Space-confined triple-singlet energy transfer enables high-efficiency deep-red afterglow in carbon dot hybrids

Deep-red afterglow materials with high emission efficiency remain fundamentally limited by inefficient intersystem crossing (ISC) and significant nonradiative decay of triplet excitons under solid-state conditions. Herein, we establish a space-confined triplet-singlet energy transfer (ET) design to achieve efficient long-wavelength afterglow emission in metal-free carbon dot hybrids. Urea-induced heteroatom engineering introduces (n, π*) states that facilitate ISC and increase triplet population, while an (3-aminopropyl) triethoxysilane-derived siloxane network rigidifies the microenvironment and suppresses vibrational relaxation, thereby stabilizing triplet excitons. Meanwhile, surface-state modulation enables favorable triplet energy alignment between the carbon core and surface-associated emissive centers, facilitating efficient triplet-mediated ET. This cooperative regulation results in bright deep-red afterglow centered at 662 nm with a photoluminescence quantum yield of 45.2%. Comparative investigations with red-emissive counterparts reveal that surface-state modulation and molecular rigidification play complementary roles in wavelength tunability and emission efficiency. The resulting materials demonstrate potential in time-resolved optical encryption and persistent afterglow lighting. This work provides mechanistic insight into triplet regulation in confined carbon systems and suggests a viable strategy for improving long-wavelength metal-free afterglow performance.