Thermally-equilibrated delayed fluorescence of gold-doped silver nanoclusters in a solid matrix

Ligand-protected metal nanoclusters (NCs) have emerged as prospective less-toxic luminescent materials which can be beneficial for numerous applications, such as bioimaging, biosensing, photo-induced catalytic process, and optoelectronics. The most intriguing properties of NCs are long photoluminescence (PL) lifetime with broadband PL emission, making them more attractive to be developed. However, low PL Quantum Yield (PLQY) and unclear excitonic dynamics mechanism strongly hinder their practical utilization in daily basis. Therefore, to address these issues, gold-doped silver nanoclusters were synthesized using one-pot synthesis method, and were covalently bonded in a solid matrix, polyvinylpyrrolidone (PVP). The fabricated gold-doped silver nanoclusters embedded in PVP matrix (AuAgNCs@PVP) have bright red emission centered at approximately 650 nm with microseconds PL lifetime. By probing the nanosecond time resolution, it was revealed that two distinct decay profile (nano- and micro-seconds) exist in AuAgNCs@PVP, showing the occurrence of Thermally Activated Delayed Fluorescence (TADF). We also found that after the absorption of photons, thermal equilibrium between singlet and triplet states were quickly reached due to fast intersystem crossing and reverse intersystem crossing (ISC/RISC) process owing to the small singlet triplet energy splitting. Hence, we proposed the mechanism behind broadband PL emission with long PL lifetime was due to Thermally Equilibrated Delayed Fluorescence (TEDF).