Synthesis and spectral properties of fluorescent photochromic diarylethenes with 6,6a-dihydropentalene-2(1H)-one ethene "bridge"

Article abstract of DOI:10.1016/j.dyepig.2014.04.040

An alternative synthetic strategy for the preparation of 6,6a-dihydropentalene-2(1H)-one derivatives comprising the stage of the regioselective α-bromination of cyclopentenone system has been proposed. The method along with the bromination process includes the alkylation of ethyl 4-aryl-3-oxobutanoate with bromocyclopentenones and intramolecular carbocyclization reaction of alkylated product. The cyclization reaction has been studied in detail and it was found that the yields of the main and side products depend strongly on alkali concentration, and the method can be also used to design 8,8a-dihydrocyclopenta[a]inden-2(1H)-one unit. The spectral properties of the compounds obtained have been studied, and it was found that pentalenone derivatives as well as starting cyclopentenones, exhibit photochromic properties; in addition, the former, unlike the latter, are also fluorescent.

Full text of DOI:10.1016/j.dyepig.2014.04.040

Dyes and Pigments 109 (2014) 105e112
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and spectral properties of fluorescent photochromic
diarylethenes with 6,6a-dihydropentalene-2(1H)-one ethene “bridge”
Dmitry V. Lonshakov, Valerii Z. Shirinian*, Igor V. Zavarzin, Andrey G. Lvov,
Mikhail M. Krayushkin
N.D. Zelinsky Institute of Organic Chemistry RAS, 47, Leninsky prosp., 119991 Moscow, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
An alternative synthetic strategy for the preparation of 6,6a-dihydropentalene-2(1H)-one derivatives
Received 18 March 2014
Received in revised form
28 April 2014
Accepted 29 April 2014
Available online 20 May 2014
comprising the stage of the regioselective a-bromination of cyclopentenone system has been proposed.
The method along with the bromination process includes the alkylation of ethyl 4-aryl-3-oxobutanoate
with bromocyclopentenones and intramolecular carbocyclization reaction of alkylated product. The
cyclization reaction has been studied in detail and it was found that the yields of the main and side
products depend strongly on alkali concentration, and the method can be also used to design 8,8a-
dihydrocyclopenta[a]inden-2(1H)-one unit. The spectral properties of the compounds obtained have
been studied, and it was found that pentalenone derivatives as well as starting cyclopentenones, exhibit
photochromic properties; in addition, the former, unlike the latter, are also fluorescent.
© 2014 Elsevier Ltd. All rights reserved.
Keywords:
Photochromic diarylethenes
Fluorescence
Cyclopent-2-en-1-one
Dihydropentalene-2(1H)-ones
Spectral properties
Thermal stability
1. Introduction
b-subtype selective ligands [18e21], while cyclopenta[a]inden-2-
ones can be employed for the treatment of brain edema [22,23].
Recently, we have found that DCPs are a new promising class of
photochromic diarylethenes with easily modifiable ethene “bridge”
[3,24e26]. Thereby, our previous work devoted to the synthesis of
DCPs by means of a convenient method for the cyclopent-2-en-1-
one ring construction urged us to use the same technique to
annelate the second cyclopent-2-en-1-one ring to DCPs molecules.
The resulting compounds e 3,4,5-triaryl-6,6a-dihydropentalene-
2(1H)-ones e could possess both above-mentioned biological and
pharmacological activities and demonstrate new improved
photochromic parameters.
The compounds containing cyclopent-2-en-1-one rings in their
structures are of great importance because of the prevalence in
natural products and other bioactive compounds as well as its ap-
plications in various aspects of science and medicine [1e5]. Thus,
the jasmons (2,3-dialkylcyclopent-2-en-1-ones) are utilized in
perfumes, cosmetics and as a food flavorings [6e9]; methyl-
enomycins (2,3-dialkyl-5-methylenecyclopent-2-en-1-one) are
known to be an antibiotics effective against both Gram-negative
and Gram-positive bacteria [10e13]; 2,3-diarylcyclopent-2-en-1-
ones (DCPs) were found to be useful for medicine as non-
steroidal anti-inflammatory [5,14,15] and antitumor [1,16,17]
agents.
2. Experimental
5-, 6-, 7-Membered cycloalkenones annelated to a cyclopentene,
cyclohexene, indanone and other similar rings are under extensive
investigation in medicinal chemistry because of their high biolog-
ical activity. For example, tetrahydrofluorenones due to its struc-
1H and 13C NMR spectra were recorded on a Bruker AM-300
spectrometer. Mass spectra were obtained on a Kratos mass spec-
trometer (70 eV) with direct sample injection into the ion source.
Melting points were measured on a Boetius hot stage and were not
corrected. High resolution mass spectra were obtained on a Bruker
maXis spectrometer.
tural similarity to 17b-estradiol are a new class of estrogen receptor
Commercially available reagents and solvents were used. 2,3-
Bis(2,5-dimethylthiophen-3-yl)cyclopent-2-en-1-one 1 [3] and 5-
bromo-2,3-bis(2,5-dimethylthiophen-3-yl)cyclopent-2-en-1-one 2
* Corresponding author.
E-mail addresses: shir@ioc.ac.ru, svbegunt@mail.ru (V.Z. Shirinian).
http://dx.doi.org/10.1016/j.dyepig.2014.04.040
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

106
D.V. Lonshakov et al. / Dyes and Pigments 109 (2014) 105e112
[24] have been prepared by the previously reported methods.
Column chromatography was performed using silica gel 60
(70e230 mesh); TLC analysis was conducted on silica gel 60 F254
plates.
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 (Acros) solutions (C ¼ 2∙10ꢀ5 mol Lꢀ1 for
absorption spectra and C ¼ 2∙10ꢀ6 mol Lꢀ1 for emission ones) at
293 K in the air presence. Photocoloration was carried out using
6 W Vilber Lourmat (France) UV-lamp model VL-6.LC (365 nm
light). The fluorescence quantum yields were determined by
134.59, 134.74, 135.30, 135.36, 135.79, 135.87, 135.90, 136.56, 136.68,
137.49, 137.54,164.40,164.47,168.00,168.98, 201.55, 201.84, 206.08,
206.11. Mass, m/z (%): 540 (98, [M]þ), 387 (100, [Mꢀ ThCH2C(O)]þ),
301 (83, [Mꢀ ThCH2C(O)CHC(O)OCH2CH3]þ). HRMS Calcd for
C
29H32O4S3 (M þ Naþ): 563.1355; Found: 563.1349.
2.4. 3,4,5-Triaryl-6,6a-dihydropentalene-2(1H)-ones (5) and 3-
(2,5-dimethylthiophen-3-yl)-8,8a-dihydrocyclopenta[a]inden-
2(1H)-one 14 (general method)
A 13%-solution of KOH (1.5 g, 26.8 mmol) in water (10 mL) was
added at once to a solution of corresponding diketoesters (4 or 13)
(0.4 mmol) in ethanol (10 mL). The reaction mixture was refluxed
for 2 h, then cooled, poured into water (100 mL) and extracted with
ethyl acetate (3 ꢁ 30 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. ester/ethyl acetate 6:1 and recrystallized from
hexane. When 2%-solution of KOH (0.2 g, 4.00 mmol) in water
(10 mL) was used in this reaction by-products 8 and 10 were also
isolated.
comparing with quinine bisulfate in 0.1 N H2SO4 (4 ¼ 0.55) [27] at
em
maxima of excitation spectra. Quantum yields of ring-closure
(4A/B) and ring-opening (4B/A) processes were calculated by
previously reported technique [3,28].
2.1. Alkylation of ethyl 4-aryl-3-oxobutanoates (3) with
bromoketones (2 and 12) (general method)
To a solution of corresponding ethyl 4-aryl-3-oxobutanoate 3
(2.0 mmol) in abs. benzene (5 mL) metallic sodium (0.05 g,
2.05 mmol) was added and the mixture was stirred overnight
(sodium dissolved entirely). To this solution under boiling point
corresponding bromoketone (2 or 12) (0.38 g, 1.0 mmol) was added
and reaction mixture was reflux for 5 h, then poured into cold water
(100 mL) and extracted with ethyl acetate (3 ꢁ 30 mL). The com-
bined organic phases were washed with water (40 mL), dried with
magnesium sulfate and evaporated in vacuum. The residue was
purified by column chromatography eluting by petrol. ester/ethyl
acetate 8:1. Diketoester 13 was used in further synthesis without
additional purification.
2.5. 4,5-Bis(2,5-dimethylthiophene-3-yl)-3-(naphthalen-1-yl)-
6,6a-dihydropentalen-2(1H)-one (5a)
Yield 0.05 g (27%), brown powder, Mp 74e76 ꢂC (ethanol). 1H
NMR (300 MHz, CDCl3, d, ppm): 1.57 (s, 3H, Me), 1.80 (s, 3H, Me),
1.97 (s, 3H, Me), 2.39 (s, 3H, Me), 2.76 (td, J ¼ 4.5, 18.8 Hz, 1H,
½CH2C(O)), 2.92e3.17 (m, 3H, ½CH2C(O), CH2CeTh), 3.73e3.91 (m,
1H, CH2CHCH2), 6.53e6.67 (m, 2H, Hthioph), 7.19e7.32 (m, 2H,
HNaph), 7.36e7.50 (m, 2H, HNaph), 7.62e7.71 (m, 2H, HNaph),
7.77e7.84 (m, 1H, HNaph).13C NMR (75 MHz, CDCl3,
d, ppm): 13.94,
14.42, 14.93, 15.13, 42.26, 42.51, 45.32, 124.60, 124.73, 124.93,
125.28, 125.41, 125.54, 125.66, 126.07, 126.50, 127.39, 127.66, 127.96,
129.45, 129.87, 131.86, 132.15, 133.28, 134.83, 136.08, 136.87, 185.50,
207.64 (C]O). Mass, m/z (%): 466 (100, [M]þ), 451 (55, [MꢀCH3]þ).
HRMS Calcd for C30H26OS2 (M þ Naþ): 489.1317; Found: 489.1303.
2.2. Ethyl 2-[3,4-bis(2,5-dimethylthiophene-3-yl)-2-oxocyclopent-
3-en-1-yl]-4-(naphthalen-1-yl)-3-oxobutanoate (4a)
Yield 0.39 g (71%), grey powder, Mp 63e64.5 ꢂC (hexane). 1H
NMR (300 MHz, CDCl3, d, ppm): 1.18, 1.22 (2t (keto-enol tautomers),
J ¼ 7.2 Hz, 3H, CH2CH3), 1.88 (m, 6H, 2Me), 2.32e2.49 (m, 6H, 2Me),
2.90 (dd, J ¼ 3.6, 18.2 Hz, ½CH2CH), 2.99e3.08 (m, 1.5H), 3.19e3.29
(m, 1H), 4.01e4.53 (m, 5H), 6.39e6.54 (m, 2H, Hthioph), 7.35e7.57
2.6. 3,4,5-Tris(2,5-dimethylthiophene-3-yl)-6,6a-dihydropentalen-
2(1H)-one (5b)
(m, 4H,Hnaph), 7.79e8.01 (m, 3H, Hnaph). 13C NMR (75 MHz, CDCl3,
d
,
Yield 0.04
(300 MHz, CDCl3,
g
(19%), amorphous brown powder. 1H NMR
d, ppm): 1.77 (s, 3H, Me), 1.96 (s, 3H, Me), 2.02 (s,
ppm): 13.67, 13.70, 13.79, 13.85, 14.34, 14.72, 14.76, 14.91, 35.19,
35.57, 44.87, 45.07, 48.26, 48.36, 56.10, 56.74, 61.29, 61.45, 123.74,
123.78, 124.75, 124.86, 125.15, 125.24, 125.51, 125.55, 126.15, 127.96,
128.02, 128.36, 128.44, 128.56, 128.59, 129.54, 129.72, 133.26,
133.51, 135.01, 135.08, 135.45, 135.57, 136.15, 136.33, 137.24, 164.04,
164.44,167.66,168.64, 201.66, 201.87, 205.85, 205.89. Mass, m/z (%):
556 (100, [M]þ), 387 (28, [Mꢀ NaphCH2C(O)]þ), 301 (19, [Mꢀ
NaphCH2C(O)CHC(O)OCH2CH3]þ). HRMS Calcd for C33H32O4S2
(M þ Naþ): 579.1634; Found: 579.1632.
3H, Me), 2.19 (s, 6H, 2Me), 2.34 (s, 3H, Me), 2.52 (dd, J ¼ 4.8, 17.2 Hz,
1H, ½CH2C(O)), 2.78 (dd, J ¼ 6.6, 17.2 Hz, 1H, ½CH2C(O)), 2.86 (dd,
J ¼ 6.2, 15.8 Hz, 1H, ½CH2CeTh), 3.00 (dd, J ¼ 7.3, 15.8 Hz, 1H,
½CH2CeTh), 3.57e3.67 (m, 1H, CH2CHCH2), 5.90 (s, 1H, Hthioph),
5.96 (s, 1H, Hthioph), 6.45 (s, H, Hthioph). 13C NMR (75 MHz, CDCl3,
d,
ppm): 13.71, 14.01, 14.86, 14.93, 14.97, 15.19, 41.77, 42.23, 45.27,
125.12, 125.69, 126.55, 126.67, 126.94, 126.99, 127.79, 130.78, 132.16,
134.09, 134.48, 134.91, 136.11, 136.63, 137.47, 151.71, 207.30 (C]O).
Mass, m/z (%): 450 (100, [M]þ), 435 (65, [MꢀCH3]þ). HRMS Calcd for
2.3. Ethyl 2-[3,4-bis(2,5-dimethylthiophene-3-yl)-2-oxocyclopent-
3-en-1-yl]-4-(1-dimethylthiophen-3-yl)-3-oxobutanoate (4b)
C
26H26OS3 (M þ Naþ): 473.1038; Found: 473.1033.
2.7. 2,3-Bis(2,5-dimethylthiophene-3-yl)-5-[3-(naphthalen-1-yl)-
2-oxopropyl]cyclopent-2-en-1-one (8a)
Yield 0.33 g (61%), grey powder, Mp 54e55 ꢂC (hexane). 1H NMR
(300 MHz, CDCl3, d, ppm): 1.22, 1.30 (2t (keto-enol tautomers),
J ¼ 7.2 Hz, 3H, CH2CH3), 1.88e1.95 (m, 6H, 2Me), 2.23e2.43 (m, 12H,
4Me), 2.94 (dd, J ¼ 3.3, 18.0 Hz, ½CH2CH), 3.01e3.12 (m, 1.5H),
3.16e3.23 (m, 1H), 3.69 (d, J ¼ 4.0 Hz, 1H), 3.85 (s, 1H), 4.08e4.16,
4.17e4.29 (2 m, 3H), 6.45 (s, 1H, Hthioph), 6.47e6.60 (m, 2H,
Yield 0.06 g (32%), dark brown powder, Mp 35e36 ꢂC (ethanol).
1H NMR (300 MHz, CDCl3,
d, ppm): 1.86 (s, 3H, Me), 1.88 (s, 3H, Me),
2.37 (s, 6H, 2Me), 2.39e2.48 (m, 1H, ½CH2CeTh), 2.66 (dd, J ¼ 8.7,
17.7 Hz, 1H, ½CHCH2), 2.90e3.02 (m, 1H, CH2CHCH2), 3.11 (d,
J ¼ 17.7 Hz, 1H, ½CHCH2), 3.15 (dd, J ¼ 2.4, 17.7 Hz, 1H, ½CH2CeTh),
4.20 (s, 2H, CH2Naph), 6.45 (s, 2H, 2Hthioph), 7.39e7.57 (m, 4H, HNaph),
Hthioph).13C NMR (75 MHz, CDCl3,
d, ppm): 13.02, 13.07, 14.11, 14.17,
14.69, 15.10, 15.14, 15.16, 15.23, 15.24, 35.52, 35.84, 43.03, 44.98,
45.27, 56.63, 57.14, 61.59, 61.75, 125.07, 125.17, 126.45, 127.38,
127.42, 128.12, 128.24,128.89, 128.94, 133.62, 133.66, 133.80, 133.93,
7.80-7.96 (m, 3H, HNaph). 13C NMR (75 MHz, CDCl3,
d, ppm): 14.17,
14.69, 15.09, 15.23, 38.34, 41.31, 42.86, 48.66, 123.90, 125.13, 125.54,

D.V. Lonshakov et al. / Dyes and Pigments 109 (2014) 105e112
107
125.91, 126.00, 126.56, 126.69, 128.84, 128.88, 129.05, 130.75,
132.28, 133.67, 134.02, 134.69, 135.22, 135.77, 136.54, 137.44, 164.18,
207.14 (C]O), 208.42 (C]O). Mass, m/z (%): 484 (100, [M]þ), 343
(37, [Mꢀ NaphCH2]þ). HRMS Calcd for C26H28O2S3 (M þ Naþ):
507.1423; Found: 507.1417.
layer of silica gel and evaporated. The residue was purified by col-
umn chromatography eluting by petrol. Ester/ethyl acetate
11:1 / 5:1 and compounds 15a-d were isolated.
2.12. 1-Bromo-4,5-bis(2,5-dimethylthiophene-3-yl)-3-
(naphthalen-1-yl)-6,6a-dihydropentalen-2(1H)-one (15a)
2.8. 2,3-Bis(2,5-dimethylthiophene-3-yl)-5-[3-(2,5-
Yield 0.02 g (17%), brown powder, Mp 73e75 ꢂC (ethanol). 1H
dimethylthiophene-3-yl)-2-oxopropyl]cyclopent-2-en-1-one (8b)
NMR (300 MHz, CDCl3, d, ppm): 1.58 (s, 3H, Me), 1.78 (s, 3H, Me),
Yield 0.02 g (10%), dark brown powder, Mp 65e67 ꢂC (ethanol).
1.96 (s, 3H, Me), 2.34 (s, 3H, Me), 3.19e3.46 (m, 2H, CH2), 4.03e4.19
(m, 1H, CH2CHCH(Br)), 4.87 (dd, 1H, J ¼ 4.9, 30.4 Hz, CHBr),
6.48e6.63 (m, 2H, Hthioph), 7.19e7.31 (m, 2H, HNaph), 7.38e7.51 (m,
1H NMR (300 MHz, CDCl3,
d, ppm): 1.87 (s, 3H, Me), 1.88 (C, 3H, Me),
2.28 (C, 3H, Me), 2.35(s, 6H, 2Me), 2.36(s, 3H, Me), 2.49 (dd, J ¼ 2.9,
18.0 Hz, 1H, ½CH2CeTh), 2.60 (dd, J ¼ 9.2, 18.0 Hz, 1H, ½CHCH2),
2.87e2.95 (m, 1H, CH2CHCH2), 3.06 (dd, J ¼ 3.7, 17.6 Hz, 1H,
½CH2CeTh), 3.15 (dd, J ¼ 7.2, 18.2 Hz, 1H, ½CHCH2), 3.55 (s, 2H,
CH2Th), 6.43 (s, 1H, Hthioph), 6.46 (s, 1H, Hthioph), 6.48 (s, H, Hthioph).
2H, HNaph), 7.62e7.69 (m, 2H, HNaph), 7.72e7.85 (m, 1H, HNaph). 13
C
NMR (75 MHz, CDCl3, d, ppm): 14.00, 14.41, 15.05, 15.13, 40.99,
52.34, 55.91, 124.49, 124.65, 124.90, 125.09, 125.22, 125.53, 125.77,
125.94, 126.27, 126.52, 127.13, 127.76, 127.94, 128.17, 128.90, 131.42,
132.08, 133.26, 134.65, 136.35, 137.95, 181.49, 198.73 (C]O). Mass,
m/z (%): 544, 546 (76, [M]þ), 466 (29, [MꢀBr]þ). HRMS Calcd for
13C NMR (75 MHz, CDCl3,
d, ppm): 13.11, 14.19, 14.72, 15.12, 15.16,
15.25, 38.40, 41.27, 43.06, 43.08, 125.08, 126.50, 127.14, 128.88,
129.02, 133.17, 133.68, 134.65, 135.18, 135.80, 136.09, 136.60, 137.41,
164.12, 206.37 (C]O), 208.42 (C]O). Mass, m/z (%): 468 (47, [M]þ),
343 (45, [Mꢀ ThCH2]þ), 315 (100, [Mꢀ ThCH2C(O)]þ). HRMS Calcd
for C26H28O2S3 (M þ Naþ): 491.1144; Found: 491.1146.
C
30H25BrOS2 (M þ Hþ): 547.0583; Found: 547.0588.
2.13. 4,5-Bis(2,5-dimethylthiophene-3-yl)-1-methoxy-3-
(naphthalen-1-yl)-6,6a-dihydropentalen-2(1H)-one (15b)
2.9. [2-Oxo-3,4-bis(2,5-dimethylthiophene-3-yl)cyclopent-3-en-1-
yl]acetic acid (10a)
Yield 0.007 g (7%), yellow powder, Mp 66e67 ꢂC (ethanol). 1H
NMR (300 MHz, CDCl3, d, ppm): 1.95 (s, 6H, 2Me), 2.36 (s, 6H, 2Me),
3.21e3.33 (m, 3H, CH2CH), 3.74 (s, 3H, OCH3), 4.43 (dd, J ¼ 4.8,
Yield 0.08 g (54%), dark gray powder, Mp 97e98.5 ꢂC (ethanol).
29.3 Hz, 1H, CHOMe), 6.50 (s, 2H, Hthioph), 7.29e7.48 (m, 3H, Hnaph),
1H NMR (300 MHz, CDCl3,
d, ppm): 1.90 (s, 3H, Me), 1.91 (s, 3H, Me),
7.62e7.83 (m, 4H, Hnaph). 13C NMR (75 MHz, CDCl3,
d, ppm): 13.97,
2.37 (s, 3H, Me), 2.38 (s, 3H, Me), 2.62 (dd, J ¼ 7.5, 15.9 Hz, 1H,
½CH2COOH), 2.77 (d, J ¼ 18.3 Hz, 1H, ½CH2COOH), 2.93e3.01 (m,
1.5H, CH2CHCH2, ¼CH2CeTh), 3.04 (d, J ¼ 4.1 Hz, 0.5H, ¼CH2CeTh),
3.23 (dd, J ¼ 6.6, 18.0 Hz, 1H, ½CH2CeTh), 6.46 (s, 1H, Hthioph), 6.56
14.41, 14.99, 15.14, 37.44, 48.82, 58.23, 76.50, 124.54, 124.70, 125.07,
125.12, 125.55, 125.75, 126.27, 126.33, 126.51, 127.81, 128.71, 130.01,
130.11, 133.50, 133.65, 134.42, 134.95, 136.11, 136.40, 136.54, 137.32,
160.68, 207.19 (C]O). Mass, m/z (%): 496 (100, [M]þ), 466 (65, [Mꢀ
OMe]þ). HRMS Calcd for C31H28O2S2 (M þ Naþ): 519.1422; Found:
519.1430.
(s, 1H, Hthioph), 9.29 (br. s, 1H, COOH). 13C NMR (75 MHz, CDCl3,
d,
ppm): 13.92, 14.52, 14.90, 15.02, 35.31, 37.86, 41.29, 124.84, 126.29,
128.90, 133.36, 134.55, 135.16, 135.70, 136.58, 137.50, 164.25, 177.47
(COOH), 207.59 (C]O). Mass, m/z (%): 360 (100, [M]þ), 345 (60,
[MꢀCH3]þ),
327
(87,
[MꢀCH3eH2O]þ),
299
(36,
2.14. 1,6a-Dibromo-4,5-bis(2,5-dimethylthiophene-3-yl)-3-
(naphthalen-1-yl)-6,6a-dihydropentalen-2(1H)-one (15c)
[MꢀCH3eHCOOH]þ). HRMS Calcd for C19H20O3S2 (M þ Naþ):
383.0746; Found: 383.0751.
Yield 0.006 g (5%), black powder, Mp 101e102 ꢂC (ethanol). 1H
NMR (300 MHz, CDCl3, d, ppm): 1.65 (s, 3H, Me), 1.73 (s, 3H, Me),
2.10. 3-(2,5-Dimethylthiophene-3-yl)-8,8a-dihydrocyclopenta[a]
inden-2(1H)-one (14)
2.08 (s, 3H, Me), 2.35 (s, 3H, Me), 3.50 (d, 2H, J ¼ 7.0 Hz, CH2), 5.41 (s,
1H, CHBr), 6.37 (s,1H, Hthioph), 6.49 (s,1H, Hthioph), 7.31e7.57 (m, 4H,
HNaph), 7.74e7.85 (m, 3H, HNaph), 10.41 (s, enolic OH). 13C NMR
Yield 0.03 g (22%, calculated from initial bromoketone 12),
amorphous brown powder. 1H NMR (300 MHz, CDCl3,
d, ppm): 2.32
(75 MHz, CDCl3, d, ppm): 13.73, 14.46, 14.96, 15.31, 29.76, 35.34,
120.61, 123.11, 124.61, 124.94, 125.70, 126.14, 126.81, 127.03, 127.72,
128.24, 128.41, 128.53, 129.20, 130.95, 131.61, 133.22, 133.58, 134.30,
135.99, 136.91, 137.15, 148.17, 205.53 (C]O). Mass, m/z (%): 622,
624, 626 (5, [M]þ), 543, 545 (6, [MꢀBr]þ), 464 (100, [Mꢀ2Br]þ).
(s, 3H, Me), 2.39 (s, 3H, Me), 2.57 (dd, J ¼ 4.4, 16.1 Hz, 1H,
½CH2C(O)), 2.81 (dd, J ¼ 8.1,17.1 Hz,1H, ½CH2C(O)), 2.93 (dd, J ¼ 6.6,
17.6 Hz, 1H, ½CH2-Ph), 3.36 (dd, J ¼ 8.5, 16.1 Hz, 1H, ½CH2-Ph),
3.62e3.77 (m, 1H, CH2CHCH2), 6.49 (s, 1H, Hthioph), 7.11e7.33 (m,
Hþ): 622.9708; Found:
3H, Harom), 7.38e7.44 (m, 1H, Harom). 13C NMR (75 MHz, CDCl3,
d,
HRMS Calcd for
622.9709.
C30H25BrOS2 (M
þ
ppm): 15.17, 15.33, 38.11, 42.54, 45.66,124.98, 125.91, 126.72,127.35,
127.50, 128.68, 128.98, 131.74, 132.39, 133.65, 136.21, 149.23, 208.02
(C]O). Mass, m/z (%): 280 (2, [M]þ), 170 (65, [Mꢀ thiophene]þ).
HRMS Calcd for C18H16OS (M þ Hþ): 281.0995; Found: 281.0999.
2.15. 1-Bromo-4,5-bis(2,5-dimethylthiophene-3-yl)-6a-methoxy-3-
(naphthalen-1-yl)-6,6a-dihydropentalen-2(1H)-one (15d)
2.11. Bromination of 3,4,5-triaryl-6,6a-dihydropentalene-2(1H)-
one (5a)
Yield 0.02 g (20%), black powder, Mp 71e72 ꢂC (ethanol). 1H
NMR (300 MHz, CDCl3, d
, ppm): 1.64 (s, 3H, Methioph), 1.74 (s, 3H,
Methioph), 1.98 (dd, 1H, J ¼ 3.3, 13.9 Hz, ½CH2), 2.09 (s, 3H, Methioph),
2.35 (s, 3H, Methioph), 2.36e2.43 (m, 1H, ½CH2), 3.74 (s, 3H, OCH3),
5.42 (s,1H, CHBr), 6.50 (s,1H, Hthioph), 6.73 (s,1H, Hthioph), 7.32e7.55
(m, 4H, HNaph), 7.69e7.85 (m, 3H, HNaph). 13C NMR (75 MHz, CDCl3,
The mixture of ketone 5a (0.1 g, 0.2 mmol) and cupric bromide
(0.11 g, 0.5 mmol) in methanol (5 mL) was stirred for 4 h under
room temperature, then 3 h under 40 ꢂC and eventually under
refluxing to maximal conversion of starting material (TLC-analysis).
The reaction mixture was poured into ice-water (50 mL) and
extracted with ethyl acetate (3 ꢁ 15 mL). The combined organic
phases were washed with water (20 mL), filtered through 1 cm
d
, ppm): 13.71,14.48,14.99,15.18, 29.77, 50.70, 75.44,121.35, 124.54,
124.68, 125.55, 125.79, 126.15, 126.77, 127.74, 127.96, 128.72, 129.55,
130.61, 133.21, 133.35, 133.55, 134.60, 136.24, 137.46, 137.52, 149.00,
168.15, 203.28 (C]O). Mass, m/z (%): 574, 576 (11, [M]þ), 544 (7,

108
D.V. Lonshakov et al. / Dyes and Pigments 109 (2014) 105e112
Ar1
COOEt
carbonyl
group
COOEt
O
O
O
O
O
CuBr2
O
III
KOH
Br
Ar2
Ar2
EtOH/H2O
a-methyl
group
Ar1
Ar2
Ar1
Ar2
Na/Benzene
I
II
IV
V
Scheme 1. Photochromic 2,3-diarylcyclopent-2-en-1-ones V synthesis.
[Mꢀ OMe]þ), 495 (82, [MꢀBr]þ), 480 (15, [MꢀBr e Me]þ). HRMS
Calcd for C31H27BrO2S2 (M þ Kþ): 613.0267; Found: 613.0259.
(Scheme 2), and at the first step the 5-bromoketone 2 has been
prepared by the previously reported technique [24] for the DCPs
regioselective bromination with copper (II) bromide.
The alkylation of ethyl 4-aryl-3-ketobutanoates 3 with 5-
bromoketones 2 in the presence of metallic sodium proved to be
different from the similar reaction in DCPs synthesis. Thus, in
contrast with the reaction between II and III (Scheme 1) which
have been shown [3] to occur at room temperature, the reaction
between compounds 2 and 3 does not proceed in these mild con-
ditions. The fact is likely to be explained by the secondary bromine
atom in bromide 2 instead of primary one in bromides II, which
provides the steric hindrance for attack of the nucleophile. The
studies of the alkylation reaction resulted in the optimal conditions,
namely, the use of 2-fold excess of ketoesters 3 and the reaction
carrying out at the boiling point. Under these conditions the in-
termediate compounds 4 are obtained with 71% (for 4a) and 61%
(for 4b) yields after column chromatography, but it was also found
that diketoesters 4 can be involved in the subsequent cyclization
reaction without additional purification.
As expected, the reaction of the cyclization of compounds 4 with
potassium hydroxide in aqueous ethanol solution is not a simple
process but is accompanied by different side reactions (Scheme 3).
The way of the cyclopentenone ring formation (compounds 5) in-
cludes the initial hydrolysis of esters 4 to form the salts 6 followed
by the dianions 7 formation under the alkali action. But the com-
pounds 6 turned out to participate also in other reactions. So, on the
one hand the decarboxylation of salt 6 can leads to diketone 8 while
the nucleophilic addition of hydroxide anion to carbonyl function of
6 seems to be a way resulting in acid 10a via dianion 9a. These side
reactions were observed when low excess (5e10 eq.) of potassium
hydroxide was used. The reaction conditions optimal for ketones 5
synthesis are the following: the use of 13% water solution of KOH
(72 eq.) in ethanol, with the ratio of EtOH/H2O being 1:1. It allows
carrying out the cyclization of compounds 4 with the yields of
ketones 5a and 5b 27% and 19%, respectively.
3. Results and discussion
3.1. Synthesis
Cyclopentenones are useful synthons for the preparation of
various cyclopentyl compounds due to the versatility of their
functionality. Cyclopent-2-en-1-one ring or cycloalkenones anne-
lated onto indanone cycle is known to be constructed by a various
ways including the ring-closing metathesis followed by the allylic
oxidative rearrangement [20,21], by PausoneKhand reaction
[29e34] as well as by different methods based on the cyclization of
1,4-dicarbonyl compounds [1,18,23,35,36] similar to the Robinson
annelation reaction [37]. But the techniques in most cases involve
peculiar and inconvenient processes or require the utilization of
reagents difficult of access or lithium and boron organic com-
pounds and, therefore, low temperatures. So, despite of above
mentioned practical importance of the cycloalkenones annelated
onto a cyclopentene, cyclohexene and indanone rings, there is no
convenient and effective method for its preparation.
At the same time the recently proposed technique for DCPs
synthesis [3] on the basis of ketones I pretends to be a convenient
way to the cyclopent-2-en-1-one ring construction. It includes the
alkylation of ethyl 4-aryl-3-ketobutanoates III [38] by
a-bromo-
ketones II as a key stage followed by the cyclization of intermediate
IV with potassium hydroxide in aqueous ethanol solution to form
DCPs V (Scheme 1). This method seems to be useful for cyclo-
pentenone ring building based actually on any compounds con-
taining in its structure (like in the structure of compound I) the
carbonyl function and methyl (or methylene) group in a-position.
As DCPs V contain the carbonyl and methylene groups in its
structure, it was then decided to extend this strategy to annelate
the second cyclopenten-2-en-1-one ring to ethene “bridge” of DCPs
V. 2,3-Bis(2,5-dimethylthiophen-3-yl)cyclopent-2-en-1-one 1 was
chosen as a starting material for the development of this method
To show that the proposed method for cyclopent-2-en-1-one
ring annelation can be useful not only in case of 2,3-
O
O
methylene
group
O
O
O
carbonyl
Br
Ar
group
O
Ar
O
CuBr2
O
KOH
3a,b
Na, PhH
O
Ar
O
MeOH
EtOH, H2O
Th
Th
Th
Th
Th
Th
Th
Th
4a (71%),
4b (61%)
5a (27%),
5b (19%)
1
2 (96%)
Ar =
;
Th =
H3C
CH3
H3C
CH3
S
S
a
b
Scheme 2. Synthetic route for the preparation of 3,4,5-triaryl-6,6a-dihydropentalene-2(1H)-ones 5.

D.V. Lonshakov et al. / Dyes and Pigments 109 (2014) 105e112
109
Ar
Ar
Ar
-
O
COO-
COOEt
Th
COO-
O
HO-
HO-
O
O
Ar
O
O
O
-H2O
-EtOH
Th
Th
Th
Th
Th
Th
Th
4a,b
6a,b
7a,b
5a,b
HO-
-CO2
Ar
Ar
HO
COOH
-
COO
O
O
O-
O
O
-
Ar
COOH
Th
Th
Th
Th
Th
Th
8a,b
9a
10a
Ar =
;
Th =
H3C
CH3
H3C
CH3
S
S
a
b
Scheme 3. The ways of formation of 3,4,5-triaryl-6,6a-dihydropentalene-2(1H)-ones 5 and by-products 8 and 10.
diarylcyclopentenones 1, the technique was extended to indanone
6 (Scheme 4). The bromoketone 11 was prepared by reported
method [39] and was employed for 4-(2,5-dimethylthiophen-3-yl)-
3-ketobutanoates 3b alkylation in the optimal conditions
mentioned above. The compound 14 has been obtained with total
22% yield by cyclization of diketoester 13.
methoxy- 15d by-products. The utilization of other brominating
reagents such as bromine and N-bromosuccinimide resulted in
complex mixture of compounds.
3.2. Spectral properties
Next, it would very attractive to continue the strategy proposed
and to annelate the third cyclopentenone ring to the molecules of
compounds 5, thus, obtaining previously undescribed compounds
with three condensed cyclopentene cycles and a carbonyl group.
Triaryldihydropentaleneone 5a was chosen for this way realization
and the first step on this way should be the bromination of ketone
3.2.1. Photochromic properties of diarylethenes
The photochromic characteristics of diarylethenes have been
measured in acetonitrile solutions at 293 K with alternating irra-
diation by UV light (lir ¼ 365 nm) and visible light (lir > 400 nm)
and are summarized in Table 1. Upon UV-irradiation, the colorless
solutions of DCPs 1 and 5a,b were converted into colored ones and
bleached back to colorless under visible light that is due to the
reversible electrocyclic reaction of hexatriene system (Scheme 6).
Analyzing the data in Table 1 one can see that the introduction
5a into the a-position to carbonyl function (compound 15a, Scheme
5). However the limiting stage of this synthetic strategy has proved
bromination reaction. The same conditions (copper (II) bromide in
methanol) were used that have been applied for bromoketone 2
synthesis but as compared with ketones 1 bromination a range of
by-products 15b-d was formed and the conversion of starting
material 5a was not full. All these resulted in desired bromide 15a
with only 17% yield and did not allow synthesizing the substances
with three condensed cyclopentene cycles. The main problem in
this case seems to be the existence of protone in 6a-position of
dihydropentaleneone molecule (shown in bold in structure 5a,
Scheme 5) which leads to the dibromo- 15c and then to bromo-
of extended
p-system into the ethene “bridge” of DCP 1 consider-
ably influences their properties. As to the spectral parameters of
open-ring isomers A it should be noted that the transition from
DCPs to corresponding dihydropentalenones results in the shift of
the absorption band of form A to the higher wavelengths region.
Whereas the highest value of absorption maximum wavelength of
the DCP 1 is detected at 309 nm, for compounds 5a and 5b it is red-
shifted to 347 nm and 351 nm respectively. Such absorption bands
shift towards the visible spectrum region is known to be a
Ar
O
O
Br
O
O
O
O
O
O
CuBr2
Na
KOH
Ar
Ar
+
CHCl3 / EtOAc
PhH
EtOH / H2O
O
O
11
12 (92%)
3b
Ar =
13
14 (22% from 12)
H3C
CH3
S
Scheme 4. The preparation of 3-(2,5-dimethylthiophen-3-yl)-8,8a-dihydrocyclopenta[a]inden-2(1H)-one 14.

110
D.V. Lonshakov et al. / Dyes and Pigments 109 (2014) 105e112
O
O
Br
OMe
O
O
Br
Br
O
Br
OMe
H
CuBr2
Ar
Ar
Ar
+
+
+
Ar
Ar
MeOH
Th
Th
Th
15c (5%)
Th
Th
Th
Th
5a
Th
Th
Th
15a (17%)
15b (7%)
15d (20%)
Ar =
;
Th =
H3C
CH3
S
Scheme 5. The bromination of triaryldihydropentaleneones 5a with copper (II) bromide.
Table 1
Spectral characteristics of initial and photoinduced forms of diarylcyclopentenones 1, 5 in acetonitrile (C ¼ 2∙10ꢀ5 mol Lꢀ1) at 293 K.
Entry
Photochromism
Compound
1
Fluorescence
lAa, nm (ε, L molꢀ1 cmꢀ1
)
lBb, nm (ε, L molꢀ1 cmꢀ1
547 (1.11∙104)
)
4A / B
4B / A
lexe, nm
lemf, nm
4
em
4UVh
c
d
openg
1
208 (1.64∙104),
245 (1.46∙104),
309 (6.43∙103)
278 (5.04∙104),
347 (1.48∙104)
251 (2.18∙104),
351 (1.33∙104)
0.27
0.065
Non fluorescent
2
3
5a
5b
522 (2.95∙104)
523 (6.40∙104)
0.03
0.07
0.001
0.001
347
351
503
509
0.26
0.17
0.04
0.03
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 (for compound 5a) or 3-Min (for compound 5b) UV-irradiation (lir ¼ 365 nm).
promising feature of compounds, as for different practical appli-
cations (for example for optical memories and fluorescent switches
[40] or medicine diagnostics [41,42]) it is significant to handle
diarylethenes which can be processed by visible rather than UV
light, because the higher the wavelength of light the lower its en-
ergy and hence the less destructive it is.
of interest to compare their properties with those of diarylethenes
5. First, it should be mentioned that the elongation of -system via
double bonds both in aryl moieties and in ethene “bridge” leads to
the same effect namely, bathochromic shift of absorption bands of
initial forms A of diarylethenes. As to photoinduced forms B the
p
influence of p-system elongation is different: the elongation in aryl
The spectral properties of closed-ring isomers B of diarylethenes
synthesized were found to be also influenced by the ethene
“bridge” structure. First, the data in Table 1 witness that the
moieties results in bathochromic shift of the absorption bands
rather than hypsochromic one as in the case of ethene “bridge”.
As for quantum yields of photochromic cyclization and cyclo-
reversion reactions, their values are also significantly affected by
“bridge” p-system elongation leads to essential hypsochromic shift
of the absorption bands of colored forms B (from 547 nm for DCP
1e522 nm or 523 nm for compounds 5a and 5b correspondingly).
the compounds structure. Namely, the introduction of extended p-
system into the ethene “bridge” of DCP 1 is seen from Table 1 to
induce the reduction of cyclization and recyclization quantum
yields (compounds 5a,b). Diarylethenes 5a,b are fluorescent sub-
stances (see Section 2.3 below) and their quantum yields reduction
therefore can be explained by the competition between photo-
chromism and fluorescence processes: light energy absorbed is
spent both to photochromic reactions and to emission, which leads
to essential suppression of the firsts [48]. In contrary, in case of
non-fluorescent DCP 1 more light energy participates in photo-
chromic reactions, thus, its quantum yields are higher than those of
compounds 5a,b.
On the other hand, the presence of extended
p-system in ethene
“bridge” was found to lead to the increase of molar extinction co-
efficients of diarylethenes. And high values of extinction co-
efficients are known to be responsible to high sensitivity of
photochromic substances and materials [40].
The compounds with the extended
p-system located in the aryl
groups are well-known from the literature [43e47] and it would be
O
O
R'
R'
R
h
h
1
2
3.2.2. Thermal stability and fatigue resistance of diarylethenes
The thermal stability and fatigue resistance of diarylethenes
R
R
R
synthesized
were
measured
in
acetonitrile
solutions
X
X
X
X
(C ¼ 2∙10ꢀ5 mol Lꢀ1) at 293 K in air and its values are summarized
A
B
in Table 2.
open form
colorless
closed (cyclic) form
It turned out that the synthesis of dihydropentaleneones 5a,b is
promising transformation of ethene “bridge” of DCP 1 which leads
to the improvement of both fatigue resistance of compounds and its
colored
Scheme 6. Photochromic reactions of diarylethenes.

D.V. Lonshakov et al. / Dyes and Pigments 109 (2014) 105e112
111
Table 2
with their bands of absorption: the difference between lB and lem
are as little as 19 nm for compound 5a and 14 nm for compound 5b.
These effective overlaps are known to be an essential factor for the
design of switches capable of fluorescence modulation.
Fatigue resistance and thermal stability of diarylcyclopentenone 1 and its derivatives
5a,b.
Entry
Compound
Fatigue resistance
Thermal stability
ꢀ
ꢁ
(number of switching cycles)a
The fluorescence quantum yields of diarylethenes 5 ð4oempenÞ were
measured using quinine bisulfate in 0.1 N H2SO4 as a reference [25].
It was found that both the emission intensity and its quantum
yields are sensitive to UV-light, namely, the parameters decrease
under UV-irradiation. Moreover, this phenomenon is the reversible
process and under visible light the emission of compounds 5 is
revived. The mentioned-above is illustrated by Fig. 1 where ab-
sorption and emission spectra of diarylethene 5b are depicted. One
can clearly see that after UV-irradiation along with the appearance
of the absorption band at visible spectrum region (523 nm, curve 2)
the 5-fold reduction in the fluorescence intensity is detected (from
approx. 2.5 a.u. (curve 3) to approx. 0.5 a.u. (curve 4)).
B/A
therm
b
t1=2
; h
#
1
2
3
1
5a
5b
50
100
75
939
3051
2657
a
Measured in air at 293 K under alternative irradiation with UV- (lir ¼ 365 nm)
and visible (l
ir > 400 nm) light.
b
Half-lifes of the closed-ring isomers measured in air at 293 K in the dark after
exposure of compounds to UV-light (lir ¼ 365 nm) (values obtained by kinetic data
processing).
thermal stability. So, the transition from compound 1 to com-
pounds 5a,b results to the 1.5÷2-fold increasing of switching cycles
number while the half-lifes of the closed-ring isomers B grow from
Such UV-induced fluorescence quenching effect is likely to be
explained by various aspects. First, considering that it is colorless
isomers A of diarylethenes 5 which possess the emission and the
closed-ring isomers B are non-fluorescent (at least have no emis-
sion bands at 503 nm for 5a and 509 nm for 5b), it can be assumed
that UV-induced emission reduction is due to the decrease in the
concentration of initial isomers A during the photocyclization.
Other causes of compounds 5 fluorescence decay under UV-light
can be the capture of the part of emission by colored isomers B
(because of broad overlap between bands of emission and ab-
sorption of colored isomers B), or the absorption of excitation
photons by forms B. All these processes are likely to proceed
simultaneously leading eventually to the fluorescence quenching
observed.
939
h (for DCP 1) up to approximately 3000 h for dihy-
dropentaleneones 5a,b.
As high thermal stability and fatigue resistance are two main
properties of photochromic compounds essential for its various
applications [40] the synthesis of dihydropentaleneones can serve
as one of the ways of the improvement of these characteristics.
3.2.3. Fluorescent properties of diarylethenes and emission
switching under UV/visible light
Dihydropentaleneones 5a,b with extended
“bridge” turned out to possess the emission. The fluorescence of
these compounds might be explained not only by the extended
p-system in ethene
p
-
system but also by the transfer of absorbed energy to the side aryl
group (thienyl or naphthyl moiety). Their fluorescent properties
have been examined in acetonitrile solutions at 293 K and the
switching parameters have been measured under alternating UV
So, the reversible emission quenching under alternative UV and
visible irradiations can be the basis for the molecular switches.
4. Conclusions
(
lir ¼ 365 nm) and visible light (lir > 400 nm) irradiation (Table 1
and Figure 1), with the excitation wavelengths (lex) being at max-
ima of corresponding excitation spectra.
A novel synthetic protocol to synthesize 6,6a-dihydropentalene-
2(1H)-one derivatives in three stages including the regioselective
bromination at a-position of cyclopentenone ring, the alkylation of
Considering the data in Table 1 and Fig. 1, first of all one can see
that the emission bands of diarylethenes 5 have a good matching
ethyl 4-aryl-3-oxobutanoate with bromocyclopentenones and
intramolecular carbocyclization reaction of alkylated product has
been proposed. The investigation of the cyclization reaction shown
that the main and side products yields depend significantly on the
concentration of alkali. It was shown that the method can also be
used to design 8,8a-dihydrocyclopenta[a]inden-2(1H)-one unit.
The photoswitching properties of the compounds obtained have
been studied, and it was found that pentalenone derivatives as well
as starting cyclopentenones, exhibit photochromic properties; in
addition, the former, unlike the latter, are also fluorescent.
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