Dynamics of p-terphenyl crystals at the phase transition temperature: a zero-field EPR study of the photoexcited triplet state of pentacene in p-terphenyl crystals

Pulsed zero-field electron paramagnetic resonance free induction decay (ZF EPR FID) techniques are employed to study the phase transition of p-terphenyl crystals (Tc = 193 K) by measuring selectively populated photoexcited triplet ZF transitions of dilute pentacene molecules in p-terphenyl single crystals. The sensitivity of ZF spectroscopy to small shifts in local magnetic fields enables the studies of guest-host configuration changes over a wide temperature range. Here we report the observation of guest pentacene (-h(14) and -d(14)) triplet ZF EPR FID spectra that disappear abruptly at Tc and of spectral broadening and shift below Tc. We interpret these spectral changes as evidence for guest couplings to host phenyl rings. Further, these data allow assignments of spectroscopic sites to crystallographic sites that occur in the phase transition.     

Singlet exciton energy transfer in tetracene-doped p-terphenyl single crystals

A picosecond laser system has been employed to study the kinetics of singlet exciton energy transfer in p-terphenyl single crystals doped with tetracene over the temperature range 77–300 K. First-order kinetics prevail and exciton migration from host to guest is controlled by thermal activation over a potential energy barrier.     

Singlet exciton migration in a random organic two-component system containing physical and chemical traps

Steady-state as well as time-dependent fluorescence studies have been carried out near 80 K on glassy mixed layers of anthracene and phenanthrene doped with tetracene and containing incipient anthracene dimers as physical traps. The efficiency of energy transfer from initially excited anthracene (“donor”) sites to tetracene (“supertrap”) has been measured as a function of donor concentration c_D. The results are interpreted in terms of a superposition of two transfer pathways: (i) Random walk of excitons across the donor ensemble to either tetracene or anthracene dimers and (ii) single step Förster transfer from the excited dimers (excimers) to tetracene. The efficiency of process (i) is in quantitative accord with the predictions of the 3D version of the Loring, Anderson and Fayer theory assuming dipole-dipole coupling.     

The nature of optical spectra of p-terphenyl

The absorption, fluorescence and fluorescence excitation spectra of p-terphenyl (TP) crystals have been investigated. The absorption spectrum of a thin (1 μm, 5 K) crystal shows several electron transitions with a strongly and weakly developed vibrational structure. It is found that even the purest TP crystals contain a large number of uncontrolled impurities absorbing in the region 331 – 390 nm, and the crystal fluorescence spectrum reported in the literature is due to the defect-impurity emission.     

Pressure induced orientational ordering in p-terphenyl

Three different interaction potentials existing in the literature along with a model proposed here have been used to model p-terphenyl under standard conditions. Of these, the model that predicts the room-temperature crystal structure well has been used to understand the behavior of p-terphenyl under pressure. Lattice parameters show good agreement with the X-ray diffraction values reported by Puschnig et al. (Puschnig, P.; Heimel, G.; Weinmeier, K.; Resel, R.; Ambrosch-Draxl, C. High Pressure Res. 2002, 22, 105). The nonplanar structure of p-terphenyl transforms to a planar structure with gradual disappearance of disorder associated with ring flipping. We show that the transformation is accompanied by a change in the potential energy profile from W-shaped to a U-shaped form, which is associated with complete planarization between 1.0 and 1.5 GPa. Our results reported here are in excellent agreement with X-ray diffraction results which also suggest the existence of a similar transition as a function of pressure in polyphenyls such as biphenyl and p-hexaphenyl. Interestingly, the amplitude of the torsional motion is largest at an intermediate pressure of 1.0 GPa. This is attributed to the rather flat potential energy landscape which occurs during the transition from W- to U-shaped potential.     

Conformation of p-terphenyl under hydrostatic pressure

The conformation of p-terphenyl (C18H14) and deuterated p-terphenyl (C18D14) has been investigated, using high-pressure infrared spectroscopy at liquid-helium temperatures. First-principles calculations, together with the experimental results, were performed to determine the structure of p-terphenyl in the twisted conformation. At low temperatures and pressures, p-terphenyl belongs to the C2 point group of symmetry. In this configuration, the central ring is twisted with respect to the plane of the outer rings. The symmetry of the molecule is nearly C2h, consistent with previous x-ray diffraction measurements.     

In-depth study into the interaction of single walled carbon nanotubes with anthracene and p-terphenyl

Solubilization of single walled carbon nanotubes (SWNT) in the presence of polycyclic aromatic hydrocarbons (PAHs) such as p-terphenyl and anthracene has been shown. The suspensions formed are stable for periods greater than 48 months but to date experimental research is scarce regarding the interactions that are taking place. Spectroscopic analysis such as Raman and fluorescence are used to probe the interactions occurring between the PAHs and the SWNT over a wide concentration range. Previous studies show the fluorescence of the PAHs is quenched on interaction with SWNT and in the case of p-terphenyl, the spectrum is red shifted. This result prompted a study of a large range of concentrations to quantify the degree of interaction between the SWNT and PAHs. It was found at high concentrations that both the PAHs and SWNT formed aggregates and at lower concentrations it was found that free PAHs and isolated SWNT were interacting. The radial breathing modes (RBMs) in Raman spectroscopy gave detail as to how diameter selective the PAH samples are when compared to the pristine SWNT modes. An increase in the wavenumber of the RBMs for both composite spectra was observed and it is believed that such a result is due to the debundling of the SWNT on interaction with the PAHs. It was also found that anthracene and p-terphenyl selectively interact with SWNT and the selected SWNT were found to be within a distinct diameter range and possessed unique physical properties.     

Pulsed electron nuclear double resonance studies of the photoexcited triplet state of pentacene in p-terphenyl crystals at room temperature

Pulsed electron nuclear double resonance (ENDOR) using a modified Davies-type [Phys. Lett. 47A, 1 (1974)] sequence is employed to study the hyperfine (HF) structure of the photoexcited triplet state of pentacene dispersed in protonated and deuterated p-terphenyl single crystals. The strong electron spin polarization and long phase memory time of triplet pentacene enable us to perform the ENDOR measurements on the S=1 spin system at room temperature. Proton HF tensor elements and spin density values of triplet pentacene are extracted from a detailed angular-dependent study in which the orientation of the magnetic field is varied systematically in two different pentacene planes. Analysis reveals that the pentacene molecule is no longer planar in the p-terphenyl host lattice. The distortion is more pronounced in the deuterated crystal where the unit cell dimensions are slightly smaller than those of the protonated crystal.     

Glassiness of thermotropic liquid crystals across the isotropic-nematic transition

The orientational dynamics of thermotropic liquid crystals across the isotropic-nematic phase transition have traditionally been investigated at long times or low frequencies using frequency domain measurements. The situation has now changed significantly with the recent report of a series of interesting transient optical Kerr effect (OKE) experiments that probed orientational relaxation of a number of calamitic liquid crystals (which consist of rod-like molecules) directly in the time domain, over a wide time window ranging from subpicoseconds to tens of microseconds. The most intriguing revelation is that the decay of the OKE signal at short to intermediate times (from a few tens of picoseconds to several hundred nanoseconds) follows multiple temporal power laws. Another remarkable feature that has emerged from these OKE measurements is the similarity in the orientational relaxation behavior between the isotropic phase of calamitic liquid crystals near the isotropic-nematic transition and supercooled molecular liquids, notwithstanding their largely different macroscopic states. In this article, we present an overview of the understanding that has emerged from recent computational and theoretical studies of calamitic liquid crystals across the isotropic-nematic transition. Topics discussed include (a) single-particle as well as collective orientational dynamics at a short-to-intermediate time window, (b) heterogeneous dynamics in orientational degrees of freedom diagnosed by a non-Gaussian parameter, (c) fragility, and (d) temperature-dependent exploration of underlying energy landscapes as calamitic liquid crystals settle into increasingly ordered mesophases upon cooling from the high-temperature isotropic phase. A comparison of our results with those of supercooled molecular liquids reveals an array of analogous features in these two important classes of soft matter systems. We further find that the onset of growth of the orientational order in the parent nematic phase induces translational order, resulting in smectic-like layers in the potential energy minima of calamitic systems if the parent nematic phase is sandwiched between the high-temperature isotropic phase and the low-temperature smectic phase. We discuss implications of this startling observation. We also discuss recent results on the orientational dynamics of discotic liquid crystals that are found to be rather similar to those of calamitic liquid crystals.     

Raman spectroscopic study of hydrogen ordered ice XIII and of its reversible phase transition to disordered ice V

Raman spectra of recovered ordered H(2)O (D(2)O) ice XIII doped with 0.01 M HCl (DCl) recorded in vacuo at 80 K are reported in the range 3600-200 cm(-1). The bands are assigned to the various types of modes on the basis of isotope ratios. On thermal cycling between 80 and 120 K, the reversible phase transition to disordered ice V is observed. The remarkable effect of HCl (DCl) on orientational ordering in ice V and its phase transition to ordered ice XIII, first reported in a powder neutron diffraction study of DCl doped D(2)O ice V (C. G. Salzmann, P. G. Radaelli, A. Hallbrucker, E. Mayer, J. L. Finney, Science, 2006, 311, 1758), is demonstrated by Raman spectroscopy and discussed. The dopants KOH and HF have only a minor effect on hydrogen ordering in ice V, as shown by the Raman spectra.     

States of molecular assembly and physical properties of crystalline long-chain compounds studied by vibrational spectroscopy

Various types of molecular assembly of long-chain compounds in solid states were investigated by means of infrared absorption, Raman and Brillouin spectroscopic methods. As for the polymorphism in even-numbered n-fatty acids, three monoclinic modifications, B, C, and E, all consisting of the orthorhombic polyethylene sublattice, give rise to their characteristic infrared and Raman spectra. A dynamical equilibrium between cis and trans conformations of the hydrogen-bonded carboxyl groups in modification C, which is related to the high-temperature stable character of this phase, is reflected to a dramatic change with temperature in the low-frequency Raman spectra. A new type of reversible solid state phase transition was found between two A-type (triclinic) modifications of myristic, palmitic, and stearic acids. The γ→α phase transition of oleic acid was found to be caused by a conformational disordering of polymethylene chains at the lamellar interfacial region. Two basic polytype structures, Mon and Orth II, of stearic acid B were investigated, and it was found that the low-frequency phonon frequencies (below 50 cm⁻¹) were strongly influenced by the polytype structure. Based on the spectroscopic considerations, Orth II was predicted as the thermodynamically stable phase around room temperature compared with Mon, and the stability is responsible for the vibrational free energy term. Some experimental findings which support this prediction were obtained. The values of the stiffness tensor elements of Mon and Orth II, measured by Brillouin scattering, indicate that the mechanical behavior of bulk crystals is very dependent on the polytype structure. The relationship between the mobility of chain molecules and the width of the spectral bands was investigated in a quantitative manner for the case of n-alkane molecules entrapped in the urea inclusion adducts. The changes in the half-width for the polarization components of various Raman bands on the transition from the orthorhombic to the hexagonal phase are interpreted in terms of the correlation functions of the Raman tensor related to the rotational motion of the alkane molecules around the chain axes.     

Thermal studies on polymorphic structures of tibolone

Tibolone polymorphic forms I (monoclinic) and II (triclinic) have been prepared by recrystallization from acetone and toluene, respectively, and characterized by different techniques sensitive to changes in solid state, such as polarized light microscopy, X-ray powder diffractometry, thermal analysis (TG/DTG/DSC), and vibrational spectroscopy (FTIR and Raman microscopy). The nonisothermal decomposition kinetics of the obtained polymorphs were studied using thermogravimetry. The activation energies were calculated through the Ozawa’s method for the first step of decomposition, the triclinic form showed a lower E _a (91 kJ mol⁻¹) than the monoclinic one (95 kJ mol⁻¹). Furthermore, Raman microscopy and DSC at low heating rates were used to identify and follow the thermal decomposition of the triclinic form, showing the existence of three thermal events before the first mass loss.     

Raman Investigation of the Low Temperature Phase Transitions in Monoclinic TlReO4

The Raman spectrum of monoclinic TlReO₄ is presented together with a factor group analysis and assignment of the bands. The phase transitions of monoclinic TlReO₄ is investigated below room temperature, and two phase transitions were identified between room temperature and 90 K. The room temperature monoclinic phase was converted to the orthorhombic phase at 170 K upon cooling, and to the tetragonal phase at 150 K. Upon heating back to room temperature the tetragonal phase persisted until the orthorhombic phase could be identified at 210 K and 220 K. From 230 K to room temperature the monoclinic phase could again be observed.     

Effect of gamma-irradiation on the thermal decomposition of pure and doped (Ba2+ caesium bromate

Effect of γ-irradiation on the isothermal decomposition of pure and doped (Ba²⁺, 0.50 mol%) caesium bromate has been studied in the temperature range 633–673 K. It is indicated that though the pure and pure irradiated crystals are immune to decomposition, doped and doped irradiated crystals undergo decomposition rapidly. There is initial rapid gas evolution representing 1–2% reaction, which is completely eleminated in doped irradiated crystals. The other stages exhibited by the crystals are, (1) acceleratory and (2) decay stages. Presence of two decay stages (one short and one long) is indicated in the doped substance, and the short decay diminishes with increase in temperature and virtually remains absent at 673 K. The acceleratory as well as decay periods of doped and doped irradiated crystals are analysed according to Prout-Tompkins, Avrami-Erofeev and Contracting square models. The rate constants in all the stages increase with increase in temperature. The energy of activation for the acceleratory periods of both the substances are almost same (± 10 kJ/mol) irrespective of the kinetic equation employed. Similar is the case with decay stages. But the energy of activation of the decay stages are higher than those of the acceleratory stages. Microscopic observation reveals that the reaction begins essentially on surfaces by the rapid formation of an interface and is followed by the penetration of the interface into the crystallite. The melting of a eutectic formed between the product CsBr and the parent material causes a marked increase in the rate.     

Zur Polymorphie von SrTeO3

Beside the transition into a ferroelectric phase at 583 K, monoclinic SrTeO₃(I) at ?1260 K transforms slowly into a high-temperature form (SrTeO₃(II)) which after quenching undergoes rapid phase transitions at 648 and 733 K. X-Ray data of triclinic α-SrTeO₃(II) (room temperature), monoclinic β-SrTeO₃(II) (660 K), and monoclinic γ-SrTeO₃(II) (770 K) are presented.     

Solid-state transitions in crystalline tetracene

Solid-state transitions in crystalline tetracene have been investigated by Raman spectroscopy and by anisotropy of the effect of a magnetic field on its prompt fluorescence. Discontinuous changes in the temperature dependence of Raman lattice modes and new phonon lines occur near 182 and 144 K, signifying the occurrence of two phase transitions. The magnetic anisotropy effect shows a gradual change with temperature, suggesting the transition at 182 K is preceded by a continuous rotation of the intermolecular plane.     

Study on the intermolecular interaction of C60 and simulations on the orientational properties of C60 in crystals

We developed the new intermolecular interaction model of C(60) with the quantitative accuracy for the molecular orientational properties in crystals. The energy difference (DeltaE) and the activation barrier (E(barrier)) between the two stable orientations (P and H orientations) in crystals are in the values of +14.7 and +260 meV in our model, respectively; these values are in fairly good agreement with the experimental values (DeltaE approximately +11 meV, E(barrier)=+235-+290 meV in experiments). The relaxation calculation for C(60) crystals using our model revealed that there is the reversal of the stable orientations between the P and H orientations under the high H-orientation occupancy (p(H)) in crystals, when p(H)>0.83, DeltaE<0. From the molecular dynamics calculations for C(60) crystals using our model, it is found that the phase transition is induced at T(C)=200-260 K, which is consistent with the experimental value of 260 K. Immediately below T(C), we found a great variety of molecular rotational jumps involving that between the P and H orientations every about 10(-9) s due to the thermal activation. In the high temperature phase (>T(C)), all molecules rotate irregularly like in Brownian motion involving the rotational "slumber" for approximately 10(-12)-10(-11) s.     

Spectroscopic properties of rare earth borohydrides: Er(BH4)3·3THF in pure and mixed crystals

The optical spectra of Er(BH₄)₃·3THF neat crystals and La, Gd, Y(BH₄)₃·3THF mixed crystals are reported and analyzed. Lanthanum borohydride is found to have a different room temperature crystal structure (triclinic) from Er, Gd, Y(BH₄)₃·3THF (Pbcn). At low temperature the Pbcn crystals undergo a phase transition to a structure with two crystallographically inequivalent sites in a unit cell. The optical spectra of Er(BH₄)₃·3THF in Er, Y, Gd(BH₄)₃·3THF crystals clearly evidence these two sites. Large vibronic intensity is observed at 1.6 K and 77 K and nine “molecular” vibrations are assigned. These modes are quite similar to those found for U(BH₄)₄. Er (BH₄)₃·3THF spectra are very different: no vibronic transitions are observed but many (often upwards of fifty for a given manifold) weak sharp “satellite” lines are found associated with pure electronic transitions. These data are discussed in terms of structural differences and comments on bonding and covalent character in lanthanide borohydrides are made.     

Structural chemistry of A3CuBO6 (A = Ca, Sr; B = Mn, Ru, Ir) as a function of temperature

Variable temperature X-ray and neutron powder diffraction techniques have been used to identify structural phase transitions in Cu-rich A(3)A'BO(6) phases. A transition from monoclinic to rhombohedral symmetry was observed by X-ray diffraction between 700 and 500 K in Sr(3)Cu(1-x)M(x)IrO(6) (M = Ni, Zn; 0 < or = x < or = 0.5). The temperature of the phase change decreased in a linear manner with Cu-content and was essentially independent of the nature of M. Ca(3.1)Cu(0.9)MnO(6) was shown to pass from a rhombohedral phase to a triclinic phase on cooling below 290 K; the structure of the triclinic phase was refined against neutron diffraction data collected at 2 K. Ca(3.1)Cu(0.9)RuO(6) undergoes a transition between a disordered rhombohedral phase and an ordered monoclinic phase when cooled below 623 K. Neutron diffraction has been used to determine the structure as a function of temperature in the range 523 < or =T/K < or = 723 and hence to determine an order parameter for the low temperature phase; the second-order transition is shown to be incomplete 100 K below the critical temperature.     

Singlet exciton trapping and heterofission in tetracene doped anthracene crystals

In tetracene doped anthracene, the magnetic field modulation of prompt tetracene fluorescence following excitation into the anthracene singlet manifold has been measured as a function of the magnetic field orientation and optical excitation energy. The results show that this modulation with low energy excitation is caused by singlet heterofission into one anthracene triplet exciton and one tetracene triplet. With higher excitation energies this modulation is due to both the singlet heterofission and also singlet homofission into a pair of anthracene triplet excitons. Heterofission occurs mainly from anthracene molecules next to a tetracene and competes with the singlet trapping. From the singlet trapping rate and from the magnetic modulation of tetracene prompt fluorescence the heterofission rate is estimated as ≈10^?11s^?1.