The synthesis of four nitronyl nitroxide (NN) biradicals is described which are conjugatively linked through p-ter-phenyl (PPP), ter-thiophene (TTT) and alternating phenylene (P) and thiophene (T) units as PTP and TPT. We first utilized Suzuki and Stille coupling reactions through protection and deprotection protocols to synthesize these (NN) biradicals. Single crystals were efficiently grown for radical precursors of 3, 5, 6, PPP-NNSi, PTP-NNSi, and final biradicals of TTT-NN, TPT-NN, and PPP-NN, whose structures and molecular packing were examined by X-ray diffraction studies. As a result, much smaller torsions between the NN and thiophene units (∼10°) in TTT-NN and TPT-NN than for NN and phenyl units (∼29°) in PPP-NN were observed due to smaller hindrance for a five vs a six membered ring. All four biradicals TTT-NN, TPT-NN, PTP-NN, and PPP-NN were investigated by EPR and optical spectroscopy combined with DFT calculations. The magnetic susceptibility was studied by SQUID measurements for TTT-NN and TPT-NN. The intramolecular exchange interactions for TPT-NN and TTT-NN were found in good agreement with the ones calculated by broken symmetry DFT calculations.

Magneto-structural correlations in stable organic biradicals have been studied on the example of weakly exchange coupled models with nitronyl nitroxide and imino nitroxide spin-carrying entities. Here, heteroatom substituted 2,2′-diaza- and 3,3′-diaza-tolane bridged biradicals were compared with the hydrocarbon analogue, while a biphenyl model with its 2,2′-bipyridine counterpart. For a 3,3′-diazatolane bridge the torsional angle between the nitronyl nitroxides and the pyridyl rings increased heavily (∼52–54°) leading to a smaller theoretical intra-dimer exchange coupling value. However, a very large antiferromagnetic coupling was obtained experimentally. This could be appropriately explained by the presence of dominating inter-dimer exchange between the molecules. For the bis(imino nitroxide) with tolane bridge a field induced ordered state between 1.8 to 4.3 T in AC-susceptibility measurements was observed. In terms of a Bose Einstein condensate (BEC) of triplons this phenomenon could be described as a magnetic field induced ordered phase with 3D character.

A fused heteroacene derivative, bis(dicyanomethylene)-end-capped-dithieno[2,3-d;2′,3′-d]benzo[2,1-b;3,4-b′]-dithiophene (4CN-DTmBDT) was synthesized. Its open-shell biradical character in the ground state is studied by a combination of electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR). Structural assignment was verified with single crystal X-ray diffraction analysis. A highly stable biradicaloid with a low energy gap and low LUMO level is achieved.

A planar heteroacene building block, dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;3,4-b′]dithiophene (DTmBDT), is reported via a facile synthetic procedure. Single-crystal X-ray diffraction of Br2-DTmBDT reveals that dodecyl chains interdigitate, still enabling close π-stacking of 3.42 Å. A very high molecular weight quasi-planar copolymer PDTmBDT-DPP exhibited a high hole mobility of 0.36 cm2 V−1 s−1 in preliminary studies of organic field-effect transistors.

A novel fluorescent dendrimer PYTPAG2, with pyrene as the interior core and triphenylamine (TPA) as the exterior periphery, is studied as a fluorescence-quenching sensor for iron (ш) ions (Fe3+), both in solution and as a film. This dendrimer-based sensor possesses preferential detection of Fe3+ by a very strong fluorescence quenching not found for other metal ions. The fluorescent detection limits of this PYTPAG2 sensor for Fe3+ in solution and thin-film are 6.5×10−7 M and 5.0×10−7 M, respectively. The possible mechanism of this process is explained by the complexation between the peripheral TPA units of PYTPAG2 and Fe3+ ions, which may disrupt the fluorescence resonance energy transfer (FRET) from the TPA groups to the pyrene core (intramolecular of PYTPAG2) and results in the fluorescence quenching. Moreover, this striking performance could not be disturbed by pH, the interference with other metal ions, counter anions, or surrounding environment. In addition, biological fluorescence imaging studies of Fe3+ in living roundworms demonstrate its valuable practical application.A novel fluorescent dendrimer PYTPAG2 studied as a fluorescence-quenching sensor to iron (ш) ions presents stable and high sensitivity and selectivity both in aqueous solutions and as films, the striking performance could not be disturbed by the real environment, such as in roundworms. It has important significance and application value for medical research and environmental monitoring.

Six thiadiazoloquinoxaline (TQ) based copolymers (P1–P6) have been synthesized using Stille coupling reaction upon varying donor moieties, substitution positions and architectures of polymer side chains. UV-vis-NIR absorption spectra indicated that all of these polymers exhibited low optical bandgaps from 0.96 to 0.75 eV. The electron affinities of these six polymers with values from −3.92 to −3.99 eV slightly changed due to the contribution of the same TQ core. However, the ionization potentials were tuned from −4.95 eV in P1 to −5.28 eV in P3 by introducing different donors. Comparing polymers P1–P4 with different donor parts, two dimensional wide-angle X-ray scattering measurements implied that P4 had a higher crystallinity because its coherence length (5.8 nm) was 2–3 times higher than those of P1–P3 (1.7–2.9 nm). This led to a best field-effect hole mobility of 0.1 cm2 V−1 s−1 for P4 among the four polymers. The polymers P4–P6 had identical molecular formulae of side chains but significant differences in device performance. In comparison with P4, the two linear alkyl chains were moved onto head to head positions of bithiophene in P5, resulting in a hole mobility of only 0.007 cm2 V−1 s−1. However, a pair of 2-decyl-tetradecyl alkyl chains was grafted onto thiophene units adjacent to the TQ core in P6, leading to the highest hole mobility up to 0.24 cm2 V−1 s−1 in this series of polymers.

A new synthetic route to prepare a centrosymmetric phenanthroline-fused azaacene derivative, TIPS-BisPhNPQ, is described. Another axialsymmetric analogue, TIPS-PhNTQ, is also synthesized for comparison. Cyclic voltammetry measurements indicate high electron affinity values of −4.03 and −4.01 eV for TIPS-PhNTQ and TIPS-BisPhNPQ, respectively. Single-crystal X-ray diffraction reveals that TIPS-PhNTQ forms dimers by intermolecular S–N and N–N interaction, while TIPS-BisPhNPQ shows a highly ordered arrangement via two-dimensional brickwork packing and intermolecular hydrogen bonding. The synthetic protocol established in this paper should be highly applicable to the preparation of more azaacene derivatives with extended π-conjugations.

Our findings provide a better understanding of the discrepancies related to the discussion in the literature on the ground state of Tschitschibabin's hydrocarbon. A series of phenylene bridged bisnitroxide molecules were designed and studied for the comparison. The simple theoretical and experimental methodologies have been developed and utilized to understand the singlet biradicaloids which exist in semi-quinoid form and exhibits characteristics of biradicaloid and quinoid form simultaneously.

Three novel thiadiazoloquinoxaline (TQ) derivatives, TIPS-APhTQ, TIPS-PhTQ, and TIPS-BDTTQ, were synthesized by introducing two triisopropylsilylethynyl groups and alternating the aromatic ring units in the condensed moiety of TQ. The synthetic route is very efficient, providing high yields. Cyclic voltammetry suggests high electron affinity values of −3.82, −3.95, and −3.99 eV for TIPS-APhTQ, TIPS-PhTQ, and TIPS-BDTTQ, respectively. Single-crystal X-ray diffraction reveals that three molecules form corresponding dimers by intermolecular S–N interaction and have very similar two-dimensional π-stacking. The π-stacking distances between them are as close as 3.34–3.46 Å.

The synthesis and characterization of fused-ring thiadiazoloquinoxaline derivatives are described. The target molecules were studied by UV-Vis absorption, emission spectroscopy and cyclic voltammetry. The optical absorption maximum λmax of the new molecules in solution were shown at 714–774 nm, with the corresponding optical gaps (Eoptg) of 1.44–1.50 eV. Density functional theory calculations were applied for the design and prediction of HOMO and LUMO variations and the corresponding optical absorptions. The thiadiazoloquinoxaline with a phenanthroline moiety showed a liquid crystalline phase as found from 2D-WAXS studies and an electron transporting behavior indicating its potential as an acceptor building block.

The positional isomers of tert-butylnitroxide (NO) substituted 4,5,9,10-tetramethoxypyrene-based mono- and biradical are synthesized. While the biradical 2,7-TMPNO in which two NO radical moieties are attached at the nodal plane of pyrene adopts a semiquinoid structure, the 1,6- and 1,8-isomers of the same exist in biradical form. The tuning of the antiferromagnetic exchange interactions is achieved by synthesizing the positional isomers of the biradical while maintaining the same radical moiety as well as the π spacer.

Two novel conjugated polymers with high molecular weight, PBDTTQ-3 and PAPhTQ, were synthesized by tuning alkyl chains and alternating the electron-donating ability of the thiadiazoloquinoxaline (TQ) moiety. Both polymers have excellent solubility in common organic solvents. UV–vis–NIR absorption and cyclic voltammetry indicate a bandgap of (0.76 eV) and high electron affinity level (−4.08 eV) for PBDTTQ-3. Two dimensional wide-angle X-ray scattering shows that both polymers are only poorly ordered in the bulk but possess a close π-stacking distance of 0.36 nm. Despite the disorder in thin film observed by grazing incidence wide-angle X-ray scattering, PBDTTQ-3 exhibits good ambipolar transport, with a maximum hole mobility of 0.22 cm2 V–1 s–1 and comparable electron mobility of 0.21 cm2 V–1 s–1.

Two new conjugated copolymers, PBDTTQ-1 and PBDTTQ-2, with a distinct linked pattern between benzodithiophene–thiadiazoloquinoxaline (BDTTQ) as acceptor and bithiophene as donor were synthesized and characterized. The difference in the linkage between donor and acceptor exerts great influence on the optoelectronic properties of the two polymers. The optical band gap decreases from 1.18 eV for PBDTTQ-1 to 1.03 eV for PBDTTQ-2, due to the lower LUMO energy level (−4.01 eV) of the latter. Moreover, density functional theory calculations demonstrate that the electron density is mainly confined on the acceptor unit in both HOMO and LUMO of PBDTTQ-1, while the electronic densities almost delocalize along the entire backbone of PBDTTQ-2, which facilitates the charge transport within the polymer chain. In contrast to PBDTTQ-1 missing any field-effect characteristics, PBDTTQ-2 exhibits ambipolar charge transporting behavior with mobilities of 1.2 × 10–3 cm2/(V s) for holes and 6.0 × 10–4 cm2/(V s) for electrons.

•π-Conjugated donor–acceptor dyes are modeled for novel blue emitters using (TD)DFT.•Meta-linkage donor-spacer-acceptor ensures disjoint frontier orbitals character.•All studied molecules absorb in the blue domain.•Criteria for molecules with prospects for TADF emitters are outlined.•Five systems with (quasi)degenerate singlet and triplet excited states are proposed.The quest for more efficient blue emitters to be applied in organic light-emitting diodes is one of the challenging tasks of contemporary nanotechnologies. An approach to enhance substantially the intrinsic efficiency of luminescent organic molecules is the so-called thermally activated delayed fluorescence. A prerequisite for its occurrence is a vanishing energy separation between the first singlet and triplet excited states. A series of donor–acceptor molecules is investigated theoretically within this study in order to validate a molecular model for design of efficient organic blue emitters with closely spaced singlet and triplet excited states. The model is based on meta-linkage of the donor and acceptor residues to a spacer ensuring frontier molecular orbitals partitioning. The optimal geometries of the molecules are obtained with density functional theory (B3LYP/6-31G*) and the singlet and triplet absorption spectra are simulated within the time-dependent density functional framework. The excited singlet-triplet energy gap is estimated and correlated to structural and energetic characteristics of the donors and acceptors. Several requirements for achieving high-energy triplet states at the molecular level in such donor–acceptor systems are outlined, the main being disjoint character of the molecular orbitals on the spacer and sufficient energy separation of the two topmost occupied orbitals. It is shown that by variation of the acceptor moiety the optical absorption transitions of the compounds can be fine-tuned in a systematic fashion. Molecules with degenerate singlet and triplet first excited states are proposed, combining bisdimethylaminotriphenylamine or phenoxazine as donors with diphenyloxadiazole or diphenyl-2,2′-bipyridine as acceptors. Bipolar molecules derived from this model could be used as prospective building blocks for efficient emissive materials in blue organic light-emitting diodes.Graphical abstract.

Considering electronic and structural interactions between fluorescent polymers and TNT, we have designed and synthesized two carbazole-based polymers (P1 and P2), possessing 2-ethylhexyl and 4-[tris-(4-octyloxyphenyl)methyl]phenyl as side chains, respectively. The distinct side chains cause dramatic differences in the polymer sensing behavior. It is demonstrated that large and more rigid side chains could reduce the interactions between polymer chains in the solid state, while supporting the diffusion of explosives through more accessible space, thereby contributing to the sensitivity. Furthermore, we have proved that bulky side chains obviously did not influence the conformation of the backbone, which is necessary for the conjugated polymers to act as fluorescent sensing material. The results suggest that P2 is a promising material for TNT detection with pronounced sensitivity, in addition they provide a new design-thought for TNT sensing materials.

We have synthesized and investigated a new biphenyl-4,4′-bis(nitronyl nitroxide) radical with intermediately strong antiferromagnetic interactions. This organic biradical belongs to a family of materials that can be used as a building block for the design of new quantum magnets. For quantum magnetism, special attention has been paid to coupled S = 1/2 dimer compounds, which when placed in a magnetic field, can be used as model systems for interacting boson gases. Short contacts between the oxygen atoms of the nitronyl nitroxide units and the hydrogen atoms of the benzene rings stabilize a surprisingly planar geometry of the biphenyl spacer and are responsible for a small magnetic interdimer coupling. The strength of the antiferromagnetic intradimer coupling constant J/kB = −14.0 ± 0.9 K, fitting the experimental SQUID-data using an isolated-dimer model. The deviations from the isolated-dimer model are attributed to a small interdimer coupling J′/kB, on the order of 1 K, consistent with the crystal structure.

Two conjugated copolymers, PPTQT and PTTQT, were developed based on thiadiazoloquinoxalines connected via ethynylene π-spacer to thiophene units. PPTQT showed maximum hole and electron mobility of 0.028 and 0.042 cm2/V s, respectively, being the first example of an ambipolar semiconducting material bearing triple bonds in the polymer backbone.

Our findings provide a better understanding of the discrepancies related to the discussion in the literature on the ground state of Tschitschibabin's hydrocarbon. A series of phenylene bridged bisnitroxide molecules were designed and studied for the comparison. The simple theoretical and experimental methodologies have been developed and utilized to understand the singlet biradicaloids which exist in semi-quinoid form and exhibits characteristics of biradicaloid and quinoid form simultaneously.

Six thiadiazoloquinoxaline (TQ) based copolymers (P1–P6) have been synthesized using Stille coupling reaction upon varying donor moieties, substitution positions and architectures of polymer side chains. UV-vis-NIR absorption spectra indicated that all of these polymers exhibited low optical bandgaps from 0.96 to 0.75 eV. The electron affinities of these six polymers with values from −3.92 to −3.99 eV slightly changed due to the contribution of the same TQ core. However, the ionization potentials were tuned from −4.95 eV in P1 to −5.28 eV in P3 by introducing different donors. Comparing polymers P1–P4 with different donor parts, two dimensional wide-angle X-ray scattering measurements implied that P4 had a higher crystallinity because its coherence length (5.8 nm) was 2–3 times higher than those of P1–P3 (1.7–2.9 nm). This led to a best field-effect hole mobility of 0.1 cm2 V−1 s−1 for P4 among the four polymers. The polymers P4–P6 had identical molecular formulae of side chains but significant differences in device performance. In comparison with P4, the two linear alkyl chains were moved onto head to head positions of bithiophene in P5, resulting in a hole mobility of only 0.007 cm2 V−1 s−1. However, a pair of 2-decyl-tetradecyl alkyl chains was grafted onto thiophene units adjacent to the TQ core in P6, leading to the highest hole mobility up to 0.24 cm2 V−1 s−1 in this series of polymers.

A fused heteroacene derivative, bis(dicyanomethylene)-end-capped-dithieno[2,3-d;2′,3′-d]benzo[2,1-b;3,4-b′]-dithiophene (4CN-DTmBDT) was synthesized. Its open-shell biradical character in the ground state is studied by a combination of electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR). Structural assignment was verified with single crystal X-ray diffraction analysis. A highly stable biradicaloid with a low energy gap and low LUMO level is achieved.

Considering electronic and structural interactions between fluorescent polymers and TNT, we have designed and synthesized two carbazole-based polymers (P1 and P2), possessing 2-ethylhexyl and 4-[tris-(4-octyloxyphenyl)methyl]phenyl as side chains, respectively. The distinct side chains cause dramatic differences in the polymer sensing behavior. It is demonstrated that large and more rigid side chains could reduce the interactions between polymer chains in the solid state, while supporting the diffusion of explosives through more accessible space, thereby contributing to the sensitivity. Furthermore, we have proved that bulky side chains obviously did not influence the conformation of the backbone, which is necessary for the conjugated polymers to act as fluorescent sensing material. The results suggest that P2 is a promising material for TNT detection with pronounced sensitivity, in addition they provide a new design-thought for TNT sensing materials.