FRET-driven red phosphorescence in lignin carbon dots for stimuli-responsive encryption

Achieving long-wavelength red room-temperature phosphorescence (RTP) in biomass-derived carbon dots (CDs) remains a fundamental challenge due to the stringent constraints of the energy gap law, which causes a sharp decline in quantum yield and lifetime as the emission red-shifts. To address this bottleneck, we propose a rational and modular energy-relay strategy. First, by modulating the electronic structure and surface states of lignin-based CDs through precursor design and nitrogen functionalization, we successfully synthesize yellow-emissive CDs with a high fluorescence quantum yield of 37.9% and a millisecond-scale phosphorescence lifetime. Subsequently, a Förster resonance energy transfer (FRET) pair is strategically constructed between these efficient donors and a red-absorbing organic dye acceptor within a rigid polymer matrix, enabling efficient triplet-to-singlet energy transfer. This process successfully generates intense red RTP centered at 584 nm with a phosphorescence lifetime exceeding 258.15 ms. Critically, this strategy decouples the requirement for a narrow-bandgap phosphor from the necessity of a stable triplet state, providing a general solution to bypass the energy gap limitation. Leveraging the stimuli-responsive dynamic bonds within the composite, we further demonstrate its utility in multilevel information encryption and develop a fast-response hydrochromic RTP ink. This study not only establishes a versatile design paradigm for sustainable long-wavelength RTP materials but also opens promising avenues for their application in advanced optical anti-counterfeiting and sensing technologies.