Tuning Crystal Ordering, Electronic Structure, and Morphology in Organic Semiconductors: Tetrathiafulvalenes as a Model Case

Tetrathiafulvalenes (TTFs) are an appealing class of organic small molecules giving rise to some of the highest performing active materials reported for organic field effect transistors (OFETs). Because they can be easily chemically modified, TTF-derivatives are ideal candidates to perform molecule-property correlation studies and, especially, to elucidate the impact of molecular and crystal engineering on device performance. A brief introduction into the state-of-the-art of the field-effect mobility values achieved with TTF derivatives employing different fabrication techniques is provided. Following, structure-performance relationships are discussed, including polymorphism, a phenomenon which is crucial to control for ensuring device reproducibility. It is also shown that chemical modification of TTFs has a strong influence on the electronic structure of these materials, affecting their stability as well as the nature of the generated charge carriers, leading to devices with p-channel, n-channel, or even ambipolar behaviour. TTFs have also shown promise in other applications, such as phototransistors, sensors, or as dopants or components of organic metal charge transfer salts used as source-drain contacts. Overall, TTFs are appealing building blocks in organic electronics, not only because they can be tailored to perform fundamental studies, but also because they offer a wide spectrum of potential applications

Neutral organic radical formation by chemisorption on metal surfaces

Organic radical monolayers (r-MLs) bonded to metal surfaces are potential materials for the development of molecular (spin)electronics. Typically, stable radicals bearing surface anchoring groups are used to generate r-MLs. Following a recent theoretical proposal based on a model system, we report the first experimental realization of a metal surface-induced r-ML, where a rationally chosen closed-shell precursor 3,5-dichloro-4-[bis(2,4,6-trichlorophenyl)methylen]cyclohexa-2,5-dien-1-one (1) transforms into a stable neutral open-shell species () via chemisorption on the Ag(111) surface. X-ray photoelectron spectroscopy reveals that the >C=O group of 1 reacts with the surface, forming a C-O-Ag linkage that induces an electronic rearrangement that transforms 1 to . We further show that surface reactivity is an important factor in this process whereby Au(111) is inert towards 1, whereas the Cu(111) surface leads to dehalogenation reactions. The radical nature of the Ag(111)-bound monolayer was further confirmed by angle-resolved photoelectron spectroscopy and electronic structure calculations, which provide evidence of the emergence of the singly occupied molecular orbital (SOMO) of 1

Kondo Effect in a Neutral and Stable All Organic Radical Single Molecule Break Junction

Organic radicals are neutral, purely organic molecules exhibiting an intrinsic magnetic moment due to the presence of an unpaired electron in the molecule in its ground state. This property, added to the low spin-orbit coupling and weak hyperfine interactions, make neutral organic radicals good candidates for molecular spintronics insofar as the radical character is stable in solid state electronic devices. Here we show that the paramagnetism of the polychlorotriphenylmethyl radical molecule in the form of a Kondo anomaly is preserved in two- and three-terminal solid-state devices, regardless of mechanical and electrostatic changes. Indeed, our results demonstrate that the Kondo anomaly is robust under electrodes displacement and changes of the electrostatic environment, pointing to a localized orbital in the radical as the source of magnetism. Strong support to this picture is provided by density functional calculations and measurements of the corresponding nonradical species. These results pave the way toward the use of all-organic neutral radical molecules in spintronics devices and open the door to further investigations into Kondo physics

Neutral organic radical formation by chemisorption on metal surfaces

Organic radical monolayers (r-MLs) bonded to metal surfaces are potential materials for the development of molecular (spin)electronics. Typically, stable radicals bearing surface anchoring groups are used to generate r-MLs. Following a recent theoretical proposal based on a model system, we report the first experimental realization of a metal surface-induced r-ML, where a rationally chosen closed-shell precursor 3,5-dichloro-4-[bis(2,4,6-trichlorophenyl)methylen]cyclohexa-2,5-dien-1-one (1) transforms into a stable neutral open-shell species () via chemisorption on the Ag(111) surface. X-ray photoelectron spectroscopy reveals that the >C=O group of 1 reacts with the surface, forming a C-O-Ag linkage that induces an electronic rearrangement that transforms 1 to . We further show that surface reactivity is an important factor in this process whereby Au(111) is inert towards 1, whereas the Cu(111) surface leads to dehalogenation reactions. The radical nature of the Ag(111)-bound monolayer was further confirmed by angle-resolved photoelectron spectroscopy and electronic structure calculations, which provide evidence of the emergence of the singly occupied molecular orbital (SOMO) of 1

Operative mechanism of hole-assisted negative charge motion in ground states of radical-anion molecular wires

Charge transfer/transport in molecular wires over varying distances is a subject of great interest. The feasible transport mechanisms have been generally accounted for on the basis of tunneling or superexchange charge transfer operating over small distances which progressively gives way to hopping transport over larger distances. The underlying molecular sequential steps that likely take place during hopping and the operative mechanism occurring at intermediate distances have received much less attention given the difficulty in assessing detailed molecular-level information. We describe here the operating mechanisms for unimolecular electron transfer/transport in the ground state of radical-anion mixed-valence derivatives occurring between their terminal perchlorotriphenylmethyl/ide groups through thiophene-vinylene oligomers that act as conjugated wires of increasing length up to 53 angstrom, The unique finding here is that the net transport of the electron in the larger molecular wires is initiated by an electron hole dissociation intermediated by hole delocalization (conformationally assisted and thermally dependent) forming transient mobile polaronic states in the bridge that terminate by an electron hole recombination at the other wire extreme. On the contrary, for the shorter radical-anions our results suggest that a flickering resonance mechanism which is intermediate between hopping and superexchange is the operative one. We support these mechanistic interpretations by applying the pertinent biased kinetic models of the charge/spin exchange rates determined by electron paramagnetic resonance and by molecular structural level information obtained from UV-vis and Raman spectroscopies and by quantum chemical modeling

Electronic and structural characterisation of a tetrathiafulvalene compound as a potential candidate for ambipolar transport properties

We report a joint experimental and theoretical study on the electronic structure and the solid-state organisation of bis(naphthoquinone)-tetrathiafulvalene (BNQ-TTF) as a promising ambipolar semiconductor. Accordingly, organic field-effect transistors (OFETs) fabricated with this material show both hole and electron transport for the first time in TTF derivatives.Peer ReviewedPostprint (published version

A compact tetrathiafulvalene-benzothiadiazole dyad and its highly symmetrical charge-transfer salt: Ordered donor p-stacks closely bound to their acceptors

A compact and planar donor-acceptor molecule 1 comprising tetrathiafulvalene (TTF) and benzothiadiazole (BTD) units has been synthesised and experimentally characterised by structural, optical, and electrochemical methods. Solution-processed and thermally evaporated thin films of 1 have also been explored as active materials in organic field-effect transistors (OFETs). For these devices, hole field-effect mobilities of µFE=(1. 3±0.5)×10-3 and (2.7±0.4)×10-3 cm2 V s-1 were determined for the solution-processed and thermally evaporated thin films, respectively. An intense intramolecular charge-transfer (ICT) transition at around 495 nm dominates the optical absorption spectrum of the neutral dyad, which also shows a weak emission from its ICT state. The iodine-induced oxidation of 1 leads to a partially oxidised crystalline charge-transfer (CT) salt {(1)2I3}, and eventually also to a fully oxidised compound {1I3} 1/2I2. Single crystals of the former CT compound, exhibiting a highly symmetrical crystal structure, reveal a fairly good room temperature electrical conductivity of the order of 2 S cm-1. The one-dimensional spin system bears compactly bonded BTD acceptors (spatial localisation of the LUMO) along its ridge. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Peer Reviewe

Structural and electronic properties of the one-dimensional organic metal bis(thiodimethylene)-tetrathiafulvalene tetracyanoquinodimethane

The synthesis and structural characterization at room temperature and 130 K of the organic charge-transfer salt bis(thiodimethylene)-tetrathiafulvalene tetracyanoquinodimethane (BTDMTTF-TNCQ) is reported. X-ray diffuse scattering as well as ir and Raman measurements show that the charge transfer in this salt is ∼0.5. BTDMTTF-TCNQ remains metallic down to very low temperatures but exhibits an anomaly at 175 K in the conductivity and thermopower which could be related to some unusual structural modification of the unit cell. Band-structure calculations show that this salt is a narrow-band one-dimensional metal. The high value of the spin susceptibility, the low value of the conductivity, and the observation of a 4 k F charge-density-wave (CDW) instability around room temperature prove that it is a highly correlated metal. In contrast with other salts of the TTF-TCNQ family, BTDMTTF-TCNQ does not exhibit critical 2 k F structural fluctuations. It is proposed that the absence of the 2 k F CDW transition is due to the stronger interlocking of the donor and acceptor molecules due to the presence of the sulfur atoms in the central position of the outer five-membered rings of the BTDMTTF donor

HOMO stabilisation in pi-Extended Dibenzotetrathiafulvalene derivatives for their application in OFETs

Three new organic semiconductors, in which either two methoxy units are directly linked to a dibenzotetrathiafulvalene (DB-TTF) central core and a 2,1,3-chalcogendiazole is fused on the one side, or four methoxy groups are linked to the DB-TTF, have been synthesised as active materials for organic field-effect transistors (OFETs). Their electrochemical behaviour, electronic absorption and fluorescence emission as well as photoinduced intramolecular charge transfer were studied. The electron-withdrawing 2,1,3-chalcogendiazole unit significantly affects the electronic properties of these semiconductors, lowering both the HOMO and LUMO energy levels and hence increasing the stability of the semiconducting material. The solution-processed single-crystal transistors exhibit high performance with a hole mobility up to 0.04 cm2¿V-1¿s-1 as well as good ambient stability.Peer Reviewe

Diradicals acting through diamagnetic phenylene vinylene bridges: Raman spectroscopy as a probe to characterize spin delocalization

We present a complete Raman spectroscopic study in two structurally well-defined diradical species of different lengths incorporating oligo p-phenylene vinylene bridges between two polychlorinated triphenylmethyl radical units, a disposition that allows sizeable conjugation between the two radicals through and with the bridge. The spectroscopic data are interpreted and supported by quantum chemical calculations. We focus the attention on the Raman frequency changes, interpretable in terms of: (i) bridge length (conjugation length); (ii) bridge conformational structure; and (iii) electronic coupling between the terminal radical units with the bridge and through the bridge, which could delineate through-bond spin polarization, or spin delocalization. These items are addressed by using the"oligomer approach" in conjunction with pressure and temperature dependent Raman spectroscopic data. In summary, we have attempted to translate the well-known strategy to study the electron (charge) structure of π−conjugated molecules by Raman spectroscopy to the case of electron (spin) interactions via the spin delocalization mechanism