Photolysis of Na<sup>+</sup>(Cryptand[2.2.2])Na<sup>-</sup>:  Photobleaching of Absorbance and Quenching of Fluorescence

Hendrickson, James Erik; Xu, Guangzhou; Pratt, and William P.; Dye, James

doi:10.1021/jp9701092

Cited by 6 publications

(5 citation statements)

References 28 publications

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“…Early optical studies were made with films produced by rapid solvent evaporation from a liquid film on an optical cell window. − This provided qualitative spectra but no control of thickness or uniformity. Quantitative spectra and four-probe conductivity of uniform films with known stoichiometry and thickness can now be obtained in favorable cases by high-vacuum codeposition of the alkali metal and the complexant. − Of course, this synthesis method provides no control over the crystallinity or crystallite orientation. One cannot, in fact, guarantee that a solid-state reaction will occur between the metal and complexant, although this usually appears to be the case.…”

Section: Synthesis Methods and Problemsmentioning

confidence: 99%

Electrides: From 1D Heisenberg Chains to 2D Pseudo-Metals

1997

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Electrides are ionic compounds in which the cations are complexed by cryptands or crown ethers and the “anions” are trapped electrons. The crystal structures of five electrides are known and are similar to the corresponding alkalides (in which the anions are alkali metal anions) except that the anionic sites are “empty”. Theory and experiment strongly support a model in which the “excess” electrons are trapped in these anionic cavities and interact with each other through connecting channels, whose geometries vary significantly from one electride to another. Measurements of optical, alkali metal NMR, and EPR spectra, magnetic susceptibilities, and conductivities provide many data that can be correlated with the structures. Three electrides have essentially 1D chains of cavities connected by channels through which the electrons communicate, as indicated by magnetic susceptibilities that are well described by a 1D Heisenberg model. The electride, K+(cryptand[2.2.2])e- has a 2D array of cavities and channels. It appears that defects, probably missing electrons (holes), are responsible for its near-metallic conductivity. The fifth electride of known structure contains Cs+ complexed by a mixed sandwich of 15-crown-5 and 18-crown-6 and has a complex cavity−channel geometry, dominated by rings of six cavities. The arguments in favor of the proposed electride model, nearly-free electrons confined as a “lattice gas” in a complex array of cavities and channels, are presented in this paper.

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Section: Synthesis Methods and Problemsmentioning

confidence: 99%

Electrides: From 1D Heisenberg Chains to 2D Pseudo-Metals

1997

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“…The method was similar to that used for the study of Na-C222 and K-C222 films. 35,[37][38][39][40] Optical absorbance and four-probe conductivities were obtained as previously described. 35 Substrate temperatures during deposition were maintained constant and ranged from -40 to -70 °C for various runs.…”

Section: Methodsmentioning

confidence: 99%

“…Thin films (2200−4200 Å thick) of stoichiometry Rb C222 were prepared on a cold sapphire substrate by high-vacuum co-deposition of Rb and C222. The method was similar to that used for the study of Na-C222 and K-C222 films. ,− Optical absorbance and four-probe conductivities were obtained as previously described . Substrate temperatures during deposition were maintained constant and ranged from −40 to −70 °C for various runs.…”

Section: Methodsmentioning

confidence: 99%

Structure and Properties of a New Electride, Rb⁺(cryptand[2.2.2])e^-

et al. 2000

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This is the sixth electride whose crystal structure has been determined and the fourth to show polymorphism. Crystals of the title electride prepared from mixed solvents have a structure similar to that of Li + (cryptand[2.1.1])e -. Electrons occupy cavities that are connected by "ladder-like" channels. The static and spin magnetic susceptibilities of polycrystalline samples that contain this polymorph (called phase R) show Heisenberg 1D antiferromagnetic behavior with -J/k B ) 30 K. Similar to other electrides with "localized" electrons, this electride is a poor conductor (σ < 10 -4 ohm -1 cm -1 ). Thin films prepared by high vacuum co-deposition of Rb metal and cryptand[2.2.2] have optical spectra and near-metallic electrical conductivity nearly identical with those of K + (cryptand[2.2.2])e -. These properties would not be expected if the film structure were the same as that obtained for crystals. Rather, they suggest that the films consist of microcrystals whose structure is similar to that of K + (cryptand[2.2.2])e -. Polycrystalline samples prepared by slow evaporation of methylamine from stoichiometric solutions at -78 °C (called phase β) have properties similar to those of K + (cryptand[2.2.2])e -. The conductivity of samples that contain phase β is more than an order of magnitude larger than those with phase R. Magnetic and spin susceptibilities show that phase β samples have much larger electron-electron interactions. As with K + (cryptand[2.2.2])e -, the magnetic susceptibility of phase β is compatible with alternating linear chain Heisenberg antiferromagnetism, with -J/k B ≈ 300 K and -J′/k B ≈ 240 K. Thin vapor co-deposited films show abrupt changes in the conductivity and optical spectrum at -12 °C that suggest a transition that may be conversion of phase β to phase R.

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“…The preparation of stoichiometric thin films of K + (C222)e - and Rb + (C222)e - by high vacuum codeposition of the alkali metal and the complexant onto a sapphire substrate has been previously described in detail. − By separate control of the oven temperatures and monitoring of the rate of vaporization with quartz thickness monitors, the composition of the films could be controlled to about ±5%. The bell jar was equipped with a rotatable substrate holder that permitted the sample to be moved (after deposition) into positions for the measurement of optical absorption spectra and/or thermionic emission.…”

Section: Methodsmentioning

confidence: 99%

Thermionic Emission from Cold Electride Films

2000

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Thermionic electron emission from 2000−4000 Å thick films of K+(cryptand[2.2.2])e- and Rb+(cryptand[2.2.2])e- was studied as a function of temperature and time. The total charge collected was generally several hundred nanocoulombs with peak currents in the range of 5−100 pA. Although electron emission was not a direct result of thermal decomposition of the films, the peak current correlated with the decomposition rate. At a given temperature, the current increased gradually to a peak value and then decreased to near zero. Interrupting the process by cooling before complete decay, followed by a return to the same temperature, restored the previous current−time behavior. Once the current had decayed to near zero, the only way to restore emission was to raise the temperature above the previous value. The process of growth and decay was then repeated. The origin of emission is unknown, but the behavior suggests the presence of surface defect sites at grain boundaries that are initially empty. Sample decomposition fills these sites, which lie only a few tenths of an electronvolt below the vacuum level. Continued decomposition destroys the sites, but new defect sites can be produced by raising the temperature, probably by forming additional grain boundaries.

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Photolysis of Na⁺(Cryptand[2.2.2])Na^-: Photobleaching of Absorbance and Quenching of Fluorescence

Cited by 6 publications

References 28 publications

Electrides: From 1D Heisenberg Chains to 2D Pseudo-Metals

Electrides: From 1D Heisenberg Chains to 2D Pseudo-Metals

Structure and Properties of a New Electride, Rb⁺(cryptand[2.2.2])e^-

Thermionic Emission from Cold Electride Films

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Photolysis of Na+(Cryptand[2.2.2])Na-: Photobleaching of Absorbance and Quenching of Fluorescence

Cited by 6 publications

References 28 publications

Electrides: From 1D Heisenberg Chains to 2D Pseudo-Metals

Electrides: From 1D Heisenberg Chains to 2D Pseudo-Metals

Structure and Properties of a New Electride, Rb+(cryptand[2.2.2])e-

Thermionic Emission from Cold Electride Films

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Product

Resources

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Photolysis of Na⁺(Cryptand[2.2.2])Na^-: Photobleaching of Absorbance and Quenching of Fluorescence

Structure and Properties of a New Electride, Rb⁺(cryptand[2.2.2])e^-