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An elaborated theoretical investigation on the optical and electronic properties of three fluorene-based platinum(II) and iridium(III) cyclometalated complexes Pt-a, Ir-a and Ir-b is reported. The geometric and electronic structures of the complexes in the ground state are studied with density functional theory and Hartree Fock approaches, while the lowest triplet excited states are optimized by singles configuration interaction (CIS) methods. At the time-dependent density functional theory (TD-DFT) level, molecular absorption and emission properties were calculated on the basis of optimized ground- and excited-state geometries, respectively. The computational results show that the appearance of triphenylamino (TPA) moiety at the 9-position of fluorene ring favors the hole-creation and leads to red-shifts of absorption and emission spectra. Moreover, Pt-a and Ir-b are nice hole-transporting materials whereas Ir-a has good charge-transfer balance, which render them useful for the realization of efficient OLEDs (Organic Light-Emitting Diodes).

The electronic structures, charge injection and transport, absorption and emission spectra, properties of two series of fluorene-based oligomers {2-[2-{2-[5-(9H-Fluoren-3-yl)-thiophen-2-yl]-vinyl}-6-(2-thiophen-2-yl-vinyl)-pyran- 4-ylidene]-malononitrile} n (FTPM) n and {2-{2-{2-[5-(9H-Fluoren-2-yl)-2-hydroxy- 3-methoxy-phenyl]-vinyl}-6-[2-(2-hydroxy-3-methoxy-phenyl)-vinyl]-pyran-4- ylidene}-malononitrile} n (FOOPM) n (n = 1–4) have been investigated by the density functional theory (DFT) approach. The ground-state geometries of (FTPM)4 and (FOOPM)4 optimized at B3LYP/6-31G(d) level exhibited zigzag arrangements. The energies of HOMO and LUMO, HOMO–LUMO energy gaps (ΔE H–L) of (FTPM) n and (FOOPM) n (n = ∞) were obtained by linear extrapolation method. Moreover, the calculations of ionization potential (IP), electronic affinity (EA), and reorganization energy (λ) were used to evaluate the charge injection and transport abilities. For (FTPM)4 and (FOOPM)4, the TDDFT calculations revealed that the absorption peaks can be characterized as π–π* transition and couple with the location of electron density distribution changes in different repeat units. All the earlier theoretical investigations are intended to establish the structure–property relationships, which can provide guidance to design the organic light-emitting diodes (OLEDs) with high performance.

A series of spiro-linked oligofluorenes and derivatives have been theoretically studied with DFT/B3LYP/6-31 G(d) associated with Z indo /SOS method. The increase of two-photon absorption (TPA) cross-sections of spiro-linked oligofluorenes with chain length can be expressed by formula lgδTPA(N) = αlgN + lgδTPA(N = 1). (N is the number of fluorene rings in the molecule). The effects of Si center and interconnection pattern of fluorene rings on TPA response are discussed. The introduction of ethyne and ethene conjugated-bridges can further increase TPA cross-sections in longer wavelength range. The maximum one-photon absorption wavelengths of all molecules are less than 400 nm, which demonstrate spiro-linked oligofluorenes and derivatives exhibit at the same time strong two-photon absorption and good transparency.

A dipolar molecule and two series of fluorene derivatives have been theoretically studied with B3LYP/6-31G(d) associated with ZINDO–SOS method. The different peripheral groups and lengthening the bridge have effect on the one-photon absorption. The maximum two-photon absorption cross sections δ max for the fluorene derivative (1B) is more than three times as that of the dipolar counterpart (S1). The δ max are obviously increased as the conjugation length increases from the series 1 molecules to the series 2 counterparts. Meanwhile, the δ max are increased with the stronger electron-donating (accepting) capability.

One serious problem associated with polyfluorene derivatives for light-emitting diodes is that they are usually much better at accepting and transporting holes than electrons. One way to achieve high electron affinity is the traditional structural modifications by the introduction of electron-withdrawing groups into π-conjugated systems. In this contribution, we apply density functional theory and the time-dependent density functional theory to investigate two series of alternating fluorene/pyridine oligomers and copolymers, namely, poly(2,7-(9,9-dimethylfluorene)-alt-3,5-pyridine) (PFPy) and poly(2,7-(9,9-dimethylfluorene-3,5-pyridine-)-alt-2,7-(9,9-dimethylfluorene)) (PFPyF) and gain a detailed understanding of the influence of pyridine units on the electronic and optical properties of fluorene derivatives. The outcomes show the incorporation of pyridine into the polyfluorene (PF) backbone resulted in a broadened energy gap and a blue shift of the absorption peaks. Most importantly, the LUMO energies of PFPy and PFPyF lower about 0.3 eV than that of their corresponding polyfluorene (PF), which directly results in the increasing of EAs than PF, indicating that the pyridine units have significantly improved the electron-accepting properties of the copolymers. In addition, the energy gap tends to broadening and the absorption peaks are gradually blue-shifted to shorter wavelengths with an increase in the pyridine content in the copolymers.

Excited state structures and spectroscopic properties of mercury(II) complexes, Hg-TFT( 1 ) and its electron-withdrawing substituents Hg-TFOT ( 2 ), Hg-TFCNT ( 3 ), where TFT = diethynylfluorenyl, TFOT = diethynyl-fluorenone, and TFCNT = diethynyl-[9-(dicyanomethylene) fluorene], were studied using singlet excitation configuration interaction (CIS) and time-dependent density functional theory(TDDFT) methods. The results of the theoretical calculations indicate that the electron-withdrawing substitutions lead to a significant decrease in the energy gap between the ground state and the first excited states. In the case of Hg-TFCNT, the second singlet excited state (S 2 ) may contribute to the luminescence because of its large S 1 - S 2 separation.

The color purity and efficiency of polyfluorene-based blue-emitting polymers is often compromised by long wavelengths, green emission bands and the charge injection difficulties. The incorporation of the phenoxazine units has been proved 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-methylphenoxazine-3,7-diyl) (PPOZ) and its fluorene copolymer poly[(10-methylphenoxazine-3,7-diyl)-alt-2,7-(9,9-dimethyl-fluorene)] (PFPOZ), and gain a detailed understanding the influence of phenoxazine units on the electronic and optical properties of fluorene derivatives. The outcomes show that the highly non-planar conformation of phenoxazine ring in the ground state preclude sufficiently close intermolecular interactions essential to forming aggregates or excimers. Furthermore, the HOMO energies lift about 0.6 eV and thus the IPs decrease about 0.6 eV in PFPOZ, suggesting the significant improved hole-accepting and transporting abilities, due to the electron-donating properties of phenoxazine ring, which results in the enhanced performances in both efficiency and brightness compared with pristine polyfluorene.

We report on a quantum-chemical study of the electronic and optical properties of Mercury(II) complexes HgTFT (1) and its electron-withdrawing substitutent HgTFOT (2), HgTFCNT (3) [where TFT = diethynylfluorenyl, TFOT = diethynylfluorenone and TFCNT = diethynyl-(9-(dicyanomethylene)fluorene)]. Our theoretical calculations indicate that the substitution of - CO , - C ( CN )2 for - CH in HgTFT significantly decreases the bond length alternation and increases the electron affinity, which would qualify HgTFOT 2 and HgTFCNT 3, especially 3, as candidates for n-type electrical conductors. The vertical transition energy associated with the low-lying excited state has been examined within the time-dependent DFT formalism for mercury diethynylfluorenyl derivatives. It is found that the electron-withdrawing substitutions lead to remarkable red shift in transition energy and alter the corresponding charge transfer manner.

One of the drawbacks for light-emitting diodes based on polyfluorene and derivatives (PFs) is the injection of electrons from the cathode due to the low electron affinity (EA) of most derivatives. Substitution by electron-accepting charge carriers on the conjugated polymer's backbone produces a remarkable influence on its electronic and optical properties. In this contribution, we apply quantum-chemical techniques to investigate a family of π-conjugated polymers poly(fluorene-alt-co-bithieno[3,2-b:2′3′-e]pyridine) (PFBTP) and poly(indenofluorene-alt-co-bithieno[3,2-b:2′3′-e]pyridine) (PIFBTP). 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 (Eg) and the maximal absorption wavelength (λabs) of PFBTP and PIFBTP are studied employing the time dependent density functional theory (TD-DFT) and semiempirical method ZINDO. The IP, EA and λabs of the polymers were also obtained by extrapolating those of the oligomers to the inverse chain length equal to zero (1/n=0). Especially, the influence of the presence of bithieno[3,2-b:2′3′-e]pyridine (BTP) groups on to the fluorene or indenofluorene moieties on the electron-accepting and -transporting is emphasized. As shown the BTP is a good electron-accepting moiety for electronic materials due to the presence of the three electron negative heteroatoms. For both PFBTP and PIFBTP, the LUMOs are significantly lower about 0.6 eV than that of their corresponding polyfluorene (PF), which results in the increasing of EAs by about 0.6 eV than PF, indicating that the bithieonpyridine units have significantly improved the electron-accepting properties of the copolymers. These cause the band gap narrower and the maximal absorption red-shifted comparing with PF.

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.

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.

A new series of π-conjugated [2,7-(9,9′-dihexylfluorene)-co-alt-5,5′-(2,2′-bipyridine)] (fluorene-co-bpy) copolymers containing fac -(bpy)Re(I)(CO) 3 Cl (bpy=2,2′-bipyridine, Re=rhenium) chromophores were synthesized and well characterized. The chelating copolymers showed characteristic MLCT absorption and emission at room temperature. Bonded Re complexes harvested excited energy efficiently through intra- and intermolecular energy from polymer backbone to Re complexes; however, both photoluminescence (PL) and electroluminescence (EL) efficiencies were unexpectedly lower. Such lower luminescent efficiency was conceivably related to charge carrier separation of localized electron-hole pairs (excitons) in fac -(bpy)Re(I)(CO) 3 Cl, which was induced by extensive π-conjugation. Further studies of density functional theory (DFT) calculation and surface photovoltaic spectra (SPS) afford proofs of charge carrier separation in fac -(bpy)Re(I)(CO) 3 Cl chelating polymers. The efficient charge carrier separation will decrease the efficiency of PL and EL but enhance the photovoltaic efficiency.

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.

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

A comparative study of electronic structure, one-photon absorption and two-photon absorption (TPA) properties of octupolar oligofluorenes and their dipolar counterparts is presented by using AM1 and ZINDO-SOS methods. The investigation of the structure–property relationship of these materials has indicated differences between octupolar oligofluorenes and dipolar ones. The results indicate that the conjugated chain length importantly affects the magnitude of TPA cross-section. The dipolar molecules reach a saturation limit for energy gap between HOMO and LUMO versus the number of fluorene rings more quickly than octupolar systems. The maximum TPA cross-section increases in proportion to N α ( α = 3.16 and 2.77), where N = 2–6 is the number of fluorene moieties in dipolar molecules and each arm of octupolar analogs. Because of the weak coupling among the branches of the octupole, an increase by a factor of less than 3 of the TPA cross-section of the octupolar molecule with respect to the dipolar molecule occurs.

A comparative study of electronic structure, one-photon absorption and two-photon absorption properties of octupolar oligofluorenes and their dipolar counterparts is presented by using AM1 and ZINDO-SOS methods. The conjugation length and molecular dimension importantly affect TPA cross-section values. The maximum TPA cross-section increases in proportion to N α ( α =2.41 and 2.24), where N is the number of fluorene moieties in dipolar molecules and each arm of octupolar analogs.

The two-photon absorption (TPA) cross-sections of a series of bifluorene molecules with different substituents (constructing three types of structures D–π–D, A–π–A and D–π–A) were calculated using ZINDO/SOS program. The results showed that the A–π–A structure with strong substituent-nitro had the largest TPA cross-section for these molecules. To verify the results, we analyzed the charge quantity of the ground and the main excited states as well as the frontier orbitals of the investigated molecules. The equilibrium geometries were obtained with AM1 method, and using ZINDO-SOS to calculate the third-order nonlinearities of the molecules, then gained the TPA cross-sections. We found that for the compounds with bifluorene as π center, the donor and acceptor strength are the important factor for the enhancement of the TPA properties, and compared with molecules with fluorene as π center, the large TPA cross-sections of bifluorene are caused by coupling effects between the two monomers.

Two important classes of organic molecules, perylene diimide (PDI) and pyrene derivatives have been found to possess relatively excellent photophysical and photochemical properties and especially high two-photon absorption cross sections (δ(T)(max)). Herein, one-photon absorption (OPA) and two-photon absorption (TPA) properties of some novel PDI and pyrene derivatives were comparatively investigated by the density functional theory (DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods. The calculated results indicate that with respect to PDI derivatives, the maximum TPA cross-sections for pyrene compounds increase obviously, the maximum peaks of OPA and TPA spectra are blue-shifted, the ΔE(H-L) (energy gaps between the highest occupied orbital and the lowest unoccupied orbital) increase. The different π-conjugated bridges (fluorene and pyrene) and terminal groups have slight effect on the OPA properties. Nevertheless, the molecules bearing 1,6-disubstituted pyrene as the π-conjugated bridge display the largest δ(T)(max) in both series of compounds 3 and 4. Moreover, the δ(T)(max) values of molecules with benzothiazole-substituted terminal groups are larger than those of the molecules with diphenylamine, which is attributed to benzothiazole groups stabilizing the planarity of the branch parts, extending the conjugated length and increasing the π-electron delocalized extent. Furthermore, the molecular size has marked effect on OPA and TPA properties. It is worthy to mention that cruciform 8 displays the largest δ(T)(max) among all the studied molecules in the range of 600-1100 nm. This research could provide a better understanding for the origin of the linear and nonlinear optical properties, and it would be helpful to gain more information about designing two-photon absorption materials with large δ(T)(max).

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.

Doubly ortho-linked cis-4,4′-bis(diarylamino)stilbene/fluorene hybrids have potential application in organic light emitting diodes. These organic molecules can function as efficient hole transport materials, with sky-blue emission and high efficiency and stability. To reveal the relationship between the properties and structures of these functional materials, we apply quantum-chemical techniques to investigate their optical properties and electronic structures. It is found that 8- and 8′-substituents give an antibonding contribution to the highest occupied molecular orbitals (HOMO), thus increasing the HOMO energy, implying an enhancement of hole-creating ability. Changes in ionization potentials confirm this presumption. The absorption and emission spectra exhibit red shifts to some extent with the various 8- and 8′-substituents, which are in good agreement with the experimental values. The Stoke shifts range moderately from 40 to 86 nm, due to the rigid spiral structure that hinders geometrical relaxation.