Triplet Excited-state Absorption Research Articles

Understanding Spin-Triplet Excited States in Carbene-Metal-Amides.

Carbene-metal-amides (CMAs) are emerging delayed fluorescence materials for organic light-emitting diode (OLED) applications. CMAs possess fast, efficient emission owing to rapid forward and reverse intersystem crossing (ISC) rates. The resulting dynamic equilibrium between singlet and triplet spin manifolds distinguishes CMAs from most purely organic thermally activated delayed fluorescence emitters. However, direct experimental triplet characterization in CMAs is underutilized, limiting our detailed understanding of the ISC mechanism. In this work, we combine time-resolved spectroscopy with tuning of state energies through environmental polarity and metal substitution, focusing on the interplay between charge-transfer (3CT) and local exciton (3LE) triplets. Unlike previous photophysical work, we investigate evaporated host : guest films of CMAs and small-molecule hosts for increased device relevance. Transient absorption reveals an evolution in the triplet excited-state absorption (ESA) consistent with a change in orbital character between hosts with differing dielectric constants. Using quantum chemical calculations, we simulate ESAs of the lowest triplet states, highlighting the contribution of only 3CT and donor-moiety 3LE states to spectral features, with no strong evidence for a low-lying acceptor-centered 3LE. Thus, our work provides a blueprint for understanding the role of triplet excited states in CMAs which will enable further intelligent optimization of this promising class of materials.

Exploring the Early Time Behavior of the Excited States of an Archetype Thermally Activated Delayed Fluorescence Molecule.

Optical pump-probe techniques allow for an in-depth study of dark excited states. Here, we utilize them to map and gain insights into the excited states involved in the thermally activated delayed fluorescence (TADF) mechanism of a benchmark TADF emitter DMAC-TRZ. The results identify different electronic excited states involved in the key TADF transitions and their nature by combining pump-probe and photoluminescence measurements. The photoinduced absorption signals are highly dependent on polarity, affecting the transition oscillator strength but not their relative energy positions. In methylcyclohexane, a strong and vibronically structured local triplet excited state absorption (3LE → 3LEn) is observed, which is quenched in higher polarity solvents as 3CT becomes the lowest triplet state. Furthermore, ultrafast transient absorption (fsTA) confirms the presence of two stable conformers of DMAC-TRZ: (1) quasi-axial (QA) interconverting within 20 ps into (2) quasi-equatorial (QE) in the excited state. Moreover, fsTA highlights how sensitive excited state couplings are to the environment and the molecular conformation.

Panchromatic Excited-State Absorption in Bis-Cyclometalated Iridium Isocyanide Complexes.

In this work, we introduce a series of cyclometalated iridium complexes and evaluate the suitability of this class of compounds in nonlinear optical (NLO) applications, with an emphasis on long-lived, panchromatic reverse-saturable absorption (RSA). The investigated complexes are represented by the general formula [Ir(C^N)2(CNArdmp)2]+, (C^N = cyclometalating ligand, CNArdmp = 2,6-dimethylphenyl isocyanide). Seven such complexes were synthesized and characterized, including in-depth analysis of their photophysical properties (UV-vis absorption, photoluminescence, and transient absorption). This series of compounds contains seven different cyclometalating ligands (2-phenylbenzothiazole (bt) (Ir1), 5-nitro-2-phenylpyridine (ppyNO2) (Ir2), 5-nitro-2-(9-phenanthryl)pyridine (phenNO2) (Ir3), 2-(benzo[b]thiophen-2-yl)quinoline (btq) (Ir4), 6-(benzo[b]thiophen-2'-yl)phenanthridine (btph) (Ir5), 2,4-diphenylquinoline (dpq) (Ir6), and 6-nitro-2,4-diphenylquinoline (NO2dpq) (Ir7)), which have profound effects on their ground-state and excited-state absorption spectra. To evaluate the effects of the isocyanide ancillary ligands, some heteroleptic bis-cyclometalated iridium(III) acetylacetonate (acac) analogue complexes are included as points of comparison. In the ground state, the bis-isocyanide complexes display UV-vis absorption with the characteristic 1LC (π → π*) band at λ < 350 nm and 1MLCT bands at 350-500 nm. Five of the complexes (Ir1, Ir2, Ir4, Ir5, and Ir6) display broad, intense triplet excited-state absorption with no ground-state bleach (GSB) over the spectral window of 400-900 nm, with excited-state lifetimes spanning three orders of magnitude from ∼32 ns to 12 μs. The photophysical data suggests that the isocyanide ancillary ligand blue-shifts the GSB transient into the UV, which is normally found in the middle of the visible region for cyclometalated iridium complexes. This study demonstrates the applicability of cationic cyclometalated iridium(III) bis-isocyanide complexes as candidate RSA materials.

Understanding the linear and nonlinear optical responses of few-layer exfoliated MoS2 and WS2 nanoflakes: experimental and simulation studies

Understanding the linear and nonlinear optical (NLO) responses of two-dimensional nanomaterials is essential to effectively utilize them in various optoelectronic applications. Here, few-layer MoS2 and WS2 nanoflakes with lateral size less than 200 nm were prepared by liquid-phase exfoliation, and their linear and NLO responses were studied simultaneously using experimental measurements and theoretical simulations. Finite-difference time-domain (FDTD) simulations confirmed the redshift in the excitonic transitions when the thickness was increased above 10 nm indicating the layer-number dependent bandgap of nanoflakes. WS2 nanoflakes exhibited around 5 times higher absorption to scattering cross-section ratio than MoS2 nanoflakes at various wavelengths. Open aperture Z scan analysis of both the MoS2 and WS2 nanoflakes using 532 nm nanosecond laser pulses reveals strong nonlinear absorption activity with effective nonlinear absorption coefficient (β eff) of 120 cm GW−1 and 180 cm GW−1, respectively, which was attributed to the combined contributions of ground, singlet excited and triplet excited state absorption. FDTD simulation results also showed the signature of strong absorption density of few layer nanoflakes which may be account for their excellent NLO characteristics. Optical limiting threshold values of MoS2 and WS2 nanoflakes were obtained as ∼1.96 J cm−2 and 0.88 J cm−2, respectively, which are better than many of the reported values. Intensity dependent switching from saturable absorption (SA) to reverse SA was also observed for MoS2 nanoflakes when the laser intensity increased from 0.14 to 0.27 GW cm−2. The present study provides valuable information to improve the selection of two-dimensional nanomaterials for the design of highly efficient linear and nonlinear optoelectronic devices.

Influence of structural and morphological features of zinc (II)-tetraphenylporphyrin thin film on its third order optical nonlinearity at pico and nano second regimes

Implications of laser pulse width on the third-order nonlinear optical properties of Zinc(II) 5, 10, 15, 20-Tetraphenyl-21H, 23H-Porphine (ZnTPP) thin films is investigated. Thin films of ZnTPP are developed on glass substrate by means of thermal vapor deposition technique. X-ray diffraction analysis illustrates polycrystalline nature of these films. Atomic force microscopy and field emission scanning electron microscope images confirm the homogeneous film surface with mean surface roughness value of 8.45 nm. UV–Visible absorption spectrum depicts signature peaks attributable to π-π* orbital transitions. Photoluminescence spectrum illustrates fluorescence emission in the wavelength range of 590 nm–650 nm from S1 state. Nonlinear optical investigations are carried out using single-beam Z-scan technique. ZnTPP thin-film at pico-second regime depicts saturable nonlinear absorption nature, attributable to the collective effect of singlet excited state absorption and filling effect of localized defect states. Whereas, at nanosecond timescale the film exhibits reverse saturable nonlinear absorption, sensitive to triplet excited state absorption. Closed aperture Z-scan measurements at pico and nano second regime highlight self-defocusing optical nonlinearity. Enhanced nonlinear optical coefficient (βeff and n2) values are strongly influenced by the heavy zinc metal ion and modified excited state dynamics due to the strong intermolecular interactions through J-aggregation.

Zinc phthalocyanine carrying sulfenyl and sulfydryl functionalized sulfur vacancies MoS2 for enhancement of the third order nonlinear optical property

Zinc phthalocyanine carrying sulfenyl and 6-sulfydryl linkers was firstly used to functionalize with sulfur vacancies MoS2. The synthesized MoS2/ZnPc composite was further prepared to be PVA (polyvinyl alcohol) thin films by a drop coating method. The microscopic structures and photophysical properties of novel MoS2/ZnPc composite were characterized. A series of analysis results indicate that zinc phthalocyanine with sulfenyl and 6-sulfydryl linkers is loaded to sulfur vacancies of MoS2 by ligand conjugation. The third order nonlinear optical properties of MoS2/ZnPc composite were studied by an open aperture Z-scan technique at 532 nm with 4 ns laser pulses width. In comparison with individual MoS2 or ZnPc, whether MoS2/ZnPc solution or MoS2/ZnPc/PVA film sample possesses larger third order nonlinear absorption coefficients (β) and lower limiting thresholds (F50). It can be ascribed to a combination of different NLO absorption mechanisms for MoS2/ZnPc composite, including one-photon absorption, two photon absorptions and nonlinear scattering of MoS2, triplet excited state absorption of ZnPc, nonlinear scattering of MoS2/ZnPc and the photo-induced electron transfer or energy transfer (PET/ET) process between MoS2 and ZnPc moieties.

Mono-/Bimetallic Neutral Iridium(III) Complexes Bearing Diketopyrrolopyrrole-Substituted N-Heterocyclic Carbene Ligands: Synthesis and Photophysics.

The synthesis and photophysics (UV-vis absorption, emission, and transient absorption) of four neutral heteroleptic cyclometalated iridium(III) complexes (Ir-1-Ir-4) incorporating thiophene/selenophene-diketopyrrolopyrrole (DPP)-substituted N-heterocyclic carbene (NHC) ancillary ligands are reported. The effects of thiophene versus selenophene substitution on DPP and bis- versus monoiridium(III) complexation on the photophysics of these complexes were systematically investigated via spectroscopic techniques and density functional theory calculations. All complexes exhibited strong vibronically resolved absorption in the regions of 500-700 nm and fluorescence at 600-770 nm, and both are predominantly originated from the DPP-NHC ligand. Complexation induced a pronounced red shift of this low-energy absorption band and the fluorescence band with respect to their corresponding ligands due to the improved planarity and extended π-conjugation in the DPP-NHC ligand. Replacing the thiophene units by selenophenes and/or biscomplexation led to the red-shifted absorption and fluorescence spectra, accompanied by the reduced fluorescence lifetime and quantum yield and enhanced population of the triplet excited states, as reflected by the stronger triplet excited-state absorption and singlet oxygen generation.

Photophysics and reverse saturable absorption of cationic dinuclear iridium(III) complexes bearing fluorenyl-tethered 2-(quinolin-2-yl)quinoxaline ligands.

The synthesis, photophysics and reverse saturable absorption of two cationic dinuclear Ir(III) complexes bearing fluorenyl-tethered 2-(quinolin-2-yl)quinoxaline (quqo) ligands are reported in this paper. The two complexes possess intense and featureless diimine ligand localized 1ILCT (intraligand charge transfer)/1π,π* absorption bands at ca. 330 and 430 nm, and a weak 1,3MLCT (metal-to-ligand charge transfer)/1,3LLCT (ligand-to-ligand charge transfer) absorption band at >500 nm. Both complexes exhibit weak dual phosphorescence at ca. 590 nm and 710 nm, which are attributed to the 3ILCT/3π,π* and 3MLCT/3LLCT states, respectively. The low-energy 3MLCT/3LLCT state also gives rise to a moderately strong triplet excited-state absorption at 490-800 nm. Because of the stronger triplet excited-state absorption than the ground-state absorption of these complexes at 532 nm, both complexes manifest a moderate reverse saturable absorption (RSA) at 532 nm for ns laser pulses. Expansion of the π-conjugation of the fluorenyl-tethered diimine ligand in Ir-1 causes a slight red-shift of the 1ILCT/1π,π* absorption bands in its UV-vis absorption spectrum and the 3MLCT/3LLCT absorption band in the transient absorption spectrum and slightly enhances the RSA at 532 nm compared to that in Ir-2. This work represents the first report on dinuclear Ir(III) complexes that exhibit RSA at 532 nm.

The Leap from Organic Light-Emitting Diodes to Organic Semiconductor Laser Diodes

In recent years, organic light-emitting device technology has expanded from organic light-emitting diodes (OLEDs) to organic semiconductor laser diodes (OSLDs) with the progress of sophisticated mo...

Cyclometalated Pt(II) complexes with tetradentate Schiff base ligands: Synthesis, photophysics, electrochemical studies and optical power limiting performance

Pt(II) complexes with tetradentate Schiff base ligands (Pt-1–Pt-5) were prepared, and their detailed photophysics were studied using steady-state, transient spectroscopies and density functional theory systematically. Pt-1–Pt-5 all exhibit 1π,π* transitions and metal-to-ligand/intraligand charge transfer transitions in the UV–vis region. Meanwhile, these complexes are emissive in solution at room temperature (λem = 400–800 nm, Φem = 0.008–0.196). Triplet transient absorption (TA) demonstrated that except Pt-3, all complexes exhibit broad and strong triplet excited-state absorption from 400 to 750 nm. What's more, nonlinear transmission experiment demonstrated that Pt-1–Pt-5 show robust reverse saturable absorption (RSA) at 532 nm ns laser pulses. Among them, Pt-5 shows the strongest nonlinear absorption performance, which could potentially be used as optical power limiting (OPL) materials.

Thermal Annealing of Singlet Fission Microcrystals Reveals the Benefits of Charge Transfer Couplings and Slip-Stacked Packing

Defects in molecular crystals, created by nonequilibrium crystal packing, can have significant influence over the energetics and dynamics of singlet fission. Though normally thought of as detrimental to the performance of organic semiconductors, in principle, defects could have crystal packings with better energetics and couplings for singlet fission than equilibrium crystal packing structures. Using two-dimensional electronic microscopy and X-ray diffraction, we monitor the population of nonequilibrium slip-stacked geometries in microcrystals of TIPS-pentacene before and after thermal annealing. We find that the presence of these slip-stacked structures enhances the singlet fission yield, as measured by triplet excited state absorption. The slip-stacked geometry matches no known polymorph of TIPS-pentacene, according to X-ray diffraction but indicates that slip-stacked molecules are present as tiny crystalline domains within the larger microcrystal. The data presented here provide evidence that slip-stacked geometries, present as defects in larger TIPS-pentacene microcrystals, enhance singlet fission yields.

Synthesis, Photophysics, and Reverse Saturable Absorption of trans-Bis-cyclometalated Iridium(III) Complexes (C^N^C)Ir(R-tpy)+ (tpy = 2,2′:6′,2″-Terpyridine) with Broadband Excited-State Absorption

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 1π,π* absorption bands at <400 nm and broad charge transfer (1CT)/1π,π* transitions at 400-600 nm. Ligand structural variations exerted a very small effect on the energies of the 1CT/1π,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 broad-band and strong triplet excited-state absorption ranging from the visible to the near-infrared (NIR) spectral regions, with both originating from the 3π,π*/3CT 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.

Improving triplet excited-state absorption and lifetime of cationic iridium(III) complexes by extending π-conjugation of the 2-(2-quinolinyl)quinoxaline ligand

The synthesis and photophysical properties (UV−vis absorption, emission, and transient absorption) of four cationic Ir(III) complexes (C^N)2Ir(R-quqo)+ (HC^N = 1-phenylisoquinoline (piq) and 1,2-diphenylpyreno[4,5-d]imidazole (dppi), quqo = 2-(2-quinolinyl)quinoxaline, R = H or fluorenyl) are reported. The UV–vis absorption and emission were simulated by time-dependent density functional theory (TDDFT). Influences of extending π-conjugation of the C^N ligand and the diimine ligand on the singlet and triplet excited-state absorption and lifetimes of these complexes were explored. All complexes exhibited intense ligand-localized 1π,π transitions, broad and structureless metal-to-ligand charge transfer (1MLCT) / ligand-to-ligand charge transfer (1LLCT) transitions, and very weak spin-forbidden 3MLCT/3LLCT/3π,π transitions in their UV–vis absorption spectra. The two complexes that bear fluorenyl-substituted quqo ligands (Ir-3 and Ir-4) also possessed a broad intraligand charge transfer (1ILCT) / 1π,π band at 430–550 nm. The predominant 3ILCT/3π,π characters of the triplet excited states of Ir-3 and Ir-4 improved their phosphorescent emission quantum yields and prolonged their triplet lifetimes compared to the weaker and short-lived emission of Ir-1 and Ir-2. In contrast to the very weak nanosecond transient absorption (TA) of Ir-1 and Ir-2, Ir-3 and Ir-4 possessed much stronger TA signals at 520−800 nm upon nanosecond laser excitation. These complexes exhibited moderate to strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses, with the RSA trend following Ir-1 >Ir-2 ≈ Ir-3 >Ir-4. Considering the long triplet excited-state lifetimes and broadband TA, complexes Ir-3 and Ir-4 could be potential broadband RSA materials.

联三吡啶铂(Ⅱ)配合物的非线性吸收和光限幅（特邀）

The reported work in 2003-2019 on the reverse saturable absorption (RSA) or two-photon absorption (TPA) and/or optical limiting (OPL) of platinum(II) terpyridine complexes was summarized in this minireview. Photophysical properties, including the ground-state absorption (GSA), excited-state absorption (ESA), excited-state lifetimes, and the quantum yields of triplet excited-state formation, RSA/OPL at 532 nm for ns laser pulses, TPA characteristics in the near-IR spectral regions, and the structure-property correlations were reviewed. This paper is composed of four sections. First, the current status of OPL materials and devices, the general requirements for reverse saturable absorbers and two-photon absorbing materials, and the different types and characteristics of square-planar platinum(II) complexes were briefly introduced. Then the photophysics and RSA/OPL of six series of Pt(Ⅱ) terpyridine-analogous complexes and the structure-property correlations were discussed. Following it the TPA of five series of Pt(Ⅱ) terpyridine complexes and the impacts of structural variations on the TPA cross sections (σ2) were reviewed. Finally, brief conclusions were drawn based on the reported studies. A general trend discovered was that the charge transfer absorption band(s) and the ESA can be readily tuned by substituents on the acetylide or the terpyridine ligand. Introducing electron-donating substituent to the acetylide or terpyridine ligand or improving the coplanarity between the aromatic substituent and the terpyridine ligand red-shifted the ground-state charge-transfer absorption band(s) at the price of decreasing/quenching the triplet ESA, which consequently reduced the RSA/OPL at 532 nm. Extending the π-conjugation of the terpyridine ligand dramatically improved the σ2 values of the Pt(Ⅱ) terpyridine complexes. Incorporation of electron-withdrawing π-conjugated aromatic substituent restrained the GSA to < 500 nm while keeping a long-lived triplet excited state with broadband ESA in the visible spectral regions and moderately strong TPA in the NIR regions. This approach could provide a solution for developing broadband OPL materials.

An Optical Power Limiting and Ultrafast Photophysics Investigation of a Series of Multi-Branched Heavy Atom Substituted Fluorene Molecules

A common molecular design paradigm for optical power limiting (OPL) applications is to introduce heavy atoms that promote intersystem crossing and triplet excited states. In order to investigate this effect, three multi-branched fluorene molecules were prepared where the central moiety was either an organic benzene unit, para-dibromobenzene, or a platinum(II)–alkynyl unit. All three molecules showed good nanosecond OPL performance in solution. However, only the dibromobenzene and Pt–alkynyl compounds showed strong microsecond triplet excited state absorption (ESA). To investigate the photophysical cause of the OPL, especially for the fully organic molecule, photokinetic measurements including ultrafast pump–probe spectroscopy were performed. At nanosecond timescales, the ESA of the organic molecule was larger than the two with intersystem crossing (ISC) promoters, explaining its good OPL performance. This points to a design strategy where the singlet-state ESA is balanced with the ISC rate to increase OPL performance at the beginning of a nanosecond pulse.

Neutral Cyclometalated Iridium(III) Complexes Bearing Substituted N-Heterocyclic Carbene (NHC) Ligands for High-Performance Yellow OLED Application.

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)2Ir(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 = NO2 (Ir1), CN (Ir2), H (Ir3), OCH3 (Ir4), N(CH3)2 (Ir5)) or C5 of the benzimidazole ring (R = NO2 (Ir6), N(CH3)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 1π,π*/1CT (charge transfer)/1d,d transitions and very weak spin-forbidden 3MLCT (metal-to-ligand charge transfer)/3LLCT (ligand-to-ligand charge transfer) transitions beyond 500 nm. Electron-withdrawing substituents caused a slight blue shift of the 1π,π*/1CT/1d,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 3π,π*/3MLCT 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 (ηc) of 71.29 cd A-1, a maximum luminance (Lmax) 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.

A New Class of Homoleptic and Heteroleptic Bis(terpyridine) Iridium(III) Complexes with Strong Photodynamic Therapy Effects.

Six homo- or heteroleptic tricationic Ir(R1-tpy)(R2-tpy)3+ complexes (Ir1-Ir6, R1/R2 = Ph, 4'-N(CH3)2Ph, 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 (1ILCT) / metal-to-ligand charge transfer (1MLCT) dominated transition(s) in their low-energy absorption bands, which red-shifted with the increased electron-releasing strength of the R1/R2 substituent. Five of the complexes exhibited ligand-centered 3 π,π*/3ILCT/3MLCT emission. With a stronger electron-releasing R1/R2 substituent, the degree of charge transfer contribution increased, leading to a decrease of the emission quantum yield. When the 4'-N(CH3)2Ph substituent was introduced on both tpy ligands, the emission of Ir3 was completely quenched. Our study on the transient absorption of these complexes demonstrated that they all possessed broadband triplet excited-state absorption in the 400-800 nm region. Pyrenyl substitution of one or more tpy ligands, as in Ir4 and Ir5, increased the lifetimes of the lowest triplet excited state and the singlet oxygen (1O2) production efficiencies. Ir1-Ir5 were nontoxic toward SK-MEL-28 cells, with photocytotoxicities that varied from 0.18 to 153 µM. Among them, Ir4 had the highest 1O2 quantum yield (0.81) in cell-free conditions, showing the largest photocytotoxicity against SK-MEL-28 cells for Ir(III) PSs to date, and was the most efficient generator of reactive oxygen species (ROS) in vitro. Ir4 possessed a very large phototherapeutic index (PI = dark EC50 / light EC50) of >1657, the largest reported for an Ir(III) complex photosensitizer upon broadband visible light (400-700 nm) activation. Ir4 also exhibited a very strong PDT effect toward MCF-7 breast cancer cells and its xenograft tumor model. Upon 450-nm light activation, Ir4 dramatically inhibited the xenograft tumor growth and exhibited negligible side effects upon PDT treatment.

Effects of Varying the Benzannulation Site and π Conjugation of the Cyclometalating Ligand on the Photophysics and Reverse Saturable Absorption of Monocationic Iridium(III) Complexes

A series of monocationic iridium(III) complexes, [Ir(C^N)2(pqu)]+PF6- [pqu = 2-(pyridin-2-yl)quinoline, C^N = 2-phenylquinoline (1), 3-phenylisoquinoline (2), 1-phenylisoquinoline (3), benzo[ h]quinoline (4), 2-(pyridin-2-yl)naphthalene (5), 1-(pyridin-2-yl)naphthalene (6), 2-(phenanthren-9-yl)pyridine (7), 2-phenylbenzo[ g]quinoline (8), 2-(naphthalen-2-yl)quinoline (9), and 2-(naphthalen-2-yl)benzo[ g]quinoline (10)], were synthesized in this work. These complexes bear C^N ligands with varied degrees of π conjugation and sites of benzannulation, allowing for elucidation of the effects of the benzannulation site at the C^N ligand on the photophysics of the complexes. Ultraviolet-visible (UV-vis) absorption and emission of the complexes were systematically investigated via spectroscopic techniques and time-dependent density functional theory calculations. Their triplet excited-state absorption and reverse saturable absorption (RSA) were studied by nanosecond transient absorption (TA) spectroscopy and nonlinear transmission techniques. The fusion of phenyl ring(s) to the phenyl ring or the 4 and 5 positions of the pyridyl ring of the C^N ligand resulted in red-shifted UV-vis absorption and emission spectra in complexes 2, 5-7, 9, and 10 compared to those of the parent complex 0, while their triplet lifetimes and emission quantum yields were significantly reduced. In contrast, the fusion of one phenyl ring to the other sites of the pyridyl group of the C^N ligand showed an insignificant impact on the energies of the lowest singlet (S1) and triplet (T1) excited states in complexes 1, 3, and 4 but noticeably affected their TA spectral features. The fusion of the naphthyl group to the 5 and 6 and positions at the pyridyl ring did not influence the S1 energy of complex 8 but altered the nature of the T1 states in 8 and 10 by switching them to the benzo[ g]quinoline-localized 3π,π* state, which resulted in completely different emission and TA spectra in these two complexes. The site-dependent variations of the ground- and excited-state absorption induced strong but varied RSA from these complexes for 4.1-ns laser pulses at 532 nm, with the RSA strength decreasing in the trend of 3 > 7 ≈ 4 ≈ 9 ≈ 6 > 8 ≈ 1 ≈ 2 ≈ 5 > 10.

Photophysical and Photobiological Properties of Dinuclear Iridium(III) Bis-tridentate Complexes.

A series of cationic dinuclear iridium(III) complexes (Ir1-Ir5) bearing terpyridine-capped fluorenyl bridging ligands and different polypyridyl or cyclometalating terminal tridentate ligands were synthesized, characterized, and evaluated for their photophysical and photobiological activities. The influence of the bridging and terminal ligands on the photophysical properties of the complexes was investigated by UV-vis absorption, emission, and transient absorption spectroscopy and simulated by TDDFT calculations. All of the complexes displayed strong bridging-ligand localized visible 1π,π* absorption and red- or near-infrared phosphorescence as well as broad triplet excited-state absorption across both visible and NIR wavelengths. These triplet states were assigned as predominantly 3π,π* for Ir1 (τ = 3.1 μs) and Ir4 (τ = 48 μs) and 3CT (charge transfer) for Ir2, Ir3, and Ir5 (τ = 1.7-2.7 μs). Complexes Ir1-Ir5 acted as in vitro photodynamic therapy (PDT) agents toward human SK-MEL-28 melanoma cells when activated with visible light, with submicromolar photocytotoxicity and phototherapeutic indices ranging from 20 to almost 300. The in vitro PDT effects with visible light did not correlate with singlet oxygen (1O2) quantum yields or DNA photocleaving capacity probed under cell-free conditions. All of the Ir(III) complexes phosphoresced brightly when associated with compromised cells (with or without light treatment) and exhibited photoactivated cellular uptake, highlighting the theranostic potential of this new class of Ir(III) complex photosensitizers.

Cyclometalated Ruthenium(II) Complexes Derived from α-Oligothiophenes as Highly Selective Cytotoxic or Photocytotoxic Agents.

The photophysical and photobiological properties of a new class of cyclometalated ruthenium(II) compounds incorporating π-extended benzo[ h]imidazo[4,5- f]quinoline (IBQ) cyclometalating ligands (C^N) bearing thienyl rings ( n = 1-4, compounds 1-4) were investigated. Their octanol-water partition coefficients (log Po/w) were positive and increased with n. Their absorption and emission energies were red-shifted substantially compared to the analogous Ru(II) diimine (N^N) complexes. They displayed C^N-based intraligand (IL) fluorescence and triplet excited-state absorption that shifted to longer wavelengths with increasing n and N^N-based metal-to-ligand charge transfer (MLCT) phosphorescence that was independent of n. Their photoluminescence lifetimes (τem) ranged from 4-10 ns for 1IL states and 12-18 ns for 3MLCT states. Transient absorption lifetimes (τTA) were 5-8 μs with 355 nm excitation, ascribed to 3IL states that became inaccessible for 1-3 with 532 nm excitation (1-3, τTA = 16-17 ns); the 3IL of 4 only was accessible by lower energy excitation, τTA = 3.8 μs. Complex 4 was nontoxic (EC50 > 300 μM) to SK-MEL-28 melanoma cells and CCD1064-Sk normal skin fibroblasts in the dark, while 3 was selectively cytotoxic to melanoma (EC50= 5.1 μM) only. Compounds 1 and 2 were selective for melanoma cells in the dark, with submicromolar potencies (EC50 = 350-500 nM) and selectivity factors (SFs) around 50. The photocytotoxicities of compounds 1-4 toward melanoma cells were similar, but only compounds 3 and 4 displayed significant phototherapeutic indices (PIs; 3, 43; 4, >1100). The larger cytotoxicities for compounds 1 and 2 were attributed to increased cellular uptake and nuclear accumulation, and possibly related to the DNA-aggregating properties of all four compounds as demonstrated by cell-free gel mobility-shift assays. Together, these results demonstrate a new class of thiophene-containing Ru(II) cyclometalated compounds that contain both highly selective chemotherapeutic agents and extremely potent photocytotoxic agents.