String-Attached Oligothiophene Substituents Determine the Fate of Excited States in Ruthenium Complexes for Photodynamic Therapy

We explore the photophysical properties of a family of Ru(II) complexes, Ru-ip-nT, designed as photosensitizers for photodynamic therapy (PDT). The complexes incorporate a 1H-imidazo[4,5-f][1,10]-phenanthroline (ip) ligand appended to one or more thiophene rings. One of the complexes studied herein, Ru-ip-3T (known as TLD1433), is currently in Phase II human clinical trials for treating bladder cancer by photodynamic therapy (PDT). The potent photocytotoxicity of Ru-ip-3T is attributed to a long-lived intraligand charge-transfer triplet state. The accessibility of this state changes upon varying the length (n) of the oligothiophene substituent. In this paper we highlight the impact of n on the ultrafast photoinduced dynamics in Ru-ip-nT, leading to the formation of the function-determining long-lived state. Femtosecond time-resolved transient absorption combined with resonance Raman data was used to map the excited-state relaxation processes from the Franck-Condon point of absorption to the formation of the lowest-energy triplet excited state, which is a triplet metal-to-ligand charge transfer ((3)MLCT) excited state for Ru-ip-0T-1T and an oligothienyl-localized triplet intraligand charge transfer ((3)ILCT) excited state for Ru-ip-2T-4T. We establish the structure-activity relationships with regard to changes in the excited state dynamics as a function of thiophene chain length, which alters the photophysics of the complexes and presumably impacts the photocytotoxicity of these photosensitizers.