Observation of the long-lived triplet excited state of perylenebisimide (PBI) in C^N cyclometalated Ir(iii) complexes and application in photocatalytic oxidation

Abstract

Perylenebisimide (PBI) was used to prepare C^N 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 (τ_T = 22.3 μs). Previously, the lifetime of the triplet state of PBI in transition metal complexes was usually shorter than 1.0 μs. 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 CC π-conjugation bond. The new complexes show strong absorption in visible region (ε = 34 200 M⁻¹ cm⁻¹ at 541 nm for Ir-2, and ε = 19 000 at 669 nm for Ir-3), compared to the model complex Ir(ppy)(bpy)[PF₆] Ir-1 (ε < 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 ³IL states were populated for Ir-2 and Ir-3 upon photoexcitation. The complexes were used as triplet photosensitizers for ¹O₂-mediated photooxidation of 1,5-dihydronaphthalene to produce juglone, an important intermediate for preparation of anti-cancer compounds. ¹O₂ quantum yields (Φ_Δ) 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.