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Ji-Kang Feng

Publications and source records attributed to Ji-Kang Feng.

17 recordsLinked to original sources
PubMed2006

Theoretical investigation of one-photon and two-photon absorption properties for multiply N-confused porphyrins.

We have theoretically investigated a series of multiply N-confused porphyrins and their Zn or Cu complexes for the first time by using DFT(B3LYP/6-31G*) and ZINDO/SOS methods. The electronic structure, one-photon absorption (OPA), and two-photon absorption (TPA) properties have been studied in detail. The calculated results indicate that the OPA spectra of multiply N-confused porphyrins are red-shifted and the OPA intensities decrease compared to normal porphyrin. The maximum two photon absorption wavelengths lambda(max) are blue-shifted and the TPA cross sections delta(max) are increased 22.7-112.1 GM when the N atoms one by one are inverted from core to beta position to form multiply N-confused porphyrins. Especially delta(max) of N3CP get to 164.7 GM. The electron donors -C6F5s at meso-position can make the TPA cross section delta(max) increase. After forming metal complexes with Cu or Zn, the TPA properties of multiply N-confused porphyrins are further increased except for N3CP, N4CP. Our theoretical findings demonstrate that the multiply N-confused prophyrins as well as their metal complexes and derivatives are promising molecules that can be assembled series of materials with large TPA cross section, and are sure to be the subject of further investigation.

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PubMed2006

Theoretical study on the optoelectronic properties of electron-withdrawing substituted diethynylfluorenyl gold(I) complexes.

We report on a quantum-chemical study of the electronic and optical properties of gold(I) complex AuTFT (1) and its electron-withdrawing substitutents, AuTFOT (2) and AuTFCNT (3) [where TFT = diethynylfluorenyl, TFOT = diethynylfluorenone and TFCNT = diethynyl-(9-(dicyanomethylene)fluorene)]. Our theoretical calculations indicate that for all systems the reorganization energies of electron and hole are in the same order of magnitude and similar to those of the well-known electrontransport material Alq3. The substitution of -CO and -C(CN)2 for -CH in AuTFT significantly decreases the bond length alternation and increases the electron affinity, which would effectively lower the energy barrier for electron injection from cathode and thus qualify AuTFOT (2) and AuTFCNT (3) as candidates for the electron transport layer (ETL) in light-emitting diodes (LEDs). The lowest lying excited-states of gold(I) diethynylfluorenyl derivatives have been studied by the singles configuration interaction (CIS) method and time-dependent density functional method (TDDFT). It is found that the electron-withdrawing substitutions evidently decrease the energy gap, leading to a remarkable red shift in transition energy and transformation in the direction of charge transfer. Our research is important in the development of new functional materials for the design of LEDs with enhanced performance.

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PubMed2006

Theoretical study of two-photon absorption properties of a series of ferrocene-based chromophores.

The electronic structures, one-photon absorption (OPA), and two-photon absorption (TPA) properties of a series of ferrocene-based chromophores with TCF-type acceptors (TCF = 2-dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran) have been studied by using the ZINDO-SOS method. The results have revealed that OPA and TPA of ferrocenyl derivatives are affected by the strength of the acceptor, especially the pi-bridge conjugation length. The TPA cross section increases with increasing acceptor strength and pi-bridge conjugation length. The TCF-type acceptor with a phenyl group can lead to a larger TPA cross section. Quadrupole molecules have the largest TPA cross sections (2000-3000 GM), which are about 4 times that of the corresponding dipolar molecules, indicating larger interactions between the top and bottom branches. Finally, the origins of the two-photon excitations for ferrocenyl derivatives are analyzed. The calculations show that ferrocenyl derivatives with TCF-type acceptors (especially quadrupole molecules) are promising candidates for TPA materials.

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PubMed2006

Search for suitable approximation methods for fullerene structure and relative stability studies: case study with C50.

Local density approximation (LDA), several popular general gradient approximation (GGA), hybrid module based density functional theoretical methods: SVWN, BLYP, PBE, HCTH, B3LYP, PBE1PBE, B1LYP, and BHandHLYP, and some nonstandard hybrid methods are applied in geometry prediction for C60 and C70. HCTH with 3-21G basis set is found to be one of the best methods for fullerene structural prediction. In the predictions of relative stability of C50 isomers, PM3 is an efficient method in the first step for sorting out the most stable isomers. HCTH with 3-21G predicts very good geometries for C50, similar to the performance of B3LYP6-31G(d). The gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital from the predictions of all the density functional theory methods has the following descending order: E(gap)(half-and-half hybrid)>E(gap)(B3LYP)>E(gap)(HCTH)(GGA)>E(gap)(SVWN)(LDA).

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PubMed2005

Theoretical studies on the structure and aromaticity of Ti2P6+.

Cationic cluster Ti2P6+ has been studied within density functional theory. The structure of this cluster is predicted to be a slightly distorted tetragonal prism. The dissociation energy of this cationic cluster is higher than that of the known sandwich compound, [(P5)2Ti]2-, because of the different bonding in these two compounds. In Ti2P6+, the hybridization of P atoms of the ring is sp3. The bonding between the metal atoms and the P ring is mainly sigma-pi. While in [(P5)2Ti]2-, the P atoms take sp2 hybridization, the bonding between the metal atom and the rings is the typical pi-pi interaction. The electronic delocalization is another stabilizing factor for Ti2P6+. The nuclear independent chemical shift indicates that Ti2P6+ is a three-dimensional aromatic molecule. The predicted infrared and NMR help to identify the Ti2P6+ conformations in experiment.

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PubMed2005

Theoretical studies of the modulation of polymer electronic and optical properties through the introduction of the electron-donating 3,4-ethylenedioxythiophene or electron-accepting pyridine and 1,3,4-oxadiazole moieties.

One serious problem associated with polyfluorene and derivatives (PFs) as blue luminescent polymers is the significant energy barrier for hole or electron injections; thus they usually face charge injection and transport difficulties with the currently available cathode and anode materials. The incorporation of an electron-donating or -accepting unit is expected to improve the recombination of the charge carriers. In this paper, we apply quantum-chemical techniques to investigate three fluorene-based copolymers, copoly(2,5-ethylenedioxythiophene-alt-9,9'-dimethylfluorene) (PEF), copoly(2,5-pyridine-alt-9,9'-dimethylfluorene) (PPyF), and poly[(fluorene-2,7-diyl)-alt-(1,3,4-oxadiazole-2,5-diyl)] (PFO), in which Delta(H)(-)(L) [the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), when n = infinity], the lowest excitation energies (E(g)), ionization potentials (IP), electron affinities (EA), and lambda(abs) and lambda(em) are fine-tuned by the regular insertion of electron-donating unit 3,4-ethylenedioxythiophene (EDOT) or electron-withdrawing units pyridine and 1,3,4-oxadiazole. The results show that the alternate incorporation of electron-donating moiety EDOT increases the HOMO energy and thus reduces the IPs, and consequently the hole injection was greatly improved. On the other hand, even though both kinds of charge carriers will improve the electron-accepting ability, the results show that electron-withdrawing moieties greatly facilitate the electron-transporting. Especially in PFO, the highly planar structural character resulted from the strong push-pull effect between the fluorene ring and the 1,3,4-oxadiazole ring and a weak interaction between the nitrogen and oxygen atoms in 1,3,4-oxadiazole ring and the hydrogen atom of the fluorene ring, significantly lowering the LUMO energy levels and thus improve the electron-accepting and transporting properties by the low LUMO energy levels.

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PubMed2005

Theoretical studies on the electronic and optical properties of two new alternating fluorene/carbazole copolymers.

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 (Delta(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 lambda(abs) of PFmCz (m = 1,2) are studied, employing the time-dependent density functional theory (TD-DFT). The properties of the two copolymers, such as Delta(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 = 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.

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PubMed2005

Theoretical study on electronic structure and optical properties of phenothiazine-containing conjugated oligomers and polymers.

The application of polyfluorenes in polymeric light-emitting diodes has been hampered because of the charge injection difficulties and the troublesome formation of a tailed emission band at long wavelengths (>500 nm) during device fabrication and operation, leading to both a color instability and reduced efficiency. The incorporation of the phenothiazine units has been proven to significantly enhance the hole injection and charge carrier balance and at the same time efficiently suppress the keto defect emission. In this contribution, we apply quantum-chemical techniques to investigate poly[10-(N-(2'-methyl)phenothiazine-3,7-diyl) and its fluorene copolymer poly[10-(N-(2'-methyl)phenothiazine-3,7-diyl)-co-alt-2,7-(9,9-dimethylfluorene)] (PFPTZ) and gain a detailed understanding the influence of phenothiazine units on the electronic and optical properties of fluorene derivatives. Density functional theory (DFT) and time-dependent DFT approaches are employed to study the neutral molecules, HOMO-LUMO gaps (Delta(H-L)), the lowest excitation energies (E(g)'s), positive and negative ions, as well as the IPs and EAs, focusing on the superiority of the electronic and optical properties attributed to the introduction of electron-donating moiety phenothiazine (PTZ) through comparing with pristine polyfluorene. The outcomes show that the highly nonplanar conformation of phenothiazine ring in the ground state preclude sufficiently close intermolecular interactions essential to forming aggregates or excimers. Furthermore, the HOMO energies lift about 0.4 eV, and thus, the IPs decrease about 0.3 eV in PFPTZ, suggesting the significant improved hole-accepting and transporting abilities, due to the electron-donating properties of phenothiazine ring by the presence of electron-rich sulfur and nitrogen heteroatoms and highly nonplanar characters, resulting in the enhanced performances in both efficiency and brightness compared with pristine polyfluorene. In addition, even though the introduction of electron-donating moiety PTZ onto fluorene leads to a slight bathochromic shift in absorption and emission spectra, the copolymer still exhibited strong blue emission.

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PubMed2005

Theoretical study on structures and aromaticities of P5(-) anion, [Ti (eta(5)-P5)]- and sandwich complex [Ti(eta(5)-P5)2]2-.

The equilibrium geometries, energies, harmonic vibrational frequencies, and nucleus independent chemical shifts (NICSs) of the ground state of P5(-) (D(5h)) anion, the [Ti (eta(5)-P5)]- fragment (C(5v)), and the sandwich complex [Ti(eta(5)-P5)2]2- (D(5h) and D(5d)) are calculated by the three-parameter fit of the exchange-correlation potential suggested by Becke in conjunction with the LYP exchange potential (B3LYP) with basis sets 6-311+G(2d) (for P) and 6-311+G(2df) (for Ti). In each of the three molecules, the P-P and Ti-P bond distances are perfectly equal: five P atoms in block P5(-) lie in the same plane; the P-P bond distance increases and the Ti-P bond distance decreases with the order P5(-), [Ti(eta(5)-P5)2]2-, and [Ti (eta(5)-P5)]-. The binding energy analysis, which is carried out according to the energy change of hypothetic reactions of the three species, predicts that the three species are all very stable, and [Ti (eta(5)-P5)]- (C(5v)), more stable than P5(-) and [Ti(eta(5)-P5)2]2- synthesized in the experiment, could be synthesized. NICS values, computed for the anion and moiety of the three species with GIAO-B3LYP, reveal that the three species all have a larger aromaticity, and NICS (0) of moiety, NICS (1) of moiety, and minimum NICS of the inner side of ring P5 plane in magnitude increase with the order P5(-), [Ti(eta(5)-P5)2]2-, and [Ti (eta(5)-P5)]-. By analysis of the binding energetic and the molecular orbital (MO) and qualitative MO correlation diagram, and the dissection of total NICS, dissected as NICS contributions of various bonds, it is the main reason for P5(-) (D(5h)) having the larger aromaticity that the P-P sigma bonds, and pi bonds have the larger diatropic ring currents in which NICS contribution are negative, especially the P-P sigma bond. However, in [Ti (eta(5)-P5)]- (C(5v)) and [Ti(eta(5)-P5)2]2- (D(5h), and D(5d)), the reason is the larger and more negative diatropic ring currents in which the NICS contributions of P-P pi bonds and P5-Ti bonds including pi, delta, and sigma bonds, especially P5-Ti bonds, are much more negative and canceled the NICS contributions of P and Ti core and lone pair electrons.

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PubMed2005

Theoretical investigation of optical and electronic property modulations of pi-conjugated polymers based on the electron-rich 3,6-dimethoxy-fluorene unit.

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 electron-donating 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 pi-conjugated polymers with substituted dimethoxy groups at the 3,6 positions of the fluorene ring, namely, poly(2,7-(3,6-dimethoxy-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 (Delta(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 lambda(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 electron-accepting abilities are improved.

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PubMed2004

An insight into a novel class of self-assembled porphyrins: geometric structure, electronic structure, one- and two-photon absorption properties.

We have theoretically investigated a series of butadiyne-linked porphyrin derivatives that exhibit large two-photon absorption (TPA) cross sections in the visible-IR range. The electronic structure, one-photon absorption (OPA), and TPA properties have been studied in detail. We found that the introduction of a butadiyne linkage and the increase of the molecular dimensionality from monomer to dimer determine the OPA intensities of Q band and Soret band, respectively. A most important role for the enhancement of the TPA cross section is played by introducing a butadiyne bridge. The complementary coordination and the combination of the terminal free base and the core zinc porphyrin are also two effective factors for the enhancement of the TPA efficiency. The dimer with two porphyrins linked at meso-positions by a butadiyne linkage results in a maximum TPA cross section (79.35 x 10(-48) cm4 s per photon). Our theoretical findings are consistent with the recent experimental observations. This series of porphyrin derivatives as promising TPA materials are the subject of further investigation.

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PubMed2004

One- and two-photon absorption of three-coordinate compounds with different centers (B,Al,N) and a 2,2'-dipyridylnitrogen functional group.

A series of three-coordinate octupolar compounds with varied centers (boron, aluminum, and nitrogen), which exhibit very large effective two-photon absorption cross sections have been theoretically studied. The ground state geometries and electronic structures are obtained using the density functional theory with the B3LYP functional and 6-31G(d) basis set, and the results are comparable to the available experimental determinations. Based on the correct geometrical and electronic structures, the one- and two-photon absorptions are predicted by the ZINDO-SOS method. Among these compounds, the boron (B) and aluminum (Al) centers act as acceptors, while the nitrogen center acts as donor according to the net charge changes during the excitation. It is found that (i) the compounds with boron and aluminum centers show two large two-photon absorption peaks, while the molecule with nitrogen center show only one two-photon absorption peak; (ii) the cross sections of the molecules with B or Al as centers are larger than that of the molecule with nitrogen as center; furthermore, the two-photon absorption cross section of the molecule with Al center is larger than that of the molecule with B center, from this point of view, our theoretical prediction provides for the experiment a good new candidate with large two-photon absorption cross section for further research; (iii) lengthening the conjugation bridge by inserting a benzene ring on the organoborane compounds (forming the investigated molecule B-2) enhances the two-photon absorption cross section, and keeping good transparency at the same time.

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PubMed2004

Theoretical studies of ground and excited electronic States in a series of rhenium(i) bipyridine complexes containing diarylethynyl-based structure.

The photophysical properties, which vary as X is varied, of Re(I)-halide complexes (X2-bpy)ReICl(CO)3 (where X=ph, DAE, DNE, and DPE; ph = phenyl (1); DAE = di(amineoethynylbenzene) (2); DPE = di(phenylethynylbenzene) (3); DNE = di(nitroethynylbenzene) (4); bpy=2,2'bipyridine), are investigated using density functional theory (DFT). The electronic properties of the neutral molecules, in addition to the positive and negative ions, are studied using B3LYP functional. Excited singlet and triplet states are examined using time-dependent density functional theory (TDDFT). The low-lying excited-state geometries are optimized at the ab initio configuration interaction singlets level. As shown, the diarylethynyl-based structure is an integral component of the bpy pi-conjugated network, which results in a good planar structure. The occupied orbitals involved in the transitions have a significant mixture of metal Re and group Cl, and the lowest unoccupied orbital is a pi orbital, which extends from ligand bpy to diarylethynyl-based substituents. The luminescence for each complex originates from the lowest triplet excited states and is assigned to the mixing of MLCT and LLCT characters. Significant insights on the effects of these diarylethynyl conjugated structure and ending substituents (NH2, ph, and NO2) on absorption and emission spectra are observed by analysis of the results of the TDDFT method. The diarylethynyl-based pi-conjugated network makes both the absorption and emission spectra red-shifted compared with simple complex (bpy)ReICl(CO)3. Furthermore, an electron-releasing group (NH2) makes absorption and emission spectra blue-shift and an electron-withdrawing group (NO2) makes them red-shift.

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PubMed2004

Calculations on the octupolar molecules with enhanced two-photon absorption cross sections based on the Zn (II) and Cu (I) as centers.

The equilibrium geometries, electronic structures, as well as one- and two-photon absorption cross sections of a series of octupolar chromophores with Zn(2+) or Cu(+) as coordinate centers and 4,4'-bis(dibutylaminostyryl)-[2,2']-bis(bipyridyl) as ligands have been determined by using B3LYP/6-31G and ZINDO methods. These molecules are designed by controlled combination of two or three bipyridyl ligands with the metal centers. The results show that Zn(2+) is an effective template for the design of octupolar structures which enable it to form tetrahedral and octahedral coordinated complexes; while Cu(+) only exists in a tetrahedral coordinated complex, comparing the tetrahedral complex with Zn(2+) as the center with that of Cu(+) as the center, it is found that the complex with the Cu(+) center is a better two-photon absorption material than the former as far as the transparency/nonlinearity is concerned. Furthermore, for the same metal center of Zn(2+), both one- and two-photon absorptions of the tetrahedral complex are redshifted relative to those of the octahedral complex, is attributed to the spiroconjugation effect in the tetrahedral complex. Our theoretical findings are consistent with recent experimental observations and provide an important foundation for the design of improved transparency-nonlinearity two-photon absorption materials.

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PubMed2002

Theoretical study on the mechanism of the 1CHCl + NO reaction.

The complex doublet potential energy surface of the CHClNO system, including 31 minimum isomers and 84 transition states, is investigated at the QCISD(T)/6-311G(d, p)//B3LYP/6-31G(d, p) level in order to explore the possible reaction mechanism of the singlet CHCl with NO. Various possible isomerization and dissociation channels are probed. The initial association between 1CHCl and NO at the terminal N-site can almost barrierlessly lead to the chainlike adducts HClCNO a (a1, a2) followed by the direct Cl-extrusion to product P9 Cl + HCNO, which is the most feasible channel. Much less competitively, a (a1, a2) undergoes a ring-closure leading to the cyclic isomer c-C(HCl)NO d followed by a concerted Cl-shift and N-O cleavage of d to form the branched isomers ClNC(H)O f (f1, f2). Eventually, f (f1, f2) may take a direct H-extrusion to produce P7 H + ClNCO or a concerted 1,2-H-shift and Cl-extrusion to form P1 Cl + HNCO. The low-lying products P2 HCl + NCO, P3 Cl + HOCN, P14 HCO + 3NCl, P6 ClO + HCN, and P13 ClNC + OH may have the lowest yields observed. Our calculations show that the product distributions of the title reaction are quite different from those of the analogous 1CHF + NO reaction, yet are similar to those of another analogous 3CH2 + NO reaction. The similarities and discrepancies among the three reactions are discussed in terms of the substitution effect. The present article may assist in future experimental identification of the product distributions for the title reaction and may be helpful for understanding the halogenated carbene chemistry.

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PubMed2002

Theoretical study on the mechanism of the (3)CH(2) + NO(2) reaction.

The complex doublet potential energy surface of the CH(2)NO(2) system is investigated at the B3LYP/6-31G(d,p) and QCISD(T)/6-311G(d,p) (single-point) levels to explore the possible reaction mechanism of the triplet CH(2) radical with NO(2). Forty minimum isomers and 92 transition states are located. For the most relevant reaction pathways, the high-level QCISD(T)/6-311 + G(2df,2p) calculations are performed at the B3LYP/6-31G(d,p) geometries to accurately determine the energetics. It is found that the top attack of the (3)CH(2) radical at the N-atom of NO(2) first forms the branched open-chain H(2)CNO(2) a with no barrier followed by ring closure to give the three-membered ring isomer cC(H(2))ON-O b that will almost barrierlessly dissociate to product P(1) H(2)CO + NO. The lesser followed competitive channel is the 1,3-H-shift of a to isomer HCN(O)OH c, which will take subsequent cis-trans conversion and dissociation to P(2) OH + HCNO. The direct O-extrusion of a to product P(3) (3)O + H(2)CNO is even much less feasible. Because the intermediates and transition states involved in the above three channels are all lower than the reactants in energy, the title reaction is expected to be rapid, as is consistent with the measured large rate constant at room temperature. Formation of the other very low-lying dissociation products such as NH(2) + CO(2), OH + HNCO and H(2)O + NCO seems unlikely due to kinetic hindrance. Moreover, the (3)CH(2) attack at the end-O of NO(2) is a barrier-consumed process, and thus may only be of significance at very high temperatures. The reaction of the singlet CH(2) with NO(2) is also briefly discussed. Our calculated results may assist in future laboratory identification of the products of the title reaction.

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