Observation of the long-lived triplet excited state of perylenebisimide (PBI) in (CN)-N-Lambda cyclometalated Ir(III) complexes and application in photocatalytic oxidation

Perylenebisimide (PBI) was used to prepare (CN)-N-Lambda cyclometalated Ir(III) complexes that show strong absorption of visible light and it is the first time the long-lived triplet excited state of PBI chromophore was observed in a transition metal complex (tau(T) = 22.3 mu s). Previously, the lifetime of the triplet state of PBI in transition metal complexes was usually shorter than 1.0 mu s. Long-lived triplet excited states are useful for applications in photocatalysis or other photophysical processes concerning triplet-triplet-energy-transfer. PBI and amino-PBI were used for preparation of cyclometalated Ir(III) complexes (Ir-2 and Ir-3), in which the PBI chromophore was connected to the coordination center via C C pi-conjugation bond. The new complexes show strong absorption in visible region (epsilon = 34 200 M-1 cm(-1) at 541 nm for Ir-2, and epsilon = 19 000 at 669 nm for Ir-3), compared to the model complex Ir(ppy)(bpy)[PF6] Ir-1 (epsilon < 5000 M-1 cm(-1) in the region beyond 400 nm). The nanosecond time-resolved transient absorption and DFT calculations indicated that PBI-localized long-lived (IL)-I-3 states were populated for Ir-2 and Ir-3 upon photoexcitation. The complexes were used as triplet photosensitizers for O-1(2)-mediated photooxidation of 1,5-dihydronaphthalene to produce juglone, an important intermediate for preparation of anti-cancer compounds. O-1(2) quantum yields (F.) up to 91% were observed for the new Ir(III) complexes and the overall photosensitizing ability is much higher than the conventional Ir(III) complex Ir-1, which shows the typical weak visible light absorption in visible region. Our results are useful for preparation of transition metal complexes that show strong absorption of visible light and long-lived triplet excited state and for the application of these complexes in photocatalysis.