Enhancing Forbidden Transition in Molecules by Toroidal Dipole Metasurface for Novel Photochemical Reactions

A molecular triplet excited state (T₁) with a long lifetime is beneficial for a mid-state of upconversion, photosensitizer, and various photochemical reactions. Because the excitation from the ground singlet state (S₀) to T₁is spin forbidden transition, T₁is usually excited via an intersystem crossing from a singlet excited state, which results in an energy loss of a few hundred meV. In this presentation, we propose an approach to excite a T₁directly from S₀ by exploiting an optical resonance of a dielectric metasurface. For this purpose, we focus on the magnetic dipole transition in molecules that allows a spin-flip transition. By exploiting the magnetic dipole transition, direct excitation of T₁ from S₀ becomes possible. However, in typical molecular systems, the magnetic dipole transition rate that depends on the square of the magnetic field intensity is much smaller than that of the electric dipole one. In this work, by developing the metasurface platform that enhances local magnetic field intensity at a molecular position, we aim direct excitation of T₁ from S₀.<br/>To enhance the local optical magnetic field, high-refractive dielectric (e.g. silicon (Si), gallium phosphide (GaP)) nanoantennas are promising because they exhibit magnetic Mie resonances in optical frequency. However, the enhanced magnetic field by the Mie resonance is localized inside the nanoantenna, which makes it difficult to harness the field for molecular excitations. In the previous work, we have realized the accessible and strong electric and magnetic field in the hexagonal array of thin Si nanodisks (diameter/height ratio ~10) exhibiting the coupled toroidal dipole (TD) resonance [1-3]. In this work, we demonstrate the enhanced photochemical reaction by exploiting the magnetic field enhancement in Si nanodisks. We first show the enhancement of excitation rate of the magnetic dipole transition in ruthenium (Ru(II)) complex molecules placed on the Si nanodisk array by photoluminescence excitation measurements. The magnetic dipole transition of Ru(II) complex molecules is approximately 40 times enhanced at the TD resonance wavelength (825 nm) of the Si nanodisk array.To demonstrate the application to photochemical reactions, we study the photosensitizing activity of Ru(II) complexes on the Si nanodisk arrays by monitoring the generation of singlet oxygen molecules. From the excitation wavelength dependence of the amounts of singlet oxygen, we successfully demonstrate the enhanced photosensitizing activity of Ru(II) complexes due to the large magnetic field by the TD resonance. The results demonstrate the singlet oxygen generation by efficient excitation of the Ru(II) complexes with much lower energy photons (~825 nm) than that of main absorption band of ~630 nm. In the presentation, we will further discuss the mechanism quantitatively by comparing the experimental results with simulation results.<br/>[1] H. Hasebe, et al., Advanced Optical Materials, 20, 2001148(2020).<br/>[2] H. Hasebe, et al., ACS Photonics, 9, 10, 3302(2022).<br/>[3] H. Sugimoto, et al., Small, 17, 47, 2104458(2021)