Dithienylethene system

A number of photochromic systems have been extensively investigated that undergo cis-trans isomerization (indigos, azo compounds) cleavage (spiropyrans), electrocyclic processes (fulgides, 1,2-diarylethenes) [8.229, 8.244, 8.245], For instance, cis-trans isomerization of a thio-indigo derivative allows the reading of pyrene excimer or monomer fluorescence [8.246]. The 1,2-dithienylethene system presents particularly attractive interconversion properties by photoreversible cyc-litation [8.245],... 

In principle any molecule able to exist in two reversible, switchable states can represents a molecular switch (bistable device) with potential to form part of molecular circuitry or act as molecular memory. An excellent component for switchable molecular devices is the 1,2-dithienylethene system, which has been exploited ingeniously by Lehn in a number of bistable systems.54 The core switching element is the transformation of the dithienylethene unit between two stable states as a function of the wavelength of incident radiation (Scheme 11.8). 

The Dithienylethene System and Related Compounds. The area of diaryl-ethenes has been the subjeet of a review.Calculations have been carried out dealing with the photochromism of such compounds.A description of the preparation of diarylethene nanoparticles has been published, and a decrease in photocyclization was attributed to aggregation. Some novel photochromic diarylethenes have been synthesized. The photochemistry exhibited by these compounds suggests that they could be of use as optical switches.Tian and Yang have reviewed the area of diarylethene photochromic switches. 1,2-Bis(2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopentene is at the basis of a new photochromic liquid crystal system. Yokoyama and his co-workers have patented details for such photochromic materials." ... 

The influence on the photochromism in dithienylethene systems of replacing the favoured perfluorocylopentene moiety by other cyclic ethene units has attracted the attention of a number of research groups. Dutch workers have shown that the use of the perhydrocyclopentene unit as in (91) has little effect on the photochromic behaviour of (92) but note that in situations where fatigue resistance and thermal stability are critical, the fluorinated moiety would be preferable. The cyclization quantum yield of (93) is reported to increase in the presence of metal ions, and more detailed accounts have been published of the interesting application of the dithienylcyclopentene-tethered jS-cyclodextrins (94) as photoswitchable hosts for meso tetrakis(4-sulfonatophenyl)porphyrin. ... 

Photochromism has also been observed when two porphyrinic groups are linked to a dithienylethene scaffold. The closed form showed an absorption band at 560 nm (01JA1784). The same behavior was observed in the system 318-319 (02AM918). Dithienylethene photochromic systems have also been described to be linked to single-walled nanotubes (07JA12590). 

Photochromic dithienylethenes have been considered as extremely promising systems to store information. The success of optical information processing requires the facile detection method which can read the stored information in a non-invasive manner. It is to say that the non-destructive readout capability is indispensable. On the basis of the ability to fine-tune the excitation wavelengths of porphyrins and the fact that they exhibit strong luminescence and attractive coordination properties, Branda covalently attached two porphyrins to the dithienylethene backbone, and synthesized a hybrid for non-destructive information processing [29], as shown in Scheme 1. It was the first example of photochromic porphyrinic dithienylethene. 

Illuminated alternately with UV and visible light, the porphyrinic dithienylethene underwent a reversible photoisomerization reaction (Scheme 1). Meanwhile, the fluorescence intensity of the porphyrin macrocycles could be regulated by alternate irradiation with UV (313nm) and visible light (longer than 480 nm). Moreover, the selective excitation band of the fluorophore was beyond the spectral band which could induce the photoisomerization reaction. Therefore, the authors asserted that this compound could act as a system for reversible data processing using fluorescence as the non-destructive detection method. 

In addition, the absorption spectra and fluorescence intensity of some porphyrins changed along with the photochromic reaction of the dithienylethene functional group, which was possible for constructing some optical chemical switching systems with emission signal. 

Recently, a ternary complex for a signal transmission system has been constructed by Aida and his co-workers [36], It involved three movable components a biaryl derivative bearing four zinc porphyrins (2), a pyridine-appended dithienylethene derivative (1), and a chiral tetrasubstituted ferrocene (3 ), as illustrated by its structure (Scheme 8). The three components were interconnected with bidentate coordination bonds. Component 2 was the bridge between 1 and 3. The well-designed triad was built up by multi-step reactions through routine methods. 

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],... 

Because the eight ferrocene moieties linking to tetraazaporphyrin magnesium moiety by ethynyl could extend the conjugation of the whole molecule system, both the B band and the Q band of TPF (Scheme 14) were redshifted in comparison with those of tetraazaporphyrin magnesium (II) with dithienylethene moieties aforementioned. 

As for the preparation method, further functionalizing of dithienylethene and assembling of multiple components into a single system often result in multi-step reactions and then decrease the overall yields greatly. Therefore, exploration of convenient and industrially available methods to synthesize these functional materials is required. 

In addition, most photochromic reactions are performed in solution. Generally, the practical uses require them to undergo photochromism in solid films or even at the level of single molecules. Therefore, it is desirable to develop new photochromic dithienylethene-phthalocyanines and their analog systems such as single crystals and polymers. 

Three major topics have dominated research activity on thiophenes since 1996 the design and synthesis of dithienylethene molecules for application as photochromic systems (Section 3.10.2.1.3) reactions brought about under transition metal catalysis (Section 3.10.2.11) and the synthesis, characterization, and reactivity of a plethora of transition metal complexes of thiophenes (Section 3.10.6). All three had received brief mention in CHEC-11(1996) (Sections 2.10.2.2.3, 2.10.4.7.3, and 2.10.6, respectively), but together account for almost one-third of the chapter now. In addition, shorter sections have been introduced to cover the following topics one-electron oxidation of thiophenes (Section 3.10.2.2) electrochemical reactions at cathodes (Section 3.10.2.7.5) sulfur-extrusion and sulfur-transfer reactions (Section 3.10.2.10) and reactivity of silicon-linked substituents (Section 3.10.4.5). 

A further innovation is to connect two (or more) dithienylethene units through another conjugating moiety attached to the thiophene units. Scheme 3 shows one such system in which the linker is a benzene ring... 

A dithienylethene-based liquid crystal material has been synthesized. This provides a morphologically stable, photoresponsive glassy nematic system in which the refractive index and optical birefringence can be modulated by photochemical means (Equation 7) <2004CSR85>. 

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