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International Journal of Photoenergy
Volume 2009 (2009), Article ID 694196, 6 pages
http://dx.doi.org/10.1155/2009/694196
Research Article

Acylated 2-(N-arylaminomethylene)benzo[b]thiophene-3(2H )-Ones: Molecular Switches with Varying Migrants and Substituents

1Southern Scientific Center of Russian Academy of Sciences, 344006 Rostov on Don, Russia
2Institute of Physical and Organic Chemistry, Southern Federal University, 344090 Rostov on Don, Russia

Received 9 February 2009; Revised 7 May 2009; Accepted 22 May 2009

Academic Editor: Mohamed Sabry Abdel-Mottaleb

Copyright © 2009 Alexander D. Dubonosov et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Synthesis and properties of photochromic acylated 2-( -arylaminomethylene)benzo[ ]thiophene-3(2 )-ones are described. Their structure largely depends on the nature of acyl migrant and in a less degree on N-aryl substituent.

1. Introduction

2-(N -Acyl-N -arylaminomethylene)benzo[b ]thiophene-3(2H )-ones represent a new family of photochromic molecular switches with fluorescent signalling [1], chiroptical [2], and chemosensor activity [35]. They may also serve as pH-sensors [6] and solar energy storage systems [7]. However the influence of the size and electronic properties of migrants and substituents in these compounds on their spectral, luminescent, and photochromic properties were insufficiently studied. To get a deeper insight into this problem we synthesized a series of novel N(O)-acylated compounds ( 2)–( 8) and studied their structure, spectral properties, and quantum yields of the N O acyl rearrangements.

2. Experimental

1H NMR spectra were recorded on a Varian Unity 300 spectrometer; δ values were measured within 0.01 ppm. IR spectra in Nujol were measured using a Specord 75IR spectrometer. UV-Vis absorption spectra in toluene were obtained with a Specord M-40 spectrophotometer. Fluorescence emission and excitation spectra were recorded on a Hitachi 650–60 spectrofluorimeter. Irradiation of solutions (  L) was carried out by filtered light of a high-pressure mercury lamp DRSh (250 W) supplied with a set of glass filters (  nm). The intensity of light used for irradiation of the solutions was photon s-1 for the spectral line 436 nm. Potassium ferrioxalate was used as the actinometer for the determination of quantum yields ( ) of the photoreactions [8]. The samples had absorbance of 0.95 at  nm (l = 1 cm, V = 0-3 L, rate of conversion of A B ≤ 5%, the experimental error in is ±5%).

3. Results and Discussion

3.1. Synthesis

Compounds ( 2), (3a), (3b), (4a), (4b), ( ), and ( ) were synthesized according to the previously described procedure [9] (Scheme 1). Compound (4c) was synthesized according to procedure [7].

694196.sch.001
Scheme 1

All the compounds obtained were divided into four groups according to the electronic and steric properties of acyl migrants and N-aryl substituents. The first group of acylated 2-(N -arylaminomethylene)benzo[b ]thiophene-3(2H )-ones consists of the compounds with the gradually increased bulkiness of the migrant: R1 = H ( ), Me (3a), Et (3b), Pri (4a), But ( ). The second one includes acylated ketoenamines possessing migrants with increasing electron-donating properties of α-C substituents: R1 = H ( ), Ph (4c), Me (3a), OMe (4b), NMe2 ( ).

The third series represents compounds with varying 4-substituents in the aroyl migrant: R3 = H (4c), 4-MeO (7a), 4-Cl (7b), 4-NO2 (7c). In the fourth group we united molecules with varying 4-substituent in N-Ar part of the molecule: R2 = NMe2 (8a), OMe (8b), Me (8c), I (8d), Br (8e), Cl (8f), F (8g), CN (8h), COMe (8i), NO2 (8j). Their synthesis is shown on Scheme 2.

694196.sch.002
Scheme 2

2-(N-aroyl-N-phenylaminomethylene)benzo[b]thiophe -ne-3(2H)-ones (7a)–(7c) (general procedure). 2-(N -phenylaminomethylene)benzo[b ]thiophene-3(2H)-one (1a) (1 mmol) was dissolved in 3 mL of acetonitrile in the presence of triethylamine and a solution of 1.2 mmol of corresponding aroyl chloride in 2 mL of acetonitrile was added at room temperature. The mixture was refluxed for 3–5 minutes. The precipitate was filtered and crystallized from toluene.

(i)2-(N-4-methoxybenzoyl-N-phenylaminomethylene) -benzo[b]thiophene-3(2H)-one (7a). (31%); mp 177–179 ; IR ( /cm-1, nujol): 1660, 1640, 1590, 1580, 1550; 1H NMR (300 MHz, CDCl3): 3.81 (3H, s, CH3), 6.80–7.84 (13H, m, Ar-H), 8.67 (1H, s, CH); Anal. Calcd for C23H17NO3S: C, 71.30; H, 4.42; N, 3.61; Found: C, 71.19; H, 4.52; N, 3.73.

(ii) 2-(N-4-chlorobenzoyl-N-phenylaminomethylene)ben -zo[b]thiophene-3(2H)-one (7b) . (64%); mp 215-216 ; IR ( /cm-1, nujol): 1640, 1580, 1560; 1H NMR (300 MHz, CDCl3): 7.16–7.85 (13H, m, Ar-H), 8.67 (1H, s, CH); Anal. Calcd for C22H14ClNO2S: C, 67.43; H, 3.60; N, 3.57; Found: C, 67.50; H, 6.73; N, 3.41.

(iii) 2-(N-4-nitrobenzoyl-N-phenylaminomethylene)ben -zo[b]thiophene-3(2H)-one (7c) . (57%); mp 211–212 ; IR ( /cm-1, nujol): 1690, 1680, 1600, 1580, 1560, 1520; 1H NMR (300 MHz, CDCl3): 7.18–8.17 (13H, m, Ar-H), 8.74 (1H, s, CH); Anal. Calcd for C22H14N2O4S: C, 65.66; H, 3.51; N, 6.96; Found: C, 65.60; H, 3.66; N, 6.84.

(iv) 2-(N-acetyl-N-arylaminomethylene)benzo[b] -thiophene-3(2H)-ones (8a)–(8j) (general procedure). Corresponding 2-(N -arylaminomethylene)benzo[b ]thiophene-3(2H )-ones (1b)–(1n) [10] (1 mmol) were dissolved in acetic anhydride (2 mL) in the presence of triethylamine and refluxed for 5–10 minutes. The precipitate was filtered and crystallized from toluene.

(v) 2- N-acetyl-N-(4-N,N-dimethylaminophenyl)amino -methylene benzo[b]thiophene-3(2H)-one (8a) . (77%); mp 245-246 ; IR ( /cm-1, nujol): 1690, 1640, 1590, 1550, 1510; 1H NMR (300 MHz, CDCl3): 2.03 (3H, s, CH3), 3.08 (6H, s, N(CH3)2), 6.77–7.82 (8H, m, Ar-H), 8.98 (1H, s, CH); Anal. Calcd for C19H18N2O2S: C, 67.43; H, 5.36; N, 8.28; Found: C, 67.56; H, 5.28; N, 8.20.

(vi) 2- N-acetyl-N-(4-methoxyphenyl)aminomethylene - benzo[b]thiophene-3(2H)-one (8b) . (43%); mp 193-194 ; IR ( /cm-1, nujol): 1690, 1660, 1570, 1540; 1H NMR (300 MHz, CDCl3): 2.08 (3H, s, CH3), 3.94 (3H, s, OCH3), 7.02–7.83 (8H, m, Ar-H), 8.92 (1H, s, CH); Anal. Calcd for C18H15NO3S: C, 66.44; H, 4.65; N, 4.30; Found: C, 66.59; H, 4.66; N, 4.18.

(vii) 2- N-acetyl-N-(4-methylphenyl)aminomethylene - benzo[b]thiophene-3(2H)-one (8c) . (83%); mp 210–212 ; IR ( /cm-1, nujol): 1700, 1650, 1580, 1550; 1H NMR (300 MHz, CDCl3): 2.05 (3H, s, COCH3), 2.52 (3H, s, CH3), 7.15–7.82 (8H, m, Ar-H), 8.95 (1H, s, CH); Anal. Calcd for C18H15NO2S: C, 69.88; H, 4.89; N, 4.53; Found: C, 69.87; H, 4.74; N, 4.65.

(viii) 2- N-acetyl-N-(4-iodophenyl)aminomethylene ben- zo[b]thiophene-3(2H)-one (8d) . (73%); mp 241-243 ; IR ( /cm-1, nujol): 1700, 1660, 1590, 1550; 1H NMR (300 MHz, CDCl3): 2.11 (3H, s, CH3), 7.03–7.94 (8H, m, Ar-H), 8.82 (1H, s, CH); Anal. Calcd for C17H12INO2S: C, 48.47; H, 2.87; N, 3.32; Found: C, 48.38; H, 2.90; N, 3.32.

(ix) 2- N-acetyl-N-(4-bromophenyl)aminomethylene - benzo[b]thiophene-3(2H)-one (8e) . (85%); mp 218-219 ; IR ( /cm-1, nujol): 1700, 1660, 1590, 1560, 1550; 1H NMR (300 MHz, CDCl3): 2.12 (3H, s, CH3), 7.19–7.83 (8H, m, Ar-H), 8.82 (1H, s, CH); Anal. Calcd for C17H12BrNO2S: C, 54.56; H, 3.23; N, 3.74; Found: C, 54.68; H, 3.17; N, 3.63.

(x) 2- N-acetyl-N-(4-chlorophenyl)aminomethylene - benzo[b]thiophene-3(2H)-one (8f) . (70%); mp 186-187 ; IR ( /cm-1, nujol): 1700, 1660, 1590, 1550; 1H NMR (300 MHz, CDCl3): 2.10 (3H, s, CH3), 7.19–7.84 (8H, m, Ar-H), 8.83 (1H, s, CH); Anal. Calcd for C17H12ClNO2S: C, 61.91; H, 3.67; N, 4.25; Found: C, 62.03; H, 3.60; N, 4.27.

(xi) 2- N-acetyl-N-(4-fluorophenyl)aminomethylene - benzo[b]thiophene-3(2H)-one (8g) . (69%); mp 213-214 ; IR ( /cm-1, nujol): 1700, 1660, 1590, 1580, 1540; 1H NMR (300 MHz, CDCl3): 2.10 (3H, s, CH3), 7.20–7.84 (8H, m, Ar-H), 8.84 (1H, s, CH); Anal. Calcd for C17H12FNO2S: C, 65.16; H, 3.86; N, 4.47; Found: C, 65.15; H, 3.95; N, 4.40.

(xii) 2- N-acetyl-N-(4-cyanophenyl)aminomethylene - benzo[b]thiophene-3(2H)-one (8h) . (57%); mp 215-216 ; IR ( /cm-1, nujol): 1690, 1670, 1580, 1560; 1H NMR (300 MHz, CDCl3): 2.21 (3H, s, CH3), 7.20–7.90 (8H, m, Ar-H), 8.73 (1H, s, CH); Anal. Calcd for C18H12N2O2S: C, 67.48; H, 3.78; N, 8.74; Found: C, 67.54; H, 3.87; N, 8.62.

(xiii) 2- N-acetyl-N-(4-acetylphenyl)aminomethylene - benzo[b]thiophene-3(2H)-one (8i) . (48%); mp 186-187 ; IR ( /cm-1, nujol): 1695, 1670, 1660, 1595, 1570, 1550; 1H NMR (300 MHz, CDCl3): 2.16 (3H, s, NCOCH3), 2.74 (3H, s, COCH3), 7.15–8.18 (8H, m, Ar-H), 8.82 (1H, s, CH); Anal. Calcd for C19H15NO3S: C, 67.64; H, 4.48; N, 4.15; Found: C, 67.69; H, 4.38; N, 4.22.

(xiv) 2- N-acetyl-N-(4-nitrophenyl)aminomethylene - benzo[b]thiophene-3(2H)-one (8j) . (37%); mp 199-200 ; IR ( /cm-1, nujol): 1700, 1670, 1600, 1580, 1560, 1530; 1H NMR (300 MHz, CDCl3): 2.22 (3H, s, CH3), 7.19–8.47 (8H, m, Ar-H), 8.74 (1H, s, CH); Anal. Calcd for C17H12N2O4S: C, 59.99; H, 3.55; N, 8.23; Found: C, 59.86; H, 3.63; N, 8.23.

3.2. Photochromic Properties

According to the performed IR and NMR spectral study compounds ( )–( ), ( ), and ( ) exist in solutions as the N-acylated isomers. Their IR-spectra contain absorption bands of the amide (1660–1730 cm-1) and exocyclic carbonyl groups (1660–1680 cm-1). 1H NMR spectra consist of characteristic signals of methine protons at 8.70–9.00 ppm which correspond to the initial Z -configuration [3]. N-Acylated ketoenamines ( )–( ), ( ), and ( ) absorb in 420–430 nm spectral region (Table 1). On the contrary, compounds ( ), ( ) correspond to O-acyl derivatives. They possess typical absorption in the UV spectral region (338–342 nm) [9] and their IR spectra contain ester carbonyl group signals (1720–1750 cm-1).

tab1
Table 1: Spectral characteristics (long-wave length maxima) of ( 2)–( 8) in acetonitrile solutions and quantum yields of the A→B rearrangement.

For the majority of N-acylated forms A of compounds ( ), ( ), ( ), ( ) irradiation (  nm) of toluene solutions results in Z E -photoisomerization around the C=C bond followed by a fast thermal N O acyl migration and S-cis S-trans isomerization of O-acyl compounds B (Scheme 3, Figure 1). Quantum yields ( ) of the A B photorearrangement include all stages shown on Scheme 3 from Z -A to S -trans -B though we consider all thermal rearrangements to be very rapid [7]. These summarized quantum yields are given in Table 1.

694196.sch.003
Scheme 3
694196.fig.001
Figure 1: Absorption spectra of (8b) in toluene solution before irradiation (1 ); after 5 seconds (2 ); 10 seconds (3 ); 20 seconds (4 ); 40 seconds (5 ); 120 seconds (6 ) of irradiation (  nm, C = 0-5 M).

In the series of compounds with varying migrant volume, the smallest migrant CHO ( ) inhibits A B photorearrangement and restricts it by the thermally reversible Z E -photoisomerization. The specific feature of this process is the formation of photostationary state depending on the wavelength of irradiation light (Figure 2). N-Acyl derivatives (3a), (3b), and (4a) reveal typical photochromic behavior (Scheme 3). The compound with the most bulky But ( ) substituent exists in the thermodinamically stable O-form B. Its photoinitiated reactions are limited by thermally reversible anti-syn isomerization around C=N bond.

694196.fig.002
Figure 2: Absorption spectra of (1a) in toluene solution before irradiation (1 ); after 120 seconds (2 ) of irradiation ( = 365 nm) and after 120 seconds (3 ) of irradiation ( = 436 nm); C = 0-5 M.

The representatives of second group ( ), (3a), (4b), (4c) with varying electronic migrant exist in N-acyl form A and demonstrate N O acyl rearrangement in accordance with Scheme 3 (except above described ketoenamine 2). The compound with the strong electron-donating α-C substituent NMe2 ( ) occurs in the form of O-acyl isomer B and shows only anti-syn isomerization around the C=N bond, similar to ( ).

Varying of 4-substituent in aroyl group (7a)–(7c) results in decrease of quantum yield A B rearrangement almost up to zero (Table 1) as correlated with the increase of electron-withdrawing properties of R3.

In the fourth series the substituents in the position 4 of the N-Ar moiety of the compounds (8b)–(8j) only slightly affect the character of the photochromic A B photorearrangement (Table 1). The photochemical transformations of (8a) with NMe2 substituent lead to formation of a mixture of O- and N-isomers with predominance of the last one due to overlap of long-wavelength absorption bands of initial compound and photoproduct.

In toluene solutions, at 293 K, N-acylated compounds ( )–( ), ( ), and ( ) exhibit weak fluorescence with maxima at 460–470 nm which extinguishes after the rearrangement into the O-acetyl isomers B due to the fast intersystem crossing processes (“on-off” process) [9]. The B A back reaction occuring when passing dried hydrogen chloride through a toluene solution of form B results in complete restoration of the initial absorption and emission spectra of form A (“off-on” process).

4. Conclusions

Series of N(O)-acylated 2-(N -arylaminomethylene)- benzo[b ]thiophene-3(2H )-ones with varying electronic and steric properties of substituents in the acyl migrants and N-aryl groups were prepared. By means of electronic absorption, vibrational, and 1H NMR spectroscopy, it was shown that ketoenamine with the smallest formyl substituent undergoes only reversible photoisomerization. The compounds containing the strongest electron-donating (NMe2) and the most bulky (But) substituents in acyl migrants represent the thermodynamically stable O-isomers. Their irradiation results only in the thermally reversible anti-syn isomerization around the C=N bond. The other compounds exhibit negative photochromism ( ) based on photoinduced photoisomerization followed by the thermal N O transfer of the acyl group.

Acknowledgments

This work was supported by Russian Foundation for Basic Research (Grants N 06-03-32561, 09-03-00052), RF Ministry of Education and Science (Grants RNP.16012 and RNP.2348) and President’s of RF Grant (NSh-363.2008.3).

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