Excimer Exciplex

Explain the deactivation of excited states by other molecules in terms of quenching processes, excimer/exciplex formation, energy transfer and electron transfer. 

Recognise situations in which the formation of excimers/exciplexes may affect the observed properties of an excited system. 

Intramolecular Folding The Excimer/Exciplex Mechanism and Dewar Isomerization (Butterfly Mechanism)... 

D. Decay Characteristics of Molecular and Excimer (Exciplex) Fluorescence 178... 

In the absence of the reverse absorption the radiative transition probability fquantum yield of fluorescence qmC) and the decay constant l/r (C)= 2 [Pg.200]

Lifetimes t°(C are available from analyses of fluorescence decay curves as described in Section II.D where, to a good approximation, 1/t (C is given as the experimental parameter Ax describing terminal decay for a system exhibiting excimer (exciplex) fluorescence only. 

Although excimer (exciplex) fluorescence is also exhibited by most dinucleotides,133 the observed phosphorescence from these systems, and from DNA, is characteristic of the lowest molecular triplet state. In the case of DNA at low temperatures this is identified132 as the triplet state of thymine which, in the absence of molecular intersystem crossing, must be populated by intermolecular energy transfer in the triplet manifold or by intersystem crossing from the XAT exciplex.134... 

It is instructive to consider the stability of other excitations in DNA, ex-cimers and exciplexes. An excimer (exciplex) is formed when two identical (nonidentical) molecules that do not interact in their ground states do so when one of the molecules is in an excited state. As a result of charge-transfer and exchange interactions of the overlapping n electrons of the two molecules on the one hand, and their mutual repulsion on the other, the molecules are drawn together in a potential minimum at a separation smaller... 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

Photocycloaddition and photoaddition can be utilized for new carbon-carbon and carbon-heteroatom bond formation under mild conditions from synthetic viewpoints. In last three decades, a large number of these photoreactions between electron-donating and electron-accepting molecules have been appeared and discussed in the literature, reviews, and books [1-10]. In these photoreactions, a variety of reactive intermediates such as excimers, exciplexes, triplexes, radical ion pairs, and free-radical ions have been postulated and some of them have been detected as transient species to understand the reaction mechanism. Most of reactive species in solution have been already characterized by laser flash photolysis techniques, but still the prediction for the photochemical process is hard to visualize. In preparative organic photochemistry, the dilemma that the transient species including emission are hardly observed in the reaction system giving high chemical yields remains in most cases [11,12]. 

We will discuss briefly the reactive species such as an exciplex and radical ion species generated by the excitation of organic molecules in the electron-donor (D)-acceptor (A) system. An exciplex is produced usually in nonpolar solvents by an interaction of an electronically excited molecule D (or A ) with a ground-state molecule A (or D). It is often postulated as an important intermediate in the photocycloaddition between D and A. In the case of D = A, an excimer is formed as an excited reactive species to cause photodimerization. In some cases, a ter-molecular interaction of an exciplex with another D or A generates a triplex, which is also a reactive intermediate for photocycloaddition. The evidence for the formation of excimers, exciplexes, and triplexes are shown in the fluorescence quenching. Excimer and exciplex emission is, in some cases, observed and an emission of triplex rarely appears. 

The importance of bioexcimers (bioexciplexes) in the photochemistry of biological compounds has been also emphasized. Computation of potential energy curves modeling the complex pheophytin-quinone shows the relevance that stabilization caused by the formation of rr-stacked excited dimers, that is, excimers (exciplexes) and the corresponding presence of conical intersections, have to provide... 

Rate constants are obtained from fluorescence decay analyses of the monomer decay profile I (t) and the excimer/exciplex profile Ig(t). These are fit to sums and differences of two exponential... 

Fig. 15. Schematic diagram showing the formation of excimer/exdplex in or nic molecule and light emission from excimer/exciplex molecule is red shifted from the excited monomer emission.

Several studies on CD complexes with aromatic molecules using steady-state and nanosecond spectroscopy have been reported. These studies aimed to understand the photophysical and photochemical behavior of organic guests such as fluorescence and phosphorescence enhancement, excimer/exciplex formation, photocleavage, charge and proton transfer, energy hopping, and cis-trans photo-... 

Excimer, Exciplex, Energy Transfer, Electron Transfer.etc. 

Keywords Luminescence Photoluminescence Chemiluminescence Electrochemiluminescence Triplet-triplet annihilation Light-emitting reaction Excimers/exciplexes... 

The proposed scheme can be deduced as follows In the LED device of SiHMOXD/Cz 10, this new red band is exhibited only in EL, but not in PL spectra. And, in the LED device of SiHMOXD/Cz 01, the blue EL band is exhibited in both EL and PL spectra. So, the blue EL color and the new red EL color come from the silicon-based copolymer with only the carbazole moiety and the silicon-based copolymer with only the oxadiazole moiety, respectively. Thus, a certain charge complex, more like excimer, exciplex, or electroplex, is formed under a strong electric field inside the device and cannot be produced by photoexcitation. From the blending study, the charge complex can be formed intramolecularly between the oxadiazole units and the carbazole units. 

With either an absorption complex or an excimer/exciplex the absorption/emission band associated with the complex will be at a different and longer wavelength than that associated with either individual molecule. The intensity of the new absorption/emission will necessarily depend on the concentrations of both species involved. For example, two Pyrene emission spectra for pyrene are shown in Figure 16.9. In dilute solution the complex between... 

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