Synthesis and spectral properties of triazine-based dendritic dithienylethenes

Article abstract of DOI:10.3184/174751911X13049474471938

Two novel triazine-based dendrimers comprising a dithienylethene core and eight and 16 ethyl groups around the periphery were synthesised in high yields in a convergent way, without using tedious protection and chromato-graphic separation steps. Their optical properties such as photochromism, fluorescence emission and film-forming behaviour were investigated. The new symmetrical dendritic dithienylethenes showed typical photochromic behaviour in solution and good film-forming performance in a poly(methyl methacrylate) (PMMA) matrix. Moreover, the dendrimers have demonstrated remarkable fluorescence enhancement and good solubility in common organic solvents, compared with the corresponding small molecule diarylethene. Thus, the new dendrimers as optoelectronic materials, have potential applications in optical storage, photo-switches, and so on.

Full text of DOI:10.3184/174751911X13049474471938

282 RESEARCH PAPER
MAY, 282–287
JOURNAL OF CHEMICAL RESEARCH 2011
Synthesis and spectral properties of triazine-based dendritic
dithienylethenes
Chuanjie Cheng, Zhongbin Liu, Liang Shen*, Long Li, Cha Ma, Shouzhi Pu and Jingkun Xu
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science &Technology Normal University, Fenglin Street, Nanchang, Jiangxi 330013,
P. R. China
Two novel triazine-based dendrimers comprising a dithienylethene core and eight and 16 ethyl groups around the
periphery were synthesised in high yields in a convergent way, without using tedious protection and chromato-
graphic separation steps. Their optical properties such as photochromism, fluorescence emission and film-forming
behaviour were investigated. The new symmetrical dendritic dithienylethenes showed typical photochromic behav-
iour in solution and good film-forming performance in a poly(methyl methacrylate) (PMMA) matrix. Moreover, the
dendrimers have demonstrated remarkable fluorescence enhancement and good solubility in common organic
solvents, compared with the corresponding small molecule diarylethene. Thus, the new dendrimers as optoelec-
tronic materials, have potential applications in optical storage, photo-switches, and so on.
Keywords: dendritic diarylethene, dendrimer, photochromism, fluorescence
Among the various types of photochromic compounds,1
diarylethenes, especially dithienylethenes, are one of the most
promising candidates as optical memory and photo-switch
materials due to their good thermal stability and impressive
feature of fatigue-resistance.2–7 Diarylethenes can undergo
cyclisation/cycloreversion (photochromic) reactions via inter-
conversion of both the open-ring and closed-ring isomers.
Great efforts have been made to synthesise new diarylethene
derivatives by altering the heteroaryl moieties or the substitu-
ents on the heteroaryl rings.8–10 So far, several research groups
have reported substituent effects on the photochromic perfor-
mance of diarylethenes.11–15 Compared with small molecules,
polymers are more advantageous as materials because of the
ease with which they form films, flakes, and/or beads. There-
fore, polymerisation techniques have been widely exploited
in the modification of diarylethenes.16 Applications have been
developed for optoelectronic devices such as optical storage
and photoswitches.7
In comparison with conventional polymers, dendrimers are
special synthetic macromolecules with the characteristics of
regular branches, high symmetry, and rigidity, which always
exhibit special properties such as good solubility and flexibil-
ity, low viscosity, and so on.17 In recent years, the potential
applications of dendrimers have been widely studied in
many fields such as photochemistry and/or photophysics,18
pharmacology,19 catalysis,20 materials,21 and nanoscience.22
Based on these advantages, we deduced that dendron-
modified diarylethenes would have good physical, chemical
and/or optical properties. For example, rigid and globular den-
dron-modified fluorescent diarylethenes are expected to reduce
or eliminate the aggregation-induced fluorescence quenching,
and enhance fluorescence emission.23–26 Also, the coupling of
dendrons with a diarylethene core is thought to improve its
solubility as there are an abundance of solublising peripheral
groups on the dendrons.27 Photochromic dendrimers such as
dendritic spiropyrans have been studied in recent years with
encouraging results.28 To the best of our knowledge, there have
been no reports on dendritic diarylethenes except Shimomura’s
publications a decade ago.29,30 However, Shimomura’s work
did not overcome the general difficulties of preparing den-
drimers. Tedious preparative thin layer chromatographic
separation processes were needed, and low yields of higher
generation dendrimers were obtained (e.g. the third generation
dendrimer in 28% yield.), a problem usually encountered
during dendrimer preparations. Moreover, no photochromic
behaviour nor fluorescent properties of the diarylethene-
containing dendrimers were described in Shimomura’s work.
Herein, we have rationally designed a new type of triazine-
based dendritic dithienylethenes. Our new dendrimers can be
efficiently prepared in high yields under mild conditions by
using a convergent method,31 and no protection/deprotection
protocols nor chromatographic separation steps were involved
in the whole synthesis. Moreover, the new symmetrical den-
dritic dithienylethenes are easily soluble in most common
organic solvents, and show a remarkable fluorescence enhance-
ment, unlike the single dithienylethene core 5 that has low
solubility and weak fluorescence.
Results and discussion
The synthetic route to the dendrimers is illustrated in
Scheme 1. First, the triazine-based dendrons 2 and 4 were syn-
thesised in a convergent way according to the literature.32
A
portion of 2.2 equiv of diethylamine was treated with cyanuric
chloride in CH2Cl2 containing an excess of diisopropylethyl-
amine (DIPEA) at 0 °C to give compound 1 in 95% yield,
which was allowed to further react with 5 equiv of piperazine
in THF at reflux to afford compound 2 in 91% yield. Com-
pound 3 was also obtained from triazine 2 in 92% yield by
reaction with cyanuric chloride. Similarly, compound 4 was
prepared in 90% yield. Compounds 7 and 8 were synthesised
from the dibromo-derivative of the diarylethene chromophore
6, which was coupled to the corresponding of triazine-based
dendrons (2.0 equiv.) in the presence of K2CO3 (2.5 equiv.).
The 1H NMR and 13C NMR spectra of the dendrimer 8 are
illustrated in Fig. 1. Typical protons on 8 have been labelled
with a–i (Fig. 1a). The triplet signal a at the chemical shift of
1.15 ppm and the quartet peak b at 3.64 ppm corresponds to
the CH3 and CH2 units, respectively of 16 ethyl groups. Pro-
tons labelled with c, d and e on piperisine groups are located
within the multiplet signals at 3.7–3.8 ppm. However, the f
protons on piperisine groups shifted to about 2.46 ppm, which
may be ascribed to the influence of the dithienylethene core.
The singlet peaks of g and i at 3.65 and 1.88 ppm are the CH2
and CH3 groups bonded to the thienyl groups, respectively.
Protons labelled with h are located at low field and correspond
to the aryl protons on the thienyl groups.
Typical carbons on 8 have been labelled with a–p (Fig. 1b).
Carbons a and b at 13.5 and 41.1 ppm are the CH3 and CH2
units respectively from the 16 ethyl groups. Carbons labelled
with c, d and g, h at about 165 ppm correspond to the carbons
of six triazine rings. At the chemical shift of around 43 ppm
are the e, f and i carbons from the piperazine rings; however,
* Correspondent. E-mail: shenliang00@tsinghua.org.cn

JOURNAL OF CHEMICAL RESEARCH 2011 283
Scheme 1 Synthesis of dendritic dithienylethenes 7a and 8a.
the chemical shift of the j carbons from the piperazines was
deshielded and resonated at 62.8 ppm, which may be attributed
to the effect of the dithienylethene core. The signals k and l at
57.2 and 14.3 ppm are due to the CH2 and CH3 groups bonded
to the thienyl moieties, respectively. Carbons of the thienyl
rings labelled with m, n, o and p are located in the range 124–
141 ppm. The carbons of the hexafluorocyclopentene ring
have no peaks in the 13C NMR spectrum. Characterisation and
1H NMR and 13C NMR spectra of compounds 1–4 and 6–8 are
available as Electronic Supplementary Information.
The photochromic reactions of dendritic dithienylethenes 7
and 8 were illustrated in Scheme 2. The colourless open-ring
isomers 7a and 8a will be converted to their purple closed-ring
isomers 7b and 8b upon irradiation with an appropriate UV
light, and a visible light irradiation will make the closed-ring
isomers return to their previous open-ring states. Details of
the absorption spectra of compounds 7 and 8 induced by
photo-irradiation at room temperature in dichloromethane
(c = 1.0 × 10−4 mol L−1) are shown in Fig. 2. The absorption
maximum of 7a was observed at 236 nm, while that of 8a
appeared at 249 nm. Upon irradiation with 297 nm light, the
colourless solution of 7a turned purple due to the appearance
of a new visible absorption band centred at 526 nm. This colour
change was attributed to the formation of closed-ring isomer
7b from its open-ring counterpart. The purple solution reverted
to colourless state upon irradiation with visible light (λ >
450 nm), which indicated that 7b returned to the initial struc-
ture 7a. Similarly, upon irradiation with 297 nm light, the
colourless solution of 8a turned purple and the absorption
maximum was observed at 526 nm, as the closed-ring isomer
8b was generated. The purple colour disappeared upon irradia-
tion with visible light (λ > 450 nm). These facts indicate that
the dendron-modified compounds 7a and 8a retain the good
photochromic properties associated with diarylethenes.
The effect of concentration upon the fluorescence emission
of 7a and 8a was examined in CH2Cl2 solution at room
temperature (Fig. 3). When the concentration of dendritic dia-
rylethene 7a ranged from 1 × 10−6 to 1 × 10−4 mol L−1, the max-
imum emission revealed a red shift, and the relative intensity
rose dramatically, especially from 5 × 10−5 to 1 × 10−4 mol L−1.
However, change of the concentration from 1 × 10−4 to 1 ×
10−3 mol L−1 caused the relative fluorescence intensity to

284 JOURNAL OF CHEMICAL RESEARCH 2011
Fig. 1 (a) The 1H NMR spectrum of the dendrimer 8; (b) The 13C NMR spectrum of the dendrimer 8.

JOURNAL OF CHEMICAL RESEARCH 2011 285
Scheme 2 Photochromism of dendritic dithienylethenes 7 and 8.
Fig. 2 Absorption spectral changes of dendrimers 7 and 8 in
dichloromethane at room temperature (1.0 × 10−4 mol L−1; 7a
and 8a: the open-ring isomers; 7b and 8b: the closed-ring
isomers).
decrease abruptly. The intensity at the concentration of 1 ×
10−3 mol L−1 was even lower than that of 1 × 10−6 mol L−1
(Fig. 3a). Compound 8a showed similar dependence on the
concentration, but the relative fluorescence intensity was lower
than that of 7a (Fig. 3b). At higher concentration, for example,
10−3 mol L−1, quite low intensity may result from molecular
aggregation and fluorescence quenching.33 Figure 4 illustrates
the fluorescence comparison of 7a, 8a and small molecule 5
in dichloromethane at room temperature. Without dendritic
modification, the intermediate dithienylethene 5 has very weak
fluorescence upon excitation at 290 nm and gives maximal
fluorescent emission at about 420 nm; when dendritic moieties
were coupled with 5, both 7a and 8a showed good fluorescence
under the same conditions. Excited at 290 nm, the fluorescence
emissions of 7a and 8a were observed at 362 nm and 349 nm.
Therefore, compared with the simple dithienylethene 5, intro-
duction of a dendritic moiety into the diarylethene core has a
positive effect on the emission of fluorescence. Further com-
parison shows that compound 5 has a very different fluores-
cence maximum to 7a and 8a, i.e., 5 has a different Stoke’s
shift to 7a and 8a, which may be ascribed to the influence of
triazinyl dendrons on the diarylethene core. Unfortunately, a
higher generation dendrimer would decrease the fluorescence
intensity of dendritic diarylethenes. This undesired phenome-
non might be explained by the higher generation dendrimer
making the conjugated structure of the dendrons twist, thus
causing lower energy transfer efficiency or increased photo-
electron transfer (PET) fluorescence quenching resulting from
the larger number of amino groups in 8a.34
Fig. 3 (a) Concentration effect on the fluorescent emission of
7a in CH2Cl2 at room temperature; (b) Concentration effect on
the fluorescent emission of 8a in CH2Cl2 at room temperature.
Good solubility of materials is often required in the case
of easy processing and manipulation. Dendrimers can be used
to enhance solubility due to their regular and tunable struc-
tures.35,36 The new dendrimer demonstrated good solubility
in common solvents such as CH2Cl2, CHCl3, THF, EtOAc,
CH3CN and DMF, in contrast to the small molecular dithienyl-
ethene 5. Moreover, the dendritic dithienylethene 8 had good
compatibility with PMMA polymer, resulting in its good film-
forming performance. In fact, a uniform thin film of compound
8 in a PMMA matrix has been prepared easily by a spin-
coating technique.

286 JOURNAL OF CHEMICAL RESEARCH 2011
CDCl3) δ 13.4, 41.0, 44.3, 46.1, 164.8, 165.5. IR (KBr) υ 3316, 2969,
2928, 1532, 1489 cm−1.
2,6-Bis{[2,6-bis(diethylamino)-1,3,5-triazin-4-yl]piperazine-1,4-
diyl}-4-chloro-1,3,5-triazine (3): A solution of 2 (2.30 g, 7.48 mmol)
and DIPEA (1.24 mL, 7.48 mmol) in CH2Cl2 (50 mL) was cooled in
an salt/ice-bath before cyanuric chloride (0.63 g, 3.40 mmol) was
added. The reaction solution was stirred in ice-bath for 1 h and at
room temperature for 24 h. The reaction mixture was washed with
5 mol L−1 aq HCl (2 × 100 mL), then water (2 × 100 mL) and dried
over sodium sulfate. The solvent was removed at reduced pressure to
give crude product 3 as a white solid, which was then recrystallised in
petroleum ether/CH2Cl2 to provide white crystals (2.32 g, 92%). M.p.
220–221 °C. 1H NMR (400 MHz, CDCl3) δ 1.14 (t, 24 H, J = 7.0 Hz),
3.54 (q, 16H, J = 7.0 Hz), 3.82 (m, 16H). 13C NMR (100 MHz, CDCl3)
δ 13.5, 41.2, 42.8, 43.1, 43.5, 164.5, 164.7, 165.5, 169.6. IR (KBr)
υ 2979, 2928, 1531, 1492 cm−1.
2,6-Bis{[2,6-bis(diethylamino)-1,3,5-triazin-4-yl]piperazine-1,4-
diyl}-4-piperazino-1,3,5-triazine (4): Similar to the synthesis of
compound 2, compound 4 was prepared from intermediate 3 (2.32 g,
3.19 mmol) and piperazine (830 mg, 9.59 mmol). After workup,
compound 4 was afforded as a white solid (2.23 g, 90%). M.p. 161–
162 oC. 1H NMR (400 MHz, CDCl3) δ 1.15 (t, 24H, J = 6.8 Hz), 1.73
(s, 1H), 2.87 (t, 4H, J = 4.8 Hz), 3.64 (q, 16H, J = 7.2 Hz), 3.74–3.75
(m, 4H), 3.75–3.80 (m, 16H). 13C NMR (100 MHz, CDCl3) δ 12.5,
40.2, 42.2, 42.3, 43.6, 45.2, 163.9, 164.57, 164.61, 164.7. IR (KBr) υ
3333, 3010, 2818, 2778, 1696, 1656, 1589, 1442, 1348, 780 cm−1.
1,2-Bis(5-bromomethyl-2-methyl-3-thienyl)-3,3,4,4,5,5-hexafluo-
rocyclopentene (6): To a stirred solution of compound 5 (200 mg,
0.466 mmol) in THF (50 mL) at 0 °C, phosphorus tribromide (0.013
mL, 0.338 mmol) was added. After stirring for 18 h and then evapora-
tion under vacuum, the residue was dissolved in dichloromethane,
washed with water (2 × 100 mL) and dried over sodium sulfate. The
solvent was removed at reduced pressure to give crude 6 as a yellow
solid. Careful recrystallisation from petroleum ether/THF afforded
white crystals (248 mg, 96%). M.p. 162–163 oC. 1H NMR (400 MHz,
CDCl3) δ 1.88 (s, 6H), 4.63 (s, 4H), 7.05 (s, 2H). 13C NMR (100 MHz,
CDCl3) δ 14.4, 25.9, 124.6, 127.5, 139.0, 143.7. IR (KBr) υ 3019,
2922, 2853, 1640, 1555, 1438, 1337, 985 cm−1.
Fig. 4 Fluorescence spectra of dendrimers 7a and 8a as well
as 5 in CH2Cl2 (5.0×10−5 mol L−1) at room temperature, excited at
290 nm.
Conclusion
In summary, we have rationally designed two new triazine-
based dendrimers containing a diarylethene core, with eight
and 16 ethyl groups in the periphery, respectively. The
products were synthesised in high yields without employing
protection/deprotection or chromatographic separation steps.
These dendrimers have good solubility in common organic
solvents. For optical properties, the two dendritic diaryleth-
enes have demonstrated good photochromic behaviour and
strong fluorescence emission compared with the correspond-
ing small diarylethene molecule 5. The results of this study
are helpful to design efficient photoactive diarylethene
derivatives.
C48H69F6N13S2 (7): A solution of intermediate 2 (554 mg, 1.80
mmol), compound 6 (100 mg, 0.180 mmol), K2CO3 (64.8 mg, 0.469
mmol) in CH2Cl2 (50 mL) was stirred for 18 h at room temperature.
The reaction mixture was washed with water (100 mL) and dried over
sodium sulfate. The solvent was removed at reduced pressure to give
Experimental
Dichloromethane was refluxed with CaH2, and distilled. Tetrahydro-
furan (THF) was refluxed with sodium and benzophenone, and dis-
tilled. All other chemicals, reagents, and solvents were used as
1
165 mg (92%) of product 7 as a purple solid. H NMR (400 MHz,
1
received from commercial sources without further purification. H
CDCl3) δ 1.13 (t, 24H, J = 6.8 Hz), 1.88 (s, 6H), 2.45 (m, 8H), 3.53
NMR and 13C NMR spectra were recorded at 400 MHz and 100 MHz
respectively on a Bruker 400 spectrometer for solutions in CDCl3,
with tetramethylsilane (TMS) as the internal standard. The FTIR
spectra were recorded via the KBr pellet method by using a Bruker
V70 FTIR spectrophotometer. The mass spectra were recorded on a
ESI-FTICR mass spectrometer (Varian 7.0T). UV–Vis absorption
spectra were measured using a Perkin Elmer Lambda-900 spectropho-
tometer. The photoluminescence were recorded on a Hitachi F-450
fluorescence spectrophotometer.
(q, 16H, J =7.0 Hz), 3.63 (s, 4H), 3.77–3.78 (m, 8H), 6.84 (s, 2H). 13
C
NMR (100 MHz, CDCl3) δ 13.4, 14.3, 41.1, 43.0, 52.8, 57.2, 124.4,
125.6, 139.9, 141.4, 164.9, 165.5. IR (KBr) υ 3073, 2971, 2930, 2862,
2811, 1706, 1666, 1539, 1491, 1432, 1371, 1330 cm−1. MS m/z (M+)
1005. Anal Calcd for C48H69F6N13S2: C, 57.29; H, 6.91; N, 18.10.
Found: C, 57.33; H, 6.89; N, 18.12%.
C91H140F6N38S2 (8): Prepared in a similar manner as 7 Reaction of
intermediate 4 (800 mg, 1.03 mmol) and compound 6 (100 mg, 0.180
mmol) provided product 8 as a purple solid (313 mg, 90%). 1H NMR
(400 MHz, CDCl3) δ 1.15 (t, 48H, J = 7.0 Hz), 1.88 (s, 6H), 2.46
(s, 8H), 3.54 (q, 32H, J = 6.8 Hz), 3.65 (s, 4H), 3.80 (m, 40H), 6.87
(s, 2H). 13C NMR (100 MHz, CHCl3) δ 13.4, 14.4, 29.7, 41.1, 43.1,
43.2, 52.8, 57.2, 60.3, 124.4, 125.7, 139.9, 141.5, 164.9, 165.4,
165.56, 165.62. IR (KBr) υ 3070, 2972, 2929, 2857, 1705, 1666,
1532, 1485, 1431, 1370, 1326 cm−1. MS m/z (M+) 1943. Anal Calcd
for C91H140F6N38S2: C, 56.21; H, 7.26; N, 27.37. Found: C, 56.24;
H, 7.28; N, 27.41%.
2,6-Bis(diethylamino)-4-chloro-1,2,4-triazine (1): A solution of
diethylamine (1.39 g, 19.04 mmol) and diisopropylethylamine
(DIPEA) (3.14 mL, 19.04 mmol) in dichloromethane were cooled
on an ice bath, followed by the addition of cyanuric chloride (1.60 g,
8.66 mmol). The solution was stirred at 0 oC for 1 h and then at room
temperature for 24 h. The mixture was washed with aqueous HCl and
water, respectively, and then dried over anhydrous Na2SO4. The sol-
vent was removed in vacuo to give product 1 (2.12 g, 95%) as a pale
1
yellow oil. H NMR (400 MHz, CDCl3) δ 0.87 (t, 12H, J = 7.0 Hz),
3.25 (m, 8H). 13C NMR (100 MHz, CDCl3) δ 12.7, 13.2, 41.3, 41.6,
164.0, 168.9. IR (KBr) υ 2981, 2942, 1559, 1548, 1465 cm−1.
This work was financially supported by the Natural Science
Foundations of China (Nos 51063002, 20962008) and the
Natural Science Foundations of Jiangxi Province (No.
2009GZH0035).
2,6-Bis(diethylamino)-4-piperazino-1,2,4-triazine (2): Compound
1 (2.12 g, 8.22 mmol) and piperazine (3.54 g, 41.09 mmol) in THF
were refluxed overnight and then evaporated. The residue was dis-
solved in dichloromethane, washed with aqueous KOH and water,
respectively, and then dried. The solvent was removed to afford prod-
1
Received 11 February 2011; accepted 19 April 2011
Paper 1100567 doi: 10.3184/174751911X13049474471938
Published online: 1 June 2011
uct 2 (2.30 g, 91%) as a colourless oil. H NMR (400 MHz, CDCl3)
δ 1.14 (t, 12H, J = 7.0 Hz), 1.92 (s, 1H), 2.86 (t, 4H, J = 5.0 Hz), 3.63
(q, 8H, J = 7.0 Hz), 3.74 (t, 4H, J = 5.0 Hz). 13C NMR (100 MHz,

JOURNAL OF CHEMICAL RESEARCH 2011 287
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