Dual photochromic properties of 4-dialkylamino-2-hydroxychalcones

Article abstract of DOI:10.1246/bcsj.74.827

4-Dialkylamino-2-hydroxychalcones [3-(4-dialkylamino-2-hydroxyphenyl)-1-(substituted phenyl)-2-propen-1-ones] undergo reversible E/Z-photoisomerization in good quantum yields (0.2-0.4) in neutral aprotic solvents. The absorption bands of the photo-Z-isome

Full text of DOI:10.1246/bcsj.74.827

c. Jpn.
© 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 827—832 (2001)
827
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Dual Photochromic Properties of 4-Dialkylamino-2-hydroxychalcones
Dual Photochromism of 2-Hydroxychalcones
R. Matsushima et al.
Ryoka Matsushima,* Shuichi Fujimoto, and Kunihiro Tokumura†
Materials Science and Chemical Engineering, Faculty of Engineering, Shizuoka University,
3-5-1 Johoku, Hamamatsu 432-8561
01
7
2
†Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-machi, Kanazawa 920-0934
(Received October 10, 2000)
SJA8
09-2673
355
tober
4-Dialkylamino-2-hydroxychalcones [3-(4-dialkylamino-2-hydroxyphenyl)-1-(substituted phenyl)-2-propen-1-
ones] undergo reversible E/Z-photoisomerization in good quantum yields (0.2–0.4) in neutral aprotic solvents. The ab-
sorption bands of the photo-Z-isomers are significantly red-shifted from the E-isomers. By addition of acid, the photo-Z-
isomers are converted to the flavylium ions with competitive reversion to the E-isomers. In aqueous ethanol solution,
photochromic interconversion between chalcones and flavylium ions takes place rather slowly and less extensively.
00
01
.5
In order to develop unique and environmentally suitable
Experimental
photochromic systems involving chalcone–flavylium intercon-
Fluorescence spectra were measured on a Hitachi 204 fluores-
cence spectrometer, and fluorescence quantum yields were esti-
mated by using aqueous acid solutions of quinine sulfate as a stan-
dard.14 Electronic absorption spectra were recorded on a Hitachi
U-3000 spectrophotometer, while 1H NMR spectra were obtained
on a EX-90 FT-NMR spectrometer (90 MHz). Melting points
were measured with a micro-melting point apparatus (Yanaco
MP-3S, Yanagimoto). The light source was a 400-W high-pres-
sure mercury lamp (Riko). Its monochromatic 405 nm beam was
selected through an interference filter (Hitachi B420), or 405 and
436 nm beams were selected through a cut-off glass filter (Toshiba
C-39B). Photoreaction quantum yields were estimated from the
plots of the consumption of E-chalcones as a function of irradia-
tion time, by using the potassium tris(oxalate)ferrate(III) solution
as a chemical actinometer under conditions that the absorbance of
each solution of the incident light was around 2.15,16 The concen-
tration of each chalcone was estimated from its molar absorption
coefficient and maximum absorbance. Reactions were carried out
without elimination of the dissolved oxygen, while fresh solvents
of guaranteed grade (Wako Chemicals) were used as received. 3-
(4-Diethylamino-2-hydroxyphenyl)-1-phenyl-2-propen-1-one
(chalcone 1) was prepared from 7-diethylaminoflavylium perchlo-
rate (1F) as follows.7 To a mixed solution containing 1F (26.5
mmol), ethanol (100 mL) and N,N-dimethylformamide (40 mL),
0.1 M aqueous sodium hydroxide solution (20 mL) was slowly
added, and the mixture was stirred for a day at room temperature
under nitrogen atmosphere. The mixture was condensed and ex-
tracted with ethyl acetate (150 mL) and water (150 mL). By re-
moval of the solvent from the extracts, a black oil was obtained;
this was subjected to column chromatography on silica gel with
hexane–ethyl acetate, giving orange needles of chalcone 1 (13%):
mp 126–132 ˚C; 1H NMR (90 MHz, in DMSO-d6) δ 1.12 (t, 6H, J
ꢀ 7.0 Hz), 3.34 (q, 4H, J ꢀ 7.0 Hz), 6.17 (sd, 1H, J ꢀ 2.4 Hz),
6.26 (dd, 1H, J ꢀ 8.8 Hz, J ꢀ 2.4 Hz), 7.49–7.62 (m, 5H), 7.96–
8.02 (m, 3H), 9.94 (s, 1H). By an essentially similar method,17,18
other chalcones 2–5 were prepared starting from the correspond-
ing flavylium perchlorates: namely, 7-diethylamino-4′-methoxy-
version, as a simplified model of the naturally occurring antho-
cyanin dyes,1–3 we have undertaken to investigate a series of
substituted 2-hydroxychalcones [3-(2-hydroxyphenyl)-1-(sub-
stituted phenyl)-2-propen-1-ones], and hitherto achieved good
photochromism at least in fluid solution. The colored forms
(flavylium ions) exhibit sharp and strong absorption bands in
the yellow,4,5 magenta,6–9 and blue10,11 regions (depending on
the substituents) which are well separated from the chalcone
bands. Among the 4′-dialkylamino-2-hydroxychalcones [3-(2-
hydroxyphenyl)-1-(4-dialkylaminophenyl)-2-propen-1-ones]
exhibit excellent photochromic reversibility and the flavylium
ions exhibit favorable color properties (high color purity and
strong intensity).7 A novel photochemical process has been
developed for the system, leading to a photon-mode erasable
optical memory with nondestructive readout ability.12,13
Recently, it was found that 4-dialkylamino-2-hydroxychal-
cones (Scheme 1) exhibit dual photochromism depending on
Scheme 1.
Structures and photoisomerization of chalcones
in neutral solution.
the medium conditions, and strong fluorescence in polar sol-
vents.10 The present paper reports on the dual photochromic
properties as well as the fluorescence of these chalcones.

828 Bull. Chem. Soc. Jpn., 74, No. 5 (2001)
Dual Photochromism of 2-Hydroxychalcones
flavylium perchlorate (2F), 7-diethylamino-4′-dimethylaminofla-
vylium perchlorate (3F), 4′,7-bis(dimethylamino)-flavylium
perchlorate (4F), and 6-(4-dimethylaminophenyl)-julolido[8,9-
b]pyrylium perchlorate (5F), respectively. Recrystallization from
ethyl acetate yielded dark blue plates of chalcone 2: mp 136 ˚C
1–5 (Scheme 1) exhibit electronic absorption bands around
410–420 nm. The H NMR coupling constants (J ꢀ 15–16
1
Hz) for the enone vinyl protons imply the E-conformation in
the unirradiated form (X-ray structural analysis of chalcone 1
also revealed an E-isomer, though data not shown here). These
chalcones undergo facile photochemical color changes (red
shifts) upon irradiation in neutral aprotic solvents, as illustrat-
ed with chalcone 3 in toluene (red shift from 408 to 471 nm)
on irradiation with 405 nm light for one minute (Fig. 1a).
While the new band of the photoisomer reverts slowly in the
dark at 30 ˚C (Fig. 1b), photochemical reversion can be more
clearly observed on irradiation with light of λ > 460 nm (Fig.
1c) at a low temperature below 5 ˚C where thermal reversion is
substantially depressed (Fig. 1d). Similar photochromic reac-
tions were found with other chalcones 1, 2, and 5, as summa-
rized in Table 1. The photochemical red-shifts are ascribable
to photochemical E/Z isomerization around the enone double
bond, as will be discussed later. The quantum yields for the
photochemical reaction are relatively high and less variable,
while the half-lives (τ1/2) of the photoisomers strongly vary
with the structure. The half-lives τ1/2 also varied with tempera-
ture. Thus, from the Arrhenius plots for the thermal reversion
of 3, the value for the apparent activation energy was estimated
1
(decomp.); H NMR (90 MHz, in DMSO-d6) δ 1.12 (t, 6H, J ꢀ
7.0 Hz), 3.34 (q, 4H, J ꢀ 7.0 Hz), 3.85 (s, 3H), 6.17 (sd, 1H, J ꢀ
2.4 Hz), 6.24 (dd, 1H, J ꢀ 8.8 Hz, J ꢀ 2.4 Hz), 7.05 (d, 2H, J ꢀ
8.8 Hz), 7.53 (d, 1H, J ꢀ 15.1 Hz), 7.61 (d, 1H, J ꢀ 8.8 Hz), 7.94
(d, 1H, J ꢀ 15.6 Hz), 8.03 (d, 2H, J ꢀ 9.3 Hz), 9.96 (s, 1H), or-
1
ange needles of chalcone 3: mp 182 ˚C (decomp); H NMR (90
MHz, in DMSO-d6) δ 1.12 (t, 6H, J ꢀ 7.0 Hz), 3.02 (s, 6H), 3.33
(q, 4H, J ꢀ 7.0 Hz), 6.17 (sd, 1H, J ꢀ 2.4 Hz), 6.24 (d, 1H, J ꢀ
8.8 Hz), 6.75 (d, 2H, J ꢀ 9.3 Hz), 7.52 (d, 1H, J ꢀ 15.6 Hz), 7.58
(d, 1H, J ꢀ 8.8 Hz), 7.88 (d, 1H, J ꢀ 15.6 Hz), 7.93 (d, 2H, J ꢀ
9.3 Hz), 9.80 (s, 1H), respectively, while recrystallization from
1
ethanol gave orange granules of chalcone 4: mp 210–212 ˚C; H
NMR (60 MHz, in DMSO-d6) δ 2.98 (s, 2H), 3.05 (s, 2H), 6.21–
8.00 (m, 10 H). The 1H NMR (DMSO-d6) spectra of the flavylium
perchlorates 1F–3F and 5F were also satisfactory. However, re-
crystallization of chalcone 5 was unsuccessful, and its crude yel-
low powders were used without further purification.
Results and Discussion
to be around 65 kJ molꢁ1
.
Photochromism in Neutral Apolar Solvents. Chalcones
Fig. 1.
Photochemical isomerization of chalcone 3 in toluene solution (30 µM) on irradiation with 405 nm light for one minute
(a), followed by thermal reversion at 30 ˚C (b), and photochemical reversion with light of λ > 460 nm 4 ˚C (c) where thermal re-
version is substantially depressed (d).

R. Matsushima et al.
Bull. Chem. Soc. Jpn., 74, No. 5 (2001) 829
Table 1.
Photochemical Isomerization and Thermal Reversion of Chalcones in Toluene Solutiona)
Quantum yieldc)
Half-life of isomerd)
Reversibilitye)
Chalcone
Before irradiation
After irradiationb)
ε
max/104 Mꢁ1 cmꢁ1
ε
max/104 Mꢁ1 cmꢁ1
No.
1
2
3
5
λ
max/nm
λ
max/nm
φEZ
τ
1/2/min
3.5
A10/A0
0.78
0.69
0.85
0.39
410
406
408
424
3.74
3.28
2.85
2.98
476
471
471
493
3.34
2.88
2.58
3.13
0.30
0.37
0.37
0.17
11
93
15
a) Initial concentration of chalcones were 8–10 ꢂ 10ꢁ5 (1 M ꢀ 1 mol dmꢁ3), irradiated with 405 nm light from a 400-W high-
pressure mercury lamp. b) Absorption maximum and appropriate molar absorption coefficient obtained from the PSS solution
immediately after irradiation. c) Quantum yield for the photochemical consumption of the initial E-chalcone, by using the
potassium tris(oxalate)ferrate(III) solution as chemical actinometer under conditions that the absorbance of each solution of
the incident light was around 2. d) Half-life for the thermal reversion of the photoisomer at 30 ˚C. e) A0 and A10 refer to the
absorbances of the photoisomer at the initial and after 10 cycles of the repetition.
Fig. 2.
Photochemical isomerization of chalcone 3 in hexane (1,1’), toluene (2,2’), acetone (3,3’), ethyl acetate (4,4’), ethanol
(5,5’), and water (6,6’). Solid curves: before irradiation, broken curves: after irradiation with sunlight for one minute. Each solu-
tion contained 8.5% of 1,2-dimethoxyethane.
Figure 2 illustrates the photochemical color changes of chal-
cone 3 in different solvents, showing large shifts in hexane or
toluene solution (Fig. 2a), but moderate one in acetone or ethyl
acetate (Fig. 2b) and no shift in hydroxylic solvents (Fig. 2c).
Similar solvent effects were found with chalcones 1, 2, and 5.
On addition of a small amount of ethanol to the photostation-
ary state (PSS) solution in toluene (2’ in Fig. 2a), the 471 nm
band rapidly disappeared to reform the 408 band of 3. There-
fore, the apparent nil spectral change in the protic solvents
(Fig. 2c) might be the result of either the rapid reversion of the
photoisomer if formed or may be due to the nil photoisomer-
ization.
Figure 3 illustrates the reversibility in neutral toluene solu-
tion toward the repeated cycles for the photochemical color
change and thermal reversion at 30 ˚C. Chalcones 1 and 3 re-
vealed fairly good reversibilities (ca. 80% retainment after 10
cycles), while chalcone 5 revealed poor reversibility (last col-
umn in Table 1) partially because of its impurity. The thermal
reversion curves in Fig. 3 become steeper (faster rate) in order
of the number of the cycles. The unusual behaviors might be
related to the complicated phenomena such as intermolecular
association and/or complexation.
We assume that the red-shifted photoisomers have the Z-
conformation about the enone double bond (Scheme 2), which
is supported by the observation that the PSS solution of 3 gave
a mixture of the flavylium ion and chalcone 3 (E-isomer) upon
addition of a strong acid. Thus, as illustrated in Fig. 4, the 471
nm band of the photoisomer disappeared and a new band
around 600 nm (flavylium band) appeared with competitive re-
version to the E-isomer 3 (408 nm), by addition of trifluoro-
acetic acid (50 µM) (1 M ꢀ 1 mol dmꢁ3). No such spectral
change was observed by addition of the acid to the solution be-
fore irradiation. The flavylium-ion formation is explained by
the reaction sequences in Scheme 2 involving photochemical
E/Z-isomerization followed by acid-catalyzed cyclization to
hemiacetals (second step) and dehydration to give flavylium
ions (last step), in analogy to the reaction sequence for the ma-
jority of other 2-hydroxychalcones in the presence of acid.4–9
Photochromism in Acid Media. Figure 5 illustrates the
photochromism of chalcone 4 in a 1:1 aqueous ethanol solu-
tion at pH 6.0. Chalcone 4 was substantially converted into
blue-colored flavylium ion upon irradiation with 405–436 nm
light, according to the assumed reaction sequence shown in
Scheme 2,4–9 while it was fairly stable in the dark. On heating

830 Bull. Chem. Soc. Jpn., 74, No. 5 (2001)
Dual Photochromism of 2-Hydroxychalcones
Fig. 4.
Absorption spectra of the photoisomer (PSS solu-
tion) of chalcone 3 in toluene immediately after irradiation
with 405 nm light (solid curve), and spectra after addition
of 50 µM trifluoroacetic acid (broken curve). No such
spectral change was observed by addition of the acid to the
solution before irradiation.
der or peak around 570 nm, implying an intermolecular associ-
ation or complexation of the flavylium ion at a higher concen-
tration in thin film. Similar spectral features, as well as large
deviations from the Beer–Lambert law, have been observed
with many flavylium ions in solution,19 often preventing accu-
rate estimation for their molar absorption coefficients. The ab-
sorption maxima (λmax) and approximate molar absorption co-
efficients (εmax/104 Mꢁ1 cmꢁ1 in parentheses) of the separately
prepared flavylium perchlorates were 1F: 522 nm (2.0), 2F:
534 nm (3.3), 3F: 600 nm (8.9), 4F: 597 nm (9.0 in aqueous
ethanol), and 5F: 613 nm (7.3), respectively, in 30 µM in etha-
nol solution. Thermal reversion of the flavylium ion 3F in the
PMMA film was very slow and substantially not irreversible.
Discussion on the Structures and Electronic Spectra.
MOPAC-PM3 calculations20 with full structure optimization
implied that the most stable E-isomers were essentially planar,
whereas the Z-isomers were nonplanar with substantially
twisted benzoyl moiety (while the cinnamoyl moiety was al-
most planar). For the Z-isomers of chalcones 1–4, intramolec-
ular hydrogen bonding between the phenolic proton and carbo-
nyl oxygen (with the H O bond distance of 1.8–2.1 Å) was
implied by the optimized structures. On the other hand, bimo-
lecular hydrogen bonds between head-to-tail pairs have been
implied for the E-isomers, which were confirmed at least in the
X-ray crystal structures of 2-hydroxychalcone [3-(2-hydroxy-
phenyl)-1-phenyl-2-propen-1-one] and 4′-methoxy-2-hydrox-
ychalcone [3-(2-hydroxyphenyl)-1-(4-methoxyphenyl)-2-pro-
pen-1-one]. Taking these results into account, PPP-CI
calculations for the E-isomers were carried out by using the
parameters for the hydrogen-bonded phenolic group, whereas
calculations for the Z-isomers were carried out by using both
the hydrogen-bonded and non-hydrogen-bonded parame-
ters.21 Table 2 lists the calculated π,π* electronic transitions
for the first bands of the E- and Z-isomers, showing fair consis-
tency with the experimental results listed in Table 1. The cal-
Fig. 3.
Reversibility toward the repeated cycles for the
photochemical isomerization (broken curves) and thermal
reversion (solid curves) of chalcones 1 (a) and 2 (b) in tolu-
ene solution at 30 ˚C. The vertical axes refers to the absor-
bance of the photoisomers.
Scheme 2.
cones and flavylium ions.
Interconversions among E/Z-isomers of chal-
at 60 ˚C for 100 min the blue-colored solution substantially
bleached to the chalcone. An isosbestic point is seen around
500 nm during the reaction. The interconversion could be re-
peated several times, as illustrated in Fig. 5 b. Under similar
conditions, chalcone 3 revealed much slower responses for
both photochemical conversion into flavylium ion (4 h) and
thermal reversion (12 h), while in PMMA film containing tan-
nic acid it underwent more facile photoreaction to give flavyli-
um ion, as illustrated in Fig. 6. The absorption spectra of the
flavylium ion around 600 nm is accompanied by a new shoul-

R. Matsushima et al.
Bull. Chem. Soc. Jpn., 74, No. 5 (2001) 831
Fig. 5.
Photochromism of chalcone 4 (10 µM) in a 1:1 aqueous ethanol solution at pH 6.0. (a) Photochemical conversion into fla-
vylium ion on irradiation with 405 and 436 nm beams from a 400-W high-pressure mercury lamp. (b) Photochromic reversibility
toward repeated cycles of photocoloration for 25 nm (solid lines) and thermal reversion on heating at 60 ˚C for 100 min (broken
lines).
Table 2.
Calcone
PPP-CI Calculations for the First Bands of the E- and Z-isomersa)
E-isomer with H-bondb) Z-isomers with H-bondb)
Z-isomers without H-bond
No.
1
λ
max/nm
396
f-value
1.05
λ
max/nm
445
f-value
0.56
λ
max/nm
442
f-value
0.55
2
389
1.09
413
0.29
434
0.59
3
384
350
1.22
0.98
427
357
0.74
0.93
423
348
0.73
0.99
6c)
a) Standard parameters were used unless otherwise noted. b) Hydrogen-bonded parameters
were used for the 2-hydroxy group. c) Chalcone 6 revealed λmax at 370 nm (εmax 3.27 ꢂ 104
M
ꢁ1 cmꢁ1) while its Z-isomer had λmax around 390 nm, in toluene solution.
culated red shifts (∆λ) from the E- to Z-isomers (H-bonded)
are in the range of 25–50 nm for chalcones 1–3 (Table 2), be-
ing comparable to the shifts observed around 65 nm (Table 1).
By contrast, the majority of 2-hydroxychalcones with RA ꢀ H
revealed blue shifts upon change from E- to Z-isomers, though
the absorption wavelength of chalcone 6 was sensitive to the
nature of the solvents. Thus, on irradiation of the E-isomer,
chalcone 6 revealed a blue shift (from 369 to 350 nm) in tet-
rahydrofuran (THF) whereas a small red shift (from 370 to 390
nm) was found in toluene. It is tentatively speculated that the
photochemically formed Z-isomer would form an intramolecu-
lar hydrogen bond in toluene which is neither proton-donating
nor -accepting, but not in THF solution, where the solvent fea-
tures a good proton acceptor and hence inhibits the hydrogen
bonding of the Z-isomer. Interestingly, chalcones 1–5 are sig-
nificantly fluorescent in solution, whereas the majority of other
2-hydroxychalcones with RA ꢀ H are generally nonfluores-
cent. This implies that the amino substituents in the A-ring
play a crucial role in the electronic transitions and/or the excit-
ed state properties. The INDO/S as well as the PPP-CI calcu-
lations implied strong intramolecular charge transfer character
(mainly in the cinnamoyl plane) in the lowest π,π* excited
states of chalcones 1–4, as compared to the much less polar na-
ture of other 2-hydroxychalcones with RA ꢀ H. Increase in the
Fig. 6.
Photochemical conversion of chalcone 3 (5 wt%)
into flavylium ion in PMMA film in the presence of 10
wt% tannic acid, on irradiation with 405 and 436 nm
beams from a 120-W high-pressure mercury lamp. Irradia-
tion time/min: (1) 0, (2) 10, (3) 30, (4) 60, and (5) 120, re-
spectively. Broken curve refers to the thermal change on
standing at room temperature for 24 h.

832 Bull. Chem. Soc. Jpn., 74, No. 5 (2001)
Dual Photochromism of 2-Hydroxychalcones
4
R. Matsushima, K. Miyakawa, and M. Nishihata, Chem.
charge transfer character in the excited states, by either struc-
tural or solvent polarity changes, tends to enhance the fluores-
cence efficiency: thus, the fluorescence quantum yields (φf) of
chalcones 1, 2, and 3 were 0.091 (λF 520 nm), 0.15 (535 nm),
and 0.22 (515 nm), respectively, on excitation at 405 nm in t-
butyl alcohol. The φf value of chalcone 3 varied with solvent:
0.25 in N,N-dimethylformamide, 0.13 in THF, 0.076 in etha-
nol, 0.014 in methanol, and 0.03 in diethyl ether. Strong
charge transfer interaction between the amino and carbonyl
groups would help to retain the planarity of the cinnamoyl
moiety, thus depressing the radiationless deactivation via
twisting motions. While φf values are usually low in ethanol
and methanol, relatively high values are obtained in t-butyl al-
cohol, whose viscosity is sufficiently high to retard the twisting
motions of the chromophores.
In summary, 4-dialkylamino-2-hydroxychalcones undergo
reversible photochemical E/Z isomerization in neutral aprotic
solvents but not in neutral protic solvents, while they are sig-
nificantly fluorescent in some polar and protic solvents. The
photo-Z-isomers, which exhibit significantly red-shifted ab-
sorption bands, are converted to the blue-colored flavylium
ions in competition with reversion to the E-isomers upon addi-
tion of small amounts of acid. In aqueous ethanol solution at
pH around 6, photochemical chalcone → flavylium conversion
and thermal reversion can be repeated with low rates.
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