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

2-(N-Acyl-N-arylaminomethylene)benzo[b]thiophene3(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.


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][4][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.

Experimental
1 H 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 highpressure mercury lamp DRSh (250 W) supplied with a set of glass filters (λ irr = 436 nm).The intensity of light used for irradiation of the solutions was 3 • 10 16 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 λ irr = 436 nm (l = 1 cm, V = 2 • 10 −3 L, rate of conversion of A→B ≤ 5%, the experimental error in ϕ is ±5%).
(i)   [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.
For the majority of N-acylated forms A of compounds (3), ( 4), ( 7), ( 8) irradiation (λ irr = 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.
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→Ephotoisomerization.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 Bu t (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.
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 NMe 2 (6) occurs in the form of O-acyl isomer B and shows only anti-syn isomerization around the C=N bond, similar to (5).

International Journal of Photoenergy
Table 1: Spectral characteristics (long-wave length maxima) of ( 2)-( 8) in acetonitrile solutions and quantum yields (ϕ) of the A→B rearrangement.
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 NMe 2 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).

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 1 H 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 (NMe 2 ) and the most bulky (Bu t ) 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 (λ max B < λ max A) based on photoinduced Z→E photoisomerization followed by the thermal N→O transfer of the acyl group.