Probing Charge Transport in π-Stacked Fluorene-Based Systems

Construction of Alkylidene Fluorene Scaffolds Using Pd-Catalyzed Direct Arene/Alkene Coupling Strategy

A versatile synthetic method for the construction of alkylidene fluorenes and their heteroarene derivatives has been developed successfully by means of a Pd(II)-catalyzed direct C-H/C-H coupling of o-alkenyl biaryls. Use of the Pd(OAc)₂ catalyst under aerobic oxidation conditions gives rise to the corresponding alkylidene fluorenes having various functional groups and diversely fused polycyclic systems. The resulting products can serve as versatile synthetic building blocks for the construction of structurally diverse polycyclic arenes and heteroarenes.

Martini 3 Coarse-Grained Model for Second-Generation Unidirectional Molecular Motors and Switches

Artificial molecular motors (MMs) and switches (MSs), capable of undergoing unidirectional rotation or switching under the appropriate stimuli, are being utilized in multiple complex and chemically diverse environments. Although thorough theoretical work utilizing QM and QM/MM methods have mapped out many of the critical properties of MSs and MMs, as the experimental setups become more complex and ambitious, there is an ever increasing need to study the behavior and dynamics of these molecules as they interact with their environment. To this end, we have parametrized two coarse-grained (CG) models of commonly used MMs and a model for an oxindole-based MS, which can be used to study the ground state behavior of MMs and MSs in large simulations for significantly longer periods of time. We also propose methods to perturb these systems which can allow users to approximate how such systems would respond to MMs rotating or the MSs switching.

Palladium-Catalyzed Cycloisomerization of 2-Ethynylbiaryls to 9-Methylidene Fluorenes

A palladium-catalyzed cycloisomerization of 2-ethynylbiaryls to 9-methylidene fluorenes is described for the first time. The cycloisomerization of 2-ethynylbiaryls proceeded smoothly in the presence of weak acid at low temperature to afford 9-methylidene fluorenes in satisfactory to high yields. This new type of cycloisomerization of 2-ethynylbiaryls is operationally simple and scalable and exhibits high functional-group tolerance. Various synthetically useful functional groups, such as halogen atoms, as well as formyl, acetyl, methoxycarbonyl, cyano, and nitro groups, remain intact during the cycloisomerization of 2-ethynylbiaryls.

Converging the Hole Mobility of Poly(2-N-carbazoylethyl acrylate) with Conjugated Polymers by Tuning Isotacticity

Nonconjugated electroactive polymers (also known as pendant or side-chain electroactive polymers) promise several potential advantages relative to conjugated polymers including enhanced mechanical durability, greater stability and synthesis via living polymerization techniques. However, most previous examples suffer from low charge carrier mobility. Here, using poly(2-N-carbazoylethyl acrylate) (PCzEA) as a model polymer, we investigate the ability of side-chain tacticity to influence hole mobility. Specifically, we investigated polymers with dyad isotacticity (m) ranging from ∼45 to ∼95% synthesized by several methods including free radical polymerization and anionic polymerization. We found that the hole mobility (μ_h) measured via the space charge limited current (SCLC) technique increased proportionally to the increasing isotacticity from 2.11 × 10^-6 cm² V^-1 s^-1 (m = 45.5%) to 4.68 × 10^-5 cm² V^-1 s^-1 (m = 94.7%) in unannealed samples and that mobilities could be boosted as high as 2.74 × 10^-4 cm² V^-1 s^-1 (m = 94.7%) with thermal annealing, which rivaled the well-known conjugated polymer poly(3-hexylthiophene) (P3HT) (μ_h = 5.8 × 10^-4 cm² V^-1 s^-1). As such, we report here clear experimental evidence that control of side chain tacticity can enhance charge carrier mobility in nonconjugated pendant electroactive polymers, converging with mobilities typically only observed in conjugated polymers.

Molecular Actuators in Action: Electron-Transfer-Induced Conformation Transformation in Cofacially Arrayed Polyfluorenes

There is much current interest in the design of molecular actuators, which undergo reversible, controlled motion in response to an external stimulus (light, heat, oxidation, etc.). Here we describe the design and synthesis of a series of cofacially arrayed polyfluorenes (^MeF nH m) with varied end-capping groups, which undergo redox-controlled electromechanical actuation. Such cofacially arrayed polyfluorenes are a model molecular scaffold to investigate fundamental processes of charge and energy transfer across a π-stacked assembly, and we show with the aid of NMR and optical spectroscopies, X-ray crystallography and DFT calculations that in the neutral state the conformation of ^MeF nH1 and ^MeF nH2 is open rather than cofacial, with a conformational dependence that is highly influenced by the local environment. Upon (electro)chemical oxidation, these systems undergo a reversible transformation into a closed fully π-stacked conformation, driven by charge-resonance stabilization of the cationic charge. These findings are expected to aid the design of novel wire-like cofacially arrayed systems capable of undergo redox-controlled actuation.

Free-Radical Copolymerization of Dibenzofulvene with (Meth)acrylates Leading to π-Stacked Copolymers

Copolymerizations of dibenzofulvene (DBF) with methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), methyl acrylate (MA), and 2-hydroxyethyl acrylate (HEA) were conducted under free radical conditions in toluene using α,α'-azobisisobutylonitrile (AIBN) as the initiator. In the copolymerizations, DBF indicated much higher reactivity than the comonomers, and the products comprised mainly of DBF units. NMR, UV, and fluorescence spectra, as well as electrochemical features indicated that the copolymers possess both isolated and rather short, sequential (meth)acrylate units, as well as π-stacked and unstacked DBF sequences. Isolated (meth)acrylate units are proposed to be sandwiched between DBF units. The ratios of π-stacked and unstacked side-chain fluorene groups of DBF units in excited states were accurately determined on the basis of fluorescent emission spectra; DBF units are mostly π-stacked in excited states as disclosed by fluorescence spectra. Two types of π-stacked sequences were suggested to be present in the ground state by electrochemical analysis. The copolymers exhibited higher solubility than pure poly(DBF).

A comprehensive study of substituent effects on poly(dibenzofulvene)s

Comparison of absorption, emission, electrochemistry and thermal properties across 14 poly(DBF)s.

Anisotropic electron-transfer mobilities in diethynyl-indenofluorene-dione crystals as high-performance n-type organic semiconductor materials: remarkable enhancement by varying substituents

In this study, the electron-transfer properties of alkynylated indenofluorene-diones with various substituents (SiMe3, SiPr3, and SiPh3) that function as n-type organic semiconductors were comparatively investigated at the first-principles DFT level based on the Marcus-Hush theory. The reorganization energies are calculated by the adiabatic potential-energy surface method, and the coupling terms are evaluated through a direct adiabatic model. The maximum value of the electron-transfer mobility of SiPr3 is 0.485 cm(2) V(-1) s(-1), which appears at the orientation angle of the conducting channel on the reference plane a-b near to 172°/352°. The predicted maximum electron mobility value of SiPr3 is nearly 26 times larger than that of SiPh3. This may be attributed to the largest number of intermolecular π-π interactions. In addition, the mobilities in all three crystals show remarkable anisotropic behavior. The calculated results indicate that SiPr3 could be an ideal candidate as a high-performance n-type organic semiconductor material. Our investigations not only give us an opportunity to completely understand the charge transport mechanisms, but also provide guidelines for designing materials for electronic applications.

Effect of donor strength of extended alkyl auxiliary groups on optoelectronic and charge transport properties of novel naphtha[2,1-b:6,5-b']difuran derivatives: simple yet effective strategy

The present study spotlights the designing of new derivatives of 2,7-bis (4-octylphenyl) naphtho [2,1-b:6,5-b'] difuran (C8-DPNDF) by substituting the alkyl groups (methyl, ethyl, propyl, butyl, pentyl, hexyl, and heptyl groups) at para position. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods are employed to optimize the molecular structures in ground and first excited states, respectively. Several electro-optical properties including hole/electron reorganization energies (λh/λe), electron affinities (EAs), ionization potentials (IPs), molecular electrostatic potentials (MEP), and frontier molecular orbitals (FMOs) have been evaluated. Furthermore their transfer integrals and intrinsic mobilities values have also been calculated. From this study, it is found that hole mobility of octyl containing derivative is raised to 4.69 cm(2) V(-1) s(-1). Moreover with attaching octyl group, hole transfer integral values have also been enhanced in newly designed derivatives. The balanced hole and electron reorganization energies, and improved transfer integrals lead to enhanced mobility in derivatives with octyl group, highlighting them as an efficient hole transfer material. Unlike the other electro-optical properties, the intrinsic hole mobility has increased because of transfer integral values of octyl containing derivative C8-DPNDF due to the dense and close crystal packing of C8-DPNDF. However, photostability of furan-based materials has not changed by increasing length of extended alkyl chain. Thus our present investigation highlights the importance of alkyl auxiliary groups that are often neglected/replaced with simple methyl group to save computation costs. Graphical Abstract The hole and electron reorganization energies of naphtho[2,1-b:6,5-b']difuran derivatives.

Effective solid electrolyte based on benzothiazolium for dye-sensitized solar cells

Thiaozole/benzothiaozole-based dicationic conductors were synthesized and applied as solid-state electrolyte in dye-sensitized solar cells (DSSCs). X-ray diffraction, scanning electron microscopy, thermal gravimetric analysis, steady-state voltammogram, photocurrent intensity-photovoltage test, and electrochemical impedance spectroscopy are used to characterize the materials and the mechanism of the cell performance. Compared to the traditional monocationic crystals, the dicationic crystals have a larger size and can provide more opportunities to fine-tune their physical/chemical properties. As a consequence, this solid-state electrolyte-based DSSC achieved photoelectric conversion efficiency of 7.90% under full air-mass (AM 1.5) sunlight (100 mW·cm(-2)).

How does the increment of hetero-cyclic conjugated moieties affect electro-optical and charge transport properties of novel naphtha-difuran derivatives? A computational approach

We have investigated computationally the effects of π-conjugation extension on naphtha[2,1-b:6,5-b'] difuran (DPNDF); where we increase the number of fused NDF (central core) and furan rings in the parent molecule. The molecular structures of all analogues have been optimized at the ground (S0) and first excited (S1) states using density functional theory (DFT) and time-dependent density functional theory (TD-DFT), respectively. Then highest occupied molecular orbitals (HOMOs), the lowest unoccupied molecular orbitals (LUMOs), photophysical properties, adiabatic/vertical electron affinities (EAa)/(EAv), adiabatic/vertical ionization potentials (IPa)/(IPv), and hole/electron reorganization energies λh/λe have been investigated. The effect of NDF and furan rings on structural and electro-optical properties has also been studied. Our calculated reorganization energies of 1a, 1b, and 2c reveal them, materials with balanced hole/electron charge transport, whereas 2a and 2b are good hole-transport materials. By increasing the number of furan rings; the photostability was augmented in 2a, 2b, and 2c.

Influence of push–pull configuration on the electro-optical and charge transport properties of novel naphtho-difuran derivatives: a DFT study

By a push–pull strategy, highly efficient and photostable naphtho-difuran derivatives were designed to get improved intrinsic electron mobility (1.13 cm² V⁻¹ s⁻¹).

THEORETICAL INVESTIGATIONS OF THE CHARGE TRANSFER PROPERTIES IN OLIGOTHIOPHENE DERIVATIVES

The structures of 5,5′-bis(naphth-2-yl)- 2,2′-bithiophene (NaT2), 5,5″-bis(naphth-2-yl)-2,2′:5′,2′-terthiophene (NaT3), 5,5‴-bis(naphth-2-yl)-2,2′:5′,2″:5″,2‴-tetrathiophene (NaT4), 5,5″″-bis(naphth-2-yl)-2,2′:5′,2″:5″,2‴:5‴,2″″-quinquethiophene (NaT5) and 5,5′″″-bis(naphth-2-yl)-2,2′:5′,2″:5″,2‴:5‴,2″″:5″″,2′″″-sexithiophene (NaT6) have been optimized at PBE1PBE/6-31G* level of theory. By increasing the chain length (thiophene units) energy gap decreases. The hole reorganization energy also decreases from NaT2–NaT6 . We have observed that mobility of NaT5 and NaT6 can be enhanced by minimizing the polarization and relaxation. The end-capped naphthyl groups have been rotated from 0°–60° in the case study of NaT2 , it was found that hole reorganization energy increases with the increment in angle. Furthermore the charge transport properties of 5,5′-bis(thionaphth-2-yl)-2,2′-bithiophene (TNT2), 5,5″-bis(thionaphth-2-yl)-2,2′:5′,2″-terthiophene (TNT3), and 5,5‴-bis(thionaphth-2-yl)-2,2′:5′,2″:5″,2‴-quaterthiophene (TNT4) have been investigated at the same level of theory. It has been studied that cis isomers have higher hole reorganization energies as compared to trans ones thus these isomers would diminish the mobility.

A chiral π-stacked vinyl polymer emitting white circularly polarized light

An optically active, π-stacked poly[2,7-bis(4-tert-butylphenyl)dibenzofulvene] having a preferred-handed helical conformation was synthesized by anionic polymerization. A thin film sample of the polymer exhibited broad-band white circularly polarized light (CPL) emission on photo excitation.

Modulation of chiroptical properties by DNA-guided assembly of fluorenes

The supramolecular organization of fluorene building blocks in a DNA scaffold is described. The molecular assembly into ordered π-aggregates leads to distinct changes in the electronic properties.

Designing of Disubstituted Derivatives of mer-Alq3: Quantum Theoretical Study

Different disubstituted derivatives of tris(8-hydroxyquinolinato)aluminium (mer-Alq3) have been designed. An electron donating group (–CH3) has been substituted at position 4; along with electron withdrawing groups (–Cl, –CN, and –F) at position 5 (i.e. tris(4-X-5-Y-8-hydroxyquinolinato)aluminium), as well as the molecules arising from reversion of these substitutions. The designed systems have been optimized at the B3LYP/6–31G* and TD-B3LYP/6–31G* level of theories, in the ground and excited states, respectively. The absorption and emission spectra have been calculated at the TD-B3LYP/6–31G* level. A significant blue shift has been observed for tris(4-methyl-5-cyano-8-hydroxyquinolinato)aluminium (2), and red shifts for each of tris(4-methyl-5-fluoro-8-hydroxyquinolinato)aluminium (3), tris(4-chloro-5-methyl-8-hydroxyquinolinato)aluminium (4), and tris(4-cyano-5-methyl-8-hydroxyquinolinato)aluminium (5). The reorganization energy in solvent best elucidates the charge transport behaviour (p- or n-type), rather than the equivalent gas phase for mer-Alq3 and its derivatives. The reorganization energies indicate that these new derivatives have better or comparable mobility when compared to the parent molecule. The electrostatic surface potential study suggested that photostability would be enhanced. On the basis of energy decomposition analysis, we have explained the distribution pattern of frontier molecular orbitals.

Efficient electron delocalization mediated by aromatic homoconjugation in 7,7-diphenylnorbornane derivatives

Efficient electron delocalization by aromatic homoconjugated 7,7-diphenylnorbornane (DPN) in alternated homoconjugated-conjugated block copolymers and reference compounds is revealed by photophysical and electrochemical measurements. The synthesis of the polymers was achieved by Suzuki polycondensation reaction. Effective electron delocalization by DPN is demonstrated by the significant red shifts observed in the absorption and emission spectra and the variation of the energy band gap of the polymers and monomeric model compounds in comparison to a series of oligophenylenes used as references (p-quaterphenyl, p-terphenyl, and biphenyl). The electron delocalization is also clearly demonstrated by the lower oxidation potential measured for homoconjugated model compound in comparison to p-terphenyl. The results show that the electron delocalization caused by two homoconjugated aryl rings is comparable to the effect produced by one conjugated aryl ring.