Excimer

About: Excimer is a research topic. Over the lifetime, 3725 publications have been published within this topic receiving 75104 citations.

Non-conjugated bichromophoric systems. Part 4. Synthesis and photochemical study of bis-9-anthryls with a four-membered chain; influence of the replacement of methylene links by oxygen atoms or dimethylsilyl groups on the formation of intramolecular excimers and photocyclomers

Abstract: Bichromophoric systems (ICC, IOO, IO1, IO2, and ISic) incorporating various four-membered links were prepared in order to study the influence of the nature of the linkages upon the photophysical and photochemical properties; these systems are bisanthracenes A–Z–A where A = 9-anthryl and Z =(CH2)4(ICC), O(CH2)2O (IOO), O(CH2)3(IO1), CH2O(CH2)2(IO2), and SiMe2(CH2)2SiMe2(ISiC). By irradiation at high dilution, each of these compounds yields a single photocyclomer PXX linked through the 9,10 positions of one ring and the 9′,10′ positions of the other, except for ISic where the bonds were formed between the 9,10 and 1′,4′ positions. The reaction quantum yields were measured at 365 nm in methylcyclohexane. The fluorescence was examined in the same solvent under stationary conditions at different temperatures and by transient kinetic analysis (single-photon counting). Three of these compounds (ICC, IOO, IO1) did not display any intramolecular excimer fluorescence; a thermodynamic study indicates that the non-radiative decay of the S1 excited state leads essentially (φi.s.c. being very small) to the photoproduct PXX and the starting material IXXvia a minimum of the potential surface (pericyclic minimum) with respective partition coefficients α and (1 –α): α= 0.13 (ICC), 0.28 (IOO), 0.27 (IO1). This shows the influence of the replacement of a CH2 group by oxygen. The other two derivatives (IO2 and ISiC) exhibit locally excited and excimer fluorescence, the latter being suggestive of large overlap between the aromatic rings. The excimers were postulated to be on the reaction pathway; the first collapses readily to the photocyclomer PO2–(φR 0.28) whereas the second, due to steric strain, leads to PSiC with a very low yield (φR 8 × 10–4;kRDca. 105 s–1). It was observed that the introduction of an oxygen atom instead of a CH2 group enhances the cycloaddition efficiency but the second oxygen did not show any further improvement. The presence of the bulky SiMe2 groups helped the excimer state to be reached (rate of formation at room temperature kDMca. 109 s–1) but strongly hindered the formation of the photocyclomer (kRDca. 105 s–7). The last result clearly shows that photocyclomerization depends first on the intra-chain interactions controlling the rate of formation of an overlapping conformation and then on possibly more severe inter-linkage and chain-ring (with the so-called ‘peri’ hydrogens) repulsions, which can be a powerful brake in the last step of the build up of cycloadduct.

Bovine Corneal Stroma Ablation Rate With 193-nm Excimer Laser Radiation: Quantitative Measurement

Abstract: Excimer lasers operating at 193 nm may become important surgical instruments in ophthalmology because of their ability to ablate tissue with excellent precision and minimal damage to adjacent tissue. However, the precision is limited by the accuracy of the measurement of the amount of tissue ablated per pulse at the fluence used. A measurement of the ablation rate of bovine corneal stroma over the range of fluences most likely to be useful for corneal surgery (50-400 mJ/cm2) is presented. The technique used produced reproducible results, the data from 47 animal eyes being averaged to further increase the precision. For this range of fluences, these results show a more precise measurement than previously published tissue ablation rate data. These results should be useful in accurately predicting the result of corneal surgery using the excimer laser if species differences are not major.

Multiresponsive Tetradentate Phosphorescent Metal Complexes as Highly Sensitive and Robust Luminescent Oxygen Sensors: Pd(II) Versus Pt(II) and 1,2,3-Triazolyl Versus 1,2,4-Triazolyl.

Abstract: Two Pd(II) complexes based on tetradentate chelate ligands with either a 1,2,4-triazolyl (Pd1) or 1,2,3-triazolyl (Pd2) unit were synthesized, and their structure-property relationships were studied. Both Pd1 and Pd2 are rare bright deep blue Pd(II) phosphors with contrasting properties. Pd1 displays stimuli-responsive luminescence in response to UV irradiation, concentration, or temperature change, which is ascribed to the facile switching of monomer to excimer emission. In contrast, a similar stimuli-responsive luminescence was not observed for Pd2. Crystal structures and time-dependent density functional theory computational studies established that the excimer formation of Pd1 is caused by electronically favored intermolecular π-π interactions and less steric protection of the Pd core because of the position of its alkyl chains, compared to Pd2. In solution, the excimer emission of Pd1 shows a much greater sensitivity toward oxygen than the monomer emission with a very large Stern-Volmer constant ( Ksv) that is more than twice that of the monomer emission. Both Pd(II) complexes are found to be outstanding oxygen sensors in ethyl cellulose films with superior sensitivity ( Ksvapp = 0.228-0.346 Torr-1) over their Pt(II) equivalents ( Ksvapp = 0.00674-0.0110 Torr-1), owing to their long phosphorescence decay lifetimes. Furthermore, Pd1 shows an excellent photostability, compared to the Pt(II) analogue, making it one of the best and highly robust oxygen sensors based on cyclometalated metal complexes.

X-Ray production by irradiation of solid targets with sub-picosecond excimer laser pulses

Abstract: A table-top excimer laser system generating sub-ps pulses was used to irradiate solid targets at intensities of up to 4×1015 W/cm2. Soft X-ray spectra of various materials were measured. The X-ray conversion efficiencies were between 1–5%. Streak camera measurements showed instrument-limited X-ray pulse duration of a few ps.