Tetrathiafulvalenes, Oligoacenenes, and Their Buckminsterfullerene Derivatives: The Brick and Mortar of Organic Electronics

New Versatile Building Block for the Construction of Tetrathiafulvalene-Based Donor−Acceptor Systems

[reaction: see text] A new dissymmetrical tetrathiafulvalene (TTF) derivative 1 was synthesized as a versatile building block to reach TTF-based donor-acceptor assemblies incorporating C(60) in triad C(60)-TTF-C(60) 2 and/or p-benzoquinone (Q) in fused dyad Q-TTF 3 and triad Q-TTF-C(60) 4.

Vibronic Coupling in Organic Semiconductors: The Case of Fused Polycyclic Benzene–Thiophene Structures

The nature of vibronic coupling in fused polycyclic benzene-thiophene structures has been studied using an approach that combines high-resolution gas-phase photoelectron spectroscopy measurements with first-principles quantum-mechanical calculations. The results indicate that in general the electron-vibrational coupling is stronger than the hole-vibrational coupling. In acenedithiophenes, the main contributions to the hole-vibrational coupling arise from medium- and high-frequency vibrations. In thienobisbenzothiophenes, however, the interaction of holes with low-frequency vibrations becomes significant and is larger than the corresponding electron-vibrational interaction. This finding is in striking contrast with the characteristic pattern in oligoacenes and acenedithiophenes in which the low-frequency vibrations contribute substantially only to the electron-vibrational coupling. The impact of isomerism has been studied as well.

Enforced Face-to-Face Stacking of Organic Semiconductor Building Blocks within Hydrogen-Bonded Molecular Cocrystals

We report a method to enforce face-to-face stacking of the aromatic rings of organic semiconductor molecules in the solid state that employs bifunctional hydrogen-bond donors, in the form of semiconductor cocrystal formers, to align semiconductor building blocks.

Organization of Acenes with a Cruciform Assembly Motif

This study explores the assembly in the crystalline state of a class of pentacenes that are substituted along their long edges with aromatic rings forming rigid, cruciform molecules. The crystals were grown from the gas phase, and their structures were compared with DFT-optimized geometries. Both crystallographic and computed structures show that a planar acene core is the exception rather than the rule. In the assembly of these molecules, the phenyl groups block the herringbone motif and further guide the arrangement of the acene core into higher order structures. The packing for the phenyl-substituted derivatives is dictated by close contacts between the C-H's of the pendant aromatic rings and the carbons at the fusions in the acene backbone. Using thiophene substituents instead of phenyls creates cofacially stacked acenes. In thin films, the thiophene-substituted derivative forms devices with good electrical properties: relatively high mobility, high ON/OFF ratios, and low threshold voltage for device activation. An unusual result is obtained for the decaphenyl pentacene when devices are fabricated on its crystalline surface. Although its acene cores are well isolated from each other, this material still exhibits good electrical properties.

Diels–Alder reaction of acenes with singlet and triplet oxygen – theoretical study of two-state reactivity

An interesting change in mechanism (from concerted to biradical) is described for the reaction of acenes (benzene through pentacene) with molecular oxygen (either singlet oxygen, 1Deltag-O2, or triplet oxygen, 3Sigmag-O2).

Monitoring the formation of TTF dimers by Na+ complexation

The formation of the two dimeric species [(TTF)2]+* and (TTF+*)2 can be monitored by complexation of Na+ on a calix[4]arene-TTF assembly.

A redox-active tri-star molecule: merging of TTF and HAT chemistry

A planar pi-conjugated heteroaromatic molecule 1 has been synthesized and fully characterized; it combines two characteristics, a charge-transfer transition originating from its inherent donor-acceptor nature in its neutral state and an intervalence charge-transfer transition in its 1(2+) mixed-valence state.

Syntheses of Soluble, π-Stacking Tetracene Derivatives

[reaction: see text] The syntheses of a series of fluorine- and alkyl/alkoxy-functionalized tetracenes are reported. These functional groups are found to improve the solubility in common organic solvents and tune molecular arrangement in solids. The crystal packing, electrochemical behavior, and UV-vis absorbance spectroscopy of these materials are discussed.

Pressure Enhanced Conductivity in Bis-1,2,3-Thiaselenazolyl Dimers

A synthetic sequence to salts of N-alkylated pyridine-bridged 1,2,3-thiaselenazolo-1,2,3-thiaselenazolylium cations [2]+ (R1 = Me, Et; R2 = H) is described. The corresponding radicals 2 (R1 = Me, Et; R2 = H) can be generated from the cations by chemical or electrochemical reduction. Crystals of the two radicals are isostructural and consist of interpenetrating pi-stacked arrays of closed-shell Se-Se sigma-bonded dimers [2]2 laced together with numerous short intermolecular Se- - -Se, Se- - -S, and Se- - -N contacts. Variable-temperature magnetic, conductivity, and near-infrared optical measurements indicate that the bulk materials behave as small band gap semiconductors with room-temperature conductivities sigma(RT) near 10(-6) S cm(-1) and thermal activation energies Ea of 0.32 eV (R1 = Me) and 0.36 eV (R1 = Et). LMTO band structure calculations on both compounds are consistent with this interpretation. The application of external pressure leads to dramatic increases in conductivity; at 4 GPa sigma(RT) reaches a value near 10(-1) S cm(-1) for R1 = Me and 10(-2) S/cm for R1 = Et. The conductivity remains activated for both compounds, but for R1 = Me the activation energy Ea is reduced to near 0.03 eV at 5 GPa, suggestive of a weakly metallic state.

Synthesis of a Thermally Stable Hybrid Acene-Thiophene Organic Semiconductor via a Soluble Precursor

An acene fused-thiophene hybrid p-semiconductor exhibiting high thermal stability has been synthesized via a soluble precursor bearing sterically interacting trimethylsilyl groups. [structure: see text]

Tetrathiafulvalene in a Perylene-3,4:9,10-bis(dicarboximide)-Based Dyad: A New Reversible Fluorescence-Redox Dependent Molecular System

A donor-acceptor dyad system involving tetrathiafulvalene (TTF) as donor attached by a flexible spacer to perylene-3,4:9,10-bis(dicarboximide) (PDI) as acceptor was synthesized and characterized. The strategy used the preliminary synthesis of an unsymmetrical PDI unit bearing an alcohol functionality as anchor group. Single-crystal analysis revealed a highly organized arrangement in which all PDI molecules are packed in a noncentrosymmetrical pattern. It was shown that the fluorescence emission intensity of the TTF-PDI dyad can be reversibly tuned depending on the oxidation states of the TTF unit. This behavior is attributed to peculiar properties of TTF linked to a PDI acceptor, which fluoresces intrinsically. Consequently, this dyad can be considered as a new reversible fluorescence-redox dependent molecular system.

Electronic Structures and Charge Transport Properties of the Organic Semiconductor Bis[1,2,5]thiadiazolo-p-quinobis(1,3-dithiole), BTQBT, and Its Derivatives

We analyze the correlation between crystal and film structures and charge transport of an important organic semiconductor, bis[1,2,5]thiadiazolo-p-quinobis(1,3-dithiole) (BTQBT), and its derivatives 4,8-bis(1,3-dithiol-2-ylidene)-4H,8H-[1,2, 5]selenadiazolo[3,4-f]-2,1,3-benzothiadiazole, 4,8-bis(1,3-diselenol-2-ylidene)-4H,8H-benzo[1,2-c:4,5-c']bis[1,2,5]thiadiazole, and tetramethyl-BTQBT. We present first-principles density functional theory (DFT) calculations that agree well with earlier angle-resolved photoelectron spectroscopy (ARPES) experiments on BTQBT films, strongly supporting that the BTQBT films adopt the same layered structure as in the single crystals. Qualitative charge transport properties based on presented DFT results agree with experiments regarding the sign of the charge carriers and the unusually small anisotropy of conductivity. These agreements indicate that accurate electronic structure calculations, when coupled with ARPES, help establish the correlation between intermolecular packing and charge transport, which is one of the central but elusive aspects of organic molecular materials. Predictions are made for derivatives of BTQBT, and calculations agree with available experimental information on the conductivities. Comparisons are made with pentacene, one of the most widely studied organic molecular materials.

Functionalized Higher Acenes: Hexacene and Heptacene

We have extended our functionalization strategy for pentacene to the higher acenes hexacene and heptacene. Provided a large enough alkyne substituent is used, these large aromatic rods are both stable and soluble and can be characterized spectroscopically as well as by single-crystal X-ray diffraction.

Third-Order Nonlinear Optical Figure of Merits for Conjugated TTF−Quinone Molecules

We have revealed a substantial enhancement of third-order optical figure of merits by the synthesis of a compact molecule possessing the tetrathiafulvalene (TTF) group with two backside C=O groups. Addition of the saturated methylene chain substantially suppresses the third-order optical figure of merits and even local optical hyperpolarizabilities at lambda = 532 nm. Another TTF-derivative molecule possessing ethylenic and acetylenic chains demonstrates large hyperpolarizabilities; however, generally, the figure of merit factor decreases due to the increasing optical losses as a consequence of enhanced linear absorption. At the same time, both of the chromophores have a large nonlinear optical response. General approaches for search and design of the third-order optical materials with improved properties are given.

Electronic Structures and Properties of Twisted Polyacenes

The effects of twisting on the electronic structures and properties of polyacenes were studied computationally using DFT methods. Singlet-triplet and HOMO-LUMO gaps and vertical S0-S1 transition energies are marginally affected as a function of end-to-end twist angle. The large twist induced by the phenyl substituents in 9,10,11,20,21,22-hexaphenyltetrabenzo[a,c,l,n]pentacene has little influence on its electronic structure.

The effect of selenium incorporation on the bandwidth and conductivity of neutral radical conductors

The first example of an undimerized pi-stacked bis-1,2,3-thiaselenazolyl radical displays improved bandwidth and conductivity relative to an isostructural bis-1,2,3-dithiazolyl.