Synthesis, enantiomeric resolution and photophysical properties of 7-cyano-14-methoxy-5-thiahexahelicene (original) (raw)
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Helicene Monomers and Dimers: Chiral Chromophores Featuring Strong Circularly Polarized Luminescence
Chemistry – A European Journal, 2019
The synthesis and chiroptical properties of a series of enantiomerically pure, C 2-symmetrical carbo[6]helicene dimers are reported. Two helicene cores are connected through a buta-1,3-diyne-1,4-diyl linker or a heteroaromatic bridge and bear arylethynyl substituents at their 15-positions. This ensures the possibility of extended electronic communication throughout the whole molecule. The new chromophores exhibit intense ECD spectra with strong bands in the UV/Vis region well above 400 nm. The anisotropy factor g abs (defined as De/e) reaches values up to 0.047, which are unusually large for single organic molecules. They also display blue fluorescence, with good quantum yields (F f~0 .25). The emitted light is circularly polarized to an outstanding extent: in some cases, the luminescence dissymmetry factor g lum = 2(I L ÀI R)/ (I L + I R) attains values of j 0.025 j. To the best of our knowledge, such values are among the highest ever reported for non-aggregated organic fluorophores. Fluorescence is a privileged physical property, and fluorescent materials find a wide range of applications, such as in medical diagnostics, chemical sensors, and optoelectronic devices (e.g., in circularly polarized OLEDs). [1] Circular polarized luminescence (CPL) has gained increasing interest for its possible application in optical information storage and transfer, where the extent of CPL can add a further dimension to the information content conveyed by light. [1] Highly CPL-active compounds were found mostly among chiral lanthanide complexes, but the interest of material scientists in chiral, molecular organic emitters also heightened more recently. [2] Helicenes are an extensively investigated class of chiral organic compounds, presenting strong UV/Vis and electronic circular dichroism (ECD) intensities, [3] but usually displaying low fluorescence quantum yields (F f). [4] Recently, polycyclic aromatic hydrocarbons, [5] with helicenes amongst them, [6] have been developed with higher fluorescence quantum yields and CPL activity, leading to their investigation for novel applications such as in spintronics. [7] Fluorescent helicene design has relied mostly on the introduction of heterocycles [8] into the helicene backbone, such as thiophene, [9] pyrrole, [10] or furan [11] rings. So far, the discovery of efficient, all-carbon helicene fluorophores has not widely been reported. The interest in the design of enantiopure fluorescent helicenes relies also on their ability to emit left-and right-circularly polarized light with different intensities (CPL). Usually, CPL is quantified through the luminescence dissymmetry factor g lum , which is defined as the ratio between the difference of the left and right circularly polarized light and the half of the total luminescence. By definition, g lum = 2(I L ÀI R)/(I L + I R), (I = intensity of circularly polarized light) can range from À2 (when the emitted light is totally right-circularly polarized) to + 2 (the emitted light is totally left-circularly polarized). In most cases, the g lum factors of helicenic fluorophores are in the 10 À3-10 À4 range, similarly to those of other non-aggregate organic molecules. [2b, 6c,d, 12] In a few remarkable cases however, helicene derivatives were found to show high g lum factors, reaching values from j 0.016 j to j 0.035 j. [6a,b, 12a] Recently, we reported a novel class of enantiomerically pure, buta-1,3-diyne-1,4-diyl linked carbo[6]helicene oligomers. [13] Dimeric (+)-(P) 2-and (À)-(M) 2-1 displayed moderate fluorescence quantum yields (F f = 0.11). Starting from dimeric (P) 2-1, we obtained by heteroaromatization [14] thiene-2,5-diyl linked (+)-(P) 2-2, [13] which displayed an increased F f value of 0.25, one of the highest F f values reported to date for a carbohelicene. We proposed that the fluorescence of (P) 2-2 originated from the push-pull substitution on the helicene cores, with the thienyl ring as electron donor and the alkynes as acceptors and thus set out to further validate this hypothesis. Here, we explore the push-pull effects by changing the thienyl to a pyrrole ring (Figure 1), resulting in N-phenylpyrrole-2,5-diyl linked compound (+)-(P) 2-and (À)-(M) 2-3. We also [a] C.
Synthesis, Stereochemical and Photophysical Properties of Functionalized Thiahelicenes
Catalysts, 2022
We report on the synthesis of a novel class of functionalized thia[6]helicenes and a thia[5]helicene, containing a benzothiophene unit and a second heteroatom embedded in the helix (i.e., nitrogen and oxygen) or a pyrene or a spirobifluorene moiety. These systems are obtained through straightforward and general procedures that involve: (i) palladium-catalyzed annulation of iodo-atropoisomers with internal alkynes and (ii) Suzuki coupling of iodo-atropoisomers with phenyl boronic acid followed by a Mallory-type reaction. Both experimental and theoretical studies on the configurational stability of some selected thia[6]helicenes confirmed their stability toward racemization at room temperature, while the pyrene-based thia[5]helicene was found to be unstable. Moreover, the configuration assignment for one representative thiahelicene was established through the comparison between experimental and theoretical circular dichroism (CD) spectra. A systematic study of the photophysical proper...
High-Spin Diradical Dication of Chiral π-Conjugated Double Helical Molecule
Journal of the American Chemical Society, 2019
We report an air-stable diradical dication of chiral D2-symmetric conjoined bis[5]diazahelicene with unprecedented high spin (triplet) ground state, singlet triplet energy gap, ΔEST = 0.3 kcal mol−1. The diradical dication possesses closed-shell (Kekulé) resonance forms with 16 π-electron perimeters. The diradical dication is monomeric in dibutylphthalate (DBP) matrix at low temperatures, and it has a half-life of more than two weeks at ambient conditions in the presence of excess oxidant. A barrier of ~35 kcal mol−1 has been experimentally determined for inversion of configuration in the neutral conjoined bis[5]diazahelicene, while the inversion barriers in its radical cation and diradical dication were predicted by the DFT computations to be within a few kcal mol−1 of that in the neutral species. Chiral HPLC resolution provides the chiral D2-symmetric conjoined bis[5]diazahelicene, enriched in (P,P)- or (M,M)-enantiomers. The enantiomerically enriched triplet diradical dication is configurationally stable for 48 h at room temperature, thus providing the lower limit for inversion barrier of configuration of 27 kcal mol−1. The enantiomers of conjoined bis[5]diazahelicene and its diradical dication show strong chirooptical properties that are comparable to [6]helicene or carbon-sulfur [7]helicene, as determined by the anisotropy factors, |g| = |Δε|/ε = 0.007 at 348 nm (neutral) and |g| = 0.005 at 385 nm (diradical dication). DFT computations of the radical cation suggest that SOMO and HOMO energy levels are near-degenerate.
Angewandte Chemie International Edition, 2008
Helicenes have attracted attention as unique inherently chiral three-dimensional aromatic compounds for several decades. Despite significant recent progress in the synthesis and applications of carbohelicenes and thiaheterohelicenes, the potential of the aza analogues with a pyridine unit (pyridohelicenes) have not been explored. There are only scattered examples of the preparation of pyridohelicenes, but with no general synthetic methodology, since the photochemical approach can fail with pyridohelicenes [4b,c,e] while non-photochemical alternatives can be difficult to adapt to the synthesis of N-heteroaromatic compounds. The properties and chemical behavior of pyridohelicenes are practically unknown apart from their basicities [4e,g] and the self-assembly of certain derivatives. Nevertheless, promising applications of pyridohelicenes in various branches of chemistry and material science might be envisaged and, therefore, further research in the field is required.
Tetrahedron Letters, 2005
Tetrahydro-as-indacene-1,8-dione 4 was prepared in 4 steps starting from 2-methyl-furan by modification of a literature procedure. Appliance of NoyoriÕs asymmetric transfer hydrogenation, resulted in (1S,8S)-1,2,3,6,7,8-hexahydroas-indacene-1,8-diol 5 in high yield (81%) and excellent enantioselectivity (>99% ee) or (8S)-8-hydroxy-3,6,7,8-tetrahydro-2Has-indacen-1-one 6 in moderate yield (58%) and equally high enantioselectivity (98.5% ee), depending on the conditions. The asymmetric reduction was expected to yield the (S)-alcohols using the (S,S)-Ts-DPEN ligand, which was confirmed by the appliance of the exciton chirality method on the corresponding bis(p-dimethylamino)benzoate 7.
Chemistry (Weinheim an der Bergstrasse, Germany), 2016
Four members of a new class of cycloborylated hexa- to deca-helicenes (1a-d) have been prepared in enantiopure forms, along with cycloplatinated deca-helicenes (1d', 1d1) further extending a family of cycloplatinated hexa- to octa-helicenes reported previously. The azabora[n]helicenes display intense electronic circular dichroism and strong optical rotations, whose dependence on the size of the helix (n = 6, 8, or 10) and number of boron atoms (1 or 2) has been examined in detail both experimentally and theoretically. The photophysical properties (unpolarized and circularly polarized luminescence) of these new fluorescent organic helicenes have been measured and compared with the corresponding organometallic phosphorescent cycloplatinated derivatives (1a1-d1).