Reversible photoisomerization

Nishiyama Kand Fujihira M 1988 Cis-trans reversible photoisomerization of an amphiphilio azobenzene derivative in its pure LB film prepared as polyion oomplexes with polyallylamine Chem. Lett. 1257-60... 

In addition to photoisomerization, there are reversible photochemical reactions of special types for asymmetrical polymethines, produciag sphopyranes (84—86) as ia equation 6, where X = NR, S, or C(CH2)2-... 

Photochromism Based on Geometric Isomerism. The simplest examples of a photochromic reaction involving reversible cis-trans isomerization is the photoisomerization of azobenzene [103-33-3] C22H2QN2 (16). 

A reactivity index which accurately predicts the site selectivity of the photoisomerization of cycloheptatrienes to their bicyclic valence tautomers fails with 1//-azepines.237 For example, for methyl 2-methyl-1//-azepine-l-carboxyIatc (14), the 1-methyl isomer 16 is the predicted major product in practice the reverse is true. 

A few examples are known of the thermally reversible, photoisomerization of 4//-azepines, e.g. 22, to l-azabicycIo[3.2.0]hepta-2,6-dienes, e.g. 23.83 The yields are high (65 95%) and the alternative isomers, the 6-azabicycIo[3.2.0]hepta-3,6-dienes, resulting from C2-C6 ring closure, do not appear to be formed. 

Photo-isomerism is also found in this group of compounds. Cis-Pt(NH3)2(H20)l+ photo-isomerizes to trans with a quantum yield of about 0.1 at 363 nm (40). Photoisomerism of Pt (glycine) 2 from cis to trans, but not its reverse is also reported. 

The iodine-catalyzed photoisomerization of all-trans- a- and (3-carotenes in hexane solutions produced by illumination with 20 W fluorescence light (2000 lux) and monitored by HPLC with diode-array detection yielded a different isomer distribution (Chen et al. 1994). Four cis isomers of [3-carotene (9-cis, 13-cis, 15-cis, and 13,15-cli-r/.v) and three cis isomers of a-carotene (9-cis, 13-cis, and 15-ri.v) were separated and detected. The kinetic data fit into a reversible first-order model. The major isomers formed during the photosensitized reaction of each carotenoid were 13,15-di-d.v- 3-carotene and 13-ds-a-carotene (Chen et al. 1994). 

Photoisomerization of pyrazolo[l,2- ]benzotriazoles 28 was studied in argon matrices at 12 K and in solution at 190 K. On irradiation at 365 nm, 28a and its dimethyl derivative 28b undergo ring closure to yield the triazasemibullvalenes 29a and 29b, respectively, which were identified by NMR and IR spectroscopy. The cyclization is reversed on warming or by irradiation at 313 nm (Equation 2) <2002PPS38>. 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

Incidentally, ICD in DPPC liposomes is observed in the temperature range below Tm but not above. Consequently, the non-linear depression of ICD will be relevant to disordering of DPPC molecular arrangement. The change in ICD is a reversible process. Reverse photoisomerization to the trans isomer restores the initial ICD. 

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 effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

Fig. 2 Possible mechanism for the reversible photoisomerization between (SP - SO3 ) and merocyanine (MC) in the interlayer of an Mg/Al LDH...

In other instances, irradiation of the d-d transition leads to no observable reaction. Examples of this behavior are found for complexes having a variety of d electron configurations and coordinative geometries square planar Ni(II) (3d)3 in Ni(CN)42 124 and mww-Ni(gIy)2 124 square planar Pd(II) in Pd(CN)42-,124 and tra -Pd(gly)2 square planar Pt(II) in Pt(CN)42" (5d)3 124 octahedral Co(III) (3d)6 in a variety of complexes (cf. Sect. III-C and III-D). A striking example of this type of behavior is afforded by the nonreversible photoisomerization of cis-Pt(gly)2 (5d)8 to trans-Pt(g y)2 [reaction (2)].124 It has been proposed that irradiation of either of these square planar complexes leads to the same tetrahedral intermediate which decays exclusively to mwj-Pt(gly)2. This behavior may be contrasted with the reversible photoisomerization shown in reaction (3).3... 

Ichimura et al. (51) have prepared silica particles showing photocontrol led reversible dispersibility in organic solvent. The silica particles incorporated spiropy-ran photochromic units, which showed reversible photoisomerization, were floccu-... 

The QC is thermodynamically unstable relative to NBD, but a catalyst is needed to revert it to NBD. The reverse reaction of QC to NBD catalytically releases heat. The storage capacity of this is 21 kcal/mol. Since unsubstituted NBD has no absorption in visible region, a sensitizer such as aromatic ketones is required for the NBD isomerization by visible light. The efficiency and stability of various storage systems based on the photoisomerization were studied100b). 

Not much is yet known about the reactivity of the recently synthesized fully unsaturated 1,3-diazepines but it has been shown that (61) undergoes a thermally reversible photoisomerization to give (142) (81H(16)137). 

G Quinkert, Thermally reversible photoisomerization , Angew Chem. Int. Edit, II, 1972, 1072. 

Studies by Nishiyama and Fujihara [149] utilizing azobenzene derivative (27) as isomerizable chromophores have demonstrated the importance of reaction cavity free volume in L-B films. The L-B films of amphiphilic derivative 4-octyl-4 -(3-carboxytrimethyleneoxy)-azobenzene (27) upon irradiation was found to be stable, no geometric isomerization of the azobennzene moiety occurred. This compound forms L-B films with water soluble polyallylamine 28 at an air-water interface. Reversible cis-trans photoisomerization occurs in the film containing 28. The reversible photoisomerization reaction in polyion complexed films is thought to occur because of the increased area per molecule provided in the film. The cross sections of molecule 27 in the pure film and in film containing 28 were estimated to be 0.28 and 0.39 nm2. Such an increased area per molecule... 

So far, various kinds of polymers which change their conformation reversibly by photoirradiation have been reported [1-6]. The polymers contain pendant or backbone photoisomerizable chromophores, and the molecular property changes, such as geometrical structure or dipole moment changes, control the conformation. The polymers change their conformation in proportion to the number of photoisomerized chromophores. Thus, when the polymers contain more photoisomerizable chromophores and absorb more photons, the conformation changes more. Physical and chemical properties associated with the conformation changes also vary with the number of absorbed photons. 

---