Polar Glass Research Articles

Solute–solvent exciplex versus twisted intramolecular charge-transfer state in anomalous fluorescence of 4-NN-dimethylaminobenzonitrile

We report on the influence which solvent deuteration and solute–solvent interactions in glassy matrices have on the fluorescence of 3,5,NN-tetramethyl-4-aminobenzonitrile and of 4-NN-dimethyl-aminobenzonitrile. The widely accepted assignment of the anomalous fluorescence of the latter compound to emission from a twisted intramolecular charge-transfer state does not explain the observations. It is pointed out that the large deuterium effect on the anomalous fluorescence may be understood in terms of the theory of radiationless electronic relaxation when the emission comes from a solute—solvent exciplex. Spectral shifts, which arise from coupling of solute electric dipole moments with the polarization of the surroundings, have been calculated for the case of a solute in a homogenous polar glassy matrix, relative to a solution in a non-polar glass. The shift found for the fluorescence of 3,5,NN-tetramethyl-4-aminobenzonitrile is not in agreement with acceptable values for molecular excited-state dipole moments. Emission from an exciplex is more in accordance with the shift. Because the amino group of 4-NN-dimethylaminobenzonitrile cannot rotate in glassy solutions, there is no exciplex emission from this compound in a polar glass. This does not exclude the formation of planar-solute exciplexes, which decay predominantly by a radiationless process. Our earlier suggestion concerning the formation of exciplexes with polar solvent molecules provides an adequate explanation of the facts for the decay of the first excited singlet states of 4-NN-dimethylaminobenzonitrile and related compounds. It may explain our observation of ion formation from these states in polar solvents, which occurs even when the amino group is restricted to be in plane with the phenyl ring and there is no anomalous fluorescence. We suggest that a planar-solute exciplex (E1) is the precursor of an emitting twisted-solute exciplex (E2). Then competition between ionic dissociation and rotational isomerization of E1 becomes an important factor, determining the quantum yield of anomalous fluorescence. A quantum chemical analysis of the consequences of overlap of atomic orbitals on the N atom of the amino group with lone-pair orbitals of the solvent molecule shows that both planar and twisted solute–solvent exciplex states may be bound states.

Time-resolved emission spectroscopy of 1,3-dimethyl indole in n-butanol

Nanosecond-time-resolved fluorescence spectra of 1,3-dimethyl indole in n-butanol have been recorded at temperatures of 85–280 K and show changes as a function of time analogous to those found on increasing the polarity of the medium. This suggests the fluorescence is due to a 1L b → 1A transition in a polar glass or fluid non-polar solvent, but that in a fluid polar medium the fluorescent state has significant charge-transfer character.

Polar glass ceramics

Abstract Pyroelectric-piezoelecric glass ceramics of polar materials like Li2Si2O5, Ba2TiGe2O8, Ba2TiSi2O8, and Various compositions in the systems Li2O-B2O3, Li2O-SiO2-ZnO, Li2O-SiO2-B2O3 have been prepared by oriented recrystallization of the glasses under a strong temperature gradient, providing a simple inexpensive process for preparing piezoelectric and pyroelectric materials. High pyroelectric responses were observed in these glass-ceramics. Values of piezoelectric d33 coefficients, frequency constants, electromechanical coupling coefficients and dielectric properties of glass-cermics were in close agreement with the values of respective single crystals.

Thermostable persilylated polar glass capillary columns by the ammonia‐etching method

AbstractStatic ammonia etching has been used to effect regular surface roughening of glass capillaries. Following such treatment, polar phases could be coated after persilylation with varioussilylation agents such as hexamethyl‐, tetramethyldiphenyl‐, and tetra‐ phenyldimethyldisilazane, respectively. This method produces columns of high thermostability, which is limited only by the maximum applicable temperature for the phase itself.

Raman scattering studies of the impurity-induced ferroelectric phase transition in KTaO3:Li

Raman scattering and optical depolarization measurements have been employed in investigations of the phase transition induced by the substitution of lithium for potassium in KTa${\mathrm{O}}_{3}$ to form ${\mathrm{K}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}\mathrm{Ta}{\mathrm{O}}_{3}$. Lithium concentrations as high as 5.4 mol% were studied. At low concentrations, $x<~0.004$, the intensities of the disorder-induced scattering features observed for pure KTa${\mathrm{O}}_{3}$ increase as the Li fraction increases. A low-frequency feature, which was previously associated with a Li resonance mode, appears to be scattering from the coupled TA and TO branches due to disorder introduced by the lithium. For $x\ensuremath{\gtrsim}0.01$, a clearly defined step in the optical depolarization is observed at a temperature ${T}_{c}$ which increases rapidly with lithium concentration and reaches a value of ${T}_{c}=70$ K at $x=0.054$. The dependence on the polarization direction of the incident light indicates that a tetragonal or orthorhombic low-temperature phase is formed with $〈100〉$ symmetry axes. New Raman-active phonons are observed near and below ${T}_{c}$. The anisotropy of the energy of the extraordinary phonon expected from polar modes is not observed in the unpoled samples. Poling can be accomplished by cooling the crystals through ${T}_{c}$ with an applied electric field, after which the polar character of these phonons is observed. The lack of anisotropy in unpoled samples is the result of the presence of ferroelectric domains with diameters smaller than optical wavelengths. Additionally, the depolarization measurements imply domain sizes of at least a few thousand \AA{}. These results clearly disagree with the previous claims that ${\mathrm{K}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}\mathrm{Ta}{\mathrm{O}}_{3}$ is not ferroelectric for $x<0.24$ and that the Li centers form a polar glass at low temperatures. Finally, the presence of the lithium impurities stiffens the TO branch at the zone center, and the ferroelectric phase results from an order-disorder transition of the off-center lithium ions.

Birefringence measurements on KTaO3:Li

Birefringence observations in a single crystal of K1-xLixTaO3 (x=0.026) indicate a first-order transition to a domain state with sections of large uniform spontaneous polarisation. Measurements under small external electric fields suggest that the main difference with usual ferroelectrics lies in the extremely long relaxation times of the present system. The relation of the author's results with the reported polar glass behaviour of the material is discussed.

Stabilisation of polarised clusters in KTaO3by Li defects: formation of a polar glass

Dielectric relaxation and electret polarisation effects in KTaO3:Li have been investigated as a function of Li concentration. The data reveal the presence of polarised clusters which are stabilised by Li impurities and permit a determination to be made of the coherence length for ferroelectric order in KTaO3. Based on the polarisation results, the KTaO3:Li system exhibits characteristics which indicate that it represents an electric dipole analogue of a spin glass and therefore should be termed a 'polar glass'.

On the geminate recombination of UV - and γ-produced electrons in a polar glass at 77 k

An ethylene glycol/water glass containing the chromophore L-tryptophan and the electron scavenger CCl 3COONa was exposed to 254 and 310 nm UV light and to γ-rays and studied by ESR methods. The total yield of radicals per absorbed quantum of radiation increases with increasing scavenger concentration. This increase is attributed to the ability of the scavenger to prevent electrons from recombining with their parent cation. The recombination probability of ejected elec- trons was estimated to 0.8 in the case of UV-induced ionization and 0.4 in the case of γ-induced ionization.

Electron Traps in Irradiated Water–Ethylene Glycol Glass at 4 and 77°K

In order to study the mechanism of electron trapping in irradiated glasses of water–ethylene glycol mixture, they were subjected to γ rays and examined by optical absorption and electron spin resonance measurements at 4 and 77°K. Most trapped electrons formed at 4°K give an optical absorption band in infrared region around 1800 nm and a single-line electron spin resonance spectrum with the width of 3 G, while the trapped electrons formed at 77°K give an absorption band in visible region (λmax = 585 nm) and an electron spin resonance spectrum with the width of 15 G. By warming the glass to 77°K after the irradiation at 4°K, the trapped electrons transformed to those observed for the irradiation at 77°K. Results indicate that the nature of trapped electrons formed at 4°K is very similar to that of trapped electrons in irradiated nonpolar alkane glasses. Even in the polar glass of water–ethylene glycol, the orientation of molecular dipoles is not needed for the formation of electron traps at 4°K, where the molecular movements are highly restricted. By raising the temperature to 77°K, the molecular dipoles in the vicinity of trapped electrons orient favorably to deepen the traps. The yield of trapped electrons, for 100 eV energy absorbed, at 4°K is estimated to be larger than 0.6. However, it is possibly less than 2.0, the yield for the irradiation at 77°K.

Luminescence and binding studies on tRNA-Phe.

The phenylalanine transfer RNA of baker's yeast (tRNA(Phe)) contains a base Y of unknown molecular structure next to the anticodon triplet. Since the base Y fluoresces at room temperature (lambda(max) = 431 nm), its emission properties offer a unique tool for studying conformational and binding properties of tRNA(Phe). The results obtained by these experiments include the following: (1) The quantum yield of fluorescence of Y in tRNA(Phe) (phiF) is 0.07 +/- 0.01 at high Mg(2+) concentrations (>10(-2)M) and about half that at 10(-3)M or less, indicating a [Mg(2+)]-dependent conformational change of the anticodon loop. (2) The fluorescence of Y isolated from tRNAPhe (Y(+)) is red-shifted by 15 nm compared to Y in tRNA(Phe) which suggests a stacked (more hydrophobic) environment for Y in the intact anticodon loop. phiF of Y(+) is 0.035. (3) The solvent isotope effect phiF(D(2)O)/phiF(H(2)O) is 1.5 for tRNA(Phe) and 1.9 for Y(+) i.e., Y in tRNA is still hydrated. (4) The temperature dependence of phiF in a polar glass shows that quenching occurs only at temperatures at which the glass has sufficiently low viscosity to permit solvent shell relaxation in the excited state. The low-temperature (80 degrees K) fluorescence is blue shifted (lambda(max) = 409 nm) and the phosphorescence has a decay time of 1.5 seconds, a threshold at 392 nm and a spectral shape like that of guanine. (5) In the presence of 10(-2)M Mg(2+) penta-uridylate, which contains the codon triplet, a small blue shift and a decrease in phiF are observed. This shift can be used to establish the formation of a binary complex between the codon and the anticodon with an association constant of 4 x 10(2)M(-1), approximately. A similar complex is formed with poly-uridylate but not with poly-cytidylate. In the absence of Mg(2+) the binary complex is not formed.

Peel Adhesion: Influence of Surface Energies and Adhesive Rheology

Abstract The adhesion properties of polymers are known to be influenced by both intermolecular forces operative at the interface and the rheological history of both bonding and unbonding. Recent adsorption and viscoelastic theories of adhesion and cohesion are implemented in a comprehensive examination of these phenomena. Eight peel force “master curves” extending over 14 decades of reduced rate and representing glassy state to flow region rheology are superimposed to provide a composite response envelope. Each master curve represents rate-temperature reduced adhesion of an alkyl acrylate adhesive (γc = 26 dyne/cm) to substrates ranging from low adhesion fluorinated polymers (γc = 15 to 17 dyne/cm) to polar poly-amide surfaces (γc = 45 dyne/cm) and glass. The rate dependent transition from interfacial to cohesive failure, a subject not treated by adsorption theory, is shown to be coincident with the onset of entanglement slippage within the polymeric adhesive. Thermodynamic criteria of polymer adhesion are shown to be applicable only to the flow region of polymeric response. This study indicates that measured surface tensions or calculated surface energies of polymeric solids do not properly account for the contributions of three dimensional network structure of the polymeric bulk phase to its total work of cohesion. Evidence of true interfacial failure of a polymer-polymer bond is supported by critical surface tension measurements.

Fluorescence Quenching and Isotope Effect of Tryptophan

The fluorescence spectra and quantum yields of trytophan (TRP) and several tryptophan derivatives in water and a polar glass have been measured over wide temperature ranges. For TRP, the wavelength of maximum emission shifts from 310 nm at 80°K to 355 nm at room temperature with almost all of the red shift occurring between 170° and 230°K, which is the temperature range where the glass softens. The successively more red-shifted spectra have no isoemissive wavelength, which supports the view that reorientation of several solvent molecules in the solvent shell of the excited TRP molecule and not a 1:1 exciplex is responsible for the red shift. The quantum yield remains constant until a temperature is reached at which solvent reorientation is virtually complete. Above that temperature, the quenching of the 355-nm emission can be fitted with a nonradiative de-excitation having an activation energy of 7 kcal/mole. A model for these spectral changes and quenching mechanism will be offered. In deuterated solvents, a large isotope effect of fluorescence yield of TRP has been reported. This effect is clearly not caused by proton transfer in the excited state since it is found to be virtually the same for TRP and 1-Me-TRP. At temperatures below those at which solvent reorientation occurs, the isotope effect vanishes, and above them it approaches an asymptotic value, the quenching activation energy being independent of the isotopic constitution of the solvent. The fluorescence of most proteins and hormones originates in their tryptophan residues. The quantum yields, isotope effects, and emission spectra of such polypeptides are compared with the corresponding parameters for solvated TRP.