Single-molecule detection of a terrylenediimide-based near-infrared emitter

Environment-sensitive fluorescent agents with near-infrared (NIR) emission are in great demand owing to their applications in biomedical and quantum technologies. We report a novel NIR absorbing (λ ^Abs _max = 734 nm) and emitting (λ ^Fl _max = 814 nm) terrylenediimide (TDI) based donor-acceptor chromophore (TDI-TPA₄), exhibiting polarity-sensitive single-photon emission. By virtue of the charge transfer (CT) character, ensemble level measurements revealed solvatochromism and NIR emission (ϕ _Fl = 26.2%), overcoming the energy gap law. The CT nature of the excited states is further validated by state-of-the-art fragment-based excited state theoretical analysis. To mimic the polarity conditions at the single-molecule level, TDI-TPA₄ was immobilized in polystyrene (PS; low polar) and poly(vinyl alcohol) (PVA; high polar) matrices, which enables tuning of the energy levels of the locally excited state and charge-separated (CS) state. Minimal blinking and prolonged survival time of the TDI-TPA₄ molecule in the PS matrix, in contrast to the PVA matrix, possibly confirms the implication of the energy gap law and polarity sensitivity of TDI-TPA₄. The existence of the CT state in nonpolar and CS state in polar solvents was confirmed by transient absorption measurements in the femtosecond regime. The current work sheds light on the design principle for NIR single-photon emitting organic chromophores for deep tissue imaging and probing the nanoscale heterogeneity.