Magnetic Circularly Polarized Luminescence in the Photoexcited States of Racemic [n]Helicenes (n=3-5,7) in Tetrahydrofuran and Dimethyl Sulfoxide Solutions.

This paper reports the first magnetic circularly polarized luminescence (MCPL) characteristics of racemic helicenes, including four unsubstituted [n]helicenes (n=3,4,5,7) and two [4]helicene derivatives bearing methoxy substituents, in tetrahydrofuran (THF) and dimethyl sulfoxide (DMSO) solutions. The value of |gMCPL | was calculated to be of the order of 10-3  T-1 within 350-430 nm under the north-up (N-up) and south-up (S-up) Faraday geometries in an external magnetic field of 1.6 T. The [n]-dependent MCPL signs were altered by the N-up and S-up geometries.

A novel Schiff base macrocycle based on 1,1'-binaphthyl for fluorescence recognition.

A novel chiral polyimine macrocycle C-1 was designed and synthesized by the self-condensation of the dialdehyde of the chiral dinaphtho[2,1-d:1',2'-f][1,3]dioxepine derivative and o-phenylenediamine by Schiff base formation, and the corresponding polyamine macrocycle C-1H was obtained by the reduction of the polyimine macrocycle. The UV-vis and fluorescence spectral studies indicated that both C-1 and C-1H form the complex with metal ions in a 1:2 ratio. The fluorescence behaviour of C-1 upon the addition of Zn2+ or Cd2+ showed a 'turn-on' response accompanied by fluorescence enhancement at 510 nm six times for Cd2+ and 13 times for Zn2+ . In contrast, C-1H revealed a 'turn-off' response upon the addition of Co2+ , Ni2+ , and Cu2+ .

Sign dependence of MCPL spectra on type and position of substituent groups of pyrene and phenanthrene derivatives.

Diamagnetic achiral pyrene and phenanthrene derivatives substituted with electron-donating hydroxyl/methoxy groups and electron-withdrawing carboxylic acid groups exhibited clear magnetic circularly polarised luminescence (MCPL) spectra at 360-460 nm in dilute solvents upon the application of N-up and S-up Faraday geometries under an external magnetic field of 1.6 T. Their MCPL signs were also susceptible upon application of the same Faraday geometry.

Optical activities of steroid ketones - Elucidation of the octant rule.

Theoretical calculations of optical activities in steroid ketones are presented by using modern semi-empirical PM7 wavefunctions. Both circular dichroism (CD) and specific rotation, which is proportional to optical rotation dispersion (ORD), are well simulated, and signs of the Cotton effect at the most long-wavelength region are fully in accordance with the experimental results. The good accordance is related to the octant rule, which is deduced within the framework of the perturbation theory. Our treatment is promising to predict the signs of the Cotton effect of large molecules, and thus, the absolute configurations can also be grasped without demanding procedures.

Band gap of carbon-sulfur [N]helicenes.

Asymmetric synthesis of (-)-[9]helicene, as well as preparation of its lower homologues, completes the series of carbon-sulfur [5]-, [7]-, [9]-, and [11]helicenes. Spectroscopic and electrochemical studies of this series provide an absorption onset-based band gap, E(g) = 3.40 eV, for a cross-conjugated (C(2)S)(n) helix; this value may be compared to E(g) = 3.59 eV obtained from TD-DFT computed excitation energies for a series of dimethyl-substituted [n]helicenes (n ≤ 31).

Oxidation of annelated diarylamines: analysis of reaction pathways to nitroxide diradical and spirocyclic products.

Oxidation of diaryldiamine 2, a tetrahydrodiazapentacene derivative, provides diarylnitroxide diradical 1 accompanied by an intermediate nitroxide monoradical and a multitude of isolable diamagnetic products. DFT-computed tensors for EPR spectra and paramagnetic (1)H NMR isotropic shifts for nitroxide diradical 1 show good agreement with the experimental EPR spectra in rigid matrices and paramagnetic (1)H NMR spectra in solution, respectively. Examination of the diamagnetic products elucidates their formation via distinct pathways involving C-O bond-forming reactions, including Baeyer-Villiger-type oxidations. An unusual diiminoketone structure and two spirocyclic structures of the predominant diamagnetic products are confirmed by either X-ray crystallography or correlations between DFT-computed and experimental spectroscopic data such as (1)H, (13)C, and (15)N NMR chemical shifts and electronic absorption spectra.

Spiro oligothiophenes.

Linear oligothiophenes, containing annelated dithieno[2,3-b:3',2'-d]thiophene units, undergo unexpected spirocyclizations upon treatment with trifluoroacetic acid. The resultant spiro oligothiophenes possess novel structures in which the spiro or dispiro skeleton is within an all-thiophene/dihydrothiophene framework. Racemic dispiro octathiophene is obtained with high diastereoselectivity and in high yield.

Radical cation of helical, cross-conjugated beta-oligothiophene.

The radical cation of carbon-sulfur [7]helicene is configurationally stable in solution at room temperature. In contrast to the radical cations of alpha-oligothiophenes, which form diamagnetic pi-dimers at low temperature, the radical cation of this helical, cross-conjugated beta-oligothiophene shows a low propensity toward dimerization.

Intramolecular cyclization of thiophene-based [7]helicenes to quasi-[8]circulenes.

Intramolecular cyclization in a series of thiophene-based dibromo[7]helicenes (4-6) with different helix structures is investigated by vacuum pyrolysis, tin- and palladium-mediated C-C bond forming reactions. The product with the cyclic structure of the annelated aromatic rings, which resembles [8]circulene devoid of an atom linkage, is referred to as quasi-[8]circulene. Vacuum pyrolysis of 4 gives insoluble, unidentified products, while 5 and 6 yield the corresponding quasi-[8]circulenes under similar conditions. Thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses for 4 indicate complex reaction pathways, while those for 5 and 6 show a single process corresponding to a loss of 1 equiv of Br(2) at about 330 degrees C. Pd-mediated reductive cyclization provides quasi-[8]circulenes for all three [7]helicenes, though only 4 gives a good isolated yield. Tributyltin hydride-mediated radical cyclization of 4-6 provides quasi-[8]circulenes in excellent yields, and it is practically insensitive to the helix structure. Experimental and calculated UV-vis absorption spectra for quasi-[8]circulenes and [8]circulenes are reported. The results suggest that the lack of atom linkage in quasi-[8]circulene does not significantly affect properties and conformation, compared to those for the corresponding [8]circulenes.

Functionalized thiophene-based [7]helicene: chirooptical properties versus electron delocalization.

The functionalized, enantiomerically pure [7]helicene 1 derived from bis(benzodithiophene) functionalized with four heptyl groups is prepared from 1,8-dibromo-4,5-diheptylbenzo[1,2-b:4,3-b']dithiophene building block 2. Such [7]helicene structure, functionalized with bromines at the terminal positions of the helicene inner rim and multiple solubilizing alkyl groups, is an attractive building block for long [n]helicenes and oligo[7]helicenes. Chirooptical properties and the degree of electron delocalization are determined and compared to those of analogous carbon-sulfur [7]helicene and [7]helicenes derived from benzodithiophene to provide a correlation between chirooptical properties and the degree of electron delocalization. [7]Helicene 1 possesses a moderately increased electron delocalization, but its chirooptical properties are similar to those for analogous [7]helicenes with relatively lower electron delocalization, indicating that chirooptical properties are not significantly affected by electron delocalization for this series of [7]helicenes. Molecular structures of racemic [7]helicene 1 and its benzodithiophene building block 2 are confirmed by single-crystal X-ray analysis. Crystals of 2 are chiral and adopt the shape of long, flexible, flat needles that can be readily bent.

Synthesis of dithieno[2,3-b:3',2'-d]thiophenes-building blocks for cross-conjugated beta-oligothiophenes.

Syntheses of annelated and functionalized beta-trithiophenes (dithieno[2,3-b:3',2'-d]thiophenes), building blocks for the helically annelated, cross-conjugated beta-oligothiophenes, are reported. UV-vis spectra reveal that the effect of octyl substituents on the onset of electronic absorption in annelated beta-trithiophenes is negligible compared to the effect of helical distortion in the higher beta-oligothiophenes.

Cross-conjugated oligothiophenes derived from the (C2S)n helix: asymmetric synthesis and structure of carbon-sulfur [11]helicene.

(-)-Sparteine-mediated asymmetric synthesis of di-n-octyl-substituted carbon-sulfur [11]helicene, a helical (C2S)n beta-undecathiophene, is described. The atom-efficient routes rely on one-step tri-annelation or two-step di- and mono-annelation to provide enantiomeric excess of (+)- or (-)-[11]helicene, respectively. X-ray structures for homologous [11] and [7]helicenes indicate similar helical curvatures. The optical band gap, Eg approximately 3.5 eV, is estimated for the (C2S)n helix polymer, with onset of electron localization at n

Helically annelated and cross-conjugated oligothiophenes: asymmetric synthesis, resolution, and characterization of a carbon-sulfur [7]helicene.

The synthesis and characterization of a novel oligothiophene, in which the thiophene rings are annelated into a [7]helicene with cross-conjugated pi-system, are described. Such [7]helicenes may be viewed as fragments of the unprecedented carbon-sulfur (C(2)S)(n)() helix, possessing sulfur-rich molecular periphery. Racemic synthesis of [7]helicene is based upon iterative alternation of two steps: C-C bond homocouplings between the beta-positions of thiophenes and annelation between the alpha-positions of thiophenes. Asymmetric synthesis is carried out using (-)-sparteine-mediated annelation of the axially chiral bis(aryllithium) with electrophilic sulfur equivalent. Alternatively, enantiomers of the [7]helicene are obtained via resolution using menthol-based chiral siloxanes. Racemic [7]helicene and four other macrocyclic products of the annelation are characterized by X-ray crystallography. One of the solvent polymorphs of the [7]helicene possesses pi-stacked columns of opposite enantiomers and multiple short intermolecular contacts, including both homochiral and heterochiral short S...S contacts, suggesting an effective intermolecular electronic coupling in two-dimensions. The [7]helicene is configurationally stable at room temperature and racemizes at 199 degrees C with a half-time of about 11 h. Selected physicochemical studies (UV-vis absorption, CD, optical rotation, and cyclic voltammetry) of the [7]helicene are described.

Chiral molecular glass: synthesis and characterization of enantiomerically pure thiophene-based [7]helicene.

Synthesis of thiophene-based [7]helicenes, which are functionalized for both design of organic chiral glasses with strong chiroptical properties and for further homologation to higher [n]helicenes, is reported. The key synthetic transformations are kinetic resolution of the intermediate diketone and the annelation step forming the center benzene ring by means of an intramolecular McMurry reaction. Based upon X-ray crystallographic determinations of the absolute configurations for (+)-enantiomers of the diketone and the [7]helicene, stereochemical correlation between the (R) axial chirality of the diketone and the (M) helical chirality of the [7]helicene is established. One such enantiopure trimethylsilyl-substituted [7]helicene possesses enchanced chiroptical properties and forms a chiral molecular glass.

Ladderlike ferromagnetic spin coupling network on a pi-conjugated pendant polyradical.

A poly(9,10-anthryleneethynylene)-based polyradical with two pendant stable phenoxyls in one anthracene skeleton was newly synthesized via polymerization of the corresponding bromoethynylanthracene monomer using a Pd(0) catalyst. The average molecular weight of the polymer reached M(n) = 5 x 10(3) and was soluble in common organic solvents. The polyradical was prepared from the corresponding hydroxyl precursor polymer and was appropriately stable at room temperature. The ESR spectrum of the corresponding monomeric radical suggested an effectively delocalized spin density distribution on the backbone anthracene. The magnetization and the static magnetic susceptibility of the polyradical were measured using a SQUID magnetometer. The large average spin quantum number (S = (5)/(2)) of the polyradical indicated that the ferromagnetic spin coupling network of the polyradical had spread throughout the pi-conjugated chain and that it was considerably insensitive to spin defects.

Average Octet Radical Polymer: A Stable Polyphenoxyl with Star-Shaped π Conjugation.

An average spin quantum number of 7/2 is displayed by the star-shaped, π-conjugated polyradical 1, even with a spin concentration of 0.8 per monomer unit. Polymer 1 is stable at room temperature, and it is a powerful candidate for the synthesis of a feasible polymer with very high spin.

High-Spin Polyphenoxyls Attached to Star-Shaped Poly(phenylenevinylene)s.

Star-shaped poly(1,2-phenylenevinylene)s 4-substituted with multiple pendant phenoxyls (2 and 3) were synthesized by polymerizing 2-bromo-4-(acetoxyphenyl)styrene in the presence of 1,3,5-triiodobenzene or 1,3,5-tris(3',5'-diiodophenyl)benzene as the core via a simple one-pot Heck reaction and subsequent hydrolysis, phenolate formation, and heterogeneous oxidation. ESR spectra indicated a delocalized spin distribution into the core parts of the star-shaped molecules. The polyradicals, 2 and 3, were in a high-spin state at low temperature, and the ferromagnetic behavior was enhanced for the polyradical with a higher molecular weight. Although an average S of 3 remained at 3/2 to 4/2, the polyradical 2 even with a spin concentration of 0.8 spin/unit revealed an average S of 7/2 to 8/2. The 1,3,5-benzene core acted as an effective magnetic coupler to align the spins of the pendant phenoxyls through the star-shaped pi-conjugated backbone.