Photogenerated isomers

The photochromic chromophores can be classified into two categories, depending on the thermal stability of the photogenerated isomers. When photogenerated isomers are unstable and revert thermally to their initial isomer state in the dark, the chromophores are classified as T-type (thermally reversible type). Most photochromic chromophores belong to this type. The photogenerated blue color of 6-nitro-l, 3, 3 -trimethylspiro-[2H-l-benzopyran-2,2 -indoline], for example, disappears in less than half an hour even in high Tg polymer matrices.181 Such thermally unstable photochromic chromophores cannot be applied in photoswitchable molecular systems, because the switched states are unstable. For those applications, the characteristic of persistence, or in other words thermal irreversibility, is indispensable. 

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. 

When we intend to apply organic molecular materials, especially photochromic dyes, to optical memory media, the indispensable condition is stability, both thermal and photochemical. The photogenerated isomers are required never to return to the initial isomers in the dark, even at elevated temperatures, e.g., 80 °C. In addition, the coloration/decoloration can be cycled many times while the photochromic performance is maintained, and the memory media are provided with nondestructive readout capability. Although several molecules which fulfill the former condition have been developed, some problems still remain to gain access to molecules and systems which fully satisfy the latter condition. 

This theory has been borne out by countless experimental studies. As an example the ring closed photogenerated isomer of l,2-bis(2-cyano-l,5-dimethyl-4-pyrrolyl)perfluorocyclopentene has a half-life of 37 s at 25 °C, whereas its thiophene counterpart (l,2-bis(2-methyl-5-phenylthiophen-3-yl)perfluorocyclopentene) has a half-life of up to 1900 years at 30 °C. 

The absorption spectrum at 0° has a maximum at 600 nm. Upon rotation of the direction of the incident polarized light by as much as 90°, the absorption intensity decreases. The anisotropy of the absorption spectra reflects the regular orientation of the photogenerated closed-ring isomers and indicates that the photochromic reaction occurred in the single-crystalline phase. The blue color disappeared by irradiation with visible light a > 480 nm). Anthracene-substituted derivatives also showed photochromic properties (01JPC(A)1741). 

On the other hand, photogenerated closed-ring isomers of diarylethenes with pyrrole, indole, or phenyl rings, which have rather high aromatic stabilization energy, are thermally unstable.1221 The photogenerated, blue, closed-ring isomer of l,2-bis(2-cyano-l,5-dimethyl-4-pyrrolyl)perfluorocyclopentene 11a disappeared in 37 s (= t1/2 ) at 25 °C. 

The thermal instability of these closed-ring isomers is ascribed to the fact that the photogenerated central carbon-carbon bonds in the closed-ring isomers are weakened by the electron-withdrawing substituents. 

The red color disappeared in 20 h at 70 °C, while no such instability was observed for the closed-ring isomer 7b, with methyl groups at its 2-positions. It is considered that the bulky substituents at the reactive carbons also weaken the photogenerated central carbon-carbon bond. 

Upon irradiation of solutions of 21a and 22a with the light in their Amax (high-pressure Hg lamp, glass filters), reversible isomerizations occur to form the photoisomers 21b and 22b. The back reactions cannot be thermally driven and proceed only on irradiation of the solutions of photogenerated colored isomers with the light corresponding to their absorption maxima. Spectral characteristics of the isomers of compounds 21 and 22 are given in Table 8.7. 

The photochemistry of a series of cis- and rra/u-PdEt2L2 (L = PMeaPh, PEt2Ph, or PEta) complexes has been well studied by Yamamoto, Grubbs, and co-workers 143). The reactions were conducted at -10 and 2 C in the presence of PhC=CPh to trap the photogenerated PdLa complexes. In the absence of added tertiary phosphine, irradiation of both the cis and trans isomers gave a 2 2 1 ratio of ethylene, ethane, and butane [Eq. (116)]. No... 

Furthermore, the transient formation of cation radicals can be observed when a colloidal titanium dioxide suspension is flashed in the presence of an olefin [56]. The flash photolysis experiments also show that the surface influences the subsequent chemistry of the photogenerated intermediate. In fact, oxygenation and isomerization dominated the chemistry observed for tran -stilbene, with the same product distribution obtained upon starting with either the cis or the trans isomer (Eq. 3). 

The photochromism of azobenzene probe molecules tells us furthermore that bulk materials differ distinctively from silica films.59 The pores in the sol-gel bulk materials were larger than those in the corresponding sol-gel films. The first-order kinetics of the thermal isomerization of the photogenerated Z-isomer revealed that the steric effects restricting the molecular motion increased in the order sol-gel... 

Figure 23.4 ORTEP drawings of diarylethene 3 upon irradiation with ultraviolet light. The open-ring isomer and the photogenerated closed-ring isomer are shown in black and gray, respectively.

---