Synthesis and optical properties of new photochromic systems based on 1,2-bis(2-methylbenzo[b]thien-3-yl) hexafluorocyclopentenes and 5,7-dihydro-1H-1,2,5,7,8-pentaaza-s-indacen-6-one or 1,7-dihydro-5-thia-1,2,7,8- tetraaza-s-indacen-6-one derivatives

Article abstract of DOI:10.1080/17415993.2013.822500

New photochromic molecules based on 1,2-bis(2-methylbenzo[b]thien-3-yl) hexafluorocyclopentenes and fluorescent 5,7-dihydro-1H-1,2,5,7,8-pentaaza-s- indacen-6-one or 1,7-dihydro-5-thia-1,2,7,8-tetraaza-s-indacen-6-one derivatives were synthesized via nucl

Full text of DOI:10.1080/17415993.2013.822500

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Synthesis and optical properties of
new photochromic systems based on
1,2-bis(2-methylbenzo[b]thien-3-
yl) hexafluorocyclopentenes and
5,7-dihydro-1H-1,2,5,7,8-pentaaza-
s-indacen-6-one or 1,7-dihydro-5-
thia-1,2,7,8-tetraaza-s-indacen-6-one
derivatives
M. M. Krayushkin a , A. M. Bogacheva b , A. N. Komogortsev a , B.
V. Lichitsky a , A. A. Dudinov a , K. S. Levchenko c , O. I. Kobeleva d
, T. M. Valova d , V. A. Barachevsky d & V. N. Charushin b
a N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of
Sciences , 47 Leninsky Pr., Moscow , 119991 , Russia
b I.Ya. Postovsky Institute of Organic Synthesis, Russian Academy
of Sciences , 20 S. Kovalevskoy, Yekaterinburg , 620990 , Russia
c Central Science Research Institute of Technology “Technomash” ,
4 Ivana Franko St., Moscow , 121108 , Russia
d Photochemistry Center of RAS , 7a, bld. 1, Novatorov, Moscow ,
119421 , Russia
Published online: 12 Aug 2013.
To cite this article: Journal of Sulfur Chemistry (2013): Synthesis and optical properties of new
photochromic systems based on 1,2-bis(2-methylbenzo[b]thien-3-yl) hexafluorocyclopentenes
and 5,7-dihydro-1H-1,2,5,7,8-pentaaza-s-indacen-6-one or 1,7-dihydro-5-thia-1,2,7,8-tetraaza-s-
indacen-6-one derivatives, Journal of Sulfur Chemistry, DOI: 10.1080/17415993.2013.822500
To link to this article: http://dx.doi.org/10.1080/17415993.2013.822500
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Journal of Sulfur Chemistry, 2013
http://dx.doi.org/10.1080/17415993.2013.822500
Synthesis and optical properties of new photochromic
systems based on 1,2-bis(2-methylbenzo[b]thien-3-yl)
hexafluorocyclopentenes and
5,7-dihydro-1H-1,2,5,7,8-pentaaza-s-indacen-6-one or
1,7-dihydro-5-thia-1,2,7,8-tetraaza-s-indacen-6-one derivatives†
M.M. Krayushkina*, A.M. Bogachevab, A.N. Komogortseva, B.V. Lichitskya, A.A. Dudinova,
K.S. Levchenkoc, O.I. Kobelevad, T.M. Valovad, V.A. Barachevskyd and V.N. Charushinb
aN.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Pr., Moscow
119991, Russia; bI.Ya. Postovsky Institute of Organic Synthesis, Russian Academy of Sciences, 20 S.
c
Kovalevskoy, Yekaterinburg 620990, Russia; Central Science Research Institute of Technology “Techno-
mash”, 4 Ivana Franko St., Moscow 121108, Russia; dPhotochemistry Center of RAS, 7a, bld. 1, Novatorov,
Moscow 119421, Russia
(Received 25 May 2013; final version received 2 July 2013)
New photochromic molecules based on 1,2-bis(2-methylbenzo[b]thien-3-yl)hexafluorocyclopentenes and
fluorescent 5,7-dihydro-1H-1,2,5,7,8-pentaaza-s-indacen-6-one or 1,7-dihydro-5-thia-1,2,7,8-tetraaza-s-
indacen-6-one derivatives were synthesized via nucleophilic substitution. Photochromic properties of the
compounds and composite mixtures of fluorophores and photochrome were investigated.
Keywords: photochromes; bis(benzothienyl)hexafluorocyclopentenes; regioselective Friedel–Crafts acy-
lation; FRET; fluorescent dyes
*Corresponding author. Email: mkray@ioc.ac.ru
†In blessed commemoration of Professor Alessandro Del’Innocenti
© 2013 Taylor & Francis

2
M.M. Krayushkin et al.
1. Introduction
Dihetarylethenes appear to be a class of very promising heterocyclic compounds because of
a variety of plausible applications in the fields of photonics and molecular electronics.[1] In
particular, compounds of this family have successfully been used for the design of light-sensitive
recording media for ultra-high-density three-dimensional optical memory systems, polymers with
photo- and electrically controlled conductivity, and optical switches.[2–5]
Nowadays, the design of new dihetarylethene skeletons with different aryl moieties is becoming
an active and very promising area of research.[6] A great deal attention has recently been paid
to the synthesis of asymmetrical structures.[7–10] Simultaneous incorporation of different func-
tional fragments in dihetarylethene molecules serves to adjust their spectral properties for specific
tasks and their use in various areas of science and technology. For example, introduction of ben-
zothiophene fragments provides a basic photochromic dihetarylethene platform that is of great
interest for the development of the fluorescence resonance energy transfer (FRET) system.[11]
Synthesis of unsymmetrical bis(benzothienyl)perfluorocyclopentenes by mono-modification of
bis(benzothienyl)perfluorocyclopentene has not, for all practical purposes, been described to date.
To the best of our knowledge, only a few examples of mononitro and monoformyl substituted
derivatives of bis(benzothienyl) perfluorocyclopentene have been reported.[12–14] It is worth
noting that reported synthesis of the above-mentioned compounds are not regioselective, and
mixtures of isomers are formed.
In the previous work, we developed a method for regioselective introduction of only one
substituent into 1,2-bis(2-methylbenzo[b]thien-3-yl)hexafluorocyclopentene 1 by using Friedel–
Crafts acylation.[15] Acylation of 1 with bromoacetyl chloride allows one to obtain mono- or
bis(bromoacetyl)dihetarylethenes depending on acylation conditions (Scheme 1).A variety of new
monosubstituted or disubstituted dihetarylethenes were prepared by heterocyclization reactions
involving bromoacetyl functionality.
Scheme 1. Acylation of 1,2-bis(2-methylbenzo[b]thien-3-yl)hexafluorocyclopentene with bro-
moacetyl chloride.
2. Results and discussion
In this paper, we report the synthesis of new photochromic systems 5 and 6 that incorporate
fluorescent moieties derived from mono-and bis(bromoacetyl)dihetarylethenes 2 and 3. The syn-
thesis is based upon nucleophilic substitution of the reactive bromine in mono- or bisbromoacetyl
bis(benzothienyl)perfluorocyclopentenes by amide nitrogen of previously synthesized fluorescent
5,7-dihydro-1H-1,2,5,7,8-pentaaza-s-indacen-6-one 4a or 1,7-dihydro-5-thia-1,2,7,8-tetraaza-s-
indacen-6-one 4b derivatives (Scheme 2).
All compounds 5 and 6 exhibited good photochromic properties and can be switched between
their ring-open (A) and ring-closed (B) forms by alternate irradiation with ultraviolet and visible
light (Scheme 3).

Journal of Sulfur Chemistry
3
Scheme 2. Synthesis of new systems based on 1,2-bis(2-methylbenzo[b]thien-3-yl)
hexafluorocyclopentenes and 5,7-dihydro-1H-1,2,5,7,8-pentaaza-s-indacen-6-one or 1,7-dihy-
dro-5-thia-1,2,7,8-tetraaza-s-indacen-6-one derivatives.
Scheme 3. The photochromic interconversions of bis(benzothienyl)perfluoro-cyclopentenes.
The results of this study are presented in Table 1 and Figures 1–4. Asymmetric 5b and
symmetric 6b dihetarylethenes containing fluorophor structural fragments exhibit photochromic
transformations and photo-induced reversible fluorescence property changes (Table 1).
For example, photo-induced spectral changes for compound 6b in toluene are shown in Figure 1.
Figure 1 shows that the initial colorless compound (Figure 1, Curve 1) is colored under UV
irradiation (Figure 1, Curve 2) and is bleached under visible light (Figure 1, Curve 3). These
photo-induced changes of absorption properties confirm that this compound is similar to another
dihetarylethenes which experience known photochromic transformations from the open form
(A) to cyclic from (B) as a result of valence photoisomerization (Scheme 3). The open form
(A) of dihetarylethene possesses fluorescence (Figure 1, Curve 4). The fluorescence intensity
changes in parallel with photochromic transformations. This assumes the reversible transition
between the fluorescent open (A) and nonfluorescent cyclic (B) isomers. The fluorescence bands
of photochromic dihetarylethenes 5b, 6b and fluorophore 4b coincide. This indicates that the
nature of the fluorescence bands is due to the introduction of the fluorescent fragments into the
dihetarylethene structure.
Similar photo-induced changes of absorption and fluorescence spectra are shown by compound
5b (Table 1).
The main distinction between compounds 5b and 6b consists in the bathochromic spectral shift
of the absorption band for the cyclic form (B) of symmetrical dihetarylethene 6b as compared
with the asymmetric compound 5b.

4
M.M. Krayushkin et al.
Table 1. Absorption – kinetic and fluorescence characteristics of organic fluorophors and photochromic dihetarylethenes
in toluene.
Compounds λmA ax (nm)a εA, (M−1 cm−1
)
λmB ax (nm)a ꢀDBphot/DAc λflAu. max (nm)d IAflu. max (a.u.)e IA∗flu.max (a.u.)f
b
4a
5a
335
290
329
324
335
320
324
322
330
320
5000
29,000
27,000
25,000
10,000
27,500
27,500
15,000
–
–
542
–
0.04
398
403
1245
20
1245
200
6a
4b
5b
6b
562
–
0.05
–
400
387
387
382
–
123
715
315
182
–
105
715
240
99
–
960
390
545
562
542
547
547
0.08
0.08
0.36
0.22
0.20
7
7 + 4a
7 + 4b
400
387
960
390
–
aAbsorption maxima of the open-ring isomers in the UV region and absorption maxima of the closed-ring isomers in the visible region.
bMolar extinction coefficients of the open-ring isomers at the absorption maxima in the UV region.
cA value of a photo-induced change of optical density at the maximum of the absorption band for the closed-ring isomers that normalized
the value of optical density at the maximum of open-ring isomers.
dA wavelength of the fluorescence band maximum for open-ring isomers of dihetarylethene or fluorophor.
eFluorescence intensity at the maximum of the fluorescence band for open-ring isomers of dihetarylethene or fluorophor.
f Fluorescence intensity at the maximum of the fluorescence band for open-ring isomers of dihetarylethene or fluorophor at the
photostationary state after UV irradiation.
Figure 1. Absorption (1–3) and fluorescence (4–6) spectra for compound 6b in toluene before (1, 4), after UV irradiation
(2, 5) and subsequent influence of visible light (3, 6). Fluorescence is recorded during excitation by light with wavelength
330 nm.
Figure 2. Absorption (1, 2) and fluorescence (3–5) spectra for the compound 5a in toluene before (1, 3), after subsequent
UV irradiation (2, 4, 5). Fluorescence is recorded during excitation by light with wavelength 335 nm.
Similar photochromic transformations were found for symmetrical 6a i and asymmetrical 5a
dihetarylethenes containing fluorophor 4a (Table 1). However, the sensitivity of these compounds
to UV irradiation was lower as compared with compounds 5b and 6b. The symmetric derivative

Journal of Sulfur Chemistry
5
Figure 3. Absorption spectra for dihetarylethene 7 in toluene before (1) and after (2) UV irradiation.
Figure 4. Absorption (1–3) and fluorescence (5) spectra of equimolar solution of the compound 7 and fluorophore 4b
in toluene before (1, 5), after UV irradiation (2, 5) and subsequent visible light irradiation (3, 5). Fluorescence is recorded
during excitation by light with wavelength 335 nm.
6a as well as dihetarylethenes 5b and 6b shows reversible changes of fluorescence intensity during
photochromic transformations (Table 1). Unlike the compound 6a, asymmetric dihetarylethene
5a manifests irreversible photo-induced increasing fluorescence intensity under UV irradiation
(Figure 2). Thus, the position of the appearing fluorescence band practically coincides with the
position of the same band of fluorophor 4a.
The observed difference of fluorescent properties for symmetric 6a and nonsymmetric 5a
dihetarylethenes can be explained by a photo-induced detachment of a fluorescent fragment from
compound 5a during UV excitation. The formation of fluorescent and non-fluorescent photoprod-
ucts is confirmed by a chromatography study carried out by us. The reason for similar distinction
is unclear and will be studied elsewhere.
For comparison to photochromes 6a and 6b, we obtained the non-fluorescent analogue 7, as
shown in Scheme 4 and also studied the spectral properties of the composite mixture of 7 and 4b
(Scheme 4).
Figure 3 shows the change in optical density during photochromic transformations.
This compound as well as the above-investigated nonsymmetrical dihetarylethenes are charac-
terized by a long-wave absorption band with a maximum at 542 nm for the photo-induced cyclic
form (B) (Table 1). Unlike 6a and 6b, dihetarylethene 7 possesses UV sensitivity that it is possible
to explain in the absence of fluorescence.

6
M.M. Krayushkin et al.
Scheme 4. Synthesis of 1,2-bis(5-(2-imidazolylacetyl)-2-methylbenzo[b]thien-3-yl)-hexafluo-
rocyclopentene.
The spectral-luminescent study of equimolar solutions of dihetarylethene 7 and fluorophor
(4a or 4b) in toluene shows the lack of the photo-induced change of fluorescence intensity for
fluorophores during photochromic transformations (Table 1, Figure 4). It testifies to practical
absence of the FRET from fluorophore to the cyclic form (B) of dihetarylethene (FRET effect)
despite some overlap of absorption bands of the dihetarylethelene cyclic form (B) and fluorescence
bands of fluorophores.
3. Conclusion
New photochromic compounds including fluorophor fragments have been synthesized.All synthe-
sized dihetarylethenes exhibit photochromic properties. For compounds 6a, 5b and 6b containing
fluorescent fragments, photo-induced reversible changes of fluorescence intensity for the open
form (A) according to photochromic transformations between open (A) and cyclic (B) photoiso-
mers were found. Dihetarylethene 5a showing photochromic transformations undergoes allegedly
irreversible photodissociation with the formation of fluorescent fragments.
4. Experimental
All reagents and solvents were used as received. Column chromatography was performed on silica
gel (Merck Kieselgel 60 H). Analytical thin-layer chromatography was carried out on aluminum-
backed plates coated with Merck Kieselgel 60 F254 that were visualized under UV light (at 254 or
365 nm). 1H NMR, 19F NMR and 13C NMR spectra were recorded on BrukerAM-300 (300 MHz)
and Bruker Avance II 300 (75 MHz) spectrometers, respectively. Chemical shifts in CDCl3 are
reported downfield from TMS (= 0) as the internal reference. ESI spectra were measured on a
Kratos MS-30 instrument.
Spectral absorption and fluorescent measurements of synthesized compounds were carried
out in toluene (Aldrich) with the use of the spectrophotometer “Cary 50 bio” (Varian) and the
spectrofluorimeter “Ñary Eclipse” (Varian), accordingly. Working concentration of compounds
in toluene made C = 4.10−5 M. Quartz cuvette of 1 cm thickness was used for measurements.
Kinetic investigations of photocoloration and photobleaching processes were carried out with
the use of the spectrophotometer “Cary 50 bio” (Varian). Working concentration of compounds
in toluene made C = 2.10−4 M. Quartz cuvette of 0.2 cm thickness was used for measurements.
During photochemical experiments, filtered irradiation from the lamp of LC-4 (Hamamatsu)
was used which was passed via light filters UFS-1 (photocoloration) and JS-16 (photobleaching).
Intensities of irradiation made 12 and 190 mW/cm2, accordingly.
4.1. General procedure for the preparation of compounds 5a and 5b
To a solution of compounds 2 (50 mg, 0.085 mmol) and 4a (29 mg, 0.085 mmol) in DMF (0.5 ml)
was added K2CO3 (12 mg, 0.085 mmol) and the mixture was stirred at RT for 24 h and poured

Journal of Sulfur Chemistry
7
into H2O. The precipitate was filtered, and purified on a chromatographic column (petroleum
ether–EtOAc, 10:5).
(5a) Yield 53%, m.p. 129–131◦C (EtOH). 1H NMR (300 MHz, CDCl3, δ, ppm): 1.52 (d, 6H,
NCHCH3CH3, J = 7.1); 2.19 (s, 3H, CH3); 2.53 (s, 3H, CH3); 3.15 (s, 3H, NCH3); 3.93 (s,
3H, OCH3); 5.11 (quartet, 1H, NCHCH3CH3, J = 7.1); 5.56 (s, 2H, CH2); 7.01–7.29 (m, 2H,
ArH); 7.36–7.57 (m, 4H, Ar); 7.69–7.83 (m, 4H, Ar); 8.10 (m, 1H, ArH); 8.12 (s, 1H, imidazole).
13C NMR (50 MHz, CDCl3, δ, ppm): 22.01 (CH3); 29.69 (NCHCH3CH3); 32.00 (NCH3); 46.30
(CH2); 48.61 (NCHCH3CH3); 55.46 (OCH3); 113.37; 114.00; 118.62; 122.10; 122.24; 122.29;
122.42; 123.09; 123.50; 124.31; 124.65; 124.73; 124.82; 125.47; 130.98; 131.10; 131.21; 138.27;
138.41; 144.90; 145.04; 155.17; 160.23; 191.04 (C O). 19F NMR (280 MHz, CDCl3, δ, ppm):
=
−110.48 – (−111.25) (m, 4F, CF2); 133.37 – (−133.54) (m, 2F, CF2). HRMS (ESI) (m/z) [MH+]
calc. for C43H33F6N5O3S2: 846.2002; found 846.1977.
(5b) Yield 50%, m.p. 130–134◦C (EtOH).1H NMR (300 MHz, CDCl3, δ, ppm): 1.51 (d,
6H, NCHCH3CH3, J = 6.6); 2.26 (s, 3H, CH3); 2.53 (s, 3H, CH3); 3.96 (s, 3H, OCH3);
4.00 (s, 3H, OCH3); 5.11 (quartet, 1H, NCHCH3CH3, J = 6.6); 5.65 (s, 2H, CH2); 7.01–
7.29 (m, 1H, ArH); 7.36–7.57 (m, 4H, Ar); 7.69–7.83 (m, 4H, Ar); 7.92 (m, 1H, ArH); 8.04
(s, 1H, thiazole). 13C NMR (50 MHz, CDCl3, δ, ppm): 21.97 (CH3); 29.72 (NCHCH3CH3);
48.12 (CH2); 48.95 (NCHCH3CH3); 56.10 (OCH3); 56.14 (OCH3); 110.90; 111.82; 112.20;
113.37; 114.00; 118.62; 122.10; 122.24; 122.29; 122.42; 123.09; 123.50; 124.31; 124.65; 124.73;
124.82; 125.47; 130.98; 131.10; 131.21; 138.27; 138.41; 144.90; 145.04; 155.17; 160.23; 191.05
(C O). 19F NMR (280 MHz, CDCl3, δ, ppm): −110.48 – (−111.25) (m, 4F, CF2); 133.37 –
=
(−133.54) (m, 2F, CF2). HRMS (ESI) (m/z) [MH+] calc. for C43H32F6N4O4S3: 879.1563; found
876.1558.
4.2. General procedure for the preparation of compounds 6a, 6b and 7
To a solution of compounds 3 (50 mg, 0.07 mmol) and 4a (47 mg, 0.014 mmol) in DMF (0.5 ml)
was added K2CO3 (10 mg, 0.07 mmol) and the mixture was stirred at RT for 24 h and poured
into H2O. The precipitate was filtered, and purified on a chromatographic column (petroleum
ether–EtOAc, 1:1).
(6a)Yield 40%, m.p. 151–155◦C (EtOH). 1H NMR (300 MHz, CDCl3, δ, ppm): 1.52 (d, 12H,
NCHCH3CH3, J = 7.1); 2.20 (s, 6H, CH3); 3.16 (s, 6H, NCH3); 3.94 (s, 6H, OCH3); 5.11 (quartet,
2H, NCHCH3CH3, J = 7.1); 5.55 (s; 4H; CH2); 7.00–7.28 (m, 2H, ArH); 7.35–7.58 (m, 4H, Ar);
7.69–7.83 (m; 4H;Ar); 8.10 (m, 2H,ArH); 8.15 (s, 2H, imidazole). 13C NMR (50 MHz, CDCl3, δ,
ppm): 22.02 (CH3); 29.70 (NCHCH3CH3); 32.02 (NCH3); 46.35 (CH2); 48.62 (NCHCH3CH3);
55.45 (OCH3); 113.39; 114.00; 118.65; 122.10; 122.25; 122.30; 122.45; 123.09; 123.50; 124.32;
124.65; 124.74; 124.83; 125.47; 130.98; 131.15; 131.22; 138.28; 138.42; 144.90; 145.05; 155.18;
160.23; 191.05 (C O). 19F NMR (280 MHz, CDCl3, δ, ppm): −110.48 – (−111.25) (m, 4F,
=
CF2); 133.37 – (−133.54) (m, 2F, CF2). HRMS (ESI) (m/z) [MH+] calc. for C62H50F6N10O6S2:
1210.3003; found 1210.3000.
(6b)Yield 45%, m.p. 150–154◦C (EtOH). 1H NMR (300 MHz, CDCl3, δ, ppm): 1.51 (d, 12H,
NCHCH3CH3, J = 6.6); 2.28 (s, 6H, CH3); 3.97 (s, 6H, OCH3); 4.01 (s, 6H, OCH3); 5.11 (quartet,
2H, NCHCH3CH3, J = 6.6); 5.65 (s, 4H, CH2); 7.01–7.29 (m, 2H, ArH); 7.36–7.57 (m, 4H, Ar);
7.69–7.83 (m, 4H, Ar); 7.92 (m, 2H, ArH); 8.04 (s, 2H, thiazole). 13C NMR (50 MHz, CDCl3, δ,
ppm): 21.97 (CH3); 29.73 (NCHCH3CH3); 48.15 (CH2); 48.96 (NCHCH3CH3); 56.10 (OCH3);
56.14 (OCH3); 110.91; 111.83; 112.25; 113.39; 114.00; 118.62; 122.10; 122.24; 122.29; 122.42;
123.09; 123.50; 124.31; 124.65; 124.73; 124.82; 125.47; 130.99; 131.10; 131.23; 138.27; 138.41;
144.90; 145.04; 155.17; 160.23; 191.05 (C O). 19F NMR (280 MHz, CDCl3, δ, ppm): −110.48
=
– (−111.25) (m, 4F, CF2); 133.37 – (−133.54) (m, 2F, CF2). HRMS (ESI) (m/z) [MH+] calc.
for C63H50F6N8O8S4: 1289.2611; found 1289.2607.

8
M.M. Krayushkin et al.
(7) Yield 50%, m.p. 140–144◦C (EtOH).1H NMR (300 MHz, CDCl3, δ, ppm): 2.29 (s, 6H,
CH3); 5.36 (s, 4H, CH2); 6.90 (m, 2H, imidazole); 7.12 (m, 2H, imidazole); 7.30–7.45 (m, 4H,
Ar); 7.60 (m, 2H, imidazole); 7.92 (m, 2H, ArH). 13C NMR (50 MHz, CDCl3, δ, ppm): 21.95
(CH3); 48.10 (CH2); 120.15; 120.30; 126.60; 129.60; 131.10; 134. 50; 138.27; 138.41; 150.17;
190.05 (C O). 19F NMR (280 MHz, CDCl3, δ, ppm): −110.49 – (−111.26) (m, 4F, CF2); 133.36
=
– (−133.55) (m, 2F, CF2). HRMS (ESI) (m/z) [MH+] calc. for C33H22F6N4O2S2: 685.6778;
found 685.6770.
Acknowledgements
The authors thank the Russian Foundation for Basic Research for financial support (Grant Nos 11-03-90736 and 13-03-
00964) and Dr A.Y. Bochkov for helpful suggestions.
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