Photochromic units

Ichimura et al. (51) have prepared silica particles showing photocontrol led reversible dispersibility in organic solvent. The silica particles incorporated spiropy-ran photochromic units, which showed reversible photoisomerization, were floccu-... 

The compound consists of two diastereomeric photobistable forms for instance, P (right-handed) and M (left-handed) helices, which can undergo photoisomerization at two different wavelengths A. and k2- Alternatively, a chiral auxiliary (X ) and a photochromic unit (A) (either chiral or achiral) may be present in systems A-X, with the auxiliary X controlling the change in chirality during the switching event. 

Photoresponsive polymers can be obtained by introducing photochromic units, such as azobenzene or spiropyran groups, into the macromolecules of polymeric compounds. As described in Chapter 1 of this book, photochromic compounds can exist in two different states, such as two isomeric structures that can be inter-converted by means of a light stimulus, and the relative concentrations of which depend on the wavelength of the incident light. For instance, in azobenzene compounds, photochromism is due to trans-cis photoisomerization around the N=N double bond, while in spiropyran compounds photochromism involves interconversion between the neutral spiro form and the zwitterionic merocyanine form (Figure 1). 

Figure 9 shows the effect of light on the absorption spectra of a poly(L-glutamate) containing 85 mol% photochromic units in the side chains. The spectrum of the... 

From this point of view, polypeptides containing photochromic units in the side chains are quite special polymers. They can exist in ordered or disordered conformations, and photoisomerization of their photochromic side chains can produce order = disorder conformational changes. These photostimulated structural variations, such as random coil a-helix, take place as highly cooperative transitions therefore photochromic polypeptides actually work as amplifiers and transducers of the primary photochemical events occurring in the photosensitive side chains. 

The combination of two photochromic units as in molecules 30, 49, 50 (Scheme 23) can result in very interesting materials. Irradiation of the mixed biphotochromic system 30 leads to a colored form absorbing in the infrared. Thus IR-sensitive materials become available. A detailed description of this new material is given in Ref. 34, 65. The charge, distribution in the colored ring open form can be schematically represented as shown in Scheme 26. 

Multicolor photochromism is useful for optoelectronic devices such as multifrequency optical memories and displays. Such systems can be obtained by mixing photochromic compounds with different colors. But an interesting possibility is to incorporate two photochromic units in the same molecule. This has now been realized <2003AGE3537, 2004CSR85> in the nonsymmetric molecule 18 in which both bis(2-thienyl)- and bis(3-thienyl)-ethenes are present, with one thiophene ring being common to both. From previous experience, it was known... 

In the EFISH experiment, a green light pump beam (wavelength 543.5 nm) from a helium-neon laser was used to initiate the photoisomerization of the photochromic units. This beam was linearly polarized either in s or p polarization tuning by a X/2 plate, and the intensity for both polarizations was about 8 mW/cm. The pump beam was incident on the sample at... 

Polymers with Covalently bound Photochromic Units... 

Synthetic polymers containing photochromic units can undergo reversible changes of their physical and chemical characteristics. (For recent reviews, see Refs. 24-26). Recent examples involving photochromic compounds other than spiropyrans are reported in Refs. 27-31. Spiropyran-containing polymers such as polyacrylates have also been prepared and were found to show photoinduced variations of their viscosity.18- 0 The change in the viscosity of the polymers partly reflects the polymer conformation. Thus, spiropyran-attached poly(L-tyro-sine) and poly(L-lysine) were synthesized by Vandewyer and Smets at the University of Louvain in 1970.32 33 No photoresponsiveness was observed however, for these modified peptides. 

The pyridine derivatives, the monodentate l-(2-methyl-5-phenyl-3-thienyl)-2-(2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene and the bidentate 1,2-bis (2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene, incorporating the DTE moiety as the photochromic unit (4-py-DTEf [50, 51] and 4-py2-DTEf [52]) and the nonfluorinated analog (4-py2-DTE) [53] have been coordinated to various transition metals, allowing the change of physical properties of metal complexes. 

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