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

Light-emitting organic nanoaggregates from functionalized p-quaterphenylenes

Functionalized para-phenylenes are versatile building blocks for generating aligned fiber-like nanostructures by a self-assembly growth process on muscovite mica substrates upon controlled vapor deposition (molecule epitaxy). Functional groups were implemented at the 1,4‴-para-positions of p-quaterphenylenes (p4P) using a Suzuki cross-coupling strategy. The nanoaggregates possess outstanding optical properties, which can be modified in a controlled manner by functionalization. Functionalization allows the fluorescence peak emission frequency to shift within the blue spectral range, and the nanoaggregates' three-dimensional shape alters depending on the substitution. In the case of asymmetrically functionalized phenylenes due to the intrinsic non-zero hyperpolarizability of push-pull functionalized oligomers and non-centrosymmetry of the respective nanofibers, they act as frequency doublers.

Synthesis and characterization of acetylene-linked bisphenalenyl and metallic-like behavior in its charge-transfer complex

We prepared and isolated a phenalenyl-based neutral hydrocarbon (1 b) with a biradical index of 14%, as well as its charge-transfer (CT) complex 1 b-F(4)-TCNQ. The crystal structure and the small HOMO-LUMO gap assessed by electrochemical and optical methods support the singlet-biradical contribution to the ground state of the neutral 1 b. This biradical character suggests that 1 b has the electronic structure of phenalenyl radicals coupled weakly through an acetylene linker, that is, some independence of the two phenalenyl moieties. The monocationic species 1 b*+ was obtained by reaction with the organic electron acceptor F4-TCNQ. The cationic species has a small disproportionation energy deltaE for the reaction 2 x 1 b*+ <==> 1 b + 1 b2+, which presumably originates from the independence of the phenalenyl moieties. The small deltaE led to a small on-site Coulombic repulsion U(eff) = 0.61 eV in the CT complex. Moreover, a very effective orbital overlap of the phenalenyl rings between molecules afforded a relatively large transfer integral t = 0.09 eV. The small U(eff)/4t ratio (= 1.7) resulted in a metallic-like conductive behavior at around room temperature. Below 280 K, the CT complex showed a transition into a semiconductive state as a result of bond formation between phenalenyl and F4-TCNQ carbon atoms.

Synthesis of poly(para-phenylene)(2-isocyano-2-tosylpropane-1,3-diyl), poly(para-phenylene)(2-oxopropane-1,3-diyl) and oligo(cyclopentadienones) via carbonylative coupling of alpha,alpha'-dibromoxylene

Poly(para-phenylene)(2-oxopropane-1,3-diyl), a potential precursor of linear graphene, is generated in low yield from hydrolysis of a previously unknown poly(para-phenylene)(2-isocyano-2-tosylpropane-1,3-diyl) generated from inexpensive, commercially available starting materials.

Alkynyl Fischer carbene complex as a traceless directing group for the regioselective cycloaddition of dithiolethiones to arylacetylene: synthesis of E-dithiafulvene thione and dithioesters

A phenylethynyl Fischer carbene complex was used as a traceless directing group for the regioselective cycloaddition of dithiolethiones to arylacetylene, which constitutes the first synthesis of E-dithiafulvene thione or dithioesters.

Electronic communication in tetrathiafulvalene (TTF)/C60 systems: toward molecular solar energy conversion materials?

The covalent connection of the electron acceptor C 60 to p-quinonoid pi-extended tetrathiafulvalenes (exTTFs) has allowed for the preparation of new photo- and electroactive conjugates able to act as artificial photosynthetic systems and active molecular materials in organic photovoltaics. The gain of aromaticity undergone by the pi-extended TTF unit in the oxidation process results in highly stabilized radical ion pairs, namely, C 60 (*-)/exTTF (*+). Lifetimes for such charge-separated states, ranging from a few nanoseconds to hundreds of microseconds, have been achieved by rationally modifying the nature of the chemical spacers. These long-lived radical pairs are called to play an important role for the conversion of sunlight into chemical or electrical power.

Extreme conformational constraints in pi-extended tetrathiafulvalenes: unusual topologies and redox behavior of doubly and triply bridged cyclophanes

Doubly and triply bridged 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (ex-TTF) derivatives have been synthesized. Key steps are the generation and macrocyclization reactions of ex-TTF-dithiolate reagents. The X-ray crystal structures of the doubly bridged cyclophanes 15 and 16 and the triply bridged system 23 show that the saddle-like conformation of the ex-TTF framework is enhanced by the short bridges between the dithiole rings. Unlike all previous ex-TTF derivatives (which display a single quasi-reversible two-electron oxidation wave, D0 --> D2+), cyclic voltammetry of the cyclophanes reveals two reversible, one-electron oxidation steps (D0 --> D*+ --> D2+), with differences between the half-wave potentials (E2(1/2) - E1(1/2)) of 0.22-0.26 V. The conformational changes and gain in aromaticity which drive the second oxidation process in unrestricted ex-TTF systems (including singly bridged cyclophanes) have been prevented by multiple bridging. The radical cation species gives rise to a very broad, low-energy band (lambdamax = 2175 and 2040 nm for 15 and 21, respectively), assigned to an intramolecular interaction. The steric constraints imposed by multiple bridging have become so extreme that the pi-framework of 15, 16, 21, and 23 exhibits remarkable optical and redox behavior which is not characteristic of ex-TTF systems.

Contrasting Photodynamics between C60-Dithiapyrene and C60-Pyrene Dyads

The photodynamics of a C60-dithiapyrene donor-acceptor conjugate were compared with the corresponding C60-pyrene conjugate. The photoinduced charge separation and subsequent charge recombination processes were examined by time-resolved fluorescence measurements on the picosecond timescale and transient absorption measurements on the picosecond and microsecond timescales with detection in the visible and near-infrared regions. We have observed quite long lifetimes (i.e., up to 1.01 ns) for the photogenerated charge-separated state in a C60-dithiapyrene dyad without the need for i) a long spacer between the two moieties, or ii) a gain in aromaticity in the radical ion pair.

Tetraazaperopyrenes: A New Class of Multifunctional Chromophores

Tetraazaperopyrene and a range of derivatives have been synthesised and their photophysical and redox-chemical properties studied. The parent compound, 1,3,8,10-tetraazaperopyrene (1), was prepared by treating 4,9-diamino-3,10-perylenequinone diimine with triethyl orthoformate, whereas the 2,9-disubstituted derivatives of 1 were obtained after treatment with the corresponding carboxylic acid chloride or anhydride (2 mol equiv). The 1,3,8,10-tetraazaperopyrene core structure was established by X-ray diffraction of 2,9-bis(2-bromophenyl)-1,3,8,10-tetraazaperopyrene (6). The UV-visible absorption spectra of the compounds have a characteristic visible pi(*)<--pi absorption band at 440 nm (log epsilon(max)=4.80) with a strong vibrational progression (Delta nu approximately 1450 cm(-1)). Diprotonation of the nitrogen atoms induces a bathochromic shift of this band from 430-440 to 470-480 nm and all four nitrogen atoms are protonated when pure H(2)SO(4) is used as the solvent. The first and second as well as the third and fourth protonations occur concomitantly, which implies that they have very similar pK(a) values and, consequently, similar proton affinities. A theoretical study of the proton affinities in the gas phase and in solution attributes this behaviour to the effects of polar solvents, which dampen the charge of a protonated site at the other end of the molecule and thus effectively decouple the two opposite pyrimidine units in the polycondensed aromatic compound. The photophysical data were modelled in a time-dependent DFT study of 1, 1H(2)(2+) and 1H(4)(4+) in both the gas phase and in a polar solvent. All the dyes show weak fluorescence in organic solvents, however, their protonated conjugate acids show dramatically increased fluorescence intensity. All of the dyes undergo two electrochemically reversible one-electron reductions with cyclovoltammetric half-wave potentials at E(red1) approximately -0.9 V and E(red2) approximately -1.3 V (vs. SCE), which are associated with characteristic spectral changes.

Double Diels−Alder Strategies to Soluble 2,9- and 2,9,6,13-Tetraethynylpentacenes, Photolytic [4 + 4] Cycloadditions, and Pentacene Crystal Packing

Four new classes of organic solvent soluble ethynylpentacene derivatives (2,9-, 2,10-, 2,6,9,13-, 2,6,10,13-) have been prepared by complementary, versatile, double Diels-Alder strategies. Functional groups on the A, C, and E rings can be manipulated to increase the solubility, modulate the electronics, and alter the solid-state packing. Cycloaddition reactions with diene 2 and 1,4,5,8-anthradiquinone (3) or ortho-quinodimethane 19 with 1-butyl-3-methylimidazolim iodide (18) as the iodide source (a significant improvement over NaI) and benzoquinone (20) followed by in situ aromatization afforded the quinones 4, 5, 21, and 22, respectively. For the 2,9- and 2,10- families, a one-pot desilylation/triflation was developed. Palladium(0) coupling and reductive aromatization afforded 2,9-di(triisopropylsilylethynyl)pentacene (10) and 2,10-di(triisopropylsilylethynyl)pentacene (11), respectively. Photodimerization of these pentacenes afforded the air-stable [4 + 4] cycloaddition pentacene precursors (tetrakisnaphthotricyclo[4.2.2.22,5]dodecanes, 12-15). Thermal cycloreversion of the dimers ( approximately 13 J/g, approximately 4 kcal/mol) produces the parent pentacenes (10 or 11). The tetrasubstituted family utilized a parallel route with extra versatility as the timing of the Grignard and palladium(0) coupling step may be varied depending upon the functional group combinations desired. The subsequent reactions provided the tetraethynylpentacenes 28-30, 33-35 (para-isomers), and 38 (meta-isomer). X-ray crystallography analysis of 28, 29, and 33 revealed various pi-pi stacked packing motifs that differ from the unfavorable herringbone pattern of pentacene.

An Original Redox-Responsive Ligand Based on a π-Extended TTF Framework

The synthesis of the first pi-extended tetrathiafulvalene (TTF) ligand featuring a furanoquinonoid spacer and pyridyl functional groups is described. This compound shows an unprecedented electrochemical sensing behavior and excellent coordinating properties toward selected divalent metal ions. Solid-state structures of the free ligand and its Ni(II)Cl2 complex are described.

Aromaticity and electronic properties of Heterosuperbenzene (Heterohexabenzocoronene)

A global electrophilicity parameter and the aromaticity of some heterocyclic polyaromatic hydrocarbons were evaluated on the basis of DFT calculations. The substitution of carbon atoms by nitrogen atoms dramatically changes the global electrophilicity of the molecules, with the fully substituted molecule being the most electrophilic with a reactivity very close to that of fullerene.

Fused Donor–Acceptor Ligands in RuII Chemistry: Synthesis, Electrochemistry and Spectroscopy of [Ru(bpy)3−n(TTF-dppz)n](PF6)2

Three ruthenium(II) polypyridine complexes of general formula [Ru(bpy)(3-n)(TTF-dppz)n](PF6)2 (n=1-3, bpy=2,2'-bipyridine), with one, two or three redox-active TTF-dppz (4',5'-bis(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2',3'-c]phenazine) ligands, were synthesised and fully characterised. Their electrochemical and photophysical properties are reported together with those of the reference compounds [Ru(bpy)3](PF6)2, [Ru(dppz)3](PF6)2 and [Ru(bpy)2(dppz)](PF6)2 and the free TTF-dppz ligand. All three complexes show intraligand charge-transfer (ILCT) fluorescence of the TTF-dppz ligand. Remarkably, the complex with n=1 exhibits luminescence from the Ru(2+)-->dppz metal-to-ligand charge-transfer ((3)MLCT) state, whereas for the other two complexes, a radiationless pathway via electron transfer from a second TTF-dppz ligand quenches the (3)MLCT luminescence. The TTF fragments as electron donors thus induce a ligand-to-ligand charge-separated (LLCS) state of the form TTF-dppz- -Ru(2+)-dppz-TTF(+). The lifetime of this LLCS state is approximately 2.3 micros, which is four orders of magnitude longer than that of 0.4 ns for the ILCT state, because recombination of charges on two different ligands is substantially slower.

Structure and Conformational Properties of three-layered Cyclophanes prepared by a one-pot Synthesis

The coupling reactions of substituted bis(bromomethyl)benzene components and tetra(mercaptomethyl)benzene derivatives have been carried out under heterogeneous dilute reaction conditions to afford the three-layered cyclophanes 3a–e as three kinds of conformational isomers in moderate yields.

An alternating pi-stacked bisdithiazolyl radical conductor

A general synthetic route to the resonance-stabilized pyrazine-bridged bisdithiazolyl framework, involving the reductive deprotection of 2,6-diaminopyrazine-bisthiocyanate and cyclization with thionyl chloride, has been developed. An N-methyl bisdithiazolyl radical, 4-methyl-4H-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyrazin-3-yl, has been prepared and characterized in solution by electron paramagnetic resonance spectroscopy and cyclic voltammetry. Its crystal structure has been determined at several temperatures. At 295 K, the structure belongs to the space group Cmca and consists of evenly spaced radicals pi-stacked in an alternating ABABAB fashion along the x-direction. At 123 K, the space group symmetry is lowered by loss of C-centering to Pccn, so that the radicals are no longer evenly spaced along the pi-stack. At 88 K, a further lowering of space group symmetry to P21/c is observed. Extended Hückel Theory band structure calculations indicate a progressive opening of a band gap at the Fermi level in the low-temperature structures. Magnetic susceptibility measurements over the range 4-300 K reveal essentially diamagnetic behavior below 120 K. Variable-temperature single-crystal conductivity (sigma) measurements indicate that the conductivity is activated, even at room temperature, with a room-temperature value sigma RT=0.001 S cm-1 and a thermal activation energy Eact=0.19 eV. Under an applied pressure of 5 GPa, sigma RT is increased by 3 orders of magnitude, but the conductivity remains activated, with Eact being lowered to 0.11 eV at 5.5 GPa.

Revisiting the Stability of Hexacenes

The Strating-Zwanenberg photodecarbonylation was used to prepare hexacene (1). Compound 1 was found to be extremely unstable in solution, undergoing dimerization and oxidation. However, when generated in a polymer matrix, 1 survived for more than 12 h under ambient conditions. Hexacenes substituted at the 6 and 15 positions with the phenyl, p-tert-butylphenyl, and mesityl groups were synthesized using the quinone reduction method, but these compounds were also shown to be unstable in solution.

Intramolecular Electron Transfer within the Substituted Tetrathiafulvalene−Quinone Dyads: Facilitated by Metal Ion and Photomodulation in the Presence of Spiropyran

Intramolecular electron transfer is observed for two new substituted tetrathiafulvalene (TTF)-quinone dyads 1 and 2 in the presence of metal ions. On the basis of the electrochemical studies of reference compound 5 and the comparative studies with dyad 3, it was proposed that the synergic coordination of the radical anion of quinone and the oligoethylene glycol chain with metal ions may be responsible for stabilizing the charge-separation state and thus facilitating the electron-transfer process. Most interestingly, the intramolecular electron-transfer processes within these two dyads can be modulated by UV-vis light irradiation in the presence of spiropyran, by taking advantage of its unique properties.

Amphiphilic perylenetretracarboxyl diimide dimer and its application in field effect transistor

A novel perylene diimide (PDI) derivative with typical amphiphilic character, 2, was designed and prepared. The spectroscopic studies on this compound in solution revealed the face-to-face dimeric configuration and effective pi-pi interaction between the two perylene rings. This amphiphilic PDI derivative was fabricated into highly ordered films by Langmuir-Blodgett (LB) technique and fabricated into an organic field effect transistor (OFET), which shows carrier mobility around 0.05 cm(2) V(-1) s(-1) and current modulation of 10(3). This OFET performance is much better than that of monomeric PDI 1 and can be attributed to the unique face-to-face structure of 2, which promotes the interactions between neighboring PDI ring in LB film as indicated by the pi-A isotherms and UV-vis absorption.

Bioinspired Nanodevice Based on the Folic Acid/Titanium Dioxide System

A new bioinspired nanomaterial has been obtained by chemisorption of folic acid onto nanocrystalline titanium dioxide. The organic chromophore is linked with the semiconductor surface via the glutamate chain and anchored with the carboxylate group. The geometry and electronic structure of the chromophore was studied in detail with DFT. Photoelectrochemical studies revealed photosensitization of the new material towards visible light. The photoelectrodes composed of the folic acid/titanium dioxide hybrid material generated photocurrent over a 300-600-nm window. Moreover, the direction of the photocurrent could be changed from anodic to cathodic and vice versa by application of the appropriate photoelectrode potential. Photoelectrochemical and spectroscopic studies allowed the elucidation of the mechanism of photocurrent switching. Photoelectrodes composed of folate-modified titanium dioxide may serve as a simple model of optoelectronic switches and may constitute the basis for molecular photoelectronic devices.