Excimers structure

How many stable excimer structures exist for naphthalene and its alkyl-substituted derivatives ... 

There is evidence both for and against the contention that only (me excimer fluorescencepeakandlifetimeispossibleforbis(l-naphthyl)compoundshavingn = 3. Studies of l,3-bis(l-naphthyl)propane9,107,lls,129) and bis(l-naphthylmethyl)ether 39.ii4.iis) jjj various solvents over a range of temperatures have found only one excimer fluorescence peak and decay rate (even though there appear to be two possible excimer structures in the ether compound 114). On the other hand, fluorescence peaks attributed to two excimer types have been recorded at 28,200 and 26,700 cm-1 for meso-bis( 1 -(1 -naphthyl)-ethyl)ether13), and at 27,000 and 24,400 cm-1 both for the compound l,3-bis(4-methoxy-l-naphthyl)propane and for l,3-bis(4-hydroxy-l-naphthyl)propane U6). 

The discovery of two excimer emitting polymorphs of the stilbene derivative 6-XXXVIII (Cohen et al. 1975) provided an opportunity to determine directly the relationship between excimer structure and its emission properties. The structure of Form A was determined crystallographically and indicated a stacking arrangement, while the structure of Form B, inferred from the cell constants and the photochemistry consistent with the topochemical principles (Section 6.4), was a pairwise arrangement... 

Fig.X.Space filling moaeis representing the TG ground state and TT excited state.The actual excimer structure must have the two pyrene groups farther apart for steric reasons.

Excimer emission is observed from many crystals when they are subjected to pressure or when they are strongly deformed in some other manner. In the deformed crystals, molecular configurations are produced which favour excimer emission. Through annealing, these defects can often be removed again. Also in vapour-deposited films which are initially amorphous at low temperatures, excimer structures can frequently be formed by a suitable annealing process. 

The typical synthetic vinyl polymers prepared by the free radical polymerisation of vinyl monomers are shown in Figure 4.3. They have a linear chain structure in which the substituents R and R are separated by three carbons. If the substituents R are aromatic chromophores, the vinyl polymers will be rich in excimer structures and strong bimolecular quenching of the fluorescence will occur. This is in agreement with the Hirayama rule [20], which says that if the chromophores are separated by 3 carbon atoms, the probability of excimer formation is the highest, assuming that the chain is flexible enough to rotate... 

Basically, in the extended conformation as well as in the three-fold helical conformation of the iPS chain, the phenyls do not form excimer states, because the distance between parallel phenyls in the three-fold helical conformation is 0.665 nm, as reported by Natta [62] and Sundararajan and co-workers [54]. This distance is too large for excimer formation of the sandwich type (which have a distance of 0.3-0.35 nm). In this situation, the excimer can be formed only outside the crystalline region, e.g., in the region of the lamellar borders, because in these areas some deformation of the helical conformation of the PS chain makes the excimer structure formation more probable. The excitation energy can effectively migrate along the helical structure [60, 63] to the lamellar border, where... 

Other topics recently studied by XPS include the effects of thermal treatment on the morphology and adhesion of the interface between Au and the polymer trimethylcy-clohexane-polycarbonate [2.72] the composition of the surfaces and interfaces of plasma-modified Cu-PTFE and Au-PTFE, and the surface structure and the improvement of adhesion [2.73] the influence of excimer laser irradiation of the polymer on the adhesion of metallic overlayers [2.74] and the behavior of the Co-rich binder phase of WC-Co hard metal and diamond deposition on it [2.75]. 

In addition to the thermal CVD reactions listed above, tungsten can be deposited by plasma CVD using Reaction(l)at350°C.[ ll P At this temperature, a metastable alpha structure (aW) is formed instead of the stable be.c. Tungsten is also deposited by an excimer laser by Reaction (1) at < 1 Torr to produce stripes on silicon substrate.P l... 

Further modification of the above nanostructures is useful for obtaining new functional materials. Thirdly, we apply the dopant-induced laser ablation technique to site-selectively doped thin diblock copolymer films with spheres (sea-island), cylinders (hole-network), and wormlike structures on the nanoscale [19, 20]. When the dye-doped component parts are ablated away by laser light, the films are modified selectively. Concerning the laser ablation of diblock copolymer films, Lengl et al. carried out the excimer laser ablation of diblock copolymer monolayer films, forming spherical micelles loaded with an Au salt to obtain metallic Au nanodots [21]. They used the laser ablation to remove the polymer matrix. In our experiment, however, the laser ablation is used to remove one component of block copolymers. Thereby, we can expect to obtain new functional materials with novel nanostmctures. 

Excimers are often characterized by a broad emission band containing no vibrational structure, occurring at longer wavelengths than emission corresponding to the monomeric singlet state/41,87-89,71-73 ... 

In the first mechanism the excimer is represented as a common intermediate for the formation of dimer and the deactivation of the excited anthracene. In the second, excimer formation is totally nonproductive with regard to dimer formation. Again as in paragraph (b) one can think of the excimer in the second mechanism as having a structure that, if dimerization proceeded, would yield the unobserved head-to-head product ... 

Kostenko E, Dobrikov M, Phshnyi D, Petyuk V (2001) 5 -Bis-pyrenylated oligonucleotides displaying excimer fluorescence provide sensitive probes of RNA sequence and structure. Nucleic Acids Res 29 3611-3620... 

Different aromatic hydrocarbons (naphthalene, pyrene and some others) can form excimers, and these reactions are accompanying by an appearance of the second emission band shifted to the red-edge of the spectrum. Pyrene in cyclohexane (CH) at small concentrations 10-5-10-4 M has structured vibronic emission band near 430 nm. With the growth of concentration, the second smooth fluorescence band appears near 480 nm, and the intensity of this band increases with the pyrene concentration. At high pyrene concentration of 10 2 M, this band belonging to excimers dominates in the spectrum. After the act of emission, excimers disintegrate into two molecules as the ground state of such complex is unstable. 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

J.-I. Lee, G. Klaerner, and R.D. Miller, Structure-property relationship for excimer formation in poly(alkylfluorene) derivatives, Synth. Met., 101 126, 1999. 

In the vapor-deposited OLED community, a number of approaches have been employed to produce white light emission. White OLEDs have been demonstrated based on multilayer structures, e.g., stacked backlights [153,168], multidoping of single-layer structures [145], phosphorescent monomer-excimer emission layers [169] and on doping of phosphorescent materials into separate bands within the emission zone, called a tri-junction [170]. The trijunction device has produced the highest white OLED efficiency of 16% external quantum efficiency demonstrated thus far [171]. 

Solubility of polymers in tetramethyl ammonium hydroxide aqueous solution was measured by dipping the wafer on which the polymer solution was spin-coated, for 1 min. at 25°C. The prebake was carried out at 90°C for 5min. Sensitivity of resists was measured after the exposure with CA 800(Cobilt) or KrF excimer laser(0.9mJ/cm2/1 pulse). The polymer structure was determined by iH-NMR, 13C-NMR(FX90Q apparatus,JEOL) and 2 Si-NMR. The molecular weight distribution was determined with a Toyo Soda Model 801 gel permeation chromatograph at 40°C. The four columns were connected in series, each packed with G-2000H8x3 and G-400H8(Toyo Soda polystylene gel), respectively. 

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