Charge-transfer Molecular Crystals Research Articles

High Piezoelectric Output Voltage from Blue Fluorescent N,N-Dimethyl-4-nitroaniline Nano Crystals in Poly-L-Lactic Acid Electrospun Fibers.

N,N-dimethyl-4-nitroaniline is a piezoelectric organic superplastic and superelastic charge transfer molecular crystal that crystallizes in an acentric structure. Organic mechanical flexible crystals are of great importance as they stand between soft matter and inorganic crystals. Highly aligned poly-l-lactic acid polymer microfibers with embedded N,N-dimethyl-4-nitroaniline nanocrystals are fabricated using the electrospinning technique, and their piezoelectric and optical properties are explored as hybrid systems. The composite fibers display an extraordinarily high piezoelectric output response, where for a small stress of 5.0 × 103 Nm-2, an effective piezoelectric voltage coefficient of geff = 4.1 VmN-1 is obtained, which is one of the highest among piezoelectric polymers and organic lead perovskites. Mechanically, they exhibit an average increase of 67% in the Young modulus compared to polymer microfibers alone, reaching 55 MPa, while the tensile strength reaches 2.8 MPa. Furthermore, the fibers show solid-state blue fluorescence, important for emission applications, with a long lifetime decay (147 ns) lifetime decay. The present results show that nanocrystals from small organic molecules with luminescent, elastic and piezoelectric properties form a mechanically strong hybrid functional 2-dimensional array, promising for applications in energy harvesting through the piezoelectric effect and as solid-state blue emitters.

Are hydrogen-bonded charge transfer crystals room temperature ferroelectrics?

We present a theoretical investigation of the anomalous ferroelectricity of mixed-stack charge transfer molecular crystals, based on the Peierls-Hubbard model, and first-principles calculations for its parametrization. This approach is first validated by reproducing the temperature-induced transition and the electronic polarization of TTF-CA, and then applied to a novel series of hydrogen-bonded crystals, for which room temperature ferroelectricity has recently been claimed. Our analysis shows that the hydrogen-bonded systems present a very low degree of charge transfer and hence support a very small polarization. A critical reexamination of experimental data supports our findings, shedding doubts on the ferroelectricity of these systems. More generally, our modeling allows the rationalization of general features of the ferroelectric transition in charge transfer crystals and suggests design principles for materials optimization.

Impact of exact exchange in the description of the electronic structure of organic charge-transfer molecular crystals

We evaluate the impact that the amount of nonlocal Hartree-Fock (%HF) exchange included in a hybrid density functional has on the microscopic parameters (transfer integrals, band gaps, bandwidths, and effective masses) describing charge transport in high-mobility organic crystals. We consider both crystals based on a single molecule, such as pentacene, and crystals based on mixed-stack charge-transfer systems, such as dibenzo-TTF--TCNQ. In the pentacene crystal, the band gap decreases and the effective masses increase linearly with an increase in the amount of %HF exchange. In contrast, in the charge-transfer crystals, while the band gap increases linearly, the effective masses vary only slightly with an increase in %HF exchange. We show that the superexchange nature of the electronic couplings in charge-transfer systems is responsible for this peculiar evolution of the effective masses. We compare the density functional theory results with results obtained within the ${G}_{0}{W}_{0}$ approximation as a way of benchmarking the optimal amount of %HF exchange needed in a given functional.

The Effects of Photochemical and Mechanical Damage on the Excited State Dynamics of Charge-Transfer Molecular Crystals Composed of Tetracyanobenzene and Aromatic Donor Molecules

Charge-transfer molecular crystals are structurally well-defined systems whose electron transfer dynamics can be studied using time-resolved spectroscopy. In this paper, five 1:1 complexes, consisting of 1,2,4,5-tetracyanobenzene as the electron acceptor and durene, 9-methylanthracene, naphthalene, phenanthrene, and pyrene as electron donors, are studied using time-resolved fluorescence and transient absorption in the diffuse reflectance geometry. Two different sample morphologies were studied: single crystals and powders prepared by pulverizing the crystals and diluting them with barium sulfate microparticles. Fluorescence lifetime and transient absorption measurements performed on the crystals and the powders yielded different results. The crystals typically exhibited long-lived monoexponential fluorescence decays, while the powders had shorter multiexponential decays. Exposure of both types of samples to high laser fluence was also shown to induce faster excited state decay dynamics as observed using fluorescence and diffuse reflectance. In addition to the more rapid decays, these molecular crystals exhibited relatively high photobleaching quantum yields on the order of 10(-4). Previous work that interpreted picosecond decays in the transient absorption as evidence for rapid recombination and charge dissociation should be re-evaluated based on the susceptibility of this class of compounds to mechanical and photochemical damage.

Capturing One-Dimensional Precursors of a Photoinduced Transformation in a Material

Achieving control of photoinduced phase transitions requires understanding how materials work during transformation induced by a laser pulse. Here we investigate the precursors of a photoinduced phase transition in the highly cooperative charge-transfer molecular crystal tetrathiafulvalene-p-chloranil and provide key insights. The photogeneration of one-dimensional nanoscale clusters was detected by time-resolved diffuse x-ray scattering with 50-ps time resolution. Such clustering of structurally relaxed electronic excitations is expected to be a common process in many materials presenting photoinduced transformations.

Probing photoinduced phase transition in a charge-transfer molecular crystal by 100 picosecond X-ray diffraction

Watching with an atomic resolution at structural changes as fast or ultra-fast photoinduced physical processes take place benefits from recent progresses in time-resolved X-ray diffraction. Molecular materials where electronic and structural changes are strongly coupled are model systems to perform such time-resolved crystallography studies. We report the structural investigation of photoinduced phase transformations between ionic and neutral states in an organic charge-transfer molecular material, using 100 picosecond (ps) synchrotron pulses. This light-induced phenomenon, triggered by an ultra-short optical pulse from a femtosecond laser, occurs by virtue of intrinsic cooperativity. Since electronic and structural changes are strongly coupled, it is of fundamental interest to perform time-resolved X-ray diffraction to obtain information at the atomic scale. We also discuss the problem of coexistence of phases and the interest of future investigations in faster timescale.

First principles investigations of a “quasi-one-dimensional” charge-transfer molecular crystal: TTF-2,5Cl2BQ

We performed first principles calculations using the projector augmented wave method in order to get a clear description at a microscopic level of the electronic distribution far from the transition for TTF-2,5Cl2BQ. Our calculations predict for the first time a low-symmetry structure (ground state) of this compound. We relate them to a simple tight-binding scheme which allows us to discuss the pertinence of the one-dimensional models used to study the phase transitions in this class of materials.

Lattice dynamical study of an internal charge transfer molecular crystal: 4-nitropyridine N-oxide and its deuterated derivative

Phonon dispersion curves, far-IR and low-frequency Raman spectra (below 200 cm −1) of 4-nitropyridine N-oxide (NPO) including results at high pressure and low temperature are reported and interpreted with the help of a lattice-dynamical calculation using atom—atom Buckingham-type potentials and a multipole expansion up to the quadrupole. The charge transfer process is analysed through its manifestations on the model parameters of some short intermolecular interactions. The good agreement between calculation and experience is confirmed by using the same model to interpret the experimental results of inelastic neutron scattering along the b ∗ direction of a deuterated NPO- d4 single crystal.

Crossover of the dynamical regime of the spt in A-TCNB under hydrostatic pressure

Abstract Anthracene-TCNB is a weak charge-transfer molecular crystal which presents a second order phase transition. At atmospheric pressure, a soft mode has been observed by inelastic neutron scattering in the high temperature phase. Under hydrostatic pressure, the transition remains continuous whereas a drastic change occurs in the dynamical regime. At 3 kbar, at the superlattice point, the soft mode-type scattering is indeed replaced by a quasielastic response. This feature indicates a continuous crossover from a displacive regime to an order-disorder regime for a modest pressure of 1.5 kbar.

Librational dynamics and instabilities in mixed-stack charge-transfer molecular crystals

Abstract The generalized susceptibility approach has been used to study temperature dependent librational dynamics and orientational instability in anthracene-tetracyanobenzene crystal.

Mean field approach to orientational transitions in charge–transfer molecular crystals

The mean field approach to orientational transitions in two-component molecular crystals has been formulated and illustrated by numerical calculations for two model systems: complexes of tetracyanobenzene (TCNB) with naphthalene (N–TCNB) and anthracene (A–TCNB). The study had addressed the characterization of the orientational disorder and mechanism of phase transitions in the crystals by calculating single particle and coupled rotational susceptibilities. It is concluded that the orientational instability in A–TCNB crystal is of the displacive type and a soft librational phonon mode at the M point of the Brillouin zone is predicted. In the N–TCNB crystal, an order–disorder type of the instability is predicted. For both systems the orientational phase transitions are found to be predominantly two dimensional.

Delayed-fluorescence ODMR and EPR studies of triplet excitons in charge-transfer molecular crystals

Triplet excitons in electron donor—acceptor charge-transfer (CT) molecular crystals are generated through the intersystem crossing process by excitation in the CT visible band and give rise to delayed fluorescence. Delayed-fluorescence optically detected magnetic resonance (DF ODMR) in magnetic field is analyzed in terms of microwave-induced transitions between energy levels of either the isolated triplet excitons or the annihilating triplet exciton pair. The spin polarization of the triplet excitons plays an important role in the described phenomena. A comparison between DF ODMR and EPR spectra of the anthracene—tetracyanobenzene and biphenyl—tetracyanobenzene systems is presented. In the former case the microwave transitions occurring between free exciton sublevels are predominantly responsible of the DF ODMR signal, whereas the transitions between energy levels of the exciton pair are the most important for biphenyl—TCNB.

Ground-state electronic properties of the charge transfer molecular crystal NMP-TCNQ. I. One-dimensional treatment including electron correlation

The Pariser-Parr-Pople Hamiltonian is used within the framework of the restricted and unrestricted Hartree-Fock crystal orbital methods to compute the energy-band structures, charge and spin density distributions in the one-dimensional TCNQ- columns of the NMP-TCNQ crystal. As a consequence of electronic correlation effects the ground state of the system is shown to be a magnetic insulator. Different mechanisms which reduce the interelectronic repulsion in organic systems are discussed and the possibility of the application of a simple Hubbard-type Hamiltonian to molecular crystals is investigated.

Electron Conduction in Charge-Transfer Molecular Crystals

Molecular crystals consisting of strong electron donors and strong electron acceptors in ordered array often possess electronic properties which are not found in other types of organic solids. Molecular orbital calculations have been carried out for the triethylammonium-(tetracyanoquino dimethane)2 crystal. The LCAO-MO method combined with a crystal-field splitting calculation resulted in a reasonably accurate value for the activation energy observed for electron conduction. On a basis of the results of this calculation, one would conclude that this type of organic solid should not be expected to exhibit the properties of conventional inorganic semiconductors. However, the results do provide a basis for some hypotheses concerning electron conduction processes and possible photoconduction mechanisms in these solids.