Ethanol Glass Research Articles

Phosphorescence emission and polarization of 3-carboxyquinoline

The purpose of this paper is to investigate the phosphorescence polarization and phosphorescence lifetime of 3-carboxyquinoline in ethanol and methylcyclohexane glasses at 77 K, in order to assign the symmetry of the lowest excited triplet state and to demonstrate the extent to which various optical criteria are affected by the breakdown of the Born-Oppenheimer approximation due to the ‘proximity effect’.

Photophysical behavior of 3-methylquinoline and 3-bromoquinoline

The phosphorescence emission and excitation spectra and their polarizations have been studied for 3-methylquinoline and 3-bromoquinoline in methylcyclohexane and ethanol glasses at 77 K. From the results obtained in 3-methylquinoline (τ p = 1.30 s in ethanol, τ p = 1.05 s in methylcyclohexane and the out-of-plane polarization) it is concluded that the emission originates from a triplet state of 3ππ ∗ character. In 3-bromoquinoline, the phosphorescence lifetimes (ca. 8 ms in a non-polar solvent and 15 ms in a polar solvent) and the negative degree of polarization indicate an emitting state of ππ ∗ character that is strongly coupled to a higher-lying triplet state of the nπ ∗ type. The most likely phosphorescence mechanisms for both molecules are proposed.

Low-temperature dynamics in an ethanol glass studied by the long-lived stimulated photon echo

The long-lived stimulated photon echo has been used to measure the dynamicsof a deuterated ethanol glass at 1.5 K. The time dependence of the effective dephasing time constant can be described by using the standard two-level system model and a 1/R distribution function for the relaxation rates of the two level systems, except for the kHz-MHz region where a gap exists. The standard TLS model, however, does not describe correctly the longitudinal decay of the stimulated photon echo during the waiting time.

The direct observation of hydrogen molecules in irradiated solid ethanol by means of Raman spectroscopy at 77 K

Hydrogen molecules produced in ethanol glass and crystal which were irradiated at 77 K were detected directly by Raman spectroscopy at 77 K. The bands at 4133 cm -1 for ethanol glass and 4155 cm -1 for ethanol crystal were ascribed to the stretching vibration of hydrogen molecules. The assignment of the bands was confirmed by spectra of irradiated ethanol-d 1 and -d 5 glasses: a new band observed at 3610 cm -1 was due to the vibration of HD molecules. The intensity of the band at 4133 cm -1 decreased in irradiated ethanol glass containing the electron scavenger, CCl 4, at 77 K. This presents further evidence that the 4133 cm -1 band is due to hydrogen molecules, since the solvated electron is a precursor of the hydrogen atom.

Dynamics in a low-temperature glass: Fast generation and detection of optical holes

High-resolution, fast optical hole-burning results are reported for the amorphous system cresyl violet in ethanol glass at 1.3 K. Holes are burned and detected using a novel technique which allows precise detection of narrow (∼0.03 cm−1 ), shallow (∼1%) holes 10 μs to 50 ms after their generation. The technique is described in detail along with careful tests demonstrating the validity of its results. The hole width is observed to increase linearly with time when plotted against log time. Using the four time correlation function description of optical hole burning, the time-dependent increase in hole width (spectral diffusion) is shown to arise from a broad distribution of fluctuation rates in the glass with the probability of having a fluctuation at rate R proportional to 1/R. The 10 μs to 50 ms data is combined with hole-width data spanning the range 100 ms to 10 000 s and with two-pulse picosecond photon echo data. The two-pulse photon echo linewidth is calculated by extrapolating the fluctuation rate distribution obtained from the hole-width data to short times. The results are in excellent agreement with experimental echo results. The combined data from the two sets of hole-burning experiments provides a detailed description of the glass dynamics over nine decades of time, 10 000 s to 10 μs. Together with the two-pulse photon echo results, the data provide information on the glass dynamical behavior over seven decades faster in time as well. The net result is a description of the dynamics in low-temperature ethanol glass on time scales spanning 16 decades.

Glass dynamics probed by the long-lived stimulated photon echo.

The dynamics in an ethanol glass at 1.5 K has been investigated from picoseconds to milliseconds by two-dimensional stimulated-photon-echo measurements on zinc porphin. In this time frame the distribution of relaxation rates exhibits a 1/R dependence except for a gap stretching from about 1 kHz to 1 MHz. Evidence is presented for a time evolution of the glass structure that is not accounted for in the standard two-level-system model.

Fractal interpretation of non-Lorentzian persistent hole shapes in organic glasses

Non-Lorentzian persistent hole shapes of chromophore in organic glasses have been explained by considering fractal structures of the organic glass hosts. The measured hole shape is well reproduced by the Fourier transform of the non-exponentially decaying function of τ, exp(− cTτ D/3 ), where c is a constant, T is the temperature, and D is the fractal dimension of the glass host. The fractal dimension D obtained by fitting the theoretical curve to the observed hole shape of oxazine 1 in polyvinyl alcohol glass is 2.3 ± 0.1 and that of oxazine 1 in ethanol glass is 2.9 ± 0.1. These values of fractal dimension are in good agreement with the values obtained in our previous experiment in both glass hosts doped with a different chromophore, HITC.

Spectral diffusion in glasses: A photon-echo study of zincporphin in ethanol

Results of picosecond photon-echo experiments on zincporphin in an ethanol glass at 1.5 K are reported and discussed. At 1.5 K, the two-pulse photon echo yields a pure dephasing time constant, a factor of 5.7 larger than the long-lived stimulated photon echo for a waiting time of 25 ms. This result accords with the prediction of the Bai—Fayer theory. However, the decay of the accumulated photon echo is much longer than expected.

Extreme environmental sensitivity of the rate of intramolecular electron transfer in mixed-valence 1',1'''-dibenzylbiferrocenium triiodide

The extreme sensitivity of the rate of intramolecular electron transfer for the mixed-valence 1',1'-dibenzylbiferrocenium cation to different environments is explored. Two different crystalline morphologies of 1',1'-dibenzylbiferronium triiodide (1) could be identified by their different crystal habits (needle and plate-like crystals). They are structurally characterized by single-crystal and powder X-ray diffraction techniques. The single-crystal X-ray structure of a parallelepiped (needle) crystal of 1 at 298 K shows that this polymorph crystallizes in the triclinic space group Pi. The unit cell parameters are a = 9.900 (1) A, b = 9.962 (1) A, c = 10.193 (2) A, CY = 112.97 (2)O, @ = 114.50 (I)O, and y = 97.80 (1)O with Z = 1. The structure is also reported for the same crystal at 135 K. An analysis of the full sphere of data again indicates Pi as the appropriate space group with unit cell constants a = 9.782 (2) A, b = 9.930 (1) A, c = 10.108 (2) A, CY = 113.08 (2)O, @ = 114.08 and y = 98.06 (1)' with 2 = 1. Refinements were carried out with 2990 (6a) and 3315 (6a) observed reflections at 298 and 135 K, respectively, to give R = 0.026 and R, = 0.037 at 298 K and R = 0.025 and R, = 0.038 at 135 K. Both the mixed-valence l', 1'-dibenzylbiferrocenium cation and triiodide anion are centrosymmetric at both temperatures. The two crystallographically equivalent metallocene moieties of the cation have dimensions intermediate between those of Fe and Fell' metallocenes. The packing of 1 consists of weakly interacting 1',1'-dibenzylbiferronium cations forming chains which are surrounded by and separated from other similar chains by the triiodide anions. The 296 K single-crystal X-ray structure of the plate-like crystals of complex 1 show that this pol morph crystallizes in the monoclinic space group P2,/n. The unit cell parameters are a = 22.286 (9) A, b = 19.413 (10) 1, c = 22.840 (5) A, and @ = 98.55 (3)O with Z = 12. Refinement was carried out with 4486 (6a) observed reflections to give R = 0.089 and R, = 0.131. There are three different l',l'- dibenzylbiferrocenium cations and three different Ip- anions in the asymmetric unit. There is not crystallographically imposed symmetry for any of the cations or anions. The temperature dependencies of the S7Fe Mossbauer spectra of the Pi and P2,/n polymorphs of complex 1 differ widely. A sample of needle crystallites has a Mossbauer spectrum characteristic of a valence detrapped mixed-valence species down to a temperature of 25 K. On the other hand, a sample of the P2,/n plate-like crystals exhibits a Mhsbauer spectrum characteristic of a valence-trapped species which remains trapped even at 300 K. It was surprising to find that mild grinding of the needle crystallites produces a valence-trapped Mossbauer signal in addition to the valence detrapped signal. Appreciable amounts of the trapped species persist for the ground Pi needles even up to 300 K. At 7 K, the EPR spectrum of a recrystallized microcrystalline sample of 1 consists of the superposition of three signals: two axial powder patterns each with its own gH and g, signals and one isotropic g = 2.16 signal. The g tensor anisotropies of the two axial signals are Ag = 1.29 and 0.76. The axial pattern with Ag = 0.76 is assigned to the Pi form, whereas, the Ag = 1.29 pattern is assigned to the P2,/n form. The two axial EPR patterns broaden with increasing temperature and the isotropic g = 2 signal gains in relative intensity, so that by 150 K it is the only signal observed. This Lorentzian g = 2.16 signal persists to high temperatures. In contrast, the variable-temperature EPR spectrum of an ethanol glass of 1 shows a single axial signal (Ag = 1.82) which broadens with increasing temperature and becomes unobservable above -77 K. The EPR properties of mixed-valence l', 1'-dibenzylbiferrocenium cation are modified by the local environments. Not only does the EPR signal change from the Pi to the P2Jn crystal forms but also there is even a sample history dependence of the g values for the two crystal forms. The g = 2.16 signal is the result of intermolecular interactions.

Probing low temperature glass dynamics by fast generation and detection of optical holes

High resolution, fast optical hole burning results are reported for the amorphous system cresyl violet in ethanol glass at 1.3 K. Holes are burned and detected using a novel technique which allows precise detection of narrow (≈ 0.03 cm −1), shallow (≈ 1%) holes 10 μ to 50 ms after their generation. The hole width is observed to change linearly when plotted against log time. Using a four point correlation model of optical hole burning, this spectral diffusion is shown to arise from a 1/ R distribution of fluctuation rates. The distribution extrapolates to the value of the two pulse photon echo linewidth measured on the same system.

Raman study of irradiated organic solids at very low temperatures—I. Ethanol glass and crystal

Raman spectra were measured at 5 K for ethanol glass containing 10 mol% CCl 4 and ethanol crystal, both of which were previously γ-irradiated at 77K. Variations in intensity of vibrational bands of ethanol and CCl 4, and appearance of new bands of ̽CCl 3 radical were observed in irradiated ethanol glass containing 10 mol% CCl 4. In irradiated ethanol crystal, the intensity of weakly hydrogen bonded OH stretching was decreased whereas that of strongly hydrogen bonded one was increased, and an enhancement of hydrogen bonding and lattice vibrations were observed.

Photophysical properties of pyridinium salts derived from purine bases

The absorption spectra, fluorescence spectra, fluorescence lifetimes, fluorescence quenching, phosphorescence spectra, phosphorescence lifetimes, and picosecond transient absorption spectra of several purinyl-pyridinium salts have been measured in several solvents at room temperature and in ethanol glasses at 77 K. It is concluded that the previously observed photochemical transformation of N-[9-(2′,3′,5′-tri-O-acetyl-β-D-ribofuranosyl)purin-6-yl]pyridinium chloride 1 into the highly fluorescent tri-O-acetylluminarosine 6 occurs via an excited triplet state. It is also shown that intersystem crossing in purinyl-pyridinium salts is induced by an intermolecular charge-transfer interaction with the counter ions. Keywords: purinyl-pyridinium salts, fluorescence, phosphorescence.

Solvation shell effects and spectral diffusion: Photon echo and optical hole burning experiments on ionic dyes in ethanol glass

Results of picosecond photon echo and optical hole burning experiments are reported for four ionic dyes in ethanol glass. At low temperatures, the dephasing times deduced from the hole widths are as much as nine times shorter than those measured by the two-pulse echo because of the effect of spectral diffusion. The temperature dependences found are of the form aTα+b exp (−ΔE/kT) due to glass two level system dynamics (T<4 K) and a process that activates exponentially at higher temperatures, possibly from a pseudolocal mode or glass optical phonon. Comparing the ratios of echo to hole burning measured dephasing times for the four dyes suggests that the dephasing is influenced by the existence of distinct local ethanol solvation shells in addition to the dynamics of the bulk solvent. A theoretical description of solvent shell effects is achieved through the use of a two spatial domain model of the glass dynamics. Calculations of dynamic perturbations from distinct solvation shell and bulk solvent regions show that the observed differences between the dyes’ dephasing ratios can be explained if the ionic chromophores alter glass dynamics locally.

Ultralow temperature effect on proton transfer reaction to benzene anions in gamma-irradiated ethanol glass

A proton transfer reaction to benzene anions in γ-irradiated ethanol was studied at 4.2 K by ESR. When an ethanol-benzene (5 mol.%) mixture is γ-irradiated at 4.2 K, benzene anions formed could be detected by ESR. Then, when the irradiated mixture is annealed at 77 K, the benzene anions change into cyclohexadienyl radicals. This observation indicates clearly the occurrence of the proton transfer reaction C 6H - 6+C 2H 5OH→C 6H 7+C 2H 5O - at 77 K. When the benzene anions are illuminated with tungsten light at 4.2 K, the anions change into cyclohexadienyl radicals even at 4.2 K. The results were interpreted in terms of a proton transfer reaction to excited benzene anions (C 6H -∗), formed by the illumination of light; C 6H -∗ 6+C 2H 5OH→C 6H 7+C 2H 5O -. The rate for the proton transfer reaction C 6H - 6+C 2H 5OH→C 6H 7+C 2H 5O - was studied by electron-pulse radiolysis at low temperatures. The proton transfer reaction to benzene anions takes place in a time shorter than 300 ns at 80 K, though the reaction is suppressed at 4.2 K.

Optical High Resolution Studies of Adsorbed and Matrixdoped Oxazine Dyes

AbstractFluorescence excitation and line narrowing spectra of oxazine dyes adsorbed on porous silica gels show maintenance of vibrational structure and small shifts in electronic absorption when compared to the spectra of the same molecules embedded in an ethanol glass. Persistent spectral holes burnt into the longwavelength tail of the S0—S1 absorption of the adsorbed species are broader than in bulk. This phenomenon is thought to be caused by low frequency adsorbate motions of the adsorbed molecules.

The nature of glass dynamics: thermal reversibility of spectral diffusion in a low temperature glass

Temperature cycling optical hole-burning experiments on cresyl violet in ethanol glass are reported. At 1.3 K a hole is burned and measured. The temperature if then raised to 2.1 K; the hole is measured. The temperature is returned to 1.3 K, and the hole is again measured. The hole is observed to be narrow at 1.3 K, broader at 2.1 K, and then narrow again when the temperature is returned to 1.3 K. Using the time-dependent hole widths and photon echo data at the two temperatures, the results can be accounted for quantitatively using the tunneling two-level system model of glasses. Models of glasses, which involve processes such as spatial diffusion of defects, cannot account for the data.

Substitution effect on the triplet state energy and decay of some disubstituted naphthalenes

Singlet- and triplet-state energy positions and triplet-state decay lifetimes of some 2,6- and 2,7-distributed naphthalene molecules, in which the substituents are OCH 3, -CN; -OCH 3, -CO 2CH 3; and -OCH 3, -COOH, have been determined in ethanol glass at 77 K. It has been found that the first excited singlet-state energy, the triplet-state energy and the triplet-state decay lifetime of the two members in each of the systems show similar dependence on the relative positions of the substituents. All the three are lower in the 2,7-compound than in the 2,6-compound.

Time evolution of non-photochemical hole burning linewidths: Observation of spectral diffusion at long times

Time-dependent non-photochemical hole burning linewidths for cresyl violet in ethanol glass are reported from 100 ms to 3000 s at 1.3 K. The linewidth increases by nearly a factor of two in this region, changing most rapidly in the range around 20 s. The increase in linewidth is directly related to the broad rate distribution of dynamic processes in the glass. Using the results of a theoretical analysis based on the four-point correlation function description of optical dephasing experiments, the fluctuation rate distribution is calculated. In the experimental time range, the distribution is characterized by a Gaussian function on a ln R scale centered at R 0 = 0.4 s −1.

Solute-solvent dynamics and interactions in glassy media: Photon echo and optical hole burning studies of cresyl violet in ethanol glass

Photon echo and non-photochemical hole burning experiments on cresyl violet in ethanol glass are reported. At 1.5 K the optical dephasing time measured by the picosecond echo is eight times longer than the time measured by hole burning because of the difference in the time scales associated with the measurements. The observed temperature dependence arises from the glass's two-level systems and from a pseudolocal phonon mode at higher temperatures. Comparing the ratios of the echo to hole burning dephasing times for cresyl violet and resornfin, suggests that their dephasing is influenced by the nature of their ethanol solvation shells in addition to the dynamics of the bulk ethanol.

Temperature-dependent fluorescence lifetime of an aldehyde derivative of azulene

The decay time of the fluorescence from the first excited singlet state of 4,6,8-trimethylazulene-1-aldehyde shows strong temperature dependence both in a frozen ethanol glass and in a poly(methyl methacrylate) matrix, over the range 77 – 180 K. The excited state decay is explained in terms of two separate non-radiative processes. The temperature-independent pathway is interpreted to be the direct internal conversion from S 1 to S 0, and the thermally activated process is assigned to vibrationally assisted internal conversion rather than intersystem crossing to the second triplet state. The thermally activated process possesses the same activation energy of 530 cm −1 in both matrices. No phosphorescence emission was detected.