Fig. 4. Stern–Volmer plots showing the quenching of the excited-state lifetime of the photocatalysts 1 (full circles and lines) and 2 (empty squares and dashed lines) in the presence of increasing amounts of the carboxylate derivatives 17 (orange) or 18 (black). Notably, no quenching is observed if the Michael acceptor 3a (red) is added to both iridium complexes. Experiments were carried out in oxygen-free acetonitrile solution at 298 K with a photocatalyst concentration of 0.015 mM, with excitation at 330 nm.