Isomerization, photon induced

The approved platinum complexes, with antineoplastic activity, differ in their photochemical stability. Among these, cisplatin, in particular, undergoes a photolysis reaction. Early studies proposed that this degradation was due to the photoaquation of the chloride ligands, and its subsequent photon-induced isomerization to trans-platin (32,33). Later studies found that the main photolysis-induced degradation product, was trichloroammineplatinate II (TCAP) (34,35). This decomposition product is limited to 1% in cisplatin for injection by the USP. 

Charra, E, Idiart, E., Kajzar, R, Nunzi, J.-M., Pfeffer, N., and Raimond, P. (1993), Two-photon induced isomerization and memory effect in functionalized polymers. SPIF 2042, 333-346. 

Two-photon induced isomerization of l,2-dicyano-l,2-bis-(2,4,5-trimethyl-3-thienyl)ethane (143) in films of poly(methyl-methacrylate) depends on the TPA... 

The slope of the logarithmic plot of absorbance change versus the logarithm of the pump intensity is 1.84, indicating the occurrence of a nonlinear absorption process. The TPA cross section of isomer A is about 0.76 GM and an increased 8 was observed for isomer B (8 = 6 GM) due to the closed ring form and higher planarity. One-photon and two-photon induced isomerization of 143 shown in Eq. (62) results in similar spectra and in the same isosbestic points observed at 329 nm, 377 nm, and 429 nm. These results clearly indicate participation of the same excited state applying either OP or TP excitation. Quantum yields are close to unity. 

Two-photon induced isomerization occurs in case of 3-[l-(l,2-dimethyl-1 H-indol-3-yl)ethylidene]-4-isopropylidene-dihydrofuran-2,5-dione (144) in toluene excited at 775 nm using a 137-nJ pulse at a 1-kHz repetition rate. A new... 

I.r. emission has been seen from the vibrationally excited CO2 product of MPD of vinylacetic and pyruvic acids, from hydrogen halides formed by chemical reaction of CI2 with H atoms produced in the MPD of various hydrocarbons, and of HBr with F and Cl atoms formed in the MPD of CFjCl. Infrared fluorescence from CjFjCl following MPA shows emission from both discrete levels and the quasicontinuum, with efficient intramolecular vibrational redistribution out of the pumped mode evident after absorption of only 2—3 photons. Emission in the i.r. has been seen following MPA in N2p4, and has been used to study the interconversion of perfluorocyclobutene to perfluoro-butadiene isomers following MPA. Further isomerization reactions induced by CO2 lasers have been reported. 

The first study of a photon-induced isomerization was reported by the same group in 2004. In this initial study the following isomerization reaction was investigated,... 

Detailed reaction dynamics not only require that reagents be simple but also that these remain isolated from random external perturbations. Theory can accommodate that condition easily. Experiments have used one of three strategies. (/) Molecules in a gas at low pressure can be taken to be isolated for the short time between collisions. Unimolecular reactions such as photodissociation or isomerization induced by photon absorption can sometimes be studied between collisions. (2) Molecular beams can be produced so that motion is not random. Molecules have a nonzero velocity in one direction and almost zero velocity in perpendicular directions. Not only does this reduce collisions, it also allows bimolecular interactions to be studied in intersecting beams and increases the detail with which unimolecular processes that can be studied, because beams facilitate dozens of refined measurement techniques. 

Not every photon absorbed by azo compounds induces isomerization. Table 1.1 shows a series of quantum yields of azobenzene collected from different authors. The spread of the values reflects the experimental problems of a seemingly simple system. 

The light-induced 13-c/,v-retinal of bacteriorhodopsin features a strained state with separated charges An additional dipole almost perpendicular to the retinal long axis arises after trans-cis isomerization (see Fig. 15b). Thus, part of the absorbed photon energy is stored. The energy difference between the 13-cis and all-trans conformations of the retinal amounts to AU 65 kJ/mol [287],... 

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