Synthesis and characterization of coumarin-based spiropyran photochromic colorants

Article abstract of DOI:10.1021/ol8018902

(Equation Presented) Two coumarin-based spiropyran derivatives were synthesized and characterized in two steps to explore their photochromic properties. Both prepared compounds are sensitive to UV light and change colors upon irradiation. The resulting photogenerated zwitterions revert to the original compounds while being heated. A new family of organic photochromic colorants is introduced.

Full text of DOI:10.1021/ol8018902

ORGANIC
LETTERS
2008
Vol. 10, No. 21
4823-4826
Synthesis and Characterization of
Coumarin-Based Spiropyran
Photochromic Colorants
Jhih-Rong Chen, Jen-Bing Wong, Pei-Yu Kuo, and Ding-Yah Yang*
Department of Chemistry, Tunghai UniVersity, 181, Taichung-Kang Road Sec. 3,
Taichung, Taiwan 407
yang@thu.edu.tw
Received August 13, 2008
ABSTRACT
Two coumarin-based spiropyran derivatives were synthesized and characterized in two steps to explore their photochromic properties. Both
prepared compounds are sensitive to UV light and change colors upon irradiation. The resulting photogenerated zwitterions revert to the
original compounds while being heated. A new family of organic photochromic colorants is introduced.
Photochromism1 refers to a reversible phototransformation
of a chemical species between two forms having different
absorption spectra. Compounds with photochromic properties
may have a wide range of applications in the area of photonic
materials2 and optical memory devices.3 The most well-
known organic photochromic colorants include spiropyrans,4
spirooxazines,5 chromenes,6 fulgides,7 and diarylethenes.8
While modifications of the existing photochromes may still
generate compounds with unprecedented properties, develop-
ment of new classes of organic photochromic dyes with novel
molecular scaffolds as suitable materials for aforementioned
applications remains desired. Recently, we reported a new
family of photochromic colorants with a coumarin-based
oxazabicycle as the molecular skeleton.9 Here, we describe
our efforts in the synthesis and characterization of two
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10.1021/ol8018902 CCC: $40.75
Published on Web 10/01/2008
2008 American Chemical Society

coumarin-based spiropyran derivatives and subsequent ex-
ploration of their photochemical properties. The photochemi-
cal interconversions between the ring-closed spiropyrans and
the proposed photogenerated ring-opened zwitterionic species
were investigated by UV-vis spectroscopy, conductivity, and
fluorescence measurements.
Scheme 2
Scheme 1 shows the two-step preparation of coumarin
Scheme 1
Upon ultraviolet (352 nm) irradiation, compound 2 changed
from colorless to green within seconds. Figure 2 shows the
spiropyran 2. It started with an acid-mediated condensation10
of m-methoxyphenol and benzoylacetone to form a green
flavylium salt 1, followed by a three-component coupling
with 4-hydroxycoumarin in the presence of triethylamine in
acetone under reflux conditions to afford the target compound
1
2. The structure of 2 was confirmed by H and 13C NMR
studies as well as by single-crystal X-ray diffraction analysis
as presented in Figure 1, which clearly reveals a spiropyran
Figure 2
.
Absorption spectra of 2 (2.5 × 10-4 M in CH2Cl2)
obtained with different exposure times (352 nm), 0-40 min, in 5
min increments.
UV-vis absorption spectra of 2 in methylene chloride with
different irradiation times. With the increase of exposure time
(352 nm), three new absorption bands with peak wavelengths
around 400, 593, and 710 nm gradually increased, along with
three isosbestic points (261, 323, and 368 nm). Although
isolation and characterization of the photogenerated product
proved to be difficult, it exhibits a green color, which
corresponds to the chromophore of the green flavylium ion
1. This observation prompted us to tentatively propose the
ring-opened zwitterionic species 6 as the photogenerated
product (Scheme 3). This proposed organic zwitterionic
species 6 can cyclize back to 2 while being heated. In
addition to imposing absorbance changes of spiropyran 2 in
the visible region upon irradiation, the pronounced structural
modifications associated with this photoinduced transforma-
tion translate also into significant dipole moment and
molecular polarizability alterations. For instance, Figure 3
shows a linear relationship between the irradiation time and
conductivity of 2 in acetonitrile at room temperature,
indicating a proportional increase in conductivity when it is
excited by UV light. However, increasing the concentration
of 2 in acetonitrile without UV irradiation does not affect
Figure 1. XRD structures of 2 and 8.
skeleton. Scheme 2 depicts the proposed mechanism for this
three-component reaction. It involved first deprotonation of
flavylium salt 1 by triethylamine to give 7-methoxy-4-
methylene-2-phenyl-4H-chromene (3) followed by reaction
with acetone to afford the alcohol 4. The subsequent coupling
of 4 with coumarin yielded 5. Final cyclization of 5 furnished
the coumarin spiropyran 2.
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4824
Org. Lett., Vol. 10, No. 21, 2008

Scheme 3
Scheme 4
the conductivity. This result not only provides the indirect
evidence to support the zwitterionic structure of 6 but also
suggests that conductivity may serve as a second output
property of 2, in addition to a distinct change in color.
Scheme 5
Figure 3. Time-dependent changes in conductivity of 2 (5 mM in
MeCN) under UV light (352 nm).
absorbance at 520 nm, which resembles to the chromophore
of the red flavylium ion 7, showed a smooth, continuous
growth and reached a plateau in approximately 80 min,
indicating the stationary conversion to the proposed zwitte-
rionic species 9. Figure 5 shows the fluorescence spectra of
the nonfluorescent 8 obtained with different exposure times
(352 nm) in methylene chloride. To our delight, the emission
intensity increases with the increase of irradiation time. The
zwitterionic species 9 is indeed fluorescent as expected.
When heated at 90 °C in toluene, this photogenerated ring-
opened form 9thermally decays away with the disappearance
of the 520 nm band with the reverse rate constant of 2 ×
10-4 s-1, along with the decrease of emission intensity (see
Fluorescence emission is considered to be one of the most
attractive signal outputs of switching materials owing to its
high sensitivity, easy detection and low-cost procedure.11 In
an effort to integrate both photochromic and fluorescent
functions into this coumarin-based spiropyran system, we
introduced an N,N-dimethylamino group onto the 4H-
chromene ring to increase the emission intensity of the
photogenerated zwitterionic species. The designed compound
8 was prepared by the similar procedure as that of 2, except
that the starting materials m-methoxyphenol and benzoylac-
etone were replaced with 4-N,N-dimethylamino-2-hydroxy-
acetophenone12 and acetophenone, respectively (Scheme 4).
The crystal structure of 8 is depicted in Figure 1, which
confirms a desired N,N-dimethylamino group at the 7-posi-
tion of 4H-chromene moiety.
Upon ultraviolet irradiation (352 nm), the spiropyran 8
changed from colorless to red (Scheme 5). With the increase
of exposure time (352 nm), a new absorption band with the
peak wavelength around 520 nm gradually increased, along
with a clear isosbestic point at 274 nm (Figure 4). The
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Figure 4
.
Absorption spectra of 8 (6.1 × 10-5 M in CH2Cl2)
obtained with different exposure times (352 nm), 0-50 min, in 5
min increments.
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Org. Lett., Vol. 10, No. 21, 2008
4825

Figure 6. HOMO/LUMO of 8.
Figure 5
.
Emission spectra of 8 (1.5 × 10-6 M in CH2Cl2) obtained
emission property of 8 can be controlled by UV irradiation.
The present work introduces a new molecular scaffold with
photochromic property. Further investigations for the devel-
opment of more sophisticated systems based on the same
molecular skeleton for colorimetric or fluorescent ion-sensing
and other applications are in progress in our laboratory.
with different exposure times (352 nm), 0-55 min, with 5 min
increments.
the Supporting Information for details). Upon UV irradiation
of the colorless product, the photoproduct turned red again
with the appearance of the 520 nm band characteristic for
9. Although the photochromic interconversion between 8 and
9 is currently limited to four cycles due to the occurrence of
side reactions, the photochromic property of these spiropy-
rans can presumably be modulated or fine-tuned by incor-
porating different substituents on the 4H-chromene or
coumarin moieties.
To gain further insight to this light-induced ring-opening
mechanism, the molecular simulation of 8 utilizing the
density function theory (DFT) method at the B3LYP/6-31G*
level13 was carried out. According to the calculation, the
electron density of HOMO is mainly located on the N,N-
dimethylamino-4H-chromene moiety. Conversely, the elec-
tron density is transferred to the coumarin group in the
LUMO (Figure 6). The net change of the electron density
during the HOMO f LUMO transition upon UV irradiation
is expected to cause the C-O bond weakening and conse-
quently cleavage, leading to the colored ring-opened zwit-
terionic species 9.
Acknowledgment. We thank the National Science Council
of the Republic of China, Taiwan, for financially supporting
this research under Contract No. NSC 96-2113-M-029-002.
Supporting Information Available: Synthesis of com-
pounds 2 and 8, experimental details, and additional spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL8018902
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In summary, two coumarin-based spiropyran derivatives
were efficiently synthesized in a two-step, three-component
reaction to study their photochromic properties. We have
demonstrated that the absorption property of 2 and the
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Org. Lett., Vol. 10, No. 21, 2008