Solubilization of Polycyclic Aromatics in Water by γ‐Cyclodextrin Derivatives

A series of hydrophilic per‐6‐thio‐6‐deoxy‐γ‐cyclodextrins (CDs) were synthesized from per‐6‐iodo‐6‐deoxy‐γ‐CD. These new hosts are able to solubilize polycyclic aromatic guests in aqueous solution to much higher extents than native CDs. Phase‐solubility diagrams were mostly linear in accordance with both 1:1 and 1:2 CD–guest complexes in aqueous solution. The stoichiometry of the inclusion complexes was further investigated by fluorescence spectroscopy, which revealed very pronounced Stokes shifts typical for 1:2 complexes. This finding was further consolidated by quantum mechanical calculations of dimer formation of the guests and space‐filling considerations by using the cross‐sectional areas of the CDs and guests. The calculated dimerization energies correlated well with the binding free energies measured for the 1:2 complexes, and provided the main contribution to the driving force of complexation in the γ‐CD cavity.     

Z‐Shaped Pyrrolo[3,2‐b]pyrroles and Their Transformation into π‐Expanded Indolo[3,2‐b]indoles

Sterically hindered 1,4‐dihydropyrrolo[3,2‐b]pyrroles possessing ortho‐(arylethynyl)phenyl substituents at positions‐2 and ‐5 were efficiently synthesized through a sila‐Sonogashira reaction. These unique Z‐shaped dyes showed relatively strong fluorescence in solution. Detailed optimization revealed that, in the presence of InCl₃, these alkynes readily undergo an intramolecular double cyclization to give hexacyclic products bearing an indolo[3,2‐b]indole skeleton in remarkable yields. Steady‐state UV–visible spectroscopy revealed that upon photoexcitation, the prepared Z‐shaped alkynes undergo mostly radiative relaxation leading to high fluorescence quantum yields. In the case of 7,14‐dihydrobenzo[g]benzo[6,7]indolo[3,2‐b]indoles, we believe that the substantial planarization of geometry in the excited state, is the underlying reason for the observed large Stokes shifts. The presence of additional electron‐withdrawing groups makes it possible to further alter the photophysical properties. The two‐photon absorption cross‐section values of both families of dyes were found to be modest and the nature of the excited state responsible for two‐photon absorption appeared to be strongly affected by the presence of peripheral groups. Serendipitous synthesis of unusual double‐Z‐shaped alkyne by Sonogashira and Glaser coupling is also reported.     

Facile Preparation of α‐Cyano‐α,ω‐Diaryloligovinylenes: A New Class of Color‐Tunable Solid Emitters

An efficient Knoevenagel condensation reaction was used to construct a series of α‐cyano‐α,ω‐diaryloligovinylenes, which show prominent fluorescence emission in the solid state. On investigating the effect of conjugation length on fluorescent properties, we found that the diene structure showed superior solid‐state luminescence. Furthermore, the emission color could be adjusted by introducing donor or acceptor functional groups at the terminal aryl groups. Full‐color emission in the visible region can be achieved by adding different functional groups to the α‐cyano‐α,ω‐diaryldivinylene moiety. The structure–property relationships were elucidated and some observations such as the substitution position effects were discussed. These compounds have potential applications as full‐color solid emissive candidates in material science and their simple structures allow them to be easily modified resulting in further interesting properties.     

Highly Efficient and Directional Homo‐ and Heterodimeric Energy Transfer Materials Based on Fluorescently Derivatized α,γ‐Cyclic Octapeptides

Cyclic octapeptides composed of α‐amino acids alternated with cis‐3‐aminocycloalkanecarboxylic acids, self‐assemble as drumlike dimers through β‐sheet‐like, backbone‐to‐backbone hydrogen bonding. Heterodimerization appears to be significantly more favored than homodimerization, and this represents a novel approach for the design and fabrication of highly stable heterodimeric assemblies. A multicomponent equilibrium network based on fluorescently derivatized self‐assembling α,γ‐cyclic octapeptides has been successfully used to form light‐harvesting/light‐converting ensembles with a distinctive organization of donor and acceptor units able to act as efficient artificial photosystems.     

γ‐Cyclodextrin Derivatives Bearing Thymolphthalein Dyes in Their Secondary Hydroxyl Side as Guest‐Responsive Color Change Indicators

Two geometrically isomeric γ‐cyclodextrin derivatives with a thymolphthalein moiety in the secondary hydroxyl side were prepared as guest‐responsive color change indicators. The isomers exhibit a pH dependence of their absorption spectra and a remarkable change in the absorbance around 610 nm upon guest addition. One of the isomers formed complexes of 2 : 1, the other of 1 : 1 stoichiometry (host/guest).     

Developing Intense Blue and Magenta Colors in α‐LiZnBO3: The Role of 3d‐Metal Substitution and Coordination

We describe the synthesis, crystal structures, and optical absorption spectra/colors of 3d‐transition‐metal‐substituted α‐LiZnBO₃ derivatives: α‐LiZn_1?xM^II_xBO₃ (M^II=Co^II (0<x<0.50), Ni^II (0<x≤0.05), Cu^II (0<x≤0.10)) and α‐Li_1+xZn_1?2xM^III_xBO₃ (M^III=Mn^III (0<x≤0.10), Fe^III (0<x≤0.25)). The crystal structure of the host α‐LiZnBO₃, which is both disordered and distorted with respect to Li and Zn occupancies and coordination geometries, is largely retained in the derivatives, which gives rise to unique colors (blue for Co^II, magenta for Ni^II, violet for Cu^II) that could be of significance for the development of new, inexpensive, and environmentally friendly pigment materials, particularly in the case of the blue pigments. Accordingly, this work identifies distorted tetrahedral MO₄ (M=Co, Ni, Cu) structural units, with a long M?O bond that results in trigonal bipyramidal geometry, as new chromophores for blue, magenta, and violet colors in a α‐LiZnBO₃ host. From the L*a*b* color coordinates, we found that Co‐substituted compounds have an intense blue color that is stronger than that of CoAl₂O₄ and YIn_0.90Mn_0.10O₃. The near‐infrared (NIR) reflectance spectral studies indicate that these compounds exhibit a moderate IR reflectivity that could be significant for applications as “cool pigments”.     

Cooperative Recognition of a Copper Cation and Anion by a Calix[4]arene Substituted at the Lower Rim by a β‐Amino‐α,β‐Unsaturated Ketone

Chemodosimeter : Upon titration with Cu^II, the fluorescence of 1 showed strong fluorescence enhancement because the metal ion helps to lock the conformation of the fluorophores. Cu^II was reduced to Cu^I by the free phenolic OH group of 1 , the phenol was then oxidised by Cu^II and assisted in the trapping of Cu^I. Ditopic behaviour was observed for 1? Cu^I, which showed further enhancement of its fluorescence intensity upon complexation with anions (as depicted, A^?=CH₃COO^? or F^?).

     

Synthesis and Photovoltaic Properties of Boron β‐Ketoiminate Dyes Forming a Linear Donor‐π‐Acceptor Structure

Organoboron complexes are of interest as chromophores for dye sensitizers owing to their light‐harvesting and carrier‐transporting properties. In this study, compounds containing boron β‐ketoiminate (BKI) as a chromophore were synthesized and used as dye sensitizers in dye‐sensitized solar cells. The new dyes were orange or red crystals and showed maximum absorptions in the 410–450 nm wavelength region on titanium dioxide substrates. These electrodes exhibited maximum efficiencies of over 80% in incident photon‐to‐current conversion efficiency spectra, suggesting that the continuous process of light absorption‐excitation‐electron injection was effectively performed. Open‐circuit photovoltages were relatively high owing to the large dipole moments of the BKI dyes with a linear molecular structure. Thus, a maximum power conversion efficiency of 5.3% was successfully observed. Comparison of BKI dyes with boron β‐diketonate dyes revealed certain differences in solution stability, spectral properties, and photovoltaic characteristics.     

β‐Cyclodextrin Pseudopolyrotaxanes with π‐Conjugated Polymer Axles

Summary: β‐Cyclodextrin (β‐CD) pseudopolyrotaxanes containing poly(thiophene‐2,5‐diyl), PTh , or poly(3‐methylthiophene‐2,5‐diyl)s, P3MeTh s, as an axle were prepared. Structures of the pseudopolyrotaxanes and their inclusion behavior with β‐CD were investigated. The UV‐vis measurements revealed that inclusion of P3MeTh s by β‐CD depended on the flexibility of the main chain and their molecular weight.

Formation of the inclusion complex of β‐CD and PTh .     

“Reduced” Coumarin Dyes with an O‐Phosphorylated 2,2‐Dimethyl‐4‐(hydroxymethyl)‐1,2,3,4‐tetrahydroquinoline Fragment: Synthesis,Spectra, and STED Microscopy

Large Stokes‐shift coumarin dyes with an O‐phosphorylated 4‐(hydroxymethyl)‐2,2‐dimethyl‐1,2,3,4‐tetrahydroquinoline fragment emitting in the blue, green, and red regions of the visible spectrum were synthesized. For this purpose, N‐substituted and O‐protected 1,2‐dihydro‐7‐hydroxy‐2,2,4‐trimethylquinoline was oxidized with SeO₂ to the corresponding α,β‐unsaturated aldehyde and then reduced with NaBH₄ in a “one‐pot” fashion to yield N‐substituted and 7‐O‐protected 4‐(hydroxymethyl)‐7‐hydroxy‐2,2‐dimethyl‐1,2,3,4‐tetrahydroquinoline as a common precursor to all the coumarin dyes reported here. The photophysical properties of the new dyes (“reduced coumarins”) and 1,2‐dihydroquinoline analogues (formal precursors) with a trisubstituted C=C bond were compared. The “reduced coumarins” were found to be more photoresistant and brighter than their 1,2‐dihydroquinoline counterparts. Free carboxylate analogues, as well as their antibody conjugates (obtained from N‐hydroxysuccinimidyl esters) were also prepared. All studied conjugates with secondary antibodies afforded high specificity and were suitable for fluorescence microscopy. The red‐emitting coumarin dye bearing a betaine fragment at the C‐3‐position showed excellent performance in stimulation emission depletion (STED) microscopy.     

Luminescent Materials: Locking π‐Conjugated and Heterocyclic Ligands with Boron(III)

Multidisciplinary research on novel organic luminescent dyes is propelled by potential applications in plastic electronics and biomedical sciences. The construction of sophisticated fluorescent dyes around a tetrahedral boron(III) center is a particular approach that has fueled the creativity of chemists. Success in this enterprise has been readily achieved with simple synthetic protocols, the products of which display unusual spectroscopic behavior. This account is a critical review of recent advances in the field of boron(III) complexes (excluding BODIPYs and acetylacetonate boron complexes) involving species displaying similar coordination features, and we outline their potential development in several disciplines.     

α‐Cyclodextrin and methylmercury chloride: a new strategy to recover organomercurials

Methylmercury (MeHg) is one of the most toxic forms of mercury in the environment. It can be accumulated in fish through the food chain; after, consumption the fish is then dangerous to fetuses and younger children, causing abnormal brain development and nervous system disorders. Cyclodextrins (CDs) are cyclic oligosaccharides consisting of six, seven or eight units of glucose. In accord with the dimensions and hydrophilic–lipophilic properties, one can obtain inclusion of hydrophobic guests in a CD cavity. In the present work we used this characteristic of CD to obtain an inclusion compound between MeHgCl and the α‐cyclodextrin, looking for a new method to reduce MeHgCl toxicity and pre‐concentration. The inclusion compound was characterized through IR, ¹H, ¹³C NMR and Raman spectroscopy. ­X‐ray diffraction and thermal analysis (TG, DTG and DSC) methods were also used. Finally, biological tests were carried out and the minimum inhibitory concentrations (MICs) for MeHgCl, α‐cyclodextrin, the MeHgCl–CD complex and a physical mixture were determined. This host–guest strategy using cyclodextrins offers an alternative and powerful method for mercury remediation. Copyright © 2000 John Wiley & Sons, Ltd.     

Inclusion Behavior of β‐Cyclodextrin with Bipyridine Molecules: Factors Governing Host‐Guest Inclusion Geometries

Guest Effect : The differences of nitrogen atom positions and the bridge bonds linked to two pyridine rings of some bipyridine guests can significantly affect the binding abilities and inclusion geometries of β‐cyclodextrin with the guests in both the solution and solid states.

     

Efficient NIR‐Light Emission from Solid‐State Complexes of Boron Difluoride with 2′‐Hydroxychalcone Derivatives

This article describes a series of nine complexes of boron difluoride with 2′‐hydroxychacone derivatives. These dyes were synthesized very simply and exhibited intense NIR emission in the solid state. Complexation with boron was shown to impart very strong donor–acceptor character into the excited state of these dyes, which further shifted their emission towards the NIR region (up to 855 nm for dye 5 b , which contained the strongly donating triphenylamine group). Strikingly, these optical features were obtained for crystalline solids, which are characterized by high molecular order and tight packing, two features that are conventionally believed to be detrimental to luminescence in organic crystals. Remarkably, the emission of light from the π‐stacked molecules did not occur at the expense of the emission quantum yield. Indeed, in the case of pyrene‐containing dye 4 , for example, a fluorescence quantum yield of about 15 % with a fluorescence emission maximum at 755 nm were obtained in the solid state. Moreover, dye 3 a and acetonaphthone‐based compounds 1 b , 2 b , and 3 b showed no evidence of degradation as solutions in CH₂Cl₂ that contained EtOH. In particular, solutions of brightly fluorescent compound 3 a (brightness: ε×Φ_f=45 000 M ^?1 cm^?1) could be stored for long periods without any detectable changes in its optical properties. All together, these new dyes possess a set of very interesting properties that make them promising solid‐state NIR fluorophores for applications in materials science.