Synthesis and photochromism of naphthopyrans bearing naphthalimide chromophore: Predominant thermal reversibility in color-fading and fluorescence switch

Article abstract of DOI:10.1021/jp208082w

Two novel photochromic naphthopyrans containing naphthalimide moieties (Nip1 and Nip2) were studied in solution under flash photolysis conditions, exhibiting highly photochromic response, rapid thermal bleaching rate and good fatigue-resistance. Owing to

Full text of DOI:10.1021/jp208082w

ARTICLE
pubs.acs.org/JPCB
Synthesis and Photochromism of Naphthopyrans Bearing
Naphthalimide Chromophore: Predominant Thermal Reversibility
in Color-Fading and Fluorescence Switch
Liwen Song,† Yuheng Yang,† Qiong Zhang,†,‡ He Tian,† and Weihong Zhu*,†
†Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science and Technology, Shanghai 200237, P. R. China
‡Department of Theoretical Chemistry and Biology, Royal Institute of Technology, AlbaNova University Centre, 10691 Stockholm,
Sweden
S Supporting Information
b
ABSTRACT: Two novel photochromic naphthopyrans con-
taining naphthalimide moieties (Nip1 and Nip2) were studied
in solution under flash photolysis conditions, exhibiting highly
photochromic response, rapid thermal bleaching rate and good
fatigue-resistance. Owing to the different N-substituted imide
groups at the naphthalimide units, the thermal bleaching rate of
Nip2 bearing phenyl on the naphthalimide unit is found to be
approximately 2 times that of Nip1 bearing n-butyl, indicating
that the photochromic properties can be modulated with intro-
duction of different functional groups on the naphthalimide unit. In Nip1 and Nip2, the strong electron-withdrawing effect of the
imide group incorporated at the naphthalimide moiety maintains several merits: (i) shifting absorption bands to longer wavelength,
(ii) beneficial to an enhancement in the ratio of transoid-cis (TC) isomer and an increase in the transformation rate from transoid-
trans (TT) to TC with respect to reference compound NP, and (iii) resulting in a preferable color bleaching rate and fading
absolutely to their colorless state with thermal reversibility. As demonstrated in the system of NP, the slow transformation process
from TT to TC might be the predominant dynamic step in thermal back process, leading to the residual color of NP being only faded
to its original colorless state by visible light irradiation. The optical densities of colored forms for Nip1 and Nip2 are dependent upon
the intensity of incident light, ensuring a possible application in the manufacture of ophthalmic lenses and smart windows.
Moreover, the fluorescence of Nip1 and Nip2 can be switched on and off by photoinduced conversion between the closed and
open forms.
’ INTRODUCTION
photographic apparatus, decorative objects, textiles, cosmetic
products, and so on.7
Photochromic materials have attracted much attention in recent
years due to their potential application in ophthalmic lenses and
molecular-level devices, such as molecular switches, optical data
storage, logic gates, and molecule imaging.1 Among various photo-
chromic systems, naphthopyrans are known to exhibit excellent
photochromic responses, good colorability, and rapid bleaching.2
Generally, the photochromic naphthopyran system undergoes
ring-opening with a color change via the photochemical cleavage
of sp3 CꢀO bond, spontaneously reverting back to their original
colorless closed form at dark or visible light irradiation when
ceasing the UV luminous irradiation. To date, naphthopyans have
become predominant in numerous naturally and biologically
active compounds, and are explored in sensors,3 polymers,4
switches5 and organic gels.6 Because of the excellent photo-
chromic response in a wide range of the visible spectrum from
yellow to red and fast bleaching rate, the commercial applica-
tion of naphthopyans in the manufacture of ophthalmic lenses
and smart windows has attracted much interest for a decade,
even has been applied in optical filters, diverse optical devices,
For commercial application in ophthalmic lenses, it is neces-
sary to keep photochromic molecules with highly efficient photo-
response, large steady-state optical density, fast bleaching speed
at ambient temperature, and excellent fatigue-resistance. Speci-
fically, all the photochromic properties are critically dependent
on their structural features with modifying different functional
groups on naphthalene and/or two aromatic rings.8 For instance,
various ring systems, such as fluorene9 and carbazole,10 fused to
the naphthopyran moiety were synthesized for the sake of modu-
lating their photochromic properties and commercial applica-
tions. With respect to 3,3-diaryl-3H-naphtho[2,1-b]pyrans ([3]H-
NP, Scheme 1), photochromic 2,2-diaryl-2H-naphtho[1,2-b]pyrans
([2]H-NP) can promote the optical density of colored forms at
ambient temperature and shift the absorption bathochromically to
Received: August 22, 2011
Revised:
October 24, 2011
Published: October 25, 2011
r
2011 American Chemical Society
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Scheme 1. Chemical Structures of [2]H-NP, [3]H-NP, and Open Forms TC and TT
neutral color range, which is quite suitable for commercial ophthal-
mic lenses. However, [2]H-NP always shows a distinct decrease in
bleaching rate. What’s even worse, the enhancement in photochro-
mic response of [2]H-NP is invariably accompanied by an increase
in the rate of photodegradation, resulting in the undesirable fatigue-
resistance.2
Scheme 2. Photochromic Reaction of Nip1 and Nip2
1,8-naphthalimide and its derivatives with a strong electron-
withdrawing imide group are a specific series of environmentally
sensitive fluorophores, widely utilized in various fields due to
their good chemical stability, large Stokes shift, and high fluo-
rescent quantum yield.11 More recently, we have incorporated
naphthalimide units into photochromic spirooxazine and diary-
lethene systems,12 especially realizing a significantly long lifetime
in the open merocyanine (MC) forms. Bearing these considera-
tions in mind, we designed and synthesized two novel naphthopyr-
ans (Nip1 and Nip2 shown in Scheme 2) bearing a naphthalimide
moiety with different N-substituted imide groups fused to the
naphthalene moiety. Essentially, several merits with good photo-
chromic properties are found in Nip1 and Nip2: (i) their colored
forms possess relatively large optical density and neutral color range,
which intrinsically corresponds to the characteristics of 2H-naphtho
[1,2-b]pyrans rather than that of 3H-naphtho[2,1-b]pyrans; (ii) the
existence of naphthalimide unit with strong electron-withdrawing
imide group can somehow eliminate the ratio of transoid-trans (TT)
form, usually a slow step, or sharply increase the transformation rate
from TT to transoid-cis (TC), thus facilitating the thermal back
absolutely to closed form with a preferable color bleaching rate and
thermal reversibility; and (iii) the fluorescence of naphthalimide
unit can be switched on and off by photoinduced conversion. More
specifically, the photochromic properties of Nip1 and Nip2 can be
modulated with different N-substitution imide groups at the
naphthalimide moiety, opening a new way to design molecular
structures with various photochromic properties.
packages purchased from Qing-dao Haiyang Chemical Com-
pany, China.
1
Instrumentation. H- and 13C-NMR spectra were recorded
on Bruker AM-400 spectrometers with tetramethylsilane (TMS)
asan internal reference andCDCl3 or DMSO-d6 as solvent. HRMS
were recorded on a Waters LCT Premier XE spectrometer with
methanol as solvent. Fluorescence spectra were recorded on a
Horiba Fluoromax 4. Photochromic reaction was induced in situ by
a continuous wavelength irradiation Hg/Xe lamp (Hamamatsu,
LC6 Lightingcure, 200 W) equipped with narrow band interfer-
ence filters of appropriate wavelengths (Semrock Hg01 for lirr =
365 nm, Semrock BrightLine FF01ꢀ575/25ꢀ25 for lirr =
575 nm). The irradiation power was measured using a photodiode
from Ophir (PD300-UV). Absorption changes were monitored
by a charge-coupled device (CCD) camera mounted on a spectro-
meter (Princeton Instruments).
Synthesis of 4-Bromo-N-butyl-1,8-naphthalimide (2a). A
mixture of 4-bromo-1,8-naphthalic anhydride 1 (8.13 g, 29.4 mmol)
and n-butylamine (6.45 g, 88.4 mmol) in acetic acid (90 mL) was
refluxed under argon for 6 h. After cooling, the mixture was poured
into water, resulting in yellow precipitate. It was filtered and
recrystallized from acetic acid to afford 7.01 g of compound 2a as
a pale yellow crystal in 71.9% yield. 1H-NMR (400 MHz, CDCl3,
ppm): δ 8.65 (d, J = 7.2 Hz, 1H, naphthaleneꢀH), 8.56 (d, J = 8.0
Hz, 1H, naphthaleneꢀH), 8.41 (d, J = 8.0 Hz, 1H, naphthaleneꢀ
H), 8.03 (d, J = 7.6 Hz, 1H, naphthaleneꢀH), 7.84 (t, J = 7.6 Hz,
1H, naphthaleneꢀH), 4.17 (t, J = 7.6 Hz, 2H, ꢀNCH2ꢀ), 1.76ꢀ
1.68 (m, 2H, ꢀNCH2CH2ꢀ), 1.50ꢀ1.40 (m, 2H, ꢀCH2CH3),
’ EXPERIMENTAL SECTION
Materials. 4-Bromo-1,8-naphthalic anhydride and propargylic
alcohol derivative 5 (Scheme 3) were commercially available and
purified before use. All other reagents were of analytical purity
and used without further treatment. Thin-layer chromatography
(TLC) analyses were performed on silica-gel plates, and flash
chromatography was conducted by using silica-gel column
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Scheme 3. Synthetic Routes of Nip1, Nip2, and Reference Compound NP
0.98 (t, J = 7.4 Hz, 3H, ꢀCH2CH3). 13C-NMR (100 MHz,
CDCl3, ppm): δ 163.52, 133.11, 131.94, 131.13, 131.03, 130.51,
130.12, 128.88, 128.03, 123.08, 122.22, 40.38, 30.16, 20.38, 13.86.
HRMS-ESI (m/z): [M + H]+ Calcd. for (C16H14BrNO2),
332.0286, found: 332.0285.
7.68 (t, J = 7.6 Hz, 1H, naphthaleneꢀH), 7.01 (d, J = 8.4 Hz, 1H,
naphthaleneꢀH), 4.16 (t, J = 7.6 Hz, 2H, ꢀNCH2ꢀ), 4.12 (s,
3H, ꢀOCH3), 1.75ꢀ1.68 (m, 2H, ꢀNCH2CH2ꢀ), 1.50ꢀ1.40
(m, 2H, ꢀCH2CH3), 0.98 (t, J = 7.3 Hz, 3H, ꢀCH2CH3). 13C-
NMR (100 MHz, CDCl3, ppm): δ 164.45, 163.88, 160.66,
133.30, 131.40, 129.23, 128.47, 125.85, 123.36, 122.36, 115.06,
105.10, 56.16, 40.07, 30.26, 20.42, 13.89. HRMS-ESI (m/z):
[M + H]+ Calcd. for (C17H17NO3), 284.1287, found: 284.1283.
Synthesis of 4-Methoxy-N-phenyl-1,8-naphthalimide (3b).
4-Bromo-N-phenyl-1,8-naphthalimide 2b (7.50 g, 21.3 mmol),
sodium methoxide (9.20 g, 0.17 mmol) and copper sulfate (0.66
g, 2.8 mmol) were mixed in dry methanol (100 mL). The reaction
mixture was refluxed under argon for 15 h. After cooling, the
resulting mixture was filtered to give yellow-green solid as a crude
product. Then the solid was washed with 10% (v/v) hydrochloric
acid (100 mL ꢁ 3) and water (100 mL ꢁ 3) to afford 6.10 g of
compound 3b as a pale yellow solid in 94.0% yield. 1H-NMR (400
MHz, CDCl3, ppm): δ 8.65ꢀ8.59 (m, 3H, naphthaleneꢀH),
7.73 (d, J = 7.6 Hz, 1H, naphthaleneꢀH), 7.58ꢀ7.53 (m, 2H,
naphthaleneꢀH), 7.48ꢀ7.45 (m, 1H, naphthaleneꢀH), 7.31 (d,
J = 7.6 Hz, 2H, naphthaleneꢀH), 7.08 (d, J = 8.0 Hz, 1H,
phenylꢀH), 4.16 (s, 3H, ꢀOCH3). 13C-NMR (100 MHz,
CDCl3, ppm): δ 164.69, 164.10, 161.09, 135.71, 133.87,
131.94, 129.79, 128.99, 128.73, 128.55, 126.04, 123.67, 122.56,
115.15, 105.32, 56.30. HRMS-ESI (m/z): [M + H]+ Calcd. for
(C19H13NO3), 304.0974, found: 304.0971.
Synthesis of 4-Bromo-N-phenyl-1,8-naphthalimide (2b).
A mixture of 4-bromo-1,8-naphthalic anhydride 1 (7.20 g, 26.0
mmol) and freshly distilled aniline (24.0 g, 0.26 mol) in acetic acid
(100 mL) was refluxed under argon for 24 h. After cooling, the
mixture was poured into water, resulting in yellow precipitate. It
was filtered and recrystallized from acetic acid to afford 7.60 g of
compound2b as a pale yellow crystal in 83.0% yield. 1H-NMR(400
MHz, CDCl3, ppm): δ 8.70 (d, J = 7.2 Hz, 1H, naphthaleneꢀH),
8.64 (d, J = 8.5 Hz, 1H, naphthaleneꢀH), 8.46 (d, J = 8.0 Hz, 1H,
naphthaleneꢀH), 8.08 (d, J = 7.6 Hz, 1H, naphthaleneꢀH), 7.89
(t, J = 7.2 Hz, 1H, naphthaleneꢀH), 7.58ꢀ7.56 (m, 2H, phenylꢀ
H), 7.51ꢀ7.48 (m, 1H, phenylꢀH), 7.31 (d, J = 7.2 Hz, 2H,
phenylꢀH). 13C-NMR (100 MHz, CDCl3, ppm): δ 163.77,
163.73, 135.10, 133.61, 132.44, 131.58, 131.22, 130.74, 130.67,
129.47, 129.31, 128.88, 128.57, 128.21, 123.21, 122.33. HRMS-ESI
(m/z): [M + H]+ Calcd. for (C18H10BrNO2), 351.9973, found:
351.9977.
Synthesis of 4-Methoxy-N-butyl-1,8-naphthalimide (3a).
4-Bromo-N-butyl-1,8-naphthalimide 2a (6.01 g, 18.1 mmol),
sodium methoxide (7.76 g, 0.14 mol) and copper sulfate (0.66
g, 2.8 mmol) were mixed in dry methanol (80 mL). The reaction
mixture was refluxed under argon for 8 h. After cooling, the
resulting mixture was filtered to give pale yellow solid as a crude
product. Then the solid was washed with 10% (v/v) hydrochloric
acid (30 mL ꢁ 3) and water (20 mL ꢁ 3) to afford 4.47 g of
compound 3a as a pale green needle-shaped crystal in 87.3%
yield. 1H-NMR (400 MHz, CDCl3, ppm): δ 8.57 (d, J = 7.2 Hz,
1H, naphthaleneꢀH), 8.52 (d, J = 8.0 Hz, 2H, naphthaleneꢀH),
Synthesis of 4-Hydroxy-N-butyl-1,8-naphthalimide (4a).
A mixture of 4-methoxy-N-butyl-1,8-naphthalimide 3a (2.08 g,
7.3 mmol) and 57% (v/v) hydroiodic acid (100 mL) was refluxed
under argon for 10 h. After cooling, the resulting mixture was
filtered, then the crude product was washed with water (50 mL ꢁ 3)
to afford yellow-green solid. The crude product was puri-
fied by column chromatography on silica (petroleum ether:ethyl
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ARTICLE
acetate = 3:1, v/v) to afford 1.16 g of compound 4a as a pale
yellow solid in 59.0% yield. H-NMR (400 MHz, DMSO-d6,
naphthaleneꢀH), 8.35 (s, 1H, naphthaleneꢀH), 7.72 (t, J = 8.0
Hz, 1H, naphthaleneꢀH), 7.55ꢀ7.51 (m, 2H, phenylꢀH),
7.48ꢀ7.44 (m, 3H, phenylꢀH), 7.41ꢀ7.35 (m, 4H, phenylꢀH),
7.32ꢀ7.27 (m, 3H, phenylꢀH), 6.88ꢀ6.84 (m, 3H, dCH,
phenylꢀH), 6.27 (d, J = 10.0 Hz, 1H, dCH), 3.78 (s, 3H,
ꢀOCH3). 13C-NMR (100 MHz, CDCl3, ppm): δ 164.56,
163.99, 159.40, 154.01, 144.23, 136.00, 135.63, 131.95, 131.06,
129.91, 129.35, 128.91, 128.78, 128.70, 128.59, 128.49, 128.44,
128.00, 126.73, 126.38, 122.91, 122.77, 122.46, 116.78, 115.21,
113.76, 85.13, 55.30. HRMS-ESI (m/z): [M + H]+ Calcd. for
(C34H23NO4), 510.1705, found: 510.1713.
Synthesis of 2-(4-Methoxyphenyl)-2-phenyl-2H-benzo-
[h]chromene (NP). In a 50 mL three-necked flask, equipped
with a DeanꢀStark apparatus, naphthalen-1-ol (1.0 g, 6.9 mmol),
propargyl alcohol derivative 5 (2.2 g, 9.2 mmol) and aluminum
oxide (10.0 g) were mixed in toluene (40 mL). The reaction
mixture was refluxed under argon for 9 h. After cooling, the
resulting mixture was filtered, and the filter cake was washed with
dichloromethane (100 mL ꢁ 3). Then the filtrate and the
scrubbing solution were combined. After concentration, the
compound was purified by column chromatography on silica
(petroleum ether:dichloromethane = 5:1, v/v) to afford 210 mg
of pale yellow solid in 8.4% yield. 1H-NMR (400 MHz, DMSO-
d6, ppm): δ 8.29 (d, J = 8.4 Hz, 1H, phenylꢀH), 7.82 (d, J = 8.0
Hz, 1H, phenylꢀH), 7.57ꢀ7.48 (m, 4H, phenylꢀH), 7.44ꢀ7.41
(m, 3H, phenylꢀH), 7.34 (t, J = 7.6 Hz, 2H, phenylꢀH),
7.30ꢀ7.23 (m, 2H, phenylꢀH), 6.90ꢀ6.84 (m, 3H, phenylꢀH,
dCH), 6.49 (d, J = 10.0 Hz, 1H, dCH), 3.69 (s, 3H, ꢀCH3).
13C-NMR (100 MHz, DMSO-d6, ppm): δ 163.75, 152.06, 150.40,
142.04, 139.31, 133.46, 133.37, 132.99, 132.77, 132.53, 131.71,
131.27, 131.17, 129.83, 129.19, 128.64, 126.51, 125.74, 120.98,
118.82, 87.44, 60.26. HRMS-ESI (m/z): [M + H]+ Calcd. for
(C26H20O2), 365.1542, found: 365.1537.
1
ppm): δ 8.53 (dd, J1 = 8.4 Hz, J2 = 1.2 Hz, 1H, naphthaleneꢀH),
8.47 (dd, J1 = 7.2 Hz, J2 = 1.2 Hz, 1H, naphthaleneꢀH), 8.36 (d,
J = 8.4 Hz, 1H, naphthaleneꢀH), 7.76 (t, J = 7.2 Hz, 1H, naphtha-
leneꢀH), 7.16 (d, J = 8.4 Hz, 1H, naphthaleneꢀH), 4.02 (t, J =
7.4 Hz, 2H, ꢀNCH2ꢀ), 1.64ꢀ1.58 (m, 2H, ꢀNCH2CH2ꢀ),
1.39ꢀ1.30 (m, 2H, ꢀCH2CH3), 0.92 (t, J = 7.3 Hz, 3H, ꢀCH2-
CH3). 13C-NMR (100 MHz, DMSO-d6, ppm): δ 163.59, 162.92,
160.17, 133.45, 131.02, 129.09, 128.78, 125.49, 122.31, 121.74,
112.55, 109.89, 40.12, 29.70, 19.78, 13.68. HRMS-ESI (m/z):
[M ꢀ H]ꢀ Calcd. for (C16H15NO3), 268.0974, found: 268.0978.
Synthesis of 4-Hydroxy-N-phenyl-1,8-naphthalimide (4b).
A mixture of 4-methoxy-N-phenyl-1,8-naphthalimide 3b (1.50 g,
4.90 mmol) and 57% (v/v) hydroiodic acid (60 mL) was refluxed
under argon for 15 h. After cooling, the resulting mixture was
filtered, then the crude product was washed with water (50 mL ꢁ
3) to afford 0.90 g of yellow-green solid. The product was directly
used without further purification for the next step.
Synthesis of 5-Butyl-10-(4-methoxyphenyl)-10-phenylbenzo-
[de]pyrano[2,3-g]isoquinoline-4,6(5H,10H)-dione (Nip1). In a
50 mL three-necked flask, equipped with a DeanꢀStark apparatus,
4-hydroxy-N-butyl-1,8-naphthalimide 4a (700 mg, 2.6 mmol), pro-
pargyl alcohol derivative 5 (834 mg, 3.5 mmol), and aluminum
oxide (7.00 g) were mixed in toluene (25 mL). The reaction
mixture was refluxed under argon for 5 h. After cooling, the result-
ing mixture was filtered, and the filter cake was washed with di-
chloromethane (50 mL ꢁ 3). Then the filtrate and the scrubbing
solution were combined. After concentration, the compound was
purified by column chromatography on silica (petroleum ether:di-
chloromethane = 1:4, v/v) to give crude product, then recrystal-
lized from ethanol to afford 360 mg of compound Nip1 as a pale
yellow solid in 28.3% yield. 1H-NMR (CDCl3, 400 MHz, ppm):
δ 8.61 (dd, J1 = 8.4 Hz, J2 = 1.2 Hz, 1H, naphthaleneꢀH), 8.56 (dd,
J1 = 7.2 Hz, J2 = 1.2 Hz, 1H, naphthaleneꢀH), 8.33 (s, 1H,
naphthaleneꢀH), 7.71 (t, J = 7.6 Hz, 1H, naphthaleneꢀH),
7.49ꢀ7.47 (m, 2H, phenylꢀH), 7.41ꢀ7.35 (m, 4H, phenylꢀH),
7.34ꢀ7.32 (m, 1H, phenylꢀH), 6.89ꢀ6.84 (m, 3H, dCH,
phenylꢀH), 6.27 (d, J = 10.0 Hz, 1H, dCH), 4.17 (t, J = 7.6
Hz, 2H, ꢀNCH2ꢀ), 3.79 (s, 3H, ꢀOCH3), 1.74ꢀ1.66 (m, 2H,
ꢀCH2CH2ꢀ), 1.47ꢀ1.41 (m, 2H, ꢀCH2CH2ꢀ), 0.98 (t, J = 7.2
Hz, 3H, ꢀCH2CH3). 13C-NMR (100 MHz, CDCl3, ppm):
δ 164.37, 163.84, 159.36, 153.63, 144.28, 136.06, 131.48, 130.67,
129.49, 128.75, 128.44, 128.39, 128.32, 127.93, 126.71, 126.25,
122.71, 122.51, 116.62, 115.25, 113.72, 84.98, 55.27, 40.08, 30.25,
20.38, 13.86. HRMS-ESI (m/z): [M + H]+ Calcd. for
(C32H27NO4), 490.2018, found: 490.2018.
’ RESULTS AND DISCUSSION
Synthesis. Naphthopyrans always play a great role in the
manufacture of ophthalmic lenses. Generally, this important appli-
cation requires that photochromic compounds possess several spe-
cific characteristics: (i) large optical density andneutral color range,
that is, the absorption spectra in the colored form almost cover the
wholevisiblespectrum, (ii) fastphotoresponse, and(iii) rapidcolor
fading rate. However, these preconditions cannot concur in tradi-
tional 2H-naphtho[1,2-b]pyrans, which always possess relatively
large optical density and neutral color range, but unfavorable fading
rate synchronously. Evans et al. reported a general method for
realizing fast and synchronizing photochromic switching for photo-
chromic dyes including naphthoprans.4c,d To further develop the
photochromic properties of naphthopyran derivatives, a naphtha-
limide unit with different N-substituted imide groups was incorpo-
rated. As illustrated in Scheme 3, the target compounds Nip1 and
Nip2 were synthesized by Claisen rearrangement of propargyl-aryl
ethers prepared from 4-hydroxy-1,8-naphthalimide (4), which was
synthesized from the raw material of 4-bromo-1,8-naphthalic
anhydride (1) via three steps (imidation, Williamson ether con-
densation, and Zeisel ether cleavage). The reference compound
NP was directly prepared by α-naphthol and propargyl alcohol
derivative 5. Notably, using excess acidic Al2O3 as a catalyst,13 the
synthesis in the Claisen rearrangement of propargyl-aryl ethers
could become straightforward with an easy purification and pref-
erable yield. Their chemical structures were well confirmed by
Synthesis of 10-(4-Methoxyphenyl)-5,10-diphenylbenzo-
[de]pyrano[2,3-g]isoquinoline-4,6(5H,10H)-dione (Nip2).
In a 50 mL three-necked flask, equipped with a DeanꢀStark
apparatus, 4-hydroxy-N-phenyl-1,8-naphthalimide 4b (400 mg,
1.38 mmol), propargyl alcohol derivate 5 (438 mg, 1.84 mmol)
and aluminum oxide (4.00 g) were mixed in toluene (20 mL). The
reaction mixture was refluxed under argon for 7 h. After cooling, the
resulting mixture was filtered, and the filter cake was washed with
dichloromethane (50 mL ꢁ 3). Then the filtrate and the scrubbing
solution were combined. After concentration, the compound was
purified by column chromatography on silica (petroleum ether:
dichloromethane = 1:2, v/v) to give crude product, then recrys-
tallized from ethanol to afford 140 mg of compound Nip2 as a pale
yellow solid in 20.0% yield. 1H-NMR (400 MHz, CDCl3, ppm): δ
8.65 (d, J = 8.0 Hz, 1H, naphthaleneꢀH), 8.58 (d, J = 7.2 Hz, 1H,
1
1H-NMR, 13C-NMR, and HRMS. The character H-NMR
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Figure 1. Absorption spectral changes upon UV irradiation at 365 nm in acetonitrile (5.0 ꢁ 10ꢀ5 M) at 293 K: (A) Nip1, (B) Nip2, (C) NP, and (D)
photographic images of Nip1, Nip2, and NP in acetonitrile before and after UV irradiation.
Table 1. Absorption Data of Nip1, Nip2, and NP before and
resonance for the vinyl proton adjacent to the carbon bearing the
heteroatom (O) can be found as doublets as 6.27 ppm for Nip1
and Nip2 and 6.49 ppm for NP with a characteristic coupling
constant (J) of 10.0 Hz. Here the specific proton resonance of
Nip1 and Nip2 shifts upfield by 0.22 ppm. In 13C-NMR, the sp3-
hybridized carbon is also characteristic, appearing at 84.98, 85.13,
and 87.44 ppm for Nip1, Nip2, and NP, respectively (see the
Experimental Section and Supporting Information for details).
Photochromic Properties. The evaluation of photochromic
behaviors for Nip1, Nip2, and NP involves the knowledge of
some relevant parameters, related to the spectra of closed and
open forms, kinetic thermal back rate, and fatigue resistance. All
photochromic properties of Nip1 and Nip2 induced by photo-
irradiation were measured in acetonitrile solution under flash
photolysis at 293 K. As shown in Figure 1, the absorption spectra
of Nip1 and Nip2 do not show substantial differences. Before UV
light irradiation, two absorption peaks are observed at 280 and
385 nm. Compared with the conventional naphthopyran NP, the
absorption band at 385 nm for Nip1 and Nip2 results from the
incorporated naphthalimide moiety (Figure 1).11 Upon irradia-
tion with UV light (365 nm), a new visible absorption band
centered at 530 nm becomes emerged, while the original peak at
280 nm is decreased to some extent, indicative of the formation
of the open MC form (Scheme 2). It is resulted from the
photochemical cleavage of the sp3 CꢀO bond that leads to the
planarization of the two originally orthogonal cycles, giving rise
to an increase in the extent of π conjugation in the MC structure.
Consequently, the colorless solution turns purple, resulting from
the open form (Figure 1D). Under the same condition, the
reference compound NP shows similar photochromic properties.
What is different, under 365 nm UV light irradiation, is that the
new visible absorption band is centered at 480 nm (Figure 1C),
after UV Irradiation in Acetonitrile (5.0 ꢁ 10ꢀ5 M)
compounds
SP form (λmax, nm)
MC form (λmax, nm)
Nip1
Nip2
NP
280, 385
280, 385
320
280, 425, 530
280, 425, 530
410, 480
which is blue-shifted by 50 nm with respect to Nip1 and Nip2.
This can be attributed to the naphthalimide unit incorporated at
the naphthopyran fragment, thus giving a strong electron-with-
drawing effect to favor the red-shifted absorption in MC form.
The optical properties of Nip1, Nip2, and NP before and after
irradiation were summarized in Table 1.
Notably, compared with NP, the red-shifted characteristics in
the absorption band of Nip1 and Nip2 are quite suitable for the
application in ophthalmic lenses.13a As illustrated in Figure 1,
their absorption spectra in the colored form almost cover the
whole visible spectrum. Moreover, upon 365 nm light irradiation,
the absorption spectra between 300 and 400 nm are also
increased. It is known that the wave band of UV light can cause
skin aging and graveness damaging. Hence, the generic nature of
Nip1 and Nip2 is beneficial to the application in eye-protective
glasses since the harmful UV light can be effectively absorbed.
Thermochromism Properties. It is known that the photo-
coloration of 2H-naphtho[1,2-b]pyran ([2]H-NP) under UV
light irradiation can induce the formation of two open forms TT
and TC: while TC isomer rapidly returns to the uncolored closed
form, the TT isomer is thermally more stable (Scheme 1).14
The colored form of Nip1 and Nip2 can be thermally back to
their closed form (CF) when the photoirradiation is ceased
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Figure 2. The biexponential (second order) decay model and experimental thermal fading kinetics in acetonitrile (5.0 ꢁ 10ꢀ5 M) at 293 K: (A) Nip1,
(B) Nip2, (C) NP (black line: data measured; cyan line: the second order decay model fitted; red line: the absorbance residuals at testing time). Inset: the
second order decay model parameters. The monitored wavelength was at 530, 530, and 480 nm for Nip1, Nip2 and NP, respectively. (D) Reverse
reaction of reference compound NP (5.0 ꢁ 10ꢀ5 M) monitored at 480 nm in acetonitrile at 293 K.
Table 2. Spectrokinetic Data of Thermal Bleaching for Nip1, Nip2, and NP (5.0 ꢁ 10ꢀ5 M in Acetonitrile) at 293 Ka
compounds
λmax (nm)
τ1/2 (s)
τ3/4 (s)
k1 (10ꢀ3 sꢀ1
)
k2 (10ꢀ3 sꢀ1
)
A0
A1
A2
Ath
Nip1
Nip2
NP
530
530
480
620
298
760
1200
540
1.11
2.48
3.25
1.14
0.30
0.757
0.626
0.917
0.762
0.707
0.549
0.032
0.080
0.004
0.032
0.026
0.408
0.0035
a k1, k2, A1, and A2 are the fitting parameters from eq 1; λmax, τ1/2, τ3/4, A0, and Ath were obtained from experimental data.
(Figure 2A,B). Two steps proceed in the color-faded course,
that is, from TC to CF form, and from TT to TC form, then
thermal back to colorless CF form. Notably, the process of TC to
CF is usually rapid; the change from TT to CF is quite slow. The
convenient measurement of decoloration rate can be described as
τ1/2 and τ3/4 values, which are the time that it takes from the
absorbance to reduce by 1/2 and 3/4 of the initial absorbance in
the colored form, respectively.15 The bleaching kinetics, deter-
mined from the absorption-time data sets, can be fitted well to the
biexponential decay (eq 1).4c
respectively; and Ath is the residual coloration (offset) at the
termination of testing time. As depicted in Figure S1 in the
Supporting Information, it was found that the fading times τ1/2
and τ3/4 for Nip2 are 298 and 540 s, respectively, which are quite
smaller than that of Nip1 (τ1/2 = 620 s, τ3/4 = 1200 s). From eq 1,
the fading rate constants (Figure 2B) can be obtained for Nip2
(k1 = 2.48 ꢁ 10ꢀ3 sꢀ1, k2 = 3.0 ꢁ 10ꢀ4 sꢀ1), while the corres-
ponding data for Nip1 (Figure 2A) is 1.11 ꢁ 10ꢀ3 and 1.14 ꢁ
10ꢀ3 sꢀ1, respectively. Obviously, the fast rate constants (k1) of
the spontaneous thermal bleaching course for Nip1 and Nip2
(Table 2) are 1.11 ꢁ 10ꢀ3 and 2.48 ꢁ 10ꢀ3 sꢀ1, respectively, indi-
cating that the N-substituted imide group at naphthalimide unit
has an effect on the thermal bleaching rate to some extent. As
illustrated in Figure 3, the purple appearance of Nip2 in acetoni-
trile was gradually bleached in about 20 min, while Nip1 still
maintained its purple color. It is found that the thermal bleaching
AðτÞ ¼ A1eꢀk τ þ A2eꢀk τ þ Ath
ð1Þ
1
2
where A(τ) is the absorbance at λmax of Nip1 and Nip2; A1 and A2
are the contributions to the initial absorbance A0; k1 and k2 are
exponential decay rate constants of fast and slow components,
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Figure 3. Photographic images of Nip1, Nip2, and NP in acetonitrile (5.0 ꢁ 10ꢀ5 M) before and after UV irradiation, and their thermal bleaching in
dark at 293 K.
rate of Nip2 in solution under dark is faster than that of Nip1,
which can be attributed to the different N-substituted imide groups
in naphthalimide unit. Also the spectrokinetic data determined for
the photochromic naphthopyrans are summarized in Table 2.
Notably, after thermal back reaction, the MC forms of Nip1 and
Nip2 are almost transformed to the colorless closed forms, and
even the color of their solution was not detectable by the naked eye.
As shown in Figure 2D, NP reached a photostationary state
(PSS) upon UV light irradiation. When the UV irradiation was
ceased, the colored form of NP commenced bleaching, and after
about 1000 s, the optical density reduced to half of its original state
value. Although the thermal back reaction time was maintained for
an additional 4000 s, the optical density value still maintained
stability. That is, the residual visible coloration cannot be sponta-
neously faded to the absolute colorless. However, upon 546 nm
light irradiation, the residual color can be efficiently faded to its
original colorless state. In order to fully understand the photo-
chromic difference, the thermal fading reaction of NP was also
investigated. As depicted in Figure 2C, the rate constants (k) of the
spontaneous thermal bleaching course are 3.25 ꢁ 10ꢀ3 and 3.54 ꢁ
successively undergo two courses through (i) thermal back reaction
and (ii) visible light irradiation.
However, despite a larger decay rate constant in the case of
reference compound NP (Table 2), its red appearance can still be
maintained for another 1 h with no more color fading. This is
because the residual color of NP is only faded to its original
colorless state by visible light irradiation. As mentioned above,
two steps proceed in the color-faded course, that is, from TC to
CF form (usually rapid), and from TT to TC form, then thermal
back to CF form (usually slow). This means that, for NP, the slow
transformation process from TT to TC form might be the
predominant dynamic step in the thermal back process. There-
fore, we consider that, in the system of Nip1 and Nip2, the
existence of the naphthalimide unit can somehow eliminate the
ratio of TT form or sharply increase the transformation rate from
TT to TC, thus facilitating the thermal back absolutely to CF
form (Figure 2A,B). By contrast, the reference compound NP
cannot bleach absolutely under the same condition (Figure 2C,
D), which can be attributed to the more contribution from stable
TT form under the same conditions, and must be irradiated by
visible light to return to its original CF state. As found in
Figure 2A,B, the residual absorption of Nip1 and Nip2 is very
low, almost reaching zero. So we did not take the irradiation with
10ꢀ6 ꢀ1, fitted with biexponential decay performed based on
s
LevenbergꢀMarquardt (LM) iteration. Accordingly, as depicted in
Figure 2D, the reverse reaction of reference compound NP can
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Table 3. The Optimized Geometries, Populations, and Re-
lative Energies ΔE for TC and TT Isomers of Nip1, Nip2, and
NP
a The relative energy of TC isomer with respect to TT isomer.
visible light to the very low residual absorption of Nip1 and Nip2.
When ceasing UV light irradiation, we can use visible light to
bleach the color form of Nip1 and Nip2.
Figure 4. Time-dependent photocoloration of Nip1 (A) and Nip2 (B)
in acetonitrile (2.19 ꢁ 10ꢀ4 M) at 293 K by UV irradiation at 365 nm,
and the subsequent thermal fading when the irradiation was ceased. The
monitored wavelength was at 530 nm.
Computational Study of Nip1, Nip2, and NP. To gain
insight into the electron-withdrawing effect of the imide group
and different N-substitution incorporated at the naphthalimide
moiety, we carried out quantum chemical calculations on Nip1,
Nip2, and the reference compound NP with the Gaussian 09
program.16 The geometries were optimized with the hybrid B3LYP
exchange-correlation functional and 6-31G(d,p) basis set. On the
basis of the optimized structures, more accurate energies were
achieved by single-point calculations with the larger basis set 6-311
+G(d,p). Frequency calculations were performed at the B3LYP/6-
31G(d,p) level of theory to obtain zero-point energies (ZPEs) and
to confirm the nature of the stationary points. The solvent effect
was taken into account within the polarizable continuum model
with acetonitrile as solvent.
and is much faster than that of the reference compound NP, an
observation in exact consistence with the observed experiment
results in the thermal bleaching decay. Obviously, in Nip1 and
Nip2 the strong electron-withdrawing effect of the imide group in-
corporated at naphthalimide moiety are preferable to an enhance-
ment in the ratio of TC isomer and an increase in the transforma-
tion rate from TT to TC with respect to reference compound NP,
thus resulting in a preferable color bleaching rate and fading abso-
lutely to their colorless state, behaving as classical thermo-reversible
photochromic photochromes. In addition, another reason account-
ing for the slower bleaching rate of Nip1 than Nip2 might be the
energy loss via the thermal vibration of the long linear alkyl chain on
N-substitution incorporated at the naphthalimide moiety in Nip1.
Photocoloration/Thermal Bleaching Cycles. The photoco-
loration investigation revealed that both Nip1 and Nip2 can
undergo coloration in several seconds with UV light irradiation.
As depicted in Figure 4, it takes only about 40 s for the optical
density of Nip2 to reach half of the value at its PSS, and about 100 s
to PSS, these values are smaller than those of the reference com-
pound NP, which need almost 400 s to reach PSS under the same
measurement conditions. Obviously, the developed naphthopyrans
Nip1 and Nip2 possess highly photoresponse, an important pre-
condition to application in ophthalmic plastic lenses. In addition,
fatigue resistance is another important factor to evaluate material
applications. A preliminary investigation of fatigue resistance for
The calculation results are summarized in Table 3. As calcu-
lated for both Nip1 and Nip2, the TT isomer is more stable than
the TC isomer. The TT isomer is calculated to be more favorable
than the TC isomer by 0.36 and 0.19 kcal molꢀ1 for Nip1 and
Nip2, respectively, while for NP, this value is 0.98 kcal molꢀ1
.
The populations for the TC and TT isomers are deduced from
the relative energies according to the Boltzmann distribution.
The populations of TC isomers are 35% and 42% for Nip1 and
Nip2, respectively. That is, in the MC forms, the portions of TC
isomer for Nip1 and Nip2 are almost 2.18ꢀ2.63 times that for
the reference compound NP, which just takes 16% potions.
Apparently, owing to the larger population of thermally unstable
TC isomer and smaller energy difference between TC and TT
isomers, the thermal bleaching rate of Nip2 is faster than Nip1,
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Figure 6. Fluorescence changes in acetonitrile (5.0 ꢁ 10ꢀ5 M) upon
365 nm irradiation at 293 K: (A) Nip1, (B) Nip2. The obtained
fluorescence was excited at 390 nm.
Figure 5. (A) Absorption changes of Nip1 in acetonitrile (5.0 ꢁ 10ꢀ5
M) at PSS upon photoirradiation power. (B) The absorbance at PSS of
Nip1 increases as a function of photoirradiation power.
naphthopyran derivatives Nip1 and Nip2 was performed, appar-
ently revealing a quite good reproducibility (Figure 4). However,
after each decoloration process, a residual visible absorbance
remained, even though the color of solution was not detectable
by the naked eye. By contrast, the optical density at PSS after suc-
cessive coloration and decolorization cycles was not altered. Con-
sequently, the low residual absorption might not result from the
photodegradation. In other investigated naphthopyrans, the resi-
dual absorption resulted from the TT isomer of the open form, and
could disappear by irradiation with visible light. This is explained by
assuming that the TT isomeric photoproduct is thermally stable
but photoreversible, and needs to be under visible light to revert to
the original colorless form.13,17
Irradiation-Dependent Optical Density. Furthermore, the
optical density at PSS is closely related to the irradiation power.
As illustrated in Figure 5, the optical density of Nip1 at PSS was
quite small when the input power was 2.96 mW. Under this con-
dition, sufficient color change cannot be detected. However, with
more powerful incident light, the optical density increased sharply,
and the value tended to be a real stationary state until about 50 mW,
then the color change of Nip1 in acetonitrile before and after
photoirradiation was very distinct. As known, naphthopyrans are
T-type photochromic dyes, which exihibit a considerable competi-
tion between forward and backward reaction. When ceasing UV
light irradiation at ambient temperature, the thermal back reaction
is rather fast when the photoirradiation power is not intense enough,
Figure 7. Fluorescence enhancement of Nip1 and Nip2 during dark
decoloration at 293 K in acetonitrile (5.0 ꢁ 10ꢀ5 M). The monitored
wavelength was at 480 nm.
and the reverse reaction has a tendency toward backward thermal
bleaching. When the irradiation lamp is powerful enough so that the
equilibrium constant of the forward reaction is larger than that of the
backward reaction, the total reaction is inclined to be coloration.
As a consequence, the more powerful the incident light, the more
intense the absorbance, and the more distinct the color change that
can be detected. Bearing this feature in mind, both Nip1 and Nip2
are suitable for industrial application in smart windows. When
sunlight is intense at noon, the intense ultraviolet radiation of sun-
light can be effectively absorbed, thus realizing the modulation
of the indoor beam and temperature according to the intensity of
sunlight.
Fluorescence Switch. It is known that 1,8-naphthalimides are
highly fluorescent moieties with good chemical stability.11 Inter-
estingly, the fluorescence of the naphthalimide moiety in Nip1
and Nip2 can be switched on and off by photoinduced conver-
sion between the closed and open forms. That is, the closed form
of Nip1 and Nip2 shows the characteristic fluorescence from
the naphthalimide moiety at 480 nm, which diminishes quickly
under the UV light irradiation. During the dark decoloration, the
fluorescence of the solution can be recovered. As depicted in Figure 6,
upon irradiation with a UV light of 365 nm, the fluorescence intensity
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of Nip1 and Nip2 reduced quickly in a few seconds. Upon irradiation
for 5 min, the fluorescence reached a stable state, and the intensity
(λex = 390 nm, λem= 480 nm) reduced by 68% and 62% for Nip1
and Nip2, respectively. Here the fluorescence quenching of the
naphthalimide moiety in the open form of Nip1 and Nip2 may
result from the electronic delocalization throughout the MC form
(Scheme 2). Again, the fluorescence intensity can be increased at
dark when ceasing the UV light irradiation (Figure 7). The process
of fluorescence on and off can be repeated between the closed and
open forms, suggestive of potential application in recording media
and other optic apparatuses.
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’ CONCLUSIONS
Two new photochromic naphthopyrans Nip1 and Nip2 bear-
ing naphthalimide units have been synthesized and investigated.
Both two compounds can reversibly change color under UV light
and the removal of irradiation. The incorporation of a naphtha-
limide unit into the classical naphthopyran structure enables
longer wavelength absorption bands. Remarkably, the electron-
withdrawing imide group of the naphthalimide unit incorporated
at naphthopyrans results in the open MC forms, exhibiting a sig-
nificantly fast thermal bleaching rate. Because of the larger popula-
tion of thermally unstable TC isomers and the low energy dif-
ference between TC and TT isomers, the thermal bleaching rate of
Nip2 bearing phenyl on the naphthalimide unit is faster than Nip1
bearing n-butyl, much faster than that of reference compound NP.
The thermal fading reversibility of Nip1 and Nip2 is substantially
complete, while the reference compound NP exhibits thermal and
photochemical reversibility in color-fading. As demonstrated in the
system of NP, the slow transformation process from TT to TC
might be the predominant dynamic step in the thermal back pro-
cess, leading to the residual color of NP being only faded to its
original colorless state by visible light irradiation. The fluorescence
of Nip1 and Nip2 can be switched on and off by photoinduced
conversion between the closed and open forms. Moreover, their
optical densities of colored forms are dependent upon the intensity
of photoinduced sources, ensuring a possible application in the
manufacture of ophthalmic lenses and smart windows.
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’ ASSOCIATED CONTENT
(8) (a) Shilova, E. A.; Pꢀepe, G.; Samat, A.; Moustrou, C. Tetrahedron
2008, 64, 9977–9982. (b) Sallenave, X.; Delbaere, S.; Vermeersch, G.;
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Supporting Information. 1H-NMR, 13C-NMR, and
S
b
HRMS spectra of Nip1, Nip2, NP, and other intermediates,
thermal back reaction of Nip1 and Nip2 at dark after 365 nm
UV light irradiation, and reverse reaction of NP upon 546 nm
light irradiation. This information is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
(10) Oliveira, M. M.; Salvador, M. A.; Delbaere, S.; Berthet, J.;
Vermeersch, G.; Micheau, J. C.; Coelho, P. J.; Carvalho, L. M.
J. Photochem. Photobiol., A: Chem. 2008, 198, 242–249.
Corresponding Author
*Fax: (+86) 21-6425-2758. E-mail: whzhu@ecust.edu.cn.
(11) (a) Gan, J. A.; Tian, H.; Wang, Z. H.; Chen, K. C.; Hill, J.; Lane,
P. A.; Rahn, M. D.; Fox, A. M.; Bradley, D. D. C. J. Organomet. Chem.
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Y.; Wei, S.; Yin, C.; Liu, L. S.; Hu, C. M.; Zhao, Y. Y.; Ye, Y. X.; Hu, X. Y.;
Fan, J. Bioorg. Med. Chem. Lett. 2011, 21, 3798–3804. (d) Georgiev, N. I.;
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V. B.; Georgiev, N. I.; Marinova, N. V. Sens. Actuators, B: Chem. 2010,
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’ ACKNOWLEDGMENT
This work was financially supported by NSFC/China, the
Oriental Scholarship, SRFDP 200802510011, and the Fundamen-
tal Research Funds for the Central Universities (WK1013002).
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