Tuneable Redox Chemistry and Electrochromism of Persistent Symmetric and Asymmetric Azine Radical Cations

Abstract

Molecular organic radicals have been intensively studied in the last decades, due to their interesting optical, magnetic and redox properties. Here we report the synthesis and characterisation of persistent organic radicals from one‐electron oxidation of redox‐active azines (RAAs), composed of two guanidinyl or related groups. By connecting two different groups together, asymmetric compounds result. In this way a series of compounds with varying redox potential is obtained that could be oxidised reversibly to the mono‐ and the dicationic charge states. The accessible redox states were fully determined by chemical redox reactions. The standard Gibbs free energy change for disproportionation of the radical monocation into the dication and the neutral molecule in solution, estimated from cyclovoltammetric measurements, varies between 43 and 71 kJ mol−1. While the neutral RAAs absorb predominately UV light, the radical monocations display strong absorptions covering almost the entire visible region and extending for some compounds into the NIR region. A detailed analysis of this highly reversible electrochromism is presented, and the fast switching characteristics are demonstrated in an electrochromic test device.

A Copper(I) Complex with Two Unpaired Electrons, Synthesised by Oxidation of a Copper(II) Complex with Two Redox‐Active Ligands

“…The synthesis of new HL1 and HL2 (protonated versions of the ligands) is sketched in Scheme 1 b, following previously established synthetic strategies by our group [31, 32] . Both molecules are built of the same aliphatic guanidinyl group attached to a different aromatic guanidinyl (HL1) or thio‐guanidinyl (HL2) group.…”

“…Thesynthesis of new HL1 and HL2 (protonated versions of the ligands) is sketched in Scheme 1b,following previously established synthetic strategies by our group. [31,32] Both molecules are built of the same aliphatic guanidinyl group attached to ad ifferent aromatic guanidinyl (HL1) or thioguanidinyl (HL2) group.Incrystals of HL1, grown by layering a1 ,2-difluorobenzene solution with n-pentane,d imeric units are formed, in which two molecules are bound by two N-H•••N hydrogen bonds (see Supporting Information). B3LYP/def2-TZVP calculations predict as ignificant binding energy (DE including zero-point energy corrections) of 85 kJ mol À1 for the dimer.The unit cell contains four different, almost planar molecules.W ith average values of 1.424 and 1.295 ,t he central NÀNs ingle and imino C=Nd ouble bond lengths are similar to those in other neutral urea azines.…”

“…B3LYP/def2-TZVP calculations predict as ignificant binding energy (DE including zero-point energy corrections) of 85 kJ mol À1 for the dimer.The unit cell contains four different, almost planar molecules.W ith average values of 1.424 and 1.295 ,t he central NÀNs ingle and imino C=Nd ouble bond lengths are similar to those in other neutral urea azines. [31,32,33,35] Ther edox activity of the protonated ligands was studied by cyclic voltammetry (see Supporting Information). The curve recorded for HL1 contains arelatively sharp oxidation/ reduction wave at À0.442 V/À0.545 Va nd ab road one at 0.102 V/À0.127 V. Similarly,f or HL2 ar elatively sharp oxidation/reduction wave at À0.235 V/À0.366 Vand ab road one at 0.508 V/0.278 Vw as observed.…”

“…We already studied the redox and coordination chemistry of neutral redox‐active urea azine ligands. [ 31 , 32 , 33 ] Cu II complexes with a neutral, peralkylated urea azine ligand were found to be stable in the solid state, but amenable to IET induced demetallation and decomposition in solution (Scheme 1 a ). [33] To avoid decomposition, we decided to strengthen the metal‐ligand bonding by using anionic urea azine ligands, generated by deprotonation of urea azines with partially alkylated substituents and N‐H groups.…”

Proton‐Coupled Electron Transfer (PCET) with 1,4‐Bisguanidino‐Benzene Derivatives: Comparative Study and Use in Acid‐Initiated C‐H Activation

“…Reaction of 2‐ methoxy‐1,4‐diaminobenzene [34] with 2‐chloro‐1,1,3,3‐tetramethylformamidinium chloride gave GFA2. For the preparation of GFA4 and GFA5, 1,4‐diaminobenzene was reacted with 2‐chloro‐1,3‐dimethyl‐1H‐benzo[d]imidazol‐3‐ium tetrafluoroborate [35] or 2‐chloro‐1,3‐dimethylperimidinium tetrafluoroborate [36] (Scheme 2 ).…”