Dramatically Enhanced Fluorescence of Heteroaromatic Chromophores upon Insertion as Spacers into Oligo(triacetylene)s

In continuation of a previous study on the modulation of π‐electron conjugation of oligo(triacetylene)s by insertion of central hetero‐spacer fragments between two (E)‐hex‐3‐ene‐1,5‐diyne ((E)‐1,2‐diethynylethene, DEE) moieties (Fig. 1), a new series of trimeric hybrid oligomers ( 14 – 18 and 22 – 24 , Fig. 2) were prepared (Schemes 1–3). Spacers used were both electron‐deficient (quinoxaline‐based heterocycles, pyridazine) and electron‐rich (2,2′‐bithiophene, 9,9‐dioctyl‐9H‐fluorene) chromophores. With 19–21 (Scheme 4), a series of transition metal complexes was synthesized as potential precursors for nanoscale scaffolding based on both covalent acetylenic coupling and supramolecular assembly. The UV/VIS spectra (Fig. 3) revealed that the majority of spacers provided hetero‐trimers featuring extended π‐electron delocalization. The new hybrid chromophores show a dramatically enhanced fluorescence compared with the DEE dimer 13 and homo‐trimer 12 (Fig. 5). This increase in emission intensity appears as a general feature of these systems: even if the spacer molecule is non‐fluorescent, the corresponding hetero‐trimer may show a strong emission (Table 2). The redox properties of the new hybrid chromophores were determined by cyclic voltammetry (CV) and rotating‐disk voltammetry (RDV) (Table 3 and Fig. 5). In each case, the first one‐electron reduction step in the hetero‐trimers appeared anodically shifted compared with DEE dimer 13 and homo‐trimer 12 . With larger spacer chromophore extending into two dimensions (as in 14 – 18 , Fig. 2), the anodic shift (by 240–490 mV, Table 3) seems to originate from inductive effects of the two strongly electron‐accepting DEE substituents rather than from extended π‐electron conjugation along the oligomeric backbone, as had previously been observed for DEE‐substituted porphyrins.     

Synthesis of Amphiphilic Fullerene Derivatives and Their Incorporation in Langmuir and Langmuir‐Blodgett Films

Various amphiphilic fullerene derivatives were prepared by functionalization of [5,6]fullerene‐C₆₀‐I_h (C₆₀) with malonate or bis‐malonate derivatives obtained by esterification of the malonic acid mono‐esters 5 – 7 . Cyclopropafullerene 10 was obtained by protection of the carboxylic acid function of 6 as a tert‐butyl ester, followed by Bingel addition to C₆₀ and a deprotection step (Scheme 2). The preparation of 10 was also attempted directly from the malonic acid mono‐ester 6 under Bingel conditions. Surprisingly, the corresponding 3′‐iodo‐3′H‐cyclopropa[1,9][5,6]fullerene‐C₆₀‐I_h‐3′‐carboxylate 11 was formed instead of 10 (Scheme 3). The general character of this new reaction was confirmed by the preparation of 15 and 16 from the malonic acid mono‐esters 13 and 14 , respectively (Scheme 4). All the other amphiphilic fullerene derivatives were prepared by taking advantage of the versatile regioselective reaction developed by Diederich and co‐workers which led to macrocyclic bis‐adducts of C₆₀ by a cyclization reaction at the C‐sphere with bis‐malonate derivatives in a double Bingel cyclopropanation. The bis‐adducts 37 – 39 with a carboxylic acid polar head group and four pendant long alkyl chains of different length were prepared from diol 22 and acids 5 – 7 , respectively (Scheme 9). In addition, the amphiphilic fullerene derivatives 45, 46, 49, 54 , and 55 bearing different polar head groups and compound 19 with no polar head group were synthesized (Schemes 11–13, 15, and 5, resp.). The ability of all these compounds to form Langmuir monolayers at the air‐water interface was investigated in a systematic study. The films at the water surface were characterized by their surface pressure vs. molecular area isotherms, compression and expansion cycles, and Brewster‐angle microscopy. The spreading behavior of compound 10 was not good, the two long alkyl chains in 10 being insufficient to prevent aggregation resulting from the strong fullerene‐fullerene interactions. While no films could be obtained from compound 19 with no polar head group, all the corresponding amphiphilic fullerene bis‐adducts showed good spreading characteristics and reversible behavior upon successive compression/expansion cycles. The encapsulation of the fullerene in a cyclic addend surrounded by four long alkyl chains is, therefore, an efficient strategy to prevent the irreversible aggregation resulting from strong fullerene‐fullerene interactions usually observed for amphiphilic C₆₀ derivatives at the air‐water interface. The balance of hydrophobicity to hydrophilicity was modulated by changing the length of the surrounding alkyl chains or the nature of the polar head group. The best results in terms of film formation and stability were obtained with the compounds having the largest polar head group, i.e. 45 and 46 , and dodecyl chains. Finally, the Langmuir films obtained from the amphiphilic fullerene bis‐adducts were transferred onto solid substrates, yielding high‐quality Langmuir‐Blodgett films.     

Protein Protection by Antioxidants: Development of a Convenient Assay and Structure‐Activity Relationships of Natural Polyphenols

In this work, a convenient test of antioxidant activity was developed, with BChE‐contaminated HSA as the target of AAPH‐induced oxidation and its esterase activity as the marker of protein integrity or degradation. The method is relatively simple, of low cost, and convenient to use. Its application to natural polyphenols showed that quercetin ( 1 ), verbascoside ( 2 ), chlorogenic acid ( 3 ), caffeic acid ( 4 ), 1,3,6,7‐tetrahydroxyxanthone ( 5 ), and mangiferin ( 6 ), are good antioxidants (IC₅₀<9 μM ). 1,5‐Dihydroxy‐3‐methoxyxanthone ( 7 ), flemichin D ( 8 ), and cordigone ( 9 ) showed modest activities (ca. 50 μM <IC₅₀<350 μM ), whereas danthrone ( 10 ) was inactive. Complementary experiments with two of the more active antioxidants, namely quercetin ( 1 ) and chlorogenic acid ( 3 ) showed that both antioxidants were better radical scavengers than chain‐breaking antioxidants. The relative adiabatic oxidation potential (ΔH_ox), the relative H‐bond dissociation energy (ΔH_abs), and the first oxidation potential measured by cyclic voltammetry were found to be related to the radical‐scavenging activity of these antioxidants.     

Facile Synthesis of Diastereoisomerically and Optically Pure 2‐Substituted Hexahydro‐1H‐pyrrolizin‐3‐ones

We report a short synthetic route that provides optically active 2‐substituted hexahydro‐1H‐pyrrolizin‐3‐ones in four steps from commercially available Boc (tert‐but(oxy)carbonyl))‐protected proline. Diastereoisomers (−)‐ 11 and (−)‐ 12 were assembled from the proline‐derived aldehyde (−)‐ 8 and ylide 9 via a Wittig reaction and subsequent catalytic hydrogenation (Scheme 3). Cleavage of the Boc protecting group under acidic conditions, followed by intramolecular cyclization, afforded the desired hexahydro‐1H‐pyrrolizinones (−)‐ 1 and (+)‐ 13 . Applying the same protocol to ylide 19 afforded hexahydro‐1H‐pyrrolizinones (−)‐ 25 and (−)‐ 26 (Scheme 5). The absolute configuration of the target compounds was determined by a combination of NMR studies (Figs. 1 and 2) and X‐ray crystallographic analysis (Fig. 3).     

N‐Arylated 3,5‐Dihydro‐4H‐dinaphtho[2,1‐c:1′,2′‐e]azepines: Axially Chiral Donors with High Helical Twisting Powers for Nonplanar Push–Pull Chromophores

Axially chiral, N‐arylated 3,5‐dihydro‐4H‐dinaphtho[2,1‐c:1′,2′‐e]azepines have been prepared by short synthetic protocols from enantiopure 1,1′‐bi(2,2′‐naphthol) (BINOL) and anilines. Alkynes substituted with two N‐phenyldinaphthazepine donors readily undergo a formal [2+2] cycloaddition, followed by retro‐electrocyclization, with tetracyanoethene (TCNE) to yield donor‐substituted 1,1,4,4‐tetracyanobuta‐1,3‐dienes (TCBDs) featuring intense intramolecular charge‐transfer (CT) interactions. A dicyanovinyl derivative substituted with one N‐phenyldinaphthazepine donor was obtained by a “one‐pot” oxidation/Knoevenagel condensation from the corresponding propargylic alcohol. Comparative electrochemical, X‐ray crystallographic, and UV/Vis studies show that the electron‐donor qualities of N‐phenyldinaphthazepine are similar to those of N,N‐dimethylanilino residues. The circular dichroism (CD) spectrum of a push–pull chromophore incorporating the chiral donor moiety features Cotton effects of exceptional intensity. With their elongated shape and the rigidity of the chiral N‐aryldinaphthazepine donors, these chromophores are effective inducers of twist distortion in nematic liquid crystals (LCs). Thus, a series of the dinaphthazepine derivatives was used as dopants in the nematic LC E7 (Merck) and high helical twisting powers (β) of the order of hundreds of μm^?1 were measured. Theoretical calculations were employed to elucidate the relation between the structure of the dopants and their helical twisting power. For the derivatives with two dinaphthazepine moieties, a strong dependence of the β‐values on the structure and conformation of the linker between them was found.     

Air‐Stable {(C5H5)Co} Catalysts for [2+2+2] Cycloadditions

Cobalt cyclopentadienyl complexes incorporating a fumarate and a CO ligand (see picture) efficiently catalyze inter‐ and intramolecular [2+2+2] cycloadditions of alkynes, nitriles, and/or alkenes to give benzenes, pyridines, or 1,3‐cyclohexadienes. Unlike catalysts such as [CpCo(CO)₂] or [CpCo(C₂H₄)₂] (Cp=C₅H₅), they are air‐stable, easy to handle, compatible with microwave conditions, and do not necessarily require irradiation to be active.

     

Contrast or assimilation: choosing camps in simple or realistic modeling

The contrast effect is a psychological phenomenon in which people exaggerate their differences. When making social comparisons, people cope with the discomforts associated with negative comparisons by criticizing others and splitting into smaller groups of similar others or by assimilation, depending on whether they perceive themselves as the member of a majority or minority group, respectively. This contrast or assimilation phenomena can explain the exaggeration of differences among computational simulation scholars. Those exaggerations are discussed in this paper as well as some realistic reasons for differences. Recognition of our status as minority group members and the virtues of assimilation and mutual support are advocated.

Synthesis and characteristics of a nonaggregating tris(tetrathiafulvaleno)dodecadehydro[18]annulene

A new tris(tetrathiafulvaleno)dodecadehydro[18]annulene with six peripheral n-hexyl substituents was prepared by oxidative Glaser-Hay cyclization of a corresponding diethynylated tetrathiafulvalene (TTF) precursor. The electronic properties of the neutral and oxidized species were studied by both UV/Vis absorption spectroscopy and electrochemistry. From these studies, it transpires that the strongly violet-colored macrocycle does not aggregate in solution to any significant degree, which was confirmed by (1)H NMR spectroscopy. This reluctance towards aggregation contrasts that observed for related TTF-annulenes containing other peripheral substitutents. Oxidation of the TTF-annulene occurs in two three-electron steps as inferred from both the peak amplitudes and the spectroelectrochemical study. We find that the tris(TTF)-fused dehydro[18]annulene is more difficult to oxidize (by +0.20 V) than the silyl-protected diethynylated mono-TTF precursor. In contrast, the first vertical ionization energy calculated at the B3 LYP/6-311+G(2d,p) level for the parent tris(TTF)-fused dehydro[18]annulene devoid of peripheral hexyl substituents is in fact lower (by 0.44 eV). Moreover, the surface morphology of 1 d drop-cast on a mica substrate was investigated by atomic force microscopy (AFM). Crystalline domains with slightly different orientations were observed. The thickness of individual layers seen in the crystalline domains and the thickness of a monolayer obtained from a very dilute solution were determined to 1.8-1.9 nm. This thickness corresponds to the diameter of the macrocycle and the layers seen in the film are apparently formed when the molecules stack in the horizontal direction relative to the substrate.     

Why are ionic liquids liquid? A simple explanation based on lattice and solvation energies

We have developed a simple and quantitative explanation for the relatively low melting temperatures of ionic liquids (ILs). The basic concept was to assess the Gibbs free energy of fusion (Delta(fus)G) for the process IL(s) --> IL(l), which relates to the melting point of the IL. This was done using a suitable Born-Fajans-Haber cycle that was closed by the lattice (i.e., IL(s) --> IL(g)) Gibbs energy and the solvation (i.e., IL(g) --> IL(l)) Gibbs energies of the constituent ions in the molten salt. As part of this project we synthesized and determined accurate melting points (by DSC) and dielectric constants (by dielectric spectroscopy) for 14 ionic liquids based on four common anions and nine common cations. Lattice free energies (Delta(latt)G) were estimated using a combination of Volume Based Thermodynamics (VBT) and quantum chemical calculations. Free energies of solvation (Delta(solv)G) of each ion in the bulk molten salt were calculated using the COSMO solvation model and the experimental dielectric constants. Under standard ambient conditions (298.15 K and 10(5) Pa) Delta(fus)G degrees was found to be negative for all the ILs studied, as expected for liquid samples. Thus, these ILs are liquid under standard ambient conditions because the liquid state is thermodynamically favorable, due to the large size and conformational flexibility of the ions involved, which leads to small lattice enthalpies and large entropy changes that favor melting. This model can be used to predict the melting temperatures and dielectric constants of ILs with good accuracy. A comparison of the predicted vs experimental melting points for nine of the ILs (excluding those where no melting transition was observed and two outliers that were not well described by the model) gave a standard error of the estimate (s(est)) of 8 degrees C. A similar comparison for dielectric constant predictions gave s(est) as 2.5 units. Thus, from very little experimental and computational data it is possible to predict fundamental properties such as melting points and dielectric constants of ionic liquids.