'Locking and unlocking control' of photochromism of naphthopyran derivative

Article abstract of DOI:10.1002/poc.1598

A 'locking and unlocking control' for a photochromic molecular system has been developed by using a photochromic naphthopyran derivative 1a as the model compound. With UV light irradiation, the colorless solution of 1a underwent ring-opening photoisomerization and converted to a purple solution 1b, which quickly faded back to a colorless solution with visible light (λ ≥ 480 nm) irradiation or in the dark. Addition of boron trifluoride diethyl etherate to the solution of 1b produced a complex compound 2b, accompanying the color change from purple to blue. It was found that 2b remained photochemically inactive. With the addition of diethanolamine, however, the 'locked' photoreaction of 2b could be unlocked and converted back to 1b, in which the photochromism was recovered. Copyright

Full text of DOI:10.1002/poc.1598

Research Article
Received: 4 May 2009,
Revised: 17 June 2009,
Accepted: 20 June 2009,
Published online in Wiley InterScience: 17 August 2009
(www.interscience.wiley.com) DOI 10.1002/poc.1598
‘Locking and unlocking control’ of
photochromism of naphthopyran derivative
Kunpeng Guoa,b and Yi Chena
*
A ‘locking and unlocking control’ for a photochromic molecular system has been developed by using a photochromic
naphthopyran derivative 1a as the model compound. With UV light irradiation, the colorless solution of 1a underwent
ring-opening photoisomerization and converted to a purple solution 1b, which quickly faded back to a colorless
solution with visible light (l ‡ 480 nm) irradiation or in the dark. Addition of boron trifluoride diethyl etherate to the
solution of 1b produced a complex compound 2b, accompanying the color change from purple to blue. It was found
that 2b remained photochemically inactive. With the addition of diethanolamine, however, the ‘locked’ photoreaction
of 2b could be unlocked and converted back to 1b, in which the photochromism was recovered. Copyright ß 2009
John Wiley & Sons, Ltd.
Keywords: boron trifluoride diethyl etherate; diethanolamine; inhibition; naphthopyran derivative; photochromism
limited.[30,34] The principle of controlling photochromism of
naphthopyran derivative with organic molecules is outlined in
INTRODUCTION
Great interest is currently devoted to bi-stable species presenting
two forms whose interconversion can be modulated by an
external stimulus.[1–5] The design of such molecular-level
switching devices is directly linked to the chemistry of signal
generation, transfer, conversion, storage and detection. Mol-
ecules with switchable properties are of considerable practical
and fundamental interest as the development of robust systems
will open up new avenues and possibilities for regulating cellular
processes with potential applications to drug delivery systems,
optical devices and sensors.[6–15]
Typical bi-stable molecules are the so-called photochromic
compounds, which undergo a reversible change induced by light
radiation, between two states of a molecule having different
absorption spectra.[16,17] Of all potential applications of photo-
chromic compounds, photoswitch is one of the most important
andattractiveapplicationfields.[18–22] Thebasicrequirements fora
photoswtich are bistability and nondestructive detection.[23–25]
Formostof thephotochromicnaphthopyrancompounds, thermal
fadingafterUVirradiationistheirintrinsiccharacteristic,[26] anditis
useless for switching purposes since the information is spon-
taneously erased after a relatively short time. Another limitation of
photochromic compounds to be used as a photoswitch is
destructive detection because two states are photochemically
active. The approach to avoid destructive detection is usually to
build dual-mode systems inwhich two reversibleprocesses can be
addressed by means of two different stimuli.[27–30]
Scheme 1 and represents a new way of manipulating the
photoswitching of a naphthopyran derivative.
EXPERIMENTAL
General methods
1H NMR spectra were recorded at 400 MHz with TMS as an
internal reference and CDCI3 as the solvent. HRMS spectra were
recorded with GC-TOF MS spectrometer. UV absorption spectra
were measured with an absorption spectrophotometer (Hitachi
U-3010). All chemicals for synthesis were purchased from
commercial suppliers, and solvents were purified according to
standard procedures. Reactions were monitored by a TLC silica
gel plate (60F-254). Column chromatography was performed on
silica gel (Merck, 70–230 mesh). A low-pressure mercury lamp
(30 W) and a Xeon light (500 W), with different wavelength filters,
were used as light sources for photocoloration and photobleach-
ing, respectively.
Synthesis of naphthopyran 1a
The synthesis route for photochromic naphthopyran 1a is
presented in Scheme 2. 1,2-Diaryl-2-propyn-1-ol[35] was obtained
by starting from 1-naphthoyl chloride, which reacted with N,N-
Currently, simple and efficient switching systems are of
considerable interest, especially in a single molecule.[31–33] In
this paper, we report a ‘locking and unlocking control’ of
photochromic systems based on a photochromic naphthopyran
derivative. In such a system, the photoisomerization of a
naphthopyran derivative can be modulated by organic mol-
ecules. As far as we know, this is the first report of control of
photochromism of a naphthopyran derivative by organic
molecules. Although pH-controlled photochromism of naphtho-
pyran has been reported, the deprotonation with NaOH resulted
in its precipitation from the organic matrix, and recycling is
*
Correspondence to: Y. Chen, Key Laboratory of Photochemical Conversion and
Optoelectronic Materials, Technical Institute of Physics and Chemistry, The
Chinese Academy of Sciences, Beijing 100190, China.
E-mail: yichencas@yahoo.com.cn
a
b
K. Guo, Y. Chen
Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, The Chinese Academy of
Sciences, Beijing 100190, China
K. Guo
Graduate School of Chinese Academy of Sciences, Beijing 100049, People’s
Republic of China
J. Phys. Org. Chem. 2010, 23 207–210
Copyright ß 2009 John Wiley & Sons, Ltd.

K. GUO AND Y. CHEN
ol (220 mg, 0.73 mmol), 2-naphthol (101 mg, 0.7 mmol),
(CH3O)3CH (0.16 ml, 0.14 mmol), and pyridinium para-toluene-
sulfonate (8.8 mg, 0.035 mmol) were dissolved into 8.0 ml of
ClCH2CH2Cl2. The mixture was refluxed till no starting material
(1,2-diaryl-2-propyn-1-ol) was detected by the TLC plate. The
reaction mixture was cooled to ambient temperature and
concentrated to 5 ml under reduced pressure; the crude product
was purified by flash column chromatography with petroleum/
ethyl acetate (10:1) as an eluent to afford target compound
naphthopyran 1a (87 mg) in 28% yield as a pale purple powder.
Characterization data of 1a: M.P. ¼ 201–2038C. IR (KBr): 2802,
Scheme 1. The principle of ‘locking and unlocking control’ of photo-
reaction of naphthopyran 1a
1610, 1357, 1231, 1189, 1081, 992, 778, 749 cmꢁ1 1H NMR
.
(400 MHz, CDCl3): d ¼ 8.31 (d, 1H, J ¼ 8.6 Hz), 7.99 (d, 1H,
J ¼ 8.5 Hz), 7.82 (d, 1H, J ¼ 8.0 Hz), 7.78 (d, 1H, J ¼ 8.1 Hz), 7.60
(d, 2H, J ¼ 7.6 Hz), 7.60 (d, 1H, J ¼ 8.9 Hz), 7.46–7.44 (t, 1H,
J1 ¼ 3.7 Hz, J2 ¼ 3.8 Hz), 7.39–7.36 (m, 2H), 7.34–7.30 (m, 5H), 7.12
(d, 1H, J ¼ 8.8 Hz), 6.66 (d, 2H, J ¼ 8.9 Hz), 6.31 (d, 1H, J ¼ 9.9 Hz),
2.91 (s, 6H). HRMS: m/z [Mþ þ 1]: 428.2. Anal. Calcd for C31H25NO:
C, 87.09; H, 5.89. Found: C, 87.48; H, 5.80.
Scheme 2. Synthesis of photochromic naphthopyran 1a
RESULTS AND DISCUSSION
Spectral photochromic properties of 1a were studied in
acetonitrile solution. 1a showed ring-opening and ring-closing
photoisomerization with UV/Vis light irradiation. The photo-
isomerization of 1a is described in Scheme 1, and changes in the
absorption spectra are presented in Fig. 1. Before irradiation,
there were two main absorption bands at 262 nm (e ¼ 5.2 ꢀ 104)
and 242 nm (3.0 ꢀ 104), respectively, which were attributed to the
ring-closing isomer 1a. Upon irradiation with UV light, a new
band at 540 nm, which corresponded to the ring-opening isomer
1b, appeared, and the absorption intensity increased onincreas-
ing the irradiation time until the photostationary state was
reached. The photostationary state was achieved when the
solution was irradiated for 90 s with UV light irradiation (30 W,
254 nm), and the process was accompanied by a change in color
of the solution, from colorless to purple. The new band at 540 nm
decreased and disappeared (Fig. 2) when the sample was
irradiated with visible light (l ꢂ 450 nm) for 15 s (500 W, Xenon
with wavelength filter) or kept in the dark for 20 min,
accompanying the color change of the solution from purple to
dimethylaniline by Friedel–Crafts reaction to afford 1,1-diarylk-
etone,[36] followed by treatment with sodium acetylide in an
ether solvent at ambient temperature. The target compound 1a
was prepared according to the literature[37,38] by the conden-
sation of 1,2-diaryl-2-propyn-1-ol with b-naphthol in the
presence of pyridinium para-toluenesulfonate (PPTS) as the
catalyst and the detailed synthesis procedure is described as
follows: (1) to a solution of anhydrous AlCl3 (0.91 g, 6.8 mmol) in
CH2Cl2 (20 ml) 1-naphthoyl chloride (1.0 ml, 6.6 mmol) was added,
the mixture was stirred for 0.5 h at ambient temperature, and
then cooled to 08C. To the cooled (08C) solution, 0.84 ml of N,N-
dimethylaniline (6.6 mmol) in CH2Cl2 (20 ml) was added dropwise.
After the addition of N,N-dimethylaniline, the reaction mixture
was stirred for 15 min, allowing the temperature to rise to
ambient temperature. The reaction solution was stirred at
ambient temperature until no starting material was detected by
the TLC plate, and then poured into 50 ml of ice water. The
product was extracted with CH2Cl2 (3 ꢀ 15 ml), the combined
organic solution was washed with a solution of NaHCO3 (5%,
30 ml), and dried (Na2SO4). After evaporation of the solvent, the
crude product was purified by flash column chromatography
with petroleum–ethyl acetate (10:1) as an eluent to afford 1,1-
diarylketone (910 mg) in 50% yield as yellow powder. (2) To the
solution of sodium acetylide (1.9 ml, 18%wt in xylene, 6.6 mmol)
in dry THF (7.0 ml), 910 mg of 1,1-diarylketone (3.3 mmol) was
added dropwise in dry THF (7.0 ml) and dry DMSO (1.65 ml) under
nitrogen at 08C. After the addition of 1,1-diarylketone and DMSO,
the reaction mixture was stirred for 15 min, allowing the
temperature to rise to ambient temperature. The reaction
solution was stirred at ambient temperature till no starting
material was detected by the TLC plate. The reaction was
quenched with ice-water (20 ml), and then poured into 20 ml of
saturated NH4Cl solution. The product was extracted with CH2Cl2
(3 ꢀ 15 ml) and the combined organic solution was dried
(Na2SO4). After evaporation of the solvent, the crude product
was purified by flash column chromatography with petroleum–
ethyl acetate (6:1) as an eluent to afford 1,2-diaryl-2-propyn-1-ol
(500 mg) in 50% yield as yellow powder. (3) 1,2-Diaryl-2-propyn-1-
Figure 1. Absorption changes of 1a (20 mM, CH3CN) with UV light
irradiation (irradiation time periods: 0, 15, 60, 90 s)
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Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 207–210

PHOTOCHROMISM OF NAPHTHOPYRAN DERIVATIVE
Figure 4. Absorption changes of 2b in CH3CN with the addition of DEA
(lines 1–4: 0, 0.2, 0.3, 0.7 mM)
Figure 2. Photobleaching of 1b upon irradiation at ꢂ400 nm (irradiation
time periods: 0, 5, 10, 15 s)
colorless. The performance of coloration and decoloration can be
repeated many times.
converted back to a purple solution of 1b, the latter was bleached
completely to a colorless solution of 1a in the dark or with visible
light irradiation. Further investigation found that the recovered
1a also underwent photoisomerization with UV/Vis irradiation
(Scheme 1), and could be recycled many times with UV/Vis light
irradiation. Control experiment showed that no significant
changes in both optical density and the rate of coloration and
decoloration were detected when DEA was added to 1a and 1b
solutions, respectively. It suggested that the addition of DEA did
not affect the photochromism of 1a.
In order to elucidate the binding site of 1b with BF3 Et2O, the
following analogues (Scheme 3) were employed in control
experiments. First, it was found that no changes in the absorption
of N,N-dimethylaniline (NP-1) were detected when BF3 Et2O was
added, suggesting that the binding site did not occur at the N
atom, which was also confirmed by the ring-closing isomer of 1a,
in which no absorption change was detected with the addition of
BF3 Et2O before UV light irradiation. Secondly, adding BF3 Et2O to
a solution of the ring-opening isomer of NP-2 produced a red
shift in the absorption of the ring-opening isomer and the color of
the solution changed from yellow to pale blue, but the pale blue
solution converted back to colorless in darkness. It indicates that
the binding site may occur at the photogenerated CO functional
group although NP-1 did not perform the ‘locked’ photochromic
behavior with the addition of BF3 Et2O. To further confirm that the
binding site occurs at the photogenerated CO functional group,
compounds NP-3, NP-4, and NP-5 were also investigated and
interesting results were obtained. It was found that no change in
Addition of 0.6 mM of boron trifluoride diethyl etherate (BF3
Et2O) to the solution of 1b in acetonitrile produced a complex
compound 2b (Scheme 1), this process was accompanied by a
color change of solution from purple to blue. As presented in
Fig. 3, the absorption at 540 nm, corresponding to 1b, was red
shifted to 605 nm, which corresponds to 2b – the conversion of
1b into 2b is quantitative with the addition of BF3 Et2O. The blue
solution of 2b could not, however, be converted back into a
colorless solution with visible light (l ꢂ 450 nm) irradiation or in
the dark, and no significant change (detected by optical density)
was detected when the blue solution of 2b was kept in the dark
for a couple of weeks or irradiated for 30 min. It suggested that 2b
is photochemically inactive and the photoreaction was ‘locked’. It
is worth noting that 2b was also obtained by adding BF3 Et2O to
the solution of 1a and then irradiating the solution mixture to a
photostationary state. In this case, the photoreaction of 2b was
also locked.
The ‘key’ for unlocking the photoreaction of 2b was
investigated. It was found that, with the addition of diethano-
lamine (DEA) to the solution of 2b, the absorption of 2b at 605 nm
decreased and shifted to 540 nm, which corresponds to 1b
(Fig. 4). Along with this process, the blue solution of 2b was
Figure 3. Absorption change of 1b (in CH3CN) with (dash line) and
without (solid line) addition of BF3 Et2O (0.6 mM)
Scheme 3. Compounds used in control experiments
J. Phys. Org. Chem. 2010, 23 207–210
Copyright ß 2009 John Wiley & Sons, Ltd.
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K. GUO AND Y. CHEN
absorption was detected when BF3 Et2O was added to the ring-
opening isomer of NP-3, and the result obtained for the
compound NP-4 was similar to that of NP-1. Only NP-5 exhibited
the ‘locking and unlocking control’ photochromic behavior with
the addition of BF3 Et2O and DEA. A possible reason is that the
strong electron-acceptor group (NO2) inhibited the CO functional
group bound with BF3 Et2O because of the decrease in the
electronic density of the CO group, and the strong electron-
donating group (N(CH3)) enhanced the CO functional group
bound with BF3 Et2O because of the increase in the electronic
density of the CO group, which resulted in an inhibition of the
bleaching of the ring-opening isomer.
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CONCLUSIONS
In summary, a ‘locking and unlocking control’ of photochromism
of the naphthopyran derivative has been demonstrated. The
photoreaction of the ring-opening isomer of the naphthopyran
derivative can be locked and unlocked via molecular modulation,
which offers a simple approach to manipulate the reversible
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Acknowledgements
This work was supported by the National Nature Science Founda-
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J. Phys. Org. Chem. 2010, 23 207–210