Gilbert Nöll

One‐ and Two‐Dimensional Electron Transfer Processes in Triarylamines with Multiple Redox Centers

Radical monocations of symmetrical triarylamines such as1 show intense intervalence charge-transfer (IV-CT) bands in the NIR spectra. These bands refer to photoinduced degenerate electron transfers (ET) between two amine redox centers. Hush analysis of the IV-CT bands revealed a very strong electronic coupling between the redox centers in 1+ compared to the total reorganization energy for the ET process. This results in a very fast thermal electron transfer.

Synthesis, (Non)Linear Optical and Redox Properties of a Donor-Substituted Truxenone Derivative

The search for better, more efficient and at the same time more transparent NLO chromophores has led to the development and investigation of new, alternative chromophore types. A new and quite general synthetic route to ring-substituted truxenones is presented, and the linear and nonlinear optical and redox properties of 1 were investigated.

Increasing the coulombic efficiency of glucose biofuel cell anodes by combination of redox enzymes.

A highly efficient anode for glucose biofuel cells has been developed by a combination of pyranose dehydrogenase from Agaricus meleagris (AmPDH) and cellobiose dehydrogenase from Myriococcum thermophilum (MtCDH). These two enzymes differ in how they oxidize glucose. AmPDH oxidizes glucose at the C(2) and C(3) carbon, whereas MtCDH at the C(1) carbon. Both enzymes oxidize efficiently a number of other mono- and disaccharides. They do not react directly with oxygen and produce no H(2)O(2). Electrodes were prepared by embedding (i) only AmPDH (in order to study this enzyme separately) and (ii) a mixture of AmPDH and MtCDH in an Os redox polymer hydrogel. Single-walled carbon nanotubes (SWCNTs) were added in order to enhance the current density. The electrodes were investigated with linear sweep and cyclic voltammetry in the presence of different substrates at physiological conditions. The electrochemical measurements revealed that the product of one enzyme can serve as a substrate for the other. In addition, a kinetic pathway analysis was performed by spectrophotometric measurements leading to the conclusion that up to six electrons can be gained from one glucose molecule through a combination of AmPDH and MtCDH. Hence, the combination of redox enzymes can lead to an enzymatic biofuel cell anode with an increased coulombic efficiency far beyond the usual yields of two electrons per substrate molecule.

Comparison of direct and mediated electron transfer for cellobiose dehydrogenase from Phanerochaete sordida.

Direct and mediated electron transfer (DET and MET) between the enzyme and electrodes were compared for cellobiose dehydrogenase (CDH) from the basidiomycete Phanerochaete sordida (PsCDH). For DET, PsCDH was adsorbed at pyrolytic graphite (PG) electrodes while for MET the enzyme was covalently linked to a low potential Os redox polymer. Both types of electrodes were prepared in the presence of single walled carbon nanotubes (SWCNTs). DET requires the oxidation of the heme domain, while MET occurs partially via the heme and the flavin domain at pH 3.5. At pH 6 MET occurs solely via the flavin domain. Most probably, the interaction of the domains decreases from pH 3.5 to 6.0 due to electrostatic repulsion of deprotonated amino acid residues, covering the surfaces of both domains. MET starts at a lower potential than DET. The midpoint potentials at pH 3.5 for the flavin (40 mV) and the heme domain (170 mV) were determined with spectroelectrochemistry. The electrochemical and spectroelectrochemical measurements presented in this work are in conformity. The pH dependency of DET and MET was investigated for PsCDH. The optimum was observed between pH 4 and 4.5 pH for DET and in the range of pH 5-6 for MET. The current densities obtained by MET are 1 order of magnitude higher than by DET. During multicycle cyclic voltammetry experiments carried out at different pHs, the PsCDH modified electrode working by MET turned out to be very stable. In order to characterize a PsCDH modified anode working by MET with respect to biofuel cell applications, this electrode was combined with a Pt-black cathode as model for a membraneless biofuel cell. In comparison to DET, a 10 times higher maximum current and maximum power density in a biofuel cell application could be achieved by MET. While CDH modified electrodes working by DET are highly qualified for applications in amperometric biosensors, a much better performance as biofuel cell anodes can be obtained by MET. The use of CDH modified electrodes working by MET for biofuel cell applications results in a less positive onset of the electrocatalytic current (which may lead to an increased cell voltage), higher current and power density, and much better long-term stability over a broad range of pH.

Intervalence charge-transfer bands in triphenylamine-based polymers

Abstract In this paper we show that bis(triarylamine) systems with conjugated bridges can be polymerised potentiodynamically. The resulting polymer films can be doped to a degree of ca. 75% per triarylamine unit. At 50% doping level the polymers show broad and intense intervalence charge-transfer bands. These IV-CT bands are quite similar to those in monomeric model compounds. Therefore, the polymers investigated can be regarded as polymeric mixed valence compounds.

A third generation glucose biosensor based on cellobiose dehydrogenase from Corynascus thermophilus and single-walled carbon nanotubes

A third generation glucose biosensor working under physiological conditions with a linear range of 0.1-30 mM, a detection limit of 0.05 mM, and a sensitivity of 222 nA µM(-1) cm(-2) has been developed by co-adsorption of cellobiose dehydrogenase (CDH) from the ascomycete Corynascus thermophilus (CtCDH) and oxidatively shortened single-walled carbon nanotubes (SWCNTs).

Optically and Thermally Induced Electron Transfer Pathways in Hexakis[4-(N,N-diarylamino)phenyl]benzene Derivatives

The optically and thermally induced electron transfer pathways of highly symmetrical (D(3)) hexaarylbenzene systems with six triarylamine redox sites have been investigated. Owing to slightly different local redox potentials, the radical trication could be selectively generated by electrochemical methods. This trication shows a strong intervalence charge-transfer band in the near infrared (NIR) that was measured by spectroelectrochemistry and analysed using multi-dimensional Mulliken-Hush theory. Quantum chemical AM1 CI calculations indicate that there is no optically induced concerted three-electron transfer that transforms the ground state into a state in which all three positively charged triarylamine moieties change place with their neutral neighbours. The potential energy surface of the ground state was constructed by using quadratic potentials. From this potential surface it is apparent that there is also no thermally allowed concerted three-electron transfer pathway. Instead, three consecutive one-electron transfer steps are necessary for this process.

Tuning of intervalence charge transfer energies by substituents in one-dimensional bis(triarylamine) systems

We synthesised a series of linear bis(triarylamine) species whose triarylamine redox centres have different local redox potentials which were tuned by substituents in the para-position. The mixed-valence (MV) radical cations of these systems were generated and investigated in situ by UV/Vis/NIR spectroelectrochemistry. The electronic coupling between the redox centres was analysed by the generalised Mulliken–Hush theory which gave a practically constant coupling. The radical cations of the species show an intervalence charge transfer (IV-CT) band in the NIR whose energy varies linearly with the electrochemical redox potential splitting. This correlation proves that the Marcus–Hush two-level model is an adequate way to describe the electronic situation in these linear MV systems.

Synthesis, Spectroscopic Properties, and Electropolymerization of Azulene Dyads

Four azulene dyads have been synthesized and studied by spectroscopic and electrochemical methods. A triarylamine, a boron-dipyrromethene (BDP or BODIPY), a porphyrin, and an isoalloxazine moiety have been linked to an extended π electron system at the 2-position of azulene, leading to the dyads 1-4, respectively. For the synthesis of 1-4, first 2-(4-ethynyl-phenyl)azulene (EPA) was prepared, which was further reacted with the halogenated chromophores by Pd-catalyzed cross-coupling reactions. The dyads 1-4 exhibit strong absorption bands in the visible range, which are dominated by the absorption spectra of the individual subchromophores. Fluorometric studies of 2-4 revealed that after excitation of the subchromophoric unit attached to the parent azulene moiety, quenching mainly through energy transfer to azulene is effective, whereas possible charge-transfer interactions play only a minor role. Potentiodynamic oxidation of the dyads 1-4 leads to the formation of polymer films, which are deposited at the electrode. The polymer film derived from 1 was further characterized by spectroelectrochemistry. During positive doping of poly-1, a strong absorption band appears at 13,200 cm(-1), which is typical for triarylamine radical cations. This band is overlapping with a broad absorption band in the low-energy region that might be caused by charge-transfer interactions within the polymer.

Charge-transfer transitions in triarylamine mixed-valence systems: the effect of temperature

Abstract The temperature dependence of inter-valence charger-transfer transitions has been investigated for three triarylamine-based mixed-valence systems: (bis-{4-[N,N-di(4-methoxyphenyl)amino]-phenyl}butadiyne, 1 + ), (4,4′-bis[N,N-di(4-methoxyphenyl)amino] biphenyl, 2 + ), and (N,N,N′,N′-tetraphenyl-1,4-phenylenediamine, 3 + ). Although the band shape of 1 + – 3 + changes with temperature, neither the position of the transition maximum nor the integral intensity are significantly affected by temperature. The shape of the absorption bands is analyzed in the framework of a dynamic vibronic model.