Size-Dependent Ultrafast Spin Dynamics in Triangulene-Based π–d Hybrid Systems

Geometric scaling provides a powerful strategy for manipulating spin properties in hybrid π–d magnetic systems. Here, we present a systematic size-dependent investigation of the static electronic structures and laser-driven ultrafast spin dynamics in Co- and Cu-doped triangulene nanoflakes ([ n ]TNFs). Static analysis reveals that varying the TNF size reshapes the interplay between transition-metal-centered magnetism and the π-magnetism of the carbon framework, leading to distinct energy-level and spin-localization regimes. In the ultrafast regime, Co-doped TNFs support robust local spin flips, while Cu-doped systems enable spin-orientation-independent reversible spin transfers. Meanwhile, the feasibility of spin transfers in Co-doped TNFs and of local spin flips in Cu-doped TNFs exhibits a strict size dependence. Moreover, a collective spin-flip mechanism is identified in Cu&[2]TNF and Co&[4]TNF, mediated by transient π–d interaction. These findings provide physical insight into π–d-assisted ultrafast spin dynamics, offering guidance for the rational design of graphene-based spintronic architectures.