Excited state, formation surfaces

Both of these reactions are chemiluminescent and the potential energy surfaces which result in either ground state or excited state formation are shown. 

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

Lochmuller and coworkers used the formation of excimer species to answer a distance between site question related to the organization and distribution of molecules bound to the surface of silica xerogels such as those used for chromatography bound phases. Pyrene is a flat, poly aromatic molecule whose excited state is more pi-acidic than the ground state. An excited state of pyrene that can approach a ground state pyrene within 7A will form an excimer Pyr +Pyr (Pyr)2. Monomer pyrene emits at a wavelength shorter than the excimer and so isolated versus near-neighbor estimates can be made. In order to do this quantitatively, these researchers turned to measure lifetime because the monomer and excimer are known to have different lifetimes in solution. This is also a way to introduce the concept of excited state lifetime. 

Although thermodynamically favorable, reductive dissolution of Fe(III)(hydr)oxides by some metastable ligands (even those, such as oxalate, that can form surface complexes) does not occur in the absence of light. The photochemical pathway is depicted in Fig. 9.3e. In the presence of light, surface complex formation is followed by electron transfer via an excited state (indicated by ) either of the iron oxide bulk phase or of the surface complex. (Light-induced reactions will be discussed in Chapter 10.)... 

Figure S.6. Schematic representation of So and Si energy profiles for DEWAR formation in TB9A and TB9ACN. 2 The excited state funnel F is very close to the ground stale surface and therefore leads to fluorescence quenching (identifiable with rate constant k). Most of the molecules return to the anthracene form via pathway a, while only a few proceed to the Dewar form (pathway b), because F is placed to the left of the ground state barrier. The steric effect of the tert-butyl substituent is indicated by the broken line. Without this prefolding" of the anthracence form. Dewar formation is not observed. The top part of the figure contains a schematic description of the butterfly-type folding process, while the bottom part contains examples of actual molecules.

The photochemical pathway is indicated by the light arrows (Fig. 17) and involves excitation of the charge transfer complex DA up to the D+A surface. The reaction complex then funnels down onto the ground state surface so as to follow the thermal route. Clearly, both pathways involve formation of the ion-pair intermediate, and are intimately related. We see therefore that the CM model constitutes a useful framework for understanding the relationship between ground state and excited state reactions. 

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