Theoretical Studies on the Structural, Electronic and Optical Properties of the New π-conjugated Copolymers Based on Carbazole and Thiophene (original) (raw)
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Polymer, 2006
The application of polyfluorenes (PFs) in polymeric light-emitting diodes (PLEDs) has been hampered because of the charge injection difficulties and the troublesome formation of a tailed emission band at long wavelengths (O500 nm) during device fabrication and operation, leading to both a color instability and reduced efficiency. The polycarbazoles have been proved to efficiently suppress the keto defect emission. In this contribution, we apply quantum-chemical techniques to investigate poly(N-methyl-2,7-carbazolediyl) (PCz), and its copolymers poly(Nmethyl-2,7-carbazolediyl-alt-2,5-thiophene) (PCzT) and poly(N-methyl-2,7-carbazoleethynylene) (PCzE), and gain a detailed understanding of the influence of carbazole units and the introduction of different charge carriers on the electronic and optical properties. The electronic properties of the neutral molecules, HOMO-LUMO gaps (D H-L ), in addition to IPs and EAs, are studied using B3LYP functional. The lowest excitation energies (E g s) and the maximal absorption wavelength l abs are studied employing the time dependent density functional theory (TDDFT). The calculated results show that the HOMO energies lift about 0.3 eV and thus the IPs decrease about 0.3 eV in all the carbazole-based polymers, suggesting the significant improved hole-accepting and transporting abilities. More important, by introducing the charge carriers thiophene ring and ethynylene, the LUMO energies in PCzT and PCzE decrease around 0.4 and 0.6 eV, respectively, which contributes to the decreasing EAs and the consequent improved electron-accepting and transporting properties. In addition, the energy gap tends to narrow and the absorption and emission peaks are gradually red-shifted to longer wavelengths with an increase in chain planarity in the copolymers. q
Theoretical Chemistry Accounts, 2010
The electronic excitation transitions of carbazole-based oligomers, (Cz-co-Cz) N , (Cz-co-Fl) N and (Cz-co-Th) N (N = 2-4) were investigated using density functional theory (DFT) and time-dependent (TD) DFT methods. Our results show that the calculated ground state geometries favor a more aromatic, planer structure, while the electronically excited geometries favor a quinoidic type structure. Absorption and fluorescence energies have been obtained from TD-B3LYP/SVP calculations performed on the S 1 optimized geometries and are in excellent agreement with experimental data. The experimental fluorescence excitation energies for (Cz-co-Cz) 4 , (Cz-co-Fl) 4 and (Cz-co-Th) 4 (2.76, 2.63, and 2.25 eV, respectively) correspond closely with the predicted S 1 transitions (2.84, 3.91 and 2.43 eV, respectively). We also report the predicted radiative lifetimes 0.52, 0.47, and 0.99 ns for (Cz-co-Cz) N , (Cz-co-Fl) N and (Cz-co-Th) N , discuss the origin of the small stoke shift of the carbazole based oligomers and the magnitude of bathochromic shifts. We conclude by discussing the benefits of theoretical calculations, which can provide critical structural and electronic understanding of excitation-relaxation phenomena that can be exploited in design of novel optical materials.
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 role of carbazole in organic light-emitting devices
Synthetic Metals, 1996
New organic oligomers and polymers based on the carbazole molecule are explored for possible applications in light-emitting devices. In one case, (butyl-or octyl-) carbazole dimers and poly(N-butyl-3,6-carbazolylene) polymer were used as the hole-transporting and lightemitting layer in multilayer light-emitting diodes (LEDs). These devices yielded bright blue light (as much as about 6000 cd m-2) with high external quantum (about 10%) and luminance efficiencies (about 21m W-1). The other case involved ([3-octylthiophene J-[bis-(Nethyl or octyl carbazolylene) ]) multiblock copolymers as the active emitting layer in single-layer LEDs. Color tuning was achieved in these devices by changing the number of monomer units contained in the thiophene chain. We also observed an increase of the external quantum efficiency in diodes based on the copolymers with short thiophene segments that we attributed to a more balanced charge injection.
Journal of Computational Chemistry, 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 = 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 (ΔH-L), in addition to the positive and negative ions, are studied using B3LYP functional. The lowest excitation energies (Egs) and the maximal absorption wavelength λabs of PFmCz (m = 1,2) are studied, employing the time-dependent density functional theory (TD-DFT). The properties of the two copolymers, such as ΔH-L, Eg, IPs, and EAs were obtained by extrapolating those of the oligomers to the inverse chain length equal to zero (1/n = 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. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 969–979, 2005
Advanced Materials Letters, 2012
In this work the geometrical and electronic properties of (Carbazole-3.4-Ethylenedioxythiophene) (Cbz-Edot) based alternating donor-acceptor conjugated oligomers were studied by the density functional theory (DFT) at the B3LYP level with 6-31G(d) basis set. The acceptors investigated include thiazole (Z), thiadiazole (D), thienopyrazine (TP), thienothiadiazole (TD), benzothiadiazole (BT) and thiadiazolothienopyrazine (TPD). A low band gap will be expected in polymers containing donor-acceptor (D-A) repeating units. In order to predict the band gaps for guiding the synthesis of novel materials with low band gaps, we apply quantum-chemical techniques to calculate the band gaps in several oligomers. The results have been compared with those of thiophene and 3,4-ethylenedioxythiophene polymers with donor-acceptor fragment. The lowest excitation energies (E ex ) and the maximal absorption wavelength (λ abs ) are studied using the time dependent density functional theory (TD-DFT), method. The electronic transitions of the absorption spectrum derived by TD-DFT method give useful structural and electronic information for designing novel conducting organic polymer materials. The theoretical results suggest that both the acceptor strength and the stable geometry contribute significantly to the electronic properties of alternating donor-acceptor conjugated copolymers.
2014
In this work, we presented a theoretical study by using DFT method on three conjugated compound based on carbazole and phenothiazine prepared by Y. Zhou et al [10] and shown in Figure 1. The HOMO, LUMO, Gap energy, λmax, λemi, Voc of these compounds have been calculated and reported in this paper. A systematic theoretical study of such compound has not been reported as we know. Thus, our aim is first, to explore their electronic and absorption properties on the basis of the DFT quantum chemical calculations. Second, we are interested to elucidate the parameters that influence the photovoltaic efficiency toward better understanding of the structure–property relationships. We think that the presented study of structural, electronic and optical properties for these compounds could help to design more efficient functional photovoltaic organic materials.
2021
In this article, we studied a series of dye-sensitized solar cells (DSSCs) type Donor-π-Acceptor involving carbazole as donors and cyanoacrylic acid as acceptors of the electrons. These cells are linked by different π-spacer unit’s, with the aim to develop new organic dyes with high-performance optoelectronic properties. Different units have been introduced in the π-bridge in order to investigate their effects on the structural and optoelectronic properties of the studied compounds, as well as their adsorbed compounds-titanium dioxide (TiO2) semi-conductor. We evaluated and assessed the important relevant parameters that influence the performance of photovoltaic cell to measure their involvement in the short-circuit photocurrent density (Jsc). Using Density Functional Theory (DFT) and Time-Dependent-BHandHLYP, the geometrical and optoelectronics properties have been predicted theoretically. The results obtained indicate that introducing the oxazole (S5) and thiazole (S6) molecules i...
Journal of Scientific Research , 2011
Opto-electronic properties of three new carbazole based co-polymers were investigated. The decay kinetics of triplet emission of the co-polymers was observed. Several findings such as optical band-gap, singlet to singlet transition energies, triplet to singlet transition energies, exchange energies (?EST) have been presented. In comparison to the other material carbazole-oxadiazole based polymer with OMe, P1, H21C10, P2 and H12C10O, P3 exchange energy is below 0.5 eV and triplet energy is significantly higher. The energy gaps between singlet and triplet excited states are 0.44-0.47 eV. From decay kinetics the triplet excited state life-times were found to be several orders of ms at 10-13 K that reduces to less than 3 ms at room temperature.
Journal of Molecular Structure: THEOCHEM, 2007
The conformational analysis and vertical excitation energies of carbazole-based molecules were carried out by B3LYP calculations with three basis sets: 6-31G, 6-31G(d) and 6-311G(d,p). Carbazole-based molecules were separated into two types, carbazole combined with five-membered (Cz-co-Thiophene (Cz-co-Th) N and Cz-co-Furan (Cz-co-F) N ) and six-membered (Cz-co-Fluorene (Cz-co-Fl) N and Cz-co-Phenyl (Cz-co-P) N ) aromatic rings. The analyses of potential energy surfaces and torsional angles for carbazole-based oligomers reveal that these copolymerized with five-membered aromatic rings are more planar than those copolymerized with six-membered ring. The obtained results indicate that TD-PBE1PBE and TD-B3LYP methods predict the excitation energies with the 6-311++G(2d,2p) basis set based on B3LYP/6-311G(d,p) optimized geometries for (Cz) N , (Cz-co-Fl) N and (Cz-co-Th) N in good agreement with experimental data. An analysis of the vertical singlet-singlet transition was also performed and the results indicated that for (Cz) N , (Cz-co-Fl) N , (Cz-co-P) N , (Cz-co-F) N and (Cz-co-Th) N oligomers, S 1 transition plays an important role. Excitation to the S 0 fi S 1 state corresponds almost exclusively to the promotion of an electron from the HOMO to the LUMO. The electronic transition derived by TD-DFT method gives useful structural and electronic information for designing novel conducting polymer materials.