Breaking the Selection Rules of Spin-Forbidden Molecular Absorption in Plasmonic Nanocavities

Controlling absorption and emission of organic molecules is crucial for efficient light-emitting diodes, organic solar cells, and single-molecule spectroscopy. Here, a new molecular absorption is activated inside a gold plasmonic nanocavity and found to break selection rules via spin–orbit coupling. Photoluminescence excitation scans reveal absorption from a normally spin-forbidden singlet to triplet state transition, while drastically enhancing the emission rate by several thousand fold. The experimental results are supported by density functional theory, revealing the manipulation of molecular absorption by nearby metallic gold atoms.