Rod-shaped gold nanoparticles (gold nanorods; GNRs) have strong light absorption in the near-infrared region. In addition, the GNR absorbance depends on the GNR orientation toward irradiation light. Therefore, GNR orientation is of great importance to control the characteristics. In this study, I aimed to actively change GNR orientations focusing on the soft properties of DNA polymer brushes. DNA polymer brushes have ability to optimize their shape flexibly depending on their interaction strength. Therefore, I tuned interaction strength between GNRs and DNA by pH via surface modification of GNRs with pH-responsive ligands. The GNR orientation was evaluated with extinction spectra. As a result, the GNRs were vertically oriented in a neutral environment, while the GNRs were notably inclined in an acidic condition. These phenomena occurred reversibly. Moreover, in the pH range from 4.0 to 7.6, the GNR orientation changed gradually in response to the pH. As a next step, this system was applied to more generous phenomena to increase application possibilities. I utilized synthetic polymer brush and its structural change to align and change GNRs’ orientation for a broader potential for application. When mildly cationized GNRs were applied to grafted poly(styrene sulfonate) (PSS) brushes, GNRs were adsorbed vertically in PSS polymer. Furthermore, when NaCl or poor solvent for PSS, which induce polymer brush shrinkage, was applied to GNRadsorbed PSS brush, GNRs were drastically tilted. These phenomena occurred reversibly.
Perovskite quantum dots (PQDs) are applicable in light-emitting diodes (LEDs) owing to their color tunability, high color purity, and excellent photoluminescence quantum yield (PLQY) in the solution state. However, the PQD film obtained through non-radiative recombination by concentration quenching and the formation of surface defects exhibited low PLQY. In this study, we focused on the energy transfer between PQDs with different energy gaps (Eg) to reduce non-radiative recombination in the film state and consequently achieve high device performance. We prepared size-controlled PQDs measuring 10.7 nm (large-size QD; LQD) and 7.9 nm (small-size QD; SQD) with different Eg and observed spectral overlap between SQD emission and LQD absorption. To investigate the FRET from SQD to LQD, we prepared SQD-LQD mixed QD (MQD). The MQD film enhanced LQD emission and exhibited a higher PLQY (52%) with a longer PL decay time (7.4 ns) than those exhibited by the neat LQD film (38% and 6.2 ns). This energy transfer was determined to FRET by photoluminescence excitation and PL decay times. Moreover, the external quantum efficiency of the MQD-based LED increased to 15%, indicating that the FRET process can enhance the PLQY of film and LED efficiency.
Protecting ligands on gold nanoclusters play important roles to determine the structures and compositions of gold cores. In this work, we designed porphyrinthiol derivatives (Por) as exchanging ligands in ligand-exchange reactions with [Au25(SR)18] – , achieving facile isolation of mono -and di-ligand exchanged products, [Au25(SR)17(Por)1] – and [Au25(SR)16(Por)2] – . The regioselectivity in [Au25(SR)17(Por)1] – was thermodynamically controlled by the steric effects of Por. The difference of steric effects of Por also regulates the structures of gold core, which were clearly revealed by X-ray spectroscopic measurements.
Oxygen quenching of photoluminescence (PL) in ligand-protected noble metal cluster (NMC) is usually used to judge whether the PL is fluorescent or phosphorescent, and energy transfer process has long been considered as the sole quenching mechanism. In this review, Rehm–Weller analysis of oxygen quenching rate constants (kq) in various NMCs reveals that a charge transfer (CT)-mediated mechanism governs the quenching process. The kq values showed a distinct dependence on the CT driving force, with a pronounced change between 105 and 1010 M–1s –1. Triplet sensitization of polycyclic aromatic hydrocarbons (PAHs) by NMC was confirmed by transient absorption spectroscopy, regardless of the degree of PL quenching by O2, demonstrating that the PL of all NMCs under investigation originates from the excited triplet state. Hence, PL quenching and triplet sensitization using PAHs as acceptors is effective approach to determine whether a cluster’s PL is fluorescent or phosphorescent.