Isomeric naphthalimides bearing pyran units: Insight into mutual relation between structure and photochromic properties

Article abstract of DOI:10.1016/j.jphotochem.2015.02.004

Two novel isomeric photochromic naphthopyrans (1 and 2) containing naphthalimide moieties were prepared and studied. In the compound 1, O-atom of pyran cycle is at C-3 position of naphthalene ring, whereas, in compound 2, O-atom of pyran cycle is at C-4 position. In the compound 2 due to para-position O-atom of pyran photochrome cycle is involved into the conjugated naphthalimide system. The variety in mutual position of pyran and naphthalimide units leads to remarkable difference in photochromic characteristics. Both compounds demonstrate the switching of the fluorescence by photoinduced conversion between the closed and open forms.

Full text of DOI:10.1016/j.jphotochem.2015.02.004

Accepted Manuscript
Title: Isomeric Naphthalimides Bearing Pyran Units: Insight
into Mutual Relation between Structure and Photochromic
Properties
Author: O.A. Fedorova A.N. Sergeeva P.A. Panchenko Yu. V.
Fedorov F.G. Erko J. Berthet S. Delbaere
PII:
S1010-6030(15)00043-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2015.02.004
JPC 9833
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
22-11-2014
2-2-2015
7-2-2015
Please cite this article as: O.A.Fedorova, A.N.Sergeeva, P.A.Panchenko,
Yu.V.Fedorov, F.G.Erko, J.Berthet, S.Delbaere, Isomeric Naphthalimides
Bearing Pyran Units: Insight into Mutual Relation between Structure and
Photochromic Properties, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2015.02.004
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Isomeric Naphthalimides Bearing Pyran Units: Insight into Mutual
Relation between Structure and Photochromic Properties
O. A. Fedorovaa, A. N. Sergeevaa, P. A. Panchenkoa, Yu. V. Fedorova, F. G. Erkob, J. Berthetb,
S. Delbaereb
aA. N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences,
119991, Vavilova str., 28, Moscow (Russia), tel. +74991359280, fax +74991357601,
e-mail: fedorova@ineos.ac.ru
b University of Lille, UDSL, CNRS UMR 8516, BP83, Lille, France
Graphical abstract
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Highlights
 Novel naphthopyrans bearing naphthalimide units were synthesized.
 The mutual position of pyran and naphthalimide units effect on the photochromism.
 Long-lived open form of photochrome is due to TT isomer is thermally stable.
 The fluorescence of naphthalimide is switched by photochromic conversion
1

Abstract
Two novel isomeric photochromic naphthopyrans (1 and 2) containing naphthalimide moieties
were prepared and studied. In the compound 1, O-atom of pyran cycle is at C-3 position of
naphthalene ring, whereas, in compound 2, O-atom of pyran cycle is at C-4 position. In the
compound 2 due to para-position O-atom of pyran photochrome cycle is involved into the
conjugated naphthalimide system. The variety in mutual position of pyran and naphthalimide
units leads to remarkable difference in photochromic characteristics. Both compounds
demonstrate the switching of the fluorescence by photoinduced conversion between the closed
and open forms.
Keywords: photoswitching fluorescence, 1,8-haphthalimide, naphthopyran, optical
spectroscopy, NMR spectroscopy
1. Introduction
Nowadays, much attention has been paid to the design, synthesis and working operation
of photochemical molecular devices. The development of a device concept to the molecular level
is of interest from the fundamental point of view as well as for the growth of supramolecular
chemistry, nanoscience and nanotechnology [1]. Molecular and supramolecular devices that
show large changes in on-off states are intensively developed as they can be modulated by
employing external stimuli such as ions, molecules, light, etc [2]. The switchable states of the
molecular-level devices refer to their luminescence, magnetic or electronic properties.
Luminescence is one of the most useful techniques to monitor the operation of molecular
devices [3]. A part of this rapidly emerging field is the development of fluorescent sensors, non-
destructive data storage, biological fluorescence labeling [4].
Photochromic molecules are able to perform binary states with different fluorescence
characteristics [5]. Regulating fluorescence intensity is one of the most attractive uses for
molecular photoswitches because of the high sensitivity, resolution, contrast and the fast
response provided by fluorescence technology. The realization of the photocontrolled
fluorophores requires the design and synthesis of the hybrid photochrome-fluorophore systems
where the ecxited state communication between two chromophores has to be effective. A
conventional approach to modulating the fluorescence relies on the integration of fluorescent dye
and photochromic unit in one molecular skeleton.
2

Keeping these considerations in mind, we designed and synthesized 3H-naphtho[2,1-
b]pyran (1) and 2H-naphtho[1,2-b]pyran (2) (Scheme 1) bearing naphthalimide units fused to
pyran moieties. Naphthalenic acid imides (1,8-naphthalimides) with electron donating
substituents attached to the naphthalene moiety are well-known fluorescent dyes high absorption
coefficients and high fluorescent quantum yields in the visible region. Such compounds have
been used in a large variety of areas, including coloration of polymers and optical brightening [6,
7], laser active media [8], electroluminescent materials [9], optical chemosensors [10] and
fluorescent markers in biology [11]. Naphthopyrans are known to exhibit excellent photochromic
responses, good colorability, and rapid bleaching [12], and are explored in sensors [13],
polymers [14], and organic gels [15].
Recently, naphthalimide units have been incorporated into photochromic spirooxazine,
diarylethene systems, especially realizing a significantly long lifetime in the open merocyanine
(MC) forms [16]. Two photochromic naphthopyrans containing naphthalimide moieties were
studied in solution under flash photolysis conditions, exhibiting predominant thermal
reversibility in color-fading and fluorescence switch [17].
The goals of the present investigations are to analyze (i) how the mutual positions of
pyran and naphthalimide units in photochromic fluorophore influences on the optical
characteristics of closed and open forms (CF and OF respectively); (ii) how the existence of
naphthalimide unit with strong electron-withdrawing imide group can somehow change the ratio
of transoid-trans (TT) and transoid-cis (TC), thus facilitating the thermal back to closed form
with a preferable color bleaching rate and thermal reversibility; and (iii) how the fluorescence of
naphthalimide unit can be switched on-off by photochromic conversion.
Scheme 1. Structures of closed and open forms of compounds 1 and 2.
3

2. Material and methods
Synthesis of 4- and 3-hydroxy-1,8-naphthalimides (Scheme 2) was carried out according to
earlier reports [18-22]. The following procedures describe the experimental details of preparation
of target compounds 1 and 2.
5,10,10-triphenylbenzo[de]pyrano[2,3-f]isoquinoline-4,6(5H,10H)-dione (1).
Method 1. The procedure was adopted from literature [18]. 3-Hydroxy-1,8-naphthalimide (1.00
g, 3.5 mmol) and 0.70 g (3.5 mmol) of 1,1-diphenylprop-2-yn-1-ol while heating to 50-55°C
were dissolved in 60 ml of toluene, and 0.06 g (0.35 mmol) of p-toluenesulfonic acid was added.
The reaction mixture was stirred at 50-60°C for 1 h and then at 70-80°C for 3-4 hours. The
solvent was evaporated. After the column chromatography (eluent PhH – EtOH, v/v = 10/1) 0.13
g of naphthopyran 1 was obtained as a yellow powder (yield was 8%).
Method 2. The procedure was adopted from literature [19]. 4-Hydroxy-1,8-naphthalimide (1.0 g,
3.5 mmol) and 0.8 g (3.5 mmol) of β-phenylcinnamaldehyde were dissolved in 60 ml of toluene,
and a solution of 1.2 g (5.2 mmol) of titanium (IV) tetraethoxide in 18 ml of toluene was added.
The mixture was stirred at 100˚C in Ar atmosphere for 6 h, cooled to 30-40˚C and 30 ml of
toluene, 0.5 g of silicagel and 1 ml of water were added to it. The resulting suspension was
stirred at 80˚C for 30 min. After cooling to ambient temperature, the precipitate was filtered off,
washed with dichloromethane several times, and the organic filtrate was evaporated. After the
column chromatography (eluent petroleum ether – ethyl acetate, v/v = 40/1) 0.08 g naphthopyran
1 was obtained as an oil (yield was 5%).
1H NMR (CD3CN, 500 MHz): δ 8.52 (d, J = 8.6 Hz, 1H), 8.42 (d, J = 7.3 Hz, 1H), 8.25 (s, 1H),
7.84 (dd, J = 8.6 and 7.3 Hz, 1H), 7.60-7.51 (m, 6H), 7.42 – 7.30 (m, 10H), 6.80 (d, J = 10.0 Hz,
1H).
13C NMR (CD3CN, 100 MHz): δ 164.0, 163.4, 152.2, 150.8, 144.1, 133.1, 129.1, 129.0, 128.9,
128.7, 128.5, 128.4, 128.3, 128.2, 127.8, 126.6, 124.5, 123.4, 121.9, 121.1, 119.3, 82.6 (22 of 33
carbons found).
Found (%): С, 82.68; Н, 4.47; N, 2.99. C33H21NO3. Calculated (%): С, 82.65; Н, 4.42; N, 2.92.
ESI-MASS, calculated, m/z: 479.52; found: 479.47 ([M+H]+).
5,10,10-triphenylbenzo[de]pyrano[2,3-g]isoquinoline-4,6(5H,10H)-dione (2) was prepared
according to the method 1 from 1.00 g (3.5 mmol) of naphthol 8, 0.70 g (3.5 mmol) of 1,1-
diphenylprop-2-yn-1-ol and 0.06 g (0.35 mmol) of p-toluenesulfonic acid. After the column
chromatography (eluent petroleum ether – ethyl acetate, v/v = 10/1) 0.15 g (10% yield) of
naphthopyran 2 was obtained as a yellow powder.
4

1H NMR (CDCl3, 400 MHz): δ 8.68 (d, J = 8.6 Hz, 1H), 8.60 (d, J = 7.3 Hz, 1H), 8.36 (s, 1H),
7.75 (dd, J = 8.6 and 7.3 Hz, 1H), 7.55 – 7.48 (m, 5H), 7.43 – 7.25 (m, 10H), 6.88 (d, J = 10.0
Hz, 1H,), 6.32 (d, J = 10.0 Hz, 1H).
13C NMR (CDCl3, 100 MHz): δ 164.5, 164.0, 153.6, 144.0, 136.0, 134.8, 133.2, 132.2, 131.0,
131.2, 130.0, 129.6, 129.2, 128.8, 128.4, 127.2, 126.8, 123.2, 123.0, 117.2, 115.6, 85.6 (22 of 33
carbons found).
Found (%): С, 82.71; Н, 4.51; N, 2.97. C33H21NO3. Calculated (%): С, 82.65; Н, 4.42; N, 2.92.
ESI-MASS, calculated, m/z: 479.52; found: 479.48 ([M+H]+).
Electronic absorption spectra were recorded on spectrophotometers Varian-Cary 5G and
Avantes AvaSpec-2048. Spectra of the colored forms were obtained when samples in the
spectrometer cell were simultaneously exposed to continuous irradiation, generated by Hg high
pressure lamp 120W equipped with optical filters (365 and 525 nm). To measure the bleaching
lifetime of the colored form 2 (OF2), sample was irradiated in the cuvette box of the
spectrophotometer. The irradiation was performed till the photostationary conditions
(equilibrium between closed and opened forms) were achieved. Then the irradiation was
interrupted and the kinetic curves corresponding to the recovery of the system to the initial
closed form were recorded. The decay kinetics was monitored at the absorption maximum of the
colored forms. To study the kinetics of the naphthopyran 1 photochromic transformation flash
photolysis was used. The probing impulse light from a Fil-105 camera flash passed through the
sample in a 1 cm quartz cell. Fluorescence spectra were recorded on spectrofluorimeters
FluoroLog-3-221 and AvaSpec-2048L. The fluorescence switch was measured using a diode
(365 nm). All spectral measurements were carried out in air-saturated solutions at 20 ± 1 °C. The
concentrations of studied compounds were of about 10-4 or 10-5 M.
LC-ESI-MS analyses were performed on a Finnigan LCQ Advantage mass spectrometer
equipped with octopole ion-trap mass-analyzer, MS Surveyor pump, Surveyor auto sampler,
Schmidlin-Lab nitrogen generator (Germany), and Finnigan X-Calibur 1.3 software for data
collecting and processing. Acetonitrile was used as the mobile phase. The isocratic elution was
maintained at a flow rate of 0.1 mL/min without column. The effluent from LC was passed
directly into the electrospray ion source without a split. Positive electrospray ionization was
achieved using an ionization voltage at 2.5 kV with temperature at 100°C.
NMR experiments were recorded on Bruker Avance 500 MHz spectrometer with a TXI
5mm probe. The experiments were carried out in toluene–d8. The concentration of solutions used
was 10-2 M. Temperature ranges used for NMR measurement and irradiation were 193K - 233 K
for 1 and 213K-233K for 2. All UV irradiations were done using a 1000W Xe-Hg high pressure
filtered short-arc lamp (Oriel), filtered by band–pass glass filter and interferential filter (365 nm).
5

The temperature of the sample was controlled with a variable temperature unit (B-VT 1000-
Bruker, 123-423K, T range).
3. Results and Discussion
3.1. Synthesis of the naphthalimides 1 and 2.
, The target compounds 1 and 2 were synthesized as illustrated in Scheme 2. The
synthesis of 3-hydroxy-1,8-naphthalimide included four steps, the order of synthetic operations
was imidation (d) - reduction (a) - diazonium salt formation (b) - hydrolysis (c) (Scheme 2). 4-
Hydroxy-1,8-naphthalimide was synthesized from the 4-nitro-1,8-naphthalic anhydride via
routine procedure which involved reduction of nitro group (a), formation of diazonium salt (b),
hydrolysis (c), and imidation (d) as it was described earlier [20-24]. 2H-Chromene derivative 1
preparation procedure is based on a ‘one-pot reaction’ starting from a 3-hydroxy-1,8-
naphthalimide. In our research, two different methods were used for obtaining of 1. Method 1
(step e in Scheme 2) involves the acid-catalyzed reaction of naphthol with propargylic alcohol
[18]. The reaction proceeds via Claisen rearrangement of alkynyl aryl ethers resulting from
naphthol ‘O-alkylation’, followed by [1,5]H sigmatropic shift and electrocyclic ring closure.
Another approach (method 2) suggests condensation of naphthol with β-phenylcinnamaldehyde
in the presence of titanium (IV) ethoxide ( step f in Scheme 2) [19]. So as the yields in methods 1
and 2 are comparable, for the synthesis of 2 only method 1 was applied. The chemical structures
and composition of compounds were well confirmed by 1H-NMR, 13C-NMR, elemental analysis
data and ESI-MS (see the Experimental Section and Supporting Information (SI) for details).
Scheme 2. Synthetic routes to the naphthopyrans 1 and 2. Reagents and conditions: (a)
SnCl2∙2H2O, HCl, AcOH or EtOH, 80°C; (b) 1) H2SO4, NaNO2, H2O, 0-5°C; 2) HBF4; (c) H2O,
H2SO4, 100°C; (d) PhNH2, AcOH, 120°C; (e) 1,1-diphenylprop-2-yn-1-ol, TsOH, MeCN, 25-
50°C; (f) 3,3-diphenyl-acrylaldehyde, Ti(OEt)4, PhCH3, 100°C.
6

3.2. Optical and photochromic properties of naphthopyrans 1 and 2
All photochromic properties of 1 and 2 induced by photoirradiation at 365 nm were
measured in acetonitrile, methylene chloride and toluene solutions under flash photolysis at 293
K. The obtained characteristics are summarized in Table 1 and Figures S1-S14 in SI.
The absorption bands of closed form in the region of 352-355 nm and 412-415 nm for 1
and in region 341-352 nm and 387-392 nm for 2 result from the incorporated naphthalimide
moiety into molecular skeleton of naphthopyran (320 nm) (Figure 1, S1-S6 in SI, Table 1) [16b].
Table 1
Spectral characteristics of compounds 1 and 2a
Solvent
Closed form
Open form
λmaxabs, nm λmaxfluor, nm φfluor
λmaxabs, nm
λmaxfluor, nm τb, s
φfluor
Naphthopyran 1
MeCN
352, 412
461
458
446
0.09
0.14
0.04
495
505
501
-
-
-
0.098
0.176
0.153
-
-
-
CH2Cl2
Toluene 355, 415
355, 415
Naphthopyran 2
MeCN
341, 388
476
471
461
0.12 336, 386, 505
0.20 336, 391, 515
0.09 343, 387, 520
534
-
525
532.3 0.005
10.3
2.7 0.003
CH2Cl2
Toluene 345, 387
352, 392
-
aAs a standard to determine the fluorescence quantum yield of closed and open forms, coumarine 6 and rhodamine
6G in ethanol were used respectively;b τ½ is the half bleaching time.
Upon irradiation of naphthopyran 1 in MeCN with UV light (365 nm), a new visible
absorption band centered at 495 nm emerged, while the original peaks in region 352, 412 nm
gradually decreased, indicative of the formation of the open MC form (Table 1, and Figures S1-
S6 in SI). The spectrum obtained after irradiation of compound 2 includes peaks (336, 386 nm)
and longwavelength peak at 505 nm belonging to the merocyanine (MC) form OF2 (Figure 1a).
Some diversity in position of absorption bands of CF or OF of the isomeric naphthopyrans is due
to difference in the structures of the chromophores 1 and 2.
The colored forms of 1 and 2 can be thermally converted back to their closed form when
the photoirradiation is ceased. The bleaching kinetics, determined from the absorption-time data
sets, can be fitted well to the monoexponential decay in MeCN and CH2Cl2 (Figures 1b, Table 1,
and Figures S7-S14 in SI). The open form of naphthopyran 2 is stable enough; its τ in MeCN is
532.2 s. Open form of compound 1 lives shorter time. The difference in values of life times of
open forms OF1 and OF2 can be explained by the fact that OF2 is stabilized by the conjugation
between the oxygen atom at C-4 position and carbonyl groups of carboxyimide moiety.
7

As one can see in Table 1 the variation of life time values in different solvents was
observed for 2. It could be connected with polar betain structure of OF2 which is more effective
stabilized in polar solvent.
a)
b)
0.5
0.4
0.3
0.2
0.1
0.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
300
400
500
/ nm
600
700
0
500
1000
1500
t / s
Fig. 1. a) Absorption spectra of CF2 (solid line), after irradiation for 6 min at 365 nm (dashed
line) (1·10-4 М) and after thermal fading (dash-dotted line) in acetonitrile at 293K; b) Thermal
fading kinetics of OF2 (1·10-4 М) in acetonitrile at 293K. The monitored wavelength was 510
nm.
The photocoloration with light irradiation can induce the formation of two open forms
Transoid-Cis (TC) and Transoid-Trans (TT): while TC isomer rapidly returns to the uncolored
closed form, the TT isomer is thermally more stable. After thermal back reaction, the MC forms
of 1 in all studied solvents and 2 in MeCN and CH2Cl2 are almost transformed to the colorless
closed forms (figs. S8, S10, S12, S14 in SI).
When the UV irradiation was ceased, the colored form of 2 in toluene commenced
bleaching, and after about 2.7 s, the optical density reduced near to half of its original state value
(fig. 2a). Although the thermal back reaction time was maintained for an additional 120 s, the
optical density value still maintained stability. That is, the residual visible coloration cannot be
spontaneously faded to the full colorless. However, upon 525 nm light irradiation, the residual
color can be efficiently faded to colorless state (fig. 2b). The similar phenomenon was mentioned
in [17] for naphthalimide derivatives with closed to 2 structure.
8

a)
0.10
0.08
0.06
0.04
0.02
0.00
0
20 40 60 80 100 120
t / s
b)
0.16
0.12
0.08
0.04
0.00
UV
UV
UV
Vis
UV
Vis
UV
Vis
Vis
Vis
0
1000 2000 3000 4000 5000
t / s
Fig. 2. a) Thermal fading kinetics of the OF2 (C = 4.2∙10-5 M) in toluene at 293K; b) time-
dependent photocoloration cycles of 2 (C = 4.2∙10-5 M) in toluene upon alternate UV (365 nm)
and visible (525 nm) light irradiation. The monitored wavelength was 515nm.
As mentioned above, two steps proceed in the color-faded course, that is, from TT to TC
form, and then thermal back to CF forms (usually slow). This means that, for 2, the thermal
relaxation from TT to TC form was extremely slow; it might be the predominant dynamic step in
the thermal back process.
Figure 2b performs the fatigue resistance of the studied compound 2. The present results
reveal the slight effect of coloration intensity decrease of as well as a residual visible absorbance
remained. We believe that the observed phenomenon might not result from the
photodegradation. This is explained by assuming that the TT isomeric photoproduct is thermally
stable but photoreversible, and needs to be under visible light to revert to the original colorless
form. Aimed to support the idea, we prepare the photocoloration-thermal bleaching cycles at 10 s
time of initial irradiation (fig.3). According to data in fig. 2b, the content of long-lived TT form
9

should be small after so short time of irradiation; this is why the time of photoirradiation was 10
s. Indeed, in this case the decoloration process is thermal; the optical density at a photostationary
state after successive coloration and decoloration cycles was not altered (fig. 3).
0.08
0.06
0.04
0.02
0.00
0
50
100
t / s
150
200
Fig. 3. Time-dependent photocoloration of 2 (C = 4.2∙10-5 M) in toluene at 293K upon UV (365
nm) light irradiation during 10 s and thermal fading cycles. The monitored wavelength was
515nm.
3.3. NMR studies of the photochromic transformation of naphthopyrans 1 and 2
To get deeper insight into the nature of processes occurring after the irradiation is ceased,
we studied photoproduct mixture by NMR spectroscopy. Irradiations of 1 were done at 193, 203,
213, 223 and 233K and thermal relaxation of solution was followed at each temperature by
measuring the peak-intensities of signals associated to new photoproducts and residual initial
forms (Figure S24). At 213K (fig 4a), three groups of signals corresponding to the residual initial
closed form and two new photoproducts were detected. The doublet signal at the most deshielded
3
chemical shift (9.41 ppm) with the transoid coupling constant J=11.4Hz and its rapid thermal
evolution allow to straightforwardly identify the H-2’ proton of the TC [25]. The second group
of new signals is deduced to correspond to the second open isomer, TT, characterized by the
3
doublet signal at 8.24 ppm, with J=11.4 Hz attributed to the H-1’ proton, and a high thermal
stability at low temperature and its decrease at highest temperature (Scheme 3).
10

Scheme 3. Photochromic reaction between 1 and TC1 and TT1 isomers with characteristic
chemical shifts  in ppm.
Thermal evolution shows that TC is rapidly decreasing while TT almost remains at the
same concentration throughout the time interval when TC is being converted to the closed form
(Table 2). From these rate constants, the thermodynamic parameters were calculated using
Arrhenius and Eyring equations (Table 3, Figs. S25-S26).
Fig. 4. 1H NMR spectra after UV irradiation of a) naphthopyran 1 at 213K and b) naphthopyran
2 at 223K, with characteristic signals of open forms TC and TT
Irradiation of naphthopyran 2 was carried out and led to the formation of two new
3
photoproducts TC characterized by a doublet signal at 9.38 ppm, with J=11.4 Hz attributed to
11

the H-2’ proton, and TT characterized by a singlet signal at 8.72 ppm attributed to H-7 (Fig. 4b,
Scheme 4). The main difference when compared with previously investigated compound 1,
concerns the highest conversion into TT isomer, and, in addition its thermal evolution being
significant, while stable in the case of TT1. This can be explained by the fact that the TT form of
2 has a lower steric repulsion between hydrogen atoms at positions 7 and 1’ in comparison with
the steric repultion between H-atoms 1 and 1’ in the TT form of compound 1 [26].
Scheme 4. Photochromic reaction between CF2 and TC2 and TT2 isomers with characteristic
chemical shifts  in ppm.
Time-evolution of concentration was followed at 213, 223, 228 and 233K by measuring the
intensities of characteristic signals for the three species (Figure S32). Using the model (eq. 1),
and fitting the concentration profiles, the rate constants k1, k2 and k3 were calculated (Table 2)
and the thermodynamic parameters extracted (Table 3, Figure S33-S34).
k2
k1
(1)
TT
2
TC
k3
Table 2
Rate constants of thermal relaxation of 1 and 2; Gibbs free energy of activation for TC2
TT2 process.
T
193 K
2.510-5
203 K
213 K
223 K
3.110-3
k1 (TC1CF1) / s‒1
1.8510-4 9.010-4
T
213 K
223 K
228 K
233 K
k1 (TC2  CF2)/ s‒1
k2 (TC2  TT2) / s‒1
k3 (TT2  TC2)/ s‒1
G# (kJmol-1)
1.2410‒5 4.1710‒5 8.8210‒5 1.7810‒4
4.3610‒5 6.8510‒5 8.1610‒5 1.4710‒4
8.4710‒6 1.8310‒5 2.3010‒5 4.6810‒5
-2.87
-2.54
-2.34
-2.20
12

For both investigated compounds 1 and 2, the thermodynamic parameters are lower than
the values generally reported for naphthopyrans. [27]. Comparison of 1 with 3-3-diphenyl-3H-
naphtho[2,1-b]pyran [28] and comparison of 2 with 2,2-diphenyl-2H-naphtho[1,2-b]pyran [29]
show the same trend, that is the predominant formation of TC isomer for 1 and the predominant
formation of TT-isomer for 2. However, the thermal stabilities of open forms are greatly affected
by the strong electron withdrawing imide groups, leading to their destabilization. Indeed, all
rates constants are accelerated.
Table 3
Thermodynamic parameters of thermal relaxation of 1 and 2
(kJ mol-1)
(J mol-1K-1)
Process Ea (kJ mol-1)
TC
TC
TC
TT
CF
CF
TT
TC
56.6
54.9
23.3
33.3
59.9
53.1
21.4
31.5
-43.6
-87.7
1 
2 
2 
2 
1
2
2
2
-225.7
-192.1
3.4. Fluorescence switchable properties
The studied compounds 1, 2 combine photochromic pyran and fluorophoric naphthalimide
fragments. The absorption of photons generates an excited singlet state from which ring-opening
of the pyran ring ensues. Fluorescence emission is also derived from a singlet excited state and it
may be that the most efficient (or rapid) energy dissipation process occurs via fluorescence
rather than the required electrocyclic bond reorganization leading to the photomerocyanine and
hence photochromism. The closed forms of 1 and 2 show the characteristic fluorescence from the
naphthalimide moiety in the region of 445-475 nm, whereas, the emission bands of open forms
lay between 525 and 535 nm. It was found that the quantum yields of fluorescence of CF1 and
CF2 are of fair values (see Table 1), thus, photoactivation causes an effective photochromic
transformation as well as fluorescence. In case of stable OF2 we were able to estimate the
quantum yield of fluorescence in MeCN and toluene (see Table 1). Its value is low. The reason
of this phenomenon could be the more high polar open form in which energy dissipation can
occur through the competitive TICT process.
Under the UV light irradiation the intensity of fluorescence diminishes quickly (fig. 5).
During the irradiation with visible light (515nm), the fluorescence of the solution can be
13

recovered. As depicted in Figure 5 and Figures S15-S18 (in SI), upon irradiation with a UV light
of 365 nm, the fluorescence intensity of 2 reduced quickly in a few seconds. Upon irradiation for
6 min, the fluorescence reached a photostationary state (λex = 365 nm, λem= 490 nm), and the
intensity reduced by 36%. The process of fluorescence intensity switch can be repeated by using
of transformation between the closed and open forms.
280
0 min
240
200
160
6 min
120
80
40
0
400
500
 / nm
600
700
Fig. 5. Changes in the fluorescence of compound 2 (C = 4.2∙10-5 M) in acetonitrile upon 365 nm
irradiation at room temperature, excitation at 365 nm.
4. Conclusions.
Two new isomeric photochromic naphthopyrans 1 and 2 bearing naphthalimide units
have been synthesized and studied. Taking into account the obtained data we are able to do
relevant conclusions on the points listed at the beginning of study. (i) The mutual position of
pyran and naphthalimide units in photochromic fluorophore influences unsubstantial on the
optical characteristics of closed and open forms, but demonstrates large effect on the
photochromic photo- and thermal transformation between CF and OF. The involvement of O-
atom of pyran photochrome cycle into the conjugated naphthalimide system provides the
substantial stabilization of open form of 2. (ii) The NMR analysis of the ratio of TT and TC
showed the existing of relatively stable TC form in case of OF1 and large amount of TT form in
OF2. So as TC isomer rapidly returns to the uncolored closed form, we observed the fast thermal
back reaction OF1 to CF1. The TT isomer of OF2 is thermally more stable, this is why it is
possible to obtain the long-lived open form of photochrome 2. (iii) The fluorescence of
naphthalimide unit can be switched on-off by photochromic conversion between the closed and
open forms.
The analysis of the ways of combination of photochromic and fluorophoric units in one
molecule prepared in this study can help in development of promising photoswitchable
14

candidates for practical applications. The design and synthesis of efficient photoswitchable
molecules or systems are essential to execute such applications as components for “single-
molecule optical memory” and “super-resolution fluorescence microscopy”.
Acknowledgments.
Financial support from the Russian Foundation for Basic Research (grants 15-03-04705, 13-03-
93107) and GDRI CNRS 93 “Phenics” (Photoswitchable Organic Molecular Systems &
Devices).
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