Theoretical Investigation Of Absorption And Emission Properties Of Novel Conjugated Copolymers Derived From Fluorene And Benzoselenazole

Organic electroluminescence have been developing in recent ten years.Recearch and exploitation of new organic light-emitting materials arebecoming one of the hotspots. Conjugated polymeric materials have manyadvantages over small inorganic and organic molecules:1. Polymaric materials are easy to manufacture film with big area.2. Conjugated materials have good adhesive ability and stability.3. The electronic structure and emission color of polymers can bemodulated by chemical structure modification.4. Polimaric light-emitting diodes have stable structures.Conjugated polymers have been a subject of considerable academic andindustrial research because of their possible applications in optoelectronicdevices such as photovoltaic devices,light-emitting diodes (LED), solar cellsand field-effect transistors. Among those polymers, polyfluorenes are animportant class of electroactive and photoactive materials. Normally,polyfluorene homopolymers have large band gaps and emit blue light.Emission color of polyfluorenes can be tuned in entire visible region byincorporating narrow band gap comonomer into polyfluorene backbone.Most widelyused narrow band gap comonomers are a variety of aromaticheterocycles with S and N atoms. Since the Se atom has a much larger sizeand less electronegativity than the S atom, it would have a more importantinfluence for the heteroatoms on the light-emitting properties of theresulting copolymers. What more, there are two different theoreticalapproaches to evaluate the band gaps of polymers. One is the polymerapproach in which the periodic structures are assumed for infinite polymers.Another one, the oligomer extrapolation technique, has acquired theincreasing popularity in this field. The oligomer extrapolation is the mainway to get the polymer band gap, which is the topic of the present work.Here we study two fluorene-based copolymers using density functionalmethods and semiclassical models. The copolymers arepoly(9,9-dioctylfluorene-co-alt-benzoselenadiazole)s , in which thecomonomer ratios of 9,9-diethylfluorene(DEF)/benzoselenadiazole(BSeD)are 2:1 and 1:1, and the corresponding copolymers are named (A)n and (B)n.And we acquire the energy gaps and the absorption spectrum byextrapolation.The ground-state and ion geometries of oligomers were fully optimizedusing the density functional theory (DFT),B3LYP/3-21G*, as implementedin Gaussian 03. In order to compare with polyfluorene(PF), theground-state geometries of the monomer are optimized byDFT/B3LYP/6-31G. The results of the optimized stuctures for theoligomeric molecules of (A)n and (B)n (n=1~4) show that the bond lengthsand angles do not suffer appreciable variation with increasing chain lengthin the series of the two oligomers. And it suggests that we can describe thebasic structures of the polymers as their oligomers. The stuctures of the twooligomers are simple. We analyse the occupied obitals, the virtual obitalsand the HOMO-LUMO gaps of the two oligomers. The occupied obitals(HOMO-4~HOMO) and the virtual obitals (LUMO~LOMO+4) appear thesame energy level with increasing chain length in the series of the twooligomers. ZINDO and TD-DFT/B3LYP calculations of the excitationenergies (Egs) and the absorption wavelengths (λabs) are then performed atthe optimized geometries of the ground states. Band gaps of thecorresponding polymers were obtained by extrapolating HOMO-LUMOgaps and the lowest excitation energies to infinite chain length, as well asthe maximal absorption wavelengths of the polymers. The oscillatorstrength of the strongest absorption is going more strongly with increasingchain length in the oligomers, so that the strongest absorption of thepolymers is from S0→S1 electronic transition composed mainly ofHOMO→LUMO transition. The band gaps abtained by HOMO-LUMOgaps, TD-DFT and ZINDO in (B)n are higher than those in (A)n. Andcontrast with (A)n, the maximal absorption wavelength of (B)n is blue shift.Contrast with F2, the lowest excitation energies in A1,B1 are lower, and themaximal absorption wavelengths are red shift.. All of IPs and EAs involvedin this paper are the energies' difference between the ions and molecules.We employed the linear extrapolation technique in this research. Thelinearity between the calculated IPs, EAs, energy gaps, maximal absorptionwavelengths of the oligomers and the reciprocal chain length is excellent forboth homologous series of oligomers. Thus, these values of the polymerscan be obtained by extrapolating the resultant linear relationship to infinitechain length. In all cases, the energy required to create a hole in the polymeris ~5.7eV, while the extraction of an electron from the anion requires ~2.2eV.The ionization potentials and electron affinities of (B)n are all lower thanthose of (A)n. It sugguest that polymers are easy to accept electron but noteasy to creat a hole with increasing concentration of BSeD. The excitedgeometries is optimized by ab initio CIS/3-21G* and the emission spectra iscomputed based on the excited geometries. The changes of the structuresduring the excitation can be prefigured from the characters of front orbitals.The structure will be tight when the antibonding changes into bonding,otherwise, the bond length will increase when the bonding changes intoantibonding. The structure of the excitation states is more quasi-panarcompare the structure of the ground states (35.3o→14.2o).The resultes of the calculations suggest that the band gaps are decreasingand maximal absorption wavelength red shift with increasing BSeDconcentration in copolymers. Compare with PF, P(DEF-BSeD) have lowerband gaps, and the maximal absorption wavelengths are red shift.