Glassy Host Research Articles

Spectral diffusion in organic glasses. Temperature dependence of permanent and transient holes

Spectral diffusion of the S 1←S 0 0—0 transition (Q y ) of bacteriochlorophyll a (BChl a) as guest in the glasses ethanol (EtOH), triethylamine (TEA), 2-methyltetrahydrofuran (MTHF) and diethylether at low temperature has been studied by transient and permanent hole burning with a single-mode GaAlAs diode laser at ≈ 780 nm. Transient holes decay on a time scale of about 100 ms, determined by the triplet-state lifetime of BChl a. Their widths are compared to those of permanent holes probed on a time scale of minutes; both follow a T 1.3±0.1 temperature dependence between 1.2 and 4.2 K and extrapolate to the fluorescence lifetime-limited value for T→0. The amount of spectral diffusion, derived from the difference in width of permanent and transient holes over a time span between ≈ 10 −5 and ≈10 2 s, strongly depends on the glassy host.

Chromophore motion in polymers for nonlinear optics by soiid-state NMR

Abstract : There is much current interest in polymer materials for nonlinear optical (NLO) applications. These materials are commonly prepared by either dissolving (doping) a suitable NLO chromophore into a glassy or liquid crystalline polymer, or by covalently attaching the chromophore to the polymer host either by direct synthesis or radiative crosslinking. Regardless of the method of preparation, macroscopic orientation of the chromophore must be induced and maintained for efficient second-harmonic generation (SHG) in frequency doubling applications. For the most common use of a glassy polymer host, chromophore orientation is achieved by the process of poling. Briefly, the material is heated above Tg, a strong electric field is applied to orient the chromophores, and the material is cooled below Tg to lock in the orientation before the field is removed. A major problem in polymeric NLO materials is loss of SHG properties over time. For example, in a study of 4-(dimethylamino)-4'-nitrostilbene in PMMA, Hampsch and coworkers reported that up to 80% of the SHG efficiency was lost after 12 h at 298 K.

Persistent IR spectral hole-burning in chalcogenide glasses

All persistent spectroscopic effects observed to date in glassy hosts have involved electronic excitation of the defect center, with the hole-burning attributed either to photochemical modification of the defect itself, or to rearrangement of the host atoms around the defect. Here we describe the production of persistent infrared spectral holes in the vibrational spectrum of molecules doped into chalcogenide glasses, the first observation of non-electronic spectral persistence in a glassy host.

Non-photochemical hole-burning in Zn-substituted myoglobin

Abstract Persistent holes were burned in the inhomogeneously broadened 595 nm absorption band of Zn-substituted myoglobin (ZnMb) at 2 K. A prominent laser-induced hole-filling effect was observed, which suggests that the burning mechanism is non-photochemical. Zn-substituted protoporphyrin IX and the surrounding polypeptide chain in this material are considered to correspond to a localized center and a glassy host in ordinary hole-burning materials. The phonon sideband hole showed a maximum around 25 cm-1 from the zero-phonon hole, and the shape of the sideband was almost the same for the freeze-dried ZnMb and for ZnMb dissolved in a glycerol-water (3:1) mixture. This result indicates that the phonon sideband represents the low frequency vibrational modes in the protein molecule. The time evolution of the hole depth during the burning process was found to obey a logarithmic law, which can be explained by assuming a large dispersion of the burning rate. The distribution of the conformational barriers between nearly-degenerate ground states was also determined by using a heat cycle method. The distribution obtained obeys a power law which has no characteristic value of energy. All of the above results suggest the glass-like character of protein molecules.

Persistent infrared spectral hole burning of the sulfur—hydrogen vibrational mode in hydrogenated As 2S 3 glass

Persistent infrared spectral hole burning is observed for the sulfur—hydrogen vibrational absorption in hydrogen-doped As 2S 3 glass, the first observation of non-electronic spectral persistence in a glassy host. A dramatic increase in hole width with decreasing burn frequency demonstrates a strong correlation between the inhomogeneous broadening perturbation at a given center and that center's excited state lifetime. Holes are observed to narrow as they spontaneously refill. The non-exponential decay of the integrated hole area over a wide dynamical range and four decades in time is well described by a Gaussian distribution of tunnelling parameters.

Excimer fluorescence as a molecular probe of polymer blend miscibility: 8. Polymeric and glassy solvent host matrices

The effects of solubility parameter differences between the guest polymer and the host matrix and of guest molecular weight on the thermodynamic miscibility and chain configuration of poly(2-vinylnaphthalene) (P2VN) dispersed in polymeric or glassy solvent hosts are investigated using excimer fluorescence. For P2VN blends with a series of poly(alkyl methacrylates) in which the difference in the guest and host solubility parameters is minimized, the small increase in the excimer to monomer fluorescence ratio, I D I M , with increasing P2VN molecular weight can be rationalized by a one-dimensional energy migration model proposed by Fitzgibbon and Frank. The results indicate the possibility of small-scale phase separation or local coil contraction for P2VN with molecular weight greater than 21 000. As the thermodynamic interaction between the guest and polymeric host is altered towards immiscibility, I D I M can be related to the extent of intermolecular aggregation in the blend. To study the efficiency of intramolecular energy migration independent of intermolecular association and free of casting solvent effects, we chose a host system consisting of a series of monomeric glassy solvents. The rate of energy transfer is investigated by transient fluorescence and treated by a one-dimensional model developed by Fredrickson and Frank. The results are in excellent agreement with independently measured photostationary-state fluorescence ratios.

X-ray photoelectron spectroscopy of a cerium-doped lanthanum aluminosilicate glass

Core-level XPS spectra of a Ce-doped La aluminosilicate glass are reported. A deconvolution of the o 1s peak into two subpeaks corresponding to bridging and non-bridging oxygens indicates that 66% of the Al ions occupy tetrahedral sites and that the number of non-bridging oxygens in a (Si, Al)O 4 4− tetrahedron is 1.20 which is consistent with the value obtained from the Si 2p chemical shift. The 3d core levels of Ce and La show main and satellite peaks. The broadening of the Ce and La peaks in the glass compared with the spectra of the corresponding sesquioxides is attributed to the random nature of the glassy host.

Electron spin resonance and optical spectroscopic investigations of the reactions of active nitrogen with a few host—guest mixed polycrystals at 77 K

Chemiluminescences are emitted by the cold hydrocarbon surface during the reaction of active nitrogen with (host) polycrystals or glasses doped with an aromatic guest and condensed on a cold finger at 77 K. These active-nitrogen-induced chemiluminescences exhibit the following features: (i) with a polycrystalline host (aromatic or not) the guest phosphorescence is emitted; (ii) no chemiluminescence is observed with a glassy host; (iii) with a methyl-substituted benzene derivative, as a host or as a guest, the derived benzyl-like radical fluorescence is emitted. Both emissions (guest phosphorescence and radical fluorescence) are proportional to the square of the nitrogen atom N( 4S) concentration. Weak deferred luminescences are emitted by the hydrocarbon polycrystal after the reaction of active nitrogen. The main feature of the electron spin resonance spectra is the presence of cyclohexadienyl-type radicals. All these spectrospopic results (chemiluminescences and deferred luminescences) can be accounted for by the following mechanism: (i) two-body recombination of nitrogen atoms on the cold hydrocarbon surface and (ii) energy transfer from the 3∑ + u state of nitrogen to either the guest triplet state or the radical excited doublet state. A parallel is suggested with the well-known characteristics of two-quanta reactions.

Paramagnetic Resonance of Copper in Amorphous and Polycrystalline GeO2

The X-band electron paramagnetic resonance spectra of Cu+ + ions doped into amorphous, tetragonal, and hexagonal GeO2 have been observed at 295°K. Optical absorption and reflection data for, respectively, the doped glass and polycrystalline samples were also obtained. From the experimental results, the symmetry of the cupric complexes and some characteristics of the metal ion's bonding are deduced. In amorphous and tetragonal GeO2, the Cu+ + ions are basically in sixfold coordination. Because these cupric complexes exhibit a significant axial distortion (elongation), however, they have been treated as having square planar symmetry. Since the host cations in amorphous GeO2 are tetrahedrally coordinated, the presence of square planar Cu+ + complexes suggests that, to a significant extent, the cupric ions determine their immediate environment in the glassy host. The Cu+ + ions in hexagonal GeO2 appear to be located within the system of oxygen channels (tunnels) which are present in this crystal.