Photoswitching molecular systems

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. 

Properties which change concomitantly with diarylethene derivative photoisomerization are geometrical structures, electronic structures, refractive indices, and chiral properties (when the molecules have chiral substituents). Table 1 shows how the above property changes are applied to various photoswitching molecular systems. Details of these photoswitching functions are described in Sections 2.3 to 2.6. 

The following section describes various photoswitching molecular systems employing diarylethenes as the switching units. 

Irie M. Photoswitchable molecular systems based on diarylethenes In Feringa BL, Ed. Molecular Switches. Weinheim Wiley-VCH, 2001 37-62. 

Irie, M. (2001) Photoswitchable molecular systems based on diarylethenes, in Molecular Switchings (ed. B.L. Feringa),... 

This review surveys our studies devoted to the photoswitchable molecular receptors based on photochromic crown ethers. Photochromic systems described in the review may be classified into three groups according to the reaction types E,Z-isomerization, [2+2]-photocycloaddition reactions and electrocyclic reaction. It has proved the groups to be an especially suitable basis for photochromic systems, and promising for the industrial applications. 

Matsuda, K. and Irie, M. (2002) Photoswitching of intermolecular magnetic interaction using photochromic compounds, in Chemistry of Nano-molecular Systems-Toward the Realization of Molecular Devices (eds... 

The rotary motion of the molecular motors has been controlled by novel methods. The molecule (26) possesses two stable TEMPO radicals which act as a highly efficient photoswitchable magnetic system. In the ( )-form, the TEMPO units were kept apart, whereas, in the (Z)-form, the two TEMPO... 

As just mentioned, phosphorus porphyrins have unique photochemical properties. Their photophysics is also interesting. Emitter-quencher assemblies based on porphyrin building blocks have attracted attention due to their potential to serve as models in photosynthetic research (see [90] for an example) or for the development of photoswitches that could be used for the fabrication of molecular electronic/optical devices. In this context, Maiya and coworkers constructed a P(VI) porphyrin system 59b with two switchable azobenzene groups positioned in the apical positions of the pseudo-octahedral phosphorus atom [92]. Photoswitch ability (luminescence on/off) was demonstrated as... 

Photoresponsive systems are seen ubiquitously in nature, and light is intimately associated with the subsequent life processes. In these systems, a photoantenna to capture a photon is neatly combined with a functional group to mediate some subsequent events. Important is the fact that these events are frequently linked with photoinduced structural changes in the photoantennae. This suggests that chemical substances that exhibit photoinduced structural changes may serve as potential candidates for the photoantennae. To date, such photochemical reactions as E/Z isomerism of azobenzenes, dimerization of anthracenes, spiropyran-merocyanine interconversion, and others have been exploited in practical photoantennae. It may be expected that if one of these photoantennae were adroitly combined with a crown ether, it would then be possible to control many crown ether family physical and chemical functions by means of an ON/OFF photoswitch. This is the basic concept underlying the designing of photoresponsive crown ethers. We believe that this is one of the earliest examples of molecular machines . 

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