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

We report a water-soluble conjugated oligoelectrolyte (COE) composed of carbazole-benzophenone, COE-CbzBP, that exhibits photogenerated spin-correlated radical pair (SCRP) behavior sensitive to static electric fields from DNA but not from lipid bilayers. The SCRP forms from a thermally activated, spin-polarized state enabled by partial π-conjugation disruption at the donor-acceptor (carbazole-benzophenone) nitrogen-carbon (N-C) junction, which facilitates a twisted intramolecular charge-transfer (TICT) geometry. This state minimizes the singlet-triplet energy gap (Δ<i>E</i>_ST = 0.12 eV), radical-pair exchange coupling (<i>J</i>_RP ∼ Δ<i>E</i>_ST/2), and charge separation free energy (Δ<i>G</i>_CS) in both DNA (-0.19 eV) and lipid bilayers (-0.55 eV). Room-temperature continuous-wave electron paramagnetic resonance (CW-EPR) reveals a photogenerated spin-polarized singlet for COE-CbzBP that splits upon DNA association, consistent with modulation of <i>J</i>_RP and hyperfine coupling (<i>A</i>_x), presumably via electric field-spin coupling. No spin-polarized signal was observed under dark, cryogenic conditions, or in liposomes, but was quenched by the spin trap 4-POBN. Transient absorption and spectroelectrochemistry confirmed magnetic-field sensitive long-lived excited-state absorption features attributed to charge-separated states ³[Cbz^•+-BP^•-]*, which were lengthened by DNA, and quenched in lipid bilayers and 4-POBN. Quantum chemical simulations show that planar geometries (lipid-like) increase Δ<i>E</i>_ST by 0.31 eV compared to TICT-optimized structures. This geometry-dependent modulation explains the absence of SCRP signatures in rigid environments, underscoring the importance of TICT states, minimized Δ<i>E</i>_ST, and favorable Δ<i>G</i>_CS for achieving room-temperature SCRP generation. These findings establish design principles for TICT-enabled molecules exhibiting qubit-like behavior that operate under ambient and biologically relevant conditions, with direct implications for quantum information science (QIS).