Trans-cis isomerization reaction

The photo-induced trans-cis isomerization reaction of certain azo-dye molecules such as Dispersed Red 1 (DRl),... 

The rapid and concomitant decay of the SE and ESA bands indicates that an efficient non-radiative process occurs from the excited state of pCA and pCM. Previous experiments on pCA gave evidence that this non-radiative process is correlated to the trans-cis isomerization reaction. The trans excited-state decay leads to the ground-state cis isomer without any detectable intermediate, following a mechanism comparable to that of trans-stilbene [11]. A similar process can be invoked for pCM because steady-state photolysis of pCM led to a stable photoproduct that we identified as the cis isomer by H NMR. In water, photoisomerization of pCM is thus found to be faster than that of pCA. The transient spectroscopy of pCM in alcohols showed a positive dynamical Stokes shift of the SE band indicating substantial photoinduced charge redistribution. Such a charge redistribution, which was not observed for pCA [11], might be the cause for a lower isomerization barrier. 

The binding energy of the He atom to rrfl 5-stilbene in the excited state is only 47 cm, so the dimer dissociation occurs at an energy well below the trans-cis isomerization reaction and well below the onset of IVR in the stilbene molecule (DeHaan et al.,... 

UV-light/visible-light illumination cycles. These cycles drove reversible trans-cis isomerization reactions along their polymer chains. 

The rate constant for the trans cis isomerization, reaction (20), was determined in aqueous acidic media containing NaC104 or KNO3, and a dependence upon [H+]... 

Orange II (Oil, Fig. 1) is an aromatic molecule which has O-H- N and N = N bonds. It shows an ESIPT and trans cis isomerization reactions. It is widely used in the dyeing of textiles, food and cosmetics, making it present in the wastewaters of the related industries. Oil exists under azoenol (AZO) and ketohydrazone (HYZ) forms (Scheme 7). Their relative populations depend on the medium. In a water solution for example, the shift of the H-atom within the O-H N intramolecular H-bond to the nitrogen site makes HYZ structure the most stable one (about 95%). While in a more polar solvent like dimethyl sulfoxide (DMSO), the HYZ population decreases to 70%, and in solid state it becomes the only populated structure. 

A kinetic scheme and a potential energy curve picture ia the ground state and the first excited state have been developed to explain photochemical trans—cis isomerization (80). Further iavestigations have concluded that the activation energy of photoisomerization amounts to about 20 kj / mol (4.8 kcal/mol) or less, and the potential barrier of the reaction back to the most stable trans-isomer is about 50—60 kJ/mol (3). 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

When the substituent groups in the polyphosphazenes were azobenzene [719] or spiropyran [720] derivatives, photochromic polymers were obtained, showing reversible light-induced trans-cis isomerization or merocyanine formation, respectively. Only photocrosslinking processes by [2+2] photo-addition reactions to cyclobutane rings could be observed when the substituent groups on the phosphazene backbone were 4-hydroxycinnamates [721-723] or 4-hydroxychalcones [722-724]. 

Compound (tht)AuC6F5 undergoes random aryl exchange with pentafluorophenylpalladium(ii) complexes, while the 3,5-dichloro-2,4,6-trifluorophenylgold complex specifically catalyzes the trans-cis isomerization of [Ar2Pd(tht)2]. The reaction appears to follow an associative mechanism.1... 

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]. 

Usual problems for the application of the PFR to organic synthesis are the occurrence of competing photochemical reactions (like trans-cis isomerization) or the presence of deactivating groups in the phenolic ring. In this subsection, it will be shown how these problems can be circumvented. 

Polyesters of cinnamic acid derivatives used as photorresists, whose main photochemical feature is [2tt -I- 2-77] dimerization, are reported to experiment PFR along with trans-cis isomerization as secondary reactions [234-240]. 

On the other hand there are certain groups of reactions that have been neglected so far, among them the photochemical behavior of the gera-dihalocyclopropanes. One of the few reported examples is by Weyerstahl who has attempted to generate hexachloro-tris-CT-homobenzene from the corresponding tris-a-homocyclo-heptatrienone [233], The latter, however, prefers to undergo trans-cis isomerization rather than decarbonylation. 

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