Simulated transient absorption reveals torsion-dependent nonradiative relaxation in a near-infrared nanographene emitter.

Functionalized nanographenes are emerging near-infrared emitters, yet the microscopic pathways underlying their ultrafast excited-state dynamics remain elusive. Here, we combine atomistic nonadiabatic simulations with transient absorption (TA) modeling to analyze the relaxation mechanisms in a donor-acceptor nanographene. We find that, during the decay of the initially populated higher-lying excited states, the transition density redistributes from the substituents to the nanographene core. This early process occurs in both fully flexible and dihedral-constrained trajectories. In contrast, relaxation into the lowest excited state is strongly torsion-dependent: freezing the donor-acceptor dihedral suppresses this nonadiabatic decay and traps population in higher-lying excited states. Decomposition of the TA signal into ground-state bleaching, stimulated emission, and excited-state absorption elucidates that access to the lowest excited state correlates with the buildup of stimulated emission in the near-infrared, while excited-state absorption remains weak in this spectral window. These results disentangle early electronic relaxation from torsion-assisted interstate decay and clarify how structural dynamics shape the spectroscopic signatures associated with efficient near-infrared emission in functionalized nanographenes. These insights provide molecular-level design principles for controlling excited-state relaxation and enhancing optical gain in near-infrared emitters.