Theoretical Investigation On The Luminescent Properties Of Poly (Fluorine-co-selenophene) And Poly (Fluorene-co-Thiophene)

There has been considerable recent interest in the optical properties ofpolymers (or oligomers) and complexes, and particularly the luminescentproperties of 7i-conjugated polyfluorene. Due to their desirable combinedproperties, such as good thermal and chemical stability, exceptionally highfluorescence quantum yield, good film-forming and hole-transportingproperties. Normally, polyfluorene homopolymers have large band-gap andemit blue light. Color tuning over entire visible region in these polymers canbe achieved by incorporating narrow band-gap comonomer intopolyfluorene backbone. The most widely used narrow band-gapcomonomers are a variety of aromatic heterocycles with S and N atoms,such as thiophene, ethylenedioxythiophene. Recently, polymers containselenophene have been reported as light-emitting materialsIn order to rationalize the experimentally observed properties of knownmaterials and to predict those of unknown ones, theoretical investigationson the structures and electronic spectra and emissive properties of thosematerials are very necessary. In the past decades, ab initio and semiempirical approaches were applied to analysis various properties of various oligomers and polymers. Recently, methods based on density functional theory(DFT) is becoming more and more popular due to its feature of including the electronic correlation in a computationally efficient manner. In its formalism, the ionization potential (IP) and electron affinity (EA) are well-defined properties that can be calculated. Time-dependent DFT(TDDFT) is a recently developed tool for computing the excitation energies, oscillator strengths and excited state compositions in terms of excitations between occupied and virtual orbitals, which has been successfully applied to calculate the band gaps and the effective conjugation length of some polymers. There are two different theoretical approaches to evaluate the band gaps of polymers. One is the polymer approach in which the periodic structures are assumed for infinite polymers. Another one the oligomer extrapolation technique, has acquired the increasing popularity in this field. In this approach, a sequence of increasing longer oligomers is calculated, and extrapolation to infinite chain length is followed. A distinct advantage of this approach is that it can provide the convergence behavior of the structural, electronic, and spectral properties ofpolymers. In practice, both the oligomer extrapolation and the polymer approaches are generally considered to be complementary to each other in understanding of the properties of the polymers. Here we studied two fluorine-based copolymers poly(fluorine-co-selenophene)(PFS) and poly(fluorene-co-thiophene)(PFT) using density functional methods and semiempirical models.Different from other investigations the main feature of this work is extrapolating the resultant linear relationship to infinite chain length to obtain the IPs and EAs of the polymers as well as energy gaps and absorption wavelengths.Optimization of the ground state geometries of the oligomers were carried out using the density functional theory (DFT), B3LYP/6-31G, as implemented in Gaussian 03. The results of the optimized structures for the oligomeric molecules of the (FS)n(n=l~4) and (FT)n(n=l~4) show that the structural changes softly with increasing chain length in the series of (FS)n, as well as (FT)n. And it suggests that we can describe the basic structures of the polymers as their oligomers. (FS)n has a stronger tendency to be coplanar compared with (FT)n ZINDO and TD-DFT/B3LYP calculations of the lowest excitation energies and the maximal absorption wavelengths (X.abs)were then performed at the optimized geometries of the ground states. The lowest excitation energies and the maximal absorption wavelengths show excellent linearity in our plots. Band gaps of the corresponding polymers were obtained by extrapolating HOMO-LUMO gaps and the lowest excitation energies to infinite chain length, as well as the maximal absorption wavelengths of the polymers. The extrapolation results of Egs and A-abs are in good agreement with the experimental data. The results of each method indicate the same conclusion that the increasing of the conjugation in the backbone of PFS makes its band gap narrow. The narrower band gap of PFS causes its longer maximal absorption wavelengths, compared with PFT. The excited geometries were optimized by ab initio CIS/3-21G and the emission spectra were computed based on the excited geometries. The changes of the structures during 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 into antibonding. All of IPs and EAs involved in this paper are the energies' difference between the ions and molecules. We employed the linear extrapolation technique in this research. The linearity between the calculated IPs, EAs, energy gap, maximal...