π-extended S₄ And C_3h Symmetric Cyclized Indole:Synthesis-Structure-Nonlinear Optical Property

Since 1991,the concept of "octupole" has been introduced into the field of organic nonlinear optics(NLO)by zyss,octupolar molecules have gradually become a research hotspot.The research on the conjugated bridges(π)and the peripheral electron acceptors(A)is relatively mature,while octupolar cores(D)are still limited to benzene,s-triazine,truxene and triindole etc.The more efficient electron-donating octupolar platforms need to be synthesized and studied urgently.Tetraindole(TTI)is a three-dimensional(3D)octupolar conjugated system with S4 symmetry,and triindole(TI)is a planar octupolar conjugated system with C3h symmetry.which have gradually attracted research interest in the field of NLO.The photoelectric properties of conjugated organic molecules are affected by many structural factors,in this thesis,two strategies had been adopted to enhance the NLO effects of the octupolar molecules:(1)introducing the electron-withdrawing groups to improve the intramolecular charge transfer property;(2)extending the π delocalization system by aromatic-fusion manner.In this thesis,the octupolar platforms of the three/two-dimensional charge transfer systems and the π-extended conjugated systems were constructed based on the S4-symmetric octupolar molecule TTI and the C3h-symmetric octupolar molecule TI.1.Three/two-dimensional charge transfer systems:synthesis,structure,NLO effects and hole transport properties.Based on TTI and TI.a series of position-isomerized TTI and TI derivatives(TTI-2-CF3 vs TTI-3-CF3 and TI-2-CF3 vs TI-3-CF3)were synthesized by introducing electronwithdrawing groups(-CF3)at 2-and 3-positions,respectively.The 3D charge transfer systems and planar charge transfer systems were successfully constructed.The crystal structure shows that the shaddle-shaped TTI-2-CF3 maintains the S4 symmetric octupolar structure.The optimized geometries of TI-2-CF3 and TI-3-CF3 possess C3-symmetry with very small dipole moments(0.195 and 0.154 Debye,respectively),indicating that TI-2-CF3 and TI-3-CF3 can be regarded as quasi-octupolar molecules.Among them,TI-2-CF3 crystallizes in the non-centrosymmetric space group Pna21,and the SHG intensity of its powder crystal is one order of magnitude higher than that of KDP(KH2PO4).The absorption and fluorescence spectra presented significantly redshifts.indicating that the charge transfer systems were initially constructed.The redshift of TI-2-CF3 is more significant.The theoretical second-order NLO coefficient βvalue and the theoretical hole mobility μ(1.1 85 cm2V-1 s-1)of TI-2-CF3 are significantly higher than that of its position-isomer TI-3-CF3,indicating that the 2-substitution site is superior to the 3-substitution site in this system.2.π-conjugation extended TTI and TI systems:synthesis,structure,NLO effects and linear photophysical properties.A series of π-extended tetraindole and triindole derivatives(TTIid vs TIid,s-TTIbt vs s-TIbt,a-TTIbt vs a-TIbt,TIph)were synthesized to construct more efficient electron donating octupolar platforms.The crystal structure shows that s-TTIbt maintains S4-symmetric octupolar architecture and crystallizes in P42/n space group.After the aromaticfusion,the absorption and fluorescence spectra showed significant redshifts,indicating that the π-conjugated system was successfully extended.The spectral redshifts of anti-isomers(a-TTIbt and a-TIbt)are larger than that of corresponding syn-isomers(s-TTIbt and s-TIbt).The two-photon absorption(TPA)cross sections(δ values)of the trimers have been measured to be 10-102 times that of the parent TI,and the δ value of the anti-isomer(a-TIbt)has been measured to be 2.7 times that of syn-isomer(s-TIbt)and 102 times that of TI.Compared with the parent TTI and TI,the theoretical NLO coefficients(β values)ofπ-extended TTI and TI derivatives are significantly improved.The theoretical β values of a-TTIbt and a-TIbt are significantly higher than those of s-TTIbt and s-TIbt,and have been calculated to be 2.1 and 4.2 times those of the parent TTI and TI,respectively.Both the experimental spectra and the theoretical calculations demonstrate that the anti-isomer(a-TIbt)formed better conjugated system than the syn-isomer(s-TIbt),which can reasonably explain the more excellent NLO properties of a-TIbt.3.N-phenyl substituted aromatic-fused TI systems:synthesis,structure and photophysical properties.A series of N-phenyl aromatic-fused triindoles(TIid-ph.s-TIbt-ph and a-TIbt-ph)were synthesized by introducing phenyl at the N-H position,which significantly improved the stability of aromatic-fused TI derivatives.These target compounds exhibited excellent fluorescence luminescence properties in solution and solid-state.Due to introducing of the phenyl ring,the absorption and fluorescence spectra displayed significantly redshifts.indicating that the benzene rings at the N atom position also contributed to the conjugation of the whole molecule.The s-TIbt-ph molecules lose the strict planarity,which is caused by the steric hindrance in s-TIbt-ph.The atoms except the N-phenyl group remain basically coplanar.and the phenyl rings on the N atom are biased towards opposite sides of the molecular plane.In summary,we have adopted three strategies to synthesize TTI and TI derivatives to tune the photophysical properties of the target compounds.The crystal structures and solution/solid-state luminescence properties were measured and studied.The nonlinear optical properties(SHG and TPA)were studied theoretically and experimentally.The charge transport properties of partial compounds were studied theoretically.In this thesis,a series of S4-and C3-symmetric octupolar platforms with more excellent NLO responses than parent TTI and TI have been constructed.These compounds can be further acted as efficient NLO octupolar platforms to build more excellent NLO materials and other photoelectric materials by modifying with electron-withdrawing groups in the future.