Transition Metal Chromophores Research Articles

Picosecond Metal-to-Ligand Charge-Transfer Deactivation in Co(ppy)3 via Jahn-Teller Distortion.

The excited-state dynamics of fac-Co(ppy)3, where ppy = 2-[2-(pyridyl)phenyl], are measured with femtosecond UV-vis transient absorption spectroscopy. The initial state is confirmed with spectroelectrochemistry to have significant metal-to-ligand charge transfer (MLCT) character, unlike other Co complexes that generally have ligand-to-metal charge transfer or ligand-field transitions in this energy range. Ground-state recovery occurs in 8.65 ps in dichloromethane. Density functional theory calculations show that the MLCT state undergoes Jahn-Teller distortion and converts to a five-coordinate triplet metal-centered state in which one Co-N bond is broken. The results highlight a potential pitfall of heteroleptic bidentate ligands when designing strong-field ligands for transition-metal chromophores.

Beyond the First Coordination Sphere─Manipulating the Excited-State Landscape in Iron(II) Chromophores with Protons.

Molecular transition metal chromophores play a central role in light harvesting and energy conversion. Recently, earth-abundant transition-metal-based chromophores have begun to challenge the dominance of platinum group metal complexes in this area. However, the development of new chromophores with optimized photophysical properties is still limited by a lack of synthetic methods, especially with respect to heteroleptic complexes with functional ligands. Here, we demonstrate a facile and efficient method for the combination of strong-field carbenes with the functional 2,2'-bibenzimidazole ligand in a heteroleptic iron(II) chromophore complex. Our approach yields two isomers that differ predominantly in their excited-state lifetimes based on the symmetry of the ligand field. Deprotonation of both isomers leads to a significant red-shift of the metal-to-ligand charge transfer (MLCT) absorption and a shortening of excited-state lifetimes. Femtosecond transient absorption spectroscopy in combination with quantum chemical simulations and resonance Raman spectroscopy reveals the complex relationship between protonation and photophysical properties. Protonation is found to tip the balance between MLCT and metal-centered (MC) excited states in favor of the former. This study showcases the first example of fine-tuning of the excited-state landscape in an iron(II) chromophore through second-sphere manipulations and provides a new perspective to the challenge of excited-state optimizations in 3d transition metal chromophores.

Time-Resolved X-ray Emission Spectroscopy and Synthetic High-Spin Model Complexes Resolve Ambiguities in Excited-State Assignments of Transition-Metal Chromophores: A Case Study of Fe-Amido Complexes.

To fully harness the potential of abundant metal coordination complex photosensitizers, a detailed understanding of the molecular properties that dictate and control the electronic excited-state population dynamics initiated by light absorption is critical. In the absence of detectable luminescence, optical transient absorption (TA) spectroscopy is the most widely employed method for interpreting electron redistribution in such excited states, particularly for those with a charge-transfer character. The assignment of excited-state TA spectral features often relies on spectroelectrochemical measurements, where the transient absorption spectrum generated by a metal-to-ligand charge-transfer (MLCT) electronic excited state, for instance, can be approximated using steady-state spectra generated by electrochemical ligand reduction and metal oxidation and accounting for the loss of absorptions by the electronic ground state. However, the reliability of this approach can be clouded when multiple electronic configurations have similar optical signatures. Using a case study of Fe(II) complexes supported by benzannulated diarylamido ligands, we highlight an example of such an ambiguity and show how time-resolved X-ray emission spectroscopy (XES) measurements can reliably assign excited states from the perspective of the metal, particularly in conjunction with accurate synthetic models of ligand-field electronic excited states, leading to a reinterpretation of the long-lived excited state as a ligand-field metal-centered quintet state. A detailed analysis of the XES data on the long-lived excited state is presented, along with a discussion of the ultrafast dynamics following the photoexcitation of low-spin Fe(II)-Namido complexes using a high-spin ground-state analogue as a spectral model for the 5T2 excited state.

Active Learning Exploration of Transition-Metal Complexes to Discover Method-Insensitive and Synthetically Accessible Chromophores.

Transition-metal chromophores with earth-abundant transition metals are an important design target for their applications in lighting and nontoxic bioimaging, but their design is challenged by the scarcity of complexes that simultaneously have well-defined ground states and optimal target absorption energies in the visible region. Machine learning (ML) accelerated discovery could overcome such challenges by enabling the screening of a larger space but is limited by the fidelity of the data used in ML model training, which is typically from a single approximate density functional. To address this limitation, we search for consensus in predictions among 23 density functional approximations across multiple rungs of "Jacob's ladder". To accelerate the discovery of complexes with absorption energies in the visible region while minimizing the effect of low-lying excited states, we use two-dimensional (2D)efficient global optimization to sample candidate low-spin chromophores from multimillion complex spaces. Despite the scarcity (i.e., ∼0.01%) of potential chromophores in this large chemical space, we identify candidates with high likelihood (i.e., >10%) of computational validation as the ML models improve during active learning, representing a 1000-fold acceleration in discovery. Absorption spectra of promising chromophores from time-dependent density functional theory verify that 2/3 of candidates have the desired excited-state properties. The observation that constituent ligands from our leads have demonstrated interesting optical properties in the literature exemplifies the effectiveness of our construction of a realistic design space and active learning approach.

YInMn blue - 200 Years in the making: New intense inorganic pigments based on chromophores in trigonal bipyramidal coordination

Materials with various colors have always fascinated humans from prehistory to the present. They bring joy to our daily life by making the world a more beautiful place to live in and influence our feelings. The important factor that decides the color of a material is its interaction with light in the visible range (400–700 nm) sensitive to human eye. Transition metal cations engender color because they have electronic transition energies that resonate with frequencies in the visible range. In general, transition metal chromophores with a coordination environment of less symmetry and more mixing between the p and d orbitals will have a greater probability of producing intense colors due to the relaxation of selection rules for the d-d transitions. This review chronicles the historical importance of inorganic pigments, reviews the origin of color in solids and highlights recent advancements in designing color pigments. Although fundamentals of the color science about gemstones and minerals are qualitatively well-understood, it is difficult to predict the color of inorganic solids before they are prepared experimentally. In spite of recent advancements in quantum mechanical theories and computational methods, predicting a crystal lattice that will produce an intense inorganic pigment of a desired color is still elusive. Serendipity often plays a role in the discovery of commercially important pigments and dyes such as Prussian Blue, Perkin's aniline dyes and recently YInMn Blue. We conclude that designing durable, nontoxic and multifunctional inorganic pigments with brilliant colors is quite challenging, and a deeper understanding of the structure-optical property relationship in this respect can be helpful.

Transition metal chromophores in glass beads of the classical and Hellenistic period: Bonding environment and colouring role

The delicate blue and green hue of ancient coloured beads from the Classical and Hellenistic period originate from the different transition metal (TM) ions added in the batch materials during glass production. The chemical composition in conjunction with the TM redox are fingerprints of the colour and vitrification process. We report on the structural and colouring/decolouring role of TM in blue and green-blue soda-lime beads from Thebes, central Greece dated back to the Classical and Hellenistic period using non-destructive X-Ray Absorption Fine Structure (XAFS) spectroscopies. Concerning the TMs redox and their consequent influence on colour, it is established that the major blue colourants are Co2+ and Cu2+, colourless Cu+ dominates in the blue beads, whereas chromophores Fe3+ and Cu2+ give rise to a green-blue hue. The Mn2+ content in the blue beads, whose occurrence is attributed to unintentional addition via the Co rich mineral colourant, counteracts the effect of the moderately higher content of Fe2+ in the raw materials. Based on the XAFS analysis results, we propose that the fractions of all TM under question (Mn, Fe, Co and Cu) were chosen and adjusted properly not only to control the desired colour, but also to ensure the stability of the glass matrix. Indeed, the integrity of the silica matrix should be further enhanced due to the presence of intermediate Fe oxides linking to Cu ions in either planar or linear geometry, while Mn and Co act as typical network formers.

Reversible thermochromic property of Cr, Mn, Fe, Co-doped Ca14Zn6Ga10O35

A new family of thermochromic materials were reported with transition metal chromophore doping in Ca14Zn6Ga10O35 host.

Inorganic pigments with transition metal chromophores at trigonal bipyramidal coordination: Y(In,Mn)O3 blues and beyond

The discovery of a brilliant-blue color through Mn3+ substitution in trigonal-bipyramidal (TBP) sites of hexagonal YInO3 has led to a search for other inorganic pigments with chromophore ions at TBP coordination. Introducing transition metal cations Fe3+, Cu2+/Ti4+ and Mn3+/Zn2+/Ti4+ into the TBP sites in YInO3 has produced various colors from orange, green to purple. These indium-containing samples are successfully prepared via several synthetic routes including solid state reactions, modified Pechini method and microwave radiation. All pigments show excellent near infrared reflectance comparable to that of commercial TiO2. Cheaper hexagonal YMO3 host materials with TBP coordination such as citrate-prepared hexagonal YAlO3 (M = Al) have failed to produce the anticipated vivid color due to the destruction of TBP sites caused by carbonate incorporation. We conclude that the resulting intense colors of pigments are due to the crystal field splitting associated with the TBP coordination and the short apical M–O bonds (M = Mn, Fe, Cu). Other colored compounds containing TBP coordination are briefly reviewed. Blue and/or bluish-purple colors are found when Mn3+, Co2+ or Cu2+ are substituted into TBP sites of metal oxides that are not hexagonal YMO3-based.

Insights into the excited state dynamics of Fe(ii) polypyridyl complexes from variable-temperature ultrafast spectroscopy.

In an effort to better define the nature of the nuclear coordinate associated with excited state dynamics in first-row transition metal-based chromophores, variable-temperature ultrafast time-resolved absorption spectroscopy has been used to determine activation parameters associated with ground state recovery dynamics in a series of low-spin Fe(ii) polypyridyl complexes. Our results establish that high-spin (5T2) to low-spin (1A1) conversion in complexes of the form [Fe(4,4'-di-R-2,2'-bpy')3]2+ (R = H, CH3, or tert-butyl) is characterized by a small but nevertheless non-zero barrier in the range of 300-350 cm-1 in fluid CH3CN solution, a value that more than doubles to ∼750 cm-1 for [Fe(terpy)2]2+ (terpy = 2,2':6',2''-terpyridine). The data were analyzed in the context of semi-classical Marcus theory. Changes in the ratio of the electronic coupling to reorganization energy (specifically, H ab 4/λ) reveal an approximately two-fold difference between the [Fe(bpy')3]2+ complexes (∼1/30) and [Fe(terpy)2]2+ (∼1/14), suggesting a change in the nature of the nuclear coordinate associated with ground state recovery between these two types of complexes. These experimentally-determined ratios, along with estimates for the 5T2/1A1 energy gap, yield electronic coupling values between these two states for the [Fe(bpy')3]2+ series and [Fe(terpy)2]2+ of 4.3 ± 0.3 cm-1 and 6 ± 1 cm-1, respectively, values that are qualitatively consistent with the second-order nature of high-spin/low-spin coupling in a d6 ion. In addition to providing useful quantitative information on these prototypical Fe(ii) complexes, these results underscore the utility of variable-temperature spectroscopic measurements for characterizing ultrafast excited state dynamics in this class of compounds.

Photodriven Oxygen Removal via Chromophore-Mediated Singlet Oxygen Sensitization and Chemical Capture.

We report a general, photochemical method for the rapid deoxygenation of organic solvents and aqueous solutions via visible light excitation of transition metal chromophores (TMCs) in the presence of singlet oxygen scavenging substrates. Either 2,5-dimethylfuran or an amino acid (histidine or tryptophan methyl ester) was used as the substrate in conjunction with an iridium or ruthenium TMC in toluene, acetonitrile, or water. This behavior is described for solutions with chromophore concentrations that are pertinent for both luminescence and transient absorption spectroscopies. These results consistently produce TMC lifetimes comparable to those measured using traditional inert gas sparging and freeze-pump-thaw techniques. This method has the added benefits of providing long-term stability (days to months); economical preparation due to use of inexpensive, commercially available oxygen scrubbing substrates; and negligible size and weight footprints compared to traditional methods. Furthermore, attainment of dissolved [O2] < 50 μM makes this method relevant to any solution application requiring low dissolved oxygen concentration in solution, provided that the oxygenated substrate does not interfere with the intended chemical process.

Colored oxides with hibonite structure II: Structural and optical properties of CaAl12O19-type pigments with chromophores based on Fe, Mn, Cr and Cu

Abstract Colored hibonite-type oxides with a general formula CaAl12-xMxO19 were prepared by solid state reactions through M-site substitution of aluminum with various di-, tri- and tetravalent cations (M = Fe, Mn, Cr, Cu, Ga, In, Ti, Ge, Sn or any combination thereof), where Fe, Mn, Cr and Cu are chromophore ions. Neutron Rietveld refinements of CaAl9.5Fe2.5O19, CaAl11.5Mn0.5O19 and CaAl10SnMnO19 compounds show that Fe3+ and Mn2+ ions prefer to occupy the tetrahedral site, while Mn3+ has an evident preference for both tetrahedral and octahedral sites and Sn4+ for octahedral site in the hibonite structure. No oxygen vacancies are found by structural analyses. Magnetic measurement confirms divalent Mn for CaAl11Sn0.5Mn0.5O19 and trivalent Mn for CaAl11.5Mn0.5O19. Diffuse reflectance spectra reveal that the d-d transitions occurring in transition metal chromophores are mainly responsible for the observed colors. The colored powders exhibit fairly high reflectance in the near-infrared region (750–2500 nm) which makes them potential candidates for cool pigment applications. Synthesis and characterization methods are briefly reviewed for hibonite type of oxides, especially those containing Fe, Mn, Cr and Cu.

Probing the Noninnocent π-Bonding Influence of N-Carboxyamidoquinolate Ligands on the Light Harvesting and Redox Properties of Ruthenium Polypyridyl Complexes.

Electronic and photophysical characterization is presented for a series of bis-heteroleptic [Ru(bpy)2(R-CAQN)](+) complexes where CAQN is a bidentate N-(carboxyaryl)amidoquinolate ligand and the aryl substituent R = p-tolyl, p-fluorobenzene, p-trifluoromethylbenzene, 3,5-bis(trifluoromethyl)benzene, or 4-methoxy-2,3,5,6-tetrafluorobenzene. Characterized by a strong noninnocent Ru(dπ)-CAQN(π) bonding interaction, density functional theory (DFT) analysis is used to estimate the contribution of both atomic Ru(dπ) and ligand CAQN(π) manifolds to the frontier molecular orbitals of these complexes. UV-vis absorption and emission studies are presented where the noninnocent Ru(dπ)-CAQN(π) bonding scheme plays a major role in defining complex electronic and photophysical properties. Oxidation potentials are tuned over a range of 0.92 V with respect to the [Ru(bpy)3](2+) reference system, hereafter referred to as 1(2+), by varying the degree of R-CAQN fluorination while maintaining consistently strong and panchromatic visible absorption properties. Electron paramagnetic resonance (EPR) spectroscopy is employed to experimentally map delocalization of the unpaired electron/electron-hole within the delocalized Ru(dπ)-CAQN(π) singly occupied valence molecular orbital of the one-electron oxidized complexes. EPR data is complemented experimentally by UV-vis-NIR spectroelectrochemistry, and computationally by molecular orbital Mulliken contributions and spin-density analysis. It is ultimately demonstrated that the CAQN ligand framework provides a simple yet broad synthetic platform in the design of redox-active transition metal chromophores with a range of electronic and spectroscopic characteristics hinting at the diversity and potential of these complexes toward photochemical and catalytic applications.

Solar Synthesis: Prospects in Visible Light Photocatalysis

Chemists have long aspired to synthesize molecules the way that plants do-using sunlight to facilitate the construction of complex molecular architectures. Nevertheless, the use of visible light in photochemical synthesis is fundamentally challenging because organic molecules tend not to interact with the wavelengths of visible light that are most strongly emitted in the solar spectrum. Recent research has begun to leverage the ability of visible light-absorbing transition metal complexes to catalyze a broad range of synthetically valuable reactions. In this review, we highlight how an understanding of the mechanisms of photocatalytic activation available to these transition metal complexes, and of the general reactivity patterns of the intermediates accessible via visible light photocatalysis, has accelerated the development of this diverse suite of reactions.

Synthesis and photophysical properties of kinetically stable complexes containing a lanthanide ion and a transition metal antenna group

A series of bimetallic complexes has been prepared in which an octadentate DOTA-monoamide pocket containing a bound lanthanide ion is linked covalently to a Re(I) or Ru(II) bipyridyl unit via an alkyl spacer group. The transition metal chromophores incorporated in this way act as effective sensitisers for lanthanide-centred luminescence. The rate and efficiency of energy transfer are dependent upon the nature of the spacer group, and upon the nature of the lanthanide acceptor. For the RuNd diad, there is a long rise-time associated with the energy-transfer step, such that energy transfer is rate determining in H(2)O, but not in D(2)O. The results also lead us to suggest that energy transfer may precede formation of the (3)M((Ru/Re))L((bpy))CT state and may be a competitive deactivation pathway for the precursor state ((1)M((Ru/Re))L((bpy))CT).

Synthesis and Spectroscopic Properties of a Prototype Single Molecule Dual Imaging Agent Comprising a Heterobimetallic Rhenium−Gadolinium Complex

A luminescent d−f hybrid containing a [Re(bpy)(Ar)(CO)3]+ chromophore and a gadolinium ion bound in a DO3A-derived binding pocket has been prepared. This complex combines long-lived luminescence from the 3MLCT state of the transition metal chromophore with high relaxivity, making it suitable for MRI imaging and luminescence imaging. The analogous complex containing ytterbium exhibits sensitized emission from both the ytterbium center and long-lived emission from the 3MLCT state.

Excited-State Absorption Properties of Platinum(II) Terpyridyl Acetylides

A comprehensive photophysical study is presented which compares the ground- and excited-state properties of four platinum(II) terpyridyl acetylide compounds of the general formula [Pt(tBu3tpy)(CCR)]+, where tBu3tpy is 4,4',4' '-tri-tert-butyl-2,2':6',2' '-terpyridine and R is an alkyl or aryl group. [Ru(tBu3tpy)3]2+ and the pivotal synthetic precursor [Pt(tBu3tpy)Cl]+ were also investigated in the current work. The latter two complexes possess short excited-state lifetimes and were investigated using ultrafast spectrometry while the other four compounds were evaluated using conventional nanosecond transient-absorption spectroscopy. The original intention of this study was to comprehend the nature of the impressive excited-state absorptions that emanate from this class of transition-metal chromophores. Transient-absorbance-difference spectra across the series contain the same salient features, which are modulated only slightly in wavelength and markedly in intensity as a function of the appended acetylide ligand. More intense absorption transients are observed in the arylacetylide structures relative to those bearing an alkylacetylide, consistent with transitions coupled to the pi system of the ancillary ligand. Reductive spectroelectrochemical measurements successfully generated the electronic spectrum of the tBu3tpy radical anion in all six complexes at room temperature. These measurements confirm that electronic absorptions associated with the tBu3tpy radical anion simply do not account for the intense optical transitions observed in the excited state of the Pt(II) chromophores. Transient-trapping experiments using the spectroscopically silent reductive quencher DABCO clearly demonstrate the loss of most transient-absorption features in the acetylide complexes throughout the UV, visible, and near-IR regions following bimolecular excited-state electron transfer, suggesting that these features are strongly tied to the photogenerated hole which is delocalized across the Pt center and the ancillary acetylide ligand.

Column: Photochemical Reactions and Photophysical Processes

Recently, interest has been aroused in the synthesis and study of optical and electronic properties of π-conjugated oligomers and polymers containing transition metal complexes. The introduction of transition metal chromophores exhibiting metal to ligand charge transfer excited state (MLCT) in the π-conjugated polymer opens new possibilities in variations of electronic and optical properties. The new materials might find wide applications such as polymeric light emitting diodes (PLED), solar energy conversion devices, nonlinear optical materials (NLOs), in materials exhibiting photorefractions, in chemical sensing, electrochromism, and electrocatalysis. The reason for this increased interest is due to the fact that the electronic and optic

Tailoring Transition Metal Complexes for Nonlinear Optics Applications. 2. A Theoretical Investigation of the Second-Order Nonlinear Optical Properties of M(CO)5L Complexes (M = Cr, W; L = Py, PyCHO, Pyz, PyzBF3, BPE, BPEBF3)

In this work, we report an ab initio investigation of second-order nonlinear optical (NLO) properties and absorption electronic spectra of push-pull transition metal chromophores of the formula [M(CO)(5)L] (M = Cr, W; L = pyridine (Py), 4-formyl-pyridine (PyCHO), pyrazine (Pyz), trans-1,2-bis(4-pyridyl)ethylene (BPE)). Pyz and BPE are considered either with one nitrogen atom free or interacting with the strong acceptor BF(3). All of the molecular properties have been calculated using two different and methodologically independent approaches: the time dependent and coupled perturbed density functional theories (TDDFT and CPDFT) and the sum-over-states (SOS) approach, where the excited states are obtained via the single configuration interaction (SCI) ab initio method. DFT results are in acceptable agreement with the experimental energy values of electronic transitions (with the exception of chromophores with the large pi-delocalization, like BPE); SCI calculations overestimate excitation energies and produce an inversion in the order of d(M) --> pi(L) and d(M) --> pi(CO) transitions. The SCI-SOS approach gives first-order hyperpolarizabilities, basically in agreement as trend and values with the experiments and seems to be a tool generally suitable for the evaluation of these properties also for transition metal complexes. On the other hand, the first-order hyperpolarizabilities computed using the CPDFT approach are consistently overestimated in comparison with the experimental results, especially in the case of a ligand with large pi-delocalization. We also show that the "two-level" approximation taking into account only the lowest energy charge transfer excitation (e.g., d(M) --> pi(L)) is not applicable to chromophores with the extended pi-delocalized ligand (BPE) coordinated to a transition metal, due to significant contributions originating from intraligand pi(L) --> pi(L) transitions. This study reports a detailed analysis and comparison of electronic NLO effects of transition metal complexes computed with DFT and ab initio SCI-SOS methodology.

The Synthesis and Characterization of Energy-Conducting Polymers with Pendant Inorganic Chromophores

ABSTRACTWe are interested in the synthesis, characterization, and performance evaluation of functional nanoscale materials comprised of a polymeric scaffold with appended cationic transition-metal lumiphores. We have developed a methodology to prepare, spectroscopically characterize, and evaluate a series of organic copolymers functionalized with inorganic chromophores. Preparation of these hybrid systems first involves the synthesis of a linear AB diblock copolymeric scaffold in which A is polystyrene (PS) and B is poly(p-tert-butoxycarbonyloxystyrene) (PStBOC), using Reversible Addition-Fragmentation chain-Transfer (RAFT) radical polymerization. The PStBOC block (B) was converted into poly(4-hydroxystyrene) by acid hydrolysis of the t-BOC moieties, and Ru(II) trisbipyridyl complexes were covalently appended using standard ester coupling reagents. These lumiphores were selected due to their strong absorbance in the visible spectrum, chemical/photochemical stability, useful redox properties, and long-lived excited state lifetimes. Attachment of the cationic transition-metal chromophores to block B of these linear AB diblock copolymeric arrays is expected to promote solid-state self-assembly into nanoscale structures. The metal-loaded macromolecular assemblies were characterized spectroscopically and the determination of the solid-state morphology of films of these materials was investigated using Transmission Electron Microscopy (TEM).

Copper Complexes Bearing Pendant, Protonatable Noncoordinating Groups: Synthesis, Characterization, and Protonation Chemistry

We have derivatized the pendant amine of (5-amino-5-methyl-3,7-diaza-2,9-nonanedioato)chlorocopper(II) with 3-pyridinecarboxaldehyde and 4-pyridinecarboxaldehyde. These new complexes, along with the starting material, comprise a set of copper complexes with chemically identical inner coordination spheres that provide protonatable nitrogen attached to the ligand framework. The 3-pyridinecarboxaldehyde adduct has been characterized by X-ray crystallography. This complex crystallized in the space group P21/n with a = 12.025(8) Å, b = 12.304(5) Å, c = 13.311(8) Å, β = 94.76(5)°, V = 1962(1) Å3, and Z = 4 (R, Rw = 0.049; 0.045). pH-dependent changes in the electronic spectroscopy and electrochemistry of these complexes have been observed. The parent complex undergoes a ∼300 cm-1 shift in λmax and a +140 mV shift in midpoint potential on going from high pH to low pH, while the pyridinecarboxaldehyde Schiff-base derivatives undergo smaller changes (78 and 110 cm-1 and 104 and 100 mV, respectively). The origins of the shift in λmax are investigated spectroscopically. A number of explanations for these shifts can be ruled out, including metal complex dimerization and axial coordination to the metal complexes. The data tend to support the suggestion that the spectroscopic shifts are largely due to the electrostatic influence of the pendant charge on the transition metal chromophore.