A Theoretical Study On Photoelectric Properties Of Fluorene Based Electroluminescent Ma Terials

The fluorene and their derivatives are seemed to be the most potentialblue light-emitting electroluminescent materials for these advantages: Goodthermal and chemical stabilities for their plane rigid biphenyl unit. Haveextremely high fluorescence quantum efficiency in solid state (about0.6~0.8). Their structure can be easily modified. And throughcopolymerization, blending, and modification we can get tunable emissioncolors. In this article, we design three kinds of fluorene based blue lightemittingmaterials. Quantum chemistry calculations are adopted toinvestigate their structure and photoelectric properties. The influences onthe structure and photoelectric properties by end-capping and substitutemodification are also investigated.We get their ground state geometries through the calculation of thedensity functional theory (DFT)-B3LYP/6-31G(d). From the results we cansee: The oligo-fluorene that end-capped symmetrically by the electronwithdrawingthienyl have higher complanation degree; the geometries of fluorene essentially unchanged and the rigid structural character ismaintained after spiro(9) modification. Fluorene that asymmetric endcappedby the electron-withdrawing thienyl and electron-deficientoxadiazole unit D-A (D=donor, A=acceptor) show that the complanationdegree of the oxadiazole with fluorene chain is higher than that of thethienyl with fluorene. From the analysis of the HOMO and LUMO frontiermolecular orbital, we can see that symmetry end-capped by the electronwithdrawingthienyl lowered the LUMO energy level accompany with thelifting of the HOMO energy level; The spiro(9) modification on thefluorene regulates the HOMO and LUMO energy levels balanced; The D-Aend-capping 9,9'-spirobifluorene have greater influence on the LUMOenergy level than the HOMO energy level.Analysis of electrical properties, namely, ionization potential, electronaffinity and reorganization energy calculations, we can see: Three kinds ofmodification lower the ionization potential and electron affinity of thefluorene, and denote that the electron and hole transportation capabilitiesare enhanced; The further analysis of the reorganization energies is madehere. The electron and hole reorganization energies are almost the same inthiophene end-capped fluorene which denote that the balance betweenelectron and hole transport; the D-A end-capping modification lead to theincreasing of the electronic transportation capabilities.Analysis of optical properties, namely, the UV-visible molecular absorption and fluorescence spectra of molecules, we can see: From thecompare of their UV-visible absorption spectra, we can see that thethiophene end-capped fluorene possess stronger absorption intensity, the DAend-capped 9,9'-spirobifluorene possess the largest maximum absorptionwavelength lower transition energies, which convenient to the excitation ofphoton. The calculation of the fluorescence spectra can give us someinformation about the excited geometry structural features. And we find thatthe complanations of their excited states are higher than their ground states.The oscillator strength and maximal absorption wavelengths of thefluorescence spectra are larger than that of the UV-visible absorptionspectra. All these materials can emit blue light with a great intensity. Thecomplanation degree of the thiophene end-capped fluorene is the highest,and the oscillator strength and maximal absorption wavelengths are thelargest.