Novel Rigidochromic and Anti-Kasha Dual Emission Fluorophores Based on D-π-A Dyads as the Promising Materials for Potential Applications Ranging from Optoelectronics and Optical Sensing to Biophotonics and Medicine.

Today we see an increasing demand for new fluorescent materials exhibiting various sensory abilities due to their broad applicability ranging from the construction of flexible devices to bioimaging. In this paper, we report on the new fluorescent pigments AntTCNE, PyrTCNE, and PerTCNE which consist of 3-5 fused aromatic rings substituted with tricyanoethylene fragments forming D-π-A diad. Our studies reveal that all three compounds exhibit pronounced rigidochromic properties, i.e., strong sensitivity of their fluorescence to the viscosity of the local environment. We also demonstrate that our new pigments belong to a very rare type of organic fluorophores which do not obey the well-known empirical Kasha'rule stating that photoluminescence transition always occurs from the lowest excited state of an emitting molecule. This rare spectral feature of our pigments is accompanied by an even rarer capability of spectrally and temporally well-resolved anti-Kasha dual emission (DE) from both higher and lowest electronic states in non-polar solvents. We show that among three new pigments, PerTCNE has significant potential as the medium-bandgap non-fullerene electron acceptor. Such materials are now highly demanded for indoor low-power electronics and portable devices for the Internet-of-Things. Additionally, we demonstrate that PyrTCNE has been successfully used as a structural unit in template assembling of the new cyanoarylporphyrazine framework with 4 D-π-A dyads framing this macrocycle (Pyr4CN4Pz). Similarly to its structural unit, Pyr4CN4Pz is also the anti-Kasha fluorophore, exhibiting intensive DE in viscous non-polar medium and polymer films, which strongly depends on the polarity of the local environment. Moreover, our studies showed high photodynamic activity of this new tetrapyrrole macrocycle which is combined with its unique sensory capacities (strong sensitivity of its fluorescent properties to the local environmental stimuli such as viscosity and polarity. Thus, Pyr4CN4Pz can be considered the first unique photosensitizer that potentially enables the real-time combination of photodynamic therapy and double-sensory approaches which is very important for modern biomedicine.

Charge Transfer Chromophores Derived from 3d-Row Transition Metal Complexes.

A series of new charge transfer (CT) chromophores of "α-diimine-MII-catecholate" type (where M is 3d-row transition metals-Cu, Ni, Co) were derived from 4,4'-di-tert-butyl-2,2'-bipyridyl and 3,6-di-tert-butyl-o-benzoquinone (3,6-DTBQ) in accordance with three modified synthetic approaches, which provide high yields of products. A square-planar molecular structure is inherent for monomeric [CuII(3,6-Cat)(bipytBu)]∙THF (1) and NiII(3,6-Cat)(bipytBu) (2) chromophores, while dimeric complex [CoII(3,6-Cat)(bipytBu)]2∙toluene (3) units two substantially distorted heteroleptic D-MII-A (where D, M, A are donor, metal and acceptor, respectively) parts through a donation of oxygen atoms from catecholate dianions. Chromophores 1-3 undergo an effective photoinduced intramolecular charge transfer (λ = 500-715 nm, extinction coefficient up to 104 M-1·cm-1) with a concomitant generation of a less polar excited species, the energy of which is a finely sensitive towards solvent polarity, ensuring a pronounced negative solvatochromic effect. Special attention was paid to energetic characteristics for CT and interacting HOMO/LUMO orbitals that were explored by a synergy of UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT study. The current work sheds light on the dependence of CT peculiarities on the nature of metal centers from various groups of the periodic law. Moreover, the "α-diimine-MII-catecholate" CT chromophores on the base of "late" transition elements with differences in d-level's electronic structure were compared for the first time.

Synthesis and Antioxidant Activity of New Catechol Thioethers with the Methylene Linker.

Novel catechol thio-ethers with different heterocyclic substituents at sulfur atom were prepared by reacting 3,5-di-tert-butyl-6-methoxymethylcatechol with functionalized thiols under acidic conditions. A common feature of compounds is a methylene bridge between the catechol ring and thioether group. Two catechols with the thio-ether group, bound directly to the catechol ring, were also considered to assess the effect of the methylene linker on the antioxidant properties. The crystal structures of thio-ethers with benzo-thiazole moieties were established by single-crystal X-ray analysis. The radical scavenging and antioxidant activities were determined using 2,2'-diphenyl-1-picrylhydrazyl radical test, ABTS∙+, CUPRAC (TEAC) assays, the reaction with superoxide radical anion generated by xanthine oxidase (NBT assay), the oxidative damage of the DNA, and the process of lipid peroxidation of rat liver (Wistar) homogenates in vitro. Most catechol-thioethers exhibit the antioxidant effect, which varies from mild to moderate depending on the model system. The dual anti/prooxidant activity characterizes compounds with adamantyl or thio-phenol substituent at the sulfur atom. Catechol thio-ethers containing heterocyclic groups (thiazole, thiazoline, benzo-thiazole, benzo-xazole) can be considered effective antioxidants with cytoprotective properties. These compounds can protect molecules of DNA and lipids from the different radical species.

Benzylidene Cyclopentanone Derivative Photoinitiator for Two-Photon Photopolymerization-Photochemistry and 3D Structures Fabrication for X-ray Application.

Micron- and submicron-scale 3D structure realization nowadays is possible due to the two-photon photopolymerization (TPP) direct laser writing photolithography (DLW photolithography) method. However, the achievement of lithographic features with dimensions less than 100 nm is in demand for the fabrication of micro-optical elements with high curvature values, including X-ray microlenses. Spectroscopic and photochemical study of a photoinitiator (PI) based on a methyl methacrylate derivative of 2,5-bis(4-(dimethylamino)benzylidene) cyclopentanone was performed. Enhanced intersystem crossing in the methyl methacrylate derivative results in increased radical generation for the subsequent initiation of polymerization. A comprehensive study of the new photocompositions was performed, with particular emphasis on photochemical constants, the degree of photopolymerization, and topology. The optimal parameters for the fabrication of mechanically stable structures were determined in this research. The threshold dose parameters for lithography (radiation power of 5 mW at a speed of 180 µm/s) when trying to reach saturation values with a conversion degree of (35 ± 1) % were defined, as well as parameters for sub-100 nm feature fabrication. Moreover, the 45 nm feature size for elements was reached. Fabrication of X-ray lens microstructures was also demonstrated.

Complexes of Metal Halides with Unreduced o-(Imino)quinones.

A series of complexes of metal halides with unreduced quinone-type ligands have been synthesized and characterized in detail. The 3,6-di-tert-butyl-o-benzoquinone (1) and 4,6-di-tert-butyl-N-aryl-substituted o-iminobenzoquinones (2-5) (aryl is 2,6-dimethylphenyl in 2, 2-methyl-6-ethylphenyl in 3, 2,6-diethylphenyl in 4, and 2,6-diisopropylphenyl in 5) were used to obtain the molecular complexes with metal 12 group halides as well as with indium(III) iodide. The molecular structures of five complexes, bearing an unreduced form of redox-active ligand, have been established by single-crystal X-ray analysis. The spectral data, electrochemical measurements, and DFT calculations indicate the significant transformations of the molecular orbitals of 1-5 upon complexation with Lewis acids. The reduction potentials of o-(imino)quinones in complexes with metal halides shift into the anodic region versus uncoordinated ones. The choice of metal halide allows varying the shift magnitude up to 1.7 V in 2·CdI2. The change of the oxidizing ability of the 1-5 upon coordination with Lewis acids enables the oxidation of mercury and ferrocene, infeasible for free ligands.

Square-Planar Heteroleptic Complexes of α-Diimine-NiII-Catecholate Type: Intramolecular Ligand-to-Ligand Charge Transfer.

Two heteroleptic NiII complexes combined the redox-active catecholate and 2,2'- bipyridine ligand platforms were synthesized to observe a photoinduced intramolecular ligand-to-ligand charge transfer (LL'CT, HOMOcatecholate → LUMOα-diimine). A molecular design of compound [NiII(3,6-Cat)(bipy)]∙CH3CN (1) on the base of bulky 3,6-di-tert-butyl-o-benzoquinone (3,6-DTBQ) was an annelation of the ligand with an electron donor glycol fragment, producing derivative [NiII(3,6-Catgly)(bipy)]∙CH2Cl2 (2), in order to influence the energy of LL'CT transition. A substantial longwave shift of the absorption peak was observed in the UV-Vis-NIR spectra of 2 compared with those in 1. In addition, the studied NiII derivatives demonstrated a pronounced negative solvatochromism, which was established using a broad set of solvents. The molecular geometry of both compounds can be ascribed as an insignificantly distorted square-planar type, and the π-π intermolecular stacking of the neighboring α-diimines is realized in a crystal packing. There is a lamellar crystal structure for complex 1, whereas the perpendicular T-motifs with the inter-stacks attractive π-π interactions form the packing of complex 2. The redox-active nature of ligand systems was clearly shown through the electrochemical study: a quasi-reversible one-electron reduction of 2,2'-bipyridine and two reversible successive one-electron oxidative conversations ("catecholate dianion-o-benzosemiquinonato radical anion-neutral o-benzoquinone") were detected.

Annulation of a 1,3-dithiole ring to a sterically hindered o-quinone core. Novel ditopic redox-active ligands.

The fused 1,3-dithiole spacer seems to be very suitable for the functionalization of sterically hindered o-quinones with additional groups capable of coordination of metal ions and/or possessing a redox activity. An effective method for the synthesis of sterically hindered o-quinones containing 1,3-diketonate, dinitrile and p-quinone-methide functional groups at the periphery of the ligand has been developed. The novel compounds have rigid and conjugated structures and exhibit properties typical of o-quinones. A study of their monoreduced semiquinone derivatives reveal that the spin density is delocalized across the whole molecule, including peripheral fragments. The first stable o-quinone derivative bearing an annulated thiete heterocycle has been isolated and characterized.

Imine-Based Catechols and o-Benzoquinones: Synthesis, Structure, and Features of Redox Behavior.

Novel sterically hindered catechols of the type 3-(RN=CH)-4,6-DBCatH2 with iminoalkyl or iminoaryl groups in the third position of the aromatic ring have been synthesized and characterized in detail. The o-benzoquinones 3-(RN=CH)-4,6-DBBQ have been synthesized by the oxidation of the corresponding catechols. The oxidation of methylimino-substituted catechol with K3[Fe(CN)6] in alkaline medium leads to the formation of two products: o-quinone and diene-dione, the product of the water addition to the corresponding o-quinone. Some o-benzoquinones react with water or methanol to yield products of water or methanol addition. A prototropic tautomerism is characteristic of catecholaldimines: a quinomethide form is observed in the case of aliphatic amine derivatives, while aryl-substituted catecholaldimines can exist both in the catechol and quinomethide forms in the crystalline state. The formation of dimeric structures motifs is observed in crystals. The electrochemical oxidation of imino-based catechols proceeds via two one-electron processes; the second wave is quasi-reversible, which is unusual for catechols.

Polyfunctional Sterically Hindered Catechols with Additional Phenolic Group and Their Triphenylantimony(V) Catecholates: Synthesis, Structure, and Redox Properties.

New polyfunctional sterically hindered 3,5-di-tert-butylcatechols with an additional phenolic group in the sixth position connected by a bridging sulfur atom-(6-(CH2-S-tBu2Phenol)-3,5-DBCat)H2 (L1), (6-(S-tBu2Phenol)-3,5-DBCat)H2 (L2), and (6-(S-Phenol)-3,5-DBCat)H2 (L3) (3,5-DBCat is dianion 3,5-di-tert-butylcatecolate)-were synthesized and characterized in detail. The exchange reaction between catechols L1 and L3 with triphenylantimony(V) dibromide in the presence of triethylamine leads to the corresponding triphenylantimony(V) catecholates (6-(CH2-S-tBu2Phenol)-3,5-DBCat)SbPh3 (1) and (6-(S-Phenol)-3,5-DBCat)SbPh3 (2). The electrochemical properties of catechols L1-L3 and catecholates 1 and 2 were investigated using cyclic voltammetry. The electrochemical oxidation of L1-L3 at the first stage proceeds with the formation of the corresponding o-benzoquinones. The second process is the oxidation of the phenolic moiety. Complexes 1 and 2 significantly expand their redox capabilities, owing to the fact that they can act as the electron donors due to the catecholate metallocycle capable of sequential oxidations, and as donors of the hydrogen atoms, thus forming a stable phenoxyl radical. The molecular structures of the free ligand L1 and complex 1 in the crystal state were determined by single-crystal X-ray analysis.

Ammonium-Charged Sterically Hindered Phenols with Antioxidant and Selective Anti-Gram-Positive Bacterial Activity.

The increase in the resistance of pathogens, in particular Staphylococcus aureus, to the action of antibiotics necessitates the search for new readily available and non-toxic drugs. In solving this problem, phenolic acylhydrazones have high potential. In this communication, the synthesis of quaternary ammonium compounds containing a differently substituted phenolic moiety has been performed. An initial study of antimicrobial activity showed that these compounds are highly selective against S. aureus and B. cereus. The highest activity (MIC 2.0 µm) was shown by hydrazones containing a catechol fragment. These compounds are more than 3-fold more active against S. aureus and 3-10-fold more active against B. cereus than norfloxacin. Low hemolytic and high antioxidant activities of all new compounds were also established.

Sensitization of NIR luminescence of Yb3+ by Zn2+ chromophores in heterometallic complexes with a bridging Schiff-base ligand.

Herein, complexes [ZnL]2 (1), {(H2O)Zn(µ-L)Yb[OCH(CF3)2]3} (2), {[(CF3)2HCO]Zn(µ-L)Yb[OCH(CF3)2](µ-OH)}2 (3), and [(H2O)Ln2(L)3] (Ln = Yb (4) and Gd (5)) containing a bridging Schiff-base ligand (H2L = N,N'-bis(3-methoxy salicylidene)phenylene-1,2-diamine) were synthesized. The compounds 1-4 were structurally characterized. The ytterbium derivatives 2-4 exhibited bright NIR metal-centred photoluminescence (PL) of Yb3+ ion under one- (λex = 380 nm) and two-photon (λex = 750 nm) excitation. The superior luminescence properties of complex 2, which was suggested as a marker for NIR bioimaging, were explained via the strong absorption of the 375 nm LMCT state of the ZnL chromophore, efficient energy transfer from ZnL towards Yb3+ through a reversible ligand-to-lanthanide electron transfer process, and absence of luminescence quenchers (C-H and O-H groups) in the first coordination sphere of the rare-earth atom.