Triplet state azobenzenes

The dimerization of phenylnitrene to form azobenzene is a characteristic reaction of the triplet state of an arylnitrene. Thus, some intermediate formed along the reaction coordinate must undergo intersystem crossing to the triplet manifold. 

The observation that azoxybenzene (19) rearranges to 2-hydroxyazobenzene (20) on direct irradiation but is reduced to azobenzene (21) in the presence of benzophenone was originally explained by a difference in reactivity between the singlet and triplet states.117 However, three new lines of evidence argue in... 

POPAM dendrimers bearing up to 32 photoswitchable azobenzene groups at the periphery were used as host compounds for eosin Y (2, 4, 5, 7 -tetrabromo-fluorescein dianion) (see also Section 5.1.2). One reason for the choice of this dye was that it shows strong fluorescence which should be influenced by being enveloped by the dendrimer. The other reason was that the energy of its lowest triplet state is higher than that of the lowest triplet state of azobenzene, leading... 

The vapor-phase pyrolysis of phenyl azide yields azobenzene in 72 % yield . The pyrolysis in aniline solution leads to the production of dibenzamil , identified as an azepine by modern techniques in benzene and p-xylene, the major products are azobenzene and aniline, presumably originating from reactions of PhN in hydrocarbon solvents the thermolysis produces aniline, alkyl-anilines, azobenzene and polymer . It has been shown that aniline and iV-alkylanilines arise from triplet state nitrene reactions with the solvent. In the gas-phase thermolysis of phenyl azide at low pressures aniline and azobenzene are the major products whereas at higher pressures the formation of l-cyano-1,3-cyclopentadiene (or 2-anilino-7H-azepine in the presence of aniline) predominates . It was deduced that aniline and azobenzene arise from triplet nitrene reactions and that at high pressures, a hot singlet nitrene is formed, which undergoes intramolecular insertion to form an azocyclopropene intermediate, viz. 

Triplet state data for azobenzene-type azo compounds are very limited. Direct absorption of a 0.51 mol solution in C7H15J in 5 cm cells has not been detectable. Neither has phosphorescence been detected. The energy of triplet states has been located only by chemical spectroscopy, i.e., the quenching of other molecules triplet states by azobenzene. Ronayette et found two relevant triplet states at about 196 and 180 kj moH... 

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... 

Isomerization of azobenzenes may also be sensitized by triplet sensitizers. Jones and Hammond and Fischer came up with different results 2% of Z-form versus 25% in the photi tationary state. A thorough study by Lemaire and coworkers showed that two triplet states at 195 and 180 kj mol in the E- and at 190 and ca. 140 kJ moT in the Z-isomer are involved in the reaction. According to Bortolus and Monti,sensitizers with high (>190 kJ moT triplet energy transfer their energy efficiently (diffusion controlled) to both E- and Z-azobenzene. Still, the isomerization yield is small— 9e- z = 0.015—in agreement with Jones and Hammond. On the other hand, the sensitized Z —> E isomerization has < -,e = 1-0. Azobenzenophane 9 also undergoes triplet-sensitized isomerization. ... 

As in the azobenzene type systems, the triplet pathway seems to be decoupled from the singlet route in pseudo-stilbenes. Little is known about the mechanism in the triplet state. The only information comes from calculations, and these show that the triplet surfaces are frequently similar in shape to the singlet surfaces. Thus, both mechanisms may be operative in the triplet state, too. 

Monti, S., Dellonte, S., and Bortolus, P. (1983). The lowest triplet state of substituted azobenzenes An energy transfer investigation. /. Photochem., 23, 249-256. 

Gorner, H., Gruen, H., and Schulte-Frohlinde, D. (1980). Laser flash photolysis study of substituted azobenzenes. Evidence for a triplet state in viscous media. /. Phys. CJjem,. 84, 3031-3039. 

The photoswitching properties associated with azobenzene and stilbene derivatives (Figure 1) are attributed to the reversible trans-cis photoisomerization of N=N and C=C double bonds, respectively. As a result of the steric constraints in the cis isomer, which reduce the n electron delocalization, the absorption and triplet-state energies for the trans-isomer are generally lower than those of the cis-counterpart, thus leading to the photochromic behavior. ... 

Azobenzenes do not emit noticeably at room temperature, but all three types show fluorescence in strong acids at 77K." Modern experimental techniques, however, allow one to study the very weak fluorescence of azobenzene and azobenzene functionalized molecules even at room temperature. Corner et al. observed a transient after laser flash photolysis of donor/acceptor-substituted azobenzenes which they assigned to the lowest energy triplet state of the E-isomer. Recent studies were directed at examining the behavior of the transient absorption of the singlet excited states of both geometric isomers with respect to time (see Section 89.4). 

The photochemistry of 4-diethylamino- and 4-diethylamino-4 -methoxyazobenzene was studied as reasonable models for commercially useful mono-azo dyes. It was concluded that the lowest singlet and triplet states of these azobenzene derivatives are only capable of geometrical isomerization, whereas hydrogen abstraction takes place from the high-lying triplet state of both the E- and Z-forms of these dyes. ... 

Baltrop and Bunce (Ref 20) employed a variety of radiation wavelengths, nitrocompds and solvents. For wavelengths less than 2900A, aniline was the main product, while above 2900A, bimolecular species such as azobenzene predominated. Since oxygen had little effect on aniline production, expts were performed in the presence of oxygen. For nitrobenzene in isopropyl alcohol, no azoxybenzene was produced as with Hurley and Testa (See above Ref 17). They concluded that the excited state abstracts H-atoms, and suggest that the nitrobenzene triplet is in tt, ti, and that nitrosobenzene is an unobserved intermediate... 

Azobenzene readily undergoes both sensitized and direct cis-trans photoisomerization. However, the sensitized photostationary cis/trans ratios with high-energy sensitizers do not predict those obtained upon direct irradiation, indicating that much of the direct photoisomerization occurs from states other than the lowest triplet.455 As with stilbene, cyclization of the cis isomer apparently is a singlet-state reaction.456... 

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

In this account httle has been said about the spin states of the carbenes and nitrenes which react, and it hcis been tacitly assumed that rearrangement occurs from singlet states, whereas amine and azobenzene formation most probably occurs from triplets. Since we are dealing with thermal reactions, the law of spin conservation 88) would predict that the first-formed species are singlets. However, this is claimed not to be absolutely necessary, for the direct formation of triplets. 

The solution phase photochemistry of phenyl azide 47 is temperature dependent/ Photolysis of 47 in the presence of diethylamine at ambient temperature yields azepine 48b. Lowering the temperature suppresses the yield of 48b and below 160K, azobenzene, the product of triplet nitrene dimerization, is produced. Thus, high temperature favors reactions of singlet state intermediates whilst low temperatures favor reactions associated with triplet phenylnitrene. 

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