3MC State Research Articles

An Increase in the Rigidity of the Environment Favors MLCT over the MC State in [Ru(bpy)2(Nicotine)2](Cl)2: A Case Study of Photolabile Ligands.

Ru(II)-complexes with photolabile ligands find a wide range of applications, e.g., in drug release and in the design of light-responsive interfaces. While light-driven ligand loss has been studied mechanistically in detail for complexes in solution, comparably few studies are present that investigate the process in a material context, i.e., in a rigid environment and in the absence of solvent. This paper adds to this underrepresented perspective by studying the excited-state dynamics of [Ru(bpy)2(nicotine)2] (Cl)2 (Ru-nico) as a model system in poly(methyl methacrylate) (PMMA) and polyacrylonitrile (PAN) matrices. Femtosecond transient absorption spectroscopy and time-resolved emission spectroscopy are employed to monitor the photodissociation of labile nicotine ligands in polymer environments. Photoexcitation within the metal-to-ligand charge transfer (MLCT) band leads to transient dissociation of the nicotine ligand when the complex is dissolved in water. However, optical excitation of the 1MLCT transition of the complexes embedded in polymer matrices does not result in photodissociation, likely due to the rigidity of the environment, which cannot solvate the undercoordinated complex after ligand dissociation and the dissociated ligand. These insights shed light on the role of the local environment when considering the photophysics of ligand loss from Ru(II)-polypyridyl complexes and, hence, their use in the light-activation of reactive molecular components in materials.

Isostructural Series of a Cyclometalated Iron Complex in Three Oxidation States.

An isostructural series of FeII, FeIII, and FeIV complexes [Fe(ImP)2]0/+/2+ utilizing the ImP 1,1'-(1,3-phenylene)bis(3-methyl-1-imidazol-2-ylidene) ligand, combining N-heterocyclic carbenes and cyclometalating functions, is presented. The strong donor motif stabilizes the high-valent FeIV oxidation state yet keeps the FeII oxidation state accessible from the parent FeIII compound. Chemical oxidation of [Fe(ImP)2]+ yields stable [FeIV(ImP)2]2+. In contrast, [FeII(ImP)2]0, obtained by reduction, is highly sensitive toward oxygen. Exhaustive ground state characterization by single-crystal X-ray diffraction, 1H NMR, Mössbauer spectroscopy, temperature-dependent magnetic measurements, a combination of X-ray absorption near edge structure and valence-to-core, as well as core-to-core X-ray emission spectroscopy, complemented by detailed density functional theory (DFT) analysis, reveals that the three complexes [Fe(ImP)2]0/+/2+ can be unequivocally attributed to low-spin d6, d5, and d4 complexes. The excited state landscape of the FeII and FeIV complexes is characterized by short-lived 3MLCT and 3LMCT states, with lifetimes of 5.1 and 1.4 ps, respectively. In the FeII-compound, an energetically low-lying MC state leads to fast deactivation of the MLCT state. The distorted square-pyramidal state, where one carbene is dissociated, can not only relax into the ground state, but also into a singlet dissociated state. Its formation was investigated with time-dependent optical spectroscopy, while insights into its structure were gained by NMR spectroscopy.

A Tetracarbene Iron(II) Complex with a Long‐lived Triplet Metal‐to‐ligand Charge Transfer State due to a Triplet‐Triplet Barrier

Mixed N‐heterocyclic carbene (NHC) / pyridyl iron(II) complexes have attracted a great deal of attention recently because of their potential as photocatalysts and light sensitizers made from Earth‐abundant elements. The most decisive challenge for their successful implementation is the lifetime of the lowest triplet metal‐to‐ligand charge transfer state (3MLCT), which typically decays via a triplet metal‐centered (3MC) state back to the ground state. We reveal by variable‐temperature ultrafast transient absorption spectroscopy that the tripodal iron(II) bis(pyridine) complex isomers trans‐ and cis‐[Fe(pdmi)2]2+with four NHC donors show 3MLCT→3MC population transfers with very different barriers and rationalize this by computational means. While trans‐[Fe(pdmi)2]2+possesses an unobservable activation barrier, the cis isomer exhibits a barrier of 492 cm–1, which leads to a nanosecond 3MLCT lifetime at 77 K. The kinetic and quantum chemical data were analyzed in the context of semi‐classical Marcus theory revealing a high reorganization energy and small electronic coupling between the two triplet states. This highlights the importance of detailed structural control and kinetic knowledge for the rational design of photosensitizers from first row transition metals such as iron.

A Tetracarbene Iron(II) Complex with a Long-lived Triplet Metal-to-Ligand Charge Transfer State due to a Triplet-Triplet Barrier.

Mixed N-heterocyclic carbene (NHC) / pyridyl iron(II) complexes have attracted a great deal of attention recently because of their potential as photocatalysts and light sensitizers made from Earth-abundant elements. The most decisive challenge for their successful implementation is the lifetime of the lowest triplet metal-to-ligand charge transfer state (3MLCT), which typically decays via a triplet metal-centered (3MC) state back to the ground state. We reveal by variable-temperature ultrafast transient absorption spectroscopy that the tripodal iron(II) bis(pyridine) complex isomers trans- and cis-[Fe(pdmi)2]2+ with four NHC donors show 3MLCT→3MC population transfers with very different barriers and rationalize this by computational means. While trans-[Fe(pdmi)2]2+ possesses an unobservable activation barrier, the cis isomer exhibits a barrier of 492 cm-1, which leads to a nanosecond 3MLCT lifetime at 77 K. The kinetic and quantum chemical data were analyzed in the context of semi-classical Marcus theory revealing a high reorganization energy and small electronic coupling between the two triplet states. This highlights the importance of detailed structural control and kinetic knowledge for the rational design of photosensitizers from first row transition metals such as iron.

Photophysical Studies of Helicate and Mesocate Double-Stranded Dinuclear Ru(II) Complexes.

The metal-ligand charge transfer (3MLCT) and phosphorescence-quenching metal-centered (3MC) states of the helicate and mesocate diastereoisomers of a double-stranded dinuclear polypyridylruthenium(II) complex have been investigated using ultrafast transient absorption spectroscopy. At 294 K, transient signals of the helicate decayed significantly slower than those of the mesocate, whereas at 77 K, no clear contrast in kinetics was observed. Contributions to excited-state decay from high-lying 3MLCT states were identified at both temperatures. Spectroscopic data (294 K) suggest that the 3MC state of the helicate lies above the 3MLCT and that the reverse is true for the mesocate; this was further validated by density functional theory calculations. The stabilization of the 3MC state relative to the 3MLCT state in the mesocate was explained by a reduction in ligand field strength due to distortion near the ligand bridge, which causes further deviation from octahedral geometry compared to the helicate. This work illustrates how minor structural differences can significantly influence excited state dynamics.

Controlling the Photophysical Properties of a Series of Isostructural d6 Complexes Based on Cr0, MnI, and FeII.

Development of first-row transition metal complexes with similar luminescence and photoredox properties as widely used RuII polypyridines is attractive because metals from the first transition series are comparatively abundant and inexpensive. The weaker ligand field experienced by the valence d-electrons of first-row transition metals challenges the installation of the same types of metal-to-ligand charge transfer (MLCT) excited states as in precious metal complexes, due to rapid population of energetically lower-lying metal-centered (MC) states. In a family of isostructural tris(diisocyanide) complexes of the 3d6 metals Cr0, MnI, and FeII, the increasing effective nuclear charge and ligand field strength allow us to control the energetic order between the 3MLCT and 3MC states, whereas pyrene decoration of the isocyanide ligand framework provides control over intraligand (ILPyr) states. The chromium(0) complex shows red 3MLCT phosphorescence because all other excited states are higher in energy. In the manganese(I) complex, a microsecond-lived dark 3ILPyr state, reminiscent of the types of electronic states encountered in many polyaromatic hydrocarbon compounds, is the lowest and becomes photoactive. In the iron(II) complex, the lowest MLCT state has shifted to so much higher energy that 1ILPyr fluorescence occurs, in parallel to other excited-state deactivation pathways. Our combined synthetic-spectroscopic-theoretical study provides unprecedented insights into how effective nuclear charge, ligand field strength, and ligand π-conjugation affect the energetic order between MLCT and ligand-based excited states, and under what circumstances these individual states become luminescent and exploitable in photochemistry. Such insights are the key to further developments of luminescent and photoredox-active first-row transition metal complexes.

Test of the orbital-based LI3 index as a predictor of the height of the [formula omitted]MLCT [formula omitted][formula omitted]MC transition-state barrier for gas-phase [Ru(N[formula omitted]N)[formula omitted]][formula omitted] polypyridine complexes

Ruthenium(II) polypyridine compounds often luminesce, with ligand-dependant lifetimes. This is the third in a series of papers devoted to finding orbital-based luminescence indices (LI) for predicting luminescence lifetimes. The third attempt (LI3) was based upon a frontier molecular orbital (FMO) like estimate of the height of the barrier to conversion from an initial phosophorescent triplet metal-ligand charge transfer (3MLCT) state to a nonluminescent triplet metal-centered (3MC) state which decays nonradiatively. A linear correlation was found between LI3 and extracted empirical average 3MLCT →3MC transition state (TS) barrier heights (Eaves) which were not believed to be quantitative but which were believed to capture the trends in the true barrier heights correctly. As it is known that Eave is a large underestimate of the true 3MLCT →3MC TS barrier height in the case of the trisbipyridine ruthenium(II) cation { [Ru(bpy)3]2+ }, but accurate TS barrier heights are difficult to obtain experimentally, it was judged useful to verify the ideas used to derive the LI3 index in the present work by calculating the energetics of the gas-phase 3MLCT →3MC reaction for a series of ruthenium(II) tris bipyridine complexes using the same density functional and basis sets used in calculating LI3. Specifically, four closely-related bipyridine complexes { [Ru(N∧N)3]2+ with N∧N = bpy (6), 4,4’-dm-bpy (70), 4,4’-dph-bpy (73), and 4,4’-DTB-bpy (74) } were used for these calculations. In the process, we examined the gas-phase trans dissociation mechanism in greater detail than has been previously done and uncovered a two part mechanism. In the first part, the electron is transferred to a single ligand rather than symmetrically to all three ligands. It is the two Ru-N bonds to this ligand which are equally elongated in the transition state. The intrinsic reaction coordinate then continues down a ridge in hyperspace and bifurcates into one of two symmetry-equivalent 3MC structures with elongated trans bonds. Interestingly, no significant difference is found for the TS barriers for the four complexes treated here. Instead, LI3 is linearly correlated with the energy difference ΔE=E(3MLCT)−E(3MC), different from what was originally intended but still consistent with the FMO arguments underlying the derivation of LI3.

Computational Modelling and Mechanistic Insight into Light‐Driven CO Dissociation of Square‐Planar Rhodium(I) Complexes

AbstractThe activation step of Vaska‐type Rh(I) complexes, such as the photocleavage of the Rh−CO bond, plays an important role in the subsequent C−H activation. To elucidate the details of the photochemistry of Vaska‐type Rh(I) complexes, such as trans‐Rh(PMe3)2(CO)(Cl), we here present a computationally derived picture as obtained at the density functional level of theory in combination with multireference wavefunction‐based methods. We have identified that the photocleavage of CO proceeds via the metal‐centered excited state (3MC, ), which is populated through intersystem crossing from the dipole‐allowed excited state S1( ‐ ). Moreover, the present study unraveled the reasons for the low C−H activation efficiency when using Rh featuring the bidentate ligand 1,2‐bis(dimethylphosphino)ethane (dmpe), namely due to its unfavorable photochemical properties, i. e., the small driving force for light‐induced CO loss and the fast deactivation of 3MC state back to the singlet ground state. In this study, we provide theoretical insight into mechanistic details underlying the light‐induced CO dissociation process, for Rh complexes featuring PMe3 and dmpe ligands.

Nickel(II) Analogues of Phosphorescent Platinum(II) Complexes with Picosecond Excited-State Decay.

Square-planar Ni(II) complexes are interesting as cheaper and more sustainable alternatives to Pt(II) luminophores widely used in lighting and photocatalysis. We investigated the excited-state behavior of two Ni(II) complexes, which are isostructural with two luminescent Pt(II) complexes. The initially excited singlet metal-to-ligand charge transfer (1MLCT) excited states in the Ni(II) complexes decay to metal-centered (3MC) excited states within less than 1 picosecond, followed by non-radiative relaxation of the 3MC states to the electronic ground state within 9 - 21 ps. This contrasts with the population of an emissive triplet ligand-centered (3LC) excited state upon excitation of the Pt(II) analogues. Structural distortions of the Ni(II) complexes are responsible for this discrepant behavior and lead to dark 3MC states far lower in energy than the luminescent 3LC states of Pt(II) compounds. Our findings suggest that if these structural distortions could be restricted by more rigid coordination environments and stronger ligand fields, the excited-state relaxation in four-coordinate Ni(II) complexes could be decelerated such that luminescent 3LC or 3MLCT excited states become accessible. These insights are relevant to make Ni(II) fit for photophysical and photochemical applications that relied on Pt(II) until now.

Nickel(II) Analogues of Phosphorescent Platinum(II) Complexes with Picosecond Excited‐State Decay

AbstractSquare‐planar NiII complexes are interesting as cheaper and more sustainable alternatives to PtII luminophores widely used in lighting and photocatalysis. We investigated the excited‐state behavior of two NiII complexes, which are isostructural with two luminescent PtII complexes. The initially excited singlet metal‐to‐ligand charge transfer (1MLCT) excited states in the NiII complexes decay to metal‐centered (3MC) excited states within less than 1 picosecond, followed by non‐radiative relaxation of the 3MC states to the electronic ground state within 9–21 ps. This contrasts with the population of an emissive triplet ligand‐centered (3LC) excited state upon excitation of the PtII analogues. Structural distortions of the NiII complexes are responsible for this discrepant behavior and lead to dark 3MC states far lower in energy than the luminescent 3LC states of PtII compounds. Our findings suggest that if these structural distortions could be restricted by more rigid coordination environments and stronger ligand fields, the excited‐state relaxation in four‐coordinate NiII complexes could be decelerated such that luminescent 3LC or 3MLCT excited states become accessible. These insights are relevant to make NiII fit for photophysical and photochemical applications that relied on PtII until now.

Mehrere Triplett-Metall-zentrierte Jahn-Teller-Isomere bestimmen die temperaturabhängigen Lumineszenzlebensdauern in [Ru(bpy)3]2.

AbstractEin genaues Verständnis der Faktoren, welche die Lumineszenzlebensdauer von Übergangsmetallverbindungen bestimmen, ist für Anwendungen in der Photokatalyse und der photodynamischen Therapie von entscheidender Bedeutung. Die im Falle von [Ru(bpy)3]2+ (bpy=2,2’‐Bipyridin) allgemein akzeptierte Theorie besagt, dass die Emissionslebensdauer durch Optimierung der Energiebarriere zwischen dem emittierenden Triplett‐Zustand des Metall‐Liganden‐Ladungstransfers (3MLCT) und dem thermisch aktivierten Triplett‐Zustand des Metall‐Zentrums (3MC), oder der Energielücke zwischen beiden Zuständen gesteuert werden kann. Hier zeigen wir, dass dies nicht allgemeingültig ist. Darüber hinaus demonstrieren wir, dass die Betrachtung eines einzelnen Relaxationspfades, der vom energetisch niedrigsten Minimum aus bestimmt wird, zu falschen Vorhersagen der temperaturabhängigen Emissionslebensdauer führt. Stattdessen erhalten wir eine ausgezeichnete Übereinstimmung mit den experimentellen temperaturabhängigen Lebensdauern, wenn ein erweitertes kinetisches Modell herangezogen wird, welches alle Pfade im Zusammenhang mit mehreren Jahn–Teller‐Isomeren und ihren effektiven Reaktionsbarrieren beinhaltet. Diese Konzepte sind für das Design weiterer lumineszierender Übergangsmetallkomplexe mit individuell angepassten Emissionslebensdauern auf der Grundlage theoretischer Vorhersagen unerlässlich.

Unraveling the Marked Differences of the Excited-State Properties of Arylgold(III) Complexes with C∧N∧C Tridentate Ligands.

Three structurally similar gold(III) complexes with C∧N∧C tridentate ligands, [1; C∧N∧C = 2,6-diphenylpyridine], [2; C∧N∧C = 2,6-diphenylpyrazine], and [3; C∧N∧C = 2,6-diphenyltriazine], have been investigated theoretically to rationalize the marked difference in emission behaviors. The geometrical and electronic structures, spectra properties, radiative and nonradiative decay processes, as well as reverse intersystem crossing and reverse internal conversion (RIC) processes were thoroughly analyzed using density functional theory (DFT) and time-dependent DFT calculations. The computed results indicate that there is a small energy difference between the lowest-energy triplet state (T1) and the second lowest-energy triplet state (T1') of complexes 2 and 3, suggesting that the excitons in the T1 state can reach the emissive higher-energy T1' through the RIC process. In addition, the non-emissive T1 states of gold(III) complexes in solution can be ascribed to the easily accessible metal-centered (3MC) state or possibly tunneling into high-energy vibrationally excited singlet states for nonradiative decay. The low efficiency of 3 is attributed to the deactivation pathway via the 3MC state. The present study elucidates the relationship between structure and property of gold(III) complexes featuring C∧N∧C ligands and providing a comprehensive understanding of the significant differences in their luminescence behaviors.

A theoretical study on the electronic structures and phosphorescence properties of light-emitting electrochemical cell champions reported in early 2022

The electronic structure, absorption properties, phosphorescence quantum efficiency and emission spectra, of a series of iridium(III) complexes containing phenyltriazole-type cyclometallated ligands (1–4) were studied using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The simulated electronic/photophysical properties of the iridium complexes are in good agreement with the experimental observations. The results of the calculations show that the lowest lying singlet absorptions for 1–4 are located at 3.83, 3.91, 3.01, and 2.98 eV, respectively. All complexes show emissions characterised as a mixture of triplet ligand-to-ligand charge transfer (3LLCT) and metal-to-ligand charge transfer (3MLCT) states. The fluorination of the attached phenyl groups (complex 2) and the introduction of –CF3 moieties (complex 3) in the phenyltriazole cyclometallated ligands results in a strong interaction in the T1 states, due to the contracted Ir–Ns bonds with the bpy ancillary ligand. Therefore, both complexes show a moderate contribution of the metal character (%Mc), a lower energy gap between the T1 and the S1 states (ΔET1-S1) together with the largest transition dipole moment (μS1), resulting in an enhanced phosphorescence quantum efficiency. The introduction of –CH3 groups (complex 4) in the phenyltriazole cyclometallated ligands stabilises the triplet metal-centred (3MC) state, resulting in a fast nonradiative decay along with a lower quantum yield. The vertical emission energies calculated at the B3LYP level for the 1–4 complexes are located at 2.60, 2.38, 2.68, and 2.62 eV, respectively. Based on these results, the theoretical calculations can provide an accurate prediction of the electronic and the photophysical properties of iridium complexes, and then can be used to guide the synthesis of new phosphorescent materials.

Low-Temperature Observation of the Excited-State Decay of Ruthenium-(Mono-2,2':6',2″-Terpyridine) Ions with Innocent Ligands: DFT Modeling of an 3MLCT-3MC Intersystem Crossing Pathway.

The synthesis, electrochemistry, and photophysical characterization of five 2,2':6',2″-terpyridine ruthenium complexes (Ru-tpy complexes) is reported. The electrochemical and photophysical behavior varied depending on the ligands, i.e., amine (NH3), acetonitrile (AN), and bis(pyrazolyl)methane (bpm), for this series of Ru-tpy complexes. The target [Ru(tpy)(AN)3]2+ and [Ru(tpy)(bpm)(AN)]2+ complexes were found to have low-emission quantum yields in low-temperature observations. To better understand this phenomenon, density functional theory (DFT) calculations were performed to simulate the singlet ground state (S0), Te, and metal-centered excited states (3MC) of these complexes. The calculated energy barriers between Te and the low-lying 3MC state for [Ru(tpy)(AN)3]2+ and [Ru(tpy)(bpm)(AN)]2+ provided clear evidence in support of their emitting state decay behavior. Developing a knowledge of the underlying photophysics of these Ru-tpy complexes will allow new complexes to be designed for use in photophysical and photochemical applications in the future.

Cover Feature: Plugging the 3MC Sink in RuII‐Based Photocatalysts (ChemCatChem 4/2023)

The Cover Feature shows a representation of a multi-step electron transfer process in a hydrogen generating, RuII intramolecular photocatalyst. A prominent deactivation pathway can occur via 3MC state population on the photocentre, resulting in undesired charge recombination. In their Research Article, S. Kupfer and co-workers use quantum chemical methods to demonstrate that coordination of biimidazole based co-ligands around the RuII photocentre centre can direct the initial photoexcitation exclusively towards the catalytic bridge. Furthermore, in the triplet manifold, 3MC population from the 3MLCT state is also reduced. These ligand motifs thereby provide a potential means to control initial photoexcitation and to reduce a possible charge-recombination pathway in photoactive transition metal complexes. More information can be found in the Research Article by S. Kupfer and co-workers.

Plugging the 3MC Sink in RuII‐Based Photocatalysts

AbstractQuantum chemical methods have been utilised to explore the kinetics and thermodynamics of a prominent charge recombination pathway in a series of RuII based molecular photocatalysts. Selective tuning of the RuII coordination sphere, replacing the tbbpy ligands of the hydrogen evolving parent photocatalyst with electron rich, biimidazole based ligands, promotes unidirectional charge transfer towards the bridging ligand during initial photoexcitation. These electronic effects are also significant in the triplet manifold, where the predicted rate of the undesired deactivation process from the 3MLCT state on the bridging ligand to a 3MC state on the ruthenium centre, is decreased relative to the parent complex, by 1–2 orders of magnitude, alongside a decrease in electronic coupling. This design methodology could be utilised to promote targeted (light‐driven) electron transfer pathways, as well as to potentially reduce 3MC deactivation pathways in commonly used polypyridyl‐based RuII photocentres, thus enhancing the quantum efficiency of light driven catalysis.

Synthesis and Theoretical and Photophysical Study on a Series of Neutral Ruthenium(II) Complexes with Donor–Metal–Accepter Configuration

In order to construct a new type of ruthenium(II) terpyridine complexes with activated triplet metal-centered (3MC) states, as well as stabilized triplet metal-to-ligand charge transfer (3MLCT) states, conducive to fine emissive performances, Ru-1, Ru-2, Ru-3, and Ru-4 were synthesized. Compared with the [Ru(terpyridine)2]2+ prototype (0.25 ns), this series of ruthenium(II) terpyridine complexes exhibit lengthened excited state lifetime (43.3 ns for Ru-1, 52.7 ns for Ru-2, 43.6 ns for Ru-3, and 53.4 ns for Ru-4). Interfragment charge transfer analysis illustrates the electron transfer direction of the four complexes, manifesting their intramolecular charge transfer characteristic. When excited, their lowest-lying triplet states are assigned as 3MLCT based on spin-density surface distribution. The singlet excited states and 3MLCT states were thoroughly studied by UV-visual absorption and nanosecond time-resolved transient absorption spectra, respectively. Photoluminescence spectra revealed their weak broadband near-infrared emission at room temperature and red phosphorescence at 77 K. The low molecular weight and the good thermal stability make Ru-1 and Ru-2 suitable for vaporization coating, while the fine solubility in common organic solvents makes Ru-3 and Ru-4 suitable for solution processing. Furthermore, the intrinsic electroneutrality and favorable energy levels endow them with new potential to be applied in the optoelectronic field.

Branching mechanism of photoswitching in an Fe(II) polypyridyl complex explained by full singlet-triplet-quintet dynamics

It has long been known that irradiation with visible light converts Fe(II) polypyridines from their low-spin (singlet) to high-spin (quintet) state, yet mechanistic interpretation of the photorelaxation remains controversial. Herein, we simulate the full singlet-triplet-quintet dynamics of the [Fe(terpy)2]2+ (terpy = 2,2’:6’,2”-terpyridine) complex in full dimension, in order to clarify the complex photodynamics. Importantly, we report a branching mechanism involving two sequential processes: a dominant 3MLCT→3MC(3T2g)→3MC(3T1g)→5MC, and a minor 3MLCT→3MC(3T2g)→5MC component. (MLCT = metal-to-ligand charge transfer, MC = metal-centered). While the direct 3MLCT→5MC mechanism is considered as a relevant alternative, we show that it could only be operative, and thus lead to competing pathways, in the absence of 3MC states. The quintet state is populated on the sub-picosecond timescale involving non-exponential dynamics and coherent Fe-N breathing oscillations. The results are in agreement with the available time-resolved experimental data on Fe(II) polypyridines, and fully describe the photorelaxation dynamics.

Efficient blue-phosphorescent trans-bis(acyclic diaminocarbene) platinum(ii) acetylide complexes.

The lack of efficient and robust deep-blue phosphorescent metal complexes remains a significant challenge in the context of electroluminescent color displays. The emissive triplet states of blue phosphors are deactivated by low-lying metal-centered (3MC) states, which can be ameliorated by increasing the σ-donating ability of the supporting ligands. Here we unveil a synthetic strategy to access blue-phosphorescent complexes with two supporting acyclic diaminocarbenes (ADCs), known to be even stronger σ-donors than N-heterocyclic carbenes (NHCs). This new class of platinum complexes has excellent photoluminescence quantum yields, with four of six complexes affording deep-blue emission. Experimental and computational analyses are consistent with a pronounced destabilization of the 3MC states by the ADCs.

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.