Excited state Isomerization scheme

In a recent paper (12) we showed that the species responsible for the green (543 nm) fluorescence of 3HF was formed kinetically from blue (413 nm) emitting species by two kinetic components. We rationalized this complex behavior by a model (Scheme 1) in which there are two pathways for excited state isomerization, i.e., a slow and a fast component. 

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). 

When the reaction was carried out on the phenoxy derivative 106, only 107 was obtained (Scheme 44) (88JHC1551). The formation of this product was rationalized assuming a heteroly tic cleavage of the O—N bond followed by isomerization (Scheme 44). If the reaction occurs in the excited triplet state of the molecule, the biradical is the most probable intermediate. 

The isomerization reaction via the excited triplet state was only found in the EE <- ZZ and EZ <-> ZE sensitized reactions. The EoZ isomerization is a basic photochemical reaction. Different excited states have different chemical behaviors. The results can be summarized i n Scheme 29 for phenyl fulgides. 

AIMD simulations have also been carried out on the chromophore present in the rhodopsin photoreceptor (retinal). In the primary event of vision, retinal passes from the ground state (GS) to an excited state (ES) and isomerizes from 11-cis to all-trans within 200 fs. A series of papers [46-50] have analyzed the GS isomerization process. More recently, calculations were extended to the first singlet ES [51] within a recently developed scheme for singlet state dynamics [52]. This work characterizes structural and energetic changes during the photoisomerization process and points to the crucial role of environment effects. 

A detailed study of the photochemical transformations of the substituted dibenzbarrelenes (297) has been carried out. Triplet excited states are involved in the transformation of the derivatives (297) into the products (298) and (299) shown in Scheme 10. The second product (299) arises from ring opening of the non-isolated isomeric semibulIvalene (300). Additional products were also reported. A discussion of the observed regioselectivity was reported. ... 

For acyclic and monocyclic 1,4-dienes, the di- r-methane rearrangement occurs in general from the singlet excited state, since loose geometries are favored in the triplet state and cis-trans isomerization is the preferred reaction, as shown in Scheme 42. 

The fluorescence decay of the locally excited state at temperatures were the excimer does not disociate back could be analysed as a sum of two exponentials. That this is not due to roational isomerism of the 1-pyrenyl group could be proven by the analysis of 2b which showed analogous behavior.The analysis of the fluorescence decay according to scheme 4 permits the determination of the ratio of Cy/Cg. 

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