Photoswitchable systems

Perhaps the most remarkable photoswitchable system based on this concept is that shown in 11.55 (Scheme 11.9), which represents a system that is both photo- and electroswitchable. As with 11.53, the open BIS-OH form is electrochemically inert. Photoswitching to the closed form enables the molecule to be oxidised easily to the bis(quinone) BIS-Oq. The quinone form is photochemically inert and cannot revert back to an open form until re-reduced. The system is of interest in the context of information storage applications. It represents a chemical approach to the EDRAW (erase direct read after write) mode... 

In this system, however, intermolecular dimerization may take place competitively with intramolecular dimerization. To rule out this possibility, compound 5, in which two anthracenes are linked by two polyether chains, was synthesized.171 It was found that intramolecular photodimerization proceeds rapidly in the presence of Na+ as the template metal cation. Compound 6 was also synthesized.181 Although this compound has not been applied in a photoswitch system, it displays a remarkable fluorescence change upon binding with RbC104 or H3N+(CH2)7NHj.[81 Yama-shita et al.[9] also synthesized 7, in which intermolecular photodimerization of anthracene is completely suppressed. The photochemically produced cyclic form 8 displayed excellent Na+ selectivity. 

Few publications on the spectroscopic and isomerization properties of simple azo compounds have appeared in the last 15 years, as compared to the decades before. There is, however, one exception Ultrashort time-resolved spectroscopy of azobenzene and its relatives has opened new access to the dynamics following pico- and femtosecond excitation. The results are most relevant for the mechanisms of the photophysical and photochemical processes, which in azoaromatic compounds primarily are isomerizations. There is, however, a host of newer investigations into the isomerization of azobenzene and its family that are directed to applications in photoswitchable systems and devices. Some of them are relevant for the understanding of the parent molecules and therefore are included in this chapter. 

FIG. 7.1 Schematic representation of an amplified photoswitchable system, demonstrating some of the design principles of such devices. 

There is growing interest in the development of fluorescence-based molecular sensors and of photoswitchable systems. These photosystems involve the design of elaborate molecular receptors for binding adventitious guests in such a way that the complexation event causes a change in the photophysical properties of the host. 

Within other applications, polysoaps have been studied as protective coatings [489, 490], photoswitchable systems [205], and proposed for chemical muscles [75]. 

Association of another light sensitive system with the [Re(CO)3(N-N)]+ chromophore leads to photoswitchable systems or photoinduced electron transfer. ... 

In the search for reversible photoswitchable systems, dithienylethenes seem promising, as the change in geometry during the photochromic reaction IS small and can prevent disorientation. A PMNIA film doped with OFl (I0"o w Av) was poled at 120°C under a corona discharge (6 kV) and sub-nutted to a sequence of alternate irradiations in the visible (514 nm) and in the U V (325 nm) (see Figure 10.9). ... 

A photochromic dianthryl molecule can act as a triplet energy transfer quencher when combining with a ruthenium diimine complex [123]. UV excitation of the systems (X = CH2O, C(O)O) results in a photoinduced cycloaddition of the dianthryl unit, leading to an increase of the luminescence intensity. Near-field optical addressing of this luminescent photoswitcheable system (X = CH2O) as a dopant in PMMA films has been demonstrated [124] (Fig. 14). 

Several reports have taken advantage of this unique property to design a variety of photoswitching systems in recent years.Moore and co-workers reported one particularly interesting example. The complexes in their study are yhc-Re(CO)3(bpy) chromophores linked by a styryl pyridine that attaches an amine or an azacrown ether group (see Scheme 4 for structures). The absorption spectra of these complexes feature an intense intraligand charge-transfer (ILCT) band, which can be removed by protonation of the amine or azacrown... 

Light-gated catalysis based on photocaged and photoswitchable systems also fascinates chemists.The and (7 ,7 )-( )/(Z)-isomers of the... 

The photochemical reversible reaction ([2-t2] -photocycloaddition and the reverse reaction) of oriented chromophores in aggregates with alternate irradiation at two independent wavelengths has attracted much attention due to the potential appHcation to photomemory, to on-off photoswitching systems, and to its availability in controlling the release of entrapped reagents in vesicles. " If the reacting monomers are colored their photochromism may be used technically in various ways. 

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