Fulgides, photocyclization

Synthesis of the dibenzofuran (27) by irradiation of grisa-3, 5 -diene-2, 3 -dione (28) is believed to involve electrocyclic ring opening followed by intramolecular cycloaddition to the ketene and elimination of carbon dioxide, as shown in Scheme 2.24 Analogous photocyclizations are responsible for the photochromism exhibited by heterocyclic fulgides such as ( )-a-3-furyl-ethy idene(isopropylidene)succinic anhydride (29), which on irradiation... 

Since a fulgide, its E, Z isomers, and photocyclized form exhibit different absorption spectra, electronic absorption spectra are often used to investigate the photoreaction of fulgides. The UV absorption spectra data of some aryl lulgides are listed in Table 4.1. 

Glaze el al. reported that fulgide 35 is a highly effective photochromic compound. But when the R is hydrogen atom 37a, no photocyclization reaction occurs,41 and only E —> Z photoisomerization takes place when 37a is irradiated by UV light, as shown in Scheme 8. 

The effects of the substituent steric hindrance of the R group in fulgide 37b-37f on the quantum yield for the photoreactions have been reported by Yokoyama et al.42-44 and Kiji et al 45 These authors demonstrated that steric hindrance has an important effect on the quantum yield of the photocyclization (0E c) and the E — Z isomerization ( E, Z). The results are shown in Table 4.7. 

The X-ray structure of two isomers of fulgide 35 have been determined by Yoshioka et al.56 In the E form, the succinic anhydride and the furyl ring were not coplanar and were considerably twisted relative to each other. This is due to the steric hindrance effect between the isopropylidene group and the 2-methyl group in the furyl moiety. The distance between the two reactive sites, which corresponds to the chemical bond formed during photocyclization, was 3.44 A. 

Table 4.21. Quantum Yield of Photocyclization Reaction of Fulgides and Fulgimides in Solution and Polymer Matrix51 91...

Another dicyanomethylene derivative of fulgide 81 was also reported. On irradiation with white light, the purple compound 81 underwent an intramolecular [4 + 4] photocyclization reaction to give colorless adducts, which underwent the reverse reaction thermally or photochemically.35 The results demonstrated that molecular modification of fulgide molecules can significantly affect the photochromic properties of fulgide family compounds. 

Indolyl fulgenate is a new kind of photochromic compound. The EvaZ photoisomerization is also involved, as shown in Scheme 32. The quantum yields of EvaZ photoisomerization are summarized in Table 4.28. In general, the quantum yields of E va Z photoisomerization of fulgide and its derivatives are much lower than those of photocyclization (see Section 4.5.3). 

Scheme 38. The mechanism of photocyclization of pyrryl fulgides. E = E-form C = C-form I = intermediate BP = by-product.

This type of photocyclization is also found in the unimolecular open form/ closed form isomerism of photochromic systems [70] as, for example, spiropyrans and spirodihydroindolazines, fulgides. The chirality in such photochromic reactions will be covered by Chap. 9 of this book an example is 47 in Scheme 9. 

The Aberchrome 540 actinometer (21) is based on the reversible photocyclization of the pale yellow furyl fulgide, I in Eq. (17), to its deep-red cyclized isomer (II), i.e.. 

Fulgides and related diarylethenes have been investigated extensively because of the long-term thermal stability of their photocyclized colored forms, which could lead to this application in erasable optical recording materials and photoswitchable optical elements. The entrapment of these photochromic molecules in polymer films is necessary for these practical applications. Picosecond laser photolysis was employed to study the electrocyclic reaction of a furylfulgide (18, Figure 8) in polymer solids and revealed that the colored structure was formed with a time constant of ca. lOps irrespective of the nature of the polymer matrix.49... 

Photochromism of fulgides is a molecular phenomenon based on the photocyclization of substituted bis-methylenesuccinic anhydrides or fulgides (328) to l,8 -dihydronaphthalene (l,8a-DHN, 329)2 (Scheme 33). 

In many cases, the CTI process competes with rearrangements and cyclizations that occur during the radiationless transition. Z-Stilbene is well known to give dihydrophenanthrene as a photoproduct along with F-stilbene (Fig. 2.4A). Reversible photocyclization is even the dominant reaction in fulgide [12] and merocya-nine [13] systems [3] (Fig. 2.4B,C). 

Fulgides (e.g., 53, see Scheme 15) are capable of photochromic reactions. Yokoyama et al. reported the preparation and subsequent photochromic isomerization of a fulgide which was designed to resist enantiotopomerization [103]. A sample of the resolved fulgide was irradiated (405 nm light in toluene solution) and was observed to come to a pss with a ratio of 19 81. Irradiation with visible light (X > 580 nm) led to the complete recovery of the initial conformation. An advance on this system was made by the same authors, who described the process of diastereomeric photochromism, in which a fulgide derivatized with a binaphthyl auxiliary was allowed to thermally equilibrate, and a photocyclization process carried out [104]. As a result of the relative populations of photoreactive... 

Fulgide derivatives have been widely used as photocontrol-lable units for the design of supramolecular switches. The photochromism of these components occurs between one of the colorless open form and the photocyclized colored form. Fulgides are not luminescent, at room temperature, unless... 

The pale yellow fulgide ( )-2-[l-(2,5-dimethyl-3-furyl)-ethyhdene]-3-isopropyl-id-ene succinic anhydride (hereafter A(E)), whose commercial name was Aberchrome 540, is considered a convenient actinometer in the near UV and visible region, because of its reversible photocyclization into the deep red cyclic valence isomer 7,7a-dihydro-2,4,7,7,7a-pentamethylbenzo(b)furan-5,6-dicarboxylic anhydride (hereafter C) [21, 22]. The quantum yield of this photocyclization (0.2) is independent of the temperature ranging from 283 to 313 K and it is also claimed to... 

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