The photophysical behavior of the photochromic naphthopyran derivative having photo-switching ability

Article abstract of DOI:10.1016/j.saa.2014.10.016

(Graph Presented) The photophysical behavior of the photochromic naphthopyran derivative 9 with photo-switching ability has been investigated in solution phase as well as in crystalline form. The proposed switching motifs might be a versatile framework in

Full text of DOI:10.1016/j.saa.2014.10.016

Accepted Manuscript
The photophysical behavior of the photochromic naphthopyran derivative hav-
ing photo-switching ability
Moon-Hwan kim, Muhammad Saleem, Ji-Su Seo, Chang-Shik Choi, Ki Hwan
Lee
PII:
S1386-1425(14)01512-1
http://dx.doi.org/10.1016/j.saa.2014.10.016
SAA 12830
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
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Revised Date:
Accepted Date:
4 July 2014
6 October 2014
8 October 2014
Please cite this article as: M-H. kim, M. Saleem, J-S. Seo, C-S. Choi, K.H. Lee, The photophysical behavior of the
photochromic naphthopyran derivative having photo-switching ability, Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.10.016
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The photophysical behaviorof the photochromic naphthopyran
derivative having photo-switching ability
Moon-Hwan kima, Muhammad Saleemb, Ji-Su Seoa, Chang-Shik Choic, Ki Hwan Leeb,*
aBiomaterial Research Center, Korea Research Institute of Chemical Technology, PO Box 107, Yuseong, Daejon
305-600, Republic of Korea
bDepartment of Chemistry, Kongju National University, Gongju, Chungnam 314-701, Republic of Korea
cDepartment of Oriental Medicine Fermentation, Far East University, Eumseong, Chungbuk 369-851, Republic of
Korea.
*Corresponding author. Fax: +82-41-856-8613; E-mail address: khlee@kongju.ac.kr
Abstract
The photophysical behavior of the photochromic naphthopyran derivative 9 with photo-switching ability has been
investigated in solution phase as well as in crystalline form. The proposed switching motifs might be a versatile
framework in the development of photonic devices whose properties can be toggled between two states on exposure
to electromagnetic radiations. The closed form of 9 undergoes reversible transformation upon irradiation with light
by changing the molecular structure from ring closed conformation 9 to the ring opened form 10. The compound 9
exhibited first absorption signal at 238 nm with shoulder peak at 261 nm and second absorption signal at 353 nm
while on photo-irradiation, the colorless solution of 9 turn to colored and there was ratiometric downfall in the signal
intensity for shoulder signal at 261 nm as well as second absorption signal at 353 nm and ultimately results in the
disappearance of these both signal after 20 min of irradiations. Meanwhile, the colorimetric change in the reaction
solution and considerable shift in the absorption and emission signal intensity as well as position suggest the
conformational changes in the molecules from less conjugate conformation 9 to the extended conjugated
conformation 10.
Key words: photochromic; naphthopyran; photo-switching; reversible transformation.
Introduction
Photochromism can be defined as a light induced reversible photoisomerization process in organic molecules
between two isomers having different absorption spectra [1,2]. Photochromic molecules undergo a reversible color
change, which originates as a results of shift in optical absorption due to changes in molecular structure or
conformation in respond to electromagnetic radiation attracts considerable attention from various fields of
chemistry, physics and material science as potential candidates for practical applications [3-7]. The photochromism
describes a change in the optical properties of a material in response to illumination proclaim growing interest in
fundamental academic research as photo-responsive materials with controllable photophysical characteristics by
adjusting photoirradiation flux. Alongside the rapid development of modern science and technologies, materials for
fast and high density data storage are in high demand. The photochromic compounds exhibits emerging applications
as elements of field effect transistors, optical memory devices, light emitting diodes, photovoltaic appliance, smart
window, ophthalmic glasses, holographic gratings, logic accessories, drug delivery vesicles, organic semiconductor,
high resolution fluorescence imaging and fluorescent sensing materials [8-18]. The photochromic molecules exhibit
shape-memory properties through temporary shape change and back recovery to their permanent shape on exposure
to the external stimuli [19,20]. The investigation of these molecules may lead to novel photoresponsive materials for
photonic applications [21]. The incorporation of photoswitches has been benevolence to researchers in the material
and biological sciences offers the conversion of external inputs to useful signals, transistor generation and
optoelectronic building blocks in molecular electronics [22-24].

In the past decade, substantial attention in the field of organic photochromic materials has been focused because of
their rapid and often instant change of physical and electronic properties such as color, luminescence, conductivity,
refractive index which exhibit fascinating applications as rewritable optical memory media, photo-optical switches
because of tunable optoelectronic features using external stimuli, such as light, temperature and electricity. Charge
transport in organic molecules is actively explored toward the realization of novel electronic circuits and
computational devices. Among various types of photochromic dyes, the naphthopyran derivatives are the most
promising molecules for electronic circuit applications due to their excellent ring-closing and ring opening isomeric
transformation upon light irradiation with adherent superiority of reversibility, fine control over fading kinetics and
high fatigue resistance. These switches are particularly valuable due to tunable properties triggered by light, the
most widely available, non-invasive, and environmentally benign external stimulus [25-31].
On the basis of these findings, the design and development of new photochromic molecules with photocontrolable
fluorescence characteristics in both solution and solid state is demanding in academic, architecture, automotive
industries, cosmetics, textiles and decoration. Herein, we report the photophysical behaviors of naphthopyran
derivative methyl-9-(dimethylamino)-2,2-bis(4-methoxyphenyl)-2H-benzo[h]chromene-5-carboxylate (9) having
photo-switching ability. From single crystal X-ray diffraction analysis, it was confirmed that the compound 9
predominantly exist in ring closed conformation 9 and electromagnetic radiation transform the molecule into ring
opened conformation 10 which revert back to the original structure in dark. The interesting photophysical properties
both in solid and solutions might be useful for practical application in optoelectronics, photochromic eyeglass
lenses, switchable color window glasses, fluorescence sign board material and photonics.
Experimental
Substrate and reagents
All the chemicals and reagents were purchased from Aldrich, Samchun Chemicals and Jin Chemical &
Pharmaceutical Company. Briefly, ethanol, methanol, chloroform, water, acetonitrile, dimethyl sulfoxide, diethyl
ether, ethyl acetate, n-hexane, toluene and 1,4-dioxane (Samchun Chemicals, Korea), H2SO4, HCl (Jin Chemical &
Pharmaceutical Co. Ltd., Korea) were used in these experiments.
Measurements and instrumentations
The reaction progress was monitored by thin layer chromatographic (TLC) analysis and Rf values were determined
by employing pre–coated silica gel aluminium plates, Kieselgel 60 F254 from Merck (Germany). Melting points were
determined on Fisher scientific (USA) melting point apparatus and are uncorrected. The UV lamp (100 W) was used
with photo-intensity 9.0 mW/cm2 having 360 nm for irradiation. The absorption spectra were measured on a Hitachi
U-3300 spectrophotometer. The fluorescence spectra were measured on a SLM8100 spectrofluorometer (Aminco,
USA) with a Xe-arc lamp light source using 4 or 8 nm band pass excitation and emission monochromators, in which
the rhodamine B solution was used as a reference to correct for variation of the Xe light source with time and
wavelength. All spectroscopic measurements were carried out in the presence of 1 x 10-5 Mdm-3 of 9. The
thermogravimetric analysis (TGA) was performed by using SDT Q600 (V20.9 Build 20) simultaneous TGA/DSC
instrument. Proton nuclear magnetic resonance spectra were recorded on a Bruker Avance 400 MHz spectrometer
with tetramethylsilane (TMS) as an internal standard. The chemical shifts are reported as sigma values in ppm
downfield from TMS of the indicated organic solution. Peak multiplicities are expressed as follows: s, singlet; d,
doublet, dd, doublet of doublets; q, quartet and m, multiplet. Single crystal data was obtained by using Bruker
SMART APEX II X–ray Diffractometer CCD equipped with MO X–ray tube and graphite monochromator at the
ambient temperature. Mass spectra were recorded on the JEOL MStation JMS–700 (USA). The schematic
representations of synthetic route adopted to obtain 9 are shown in Scheme 1.

Scheme 1: Synthesis of 9: Reagents and conditions: (i) Na, EtOH, reflux, N2 gas; (ii) Anh. Sodium acetate, acetic
anhydride; (iii) NaOH, EtOH; (iv) Conc. H2SO4, MeOH; (v) 18 W % sodium acetylide/Xylene, anh. THF; (vi)
Al2O3, toluene, reflux.
Synthesis of (E)-4-[4-(dimethylamino)phenyl]-3-(ethoxycarbonyl)but-3-enoic acid (3)
N,N-Dimethyl amino benzaldehyde (100 g, 0.67 mL, 1 eq) and diethyl succinate (140 g, 1.2 eq) were completely
dissolved in 500 mL anhydrous ethanol at 40-50 °C. Sodium (18.5 g, 1.2 eq) was separately dissolved in anhydrous
ethanol and added slowly in the reaction vessel with the help of dropping funnel keeping low temperature. The quick
addition may generate solid in the reaction mixture which hinder stirring. The reaction mixture was heated under
reflux at 80-90 °C after complete addition into the reaction mixture followed by constant monitoring the reaction
progress with the help of TLC. On complete consumption of starting materials, the reaction mixture was cooled,
removed ethanol on reduced pressure to left behind high viscous red color fluid. The organic compound was
extracted by using solvent extraction (ethyl acetate: H2O = 1:1, 500 mL). The pH was set at 5.5 using 2 N HCl and
organic layer was extracted twice and dehydrated by putting MgSO4 followed by evaporation of excess solvent
1
under reduced pressure leave behind 3 with high yield (157 g, 85 %). The structure of 3 was characterized by H
NMR analysis and 1H NMR data are given below
1H NMR (CDCl3, ppm): δ 1.23 (t, 3H), 2.94 (s, 6H), 3.45 (s, 3H), 4.15 (q, 2H), 6.73 (d, 2H), 7.29 (d, 2H), 7.63 (s,
1H), 12.20 (s, 1H).
Synthesis of ethyl 4-acetoxy-6-(dimethylamino)-2-naphthoate (4)
Compound 3 (100 g, 0.36 mol, 1 eq) and anhydrous sodium acetate (29.5 g, 1 eq) were added in acetic anhydride
(500 mL) and heated under reflux for 4 h. On completion, the reaction mixture was cooled to room temperature and
poured in 3 L of cold water which afford brown color solid. The solid product was filter and removes the moisture
on vacuum dry oven for one day.
1H NMR (CDCl3, ppm): δ 1.42 (t, 3H), 2.47 (s, 3H), 3.06 (s, 6H), 4.40 (q, 2H), 6.73 (s, 1H), 7.15 (d, 1H), 7.71 (s,
1H), 7.80 (d, 1H), 8.35 (s, 1H).
Synthesis of 6-(dimethylamino)-4-hydroxy-2-naphthoic acid (5)
Compound 4 (100 g, 0.33 mol, 1 eq) was dissolved in 200 mL ethanol and sodium hydroxide (80 g, 6 eq) was
separately dissolved in 1 L water. Both solution was mixed and stirred for 2 h. at 80-90 °C. After that ethanol layer
was removed on reduced pressure and set the pH 6 of reaction mixture by using 2 N HCl and extracted organic layer

using ethyl acetate, dried with MgSO4, finally removed the excess solvent on rotary which left behind deep brown
product, yield (70 g, 85 %).
1H NMR (CDCl3, ppm): δ 3.32 (s, 6H), 7.07 (s, 1H), 7.23 (s, 1H), 7.87 (s, 1H), 10.00 (s, 1H).
Synthesis of methyl 6-(dimethylamino)-4-hydroxy-2-naphthoate (6)
Compound 5 (100 g, 0.43 mol, 1 eq) dissolved in 2 L methanol and slowly added 25 mL concentrated H2SO4 inside
the reaction mixture and reflux for 5 h. After that methanol was evaporated on rotary and reaction mixture was
neutralized using NaOH, extracted with ethyl acetate and water (due to low solubility product, ethyl acetate and
water ratio was kept 1.5:1), dried the ethyl acetate layer with MgSO4 evaporated the excess solvent on rotary left
behind solid product of 6, yield (85 g, 80 %).
1H NMR (CDCl3, ppm): δ 3.13 (s, 6H), 3.94 (s, 3H), 6.05 (s, 1H), 7.15 (s, 1H), 7.20 (d, 1H), 7.75 (s, 1H), 7.75 (d,
1H), 8.08 (s, 1H).
Synthesis of 1,1-bis(4-methoxyphenyl)prop-2-yn-1-ol (8)
18 W % sodium acetylide in xylene/mineral oil 121.1 mL (0.45 mol, 1 eq) was dissolved in anhydrous THF (1L)
saturated with acetylene gas. After that 4,4-dimethoxybenzophenone 100 g (0.41 mol, 1 eq) was put inside the
reaction mixture under nitrogen and stirred for 24 h. at room temperature. The product was extracted by solvent
extraction using ethyl acetate and water, organic layer was dried with MgSO4 and solid product on vacuum drier.
After complete drying, the product again dissolve in ethyl acetate and poured into 1 L hexane slowly under stirring
which afford 8 as white color solid, yield (99 g, 90 %).
1H NMR (CDCl3, ppm): δ 2.70 (s, 1H), 2.85 (s, 1H), 3.78 (s, 6H), 6.85 (d, 4H), 7.48 (d, 4H).
Synthesis of methyl 9-(dimethylamino)-2,2-bis(4-methoxyphenyl)-2H-benzo [h] chromene-5-carboxylate (9)
Compound 6 (100 g, 0.41 mol, 1 eq), compound 8 (87 g, 0.8 eq) and Al2O3 (415 g, 10 eq) was put inside the reactor
(connected with Dean-Stock device) containing toluene (3 L). The reaction mixture was deoxygenized with the
stream of nitrogen gas followed by refluxing the reaction mixture. On completion, the reaction mixture was cooled
to room temperature, washed with NaOH solution and removed the toluene on rotary. After that solid product was
mixed with ethyl acetate: water and organic layer was separated, removed excess solvent on rotary to afford 9 which
was purified by column chromatography using hexane: ethyl acetate: toluene (5:1:1) solvent system. Fine yellow
crystals of 9 were collected on mixing it with ethyl acetate [32-38].
1H NMR (CDCl3, ppm): δ 3.09 (s, 6H), 3.76 (s, 6H), 3.9 (s, 3H), 6.09 (d, 1H), 6.79 (d, 4H), 7.05 (dd, 1H), 7.25 (d,
1H), 7.35 (d, 4H), 7.60 (dd, 2H), 7.95 (s, 1H).
Crystal structure determination of compound 9
In order to get better insight about the predominant conformation of compound 9, the fine crystal of 9 was grown on
ethyl acetate and characterized by single crystal X–ray diffraction analysis. From the single crystal obtained was
confirmed that molecule 9 predominantly exist in ring closed conformation (Fig. 1), the closed form shows that both
aromatic ring and naphthopyran moieties of the molecule consist of orthogonal nonplanar structure. Thus, the closed
form has a stacking rearrangement with the adjacent phenyl rings [39].

Fig. 1: X–Ray crystal structure of 9.
Crystal data
The crystallographic data obtained for compound 9 are shown in Table 1
Table 1. Crystallographic data for compound 9
Description
Empirical formula
Experimental values
C31H29NO5
495.55
Description
Experimental values
112.334 (1)°
β
Formula weight
93.484 (1)°
γ
Volume
3
Temperature
100 K
1211.85 (3) Å
Appearance
Crystal system
Pale yellow
Triclinic
z
2
3
Density (calculated)
Absorption coefficient
F(000)
1.358 Mg/m
-1
Space group
P1
0.09 mm
a
b
c
9.8923 (1) Å
10.9535 (1) Å
12.1720 (2) Å
93.860 (1)°
524
3
Crystal size
Theta range for data collection
Refinement method
0.20 x 0.16 x 0.16 mm
2.2–28.4°
2
α
Full-matrix least-squares on F
Structure solution and refinement
Data collection: APEX II (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker,
2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure:
SHELXL97 (Sheldrick, 2008).
Results and discussions
Photoisomerization mechanism
The schematic representation for the photoisomerization mechanism of 9 are shown in Fig. 2.The compound 9 in
solution form was almost colorless in dark while it turns to color on photoirradiation with considerable enhancement
in the fluorescence emission intensity suggesting the photoisomerization of less conjugated conformation 9 into
extended conjugated conformation 10. As shown in Fig. 2, there was π electronic delocalization up to higher extent

in the ring opened conformation 10 in comparison to the ring closed conformation 9. Moreover, the electron
withdrawing methoxy groups attached to para position of phenyl ring in the core structure of 9 facilitate the
photoisomerization from less conjugated conformation 9 into extended conjugated conformation 10. As soon as
molecule was put in dark condition, it reverts back to the more stable ring closed conformation while the ring
opening process can only be triggered by external stimuli (light in this case).
OCH3
OCH3
O
OCH3
OCH3
O
hv
O
dark
O
OCH3
N
OCH3
N
Ring open conformation
9
10
Ring closed conformation
Fig. 2: The photoisomerization mechanism of 9. The left side snapshot of reaction solution was taken in day light
while right side image was obtained under UV-lamp by illuminating at 365 nm.
Spectroscopic properties
The spectroscopic properties of 9 were performed in 1,4-dioxane at 25 °C and the corresponding spectra are shown
in Fig. 3. The solution of closed form of compound 9 was colorless as shown by inset of Fig. 3 and it exhibited first
absorption signal at 238 nm with shoulder peak at 261 nm and second absorption signal at 353 nm with considerable
molar absorption coefficient values of 6.3×104 M-1cm-1, 3.7×104 M-1cm-1 and 1.3×104 M-1cm-1, respectively.
Interestingly, on photo-irradiation, the solution color of 9 turn to colored and there was ratiometric downfall in the
signal intensity for shoulder signal at 261 nm as well as second absorption signal at 353 nm and ultimately results in
the disappearance of these both signal after 20 min of irradiations. The change in color of solution from colorless to
color is the indication of structural changes in the target molecule 9 from less conjugated conformational structure to
the extensive conjugated conformational structural 10 as the compound 9 have fragile carbon to oxygen ether
linkage in the core skeleton and photo-irradiation cause breakage of ether linkage with subsequent transformation
into new conformation 10 with emergence of carbonyl functionality as well as new double bond which facilitate the
delocalization of electronic cloud from one side of the molecule to the other side.

o
Fig. 3: Electronic absorption spectrum of compound 9 before and after irradiation in 1,4-dioxane at 25 C on
periodic exposure to xenon lamp after regular interval of 1 min; inset snapshot represent the visual color change of
compound 9 solution on photo-irradiation with xenon lamp.
When the open form sample put in the dark room, it returned to closed form. In the case of closed form 9, there was
a little emission at 422 nm in 1,4-dioxane at ambient temperature. However, a remarkable emission appeared at 440
nm when the ring closed form 9 changed to the ring open form 10 upon irradiation with ILC PS300-1 Xenon lamp.
In particular, there was constant increment in the fluorescence emission intensity with gradual bathochromic shift by
increasing the irradiation time from 0 min to 20 min. The bathochromic shift in the fluorescence spectrum of 9
reflect the decrease in energy gape between HOMO to LUMO transition harvesting low energy emission (longer
wavelength) spectra due to ring opened conformation 10 triggered by photo irradiation as shown in Fig. 4 while
emission intensity was found to decreased gradually by keeping the sample solution in dark room.
Fig. 4: Fluorescence emission spectra of 9 (1×10-5 molL-1) in 1,4-dioxane at 25 °C; (a) before irradiation and (b)
after irradiation with ILC PS300-1 Xenon lamp on different time interval from 0-20 min.
Solid state photochromism
In the past decade, a number of photochromic molecules have been proposed to improve their photochromic
properties such as fatigue resistance, reversibility and stability. Unfortunately, most photochromic dyes do not show
photoisomerization in the crystalline state and the photochromic properties have been studied in the solution phase.
On this basis, the design and development of photochromic molecules capable of undergoing photochromism in
different states of the matter is a wide topic. The present research was conducted to develop molecules with
interesting photochromic properties in solution phase and in bulk solid state. From a practical viewpoint, the solid-
state photochromism exhibit prospective potential applications, therefore, optical switches based on materials in
solid state and even in single crystalline state is indispensable and first important steps for the fundamental studies
concerning development of photoelectronic systems for designing high efficient optical memories, solid-state
photonic switches, information storage, molecular machines, recording digital information, nonlinear optics and
photochromic ophthalmic lenses [40-46]. Keeping in mind the above findings, beside photochromic behavior
investigation in solution phase, we have further investigated the photoluminescence measurement of compound 9 in
solid state and the results are shown in Fig. 5. The compound 9 showed a characteristic photoluminescence emission
band at 467 nm and this photoluminescence emerged by the transformation of ring closed conformation 9 to the ring
opened conformation 10 triggered by the light source of the spectrophotometer. The delightful photoluminescence
properties of the target compound might be utilized for practical development of photonic materials and
photoswitchable accessories.

Fig. 5: Solid state photoluminescence emission spectrum of the target compound at 25 °C; Inset photograph showed
the visual appearance of 9 in day light (inset, left side photograph) and under UV-lamp by illuminating at longer
wavelength of 365 nm (inset, right side photograph).
Fluorescence life time measurement
The measurement of fluorescence life time for the target compound was performed with TRPL method using iHR
320 imaging spectrometer (iHR series spectrometer, Horiba scientific, Japan). The fluorescence decay of open form
was found to be practically mono-exponential, which confirmed that this compound possessed only one fluorescent
species with a single emission process. As can be seen in Fig. 6, the fluorescence decay curve of open form was
clearly mono-exponential with τF value of 0.673 ns. This decay curve was due to open conformation of the target
compound and the conformational changes from ring closed conformation 9 to the ring opened conformation 10 was
triggered by the light source of the spectrophotometer. Thus, this result showed a clear correlation between the
fluorescence lifetime and electronic absorption, demonstrating that there exists only one open form having a very
short fluorescence lifetime.
Fig. 6: The photoluminescence decay curve for the compound 10 deduced from the exponential decay of the TRPL
signal.

Solvatochromism study
To evaluate the effect of solvent polarities on the photochromic behavior of compound 9, the fluorescent spectral
measurements was carried out in various solvents of different polarities by photoirradiating the sample for 2 min
followed by subsequent recording of emission spectra and the results obtained are tabulated in Table 2. Correlation
of the fluorescence emission response of compound 9 with different solvents revealed that there was slight
bathochromic shift in the emission spectrum by increasing solvent polarity. The solvent with lower values of π*
stabilized the energy level of the target molecule, hence their emission spectra appear at shorter wavelength as the
stabilized energy level need relatively higher energy for electronic transition. Meanwhile, the fluorescence intensity
partially depending on the β values of the indicated solvents. There was successive increment in the fluorescence
intensity for the polar aprotic solvent while there was slight downfall in the fluorescence intensity in case of the
polar protic solvent due to involvement in the hydrogen bonding.
Table 2: Effect of solvent polarities on the photophysical behavior of compound 9
S. No.
Solvent
Diethyl ether
Cyclohexane
Toluene
1,4-Dioxane
Acetonitrile
Water
aλmax (nm)
420
bF/Fₒ
2.23
2.18
2.27
2.71
3.02
2.43
cπ* [47]
0.27
0
0.54
0.55
0.75
1.09
dβ [47]
0.47
0
0.11
0.37
0.31
0.18
1
2
3
4
5
6
428
434
440
443
453
aFluorescence emission maxima.
bF/Fₒ is determined as a ratio between the maximum fluorescence intensity (F, after 2 min irradiation of sample) and
minimum fluorescence intensity (Fₒ, without photoirradiation).
cIndex of solvent polarity-polarizability.
dParameter related to the solvent's ability to accept a proton in a solute-solvent hydrogen bonding.
Thermogravimetric analysis
The thermal stability of fluorescent organic materials is prerequisite for the durability, lifetime and efficiency of
photochromic materials. Degradation of fluorescent amorphous organic materials may reduce the broad spectrum
applications of the photoswitching molecules. To assess the thermal stability of 9, thermogravimetric analysis
(TGA) was performed by flowing dry nitrogen atmosphere at the heating rate of 10 °C/min using 2.4 mg sample in
the temperature range of 25–700 °C. The compound 9 shows appreciable thermal stability up to 285 °C without any
weight loss, afterward, there was continuous weight loss and molecules loses its 80 % of weight at 400 °C and it was
not completely decomposed up to 700 °C (Fig. 7). Due to high thermal stability, the compound 9 can be used as
dopant in the manufacturing of photochromic window glasses, photochromic car windows and sign board material
where it meet the requirement of sustaining high temperature without thermal decomposition.

Fig. 7: Thermogravimetric analysis results for compounds 9.
Conclusion
The photophysical behavior of photochromic naphthopyran derivative 9 with photo-switching ability has been
investigated in solution phase as well as in crystalline form. The compound 9 exhibited photoswitching
characteristics assessed by spectrophotometric measurement. The target compound exhibited first absorption signal
at 238 nm with shoulder peak at 261 nm and second absorption signal at 353 nm with considerable molar absorption
coefficient values of 6.3×104 M-1cm-1, 3.7×104 M-1cm-1 and 1.3×104 M-1cm-1, respectively. Interestingly, on photo-
irradiation, the colorless solution of 9 turn to colored and there was ratiometric downfall in the signal intensity for
shoulder signal at 261 nm as well as second absorption signal at 353 nm and ultimately results in the disappearance
of these both signal after 20 min of irradiations. The change in color of solution from colorless to color is the
indication of structural changes in the target molecule 9 from less conjugated conformational structure to the
extensive conjugated conformational structural 10. Meanwhile, in the case of fluorescence emission spectral
measurement, there was a low intensity signal at 422 nm recorded in 1,4-dioxane. However, a remarkable emission
appeared at 440 nm when the ring closed form 9 changed to the ring open form 10 upon irradiation with ILC PS300-
1 Xenon lamp by turning the sample solution from colorless to the colored. Concurrently, in the crystalline form, the
compound 9 presents an intense and broad emission band at ꢀmax 467 nm. It indicates that open form is linked to the
naphthopyran part conjugated with the photochromic skeleton. Therefore, photochromic properties play an
important role for the extension of π-conjugation state. The solvatochromism study of the compound 9 in variety of
organic solvents with different polarities showed slight bathochromic shift on increasing solvent polarities suggest
that the excited state of 9 is more polar than the ground state. The fluorescence decay of open form was found to be
practically mono-exponential, which confirmed that this compound possessed only one fluorescent species with a
single emission process. The target compound exhibited considerable thermal stability up to 285 °C without any
weight loss assessed by thermogravimetric analysis.
Acknowledgments
This research was supported by the Ministry of Commerce, Industry and Energy (MOCIE-2005-10023441) and the
Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry
of Education (NRF- 2011-0015056).
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Highlights


The photophysical behavior of the photochromic naphthopyran derivative 9 has been investigated in
solution phase as well as in crystalline form.
The closed form of 9 undergoes reversible transformation upon irradiation with light by changing the
molecular structure from ring closed conformation 9 to the ring opened form 10.
The compound 9 exhibited an auspicious ratiometric fluorescence response on exposure to white light.
The bathochromic shift on increasing solvent polarities suggests that the excited state of 9 is more polar
than the ground state.



Graphical abstract
The photophysical behavior of the photochromic naphthopyran
derivative having photo-switching ability
Moon-Hwan kima, Muhammad Saleemb, Ji-Su Seoa, Chang-Shik Choic, Ki Hwan Leeb,*
aBiomaterial Research Center, Korea Research Institute of Chemical Technology, PO Box 107, Yuseong, Daejon
305-600, Republic of Korea
bDepartment of Chemistry, Kongju National University, Gongju, Chungnam 314-701, Republic of Korea
cDepartment of Oriental Medicine Fermentation, Far East University, Eumseong, Chungbuk 369-851, Republic of
Korea.
*Corresponding author. Fax: +82-41-856-8613; E-mail address: khlee@kongju.ac.kr
The photophysical behavior of the photochromic naphthopyran derivative has been investigated in solution phase as
well as in crystalline form.