Symmetrical photochromic

Chinese researchers reported a synthetic route to photochrome 16 (Scheme 7), in which the aldehyde functions are then transformed into cyclic acetals and thioacetals, methylol and dicyanoethylene groups (07T5437, 08T2576). Scheme 7 also gives (at the bottom) symmetrical photochrome 17 (where R are ferrocenyl substituents), which does not exhibit fluorescence in the initial state, but shows fluorescence in the cyclic form and which is also synthesized starting from dialdehyde 16 (08AFM302). 

Scheme 12 New synthetic route to symmetrical photochromic diarylperfluorocyclopentenes.

Even thienyl bromide 33 with the electron-withdrawing and fairly bulky benzothia-zole fragments enters the reaction to form the photochromic product 34 (Scheme 9) [47], As a rule, the yields of the symmetrical photochromes are 30-50 %. 

Lucas LN, van Esch JH, Kellogg RM, FeringaBL (1999) A new synthetic route to symmetrical photochromic diarylperfluorocyclopenthenes. Tetrahedron Lett 40 1775-1778. doi 10.1016/ 80040-4039(98)02688-4... 

Following similar trials with the formation of diarylbenzenes [63-65], the same research group has reported a multistep synthesis of photochromic diarylethenes using a microflow system that contained two linked micromixers and microreactors (MRi 2) [66]. Similarly to the previously reported linked microreactors, the reactors used in this setup were made of stainless steel tubes and T-shaped micromixers. Initial experiments were conducted in two steps in a continuous sequence to afford symmetrical octasubstitued diaryUiexafluoro cyclopentene (Scheme 29). 

The most widely used method for the preparation of perfluorocyclopen-tene-containing DTEs is based on the reactions of lithium derivatives of thiophene with octafluorocyclopentene (98KGS927, 99IZV979, 99JP183). This method is used for the assembly of photochromic products from components indifferent to butyllithium or for the formation of the core of the molecules for their subsequent functionalization. This approach is suitable for the synthesis of both symmetrical and unsymmetrical photochromes. 

Here typical syntheses of symmetrical products are considered. In Scheme 2, the treatment of bromide 1 with butyllithium under standard conditions in the presence of perfluorocyclopentene affords photochrome 2 (31 %) (99IZV979). Derivatives 3 and 4 were synthesized in a similar way (98KGS927). 

From numerous examples of the synthesis of symmetrical and unsym-metrical dithienylperfluorocyclopentenes here are given the scheme for the preparation of photochromic products containing the terminal thiol group 40 and 41 on the base of trimethylsilyl derivative 39 for the subsequent synthesis of gold and silver nanoparticles (04CL456, 06BCJ1413, 07CC1355) (Scheme 13). 

Unlike Scheme 15, which shows the synthesis of symmetrical products 45 and 46, Scheme 16 illustrates the approach to the synthesis of unsym-metrical photochromes 49 and 50 starting from mononitro derivative 47, prepared in 75% yield from 44 (in a mixture with dinitro derivative 48). 

Product 117 is a convenient starting compound for the subsequent modification of photochromes. Publication (09TL1614) gives an efficient synthetic route to both symmetrical 118 and unsymmetrical 119 phenyl-substituted dihetarylethenes bearing amino, hydroxy, or carboxy groups based on a Suzuki reaction of dichloride 117 with commercially available substituted boronic acids (or their pinacol esters) in a dimethyl ether (DME)-H20 mixture (4 1). For the symmetrical products, the yields are 85-95% for the unsymmetrical products, they are 60%. 

The chlorine atoms in the key starting 121 can be easily replaced by metal in the presence of BuLi at 20°C, which offers considerable promise in preparing various symmetrical and unsymmetrical derivatives. In solution, this product exhibits typical photochromic properties. However, an X-ray diffraction study of 121 showed that the thiophene rings in the molecule are parallel to each other, which is responsible for the absence of the photochromic properties in the crystalline state (05AC (E)o951). 

The transformations of 121 into various symmetrical and unsymmetrical perhydrocyclopentene-bridge photochromes, for example, 122-127, were documented (98CC2313,03EJOC155, 04CSR85, 07CC1698). 

A series of symmetrical and unsymmetrical photochromic 3,4-dihe-taryl-2,5-dihydropyrroles 147-150 containing the furan, thiophene, oxa-zole, or indole rings were synthesized as described above in 60-80% yields starting from p-anisidine (05JOC5001). 

Symmetrical and unsymmetrical thenoines, diketones, and chloro ketones 213-225 containing substituted thienyl, benzothiophene, and other moieties are versatile precursors of structures containing various heterocycles as bridges. Chloro ketones were used, for example, in the synthesis of photochromic thiazoles 226 (01IZV113) and tetrathiafulva-lenes 227 (99CL1071) (Scheme 65). 

Table 1.8 The absorption maxima of non-symmetrical maleic anhydride based photochromes...

Over the past few years, in terms of atom economy and extension the conjugated system of dithienylethene, we fused photochromic dithienylethene directly to the position of the pyrrole units of porphyrazines (tetra-azaporphyrines) or phthalocyanines, and developed a class of unsymmetrical and symmetrical phthalocyanine and porphyrazine hybrids containing 2,4, 6, and 8 thiophenyl groups (BTE-TAPs), as illustrated in Scheme 9 [37-39],... 

Applying one of the four synthetic approaches (a-d) (see Sections 6.2.1.1 to 6.2.1.4) in combination with the same or a second approach, can lead to biphotochromic systems.30-33 In principle, these are symmetrical systems such as DHI-DHI and unsymmetrical systems such as DHI-a-DHI-b DHI, photochromic systems-c are possible. 

A series of N-salicylidene aniline (SA) derivatives (Figure 10.10) have been investigated for their photochromic and NLO properties. For many years, scientists have known of the photochromism of some of these molecules in the crystalline state. Among them, those that crystallize in a nonGentro-symmetric space group were selected, and the NL[Pg.322]

Other examples in which theoretical calculations were applied in the study of photochromism include an MNDO/3 study on dihydroindolizines,15061 the use of PPP electron densities as an aid for estimating the linear dichromism (LD) and magnetic circular dichromism (MCD) of symmetric dialkoxyanthracenes,62 and studies of the structures of unusual twisted cis zwitterionic azomethines,63 a... 

A. Grabowska, K. Kownacki, andL. Kaczmarek, Proton transfer along the internal hydrogen bonds in excited Schiff bases. Photochromism in symmetric systems with two equivalent reaction sites, /. Luminescence 60/61, 886-890 (1994). 

Unlike the symmetrically substituted analogue phenl, the unsymmetrically substituted DTE containing 1,10-phenanthroline ligand pheu2 and the corresponding Ru-pheu2 exhibit no photochromic properties [75]. 

Naturally, the reactions of OFCP accompanied by the formation of C-C bonds are most similar to the methods of preparation of photochromes described in Sect. 3. The treatment of 1 with organolithium reagents gave the corresponding symmetrical disubstituted perfluorocyclopentenes B (Scheme 4) in good to high yields. 

The main method for the preparation of perfluorocyclopentene DHE used in the majority of studies is the reaction of the lithium derivatives of thiophene with octafluorocyclopentene [43 5, 41]. The photochromic products are thus assembled from the components indifferent to butyllithium or the molecule backbone is formed for further functionalization. This approach allows one to synthesize both symmetrical and unsymmetrical photochromes. 

Typical syntheses of the symmetrical products are demonstrated in Scheme 8. The treatment of bromides 29 and 31 with butyllithium, under standard conditions, in the presence of perfluorocyclopentene affords photochromes 30 [46] and 32 [44],... 

The product showed no photochromism in the crystalline phase as compared with similarly structured l,2-bis(2-methyl-6-nitro-l-benzothiophen-3-yl)perfluorocyclopen-tene 120 [ 131 ]. The unit cell of a crystal of 58A includes two symmetrically independent... 

Hermes S, DassaG, Toso G, Bianco A, BettaieUi C, ZerM G (2009) New fast synthesis route for symmetric and asymmetric phenyl substituted photochromic ditMenylethenes beMng functional groups such as alcohols, carboxylic adds, or amines. Tetrahedron Lett 50 1614—1617. doi 10.1016/j.tetlet.2009.01.102... 

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