Theoretical Studies of the Absorption and Emission Properties of the Fluorene-Based Conjugated Polymers (original) (raw)
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Journal of Organic Chemistry, 2005
Poly(fluorene)-type materials are widely used in polymer-based emitting devices. One of the drawbacks of light-emitting diodes based on polyfluorene derivatives is the injection of holes from the anode due to the high ionization potential (IP) of most derivatives. Substitution by electrondonating alkoxy substituents or by adding charge carriers on the conjugated polymer's backbone produces a remarkable influence on its electrical and optical properties. In this contribution, we apply quantum-chemical techniques to investigate a family of π-conjugated polymers with substituted dimethoxy groups at the 3,6 positions of the fluorene ring, namely, poly(2,7-(3,6dimethoxy-fluorene)(PDMOF), poly(2,7-(3,6-dimethoxy-fluorene)-co-alt-fluorene (PDMOFF), and poly(2,7-(3,6-dimeth-oxy-fluorene)-co-alt-2,5-thiophene (PDMOFT). The electronic properties of the neutral molecules, HOMO-LUMO gaps (∆ H-L ), in addition to the positive and negative ions, are studied using the B3LYP functional. The lowest excitation energies (E g ) and the maximal absorption wavelength λ abs of PDMOF, PDMOFF, and PDMOFT are studied by employing time-dependent density functional theory (TD-DFT) and the ZINDO semiempirical method. The IP, EA, and E g values of each polymer were obtained by extrapolating those of the oligomers to the inverse chain length equal to zero ( 1 / n ) 0). The influence of the presence of methoxy groups on the fluorene moiety on the ionization potential is especially emphasized. The outcomes show that the HOMO energies of these systems under study increase by about 0.4 eV and the IP values decrease by about 0.3 eV compared to those of the corresponding polyfluorene. Both effects result in a reduction of the energy barrier for the injection of holes in related polymeric light-emitting devices and should contribute to the enhancement of their performances. Because of the cooperation with thiophene in PDMOFT, which results in a good planar conformation, both the hole-creating and electronaccepting abilities are improved.
International Journal of …, 2011
Density functional theory (DFT) and time-dependent DFT (TDDFT) were employed to study ground-state properties, HOMO-LUMO gaps (Δ H-L ), excitation energies (E g ), ionization potentials (IPs), and electron affinities (EA) for PFV-alt-PDONV and PFV-alt-PDIH-PPV having different alternating groups. Excited-state properties were investigated using configuration interaction singles (CISs) while fluorescence energies were calculated using TDDFT. The results show that PFV-alt-PDONV exhibits blueshifted energies for both HOMO-LUMO gaps (Δ H-L ) and excitation energies (E g ) compared with PFV-alt-PDIH-PPV. The predicted IP and EA clearly indicate that PFV-alt-PDIH-PPV has both easier hole creation and electron injection than that of PFV-alt-PDONV. The maximal absorption wavelengths of all polymers are strongly assigned to π → π * transition. The predicted radiative lifetimes of PFV-alt-PDONV and PFV-alt-PDIH-PPV for B3LYP/6-31G(d) are 0.36 and 0.61 ns, respectively, indicating that PFV-alt-PDIH-PPV should have a better performance for long-time emission than that of PFV-alt-PDONV.
Polymer, 2005
Poly(fluorene)-type materials are widely used in polymer-based emitting devices. During operation there appears, however, an additional emission peak at around 2.3 eV, leading to both a color instability and reduced efficiency. The incorporation of the carbazole units has been proven to efficiently suppress the keto defect emission. In this contribution, we apply quantum-chemical techniques to investigate two series of alternating fluorene/carbazole oligomers and copolymers poly[2,7-(N-(2-methyl)-carbazole)-co-alt-2,7-m(9,9-dimethylfluorene)], namely, PFmCz (m ϭ 1,2) and gain a detailed understanding of the influence of carbazole units on the electronic and optical properties of fluorene derivatives. The electronic properties of the neutral molecules, HOMO-LUMO gaps (⌬ H-L ), in addition to the positive and negative ions, are studied using B3LYP functional. The lowest excitation energies (E g s) and the maximal absorption wavelength abs of PFmCz (m ϭ 1,2) are studied, employing the time-dependent density functional theory (TD-DFT). The properties of the two copolymers, such as ⌬ H-L , E g , IPs, and EAs were obtained by extrapolating those of the oligomers to the inverse chain length equal to zero (1/n ϭ 0). The outcomes showed that the carbazole unit is a good electron-donating moiety for electronic materials, and the incorporation of carbazole into the polyfluorene (PF) backbone resulted in a broadened energy gap and a blue shift of both the absorption and photoluminescence emission peaks. Most importantly, the HOMO energies of PF1Cz and PF2Cz are both a higher average (0.4 eV) than polyfluorene (PF), which directly results in the decreasing of IPs of about 0.2 eV more than PF, indicating that the carbazole units have significantly improved the hole injection properties of the copolymers. In addition, the energy gap tends to broaden and the absorption and emission peaks are gradually blue-shifted to shorter wavelengths with an increase in the carbazole content in the copolymers. This is due to the interruption of the longer conjugation length of the backbone in the (F1Cz) n series.
The Journal of Chemical Physics, 2003
We report a detailed quantum-chemical characterization of the electronic and optical properties of polyfluorene chains and compare them to those in copolymers containing alternating fluorene and benzothiadiazole or ethylenedioxythiophene units. The introduction of the comonomer can strongly modify the excitonic properties as well as the efficiency of charge-and energy-transfer processes. The choice of the comonomer is thus critical in targeting specific optical properties while maintaining good transport properties.
The Journal of Physical Chemistry B, 2002
A detailed analysis of the optical and photophysical properties of 2,7-bis(phenylene)-9,9-dioctylfluorene (PFP), 2,7-bis(biphenylene)-9,9-dioctylfluorene (BPFBP), 2,7-bis(2-thienyl)-9,9-dioctylfluorene (TFT), and 2,7-bis-(2,2 \-bithien-5-yl)-9,9-dioctylfluorene (BTFBT) in various environments are reported. The optical properties of the free molecules isolated in an alkane matrix are obtained and discussed in terms of the conformation adopted by each derivative in the electronic ground and first excited states. Also, conformational changes are responsible for the optical changes observed at high concentrations in an isopentane glass at 77 K. High quantum yields of all the oligofluorenes at 77 K indicate the absence of quenching effects such as excitonic or aggregation effects. The similar spectral and photophysical properties in matrix and glass environments are explained by the disorder introduced in oligofluorenes by long octyl chains at the C-9 position of the fluorene moiety. To study the effect of intermolecular interactions in the solid state, we recorded the spectra of thin films of these derivatives. The much red-shifted emission band in the solid state cannot be explained by conformational changes and has its origin in the π-stacking of conjugated oligomers in their relaxed S 1 state. As an evidence to show the importance of the role played by octyl chains at the C-9 position of the fluorene moiety, we synthesized two new model compounds: one, without octyl chains at the C-9 position of the fluorene moiety, 2,7-bis(2-thienyl)fluorene (TFTWC) and another with more octyl chains, 1,4-bis(9,9dioctylfluoren-2-yl)phenyl (FPF). The spectral properties of these derivatives have been studied at room temperature and at 77 K. These systems serve as excellent examples to show the effect of intermolecular interactions on optical properties of oligofluorenes.
The electronic states of polyfluorene copolymers with alternating donor-acceptor units
The Journal of Chemical Physics, 2004
We calculate the electronic states of the low bandgap polyfluorene-based copolymer DiO-PFDTBT, which consists of alternating 9,9-dioctyl-9H-fluorene and 4,7-di-thiophen-2-yl-benzo͓1,2,5͔thiadiazole ͑TBT͒ units, and compare with the steady-state absorption, emission, and excitation spectrum. Using the semiempirical quantum-chemical ͑ZINDO͒ method we can assign the characteristic bands of the ''camel-back'' absorption spectrum to one charge transfer state at lower energy localized on the TBT unit, and one delocalized excitonic state at higher energy corresponding to the -conjugated electron system. Additional ''dark'' charge transfer states in the gap between these bands have been revealed. Calculations are also made on the red light emitting polyfluorene-based copolymer poly͑fluorene-co-benzothiadiazole͒ ͑F8BT͒, which contains benzo͓1,2,5͔thiadiazole instead of TBT. The nature of the electronic states in F8BT and DiO-PFDTBT are found to be qualitatively the same.
Characterization of two- and three-photon absorption of polyfluorene derivatives
Journal of Polymer Science Part B: Polymer Physics, 2014
Fluorene-based polymer derivatives are promising materials for organic electronic devices because of their photoluminescence and electroluminescence, good film-forming ability, and favorable chemical and thermal properties. Although optical properties of polyfluorene have already been reported, most of the studies focused on the linear optical properties, whereas nonlinear optical characteristics have only recently received more detailed attention. Here, we report on two polyfluorene derivatives, poly(9,9 0 -n-dihexyl-2,7-fluorenediyl) (LaPPS 10) and poly(9,9 0 -n-dihexyl-2,7-fluorene-diyl-vinylene) (LaPPS 38), which present intense nonlinear absorption and fluorescence. Two-photon absorption cross-section properties of both polymers were characterized in the spectral range from 500 nm up to 900 nm, reaching peak values around 2000 G€ oppert Mayer units. Optical limiting behavior and twophoton-induced fluorescence of both polymers have also been investigated. Furthermore, the first molecular hyperpolarizability of the polymers was also studied using hyper-Rayleigh scattering. In addition, the three-photon absorption (3PA) spectra of both materials were also investigated, and 3PA crosssection values in the order of 1 3 10 278 cm 6 s 2 photon 22 were observed.