Excitation intra-molecular

Figure B2.5.18 compares this inter molecular selectivity with intra molecular or mode selectivity. In an IR plus UV, two-photon process, it is possible to break either of the two bonds selectively in the same ITOD molecule. Depending on whether the OFI or the OD stretching vibration is excited, the products are either IT -t OD or FIO + D [24]- hr large molecules, mirmnolecular selectivity competes with fast miramolecular (i.e. unimolecular) vibrational energy redistribution (IVR) processes, which destroy the selectivity. In laser experiments with D-difluorobutane [82], it was estimated that, in spite of frequency selective excitation of the...

In (1), Hq yields the total energy of system in which the molecules and the lattice are excited, yet there are no interactions between molecules and the lattice. The transfer of an electron from site m to site n is given by //j. Polaronic effects, i.e., effects due to the interaction of the electronic excitation and the lattice, are given by H2 and H. hi H2, the energy of the site is reduced by the interaction with the lattice vibration. In H, the lattice vibration alters the transition probability amplitude from site m to n. The term lattice vibration may refer to inter-molecular or intra-molecular vibrations. Static disorder effects are considered in H4, which describes the changes to the site energy or transition probabihty amplitude by variations in the structure of the molecular sohd. 

If definite stoichiometry is maintained in the exciplex formation, an isoemissive point similar to isosbestic point in absorption miy be observed. An interesting example of intra-molecular exciplex formation has been reported foi 9-methoxy-10-phenanthrenecarboxanil. The aniline group is not necessarily coplanar with the phenanthrene moiety but is oriented perpendicular to it. The u-elcctron located on its N-atom interacts with the excited -electron system and an intramolecular exciplex with T-bone type structure is formed in rigid glassy medium where rotation is restricted. Temperature dependence of fluorescence of this compound in methylcyclohexane-isopentane (3 1) solvent shows a definite isoemissive point (Figure 6.8). As the solvent melts and movement is restored to the molecule, structured fluorescence reappears. 

Photophysical processes include radiative transition in which excited molecule emits light in the form fluorescence or phosphorescence and returns to the ground state, intra-molecular non-radiative transitions, in which some of the energy of the absorbed photon ultimately gets converted to heat. 

Figure 1.8 Three possible intra-molecular energy transition mechanisms [9]. (Reprinted from Coordination Chemistry Reviews, 99, G.E. Buono-core, H. Li, and B. Marciniak, Quenching of excited states by lanthanide ions and chelates in solution, 55-87, 1990, with permission from Elsevier.)...

Resonance Raman (RR) experiments have also provided valuable data on the structure of the electron. RR spectra of aqueous solvated electrons revealed enhancements of the water inter- and intra-molecular vibrations demonstrating that electronic excitation was significantly coupled to these modes. Frequency downshifts of the resonantly enhanced H2O bend and stretch were explained by charge donation into solvent frontier orbitals. RR spectra in primary alcohols (methanol, ethanol, propan-l-ol) revealed strong vibronic coupling of the solvated electron to at least five normal modes of the solvent. The spectra showed enhancements of the downshifted OH stretch. 

Bartkowiak reviewed the connection between the NLO response and solva-tochromlc behavior of donor-acceptor Tr-conjugated molecules. The marked NLO properties of these molecules are associated with an intra-molecular charge-transfer excited state. This author points out that the environmental interactions may have a very significant effect on the hyperpolarizabilities (they may even lead to a change of sign). Bartkowiak shows that a simple two-state model combined with the solvatochromic methods may allow the prediction of changes of in molecular hyperpolarizabilities as a function of the solvent polarity. 

UV absorbers have been shown to undergo photolysis reactions in polymer materials, the loss being quite critical depending upon the nature and form of the material. Spectroscopic methods have been developed to design more effective UV absorbers which function via intra-molecular proton transfer in the excited state. Other effective stabilizers for polymers includes thioureas for PMMA, aryl nitrenes for PP, diphenylnitrone for polyethylene, zinc glycerolate in PVC o and hexazoclanes for cellulosics and nylons. ... 

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