Twisted excited state

As other examples one may quote the symmetry-breaking of the CASSCF (4e in 4MO) calculation of the inn twisted excited state of ethylene (G. Trinquier and Malrieu, in "The structirre of Double Bond". Patai ed., John Wiley (1990) p 1, or the symmetry-breaking in electron transfer problems (A. Faradzed, M. Dupuis, E. dementi and A. Aviram, J. Amer. Chem. Soc. 112, 4206 (1992). 

I /> ) twisted excited state intermediates. The latter was hrst proposed by Dauben and Ritscher and subsequently received considerable theoretical support " (however, see Reference 83). Experimental support for zwitterionic twisted intermediates in diene " and triene" photoisomerizations has come from studies of unsymmetrically substituted systems, albeit mostly aryl-substituted ones. For example, Squillacote and Semple showed that the direct irradiation of -l,3-pentadiene-Z7rf (E-lzid) with 229 nm light results in the formation of fi-TS-pentadiene-Efd and Z-1,3-... 

Three emission bands (a, p and y in the order of decreasing energy) are observed in CH2CI2 solution and are found to be the emission from the excited state of 1, from the excited state of a solute-solvent complex and from a relaxed twisted excited state of the solute-solvent complex, respectively. Model compound studies show that squaraine forms strong solute-solvent complexes with alcoholic solvent molecules. Analogous complexation process between 1 and the OH groups in PVF is also shown to occur. A model for the stabilization of particles of 1 in polymer solution is put forward where we propose that the stabilization mechanism is a steric effect achieved by adsorption of PVF macromolecules onto particles of 1 via the formation of the PVF 1 complex. 

The observed signals after photoexcitation of tetraphenylethylene (Fig. 17) were analyzed in terms of the population grating due to the twisted excited singlet state, the thermal contribution, and the molecular volume change. The fast rising component of the thermal grating represents the vibrational relaxation in the excited state, and the slower one reflects the heat released by the relaxation from the twisted excited state. The decay rate constant of the twisted singlet state was determined from the temporal... 

Double Activated Mechanism. For 4,4 -NDS in toluene (as an example of a solvent within a certain polarity range), a new double activated mechanism for trans - cis photoisomerization has been suggested [152,159], The two consecutive activated steps are (1) population of an excited state (A ), having strong CT character and essentially transoid geometry, in competition with fluorescence, and (2) rotation about the C=C double bond to a twisted excited state (probably p ). At high temperatures the cis form is formed via the sequence... 

Photoisomerization of Stilbene Especially detailed study of the mechanism of photochemical configurational isomerism has been done on Z- and E-stilbene. The isomerization involves a twisted singlet state that can be attained from either the Z- or the E-isomer. Spectroscopic data have established the energies of the singlet and triplet states of both Z- and E-stilbene and of the twisted excited states that are formed from both isomers. Some features of the potential energy diagram are due to structural... 

The interpretation of the A - X band-system in terms of a twisted excited state has a long history, but there is still no consensus as to the value of the angle of twist in the excited state. References to earlier work with discussions are given in Ref. [1]. 

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