Thermally irreversible photochromic dithienylethenes

Abstract. A lot of previously unknown derivatives of dithienylperfluorocyclopentene (DTPFCP) were synthesized. It was shown that 2,2′-dialkylsubstituted DTPFCP’S are phtochromes. The quantum yields of forward and backward photochemical reactions and spectral parameters of open and cyclic forms of the photochromes synthesized were measured. An attempt to obtain fluorescing phtochromes by means of introduction of benzoxazolyl, benzthiazolyl and 1,2,4-oxadiazolyl moieties in 5 and 5′ positions of DTPFCP failed; these compounds were synthesized but they do not fluoresce.


INTRODUCTION
Dithienylethenes are studied during last years as photochromes very actively [1].Previously unknown alkylthio-or alkylsulphonyl derivatives of dithienylethenes as well as dithienylethenes with heterocycles as potential fluorophoric fragments are the objects of this study.
It is known that alkylthio groups activate thiophene ring strongly in the reaction of electrophilic substitution and metallation.These groups promote substantially the introduction of various functional substituents into free positions of the heterocycle and allow one to create fused systems based on thiophene with longer chains of conjugation.
The introduction of "fluorophoric" substituents can help to solve the problem of the read out without essential effect on interconversion of cyclic and open forms of dithienylethene photochromes.

SYNTHESIS OF DITHIENYLETHENES
Synthesis of compounds with sulphur containing groups in positions 2 of thiophene rings was conducted according to Scheme 1 starting from 2ethylthiothiophene.
The bromination of the latter and further debromination of dibromosulphide II at −70 • C result in 3bromo-2-ethylthiothiophene III in a good yield.Bissulphide I was synthesized in the yield 60% by the successive action of BuLi and octafluorocyclopentene on bromide III in ether.The compound I is transformed quantitatively into bis-sulphone IV by the action of excess 30% H 2 O 2 .Bis-sulphide I is easily metallated by BuLi in ether during several minutes into either thiophene cycles and then after the treatment by DMF or CO 2 is transformed in good yields into dialdehyde V or into dicarboxylic acid VI, respectively.
The use of 5-ethylthio-2-ethylthiophene VII (Scheme 2) as starting compound enables to obtain not only alkylated analogs of I and IV, but also their isomers with sulphur-containing groups in position 5 of thiophene cycles.
Dibromide VIII is formed in high yield by the bromination of sulphide VII using two equivalents of Br 2 in glacial AcOH.The treatment of VIII with one equivalent of BuLi in Et 2 O results in the mixture of isomeric monobromides IX and X in the ratio ∼ 1 : 4. The pure 3-bromo-5-ethylthio-2-ethylthiophene X was isolated by column chromatography and it was used for further synthesis.Isomeric 1,2-bis(2-ethyl-5-ethylthio-3thienyl)hexafluorocyclopentene XI and 1,2-bis(5-ethyl-2-ethylthio-3-thienyl)hexafluorocyclopentene XII were obtained by successive action of BuLi and octafluorocyclopentene at −70 • C on monobromides X and IX, respectively.Isomers XI and XII were oxidized with 30% H 2 O 2 into corresponding bis-sulphones XIII and XIV.
Alternative method to obtain the dithienylethene XI starting with 2-ethylthiophene was also proposed (Scheme 3).
This approach allows to obtain X in 75% yield.This compound is necessary for synthesis of XI and which does not contain isomeric 4-bromo-5-ethylthio-2-ethylthiophene (in contrast to methods shown in Scheme 2).The compound XI is easily brominated with 2 equivalents of Br 2 in chloroform or with bromidebromate mixture quantitatively into dibromide XV, which is transformed into bis-sulphone XVI by oxidation with 40% H 2 O 2 in AcOH.Dicarboxylic acid XVII is formed in 70% yield by the interaction between XV and BuLi in Et 2 O with subsequent carbonization.
As it was stated in [2], reading out can be realized by means of fluorescence excited in the spectral region where the interconversion of two dithienylethenes is absent.The synthesis of the compounds containing potential "fluorophor" fragments to provide fluorescing photochromes was performed to create the substances with properties stated above.Benzoxazole cycle, benzthiazole cycle and aryl derivatives 1,3,4oxadiazole were chosen as such fragments.Recall for instance that 2,5-bis(2-benzoxazolyl)thiophene is used as fluorophor [3].1,2-Bis[5-(2-benzoxazolyl)-2-methyl-3-thienyl]hexafluorocyclopentene XXIII was obtained according to the Scheme 4.
2-(4-Bromo-5-methyl-2-thienyl)benzoxazole XIX was formed using condensation of 4-bromo-5-methyl-2thiophenecarboxylic acid XX with o-aminophenol in xylene in the presence of H 3 BO 3 in 45% yield.The exchange of Br for Li in bromide XIX under the action of BuLi at −70 • C resulted in Li-derivative XXI.The interaction of XXI with octafluorocyclopentene resulted in the adduct XXII (49% yield).The interaction of this adduct with Li-derivative XXI resulted in the product XXIII in 31% yield.The acid XX was the starting compound also in the preparation of the benzthiazole photochrome XXV according to Scheme 5.

PHOTOCHROMISM OF DITHIENYLETHENES
The study of photochromic properties of synthesized dithienylethenes has been shown that the compounds XI, XIII, XV, XVI, XVII, XVIII, XXIII, XXV, XXIXa and XXIXb are photochromes.In the case of compounds bearing in are not changed.The ratio of concentrations of A and B forms in the photostationary state depends on irradiation wavelength and inversely proportional to the ratio of extinction coefficients of these forms at certain wavelength.
The isosbestic points in absorption spectra of solution of these photochromic compounds are observed.The coincidence of their position for forward and backward reactions testifies the complete photoreversibility of photocyclization and the absence of side processes.
The dark reaction A → B is absent, the dark reaction B → A is also absent.The quantum yields of forward and backward phtochemical reactions and spectral data of open and cyclic forms are given in Table 1.
For 5, 5 -disulphonyl substituted dithienylethenes XIII and XVI the formation of coloured form B, accompanied by the decrease of the optical density at the absorption maximum of A, is observed under the irradiation of ethanolic solutions with UV-light (λ = 313 nm).However, under the subsequent irradiation of these solutions with the light of λ = 578 nm the isosbestic points in the absorption spectra of the solutions are not observed (Figures 8 and 9).It proves that photochromic reactions of compounds XIII and XVI are accompanied by some other transformations which are not related with photocyclization.
The dithienylethenes, containing alkylthio-or alkylsulphonyl groups in positions 2 of thiophene cycles, but not alkyl ones (i.e.I, IV, V, VI, XII and XIV) do not change their absorption spectra under UV-irradiation (λ = 313 nm), that is, they are not photochromes.
We have studied the fluorescence properties of

EXPERIMENTAL PART
The irradiation of samples was conducted with resonance mercury lamp (λ = 254 nm) and mercury lamp DRSh-500 supplied with filters for selection of mercury spectrum lines at wavelengths 313, 546 and 578 nm.The intensity of DRSh-500 irradiation was measured by means of photocell F4, calibrated by ferrioxalate actinometer [4] for λ = 313 nm and λ = 546 nm and actinometer based on Reynecke salt [5] for λ = 578 nm.Absorption spectra were registered by spectrophotometer "Shimadzu-2101PC".The fluorescence was studied by means of spectrofluorimeter "Perkin-Elmer LS-50".
To determine the quantum yields of forward and backward reactions the ethanolic solution of a substance was irradiated with the light λ = 313 nm (forward reaction) and λ = 546 nm or λ = 578 nm (backward reaction).The duration of irradiation was varied from 5 s till 1-2 min (7-10 experimental points).The absorption spectrum of irradiated solution was registered after each exposition.
The quantum yield (Φ) was calculated by means of the following equation where V -the volume of irradiated solution, ∆D B (t)-the change of solution optical density at absorption spectrum maximum of B during time t, ε B -extinction coefficient of B, l-the length of optical way, J abs (t)-the quantity of light, absorbed by the solution during time t.For backward reaction the dependence of ∆D B (t) on J abs (t) is linear, therefore ∆D B (t)/J abs (t) = const and thus Φ does not depend on t; for forward reaction ∆D B (t)/J abs (t) was determined as the tangent of the slope to the curve ∆D B (t) − J abs (t) at the origin of the coordinates.
To calculate the extinction coefficient of B(ε B ) it was supposed that at small expositions (small conversions) the contribution of B absorption is negligible small.Then Scheme 1

C 1 −∆t 1 −
B (t) = C A (t 0 ) − C A (t) = D A (t 0 ) − D A (t) ε A l = ∆D A (t) ε A l ,where C A (t) and C B (t)-the molar concentrations of A and B, respectively, after the irradiation during time t, C A (t 0 )-the molar concentration of A in non-irradiated solution, D A (t)-the optical density of the solution after the irradiation during time t, D A (t 0 )-the optical density of non-irradiated solution at the maximum of absorption spectrum of A. By substituting of the expression for C B (t) into the equationε B = D B /(C B l) we get ε B = ε A D B (t)/∆D A (t).The quantity of the absorbed light (J abs ) was determined by means of equationJ abs = J 0 t 0 10 −Dλ dt = J 0 10 −Dλ(∆t) ∆t,where J 0 -the light intensity at the irradiation wavelength, D λ -the optical density of the solution at the irradiation wavelength.

Table 1 .
The quantum yields of forward and backward photochemical reactions and spectral data of open and cyclic forms of photochromic dithienylperfluorocyclopentenes.