Towards Understanding the Reactivity and Optical Properties of Organosilicon Sulfide Clusters

Article abstract of DOI:10.1002/anie.202011370

We report the extension of the class of organotetrel sulfide clusters with further examples of the still rare silicon-based species, synthesized from RSiCl₃ with R=phenyl (Ph, I), naphthyl (Np, II), and styryl (Sty, III) with Na₂S. B

Full text of DOI:10.1002/anie.202011370

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Accepted Article
Title: Towards Understanding the Reactivity and Optical Properties of
Organosilicon Sulfide Clusters
Authors: Stefanie Dehnen, Katharina Hanau, Sebastian Schwan,
Moritz Schäfer, Marius Müller, Christof Dues, Niklas Rinn,
Simone Sanna, Sangam Chatterjee, and Doreen Mollenhauer
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202011370
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10.1002/anie.202011370
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RESEARCH ARTICLE
Towards Understanding the Reactivity and Optical Properties of
Organosilicon Sulfide Clusters
Katharina Hanau,[a] Sebastian Schwan,[b,c] Moritz R. Schäfer,[b,c] Marius J. Müller,[d] Christof Dues,[e]
Niklas Rinn,[a] Simone Sanna,*[e] Sangam Chatterjee,*[d] Doreen Mollenhauer,*[b,c] and Stefanie
Dehnen*[a]
[a]
Prof. Dr. S. Dehnen, K. Hanau
Fachbereich Chemie und Wissenschaftliches Zentrum für Materialwissenschaften (WZMW)
Philipps-Universität Marburg
Hans-Meerwein-Str. 4, 35043 Marburg, Germany
E-mail: dehnen@chemie-uni-marburg.de.
[b]
Prof. Dr. Doreen Mollenhauer, Sebastian Schwan, Moritz R. Schäfer
Institute of Physical Chemistry
Justus Liebig University Giessen
Heinrich-Buff-Ring 17, 35392 Giessen, Germany
E-mail: Doreen.Mollenhauer@phys.Chemie.uni-giessen.de
[c]
[d]
Prof. Dr. Doreen, Mollenhauer, Sebastian Schwan, Moritz R. Schäfer
Center for Materials Research (LaMa)
Justus-Liebig University Giessen
Heinrich-Buff-Ring 16, 35392 Giessen, Germany
Prof. Dr. S. Chatterjee, M. J. Müller
Institute of Experimental Physics I and Center for Materials Research (LaMa)
Justus Liebig University Giessen
Heinrich-Buff-Ring 17, 35392 Giessen, Germany
E-mail: Sangam.Chatterjee@exp1.physik.uni-giessen.de
[e]
Prof. Dr. S. Sanna, C. Dues
Institut für Theoretische Physik and Center for Materials Research (LaMa)
Justus-Liebig-Universität Gießen,
Heinrich-Buff-Ring 16, 35392 Gießen, Germany
E-mail: simone.sanna@theo.physik.uni-giessen.de
Supporting information for this article is given via a link at the end of the document.
Abstract: We report about the successful extension of the class of
organogtetrel sulfide clusters by further examples of the still rare
silicon-based species, synthesized from RSiCl3 with R = phenyl (Ph,
I), naphthyl (Np, II), and styryl (Sty, III) with Na2S. Besides known
[(PhSi)4S6] (IV), new compounds [(NpSi)4S6] (1) and [(StySi)4S6] (2)
were obtained, the first two of which underwent follow-up reactions
with [AuCl(PPh3)] to form ternary complexes. Density functional
theory (DFT) studies of cluster dimers helped to understand
fundamental differences between the habitus of {Si4S6}-based vs.
{Sn4S6}-based compounds. Investigations of the optical properties of
crystalline compound 1 and intrinsically amorphous 2 indicated the
former to show a pronounced nonlinear optical response, i.e., second-
harmonic generation, while for the latter, the chemical damage
threshold seems to inhibit a corresponding observation that can be
clearly put down to nonlinear optical effects. Calculations within the
independent particle approximation (IPA) served to rationalize and
compare electronic and optical excitations of [(RSi)4S6] clusters (R =
Ph, Np). The calculations reproduced the measured data and allowed
for the interpretation of the main spectroscopic features.
Introduction
The chemical and structural properties of tetrel chalcogenide
clusters with organic substituents have been extensively studied
in the past.[1] In particular, a large variety of compounds has been
reported for the element combinations Sn/E[1a,2] (E = S, Se, Te)
and Ge/E,[1c,3] whereas significantly fewer compounds with Si/E-
based cluster cores have been known so far.[1b,4] Several studies
investigating the reactivity and properties of tin chalcogenide
clusters were undertaken, showing the possible derivatization of
the organic substituents[5,2f] as well as the formation of ternary
inorganic cluster cores by introducing transition metal
complexes.[6]
In addition to the chemical features of such clusters, the styryl-
substituted cluster [(StySn)4S6] (Sty = 4-vinylphenyl), which was
determined via DFT calculations to have a heteroadamantane-
type molecular structure, was recently shown to possess an
extreme nonlinear optical behavior.[7] These findings led to the
investigation of further compounds of the type [(RSn)4S6] with
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cyclic and/or aromatic substituents R to understand how the
substituents influence the compounds’ properties. Generally, it
was shown that distinct order within the solid material (crystallinity
or pronounced π-stacking interactions) leads to second-harmonic
generation (SHG), while a high degree of amorphousness results
in white-light generation (WLG).[8]
diffraction. As expected, the Si atom exhibits a trigonal pyramidal
coordination environment. Furthermore, π-interactions between
the naphthyl substituents are observed in the unit cell, alternating
between parallel-displaced π-stacking and T-shaped CH/π-
interactions (Figure 1, right). The distance between the adjacent
naphthyl rings with π-stacking is slightly larger (3.4397(18) Å)
than the distance between the layers in graphene (3.35 Å),[9]
whereas the distance between the center of one of the naphthyl
rings to the closest H atom of the next perpendicular naphthyl ring
(2.9371(1) Å) is in the typical range of strong CH/π-interactions.[10]
In this context, we intended to find out, whether the principles of
Sn/E chemistry can also be applied to the Si/E elemental
combination, and whether a replacement of Sn atoms with Si
atoms leads to changes in the cluster’s reactivity and properties.
Herein, we present the first results of these studies.
The preparation of corresponding silicon analogs from RSiCl3 (R
= organic substituent) and a corresponding chalcogen source is
much more challenging than the corresponding tin or germanium
chemistry, as the use of the most appropriate reactant, E(SiMe3)2
(E = S, Se, Te), logically lacks the driving force of Si-Cl bond
formation. Here, a Si-Cl bond needs to be cleaved at the same
time in the other reactant. So, the use of binary chalcogen salts
such as A2E (A = alkali metal) is required, which leads to a more
complicated work-up. However, besides the reproduction of
known [(PhSi)4S6] (IV) we herein report on the successful
synthesis of two new organosilicon sulfide compounds,
[(NpSi)4S6] (1) and [(StySi)4S6] (2), their characterization and
follow-up chemistry with gold complexes to form ternary
complexes [{RSi(µ-S)}2{AuPPh3(µ-S)}2] (3: R = Ph, 4: R = Np). We
used density functional theory (DFT) methods to gain insight in
structural features of the adamantane-based compounds, in
particular regarding inter-cluster interactions that contribute to the
macroscopic habitus of the solid. All new compounds were
investigated regarding their linear and nonlinear optical
properties, which were additionally studied by means of quantum
chemistry. The calculations explain the preferred habitus of the
compounds, and reveal the deep impact of the ligands in the
optical response
Figure 1. Molecular structure (left) and crystal structure (right) of II. Ellipsoids
drawn at 50% probability level. H atoms omitted for clarity; π-stacking and CH/
π-interactions are indicated as dashed lines. Selected distances and angles [Å,
°]: Si−Cl 2.0313(7) – 2.0385(7)), Si−C 1.8426(18), Cl−Si−Cl 106.42(3) -
107.58(3), Cl−Si−C 111.42(6) – 112.56(6), CNp···π (red) 3.4397(18), CNp−H···π
(blue) 2.9371(1).
The reaction of II with 1.5 equivalents of Na2S affords a crystalline,
colorless solid. Upon dissolution in toluene and cooling to −25 °C,
single crystals of [(NpSi)4S6]∙0.5C7H8 (1∙0.5C7H8) suitable for X-
ray diffraction were obtained. The molecular structure of 1 is
ꢀ
shown in Figure 2. 1 crystallizes in the triclinic space group P1
with two molecules per asymmetric unit. Like [(PhSi)4S6], 1 is an
adamantane-type Si/S cluster with four organic substituents, each
bound to one Si atom. The molecule shows minor deviations from
Td symmetry, due to slightly different angles in the inorganic core
and different orientations of the organic substituents.
Furthermore, the two individuals differ greatly in the orientation of
the organic substituents (see Figure 2, bottom), indicating a very
low energy barrier for rotation of the organic substituents about
the Sn−C bond, which in most cases leads to intrinsic
amorphousness. Notably, while both [(PhSn)4S6] and [(NpSn)4S6]
are amorphous (with the Np compound comprising a considerable
amount of order according to its optical response, see also
below),[7b] the two Si analogs are crystalline. Initially, this was
ascribed to the smaller radius of the Si/S core with respect to the
Sn/S core only, as the corresponding smaller volume per given
number of cluster molecules causes the aromatic rings of
neighboring clusters to approach more closely overall, and thus
form more efficient π-stacking interactions. In this work, we
present a refined version of this interpretation (see below).
Results and Discussion
Syntheses, crystal structures and spectroscopic data. The
reactions of RSiCl3 with R = phenyl (Ph, I), naphthyl (Np, II), and
styryl (Sty, III) with Na2S discussed herein are summarized in
Scheme 1.
PPh3
R
Au
+ 4 [AuCl(PPh3)]
+ 2 Na2S
Si
S
+ 6 Na2S
- 12 NaCl
S
S
S
S
R
S
4 RSiCl3
2
Si
Si
S
Si
Si
Si
R
R
S
R
S
S
Au
Ph3P
R
R = Ph (I),
R = Ph (IV),
R = Ph (3),
II
1
4
Np ( ),
Np ( ),
Np ( ),
Sty (III)
Sty (2)
Scheme 1. Summary of reactions done to produce compounds IV and 1 - 5.
While synthesizing NpSiCl3 (II) as a precursor for [(NpSi)4S6] (1),
single crystals were obtained upon distillation, and the molecular
structure (Figure 1, left) was determined via single crystal X-ray
2
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10.1002/anie.202011370
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taken place. Hence, the NMR data indicate the formation of a
compound comprising individual adamantane-type clusters
[(StySi)4S6]. Yet we assume that they start to unstoppably form
oligo-/polymers shortly upon synthesis.
Further studies on the reactivity of the adamantane-like
orgnaosiliconsulfide clusters were undertaken. The addition of
Na2S and [AuCl(PPh3)], leads to the fragmentation of the clusters
and the formation of the four-membered rings [{RSi(µ-
S)}2{AuPPh3(µ-S)}2] (3: R = Ph, 4: R = Np). Colorless single
crystals of 3·CH2Cl2 and 4 were obtained by layering the
respective reaction solutions with n-hexane. Their molecular
structures are shown in Figure 3. 3·CH2Cl2 crystallizes in the
monoclinic space group C2/c, and 4 crystallizes in the triclinic
ꢀ
space group P1, both with half a molecule per asymmetric unit.
The molecules in these compounds have an inversion center in
the center of an {RSi(µ-S)}2 ring, with nearly linear {Au(PPh3)(µ-
S)} substituents attached to the Si atoms. Although the structural
parameters of the central unit in both compounds are quite similar,
they differ significantly in the orientation of the {Au(PPh3)(µ−S)}
units: whereas they point away from the central {Si2S2} ring in 3
with a C1−Si1−S2−Au1 cis arrangement, the corresponding
atoms in 4 show a trans arrangement. Thus, while the interatomic
distances in these compounds are similar, the Si1−S2−Au1 and
S2−Au1−P1 bond angles are different. Homologous Sn/S
compounds exhibiting this structural motif as well as a cis or trans
arrangement depending on the organic moiety at the tetrel atom
were described previously, showing the same trends in the
T−S−Au and S−Au−P bond angles.[12]
Figure 2. Molecular structure of 1 (top) and visualization of the two different
orientations of the organic substituents in the two individuals within the
asymmetric unit (bottom); the structure at the top refers to the bottom right
individual. Ellipsoids drawn at 50% probability level. H atoms omitted for clarity.
Selected distances and angles [Å, °]: Si−S 2.111(3) – 2.153(3), Si−C 1.837(8)
– 1.876(8), Si−S−Si 103.42(11) – 105.64(13), S−Si−S 110.31(12) – 113.28(12),
S−Si−C 104.1(3) – 110.1(2).
The reaction of StySiCl3 (III) with 1.5 equivalents of Na2S should
proceed similarly and lead to the formation of [(StySi)4S6] (2).
However, only the crude product could be isolated upon
evaporation of the solvent in form of a colorless “solid” with the
consistency of cotton candy or honey – depending on the duration
of the reaction and drying process. We attribute this to the high
polymerization tendency of the styryl substituents, and the
corresponding oligomerization/polymerization that proceeds
visibly as time goes by. Notably, this was not the case for the Sn
analogs, which we take as a further hint for the correlation of the
nature of the T/E cluster core with the properties of the organic
substituents: in the Sn/S clusters, the styryl substituents of
neighboring cluster units are obviously not close enough to allow
for a chemical reaction of neighboring vinyl groups. As a
consequence of the different situation with an underlying Si/S
cluster core, 2 could neither be obtained as single crystals, nor
could individual clusters be detected by means of liquid injection
field desorption/ionization (LIFDI) or electrospray ionization (ESI)
mass spectrometry. However, we were able to collect 29Si NMR
data of a freshly prepared sample of 2 that turned out to be
sufficiently soluble. A downfield shift of ~9 ppm of the observed
singlet with respect to the precursor signal is in accordance with
the NMR data of other Si/S adamantane-type clusters and their
corresponding organosilicon trichloride precursors reported
previously, such as [(EtSi)4S6]/EtSiCl3, [(PhSi)4S6]/PhSiCl3, or
[(CH2=CHSi)4S6]/CH2=CHSiCl3.[11] Another indication is given by
the 1H and 13C NMR spectra, which clearly show the signals of the
styryl substituent at a significant downfield-shift compared to the
starting compound III, with the ipso- and meta-atoms being the
most affected, which indicates a reaction at the Si atom to have
Figure 3. Molecular structure of 3 (left) and 4 (right) Ellipsoids drawn at 50%
probability level. H atoms and P-bound phenyl groups omitted for clarity.
Selected distances and angles [Å, °]: 3: Si−S 2.091(2) – 2.144(2), S−Au
2.3136(16), Au−P 2.2534(16), Si−C 1.870(7), Si−S−Si 82.40(9), S−Si−S
97.60(9) – 115.54(10), Si−S−Au 90.97(7), S−Au−P 178.03(6). 4: Si−S 2.095(2)
– 2.1589(3), S−Au 2.297(2), Au−P 2.256(2), Si−C 1.865(8), Si−S−Si 82.58(10),
S−Si−S 97.42(10) – 115.64(10), Si−S−Au 103.57(9), S−Au−P 173.35(7).
Upon fragmentation of the adamantane-type cluster and
formation of 4, a further downfield shift by 5.6 ppm was observed
in the 29Si NMR spectrum. Table 1 summarizes the respective
NMR data for compounds I – IV, 1, 2, and 4. Note that NMR data
of compound 3 were not obtained owing to very poor solubility of
the crystals.
3
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Table 1. 29Si NMR shifts of compounds I – IV, 1, 2, and 4 reported in this
work..
Compound
I
II
III
IV
1
2
4
δ (29Si) /ppm
−0.9
−0.4
−0.9
10.9
6.8
8.7
12.4
Quantum chemical investigation of structural features. It was
experimentally shown, that [(PhSi)4S6] (IV) and [(NpSi)4S6] (1) are
observed in a crystalline phase, while [(PhSn)4S6] and [(NpSn)4S6]
are intrinsically amorphous. Nevertheless, is was suggested that
[(NpSn)4S6] features a structure with a higher amount of order
compared to [(PhSn)4S6], owing to more efficient π-stacking
interactions between the Np substituents as compared to the Ph
substituents.[7] This assumption was based on the different
nonlinear optical responses of the two compounds, which showed
SHG (requiring phase matching) for the Np compound but not for
the Ph analog, for which WLG was reported.
Figure 4. Scan of the phenyl group (top) and naphthyl group (bottom) rotations
by variation of the dihedral angle for corresponding clusters [(PhSi)4S6] and
[(NpSi)4S6] calculated at the BP86-D3/cc-pVDZ(-PP) level of theory. Structures
at the minimum and maximum positions of the scan are shown on the hand right
side. The corresponding scans for the Sn/S core cluster analoges are given in
Figure S12.
To explain the general difference observed in the [(RSn)4S6] and
[(RSi)4S6] cluster compounds with R = Ph, Np, one may consider
the smaller radius of the {Si4S6} core with respect to the {Sn4S6}
core to enable a stronger interaction of the neighboring clusters
through more efficient π-stacking interactions. To investigate this
hypothesis, quantum chemical calculations of the interaction in
cluster dimers as a minimal model were carried out, which already
provide valuable insights into the cluster interaction that can be
transferred to the extended crystalline or amorphous material.
In order to classify the flexibility of the substituents, we initially
calculated the rotation barriers of the substituents for both cluster
cores. Rotation barriers of the substituent at the single cluster are
hardly present (<1 kJ/mol) for the clusters with phenyl
substituents, while for the clusters with naphthyl substituents, a
barrier height of 17 kJ/mol is obtained for the [(NpSi)4S6] cluster
and 9 kJ/mol for the [(NpSn)4S6] cluster (see Figure 4 and Figure
S12). The larger rotation barrier for the cluster with an {Si4S6} core
as compared to the cluster with an {Sn4S6} core can indeed be
explained by the smaller core radius. The smaller Si−C distance
in this cluster (1.889 Å) in comparison with the corresponding
Sn−C distance (2.088 Å) increases the barrier by a stronger
interaction between the hydrogen atom of the substituent and the
sulfur atoms of the cluster. The relative positions of S and H atoms
at the rotation barrier are visualized in Figure 5.
Figure 5. Comparison of the S-H distances at the energetic maximum of the
rotational scan for [(NpSi)4S6] and [(NpSn)4S6] calculated at the BP86-D3/cc-
pVDZ(-PP) level of theory.
The investigation of the cluster dimer structures was performed to
analyze the interaction between the molecules with different
cluster cores and substituents. The cluster dimer structures were
determined by two computational approaches and their results
were combined to obtain a larger number of possible conformers.
The dissociation energies of all cluster dimers are plotted against
their core−core distance in Figure 6, and selected cluster dimer
structures are plotted in Figure 7.
Furthermore, the substituent−substituent interaction can increase
the rotation barrier, however this is of minor importance for the
considered conformers. In summary, the calculations indicate a
high orientational flexibility for phenyl substituents on both Si/S
and Sn/S clusters, as opposed to a comparatively lower freedom
of orientation for naphthyl substituents. The latter show a higher
orientational flexibility for the cluster with larger core radius (Sn/S)
than for those with smaller core radius (Si/S), which is in
agreement with the tendency for (a) higher order in compounds
with R = Np than in compounds with R = Ph, and (b) higher order
in {Si4S6}-based clusters than in {Sn4S6}-based clusters.
4
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structures (purple region in Figure S13). Since we do not consider
them to be relevant for the solid-state structures of this study, we
have not considered the fused clusters further.
Figure 6. Dissociation energy of cluster dimers, plotted against the
corresponding core−core distance calculated at the BP86-D3/cc-pVDZ(-PP)
level of theory. Medium core−core distances are indicated by an orange
background, large core-core distances by a green background. The region of
small core−core distances is plotted in Figure S13.
First, we discuss the obtained cluster dimers in regard of their
core−core distances and geometric structures. Most of the cluster
dimers have a distance of the two adamantane cores between 5.9
and 6.5 Å (orange region in Figure 6). The [(PhSi)4S6] cluster
dimers show a core−core distance range that is shifted to slightly
smaller values than the range calculated for [(NpSi)4S6]. The
same trend, but even more pronounced, is observed for the
{Sn4S6}-based cluster dimers. Cluster dimers in this region of
inter-cluster distances yielded stacked or alternating substituents
and different orientations of the cluster cores (Figure 7). In all of
them, a minimum of two substituents of one cluster interact with
at least two substituents of the other cluster. In the “stacked
dimer” conformers, the substituents are arranged directly towards
each other. In the “alternating dimer” conformers, the substituents
of one cluster are located in a void between the substituents of
the other cluster. Interestingly, the “alternating dimer” structure is
slightly preferred on average for clusters with phenyl substituents,
while for naphthyl substituents the “stacked dimer” structure is
slightly preferred. The core−core distances between Si/S and
Sn/S clusters, respectively, differ only slightly for analogous
cluster dimers (by ~0.15 Å on average). The “alternating dimers”
show a high similarity to the arrangement of the cluster in the
crystal structures, as demonstrated for [(PhSi)4S6] in Figure 8.
Although similar structural units are present, the core−core
distance in the crystal structure (closest core−core distances:
7.05 – 7.46 Å in IV, 7.44 – 7.58 Å in 1) is much larger (by 0.5 to
1.0 Å) as compared to the calculated cluster dimers, as in the
crystal, the interaction is shared between more than two
clusters.Besides the majority of cluster dimers with a core−core
distance between 6.0 and 6.5 Å, there are a few cluster dimers
with core−core distances between 7.2 and 8.0 Å, with a poor
relative orientation of the clusters to each other (green region in
Figure 6; example of a structure shown in Figure 7, top). The Si/S
cluster dimers with phenyl and naphthyl substituents listed in this
green region show larger core−core distances than the
corresponding crystal structures. On the other hand, for some of
the Sn/S-based cluster dimers, we found a fusion of cluster
Figure 7. Examples for cluster dimers emerging from the calculations, shown
for dimers of [(PhSi)4S6] calculated at the BP86-D3/cc-pVDZ(-PP) level of
theory. Top: cluster dimer with a large core−core distance. Centre: cluster dimer
with medium core−core distance and alternating substituents (two views).
Bottom: cluster dimer with medium core−core distance and stacked substituents
(two views).
Figure 8. Comparison of the relative orientation of [(PhSi)4S6], as an example
of the generated dimer structures (green, core−core distance 6.37 Å) calculated
at the BP86-D3/cc-pVDZ(-PP) level of theory, and a dimer extracted from the
crystal structure of [(PhSi)4S6][7a] (blue, core−core distance 7.05 Å), indicating
the good agreement of the model with the experimental data.
Second, we discuss the obtained cluster dimers depending on
their dissociation energy. Considering each individual cluster
composition, the majority of dimer clusters with core−core
distances between 5.9 and 6.5
Å yield higher absolute
dissociation energies as compared to the absolute dissociation
energies of the cluster dimers with core−core distances between
7.2 and 8.0 Å. The difference in dissociation energies between
the medium and large core−core cluster dimers is smaller for
clusters with Np ligands than for those with Ph ligands.
Considering the cluster dimers with core−core distances between
5.9 and 6.5 Å, there are 2 - 3 regions of dissociation energies for
the individual cluster compositions in Figure 6, each of which
correlates with a certain number of substituents interacting with
5
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each other. Thus, the highest dissociation energies were obtained
for cluster dimers in which three substituents of one cluster
interact with three substituents of the other one. The “stacked
dimer” and “alternating dimer” conformers for individual cluster
compositions yield similar dissociation energies. Among the
cluster dimers with highest dissociation energies, [(NpSi)4S6]
exhibits a dissociation energy that is by about 65 kJ/mol higher
than that of [(PhSi)4S6]. A similar, but slightly smaller, difference
in dissociation energies is calculated for the corresponding
{Sn4S6}-based cluster dimers (~53 kJ/mol). Obviously, the larger
Np substituents lead to a stronger interaction between cluster
dimers than Ph substituents. Other than originally anticipated, the
investigation of the cluster dimers did not indicate larger
dispersive interaction for clusters comprising a smaller radius of
the cluster core. The {Sn4S6}-based clusters have a higher
absolute dissociation energy than the {Si4S6}-based cluster
dimers, which illustrates a stronger interaction overall. To
understand this result, we performed a decomposition analysis of
the cluster dimer binding energy contributions into the
substituent−substituent interaction, the substituent−core and
core−core interaction (see Figure 9).
In conclusion, we propose that the predominance of the relatively
isotropic core−core interactions in [(PhSn)4S6] outplay the rather
directional interactions involving the substituents, which explains
why this compound shows a distinctly lower tendency for order in
the solid than the crystalline {Si4S6}-based homologue. This trend
is also visible for the clusters with Np substituent, yet for
[(NpSn)4S6] the relatively similar strengths of core−core and
substituent−substituent interactions allow for a higher degree of
intermolecular order, which is in agreement with the (virtually
contradicting) experimental findings: the powder produces SHG
(as a sign of phase matching) in spite of an apparently amorphous
nature according to X-ray diffraction. The results of this study thus
contribute significantly to the overall understanding of the origin of
(dis)order in compounds of the type [(RT)4E6] (T = Si, Ge, Sn; E =
S, Se, Te).
Optical properties. We explored the optical response of
compounds 1 − 4 by optical spectroscopy. The absorption
and emission spectra are summarized in Figure 10.
The decomposition of the binding energy reveals similar
substituent−substituent binding energies for cluster dimers with
the same substituent. The larger naphthyl substituents show a
higher binding energy than the smaller phenyl substituents due to
larger dispersive interactions. In contrast, the core−core binding
energy is larger for the Sn/S-based cluster dimers than for the
Si/S based cluster dimers due to the larger adamantane core.
This stronger core−core interaction for the larger Sn/S-based
cluster dimers leads to higher absolute values of the dissociation
energies than observed for the Si/S-based analogs. For the
cluster dimers with larger core−core distances (green region in
Figure 6), the same trend in dissociation energies is observed,
although larger core−core distances are found here than for
crystalline [(PhSi)4S6] and [(NpSi)4S6]. We therefore believe that
these insights can be transferred to crystalline and amorphous
cluster materials with core−core distances between the medium
and large core−core distance regions of Figure 6.
Figure 10. Linear absorption spectra in solution (yellow line) and
photoluminescence spectra (solid: blue line, solution: gray line) of compounds
(1 − 4)
Clearly, the emission and absorption data are mirror-image-like
for the solution data. The emission spectra of the solid phase
show an additional resonance about 500 meV below the emission
maximum in solution. Such features are commonly attributed to
excitons or an ensemble thereof. Notably, the higher-energy
emission maximum persists in the condensed matter phase for all
samples. However, it is significantly quenched by the existence of
Au/S moieties in compounds 3 and 4 in contrast to 1 and 2). This
apparently enhances the lower-energy emission channels. In
particular, for compound 4, several distinct emission channels are
observed which are tentatively attributed to transitions involving
atomic orbitals of the Au(I) atom.
The crystalline compound 1 shows a clear nonlinear optical
response. In agreement with the observation made for IV,[7b] we
observe SHG here in spite of the centrosymmetric space group,
which therefore is attributed to surface effects or defects of the
crystal (see Figure 11). The other compounds, in general, do not
show a nonlinear response that is clearly distinguishable from
luminescence effects owing to chemical transformations. Hence,
Figure 9. Decomposition of the binding energy contributions of cluster dimers
into substituent-substituent interaction, the substituent−core and core−core
interaction calculated at the BP86-D3/cc-pVDZ(-PP) level of theory.
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we can neither confirm nor exclude that the adamantane-based
compound 2 exhibits WLG, but its significantly higher chemical
sensitivity to oxygen and humidity as compared to the heavier
homologue hampers a clear statement to date (note that it is
technically not possible to prevent that the samples are exposed
to air for a very short moment prior to the measurement).
nearly canceled out by the electron-hole attraction. Thus, as
many-body effects due to the electron−electron and the
electron−hole interaction counterbalance, the IPA-calculated
optical excitation spectra are expected to yield a reasonable
description of the measured optical response, at least for the low-
energy excitations.
Figure 11. Nonlinear optical response of compound 1. Excitation power
dependency of SHG. SHG spectrum from excitation at 800 nm (inset).
Calculation of optical properties. We investigated the role of
the substituents (phenyl vs. naphthyl) on the optical response of
the molecular clusters. Thereby, we modeled two of the
synthesized clusters with the same core but different substituents,
[(PhSi)4S6] and [(NpSi)4S6]. The structural relaxation of single
clusters and molecular crystals performed by the plane-wave
implementation of the DFT within the periodic supercell method
leads to geometries of the same symmetry and in overall close
agreement with the structures calculated with a localized basis.
The optimized geometries were employed for the calculation of
the electronic and optical excitations.
Figure 12. Optimized molecular structures and orbital characters of the HOMO
and LUMO states of [(PhSi)4S6] (left) and [(NpSi)4S6] (right) single clusters.
The extinction coefficient of [(PhSi)4S6] (blue line) and [(NpSi)4S6]
(black line) single clusters is shown in Figure 13. The components
of the dielectric tensor are averaged to allow for a direct
comparison with experimental data later in the manuscript. The
general shapes of the dielectric functions are rather different,
although the structures of the two clusters are similar. In order to
rationalize these differences and understand their origin, we
analyzed the electronic transitions of the two clusters. Indeed,
spectral resonances can be directly related to electronic
transitions between occupied and empty states at the IPA level.
Figure 12 shows the relaxed geometry as well as the HOMO and
LUMO states for the [(PhSi)4S6] and [(NpSi)4S6] clusters. The
structural differences between the cluster cores of the two
molecules are limited. The Si−S distance amounts to 2.15 Å in
both clusters, while the S−C bond length is 1.87 Å in [(PhSi)4S6]
and 1.88 Å in [(NpSi)4S6]. However, the different substituents
have a deeper impact on the molecular electronic structure. The
(degenerate) HOMO of the [(PhSi)4S6] molecule is localized at the
S atoms in the cluster core, while the HOMO of the [(NpSi)4S6]
cluster is localized on the naphthyl rings. The LUMO of both
systems is similar and localized on the substituents (see Figure
12). Thus, the DFT-calculated HOMO-LUMO gap of the two
systems is rather different and amounts to 3.76 eV for [(PhSi)4S6]
and 2.96 eV for the [(NpSi)4S6] cluster. Quasiparticle effects,
calculated in an approximate manner by the ΔSCF method as
described in Ref. [13], open up the fundamental independent-
particle-approximation (IPA) gap to a value of 6.54 eV for
[(PhSi)4S6] and 5.09 eV for the [(NpSi)4S6] cluster (see Figure S15
in the Supporting Information). The energy of the lowest excitonic
excitation, describing the transition of one electron to the LUMO
leaving a hole behind in the HOMO, is calculated following the
procedure described in Ref. [13]. Excitonic excitations of 3.92 eV
for [(PhSi)4S6] and 2.89 eV for the [(NpSi)4S6] cluster,
respectively, are predicted. These energies are rather close to the
HOMO-LUMO gap, suggesting that the quasiparticle shifts are
Figure 13. Extinction coefficients calculated in the independent-particle
approximation for isolated [(PhSi)4S6] (blue line) and [(NpSi)4S6] (black line)
clusters.
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The onset of the optical absorption corresponds exactly to the
DFT-calculated HOMO-LUMO transition in the case of the
[(NpSi)4S6] cluster (black line). Structures in the first peak match
quite well the energy of the HOMO-LUMO+1,2 (degenerate) and
HOMO-LUMO+3. The main peak at about 4 eV is mainly due to
transitions from the HOMO to a group of energetically close states
localized at the C atoms of the naphthyl rings. Instead, in the case
of the [(PhSi)4S6] cluster (blue line), the onset of the optical
absorption is located at much higher energies than the DFT-
calculated HOMO-LUMO difference. Indeed, the HOMO-LUMO
transition probability is small, due to the different spatial
localization and orbital character of the two states. We assign the
first spectral peak to transitions from the HOMO to three
molecular orbitals about 4.4 eV above the HOMO and with a
spatial extension partially overlapping the HOMO. Summarizing,
the adsorption spectra of [(PhSi)4S6] and [(NpSi)4S6] clusters are
rather different concerning the positions and line shapes of the
spectral features. The deviations in the optical response origin
from the qualitatively different and differently localized electronic
states involved in the transitions resulting in the main spectral
peaks of the two compounds.
The optical answer of the isolated clusters represents the basis
for understanding the absorption spectra of the solid material
comprising the molecules. According to the calculations, which
started from the experimental crystal data, the optimized
geometries show only small modifications of the geometrical
parameters determined for the isolated molecules. The largest
bond length deviation is 1.4 pm, while the bond angles of the
substituents are rotated by 13.2° with respect to the single
molecules due to the presence of toluene. When the single
clusters aggregate to form the crystalline material, the discrete
energy levels broaden to become energy bands. These have
been calculated in the IPA approximations along the directions in
reciprocal space shown in Figure S16, and are illustrated in Figure
14. The electronic band gap of the crystals is slightly smaller than
the HOMO-LUMO energy differences of the corresponding
molecules; however, the orbital character of valence and
conduction edges closely resembles that of the HOMO and LUMO
of the parent molecules (see Figure 12). As the band dispersion
is rather flat and no major rearrangement is expected upon
inclusion of quasiparticle effects, self-energy correction mainly
serves to widen the band gap, and may be replaced by a
numerically less costly scissors-shift for the calculation of the
optical response.
Figure 14. Electronic structure of the crystals comprising [(PhSi)4S6] and
[(NpSi)4S6] molecules calculated within the IPA. The fundamental band gap is
indicated by yellow color.
We now proceeded with the optical characterization of the
synthesized crystalline compounds. Owing to the synthesis
method, the experimental crystal structure of [(NpSi)4S6] (1)
comprises toluene molecules (0.5 C7H8 per formula unit of 1, see
Table S1). As illustrated in Figure S17, the extinction coefficient
calculated with and without toluene does not substantially differ,
demonstrating that the optical response of the molecular cluster
is not affected by the presence of solvent. The extinction
coefficient calculated within the IPA for the [(PhSi)4S6] (IV) derived
and [(NpSi)4S6] (1) derived molecular crystals is shown in Figure
15.
Figure 15. Extinction coefficient of the [(PhSi)4S6] derived (blue line) and
[(NpSi)4S6] derived (black line) molecular crystals calculated within the IPA at
the DFT equilibrium geometry.
For both compounds, the spectral signatures of the isolated
clusters are clearly recognizable in the optical answer of the
corresponding molecular crystals. They barely shift in energy, and
the relative intensity compares well with that predicted for the
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single clusters. It can thus be concluded that the transitions
leading to the absorption peaks of the single clusters are chiefly
responsible for the major peaks in the dielectric function of the
semiconducting crystals. The main difference between molecules
and crystals comprising these molecules is that the crystal
absorption features are broadened with respect to the molecular
peaks due to the energy dispersion of the molecular states upon
aggregation. As the onset of the molecular and crystalline
absorption coincides, we can exclude new intermolecular low-
energy electronic transitions to occur when the clusters aggregate
to form crystals. Thus, for the investigated compounds, the
molecules seem to individually interact with light rather than
jointly. However, the relative orientation of the molecules in the
solids (ordered vs. non-ordered) controls the nature of the
nonlinear optical response (with or without phase matching).
attraction by means of the Bethe-Salpeter equation might further
improve the agreement with the experiment.
The calculation of the second-order polarizability tensor for the
crystalline phases of [(PhSi)4S6] (IV) and [(NpSi)4S6] (1) shows
vanishing optical nonlinearities for all tensor components, as
expected for a centrosymmetric crystal structure. Deviations from
this behavior in experiments thus arise from structural defects and
amorphous regions in the samples, as well as from surface-
related contributions. At least for centrosymmetric crystals, they
are not inherent bulk properties.
Conclusions
We presented the synthesis of new members of the rare class of
organosilicon sulfide clusters of the general formula [(RSi)4S6].
We added new members with R = naphthyl (Np, 1) and styryl (Sty,
2), which were investigated with respect to their reactivity towards
[AuCl(PPh3)], their structures, and their optical properties by a
combination of comprehensive experimental and theoretical
studies. The investigations contributed to characterize and further
understand the chemical behavior of such clusters, which showed
to behave differently from the much better known germanium and
tin homologues. Our studies indicate a much more pronounced
tendency to polymerization in case of 2 than observed for its Sn
homologue, and confirm the expected, much higher air- and water
sensibility of the Si compounds. Second, the observation that
clusters with the combination Ph/Si/S (reported previously) and
Np/Si/S (1) are crystalline, while the Sn homologues are
amorphous was studied on the bases of cluster dimer models
calculated with DFT methods. Different types of intra-cluster
interactions were found, that led to different dissociation energies
as a function of the nature of the substituents, their relative
orientation, and the core compositions {Si4S6} versus {Sn4S6}. The
detailed study ultimately helped to explain the experimental
findings on the basis of much stronger isotropic interactions in the
{Sn4S6}-based cases in comparison with directional interactions
that play a much more notable role in the {Si4S6}-based materials.
Regarding the optical properties of the new compounds and the
products of their reaction with the gold complex, we could show
that the emission is quenched upon fragmentation and formation
of the gold compounds. Moreover, the crystalline compound 1
was proved to exhibit SHG as a consequence of high order that
allows for phase matching; however the SHG signal does not
stem from the crystalline bulk but should be attributed to surface
phenomena or structural defects. The behavior of 2 is clearly
different, but its chemical nature inhibited a statement regarding
the compound’s nonlinear response. The experimental findings
were corroborated by static and time-dependent DFT calculations
of both the molecules and the crystalline material, which are in full
agreement with the measurements and therefore indicate the
applicability of the used methods and models.
In order to compare the calculated spectra of the crystalline
material comprising [(NpSi)4S6] cluster (1) with the measured
data, we accounted for many-body effects in an approximated
manner by means of a scissors-shift.[14] We chose a scissor-shift
of 0.7 eV, correcting the DFT underestimation of the
experimentally measured bandgap of the molecular crystals. The
corresponding results are shown by the blue line Figure 16
together with experimental results (yellow line) from Figure 10.
The comparison with the experimental data shows that
calculations at the IPA level with scissors-shifts already agree with
the measurements.
Figure 16. Extinction coefficient of the crystals comprising [(NpSi)4S6]
molecules, calculated within the IPA with scissor-shifts of 0.7 eV (blue line). The
measured curve (yellow line) as well as the IPA calculation without scissors-
shift (gray line) are shown for comparison.
The calculations fairly describe the main adsorption peak,
originating from transitions between the valence band edge and
bands localized at the C atoms of the naphthyl rings, including the
shoulder observed at about 4.6 eV. However, the intensity of the
peak at about 3.5 eV corresponding to the transition from the
valence band maximum to the conduction band minimum is
overestimated in the calculations in comparison with the
measured spectra. More refined and computationally demanding
Experimental Section
All details regarding experimental and theoretical methods are
provided in the Supporting Information.
approaches taking in account quasiparticle effects in
perturbative manner and accounting for the electron-hole
a
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Schwedtmann, A. Hepp, K. Schwedtmann, J. J. Weigand, F. Lips, Eur.
J. Inorg. Chem. 2019, 4719–4726; h) D. Reiter, P. Frisch, D. Wendel, F.
M. Hörmann, S. Inoue, Dalton Trans. 2020, 49, 7060–7068.
Acknowledgements
This
work
was
supported
by
the
Deutsche
Forschungsgemeinschaft within the framework of GRK 1782 and
FOR 2824. S.C. acknowledges personal support through the
Heisenberg Programme under contracts (CH660/02 and
CH660/08). The Höchstleistungrechenzentrum Stuttgart (HLRS)
is gratefully acknowledged for grants of high-performance
computer time. The authors acknowledge the computational
resources provided by the HPC Core Facility and the HRZ of the
Justus-Liebig-Universität Gießen.
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Keywords: Organosilicon chalcogenide clusters • Gold • X-ray
diffraction • DFT calculations • Nonlinear optical properties
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Entry for the Table of Contents
With the aim of gaining further insight in the nonlinear optical properties and reactivity of organotetrel chalcogenide clusters, we
studied the yet widely unexplored silicon homologues, [(RSi)4S6] (R = Ph, Np, Sty), and products upon reaction with [AuCl(PPh3)],
[{RSi(µ-S)}2{AuPPh3(µ-S)}2]. In-depth quantum chemical investigations of cluster dimers allowed to understand differences in the
habitus of silicon and tin compounds, and DFT calculations of crystalline models confirmed the applicability of these methods in
comparison with the experiments.
Institute and/or researcher Twitter usernames: @DehnenStefanie
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