Excimer defined

The correlation time tc of the motions involved in intramolecular excimer formation is defined as the reciprocal of the rate constant ki for this process. Its temperature dependence can be interpreted in terms of the WLF equationb) for polymers at temperatures ranging from the glass transition temperature Tg to roughly Tg +100° ... 

The heparin and poly-L-glutamate titrations show a markedly different behavior than do the DNA titrations. As polyanion is added, the fluorescence of the an-thrylpolyamine solution decreases until a well-defined minimum is reached. A new emission at 510 nm, which we assign to the anthracene excimer of 14, increases and decreases coincidently with the titrated fluorescence minimum. Likewise, the UV spectrum of 10 fiM 14 with added heparin shows hypochromism that occurs and disappears coincidently with the fluorescence minimum and a 2-nm red shift. We have proposed template-directed excimer formation as the physical basis for these observations. In the absence of heparin, fluorescence of the unassociated probe is observed. As heparin is added, the fluorescence decreases as a result of heparin-directed interaction between probe molecules. Additional heparin permits the fluorophore population to diffuse over the length of the poly anion, thus avoiding excimer formation and yielding a net CHEF. 

Excimer formation can serve as a sensitive probe of group proximities Excimers make evident the interaction of an excited molecule M, (typically an aromatic hydrocarbon), with a molecule in the ground state M producing an excited dimer Mf (or D ). The dimer must be formed within the lifetime of the excited species (e.g., for pyrene derivatives, about 100 nsec). For molecules such as pyrene, excimer formation and fluorescence are contingent on attainment of a well-defined steric arrangement in the dimer.41... 

From the stationary state approximation, d[A ]/dt <= 0 and d[A ]/dt = 0, the expressions for the steady state concentration of [A ] and [A ] can be set up. We can further define, 6m, and as quantum efficiencies of emission from dilute solution, from concentrated solution with quenching, and of excimer emission, respectively ... 

In some cases excited state chromophores form supramolecular complexes either with ground state chromophores on the same molecule or one nearby resulting in the formation of an excimer (excited state dimer) or, if the two chromophores are different to one another an exciplex. Formally an excimer as defined as a dimer which is associated in an electronic excited state and which is dissociative in its ground state.1 Formation of the pyrene excimer is illustrated in Figure 11.3. 

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 ). 

Recent studies in our laboratory were aimed at defining more closely the conditions governing Intramolecular excimer formation in dilute polymer solutions (15). An alternating copolymer of styrene with maleic anhydride or methylmethacrylate showed no excimer emission, confirming that interactions of other than neighboring phenyl residues made no significant contribution to... 

As a fluorescence probe, a TICT compound possesses several merits. It can probe both microviscosity and micropolarity and furthermore, the mode of molecular motion necessary to the TICT phenomenon is well defined and limited to the rotation around a particular bond. This is a difference from intramolecular exciplex or excimer formation in which multiple modes of bond rotation and bending are compounded [7]. Fluorescence polarization studies also can not be free from ambiguity of the mode of rotation [8]. 

These results allow the conclusion that in a bichromophorlc molecule linked by three carbon-carbon bonds or in the ether analog with a well defined conformation of the chain, a symmetric substitution at the chromophore and in absence of interactions in the ground state excimer formation can be described by a intramolecular versiom of the kinetic scheme 1. 

Particular emphasis is placed on the side effects of the procedures. The thermal, mechanical, and in particular the chemical effects are discussed from a fimdamental/mechanistic standpoint and are exemplified in the case of the laser-based restoration of painted artworks. Certainly, the interaction of intense laser pulses with organic substrates is highly complex. In fact, in many respects, conventional photochemical concepts would suggest that the laser procedures would be particularly damaging for molecular/organic substrates. Yet these limitations or deleterious influences have been shown, at least empirically, to be avoidable to a large extent. This is best illustrated in the laser restoration of painted artworks. Despite their chemical and structural complexity, excimer lasers can be applied successfully to restoration, and irradiation conditions can be defined under which potentially damaging side effects to the substrate are minimal or at least inconsequential. 

Threshold Behavior. A closer inspection of the Figs. 25 and 26 shows that a different behavior is observed for 248- and 308-nm irradiation. The ablation at 308 nm, where the polymer has a higher absorption coefficient (Fig. 24), shows a very clear and well-defined threshold fluence. This type of sharp threshold was previously found only for irradiation of polyimide with an ArF excimer laser (193 nm). In a study of polyimide laser ablation at various wavelengths, this behavior was assigned to a photochemical process [63]. In this study only single-pulse data were employed and the thresholds of ablation were analyzed by using the QMB technique. 

An excimer is defined as a molecule which exists only in a highly excited state and possesses no stable ground state (see section 2.27). 

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