Modification of a photochromic 3-aryl-3-(α-naphthalene)-3H-naphtho[2, 1-b]pyran system with a fast fading speed in solution and in a rigid polymer matrix

Article abstract of DOI:10.1039/c1jm10139k

We have demonstrated that 3-aryl-3-(α-naphthalene)-3H-naphtho[2,1-b] pyrans photochromic system shows large optical density at photosteady state at ambient temperature. In this paper, we describe a strategy for modifying this photochromic system with a fast fading speed in both solution and in a rigid polymer matrix. It is found that the nature and position of the substituted groups attached to the aryl moiety at the 3-position play a key role in determining the fading speed of 3-aryl-3-(α-naphthalene)-3H-naphtho[2,1-b] pyrans at ambient temperature. The fading speed of colored forms increase significantly when the electron-donating groups are attached to the para-position of aryl moieties at the 3-position. Further investigations find that (1) a strong electron-donating group is better than a weak electron-donating group for a fast fading speed, (2) an electron-donating group attached to the para-position of the naphthalene ring is better than one attached to the para-position of a benzene ring, and (3) with electron-donating groups at both para-positions of the naphthalene and benzene rings, the fading speed is dramatically increased in both solution and in the rigid polymer matrix. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1jm10139k

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PAPER
Modification of a photochromic 3-aryl-3-(a-naphthalene)-3H-naphtho[2,1-b]
pyran system with a fast fading speed in solution and in a rigid polymer matrix
*
Shulie Han and Yi Chen
Received 11th January 2011, Accepted 26th January 2011
DOI: 10.1039/c1jm10139k
We have demonstrated that 3-aryl-3-(a-naphthalene)-3H-naphtho[2,1-b]pyrans photochromic system
shows large optical density at photosteady state at ambient temperature. In this paper, we describe
a strategy for modifying this photochromic system with a fast fading speed in both solution and in
a rigid polymer matrix. It is found that the nature and position of the substituted groups attached to the
aryl moiety at the 3-position play a key role in determining the fading speed of 3-aryl-3-(a-
naphthalene)-3H-naphtho[2,1-b]pyrans at ambient temperature. The fading speed of colored forms
increase significantly when the electron-donating groups are attached to the para-position of aryl
moieties at the 3-position. Further investigations find that (1) a strong electron-donating group is better
than a weak electron-donating group for a fast fading speed, (2) an electron-donating group attached to
the para-position of the naphthalene ring is better than one attached to the para-position of a benzene
ring, and (3) with electron-donating groups at both para-positions of the naphthalene and benzene
rings, the fading speed is dramatically increased in both solution and in the rigid polymer matrix.
showed a large optical density at a photosteady state and a slow
Introduction
fading speed in both solution and in a rigid polymer matrix at
ambient temperature by comparison with 3,3-diphenyl-3H-
naphtho[2,1-b]pyran (Py) in the same conditions (Scheme 1). In
this paper, we extend our research on structure–photochromic
activity relationships so as to develop a 3H-naphtho[2,1-b]pyran-
based practical system with a large optical density at a photo-
steady state and a fast fading speed at ambient temperature. In
terms of early reports32,33 and our previous findings31 that an
electron-donating group in the para-position on the phenyl or
naphthalene group of 3,3-diaryl-3H-naphtho[2,1-b]pyrans
results in rapid fading, we have designed and prepared a class of
3-phenyl-3-(a-naphthalene)-3H-naphtho[2,1-b]pyrans with elec-
tron-donating groups in the para-position on both the benzene
ring and the naphthalene ring as an extension of the 3H-naphtho
[2,1-b]pyran system (Scheme 2). With such a system, a large
optical density of the photosteady state and a fast fading speed
were obtained in both solution and in a rigid polymer matrix at
Organic photochromic materials have attracted considerable
interest due to their color change upon irradiation with light.1–3
In particular, thermally reversible photochromic molecules offer
the opportunity to change and reset molecular properties by
simply turning a light source on and off.4–7 Recently, novel
photochromic molecules with instantaneous coloration upon
exposure to UV light and rapid fading in the dark have attracted
great attention for the development of optical switching devices,
and a great progress has been achieved.8–11
Photochromic naphthopyrans are being extensively explored
for applications as optical switches12–16 and ophthalmic lenses,17–22
where a large steady-state optical density and rapid fading at
ambient temperature is highly desired.23–26 It is known that
photochromic 3,3-diaryl-3H-naphtho[2,1-b]pyrans show thermal
unstable colored forms at ambient temperature and rapidly fade
back to a colorless form. Therefore, the optical density of colored
forms is usually small at ambient temperature.27–30 The low
optical density at ambient temperature is the main drawback of
this system; therefore, the construction of a 3,3-diaryl-3H-
naphtho[2,1-b]pyran system with a large colored optical density
and a rapid fading speed is crucial for their potential industrial
applications. Recently, we reported31 that a 3H-naphtho[2,1-b]
pyran bearing an a-naphthalene group at the 3-position (NPy)
Laboratory of Organic Optoelectronic Functional Materials and Molecular
Engineering, Technical Institute of Physics and Chemistry, The Chinese
Academy of Science, Beijing, 100190, China. E-mail: yichencas@yahoo.
com.cn; Fax: +86 10 6487 9375; Tel: +86 10 8254 3595
Scheme 1 The structure of two 3-phenyl-3-(a-naphthalene)-3H-naph-
tho[2,1-b]pyrans.
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Scheme 3 Synthetic route to 3,3-diaryl-3H-naphtho[2,1-b]pyrans.
(1.0 equiv.) in 1,2-dichloroethane with 2 equiv. of (MeO)3CH
and 5 mol% PPTS (pyridinium para-toluensulfonate) furnished
the desired product.
1a: Yield: 72%. 1H NMR (400 MHz, CDCl3, d): 8.29 (d, J ¼ 8.6
Hz, 1H), 8.00 (d, J ¼ 8.4 Hz, 1H), 7.82 (d, J ¼ 7.8 Hz, 1H), 7.79
(d, J ¼ 8.2 Hz, 1H), 7.71–7.68 (m, 2H), 7.60 (d, J ¼ 8.8 Hz, 1H),
7.47 (t, 1H), 7.41–7.31 (m, 7H), 7.11 (d, J ¼ 8.8 Hz, 1H), 6.84–
6.82 (m, 2H), 6.28 (d, J ¼ 9.9 Hz, 1H), 3.76 (s, 3H). MS: m/z [M+]:
414. Anal. calc. for C30H22O2: C, 86.93; H, 5.35. Found: C, 86.72;
H, 5.49%.
2a: Yield: 28%. 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 [M+ +
1]: 428.2. Anal. calc. for C31H25NO: C, 87.09; H, 5.89. Found: C,
87.48; H, 5.80%.
3a: Yield: 76%. 1H NMR (400 MHz, CDCl3, d): 8.27 (dd, J1 ¼
8.3 Hz, J2 ¼ 7.3 Hz, 2H), 8.01 (d, J ¼ 8.5 Hz, 1H), 7.70 (d, J ¼ 8.1
Hz, 1H), 7.59 (dd, J1 ¼ 8.1 Hz, J1 ¼ 8.8 Hz, 2H), 7.51–7.45 (m,
3H), 7.46–7.24 (m, 7H), 7.10 (d, J ¼ 8.8 Hz, 1H), 6.67 (d, J ¼ 8.2
Hz, 1H), 6.21 (d, J ¼ 9.9 Hz, 1H), 3.95 (s, 3H). MS: m/z [M+]:
414. Anal. calc. for C30H22O2: C, 86.93; H, 5.35. Found: C, 86.57;
H, 5.21%.
Scheme 2 The structures of the target compounds.
ambient temperature, which provides a useful strategy for the
design of 3H-naphtho[2,1-b]pyran-based naphthopyrans and
benefits their future practical applications.
4a: Yield: 38%. 1H NMR (400 MHz, CDCl3, d): 8.32 (d, J ¼ 8.6
Hz, 1H), 8.23 (d, J ¼ 8.4 Hz, 1H), 7.99 (d, J ¼ 8.5 Hz, 1H), 7.69
(d, J ¼ 8.1 Hz, 1H), 7.58 (d, J ¼ 8.8 Hz, 1H), 7.54–7.27 (m, 11H),
7.10 (d, J ¼ 8.8 Hz, 1H), 6.91 (d, J ¼ 8.0 Hz, 1H), 6.22 (d, J ¼ 9.9
Hz, 1H), 2.85 (s, 6H). MS: m/z [M+ + 1]: 428. Anal. calc. for
C31H25NO: C, 87.09; H, 5.89. Found: C, 87.34; H, 5.78%.
5a: Yield: 70%. 1H NMR (400 MHz, CDCl3, d): 8.28 (d, J ¼ 9.8
Hz, 2H), 7.99 (d, J ¼ 8.4 Hz, 1H), 7.70 (d, J ¼ 8.0 Hz, 1H), 7.59
(d, J ¼ 8.8 Hz, 1H), 7.57 (d, J ¼ 8.2 Hz, 1H), 7.47 (t, 1H), 7.45–
7.29 (m, 6H), 7.09 (d, J ¼ 8.8 Hz, 1H), 6.84 (d, J ¼ 8.8 Hz, 2H),
6.67 (d, J ¼ 8.2 Hz, 1H), 6.23 (d, J ¼ 9.9 Hz, 1H), 3.95 (s, 3H),
3.77 (s, 3H). MS: m/z [M+]: 444. Anal. calc. for C31H24O3: C,
83.76; H, 5.44. Found: C, 83.83; H, 5.38%.
6a: Yield: 10%. 1H NMR (400 MHz, CDCl3, d): 8.31 (d, J ¼ 8.5
Hz, 1H), 8.25 (d, J ¼ 8.4 Hz, 1H), 7.99 (d, J ¼ 8.5 Hz, 1H), 7.71
(d, J ¼ 8.6 Hz, 1H), 7.60 (d, J ¼ 8.8 Hz, 1H), 7.53 (d, J ¼ 7.9 Hz,
1H), 7.47 (t, 1H), 7.45–7.29 (m, 6H), 7.11 (d, J ¼ 8.8 Hz, 1H),
6.93 (d, J ¼ 7.9 Hz, 1H), 6.83 (d, J ¼ 8.8 Hz, 2H), 6.25 (d, J ¼ 9.9
Hz, 1H), 3.79 (s, 3H), 2.85 (s, 6H). MS: m/z [M+]: 457. Anal. calc.
for C32H27NO2: C, 84.00; H, 5.95. Found: C, 84.43; H, 5.63%.
Experimental
General
1H NMR spectra were recorded at 400 MHz with tetrame-
thylsilane (TMS) as an internal reference and CDCl3 as the
solvent. Mass spectra were recorded using a Trio-2000 GC-MS
spectrometer. UV absorption spectra were measured on an
absorption spectrophotometer (Hitachi U-3010). Coloration was
carried out with a UV light (l ¼ 254 nm, intensity: 4.3 mW cmꢀ2).
All chemicals for synthesis were purchased from commercial
suppliers, and solvents were purified according to standard
procedures. Reactions were monitored by TLC silica gel plates
(60F-254). Column chromatography was performed on silica gel
(Merck, 70–230 mesh). PMMA thin films were prepared as
follows: the naphthopyran (1.0 mg) was dissolved in 1.0 mL of
a PMMA–cyclohexanone solution (10%, w/w). A film was
obtained by spin coating on quartz glass (diameter: 1.3 mm) with
ꢁ
a gradient of 700 rpm (10 s), followed 1200 rpm (30 s) (25 C),
and dried in air and kept in the dark at room temperature. The
concentration of the thin film was about 1.2 ꢂ 10ꢀ5 mol gꢀ1
.
Material
Results and discussion
Naphthopyrans 1a–6a were synthesized according to the litera-
ture34 using the synthetic method described in Scheme 3. The
general procedure for their preparation is as follows. The treat-
ment of the 1,2-diaryl-2-propyn-1-ol (1.1 equiv.) and b-naphthol
Compounds 1a–6a showed typical photochromism in both
solution and in a rigid polymer matrix upon irradiation with UV
light. As a model compound, the photochromism and absorption
spectral changes of 5a in acetonitrile (A) and a PMMA matrix
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Table 1 Kinetics datab of 1a–6b in MeCN solution (2 ꢂ 10ꢀ5 M) at 10 ꢁC
Compound lmax/nm O. D. T /s
½
T
3/4 /s K1 (ꢂ10ꢀ2
)
K2 (ꢂ10ꢀ4
)
NPy(b)a
1b
2b
3b
4b
422
454
540
446
414
466
466
0.384 125
0.456 106
0.554 51
0.336 48
0.215 43
0.527 30
0.311 25
405
305
118
110
104
72
0.33
0.58
1.18
1.21
1.54
1.89
2.09
7.20
2.32
8.06
4.33
4.46
1.66
2.68
5b
6b
68
NPy used as a reference compound (2 ꢂ 10ꢀ5 M, in MeCN). b Samples
initially irradiated at 254 nm until in a photosteady state (40 s). Thermal
decoloration was then monitored at the lmax of the colored form at 10 ꢁC
in the dark.
a
Scheme 4 The photochromism of 5a in both solution and in a PMMA
thin film.
thin film (B) with UV light irradiation are presented in Scheme 4
and Fig. 1, respectively. The absorption of the photosteady state
(5b) appeared at 466 nm (in solution) and 476 nm (in PMMA
matrix), respectively. Compared with the absorption in solution,
5b showed a 10 nm red-shift in the PMMA matrix, which is
probably due to the polarity of the medium. Similar results were
obtained with other compounds, and the absorption data of 1a–
6b in both solution and in a PMMA matrix are presented in
Table 1 and Table 2, respectively.
FC. The process of TC / FC (k1) is usually rapid and that of
TT / TC (k2) is slow.35–37 Convenient measures of the fading
speed are the T and T
½
for the optical density to reduce by ½ and 3/4 of the initial optical
/
values,38,39 which are the time it takes
3 4
density of the colored form, and the smaller are T and T3/4 , the
½
faster is the fading. Fig. 2 representedꢁthe fading kinetics of 5b in
CH3CN at ambient temperature (10 C). It was found that the
There are two open forms, TC (transoid-cis) and TT (transoid-
trans), when 3H-naphtho[2,1-b]pyrans are irradiated with UV
light to a photosteady state (Scheme 5); the TC form is the major
and the TT form is the minor. The fading of the colored form
back to the FC (colorless form) involve two steps, TT / TC /
fading times T and T
½
fading rate constant (k1 ¼ 1.89 ꢂ 10ꢀ2 sꢀ1, k2 ¼ 1.66 ꢂ 10ꢀ4 sꢀ1
/ were 30 and 72 s, respectively, and the
3 4
)
was obtained using the biexponential decoloration model
according to the standard biexponential equation.18,19,25 The
fading kinetics of other compounds was analyzed using the same
method, and the data are listed in Table 2. Compared with their
parent compound, NPy, all the compounds showed a fast fading
speed and a large fading rate constant, indicating that the elec-
tron-donating group attached to para-position of the aryl moiety
at the 3-position improved the fading speed of the photosteady
state of these naphthopyrans. A further investigation found the
following results. First, the fading speed of 3b (T ¼ 48 s, T
½
/
3 4
¼
110 s) and 4b (T ¼ 43 s, T3/4 ¼ 104 s) were faster than that of 1b
½
(T ¼ 106 s, T
/
3 4
¼ 305 s) and 2b (T ¼ 51 s, T
/
3 4
¼ 118 s),
½
½
respectively, which suggests that the electron-donating group
attached to the para-position of the naphthalene ring is better
than that attached to the para-position of the benzene ring.
Second, comparing 2b with 1b and 4b with 2b, it is found that the
fading time of 2b is faster than that of 1b, and that of 4b is faster
than that of 3b. This indicates that the fading speed is influenced
by the strength of the electron-donating groups, and that the
stronger the electron-donating group, the faster the fading speed.
Third, by comparison of the data listed in Table 1, both 5b and 6b
showed the fastest fading speed, which reveals that bearing
electron-donating groups at both the para-position of the
naphthalene ring and the benzene ring dramatically increases the
fading speed.
The fading speed of 1a–6b in a rigid polymer matrix were also
investigated by doping naphthopyrans 1a–6a in a polymethyl
methacrylate (PMMA) matrix. A naphthopyran–PMMA thin
film was obtained by spin coating a mixed solution of the
naphthopyran and PMMA in cyclohexanone. As compared with
NPy (b), the fading speed of all the target compounds, 1a–6b,
Fig. 1 Absorption changes of 5b in acetonitrile (2 ꢂ 10ꢀ5 M, irradiating
light: l ¼ 254 nm, intensity: 4.3 mW cmꢀ2, irradiating periods: 0, 5, 10, 15,
20 and 25 s) and in a PMMA thin film (1.2 ꢂ 10ꢀ5 mol gꢀ1, irradiating
light: l ¼ 254 nm, intensity: 4.3 mW cmꢀ2, irradiating periods: 0, 30, 60,
90, 120, 150, 180 and 210 s).
increased significantly, and both T and T
½
/ decreased greatly
3 4
(Table 3), which reveals that the electron-donating group
attached to the para-position of the aryl moiety at the 3-position
increases the fading speed of the photosteady state of
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Table 2 Kinetics datab of 1a–6b in a PMMA matrix (1.2 ꢂ 10ꢀ5 mol gꢀ1) at 10 ꢁC
Compound
lmax/nm
O. D.
T /s
½
T
3/4 /s
K1 (ꢂ10ꢀ2
)
K2 (ꢂ10ꢀ4
)
NPy(b)a
1b
2b
3b
4b
426
462
548
452
420
476
476
0.73
300
186
65
56
50
>1000
800
150
145
125
120
94
0.32
0.52
1.03
1.14
1.20
1.38
1.58
2.43
3.50
5.59
5.17
4.62
4.06
3.43
1.114
1.330
1.083
0.780
1.057
0.769
5b
6b
45
40
a
NPy used as a reference compound (1.2 ꢂ 10ꢀ5 mol gꢀ1, in PMMA). b Samples initially irradiated at 254 nm until in a photosteady state (1.5 min).
Thermal decoloration was then monitored at the lmax of the colored form at 10 ꢁC in the dark.
Scheme 5 The structures of the FC, TC and TT isomers.
Fig. 3 The fatigue resistance of 5a in a PMMA thin film.
showed very good fatigue resistance in a PMMA thin film. As
presented in Fig. 3, after 10 cycles of coloration/decoloration, no
distinct degradation was detected by UV-vis absorption for 5a
(detection optical density at lmax ¼ 476 nm). Other compounds
also showed excellent fatigue resistance in PMMA thin films and
less than 10% degradation was detected after 10 cycles.
Conclusion
A class of artificial photochromic 3-aryl-3-(a-naphthalene)-3H-
naphtho[2,1-b]pyrans with fast fading speeds in both solution
Fig. 2 The fading kinetics of 5b in MeCN solution at 10 ꢁC.
and in rigid polymer matrices has been developed. It was
demonstrated that both the electronic properties and the position
of substituents have a great influence over the fading speed of
3-aryl-3-(a-naphthalene)-3H-naphtho[2,1-b]pyrans, with the
attachment of strong electron-donating groups at the para-
position of both the a-naphthalene ring and the benzene ring
significantly increasing their fading speed at ambient tempera-
ture.
naphthopyrans. As presented in Table 3, the values of k of 5b and
6b are larger than those of 1b–4b in a PMMA thin film, resulting
in the fading speed of 5b and 6b being faster than those of 1b–4b.
Also, it was found that the fading speed of 3b and 4b are faster
than those of 1b and 2b, respectively, and that of 2b and 4b are
faster than those of 1b and 3b, respectively, which confirms that
the fading speed of 3-aryl-3-(a-naphthalene)-3H-naphtho[2,1-b]
pyrans with a strong electron-donating group attached to the
para-position of the aryl ring at the 3-position is dramatically
increased; this result is in agreement with that obtained in solu-
tion. It is worth noting that although the switching performance
of naphthopyrans is profoundly influenced by the local host
environment and the media that incorporate them,30,40 both the
electronic and structural nature of naphthopyrans decides their
intrinsic properties and plays a key role in their switching
performance.
Acknowledgements
This work was supported by the National Basic Research
Program of China (2010CB934103) and the National Nature
Science Foundation of China (21073214).
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