Terphenyl Crystals Research Articles

Large-Sized Nitrogen-Containing Biphenyl-Doped p-Terphenyl Crystals with Tunable Luminescence for Organic Scintillators

Large-Sized Nitrogen-Containing Biphenyl-Doped <i>p</i>-Terphenyl Crystals with Tunable Luminescence for Organic Scintillators

QM/MM optimization with quantum coupling: Host-guest interactions in a pentacene-doped p-terphenyl crystal.

In this work, we present a novel force-based scheme to perform hybrid quantum mechanics/molecular mechanics (QM/MM) computations. The proposed scheme becomes especially relevant for the simulation of host-guest molecular systems, where the description of the explicit electronic interactions between a guest molecule and a classically described host is of key importance. To illustrate its advantages, we utilize the presented scheme in the geometry optimization of a technologically important host-guest molecular system: a pentacene-doped p-terphenyl crystal, a core component of a room-temperature MASER device. We show that, as opposed to the simpler and widely used hybrid scheme ONIOM, our Quantum-Coupling QM/MM scheme was able to reproduce explicit interactions in the minimum energy configuration for the host-guest complex. We also show that, as a result of these explicit interactions, the host-guest complex exhibits an oriented net electric dipole moment that is responsible for red-shifting the energy of the first singlet-singlet electronic excitation of pentacene.

Strongly Correlated Ladders in K-Doped p-Terphenyl Crystals.

Potassium-doped terphenyl has recently attracted attention as a potential host for high-transition-temperature superconductivity. Here, we elucidate the many-body electronic structure of recently synthesized potassium-doped terphenyl crystals. We show that this system may be understood as a set of weakly coupled one-dimensional ladders. Depending on the strength of the interladder coupling, the system may exhibit insulating spin-gapped valence-bond solid or antiferromagnetic phases, both of which upon hole doping may give rise to superconductivity. This terphenyl-based ladder material serves as a new platform for investigating the fate of ladder phases in the presence of three-dimensional coupling as well as for novel superconductivity.

Growth of Pentacene-Doped p-Terphenyl Crystals by Vertical Bridgman Technique and Doping Effect on Their Characterization

High quality pentacene-doped p-terphenyl crystals were successfully grown by the modified vertical Bridgman technique with a specially designed double-walled ampule. Upon the dopant addition, the growth crystal exhibits a color change from colorless to purple due to the guest-induced absorption changes. Powder X-ray diffraction, Fourier transform infrared (FTIR), 1H nuclear magnetic resonance (NMR), and fluorescence spectra studies of the doped crystal were carried out and compared with those of a pure p-terphenyl crystal. The decrease in both crystallinity and fluorescent intensity of the doped crystal clearly indicates that the guest molecules appear as defects in the form of irregularly oriented molecules which do not significantly distort the crystal structures. No significant changes on FTIR and 1H NMR spectral features of the doped crystal in relation to the corresponding features observed for the single crystal shows that the crystal doping in such small amounts does not lead to noticeable changes ...

ChemInform Abstract: Detection and Spectroscopy of Single Pentacene Molecules in a p- Terphenyl Crystal by Means of Fluorescence Excitation.

Recent advances in fluorescence excitation spectroscopy with high efficiency have produced greatly improved optical spectra for the first electronic transition of individual single molecules of pentacene in p‐terphenyl crystals at low temperatures (1.5 to 10 K). Two classes of single molecule behavior are observed: class I molecules have time‐independent resonance frequencies, and class II molecules show a diffusive motion among several resonant frequencies with time which we term ‘‘spectral diffusion’’ by analogy with a similar effect which is common in amorphous materials. The temperature dependence of the linewidth and the power dependence of the fluorescence emission rate and of the linewidth are reported and analyzed. Various forms of the surprising class II behavior are described, including jumping among several discrete frequencies, creeping toward the center of the inhomogeneous line in many small steps, and a wandering among many possible resonance frequencies. The occurrence of class II behavior is restricted to the wings of the inhomogeneous line suggesting that the effect is correlated with some form of local disorder. The spectral diffusion rate increases with increasing temperature, suggesting that the effect may be due to phonon‐assisted transitions of local degrees of freedom around the pentacene defect whose source remains to be identified conclusively.

Enhanced energy resolution ( 22Na, 137Cs and 241Am) and alpha–gamma ( 241Am + 60Co) and neutron–gamma ( 241Am–Be) pulse shape discrimination properties of p-terphenyl crystals prepared by modified selective self-seeded vertical Bridgman technique (SSVBT)

p -Terphenyl crystals were grown by modified selective self-seeded vertical Bridgman technique (SSVBT). This new technique has been adopted for nucleation and growth of seed crystals and the bulk crystals were grown by selective self-seeding method. The scintillation properties of the grown p -terphenyl crystal by the aforementioned method have been studied for different radioactive isotopes. Its particle-detection ability has been brought out by pulse-shape discrimination studies for alpha–gamma ( Am 241 + Co 60 ) and neutron–gamma ( 241Am–Be) sources. The results show that there is an enhancement in the scintillation characteristics of the SSVBT p -terphenyl crystals when compared to the commercially available p -terphenyl crystals.

Effects of anisotropy and the proximity to the surface on the spontaneous emission rate of pentacene in p-terphenyl crystal

Due to the anisotropy of a birefringent host crystal, the spontaneous emission rate of a single-impurity molecule depends on the orientation of the transition dipole moment relative to the host crystal. In p -terphenyl crystal, the transition dipole moment of a single pentacene impurity can have four different orientations. Here the relative spontaneous emission rate of the electric quadrupole transition is calculated depending on the unknown component of the transition dipole moment normal to the a ′/ b ′-plane. Maximum difference in the spontaneous emission rate of the electric quadrupole transition is up to 9%. For comparison, the effects caused by the proximity to the plane surface of the crystal are calculated.

High-pressure low-temperature phase transition in a dopedpara-terphenyl crystal: A spectral-hole-burning study

The homogeneous width of zero-phonon lines in a chlorin-doped para-terphenyl crystal is determined by spectral hole burning at $T=4.2--14\mathrm{K}$ under various hydrostatic pressures up to 7 kbar. A drastic broadening of spectral holes with pressure occurs on approaching the order-disorder (triclinic-to-monoclinic) phase transition which takes place under $P\ensuremath{\approx}5.5\mathrm{kbar}$ at $4.2l~Tl~14\mathrm{K}.$ The thermal broadening of spectral holes at fixed pressures is an activated process with the activation energy depending on pressure and having a deep minimum at $P\ensuremath{\approx}5.5\mathrm{kbar},$ i.e., in the phase-transition region.

Optical spectroscopic study of tetracene guest site configurations in perdeuterated and partially deuterated p-terphenyl host crystals

Abstract The nature of the stable guest sites, O 1 and O 2 , and the conformational relaxations in the host cage of the metastable sites, O 3 and O 4 , of tetracene in partially deuterated and perdeuterated p -terphenyl crystals are investigated using stationary optical spectroscopy. Deuteration of the p -terphenyl host matrix leads to blue-shift the electronic origins of the guest spectra and a decrease of the splitting between the spectral lines of the O 1 and O 2 sites. The phonon sidebands in the excitation spectra of the metastable O 3 and O 4 sites in perdeuterated p -terphenyl were found to have a significantly smaller width as compared to that of the perprotonated host. This difference is shown to reflect the relatively smaller conformational changes of the perdeuterated host molecules surrounding the guests in the O 3 and O 4 sites.

Pump-Probe Experiments with a Single Molecule: ac-Stark Effect and Nonlinear Optical Response.

We have measured the ac-Stark effect on single terrylene molecules in $p$-terphenyl crystals at superfluid helium temperatures. With the pump beam far enough from resonance, the molecular transition shifts as expected, i.e., proportional to the pump intensity and to the inverse pump detuning. Experiments closer to resonance with stronger beams show Autler-Townes-like structures that can be well reproduced by means of Bloch equations with two intense laser fields. These experiments are a demonstration of nonlinear optical effects with the localized electronic states of a single molecule trapped in a solid.

Spectral diffusion of individual pentacene molecules in P-terphenyl crystal: Stochastic theoretical model and analysis of experimental data

We propose a microscopic theoretical model to explain recent experiments involving the spectral diffusion of individual pentacene molecules embedded in p-terphenyl crystal. The experimental spectral diffusion trajectories are analyzed in terms of three stochastic characterizations: The time autocorrelation of transition frequency fluctuations, the time-dependent distribution of spectral jumps, and the equilibrium distribution of frequencies. The observed spectral diffusion is well described by our model, and we determine detailed quantitative information about the localized excitations that are responsible for the fluctuations in the pentacene transition frequency. We find that the spectral diffusion of the pentacene transition is caused by the reorientation of the central phenyl ring in the p-terphenyl molecule, and that this reorientation can only occur at a wall between domains of different central phenyl ring ordering. Furthermore, we find that only those pentacene molecules that reside within a few lattice spacings of these domain walls exhibit spectral diffusion.

Detection and spectroscopy of single pentacene molecules in a p-terphenyl crystal by means of fluorescence excitation

Recent advances in fluorescence excitation spectroscopy with high efficiency have produced greatly improved optical spectra for the first electronic transition of individual single molecules of pentacene in p-terphenyl crystals at low temperatures (1.5 to 10 K). Two classes of single molecule behavior are observed: class I molecules have time-independent resonance frequencies, and class II molecules show a diffusive motion among several resonant frequencies with time which we term ‘‘spectral diffusion’’ by analogy with a similar effect which is common in amorphous materials. The temperature dependence of the linewidth and the power dependence of the fluorescence emission rate and of the linewidth are reported and analyzed. Various forms of the surprising class II behavior are described, including jumping among several discrete frequencies, creeping toward the center of the inhomogeneous line in many small steps, and a wandering among many possible resonance frequencies. The occurrence of class II behavior is restricted to the wings of the inhomogeneous line suggesting that the effect is correlated with some form of local disorder. The spectral diffusion rate increases with increasing temperature, suggesting that the effect may be due to phonon-assisted transitions of local degrees of freedom around the pentacene defect whose source remains to be identified conclusively.

Comment on "Single pentacene molecules detected by fluorescence excitation in a p-terphenyl crystal"

A Comment on the Letter by M. Orrit and J. Bernard, Phys. Rev. Lett. 65, 2716 (1990).Received 11 December 1990DOI:https://doi.org/10.1103/PhysRevLett.66.1376©1991 American Physical Society

Single pentacene molecules detected by fluorescence excitation in a p-terphenyl crystal.

Experiments clearly prove that narrow peaks in the fluorescence-excitation spectrum of a pentacene-doped p-terphenyl crystal stem from single molecules. This claim is supported by the distribution, width, and height of the peaks, as well as by the correlation of the emitted light and the sudden drops and surges of the emission of certain peaks. We attribute these to the hole burning of a single molecule. These results show the feasibility of the optical study of a single molecule and its local environment.

Charge carrier generation in p-terphenyl crystal. Autoionization of higher excited states

Photoionization of singlet excitons in p-terphenyl crystal is studied with nanosecond laser excitation. The photoionization spectrum and the photoabsorption spectrum of the lowest excited state are measured. The ratio of the two spectra gives the efficiency with which higher excited states, generated by the two-step excitation, ionize. The observed efficiency is constant below 7.3 eV and is remarkably high (∼10−2 ). The results do not support the view that the excess energy is converted directly to the kinetic energy of the generated electrons in the crystal and indicate that the initially prepared higher excited states relax rapidly and the succeeding autoionization yields electrons with a fixed kinetic energy.

Multiply enhanced site selective CARS spectroscopy

Three tunable lasers are used to perform site selective coherent anti-Stokes Raman (CARS) spectroscopy in pentacene:p-terphenyl crystals at 2 K. The vibrational and vibronic levels of pentacene in the individual substitutional sites of p-terphenyl can be selectively examined by matching the resonances of a particular site. The signals from a site and the selectivity between sites are much higher than CARS performed with two tunable lasers. The spectra are dominated by lines from processes where the vibrational and vibronic states involved in the four-wave mixing are associated with the same molecular mode. There are also important lines from processes where different vibrational and vibronic modes are involved in the nonlinear mixing. The intensities of lines from the resonant site are strongly dependent on the laser energy because of saturation of the electronic transition.

EPR study of acceptor doped p-terphenyl crystals: The oriented radical cation precursor for a conducting polymera)

Upon reacting gaseous SO3 or AsF5 with single crystals of p-terphenyl, the resulting π-radical cation is displaced from its original molecular position by a rotation of 64° about the rod axis and a tilt of 42° toward the c* crystal axis. Such action causes an eventual destruction of the crystal. This observation suggests that polymerization of p-terphenyl occurs between monomers related by a 21 symmetry axis.

An electron spin echo study of the photoexcited triplet state of tetracene in p-terphenyl crystals at room temperature

Room temperature magnetic resonance studies of the photoexcited triplet state of tetracene in p-terphenyl crystals have been carried out by the electron spin echo technique. The studies yield the following structural and dynamical information: (1) zero-field splittings; D/hc=±0.0551(1) and E/hc=∓0.0042(1) cm−1; (2) spin densities: ρ1(α)=0.055, ρ2(β)=0.016, ρ5(γ)=0.217, ρ13=−0.025, and ρ15=−0.027; and (3) relative triplet population rates: px≊py≳pz.

Optical Dephasing Due to Virtual Transfer of Excitation Energy as Observed in Photon Echo Experiments of Pentacene Molecules in Crystals

Optical dephasing due to intermolecular interaction is studied for pentacence molecules introduced in naphthalene and p -terphenyl crystals by measuring the ordinary photon echo and the accumulated three-pulse echo. It is found that the reciprocal of pure dephasing time increases linearly with the guest molecule concentration at low temperature where pure dephasing is known to be temperature-independent. A new model based on virtual transfer of excitation energy, i.e. energy transfer without phonon asistance, between guest molecules is presented to interpret this dephasing process. It is shown that observed linear dependence of the line width representing pure dephasing is well interpreted by this model, and that the calculated relationship between the line width and concentration agrees with the experimental one within a factor of about two.

Singlet Energy Transfer in p-Terphenyl Crystal Doped with Tetracene

Abstract The transfer of singlet energy in p-terphenyl crystal was measured as a function of the dopant (tetracene) concentration and the temperature. The simple hopping model yields a value of 3×10−14 s. This value is believed to be correct within a factor of 5. In the low temperature phase of the crystal, the energy transfer is temperature dependent with an activation energy of 66 cm−1 in undeuterated p-terphenyl and 90 cm−1 in perdeuterated p-terphenyl. It seems probable that in the low temperature phase the freedom of the conformational change of the molecule plays an important role in the energy migration.