It is well known that azobenzene exhibits photochemical trans-cis and cis-trans isomerizations upon irradiation of UV and visible light, respectively. Usually the cis isomer is thermally less stable, so cis-trans thermal isomerization takes place in the dark. Utilizing these photo- and thermal properties of azobenzene,numerous photofunctional molecules and photoswithing devices had been designed. Controlling the photo- and thermal reactivity of azobenzene is a prerequisite to optimize the performance of these photofunctional devices. Especially, controlling the volatility of the molecular information generated by light irradiation due to thermal instability of cis-azobenzene is of particular interest and is crucial to many proposed practical applications. In literature, some reports can be found that cis isomers of azobenzenes are thermodynamically stable but this is achieved only by compromising its photoresponsive properties. Fujita et al. showed that thermal and photochemical cis-trans isomerizations can be suppressed by incorporating cis-azobenzene into a supramolecular host molecule

Fig. 1 Molecular formula (a) and crystal structure (b–d) viewed from three different directions of the most thermodynamically stable isomer (cis–cis) of 1.z Note that both NQN bonds are in cis configuration. Yasuo Norikane, Ryuzi Katohb and Nobuyuki Tamaoki

UV-Vis absorption spectra of 1 upon irradiation with an Hg lamp (436 nm, 20 mW/cm2, 20 min) or with a laser (430 nm, 10 Hz) (a),y difference absorption spectrum between before and after the laser irradiation (b), and absorption changes observed at 314 nm after alternating irradiations at 436 and 365 nm over 10 complete cycles (c).

First report of thermodynamically stable cis isomer of an azobenzene derivative that exhibits unusual thermal trans-cis isomerization and photochemically reversible isomerization.

The photochromic properties of AB are, however, far from ideal, because the S1(nπ*) electronic absorption bands of its (E) and (Z) isomers peak at practically the same wavelength (λ ) 450-440 nm) and differ essentially only in their oscillator strengths. Switching AB forth and back therefore requires cycling between irradiation in the S1(nπ*) and S2(ππ*) bands, although the latter is located in the less easily accessible UV region, where other molecular moieties attached to the AB unit may suffer photodamage.

The single crystal X-ray diffraction structure confirms the calculated slightly disturbed (Z) conformation of the central CNNC moiety, which is almost coplanar with a CNNC dihedral angle of only 6.4° and NNC angles of 121° just slightly larger than the ideal sp2 angle. The ethylenic bridge shows some conformational flexibility. Due to the ring strain, 1Z and not 1E is the thermodynamically stable form at room temperature, in complete contrast to normal ABs.

In conclusion, The bridged AB derivative 1 has much superior spectroscopic properties compared to the parent AB molecule and other commonly used AB derivatives. Most importantly, the respective S1(nπ*) bands of 1Z and 1E are well resolved (peak absorptions at 404 and 490 nm, in striking contrast to AB, where the (Z) and (E) nπ* bands are practically coincident. Efficient switching of 1 in the (Z) to (E) and (E) to (Z) directions can therefore be accomplished with visible light via one or the other nπ* band, whereas conversion between the (E) and (Z) isomers of AB requires irradiation in the nπ* band in the visible in one direction and in the ππ* band in the UV in the other. The photoisomerization quantum yields of 1 are substantially higher in both directions than in the case of AB, where ΦZ-E ) 53% and ΦE-Z ) 24%. Moreover, a >90% photoconversion yield cannot be achieved in the case of AB to our knowledge. Unlike AB, the nπ* absorption of the (E) isomer of 1 is stronger than that of the (Z) isomer; this was reconciled by time-dependent density functional calculations on the excited states. The 4.5 h thermal lifetime of 1E at room temperature is not a drawback especially for applications involving fast repeated forward and backward switching cycles or at lower temperatures.