David H. Metcalf

Measurement and analysis of excited-state decay kinetics and chiroptical activity in the 6HJ ← 4G52 transitions of Sm3+ in trigonal Na3[Sm(C4H4O5)3] · 2NaClO4 · 6H2O

Abstract Axial total luminescence (TL) and circularly polarized luminescence spectra are presented for the 6 H J (J = 5 2 , 7 2 , 9 2 ) ← 4 G 5 2 transition regions of Na3[Sm(oxydiacetate)3]·2NaClO4·6H2O (SmODA) for sample temperatures of 77 K and room temperature (∼ 300 K). TL and CPL data are rationalized in terms of dipole strengths and rotatory strengths of transitions between the individual crystal-field levels of Sm3+. The calculation of dipole- and rotatory strengths was accomplished using crystal-field wave functions and electric-dipole intensity parameters determined in previous studies of the polarized absorption spectra of Sm3+ in SmODA. Calculated dipole strengths and rotatory strengths for crystal-field transitions in the 6 H J (J = 5 2 , 7 2 , 9 2 ) ← 4 G 5 2 regions are given. Simulated TL/CPL spectra are generated from calculated dipole- and rotatory strengths, and are compared to observed data. Excellent agreement is found between theory and observation. Non-radiative deactivation of 4 G 5 2 emission in SmODA and 5D0 emission in EuODA is investigated. It is shown that two vibrational modes are active in non-radiative deactivation from these states: the C-H stretching mode of the oxydiacetate ligand; and the O-H stretching mode of lattice waters. Multiphonon emission due to interaction with both vibrational modes is analyzed in terms of the modified energy-gap law of Van Dijk and Schuurmans. It is found that both modes couple with near-equal strengths to the lanthanide ion. It is suggested that the processes promoting the multiphonon transitions may have different efficiencies for the two active vibrational modes in these systems.

Chiral discrimination in electronic energy-transfer processes in solution. Effects of temperature and solution properties on chirality-dependent rate parameters

Abstract Time-resolved chiroptical luminescence (TR-CL) measurements are used to study the kinetics of chilality-dependent excited-state quenching processes in solution. Solution samples contain a racemic mixture of chiral luminophore molecules (L) and a small, optically-resolved concentration of chiral quencher molecules (Q) in an achiral solvent (H 2 O, D 2 O, or an H 2 O-ethylene glycol mixture). The luminophores are excited with a pulse of unpolarized light to create an initially racemic excited-state population of ΔL ∗ and ΛL ∗ enantiomers, and TR-CL measurements are then used to monitor the differential decay kinetics of the ΔL ∗ and ΛL ∗ subpopulations. Observed differences between the ΔL ∗ and ΛL ∗ decay kinetics differential rate processes and efficiencies for ΔL ∗ -Q versus ΛL ∗ -Q quenching actions, and they are diagnostic of chiral discriminatory interactions between the luminophore and quencher molecules. In each of the systems examined here, quenching occurs via electronic energy-transfer processes in transient (ΔL ∗ -Q) and (ΛL ∗ -Q) 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. Both quenching rates and enantioselectivity are observed to increase with increasing temperature over the temperature ranges represented in this study (253–353 K for H 2 O-ethylene glycol solutions and 273–353 K for H 2 O and D 2 O solutions), and differences between the free-energies of activation for ΔL ∗ -Q versus ΛL ∗ -Q quenching processes increase from ≈ 2.5 to 4.8 kJ/mol over the 273–353 K temperature range. Both the luminophore and quencher molecules examined in this study have three-bladed propeller-like structures with very similar shapes and sizes, and differential steric constraints in (ΔL ∗ -Q) versus (ΛL ∗ -Q) contact pairs are small. However, at room temperature and above, the relative efficiencies of ΔL ∗ -Q versus ΛL ∗ -Q quenching processes differ by factors > 3.2, indicating an extraordinary degree of chiral discrimination in the underlying interaction processes.

Chirality-dependent intermolecular interactions probed by time-resolved chiroptical luminescence measurements of enantio-differential excited-state quenching kinetics

Abstract Time-resolved chiroptical luminescence (TR-CL) spectroscopy is used to measure the chirality dependence of dynamic excited-state quenching processes in solution. Measurements are performed on a series of luminophore-quencher systems in which the interacting molecular species have similar overall shapes and sizes, but differ with respect to one or more well-defined (and incrementally varied) elements of structural chirality. The measurements yield rate constants for competing enantiospecific quenching processes, and the relative values of these rate constants provide information about both the degree and sense (handedness) of chiral discriminatory interactions between the luminophore and quencher species. It is shown that both the degree and sense of chiral recognition/discrimination in the quenching processes are modulated by small changes in quencher structure.

Circularly polarized luminescence (CPL) spectra of Samarium(III) in trigonal Na3[Sm(oxydiacetate)3]· 2NaClO4·6H2O

Abstract Circularly polarized luminescence (CPL) from Sm(III) in trigonal Na3[Sm(oxydiacetate)3]· 2NaClO4· 6H2O is reported. This luminescence occurs from the 4G 5 2 multiplet of Sm(III) and, in the spectra reported here, it spans the 4G 5 2 →6HJ(J= 5 2 , 7 2 , and 9 2 ) transition regions of Sm(III). Rotatory and dipole strengths are calculated for all crystal-field transitions lying within these regions, and calculated spectra are produced to help interpret the observed spectra. Comparisons between calculated and observed spectra permit an assessment of the energy and intensity parameters derived from a previous analysis of Na3[Sm(oxydiacetate)3] · 2NaClO4· 6H2O absorption spectra.

Optical absorption and emission spectra of Tb3+ in hexagonal crystals of Na3[Tb(pyridine-2,6-dicarboxylate)3] · NaClO4 · 10H2O

We have measured and analyzed the polarized orthoaxial luminescence and absorption spectra of Tb3+ and Eu3+ in single crystals of trigonal Na3[Ln(dpa)3]·NaClO4·10H2O (dpa = 2,6-pyridinedicarboxylate, the dianion of dipicolinic acid). Differential linear polarization (σvs. π) is measured in the orthoaxial luminescence and absorption spectra. A semiempirically derived energy level scheme is developed for the crystal field components of the Tb3+ (7Fj (J = 0–6), 6D4, 6D3, 5G8, 6L10, 6D2) 6G5) and the Eu3+ (7Fj (J = 0–6), 6Dj (J = 0–3) multiplets.

Stereochemical dynamics and recognition probed by lanthanide chiroptical spectroscopy

Abstract A mechanistic model for enantioselective excited-state quenching kinetics is described for systems in which quenching occurs via electronic energy transfer between chiral lanthanide (donor) and transition-metal (acceptor) complexes in solution. This model is applied to systems in which quenching rates are slow compared with translational and rotational diffusion rates of the donor and acceptor species in solution. The rate parameters derived from enantioselective quenching measurements are expressed in terms of both electronic and stereoselective contributions to intermolecular chiral discrimination, and applications are illustrated for several lanthanide (luminophore)-transition metal (quencher) systems.

Optical activity of Na3[Dy(oxydiacetate)3] · 2NaClO4 · 6H2O☆

Abstract Circular dichroism (CD) and circularly polarized luminescence (CPL) spectra are reported for single crystals of Na 3 [Dy(ODA) 3 ]·2NaClO 4 · 6H 2 O (ODA ≡; oxydiacetate) at 10 K. The spectra for transitions with significant magnetic dipole intensity (selection rule ¦ΔJ¦⩽1 ) show large amounts of optical activity, as expected. Optical activity observed in other transitions is dependent on the degree of mixing of other levels into the states involved.

Chirality-dependent excited-state quenching of Eu(dpa)33− and Tb(dpa)3+3 luminophores by diastereomeric structures of Δ-[Co(trans-1,2-cyclohexanediamine)3]3+ in aqueous solution

Abstract Four diastereomers of Δ-[Co( trans -1,2-cyclohexanediamine) 3 ] 3+ are examined as enantioselective excited-state quenchers of chiral Eu(dpa) 3 3− and Tb(dpa) 3 3− luminophores (dpadipicolinate dianion) in solution. In their quenching actions, all four diastereomers can distinguish between and interact differentially with the Λ and Δ enantiomers of the luminophores, but their enantiomeric preferences show interesting variations that may be correlated with chelate-ring conformational structure.

The determination of chiroptical dissymmetry factors and electric-vs. magnetic-dipole transition amplitude ratios from circularly polarized excitation and emission measurements on racemic Eu(dpa)33− in aqueous solution

Abstract A combination of circularly polarized excitation and emission measurements is used to determine the chiroptical properties of Eu(dpa)3−3 complexes in aqueous solution (dpadipicolinate dianionpyridine-2,6-dicarboxylate). Intrinsic chiroptical dissymmetry factors are measured for four absorptive and three emissive transitions, and the results are used to calculate ratios of electric- versus magnetic-dipole transition amplitudes.

Time-resolved chiroptical luminescence spectra of UO22+ in cubic Na[UO2(CH3COO)3] crystals: evidence for circular dichroic self-absorption

Abstract Time-resolved chiroptical luminescence (TR-CL) measurements are reported for cubic crystals of Na[UO 2 (CH 3 COO) 3 ]. These crystals belong to the enantiomorphic space group P 2 1 3, and they exhibit strong chiroptical activity within the Σ g + ← Π g emission spectrum of UO 2 2+ . Circular polarization is observed in the origin line of this spectrum, as well as in a vibronic progression from the origin based on the symmetric stretching mode ( ν S ) of UO 2 2+ . The TR-CL results are used to confirm the presence of circular dichroic self-absorption within the origin transition.