M. Carmen Ruiz Delgado

Tuning the Charge-Transport Parameters of Perylene Diimide Single Crystals via End and/or Core Functionalization: A Density Functional Theory Investigation

Perylene tetracarboxylic diimide (PTCDI) derivatives stand out as one of the most investigated families of air-stable n-type organic semiconductors for organic thin-film transistors. Here, we use density functional theory to illustrate how it is possible to control the charge-transport parameters of PTCDIs as a function of the type, number, and positions of the substituents. Specifically, two strategies of functionalization related to core and end substitutions are investigated. While end-substituted PTCDIs present the same functional molecular backbone, their molecular packing in the crystal significantly varies; as a consequence, this series of derivatives constitutes an ideal test bed to evaluate the models that describe charge-transport in organic semiconductors. Our results indicate that large bandwidths along with small effective masses can be obtained with the insertion of appropriate substituents on the nitrogens, in particular halogenated aromatic groups.

Transition from Tunneling to Hopping Transport in Long, Conjugated Oligo-imine Wires Connected to Metals

We report the electrical transport characteristics of conjugated oligonaphthalenefluoreneimine (ONI) wires having systematically varied lengths up to 10 nm. Using aryl imine addition chemistry, ONI wires were built from gold substrates by extending the conjugation length through imine linkages between highly conjugated building blocks of alternating naphthalenes and fluorenes. The resistance and current-voltage characteristics of ONI wires were measured as a function of molecular length, temperature, and electric field using conducting probe atomic force microscopy (CP-AFM). We have observed a transition in direct current (DC) transport from tunneling to hopping near 4 nm as previously established for oligophenyleneimine (OPI) wires. Furthermore, we have found that long ONI wires are less resistive than OPI wires. The single-wire conductivity of ONI wires is approximately 1.8 +/- 0.1 x 10(-4) S/cm, a factor of approximately 2 greater than that of OPI wires, and consistent with the lower transport activation energy ( approximately 0.58 eV versus 0.65 eV or 13 versus 15 kcal/mol). Quantum chemical calculations reveal that charge is preferentially localized on the fluorene subunits and that the molecules are substantially twisted. Overall, this work confirms that imine addition chemistry can be used to build molecular wires long enough to probe the hopping transport regime. The versatility of this chemistry, in combination with CP-AFM, opens up substantial opportunities to probe the physical organic chemistry of hopping conduction in long conjugated molecules.

Impact of Perfluorination on the Charge-Transport Parameters of Oligoacene Crystals

The charge-transport parameters of the perfluoropentacene and perfluorotetracene crystals are studied with a joint experimental and theoretical approach that combines gas-phase ultraviolet photoelectron spectroscopy and density functional theory. To gain a better understanding of the role of perfluorination, the results for perfluoropentacene and perfluorotetracene are compared to those for their parent oligoacenes, that is, pentacene and tetracene. Perfluorination is calculated to increase the ionization potentials and electron affinities by approximately 1 eV, which is expected to reduce significantly the injection barrier for electrons in organic electronics devices. Perfluorination also leads to significant changes in the crystalline packing, which greatly affects the electronic properties of the crystals and their charge-transport characteristics. The calculations predict large conduction and valence bandwidths and low hole and electron effective masses in the perfluoroacene crystals, with the largest mobilities expected along the pi-stacks. Perfluorination impacts as well both local and nonlocal vibrational couplings, whose strengths increase by a factor of about 2 with respect to the parent compounds.

Hexaazatriphenylene (HAT) versus tri-HAT: The Bigger the Better?

A new hexaazatriphenylene (HAT) derivative formed by the fusion of three HAT units has been prepared and its electronic and molecular structures have been fully characterized by optical and vibrational Raman spectroscopy, electrochemistry, solid-state UV and inverse photoemission spectroscopy (UPS and IPES), and by quantum-chemical calculations. A comparative HAT versus tri-HAT study was performed. The fusion of three HAT molecules causes modifications in the optical and electrochemical properties consistent with enhanced π-electron delocalization attained in a bigger planar core. Such combined experimental and theoretical studies are useful to balance chemical design with supramolecular engineering directed to find enhanced characteristics for new organic semiconductor applications.

Zethrene biradicals: How pro-aromaticity is expressed in the ground electronic state and in the lowest energy singlet, triplet, and ionic states

A analysis of the electronic and molecular structures of new molecular materials based on zethrene is presented with particular attention to those systems having a central benzo-quinoidal core able to generate Kekulé biradicals whose stability is provided by the aromaticity recovery in this central unit. These Kekulé biradicals display singlet ground electronic states thanks to double spin polarization and have low-energy lying triplet excited states also featured by the aromaticity gain. Pro-aromatization is also the driving force for the stabilization of the ionized species. Moreover, the low energy lying singlet excited states also display a profound biradical fingerprint allowing to singlet exciton fission. These properties are discussed in the context of the size of the zethrene core and of its substitution. The work encompasses all known long zethrenes and makes use of a variety of experimental techniques, such as Raman, UV-Vis-NIR absorption, transient absorption, in situ spectroelectrochemistry and quantum chemical calculations. This study reveals how the insertion of suitable molecular modules (i.e., quinoidal) opens the door to new intriguing molecular properties exploitable in organic electronics.

Delocalization-to-Localization Charge Transition in Diferrocenyl- Oligothienylene-Vinylene Molecular Wires as a Function of the Size by Raman Spectroscopy

In going from short to large size thienylene-vinylene diferrocenyl cations, the transition from a charge delocalized to a localized state is addressed by resonance Raman spectroscopy and supported by theoretical model chemistry. The shorter members, dimer and tetramer, display conjugated structures near the cyanine limit of bond length equalization as a result of the strong interferrocene charge resonance, producing a full charge delocalized mixed valence system. In the longest octamer, charge resonance vanishes and the cation is localized at the bridge center (the mixed valence property disappears). The hexamer is at the delocalized-to-localized turning point. Solvent and variable-temperature Raman measurements highlight this borderline property. A detailed structure-property correlation of bond length alternation data and Raman frequencies is proposed to account for the whole set of spectroscopic properties, with emphasis on the changes observed with the size of the molecular wire.

α-Oligofurans show a sizeable extent of π-conjugation as probed by Raman spectroscopy

A Raman spectroscopic analysis revealed that π-conjugation does not reach saturation at least up to the octamer in long α-oligofurans and spreads over 14-15 furan units in the polyfuran. Comparing DFT calculations with experimental results suggests that a considerable amount of HF exchange is required to reproduce computationally the observed conjugation.

Ferrocenyl‐Ended Thieno–Vinylene Oligomers: Donor–Acceptor Polarization and Mixed‐Valence Properties with Emphasis on the Raman Mapping of Localized‐to‐Delocalized Transitions

Whats your role? New oligothiophene-vinylene compounds have been synthesized to study the role of the conjugated chain in two different cases (see scheme; MV=mixed valence). The electronic and molecular structures were analyzed by means of electronic, X-ray photoelectron, and Raman spectroscopy, together with thermo spectroscopy, electrochemistry, and DFT calculations.New oligothiophene-vinylene compounds have been synthesized in order to study the role of the conjugated chain in two different cases: 1) when push-pull action operates between an electron-donor and an electron-acceptor group at the ends of the thiophene-vinylene conjugated chain, and 2) when mixed-valence action is induced by single oxidation of the same chain functionalized at both terminal positions with ferrocene groups leading to competition between the donor groups. The electronic and molecular structures are analyzed by means of electronic, X-ray photoelectron and Raman spectroscopies, together with thermospectroscopy, electrochemistry and density functional theory calculations. The cyclic voltammetry processes have been followed by spectrochemistry. It is shown that the radical cation of the diferrocenyl derivative is a class III mixed-valence system (i.e., fully delocalized) according to its Raman spectrum. Moreover, by Raman thermo-spectroscopy the thermal transition of this radical cation from a delocalized (class III, room temperature) to a localized (class II, -160 degrees C) state is scanned. In all cases the Raman study is paralleled by an electronic absorption spectroscopic analysis. Structure-property relationships are proposed for molecules of two important fields of very active research as that of the non-linear optics (i.e., organic optoelectronic) and that of the mixed-valence systems (i.e., charge-transfer processes).

Oxidation of end-capped pentathienoacenes and characterization of their radical cations.

A detailed investigation of the optical and electrochemical properties of two pentathienoacene derivatives, 2,6-bis(trimethylsilyl)-alpha-pentathienoacene (TMS-T5-TMS) and 2,6-bis(triisopropylsilyl)-alpha-pentathienoacene (TIPS-T5-TIPS), as the neutral and oxidized species was performed in the temperature range of 80-300 K. The experimental solution UV/Vis and solid-state Raman spectra were interpreted by using time-dependent DFT and DFT quantum chemical calculations at the B3LYP/6-31G** level. Bond lengths, HOMO-LUMO positions, and charge distribution were also predicted by computational methods for both the neutral and oxidized states of each thienoacene. As evidenced by ESR and spectroelectrochemical data, upon oxidation the pentathienoacene derivative with the less sterically hindering trimethylsilyl solubilizing groups, TMS-T5-TMS, undergoes pi dimerization to form [TMS-T5-TMS](2) (2+). In contrast, TIPS-T5-TIPS, with the more bulky triisopropylsilyl solubilizing groups, was oxidized to the radical cation but dimerization was prevented due to steric interactions. These experimental observations are supported by DFT calculations, which were used to investigate [TMS-T5-TMS](2) (2+) and [TIPS-T5-TIPS](2) (2+) pi dimers in the solid state and in solution. The redox potentials and absorption peak locations corresponding to the radical cations and pi dimer of TMS-T5-TMS were identified experimentally.

Substituent and counterion effects on the formation of π-dimer dications of end-capped heptathienoacenes

We have investigated the impact of the functionalization and the chemical nature of counterions on the π-dimer dications formation in two end-capped heptathienoacenes. Radical cations of an α-substituted heptathienoacene with triisopropylsilyl groups do not π-dimerize, while those of an α,β-substituted heptathienoacene with four n-decyl side chains show a high propensity toward π-dimerization, increased by PF(6)(-) counterions.