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

Helicene Appended Benzothiadiazoles as Chiral Emitters

A homologous series of 4,7-bis(aryl) substituted benzothiadiazole (BTD) compounds, containing the helicenic derivatives bis([4]helicene), bis([5]helicene) and bis([6]helicene), have been prepared upon a double Suzuki coupling between 3,6-bis(pinacolyl-borane)-BTD and the corresponding bromo-aryl precursors. The single crystal X-ray structure of the bis([4]helicene) compound shows the existence of both helicities (M) and (P) on the same molecule. All the compounds of the series are highly emissive in solution, with quantum yields of the emission ranging from 50 to 91 %. The enantiopure compounds (M,M) and (P,P) for the BTD-bis([6]helicene) have been prepared from the corresponding enantiopure 2-bromo-[6]helicene precursors. Their chiroptical properties have been investigated in correlation with density functional theory (DFT) calculations, which allowed to confidently assign the absolute configuration of the helicene arms and to characterize the different electronic transitions, including the low energy charge transfer excitation from helicenes to BTD. The enantiomerically pure fluorophores (M,M)- and (P,P)-BTD-bis([6]helicene), which exist in solution as two main conformers, according to the DFT calculations, show CPL activity in solution, with glum factors of ≈1.7×10-3 at λem=525 nm, and also in the solid state, with glum factors of ≈1.2×10-3 in spite of the strong decrease of the quantum efficiency.

High pressure behaviour of the organic semiconductor salt (TTF-BTD)2I3

This study focuses on the effect of structure compression and cooling on the stereoelectronic properties of the planar π-conjugated TTF-BTD (TTF = tetrathiafulvalene; BTD = 2,1,3-benzothiadiazole) molecule, a prototypical example in which an electron-donor moiety is compactly annulated to an electron-acceptor moiety. Its partially oxidised iodine salt (TTF-BTD)2I3 is a crystalline semiconductor featuring segregated columns of TTF+0.5 units stacked via alternating short and long π-π interactions. We studied TTF-BTD at temperatures ranging from 300 K to 90 K and at pressures up to 7.5 GPa, using both X-ray diffraction and Raman spectroscopy to determine the properties of the compressed samples. Periodic DFT calculations and several theoretical tools were employed to characterize the calculated structural modifications and to predict the structural changes up to 60 GPa. The existence of an unprecedented new phase is predicted above 20 GPa, following a covalent bond formation between two neighbouring TTF-BTD units.

Field-induced mononuclear cobalt(II) single-molecule magnet (SMM) based on a benzothiadiazole-ortho-vanillin ligand

A unique π-conjugated benzothiadiazole-ortho-vanillin ligand (HL), characterized by single crystal X-ray diffraction and DFT calculations, has been prepared by condensation between 4-amino-benzothiadiazole (BTD) and ortho-vanillin. Its reaction with cobalt(II) acetate...

Fluorescent Benzothiadiazole Derivatives as Fluorescence Imaging Dyes: A Decade of New Generation Probes

The current review describes advances in the use of fluorescent 2,1,3-benzothiadiazole (BTD) derivatives after nearly one decade since the first description of bioimaging experiments using this class of fluorogenic dyes. The review describes the use of BTD-containing fluorophores applied as, inter alia, bioprobes for imaging cell nuclei, mitochondria, lipid droplets, sensors, markers for proteins and related events, biological processes and activities, lysosomes, plasma membranes, multicellular models, and animals. A number of physicochemical and photophysical properties commonly observed for BTD fluorogenic structures are also described.

Dimensionality Control in Crystalline Zinc(II) and Silver(I) Complexes with Ditopic Benzothiadiazole-Dipyridine Ligands

Three 2,1,3-benzothiadiazole-based ligands decorated with two pyridyl groups, 4,7-di(2-pyridyl)-2,1,3-benzothiadiazol (2-PyBTD), 4,7-di(3-pyridyl)-2,1,3-benzothiadiazol (3-PyBTD) and 4,7-di(4-pyridyl)-2,1,3 benzothiadiazol (4-PyBTD), generate ZnII and AgI complexes with a rich structural variety: [Zn(hfac)2(2-PyBTD)] 1, [Zn2(hfac)4(2-PyBTD)] 2, [Ag(CF3SO3)(2-PyBTD)]23, [Ag(2-PyBTD)]2(SbF6)24, [Ag2(NO3)2(2-PyBTD)(CH3CN)] 5, [Zn(hfac)2(3-PyBTD)] 6, [Zn(hfac)2(4-PyBTD)] 7, [ZnCl2(4-PyBTD)2] 8 and [ZnCl2(4-PyBTD)] 9 (hfac = hexafluoroacetylacetonato). The nature of the resulting complexes (discrete species or coordination polymers) is influenced by the relative position of the pyridyl nitrogen atoms, the nature of the starting metal precursors, as well as by the synthetic conditions. Compounds 1 and 8 are mononuclear and 2, 3 and 4 are binuclear species. Compounds 6, 7 and 9 are 1D coordination polymers, while compound 5 is a 2D coordination polymer, the metal ions being bridged by 2-PyBTD and nitrato ligands. The solid-state architectures are sustained by intermolecular π–π stacking interactions established between the pyridyl group and the benzene ring from the benzothiadiazol moiety. Compounds 1, 2, 7–9 show luminescence in the visible range. Density Functional Theory (DFT) and Time Dependent Density Functional Theory (TD-DFT) calculations have been performed on the ZnII complexes 1 and 2 in order to disclose the nature of the electronic transitions and to have an insight on the modulation of the photophysical properties upon complexation.

Crystal structures, red-shifted luminescence and iodide-anion recognition properties of four novel D-A type Zn(ii) complexes

Four D-A type Zn(ii) coordination complexes, [Zn(C29H29N3O2)·(CH3OH)]·(CH3OH) (1), Zn2(C74H90N6O4)·(CH3OH) (2), [Zn(C30H28N4O2)·(CH3OH)]·(CH3OH) (3) and [Zn(C38H44N4O2)·(C2H5OH)]·(C2H5OH) (4), were designed, synthesized, and studied. Their fluorescence properties in the solid state and in THF solution were comprehensively analysed based on their single-crystal structures. The results showed that the red-shift of fluorescence emission from complexes 1 to 4 was successfully achieved via the strategy of enhancing intramolecular charge transfer (ICT) effects by increasing the number of electron-pulling and pushing groups gradually. Meanwhile, because of the fluorescence recognition abilities of these four complexes towards iodide anions in THF, they could be regarded as potential fluorescent sensors for I- in this organic solution in the future.

Gold-linked strings of donor–acceptor dyads: on-surface formation and mutual orientation

Strings of fused donor–acceptors with their dipoles following a complicated correlation driven partially by next-nearest-neighbor effects on Au(111).

Dipole Moment and Polarizability of Tunable Intramolecular Charge Transfer States in Heterocyclic π-Conjugated Molecular Dyads Determined by Computational and Stark Spectroscopic Study

The annulation of two redox-active molecules into a compact and planar structure paves the way toward a new class of electronically versatile materials whose physical properties can be tuned via a substitution of one of the constituting moieties. Specifically, we present tetrathiafulvalene-benzothiadiazole donor-acceptor molecules. The critical role played by the dielectric properties of these molecules is evident by the large spectral shifts of the ground-state absorption spectra in a range of solvents. Stark spectroscopy is performed to determine experimentally dipole and polarizability change over transitions in the visible range with particular attention to the transition from the highest-occupied molecular orbital (HOMO) to the lowest-unoccupied molecular orbital (LUMO). The experimental results are compared to the results of time-dependent density functional theory calculations, and we reciprocally validate results from calculation and experiment. This allows us to filter out effective models and reveal important insights. The calculations are initially performed in the gas phase and subsequently a polarizable continuum model is adopted to probe the influence of the solvent on the molecular dielectric properties. The results show a large charge displacement from the HOMO to the LUMO and confirm the intramolecular charge transfer nature of the lowest-energy transition. Substitution of the acceptor moiety with electron-withdrawing groups results in changes to the experimentally determined molecular properties consistent with the effects predicted by computational results. The dominant contribution to the electroabsorption signal is due to the change in dipole moment, which is measured to be roughly 20 D for all samples and forms a small angle with the transition dipole moment in a toluene solvent environment.

Donor-Acceptor Properties of a Single-Molecule Altered by On-Surface Complex Formation

Electron donor-acceptor molecules are of outstanding interest in molecular electronics and organic solar cells for their intramolecular charge transfer controlled via electrical or optical excitation. The preservation of their electronic character in the ground state upon adsorption on a surface is cardinal for their implementation in such single-molecule devices. Here, we investigate by atomic force microscopy and scanning tunneling microscopy a prototypical system consisting of a π-conjugated tetrathiafulvalene-fused dipyridophenazine molecule adsorbed on thin NaCl films on Cu(111). Depending on the adsorption site, the molecule is found either in a nearly undisturbed free state or in a bound state. In the latter case, the molecule adopts a specific adsorption site, leading to the formation of a chelate complex with a single Na⁺ alkali cation pulled out from the insulating film. Although expected to be electronically decoupled, the charge distribution of the complex is drastically modified, leading to the loss of the intrinsic donor-acceptor character. The chelate complex formation is reversible with respect to lateral manipulations, enabling tunable donor-acceptor molecular switches activated by on-surface coordination.

TTF–PTM dyads: from switched molecular self assembly in solution to radical conductors in solid state

Dyads formed by tetrathiafulvalene (TTF) linked to perchlorotriphenylmethyl (PTM) radicals exhibit interesting physical properties such as bistability in solution or conductivity in solid state.

Stimuli-responsive NLO properties of tetrathiafulvalene-fused donor–acceptor chromophores

Tetrathiafulvalene-fused electron donor–acceptor dyads display second order nonlinear optical properties that can be triggered by a pH or a redox stimulus.

Thiazole-induced rigidification in substituted dithieno-tetrathiafulvalene: the effect of planarisation on charge transport properties

Two novel tetrathiafulvalene (TTF) containing compounds 1 and 2 have been synthesised via a four-fold Stille coupling between a tetrabromo-dithienoTTF 5 and stannylated thiophene 6 or thiazole 4. The optical and electrochemical properties of compounds 1 and 2 have been measured by UV–vis spectroscopy and cyclic voltammetry and the results compared with density functional theory (DFT) calculations to confirm the observed properties. Organic field effect transistor (OFET) devices fabricated from 1 and 2 demonstrated that the substitution of thiophene units for thiazoles was found to increase the observed charge transport, which is attributed to induced planarity through S–N interactions of adjacent thiazole nitrogen atoms and TTF sulfur atoms and better packing in the bulk.

Exploring the electronic structure of an organic semiconductor based on a compactly fused electron donor-acceptor molecule

A Mott-type semiconductor based on a compactly fused and partially oxidized electron donor-acceptor (D-A) molecule was recently prepared and identified to exhibit a large room-temperature conductivity of 2 S cm(-1) . In a marked contrast to the organic conductors characterized by relatively well decoupled and segregated uniform stacks of D and A moieties, the formally half-oxidized tetrathiafulvalene donors of the actual compound are organized in columnar π stacks only, whereby the coplanar electron-acceptor units, namely benzothiadiazole, are closely annulated along their ridges. Herein, we present a theoretical study that explores the electronic structure of this novel type of organic semiconductor. The highly symmetric-solid state material behaves as a one-dimensional electronic system with strong antiferromagnetic interactions (coupling constant>200 cm(-1) ). The unique shape and local dipole of this redox-active fused electron D-A molecule lays the basis for further investigations of the collective electronic structure, mainly in the function of different counterions embedded in the crystalline lattice.

Coordination-directed self-assembly of a simple benzothiadiazole-fused tetrathiafulvalene to low-bandgap metallogels

Coordination-driven gelation of a benzothiadiazole-fused tetrathiafulvalene (TTF) is for the first time demonstrated for the construction of low-bandgap metallogels.

Current advances in fused tetrathiafulvalene donor–acceptor systems

Development and applications of tetrathiafulvalene-fused electron donor–acceptor systems will be covered within this tutorial review.

Modulation of the charge transfer and photophysical properties in non-fused tetrathiafulvalene-benzothiadiazole derivatives

Stille and Sonogashira coupling strategies allowed the preparation of luminescent TTF-benzothiadiazole donor–acceptor compounds with direct linkage or an acetylenic spacer.

HOMO stabilisation in π-extended dibenzotetrathiafulvalene derivatives for their application in organic field-effect transistors

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 cm(2)  V(-1)  s(-1) as well as good ambient stability.