Synthesis And Optoelectronics Properties Of Three-Dimensional Bay-Annulated Indigo Derivatives

With the arriving of electronic age in the 20th century,as well as the developing of molecular structure theory,dyes have been gradually applied from coloration to photoelectric conversion.That is,dye molecules are focused on the radiation transition or charge separation based on their excited states,generating through light or electricity-inducing.This has resulted in a series of organic electronics applications.In the field of organic electronics,a number of molecular materials have been designed and synthesized by introducing chromophore groups.By this approach,not only the structural diversity but also the bipolar transporting characteristics with high carrier mobility of the correponding materials can be obtained.Indigo,possessing highly planar structure and strong intermolecular?-?interaction,shows excellent carrier mobility in crystalline state.Through an annulation reaction at the bay position of indigo,the energy level,band gap and processability of the bay-annulated indigo(BAI)can be further improved.Based on such an enhanced planar structure,exploring BAI-based molecules with three-dimensional(3D)conformation for doped organic optoelectronic devices will expand the applying field of BAI unit.The spiro aromatic backbone,spiro[fluorene-9,9?-xanthene](SFX),with the synthetic advantage of“one-pot”synthesis,3D conformation and rigid steric structure,is beneficial to suppress the strong aggregation tendency of highly planar units.Hence,two BAI dimers,2,7-BAI-SFX and 2',7'-BAI-SFX,and a BAI tetramer,2,7,3',6'-BAI,attaching on SFX core,were designed and synthesized.Hydrogen nuclear magnetic resonance(~1H NMR)and high-resolution mass spectrometry(HRMS)measurements were carried out to confirm the structure of the three compounds.Their photophysical and electrochemical properties were determined by UV-Vis absorption spectroscopy and cyclic voltammetry.Then,the optimized molecular conformations and the distributions of the frontier molecule orbitals(FMOs)of the resulting compounds were performed.The results show that the optical band gaps of 2,7-BAI-SFX and2',7'-BAI-SFX are 1.75 eV and 1.88 eV;the HOMO levels and LUMO levels are-5.27 eV,-5.25 eV and-3.60 eV,-3.50 eV,respectively.The optical band gap of 2,7,3',6'-BAI-SFX is 1.80 eV;the HUMO level and LUMO levels are-5.67 eV and-3.92 eV,respectively.The measured optical band gap is consistent with the electrochemical band gap.Moreover,the absorption spectra of the spin-coated films of the three compounds were compared with the absorption spectra of the dilute solution of chloroform.The maximum absorption wavelength did not show significant shift,indicating a weakened aggregation tendency between the molecules.According to the low-lying LUMO energy level,narrow band gap(1.70~1.80 eV)and the weak aggregation characteristics generated from the 3D structure,we speculate that these molecules are promising n-type semiconductor material for organic optoelectronics.