Oligothiophene Bipyridine Alternate Copolymers and Their Ruthenium Metalated Analogues:  In Situ ESR and UV−Vis Investigations of Metal−Chain Interactions

Abstract

In situ electron spin resonance (ESR) and UV-vis spectro-electrochemical studies have been performed on two copolymers consisting of alternating subunits of regioregular head to tail (HT) coupled 3-octylthiophene tetramer and 2,2'-bipyridine subunits (P4) or 3-octylthiophene hexamer subunits of the same regioregularity and 2,2'-bipyridine subunits (P6). Both P4 and P6 have been investigated in their metal-free form as well as in the ruthenium(II) metalated form (P4-Ru and P6-Ru). P4 and P6 in the p-doped state exhibit a clear ESR signal characteristic of the presence of polarons in the oligothienylene subunits. In the case of P4, no recombination of polarons into bipolarons is observed, whereas the recombination process takes place in P6. The formation of bipolarons is well-rationalized in terms of the conjugation length, and it seems clear that the higher length of the oligothiophene subunit in P6( )()stabilizes bipolarons(.)() The same effect, is induced by the coordination of -Ru(bpy)(2)(2+) to the bipyridine unit in the metalated form of both polymers, which results in an increase of the conjugation length. Important information is gained from the analysis of the ESR spectra of both nonmetalated and metalated in the oxidized (p-doped) and reduced (n-doped) forms. In the p-doped state both nonmetalated and metalated polymers reveal the presence of a narrow ESR line characteristic of the mobile spin carriers in the polymer matrix. The oxidation of the metal center occurs at higher potentials and leads to an irreversible destruction of the system. To the contrary, in the reduced (n-doped) state the ESR lines of the nonmetalated and metalated polymers markedly differ. A significant line broadening with simultaneous change of the g-value is caused by spin-orbit coupling phenomenon induced by the presence of the coordinating metal. Finally, the observation of a clear polaronic band in the UV-vis spectrum of p-doped P4 and its strong dependence on the applied potential can be clearly correlated with the potential induced changes in the ESR spin density. The same applies to P4-Ru, where the changes in the polaronic and bipolaronic bands can also be correlated with the ESR spin density changes.

Polaron dynamics in two types of long oligothiophenes revealed byQ- andX-band ESR measurements

“…The result g Ͼ g e observed in h-16T is thus characteristic of p-doped materials and has actually been reported on p-doped polypyrroles ͑PPy͒ 10,27 and PT derivatives. [28][29][30][31][32] Similarly, a g value smaller than g e is typically observed in n-doped materials. Indeed, the electrochemically n-doped PT derivatives were reported to have g values smaller than g e .…”

“…Indeed, the electrochemically n-doped PT derivatives were reported to have g values smaller than g e . 32 In this respect, the g av values of p-doped o-20T, being smaller than g e , are somewhat unexpected. Similar features in g values were also reported for some p-doped PATs with some crystalline morphology.…”

Functional Oligothiophenes: Molecular Design for Multidimensional Nanoarchitectures and Their Applications

“…From ESR measurements, strong spin-orbit coupling was observed in the reduced (ndoped) state of these complexes, indicating that the injection of charge occurs preferentially on the metalated bipyridine units of the polymer backbone, which acts as an electronic gate. 367 Pappenfus and Mann prepared bipyridine-capped oligothiophene ligands 2.281 as well as their respective binuclear Ru(II) complexes 2.282 (Scheme 2.51). 368 Precursor sexithiophene 2.280 was synthesized by oxidative coupling of terthiophene with BuLi/Fe(acac) 2 .…”

Electronic interactions within composites of polyanilines formed under acidic and alkaline conditions. Conductivity, ESR, Raman, UV-vis and fluorescence studies

“…This was close to the free electron g value (2.0023) indicative of resonance that comes from electrons delocalised in the p-system of the carbon atoms forming the polymer backbone in the main chain. [38][39][40][41][42][43] The ESR spectra showed no hyperfine structure, which is characteristic of delocalised free radicals that exist along the polymer backbone. 44 As with previously published work, the intensity was comparable to those observed for primary doped PAni, with the peak to peak line-widths also indicating a high electronic delocalisation which is consistent with a more extended conformation of the polymeric chains.…”