Six homo-or heteroleptic tricationic Ir(R 1-tpy)(R 2-tpy) 3+ complexes (Ir1-Ir6, R 1 /R 2 = Ph, 4′-N(CH 3) 2 Ph, pyren-1-yl, or 4′-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}Ph, tpy = 2,2';6',2"terpyridine) were synthesized and tested for photodynamic therapy (PDT) effects. The ground-and excited-state characteristics of these complexes were studied systematically via spectroscopic methods and quantum chemistry calculations. All complexes possessed intraligand charge transfer (1 ILCT) / metal-to-ligand charge transfer (1 MLCT) dominated transition(s) in their low-energy absorption bands, which red-shifted with the increased electron-releasing strength of the R 1 /R 2 substituent. Five of the complexes exhibited ligand-centered 3 π,π*/ 3 ILCT/ 3 MLCT emission. With a stronger electron-releasing R 1 /R 2 substituent, the degree of charge transfer contribution increased, leading to a decrease of the emission quantum yield. When the 4′-N(CH 3) 2 Ph substituent was introduced on both tpy ligands, the emission of Ir3 was completely quenched. Our *
Five heteroleptic tris-diimine ruthenium(II) complexes [RuL(N^N)](PF) (where L is 3,8-di(benzothiazolylfluorenyl)-1,10-phenanthroline and N^N is 2,2'-bipyridine (bpy) (1), 1,10-phenanthroline (phen) (2), 1,4,8,9-tetraazatriphenylene (tatp) (3), dipyrido[3,2-a:2',3'-c]phenazine (dppz) (4), or benzo[i]dipyrido[3,2-a:2',3'-c]phenazine (dppn) (5), respectively) were synthesized. The influence of π-conjugation of the ancillary ligands (N^N) on the photophysical properties of the complexes was investigated by spectroscopic methods and simulated by density functional theory (DFT) and time-dependent DFT. Their ground-state absorption spectra were characterized by intense absorption bands below 350 nm (ligand L localized π,π* transitions) and a featureless band centered at ∼410 nm (intraligand charge transfer (ILCT)/π,π* transitions with minor contribution from metal-to-ligand charge transfer (MLCT) transition). For complexes 4 and 5 with dppz and dppn ligands, respectively, broad but very weak absorption (ε < 800 M cm) was present from 600 to 850 nm, likely emanating from the spin-forbidden transitions to the triplet excited states. All five complexes showed red-orange phosphorescence at room temperature in CHCl solution with decreased lifetimes and emission quantum yields, as the π-conjugation of the ancillary ligands increased. Transient absorption (TA) profiles were probed in acetonitrile solutions at room temperature for all of the complexes. Except for complex 5 (which showed dppn-localized π,π* absorption with a long lifetime of 41.2 μs), complexes 1-4 displayed similar TA spectral features but with much shorter triplet lifetimes (1-2 μs). Reverse saturable absorption (RSA) was demonstrated for the complexes at 532 nm using 4.1 ns laser pulses, and the strength of RSA decreased in the order: 2 ≥ 1 ≈ 5> 3 > 4. Complex 5 is particularly attractive as a broadband reverse saturable absorber due to its wide optical window (430-850 nm) and long-lived triplet lifetime in addition to its strong RSA at 532 nm. Complexes 1-5 were also probed as photosensitizing agents for in vitro photodynamic therapy (PDT). Most of them showed a PDT effect, and 5 emerged as the most potent complex with red light (EC = 10 μM) and was highly photoselective for melanoma cells (selectivity factor, SF = 13). Complexes 1-5 were readily taken up by cells and tracked by their intracellular luminescence before and after a light treatment. Diagnostic intracellular luminescence increased with increased π-conjugation of the ancillary N^N ligands despite diminishing cell-free phosphorescence in that order. All of the complexes penetrated the nucleus and caused DNA condensation in cell-free conditions in a concentration-dependent manner, which was not influenced by the identity of N^N ligands. Although the mechanism for photobiological activity was not established, complexes 1-5 were shown to exhibit potential as theranostic agents. Together the RSA and PDT studies indicate that developing new agents with long intrinsic triplet lifetimes, high yields for...
Three heteroleptic bis-terpyridine ruthenium(II) complexes (Ru1−Ru3) [Ru(tpy-R 1 )(tpy-R 2 )] 2+ (tpy = 2,2′:6′,2″-terpyridine, R 1 /R 2 = phenyl, 4-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}phenyl, pyren-1-yl, or 4phenyl-BODIPY (boron dipyrromethene)) were synthesized and investigated for their potential applications as photosensitizers (PSs) for photodynamic therapy. All complexes displayed broad and intense absorption band in the green spectral regions (450−600 nm), which arose from the spin-allowed charge-transfer transitions mixed with ligand-localized 1 π,π* transitions. All complexes show weak green emission at 513−549 nm and/or even weaker red emission at 646−674 nm at room temperature depending on the excitation wavelength and the solvent used. Incorporating the BODIPY motif to the 4′position of one of the tpy ligands in Ru2 and Ru3 drastically prolonged the lifetimes of the lowest triplet excited states (T 1 ) of Ru2 and Ru3 to tens of microseconds. This promoted the singlet oxygen formation sensitized by Ru2 and Ru3 upon green light activation, which in turn induced significant photocytotoxicity toward the A549 human lung cancer cell line with an EC 50 value of 1.50 μM for Ru2 and 7.41 μM for Ru3 under 0.48 J•cm −2 500 nm light irradiation. Laser confocal scanning microscopy imaging revealed that Ru2 mainly distributed to lysosomes upon cell uptake. Upon 500 nm light activation, Ru2 induced lysosomal damage and subsequent mitochondrial membrane potential decrease. The dominant cell death pathway was apoptosis. These results demonstrated the potential utilization of [Ru(tpy-R 1 )(tpy-R 2 )] 2+ complexes as PSs for PDT.
Extending the bandwidth of triplet excited-state absorption in transition-metal complexes is appealing for developing broadband reverse saturable absorbers. Targeting this goal, five bis-terdentate iridium(III) complexes (Ir1-Ir5) bearing trans-bis-cyclometalating (C^N^C) and 4′-R-2,2′:6′,2″-terpyridine (4′-R-tpy) ligands were synthesized. The effects of the structural variation in cyclometalating ligands and substituents at the tpy ligand on the photophysics of these complexes have been systematically explored using spectroscopic methods (i.e., UV−vis absorption, emission, and transient absorption spectroscopy) and time-dependent density functional theory (TDDFT) calculations. All complexes exhibited intensely structured 1 π,π* absorption bands at <400 nm and broad charge transfer ( 1 CT)/ 1 π,π* transitions at 400−600 nm. Ligand structural variations exerted a very small effect on the energies of the 1 CT/ 1 π,π* transitions; however, they had a significant effect on the molar extinction coefficients of these absorption bands. All complexes emitted featureless deep red phosphorescence in solutions at room temperature and gave broadband and strong triplet excited-state absorption ranging from the visible to the near-infrared (NIR) spectral regions, with both originating from the 3 π,π*/ 3 CT states. Although alteration of the ligand structures influenced the emission energies slightly, these changes significantly affected the emission lifetimes and quantum yields, transient absorption spectral features, and the triplet excited-state quantum yields of the complexes. Except for Ir3, the other four complexes all manifested reverse saturable absorption (RSA) upon nanosecond laser pulse excitation at 532 nm, with the decreasing trend of RSA following Ir2 ≈ Ir4 > Ir1 > Ir5 > Ir3.The RSA trend corresponded well with the strength of the excited-state and ground-state absorption differences (ΔOD) at 532 nm for these complexes.
The synthesis, crystal structure, and photophysics of a series of neutral cyclometalated iridium(III) complexes bearing substituted N-heterocyclic carbene (NHC) ancillary ligands ((C ∧ N) 2 Ir(R-NHC), where C ∧ N and NHC refer to the cyclometalating ligand benzo[h]quinoline and 1-phenylbenzimidazole, respectively) are reported. The NHC ligands were substituted with electron-withdrawing or -donating groups on C4′ of the phenyl ring (R = NO 2 (Ir1), CN (Ir2), H (Ir3), OCH 3 (Ir4), N(CH 3 ) 2 (Ir5)) or C5 of the benzimidazole ring (R = NO 2 (Ir6), N(CH 3 ) 2 (Ir7)). The configuration of Ir1 was confirmed by a single-crystal X-ray diffraction analysis. The ground-and excited-state properties of Ir1− Ir7 were investigated by both spectroscopic methods and time-dependent density functional theory (TDDFT) calculations. All complexes possessed moderately strong structureless absorption bands at ca. 440 nm that originated from the C ∧ N ligand based 1 π,π*/ 1 CT (charge transfer)/ 1 d,d transitions and very weak spin−forbidden 3 MLCT (metal-to-ligand charge transfer)/ 3 LLCT (ligand-to-ligand charge transfer) transitions beyond 500 nm. Electron-withdrawing substituents caused a slight blue shift of the 1 π,π*/ 1 CT/ 1 d,d band, while electron-donating substituents induced a red shift of this band in comparison to the unsubstituted complex Ir3. Except for the weakly emissive nitro-substituted complexes Ir1 and Ir6 that had much shorter lifetimes (≤160 ns), the other complexes are highly emissive in organic solutions with microsecond lifetimes at ca. 540−550 nm at room temperature, with the emitting states being predominantly assigned to 3 π,π*/ 3 MLCT states. Although the effect of the substituents on the emission energy was insignificant, the effects on the emission quantum yields and lifetimes were drastic. All complexes also exhibited broad triplet excited-state absorption at 460−700 nm with similar spectral features, indicating the similar parentage of the lowest triplet excited states. The highly emissive Ir2 was used as a dopant for organic light-emitting diode (OLED) fabrication. The device displayed a yellow emission with a maximum current efficiency (η c ) of 71.29 cd A −1 , a maximum luminance (L max ) of 32747 cd m −2 , and a maximum external quantum efficiency (EQE) of 20.6%. These results suggest the potential of utilizing this type of neutral Ir(III) complex as an efficient yellow phosphorescent emitter.
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