Excimer characterized

H. (2001) Characterization of water contribution to excimer laser ablation of collagen. J. Photochem. Photobiol. A, 145, 195-200. 

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 ... 

It is confirmed that the polymer matrix around ablated area was also affected strongly by laser ablation. The change of the matrix properties are brought about over a few tens of pin. This type of information is basically important and indispensable for practical applications such as excimer laser lithography. The time-resolved fluorescence spectroscopy is one of the powerful characterization methods for ablated polymer matrix. 

The steady-state fluorescence intensities are obtained by integration of Eqs (4.43) and (4.44). The ratio of the fluorescence intensities of the excimer and monomer bands, Ie/Im (Figure 4.6), is often used to characterize the efficiency of excimer formation. This ratio is given by... 

The time-resolved spectroscopy is a sensitive tool to study the solute-solvent interactions. The technique has been used to characterize the solvating environment in the solvent. By measuring the time-dependent changes of the fluorescence signals in solvents, the solvation, rotation, photoisomerization, or excimer formation processes of a probe molecule can be examined. In conventional molecular solutions, many solute-solvent complexes. 

The further assumption that 3M is degenerate with the correlating molecular triplet state 3M provides an estimate of the energy (3M ) of this state in the region (XM ) > (3M ) > E(3M ) which may be spectroscopically inaccessible. Double intersystem crossing to different molecular triplet states of naphthalene87 is also apparently exhibited by the excimer of 1,6-dimethylnaphthalene40 in which the nonradiative process is characterized by a rate constant kf which is the sum of temperature-dependent and temperature-independent terms. The value of the latter is also consistent with a spin-prohibited process (Table XVI). 

The objective of this review is to characterize the excimer formation and energy migration processes in aryl vinyl polymers sufficiently well that the excimer probe may be used quantitatively to study polymer structure. One such area of application in which some measure of success has already been achieved is in the analysis of the thermodynamics of multicomponent systems and the kinetics of phase separation. In the future, it is likely that the technique will also prove fruitful in the study of structural order in liquid crystalline polymers. 

In conclusion, there is much to be done in characterizing the photophysics of naphthalenophanes. The fact that the eclipsed isomers emit at lower energies relative to the noneclipsed isomers is in accord with the assignment of the parallel-plane sandwich structure to the naphthalene solution excimer. 

While the photodimerization of bis(l-naphthylmethyl) ether was acknowledged somewhat earlier 39), the photodimers were first characterized and the quantum yield of the dimerization determined by Todesco et al. U2). Both the syn- and anti-photodimers were formed in roughly equal amounts, and the quantum yield for formation of the anti-dimer was independent of solvent. However, the quantum yields for formation of the syn-dimer and for excimer fluorescence were found to vary with solvent such that their sum was independent of solvent. The fact that irradiation of l,3-bis(l-naphthyl)-1-propanol yields only the syn-photodimer 113> indicates that the conformational properties of oxygen are largely responsible for anii-dimerization in the ether compound. The possibility of photodimerization was unfortunately not considered in the fluorescence studies of protonated bis (1-naphthylmethyl) amine 115>, l,3-bis(4-methoxy-l-naphthyl) propane 116>, and meso-bis( 1 -(1 -naphthyl-ethyl) ether 13). 

The quantum yields and decay rates of the intermolecular excimer of naphthalene and its derivatives are given in Table 8. The solvent ethanol water 95 5 v/v is one of the few solvents in which the fluorescence of these compounds has been completely characterized. Examination of the values of kD and QM for other solvents shows that 95 % EtOH does not belong in the same class as the hydrocarbon solvents, or even anhydrous ethahol. In the latter solvents, kD/kM falls between 0.8 for 1,6-dimethylnaphthalene and 1.4 for naphthalene. Although the quantity k /k has been measured only once for a naphthyl compound in a hydrocarbon solvent (see Table 5), the values 0.3 and 0.4 seem appropriate for 1,6-dimethylnaphthalene and naphthalene, respectively, in hydrocarbon solvents. Since QD/QM = (kpD/kpM) -s-(kD/kM), we obtain QD/QM = 0.4 for 1,6-dimethylnaphthalene and 0.3 for naphthalene. The intrinsic quantum yield ratio as determined in 95 % EtOH solvent is about seven... 

As noted earlier, the limiting lifetime of pyrene excimer fluorescence from concentrated solutions in PS and PMMA glasses was found to be the same as that of pyrene in cyclohexane solution. There have been no similar studies of naphthyl compounds in rigid glasses. Values of k and Q for the [2,6]-naphthalenophanes have not yet been determined for any solvent system. The bis(2-naphthyl) compounds have not been quantitatively characterized in rigid matrices, probably because excimer fluorescence is weak and difficult to detect under such conditions. Given such limited data, it can only be assumed that the values of QD and kD of 2-naphthyl excimers remain the same in rigid solution as in fluid solution. 

Excimer fluorescence from polychromophoric compounds in rigid systems, while easy to detect, is difficult to interpret. The transient response of the excimer can be empirically characterized by the limiting lifetime T p. In the absence of processes which convert D to M, this limiting lifetime is the reciprocal of k . We will examine xa]D for PS, P2VN, and other aromatic polymers to see if there is any difference between fluid and rigid solution at room temperature. 

The fluorescence of phenyl or 2-naphthyl excimers is characterized by a single broad peak for both inter- and intramolecular formation. The parallel sandwich arrangement of chromophores is the only possible configuration for the excimer. 

On the other hand, pyrenyl-L-alanine 184 has also been used as a conformational probe in the characterization of an artificial 4-a-helix bundle protein.11,121 The 53-residue peptide 186 incorporating one residue of 184 in each of two different helical segments was synthesized by solid-phase synthesis using a segment condensation strategy and the oxime resin. Boc-pyrenyl-L-alanine 191 was coupled just like any other amino acid by the BOP/HOBt method in DMF. CD and fluorescence studies demonstrated that the two pyrene groups were in close proximity forming an excimer complex, which is possible only when the polypeptide chain folds into a 4-a-helix bundle structure. 

The excited state properties of bis-a-9-anthrylmethyl ethers 23 closely resemble those of l,3-di-9-anthrylpropane derivatives. The photoexcited parent compound 23a deactivates by fluorescence (0 = 0.03) from the locally excited state only, and it isomerizes by intramolecular 4ji+4ji cycloaddition with a quantum yield of 0.32 [66]. By contrast, excimer emission (see Table 4) does characterize the excited state properties of the 10,10 -diphenyl derivative 23b, which does not undergo intramolecular cycloaddition for steric reasons [66,67]. 

Exceptional fluorescence properties also characterize the ri.s-isomer 38e. Unsubstituted cis-l,2-di-9-anthrylethylene 38a and its monosubstituted derivatives such as 38b are nonfluorescent at room temperature. By contrast, cis-dianthrylethylene 38e does fluoresce with quantum yields of 0.0018, 0.0042, and 0.0064 in cyclohexane, dichloromethane, and acetonitrile, respectively. The emission is structureless (see Figure 18), and is associated with a solvent-independent Stokes shift of about 6000cm-1. As the molecular geometry of 38e is characterized by overlapping anthracene systems [80], the structureless emission may be attributable to an intramolecular excimer state. 

Samples of 1 (200 mg) were sealed in evacuated Pyrex ampoules (inner diameter 4 mm) and immersed in a 500-mL Pyrex beaker filled with ice and water in such a way that no ice blocked the laser beam. The beam of an excimer laser (Lambda Physics, EMC 201 XeCl 17 ns pulses 50 Hz repetition rate 3 h X = 308 nm) was positioned vertically using two dielectric mirrors and focused to the desired intensity by a quartz-lens with a focal length of 20 cm. For low intensity irradiations, the ampoules were placed in front of a mercury arc at a distance of 5 cm. The product ratio depended on the light intensity. The compounds 1, 2, 3 and 4 were separated by gas chromatography or HPLC on RP18 and spectroscopically characterized after 93-97% conversion to 3 and 4. 

In 1977, Scharf and Mattay [123] found that benzene undergoes ortho as well as meta photocycloaddition with 2,2-dimethyl-1,3-dioxole and, subsequently, Leismann et al. [179,180] reported that they had observed exciplex fluorescence from solutions in acetonitrile of benzene with 2,2-dimethyl-l,3-dioxole, 2-methyl-l,3-dioxole, 1,3-dioxole, 1,4-dioxene, and (Z)-2,2,7,7-tetram-ethyl-3,6-dioxa-2,7-disilaoct-4-ene. The wavelength of maximum emission was around 390 nm. In cyclohexane, no exciplex emission could be detected. No obvious correlation could be found among the ionization potentials of the alkenes, the Stern-Volmer constants of quenching of benzene fluorescence, and the fluorescence emission energies of the exciplexes. Therefore, the observed exciplexes were characterized as weak exciplexes with dipole-dipole rather than charge-transfer stabilization. Such exciplexes have been designated as mixed excimers by Weller [181],... 

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]. 

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