Synthesis And Photophysical Properties Of Pyrazinothiophene-based Triple Helicenes

Helicene is polycyclic aromatics formed by ortho-fused multiple aromatic rings,and its whole molecule presents a screw-shaped skeleton.As an important part of heterohelicene,thiohelicene has made great progress in both construction of molecular structures and photoelectric properties.However,thiohelicene skeleton contains multiple sulfur atoms,which heavy atom effect is very remarkable.Since the heavy atom effect directly affects the precess of intersystem crossing(ISC),the fluorescence quantum yield of thiohelicene is generally low.In this thesis,we attempted to construct functionalized novel thiophene-based triple helicenes with low the heavy atom effect by introducing a strong electron-withdrawing group such as pyrazine into the helicene skeleton to design D-A compound or using metal to ligand charge transfer(MLCT)effect to construct Ru(II)metal-coordination compound.A series of single helicene and triple helicene compounds(DPPz[7]H,TCPz[7]TH)with D-A structures,and Ru(II)-directed triple helicates(DPPz[7]HRu)were designed and synthesized.Besides the synthetic work,the absorption and fluorescence spectroscopic behaviors are also explored.The main content is divided into the following two parts.1.Synthesis and photophysical properties of pyrazine-thiophene-based helicenesUsing 3-bromothiophene as the starting material,diketodithiophene 4 and diketo[7]thiohelicene 9 were synthesized by multi-stepped moisture sensitive reactions,such as Cu Cl₂-promoted oxidative reaction,trimethylsilyl group protection,cyclization with(Ph SO₂)₂S,bromine migration,dimethyl oxalate cyclization,etc.In the overall yieldof 32%,5%,respectively.Then diketone 4 and 9 reacted with o-phenylenediamine and 2,3,6,7,14,15-hexaaminotriptycene hexahydrochloride,respectively by amine-ketone condensation to obtain the four pyrazine-thiophene fused aromatic compounds,DTPz,TCPz,Pz[7]H,and TCPz[7]TH with total yields of 31%,25%,4%,and 3%,respectively.Here,Pz[7]H is pyrazine-thiophene-based mono-helicene,and TCPz[7]TH is a triptycene-cored triple helicene.All intermediates and target products were characterized.The single crystal structure of the compound TCPz exhibits the typical skeleton characteristics of triptycene in the center of molecule with C₃-axis symmetric.In the crystal packing of TCPz,multiple intermolecular interactions,such as C···C and S···C were found.The single crystal structure of 9 reveals that the molecular skeleton of the helicenedione has a helical configuration with the helix climbing height of 3.62?and the turns in-plane angle of237.5°.In the crystal packing of 9,multiple intermolecular interactions,such as C···O,S···C,C···C were observed.The UV-vis absorption spectra of DTPz,TCPz,Pz[7]H,and TCPz[7]TH show typical D-A conjugation effect,showing strong absorption band at long wavelength.These compounds exhibit typical intramolecular charge transfer(ICT)state emission with significant solvent polarity effects at room temperature in solution.In the frozen state(77 K),they show both fluorescence and phosphorescence emissions.These spectroscopic properties reveal remarkable relationship with the conjugation of molecular structures,and show the fact that introduction of pyrazine unit in thiophene-based helicene can greatly increase the emission ability of helicenes,including Pz[7]H,and TCPz[7]TH.Moreever,the synthesized triptycene-cored triple helicene(TCPz[7]TH)a new and novel member benefiting the helicene family.2.Synthesis and photophysical properties of ruthenium(II)coordinated pyrazine-thiophene-based triple heliceneDiketone compounds 4 and 9 reacted respectively with 1,10-phenanthroline-5,6-diamine by amine-ketone condensation reaction to afford ligands DTDPPz and DPPz[7]H.Complex DTDPPz Ru and DPPz[7]HRu were synthesized by metal-coordination between Ru(II)and the ligands in the total yields of 20%,2%,respectively.A novel triple helicene,DPPz[7]HRu was successfully synthesized.All intermediates and target products were characterized.The single crystal structure of DTDPPz Ru shows that the molecule has three dithienophenazinophenanthroline building units by six-coordinated bonds in the perpendicular manner to each other.The multiple intermolecular interactions,such as S···C,C···C,N···N,N···C were observed in the crystal.Both UV-vis absorption and fluorescence spectra of ligands DTDPPz and DPPz[7]H show a significant conjugated moleular structures and properties relationship.Complex DTDPPz Ru and DPPz[7]HRu exhibit the strong metal to ligand charge transfer(MLCT)effect in both absorption and fluorescene emission.Althrough DPPz[7]HRu does not show the obvious increase of emission,the obtained ruthenium(II)directed triple helicene itself is a novel member,which benifits both helicene chemistry and coordination chemistry.More work is in the progress.