Synthesis and photochromism of a spirooxazine derivative featuring a carbazole moiety: Fast thermal bleaching and excellent fatigue resistance

Article abstract of DOI:10.1016/j.dyepig.2014.04.004

A novel photochromic spirooxazine derivative bearing a carbazole moiety (SOC) was synthesized and studied in solution under flash photolysis conditions. It is found to exhibit excellent characteristics like high photochromic response, large steady-state optical density, fast thermal bleaching rate and good fatigue-resistance. The effect of different solvents on the photochromic properties of the compound was evaluated, revealing that the photochromic properties can be modulated by different solvents based on the corresponding polarity. The mechanism and kinetics of the thermal fading process of compound SOC were additionally investigated by theoretical simulations, where the isomerization pathway from the trans-trans-cis conformation was found to be several times faster than that from the cis-trans-cis conformation. This type of fast-bleaching and fatigue-resistent photochromic compounds is expected to pave an exciting avenue in future development of high-performance photochromic materials.

Full text of DOI:10.1016/j.dyepig.2014.04.004

Dyes and Pigments 107 (2014) 174e181
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and photochromism of a spirooxazine derivative featuring
a carbazole moiety: Fast thermal bleaching and excellent fatigue
resistance
,
b
,
c
,
b
a
c
Qi Zou a , Xin Li
, Ji Zhou , Kangkang Bai , Hans Ågren
*
**
a College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China
b Key Laboratory for Advanced Materials, East China University of Science and Technology, Shanghai 200237, PR China
c Division of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute of Technology, SE-10691 Stockholm, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel photochromic spirooxazine derivative bearing a carbazole moiety (SOC) was synthesized and
studied in solution under flash photolysis conditions. It is found to exhibit excellent characteristics like
high photochromic response, large steady-state optical density, fast thermal bleaching rate and good
fatigue-resistance. The effect of different solvents on the photochromic properties of the compound was
evaluated, revealing that the photochromic properties can be modulated by different solvents based on
the corresponding polarity. The mechanism and kinetics of the thermal fading process of compound SOC
were additionally investigated by theoretical simulations, where the isomerization pathway from the
trans-trans-cis conformation was found to be several times faster than that from the cis-trans-cis
conformation. This type of fast-bleaching and fatigue-resistent photochromic compounds is expected to
pave an exciting avenue in future development of high-performance photochromic materials.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 14 February 2014
Received in revised form
29 March 2014
Accepted 2 April 2014
Available online 13 April 2014
Keywords:
Photochromism
Spirooxazine
Thermal fading kinetics
Theoretical simulations
Carbazole
Solvation
1. Introduction
moieties (indoline and naphthooxazine) linked by a tetrahedral
spiro-carbon which prevents the two
p-electron systems from
Considerable effort has been made to develop organic photo-
chromic materials that change their chemical and physical properties
upon irradiation with certain wavelengths of light and containing
photo-generated species that can be reversed to the initial species
either thermally or by irradiation with a different wavelength of light
[1]. In particular, thermally reversible photochromic compounds
facilitate setting and resetting physicochemical properties of the
compounds by simply turning a light source on and off [2].
Among thermally reversible photochromic compounds, spi-
roindolinonaphthooxazine (SO) derivatives are most promising
candidates for applications as multifunctional optoelectronic ma-
terials, such as smart windows, ophthalmic lenses, optical data
processing and optical switching, as well as displays [3]. This owes to
their good coloring power and remarkable fatigue resistance. These
compounds are composed of two heterocyclic nearly planar
conjugation. Such structural characteristics result in colorless or
pale yellow solutions of the spiro-compounds since the lowest
electronic transition of the molecule occurs in the near ultraviolet
(UV) region [4]. As illustrated in Scheme 1, the photo-cleavage of the
CeO spiro-bond takes place upon exposure to UV light and subse-
quent rotation around the CeC bond to give rise to an open mer-
ocyanine (MC) structure that absorbs in the visible light region along
with the intensively colored solution [5]. The MC isomer is denoted
here by its trans-trans-cis (TTC) conformation [1a]. The solution
returns thermally back to the original state when the UV light irra-
diation ceases.
For commercial application of multifunctional optoelectronic
materials, it is essential to maintain photochromic molecules with
highly efficient photoresponse, large steady-state optical density
and fast-bleaching speed at ambient temperature, as well as
fatigue-resistance [6]. Herein, we report a convenient synthesis of a
novel spirooxazine derivative SOC (Scheme 2) featuring a carbazole
moiety that is usually applied in effective electron-donor and hole-
transporting materials [7], for the sake of expanding the photo-
chromic properties and possibly the commercial applications of
* Corresponding author. Tel./fax: þ86 21 35303544.
** Corresponding author. Tel.: þ46 8 55378417; fax: þ46 8 55378590.
E-mail addresses: qzou@shiep.edu.cn (Q. Zou), lixin@theochem.kth.se (X. Li).
http://dx.doi.org/10.1016/j.dyepig.2014.04.004
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

Q. Zou et al. / Dyes and Pigments 107 (2014) 174e181
175
internal standard. Mass spectra (MS) were recorded on a Waters
LCT Premier XE spectrometer with methanol as solvent. Photo-
chromic reaction was induced in situ by a continuous wavelength
irradiation Hg/Xe lamp (Hamamatsu, LC6 Lightingcure, 200W)
equipped with narrow band interference filters of appropriate
wavelengths (Semrock Hg01 for lirr ¼ 365 nm, Semrock BrightLine
FF01-575/25-25 for lirr ¼ 575 nm). UVevis absorption changes
were monitored by a charge-coupled device (CCD) camera moun-
ted on a spectrometer.
Scheme 1. The structural interchange between the SO and MC isomers.
2.2. Kinetic measurements in solution
spirooxazine derivatives. Several merits with good photochromic
properties turn out inherent in compound SOC: (i) its intensively
colored form (MC isomer) possesses relatively large optical density;
(ii) it can interchange rapidly between the SO and MC forms when
luminous UV irradiation is switched on and off; (iii) it tolerates
hundred switching cycles without decomposing, indicative of an
excellent fatigue resistance; (iv) its photochromic properties
regularly change with respect to the polarity of different solvents,
suggesting that the photochromic properties can be modulated by
solvent interaction. These merits are highly desired for use in op-
toelectronic materials, for instance, ophthalmic lenses and smart
windows, and provide insight for the future design of functional
photochromic materials.
A solution of compound SOC in the appropriate solvent, with a
concentration of 6.0 ꢀ 10ꢁ5 M was measured in a well-stirred
quartz cell (light-path length: 10 mm). The thermal decay at lmax
was studied for all of the samples until 3 s after photoirradiation.
The thermal decay curves were fitted with exponential functions of
Origin 8.0 software to obtain the pre-exponential constants (Ath, A1,
and A2) and kinetic constants (k1 and k2) of the biexponential ki-
netic equation to study the photochromic kinetics of compound
SOC.
2.3. Computational methods
The thermal fading (ring-closure) process of the spirooxazine
derivative (compound SOC) was investigated by first-principles
calculations. The geometries of the spirooxazine isomers and
transition states along reaction coordinates were optimized by
density functional theory (DFT) calculations, using the M06 func-
tional [8] and the 6-31G** basis set [9] as implemented in the
Gaussian 09 program package [10]. Solvent effects of tetrahydro-
furan (THF) were taken into account by the polarizable continuum
model (PCM) [11] in all calculations. At optimized geometries, fre-
quency analyses were performed and thermodynamic data were
computed at 290 K.
2. Experimental section
2.1. Materials and instrumentations
2,7-Dihydroxynaphthalene and 3,3-dimethyl-2-methylene-1-
phenylindoline were purchased from SigmaeAldrich and used
without further purification. Other starting materials 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 packages purchased from Qing-dao Haiyang Chemical
Company, China.
2.4. Synthesis
1H NMR and 13C NMR spectra in CDCl3 were recorded on Brucker
AM-400 spectrometers with tetramethylsilane (TMS) as the
The synthesis route of compound SOC is shown in Scheme 2.
Scheme 2. Synthetic routine of photochromic compound SOC.

176
Q. Zou et al. / Dyes and Pigments 107 (2014) 174e181
2.4.1. Synthesis of 7-bromonaphthalen-2-ol (2)
2.4.4. Synthesis of 7-(9H-carbazol-9-yl)naphthalen-2-ol (5)
9-(7-methoxynaphthalen-2-yl)-9H-carbazole (0.356 g,
A synthetic procedure was based on the literature method
[12]. To a vigorously stirred mixture of triphenylphosphine
(31.5 g, 120 mmol) in acetonitrile (50.0 mL), bromine (19.2 g,
120 mmol) was added dropwise at 0 ꢂC. The reaction mixture was
allowed to reach room temperature, and 2,7-dihydroxynaph
thalene 1 (16.0 g, 100 mmol) was added in one portion. The
mixture was heated to 70 ꢂC for 30 min, after which the solvent
was removed by rotary evaporation. The flask was equipped with
a gas trap, and the black residue was heated to 250 ꢂC for 1 h.
After cooling to room temperature, the mixture was dissolved in
200 mL of dichloromethane and the viscous liquid was obtained
after column chromatography (silica gel, petroleum ether/
dichloromethane 1:1). The crude product was purified by column
chromatography (silica gel, petroleum ether/dichloromethane
3:2) to give the compound 2 (18.8 g, 84.3 mmol, 84%) as a beige
4
1.10 mmol) was dissolved in anhydrous dichloromethane (20.0 mL)
and boron tribromide (4.00 mL of 1 M solution in dichloromethane)
was added dropwise under argon for 20 min at 0 ꢂC using a syringe.
The solution was stirred for 4 h at 0 ꢂC, before it was poured into icy
water (20.0 mL). The mixture was extracted with dichloromethane.
The organic layer was dried over MgSO4, filtrated, and concen-
trated. The residue was purified by column chromatography (silica
gel, petroleum ether/dichloromethane 1:1) to give the compound 5
(0.174 g, 51%) as a white solid. 1H NMR (400 MHz, CDCl3)
d (ppm):
5.31 (s, 1H, eOH), 7.20 (d, 2H, J 7.6 Hz, carbazole-H), 7.30 (t, 2H, J
7.4 Hz, carbazole-H), 7.41 (t, 2H, J 7.6 Hz, carbazole-H), 7.47 (d, 2H, J
8.0 Hz, naphthalene-H), 7.51 (d, 1H, J 8.4 Hz, naphthalene-H), 7.88
(d, 2H, J 6.4 Hz, naphthalene-H), 7.98 (d, 1H, J 8.4 Hz, naphthalene-
H), 8.17 (d, 2H, J 8.0 Hz, carbazole-H). 13C NMR (100 MHz, CDCl3)
powder. 1H NMR (400 MHz, CDCl3)
d
(ppm): 5.09 (s, 1H, eOH),
d (ppm): 109.5, 109.9, 118.3, 120.0, 120.4, 123.0, 123.5, 123.8, 126.0,
7.06 (d, 1H, J 2.4 Hz, naphthalene-H), 7.10 (dd, 1H, J 8.8, 2.8 Hz,
naphthalene-H), 7.24 (dd, 1H, J 8.8, 2.0 Hz, naphthalene-H), 7.63
127.9, 129.7, 130.0, 135.3, 135.9, 141.0, 154.2. HR-ESIMS m/z:
[M þ H]þ calcd. for C22H16NO, 310.1232; found, 310.1234.
(d, 1H,
J 8.8 Hz, naphthalene-H), 7.72 (d, 1H, J 8.8 Hz,
naphthalene-H), 7.84 (d, 1H, J 1.2 Hz, naphthalene-H). 13C NMR
2.4.5. Synthesis of 7-(9H-carbazol-9-yl)-1-nitrosonaphthalen-2-ol
(6)
(100 MHz, CDCl3)
d (ppm): 108.7, 118.1, 120.8, 127.0, 127.3, 128.3,
129.4, 129.9, 135.7, 154.0. HR-ESI-MS m/z: [MꢁH]ꢁ calcd. for
7-(9H-carbazol-9-yl)naphthalen-2-ol 5 (0.174 g, 0.560 mmol)
was dissolved in the solution of acetic acid (10.0 mL) and water
(3.00 mL). Then the solution of sodium nitrite (0.039 g, 0.780 mmol)
inwater (1.00 mL) was added dropwise over 15 min at 0e5 ꢂC using a
syringe. The mixture was stirred for 2 h at 0 ꢂC, before the reaction
mixture was filtered. The residue was washed with dilute acetic acid
in water (5.00 mL), then washed with deionized water (50.0 mL) to
give the compound 6 (0.084 g, 44%) as a red solid, which was used
directly in the synthesis of compound SOC.
C
10H6BrO, 220.9602; found, 220.9601.
2.4.2. Synthesis of 7-bromo-2-methoxynaphthalene (3)
A procedure slightly modified from the one described in
Ref. [13] was followed. To a mixture of 7-bromonaphthalen-2-ol 2
(3.00 g, 13.0 mmol) and potassium carbonate (5.38 g, 39.0 mmol) in
acetone (80.0 mL), iodomethane (9.00 g, 65.0 mmol) was added
under stirring. The reaction mixture was refluxed under argon for
10 h. After cooling to room temperature, the mixture was filtered,
and the filtrate was concentrated under vacuum. The crude product
was purified by column chromatography (silica gel, petroleum
ether) to give the compound 3 (2.87 g, 12.1 mmol, 93%) as a white
2.4.6. Synthesis of 1-phenyl-3,3-dimethyl-90-(9H-carbazol-9-yl)-
spiroindolinenaphthoxadine (SOC)
7-(9H-carbazol-9-yl)-1-nitrosonaphthalen-2-ol
6
(1.40 g,
solid. 1H NMR (400 MHz, CDCl3)
d
(ppm): 3.92 (s, 3H, eOCH3), 7.03
4.14 mmol) was gently refluxed in absolute ethanol (50.0 mL) and
to the hot solution was added dropwise over 20 min a solution of
3,3-dimethyl-2-methylene-1-phenylindoline (1.00 g, 4.14 mmol) in
absolute ethanol (20.0 mL) with triethylamine (2.09 g, 20.7 mmol).
The mixture was then refluxed for 24 h. After cooling to room
temperature, the mixture was concentrated under vacuum. The
crude product was purified by column chromatography (silica gel,
petroleum ether/dichloromethane 4:1) to give the compound SOC
(0.736 g, 32%) as a yellow solid. IR (KBr) nmax (cmꢁ1): 749, 984, 1019,
1081, 1092, 1384, 1400, 1447, 1477, 1498, 1594, 1619, 2962, 3145,
(d, 1H, J 2.4 Hz, naphthalene-H), 7.15 (dd, 1H, J 8.8, 2.8 Hz,
naphthalene-H), 7.40 (dd, 1H, J 8.8, 2.0 Hz, naphthalene-H), 7.63 (d,
1H, J 8.8 Hz, naphthalene-H), 7.70 (d, 1H, J 8.8 Hz, naphthalene-H),
7.90 (d, 1H, J 1.6 Hz, naphthalene-H). 13C NMR (100 MHz, CDCl3)
d
(ppm): 55.8, 104.6, 119.1, 120.4, 127.5, 129.0, 129.6, 130.2, 132.6,
158.3. HR-ESI-MS m/z: [M þ H]þ calcd. for C11H10BrO, 236.9915;
found, 236.9912.
2.4.3. Synthesis of 9-(7-methoxynaphthalen-2-yl)-9H-carbazole
(4)
3422. 1H NMR (400 MHz, CDCl3)
d (ppm): 1.44 (d, 6H, J 6.0 Hz, e
A
mixture of 7-bromo-2-methoxynaphthalene
3
(1.00 g,
CH3), 6.73 (d, 1H, J 8.0 Hz, benzene-H), 6.95 (t, 1H, J 7.4 Hz, N-
benzene-H), 7.11e7.19 (m, 4H, benzene-H and carbazole-H), 7.28e
7.35 (m, 6H, N-benzene-H and carbazole-H), 7.42 (t, 2H, J 7.6 Hz,
carbazole-H), 7.50 (d, 2H, J 8.0 Hz, naphthalene-H), 7.55 (dd, 1H, J
8.4, 2.0 Hz, naphthalene-H), 7.62 (s, 1H, eN]CH), 7.76 (d, 1H, J
8.8 Hz, benzene-H), 7.93 (d, 1H, J 8.4 Hz, naphthalene-H), 8.16 (d,
4.20 mmol), carbazole (2.10 g, 13.0 mmol), copper powder
(0.270 g, 4.20 mmol), and potassium carbonate (3.00 g,
21.0 mmol) in trichlorobenzene (50.0 mL) was refluxed under
argon for 24 h. After cooling to room temperature, the mixture
was filtered, and the filtrate was concentrated under vacuum. The
residue was purified by column chromatography (silica gel, pe-
troleum ether/dichloromethane 4:1) to give the compound 4
(0.360 g, 1.10 mmol, 27%) as a yellow solid. 1H NMR (400 MHz,
2H, J 7.6 Hz, carbazole-H), 8.65 (d, 1H, J 1.6 Hz, naphthalene-H). 13
NMR (100 MHz, CDCl3) (ppm): 19.6, 24.6, 52.1, 97.3, 107.1, 109.0,
C
d
116.4,118.4,118.9,119.2,119.5,121.0,122.2,122.3,122.4,124.6,124.9,
124.5, 126.8, 127.1, 128.3, 128.5, 128.9, 130.7, 134.7, 135.3, 138.2,
140.0, 143.3, 144.9, 150.1. HR-ESI-MS m/z: [M þ H]þ calcd. for
CDCl3)
d (ppm): 3.85 (s, 3H, eOCH3), 7.15 (d, 2H, J 5.2 Hz,
carbazole-H), 7.21 (t, 2H, J 7.2 Hz, carbazole-H), 7.33 (t, 2H, J
5.4 Hz, carbazole-H), 7.38 (d, 2H, J 8.0 Hz, naphthalene-H), 7.42
(dd, 1H, J 8.6, 1.8 Hz, naphthalene-H), 7.83 (d, 1H, J 1.6 Hz,
naphthalene-H), 7.87 (d, 1H, J 8.4 Hz, naphthalene-H), 7.98 (d, 1H,
J 8.0 Hz, naphthalene-H), 8.08 (d, 2H, J 7.6 Hz, carbazole-H). 13C
C39H30N3O, 556.2389; found, 556.2390.
3. Results and discussion
NMR (100 MHz, CDCl3)
d
(ppm): 55.5, 109.5, 110.0, 119.5, 120.4,
3.1. Synthesis and characterization
120.6, 123.6, 123.8, 123.9, 126.2, 128.0, 128.9, 129.1, 134.9, 140.9,
158.4. HR-ESI-MS m/z: [M þ H]þ calcd. for C23H18NO, 324.1388;
found, 324.1384.
The target compound SOC was synthesized in six steps (sub-
stitution, etherification, Ullmann condensation, Zeisel ether

Q. Zou et al. / Dyes and Pigments 107 (2014) 174e181
177
cleavage, nitrosylation, condensation), starting from commercial
and known precursors, as illustrated in Scheme 2. In order to pre-
pare the designed compound SOC, we devised a new protocol to
exploit the key intermediate 6 (1-nitrosonaphthalen-2-ol with a
carbazole appendage at C-7 position), and elaborately synthesized
it in five steps first. The chemical structures of compound SOC and
other key intermediate products were well confirmed by 1H NMR,
13C NMR, and HRMS (see the Experimental section and Supporting
Information for details).
3.2. Photochromic properties
The evaluation of photochromic behavior for compound SOC
involves some relevant parameters, related to the absorption
spectra of the closed and open forms, kinetic thermal bleaching rate
and fatigue resistance, as well as solvent effects. It is well-known
that the photochromic reaction is caused by the reversible spiro-
heterolytic cleavage of the Csp3-O bond under UV irradiation,
yielding the open MC form which can return to the closed SO form
upon irradiation at visible light or in the dark [1a]. Under contin-
uous irradiation with the light of 365 nm, the tetrahydrofuran (THF)
solution of compound SOC very quickly reached the photosta-
tionary equilibrium. The color of the solution changed from light-
yellow to blue, indicative of the formation of the open MC form.
The optical density of compound SOC at the photostationary state
was as high as 0.94, while its intensively colored form was ther-
mally unstable at ambient temperature. The measurement of the
transient absorption spectra was initiated immediately after ceas-
ing the irradiation when a photostationary state was reached under
illumination with 365 nm light at 290 K. As depicted in Fig. 1, the
absorption band centered at 602 nm decreased rapidly after ceasing
the irradiation, and a complete fading was achieved within 10 s,
accompanied by the blue solution turning back to light-yellow,
resulting from the closed SO form. Notably, upon irradiation with
365 nm light, the absorption spectra in the intensively colored form
almost covered the whole visible spectrum with a broad absorption
band between 300 and 400 nm. This generic nature enabled
compound SOC to serve as a type of promising materials for
extensive applications in the field of eye-protective glasses and
smart windows, because both the visible and UV light could be
effectively absorbed [14].
Fig. 2. The biexponential (second order) decay model and experimental thermal
fading kinetics in THF solution (6.0 ꢀ 10ꢁ5 M) at 290 K (black line: data measured; red
line: the second order decay model fitted). Inset: the second order decay model pa-
rameters. The monitored wavelength was at 602 nm.(For interpretation of the refer-
ences to color in this figure legend, the reader is referred to the web version of this
article.)
The photochromic property of compound SOC was also inves-
tigated by transient UVevis absorption spectroscopy in a series of
solvents with varying polarity index, namely toluene, acetonitrile,
and methanol. The UVevis absorption spectral changes of com-
pound SOC in other representative solutions were similar to in THF
solution, along with the rapid fading rates. The maximum wave-
length of the absorption band (lmax) at the photostationary state
and corresponding absorbance (Aabs) are listed in Table S1. Inter-
estingly, slight bathochromic shifts of the absorption band derived
from the MC form were observed with increasing solvent polarity
from 601 nm in the weakly polar toluene to 607 nm in the
moderately polar acetonitrile and to 618 nm in the highly polar
methanol. The bathochromic shift shown in Table S1 at higher
solvent polarity is indicative of a better stabilization of the excited
state of the photomerocyanines relative to the ground state, i.e. of
an increasing dipole moment upon electronic excitation [4].
Therefore, the ground-state weakly polar molecule should
approach the configuration of the quinoid form and the excited
state should have a prevalent zwitterionic character, which is in
accordance with the
the open form [15]. The quantum yields (
p
e
p
* character of the singlet excited state of
) of the SP / MC isomers
F
of compound SOC in various solvents were evaluated by the stan-
dard procedures [16]. It turned out that the quantum yields grad-
ually increased when the polarity of the solvents decreased in the
sequence of methanol (
F
¼ 0.251), acetonitrile (
F
¼ 0.438), THF
(F
¼ 0.677), toluene ( ¼ 0.701). In addition, Table S1 showed that
F
Table 1
Spectrokinetic dataa of thermal fading for compound SOC (6.0 ꢀ 10ꢁ5 M) in different
solvents at 290 K.
Solvent
s1/2 (ms) s3/4 (ms) k1 (sꢁ1
)
k2 (sꢁ1
)
A1
A2
Ath
Toluene
THF
Acetonitrile 363
885
675
1820
1742
786
0.372
2.05
4.76
4.85
0.361
0.60
0.55
4.63
0.366 0.392 0.0121
0.369 0.567 0.0046
0.244 0.555 0.006
0.182 0.212 0.0197
Methanol
185
331
Fig. 1. Transient UVevis absorption changes of compound SOC in THF solution
(6.0 ꢀ 10ꢁ5 M) at 290 K after irradiation with 365 nm light in a well-stirred quartz cell
(light-path length: 10 mm). Each of the spectra was recorded at 650 ms intervals
within a 10 s period.
a
Samples initially irradiated at UV light of 365 nm until in the photostationary
state (3 s). Thermal decoloration was then monitored at the lmax of the intensively
colored form (MC form) at 290 K in the dark.

178
Q. Zou et al. / Dyes and Pigments 107 (2014) 174e181
Fig. 3. Energy barrier for isomerization between TTC and CTC.
the absorbance of compound SOC in acetonitrile solution at the
photostationary state was as large as 1.25, which was about 2.6
times larger than that of methanol solution (0.49) under the same
conditions, indicating that the solvent played a key role in obtain-
ing large photosteady-state optical density of compound SOC.
order to fully understand the impact of the solvent polarity on the
fading thermal kinetics, the fading kinetics of compound SOC was
analyzed for other solvents using the same method - the spec-
trokinetic data were summarized in Table 1. By comparison of the
data listed in Table 1, it was found that the fading speeds of the MC
form in different solvents were dramatically accelerated as the
solvent polarity increased in the sequence of toluene
(s1/
3.3. Thermal fading kinetics
¼ 885 ms, s3/4 ¼ 1820 ms), THF (s1/2 ¼ 675 ms, s3/4 ¼ 1742 ms),
2
acetonitrile (s1/2 ¼ 363 ms, s3/4 ¼ 786 ms), methanol (s1/2 ¼ 185 ms,
s3/4 ¼ 331 ms). The effects of solvent on the kinetic and thermo-
dynamic parameters of spirooxazine ring opening and closing are
generally interpreted in terms of solvation effects. The positive
solvatochromism reveals that the thermal bleaching course can be
tuned by choosing an appropriate solvent.
Kinetic studies are important as they concern the behavior of
photochromic molecules of applicative interest. The thermal fading
kinetics that is determined from the absorption-time data sets was
analyzed using the following biexponential equation [17]:
1
2
Að
s
Þ ¼ A1eꢁk s þ A2eꢁk s þ Ath
(1)
As mentioned above, the fast fading kinetics made it possible to
change the color of the solution restricted to the position where it
was irradiated with UV light, since the diffusion rate of the inten-
sively colored species was slightly slower than the thermal
bleaching rate at ambient temperature. Due to the short half-
lifetime (s1/2 ¼ 185 ms) of compound SOC in methanol solution at
290 K, the blue block of the solution only appeared where 365 nm
light beam irradiated, other part of the solution remained light-
yellow (see Figure S1). Such fast fading kinetics is indispensable
for applications in light modulators and optical data processing.
where A (
s
) is the absorbance at lmax of compound SOC; A1 and A2
are the contributions to the initial absorbance A0; k1 and k2 are
exponential decay rate constants of fast and slow components,
respectively; and Ath is the residual coloration (offset) at the
termination of testing time.
This is the most common model used for the analysis of the
photochromic system and makes is beneficial to compare with
other literature reported values [18]. It was found that the thermal
fading kinetics of compound SOC is well described by the biexpo-
nential function with correlation coefficients >0.99 in all analyses.
Additionally, the convenient measurement of thermal fading rate
3.4. Density functional theory calculations
can be described by the s1/2 and s3/4 values, which are the time that
takes for the absorbance to reduce by 1/2 and 3/4 of the initial
absorbance in the MC form, respectively [6].
From equation (1) and Fig. 2, s1/2 and s3/4 obtained for com-
pound SOC in THF solution were 675 ms and 1742 ms with the
fading rate constants (k1 ¼ 2.05 sꢁ1, k2 ¼ 0.60 sꢁ1), respectively. In
According results presented in the literature [19], the open-ring
isomer of spirooxazine derivatives has a number of conformations,
among which the two most stable ones are termed as TTC and CTC.
Here “T” and “C” denote the trans/cis conformation of the three
dihedral angles (1-2-3-4, 2-3-4-5 and 3-4-5-6, see Fig. 3). We
Fig. 4. Energy barrier for ring-closure isomerization between TTC and SOt.

Q. Zou et al. / Dyes and Pigments 107 (2014) 174e181
179
Fig. 5. Energy barrier for ring-closure isomerization between CTC and SOc.
examined the energy barrier for isomerization between TTC and
CTC by scanning the potential energy surface along the rotation of
dihedral angle 1-2-3-4 (see Fig. 3 and Table S2), and found that the
barrier is actually larger than 84 kJ/mol (or 20 kcal/mol), which is
not accessible under ambient temperature. This means that the two
open-ring isomers, TTC and CTC, are not interconvertible. This is
also connected to the observation that the thermal fading kinetics
of open-ring isomer usually exhibits biexponential decay [17,18].
We further investigated the ring-closure isomerization path-
ways from TTC and CTC, respectively. As shown in Fig. 4, the rota-
tion of the dihedral angle 2-3-4-5 gives rise to the closed-ring
isomer SOt, which is more stable than its open-ring counterpart
TTC. This process takes place via a transition state with an energy
barrier of 72.6 kJ/mol (Table S3). Differently, the ring-closure pro-
cess of CTC goes through two transition states and one intermediate
CCC (Fig. 5). The rotation of the dihedral angle 2-3-4-5 in CTC has to
Fig. 6. Time-dependent photocoloration of compound SOC (6.0 ꢀ 10ꢁ5 M) in different solutions: (a) THF; (b) toluene; (c) acetonitrile; (d) methanol at 290 K by UV irradiation at
365 nm, and the subsequent thermal fading when the irradiation was ceased. The monitored wavelength was at the lmax of the intensively colored form (MC form), respectively.

180
Q. Zou et al. / Dyes and Pigments 107 (2014) 174e181
overcome an energy barrier of 76.8 kJ/mol to reach CCC where all
the three dihedral angles are all in their cis conformations
(Table S4). Intermediate CCC then overcomes a small energy barrier
of 10.9 kJ/mol to reach the ring-closure isomer SOc through
shortening of the CeO distance (Fig. 5 and Table S4).
temperature. Moreover, compound SOC was found to exhibit reg-
ular change on the photochromic properties depending on solvent
polarity, indicating solvent-tunable photochromic behavior. Den-
sity functional theory calculations provided insight into the
mechanism and kinetics of the thermal fading process of the spi-
rooxazine derivative SOC. In particular, the TTC / SOt pathway
was found to be several times faster than CTC / SOc with the latter
pathway containing an intermediate state where the spirooxazine
is in its cis-cis-cis conformation. This work is expected to provide a
promising, robust platform to develop the photochromic materials
with promising photoresponsive characteristics, such as rapid
switching speed and remarkable stability.
Based on the energy barriers and Eyring equation, we computed
the rate constants for the two ring-closure isomerization pathways,
namely TTC / SOt and CTC / SOc, as shown in Table S5. The
TTC / SOt ring-closure pathway is found to have a lower energy
barrier and is hence several times faster than CTC / SOc. The
computed rate constants are 0.50 and 0.09 sꢁ1 for TTC / SOt and
CTC / SOc, respectively, which are in qualitative agreement (albeit
smaller) compared with the experimental observation of 2.05 and
0.60 sꢁ1. According to the Eyring equation, the experimental energy
barriers for the two ring-closure isomerization pathways can be
estimated as 69.2 and 72.2 kJ/mol, respectively. We can see that the
computed energy barriers (72.6 and 76.8 kJ/mol) overestimate the
barriers by w4 kJ/mol (or w1 kcal/mol), which is reasonable for a
parameterized density functional (M06) and a double-zeta basis set
(6-31G**). We further refined the energy barrier for the TTC / SOt
ring-closure pathway by recomputing the self-consistent field en-
ergy using the triple-zeta basis set 6-311þþG** and obtained an
energy barrier of 71.6 kJ/mol, which shows better agreement with
experimental result.
Acknowledgments
This work was financially supported by Supported by Innovation
Program of Shanghai Municipal Education Commission (14YZ128),
and the Science and Technology Commission of Shanghai Munici-
pality (14YF1410900), PR China.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.04.004.
3.5. Photocoloration/thermal bleaching cycles
References
The photocoloration investigation showed that compound SOC
in various solutions could undergo photochemical reaction within
3 s upon UV light irradiation. As depicted in Fig. 6, it took only about
0.92 s in methanol, 1.30 s in acetonitrile, 2.07 s in THF and 2.52 s in
toluene, respectively, for compound SOC to reach the photosta-
tionary state under the same measurement conditions, which was
consistent with its thermal fading rate. Obviously, compound SOC
possesses high photoresponse, which is an important precondition
in practical applications for ophthalmic plastic lenses and smart
windows, as well as optical switching devices. Additionally, fatigue
resistance is another important factor to consider for applicable
photochromic materials. A preliminary investigation of fatigue
resistance for compound SOC in different solvents was performed
by simply turning UV light of 365 nm on and off. As presented in
Fig. 6, after 10 cycles of coloration/bleaching, no distinct degrada-
tion was detected by UVevis absorption for compound SOC in THF
and methanol solution. Compound SOC also showed good fatigue
resistance in toluene solution, and less than 15% degradation was
detected after 10 cycles. The fatigue resistance of compound SOC in
THF and methanol solution was further tested by subjecting it to
100 colorationedecoloration cycles, respectively. As can be seen
from Figure S2, there is no sign of degradation after one hundred
switching cycles, even in the presence of molecular oxygen,
demonstrating very good fatigue resistance of compound SOC in
THF and methanol solution, respectively.
[1] (a) Tamai N, Miyasaka H. Ultrafast dynamics of photochromic systems. Chem
Rev 2000;100:1875e90;
(b) Irie M. Diarylethenes for memories and switches. Chem Rev 2000;100:
1685e716;
(c) Tian H, Yang SJ. Recent progresses on diarylethene based photochromic
switches. Chem Soc Rev 2004;33:85e97;
(d) Zhang JJ, Zou Q, Tian H. Photochromic materials: more than meets the eye.
Adv Mater 2013;25:378e99;
(e) Zhang JJ, Wang JX, Tian H. Taking orders from light: progress in photo-
chromic bio-materials. Mater Horiz 2014;1:169e84;
(f) Zou Q, Zhang JJ, Tian H. Multi-functional molecular switches based on
photochromic dithienylethenes. Prog Chem 2012;24:1632e45;
(g) Pu SZ, Liu G, Wang RJ, Chen B. Modulation of a fluorescence switch based
on
a photochromic diarylethene and a rhodamine B dye. Dyes Pigment
2013;98:238e45;
(h) Liu G, Pu SZ, Wang RJ. Photochromism of asymmetrical diarylethenes with
a pyrrole unit: effects of aromatic stabilization energies of aryl rings. Org Lett
2013;15:980e3;
(i) Pu SZ, Zheng CH, Sun Q, Liu G, Fan CB. Enhancement of cyclization quantum
yields of perfluorodiarylethenes via weak intramolecular interactions. Chem
Commun 2013;49:8036e8;
(j) Harada Y, Hatano S, Kimoto A, Abe J. Remarkable acceleration for back-
reaction of a fast photochromic molecule. J Phys Chem Lett 2010;1:1112e5;
(k) Zou Q, Li X, Zhang JJ, Zhou J, Sun BB, Tian H. Unsymmetrical diarylethenes
as molecular keypad locks with tunable photochromism and fluorescence via
Cu2þ and CNꢁ coordinations. Chem Commun 2012;48:2095e7.
[2] (a) Kawai S, Nakashima T, Atsumi K, Sakai T, Harigai M, Imamoto Y, et al. Novel
photochromic molecules based on 4,5-dithienyl thiazole with fast thermal
bleaching rate. Chem Mater 2007;19:3479e83;
(b) Fujita K, Hatano S, Kato D, Abe J. Photochromism of a radical diffusion-
inhibited hexaarylbiimidazole derivative with intense coloration and fast
decoloration performance. Org Lett 2008;10:3105e8;
(c) Kishimoto Y, Abe J. A fast photochromic molecule that colors only under
UV light. J Am Chem Soc 2009;131:4227e9;
(d) Prostota Y, Coelho PJ, Pina J, de Melo JS. Fast photochromic sterically
hindered benzo[1,3]oxazines. J Photochem Photobiol A 2010;216:59e65;
(e) Guo KP, Chen Y. A strategy for the design of photochromic naphthopyrans
with large optical density at photosteady state and fast fading speed at
ambient temperature in the dark. J Mater Chem 2010;20:4193e7;
(f) Deniz E, Tomasulo M, Cusido J, Sortino S, Raymo FM. Fast and stable
photochromic oxazines for fluorescence switching. Langmuir 2011;27:
11773e83.
4. Conclusions
A spirooxazine derivative SOC featuring with a carbazole unit,
for the first of their kind, was synthesized and characterized. Its
photochromic behaviors were investigated in solvents of different
polarity. The compound underwent reversible ring-opening/
closure isomerization between the closed SO form and the open
MC form upon irradiation with 365 nm light and in the dark.
Benefiting from a special design of molecular structure, the com-
pound displayed fast coloration rate under UV light illumination,
large steady-state optical density, and rapid bleaching under dark
condition, as well as excellent fatigue resistance at ambient
[3] (a) Minkin VI. Photo-, thermo-, solvato-, and electrochromic spiroheterocyclic
compounds. Chem Rev 2004;104:2751e76;
(b) Wang S, Yu CY, Choi M-S, Kim S-H. A switching fluorescent photochromic
carbazoleespironaphthoxazine copolymer. Dyes Pigment 2008;77:245e8;
(c) York M, Evans RA. Synthesis and properties of 1,3,3-trimethylspiro[indo-
line-2,30-naphtho[2,1-b][1,4]oxazin]-60-amine, a novel, red colouring photo-
chromic spirooxazine. Tetrahedron Lett 2010;51:2195e7;

Q. Zou et al. / Dyes and Pigments 107 (2014) 174e181
181
(d) Tian ZY, Stairs RA, Wyer M, Mosey N, Dust JM, Kraft TM, et al. Spirooxazine
to merooxazine interconversion in the presence and absence of zinc:
approach to a bistable photochemical switch. J Phys Chem A 2010;114:
11900e9;
[11] Tomasi J, Mennucci B, Cammi R. Quantum mechanical continuum solvation
models. Chem Rev 2005;105:2999e3094.
[12] Pijper D, Jongejan MGM, Meetsma A, Feringa BL. Light-controlled supramo-
lecular helicity of a liquid crystalline phase using a helical polymer func-
(e) Gentili PL. The fundamental fuzzy logic operators and some complex
Boolean logic circuits implemented by the chromogenism of a spirooxazine.
Phys Chem Phys 2011;13:20335e44;
tionalized with
2008;130:4541e52.
a single chiroptical molecular switch. J Am Chem Soc
[13] Kohout M, Svoboda J, Novotná V, Pociecha D, Glogarová M, Gorecka E.
A nematic-polar columnar phase sequence in new bent-shaped liquid crystals
(f) Christie RM, Morgan KM, Rasheed A, Aldib M, Rosair G. The molecular
design, synthesis and photochromic properties of spirooxazines containing a
permanent azo (hydrazone) chromophore. Dyes Pigment 2013;98:263e72.
[4] Minkovska S, Jeliazkova B, Borisova E, Avramov L, Deligeorgiev T. Substituent
based on
2009;19:3153e60.
[14] Queiroz M, Abreu AS, Ferreira PMT, Oliveira MM, Dubest R, Aubard J, et al.
A thieno-2H-chromene -amino acid derivative: synthesis and photochromic
a 7-hydroxynaphthalene-2-carboxylic acid core. J Mater Chem
and solvent effect on the photochromic properties of
a
series of spi-
a
roindolinonaphthooxazines. J Photochem Photobiol A 2004;163:121e6.
[5] (a) Paramonov SV, Lokshin V, Fedorova OA. Spiropyran, chromene or spi-
rooxazine ligands: insights into mutual relations between complexing and
photochromic properties. J Photochem Photobiol C 2011;12:209e36;
(b) Watkins DL, Fujiwara T. Synthesis, characterization, and solvent-
independent photochromism of spironaphthooxazine dimers. J Photochem
Photobiol A 2012;228:51e9.
[6] (a) Han SL, Chen Y. 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. J Mater Chem 2011;21:4961e5;
properties. Org Lett 2005;7:4811e4.
[15] Bohne C, Fan MG, Li ZJ, Liang YC, Lusztyk J, Scaiano JC. Laser photolysis studies
of photochromic processes in spirooxazines: solvent effects on photo-
merocyanine behavior. J Photochem Photobiol A 1992;66:79e90.
[16] Favaro G, Malatesta V, Mazzucato U, Ottavi G, Romani A. Thermally reversible
photoconversion of spiroindoline-naphtho-oxazines to photomerocyanines: a
photochemical and kinetic study. J Photochem Photobiol A 1995;87:235e41.
[17] (a) Lee DK, Cha HG, Pal U, Kang YS. Kinetics of decolorization of
spironaphthooxazine-doped photochromic polymer films. J Phys Chem
2009;113:12923e7;
B
(b) Song LW, Yang YH, Zhang Q, Tian H, Zhu WH. Synthesis and photochro-
mism of naphthopyrans bearing naphthalimide chromophore: predominant
thermal reversibility in color-fading and fluorescence switch. J Phys Chem B
2011;115:14648e58.
(b) Malic N, Campbell JA, Ali AS, York M, D’Souza A, Evans RA. Controlling
molecular mobility in polymer matrices: synchronizing switching speeds of
multiple photochromic dyes. Macromolecules 2010;43:8488e501.
[18] (a) Evans RA, Hanley TL, Skidmore MA, Davis TP, Such GK, Yee LH, et al. The
generic enhancement of photochromic dye switching speeds in a rigid poly-
mer matrix. Nat Mater 2005;4:249e53;
[7] Wang S, Yu CY, Choi M-S, Kim SH. Synthesis and switching properties of
photochromic carbazoleespironaphthoxazine copolymer. J Photochem Pho-
tobiol A 2007;192:17e22.
[8] Zhao Y, Truhlar DG. The M06 suite of density functionals for main group
thermochemistry, thermochemical kinetics, noncovalent interactions, excited
states, and transition elements: two new functionals and systematic testing of
four M06 functionals and 12 other functionals. Theor Chem Acc 2008;120:
215e41.
[9] Hehre WJ, Ditchfield R, Pople JA. Self-consistent molecular orbital methods.
XII. Further extensions of Gaussian-type basis sets for use in molecular orbital
studies of organic molecules. J Chem Phys 1972;56:2257e61.
[10] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al.
Gaussian 09, Revision A.2. Wallingford, CT: Gaussian, Inc.; 2009.
(b) Sriprom W, Néel M, Gabbutt CD, Heron BM, Perrier S. Tuning the color
switching of naphthopyrans via the control of polymeric architectures. J Mater
Chem 2007;17:1885e93;
(c) Moorthy JN, Venkatakrishnan P, Samanta S, Kumar DK. Photochromism of
arylchromenes: remarkable modification of absorption properties and life-
times of o-quinonoid intermediates. Org Lett 2007;9:919e22.
[19] (a) Nakamura S, Uchida K, Murakami A, Irie M. Ab initio MO and proton NMR
NOE studies of photochromic spironaphthoxazine. J Org Chem 1993;58:5543e5;
(b) Liu FY, Morokuma K. Multiple pathways for the primary step of the spi-
ropyran photochromic reaction: a CASPT2//CASSCF study. J Am Chem Soc
2013;135:10693e702.