Synthesis Of Benzene-cyclooctatetrathiophene And Triazine-cyclooctatetrathiophene Derivatives And Their AIE Properties

Aggregation-induced emission（AIE）is an abnormal phenomenon with certain organic luminophores.These luminescent molecules have no fluorescence or weak fluorescence in the solution while they show strong fluorescence in the aggregated state or in the crystalline state.Since this concept has been brought in2001,it has been widely applied in ion detection,bio-imaging,photodynamic therapy and so on.As for the current reported AIE systems,the AIE mechanism is generally classified into the restriction of intramolecular rotations（RIR）and/or the restriction of intramolecular vibrations（RIV）,However,only a few examples based on RIV are reported.So it would be interesting to find some new type of RIV examples and increase AIE emission.Cyclooctatetrathiophene（COTh）is one of the cyclic-typed oligothiophenes possessing a“saddle shaped”structure and more reactive positions for derivation and functionalization.Our recent study demonstrates that COTh also has the AIE properties,which can be classified to the RIV mechanism.Based on the previous finding,COTh has been selected as a building block in this study and two series molecules are designed and synthesized with benzene and triazine as cores.The photophysical properties and their AIE activities have been studied and the main works are summarized as following:1.Synthesis and AIE Properties of the Benzene-COTh DerivativesUsing 3TCOTh as starting material,Ph-COTh,Ph-2COTh,and Ph-3COTh were synthesized through nucleophilic substitution reaction and Suzuki reaction.Their total yields were 72%,73%and 69%,respectively.All the three molecules（Ph-COTh,Ph-2COTh,and Ph-3COTh）displayed four absorption peaks during UV-vis absorption characterization and the two absorption peaks in the long wavelength of the three compounds are attributed to intramolecular charge transfer（CT）absorption based on the quantum theoretical calculation.In solution,all molecules exhibited single intramolecular charge transfer（ICT）state emission peaks（550-565 nm）.However,local emission of COTh peaked at 400 nm and ICT state emission peaked in region of 470-480 nm are observed in rigid solution for these molecules.Their quantum yields（QY）enhanced with the increased number of COTh units in each molecule.In the THF-H₂O binary solvent system,AIE phenomena were observed for all the three molecules and the AIE emission showed up with the emission peaks were 500,508 and 520 nm,respectively for Ph-COTh,Ph-2COTh,and Ph-3COTh,when the corresponding water volume fraction reached up to70%,50%,and 40%,respectively.The structure of Ph-COTh has been confirmed by the single crystal analysis and the results indicate that H-H interactions are existing between the molecules,all the results indicated the restriction of molecular skeleton vibrations and intramolecular single bands rotations are the main reasons for the AIE phenomena of these compound.The chiral resolution of Ph-COTh was done by chiral HPLC.Two enantiomers,（+）-Ph-COTh and（-）-Ph-COTh were obtained efficiently.Their CV spectra,optical rotations and half-lifes were measured.（-）-Ph-COTh showed clear AIE phenomenon in the THF-H₂O binary solvent system.However,its AIE intensity is weaker than Ph-COTh due to weak intermolecular interaction in aggregation.2.Synthesis and AIE properties of Triazine-COTh DerivativesUsing the 3TCOTh as a starting material,T-COTh,T-2COTh,and T-3COTh were synthesized via Kumada-type reaction.The two absorption peaks in the long wavelength of the three compounds are attributed to intramolecular charge transfer（CT）absorption according to the quantum theoretical prediction.All molecules exhibited single intramolecular charge transfer（ICT）emission peaks（560-570 nm）.However,local emission of COTh peaked at 400 nm and ICT state emission peaked in region of 480-500 nm are observed for these molecules at the frozen state.Meanwhile,the QY for these molecules elevated with the increased number of COTh units in each molecule.In the THF-H₂O binary solvent system,AIE phenomena were observed for all the three molecules and the AIE emission start to show up with the emission peaks were 545,550,and 530 nm for Ph-COTh,Ph-2COTh,and Ph-3COTh when the corresponding water volume fraction reached up to 80%,60%,and 40%,respectively.The structures of T-COTh and T-2COTh have been confirmed by the single crystal analysis and the results indicate that the thiophene ring and the neighboring triazine ring tend to present near-plane conformations,which may be attributed to the N-S intramolecular interactions between the triazine ring and its adjacent thiophene ring.Such N-S intramolecular interactions would facilitate the ICT emission and the increase of the fluorescence QY of these Triazine-COTh derivatives.In addition,the coexistence of the C-C and O-H interactions would restrict the free rotations of single bonds between the thiophene ring and the triazine ring in the molecule,which facilitates the generation of the AIE phenomenon.Similarly,all the results indicated the restriction of molecular skeleton vibrations and intramolecular single bands rotations are the main reasons for the AIE phenomena of these compound.