Fluorene Molecules Research Articles

Correlation Between Molecular Conformation, Packing, and Dynamics in Oligofluorenes: A Theoretical/Experimental Study

Fluorene-based systems have shown great potential as components in organic electronics and optoelectronics (organic photovoltaics, OPVs, organic light emitting diodes, OLEDs, and organic transistors, OTFTs). These systems have drawn attention primarily because they exhibit strong blue emission associated with relatively good thermal stability. It is well-known that the electronic properties of polymers are directly related to the molecular conformations and chain packing of polymers. Here, we used three oligofluorenes (trimer, pentamer, and heptamer) as model systems to theoretically investigate the conformational properties of fluorene molecules, starting with the identification of preferred conformations. The hybrid exchange-correlation functional, OPBE, and ZINDO/S-CI showed that each oligomer exhibits a tendency to adopt a specific chain arrangement, which could be distinguished by comparing their UV/vis electronic absorption and (13)C NMR spectra. This feature was used to identify the preferred conformation of the oligomer chains in chloroform-cast films by comparing experimental and theoretical UV/vis and (13)C NMR spectra. Moreover, the oligomer chain packing and dynamics in the films were studied by DSC and several solid-state NMR techniques, which indicated that the phase behavior of the films may be influenced by the tendency that each oligomeric chain has to adopt a given conformation.

Nonlinear Optical Properties of a Series of 6,12-Diethynylindeno[1,2-<em>b</em>]fluorene Derivatives

The polarizabilities (αs) and second hyperpolarizabilities (γs) of a series of 6,12-diethynylindeno (1,2-b)fluorene derivatives were investigated by the density functional theory CAM-B3LYP method. The calculated results indicate that these molecules possess considerably large second hyperpolarizabilities. Replacing the 6,12-hydrogen atoms on indeno(1,2-b)fluorene molecules by ethynyl silyl or oxygen atoms results in a change in the geometry of the molecular structure, which affects the nonlinear optical (NLO) properties. Introducing ethynyl silyl groups into the molecules can increase the αs and γs values, while these values decrease introducing oxygen atoms into the molecules. Also, the γs values depend on the 2, 8-disubstituted R groups (R=H, F, CH3) of the indeno(1,2-b)fluorene molecules. When R is methyl, the molecule has much larger αs and γs values. Moreover, according to time-dependent density functional theory calculations on the indeno(1,2-b)fluorene series, the maximum absorption wavelength of the ethynyl silyl derivatives display a bathochromic shift due to increasing conjugation, while a blue shift of the maximum absorption wavelengths are observed in the oxygen-substituted derivatives because the conjugation decreases as the molecular structure is distorted.

Competing photodehydration and excited-state intramolecular proton transfer (ESIPT) in adamantyl derivatives of 2-phenylphenols

2-Phenylphenol derivatives strategically substituted with a hydroxyadamantyl substituent were synthesized and their photochemical reactivity was investigated. Derivatives 9 and 10 undergo competitive excited-state intramolecular proton transfer (ESIPT) from the phenol to the carbon atom of the adjacent phenyl ring and formal ESPT from the phenol to the hydroxyl group coupled with dehydration. These two processes (both via S1) give rise to two classes of quinone methides (QMs) that revert to starting material or react with nucleophiles, respectively. ESIPT to carbon atoms was studied by performing photolyses in the presence of D2O, whereupon deuterium incorporation to the adjacent phenyl ring was observed ([Formula: see text] = 0.1–0.2). The competing formal ESPT and dehydration takes place with quantum yields that are an order of magnitude lower and was studied by isolation of photomethanolysis products. Derivative 8 did not undergo ESIPT to carbon atom. Owing to the presence of an intramolecular H bond, an efficient ESIPT from the phenol to the hydroxyl group coupled with dehydration gives a QM that efficiently undergoes electrocyclization (overall [Formula: see text] = 0.33), to give chroman 16. In addition, spiro[adamantane-2,9′-(4′-hydroxy)fluorene] (12) undergoes ESIPT, unlike the previously reported unreactive parent 2-hydroxyfluorene. The reactive singlet excited states of the prepared biphenyl and fluorene molecules were characterized by fluorescence spectroscopy, whereas laser flash photolysis (LFP) was performed to characterize the longer lived QM intermediates.

The Electrical Properties of Biphenylenes

The effect of the partial antiaromaticity of biphenylene on its substitution chemistry, its oxidation potential, and its single-molecule conductance is explored. Biphenylene and fluorene molecules with linkers of two amino groups or two cyclic thioether groups were synthesized and their conduction properties were investigated using scanning tunneling microscopy (STM) break-junction techniques and DFT calculations. Despite the partial antiaromaticity of biphenylene, which causes the biphenylenes to be much more easily oxidizable, no significant increase in molecular conductance was found.

Two-photon polymerization of a diacrylate using fluorene photoinitiators–sensitizers

Two-photon polymerization (TPP) of a diacrylate (monomer) using two different fluorene molecules as photoinitiators–sensitizers is reported. The fluorene molecules contain e − donating (triphenylamine) or e − withdrawing (phthalimide) edge substituents. An amine co-initiator is also used lowering the polymerization threshold. The dependence of the polymerization properties (threshold, polymerization rate, lateral and axial resolution) on the type of resin, the writing power and speed as well as on the NA of the focusing lens is studied in detail. The fluorene with triphenylamine substituent exhibits favorable properties as TPP photoinitiator–sensitizer compared to the fluorene with phtalimide one, although its two-photon absorption cross-section is smaller. Polymerization of the pure monomer or monomer-amine composite (i.e. without fluorene) is also reported.

New fluorene-substituted TTF derivatives as photofunctional materials

We have investigated photoinduced conductors and photoelectric conversion materials using new tetrathiafulvalene (TTF) derivatives containing a fluorescent part. In this study, we focused on new TTF molecules 1-3 in which a fluorene molecule was used as a fluorescent part and was connected to the TTF moiety through alkyl or alkene spacers. We discuss the synthesis, electrochemical properties, and optical properties of molecules 1-3. We also investigated the photoelectric conversion functionality by photoelectrochemical method using the thin films of molecules 1-3 that were spin-coated on ITO-coated glass substrate. We observed electric current generations that depend on the absorption spectra of thin films, suggesting that absorbed photons are converted to electric currents on the thin film/ITO electrode.

Photoluminescence properties of porous silicon/fluorene dye composites

We report on the fabrication and the photoluminescence (PL) study of hybrid nanocomposites formed by embedding fluorene dye molecules in the vertical cylindrical nanopores of a nanoporous silicon layer. The pores had a diameter of ∼20 nm and were homogeneously filled with the fluorene molecules as evidenced by scanning electron microscopy images. Efficient PL in the blue spectral region, attributed to the fluorene dyes embedded into the pores, was obtained from the composites. The PL spectrum of the dyes in porous silicon is blue-shifted compared to their PL spectrum in solution due to the confinement and isolation of the molecules into the pores. This blue-shift was enhanced when the dyes were initially dissolved in an aromatic solvent.

Photoluminescence in the blue spectral region from fluorene molecules embedded in porous anodic alumina thin films on silicon

The photoluminescence (PL) in the blue spectral region from fluorene molecules embedded in the pores of anodic alumina thin films on silicon was investigated in detail. It was found that the PL was strongly dependent on the diameter and depth of the pores, as well as on the solvent used to dissolve the fluorene molecules. A photoluminescence blue shift with a maximum value of 30 nm was observed when the fluorenes were embedded into 40 nm pores of the alumina film compared to the corresponding spectrum in solution. The results have shown that the penetration of the molecules into the pores, as well as the formation of nanosize aggregates are favored when the molecules are dissolved in an aromatic solvent.

Hydrotreated-LCO oxidation in a transported slurry reactor-hydrocyclon system reactor for a low emission fuel oil production: I Kinetic of reactions

Tetralin, indane, and fluorene molecules in a hydrogenated diesel were selectively oxidized with air to determine their kinetic behavior during the reaction. The oxidation of model molecules was performed in a continuous stirred tank reactor to obtain data to simulate diesel oxidation in a slurry (three-phase) reactor. The products of reactions were analyzed by elution in a silica column followed by GC–MS analysis of the fractions to determine the amount of product converted. The analytical procedure was calibrated using the model molecules. A lump model of 11 main reactions is considered here to evaluate the conversion of tetralin, indane, and fluorene into oxygenated compounds. The kinetic study was performed at different temperatures, space velocities, and reactant concentrations. A program using a genetic algorithm optimization tool was developed to calculate the values of the kinetics rate constants. The results indicated that the reaction can be modeled by a simple apparent first order of reaction with respect to hydrocarbon and around 0.3 orders of reaction respects to oxygen. The reactivity followed the order tetralin, indane, and then fluorene. The heterogeneous kinetic rate expressions of one active site represent the experimental results in the range of operating conditions studied. Under the experimental conditions, the ketones and ketols formed are strongly adsorbed in the active sites. Production of alcohols generates some poly-oxygenated compounds that reduce the storage stability of the fuel.

Synthesis, Structure, Electronic State, and Luminescent Properties of Novel Blue‐Light‐Emitting Aryl‐Substituted 9,9‐Di(4‐(di‐p‐tolyl)aminophenyl)fluorenes

AbstractNovel blue‐light‐emitting fluorene derivatives 5a–c and 7a–c containing bulky and highly emissive groups, namely pyrene, 10‐phenylanthracene‐9‐yl and 10‐(4′‐diphenylaminophenyl)anthracene‐9‐yl groups, as well as hole‐injecting/transporting triarylamines were synthesized. Single crystals of compounds 5a, 5c, 7a, and 7c were grown and their crystal structures were determined by X‐ray diffraction. The four fluorene derivatives have nonplanar molecular structures, which reduce the intermolecular interaction and the likelihood of molecular aggregation or excimer formation. No unwanted long‐wavelength emission was observed in the photoluminescence (PL) spectra of the 5a–c and 7a–c thin films. Their PL spectra reveal excellent thermal stability after annealing treatment under air and ambient light. All of the six compounds show high fluorescence quantum yields and outstanding thermal stabilities. The 2‐aryl and 2,7‐diaryl substituents at the fluorene molecule have a significant effect on the photophysical properties and the thermal characteristics. The six compounds show almost the same energy levels for the highest occupied molecular orbitals (HOMOs) of about −5.20 eV, which allows effective hole injection. The C2‐ and C7‐aryl substituents play a relatively less‐important role in the HOMO energy levels, which depend mainly on the triphenylamino groups at the C9 position. The molecular orbitals, excitation energy, and emission energy were calculated to explain the real origin of their photophysical characteristics. The HOMOs are mainly localized on the triphenylamino groups at the C9 position, while the lowest unoccupied molecular orbitals (LUMOs) have a significant orbital density at the C2‐ and/or C7‐aryl substituents. Pure‐blue‐light‐emitting diodes based on 2,7‐diaryl‐9,9‐di(triarylamino)fluorenes were fabricated.

The role of intermolecular vibrations and reorganization of a reaction system in tunneling reactions with H atom transfer. A Debye model for the medium

Mechanisms of temperature dependence of the rate constants for two types of solid-state tunneling chemical reactions, namely, transfer of an H atom between two molecules and intramolecular transfer, are analyzed. To this end, an analytical expression for the rate constant for tunneling atom transfer in solids is derived in the framework of a modified theory of nonradiative transitions. The mechanisms of the temperature dependence of the rate constant considered in this work include oscillations of the potential barrier to chemical reaction in intermolecular fluctuations and reorganization of the medium. The effect of pressure on the distance between reactants and on the frequency of intermolecular vibrations is taken into account. The theory developed is used to interpret experimental data on tunneling transfer of an H atom in two reactions: a) intramolecular hydrogen transfer in a matrix-isolated formic acid molecule entrapped in an argon crystal and b) H atom transfer from a fluorene molecule to an excited acridine molecule in a fluorene crystal.

Strong Two Photon Absorption and Photophysical Properties of Symmetrical Chromophores with Electron Accepting Edge Substituents

Two photon absorption (TPA) and photophysical properties of three new symmetrical chromophores with electron accepting phthalimide edge substituents have been studied. The three chromophores contain fluorene, alcoxy-substituted divinyl benzene, and carbazole moieties as central cores, respectively. The femtosecond time-resolved fluorescence upconversion spectroscopy and two photon excited fluorescence technique have been carried out. The effect of solvent polarity on TPA and on photophysics has also been determined. Ultrafast fluorescence dynamics, with decay times ranging from 1 to 13 ps, are revealed in polar solvents. This is attributed to the relaxation of the chromophores to the intramolecular charge transfer state. The chromophore bearing fluorene central core, being of the type A-pi-A, is the most efficient concerning TPA. Strong TPA, with a cross section value as high as 2100 GM at an excitation wavelength of 770 nm is found in acetophenone which is a solvent of intermediate polarity. The TPA spectra were also reproduced using a sum over states three-state model. A study of the TPA induced photobleaching of the fluorene molecule, doped in a solid poly(methyl-methacrylate) film, has shown that this material is very promising for efficient TPA optical data storage.

Effect of Pore Confinement and Molecular Orientation on Hydrogen Transfer during a Free-Radical Reaction in Mesoporous Silica

Mesoporous silicas with controllable pore size are providing important platforms finding a multitude of applications such as in catalysis, separations, and as nanoreactors for molecular transformations. In this study, we probe the influence of a coattached hydrogen donor molecule, fluorene, on the free-radical pyrolysis of 1,3-diphenylpropane (DPP) in MCM-41 silica as a function of pore size (1.6−2.8 nm). The influence of surface orientation of the fluorene molecule on the pyrolysis rate is examined through use of isomeric 2-hydroxyfluorene (2-FL) and 3-hydroxyfluorene (3-FL) precursors for surface attachment. The DPP pyrolysis rates are found to be sensitive to both the surface orientation of the fluorene molecule and the pore size. Furthermore, whereas the 2-FL led to faster DPP pyrolysis rates compared with 3-FL on the exterior surface of a nonporous silica nanoparticle (Cabosil), the opposite effect was observed in the mesoporous silica with the smallest pore size. The results are interpreted based on the influence of molecular orientation of the isomeric fluorene molecules on the key bimolecular hydrogen transfer steps to intermediate free-radicals on the surface and suggest that differences in surface curvature between the Cabosil particles and MCM-41 cylindrical pores may play a key role.

A new strategy of enhancing two-photon absorption in benzothiazole-containing fluorene molecules: Introduction of auxiliary donor–acceptor effects

A series of benzothiazole-containing fluorene molecules have been designed and their one- and two-photon absorption properties have been investigated theoretically by using the ZINDO method. The effects of electron-excessive/deficient heterocyclic bridges as auxiliary donors (auxD)/ acceptors (auxA) on TPA cross-sections were studied. The results show that the molecules with D–π–auxA-A, D-auxD–π–A, or D-auxD–π–auxA-A structure types have large TPA cross-section, which can be a valuable strategy in the design of two-photon absorption materials. Also, a linear relationship between the first hyperpolarizability and the TPA cross-section is observed.

Arene–perfluoroarene interactions in crystal engineering. Part 14. 1:1 Complexes of octafluoronaphthalene with fluorene and 9,10-dihydrophenanthrene

The 1:1 molecular complexes of octafluoronaphthalene (OFN) with fluorene (1) and with 9,10-dihydrophenanthrene (2) were prepared by cocrystallization of the components and characterized by single-crystal X-ray diffraction. The structure of 1 can be described as a mixed-stack or a laminar checkerboard motif of alternating OFN and fluorene molecules, whilst slanted mixed-stack, herringbone, and face-to-face heteromolecular dimer motifs can all be recognised in 2.Key words: single crystal, X-ray structure, π stacking, fluoroarene.

Theoretical Study of Vibrational and Optical Spectra of Methylene-Bridged Oligofluorenes

We report a systematic characterization of methylene-bridged fluorene oligomers constructed of two, four, six, and eight aromatic rings using time-dependent density functional theory (TDDFT), the ab initio approximate coupled-cluster singles and doubles (CC2) method, and semiempirical spectroscopic Zerner's intermediate neglect of differential overlap method (ZINDO/S). Geometry optimizations have been performed for the ground state and for the first electronically excited state. Vertical excitations and the fluorescence transition from the lowest excited state have been calculated. Computed ground-state geometries and infrared spectra for fluorene are in good agreement with experimental results. The RI-CC2 and ZINDO/S absorption and fluorescence spectra agree very well with the available experimental data for studied fluorene oligomers and for para oligophenylenes films. On the other hand, TDDFT calculations underestimate excitation and fluorescence energies systematically for larger systems (N > 4) in comparison with the above-mentioned results. The effective conjugation length was estimated to 13-14 repeat units. The computed radiative lifetimes for the fluorene molecule show good agreement with experiment within realistic expectations. The decrease of the radiatiave fluorescence lifetime with the increase in the conjugation length has been discussed also.

Two-photon anisotropy: Analytical description and molecular modeling for symmetrical and asymmetrical organic dyes

One- and two-photon anisotropy spectra of a series of symmetrical and asymmetrical polymethine (PD) and fluorene molecules were measured experimentally and discussed theoretically within the framework of three-state and four-state models. For all the molecules discussed in this paper, the experimental two-photon anisotropy values, r 2PA, lie in the relatively narrow range from 0.47 to 0.57 and remain almost independent of wavelength over at least two electronic transitions. This is in contrast with their one-photon anisotropy, which shows strong wavelength dependence, typically varying from ≈0 to 0.38 over the same transitions. A detailed analysis of the two-photon absorption (2PA) processes allows us to conclude that a three-state model can explain the 2PA anisotropy spectra of most asymmetrical PDs and fluorenes. However, this model is inadequate for all the symmetrical molecules. Experimental values of r 2PA for symmetrical polymethines and fluorenes can be explained by symmetry breaking leading to the deviation of the orientation of the participating transition dipole moments from their “classical” orientations.

Development of Distributed-Feedback Tunable Blue-Violet Waveguide Plastic Laser Based on Fluorene Compound

The distributed feedback plastic lasers for blue-violet were developed. The developed fluorene molecules for electroluminescence devices were adopted and doped in poly(methymethacrylate:2-hydroxyethylemethacrylate). Laser operation was confirmed at a wavelength of about 410 nm using frequency-tripled and passively Q-switched Nd:Cr:YAG laser. A laser threshold of 70 nJ and the longest lifetime of 3000 shots were demonstrated.

Red‐Emitting Fluorenes as Efficient Emitting Hosts for Non‐Doped, Organic Red‐Light‐Emitting Diodes

AbstractRare red‐fluorescent fluorene derivatives have been designed and synthesized. The long‐wavelength red fluorescence is achieved by incorporating a di(4‐tolyl)amino or diphenylamino electron donor and a dicyanovinyl electron acceptor. The single‐crystal X‐ray structures of the di(4‐tolyl)amino (pTSPDCV) and diphenylamino (PhSPDCV) compounds indicate only weak non‐π van der Waals contacts in addition to long‐distance dipole–dipole interactions of the red‐emitting fluorene molecules in the solid state. The aggregation of the dipolar fluorene is largely suppressed by introducing bulky 9,9‐substituents (spiro‐fused bifluorene) as well as a non‐planar di(4‐tolyl)amino or diphenylamino group. In the solid state, these fluorene derivatives all show red fluorescence that is much brighter than with the red dopants Nile Red and DCM (4‐(dicyanomethylene)‐2‐methyl‐6‐[4‐(dimethylaminostyryl)‐4H‐pyran]). The unique photophysical properties of red‐emitting fluorene derivatives differ from other known red dopants and facilitate the fabrication of non‐doped red organic light‐emitting diodes (OLEDs). Authentic red (CIE, x = 0.65, y = 0.35) electroluminescence with a brightness of more than 12 000 cd m–2 (greater than 600 cd m–2 at 20 mA cm–2) and a remarkable external quantum efficiency as high as 3.6 % have been observed for the red‐emitting OLEDs with pTSPDCV or PhSPDCV as the sole emitting host.

Synthesis of a constrained ligand comprising carboxylate and amine donor groups via direct 1,8-functionalization of positionally protected fluorene

The synthesis of 1,8-bis(dimethylaminomethylethynyl)-3,6-di( tert-butyl)fluorene-9-yl-acetic acid, a potentially dinucleating ligand containing two N-donor and bridging carboxylate groups, is described. The electronically disfavored 1,8-disubstitution of the fluorene ring system was achieved by using tert-butyl protecting groups in the 3- and 6-positions of the fluorene molecule in combination with mercury(II) as a sterically demanding electrophile. The straightforward synthesis of a 1,8-diiodofluorene derivative provides simple general access to 1,8-disubstituted fluorene molecules.