Multispin Photoexcited State and Generation of Dynamic Electron Spin Polarization Observed in Anthracene–Radical-Linked Systems with Extended π-Conjugated Spin Coupler

Understanding multispin photoexcited systems and their excited-state dynamics is crucial for modern technological applications of organic radicals and related spin systems. In this study, the multispin photoexcited states and excited-state dynamics of anthracene-radical-linked π-conjugated spin systems (OV2 and IN2) with an extended π-conjugated spin coupler were investigated using time-resolved electron spin resonance (TRESR), pulse ESR, and transient absorption measurements. The results were compared to those of a system with a phenylene spin-coupler (OV1). Solution- and solid-phase TRESR measurements revealed the occurrence of electron spin polarization transfer (ESPT) from the doublet photoexcited state (D; trip-doublet state) to the doublet ground state (DG state) in OV2 and IN2. The solid-phase TRESR measurements identified a doublet-quartet (D-Q) mixed state (Q; trip-quartet state) in OV2 and IN2 in addition to the spin-polarized DG state, and ESPT from the photoexcited state to the DG state was directly observed as a time-dependent resonance field shift of the central emissive peak (g ≈ 2.0). Pulse ESR experiments showed that the decay time of dynamic electron spin polarization (DEP) and the characteristic time of DEP generation strongly depended on the expected magnitude of the intramolecular exchange coupling between the triplet-state chromophore and the radical substituent. This finding supports that DEP generation in the examined π-radical systems proceeds predominantly via radical spin-orbit borrowing-enhanced intersystem crossing.