Synthesis, characterization, and solvent-independent photochromism of spironaphthooxazine dimers

Article abstract of DOI:10.1016/j.jphotochem.2011.11.011

Specifically angled, conjugated spiroindolinonaphthooxazine dimers (SNOD) have been synthesized. The photochromic reactions of two types of SNOD were studied under continuous UV irradiation in solvents of different polarity. Comparison of these results wi

Full text of DOI:10.1016/j.jphotochem.2011.11.011

Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 51–59
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis, characterization, and solvent-independent photochromism of
spironaphthooxazine dimers
Davita L. Watkins, Tomoko Fujiwara∗
Department of Chemistry, The University of Memphis, 213 Smith Chemistry Bldg., Memphis, TN 38152, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 June 2011
Received in revised form
24 November 2011
Accepted 26 November 2011
Available online 13 December 2011
Specifically angled, conjugated spiroindolinonaphthooxazine dimers (SNOD) have been synthesized. The
photochromic reactions of two types of SNOD were studied under continuous UV irradiation in solvents
of different polarity. Comparison of these results with the single unit provides the examination of the
specific effect of substituents on their photochromic properties and relaxation kinetics. The photome-
rocyanine isomers showed positive solvatochromism, supporting the premise for a less polar quinoidal
structure. The thermal closing rate at 25 ◦C ranged from 0.2 to 1.6 s−1 depending on the compound and
solvent. Photochromism of these new compounds showed little dependency on solvent polarity and
stable cyclability.
Keywords:
Spirooxazines
Biphotochromic
Photokinetics
Solvatochromism
Photochromism
Merocyanine
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
thermal reversibility, yielding different isomers from the starting
material, and/or extensive photodegradation [7,8]. In addition, bi-
Photochromic materials have shown practical applications in
the latest technologies such as optical switching, sensor, drug deliv-
ery, data storage, and recording [1,2]. The use of spirooxazine based
materials has become of increasing interest due to their excellent
photostability, compatibility in a variety of matrices, and distinc-
tive changes in structure and absorption spectra upon irradiation.
The photoresponsive site of the spirooxazine is the center sp3 spiro
carbon. When irradiated with UV light, the carbon oxygen bond
cleaves and achieves sp2 hybridization, yielding a conjugated zwit-
terionic or quinoidal system [3,4]. This conjugated isomer, called
merocyanine (MC) is metastable and readily isomerizes via ring
closing into the spiro (SP) form either thermally in dark conditions
or under visible light [5].
As an approach to improve the applicability of spirooxazine
based compounds, joining two of the species through a spacer offers
unique bi-photochromic molecules with distinctive photokinetic
properties [6–8]. A challenge faced with designing bi-photochromic
molecules is that these dimer systems often exhibit different or
deficient photochemical properties from those of the single com-
ponents. Photochromic properties and the rate of thermal closing
are affected by the nature of the spacer, e.g., inducing a loss of
photochromic systems have shown to be either too rigid or flexible
for further application.
Recently, we reported a shape specific photochromic dimer,
called SPOD, consisting of two spiro-phenanthro-oxazine moieties
connected via triple bond at the indoline portion of the spirooxazine
moieties [9]. This SPOD has shown interesting binding ability with
palladium catalyst. The open MC form of this molecule showed a
bathochromic shift at the maximum wavelength of absorption and
higher colourability due to the increased conjugation. It exhibited
excellent reversibility; however, the data from thermal bleaching
illustrates that the rate of thermal closure increased dramatically
with increasing solvent polarity. Thermal closing rates ranged from
0.12 s−1 in nonpolar methylcyclohexane to 4.48 s−1 in polar solvent,
dimethylformamide. Reports suggest that the solvent dependency
for phenanthrene systems is due to a loss in pseudo pi-conjugation
between the indoline and oxazine moieties in its transition state. As
the electronic charge on the indoline nitrogen and oxazine oxygen
atoms increases, the molecular dipole moment increases causing
the accelerated closing with polarity of solvent [10,11].
In this work, we have improved our bi-photochromic sys-
tems by replacing the phenanthro-oxazine with naphtho-oxazine
to create molecules having environmentally independent opti-
cal properties. We report the synthesis and photochromic
properties of two types of spiro-naphtho-oxazine dimers: Debald-
SNOD(2) and Deba-SNOD(3). The chemical structures and typical
∗
Corresponding author. Tel.: +1 901 678 5558; fax: +1 901 678 3447.
E-mail address: tfjiwara@memphis.edu (T. Fujiwara).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.11.011

52
D.L. Watkins, T. Fujiwara / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 51–59
N
I
N
O
O
O
1
UV
heat, vis
N
N
N
N
N
N
O
O
N
O
N
O
SP
MC
2
HO
HO
UV
heat, vis
N
N
N
O
N
O
O
O
N
N
N
N
SP
MC
3
Fig. 1. The structure of I-SNO(1) and typical photoreactions of Debald-SNOD(2) and Deba-SNOD(3).
NO
photoreactions for these compounds are given in Fig. 1. Two SNO
units in these dimers were connected to different positions of a
phenyl ring spacer, which would create different conjugation sys-
tems. Additionally, both dimers possess a functional group on the
center phenyl ring to offer further modifications such as grafting on
the surface and polymers. The investigation in this report includes
studies on the effects of solvent and structure on the absorption
properties, fatigue resistance, and closing rate kinetics of the mero-
cyanine of the dimers as compared to the single unit, Iodo-SNO(1).
OH
I
NHNH2
+
i. a
ii. b, c
O
O
I-SNO
(1)
N
I
O
i. d
1
Debald-SNOD
(2)
f
ii. e
Br
Br
2. Experimental
HO
HO
2.1. General methods
i. d
1
All reactants (Fisher Scientific, Pittsburg, PA, USA) and deuter-
ated solvents (99.9 atom% D, Sigma–Aldrich, St. Louis, MO, USA)
were purchased and used as received. The reaction solvents were
purified by Vacuum Atmospheres solvent purifier system. Flash col-
umn chromatography was performed on silica gel (200–400 mesh,
60A, Fisher Scientific). A 200 W Mercury Xenon lamp housed in
a light box containing a 340 nm colored glass filter (FSR-U340)
was directed at a right angle towards a cuvette containing a 10 mL
solution of photochromic species maintained at room temperature.
Absorption spectra were recorded with an Ocean Optics USB4000
Fiber Optic Spectrometer with UV–vis Dip Probe, capable of record-
ing spectra in the UV to visible region every 0.1–1 s. 1H and 13C
spectra were recorded for CDCl3 solutions on JOEL 270 MHz and
Varian 500 MHz instruments. Mass spectroscopy data were col-
lected with positive mode of the ESI source on an LCQ-Advantage.
Deba-SNOD
(3)
f
ii. e
Br
Br
Scheme 1. Synthetic route of I-SNO(1), Debald-SNOD(2), and Deba-SNOD(3).
Reagents and conditions: (a) H2SO4, EtOH; (b) MeOTf, ACN; (c) NaOH; (d)
Pd(PPh3)2Cl2, CuI, Et3N; (e) TBAF, THF; (f) Pd(PPh3)2Cl2, CuI, NMP; and (g) K2CO3,
MeOH.
2.2. Synthesis of photochromic compounds 1, 2, and 3 (Scheme 1)
The synthetic methods shown in Scheme
1 are adapted
and modified from a previously published procedure [9]. The
bi-photochromic systems were synthesized in five steps with
an overall yield of 29% for Debald-SNOD(2) and 38% for
Deba-SNOD(3). The synthesis started with the conversion of

D.L. Watkins, T. Fujiwara / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 51–59
53
4-iodophenylhydrazine to 5-iodo-2,3,3-trimethylindole via Fischer
2.2.5. 3,4-Diethynylbenzaldehyde
cyclization followed by a methyl triflate in acetonitrile. The product
was treated with base and reacted with 1-nitrosonaphthalen-
2-ol in trichloroethane to yield I-SNO(1). The single unit
spirooxazine was then reacted with 3,4-diethynylbenzalcohol or
3,4-diethynylbenzaldehyde using Sonogashira coupling to gener-
ate the respective spirooxazine dimer.
3,4-Bis(trimethylsilylethynyl)benzaldehyde (0.45 g, 2.9 mmol)
and K2CO3 (167 mg, 1.2 mmol) was dissolved in methanol (12 mL)
at room temperature. The reaction was stirred for 1 h, diluted with
dichloromethane (15 mL), and washed with aqueous sodium bicar-
bonate (15 mL),and deionized water (2 × 15 mL). The organic layer
was collected, dried over Na2SO4, and evaporated to yield a yellow
solid (0.185 g, 79.6%). 1H NMR (270 MHz, CDCl3): ı 9.97 (s, 1H), 7.98
(s, Ph H, 1H), 7.78 (dd, Ph H, 1H), 7.63 (d, Ph H, 1H).
2.2.1. 5-Iodo-2,3,3-trimethylindolenine
Isopropylmethylketone (1.8 mL, 16.7 mmol) followed by conc.
H2SO4 (0.6 mL) was added to a solution of 4-iodophenylhydrazine
(1.8 g, 7.57 mmol) and ethanol (20 mL) at room temperature. The
reaction was allowed to reflux for 12 h. The reaction mixture was
allowed to cool to room temperature and 10% sodium bicarbon-
ate was added dropwise until a biphasic mixture was formed. The
mixture was extracted with diethyl ether (2 × 20 mL) and washed
with deionized water (20 mL). The organic layer was dried over
Na2SO4 and evaporated to yield a brown vicious liquid. The prod-
uct was purified using column chromatography with 1:1 diethyl
2.2.6. 3,4-Bis((1,3,3-trimethylspiro[indoline-
2,3ꢀ-naphtho[2,1-b][1,4]oxazine]-5-yl)ethynyl)benzaldehyde
(Debald-SNOD(2))
N-Methylpyrrolidine (15 mL) was added to a reaction ves-
sel containing 3,4-diethynylbenzaldehyde (0.18 g, 1.17 mmol),
I-SNO(1) (0.797 g, 1.76 mmol), CuI (32.0 mg, 0.117 mmol), and
Pd(PPh3)2Cl2 (79.8 mg, 0.117 mmol) under nitrogen atmosphere.
The reaction mixture was stirred at 85 ◦C for 12 h, cooled to
room temperature, and diluted with dichloromethane (20 mL). The
product was washed with 2% HCl followed by water (2 × 20 mL)
and further purified using column chromatography with 2:1
dichloromethane and hexane to give a green solid (0. 274 g, 28.3%).
1H NMR (500 MHz, CDCl3, ı): 10.0 (s, 1H), 8.55 (d, 2H, J = 8 Hz), 8.03
(s, 1H), 7.77 (d, 1H, J = 8 Hz), 7.75 (d, 2H, J = 8 Hz), 7.73 (s, 2H), 7.71 (s,
1H, J = 8 Hz), 7.66 (d, 2H, J = 7.75 Hz), 7.65 (d, 1H, 7 Hz), 7.58–7.61 (m,
3H), 7.52–7.56 (m, 3H), 7.41 (m, 2H), 7.32 (s, 2H, J = 8 Hz), 7.01 (d, 2H,
J = 8.5 Hz), 6.92 (dd, 2H, J = 9 Hz, 2 Hz), 6.54 (d, 2H, J = 7.75 Hz), 2.79
(s, 6H), 1.37 (m, 12H, J = 16.5 Hz). MS (m/z): calcd. [M+H]+ 807.3;
found 807.3.
ether and hexane to give a brown vicious liquid (1.72 g, 76.2%). 1
H
NMR (270 MHz, CDCl3, ı): 7.63–7.59 (m, 2H), 7.28 (d, 1H, J = 8 Hz),
2.26 (s, 3H), 1.29 (s, 6H).
2.2.2. 1,2,3,3-Tetramethyl-5-iodo-3H-indolium triflate
Methyl triflate (1.4 mL, 13.0 mmol) was added to a solution of 5-
iodo-2,3,3-trimethylindolenine (1.7 g, 5.89 mmol) and acetonitrile
(ACN) (20 mL) under nitrogen atmosphere and allowed to reflux for
8 h. The solvent was evaporated and the residue was washed with
diethyl ether (50 mL) to yield a reddish solid (1.3 g, 73.8%). 1H NMR
(270 MHz, CDCl3, ı): 8.14 (d, 1H, J = 1.5 Hz), 8.01 (dd, 1H, J = 8.5 Hz),
7.53 (d, 1H, J = 8.5 Hz), 3.91 (s, 3H), 2.69 (s, 3H), 1.56 (s, 6H).
2.2.7. 3,5-Bis(trimethylsilylethynyl)benzalcohol
Trimethylsilylacetylene (4.5 mL, 31.4 mmol) was added to
a
solution of 3,5-dibromobenzalcohol (1.27 g, 4.78 mmol),
2.2.3. 5-Iodo-1,3,3-trimethylspiro[indoline-
Pd(PPh3)2Cl2 (35.2 mg, 0.050 mmol), and CuI (8.81 mg,
0.0458 mmol) in Et3N (10 mL) at room temperature under
nitrogen atmosphere. After stirring for an hour, the reaction tem-
perature was increased to 85 ◦C and stirred for 12 h. The reaction
mixture was allowed to cool to room temperature, diluted with
diethyl ether (20 mL), poured into deionized water (20 mL), to
give a biphasic mixture which was extracted with diethyl ether
(3 × 20 mL). The organic layer was dried over Na2SO4 and evapo-
rated to yield a yellow film (1.83 g). 1H NMR (300 MHz, CDCl3): ı
7.51 (t, Ph H, 1H), 7.41 (m, Ph H, 2H), 4.63 (s, Ph CH2, 2H), 0.23
(s, Si (CH3)3, 18H).
2,3ꢀ-naphtho[2,1-b][1,4]oxazine] (I-SNO(1))
A solution of 1,2,3,3-tetramethyl-5-iodo-3H-indolium triflate
(1.3 g, 4.34 mmol) in aqueous NaOH (1 M, 100 mL) was stirred
to 15 min at room temperature. The product was extracted with
dichloromethane. The organic layer was dried over Na2SO4 and
evaporated to yield a bright pink solid. The red solids (1.71 g,
5.68 mmol) and 1-nitrosonaphthalen-2-ol (1.21 g, 6.99 mmol) were
taken into a reaction vessel. Trichloroethane was added to the
reaction vessel under nitrogen atmosphere. The reaction mixture
was refluxed for 12 h and cooled to room temperature. After the
solvent was evaporated, the product was purified using column
chromatography with 1:2 dichloromethane and hexane to give
a green solid (1.45 g, 56.4%). 1H NMR (270 MHz, CDCl3, ı): 8.52
(dd, 1H, J = 8.5 Hz), 7.76 (d, 1H, J = 8 Hz), 7.73 (s, 1H), 7.65 (d, 1H,
J = 8.5 Hz), 7.57 (t, 1H, J = 7.3 Hz), 7.46 (dd, 1H, J = 8 Hz), 7.39 (t, 1H,
J = 8 Hz), 7.31 (s, 1H), 6.98 (d, 1H, 8.5 Hz), 6.36 (d, 1H, J = 8 Hz), 2.73
(s, 3H), 1.36 (d, 6H, J = 19 Hz). MS (m/z): calcd. [M+H]+ 455.1; found
454.2.
2.2.8. 3,4-Diethynylbenzalcohol
Tetrabutylammonium fluoride (TBAF: 1.0 M solution of THF,
13.7 mmol) was added to 3,5-bis(trimethylsilylethnyl)benzalcohol
(1.83 g, 6.09 mmol) in anhydrous tetrahydrofuran (THF) (25 mL) at
room temperature under nitrogen atmosphere. The reaction was
stirred for 8 h, diluted with diethyl ether (25 mL), then washed with
2% HCl (25 mL), brine (25 mL), and deionized water (2 × 25 mL).
The organic layer was collected, dried over Na2SO4, and evapo-
rated to yield a brown solid. The product was further purified
using column chromatography with 3:7 ethyl acetate and hexane
to give a golden solid (0.360 g, 33.5%). 1H NMR (270 MHz, CDCl3):
ı 7.54 (t, Ph H, 1H), 7.48 (s, Ph H, 2H), 4.67 (s, Ph CH2, 2H),
2.2.4. 3,4-Bis(trimethylsilylethynyl)benzaldehyde
Trimethylsilylacetylene (4 mL, 28 mmol) was added to a solu-
tion of 3,4-dibromobenzaldehyde (0.77 g, 2.9 mmol), Pd(PPh3)2Cl2
(102 mg, 0.15 mmol), and CuI (43.4 mg, 0.23 mmol) in Et3N (10 mL)
at room temperature under nitrogen atmosphere. After stirring for
an hour, the temperature was increased to 85 ◦C and the reaction
continued for 12 h. The reaction mixture was allowed to cool to
room temperature, diluted with diethyl ether (20 mL), poured into
deionized water (20 mL), to give a biphasic mixture which was
extracted with diethyl ether (3 × 20 mL). The organic layer was
dried over Na2SO4 and evaporated to yield a yellow film (0.564 g,
64.8%). 1H NMR (270 MHz, CDCl3): ı 9.97 (s, 1H), 7.59 (d, Ph H, 1H),
7.71 (dd, Ph H, 1H), 7.93 (s, Ph H, 1H), 0.23 (s, Si (CH3)3, 18H).
3.09 (s,
C CH, 2H).
2.2.9. 3,5-Bis((1,3,3-trimethylspiro[indoline-
2,3ꢀ-naphtho[2,1-b][1,4]oxazine]-5-yl)ethynyl)phenylmethanol
(Deba-SNOD(3))
N-Methylpyrrolidine (15 mL) was added to a reaction vessel
containing I-SNO(1) (0.63 g, 1.39 mmol), 3,4-diethynylbenzalcohol
(0.145 g, 0.929 mmol), CuI (17.2 mg, 0.092 mmol), and Pd(PPh3)2Cl2
(64.1 mg, 0.092 mmol) under nitrogen atmosphere. The reaction

54
D.L. Watkins, T. Fujiwara / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 51–59
Fig. 2. 1H NMR spectra of (a) Debald-SNOD(2) and (b) Deba-SNOD(3) (500 MHz, room temp., CDCl3).
mixture was stirred at 85 ◦C for 12 h, cooled to room tempera-
ture, and diluted with dichloromethane (20 mL). The product is
washed with 2% HCl followed by water (2 × 20 mL) and further
purified using column chromatography with 3:7 ethyl acetate and
hexane to give a green solid (0.423 g, 37.6%). 1H NMR (500 MHz,
CDCl3, ı): 8.56 (d, 2H, J = 8 Hz), 7.76 (d, 2H, J = 8 Hz), 7.74 (s, 2H),
7.68 (d, 2H, J = 8 Hz), 7.60 (m, 3H), 7.47 (s, 2H), 7.40–7.44 (m,
4H), 7.02 (d, 2H, J = 8 Hz), 6.55 (d, 2H, J = 8 Hz), 4.70 (s, 2H), 2.80
(s, 6H), 1.38 (d, 6H, J = 10.5 Hz). MS (m/z): calcd. [M+H]+ 809.3;
found 809.3.
3. Results and discussion
3.1. Synthesis
The approach to improve the photochromic abilities of spiro-
compounds involved connecting the two spirooxazines directly
through a benzyl spacer. The major difference in the two dimers
is seen in the positioning of the spirooxazine units. The two pho-
tochromic moieties are linked ortho to each in Debald-SNOD(2)
while they are meta to each other in Deba-SNOD(3). The ortho posi-
tioning may offer a longer conjugated system in Debald-SNOD(2)
which would facilitate in simultaneous activation of both moieties
upon irradiation [7]. Both dimers have been designed to have spe-
cific angles, so that the photochromic units of the MC form create
a cavity for further applications such as sensing or binding.
The new dimers were obtained with high purity and success-
fully characterized using 1H, 13C, and 2D-NMR analyses. The 1H
NMR spectra of the compounds 2 and 3 are shown in Fig. 2. Both
integration of characteristic spirooxazine signals at 1.37, 2.81, and
7.71 ppm and chemical shifts of protons near triple bond connec-
tions assisted in identification of the dimeric structure for each
compound. Downfield shifts to 6.5 and 7.3–7.4 ppm in these dimers
from the peaks in single I-SNO(1) confirm connection to diethynyl-
benzyl spacer. NMR data offer distinctive difference in structure
between asymmetric and symmetric dimers particularly splitting
doublets at 7 ppm in Debald-SNOD indicating complex coupling
that is not seen in Deba-SNOD.
2.3. Procedures for the determination of photochromic properties
A 10 mL solution was prepared in a vial containing a micro stir
bar, maintained at 25 ◦C. Light from a 200 W Mercury Xenon lamp,
filtered by 340 nm colored glass filter (FSR-U340), was directed per-
pendicular to the side of the vial. The absorbance was recorded
using the Ocean Optics software. The solution was irradiated and
stirred while monitoring the change in absorbance in the visible
region.
Solvatochromism was studied using the spiro compounds
(10−4 M) in solvents of various polarity. The wavelength at the
maximum MC absorbance was recorded in each solution. The kinet-
ics of thermal bleaching was studied following the disappearance
of the colored form at the wavelength of maximum absorbance.
After absorbance maximum was reached for the solutions (10−4 M),
irradiation was discontinued, and thermal decay was recorded
and plotted as absorbance versus time. The measurements were
performed at room temperature. First-order rate constants were
obtained from the linear dependences of ln A/Ao versus time. Cycla-
bility was evaluated by plotting absorbance of merocyanine at the
wavelength maxima versus time upon repeated irradiation in an
on and off manner.
3.2. Photoisomerization in solution
Low temperature NMR spectroscopy was used to observe the
merocyanine isomerization of a single SNO unit and the dimers
[11–13]. Fig. 3 shows 1H NMR spectra of I-SNO(1) and Deba-
SNOD(3), before and after 1 min UV irradiation (340 nm) in THF-d6

D.L. Watkins, T. Fujiwara / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 51–59
55
Fig. 3. Low temperature 1H NMR spectra of (a) I-SNO(1) and (b) Deba-SNOD(3) in THF-d8 at −30 ◦C; before (upper) and after (lower) UV irradiation of each compound. The
photographs show the NMR samples.
at −30 ◦C. Upon photo-exposure, similar changes occurred in both
structures of 1 and 3. The opening of spirooxazine is characterized
by a peak appearance at 1.88 ppm for two methyl groups as a sin-
glet and for aromatic H at 9.95 ppm for I-SNO (Fig. 3a, after UV).
In the closed form, these protons correspond to two methyl sin-
glets at 1.35 and 1.32 ppm, and a singlet at 7.7 ppm, respectively.
In addition, the merocyanine peaks of doublets at 6.43 ppm and
7.12 ppm were observed after irradiation shown in lower spec-
tra of Fig. 3a. The doublet signals correspond to aromatic peaks
of the closed form meta to the indoline nitrogen and oxazine oxy-
gen, respectively. These doublets shifting upfield from original SP
signals to 6.43 ppm and downfield to 7.12 ppm suggest a change in
electron distribution indicative of the open form. Besides appear-
ance of these resonance peaks of the MC form, some SP signals were
also shifted after UV irradiation. Similar peaks of the MC isomers
were observed for the dimers, 2 (data not shown) and 3 (Fig. 3b)
after UV treatment. Peaks of low intensity indicate a small con-
version to the expected photomerocyanines at this temperature
(−30 ◦C) and conditions as consistent with lower quantum yields
reported on general SNO series [3,11,14,15].
To examine photochromism and reversibility of compounds 1,
2, and 3, the absorption spectra were collected. A typical spectrum
of each compound in solution resulted in a strong band in the UV
region representing the closed form. The absorption spectra for the
two dimers (2, 3) in THF are shown in Fig. 4. Photo irradiation with
a UV light (340 nm) resulted in the formation of colored MC isomer
with ꢀmax in the range of 550–630 nm. When the light is removed,
the absorption spectrum prior to excitation is restored concluding
that isomerization is reversible.
moieties of the molecule. The absorbance spectra of Debald-
SNOD(2) exhibit large broad peaks from 300 to 420 nm while the
single unit (SNO) and Deba SNOD(3) compounds have narrow
absorption bands at 300–360 nm. The difference is based on the
structure in that the spirooxazine units are linked at ortho posi-
tion to each other in 2 while they are meta to each other in 3. The
a)
2
1.5
UV
(340nm)
vis or
dark
1
0.5
0
210
310
410
510
610
710
2
1.5
1
b)
UV
(340nm)
vis or
dark
0.5
0
210
310
410
510
610
710
Wavelength (nm)
The absorbance in the spectrum of the spiro form consists of
localized ␲–␲* transitions in UV region of the indoline and oxazine
Fig. 4. Absorbance spectra of (a) Debald-SNOD(2) and (b) Deba-SNOD(3) in THF,
1 × 10−4 M upon irradiation of UV light (340 nm) at room temperature.

56
D.L. Watkins, T. Fujiwara / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 51–59
Table 1
ortho position offers longer conjugated system between two indo-
line units of SNO moieties, which absorbs longer wavelength at the
peak about 390 nm.
The wavelength of the MC form absorption of I-SNO(1), Debald-SNOD(2), and Deba-
SNOD(3).a
Solvent
Brooker’s red
ꢀMC (nm)b
The open form of spirooxazine is characterized by an absorp-
tion band in the visible region. The MC absorbance of the dimers
2 and 3 in THF (609 and 606 nm, respectively) showed a shift to
longer wavelength than that of the single unit 1 (589 nm) due to
increased conjugation. The intensities of the MC forms in those
compounds were observed to be lower than previously reported
studies of spiro-naphtho-oxazines (SNO) [14,16,17]. The molar
extinction coefficient of the MC form in SNOD is still unknown;
however, a possible factor for the low intensity is possibly due to a
temperature increase of the solution during irradiation. Generally,
the ring opening reaction of spirooxazines is temperature sensitive
[18]. In addition, substituents exerting an inductive effect on the
indoline moiety of spirooxazine affect to destabilize the open form.
The structural differences of the two dimers play a significant
role in the opening and closing of each moiety upon irradia-
tion. We speculated that by increased conjugation through two
spirooxazine units, the simultaneous activation of both units would
occur upon irradiation [7]. To investigate this hypothesis, we
extended the UV exposure time and examined the opening and
closing upon irradiation in nonpolar and polar solvents. Fig. 5
shows plots of the absorption maxima of the merocyanine of
Debald-SNOD(2) and Deba-SNOD(3) in nonpolar methylcyclohex-
ane (5a) and polar dimethylformamide (5b) versus time. Upon
initial excitation, a rapid increase in absorbance followed by
decrease is observed in both solvents, which is reported as ‘char-
acteristic’ for the spironapthooxazines, yet the reason is unknown
[19]. After this initial projection, the merocyanine absorbance of
both dimers in methylcyclohexane (5a) increased stepwise upon
parameters (ꢁR)
1
2
3
Methylcyclohexane
Diethyl ether
Toluene
Tetrahydrofuran
Acetone
50.1
48.3
47.2
46.6
45.7
43.7
553
565
587
589
590
610
589
597
607
609
614
621
583
593
603
606
612
619
Dimethylformamide
At 25 ◦C, concentration of the solution: 1 × 10−4 M.
a
b
Obtained under UV irradiation at 340 nm.
sustained exposure to UV light. The plot of Debald-SNOD(2) shows
a shoulder before maximum absorbance is attained, and then
maintains a steady absorbance. While, opening to the MC form
of Deba-SNOD(3) shows a slow two-step process to reach stable
absorbance. It is again, unknown why the MC absorbance decreases
after the major increase. The difference between Debald-SNOD and
Deba-SNOD can be explained by the conjugation of two units of
the Debald-SNOD, whose second spirooxazine unit opens faster
than non-conjugated Deba-SNOD. In contrast to the behavior in
methylcyclohexane, in polar dimethylformamide, Debald-SNOD(2)
maintains a maximum absorbance of the merocyanine form upon
initial opening, while Deba-SNOD(3) displays a gradual increase
to reach absorbance maximum. After 150 s, the light source was
turned off so that isomerization to the closed form could be
observed. Upon discontinuation of irradiation, a dual step closing
process is observed distinctively in methylcyclohexane (Fig. 5a)
in which the dimers generally demonstrate a slower isomeriza-
tion to the SP form. The decline in intensity is evidently faster
for the conjugated dimer, Debald-SNOD than that of Deba-SNOD.
This is probably due to the sequential closing of two moieties. In
dimethylformamide, the cyclization appears faster due to the non-
polar nature of the merocyanine form and its stability in this media
[16]. Thus, multistep closing was not observed.
The solvatochromism of the photomerocyanine-based ␲–␲*
absorption band of 1, 2, and 3 were examined in a series of sol-
vents. The wavelength maxima were correlated with the Brooker’s
red parameters (ꢁR) [18,20], which takes into account solvation
effects such as dispersion forces and hydrogen bonding, rather than
simple solvent polarity parameters. Spectral data indicate positive
solvatochromism as wavelength maxima of the MC isomers shift
to longer wavelengths with increasing relative solvent polarity as
summarized in Table 1. This bathochromic shift indicates that the
lowest excited state of the MC is better stabilized by polar solvents
resulting in a decrease in transition energy. The normalized absorp-
tion spectra of Deba-SNOD(3) under UV irradiation are shown in
Fig. 6a, along with the plot of wavenumbers of absorption maxima
as a function of Brooker’s red parameters in Fig. 6b. In addition to
bathochromic shift in ꢀmax, a decrease in shoulder intensity with
increasing solvent polarity was observed. These data suggest that
the quinoidal form of the MC isomer is favored over the zwitterions.
Examination of the absorbance of the photomerocyanine forms of
single SNO and dimers confirmed positive solvatochromism, but
both dimers (2, 3) showed smaller shift compared to the shift seen
in I-SNO.
3.3. Thermal fading kinetics of the MC form
Fig. 7 shows a thermal fading of Deba-SNOD(3) in ether; the MC
absorbance at the wavelength maxima collected immediately after
the UV irradiation was terminated. The inserted plot of ln(A/A0)
versus time exhibited the first order kinetics of the closing rate
and gave a rate constant with the relaxation time obtained using
Fig. 5. Plots of the absorption maxima of the merocyanine of Debald-SNOD(2)
and Deba-SNOD(3) in methylcyclohexane (a) and dimethylformide (b), 1 × 10−4
M
versus time subjected to UV irradiation at room temperature.

D.L. Watkins, T. Fujiwara / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 51–59
57
Time (s)
0.1
0.08
0.06
0.04
0.02
0
0
-1
-2
-3
-4
0
2
4
0
1
2
3
4
5
6
Time (s)
Fig. 7. The thermal decay of the MC absorbance at 593 nm of Deba-SNOD(3) in
diethyl ether at 25 ◦C ([3] = 1.0 × 10−4 M). Embedded graph depicts the first-order
kinetic plot of the thermal decay.
moieties, compounds 2 and 3 show relatively similar closing rates
with the exception of toluene. The cyclization rate of the MC form
in the dimers show smaller dependence upon solvent polarity com-
pared to that of I-SNO.
In general, thermal closing rate varies depending both on struc-
tural characteristics and solvent polarity [4,16,18]. It has been
reported that substituents influence the photochromism through
both electronic and steric effects. The general stability of the MC
dimers (2, 3) in solution compared to the single form (1) may be due
to the inductive effect of the halogen into the MC indoline moiety
destabilizing the merocyanine form of 1.
Fig. 6. (a) Absorbance spectra of the MC form of Deba-SNOD(3) in various solvents,
and (b) a plot of wavenumbers (cm−1) of the MC maxima as a function of Brooker’s
red parameters of all solvents.
In our previous report on spiro-phenanthro-oxazine dimer
(SPOD), the closing rate constants were extremely different from
non-polar to polar solvents [9]. For comparison, the rate con-
stants of SPOD and two SNODs, 2 and 3 were plotted against log
scale of solvent dielectric constants in Fig. 8. The closing rate of
t = 1/k. Data from elongated irradiation revealed a sequential clos-
ing of the two spirooxazine moieties (Fig. 5); however, overall
thermal closing in all solvents was evaluated to obey first order
kinetics. Table 2 summarizes calculated rate constants of the three
compounds in various solvents. The relaxation half-life of the mero-
cyanines (ꢂMC–SO) given in Table 2 was obtained from the first
order rate constant using the expression t1/2 = ln 2/k. The results
of rate constants indicate relatively slow thermal decay of the
MC isomer in methylcyclohexane for all compounds. The single
unit, I-SNO(1) showed increased closing rates with an increase in
polarity of solvents with the exception of toluene. Thermal closure
for the single unit is generally faster than that of the dimers (2,
3) with the exception of diethyl ether, which is consistent with
previous studies on other spiro-dimers and partially due to the
delay in opening and closing of one moiety of the biphotochromic
system [9]. Despite their differences in conjugation between
SPOD (green marks) in methylcyclohexane (ε = 2.1) was 0.12 s−1
,
and in dimethylformamide (ε = 38) was 4.48 s−1, and thus, had
a significant solvent dependency. It is suggested that for spiro-
phenanthro-oxazine systems, the transition state of the ring closing
reaction is more polar than either the SP or MC forms [10,11]. The
polarity of the transition state is due to a loss of pseudo ␲ con-
jugation upon rotation of indoline and oxazine moieties, which
increases the electronic charge on both the indoline nitrogen and
oxazine oxygen. The MC isomer being planar results in delocal-
ization of the charge throughout the molecule and thus loses the
bipolar nature. As the polarity of the solvent increases, the quinoidal
MC form is destabilized to decrease energy gap with the transition
state, and the closing rate is drastically increased for SPOD.
Table 2
Thermal fading ratesa of compounds 1, 2, and 3 (1.0 × 10−4 M) in different solvents of varying polarity.
Solvent
Dielectric constant, ε
Rate constant, kT/s−1
b(ꢂMC–SO/s)
I-SNO(1)
Debald-SNOD(2)
Deba-SNOD(3)
2.1
2.4
4.3
7.5
21
0.82
(0.9)
1.92
(0.4)
0.67
(1.0)
2.90
(0.2)
1.51
(0.5)
1.84
(0.4)
0.41
(1.7)
1.02
(0.7)
0.77
(0.9)
0.57
(1.2)
1.18
(0.6)
1.59
(0.4)
0.27
(2.6)
0.22
(3.2)
0.56
(1.2)
0.48
(1.4)
1.44
(0.5)
1.03
(0.7)
Methylcyclohexane
Toluene
Diethyl
ether
Tetrahydrofuran
Acetone
38
Dimethylformamide
a
At 25 ◦C.
b
The relaxation half-life of the merocyanines.

58
D.L. Watkins, T. Fujiwara / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 51–59
O
5
4
3
2
1
0
Debald-SNOD (2)
Deba-SNOD (3)
Debald-SNOD
SPOD
N
N
N
N
N
O
HO
HO
O
Deba-SNOD
SPOD
N
O
O
N
N
N
N
O
O
N
N
1
10
100
log (ε)
Fig. 8. Solvent dependency of thermal closing rate of Debald-SNOD (diamond), Deba-SNOD (square), and SPOD (triangle) [9] upon solvent dielectric constant.
Fig. 9. Cycles of UV irradiation of Debald-SNOD ([2] = 1 × 10−4 M) at 25 ◦C followed by thermal closing; absorption maxima for the MC isomers at 581 nm in methylcyclohexane
(higher absorbance) and 621 nm in DMF (lower absorbance) were plotted.
For the spiro-naphtho-oxazine dimers in this study, the removal
of the ring on the oxazine portion of phenanthrene (SPOD) to naph-
thalene (SNOD) changes the polarity of the molecule itself. Though
the linker can exert a substituent effect which increases the elec-
tronic charge on the indoline nitrogen, the electron density at the
oxazine oxygen is reduced. The loss of electron density upon the
rotation for closing makes the transition state of the cyclization
reaction less polar, which decreases the polarity gap with either the
SP or MC forms. This similarity in compound polarity through pho-
tocyclization reaction is a plausible explanation to make the closing
rate of SNOD significantly less sensitive to the solvent polarity as
Fig. 8 shows.
relatively polar MC forms such as isomer of spiropyrans [15,21].
Except in this particular case, the results of cyclability tests in
various solvents indicated no appreciable fatigue or irreversibil-
ity under the experimental conditions over 20 min. Deba-SNOD(3)
also showed excellent cyclability.
4. Conclusions
The photochromic reactions of two spironaphthooxazine
dimers were studied under continuous UV irradiation in sol-
vents of different polarity. The investigation examined the effects
of solvent and structure on the absorption properties and clos-
ing rate kinetics of the merocyanine. The structural difference
of these dimers influenced the activation and deactivation pro-
cesses of two moieties. The dimer with complete conjugation
through the two spirooxazine units showed faster opening and
closing of the second unit compared to the dimer whose conju-
gation was interrupted between two units as we hypothesized.
A bathochromic shift and plotting of the absorbance of the pho-
tomerocyanine forms of both dimers against empirical Brooker’s
parameters confirmed a quinoidal merocyanine structure. The rate
of cyclization of MC form of the dimers showed very little depen-
dence upon solvent polarity unlike that of the previously examined
spirophenanthrooxazine dimer. In summary, spironaphthooxazine
dimers prepared in this study exhibited an excellent response
to photo induction, reversible isomerization, and high fatigue
3.4. Fatigue resistance
To investigate the resistance of SNOD series to photodegra-
dation, solutions of each compound in varying solvents were
alternately UV-irradiated and left in the dark. Plots of the
absorption maxima of the merocyanine of Debald-SNOD(2) in
methylcyclohexane and dimethylformide versus time subjected
to an off and on manner of irradiation are shown in Fig. 9.
Debald-SNOD in methylcyclohexane exhibited small decrease of
MC absorbance over the long, repeated irradiation. In this highly
non-polar solvent, precipitation was observed after extended UV
exposure, which could be explained as a result of aggregation of the
planar MC isomers by ␲–␲ stacking. This is typical phenomenon for

D.L. Watkins, T. Fujiwara / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 51–59
59
resistance which are deemed valuable traits for practical applica-
tion. Further modification of SNOD molecules such as conjugation
on substrate surfaces or to polymers, and investigation of binding
interaction of SNOD with guest compounds are underway.
Journal of Photochemistry and Photobiology A: Chemistry 189 (2007) 224–231.
[11] J. Berthet, S. Delbaere, V. Lokshin, C. Bochu, A. Samat, R. Guglielmetti, G.
Vermeersch, Studies of polyphotochromic behaviour of supermolecules by
NMR spectroscopy. Part 1.
A bis-spirooxazine with a (Z)-ethenic bridge
between each moiety, Photochemical & Photobiological Sciences 1 (2002)
333–339.
[12] J. Berthet, S. Delbaere, D. Levi, P. Brun, R. Guglielmetti, G. Vermeersch,
Investigations by NMR spectroscopy of a polyphotochromic system involv-
Acknowledgement
ing two entities, spirooxazine and naphthopyran, linked by
bridge, Journal of the Chemical Society, Perkin Transactions
2118–2124.
a
Z-ethenic
(2002)
2
This work was partly supported by Petroleum Research Fund
#46386-GB7.
[13] J. Berthet, J.-C. Micheau, A. Metelitsa, G. Vermeersch, S. Delbaere, Multistep
thermal relaxation of photoisomers in polyphotochromic molecules, The Jour-
nal of Physical Chemistry A 108 (2004) 10934–10940.
[14] F. Maurel, J. Aubard, P. Millie, J.P. Dognon, M. Rajzmann, R. Guglielmetti,
A. Samat, Quantum chemical study of the photocoloration reaction in
the napthoxazine series, The Journal of Physical Chemistry A 110 (2006)
4759–4771.
[15] N.A. Voloshin, A.V. Metelitsa, J.C. Micheau, E.N. Voloshina, S.O. Besugliy, N.E.
Shelepin, V.I. Minkin, V.V. Tkachev, B.B. Safoklov, S.M. Aldoshin, Spiropyrans
and spirooxazines. 2. Synthesis, structures, and photochromic properties of 6-
cyano-substituted spironaphthooxazines, Russian Chemical Bulletin 52 (2003)
2038–2047.
[16] S. Minkovska, B. Jeliazkova, E. Borisova, L. Avramov, T. Deligeorgiev, Substituent
and solvent effect on the photochromic properties of a series of spiroindolinon-
aphthooxazines, Journal of Photochemistry and Photobiology A: Chemistry 163
(2004) 121–126.
[17] N. Tamai, H. Masuhara, Femtosecond transient absorption spectroscopy of
a spirooxazine photochromic reaction, Chemical Physics Letters 191 (1992)
189–194.
[18] V.A. Lokshin, A. Samat, A.V. Metelitsa, Spirooxazines: synthesis, structure,
spectral and photochromic properties, Russian Chemical Reviews 71 (2002)
893–916.
[19] F. Ortica, D. Levi, P. Brun, R. Guglielmetti, U. Mazzucato, G. Favaro, Pho-
tokinetic behaviour of biphotochromic supramolecular systems. Part 2. A
bis-benzo-[2H]-chromene and a spirooxazine–chromene with a (Z-)ethenic
bridge between each moiety, Journal of Photochemistry and Photobiology A:
Chemistry 139 (2001) 133–141.
[20] L.G.S. Brooker, A.C. Craig, D.W. Heseltin, P.W. Jenkins, L.L. Lincoln, Color and
constitution. 13. Merocyanines as solvent property indicators, Journal of the
American Chemical Society 87 (1965) 2443.
References
[1] B.L. Feringa, R.A. van Delden, N. Koumura, E.M. Geertsema, Chiroptical molec-
ular switches, Chemical Reviews 100 (2000) 1789–1816.
[2] E.B. Gaeva, V. Pimienta, S. Delbaere, A.V. Metelitsa, N.A. Voloshin, V.I. Minkin,
G. Vermeersch, J.C. Micheau, Spectral and kinetic properties of a red-blue
pH-sensitive photochromic spirooxazine, Journal of Photochemistry and Pho-
tobiology A: Chemistry 191 (2007) 114–121.
[3] A.K. Chibisov, H. Gorner, Photoprocesses in spirooxazines and their merocya-
nines, The Journal of Physical Chemistry A 103 (1999) 5211–5216.
[4] V.S. Marevtsev, N.L. Zaichenko, Peculiarities of photochromic behaviour of
spiropyrans and spirooxazines, Journal of Photochemistry and Photobiology
A: Chemistry 104 (1997) 197–202.
[5] G. Berkovic, V. Krongauz, V. Weiss, Spiropyrans, spirooxazines for memories
and switches, Chemical Reviews 100 (2000) 1741–1754.
[6] G. Favaro, D. Levi, F. Ortica, A. Samat, R. Guglielmetti, U. Mazzucato, Photokinetic
behaviour of bi-photochromic supramolecular systems. Part 3. Compounds
with chromene and spirooxazine units linked through ethane, ester and acety-
lene bridges, Journal of Photochemistry and Photobiology A: Chemistry 149
(2002) 91–100.
[7] F. Ortica, D. Levi, P. Brun, R. Guglielmetti, U. Mazzucato, G. Favaro, Pho-
tokinetic behaviour of biphotochromic supramolecular systems. Part 1. A
bis-spirooxazine with a (Z)ethenic bridge between each moiety, Journal of
Photochemistry and Photobiology A: Chemistry 138 (2001) 123–128.
[8] A. Samat, V. Lokshin, K. Chamontin, D. Levi, G. Pepe, R. Guglielmetti, Syn-
thesis and unexpected photochemical behaviour of biphotochromic systems
involving spirooxazines and naphthopyrans linked by an ethylenic bridge,
Tetrahedron 57 (2001) 7349–7359.
[21] K. Fukushima, A.J. Vandenbos, T. Fujiwara, Spiropyran dimer toward
photo-switchable molecular machine, Chemistry of Materials 19 (2007)
644–646.
[9] S. Kumar, D.L. Watkins, T. Fujiwara, A tailored spirooxazine dimer as a photo-
switchable binding tool, Chemical Communications (2009) 4369–4371.
[10] K.M. Louis, T. Kahan, D. Morley, N. Peti, R.S. Murphy, Photochromism of spiroox-
azines with elements of lipid complementarity in solution and liposomes,