Reaction crystalline-state photochromism

Reaction Dynamics Studies on Crystalline-State Photochromism of Rhodium Dithionite Complexes 487... 

Crystalline-state photochromism usually proceeds with considerably lower interconversion ratios of less than 15% because the light penetration into the bulk crystal is prohibited by the absorption of the photo-generated isomer (inner-filter effects) [3,4]. The fully reversible crystalline-state photochromism of 1 can be partly attributed to its photochromic property. The rhodium dithionite complex 1 belongs to a unique class of photochromic compounds, which exhibits a unimolecular type T inverse photochromism [13]. The type T inverse photochromism means that the back reaction occurs thermally and the A.max of the absorption spectrum of 1 is longer than that of 2. If the back reaction occurs photochemically and the XmaK of the initial absorption spectrum is shorter than that of the photo-generated isomer, it is called type P positive photochromism and is known as a common photochromic system. 

In this chapter, crystalline-state photochromic dynamics of rhodium dithionite complexes are reviewed. The chemistries described here have been achieved not only by recent developments of the analytical technique but also by discovery of a new class of transition-metal based photochromic compounds. One of the advantages of transition-metal complexes is structural diversity. In order to find the rule of an exquisite combination of metal ions and ligands, we are currently synthesizing various dithionite derivatives with other metal ions and/or modified Cp ligands. As shown in this chapter, dithionite complexes are a very useful photochromic system to investigate crystalline-state reaction dynamics. We believe that dynamics studies of newly synthesized dithionite derivatives provide useful insight into the construction of sophisticated molecular switches. A dithionite complex may appear in a practical application field in the near future. 

H. Nakai and K. Isobe, Crystalline-State Photochromism of Rhodium Dinuclear Complexes Having a Dithionite Group and Its Reaction Dynamics , Nippon Kessho Gakkaishi, 2008, 50, 348. 

Photochromism Based on Redox Reactions. Although the exact mechanism of the reversible electron transfer is often not defined, several viologen salts (pyridinium ions) exhibit a photochromic response to uv radiation in the crystalline state or in a polar polymeric matrix, for example,... 

In a series of studies, substituents were introduced or varied at the 2-positions of the thiophene rings of di(benzothienyl)ethenes. The reactions of 3-bromo-2(n-aIkyl)-l-benzothiophene with BuLi and octafluorocyclo-pentene gave bis(2-n-alkyl-l -benzothiophen-3-yl)perfluorocyclo-pentenes 60 containing alkyl substituents with a different chain length at position 2 of the benzothiophene ring. The authors stated that the Et-, Pr-, and Bu-containing derivatives exhibit photochromic properties even in the crystalline state (06JPP(A)162). 

As concerns photochromes in a solid matrix, a question that immediately arises is to what extent the nature of the matrix impedes the photochromic reaction. This problem has been studied in detaih but it is beyond the scope of this review. There is a general rule that states photochromic reactions are sluggish in polymer matrices compared to fluid solutions. This statement is true for some stilbene derivatives, but it is not true for azo derivatives, especially for push-pull azobenzene derivatives like DRl, for which the trans->cis quantum yield equals 0.11 in PMMA at 20°C compared to 0.24 in a liquid hydrocarbon mixture at -110°C. Photochromism of spiropyrans shows an important matrix effect as the quantum yield for the conversion between the spiropyran and the photomerocyanin is equal to 0.8 in ethyl acetate and decreases to 0.102 in PMMA at room temperature. The same decrease is observed for the back photochemical reaction efficiency 0.6 in ethyl acetate, compared with 0.02 in PMMA at room temperature. Conversely, the matrix effect is much less for furylfulgides the quantum yields are almost the same in solutions as in polymer matrices. Although most of photochromic molecules exhibit photochromism in polymers and sol-gels, few of them exhibit this property in the crystalline state, due to topochemical reasons. However, some anils and dithienylethenes are known to be photochromic in the crystalline state. 

Liquid-crystalline phases with two-dimensional order (Table 1) influence photochromic processes because their molecular orientation is reversibly altered by photochromic reactions owing to the marked structural changes of the photoactive molecules.3 Intensive interest has been focused on the photoinduced alteration of mesophases admixed with photochromic compounds, since photochromism can be transformed to the reversible modification of the optical anisotropy of liquid-crystalline states. This phenomenon is applicable to optical storage of information as nondestructive readout can be performed with light of wavelengths far from those causing the photochromic reactions. 

The photochromism of 1 is achieved by atom rearrangement into the intramolecular oxygen-atom insertion product 2, in contrast to the frequently reported organic photochromic systems based on photo-induced cyclization, cis/trans isomerization, or H atom transfer [1]. The quantum yield of the photoreaction from 1 to 2 at 509 nm in acetonitrile without 02 is 0.14 0.01. In solution, photoreaction of 1 causes the oxidation reaction by atmospheric oxygen, resulting in a mixture of 2 and further oxidation products such as [(RhCp )2((i-CH2)2( i-S03)] and [(RhCp )2 ( r-CH2)2( t-S04)]. In contrast, in the crystalline state, the photochromic system between 1 and 2 is stable and repeatable with essentially 100% interconversion ratio. 

A detailed account of the (2 + 2)-photocycloaddition reactions of homo-quinones has been published. The addition reactions afford regio and exo.endo isomers of tricyclic diones. The major product is thought to arise from the more stable triplet 1,4-biradical intermediate.A review has reported on the addition of alkenes and alkynes to the enone system of homobenzoquinones. Dibromo-9-(3-oxo-3-phenylprop-l-ynyl)-[9 -fluoren-9-yl-3-fluoren-9-ylidene-1-phenylpropenone is photochromic in the crystalline state. 

The photoreactivity of DTE derivatives in the crystalline state is of special interest because of their potential usefulness for holographic and three-dimensional memories. In crystals, molecules are regularly oriented and packed in fixed conformations. In many cases, free rotation is inhibited. Therefore, the photoreactivity in the crystalline phase is dependent on the space for free rotation of the thienyl rings and the conformation formed in the crystal lattice. Several reports on the synthesis of metal complexes of photochromic diarylethenes and their photo-reactivity in the single-crystalline phase or the photoswitching of the coordination structure have been reported. These studies demonstrate that complexation to metal ions does not prohibit the photochromic reactions of the diarylethene units in the single-crystalline phase. 

Some selected examples of 1,2-diarylethenes were shown in this review. Miyasaka and Irie et al. revealed the three-photon cyclization and two-photon cycloreversion of diarylethens (223) by a near-infrared femtosecond laser pulse at 1.28 pm. They also found single-molecule fluorescence photoswitching of a diarylethene-perylenebisimide dyad (225)." " Photomechanical effects of two-component cocrystal (226) composed of 1,2-diarylethene derivative and perfluoronaphthalene or hydrogen-bonded diarylethene-imidazoline crystalline (227) were examined. A variety of photochromic reactions of 1,2-diarylethenes in crystalline states were reported by Iiie et al. ... 

As the geomeMcal structural changes are relatively small in the photochromic reactions, most of these reversible photoreactions could also proceed in the crystalline state. Thus, diarylethenes also exhibit crystalline state photochro-2.3 Benzopyran and naphthopyran (chromene) mism. Single crystals of the cocrystals with two or... 

Typical examples of this type include the cleavage reactions of octaphenyl-l,l -bipyrrolyl and of hexaphenyl-l,l -biimidazolyl, which on exposure to light yield the colored free radicals 49 and 50, respectively. These reactions form the basis of a photochromic system. Photochromism is observed both in the crystalline state and in solution. 

Situations where the absorption spectra of reactants and products are not conducive to high yield solid-state reactions may occur in classic photochromic systems. In a recent study on the crystalline photochromism of l,2-bis(2,4-dimethyl-3-thienyl)perfluorocyclopentane (55, Scheme 35) by Irie et al., it was... 

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