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The first cyclometalated dibenzothiophene S,S-dioxide derivative, namely [fac-2-(5,5-dioxidodibenzothiophen-3-yl)pyridine]iridium(III) (3) has been synthesised in high yield and characterised by X-ray crystallography, solution electrochemistry, absorption and emission spectroscopy. The Ir atom has a fac-octahedral coordination with three chelating ligands (A, B and C); each Ir–N bond is in trans position to an Ir–C bond. In cyclic voltammetry experiments in dichloromethane, complex 3 undergoes a reversible metal-centred Ir^III/Ir^IV oxidation at E_1/2^ox = +1.04 V vs. Ag/Ag⁺ reference electrode. Complex 3 exhibits bright green photoluminescence (λ_max = 525 nm in toluene) from mixed ³MLCT/³ππ* states with quantum yield (Φ_PL) of 0.26 in toluene solution. The phosphorescence emission decay follows first order kinetics, with a lifetime of 4.9 μs. A comparison of complex 3 with analogues 1 and 2, where the dibenzothiophene S,S-dioxide unit is replaced by 9,9-dihexylfluorene and N-hexylcarbazole respectively, establish that the substituent para to the Ir metal atom, i.e. CR₂ in 1, NR in 2 and SO₂ in 3, has a major influence on the redox and emission properties in this series. Organic light-emitting diodes (OLEDs) were fabricated by spin-coating techniques using a polyspirobifluorene copolymer (PSBF) as a host and complex 3 as dopant.In a single-layer blend configuration ITO/PEDOT:PSS/PSBF:3(8 wt.-%)/Ba/Al OLEDs showed pale blue/white light emission (CIE coordinates: x = 0.29, y = 0.31) arising from a combination of fluorescence from the host copolymer (λ_max = 450 nm) and phosphorescence from 3 (λ_max = 530 nm). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

We report the synthesis of 2-chloro-5-pyrimidylboronic acid (6) and 2-amino-5-pyrimidylboronic acid (8) by lithium–halogen exchange followed by reaction with triisopropylborate. Their reactivity with heteroaryl halides in Suzuki–Miyaura cross-coupling reactions has been evaluated. New highly functionalized 5-heteroarylpyrimidine derivatives 24–33 (heteroaryl = quinoline, pyridine, pyrimidine, pyrazine, thiophene, benzothiazole) have been obtained in synthetically useful yields. The X-ray structure of 6 reveals extensive intermolecular O–H···N hydrogen bonding in the crystal. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The synthesis of new conjugated aryleneethynylene derivatives of up to ca. 8 nm molecular length (compound 16) with terminal alkyne substituents and 9,9-dihexylfluorene units in the backbone is described. Key synthetic steps are Pd-mediated Sonogashira coupling methodology combined with regioselective removal from the terminal alkyne units of 2-hydroxy-2-propyl protecting groups in the presence of trimethylsilyl groups. The structural and electronic properties of 16 were obtained from DFT calculations: the intramolecular terminal C···C′ distance in its relaxed conformation was found to be 7.8 nm. The calculated distribution of HOMO and LUMO orbitals and the strong blue fluorescence observed for 16 (λ_max = 420, 443 nm in CHCl₃ solution) are consistent with a highly conjugated penta[(9,9-dihexyl-2,7-fluorenylene)ethynylene] structure. Molecule 16 possesses multifunctionality and is of interest for future molecular electronic device applications.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The synthesis and photophysical properties are described for a series of porphyrin, phthalocyanine and pyrazinoporphyrazine derivatives which bear four or eight peripheral fluorenyl substituents as antennae. Representative examples are 5,10,15,20-tetra(9,9-dihexyl-9H-fluoren-2-yl)porphyrin (2), 5,10,15,20-tetrakis[4-(9,9-dihexyl-9H-fluoren-2-yl)phenyl]porphyrin (3), 2,3,9,10,16,17,23,24-octakis(9,9-dihexyl-9H-fluoren-2-yl)-29H,31H-phthalocyanine (8) and 2,3,9,10,16,17,23,24-octakis[4-(9,9-dihexyl-9H-fluoren-2-yl)phenyl]-29H,31H-tetrapyrazinoporphyrazine (9). Palladium-mediated Suzuki–Miyaura cross-coupling reactions have been key steps for attaching the substituents. The compounds are deep-red emitters: λ_max(em)=659 (3), 737 (8) and 684 nm (9). Their absorption and emission spectra, their fluorescence lifetimes and quantum yields are correlated with the structures of the macrocycles and the substituents. The solution fluorescence quantum yields of porphyrin derivatives substituted with fluorene (2–4) and terphenyl substituents (7) (Φ_f=0.21–0.23) are approximately twice that of tetraphenylporphyrin. For phthalocyanine derivative 8, Φ_f was very high (0.88). Specific excitation of the fluorene units of 8 produced emission from both of them (λ_max=480 nm) and also from the phthalocyanine core (λ_max=750 nm), indicating a competitive rate of energy transfer and radiative decay of the fluorenes. Organic light-emitting devices (OLEDs) were made by spin-coating techniques by using a polyspirobifluorene (PSBF) copolymer as the host blended with 3 (5 wt. %) in the configuration ITO/PEDOT:PSS/PSBF copolymer:3/Ca/Al. Deep-red emission (λ_max=663 nm; CIE coordinates x=0.70, y=0.27) was observed with an external quantum efficiency of 2.5 % (photons/electron) (at 7.5 mA cm⁻²), a low turn-on voltage and high emission intensity (luminance) of 5500 cd m⁻² (at 250 mA/ m²).

Using ligands synthesized by Suzuki cross-coupling methodology, new phosphorescent homoleptic tris-cyclometalated complexes have been obtained, namely fac-[Ir(Cz-2-Fl_nPy)₃] (1 d–f) and fac-[Ir(Cz-3-Fl_nPy)₃] (2 d–f), which are solution-processible triplet emitters (Cz denotes N-hexylcarbazole, n is the number of 9,9′-dihexylfluorene (Fl) units (n=0,1,2) and Py is pyridine). In all cases, Py and Fl are substituted at the 2- and 2,7-positions, respectively, and Cz moieties are substituted by either Py or Fl at the 2- or 3-positions, in series 1 and 2, respectively. The oxidation potential of 1 d studied by cyclic voltammetry (=0.14 V, versus Ag/AgNO₃, CH₂Cl₂) is less positive (i.e. raised HOMO level) compared to that of the isomer 2 d (=0.30 V), where the Cz-nitrogen is meta to the Ir center. Ligand-centered oxidations occur at more positive potentials, leading to 7+ oxidation states with good chemical reversibility and electrochemical quasi-reversibility, for example, for 2 f =0.45 (1e), 0.95 (3e), 1.24 V (3e). Striking differences are seen in the solution-state photophysical data between complexes [Ir(Cz-2-Py)₃] (1 d) and [Ir(Cz-3-Py)₃] (2 d), in which the Cz moiety is bonded directly to the metal center: for the latter there is an 85 nm blue-shift in emission, a decrease in the luminescence lifetime and an increase in the PLQY value. Organic light emitting devices were made by spin-coating using polyspirobifluorene:bis(triphenyl)diamine (PSBF:TAD) copolymer as host and the complexes 1 d or 2 d as dopants. Turn-on voltages are low (3–4 V). With 1 d orange light is emitted at λ_max=590 nm with an EQE of 1.3 % (at 7.5 mA cm⁻²) and an emission intensity (luminance) of 4354 cd m⁻² (at 267 mA m⁻²). The green emission from 2 d devices (λ_max=500 nm) is due to the reduced electron-donating ability of the carbazole unit in 2 d. Recording the EL spectra of the 1 d device at 6 V (current density, 100 mA cm⁻²) established that the time to half brightness was about 9 h under continuous operation with no change in the spectral profile, confirming the high chemical stability of the complex.

Derivatives of 9-(1,3-dithiol-2-ylidene)fluorene (9) and 9-(1,3-dithiol-2-ylidene)thioxanthene (10) have been synthesised using Horner–Wadsworth–Emmons reactions of (1,3-dithiol-2-yl)phosphonate reagents with fluorenone and thioxanthen-9-one. X-ray crystallography, solution electrochemistry, optical spectroscopy, spectroelectrochemistry and simultaneous electrochemistry and electron paramagnetic resonance (SEEPR), combined with theoretical calculations performed at the B3P86/6-31G** level, elucidate the interplay of the electronic and structural properties in these molecules. These compounds are strong two-electron donors, and the oxidation potentials depend on the electronic structure of the oxidised state. Two, single-electron oxidations (<) were observed for 9-(1,3-dithiol-2-ylidene)fluorene systems (9). In contrast, derivatives of 9-(1,3-dithiol-2-ylidene)thioxanthene (10) display the unusual phenomenon of inverted potentials (>) resulting in a single, two-electron oxidation process. The latter is due to the aromatic structure of the thioxanthenium cation (formed on the loss of a second electron), which stabilises the dication state (10²⁺) compared with the radical cation. This contrasts with the nonaromatic structure of the fluorenium cation of system 9. The two-electron oxidation wave in the thioxanthene derivatives is split into two separate one-electron waves in the corresponding sulfoxide and sulfone derivatives 27–29 owing to destabilisation of the dication state.

Derivatives of 9-[2-(1,3-dithiol-2-ylidene)ethylidene]thioxanthene have been synthesized using Horner–Wadsworth–Emmons reactions of (1,3-dithiol-2-yl)phosphonate reagents with thioxanthen-9-ylidene-acetaldehyde (5). Further reactions lead to the sterically crowded cross-conjugated “vinylogous tetrathiafulvalene” derivative 9-[2,3-bis-(4,5-dimethyl-1,3-dithiol-2-ylidene)-propylidene]thioxanthene (10). X-ray crystallography, solution electrochemistry, optical spectroscopy, spectroelectrochemistry, and simultaneous electrochemistry and electron paramagnetic resonance spectroscopy, combined with theoretical calculations performed at the B3LYP/6-31G(d) level, elucidate the interplay of the electronic and structural properties in these molecules. For compound 10, multistage redox behavior is observed: the overall electrochemical process can be represented by 1010^.+10²⁺10⁴⁺ with good reversibility for the 1010^.+10²⁺ transformations. At the tetracation stage there is the maximum gain in aromaticity at the dithiolium and thioxanthenium rings. Theory predicts that for 10, 10^.+, and 10²⁺ the trans isomers are more stable than the cis isomers (by ca. 2–18 kJ mol⁻¹), whereas for 10⁴⁺ the cis isomer becomes more stable than the trans isomer (by ca. 25 kJ mol⁻¹) [trans and cis refer to the arrangement of the two dithiole moieties with respect to the central C(R)C(H) fragment]. These data explain the detection in cyclic voltammograms of both trans and cis isomers of 10 and 10^.+ during the reduction of 10⁴⁺ at fast scan rates (>100 mV s⁻¹) when the cis–trans isomerization is not completed within the timescale of the experiment. The X-ray structure of the charge-transfer complex (CTC) of 10 with 2,4,5,7-tetranitrofluorene-9-dicyanomethylenefluorene (DTeF) [stoichiometry: 10^.+⋅(DTeF)₂^.−⋅2 PhCl] reveals a twisted conformation of 10^.+ (driven by the bulky thioxanthene moiety) and provides a very rare example of segregated stacking of a fluorene acceptor in a CTC.

The first π-extended tetrathiafulvalene (exTTF) dimer in which the two exTTF units are covalently connected by 1,3-dithiole rings has been obtained in a multistep synthetic procedure involving the Ullmann cross-coupling reaction by using copper(I) thiophene-2-carboxylate (CuTC). The electronic spectrum reveals a significant electronic interaction between the exTTF units. The electrochemical study carried out by cyclic voltammetry in solution and in thin-layer conditions, and the electrochemical simulation and spectroelectrochemical (SEC) measurements confirm the electronic communication and show that the oxidation of dimer 14 occurs as two consecutive 2 e⁻ processes D⁰–D⁰D²⁺–D⁰D²⁺–D²⁺. Theoretical calculations, performed at the B3P86/6-31G* level, confirm the experimental findings and predict that 14²⁺ exists as a delocalized D^.+–D^.+ species in the gas phase and as a localized D²⁺–D⁰ species in solution (CH₃CN or CH₂Cl₂). Oxidation of 14²⁺ forms the tetracation 14⁴⁺ which is constituted by two aromatic anthracene units bearing four aromatic, almost orthogonal 1,3-dithiolium cations.

Se ha sintetizado el primer dímero derivado de TTF π-extendido (exTTF) en el que las dos unidades de exTTF se encuentran unidas mediante un enlace covalente a través de los anillos de 1,3-ditiol. El procedimiento sintético consta de varios pasos e implica una reacción de acoplamiento cruzado de tipo Ullmann empleando 2-tiofencarboxilato de cobre(I) (CuTC). Los espectros electrónicos revelan una interacción electrónica significativa entre las unidades de exTTF. Se ha realizado el estudio electroquímico mediante voltamperometría cíclica en disolución y en condiciones de capa fina, así como la simulación electroquímica y las medidas espectroelectroquímicas. Los datos obtenidos confirman la comunicación electrónica entre ambas unidades, y muestran que la oxidación del dímero 14 ocurre como dos procesos que involucran dos electrones D⁰–D⁰ D²⁺–D⁰D²⁺–D²⁺. Cálculos teóricos B3P86/6-31G* confirman los hechos experimentales y predicen que 14²⁺existe como una especie deslocalizada D^.+–D^.+en fase gas y como una especie localizada D²⁺–sD⁰en disolución (CH₃CN o CH₂Cl₂). La oxidación de 14²⁺forma la especie tetracatiónica 14⁴⁺, constituida por dos unidades de antraceno aromáticas con cuatro cationes 1,3-ditiolio aromáticos en disposición prácticamente ortogonal a las unidades de antraceno.

Fast lückenlos: Kürzlich wurden kleine organische Moleküle mit HOMO-LUMO-Energielücken unter 0.5 eV synthetisiert, indem ein konjugiertes System erweitert und eine Donor- kovalent mit einer Acceptor-Einheit verknüpft wurde (siehe Bild). Diese Verbindungen zeigen thermisch angeregten Elektronentransfer (in Lösung) und metallische Leitfähigkeit (als Festkörper).

Mind the gap: HOMO–LUMO gaps less than 0.5 eV have been recently engineered in small organic molecules through extended conjugation and donor–acceptor covalent coupling approaches (see picture). Such compounds reveal a number of unusual electronic properties such as thermo-excited intramolecular electron transfer (in solution) and metallic conductivity (in the solid state).

The syntheses of a series of novel tetrathiafulvalene derivatives substituted with four side-chains terminating in hydrophilic amidoalcohol (arborol) groups are reported, for example the tetrathiafulvalene derivative 15. Some of these arborol derivatives formed thin films by spin-coating from methanol solution onto solid supports such as glass, ITO-coated glass and gold. The quality of the films improved markedly with an increasing number of alcohol substituents at the periphery of the molecule. Optical absorption spectroscopy showed that oxidative doping of these films could be achieved chemically and electrochemically. The observation of low-energy bands in the UV/Vis absorption spectra (λ_max = 820 nm tailing to ca. 1300 nm) and the moderate levels of conductivity (σ_rt ≈ 10⁻⁴ S cm⁻¹) in these doped films strongly suggest that a significant degree of supramolecular order is present, with π-π stacking of the TTF cores. To the best of our knowledge this is the first time that semiconducting behaviour has been achieved in molecular arborol systems that possess an electroactive core unit. This work offers the prospect of using spin-coated films of TTF-arborols as semiconducting charge-transport layers in optoelectronic devices. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Fast 30 Jahre nach der theoretischen Vorhersage wurde eine stabile und gut charakterisierte Verbindung synthetisiert, in der Tetrathiafulvalen- und Tetracyanchinodimethan-Einheiten kovalent verknüpft sind (1, siehe Schema). Die extrem kleine HOMO-LUMO-Lücke von 1 (0.17 eV) ermöglicht erstmals, einen thermisch angeregten Elektronentransfer in einer Donor-Acceptor-Diade zu verfolgen. Langmuir-Blodgett-Filme von 1 sind nichtleitend.

Nearly 30 years after theoretical prediction, a stable and well-characterized compound that covalently links tetrathiafulvalene and tetracyanoquinodimethane fragments has been synthesized (1, see scheme). The extremely low HOMO–LUMO gap of 1 (0.17 eV) allows the first observation of thermoexcited electron transfer in a donor–acceptor diad. Langmuir–Blodgett films of 1 are not electrically conducting.

Novel R₃TTF–σ–A compounds 14, 16 and 19 (R₃TTF=trialkyltetrathiafulvalene, σ=saturated spacer, A=polynitrofluoren-9-dicyanomethylene acceptor) incorporating very strong donor and acceptor moieties have been synthesized by condensation of the corresponding R₃TTF–σ-fluoren-9-one diads with malononitrile. Reversible five-step amphoteric redox behavior has been observed with an extremely low HOMO–LUMO gap (≈0.3 eV). For compound 14 a strong EPR signal is observed in the solid state, ascribed to intermolecular complexation: a less intense signal is seen in solution, corresponding to ca. 2 % of the molecules existing in a radical form at room temperature. Intramolecular charge transfer in diads 14 and 16 is manifested in strong absorption bands in the near-IR region of their electronic spectra. Spectroelectrochemical data reveal marked electrochromic behavior in the visible and near-IR region of both compounds. The first X-ray crystal structure of a fluorene radical-anion salt is reported, namely the copper salt of 2,4,5,7-tetranitro-9-dicyanomethylenefluorene (1:1 stoichiometry).

The syntheses are reported of the title polymeric alkoxyPBD derivative 5 and the dipyridyl analogue 12 using Suzuki coupling reactions of 1,4-dialkoxybenzene-2,5-diboronic acid with 2,5-bis(4-bromophenyl)-1,3,5-oxadiazole, and its dipyridyl analogue, respectively. Thermal gravimetric analysis shows that polymers 5 and 12 are stable up to 370 °C and 334 °C, respectively. Films of polymer 5 spun from chloroform solution show an absorption at λ_max = 367 nm, and a weaker band at 312 nm, and strong blue photoluminescence at λ_max = 444 nm. The photoluminescence quantum yield (PLQY) was found to be 27 ± 3 %. For polymer 12, the absorption spectra reveal bands of equal intensity at λ_max = 374 and 312 nm, with PL at λ_max = 475 nm. Device studies using polymer 12 were hampered by its instability under illumination and/or electrical excitation. Polymer 5 is stable under these conditions and acts as an efficient electron-transporting/hole-blocking layer. For devices of configuration ITO/PEDOT/MEH-PPV/polymer 5/Al an external quantum efficiency of 0.26 % and brightness of 800 cd/m² was readily achieved: orange emission was observed, identical to the MEH-PPV electroluminescence.

The synthesis of new derivatives of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene is reported. ¹H NMR studies on compound 4 are consistent with two conformers of the saddle-shaped molecule which interconvert by a boat−boat flipping of the central ring at high temperature. The hydroxymethyl substituent of 4 is readily esterified to yield the benzoyl and 2-naphthaloyl ester derivatives 5a and 5b, respectively, and the dimer and the trimer structures 6 and 7, respectively, by reaction with benzoyl chloride, 2-naphthaloyl chloride, 1,4-benzenedicarbonyl chloride and 1,3,5-benzenetricarbonyl chloride. Deprotonation of 9-(4,5-dimethyl-1,3-dithiol-2-ylidene)-10-(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (8) using lithium bis(trimethylsilyl) amide (LHMDS) followed by in situ quenching of the lithiated intermediate with methylchloroformate gave the diester derivative 10 (96% yield), whereas using LDA gave the monoester 9 as the major product (32% yield). Diester 10 was reduced to the di(hydroxymethyl) derivative 11. The solution electrochemistry of these new compounds has been studied by cyclic voltammetry: the ester substituents of 9 and 10 are conjugated to the π-framework, causing a positive shift of E^ox. The X-ray crystal structures of compounds 4, 5a, 5b·CH₂Cl₂, 10 and 11 are reported. The molecules adopt a saddle-like conformation; the bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene system is U-shaped due to the boat conformation of the central quinodimethane ring and folding of both the 1,3-dithiole rings. Each compound displays a dimeric packing motif which is most pronounced for 5a and 5b·CH₂Cl₂. Both these structures contain “oligomeric” acceptor···donor stacks: 4-layer ADDA in 5a and 6-layer DADDAD in 5b (D = dithiole; A = benzoyl and naphthoyl, respectively) but neither contains infinite stacks.

The synthesis of 4,4′,5-trimethyl-5′-(4-pyridyl)tetrathiafulvalene (3), has been accomplished by reaction of the stannylated precursor 2 with 4-bromopyridine. Alkylation of 3 affords the N-methylpyridinium salt 4 which displays intramolecular charge transfer properties in solution as deduced from UV/Vis spectra, cyclic voltammetric data, and semiempirical quantum mechanical calculations. The X-ray crystal structures of neutral molecule 3, the salt (4⁺)I⁻ and the charge transfer salt (4⁺)₂(TCNQ₃)^2−· are reported. The positive charge in (4⁺)I⁻ and (4⁺)₂(TCNQ₃)^2−· is predominantly confined on the pyridinium ring. The structure of (4⁺)₂(TCNQ₃)^2−· comprises segregated stacks of cations and TCNQ radical anions, with the anions overlapping in the usual ring-over-bond fashion. This salt is a semiconductor (σ_rt = 10⁻² S cm⁻¹).

Electron donor−π−acceptor chromophores 5, 9, 11, 18−20, 21, 22, 27, 28a, 28c, 31, 32, 34−36, 38a−c, 41a, 41c, and 42 have been synthesised. The donor units are 1,3-dithiole and ferrocene; conjugated ethylenic, phenyl, phenylenevinylene, thienyl, bithienyl, terthienyl, or thienylenevinylene linkers act as a central π-electron relay unit, and dicyanomethylene and polynitrofluorene groups as the acceptor unit. The electronic absorption spectra display a broad low-energy intramolecular charge transfer band in the visible region (500−700 nm) the energy (hν_ICT ≈ 1.7−2.5 eV) and intensity (ϵ ≈ 5000−50000 M⁻¹cm⁻¹) of which depend substantially on the nature of both D and A moieties and on the structure of the linker unit. Nonlinear optical properties have been evaluated using the EFISH technique: the highest μβ(0) values are observed for 38b [(900±300)×10⁻⁴⁸ esu] and 42 [(1800±300)×10⁻⁴⁸ esu] establishing that polynitrofluorene is a promising acceptor terminal moiety in this context. The molecular and electronic structures of 49 and 50 have been calculated by the RHF/6-31G(d)//RHF/6-31G(d) ab initio method. The HOMO is located mostly in the 1,3-dithiolium ring, and the LUMO mostly at the dicyanomethylene fragment (and the phenyl ring of 50) although the electronic population at C2 of the 1,3-dithiolium rings is also considerable. The X-ray crystal structures of 9, 18 and 27 are reported. In all three structures the conjugated π-systems are effectively planar with extensive π-electron delocalisation between the donor and acceptor moieties. The planar conformation of 18 gives rise to a close intramolecular S···S contact of 3.095(3) Å between the dithiole and thiophene units.

We have synthesised new conjugated donor-π-acceptor (D-π-A) chromophores 7, 9, and 12−15 in which monosubstituted tetrathiafulvalene (TTF) and trimethyl-TTF units are the donor moieties, connected by ethylenic bridges to electron-deficient benzene derivatives as the acceptor moieties. These compounds display a broad intramolecular charge transfer (ICT) band in their solution UV/Vis spectra at λ_max = ca. 500 nm. The second order nonlinear optical (NLO) properties of these derivatives have been studied using the EFISH technique and calculated by semiempirical and ab initio theoretical methods. The effect of methyl substituents in the TTF moiety and the nature of the conjugated bridge are discussed. Analysis of the bond lengths obtained by an X-ray diffraction study of compound 9 reveal ICT in the solid state.

The convenient one pot synthesis of pyrazolo[3,4-c]isoquinolines 1 from 5-aminopyrazoles 2 and paraformaldehyde in formic or trifluoroacetic acids is described.

Flash photolysis of bis[4,5-di(methylsulfanyl)1,3-dithiol-2-ylidene]-9,10-dihydroanthracene (1) in chloroform leads to formation of the transient radical cation species 1^.+ which has a diagnostic broad absorption band at λ_max≈650 nm. This band decays to half its original intensity over a period of about 80 μs. Species 1^.+ has also been characterised by resonance Raman spectroscopy. In degassed solution 1^.+ disproportionates to give the dication 1²⁺, whereas in aerated solutions the photodegradation product is the 10-[4,5-di(methylsulfanyl)1,3-dithiol-2-ylidene]anthracene-9(10 H)one (2). The dication 1²⁺ has been characterised by a spectroelectrochemical study [λ_max (CH₂Cl₂)=377, 392, 419, 479 nm] and by an X-ray crystal structure of the salt 1²⁺(ClO₄⁻)₂, which was obtained by electrocrystallisation. The planar anthracene and 1,3-dithiolium rings in the dication form a dihedral angle of 77.2°; this conformation is strikingly different from the saddle-shaped structure of neutral 1 reported previously.

The synthesis of 2,6-dialkoxy-9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene derivatives 15 and 16 is described. Solution electrochemistry shows that 15 and 16 display three redox waves, representing the sequential formation of the dication, radical trication and tetracation species in an E_qE_qE_q process. The X-ray crystal structures of neutral compounds 15 and 16 and the charge transfer complex (15)²⁺(TCNQ^−•)₂· 2MeCN are reported. The neutral molecules adopt a saddle-like conformation; the bis(1,3-dithiole)benzoquinone system is U-shaped through an ‘accumulating bend’ comprising the boat conformation of the central (quinonoid) ring and folding of both 1,3-dithiole rings. In the complex (15)²⁺(TCNQ^−•)₂· 2MeCN the anthracene system is planar and aromatic; the dithiolium cations form a dihedral angle of 78° with the anthracene plane. The TCNQ anion radicals form a stack of dimers with interplanar separations of 3.15 Å within a dimer and 3.50 Å between the dimers. The structure contains unusually short intermolecular S···N contacts [2.865(3) Å].

The synthesis of new derivatives of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene has been achieved by two different routes. Deprotonation of 8 using LDA in THF at −78 °C, followed by in situ quenching of the lithiated intermediate 9 with N,N-dimethylformamide, N-methyl isothiocyanate and methyl chloroformate gave aldehyde, thioamide and methyl ester derivatives 10-12, respectively. Sulfur insertion into the lithiated species 9 followed by reaction of the transient thiolate anion with benzoyl chloride gave the thioester derivative 13 which served as a convenient shelf-stable precursor of other mono-functionalised derivatives of 8. Debenzoylation of 13 and trapping of the transient thiolate anion with iodomethane and 6-bromohexan-1-ol yielded 14 and 15, respectively. Reaction of cation salt 17 with the anion of anthrone 18 gave compound 20, the thiolate anion of which reacted with 6-bromohexan-1-ol to afford the alcohol derivative 21. Subsequent reactions gave alcohol derivative 25 of the title system. The unexpected product 29 was obtained from reaction of 28 with triethyl phosphite. The X-ray crystal structures of compounds 12, 14, 28, and 29 are reported. The molecules adopt a saddle-like conformation; the bis(1,3-dithiole)benzoquinone system is U-shaped through an ‘accumulating bend’ comprising the boat conformation of the central (quinonoid) ring, folding of both 1,3-dithiole rings along S···S vectors, and out-of-plane tilting of the exocyclic CC bonds, all in the same (inward) direction.

New [3]- and [4]-dendralenes bearing electron-donor 1,3-dithiole and ferrocene substituents have been synthesised. Compounds 8, 15 and 17 have been characterised by single-crystal X-ray diffraction. Two of the dithiole rings of 8 are conjugated (dihedral angle 9°), while the third dithiole ring is almost orthogonal to this plane, and hence its π-electron system is isolated. For the dendralene precursor molecule 15, the substituted cyclopentadienyl ring, two C=C bonds and fused dithiole and dithiine rings comprise an extended π-conjugated system. In molecule 17 the potential conjugation path C(6)C(3)C(4)C(5)-C₅H₅ is distorted by an 8° twist around the C(3)−C(4) bond and a 7° twist around the C(5)−C(21) bond, and the delocalisation along the chain is insignificant. Solution electrochemical data demonstrate that the dendralenes are strong π-electron donors, which give rise to dication, radical trication or tetracation species. Spectroelectrochemical studies on compounds 7 and 10 suggest that the radical species are situated within the linear 1,2-ethylenediylidene moieties and that a conformational change may occur at the dication redox stage. UV/Vis spectroscopic data are consistent with poor cross-conjugation in these systems.

Two conceptually different routes to transient 1,2-diselones are reported: 1) via ring fragmentation of the 1,4,2-diselenazine system 6, and 2) by the tributylphosphane-induced depolymerisation of the shelf-stable organoselenium polymer 15. Evidence for the intermediacy of 1,2-diselone species 7 and 16 is provided in both cases by in situ trapping with dimethyl acetylenedicarboxylate (DMAD) to yield 1,4-diselenin derivatives. The route via 15 is especially expedient and trapping of 16 is efficient. Subsequent reactions of adduct 17 afford [1,2-ethanediylbis(diphenylphosphane)][5,6-bis(methoxycarbonyl)-1,4-diselenin-2,3-dithiolato]nickel(IV) (20). Theoretical calculations at Hartree-Fock (HF) and Møller-Plesset electron-correlated levels (MP2) suggest that the cyclic 1,2-diselenete structure 7 c is significantly more stable than the acyclic 1,2-diselone structure 7 a. For the bicyclic system 16, the difference in energy between the cyclic and acyclic structures is considerably reduced due to the conformational rigidity imposed by the fused 1,3-dithiole ring. In contrast, the acyclic structures of the 1,2-dithione analogues 13 a and 25 a are computed to be more stable than their corresponding cyclic 1,2-dithiete structures 13 c and 25 c.

Photolysis of 1,2-dithiole-3-thione derivative 1 results in the formation of the 1,2,4-trithiolane derivative 2, the structure of which was established by single crystal X-ray analysis. Interesting bond delocalisation is observed in the molecular structure. A mechanism involving dimerisation of 1 followed by loss of sulfur is proposed.

The pyrazinoporphyrazine system 13 (metal-free, zinc and copper derivatives) has been synthesised by tetramerisation of 2,3-dicyanopyrazine monomer unit 10. The structure of 13a-c has been established by ¹H NMR spectroscopy, UV/Vis spectrophotometry, MALDI-TOF mass spectrometry, cyclic voltammetry and differential pulse voltammetry. The electrochemical redox behaviour of 13a-c is strongly solvent dependent. The expected two-stage oxidation of the tetrathiafulvalene (TTF) units of 13a-c was observed in a range of solvents; in addition, oxidation and reduction of the pyrazinoporphyrazine core of the metal-free derivative 13a was detected in benzonitrile. On excitation of 13 in the Q-band region no fluorescence was observed, which is presumably the consequence of intramolecular charge transfer between the TTF moieties and the excited state of the central porphyrazine. Molecular modelling studies on 13a and 13c are reported. During the course of this work, the novel TTF macrocycles 11 and 20 were synthesised; their X-ray crystal structures reveal severely bent TTF units, the conformations of which are discussed in detail. The X-ray crystal structures of the bis(1,3-dithiole) systems 15 and 18 have also been determined.