Harry P. J. M. Dekkers

Optical Artifacts in Circularly Polarized Luminescence Spectroscopy

The source of optical artifacts in circularly polarized luminescence is examined by application of the method of Mueller matrices. Expressions for the circularly polarized luminescence intensity are developed that illustrate that the source of the artifact in most cases is due to the small inherent birefringence in the photoelastic modulator employed in the experimental measurement. Experimental results are presented that are consistent with this conclusion.

Chiroptical properties of an optically pure dicopper(I) trefoil knot and its enantioselectivity in luminescence quenching reactions

Chiroptical spectroscopy is used to investigate the properties of an optically pure dinuclear copper(I) trefoil knot. For the metal-to-ligand charge tranfer (MLCT) transition in the visible region (520 nm), the electric and magnetic transition dipole moments are determined from absorption and circular dichroism spectra: 2.8 Debye and 0.5 Bohr magneton (muB). Circular polarization in the luminescence (CPL) of the knot is determined and this allows the electric and magnetic transition dipole moments in emission to be calculated: 0.02 Debye and 0.003 muB. The large difference between the moments in absorption and emission shows that the emission observed does not originate directly from the 1MLCT state. Given the low probability for radiative decay we assign the long-lived emitting excited state to a 3MLCT state. The copper(I) trefoil knot is found to quench the emission from TbIII and EuIII(dpa)3(3)-(dpa = pyridine-2,6-dicarboxylate) with a bimolecular rate constant of 3.2 and 3.3 x 10(7)M(-1)S(-1), respectively, at room temperature in water-acetonitrile (1:1 by volume). Experimental results indicate that the (lambda)-knot preferentially quenches the lambda enantiomer of the lanthanide complex with an enantioselectivity (ratio of quenching rate constants for lambda and lambda: kqlambda/kqdelta) of 1.012+/-0.002 for EuIII and 1.0180+/-0.003 for TbIII.

Discrimination between 3ππ* and 3nπ* states in organic molecules by circular polarization of phosphorescence

Abstract The degree of circular polarization g in electronic transitions of chiral molecules is a measure of the ratio of the magnetic and electric dipole transition moments. From theory an upper bound for |g| as a function of dipole strength D can be found. This offers the possibility to distinguish 3ππ* from 3nπ* states in organic molecules. For molecules of approximate C2v symmetry it can be shown that large absolute values of glum (order 10-1) can arise only in the case of 3ππ* phosphorescence, whereas emission from 3nπ* states yields low values of glum (order 10-3). This conclusion is substantiated with experimental data on three examples: α-santonin, (1S)-(-)-1-bromo-α-fenchocamphoronequinone and (+)-thicamphor.

Linearly polarized luminescence spectra of Eu(2,6-pyridine-dicarboxylate)3−3 in hydroxylic solution

Emission spectra and linearly polarized emission spectra are reported for Eu(2,6-pyridine-dicarboxylateDPA)3−3 in a hydroxylic glass (water/ethylene glycol, 1:2 by volume) at 157 K. The splitting pattern of the 7FJ←5D0(J=1, 2, 4) emission bands (in order of decreasing energy A2, E; E, E and E, A2, E, E) and their polarization anisotropies upon 465.8 nm irradiation are consistent with the complex having approximate D3 symmetry. The two 7F1←5D0 bands have predominantly (≳80%) magnetic dipole character. The polarization data upon excitation in the 290 nm absorption show that this band involves π→π* ligand transitions, polarized perpendicular to the C2 axis of the DPA ligand. The angle between the C3 axis of the complex and the normal vector to the plane of the pyridine ring is found to be 41°±5°.

Time-resolved polarization of luminescence spectroscopy : an accurate and versatile digital instrument for the sub-μs time domain

An instrument is described for the measurement of circular and linear polarizations of the luminescence with a time resolution up to 50 ns. The spectrometer, which makes use of a 50-kHz photoelastic modulator and a differential photon-counting technique, is accurate and absolute and is capable of detecting small degrees of polarization. Making use of excitation light pulses which lie randomly in the modulators duty cycle, the instrument can be readily interfaced with a variety of externally triggered or free running pulsed light sources.

Measurement of the Circular Polarization of the Luminescence of Photoselected Samples Under Artifact-Free Conditions

The measurement of the Circular Polarization of the Luminescence (CPL) in viscous solvents is hampered by the occurrence of artifacts. These arise when the emitted radiation is partially linearly polarized due to the photoselection effect. It is shown that there are two configurations of the CPL spectrometer in which such linear polarization artifacts are virtually negligible: one requires perpendicular excitation and emission channels as well as horizontally polarized incident radiation; the other involves a 180° geometry and unpolarized excitation light. In both options, essentially artifact-free CPL data are obtained, enabling the study of the directional dependence of the optical activity in emission. This is illustrated for the case of the 3ππ* → S0 phosphorescence of the cross-conjugated dienone α-santonin.

pH dependence of the enantioselective excited-state quenching of Λ,Δ-Tb(III) and Λ,Δ-Eu(III)tris(pyridine-2,6-dicarboxylate) chelates by ferricytochrome c from horse heart and ferricytochrome c-550 from Paracoccus versutus

Abstract The pH dependence of the dynamic quenching of the luminescence from Tb(III) and Eu(III) tris(pyridine-2,6-dicarboxylate≡DPA) chelates by the title proteins is studied. For Tb(DPA)33– also the quenching by the Lys 14→Glu and Lys99→Glu mutants of cytochrome c-550 (cytc-550) is investigated. The rate constants for quenching of the electronically excited Λ and Δ enantiomers of the luminophore by equine cytochrome c show a sharp decrease upon increasing the pH from 7 to 10, which can be described phenomenologically by deprotonation of a single acidic group with pKa of 9.2±0.1 for Eu and 9.4±0.1 for Tb. These values are similar to that found for the alkaline transition of the protein. The alkaline conformer(s) of the protein at pH>10 is found to be a very inefficient quencher of the lanthanide luminescence. For Tb, but not for Eu, a significant lowering of the degree of enantioselectivity (Eq) in the quenching is found along with a reduction of the quenching rates. For cytc-550, the decrease of the quenching rate constants with increasing pH is described by pKa=9.8±0.1 and for the two mutants the same value is obtained. For the cytc-550 proteins the change of the quenching rates does not correlate with the alkaline transition, for which a pKa of 11.2 has been reported by other workers. For all proteins, the reduction of the quenching rates at high pH is ascribed to a reduction of the binding affinity of the excited lanthanide complex to the surface area of the protein near the exposed heme edge, caused by deprotonation of (presumably) several lysine residues.

Direct determination of enantiomeric purity using CPL spectroscopy

Abstract The dependency of the Circular Polarization of the Luminescence (CPL) on the handedness of the excitation light potentially provides a means to determine enantiomeric purity. In literature two procedures have been outlined for such determinations, but as yet they have not been experimentally tested. These procedures, along with a third one we propose here, are evaluated from theory and confronted with experiment. It appears that the CPL technique provides a valuable tool for the determination of enantiomeric purity. The scope and limitations of the method are discussed.

Enantioselective outer-sphere complexation of lanthanide(III) tris (pyridine-2,6-dicarboxylate) chelates with vitamin B12 derivatives. An NMR and CD study

Abstract The binding of three corrinoids, cyanocobalarnin (vitamin B12, CNCbl), aquacobalamin (vitamin B12a, H2OCbl+) and dicyanocobinamide ((CN)2Cbi), collectively denoted by Q, to the Δ- and Λ-enantiomer of Ln(DPA)33− complexes (Ln=lanthanide, DPA=2,6-pyridine dicarboxylate) in aqueous solution at room temperature is investigated by NMR and Circular Dichroism (CD) spectroscopy. Tb(DPA)33− induces shifts of the 1H-NMR resonances of Q. A formula, describing the dipolar interaction between the proton spin and the paramagnetic moment of the lanthanide complex, is fitted to the lanthanide induced shifts, yielding the position of Tb(DPA)33− and the orientation of its magnetic axis with respect to the corrinoid. The magnitude of the shifts provide the value of Kavg, an average of the association constants of the two diastereomeric complexes {Δ-Tb(DPA)33−Q) and {Λ-Tb(DPA)33−Q}. We find Kavg=4±0.5 (B12) and 0.8±0.2 M−1 ((CN)2Cbi). The value for H2OCbl+ depends strongly on the solution’s ionic strength I: Kavg=(7±1)×101, 22±5 and 16±7 M−1 at I=0.007, 0.12 and 0.22 M, respectively. The addition of the corrinoids to a (racemic) solution of Nd(DPA)33−, induces CD in the electronic Nd(III) transitions in the 700–800 nm region (ICD). This Pfeiffer effect is attributed to an excess of Λ-Nd(DPA)33− in the enantioselective, outer-sphere association with the corrinoids. From the magnitude of the ICD as a function of [Q], in conjunction with the (indirectly) determined CD effect of Λ-Nd(DPA)33−, values are determined for KΔ−KΛ. We find KΔ−KΛ=−24±2 M−1 for H2OCbl+ at pH 6.7 and I=0.12 M and −12±2 M−1 at I=0.22 M; −4.4±0.5 M−1 for CNCbl and −0.9±0.3 M−1 for (CN)2Cbi. Adopting the view that in the lanthanide series Ln(DPA)33− chelates are isostructural, the results of the NMR and CD experiments are combined to estimate the enantioselectivity in the binding Eb, defined as (KΔ−KΛ)/2Kavg. We find Eb=−0.5±0.2 for H2OCbl+, −0.7±0.2 for CNCbl and −0.7±0.3 for (CN)2Cbi. On the basis of the NMR data, a molecular structure is proposed for the two diastereomeric {Ln(DPA)33−Q} complexes. The bonding involves hydrogen bonds between the amide proton on the a and g side chain of the corrinoid and the carboxylate oxygen of two DPA ligands.

Enantioselective excited-state quenching of racemic Tb (III) and Eu (III) Tris (pyridine-2,6-dicarboxylate) by vitamin B12 derivatives

Abstract Enantioselectivity in the dynamic quenching of the luminescence of the Δ and Λ enantiomers of racemic Tb(III)(pyridine-2,6-dicarboxylate=DPA) 3 3− and Eu(DPA) 3 3− by a series of corrinoids is demonstrated by time resolved luminescence and circular-polarization-of-luminescence (CPL) spectroscopy. Studied are cyanocobalamin (vitamin B 12 ), aquacobalamin (B 12a ) and its conjugated base hydroxocobalamin (HOCbl), dicyanocobinamide ((CN) 2 Cbi) and the heptamethyl ester of dicyanocobyrinic acid ((CN) 2 Cby(OMe) 7 ). For this set of quenchers (Q), the diastereomeric quenching rate constants ( k q Δ and k q Λ ) are reported together with the degree of enantioselectivity E q =( k q Δ − k q Λ )/( k q Δ + k q Λ ). In the systems with Tb, values of the average rate constant k q avg (=( k q Δ + k q Λ )/2) are 1.0, 2.9 and 0.53 10 8 M −1 s −1 for CNCbl, (CN) 2 Cbi, (CN) 2 Cby(OMe) 7 with E q =−0.24, −0.20, +0.01 (standard error of E q is 0.01). The quenching by B 12a is strongly dependent on pH and ionic strength ( I ); when I =12 mM we find k q avg =5.3, E q =−0.23 at pH 6.7 and k q avg =1.3, E q =−0.27 at pH 8.9. Corresponding rates for Eu are 0.41, 27, 3.4 10 7 M −1 s −1 and for B 12a , 7.3 and 1.2 10 7 M −1 s −1 , corresponding values for E q −0.27, −0.29, +0.02, −0.21 and −0.29. The quenching reaction is modeled as a pre-equilibrium involving the formation of an encounter complex (association constant K ) followed by the actual electronic energy transfer step (rate k et ). By relating the quenching data with molecular structure it is argued that the binding in the encounter complex involves two hydrogen bonds between the uncoordinated carboxylate oxygen atom of two DPA ligands of Ln(DPA) 3 3− and two amide groups of the corrinoid, presumably involving the a and g, the a and b, or the b and g side chains. For some corrinoid/Ln(DPA) 3 3− complexes the association constants and enantioselectivities in the ground state are known (Spectrochimica Acta 55A (1999) 1837–1855), which allows for an estimate of the average rate of energy transfer, k et avg (i.e. ( k et Δ + k et Λ )/2). The enantioselectivity in the quenching reaction is lower than in the ground state association which is interpreted in terms of different values of k et in the two diastereomeric Ln-corrinoid complexes; for both Tb and Eu we find k et Δ / k et Λ ∼0.3 with CNCbl, B 12a at pH 6.7 and (CN) 2 Cbi. These data imply that the chiral discrimination in the energy transfer is considerable and counteracts that in the binding but does not dominate it.