A Conjugated Oligoelectrolyte Exhibiting Room Temperature Spin-Correlated Radical Pair Character for Biological Sensing

We report a water-soluble conjugated oligoelectrolyte (COE), Quantum-COE, exhibiting photogenerated spin-correlated radical pair (SCRP) behavior sensitive to the electric field of DNA, but not to that of lipid bilayers. The spin polarization is thermally activated and arises from partial π-conjugation disruption at the donor-acceptor (carbazole-benzophenone) junction. This design promotes a twisted intramolecular charge-transfer (TICT) state with a minimized singlet-triplet energy gap (ΔEST = 0.12 eV), exchange coupling constant (J ≈ ΔEST/2), and a thermodynamically favorable radical pair formation step (ΔGCS) in DNA (- 0.19 eV) and liposomes (- 0.55 eV). Continuous-wave EPR at room temperature revealed a photogenerated spin-polarized signal from Quantum-COE that split upon DNA association, consistent with electric-field-induced perturbation of its transition dipole moment, resulting in Stark-like modulation of the J and hyperfine coupling (Ax). No EPR signal was observed for Quantum-COE under dark, cryogenic conditions, or lipid bilayers, but was sequestrated by spin trap 4-POBN. Transient absorption spectroscopy and spectroelectrochemistry confirmed long-lived excited-state absorption features assigned to [Cbz•+] and [BP•-], which were enhanced by DNA and quenched in lipid bilayers and 4-POBN. Quantum chemical simulations showed that planar (in lipids) geometries increase ΔEST by 0.31 eV compared to TICT-optimized structures. This geometry-dependent behavior explains the absence of SCRP signals in rigid environments, underscoring the importance of TICT-states, minimized ΔEST, and favorable ΔGCS for yielding room-temperature photogenerated SCRPs. Overall, this work establishes a design strategy for TICT-enabled qubit-like properties in water-soluble probes for sensing the electric fields of freely diffusing biomolecules under anoxic and ambient conditions.