Alberto Girlando

Vibrational spectroscopy of mixed stack organic semiconductors: Neutral and ionic phases of tetrathiafulvalene–chloranil (TTF–CA) charge transfer complex

The infrared and Raman spectra for the room temperature, quasineutral, and the low temperature, quasi‐ionic, phases of the mixed stack charge transfer complex tetrathiafulvalene–chloranil (TTF–CA) are reported. The analysis of the analogous data for a newly synthesized room temperature phase point to a dimerized segregated stack structure. All the vibrational data are interpreted and exploited through a clear identification of the differences, for the two types of stacks, in the spectroscopic effects due to the vibronic interaction, i.e., the coupling between electron and molecular vibration (e‐mv). It is shown that for distorted mixed stack complexes both Raman and infrared spectra can be substantially influenced by the vibronic interaction, whereas the dimerized segregated stack complexes, as already known, display striking vibronic effects only in infrared. The theoretical model which explains the origin of these effects is briefly summarized and its extension to mixed stack structures successfully use...

Vibronic structure of PTCDA stacks: the exciton–phonon-charge-transfer dimer

Abstract Perylenetetracarboxylic acid dianhydride (PTCDA) stacks face-to-face in crystals and multiple quantum wells (MQWs). Excitations of PTCDA stacks are mixed molecular (Frenkel) and charge-transfer (CT) states coupled to a molecular vibration. Eclipsed stacks and molecular conjugation imply strong Frenkel–CT mixing in absorption and electroabsorption, with k=0 at the top of the exciton band, and negligible mixing at k=π for emission from the bottom. The exciton–phonon-CT dimer developed for k=0 processes is a nonadiabatic approximation for narrow CT bands. Quantitative dimer spectra are obtained in the vibronic basis of displaced harmonic oscillators for excited PTCDA and radical ions. We present a joint analysis of absorption and emission in PTCDA stacks and MQWs using parameters from solution, molecular calculations, and related conjugated systems. Polarized single-crystal absorption decisively relates the entire 2–3 eV system to molecular π–π* transitions, while electroabsorption with field along the stack implicates adjacent ions in the stack. The simple structure and extensive PTCDA spectra make possible detailed modelling of mixed Frenkel–CT vibronics that were far less accessible in previous organic molecular crystals. Since the coupled mode is closely related to polyenes and conjugated polymers, PTCDA provides a bridge between molecular insulators and extended systems capable of charge transport.

Vibrational analysis of spectra of quinonoid molecular ions. Part 3.—Vibrational spectra and assignment of 7,7,8,8-tetracyanoquinodimethane radical anion

The Raman and infrared spectra of TCNQ and [2H4]TCNQ radical anions in solution are presented for the first time. The successful assignment of most of the totally symmetric ag modes, obtained directly from the experimental Raman depolarization ratios, corroborates the assignment of the infrared active modes which is based on comparison with the neutral molecules.Radical–radical dimerization in water solution has no relevant influence on the intramolecular fundamentals. The Raman and infrared powder spectra of LiTCNQ and RbTCNQ and of the analogous fully deuterated salts are also reported, and the solid state–solution shifts in the band positions are found not to exceed 15 cm–1.Once the effect of environment on TCNQ– vibrational frequencies had been identified, we were able to evaluate the frequency shifts due to the addition of the extra electron. These shifts of up to about seventy wave numbers are reproduced by a normal coordinate analysis, based on a set of judicious adjustments of the neutral molecule empirical Valence Force Field.The vibrational spectra have been used successfully, as previously with Cs2(TCNQ)3, to demonstrate that in crystalline Cu(TCNQ)2 both neutral and negatively charged TCNQ units are present.

Infrared and Raman spectra of TTF and TTF-d4

Abstract The infrared and Raman spectra of TTF and TTF- d 4 as powder and solution samples are reported. An extensive assignment of the fundamental vibrational modes is presented and discussed in terms of a normal coordinate analysis of the inplane modes. The results obtained allow the calculation of the cartesian displacements of the totally symmetric modes, which are of relevance for the evaluation of the electron-intramolecular vibration linear coupling constants.

Raman phonon spectra of pentacene polymorphs

We report for the first time lattice phonon Raman spectra of pentacene measured by means of a Raman microprobe technique. We experimentally prove the existence of two polymorphs, as expected from recent structural studies. A comparison with Quasi Harmonic Lattice Dynamics calculations, previously performed starting from the available X-ray data, help us in identifying the phase to which each crystal belongs.

Probing polymorphs of organic semiconductors by lattice phonon Raman microscopy

Using micro-Raman techniques to investigate crystal polymorphism is an efficient method, capable of monitoring physical modifications and phase inhomogeneities in crystal domains at the micrometre scale. In the presence of polymorphism, phase mixing is a common occurrence which becomes a crucial issue in structured organic materials tailored for applications in molecular electronics and photonics. A good phase homogeneity is, in fact, required for optimal and reproducible device performance. We tackle the problem of polymorphism in organic semiconductors by combining experimental and theoretical methods. Experimentally we have found that different crystalline polymorphs may be conveniently investigated using their Raman spectra in the region of the lattice phonons, whose frequencies probe intermolecular interactions and are very sensitive to differences in molecular packing. We propose lattice phonon confocal micro-Raman mapping as a fast and reliable diagnostic tool for in-situ characterization of the phase purity. The theoretical approach aims to predict crystal structures and possible coexistence of polymorphs by ranking them in energy and proving that the deepest calculated minima actually correspond to the experimental X-ray diffraction structures of bulk crystals. This combined spectroscopic and theoretical approach to the dynamical properties of a crystal lattice provides a unique body of information on crystal structure recognition of molecular crystals.

Inherent structures of crystalline pentacene

Using a quasi-Monte Carlo scheme, we search the potential energy surface of crystalline pentacene to sample its local minima, which represent the “inherent” structures, i.e., the possible configurations of mechanical equilibrium. The system is described in terms of rigid molecules interacting through a standard atom–atom potential model. Several hundreds of distinct minima are encountered, with a surprising variety of structural arrangements. We find that deep minima are easily accessible because they exhibit a favorable energy distribution and their attraction basins tend to be wide. Thanks to these features of the potential surface, the localization the global minimum becomes entirely feasible, allowing reliable a priori predictions of the crystallographic structures. The results for pentacene are very satisfactory. In fact, the two deepest minima correspond to the structures of the two known experimental polymorphs, which are described correctly. Further polymorphs are also likely to exist.

Accurate electron-molecular vibration coupling constants from powders optical spectra: TCNQ and TTF

Abstract A simple but accurate method is proposed for the experimental determination of electron — molecular vibration (e-mv) coupling constants from the optical spectra of powdered samples of dimerized, segregated stack organic quasi-one-dimensional radical salts. By re-examining various TCNQ-based compounds a transferable set of coupling constants is obtained for this key molecular structure. The application of our method to TTF Br salt leads to TTF e-mv coupling constants which nicely compare with those determined from TTF-Chloranil.

Phonons and structures of tetracene polymorphs at low temperature and high pressure

Crystals of tetracene have been studied by means of lattice phonon Raman spectroscopy as a function of temperature and pressure. Two different phases (polymorphs I and II) have been obtained, depending on sample preparation and history. Polymorph I is the most frequently grown phase, stable at ambient conditions. A pressure induced phase transition, observed above 1 GPa, leads to polymorph II, which is also obtained at temperatures below 140 K. Polymorph II can also be maintained at ambient conditions. We have calculated the crystallographic structures and phonon frequencies as a function of temperature, starting from the configurations of the energy minima found by exploring the potential energy surface of crystalline tetracene. The spectra calculated for the first and second deepest minima match satisfactorily those measured for polymorphs I and II, respectively. All published x-ray structures, once assigned to the appropriate polymorph, are also reproduced.

Influence of the intermolecular charge transfer interaction on the solution and solid state infrared spectra of 7,7,8,8-tetracyanoquinodimethane (TCNQ) alkaline salts

The results of a study of the effect of concentration on the electronic and infrared absorption spectra of LiTCNQ and Li[2H4]TCNQ in dimethylsulphoxide, as well as the Raman and infrared spectra of aqueous solutions of the same salts, are reported. The formation of the self-dimer of the TCNQ radical anion is accompanied by the appearance of some peculiar absorptions in the infrared. They are attributed to a vibronic intensity enhancement of some vibrational modes of the dimer, corresponding to the out-of-phase coupling of totally symmetric modes of the isolated radicals. The phenomenon is interpreted in terms of the Ferguson–Person vibronic model, which embodies the basic concepts of the charge transfer (CT) interaction.The comparison of the vibronic effects discovered for the TCNQ radical dimer in solution with those reported in the literature for the infrared spectrum of crystalline KTCNQ shows that the dimeric unit essentially contains the same vibronic features induced by the CT interaction in the solid state. The infrared spectra of the two polymorphic forms RbTCNQ(I) and RbTCNQ(II) are also reported. Their comparison shows that the intensity of the vibronic features is sensitive to different types of anion stacking in the two structures.An overall analysis of the reported data allows us to give the complete list of the infrared absorptions of vibronic origin and to reassign some infrared active fundamentals of the monomeric TCNQ radical anion.