Skeletal Phosphorus Editing in Large N,P-Codoped Fused Nanographenes Enables Tunable Excited-State Absorption

While nitrogen doping into large fused π-conjugated frameworks is relatively facile, the skeletal incorporation of both nitrogen and phosphorus into large fused π-conjugated frameworks remains synthetically demanding owing to the geometric mismatch and chemical lability associated with embedded phosphorus centers. Herein, we report a sequential electrophilic phosphination/sulfur-trapping strategy that provides access to a rigid ten-ring-fused N,P-codoped nanographene containing two embedded azaphosphinine units. The initially isolated P=S framework can be chemoselectively converted into the corresponding P=O analogue without altering the fused carbon-nitrogen backbone, furnishing a structurally matched P=S/P=O molecular pair. Single-crystal X-ray diffraction reveals that P=S to P=O exchange modulates local distortion around azaphosphinine units. Spectroscopic measurements and theoretical calculations show that this localized phosphorus modification redistributes frontier orbital density and electrostatic polarization. Femtosecond transient absorption spectroscopy reveals comparable excited-state lifetimes for the two derivatives (τ = 1.46 and 1.62 ns), but a stronger excited-state absorption band for the P=S derivative. Open-aperture Z-scan measurements at 532 nm further confirm that the enhanced excited-state absorption of the P=S derivative translates into stronger reverse saturable absorption than the P=O analogue, with a βeff value comparable to benchmark C60. This work establishes synthetic access to large skeletal N,P-codoped nanographene and shows that local phosphorus chemistry can regulate excited-state absorption.