Chiral Molecular Ruby [Cr(dqp)2]3+ with Long-Lived Circularly Polarized Luminescence

Optical Property Control by the Interligand Charge Transfer Excited State in Brominated Homoleptic and Heteroleptic Aluminum Dinuclear Triple-Stranded Helicates

The utilization of aluminum, an abundant and inexpensive element, for the synthesis of novel functional complexes is extremely important, but the design and control of photofunctionality are still unexplored. In this study, we focused on our previously developed dinuclear triple-stranded helicates incorporating two aluminum ions (ALPHY) to synthesize both homoleptic and heteroleptic complexes with bromine atoms at the 3-position of the pyrrole moiety in the Schiff base ligands. The brominated Schiff base ligands were reacted with AlCl3 to synthesize homoleptic complexes, while different ligands were mixed to prepare heteroleptic complexes. Single-crystal X-ray structural analysis revealed the structures of these novel complexes. We found that increasing the degree of bromination resulted in a tunable emission color, shifting progressively from 550 (yellow) to 566 nm (orange). Optical resolution of the complexes facilitated the observation of mirror-image circular dichroism and circularly polarized luminescence. Furthermore, employing ultrafast spectroscopy techniques, we have elucidated that the optical properties are governed by the interligand charge transfer (ILCT) among the three ligands. The formation of heteroleptic complexes induces the ILCT state even in nonpolar environments, thereby accelerating nonradiative decay and intersystem crossing. These findings mark significant advancements in photofunctional materials based on multinuclear complexes.

Ferromagnetically Coupled Chromium(III) Dimer Shows Luminescence and Sensitizes Photon Upconversion

There has been much progress on mononuclear chromium(III) complexes featuring luminescence and photoredox activity, but dinuclear chromium(III) complexes have remained underexplored in these contexts until now. We identified a tridentate chelate ligand able to accommodate both meridional and facial coordination of chromium(III), to either access a mono- or a dinuclear chromium(III) complex depending on reaction conditions. This chelate ligand causes tetragonally distorted primary coordination spheres around chromium(III) in both complexes, entailing comparatively short excited-state lifetimes in the range of 400 to 800 ns in solution at room temperature and making photoluminescence essentially oxygen insensitive. The two chromium(III) ions in the dimer experience ferromagnetic exchange interactions that result in a high spin (S=3) ground state with a coupling constant of +9.3 cm-1. Photoinduced energy transfer from the luminescent ferromagnetically coupled dimer to an anthracene derivative results in sensitized triplet-triplet annihilation upconversion. Based on these proof-of-principle studies, dinuclear chromium(III) complexes seem attractive for the development of fundamentally new types of photophysics and photochemistry enabled by magnetic exchange interactions.

Chiral Tetrakis Eu(III) Complexes with Ammonium Cations for Improved Circularly Polarized Luminescence

Large dissymmetry factor of the circularly polarized luminescence (gCPL) was observed in ligand and coordination tuned chiral tetrakis europium (Eu(III)) complexes with ammonium cations. The gCPL value was estimated to be -1.54, which is the largest among chiral luminescent molecules. Through photophysical measurements, single crystal X-ray structural analyses and quantum chemical calculations, changes in the geometric and electronic structures were observed for a series of chiral tetrakis Eu(III) complexes which enhanced the gCPL value. The emission quantum yield and photosensitized energy transfer efficiencies of chiral Eu(III) complexes with ammonium cations were also larger than those of chiral Eu(III) complex with Cs+. Based on the systematic modifications and analyses for chiral tetrakis Eu(III) complex, effect of the ammonium cation on enhanced CPL brightness is reported.

A new era of LMCT: leveraging ligand-to-metal charge transfer excited states for photochemical reactions

Ligand-to-metal charge transfer (LMCT) excited states are capable of undergoing a wide array of photochemical reactions, yet receive minimal attention compared to other charge transfer excited states. This work provides general criteria for designing transition metal complexes that exhibit low energy LMCT excited states and routes to drive photochemistry from these excited states. General design principles regarding metal identity, oxidation state, geometry, and ligand sets are summarized. Fundamental photoreactions from these states including visible light-induced homolysis, excited state electron transfer, and other photoinduced chemical transformations are discussed and key design principles for enabling these photochemical reactions are further highlighted. Guided by these fundamentals, this review outlines critical considerations for the future design and application of coordination complexes with LMCT excited states.

Linker-dependent control of the chiroptical properties of polymethylene-vaulted trans-bis[(β-iminomethyl)naphthoxy]platinum(II) complexes

The effects of polymethylene bridges on the chiroptical properties of trans-bis[(β-iminomethyl)naphthoxy]platinum(II) platforms were examined both experimentally and theoretically using newly designed planar chiral Pt analogues (1) having three-dimensional superstructures. A series of optically pure polymethylene-vaulted Pt complexes (R)- and (S)-1 were synthesized and characterized with regard to the chiroptical behaviour of the trans-bis[(β-iminomethyl)naphthoxy]platinum(II) platforms. These complexes were found to exhibit structure-dependent chiroptical characteristics in solution, such that the absolute values of specific rotation, the circular dichroism dissymmetry factor (gabs) and the circularly polarized luminescence dissymmetry factor (glum) all increased upon shortening the polymethylene bridges. Density functional theory and time dependent density functional theory calculations were used to analyse vaulted and non-vaulted complexes, which demonstrated that the present linker-dependent chiroptical properties resulted from constraint-induced changes in the square planar Pt coordination centres rather than from chiral distortion along the coordination platforms.

Recent Advances of Chiral Isolated and Small Organic Molecules: Structure and Properties for Circularly Polarized Luminescence

This paper explores recent advancements in the field of circularly polarized luminescence (CPL) exhibited by small and isolated organic molecules. The development and application of small CPL molecule are systematically reviewed through eight different chiral skeleton sections. Investigating the intricate interplay between molecular structure and CPL properties, the paper aims at providing and enlighting novel strategies for CPL-based applications.

Augmentation of NIR Circularly Polarized Luminescence Activity in Shibasaki-Type Lanthanide Complexes Supported by the Spirane Sphenol

We report two new circularly polarized luminescence (CPL)-active lanthanide complexes emissive in the near-infrared (NIR) region; using sphenol as a supporting ligand, we provide the first reported example of an NIR-emissive lanthanide complex supported by a chiral spirane. Inclusion of a quaternary carbon to impart axial chirality results in dramatic augmentation of the CPL strength of the resultant sphenolate complexes (glum ≤ 0.77 for [(sphenol)3ErNa3(thf)6]) compared to that of their contemporary biaryl-based axially chiral analogues (glum ≤ 0.47 for [(binol)3ErNa3(thf)6]). Despite similar structural parameters, the rigid spiro carbon of sphenol enables the strongest dissymmetry factors observed to date from Shibasaki-type complexes for both Yb and Er. We also demonstrate the sensitivity of the reported chiroptical measurements to small variations in instrumental parameters, such as bandpass, and suggest a standardized method or at least that additional detail should be included in future reports to allow for direct comparisons between newly published CPL emitters.

Iron(III) Carbene Complexes with Tunable Excited State Energies for Photoredox and Upconversion

Substituting precious elements in luminophores and photocatalysts by abundant first-row transition metals remains a significant challenge, and iron continues to be particularly attractive owing to its high natural abundance and low cost. Most iron complexes known to date face severe limitations due to undesirably efficient deactivation of luminescent and photoredox-active excited states. Two new iron(III) complexes with structurally simple chelate ligands enable straightforward tuning of ground and excited state properties, contrasting recent examples, in which chemical modification had a minor impact. Crude samples feature two luminescence bands strongly reminiscent of a recent iron(III) complex, in which this observation was attributed to dual luminescence, but in our case, there is clear-cut evidence that the higher-energy luminescence stems from an impurity and only the red photoluminescence from a doublet ligand-to-metal charge transfer (2LMCT) excited state is genuine. Photoinduced oxidative and reductive electron transfer reactions with methyl viologen and 10-methylphenothiazine occur with nearly diffusion-limited kinetics. Photocatalytic reactions not previously reported for this compound class, in particular the C-H arylation of diazonium salts and the aerobic hydroxylation of boronic acids, were achieved with low-energy red light excitation. Doublet-triplet energy transfer (DTET) from the luminescent 2LMCT state to an anthracene annihilator permits the proof of principle for triplet-triplet annihilation upconversion based on a molecular iron photosensitizer. These findings are relevant for the development of iron complexes featuring photophysical and photochemical properties competitive with noble-metal-based compounds.

Additional Insights into the Design of Cr(III) Phosphorescent Emitters Using 6-Membered Chelate Ring Bis(imidazolyl) Didentate Ligands

The interest in Cr(III) complexes has been renewed over the past decades for building practical guidelines in the design of efficient earth-abundant phosphorescent near-infrared emitters. In that context, we report the first family of homoleptic tri(didentate) Cr(III) complexes [CrL3]3+ based on polyaromatic ligands inducing 6-membered chelate rings, namely, the bis(1-methylimidazol-2-yl)ketone (L = bik), bis(1-methylimidazol-2-yl)methane (L = bim), and bis(1-methylimidazol-2-yl)ethane (L = bie) ligands. The programmed close-to-perfect octahedral microsymmetry of {CrIIIN6} chromophores found in [Cr(bik)3](OTf)3 (1), [Cr(bim)3](OTf)3 (2), and [Cr(bie)3](BF4)3 (3) ensures a ligand-field strength large enough to induce intense and long-lived Cr-based phosphorescence. Impressive excited-state lifetimes (5.0-8.2 ms) were obtained at low temperatures for the [Cr(L)3]3+ series. Additionally, the photoluminescent quantum yield climbs to 0.8% for compound 1 in deaerated solutions. Moreover, the photophysical features of the three homoleptic complexes are barely influenced by the presence of dioxygen presumably because of the poor overlap between the Cr-based phosphorescence spectra (ca. 14100 cm-1) and the 1Σg+ ← 3Σg- transition in the absorption spectrum of dioxygen (13100 cm-1). The multiredox electrochemical pattern of 1 is evidenced by cyclic voltammetry as well as its strong photooxidant behavior. The pH sensitivity of 2 and 3 luminescence is discussed, along with the reactivity of their β-diketiminate derivatives.

Luminescence and Excited-State Reactivity in a Heteroleptic Tricyanido Fe(III) Complex

Harnessing sunlight via photosensitizing molecules is key for novel optical applications and solar-to-chemical energy conversion. Exploiting abundant metals such as iron is attractive but becomes challenging due to typically fast nonradiative relaxation processes. In this work, we report on the luminescence and excited-state reactivity of the heteroleptic [FeIII(pzTp)(CN)3]- complex (pzTp = tetrakis(pyrazolyl)borate), which incorporates a σ-donating trispyrazolyl chelate ligand and three monodentate σ-donating and π-accepting cyanide ligands. Contrary to the nonemissive [Fe(CN)6]3-, a broad emission band centered at 600 nm at room temperature has been recorded for the heteroleptic analogue attributed to the radiative deactivation from a 2LMCT excited state with a luminescence quantum yield of 0.02% and a lifetime of 80 ps in chloroform at room temperature. Bimolecular reactivity of the 2LMCT excited state was successfully applied to different alcohol photo-oxidation, identifying a cyanide-H bonding as a key reaction intermediate. Finally, this research demonstrated the exciting potential of [Fe(pzTp)(CN)3]- as a photo-oxidant, paving the way for further exploration and development of emissive Fe-based photosensitizers competent for photochemical transformations.

Improved transition metal photosensitizers to drive advances in photocatalysis

To function effectively in a photocatalytic application, a photosensitizer's light absorption, excited-state lifetime, and redox potentials, both in the ground state and excited state, are critically important. The absorption profile is particularly relevant to applications involving solar harvesting, whereas the redox potentials and excited-state lifetimes determine the thermodynamics, kinetics, and quantum yields of photoinduced redox processes. This perspective article focuses on synthetic inorganic and organometallic approaches to optimize these three characteristics of transition-metal based photosensitizers. We include our own work in these areas, which has focused extensively on exceptionally strong cyclometalated iridium photoreductants that enable challenging reductive photoredox transformations on organic substrates, and more recent work which has led to improved solar harvesting in charge-transfer copper(i) chromophores, an emerging class of earth-abundant compounds particularly relevant to solar-energy applications. We also extensively highlight many other complementary strategies for optimizing these parameters and highlight representative examples from the recent literature. It remains a significant challenge to simultaneously optimize all three of these parameters at once, since improvements in one often come at the detriment of the others. These inherent trade-offs and approaches to obviate or circumvent them are discussed throughout.

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

Square-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 (1 MLCT) excited states in the NiII complexes decay to metal-centered (3 MC) excited states within less than 1 picosecond, followed by non-radiative relaxation of the 3 MC states to the electronic ground state within 9-21 ps. This contrasts with the population of an emissive triplet ligand-centered (3 LC) excited state upon excitation of the PtII analogues. Structural distortions of the NiII complexes are responsible for this discrepant behavior and lead to dark 3 MC states far lower in energy than the luminescent 3 LC 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 3 LC or 3 MLCT excited states become accessible. These insights are relevant to make NiII fit for photophysical and photochemical applications that relied on PtII until now.

Fundamentals, Advances, and Artifacts in Circularly Polarized Luminescence (CPL) Spectroscopy

Objects are chiral when they cannot be superimposed with their mirror image. Materials can emit chiral light with an excess of right- or left-handed circular polarization. This circularly polarized luminescence (CPL) is key to promising future applications, such as highly efficient displays, holography, sensing, enantiospecific discrimination, synthesis of drugs, quantum computing, and cryptography. Here, a practical guide to CPL spectroscopy is provided. First, the fundamentals of the technique are laid out and a detailed account of recent experimental advances to achieve highly sensitive and accurate measurements is given, including all corrections required to obtain reliable results. Then the most common artifacts and pitfalls are discussed, especially for the study of thin films, for example, based on molecules, polymers, or halide perovskites, as opposed to dilute solutions of emitters. To facilitate the adoption by others, custom operating software is made publicly available, equipping the reader with the tools needed for successful and accurate CPL determination.

Enhancing Circularly Polarized Phosphorescence via Integrated Top-Down and Bottom-Up Approach

In the dynamic domain of chiroptical technologies, it is imperative to engineer emitters endowed with circularly polarized luminescence (CPL) properties. This research demonstrates an advancement by employing a combined top-down and bottom-up strategy for the simultaneous amplification of photoluminescence quantum yield (Φ) and the luminescence dissymmetry factor (glum ). Square-planar Pt(II) complexes form helical assemblies, driven by torsional strain induced by bis(nonyl) chains. Integration of chiral anions leads these assemblies to prefer distinct helical sense. This arrangement activates the metal-metal-to-ligand charge transfer (MMLCT) transition that is CPL-active, with Φ and |glum | observing an upswing contingent on the charge number and aryl substituents in chiral anions. Utilizing the soft-lithographic micromolding in capillaries technique, we could fabricate exquisitely-ordered, one-dimensional co-assemblies to achieve the metrics to Φ of 0.32 and |glum | of 0.13. Finally, our spectroscopic research elucidates the underlying mechanism for the dual amplification, making a significant stride in the advancement of CPL-active emitters.

Electronic Structure and Excited-State Dynamics of the NIR-II Emissive Molybdenum(III) Analogue to the Molecular Ruby

Photoactive chromium(III) complexes saw a conceptual breakthrough with the discovery of the prototypical molecular ruby mer-[Cr(ddpd)2]3+ (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine), which shows intense long-lived near-infrared (NIR) phosphorescence from metal-centered spin-flip states. In contrast to the numerous studies on chromium(III) photophysics, only 10 luminescent molybdenum(III) complexes have been reported so far. Here, we present the synthesis and characterization of mer-MoX3(ddpd) (1, X = Cl; 2, X = Br) and cisfac-[Mo(ddpd)2]3+ (cisfac-[3]3+), an isomeric heavy homologue of the prototypical molecular ruby. For cisfac-[3]3+, we found strong zero-field splitting using magnetic susceptibility measurements and electron paramagnetic resonance spectroscopy. Electronic spectra covering the spin-forbidden transitions show that the spin-flip states in mer-1, mer-2, and cisfac-[3]3+ are much lower in energy than those in comparable chromium(III) compounds. While all three complexes show weak spin-flip phosphorescence in NIR-II, the emission of cisfac-[3]3+ peaking at 1550 nm is particularly low in energy. Femtosecond transient absorption spectroscopy reveals a short excited-state lifetime of 1.4 ns, 6 orders of magnitude shorter than that of mer-[Cr(ddpd)2]3+. Using density functional theory and ab initio multireference calculations, we break down the reasons for this disparity and derive principles for the design of future stable photoactive molybdenum(III) complexes.

Towards Fast Circularly Polarized Luminescence in 2-Coordinate Chiral Mechanochromic Copper(I) Carbene Complexes

A series of chiral mechanochromic copper(I) cAAC (cAAC=cyclic (alkyl)(amino)carbene) complexes with a variety of amide ligands have been studied with regard to their photophysical and chiroptical properties to elucidate structure-property relationships for the design of efficient triplet exciton emitters exhibiting circularly polarized luminescence. Depending on the environment, which determines the excited state energies, either thermally activated delayed fluorescence (TADF) from 1/3 LLCT states or phosphorescence from 3 LLCT/LC states occurs. However, neither chiral moieties at the carbene nor at the carbazolate ligands provide detectable luminescence dissymmetries glum . An exception is [Cu(phenoxazinyl)(cAAC)], showing orange to deep red TADF with λmax =601-715 nm in solution, powders and in PMMA. In this case, the amide ligand can undergo distortions in the excited state. This design motif leads to the first linear, non-aggregated CPL-active copper(I) complex with glum of -3.4 ⋅ 10-3 combined with a high radiative rate constant of 6.7 ⋅ 105  s-1 .

Chiroptical Response Control of Planar and Axially Chiral Polymethylene-Vaulted Platinum(II) Complexes Bearing 1,1'-Binaphthyl Frameworks

In this study, the synthesis, structure, and chiroptical response control of planar chiral polymethylene-vaulted trans-bis[(β-iminomethyl)aryloxy]platinum(II) complexes bearing axially chiral 1,1'-binaphthyl ligands are described. A series of enantiopure polymethylene (n = 4-10)-vaulted complexes were prepared in 6 steps using commercially available (R)- or (S)-BINOL as the starting material without an optical resolution process. The trans-coordination and three-dimensional vaulted structures of the platinum complexes were elucidated from X-ray diffraction (XRD) studies. The complexes were found to show structural dependence of chiroptical responses in the dilute solution state such that the absolute values of [α]D, dissymmetry factors gabs in circular dichroism (CD), and glum in circularly polarized luminescence (CPL) increased upon shortening the length of the polymethylene bridges. The enhanced chiroptical responses were theoretically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, and the results are discussed in terms of the molecular structures and transition dipole moments of the ground states. The structural dependence of the chiroptical responses was ascribed to the distortion of the coordination platforms caused by restriction of the vaulting methylene linkers.

Better Covalent Connection in a Molecular Triad Enables More Efficient Photochemical Energy Storage

Numerous studies have explored the kinetics of light-induced charge separation and thermal charge recombination in donor-acceptor compounds, but quantum efficiencies have rarely been investigated. Here, we report on two essentially isomeric molecular triads, both comprising a π-extended tetrathiafulvalene (ExTTF) donor, a ruthenium(II)-based photosensitizer, and a naphthalene diimide (NDI) acceptor. The key difference between the two triads is how the NDI acceptor is connected. Linkage at the NDI core provides stronger electronic coupling to the other molecular components than connection via the nitrogen atoms of NDI. This change in molecular connectivity is expected to accelerate both energy-storing charge separation and energy-wasting charge recombination processes, but it is not a priori clear how this will affect the triad's ability to store photochemical energy; any gain resulting from faster charge separation could potentially be (over)compensated by losses through accelerated charge recombination. The new key insight emerging from our study is that the quantum yield for the formation of a long-lived charge-separated state increases by a factor of 5 when going from nitrogen- to core-connected NDI, providing the important proof of concept that better molecular connectivity indeed enables more efficient photochemical energy storage. The physical origin of this behavior seems to root in different orbital connectivity pathways for charge separation and charge recombination, as well as in differences in the relevant orbital interactions depending on NDI connection. Our work provides guidelines for how to discriminate between energy-storing and energy-wasting electron transfer reactions in order to improve the quantum yields for photochemical energy storage and solar energy conversion.

Direct Determination of the Rate of Intersystem Crossing in a Near-IR Luminescent Cr(III) Triazolyl Complex

A detailed understanding of the dynamics of photoinduced processes occurring in the electronic excited state is essential in informing the rational design of photoactive transition-metal complexes. Here, the rate of intersystem crossing in a Cr(III)-centered spin-flip emitter is directly determined through the use of ultrafast broadband fluorescence upconversion spectroscopy (FLUPS). In this contribution, we combine 1,2,3-triazole-based ligands with a Cr(III) center and report the solution-stable complex [Cr(btmp)₂]³⁺ (btmp = 2,6-bis(4-phenyl-1,2,3-triazol-1-yl-methyl)pyridine) (1³⁺), which displays near-infrared (NIR) luminescence at 760 nm (τ = 13.7 μs, ϕ = 0.1%) in fluid solution. The excited-state properties of 1³⁺ are probed in detail through a combination of ultrafast transient absorption (TA) and femtosecond-to-picosecond FLUPS. Although TA spectroscopy allows us to observe the evolution of phosphorescent excited states within the doublet manifold, more significantly and for the first time for a complex of Cr(III), we utilize FLUPS to capture the short-lived fluorescence from initially populated quartet excited states immediately prior to the intersystem crossing process. The decay of fluorescence from the low-lying ⁴MC state therefore allows us to assign a value of (823 fs)^-1 to the rate of intersystem crossing. Importantly, the sensitivity of FLUPS to only luminescent states allows us to disentangle the rate of intersystem crossing from other closely associated excited-state events, something which has not been possible in the spectroscopic studies previously reported for luminescent Cr(III) systems.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

Many coordination complexes and organometallic compounds with the 4d⁶ and 5d⁶ valence electron configurations have outstanding photophysical and photochemical properties, which stem from metal-to-ligand charge transfer (MLCT) excited states. This substance class makes extensive use of the most precious and least abundant metal elements, and consequently there has been a long-standing interest in first-row transition metal compounds with photoactive MLCT states. Semiprecious copper(I) with its completely filled 3d subshell is a relatively straightforward and well explored case, but in 3d⁶ complexes the partially filled d-orbitals lead to energetically low-lying metal-centered (MC) states that can cause undesirably fast MLCT excited state deactivation. Herein, we discuss recent advances made with isoelectronic Cr⁰, Mn^I, Fe^II, and Co^III compounds, for which long-lived MLCT states have become accessible over the past five years. Furthermore, we discuss possible future developments in the search for new first-row transition metal complexes with partially filled 3d subshells and photoactive MLCT states for next-generation applications in photophysics and photochemistry.

Substituents make a difference: 6,6″-modified terpyridine complexes with helix configuration and enhanced emission

A series of complexes L22-M (L2: 6,6″-bis(4-methoxyphenyl)-4'-phenyl-2,2':6',2″-terpyridine, M: Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) were synthesized by coordinating p-methoxyphenyl 6,6″-substituted terpyridine ligand with first-row transition metal ions and characterized by NMR, ESI-MS, and X-ray single crystal diffraction techniques. Single-crystal structures demonstrated that the steric hindrance of p-methoxyphenyl substituents endowed complexes L22-M with obvious longer coordination bond lengths and larger bond angles and dihedral angles compared with unmodified L12-M (L1: 4'-phenyl-2,2':6',2″-terpyridine). The chiral helix geometry was observed for L22-M, in which 2,2':6',2″-terpyridine moiety dramatically twisted to a spiral form in comparison to the nearly coplanar structure of the parent L12-M, resulting in plentiful intramolecular and intermolecular π-π interactions. Also, the appealing racemic (P and M) double helix packed structure for 6,6″-modified bisterpyridine complex L22-Cu was formed in the crystal. The consequent appealing charge transfer (CT) emission for L22-Zn in the solution and solid were investigated via UV-vis and fluorescence spectroscopy techniques and time-dependent density functional theory (TD-DFT) calculations. This work afforded a new method to achieve intriguing chiral geometry and CT optical properties via the subtle design and modification of terpyridine ligands.

Mechano-Stimulus and Environment-Dependent Circularly Polarized TADF in Chiral Copper(I) Complexes and Their Application in OLEDs

Molecular emitters that combine circularly polarized luminescence (CPL) and high radiative rate constants of the triplet exciton decay are highly attractive for electroluminescent devices (OLEDs) or next-generation photonic applications, such as spintronics, quantum computing, cryptography, or sensors. However, the design of such emitters is a major challenge because the criteria for enhancing these two properties are mutually exclusive. In this contribution, we show that enantiomerically pure {Cu(Cbz^R)[(S/R)-BINAP]} [R = H (1), 3,6-tBu (2)] are efficient thermally activated delayed fluorescence (TADF) emitters with high radiative rate constants of k_TADF up to 3.1 × 10⁵ s^-1 from ^1/3LLCT states according to our temperature-dependent time-resolved luminescence studies. The efficiency of the TADF process and emission wavelengths are highly sensitive to environmental hydrogen bonding of the ligands, which can be disrupted by grinding of the crystalline materials. The origin of this pronounced mechano-stimulus photophysical behavior is a thermal equilibrium between the ^1/3LLCT states and a ³LC state of the BINAP ligand, which depends on the relative energetic order of the excited states and is prone to inter-ligand C-H···π interactions. The copper(I) complexes are also efficient CPL emitters displaying exceptional dissymmetry values g_lum of up to ±0.6 × 10^-2 in THF solution and ±2.1 × 10^-2 in the solid state. Importantly for application in electroluminescence devices, the C-H···π interactions can also be disrupted by employing sterically bulky matrices. Accordingly, we have investigated various matrix materials for successful implementation of the chiral copper(I) TADF emitters in proof-of-concept CP-OLEDs.

The energy gap law for NIR-phosphorescent Cr(III) complexes

A series of homoleptic Cr(III) complexes containing substituted anionic 1,3-bis(pyridin-2-ylimino)isoindolin-2-ide ligands are phosphorescent with λ_max in the 777-970 nm range in degassed fluid solutions. The energy gap law has been successfully applied to the doublet excited states of Cr(III) complexes.

Conglomerate, Racemate, and Achiral Crystals of Polymetallic Europium(III) Compounds of Bis- or Tris-β-diketonate Ligands and Circularly Polarized Luminescence Study

This work reports (a) conglomerate and racemic crystal structures of [(Δ,Δ,Δ,Δ,Δ,Δ)- or/and (Λ,Λ,Λ,Λ,Λ,Λ)-Eu^III ₆(TTP)₈(OH₂)₆Na₄] _n coordination polymers, (b) racemic crystal structures of (Δ,Δ,Δ,Δ)-/(Λ,Λ,Λ,Λ)-Eu^III ₄(TTP)₄(bipy)₄(MEK)₂(OH₂)₂ tetrahedral clusters, and (c) the achiral crystal structure of the [Eu^III ₂(BTP)₄(OH₂)₂Na₂] _n coordination polymer (where BTP = dianionic bis-β-diketonate, TTP = trianionic tris-β-diketonate, and bipy = 2,2'-bipyridine). The screw coordination arrangement of the TTP ligand has led to the formation of homoconfigurational racemic Eu^III products. The conglomerate crystallization of [Eu^III ₆(TTP)₈(OH₂)₆Na₄] _n appears to be caused by the presence of the sodium, Na⁺ counterions, and interactions between oxygen atoms and the trifluoromethyl unit of the TTP ligand and Na⁺ ions. All the Eu^III compounds exhibit characteristic red luminescence (⁵D₀ → ⁷F_J, J = 0-4) in solution or in the solid crystalline state. Circularly polarized luminescence (CPL) was observed in the chiral Eu^III ₆(TTP)₈(OH₂)₆Na₄] _n species, displaying a |g _lum| value in the range of 0.15 to 0.68 at the ⁵D₀ → ⁷F₁ emission band. Subtle changes of the [Eu^III ₆(TTP)₈(OH₂)₆Na₄] _n structure which may be due to selection of twinned crystals or crystals that do not correspond to a perfect spontaneous resolution, are considered to be responsible for the variation in the observed CPL values.

Towards Luminescent Vanadium(II) Complexes with Slow Magnetic Relaxation and Quantum Coherence

Molecular entities with doublet or triplet ground states find increasing interest as potential molecular quantum bits (qubits). Complexes with higher multiplicity might even function as qudits and serve to encode further quantum bits. Vanadium(II) ions in octahedral ligand fields with quartet ground states and small zero-field splittings qualify as qubits with optical read out thanks to potentially luminescent spin-flip states. We identified two V²⁺ complexes [V(ddpd)₂ ]²⁺ with the strong field ligand N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine (ddpd) in two isomeric forms (cis-fac and mer) as suitable candidates. The energy gaps between the two lowest Kramers doublets amount to 0.2 and 0.5 cm^-1 allowing pulsed EPR experiments at conventional Q-band frequencies (35 GHz). Both isomers possess spin-lattice relaxation times T₁ of around 300 μs and a phase memory time T_M of around 1 μs at 5 K. Furthermore, the mer isomer displays slow magnetic relaxation in an applied field of 400 mT. While the vanadium(III) complexes [V(ddpd)₂ ]³⁺ are emissive in the near-IR-II region, the [V(ddpd)₂ ]²⁺ complexes are non-luminescent due to metal-to-ligand charge transfer admixture to the spin-flip states.

Photodynamics of the Molecular Ruby [Cr(ddpd)2]3

The introduction of strong-field ligands can enable luminescence in first-row transition-metal complexes. In this way, earth-abundant near-infrared emitters can be obtained using early 3d metals. A prime example is the molecular ruby [Cr(ddpd)₂]³⁺ (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine) that can achieve high phosphorescence quantum yields at room temperature in aqueous solution. To understand these remarkable properties, here, we simulate its photodynamics in water using trajectory surface hopping on linear vibronic coupling potentials parametrized from multiconfigurational CASSCF/CASPT2 calculations. We find that after excitation to the second absorption band, a relaxation cascade through metal-centered states occurs. After an initial back-and-forth intersystem crossing with higher-lying doublet states, the complex relaxes through a manifold of quartet metal-centered states to the low-lying doublet metal-centered states which are responsible for the experimentally observed emission. These electronic processes are driven by an elongation of the Cr-ligand bond lengths as well as the twisting motion of the trans-coordinated pyridine units in the ddpd ligands. The low-lying doublet states are reached within 1-2 ps and are close in geometry to the doublet minima, thus explaining the high phosphorescence quantum yield of the molecular ruby [Cr(ddpd)₂]³⁺.

Circularly polarized near-infrared phosphorescence of chiral chromium(III) complexes

Homoleptic Cr(III) complexes containing anionic tridentate 1,8-(bisoxazolyl)carbazolide ligands are phosphorescent in deaerated solutions with peak maxima in the range of 813-845 nm. The ligand carbon-centred chirality has been transferred to the helical chirality of the complexes and hence induced circularly polarized NIR-emissions with dissymmetry factor in the scale of 2.0 × 10^-3.

Circularly Polarized Luminescence of Cyclometalated Platinum(II) Complex Excimers: Large Difference between Isomers

A series of platinum(II) complexes bearing a chiral β-diketonato ligand and a cyclometalated ligand have been prepared. The platinum(II) complexes, (SP-4-3)-[Pt(ppy)(D-tac)] (ppy = 2-phenylpyridine, D-tac = 3-trifluoroacetyl-(D)-camphor), (SP-4-4)-[Pt(ppy)(D-tac)], (SP-4-3)-[Pt(ppy)(D-pbc)] (D-pbc = 3-perfluorobutyryl-(D)-camphor), and (SP-4-4)-[Pt(ppy)(D-pbc)], and their enantiomers were isolated and characterized by elemental analysis, NMR, and X-ray structural analysis. Photoisomerization between SP-4-3 (trans) and SP-4-4 (cis) isomers was observed. Green emission due to the monomer was observed in diluted solutions for all complexes. Higher quantum yields and longer lifetimes of green emission were observed in nonpolar solvents compared to polar solvents. The two geometrical isomers had surprisingly different excimer formation efficiencies. For the trans isomers, orange emission due to the excimers was observed in nonpolar solvents at high concentrations, whereas negligible intensities of the excimer emission were observed for the cis isomers. The formation of the excimers was evaluated by emission decay and time-resolved emission spectra. For the trans isomers, the green emission due to the monomer showed negligible CPL signals, but the orange emission gave pronounced CPL signals. The dissymmetry factors, g-values, of the excimer CPL (g_lum = 0.002) were enhanced over those of the circular dichroism (g_abs = 0.0002, g_lum/g_abs = 10). The intensities of the emission and the CPL of the excimer under oxygen were very small, although those under an argon atmosphere were very strong. Therefore, the emission color of the trans-isomers was changed from green to orange by deoxygenation.

Luminescent and Photofunctional Transition Metal Complexes: From Molecular Design to Diagnostic and Therapeutic Applications

There has been emerging interest in the exploitation of the photophysical and photochemical properties of transition metal complexes for diagnostic and therapeutic applications. In this Perspective, we highlight the major recent advances in the development of luminescent and photofunctional transition metal complexes, in particular, those of rhenium(I), ruthenium(II), osmium(II), iridium(III), and platinum(II), as bioimaging reagents and phototherapeutic agents, with a focus on the molecular design strategies that harness and modulate the interesting photophysical and photochemical behavior of the complexes. We also discuss the current challenges and future outlook of transition metal complexes for both fundamental research and clinical applications.

Shedding Light on the Oxidizing Properties of Spin-Flip Excited States in a CrIII Polypyridine Complex and Their Use in Photoredox Catalysis

The photoredox activity of well-known Ru^II complexes stems from metal-to-ligand charge transfer (MLCT) excited states, in which a ligand-based electron can initiate chemical reductions and a metal-centered hole can trigger oxidations. Cr^III polypyridines show similar photoredox properties, although they have fundamentally different electronic structures. Their photoactive excited state is of spin-flip nature, differing from the electronic ground state merely by a change of one electron spin, but with otherwise identical d-orbital occupancy. We find that the driving-force dependence for photoinduced electron transfer from 10 different donors to a spin-flip excited state of a Cr^III complex is very similar to that for a Ru^II polypyridine, and thereby validate the concept of estimating the redox potential of d³ spin-flip excited states in analogous manner as for the MLCT states of d⁶ compounds. Building on this insight, we use our Cr^III complex for photocatalytic reactions not previously explored with this compound class, including the aerobic bromination of methoxyaryls, oxygenation of 1,1,2,2-tetraphenylethylene, aerobic hydroxylation of arylboronic acids, and the vinylation of N-phenyl pyrrolidine. This work contributes to understanding the fundamental photochemical properties of first-row transition-metal complexes in comparison to well-explored precious-metal-based photocatalysts.

Complex-as-Ligand Strategy as a Tool for the Design of a Binuclear Nonsymmetrical Chromium(III) Assembly: Near-Infrared Double Emission and Intramolecular Energy Transfer

The chromium(III) polypyridyl complexes are appealing for their long-lived near-infrared (NIR) emission reaching the millisecond range and for the strong circularly polarized luminescence of their isolated enantiomers. However, harnessing those properties in functional polynuclear Cr^III devices remains mainly inaccessible because of the lack of synthetic methods for their design and functionalization. Even the preparation and investigation of most basic nonsymmetrical Cr^III dyads exhibiting directional intramolecular intermetallic energy transfer remain unexplored. Taking advantage of the inertness of heteroleptic chromium(III) polypyridyl building blocks, we herein adapt the "complex-as-ligand" strategy, largely used with precious 4d and 5d metals, for the preparation of a binuclear nonsymmetrical Cr^III complex (3d metal). The resulting [(phen)₂Cr(L)Cr(tpy)]⁶⁺ dyad shows dual long-lived NIR emission and a directional intermetallic energy transfer that is controlled by the specific arrangements of the different coordination spheres. This strategy opens a route for building predetermined polynuclear assemblies with this earth-abundant metal.

Efficient Triplet-Triplet Annihilation Upconversion Sensitized by a Chromium(III) Complex via an Underexplored Energy Transfer Mechanism

Sensitized triplet-triplet annihilation upconversion (sTTA-UC) mainly relies on precious metal complexes thanks to their high intersystem crossing (ISC) efficiencies, excited state energies, and lifetimes, while complexes of abundant first-row transition metals are only rarely utilized and with often moderate UC quantum yields. [Cr(bpmp)₂ ]³⁺ (bpmp=2,6-bis(2-pyridylmethyl)pyridine) containing earth-abundant chromium possesses an absorption band suitable for green light excitation, a doublet excited state energy matching the triplet energy of 9,10-diphenyl anthracene (DPA), a close to millisecond excited state lifetime, and high photostability. Combined ISC and doublet-triplet energy transfer from excited [Cr(bpmp)₂ ]³⁺ to DPA gives ³ DPA with close-to-unity quantum yield. TTA of ³ DPA furnishes green-to-blue UC with a quantum yield of 12.0 % (close to the theoretical maximum). Sterically less-hindered anthracenes undergo a [4+4] cycloaddition with [Cr(bpmp)₂ ]³⁺ and green light.

Manganese(I) Complex with Monodentate Arylisocyanide Ligands Shows Photodissociation Instead of Luminescence

Recently reported manganese(I) complexes with chelating arylisocyanide ligands exhibit luminescent metal-to-ligand charge-transfer (MLCT) excited states, similar to ruthenium(II) polypyridine complexes with the same d⁶ valence electron configuration used for many different applications in photophysics and photochemistry. However, chelating arylisocyanide ligands require substantial synthetic effort, and therefore it seemed attractive to explore the possibility of using more readily accessible monodentate arylisocyanides instead. Here, we synthesized the new Mn(I) complex [Mn(CNdippPh^OMe2)₆]PF₆ with the known ligand CNdippPh^OMe2 = 4-(3,5-dimethoxyphenyl)-2,6-diisopropylphenylisocyanide. This complex was investigated by NMR spectroscopy, single-crystal structure analysis, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) measurements, IR spectroscopy supported by density functional theory (DFT) calculations, cyclic voltammetry, and time-resolved as well as steady-state UV–vis absorption spectroscopy. The key finding is that the new Mn(I) complex is nonluminescent and instead undergoes arylisocyanide ligand loss during continuous visible laser irradiation into ligand-centered and charge-transfer absorption bands, presumably owed to the population of dissociative d–d excited states. Thus, it seems that chelating bi- or tridentate binding motifs are essential for obtaining emissive MLCT excited states in manganese(I) arylisocyanides. Our work contributes to understanding the basic properties of photoactive first-row transition metal complexes and could help advance the search for alternatives to precious metal-based luminophores, photocatalysts, and sensors.

We report the synthesis, characterization, and X-ray crystal structure of an octahedral manganese(I) complex with six monodentate arylisocyanide ligands that undergoes photoinduced ligand loss.

A Low-Temperature-Resistant Flexible Organic Crystal with Circularly Polarized Luminescence

Flexible organic crystals with unique mechanical properties and excellent optical properties are of paramount significance for their wide applications in various research fields such as adaptive optics and soft robotics. However, low-temperature-resistant flexible organic crystal with circularly polarized luminescence (CPL) has never been reported. Herein, chiral organic crystals with CPL activity and low-temperature flexibility (77 K) are fabricated by the solvent diffusion method from chiral Schiff base, S(R)-4-bromo-2-(((1-phenylethyl)imino)methyl)phenol (S(R)-BPEMP). The corresponding chirooptical properties for the two enantiomeric crystals were thoroughly investigated, including the measurements of circular dichroism (CD) and CPL. To the best of our knowledge, this is the first report on low-molecular-weight flexible organic crystals with CPL activity, and we believe that the results will give a new impetus to the research of organic crystals.

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 (L^CNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(L^CNC)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(L^CNC)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(L^CNC)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.

Molecular Rubies in Photoredox Catalysis

The molecular ruby [Cr(tpe) ₂ ] ³⁺ and the tris(bipyridine) chromium(III) complex [Cr(dmcbpy) ₃ ] ³⁺ as well as the tris(bipyrazine)ruthenium(II) complex [Ru(bpz) ₃ ] ²⁺ were employed in the visible light-induced radical cation [4+2] cycloaddition (tpe = 1,1,1-tris(pyrid-2-yl)ethane, dmcbpy = 4,4'-dimethoxycarbonyl-2,2'-bipyridine, bpz = 2,2'-bipyrazine), while [Cr(ddpd) ₂ ] ³⁺ serves as a control system (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine). Along with an updated mechanistic proposal for the Cr^III driven catalytic cycle based on redox chemistry, Stern-Volmer analyses, UV/Vis/NIR spectroscopic and nanosecond laser flash photolysis studies, we demonstrate that the very weakly absorbing photocatalyst [Cr(tpe) ₂ ] ³⁺ outcompetes [Cr(dmcbpy) ₃ ] ³⁺ and even [Ru(bpz) ₃ ] ²⁺ in particular at low catalyst loadings, which appears contradictory at first sight. The high photostability, the reversible redoxchemistry and the very long excited state lifetime account for the exceptional performance and even reusability of [Cr(tpe) ₂ ] ³⁺ in this photoredox catalytic system.

Heteroleptic mer-[Cr(N∩N∩N)(CN)3] complexes: synthetic challenge, structural characterization and photophysical properties

The substitution of three water molecules around trivalent chromium in CrBr₃·6H₂O with the tridentate 2,2′:6′,2′′-terpyridine (tpy), N,N′-dimethyl-N,N′-di(pyridine-2-yl)pyridine-2,6-diamine (ddpd) or 2,6-di(quinolin-8-yl)pyridine (dqp) ligands gives the heteroleptic mer-[Cr(L)Br₃] complexes. Stepwise treatments with Ag(CF₃SO₃) and KCN under microwave irradiations provide mer-[Cr(L)(CN)₃] in moderate yields. According to their X-ray crystal structures, the associated six-coordinate meridional [CrN₃C₃] chromophores increasingly deviate from a pseudo-octahedral arrangement according to L = ddpd ≈ dpq ≪ tpy; a trend in line with the replacement of six-membered with five-membered chelate rings around Cr^III. Room-temperature ligand-centered UV-excitation at 18 170 cm⁻¹ (λ_exc = 350 nm), followed by energy transfer and intersystem crossing eventually yield microsecond metal-centered Cr(²E → ⁴A₂) phosphorescence in the red to near infrared domain 13 150–12 650 cm⁻¹ (760 ≤ λ_em ≤ 790 nm). Decreasing the temperature to liquid nitrogen (77 K) extends the emission lifetimes to reach the millisecond regime with a record of 4.02 ms for mer-[Cr(dqp)(CN)₃] in frozen acetonitrile.

The heteroleptic mer-[Cr(L)(CN)₃] (L = tpy, ddpd, dqp) complexes with their C_2v-symmetrical [CrC₃N₃] luminescent chromophores represent the missing links between pseudo-octahedral [CrN₆] and [CrC₆] units found in their well-known homoleptic parents.

The overlooked NIR luminescence of Cr(ppy)3

Cr(ppy)₃, a structural analog of the green phosphorescent Ir(ppy)₃, emits even in solution at room temperature from a weakly distorted spin-flip state at 910 nm (Hppy = 2-phenylpyridine). The low energy arises from an enhanced covalence of the Cr-C bonds as compared to Cr-N bonds. Lower temperature reduces thermally activated decay increasing the emission intensity.

Spin-flip luminescence

In molecular photochemistry, charge-transfer emission is well understood and widely exploited. In contrast, luminescent metal-centered transitions only came into focus in recent years. This gave rise to strongly phosphorescent Cr^III complexes with a d³ electronic configuration featuring luminescent metal-centered excited states which are characterized by the flip of a single spin. These so-called spin-flip emitters possess unique properties and require different design strategies than traditional charge-transfer phosphors. In this review, we give a brief introduction to ligand field theory as a framework to understand this phenomenon and outline prerequisites for efficient spin-flip emission including ligand field strength, symmetry, intersystem crossing and common deactivation pathways using Cr^III complexes as instructive examples. The recent progress and associated challenges of tuning the energies of emissive excited states and of emerging applications of the unique photophysical properties of spin-flip emitters are discussed. Finally, we summarize the current state-of-the-art and challenges of spin-flip emitters beyond Cr^III with d², d³, d⁴ and d⁸ electronic configuration, where we mainly cover pseudooctahedral molecular complexes of V, Mo, W, Mn, Re and Ni, and highlight possible future research opportunities.

Enhancement of Chiroptical Responses of trans-Bis[(β-iminomethyl)naphthoxy]platinum(II) Complexes with Distorted Square Planar Coordination Geometry

The relationship between the coordination geometry and photophysical properties of trans‐bis[(β‐iminomethyl)naphthoxy]platinum(II) was investigated both experimentally and theoretically. A series of platinum(II) complexes with differently substituted iminomethyl groups were synthesized, and their photophysical properties were examined in solution, in the crystalline, and in the PMMA film‐dispersed state, respectively (PMMA=poly(methyl methacrylate)). These complexes showed structure‐dependent emission spectra, in which the color of the luminescence in the crystalline state varied over a range of about 40 nm depending on the specific bowl‐shaped molecular structure. The chiral complexes with (R,R)‐ and (S,S)‐configurations were found to have structure‐dependent chiroptical properties both in solution and the PMMA film‐dispersed state such that the intensity of circular dichroism (CD) and circularly polarized luminescence (CPL) were enhanced with bulky cyclic substituents at the nitrogen atoms. A theoretical study using density functional theory (DFT) and time‐dependent (TD)‐DFT calculations revealed that the enhancement of chiroptical responses is due to the amplification of the magnetic dipole moment caused by the distortion of the square planar geometry.

Bulky ligands protect molecular ruby from oxygen quenching

Steric protection strongly reduces phosphorescence quenching of excited molecular rubies by oxygen. The most bulky ligand enables photoluminescence quantum yields up to 5.1% and lifetimes up to 518 µs in air-saturated acetonitrile.