Fig. 3: A plausible mechanism for abnormal thermally-stimulated dynamic phosphorescence.

a Normalized phosphorescence spectra of FPO molecule in m-THF solution and PMMA film at 77 K. b Molecular arrangement of FPO molecules viewed along a axis (left) and π-π stacking (right) in crystal. c Thermal expansion coefficient of FPO crystal along the principal axes (a–c) at different temperature from 90 to 360 K. d Natural transition orbitals (NTOs) contributing to the lowest-energy triplet transitions of FPO trimer and dimer models in crystal. The inset structures show the molecular conformation in excited state of trimer (left) and dimer (right) models. e Proposed energy transfer processes for thermally-stimulated dynamic phosphorescence. Note that molecules at ground state (S₀) reaches to S₁ (step 1) after absorption of photons, and then transform to T₁ through ISC (step 2). Under lower temperature, the excited molecules at T₁ are further stabilized by the triplet excited state (¹T₁^*) of trimer with a lower energy level (step 3), enabling long-lived yellow phosphorescence (step 4). As temperature increases, the excited molecule experiences large conformation adjustment in an environment of uneven intermolecular interactions, which was further stabilized for ²T₁^* (step 5) of dimer. Finally, a bright cyan-blue phosphorescence was observed via radiative transition from ²T₁^* (step 6).