Diarylethenes temperature

Such thermally irreversible photochromic chromophores represent the other class, classified as P-type (photochemically reversible type). Although many photochromic compounds have been so far reported, P-type chromophores are very rare. Only two families, furylfulgide derivatives and diarylethene derivatives, exhibit this reactivity.19 101 The photogenerated isomers of these derivatives are thermally stable and never revert to their initial isomers even at elevated temperatures (-100 °C). The thermally stable photochromic compounds offer potential for various applications in photoswitching and memory devices. 

The blue color was disappeared by irradiation with visible (X > 500 nm) light. In the dark, however, the blue color remained stable and at room temperature never reverted to the colorless form. In toluene, the colored isomer was found to be stable even at 100 °C. The stable, colored isomer was isolated by HPLC and its molecular structure was analyzed by NMR and X-ray crystallography. Both indicated that the blue colored isomer was the closed-ring form. Therefore, the photochromism of the diarylethene derivative was ascribed to the following photocydization and cycloreversion reactions. 

With conventional protocols requiring low reaction temperatures, typically —78 °C, to prevent side reactions from occurring, scaling the reaction for industrial production of such compounds has proved difficult. As such, the authors evaluated the process under continuous flow, proposing that the effective temperature control and accurate residence times attainable within miniaturized flow reactors would enable the synthesis of diarylethenes at temperatures above — 78 °C and thus facilitate the large-scale synthesis of such compounds. 

TABLE 17.1 Glass Transition Temperatures of Amorphous Diarylethenes... 

FIG. 17.4 DSC profile of diarylethene 2 at second and subsequent heating scans. The temperature scan rate is IO°C/min.The sample weight is 9.08 mg. 

State was attained. After UV-light irradiation, about 80% of the absorbance at 580 nm was recovered. The conversion of 80% from 2a to 2b was almost the same as that in the solution phase. The conversion of the film prepared from 2a solution was 40%, which is half of the conversion of the film prepared from a solution of the closed-ring form isomer. This difference in the maximum conversion to 2b is caused by the conformation of the open-ring form isomers. The isomer has two conformations, anti-parallel and parallel conformations. The former is photoactive whereas the latter is inactive, and half of the open-ring form isomers are in the inactive parallel conformations in solution. Half of 2a molecules in the film prepared from 2a solution are in the inactive parallel conformation. The conformational change is difficult in the amorphous film below Tg. In contrast, 2a in the bleached film prepared from the solution of 2b is in an anti-parallel conformation, and the maximum conversion to 2b at the photostationary state is about two times larger than that in the film prepared from 2a solution. A similar increase in the conversions in the film prepared from the closed-form isomer has been observed in amorphous diarylethenes, 3-10. It should be noted that heat treatment of the bleached 7a film at a temperature above Tg resulted in a decrease in the maximum conversion, which indicates that conformational change takes place at temperatures above Tg. 

As described above, the diarylethene synthesis should be carried out at low temperatures, such as —78 °C, in a macrobatch reactor to avoid any undesirable side reactions. However, the requirement of such low temperatures has been an obstacle to industrial-scale applications. For example, the reaction shown in Scheme 10.3 needs to be carried out at —70 °C and the optimized yield on an industrial scale is 55%. Carrying out the reaction at 0°C leads to the formation of a complex mixture. 

The optimized yield is 81 % at 0 °C and the total residence time is ca. 6 s. Therefore, the synthesis of diarylethenes can be accomplished at 0°C, which is easily attained in industry, by virtue of an effective temperature... 

Figure 10.3 Effect of temperature on the synthesis of diarylethene in a microflow...

The first example of diastereoselective photochromism of a diarylethene was reported by Me s group for the bisbenzothienylethene D-1 possessing a-(/)-or ( /)-menthyloxy substitutent on C-2 of one of the two benzothiophene rings [32]. The diastereoselectivity was dependent on solvent polarity and temperature... 

As revealed in the late 1980s, photochromic dihetarylethenes (DHE) I (often named in hterature as diarylethenes) have a unique property, namely, in the absence of photoirradiation, their initial lA and cycUzed IB forms are stable, in the most part of cases, up to decomposition temperatures [1 ]. 

When conformational homogeneity is not ensured in a cis-diarylethene, the photoisomerization pathways could be rather complex. For example, for l,2-P,P -dinaphthylethene (10), the cis isomer should exist in three possible conformations (ds-lOa, ds-lOb, cis-lOc), as shown below. HT of these conformers should lead to three conformers of the trans products (traws-lOa, -10b, and -10c) (solid lines) with comparable internal steric strain. The same conformer products are also expected from OBF processes (dashed line), albeit not from the same starting conformers as in HT processes. While it is clear that the product ratio is dependent on a number of factors, this ambiguity renders such systems uninformative for elucidation of the exact nature of their photoisomerization pathways. On the other hand, it should be clear that photoisomerization, e.g., by HT, is not hkely to produce the same equilibrated mixture of conformers of the trans isomer as in the equilibrated mixture, i.e., obtained fi"om irradiation at room temperature. Thus, the claim of possible preparation of a nonequilibrated trans isomer in organic glass is also consistent with, but not a proof for, involvement of HT. 

Other ds-diarylethene systems reported to give nonequilibrated trans products at low temperature in organic glasses were l-naphthyl-2-phenylethenes, l-a-naphthyl-2-P-naphthylethene and 1-P-naph-thyl-3-phenanthrylethene. But, none of these cis reactants is likely to be conformationally homogeneous. 

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