Nitronyl Nitroxide Bifunctionalized Electron-Poor Chromophores: Synthesis of Stable Dye Biradicals by Lewis Acid Promoted Desilylation

Abstract

Open shell organic molecules bearing π-cores are of great interest for optical, electronic, and magnetic applications but frequently suffer fast decomposition or lack synthetic accessibility. In this regard, nitronyl nitroxides are promising candidates for stable (bi-)radicals due to their high degree of spin delocalization along the O−N−C−N−O pentad unit. Unfortunately, they are limited to electron-rich systems so far. To overcome this limitation, we developed a synthetic procedure for the twofold spin decoration of electron-poor chromophores (E red = −1158 mV) with nitronyl nitroxide radical moieties via selective deprotection/ oxidation of the respective silylated precursors with boron fluoride and subsequent quenching with tetraethyl orthosilicate. Nitronyl nitroxide biradicals PBI−NN, IIn−NN, PhDPP−NN, ThDPP−NN, and FuDPP−NN bridged by perylene bisimide (PBI), isoindigo (IIn), and diketopyrrolopyrrole (DPP) pigment colorants were finally obtained as bench stable compounds after periodate oxidation with yields of 60−81%. The absorption spectral signatures of the chromophores remain preserved in the open shell state and match the ones of the pristine parent compounds, which allowed an a priori prediction of their optical properties. Consequently, we achieved twofold spin labeling while keeping the intrinsic properties of the electron deficient chromophores intact.

Synthesis and photoinduced behavior of DPP-anchored nitronyl nitroxides: a multifaceted approach

“…1 ) were obtained and partially characterized. 4 In addition to the listed diradicals ( Fig. 1 ), perylene bisimide (PBI) and iso-indigo (IIn) conjugated diradicals were also synthesized.…”

“…1 ), perylene bisimide (PBI) and iso-indigo (IIn) conjugated diradicals were also synthesized. 4 Although all diradicals retain the intrinsic optical properties of the dyes or, more specifically, pi-conjugated chromophore, at the same time they exhibit an indirect spin coupling between two distant paramagnetic centres. Room temperature EPR data 4 provided only nearly identical 9-line spectra, demonstrating that the exchange coupling parameter J is much larger than twice the hyperfine coupling constant.…”

“… 4 Although all diradicals retain the intrinsic optical properties of the dyes or, more specifically, pi-conjugated chromophore, at the same time they exhibit an indirect spin coupling between two distant paramagnetic centres. Room temperature EPR data 4 provided only nearly identical 9-line spectra, demonstrating that the exchange coupling parameter J is much larger than twice the hyperfine coupling constant. The distances between the radical units were very large (C2–C2′ distance in the range 2.2–2.4 nm for DPP-based diradicals) due to the additional phenyl rings connecting the NN moieties to the DPP core.…”

“…The distances between the radical units were very large (C2–C2′ distance in the range 2.2–2.4 nm for DPP-based diradicals) due to the additional phenyl rings connecting the NN moieties to the DPP core. 4 To date, comprehensive temperature-dependent studies of liquid and frozen solutions via EPR spectroscopy, as well as magnetization measurements of polycrystalline powder, have not been conducted. These analyses are crucial for a deeper understanding and clear differentiation of the magnetic properties of such diradicals.…”