Polarized Optical Absorption and Emission Spectra and the Electronic Energy-Level Structure of Tb(dpa)33- Complexes in Na3[Yb0.95Tb0.05(dpa)3]·NaClO4·10H2O

Molecular light-upconversion: we have had a problem! When excited state absorption (ESA) overcomes energy transfer upconversion (ETU) in Cr(III)/Er(III) complexes

Nine-coordinate [ErN₉] or [ErN₃O₆] chromophores found in triple helical [Er(L)₃]³⁺ complexes (L corresponds to 2,2′,6′,2′′-terpyridine (tpy), 2,6-(bisbenzimidazol-2-yl)pyridine (bzimpy), 2,6-diethylcarboxypyridine (dpa-ester) or 2,6-diethylcarboxamidopyridine (dpa-diamide) derivatives), [Er(dpa)₃]³⁻ (dpa is the 2,6-dipicolinate dianion) and [GaErGa(bpb-bzimpy)₃]⁹⁺ (bpb-bzimpy is 2,6-bis((pyridin-2-benzimidazol-5-yl)methyl-(benzimidazol-2-yl))pyridine) exhibit NIR (excitation at 801 nm) into visible (emission at 542 nm) linear light upconversion processes in acetonitrile at room temperature. The associated quantum yields 5.5(6) × 10⁻¹¹ ≤ ϕ^up_tot(ESA) ≤ 1.7(2) × 10⁻⁹ appear to be 1–3 orders of magnitude larger than those predicted by the accepted single-center excited-state absorption mechanism (ESA). Switching to the alternative energy transfer upconversion mechanism (ETU), which operates in multi-centers [CrErCr(bpb-bzimpy)₃]⁹⁺, leads to an improved quantum yield of ϕ^up_tot(ETU) = 5.8(6) × 10⁻⁸, but also to an even larger discrepancy by 4–6 orders of magnitude when compared with theoretical models. All photophysical studies point to Er(⁴I_13/2) as being the only available ‘long-lived’ (1.8 ≤ τ ≤ 6.3 μs) and emissive excited state, which works as an intermediate relay for absorbing the second photon, but with an unexpected large cross-section for an intrashell 4f → 4f electronic transition. With this in mind, the ETU mechanism, thought to optimize upconversion via intermetallic Cr → Er communication in [CrErCr(bpb-bzimpy)₃]⁹⁺, is indeed not crucial and the boosted associated upconversion quantum yield is indebted to the dominant contribution of the single-center erbium ESA process. This curious phenomenon is responsible for the successful implementation of light upconversion in molecular coordination complexes under reasonable light power intensities, which paves the way for applications in medicine and biology. Its origin could be linked with the presence of metal–ligand bonding.

Near-infrared to visible molecular upconversion exhibits quantum yields which are 2–6 orders of magnitude larger than those modeled with the accepted linear excited state absorption (ESA) or energy transfer (ETU) mechanisms: we have had a problem!

A Simple Thermodynamic Model for Rationalizing the Formation of Self-Assembled Multimetallic Edifices: Application to Triple-Stranded Helicates

Reaction of the bis-tridentate ligand bis[1-ethyl-2-[6'-(N,N-diethylcarbamoyl)pyridin-2'-yl]benzimidazol-5-yl]methane (L2) with Ln(CF(3)SO(3))(3).xH(2)O in acetonitrile (Ln = La-Lu) demonstrates the successive formation of three stable complexes [Ln(L2)(3)](3+), [Ln(2)(L2)(3)](6+), and [Ln(2)(L2)(2)](6+). Crystal-field independent NMR methods establish that the crystal structure of [Tb(2)(L2)(3)](6+) is a satisfying model for the helical structure observed in solution. This allows the qualitative and quantitative beta23 (bi,Ln1,Ln2)characterization of the heterobimetallic helicates [(Ln(1))(Ln(2))(L2)(3)](6+). A simple free energy thermodynamic model based on (i) an absolute affinity for each nine-coordinate lanthanide occupying a terminal N(6)O(3) site and (ii) a single intermetallic interaction between two adjacent metal ions in the complexes (DeltaE) successfully models the experimental macroscopic constants and allows the rational molecular programming of the extended trimetallic homologues [Ln(3)(L5)(3)](9+).

The Solution Structure of Rhombic Lanthanide Complexes Analyzed with a Model-Free and Crystal-Field Independent Paramagnetic NMR Method: Application to Nonaxial Trimetallic Complexes [LnxLu3-x(TACI-3H)2(H2O)6]3+ (x = 1−3)

The model-free approach has been extended with the derivation of a novel three-nuclei crystal-field independent method for investigating isostructurality in nonaxial (i.e., rhombic) complexes along the lanthanide series. Application of this technique to the heterotrimetallic sandwich complexes [LnLu2(TACI-3H)2(H2O)6]3+, which possess a single C2v-symmetrical paramagnetic center, unambiguously evidences isostructurality for Ln = Pr-Yb, while the variation of the second-rank crystal-field parameters and along the series prevents reliable structural analyses with the classical one-nucleus equation. Extension toward polymetallic magnetically noncoupled rhombic lanthanide complexes in [Ln2Lu(TACI-3H)2(H2O)6]3+ (two paramagnetic centers with Cs microsymmetry) and [Ln3(TACI-3H)2(H2O)6]3+ (three paramagnetic centers with C2v microsymmetry) requires only minor modifications of the original three-nuclei equation. Isostructurality characterizes [Ln2Lu(TACI-3H)2(H2O)6]3+ (Ln = Pr-Yb), while [Ln3(TACI-3H)2(H2O)6]3+ exhibit a structural change between Eu and Tb which results from the concomitant contraction of the three metallic centers. Particular attention has been focused on (i) the stepwise increase of contact (i.e., through-bond) and pseudocontact (i.e., through-space) contributions when the number of paramagnetic centers increases, (ii) the assignment of 13C resonances in the strongly paramagnetic complexes [Ln3(TACI-3H)2(H2O)6]3+ (Ln = Tb-Yb) for which reliable T1 measurements and [1H-13C] correlation spectra are not accessible, and (iii) the combination of crystal-field dependent and independent methods for analyzing the paramagnetic NMR spectra of axial and nonaxial lanthanide complexes.

Chirality-dependent interactions between molecular propeller structures in solution. Chiral recognition and discrimination processes modulated by temperature and incremental changes in structural chirality

Time-resolved chiroptical luminescence (TR-CL) measurements are used to study chirality-dependent intermolecular interactions in dynamic excited-state quenching processes. The measurements are carried out on solution samples that contain a racemic mixture of chiral luminophore molecules (with enantiomeric structures denoted by LambdaL and DeltaL) and a small, optically resolved concentration of chiral quencher (CQ) molecules. The luminophores are excited with a pulse of linearly polarized laser radiation to produce an initially racemic excited-state population of LambdaL* and DeltaL* enantiomers, and TR-CL measurements are then used to monitor the differential decay kinetics of the LambdaL* and DeltaL* subpopulations. Observed differences between the LambdaL* and DeltaL* decay kinetics reflect differential rate processes and efficiencies for LambdaL*-CQ vs. DeltaL*-CQ quenching actions, and they are diagnostic of chiral discriminatory interactions between the luminophore and quencher molecules. Twelve different luminophore-quencher systems are examined, in both H(2)O and D(2)O solutions, and in each case the quenching kinetics are measured over the 273-308 K temperature range. In all of the systems examined here, quenching occurs via electronic energy-transfer processes in transient (LambdaL*-CQ) and (DeltaL*-CQ) encounter complexes, and the chiral discriminatory rate parameters reflect the relative stabilities and lifetimes of these complexes as well as their structures and internal (electronic and nuclear) dynamics. All of the luminophore and quencher molecules examined in this study have three-bladed propeller-like structures that are very similar in overall shape and size. However, they exhibit small differences in the structural details of their propeller blades, and it is found that these small differences in structure can produce both qualitative and very substantial quantitative differences in their chiral recognition and discrimination properties.

Medium effects on the photophysical properties of terbium(III) complexes with pyridine-2,6-dicarboxylate

Emission quantum yields and excited-state lifetimes of mono-, bis- and tris-complexes of terbium(III) with pyridine-2,6-dicarboxylate (DPA) have been measured for aqueous solutions as a function of pH and in the presence of six different anions: bromide, chloride, sulfate, perchlorate, acetate and nitrate. The state of ligation of the terbium ion had a more dramatic effect on photophysical properties of the complexes in comparison with the alteration of other medium parameters. An increase in the number of DPA ligands, coordinated with Tb3+ (from zero to three) resulted in an increase in the emission quantum yield from 0.06 to approximately 0.4, with concurrent prolongation of the excited-state lifetime from 0.5 to approximately 2 ms. It was determined that both an increase in the non-radiative decay rates and a decrease in the values of the radiative decay rate constants, contribute to the luminescence quenching that was detected when the number of DPA ligands was decreased and/or the media acidified. The purpose of this investigation is to aid in the establishment of optimal conditions for conducting terbium(III)-based emission assays for DPA, which is found in the coats of endospores.

Chiral recognition phenomena probed by steady-state luminescence measurements: Stern-Volmer analysis of chiral discriminatory behavior

The chiral recognition phenomenon observed in enantioselective excited-state energy transfer processes currently requires the use of chiroptical spectroscopic techniques to probe the magnitude and sense of the discriminatory interactions. The use of chiroptical spectroscopic techniques limits the study of chiral recognition to those molecular species with strong absorption or emission dissymmetry factors. This study presents the theoretical and experimental methodology to determine the magnitude of chiral discriminatory interactions with unpolarized, steady-state luminescence measurements. Based on bimolecular luminescence quenching kinetics for a system containing chiral molecules, the Stern-Volmer equation is derived and contains a chiral discriminatory term for a system containing a chiral but racemic luminophore and an enantiomerically resolved quencher species. The utility of this methodology is confirmed by examining the enantioselective excited-state quenching between several Ln(dpa)(3)(3-) complexes (where Ln = Eu(3+), Tb(3+), or Dy(3+) and dpa = pyridine-2,6-dicarboxylate) acting as the energy donor and either racemic or enantiomerically resolved [Co(dach)(3)](3+) (where dach = trans-1R,2R (or 1S,2S)-diaminocyclohexane) acting as the energy acceptor in an aqueous solution. The results of this study confirm the utility of unpolarized, steady-state luminescence measurements as a probe of chiral discriminatory behavior.

Analysis of paramagnetic NMR spectra of triple-helical lanthanide complexes with 2,6-dipicolinic acid revisited: a new assignment of structural changes and crystal-field effects 25 years later

Variable-temperature (1)H and (13)C NMR measurements of the D(3)-symmetrical triple-helical complexes [Ln(L1-2H)(3)](3)(-) (L1 = pyridine-2,6-dicarboxylic acid; Ln = La-Lu) show evidence of dynamic intermolecular ligand-exchange processes whose activation energies depend on the size of the metal ion. At 298 K, the use of diastereotopic probes in [Ln(L3-2H)(3)](3)(-) (L3 = 4-ethyl-pyridine-2,6-dicarboxylic acid) shows that fast intramolecular P <==> M interconversion between the helical enantiomers occurs on the NMR time scale. Detailed analyses of the paramagnetic NMR hyperfine shifts according to crystal-field independent techniques demonstrate the existence of two different helical structures, one for large lanthanides (Ln = La-Eu) and one for small lanthanides (Ln = Tb-Lu), in complete contrast with the isostructurality proposed 25 years ago. A careful reconsideration of the original crystal-field-dependent analysis shows that an abrupt variation of the axial crystal-field parameter A(0)2<r (2)> parallels the structural change leading to some accidental compensation effects that prevent the detection of structural variations according to the classical one-nucleus method. Crystal structures in the solid state and density functional theory calculations in the gas phase provide structural models that rationalize the paramagnetic NMR data. A regular triple-helical structure is found for small lanthanides (Ln = Tb-Lu) in which the terdentate chelating ligands are rigidly tricoordinated to the metals. A flexible and distorted structure is evidenced for Ln = La-Eu in which the central pyridine rings interact poorly with the metal ion. The origin of the simultaneous variation of structural parameters and crystal-field and hyperfine constants near the middle of the lanthanide series is discussed together with the use of crystal-field-independent techniques for the interpretation of paramagnetic NMR spectra in axial lanthanide complexes.

Electronic State Structure and Optical Properties of Tb(oda)(3)(3)(-) Complexes in Trigonal Na(3)[Tb(oda)(3)].2NaClO(4).6H(2)O Crystals

Polarized optical absorption and emission measurements are used to locate and assign 95 crystal-field energy levels split out of the 4f( )(8) electronic configuration of Tb(3+) in single crystals of Na(3)[Tb(oda)(3)].2NaClO(4).6H(2)O (where oda denotes an oxydiacetate ligand). The absorption measurements span the 235-490 nm wavelength range, and the emission measurements span the 485-685 nm wavelength range. The combined absorption and emission spectra measurements provide access to the energy-level structures of 46 different 4f( )(8)[SL]J multiplet manifolds of Tb(3+) (all multiplet manifolds with baricenter energies <42 400 cm(-)(1) above ground). The site symmetry of the Tb(3+) ions in Na(3)[Tb(oda)(3)].2NaClO(4).6H(2)O is D(3), and the point-group symmetry of the tris-terdentate Tb(oda)(3)(3)(-) coordination complexes is also D(3). The Tb(oda)(3)(3)(-) complexes are chiral, and they exist in just one, fully resolved enantiomeric form in single crystals of Na(3)[Tb(oda)(3)].2NaClO(4).6H(2)O. The crystals exhibit strong chiroptical activity in their absorption and emission spectra, and results obtained from both circularly polarized and linearly polarized optical spectra measurements are used in making transition line assignments. The energy-level data acquired from the spectroscopic measurements are analyzed in terms of a model Hamiltonian that includes consideration of both isotropic and nonisotropic 4f electron/crystal-field interactions, and the interaction parameters derived from this analysis are discussed and then compared with those obtained for other Na(3)[Ln(oda)(3)].2NaClO(4).6H(2)O systems and for Tb(3+) in other crystalline hosts.