Structural Flexibility-Driven Dual Emission Switching in Ultrathin Gold Nanorod Clusters.

Recently, the unique dual emission phenomena of gold nanoclusters (AuNCs) have been reported, but the relationship between their emission characteristics and cluster structure and size remains unclear. This study focuses on atomically precise one-dimensional (1D) ultrathin rod-shaped AuNC systems (Au₂₄ , Au₄₂ , and Au₆₀ ), revealing the structure-size dependency of their dual emission mechanisms using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. The results show that as the aspect ratio (AR) increases (from 3.1 to 6.2 to 9.4), the core structure transitions from flexible to rigid, with enhanced x-axis transition dipole moments (μ _x , from 0.58 or 1.00 to 6.31 and then to 10.73 D), narrowing the adiabatic energy gap (ΔE _ST) between singlet (S₁) and triplet (T₁) states (from 0.50 or 0.81 eV to 0.37-0.57 eV). This leads to elevated fluorescence radiative rate constants (k _r ^F) and intersystem crossing rate constants (k _ISC) in the 10⁸ s^-1 regime. Consequently, shorter clusters exhibit dual fluorescence emission (F1+F2), while elongated systems show fluorescence-phosphorescence dual emission (F + P). Electronic structure analysis reveals that increased size weakens charge transfer excitation while enhancing excited state localization. This work establishes a quantitative framework linking size, flexibility, and dual emission mechanisms in anisotropic AuNCs, offering design principles for tunable dual emission probes in optical imaging and sensing applications.