Molecular engineering for panchromatic absorbing oligothiophene donor-π-acceptor organic semiconductors

A series of three novel organic semiconductor molecules based on the donor-π-bridge-acceptor (D-π-A) modular design have been designed, synthesised and characterised. These small organic molecules have a common donor (triphenylamine), π-bridge (an alkylated tetrathiophene) and various acceptors to provide tuning of optical and electronic properties. The examined acceptors were dicyanovinylidene, cyanopyridone and oxoindenemalononitrile groups. These compounds were highly soluble in common organic solvents, such as chloroform, toluene and chlorobenzene. As acceptor strength increased from dicyanovinylidene to cyanopyridone to oxoindenemalononitrile groups, the wavelength of the longest wavelength absorption maximum increased (518, 587, 619 nm (solution)), the respective extinction coefficients increased (4.9×10⁴, 6.3×10⁴, 7.4×10⁴ M⁻¹ cm⁻¹) and with increasing acceptor strength the band gap narrowed (1.63, 1.42 1.38 eV). Panchromatic absorbance was observed for the compounds comprising the cyanopyridone and oxoindenemalononitrile acceptors.     

Design and photophysical properties of zinc(II) porphyrin-containing dendrons linked to a central artificial special pair

The click chemistry synthesis and photophysical properties, notably photo-induced energy and electron transfers between the central core and the peripheral chromophores of a series of artificial special pair-dendron systems (dendron = G1, G2, G3; Gx = zinc(II) tetra-meso-arylporphyrin-containing polyimides) built upon a central core of dimethylxanthenebis(metal(II) porphyrin) (metal = zinc, copper), are reported. The dendrons act as singlet and triplet energy acceptors or donors, depending on the dendrimeric systems. The presence of the paramagnetic d(9) copper(II) in the dendrimers promotes singlet-triplet energy transfer from the zinc(II) tetra-meso-arylporphyrin to the bis(copper(II) porphyrin) unit and slow triplet-triplet energy transfer from the central bis(copper(II) porphyrin) fragment to the peripheral zinc(II) tetra-meso-arylporphyrin. If bis(zinc(II) porphyrin) is the central core, evidence for chain folding is observed; this is unambiguously demonstrated by the presence of triplet-triplet energy transfer in the heterobimetallic systems, a process that can only occur at short distances.     

Hybrid gold-nanoparticle-cored conjugated thiophene dendrimers: synthesis, characterization, and energy-transfer studies

A series of hybrid Au-nanoparticle-dendrimer materials: nanoparticle-cored thiophene dendrimers (NCTDs) were synthesized, characterized, and investigated for their energy-transfer properties. These hybrid nanoparticles were obtained by the simultaneous and in situ reduction of gold(III) chloride and self-assembly of the thiol-containing thiophene dendritic ligands. The dendron ligands were radially attached to the gold nanoparticles and were analyzed by TEM, UV/Vis, (1)H NMR, and FTIR spectroscopies. The solution fluorescence of the attached thiophene dendrons are quenched progressively. Both alkyl-chain length and dendron size have significant influence on the energy-transfer efficiency, as well as on core sizes and size distribution of the Au nanoparticles. In spite of the phenomenon's dependence on nanoparticle size, the energy transfer generally follows the 1/d(2) distance dependence. Single NCTD nanoparticles were also adsorbed on highly ordered pyrolytic graphite (HOPG) and uniform aggregates were observed on mica flat substrates.     

2,2′:3′,2′′‐Terthiophene‐Based all‐Thiophene Dendrons and Dendrimers: Synthesis,Structural Characterization,and Properties

The synthesis of generational dendritic oligothiophenes (DOTs) has been successfully achieved by a divergent/convergent approach that involves halogenation, boronation, and palladium‐catalyzed Suzuki coupling reactions. The key point in the presented synthetic approach is the use of trimethylsilyl (TMS) protecting groups, which allow for the core‐lithiation and subsequent boronation of the dendrons and for the peripheral ipso‐substitution with iodine monochloride or N‐bromosuccimide. In addition, the TMS protecting groups can be completely removed by using tetrabutylammonium fluoride, thus yielding only‐thiophene‐based dendrons and dendrimers. Due to their highly branched structure, all these synthesized DOTs are soluble in organic solvents. Chemical structures were confirmed by NMR spectroscopic, mass spectrometric, and elemental analysis. Concentration‐dependent ¹H NMR spectroscopic investigations revealed that the higher generation compounds tend to aggregate in solution. Such an aggregation behavior was further confirmed by measuring with MALDI‐TOF MS. Both MALDI‐TOF MS and gel‐permeation chromatography (GPC) analyses confirmed the monodispersity of the DOTs. Furthermore, GPC results revealed that these DOT molecules adopt a condensed globular molecular shape. Their optical and electronic properties were also investigated. The results indicated that these DOTs comprise various conjugated α‐oligothiophenes with different chain lengths, which results in the higher generation compounds showing broad and featureless UV/Vis absorption spectra and ill‐defined redox waves.     

Solubilization of a π‐Conjugating Hydrophobic Dendrimer in Aqueous Media

Phenylazomethine dendrimers, which can be used as the template for precise metal nanoparticles, are soluble only in aprotic media. The use of a specific surfactant enabled their solubilization in aqueous solutions. Under dilute condition, they formed discrete micelles with a size close to the that of the dendrimer. Condensation resulted in their aggregation; however, they remained in a homogeneous solution without precipitation. Furthermore, each micelle was well isolated. The triplet‐excited state of the zinc porphyrin core in the micelles was significantly stable, suggesting the formation of rigid core‐shell micelles preventing any external molecules from approaching to the core.     

Progresses in organic field-effect transistors and molecular electronics

In the past years, organic semiconductors have been extensively investigated as electronic materials for organic field-effect transistors (OFETs). In this review, we briefly summarize the current status of organic field-effect transistors including materials design, device physics, molecular electronics and the applications of carbon nanotubes in molecular electronics. Future prospects and investigations required to improve the OFET performance are also involved. __________ Translated from Huaxue Tongbao (Chemistry), 2006, 69(6) (in Chinese)     

Preparation of synthons for carborane containing macromolecules

The modification of para-carborane with appropriate functionalities for incorporation within a dendrimer framework was accomplished by functionalizing the carbon centers with protected alcohol and free acid groups. These compounds are excellent candidates for utilization as functional linkers between two generations of an aliphatic polyester dendrimer structure. Future assembly of these structures will result in dendritic macromolecules containing carboranes within their interior and will be enveloped by hydrophilic groups (hydroxyls) to maintain their water solubility and biocompatibility. These structures have potential applications in Boron Neutron Capture Therapy and Synovectomy. Additionally, carboranes were coupled to polymerizable acrylate structures, and it was shown that the resulting carborane monomers could be polymerized using living free-radical polymerization techniques.     

Self-assembled gelators for organic electronics

Nature excels at engineering materials by using the principles of chemical synthesis and molecular self-assembly with the help of noncovalent forces. Learning from these phenomena, scientists have been able to create a variety of self-assembled artificial materials of different size, shapes, and properties for wide ranging applications. An area of great interest in this regard is solvent-assisted gel formation with functional organic molecules, thus leading to one-dimensional fibers. Such fibers have improved electronic properties and are potential soft materials for organic electronic devices, particularly in bulk heterojunction solar cells. Described herein is how molecular self-assembly, which was originally proposed as a simple laboratory curiosity, has helped the evolution of a variety of soft functional materials useful for advanced electronic devices such as organic field-effect transistors and organic solar cells. Highlights on some of the recent developments are discussed.     

Study of hopping transport in long oligothiophenes and oligoselenophenes: dependence of reorganization energy on chain length

Internal reorganization energies for self-exchange hole-transfer process were calculated at the B3LYP/6-31G(d) level of theory for a series of oligothiophenes and oligoselenophenes up to the 50-mers. This is the first study of reorganization energy in very long pi-conjugated systems. We observed a linear correlation between reorganization energy and the reciprocal chain length for these long pi-conjugated heterocyclic oligomers, which can be explained by the changes in bond length that occur between the neutral and cation radical species and by the charge distribution in the cation radicals. In contrast to the saturation behavior observed for the HOMO-LUMO gaps of long pi-conjugated heterocyclic oligomers, the reorganization energy does not show saturation behavior for any length of the oligomers in this study, even up to the 50-mers. Interestingly, the reorganization energy approaches zero for infinite numbers of oligomer units (at the B3LYP/6-31G(d) level of theory), that is, for polythiophene and polyselenophene. The absolute values of the reorganization energies of oligoselenophenes, and the changes that occur in those energies with chain length, are similar to those found for oligothiophenes.     

Dendronized perylenetetracarboxdiimides with peripheral triphenylamines for intramolecular energy and electron transfer

Novel perylene-3,4,9,10-tetracarboxdiimides (PDI) dyes functionalized with polyphenylene dendrimers attached at the bay region are reported. Derivatives of PDI bearing polyphenylene dendrimers up to the second generation, substituted with an increasing number of triphenylamine (TPA) moieties at the periphery, as well as a related nondendronized model compound were prepared. Intramolecular energy transfer was demonstrated by the observation of PDI emission on excitation of the triphenylamines, and electron transfer was detected by comparing photoluminescence quenching in solvents of different polarity.     

Synthesis and Characterization of New Conjugated Fluorenyl‐Porphyrin Dendrimers for Optics

A new family of conjugated meso‐tetraphenylporphyrin‐based dendrimers with four ( TPP1 , TPP2 ), eight ( TPP3 , TPP4 , TPP5 ) and up to sixteen ( TPP6 ) fluorenyl groups has been synthesized and fully characterized. These tetraphenylporphyrin‐cored dendrimers present peripheral alkynyl π‐conjugated dendrons with fluorenyl termini. The meso‐aryl rings of these porphyrins are functionalized either in para‐ ( TPP1 , TPP2 , and TPP3 ) or meta‐positions ( TPP4 , TPP5 , and TPP6 ). Their detailed luminescence properties are discussed in reference to two porphyrins lacking fluorenyl dendrons ( TPP ‐ H_1,2,3 and TPP ‐ H_4,5,6 ). A strong dependence of their luminescence quantum yield and lifetime on their structures is stated, their nonlinear optical properties were also discussed.     

Concise Synthesis of Well‐Defined Linear and Branched Oligothiophenes with Nickel‐Catalyzed Regiocontrolled Cross‐Coupling of 3‐Substituted Thiophenes by Catalytically Generated Magnesium Amide

The synthesis of linear and branched oligothiophenes of well‐defined structures is performed with regioselective deprotonation of 3‐substituted thiophenes and nickel‐catalyzed cross‐coupling of the thus formed metalated species with a bromothiophene. The reaction of 3‐hexylthiophene with EtMgCl and 2,2,6,6‐tetramethylpiperidine (TMP‐H, 10 mol %) induces the metalation selectively at the 5‐position by use of the catalytically generated hindered magnesium amide (TMPMgCl) and the subsequent reaction of a 2‐halo‐3‐hexylthiophene (bromide or chloride) in the presence of a nickel catalyst affords a head‐to‐tail (HT)‐type dimer. By repeating the same sequence, the linear oligothiophene up to a 4‐mer is synthesized in good yield. The reaction of 3‐hexylthiophene with 2,3‐dibromothiophene also takes place to afford a branched oligothiophene 3‐mer in quantitative yield. The obtained 3‐mer is also metalated at the sterically less‐hindered position in a regioselective manner furnishing a 7‐mer in >99 % yield after a further coupling reaction with 2,3‐dibromothiophene. These dendrimers react with several multifunctionalized organic electrophiles, leading to a variety of branched oligothiophenes.     

60]Fullerene: a versatile photoactive core for dendrimer chemistry

Owing to their special photophysical properties, fullerene derivatives are good candidates to demonstrate dendritic effects. In particular, the triplet lifetimes of a C(60) core can be used to evaluate its degree of isolation from external contacts. On the other hand, the fullerene core can act as a terminal energy receptor in dendrimer-based light-harvesting systems. When a fullerodendrimer is further functionalized with a suitable electron donor, it may exhibit the essential features of a multicomponent artificial photosynthetic system in which photoinduced energy transfer from the antenna to the C(60) core is followed by electron transfer.     

Excited state properties of novel p- and n-type organic semiconductors with an anthracene unit

Photoinduced dynamics of novel p- and n-type organic semiconductors with an anthracene unit are theoretically investigated with quantum chemistry methods. The calculated vertical absorption and fluorescence frequencies of them are consistent with the experimental data. The changing tendencies of the dihedral angles between anthracene unit and trifluoromethylphenyl (or thiophene) in the photoinduced dynamics processes (vertical absorption and vertical fluorescence) are examined from the geometries of optimized ground and excited states. To study the influence of the individual units of the derivatives to the excited state properties of the derivatives, the energies and densities of frontier orbital HOMOs and LUMOs of the individual unit and the derivatives are studied in the processes of vertical absorption and fluorescence. The excited state properties of the two derivatives in the processes of vertical absorption and fluorescence are studied with 2D and 3D real space analysis methods, which are employed to study the electron–hole coherence and the excitation delocalization (with transition density matrix method), and charge and energy transfer (with transition and charge difference density method). Overall, the computed results remain in good agreement with the relevant experimental data, and the theoretical results promote deeper understanding to the optical and electronic properties of the semiconductor in the process of photoinduced dynamics.     

Divergent synthesis of dendrimer‐like macromolecules through a combination of atom transfer radical polymerization and click reaction

This article describes a divergent strategy to prepare dendrimer‐like macromolecules from vinyl monomers through a combination of atom transfer radical polymerization (ATRP) and click reaction. Firstly, star‐shaped polystyrene (PS) with three arms was prepared through ATRP of styrene starting from a three‐arm initiator. Next, the terminal bromides of the star‐shaped PS were substituted with azido groups. Afterwards, the azido‐terminated star‐shaped PS was reacted with propargyl 2,2‐bis((2′‐bromo‐2′‐methylpropanoyloxy)methyl)propionate (PBMP) via click reaction. Star‐shaped PS with six terminal bromide groups was afforded and served as the initiator for the polymerization of styrene to afford the second‐generation dendrimer‐like PS. Iterative process of the aforementioned sequence of reactions could allow the preparation of the third‐generation dendrimer‐like PS. When the second‐generation dendrimer‐like PS with 12 bromide groups used as an initiator for the polymerization of tert‐butyl acrylate, the third‐generation dendrimer‐like block copolymer with a PS core and a poly (tert‐butyl acrylate) (PtBA) corona was afforded. Subsequently PtBA segments were selectively hydrolyzed with hydrochloric acid, resulting an amphiphilic branched copolymer with inner dendritic PS and outer linear poly(acrylic acid) (PAA). Following the same polymerization procedures, the dendrimer‐like PS and PS‐block‐PtBA copolymers of second generation originating from six‐arm initiator were also synthesized. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3330–3341, 2007     

Designing systems for a multiple use of light signals.

The photochemical and photophysical behavior of two dendrimers consisting of a benzophenone core and branches that contain dimethoxybenzene units has been investigated. Such dendrimers can undergo a variety of photochemical and photophysical processes, namely: 1) quenching of the fluorescence and phosphorescence of the dimethoxybenzene units by energy transfer to the benzophenone core (antenna effect), 2) direct and sensitized phosphorescence (and delayed fluorescence) of the benzophenone core, 3) hydrogen abstraction by the triplet excited state of the benzophenone core from solvent molecules, 4) intramolecular hydrogen abstraction by the triplet excited state of the benzophenone core from the dendrimer branches, 5) quenching of the phosphorescence and hydrogen abstraction reaction of the benzophenone core by energy transfer to terbium ions and dioxygen; 6) conversion of the UV light absorbed by the dendrimer branches into visible (Tb3+) or near infrared (O2) emission via the sequence of processes 1) and 5). The results obtained emphasize the great potential of suitably designed dendrimers for a multiple use of light signals.     

Synthesis and characterization of dendritic multichromophores based on rylene dyes for vectorial transduction of excitation energy

The synthesis of dendritic multichromophores based on a rigid polyphenylene scaffold is presented. Different rylene chromophores are incorporated into the core, the branches, and the surface of the dendrimer. In this way, two generations of dendritic dyads consisting of a terrylenediimide core, a stiff polyphenylene scaffold, and a perylenemonoimide periphery were obtained. Furthermore, the first synthetic approach to a dendritic triad is introduced. The outer sphere of this macromolecule is formed by naphthalenemonoimide chromophores, whereas perylenemonoimide groups are located in the dendritic scaffold, and the terrylenediimide chromophore serves as a core molecule. This multichromophore absorbs over the whole range of the visible spectrum and shows well-separated absorption envelopes. In the course of dendrimer synthesis new attempts towards a straightforward functionalization strategy for rylene dyes are also presented.     

Core‐and‐Surface‐Functionalized Polyphenylene Dendrimers for Solution‐Processed,Pure‐Blue Light‐Emitting Diodes Through Surface‐to‐Core Energy Transfer

Several pyrene‐based polyphenylene dendrimers (PYPPDs) with different peripheral chromophores (PCs) are synthesized and characterized. Deep blue emissions solely from the core are observed for all of them in photoluminescence spectra due to good steric shielding of the core and highly efficient surface‐to‐core Förster resonant energy transfers (FRETs). Device performances are found in good correlation with the energy gaps between the work function of the electrodes and the frontier molecular orbital (FMO) levels of the PCs. Pure blue emission, luminance as high as 3700 cd m⁻² with Commission Internationale de l'Éclairage 1931 (CIE_xy) = (0.16, 0.21), and a peak current efficiency of 0.52 cd A⁻¹ at CIE_xy = (0.17, 0.20) are achieved. These dendrimers are among the best dendritic systems so far for fluorescent blue light‐emitting materials.

     

Dendrimers as Nd3+ ligands: Effect of Generation on the Efficiency of the Sensitized Lanthanide Emission

We have designed two novel dendrimers with cyclam cores with appended poly(amido amine) (PAMAM) dendrons, decorated at the periphery with four and eight dansyl chromophores, respectively. The photophysical properties of the dendrimers and their Nd³⁺ complexes have been investigated. The energy‐transfer efficiency to the lanthanide ions from these dendrimers has been studied as a function of the generation. It has been observed that an increase in the dendrimer generation as well as the number of amide units enhances the energy transfer to the lanthanide ion.