Combined spectroelectrochemical and theoretical study of electron-rich dendritic 2,5-diaminothiophene derivatives: N,N,N',N'-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine.

The in situ spectroelectrochemical and electron spin resonance (ESR) behavior of the recently prepared N,N,N',N'-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine 11 is presented. The results are compared to the ones of the parent 2,5-bis-diphenylamino-thiophene 41 as well as to the corresponding high-molar third dendrimer generation 8 containing the same thiophene-2,5-diamine core. The dendritic compound 11 can be reversibly oxidized in three separated steps to yield the corresponding stable monocation 11(•+), dication 11(2+), and tetracation 11(4+). A well resolved ESR spectrum of the corresponding cation radical 11(•+) with dominating splittings from two nitrogen atoms and two hydrogen atoms was observed at the first oxidation peak similar to 41(•+). The shape of the SOMOs orbitals very well correlates with the proposed distribution of the unpaired electron mainly on the thiophene center and neighboring nitrogen atoms. The spin delocalization on the central thiophene moiety in the monocations for all three model compounds 41(•+), 11(•+), and 8(•+) was confirmed. The computed single occupied molecular orbital (SOMO) for trication 11(•3+) is completely different compared to the SOMO of the corresponding monocation 11(•+), and it confirms a largely delocalized unpaired spin density. Dominating diamagnetic product was determined at the third oxidation peak, confirming the formation of a tetracation by a two electron oxidation of ESR silent dication. The positive charge is fully delocalized over the lateral parts of the molecule leading to the high stability of tetracation 11(4+). The estimated theoretical limit energy of the lowest optical transition S0 → S1 is 2.90 eV, and it can be achieved for the 3D dendrimer generation.

The charging of the carbon nanotube/oligothiophene interphase as studied by in situ electron spin resonance/UV-Vis-NIR spectroelectrochemistry.

A detailed in situ Electron Spin Resonance (ESR)/UV-Vis-NIR spectroelectrochemical study of the oligothiophene/single walled carbon nanotube (SWCNT) interphase is presented to provide an insight into the interaction of nanotubes with oligothiophenes. Used as electrode materials these composites are followed in situ with respect to the paramagnetic and diamagnetic states formed upon electrochemical charging. The variation of the oligomer chain length and the type, position and number of substituents at the oligomer is used to understand the structural influence on the formation of the charged states in the material upon electrochemical reaction. For ß,ß'-dihexylsexithiophene (ß,ß'-DHST)-SWCNT the enlarged current in the composite and a decreased radical cation concentration can be explained by the formation of π-dimers. By interaction with SWCNTs the π-dimerization of oligothiophenes and the formation of multi π-stack structures occur. For α,ω-dicyano-ß,ß'-dibutylquaterthiophene (DCNDBQT)-SWCNT a new paramagnetic structure of the oligomer is formed as an intermediate which undergoes follow-up reactions. Using different substituted oligothiophenes their interaction with nanotubes can be understood with respect to the structure of the oligomer.

Charged states of α,ω-dicyano ß,ß'-dibutylquaterthiophene as studied by in situ ESR UV-vis NIR spectroelectrochemistry.

The influence of the molecular structure on the stabilization of charged states was studied in detail by in situ ESR UV-vis NIR spectroelectrochemistry at a novel α,ω-dicyano substituted ß,ß'-dibutylquaterthiophene (DCNDBQT) and the electrochemically generated cation and anion radicals have been proved for the first time. The voltammetry of DCNDBQT results in two separate oxidation steps with the reversible first one. The experimental absorption maxima at 646 and 1052 nm together with the calculated ones (by DFT method) as well as an ESR signal at the first anodic step prove the presence of a radical cation. Three additional optical bands (554, 906, and 1294 nm for CT-transition) can be attributed to the formation of cation radical dimer. The dicationic structure formed in the second oxidation step is not stable. The stabilization proceeds via a dimer formation in two chemical follow-up reactions. The existence of the dimeric structures was proved by ex situ MALDI TOF mass spectrometry. As the substitution by cyano groups opens the route to cathodic reductions, DCNDBQT shows a single quasi-reversible reduction step. Here, the in situ ESR UV-vis NIR spectroelectrochemical measurements and theoretical calculations let us confirm the electrochemical generation of an anion radical. As we found a low number of anion radicals by quantitative ESR spectroelectrochemistry and an appearance of additional bands in the UV-vis NIR absorption spectra, the formation of dimeric structures must be considered and was corroborated by mass spectrometry. The role of dimerization in the reaction mechanism of the DCNDBQT oxidation and reduction are discussed in general. The experimental results were interpreted using the quantum chemical calculations based on density functional theory.

The route to functional graphene oxide.

We report on an easy-to-use, successful, and reproducible route to synthesize functionalized graphite oxide (GO) and its conversion to graphene-like materials through chemical or thermal reduction of GO. Graphite oxide containing hydroxyl, epoxy, carbonyl, and carboxyl groups loses mainly hydroxyl and epoxy groups during reduction, whereas carboxyl species remain untouched. The interaction of functionalized graphene with fluorescent methylene blue (MB) is investigated and compared to graphite, fully oxidized GO, as well as thermally and chemically reduced GO. Optical absorption and emission spectra of the composites indicate a clear preference for MB interaction with the GO derivatives containing a large number of functional groups (GO and chemically reduced GO), whereas graphite and thermally reduced GO only incorporate a few MB molecules. These findings are consistent with thermogravimetric, X-ray photoelectron spectroscopic, and Raman data recorded at every stage of preparation. The optical data also indicate concentration-dependent aggregation of MB on the GO surface leading to stable MB dimers and trimers. The MB dimers are responsible for fluorescence quenching, which can be controlled by varying the pH value.