MLCT State Research Articles

Not All 3MC States Are the Same: The Role of 3MCcis States in the Photochemical N∧N Ligand Release from [Ru(bpy)2(N∧N)]2+ Complexes.

Ruthenium(II) complexes feature prominently in the development of agents for photoactivated chemotherapy; however, the excited-state mechanisms by which photochemical ligand release operates remain unclear. We report here a systematic experimental and computational study of a series of complexes [Ru(bpy)2(N∧N)]2+ (bpy = 2,2'-bipyridyl; N∧N = bpy (1), 6-methyl-2,2'-bipyridyl (2), 6,6'-dimethyl-2,2'-bipyridyl (3), 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (4), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole (5), 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl (6)), in which we probe the contribution to the promotion of photochemical N∧N ligand release of the introduction of sterically encumbering methyl substituents and the electronic effect of replacement of pyridine by 1,2,3-triazole donors in the N∧N ligand. Complexes 2 to 6 all release the ligand N∧N on irradiation in acetonitrile solution to yield cis-[Ru(bpy)2(NCMe)2]2+, with resultant photorelease quantum yields that at first seem counter-intuitive and span a broad range. The data show that incorporation of a single sterically encumbering methyl substituent on the N∧N ligand (2 and 5) leads to a significantly enhanced rate of triplet metal-to-ligand charge-transfer (3MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors (4 and 6) leads to a similar rate of 3MLCT deactivation but with much greater photochemical reactivity. The data reported here, discussed in conjunction with previously reported data on related complexes, suggest that monomethylation in 2 and 5 sterically inhibits the formation of a 3MCcis state but promotes the population of 3MCtrans states which rapidly deactivate 3MLCT states and are prone to mediating ground-state recovery. On the other hand, increased photochemical reactivity in 4 and 6 seems to stem from the accessibility of 3MCcis states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.

Molecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes.

Square-planar NiII complexes and their electronically excited states play key roles in cross-coupling catalysis and could offer new opportunities to complement well-known isoelectronic PtII luminophores. Metal-to-ligand charge transfer (MLCT) excited states and their deactivation pathways are particularly relevant in these contexts. We sought to extend the lifetimes of 3MLCT states in square-planar NiII complexes by creating coordination environments that seemed particularly well adapted to the 3d8 valence electron configuration. Using a rigid tridentate chelate ligand, in which a central cyclometalated phenyl unit is flanked by two coordinating N-heterocyclic carbenes, along with a monodentate isocyanide ligand, a very strong ligand field is created. Bulky substituents at the isocyanide backbone furthermore protect the NiII center from nucleophilic attack in the axial directions. UV-Vis transient absorption spectroscopies reveal that upon excitation into 1MLCT absorption bands and ultrafast intersystem crossing to the 3MLCT excited state, the latter relaxes onward into a metal-centered triplet state (3MC). A torsional motion of the tridentate ligand and a NiII-carbon bond elongation facilitate 3MLCT relaxation to the 3MC state. The 3MLCT lifetime gets longer with increasing ligand field strength and improved steric protection, thereby revealing clear design guidelines for square-planar NiII complexes with enhanced photophysical properties. The longest 3MLCT lifetime reached in solution at room temperature is 48 ps, which is longer by a factor of 5-10 compared to previously investigated square-planar NiII complexes. Our study contributes to making first-row transition metal complexes with partially filled d-orbitals more amenable to applications in photophysics and photochemistry.

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.

Acetonitrile Ligand Photosubstitution in Ru(II) Complexes Directly from the 3MLCT State.

The excited states of the series [Ru(tpy)(L)(CH3CN)]n+ (1-11) (n = 1, 2) containing bidentate ligands L with varying electron-donating ability were characterized through Arrhenius analysis of the temperature dependence of their excited-state lifetimes. Complexes 1-11 undergo photoinduced CH3CN dissociation upon 450 nm irradiation with ligand exchange quantum yields that increase with the energy barrier to populating a dissociative triplet ligand field (3LF) state from the lowest-energy triplet metal-to-ligand charge transfer (3MLCT) excited state. Combined with DFT calculations, the data indicate that ligand photodissociation in 1-11 occurs directly from the 3MLCT state instead of a 3LF state. This finding is in contrast to the generally accepted mechanism for ligand photodissociation in Ru(II) complexes and indicates that alternative pathways for photoinduced ligand dissociation are available. These results can widely impact design principles for applications that require ligand photodissociation, such as photochemotherapy and photocatalysis, as well as for those where photosubstitution is undesirable, such as solar energy conversion.

Distinctive Aspects in Aquation, Proton-Coupled Redox, and Photoisomerization Reactions between Geometric Isomers of Mononuclear Ruthenium Complexes with a Large-π-Conjugated Tetradentate Ligand.

Geometric isomers of mononuclear ruthenium(II) complexes, distal-/proximal-[Ru(tpy)(dpda)Cl]+ (d-/p-RuCl, tpy = 2,2':6',2″-terpyridine, dpda = 2,7-bis(2-pyridyl)-1,8-diazaanthracene), were newly synthesized to comprehensively investigate the geometric and electronic structures and distinctive aspects in various reactions between isomers. The ultraviolet (UV)-visible absorption spectra of d-/p-RuCl isomers show intense bands for metal-to-ligand charge transfer (MLCT) at close wavelengths of 576 and 573 nm, respectively. However, time-dependent density functional theory (TD-DFT) calculations suggest that the MLCT transition of d-RuCl involves mainly single transitions to the π* orbital of the dpda ligand in contrast to mixing of the π* orbitals of the dpda and tpy ligands for p-RuCl. The aquation reaction (1.5 × 10-3 s-1) of p-RuCl to yield proximal-[Ru(tpy)(dpda)(OH2)]2+ (p-RuH2O) is faster than that (5.3 × 10-6 s-1) of d-RuCl in D2O/CD3OD (4:1 v/v) by three orders of magnitude, which resulted from the longer Ru-Cl bond by 0.017 Å and the distorted angle (100.2(3)°) of Cl-Ru-N (a nitrogen of dpda, being on a tpy plane) due to the steric repulsion between Cl and dpda for p-RuCl. Electrochemical measurements showed that d-RuH2O undergoes a 2-step oxidation reaction of 1H+-coupled 1e- processes of RuII-OH2/RuIII-OH and RuIII-OH/RuIV═O at pH 1-9, whereas p-RuH2O undergoes a 1-step oxidation reaction of a 2H+-coupled 2e- process of RuII-OH2/RuIV═O in the pH range of pH 1-10. The irreversible photoisomerization from d-RuH2O to p-RuH2O was observed in aqueous solution with an internal quantum yield (Φ) of 5.4 × 10-3% at 520 nm, which is lower compared with Φ = 1.1-2.1% of mononuclear Ru(II) aquo complexes with similar bidentate ligands instead of dpda by three orders of magnitude. This is possibly ascribed to the faster nonradiative decay rate from the excited 3MLCT state to the ground state for d-RuH2O due to the lower π* level of dpda ligands according to the energy-gap law: the rate decreases exponentially with the increasing energy gap.

Electronic structure analysis of copper photoredox catalysts using the quasi-restricted orbital approach.

In this computational study, the electronic structure changes along the oxidative and reductive quenching cycles of a homoleptic and a heteroleptic prototype Cu(I) photoredox catalyst, namely, [Cu(dmp)2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline) and [Cu(phen)(POP)]+ (POP = bis [2-(diphenylphosphino)phenyl]ether), are scrutinized and characterized using quasi-restricted orbitals (QROs), electron density differences, and spin densities. After validating our density functional theory-based computational protocol, the equilibrium geometries and wavefunctions (using QROs and atom/fragment compositions) of the four states involved in photoredox cycle (S0, T1, Dox, and Dred) are systematically and thoroughly described. The formal ground and excited state ligand- and metal-centered redox events are substantiated by the QRO description of the open-shell triplet metal-to-ligand charge-transfer (3MLCT) (d9L-1), Dox (d9L0), and Dred (d10L-1) species and the corresponding structural changes, e.g., flattening distortion, shortening/elongation of Cu-N/Cu-P bonds, are rationalized in terms of the underlying electronic structure transformations. Among others, we reveal the molecular-scale delocalization of the ligand-centered radical in the 3MLCT (d9L-1) and Dred (d9L-1) states of homoleptic [Cu(dmp)2]+ and its localization to the redox-active phenanthroline ligand in the case of heteroleptic [Cu(phen)(POP)]+.

Real-Time and In Situ Viscosity Monitoring in Industrial Adhesives Using Luminescent Cu(I) Phenanthroline Molecular Sensors.

Monitoring the viscosity of polymers in real-time remains a challenge, especially in confined environments where traditional rheological measurements are hard to apply. In this study, we have utilized the luminescent complex [Cu(diptmp)2]+ (diptmp = 2,9-diisopropyl-3,4,7,8-tetramethyl-1,10-phenanthroline) as an optical probe for real-time sensing of viscosity in various adhesives during the curing process (viscosity increases). The emission lifetime of the triplet metal-to-ligand charge transfer (3MLCT) state of [Cu(diptmp)2]+ in epoxy adhesive increased exponentially during curing, similar to viscosity values obtained from oscillatory rheology. The longer lifetime in higher viscosity materials was attributed to changes in the excited-state deactivation processes from a known Jahn-Teller distortion in the Cu(I) geometry from tetrahedral in the ground state to square planar in the excited state. The real-time viscosity was also monitored reversibly by emission lifetime during polymer swelling (viscosity and lifetime decrease) and unswelling (viscosity and lifetime increase). Monitoring emission lifetime, unlike measuring the excited-state lifetime via transient absorption measurements in our previous study, allowed us to measure viscosity in opaque samples which scatter light. The optical probe [Cu(diptmp)2]+ in Gorilla Glue adhesive showed a clear correlation of the emission intensity or lifetime to viscosity during the curing process. We have also compared these lifetime changes using [Ru(bpy)3]2+ (bpy = bipyridine) as a control. [Cu(diptmp)2]+ showed not only a higher emission lifetime but also more ubiquity as a real-time viscosity sensor.

(Invited) Ultrafast Solar Energy Conversion with Earth-Abundant Complexes

Transition-metal complexes are widely used as photosensitizers and photocatalysts, and in light-emitting devices.[1] We have studied ultrafast proceses in a variety of DSSC and photocatalytic aplications.[2,3] More recently we demonstrated the first iron complex [FeIII(btz)3]3+ exhibiting room temperature photoluminescence from a CT state and show that it has a 100 ps excited state CT lifetime which is unprecedented for any iron complex.[4] This was characterized as a rare low-spin FeIII d5 complex with emission from a long-lived doublet ligand-to-metal charge transfer (2LMCT) state. Absence of intersystem crossing (ISC) in this complex avoids significant excited state energy losses as encountered in the prevailing class of d6 TM complexes of e.g. FeII and RuII. Furthermore, we have also shown [FeII(btz)3]2+ low-spin complex [5], a FeII analogue to [FeIII(btz)3]3+. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited state lifetime in CH3CN solution that is more than one order of magnitude longer compared to the MLCT lifetime of any previously reported FeII complex. Together, these results show that the FeII and FeIII oxidation states of the same Fe(btz)3 complex feature long lived MLCT and LMCT states, respectively. This demonstrates the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of photophysical and photochemical applications, and on-going efforts for further improvements of the excited state dynamics of such systems will be outlined, including the very latest development of an iron compound [Fe(phtmeimb)2]+ with an excited state stable enough to emit light for 2 ns and able to engage in bimolecular electron transfer [6].1 Ponseca, C. S., Jr.; Chabera, P.; Uhlig, J.; Persson, P.; Sundstrom, V. Ultrafast Electron Dynamics in Solar Energy Conversion. Chem Rev 2017, 117 (16), 10940.2 H. Imahori et al, Photoinduced Charge Carrier Dynamics of Zn−Porphyrin−TiO2 Electrodes: The Key Role of Charge Recombination for Solar Cell Performance, J. Phys. Chem. A 2011, 115, 16, 3679-36903 M. Yamamoto et al, Visible light-driven water oxidation with a subporphyrin sensitizer and a water oxidation catalyst, Chem. Commun. 2016,52, 13702-137054 Chabera, P. et al. A low-spin Fe(III) complex with 100-ps ligand-to-metal charge transfer photoluminescence. Nature 2017, 543 (7647), 695.5 Chabera, P. et al. Fe(II) Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime, J Phys Chem Lett 2018, 9, 459−463.6 Kjær, K. S. et al. Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime. Science 2019, 363 (6424), 249-253. Figure 1

Resolving Femtosecond Solvent Reorganization Dynamics in an Iron Complex by Nonadiabatic Dynamics Simulations.

The ultrafast dynamical response of solute–solvent interactions plays a key role in transition metal complexes, where charge transfer states are ubiquitous. Nonetheless, there exist very few excited-state simulations of transition metal complexes in solution. Here, we carry out a nonadiabatic dynamics study of the iron complex [Fe(CN)4(bpy)]2– (bpy = 2,2′-bipyridine) in explicit aqueous solution. Implicit solvation models were found inadequate for reproducing the strong solvatochromism in the absorption spectra. Instead, direct solute–solvent interactions, in the form of hydrogen bonds, are responsible for the large observed solvatochromic shift and the general dynamical behavior of the complex in water. The simulations reveal an overall intersystem crossing time scale of 0.21 ± 0.01 ps and a strong reliance of this process on nuclear motion. A charge transfer character analysis shows a branched decay mechanism from the initially excited singlet metal-to-ligand charge transfer (1MLCT) states to triplet states of 3MLCT and metal-centered (3MC) character. We also find that solvent reorganization after excitation is ultrafast, on the order of 50 fs around the cyanides and slower around the bpy ligand. In contrast, the nuclear vibrational dynamics, in the form of Fe–ligand bond changes, takes place on slightly longer time scales. We demonstrate that the surprisingly fast solvent reorganizing should be observable in time-resolved X-ray solution scattering experiments, as simulated signals show strong contributions from the solute–solvent scattering cross term. Altogether, the simulations paint a comprehensive picture of the coupled and concurrent electronic, nuclear, and solvent dynamics and interactions in the first hundreds of femtoseconds after excitation.

Upconversion in a d-f [RuYb3] Supramolecular Assembly.

We have prepared a hetero-tetrametallic assembly consisting of three ytterbium ions coordinated to a central [Ru(bpm)3]2+ (bpm = 2,2'-bipyrimidine) motif. Irradiation into the absorption band of the peripheral ytterbium ions at 980 nm engenders emission of the 3MLCT state of the central [Ru(bpm)3]2+ core at 636 nm, which represents the first example of f → d molecular upconversion (UC). Time-resolved measurements reveal a slow rise of the UC emission, which was modeled with a mathematical treatment of the observed kinetics according to a cooperative photosensitization mechanism using a virtual Yb centered doubly excited state followed by energy transfer to the Ru centered 1MLCT state.

Proton-Coupled Electron Transfer in a Ruthenium(II) Bipyrimidine Complex in Its Ground and Excited Electronic States.

Proton-coupled electron transfer (PCET) was studied for the ground and excited electronic states of a [Ru(terpy)(bpm)(OH2)(PF6)2] complex, Ru-bpm. Cyclic voltammetry measurements show that the Ru(II)-aqua moiety undergoes PCET to form a Ru(IV)-oxo moiety in the anodic region, while the bpm ligand undergoes PCET to form bpmH2 in the cathodic region. The photophysical behavior of Ru-bpm was studied using steady-state and femtosecond transient UV-vis absorption spectroscopy, coupled with density functional theory (DFT) calculations. The lowest-lying excited state of Ru-bpm is described as a (Ru → bpm) metal-to-ligand charge-transfer (MLCT) state, while the metal-centered (MC) excited state was found computationally to be close in energy to the lowest-energy bright MLCT state (MC state was 0.16 eV above the MLCT state). The excited-state kinetics of Ru-bpm were found via transient absorption spectroscopy to be short-lived and were fit well to a biexponential function with lifetimes τ1 = 4 ps and τ2 = 65 ps in aqueous solution. Kinetic isotope effects of 1.75 (τ1) and 1.61 (τ2) were observed for both decay components, indicating that the solvent plays an important role in the excited-state dynamics of Ru-bpm. Based on the pH-dependent studies and the results from prior studies of similar Ru-complexes, we hypothesize that the 3MLCT state forms an excited-state hydrogen-bond adduct with the solvent molecules and that this process occurs with a 4 ps lifetime. The formation of such a hydrogen-bond complex is consistent with the electronic density accumulation at the peripheral N atoms of the bpm moiety in the 3MLCT state. The hydrogen-bonded state 3MLCT decays to the ground state with a 65 ps lifetime. Such a short lifetime is likely associated with the efficient vibrational energy transfer from the 3MLCT state to the solvent.

Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds.

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d6 valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (LCNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(LCNC)2](PF6), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(LCNC)2](PF6) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(LCNC)2](PF6), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

Nonaromatic Organonickel(II) Phototheranostics.

Earth-abundant metal-based theranostics, agents that integrate diagnostic and therapeutic functions within the same molecule, may hold the key to the development of low-cost personalized medicines. Here, we report a set of O-linked nonaromatic benzitripyrrin (C^N^N^N) macrocyclic organonickel(II) complexes, Ni-1-4, containing strong σ-donating M-C bonds. Complexes Ni-1-4 are characterized by a square-planar coordination geometry as inferred from the structural studies of Ni-1. They integrate photothermal therapy, photothermal imaging, and photoacoustic imaging (PAI) within one system. This makes them attractive as potential phototheranostics. Relative to traditional Ni(II) porphyrins, such as F20TPP (tetrapentafluorophenylporphyrin), the lowest energy absorption of Ni-1 is shifted into the near infrared region, presumably as a consequence of Ni-C bonding. Ultrafast transient absorption spectroscopy combined with theoretical calculations revealed that, upon photoexcitation, a higher population of ligand-centered and 3MLCT states is seen in Ni-1 relative to NiTPBP (TPBP = 6,11,16,21-tetraphenylbenziporphyrin). Encapsulating Ni-1 in 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DSPE-PEG2000) afforded nanoparticles, Ni-1@DSPE, displaying red-shifted absorption features, as well as good photothermal conversion efficiency (∼45%) in aqueous media. Proof-of-principle experiments involving thrombus treatment were carried out both in vitro and in vivo. It was found that Ni-1@DSPE in combination with 785 nm photo-irradiation for 3 min (0.3 W/cm2) proved successful in removing blood clots from a mouse thrombus model as monitored by photoacoustic imaging (PAI). The present work highlights the promise of organonickel(II) complexes as potential theranostics and the benefits that can accrue from manipulating the excited-state features of early transition-metal complexes via, for example, interrupting π-conjugation pathways.

Red-emitting heteroleptic iridium(III) complexes: photophysical and cell labeling study.

Two red-emitting heteroleptic iridium(III) complexes (Ir-p and Ir-q) were synthesized and their photophysical and biological properties were analyzed. After their structures have been confirmed by several techniques, such as 1H NMR, 13C NMR, FT-IR, UV-Vis, and MALDI TOF analyses, their luminescence behavior was investigated in ethanol and PBS (physiological medium, pH ~ 7.4) solutions. Emission spectra of both complexes are dominated by 3MLCT states at room temperature in ethanolic solution, but at 77K the Ir-q exhibits the 3LC as the dominant emission state. The Ir-q complex shows one of the highest emission quantum yields, 11.5%, for a red emitter based on iridium(III) complexes in aerated PBS solution, with color coordinates (x;y) of (0.712;0.286). Moreover, both complexes display high potential to be used as a biological marker with excitation wavelengths above 400nm, high water solubility (Ir-p 1838μmol L-1, Ir-q 7601μmol L-1), and distinct emission wavelengths from the biological autofluorescence. Their cytotoxicity was analyzed in CHO-k1 cells by MTT assays, and the IC50 was estimated as being higher than 131μmol L-1 for Ir-p, and higher than 116μmol L-1 for Ir-q. Concentrations above 70% of viability were used to perform cell imaging by confocal and fluorescence microscopies and the results suggest that the complexes were internalized by the cell membrane and they are staining the cytoplasm region.

Triplet‐Triplet Annihilation Upconversion from Ru(II) Phenanthroline Complexes and 2‐Substituted Anthracene Derivatives

AbstractTwo Ru(II) phenanthroline complexes (Ru‐ph and Ru‐py) were synthesized and their photophysical properties were systematically investigated. For Ru‐py, the conjugation of pyrene at 3‐ and 8‐positions of auxiliary ligand 1,10‐phenanthroline can increase the phosphorescence quantum yield, prolong the triplet lifetime and reduce the triplet energy level. The extended triplet lifetime of Ru‐py was revealed by using the density function theory calculations, which is originated from the triplet state equilibrium between the metal‐to‐ligand charge‐transfer (3MLCT) state and the intraligand (3IL) state. The upconversion fluorescence of 2‐substituted anthracene derivatives (DTACl, DTACN and DNACN) sensitized by Ru‐ph and Ru‐py in DMF was investigated under monochromatic 488 nm excitation with an ultralow power density of 40 mW/cm2. Among them, Ru‐py/DTACl presents the highest upconversion quantum yield (7.4 %). It is found that a small energy gap between the triplet states of sensitizer and emitter helps to increase upconversion quantum yield besides the high triplet‐triplet annihilation efficiency and fluorescence radiation of emitter and the long triplet lifetime of sensitizer.

A Photoreactive Iron(II) Complex Luminophore.

Controlling the order and lifetimes of electronically excited states is essential to effective light-to-potential energy conversion by molecular chromophores. This work reports a luminescent and photoreactive iron(II) complex, the first performant group homologue of prototypical sensitizers of ruthenium. Double cyclometalation of a phenylphenanthroline framework at iron(II) favors the population of a triplet metal-to-ligand charge transfer (3MLCT) state as the lowest energy excited state. Near-infrared (NIR) luminescence exhibits a monoexponential decay with τ = 2.4 ns in the solid state and 1 ns in liquid phase. Lifetimes of 14 ns at 77 K are in line with a narrowing of the NIR emission band at λem,max = 1170-1230 nm. Featuring a 3MLCT excited-state redox potential of -2 V vs the ferrocene/ferrocenium couple, the use of the Fe(II) chromophore as a sensitizer for light-driven synthesis is exemplified by the radical cross-coupling of 4-chlorobromobenzene and benzene.

Deciphering the Ligand's geometric effect on the photophysical properties of osmium complex and its application in triplet-triplet annihilation upconversion

Osmium complex, which featured with direct S0→T1 absorption, are of particular interest in several photochemical applications including triplet-triplet annihilation (TTA) upconversion but less investigated as compared to Ru(II) analogues and the structural correlation to the photophysical properties, especially intersystem crossing (ISC) efficiency is still ambiguous. Herein, a series of Os(II) complexes (Os-1, Os-2, Os-3 and Os-4) bearing different ligands (bipyridine, terpyridine and 2,6-bis(8′-quinolinyl)-pyridine) have been prepared to explore the effect of structural variation/geometry on the photophysical properties. The photophysics of current Os(II) complexes were studied using steady-state as well as transient absorption spectroscopy and theoretical computations. Os-2 complex, bearing the distorted octahedral geometry, showed the higher singlet oxygen sensitizing ability (ФΔ = 68%) and longer triplet lifetime (τT = 281 ns) compared to Os-3 which exhibited the fastest triplet decay (τT = 57 ns), although it is with close-to-perfect octahedral coordination geometry. The large difference in excited-state features of Os(II) complexes is attributed to a strong influence of steric interactions on the ligand field strength, which in turn affects the energy gap between 3MLCT and 3MC states. Our results show that, in contrast to Ru(II) analogues, the perfect octahedral coordination geometry in Os(II) complexes leads to a decrease in the energy gap between 3MLCT and 3MC by destabilizing the 3MLCT energy, which resulted in drastically different photophysical properties. Finally, Os(II) complexes with different geometric features were implemented in triplet-triplet annihilation upconversion and a moderate upconversion quantum yield of 1.4% was observed. This study will be helpful for the future designing of osmium derived triplet photosensitizers.

Comprehensive Picture of the Excited State Dynamics of Cu(I)- and Ru(II)-Based Photosensitizers with Long-Lived Triplet States.

Ru(II)- and Cu(I)-based photosensitizers featuring the recently developed biipo ligand (16H-benzo-[4',5']-isoquinolino-[2',1',:1,2]-imidazo-[4,5-f]-[1,10]-phenanthrolin-16-one) were comprehensively investigated by X-ray crystallography, electrochemistry, and especially several time-resolved spectroscopic methods covering all time scales from femto- to milliseconds. The analysis of the experimental results is supported by density functional theory (DFT) calculations. The biipo ligand consists of a coordinating 1,10-phenanthroline moiety fused with a 1,8-naphthalimide unit, which results in an extended π-system with an incorporated electron acceptor moiety. In a previous study, it was shown that this ligand enabled a Ru(II) complex that is an efficient singlet oxygen producer and of potential use for other light-driven applications due to its long emission lifetime. The goal of our here presented research is to provide a full spectroscopic picture of the processes that follow optical excitation. Interestingly, the Ru(II) and Cu(I) complexes differ in their characteristics even though the lowest electronically excited states involve in both cases the biipo ligand. The combined spectroscopic results indicate that an emissive 3MLCT state and a rather dark 3LC state are populated, each to some extent. For the Cu(I) complex, most of the excited population ends up in the 3LC state with an extraordinary lifetime of 439 μs in the solid state at 20 K, while a significant population of the 3MLCT state causes luminescence for the Ru(II) complex. Hence, there is a balance between these two states, which can be tuned by altering the metal center or even by thermal energy, as suggested by the temperature-dependent experiments.

Ruthenium(II) Polypyridine Complex-Based Aggregation-Induced Emission Luminogen for Rapid and Selective Detection of Phosgene in Solution and in the Gas Phase.

A bis-heteroleptic ruthenium(II) complex, Ru-1, of 4,7-bis(2-aminoethylamino)-1,10-phenanthroline for selective "turn-on" detection of highly toxic chemical warfare agent phosgene is presented. Probe Ru-1 exhibits aggregation-induced emission (AIE), and the restricted intramolecular motion is responsible for the AIE activity. In a CHCl3/CH3CN [95:5 (v/v)] solvent mixture, a unique self-assembled vesicular structure was formed after aggregation, which was supported by transmission electron microscopy, field emission scanning electron microscopy, and atmoic force microscopy studies. Probe Ru-1 showed a rapid and highly selective luminescence turn-on response for phosgene over other competitive chemical warfare agents with a low detection limit (13.9 nM) in CH3CN. The 2-aminoethylamino groups in Ru-1 act as a reacting site for nucleophilic addition to the carbonyl center of phosgene and undergo intramolecular cyclization. The final product of the phosgene-mediated reaction, Ru-1-Phos, contains 2-imidazolidinone groups, which has been confirmed by electrospray ionization mass spectometry and 1H nuclear magnetic resonance (NMR) spectroscopy. 1H NMR titration of Ru-1 with phosgene supported the reaction mechanism and also pointed to the simultaneous reaction of phosgene at two 2-aminoethylamino sites. For the first time, the crystal structure of the phosgene reaction product, Ru-1-Phos, containing the cyclized 2-imidazolidinone group was confirmed by single-crystal X-ray diffraction, which indubitably validates the reaction mechanism. Triplet state time-dependent density functional theory calculations showed that the weak luminescence of Ru-1 was mostly due to the population of the non-emissive 3MC state. The cyclization reaction with phosgene and the corresponding 2-imidazolidinone product formation populated the emissive 3MLCT state in Ru-1-Phos and is the key reason for the enhanced luminescence. Furthermore, a low-cost portable test paper strip has been fabricated with Ru-1 for the real-time selective monitoring of phosgene gas at the nanomolar level.

Cationic Ir(III) Complexes Featuring Phenylimidazole-type Cyclometalated Ligands: Fluorine-Free Blue Phosphorescent Emitters for Light-Emitting Devices.

The development of blue emissive cationic Ir(III) complexes with no fluorine substitutions but with sufficient blue color purity and high phosphorescence efficiency has remained challenging. Here, fluorine-free cyan to deep blue emissive cationic Ir(III) complexes with phenylimidazole-type cyclometalated ligands (C∧N) are reported, which are [Ir(dphim)2(dmapzpy)]PF6 (1), [Ir(ipr-dphim)2(dmapzpy)]PF6 (2), [Ir(ipr-dphim)2(bipz)]PF6 (3), and [Ir(ipr-dphim)2(bicb)]PF6 (4). 1,2-Diphenyl-1H-imidazole (dphim) and 1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole (ipr-dphim) are the phenylimidazole-type C∧N ligands, and 4-dimethylamino-2-(1H-pyrazol-1-yl)pyridine (dmapzpy), di(1H-pyrazol-1-yl)methane (bipz), and 3,3'-methylenebis(1-methyl-1H-imidazol-3-ium-2-ide) (bicb) are the neutral ancillary ligands (A∧A). In both solution and diluted films, complex 1 shows a cyan emission with the emission maxima at ∼472 and 495 nm, and complexes 2-4 provide a deep blue emission with the emission maxima at ∼460 and 480 nm. While the complexes exhibit low to moderate phosphorescence efficiencies (0.05-0.35) in a degassed CH3CN solution, they exhibit high phosphorescence efficiencies (up to 0.82) in diluted films. Theoretical calculations revealed that the mixed 3π-π* (C∧N-centered)/3MLCT (Ir → C∧N) states are responsible for the emission afforded by complexes 1-4, which undergo nonradiative deactivations induced by different types of metal-centered states. Organic light-emitting diodes with complexes 1-4 as phosphorescent dopants are fabricated by a solution process, which affords a blue-green to blue emission with the emission maxima at ∼460 and 490 nm for the blue devices and a high current efficiency at 28.1 cd A-1 for the blue-green device. Solid-state light-emitting electrochemical cells are also fabricated with complexes 1-2 as phosphorescent dopants, which provide green-blue to blue emission with a high luminance (up to 840 cd m-2) and current-efficiency (up to 16.8 cd A-1) under a constant-current driving. The work reveals that, by using phenylimidazole-type C∧N ligands and optimized A∧A ligands, blue emissive cationic Ir(III) complexes with no fluorine substitutions but with sufficient blue-color purity and a high phosphorescence efficiency can be developed.