Photo- and PH-switchable fluorescent diarylethenes based on 2,3-diarylcyclopent-2-en-1-ones with dialkylamino groups

Article abstract of DOI:10.1016/j.dyepig.2015.09.027

New dialkylamino groups-comprising diarylethenes of 2,3-diarylcyclopent-2-en-1-one (DCP) series have been synthesized and its photochromic, fluorescent as well as acidochromic features have been investigated. It was shown that photochromic properties of t

Full text of DOI:10.1016/j.dyepig.2015.09.027

Dyes and Pigments 124 (2016) 258e267
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Photo- and PH-switchable fluorescent diarylethenes based on 2,3-
diarylcyclopent-2-en-1-ones with dialkylamino groups
,
Valerii Z. Shirinian a *, Dmitry V. Lonshakov a, Andrey G. Lvov a, Alexey M. Kavun b,
Anton V. Yadykov b, Mikhail M. Krayushkin a
a N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47, Leninsky Prosp., 119991 Moscow, Russian Federation
b Higher Chemical College of the Russian Academy of Sciences, 9, Miusskaya Pl., 125047 Moscow, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
New dialkylamino groups-comprising diarylethenes of 2,3-diarylcyclopent-2-en-1-one (DCP) series have
been synthesized and its photochromic, fluorescent as well as acidochromic features have been inves-
tigated. It was shown that photochromic properties of the substances depend strong on their structures;
the non-symmetry of the photochromic molecule is a powerful tool to properly control and tune the
parameters of diarylethenes. It has been found that the most promising of DCPs synthesized are those
containing dialkylamino groups in ethene “bridge” (rather than in aryl moieties) because they possess
photo- and pH-switchable emission along with thermal stability. It has been established that the
introduction at the second position of the cyclopentenone ring the benzene residue leads to the
disappearance of the photochromic properties.
Received 18 May 2015
Received in revised form
12 August 2015
Accepted 17 September 2015
Available online 3 October 2015
Keywords:
Photochromic diarylethenes
Fluorescence
Diarylcyclopent-2-en-1-one
Spectral properties
Thermal stability
Synthesis
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
structures of compounds and their spectral properties, which, in
turn, require the synthesis and modification of the structures.
Design and synthesis of functional organic and hybrid materials
has a high priority in the development of new technologies and
smart devices [1e3]. Among them photochromic materials have
been the focus of much attention because of their potential tech-
nological applications in display, high-density memory devices and
other high technology fields [4e6]. One of promising classes of
photochromic organic dyes are diarylethenes due to excellent
thermal stability of its ring-open and ring-closed isomers, high
fatigue resistance, rapid response, and photochromism in crystals
[7e10].
The possible applications of diarylethenes are connected with
design of photonic devices (optical memories and molecular pho-
toswitches) [5,7], chemosensors [11e14], write-by-light/erase-by-
heat recording systems [15,16], cell markers for biochemistry and
medicine [17], smart devices and materials, etc [18,19]. To gain
compounds with properties appropriate for definite purposes it is
necessary first of all to study the correlations between chemical
One of interesting modifications of diarylethenes structures
seems to be introduction into the molecules of electron donating
dialkylamino groups including those forming merocyanine systems
(conjugation between donating and withdrawing atoms e push-
pull-system). To the best of authors' knowledge, a few of similar
substances and systems have been synthesized as multi-
chromophoric fluorescent switches [20e23] (including pH-
sensitive ones [24e27]) and systems switching between P- and T-
types [26]. In most cases dialkylamino-substituents, merocyanine
dye [20] or spiropyran moieties [21] were attached to aryl groups
(covalently or by hydrogen bonds [13]); there is also the only
example of diarylethene with dimethylamino group in thiazole
ethene “bridge” [25].
To extend the number of known diarylethenes with
dialkylamino-substituents (essentially in ethene “bridge”, which
actually has not been described) and to study thoroughly their
properties and “structure-property” relationships it was proposed
to synthesize the compounds based on photochromic 2,3-
diarylcyclopent-2-en-1-ones (DCPs). DCPs have been reported
earlier [28,29] to be promising class of photochromic diarylethenes
with easily modifiable ethene “bridge” [30e34].
* Corresponding author.
E-mail addresses: shir@ioc.ac.ru, svbegunt@mail.ru (V.Z. Shirinian).
http://dx.doi.org/10.1016/j.dyepig.2015.09.027
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

V.Z. Shirinian et al. / Dyes and Pigments 124 (2016) 258e267
259
Here, we have performed the design of three types of photo-
switchable compounds with dialkylamino-groups. It was pre-
pared new photochromic and fluorescent substances 4 and 6
(Scheme 1) comprising various electron donating dialkylamino
groups both in aryl moieties (compounds 4) and in cyclopentenone
“bridge” (compounds 6).
were obtained from a TOF mass Bruker maXis spectrometer with an
ESI source. Microanalyses were obtained using a PerkinElmer 2400
Series II CHNS/O Elemental Analyzer.
Electronic absorption spectra were recorded on a LOMO SF-56
spectrophotometer. Fluorescence spectra were measured using a
Fluorat®-02-Panorama spectrofluorometer. The experiments were
performed in acetonitrile solutions (C ¼ 2$10ꢀ5 mol Lꢀ1 for ab-
sorption spectra and C ¼ 2$10ꢀ6 mol Lꢀ1 for emission ones) at
293 K in the air presence. Photocoloration was carried out using
6W Vilber Lourmat (France) UV-lamp model VL-6.LC (365 nm
light); the emission was induced at maxima of excitation spectra.
It is important to note that the introduction in diarylethene
molecule of an additional conjugated
p-system along with hexa-
triene is an effective way to design high-efficiency multifunctional
photosensitive substances (materials). DCPs 4aed and 6aee
include merocyanine system (conjugation between donating ni-
trogen atom and withdrawing oxygen atom) along with hexatriene
system responsible to photochromic reaction. While in compounds
6aee merocyanine and hexatriene systems are separated from each
other, in diarylethenes 4aed two systems intersect, whereas in
DCPs 4e,f merocyanine system is absent at all. The differences are
certain to influence on photochromic and fluorescent characteris-
tics of diarylethenes; the properties of dialkylamino groups-
containing DCPs as well as their switching parameters have been
investigated and are discussed below.
Quantum yields of ring-closure (4A/B) and ring-opening (4
)
B/A
processes were calculated by previously reported technique
[28,35]. The fluorescence quantum yields were determined by
using as references the solutions of 4-dimethylamino-40-nitro-
stilbene in benzene (4 ¼ 0.53) [36] (for compounds 6b,e), quinine
f
in 0.1 N sulfuric acid (4 ¼ 0.55) [37] (for compounds 4b,c,g),
f
naphthalene in methylcyclohexane (4 ¼ 0.23) [38] (for com-
f
pounds 4e,f), coumarin-1 in ethanol (4 ¼ 0.73) [39] (for com-
f
pounds 4a,d) or coumarin-30 in acetonitrile (4 ¼ 0.67) [39] (for
f
compounds 6a,c,d).
Commercially available reagents and solvents were used. Col-
umn chromatography was performed using silica gel 60
(70e230 mesh); TLC analysis was conducted on silica gel 60 F254
plates.
2. Experimental
2.1. Materials and methods
1H and 13C NMR spectra were recorded in deuterated solvents
on a Bruker AM-300 spectrometer working at 300 MHz for 1H,
75 MHz for 13C. Mass spectra were obtained on a Kratos mass
spectrometer (70 eV) with direct sample injection into the ion
source. Melting points were measured on a Boetius hot stage and
were not corrected. IR spectra were obtained on a Specord M80 or
M82 spectrometer in KBr pellets. High resolution mass spectra
2.2. Synthesis
1-Arylethanones 1 based on 2-methyl-1-benzothiophen and 2-
heptyl-1-benzothiophen were obtained by their successive 3-
acetylation [40]. 1-[4-(Dialkylamino)phenyl]ethanones were syn-
thesized from commercial fluorobenzene by its 4-acetylation [41]
Dialkylamino group in ethene bridge:
Dialkylamino group in aryl moiety:
merocyanine system is present
only in open form A
Dialkylamino group in aryl moiety:
merocyanine system is present
merocyanine system is present neither
both in open form A and in closed form B
in open form A nor in closed form B
R2
N
R3
..
O
O
O
R1
R1
S
S
..
N
R2
..
R3
R2
Me
N
R3
S
S
Me
Me
R2
Me
4a-d open form A
4e,f open form A
6a-e open form A
no
photochromism
N
..
R3
O
O
R1
O
R1
S
R3
N
..
S
..
R2
R2
Me
N
R3
Me
Me
S
S
Me
4a-d closed form B
4e,f closed form B
6a-e closed form B
Scheme 1. DCPs containing dialkylamino groups in ethene “bridge” or in aryl moieties.

260
V.Z. Shirinian et al. / Dyes and Pigments 124 (2016) 258e267
and subsequent efficient nucleophilic substitution of fluorine atom
in water by corresponding secondary amine by earlier developed
method [42].
2.2.2. 2-(2-Methyl-1-benzothiophen-3-yl)-3-(4-pyrrolidin-1-
ylphenyl)cyclopent-2-en-1-one (4a)
Yield 0.50 g (45%), yellow powder, M.p. 80e82 ꢁC (ethanol). 1H
b-Ketoesters 2 were prepared by previously described way [43]
NMR (300 MHz, CDCl3, d
, ppm): 1.94e2.02 (m, 4H, CHp2yrrol), 2.28 (s,
based on acylation of Meldrum's acid with corresponding arylacetic
acids obtained in turn by the reduction of ethyl-2-(2-
alkylbenzothiophene-3-yl)-2-oxoacetates with triethylsilane [44]
or by WillgerodteKindler reaction [45] of 1-[4-(dialkylamino)
phenyl]ethanones.
3H, CH3), 2.71e2.77 (m, 2H, CHc2yclopent), 3.18e3.31 (m, 6H, 2CH2pyrrol
,
CHc2yclopent), 6.34 (d, 2H, Harom, J ¼ 8.8 Hz), 7.19e7.32 (m, 5H, Нarom),
7.76 (d, 1H, Harom, J ¼ 7.7 Hz). 13C NMR (75
МHz, CDCl3, d, ppm):
14.82, 25.46, 28.89, 34.43, 47.46, 110.85, 111.32, 122.02, 122.58,
123.64,124.05, 126.57, 129.50, 129.66,137.89, 138.90,139.25,149.34,
170.12, 207.26 (C]O). Mass, m/z (%): 373 (100, [M]þ). HRMS:
Calculated for C24H23NOS (M þ Hþ): 374.1573. Found: 374.1572.
a-Bromoketones 3 were synthesized by 5-min refluxing of
mixture of corresponding 1-arylethanone 1 (1.0 eq.) with copper
(II) bromide (2.5 eq.) in methanol (the technique similar to that for
2,3-diarylcyclopent-2-en-1-ones bromination [30]).
2,3-Diarylcyclopent-2-en-1-ones 4 were obtained by alkylation
of b-ketoesters 2 with bromoketones 3 [28] (Scheme 2) and aryl-
methylidenecyclopentenones 6 were prepared by the reactions of
compound 4h with benzaldehydes 5 [32] (Scheme 3), as following.
The cyclopentenones 4h [28], 4i [28], 4j [28], 4k [46], 4l [46] and
4m [47] were synthesized from corresponding ketoester 2 and
bromoketones 3 by the method used in Ref [28].
2.2.3. 2-(2-Methyl-1-benzothiophen-3-yl)-3-(4-piperidin-1-
ylphenyl)cyclopent-2-en-1-one (4b)
Yield 0.23 g (20%), red powder, M.p. 80e83 ꢁC (ethanol). 1H NMR
(300 MHz, CDCl3, d
, ppm): 1.52e1.65 (m, 6H, 3CHp2iper), 2.27 (s, 3H,
CH3), 2.71e2.78 (m, 2H, CHc2yclopent), 3.15e3.27 (m, 6H, 2CH2piper
,
CHc2yclopent), 6.67 (d, 2H, Harom, J ¼ 9.2 Hz), 7.19e7.31 (m, 5H, Harom),
7.76 (d, 1H, Harom, J ¼ 7.0 Hz). 13C NMR (75
МHz, CDCl3, d, ppm):
14.82, 24.36, 25.43, 28.93, 34.45, 48.70, 113.06, 114.08, 121.24,
122.06, 122.52, 123.72, 123.95, 124.09, 129.43, 131.10, 138.90, 139.15,
152.75, 169.73, 207.44 (C]O). Mass, m/z (%): 387 (100, [M]þ).
HRMS: Calculated for C25H25NOS (M þ Hþ): 388.1730. Found:
388.1730.
2.2.1. Synthesis of diarylcyclopentenones (4) (general procedure)
To a solution of ketoester 2 (10 mmol) in abs. benzene (30 mL)
sodium (0.24 g, 10.5 mmol) was added, and the mixture was stirred
for 3e4 h until sodium dissolved. To a cooled to 10 ꢁC reaction
mixture a solution of bromoketone 2 (10 mmol) in abs. benzene
(15 mL) was added over 0.5 h. The solution was stirred overnight at
room temperature, then poured into cold water (150 mL) and
extracted with ethylacetate (3 ꢂ 50 mL). The combined organic
phases were washed with water (100 mL), filtered through 1 cm
layer of silica gel and evaporated. The intermediate e alkylated
ketoester e was used further without additional purification.
A solution of KOH (0.84 g, 15 mmol) in water (11 mL) was added
at once to a solution of crude alkylated ketoester (3 mmol) in
ethanol (11 mL). The reaction mixture was refluxed for 4 h, then
cooled, poured into water (10 mL) and extracted with ethyl acetate
(3 ꢂ 20 mL). The combined extracts were washed with water
(50 mL), dried with magnesium sulfate and evaporated in vacuum.
The residue was purified by column chromatography eluting by
petrol. ether/ethyl acetate 4:1 and recrystallized from ethanol. The
physical and chemical properties including spectral characteristics
of DCPs 4hej [28], 4k,l [46] and 4m [30,47] have been reported
previously.
2.2.4. 2-(2-Methyl-1-benzothiophen-3-yl)-3-(4-morpholin-4-
ylphenyl)cyclopent-2-en-1-one (4c)
Yield 0.47 g (40%), brown powder, M.p. 75e77 ꢁC (ethanol). 1H
NMR (300 MHz, CDCl3, d, ppm): 2.26 (s, 3H, CH3), 2.73e2.79 (m, 2H,
CH2cyclopent), 3.14e3.20 (m, 4H, 2CH2morph), 3.20e3.26 (m, 2H, CHc2y-
clopent), 3.75e3.83 (m, 4H, 2CH2morph), 6.68 (d, 2H, Harom, J ¼ 8.9 Hz),
7.17e7.25 (m, 5H, Нarom), 7.77 (d, 1H, Harom, J ¼ 7.9 Hz). 13C NMR (75
МHz, CDCl3, d, ppm): 14.82, 29.76, 34.46, 47.67, 66.62, 113.39,
113.99, 122.06, 122.44, 123.75, 124.11, 129.27, 130.13, 138.14, 138.88,
139.04, 152.35, 169.34, 207.26 (C]O). Mass, m/z (%): 389 (47, [M]þ),
331 (17, [M-C3H6O]þ), 190 (100, [C11H10OS]þ), 148 (42, [C9H8S]þ),
132 (48, [C9H10N]þ). HRMS: Calculated for C24H23NO2S (M þ Naþ):
412.1342. Found: 412.1338.
2.2.5. 3-(4-Azepan-1-ylphenyl)-2-(2-methyl-1-benzothiophen-3-
yl)-2-en-1-one (4d)
Yield 0.54 g (45%), red powder, M.p. 90e92 ꢁC (ethanol). 1H NMR
(300 MHz, CDCl3,
d
, ppm): 1.42e1.75 (m, 8H, 4CHа2зеп), 2.26 (s, 3H,
Ar1
COOEt
O
2
O
1. Na, benzene
O
+
Ar1(Ar2)
2. KOH, EtOH/H2O
O
Ar1
Ar2
1
4a-g
Br
Ar2
3
a
b
c
d
e
f
g
Ar1
Ar2
N
N
N
Me
S
Me
Me
Me
S
S
S
O
O
O
N
N
N
N
N
Me
H C
S
S
O
Scheme 2. Synthetic route for the preparation of dialkylamino-containing DCPs 4aeg.

V.Z. Shirinian et al. / Dyes and Pigments 124 (2016) 258e267
261
Ar
H
Ar
O
O
O
5a-e
NaOH
EtOH, H2O
Me
Me
Me
S
S
S
S
Me
Me
Me
Me
4h
Me
6a-e
Ar =
N
N
N
N
N
O
a
b
c
d
e
Scheme 3. Condensation of dithienylcyclopentenone 4h with substituted benzaldehydes 5aee.
CH3), 2.67e2.77 (m, 2H, CHc2yclopent), 3.15e3.25 (m, 2H, CHc2yclopent),
3.32e3.44 (m, 4H, 2CH2азеп), 6.44 (d, 2H, Harom, J ¼ 8.8 Hz), 7.12e7.31
Hphenyl, J ¼ 8.8 Hz). 13C NMR (75
М
Hz, CDCl3,
d
, ppm): 28.92, 29.68,
47.94 (N(CH2)2morph), 49.06 (N(CH2)m2 orph), 66.65 (O(CH2)m2 orph),
66.91 (O(CH2)m2 orph), 113.96, 115.50, 124.48, 126.41, 129.57, 130.39,
137.34, 150.48, 150.86, 166.17, 208.80 (C]O). Mass, m/z (%): 404 (93,
[M]þ). Calculated for C25H28N2O3, %: C, 74.23, H, 6.98, N, 6.93.
(m, 5H, Нarom), 7.74 (d, 1H, Harom, J ¼ 7.7 Hz). 13C NMR (75
МHz,
CDCl3,
d, ppm): 14.83, 26.91, 27.44, 28.80, 34.41, 49.26, 110.74,
121.60, 122.02, 122.59, 123.56, 124.06, 126.61, 129.48, 129.85, 137.50,
138.90, 139.27, 150.62, 169.74, 207.24 (C]O). Mass, m/z (%): 401
(100, [M]þ). HRMS: Calculated for C26H27NOS (M þ Naþ): 424.1706.
Found: 424.1710.
Найдено, %: C, 73.85, H, 7.05, N, 6.98.
2.2.9. Synthesis of 5-arylmethylidenecyclopent-2-en-1-ones (6)
(general method)
2.2.6. 3-(2-Methyl-1-benzothiophen-3-yl)-2-(4-piperidin-1-
ylphenyl)cyclopent-2-en-1-one (4e)
(Scheme 3). The mixture of dithienylcyclopentenone 4h
(0.33 mmol) and corresponding aldehyde 5 (0.43 mmol) in mixture
of ethanol (1 mL) and 10%-solution of NaOH in water (1 mL) was
refluxed for 1e3 h to maximal conversion of initial cyclopentenone
4h (TLC-control) and poured into ice-water (60 mL). The residue
was filtered off, washed with water (2 ꢂ 20 mL), petrol ether
(2 ꢂ 20 mL) and recrystallized from ethanol.
Yield 0.35 g (30%), pale red powder, M.p. 146e149 ꢁC (ethanol).
1H NMR (300 MHz, CDCl3, , ppm): 1.44e1.74 (m, 6H, 3CHp2iper), 2.10
d
(s, 3H, CH3), 2.65e2.77 (m, 2H, CHc2yclopent), 2.97e3.24 (m, 6H,
2CHp2iper, CH2cyclopent), 6.70 (d, 2H, Harom, J ¼ 8.4 Hz), 7.11 (d, 2H,
Harom, J ¼ 8.4 Hz), 7.29 (t, 2H, Harom, J ¼ 3.7 Hz), 7.48e7.56 (m, 1H,
Harom), 7.70e7.80 (m, 1H, Harom). 13C NMR (75
МHz, CDCl3, d, ppm):
14.77, 24.33, 25.67, 30.76, 35.53, 50.04, 115.63, 122.03, 122.28,
124.02, 124.52, 129.22, 129.90, 136.78, 138.75, 138.92, 142.09,
163.40, 207.97 (C]O). Mass, m/z (%): 387 (95, [M]þ). HRMS:
Calculated for C25H25NOS (M þ Hþ): 388.1730. Found: 388.1730.
2.2.10. 2,3-Bis(2,5-dimethylthiophen-3-yl)-5-{[4-(dimethylamino)
phenyl]methylidene} cyclopent-2-en-1-one (6a)
Yield 0.13 g (91%), yellow powder, M.p. 198e199 ꢁC (ethanol). 1H
NMR (300 MHz, CDCl3, d, ppm): 1.92 (s, 3H, Me), 1.95 (s, 3H, Me),
2.40 (s, 3H, Me), 2.43 (s, 3H, Me), 3.04 (s, 6Н, 2NCH3), 3.81 (s, 2H,
CHc2yclopent), 6.58 (s, 1H, Hthioph), 6.66 (s, 1H, Hthioph), 6.74 (d,
J ¼ 8.4 Hz, 2Н, Нphenyl), 7.47 (s, 1Н, СНaldeh), 7.55 (d, J ¼ 8.4 Hz, 2Н,
2.2.7. 3-(2-Heptyl-1-benzothiophen-3-yl)-2-(4-morpholin-4-
ylphenyl)cyclopent-2-en-1-one (4f)
Yield 0.35 g (25%), gray powder, M.p. 160e163 ꢁC (ethanol). 1H
Н d, ppm): 14.35,14.68,15.24,15.35,
phenyl). 13C NMR (75 MHz, CDCl3,
NMR (300 MHz, CDCl3,
d
, ppm): 0.84e0.94 (m, 3H, CH3), 1.16e1.32
37.62 (CH2cyclopent), 40.19 (2NCH3), 112.09, 123.57, 125.27, 126.94,
128.68, 129.90, 132.16, 132.38, 133.89, 135.07, 135.60, 136.69, 136.97,
137.50, 151.05, 157.48, 195.54 (C]O). Mass, m/z (%): 433 (85, [M]þ),
418 (100, [MꢀCH3]þ), 390 (32, [Mꢀ N(CH3)2]þ). HRMS: Calculated
for C26H27NOS2 (M þ H)þ: 434.1607. Found: 434.1601. IR (KBr),
cmꢀ1: 2951 (СeHarom), 2914 (СeHarom), 2864 (СeHarom), 1676 (C]
O), 1600, 1526, 1443, 1371, 1291 (CaromeN), 1189 (CalipheN), 1139,
815 (СeHphenyl), 528.
(m, 10H, 5CH2), 2.37e2.63 (m, 2H, CH2), 2.76e2.81 (m, 2H, CH2cy-
clopent), 3.05e3.12 (m, 4H, 2CHm2 orph), 3.14e3.25 (m, 2H, CH2cyclopent),
3.77e3.84 (m, 4H, 2CH2morph), 6.70 (d, 2H, Harom, J ¼ 8.9 Hz), 7.16 (d,
2H, Harom, J ¼ 8.9 Hz), 7.30e7.38 (m, 2H, Нarom), 7.49e7.56 (m, 1H,
Н
arom), 7.78e7.84 (m, 1H, Нarom). 13C NMR (75
МHz, CDCl3, d, ppm):
14.14, 22.70, 29.09, 29.41, 29.45, 30.95, 31.25, 31.75, 35.53, 48.86,
66.86, 114.99, 122.01, 122.46, 122.76, 124.05, 124.54, 129.11, 129.47,
138.65, 138.96, 142.00, 142.84, 150.73, 164.40, 207.89 (C]O). Mass,
m/z (%): 473 (100, [M]þ), 388 (10, [MꢀC6H13]þ). Calculated for
2.2.11. 2,3-Bis(2,5-dimethylthiophen-3-yl)-5-[(4-pyrrolidin-1-
ylphenyl)methylidene]cyclopent-2-en-1-one (6b)
C
30H35NO2S, %: C, 76.07, H, 7.45. Найдено, %: C, 73.15, H, 7.48.
Yield 0.13 g (90%), yellow powder, M.p. 160e163 ꢁC (ethanol). 1H
2.2.8. 2,3-Bis(4-morpholin-4-ylphenyl)cyclopent-2-en-1-one (4g)
Yield 0.38 g (32%), yellow powder, M.p. 224e224.5 ꢁC (ethanol).
NMR (300 MHz, CDCl3,
d, ppm): 1.91 (s, 3H, CH3); 1.95 (s, 3H, CH3);
2.00e2.13 (m, 4H, CH2); 2.40 (s, 3H, CH3); 2.43 (s, 3H, CH3);
1H NMR (300 MHz, CDCl3,
d
, ppm): 2.61e2.69 (m, 2H, CHc2yclopent),
3.23e3.34 (m, 4H, CH2); 3.80 (s, 2H, CH2); 6.55e6.64 (s, 1H,1Нthioph
,
2.95e3.04 (m, 2H, CHc2yclopent), 3.14e3.27 (m, 4H, N(CH2)m2 orph),
3.80e3.93 (m, 4H, O(CH2)m2 orph), 6.76 (d, 2H, Hphenyl, J ¼ 8.8 Hz), 6.91
(d, 2H, Hphenyl, J ¼ 8.8 Hz), 7.18 (d, 2H, Hphenyl, J ¼ 8.8 Hz), 7.37 (d, 2H,
д, 2H, Hphenyl, J ¼ 6.9 Hz); 6.66 (s, 1H, Hthioph); 7.47 (s, 1H, CHaldeh);
7.54 (d, 2H, Hphenyl, J ¼ 6.9 Hz). 13C Я
МР (75 МHz, CDCl3, d, ppm):
14.31, 14.64, 15.20, 15.31, 25.54 (CH2), 37.66 (CH2), 47.60 (2CH2),

262
V.Z. Shirinian et al. / Dyes and Pigments 124 (2016) 258e267
111.89, 122.87, 125.25, 126.91, 128.00, 129.93, 132.22, 132.55, 133.89,
134.99, 135.51, 136.61, 136.83, 137.47, 148.59, 157.28, 195.52 (C]O).
Mass, m/z (%): 459 (100, [M]þ); 444 (96, [MꢀCH3]þ). HRMS:
Calculated for C28H29NOS2 (M þ Н)þ: 460.1763. Found: 460.1749. IR
(KBr), cmꢀ1: 2965 (СeHarom), 2914 (CH3, СeHarom), 2841 (CH3,
СeHarom), 1678 (C]O), 1601 (C]C), 1526, 1444, 1390, 1292
(CaromeN), 1185 (CalipheN), 1140, 808 (СeHphenyl).
irradiation by UV light
(
lir
¼
365 nm) and visible light
(l
ir > 400 nm) and are summarized in Table 1. Upon UV-irradiation,
the colorless solutions of DCPs 4 and 6 were converted into colored
ones and bleached back to colorless under visible light that is due to
the reversible electrocyclic reaction of hexatriene system from
colorless form A to cyclic (colored) form B (Scheme 4).
Analyzing the data in Table 1 one can see that the properties of
DCPs 4aeg and 6aee containing electron donating dialkylamino-
groups depend considerably on their structures. Some correla-
tions between diarylethenes structures and its fluorescent and
photochromic parameters have been revealed.
2.2.12. 2,3-Bis(2,5-dimethylthiophen-3-yl)-5-[(4-piperidin-1-
ylphenyl)methylidene]cyclopent-2-en-1-one (6c)
Yield 62 Мг (40%), yellow powder, M.p. 156e158 ꢁC (ethanol). 1H
NMR (300 MHz, CDCl3, d, ppm): 1.58e1.77 (m, 6H, CH2), 1.91 (s, 3H,
CH3), 1.94 (s, 3H, CH3), 2.40 (s, 3H, CH3), 2.43 (s, 3H, CH3), 3.27e3.67
(m, 4H, CH2), 3.81 (s, 2H, CH2), 6.58 (s, 1H, Hthioph), 6.66 (s, 1H,
Hthioph), 6.92 (d, 2H, Hphenyl, J ¼ 8.2), 7.46 (s,1H, CHaldeh), 7.54 (d, 2H,
3.1. Photochromic properties of DCPs
From the structures of the DCPs synthesized one can see that the
compounds 4aed,g and 6aee are bichromophoric, with a mer-
ocyanine system fused to a photochromic one. While in DCPs
4aed,g these systems intersect, in compounds 6aee they are
separated; compounds 4e,f in the same time lack merocyanine
system at all (Scheme 1). The structural differences are certain to
affect the photochromic properties of DCPs.
Hphenyl, J ¼ 8.2). 13C NMR (75
МHz, CDCl3, d, ppm): 14.32, 14.66,
15.21, 15.32, 24.42 (CH2), 25.57 (2CH2), 37.51 (CH2), 49.17 (2CH2),
115.01, 125.19, 125.27, 126.85, 129.39, 129.78, 131.78, 132.20, 133.79,
135.08, 135.61, 136.70, 137.04, 137.42, 152.16, 157.67, 195.54 (C]O).
Mass, m/z (%): 473 (100, [M]þ); 458 (27, [MꢀCH3]þ). HRMS:
Calculated for C29H31NOS2 (M þ Н)þ: 474.1920. Found: 474.1910. IR
(KBr), cmꢀ1
:
2966 (СeHarom), 2935 (СeHarom), 2915 (CH3,
As to the values of absorption bands maxima of open-ring iso-
mers A of DCPs it should be noticed that the presence of dia-
lkylamino groups in the molecules causes, as a rule, the appearance
of the bands of forms A in the fields of long wave UV light and/or
visible light (Table 1). For example, for DCPs 4aed,g the maxima are
ranged in the 344e380 nm, while for compounds 6aee they are
strongly shifted up to the 386e422 nm (in contrast with parent
dithienyl-derivative 4h: the most long-wavelength band is only at
309 nm). Only in case of DCPs 4e,f absorption bands of colorless
forms A were observed in the fields of short wave UV light (at
261e262 nm). It is of interest to see that these observations
correlate with the above-mentioned structural features of com-
pounds: thus, it seems to be merocyanine system in the initial
forms A of DCPs 4aed,g, 6aee which induces the appearance of
absorption bands in long wave UV or visible region; and on the
contrary, DCPs 4e,f (without merocyanine system) lack long
wavelength absorption bands. The absorption bands shifts towards
visible spectrum region (like in case of DCPs 6aee) is known to be a
promising feature of compounds as for different practical applica-
tions it is significant to handle diarylethenes which can be pro-
cessed by less destructive visible light rather than UV [48].
Absorption bands maxima of photoinduced isomers B of DCPs
4aed are ranged in the 455e504 nm. The introduction of dia-
lkylamino groups into the ethene “bridge” leads to bathochromic
shifts of the bands of colored forms B: from 547 nm for compound
4h to 568e580 nm region for its derivatives 6aee. Talking about
compounds 4 it is worthwhile to notice the entire absence of
photochromic properties of DCPs 4eeg; one might try to explain
the observation by the above-mentioned lack of merocyanine
system in the diarylethenes structures.
СeHarom), 2854 (CH3, СeHarom), 2822 (СeHarom), 1682 (C]O),
1635, 1603 (C]C), 1519, 1443, 1385, 1291 (CaromeN), 1252, 1223,
1190 (CalipheN), 1140, 1125, 923, 883, 815 (СeHphenyl).
2.2.13. 2,3-Bis(2,5-dimethylthiophen-3-yl)-5-[(4-morpholin-4-
ylphenyl)methylidene] cyclopent-2-en-1-one (6d)
Yield 0.10 g (82%), yellow powder, M.p. 211e213 ꢁC (ethanol). 1H
NMR (300 MHz, CDCl3, d, ppm): 1.88 (s, 3H, Me), 1.92 (s, 3H, Me),
2.37 (s, 3H, Me), 2.40 (s, 3H, Me), 3.24 (s, 4Н, 2CHm2 orph), 3.78 (s, 2H,
CHc2yclopent), 3.84 (s, 4Н, 2CH2morph), 6.54 (s, 1H, Hthioph), 6.63 (s, 1H,
Hthioph), 6.89 (d, J ¼ 8.1 Hz, 2Н, Нphenyl), 7.43 (s, 1Н, СНaldeh), 7.53 (d,
J ¼ 8.1 Hz, 2Н, Нphenyl). 13C NMR (75 MHz, CDCl3,
d, ppm): 14.23,
14.59, 15.12, 15.22, 37.34 (CHc2yclopent), 48.06 (CH2morph), 66.66
(CHm2 orph), 114.73, 125.06, 126.60, 126.72, 129.58, 130.13, 131.28,
131.99, 133.61, 135.05, 135.58, 136.69, 137.10, 137.29, 151.58, 157.81,
195.37 (C]O). Mass, m/z (%): 475 (100, [MꢀH]þ), 460 (55,
[MꢀCH3]þ). HRMS: Calculated for C28H29NO2S2 (M þ H)þ: 476.1712.
Found: 476.1709. IR (KBr), cmꢀ1: 2984 (СeHarom), 2908 (СeHarom),
2856 (СeHarom), 1680 (C]O), 1636, 1596, 1520, 1448, 1296
(CaromeN), 1244, 1188 (CalipheN), 1124, 1112, 928, 836, 820
(СeHphenyl).
2.2.14. 5-[(4-Azepan-1-ylphenyl)methylidene]-2,3-bis(2,5-
dimethylthiophen-3-yl)cyclopent-2-en-1-one (6e)
Yield 0.13 g (84%), brown powder, M.p. 166e168 ꢁC (ethanol). 1H
NMR (300 MHz, CDCl3, d, ppm): 1.50e1.61 (m, 8H, CH2), 1.92 (s, 3H,
CH3), 1.94 (s, 3H, CH3), 2.40 (s, 3H, CH3), 2.43 (s, 3H, CH3), 3.49e3.58
(m, 4H, CH2), 3.80 (s, 2H, CH2), 6.58 (s, 1H, Hthioph), 6.65 (s, 1H,
Hthioph), 6.72 (d, 2H, Hphenyl, J ¼ 8.8 Hz), 7.45 (s, 1H, CHaldeh), 7.52 (d,
2H, Hphenyl, J ¼ 8.8 Hz). Я
М
Р
13C (75
МHz, CDCl3,
d, ppm): 14.32,
However then it was determined that this effect is not due to the
presence or absence of dialkylamino groups in DCPs molecules but
caused by the presence of the phenyl moiety in the structures. It
was shown by the several examples (Chart 1) that when phenyl ring
is in the 3rd position of ethene “bridge”, DCPs (compounds 4b,h,i,k)
are basically photochromic diarylethenes but when phenyl group is
attached to the second position, DCPs (compounds 4e,g,j,l,m) do
not undergo photocyclization at all. It should be emphasized that
the nature of the second aryl group (thienyl, benzothienyl or oxa-
zolyl) in the structures as well as the presence or absence of sub-
stituents in phenyl ring does not matter: this regularity is valid for
DCPs with different kind of aryl moieties (Chart 1).
14.66, 15.22, 15.33, 27.01 (2CH2), 27.59 (2CH2), 37.64 (CH2), 49.62
(2CH2), 111.35, 122.76, 125.25, 126.91, 128.04, 129.92, 132.27, 132.70,
133.89, 134.99, 135.53, 136.60, 136.85, 137.47, 149.79, 157.24, 195.55
(C]O). Mass, m/z (%): 487 (100, [M]þ); 472 (20, [MꢀCH3]þ). HRMS:
Calculated for C30H33NOS2 (M þ Н)þ: 488.2076. Found: 488.2062.
IR (KBr), cmꢀ1: 2930 (СeHarom), 2854 (CH3, СeHarom), 1676 (C]O),
1579 (C]C), 1520, 1469, 1397, 1291 (CaromeN), 1260, 1186
(CalipheN), 1165, 1111, 917, 811 (СeHphenyl).
3. Results and discussion
The photochromic characteristics of diarylethenes have been
measured in acetonitrile solutions at 293 K with alternating
The reason for the lack of photochromic activity may be
different, including competition between photochromic properties

V.Z. Shirinian et al. / Dyes and Pigments 124 (2016) 258e267
263
Table 1
Photochromic and fluorescent characteristics of DCPs synthesized in acetonitrile (C ¼ 2$10ꢀ5 mol Lꢀ1) at 293 K in air.
Entry
Photochromism
Compound
4a
Fluorescence
lexe, nm
378
lAa, nm (ε, L$molꢀ1$cmꢀ1
)
lBb, nm (ε, L$molꢀ1$cmꢀ1
504 (1.53$104)
)
4A/B
4B/A
lemf, nm
4
em
4UVh
c
d
openg
1
2
3
4
5
238 (7.31$104)
378 (4.50$104)
235 (6.34$104)
355 (3.62$104)
238 (7.84$104)
344 (3.63$104)
230 (4.57$104)
356 (6.65$103)
234 (6.50$104)
380 (4.08$104)
262 (5.12$104)
261 (2.64$104)
361 (3.55$104)
254 (3.45$104)
348 (3.10$104)
283 (3.03$104)
208 (1.64$104),
245 (1.46$104),
309 (6.43$103)
0.27
0.001
0.001
0.001
0.003
0.001
464
0.13
e
4b
478 (1.13$104)
455 (5.52$103)
0.11
0.16
0.07
0.15
350
350
456
443
0.13
e
e
e
e
4c
0.30
4c þ HCl
398 (2.55$104), 545 (2.40$104)
494 (1.38$104)
~5$10ꢀ3
0.17
4d
380
449
6
7
8
9
4e
4f
Non photochromic
Non photochromic
Non photochromic
Non photochromic
261
261
539
538
0.09
e
e
e
e
0.15
4f þ HCl
~3$10ꢀ3
0.09
4g
348
549
10
11
4g þ HCl
Non photochromic
~2$10ꢀ3
e
4h
547 (1.11$104)
0.27
0.065
0.001
Non fluorescent
12
6a
233 (6.48$104)
411 (3.93$104)
296 (5.02$104)
341 (3.75$104)
421 (2.88$104)
372 (1.25$105)
240 (4.51$104)
402 (2.83$104)
367 (1.86$105)
252 (1.90$104)
386 (1.62$104)
311 (3.15$104),
359 (2.23$104)
341 (3.75$104)
422 (1.98$104)
308 (1.04$105),
357 (3.90$104)
577 (br.) (2.65$104)
0.01
0.31
411
421
402
386
542
0.34
0.17
13
14
6a þ HCl
605 (1.00$104)
0.010
0.0003
~2$10ꢀ3
<2$10ꢀ3
6b
578 (br.) (2.12$104)
0.005
548
552
537
0.24
0.15
15
16
6b þ HCl
602 (3.80$104)
580 (4.45$104)
0.005
0.01
0.0003
0.001
~1$10ꢀ3
<1$10ꢀ3
6c
0.17
0.05
17
18
6c þ HCl
590 (2.25$104)
571 (7.90$103)
0.02
0.04
0.001
0.004
~1$10ꢀ3
<1$10ꢀ3
6d
0.16
0.04
19
20
21
6d þ HCl
6e
604 (1.53$104)
0.11
0.004
0.0001
0.001
1$10ꢀ3
0.33
<1$10ꢀ3
0.22
568 (br.) (2.45$104)
583 (4.00$104)
0.005
0.003
421
543
6e þ HCl
~1$10ꢀ3
<1$10ꢀ3
a
Absorption maxima (extinction coefficients) of open-ring isomers of DCPs.
Absorption maxima (extinction coefficients) of closed-ring isomers DCPs.
Quantum yields of ring-closure reactions of DCPs.
Quantum yields of ring-opening reactions of DCPs.
Emission excitation wavelengths of DCPs.
b
c
d
e
f
Emission maxima wavelengths of DCPs.
g
h
Fluorescence quantum yields of ring-open isomers of DCPs.
Fluorescence quantum yields of DCPs after 2-Min UV-irradiation (lir ¼ 365 nm).
and fluorescence emission; moreover, in some cases the latter can
completely suppress the photoswitching process. Another possible
reason may be an intermolecular charge transfer with the forma-
tion of TICT (twisted intermolecular charge transfer interaction)
states, which, according to the literature, often leads to the com-
plete disappearance of photochromic properties [49,50]. The
possibility of such an effect could be related with the conjugation
between an electron-acceptor carbonyl and electron-donor dia-
lkylamino groups of diarylethene molecule. Other factors may also
contribute to lack of photochromism. So, we have recently found
that diarylethenes comprising oxazole and phenyl rings as aryl
moieties under UV-irradiation (
l
¼ 365 nm) undergo photo-
isomerization with skeletal rearrangement, giving the naphthalene
derivatives (Scheme 5) [46]. This photoreaction proceeds also
through the formation of cyclic form, although in the case of 2-
phenyl-substituted diarylcyclopentenone (compound 4l) the for-
mation of the cyclic form is also unable to register, but the change of
the absorption spectrum in the UV region is observed (see Ref. [46]
where this phenomenon has been described in detail).
In the case of benzothiophene derivative 4e the photochromic
transformation has not also been observed but in contrast of
compounds 4g,m it is observed the absorption spectra change in
the UV-region without the appearance of the absorption band in
the visible region (Scheme 6 and Fig. 1). For this reaction the
product structure has not yet been proved, but the reaction most
likely proceeds with the opening of the benzothiophene ring,
similar to that described most recently by Kawai et al. [51].
Scheme 4. Photochromic reactions of DCPs.

264
V.Z. Shirinian et al. / Dyes and Pigments 124 (2016) 258e267
O
O
O
S
S
Me
Me
N
R
N
R
4g (R = morpholin-1yl)
4m (R = H)
4e
4b
NON photochromic
Photochromic
NON photochromic
NON photochromic
O
O
O
Me
S
S
Me
Me
4j
NON photochromic
Me
S
S
Me
Me
Me
Me
4h
4i
Photochromic
Photochromic
O
O
N
N
O
O
Ph
Me
Me
4l
Ph
4k
Photochromic
NON photochromic
Photorearrangement
Photorearrangement
Chart 1. The influence of non-symmetry of DCPs molecules on its photochromic properties (presence or absence of photochromism).
O
O
O
UV
H
H
N
N
N
Ph
O
O
Me
Ph
Me O
Ph
Me
7
4l A
4l B
Scheme 5. Photorearrangement of diarylethene 4l.
For each of these compounds it is required further more detailed
study, but in this work we have investigated the patterns of the
influence of dialkylamino group at the different positions of
photochromic molecules on spectral and kinetic characteristics.
Among the “structure-property” relationships the influence of
dialkylamino groups introduction on the quantum yields of pho-
toreactions could also be mentioned. The typical changes observed
in the absorption spectra of the photochromic reactions of diary-
lethenes (for compound 6e) in acetonitrile are shown in Fig. 2. The
spectral characteristics and quantum yields of the photoreactions
are summarized in Table 1. One can see that the most significant
effect is observed in case of transition from DCP 4h to its derivatives
6aee. Such ethene “bridge” modification (introduction of addi-
O
O
HS
UV
S
C7H15
C7H15
N
N
?
O
O
4e
8
tional
p-system with electron donating dialkylamino groups and
therefore creation of merocyanine system in the molecule) results
Scheme 6. Photoreaction of diarylethene 4e and possible structure of photoproduct 8.

V.Z. Shirinian et al. / Dyes and Pigments 124 (2016) 258e267
265
Table 2
Kinetic data of thermal bleaching reactions of DCPs synthesized (acetonitrile solu-
tions, C ¼ 2$10ꢀ5 mol Lꢀ1 at 293 K in air).
kB/Atherma, sꢀ1
t
b, h
B/A
1=2 therm
Entry
Compound
1
2
3
4
5
6
7
8
9
4a
4b
4d
4h
6a
6b
6c
6d
6e
2.69$10ꢀ5
3.11$10ꢀ5
1.63$10ꢀ5
2.05$10ꢀ7
6.84$10ꢀ8
3.29$10ꢀ7
3.39$10ꢀ7
3.61$10ꢀ7
3.70$10ꢀ7
7.2
6.2
12
939
2810
585
568
533
520
a
Thermal cycloreversion reaction rate constants (obtained from kinetic curves).
Half-lifes of the ring-closed isomers in the dark.
b
It was found that all compounds containing dialkylamino
groups (in aryl moieties or in ethene “bridge”) are capable of
emitting fluorescence under UV-light excitation. The emission pa-
rameters depend strong on the compounds structure. So, among
DCPs 4aeg the influence of the molecules non-symmetry should be
again mentioned: compounds 4aed (with merocyanine system in
open forms A) emit in violet-blue region of spectrum
(443e464 nm) while in case of non-photochromic DCPs 4eeg (no
merocyanine system) the fluorescence bands are bathochromically
shifted to green spectrum region (538e549 nm). The introduction
of merocyanine system into cyclopentenone “bridge” of DCPs
turned out to be the efficient way to obtain fluorescent diary-
lethenes: while parent compound 4h is non-fluorescent its de-
rivatives 6aee possess emission at the 537e552 nm region.
The dialkylamino group influence on this property is not
entirely obvious. It is only could be mentioned that in a series of
DCPs 4aed,g (with merocyanine system in colorless form A) and
4e,f (without merocyanine system) the highest fluorescence
quantum yields are observed for morpholine-containing com-
pounds 4c (0.30) and 4f (0.17) correspondingly, although the
quantum yield of bis(morpholinophenyl) derivatives (DCP 4g) is as
little as 0.09.
Fig. 1. Absorption spectra changes of diarylethene 4e in acetonitrile solution
(C z 2*10ꢀ5 M) before and after irradiation with UV (365 nm).
in the essential decrease of photocyclization efficiency: photo-
coloration quantum yield is diminished from 0.27 for diarylethene
4h to 0.005e0.04 for compounds 6aee. Diarylethenes 6aee are
fluorescent substances (in contrast to 4h, see below) and their
photocyclization quantum yields reduction therefore can be
explained by the competition between photochromiс and emission
processes, which leads to essential suppression of the former.
For several DCPs synthesized thermal stability of its photoin-
duced forms B has been estimated by calculation of rate constants
of thermal cycloreversion reaction and half-lifes of the ring-closed
isomers B in the dark (Table 2). Compounds 4a,b,d containing
phenyl moieties in the structures were assumed to be actually
thermally unstable: their colored isomers became colorless in the
dark during as little as 6.2e12 h. The insertion of merocyanine
system into ethene “bridge” of DCP 4h proved to lead as a rule to
slight decrease of stability of cyclic forms B from 939 h for com-
pound 4h to 520e585
h for DCPs 6bee. Only in case of
dimethylamino-derivative 6a 3-fold increase of stability was
detected.
The fluorescence of compounds 6aee was shown to be light-
sensitive, with emission decaying (its quantum yields decrease)
under UV-irradiation (Table 1 and Fig. 3). It is worthwhile to note
that the emission reduction is reversible and under irradiation with
visible light the emission is revived again. The photosensitivity of
fluorescence of DCPs 4 was not studied because of low fatigue
resistance of the substances.
3.2. Fluorescent properties of DCPs
In the paper fluorescent properties of DCPs synthesized have
also been studied; such characteristics as wavelengths of emission
bands maxima and fluorescence quantum yields were measured
and are represented in Table 1.
3.3. Acidochromism of DCPs
The addition of acid to the solutions of dialkylamino-containing
DCPs was expected to induce essential changes both in photo-
chromic and in fluorescent properties of compounds. The acidifi-
cation of solutions was carried out by the passing high excess of
hydrogen chloride by means of syringe.
It was already noted above that the introduction of merocyanine
system into the ethene “bridge” of the molecules leads to the
appearance of the absorption bands of DCPs isomers A in range of
visible light (compounds 6aee). Therefore it is not surprising to see
that the passing HCl through acetonitrile solutions of these diary-
lethenes results in vanishing of the long-wavelength bands because
of dimethylamino group protonation, which destroys its electron
donating ability and break down the merocyanine system on the
whole (compare data for colorless forms A of pure compounds 6
and after addition of HClg in Table 1 and Fig. 3). In case of DCPs 4 the
influence of acid addition on initial isomers A properties is more
complex: in case of compounds 4c (with merocyanine system in the
Fig. 2. Absorption spectra changes of diarylethene 6e in acetonitrile solution
(C z 2*10ꢀ5 M) before and after irradiation with UV (365 nm) and visible light.

266
V.Z. Shirinian et al. / Dyes and Pigments 124 (2016) 258e267
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Fig. 3. Spectral changes of fluorescent switch 6d under UV-irradiation. Absorption
spectra (left axis) of DCP 6d (C ¼ 2.0 ꢂ 10ꢀ5 mol Lꢀ1 in CH3CN) at 293 K: before
irradiation (curve 1), after 2 Min UV irradiation (lir ¼ 365 nm, curve 2), with HClg
passing through before irradiation (curve 3) and with HClgas passing through after 20s
UV irradiation (lir ¼ 365 nm, curve 4). Emission spectra (right axis) of DCP 6d
(C ¼ 2.0 ꢂ 10ꢀ6 mol Lꢀ1 in CH3CN) at 293 K with emission excitation at lex ¼ 386 nm:
before irradiation (curve 5) and after 2 Min UV irradiation (lir ¼ 365 nm, curve 6).
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initial values.
4. Conclusion
A series of new fluorescent diarylethenes with cyclopentenone
ethene “bridge” containing electron donating dialkylamino groups
in ethene fragment or in aryl moieties of 2,3-diarylcyclopent-2-en-
1-ones have been synthesized. It was shown that the compounds
possess three types of activities e photochromic, fluorescent,
acidochromic e which leads to photo- and pH-switchable emission
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This work was supported by the Russian Science Foundation
(RSF grant 14-50-00126).

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