# Acylated 2-(N-arylaminomethylene)benzo[b]thiophene-3(2H

Authors: Alexander D. Dubonosov; Vladimir P. Rybalkin; Lydia L. Popova; Alexey V. Tsukanov; Yurii V. Revinsky; Vladimir A. Bren; Vladimir I. Minkin
Journal: International Journal of Photoenergy
 (2009)

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## Abstract

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


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## Body

## 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 [3–5]. 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 (V=2·10-3 L) was carried out by filtered light of a high-pressure mercury lamp DRSh (250 W) supplied with a set of glass filters (λirr=436 nm). The intensity of light used for irradiation of the solutions was 3·1016 photons·s-1 for the spectral line 436 nm. Potassium ferrioxalate was used as the actinometer for the determination of quantum yields (φ) of the photoreactions [8]. The samples had absorbance of 0.95 at λirr=436 nm (l = 1 cm, V = 2·10-3 L, rate of conversion of A→B ≤ 5%, the experimental error in φ is ±5%).


## 3. Results and Discussion

### 3.1. Synthesis

Compounds ( 2), (3a), (3b), (4a), (4b), (5), and (6) were synthesized according to the previously described procedure [9] (Scheme 1). Compound (4c) was synthesized according to procedure [7].Scheme 1All 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 (2), Me (3a), Et (3b), Pri (4a), But (5). The second one includes acylated ketoenamines possessing migrants with increasing electron-donating properties of α-C substituents:  R1 = H (2), Ph (4c), Me (3a), OMe (4b), NMe2 (6).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.Scheme 22-(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 177–179C°; IR (υmax/cm-1, nujol): 1660, 1640, 1590, 1580, 1550; 1H NMR (300 MHz, CDCl3): δ3.81 (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 215-216C°; IR (υmax/cm-1, nujol): 1640, 1580, 1560; 1H NMR (300 MHz, CDCl3): δ7.16–7.85 (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 211–212C°; IR (υmax/cm-1, nujol): 1690, 1680, 1600, 1580, 1560, 1520; 1H NMR (300 MHz, CDCl3): δ7.18–8.17 (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 245-246C°; IR (υmax/cm-1, nujol): 1690, 1640, 1590, 1550, 1510; 1H NMR (300 MHz, CDCl3): δ2.03 (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 193-194C°; IR (υmax/cm-1, nujol): 1690, 1660, 1570, 1540; 1H NMR (300 MHz, CDCl3): δ2.08 (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 210–212C°; IR (υmax/cm-1, nujol): 1700, 1650, 1580, 1550; 1H NMR (300 MHz, CDCl3): δ2.05 (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 241-243C°; IR (υmax/cm-1, nujol): 1700, 1660, 1590, 1550; 1H NMR (300 MHz, CDCl3): δ2.11 (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 218-219C°; IR (υmax/cm-1, nujol): 1700, 1660, 1590, 1560, 1550; 1H NMR (300 MHz, CDCl3): δ2.12 (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 186-187C°; IR (υmax/cm-1, nujol): 1700, 1660, 1590, 1550; 1H NMR (300 MHz, CDCl3): δ2.10 (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 213-214C°; IR (υmax/cm-1, nujol): 1700, 1660, 1590, 1580, 1540; 1H NMR (300 MHz, CDCl3): δ2.10 (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 215-216C°; IR (υmax/cm-1, nujol): 1690, 1670, 1580, 1560; 1H NMR (300 MHz, CDCl3): δ2.21 (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 186-187C°; IR (υmax/cm-1, nujol): 1695, 1670, 1660, 1595, 1570, 1550; 1H NMR (300 MHz, CDCl3): δ2.16 (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 199-200C°; IR (υmax/cm-1, nujol): 1700, 1670, 1600, 1580, 1560, 1530; 1H NMR (300 MHz, CDCl3): δ2.22 (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 (2)–(4), (7), and (8) 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 (2)–(4), (7), and (8) absorb in 420–430 nm spectral region (Table 1). On the contrary, compounds (5), (6) 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).Table 1
Spectral characteristics (long-wave length maxima) of ( 2)–( 8) in acetonitrile solutions and quantum yields(φ) of the A→B rearrangement.
Comp.Absorption of (Z-A), λmax (nm),φA→BAbsorption of (S-trans-B), λmax (nm),(ε×10-4, dm3 mol-1 cm-1)(ε×10-4, dm3 mol-1 cm-1)2*424 (1.10)——3a*427 (1.04)0.60338 (2.32)3b425 (1.24)0.63339 (2.27)4a427 (1.28)0.62342 (2.24)4b*422 (1.17)0.41333 (0.81)4c*423 (1.36)0.52340 (2.14)7a430 (1.42)0.48342 (2.19)7b430 (1.32)0.40343 (2.09)7c431 (1.34)0.002—8a430 (1.41)——8b425 (1.22)0.50353 (2.20)8c425 (1.26)0.50345 (2.25)8d425 (1.23)0.47350 (2.56)8e424 (1.22)0.51347 (2.51)8f424 (1.23)0.49347 (2.51)8g424 (1.19)0.41342 (2.30)8h424 (1.13)0.48346 (2.52)8i424 (1.29)0.45350 (2.74)8j424 (1.14)0.29356 (2.65)5——338 (2.36)6——342 (2.14)*See [9].For the majority of N-acylated forms A of compounds (3), (4), (7), (8) irradiation (λirr=436 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 (φ) 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.Scheme 3Figure 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 (λirr=436 nm, C = 2.5·10-5 M).In the series of compounds with varying migrant volume, the smallest migrant CHO (2) 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 (5) substituent exists in the thermodinamically stable O-form B. Its photoinitiated reactions are limited by thermally reversible anti-syn isomerization around C=N bond.Figure 2
Absorption spectra of (1a) in toluene solution before irradiation (1); after 120 seconds (2) of irradiation (λirr = 365 nm) and after 120 seconds (3) of irradiation (λirr = 436 nm); C = 2.5·10-5 M.The representatives of second group (2), (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 (6) occurs in the form of O-acyl isomer B and shows only anti-syn isomerization around the C=N bond, similar to (5).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 (2)–(4), (7), and (8) 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).


## 3.1. Synthesis

Compounds ( 2), (3a), (3b), (4a), (4b), (5), and (6) were synthesized according to the previously described procedure [9] (Scheme 1). Compound (4c) was synthesized according to procedure [7].Scheme 1All 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 (2), Me (3a), Et (3b), Pri (4a), But (5). The second one includes acylated ketoenamines possessing migrants with increasing electron-donating properties of α-C substituents:  R1 = H (2), Ph (4c), Me (3a), OMe (4b), NMe2 (6).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.Scheme 22-(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 177–179C°; IR (υmax/cm-1, nujol): 1660, 1640, 1590, 1580, 1550; 1H NMR (300 MHz, CDCl3): δ3.81 (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 215-216C°; IR (υmax/cm-1, nujol): 1640, 1580, 1560; 1H NMR (300 MHz, CDCl3): δ7.16–7.85 (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 211–212C°; IR (υmax/cm-1, nujol): 1690, 1680, 1600, 1580, 1560, 1520; 1H NMR (300 MHz, CDCl3): δ7.18–8.17 (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 245-246C°; IR (υmax/cm-1, nujol): 1690, 1640, 1590, 1550, 1510; 1H NMR (300 MHz, CDCl3): δ2.03 (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 193-194C°; IR (υmax/cm-1, nujol): 1690, 1660, 1570, 1540; 1H NMR (300 MHz, CDCl3): δ2.08 (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 210–212C°; IR (υmax/cm-1, nujol): 1700, 1650, 1580, 1550; 1H NMR (300 MHz, CDCl3): δ2.05 (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 241-243C°; IR (υmax/cm-1, nujol): 1700, 1660, 1590, 1550; 1H NMR (300 MHz, CDCl3): δ2.11 (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 218-219C°; IR (υmax/cm-1, nujol): 1700, 1660, 1590, 1560, 1550; 1H NMR (300 MHz, CDCl3): δ2.12 (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 186-187C°; IR (υmax/cm-1, nujol): 1700, 1660, 1590, 1550; 1H NMR (300 MHz, CDCl3): δ2.10 (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 213-214C°; IR (υmax/cm-1, nujol): 1700, 1660, 1590, 1580, 1540; 1H NMR (300 MHz, CDCl3): δ2.10 (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 215-216C°; IR (υmax/cm-1, nujol): 1690, 1670, 1580, 1560; 1H NMR (300 MHz, CDCl3): δ2.21 (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 186-187C°; IR (υmax/cm-1, nujol): 1695, 1670, 1660, 1595, 1570, 1550; 1H NMR (300 MHz, CDCl3): δ2.16 (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 199-200C°; IR (υmax/cm-1, nujol): 1700, 1670, 1600, 1580, 1560, 1530; 1H NMR (300 MHz, CDCl3): δ2.22 (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 (2)–(4), (7), and (8) 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 (2)–(4), (7), and (8) absorb in 420–430 nm spectral region (Table 1). On the contrary, compounds (5), (6) 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).Table 1
Spectral characteristics (long-wave length maxima) of ( 2)–( 8) in acetonitrile solutions and quantum yields(φ) of the A→B rearrangement.
Comp.Absorption of (Z-A), λmax (nm),φA→BAbsorption of (S-trans-B), λmax (nm),(ε×10-4, dm3 mol-1 cm-1)(ε×10-4, dm3 mol-1 cm-1)2*424 (1.10)——3a*427 (1.04)0.60338 (2.32)3b425 (1.24)0.63339 (2.27)4a427 (1.28)0.62342 (2.24)4b*422 (1.17)0.41333 (0.81)4c*423 (1.36)0.52340 (2.14)7a430 (1.42)0.48342 (2.19)7b430 (1.32)0.40343 (2.09)7c431 (1.34)0.002—8a430 (1.41)——8b425 (1.22)0.50353 (2.20)8c425 (1.26)0.50345 (2.25)8d425 (1.23)0.47350 (2.56)8e424 (1.22)0.51347 (2.51)8f424 (1.23)0.49347 (2.51)8g424 (1.19)0.41342 (2.30)8h424 (1.13)0.48346 (2.52)8i424 (1.29)0.45350 (2.74)8j424 (1.14)0.29356 (2.65)5——338 (2.36)6——342 (2.14)*See [9].For the majority of N-acylated forms A of compounds (3), (4), (7), (8) irradiation (λirr=436 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 (φ) 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.Scheme 3Figure 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 (λirr=436 nm, C = 2.5·10-5 M).In the series of compounds with varying migrant volume, the smallest migrant CHO (2) 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 (5) substituent exists in the thermodinamically stable O-form B. Its photoinitiated reactions are limited by thermally reversible anti-syn isomerization around C=N bond.Figure 2
Absorption spectra of (1a) in toluene solution before irradiation (1); after 120 seconds (2) of irradiation (λirr = 365 nm) and after 120 seconds (3) of irradiation (λirr = 436 nm); C = 2.5·10-5 M.The representatives of second group (2), (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 (6) occurs in the form of O-acyl isomer B and shows only anti-syn isomerization around the C=N bond, similar to (5).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 (2)–(4), (7), and (8) 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 Z→E 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 (λmaxB<λmaxA) based on photoinduced Z→E photoisomerization followed by the thermal N→O transfer of the acyl group.



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*Source: 694196-2009-08-18.xml*