Excited molecular complexes

The competition between coherence and incoherence is readily illustrated by the dimer, which serves as a model of a two-level system. If molecule 1 is initially excited at time t = 0, the subsequent populations of molecules 1 and 2 are, 

Equations (9.48) and (9.49) indicate that the system exhibits the classical populations after a relaxation (or decoherence) time, Evidently, we also see that the system is overdamped when 2 J /ft fc - 7]. 

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Here Icq /fQ2 because (A3.4.52) proceeds tlu-ough a highly-excited molecular complex iwith particularly... 

Excited molecular complexes of the donor-acceptor type are called excimers if formed from identical molecules and exiplexes if originated from different molecules. From the theory, it is concluded that photochemical influence will more readily accelerate electron transfer in a weak donor-acceptor pair than in a strong pair (Juillard and Chanon 1983). An organic molecule in an electron-excited state is a more active oxidant or stronger reducer than the same molecule in a ground state. 

A molecular complex may be dissociated in the ground state, and yet be associated in an excited electronic state. An excited molecular complex of defined stoichiometry, which dissociates in the ground state, is known as an exciplex [110,111], the term being derived from exci (ted comp) plex by analogy to excimer (= exci (ted di) mer ). 

The term exciplex (excited complex) is used to describe an electronically excited molecular complex of definite stoichiometry. Complexes which fall into this broad classification include ... 

An excimer is an electronically excited molecular complex formed between two suitably oriented aromatic rings when one of them has been promoted to an excited state by absorption of energy. The normal characterization parameter is the ratio of the excimer fluorescence intensity to the monomer fluorescence intensity, R, where the monomer refers to the uncomplexed aromatic ring. Of importance for our work is that it is a common feature of the photophysical behavior of the aryl vinyl polymers, as described in a recent review [48]. The objective of this section is to demonstrate the sensitivity of excimer fluorescence to those variables expected to influence the free energy of mixing in polymer blends solubility parameters of the two components, concentration, temperature and molecular weight. 

S-S annihilation phenomena can be considered as a powerful tool for investigating tire exciton dynamics in molecular complexes [26]. However, in systems where tliat is not tire objective it can be a complication one would prefer to avoid. To tliis end, a measure of suitably conservative excitation conditions is to have tire parameter a< )T < 0.01. Here x is tire effective rate of intrinsic energy dissipation in tire ensemble if tire excitation is by CW light, and T = IS tire... 

Molecular Interaction. The examples of gas lasers described above involve the formation of chemical compounds in their excited states, produced by reaction between positive and negative ions. However, molecules can also interact in a formally nonbonding sense to give complexes of very short lifetimes, as when atoms or molecules collide with each other. If these sticky collisions take place with one of the molecules in an electronically excited state and the other in its ground state, then an excited-state complex (an exciplex) is formed, in which energy can be transferred from the excited-state molecule to the ground-state molecule. The process is illustrated in Figure 18.12. 

Through direct excitation of a monomeric or polymeric molecule or of a molecular complex (A) followed by a reaction producing an initiating species ... 

It is apparent from the quantity of material included in this chapter that there is an extensive body of work concerning the utilization of diene and polyene photochemistry in a synthetic setting. The unique behavior of the excited chromophores permits the application of powerful new methods for the construction of complex molecules. Unusual photochemical rearrangements and photocycloaddition pathways often lead to substantial increases in molecular complexity, allowing such processes to serve as key strategic steps in target oriented syntheses. 

Exchange reactions, hydrogen isotope, of organic compounds in liquid ammonia, 1, 156 Exchange reactions, oxygen isotope, of organic compounds, 2, 123 Excited complexes, chemistry of, 19, 1 Excited molecular, structure of electronically, 3, 365... 

H. Beens and A. Weller, Excited molecular rc-complexes in solution, in Organic Molecular Photophysics, Vol. 2 (J. B. Birks, ed.), John Wiley Sons, London (1975). 

Photochemical deoxygenation of nitrobenzene to nitrosobenzene with cyanide ions 38,40) or by molecular complexation with boron trichloride have been reported. No experiments to elucidate the multiplicity of the reacting excited state have been described, however. 

Beens H, Knibbe H, Weller A (1967) Dipolar nature of molecular complexes formed in excited state. J Chem Phys 47 1183... 

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