Tetraphenylbenzidine Research Articles

New semiconductors based on triphenylamine with macrocyclic architecture: synthesis, properties and applications in OFETs

Two novel macrocycles based on triphenylamine (TPA) have been synthesized by McMurry coupling reactions. The cyclic compound 2 consisted of two triphenylamines linked with ethylene bridges bearing two n-butyl chains. The compound 3 was based on N,N,N′,N′-tetraphenylbenzidine (TPD) units with macrocyclic architecture. They were fully characterized by cyclic voltammetry, UV-vis absorption and self-assembly properties. The crystal structure of 2 was determined by X-ray analysis. Atomic force microscope and scanning electron microscope images showed that compound 3 could form interesting fiber-like nanostructures by self-assembly. Both of the compounds can be used as active layers for p-type OFETs. The OFET device based on 2 prepared via a vacuum-deposit method gave a mobility of 2.3 × 10–3 cm2 V–1 s–1 and a current on/off ratio of 105. High quality thin films of 3 were fabricated by spin coating from solution, and gave a mobility of 2.0 × 10–3 cm2 V–1 s–1 with a current on/off ratio of 2 × 105. The results showed that the TPA derivatives with cyclic structures might fit better for OFETs. They may provide promising new choices for organic semiconductors.

Synthesis and Characterization of Donor−Bridge−Acceptor Molecule Containing Tetraphenylbenzidine and Perylene Bisimide

A novel donor acceptor dyad consisting of tetraphenylbenzidine (TPD) and perylene bisimide was synthesized by linking these moieties using a dodecyl spacer. HOMO and LUMO values of the dyad were aquired by cyclic voltammetry. Photophysical properties were studied by steady-state UV−vis and fluorescence spectroscopy. After selective excitation of the TPD moiety, a quenching of donor fluorescence and the appearance of acceptor fluorescence was observed. This proves nonradiative energy transfer from the donor to the acceptor group. The energy transfer was 4 times more efficient in DBA than in a mixture of D and A. The energy transfer efficiency in the dyad is also independent of the concentration, indicating intramolecular transfer mechanism. However, the direct excitation of the acceptor in the dyad exhibited reduced fluorescence emission of the acceptor, indicating electron transfer between the moieties. Thus, this DBA is an excellent model system for the study of energy- and electron-transfer processes in organic semiconductors.

Fabrication and characteristics of hole transporting materials–transition metal nanoparticle composites

A new concept of inorganic nanoparticle dopant into organic hole transporting materials (HTM) is proposed for the design of semiconducting composites. Composites consisting of HTM and TiO 2 nanoparticles were prepared using a polymer as a matrix. TiO 2 nanoparticles were prepared by sol–gel method starting from titanium tetraisopropoxide. The particle size of TiO 2 ranged from 2 to 5 nm. It was found that electrons were transferred from HTM to TiO 2, leaving holes in HTM. The composites showed semiconducting characteristics with the conductivity on the order of 10 −8 S/cm. A device consisting of ITO/composite/Al showed rectifying ability. The number of the holes was approximately one per 7×10 5 molecules of N, N, N′, N′-tetraphenylbenzidine (TPD) for the composite consisting of polycarbonate, TPD and TiO 2 with the weight ratio of 100:100:3.

Semiconducting nanocomposite from titanium dioxide and organic charge transporting compound

Semiconducting nanocomposite was prepared from tetraphenylbenzidine (TPD) and titanium oxide using polycarbonate as a binder. Titanium oxide was prepared by sol–gel process in the presence of binder and TPD. The conductance of the composite reached ca. 3×10 −8 S/cm with the titanium oxide content of 3% of TPD. The film containing only TPD or titanium oxide had conductivity lower than 10 −13 S/cm.

Preparation of New Hole Transport Polymers via Copolymerization ofN,N ′-Diphenyl-N,N′-bis(4-alkylphenyl)benzidine (TPD) Derivatives with 1,4-Divinylbenzene

Monomers containing N,N ′,N,N ′-tetraphenylbenzidine (TPD) were prepared by introducing two alkyl or alkoxy substituents into the p-position of two of the four phenyl groups in the triphenylamine unit. The monomers were copolymerized with 1,4-divinylbenzene (DVB) in the presence of p-toluenesulfonic acid and Lewis acids, such as SnCl4 and BF3. The polymerization conditions and the catalyst types showed a marked influence on yield, molecular weight and even polymer structure. 1H NMR measurements revealed that, by using p-toluenesulfonic acid as a catalyst, the linkage between the TPD unit and DVB occurs at the p-position of the phenyl group and at the m-position of the tolyl group in the TPD unit, whereas, with SnCl4 or BF3 catalyst, substitution occurs exclusively at the p-position of TPD. All polymers are soluble in common organic solvents, which allows the preparation of thin films of good quality. The rigid structure of the polymeric backbone results in a high thermal stability up to 400°C. The results of ionization potential measurements and the redox behavior showed that the introduction of the TPD unit into this kind of polymer backbone does not change its electronic structure. On the basis of high electroactivity, the polymers were successfully used as hole transport layers in two-layer electroluminescence devices.

Enhancement of Ionization Efficiency by Electrochemical Reaction Products in On-Line Electrochemistry/Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

A miniaturized two-electrode electrochemical (EC) cell was developed and was coupled on-line with an electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer (ESI-FTICR MS). Electrochemistry on-line with mass spectrometry, EC/ESI-FTICR MS, of triphenylamine (TPA), which undergoes one-electron oxidation to form a radical cation (TPA*+), demonstrates a significant sensitivity enhancement compared to ESI-FTICR MS. The on-line EC cell configuration with a stainless steel ES needle as the working electrode produces the highest sensitivity in EC/ESI-MS. The results provide evidence that, during the ES ionization, electrolytic reactions occur mainly in the ES tip region, as previously predicted. The results demonstrate that ESI-MS signal suppression by tetrabutylammonium perchlorate electrolyte, which can be a problem, is minimized in EC/ESI-MS. TPA*+ dimer tetraphenylbenzidine (TPB) can be detected by EC/ESI-MS, together with TPA*+, as TPB*+ and TPB2+. The high mass resolving power of FTICR MS was exploited to identify TPB2+ dication in the presence of [TPA*+ - H*]+ ions of the same m/z, from their respective isotopic distributions. The dimer dication TPB2+ can be detected only in EC/ESI-MS.

An Intermediate State of the Triphenylamine Cation Radical Revealed Using an Electron-Transfer Stopped-Flow Method

Using an electron-transfer stopped-flow method, the triphenylamine cation radical (TPA. + ) was generated in acetonitrile via the electron transfer between TPA and the tris(p-bromophenyl) amine cation radical (TBPA. + ), and the changes in the absorption spectra were observed. The well-known decay process of TA. + was observed in the absence of TPA, reflecting the dimerization reaction of TPA. + and the consecutive electron-transfer reactions of the dimer compound, tetraphenylbenzidine (TPB). When TBPA. + was mixed with a large excess amount of TPA, new absorption peaks appeared around 400 and 850 nm. No time changes were observed for these peaks, while the decrease in TPA. + and the increase in TPB. were observed overlapping on these peaks. The new absorption spectrum can be assigned to that of(TPA) 2 . + , or, more presumably, (TPA) n + . Although (TPA) n + has not been recognized as an intermediate in the electrochemical oxidation of TPA, it has been ascertained that (TPA) n + is electrochemically generated under certain conditions, e.g., during the initial stage of the oxidation of TPA in a stirred solution containing a sufficient amount of TPA.

Preparation of hole transporting polymers by condensation polymerization of triphenylamine derivatives

Hole transporting polymers were prepared by condensation polymerization of triphenylamine and N,N,N',N'-tetraphenylbenzidine (TPD) having alkyl group with aldehydes in the presence of p-toluenesulfonic acid. The obtained polymers had molecular weight higher than 10,000 and good film formation ability. It was found that the aromatic amine monomers were connected with aldehyde monomer at the p-position of the phenyl group. TPD-aldehyde polymers had almost the same UV absorption and redox potentials as those of TPD monomer indicating that the electronic structure of amine unit did not change by the polymerization. The hole transporting mobility was in the range of 10−3-10−6cm2/Vs. The electroluminescent device consisting of ITO/TPD polymer/Alq/Mg-Ag had a maximum luminance of 9000 cd/m2.

Molecular orientation-photoconductivity relationship study of phthalocyanine polymer-oriented thin films

The molecular orientation–photoconductivity relationships of several kinds of phthalocyanine polymer (PPc)-oriented thin films have been studied in double-layered photoreceptor devices, where the charge-generation layers (CGLs) are phthalocyanine polymer-oriented thin films and the charge-transportation layers (CTLs) are composed of hole transporting materials of tetraphenyl benzidine or hydrazone. The oriented thin films containing PPc dispersed in polyvinyl difluoride (PVDF) were prepared by the electric field orientation. The results showed that the photosensitivities of the phthalocyanine polymer (PPcs)-oriented thin films were higher than those of the unoriented PPcs thin films, and varied with their molecular structures and the molecular stacking in the films. This was thought to be due to the molecular orientation effect, which was demonstrated by the analyses of the polarized fluorescence, DSC, FTIR reflection absorption spectroscopy (FTIR-RAS), and angle-dependent XPS. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2331–2339, 2000

Synthesis of polymers for hole and electron transport materials in organic electroluminescent devices

Styrene-type polymers having tetraphenylbenzidine (TPD) or tetraphenylphenyldiaminobenzene unit (PDA) and a oxadiazole unit (PBD) on the side chain were prepared as hole and electron transport materials, respectively, of an electroluminescent (EL) device. The device structures employed were [indium-tin-oxide (ITO)/hole transport layer (HTL)/Al] (type I), or [ITO/hole transport layer (HTL)/electron transport layer (ETL)/Al] (type II). Type I devices provided current density higher than 100 mA/cm/sup 2/ but no luminescence was observed. Type II devices emitted luminescence of about 10 cd/m/sup 2/ at the current density of about 170 mA/cm/sup 2/. The emission maximums of these devices were 460 and 530 nm for the device using TPD and PDA, respectively.

Fabrication and characteristics of ZnS nanocrystals/polymer composite doped with tetraphenylbenzidine single layer structure light-emitting diode

The hexagonal ZnS nanocrystals were synthesized in polymer matrix. The ZnS polymer composite doped with tetraphenylbenzidine (TPB) as light-emitting layer was used to fabricate a single layer structure light-emitting diode which has a low turn on voltage such as 2.5 V. The electroluminescence spectrum was obtained at room temperature. Owing to the effect that TPB interacted with ZnS nanocrystal to form the luminescent center, the emission was peaking at 520 nm which shifts to the lower energy compared with that of ZnS, and the half-width of the emission was about 20 nm.

Resonance Raman measurement of electrochemically generated short-lived intermediates by means of a pulse electrolysis stopped flow method

Resonance Raman (RR) measurement with a pulse electrolysis stopped flow method was developed for the purpose of the measurement of electrochemically generated short-lived intermediates. In this method the solution, electrolysed by imposing a current pulse on a column electrode, is delivered to an optical flow cell by means of a stopped flow technique and then the RR spectrum is observed at the optical cell. The RR spectrum of the triphenylamine cation radical (TPA .+), which dimerizes rapidly to produce the dimeric compound tetraphenylbenzidine (TPB), was successfully measured selectively without any interference by the oxidation products of TPB generated in subsequent electron transfer reactions. The RR spectrum of TPA .+ obtained had a different profile from that of the dimer cation radical (TPB .+) but was quite similar to that of the dimer dication (TPB 2+). This result shows the similarity between the vibrational structures of TPA .+ and TPB 2+, which can be explained by their canonical structures. By using the proposed method, it should become possible to observe the RR spectra of electrogenerated intermediates whose half-lives are several tens of milliseconds.