Optical dephasing in a glass-like system: a photon echo study of pentacene in benzoic acid

Optical absorption and picosecond photon echo experiments are used to study the dephasing of pentacene in benzoic acid. It is shown that, while the absorption spectrum of pentacene is effected by proton transfer in the benzoic acid dimer, the dephasing is caused by elastic and inelastic phonon scattering processes.     

Picosecond tri-transition and two-color photon echoes in a doped molecular solid

Picosecond tri-level photon echoes are generated among vibronic transitions of pentacene doped into a naphthalene host. The echoes are generated with three excitation pulses of which the first one, at ω₁, always excites a vibronic transition in the pentacene molecule. With the second excitation pulse at ω₂ and the third at ω₁, a tri-transition echo (TTE) is formed. With the time ordering of the second and third pulse reversed, a connected two-color stimulated echo (C2CSE) is generated. It is shown that, for small pulse angles, the low-temperature decay, of both echo effects is identical and that a smooth transition of one echo effect into the other occurs at the overlap in time between the second and third excitation pulse. Observation of these echoes further indicates that the inhomogeneous broadening at the selected transitions is strongly correlated.     

Optical dephasing of molecular dimers in mixed crystals: Picosecond photon echo experiments

Picosecond photon echo experiments are used to examine optical dephasing of substitutional dimers and monomers of tetracene and pentacene in p-terphenyl host crystals. A comparison of experiments on tetracene and pentacene dimers permits the mechanism responsible for temperature-dependent optical dephasing to be determined. It is shown that excitation of librations rather than scattering between delocalized dimer states is the principal mechanism.     

Delocalized electronic excitations of pentacene dimers in a p-terphenyl host: picosecond photon echo experiments

Photon echo experiments are performed on delocalized electronic excitations of optical dimer states. The delocalized dimer states are found to have very long coherence times, close to the coherent limit The dimer dephasing is qualitatively similar to dephasing of pentacene monomers.     

A new type of soluble pentacene precursor for organic thin-film transistors

A new type of soluble pentacene precursor is synthesized, which extrudes a unit of CO upon heating at 150 degrees C, to produce pentacene in nearly quantitative yield.     

Photon echoes stimulated from long-lived ordered populations in multi-level systems. The effect of intersystem crossing and optical branching

It is shown that the three pulse stimulated photon echo (3PSE) may be used to determine the yield for intersystem crossing in a guest molecule doped into a host lattice. An exact expression is derived for the intensity of the 3PSE for the case when all three spin sublevels participate in the optical pumping cycle. Yields for intersystem crossing in the systems naphthalene in durene (45%) and pentacene in naphthalene (15%) were obtained from the stimulated photon echo decay. In addition, the 3PSE formation is discussed in pseudo two-level systems where the level structure is caused by magnetic interactions. As an example, the 3PSE decay of the system triphenylmethyl in triphenylamine is analyzed.     

Ultrafast optical dephasing in a low-temperature organic glass

The optical dephasing of pentacene in an o-terphenyl glass at 1.5 K is examined with picosecond photon echo and fluorescence line narrowing experiments. The homogeneous T₂ is found to be 1–3 ps, in contrast to all previous measurements on glass—chromophore systems in which the homogeneous T₂are two to three orders of magnitude longer.     

Intersystem crossing from singlet states of molecular dimers and monomers in mixed molecular crystals: picosecond stimulated photon echo experiments

An experimental study of the excited-state dynamics of pentacene dimers and monomers in p-terphenyl host crystals is presented. Picosecond stimulated photon echoes, picosecond photon echoes, and fluorescence lifetime measurements are used to study intersystem crossing and homogeneous dephasing of delocalized dimers and monomers at 1.8 K. It is found that the dimer states can have intersystem crossing rate constants which are orders of magnitude greater than the corresponding monomers. Three mechanisms are considered to explain the differences between dimer and monomer intersystem crossing. Fluorescence lifetime measurements and photon echo measurements demonstrate that the only source of homogeneous line broadening at 1.8 K is population relaxation. These measurements combined with the stimulated echo measurements show that differences in lifetimes exhibited by the various dimer and various monomer sites are due solely to differences in intersystem crossing rate constants.     

Thermally induced solid-state phase transition of bis(triisopropylsilylethynyl) pentacene crystals

Bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) is a functionalized pentacene derivative designed to enhance both the solution solubility and solid-state packing of pentacene. In this paper, we report our observations of a solid-state phase transition in TIPS pentacene crystals upon heating or cooling. Evidence from differential scanning calorimetry (DSC), hot-stage optical microscopy, as well as high-temperature X-ray and electron diffraction are presented. A reasonable match with experimental data is obtained with molecular modeling. Our results reveal that the transition is associated with a conformational reorganization of the TIPS side groups, accompanied by a slight decrease in the acene-to-acene spacing and a shift of the overlap between the neighboring pentacene units. The observed cracking should be avoided or minimized in TIPS pentacene-based thin film transistors to maintain their relatively high charge carrier mobility.     

Singlet Heterofission in Tetracene–Pentacene Thin-Film Blends

Heterofission is a photophysical process of fundamental and applied interest whereby an excited singlet state is converted into two triplets on chemically distinct chromophores. The potential of this process lies in the tuning of both the optical band gap and the splitting between singlet and triplet energies. Herein, we report the time-domain observation of heterofission in mixed thin films of the prototypical singlet fission chromophores pentacene and tetracene using excitation wavelengths above and below the tetracene band gap. We found a time constant of 26 ps for endothermic heterofission of a singlet exciton on pentacene in blends with low pentacene fractions, which was outcompeted by pentacene homofission for increasing pentacene concentrations. Direct excitation of tetracene lead to fast energy transfer to pentacene and subsequent singlet fission, which prevented homo- or heterofission of a singlet exciton on tetracene.     

The vibrational reorganization energy in pentacene: molecular influences on charge transport

The reorganization energy in pentacene is reported on the basis of a joint experimental and theoretical study of pentacene ionization using high-resolution gas-phase photoelectron spectroscopy, semiempirical intermediate neglect of differential overlap calculations, and first-principles correlated quantum-mechanical calculations at MP2 and density functional theory levels. The reorganization energy upon positive ionization of pentacene is determined both experimentally and theoretically to be remarkably low. This is one key element that allows one to rationalize the extremely high hole mobilities recently measured in pentacene single crystals.     

Electrostatic field and partial Fermi level pinning at the pentacene-SiO(2) interface

Monolayer islands of pentacene deposited on silicon substrates with thermally grown oxides were studied by electric force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) in ultrahigh vacuum (UHV) after prior 10 min exposure to atmospheric ambient. On 25-nm-thick oxides, the pentacene islands are 0.5 V higher in electrostatic potential than the silicon dioxide background because of intrinsic contact potential differences. On 2-nm-thin oxides, tunneling across the oxides allows Fermi level equilibration with pentacene associated states. The surface potential difference depends on the doping of the underlying Si substrates. The Fermi level movement at the pentacene SiO(2) interface was restricted and estimated to lie between 0.3 and 0.6 eV above the pentacene valence band maximum. It is proposed that hole traps in the pentacene or at the pentacene-oxide interface are responsible for the observations.     

Ubiquitous pentacene monolayer on metals deposited onto pentacene films

Photoelectron spectroscopy (XPS and UPS) was used to study the deposition of metal layers (Ag, Cu, and Au) onto pentacene films. Very low work functions were measured (PhiAg = 3.91 eV, PhiCu = 3.93 eV, and PhiAu = 4.3 eV) for all of the metals, in agreement with results from the literature. The intensities of the C 1s core-level signals from pentacene that were monitored during stepwise metal deposition leveled off at a value of about 30% of a thick pentacene film. This C 1s intensity is comparable to that of one monolayer of pentacene deposited onto the respective metal. The valence band spectra of metals deposited onto pentacene and spectra collected for pentacene deposited onto bare metal surfaces are very similar. These findings lead to the conclusion that approximately one monolayer of pentacene is always present on top of the freshly deposited metal film, which explains the very low work function of the metals when they are deposited onto organic films. We expect similar behavior with other nonreactive metals deposited onto stable organic layers.     

Computationally derived rules for persistence of C60 nanowires on recumbent pentacene bilayers

The tendency for C(60) nanowires to persist on two monolayers of recumbent pentacene is studied using molecular dynamics (MD) simulations. A review of existing experimental literature for the tilt angle adopted by pentacene on noble metal surfaces shows that studies cover a limited range from 55° to 90°, motivating simulation studies of essentially the entire range of tilt angles (10°-90°) to predict the optimum surface tilt angle for C(60) nanowire formation. The persistence of a 1D nanowire depends sensitively on this tilt angle, the amount of initial tensile strain, and the presence of surface step edges. At room temperature, C(60) nanowires oriented along the pentacene short axes persist for several nanoseconds and are more likely to occur if they reside between, or within, pentacene rows for ? ≤ ～60°. The likelihood of this persistence increases the smaller the tilt angle. Nanowires oriented along the long axes of pentacene molecules are unlikely to form. The limit of stability of nanowires was tested by raising the temperature to 400 K. Nanowires located between pentacene rows survived this temperature rise, but those located initially within pentacene rows are only stable in the range ?(1) = 30°-50°. Flatter pentacene surfaces, that is, tilt angles above about 60°, are subject to disorder caused by C(60) molecules "burrowing" into the pentacene surface. An initial strain of 5% applied to the C(60) nanowires significantly decreases the likelihood of nanowire persistence. In contrast, any appreciable surface roughness, even by half a monolayer in height of a third pentacene monolayer, strongly enhances the likelihood of nanowire formation due to the strong binding energy of C(60) molecules to step edges.     

Substituent effects on the electronic characteristics of pentacene derivatives for organic electronic devices: dioxolane-substituted pentacene derivatives with triisopropylsilylethynyl functional groups

The intramolecular electronic structures and intermolecular electronic interactions of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene), 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]-pentacene (TP-5 pentacene), and 2,2,10,10-tetraethyl-6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (EtTP-5 pentacene) have been investigated by the combination of gas-phase and solid-phase photoelectron spectroscopy measurements. Further insight has been provided by electrochemical measurements in solution, and the principles that emerge are supported by electronic structure calculations. The measurements show that the energies of electron transfer such as the reorganization energies, ionization energies, charge-injection barriers, polarization energies, and HOMO-LUMO energy gaps are strongly dependent on the particular functionalization of the pentacene core. The ionization energy trends as a function of the substitution observed for molecules in the gas phase are not reproduced in measurements of the molecules in the condensed phase due to polarization effects in the solid. The electronic behavior of these materials is impacted less by the direct substituent electronic effects on the individual molecules than by the indirect consequences of substituent effects on the intermolecular interactions. The ionization energies as a function of film thickness give information on the relative electrical conductivity of the films, and all three molecules show different material behavior. The stronger intermolecular interactions in TP-5 pentacene films lead to better charge transfer properties versus those in TIPS pentacene films, and EtTP-5 pentacene films have very weak intermolecular interactions and the poorest charge transfer properties of these molecules.     

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.     

Probing interfacial characteristics of rubrene/pentacene and pentacene/rubrene bilayers with soft X-ray spectroscopy

The electronic structure of rubrene/pentacene and pentacene/rubrene bilayers has been investigated using soft X-ray absorption spectroscopy, resonant X-ray emission spectroscopy, and density-functional theory calculations. X-ray absorption and emission measurements reveal that it has been possible to alter the lowest unoccupied and the highest occupied molecular orbital states of rubrene in rubrene/pentacene bilayer. In the reverse case, one gets p* molecular orbital states originating from the pentacene layer. Resonant X-ray emission spectra suggest a reduction in the hole-transition probabilities for the pentacene/rubrene bilayer in comparison to reference pentacene layer. For the rubrenepentacene structure, the hole-transition probability shows an increase in comparison to the rubrene reference. We also determined the energy level alignment of the pentacene-rubrene interface by using X-ray and ultraviolet photoelectron spectroscopy. From these comparisons, it is found that the electronic structure of the pentacene-rubrene interface has a strong dependence on interface characteristics which depends on the order of the layers used.     

Synthesis of a Soluble Pentacene Derivative: 6,13-bis(m-trifluoromethyl phenyl ethenyl) Pentacene †

We report the synthesis and characterization of a new pentacene derivative, 6,13- bis(m-trifluoromethyl phenylethynyl)pentacene. This pentacene derivative is soluble in common organic solvents and found to have the HOMO energy level at ?5.45 eV in solution.     

Electrochemical hydrogen storage in Li-doped pentacene

Porous metal-incorporated nanostructured organic compounds represent an innovative concept in hydrogen storage. This work reports in detail the first practical realization of electrochemical hydrogen storage in such a material, Li-doped pentacene. Although lithium doping is at a very low level the conductivity of pentacene increases considerably after the doping of lithium, which makes ordered layer-stacked Li-doped pentacene very promising as electrode material for electrochemical hydrogen storage. A discharge capacity of ca. 238 mA h g(-1) has been achieved in 6M KOH for the Li-doped pentacene electrode, which is comparable to that obtained from commonly used electrode material, LaNi5 alloy for electrochemical hydrogen storage. Furthermore, a great improvement in the hydrogen storage capacity can be expected from the Li-doped pentacene by simply raising the lithium doping level.     

Effect of dielectric roughness on performance of pentacene TFTs and restoration of performance with a polymeric smoothing layer

The morphology, structure, and transport properties of pentacene thin film transistors (TFTs) are reported showing the influence of the gate dielectric surface roughness. Upon roughening of the amorphous SiO2 gate dielectric prior to pentacene deposition, dramatic reductions in pentacene grain size and crystallinity were observed. The TFT performance of pentacene films deposited on roughened substrates showed reduced free carrier mobility, larger transport activation energies, and larger trap distribution widths. Spin coating roughened dielectrics with polystyrene produced surfaces with 2 A root-mean-square (rms) roughness. The pentacene films deposited on these coated surfaces had grain sizes, crystallinities, mobilities, and trap distributions that were comparable to the range of values observed for pentacene films deposited on thermally grown SiO2 (roughness also approximately 2 A rms).