Directional Construction And Spectroscopic Behaviors Of Chalcogenohelicenes

Helicenes have received more and more attention in the fields of chiral supramolecular chemistry and circularly polarized luminescence（CPL）due to their unique helical structures and high specific rotations.Chalcogenohelicene is an important branch of helicene,which has five-membered heteroaromatic rings,chalcogenophenes（namely furan,thiophene,selenophene and tellurophene）as building blocks.Chalcogenohelicene shows the lack of species and limited synthetic method due to the different chemical reactivity of chalcogenophenes themselves,resulting in poor selectivity of bond formation among them.In this thesis,two types of chalcogenohelicenes,namely benzo-chalcogeno[7]helicenes and phenanthrochalcogeno[9]helicenes will be designed and synthesized by selective protection of the chalcogenophenes and subsequent ordered coupling reactions.The spectral property,intermolecular interaction mode,molecular stacking pattern,energy level,and energy gap of these chalcogenophenes were studied by the analyses of crystal structures,spectroscopic behaviors,cyclic voltammetry and theoretical calculation.The results showed that the composition change of chalcogen atoms in the molecular skeleton not only effects conjugation degree,spectroscopic behavior,energy level and energy gap,but also modulates intermolecular interaction mode,stacking pattern and crystal forms of the chalcogenophenes.The specific research contents are listed as following:1.The directional construction of chalcogen fused triheteroaromatic rings1)With（4-bromofuran-2-yl）trimethylsilane and trimethyl（4-（4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl）selenophen-2-yl）silane as starting materials,2,5-bis（trimethyllsilyl）-selenopheno[3’,2’:4,5]thieno[2,3-b]furan（bb-STF）was directionally synthesized through Suzuki cross-coupling,n-Bu Li deprotonation,and cyclization with bis（phenylsulfonyl）sulfide（（Ph SO₂）₂S）,the total yield is 6%.2)The 2,5-bis（trimethyllsilyl）-selenopheno[2,3-b]thieno[3,2-d]thiophene（bb-STT）was directed synthesis from trimethyl（4-（5-（trimethylsilyl）selenophen-3-yl）thiophen-2-yl）silane,through Suzuki cross-coupling,n-Bu Li deprotonation,and cyclization with（（Ph SO₂）₂S）.2,5-Bis（trimethyllsilyl）-selenopheno[3’,2’:4,5]selenopheno[2,3-b]thiophene（bb-SST）and2,5-bis（trimethyllsilyl）-selenopheno[3’,2’:4,5]telluropheno[2,3-b]thiophene（bb-STeT）were directionally synthesized from the cyclization with selenium/tellurium powder,and following reduction with Cu powder.The total yields were5%,5%and 1%,respectively.3)With 5,5’-bis（trimethylsilyl）-3,3’-bithiophene/5,5’-bis（trimethylsilyl）-3,3’-biselenophene as starting materials,2,5-bis（trimethylsilyl）-telluropheno[2,3-b:5,4-b’]dithiophene（bb-TeDT）and 2,5-bis（trimethylsilyl）-telluropheno[2,3-b:5,4-b’]diselenophene（bb-TeDS）were directionally synthesized,through n-Bu Li deprotonation,the cyclization with tellurium powder,and following reduction with Cu powder.The total yields were 6%and 0.9%,respectively.2.The directional construction of chalcogenohelicenes1)Directional synthesis of benzochalcogeno[7]helicenes:With bb-DTS,bb-STT and bb-DSS as starting materials,benzochalcogeno[7]helicenes,[7]BH-1,[7]BH-2,[7]BH-3 were directionally synthesized through bromination,Br/Li exchange,DMF,intermolecular Mc Murry reaction,and oxidation photocyclization.The total yields were 6%,3%and 1%,respectively.2)Directional synthesis of phenanthrochalcogeno[9]helicenesWith bb-DTT,bb-STT and bb-DST as starting materials,phenanthrochalcogeno[9]helicenes,[9]BH-1,[9]BH-2,[9]BH-3 were directionally synthesized through bromination,Br/Li exchange,DMF aldehyde,intermolecular Wittig reaction,and oxidation photocyclization.The total yields were 14%,0.03%and 0.05%,respectively.3.Crystal analysis of chalcogen fused chalcogenoaromaticsThe crystal analysis showed that the fused trichalcogenoaromatic compounds bb-SST and bb-TeDT possess a nearly planar molecular geometry.With the introduction of selenium and tellurium atoms,the bond length of C-X increases and the bond angle of C-X-C decreases,meanwhile,there are multiple non-covalent interactions,such as C…Te,Te…Teand Te…S were observed.With the gradual replacement of sulfur atoms by selenium atoms in benzochalcogeno[7]helicenes,[7]BH-1,[7]BH-2,[7]BH-3 and phenanthrochalcogeno[9]helicenes,[9]BH-1,[9]BH-2,[9]BH-3,the inner helix climbs and turns in-plane gradually become larger.The inner helix climbs were recorded as 2.90?,2.92?,2.98?,4.37?,4.56?and 4.67?,respectively,the turns in-plane were recorded as 313.7°,315.4°,322.7°,403.7°,411.6°and 423.3°,respectively.There are multiple non-covalent interactions,for example C…Se,Se…Se,C…S and S…S were observed.The impacts on the intermolecular interaction and molecular stacking mode induced by the gradual change of sulfur atom,selenium atom to tellurium atom were explored by examining these three types of compounds.Thus,the intermolecular interaction mode and molecular stacking pattern can be well-tuned by changing the composition of the chalcogen atoms in the molecular skeletons of chalcogenohelicenes.4.Spectroscopic behaviors of chalcogenohelicenesWith the increase of replacement from sulfur atom to selenium atom in chalcogenohelicenes of both[7]BH-1,[7]BH-2,[7]BH-3 and[9]BH-1,[9]BH-2,[9]BH-3,their UV-vis absorption and fluorescence spectra showed gradually red-shift due to the increase of molecular polarizability.However,the fluorescence quantum yields decreased due to the increasing heavy atom effect.At 77 K,chalcogenohelicenes of both[7]BH-1,[7]BH-2,[7]BH-3 and[9]BH-1,[9]BH-2,[9]BH-3 showed phosphorescence emission in the long wavelength region with fine structures,and the transient phosphorescence emissions exhibited long-lived lifetime at the millisecond level.Furthermore,[9]BH-1 gave an amazing long afterglow with 3s.From these photophysical behaviors of the chalcogenohelicenes,it is clear that their properties can be tuned by the change of chalcogen atoms composition of molecular skeleton.Based on the cyclic voltammetric characteristics of these chalcogenohelicenes,the band gaps were gradually narrowed down with the change of the chalcogen atoms（O,S,Se,Te）due to the increased HOMO and decreased LUMO.The results showed that the energy level and energy gap of the molecules can be tuned by changing the composition of the chalcogen atoms from the chalcogenohelicenes.