Furyl Fulgides

Compound 35 is pale yellow (A,max = 343 nm in toluene) 36 has an absorption minimum in the near-UV region and the absorption maximum wavelength (A,max) is 494 nm in toluene. The UV/VIS absorption spectra of 35 and 36 have been reported in Ref. 37). 

Substituent Effects on the Quantum Yield of Photochromic Reactions of 

Heller and Langan38 reported that the quantum yield for photocoloration ( bE c) of fulgide 35 to the 7,7a-DHBF (36) in toluene was 0.20, and the J E- c value appeared to be wavelength independent over the range 313-366 nm. Temperature (10-40°C) had little effect on the quantum yield for photocoloration. Furthermore, the cycles of photochromism of 35-36-35 did not affect the ( J E - c) value. These results showed that fulgide 35 is well suited for chemical actinometry in the near-UV and visible spectral region. 

A highly efficient photochromic process is essential for organic photochromic compounds used as optical recording materials.39,40 That means that the quantum yields for the photocoloring and bleaching reactions should be high, but the quantum yield for the side reactions should be as low as possible. In order to solve the problems mentioned earlier, extensive studies have been carried out, and some promising results have been obtained so far. 

Structure modifications of fulgide molecules play an important role in increasing the values of quantum yields of the photochromic reactions. These modifications 

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The furyl fulgide 172 has found use as a stable, recyclable actinometer for conventional photochemical experiments in the 313-366 mn wavelength range, where 173 = 0.20 and is independent of temperature and concentration245. It has also been developed as an actinometer in one- and two-laser flash photolysis experiments246. The colored form 173 can be converted back to 172 by simple exposure to visible light (equation 65). 

Steric Effects on the Photochromic Behavior of the Alkylidene Group of Furyl Fulgides... 

Table 4.8. Absorption Spectra Data and Quantum Yield of Furyl Fulgides 39 (E,Z) and its Colored Form (40) (C) with Bulky Alkylidene Substituents in Toluene...

Replacement of the 3-fuiyl group by a 3-thienyl group led to the formation of thienyl-substituted fulgides,58 which have photochromic properties similar to the furyl fulgides. Glaze et al.58 reported that the main difference is a slight bath-ochromic shift of the absorption band of the colored form, 7,7a-dihydrobenzothio-phene (7,7a-DHBT, 48). The photochemical reaction is shown in Scheme 13. 

Kurita et al. 2 Takeda et al. 3 and Parthenopoulos and Rentzepis11 used picosecond laser photolysis techniques to study the photochromic processes of furyl fulgide. They found that the excited states of furyl fulgide and its colored form were singlet states and had n,n characteristics. Takeda et al.13 reported from theoretical studies that the values of the oscillator strength and the radiation lifetime (/r) were... 

A typical representation is that for furyl fulgides, as shown in Scheme 30. The quantum yields of Eft Z photoisomerization are summarized in Table 4.26. 

Table 4.26. Quantum Yields of Ef>Z Photoisomerization of Furyl Fulgides 84 (Irradiated at 366 nm)...

Photochromic reactions of the heterocyclic fulgides are also in accordance with the Woodward-Hoffmann selection rules. The molecular structures have dramatic effects on the quantum yields of photoinduced ring-closure and ring-opening reactions of fulgides. The photochemical reactions of furyl fulgides are shown in Scheme 35 and the quantum yields are summarized in Table 4.29.44 45 104... 

Table 4.29. Quantum Yields of Ring-Closure and Ring-Opening Reactions of Furyl Fulgides 87...

The third possibility is to design photochromic materials in which the photochemical bleaching reaction of the colored form has an activity energy barrier that could act as a threshold. Picosecond laser photolysis studies on furyl fulgides12 demonstrated that the transformation from the excited state of the colored form to the S o potential surface of furyl fulgides has an activity energy barrier. 

Heller109 and Wintgens etai 110 studied furyl fulgides, for example, Aber-chrome-540 (35), as actinometers. The photoreversible reactions are shown in Scheme 7. 

Several inks111 112 have been developed for paints and labels of security masks. Furyl fulgide was spun into fibers and woven into a fabric that was used to manufacture photochromic labels. In 1995, the Color Change Corporation (U.S.) exhibited several different color photochromic fabrics as commercial products at the third international symposium on the chemistry of functional dyes. 

S. Kurita, A. Kashiwagi, Y. Kurita, H. Miyasaka, and N. Mataga, Picosecond laser photolysis studies on the photochromism of a furyl fulgide, Chem. Phys. Lett., 171, 553-557 (1990). 

J. Takeda, T. Tayu, S. Kurita, Y. Yokoyama, Y. Kurita, T. Kuga, and M. Matsuoka, Radiative and non-radiative decay processes of the excited state of the colored form of photochromic furyl fulgide,... 

Y. Yokoyama, T. Goto, T. Inoue, M. Yokoyama, and Y. Kurita, Fulgides as efficient photochromic compounds role of the substituent on fiiryl alkylidene moiety of furyl fulgides in the photoreaction,... 

M. Hawkins and A. G. Bowyer, Furyl fulgide as inks for some printing, Eur. Pat. Appl. EP279600 (1988). 

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.. 

For actinometric measurements, a 1cm cuvette containing a =5mM solution of the furyl fulgide (Aberchrome 540) in toluene is used, which completely absorbs UV radiation between 313 and 366 nm. The color change from yellow to red due to the photoinduced (conrotatory) ring closure is monitored at 494 nm, and the resulting linear increase in absorbance (AA ) with irradiation time (Af) is evaluated using Eq. (18) to calculate the intensity (l ) of the incident UV radiation, i.e.. 

Yokoyama Y, Hayata H, Ito H, Kurita Y. Photochromism of a furyl fulgide, 2-[l-(2,5-dimethyl-3-furyl)ethylidene]-3-isopropylidene succinic anhydride in solvents and polymer films. Bull Chem Soc Jpn 1990 63 1607-1610. 

Furyl fulgide 20 (Eq. (7)) is the first known example of thermally irreversible photochromic molecules. Although phenyl fulgides such as 21 have been known to show photochromic reactions, photogenerated dihydronaphthalene derivatives such as 21C (Eq. (8)) are thermally unstable and undergo irreversible side reactions, i.e., 1,5-hydrogen shifts to produce 1,2-dihydonaphthalene derivatives [23-26]. This... 

Upon UV irradiation the furyl fulgide undergoes not only cyclization but also an E-Z photoisomerization reaction (Scheme 6). The ratio between the cyclization and the E-Z isomerization depends on the steric hindrance of substituent R. Furyl fulgide derivatives with various R substituents were prepared and the quantum yields were compared [31-34]. The results are summarized in Table 4. 

Table 4. Quantum yields of furyl fulgide derivatives in toluene.

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