Studies on piezochromic photochromic (E)-5-dicyanomethylene-3[1-(2,5-dimethyl-3-furyl)ethylidene]-4-diphenylmethylene tetra-hydrofuran-2-one and related photochromic compounds

Article abstract of DOI:10.1039/b002414g

Yellow crystals of the title compound 2b gave a dark red powder on mechanical grinding. The yellow and red solids turned deep blue on irradiation (366 nm) and the red and blue solids turned yellow on exposure to white light. The yellow crystals gave a dee

Full text of DOI:10.1039/b002414g

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Studies on piezochromic photochromic (E)-5-dicyanomethylene-3-
[1-(2,5-dimethyl-3-furyl)ethylidene]-4-diphenylmethylenetetra-
hydrofuran-2-one and related photochromic compounds
1
Abdullah M. A. Asiri, Alison Cleeves and Harry G. Heller*
Chemistry Department, Cardiff University, PO Box 912, Cardiff, UK CF1 3TP
Received (in Cambridge, UK) 27th March 2000, Accepted 2nd June 2000
Published on the Web 5th July 2000
Yellow crystals of the title compound 2b gave a dark red powder on mechanical grinding. The yellow and red
solids turned deep blue on irradiation (366 nm) and the red and blue solids turned yellow on exposure to white
light. The yellow crystals gave a deep red solution in toluene, which turned blue on irradiation with ultraviolet
light. The blue solution reverted to red on exposure to white light. The chemistry of these colour changes is
discussed. Related photochromic compounds 2a, 2c, 2d, 4a, 4b, 7a and 7, with a lesser degree of molecular
overcrowding, are not piezochromic. The related non photochromic compound 1, with the greatest degree of
overcrowding, is tribochromic.
We introduced the term ‘tribochromism’ to describe the new
phenomenon of metastable crystals that undergo a major and
irreversible colour change on mechanical grinding. The col-
oured form is stable and does not undergo the reverse change
when kept in the dark, on heating or when dissolved in an
organic solvent. The phenomenon was illustrated by crystals of
the folded yellow dicyanomethylene compound 1y, which
gave the deep red twisted form 1r on mechanical grinding.1
Piezochromic compounds undergo reversible colour changes.2
5-dicyanomethylene-3-[1-(2,5-dimethyl-3-furyl)ethylidene]-4-
diphenylmethylenetetrahydrofuran-2-one 2b which turned out
to be piezochromic and not tribochromic as expected. The
difference in properties is attributed to the lesser degree of
molecular overcrowding. If overcrowding is reduced further, the
piezochromic properties are lost also.
Compound 2b was prepared by the method of Moore and
Khim4 by condensation of (E)-fulgide 2a with malononitrile in
the presence of diethylamine in THF, followed by cyclisation of
the resulting salt with acetyl chloride.
Light-sensitive yellow crystals of compound 2b turned blue
on exposure to ultraviolet light and red on grinding. By analogy
with the tribochromic compounds previously reported, the
yellow form is assumed to have the folded structure 2by and the
red form to have the twisted structure 2br. Both blue and red
forms reverted to yellow on irradiation with white light.
We have reported on the photochromic properties of
5-dicyanomethylene-3-[1-(2,5-dimethyl-3-furyl)ethylidene]-4-
isopropylidenetetrahydrofuran-2-one 2d which, on photocyclis-
ation, gave blue photochrome 3d (λmax 610 nm) showing a major
bathochromic shift compared to photochrome 3c (λmax 496
nm) from photocyclisation of the corresponding fulgide† 2c
(Scheme 1).3
When the yellow crystals were dissolved in toluene, a red
solution (λmax 514 nm) was obtained which, on irradiation (366
nm) or boiling (110 ЊC), turned deep blue (λmax 610 nm). (The
spectra are similar to those shown in Fig. 1.) When the blue
solution was exposed to white light, it turned red. Removal of
solvent and crystallisation of the residue gave yellow crystals.
The stereochemistry of compound 2b follows from its NMR
spectra, which showed a methyl group, deshielded by the
carbonyl group (δ 2.44 ppm). The assignment of its structure as
the 5-dicyanomethylenetetrahydrofuran-2-one derivative 2b
(and not as the 2-dicyanomethylenetetrahydrofuran-5-one
derivative 8b, Y = O) is based on the observation1 that crystals
of all eleven yellow folded tribochromic compounds and red
twisted forms had adjacent diphenylmethylene and dicyano-
methylene groups and gave red solutions.
Scheme 1
Replacement of the isopropylidene group in compound 2d by
the diphenylmethylene group gave photochromic crystals of
† IUPAC term for fulgide is dialkylidenesuccinic anhydride.
DOI: 10.1039/b002414g
J. Chem. Soc., Perkin Trans. 1, 2000, 2741–2744
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Fig. 1 The UV-Vis spectra of (Z)-lactone 4b (1 × 10Ϫ4 molar solu-
tion in toluene) (a) before and (b) after heating at 110 ЊC.
Blue photochrome, 2,4,8a-trimethyl-8,8-diphenyl-7-dicyano-
methylene-5-oxo-5,7,8,8a-tetrahydrobenzo[1,2-b:4,5-c]difuran
3b showed a characteristic high field 8a methyl signal at δ 0.90
ppm in its NMR spectrum.
Scheme 3
which cyclised to the blue photochrome 9b (λmax 606 nm)
(Scheme 3). (Photochrome 9a shows a value of λmax 536 nm for
its long wavelength absorption band.)
(Z)-5-Dicyanomethylene-3-[1-(2,5-dimethyl-3-furyl)ethyl-
idene]-4-bis(4-fluorophenyl)methylenetetrahydrofuran-2-one
4b, red crystals, was prepared by base-catalysed condensation
of the corresponding (Z)-fulgide 4a with malononitrile fol-
lowed by cyclisation of the resulting salt with acetyl chloride,
illustrating that 5-dicyanomethylene derivatives can be
obtained from either (E)- or (Z)-diarylfulgides 2a or 4a. The
stereochemistry of (Z)-lactone 4b follows from its high field
methyl singlet at 1.5 ppm, due to shielding of a methyl group by
the adjacent phenyl group. (Z)-Lactone 4b gave a red solution
in toluene (λmax 520 nm). On irradiation (366 nm) of the solu-
tion at room temperature or on boiling (110 ЊC), (Z)-lactone 4b
isomerised to the (E)-isomer 5b (Scheme 2), which cyclised to
the blue photochrome 6b (λmax 608 nm) (Fig. 1).
(Z)-Lactone 10, which gave a red solution in toluene (λmax
333 and 508 nm, ε 6 290 and 14 300 dm3 molϪ1 cmϪ1; λmin 450,
ε 4 957 dm3 molϪ1 cmϪ1), did not photoisomerise and photo-
cyclise like (Z)-lactone 7b did. On prolonged irradiation at 366
nm, it gave the maleic anhydride derivative 11 (Scheme 4),
presumably by a photochemical 1,5-antarafacial H-shift. The
structure of the latter was assigned from its NMR spectrum,
which showed four signals at 5.42, 5.43, 6.07 and 6.13 ppm
assigned to methine, two olefinic and the thienyl hydrogens, and
also loss of a methyl signal.
Scheme 4
Experimental
Melting points were determined on a Reichert hot-stage appar-
atus and are uncorrected. Ultraviolet and visible spectra were
measured in toluene on a Cecil CE6600 spectrophotometer;
infrared spectra were measured for Nujol mulls on a Perkin-
Elmer 1600 FTIR spectrometer; NMR spectra were obtained
with a Bruker WM360 (360 MHz) spectrometer using CDCl3
solutions; mass spectra were recorded on a Varian Mat.
CH5 spectrometer; and microanalyses were carried out using a
Perkin-Elmer 240B analyser. Petrol refers to the fraction of bp
60–80 ЊC. Solvents were dried before use. Photocoloration (366
nm) was carried out using a 200 W medium pressure mercury
lamp and an OX1A filter and photobleaching was achieved
using a tungsten filament lamp.
Scheme 2
Condensation of (Z)-fulgide 7a with malononitrile gave
yellow crystals of (Z)-2-dicyanomethylene-3-[1-(2,5-dimethyl-3-
thienyl)ethylidene]-4-diphenylmethylenetetrahydrofuran-5-one
7b and dark red crystals of (Z)-5-dicyanomethylene-3-[1-(2,5-
dimethyl-3-thienyl)ethylidene]-4-diphenylmethylenetetrahydro-
furan-2-one 10, illustrating that both 2- and 5-dicyano-
methylene derivatives can be obtained in the same reaction.
Their stereochemistry followed from their NMR spectra, which
showed high field singlets at 1.50 and 1.44 ppm respectively
due to shielding of a methyl group by a phenyl group in each
case. (Z)-Lactones 7b and 10 gave yellow and red solutions in
toluene respectively. On irradiation (366 nm) (Z)-lactone 7b in
toluene (λmax 356 nm) isomerised to the (E)-isomer 8b, Y = S,
Preparation of fulgides
First Stobbe condensation. Dimethyl succinate and ketone in
toluene were added dropwise to a stirred suspension of potas-
sium tert-butoxide in toluene. The mixture was stirred at room
temperature (ca. 12 hours) and poured into water. The aqueous
layer was acidified with hydrochloric acid and the liberated
half-ester extracted with ether, dried and the solvent was
removed.
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Re-esterification. Acetyl chloride (1.5 equiv.) was added to
the half-ester in methanol and the mixture boiled (6 h). Solvent
was removed under reduced pressure and the residue dissolved
in ether, washed with water and with sodium bicarbonate solu-
tion, dried and the solvent removed. The diester was purified by
vacuum distillation.
yield) was obtained as orange crystals (from methanol), mp
196–198 ЊC. Found: C, 75.17; H, 5.14; Mϩ 400. C25H20O3S
required C, 75.01; H, 5.00%; M 400. λmax 388 nm, ε 9 333;
δH 2.37 (3H, s, CH3), 2.39 (3H, s, CH3), 1.43 (3H, s, CH3 trans to
C᎐O), 6.41 (1H, s, 4-thienyl proton) and 7.10–7.50 (10H, m,
᎐
aromatics).
Dimethyl (Z)-[1-(2,5-dimethyl-3-thienyl)ethylidene]succinate
Second Stobbe condensation. The diester from the first con-
densation and ketone in toluene were added dropwise to a
stirred suspension of potassium tert-butoxide in toluene. The
mixture was stirred at room temperature (6 h) and poured into
water. The aqueous layer was acidified with hydrochloric acid
and the liberated half-ester was extracted with ether, dried, and
the solvent removed.
Dimethyl succinate (35 g, 0.24 mol) and 3-acetyl-2,5-dimethyl-
thiophene (37 g, 0.24 mol) with t-BuOK (32 g. 0.28 mol) in
toluene (150 cm3) gave the half-ester which on esterification
gave a black oil. Distillation at 140–150 ЊC at 0.1 mmHg gave
pure (Z)-diester as a yellow oil (43 g, 64% yield).
(E)-5-Dicyanomethylene-3-[1-(2,5-dimethyl-3-furyl)ethylidene]-
4-diphenylmethylenetetrahydrofuran-2-one 2b
Hydrolysis and cyclisation. The half-ester was boiled with
10% ethanolic KOH (6 h) and the ethanol removed under
reduced pressure. The residual potassium salt was dissolved in
water and the diacid liberated by addition of hydrochloric acid.
The diacid was dissolved in ether, dried, and the solvent
removed. Diacid in dichloromethane (DCM) was stirred with
acetyl chloride at room temperature (3 h). Solvent was removed
and the fulgide purified by column chromatography on silica gel
using DCM and petrol as eluant followed by recrystallisation.
(E)-[1-(2,5-Dimethyl-3-furyl)ethylidene]diphenylmethylene-
succinic anhydride 2a (1 g, 2.6 mmol) and malononitrile (0.17 g,
2.6 mmol) were added to diethylamine (0.38 g, 5.15 mmol) in
THF (10 cm3) at 0 ЊC and the solution was stirred at room
temperature (12 h). Solvent was removed and the residue dis-
solved in DCM (10 cm3) was treated with acetyl chloride (10
cm3). Acetyl chloride was removed under reduced pressure and
the residue crystallised (from ethyl acetate and petrol 1:1) giving
(E)-tetrahydrofuran-2-one 2b as light-sensitive yellow crystals
(0.2 g, 18% yield), mp 198–200 ЊC (dark red melt). Found: C,
77.99; H, 4.46; N, 6.5; Mϩ 432. C28H20O3N2 requires C, 77.79;
H, 4.62; N, 6.47%; M 432. λmax 344 nm, ε 12 020; δH 1.90 (3H, s,
(E)-[1-(2,5-Dimethyl-3-furyl)ethylidene]diphenylmethylene-
succinic anhydride 2a
Dimethyl diphenylmethylenesuccinate (30 g, 0.1 mol) and 2,5-
dimethyl-3-acetylfuran (13.35 g, 0.1 mol) with t-BuOK (11.2 g,
0.1 mol) in toluene (150 cm3) gave the half-ester as a dark gum
which was hydrolysed and cyclised. (E)-Fulgide 2a (1.5 g, 4%
yield) was obtained as orange crystals (from petrol), mp 178–
180 ЊC; δH 1.95 (3H, s, CH3), 1.99 (3H, s, CH3), 2.43 (3H, s, CH3
CH ), 2.06 (3H, s, CH ), 2.40 (3H, s, CH cis to C᎐O), 5.28 (1H,
᎐
3
3
3
s, 4-furyl proton) and 6.61–7.47 (10H, m, aromatics); νmax/cmϪ1
2224 (CN) and 1706 C᎐O).
᎐
(Z)-5-Dicyanomethylene-3-[1-(2,5-dimethyl-3-furyl)ethylidene]-
4-bis(4-fluorophenyl)methylenetetrahydrofuran-2-one 4b
cis to C᎐O), 5.32 (1H, s, 4-furyl proton) and 7.10–7.44 (10H, m,
᎐
aromatics).
(Z)-[1-(2,5-Dimethyl-3-furyl)ethylidene]diphenylmethylene-
succinic anhydride 4a (0.71 g, 1.69 mmol) and malononitrile
(0.12 g, 1.82 mmol) were added to diethylamine (0.25 g, 3.42
mmol) in THF (15 cm3) at 0 ЊC and the solution was stirred at
room temperature (12 h). The solvent was removed and the
colourless residue dissolved in DCM (10 cm3) was treated with
acetyl chloride (10 cm3). Acetyl chloride and solvent were
removed under reduced pressure and the residue was crystal-
lised (from DCM and petrol) to give dark red crystals of (Z)-
tetrahydrofuranone 4b (0.45 g, 57% yield), mp 172–174 ЊC.
Found: C, 69.20; H, 3.96; N, 5.96; Mϩ 468. C28H18F2O3N2
requires C, 77.79; H, 4.62; N, 6.47%; M 468. λmax 400, 520 nm,
ε 14 790 and 8 913; δH 2.24 (3H, s, CH3), 2.28 (3H, s, CH3), 1.50
Dimethyl diphenylmethylenesuccinate
Dimethyl succinate (60 g) and benzophenone (70 g) with
t-BuOK (50 g) in toluene (250 cm3) gave the half-ester, which
was esterified to give dimethyl diphenylmethylenesuccinate as
yellow crystals (from petrol) (79.6 g, 75% yield), mp 78–80 ЊC.
(Z)-Bis(4-fluorophenyl)methylene[1-(2,5-dimethyl-3-furyl)-
ethylidene]succinic anhydride 4a
Dimethyl (E)-[1-(2,5-dimethyl-3-furyl)ethylidene]succinate (14
g, 0.07 mol) and 4,4Ј-difluorobenzophenone (15 g, 0.07 mol)
with t-BuOK (11.2 g, 0.1 mol) in toluene (130 cm3) gave the
half-ester as a dark gum which was hydrolysed and cyclised.
(Z)-Fulgide 4a (1.5 g, 6.7% yield) was obtained as orange
crystals (from methanol), mp 200–202 ЊC. Found: C, 71.30; H,
4.36; Mϩ 420. C25H18O4F2 required C, 71.44; H, 4.28%; M 420.
λmax 405 nm, ε 5 495; δH 2.22 (3H, s, CH3), 2.30 (3H, s, CH3),
1.50 (3H, s, CH trans to C᎐O), 5.88 (1H, s, 4-furyl proton) and
(3H, s, CH , trans to C᎐O), 5.90 (1H, s, 4-furyl proton) and
᎐
3
7.08–7.44 (8H, m, aromatics); νmax/cmϪ1 2226 (CN) and 1710
(C᎐O).
᎐
(Z)-5-Dicyanomethylene-3-[1-(2,5-dimethyl-3-thienyl)ethyl-
idene]-4-diphenylmethylenetetrahydrofuran-2-one 7b and (Z)-5-
dicyanomethylene-3-[1-(2,5-dimethyl-3-thienyl)ethylidene]-4-
diphenylmethylenetetrahydrofuran-2-one 10
᎐
3
7.0–7.30 (8H, m, aromatics); νmax/cmϪ1 1810 (C᎐O) and 1763
᎐
(C᎐O).
᎐
(Z)-[1-(2,5-Dimethyl-3-thienyl)ethylidene]diphenylmethylene-
succinic anhydride 7a (1.5 g, 3.75 mmol) and malononitrile (0.25
g, 3.78 mmol) were added to diethylamine (0.85 g, 11.5 mmol)
in THF (10 cm3) at 0 ЊC and the solution was stirred at room
temperature (ca. 12 h). The solvent was removed and the resi-
due dissolved in DCM (10 cm3) was treated with acetyl chloride
(10 cm3). Acetyl chloride and solvent were removed under
reduced pressure and the residue was crystallised from chloro-
form and petrol (1:1). The first crop of yellow crystals was (Z)-
tetrahydrofuranone 7b (0.1 g, 6% yield), mp 180–182 ЊC.
Found: C, 74.84; H, 4.60; N, 6.03. C28H20O2N2S requires C,
75.02; H, 4.46; N, 6.24%. λmax 356 nm, ε 21 380; δH 2.15 (3H,
Dimethyl (E)-[1-(2,5-dimethyl-3-furyl)ethylidene]succinate
Dimethyl succinate (50 g, 0.34 mol) and 3-acetyl-2,5-dimethyl-
furan (47 g, 0.34 mol) with potassium tert-butoxide (38 g, 0.34
mol) in toluene (100 cm3) gave the half-ester which on esterifi-
cation gave a black gum. Distillation at 150–170 ЊC at 0.4
mmHg gave pure (E)-diester as a yellow oil (41 g, 45% yield).
(Z)-[1-(2,5-Dimethyl-3-thienyl)ethylidene]diphenylmethylene-
succinic anhydride 7a
Dimethyl (Z)-[1-(2,5-dimethyl-3-thienyl)ethylidene]succinate
(15 g, 0.064 mol) and benzophenone (11.6 g, 0.064 mol) with
t-BuOK (10 g, 0.09 mol) in THF (150 cm3) gave the half-ester
which was hydrolysed and cyclised. (Z)-Fulgide 7a (1.5 g, 6.7%
s, CH ), 2.21 (3H, s, CH ), 1.50 (3H, s, CH trans to C᎐O),
᎐
3
3
3
6.22 (1H, s, 4-thienyl proton) and 6.74, 7.13–7.57 (10H, m,
J. Chem. Soc., Perkin Trans. 1, 2000, 2741–2744 2743

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aromatics). The second crop of dark red crystals was (Z)-
tetrahydrofuranone 10 (0.2 g, 12%), mp 173-175 ЊC (after
recrystallisation from DCM and petrol). Found: C, 75.08; H,
4.27; N, 6.34; Mϩ 448. C28H20O2N2S requires C, 75.02; H, 4.46;
N, 6.24%; M 448. λmax 330 nm, ε 16 220; δH 2.32 (3H, s, CH3),
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᎐
3
3
4-thienyl proton) and 6.74, 7.28–7.56 (10H, m, aromatics);
νmax/cmϪ1 2225 (CN) and 1706 (C᎐O).
᎐
Photorearrangement of (Z)-5-dicyanomethylene-3-[1-(2,5-
dimethyl-3-thienyl)ethylidene]-4-diphenylmethylenetetrahydro-
furan-2-one 10
An NMR tube containing a deep red solution of (Z)-lactone 10
in deuteriochloroform was irradiated (3 days) at 366 nm (using
the focussed beam of the 200 W mercury lamp) and the NMR
spectrum measured. The spectrum showed that the solution
contained a ca. 3:2 mixture of the maleic anhydride derivative
11 and starting (Z)-lactone 10.
Acknowledgements
We thank King Abdul Aziz University, Saudi Arabia for
financial support to A. M. A. A.
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2744
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