New photochromic bisthienylethene derivatives containing carbazole

Article abstract of DOI:10.1002/poc.1200

Some new photochromic bisthienylethene (BTE) derivatives containing carbazole were synthesized by Suzuki coupling method. All compounds were characterized by ¹H NMR spectra, ¹³C NMR, mass spectra, etc. Their optical, photochromic, an

Full text of DOI:10.1002/poc.1200

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 968–974
Published online 22 August 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1200
New photochromic bisthienylethene derivatives
containing carbazole
1
1
2
1
2
1
2**
*
Yanli Feng, Wei Shen, Wei Huang, Wenjuan Tan, Changgui Lu, Yiping Cui
and He Tian
1Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai
200237, People’s Republic of China
2Advanced Photonics Center, Southeast University, Nanjing 210096, People’s Republic of China
Received 19 December 2006; revised 20 March 2007; accepted 21 March 2007
ABSTRACT: Some new photochromic bisthienylethene (BTE) derivatives containing carbazole were synthesized by
Suzuki coupling method. All compounds were characterized by 1H NMR spectra, 13C NMR, mass spectra, etc. Their
optical, photochromic, and electrochemical properties were described. Moreover, their two-photon absorption (TPA)
properties were investigated both in open and closed forms, finding observable distinction in the TPA of the ring-open
and ring-closed forms. These compounds could be used as two-photon switches and for the potential applications in
three-dimensional optical storage. Copyright # 2007 John Wiley & Sons, Ltd.
Supplementary electronic material for this paper is available in Wiley InterScience at http://www.mrw.interscience.
wiley.com/suppmat/0894-3230/suppmat/
KEYWORDS: synthesis; photochromic; bisthienylethene; carbazole; two-photon
INTRODUCTION
The open and closed-ring isomers of photochromic
BTE derivatives differ from each other not only in their
absorption but also in various physical and chemical
properties, such as luminescence, refractive indices,
oxidation/reduction potentials, chiral properties, mag-
netic interactions, and two-photon properties.1–3,7,8 On
the basis of our earlier work,9–11 herein we incorporated
the photochromic BTE unit and carbazole units via a
Suzuki coupling reaction for the purpose of developing
photochromic compounds with large two-photon cross-
sections. Photochromic compounds C-BTE and C-BTE-C
(Scheme 1) with good solubility in common organic
solvents could be synthesized with structures between the
colorless ring-opened and colored ring-closed forms by
alternating irradiation with 254 nm and visible light
(>500 nm) in THF. Moreover, two-photon as well as
electrochemical properties of these new photochromic
compounds were also discussed, which showed that
C-BTE and C-BTE-C compounds could work as
photoswitches and two-photon switches.
Photochromic materials are potentially useful for advan-
ced optoelectronic devices such as optical memory,
optical switching, and displays.1 Among various types of
photochromic compounds, bisthienylethene (BTE) deriva-
tives are the most promising compounds because of their
excellent fatigue resistance and thermal stability in both
isomeric forms, picosecond switching times, and a high
photochemical quantum yields.2–4 Currently, optical data
can be stored in two-dimensional volume of photo-
chromic materials; however, the techniques have funda-
mental limitations in the available memory density owing
to their two-dimensional nature. The constraint can be
overcome if memory can be stored within the three-
dimensional volume of the material, increasing the data
holding capacity by a factor proportional to the thickness
of the medium. Within this theme, the two-photon
absorption (TPA) of photochromic compounds can be
utilized to achieve higher spatial resolution than
one-photon absorption (OPA). Thus, it is desirable to
synthesize and characterize active organic photochromic
materials with large TPA cross-section values using
high-density multilayer storage devices.5,6
RESULTS AND DISCUSSIONS
Molecular design and synthesis
*Correspondence to: H. Tian, Key Laboratory for Advanced Materials
and Institute of Fine Chemicals, East China University of Science &
Technology, Shanghai 200237, People’s Republic of China.
E-mail: tianhe@ecust.edu.cn
**Correspondence to: Y. Cui, Advanced Photonics Center, Southeast
University, Nanjing 210096, P. R. China. E-mail: cyp@seu.edu.cn
Here, the Suzuki coupling method is used to synthesize
symmetric and unsymmetric BTE derivatives by con-
trolling the ratio of starting materials and reagents.12–14
Copyright # 2007 John Wiley & Sons, Ltd.
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BISTHIENYLETHENE DERIVATIVES
969
Scheme 1. The photochromic process of C-BTE and C-BTE-C (bisthienylethenes containing carbazole units)
1,2-Bis(5-chloro-2-methylthien-3-yl) cyclopentene was
converted into the bis(boronic) esters of BTE via n-BuLi/
B(OBu)3, and then directly used in the next Suzuki
reaction without any workup because during isolation the
bis(boronic) esters are easily deboronized. Suzuki
coupling with N-(4-bromobenzyl)-carbazole gave the
corresponding unsymmetric C-BTE and symmetric
C-BTE-C compounds by controlling the ratio of starting
materials and reagents (Scheme 2).
Absorption spectra
All synthesized compounds could readily dissolve in
common organic solvents, such as chloroform, dichlor-
omethane, and THF. These BTE derivatives show typical
photochromism in solutions. The THF solution of C-BTE
(ring-open form) was colorless. Upon irradiation with
light of 254 nm, the solution gradually turned pink and
new absorption bands at 339 and 490 nm appeared from
Scheme 2. Routes for synthesizing C-BTE and C-BTE-C. Reagents: (i) n-BuLi (1 eq), B(OBu)3, THF, ꢀ788C; (ii) 2 M Na2CO3,
Pd(PPh3)4, THF, 608C, 43%; (iii) n-BuLi (2 eq), B(OBu)3, THF, ꢀ788C; (iv) 2 M Na2CO3, Pd(PPh3)4, THF, 608C, 34.2%
Copyright # 2007 John Wiley & Sons, Ltd.
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Y. FENG ET AL.
obtained as 1.0:0.6 from the ratio of the integrated area of
HPLC peaks. Similarly, the ratio of C-BTE ring-open and
closed forms in the PSS was 1.0:0.56.
Fluorescence spectra
Typical fluorescence spectral changes of these new BTE
derivatives were obtained for the photochromic reaction.
The emission spectra excited at 300 nm for C-BTE and
315 nm for C-BTE-C are shown in Fig. 3. In the open
form of C-BTE (Fig. 3(a)), the fluorescence intensity
peaks are located at 349 and 363 nm. After irradiation at
254 nm to reach the PSS, the fluorescence intensity of
C-BTE in THF increased. A similar result was obtained
from Fig. 3(b), when C-BTE-C was excited at 315 nm,
and the fluorescence intensity appeared at 349, 364, and
385 nm, and after irradiation at 254 nm, the fluorescence
intensity of C-BTE-C was also significantly increased. In
Fig. 3(a), the fluorescence peaks at 349 and 363 nm were
ascribed to carbazole and the fluorescence intensity of
BTE was quenched due to the Cl group, which was a
fluorescence quenching group. After irradiation at
254 nm, the absorption of the open form of BTE declined.
Therefore, when excited at 300 nm, the amount of
Figure 1. Absorption changes of C-BTE in THF (1.0 ꢁ
10ꢀ5 mol Lꢀ1) under different irradiation times by 254 nm
light
the ring-closed form of BTE (Fig. 1). A similar result was
also obtained from Fig. 2. When symmetric C-BTE-C in
THF was irradiated at 254 nm, the solution gradually
turned purple and the new absorption bands appeared at
366 and 540 nm, which were ascribed to the closed form
isomer. The maximal absorption band of the closed-ring
form of C-BTE-C was red-shifted with respect to that of
C-BTE in THF, due to the formation of an extended
p-conjugation system.15,16
Both of the two compounds in THF attained a
photo-stationary state (PSS) at about 15 min. Especially,
the colored isomers (closed forms) of these compounds
are very stable in dark at the room temperature.
Irradiation of their PSS with >500 nm light could lead
to a complete recovery of the initial absorption signal, and
the pink or purple solution turned colorless.
HPLC is a suitable way to measure the ratios since the
ring-open and closed forms of BTE derivatives are
difficult to be separated by silica column chromatography.
The ratio between the ring-open and closed forms can be
Figure 2. Absorption changes of C-BTE-C in THF (1.0 ꢁ
10ꢀ5 mol Lꢀ1) under different irradiation times by 254 nm
light
Figure 3. Fluorescence changes of C-BTE (lex ¼ 300 nm)
and C-BTE-C (lex ¼ 315 nm) in THF (1.0 ꢁ 10ꢀ5 mol Lꢀ1
)
under different irradiation times by 254 nm light
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BISTHIENYLETHENE DERIVATIVES
971
photons absorbed by the carbazole molecule was
increased and fluorescence intensity of carbazole
increased.17 For C-BTE-C, the fluorescence peaks at
349 and 363 nm were ascribed to carbazole and the peak
at 385 nm was ascribed to BTE. When irradiated at
254 nm, fluorescence intensity of carbazole units
increased. The enhanced luminescence of the closed
ring of C-BTE and C-BTE-C upon irradiation with
254 nm light might be attributed to the formation of an
extended p-conjugation system that results from the
photocyclization18 and the very small spectral overlap
between the emission and the absorption bands of the
closed-ring form.19
The fluorescence lifetimes of C-BTE and C-BTE-C
(open forms) were obtained by an interactive nonlinear
deconvolution fitting procedure and the fluorescent
lifetimes were well fitted with a single exponential
function. The fluorescence of open form of C-BTE in
THF decayed with a lifetime of 1.86 ns (x2 ¼ 1.112,
excited at 360 nm) and the fluorescence lifetime of the
open form of C-BTE-C was 2.11 ns (x2 ¼ 1.185, excited
at 360 nm).
Notably, irradiation of C-BTE and C-BTE-C with
>500 nm light could lead to a complete recovery of the
initial fluorescence signal. Figure 4 shows that the
fluorescence intensity of C-BTE-C at 349 nm changed
reversibly upon excitation at 315 nm by alternating
irradiation with 254 nm and >500 nm light. This cycle
could be repeated more than 10 times, indicative of their
good fatigue resistance.
Figure 5. Cyclic voltammograms of C-BTE-C in dichloro-
methane containing tetrabutylammonium tetrafluoroborate
(0.1 mol Lꢀ1) before (solid line) and after (dashedꢀli1ne) irradia-
tion with 254 nm light at the scan of 100 mV s
fluoroborate (0.1 mol Lꢀ1). The oxidation levels of
C-BTE-C were 1.42 and 1.22 V before the irradiation,
and after subsequent irradiation with 254 nm to reach the
PSS the oxidation states appeared at 1.42 and 1.11 V.
Similarly, in C-BTE, the open-ring isomer of C-BTE
showed three oxidation states at 1.60, 1.47, and 1.34 V.
After 20 min irradiation with 254 nm to reach the PSS, the
new oxidation states appeared at 1.15, 0.79, and 0.67 V.
The oxidation states of the ring-closed form appeared at
lower potentials with respect to that of the ring-open
form, which clearly indicates that the ring-closed isomers
of C-BTE and C-BTE-C are easily oxidized than the
ring-open isomers.18
Electrochemical properties
Electrochemical studies were performed on C-BTE and
C-BTE-C by a VersaStar II electrochemical analyzer.
Figure 5 shows cyclic voltammograms of C-BTE-C in
dichloromethane containing tetrabutylammonium tetra-
Two-photon properties
Recently, extensive efforts have been concentrated on the
synthesis of organic materials having large TPA cross-
section, and the investigation of their chemical structure
and TPA property relationships. It is known that TPA can
be enhanced either by increasing the conjugation length
or by an appropriate combination of electron donors and
acceptors.
A typical TPA optical power limiting curve (e.g.,
taking C-BTE-C (open form) at 600 nm) is shown in
Fig. 6. Value of TPA cross-section (given in
GM ¼ 10ꢀ50 cm4 s photonꢀ1 moleculeꢀ1) for C-BTE-C
(open form) was measured to be 5506.64 GM, and the
coefficient (b) was 1.0 cm/GW. TPA cross-section values
for C-BTE and C-BTE-C (open and closed form) were
measured at different wavelength of the laser pulses, as
shown in Fig. 7. For C-BTE, TPA cross-section values
were measured at every 25 nm interval between 575 and
750 nm. If the wavelengths were shorter than 575 nm, the
transmission of the closed form was too low, and if the
wavelengths were longer than 750 nm, the robustness of
Figure 4. Modulated emission intensity at peak of 349 nm
(excited at 315 nm) of C-BTE-C in THF during alternating
irradiation at the wavelength of 254 nm and >500 nm light,
respectively
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Y. FENG ET AL.
the laser energy would make big errors in the
measurements. For C-BTE-C, TPA cross-section values
were measured at every 25 nm interval between 650 and
750 nm. TPA cross-section values of C-BTE-C are larger
than those of C-BTE, since carbazole is a strong
electron-donating group and the C-BTE-C has a
D-P-D structure. Also TPA was enhanced by the
increased conjugation length.20 There are observable
distinctions in the TPA of the ring-open and ring-closed
forms of the two compounds at the same wavelength,
which can be used as a two-photon switch. Compared to
TPA values of the reported BTE derivatives,6 the two
compounds have significant TPA values, especially for
C-BTE-C, indicating the potential utility of these
compounds as three-dimensional information storage
media.
CONCLUSION
Figure 6. Measured output intensity versus input intensity
of the 600 nm laser pulses of compound C-BTE-C (open
form)
In summary, new photochromic C-BTE and C-BTE-C
were successfully synthesized via the Suzuki coupling
method by controlling the ratios of the starting materials
and reagents. They show good photochromic properties,
excellent fatigue resistance, solubility, electrochemical
properties, and large TPA cross-sections. There are
observable distinctions in the TPA of the ring-open and
ring-closed forms of the two compounds. These photo-
chromic compounds could be used as two-photon
switches and have potential application for three-
dimensional optical data storage. Further investigation
of the applications is currently in progress.
EXPERIMENTAL
General procedure
1H NMR and 13C NMR spectra were recorded on a Bruker
AM 500 spectrometer with tetramethyl silane (TMS) as
the internal reference. MS were recorded by EI mass
spectroscopy. Absorption spectra were measured by a
Varian Cary500 UV–Vis spectrophotometer. Fluor-
escence spectra were measured with a Varian Cary
Eclipse Fluorescence spectrophotometer. Fluorescence
lifetimes were measured by an Edingburg Lifespec-Ps
spectrofluorometer. The optical switch experiments were
carried out using a photochemical reaction apparatus with
a 200 W Hg lamp. Cyclic voltammetry measurements
were carried out with a platinum electrode using
millimolar solutions in CH2Cl2 containing 0.1 M of the
support electrolyte, tetrabutylammonium tetrafluorobo-
rate, in a three-electrode cell and potentiostat assembly by
VersaStarII electrochemical analyzer. HPLC analyses
were determined by Agilen 1100 and column Extend-C18
(5 mm, Column 4.6 ꢁ 150 mm) eluted by methanol/
Figure 7. TPA cross-section values for C-BTE and C-BTE-C
were measured at different wavelengths of the laser pulses
acetonitrile (1.5:1) at a flow rate of 0.6 ml minꢀ1
.
Copyright # 2007 John Wiley & Sons, Ltd.
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BISTHIENYLETHENE DERIVATIVES
973
Linear transmission spectra were recorded on a
Shimadzu UV-3600 PC spectrophotometer and TPA
cross-sections were measured by the nonlinear transmit-
tance method. The excitation source was an OPO system
(Continuum, Pather OPO) pumped by the triple frequency
output of a Q-switched Nd:YAG laser system (Con-
tinuum, Precision II 8010) producing ꢂ7 ns-duration,
ꢂ1 cmꢀ1 spectral-width laser pulses with a repetition rate
of 10 Hz. The wavelengths can be tuned from 450 to
1025 nm. The concentration of samples was fixed at
0.01 M dichloromethane, and quartz cells having a path
length of 1 cm were used for all measurements.
A mixture of N-(4-bromobenzyl)-carbazole (0.51 g,
1.52 mmol), Pd(PPh3)4 (0.14 g), and THF (10 ml) was
stirred for 15 min at room temperature. Then
aqueous Na2CO3 (10 ml, 2 M) was added. The reactive
mixture was heated at a temperature of 608C and the
solution of bis(boronic) esters of BTE was added
dropwise via a syringe. Subsequently, the mixture was
refluxed for 20 h and cooled to room temperature. The
reactive mixture was poured into H2O and extracted with
ether and dried with anhydrous Na2SO4. After concen-
trating, the compound was purified by column chroma-
tography on silica (petroleum ether/ethyl acetate ¼
10:1 v/v) to yield C-BTE (yield 43%).
1
C-BTE: H NMR (500 MHz, DMSO, ppm): d ¼ 1.77
Materials
(s, 3H, —CH3), 1.84 (s, 3H, —CH3), 1.91–1.95 (m, 2H,
—
—CH —), 2.66–2.72 (m, 4H, —CH C CCH —),
—
2
2
2
Reagents and starting materials were used as received.
Solvents were distilled and dried before use.
1,2-Bis(5-chloro-2-methylthien-3-yl) cyclopentene was
synthesized and purified according to the established
procedures.21,22
5.61(s, 2H, —CH2—), 6.77 (s, 1H, thiophene-H), 7.10
(s, 1H, thiophene-H), 7.13 (d, J ¼ 8.1 Hz, 2H, Ph-H), 7.20
(t, J1 ¼ 7.3 Hz, J2 ¼ 7.6 Hz, 2H, Ph-H),7.37 (d,
J ¼ 7.9 Hz, 2H, Ph-H), 7.40 (t, J1 ¼ 8.1 Hz, J2 ¼
7.2 Hz, 2H, Ph-H), 7.59 (d, J ¼ 8.2 Hz, 2H, Ph-H), 8.16
(d, J ¼ 7.7 Hz, 2H, Ph-H). 13C NMR (d6-DMSO, ppm):
140.53, 139.08, 137.29, 136.76, 135.61, 135.33, 134.06,
133.78, 133.41, 133.10, 127.90, 127.72, 126.32, 125.47,
124.52, 123.95, 122.68, 120.80, 119.53, 109.94, 45.69,
38.37, 38.20, 22.65, 14.34, 14.17. MS (m/z) [Mþ] Calcd.
for C34H28ClNS2: 549.1, Found: 549.1.
Synthesis of N-(4-bromobenzyl)-carbazole
A mixture of carbazole (1 g, 6.0 mmol), KOH (0.6 g,
10.8 mmol), and DMSO (30 ml) was stirred for 30 min
at room temperature under nitrogen. Then the solution of
4-bromobenzyl-bromide (1.8 g, 7.2 mmol) in DMSO
(10 ml) was added dropwise for about 2 h. Subsequently,
the reactive mixture was heated at a temperature of 558C
for another 2 h and cooled to the room temperature. The
reactive mixture was poured into H2O (100 ml) to a solid
sample. The resulting solid was recrystallized from
ethanol to get N-(4-bromobenzyl)-carbazole as a white
powder (yield 75%), m.p. 184–1868C.
Synthesis of C-BTE-C
Compounds C-BTE-C were prepared in the same manner
as described above for the preparation of C-BTE. To the
solution of 1,2-bis(5-chloro-2-methylthien-3-yl) cyclo-
pentene (0.50 g, 1.52 mmol) in anhydrous THF (10 ml)
was added n-BuLi (2.5 ml of 1.6 M solution in hexane,
4.0 mmol) using a syringe in two portions under nitrogen
at ꢀ788C. Then B(OBu)3 (1 ml, 3.4 mmol) was added to
one portion. At the same reaction condition as for BTE,
the solution of the symmetric bis(boronic) esters of BTE
was obtained. Then the mixture of N-(4-bromobenzyl)-
carbazole (1.2 g, 3.1 mmol), Pd(PPh3)4 (0.30 g), and THF
(10 ml) was stirred for 15 min at room temperature. Then
aqueous Na2CO3 (10 ml, 2 M) was added. The reactive
mixture was heated at 608C and the solution of
bis(boronic) esters of BTE was added dropwise via a
syringe. C-BTE-C was similarly obtained (petroleum
ether/ethyl acetate ¼ 6:1 v/v) with a yield of 34.2%.
C-BTE-C: 1H NMR (500 MHz, DMSO, ppm): d ¼ 1.83
(s, 6H, —CH3), 1.95–1.98 (m, 2H, —CH2—), 2.73–2.76
1H NMR (500 MHz, CDCl3, ppm): d ¼ 5.63 (s, 2H,
—CH2—), 7.08 (d, J ¼ 8.4 Hz, 2H, Ph-H), 7.21 (t,
J1 ¼ 7.1 Hz, J2 ¼ 7.2 Hz, 2H, Ph-H), 7.40–7.46 (m, 4H,
Ph-H), 7.58 (d, J ¼ 8.2 Hz, 2H, Ph-H), 8.17 (d, J ¼ 7.7 Hz,
2H, Ph-H). 13C NMR (CDCl3, ppm): 140.46, 136.18,
131.90, 128.11, 125.94, 123.08, 121.31, 120.48, 119.41,
108.72, 46.01. MS (m/z) [Mþ] Calcd. for C19H14BrN:
335.0, Found: 335.0.
Synthesis of C-BTE
To the solution of 1,2-bis(5-chloro-2-methylthien-3-yl)
cyclopentene (0.50 g, 1.52 mmol) in anhydrous THF
(10 ml) n-BuLi (1 ml of 1.6 M solution in hexane,
1.6 mmol) was added using a syringe in two portions
under nitrogen at ꢀ788C. This solution was stirred for
30 min at room temperature, then B(OBu)3 (0.5 ml,
1.7 mmol) was added to one portion. This reddish solution
was stirred for 1 h at room temperature and was then used
in the Suzuki cross coupling reaction without any workup
because the product was deboronized during isolation.
—
(t, 4H, —CH C CCH —), 5.62(s, 4H, —CH —),7.10
2
—
2
2
(s, 2H, thiophene-H), 7.12 (d, J ¼ 8.1 Hz, 4H, Ph-H), 7.24
(t, J1 ¼ 7.3 Hz, J2 ¼ 7.5 Hz, 4H, Ph-H), 7.37 (d, J ¼
8.2 Hz, 4H, Ph-H), 7.42 (t, J1 ¼ 7.3 Hz, J2 ¼ 8.1 Hz, 4H,
Ph-H), 7.61 (d, J ¼ 8.2 Hz, 4H, Ph-H), 8.17 (d, J ¼ 7.7 Hz,
4H, Ph-H). 13C NMR (d6-DMSO, ppm): 140.52, 138.88,
137.23, 137.00, 134.62, 133.99, 133.16, 127.88, 126.32,
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Y. FENG ET AL.
125.46, 124.66, 122.67, 120.80, 119.53, 109.94, 45.67,
41.77, 41.56, 14.36. MS (m/z) [Mþ] Calcd. for
C53H42N2S2: 770.3, Found: 770.1.
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Acknowledgements
This work was supported by National Foundation of
China (90401026 and 20476027) and Scientific Commit-
tee of Shanghai, and also by the National Natural Science
Foundation of China (No. 10374013) and Jiangsu Pro-
vince Nature Science Foundation of China (No.
BK2006553).
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J. Phys. Org. Chem. 2007; 20: 968–974
DOI: 10.1002/poc