E-azobenzene

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... 

In the 275-340 nm wavelength range, the concentrated solution of E-azobenzene absorbs the incident radiation completely (100% absorption). Thus the E Z isomerization, with a quantum yield of [Pg.147]

A major feature of the azo group is its capability to isomerize this is the property used widely in photoresponsive organic thin films. The E-Z photoisomerization of azobenzene was detected by Flartley, who created the Z-form by irradiation of E-azobenzene. Generally, the E-forms of azo compounds are more stable than the Z-forms. The parent E-azobenzene is by ca. 50 kj moT the more thermodynamically stable isomer. Isomerization is the main photoreaction of most aromatic azo compounds. Other thermal- and photoreactions lose the competition with isomerization. 

In the spectrum of E-azobenzene, the unstructured low intensity and low energy n —> Jt band is identified in the 400 to 500 nm region (n-hexane A. ax = 449 nm, = " 05 1 mol" cm" ). This band corresponds to an n. —> K excitation and is forbidden under the symmetry C2h of E-azobenzene. Compared to n —> n transitions in other molecules that of azobenzene is very intense. This may be due to nonplanar distortion and vibrational coupling. The n —> Tt" intensity is stolen from the relatively far-off first K n band, as shown by their common direction of polarization. e of the n,jt state decreases by about 20% when the solution is frozen at 77K, which maybe interpreted as better planarity at low temperature. The n —> 7t band of the E-compound is continuous a case with vibrational features has never been found. This band extends to 620 nm, where e becomes smaller than 510" 1 mol" cm", which gives a state energy of ca. 17 500 cm" (2.1 eV, 205 kjmol" ). ... 

In the spectrum of Z-azobenzene, the n —> 71 transition is allowed under the symmetry of C2y. The n —> tu band maximum is 440 nm with e = 1250 1 mol cm", much higher than that of E-azobenzene. This is relevant to the E-Z isomerization reaction. This band is also continuous, as in all nonrigid Z-azo compounds. 

E-azobenzene) and about 192 and 142 kJ moH (Z-azobenzene). Monti et al. located triplet states at 146 (E-azobenzene) and 121 kJ mol (Z-azobenzene). From their kinetic results, they inferred that the azobenzene acceptor should be twisted (phantom triplet) when accepting the energy and calculated... 

The relevant vibrations for this review are the N=N and C-N (Ph-N) stretching vibrations and, perhaps, torsional vibrations around the C-N bond. The E-azobenzene molecule has a center of inversion, and therefore the N=N vibration is infrared-inactive, but Raman-active, and has been found to be at 1442 cm". By IR spectroscopy, Kiibler et al. located the symmetric C-N stretching vibration at 1223 cm" in E- and at 866 cm in Z-azobenzene. The N=N vibration in Z-azobenzene is at 1511 cm" (in KBr pellets). These numbers are confirmed by newer work Biswas and Umapathy report 1439 and 1142 cm for the N=N and C-N vibrations (in CCE), and Fujino and Tahara found nearly identical results (1440 cm" and 1142 cm ). A thorough vibrational analysis of the E-isomer is given by Amstrong et al. The vibrations in the (n,7t ) excited state are very similar 1428 cm" and 1130 cm"h... 

In the experimental realm, picosecond and femtosecond experiments contribute to the understanding of isomerization. A review of the ultrafast dynamics of photochromic systems appeared in 2000. A study of the photochemistry of E-azobenzene on excitation of the S2 state [ (71,71 )] was... 

The relevant vibrations for this review are the N=N and C-N (Ph-N) stretching vibrations and, perhaps, torsional vibrations around the C-N bond. The E-azobenzene molecule has a center of inversion, and therefore the N=N vibration is infrared-inactive, but Raman-active, and has been found to be at 1442 By IR spectroscopy, Kiibler et al. located the symmetric... 

Scheme 11.10 E azobenzene derivative fits into a diazapyrenilium cavity, the Z isomer does not...

Photocyclisation - A wide variety of ring-forming reactions has again been reported. Irradiation of azepine derivative (16) results in 4-ju-electrocyclisation to a mixture of the corresponding exo and endo cyclobutenes. " 6-ju-Electrocyclisa-tion has been employed in a scaled-up synthesis (>300g) of 6-aza-l,10-phenan-throic anhydride (18) from the stilbazole (17). E-Azobenzene, incorporated into water-swollen acid-form Nafion fluorocarbon membranes, exists as the proto-nated form (19) and exhibits ambient temperature fluorescence, previously... 

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