Poly(triacetylene) Oligomers: Conformational Analysis by X‐Ray Crystallography and Synthesis of a 17.8‐nm‐Long Monodisperse 24‐mer

Starting from the octameric poly(triacetylene) (PTA) oligomer 1e as a large `macromonomer', the monodisperse tetracosamer (24‐mer) 1h was prepared by a previously introduced statistical deprotection‐oligomerization sequence (Scheme). It is the longest known molecular rod featuring a fully conjugated, non‐aromatic all‐carbon backbone. Matrix‐assisted laser‐desorption‐ionization time‐of‐flight (MALDI‐TOF) mass spectrometry was particularly useful in the characterization of oligomer 1h and clearly demonstrated its monodispersity (Fig. 1). In an effort to further clarify the conformational preferences of PTA oligomers, the X‐ray crystal structure of the 3.2‐nm‐long tetramer 1c was solved (Figs. 2 – 4). In the solid state, the C=C bonds in 1c all adopt the s‐trans conformation with respect to the buta‐1,3‐diynediyl moieties. The π‐conjugated system is perfectly planar, with the squared sum of the deviations of the backbone C‐atoms from the best plane amounting to 0.077 Ų. Analysis of the crystal lattice revealed a layered structure, in which the π‐conjugated backbone of one oligomer is insulated by the trialkylsilyl groups of adjacent oligomers in neighboring layers.     

FT Raman and DFT Study on a Series of All‐anti Oligothienoacenes End‐Capped with Triisopropylsilyl Groups

Herein, we study the π‐conjugational properties of a homologous series of all‐anti oligothienoacenes containing four to eight fused thiophene rings by means of FT Raman spectroscopy and DFT calculations. The theoretical analysis of the spectroscopic data provides evidence that selective enhancement of a very limited number of Raman scatterings is related to the occurrence in these oligothienoacenes of strong vibronic coupling between collective ν(C?C) stretching modes in the 1600–1300 cm^?1 region and the HOMO/LUMO frontier orbitals (HOMO=highest occupied molecular orbital; LUMO=lowest unoccupied molecular orbital). The correlation of the Raman spectroscopic data and theoretical results for these all‐anti oligothienoacenes with those previously collected for a number of all‐syn oligothienohelicenes gives further support to the expectation that cross‐conjugation is dominant in heterohelicenes. Fully planar all‐anti oligothienoacenes display linear π conjugation which seemingly does not reach saturation with increasing number of annulated thiophene rings in the oligomeric chain at least up to the octamer.     

Synthesis and Photophysical Properties of Dimethoxybis(3,3,3‐trifluoropropen‐1‐yl)benzenes: Compact Chromophores Exhibiting Violet Fluorescence in the Solid State

Dimethoxybis(3,3,3‐trifluo‐ropropen‐1‐yl)benzenes were prepared through palladium‐catalyzed double cross‐coupling reactions of diiododimethoxybenzenes with CF₃C≡CZnCl, followed by reduction of CF₃C≡C groups with LiAlH₄ or H₂ in the presence of the Lindlar catalyst. The edges of the absorption spectra of 1,2‐(MeO)₂‐4,5‐(CF₃CHC=CH)₂benzenes 1 and 1,3‐(MeO)₂‐4,6‐(CF₃CH=CH)₂benzenes 2 in cyclohexane ranged from 348 to 360 nm, whereas the absorption spectra of 1,4‐(MeO)₂‐2,5‐[(E)‐CF₃CH=CH]₂ benzene ((E)‐ 3 ) ended at 406 nm. These findings indicate that the effective conjugation length of (E)‐ 3 was significantly larger than those of 1 and 2 . Consistently, 1 and 2 in cyclohexane exhibited fluorescence with emission maxima in the UV region, whereas (E)‐ 3 in cyclohexane emitted violet light with an emission maximum at 407 nm. All the fluorescence spectra of 1 – 3 in various solvents redshifted as the solvent polarity increased. The photoluminescence of 1 , E‐1 , Z‐1 , 2 , E‐2 , E‐2H , Z‐2 , E‐3 , E‐3H , Z‐3 in the solid states was also observed with emission maxima in the violet region. It is important to note that the quantum yields of (E)‐ 3 in a neat thin film and in a doped polymer film were 0.37 and 0.49, respectively. Density functional theory calculations suggested that the fluorine atoms contribute to a slight extension of both the HOMOs and the LUMOs, as well as narrowing of the HOMO–LUMO gaps when compared with the corresponding fluorine‐free analogues. In the case of (E)‐ 3 , it is suggested that the HOMO–LUMO transition includes charge transfer from the ethereal oxygen atoms to the C(sp²) CF₃ moieties.     

Opp‐Dibenzoporphyrins as a Light‐Harvester for Dye‐Sensitized Solar Cells

New opp‐dibenzoporphyrins were prepared in a concise method that was based on a Pd⁰‐catalyzed cascade reaction. These porphyrins, which contained carboxylic‐acid linker groups on benzene rings that were fused to the porphyrin at their β,β′‐positions, were examined as sensitizers for dye‐sensitized solar cells for the first time. Whereas all of the porphyrins showed solar‐energy‐to‐electricity conversion, an opp‐dibenzoporphyrin with conjugated carboxylic‐acid linkers displayed the highest conversion efficiency and an exceptionally high J_sc value. Cyclic voltammetry of these porphyrins suggested that the fusion of two aromatic benzene rings onto the periphery of the porphyrin lowered the HOMO–LUMO energy gap; the incorporation of a conjugated carboxylic‐acid linker group decreased the HOMO–LUMO gap even further. These CV data are consistent with DFT calculations for these porphyrins and agree well with the UV/Vis absorption‐ and fluorescence spectra of these porphyrins.     

Synthesis,Characterization, and Computational Studies of Selenium Derivatives of 3,5‐Dichloropyridine

A methodology that can be tailored to incorporate selenium at the C‐2 or C‐4 position of 3,5‐dichloropyridine ( 1 ) was developed. For this, the lithiation of 1 with and without prior boron trifluoride complexation was utilized. The use of 1.3 equiv of LDA in the reaction did not give the desired product; however, the use of 2.3 equiv of LDA successfully inserted selenium into the C─Li bond. The observed regioselectivity in these reactions has been explained in light of relative stability of the lithiated species formed in dimethyl ether solution. Quantum chemical analysis was used to calculate the deprotonation energy and pK _a values and correlated with the observed regioselectivity. Theoretical analysis (B3LYP/6‐311++G(d,p)) of the synthesized compounds was performed to predict the effect of structural variations on the molecular properties of pyridylselenium derivatives. Various thermodynamic parameters and HOMO‐LUMO energies in the gas and solvent phases were calculated. When compared with 1 , the insertion of selenium into a pyridine moiety drastically reduces the HOMO‐LUMO energy gap, which clearly explains photochemical liability of selenium‐containing pyridine derivatives. The ¹H‐ and ¹³C‐NMR chemical shifts were also calculated by using gauge‐including atomic orbital method, and the results were validated with the experimental data.     

Controlling the Conformational Changes in Donor–Acceptor [4]‐Dendralenes through Intramolecular Charge‐Transfer Processes

The synthesis of two [4]‐dendralene compounds incorporating thiophene‐(p‐nitrophenyl) donor–acceptor units is presented. The dendralenes adopt two different conformers in solution and solid state and the transformation between the structures can be controlled by light and heat. The electron‐donating components of the dendralenes are represented by bromothienyl (in 13 ) and ethylenedioxythiophene(EDOT)‐thienyl (in 15 ) end‐groups. The most facile transformation involves the isomerisation of donor–acceptor conjugated systems ( a conformers) into structures in which only the thiophenes are conjugated ( b conformers), and this process is driven by ambient light. The structures of the two conformers of compound 13 are confirmed by single‐crystal X‐ray diffraction studies and the structural changes in both compounds have been monitored by ¹H NMR spectroscopy and absorption studies. The transformations were found to be first‐order processes with rate constants of k=0.0027 s^?1 and k=0.00022 s^?1 for 13 and 15 , respectively. Density functional theory calculations at the B3LYP/6‐31G* level give credence to the proposed mechanism for the a → b conversion, which involves photoinduced intramolecular charge transfer (ICT) as the key step. The EDOT derivative ( 15 ) can be polymerised by electrochemical oxidation and a combination of cyclic voltammetry and UV/Vis spectroelectrochemical experiments indicate that the a conformer can be trapped and stabilised in the solid state.     

Unexpected Thermal Transformation of Aryl 3‐Arylprop‐2‐ynoates: Formation of 3‐(Diarylmethylidene)‐2,3‐dihydrofuran‐2‐ones

A number of aryl 3‐arylprop‐2‐ynoates 3 has been prepared (cf. Table 1 and Schemes 3 – 5). In contrast to aryl prop‐2‐ynoates and but‐2‐ynoates, 3‐arylprop‐2‐ynoates 3 (with the exception of 3b ) do not undergo, by flash vacuum pyrolysis (FVP), rearrangement to corresponding cyclohepta[b]furan‐2(2H)‐ones 2 (cf. Schemes 1 and 2). On melting, however, or in solution at temperatures >150°, the compounds 3 are converted stereospecifically to the dimers 3‐[(Z)‐diarylmethylidene]‐2,3‐dihydrofuran‐2‐ones (Z)‐ 11 and the cyclic anhydrides 12 of 1,4‐diarylnaphthalene‐2,3‐dicarboxylic acids, which also represent dimers of 3 , formed by loss of one molecule of the corresponding phenol from the aryloxy part (cf. Scheme 6). Small amounts of diaryl naphthalene‐2,3‐dicarboxylates 13 accompanied the product types (Z)‐ 11 and 12 , when the thermal transformation of 3 was performed in the molten state or at high concentration of 3 in solution (cf. Tables 2 and 4). The structure of the dihydrofuranone (Z)‐ 11c was established by an X‐ray crystal‐structure analysis (Fig. 1). The structures of the dihydrofuranones 11 and the cyclic anhydrides 12 indicate that the 3‐arylprop‐2‐ynoates 3 , on heating, must undergo an aryl O→C(3) migration leading to a reactive intermediate, which attacks a second molecule of 3 , finally under formation of (Z)‐ 11 or 12 . Formation of the diaryl dicarboxylates 13 , on the other hand, are the result of the well‐known thermal Diels‐Alder‐type dimerization of 3 without rearrangement (cf. Scheme 7). At low concentration of 3 in decalin, the decrease of 3 follows up to ca. 20% conversion first‐order kinetics (cf. Table 5), which is in agreement with a monomolecular rearrangement of 3 . Moreover, heating the highly reactive 2,4,6‐trimethylphenyl 3‐(4‐nitrophenyl)prop‐2‐ynonate ( 3f ) in the presence of a twofold molar amount of the much less reactive phenyl 3‐(4‐nitrophenyl)prop‐2‐ynonate ( 3g ) led, beside (Z)‐ 11f , to the cross products (Z)‐ 11fg , and, due to subsequent thermal isomerization, (E)‐ 11fg (cf. Scheme 10), the structures of which indicated that they were composed, as expected, of rearranged 3f and structurally unaltered 3g . Finally, thermal transposition of [¹⁷O]‐ 3i with the ¹⁷O‐label at the aryloxy group gave (Z)‐ and (E)‐[¹⁷O₂]‐ 11i with the ¹⁷O‐label of rearranged [¹⁷O]‐ 3i specifically at the oxo group of the two isomeric dihydrofuranones (cf. Scheme 8), indicating a highly ordered cyclic transition state of the aryl O→C(3) migration (cf. Scheme 9).     

A Novel BODIPY‐Based Low‐Band‐Gap Small‐Molecule Acceptor for Efficient Non‐fullerene Polymer Solar Cells

We report herein an efficient A¹‐C≡C‐A²‐C≡C‐A¹ type small‐molecule 4,4'‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐ indacene (BODIPY) acceptor (A¹=BODIPY and A²=diketopyrrolopyrrole (DPP)) by following the A‐to‐A excited electron delocalization via the BODIPY meso ‐position, the inherent directionality for the excited electron delocalization. The lowest unoccupied molecular orbital (LUMO) delocalizes across over whole the two flanking A¹ and the central A², and the highest occupied molecular orbital (HOMO) localizes dominantly on the ‐C≡C‐DPP‐C≡C‐ segment. The excited electron upon light excitation of the DPP segment delocalizes over both the BODIPY and DPP segments. The acceptor in chloroform shows an unprecedented plateau‐like broad absorption between 550 and 700 nm with a large FWHM value of 195 nm. Upon transition into solid film, the acceptor shows absorption in the whole near ultraviolet‐visible‐near infrared wavelength region (300‐830 nm) with a low band gap of 1.5 eV and a maximum absorptivity of 0.85×10⁵ cm^‐1. Introduction of the ethynyl spacer between the A¹ and A² and the close BODIPY‐to‐DPP LUMO energy levels are crucial for the excited π−electron delocalization across over whole the conjugation backbone. A power conversion efficiency of 6.60% was obtained from the ternary non‐fullerene solar cell with PTB7‐Th:p ‐DTS(FBTTh₂)₂ (0.5 : 0.5) as the donor materials, which is the highest value among the non‐fullerene organic solar cells with BODIPY as the electron acceptor material.     

The Pi‐Nature of Methyl Obtained by the Natural Bond Orbital Method: Orbital‐Based Rationalizations of Site‐Dependent Substitution Effects on Fine Color‐Tuning of Luminescence

Both C‐H bonding and antibonding (σ_CH and σ*_CH) of a methyl group would contribute to the highest occupied or lowest unoccupied molecular orbitals (HOMO or LUMO) in methylated derivatives of Ir(ppz)₂ 3 iq (ppz = 1‐phenylpyrazole and 3iq = isoquinoline‐3‐carboxylate). This is found by analysis of HOMO (or LUMO) formed by linear combination of bond orbitals using the natural bond orbital (NBO) method. The elevated level of HOMO (or LUMO) uniformly found for each methylated derivative, indicating the σ_CH‐destabilization outweighs the σ*_CH‐stabilization. To broaden the HOMO‐LUMO gap, methylation at a carbon having smaller contribution to HOMO and/or larger contribution to LUMO is suggested.     

Regioselective One‐Step Synthesis and Topological Chirality of trans‐3, trans‐3,trans‐3 and e,e,e [60]Fullerene‐Cyclotriveratrylene Tris‐adducts: Discussion on a Topological meso‐Form

The C₃‐symmetrical [60]fullerene‐cyclotriveratrylene (CTV) tris‐adducts (±)‐ 1 (with a trans‐3,trans‐3,trans‐3 addition pattern) and (±)‐ 2 (with an e,e,e addition pattern) were prepared in 11 and 9% yield, respectively, by the regio‐ and diastereoselective tether‐directed Bingel reaction of C₆₀ with the tris‐malonate‐appended CTV derivative (±)‐ 3 (Scheme). This is the first example for tris‐adduct formation by a one‐step tether‐directed Bingel addition. Interchromophoric interactions between the electron‐rich CTV cap and the electron‐attracting fullerene moiety have a profound effect on the electrochemical behavior of the C‐sphere (Fig. 4 and Table 1). The fullerene‐centered first reduction potentials in compounds (±)‐ 1 and (±)‐ 2 are by 100 mV more negative than those of their corresponding tris[bis(ethoxycarbonyl)methano][60]fullerene analogs that lack the CTV cap. A particular interest in (±)‐ 1 and (±)‐ 2 arises from the topological chirality of these molecules. A complete topology study is presented, leading to the conclusion that the four possible classical stereoisomers of the e,e,e regioisomer are topologically different, and, therefore, there exist four different topological stereoisomers (Fig. 6). Interestingly, in the case of the trans‐3,trans‐3,trans‐3 tris‐adduct, there are four classical stereoisomers but only two topological stereoisomers (Fig. 7). An example of a target molecule representing a topological meso‐form is also presented (Fig. 8).     

meso‐3,5‐Bis(trifluoromethyl)phenyl‐Substituted Expanded Porphyrins: Synthesis,Characterization, and Optical,Electrochemical, and Photophysical Properties

Trifluoroacetic acid‐catalyzed condensation of pyrrole with electron‐deficient and sterically hindered 3,5‐bis(trifluoromethyl)benzaldehyde results in the unexpected production of a series of meso‐3,5‐bis(trifluoromethyl)phenyl‐substituted expanded porphyrins including [22]sapphyrin 2 , N‐fused [22]pentaphyrin 3 , [26]hexaphyrin 4 , and intact [32]heptaphyrin 5 together with the conventional 5,10,15,20‐tetrakis(3,5‐bis(trifluoromethyl)phenyl)porphyrin 1 . These expanded porphyrins are characterized by mass spectrometry, ¹H NMR spectroscopy, UV/Vis/NIR absorption spectroscopy, and fluorescence spectroscopy. The optical and electrochemical measurements reveal a decrease in the HOMO–LUMO gap with increasing size of the conjugated macrocycles, and in accordance with the trend, the deactivation of the excited singlet state to the ground state is enhanced.     

Supramolecular Fullerene Chemistry: A Comprehensive Study of Cyclophane‐Type Mono‐ and Bis‐Crown Ether Conjugates of C70

The covalently templated bis‐functionalization of C₇₀, employing bis‐malonate 5 tethered by an anti‐disubstituted dibenzo[18]crown‐6 (DB18C6) ether, proceeds with complete regiospecificity and provides two diastereoisomeric pairs of enantiomeric C₇₀ crown ether conjugates, (±)‐ 7a and (±)‐ 7b , featuring a five o'clock bis‐addition pattern that is disfavored in sequential transformations (Scheme 1). The identity of (±)‐ 7a was revealed by X‐ray crystal‐structure analysis (Fig. 6). With bis‐malonate 6 containing a syn‐disubstituted DB18C6 tether, the regioselectivity of the macrocylization via double Bingel cyclopropanation changed completely, affording two constitutionally isomeric C₇₀ crown ether conjugates in a ca. 1 : 1 ratio featuring the twelve ( 16 ) and two o'clock ((±)‐ 15 ) addition patterns, respectively (Scheme 3). The X‐ray crystal‐structure analysis of the twelve o'clock bis‐adduct 16 revealed that a H₂O molecule was included in the crown ether cavity (Figs. 7 and 8). Two sequential Bingel macrocyclizations, first with anti‐DB18C6‐tethered ( 5 ) and subsequently with syn‐DB18C6‐tethered ( 6 ) bis‐malonates, provided access to the first fullerene bis‐crown ether conjugates. The two diastereoisomeric pairs of enantiomers (±)‐ 28a and (±)‐ 28b were formed in high yield and with complete regioselectivity (Scheme 9). The cation‐binding properties of all C₇₀ crown‐ether conjugates were determined with the help of ion‐selective electrodes (ISEs). Mono‐crown ether conjugates form stable 1 : 1 complexes with alkali‐metal ions, whereas the tetrakis‐adducts of C₇₀, featuring two covalently attached crown ethers, form stable 1 : 1 and 1 : 2 host‐guest complexes (Table 2). Comparative studies showed that the conformation of the DB18C6 ionophore imposed by the macrocyclic bridging to the fullerene is not particularly favorable for strong association. Reference compound (±)‐ 22 (Scheme 4), in which the DB18C6 moiety is attached to the C₇₀ sphere by a single bridge only and, therefore, possesses higher conformational flexibility, binds K⁺ and Na⁺ ions better by factors of 2 and 20, respectively. Electrochemical studies demonstrate that cation complexation at the crown ether site causes significant anodic shifts of the first reduction potential of the appended fullerene (Table 3). In case of the C₇₀ mono‐crown ether conjugates featuring a five o'clock functionalization pattern, addition of 1 equiv. of KPF₆ caused an anodic shift of the first reduction wave in the cyclic voltammogram (CV) by 70 to 80 mV, which is the result of the electrostatic effect of the K⁺ ion bound closely to the fullerene core (Fig. 14). Addition of 2 equiv. of K⁺ ions to C₇₀ bis‐crown ether conjugates resulted in the observation of only one redox couple, whose potential is anodically shifted by 170 mV with respect to the corresponding wave in the absence of the salt (Fig. 16). The synthesis and characterization of novel tris‐ and tetrakis‐adducts of C₇₀ are reported (Schemes 5 and 6). Attempts to prepare even more highly functionalized derivatives resulted in the formation of novel pentakis‐ and hexakis‐adducts and a single heptakis‐adduct (Scheme 7), which were characterized by ¹H‐ and ¹³C‐NMR spectroscopy (Fig. 10), as well as matrix‐assisted laser‐desorption‐ionization mass spectrometry (MALDI‐TOF‐MS). Based on predictions from density‐functional‐theory (DFT) calculations (Figs. 12 and 13), structures are proposed for the tris‐, tetrakis‐, and pentakis‐adducts.     

Synthesis of poly(N‐9‐ethylcarbazole‐exo‐norbornene‐5,6‐dicarboximide) for hole‐transporting layer in hybrid organic light‐emitting devices

We synthesized new polynorbornene dicarboximide (PCaNI) functionalized with hole‐transporting carbazole moieties and its copolymer (PCaNA) by ring‐opening metathesis polymerization (ROMP), where the PCaNA was further reacted with 3‐amino‐triethoxysilane to prepare PCaNI/silica hybrid. We also investigated the feasibility of PCaNI and PCaNI/silica hybrid (PCaSi) as a hole‐transporting material for hybrid organic light emitting devices (HOLEDs). To improve the performance of the PCaNI‐based HOLEDs, N,N′‐diphenyl‐N,N′‐(3‐methylphenyl)‐[1,1′‐biphenyl]‐4‐4′‐diamine (TPD) was also introduced into the PCaNI matrix. Results showed that PCaNI exhibited high glass transition temperature (～260 °C) and high optical transparency in the visible region. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of PCaNI were measured as 5.6 and 2.2 eV, while the TPD‐doped PCaNI showed 5.7 eV (HOMO) and 2.6 eV (LUMO). The PCaNI/silica hybrid nanolayers showed excellent solvent resistance due to the formation of covalent bonds between ITO and PCaNI. The HOLEDs with PCaNI/TPD or PCaSi/TPD hybrid nanolayers exhibited relatively higher luminance (～10,000 cd/m²), lower operating voltage (～6.5 V at 300 cd/m²), and higher current efficiency (～2.7 cd/A). © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Ab initio investigation of 2,2′‐bis(4‐trifluoromethylphenyl)‐5,5′‐bithiazole for the design of efficient organic field‐effect transistors

The initial molecular structure of 2,2′‐bis(4‐trifluoromethylphenyl)‐ 5,5′‐bithiazole has been optimized in the ground state using density functional theory (DFT). The distribution patterns of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have also been evaluated. To shed light on the charge transfer properties, we have calculated the reorganization energy of electron λ_e, the reorganization energy of hole λ_h, adiabatic electron affinity (EA_a), vertical electron affinity (EA_v), adiabatic ionization potential (IP_a), and vertical ionization potential (IP_v) using DFT. Based on the evaluation of hole reorganization energy, λ_h, and electron reorganization energy, λ_e, it has been predicted that 2,2′‐bis(4‐trifluoromethylphenyl)‐5,5′‐bithiazole would be a better electron transport material. Finally, the effect of electric field on the HOMO, LUMO, and HOMO–LUMO gap were observed to check its suitability for the use as a conducting channel in organic field‐effect transistors. © 2015 Wiley Periodicals, Inc.     

Cleft‐Type Diamidinium Receptors for Dicarboxylate Binding in Protic Solvents

A series of potential cleft‐type receptors for dicarboxylate substrates were prepared by attachment of two phenylamidinium ions to either naphthalene or 1,1′‐binaphthalene scaffolds. Their synthesis (Schemes 1 – 4) involved the Pd⁰‐catalyzed cross‐coupling of aryl nitriles to the central scaffold, followed by transformation of the nitrile into amidinium groups using the Garigipati reaction. The 1,1′‐binaphthalene derivative (±)‐ 1 with phenylamidinium residues attached to the 6,6′‐positions in the major groove was found to be a highly efficient receptor for dicarboxylate guests, such as glutarate and isophthalates, even in competing protic solvents such as CD₃OD (Table 1). The van't Hoff analysis of variable‐temperature ¹H‐NMR (VT‐NMR) titrations (Table 2 and Fig. 3) and isothermal microcalorimetry (ITC; Table 3 and Fig. 4) revealed that complexation in MeOH is strongly entropically driven with an unfavorable enthalpic change, which partially compensates the entropic gain. These thermodynamic quantities are best explained by a particularly favorable solvation of the binding partners in the unbound state and the release of the MeOH molecules, which solvate the free ions into the bulk upon complexation. Receptor (±)‐ 1 binds flexible glutarate and rigid isophthalates with similar association strength. This lack in response to guest preorganization and reduced guest selectivity is explained with the non‐directionality of the coulombic charge‐charge interactions in the complexes.     

o‐Carborane‐based Biphenyl and p‐Terphenyl Derivatives

The synthesis and properties of biphenyl‐ and p‐terphenyl‐fused o‐carboranes are described. Aryl rings in the biphenyl and p‐terphenyl skeletons are highly coplanar because of the presence of the o‐carborane unit. o‐Carborane exhibits an electron‐withdrawing character via the inductive effect, resulting in a decrease in both the HOMO and LUMO levels of oligophenyls without causing electronic perturbation.     

Investigations on regio‐ and stereoselectivities in cycloadditions involving α‐ (3‐pyridyl)‐N‐phenylnitrone: Development of an efficient route to novel nicotine analogs

Thermal reactions of hitherto α‐(3‐pyridyl)‐N‐phenylnitrone ( 1 ) with mono‐substituted electron‐rich and electron‐neutral dipolarophiles are regio‐, and stereo‐selective (exo‐selective), controlled by LUMO ‐ dipole ‐ HOMO‐ dipolarophile interaction, and furnish syn‐5‐substituted‐3‐(3‐pyridyl)‐isoxazolidines ( 5 ) in high yields. With electron deficient dipolarophiles such as acrylonitrile there is observed a loss of regioselectivity as well as stereoselectivity and the regioselectivity is reversed in reactions with methyl vinyl ketone and methyl acrylate, due to intervention of HOMO‐dipole ‐ LUMO‐dipolarophile interaction, affording 4‐substi‐tuted‐3‐(3‐pyridyl)‐isoxazolidines ( 7 ) as major products. Reactions of nitrone ( 1 ) with disubstituted dipolarophiles such as methyl methacrylate and ethyl coronate furnish methyl syn‐5‐methy‐3‐pyridyl‐1‐phenyl‐isoxazolidine‐5‐carboxylate ( 8 ) and ethyl anti‐5‐methy‐3‐pyridyl‐1‐phenyl‐isoxazolidine‐4‐carboxylate ( 10 ), respectively, in high yields. Reaction with N‐Phenylmaleimide affords novel isoxazolidino‐pyrro‐lidinediones bearing a 3‐pyridyl moiety ( 11, 12 ). A mechanistic rationalization of the obtained results in terms of electronic, steric and secondary interactions is proffered.     

Solid‐Phase Synthesis of Oligo(triacetylene)s and Oligo(phenylenetriacetylene)s Employing Sonogashira and Cadiot?Chodkiewicz‐Type Cross‐Coupling Reactions

We describe the first polymer‐supported synthesis of poly(triacetylene)‐derived monodisperse oligomers, utilizing Pd⁰‐catalyzed Sonogashira and Cadiot? Chodkiewicz‐type cross‐couplings as the key steps in the construction of the acetylenic scaffolds. For our investigations, Merrifield resin functionalized with a 1‐(4‐iodoaryl)triazene linker was chosen as the polymeric support ( R2 ; Figure and Scheme 3). The linker selection was made based on the results of several model studies in the liquid phase (Schemes 1 and 2). For the solid‐support synthesis of the oligo(phenylene triacetylene)s 7b – 7d , a set of only three reactions was required: i) Pd⁰‐catalyzed Sonogashira cross‐coupling, ii) Me₃Si? alkyne deprotection by protodesilylation, and iii) cleavage of the linker with liberation of the generated oligomers (Scheme 5). The longest‐wavelength absorption maxima of the oligo(phenylene triacetylene)s 7a – 7d shift bathochromically with increasing oligomeric length, from λ_max 337 nm (monomer 7a ) to 384 nm (tetramer 7d ; Table 2). Based on the electronic absorption data, the effective conjugation length (ECL) of the oligo(phenylene triacetylene)s is estimated to involve at least four monomer units and 40 C‐atoms. π‐Electron conjugation in these oligomers is less efficient than in the known oligo(triacetylene)s 14a – 14d (Table 2) due to poor transmittance of π‐electron delocalization by the phenyl rings inserted into the oligomeric backbone. Similar conclusions were drawn from the electrochemical properties of the two oligomeric series as determined by cyclic (CV) and rotating‐disk voltammetry (RDV; Table 3). In sharp contrast to 14b – 14d , the oligo(phenylene triacetylene)s 7b – 7d are strongly fluorescent, with the highest quantum yield Φ_F=0.69 measured for trimer 7c (Table 2). Whereas the Sonogashira cross‐coupling on solid support proceeded smoothly, optimal conditions for alkyne? alkyne cross‐coupling reactions employing Pd⁰‐catalyzed Cadiot? Chodkiewicz conditions still remain to be developed, despite extensive experimentation (Scheme 7 and Table 1).     

Synthesis and isolation of chloro‐β‐carbolines obtained by chlorination of β‐carboline alkaloids in solution and in solid state

β‐Carbolines (1‐5) undergo electrophilic aromatic substitution with N‐chlorosuccinimide and N‐chlorobenzotriazole under different experimental conditions. Although 6‐chloro and 8‐chloro‐nor‐har‐mane ( 1a and 1b ) and 6‐chloro and 8‐chloro‐harmane ( 2a and 2b ) obtained by chlorination with sodium hypochlorite of nor‐harmane (1) and harmane (2) were isolated and fully characterized recently, other chloroderivatives of nor‐harmane and harmane have never been described. The preparation and subsequent isolation, purification and full characterization of the dichloroderivatives 1c and 2c are reported (mp, R_f, ¹H nmr, ¹³C nmr and ms) together with the preparation, isolation and charaterization, for the first time, of the chloroderivatives obtained from harmine (3a‐3c) , harmol (4a‐4b) and 7‐acetylharmol (5a‐5c) . As chlorinating reagent N‐chlorosuccinimide and N‐chlorobenzotriazole in solution as well as the β‐carboline ‐N‐chlorosuccinimide solid mixture have been used and their uses have been compared. Gc (t_R) and gc‐ms (m/z) data for other monochloro derivative of nor‐harmane (1d) and monochloro‐ and dichloroderivatives of harmane ( 2d and 2e‐2f ), obtained in trace amounts, are also included (Scheme 1 and Table I). Semiempirical AM1 and PM3 calculations have been performed in order to predict reactivity in terms of the energies of HOMO‐LUMO difference and in terms of the charge density of β‐carbolines (1‐5) and chloro‐β‐carbolines ( 1a‐1c, 2a‐2c, 3a‐3c, 4a‐4b , and 5a‐5c ) (Scheme 1). Theoretical and experimental results are discussed briefly.     

Building Molecular Complexity from Quinizarin: Conjoined Coumarins and Coronene Analogs

The double Knoevenagel condensation of 1,4‐dibenzoyloxyanthraquinone with methyl esters of arylacetic acids affords a series of compounds based upon a previously unknown 1,8‐dioxa‐benzo[e]pyrene‐2,7‐dione heterocyclic core. The aryl groups incorporated in the 3‐ and 6‐positions can be oxidatively coupled to the π‐expanded backbone to produce a further new heterocyclic core: 1,10‐dioxa‐dibenzo[dj]coronene‐2,9‐dione. The intriguing optical properties of these π‐expanded coumarin derivatives are discussed and rationalized through quantum chemical calculations. The broad absorption bands of 1,8‐dioxa‐benzo[e]pyrene‐2,7‐dione‐based dyes are attributed to both HOMO?1→LUMO and HOMO→LUMO transitions, which have a similar energy. Weakly coupled electron‐donating aryl substituents result in a moderate bathochromic shift of both the absorption and emission by 30–60 nm in toluene. The emissive properties of these compounds are in part determined by the oscillator strength of the main transition, lifetimes of the excited state, and by the energy match of the excited state with a triplet state of a similar energy. The 1,10‐dioxa‐dibenzo[dj]coronene‐2,9‐dione displays a much smaller Stokes shift, yet a markedly increased fluorescence quantum yield of 90 % owing to the increased rigidity compared with the 1,8‐dioxa‐benzo[e]pyrene‐2,7‐dione core.