Charge Delocalization in a Homologous Series of α,α′-Bis(dianisylamino)-Substituted Thiophene Monocations

Abstract

A homologous series of three molecules containing thiophene, bithiophene and terthiophene bridges between two redox-active tertiary amino-groups was synthesized and explored. Charge-delocalization in the one-electron oxidized forms of these molecules was investigated by a combination of cyclic voltammetry, near-infrared optical absorption spectroscopy, and EPR spectroscopy. All three cation radicals can be described as organic mixed-valence species, and for all of them the experimental data is consistent with strong delocalization of the unpaired electron. Depending on what model is used for 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 analysis of the optical absorption data, estimates for the electronic coupling matrix element (H AB ) range from ∼5000 cm -1 to ∼7000 cm -1 for the shortest member of the homologous series. According to optical absorption and EPR spectroscopy even the terthiophene radical appears to belong either to Robin-Day class III or to a category of radicals commonly denominated as borderline class II / class III systems. The finding of such a large extent of charge delocalization over up to three adjacent thiophene units is remarkable.

Charge Delocalization in an Organic Mixed Valent Bithiophene Is Greater Than in a Structurally Analogous Biselenophene

“…22 The hyperfine structure of all three radical cations is due to the interaction of the unpaired electron with nuclear spins of nitrogen and hydrogen, as previously reported for the structurally identical thiophene compounds I + and II + . 23 The hyperfine coupling constants are slightly higher for 1 + , 2 + , and 3 + than the values reported for their thiophene analogs I + and II + ( Table 5). The slight difference in hyperfine interaction might be due to the higher polarity of acetonitrile (used as solvent in the case of 1 + , 2 + , and 3 + ) compared to that of dichloromethane (employed in our prior study of I + and II + ).…”

“…Qualitatively, the spectra of 1 + and 2 + are very similar to those of I + and II + that were measured previously under identical conditions. 23 Upon oxidation of the bis(4-chlorophenyl)amino-decorated selenophene (3), there is a new band at ∼15 200 cm −1 (purple trace in Figure 2c) which we interpret as the analog to the 14 000 cm −1 band of 1 + (Figure 2a). Replacement of methoxy groups on the amines by chloro substituents thus induces a blue shift of the lowestenergetic monocation absorption.…”

“…b From ref23. β is the distance decay constant as defined in eq 4. β n is the dimensionless decay constant characterizing the decrease of H AB per σ-bond between the two nitrogen atoms.…”

Photoinduced Electron and Proton Transfer with Metal Complexes and Organic Molecules

“…Based on a combination of electrochemical, optical spectroscopic and EPR investigations 4 + was found to be a class III mixed-valence species while the longer congeners 5 + and 6 + seem to be at the borderline of class III and class II. [12] The analogous phenylene-bridged systems is already a fully charge-localized class I species. [8,9] Such investigations of charge delocalization phenomena in oligothiophenes and related heteroaromatics are relevant in the context of organic light-emitting diodes.…”

Ferrocenes Bridged by Ethylenediamino Thiophene: Varying Charge Transfer Properties in a Series of 3,4-Di-N-substituted 2,5-Diferrocenyl Thiophenes

“…Hence, the investigation of the electronic coupling H ab is more accurate by using the transition dipole moment, which can be determined from the experimental spectra through integration of the IVCT absorption band. 94,110,112 However, the observation of a couple of NIR absorptions close together, as typical for metalcontaining mixed-valent species, makes such a spectra analysis difficult (vide supra). 30,37,105,106,108,109,113 Furthermore, due to the complex capture of the effective charge transfer distance r ab , the geometrical distance of the interacting redox centers is (Table 2) roughly verify the tendency of the increasing electronic interaction in the series of thiophenes 1−4 and in comparison with 2,5-diferrocenyl thiophene as well as 2,5-DiFcEDOT.…”