Photochromism of asymmetrical diarylethenes with a pyrrole unit: Effects of aromatic stabilization energies of aryl rings

Article abstract of DOI:10.1021/ol400188h

Four novel asymmetrical diarylethenes with a pyrrole and a variable aryl unit have been synthesized. Their crystal structures and photochromic performance were systematically studied to elucidate the effects of the aromatic stabilization energy (ASE) of aryl units. Experimental results showed that their thermal stability as well as cyclization quantum yield, photoconversion ratio, fluorescence modulation efficiency, and fatigue resistance exhibited a strong correlation with ASE.

Full text of DOI:10.1021/ol400188h

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Photochromism of Asymmetrical
Diarylethenes with a Pyrrole Unit:
Effects of Aromatic Stabilization
Energies of Aryl Rings
Gang Liu, Shouzhi Pu,* and Renjie Wang
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal
University, Nanchang 330013, P. R. China
pushouzhi@tsinghua.org.cn
Received November 9, 2012
ABSTRACT
Four novel asymmetrical diarylethenes with a pyrrole and a variable aryl unit have been synthesized. Their crystal structures and photochromic
performance were systematically studied to elucidate the effects of the aromatic stabilization energy (ASE) of aryl units. Experimental results
showed that their thermal stability as well as cyclization quantum yield, photoconversion ratio, fluorescence modulation efficiency, and fatigue
resistance exhibited a strong correlation with ASE.
Photochromic materials have attracted considerable
attention due to their wide applications in optical memory
media and photo-optical switching devices.1 Among var-
ious photochromic materials, the diarylethenes with het-
eroaryl groups are one of the most promising candidates
for their excellent thermal stability and photochemical
reactivity.2 The structural diversity originating from a
variation of a bridge and two aryl units offer a unique
chance for chemists to expand the library of diarylethenes
and seek the ones with ideal photochromic properties.
In the past several decades, significant advancement has
been made in the synthesis of novel diarylethene struc-
tures.3 It has been revealed that the nature of the aryl ring
imposes significant influenceontheir properties.4 Based on
the study of a series of symmetrical diarylethenes with two
phenyl, pyrrolyl, furyl, and thienyl groups, Irie pointed
out that aromaticity of the aryl groups, which well corre-
lates with the ground-energy difference of the open- and
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2008, 64, 2576. (e) Higashiguchi, K.; Matsuda, K.; Tanifuji, N.; Irie, M.
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K.; Tian, H.; Zhu, W. H. Chem.;Eur. J. 2012, 18, 11685. (h) Kitai, J.;
Kobayashi, T.; Uchida, W.; Uchida, K. J. Org. Chem. 2012, 77, 3270. (i)
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Commun. 2012, 48, 11951.
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Pu, S. Z.; Yang, T. S.; Xu, J. K.; Chen, B. Tetrahedron Lett. 2006, 47,
6473. (c) Morimitsu, K.; Shibata, K.; Kobatake, S.; Irie, M. J. Org.
Chem. 2002, 67, 4574. (d) Pu, S. Z.; Liu, G.; Shen, L.; Xu, J. K. Org. Lett.
2007, 9, 2139. (e) Yamaguchi, T.; Irie, M. J. Photochem. Photobiol. A
2006, 178, 162. (f) Morimitsu, K.; Kobatake, S.; Irie, M. Tetrahedron
Lett. 2004, 45, 1155. (g) Sysoiev, D.; Fedoseev, A.; Kim, Y.; Scheer, E.;
Groth, U.; Steiner, U. E. Chem.;Eur. J. 2011, 17, 6663. (h) Uchida, K.;
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Angew. Chem., Int. Ed. 2012, 51, 1.
r
10.1021/ol400188h
XXXX American Chemical Society

closed-ring forms, is the key molecular property that deter-
mines the thermal stability of the closed-ring isomers.1a
While the diarylethenes with two thiophenes, furans, and
thiazoles exhibit excellent thermal stability, the ones with
two indoles, pyrrols, and benzenes are thermally unstable.
It was concluded that introduction of heteraryl groups
with low aromatic stabilization energy (ASE) is the design
principle of thermally irreversible diarylethenes. But the
effects of the ASE of aryl units on more complicated
unsymmetrical diarylethenes, especially those containing
aryl groups of both high and low aromatic stabilization
energies, remain unclear.
Analysis of our previous research on novel asymmetrical
perfluorodiarylethenes indicated that the heteroaryl groups
with low aromaticity stabilization energies (e.g., thiazyl and
thienyl groups) affected not only thermal stability but
also, more importantly, certain photochromic properties
of asymmetcial diarylethenes, such as cyclization quantum
yield, photoconversion ratio, and fluorescence modulation
efficiency.5 For instance, a hybrid of a thiazole and benzene
possessed significantly improved thermal stability and
photochromic features compared with the diphenylethene,
which is unreactive to photoirradiation.5a Similarly, when
one pyrrol unit of thermally unstable dipyrrolylethene was
replaced with a phenylthiophene, the new diarylethene ex-
hibited much better properties.4d However, the effects of the
aromatic stabilization energy of aryl moieties on the photo-
chromic performance of asymmetrical diarylethenes has not
been systematically investigated.
Figure 1. Thermal fading of 1cꢀ4c at 293 K in ethyl acetate.
Fishtik’s research,1a,6 the aromatic stabilization energy
of the aryl units increases in order of thiazyl < thienyl
(4.7 kcal/mol) < pyrrolyl (13.8 kcal/mol) < pyridyl < phenyl
(27.7 kcal/mol). As shown in Figure 1, the thermal cyclo-
reversion rates of 1cꢀ4cat 293 K accelerated, when ASE of
the variable aryl unit is elevated. This result indicated that
Irie’s design principle of thermally irreversible diaryl-
ethenes is very applicable to asymmetrical ones.
In this research work, we designed and synthesized four
novel asymmetrical pyrrole-containing diarylethenes, which
have a thiazole, thiophene, pyridine, and benzene moiety,
respectively (Scheme 1). Their photochromic properties in
different states and crystallographic structures were studied
and compared to elucidate the effects of the ASE of aryl
rings on the thermal stability and photochromism of asym-
metrical diarylethenes.
Scheme 1. Photochromism of Diarylethenes 1oꢀ4o
Figure 2. Absorption spectrum and the color changes of diaryl-
ethene 3 with stimulation of acid/base and light in hexane (2.0 ꢁ
10ꢀ5 mol Lꢀ1) at room temperature.
The absorption spectrum of 1oꢀ4o in both hexane and
PMMA films were recorded with a UV/vis spectrometer
(Figure S2, SI). The absorption maxima of both open-ring
and closed-ring isomers of 1ꢀ4 were summarized in Table 1.
As shown in Figure 2, 3o exhibited a sharp absorption peak
at 291 nm in hexane corresponding to a πfπ* transition.7
Upon irradiation with UV light, a new visible absorption
band centered at 573 nm emerged due to the formation of
closed-ring isomer 3c, with a solution color change from
colorless to blue. The blue color was bleached by irradiation
with visible light (λ> 450 nm), and the absorption spectrum
Diarylethenes 1oꢀ4o were synthesized according to known
procedures4d and characterized by NMR, elemental analysis,
and IR spectroscopy. All four diarylethenes cyclized with
notable solution color change upon UV irradiation. The
thermal stability of the closed-ring isomers (1cꢀ4c) was
evaluated in ethyl acetate at different temperatures (Figure S1,
Supporting Information (SI)). According to Irie’s and
(6) Fishtik, I.; Stefan, G. Phys. Chem. Chem. Phys. 2012, 14, 15888.
(7) Li, Z. X.; Liao, L. Y.; Sun, W.; Xu, C. H.; Zhang, C.; Fang, C. J.
J. Phys. Chem. C 2008, 112, 5190.
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Pu, S. Z.; Yan, P. J.; Liu, G. Tetrahedron Lett. 2011, 52, 143.
B
Org. Lett., Vol. XX, No. XX, XXXX

conformations, and the distances between the two reactive
˚
˚
˚
carbons were shorter than 4.2 A (3.589 A for 1o and 3.628 A
for 2o, Table S2, SI). As expected, the crystals of 1o and 2o
exhibited a notable color change upon irradiation with 297
nm light.8 In contrast, exposure to visible light entirely
bleached the colors of 1c and 2c. The photochromism of 1
and 2 was highly consistent in the crystalline phase even
after 200 repeated cycles, which is favorable for applications
in optoelectronic devices.9 Though the crystals of 3o and 4o
were also packed in anti-parallel conformations and the
˚
distances between bond-forming carbons were 4.054 A for
˚
3o and 3.829 A for 4o, they still showed no photochromism.
The above results showed that other than the conforma-
tion and distance factors, the heteroaryl unit with low ASE
played a pivotal role in the photochromism of asymmetrical
diarylethenes in the crystalline phase. Our previous research
on benzene-based diarylethenes is also consistent with this
point.10 When both aryl units are of high ASE, such as
diphenylethene and 4o, their crystals showed no photo-
reactivity. When aryl units in the diarylethene system
were a low ASE thiophene and a high ASE pyridine, their
crystals exhibited excellent photochromic activity.5b
Figure 3. Photoconversion ratios of 1ꢀ4 in the photostationary
state in hexane by HPLC analysis.
returned to the initial state of 3o. Furthermore, addition
of trifluoroacetic acid to the solution of 3c produced the
protonated 3c0 whose absorption maximum shifted from
573 to 382 nm, accompanied with a notable solution color
change from blue to pale yellow.
As shown in Table 1, cyclization quantum yields (Φo‑c
)
and photoconversion ratios in the photostationary state
(PR) of1ꢀ4showed trends that were similar to their thermal
stability. The Φo‑c of 1ꢀ4 decreased (0.15 for 1, 0.13 for 2,
0.065 for 3, and 0.041 for 4) incrementally with the ASE of
the variable aryl group. In the photostationary state, the
photoconversion ratios of 1ꢀ4 from the open-ring to closed-
ring isomers were determined by HPLC analysis with values
of 79% for 1, 78% for 2, 48% for 3, and 16% for 4, which is
in accordance with the increasing order of the ASE of the
variable aryl rings (Figure 3). It is noteworthy that when the
asymmetrical diarylethenes are composed of both low and
high ASE aryl units, their Φo‑c and PR are significantly
better than those with two high ASE aryl rings.
Table 1. Photochromic Parameters of Diarylethenes 1ꢀ4 in
Hexane (2.0 ꢁ 10ꢀ5 mol Lꢀ1) at Room Temperature
Φc
λ
c,max/nma
(ε/L molꢀ1 cmꢀ1
λ
c,max/nmb
(ε/L molꢀ1 cmꢀ1
)
)
Φo‑c
Φc‑o PRd (%)
243 (2.36 ꢁ 104)
238 (2.59 ꢁ 104)
291 (8.95 ꢁ 103)
290 (1.26 ꢁ 104)
509 (4.36 ꢁ 103) 0.15
553 (4.39 ꢁ 103) 0.13
0.23
0.14
79
78
48
16
Figure 4. ORTEP drawings (ellipsoids are drawn at 30% level):
(A) 1o; (B) 2o; (C) 3o; (D) 4o; (E) photographs demonstrating
their photochromic processes in the crystalline phase.
1
2
3
4
573 (1.65 ꢁ 103) 0.065 0.30
572 (1.08 ꢁ 103) 0.041 0.27
a Absorption maxima of open-ring isomers. b Absorption maxima of
The fluorescence spectra of 1oꢀ4o (Figure 5) and the
closed-ring isomers. c Quantum yields of cyclization reaction (Φo‑c) and
cycloreversion reaction (Φc‑o). d Photoconversion ratios of diarylethenes
1ꢀ4 in the photostationary state.
emission intensity changes induced by photoirradiation
(8) (a) Morimoto, M.; Irie, M. Chem. Commun. 2005, 3895. (b)
Kobatake, S.; Irie, M. Chem. Commun. 2002, 23, 2804.
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Single crystals of 1oꢀ4o were obtained via slow evapora-
tion of their solutions and subjected to X-ray diffraction
analysis (Figure S3 and Table S1, SI). Their ORTEP draw-
ings and photochromic processes in the crystalline phase are
shown in Figure 4. 1o and 2o were packed in anti-parallel
2006, 18, 2910. (b) Berberich, M.; Krause, A. M.; Orlandi, M.; Scandola,
€
F.; Wurthner, F. Angew. Chem. 2008, 120, 6718. (c) Norsten, T. B.;
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Org. Lett., Vol. XX, No. XX, XXXX
C

(FigureS4, SI) weremeasuredin hexane and PMMA films.
The parameters of their fluorescence behaviors (Table 2)
showed that 1oisthe best in terms of emission intensity and
fluorescence quantum yield, 2o and 3o are of medium level,
and 4o has the lowest values in both hexane and PMMA
films. The results suggested that introduction of a nitrogen
atom to the variable aryl ring (1o vs 2o, 3o vs 4o) has a re-
markable impact on their fluorescence performance. Due
to the fact that the fluorosecence change of diarylethene
upon UV irradiation is dependent on photoconversion
ratio,11 the fluorescence modulation efficiency (η) of 1ꢀ4
should also correlate with the ASE of the variable hetero-
aryl unit. The η values determined for 1ꢀ4 in both hexane
and PMMA films demonstrated this hypothesis well. The
fluorescence modulation efficiencies of the diarylethenes
with at least one low ASE unit (74/79% for 1o, 69/69% for
2o) are significantly higher than those of the diarylethenes
with two high ASE units (21/30% for 3o, 16/21% for 4o).
In addition, the fatigue resistance of 1ꢀ4 was also deter-
mined (Figure S5, SI). In PMMA films, the diarylethenes
with one low ASE unit exhibited much better fatigure re-
sistance (93% for 1o and 91% for 2o after 150 repeat cycles)
than those with two high ASE units (84% for 3o and 80%
for 4o after 150 repeat cycles) due to the lowered ground
state energy of the closed-ring isomers. When exposed to air
in the solution phase, the fatigue resistance of 1ꢀ4 was
remarkably reduced, which could be possibly ascribed to the
presence of a pyrrole unit in these compounds.12
Figure 5. Fluorescence emission spectra of diarylethenes 1oꢀ4o
in both hexane (A) and PMMA films (B) at room temperature.
Table 2. Fluorescence Parameters of Diarylethenes 1ꢀ4 in Both
Hexane (2.0 ꢁ 10ꢀ5 mol Lꢀ1) and PMMA Films
hexane
PMMA
a
λem
Ifb
ηc (%)
λem
If
η (%)
Φf
1o
2o
3o
4o
448
409
426
411
1500
817
725
105
74
69
21
16
421
408
414
406
7449
4088
3145
657
79
69
30
21
0.085
0.054
0.039
0.031
In summary, four novel diarylethenes with a pyrrole unit
and a variable aryl unit have been synthesized to study the
effects of ASE of aryl rings on their thermal and photo-
chromic behaviors. Experimental results showed that the
thermal stability of 1ꢀ4 correlates with the ASE of the
variable aryl rings. Thermal fading of the closed-ring
isomer was significantly suppressed by the introduction
of a low ASE aryl unit, which is in good accordance with
Irie’s principle. Furthermore, their photochromic proper-
ties closely related to the ground-energy difference of the
open- and closed-ring isomers, such as cyclization quan-
tum yield, photoconversion ratio, photochromism in the
crystalline phase, fluorescence modulation efficiency, and
fatigue resistance, also exhibited a similar dependence on
ASE. Although a more detailed investigation on more
structural analogs is required to completely elucidate the
structureꢀproperties relationship for diarylethenes, the re-
sults of our research provided new insights into the effects of
a Emission peak. b Emission intensity. c Fluorescence modulation
efficiency in the photostationary state.
aryl moieties, which is valuable for the design of novel
asymmetrical diarylethenes with ideal thermal and photo-
chromic properties.
Acknowledgment. We thank the NSFC of China
(21262015, 21162011), the Project of Jiangxi Youth Scien-
tist (2010DQ00200), the Project of Jiangxi Advantage Sci-
Tech Innovative Team (20113BCB24023), and the Project
of the Science Funds of Jiangxi Education Office (KJLD-
12035, GJJ12587) for financial support.
Supporting Information Available. Experimental pro-
cedures, characterization data, and CIF files for 1ꢀ4.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(11) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T.
Nature 2002, 759.
(12) Higashiguchi, K.; Matsuda, K.; Yamada, T.; Kawai, T.; Irie, M.
Chem. Lett. 2000, 1358.
The authors declare no competing financial interest.
D
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