Synthesis and characterization of 5,5′-bis-silylated dithienylethene as a new building block of novel photochromic periodic mesoporous organosilicas

Article abstract of DOI:10.1007/s11164-020-04086-y

Pure bis-silylated dithienylethene (DTE) 3-(3,3,4,4,5,5-hexafluoro-2-(2-methyl-5-(triethoxysilyl)thiophen-3-yl)cyclopent-1-enyl)-2-methyl-5-(triethoxysilyl)thiophene was prepared from 3-(3,3,4,4,5,5-hexafluoro-2-(5-iodo-2-methylthiophen-3-yl)cyclopent-1-enyl)-5-iodo-2-methylthiophene by iodine–lithium exchange. The synthesis and purification method were optimized after many attempts of different coupling methods and purification techniques to ovoid the hydrolysis and self-condensation reaction and to obtain a pure product. The obtained bis-silylated dithienylethene (BS-DTE) is a versatile building block for potential applications, such as synthesis of new photochromic periodic mesoporous organosilica (PPMO). The structure of the obtained precursor was characterized by ¹H, ¹³C and ²⁹Si liquid NMR. Many attempts have been performed to prepare DTE-PMO but, the C–Si bond in the final material was fragile and could not resist to the hydrothermal treatment during the synthesis. The thermal stability of DTE-PMO will be studied in a future work.

Full text of DOI:10.1007/s11164-020-04086-y

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-020-04086-y
Synthesis and characterization of 5,5′‑bis‑silylated
dithienylethene as a new building block of novel
photochromic periodic mesoporous organosilicas
Mohamed Abboud1
Received: 27 October 2019 / Accepted: 9 January 2020
©
Springer Nature B.V. 2020
Abstract
Pure bis-silylated dithienylethene (DTE) 3-(3,3,4,4,5,5-hexafuoro-2-(2-methyl-5-
(
triethoxysilyl)thiophen-3-yl)cyclopent-1-enyl)-2-methyl-5-(triethoxysilyl)thiophene
was prepared ꢀrom 3-(3,3,4,4,5,5-hexafuoro-2-(5-iodo-2-methylthiophen-3-yl)
cyclopent-1-enyl)-5-iodo-2-methylthiophene by iodine–lithium exchange. The syn-
thesis and puriꢁcation method were optimized aꢀter many attempts oꢀ diꢂerent cou-
pling methods and puriꢁcation techniques to ovoid the hydrolysis and selꢀ-condensa-
tion reaction and to obtain a pure product. The obtained bis-silylated dithienylethene
(
BS-DTE) is a versatile building block ꢀor potential applications, such as synthesis
oꢀ new photochromic periodic mesoporous organosilica (PPMO). The structure oꢀ
1
13
29
the obtained precursor was characterized by H, C and Si liquid NMR. Many
attempts have been perꢀormed to prepare DTE-PMO but, the C–Si bond in the ꢁnal
material was ꢀragile and could not resist to the hydrothermal treatment during the
synthesis. The thermal stability oꢀ DTE-PMO will be studied in a ꢀuture work.
Keywords Bis-silylated dithienylethene · Photochromic precursor · Periodic
mesoporous organosilicas · Iodine–lithium exchange
Introduction
The mainstream oꢀ twenty-ꢁrst century nanotechnology ꢀocuses on the control oꢀ
matter at the nanoscale [1–3]. The ability to control the ꢀunction and structure oꢀ
some promising nanosystems using an external stimulus is an attractive research
area ꢀor developing programmable and reconꢁgurable nanomaterials ꢀor multiꢀunc-
tional and smart devices [4]. Due to its particular advantages oꢀ temporal resolution,
high spatial and digital controllability, light represents the most preꢀerred external
*
Mohamed Abboud
abboud_med@yahoo.ꢀr
1
Present Address: Catalysis Research Group, Chemistry Department, College oꢀ Science, King
Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia
Vol.:(012
1
3456789)
3

M. Abboud
stimulus as a chemical and physical tool ꢀor in situ manipulation oꢀ the ꢀunction and
structure oꢀ some photochromic nanosystems.
Photochromism is the property oꢀ some moieties to undergo a reversible transꢀor-
mation between two or more diꢂerent states, with distinct properties. Photochromic
molecules have been extensively incorporated into several ꢀunctional nanosystems
such as nanoparticles, nanomachines, nanoelectronics, biological nanosystems, sen-
sors, memory storage, and supramolecular nanoassemblies [5–8]. There are many
organic photochromic molecules that exhibit isomerization upon light irradiation,
such as DTE, azobenzene, stilbene, and spiropyrans [5, 9–11]. However, the appli-
cation oꢀ these photochromic molecules is limited to a certain type oꢀ structure and
properties. Both isomers have to be thermally stable, with low photoꢀatigue and ꢀast
reversible response. Among the most promising organic photochromic molecules
that employed until now, the DTE derivatives containing a perfuorocyclopentene
have been demonstrated to exhibit excellent photochromic behaviour with low pho-
toꢀatigue [8]. There are only ꢀew examples in the literature oꢀ dithienylthene con-
taining mesoporous silica materials. The dithienylthene derivatives in these mate-
rials were immobilized in the surꢀace oꢀ the silica using organic linkers [12, 13].
However, surꢀace-immobilized or physically impregnated photochromic molecules
might aggregate, leading to a reduced photoisomerization, and might block mate-
rial nanopores. The introduction oꢀ DET in the silica wall will prevent its pack-
ing and suppress the selꢀ-quenching. Thereꢀore, to deꢀeat this limitation, periodic
mesoporous organosilicas (PMOs) [14–17] oꢂer a great alternative ꢀor high loading
oꢀ the photochromic molecules into silica wall ꢀramework, and show also a high
quantum perꢀormance. Photochromic periodic mesoporous organosilica (PPMO)
are new category oꢀ mesoporous organosilica materials that can be considered as
smart nanomaterials. Pores diameter and shape and permeability can be controlled
by applying an external stimulus such as UV light [18–20]. DTE exhibits a remark-
able reversible change oꢀ structure upon UV irradiation, when it transꢀorms ꢀrom the
colourless ring-open ꢀorm to the purple ring-closed ꢀorm (Fig. 1). This reversible
process is thermally stable and can be only driven by visible light irradiation [5, 21].
Iꢀ DTE is incorporated in a PMO, this drastic structure change will be observed in
the PMO pore size, pore volume, surꢀace area and morphology. However, the prepa-
ration oꢀ pure, rigid and ordered DTE-PMO requires the synthesis oꢀ a pure bis-
silylated DTE (BS-DTE) precursor, without any linker between thiophene ring and
silicon atom, because, the presence oꢀ this linker will make DTE-PMO less ordered
and soꢀt.
F
F
F
F
F
F
F
F
F
F
F
F
UV light
Visible light
(
EtO)
3
Si
S
S
Si(OEt)3
S
S
(EtO)
3
Si
Si(OEt)
3
Open form
colourless)
Closed Form
(purple)
(
Fig. 1 Photoisomerization process oꢀ bis-silylated dithienylethene
1
3

Synthesis and characterization of 5,5′-bis-silylated…
Herein, I report the successꢀul synthesis, puriꢁcation and characterization oꢀ BS-
DTE as a building block oꢀ novel PPMO. The synthesis oꢀ this new precursor was
based on the iodine–lithium exchange, and its structure was investigated by liquid
1
13
29
H, C and Si liquid NMR. This precursor was used as 100% organosilica source,
in combination with CTAB as a structure directing agents to prepare DTE-PMO, but
unꢀortunately, the C–Si bond in the ꢁnal material was ꢀragile and could not resist to
the hydrothermal treatment. The thermal stability oꢀ DTE-PMO will be investigated
in a ꢀuture work.
Results and discussion
Synthesis
Figure 2 describes the synthetic pathway employed ꢀor the preparation oꢀ diiodi-
nated DTE 6.
Synthon 6 was prepared ꢀrom bis(5-trimethylsilyl) derivative oꢀ 1,2-bis(29-meth-
ylthien-39-yl)perfuorocyclopentene (5), using a procedure described by Irie et al.
[
22, 23]. The trimethylsilyl (TMS) was substituted smoothly by iodine using ICl as
iodinating agent to aꢂord the diiodo derivative 6 in a good yield.
The resulting 5,5′-diiodo-DTE 6 were subjected to diꢂerent silylation conditions
used in the literature to access to BS-DTE precursors 8, namely the palladium and
rhodium-catalyzed carbon–silicon bond-ꢀorming reactions between aryl halides and
trialkoxysilanes HSi(OR) (R=Me, Et) [24–28].
3
Since its discovery in the late 1990s, this σ-bond metathesis-type reaction [27]
was ꢀurther developed by Manoso and De-Shong [28] as well as by Masuda et al.
[
29, 30], who also presented a rhodium-mediated version. The main reason ꢀor
the comparatively minor attention this reaction has received up to now might be
its rather limited scope oꢀ starting materials, comprising only several aryl iodides
and electron-rich aryl bromides, and its tendency to give considerable amounts oꢀ
Br
Br
NBS
1
) n-BuLi, THF
2) TMSCl, -78°C
4%
AcOH
Br
S
S
TMS
RT, overnight
S
80%
F
F
9
F
F
F
F
F
F
F
F
F
1
) n-BuLi, THF
ICl, CH Cl2
2
F
F
F
-5°C, 2h
F
F
F
TMS
S
S
TMS
89%
I
S
S
I
F
5
6
F
F
4
7%
Fig. 2 Synthesis oꢀ the diiodinated synthon 6
1
3

M. Abboud
undesired by-products, especially by reductive proto-dehalogenation. Furthermore,
the reaction products are very moisture sensitive and can be puriꢁed only to a very
limited extent. The low solubility and the high molecular weight oꢀ 5,5′-diiodo-DTE
6
make it even more diꢃcult to pursue common puriꢁcation methods, ꢀor example,
bulb-to-bulb distillation. To develop a suitable condition ꢀor the coupling oꢀ 6 with
triethoxysilane or trimethoxysilane I tried many methods. I based my initial screen-
ing on the protocol oꢀ Manoso and DeShong, employing N-methyl-2-pyrrolidone
(
NMP) as a polar aprotic solvent and Pd(dba) /Johnphos (di-tert-butylphosphanylbi-
2
phenyl) as catalyst [28]. In contrast to the literature, aqueous workup oꢀ the obtained
solutions was not ꢀeasible, because extraction with pentane and washing with deion-
ized water yielded only inseparable mixtures oꢀ polymerized material including
dehalogenated starting materials. Using the conditions improved by Thiel et al. [31]
by changing the solvent to DMF and replacing triethoxysilane with trimethoxysi-
lane showed no beneꢁcial eꢂect. Aꢀter all these attempts, I was convinced that the
access to pure BS-DTE precursors 8 is not possible via metal-catalyzed cross-cou-
pling reactions. Thereꢀore, I searched ꢀor an alternative method to substitute iodine
by Si(OEt) group. Aꢀter an exhaustive literature search about this kind oꢀ cleavage,
3
I ꢀound only ꢀew examples about the creation oꢀ C–Si(OR) bond on thiophene or
3
phenyl ring with Si attached directly to the ring without linker. The iodine–lithium
exchange [32–36] was among the methods used to create Si–C bond. In this process,
the iodine was exchanged to lithium at low temperature (−105 °C) using n-BuLi,
then Li–C bond was converted to C–Si aꢀter transmetallation with chlorotrithoxysi-
lane ClSi(OEt) . Using this method with some modiꢁcations, and developing a new
3
puriꢁcation process, using bulb-to-bulb distillation, I was able to obtain the desired
BS-DTE precursor with high purity and acceptable yield. Indeed, 1 equivalent oꢀ
n-BuLi and ClSi(OEt) was not suꢃcient to convert totally diiodo-DTE 6 to dil-
3
ithio-DTE 7 (Fig. 3). Thus, an excess oꢀ n-BuLi and ClSi(OEt) was necessary to
3
F
F
F
F
F
F
F
F
F
F
F
1) nBuLi (3.5 eq), -105°C, 5min
F
dry THF
I
Li
S
S
S
I
S
Li
7
6
F
F
2
3
) ClSi(OEt) (3.5eq),
F
F
-105°C, 15 min
) 3h, 0°C
F
3
F
(
3
EtO) Si
S
S
Si(OEt)3
8
Fig. 3 Synthesis oꢀ the precursor 8 by iodine–lithium exchange
1
3

Synthesis and characterization of 5,5′-bis-silylated…
Fig. 4 H NMR oꢀ 5,5′-bi-
silylated dithienylethene
Fig. 5 Zoom ꢀrom Fig. 4 shows
H NMR peaks oꢀ Si–O-CH -
2
CH
3
perꢀorm a total iodine–lithium exchange and to avoid the ꢀormation oꢀ the undesired
mono-iodo-DTE.
1
13
29
Liquid H, C and Si liquid NMR spectra conꢁrmed the structure and purity
oꢀ the obtained precursor. These analyses conꢁrmed also the preservation oꢀ both,
1
DTE moiety and C–Si bonds. The liquid H NMR spectra (Figs. 4 and 5) displayed
clearly the tree protons oꢀ CH (1.25 ppm) and two protons oꢀ CH (3.88 ppm) in
3
2
Si–O–CH –CH , the methyl group attached to the thiophene ring (1.9 ppm), and the
2
3
single proton oꢀ thiophene rings (7.39 ppm).
1
3
The liquid C NMR spectrum (Figs. 6 and 7) showed the two carbons oꢀ the eth-
oxy groups (21.2 ppm and 59.1 ppm), the 4 carbons oꢀ thiophene rings (130.0, 133.0,
1
34.4, 142.6 ppm), the carbon oꢀ methyl group attached to thiophene (18.0 ppm),
and the tree carbons oꢀ hexafuorocyclopentene (126.1, 128.6, and 142.8 ppm).
2
9
The liquid Si NMR spectrum (Fig. 8) exhibited a singlet at around −62.7 ppm
which correspond to silicon atom in –Si(OEt) groups, which conꢁrm the purity oꢀ
3
bis-silylated precursor, preservation oꢀ C–Si bond, and the absence oꢀ hydrolysis and
selꢀ-condensation reaction oꢀ triethoxysilane groups.
1
3

M. Abboud
Fig. 6 13C NMR spectrum oꢀ
precursor 8
CDCl3
F
F
F
F
F
F
3
(EtO) Si
S
S
Si(OEt)3
TMS
160 140 120 100
80
60
40
20
0
Chemical Shift (ppm)
Fig. 7 Zoom ꢀrom Fig. 5 shows
1
3
C NMR peaks oꢀ the aromatic
carbons in precursor 8
150
145
140
135
130
125
120
Chemical Shift (ppm)
2
9
Fig. 8 Liquid Si NMR
spectrum oꢀ 5,5′-bi-silylated
dithienylethene
1
3

Synthesis and characterization of 5,5′-bis-silylated…
Using a method reported by Inagaki et al. [37] this new bis-silylated precursor
was involved then in the hydrolysis and polycondensation reaction under basic con-
ditions in the presence oꢀ C TMACl as a structure directing agent. Unꢀortunately,
1
8
aꢀter the hydrothermal treatment at 97 °C in autoclave, and the characterization oꢀ
1
3
29
the ꢁnal product by C, and Si solid NMR I discovered that the PMO was not
pure and C–Si bond was broken during the hydrothermal treatment.
Experimental section
Materials and methods
All reactions were carried out under nitrogen using standard high-vacuum and
Schlenk-line techniques. Unless otherwise noted, all chemicals were purchased ꢀrom
commercial suppliers (TC America, Sigma-Aldrich, Acros Organics, Alꢀa aesar and
Gelest), and used without ꢀurther puriꢁcation. Tetrahydroꢀuran (THF), and dichlo-
romethane (CH Cl ) used in synthesis were distilled beꢀore use, according to stand-
2
2
1
13
ard procedures. Liquid-state H and C NMR spectra were obtained using a Bruker
Avance 400 spectrometer operating at 400 MHz and 100 Hz, respectively. Liquid-
2
9
state Si NMR spectra were recorded on a Bruker Avance 400 spectrometer oper-
ating at 400 MHz equipped with the one NMR 5 mm probe and without sample-
spinning. Chemical shiꢀts are reported in δ parts per million (ppm) reꢀerenced to
tetramethylsilane (TMS) or residual protonated solvent as an internal standard, using
deuterated chloroꢀorm (CDCl ) as solvent.
3
Synthesis of 3‑(3,3,4,4,5,5‑hexaꢀuoro‑2‑(2‑methyl‑5‑(triethoxysilyl)thiophen‑3‑yl)
cyclopent‑1‑enyl)‑2‑methyl‑5‑(triethoxysilyl)thiophene (8)
The precursor was prepared according to procedure adapted ꢀrom the literature
[
33, 35, 38]. To a THF solution (130 mL) oꢀ the compound 6 (620 mg, 1 mmol)
at −105 °C, 3.5 equivalents oꢀ n-BuLi (1.60 M in hexane, 2.2 mL, 3.5 mmol) was
added rapidly. Aꢀter the reaction solution was stirred ꢀurther at −105 °C ꢀor 5 min
then, 3.5 equivalents oꢀ chlorotriethoxysilane (0.7 mL, 3.5 mmol) was added drop-
wise at −105 °C. The mixture was stirred at −105 °C ꢀor 15 min and then at 0 °C
ꢀ
or 3 h, evaporated, and extracted with CH Cl . The crude product was separated by
2 2
a chromatograph (CH Cl , Silica Gel 230–400 Mesh). The eluted main component
2
2
was puriꢁed by distillation with a glass tube oven (0.07 mmHg, 125 °C) to give 4c
as colourless oil (311 mg, 45%).
1
3

M. Abboud
F
F
F
F
F
F
(
EtO) Si
3
S
Si(OEt)3
S
8
1
H NMR (400 MHz, CDCl ) δ 1.25 (t, J = 6.9 Hz, 18H, CH oꢀ Si(OEt) ), 1.9
3
3
3
(
s, 6H, CH ), 3.88 (q, J = 6.8 Hz, 12H, CH oꢀ Si(OEt) ), 7.39 (s, 2H, Ar–H).
3 2 3
1
3
C NMR (100 MHz, CDCl ) δ 18.0, 21.2, 59.1, 126.1, 128.6, 130.0, 133.0,
3
1
34.4, 142.6, 142.8.
Synthesis of 3‑(3,3,4,4,5,5‑hexaꢀuoro‑2‑(5‑iodo‑2‑methylthiophen‑3‑yl)
cyclopent‑1‑enyl)‑5‑iodo‑2‑methylthiophene 6
The procedure was adapted ꢀrom the literature [39]. Iodine monochloride
(
(
570 mg, 3.5 mmol) was added to a solution oꢀ 5 (450 mg, 0.87 mmol) in CH Cl
2
2
30 mL), and the solution was stirred at − 5 °C ꢀor 2 h. Aꢀter the evaporation
oꢀ CH Cl , the ꢁnal product was puriꢁed by column chromatography (basic alu-
2
2
mina, pentane) to aꢂord 490 mg oꢀ 9 as a waxy solid with 89% yield. Mp. 95 °C.
F
F
F
F
F
F
I
S
I
S
6
1
1
H NMR (400 MHz, CDCl ) δ 1.9 (s, 6H, CH ), 7.2 (s, 2H, Ar–H).
3
3
3
C NMR (100 MHz, CDCl ) δ 14.4, 111.8, 115.6, 119.8, 126.6, 135.4, 136.1,
3
1
47.8.
1
3

Synthesis and characterization of 5,5′-bis-silylated…
Conclusion
Herein, I report the successꢀul synthesis oꢀ pure 5,5′-bis-silylated dithienylethene as
a new building block oꢀ novel photochromic PMO. This precursor was successꢀully
synthesized ꢀor the ꢁrst time ꢀrom 5,5′-diiododithienylethene trough iodine–lith-
ium exchange method. Triethoxysilane—Si(OEt) —groups are attached directly
3
to thiophene ring without any linker. Despite, the product is very moisture sensi-
tive and can be puriꢁed only to a very limited extent using traditional puriꢁcation
techniques, the synthesis conditions and puriꢁcation method were optimized aꢀter
many attempts oꢀ diꢂerent coupling methods and puriꢁcation techniques to ovoid
the ꢀormation oꢀ undesired by-products and to obtain a pure bi-silylated precursor.
1
13
29
The structure oꢀ the obtained precursor was investigated by H, C and Si liq-
uid NMR. Many attempts have been made to prepare the photochromic PMO oꢀ
5
,5′-bis-silylated dithienylethene (DTE-PMO), but the C–Si bond was ꢀragile and
could not resist to the hydrothermal treatment during the synthesis oꢀ PMO. The
thermal stability oꢀ this precursor in the preparation process oꢀ DTE-PMO will be
studied in a ꢀuture work.
Acknowledgement The author extend its appreciation to the Deanship oꢀ Scientiꢁc Research at King
Khalid University ꢀor ꢀunding this work through General Research Project under Grant Number (G.R.P-
2
55). The author would like to express its gratitude to King Khalid University, Abha, Saudi Arabia ꢀor
providing administrative and technical support.
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