The effect of the formyl group position upon asymmetric isomeric diarylethenes bearing a naphthalene moiety

Article abstract of DOI:10.3762/bjoc.8.114

Three new isomeric asymmetric diarylethenes with a naphthyl moiety and a formyl group at the para, meta or ortho position of the terminal benzene ring were synthesized. Their photochromism, fluorescent-switch, and electrochemical properties were investiga

Full text of DOI:10.3762/bjoc.8.114

The effect of the formyl group position upon
asymmetric isomeric diarylethenes
bearing a naphthalene moiety
Renjie Wang, Shouzhi Pu*§, Gang Liu and Shiqiang Cui
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1018–1026.
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science &
Technology Normal University, Nanchang 330013, PR China
doi:10.3762/bjoc.8.114
Received: 20 February 2012
Accepted: 19 June 2012
Published: 05 July 2012
Email:
Shouzhi Pu* - pushouzhi@tsinghua.org.cn
* Corresponding author
This article is part of the Thematic Series "Molecular switches and cages".
Guest Editor: D. Trauner
§ Tel.: +86-791-83831996; Fax: +86-791-83831996
Keywords:
chemical diversity; diarylethene; electrochemistry; formyl group;
photochromism; substituent position effect
© 2012 Wang et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Three new isomeric asymmetric diarylethenes with a naphthyl moiety and a formyl group at the para, meta or ortho position of the
terminal benzene ring were synthesized. Their photochromism, fluorescent-switch, and electrochemical properties were investi-
gated. Among these diarylethenes, the one with a formyl group at the ortho position of benzene displayed the largest molar absorp-
tion coefficients and fluorescence quantum yield. The cyclization quantum yields of these compounds increased in the order of
para < ortho < meta, whereas their cycloreversion quantum yields decreased in the order of meta > para > ortho. Additionally, all of
these diarylethenes functioned as effective fluorescent switches in both solution and PMMA films. Cyclic voltammograms
proved that the formyl group and its position could effectively modulate the electrochemical behaviors of these diarylethene deriva-
tives.
Introduction
In the past decade, photochromic materials have received much Light is a convenient and powerful trigger to control the reactiv-
attention because of their applications in potential photoswitch- ity of biomolecules in organisms, exemplified by its use in
able, molecular devices and optical memory storage systems fluorescent probes, fluorescence imaging, and molecular
[1]. Among these materials, the utilization of diarylethene switches in logic circuits [15-22]. In general, in order to effi-
derivatives in molecular electronics, optical memory, and vari- ciently realize the artificial induction of photosensitivity in
able-transmission filters has been well documented [1-3]. So biomolecules, the photoactive molecules must possess the
far, a huge number of studies concerning the photochromic following properties: (1) low cytotoxicity, (2) high sensitivity,
properties of dithienylethene derivatives have been reported (3) easy chemical modification. In the past few decades, various
[4-14].
types of photochromic molecules, such as fulgides, spiro-
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Beilstein J. Org. Chem. 2012, 8, 1018–1026.
pyranes, azobenzenes, and diarylethenes, have been developed The results revealed that these molecules have excellent
[2,23-31].
photochromism with good fatigue resistance and thermal
stability [37]. In this study, in order to further elucidate the
Among these compounds, diarylethene is one of the most substituent position effects on the photochromic features of
promising photoswitchable units within the photochromic naphthalene-containing diarylethenes, we synthesized three new
system, and the successful use of diarylethenes as a fluores- isomeric diarylethenes with a formyl group at the para, meta,
cence modulation center to realize a photoswitchable probe for and ortho position on the terminal benzene ring (1–3). The
imaging living cells was reported. For example, Zou et al. photochromic scheme of 1–3 is shown in Scheme 1.
reported an amphiphilic molecule with hydrophilic and
hydrophobic chains on two ends of a rigid diarylethene core. Results and Discussion
This compound can form stable vesicle nanostructures in The synthesis route for diarylethenes 1o–3o is shown in
aqueous solution, and exhibits excellent switchable fluores- Scheme 2. First, the benzaldehydethiophene derivatives 5a–c
cence between open and closed states in the living cells, with were prepared by Suzuki coupling of three bromobenzaldehyde
low cytotoxicity [32]. Piao et al. developed a multiresponsive derivatives with a thiopheneboronic acid 4 [38-42]. Second,
fluorescent molecular switch containing terpyridine. This di- 1,3-dioxolane-phenylthiophene derivatives 6a–c were prepared
arylethene can serve as a detector for metal-ion transmembrane by the reported method [39-41,43]. Then, 1,3-dioxolane-
transport [33]. Singer et al. explored a novel diarylethene with a phenylthiophene derivatives 6a–c were separately lithiated and
7-deazaadenosine, which led to new research in photochromic coupled with (2-methylnaphth-1-yl)perfluorocyclopentene [37]
nucleosides and molecular recognition properties of nucleic to give the asymmetric diarylethene derivatives 7a–c [44].
acids with the light sensitivity of diarylethenes [34]. Recently, Finally, compounds 1o–3o were prepared by hydrolyzing com-
Wu et al. designed and synthesized a novel diarylethene- pounds 7a–c in the presence of pyridine and p-toluenesulfonic
containing dithiazolethene, which exhibited a gated photo- acid in acetone/water. The structures of 1o–3o were confirmed
chromic reactivity controlled by complexation/dissociation with by elemental analysis, NMR, and IR (Supporting Information
BF3 [35]. All of the above research revealed that the explored File 1).
novel diarylethenes have versatile applications and are still
important and attractive.
Photoisomerization of diarylethenes 1–3
Diarylethenes 1−3 showed good photochromic properties and
Diarylethene derivatives, especially those containing a per- could be toggled between their colorless open-ring isomers
fluorocyclopentene bridge are one of the most promising photo- (1o−3o) and colored closed-ring isomers (1c−3c) by alternate ir-
chromic compounds due to their high fatigue resistance and radiation with UV and visible light (λ > 500 nm). As shown in
thermal stability [2,36]. In general, the photochromic reactivity Figure 1A, diarylethene 1o exhibited a sharp absorption peak at
of diarylethenes mainly depends on heteroaryl groups and 323 nm (ε, 2.82 × 104 L mol−1 cm−1) in hexane, which arose
different electron-donor/acceptor substituents. The formyl from the π→π* transition [45]. Upon irradiation with 297 nm
group can be modified to form various chemical groups and it light, the colorless solution of 1o gradually turned red, and a
can also be connected with different fluorophores via a Schiff new absorption band was observed in the visible region
base structure. In a previous work, we developed a new class of centered at 524 nm (ε, 1.40 × 104 L mol−1 cm−1) due to the for-
diarylethenes with a naphthalene group and a thiophene group. mation of the closed-ring isomer 1c. Alternatively, the red-
Scheme 1: Photochromism of diarylethenes 1–3.
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Beilstein J. Org. Chem. 2012, 8, 1018–1026.
Scheme 2: Synthetic route for diarylethenes 1–3.
colored solution could be bleached to become colorless by and 268 nm, respectively. The color changes of diarylethenes
re-production of the open-ring isomer 1o upon irradiation with 1–3 by alternating irradiation with UV and visible light
visible light (λ > 500 nm). In the photostationary state, a clear (λ > 500 nm) in hexane are shown in Figure 2A.
isosbestic point of diarylethene 1 was observed at 349 nm,
which supported the reversible two-component photochromic In PMMA amorphous films, diarylethenes 1–3 also showed
reaction scheme [46]. Similarly 1o, compounds 2o and 3o also similar photochromic activity to that in hexane. The absorption
showed good photochromism in hexane (Figure 1B). The color- maxima of closed-ring isomers of diarylethenes 1c–3c in
less solutions of 2o and 3o turned pink and magenta due to the PMMA films were at longer wavelengths. The values of the
formation of the closed-ring isomers 2c and 3c, when irradiated absorption maxima of the ring-closed isomers are 14 nm for 1c,
with 297 nm light. Their absorption maxima appeared at 512 7 nm for 2c, and 21 nm for 3c. The redshift phenomena may be
and 496 nm respectively. The colored solutions of 2c and 3c can ascribed to a polar effect of the polymer matrix and the stabi-
also be decolorized upon irradiation with visible light lization of the molecular arrangement in the solid medium
(λ > 500 nm), and their isosbestic points were observed at 277 [47,48]. The color changes of diarylethenes 1–3 upon alter-
Figure 1: Absorption spectral changes of diarylethenes 1–3 by photoirradiation with UV–vis in hexane (2.0 × 10−5 mol/L) at room temperature: (A) 1;
(B) 2 and 3.
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Beilstein J. Org. Chem. 2012, 8, 1018–1026.
Figure 2: The color changes of diarylethene 1–3 by photoirradiation at room temperature: (A) in hexane; (B) in PMMA films.
nating irradiation with UV and visible light in PMMA films are The photochromic features of compounds 1–3 are summarized
shown in Figure 2B. The photoconversion ratios from open-ring in Table 1. The results indicate that the position of the formyl
to closed-ring isomers of 1–3 were analyzed by HPLC in the group at the terminal benzene significantly affects the photo-
photostationary state (Figure 3). It was calculated that their chromic properties of these diarylethenes, such as the absorp-
photoconversion ratios in the photostationary state were 82% tion maxima, molar absorption coefficients, and quantum yields
for 1, 79% for 2, and 81% for 3.
of cyclization and cycloreversion. For the isomeric
diarylethenes 1–3, the absorption maxima of both the open-ring
and closed-ring isomers exhibited a remarkable hypochromatic
shift when the formyl group was moved from the para to the
meta, then to the ortho position in both hexane and PMMA
films, whereas the molar absorption coefficients of
diarylethenes 1–3 increased in order of meta < para < ortho
substitution by the formyl group in hexane. The results were in
agreement with those of the reported diarylethenes containing
an electron-withdrawing cyano group [49], but were different
from those with an electron-donating methoxy group
[14,50,51]. The cycloreversion quantum yields of diarylethenes
1–3 increased in the order of ortho (Φc-o = 0.11) < para (Φc-o =
0.12) < meta substitution (Φc-o = 0.15) by the formyl group.
However, the cyclization quantum yield of the para-substituted
derivative 1 was the largest (Φc-o = 0.35), while that of the
meta-substituted derivative 2 was the lowest (Φc-o = 0.21).
Compared to the electron-donating methoxy group or electron-
withdrawing cyano group [49,50], the position of the electron-
Figure 3: The photoconversion ratios of diarylethenes 1–3 in the
photostationary state as analyzed by HPLC.
Table 1: Absorption spectral properties of diarylethenes 1–3 in hexane (2.0 × 10−5 mol L−1) and in PMMA films (10%, w/w) at room temperature.
λo,max/nma (ε/L mol−1 cm−1)
λc,max/nmb (ε/L mol−1 cm−1)
Φc
conversion at PSS in
hexane
compound
hexane
PMMA film
hexane
PMMA film
Φo-c
Φc-o
1
2
3
323 (2.82 × 104)
256 (2.75 × 104)
245 (3.48 × 104)
326
288
248
524 (1.40 × 104)
512 (1.01 × 104)
496 (1.57 × 104)
538
519
517
0.35
0.21
0.25
0.12
0.15
0.11
82
79
81
aAbsorption maxima of ring-open isomers. bAbsorption maxima of ring-closed isomers. cQuantum yields of ring-open (Φo-c) and ring-closed isomers
(Φc-o), respectively.
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Beilstein J. Org. Chem. 2012, 8, 1018–1026.
withdrawing formyl group can effectively modulate the absorp- temperature [2,52]. As shown in Figure 4, the coloration and
tion maxima of diarylethenes, which may be a novel strategy decoloration cycle of 1–3 can be repeated more than 100 times
for exploring photochromic diarylethenes at shorter wave- in hexane with less than 5% degradation of 1c–3c. In PMMA
lengths.
films, 1–3 also exhibited excellent photochromic properties
after 200 cycles with only ca. 6–10% degradation of 1c–3c. The
The thermal stabilities of the open-ring and closed-ring isomers results showed that all three isomeric diarylethenes 1–3 had
of 1–3 were tested by storing the compounds at both room good fatigue resistance in both hexane and PMMA films.
temperature in hexane and at 351 K in ethanol. The hexane
solutions were kept at room temperature in the dark, and Fluorescence of diarylethenes 1–3
exposed to air for more than two months. No changes in the Fluorescence can be used not only in molecular-scale optoelec-
UV–vis spectra were observed for 1–3. At 351 K, diarylethenes tronics but also in digital photoswitching [22,53,54]. Like most
1–3 also showed excellent thermal stability for more than 12 h of reported diarylethenes, diarylethenes 1–3 exhibited notable
in ethanol. Fatigue resistance is a critical factor for practical fluorescence in both hexane and PMMA films. Their fluores-
applications in optical devices, and the fatigue resistances of cence spectra were measured at room temperature with a
diarylethenes 1–3 were examined in both hexane and PMMA Hitachi F-4500 spectrophotometer (Figure 5). In hexane, the
films by alternate irradiation with UV and visible light at room emission peaks of 1o–3o were observed at 384, 387, and
Figure 4: Fatigue resistance of diarylethenes 1–3 in hexane in air atmosphere at room temperature: (A) in hexane; (B) in PMMA films. Initial
absorbance of the sample was fixed to 1.0.
Figure 5: Fluorescence emission spectra of diarylethenes 1–3 at room temperature: (A) in hexane solution (2.0 × 10−5 mol L−1); (B) in PMMA films
(10%, w/w).
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Beilstein J. Org. Chem. 2012, 8, 1018–1026.
389 nm, when excited at 307, 315, and 300 nm, whereas those photostationary state was 41% in hexane and 65% in a PMMA
in PMMA films were observed at 438, 441, and 427 nm, when film. Similarly, in the photostationary state, the fluorescence
excited at 334, 300, and 300 nm respectively. In comparison modulation efficiencies of diarylethenes 2 and 3 in hexane were
with those of 1o–3o in hexane, the fluorescence emission peaks 73 and 78%, and those in PMMA films were 38 and 42%, res-
of 1o–3o in PMMA films consistently exhibited a remarkable pectively. The residual fluorescence for 1–3 in the photosta-
bathochromic shift. The emission intensity of the ortho-substi- tionary state may be attributed to an incomplete cyclization
tuted derivative 3o was the strongest, while that of the para- reaction and the existence of parallel conformations [55,56].
substituted derivative 1o was the weakest in both hexane and Among the three isomeric derivatives, the fluorescent modula-
PMMA films. Compared to the unsubstituted parent diaryl- tion efficiency of diarylethene 2 was the largest and that of 1
ethene, 1-(2-methylnaphth-1-yl)-2-[2-methyl-5-phenylthien-3- was the smallest in PMMA films, suggesting that the diaryl-
yl]perfluorocyclopentene (Φf = 0.011) [37], the fluorescence ethene 2 is the best candidate for the fluorescence photo-
emission intensities of diarylethenes 1o and 2o were decreased, switching material.
but that of 3o was evidently increased. When anthracene was
Electrochemical properties of diarylethenes
used as the reference, the fluorescence quantum yields of 1o–3o
were determined to be 0.012, 0.018, and 0.059, respectively, 1–3
indicating that the formyl group on the terminal benzene ring The electrochemical behaviors of diarylethene derivatives have
could notably enhance the fluorescence quantum yield and attracted much attention because of their potential applications
remarkably influence the fluorescence emission intensity of in molecular-scale electronic switches [57-62]. The electro-
diarylethenes with a naphthalene moiety.
chemical properties of 1–3 were evaluated by cyclic voltam-
metry (CV) under the same experimental conditions reported
Diarylethenes 1–3 exhibited an evident fluorescence switching previously [13]. The CV curves of diarylethenes 1–3 are shown
capability upon changing from the open-ring to the closed-ring in Figure 7. The onset potentials (Eonset) of oxidation and reduc-
isomers by photoirradiation in both hexane and PMMA films. tion for 1o were initiated at +1.79 and −0.89 V, and those of 1c
When irradiated by UV light, the photocyclization reaction at +1.76 and −0.91 V, respectively. According to the reported
yielded the nonfluorescent closed-ring isomers 1c–3c, resulting method [63,64], the ionization potential and electron affinities
in a decrease in emission intensity as compared to the open-ring of 1o were calculated to be −6.59 and −4.09 eV, and those of 1c
isomers 1o–3o. The back irradiation by visible light of appro- were −6.56 and −3.89 eV. Based on the highest occupied mole-
priate wavelength (λ > 500 nm) regenerated the open-ring cular orbital (HOMO) and lowest unoccupied molecular orbital
isomers 1o–3o, and recovered their original emission intensity. (LUMO) energy level, the band gap Eg (Eg = LUMO–HOMO)
As shown in Figure 6, upon irradiation with UV light, the emis- of 1o and 1c can be determined to be +2.50 and +2.67 eV. Simi-
sion intensity of 1 decreased, and was quenched to ca. 59% in larly, the oxidation potential of 2o and 3o is initiated at +1.76
hexane and 35% in a PMMA film, when it arrived at the photo- and +1.83 V, and that of 2c and 3c is initiated at +1.74 and
stationary state. The fluorescent modulation efficiency in the +1.79 V. The results indicate that the oxidation process for the
Figure 6: Emission intensity changes of diarylethene 1 upon irradiation with UV light at room temperature: (A) in hexane (excited at 307 nm), (B) in a
PMMA film (excited at 324 nm).
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Figure 7: Cyclic voltammetry of diarylethenes 1–3 in acetonitrile with a scanning rate of 50 mV/s.
open-ring isomers 1o–3o occurs at higher potentials than in the
Table 2: Electrochemical properties of diarylethenes 1–3.
corresponding closed-ring isomers 1c–3c. This was because the
longer conjugation length of the closed-ring isomers generally
leads to a less positive potential [62,65]. The cyclization reac-
tion allows the π-conjugation to extend across the perfluoro-
cyclopentene ring causing a lower oxidation onset. As shown in
Table 2, for the band gap of diarylethenes 1 and 2, the values of
Eg of the open-ring isomers were lower than those of the
closed-ring isomers, with the exception of 3. Among these com-
pounds, the Eg of 1o was the smallest, which implies that the
charge transfer of 1o was faster compared to that in others [66].
All these data suggest that the position of the formyl group at
the terminal benzene ring has a remarkable effect on the electro-
chemical behaviors of these diarylethenes, but further work is
required to quantify these effects.
compound
oxidation
reduction
band gap
Eonset
IP
Eonset
EA
Eg
(V)
(eV)
(V)
(eV)
1o
1c
2o
2c
3o
3c
+1.79
+1.76
+1.76
+1.74
+1.83
+1.79
−6.59
−6.56
−6.56
−6.54
−6.63
−6.59
−0.89
−0.91
−0.83
−1.01
−0.87
−0.80
−4.09
−3.89
−3.97
−3.79
−3.93
−4.00
2.50
2.67
2.59
2.75
2.70
2.59
Supporting Information
Supporting Information File 1
Conclusion
Experimental procedures and spectral data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-114-S1.pdf]
Three new asymmetric isomeric diarylethenes containing a
formyl group at either the para, meta, or ortho position of the
terminal benzene ring were synthesized for the investigation of
the effect of substituent position on their optical and electro-
chemical properties. The results revealed that the formyl group Acknowledgements
and its position had significant effects on the properties of these This work was supported by the National Natural Science Foun-
isomeric diarylethenes. The electron-withdrawing formyl group dation of China (20962008, 21162011), the Project of Jiangxi
endowed these diarylethenes with some new properties, which Academic and Technological Leader (2009DD00100), the Pro-
were different from those of diarylethenes with a methoxy ject of Jiangxi Youth Scientist, and the Project of the Science
group or halide at the terminal benzene.
Funds of Jiangxi Education Office (GJJ10241, GJJ09646).
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Beilstein J. Org. Chem. 2012, 8, 1018–1026.
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