Lattice Dynamics of Quinacridone Polymorphs: A Combined Raman and Computational Approach.

Polarized low-frequency Raman microscopy and a posteriori dispersion-corrected density functional simulations are combined to investigate the lattice vibrations of the αI, ß, and γ polymorphs of the model organic semiconductor quinacridone, which are known to display different optical and electronic properties. The comparison between experiments and calculations allows for unambiguous mode assignment and identification of the scattering crystal faces. Conversely, the agreement between simulations and experiments validates the adopted computational methods, which correctly describe the intermolecular interaction of the molecular material. The acquired knowledge of quinacridone lattice dynamics is used to describe the αI to ß thermal transition and, most consequentially, to reliably characterize the electron-lattice phonon coupling strength of the three polymorphs, obtaining hints about the electrical transport mechanism of the material.

Self-assembled architectures with segregated donor and acceptor units of a dyad based on a monopyrrolo-annulated TTF-PTM radical.

An electron donor-acceptor dyad based on a polychlorotriphenylmethyl (PTM) radical subunit linked to a tetrathiafulvalene (TTF) unit through a π-conjugated N-phenyl-pyrrole-vinylene bridge has been synthesized and characterized. The intramolecular electron transfer process and magnetic properties of the radical dyad have been evaluated by cyclic voltammetry, UV/Vis spectroscopy, vibrational spectroscopy, and ESR spectroscopy in solution and in the solid state. The self-assembling abilities of the radical dyad and of its protonated non-radical analogue have been investigated by X-ray crystallographic analysis, which revealed that the radical dyad produced a supramolecular architecture with segregated donor and acceptor units in which the TTF subunits were arranged in 1D herringbone-type stacks. Analysis of the X-ray data at different temperatures suggests that the two inequivalent molecules that form the asymmetric unit of the crystal of the radical dyad evolve into an opposite degree of electronic delocalization as the temperature decreases.

Phonon dynamics and electron-phonon coupling in pristine picene.

The paper reports a complete analysis of the phonon structure of crystalline picene, a recently announced organic semiconductor. Both lattice and intramolecular vibrations are investigated. An exhaustive assignment of lattice phonons is obtained through polarized Raman spectra assisted by lattice dynamics calculations based on a well tested atom-atom potential model. Raman, infrared spectra and density functional (DFT) calculations are used for the characterization of intramolecular modes. Coupling between low-frequency molecular vibrations and lattice phonons is accounted for. Molecule-to-molecule transfer integrals, as well as the Peierls and Holstein (non-local and local) coupling constants, are evaluated through the semiempirical method INDO/S (Intermediate Neglect of Differential Overlap with Spectroscopic parametrization).

Interaction of charge carriers with lattice and molecular phonons in crystalline pentacene.

The computational protocol we have developed for the calculation of local (Holstein) and non-local (Peierls) carrier-phonon coupling in molecular organic semiconductors is applied to both the low temperature and high temperature bulk crystalline phases of pentacene. The electronic structure is calculated by the semimpirical INDO/S (Intermediate Neglect of Differential Overlap with Spectroscopic parametrization) method. In the phonon description, the rigid molecule approximation is removed, allowing mixing of low-frequency intra-molecular modes with inter-molecular (lattice) phonons. A clear distinction remains between the low-frequency phonons, which essentially modulate the transfer integral from a molecule to another (Peierls coupling), and the high-frequency intra-molecular phonons, which modulate the on-site energy (Holstein coupling). The results of calculation agree well with the values extracted from experiment. The comparison with similar calculations made for rubrene allows us to discuss the implications for the current models of mobility.

Polarized absorption, spontaneous and stimulated blue light emission of J-type tetraphenylbutadiene monocrystals.

Blue amplified spontaneous emission at room temperature is demonstrated from the exposed face of the strongly emitting organic semiconductor 1,1,4,4-tetraphenyl-1,3-butadiene in single crystal form. The symmetry of the crystal and calculation of lattice sums indicate the J-type organization of the molecular transition moments. The minimum in the lowest exciton dispersion branch, from which emission takes place, is found at the edge of the Brillouin zone leading to a dominant vibronic emission since the zero-phonon line is forbidden. The observed gain narrowed line is attributed to the vibronic replica which becomes amplified with increased pumping. The reported emission is along the normal to the exposed crystal face, important for the development of vertical cavity geometry lasers based on organic single crystals. The threshold excitation fluence of 400 microJ cm(-2) is comparable to other organic crystalline systems, even if the amplification path is much reduced as a consequence of the vertical geometry. Considering these relevant aspects, the optical characterization of this material is provided. The polarized absorption spectra are reported and the properties of the lowest-energy excitonic state investigated. Calculation of the electronic transitions for the isolated molecule, lattice sums for the transition at lowest energy, and the symmetry of the crystal allow attributing the largest face of the samples and the observed optical bands in the spectra. Polarized time-resolved spectra are also reported allowing to identify the intrinsic excitonic emission.

Molecular dynamics simulations for a pentacene monolayer on amorphous silica.

Molecular dynamics simulations are presented for "bulklike" and "filmlike" monolayers of pentacene deposited on a slab of amorphous silica. The two simulated systems, which mainly differ in the tilt angle between the pentacene molecules and the silica surface, exhibit structural and energetic properties that match the available measurements. The bulklike monolayer, the structure of which corresponds to that of the low-temperature polymorph of crystalline pentacene, is stable. The filmlike monolayer, in which the molecules are most closely normal to the surface, is instead thermodynamically metastable, in agreement with the experimental evidence.

Polymorphism and phonon dynamics of alpha-quaterthiophene.

Low-frequency (10-150 cm(-1)) Raman spectra of the low-temperature (LT) and high-temperature (HT) polymorphs of the organic semiconductor alpha-quaterthiophene at 300 and 10 K are reported. Polarized spectra, assisted by quasi-harmonic lattice dynamics (QHLD) calculations, allow characterization of the lattice phonon dynamics and identification of the two phases spectroscopically. The experimental data can be explained by taking into account the coupling between intermolecular (lattice) and low-frequency intramolecular modes. Finally, Raman mapping is used to investigate the phase purity.

Are crystal polymorphs predictable? The case of sexithiophene.

Using sexithiophene as a benchmark compound, we present a very effective strategy for searching the potential energy minima of a crystalline material, described in terms of rigid molecules with Coulombic and atom-atom interactions. The strategy involves uniform sampling of the many-body energy hypersurface, mechanical identification of all constraints deriving from the crystallographic symmetry, and a "sight-resight" method, originally introduced in wildlife ecology, for assessing the completeness of the search. Thousands of distinct potential energy minima, with a surprising variety of structural arrangements, are identified for sexithiophene. Despite the large number of competing minima, the system presents a small number of deep minima, with very different structures and not particularly congested in energy or density. The two deepest minima correspond to the structures of the two known experimental polymorphs, which are satisfactorily described.

Do computed crystal structures of nonpolar molecules depend on the electrostatic interactions? The case of tetracene.

We present a strategy for comparing the global properties of competing potential models. By systematically sampling the potential energy surface of crystalline tetracene, we assess how the number, energy and structure of its minima are modified by switching on (or off) the Coulombic interactions. The increased complexity of the Coulombic potential leads to a more "rugged" potential energy surface with a larger number of minima, but the effect is not large. In fact, we find a subset of minima stable only in presence of the Coulombic interactions, a smaller subset stable only in their absence, and a large majority stable in both cases. Among these, there is a very good, but not perfect, correlation between the energies and the structures computed with and without the electrostatic interactions. Although electrostatic interactions play a role even in a rigid nonpolar molecule such as tetracene, they are not as crucial as often believed, because altering the electrostatic model (or switching it off completely) leads, in most cases, to equivalent results.

Anomalous dispersion of optical phonons at the neutral-ionic transition: evidence from diffuse x-ray scattering.

Diffuse x-ray data for mixed-stack organic charge-transfer crystals approaching the neutral-ionic phase transition can be quantitatively explained as due to the softening of the optical phonon branch. The interpretation is fully consistent with vibrational spectra, and underlines the importance of electron-phonon coupling in low-dimensional systems with delocalized electrons.

Charge-sensitive vibrations in p-chloranil: the strange case of the C=C antisymmetric stretching.

We have combined DFT calculations with single-crystal polarized infrared spectra to reinvestigate the assignment of the C=C antisymmetric stretching mode b(2u)nu(18) of p-chloranil (CA). The frequency of this mode indeed seems to display a nonlinear dependence on the average charge on the CA molecule (rho), at variance with the behavior of the antisymmetric C=O stretching frequency. The DFT calculations show that the origin of the problem is a drastic, 2 orders of magnitude decrease of the infrared intensity of the C=C antisymmetric stretching upon electron addition. Therefore, no infrared band can be easily associated to this mode in charge-transfer (CT) solids with rho approximately > 0.5. On the other hand, a linear relationship between rho and the b(2u)nu(18) frequency is found in quasi-neutral CT complexes of CA.

In situ spectroscopic characterization of rectifying molecular monolayers self-assembled on gold.

We report visible, Raman, and infrared spectra of self-assembled monolayers (SAMs) formed by the donor-(pi-bridge)-acceptor chromophore, Z-beta-[N-(omega-acetylthioalkyl)-4-quinolinium]-alpha-cyano-4-styryldicyanomethanide (CH3CO-S-CnH2n-Q3CNQ where n=8, 10), on gold-coated substrates. The data are compared with the spectra collected for the same compound in solution and in the solid state, and with those obtained for a Langmuir-Blodgett (LB) monolayer of C16H33-Q3CNQ deposited on gold. Spectral analysis confirms that in solution, in the solid state and in the LB film the chromophore has a zwitterionic (D+-pi-A-) ground state. At variance with this well-known result, our data show that in SAMs deposited on gold the chromophore has a more neutral, quinoid ground state. We relate this difference to the different packing of the molecules in the two different films: in SAMs in fact the chromophores stand almost vertical with respect to the substrate, whereas in LB films they make an angle of about 45 degrees. The Q3CNQ molecule is a well-known molecular rectifier, and for SAMs we were able to check the direction of electron flow at forward bias on the same samples that have been characterized spectroscopically, shedding light on the rectification mechanism.

Inherent structures of crystalline tetracene.

We have systematically sampled the potential energy surface of crystalline tetracene to identify its local minima. These minima represent all possible stable configurations and constitute the "inherent structures" of the system. The crystal is described in terms of rigid molecules with Coulombic and atom-atom interactions. Hundreds of distinct minima are identified, mostly belonging to the space groups P (triclinic) and P2(1)/c (monoclinic), with a variety of structural arrangements. The deepest minimum corresponds to the high temperature-low pressure polymorph. This is the only polymorph with a completely described X-ray structure, which is satisfactorily described by the calculations. The next deep minimum is likely to correspond to the low temperature-high pressure polymorph, which has been experimentally identified but not yet fully described.

Novel fused D-A dyad and A-D-A triad incorporating tetrathiafulvalene and p-benzoquinone.

Novel fused donor-acceptor dyad (TTF-Q or D-A) and acceptor-donor-acceptor triad (Q-TTF-Q or A-D-A) incorporating the donor tetrathiafulvalene (TTF) and the acceptor p-benzoquinone (Q) have been synthesized. The solution UV-vis spectra of these molecules display a low-energy absorption band that is attributed to an intramolecular charge transfer between both antagonistic units. The presence of reversible oxidation and reduction waves for the donor and acceptor moieties was shown by cyclic voltammetry, in agreement with the ratio TTF/quinone(s) units. The successive generation from these compounds of the cation radical and anion radical obtained upon (electro)chemical oxidation and reduction, respectively, was monitored by optical and ESR spectroscopies. The anion radical Q-TTF-Q(-.) triad was demonstrated to be a class II mixed-valence system with the existence of a temperature-dependent intramolecular electron transfer. The crystallographic tendency of these fused systems to overlap in mixed stacks of alternating A-D-A units is also discussed.

Probing pentacene polymorphs by lattice dynamics calculations.

We have performed a lattice dynamics calculation to compute the "inherent structures" of minimum potential energy for pentacene, starting from available X-ray data. The calculation shows that two distinct bulk crystalline phases of pentacene exist, with very subtle structural differences but clearly different phonon spectra. The method of crystal growth (from solution or vapor) is not the determining factor for obtaining either structure.