Novel group of negative photochromes containing a nitrile-rich acceptor: synthesis and photochromic properties

Article abstract of DOI:10.1007/s11164-019-03853-w

An approach to the synthesis of novel group of negative photochromes has been developed based on the interaction of the tricycanofuran TCF or hydroxytricyanopyrrole HTCP acceptors with methoxy-substituted 2-hydroxybenzaldehydes in ethanol in the presence of ammonium acetate. An essential part of the obtained photochromes is a combination of a phenol fragment and a butadienetricarbonitrile BDTC acceptor connected via a vinylene bridge. The synthesized compounds are sensitive to visible light T-type photochromes. A transformation degree of the initial form, as well as the rate of the reverse dark reaction, can be varied by changing both the position of a methoxy group at aromatic ring and the type of the heterocycle with the BDTC fragment.

Full text of DOI:10.1007/s11164-019-03853-w

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03853-w
Novel group of negative photochromes containing
a nitrile‑rich acceptor: synthesis and photochromic
properties
Mikhail Yu. Belikov1 · Mikhail Yu. Ievlev1 · Sergey V. Fedoseev1 · Oleg V. Ershov1
Received: 2 March 2019 / Accepted: 8 May 2019
© Springer Nature B.V. 2019
Abstract
An approach to the synthesis of novel group of negative photochromes has been
developed based on the interaction of the tricycanofuran TCF or hydroxytricyano-
pyrrole HTCP acceptors with methoxy-substituted 2-hydroxybenzaldehydes in etha-
nol in the presence of ammonium acetate. An essential part of the obtained pho-
tochromes is a combination of a phenol fragment and a butadienetricarbonitrile
BDTC acceptor connected via a vinylene bridge. The synthesized compounds are
sensitive to visible light T-type photochromes. A transformation degree of the initial
form, as well as the rate of the reverse dark reaction, can be varied by changing both
the position of a methoxy group at aromatic ring and the type of the heterocycle
with the BDTC fragment.
Keywords Negative photochromism · Tricyanofuran TCF · Hydroxytricyanopyrrole
HTCP · Butadienetricarbonitrile fragment BDTC · Cyano groups · Donor–acceptor
chromophores
Introduction
Photochromic compounds are intensively studied objects because of the great oppor-
tunities for their use as photo-switching components for biological research [1–5],
various materials [6–8], nanosystems [9] and supramolecular chemistry [10, 11].
Most of the works in this feld of research are related to positive photochromism.
This is the phenomenon when the photo-induced form of a molecule has a deeper
color than the original one. The reverse (negative) photochromism, when the colored
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-019-03853-w) contains supplementary material, which is available to authorized users.
*
Mikhail Yu. Ievlev
hiliam@bk.ru
1
Ulyanov Chuvash State University, Moskovsky pr., 15, Cheboksary, Russia 428015
Vol.:(0123456789)
1 3

M. Y. Belikov et al.
OH
R
OH
MeO
CN
CN
CN
CN
O
Vis.
∆
CN
CN
CN
CN
R
O
=
O
X
NH
X
CN
O,
R
R
R
A
B
A*
this work
Fig. 1 Known representatives of TCF photochromes A and B compounds studied in this work
Fig. 2 The general formula
of a new group of negative
photochromes containing a
nitrile-rich acceptor and a
2-vinylphenol fragment
initial form of a molecule reversibly becomes less colored under the action of irra-
diation (mostly by visible light), is much less studied.
An obvious advantage for the practical use of negative photochromes is the possi-
bility to use visible light, which is less destructive for organic compounds and living
systems than ultraviolet radiation. In this regard, research on new groups of reverse
photochromes with widely varying practically signifcant properties is an important
task.
There are several known groups of negative photochromes such as spiropyrans
[12–15], 1,1′-binaphthyl-bridged imidazole dimers [16–18], dihydropyrenes [19–21]
and Stenhouse adducts [22–25]. Results of the research on the reverse photochromes
from all over the world have been recently summarized and collected in two review
articles in 2017 and 2018 [26, 27]. However, there is only a little information about
a new type of reverse photochrome A, which consists of a tricyanofuran (TCF) moi-
ety (Fig. 1).
Only two examples of compounds A have been described, and an interaction of
2-(3-cyano-4-methylfuran-2(5H)-ylidene)malononitriles (TCF) with salicylic alde-
hyde was used for the synthesis [28, 29]. During the photochromic transformation,
a colored form, A, becomes colorless under the action of visible light due to the
formation of a spiro derivative A*, which is able to thermally transform back into
the original form A. Based on these single data, it can be assumed that a system C
consisting of a butadienetricarbonitrile (BDTC) acceptor and a phenolic fragment,
bridged by a double bond, is necessary for photochromic transformations (Fig. 2).
We believe that an investigation of compound C is promising for discovering of
a new wide group of negative photochromes, due to a variety of substituents being
possible in the phenolic fragment. The literature on the chemistry of photochromes
of this series does not reveal the efect of the substituent at the aryl moiety on the
photochromic behavior and other physicochemical properties of compound A, since
1 3

Novel group of negative photochromes containing a nitrile-rich…
only unsubstituted salicylic aldehyde was used. It is also important that only the
TCF fragment is reported as the BDTC acceptor, and there is no information about
the use of other acceptors in structures of type C.
In this regard, the purpose of this work is to show the possibility of varying the
position of the substituent at the aryl ring, as well as changing the nitrile-rich accep-
tor, to synthesize novel negative photochromes with easily adjustable physicochemi-
cal characteristics.
Experimental
Materials and methods
The progress of the reactions and the purity of the products were monitored by TLC
on Sorbfl plates (spots were visualized under UV light, by treatment with iodine
vapor, or by heating). The IR spectra were recorded on a FSM-1202 spectrometer
with Fourier transform from samples dispersed in mineral oil. The NMR spectra
were measured in DMSO-d6 on a Bruker DRX-500 spectrometer using tetramethyl-
silane as an internal reference. Elemental analyses were performed using a Per-
kin–Elmer 2400 CHN elemental analyzer. The mass spectra were obtained on a gas
chromatograph mass spectrometer (Shimadzu GCMS-QP2010S). The UV spectra
were recorded on an Agilent Cary 60 UV–Vis Spectrophotometer. Melting points
were determined on an OptiMelt MPA100 device.
Synthesis and spectral data
Typical procedure for the preparation of methoxy substituted (E)‑2‑(3‑cyano‑4‑(2‑hydr
oxystyryl)‑5,5‑dimethylfuran‑2(5H)‑ylidene)malononitriles 2a–d. To a suspension of
0.199 g (1 mmol) of TCF acceptor 1a in 2 mL of 96% ethanol 1.05 mmol of an
appropriate methoxy substituted 2-hydroxybenzaldehyde and 0.077 g (1 mmol) of
dry ammonium acetate were added. The resulting mixture was stirred at 40–50 °C
for 10 min, then for 1–2 h at room temperature (TLC controlled). After the reac-
tion completion, the mixture was cooled to 0–5 °C then the precipitated product was
fltered, washed with cold ethanol and dried in a vacuum desiccator over CaCl2 to
constant weight.
(E)‑2‑(3‑cyano‑4‑(2‑hydroxy‑3‑methoxystyryl)‑5,5‑dimethylfuran‑2(5H)‑ylidene)malo-
1
nonitrile 2a. Mp 244–245 °C (dec.). H NMR (500.13 MHz, DMSO-d6): δ 1.76 (6H,
s, 2CH3), 3.84 (3H, s, OCH3), 6.89 (1H, t, J=8.0 Hz, C6H3), 7.12 (1H, dd, J=8.0,
1.2 Hz, C6H3), 7.33 (1H, d, J=16.5 Hz, CH=), 7.45–7.48 (1H, m, C6H3), 8.27 (1H,
d, J=16.5 Hz, CH=), 10.04 (1H, s, OH). 13C NMR (125.67 MHz, DMSO-d6): δ
25.09, 53.88, 56.19, 97.31, 99.22, 111.36, 112.01, 112.88, 115.09, 115.38, 119.52,
121.02, 121.50, 143.49, 147.92, 148.15, 176.07, 177.45. MS, (EI, 70 eV): m/z (%)
333 [M]+ (100), 318 [M–Me]+ (17), 248 (76). IR (mineral oil, cm−1): 3224 (OH),
1 3

M. Y. Belikov et al.
2228 (CN). 1631 (C=C). Anal. Calcd for C19H15N3O3: C, 68.46; H, 4.54; N, 12.61.
Found: C, 68.32; H, 4.59; N, 12.75.
(E)‑2‑(3‑cyano‑4‑(2‑hydroxy‑4‑methoxystyryl)‑5,5‑dimethylfuran‑2(5H)‑ylidene)malo-
1
nonitrile 2b. Mp 234–235 °C (dec.). H NMR (500.13 MHz, DMSO-d6): δ 1.75 (6H,
s, 2CH3), 3.81 (3H, s, OCH3), 6.48 (1H, d, J=2.4 Hz, C6H3), 6.57 (1H, dd, J=8.9,
2.5 Hz, C6H3), 7.27 (1H, d, J=16.3 Hz, CH=), 7.83 (1H, d, J=8.9 Hz, C6H3), 8.16
(1H, d, J=16.3 Hz, CH=), 11.07 (1H, s, OH). 13C NMR (125.67 MHz, DMSO-d6):
δ 25.34, 52.54, 55.47, 94.72, 98.53, 100.86, 107.64, 111.47, 112.02, 112.11, 112.97,
115.07, 132.48, 144.54, 160.81, 164.72, 176.62, 177.28. MS, (EI, 70 eV): m/z (%)
333 [M]+ (100), 318 [M–Me]+ (23), 248 (100). IR (mineral oil, cm−1): 3246 (OH),
2226 (CN). 1624 (C=C). Anal. Calcd for C19H15N3O3: C, 68.46; H, 4.54; N, 12.61.
Found: C, 68.39; H, 4.62; N, 12.70.
(E)‑2‑(3‑cyano‑4‑(2‑hydroxy‑5‑methoxystyryl)‑5,5‑dimethylfuran‑2(5H)‑ylidene)malo-
1
nonitrile 2c. Mp 258–259 °C (dec.). H NMR (500.13 MHz, DMSO-d6): δ 1.77 (6H,
s, 2CH3), 3.76 (3H, s, OCH3), 6.98 (1H, d, J=8.9 Hz, C6H3), 7.01 (1H, dd, J=8.9,
3.1 Hz, C6H3), 7.39 (1H, d, J=3.1 Hz, C6H3), 7.43 (1H, d, J=16.4 Hz, CH=), 8.18
(1H, d, J=16.4 Hz, CH=), 10.42 (1H, s, OH). 13C NMR (125.67 MHz, DMSO-
d6): δ 25.20, 53.74, 55.74, 97.24, 99.16, 111.34, 112.04, 112.91, 113.08, 115.26,
117.57, 121.40, 121.67, 143.96, 152.45, 153.04, 176.28, 177.41. MS, (EI, 70 eV):
m/z (%)333 [M]+ (72), 318 [M–Me]+ (18), 248 (100). IR (mineral oil, cm−1): 3290
(OH), 2222 (CN). 1607 (C=C). Anal. Calcd for C19H15N3O3: C, 68.46; H, 4.54; N,
12.61. Found: C, 68.30; H, 4.64; N, 12.77.
(E)‑2‑(3‑cyano‑4‑(2‑hydroxy‑6‑methoxystyryl)‑5,5‑dimethylfuran‑2(5H)‑ylidene)malo-
1
nonitrile 2d. Mp 282–283 °C (dec.). H NMR (500.13 MHz, DMSO-d6): δ 1.76 (6H,
s, 2CH3), 3.89 (3H, s, OCH3), 6.57–6.60 (2H, m, C6H3), 7.33 (1H, t, J=8.3 Hz,
C6H3), 7.63 (1H, d, J=16.5 Hz, CH=), 8.15 (1H, d, J=16.5 Hz, CH=), 11.04 (1H,
s, OH). 13C NMR (125.67 MHz, DMSO-d6): δ 25.87, 53.48, 56.29, 97.23, 98.48,
102.43, 108.88, 110.77, 111.18, 112.11, 112.93, 116.60, 134.92, 140.09, 159.98,
160.62, 177.22, 177.28. MS, (EI, 70 eV): m/z (%) 333 [M]+ (93), 318 [M–Me]+
(20), 248 (100). IR (mineral oil, cm−1): 3226 (OH), 2231 (CN). 1611 (C=C). Anal.
Calcd for C19H15N3O3: C, 68.46; H, 4.54; N, 12.61. Found: C, 68.42; H, 4.62; N,
12.73.
Typical procedure for the preparation of methoxy substituted (E)‑2‑(3‑cyano‑5‑hydro
xy‑4‑(2‑hydroxystyryl)‑5‑methyl‑1H‑pyrrol‑2(5H)‑ylidene)malononitriles
2e–h. To
a suspension of 0.2 g (1 mmol) of 2-(3-cyano-5-hydroxy-4,5-dimethyl-1H-pyrrol-
2(5H)-ylidene)malononitrile 1b HTCP in 2 mL of 96% ethanol 1.05 mmol of an
appropriate methoxy substituted 2-hydroxybenzaldehyde and 0.077 g (1 mmol) of
dry ammonium acetate were added. The resulting mixture was strirred at room tem-
perature for 4–8 h (TLC controlled). After the reaction completion, the mixture was
cooled to 0–5 °C then precipitated product was fltered, washed with cold ethanol
and dried in a vacuum desiccator over CaCl2 to constant weight.
1 3

Novel group of negative photochromes containing a nitrile-rich…
(E)‑2‑(3‑cyano‑5‑hydroxy‑4‑(2‑hydroxy‑3‑methoxystyryl)‑5‑methyl‑1H‑pyr-
1
rol‑2(5H)‑ylidene)malononitrile 2e. Mp 171–172 °C (dec.). H NMR (500.13 MHz,
DMSO-d6): δ 1.67 (3H, s, CH3), 3.84 (3H, s, OCH3), 6.87 (1H, t, J=8.0 Hz,
C6H3), 7.09 (1H, dd, J=8.0, 1.2 Hz, C6H3), 7.19 (1H, s, OH pyrrol), 7.23 (1H,
d, J=16.4 Hz, CH=), 7.28–7.31 (1H, m, C6H3), 8.17 (1H, d, J=16.4 Hz, CH=),
9.80 (1H, s, OH), 10.53 (1H, s, NH). 13C NMR (125.67 MHz, DMSO-d6): δ 25.90,
44.17, 56.13, 93.61, 99.06, 111.92, 114.13, 114.58, 115.34, 115.96, 119.61, 120.02,
121.75, 141.69, 147.21, 148.12, 160.40, 171.27. MS, (EI, 70 eV): m/z (%) 334 [M]+
(4), 318 (100), 316 [M-H2O]+ (81). IR (mineral oil, cm−1): 3350–3220 (OH, NH),
2223 (CN). 1593 (C=C). Anal. Calcd for C18H14N4O3: C, 64.66; H, 4.22; N, 16.76.
Found: C, 64.50; H, 4.31; N, 16.55.
(E)‑2‑(3‑cyano‑5‑hydroxy‑4‑(2‑hydroxy‑4‑methoxystyryl)‑5‑methyl‑1H‑pyr-
1
rol‑2(5H)‑ylidene)malononitrile 2f. Mp 163–164 °C (dec.). H NMR (500.13 MHz,
DMSO-d6): δ 1.65 (3H, s, CH3), 3.79 (3H, s, OCH3), 6.49 (1H, t, J=2.4 Hz,
C6H3), 6.54 (1H, dd, J=8.8, 2.4 Hz, C6H3), 7.11 (1H, s, OH pyrrol), 7.18 (1H, d,
J=16.2 Hz, CH=), 7.64 (1H, d, J=8.8 Hz, C6H3), 8.07 (1H, d, J=16.2 Hz, CH=),
10.36, 10.78 (2H, br s, NH, OH). 13C NMR (125.67 MHz, DMSO-d6): δ 26.50,
43.88, 55.80, 93.73, 97.01, 101.84, 107.66, 112.61, 113.65, 114.64, 115.58, 115.86,
131.58, 142.89, 160.29, 161.06, 164.15, 172.40. MS, (EI, 70 eV): m/z (%) 334 [M]+
(3), 318 (50), 316 [M-H2O]+ (85). IR (mineral oil, cm−1): 3355–3230 (OH, NH),
2221 (CN). 1590 (C=C). Anal. Calcd for C18H14N4O3: C, 64.66; H, 4.22; N, 16.76.
Found: C, 64.54; H, 4.34; N, 16.60.
(E)‑2‑(3‑cyano‑5‑hydroxy‑4‑(2‑hydroxy‑5‑methoxystyryl)‑5‑methyl‑1H‑pyr-
1
rol‑2(5H)‑ylidene)malononitrile 2g. Mp 156–157 °C (dec.) H NMR (500.13 MHz,
DMSO-d6): δ 1.68 (3H, s, CH3), 3.75 (3H, s, OCH3), 6.88 (1H, d, J=8.9 Hz,
C6H3), 6.96 (1H, dd, J=8.9, 3.0 Hz, C6H3), 7.15 (1H, s, OH pyrrol), 7.20 (1H, d,
J=3.0 Hz, C6H3), 7.33 (1H, d, J=16.4 Hz, CH=), 8.09 (1H, d, J=16.4 Hz, CH=),
10.18 (1H, s, OH), 10.50 (1H, s, NH). 13C NMR (125.67 MHz, DMSO-d6): δ 25.87,
44.14, 55.69, 93.59, 98.83, 111.96, 112.08, 114.12, 115.35, 116.21, 117.49, 120.52,
121.73, 142.04, 152.29, 152.50, 160.47, 171.44. MS, (EI, 70 eV): m/z (%) 334 [M]+
(2), 318 (71), 316 [M-H2O]+ (35). IR (mineral oil, cm−1): 3340–3205 (OH, NH),
2226 (CN). 1586 (C=C). Anal. Calcd for C18H14N4O3: C, 64.66; H, 4.22; N, 16.76.
Found: C, 64.48; H, 4.35; N, 16.58.
(E)‑2‑(3‑cyano‑5‑hydroxy‑4‑(2‑hydroxy‑6‑methoxystyryl)‑5‑methyl‑1H‑pyr-
1
rol‑2(5H)‑ylidene)malononitrile 2h. Mp 152–153 °C (dec.). H NMR (500.13 MHz,
DMSO-d6): δ 1.67 (3H, s, CH3), 3.86 (3H, s, OCH3), 6.57 (2H, d, J=8.3 Hz, C6H3),
7.27 (1H, t, J=8.3 Hz, C6H3), 7.13 (1H, s, OH pyrrol), 7.72 (1H, d, J=16.4 Hz,
CH=), 8.27 (1H, d, J=16.4 Hz, CH=), 10.45 (1H, s, OH), 10.78 (1H, s, NH).
13C NMR (125.67 MHz, DMSO-d6): δ 26.26, 43.69, 56.13, 93.36, 97.94, 102.37,
108.88, 111.07, 112.14, 114.27, 115.48, 118.13, 133.58, 138.72, 159.44, 160.27,
160.55, 172.93. MS, (EI, 70 eV): m/z (%) 334 [M]+ (2), 318 (15), 316 [M-H2O]+
(46). IR (mineral oil, cm−1): 3355–3160 (OH, NH), 2214 (CN). 1597 (C=C). Anal.
1 3

M. Y. Belikov et al.
Calcd for C18H14N4O3: C, 64.66; H, 4.22; N, 16.76. Found: C, 64.53; H, 4.29; N,
16.63.
Results and discussion
In this work, a methoxy group was used as a model substituent; its position was var-
ied across the entire ring of the 2-vinylphenol fragment (Scheme 1). TCF 1a-based
tricyanofuran derivative, as well as HTCP 1b-based hydroxytricyanopyrrole deriva-
tive, were used as a nitrile-rich acceptor. These two BDTC acceptors were chosen
because the TCF derivatives had been already used for the synthesis of negative
photochromes of the studied group [28, 29], and the HTCP molecules are the closest
structural analogues of TCF from all the structures described in the literature [30].
The similarities between TCF and HTCP are the presence of the BDTC acceptor
and the geometry of the substituents in heterocycles. The bigger reactivity is another
reason to use the less-studied HTCP acceptors for the synthesis of negative pho-
tochromes. They can be used to adjust the physicochemical properties of chromo-
phores if necessary [31].
The starting acceptors are synthetically readily available. TCF 1a was obtained in
52% yield from malononitrile and the hydroxyketone in pyridine in the presence of
acetic acid [32], HTCP 1b was synthesized in 78% yield by the reaction of malon-
onitrile dimer with diacetyl in aqueous ethanol in the presence of sodium hydroxide
[30]. The synthesis of the desired chromophores 2a–h was carried out by the inter-
action of acceptors 1a, b with methoxy-substituted salicylic aldehydes in ethanol in
the presence of ammonium acetate. The transformation occurred within 1–8 h and
resulted in a series of unknown compounds 2a–h with moderate to good yields of
48–90% (Scheme 1; Table 1).
1
The structure of compounds 2 is considered with the data of IR, H and 13C
NMR spectroscopy, mass spectrometry and elemental analysis. A characteristic
1
feature of the H NMR spectra of compounds 2 is the presence of signals of two
vicinal vinyl protons in the 7.27–8.27 ppm region with the coupling constant of
16.2–16.5 Hz, showing the E-confguration of a double bond. The presence of a
phenolic hydroxyl is proved by the signal at 9.80–11.07 ppm, while the protons
of a methoxy group appear at 3.75–3.89 ppm. The characteristic of 13C NMR
Scheme 1 Synthesis of compounds 2a–h
1 3

Novel group of negative photochromes containing a nitrile-rich…
Table 1 Synthesis of
Entry Substrate MeOC6H3
R
X
Product Yield %a
compounds 2a–h
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1b
1b
1b
1b
3-MeO
4-MeO
5-MeO
6-MeO
3-MeO
4-MeO
5-MeO
6-MeO
Me
Me
Me
Me
O
O
O
O
2a
2b
2c
2d
87
70
66
90
57
62
71
48
OH NH 2e
OH NH 2f
OH NH 2g
OH NH 2h
aIsolated yield
spectra of compounds 2 is an appearance of signals of three cyano groups at
111.18–115.48 ppm and of small intense signals of C-atoms of C(CN)2 fragments
at 43.69–53.88 ppm. The mass spectra are characterized by the presence of peaks
of molecular ions [M]+ with 2–100% intensity.
The light absorption properties of all compounds 2 were studied in THF solu-
tions at 2.5 × 10−5 M. Absorption bands were characterized with maxima in the
range of 414–470 nm and the molar extinction coefcient about 2.98–4.59× 104
(Fig. 3, Table 2). The shortest wavelength absorption (414–431 nm) was observed
for the 3- and 6-MeO-derivatives, while 4- and 5-methoxy-substituted chromo-
phores were absorbed at the longest wavelength region (450–470 nm). A type
of the heterocycle also afected the absorption properties: the HTCP derivatives
2e–h have a hypsochromic shift of the absorption maxima by 8–18 nm in com-
parison with the corresponding TCF compounds 2a–d.
Fig. 3 Absorption spectra of compounds 2a–d (left) and 2e–h (right) in THF (C=2.5×10−5 M)
1 3

M. Y. Belikov et al.
Table 2 Light absorption and photochromic characteristics of compounds 2a–h
Comp.
X
MeOC6H3
λmax (nm)
ε×104
Rate constant, ka
Half-life, τ1/2 (s)b
Abs.
(L×mol−1×cm−1
)
decrease,
c
%
2a
2b
2c
2d
2e
2f
O
O
O
O
3-MeO
4-MeO
5-MeO
6-MeO
422
467
470
431
414
450
452
419
3.91
5.54
2.91
4.59
4.00
3.82
2.98
4.07
2.20×10−3
2.27×10−1
315
3
32
1765
283
40
3150
1145
36
49
29
52
31
79
87
90
1.14×10−7
3.67×10−9
2.45×10−3
1.75×10−2
2.20×10−4
9.65×10−9
*
*
NH 3-MeO
NH 4-MeO
NH 5-MeO
NH 6-MeO
2g
2h
*
aRate constant of the reverse thermal reaction is in s−1 (frst-order) or in mol×L−1 ×s−1 (pseudo-zero-
order) if marked with an asterisk (*)
bHalf-life period of the reverse thermal reaction was found as 0.693/k (frst-order) or as [C2]/2k (pseudo-
zero-order)
cPercentage of the decrease for the long-wavelength absorption band in the photostationary state
The behavior of the THF solutions of compounds 2a–h under irradiation was
studied to evaluate the photochromic properties of the synthesized molecules. We
found that all the compounds 2a–h show negative photochromism under non-fltered
visible light irradiation at 20 °C (Scheme 2).
Colored solutions were irradiated with visible light for 3 min to reach a less-
colored photo-stationary state (PSS) (Fig. 4). The observed decrease of the long-
wave absorbance in the photostationary state varied from 29% for 5-MeOTCF 2c to
90% for 6-MeOHTCP 2h (Table 2). An initial absorption was restored in the dark at
room temperature apparently due to a reverse thermal transformation of the photo-
induced form 2* to the starting one 2. Therefore, all the compounds were found to
be thermally unstable (T-type) negative photochromes. We analyzed an infuence of
the methoxy group position to the photo-transformation degree of compounds 2 and
found that the largest decrease of an absorption band among two groups of the TCF
2a–d and HTCP 2e–h derivatives is observed for 6-MeO-substituted compounds
2d—52% and 2h—90%. Moreover, it can be noted that the HTCP derivatives gener-
ally have a better degree of photo-transformation of the initial form in comparison
to TCF.
Scheme 2 Photo-induced transformation of compounds 2a–h
1 3

Novel group of negative photochromes containing a nitrile-rich…
Fig. 4 Absorption spectra of the compound 2h in THF (C=2.5×10−5 M) before and after irradiation
The kinetic curves of the reverse thermal reaction were recorded. It was found that
the half-life period (τ1/2) of the photo-induced form 2* transformation (Scheme 2)
depends on the position of a methoxy group and the type of the heterocycle. We
found that τ1/2 is varied in the range from 3 s for 4-MeOTCF 2b to 52.5 min for
5-MeOHTCP 2g (Table 2). The smallest thermal stability was observed for a couple
of 4-MeO substituted chromophores 2b and 2f (τ1/2 are 3 s and 40 s, respectively). A
photo-induced form of 6-MeO-substituted compounds 2d and 2h showed a signif-
cantly larger half-life period of about 19–29 min. Accordingly, 3-MeO-substituted
chromophores 2a and 2e are intermediate in τ1/2 values which are around 5 min. For
these three couples of chromophores, the type of heterocycle afects τ1/2 insignif-
cantly in comparison with the infuence of the methoxy group position. The only
exceptions are 5-MeO-substituted compounds 2c and 2g for which τ1/2 values difer
dramatically—about 0.5 min and 52.5 min, respectively, and the latter is the highest
τ1/2 that was observed for chromophores 2 (Table 2).
An interesting feature was the observed diference in the kinetics of the reverse
thermal transformation. Assuming that there are only two forms in the solution (2
and 2*), the concentration of the photo-induced form 2* [C2] was evaluated as the
diference between the initial concentration (C0 =2.5×10−5 M) and the concen-
tration of the form 2 [C1] during the reverse thermal reaction, in which [C1] was
found using the optical density values and known molar absorptivity (ε) from the
Beer–Lambert–Bouguer law. Thus, for the thermal coloration of compounds 2c,
d, h, the concentrations of the 2* forms were ftted in a pseudo-zero-order kinetic
equation (Fig. 5, e.g., compound 2d), whereas for chromophores 2a, b, e–g, the con-
centrations of the photo-induced forms 2* were ftted for frst-order kinetics (Fig. 6;
e.g. compound 2e).
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M. Y. Belikov et al.
Fig. 5 Kinetics of the reverse thermal transformation for the compound 2d (left), the plot of ftting the
concentration of photo-induced form (C2) in a pseudo-zero-order kinetic equation (right)
Fig. 6 Kinetics of the reverse thermal transformation for the compound 2e (left), the plot of ftting the
concentration of photo-induced form (C2) in a frst-order kinetic equation (right)
A diferent rate of the reverse thermal reaction is probably related to the struc-
tural features of photo-induced forms 2*, which are afected by the position of a
methoxy group at the aryl ring and the type of a fve-membered heterocycle. It
also should be noted that, for the known analogue of structures 2*, a CH-acidity
and a dissociation were described [28]. THis probably afects the kinetics of the
reverse reaction in the case of the compounds studied in this work.
An important characteristic of the photochromic compounds, which largely
defnes their practical use, is the cyclicity of the transformations. The HTCP
acceptor has not previously been reported in any photochromic system, and
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Novel group of negative photochromes containing a nitrile-rich…
Fig. 7 The photo-thermal cycles
of 2f. For each cycle, the solu-
tion of 2f was irradiated with
visible light for 2.5 min, then
kept in the dark for 6 min
therefore we studied the cyclicity of the photo-thermal transformations for
chromophore 2f (Fig. 7).
It was found that THF solution of compound 2f after irradiation with visible light
until PSS is reached, followed by keeping the solution in the dark for 6 min, restores the
initial absorbance (Fig. 7). This process was repeated 10 times, and after that, the solu-
tion was kept in dark for about 10 min, and the fnal solution showed almost no decrease
in the optical density in comparison with the initial solution. This result indicates a
good fatigue resistance of compound 2f as well as a resistance to photodegradation.
Conclusion
Thus, a series of the TCF and HTCP chromophores have been synthesized, con-
taining a methoxy-substituted 2-vinylphenol fragment. All the obtained compounds
were found to be thermally reversible (T-type) negative photochromes in solution,
for which a photo-induced form could be generated by the action of visible light. It
was found that a degree of photochromic transformation, a thermal stability of the
photo-induced form, and the kinetics of the reverse thermal reaction can be varied
both by changing the position of the methoxy group at the aryl moiety, and by the
type of a nitrile-rich acceptor.
Acknowledgements The research was performed under the fnancial support of the Russian Science
Foundation (Grant No. 18-73-10065).
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