Photoswitching devices

Most switching devices studied make use of light for interconverting a molecule between two different states. Both forward and backward processes may be photo-induced or one of them, usually the reverse transformation, may be thermal. 

A number of photochromic systems have been extensively investigated that undergo cis-trans isomerization (indigos, azo compounds) cleavage (spiropyrans), electrocyclic processes (fulgides, 1,2-diarylethenes) [8.229, 8.244, 8.245], For instance, cis-trans isomerization of a thio-indigo derivative allows the reading of pyrene excimer or monomer fluorescence [8.246]. The 1,2-dithienylethene system presents particularly attractive interconversion properties by photoreversible cyc-litation [8.245], 

Electro-photoswitching devices possess a dual mode operation resulting from different electrochemical properties in the two photo-interconvertible states. They may display both photochromism and electrochromism. They allow the photomodulation of electrochemical properties as was shown to occur in photochromic dihydroazulene derivatives [8.247]. Separate photochemical and electrochemical interconversion has been realized in systems containing a photoactive azo [8.248, 8.249] or dihydroazulene [8.250] unit appended with an anthraquinone moiety. 

Similarly, 12 lb possesses markedly enhanced optical non-linearity compared with 121a, thus displaying a photoswitching of NLO properties. 

Photochemical switching of the electrical properties of conductive Langmuir-Blodgett films results from photoinduced conformational changes produced by an azobenzene unit [8.253]. 

---

The occurrence of two different structures that can be interconverted through the agency of an external light stimulus can be the basis of a molecular switch. Moreover, when photochromic molecules are incorporated into polymeric compounds, their photoisomerization can affect the structure and the physical properties of the attached macromolecules. Therefore, photochromic polymers may be highly promising materials for application in optical technologies, as well as in the design of photoswitchable devices. 

It should be said, however, that the initial light signal associated with the photoisomerization of the photochromic moiety is usually a weak effect, and requires amplification in order to construct photoswitchable devices. The greater the amplification factor, the greater is the sensitivity of the system. Substantial amplification can be achieved when the primary photochemical reaction is coupled with a subsequent event that occurs after absorption of light. 

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... 

Ushakov, E.N., Gromov, S.P., Buevich, A., Baskin, I.I., Fedorova, O.A., Vedernikov, A.I., Alfimov, M.V., Eliasson, B., Edlund, U. (1999) Crown-containing styryl dyes cation-induced self-assembly of multiphotochromic 15-crown-5 ethers into photoswitchable molecular devices, J. Chem. Soc., Perkin Trans. 2, 601-607. 

Such thermally irreversible photochromic chromophores represent the other class, classified as P-type (photochemically reversible type). Although many photochromic compounds have been so far reported, P-type chromophores are very rare. Only two families, furylfulgide derivatives and diarylethene derivatives, exhibit this reactivity.19 101 The photogenerated isomers of these derivatives are thermally stable and never revert to their initial isomers even at elevated temperatures (-100 °C). The thermally stable photochromic compounds offer potential for various applications in photoswitching and memory devices. 

Redox enzymes are the active component in many electrochemical enzyme electrode biosensor devices.1821 The integration of two different redox enzymes with an electrode support, in which one of the biocatalysts is photoswitchable between ON and OFF states, can establish a composite multisensor array. The biomaterial interface that includes the photoswitchable enzyme in the OFF state electrochemi-cally transduces the sensing event of the substrate corresponding to the nonphoto-switchable enzyme. Photochemical activation of the light-active enzyme leads to the full electrochemical response, corresponding to the analysis of the substrates of the two enzymes. As a result, the processing of the signals transduced by the composite biomaterial interface in the presence of the two substrates permits the assay of the... 

Figure 17.9 Schematic representation of the surface-bound photoswitchable rotaxane 10.104 Such a device is capable of transducing an optical signal into an electronic signal by means of the photocontrolled ring shuttling in the rotaxane molecules. (Adapted with permission from V. Balzani et al., ChemPhysChem 2008, 9, 202-220. Copyright Wiley-VCH Verlag GmbH Co. KGaA.)...

G. M. Tsivgolulis, and J.-M. Lehn, Photonic molecular devices reversibly photoswitchable fluorophores for nondestructive readout for optical memory, J Chem. Soc. Chem. Commun. 1995, 1119-1122. 

Abraham, Wlosnewski, Buck, and Jacob [95] invented a new type of photoswitchable rotaxane based on the above principles where the diaryl-methoxy-cycloheptatriene unit is replaced by 9-aryl-9-methoxy-acridanes (Fig. 9b4) that undergo photoheterolysis (313 nm light) with formation of acridinium ions (Fig. 9b5). Duo, Jacob, and Abraham [96] demonstrated that such devices can be deposited and can operate on gold nanoparticles. 

The biaryls are useful in rational designing functional molecules and materials. The large steric hindrance and the semirigid structure with restrict rotation provided various functional biaryls, such as arylporphyrins [162,170], molecular-scale motors rotate by chemical power or light [73,163], a photoswitchable electron transfer aromatic compounds for the design of molecular photonic devices [171,172],a stable thioaminyl radicals [173],phenylnitroxide-substitut-ed Zn(II) porphyrins [174], and polycyclic aromatic compounds [175-177]. 

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. 

Willner, L, and Willner, B. Photoswitchable biomaterials as grounds for optobioclcctronic devices. Bioelectrochem. Bioenerg. 1997, 42, 43-57. 

---