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

Understanding and controlling spin dynamics in organic dyes is of significant scientific and technological interest. The investigation of 2,5-dihydropyrrolo[4,3-c]pyrrolo-1,4-dione derivatives (DPPs), one of the most widely used dyes in many fields, has so far been limited to closed-shell molecules. We present a comprehensive joint experimental and computational study of DPP derivatives covalently linked to two nitronyl nitroxide radicals (DPP^Th-NN₂). Synthesis, single crystal X-ray diffraction study, photophysical properties, magnetic properties established using steady-state and pulse EPR, fast spin dynamics, and computational modelling using density functional theory and ab initio methods of electronic structure and spectroscopic properties of DPP^Th-NN₂ are presented. The single-crystal X-ray diffraction analysis of DPP^Th-NN₂ and computational modeling of its electronic structure suggest that effective conjugation along the backbone leads to noticeable spin-polarization transfer. Calculations using ab initio methods predict a weak exchange interaction of radical centers through a singlet ground state of DPP^Th with a small singlet-triplet splitting (ΔE_ST) of about 25 cm^-1 (∼0.07 kcal mol^-1). In turn, a strong ferromagnetic exchange interaction between the triplet state of DPP^Th chromophore and nitronyl nitroxides (with J ∼ 250 cm^-1) was predicted.