Rikkyo University

Triplet-triplet annihilation photon upconversion (TTA-UC) is a method of converting long-wavelength light into short-wavelength light that has attracted attention as a light energy conversion technology that could have wide applications. TTA-UC is carried out using a system that combines a sensitizer (S) and an emitter (E), with the sensitizer absorbing incident light and transferring its energy to the emitter (Figure 1). This process of transferring excitation energy is called triplet energy transfer (TET). The higher the efficiency of TET, the more excited triplet states (³E*) are generated in the emitter. This promotes TTA, which generates large numbers of excited singlet states (¹E*) in the emitter, thus creating UC light (hv_UC) with shorter wavelengths (higher energy) than the incident light (hv_a) (i.e., hv_a <hv_UC).

Because the TTA-UC mechanism is a two-photon process based on TTA occurring between 2 emitter molecules, the maximum UC efficiency (Φ_UC) is 50%. The key to raising efficiency levels is increasing the sensitizer’s triplet generation quantum yield (Φ_T), the TET quantum yield (Φ_TET) between the sensitizer and the emitter, and the TTA quantum yield (Φ_TTA) of the emitter. In their study, the researchers developed a new sensitizer – Au₂Cu₆(S-Adm)₆[P(DPA)₃]₂ (S-Adm = 1-adamanthanethiolate; DPA = 9,10-diphenylanthracene) – aimed at achieving 100% Φ_T and Φ_TET (Figure 1).

Figure 1: Schematic diagram of photon upconversion by an Au₂Cu₆(S-Adm)₆[P(DPA)₃]₂ cluster sensitizer, in which the Au₂Cu₆(S-Adm)₆ structure is modified with a P(DPA)₃ triplet mediating ligand. The Au₂Cu₆ core absorbs incident light (hv_a) and rapidly transfers its energy to the triplet mediator ligand, where it can be retained for a long duration. Conversion to blue light (hv_UC) is achieved when the energy is transferred to the emitter.

To solve this problem, the research team designed and synthesized a new phosphine derivative P(DPA)₃ (DPA = 9,10-diphenylanthracene) to serve as a triplet mediator ligand (TL) and inserted it in place of the PPh₃ ligand to create Au₂Cu₆(S- Adm)₆[P(DPA)₃]₂ (Figure 2, right). In this cluster, when the Au₂Cu₆ core (M) absorbs red light (640 nm), it induces charge transfer between it and the P(DPA)₃ site, and this charge-separated state (M^+-TL⁻)* rapidly transfers triplet energy to the P(DPA)₃ site, generating a long-lived (150 µs) triplet TL state (M^-3TL*) at 100% efficiency. This demonstrates that Au₂Cu₆(S-Adm)₆[P(DPA)₃]₂ is much better at sensitizing the triplet state of the emitter compared to Au₂Cu₆(S-Adm)₆(PPh₃)₂.

In addition, a record-setting level of efficiency for converting red light (640 nm) to blue light (433 nm) with UC was achieved in a degassed solution of Au₂Cu₆(S-Adm)₆[P(DPA)₃]₂ sensitizers and DPA emitters (E), with Φ_UC = 20.7% (threshold intensity I_th = 0.036 Wcm^-2). This is a dramatic improvement over Au₂Cu₆(S-Adm)₆(PPh₃)₂ (Φ_UC = 0.36%, I_th > 5 Wcm^-2). They also found that the system is capable of 20% conversion efficiency even at the standard level of sunlight illumination (1 sun) seen on the Earth's surface (Figure 3). This demonstrates that modifying the Au₂Cu₆(S-Adm)₆ structure with a triplet mediator ligand P(DPA)₃ creates an excellent metal cluster sensitizer that combines excellent photostability with UC efficiency.

Figure 3: Photograph and spectra of UC emission observed when a degassed solution of Au₂Cu₆(S-Adm)₆[P(DPA)₃]₂ sensitizers and DPA emitters is irradiated with 1 sun (0.10 Wcm^-2) of simulated sunlight (limited to 540-700 nm).