Syntheses, Crystal Structures And Properties Of9-Fluorenyl Schiff Bases Derivatives

Fluorene and its derivatives are excellent organic blue light-emitting materials. Theirresearch had caused more and more attention. Organic crystals provide important structuralinformation for the properties of the materials. In order to further explore the relationshipbetween the crystal structures and properties of fluorene derivatives, we synthesized a seriesof novel fluorene derivatives with Schiff base and spiro structures. We systematically studiedthe crystal structure, thermodynamic properties and photoelectric performance. We used thequantum chemical calculation methods to study their electronic structure and spectralcharacteristics, which would explain the relationship between the molecular structures andproperties. The main contents and results are as follows:1. We synthesized nine novel9-substituted fluorene derivatives. Their molecular structurescan be divided into four types: Four para-aryl-9-sustituted fluorenyl bi-Schiff bases; Twopara-alkyl-9-sustituted fluorenyl bi-Schiff bases; Two ortho-aryl-9-sustituted fluorenyl bi-Schiff bases; One9-spiro fluorene derivative. The target products were fully characterized byIR,1H NMR,13C NMR, mass spectrometry and elemental analysis. Single crystals of thecompounds were obtained by the method of slowly-solution-volatile. Crystal structural datawere determined by X-ray single crystal diffraction experiments.2. The thermodynamic, optical and electrochemical properties of all the compounds werestudied. We calculated the optimized structure, frontier molecular orbitals, electron energylevels, absorption spectra and emission properties using theory of quantum chemistry. Thecalculated results were analyzed and compared with the experimental results. Details are asfollows:1) Para-aryl-9-sustituted fluorenyl bi-Schiff bases:Thermodynamic analysis showed that all the compounds have high thermaldecomposition termperatures, and there are no glass transitions. The absorption spectrum hadtwo bands (one strong and the other weak), representing π-π~*and n-π~*transition, respectively.The fluorescence spectra showed that these compounds had blue light emission and goodfluorescence quantum efficiency. Cyclic voltammetry was used to determine the HOMO andLUMO energy levels of the compounds. Scanning electron microscopy (SEM) and scanning tunneling microscopy (STM) were used to characterize the morphology of the compounds.We fabricated some electroluminescent devices using the four compounds, and measured therelationship between the voltage and brightness.2) Para-alkyl-9-sustituted fluorenyl bi-Schiff bases:The thermodynamic analysis showed that compounds containing alkyl chains exhibitedworse thermal stability than their aryl-substituted analogues. The absorption spectrum hadtwo bands (one strong and the other weak), representing π-π~*and n-π~*transition, respectively.The fluorescence spectra showed that the two compounds had blue light emission andmoderate fluorescence quantum efficiency.3) Ortho-aryl-9-sustituted fluorenyl bi-Schiff bases:The thermodynamic analysis indicated that these two compounds had glass transitionprocess and had good thermal stability. The absorption spectrum had two bands (one strongand the other weak), representing π-π~*and n-π~*transition, respectively. Molecules in solidfilms had higher fluorescence quantum efficiency than in solution.4)9-Spiro fluorene derivative:The thermodynamic analysis showed that1’,3’-dihydrospiro[fluorene-9,2’-perimidine]with rigid spiro structure exhibited excellent thermal stability. The absorption maximumwavelength was352nm, which was attributed to π-π~*transition. Molecules in the solid filmshowed high fluorescence quantum efficiency. Cyclic voltammetry was used to determine theHOMO and LUMO energy levels.3. Three kinds of fluorenone co-crystals based on N-H…O hydrogen bonds were preparedby simple recrystallization. Crystal structural data were determined by X-ray single crystaldiffraction experiments. Intermolecular interactions were analyzed and compared within thethree co-crystals. The type and size of the hydrogen bond were characterized by IR spectra.The thermodynamic study showed that there was only one melting point in each co-crystal.