Promoting Excitation of Triplet State of Molecule by Enhanced Magnetic Field of Dielectric Metasurfaces

Efficient excitation of a triplet (T₁) state of a molecule has far-reaching effects on photochemical reaction and energy conversion systems. Since optical transition from a ground singlet (S₀) to a T₁ state is spin-forbidden, it is generated <i>via</i> intersystem crossing (ISC) from an excited singlet (S₁) state. Although the ISC is enhanced by utilizing a heavy atom effect, energy loss during ICS, i.e., S₁→T₁ relaxation, is inevitable in the process. Here, we propose a general approach to directly excite a T₁ state from a ground S₀ state <i>via</i> magnetic dipole transition, which is boosted by enhanced magnetic field induced by a dielectric metasurface. The enhanced magnetic field promotes the magnetic dipole transition that allows the transition between different multiplicities (<i>i.e.</i>, S₀→T₁). As a dielectric metasurface, we propose a hexagonal array of silicon (Si) nanodisks; the nanodisk array induces strongly enhanced magnetic field on the surface due to the coupled toroidal dipole (TD) resonance.[1] Ru(II) complexes (RuPc(py)₂) are placed on the dielectric metasurface as a benchmark to monitor the S₀→T₁ transition enhancement by the optical resonance of the metasurface. We prove this concept by measuring the phosphorescence (T₁→S₀ transition) spectra under different excitation wavelengths corresponding to the S₀→T₁ energy and demonstrate a large enhancement of the phosphorescence of the molecule by tuning the optical resonance of metasurfaces to the S₀→T₁ transition wavelength.[2]<br/>A Si nanodisk array is fabricated by nanosphere lithography[1] and on top of it, a RuPc(py)₂ doped PMMA layer (30 nm in thickness) is deposited. The metasurface exhibits TD resonance at 805 nm. The PLE spectra detected at phosphorescence emission of the RuPc(py)₂ at 860 nm are recorded for metasurfaces and a reference sample. Compared to the featureless PLE spectrum of the reference sample, RuPc(py)₂ molecules on the metasurface show a distinct PLE peak around 805 nm; the phosphorescence intensity is 35-fold enhanced. The peak corresponds to the TD resonance wavelength of the metasurface. Because 805 nm is very far from the peak of the S₀→S₁ transition (630 nm) of RuPc(py)₂ molecules, the efficient excitation around 805 nm is most likely <i>via</i> the S₀T₁ magnetic dipole transition enhanced by the intense magnetic field under TD resonance. Excitation to the T₁ state by 805 nm light means that a photon energy of more than 400 meV is saved compared to the process involving the ISC. In the presentation, we will further discuss the validity of the model by quantitatively comparing the experimental results with electromagnetic numerical simulations.<br/>[1] H. Hasebe, H. Sugimoto, T. Hinamoto, M. Fujii, <i>Adv. Opt. Mater.</i> <b>2020</b>, <i>8</i>, 2001148.<br/>[2] H. Sugimoto, H. Hasebe, T. Furuyama, M. Fujii, <i>Small </i><b>2021</b>, 2104458.