Photo- and thermochromic spiropyrans 42.*The effect of structural factors on the photochromic properties of indolinospiro-pyrans containing a condensed furan fragment

Article abstract of DOI:10.1007/s10593-014-1528-x

Syntheses are reported for a series of N-substituted indolinospiropyrans containing a fused furan fragment in the benzopyran part of the molecule. The structure of these compounds was established by IR and multinuclear NMR spectroscopy. A study was carrie

Full text of DOI:10.1007/s10593-014-1528-x

Chemistry of Heterocyclic Compounds, Vol. 50, No. 5, August, 2014 (Russian Original Vol. 50, No. 5, May, 2014)
PHOTO- AND THERMOCHROMIC SPIROPYRANS
42.* THE EFFECT OF STRUCTURAL FACTORS ON THE
PHOTOCHROMIC PROPERTIES OF INDOLINOSPIRO-
PYRANS CONTAINING A CONDENSED FURAN FRAGMENT
R. V. Tyurin1, B. S. Lukyanov2**, A. V. Chernyshev2, G. S. Borodkin2,
K. N. Khalanskii2, L. V. Chepeleva3, and M. B. Lukyanova2
Syntheses are reported for a series of N-substituted indolinospiropyrans containing a fused furan
fragment in the benzopyran part of the molecule. The structure of these compounds was established by
IR and multinuclear NMR spectroscopy. A study was carried out on the effect of the fused benzene ring
and bulky N-benzyl substituent on the photochromic properties of these compounds in solution and in a
polymer matrix. Dynamic NMR spectroscopy was used to study the thermally induced isomerization of
the N-benzyl derivative.
Keywords: furo[3,2-f]chromene, indolinospiropyrans, multinuclear NMR spectroscopy, photo-
chromism, photoisomerization.
The major factors affecting the spectrokinetic behavior of photochromic compounds with an indoline
spiro ring are the structure of the indoline segment and the nature of the substituent at the nitrogen atom. Both
these factors affect the stability of the open form and, thereby, the photochromic properties such as the solution
color intensity and lifetime of the open form [2].
In previous work [3], we obtained N-methylindolinospiropyrans 1a,b. A spectrokinetic study showed
that the equilibrium is shifted toward formation of the colored photoinduced form upon the irradiation of
ethanolic solutions of spiropyrans 1a,b at 365 nm at room temperature.
In this work, in the framework of systematic studies we investigated the effect of the steric effect of the
bulky N-benzyl substituent as well as of isomeric benzo-fused N-methylindolinospiropyrans, which were
thought capable of hindering the photochromic transformations.
_______
*For Communication 41, see [1].
**To whom correspondence should be addressed, e-mail: bluk@ipoc.sfedu.ru.
1Southern Science Center, Russian Academy of Sciences, 41 Chekhov Ave., Rostov-on-Don 344006, Russia;
e-mail: wingerover@yandex.ru.
2Physical and Organic Chemistry, Southern Federal University, 194/2 Stachki Ave., Rostov-on-Don 344090,
Russia.
3Chemistry Research Institute, V. N. Karazin Kharkiv National University, 4 Svobody Sq., Kharkiv 61077,
Ukraine; e-mail: chepelev2002@ukr.net.
_________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 797-805, May, 2014. Original
article submitted March 25, 2014.
734
0009-3122/14/5005-0734©2014 Springer Science+Business Media New York

For this purpose, we obtained spiropyrans 1a-c through brief heating of equimolar amounts of the
corresponding N-substituted 2,3,3-trimethylindolenylium perchlorates 2a-c, hydroxyaldehyde 3, and piperidine
in 2-propanol at reflux. Under analogous conditions, benzo-fused derivatives 6 and 7 were obtained from
1,2,3,3-tetramethylbenzo[e]indolinium (4) and 1,2,3,3-tetramethylbenzo[g]indolinium iodides (5).
The structures of products 1, 6, 7 were confirmed by elemental analysis, IR and 1H NMR spectroscopy.
In addition, the structure of spiropyran 1c was further confirmed by 13C and 15N NMR spectroscopy. The IR
spectra of these compounds have bands at 1600-1620 (υ C=C), 1250-1290 (υ C-N), and 930-950 cm-1 (υ C-O).
R1
Me
O
R1
Me
Me
Me
Me
Ph
N
HO
+
+
2-PrOH
, 1 h
Ph
N
–
Ph
N
ClO4
O
R
O
R
3
2a–c
Ph
O
1a–c
1, 2 a R = Me, R1 = H; b R = Me, R1 = Cl; c R = CH2Ph, R1 = H
Me
Me
Me
Me
Me
N
+
3
2-PrOH
, 1 h
+
Ph
N
N
O
Me
I–
Me
Ph
4
O
6
Me
Me
Me
Me
N
Me
I–
N
Ph
+
+
3
N
Me
2-PrOH
, 1 h
O
Me
Ph
O
5
7
The protons of the gem-dimethyl groups at position 3 of the indoline fragment in the 1H NMR spectra of
spiropyrans 1a-c, 6, 7 are seen at 1.08-1.26 and 1.32-1.65 ppm due to their magnetic inequivalence, which is
1
evidence for the spirocyclic structure of these products. The H NMR spectra of spiropyran 1c shows two
doublets for the benzyl substituent methylene group at 4.12 and 4.53 ppm in addition to the two three-proton
singlets from the gem-dimethyl group. In order to refine the structure of these compounds, a series of two-
dimensional NMR spectral experiments were carried out for spiropyran 1c (Supplementary material to this
article).
1
Our assignment of the signals in the H NMR spectra of spiropyran 1c was supported by the two-
1
1
dimensional H1H COSY correlation spectrum. The H13C HSQC single-quantum heteronuclear correlation
spectrum was used to identify the signals in the 13C NMR spectrum of spiropyran 1c. H13C HMBC and
1
1H15N HMBC techniques for long-range heteronuclear correlation were used for the assignment of the
chemical shifts of the carbon atoms not attached to hydrogen atoms.
735

TABLE 1. Absorption Spectral and Kinetic Properties of Spiropyrans
1a-c, 6 and 7 in Ethanol
abs
max
abs
τ1/220°С, s
λ
(SP), nm (ε, М–1×cm–1)
λ
(MC), nm
max
Compound
245, 320, 362 sh
252, 320, 361 sh
244 (22100), 323 (29300), 363 (4500) sh
246 (52100), 320 (47850), 365 (7560) sh
249 (55400), 324 (48300), 368 (9100) sh
476, 586
3.09
1.16
0.69
3.59
2.42
1a
1b
1c
6
484, 598
482, 596
487, 601
481, 609
7
Fig. 1. Absorption spectra of spiropyran 1c in ethanol upon steady-state irradiation at  365 nm at 20°C (the
interval between the spectra was 1 s) (a) and at -70°C (the interval between the spectra was 10 s) (b).
736

The long-range correlation spectra permitted the unequivocal identification of the carbon atoms in
spiropyran 1c not attached to hydrogen atoms and fully confirmed the structure of this compound.
The signal at 104.2 ppm was reliably assigned to the spiro atom C-2(7'), which is in complete accord
with second-order coupling constants reflecting coupling of the carbon atom with the proton H-8' and third-
order coupling constants reflecting coupling of the carbon atom C-2(7') with the protons of the methyl groups at
position 3' as well as with the methylene group at the nitrogen atom.
The results of the photochemical investigation of ethanolic solutions of photochromic spiropyrans 1a-c,
6, and 7 (Table 1) showed that the intensity of the absorption band of the open form of benzyl derivative 1c is
much lower than for the analogous parameter of spiropyrans 1a,b after photoirradiation at 365 nm at room
temperature (Fig. 1a). Careful examination of the kinetic curve for the absorption intensity when taking the
spectra for spiropyrans 1a-c at an interval of 1 s showed that they have a similar growth rate only in the first
stage, while the absorption intensity of the solution of the benzyl derivative upon further irradiation almost stops
increasing.
This finding may indicate either steric hindrance related to opening of the pyran heterocycle due to the
bulky substituent or photodecomposition of the open form, whose accumulation in the irradiated solution would
lead to steady enhancement of coloration. On the other hand, the parameter characterizing the lifetime of the
open form (1/2) of benzyl derivative 1c is 0.7 s, which is comparable to the analogous value for methyl
derivatives 1a,b. Similar time characteristics are also found for solutions of spiropyrans 1a-c upon taking the
spectra at -70°C (60, 60, and 45 s for compounds 1a, 1b, and 1c, respectively). However, the rates of increase of
the absorption intensity of the benzyl derivative are not discernible on the background of the other
photochromic products, which is shown in the case of compound 1c (Fig. 1b). These results indicate
photodecomposition of the open form of compound 1c at room temperature. In order to confirm this hypothesis,
we irradiated a solution of compound 1c with ultraviolet light at 365 nm for 2 h at 24°C. No decoloration of the
solution was observed in this case. This finding indicated accumulation of a photodecomposition product having
constant coloration. However, the presence of impurities in significant amounts was not observed using NMR
spectroscopy, indicating that, despite formation of a colored photodecomposition product leading to a shift of
the photoisomerization equilibrium toward formation of the open form, ring opening is slow and rate-
determining for formation of the photodecomposition product of the merocyanine form.
Fig. 2. Absorption spectra of a solution of spiropyran 7 in ethanol upon irradiation at  365 nm at 20°C (the
interval between the spectra was 1 s). The insert shows visible spectrum on an enlarged scale.
737

A study of the photochromic properties of isomeric benzo-fused n-methylindolinospiropyrans 6 and 7
showed that the steric hindrance arising due to interaction of the N-methyl group of spiropyran 7 with the peri
position of the naphthalene fragment of benzo[g]indole leads to deformation and change in electronic properties
of the heterocyclic system. This finding is reflected in the spectrokinetic behavior of this compound 7.
The absorption intensity of spiropyran 7 upon steady state irradiation at  365 nm is four times less than for
isomer 6 (Figs. 2 and 3). The lifetime of the open form of spiropyran 7 (1/2) under these conditions is 2.42 s, which
is 1.5 times less than the lifetime of the sterically unhindered isomer 6. Hence, the structure of spiropyran 7 is less
conducive to formation of the merocyanine form than the structure of isomer 6. The steric hindrance due to the
methyl group in the benzo[g]indoline skeleton leads to more rapid closing of the pyran heterocycle in spiropyran 7.
The photochemical transformations of spiropyran 1c were also studied in a polystyrene film obtained by
evaporation of a dichloromethane solution of polystyrene and compound 1c over 48 h in the dark.
The absorption spectra of the polystyrene film were recorded upon irradiation at 365 nm (Fig. 4). The
intensity of the long-wavelength absorption band at 350 nm steadily decreased over 50 min and a slight increase
in the optical density at 483 nm was observed. No significant absorption was observed at longer wavelengths
(550-700 nm).
Photo- and thermochromic spiropyrans containing diastereotopic groups may be objects for study of
thermally induced bond isomerization using dynamic NMR spectroscopy. Such groups in spiropyrans 1 are the
gem-dimethyl groups at position 3 of the indoline fragment. The rate constants and activation parameters may
be determined using the temperature dependence of the NMR spectra of exchanging diastereotopic groups.
When there is no exchange, the spectra show two singlets of the inequivalent methyl groups at position 3 and
two doublets for the methylene group (Fig. 5).
Fig. 3. Absorption spectra of a solution of spiropyran 6 in ethanol upon irradiation at  365 nm at 20°C. The
intervals between the spectra were 1 s. The insert shows the visible portion of the spectrum on an enlarged scale.
The dynamics was studied in a solution of nitrobenzene-d5 for spiropyran 1c (Fig. 5). Upon heating, the
1
peaks of the H NMR signals for the protons characterizing spirocyclic form 1c begin to broaden and coalesce
but complete coalescence of both the methyl and methylene protons is not achieved under the experimental
conditions. This finding suggests that the free energy for conversion of the closed form of the spiropyrans is
rather high and a special calculation apparatus must be used to determine the theoretical coalescence
temperature and, thus, the activation parameters of the thermally induced bond isomerization of spiropyran 1c.
738

Fig. 4. Absorption spectra of a polystyrene film of compound 1c upon irradiation at 365 nm in the absorption
range of the photoproduct. Irradiation times: 1) 180 s, 2) 480 s, 3) 1080 s, and 4) 3000 s.
1
Fig. 5. Dynamics of the signals of the protons of the methyl and methylene groups in the H NMR spectrum of
compound 1c at 30-180°C in nitrobenzene-d5.
Thus, our study has shown that fusion of a benzene ring to the indoline fragment of the molecule in the
case of N-substituted benzo[e]indoline system leads to a slight increase in absorption in the ultraviolet spectral
region, while the steric hindrance due to the N-methyl substituent in the benzo[g]indoline derivative leads to
poorer photochromic characteristics. We also discovered that the introduction of a bulky substituent at the
nitrogen atom such as a benzyl group does not lead to marked change in the photochromic properties of the
benzofuran derivatives of indolinospiropyrans at reduced temperature, while ultraviolet irradiation at room
temperature leads to photodecomposition of the open form. This finding may be attributed to the high lability of
the methylene group hydrogen atom.
739

EXPERIMENTAL
IR spectra were recorded on a Varian Excalibur 3100 FT-IR (frustrated total internal reflection).
Electronic absorption spectra of solutions of the products before and after irradiation were recorded on an
Agilent 5483 spectrophotometer. A DRSh-250 mercury lamp with a light filter separating out the mercury
1
spectrum line at 365 nm was used as the source of photoactive UV light. H NMR spectra were recorded on a
Bruker Avance DPX 250 spectrometer at 250 MHz in a Fourier pulse mode in CDCl3. The 13C and two-
1
13
15
dimensional H, C, and N NMR spectra were recorded (600, 125, and 60 MHz, respectively) in CDCl3 and
1
also dynamic H NMR spectra in nitrobenzene-d5 were recorded on a Bruker Avance-600 spectrometer. The
signals were assigned relative to the signals of the residual proton signals of the CDCl3 and nitrobenzene-d5
samples (7.24 and 8.22 ppm, respectively). Elemental analysis was carried out by the classical microanalysis
method [4]. Melting points were determined on a Fisher–Jones instrument from Fisher Scientific.
1-Benzyl-3,3-dimethyl-1',2'-diphenylspiro[indoline-2,7'-furo[3,2-f]chromene] (1c). Piperidine (0.1 ml)
was added to a suspension of indolinium salt 2c (0.349 g, 1 mmol) in 2-propanol (10 ml) at 50°C, and then
aldehyde 3 [5] (0.314 g, 1 mmol) was added to the resultant solution. The solution was maintained at reflux for
1 h. After cooling, the resultant colorless precipitate was filtered off, washed with ethanol, and dried. Yield 0.5 g
(92.1%). Mp 179-183°C (EtOH). IR spectrum, ν, cm-1: 1639, 1604 (υ С=С), 1286, 1252 (υ С–N), 952, 910
1
(υ С–O). H NMR spectrum, δ, ppm (J, Hz): 1.21 (3Н, s, СН3); 1.35 (3Н, s, СН3); 4.12 (1Н, d, J = 16.6) and
4.53 (1Н, d, J = 16.6, NСН2); 5.49 (1Н, d, J = 10.4, Н-8'); 6.22 (1Н, d, J = 7.7, J = 1.3, Н-7); 6.42 (1Н, d, J = 10.5,
Н-9'); 6.73 (1Н, d, J = 7.6, Н-5'); 6.80 (1Н, td, J =7.5, J = 1.3, Н-5); 7.00 (1Н, td, J = 7.6, J = 0.9, Н-6); 7.08
(1Н, dd, J = 7.2, J = 0.9, Н-4); 7.13-7.25 (8Н, m, Н Ph); 7.24 (1Н, d, J = 7.6, Н-4'); 7.40-7.50 (7Н, m, Н Ph).
13C NMR spectrum, δ, ppm: 20.2 (CH3); 26.0 (CH3); 47.7 (CH2Ph); 52.1 (CMe2); 104.2 (C-2(7')); 107.6 (C-7);
110.8 (C-9'b); 111.5 (C-4'); 113.0 (C-5); 117.8 (C-3a); 118.3 (C-8'); 119.2 (C-5); 121.5 (C-4); 125.4 (C-9');
125.9 (C-9'a); 127.3 (C-6); 139.5 (C-1'); 147.5 (C-7a); 148.5 (C-5'a); 150.5 (C-3'a); 151.7 (C-2'). Found, %:
C 85.82; H 5.74; N 2.59. C39H31NO2. Calculated, %: C 85.84; H 5.73; N 2.57.
1,3,3-Trimethyl-1',2'-diphenylspiro[benzo[e]indoline-2,7'-furo[3,2-f]chromene] (6) was obtained
analogously to spiropyran 1c from indolinium salt 4 and aldehyde 3. Yield 91%. Mp 225-227°С (EtOH).
1
IR spectrum, ν, cm-1: 1644, 1592 (υ С=С), 1270, 1259 (υ С–N), 952, 942 (υ С–O). H NMR spectrum, δ, ppm
(J, Hz): 1.26 (3Н, s, 3-СН3); 1.65 (3Н, s, 3-СН3); 2.78 (3Н, s, NСН3); 5.49 (1Н, d, J = 10.5, Н-8'); 6.54 (1Н, d,
J = 10.5, Н-9'); 6.66 (1Н, d, J = 9.1, Н Ar); 6.94 (1Н, d, J = 8.6, Н Ar); 7.18-7.27 (5Н, m, Н Ar); 7.38 (1Н, td,
J = 8.4, J = 1.3, Н Ar); 7.43-7.62 (7Н, m, Н Ar); 7.72 (1Н, d, J = 8.6, Н Ar); 7.78 (1Н, d, J = 7.9 Н Ar); 7.92
(1Н, d, J = 8.5, Н Ar). Found, %: C 85.51; H 5.52; N 2.72. С37H29NO2. Calculated, %: C 85.55; H 5.63; N 2.70.
1,3,3-Trimethyl-1',2'-diphenylspiro[benzo[g]indoline-2,7'-furo[3,2-f]chromene] (7) was obtained
analogously to spiropyran 1c from indolinium salt 5 and aldehyde 3. Yield 74%. Mp 168-173°С (EtOH).
1
IR spectrum, ν, cm-1: 1642, 1606 (υ С=С), 1309, 1257 (υ С–N), 962, 937 (υ С–O). H NMR spectrum, δ, ppm
(J, Hz): 1.08 (3Н, s, 3-СН3); 1.37 (3Н, s, 3-СН3); 3.30 (3Н, s, NСН3); 5.49 (1Н, d, J = 10.5, Н-8); 6.52 (1Н, d,
J = 10.5, Н-9); 6.67 (1Н, d, J = 8.8, Н Ar); 7.27-7.62 (15Н, m, Н Ar); 7.79 (1Н, m, Н Ar); 8.30 (1Н, d, J = 9.8,
Н Ar). Found, %: C 85.54; H 5.57; N 2.69. С37H29NO2. Calculated, %: C 85.55; H 5.63; N 2.70.
A file with supplementary information is available on the site http://hgs.osi.lv and contains the two-
dimensional NMR spectra of spiropyran 1c.
This work was carried out with the financial support of the Russian Foundation for Basic Research (grant
No. 13-03-0631, K. N. Khalanskii; grant No. 13-03-90437, B. S. Lukyanov), State Fund of Fundamental
Investigation of Ukraine (grant 53.3/006, L. V. Chepeleva) as well as in the framework of the Basic Part of the State
Assignment in Scientific Activity (Physical and Organic Chemistry Research Institute of the Southern Federal
University (A. V. Chernyshev, G. S. Borodkin, and M. B. Lukyanov) and the Grant Council of the President of the
Russian Federation (grant NSh-274.2014.3).
740

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