Exploring Differences in Lanthanide Excited State Reactivity Using a Simple Example: The Photophysics of La and Ce Thenoyltrifluoroacetone Complexes.

A series of four lanthanide thenoyltrifluoroacetone (TTA) complexes consisting of two f0 (La3+ and Ce4+) and two f1 (Ce3+) complexes was examined using steady-state and time-resolved spectroscopic techniques. The wide range of spectroscopic techniques presented herein have enabled us to discern the nature of the excited states (charge transfer, CT vs ligand localized, LL) as well as construct a Jablonski diagram for detailing the excited state reactivity within the series of molecules. The wavelength and excitation power dependence for these series of complexes are the first direct verification for the presence of simultaneous competing, noninteracting CT and LL excited states. Additionally, a computational framework is described that can be used to support spectroscopic assignments as a guide for future studies. Finally, the relationship between the obtained photophysics and possible photochemical separation mechanisms is described.

Trifluoromethylated Phenanthroline Ligands Reduce Excited-State Distortion in Homoleptic Copper(I) Complexes.

We report the synthesis and excited-state dynamics for a series of homoleptic copper(I) trifluoromethylated phenanthroline complexes with two, three, and four trifluoromethyl functional groups. Our analysis of the steady-state absorbance and emission, transient-absorption spectroscopy, and electronic-structure-theory calculations results enable in-depth analysis of the pseudo-Jahn-Teller distortion inhibition from increased steric hindrance of the trifluoromethyl functional group relative to the prototypical dimethyl phenanthroline complex. Surprisingly, our results demonstrate that the greatest degree of pseudo-Jahn-Teller distortion inhibition is achieved with trifluoromethylation of only the 2 and 9 positions by an unusual combination of steric hindrance and stabilization of a nondistorted 1MLCT manifold observed by transient kinetic lifetimes and optimized excited-state structures. The intersystem-crossing (ISC) lifetime for the 2,9-bis(trifluoromethyl)-1,10-phenanthroline Cu(I) complex is 69 ps, while the triplet excited-state lifetime and emission quantum yield are 106 ns and 4 × 10-3, respectively. Further trifluoromethylation of the phenanthroline yields a greater σ bond inductive withdrawing force on the phenanthroline nitrogens, ultimately resulting in weaker coordination to the copper. Last, the surprising success of the 2,9-bis(trifluoromethyl)-1,10-phenanthroline Cu(I) complex by adjusting both ligand sterics and electronic properties outlines a new strategy for developing long-lived Cu(I) charge-transfer complexes.

Picosecond to Nanosecond Manipulation of Excited-State Lifetimes in Complexes with an FeII to TiIV Metal-to-Metal Charge Transfer: The Role of Ferrocene Centered Excited States.

Time-resolved transient absorption spectroscopy and computational analysis of D-π-A complexes comprising FeII donors and TiIV acceptors with the general formula RCp2Ti(C2Fc)2 (where RCp = Cp*, Cp, and MeOOCCp) and TMSCp2Ti(C2Fc)(C2R) (where R = Ph or CF3) are reported. The transient absorption spectra are consistent with an FeIII/TiIII metal-to-metal charge-transfer (MMCT) excited state for all complexes. Thus, excited-state decay is assigned to back-electron transfer (BET), the lifetime of which ranges from 18.8 to 41 ps. Though spectroscopic analysis suggests BET should fall into the Marcus inverted regime, the observed kinetics are not consistent with this assertion. TDDFT calculations reveal that the singlet metal-to-metal charge-transfer (1MMCT) excited state for the FeII/TiIV complexes is not purely MMCT in nature but is contaminated with the higher-energy 1Fc (d-d) state. For the diferrocenyl complexes, RCp2Ti(C2Fc)2, the ratio of MMCT to Fc centered character ranges from 57:43 for the Cp* complex to 85:15 for the MeOOCCp complex. For the diferrocenyl and monoferrocenyl complexes investigated herein, the excited-state lifetimes decrease with increased 1Fc character. The effect of CuI coordination was also analyzed by time-resolved transient absorption spectroscopy and reveals the elongation of the excited-state lifetime by 3 orders of magnitude to 63 ns. The transient spectra and TDDFT analysis suggest that the long-lived excited state in Cp2Ti(C2Fc)2·CuX (where X is Cl or Br) is a triplet iron species with an electron arrangement of TiIV-3FeII-CuI.

Controlling Photoisomerization Reactivity Through Single Functional Group Substitutions in Ruthenium Phosphine Sulfoxide Complexes.

We report the crystallography, emission spectra, femtosecond pump-probe spectroscopy, and density functional theory computations for a series of ruthenium complexes that comprise a new class of chelating triphenylphosphine based ligands with an appended sulfoxide moiety. These ligands differ only in the presence of the para-substitutent (e.g., H, OCH3, CF3). The results show a dramatic range in photoisomerization reactivity that is ascribed to differences in the electron density of the phosphine ligand donated to the ruthenium and the nature of the excited state.

Unravelling the enigma of ultrafast excited state relaxation in non-emissive aggregating conjugated polymers.

We investigate a class of non-emissive conjugated polymers with very short excited state lifetimes believed to undergo singlet fission and relaxation to mid-gap forbidden excited states. Poly(3-decylthieneylenvinylene) (P3DTV) and its heavy atom analog, poly(3-decylseleneylenvinylene) (P3DSV), are strongly aggregating conjugated polymers that experience large excited state displacements along multiple vibrational modes. We demonstrate this Franck-Condon vibrational activity effectively disperses excitation energy into multiple non-radiative channels that can be explained using a simple, two-state potential energy surface model. Resonance Raman spectroscopy is sensitive to early Franck-Condon vibrational activity and we observe rich harmonic progressions involving multiple high frequency CC backbone symmetric stretching motions (∼1000-1600 cm-1) in both systems reflecting mode-specific excited state geometrical displacements. Transient absorption spectra confirm that efficient non-radiative processes dominate excited state relaxation dynamics which are confined to π-stacked aggregated chains. Surprisingly, we found little influence of the heteroatom consistent with efficient vibrational energy dissipation. Our results highlight the importance of aggregation and multi-dimensional Franck-Condon vibrational dynamics on the ability to harvest excitons, which are not usually considered in materials design and optimization schemes.

"Roller-Wheel"-Type Pt-Containing Small Molecules and the Impact of "Rollers" on Material Crystallinity, Electronic Properties, and Solar Cell Performance.

We report the synthesis, characterization, and detailed comparison of a series of novel Pt-bisacetylide containing conjugated small molecules possessing an unconventional "roller-wheel" shaped structure that is distinctly different from the "dumbbell" designs in traditional Pt-bisacetylide containing conjugated polymers and small molecules. The relationships between the chemical nature and length of the "rollers" and the electronic and physical properties of the materials are carefully studied by steady-state spectroscopy, cyclic voltammetry, differential scanning calorimetry, single-crystal X-ray diffraction, transient absorption spectroscopy, theoretical calculation, and device application. It was revealed that if the roller are long enough, these molecules can "slip-stack" in the solid state, leading to high crystallinity and charge mobility. Organic solar cells were fabricated and showed power conversion efficiencies up to 5.9%, out-performing all existing Pt-containing materials. The device performance was also found to be sensitive to optimization conditions and blend morphologies, which are a result of the intricate interplay among materials crystallinity, phase separation, and the relative positions of the lowest singlet and triplet excited states.

Extreme Ultraviolet Reflection Spectroscopy of Lanthanides and Actinides Using a High Harmonic Generation Light Source.

Absorption spectroscopy probing transitions from shallow-core d and f orbitals in lanthanides and actinides reveals information about bonding and the electronic structure in compounds containing these elements. However, spectroscopy in this photon energy range is challenging because of the limited availability of light sources and extremely short penetration depths. In this work, we address these challenges using a tabletop extreme ultraviolet (XUV), ultrafast, laser-driven, high harmonic generation light source, which generates femtosecond pulses in the 40-140 eV range. We present reflection spectroscopy measurements at the N4,5 (i.e., predominantly 4d to 5f transitions) and O4,5 (i.e., 5d to 5f transitions) absorption edges on several lanthanide and uranium oxide crystals. We compare these results to density functional theory calculations to assign the electronic transitions and predict the spectra for other lanthanides. This work paves the way for laboratory-scale XUV absorption experiments for studying crystalline and molecular f-electron systems, with applications ranging from surface chemistry, photochemistry, and electronic or chemical structure determination to nuclear forensics.

Influence of Linker Identity on the Photochemistry of Uranyl-Organic Frameworks.

While uranyl-based metal-organic frameworks (MOFs) boast impressive photocatalytic abilities, significant questions remain regarding their excitation pathways and methods to fine-tune their performance due to the lack of information regarding heterogeneous uranyl catalysis. Herein, we investigated how linker identity and photoexcitation impact uranyl photocatalysis when the uranyl coordination environment remains constant. Toward this end, we prepared three uranyl-based MOFs (NU-1301, NU-1307, and ZnTCPP-U2) and then examined the structural and photochemical properties of each through X-ray diffraction, X-ray absorption, and photoluminescence. We then correlated our observations to the photocatalytic performance for fluorination of cyclooctane. The excitation profile from NU-1301 and NU-1307 exhibited spin-forbidden linker transitions and uranyl vibronic progressions, with uranyl excitation and emission being most dominant in NU-1301. Consequently, NU-1301 was a more active photocatalyst than NU-1307. In contrast, the excitation profile from ZnTCPP-U2 contained transitions associated with the porphyrin linker exclusively. Photocatalytic activity from ZnTCPP-U2 significantly underperformed in comparison to that of the other two MOFs. These data suggest that linkers' photophysical properties can be used to predict the photocatalytic behavior of uranyl-containing MOFs.

Synthesis, solid-state, solution, and theoretical characterization of an "in-cage" scandium-NOTA complex.

Developing chelators that strongly and selectively bind rare-earth elements (Sc, Y, La, and lanthanides) represents a longstanding fundamental challenge in inorganic chemistry. Solving these challenges is becoming more important because of increasing use of rare-earth elements in numerous technologies, ranging from paramagnets to luminescent materials. Within this context, we interrogated the complexation chemistry of the scandium(III) (Sc3+) trication with the hexadentate 1,4,7-triazacyclononane-1,4,7-triacetic acid (H3NOTA) chelator. This H3NOTA chelator is often regarded as an underperformer for complexing Sc3+. A common assumption is that metalation does not fully encapsulate Sc3+ within the NOTA3- macrocycle, leaving Sc3+ on the periphery of the chelate and susceptible to demetalation. Herein, we developed a synthetic approach that contradicted those assumptions. We confirmed that our procedure forced Sc3+ into the NOTA3- binding pocket by using single crystal X-ray diffraction to determine the Na[Sc(NOTA)(OOCCH3)] structure. Density functional theory (DFT) and 45Sc nuclear magnetic resonance (NMR) spectroscopy showed Sc3+ encapsulation was retained when the crystals were dissolved. Solution-phase and DFT studies revealed that [Sc(NOTA)(OOCCH3)]1- could accommodate an additional H2O capping ligand. Thermodynamic properties associated with the Sc-OOCCH3 and Sc-H2O capping ligand interactions demonstrated that these capping ligands occupied critical roles in stabilizing the [Sc(NOTA)] chelation complex.

Understanding the Photochemical Properties of Polythiophene Polyelectrolyte Soft Aggregates with Sodium Dodecyl Sulfate for Antimicrobial Activity.

The threat of antibiotic-resistant bacteria is an ever-increasing problem in public health. In this report, we examine the photochemical properties with a proof-of-principle biocidal assay for a novel series of regio-regular imidazolium derivative poly-(3-hexylthiophene)/sodium dodecyl sulfate (P3HT-Im/SDS) materials from ultrafast sub-ps dynamics to µs generation of reactive oxygen species (ROS) and 30 min biocidal reactivity with Escherichia coli (E. coli). This broad series encompassing pure P3HT-Im to cationic, neutral, and anionic P3HT-Im/SDS materials are all interrogated by a variety of techniques to characterize the physical material structure, electronic structure, and antimicrobial activity. Our results show that SDS complexation with P3HT-Im results in aggregate materials with reduced ROS generation and light-induced anti-microbial activity. However, our characterization reveals that the presence of non-aggregated or lightly SDS-covered polymer segments is still capable of ROS generation. Full encapsulation of the P3HT-Im polymer completely deactivates the light killing pathway. High SDS concentrations, near and above critical micelle concentration, further deactivate all anti-microbial activity (light and dark) even though the P3HT-Im regains its electronic properties to generate ROS.

Using molten salts to probe outer-coordination sphere effects on lanthanide(III)/(II) electron-transfer reactions.

Controlling structure and reactivity by manipulating the outer-coordination sphere around a given reagent represents a longstanding challenge in chemistry. Despite advances toward solving this problem, it remains difficult to experimentally interrogate and characterize outer-coordination sphere impact. This work describes an alternative approach that quantifies outer-coordination sphere effects. It shows how molten salt metal chlorides (MCln; M = K, Na, n = 1; M = Ca, n = 2) provided excellent platforms for experimentally characterizing the influence of the outer-coordination sphere cations (Mn+) on redox reactions accessible to lanthanide ions; Ln3+ + e1- → Ln2+ (Ln = Eu, Yb, Sm; e1- = electron). As a representative example, X-ray absorption spectroscopy and cyclic voltammetry results showed that Eu2+ instantaneously formed when Eu3+ dissolved in molten chloride salts that had strongly polarizing cations (like Ca2+ from CaCl2) via the Eu3+ + Cl1- → Eu2+ + ½Cl2 reaction. Conversely, molten salts with less polarizing outer-sphere M1+ cations (e.g., K1+ in KCl) stabilized Ln3+. For instance, the Eu3+/Eu2+ reduction potential was >0.5 V more positive in CaCl2 than in KCl. In accordance with first-principle molecular dynamics (FPMD) simulations, we postulated that hard Mn+ cations (high polarization power) inductively removed electron density from Lnn+ across Ln-Cl⋯Mn+ networks and stabilized electron-rich and low oxidation state Ln2+ ions. Conversely, less polarizing Mn+ cations (like K1+) left electron density on Lnn+ and stabilized electron-deficient and high-oxidation state Ln3+ ions.

Advancing understanding of actinide(iii) (Ac, Am, Cm) aqueous complexation chemistry.

The positive impact of having access to well-defined starting materials for applied actinide technologies - and for technologies based on other elements - cannot be overstated. Of numerous relevant 5f-element starting materials, those in complexing aqueous media find widespread use. Consider acetic acid/acetate buffered solutions as an example. These solutions provide entry into diverse technologies, from small-scale production of actinide metal to preparing radiolabeled chelates for medical applications. However, like so many aqueous solutions that contain actinides and complexing agents, 5f-element speciation in acetic acid/acetate cocktails is poorly defined. Herein, we address this problem and characterize Ac3+ and Cm3+ speciation as a function of increasing acetic acid/acetate concentrations (0.1 to 15 M, pH = 5.5). Results obtained via X-ray absorption and optical spectroscopy show the aquo ion dominated in dilute acetic acid/acetate solutions (0.1 M). Increasing acetic acid/acetate concentrations to 15 M increased complexation and revealed divergent reactivity between early and late actinides. A neutral Ac(H2O)6 (1)(O2CMe)3 (1) compound was the major species in solution for the large Ac3+. In contrast, smaller Cm3+ preferred forming an anion. There were approximately four bound O2CMe1- ligands and one to two inner sphere H2O ligands. The conclusion that increasing acetic acid/acetate concentrations increased acetate complexation was corroborated by characterizing (NH4)2M(O2CMe)5 (M = Eu3+, Am3+ and Cm3+) using single crystal X-ray diffraction and optical spectroscopy (absorption, emission, excitation, and excited state lifetime measurements).

An excited state dynamics driven reaction: wavelength-dependent photoisomerization quantum yields in [Ru(bpy)2(dmso)2]2.

We report the excited-state behavior of a structurally simple bis-sulfoxide complex, cis-S,S-[Ru(bpy)2(dmso)2]2+, as investigated by femtosecond pump-probe spectroscopy. The results reveal that a single photon prompts phototriggered isomerization of one or both dmso ligands to yield a mixture of cis-S,O-[Ru(bpy)2(dmso)2]2+ and cis-O,O-[Ru(bpy)2(dmso)2]2+. The quantum yields of isomerization of each product and relative product distribution are dependent upon the excitation wavelength, with longer wavelengths favoring the double isomerization product, cis-O,O-[Ru(bpy)2(dmso)2]2+. Transient absorption measurements on cis-O,O-[Ru(bpy)2(dmso)2]2+ do not reveal an excited-state isomerization pathway to produce either the S,O or S,S isomers. Femtosecond pulse shaping experiments reveal no change in the product distribution. Pump-repump-probe transient absorption spectroscopy of cis-S,S-[Ru(bpy)2(dmso)2]2+ shows that a pump-repump time delay of 3 ps dramatically alters the S,O : O,O product ratio; pump-repump-probe transient absorption spectroscopy of cis-O,O-[Ru(bpy)2(dmso)2]2+ with a time delay of 3 ps uncovers an excited-state isomerization pathway to produce the S,O isomer. In conjunction with low-temperature steady-state emission spectroscopy, these results are interpreted in the context of an excited-state bifurcating pathway, in which the isomerization product distribution is determined not by thermodynamics, but rather as a dynamics driven reaction.

Generating Photonastic Work from Irradiated Dyes in Electrospun Nanofibrous Polymer Mats.

For solar-driven macroscopic motions, we assert that there is a local heating that facilitates large-scale deformations in anisotropic morphologic materials caused by thermal gradients. This report specifically identifies the fate of heat generation in photonastic materials and demonstrates how heat can perform work following excitation of a nonisomerizing dye. Utilizing the electrospinning technique, we have created a series of anisotropic nanofibrous polymer mats that comprise nonisomerizing dyes. Polymers are chosen because of their relative glass transition temperatures, elastic moduli, and melting temperatures. Light irradiation of these polymer mats with an excitation wavelength matching the absorption characteristics of the dye leads to macroscopic deformation of the mat. Analysis of still images extracted from digital videos provides plots of angular displacement vs power. The data were analyzed in terms of a photothermal model. Analyses of scanning electron microscopy micrographs for all samples are consistent to local melting in low Tg polymers and softening in high Tg polymers. Dynamic mechanical analysis allowed for quantification of the modulus change under a given light fluence. We employ these data to calculate a energy conversion efficiency. These efficiencies for the polymer mats are compared to other nonmuscular systems, including a few natural, biological samples.

A "roller-wheel" Pt-containing small molecule that outperforms its polymer analogs in organic solar cells.

A novel Pt-bisacetylide small molecule (Pt-SM) featuring "roller-wheel" geometry was synthesized and characterized. When compared with conventional Pt-containing polymers and small molecules having "dumbbell" shaped structures, Pt-SM displays enhanced crystallinity and intermolecular π-π interactions, as well as favorable panchromatic absorption behaviors. Organic solar cells (OSCs) employing Pt-SM achieve power conversion efficiencies (PCEs) up to 5.9%, the highest reported so far for Pt-containing polymers and small molecules.

Comparison of vapor formation of water at the solid/water interface to colloidal solutions using optically excited gold nanostructures.

The phase transformation properties of liquid water to vapor is characterized by optical excitation of the lithographically fabricated single gold nanowrenches and contrasted to the phase transformation properties of gold nanoparticles located and optically excited in a bulk solution system [two and three dimensions]. The 532 nm continuous wave excitation of a single gold nanowrench results in superheating of the water to the spinodal decomposition temperature of 580 ± 20 K with bubble formation below the spinodal decomposition temperature being a rare event. Between the spinodal decomposition temperature and the boiling point liquid water is trapped into a metastable state because a barrier to vapor nucleation exists that must be overcome before the thermodynamically stable state is realized. The phase transformation for an optically heated single gold nanowrench is different from the phase transformation of optically excited colloidal gold nanoparticles solution where collective heating effects dominates and leads to the boiling of the solution exactly at the boiling point. In the solution case, the optically excited ensemble of nanoparticles collectively raises the ambient temperature of water to the boiling point where liquid is converted into vapor. The striking difference in the boiling properties of the single gold nanowrench and the nanoparticle solution system can be explained in terms of the vapor-nucleation mechanism, the volume of the overheated liquid, and the collective heating effect. The interpretation of the observed regimes of heating and vaporization is consistent with our theoretical modeling. In particular, we explain with our theory why the boiling with the collective heating in a solution requires 3 orders of magnitude less intensity compared to the case of optically driven single nanowrench.