Aggregation/Crystallization-Induced Emission in Naphthyridine-Based Carbazolyl-Modified Donor-Acceptor Boron Dyes Tunable by Fluorine Atoms.

Four donor-acceptor boron difluoride complexes based on the carbazole electron donor and the [1,3,5,2]oxadiazaborinino[3,4-a][1,8]naphthyridine acceptor were designed, synthesized, and systematically spectroscopically investigated in solutions, in dye-doped polymer films, and in the solid states. The dyes exhibit an intense blue to red solid-state emission with photoluminescence quantum yields of up to 59 % in pure dye samples and 86 % in poly(methyl methacrylate) films. All boron complexes show aggregation-induced emission and reversible mechanofluorochromism. The optical properties of these dyes and their solid state luminescence can be tuned by substitution pattern, i. e., the substituents at the naphthyridine unit. Exchange of CH3- for CF3-groups does not only increase the intramolecular charge transfer character, but also provides a crystallization-induced emission enhancement.

Anion Recognition by a Pincer-Type Host Constructed from Two Polyamide Macrocyclic Frameworks Jointed by a Photo-Addressable Azobenzene Switch.

A sterically crowded light-responsive host 1 was synthetized with a 93% yield by applying a post-functionalization protocol utilizing the double amidation of 4,4'-azodibenzoyl dichloride with a readily available 26-membered macrocyclic amine. X-ray structures of two hydrates of trans-1 demonstrate a very different alignment of the azobenzene linkage, which is involved in T-shape or parallel-displaced π⋯π stacking interactions with the pyridine-2,6-dicarboxamide moieties from the macrocyclic backbone. Despite the rigidity of the macrocyclic framework, which generates a large steric hindrance around the azobenzene chromophore, the host 1 retains the ability to undergo a reversible cis⟷trans isomerization upon irradiation with UVA (368 nm) and blue (410 nm) light. Moreover, thermal cis→trans back-isomerization (ΔG0 = 106.5 kJ∙mol-1, t½ = 141 h) is markedly slowed down as compared to the non-macrocyclic analog. 1H NMR titration experiments in DMSO-d6/0.5% water solution reveal that trans-1 exhibits a strong preference for dihydrogenphosphate (H2PO4-) over other anions (Cl-, MeCO2-, and PhCO2-), whereas the photogenerated metastable cis-1 shows lower affinity for the H2PO4- anion.

Stabilization of Near Identical Hydrogen Bonded Octameric Water Clusters in Crystal Structures of Three Distinct Non-Charged Polyamide Macrocyclic Host Molecules.

In this paper, we present a comparative analysis of the solid state structures of three well-resolved hydrates of macrocyclic host molecules 1a, 1b, and 2 containing an intrannular amide-aryl substituent (lariat arm) connected to a fixed 26-membered ring in a normal (-NHCOAr, hosts 1a and 1b) or reverse manner (-CONHAr, host 2). Despite different chemical structures, these hosts crystallize as isostructural tetrahydrates in the same P-1 space group. Moreover, their crystals exhibit identical hydrogen bond motifs resulting in a stabilization of an almost identical unusual octameric water cluster built from the cyclic tetramer core and four water molecules, attached sequentially in an "up-and-down" manner. Further analysis reveals that, among the series, the structure of host 2 provides the most suitable environment for the accommodation of this type of water cluster.

Towards More Photostable, Brighter, and Less Phototoxic Chromophores: Synthesis and Properties of Porphyrins Functionalized with Cyclooctatetraene.

Free base and zinc porphyrins functionalized with cyclooctatetraene (COT), a molecule known as a good triplet-state quencher, have been obtained and characterized in detail by structural, spectral, and photophysical techniques. Substitution with COT leads to a dramatic decrease of the intrinsic lifetime of the porphyrin triplet. As a result, photostability in oxygen-free solution increases by two to three orders of magnitude. In non-degassed solutions, improvement of photostability is about tenfold for zinc porphyrins, but the free bases become less photostable. Similar quantum yields of photodegradation in free base and zinc porphyrins containing the COT moiety indicate a common mechanism of photochemical decomposition. The new porphyrins are expected to be much less phototoxic, since the quantum yield of singlet oxygen formation strongly decreases because of the shorter triplet lifetime. The reduction of triplet lifetime should also enhance the brightness and reduce blinking in porphyrin chromophores emitting in single-molecule regime, since the duration of dark OFF states will be shorter.

Organolithium-Mediated Postfunctionalization of Thiazolo[3,2-c][1,3,5,2]oxadiazaborinine Fluorescent Dyes.

An effective method for transition-metal-free postfunctionalization of thiazolo[3,2-c][1,3,5,2]oxadiazaborinine dyes via direct lithiation of the 1,3-thiazole ring was developed. The reaction allows valuable regioselective C-H modification of these N,O-chelated organoboron chromophores incorporating different groups, including C-, Hal-, Si-, S-, Se-, and Sn-substituents. As a result, a library of novel fluorescent 1,3-thiazole-based organoboron complexes has been synthesized and characterized. The influence of the donor/acceptor strength of the substituent E on the photophysical properties has been established. The compound with a bulky lipophilic substituent (SnBu3) exhibits a relatively high solid-state photoluminescence quantum yield of 44%.

Application of the Suzuki-Miyaura Reaction for the Postfunctionalization of the Benzo[4,5]thiazolo[3,2- c][1,3,5,2]oxadiazaborinine Core: An Approach toward Fluorescent Dyes.

A fluorescent dye based on the 8-brominated benzo[4,5]thiazolo[3,2- c][1,3,5,2]oxadiazaborinine core was synthesized from benzo[ d]thiazol-2-amine. The new boron complex can be effectively modified by a palladium-catalyzed Suzuki-Miyaura cross-coupling reaction with (het)arylboronic acids. This reaction allows a valuable regioselective postfunctionalization of 1,3,5,2-oxadiazaborinine chromophores with different aromatic substituents. The solutions of obtained target complexes in organic solvents demonstrate high fluorescence quantum yields. The compound with a 4-cyanophenyl group at benzothiazole unit (Ar = 4-C6H4CN) exhibits a comparatively high fluorescence quantum yield of 0.31 in the solid state.

Complexation of tropane alkaloids by cyclodextrins.

Complexes of atropine, homatropine, scopolamine, and ipratropium with cyclodextrins were investigated by NMR and capillary electrophoresis. It has been demonstrated that tropane alkaloids form complexes with ß- and γ-cyclodextrins of 1:1 stoichiometry. NMR measurements indicate the formation of complexes where both aliphatic and aromatic parts of tropane alkaloids interact with ß-cyclodextrin. The stability constants of the investigated alkaloids with ß- and γ-cyclodextrins were determined by capillary electrophoresis. It has been found that ß-cyclodextrin forms ten times more stable complexes than γ-cyclodextrin. Moreover, the analysis of the obtained crystal structure of ß-cyclodextrin/(-)-hyoscyamine complex reveals that two molecules of (-)-hyoscyamine oriented in head-to-tail mode are tightly fitted inside head-to-head ß-cyclodextrin dimer. Conformation of (-)-hyoscyamine as well as scopolamine changes substantially upon complexation adapting to the cavity of ß-cyclodextrin as shown by X-ray analysis, NMR and DFT calculations data.

Benzo[4,5]thiazolo[3,2- c][1,3,5,2]oxadiazaborinines: Synthesis, Structural, and Photophysical Properties.

A family of highly emissive benzo[4,5]thiazolo[3,2- c][1,3,5,2]oxadiazaborinines, conjugated with the donor 4-dimethylaminophenyl group, was designed and synthesized. Their photophysical, both in solution and in the solid state, and structural properties were investigated. The influence of donor and acceptor substituents (R) in the benzothiazole unit on photophysical properties of complexes was found out. The tetrafluorobenzothiazole analogue exhibits nonbonded nuclear spin-spin coupling between fluorines from the BF2 group and α-fluorine atom at the benzene ring. Additionally, this boron complex demonstrates a comparatively high solid-state fluorescence quantum yield (Φ = 0.34).

N,O π-Conjugated 4-Substituted 1,3-Thiazole BF2 Complexes: Synthesis and Photophysical Properties.

A series of 1,3-thiazole-based organoboron complexes has been designed and synthesized by acylation of 2-amino 4-subsituted 1,3-thiazoles with (4-dimethylamino)benzoyl chloride and the subsequent BF2 complexation reaction. The influence of substituents in position 4 of the thiazole ring on photophysical properties of the complexes has been investigated. Synthesized thiazolo[3,2-c][1,3,5,2]oxadiazaborinines mainly showed intensive fluorescence in solutions. Complex with a 4,5-unsubstituted thiazole unit demonstrated an aggregation induced emission (AIE) effect and a very high fluorescent quantum yield (94%) in the solid state because of the inhibition of π-π/π-n interactions in the molecular packing.

Comparative study of molecular recognition of folic acid subunits with cyclodextrins.

The complexation of pteroic acid and pterine, subunits of folic acid, with native cyclodextrins (α‒, ß‒, and γ‒CDs) was studied in solution (UV-vis), and in the solid state (thermal analysis, IR and Raman). UV-vis titrations at pH = 7.4 provided data regarding stoichiometry of the formed complexes as well as their associations constants. Stability of the complexes increases in the series: γ‒CD < ß‒CD << α‒CD for pterine, and γ‒CD < α‒CD << ß‒CD for pteroic acid. The mode of complexation was further exploited by molecular modeling studies (docking studies, PM6) showing additionally changes in conformation of pteroic acid upon complexation. Comparison of the association constants of the complexes of pterine and pteroic acid with native cyclodextrins with data obtained for analogous complexes with folic acid shows that all folic acid complexes are less stable than those formed from its subunits.

Structural diversity in the host-guest complexes of the antifolate pemetrexed with native cyclodextrins: gas phase, solution and solid state studies.

The complexation of the antifolate pemetrexed (PTX) with native cyclodextrins was studied. This process, along with the findings gathered for the structurally related folic acid was treated as a model for exploiting host-guest interactions of this class of guest molecules in the gas phase, in solution and in the solid state. Mass spectrometry was employed for the investigation of the architecture and relative gas-phase stabilities of these supramolecular complexes. The mode of complexation was further tracked by 1D and 2D NMR proving the formation of the exclusion-type complex with α-CD and pseudorotaxane inclusion-type complexes with ß-, and γ-CDs. UV-vis titrations at pH 7.4 gave association constants for the obtained complexes. The stability of the complexes increases in the series: α-CD/PTX < γ-CD/PTX << ß-CD/PTX. The association of PTX with a monomer cyclodextrin equivalent - methyl α-D-glucopyranoside - was investigated for a deeper understanding of the type of host-guest interactions. Solid state studies of PTX/CDs were performed using FTIR-ATR and Raman spectroscopy techniques.

Folate appended cyclodextrins for drug, DNA, and siRNA delivery.

Drug, DNA, and siRNA delivery systems based on cyclodextrin (CD) core and connected with folate (FA) via various linkers are presented. They include simple mono-derivatized cyclodextrins as well as cyclodextrins with higher degree of substitution, both in their primary and secondary sides. Examples of simple polymers and dendrimers are also discussed. Such carriers possess properties inherent to both of their components. Cyclodextrin provides the ability to encapsulate organic molecules in its inner cavity, thus improving their solubility in water, bioavailability, and stability, while FA assures targeting folate receptor overexpressing cancer cells. Drug delivery systems loaded with drugs such as e.g. methotrexate, doxorubicin, paclitaxel, vinblastine, and docetaxel were found to have superior properties as compared to the free drug. Dendritic folate appended cyclodextrins were also found to be good sustained release systems for DNA and siRNA. The recent progress in the synthesis and drug, DNA, and siRNA delivery application of folate appended cyclodextrins is presented.

Structural elucidation of specific noncovalent association of folic acid with native cyclodextrins using an ion mobility mass spectrometry and theoretical approach.

The combination of ion mobility mass spectrometry studies and theoretical calculations including docking studies permitted a detailed structural description of noncovalent complexes of folic acid (FA) and native cyclodextrins (α-CD, ß-CD, and γ-CD). The mode of noncovalent association depended on the cavity size of the cyclodextrin. The structure of FA/α-CD represented the exclusion complex in which the aminobenzoic moiety and the aromatic pteridine ring of folic acid remain outside the cyclodextrin cavity, while the glutamate residue is anchored in the interior of the α-cyclodextrin. A rotaxane-type structure was proposed for the FA/ß-CD complex with the aminobenzoic part of FA being trapped in the central cavity of ß-CD. The glutamate residue and the aromatic pteridine ring interact with the primary and secondary rim hydroxyl residues, respectively, enhancing complex stability. Two possible structures of FA/γ-CD were suggested, the first one being analogous to the FA/ß-CD complex and the second one being more stable-in which the aromatic pteridine ring penetrates into the CD cavity while the glutamate residue with the aminobenzoic part of FA is exposed to the cone exterior of CD at its wider edge. Further insight into the association behavior of the folic acid toward cyclodextrins evaluated by thermodynamic calculations indicates that the process is highly exothermic. The complex stability increased in the order FA/α-CD < FA/ß-CD < FA/γ-CD. This order is consistent with the previously determined relative gas-phase stability established based on the dissociation efficiency curves of the FA/CD complexes.

Synthesis of polyhydroxylated quinolizidines and azaspiro[4.5]decanes from D-xylose.

The synthesis of novel polyhydroxylated quinolizidines and azaspiro[4.5]decanes is reported. A key step of this transformation involved an addition of allylmagnesium bromide to an ω-bromonitrile derived from D-xylose followed by an intramolecular displacement of a bromide. The resulting cyclic imine was treated either with allylmagnesium bromide or with NaBH4, to provide 2,2-diallyl- or 2-allylpiperidine, respectively. The desired bicyclic framework was constructed via a ring-closing metathesis reaction. The Ru catalysts were reused in the following syn-dihydroxylation step.

ß-Cyclodextrin as the suitable molecular container for isopulegol enantiomers.

Isopulegol, an insoluble in water and highly volatile compound, due to its neuroactive properties is a potentially important agent for medical applications. Formation of "host-guest" molecular complexes with cyclodextrins would lead to the increase of its water solubility and bioavailability. Interactions between native cyclodextrins (α, ß and γ) and isopulegol enantiomers were studied in solution proving the formation of inclusion complexes for ß- and γ-cyclodextrins. For the more stable complexes with ß-cyclodextrin crystal structures were obtained showing the formation of molecular capsules forming molecular container able to accommodate two guest molecules.

Interactions with ß-cyclodextrin as a way for encapsulation and separation of camphene and fenchene.

The separation of isomeric monoterpenes, camphene and fenchene by complexation with ß-cyclodextrin is presented. Both of the monoterpenes form complexes with ß-cyclodextrin (as shown by both gas chromatography and (1)H NMR) with similar stability constants nevertheless it is possible to separate them by re-crystallization. The crystal structure of ß-cyclodextrin with fenchene was also studied by X-ray diffraction.

Structural diversity in native cyclodextrins/folic acid complexes--from [2]-rotaxane to exclusion compound.

The formation of different complexes of folic acid depending on the size of the host cyclodextrin resulting in either an exclusion compound (with the smallest α-cyclodextrin) or 2-rotaxane, where cyclodextrin is threaded over folic acid (with ß- and γ-cyclodextrins), is presented. The formation is carried out in water which allows both possible application in pharmaceutical sciences and usage of environmentally friendly "green chemistry". The obtained compounds are thoroughly characterized using one and two dimensional NMR, mass spectrometry, differential scanning calorimetry and thermogravimetric analysis.