Theoretical Investigation On Luminescent Properties Of Polycarbazole And Polyfluorene Derivatives

Since the initial report of polymer light-emitting diodes(PLEDs) based on poly(p-phenylenevinylene) (PPV) by theCambridge group, the dissoluble conjugated polymers havereceived considerable attention because they have many strongpoints such as easy to be dealt with, flexile, membranous and theirenergy gaps can be adjusted by chemical technologies. In the pastdecade, a lot of investigators have devoted themselves to studyingand develop new types of luminescent devices with high efficiency,high stability, high brightness in order to obtain three primary(Red, Green and Blue) colors. Many kinds of conjugated polymershave been synthesized for the electroluminescence, such as PPV,poly thiophene (PTh), poly perinaphthalene (PPP), poly fluorene(PF) and their ramifications and so on. Among these polymers,polyfluorene derivatives show interesting and unique chemical andphysical properties because they contain a rigid planar biphenylunit and the facile substitution at the remote C-9 position, and alsocan improve the solubility and process stability of the polymerswithout significantly increasing the steric interactions in thepolymer backbone. As a result, polyfluorene and its derivativeshave emerged as the most promising light-emitting materials dueto their emission at wavelength spanning the entire visiblespectrum, high fluorescence efficiency, and good thermal stability.Indeed, many blue-and green-light-emitting polyfluorenederivatives have been reported and recently, the search ofred-light-emitting diodes is being interested. However, most ofthese red-emitting polymers synthesized to date have low electronaffinity. It is established that high quantum efficiency PLEDs canbe obtained by achieving both efficient charge injection andbalanced mobility of both charge carriers inside the emissivematerials.The oligomer extrapolation is the main way to get thepolymer band gap, which has proved practicable. In this paper, wemake use of the oligomer extrapolation technique scores of times.To study the properties of polymers using the oligomerextrapolation technique, we consider the varieties of Ionizationpotentials, Electron Affinities, the gap between HOMO andLUMO, the maximal absorption wavelengths along with accretionof conjugated chain length n.We have investigated poly(3,9-carbazole) and polyfluorenederivatives containing 2-pyran-4-ylidenemalononitrile (PM). Theground-state geometries of the oligomers were fully optimizedusing the density functional theory (DFT), B3LYP/6-31G, asimplemented in Gaussian. The results of the optimized structuresfor the oligomeric molecules of every polymer system show thatthe structural changes softly with increasing chain length n. Wecan obtain HOMO energy, LUMO energy and the gap between thetwo orbits and also compute Ionization potentials, ElectronAffinities based on the ground-state geometries of oligomers.ZINDO and TD-DFT/B3LYP calculations of the maximalabsorption wavelengths (λabs) were then performed at theoptimized geometries of the ground states. The maximalabsorption wavelengths and IPs, EAs show excellent linearity inthe plots. Band gaps of the corresponding polymers were obtainedby extrapolating HOMO-LUMO gaps. The excited geometrieswere optimized by ab initio CIS and the emission spectra werecomputed based on the excited geometries. The excited geometriesare compared to the ground geometries computed by HF.The correlative systems and the main contents are followed:(1) The polymer of 2, 7-carbazole and its derivatives havebeen investigated and reported by our research group. Thepolymer of 3, 9-carbazole was synthesized three years ago inPolymer. Poly(3,9-carbazole) has some good performances such ashigh HOMO energy, high thermal stability and high temperature ofglass transition. The systemic theoretical investigation of thispolymer has been taken. The HOMO energies, LUMO energies, thegaps of HOMO-LUMO, and the maximal absorption wavelengthshave shown excellent linearity as function of reciprocal conjugatedchain length n. The extrapolation results of many polymersproperties are in good agreement with the experimental data. Therange of absorption spectra is small that may be due to the baddishconjugation. And the fluorescence spectra are blue-shiftcomparing with poly(2,7-carbazole) and polyfluorene. Thematerials with IP of 4.8-5.8eV can be used as hole transportmaterials in PLEDs.(2) In general, many light-emitting polymers inject andtransport holes more efficiently than electrons due to their inherentrichness of π-electrons. So it is a challenging task to develop newred-light-emitting polymers with high electron affinity, especiallyfor the polyfluorene derivatives. In this paper, we try to advancethe EA and fluorescence efficiency of polymer materials byintroducing 2-pyran-4-ylidene-malononitrile (PM) in polyfluoreneto change the structures of the polymers.So we chose the PFPM and PFOPM as our topic. We havestudied many sorts of rules and properties from oligomer topolymers including the HOMO energies, the LUMO energies, thegaps of HOMO-LUMO, IPs and EAs. The results indicate it is notfavorable to transport the holes because the IPs of PFPM andPFOPM are bigger 0.5eV than polyfluorene, but it is contrarilyfavorable to transport electrons because the EAs are bigger1.2-1.5eV than polyfluorene. The ability on inpouring ortransmitting electron is boost up, and it is especially important thatthe balance of transmission speed between electron and hole isfounded. Electron-deficiency is added due to the debasement ofthe LUMO energy and the increasing of the EA. It is useless forPFOPM by increasing the EAs to introduce aetheroxyl comparingto PFPM. The gaps of HOMO-LUMO are been reduced along withthe increasing of conjugated chain length n. we also have studiedthe spectra properties of the two Polyfluorene Derivatives. Theabsorption spectra and fluorescence spectra of PFPM and PFOPMare red-shift comparing to polyfluorene corresponding with thegaps of HOMO-LUMO. The circumgyration between fluorene andPM is limited because of the presence of aetheroxyl due to theplanar structure of the excited geometry of PFOPM.The research above indicate the poly(3,9-cabazole) differ frompoly(2,7-carbazole) can be used as hole transport materials inPLEDs. The conjugation of poly(3,9-carbazole) is broken and thespectra of it are blue-shift relative to polyfluorene. The EA ofpolyfluorene derivatives (PFPM and PFOPM) is advanced with awide range by introducing 2-pyran-4-ylidene-malononitrile (PM)and the ability on transmitting electron is boost up due to theimprovement of fluorescence materials. Plenty of results haveshown that the structures of the polymer can be modified in orderto obtain the desired IP, EA or the maximal emission wavelengths.