The photophysical properties of naphthalene bridged disilanes

Article abstract of DOI:10.1039/d1ra02961d

Structural isomers of naphthalene-bridged disilanes were preparedviacatalytic intramolecular dehydrocoupling of disilyl precursors using Wilkinson's catalyst. Interestingly, it was observed that interchanging the side groups on the silicon atoms altered t

Full text of DOI:10.1039/d1ra02961d

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The photophysical properties of naphthalene
bridged disilanes†
ab
Cite this: RSC Adv., 2021, 11, 21343
c
d
bd
Vipin B. Kumar, Cassandra L. Fleming, Sai Shruthi Murali, Paul A. Hume,
bd
ab
ab
Nathaniel J. L. K. Davis,
Tilo Sohnel
and Erin M. Leitao
*
¨
Structural isomers of naphthalene-bridged disilanes were prepared via catalytic intramolecular
dehydrocoupling of disilyl precursors using Wilkinson's catalyst. Interestingly, it was observed that
interchanging the side groups on the silicon atoms altered the photophysical properties of the bridged
disilanes. Herein, we report the first example of naphthalene bridged disilanes forming excimers in non-
polar solvents. Cyclic voltammetry experiments and DFT calculations were performed to analyse the
band gaps of the compounds and s–p mixing in the bridged disilanes.
Received 16th April 2021
Accepted 8th June 2021
DOI: 10.1039/d1ra02961d
rsc.li/rsc-advances
UV radiation, therefore limiting the applications of these
materials. For example, successive blue shis were observed in
Introduction
5
The unique sigma electron delocalisation (s-conjugation) the UV spectra of oligosilanes over time indicating photode-
16
present across catenated Si atoms grants polysilanes useful composition. Sakurai et al. proposed a radical mechanism for
properties, such as absorption in the UV region and semi- the photodecomposition of the Si–Si bonds as they successfully
conducting capability, when compared to the analogous carbon- isolated the silyl radicals, as detected by electron paramagnetic
1
–3
17
based polymers. This feature is attributed to the fact that silicon resonance spectroscopy, for an aryldisilane in cyclohexane.
has a lower ionization energy and a higher electron affinity
We propose that reinforcing the Si–Si bond in a disilane,
*
compared to carbon, resulting in a lower singlet ssisi ꢀ s excita- using a tether in the form of a covalent bridge, is a potential way
SiSi
*
tion energy in Si–Si bonds compared with the sCC ꢀ s excitation to support the weak Si–Si bonds. In the presence of a bridge,
CC
4
energy in C–C bonds. Polysilanes typically absorb in the range 250– upon UV irradiation, the silyl radicals formed during photode-
3
50 nm and the absorption is greatly inuenced by the side groups composition will remain in close proximity to each other,
attached to the silicon atoms. Previous reports have shown that the therefore creating a greater chance for the two silyl radicals to
presence of aromatic groups on the silicon atoms cause a red shi of recombine, reforming the covalent Si–Si bond. The bridge could
2
5–35 nm in the UV absorption spectra and enhance s–p mixing be either alkyl or aromatic. Since aromatic groups attached to
5
thereby increasing the conjugation across the system. Organo- the silicon atoms leads to s–p mixing, attaching aromatic
silanes have a number of applications in both medicinal chem- substituents will presumably enhance the semi-conducting
6–8
9
istry and as energy materials. The photoluminescent properties of properties of the material owing to increased conjugation.
silicon based materials have provided a new platform for polysilanes Evidence for this was observed in studies conducted by Klausen
18
to act as chemo sensors for detecting explosive materials such as et al. wherein the conductivity of the disilanes containing
10–12
2,4,6-trinitrotoluene (TNT) and picric acid.
Being photoactive in sulfur tethers were measured by a scanning tunnelling micro-
nature, long chain organosilanes have also been tested for photo- scope (STM) break junction technique.
13–15
conductivity and photocurrent generation.
In our quest to prepare oligosilanes that are robust, with
Despite having the potential to be used in various applica- increased conjugation that can ultimately be used for opto-
tions, the Si–Si sigma bonds cleave upon excessive exposure to electronics, substituted naphthalene bridged disilanes seemed
19
to be the best candidates. We have previously reported that
naphthalene disilanes increase the overall conjugation owing to
the s–p mixing and also benet the geometry of the resulting
a
School of Chemical Sciences, The University of Auckland, Private Bag, 92019,
Auckland, 1142, New Zealand. E-mail: erin.leitao@auckland.ac.nz
b

ve membered ring system as the Si–Si bond length matches
The MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand
with that of the 1,8-substitution of naphthalene. In this study,
we investigated the effects of modifying the substitution around
the silicon atoms in the naphthalene bridged disilanes on their
photophysical properties in organic solvents of varying polarity.
c
Centre for Biomedical and Chemical Sciences, School of Science, Auckland University
of Technology, Private Bag 92006, Auckland 1142, New Zealand
d
School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box
6
006140, New Zealand
†
Electronic supplementary information (ESI) available. CCDC 2060937 and The three target bridged disilanes (1b, 2b and 3b, Fig. 1), two of
2
060939. For ESI and crystallographic data in CIF or other electronic format see
which are structural isomers of each other, were prepared.
DOI: 10.1039/d1ra02961d
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Results and discussion
The target naphthalene bridged disilane precursors 1a, 2a and
3
a (Scheme 1) were synthesised using previously reported
19–21
methodologies (Fig. S1–S7†).
A two-step lithiation process to
synthesize 1a was used in order to introduce the two different
silanes. 1,8-Dibromonapthalene was monolithiated with n-
butyllithium and further reacted with a chlorodimethylsilane
(
ClSiMe H) to form 1. Aer purication, 1 was further lithiated
2
and reacted with chlorodiphenylsilane (ClSiPh H) to produce 1a
2
as a white solid. The disilyl naphthalene derivatives (2a and 3a)
were prepared from 1,8-dibromonapthalene, which was reacted
with n-butyllithium to form 1,8-dilithiated naphthalene in situ.
Followed by a slight excess of the respective chlorosilanes (e.g.
Scheme 1 Synthesis of disilyl precursors 1a, 2a and 3a.
2
PhMeSiClH or Me SiClH) the formation of the required chiral
and tetramethyl substituted disilyl naphthalene derivatives, 2a
and 3a respectively, were obtained.
The disilyl precursors (1a, 2a and 3a) were subjected to
3 3
intramolecular dehydrocoupling using 5 mol% of RhCl(PPh )
For the studies of the photophysical properties, naphthalene
was selected as the parent compound. Absorption and emission
spectra of naphthalene and the three-bridged disilanes (1b, 2b
and 3b) were obtained in organic solvent of varying polarity
(Table 1 and Fig. S17–S20†). In comparison to naphthalene, the
in toluene to produce the asymmetric naphthalene bridged
disilane 1b (Fig. S8–S10†) and the symmetric naphthalene
bridged disilane 2b and 3b (Scheme 2). Interestingly, it was
observed that 1b and 3b existed as a crystalline solid, whereas,
the chiral 2b was obtained as a waxy substance. Product 1b was
absorption spectra of the bridged disilanes 1b, 2b and 3b dis-
played red-shied absorption, thereby indicating the extended
conjugation in the bridged systems. The absorption maxima of
the naphthalene and compounds 1b–3b showed very little
dependency on solvent polarity. The emission proles were
recorded in the same set of solvents (Fig. 3) at an excitation
wavelength of 305 nm for 1b, 2b and 3b and 286 nm for
naphthalene. In THF and acetonitrile, the emission spectra of
the bridged silanes (1b, 2b and 3b) exhibited vibronic ne
structure (maxima at ca. 335 and 345 nm), displaying a close
resemblance to that of naphthalene. However, upon moving to
non-polar solvents (cyclohexane and toluene), an additional
broad emission band at longer wavelengths (>400 nm) is
observed, suggesting the formation of excimers (excited state
dimers, Fig. 3). The presence of this additional emission band
in non-polar solvents was not observed for naphthalene.
ꢁ
dissolved in pentane and stored at ꢀ20 C to grow single crys-
tals (Fig. 2). However, repeated attempts to crystallise 2b, such
ꢁ
ꢁ
as freezing at ꢀ20 C and ꢀ40 C in pentane for 24 h and 2 h,
respectively, did not yield any crystalline solids, leaving the waxy
substance aer removal of pentane.
The ve membered ring in 1b was observed to be slightly
distorted from planarity. The C13–Si1–Si2 bond angle was
ꢁ
ꢁ
9
3.02 compared with the C21–Si2–Si1 bond angle of 91.86
indicating the effect of steric bulk on the two silicon centres
Table S1†). The bond angles around the silicon centre bearing
(
the bulkier phenyl rings were narrower compared with the
silicon centre containing methyl groups. The Si1–Si2 bond
19
˚
distance was 2.33 A which was similar to the 3b derivative.
NMR spectroscopy and GC-MS analysis of 2b (Fig. S11–S15†)
conrmed that the product obtained was a mixture of two
isomers. The cis and trans isomers of the product 2b could not
be separated by silica gel column chromatography or high-
pressure ash chromatography as the mixture showed no
separation on TLC despite varying the solvent polarities.
Fig. 1 Substituted disilyl naphthalene precursors (1a, 2a and 3a) and Scheme 2 Intramolecular dehydrocoupling of 1a, 2a and 3a to
naphthalene bridged disilanes (1b, 2b, and 3b).
produce the bridged disilanes 1b, 2b and 3b.
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these results (Table 1). In THF, the three derivatives exhibit
weak to moderate uorescence (compounds 1b and 3b 4 <0.01;
F
compound 2b 4F ¼ 0.09). However, in non-polar solvents
(cyclohexane and toluene) all three compounds show an
increase in uorescence quantum yields (4
the formation of excited state dimers. Interestingly, for 2b,
F
¼ 0.15–0.32) due to
a high uorescence quantum yield in acetonitrile (4
also observed.
F
¼ 0.49) is
Several literature examples of excimer formation in mole-
22–25
cules containing a naphthalene moiety have been reported.
Pandeeswer et al. and Boer et al. observed broad peaks in the
Fig. 2 Molecular structures of 1a (left) and 1b (right, only one of the
two crystallographically independent molecules is shown) with
thermal ellipsoids shown at 50% probability level.
visible regions for naphthalene diimides in polar as well as non-
22,25
polar solvents.
Similarly, L'Her et al. observed broad peaks in
23
the visible regions for imidazolium-naphthalene salts in DCM.
However, to the best of our knowledge, this is the rst example
where excimers are observed through intermolecular interac-
tions between naphthalene in bridged disilane compounds. It is
also important to note that this phenomenon was not observed
in the disilyl naphthalene precursors, prior to the formation of
the Si–Si bond. This was evident from the detailed photo-
physical study conducted by Maeda et al. on various silyl
naphthalene compounds, similar to 1a–3a in non-polar
Excimers form between identical molecules, when a mono-
mer in the excited state interacts with another monomer in the
ground state, which subsequently relaxes by dissociation and
involves emission of photons. Concentration studies were
conducted in cyclohexane (Fig. 4) and THF (Fig. S21–S24†) to
provide further evidence for the presence of excimers in non-
polar solutions. As excimers are favoured at higher concentra-
tions, it was anticipated that the broad band observed at
22
26
solvents. As such, it appears that the Si–Si bond plays
a key role in the formation of excimers in the compounds 1b–
>400 nm in cyclohexane would decrease upon dilution. As
3
b. The work of Karatsu et al. also highlighted the signi-
illustrated in Fig. 4(ii), the emission spectrum of compound 1b
cance the presence of the Si–Si bond has on excimer forma-
in cyclohexane is highly dependent on concentration. Upon
27–30
tion.
of a series of permethyloligosilanes of varying length
[Me Si]
In their investigation, the photophysical properties
ꢀ
4
dilution of a 2.54 ꢂ 10 M solution of 1b, the broad emission
band at 407 nm signicantly decreases. The same trend was
also observed for compounds 2b and 3b (Fig. 4(iii and iv)). In
contrast, the emission spectrum of naphthalene in cyclohexane
at varying concentrations remained unchanged (Fig. 4(i)).
Measurements of the photoluminescence quantum yields of
the naphthalene bridged disilanes (1b, 2b and 3b) corroborated
(
2
n
, n ¼ 1–4, 6), containing naphthalene or anthracene
groups on the terminal silicon atoms, were explored. The two
aromatic groups on the ends of the oligosilane chains have
the ability interact via p-stacking, forming an intramolecular
sandwich. In cases where the number of silicon atoms in the
chain was greater than 1, intramolecular excimer formation
a
Table 1 Spectroscopic data for naphthalene and compounds 1b, 2b, 3b
b
ꢀ1
ꢀ1
c
d
Compound
Solvent
l
abs (nm)
3
max (M cm
)
l
em (nm)
F
F
Naphthalene
Cyclohexane
Toluene
THF
276
nd
5976
nd
4499
nd
4230
nd
9017
nd
6797
nd
7097
nd
10 635
nd
8345
nd
324
nd
nd
nd
nd
nd
276
274
292
293
293
292
292
293
292
292
290
289
291
290
323
322
407
427
335
336
335, 413f
335, 427
336
335
376
405
335
ACN
1b
2b
3b
Cyclohexane
Toluene
THF
0.17
0.17
0.007
0.05
0.19
0.15
0.09
0.49
0.20
0.32
0.008
0.13
ACN
e
Cyclohexane
Toluene
THF
ACN
Cyclohexane
Toluene
THF
ACN
335
a
b
c
nd ¼ not determined. Wavelength of the absorption maximum. Wavelength of the emission maximum; for naphthalene, excitation wavelength
d
was 286 nm. For compounds 1b–3b, excitation wavelength was 305 nm. Fluorescence quantum yield for compounds 1b–3b in different solvent,
excitation wavelength range 285–295 nm. Emission band ascribed to the formation of excimers. Emission band ascribed to the formation of
e
f
excimers. Naphthalene was not measured in toluene due to the strong absorption of toluene in the UV region in which naphthalene absorbs.
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Fig. 3 Absorption (dashed) and emission (solid) profiles of (i) naphthalene, (ii) 1b, (iii) 2b, and (iv) 3b in cyclohexane (black), toluene (red), THF
blue), acetonitrile (green). Absorption spectrum is that recorded in THF.
(
was observed in cyclohexane. Interestingly, when the oligo- groups (i.e. naphthalene or anthracene). Furthermore, unlike
silane chain was tetramethyl disilane (n ¼ 2), the strongest in our study, excimer formation occurred for the shorter
2
7
excimer uorescence was observed, indicating the disilane chain oligomers (n ¼ 2, 3 for naphthalene and n ¼ 2–4 for
tether provides the optimal distance between the two end anthracene) in polar solvent (e.g. acetonitrile) as well.
ꢀ
4
ꢀ5
Fig. 4 Concentration dependent emission of (i) naphthalene upon excitation at 286 nm (1.76 ꢂ 10 M (black) / 2.33 ꢂ 10 M (blue)), (ii) 1b
ꢀ4
ꢀ6
ꢀ4
upon excitation at 305 nm (2.54 ꢂ 10 M (black) / 3.10 ꢂ 10 M (blue)), (iii) 2b upon excitation at 305 nm (1.36 ꢂ 10 M (black) / 1.61 ꢂ
ꢀ6
ꢀ5
ꢀ6
1
0
M (blue)) and (iv) 3b upon excitation at 305 nm (3.20 ꢂ 10 M (black) / 1.20 ꢂ 10 M (blue)) in cyclohexane.
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Fig. 5 Optimized geometries of 1b, 2b(cis/trans) and 3b. (Hydrogen
atoms are omitted for clarity).
The electrochemical behaviour of the bridged disilanes (1b,
2b, 3b) was determined using cyclic voltammetry (CV) in dry
acetonitrile under inert atmosphere with ferrocene as an
internal standard to calibrate the potentials (Fig. S25–S27†).
The electrochemical oxidation (Eox) and reduction (Ered
)
potentials were used to calculate the HOMO–LUMO band
gaps of the three bridged disilanes relative to ferrocene/
Fig. 6 HOMO diagrams from the optimised structures of 1b, 2b(cis/
trans) and 3b.
3
1,32
ferrocenium (4.8 eV below the vacuum level).
gaps for 1b and 2b were found to be similar (3.62 eV and
.67 eV, respectively) and lower than the calculated value for
The band
3
extended the overall conjugation of the system. This
extended conjugation, a result of the s–p mixing responsible
for the reduction in the band gaps, is well established in the
compound 3b (3.85 eV) (Table S2†). This was expected due to
the extra p-conjugation afforded by the phenyl substituents
in compounds 1b and 2b.
3
4
literature. Another way to lower the band gap is to increase
DFT calculations were performed using Gaussian 09 so-
1
6
33
the number of Si atoms in the chain. It was observed that
the electronic contribution from the naphthalene rings was
signicantly more substantial than the phenyl rings attached
to the Si centres, and that all the aromatic rings contributed
to the s–p mixing therefore explaining the difference in the
band gap values between the various bridged silanes (1b and
ware package, for 1b, the two isomers of 2b (2b-cis and 2b-
trans, Fig. 5), 3b and naphthalene to calculate their band gaps
in the gas phase and in solution phases using B3LYP/6-
3
4
1++G**. The band gaps of 1b and 2b isomers varied form
.01 eV to 4.07 eV and were lower in value than 3b and naph-
thalene (Table 2) similar to the trend observed by the CV
experiments. The lower relative band gaps for compounds 1b
and 2b can be attributed to the presence of phenyl substit-
uents on each silicon atom, increasing the overall conjuga-
tion. The diagrams depicting the HOMO of the same set of
compounds (Fig. 6) indicated that the naphthalene also
2
b) and 3b. Similar results were obtained for the other
naphthalene based bridged systems recently reported by our
1
9
group.
35
Table 2 Calculated band gaps of 1b, 2b-cis, 2b-trans and 3b in both gas and solution phases by TD-SCF(DFT).
Gas phase
THF
Cyclohexane
l
max
l
max
l
max
Compound
Band gap (eV)
(nm)
Band gap (eV)
(nm)
Band gap (eV)
(nm)
1
2
2
3
b
4.06
4.07
4.04
4.14
4.32
305.0
304.2
306.4
299.0
286.5
4.02
4.05
4.01
4.10
4.29
308.0
306.0
308.5
302.0
289.0
4.02
4.05
4.02
4.10
4.28
308.0
306.0
308.0
302.0
289.4
b-cis
b-trans
b
Naphthalene
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resolution mass spectrometry measurements were made on
a Bruker microTOF-QII mass spectrometer, equipped with a KD
Conclusions
A series of naphthalene bridged disilanes (1b, 2b, 3b) contain- Scientic syringe pump, in positive ion ESI mode. Hard ioni-
ing different arrangements of methyl and phenyl substituents at zation mass spectrometry analysis was performed on Agilent
silicon were synthesised, characterised and the photophysical 7890A GC + 5975C EI-MS with Agilent auto-sampler. X-ray
properties were investigated. Experimental results and DFT diffraction analysis of single crystals of 1a and 1b were per-
calculations suggested that the Si–Si bonds as well as the formed on a Rigaku Oxford Diffraction XtaLAB-Synergy-S single
presence of aromatic groups, particularly naphthalene, crystal diffractometer with a PILATUS 200 K hybrid pixel array
˚
attached to the Si atoms contributed to the lowering of the band detector using Cu Ka radiation (l ¼ 1.54184 A; ESI Table 1†).
gaps. For example, lmax was higher for 1b and 2b than 3b. The data was processed with the SHELX2018/3 and Olex2 1.3
36,37
Furthermore, interchanging the substitution on the silicon soware packages.
All non-hydrogen atoms were rened
atoms in the disilane derivatives resulted in signicant changes anisotropically. Hydrogen atoms were inserted at calculated
in the photophysical properties that were observed. The naph- positions and rened with a riding model or located in the
thalene disilane derivatives 1b (asymmetric with SiMe
SiPh
substitution) and 3b (no Ph substituents) both absorbed hydrogen atoms). Mercury 2020.3.1 was used to visualize the
2
and difference Fourier map and rened without restrictions (silane
2
38
in the UV region and showed weak emission in the near-UV molecular structures. UV-Vis spectra were recorded on a Shi-
region, similar to naphthalene. In contrast, the symmetric, madzu UV-3600 Plus spectrophotometer and the uorescence
naphthalene disilane mixture 2b(cis/trans) showed an intense emission spectra were recorded on a Jasco FP-8600 spectrou-
emission peak in polar solvents. On switching to non-polar orometer. Samples were recorded in a macro quartz cuvette
solvents such as cyclohexane and toluene, broadening in the (light path ¼ 10 ꢂ 10 mm). Fluorescence quantum yields were
visible region, corresponding to the presence of excimers, was determined using an Edinburgh Photonics FLS-980 spectro-
observed in the emission prole for all three disilane molecules. photometer equipped with an integrating sphere, using a 450 W
As expected, concentration studies performed showed that ozone free xenon arc lamp as the light source. CV experiments
a decrease in the concentration led to a decrease in the broad were performed using a BAS CGME controlled mercury stand
band at ca. 430 nm. The photoluminescence observed for 2b cell. All the DFT calculations were performed using Gaussian
33
presents opportunities to use this compound in applications 09. The geometries of all the subject molecules were optimised
39–41
such as light emitting devices or as a photocurrent generator in using the B3LYP functional and 6-311++G** basis set.
solar cells. Further derivatising these molecules as well as
studying their potential in applications is ongoing in our
laboratory.
Preparation of 1a
1
a was synthesized via a literature procedure and the NMR data
21
was in excellent agreement with the published compound.
Experimental
General information
Preparation of 2a
All the experiments and manipulations were performed under
nitrogen atmosphere in an MBraun Unilab 1200/780 glovebox
or using conventional Schlenk techniques. All glassware was
To a 100 mL Schlenk ask equipped with a stir bar was added
1,8-dibromonaphthalene (2 mmol, 0.571 g) and 15 mL THF
ꢁ
ꢁ
solvent. The solution was cooled to ꢀ78 C using a dry ice-
acetone bath, to which was added n-BuLi (2.0 M in cyclo-
hexane, 3.0 equiv., 6 mmol, 3 mL) dropwise and the mixture was
oven-dried overnight in a 135 C oven. 1,8-Dibromonaphthalene
and Wilkinson's catalyst were purchased from AK Scientic,
chlorodimethylsilane and chlorodiphenylsilane were purchased
from Sigma-Aldrich, chloromethylphenylsilane was purchased
from Gelest. Materials were used directly as received. Ether and
toluene were obtained from solvent purication system and
later dried over activated molecular sieves (3 A˚ ).
ꢁ
stirred at ꢀ78 C for 1 hour. To the lithiated mixture was added
chloromethylphenylsilane (3 equiv., 6 mmol, 0.9 mL) and was
ꢁ
stirred at 22 C overnight. The reaction mixture was quenched
3
with saturated solution of NaHCO (5 mL) and the aqueous
layer was extracted with diethyl ether (3 ꢂ 10 mL). All the
organic layers were combined and dried over Na SO . The crude
2
4
Instrumentation
product was obtained upon removal of solvents under vacuum
1
13
29
1
1
29
H, C and Si{ H} DEPT NMR, H– Si HSQC NMR spectra as a yellow gummy substance. The crude was puried over silica
were recorded on Bruker DPX-400 (400 MHz) spectrometer. gel column, using 100% hexanes as the eluent, to obtain non-
Chemical shis were reported in parts per million (ppm) rela- separable diastereomers of 2a as a blueish-yellow gum in 64.3%
tive to tetramethylsilane and are referenced to residual protium yield.
1
in the NMR solvent (e.g., CHCl
3
¼ 7.26 ppm) as the internal
3
H NMR (400 MHz, CDCl ): d 7.92–7.26 (m, Ph, 16H), 5.59 (p,
1
3
1
standard. The silicon NMR resonances were determined with SiH, 2H), 0.69 (dd, CH
, 6H). C{ H} NMR (100.6 MHz, CDCl ):
3
3
a DEPT pulse sequence. Data are represented as follows: d 144.19, 139.80, 139.68, 139.59, 137.46, 137.42, 137.05, 137.00,
2
9
1
chemical shi (d), multiplicity (s ¼ singlet, d ¼ doublet, t ¼ 135.88, 133.66, 131.10, 129.85, 129.79, 126.53, ꢀ0.08. Si{ H}
triplet, q ¼ quartet, p ¼ quintet, sep ¼ septet, m ¼ multiplet), NMR (79.5 MHz, CDCl ): d ꢀ20.37. EI: Found for C24
coupling constants in Hertz (Hz) and integration. High 368.19 m/z, calc'd: 368.14 m/z.
3
2
H24Si :
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Preparation of 1b
and Employment and the Royal Society of New Zealand. We
thank Tatiana Grousto for collecting the single crystal X-ray
diffraction data and Dr David Ware and David Goodman for
their assistance with the CV measurements. The theoretical
studies were enabled by the services provide by New Zealand e-
Science Infrastructure (NeSI).
To a 50 mL Schlenk ask was added 1a (1 mmol, 0.37 g), Wil-
kinson's catalyst (5 mol%, 0.05 mmol, 46 mg) and 10 mL
toluene. The mixture was heated at 90 C for 72 hours. To the
mixture was added 15 mL hexanes and the mixture was ltered
through a Florisil column. The colorless solution was concen-
trated under high vacuum to remove the solvents and obtain the
ꢁ
crude product. The crude was re-dissolved in pentane (7 mL) Notes and references
ꢁ
and was stored at ꢀ20 C overnight to obtain colorless crystals
1
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of 1b in 30.0% yield.
1
H NMR (400 MHz, CDCl
3
): d 8.02–7.38 (m, Ph, 16H), 0.56 (s,
): d 141.86, 134.8,
33.47, 133.11, 131.78, 130.89, 128.62, 128.16, 126.94, 124.94,
1
3
1
3 3
CH , 6H). C{ H} NMR (100.6 MHz, CDCl
2
1
1
2
9
1
24.90, ꢀ3.54. Si{ H} NMR (79.5 MHz, CDCl
3
): d ꢀ20.62. EI:
3
4
5
6
found for C24H22Si
2
: 366.14 m/z, calc'd: 366.12 m/z.
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Preparation of 2b
To a 50 mL Schlenk ask was added 2a (1.2 mmol, 0.47 g),
Wilkinson's catalyst (5 mol%, 0.06 mmol, 55 mg) and 10 mL
toluene. The mixture was heated at 90 C for 72 hours. To the
ꢁ
7
8
9
A. K. Franz and S. O. Wilson, J. Med. Chem., 2013, 56, 388–
mixture was added 15 mL hexane and the mixture was ltered
through a Florisil column. The colorless solution was dried
under high vacuum to obtain the crude product. The crude was
washed with cold pentane (2 ꢂ 5 mL) to obtain pure 2b as
blueish-yellow gum in 24.5% yield.
From the proton NMR spectrum, it was evident that the
product was a mixture of cis and trans isomers in 2 : 1 ratio
respectively. Similar results were observed by Kim et al., and
based on their ndings, it is concluded that the cis isomer was
present in higher amount than the trans isomer.
H NMR (400 MHz, CDCl ): d 7.90–7.10 (m, Ph, 24H), 0.75 (s,
cis-CH , 6H), 0.60 (s, trans-CH , 3H). C{ H} NMR (100.6 MHz,
): d 145.70, 140.09, 140.03, 135.47, 133.87, 133.70, 133.02,
32.09, 131.71, 129.83, 128.70, 128.40, 127.87, 126.87, 126.87,
405.
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1
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7
42
103.
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1
1
1
2
1
3
1
3
1
3
3
CDCl
3
1
1
2
9
1
26.61, 125.00, 123.89, ꢀ5.71, ꢀ6.09. Si{ H} NMR (79.5 MHz,
CDCl
3
): d ꢀ22.90, ꢀ23.33. EI: found for C24
H22Si
2
: 366.12 m/z.
14 R. G. Kepler, J. Zeigler, L. A. Harrah and S. R. Kurtz,
Photocarrier generation and transport in -bonded polysilanes,
calc'd: 366.16 m/z.
1
987.
Preparation of 3a and 3b
1
5 A. O. Kawashima, T. Oku, A. Suzuki, K. Kikuchi and
3
a and 3b were synthesized via a literature procedure and the
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19
NMR data was in good agreement with what was reported.
1
7 H. Sakurai, Y. Nakadaira, M. Kira, H. Sugiyama, K. Yoshida
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Conflicts of interest
There are no conicts to declare.
18 R. S. Klausen, J. R. Widawsky, M. L. Steigerwald,
L. Venkataraman and C. Nuckolls, J. Am. Chem. Soc., 2012,
1
34, 4541–4544.
Acknowledgements
1
9 K. M. Rabanzo-Castillo, M. Hanif, T. S ¨o hnel and E. M. Leitao,
The authors would like to acknowledge the support provided by
Dalton Trans., 2019, 48, 13971–13980.
the School of Chemical Sciences, University of Auckland and the 20 W. Ando, T. Wakahara, T. Akasaka and S. Nagase,
MacDiarmid Institute. VBK, SM, PAH, and EML thank Royal Organometallics, 1994, 13, 4683–4685.
Society of New Zealand Marsden Fast-Start grant for the nan- 21 N. L u¨ hmann, H. Hirao, S. Shaik and T. M u¨ ller,
cial support and doctoral scholarships for VBK and SM. NJLKD Organometallics, 2011, 30, 4087–4096.
acknowledges research funding from the Victoria Research 22 S. A. Boer, R. P. Cox, M. J. Beards, H. Wang, W. A. Donald,
Trust, the Science for Technological Innovation Science Chal-
lenges, the Marsden Fund, the Ministry of Business, Innovation
T. D. M. Bell and D. R. Turner, Chem. Commun., 2019, 55,
663–666.
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2021 The Author(s). Published by the Royal Society of Chemistry
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