Synthesis, photochromic and fluorescent properties of hybrid compounds of fulgimides and benzothiazolylthienothiophene

Article abstract of DOI:10.1007/s10593-011-0745-9

The precursors and hybrid compounds, including photochromic fulgimides and fluorescent benzothiazolylthienothiophene units, have been synthesized. Spectroscopic investigations by absorption and fluorescence methods showed that fulgimide hybrids display ph

Full text of DOI:10.1007/s10593-011-0745-9

Chemistry of Heterocyclic Compounds, Vol. 47, No. 2, May, 2011 (Russian Original Vol. 47, No. 2, February, 2011)
SYNTHESIS, PHOTOCHROMIC AND FLUORESCENT
PROPERTIES OF HYBRID COMPOUNDS OF FULGIMIDES
AND BENZOTHIAZOLYLTHIENOTHIOPHENE
L. K. Karabaeva1, I. A. Platonova1, I. V. Zavarzin1, S. I. Luiksaar1, V. N. Yarovenko1,
B. V. Nabatov1, M. M. Krayushkin1*, and V. A. Barachevski2
The precursors and hybrid compounds, including photochromic fulgimides and fluorescent
benzothiazolylthienothiophene units, have been synthesized. Spectroscopic investigations by absorption
and fluorescence methods showed that fulgimide hybrids display photochromism, but do not display
fluorescence properties. A possible explanation for this fact is proposed and also the decreased
effectiveness of photochromic conversion.
Keywords: benzothiazolylthienothiophenes, hybrid photochromic fulgimides, fulgimides, synthesis,
fluorescence properties, photochromism.
Currently photochromic materials are actively under consideration as potential recording media for
three-dimensional storage [1]. Important properties which determine their utility for these uses are
nondestructive sensing of optical information, which is possible only in those cases when the sensed irradiation
is not absorbed by two forms of photochromic compounds mutually transformed under the influence of light.
One of the most fruitful routes for the solution of this problem is the creation of photochromic materials capable
of sensing optical information by the fluorescence method [2, 3]. For this objective in particular hybrid
photochromic compounds were synthesized which contained in their structure both photochromic and
fluorophoric units [4-9] in which the absorption bands of the fluorescent part of the molecule were located in
addition to the adsorption bands of the two forms of the photochromic part, but the fluorescent spectrum
coincides with the absorption spectrum of one of the forms of the photochromic compound.
In the present work an attempt was made to synthesize new hybrid compounds based on photochromic
amino-substituted fulgimides and fluorescent derivatives of benzothiazolylthienothiophene.
_______
Dedicated to our colleague L. I. Belen'kii on his splendid Jubilee.
* To whom correspondence should be addressed, e-mail: mkray@mail.ioc.ac.ru.
1N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Pr., Moscow
119991, Russia.
2Photochemistry Center, Russian Academy of Sciences, 7a, bdq 1 Novatorov St., Moscow 119421, Russia; e-
mail: barva@photonics.ru.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No.2, 287-295, February, 2011. Original
article submitted January 19, 2011.
0009-3122/11/4702-0229©2011 Springer Science+Business Media, Inc.
229

The objects of the study were two hybrid photochromic compounds: (3Z)-1-(4-{[5-(1,3-benzothiazol-
2-yl)-6-pentylthieno[3,2-b]thien-2-yl]methylideneamino}phenyl)-3-[1-(2,5-dimethylthien-3-yl)ethylidene]-
4-(1-methylidene)-2,5-pyrrolidinedione (1a) and (3Z)-1-(4-{[5-(1,3-benzothiazol-2-yl)-6-pentylthieno[3,2-b]-
thien-2-yl]methylideneamino}phenyl)-3-[1-(2-methylbenzo[b]thien-3-yl))ethylidene]-4-(1-methylethylidene)-
2,5-pyrrolidinedione (1b), synthesized in high yield from the photochromic compounds 2a,b and the fluorescent
compound 5-(1,3-benzothiazol-2-yl)-6-pentylthieno[3,2-b]thiophene-2-carbaldehyde (3) by the following
general scheme:
R1
Me
O
O
N
S
S
DMF
R
N
NH2
+
S
S
Me
Me
2a,b
C5H11
O
Me
R
3
R1
Me
O
N
O
N
S
S
S
N
S
C5H11
Me
Me
1a,b
Me
1, 2 a R = Me, R1 = H; b R+R1 =
1a (62%); 1b (79%)
The synthesis of 2-(3-pentylthieno[3,2-b]thien-2-yl)-1,3-benzothiazole (4) – the starting compound for
the synthesis of aldehyde 3 – was accomplished by the following scheme:
O
O
Br
BuLi, S8,
ClCH2CO2Et
S
C5H11COCl,
SnCl4
S
OEt
OEt
EtONa
O
S
S

S
–70°C
5
C5H11
6
8
O
SOCl2,
o-NH2C6H4SH
S
N
S
S
OH
LDA, DMF, THF
–78°C
3
S
S
C5H11
C5H11
7
4
TABLE 1. Absorption and Fluorescent Characteristics* of Benzothiazoles
in Toluene
Com-
pound
λ
max ab., nm
Dmax ab
λmax ex. fl, nm
λmax fl, nm
Δλ, nm
Imax fl, nm
378
353
0.25
0.21
377
353
438
413
60
60
145
3
4
1140
_______
* λmax ab, λmax ex fl, and λmax fl are the wavelengths of maxima of the waves of
absorption, excited fluorescence, and fluorescence, respectively; Dmax ab is the
optical density at the maximum of the absorption bands; Δλ = λmax fl – λmax ab
max fl is the intensity of fluorescence at the maximum of the fluorescence band.
;
I
230

Ester 5 was synthesized from 6-bromothiophene (6) using ethyl monochloroacetate. Its acylation with
capronyl chloride and subsequent hydrolysis gave 3-pentylthieno[3,2-b]thiophene-2-carboxylic acid (7) which
was obtained by an analogous method [10]. The synthesis of the corresponding acid chloride, and its reaction
with o-aminothiophenol in N-methylpyrrolidone with the formation of the thiazole derivative of 3 occurred in
high yield.
In contrast from most monothienyl derivatives, formylation of compound 4 under typical conditions of
the Vilsmeier reaction did not give the aldehyde 3. The latter was successfully obtained in high yield by the
reaction of compound 4 with lithium diisopropylamide (LDA) in THF at -78°C with subsequent treatment with
DMF of the first formed organolithium derivative.
The results of absorption and luminescence investigations of the fluorophores 3 and 4 are presented in
Table 1 and Fig. 1.
Fig. 1. Absorption spectra (1), excited (2), and emission (3) fluorescence of compound 3 in toluene.
It is seen from the data of Table 1 that introduction of a carbonyl group into benzthiazole 4 led to a
bathochromic shift of the absorption, excitation, and fluorescence bands by 24-25 nm. In the same way
fluorescence of compound 3 is considerably reduced.
Analysis of the properties of fulgimides 2a and 2b, precursors of the hybrid compounds, shows that they
possess reversible photoconversion which is normal for fulgimides [1]. It is explained by photoinduced valence
photoisomerization, as a result of which the open form A is converted into the cyclic form B.
R1
R1
Me
Me
O
O
hv
R
R
R2
N
R2
N
S
hv1
S
Me
Me
Me
O
O
Me
Me
Me
A
B
231

TABLE 2. Absorption Characteristics* of Fulgimides and Hybrid Compounds
in Toluene
Com-
pound
phot
phot
λ
А max, nm
DА max
λВ max, nm
ΔDВ
ΔDВ ab /DА max
ab.
332 sh
316 sh
404
0.35
0.29
3.23
2.78
525
490
0.18
0.18
0.10
0.06
0.51
0.62
0.03
0.02
2a
2b
1a
1b
528
404
~490
_______
* λA max and λB max are the wavelengths of the absorption maxima of the open
form A and the photoinduced closed B form; DA max is the optical density at the
phot
maximum of the absorption bands of the open form A; ΔDB
is the photo-
ab
induced change in the optical density at the maximum of absorption bands of
the cyclic form B in the visible region of the spectrum in the photo-equilibrium
state; sh is the shoulder in the absorption spectrum.
Results of the investigation of the absorption properties of the photochromic compounds 2a and 2b in
toluene are shown in Table 2 and Fig. 2.
Fig. 2. Absorption spectra of fulgimide 2a in toluene before (1) and after irradiation
successively with UV (2) and visible light (3).
The results of the spectral-kinetic investigations of the amide-substituted fulgimides 2a and 2b agree
with the data presented previously [1].
Fluorescence was not observed for either the open form nor for the closed form of the amino derivatives
of the fulgimides studied.
The results of the spectral-kinetic investigations of the photochromy of the conversions of the
synthesized hybrid fulgimides 1a and 1b in toluene solution are shown in Table 2 and Fig. 3.
232

It follows from the data of Table 2 that the hybrid fulgimides are typical photochromes. The initial open
forms of these hybrid compounds are characterized by absorption bands, the maxima of which occur in the
region of 404 nm, significantly shifted to the longwave length region of the spectrum in comparison with their
fulgimide precursors (see Fig. 2). Such shifts of the absorption bands of the open forms are evidently explained
by the absorption of the benzothiazolylthienothiophene units (see Fig. 1). When hybrid fulgimides are irradiated
long enough with filtered UV light with alternate separation of the different mercury lines new broad absorption
bands appear in the visible region of the spectrum with maxima in the 490-528 nm range (Fig. 3), which
practically coincide with the absorption bands of the cyclic forms of the fulgimide precursors.
Fig. 3. Absorption spectra of the hybrid fulgimide 1a in toluene before (1) and after
irradiation in UV (2) and visible light (3).
Comparison of the results of spectral investigations carried out with data obtained for the fulgimide
precursors permits the conclusion that the observed reversible photoconversion of the hybrid fulgimides is also
explained by photoinduced valence isomerization, but the process of photocoloring is sharply reduced after
introduction into the structure of the precursors of the fluorescent units. In this respect it was observed that in the
hybrid fulgimides 1a and 1b, as in the fulgimide precursors, neither the open nor cyclic forms possess
fluorescent properties. The decrease in the effectiveness of the photocoloring process can be explained, in our
view, by transformation of the activating radiation of the nonphotochromic fragment or the deactivation of the
photoexciting energy into heat as a result of additional vibration processes, explained by the presence of non-
photochromic fragments which may undergo cis-trans isomerization. The last factor is possibly the reason for
the absence of fluorescence in the hybrid fulgimide.
So as a result of carrying out this study, two new hybrid fulgimides were synthesized with photochromic
properties, but not fluorescence, despite the presence of fluorescent fragment in their structures. The observed
decrease in the effectiveness of the photochromic conversion and the absence of fluorescent properties, it is
proposed, is connected to the optical properties and structural factors of the benzothiazolyl-thienothiophene
fragments.
233

EXPERIMENTAL
Spectrophotometric measurements (photostationary spectra) and also kinetics of the processes of
photocoloring, photoachromatism and photodegradation of photoinduced cyclic forms of the compounds studied
in toluene solution were carried out with a Varian Cary 50 Bio (Australia) spectrophotometer.
Spectrofluorometric measurements of the initial and cyclic forms were carried out on a Varian Cary Eclipse
Fluorescence spectrophotometer (Australia).
Quartz cuvettes with a width of 2 mm were used to measure absorption characteristics (kinetic
measurements) and cuvettes with a width of 10 mm were used for spectrophotometric studies. To record
absorption, excited and emission spectra of compounds 1a,b, 2a,b a solution concentration of 4·10-5 mol/l was
chosen, and for compounds 3 and 4 a solution concentration 5·10-6 mol/l was chosen. Radiation was carried out
with the light from a mercury-xenon lamp LC4 100−240 V−300 VA (Japan) on a HAMAMATSU lightning
cureTM irradiation set-up using UFS-1, ZhS-11, ZhS-16, and ZhS-17 filters.
1H NMR spectra were recorded with a Bruker AC-300 (300 MHz) spectrometer of CDCl3 solutions
(compounds 1a,b, 7) and DMSO-d6 solutions (compounds 3-5, 8) with residual protons of the deuterated
solvents as internal standards. Mass spectra were recorded on a Kratos instrument with direct insertion of the
sample into the ionizing chamber with an ionization energy of 70 eV and an control voltage of 1.76 kV. Melting
points were measured on a Boetius block and were not corrected.
Commercial available reagents from Acros and toluene solvent from Aldrich were used in this study.
Ethyl (3-Thienylmercapto)acetate (5) was obtained by metallation [11] of 3-bromothiophene (6) [12, 13]
with butyllithium at -70°C with subsequent treatment of the organolithium derivative formed with sulfur and
20
1
ethyl monochloroacetate. Yield 79%; bp 146-150°C (12 Torr), (bp 140-142°C (9 Torr) [14]), nD 1.5610. H
NMR spectrum, δ, ppm (J, Hz): 1.15 (3H, t, J = 7.1, CH3); 3.75 (2H, s, CH2); 4.10 (2H, q, J = 4.55, CH2); 7.15
(1H, s, H-2); 7.65 (2H, m, H-4,5). Mass spectrum, m/z (Irel, %): 202 [M+] (67), 129 (100). C8H10O2S2.
Ethyl [(2-Hexanoylthien-3-yl)mercapto]acetate (8). A solution of tin(IV) chloride (7.2 ml) in benzene
(10 ml) was added dropwise at -5°C to a solution containing compound 5 (12.14 g, 60 mmol) and capronoyl
chloride (8.76 g, 65 mmol) in benzene (80 ml). The mixture was stirred for 2 h at room temperature. The
reaction mixture was cooled to -5°C and 70 ml of 10% HCl was added. It was cooled again. The aqueous layer
was separated and extracted with benzene. The combined benzene extract was washed with water and dried over
magnesium sulfate. The benzene was evaporated. Yield 82%; mp 55-56°C (ethanol). 1H NMR spectrum, δ, ppm
(J, Hz): 0.90 (3H, t, J = 6.9, CH3); 1.25 (3H, t, J = 7.0, CH3); 1.45 (4H, m, 2CH2); 1.75 (2H, m, CH2); 2.80 (2H,
t, J = 6.9, CH2); 3.75 (2H, s, CH2); 4.20 (2H, q, J = 4.7, CH2CH2); 7.15 (1H, d, J = 5.25, H-4); 7.50 (1H, d, J =
5.2, H-5). Mass spectrum, m/z (Irel, %): 300 [M+] (11), 156 (90), 71 (100). Found, %: C 55.85; H 6.80; S 21.43.
C14H20O3S2. Calculated, %: C 55.97; H 6.71; S 21.35.
3-Pentylthieno[3,2-b]thiophene-2-carboxylic Acid (7). A solution of ester 8 (14.814 g, 49 mmol) in
absolute ethanol (45 ml) was added to a solution of sodium ethoxide (3.97 g, 172 mmol) in absolute ethanol
(100 ml). The reaction mixture was boiled for 5 h, the ethanol was evaporated, the residue was dissolved in
water (200 ml) and acidified with HCl. The yellowish precipitate was filtered off, washed with water and dried
in a vacuum dessicator. Yield 80%; mp 105-107°C (ethanol). 1H NMR spectrum, δ, ppm (J, Hz): 0.89 (3H, t, J =
5.9, CH3); 1.35 (4H, m, 2CH2); 1.80 (2H, m, CH2); 3.75 (2H, t, J = 7.6, CH2); 7.15 (1H, d, J = 5.0, H-6); 7.53
(1H, d, J = 5.0, H-5). Mass spectrum, m/z (Irel, %): 254 [M+] (16), 198 (100). Found, %: C 56.58; H 5.46;
S 25.32. C12H14O2S2. Calculated, %: C 56.66; H 5.55; S 25.21.
2-(3-Pentylthieno[2,3-b]thien-2-yl)-1,3-benzothiazole (4). A solution of compound 7 (4.4 g, 17 mmol)
in N-methylpyrrolidone (85 ml) was cooled to 0°C and SOCl2 (1.46 ml, 21 mmol) was added with stirring. The
mixture was removed from the cold bath, the temperature of the mixture was allowed to rise to 20°C, kept at that
temperature for 20 min, and o-aminothiophenol (2.00 ml, 18.2 mmol) was added. After this the mixture was
234

boiled for 4 h 30 min, then cooled to 5°C, 1 N NaOH solution (120 ml) was added, and stirred for 30 min at 20°C.
The precipitate was filtered off, washed with 2% NaOH solution and water and then dried. The residue was
chromatographed on silica gel with a 3:1 petroleum ether–ethyl acetate mixture. Yield 77%; mp 85-86°C (ethanol).
1H NMR spectrum, δ, ppm (J, Hz): 0.88 (3H, t, J = 6.9, CH3); 1.40 (4H, m, 2CH2); 1.80 (2H, m, CH2); 3.20 (2H, t,
J =7.8, CH2); 7.52 (3H, m, H-6, 2H arom); 7.89 (1H, d, J = 5.25, H-5); 8.03 (1H, d, J = 7.9, H arom); 8.18 (1H, d,
J = 7.5, H arom). Mass spectrum, m/z (Irel, %): 343 [M]+ (3), 314 [M+H-Et]+ (8), 300 [M+H-Et-Me]+ (100). Found,
%: C 62.69; H 5.16; N 4.29; S 27.86. C18H17NS3. Calculated, %: C 62.93; H 4.99; N 4.08; S 28.00.
5-(1,3-Benzothiazol-2-yl)-6-pentylthieno[3,2-b]thiophene-2-carbaldehyde (3). Compound 4 (3 g,
8.73 mmol) was dissolved in dry THF (100 ml) in an atmosphere of argon, cooled to -78°C and lithium
diisopropylamide (4.8 ml – 2 mol/l in THF) was added. The mixture was removed from the cold bath and the
mixture temperature allowed to rise to 0°C, kept at this temperature for 5 min, then cooled again to -78°C and a
solution of DMF (1 ml) in THF (8 ml) was added. The mixture was removed from the cooling bath and the
temperature allowed to reach room temperature. After 12 h the reaction mixture was added to 1% HCl (300 ml),
extracted with methylene chloride, washed with water, the extract was dried over magnesium sulfate, and
evaporated. The residue was chromatographed on silica gel in a 4:1 hexane–ethyl acetate mixture. Yield 73%;
1
mp 121-123°C (ethanol). H NMR spectrum, δ, ppm (J, Hz): 0.88 (3H, t, J = 6.55, CH3); 1.42 (4H, m, 2CH2);
1.80 (2H, m, CH2); 3.20 (2H, t, J = 7.7, CH2); 7.53 (2H, m, 2H arom); 8.08 (1H, J = 8.1, H arom); 8.18 (1H, d,
J = 8.1, H arom); 8.42 (1H, s, H-3); 10.02 (1H, s, COH). Mass spectrum, m/z (Irel, %): 371 [M]+ (40), 342
[M+H-Et]+ (17), 328 [M+H-Et-Me]+ (100). Found, %: C 61.65; H 4.39; N 3.95; S 25.68. C19H17NOS3.
Calculated, %: C 61.42; H 4.61; N 3.77; S 25.89.
Schiff Bases 1a,b (General Method). Compounds 2a,b (0.41 mmol) were dissolved in DMF (5 ml) and
aldehyde 3 (0.15 g, 0.41 mmol) was added and the mixture was kept at room temperature for 12 h with heating.
The formed yellow crystals were filtered off, washed on the filter with ethanol, and dried in a vacuum
dessicator.
(3Z)-1-(4-{[5-(1,3-Benzothiazol-2-yl)-6-pentylthieno[3,2-b]thien-2-yl]methylideneamino}phenyl-3-[1-
(2,5-dimethylthien-3-yl)ethylidene]-4-(1-methylethylidene)-2,5-pyrrolidinedione (1a). Yield 62%, yellow
crystals; mp 210-212°C. 1H NMR spectrum, δ, ppm (J, Hz): 0.88-0.97 (3H, m, CH3); 1.36-1.58 (4H, m, 2CH2);
1.80-1.95 (2H, m, CH2); 2.07 (3H, s, CH3); 2.16 (3H, s, CH3); 2.38 (3H, s, CH3); 2.43 (3H, s, CH3); 2.52 (3H, s,
CH3); 3.18-3.27 (2H, m, CH2); 6.55 (1H, s, H-4); 7.21-7.32 (2H, m, H arom); 7.34-7.45 (3H, m, H arom);
7.48-7.51 (1H, m, H arom); 7.64 (1H, s, H-3'); 7.88-7.93 (1H, m, H arom); 8.05-8.12 (1H, m, H arom); 8.58
(1H, s, −N=CH−). Mass spectrum, m/z (Irel, %): 720 [M]+ (100). Found, %: C 66.64; H 5.13; N 5.81; S 17.72.
C40H37N3O2S4. Calculated, %: C 66.73; H 5.18; N 5.84; S 17.81.
(3Z)-1-(4-{[5-(1,3-Benzothiazol-2-yl)-6-pentylthieno[3,2-b]thien-2-yl]methylideneamino}phenyl-3-[1-
(2-methylbenzo[b]thien-3-yl)ethylidene]-4-(1-methylethylidene)-2,5-pyrrolidinedione (1b). Yield 79%, yellow
1
crystals; mp 252-259°C. H NMR spectrum, δ, ppm (J, Hz): 0.87-1.00 (3H, m, CH3); 1.33-1.64 (4H, m, 2CH2);
1.79-1.94 (2H, m, CH2); 2.16 (3H, s, CH3); 2.20 (3H, s, CH3); 2.48 (3H, s, CH3); 2.55 (3H, s, CH3); 3.13-3.27 (2H,
m, CH2); 7.18-7.54 (8H, m, H arom); 7.61 (1H, s, H-3); 7.68-7.77 (1H, m, H arom); 7.87-7.94 (1H, m H arom); 8.02-
8.11 (1H, m, H arom); 8.54 (1H, s, −NH=CH−). Mass spectrum, m/z (Irel, %): 756 [M]+ (100). Found, %: C 68.33; H
4.86; N 5.51; S 16.81. C43H37N3O2S4. Calculated, % : C 68.31; H 4.93; N 5.56; S 16.96.
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