Vibrational anisotropy decay resolves rare earth binding induced conformational change in DTPA.

Elucidating the relationship between metal-ligand interactions and the associated conformational change of the ligand is critical for understanding the separation of lanthanides via ion binding. Here we examine DTPA, a multidentate ligand that binds lanthanides, in its free and metal bound conformations using ultrafast polarization dependent vibrational spectroscopy. The polarization dependent pump-probe spectra were analyzed to extract the isotropic and anisotropic response of DTPA's carbonyl groups in the 1550-1650 cm-1 spectral region. The isotropic response reports on the population relaxation of the carbonyl stretching modes. We find that the isotropic response is influenced by the identity of the metal ion. The anisotropy decay of the carbonyl stretching modes reveals a faster decay in the lanthanide-DTPA complexes than in the free DTPA ligand. We attribute the anisotropy decay to energy transfer among the different carbonyl sites - where the conformational change results in an increased coupling between the carbonyl sites of metal-bound DTPA complexes. DFT calculations and theoretical simulations of energy transfer suggest that the carbonyl sites are more strongly coupled in the metal-bound conformations compared to the free DTPA. The stronger coupling in the metal bound DTPA conformation leads to efficient energy transfer among the different carbonyl sites. Comparing the rate of anisotropy decay across the series of metal bound DTPA complexes we find that the anisotropy is sensitive to the charge density of the central metal ion, and thus can serve as a molecular scale reporter for lanthanide ion binding.

Observing Similarities and Differences in the Properties of Isostructural Niobium(V)/Tantalum(V) Coordination Compounds with Strong Pi-Donor Ligands.

While niobium and tantalum are found together in their mineral ores, their respective applications in technology require chemical separation. Nb/Ta separations are challenging due to the similar reactivities displayed by these metals in the solution phase. Coordination complexes of these metals have been studied in the contexts of catalysis, small-molecule activation, and functional group insertion reactivity; relatively few studies exist directly comparing the properties of isostructural Nb/Ta complexes. Such comparisons advance the development of Nb/Ta separation chemistry through the potential for differential reactivity. Here, we explore fundamental physicochemical properties in extensively characterized Nb/Ta coordination complexes [Na(DME)3][MClamp], (Clamp6- = tris-(2-(3',5'-di-tert-butyl-2'-oxyphenyl)amidophenyl)amine; M = Nb, Ta) to advance the understanding of the different electronic, optical, and excited-state properties that these metals exhibit in pi-loaded coordination complexes.

Resolving the Impact of Hydrogen Bonding on the Phylloquinone Cofactor through Two-Dimensional Infrared Spectroscopy.

Two-dimensional infrared spectroscopy (2DIR) was applied to phylloquinone (PhQ), an important biological cofactor, to elucidate the impact of hydrogen bonding on the ultrafast dynamics and energetics of the carbonyl stretching modes. 2DIR measurements were performed on PhQ dissolved in hexanol, which served as the hydrogen bonding solvent, and hexane, which served as a non-hydrogen bonding control. Molecular dynamics simulations and quantum chemical calculations were performed to aid in spectral assignment and interpretation. From the position of the peaks in the 2DIR spectra, we extracted the transition frequencies for the fundamental, overtone, and combination bands of hydrogen bonded and non-hydrogen bonded carbonyl groups of PhQ in the 1635-1680 cm-1 region. We find that hydrogen bonding to a single carbonyl group acts to decouple the two carbonyl units of PhQ. Through analysis of the time-resolved 2DIR data, we find that hydrogen bonding leads to faster vibrational relaxation as well as an increase in the inhomogeneous broadening of the carbonyl groups. Overall, this work demonstrates how hydrogen bonding to the carbonyl groups of PhQ presents in the 2DIR spectra, laying the groundwork to use PhQ as a 2DIR probe to characterize the ultrafast fluctuations in the local environment of natural photosynthetic complexes.

Probing Ligand Effects on the Ultrafast Dynamics of Copper Complexes via Midinfrared Pump-Probe and 2DIR Spectroscopies.

The effects of ligand structural variation on the ultrafast dynamics of a series of copper coordination complexes were investigated using polarization-dependent mid-IR pump-probe spectroscopy and two-dimensional infrared (2DIR) spectroscopy. The series consists of three copper complexes [(R3P3tren)CuIIN3]BAr4F (1PR3, R3P3tren = tris[2-(phosphiniminato)ethyl]amine, BAr4F = tetrakis(pentafluorophenyl)borate) where the number of methyl and phenyl groups in the PR3 ligand are systematically varied across the series (PR3 = PMe3, PMe2Ph, PMePh2). The asymmetric stretching mode of azide in the 1PR3 series is used as a vibrational probe of the small-molecule binding site. The results of the pump-probe measurements indicate that the vibrational energy of azide dissipates through intramolecular pathways and that the bulkier phenyl groups lead to an increase in the spatial restriction of the diffusive reorientation of bound azide. From 2DIR experiments, we characterize the spectral diffusion of the azide group and find that an increase in the number of phenyl groups maps to a broader inhomogeneous frequency distribution (Δ2). This indicates that an increase in the steric bulk of the secondary coordination sphere acts to create more distinct configurations in the local environment that are accessible to the azide group. This work demonstrates how ligand structural variation affects the ultrafast dynamics of a small molecular group bound to the metal center, which could provide insight into the structure-function relationship of the copper coordination complexes and transition-metal coordination complexes in general.

A Metal-Free, Photocatalytic Method for Aerobic Alkane Iodination.

Halogenation is an important alkane functionalization strategy, but O2 is widely considered the most desirable terminal oxidant. Here, the aerobic iodination of alkanes, including methane, was performed using catalytic [nBu4N]Cl and light irradiation (390 nm). Up to 10 turnovers of CH3I were obtained from CH4 and air, using a stop-flow microtubing system. Mechanistic studies using cyclohexane as the substrate revealed important details about the iodination reaction. Iodine (I2) serves multiple roles in the catalysis: (1) as the alkyl radical trap, (2) as a precursor for the light absorber, and (3) as a mediator of aerobic oxidation. The alkane activation is attributed to Cl• derived from photofragmentation of the electron donor-acceptor complex of I2 and Cl-. The kinetic profile of cyclohexane iodination showed that aerobic oxidation of I3- to produce I2 in CH3CN is turnover-limiting.

A contorted nanographene shelter.

Nanographenes have kindled considerable interest in the fields of materials science and supramolecular chemistry as a result of their unique self-assembling and optoelectronic properties. Encapsulating the contorted nanographenes inside artificial receptors, however, remains challenging. Herein, we report the design and synthesis of a trigonal prismatic hexacationic cage, which has a large cavity and adopts a relatively flexible conformation. It serves as a receptor, not only for planar coronene, but also for contorted nanographene derivatives with diameters of approximately 15 Å and thicknesses of 7 Å. A comprehensive investigation of the host-guest interactions in the solid, solution and gaseous states by experimentation and theoretical calculations reveals collectively an induced-fit binding mechanism with high binding affinities between the cage and the nanographenes. Notably, the photostability of the nanographenes is improved significantly by the ultrafast deactivation of their excited states within the cage. Encapsulating the contorted nanographenes inside the cage provides a noncovalent strategy for regulating their photoreactivity.

Three-Photon Spectroscopy of Porphyrins.

Recent advances in laser technology have made three-photon (3P) microscopy a real possibility, raising interest in the phenomenon of 3P absorption (3PA). Understanding 3PA of organic chromophores is especially important in view of those imaging applications that rely on exogenous probes, whose optical properties can be manipulated and optimized. Here, we present measurements and theoretical analysis of the degenerate 3PA spectra of several phosphorescent metalloporphyrins, which are used in the construction of biological oxygen probes. The effective 3PA cross sections (σ(3)) of these porphyrins near 1700 nm, a new promising biological optical window, were found to be on the order of 1000 GM3 (1 GM3 = 10-83 cm6 s2), therefore being among the highest values reported to date for organic chromophores. To interpret our data, we developed a qualitative four-state model specific for porphyrins and used it in conjunction with quantitative analysis based on the time-dependent density functional theory (TDDFT)/a posteriori Tamm-Dancoff approximation (ATDA)/sum-over-states (SOS) formalism. The analysis revealed that B (Soret) state plays a key role in the enhancement of 3PA of porphyrins in the Q band region, while the low-lying two-photon (2P)-allowed gerade states interfere negatively and diminish the 3PA strength. This study features the first systematic examination of 3PA properties of porphyrins, suggesting ways to improve their performance and optimize them for imaging and other biomedical applications.

Extracting the Frequency-Dependent Dynamic Stokes Shift from Two-Dimensional Electronic Spectra with Prominent Vibrational Coherences.

The dynamic Stokes shift is a common means for characterizing ultrafast solvation dynamics of electronically excited states. Here we extract the excitation frequency-dependent dynamic Stokes shift from two-dimensional electronic spectra (2DES) of cresyl violet, a molecule with a well-defined vibronic progression. The extracted dynamic Stokes shift function, S(t), exhibits oscillatory behavior, and the oscillatory components are assigned to intramolecular vibrational modes through DFT and TD-DFT calculations. The well-characterized oscillations are incorporated into the fitting procedure of S(t). The excitation frequency dependence of the ultrafast response is examined through the analysis of S(t) obtained from slices taken at different excitation frequencies of the 2DES spectra. The extracted ultrafast timescales range from 36 to 98 fs, and we interpret the frequency dependence of the timescales in the context of other dynamic processes that also lead to lineshape changes in the 2DES spectrum, such as vibrational energy relaxation and spectral diffusion. Through comparison of the extracted timescales, we find that the fastest timescales are extracted over a range of excitation frequencies, where contributions from vibrational relaxation and spectral diffusion can be minimized.

Halogen Bonding Facilitates Intersystem Crossing in Iodo-BODIPY Chromophores.

BODIPY chromophores can serve as organic-based triplet photosensitizers for a wide range of applications. To perform this function, the formation of the triplet state is critical, and a better understanding of how to modulate the formation of the triplet state could lead to further advances in BODIPY-based sensitizers for solar energy conversion and photodynamic therapy. In this work we investigate the ability of halogen bonding, a noncovalent solvent interaction, to facilitate intersystem crossing in a diiodo-BODIPY. Ultrafast transient absorption spectroscopy is applied to diiodo-BODIPY in the presence of pyridine-based halogen bonding solvent molecules to determine the rate constants for intersystem crossing. We find that halogen bonding facilitates the formation of the triplet state by increasing the intersystem crossing rate constant of diiodo-BODIPY. The results are interpreted in terms of the Marcus expression for intersystem crossing. Quantum chemical calculations show that halogen bonding acts to alter both the spin-orbit coupling terms and the relative energetics of the singlet and triplet states.

Simple and Efficient Theoretical Approach To Compute 2D Optical Spectra.

A highly efficient scheme is proposed and benchmarked to compute 2D optical spectra. This scheme is ideally designed for electronic spectroscopy; however, the method can be applied in a straightforward way to vibrational spectroscopy as well. Our scheme performs dynamics only for the t2 duration, eliminating explicit t1 and t3 coherent dynamics and thus can achieve dramatic improvements in efficiency. To gain this efficiency, we assume the system is in the inhomogeneous regime and that there is no significant nonadiabatic transfer of population during the t1 and t3 coherence times. Preliminary results are presented for the Frenkel Hamiltonian. We obtain excellent agreement with numerically exact results (which are possible for this simplistic model Hamiltonian), capturing all relevant trends at least qualitatively (and sometimes quantitatively).

Unraveling Confined Dynamics of Guests Trapped in Self-Assembled Pd6L4 Nanocages by Ultrafast Mid-IR Polarization-Dependent Spectroscopy.

Self-assembled coordination cages form host-guest complexes through weak noncovalent interactions. Knowledge of how these weak interactions affect the structure, reactivity, and dynamics of guest molecules is important to further the design principles of current systems and optimize their specific functions. We apply ultrafast mid-IR polarization-dependent pump-probe spectroscopy to probe the effects of two Pd6L4 self-assembled nanocages on the properties and dynamics of fluxional group-VIII metal carbonyl guest molecules. We find that the interactions between the Pd6L4 nanocages and guest molecules act to alter the ultrafast dynamics of the guests, restricting rotational diffusional motion and decreasing the vibrational lifetime.

Ultrafast Energy Transfer Involving the Red Chlorophylls of Cyanobacterial Photosystem I Probed through Two-Dimensional Electronic Spectroscopy.

Photosystem I (PSI) is a naturally occurring light-harvesting complex that drives oxygenic photosynthesis through a series of photoinitiated transmembrane electron transfer reactions that occur with a high quantum efficiency. Understanding the mechanism by which this process occurs is fundamental to understanding the near-unity quantum efficiency of PSI and in turn could lead to further insight into PSI-based technologies for solar energy conversion. In this article, we have applied two-dimensional electronic spectroscopy to PSI complexes isolated from two different cyanobacterial strains to gain further insight into the ultrafast energy transfer in PSI. The PSI complexes studied differ in the number and absorption of the red chlorophylls, chlorophylls that lie to lower energies than the reaction center. By applying a global analysis to the 2D electronic spectra of the PSI complexes we extract 2D decay associated spectra (2D-DAS). Through analysis of the 2D-DAS we observe a 50 fs relaxation among the bulk antenna chlorophylls in addition to two pathways of energy equilibration involving the red chlorophylls: a fast 200 fs equilibration followed by a 2-4 ps equilibration. As demonstrated with a model system, the λ1, λ3 coordinates of the cross-peaks in the 2D-DAS spectra indicate that the two equilibration pathways involve different chlorophyll molecules.

Understanding and Controlling the Emission Brightness and Color of Molecular Cerium Luminophores.

Molecular cerium complexes are a new class of tunable and energy-efficient visible- and UV-luminophores. Understanding and controlling the emission brightness and color are important for tailoring them for new and specialized applications. Herein, we describe the experimental and computational analyses for series of tris(guanidinate) (1-8, Ce{(R2N)C(N iPr)2}3, R = alkyl, silyl, or phenyl groups), guanidinate-amide [GA, A = N(SiMe3)2, G = (Me3Si)2NC(N iPr)2], and guanidinate-aryloxide (GOAr, OAr = 2,6-di- tert-butylphenoxide) cerium(III) complexes to understand and develop predictive capabilities for their optical properties. Structural studies performed on complexes 1-8 revealed marked differences in the steric encumbrance around the cerium center induced by various guanidinate ligand backbone substituents, a property that was correlated to photoluminescent quantum yield. Computational studies revealed that consecutive replacements of the amide and aryloxide ligands by guanidinate ligand led to less nonradiative relaxation of bright excited states and smaller Stokes shifts. The results establish a comprehensive structure-luminescence model for molecular cerium(III) luminophores in terms of both quantum yields and colors. The results provide a clear basis for the design of tunable, molecular, cerium-based, luminescent materials.

Ultrafast Solvation Dynamics and Vibrational Coherences of Halogenated Boron-Dipyrromethene Derivatives Revealed through Two-Dimensional Electronic Spectroscopy.

Boron-dipyrromethene (BODIPY) chromophores have a wide range of applications, spanning areas from biological imaging to solar energy conversion. Understanding the ultrafast dynamics of electronically excited BODIPY chromophores could lead to further advances in these areas. In this work, we characterize and compare the ultrafast dynamics of halogenated BODIPY chromophores through applying two-dimensional electronic spectroscopy (2DES). Through our studies, we demonstrate a new data analysis procedure for extracting the dynamic Stokes shift from 2DES spectra revealing an ultrafast solvent relaxation. In addition, we extract the frequency of the vibrational modes that are strongly coupled to the electronic excitation, and compare the results of structurally different BODIPY chromophores. We interpret our results with the aid of DFT calculations, finding that structural modifications lead to changes in the frequency, identity, and magnitude of Franck-Condon active vibrational modes. We attribute these changes to differences in the electron density of the electronic states of the structurally different BODIPY chromophores.

Light Absorption and Energy Transfer in the Antenna Complexes of Photosynthetic Organisms.

The process of photosynthesis is initiated by the capture of sunlight by a network of light-absorbing molecules (chromophores), which are also responsible for the subsequent funneling of the excitation energy to the reaction centers. Through evolution, genetic drift, and speciation, photosynthetic organisms have discovered many solutions for light harvesting. In this review, we describe the underlying photophysical principles by which this energy is absorbed, as well as the mechanisms of electronic excitation energy transfer (EET). First, optical properties of the individual pigment chromophores present in light-harvesting antenna complexes are introduced, and then we examine the collective behavior of pigment-pigment and pigment-protein interactions. The description of energy transfer, in particular multichromophoric antenna structures, is shown to vary depending on the spatial and energetic landscape, which dictates the relative coupling strength between constituent pigment molecules. In the latter half of the article, we focus on the light-harvesting complexes of purple bacteria as a model to illustrate the present understanding of the synergetic effects leading to EET optimization of light-harvesting antenna systems while exploring the structure and function of the integral chromophores. We end this review with a brief overview of the energy-transfer dynamics and pathways in the light-harvesting antennas of various photosynthetic organisms.

The Hexachlorocerate(III) Anion: A Potent, Benchtop Stable, and Readily Available Ultraviolet A Photosensitizer for Aryl Chlorides.

The hexachlorocerate(III) anion, [CeIIICl6]3-, was found to be a potent photoreductant in acetonitrile solution with an estimated excited-state reduction potential of -3.45 V versus Cp2Fe0/+. Despite a short lifetime of 22.1(1) ns, the anion exhibited a photoluminescence quantum yield of 0.61(4) and fast quenching kinetics toward organohalogens allowing for its application in the photocatalytic reduction of aryl chloride substrates.

Cerium Photosensitizers: Structure-Function Relationships and Applications in Photocatalytic Aryl Coupling Reactions.

Two complete mixed-ligand series of luminescent Ce(III) complexes with the general formulas [(Me3Si)2NC(N(i)Pr)2]xCe(III)[N(SiMe3)2]3-x (x = 0, 1-N; x = 1, 2-N, x = 2, 3-N; x = 3, 4) and [(Me3Si)2NC(N(i)Pr)2]xCe(III)(OAr)3-x (x = 0, 1-OAr; x = 1, 2-OAr, x = 2, 3-OAr; x = 3, 4) were developed, featuring photoluminescence quantum yields up to 0.81(2) and lifetimes to 117(1) ns. Although the 4f → 5d absorptive transitions for these complexes were all found at ca. 420 nm, their emission bands exhibited large Stokes shifts with maxima occurring at 553 nm for 1-N, 518 nm for 2-N, 508 nm for 3-N, and 459 nm for 4, featuring yellow, lime-green, green, and blue light, respectively. Combined time-dependent density functional theory (TD-DFT) calculations and spectroscopic studies suggested that the long-lived (2)D excited states of these complexes corresponded to singly occupied 5dz(2) orbitals. The observed difference in the Stokes shifts was attributed to the relaxation of excited states through vibrational processes facilitated by the ligands. The photochemistry of the sterically congested complex 4 was demonstrated by C-C bond forming reaction between 4-fluoroiodobenzene and benzene through an outer sphere electron transfer pathway, which expands the capabilities of cerium photosensitizers beyond our previous results that demonstrated inner sphere halogen atom abstraction reactivity by 1-N.

Luminescent Ce(III) Complexes as Stoichiometric and Catalytic Photoreductants for Halogen Atom Abstraction Reactions.

Luminescent Ce(III) complexes, Ce[N(SiMe3)2]3 (1) and [(Me3Si)2NC(RN)2]Ce[N(SiMe3)2]2 (R = (i)Pr, 1-(i)Pr; R = Cy, 1-Cy), with C(3v) and C(2v) solution symmetries display absorptive 4f → 5d electronic transitions in the visible region. Emission bands are observed at 553, 518, and 523 nm for 1, 1-(i)Pr, and 1-Cy with lifetimes of 24, 67, and 61 ns, respectively. Time-dependent density functional theory (TD-DFT) studies on 1 and 1-(i)Pr revealed the (2)A1 excited states corresponded to singly occupied 5d(z(2)) orbitals. The strongly reducing metalloradical character of 1, 1-(i)Pr, and 1-Cy in their (2)A1 excited states afforded photochemical halogen atom abstraction reactions from sp(3) and sp(2) C-X (X = Cl, Br, I) bonds for the first time with a lanthanide cation. The dehalogenation reactions could be turned over with catalytic amounts of photosensitizers by coupling salt metathesis and reduction to the photopromoted atom abstraction reactions.

Monitoring equilibrium reaction dynamics of a nearly barrierless molecular rotor using ultrafast vibrational echoes.

Using rapidly acquired spectral diffusion, a recently developed variation of heterodyne detected infrared photon echo spectroscopy, we observe ∼3 ps solvent independent spectral diffusion of benzene chromium tricarbonyl (C6H6Cr(CO)3, BCT) in a series of nonpolar linear alkane solvents. The spectral dynamics is attributed to low-barrier internal torsional motion. This tripod complex has two stable minima corresponding to staggered and eclipsed conformations, which differ in energy by roughly half of kBT. The solvent independence is due to the relative size of the rotor compared with the solvent molecules, which create a solvent cage in which torsional motion occurs largely free from solvent damping. Since the one-dimensional transition state is computed to be only 0.03 kBT above the higher energy eclipsed conformation, this model system offers an unusual, nearly barrierless reaction, which nevertheless is characterized by torsional coordinate dependent vibrational frequencies. Hence, by studying the spectral diffusion of the tripod carbonyls, it is possible to gain insight into the fundamental dynamics of internal rotational motion, and we find some evidence for the importance of non-diffusive ballistic motion even in the room-temperature liquid environment. Using several different approaches to describe equilibrium kinetics, as well as the influence of reactive dynamics on spectroscopic observables, we provide evidence that the low-barrier torsional motion of BCT provides an excellent test case for detailed studies of the links between chemical exchange and linear and nonlinear vibrational spectroscopy.

Energy transfer pathways in light-harvesting complexes of purple bacteria as revealed by global kinetic analysis of two-dimensional transient spectra.

Excited state dynamics in LH2 complexes of two purple bacterial species were studied by broad-band two-dimensional electronic spectroscopy. The optical response was measured in the 500-600 nm spectral region on the 0-400 fs time scale. Global target analysis of two-dimensional (2D) transient spectra revealed the main energy transfer pathways between carotenoid S2, 1Bu(-) and S1 states and bacteriochlorophyll Qx state. Global analysis ascertained the evolutionary and vibration-associated spectra, which also indicated the presence of a higher-lying vibrational level in the carotenoid S1 state. The estimation of the spectral overlap between the 1Bu(-) state and the Qx state indicated a significant contribution of the 1Bu(-) state to the overall S2-to-Qx excitation energy transfer.