The synthesis, photophysics, reverse saturable absorption, and photodynamic therapeutic effect of six cationic biscyclometalated Ir(iii) complexes (1-6) with extended π-conjugation on the diimine ligand and/or the cyclometalating ligands are reported in this paper. All complexes possess ligand-localized π,π* absorption bands below 400 nm and charge-transfer absorption bands above 400 nm. They are all emissive in the 500-800 nm range in deoxygenated solutions at room temperature. All complexes exhibit strong and broad triplet excited-state absorption at 430-800 nm, and thus strong reverse saturable absorption for ns laser pulses at 532 nm. Complexes 1-4 are strong reverse saturable absorbers at 532 nm, while complex 6 could be a good candidate as a broadband reverse saturable absorber at 500-850 nm. The degree of π-conjugation of the diimine ligand mainly influences theπ,π* transitions in their UV-vis absorption spectra, while the degree of π-conjugation of the cyclometalating ligand primarily affects the nature and energies of the lowest singlet and emitting triplet excited states. However, the lowest-energy triplet excited states for complexes 3-6 that contain the same benzo[i]dipyrido[3,2-a:2',3'-c]phenazine (dppn) diimine ligand but different cyclometalating ligands remain the same as the dppn ligand-localized π,π* state, which gives rise to the long-lived, strong excited-state absorption in the visible to the near-IR region. All of the complexes exhibit a photodynamic therapeutic effect upon visible or red light activation, with complex 6 possessing the largest phototherapeutic index reported to date (>400) for an Ir(iii) complex. Interactions with biological targets such as DNA suggest that a novel mechanism of action may be at play for the photosensitizing effect. These Ir(iii) complexes also produce strong intracellular luminescence that highlights their potential as theranostic agents.

We report the synthesis, photophysics, and reverse saturable absorption together with time-dependent density functional theory modeling of seven cationic iridium(III) complexes bearing one 2,2′-bipyridine ligand and two cyclometalating ligands (C^N ligand) with varied degrees of π-conjugation (HC^N = benzo[H]quinoline in 1, 1-phenylisoquinoline in 2, 1-(2-pyridyl)naphthalene in 3, 2-(2-pyridyl)naphthalene in 4, 1-(2-pyridyl)pyrene in 5, 1,2-diphenyl-pyreno[4,5-d]imidazole in 6, and 3-(2-pyridyl)perylene in 7). All complexes possess ligand-localized 1π,π* transitions as the major absorption bands and lower-energy 1MLCT (metal-to-ligand charge transfer)/1LLCT (ligand-to-ligand charge transfer) transitions in their ultraviolet–visible absorption spectra. The extended π-conjugation in the cyclometalating ligands of complexes 5–7 causes a significant red-shift of the major absorption bands with increased molar extinction coefficients with respect to those of complexes 1–4 that contain less conjugated C^N ligands. All complexes are emissive in solutions at room temperature and in glassy matrix at 77 K. The emitting states are assigned to 3π,π* (C^N ligand localized) /3MLCT for 1, 3π,π*/3MLCT/3LMCT (ligand-to-metal charge transfer) for 2–4, pure 3π,π* transitions for 5 and 6, and 3π,π*/3MLCT/3LMCT/3LLCT for 7. Complex 5 possesses the lowest emission energy because the larger conjugation and the most delocalized character of the 3π,π* transition within the C^N ligand in this complex. Complexes 1, 4, and 7 possess larger contribution of charge transfer characters in their lowest triplet excited states. Therefore, the transient absorption of these three complexes is broad but short-lived (90–300 ns). In contrast, complexes 2, 3, 5, and 6 all give long-lived (2.0–19.5 μs) triplet transient absorption in the visible spectral region of ca. 450–700 nm, which can be regarded as emanating predominantly from the C^N ligand-centered 3π,π* state. The reverse saturable absorption (RSA) of these complexes was evaluated at 532 nm for nanosecond laser pulses. The results demonstrate that these complexes, except for 7, all exhibit strong RSA for nanosecond laser pulses at 532 nm, with a trend of 7 < 1 < 4 < 6 < 5 ≈ 2 ≈ 3.