Tunable luminescence of new photochromic bisthienylethenes containing triphenylamine

Article abstract of DOI:10.1071/CH04238

Two new photochromic 1,2-bisthienylethenes containing triphenylamine were conveniently prepared; large cyclization quantum yields and obvious fluorescent changes regulated by the photoisomerization were observed. The substitution effect on the photochromic process is discussed based on these systems. CSIRO 2005.

Full text of DOI:10.1071/CH04238

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2005, 58, 321–326
Tunable Luminescence of New Photochromic Bisthienylethenes
Containing Triphenylamine
Qianfu Luo,A Saihong Sheng,A Saihe Cheng,A and He TianA,B
A Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science
and Technology, Shanghai 200237, China.
B Corresponding author. Email: tianhe@ecust.edu.cn
Two new photochromic 1,2-bisthienylethenes containing triphenylamine were conveniently prepared; large cycliza-
tion quantum yields and obvious fluorescent changes regulated by the photoisomerization were observed. The
substitution effect on the photochromic process is discussed based on these systems.
Manuscript received: 11 October 2004.
Final version: 28 February 2005.
Introduction
rings could increase the coloration quantum yields.[24] In
addition, introduction of triphenylamine at both positions
in the thiophene ring resulted in photochromic amorphous
materials.[25]
Recently, there has been increasing interest in the synthesis,
design, and investigation of organic photochromic material as
a potential technological application for the optical recording
for storage of information and optical switching.[1−4] Prac-
tical applications are also well established, such as in optical
transmission or in holography. Among these photochromic
molecules 1,2-bisthiophenylethenes (BTEs)[5] have become
promising candidates[6−14] because these compounds com-
binemanyfavourableproperties, namelythermalirreversibil-
ity, high resistant fatigue, and large changes in absorption
wavelength between the two isomers.[5,15,16] Moreover, the
photochemical and photophysical properties of BTEs could
be selectively controlled by substitution and by electronic
coupling of the two switching units.[17,18] Photoregulated
emission is also possible using photochromic compounds
when there is a discriminating interaction between the excited
state of the fluorescent dye and the ground state of one of
the photochromic isomers. This interaction can be through
photo-induced energy or electron-transfer processes.[19−21]
By choosing a suitable combination of a fluorescent dye and
a photochromic compound, complete on–off switching of
fluorescence could be realized. The switching characteris-
tics are well controlled by changing the concentration of the
photochromic compound.[22] Since the ring-opened form of
diarylethene has parallel and anti-parallel conformations, the
balance between the two conformations limits the maximum
cyclization quantum yield to 0.5,[5a] which consequently lim-
its the switching efficiency for various potential applications.
By increasing the ratio of the anti-parallel conformation by
inclusion of a photochromic diarylethene in a cyclodextrin
cavity, Takeshita and Choi[23] reported enhancement of the
photocyclization yield. It was also reported that some bulky
substituents attached to the 4- or 4ꢀ-position of the thiophene
It is well known that the incorporation of bulky and
heavy substituents makes glass formation easier, leading
to enhanced stability of the resulting amorphous glass.
Triphenylamine is the substitute most often chosen for this
modification.[25] The substituents introduced in the 2,2ꢀ-
positions of BTEs strongly influence the distribution between
the parallel (p) and antiparallel (ap) conformers (active form
for electrocyclization) present in diarylethene derivatives.[26]
In this paper, the substituted triphenylamines, whose the
n-butyl substitutes can remarkably increase the solubility of
the title compound, were introduced to the 5- or 5ꢀ-position
of the thiophene rings to provide two new photochromic
1,2-dicyanobisthienylethenes. As might be expected, their
cyclization quantum yields have been drastically improved.
In addition, the different number (one or two) of fluo-
rescent chromophores of triphenylamine anchored to the
bisthienylethenes can lead to quite different reversible lumi-
nescence changes in their photochromic processes.
Results and Discussion
Compounds 1 and 2 were prepared by the reaction of
boric acid 3 and 1,2-bis(5-bromo-2-methyl-3-thienyl)-1,2-
dicyanoethene 4 with Pd(PPh3)4 as catalyst in THF as shown
1
in Scheme 1. The H NMR (500 MHz, CDCl3) spectra of
compounds 1 and 2 are shown in Fig. 1. For compound 1, the
signals of the two methyl protons at the 2- and 2ꢀ-positions
of the two thiophene rings appeared at 2.09 and 2.27 ppm,
respectively. The thienyl protons at the 4- and 4ꢀ-positions
appeared at 7.03 and 6.78 ppm. The doublet signals at 6.95,
7.00, 7.14, and 7.20 ppm could be assigned to the phenyl
© CSIRO 2005
10.1071/CH04238
0004-9425/05/050321

322
Q. Luo et al.
HO
OH
B
Br
Li
NH
Br
2
NH2
I
NaNO2
HCl
n-BuLi
B(OMe)3
H3Oϩ
KI
N
N
N
H2O
THF
5
6
7
3
NC
S
CN
O
O
Cl
CN
NBS
(n-C4H9)4NϩBrϪ
50% NaOH
O
(n-C4H9)4NϩBrϪ
Br
Br
Br
Br
Br
ZnCl2
KCN
S
S
S
S
S
8
9
10
4
NC
CN
S
NC
S
CN
S
(PPh3)4Pd/THF
2 M Na2CO3
Br
S
ϩ
N
N
N
1
2
Scheme 1. The synthesis of compounds 1 and 2.
region considerably increases the extinction coefficient of
the photochromic systems at the wavelength of irradiation, as
shown in Fig. 2. Upon irradiation of the solution of compound
1 with 365 nm light, the absorption peaks at 304 and 358 nm
gradually decreased, and a new absorption band with a maxi-
mum at around 600 nm appeared. During the photochromism
there exists an isosbestic point owing to the formation of
the closed form of compound 1 by photocyclization. The
colour of 1 in CH2Cl2 changed from yellow to green. The
system finally reached a photostationary state after irradi-
ation for 160 s. Irradiation of the ring-closed isomer of 1
with 600 nm light induced a rapid ring-opening photoreac-
tion, and the original absorption spectrum corresponding to
the ring-opened form recovered (Scheme 2). Compound 2
exhibited similar photochromism to compound 1, as shown
in Fig. 2. Compound 2 (ring-opened form) has an absorption
maximum at 356 nm in CH2Cl2. When this compound was
irradiated by 365 nm light, a new absorption peak appeared
at 620 nm. The absorption maximum shifts to longer wave-
length by comparison with its opened form (shown in Fig. 2
and Scheme 2). Upon exposure of the closed form (green
solution) to visible light (600 nm), the solution again became
yellow and the initial absorption was restored. For compound
2, the time taken to reach the chromic photostationary state
was only 18 s.
Compound 1
Compound 2
2
8
7
6
5
4
3
1
ppm
Fig. 1. The 1H NMR (500 MHz, CDCl3) spectra of compounds 1
and 2.
protons of the substituted triphenylamine. Although there
are also two methyl groups in the two thiophenes of com-
pound 2, the symmetry of the molecule placed them in the
same chemical and magnetic environment. Therefore, the
methyl protons are considered chemically equivalent and
appeared at 2.24 ppm, while the thienyl protons appeared at
7.09 ppm. The phenyl proton signals of the triphenylamine
were observed at 6.93, 6.99, 7.05, and 7.22 ppm.
The photochromic properties of compounds 1 and 2 in
CH2Cl2 are shown in Fig. 2. Compound 1 (open form) has
the absorption bands at 304 and 358 nm. The introduction
of triphenylamine groups that absorb strongly in the UV
The cyclization quantum yields and the coefficients of the
compounds1and 2weremeasuredinCH2Cl2 at25◦C(shown
in Table 1). As listed in Table 1, the coefficients are relatively
large in the UV region, and the quantum yields are more
than 40%. This may be due to the substituent effect at the

Tunable Luminescence of Photochromic Bisthienylethenes
323
Compound 1
Table 1. Absorption maximum and coefficients of the ring-
closed form, and the cyclization quantum yields of 1 and 2
in CH2Cl2
0
5 s
20 s
800
600
400
200
0
0.4
0.3
0.2
0.1
0.0
40 s
A
60 s
Compound
λmax [nm]
(ring-closed)
ε
Aps (ꢀo–c)
100 s
160 s
[104 M−1cm−1
]
([%])
1
2
598
620
1.2 (358 nm)
5.4 (303 nm)
0.05 (41.2)
0.26 (48.9)
A
300
350
400
450
500
550
600
Aps is the longer wavelength absorption maximum of the photo-
stationary state under irradiation with 365 nm light.[28] ꢀo–c is the
quantum yield for the photocyclization of the open form to the closed
form, calculated by a comparable method.[29]
Wavelength/nm
forms.[21] The conversion of the photocyclization reaction,
αps, at the photostationary state can be determined as 0.21 and
0.55 for compounds 1 and 2, respectively. The enhancement
in the conversion can be explained by active participation of
the bulky substituents in the photochromic process. However,
the quantum yields for the ring-opening reactions of related
compounds with extended π-conjugation are generally very
small.
300
400
500
600
700
800
Wavelength/nm
Compound 2
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
70
18s
16s
0
5 s
10 s
30 s
60 s
360 s
60
50
14s
40
12s
30
10s
8s
6s
4s
2s
0
20
Both compounds 1 and 2 emit luminescence with vary-
ing intensity when excited at 303 nm. It is interesting to
note that the fluorescence intensity greatly depends on the
photochromic reactions of compounds 1 and 2 (insets in
Fig. 2). In the open form, compound 1 displays signifi-
cant fluorescence intensity at around 455 nm, which is the
emission character of triphenylamine group.[25] Irradiation
of this compound with 365 nm light produced the ring-closed
form, and the fluorescence intensity was reduced to less than
5% of the initial value for approx. 3 min. The greater-than-
95% fluorescence quenching of the triphenylamine group
by the ring-closed form might result from one main path-
way for energy loss, namely the efficient energy transfer
from the excited triphenylamine to the ring-closed form of
bisthienylethene (Förster or photochromic fluorescence res-
onance energy transfer, pcFRET).[21,30] The irradiation by
light of 600 nm regenerates the ring-opened form of com-
pound 1, stops the energy transfer mentioned above, and
then restore the original emission spectrum. Therefore, effi-
cient fluorescent switching can take place between the on
and off states by irradiation with 365 and 600 nm light. FRET
can be activated and deactivated reversibly by switching on
and off the closed form of the bisthienylethenes, as demon-
strated by the synchronous change in absorbance of the closed
form and donor fluorescence intensity induced by the UV-
vis irradiation cycles. Compared with that for compound 1,
the fluorescence decay of compound 2 is not so significant
(only about 20% of the initial value). In theory, the efficient
pcFRET from the excited triphenylamine to the ring-closed
bisthienylethene is due to overlapping of the emission band of
its opened form and the absorption spectra of the ring-closed
form.[21] The overlapping for compound 1 is more efficient
than that for compound 2, and the fluorescence quenching
of compound 1 is more significant than compound 2. How-
ever, the quite large differences between 1 and 2 could not be
explained satisfactorily by pcFRET only.
10
0
350
400
450
500
550 600
Wavelength/nm
300
400
500
600
700
800
Wavelength/nm
Fig. 2. The absorption changes of 1 and 2 in CH2Cl2 using differ-
ent irradiation times at 365 nm. The inset shows the emission spectra
(excited at 303 nm) changes of 1 and 2 in CH2Cl2 (1.2 × 10−5 mol L−1
)
with different irradiation times at 365 nm.
NC
CN
NC
CN
365nm
Ͼ600nm
Ј
R
R
R
RЈ
S
S
S
S
Ј
1 R ϭ Br
2 R ϭ R
Ј
Rϭ
N
Bun
Bun
Scheme 2. The interconversion of the ring-closed and ring-opened
forms of 1 and 2.
5,5ꢀ-position of the thiophene rings. Owing to the introduc-
tion of bulky substituents (such as triphenylamine) and heavy
atoms (Br) at the 5,5ꢀ-position of the thiophene rings, the
population of the anti-parallel conformation might increase,
increasing the quantum yields in turn.[5a,27] The systems dis-
play well defined isosbestic points. Thus, one can calculate
the relative conversion of the photocyclization reaction, αps,
following the expressions based on the absorbance of the two
Since the photochromic bisthienylethene unit studied here
contains cyano groups and bromine for compound 1, it

324
Q. Luo et al.
6.0 min
5.0 min
4.5 min
4.0 min
3.5 min
3.0 min
2.5 min
2.0 min
1.5 min
1.0 min
0.5 min
0
to lead to excellent photo-switchable luminescence in differ-
ing degrees, which has potential applications in the areas of
fluorescent probes and optical readouts for erasable memory
media.
2
Experimental
Materials and General Procedures
CH2Cl2 was distilled over calcium hydride under argon. All other sol-
vents were used as received. All other reagents and starting materials
werepurchasedfromAcros. SolventsforNMRanalysis(purchasedfrom
Aldrich) were used as received.
1H NMR spectra were obtained with a Brucker AM-500 spec-
trometer. Mass spectra were determined on a Hitachi-80 spectrometer.
UV-Visible spectra were determined with a Varian Cary 500 spec-
trometer. Fluorescent spectra were recorded on a Varian Cary Eclipse
spectrometer.
0
300
400
500
600
Wavelength/nm
Fig. 3. Absorption changes of compound 4 in CH2Cl2 upon irradiation
at 365 nm.
Preparation of Compounds 1 and 2
1-Butyl-4-iodobenzene 5
acts as a strong electron acceptor. The luminescence of
triphenylamine in both compounds is effectively regulated
by photochromic processes of the BTE subunit, attributed
to another pathway, namely the change in photo-induced
electron transfer (PET) between triphenylamine and each
form of BTE subunit.[19,31] Obviously, the BTE subunit
containing bromine in compound 1 has stronger electron-
accepting ability than that for compound 2, where both ends
of the BTE subunit are anchored the same electron donor
triphenylamine groups. Therefore, more than 95% of the flu-
orescence quenching of compound 1 by the photochromic
process results from two pathways, namely the efficient
energytransfer(FörsterorpcFRET)andPETfromtheexcited
triphenylamine to the ring-closed form of bisthienylethene.
In other words, the photochromic process for compound 1 is
assisted by a FRET effect and PET.[26] Although the conver-
sion efficiency of compound 1 is relative small, the quantum
yield is more than 40%. This is another explanation for the
high quantum yields, besides the substituent effect at the
5,5ꢀ-position of the thiophene rings.
Concentrated HCl (300 mL), 4-butylaniline (150 g), and sodium
nitrite (80 g) were agitated by a mechanical stirrer at below 5◦C. An
aqueous solution of potassium iodide (170 g) was added, and the reaction
mixture was allowed to stir overnight. After completion of the reac-
tion, the mother liquor was distilled under reduced pressure and then
steam-distilled to give the pure product.
N-(4-Bromophenyl)-N,N-bis(4-butylphenyl)benzenamine 6
In a 250 mL round-bottomed flask were placed o-dichlorobenzene
(150 mL), 1-butyl-4-iodobenzene (35 g, 0.135 mol), p-bromoaniline
(8.6 g, 0.05 mol), potassium carbonate (30 g, 0.22 mol), bronze powder
(2.2 g), and polyethylene glycol (1 g). The mixture was stirred at reflux
overnight under nitrogen, then cooled to room temperature, and fil-
tered. The mother liquor was distilled under reduced pressure to remove
dichlorobenzene. Compound 6 was isolated in 40% yield by column
chromatography on silica gel. δH (500 MHz, CDCl3) 0.93 (t, 6H, CH3),
1.36 (m, 4H, CH2), 1.58 (m, 4H, CH2), 2.56 (t, 4H, CH2), 6.89 (d, 2H, J
7.02), 6.97 (d, 4H, J 8.41), 7.03 (d, 4H, J 8.41), 7.26 (d, 2H, J 7.02). m/z
435.2 (M+, 96%), 437.2 (M+ + 2, 92), 392.1 (M − C3H7, 100), 394.1
(M + 2 − C3H7, 97).
4-[Bis(4-butylphenyl)amino]phenylboronic Acid 3
A 100 mL, two-necked, round-bottomed flask was flushed with dry
nitrogen and filled with dry tetrahydrofuran (50 mL). Compound 6
(7.08 g, 16 mmol) was added and the solution was cooled to −78◦C
in a liquid nitrogen bath. n-BuLi (10 mL) was added dropwise with stir-
ring. After 30 min, methyl borate (3 mL) was added and, on completion
of the reaction, the mixture was allowed to warm to room temperature
(overnight) with stirring. HCl (10 M, 10 mL) was then added until the
layers separated, the mixture was stirred for 5 h, and then poured into
water.TheTHF layer was separated and the aqueous layer extracted three
times with ether.The organic phases were combined and the mixture was
distilled under reduced pressure. Finally, silica gel column chromatog-
raphy [CH2Cl2/petroleum (1 : 1)] yielded the pure product (34%), mp
158–160◦C. δH [500 MHz, (CD3)2SO] 0.91 (t, 6H, CH3), 1.32 (m, 4H,
CH2), 1.54 (m, 4H, CH2), 2.53 (t, 4H, CH2), 6.81 (d, 2H, J 8.30), 6.93
(d, 4H, J 8.15), 7.12 (d, 4H, J 8.15), 7.63 (d, 2H, J 8.30), 7.82 (s, 2H,
OH). m/z 435.2 (M+, 96%), 437.2 (M+ + 2, 92).
In addition, the intermediate product, compound 4 shown
in Scheme 1, also exhibited good photochromism properties
(Fig. 3). When compound 4 was irradiated at 365 nm, a new
absorption band appeared at 440 nm. Fig. 3 shows a typical
change in absorption of diarylethene derivatives in solvents.
From this figure, it can also be clearly seen that an isosbestic
point appeared at 400 nm, the same wavelength of the isos-
bestic point in compounds 1 and 2. Moreover, compared to
most of the known diarylethene derivatives, the absorption
maximum of the closed form for compound 4 was in the
region of shorter wavelength (yellow), which may be useful
in photon-mode full colour tuning and rewritable full colour
print.[32,33]
Conclusions
2-Bromo-5-methylthiophene 8
In summary, new photochromic bisthienylethenes (BTEs)
bearing triphenylamine units have been conveniently pre-
pared. They undergo open-to-closed and closed-to-open ring
photochromism in relative large quantum yields by alternate
irradiation with light of 365 and 600 nm. The introduction of
a fluorescent chromophore on the thiophene rings was shown
In the absence of light, 2-methylthiophene (49 g, 0.5 mol) was added
to N-bromosuccinimide (95 g, 0.53 mol) in chloroform/glacial acetic
acid [500 mL of a 50 : 50 (v/v) mixture], and the resulting solution mildly
heated to reflux for approx. 1 h. The mixture was then allowed to warm
to room temperature and poured into aqueous NaOH (3 M, 500 mL).
The organic layer was separated and washed with H2O. The oily crude
product was distilled under reduced pressure (66–68◦C/13 mmHg), and

Tunable Luminescence of Photochromic Bisthienylethenes
325
2-bromo-5-methylthiophene was obtained (74.3 g, 84%). δH (500 MHz,
CDCl3) 2.42 (s, 3H), 6.51 (d, 1H, J 3.63), 6.82 (d, 1H, J 3.63).
6.99 (d, 8H, J 8.42, Ar–H), 7.05 (d, 8H, J 8.54, Ar–H), 7.22 (d, 4H,
J 8.54, Ar–H). m/z 980.6 (M+).
5-Bromo-3-chloromethyl-2-methylthiophene 9
Acknowledgments
A stream of dry HCl gas was purged into a vessel containing a vig-
orously stirred mixture of 2-bromo-5-methylthiophene (54 g, 0.3 mol),
trioxane (15 mol), and anhydrous zinc chloride in CCl4 (150 mL) for 1 h
at room temperature, and for another 4 h at 45◦C. The resulting solution
was poured into water; the organic layer was separated and washed with
H2O and saturated NaHCO3. After the solvent was removed, distilla-
tion of the resulting oil under reduced pressure (132–134◦C/13 mmHg)
gave a yellow oily liquid (64%). δH (500 MHz, CDCl3) 2.39 (s, 3H),
4.46 (s, 2H), 6.93 (s, 1H). m/z 225.9 (M+, 42%), 223.9 (M+ − 2, 31),
191 (M − Cl, 92), 189 (M − Cl − 2, 100).
The authors acknowledge the support from NSFC, China
(20476027, 90401026), the Education Committee of
Shanghai, and the Scientific Committee of Shanghai.
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2,3-Bis(5-bromo-2-methylthiophen-3-yl)maleonitrile 4
Aqueous NaOH solution (50%, 30 mL) containing (n-Bu)4N+Br−
(0.42 g, 2 mmol) was added to a mixture of compound 10 and CCl4
(120 mL) at 40◦C, and the resulting solution was stirred for 1.5 h. The
reaction mixture was poured into water, and the product extracted with
ether and chloroform. After the solvent was removed, chromatography
(petroleum ether/CH2Cl2, 2 : 1) gave pure trans- and cis-product.
Cis-form (RF 0.2), yellow solid, mp 123–125◦C. δH (500 MHz,
CDCl3) 2.20 (s, 6H, CH3), 6.63 (s, 2H, CH). m/z 430.9 (M+, 63%),
432.9 (M+, 27).
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Trans-form (RF 0.6), yellow powder, mp 147–149◦C. δH (500 MHz,
CDCl3) 2.54 (s, 3H, CH3), 2.55(s, 3H, CH3), 7.16 (s, 2H, CH). m/z
430.9 (M+), 432.9 (M+).
2-(5-{4-[Bis(4-butylphenyl)amino]phenyl}-2-methylthiophen-3-
yl)-3-(5- bromo-2-methylthiophen-3-yl)maleonitrile 1 and
2,3-Bis(5-{4-[bis(4-butylphenyl)amino]phenyl}-2-
methylthiophen-3-yl)maleonitrile 2
The resulting boric acid (1.5 g, 3.5 mmol) was added to a mixture
of aqueous Na2CO3 (2 M, 10 mL) and THF (30 mL). 2,3-Bis(5-bromo-
2-methylthiophen-3-yl)maleonitrile (0.52 g, 1.3 mmol) and Pd(PPh3)4
catalyst were then added. The reaction mixture was stirred at reflux
under nitrogen overnight. After completion of the reaction, the mixture
was poured into water, the organic layer separated, and the aqueous layer
extracted with ether three times.The combined organic phases were con-
centrated under reduced pressure, subjected to column chromatography
(CH2Cl2/petroleum ether, 1 : 3), and the products (1 and 2, shown in
Scheme 1) were obtained.
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Compound 1 was obtained as an orange solid (0.22 g, 24%), RF 0.22
(Calc. for C40H38BrN3S2: C 68.2, H 5.4, N 6.0. Found: C 68.3, H 5.4,
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Compound 2 was obtained as a yellow solid (0.14 g, 12.4%), RF
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