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Theoretical Study On Conjugated Polymers Containing Heterocycle For Luminescent Materials

Posted on:2008-02-14Degree:MasterType:Thesis
Country:ChinaCandidate:H L WangFull Text:PDF
GTID:2121360218457945Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Recently, the molecular design, synthesis and the luminescent mechanism of the luminescent polymer materials become one of the most active fields. The paper applied the quantum chemistry methods and systematically studied the following 4 systems: (1) poly(alkylthiophene-co-pyridine)s; (2) carbazolyl-pyridinyl alternating copolymers; (3) 2,7- linked and 3,6-linked carbazole trimers (4) phenol-pyridyl boron complexes.Firstly, quantum-chemical techniques were applied to investigate a series of conjugated polymers: poly(3-octylthien-2,5-ylene-co-pyrid-2,6-ylene) (p1), poly[(pyrid-2,6-ylenebis(3-octylthien-2,5-ylene)) (p2) and poly[pyrid-2,5-ylenebis(3-octylthien-2,5-ylene)] (p3), comprising alternatingπ-excessive 3-alkylthiophene andπ-deficient meta- or para-linked pyridine moieties. Their ground state and excited state structures were optimized with density functional theory B3LYP method at 3-21G* level, and the optical properties were calculated by the time-dependent density functional theory (TD-DFT) and ZINDO/S methods. Their HOMO-LUMO gaps (△H-L), the lowest excitation energies (Eg), ionization potentials (IP) and electron affinities (EA) were obtained by extrapolating those of the polymers to the inverse chain length equal to zero (1/n=0). The calculated results showed that the decrease of the pyridylene content and the para-linked pyridylene in the backbone of the polythiophene resulted in a narrowed energy gap and bathochromic absorption and emission peaks.Secondly, three A-B-type fluorescent copolymers comprised of alternating carbazolyl and pyridinyl units, poly[(2,7-(N-(2-ethylhexyl)carbazolyl)-alt-(3,5-pyridinyl))](PEHCP-35), poly[(2,7-(N-(2-ethylhexyl)carbazolyl)-alt-(2,6-pyridinyl))](PEHCP-26) and poly[(2,7-(N-(2-ethyl- hexyl)carbazolyl)-alt-(2,5-pyridinyl))](PEHCP-25), are studied by means of the density functional theory (DFT/B3LYP/6-31G). Based on the optimized geometries, the optical properties are calculated by employing time dependent density functional theory (TD-DFT). The band gaps and optical properties are saturated quickly in PEHCP-35 and PEHCP-26. It is known from experiment that PEHCP-25 is actually an oligomer with a polymerization degree of 4. So the tetramers of PEHCP-35, PEHCP-26 and PEHCP-25 are adopted to study the electronic and optical properties, and the calculated results are in close agreement with experiment. The calculated band gaps of copolymers obtained from two ways, namely, HOMO-LUMO gaps and the lowest excitation energies, decrease in the following order PEHCP-35 > PEHCP-26 > PEHCP-25, the same trend as the data obtained from the edge of the electric band but different from the electrochemically obtained data from experiment (PEHCP-25 > PEHCP-26 > PEHCP-35). The outcomes showed that when excited a charge transfer from carbazolyl unit to pyridinyl unit occurs and the lumophor is mainly carbazolyl units. The UV absorption and emission wavelengths both exhibit bathochromic shifts: PEHCP-35 < PEHCP-26 < PEHCP-25.Thirdly, electronic properties, such as HOMO and LUMO energies, band gaps, the ionization potential (IP) and electron affinity (EA) of 2, 7-linked and 3, 6-linked carbazole trimers, two conjugated oligomers with different linkage of carbazole, were studied by the density functional theory with Becke–Lee–Young–Parr composite exchange correlation functional (B3LYP). The absorption spectra of these compounds were also investigated by time dependent density functional theory (TD-DFT) with 6-31G* basis set. The calculated results indicated that the HOMO and LUMO of the 2, 7-linked and 3, 6-linked carbazole trimers are both slightly destabilized on going from methyl substitution to sec-butyl substitution. Seen from IP and EA, they all exhibit good hole-transporting but poor electron-accepting ability. The presence of alkyl groups on the nitrogen atoms does not affect the intrachain electronic delocalization along the molecular frame, thus no significant effect on the band gap and absorption spectra of compounds.At last, electronic properties, such as HOMO and LUMO energies, band gaps and the ionization potentials (IP) of five boron complexes containing mixed phenol-pyridine donor set, were studied by the density functional theory with Becke–Lee–Young–Parr composite exchange correlation functional (B3LYP). The absorption and emission spectra of these complexes were also investigated by time dependent density functional theory (TD-DFT) with 6-31G* basis set. The calculated results indicated that the HOMO and LUMO of the complexes are both destabilized by electron-donating substituents (CH3 and NH2). Seen from IPs, they all exhibit good hole-transporting ability, and their correlations with HOMO energies is linear. The TD-DFT calculations showed that different substituents on the phenol-pyridyl unit lead to nominal change on the absorptions of these boron complexes. The calculations on the structural, electronic and optical properties of the compounds are in good agreement with the experimental data. The electronic and luminescent properties of a series of heterocyclic polymers were analyzed and investigated theoretically. The results indicate that density functional theory and time dependent density functional theory can provide reasonable geometry and electronic properties, and we can apply it to the design of new luminescent polymer materials.
Keywords/Search Tags:Time-dependent density functional theory, ionization potential, electron affinity, absorption spectrum, emission spectrum
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