Diarylethene photoswitches

Table 9.1 Magnetic interaction between two nitronyl nitroxide connected by diarylethene photoswitches.

Miklailov et have employed TDDFT to design two-photon operated molecular switches built from the diarylethene photoswitching unit and a two-photon absorbing pendant substituent. Both the one- and two-pho-ton absorption spectra were simulated and analyzed in order to understand... 

Matsuda K, Yamaguchi H, Sakano T, Ikeda M, Tanifuji N, Irie M (2008) Conductance photoswitching of diarylethene gold nanoparticle network induced by photochromic reaction. J Phys Chem C 112(43) 17005-17010... 

A new class of photochromic diarylethenes containing thieno[3,2-b] thiophenes, dithieno[3,2-b 2, 3 -d]thiophene (08CC5203), and dithieno [3,2-fc 2, 3 -d]pyrroles (09CEJ10005) with photoswitchable luminescence properties were synthesized by Suzuki cross-coupling reactions (Scheme 48). 

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. 

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. 

Fig. 5 Concept for photoswitchable ion tweezers possessing a diarylethene switching unit.

Fig. 7 Concept for photoswitchable saccharide tweezers possessing a diarylethene unit.

Fig. 9 Structure of sandwich cells for measurement of photoswitching of hole injection, (a) metal-diarylethene-Au, (b) ITO-diarylethene-organic hole transport layer.

Fig. 11 shows the photoswitching of the injection current. Upon UV irradiation, the hole injection current increased, while decreasing to zero on irradiation with visible light. Very thin amorphous diarylethene film as thin as 0.2 pm could also control the hole injection to the organic hole transport layer (Fig. 9b). These results are potentially applicable to optical memory-type organic photoconductors.

Fig. 11 Photoswitching ofthe injection current for diarylethene lSdispersed in polystyrene film (40wt%) under a constant electric field of 60 V/...

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... 

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. 

Diarylethene as a Photoswitching Unit of Intramolecular Magnetic Interaction... 

In the previous photoswitches, a radical unit is placed at each side of the diarylethene photo switching unit and separated by an extended it-conjugated chain. When the jr-conjugated chain length between the radical becomes longer, both photocyclization and cycloreversion reactivities are reduced. This is attributed to the reduced excitation density at the central diarylethene unit [69]. The excitation... 

Attempts to photoswitch the intramolecular magnetic interaction based on photo-chromism of diarylethenes have been overviewed. The switching between the disjoint and non-disjoint structures caused a change in the interaction between the separated spins. Photochromic diarylethene is one of the most favorable photoswitching units for magnetic interactions. This system has the possibility to be applied to the molecular-scale information processing system [73-77]. 

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