Organotetrel Chalcogenide Clusters: Between Strong Second-Harmonic and White-Light Continuum Generation

Article abstract of DOI:10.1021/jacs.6b10738

Highly directional white-light generation was recently reported for the organotin sulfide cluster [(StySn)₄S₆] (Sty = p-styryl). This effect was tentatively attributed to the amorphous nature of the material in combination with the s

Full text of DOI:10.1021/jacs.6b10738

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Communication
Organotetrel Chalcogenide Clusters: Between Strong
Second-Harmonic and White-Light Continuum Generation
Nils W. Rosemann, Jens P. Eußner, Eike Dornsiepen, Sangam Chatterjee, and Stefanie Dehnen
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b10738 • Publication Date (Web): 29 Nov 2016
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Organotetrel Chalcogenide Clusters: Between Strong Second-
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‡§
†
†
‡§
Nils W. Rosemann, Jens P. Eußner, Eike Dornsiepen, Sangam Chatterjee, and Stefanie
Dehnen, *
†
†
Fachbereich Chemie und Wissenschaftliches Zentrum für Materialwissenschaften (WZMW), Philipps-Universität
Marburg, Hans-Meerwein-Straße, DE-35043 Marburg, Germany
‡
Fachbereich Physik und Wissenschaftliches Zentrum für Materialwissenschaften (WZMW), Philipps-Universität
Marburg, Renthof 5, DE-35032 Marburg, Germany
§
Institute of Experimental Physics I, Justus-Liebig-University Giessen, DE-353992 Giessen, Germany
Supporting Information Placeholder
damental incident light to introduce a nonlinear
polarization in the crystal. For phase-matching con-
ditions, i.e., when the refractive index of fundamen-
tal and second-harmonic light are identical, both can
exit the nonlinear medium and may later be separat-
ABSTRACT: Highly directional white-light generation
was recently reported for the organotin sulfide clus-
ter [(StySn) S ] (Sty = para-styryl). This effect was
tentatively attributed to the amorphous nature of the
material in combination with the specific combina-
tion of an inversion symmetry-free T/E cluster core
4
6
3b
ed. Currently, the search for advanced light conver-
sion media is aiming at increased conversion effi-
(T = tetrel, E = chalcogen) with the attachment of
4
ciencies and extend accessible frequency regimes.
ligands that allow for π-delocalization of the electron
density. Systematic variation of T and the organic
ligand (R) that runs from T = Si through Ge to Sn
and from R = methyl through phenyl and para-styryl
to 1-naphthyl allows for a more comprehensive view.
According to powder X-ray data, only [(PhSi) S ] is
Another class of highly nonlinear light-converting
materials enhance effects, such as self-phase modu-
lation. These are used in extreme nonlinear process-
es such as the generation of highly brilliant, i.e., di-
rectional broadband supercontinua. However, this
process commonly requires intense, short-pulsed
laser sources, confining their use to mostly scientific
4
6
single-crystalline among the named combinations.
Here, we demonstrate the fine-tuneability of the
nonlinear response, i.e., changing from white-light
generation to second-harmonic generation (SHG) as
well as controlling of the white-light properties. The-
se are investigated as function of the parameters T, π-
delocalization of the electron density within R, and
the orderliness within the molecular solids.
5
applications.
In the course of our current research on tetrel chal-
cogenide hybrid compounds, clusters and functional
6
networks, we recently showed that the organotin
sulfide cluster compound [(StySn) S ] (Sty = para-
4
6
styryl, 4-H C=CH–C H ) enables spectrally broad-
s
6
4
band and highly brilliant white-light. The observed
vast optical nonlinearity even allows this process for
low-fluency steady-state illumination generated
from cost-effective near-infrared continuous-wave
laser diodes. Tentatively, the white-light generation
was identified as emission from coherently accelerat-
ed electrons in the delocalized π-electron system of
the organic ligands, combined with a strong transi-
tion dipole that is provided by the tetrel chalco-
Light-converting materials are continuously devel-
oped for a multitude of applications including ad-
vanced phosphors for bright white-light emission in
1
light-emitting diodes. These are nowadays omni-
present in diffuse illumination and display applica-
tions. Other light-converting materials target non-
linear optical processes such as second-harmonic-
7
2
genide cluster-core, and a lack of phase-matching.
generation (SHG). Here, typically coherent laser
radiation is up-converted into light of half the wave-
length of the incident radiation. This is commonly
achieved in crystalline materials lacking inversion
For a more comprehensive view of the nonlinear
optical response, we studied the effect of slight varia-
tions of the system. For this, we prepared two series
3a
symmetry. Microscopically, this enables the fun-
of compounds of the general formula [(RT) S ] (R =
4 6
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organic ligand; T = Si, Ge, Sn). All of these com- crystallographic symmetry is reduced by different
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pounds were expected to exhibit (very) strong non-
linear optical properties, as all of them have similar
compositions and structures as the proof-of-
principle compound, included in this study as com-
pound 3 for comparison.
orientations of the organic groups and by slight de-
formations of the inorganic core.
One series addressed the effect of the ligands on
[
=
(
(RSn) S ] scaffolds. The ligands were varied from R
4 6
8a
Me (1) through R = Np (1-naphthyl, 2) and Sty
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3), to R = Ph (4) for exploration of the impact of
the presence and the nature of the π-electron system.
For this, it was either completely absent (ꢀ Me) or
decreased in size (Np ꢀ Sty ꢀ Ph). Changing the
ligand particularly changes the intermolecular inter-
actions including the tendency of the solid material
to exhibit no, short-range or long-range order, which
determines its amorphousness versus crystallinity.
The second series addressed the composition of the
inorganic cluster core. This affects the optical gap of
the clusters, starting out from the Sn/S combination
Figure 1. Fragment of the crystal structure (left; wire
representation, shown without H atoms) and molecular
structure (right; ellipsoids drawn at the 70% probability
level) of [(PhSi) S ] (6) according to X-ray diffraction
4
6
studies. Selected structural parameters [Å, °]: Si–S:
.1184(5)–2.1476(5), C–Si: 1.854(1)–1.855(1),ꢀ Si–S–Si:
2
102.86(2)–104.25(2), S–Si–S: 111.29(2)–113.27(2), C–Si–S:
103.85(5)–108.50(5).
9
in 4 through Ge/S (5) to Si/S (6). Overall, the two
Next, we investigated the nonlinear optical proper-
ties of the compounds. The samples were kept under
high-vacuum condition during laser irradiation. For
excitation, a continuous wave diode laser operating
at a central wavelength of 980 nm was used. The
optical power of 200 mW was focused on the sample
in a confocal setup featuring a reflective microscope
objective to avoid any chromatic aberrations (see
Supporting Information). This setup yielded a spot
size of less than 10 µm in diameter on the sample. For
sample series firstly reveal if and how the T/E/R
combinations affect its orderliness or potential crys-
tallization, and secondly, how this affects the nonlin-
ear optical properties.
All compounds were obtained upon reactions of the
respective organotetrel halides RTCl with sodium
3
sulfide in an acetone/water solvent mixture or tetra-
hydrofurane at room temperature with the for-
mation of NaCl being the driving force of the cluster
formation. Except for compound 6, the compounds
were obtained as amorphous powders under a large
variety of reaction and crystallization conditions
detection,
a
Czerny-Turner-type spectrometer
equipped with a thermoelectrically cooled back-
illuminated deep-depletion Si-charge-coupled device
camera was used. Residual scattered pump-laser was
attenuated using a heat absorbing glass filter (Schott
KG3).
2
tested until now. Hence, the molecular structures of
the latter were elucidated via density functional the-
ory (DFT) calculations (see Supporting Infor-
mation). According to these, all compounds are
based on hetero-adamantane type [T S ] scaffolds,
Characteristic spectra for the first sample series
(compounds 1-4) are given in Fig. 2. Clearly, the na-
ture and the extent of the π-electron systems crucial-
ly influence the optical nonlinearities. Compounds 3
and 4 show pronounced white-light emission, while
compounds 1 and 2 show intense SHG. The latter
requires phase-matching (which is no precondition
for white-light emission). Hence, the optical experi-
ments infer a certain degree of order in the powders
obtained for 1 and 2. In 1, both the low steric demand
and the low flexibility of the ligand seems to allow
for order – that can even lead to the formation of
4
6
which are energetically favored over “double-
decker”-type isomers by 19.2–36.4 kJ/mol (see Table 1
and Supporting Information). The organic substitu-
ents point away from the center of the inorganic
cluster core in pseudo-tetrahedral fashion. As an
exception, compound 6 could be obtained as single-
crystals. Thus, the structure of this compound was
determined by single crystal X-ray diffraction, there-
8c
by confirming the previous prediction for it. The
structure was solved and refined in the monoclinic
space group P2 /c with Z = 4 (see Figure 1 and Sup-
1
8a
single-crystals – although it is not visible in the X-
porting Information). In agreement with DFT calcu-
lations, the molecular structure of 6 is based on an
inversion-free hetero-adamantane type inorganic
ray diffraction diagrams of the powder studied herein
(Figure S3). In the case of 2, the pronounced π-π-
stacking capability of the Np ligands apparently
causes a relatively high degree of order in the, albeit,
scaffold with (idealized) T symmetry. The actual
d
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comparatively small-sized electronic system, exhibits
amorphous powder. Hence, both ligands, although
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being significantly different in size and electronic
nature, provoke a similar non-linear response.
SHG, as the Me groups are in close proximity to the
inorganic core.
The situation changes upon attachment of ligands
with a medium-size extension of the π-electron sys-
tem. Obviously, an effective intermolecular interac-
tion does not take place between the cluster mole-
cules in 3 (R = Sty), which we put down on both a
reduced π-stacking capability, when compared with
Np ligands, and a higher conformational flexibility of
the Sty ligands as compared to all other ligands in-
volved herein. The smallest π-electron system in this
series is present in the Ph groups in compound 4.
Like the Sty ligands in 3, they hamper any intermo-
lecular order when being attached to an [Sn S ] core.
The second group of samples comprises compounds
4, 5, and 6. Here, the inorganic cluster core is varied,
while the Ph ligands, which proved as most suitable
for white-light generation in the first series, remain
unchanged. In striking difference to the other two
Ph-decorated compounds, 6 is obtained in clear and
sizeable single crystals (Figure S5). This difference in
appearance is directly reflected on the measured
spectra shown in Figure 3. Here, exclusively SHG is
observed for the crystalline compound, while the
other two exhibit broadband white-light emission –
in perfect agreement with the assumption given
above.
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Hence, phase-matching is ruled out and the nonlin-
ear optical response changes from SHG to white-light
generation.
The subtle, yet crucial role of microscopic molecular
order is further underlined by the slightly reduced
white-light generation in 3 compared to 4. Here, the
delocalization of the π-electron systems is virtually
identical. Nevertheless, the Sty ligands in 3 provide
an increased potential for dispersive interactions
between neighboring molecules, thus a potentially
higher degree of order in the material, however
without reaching long-range order or crystallinity.
Thus, the strength of the nonlinear response might,
in turn, be used as a measure for the degree of in-
termolecular order in a macroscopically amorphous
compound.
Figure 3. Spectra measured for an excitation wavelength
of 980 nm. The amorphous compounds 4 (blue) and 5
(light blue, dashed) exhibit strong white-light emis-
sions. The crystalline compound 6 (dark blue) exhibits
strong SHG.
The fact that Ph ligands, which perfectly exclude
crystallization in the case of compounds 4 and 5, do
allow for the longer-range order in a crystal lattice in
6, can be put down on the different relative sizes of
the three components, T, E, and R. Obviously, four
Ph ligands match well the steric demands of the
[
Si S ] scaffold to form a stable crystal structure with
4 6
interdigitating Ph groups that, however, do not fea-
ture any typical π-stacking orientations to each oth-
er. In contrast, a suitable crystal lattice seems to be
lacking in the T/E/R = Sn/S/Ph or Ge/S/Ph combina-
tion in 4 or 5, respectively.
Figure 2. Emission spectra measured for an excitation
wavelength of 980 nm. Compounds 1 (black) and 2
(
green) exhibit intense SHG. Compounds 3 (red) and 4
Comparing the spectra of 4 and 5 reveals a blue shift
of the maximum and of the high-energy flank when
replacing Sn with Ge. The white-light spectrum in
both compounds is limited by reabsorption from the
fundamental electronic transition of the cluster core.
(blue) exhibit strong white-light emissions (scaled by a
factor of 3).
The strong nonlinear response found for all four
compounds underlines the efficient enhancement of
the nonlinearity by the transition dipole moment of
the cluster core. Hence, even compound 1, with its
In accordance with the band gaps of SnS
2.18–2.44 eV and 3.2 eV, respectively , the reabsorp-
and GeS
,
2
2
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tion limit in compound 5 is blue shifted compared to
. This is confirmed by UV/VIS absorption spectros-
This work was supported by the Deutsche For-
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schungsgemeinschaft within the framework of grant
GRK1782. We thank Jan Christman for his help with the
synthesis and Vanessa Dahmen for her help with the
spectroscopic experiments.
copy (Figure S2). This is confirmed by UV/VIS ab-
sorption spectroscopy (Figure S2). The white-light
spectrum in both compounds is limited by reabsorp-
tion from the fundamental electronic transition of
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PXRD, UV/Vis and IR spectroscopy) and CIF. This ma-
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AUTHOR INFORMATION
Corresponding Authors
dehnen@chemie.uni-marburg.de
4
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(
Author Contributions
1
N.W.R, J.P.E., and E.D. contributed equally.
ACKNOWLEDGMENT
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