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

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.

Probing Charge‐Transfer Processes in a Covalently Linked [Ge9]‐Cluster Imine Dyad

“…However, a targeted synthesis of oxidized [Ge 9 ] species is hampered by the formation of by‐products and low yields. An investigation of the optical properties of silylated clusters [Ge 9 {Si(TMS) 3 } 3 ] − by means of ultrafast transient absorption spectroscopy (TA) revealed a photoexcitation‐induced transfer of cluster electrons to the solvent or ligand environment, and, as a result, the generation of a transient [Ge 9 ] 0 species [24, 25] . Similar interesting findings were reported for the transition metal‐ decorated species [Ge 9 {Si(TMS) 3 } 3 FeCp(CO) 2 ] [26] …”

Intersystem Crossing Rates in Photoexcited Rose Bengal: Solvation versus Isolation

“…Transient absorption (TA) spectra in the UV–vis and near-infrared (NIR) spectral regions in aqueous solution at room temperature were recorded according to the setups and procedures described elsewhere. , In brief, for detection in the UV–vis range, a small portion of a Ti/sapphire laser system output (Astrella, Coherent, 800 nm, 1 kHz, 35 fs, 7.2 mJ, also used to provide excitation pulses for tr-PES measurements) was employed to pump a non-collinear optical parametric amplifier (NOPA). Excitation pulses centered at a wavelength of 490 nm with magic angle polarization and an energy of 0.4 μJ were obtained from the NOPA.…”

“…Recording of TA spectra in the NIR spectral range was accomplished by using a CPA 2210 (Clark-MXR, 775 nm, repetition rate 1 kHz, 150 fs, 1.3 mJ, Ti/sapphire regenerative amplifier of a frequency doubled Er-Fibre oscillator). 13 The excitation wavelength was set to 548 nm (NOPA, magic angle) at an energy of 1.9 μJ per pulse. Probe pulses between 900 and 1600 nm were generated using a YAG crystal after seeding at 775 nm.…”

Untersuchung von Ladungstransferprozessen in einer kovalent verknüpften [Ge9]‐Cluster‐Imin Dyade

“…Erwähnenswert ist, dass sich das Spektrum von 1 À nicht wesentlich von denen des Vorläuferliganden DAB(II) Dipp Br, dem des Reaktanden [Ge 9 {Si(TMS) 3 } 2 ] 2À oder dem von Spezies mit anderen aromatischen oder heterocyclischen Liganden unterscheidet (Hintergrundinformationen, Abbildungen S18-S23). [24,25] Das beobachtete Spektrum kann also sowohl durch clusterzentrierte als auch durch DAB(II)-zentrierte Übergänge erklärt werden. Eine weiterführende Analyse der beteiligten Spezies und deren angeregter Zustände ist lediglich aufgrund von Absorptionsspektren nicht möglich.…”

“…Eine Untersuchung der optischen Eigen-schaften der silylierten Cluster [Ge 9 {Si(TMS) 3 } 3 ] À mittels ultraschneller transienter Absorptionsspektroskopie (TA) zeigte einen durch Lichtabsorption induzierten Transfer von Clusterelektronen auf das Lösemittel oder die Ligandumgebung und -als Ergebnis -die Bildung einer transienten [Ge 9 ] 0 -Spezies. [24,25] Ähnliche interessante Ergebnisse wurden für die Übergangsmetall-dekorierte Spezies [Ge 9 {Si-(TMS) 3 } 3 FeCp(CO) 2 ] berichtet. [26] Eine Erweiterung des elektronischen Systems wurde in den sogenannten Zintl-Triaden realisiert, [5][6][7] in denen -wie in [R-Ge 9 -(CH=CHÀ CH=CH)À Ge 9 -R] 4À (R = CH=CH 2 ) [7] zwei [Ge 9 ]-Cluster über eine konjugierte Kohlenwasserstoffkette miteinander verbunden sind.…”