Photoswitching reversed

Figure 5.40 Cyclic voltammetry of (a) 3,4-hyd roxyphenylacetic acid, and (b) dopamine at pho-toswitchable nitrospiropyran electrodes the insets show the corresponding photoswitched reversible induced currents monitored at 470 mV. Reprinted with permission from A. N. Shipway and I. Willner, Acc. Chem. Res., 34,6 (2001). Copyright (2001) American Chemical Society...

Olsen S, Lamothe K, MartiAAnez TJ (2010) Protonic gating of excited-state twisting and charge localization in GFP chromophores a mechanistic hypothesis for reversible photoswitching. J Am Chem Soc 132 1192-1193... 

Andresen M, Wahl MC, Stiel AC, Grater F, Schafer LV, Trowitzsch S, Weber G, Eggeling C, Gmbmuller H, Hell SW, Jakobs S (2005) Structure and mechanism of the reversible photoswitch of a fluorescent protein. Proc Natl Acad Sci USA 102 13070-13074... 

Bizzarri R, Serresi M, Cardarelli F, Abbruzzetti S, Campanini B, Viappiani C, Beltram F (2009) Single amino acid replacement makes Aequorea victoria fluorescent proteins reversibly photoswitchable. J Am Chem Soc 132 85-95... 

Hofmann M, Eggeling C, Jakobs S, Hell SW (2005) Breaking the diffraction barrier in fluorescence microscopy at low light intensities by using reversibly photoswitchable proteins. Proc Natl Acad Sci USA 102 17565-17569... 

Habuchi, S., Ando, R., Dedecker, P., Verheijen, W., Mizuno, H., Miyawaki, A. and Hofkens, J. (2005). Reversible single-molecule photoswitching in the GFP-like fluorescent protein Dronpa. Proc. Natl. Acad. Sci. USA 102, 9511-6. 

Vogtle and co-workers first reported a photoswitchable dendrimer [33] with six peripheral azobenzene groups, which took advantage of the efficient and fully reversible photoisomerization reaction of azobenzene-type compounds (Scheme 7). In a follow-up study [34], polypropylene imine) dendrimers bearing azobenzene moieties (p-Im-Gn, n = 1-4) on the periphery were synthesized. These dendrimers displayed similar photoisomerization properties as the azobenzene monomers. Irradiation of the all-E azobenzene dendrimers at 313 nm led to the Z-form dendrimers, while irradiation at 254 nm or heating could convert the Z-form dendrimers back to the E-form dendrimers. The observation that the... 

McGrath and Junge [36] reported a photoresponsive poly(aryl ether) dendrimer with azobenzene as the dendrimer core. These dendrimers exhibited reversible trans to cis photoisomerization by irradiation at 350 nm. The authors proposed the use of this type of dendrimer as novel photoswitchable transport vectors. This is based on the expected ability of dendrimers to encapsulate or eject small molecules reversibly upon light perturbation. 

The quinone-hydroquinone redox couple built into complexes 124 and 125 fulfils the requirements for the design of a bistable electro-photoswitch both the oxidized and reduced forms are isolable and stable the reduced form 125 is luminescent, whereas the oxidized form 124 is quenched the electrochemical interconversion of the two species is reversible [8.256]. 

Reversible photoisomerisation [23] numbers among the readiest reactions in photochemistry. The presence of a photoswitchable moiety is a prerequisite for this reaction. Since photoisomerisation commands comparable interest in physics and chemistry - as well as biology - it will be considered here at the conclusion of the photophysical chapter and before the beginning of the following chapter on chemical reactions. 

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. 

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. 

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

Scheme 8 Assembly of a photoisomerizable glucose oxidase monolayer electrode and the reversible photoswitchable activa-tion/deactivation of the bioelectrocatalytic functions of the enzyme electrode.

Scheme 10 Reversible photoswitchable activation/deactivation of the electrical contact between cytochrome c and the electrode and the secondary activation/deactivation of the COx-biocata-lyzed reduction of oxygen using a thiolated nitrospiropyran and thiolated pyridine mixed monolayer as a command interface.

Table 2 summarizes different possible applications of photoswitchable biomaterials, while detailing the nature of the biomaterial, the area of application, and, when possible, specific examples. Reversible light-induced activation and deactivation of redox proteins (enzymes) corresponds to write - read - erase functions. The photonic activation of the biomaterial corresponds to the write function, whereas the amperometric transduction of the recorded optical information represents the read function of the systems. Switching off of the redox functions of the proteins erases the stored photonic information and regenerates the photosensory biomaterial. These integrated, photoswitchable redox enzyme electrode assemblies mimic logic functions of computers, and may be considered as first step into the era of biocomputers. 

Photoswitchable antigen/antibody (substrate/ receptor) complexes 1. Reversible immunosensors 2. Patterning of surfaces with biomaterials using antigen/antibody-biomaterial conjugates (Design of biosensor arrays, biochips) 1. Immobilization of systems on electronic transducers (electrodes, piezoelectric crystals, FET) or the assembly of biomaterials on inert supports by non-covalent interactions (eg. glass, polymers)... 

Diarylethenes are promising synthetic photoswitchable molecules because of their outstanding fatigue-resistance in reversible light-induced transformation between two isomers with different absorption spectra.21,22 The tt-conjugation extends over the entire molecular in the closed form, whereas it is restricted to each half of the molecule in the open form. As a consequence, the closed form is expected to exhibit better electrical conductance than the open form. The two forms are referred to as the on and off states of the switch, respectively. The UV-vis spectra of this molecular switch in toluene show that the wavelengths that can be used for on to off switching are 420 < A< 650 nm, and 300 < A< 350 nm for the reverse operation. 

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