moment is small and shows increased metal-centered character and another with a large static dipole moment, where the electron density is transferred from the tpy to pyz-NHC ligand. Coherent oscillations of 400 fs are observed between these two sets of triplet populations, until the mixture equilibrates to a ratio of 60:40. Finally, the importance of selecting suitable normal modes is highlighted—a choice that can be far from straightforward in transition-metal complexes with hundreds of degrees of freedom.

Highlights

  • A major challenge pressing modern society is to satisfy the continually increasing global energy demand.[1]

  • There are three excited states with notable oscillator strength close to the first absorption maximum: S6, S7, and S9. Out of these three states, the S7 state possesses by far the largest oscillator strength; i.e., loosely speaking, it will contribute the majority to the intensity of the first absorption maximum. This peak is found at slightly higher energies than the absorption maximum of the Wigner averaged LC-BLYP spectrum, which is due to the typical red shift introduced by vibrational motion.[67,68]

  • We have investigated the excited-state dynamics of [FeII(tpy)(pyz-Nheterocyclic carbene (NHC))]2+ using a linear vibronic coupling (LVC) model within the framework of trajectory SH

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Summary

Introduction

A major challenge pressing modern society is to satisfy the continually increasing global energy demand.[1] This can be met by harvesting the energy reaching the Earth in the form of sunlight if this energy can be efficiently captured at a large scale. The solar cell technologies most widely employed for this purpose are based on crystalline silicon semiconductors,[2] which, despite possessing the highest energy conversion efficiency, are still expensive. Low-cost alternatives are dye-sensitized solar cells,[3] which often use organic or organometallic dyes attached to nanoporous metal oxides to absorb sunlight.[4] Efficient organometallic dyes are complexes consisting of rare metals, such as ruthenium, but those render large-scale application costly. Many efforts are directed to find more sustainable and Earth-abundant and cheaper alternatives such as iron-based[5] or other 3d metalbased[6,7] complexes

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