Synthesis, X-ray Structure, and Electrochemical Oxidative Coupling Reactions of 1,5- and 2,6-Bis(1,4-dithiafulven-6-yl)naphthalenes

Fluorescent and Magnetic Radical Dendrimers as Potential Bimodal Imaging Probes

Dual or multimodal imaging probes have emerged as powerful tools that improve detection sensitivity and accuracy in disease diagnosis by imaging techniques. Two imaging techniques that are complementary and do not use ionizing radiation are magnetic resonance imaging (MRI) and optical fluorescence imaging (OFI). Herein, we prepared metal-free organic species based on dendrimers with magnetic and fluorescent properties as proof-of-concept of bimodal probes for potential MRI and OFI applications. We used oligo(styryl)benzene (OSB) dendrimers core that are fluorescent on their own, and TEMPO organic radicals anchored on their surfaces, as the magnetic component. In this way, we synthesized six radical dendrimers and characterized them by FT-IR, ¹H NMR, UV-Vis, MALDI-TOF, SEC, EPR, fluorimetry, and in vitro MRI. Importantly, it was demonstrated that the new dendrimers present two properties: on one hand, they are paramagnetic and show the ability to generate contrast by MRI in vitro, and, on the other hand, they also show fluoresce emission. This is a remarkable result since it is one of the very few cases of macromolecules with bimodal magnetic and fluorescent properties using organic radicals as the magnetic probe.

Studies of a bola-type bis(dithiafulvene) molecular system: synthesis, crystal structure, and electrochemical properties

A bis(dithiafulvene) compound that contains a 1,3-diphenoxypropane central unit was designed and investigated in this work.

Donor-Donor'-Acceptor Triads Based on [3.3]Paracyclophane with a 1,4-Dithiafulvene Donor and a Cyanomethylene Acceptor: Synthesis, Structure, and Electrochemical and Photophysical Properties

Donor-donor'-acceptor triads (1, 2), based on [3.3]paracyclophane ([3.3]PCP) as a bridge, with electron-donating properties (D') using 1,4-dithiafulvene (DTF; TTF half unit) as a donor and dicyanomethylene (DCM; TCNE half unit) or an ethoxycarbonyl-cyanomethylene (ECM) as an acceptor were designed and synthesized. The pulse radiolysis study of 1 a in 1,2-dichloroethane allowed the clear assignment of the absorption bands of the DTF radical cation (1 a^.+ ), whereas the absorption bands due to the DCM radical anion could not be observed by γ-ray radiolysis in 2-methyltetrahydrofuran rigid glass at 77 K. Electrochemical oxidation of 1 a first generates the DTF radical cation (1 a^.+ ), the absorption bands of which are in agreement with those observed by a pulse radiolysis study, followed by dication (1 a²⁺ ). The ESR spectrum of 1 a^.+ showed a symmetrical signal with fine structure and an ESR simulation predicted that the spin of 1 a^.+ is delocalized over S and C atoms of the DTF moiety and the central C atom of the trimethylene bridge bearing the DTF moiety. Pulse radiolysis, ESR, and electrochemical studies indicate that the DTF radical cation of 1 a^.+ is more stable than that of 6^.+ , and the latter shows a strong tendency to dimerize. This result indicates that the [3.3]PCP moiety as a bridge can stabilize the DTF radical cation more than the 1,3-diphenylpropane moiety because of kinetic stability due to its rigid structure and the weak electronic interaction of DTF and DCM moieties through [3.3]PCP.

Tuning Spin-Spin Interactions in Radical Dendrimers

Two generations of polyphosphorhydrazone (PPH) dendrimers were synthesized and fully functionalized with TEMPO radicals via acrylamido or imino group linkers to evaluate the impact of the linker substitution on the radical-radical interactions. A drastic change in the way that the radicals interacted among them was observed by EPR and CV studies: while radicals in Gn -imino-TEMPO dendrimers presented a strong spin-spin interaction, in the Gn -acrylamido-TEMPO ones they acted mainly as independent radicals. This shows that these interactions could be tuned by the solely substitution of the radical linker, opening the perspective of controlling and modulating the extension of these interactions depending on each application. The chemical properties of the linker strongly influence the spin-spin exchange between pendant radicals.

Synthesis, X-Ray Structure, Magnetic Properties, and a Study of Intra/Intermolecular Radical-Radical Interactions of a Triradical TEMPO Compound

A novel triradical compound with a P=S core and three branches functionalized with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals is synthesized and characterized by IR, (1) H NMR, (31) P NMR, and EPR spectroscopy and MALDI-TOF mass spectrometry, and its chemical structure is confirmed by X-ray diffraction analysis. The triradical shows neither spin exchange interactions between its radical units nor detectable dipolar interactions. This is consistent with the separation between the radical units found in its X-ray diffraction structure, and discounts the existence of intramolecular interactions. This conclusion is confirmed by an EPR concentration study. The concentration at which intermolecular interactions start to appear is determined (5×10(-3)  m) and this concentration should be taken into account as a higher concentration limit when studies on intramolecular radical-radical interactions in polyradicals with similar structure are required. SQUID magnetometry analysis of the compound shows antiferromagnetic interactions between the spin carriers of different molecules; that is, antiferromagnetic intermolecular interactions.

Tetrathiafulvalene Vinylogue-Fluorene Co-oligomers: Synthesis, Properties, and Supramoleclar Interactions with Carbon Nanotubes

A series of bis(dithiafulvenyl)-end-capped fluorene derivatives was prepared and subjected to a one-pot iodine-promoted oxidative polymerization to yield π-conjugated co-oligomers containing tetrathiafulvalene vinylogue and fluorene repeat units. The resulting π-oligomers were characterized to take either acyclic or cyclic molecular structures, depending on the π-conjugation length of the monomer used for the polymerization. Electronic and electrochemical redox properties were examined by UV-vis spectroscopic and cyclic voltammetric analyses, while the supramolecular interactions of the π-oligomers with single-walled carbon nanotubes were investigated by UV-vis-NIR and Raman spectroscopy.

Crown Ether Vinylogous Tetrathiafulvalene Receptors: Complexation Interference on the Molecular Movements Triggered by Electron Transfer

The synthesis of a series of crown ether substituted vinylogous tetrathiafulvalenes (TTFVs) has been carried out through oxidative coupling of bisdithiafulvenes. These new receptors have been fully characterized using electrochemical, spectroelectrochemical, and molecular modeling experiments. These studies show that, upon oxidation, either a clip movement (TTFVs 3a,b) or a stretch movement (TTFV 3c) occurs, depending on the length of the crown ether chains. Preliminary electrochemical studies, undertaken on TTFV 3c in dichloromethane, show a little shift of the first standard oxidation potential toward more positive values upon addition of the Pb(2+) ion, but a considerable variation of the electron-transfer kinetics. This result introduces an interesting concept for the preparation of sensors not based on thermodynamic variations but on kinetic modifications of the electron transfer.

Synthesis and radical coupling of pyridine-bridged π-extended tetrathiafulvalene (TTF)-type donors and push–pull analogues

A new family of pi-extended TTF analogues (3a-c) and D-pi-A chromophores (5a-c), in which the electroactive units (1,3-dithiole rings and 2,2-dicyanovinyl groups) are connected through a pyridine bridge with a meta substitution pattern, is reported. The redox behavior of these compounds has been investigated by cyclic voltammetry and theoretical calculations performed at the B3P86/6-31G** level. Unlike many pi-extended TTF derivatives, the 1,3-dithiole rings in compounds 3a-c do not behave independently and two oxidation processes are observed with an anodic separation ranging from 50 to 150 mV. Calculations show that electrons are equally extracted from both dithiole rings. A biradical structure is predicted for the dication state due to the near-degeneracy of the HOMO and HOMO - 1 orbitals. The presence of both donor (D) and acceptor (A) fragments in conjugates results in irreversible oxidation and reduction processes associated with the 1,3-dithiole ring and with the 2,2-dicyanovinyl moiety, respectively. An electrochemical-chemical-electrochemical (ECE) process takes place for all the compounds reported. The chemical process implies the dimerization of the radical cation for compounds 5 and the oligomerization of the biradical dication for compounds 3. The ECE process therefore generates new neutral dimeric (5) or oligomeric (3) species that incorporate the TTF vinylogue core.

Remarkable Interplay of Redox States and Conformational Changes in a Sterically Crowded, Cross-Conjugated Tetrathiafulvalene Vinylog

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 10-->10(.+)-->10(2+)-->10(4+) with good reversibility for the 10-->10(.+)-->10(2+) 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 10(2+) the trans isomers are more stable than the cis isomers (by ca. 2-18 kJ mol(-1)), whereas for 10(4+) 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 10(4+) 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.

Salts of extended tetrathiafulvalene analogues: relationships between molecular structure, electrochemical properties and solid state organisation

By considering the structures of many salts derived from extended TTF analogues, relationships between the molecular architecture of the donors with their electrochemical properties and their stacking mode in the salts are presented in this critical review. Three categories of donors corresponding to their extension modes have been considered. Firstly, for linearly extended TTFs the crucial role of the spacer in modifying the electrochemical properties and the packing mode in the salts is presented. Secondly, bidimentional extension of the donors obtained by linking several dithiafulvenyl units on a TTF core led to materials with increased dimensionality. Finally, the last class corresponds to the fusion, directly or across a benzene ring, of TTF frameworks. The former are the base of many salts with metallic behaviour. (148 references.).

Cyclic vinylogous TTF: a potential molecular clip triggered by electron transfer

Cyclic vinylogous tetrathiafulvalenes (TTFs) where the two dithiole rings are linked through the outer sulfur atoms with an alkyl chain of various lengths were synthesized by oxidative intramolecular coupling of bis(dithiafulvenes). Upon oxidation, these systems exhibit large molecular movements associated with electron transfers. Their electrochemical behaviors were investigated together with the X-ray crystallographic structures of several derivatives and compared with molecular geometry calculations. Dependent on the length of the alkyl chain, either a stretch or a clip movement can be observed. These conformational modifications were found to be fast and fully reversible and required only low oxidation potentials. The movements seem to be almost insensitive to the nature of the substituents or its steric hindrance on the central conjugated spacer.

New Concepts in Tetrathiafulvalene Chemistry

Tetrathiafulvalene (TTF) and its derivatives were originally prepared as strong electron-donor molecules for the development of electrically conducting materials. This Review emphasizes how TTF and its derivatives offer new and in some cases little-exploited possibilities at the molecular to the supramolecular levels, as well as in macromolecular aspects. TTF is a well-established molecule whose interest goes beyond the field of materials chemistry to be considered an important building block in supramolecular chemistry, crystal engineering, and in systems able to operate as machines. At the molecular level, TTF is a readily available molecule which displays a strong electron-donor ability. However, its use as a catalyst for radical-polar crossover reactions, thus mimicking samarium iodide chemistry, has only recently been addressed. Important goals have been achieved in the use of TTF at the macromolecular level where TTF-containing oligomers, polymers, and dendrimers have allowed the preparation of new materials that integrate the unique properties of TTF with the processability and stability that macromolecules display. The TTF molecule has also been successfully used in the construction of redox-active supramolecular systems. Thus, chemical sensors and redox-switchable ligands have been prepared from TTF while molecular shuttles and molecular switches have been prepared from TTF-containing rotaxanes and catenanes. A large synthetic effort has been devoted to the preparation of the so-called organic ferromagnets, many of which are derived from TTF. The main task in these systems is the introduction of ferromagnetic coupling between the conduction electrons and localized electrons. TTF has also played a prominent role in molecular electronics where TTF-containing D-sigma-A molecules have allowed the preparation of the first confirmed unimolecular rectifier. Recently, it has been confirmed that TTF can display efficient nonlinear optic (NLO) responses in the second and third harmonic generation as well as a good thermal stability. These findings can be combined with the redox ability of TTF as an external stimuli to provide a promising strategy for the molecular engineering of switchable NLO materials. Fullerenes endowed with TTF exhibit outstanding photophysical properties leading to charge-separated (CS) states that show remarkable lifetimes.