Spiropyran-azobenzene-DBU system as solvent indicator

Article abstract of DOI:10.1016/j.tetlet.2018.07.025

A dyad bearing azobenzene and spiropyran units was synthesized and its applications in indicating the polarity and protic or aprotic properties of a solvent were explored. The spiropyran-azobenzene derivative (SPAB) can be induced to different forms in different miscellaneous solvents accompanied with different color changes and spectral characteristics at the presence of organic base DBU. In a nonpolar or low-polar solvent, SPAB exists in thermostable spiropyran form with yellow color output. While in an aprotic polar solvent, the spiropyran part isomerized to merocyanine form giving a blue color. When SPAB is subjected to a protic solvent, the alkylation reaction occurs at the oxygen generating the alkylated-SPAB with red color. This solvent-dependent property can be used for discriminating solvent type.

Full text of DOI:10.1016/j.tetlet.2018.07.025

Accepted Manuscript
Spiropyran-azobenzene-DBU System as Solvent Indicator
Zhiping Xu, Shen Li, Yujun Shen, Meijun Chen, Xusheng Shao
PII:
S0040-4039(18)30885-2
https://doi.org/10.1016/j.tetlet.2018.07.025
TETL 50134
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
16 May 2018
3 July 2018
9 July 2018
Please cite this article as: Xu, Z., Li, S., Shen, Y., Chen, M., Shao, X., Spiropyran-azobenzene-DBU System as
Solvent Indicator, Tetrahedron Letters (2018), doi: https://doi.org/10.1016/j.tetlet.2018.07.025
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Spiropyran-azobenzene-DBU System as Solvent Indicator
Zhiping Xub, Shen Lib, Yujun Shenb, Meijun Chenb, Xusheng Shaoa,b,*
aState Key Laboratory of Bioreactor Engineering, East China University of Science and Technology, Shanghai 200237, China
bShanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China University of Science and Technology,
Shanghai, 200237, China
Abstract: A dyad bearing azobenzene and spiropyran units was synthesized and its applications in
indicating the polarity and protic or aprotic properties of a solvent were explored. The
spiropyran-azobenzene derivative (SPAB) can be induced to different forms in different miscellaneous
solvents accompanied with different color changes and spectral characteristics at the presence of organic
base DBU. In a nonpolar or low-polar solvent, SPAB exists in thermostable spiropyran form with yellow
color output. While in an aprotic polar solvent, the spiropyran part isomerized to merocyanine form giving
a blue color. When SPAB is subjected to a protic solvent, the alkylation reaction occurs at the oxygen
generating the alkylated-SPAB with red color. This solvent-dependent property can be used for
discriminating solvent type.
Key words: spiropyran; solvent; solvatochromism.
1. Introduction
Spiropyran and azobenzene are important photochromic compounds because of their potential
applications in a variety of fields such as molecular machines, organic synthesis, material science and
biological function modulation.1-5 Spiropyrans are highly sensitive compounds that can be reversibly
switched forth and back between colorless spiropyran (SP) and colored merocyanine (MC) form upon
6
external stimuli such as heat, light, acid and mechanical press. SP and MC forms have dramatically
different polarities, charges, and molecular conformations. Typical photochromic reaction of spiropyrans
7
is the reversible photochemical cleavage of the C-O bond in the spiropyran ring. This reversible
transformation has been widely used in numerous areas including self-assembly,8 modulation of
9-12
13-14
fluorescence of nanoparticles,
recognition and quantification of amino acids,
electro-optical
devices, 15molecular logic switches,17-18 proton transfer,19 photoswitchable biomaterials
high-density photochemical erasable memories. 22
and
20-21
Figure 1. Photochromic transformation of spiropyrans and azobenzene and molecular design of SPAB
Spiropyran can exhibit solvatochromism behavior upon different polarity of solvents. The highly polar
MC form can cause hypsochromic or bathochromic shifts depending on the solvent polarities. Therefore,
spiropyrans has been developed as indicators for solvent polarity due using its negative
solvatochromism .23 Azobenzenes are widely-used dyes and photochromic molecules with a broad

application prospects in many areas.24-27 Azobenzene dyes also find extensive applicability in analytical
chemistry as acid-base, redox and metallochromic indicators. As developing novel chemosensors is highly
desirable28-31, herein, we report a colorimetric chemodosimeter which can be used to distinguish different
miscellaneous solvents by blending spiropyran and azobenzene structure together. The prepared
spiropyran-azobenzene (SPAB) derivative has different color responsive towards different polarity of
solvents. Furthermore, as a first example, SPAB can determine the protic or aprotic property of a solvent.
2. Results and discussions
To design a probe for distinguishing different miscellaneous solvents, our idea is to make use of
reversible isomerization of spiropyrans in polar solvents with the introduction of the alkali (Figure. 2). In
contrast, the isomerization is difficult to occur in nonpolar solvents. Furthermore, in order to improve
sensibility of spiropyran, p-nitro azo moiety was introduced here as electron-withdrawing moiety to
enlarge the conjugation of benzopyran ring. Besides, azobenzene is also a good chromophore and
indicator for pH and polarity variations. We envisioned that nucleophilic addition reaction of protic
solvents (e.g. methanol and ethanol) with highly electrophilic indolium fragment of the MC form would
occur under alkaline conditions. While for aprotic solvent, such as DMF and DMSO, such nucleophilic
addition reaction cannot undergo at presence of alkaline. Therefore, discrimination of protic and aprotic
solvents can be realized by using this principle.
Figure 2. Principle of distinguishing different miscellaneous solvents.
The synthetic route for SPAB derivatives is depicted in Figure 3. Target nitro-substituted SPAB1 was
prepared firstly and SPAB2 and hydroxyethyl-substituted SPAB3 were then synthesized as controls.
SPAB1-2 were prepared starting from aniline through diazotization and cyclization with intermediate 5.
The SPAB3 were synthesized through three-step reactions from 2,3,3-trimethyl-3H-indole 6.

Figure 3. Synthesis routes of SPAB derivatives
After successful preparation of the SPABs, we then investigated the sensitivity and applicability of the
probe towards different solvents (Figure 4A). The solution of SPAB1 in low-polar chloroform (1 mol/L)
presented an orange-yellow color. When approximately equimolar amount of DBU was added to the
above solution, no obvious changes on color and UV-Vis absorption spectrum can be observed even the
solution stayed overnight. The results indicated that cleavage of the C-O bond of SPAB1 cannot occur in
chloroform solution upon addition of DBU. While in polar solvents acetonitrile, DMF or DMSO (1 mol/L),
the solution color was green, dark green and blue corresponding to the maximum UV absorption
wavelengths of 590 nm, 630 nm and 635 nm, respectively. The red shift occurred with increase the
polarity of the solvents. Therefore, the molecular probe can recognize acetonitrile, DMF and DMSO. The
appearance of absorption bands around 600 nm was corresponding to the absorption of MC, verifying
that SPAB1 turned to its open MC form. Same treatment of SPAB1 in methanol (1 mol/L) led to the color
change from yellow to pale orange immediately. UV-Vis spectrum indicated the appearance of an
absorption band at 540 nm. This can be explained by the fact that methanol reacted with the electrophilic
indolium fragment of MC form under the catalysis of DBU. The iminium carbon atom adjacent to the
positively charged nitrogen in the indolium fragment of the zwitter ionic MC species is highly electrophilic
and susceptible to nucleophilic attack. When SPAB1 was subjected to protic solvent ethanol (1 mol/L), the
solution color is brown with the absorption value of 500 nm. The UV-absorbing of solution in methanol
occurs to red shift compared to ethanol, therefore, the molecular probes is able to distinguish methanol
and ethanol.
2.0
MeCN DMF DMSO
MeOH EtOH
CHCl3
1.5
1.0
0.5
0.0
CHCl3
EtOH
MeOH
CH3CN
DMF
DMSO
300
400
500
600
700
800
Wavelength (nm)
Figure 4. Color changes and UV absorption spectra of SPAB1 in different solvents containing DBU (A), principles for color changes of SPAB1-DBU
system (B).
Table 1. The maximum absorption wavelength of SPAB1-DUB system in different solvents.
Solvent
λ1max (nm)
λ2max (nm)
chloroform
acetonitrile
methanol
ethanol
DMF
396
/
/
583
390
525
396
496
401
623
DMSO
399
626
In order to further identify the rationale behind this discrimination process, two control compounds
SPAB2 and SPAB3 were prepared. Replacement of electron-withdrawing nitro group by hydrogen

generated SPAB2. When SPAB2 was treated by different solvents together with DBU, no obvious color or
UV-Vis absorption changes were detected (Figure 5A), that is, the SPAB2 stayed in its closed form. This
phenomenon confirmed that nitro substituent played a key role in stabilizing opened MC form.
1.5
SPAB2
1.0
A
SPAB3
MeOH DMSO
DMSO+DBU
B
MeOH+DBU
MeOH
1.0
CHCl
3
DMF
0.5
0.0
0.5
0.0
300
400
500
600
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Figure 5. (A) UV absorption spectra of SPAB2 in three kinds of solvent containing DBU. (B) Color changes and UV absorption spectra of SPAB3 in
1
1
1
methanol and DMSO containing DBU, (B) H NMR tracking of SPAB3, (a) H NMR spectra of SPAB3 in DMSO-d6, (b) H NMR spectra of SPAB3 in
DMSO-d6 after adding DBU.
Hydroxyethyl derivative SPAB3 was prepared with a view to examine the mechanism behind the color
change in protic solvents. Treatment of SPAB3 with DBU in methanol caused slight color change even the
solution stayed for overnight (Figure 5B). This trend is similar to that observed on SPAB2 without
electron-withdrawing nitro groups, that is, the nucleophilic addition reaction of methanol with SPAB3 was
difficult to proceed. Interestingly, when SPAB3 was subjected to DMSO and DBU, the color of the solution
changed from orange-yellow to red. Correspondingly, the appearance of an absorption band at 513 nm
was observed in UV-Vis spectrum. This is due to the intramolecular attack of hydroxyl group to iminium
carbon atom adjacent to positively charged nitrogen in indolium fragment of zwitter ionic MC species
1
catalyzed by DBU (Figure 5C). This process was further identified by H NMR tracking of the reaction
(Figure 5C). The DBU-induced absorption change is caused by the formation of new production. The signal
of proton Hc disappears after addition of DBU because hydroxyl reacted with the spirocarbon to form a
new heterocycle. The low-field shift of Ha is caused by deshielding effect of the olefin. The high-field
change of Hb proton in ortho position of phenolate oxygen indicated that reaction of SPAB3 with DBU
produced a spirocycle-opened species. The above observations further verified the rationale behind the
color change of SPAB-DBU in methanol and ethanol.
Conclusion
Based on the solvatochromism behavior of spiropyran and azobenzene,
a
hybrid
spiropyran-azobenzene derivative was designed and prepared as a solvent probe. Together with base

DBU, the discrimination of non-polar solvent, aprotic polar solvent and protic polar solvent was achieved
using the designed SPAB. Different types of solvents present different color output towards SPAB-DBU
system and the rationale behind this was also investigated. The results described here can be used as an
auxiliary method for fast detection of solvent types
Experimental
Chemicals and instrumentations
Melting points (mp) were recorded on Büchi B540 apparatus (Büchi Labortechnik AG, Flawil, Switzerland) and
1
are uncorrected. H NMR and 13C NMR spectra were recorded on Bruker AM-400 (400 MHz) spectrometer with
CDCl3 or DMSO-d6 as the solvent and TMS as the internal standard. Chemical shifts are reported in δ (parts per
million) values. Electrospray ionization (ESI) mass spectrometry was performed in a HP 1100 LC-MS spectrometer.
Analytical thin layer chromatography (TLC) was carried out on precoated plates (silica gel 60 F254), and spots were
visualized with ultraviolet (UV) light. Column chromatography was performed using silica gel (Hailang, Qingdao)
200-300 mesh. Unless otherwise noted, reagents and solvents were used as received from commercial suppliers.
Yields were not optimized. All reactions were carried out under a protective atmosphere of drying nitrogen or
utilizing a calcium chloride tube.
Synthesis the target compounds
Synthesis of 2-hydroxy-5-((4-nitrophenyl)diazenyl)benzaldehyde (4a). To a solution of 4-nitroaniline 1a (0.05 mol)
in distilled water (100 mL), 50 mL of 3 M hydrochloride was added dropwisely under the condition of ice bath. Then
3.6 M sodium nitrite aqueous solution was added to the above solution slowly. The mixture was kept on vigorously
stirring for 1 h. After that a mixture of salicylic aldehyde 25 mL (0.05 mol) and 1.1 M sodium carbonate aqueous
solution were added into the reaction mixture. The reaction stirred for 4 h at room temperature and neutralized
with hydrochloride aqueous solution. The precipitate was washed with water and dried. Recrystallization of the
crude product with ethanol gave 4a. Yield, 64%; Melting point, 196.5-197.8 °C; 1H NMR (400 MHz, DMSO-d6) δ11.96
(s, 1H), 10.38 (s, 1H), 8.41 (d, J = 8.7 Hz, 2H), 8.23 (s, 1H), 8.18-8.09 (m, 1H), 8.04 (d, J = 8.6 Hz, 2H), 7.32-7.18 (m, 1H);
13C NMR(101 MHz, DMSO-d6) δ 160.1, 135.9, 134.1, 130.3, 129.4, 128.6, 121.54,120.2, 117.7, 112.2, 111.0.
Synthesis of 2-hydroxy-5-(phenyldiazenyl)benzaldehyde (4b). The 4b was synthesized according to a similar
procedure for preparing 4a. Yield, 67%; Melting point, 128.1-129.3 °C; 1H NMR (400 MHz, DMSO-d6) δ 11.55 (s, 1H),
10.39 (s, 1H), 8.20 (s, 1H), 8.11 (d, J = 8.8 Hz, 1H), 7.87 (d, J = 7.3 Hz, 2H), 7.57 (m, 3H), 7.21 (d, J = 8.8 Hz, 1H); 13C
NMR (101 MHz, DMSO-d6) δ 190.52, 163.39, 151.82, 144.7, 131.1, 129.7, 129.4, 123.7, 122.6, 122.3, 118.4.
Synthesis of 9,9,9a-trimethyl-2,3,9,9a-tetrahydrooxazolo[3,2-a]indole (8). A solution of 2,3,3-trimethyl-3H-indole
(12.56 mmol) and 2-iodoethyl alcohol (12.56 mmol) in acetonitrile (30 mL) was refluxed for 24 h under nitrogen
protection. After cooling down to room temperature, the solvent was removed under reduced pressure. The residue
was suspended in hexane (30 mL) and the mixture was filtered. The resulting solid was recrystallized from
chloroform (40 mL) to afford 1-(2-hydroxyethyl-2,3,3-trimethyl-3H-indolium iodine (7). A solution of 7 (4.89 mmol)
and KOH (7.83 mmol) in water (15 mL) was stirred at 25 °C for 20 min, and then was extracted with diethyl ether (3 ×
30 mL). The organic phase was concentrated under reduced pressure to afford 8. Yield, 67%; Melting point,
44.3-45.1 °C; 1H NMR (CDCl3, 400 MHz) δ 7.19-7.08 (m, 2H), 6.98-6.92 (m, 1H), 6.78 (d, 1H, J = 6.6 Hz), 3.90-3.49 (m,
4H), 1.47 (s, 3H), 1.42 (s, 3H), 1.21 (s, 3H); 13CNMR (CDCl3, 101MHz) δ 150.6, 140.0, 127.5, 122.4, 121.7, 112.0, 109.0,
63.0, 50.1, 47.0, 28.1, 20.8, 17.6.
Synthesis of 1',3',3'-trimethyl-6-((4-nitrophenyl)dizenyl)spiro[chromene-2,2'-indo-line] (SPAB1). 4a (1.0 mmol)
was dissolved in 10 mL ethanol. The mixture was heated to reflux for 30 min. Then fisher base 5 (1.0 mmol) was
added into mixture drop wise. The reaction was held under Argon atmosphere for 12 h. After completion of the
reaction, the mixture was cooled down to room temperature. The crude product was obtained by filtering and

washed with ethanol. Recrystallization by ethanol afforded the final product as yellow solid. Yield, 44%; Melting
point, 209.8-210.6 °C; 1HNMR (400 MHz, DMSO-d6) δ 1.13 (s, 3H), 1.25 (s, 3H), 2.70 (s 3H), 5.94 (d, J = 10.28 Hz, 1H),
6.60 (d, J = 8.28 Hz, 1H), 6.79 (t, J = 7.52 Hz, 1H), 6.92 (d, J = 8.72 Hz, 1H), 7.12 (m, 2H), 7.22 (d, J = 10.28 Hz, 1H), 7.78
(dd, J = 8.68 Hz, 2.44 Hz, 1H), 7.87 (d, J = 2.4 Hz, 1H), 7.94 (d, J = 8.72 Hz, 2H), 8.04 (d, J = 8.68 Hz, 2H); 13C NMR (101
MHz, DMSO-d6) δ 161.5, 158.1, 152.9, 147.5, 135. 8, 128.8, 127.6, 126.3, 124.9, 123.2, 121.6, 120.7, 119.1, 115.6,
+
109.7, 106.8, 105.5, 86.3, 51.5, 28.6, 25.7, 19.8; HRMS (ESI), m/z [M+H]+ calcd for C25H25N4O3 , 427.1770; found,
427.1771.
Synthesis of 1',3',3'-trimethyl-6-(phenyldiazenyl)spiro[benzofuran-2,2'-indole] (SPAB2). 4a (1.0 mmol) was
dissolved in 10 mL ethanol. The mixture was heated to reflux for 30 min. Then fisher base 5 (1.0 mmol) was added
into mixture drop wise. The reaction was held under Argon atmosphere for 12 h. After completion of the reaction,
the mixture was cooled down to room temperature. The crude product was obtained by filtering and washed with
ethanol. Recrystallization by ethanol afforded the final product as yellow solid. Yield, 56%; Melting point,
166.4-166.7 °C; 1H NMR (400 MHz, DMSO-d6) δ 7.86-7.79 (m, 3H), 7.74 (d, J = 8.6 Hz, 1H), 7.55 (dt, J = 21.6, 7.1 Hz,
3H), 7.21 (d, J = 10.3 Hz, 1H), 7.15-7.07 (m, 2H), 6.88 (d, J = 8.6 Hz, 1H), 6.81 (t, J = 7.3 Hz, 1H), 6.61 (d, J = 7.6 Hz, 1H),
6.61 (d, J = 7.6 Hz, 1H), 5.91 (d, J = 10.3 Hz, 1H), 2.70 (s, 3H), 1.25 (s, 3H), 1.13 (s, 3H); 13C NMR (101 MHz, DMSO-d6)
δ 156.9, 151.9, 147.5, 145.9, 136.1, 130.8, 129.4, 129.0, 127.5, 125.4, 122.2, 121.4, 120.9, 120.5, 119.1, 115.3, 106.9,
105.0, 51.6, 28.5, 25.4, 19.7. HRMS (ESI), m/z [M+H]+ calcd for C25H24N3O+, 382.470; found. 382.472.
Synthesis of 2-[3',3'-dimethyl-6-(phenyldiazenyl)spiro[benzofuran-2,2'-indole]-1'-yl)ethan-1-ol (SPAB3). The
mixture of 9,9,9a-trimethyl-2,3,9,9a-tetrahydrooxazolo[3,2-a]indole (8) (2.0 mmol) and 4b (2.0 mmol) was dissolved
in ethanol (20 mL). Then the reaction was refluxed under argon overnight. After completion of the reaction, half of
the solvent was removed in vacuum, cooled and filtered to give the crude product. Further purification by column
chromatography (petroleum ether/ethyl acetate = 10: 1) afforded the target product as reddish brown solid. Yield,
47%; Melting point, 194.3-195.1 °C; 1H NMR (400 MHz, CDCl3) δ 7.86 (d, J = 7.4 Hz, 2H), 7.76 (dd, J = 8.6, 2.3 Hz, 1H),
7.68 (d, J = 2.2 Hz, 1H), 7.50 (t, J = 7.4 Hz, 2H), 7.43 (t, J = 7.2 Hz, 1H), 7.25 (s, 1H), 7.18 (dd, J = 8.0, 7.3 Hz, 1H), 7.10
(d, J = 7.2 Hz, 1H), 6.94 (d, J = 10.3 Hz, 1H), 6.88 (dd, J = 12.8, 5.5 Hz, 1H), 6.82 (d, J = 8.7 Hz, 1H), 6.66 (d, J = 7.8 Hz,
1H), 5.73 (d, J = 10.3 Hz, 1H), 3.88-3.69 (m, 2H), 3.60-3.45 (m, 1H), 3.34 (ddd, J = 15.5, 10.3, 5.6 Hz, 1H), 1.32 (s, 3H),
1.18( s, 3H); 13C NMR (101 MHz, CDCl3) δ 156.6, 152.7, 147.3, 146.8, 136.2, 130.4, 129.4 129.0, 127.7, 125.9, 122.5,
121.9, 121.00, 120.6, 119.6, 118.8, 115.6, 106.8, 105.6, 60.9, 52.5, 46.1, 25.9, 20.2. HRMS (ESI), m/z [M+Na]+ calcd
for C26H25N3O2Na+, 412.203; found, 412.226.
Characterization.
UV–vis absorption spectroscopy was used to detect the change of absorption profiles of target compounds.
Absorption spectra were recorded on a Lambda 25 UV/vis spectrometer (PerkinElmer, Shanghai). Target compounds
were dissolved in different solvents using a microcuvette and subjected to the test. The changes of the SPAB3 before
and after adding DBU was investigated in detail by 1H NMR spectroscopy.
Acknowledgments
This work was financial supported by National Key Research and Development Program of China (2018YFD0200100),
Science and Technology Commission of Shanghai Municipality (16391902300), Innovation Program of Shanghai
Municipal Education Commission (2017-01-07-00-02-E00037) and the Fundamental Research Funds for the Central
Universities (222201718004).

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Highlights:
1.
2.
3.
Photochromic spiropyran and azobenzene were linked together by covalent bond.
Spiropyran-azobenzene can be induced to different forms in different solvents.
Different spiropyran-azobenzene form gave different color output as a solvent indicator.

Graphical Abstract：
32.