Suppression of kernel vibrations by layer-by-layer ligand engineering boosts photoluminescence efficiency of gold nanoclusters

Abstract

The restriction of structural vibration has assumed great importance in attaining bright emission of luminescent metal nanoclusters (NCs), where tremendous efforts are devoted to manipulating the surface landscape yet remain challenges for modulation of the structural vibration of the metal kernel. Here, we report efficient suppression of kernel vibration achieving enhancement in emission intensity, by rigidifying the surface of metal NCs and propagating as-developed strains into the metal core. Specifically, a layer-by-layer triple-ligands surface engineering is deployed to allow the solution-phase Au NCs with strong metal core-dictated fluorescence, up to the high absolute quantum yields of 90.3 ± 3.5%. The as-rigidified surface imposed by synergistic supramolecular interactions greatly influences the low-frequency acoustic vibration of the metal kernel, resulting in a subtle change in vibration frequency but a reduction in amplitude of oscillation. This scenario therewith impedes the non-radiative relaxation of electron dynamics, rendering the Au NCs with strong emission. The presented study exemplifies the linkage between surface chemistry and core-state emission of metal NCs, and proposes a strategy for brighter emitting metal NCs by regulating their interior metal core-involved motion.

Triple Ligand Engineered Gold Nanoclusters with Enhanced Fluorescence and Device Compatibility for Efficient Electroluminescence Light-Emitting Diodes

“…Further global fitting of ESA kinetic traces of Au NCs presents four decay components of 0.58 ps, 5.03 ps, 72.5 ps, and >2 ns for ATT-Au NCs, 0.53 ps, 5.25 ps, 80.7 ps, and >2 ns for Arg/ATT-Au NCs, and 0.51 ps, 5.78 ps, 92.3 ps, and >2 ns for TOAB/Arg/ATT-Au NCs (Table S2). As shown in Figure S9, the decay components with long lifetimes (>2 ns) are attributed to the radiative transition from S 1 excited state to S 0 ground state. ,, The fractional picosecond decay components can be assigned to the internal conversion of hot electrons excited by high-energy pump beam from S n state to S 1 state . Since there is no activated triplet state, the decay components with few picoseconds and dozens of picoseconds are attributed to the solvent relaxation induced by solute–solvent interaction, and delayed vibrational relaxation (VR) in S 1 state, respectively. , It should be mentioned that the delayed VR is the main nonradiative pathway of the Au NCs, which redistributes the density of excited electrons by cooling them to the lowest S 1 state. , The TOAB/Arg/ATT-Au NCs exhibit the slowest delayed VR process, indicating that the triple ligand engineering strategy effectively inhibits the vibrational relaxation of the excited state, thus enhancing the PLQYs of Au NCs and strengthening the competitiveness in EL-LED applications.…”

“…The transient PL decay curves exhibit average lifetimes of 4.9 ns for ATT-Au NCs, 45 ns for Arg/ATT-Au NCs, and 110 ns for TOAB/Arg/ATT-Au NCs. According to the nanosecond-scale decay time, narrow fwhm, and small Stokes shift, the PL emission is attributed to the Au core-state. , In addition, by combining PLQYs and PL lifetimes, the radiative decay rates ( k r ) and nonradiative decay rates ( k nr ) are calculated. As indicated in Table S1, the k r of ATT-Au NCs shows a 3.4-fold increment, while the k nr presents a 75-fold decline after triple ligand modification, suggesting that triple ligand engineering can greatly suppress the nonradiative charge transfer. , …”

“…These results demonstrate that the modification of Arg initiates the re-excitation channels from high excited states to relatively low ones. 40,41 This process effectively increases the absorption cross-section and the radiative decay rates of Arg/ ATT-Au and TOAB/Arg/ATT-Au NCs. 30 The normalized ESA and GSB kinetic traces show that the ATT-Au NCs exhibit the fastest decays and TOAB/Arg/ATT-Au NCs exhibits the slowest decays (Figure 2d S2).…”

“…41,43 It should be mentioned that the delayed VR is the main nonradiative pathway of the Au NCs, which redistributes the density of excited electrons by cooling them to the lowest S 1 state. 41,43 The TOAB/Arg/ATT-Au NCs exhibit the slowest delayed VR process, indicating that the triple ligand engineering strategy effectively inhibits the vibrational relaxation of the excited state, 43 thus enhancing the PLQYs of Au NCs and strengthening the competitiveness in EL-LED applications.…”

“…As indicated in Table S1, the k r of ATT-Au NCs shows a 3.4-fold increment, while the k nr presents a 75-fold decline after triple ligand modification, suggesting that triple ligand engineering can greatly suppress the nonradiative charge transfer. 16,41 The mechanism of PL enhancement after triple ligand engineering is further investigated by femtosecond transient absorption (TA) measurements for ATT-Au, Arg/ATT-Au, and TOAB/Arg/ATT-Au NCs. With the pump beam at 350 nm, two ground-state bleach (GSB) peaks centered at 404 and 480 nm along with two excited state absorption (ESA) peaks centered at 435 and 521 nm are observed in the TA spectra of ATT-Au NCs (Figure 2a).…”