Fluorescent photochromic complex of 1,8-naphthalimide derivative and benzopyrane containing benzo-18-crown-6 ether

Article abstract of DOI:10.1016/j.jphotochem.2020.112975

We present here design, preparation, characterization, and properties of a photochromic supramolecular complex of benzopyran containing benzo-18-crown-6-ether 1 with 4-aminonaphthalimide 2 as a fluorophore. Phototransformation of the supramolecular complex 1?2 is accompanied by the appearance of a predominantly open TC form, which is consistent with the monoexponential bleaching curve of an open form at room temperature. Compared with free benzopyran 1, the bleaching rate is increased by 5–6 times in the presence of 2. From a synthetic point of view, varying the components in the supramolecular complex is more preferable to obtain a system with the desired characteristics. Thus, this study demonstrates that the supramolecular approach is promising for systems that contain photochromic and fluorescent components that influence each other.

Full text of DOI:10.1016/j.jphotochem.2020.112975

Journal of Photochemistry & Photobiology, A: Chemistry 405 (2021) 112975
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Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Fluorescent photochromic complex of 1,8-naphthalimide derivative and
benzopyrane containing benzo-18-crown-6 ether
,
,
,
,d
O.A. Fedorovaa b, A.N. Arkhipovaa, P.A. Panchenkoa b, J. Berthetc d, S. Delbaerec
,
S. Minkovskae, Yu. V. Fedorova *
,
a A. N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences, 119991, Vavilova str., 28, Moscow, Russia
b Dmitry Mendeleev University of Chemical Technology of Russia, 125047, Miusskaya sqr. 9, Moscow, Russia
c University of Lille, UDSL, CNRS UMR 8516, BP83, Lille, France
d Univ Lille, INSERM, CHU Lille, UMR-S 1172, Lille Neuroscience and Cognition Research Center, F-59000, Lille, France
e Institute of Catalysis Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bldg. 11, Sofia, 1113, Bulgaria
A R T I C L E I N F O
A B S T R A C T
Keywords:
We present here design, preparation, characterization, and properties of a photochromic supramolecular com-
plex of benzopyran containing benzo-18-crown-6-ether 1 with 4-aminonaphthalimide 2 as a fluorophore. Pho-
totransformation of the supramolecular complex 1⋅2 is accompanied by the appearance of a predominantly open
TC form, which is consistent with the monoexponential bleaching curve of an open form at room temperature.
Compared with free benzopyran 1, the bleaching rate is increased by 5–6 times in the presence of 2. From a
synthetic point of view, varying the components in the supramolecular complex is more preferable to obtain a
system with the desired characteristics. Thus, this study demonstrates that the supramolecular approach is
promising for systems that contain photochromic and fluorescent components that influence each other.
1,8-naphthalimide
Benzopyrane
NMR spectroscopy
Supramolecular complex
Fluorophore
1. Introduction
fluorescence is achieved by changing in electron distribution upon
photochromic transformation, energy transfer, or electron transfer be-
Fluorescent molecules are widely used in chemistry, biology, medi-
cine as chemical sensors [1] and biological labels [2]. The possibility of
photo-switching of a fluorescent signal using light can significantly
expand their field of application. Fluorescent photoswitchable mole-
cules have attracted much attention because of their practical impor-
tance in bioimaging [3], logic gates [4], information storage [5] and
optoelectronic devices [6]. One of the possibilities to realize
photo-switching of a fluorescent signal using light is the combination of
fluorescent and photochromic fragments in a molecule. Typical organic
photochromic compounds include diarylethenes [7], azobenzenes [8],
spiropyrans [9], fulgides [10], naphthopyrans [11], spirooxazines [12].
Photochromic fragments can reversibly change their geometry and
electronic structure under excitation with light, thus influencing the
fluorescent properties of the whole molecule. The introduction of a
fluorescent fragment into the photochromic system can be realized
through the covalent attachment of a fluorophore to photochrome
[13–15] or much less often through supramolecular association. Upon
irradiation with UV or visible light, the enhancement or quenching of
tween fluorophores and photochromic groups.
1,8-Naphthalimide derivatives are of interest due to their promising
photophysical and biological properties [16]. They are particularly
attractive as fluorophores. They have high quantum yields and good
photostability, and their fluorescence varies from blue to red depending
on substituents. 1,8-Naphthalimide-derived fluorescent pigments and
fluorescent compounds can be used as sensors for certain metal cations,
and as fluorescent markers [17,18].
There are some examples of naphthalimide-containing photochromic
systems in literature. When using a 1,8-naphthalimide fluorophore in
combination with an indoline-based “donor” and a barbituric acid
“acceptor”, the donor-acceptor Stenhouse adduct (DASA) shows a
switch of the fluorescence quantum yield from 0.21 to 0.56 [19]. A series
of naphthalimides containing dithienylethene fragments in different
position of fluorophore molecules was synthesized and their photo-
chromic properties were studied [20–22]. When thienyl fragments are in
the 3rd and 4th positions of the naphthalene ring of the naphthalimide
derivative, the photochromic conversion to the cyclic form does not
* Corresponding author.
E-mail address: fedorov@ineos.ac.ru (Yu.V. Fedorov).
https://doi.org/10.1016/j.jphotochem.2020.112975
Received 3 July 2020; Received in revised form 1 September 2020; Accepted 9 October 2020
Available online 14 October 2020
1010-6030/© 2020 Elsevier B.V. All rights reserved.

O.A. Fedorova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 405 (2021) 112975
Scheme 1. Synthetic route to the naphthalimide 2, and structures of compounds 1, 2 and complex 1⋅2.
affect the fluorescence characteristics of naphthlimide [20]. In the next
research, the two prepared photochromic bisthienylethene derivatives
with a six-membered aryl ring of the fluorescent naphthalimide moiety
as the center ethene bridging unit differ from the previous one by OMe
group in thienyl heterocycles [21]. They exhibit considerably high
cyclization quantum yield and good fatigue resistance. Interestingly, the
fluorescence arising from the naphthalimide unit could be well modu-
lated by photochromism and solvatochromism. To the similar photo-
chromic bisthienylethene system based on naphthalimide, the ferrocene
unit was added as N-substituent. The system demonstrates
fluorescence-modulation through photoinduced electron transfer (PET),
a decrease in the photochromic cyclization quantum yield, and a se-
lective two-step oxidation process [22].
form can respond to various combinations of chemical and photo-
chemical stimuli and displays multiple states with distinguishable
fluorescence output. The PET process between the spiro-unit and the
naphthalimide fluorescent unit has been observed and found to be
capable of switching by acid-base stimuli. In the merocyanine form
(MC), both the PET and the electronic energy transfer (EET) processes
between the MC fragment and the fluorescent naphthalimide fragment,
which can be chemically controlled, were detected.
In this paper, we report on the design, preparation, characterization,
and properties of a supramolecular complex between benzopyran con-
taining benzo-18-crown-6-ether 1 and 4-aminonaphthalimide 2 as a
fluorophore (Scheme 1). Our research is aimed to reveal the mutual
influence of photochromic and fluorophore units on the properties of the
obtained supramolecular system.
Photochromic E,Z-isomerization in 4-styryl-1,8-naphthalimides does
not significantly affect their fluorescent properties [23,24].
In the two photochromic spirooxazines-naphthalimides, the imide
group of naphthalimide unit gives strong electron-withdrawing effect
favoring the long-lived merocyanine in the dark and good colorability in
solution [25]. Remarkably, their open merocyanine forms exhibit
significantly long lifetimes, almost three magnitudes longer than that of
unsubstituted spironaphthoxazine. Moreover, the fluorescence of
naphthalimide unit can be switched on and off by photoinduced con-
version between the open and closed forms.
2. Experimental sections
2.1. Synthesis of compounds
Benzocrown ester-containing pyran 1 was obtained by the reaction
of commercially available phenol with β-phenylcinnamic aldehyde in
the presence of titanium ethoxide (IV) in toluene as it was described
earlier. [29] Naphthalimide derivative 2 was prepared as shown in
Scheme 1. The synthesis includes imidation of 4-nitro substituted 1,
8-naphthalic anhydride 3 by ethylenediamine, in which one of the
amino groups was protected with di-tert-butyl dicarbonate. [30,31] The
nitro group reduction in 4 was carried out with tin (II) chloride in the
presence of acid to simultaneously deprotect the amino group in the
Two photochromic naphthopyrans containing naphthalimide moi-
eties were studied in solution under flash photolysis conditions [26].
Photochromic naphthopyrans exhibit an excellent photochromic
response, rapid thermal bleaching rate and good fatigue-resistance. It
was shown that the photochromic properties can be modulated with the
introduction of different functional groups on the naphthalimide unit
and the fluorescence of naphthalimide moiety can be switched on and
off by photoinduced conversion between the closed and open forms.
Studies of two isomeric photochromic naphthopyrans connected with
naphthalimide moieties in different manner showed that the variety in
mutual arrangement of pyran and naphthalimide units leads to
remarkable difference in photochromic characteristics [27].
N-alkyl
fragment.
[32]
The
protonation
of
4-amino--
N-aminoethylnaphthalimide 5 was performed in acetonitrile using
perchloric acid to give perchlorate 2.
Compound 5. To a stirring mixture of compound 4 (200 mg, 0.52
mmol) and ethanol (10 ml) a solution of SnCl2∙2H2O (780 mg, 2.55
mmol) in concentrated hydrochloric acid (5.0 ml,
ρ =1.18 g/mL) was
added dropwise at 50 ◦C. The reaction mixture was refluxed for 2.5 h,
cooled to ambient temperature, diluted with water and then 5 mass. %
sodium hydroxide aqueous solution was added until slightly alkaline.
The product was extracted thrice with dichloromethane, the combined
organic extracts were washed with water, dried and evaporated in
vacuum to give 100 mg of 5 (yield 75 %). M.p. 170–173 ◦C. 1H NMR
(400 MHz, DMSO-d6, 21 ◦C, δ / ppm, J / Hz): 2.77 (t, 2H, CH2, J = 7.0)
4.03 (t, 2H, CH2, J = 7.0), 6.84 (d, 1H, ArH, J = 8.4), 7.44 (br. s, 2H,
NH2), 7.64 (dd, 1H, ArH, J1 = 7.3, J2 = 8.4), 8.18 (d, 1H, ArH, J = 8.4),
In the above cases, the naphthalimide fluorophore was conjugated
with the photochromic molecule or covalently linked to it via a spacer
forming dyad compounds. As a review of the literature shows, a com-
bination of photochrome and fluorophore in a supramolecular fashion is
rarely realized. It was reported on the design, preparation, character-
ization and properties of the mechanically interlocked complex of
bisspiropyran-containing [2]rotaxane with 4-morpholin-naphthalimide
fluorophore [28]. Rotaxane with photochromic fragments in spiro
2

O.A. Fedorova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 405 (2021) 112975
8.42 (dd, 1H, ArH, J1 = 7.3, J2 = 1.1), 8.61 (dd, 1H, ArH, J1 = 8.4, J2 =
1.1). Found (%): C, 65.91; H, 5.19; N, 16.51. Calculated for C14H13N3O2
(%): C, 65.87; H, 5.13, N, 16.46.
Compound 2. Compound 5 (50 mg, 0.19 mmol) was dissolved in
acetonitrile (15 ml). To a resulting mixture 70 mass. % perchloric acid
(14 μl, 0.16 mmol) was added. The precipitate was filtered off, recrys-
tallized from ethanol and dried at 60 ◦C to give 60 mg of 2 (yield 86 %).
M.p. 88 ◦C (decomp.). 1H NMR (400.13 MHz, DMSO-d6, 21 ◦C, δ / ppm, J
/ Hz): 3.06–3.16 (m, 2H, CH2), 4.27 (t, 2H, CH2, J = 5.7), 6.86 (d, 1H,
ArH, J = 8.4), 7.64–7.72 (m, 1H, ArH), 7.75 (br. s, 3H, NH+3 ), 8.21 (d,
1H, ArH, J = 8.4), 8.45 (d, 1H, ArH, J = 7.0), 8.64 (d, 1H, ArH, J = 8.0).
Found (%): C, 47.34; H, 4.00; N, 11.86. Calculated for C14H14ClN3O6
(%): C, 47.27; H, 3.97, N, 11.81.
2.2. Optical spectroscopy
Electronic absorption spectra were recorded on spectrophotometers
Varian-Cary 5 G 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 120 W equipped with optical filter 313 nm. To measure
the dark lifetime of the colored form of benzopyran 1, solutions of 1 in
acetonitrile or dichloromethane were irradiated in the cuvette box of the
spectrophotometer Avantes AvaSpec-2048 with 313 nm light. The
irradiation was performed till the photostationary conditions (equilib-
rium between closed and opened forms) were achieved. Then the irra-
diation 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
form at 384 nm.
Fluorescence spectra were recorded on spectrofluorimeters
FluoroLog-3-221 and AvaSpec-2048 L. 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 10ꢀ 4 or 10-5 M.
Fig. 1. (a) Absorption spectra of the benzopyran 1 (A) (C1 = 2.29⋅10ꢀ 5 M),
naphthalimides 5 (B) and 2 (C) (C5 = C2 = 1.14⋅10ꢀ 5 M); (b) fluorescence
spectra of 5 alone (C5 = 1.14⋅10ꢀ 5 M) (A) and in the presence of various
amounts of perchloric acid: (B) – 0.285⋅10ꢀ 5 M, (C) – 0.57⋅10ꢀ 5 M, (D) –
0.855⋅10ꢀ 5 M, (E) – 1.14⋅10ꢀ 5 M, (F) – 1.425⋅10ꢀ 5 M. Acetonitrile, 22 ◦C, λex
=380 nm.
2.3. Determination of fluorescence quantum yields
Coumarin 481 in acetonitrile (φfl = 0.08) was used as a reference for
the fluorescence quantum yield measurements. [33]
The fluorescence quantum yields of the individual compounds 2 and
5, and the complex 1∙2 were determined in acetonitrile at 25 ◦C and λex
= 340 nm according to equation (1) [34]:
ꢀ
)
1 ꢀ 10ꢀ A n2
R
S
A
A2
A1
A2
fl
= φfl
∙
∙
ꢀ
∙φf2l
(2)
ꢀ
)
φ
1⋅2
RSR
2
1 ꢀ 10ꢀ A n2R
R
S
(1 ꢀ 10ꢀ A )n
φfl = φfl
∙
(1)
RSR (1 ꢀ 10ꢀ A)n2R
wherein φf2l is the fluorescence quantum yield of the naphthalimide 2;
A = A1 + A2 + A3 is the absorption at 340 nm of the mixture of 1 and 2,
where A1, A2 and A3 are the absorptions of complex 1∙2, free naph-
thalimide 2 and free benzopyran 1 respectively; is the area underneath
the curve of the fluorescence spectrum of the studied solution containing
the mixture of 1 and 2. A2 and A3 values were calculated by the
Lambert-Beer law from the known concentrations of the free naph-
wherein φfl and φfRl are the fluorescence quantum yields of the studied
solutions and the standard compound respectively, A and AR are the
absorptions at 340 nm of the studied solutions and the standard
respectively, S and SR are the areas underneath the curves of the fluo-
rescence spectra of the studied solutions and the standard respectively, n
and nR are the refraction indices of the solvents for the substance under
study and the standard compound.
thalimide 2 and benzopyran 1 (5.3 μM and 29 μM respectively, see
above) and their molar extinction coefficients at excitation wavelength
340 nm ε340 (2) and ε340 (1) (780 and 2970 M–1 cm–1). A1 was found as
the difference A ꢀ A2 ꢀ A3. Derivation of Eq. (2) is presented in sup-
plementary information.
To determine the fluorescence quantum yield of complex 1∙2, we
used an acetonitrile solution containing 33.8 μM of 1 and 10.1 μM of 2.
Considering that logarithms of stability constants (log K) for the com-
plexes of 1 and protonated forms of some aliphatic aminoacids (γ-ami-
Fluorescence quantum yield of naphthalimide chromophore in the
nobutyric acid (C4), ε-aminocaproic acid (C6) and ω-aminocaprylic acid
fl
complex 1∙2 (φ ) was estimated by the Eq. (3):
(C8)) in MeCN are in the range 4.42–4.68, [35] we estimated the con-
centration of 1∙2 species in the solution to be as high as 4.8 M (ob-
NI
μ
A1
fl
fl
φ
= φ
(3)
tained using log K = 4.5), and concentrations of free naphthalimide 2
NI
1⋅2
A
NI
and benzopyran 1 are 5.3
μ
M and 29
μ
M respectively. The fluorescence
fl
quantum yield of complex 1∙2 (φ ) was calculated by the Eq. (2):
wherein ANI is the absorption at 340 nm of naphthalimide chromophore
in the complex 1⋅2. ANI was calculated under the assumption that the
values ε340 (2) and ε340 (1) do not change upon going from individual
1⋅2
3

O.A. Fedorova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 405 (2021) 112975
Fig. 2. 1H NMR spectra of solutions of benzopyran 1, an equimolar mixture of 1 with 2, and naphthalimide 2 (acetonitrile, 25 ◦C). The prime and double prime
indicate the proton signals of phenyl groups in the compound 1. Numbering of carbon atoms of the crown ether cavity of 1 is similar to that shown in Fig. 3.
Fig. 3. Aliphatic part of 1H NMR spectrum (a) and 1D NOESY spectrum (b) of an equimolar mixture of 1 with 2 (acetonitrile, 25 ◦C). Arrows in the structure of
complex 1∙2 show the protons of the crown ether fragment which are close enough in space to the protons of CH2 group of the naphthalimide N-alkyl substituent to
exhibit NOE.
4

O.A. Fedorova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 405 (2021) 112975
a 1000 W Xe-Hg high pressure short-arc lamp (Oriel), filtered by inter-
ferential filter (λ = 313 nm).
2.5. Computational details
Quantum chemical calculations were carried out by the MOPAC
2016 program package using the PM7 semiempirical method. Calcula-
tions were performed at optimized geometries, which reached gradient
variations less than 0.01 kcal/mol. The solvent effect was included in
geometry optimizations following the “COnductorlike Screening
MOdel” (COSMO) implemented in MOPAC 2016. A dielectric constant
of
ε
= 40 and a refraction index of solvent (n) such that n2 = 2 were used.
3. Results and discussion
3.1. Optical properties of naphthalimides 5, 2 and benzopyran 1
Compound 5 exhibits the long wavelength absorption band at 416
nm in acetonitrile solution (Scheme 1, Fig. 1a). The addition of
perchloric acid causes the formation of 2 whose long wavelength ab-
sorption band is shifted bathochromically by 6 nm and appeared at 424
nm. Compound 5 emits light at 514 nm with quantum yield φfl = 0.48,
whereas compound 2 emits at 533 nm with φfl = 0.31. The quantum
yield of fluorescence of 5 is higher than for 2 (Fig. 1b), which may be due
to the presence of an intramolecular hydrogen bond in the protonated
naphthalimide 2 (Scheme 1). [36] Benzopyran 1 has a long wavelength
absorption band in the UV region of the spectrum at 325 nm in aceto-
nitrile solution and does not fluoresce.
3.2. Coordination of benzopyran 1 with naphthalimide 2
It is known that protonated amino group is able to coordinate with
benzopyran 18-crown-6 ether moiety with stability constant log K11
=
4.6 in acetonitrile. [35] It can be assumed that the naphthalimide de-
rivative having an ammonium group in the N-substituent will be able to
coordinate with the crown ether fragment of the photochromic benzo-
pyran molecule, thereby ensuring the close position of both units in the
supramolecular complex. Due to the supramolecular organization of two
chromophoric units, the occurrence of PET or EET processes influencing
the photochromic and fluorophore charateristics of supramolecular
complex can be expected.
Fig. 4. Absorption (a) and fluorescence (b) spectra of 2 (A) (C2 = 1.01⋅10ꢀ 5 M)
and mixture of 1 with 2 (B) (C1 = 3.38⋅10ꢀ 5 M and C2 = 1.01⋅10ꢀ 5 M), λex
=
340 nm, acetonitrile, 25 ◦C.
components 1 and 2 to complex 1∙2, i.e. ANI = A1∙ε340(2)/(ε340(2) +
ε340(1))
We have confirmed the formation of supramolecular complex of
benzopyran 1 with naphthalimide 2 by NMR spectroscopy. It was shown
that the addition of naphthalimide 2 to the solution of crown-containing
benzopyran 1 causes changes in the position of the proton signals of the
macrocyclic methylene groups, which confirms the interaction of the
ammonium group of 2 with the macrocyclic moiety of 1 (Fig. 2). 1D
2.4. NMR experiments
NMR spectra were record on Bruker Avance 500 MHz spectrometer
equipped with a TXI probe, using standard sequences. Data acquisition
and processing were performed with Topspin 2.1 software (Bruker). The
experiments were carried out in CD3CN. Temperature ranges used for
NMR measurement and irradiation were 243 K–298 K. The temperature
of the sample was controlled with a variable temperature unit (B-VT
1000-Bruker, 123ꢀ 423 K, T range). All UV irradiations were done using
NOESY spectrum (Fig. 3) evidenced the dipolar correlations between the
+
–
–
methylene groups of crown ether fragment and the CH2N H3 , which
means that CH2N H+3 group is placed into the macrocyclic cavity.
–
–
The absorption spectrum of a mixture of the closed form of
Table 1
Spectral properties of the supramolecular complex 1∙2 and its individual components and the lifetime of the open form of benzopyran 1 and complex 1∙2 in
acetonitrile and dichloromethane at 22 ◦C.
Acetonitrile
Dichloromethane
Species
abs
fl
abs
fl
φfl
τ
/s
τ/s
λ
/nm
λ
/nm
λ
/nm
λ
/nm
max
max
max
max
1(CF)
325
–
–
326
–
1(OF)
2
384; 470
424
–
–
174
–
383; 464
421
–
3200
408
533
532
532
0.31
537
513
520
1(CF)þ2
423
0.02 (0.05**; 0.24***)
–
415
1(OF)þ2*
423
–
87
408
*
C1 = 2.3∙10ꢀ 5 M, C2 = 9.2∙10ꢀ 5 M.
**
***
Apparent fluorescence quantum yield of complex 1⋅2.
Apparent fluorescence quantum yield of naphthalimide chromophore in complex 1⋅2 estimated by the Eq. (3).
5

O.A. Fedorova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 405 (2021) 112975
Fig. 5. Energy diagram of the frontier molecular orbitals of the closed form of complex 1∙2 (MOPAC 2016, PM7 method).
Scheme 2. Photochromic equilibrium of benzopyran 1.
benzopyran 1 and naphthalimide 2 in acetonitrile (Fig. 4a) is close to the
sum of the spectra of individual components (Fig. 1a), which indicates
that no noticeable spectral changes occur during the formation of the
supramolecular complex 1 with 2. More significant changes were found
in the fluorescence spectra upon excitation at 340 nm when adding 1 to
2. The interaction of 1 with 2 results in decrease of fluorescence
(Fig. 4b), the apparent quantum yield of the solution containing the
species in their closed and open forms. The solubility of complex 1∙2 in
CH2Cl2 was found to be not sufficient for the NMR experiments.
Therefore, only optical studies were carried out in both solvents.
As shown in Scheme 2, UV light irradiation of benzopyran 1 causes
heterolytic cleavage of the pyran carbon-oxygen bond with the forma-
tion of an open ring merocyanine structure represented by two isomers:
transoid-cis (TC) and transoid-trans (TT). The changes in the absorption
spectra in MeCN and CH2Cl2 are shown in Figs. 6a and S1a. The relax-
ation kinetics of the open form of benzopyran in acetonitrile or
dichloromethane at ambient temperature is described by a mono-
mixture of 2 (10.1 μM) and 1 (33.8 μM) is 0.02 which is much lower than
that of 2 (0.31, see Table 1). In part, this could be explained by the
relatively high absorption at 340 nm of the non-fluorescent chromene
species. However, some reduction in emission efficiency of the naph-
thalimide chromophore (from 0.31 to 0.24) on going from compound 2
to the complex 1∙2 was found (see Experimental part for the details of
the quantum yield calculations). To explain this effect, semiempirical
calculations of the frontier molecular orbitals (HOMO and LUMO) for
1∙2 were performed by the PM7 method. As Fig. 5 shows, the HOMO is
localized over the benzopyran fragment and the highest molecular
orbital of naphthalimide fluorophore appears to be HOMO(–1). In such a
situation, PET between benzopyran HOMO and singly occupied HOMO
(–1) of naphthalimide in the excited state would be thermodynamically
feasible which leads to some decrease in the fluorescence quantum yield
of naphthalimide chromophore in the 1∙2 complex.
exponential dependence with a lifetime of τ = 174 s (MeCN) and 3200 s
(CH2Cl2), which apparently corresponds to the TC isomer (Fig. 6b,
Fig. S1b). [35] TT-isomer is significantly more stable as evidenced by the
presence of residual absorption exceeding absorption of benzopyran 1
closed form even after prolonged relaxation (Fig. 6b).
1H NMR spectrum of benzopyran 1 recorded in acetonitrile solution
at –30 ◦C after irradiation with UV light (λ = 313 nm) highlighted the
conversion of free benzopyran 1 into the two photomerocyanines TC and
TT (Fig. 7). More particularly, the formation of TC isomer is predomi-
nant and well characterized by the downfield doublet at 8.6 ppm for H-3
with the coupling constant 3J = 12.2 Hz, whereas the presence of TT is
identified by the singlet at 5.7 ppm. Study of the relaxation kinetics of
benzopyran 1 open form at low temperatures (–30 ◦C and ꢀ 10 ◦C) using
NMR spectroscopy has allowed to determine the lifetime of TC isomer
which was 77 h at –30 ◦C and 6 h at ꢀ 10 ◦C (Table 2). In this case, the
minor TT form of benzopyran 1 is more stable and lives for several days
at room temperature. Taking into account these findings and the known
behavior of chromenes, [10,26,35] we conclude that (i) the TC form is
generated first, (ii) the TT form accumulation requires prolonged or/and
high intensity irradiation, and (iii) the TC form exhibits bleaching rate
considerably higher than that of the TT form.
3.3. Photochromic behavior of 1 and complex 1∙2
The photochromic transformations of 1 and complex 1∙2 were
studied in acetonitrile and dichloromethane solutions. Dichloromethane
was chosen as a solvent because the stability constants for the complexes
of ammonium derivatives with 18-crown-6-containing ligands are
higher in CH2Cl2 than in MeCN. [35,37] On the other hand, compounds
1 and 2 as well as their complex 1∙2 are well soluble in MeCN solution,
which is appropriate for NMR analysis of the structures of photochromic
Upon irradiation of supramolecular complex 1⋅2 with 313 nm light,
6

O.A. Fedorova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 405 (2021) 112975
and 408 s in CH2Cl2 at 22 ◦C (Table 1, Fig. 8b), which is lower than the
corresponding values for the compound 1 (176 and 3200 s, see above).
The study of the phototransformation of the supramolecular complex
1∙2 by NMR spectroscopy at –30 ◦C in acetonitrile showed the
appearance of an open form of benzopyran 1 upon UV irradiation
(Fig. 9). Analysis of the spectra of the open form revealed the presence of
a predominant TC form, which is consistent with the monoexponential
bleaching curve of the open form at room temperature. By analogy with
the starting benzopyran 1 for complex 1∙2, kinetic studies were carried
out and the kinetic characteristics of bleaching were obtained.
Compared with free benzopyran 1, in the presence of 2, the bleaching
rate is increased by 5–6 times (Table 2).
In order to elucidate the difference in bleaching rates for the open
forms of complex 1∙2 and compound 1, we calculated the charges on the
carbonyl oxygen atom O(1) and carbon atom C(2) of the CPh2-group in
the TC forms of 1 and 1∙2 responsible for thermal relaxation (see Fig. 2
for the atom numbering in the chromene fragment). Calculations were
performed using PM7 method. The results show that in the 1-TC, these
charges are both negative: –0.026 for C(2) and –0.597 for O(1). As a
result of π-conjugated push-pull character of the chromene open form,
complexation with the positively charged CH2NH+3 fragment of 2 de-
creases to some extent the charge on O(2) atom, which appears to be
–0.588 in 1∙2-TC, and also brings a slight positive charge (+0.010) to C
(2). Hence, the ring closure by the nucleophilic attack of carbonyl ox-
ygen on C(2) atom would be more complicated in the case of individual
chromene 1 compared with its complex 1∙2. This is consistent with the
faster kinetics of decoloration for 1∙2 in our experiments.
3.4. Fluorescence photoswitching behavior
It is known that 1,8-naphthalimides are highly fluorescent with high
chemical stability. Interestingly, the fluorescence intensity of naph-
thalimide molecule 2 in complex 1∙2 can be modulated by photoin-
duced conversion between the open and closed forms of benzopyran 1.
Fig. 6. Changes in the absorption spectrum of the benzopyran 1 during UV
irradiation at 313 nm (a) and the relaxation kinetics of its open form (b) in
acetonitrile at 22 ◦C (C = 2.3 ∙ 10ꢀ 5 M, λ =384 nm).
Table 2
The lifetime of the TC isomer of benzopyran 1 alone and in the complex 1∙2, kTC
- relaxation rate constant of TC form in acetonitrile.
Temperature / ◦
C
kTC / sꢀ 1
τ / h
the formation of the colored open form of benzopyran 1 has been also
observed (Fig. 8a, Fig. S2). It was found that complexation significantly
affects the lifetime of the open form of benzopyran 1. A study of the
bleaching kinetics using optical spectroscopy showed that the relaxation
of the open form is monoexponential, and its lifetime is 87 s in MeCN
Species
–30
–10
–30
–10
2.51⋅10ꢀ 6
3.17⋅10ꢀ 5
1.12⋅10ꢀ 5
1.87⋅10ꢀ 4
77
6
1
17
1
•
1 2
1
Fig. 7. H NMR spectra of the benzopyran 1 a) before irradiation; b) after UV irradiation at 313 nm for 60 min (acetonitrile, –30 ◦C).
7

O.A. Fedorova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 405 (2021) 112975
That is, the closed form of complex shows the characteristic fluorescence
from naphthalimide unit at 520 nm, which decreases quickly under the
UV irradiation in CH2Cl2 (Fig. 10) and in MeCN (Fig. S3). During the
decoloration in the dark, the fluorescence of the solution is also recov-
ered. The effect of luminescence quenching of naphthalimide derivative
is more pronounced in non-polar CH2Cl2. It could be due to the more
stable and long-lived open form of benzopyran in this solvent if
compared with MeCN. Considering the high degree of overlap between
the absorption spectrum of the compound 1 open form and fluorescence
band of 2 (see Fig. S4 for the graphical representation), the emission
quenching at 520 nm can be rationalized in terms of resonance energy
transfer from the excited naphthalimide fluorophore to the acceptor
merocyanine form of chromene unit.
4. Conclusions
Supramolecular complex of the photochromic benzopyran 1 and
naphthalimide derivative 2 has been prepared and investigated. Com-
plex 1⋅2 can reversibly change color under UV light and the removal of
Fig. 8. Changes in the absorption spectrum of the supramolecular complex 1∙2
during UV irradiation at 313 nm (a); and the relaxation kinetics of its open form
(b) in acetonitrile at 22 ◦C (C1 = 2.3 ∙ 10ꢀ 5 M, C2 = 9.2 ∙ 10ꢀ 5 M, λ=384 nm).
Fig. 10. The fluorescence spectra of a solution of complex 1⋅2 before (denoted
as «1-CF + 2») and after (denoted as «1-OF + 2») irradiation (313 nm, 25 min)
in dichloromethane (C1 = 2.3∙10ꢀ 5 M, C2 = 2.3∙10ꢀ 5 M, λex =355 nm).
Fig. 9. 1H NMR spectra of supramolecular complex 1∙2 a) before UV irradiation; b) after UV irradiation at 313 nm during 1 h (acetonitrile, –30 оC).
8

O.A. Fedorova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 405 (2021) 112975
irradiation. The combination of two functional molecules in supramo-
lecular complex affects on both photochromic and fluorescence char-
acteristics of the components. Thus, the open form of benzopyran 1 in
complex 1⋅2 exhibits a significantly higher rate of thermal bleaching
than in free benzopyran. The obtained results are similar to what was
observed earlier for naphthopyrans covalently linked to naphthalimide
chromophore. [26] The fluorescence of complex 1⋅2 can be switched on
and off by photoinduced conversion between the closed and open forms.
Moreover, the changes in fluorescence intensity between closed and
open forms are more pronounced in non-polar solvent CH2Cl2. The
proposed reason of the fluorescence photoswitching is PET process be-
tween benzopyran and naphthalimide components of the supramolec-
ular complex and, in part, additional absorption of non-fluorescent
chromene species at the excitation wavelength.
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