Origin of abnormal fluorescence in organometallic complexes: Weak heavy-atom effect and large energy gap

Organometallic complexes typically exhibit phosphorescence, which originate from the heavy-atom effect (HAE) that facilitates spin-forbidden transitions via enhanced spin–orbit coupling (SOC). However, certain complexes display anomalous fluorescence, challenging conventional understanding. Here, we investigated two representative fluorescent organometallic complexes, Per-B-Au and PMI-Pt, using time-dependent density functional theory coupled with the thermal vibration correlation function method. Our results demonstrate that the abnormal fluorescence originates from the weakened HAE and large energy gap. In Per-B-Au, the presence of bridging unit decouples the chromophore from the metal center, effectively suppressing SOC and thereby reducing the ISC rate. In contrast, PMI-Pt exhibits moderate SOC enhancement due to covalent bonding between Pt atom and chromophore. Nevertheless, the ISC rate (∼10⁷ s⁻¹) in PMI-Pt remains lower than the radiative decay rate (k_f ∼10⁸ s⁻¹), which is attributed to the distinct excited-state features of the chromophore, including a large oscillator strength, an energetically lower S₁ relative to T₂, and a large singlet-triplet energy gap (ΔE_ST). Our proposed mechanism is also tenable for other reported fluorescent organometallic complexes. These findings provide fundamental insights into the luminescence mechanisms in organometallics and offer valuable guidance for designing novel fluorescent materials.