Ligand Charge Transfer States Research Articles

Blue Thermally Activated Delayed Fluorescent OLEDs With EQE up to 25% via 3MMLCT‐Induced and Short‐Lived Phosphorescent Dinuclear Pt(II) Sensitizers

AbstractThe novel dinuclear Pt(II) complexes, Di‐Pt‐CH3 and Di‐Pt‐CD3 with non‐fluorinated n‐hetero cyclic (NHC) ligands are developed. They exhibit phosphorescent emission in the range of 440–460 nm in film state with ≈60% photoluminescence quantum yield (PLQY) and a shorter lifetime due to a moderate Pt‐Pt distance of 3.21 Å. By suitably combining with blue multiresornance thermally activated delay fluorescence ( MR‐TADF) emitters, t‐DABNA and ν‐DABNA, efficient energy transfer is achieved from the triplet intraligand state (3IL) and triplet metal ligand change transfer (3MLCT) mixed states of Pt(II) complexes to the singlet state of the emitters. Importantly, the delayed triplet lifetime of the TADF emitter is shortened through the fast relaxation of triplet metal‐metal to ligand charge transfer (3MMLCT) states, possessing 0.07 eV lower energy compared to the triplet states of the TADF emitters. Di‐Pt‐CH3 and Di‐Pt‐CD3 are employed in phosphorescent and phosphorescent sensitized TADF (PS‐TADF) blue OLEDs, resulting in high external quantum efficiency (EQE) of 18.8% and 25.4%, respectively. An extremely low roll‐off characteristic of 9.8% is observed in the PS‐TADF OLED. Additionally, deuterium substitution of the methyl group improved phosphorescent device lifetime by 2.6 times. Notably, Di‐Pt‐CD3 resulted in significant lifetime enhancements: 4.7 times in phosphorescent devices and 6.6 times in PS‐TADF devices, compared with Ir(cb)3‐based devices. The mechanism for the increased lifetime is extensively studied through the magneto‐electroluminescence (MEL) and transient electroluminescence (TrEL) measurements.

Sonoinduced Tumor Therapy and Metastasis Inhibition by a Ruthenium Complex with Dual Action: Superoxide Anion Sensitization and Ligand Fracture.

Photoresponsive ruthenium(II) complexes have recently emerged as a promising tool for synergistic photodynamic therapy and chemotherapy in oncology, as well as for antimicrobial applications. However, the limited penetration power of photons prevents the treatment of deep-seated lesions. In this study, we introduce a sonoresponsive ruthenium complex capable of generating superoxide anion (O2•-) via type I process and initiating a ligand fracture process upon ultrasound triggering. Attaching hydroxyflavone (HF) as an "electron reservoir" to the octahedral-polypyridyl-ruthenium complex resulted in decreased highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps and triplet-state metal to ligand charge transfer (3MLCT) state energy (0.89 eV). This modification enhanced the generation of O2•- under therapeutic ultrasound irradiation at a frequency of 1 MHz. The produced O2•- rapidly induced an intramolecular cascade reaction and HF ligand fracture. As a proof-of-concept, we engineered the Ru complex into a metallopolymer platform (PolyRuHF), which could be activated by low-power ultrasound (1.5 W cm-2, 1.0 MHz, 50% duty cycle) within a centimeter range of tissue. This activation led to O2•- generation and the release of cytotoxic ruthenium complexes. Consequently, PolyRuHF induced cellular apoptosis and ferroptosis by causing mitochondrial dysfunction and excessive toxic lipid peroxidation. Furthermore, PolyRuHF effectively inhibited subcutaneous and orthotopic breast tumors and prevented lung metastasis by downregulating metastasis-related proteins in mice. This study introduces the first sonoresponsive ruthenium complex for sonodynamic therapy/sonoactivated chemotherapy, offering new avenues for deep tumor treatment.

Bifurcation of Excited-State Population Leads to Anti-Kasha Luminescence in a Disulfide-Decorated Organometallic Rhenium Photosensitizer.

We report a rhenium diimine photosensitizer equipped with a peripheral disulfide unit on one of the bipyridine ligands, [Re(CO)3(bpy)(S-Sbpy4,4)]+ (1+, bpy = 2,2'-bipyridine, S-Sbpy4,4 = [1,2]dithiino[3,4-c:6,5-c']dipyridine), showing anti-Kasha luminescence. Steady-state and ultrafast time-resolved spectroscopies complemented by nonadiabatic dynamics simulations are used to disclose its excited-state dynamics. The calculations show that after intersystem crossing the complex evolves to two different triplet minima: a (S-Sbpy4,4)-ligand-centered excited state (3LC) lying at lower energy and a metal-to-(bpy)-ligand charge transfer (3MLCT) state at higher energy, with relative yields of 90% and 10%, respectively. The 3LC state involves local excitation of the disulfide group into the antibonding σ* orbital, leading to significant elongation of the S-S bond. Intriguingly, it is the higher-lying 3MLCT state, which is assigned to display luminescence with a lifetime of 270 ns: a signature of anti-Kasha behavior. This assignment is consistent with an energy barrier ≥ 0.6 eV or negligible electronic coupling, preventing reaction toward the 3LC state after the population is trapped in the 3MLCT state. This study represents a striking example on how elusive excited-state dynamics of transition-metal photosensitizers can be deciphered by synergistic experiments and state-of-the-art calculations. Disulfide functionalization lays the foundation of a new design strategy toward harnessing excess energy in a system for possible bimolecular electron or energy transfer reactivity.

Bis-oxazoline derivatives as ancillary ligands for bis-cyclometalated iridium complexes

Organometallic iridium complexes with two cyclometalated ligands (C^N) and one bis-oxazoline derived ancillary ligand (L^X), i.e. (C^N)2Ir(L^X), are reported. The C^N ligands are 1-phenylpyrazoline (ppz), 2-(4,6-difluorophenyl)pyridine (F2ppy), 2-phenylpyridine (ppy), 1-phenylisoquinoline (piq). The box ligand is (4S-+)-phenyl-α-[(4S)-phenyloxazolidin-2-ylidene]-2-oxazoline-2-acetonitrile. The emission of these complexes span across the visible and into the near-ultraviolet region of the electromagnetic spectrum with moderate to high photoluminescence quantum yields (ΦPL = 0.45–1.0). These complexes were found to emit from a metal-ligand to ligand charge transfer (ML'LCT) state and have lifetimes (1.3–2.1 μs), radiative rates (105 s−1), and nonradiative rates (104–105 s−1) comparable to state-of-the-art iridium emitters. The (ppy)2Ir(BOX-CN) complexes were resolved into the Δ- and Λ- diastereomers using differences in their solubility and additionally characterized by x-ray crystallography, stability, and chiroptic studies. The high ΦPL of these isomers results in the best to date brightness for circularly polarized luminescence (CPL) from iridium complexes (7.0 M−1 cm−1), with dissymmetry factors of −0.57 × 10−3 and +1.9 × 10−3 for 3Δ and 3Λ, respectively. The significant difference in CPL magnitude between 3Δ and 3Λ likely arises from interligand interactions (edge-to-face arrangement versus strong π-π interaction) for the pendant phenyl ring of the BOX-CN ligand which differ for the two isomers.

Tuning Emission Lifetimes of Ir(C^N)2(acac) Complexes with Oligo(phenyleneethynylene) Groups

Emissive compounds with long emission lifetimes (μs to ms) in the visible region are of interest for a range of applications, from oxygen sensing to cellular imaging. The emission behavior of Ir(ppy)2(acac) complexes (where ppy is the 2-phenylpyridyl chelate and acac is the acetylacetonate chelate) with an oligo(para-phenyleneethynylene) (OPE3) motif containing three para-rings and two ethynyl bridges attached to acac or ppy is examined here due to the accessibility of the long-lived OPE3 triplet states. Nine Ir(ppy)2(acac) complexes with OPE3 units are synthesized where the OPE3 motif is at the acac moiety (aOPE3), incorporated in the ppy chelate (pOPE3) or attached to ppy via a durylene link (dOPE3). The aOPE3 and dOPE3 complexes contain OPE3 units that are decoupled from the Ir(ppy)2(acac) core by adopting perpendicular ring-ring orientations, whereas the pOPE3 complexes have OPE3 integrated into the ppy ligand to maximize electronic coupling with the Ir(ppy)2(acac) core. While the conjugated pOPE3 complexes show emission lifetimes of 0.69-32.8 μs similar to the lifetimes of 1.00-23.1 μs for the non-OPE3 Ir(ppy)2(acac) complexes synthesized here, the decoupled aOPE3 and dOPE3 complexes reveal long emission lifetimes of 50-625 μs. The long lifetimes found in aOPE3 and dOPE3 complexes are due to intramolecular reversible electronic energy transfer (REET) where the long-lived triplet-state metal to ligand charge transfer (3MLCT) states exchange via REET with the even longer-lived triplet-state localized OPE3 states. The proposed REET process is supported by changes observed in excitation wavelength-dependent and time-dependent emission spectra from aOPE3 and dOPE3 complexes, whereas emission spectra from pOPE3 complexes remain independent of the excitation wavelength and time due to the well-established 3MLCT states of many Ir(ppy)2(acac) complexes. The long lifetimes, visible emission maxima (524-526 nm), and photoluminescent quantum yields of 0.44-0.60 for the dOPE3 complexes indicate the possibility of utilizing such compounds in oxygen-sensing and cellular imaging applications.

Synthesis and conjugation properties of alkynyl functionalized salicylidene Ni(II) and Zn(II) phosphine complexes and their use as a precursor for preparation of NiO and ZnO nanoparticles

The complexes of type [Ni(L1/L2 )(PPh3)2/dppe]NO3 (1a-4a) and [Zn(L1/L2 )(PPh3)2/(dppe)]NO3 (1b-4b) [where L1 = 2-((E)-(4-(2-(trimethylsilyl)ethynyl)phenylimino)methyl)-4-(2-(4-bromophenyl)ethynyl) phenol; L2 = 2-((E)-(4-(2-(trimethylsilyl)ethynyl)phenylimino)methyl)-4-(2-(4-methoxyphenyl)ethynyl) phenol; PPh3 = triphenylphosphine; and dppe = 1, 2-bis(diphenylphosphino)ethane] have been synthesized and characterized by elemental analyses, IR, UV–Vis, 1H NMR, and 31P NMR spectral studies. The ancillary phosphine ligands significantly perturb the metal to ligand charge transfer (MLCT) state of Ni(II) and Zn(II) complexes and associated with their electron donating character. The room temperature luminescence is observed for all complexes which corresponds to π→π* ILCT transition with some MLCT character. The second harmonic generation (SHG) efficiency of the complexes was determined by Kurtz-powder technique indicating that all complexes displayed the SHG property. Metal oxide nanoparticles were obtained by calcinations of representative complexes 1a and 4b at 600 °C and are characterized by FTIR, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and selected area electron diffraction techniques.

Participation of fractional charge transfer on the efficiency of singlet oxygen production: Heteroleptic ruthenium (II) bipyridine derivatives

The photophysical properties of some synthesized heteroleptic polypyridyl ruthenium(II) complexes of the form [Ru(bpy)2L](PF6)2 where L = 2,2′-bipyridine, 4,4′-dimethoxy-2,2′-bipyridine, 1,10-phenanthroline, 4,4′-dimethyl-2,2′-bipyridine, 4,7-diphenyl-1,10-phenanthroline and 3,4,7,8-tetramethyl-1,10-phenanthroline, are investigated in acetonitrile. The excited state lifetime of the metal to ligand charge transfer (3MLCT) states of these complex ions are found to be ligand dependent and were found to be in the range 0.56 μs to 1.5 μs. Oxygen quenching of the excited states rate constants, kq, were found to be in the range 1.50–2.24 × 109 M−1s−1. Singlet oxygen quantum yield values, ΦΔ, are found to vary from 0.40 to 0.60 and the efficiency, fΔT, with which excited singlet oxygen, O2 (1Δg), is thereby produced was found to be in the range 0.55–0.85. Treatment of the data with charge transfer and non-charge transfer pathways using Schmidt’s model shows that partial charge transfer parameter was in the range of 0.18 to 0.58. It has also been found that fΔT decreases and oxygen quenching rate constants, kq, increases as pCT increases.

Luminescent Soft Crystals That Exhibit Color Changes in Response to Vapor

It is well-known that square-planar Pt(II) complexes exhibit characteristic luminescence when the complex units are stacked with Pt···Pt short contacts. Such assembled systems have interesting properties that the luminescence color can change sensitively in response to the surroundings such as temperature, pressure, and vapor. Therefore, the Pt(II) complexes are attractive as flexible and ordered sensing materials called soft crystals. In this presentation, the latest topics of vapochromic Pt(II) complexes are introduced.1. Wide-range color tuning of luminescence based on the fine control of the Pt···Pt interactions.1) A series of assembled Pt(II) complexes comprising N-heterocyclic carbene and cyanide ligands was constructed using different substituent groups (Figure 1). All the complexes exhibited highly efficient photoluminescence with an emission quantum yield in the solid state at room temperature, originating from the triplet metal-metal-to ligand charge transfer (3MMLCT) state. Their emission colors cover the entire visible region from red to blue by changing the substituents. It is notable that blue 3MMLCT emission was realized firstly in this system. In addition, vapochromic response was observed based on the single-crystal-to-single-crystal transformation.2. Fast and stable vapochromic response induced through nanocrystal formation of a luminescent platinum(II) complex on periodic mesoporous organosilica.2) A hybrid vapoluminescent system exhibiting fast and repeatable response was constructed using periodic mesoporous organosilica with bipyridine moieties (Bpy-PMO) and a Pt(II) complex bearing a potentially luminescent 2-phenylpyridinato ligand. An intense red luminescence appeared when the Pt(II)-complex immobilized Bpy-PMO was exposed to methanol vapor and disappeared on exposure to pyridine vapor. The ON-OFF vapochromic behavior occurred repeatedly in a methanol/ pyridine/heating cycle. Interestingly, a rapid response was achieved in the second cycle and cycles thereafter. The unique vapoluminescence triggered by the unprecedented protodesilylation on exposure to protic solvent vapor at room temperature is attributable to Bpy-PMO being a giant ligand and an effective vapor condenser. Consequently, this hybrid system presents a new strategy for developing sensors using bulk powdery materials. 1) D. Saito et al., Angew. Chem. Int. Ed. 2020, 59, 18723–18730. 2) H. Matsukawa et al., Sci. Rep. 2019, 9:15151. Figure 1

Modulation of Osmium(II) Tris(2,2'-bipyridine) Photophysics through Encapsulation within Zinc(II) Trimesic Acid Metal Organic Frameworks.

The Os(II) tris(2,2'-bipyridine) (OsBpy) complex exhibits optical properties that are particularly attractive for light harvesting systems due to the broad absorption spectrum extending throughout the solar spectrum. However, the relatively short lifetime of the triplet metal to ligand charge transfer state (3MLCT) relative to the related Ru(II)tris(2,2'-bipyridine) (RuBpy) has limited applications. Here, the encapsulation of OsBpy within two distinct Zn(II)-trimesic acid MOFs, HKUST-1(Zn) and USF-2 is demonstrated in an effort to extend the 3MLCT lifetime. Encapsulation results in a hypsochromatic shift of the steady-state emission band in both frameworks resulting from a destabilization of the 3MLCT. The encapsulated OsBpy also exhibits extended emission lifetimes in both HKUST-1(Zn) (104 ns in MOF vs 50 ns in methanol) and USF-2 (81 ns in MOF vs 50 ns in methanol) arising from changes in the nonradiative decay constants in both systems. The data are also consistent with vibronic perturbations involved in mixing between higher energy 3MLCT* and ligand field states.

Photophysical and Electrocatalytic Properties of Rhenium(I) Triazole-Based Complexes

A series of [Re(N^N)(CO)3(Cl)] (N^N = diimine) complexes based on 4-(pyrid-2-yl)-1,2,3-triazole (1), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (2), and 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (3) diimine ligands were prepared and their photophysical and electrochemical properties were characterized. The ligand-based reduction wave is shown to be highly sensitive to the nature of the triazole-based ligand, with the peak potential shifting by up to 600 mV toward more positive potential from 1 to 3. All three complexes are phosphorescent in solution at room temperature with λmax ranging from 540 nm (1) to 638 nm (3). Interestingly, the complexes appear to show inverted energy-gap law behaviour (τ = 43 ns for 1 versus 92 ns for 3), which is tentatively interpreted as reduced thermal accessibility of metal-centred (3MC) states from photoexcited metal to ligand charge transfer (3MLCT) states upon stabilisation of the N^N-centred lowest unoccupied molecular orbital (LUMO). The photophysical characterisation, supported by computational data, demonstrated a progressive stabilization of the LUMO from complex 1 to 3, which results in a narrowing of the HOMO–LUMO energy gap (HOMO = highest occupied molecular orbital) across the series and, correspondingly, red-shifted electronic absorption and photoluminescence spectra. The two complexes bearing pyridyl (1) and pyrimidyl (2) moieties, respectively, showed a modest ability to catalyse the electroreduction of CO2, with a peak potential at ca. −2.3 V versus Fc/Fc+. The catalytic wave that is observed in the cyclic voltammograms is slightly enhanced by the addition of water as a proton source.

DFT/TD-DFT investigation on the photoinduced electron transfer of diruthenium and viologen complexes

Photoinduced electron transfer, chemical stability, and redox property of diruthenium−viologen complexes were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Target systems include a series of diruthenium and viologen complexes consisting of Ru(bpy)3-viologen-Ru(bpy)3; Ru–V–Ru, Ru(bpy)3-viologen-Ru(bpy)(dcbpy)2; Ru–V–RuCOOH, and Ru(bpy)3-viologen-viologen-Ru(bpy)3; Ru–V–V–Ru, where bpy = 2,2′-bipyridyl and dcbpy = 4,4′-dicarboxyl-2,2-bipyridyl. The B3LYP functional was used to investigate the ground state properties for the three diruthenium and viologen complexes and for two parent complexes: ruthenium-bipyridine (Ru(bpy)32+) and methyl viologen (MV2+) as well as their oxidized states. Singlet excited states were examined using the TD-DFT calculations with CAM-B3LYP functional in acetonitrile using conductor polarizable continuum model (CPCM). The TD-DFT calculations reproduced the experimentally observed variation of the absorption spectra due to oxidation states. The correlation between the calculated UV–Vis transition and molecular orbitals for the excited state clearly established that the viologen dication (V2+) changed their reductive product into the radical cation (V•+) and neutral (V0), whereas ruthenium retained its formal Ru(bpy)32+ oxidation state. The metal to ligand charge transfer state of Ru(bpy)32+ was shown to undergo electron transfer quenching by methyl viologen dication (MV2+). These findings are relevant for the development of novel optoelectronic materials owing to a combination of chemical stability, redox activity, and long-lived excited states.

The combination of multipartitioning of the Hamiltonian with canonical Van Vleck perturbation theory leads to a Hermitian variant of quasidegenerate N-electron valence perturbation theory.

Many recent developments in the area of multistate multireference perturbation theories focused on methods that use a state-averaged 0th order Hamiltonian. We recently found that the dynamic correlation dressed complete active space method fails in describing ligand field and charge transfer states in a balanced way precisely because it uses a state-averaged 0th order Hamiltonian [L. Lang and F. Neese, J. Chem. Phys. 150, 104104 (2019)]. The multipartitioning idea allows the use of state-specific 0th order Hamiltonians in a multistate framework and could therefore alleviate the mentioned problem. However, the effective Hamiltonian is non-Hermitian in the traditional formulation of multipartitioning, which can lead to unphysical behavior, especially for nearly degenerate states. In order to achieve a more balanced treatment of states with different physical character and at the same time have a Hermitian effective Hamiltonian, we combine in this work multipartitioning with canonical Van Vleck perturbation theory. At the 2nd order, the result is a Hermitian variant of multipartitioning quasidegenerate N-electron valence state perturbation theory. The effect of model space noninvariance of the method is discussed and the benefit of a Hermitian formulation is highlighted with numerical examples. The method is shown to give good results for the calculation of electronic transitions of the [CuCl4]2 -complex and for the calculation of electron paramagnetic resonance parameters, which are two examples where the balance between ligand field and charge transfer configurations is of utmost importance.

A new series of three-coordinate cuprous complexes formed by steric hindrance of a phosphine ligand: Synthesis, structure characterization, properties and TD-DFT calculations

Four new three-coordinate cuprous complexes with a CuN2P core, 1-QBO, 2-Phen, 3-MePBO, 4-QBI, were designed and synthesized by utilizing a steric hindrance of phosphine ligand o-Anisyl3P [QBO = 2-(2′-quinolyl)benzoxazole, Phen = 1,10-Phenanthroline, MePBO = 5-methyl-2-(2′-pyridyl)-benzoxazole, QBI = 2-(2′-quinolyl)benzimidazole, o-Anisyl3P = tri(2-methoxyphenyl)-phosphine]. As a counterpart to 1-QBO, a four-coordinate complex 5-QBOP2 has also been synthesized with a CuN2P2 core. All complexes were characterized by single-crystal X-ray diffraction, spectroscopic analysis (IR, UV–Vis), elemental analysis, and photoluminescence study. Single-crystal X-ray diffraction revealed that complexes 1–4 all adopt trigonal CuN2P coordination geometry with one phosphine and one diimine ligand. Their UV–Vis absorption spectra exhibit concentration dependences of absorption edge shift. Time-dependent density functional theory (TD-DFT) calculations reveal that their S1 states and the peak transition states can be mainly assigned as ligand–ligand & metal–ligand charge transfer (L′LCT + MLCT) and intra-ligand charge transfer (ILCT) states, respectively. It is noteworthy that all three-coordinate complexes 1–4 do not display obvious photoluminescence (PL), whereas the PL of four-coordinate complex 5 is turned on by an extra coordination of a phosphine ligand to 1. This PL complex has also been synthesized and characterized as 5-QBOP2 from 1-QBO. This model of three-coordinate to four-coordinate change with PL turn-on behavior could be used for sensing volatile organic compounds (VOCs).

Photophysical study of [Ru(2,2′-bipyridine)3]2+ and [Ru(1,10-phenanthroline)3]2+ encapsulated in the Uio-66-NH2 metal organic framework

Metal organic frameworks are stable porous materials that provide an excellent environment for the encapsulation of photoactive guests. The ruthenium(II) polyimine complexes, in particular, ruthenium(II) tris-(1,10-phenanthroline) (RuPhen) and ruthenium(II) tris-(2,2′-bipyridine) (RuBpy) are ideal candidates for encapsulation due to their unique photophysics. Here the excited state properties of RuPhen and RuBpy encapsulated within the zirconium oxide based MOF Uio-66 and the amine derivatized MOF Uio-66-NH2 are reported. The encapsulated complexes exhibit stabilization of the excited triplet metal to ligand charge transfer state (3MLCT) resulting in a bathochromatic shift of the emission band and extension of the emission lifetime. The introduction of an amine functional group into the Uio-66 framework gives rise to a reduction in lifetimes relative to the parent Uio-66 framework materials that may arise from perturbation to low lying singlet in character MLCT states.

Light-activated ruthenium (II)-bicalutamide prodrugs for prostate cancer

Targeted delivery of clinically approved anticancer drug to tumor sites is an effective way to achieve enhanced drug efficacy as well as reduced side effects and toxicity. Here bicalutamide is caged by the Ru(II) center through the nitrile group, and three photoactive Ru(II) complexes were designed and synthesized. Docking study showed that the ruthenium(II) fragments can effectively block the binding of complexes 1–3 with AR (androgen receptor) owing to the large steric structures, thus bicalutamide in complexes 1–3 could not interact with AR-LBD (ligand binding domain). Once irradiation with blue light (465nm), complexes 1–3 can release bicalutamide and anticancer Ru(II) fragments, which possesses dual-action of AR binding and DNA interaction simultaneously. In vitro cytotoxicity study on these complexes further confirmed that complexes 1–3 exhibited considerable cytotoxicity upon irradiation with blue light. Significantly, complex 3 could be activated at 660nm, which greatly increases the scope of complex 3 to treat deeper within tissue. Theoretical calculations showed that the lowest singlet excitation energy of complex 3 is lower than those of complexes 1–2, which explains the experimental results well. Moreover, the 3MC (metal centered) states of these complexes are more stable than their 3MLCT (metal to ligand charge transfer) states, indicating that the photoactive processes of these complexes are likely to result in ligand dissociation.

Synthesis and characterization of phosphorescent isomeric iridium complexes with a rigid cyclometalating ligand

The synthesis, structures and photophysical properties of three phosphorescent isomeric heteroleptic iridium complexes are reported. The complexes are bis-cyclometalated with a rigid 9-tert-butyl-pyrazolo[1,5-f]phenanthridine (tpzp) ligand. The isomers (I1, I2 and I3) of the heteroleptic complex Ir(tpzp)2pic (pic = picolinate) were isolated from reactions and characterized by 2D NMR experiments and X-ray crystallography. The tpzp ligands in isomer I1 have a trans-N,N configuration, whereas the tpzp ligands in isomers I2 and I3 both have a cis-N,N configuration, with the three coordinated nitrogens being facial in I2 and meridinal in I3. The I1 complex was found to isomerize during the sublimation under high vacuum (0.5 × 10−6 torr) at 300 °C, giving a ratio of I1:I2:I3 = 3:1:6. Reduction potentials and DFT calculations of the Ir(tpzp)2pic isomers indicate that the LUMO for these heteroleptic complexes is localized on the picolinate moiety. The Ir(tpzp)2pic isomers all display broad, featureless emission at room temperature (λem ∼ 530 nm) consistent with emission from a triplet metal–ligand to ligand (Ir-tpzp to picinolate) charge transfer (3MLLCT) state. Broad 3MLLCT emission (λem ∼ 520 nm) is still observed from isomers I2 and I3 in rigid media at 77 K. However, under the same conditions structured blue phosphorescence (λem = 452 nm) is observed from isomer I1, consistent with emission from a metal perturbed 3LC state on the tpzp ligand.

Theoretical Study of the Light-Induced Spin Crossover Mechanism in [Fe(mtz)6]2+ and [Fe(phen)3]2.

The deactivation pathway of the light-induced spin crossover process in two Fe(II) complexes has been studied by combining density functional theory calculations for the geometries and the normal vibrational modes and highly correlated wave function methods for the energies and spin-orbit coupling effects. For the two systems considered, the mechanism of the photoinduced conversion from the low-spin singlet to the high-spin quintet state implies two intersystem crossings through intermediate triplet states. However, for the [Fe(mtz)6]2+ complex, the process occurs within few picoseconds and involves uniquely metal-centered electronic states, whereas for the [Fe(phen)3]2+ system the deactivation channel involves both metal to ligand charge transfer and metal-centered states and takes place in a femtosecond time scale.

Ultrafast dynamics of two copper bis-phenanthroline complexes measured by x-ray transient absorption spectroscopy

Ultrafast structural dynamics of the metal to ligand charge transfer (MLCT) states of two copper bis-phenanthroline complexes were captured by using x-ray transient absorption (XTA) spectroscopy at the Linac Coherent Light Source and further described by theoretical calculations. These complexes have the general formula [Cu(I)(R)2]+, where R = 2,9-dimethyl-1,10-phenanthroline (dmp) and 2,9-diphenyl-1,10-phenanthroline disulfonic acid disodium salt (dpps). [Cu(I)(dmp)2]+ has methyl groups at the 2,9 positions of phenanthroline (phen) and adopts a pseudo-tetrahedral geometry. In contrast, [Cu(I)(dpps)2]+ possesses two bulky phenyl-sulfonate groups attached to each phen ligand that force the molecule to adopt a flattened tetrahedral geometry in the ground state. Previously, optical transient absorption (OTA) and synchrotron based XTA experiments with 100 ps time resolution have been employed to study the relationship between structural distortions and excited state relaxation pathways in the two complexes. However, the dynamics of the MLCT transition during the first few picoseconds after excitation in these complexes remained unclear because of limitations in element specificity in OTA and in the time resolution of synchrotron sources in XTA. In this experiment, the local coordination geometry and oxidation state of copper were probed with a temporal resolution of ∼300 fs. Unexpectedly, the depletion of the Cu(I) signal due to the MLCT transition was found to be non-impulsive in the case of [Cu(I)(dpps)2]+ with a time constant of 0.6 ps, while the Cu(I) depletion in [Cu(I)(dmp)2]+ was instantaneous within the 300 fs instrument response time. The slower Cu(I) depletion kinetics in [Cu(I)(dpps)2]+, previously unobserved in femtosecond OTA experiments, is likely due to intramolecular motions on the sub-picosecond time scale that could alter the localization of the transferred electron in the phen ligands.

Photo-physical studies of ruthenium(II) tris(1,10-phenanthroline) confined within a polyhedral zinc(II)-trimesic acid metal organic framework

The development of new classes of photo-active porous materials is of specific interest in photocatalysis and light harvesting applications. Metal organic frameworks (MOFs) provide an excellent platform through which to develop photo-active materials as the large pores can serve as hosts for photo-active guests. Here the photophysical properties of [Ru(1,10-phenanthroline)3]2+ (RuPhen) encapsulated within a polyhedral Zn(II)-trimesic acid metal organic framework material, hereafter, USF2, are reported. Encapsulation alters the photophysical properties of the RuPhen exhibiting a biphasic decay of the triplet metal to ligand charge transfer state (3MLCT) with lifetimes of 142ns and 966ns at 20°C relative to 320ns for RuPhen in deaerated ethanol solution. In addition, the encapsulated RuPhen displays a hypsochromic shift in the steady state emission relative to RuPhen in ethanol from 586nm to 579nm. These results are quite distinct from the corresponding RuBpy@USF2 and it is suggested that RuBpy and RuPhen encapsulate within different cavities of the USF2 framework.

Self-Assembly of Tunable Heterometallic Ln-Ru Coordination Polymers with Near-Infrared Luminescence and Magnetocaloric Effect.

A series of heterometallic lanthanide (Ln)-Ru coordination polymers, denoted Gd-1, Yb-2, and Nd-3, were prepared by solvothermal reaction of a carboxylate derivative of [Ru(bpy)3 ]2+ (Rubpy, bpy=2,2'-bipyridine), oxalic acid, and Ln(OAc)3 by using the metalloligand strategy. Single-crystal X-ray diffraction indicated that the resulting isostructural heterometallic complexes have 1D butterfly-shaped Ln-Ru-based coordination chains but show slight differences in the coordination environments of the Ln centers. The introduced Ru(bpy) metalloligands could act as good light-harvesting antennas to effectively sensitize near-infrared (NIR) luminescence by energy transfer from the triplet metal-to ligand charge transfer state of Rubpy units to Ln (Yb or Nd) under the excitation in the visible-light region. Additionally, dopant-concentration-dependent behavior of the Ru-based emission and sensitized NIR emission was demonstrated in Gd-1. Finally, the magnetocaloric effect of Gd-1 was studied. The preparation of such heterometallic coordination polymers offers a versatile platform to investigate dimensionally controlled properties.