Gated Photoisomerization

A highly desirable property in information storage systems based on molecular switches is gated response.1491 Gated photochemical reactivity implies that no change occurs upon irradiation unless another external stimulus, either physical or chemical, is applied to the system. Scheme 18 shows a typical write-lock-unlock-erase cycle involving photoisomerization and protonation. 

A major advantage is the potential to lock (and protect) written information in the photobistable material. A number of chemical gated systems involving mutual regulation of the photochromic event and, for instance, fluorescence, ion binding, or electrochemical properties have been reported.1501 Scheme 19 illustrates a chiral gated response system based on donor-acceptor substituted alkene 17.[511 The photochemical isomerization process of both the M-ds and the P-trans form was effectively blocked by the addition of trifluoroacetic acid. Protonation of the dimethyl-amine donor unit of M-rfs-17a and P-trons-17b resulted in an ineffective acceptor-acceptor (nitro and ammonium) substituted thioxanthene lower half. Since the stereoselective photoisomerization of 17 relies on the presence of both a donor and acceptor unit, photochemical switching could be restored by deprotonation by the addition of triethylamine. 

Spiropyran/merocyanine photoisomerization was also used at the alkylthiol SAM-modified gold electrodes to gate/ungate electron transfer [130-133]. WiUner etal. constructed a spiropyran/merocyanine-... 

Multi-mode molecular switching properties and functions of azo-conju-gated terpyridine complexes of transition metals have been studied [47-49]. The dependences of the photoisomerization behavior on the metal center and its oxidation state were investigated using azobenzene-attached terpyridine complexes of four metals Fe, Co, Ru, and Rh. As for the Fe and Ru complexes, 60-62, photoisomerization is totally inhibited when UV light irradiation excites the n -n band of the azobenzene moiety in the trans form due to the occurrence of energy transfer from the azobenzene unit to the complex unit [50-53]. 

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