Photoexcitation of Ge9- Clusters in THF: New Insights into the Ultrafast Relaxation Dynamics and the Influence of the Cation.

Nadine C Michenfelder,Melina Dilanas,Andreas Schnepf,Christian Gienger,Andreas-Neil Unterreiner

doi:10.3390/molecules25112639

Nadine C Michenfelder, Melina Dilanas + Show 3 more

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https://doi.org/10.3390/molecules25112639

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Abstract

We present a comprehensive femtosecond (fs) transient absorption study of the [Ge9(Hyp)3]− (Hyp = Si(SiMe3)3) cluster solvated in tetrahydrofuran (THF) with special emphasis on intra- and intermolecular charge transfer mechanisms which can be tuned by exchange of the counterion and by dimerization of the cluster. The examination of the visible and the near infrared (NIR) spectral range reveals four different processes of cluster dynamics after UV (267/258 nm) photoexcitation related to charge transfer to solvent and localized excited states in the cluster. The resulting transient absorption is mainly observed in the NIR region. In the UV-Vis range transient absorption of the (neutral) cluster core with similar dynamics can be observed. By transferring concepts of: (i) charge transfer to the solvent known from solvated Na− in THF and (ii) charge transfer in bulk-like materials on metalloid cluster systems containing [Ge9(Hyp)3]− moieties, we can nicely interpret the experimental findings for the different compounds. The first process occurs on a fs timescale and is attributed to localization of the excited electron in the quasi-conduction band/excited state which competes with a charge transfer to the solvent. The latter leads to an excess electron initially located in the vicinity of the parent cluster within the same solvent shell. In a second step, it can recombine with the cluster core with time constants in the picosecond (ps) timescale. Some electrons can escape the influence of the cluster leading to a solvated electron or after interaction with a cation to a contact pair both with lifetimes exceeding our experimentally accessible time window of 1 nanosecond (ns). An additional time constant on a tens of ps timescale is pronounced in the UV-Vis range which can be attributed to the recombination rate of the excited state or quasi conduction band of Ge9−. In the dimer, the excess electron cannot escape the molecule due to strong trapping by the Zn cation that links the two cluster cores.

Highlights

The observation/tracking of electrons in photoexcited molecules, nanoclusters or particles is a very interesting research field in chemistry involving intra- and intermolecular electron transfer, conduction mechanisms or magnetic properties of molecules with regard to photochemical reactions and photocatalysis [1,2,3,4,5,6,7,8,9,10]
No absorption bands could be found in the near infrared (NIR) spectral range
We investigated the relaxation channels after photoexcitation of the [Ge9 (Hyp)3 ]− clusters with different cations (K+, Li+ and K(crypt)+ ) and as dimer with fs pump probe broadband absorption spectroscopy in THF solution

Summary

Introduction

The observation/tracking of electrons in photoexcited molecules, nanoclusters or particles is a very interesting research field in chemistry involving intra- and intermolecular electron transfer, conduction mechanisms or magnetic properties of molecules with regard to photochemical reactions and photocatalysis [1,2,3,4,5,6,7,8,9,10]. It was used to understand the redox or electron transfer [12]. It was used to understand the redox photochemistry of a molybdenum(II) cluster [13] or to find appropriate building units for photochemistry of a molybdenum(II) cluster [13] or to find appropriate building units for photoactive photoactive metal-organic-frameworks (MOFs) [14]. Charged compounds offer good access via metal-organic-frameworks (MOFs) [14]. Charged compounds offer good access via charge transfer charge transfer transitions to their electronic structure in the excited state [15]. A charge transfer can transitions to their electronic structure in the excited state [15].

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