Stable Organic (Bi)Radicals by Delocalization of Spin Density into the Electron‐Poor Chromophore Core of Isoindigo

Abstract

The first isoindigo (bi)radicals were obtained by proton coupled oxidation of their 4-hydroxyaryl substituted precursors. Optical and magnetic spectroscopic studies revealed a singlet open-shell biradicaloid electronic ground state for the bisphenoxyl-isoindigo (<s >=1.20) with a small singlet-triplet energy gap of 0.065 eV and a large biradical character of y=0.79 that was corroborated by temperature-dependent EPR spectroscopy and quantum chemical calculations. The concept of kinetic blocking of the radical centers and delocalization of spin density into the electron-withdrawing chromophore core of isoindigo offers an entry into a new class of exceptionally stable open-shell functional materials based on organic colorants.

Tuning phenoxyl-substituted diketopyrrolopyrroles from quinoidal to biradical ground states through (hetero-)aromatic linkers

“…In compliance with the increasing amount of triplet species at higher temperature, also a pronounced broadening of resonance signals in the aromatic region of the respective 1 H NMR spectra is observed, thereby conrming the substantial paramagnetic character of DPP1cc at room temperature. 18,19,25 Accordingly, the 1 H NMR spectrum of DPP1cc in CD 2 Cl 2 at 298 K shows very broad signals (Fig. 4b), which signicantly gain intensity upon decreasing the temperature to 180 K (Fig.…”

“…Such CT bands are commonly observed in halochromic systems. 18,19,25 Notably, the immediate addition of an equimolar amount of triuoroacetic acid to freshly prepared solutions of the dianions leads to a complete recovery of the absorption spectral signatures of the corresponding phenols (Fig. S7, ESI †) and hence reveals the reversibility of the deprotonation process.…”

“…45,69,70 Dehydrogenation of bisphenols occurs easily using proton coupled oxidation which follows a concerted deprotonationoxidation mechanism. 18,19,25 Therefore, solutions of DPP1-3 were treated with basic tetra-n-butylammonium uoride (TBAF) in order to study the occurring spectral changes. Gradual changes in the respective absorption spectra were monitored by UV/vis/NIR spectroscopy (Fig.…”

“…[14][15][16][17] Within this material class, biradical(oid)s composed of p-systems substituted with two phenoxyl units are among the most outstanding representatives, due to the broad variety of accessible structures, which makes them an ideal model system for experimental and theoretical investigations (Scheme 1). [18][19][20][21][22][23][24][25] In order to gain insight into the biradical character of these organic compounds, it is crucial to experimentally distinguish the biradical and quinone singlet state. As energy differences between these states tend to be very narrow, an even more careful choice of analytical methods is essential to uncover hidden discrepancies and draw solid conclusions.…”

A BODIPY-Bridged Bisphenoxyl Diradicaloid: Solvent-Dependent Diradical Character and Physical Properties

“…Usually, the diradical character increases with extension of the molecular length due to the recovery of more aromatic sextet rings in the diradical form. However, the parent extended quinones are very reactive, and thus various modifications have been conducted to obtain kinetically and thermodynamically stabilized quinone-based diradicaloids with different spacers, e.g., oligothiophenes [10], corannulene [11], perylenes [12,13], porphyrin [14], hexa- peri -hexabenzocoronene [15], naphthalene bisimide [16], and isoindigo [17]. Notably, these extended quinones exhibited intense electronic absorption in the near-infrared (NIR) region and thermally populated paramagnetic activity.…”