Photoexcitation of Ge9- Clusters In

molecules

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

Nadine C. Michenfelder 1, Christian Gienger 2, Melina Dilanas 1 , Andreas Schnepf 2,* and Andreas-Neil Unterreiner 1,*

1 Institut für Physikalische Chemie, Karlsruher Institut für Technologie (KIT), Kaiserstr. 12, 76131 Karlsruhe, Germany; [email protected] (N.C.M.); uxﬀ[email protected] (M.D.) 2 Institut für Anorganische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany; [email protected] * Correspondence: [email protected] (A.S.); [email protected] (A.-N.U.); Tel.: +49-7071-29-76635 (A.S.); +49-721-608-47807 (A.-N.U.); Fax: +49-(7071)-28-2436 (A.S.) Received: 16 April 2020; Accepted: 3 June 2020; Published: 5 June 2020

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.

Keywords: ultrafast dynamics; metalloid cluster; germanium; photoexcitation; excess charge

Academic Editors: Constantina Papatriantafyllopoulou and George E. Kostakis

1. 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 [1–10]. Transient absorption spectroscopy is a versatile tool to investigate photophysical and chemical mechanisms and processes, for example (bi-)radical lifetimes [11]

Molecules 2020, 25, 2639; doi:10.3390/molecules25112639 www.mdpi.com/journal/molecules Molecules 2020, 25, 2639 2 of 14 Molecules 2019, 24, x FOR PEER REVIEW 2 of 22 electron transfer [12]. Within inorganic chemistry, it was also used to understand the redox or electron transfer [12]. Within inorganic chemistry, it was also 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]. A charge transfer can occur intra- occur intra- or intermolecularly or as a charge transfer to the solvent, when working in solution or intermolecularly or as a charge transfer to the solvent, when working in solution [16,17]. Most [16,17]. Most excited inorganic clusters and complexes show intramolecular charge transfer excited inorganic clusters and complexes show intramolecular charge transfer [8,12,18,19] or triplet [8,12,18,19] or triplet state dynamics [20–24]. The metalloid cluster [Ge9(Hyp)3]− (Hyp = Si(SiMe3)3) state dynamics [20–24]. The metalloid cluster [Ge9(Hyp)3]− (Hyp = Si(SiMe3)3)[25] (see Figure1) [25] (see Figure 1) with its high-yield synthesis, high stability under inert conditions and good with its high-yield synthesis, high stability under inert conditions and good solubility in organic solubility in organic solvents was used for, among others, build up reactions which give access to solvents was used for, among others, build up reactions which give access to cluster aggregates and cluster aggregates and chain compounds with definite compositions [25–33]. The Ge9− entity can be chain compounds with definite compositions [25–33]. The Ge9− entity can be linked, for example, linked, for example, by transition metal ions like Cu+, Ag+, Au+ [26] or Zn2+, Cd2+, Hg2+ [27] to build by transition metal ions like Cu+, Ag+, Au+ [26] or Zn2+, Cd2+, Hg2+ [27] to build up a neutral up a neutral dimer in the latter case. The chemical variety of this compound opens up a large dimer in the latter case. The chemical variety of this compound opens up a large playground to playground to understand its chemical and physical properties. This cluster differs from most understand its chemical and physical properties. This cluster differs from most inorganic clusters inorganic clusters as it can probably completely abstract an electron from the cluster core [2] which as it can probably completely abstract an electron from the cluster core [2] which allows both intra- allows both intra- and intermolecular charge transfer. This approach gives access to different photo and intermolecular charge transfer. This approach gives access to different photo induced reactions induced reactions simultaneously. To observe the processes after photoexcitation, e.g., the simultaneously. To observe the processes after photoexcitation, e.g., the detachment of an electron detachment of an electron from the cluster, fs transient absorption (TA) spectroscopy was applied. In from the cluster, fs transient absorption (TA) spectroscopy was applied. In solution, such an abstracted solution, such an abstracted electron can dissolve as a so-called solvated electron [34]. In presence of electron can dissolve as a so-called solvated electron [34]. In presence of a cation a contact pair a cation a contact pair with the solvated electron forms [35,36]. The well-known absorption spectra with the solvated electron forms [35,36]. The well-known absorption spectra of the solvated electron + − of the solvated electron (λmax = 2120 nm) [37] and the cation-electron+ contact pairs (e.g., [Na ,e ]THF: (λmax = 2120 nm) [37] and the cation-electron contact pairs (e.g., [Na ,e−]THF:(λmax = 890 nm) [38], + − + − (λmax+ = 890 nm) [38], [Li ,e ]THF (λmax = 1180+ nm) [39] and [K ,e ]THF (λmax = 1125 nm) [40]) peak in the [Li ,e−]THF (λmax = 1180 nm) [39] and [K ,e−]THF (λmax = 1125 nm) [40]) peak in the NIR spectral range. NIR spectral range. The Ge9− cluster moiety can be excited in the UV-Vis range as presented in The Ge9− cluster moiety can be excited in the UV-Vis range as presented in reference [2]. With single reference [2]. With single wavelength probing in the near infrared (NIR) region, indications for wavelength probing in the near infrared (NIR) region, indications for excess electron formation were excess electron formation were found whereas in the UV-Vis regime cluster dynamics were found whereas in the UV-Vis regime cluster dynamics were observed [2]. In another publication, observed [2]. In another publication, we also investigated a neutral complex [Ge9(Hyp)3FeCp(CO)2] we also investigated a neutral complex [Ge9(Hyp)3FeCp(CO)2] which gave new insights into ultrafast which gave new insights into ultrafast dynamics of this cluster-type [5]. Surprisingly, no long-lived dynamics of this cluster-type [5]. Surprisingly, no long-lived excited states exceeding lifetimes of excited states exceeding lifetimes of more than several hundreds of picoseconds were observed, more than several hundreds of picoseconds were observed, which was attributed to the special role of which was attributed to the special role of cationic iron. cationic iron.

Figure 1. Molecular structure of LiGe9 (left) and Ge9ZnGe9 (right) from crystallographic data without Figure 1. Molecular structure of LiGe9 (left) and Ge9ZnGe9 (right) from crystallographic data without hydrogen atoms. Color-code: Ge(blue); Zn(cyan); Si(black); C(grey); Li(pink); O(red). hydrogen atoms. Color-code: Ge(blue); Zn(cyan); Si(black); C(grey); Li(pink); O(red).

In this contribution, we present a comprehensive study on the [Ge9(Hyp)3]− cluster dynamics In this contribution, we present a comprehensive study on the [Ge9(Hyp)3]− cluster dynamics after photoexcitation with special emphasis on the influence of different counter ions like K(crypt)+ + after photoexcitation with special emphasis on the influence+ of different +counter ions like K(crypt) (crypt = [2.2.2]cryptant) (K(crypt)Ge9) in comparison to Li (LiGe9) or K (KGe9). Complementing (crypt = [2.2.2]cryptant) (K(crypt)Ge9) in comparison to Li+ (LiGe9) or K+ (KGe9). Complementing studies on [(Hyp)3Ge9ZnGe9(Hyp)3][27,33] (Ge9ZnGe9) show a strong influence of dimerization studies on [(Hyp)3Ge9ZnGe9(Hyp)3] [27,33] (Ge9ZnGe9) show a strong influence of dimerization on the long timescale dynamics of the cluster which is crucial for the understanding of possible

Molecules 2020, 25, 2639 3 of 14 Molecules 2019, 24, x FOR PEER REVIEW 3 of 22 relaxationon the long pathways. timescale This dynamics study also of theincludes cluster results which obtained is crucial from for a the new understanding experimental setup of possible [5,41] thatrelaxation now allows pathways. long Thistimescale study (1 also ns) includesbroadband results probe obtained in the UV from-Vis a newas well experimental as in the NIR setup spectral [5,41] rangethat now. This allows should long allow timescale the observation (1 ns) broadband of at least probe a section in the of UV-Vis the high as- wellenergy as intail the of NIRthe solvated spectral electronrange. This spectrum should in allow THF the, which observation still peaks of at (ca. least 2100 a section nm) beyond of the high-energy our experimentally tail of the accessib solvatedle rangeelectron of spectrum 1400 nm in. THF,The which analysis still results peaks (ca. in a 2100 concept nm) beyond for the our charge experimentally transfer accessibleprocesses range after photoexcitationof 1400 nm. The of analysis the metalloid results in[Ge a9 concept(Hyp)3]− for cluster the charge and the transfer role of processescounter ions after and photoexcitation dimerization of. the metalloid [Ge9(Hyp)3]− cluster and the role of counter ions and dimerization. 2. Results and Discussion 2. Results and Discussion 2.1. Steady State Spectroscopy in Solution 2.1. Steady State Spectroscopy in Solution Stationary absorption spectra for K(crypt)Ge9, Ge9ZnGe9, LiGe9 and KGe9 are shown in Figures Stationary absorption spectra for K(crypt)Ge , Ge ZnGe , LiGe and KGe are shown in Figure2 2 and S1 in the Supplementary Information (SI). For9 the9 monomers9 , 9a spectral broad9 absorption band and Figure S1 in the Supplementary Information (SI). For the monomers, a spectral broad absorption in the UV spectral region was observed, which extends into the Vis regime. No absorption bands band in the UV spectral region was observed, which extends into the Vis regime. No absorption bands could be found in the near infrared (NIR) spectral range. The results nicely match the known could be found in the near infrared (NIR) spectral range. The results nicely match the known spectrum spectrum of the LiGe9 cluster [2], whereby the spectrum does not significantly differ with K+ or of the LiGe cluster [2], whereby the spectrum does not signiﬁcantly diﬀer with K+ or K(crypt)+ as K(crypt)+ as9 counterion. As known from time dependent density functional theory (TD-DFT) counterion. As known from time dependent density functional theory (TD-DFT) calculations using calculations using a grid-based projector-augmented wave code [2], the transitions in the visible a grid-based projector-augmented wave code [2], the transitions in the visible region primarily originate region primarily originate from atomic orbital contributions of the Ge9− cluster core. For the from atomic orbital contributions of the Ge9− cluster core. For the Ge9ZnGe9 cluster, enhanced bands Ge9ZnGe9 cluster, enhanced bands around 390 nm and 490 nm were observed [33]. Known from around 390 nm and 490 nm were observed [33]. Known from TD-DFT calculations in the gas phase, TD-DFT calculations in the gas phase, the transitions from the HOMO−1 to the LUMO were found at the transitions from the HOMO 1 to the LUMO were found at 500 nm. Around 400 nm multiple 500 nm. Around 400 nm multiple− transitions from the HOMO−4, HOMO−3 and HOMO−5 to the transitions from the HOMO 4, HOMO 3 and HOMO 5 to the LUMO+2, LUMO+3, LUMO+4 and LUMO+2, LUMO+3, LUMO+4− and LUMO+5− contribute− to the peak [33]. LUMO+5 contribute to the peak [33].

K(crypt)Ge9

Ge9ZnGe9

0 300 400 500 600 800 1200 1600 l / nm

Figure 2. Absorption spectra for K(crypt)Ge9 and Ge9ZnGe9 between 250 and 1600 nm in THF Figure 2. Absorption spectra for K(crypt)Ge9 and Ge9ZnGe9 between 250 and 1600 nm in THF solution solution with a concentration of about 1 mmol/L. Spectra of KGe9 and LiGe9 are very similar to with a concentration of about 1 mmol/L. Spectra of KGe9 and LiGe9 are very similar to K(crypt)Ge9 Kand(crypt) canGe be9 found and can in thebe found SI Figure in the S1. SI Figure S1.

2.2.2.2. Transient Transient A Absorptionbsorption S Spectroscopypectroscopy in S Solutionolution

2.2.1.2.2.1. Dynamics Dynamics of the the M Monomeronomer After excitation into the UV band (267 nm) of K(crypt)Ge , where K+ is trapped by a [2.2.2]cryptand, After excitation into the UV band (267 nm) of K(crypt)Ge9 9, where K+ is trapped by a [2.2.2]cryptand,TA spectra as shown TA spectra in Figure as3 shown were obtained. in Figure For 3 were this compound,obtained. For a spectral this compound broad TA, a band spectral extending broad TAfrom band the visibleextending to the from NIR range the visible was observed. to the NIR In therange UV region,was observed. it is superimposed In the UV by region, the ground it is superimposedstate bleach (GSB), by the which ground leads state to negative bleach transient(GSB), which response leads around to negative 400 nm. transient Ground resp stateonse population around 400 nm. Ground state population does not recover within the experimental time window (1 ns). Comparably long-lived absorption bands dominate throughout the blue to the NIR spectral region.

Molecules 2020, 25, 2639 4 of 14

Molecules 2019, 24, x FOR PEER REVIEW 4 of 22 doesMolecules not 2019 recover, 24, x FOR within PEERthe REVIEW experimental time window (1 ns). Comparably long-lived absorption4 of 22 bandsThe transient dominate response throughout on a longer the blue timescale to the NIR shows spectral a steady region. increase The transientfrom visible response to NIR, on which a longer can timescalebeThe interpreted transient shows response as a a steady blue on tailing increase a longer end from timescaleof an visible absorption shows to NIR, aband whichsteady that can increase will be interpretedbe fromdiscussed visible as below a to blue NIR,. tailing which end can of anbe absorptioninterpreted band as a blue that tailing will be end discussed of an absorption below. band that will be discussed below. 1.6

1.41.6 0.3 1.21.4 0.3 1.01.2 0.2 0.81.0

-3 0.2 0.60.8

-3

A / 10 / A 0.1 D 0.40.6

A / 10 / A t / ps 0.1 D 0.20.4 2.0 t / ps 0.00.2 5.0 0.0 102.0 -0.20.0 K(crypt)Ge 505.0 0.0 9 10010 -0.2 K(crypt)Ge 110050 -0.4 9 100 -0.1 1100 -0.4 400 500 600 900 1000 1100 1200 -0.1 400 500 600 l /900 nm 1000 1100 1200

Figure 3. Broad transient absorption spectra for K(crypt)Gel / nm 9 in THF after excitation at 267/258 nm at

givenFigure delay 3. Broad times transient. The gray absorption line shows spectra the ground for K(crypt)Ge state absorption9 in THF spectrum after excitation with inverted at 267/258 OD nm axis at. Figure 3. Broad transient absorption spectra for K(crypt)Ge9 in THF after excitation at 267/258 nm atThegiven given lowered delay delay times absolute times.. The Thetransient gray gray line lineresponseshows shows the indicat ground the groundes stateabsorption state absorption absorption of the spectrum white spectrum-light with probe withinverted invertedbeam OD by axis ODthe. axis.sampleThe lowered The between lowered absolute 350 absolute and transient 390 transient nm response. response indicat indicateses absorption absorption of the of white the white-light-light probe probe beam beam by the by thesample sample between between 350 350and and 390 390nm nm.. This can be further visualized by inspecting single transients at different wavelengths (see FigureThisThis 4). can canDue be be further to further different visualized visualized techniques, by inspecting by the inspec time singleting resolution single transients transients of at the diﬀ UVerent at -Vis different wavelengths excited wavelengths experiments (see Figure (see4 is). DueconsiderablyFigure to di4).ﬀ erentDue shorter to techniques, different than in techniques, the the time NIR resolution region the time as of obresolution theservable UV-Vis ofby excited the a faster UV experiments-Vis rise excited of the is experimentscorresponding considerably is shortertransientsconsiderably than on inan shorter theultrashort NIR than region intimescale. the as NIR observable The region transients as by ob aservable fasterfor the rise highest by of a thefaster wavelengths corresponding rise of the tend corresponding transients to be among on anthosetransients ultrashort with on highest timescale. an ultrashort induced The transientstimescale. absorption forThe while the transients highest showing wavelengths for a the continuous highest tend wavelengths toincrease be among in the thosetend NIR to with beon highest amonga long inducedtimescale.those with absorption highest induced while showing absorption a continuous while showing increase a incontinuous the NIR on increase a long timescale.in the NIR on a long timescale. 3.5 0.7 500 nm 3.5 K(crypt)Ge9 700 nm 0.7 3.0 500 nm 0.6 900 nm K(crypt)Ge9 700 nm 3.0 1000 nm 0.6 900 nm 2.5 1100 nm 0.5 1000 nm 1200 nm 2.5 1100 nm 0.5

-3 2.0 0.4 1200 nm

/10 -3 2.0 0.4

A 1.5 0.3 D

/10

A 1.5 0.3 D 1.0 0.2

1.0 0.2 0.5 0.1

0.5 0.1 0.0 0.0

0.0 0.0 -0.5 -0.1 0 2 4 6 8 200 400 600 800 1000 -0.5 -0.1 0 2 4 6 8 t / ps 200 400 600 800 1000

t / ps Figure 4. 4. TransientTransient response response after after 267/258 267/258 nm nm excitation at at different diﬀerent wavelengths (circles) for K(crypt)Ge − in THF. On a long timescale one can see that the values continuously increase with longer K(crypt)GeFigure 4. 9Transient9− in THF . responseOn a long after timescale 267/258 one nm can excitation see that the at different values continuously wavelengths increase (circles) with for wavelengths. Fit curves are displayed as lines. longerK(crypt)Ge wavelengths.9− in THF F. itOn curves a long are timescale displayed one as lines. can see that the values continuously increase with longer wavelengths. Fit curves are displayed as lines. To get a closer look on the dynamics and time constants of the processes, global analysis was doneTo with get a an closerin-house look written on the programdynamics in and the time MATLAB constants environment of the processes,. The transients global analysis were fitted was done with an in-house written program in the MATLAB environment. The transients were fitted

Molecules 2019, 24, x FOR PEER REVIEW 5 of 22

(see SI Fit function) with four time constants, which were essential to reproduce the data adequately and are given in Table 1 for all compounds and the corresponding amplitudes are presented as decay associated spectra (DAS) (see Figure 5 for K(crypt)Ge9). The DAS show the absorption spectra of the excited states, which correspond to the respective time constant.

Table 1. Time constants from global analysis after UV (267 nm) and additionally 400 nm excitation

for Ge9ZnGe9.

K(crypt)Ge9 KGe9 LiGe9 Ge9ZnGe9 Ge9ZnGe9

Molecules 2020, 25, 2639 (267 nm) (267 nm) (267 nm) (267 nm) (400 nm)5 of 14

τ1/ps 0.1 0.5 0.5−0.7 0.3 0.4 Toτ2 get/ps a closer look2.5 on−3.5 the dynamics and3.3 time constants2.4 of the processes,2.5− global5.8 analysis was1.8 done τ3/ps 120−140 30−50 40−60 30 30 with an in-house written program in the MATLAB environment. The transients were ﬁtted (see SI Fit τ4/ps >1000 >1000 >1000 700−1200 400 function) with four time constants, which were essential to reproduce the data adequately and are given The short timescale dynamics, which are pronounced mainly in the visible part of the spectrum in Table1 for all compounds and the corresponding amplitudes are presented as decay associated can be attributed to the cluster core dynamics [2]. A more detailed analysis is challenging as suitable spectra (DAS) (see Figure5 for K(crypt)Ge ). The DAS show the absorption spectra of the excited reference systems are unknown and theoretical9 calculations do not provide dynamic information. states, which correspond to the respective time constant. On the other hand, absorption bands of long-lived species in the NIR are often triplet states or charge transferTable species 1. Time like constants solvated from electrons. global analysis Triplet after states UV in (267 this nm) system and additionally are unlikely 400 to nm be excitation populated as previous calculations did not show energies of triplet states that may be accessible under our for Ge9ZnGe9. experimental conditions [2]. Therefore, we concentrate on the excess charge dynamics as potential candidates. The long-livedK(crypt)Ge process9 (A4,KGe τ4) 9can be LiGemainly9 observedGe9ZnGe as9 GSBGe around9ZnGe9 400 nm and as (267 nm) (267 nm) (267 nm) (267 nm) (400 nm) induced absorption in the NIR spectral region. It is known from gas phase experiments that the τ /ps 0.1 0.5 0.5 0.7 0.3 0.4 vertical electron1 detachment energy is 3.37 eV [2],− which somewhat lowers in solution [42,43] eventually leadingτ /ps to a charge 2.5 3.5 transfer to 3.3 solvent (CTTS). 2.4 Therefore, 2.5 5.8 we compared 1.8 the absorption 2 − − spectra of the solvatedτ /ps electron 120 140 in THF 30[37]50 with the 40 long60-lived DAS 30 (A4) of the 30 Ge9−-cluster (see the 3 − − − orange dashed curveτ /ps in Figure>1000 5). The match>1000 is within>1000 the error 700 tolerance.1200 400 4 −

3.5 0.7

3.0 0.6 2.5 A1 (0.1 ps) A (2.5-3.5 ps) 0.5 2.0 2

-3 A3 (120-140 ps) A (> 1 ns) 1.5 4 0.4

/ 10 /

A 1.0 0.1 0.5 0.0 0.0 K(crypt)Ge9 -0.5 -0.1 400 500 600 900 1000 1100 1200 l / nm

FigureFigure 5. 5.Decay Decay associated associated spectra spectra from from global global ﬁt (further fit (further information information is given in is the given SI)for in K(crypt)Ge the SI) for9 inK(crypt)Ge THF. A section9 in THF of. theA section absorption of the spectrum absorption of s thepectrum solvated of the electron solvated in THFelectron is given in THF as anis given orange as dashedan orange line. dashed Further line spectra. Further can spectra be found can in be the found SI Figure in the S2. SI Figure S2.

TheAs a short result timescale of this agreement, dynamics, we which assume are pronounceda formation of mainly a solvated in the electron visible partin THF of the solution spectrum that canwas be abstracted attributed from to the the cluster excit coreed Ge dynamics9− cluster [ 2after]. A morephotoexcitation detailed analysis. To understand is challenging the underlying as suitable referenceprocesses, systems we apply are a unknown mechanism and which theoretical was initially calculations proposed do not by provide Barthel dynamicet al. [16] information. to describe Ondynamics the other after hand, photoexcitation absorption bands of Na of− in long-lived THF solution. species In inthis the scenario, NIR are t oftenhe excited triplet Na states−* can or undergo charge transfer species like solvated electrons. Triplet states in this system are unlikely to be populated as previous calculations did not show energies of triplet states that may be accessible under our experimental conditions [2]. Therefore, we concentrate on the excess charge dynamics as potential candidates. The long-lived process (A4, τ4) can be mainly observed as GSB around 400 nm and as induced absorption in the NIR spectral region. It is known from gas phase experiments that the vertical electron detachment energy is 3.37 eV [2], which somewhat lowers in solution [42,43] eventually leading to a charge transfer to solvent (CTTS). Therefore, we compared the absorption spectra of Molecules 2020, 25, 2639 6 of 14

the solvated electron in THF [37] with the long-lived DAS (A4) of the Ge9−-cluster (see the orange dashed curve in Figure5). The match is within the error tolerance. As a result of this agreement, we assume a formation of a solvated electron in THF solution that was abstracted from the excited Ge9− cluster after photoexcitation. To understand the underlying processes, we apply a mechanism which was initially proposed by Barthel et al. [16] to describe dynamics after photoexcitation of Na− in THF solution. In this scenario, the excited Na−* can undergo a CTTS reaction within 0.7 ps. The resulting excess electron is initially located in the vicinity of the parent Na0, leading to absorption bands of Na0 (peaking near 890 nm [16,38,44]) and the solvated electron (e−solv, 2120 nm in THF solution [37]). Partially, geminate recombination is possible that 0 results in a decay of the Na and the e−solv bands on a timescale of 1.5 ps. The remaining excess electrons can escape the parent Na0 to survive some ns until further recombination reaction occurs. Our system is of course much larger involving more degrees of freedom, but nevertheless there are some similarities. The following chemical equations show the adapted mechanism from Na− [16] onto the Ge9− cluster: hν [Ge ] [Ge ] 9− → 9− ∗ CTTS [ ] [ 0] Ge9− ∗ Ge9 esolv− −−−−→ · (1) recomb. 0 [Ge ] e− [Ge−] 9 · solv −−−−−→ 9 escape 0 0 [Ge ] e− [Ge ] + e− 9 · solv −−−−−→ 9 solv The negatively charged cluster with its relatively low electron detachment energy releases an electron after photoexcitation. Subsequently, this electron is transferred to the solvent (CTTS) which can be observed as an ultrafast decay of the excited Ge9− species with time constant τ1. In line with earlier studies on a LiGe9 cluster [2,45], vibrational relaxation probably occurs on this timescale, too. 0 The second decay process (τ2) can be attributed to the recombination with the parent Ge9 cluster of the solvated electrons localized nearby. The third time constant in this simple model has no relating process. We will return to this issue in the further course of the Discussion section. Solvated electrons that escape the interaction area can survive longer than our experimental delay range (1.2 ns) [37]. For K(crypt)Ge9 the absorption spectrum after 1 ns and the decay associated absorption spectrum of τ4 matches the high energy tail of the solvated electron absorption spectrum in THF with its maximum at 2120 nm [37] (see SI Figure S2).

2.2.2. Impact of the Counter Ion Next, we concentrate on the influence of different counterions. It is known, that the cation (K+) can be bound to the cluster core [46] if not trapped by crypt. Therefore, it can be assumed for KGe9 and LiGe9 that the distance between the cluster core and the cation is small enough for an interaction to take place, which is expected to influence the relaxation dynamics. At first glance, the transient spectra of KGe9 and LiGe9 (see Figure S3) are similar to the one of K(crypt)Ge9 showing a spectral broad long-lived transient absorption throughout the entire experimentally observed spectral range. A closer look, however, reveals subtle differences. For example, time constants from global fitting as described above are in the same order of magnitude except for τ3, which is much shorter for the free cation species (Table1). In addition, the longest timescale, designated with time constant τ4, shows a somewhat different spectral signature, which does not have much similarity with the solvated electron spectrum in THF. While the UV-Vis range again shows typical spectral features assigned to cluster dynamics, there is a small, but reproducible maximum around 1050 nm in the DAS (Figure6). Molecules 20192020,, 2524,, 2639x FOR PEER REVIEW 157 of 1422

16 + - A1 (0.2-0.5 ps) [K ,e ] 0.24 A (3-4 ps) e- 12 2 solv A3 (30 ps) 0.20

A4 (>1 ns) 8 0.16

-3 4 0.12

/ 10 / i 1.5 0.06

1.0 0.04

0.5 0.02 KGe9

0.0 0.00 400 500 600 900 1000 1100 1200 l / nm

+ Figure 6.6. DASDAS after after 267 267/258/258 nm nm excitation excitation for for KGe KGe9 in9 THF. in THF Dashed. Dashed blue andblue orange and orange lines show lines a show[K ,e − a] contact[K+,e−] contact pair [40 ] pair and [40] a section and of a section the absorption of the absorptionof the solvated of the electron solvated in THF electron [37]. The in THF corresponding [37]. The

transientcorresponding spectra transient and all spectra forand LiGe all spectra9 can be for found LiGe in9 can SI Figuresbe found S3–S5. in SI Figures S3–5.

TheThis absorption feature nicely spectrum matches of anothera contact form pair of of charge the electron transfer with species, K(crypt) which+ is cannot beknown attributed but one to woulda contact expect pair it between at longer an wavelengths electron and as K it+ should[40]. Again, be weaker we apply bound concepts than the from free suitableion contact reference pairs. + Thereforesystems. For, the example, spectra it is of known the contact that I− releases pairs and an electron the solvated which electron forms a contact may superimpose. pair with Na , The i.e., + + + formation[Na ,e−], inprocess aqueous of the NaI solvated [1]. The electron corresponding and the contact cation-electron pair absorption contact maximapair cannot for be Li observedand K liein ourat 1180 case. [39 T]he and cation 1125 has [40] only nm (seeweak also influence SI Figure on S2). the short timescale dynamics of the cluster (τ1 and + τ2), butThe on absorption the third process spectrum and of on a contactthe charge pair transfer of the electron species with(τ4). The K(crypt) chargeis transfer not known species but are one a wouldstrong expecthint that it atphotoexcitation longer wavelengths of the Ge as it9− shouldspecies beleads weaker to an bound electron than detachment the free ion and contact finally pairs. to a Therefore,release of this the spectraelectron of to the the contact solvent pairs, which and is the in solvatedline with electronearlier analysis may superimpose. by our groups The [2] formation. In this processscenario, of the the lo solvatedw intensity electron band around and the 460 cation-electron nm of A4 forcontact KGe9 is pair assign cannoted to bedynamics observed of ina Ge our90 speciescase. The. It cation remains has from only weakthe electron inﬂuence detach on thement short process. timescale All dynamicstransient response of the cluster in this (τ1 spectraland τ2), regionbut on thecan third be related process to and cluster on the dynamics charge transfer of Ge9 species0 and unaltered (τ4). The chargeGe9− moieties, transfer speciesi.e., clusters are a strongwhere photodetachmenthint that photoexcitation did not of occur the Ge. 9− species leads to an electron detachment and ﬁnally to a release of this electron to the solvent, which is in line with earlier analysis by our groups [2]. In this scenario, 0 2.2.3.the low Dynamics intensity of band the D aroundimer 460 nm of A4 for KGe9 is assigned to dynamics of a Ge9 species. It remains from the electron detachment process. All transient response in this spectral region can be To further substantiate this analysis, excitation of the cluster dimers such as Ge9ZnGe9 is related to cluster dynamics of Ge 0 and unaltered Ge moieties, i.e., clusters where photodetachment helpful. After excitations at 267/2589 nm the transient9− spectra (see Figure 7) reveal some similar did not occur. features as the monomer clusters and can be summarized as follows: i) The broad TA bands from Vis to2.2.3. NIR Dynamics are superimposed of the Dimer by the negative transient response of the GSB around 390 and 480 nm. ii) Global analysis reveals four-time constants (see Table 1), which are still in the same order of magnitudeTo further as for substantiate the monomers. this analysis, In contrast excitation to the ofmonomeric the cluster compounds, dimers such t ashe Gelong9ZnGe-lived9 istransient helpful. Afterresponse excitations in the NIR at 267does/258 neither nm the match transient the solvated spectra electron (see Figure spectrum7) reveal in THF some nor similar a cation features/electron as contacthe monomert pair. Instead clusters, the and spectrum can be summarized shows more assimilarity follows: with (i) The the broadexcited TA cluster bands core from spectra Vis to such NIR asare there superimposed are stronger by amplitudes the negative towards transient the response blue spectral of the GSBpart aroundcompared 390 to and red 480 ones. nm. Finally, (ii) Global we pointanalysis out reveals that the four-time overall constantstransient (seeresponse Table 1almost), which completely are still in recovers the same after order our of magnitudemaximum asdelay for timethe monomers. of 1 ns. As Ina result, contrast this to makes the monomeric a CTTS less compounds, likely for thethe long-liveddimeric form transient. response in the NIR does neither match the solvated electron spectrum in THF nor a cation/electron contact pair. Instead, the spectrum shows more similarity with the excited cluster core spectra such as there are stronger amplitudes towards the blue spectral part compared to red ones. Finally, we point out that the overall

Molecules 2020, 25, 2639 8 of 14

transient response almost completely recovers after our maximum delay time of 1 ns. As a result, this Molecules 2019, 24, x FOR PEER REVIEW 16 of 22 makes a CTTS less likely for the dimeric form.

0.4 5 t / ps 2.2 4 5.0 0.3 10 3 20 50 0.2 100 2

-3 1000 0.1 1

A / 10 / A

D 0 0.0

-1 -0.1

-2 Ge9ZnGe9 -0.2 -3 350 400 450 500 550 600 900 1000 1100 1200 l / nm

Figure 7. Transient spectra at given delay times for Ge ZnGe in THF after 267 nm excitation. Ground Figure 7. Transient spectra at given delay times for Ge99ZnGe99 in THF after 267 nm excitation. Ground statestate absorption absorption spectrum spectrum in in gray gray is is inverted. inverted.

In addition to the UV excitation, the stationary absorption spectrum of Ge9ZnGe9 allows another In addition to the UV excitation, the stationary absorption spectrum of Ge9ZnGe9 allows excitation regime at 400/388 nm (3.09/3.20 eV). This is, of course, slightly lower than the photodetachment another excitation regime at 400/388 nm (3.09/3.20 eV). This is, of course, slightly lower than the energy known from gas phase experiments. On the other hand, it is known for water [42] and THF [43] photodetachment energy known from gas phase experiments. On the other hand, it is known for that the ionization/photodetachment energy decreases by almost 1 eV in solution compared to the gas water [42] and THF [43] that the ionization/photodetachment energy decreases by almost 1 eV in phase making a photodetachment feasible. Indeed, the transient spectra (Figure S3) resemble the one solution compared to the gas phase making a photodetachment feasible. Indeed, the transient after UV excitation as do the first three-time constants (see Table1). The fourth time constant, however, spectra (Figure S 3) resemble the one after UV excitation as do the first three-time constants (see is significantly shorter. Table 1). The fourth time constant, however, is significantly shorter. So far, we attribute the short timescale processes τ1 to vibrational relaxation and CTTS and τ2 to So far, we attribute the short timescale processes τ1 to vibrational relaxation and CTTS and τ2 to geminate recombination processes for all compounds. For the monomer, the charge transfer leads to geminate recombination processes for all compounds. For the monomer, the charge transfer leads to a solvated electron which can escape out of the interaction sphere of the parent cluster. It can combine a solvated electron which can escape out of the interaction sphere of the parent cluster. It can with the counterion. Both leads to long lived TA in the NIR spectral range. For the dimer an abstraction combine with the counterion. Both leads to long lived TA in the NIR spectral range. For the dimer an of the electron cannot be observed. In the visible range, the transient response is attributed to the Ge9− abstraction0 of the electron cannot be observed. In the visible range, the transient response is and Ge9 species. attributed to the Ge9− and Ge90 species. τ + Finally, we address the interpretation of the third time constant 3. The monomer with K(crypt)+ Finally, we address the interpretation of the third time constant τ3. The monomer+ with+ K(crypt) has a τ3 of 120 to 140 ps, whereas it decreases to 40 and 30 ps for the cations Li and K , respectively. has a τ3 of 120 to 140 ps, whereas it decreases to 40 and 30 ps for the cations Li+ and K+, respectively. Indeed, the shortest value can be found for the K+ cation and for the cluster dimer (30 ps). An influence Indeed, the shortest value can be found for the K+ cation and for the cluster dimer (30 ps). An of the dimerization on the third time constant is not present. This differs from the fourth process which influence of the dimerization on the third time constant is not present. This differs from the fourth is shorter for lower excitation energy and is directly linked to the charge transfer and therefore to process which is shorter for lower excitation energy and is directly linked to the charge transfer and the electron detachment process. In addition, the transient spectra of the dimer suggest that this process therefore to the electron detachment process. In addition, the transient spectra of the dimer suggest is independent of the excitation wavelength. One can conclude that the state, which we describe with τ , that this process is independent of the excitation wavelength. One can conclude that the state, which3 is not involved into the charge transfer process. Nevertheless, the cation, especially the trapped cation, we describe with τ3, is not involved into the charge transfer process. Nevertheless, the cation, interacts with the process we are looking for by e.g. electron withdrawing effects. In all investigated especially the trapped cation, interacts with the process we are looking for by e.g. electron compounds, the DAS (A3) are similar to the spectra, which can be attributed to the cluster in its excited withdrawing effects. In all investigated compounds, the DAS (A3) are similar to the spectra, which state with or without the detached electron. To explain these experimental findings, we take into can be attributed to the cluster in its excited state with or without the detached electron. To explain account another concept known for charge transfer processes after photoexcitation, which was applied these experimental findings, we take into account another concept known for charge transfer to explain excited state dynamics of [Ge (Hyp) FeCp(CO) ][5]. In this case, no solvated electron or processes after photoexcitation, which9 was 3 applied to2 explain excited state dynamics of long-lived states occur, but a trap state exists with a lifetime of ~150 ps that can be attributed to a charge [Ge9(Hyp)3FeCp(CO)2] [5]. In this case, no solvated electron or long-lived states occur, but a trap state exists with a lifetime of ~150 ps that can be attributed to a charge transfer state. This is possible for Ge9ZnGe9 but is unlikely for the monomers as their counterion is not directly linked to the cluster and the contact pair bands are pronounced in the NIR spectral range. In bulk materials like α-Fe2O3 nanoparticles, thin films, colloidal solutions [47–49], or Fe10Ln10 clusters [8], photoexcitation into the

Molecules 2020, 25, 2639 9 of 14

transfer state. This is possible for Ge9ZnGe9 but is unlikely for the monomers as their counterion is not directly linked to the cluster and the contact pair bands are pronounced in the NIR spectral Molecules 2019, 24, x FOR PEER REVIEW 17 of 22 range. In bulk materials like α-Fe2O3 nanoparticles, thin films, colloidal solutions [47–49], or Fe10Ln10 quasiclusters-conduction [8], photoexcitation band leads to into a charge the quasi-conduction transfer from oxygen band leadsto iron to [17] a charge. Three transfer-time constants from oxygen can beto found iron [ 17on]. a Three-timesub ps, ps, constantsand tens of can ps be timescale found on that a subare ps,attributed ps, and to tens vibrational of ps timescale relaxation that and are trapping,attributed gemina to vibrationalte recombination relaxation, and and relaxation trapping, from geminate trap recombination,states, respectively and. relaxationBy analogy from, on trapthe states, respectively. By analogy, on the timescale of the CTTS (τ ) a concurrent process is therefore timescale of the CTTS (τ1) a concurrent process is therefore the localization1 around the cluster core in the localization around the cluster core in a low excited state or lower edge of a quasi-conduction band. a low excited state or lower edge of a quasi-conduction band. τ3 can be attributed to the relaxation τ can be attributed to the relaxation from the quasi conduction band or the excited state. Therefore, from3 the quasi conduction band or the excited state. Therefore, the influence of the cation on τ3 is τ relatedthe influence to the change of the cation of the on potential3 is related energy to thesurface change of the of theexcited potential state energydue to surfaceinteraction of the with excited the electrostaticstate due to interaction field of the with cat theion. electrostatic In Figure field 8 of an the overview cation. In Figure on the8 an possible overview processes on the possible after photoexcitationprocesses after photoexcitationis given. is given.

Figure 8. Illustration on the processes after photoexcitation (hν) of the Ge9−-cluster compounds. Figure 8. Illustration on the processes after photoexcitation (hν) of the Ge9−-cluster compounds. RecombinationRecombination processes processes are are depicted depicted as as violet violet arrows arrows all all other other processes processes in in orange. orange.

ThisThis concept concept is is also also helpful helpful for for the the understanding understanding of of the the long long time time-scale-scale dynamics dynamics of of the the cluster cluster dimer.dimer. For For 400 400 nm nm excitation excitation ττ4 4amountsamounts to to only only 400 400 ps ps and and is is significant signiﬁcantlyly shorter shorter than than after after 267 267 nm nm excitation.excitation. Interestingly, the the DAS DAS for for this this time time constant constant are quite are similar.quite similar. The ground The state ground completely state completelyrecovers within recovers the within experimental the experimental time window time of window 1.2 ns (after of 1.2 400 ns nm(after excitation). 400 nm excitation) The long-lived. The longtransient-lived absorption transient absorption in the NIR does in the not NIR match does the not solvated match electron the solvated absorption electron spectrum, absorption neither spectrum,after UV norneither after after 400 UV nm nor excitation. after 400 As nm the excitation. cation is As part the of cation the metalloid is part of cluster the metalloid entity, a cluster contact entity,pair spectrum a contact is, pair as expected,spectrum notis, as observable. expected, not Therefore, observable. no evidence Therefore, for no a release evidence of thefor electrona release into of thethe electron solvent caninto bethe found. solvent It can is still be possiblefound. It that is still because possible of a that very because low concentration, of a very low a smallconcentration, amount of asolvated small amount electrons of may solvated not be electrons observable, may but notthe complete be observ recoveryable, but of the ground complete state recovery bleachdoes of the not groundpromote state any bleacspeciesh does left after not 1promote ns. Consulting any species the concept left after from 1 bulk-like ns. Consulting materials the and concept results from from bulk[Ge9-like(Hyp) materials3FeCp(CO) and2 ][results5], a trap from state [Ge resulting9(Hyp)3FeCp(CO) from a charge2] [5], transfera trap state between resulting the clusterfrom a core charge and transferthe Zn cationbetween like inthe the monomer cluster core complex and [Gethe9(Hyp) Zn 3FeCp(CO)cation like2][ 5 in], may the serve monomer as an explanation complex [Gewhy9(Hyp) the electron3FeCp(CO) cannot2] [5], completely may serve be as released an explanation from the why cluster. the electronIndeed, its cannot lifetime completely depends be on releasedexcitation from wavelength, the cluster. i.e., Indeed, on the excessits lifetime energy depends deposited on inexcitation the excited wavelength, state. As wei.e. are, on not the aware excess of energysystematic deposited energy-depending in the excited studies state. As on we such are systems, not aware the of wavelength systematic dependentenergy-depending lifetimes studies are still onreminiscent such systems, to studies the ofwavelength excess electrons dependent in THF lifetimes [43] or liquid are still ammonia reminiscent [7,50] whereby to studies excess of excess energy electronsin the excited in THF state [43] determines or liquid ammonia survival [7,50 probability.] whereby excess energy in the excited state determines survival probability.

3. Materials and Methods

3.1. Sample Preparation

Molecules 2020, 25, 2639 10 of 14

3. Materials and Methods

3.1. Sample Preparation All reactions were carried out under a nitrogen atmosphere using standard Schlenk techniques or under argon atmosphere in a glove box. Toluene, hexane, and THF were dried with sodium, acetonitrile was dried with phosphorus(V)oxide, and pentane was dried with calcium hydride and all were distilled prior to use.

3.1.1. K4Ge9 K (1.54 g, 39.39 mmol) and Ge (5.0 g, 68.84 mmol) were heated in an evacuated quartz glass vial to 650 ◦C with a ramp of 100 ◦C per hour and then held at 650 ◦C for 72 h. After cooling down to room temperature the vial was opened in a glove box and K4Ge9 was obtained as a grey solid and characterized via X-Ray powder diﬀraction on a Stadi-P (STOE, Darmstadt, Germany) powder diﬀractometer using germanium monochromated Cu-Kα1 radiation (λ = 154.06 pm and a Mythen 1K detector (see Figure S6)).

3.1.2. KGe9Hyp3

K4Ge9 (3.0 g, 3.07 mmol) was suspended in 150 ml of THF. ClSi[Si(CH3)3]3 (3.0 mL, 2.56 g, 9.056 mmol) was heated to 60 ◦C to melt and then added quickly via syringe to the suspension. The mixture is stirred at ambient temperature for three days and after that the solvent is removed in vacuo. The residue is washed twice with 100 mL of pentane and then extracted with THF. After removal of the solvent the product is obtained as an orange solid with a yield of 38%. KGe9Hyp3 1 was used NMR pure or crystallized from toluene at 30 ◦C in the shape of orange needles. H-NMR 13 − 29 (C6D6, 300 MHz): δ 0.52 (s, 81H, SiMe3), C-NMR (C6D6, 62.9 MHz): δ 3.04 (SiMe3), Si-NMR (C6D6, 49.7 MHz, inept-nd): δ 8.48 (decet, Si(SiMe ) , 2J Si-H = 6.5 Hz), 105.59 (s, Si(SiMe ) ). − 3 3 − 3 3

3.1.3. K(crypt-2,2,2)Ge9Hyp3

KGe9Hyp3 (500 mg, 0.35 mmol) was dissolved in acetonitrile and layered with hexane. [2,2,2]-cryptand was dissolved in hexane and added to the hexane layer. Orange crystals formed 1 13 at room temperature at the interface. H-NMR (thf-d8, 300 MHz): δ 0.52 (s, 81H, SiMe3), C-NMR (thf-d , 62.9 MHz): δ 3.04 (SiMe ), 29Si-NMR (thf-d , 49.7 MHz, inept-nd): δ 8.48 (decet, Si(SiMe ) , 6 3 8 − 3 3 2J Si-H = 6.5 Hz), 105.59 (s, Si(SiMe ) ). − 3 3

3.1.4. LiGe9Hyp3

KGe9Hyp3 (1.00 g, 0.7 mmol) and LiBr (61 mg, 0.7 mmol) were dissolved in THF at room temperature and stirred overnight. After removing the solvent in vacuo the product was washed with pentane and then extracted with THF. It was obtained in the form of red crystals at 30 ◦C with 95% 1 13 − yield. H-NMR (thf-d8, 300 MHz): δ 0.24 (s, 81H, SiMe3), C-NMR (thf-d8, 62.9 MHz): δ 3.19 (SiMe3), 29 Si-NMR (thf-d8, 49.7 MHz, inept-nd): δ 9.85 pm (decet, Si(SiMe3)3, 2J Si-H = 6.6 Hz), 105.59 ppm 7 − − (s, Si(SiMe3)3), Li-NMR (thf-d8, 116.64 MHz): δ 0.39 (s, Li).

3.1.5. ZnGe18Hyp6

ZnCl2 (13 mg, 95 µmol) and KGe9Hyp3 (256 mg, 178 µmol) were separately dissolved in THF and cooled to -78 ◦C. While stirring the ZnCl2 solution was added dropwise to the KGe9Hyp3 solution and afterwards the reaction solution was slowly warmed to room temperature overnight. The solvent was then removed in vacuo and the product extracted with pentane. ZnGe18Hyp6 crystallizes in the form of dark red needles at 30 C with 80% yield. 1H-NMR (C D , 300 MHz): δ 0.54 (s, 162H, SiMe ), − ◦ 6 6 3 13C-NMR (C D , 62.9 MHz): δ 3.25 (SiMe ), 29Si-NMR (C D , 49.7 MHz, inept-nd): δ 9.59 (decet, 6 6 3 6 6 − Si(SiMe ) , 2J Si-H = 6.6 Hz), 105.30 (s, Si(SiMe ) ). 3 3 − 3 3 Molecules 2020, 25, 2639 11 of 14

Under exclusion of oxygen and water THF (abs.) was added as solvent for stationary and transient absorption measurements. Measurements were done in cuvettes optimized for the use under inert conditions.

3.2. Steady-State Spectroscopy in Solution Absorption spectra were obtained with an UV/Vis/NIR spectrometer Cary 500 (Varian, Palo Alto, CA, USA) in THF as solvent in a wavelength range between 200 and 2000 nm. Spectra were measured at room temperature in cuvettes made of fused silica (Hellma, Nürnberg, Germany) with 1 mm optical path length. Concentrations are about 1 mmol/L.

3.3. Transient Absorption Spectroscopy in Solution To obtain time resolved spectra in the UV-Vis wavelength regime, an experimental setup described elsewhere [41] was used. Brieﬂy, one small part (2–3 µJ) of the 800 nm (Astrella (Coherent, Santa Clara, CA, USA), 7 mJ, 35 fs, repetition rate 1 kHz) laser output was focused into a movable 2 mm CaF2 crystal (nortus optronic GmbH, Wörth am Rhein, Germany) to generate a white-light continuum between 350 and 720 nm. After passing the sample, the white-light is refracted by a fused silica prism and recorded by a CCD Camera (Series 2000, Si Photodetector, Entwicklungsbüro Stresing, Berlin, Germany). Pump wavelengths at 400 and 267 nm were generated by second harmonic generation and sum frequency mixing of the 800 and 400 nm pulses in BBO crystals, respectively. The spot size in the sample was about 200 µm, which was more than twice the white-light spot size. Excitation energies were for all wavelengths 400 nJ per pulse. Delay of the pump pulse was managed by a computer-controlled translation stage (maximum delay ~1.2 ns, Thorlabs, Newton, NJ, USA), whereby every second pulse was blocked with an optical chopper (Thorlabs), resulting in spectra with and without excitation. Diﬀerentiation results in ∆A spectra with a time resolution better than 100 fs. Data were collected with an in-house written Labview program. For recording TA spectra in the NIR spectral range, as described in an earlier publication [5], a CPA 2210 (Clark-MXR, Dexter, MI, USA, 775 nm, repetition rate 1 kHz, 1.3 mJ, 150 fs) was used. One part of the 775 nm beam propagated through a computer-controlled translation stage (maximum delay range 1.4 ns, Physical Instruments PI, Karlsruhe, Germany) and was used to generate an NIR white-light continuum between 900 and 1600 nm in a YAG crystal (nortus Optronic GmbH). The intensity of the refracted pulse (SF10 prism) was collected with a CCD Camera (Series 2000, InGaAs Photodetector, Entwicklungsbüro Stresing). Data were processed by the same program like in the other system adapted to the needs for NIR detection. Pump pulses were generated by second (388 nm) and third (258 nm) harmonic generation of the fundamental laser wavelength. Excitation energies were in the range of 1 µJ per pulse and spot sizes in the sample around 500 µm which, again, corresponded roughly two times the white-light spot size. Longer pulse duration of the fundamental and group velocity mismatch between UV/Vis pump pulses and NIR white-light detection resulted in a time resolution of roughly 200 fs obtained by data analysis.

4. Conclusions

We investigated the relaxation channels after photoexcitation of the [Ge9(Hyp)3]− clusters with different cations (K+, Li+ and K(crypt)+) and as dimer (bound through Zn2+) with fs pump probe broadband absorption spectroscopy in THF solution. In analogy to the charge transfer of bulk-like materials and charge transfer to solvent processes in Na−, we identified four different processes: Within 0.5 ps after photoexcitation with a 267 nm laser pulse an electron transfer to the solvent takes place, whereas localization in the quasi conduction band and vibrational relaxation occur on a similar timescale. The geminate recombination between the excess electron and the parent cluster occurs within about 2 ps. On a tens of ps timescale one can observe the relaxation from the quasi conduction band. Some excess electrons can escape the cluster influence, leading to solvated electrons and contact pairs with the cations that show characteristic spectra in the NIR with lifetimes longer one ns. For the Molecules 2020, 25, 2639 12 of 14 dimer, the excess electron cannot escape the trapping field of the Zn atom. This leads to a charge transfer state, which decays on a shorter timescale depending on the excitation wavelength. Hence, the dimer shows a normal relaxation pathway for an inorganic cluster in opposition to the monomer, which can completely release an electron into the solvent, which can act as reducing agent. In future work, the influence of different bound cations in form of dimers like Ge9ZnGe9 or complexes like [Ge9(Hyp)3FeCp(CO)2] and on the other hand of different sizes of substituents of the cluster seem to be promising targets for investigations.

Supplementary Materials: The following are available online, Figures S1–S2: Absorption spectra, Figures S3–S4: Transient absorption spectra and decays, Figure S5: Decay associated spectra, Figure S 6: PXRD Pattern. Author Contributions: Data curation, N.C.M., C.G. and M.D.; Formal analysis, N.C.M. and M.D.; Project administration, A.S. and A.-N.U.; Supervision, A.S. and A.-N.U.; Writing—original draft, N.C.M. and C.G.; Writing—review & editing, A.S. and A.-N.U. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Deutsche Forschungsgemeinschaft, projects UN108/6-1 and SCHN738/9-1. Acknowledgments: The authors acknowledge financial support by Karlsruhe Institute of Technology (KIT), the University of Tübingen and the Deutsche Forschungsgemeinschaft (DFG). The authors would also like to thank the reviewers for their helpful comments. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Bragg, A.E.; Schwartz, B.J. Ultrafast Charge-Transfer-to-Solvent Dynamics of Iodide in Tetrahydrofuran. 2. Photoinduced Electron Transfer to Counterions in Solution. J. Phys. Chem. A 2008, 112, 3530–3543. [CrossRef] [PubMed] 2. Klinger, M.; Schenk, C.; Henke, F.; Clayborne, A.; Schnepf, A.; Unterreiner, A.N. UV photoexcitation of

a dissolved metalloid Ge9 cluster compound and its extensive ultrafast response. Chem. Commun. 2015, 51, 12278–12281. [CrossRef][PubMed] 3. Dotti, N.; Heintze, E.; Slota, M.; Hübner, R.; Wang, F.; Nuss, J.; Dressel, M.; Bogani, L. Conduction mechanism of nitronyl-nitroxide molecular magnetic compounds. Phys. Rev. B 2016, 93, 165201. [CrossRef] 4. Chen, R.; Hong, Z.-F.; Zhao, Y.-R.; Zheng, H.; Li, G.-J.; Zhang, Q.-C.; Kong, X.-J.; Long, L.-S.; Zheng, L.-S. Ligand-Dependent Luminescence Properties of Lanthanide–Titanium Oxo Clusters. Inorg. Chem. 2019, 58, 15008–15012. [CrossRef][PubMed] 5. Michenfelder, N.C.; Gienger, C.; Schnepf, A.; Unterreiner, A.-N. The inﬂuence of the FeCp(CO)2+ moiety

on the dynamics of the metalloid [Ge9(Si(SiMe3)3)3]− cluster in thf: Synthesis and characterization by time-resolved absorption spectroscopy. Dalton Trans. 2019, 48, 15577–15582. [CrossRef] 6. Liedy, F.; Eng, J.; McNab, R.; Inglis, R.; Penfold, T.J.; Brechin, E.K.; Johansson, J.O. Vibrational coherences in manganese single-molecule magnets after ultrafast photoexcitation. Nat. Chem. 2020, 12, 452–458. [CrossRef] 7. Vogler, T.; Vöhringer, P. Probing the band gap of liquid ammonia with femtosecond multiphoton ionization spectroscopy. Phys. Chem. Chem. Phys. 2018, 20, 25657–25665. [CrossRef] 8. Baniodeh, A.; Liang, Y.; Anson, C.E.; Magnani, N.; Powell, A.K.; Unterreiner, A.-N.; Seyﬀerle, S.; Slota, M.; Dressel, M.; Bogani, L.; et al. Unraveling the Inﬂuence of Lanthanide Ions on Intra- and Inter-Molecular

Electronic Processes in Fe10Ln10 Nano-Toruses. Adv. Funct. Mater. 2014, 24, 6280–6290. [CrossRef] 9. Tungulin, D.; Leier, J.; Carter, A.B.; Powell, A.K.; Albuquerque, R.Q.; Unterreiner, A.N.; Bizzarri, C. Chasing BODIPY: Enhancement of Luminescence in Homoleptic Bis(dipyrrinato) ZnII Complexes Utilizing Symmetric and Unsymmetrical Dipyrrins. Chem. Eur. J. 2019, 25, 3816–3827. [CrossRef] 10. Kloepfer, J.A.; Vilchiz, V.H.; Lenchenkov, V.A.; Bradforth, S.E. Femtosecond dynamics of photodetachment of the iodide anion in solution: Resonant excitation into the charge-transfer-to-solvent state. Chem. Phys. Lett. 1998, 298, 120–128. [CrossRef] 11. Bakker, M.J.; Hartl, F.; Stufkens, D.J.; Jina, O.S.; Sun, X.Z.; George, M.W. Alkene-Stabilized Biradical and

Zwitterionic Photoproducts of the Clusters [Os3(CO)10(α-diimine)]: A Time-Resolved Transient Absorption and Infrared Study. Organometallics 2000, 19, 4310–4319. [CrossRef] Molecules 2020, 25, 2639 13 of 14

12. Kahnt, A.; Heiniger, L.-P.;Liu, S.-X.; Tu,X.; Zheng, Z.; Hauser, A.; Decurtins, S.; Guldi, D.M. An Electrochemical and Photophysical Study of a Covalently Linked Inorganic–Organic Dyad. ChemPhysChem 2010, 11, 651–658. [CrossRef]

13. Maverick, A.W.; Gray, H.B. Luminescence and redox photochemistry of the molybdenum(II) cluster Mo6Cl142. J. Am. Chem. Soc. 1981, 103, 1298–1300. [CrossRef] 14. Yuan, S.; Qin, J.-S.; Xu, H.-Q.; Su, J.; Rossi, D.; Chen, Y.; Zhang, L.; Lollar, C.; Wang, Q.; Jiang, H.-L.; et al.

[Ti8Zr2O12(COO)16] Cluster: An Ideal Inorganic Building Unit for Photoactive Metal–Organic Frameworks. ACS Cent. Sci. 2018, 4, 105–111. [CrossRef][PubMed] 15. Michenfelder, N.C.; Ernst, H.A.; Schweigert, C.; Olzmann, M.; Unterreiner, A.N. Ultrafast stimulated emission of nitrophenolates in organic and aqueous solutions. Phys. Chem. Chem. Phys. 2018, 20, 10713–10720. [CrossRef] 16. Barthel, E.R.; Martini, I.B.; Schwartz, B.J. Direct observation of charge-transfer-to-solvent (CTTS) reactions:

Ultrafast dynamics of the photoexcited alkali metal anion sodide (Na−). J. Chem. Phys. 2000, 112, 9433–9444. [CrossRef] 17. Fan, H.M.; You, G.J.; Li, Y.; Zheng, Z.; Tan, H.R.; Shen, Z.X.; Tang, S.H.; Feng, Y.P. Shape-Controlled Synthesis

of Single-Crystalline Fe2O3 Hollow Nanocrystals and Their Tunable Optical Properties. J. Phys. Chem. C 2009, 113, 9928–9935. [CrossRef] 18. Yam, V.W.-W.; Lo, K.K.-W.; Wang, C.-R.; Cheung, K.-K. Synthesis, Photophysics, and Transient Absorption Spectroscopic Studies of Luminescent Copper(I) Chalcogenide Complexes. Crystal Structure of

[Cu4(µ-dtpm)4(µ4-S)](PF6)2 {dtpm = Bis[bis(4-methylphenyl)phosphino]methane}. J. Phys. Chem. A 1997, 101, 4666–4672. [CrossRef] 19. Zhou, M.; Long, S.; Wan, X.; Li, Y.; Niu, Y.; Guo, Q.; Wang, Q.-M.; Xia, A. Ultrafast relaxation dynamics

of phosphine-protected, rod-shaped Au20 clusters: Interplay between solvation and surface trapping. Phys. Chem. Chem. Phys. 2014, 16, 18288–18293. [CrossRef] 20. Rothfuss, H.; Knöfel, N.D.; Tzvetkova, P.; Michenfelder, N.C.; Baraban, S.; Unterreiner, A.-N.; Roesky, P.W.; Barner-Kowollik, C. Phenanthroline—A Versatile Ligand for Advanced Functional Polymeric Materials. Chem. Eur. J. 2018, 24, 17475–17486. [CrossRef]

21. Ruan, L.; Gao, X.; Zhao, J.; Xu, C.; Liang, D. Preparation and characteristics of Eu(DBM)3phen: Synthesis, single-crystal structure and spectroscopic analysis. J. Mol. Struct. 2017, 1149, 265–272. [CrossRef] 22. Yuetao, Y.; Qingde, S.; Guiwen, Z. Photoacoustic spectroscopy study on lanthanide ternary complexes with dibenzoylmethide and phenanthroline. Spectrochim. Acta A 1999, 55, 1527–1533. [CrossRef] 23. Thielemann, D.T.; Klinger, M.; Wolf,T.J.A.; Lan, Y.; Wernsdorfer, W.; Busse, M.; Roesky,P.W.; Unterreiner, A.-N.; Powell, A.K.; Junk, P.C.; et al. Novel Lanthanide-Based Polymeric Chains and Corresponding Ultrafast Dynamics in Solution. Inorg. Chem. 2011, 50, 11990–12000. [CrossRef][PubMed] 24. Cannizzo, A.; Blanco-Rodríguez, A.M.; El Nahhas, A.; Šebera, J.; Záliš, S.; Vlˇcek,A.; Chergui, M. Femtosecond Fluorescence and Intersystem Crossing in Rhenium(I) Carbonyl Bipyridine Complexes. J. Am. Chem. Soc. − 2008, 130, 8967–8974. [CrossRef]

25. Schnepf, A. [Ge9{Si(SiMe3)3}3]−: Ein löslicher polyedrischer Ge9-Cluster, stabilisiert durch nur drei Silylliganden. Angew. Chem. 2003, 115, 2728–2729. [CrossRef]

26. Schenk, C.; Henke, F.; Santiso-Quiñones, G.; Krossing, I.; Schnepf, A. [Si(SiMe3)3]6Ge18M (M = Cu, Ag, Au): Metalloid cluster compounds as unusual building blocks for a supramolecular chemistry. Dalton Trans. 2008, 4436–4441. [CrossRef]

27. Henke, F.; Schenk, C.; Schnepf, A. [Si(SiMe3)3]6Ge18M (M = Zn, Cd, Hg): Neutral metalloid cluster compounds of germanium as highly soluble building blocks for supramolecular chemistry. Dalton Trans. 2009, 9141–9145. [CrossRef] 4 28. Li, F.; Sevov, S.C. Rational Synthesis of [Ge−9{Si(SiMe3)3}3]− from Its Parent Zintl Ion Ge−9 −. Inorg. Chem. 2012, 51, 2706–2708. [CrossRef] 29. Kysliak, O.; Schnepf, A. Metalloid Germanium Clusters as Starting Point for New Chemistry. In Reference Module in Chemistry, Molecular Sciences and Chemical Engineering; Elsevier: Amsterdam, The Netherlands, 2017.

30. Schenk, C.; Schnepf, A. [AuGe18{Si(SiMe3)3}6]−: A Soluble Au–Ge Cluster on the Way to a Molecular Cable? Angew. Chem. Int. Ed. 2007, 46, 5314–5316. [CrossRef] 31. Geitner, F.S.; Fässler, T.F. Introducing Tetrel Zintl Ions to N-Heterocyclic Carbenes – Synthesis of Coinage – Metal NHC Complexes of [Ge9{Si(SiMe3)3}3] . Eur. J. Inorg. Chem. 2016, 2016, 2688–2691. [CrossRef] Molecules 2020, 25, 2639 14 of 14

32. Geitner, F.S.; Giebel, M.A.; Pöthig, A.; Fässler, T.F. N-Heterocyclic Carbene Coinage Metal Complexes of 2 the Germanium-Rich Metalloid Clusters [Ge9R3]− and [Ge9RI2] − with R = Si(iPr)3 and RI = Si(TMS)3. Molecules 2017, 22, 1204. [CrossRef][PubMed]

33. Kysliak, O.; Nguyen, D.D.; Clayborne, A.Z.; Schnepf, A. [PtZn2Ge18(Hyp)8] (Hyp = Si(SiMe3)3): A Neutral Polynuclear Chain Compound with Ge9(Hyp)3 Units. Inorg. Chem. 2018, 57, 12603–12609. [CrossRef] 34. Dorfman, L.M.; Jou, F.Y.; Wageman, R. Solvent Dependence of Optical Absorption Spectrum of Solvated Electron. Berich. Bunsen. Phys. Chem. 1971, 75, 681–685. [CrossRef] 35. Giling, L.J.; Kloosterboer, J.G.; Rettschnick, R.P.H.; van Voorst, J.D.W. Flash photolysis of negative aromatic ions in liquid solutions. Chem. Phys. Lett. 1971, 8, 457–461. [CrossRef] 36. Kloosterboer, J.G.; Giling, L.J.; Rettschnick, R.P.H.; van Voorst, J.D.W. Flash photolysis of solutions of sodium in ethers. Chem. Phys. Lett. 1971, 8, 462–466. [CrossRef] 37. Jou, F.Y.; Dorfman, L.M. Pulse radiolysis studies. XXI. Optical absorption spectrum of the solvated electron in ethers and in binary solutions of these ethers. J. Chem. Phys. 1973, 58, 4715–4723. [CrossRef] 38. Bockrath, B.; Dorfman, L.M. Pulse radiolysis studies. XXII. Spectrum and kinetics of the sodium cation-electron pair in tetrahydrofuran solutions. J. Phys. Chem. 1973, 77, 1002–1006. [CrossRef] 39. Bockrath, B.; Dorfman, L.M. Ionic Aggregation of Solvated Electron with Lithium Cation in Tetrahydrofuran Solution. J. Phys. Chem. 1975, 79, 1509–1512. [CrossRef] 40. Salmon, G.A.; Seddon, W.A.; Fletcher, J.W. Pulse Radiolytic Formation of Solvated Electrons, Ion–pairs, and Alkali Metal Anions in Tetrahydrofuran. Can. J. Chem. 1974, 52, 3259–3268. [CrossRef] 41. Schweigert, C.; Babii, O.; Afonin, S.; Schober, T.; Leier, J.; Michenfelder, N.C.; Komarov, I.V.; Ulrich, A.S.; Unterreiner, A.N. Real-Time Observation of Diarylethene-Based Photoswitches in a Cyclic Peptide Environment. ChemPhotoChem 2019, 3, 403–410. [CrossRef] 42. Stähler, J.; Deinert, J.-C.; Wegkamp, D.; Hagen, S.; Wolf, M. Real-Time Measurement of the Vertical Binding Energy during the Birth of a Solvated Electron. J. Am. Chem. Soc. 2015, 137, 3520–3524. [CrossRef][PubMed] 43. Martini, I.B.; Barthel, E.R.; Schwartz, B.J. Mechanisms of the ultrafast production and recombination of solvated electrons in weakly polar ﬂuids: Comparison of multiphoton ionization and detachment via

the charge-transfer-to-solvent transition of Na− in THF. J. Chem. Phys. 2000, 113, 11245–11257. [CrossRef] 44. Piotrowiak, P.; Miller, J.R. Spectra of the solvated electron in the presence of sodium cation in tetrahydrofuran and in its.alpha.,.alpha.’-methylated derivatives. J. Am. Chem Soc. 1991, 113, 5086–5087. [CrossRef]

45. Andre Clayborne, P.; Hakkinen, H. The electronic structure of Ge9[Si(SiMe3)3]3−: A superantiatom complex. Phys. Chem Chem Phys. 2012, 14, 9311–9316. [CrossRef] 2 46. Kysliak, O.; Schnepf, A. {Ge9[Si(SiMe3)3]2} −: A starting point for mixed substituted metalloid germanium clusters. Dalton Trans. 2016, 45, 2404–2408. [CrossRef] 47. Kiwi, J.; Denisov, N.; Gak, Y.; Ovanesyan, N.; Buﬀat, P.A.; Suvorova, E.; Gostev, F.; Titov, A.; Sarkisov, O.; Albers, P.; et al. Catalytic Fe3+ clusters and complexes in Naﬁon active in photo-Fenton processes. High-resolution electron microscopy and femtosecond studies. Langmuir 2002, 18, 9054–9066. [CrossRef] 48. Baskoutas, S.; Terzis, A.F. Size-dependent band gap of colloidal quantum dots. J. Appl. Phys. 2006, 99, 013708. [CrossRef]

49. Joly, A.G.; Williams, J.R.; Chambers, S.A.; Xiong, G.; Hess, W.P.; Laman, D.M. Carrier dynamics in α-Fe2O3 (0001) thin ﬁlms and single crystals probed by femtosecond transient absorption and reﬂectivity. J. Appl. Phys. 2006, 99, 053521. [CrossRef] 50. Vohringer, P. Ultrafast dynamics of electrons in ammonia. Annu. Rev. Phys. Chem. 2015, 66, 97–118. [CrossRef]

Sample Availability: Samples of the compounds are available from the authors.

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