30th Helsinki Winter School in Theoretical Chemistry
F-element optical and NMR spectroscopy-insights from experiment and theory 30th Helsinki Winter School inTheoretical Chemistry 17/12/2014 Optical Spectroscopy 1: Lanthanides
Louise Natrajan Research Themes Transition metal polypyridyls: two photon spectroscopy
Pure oxidation state actinide compounds and fundamental chemistry Geochemistry of Uranyl Lanthanide upconverting nanoparticle enzyme biosensors and FRET Optical Spectroscopy X-ray diffraction UV-vis-nIR Emission
IR and Raman NMR EPR XAS (EXAFS, XANES) Builds on coordination chemistry courses (transition metals, spectroscopy) Electronic spectroscopy Quantum chemistry (terms and states) F-block course Dave Mills course will build upon this Recommended reading: Chemistry of the f-block elements-Helen Aspinall (Gordon and Breach) The f elements-Nikolas Kaltsoyannis and Peter Scott (Oxford Chemistry Primers) Principles of Fluorescence Spectroscopy-Joseph Lackowicz (Springer) Chemical Reviews special edition, 2002, 102, (6), 1805 2476. Useful Properties of the Lanthanides
Resulting from the core like nature of the f-orbitals: Catalytic convertors in car exhausts (CeO2) CeO2-good catalysts-hard Lewis acids (less competing deactivation mechanisms such as beta hydride elimination as bonding is ionic and not covalent) F-electronic configuration-long lived f-f- transitions results in very bright emission that can be used in lighting and imaging (as we will see later on in the course) F electron configuration and ground state term symbol means Gd can effectively shorten the T1 (longitudinal relaxation time) of protons (and water protons) close in space to the Gd centre-providing contrast between healthy and diseased tissue in imaging of the body. Does anyone know of any other uses of Lns? (give them a minute to think about)LASERS Nd-YAG-laser pen Optics and lighting - Polishing powders - Protection against sun (sunglasses) - Lasers, particularly Nd YAG - Phosphors for displays (incl. electrolumin. displays) - Fluorescent lamps Medicine - Seasickness (Ce oxalate), thromboses (Nd oxalate) - Renal insufficiency (La2(CO3)3.4H2O)- X-ray intensifying screens - NMR imaging - Cancer radio- and photo-therapy - Laser surgery (Nd YAG laser) - Luminescent immunoassays Science - Shift reagents, luminescent and magnetic probes - Catalysts for organic chemistry Lighting-fluorescent lamps Imaging in medicine: MRI (Gd), luminescent cellular probes (e.g. Eu) Lanthanide Complexes as Luminescent Sensors and Probes
Applications: luminescent phosphors: lighting, displays, security encoding Biological Uses: Structural probes (site symmetry, coordination number) Analytical probes (determining concentrations of analytes) Imaging probes for medical diagnosis e.g. tumour cell imaging Refs. T. Gunnlaugsson et al., Topics in Current Chemistry, 2007, 281, pg J. Yuan et al., Journal of Fluorescence, 2005, 15, pg Near Infra-red Excitation/Emission
> 500 microns depth penetration of near infra-red light Yb3+, Nd3+ Excitation in the red or nIR is advantageous-Ln, Yb, Nd especially 3D optical imaging Biological imaging Photochemistry Re-cap:
Absorption of light:Electronic transitions organic molecules, metal complexes (high ) Vertical absorption Vibrational relaxation (VR) Non-radiative de-activation excited state Vertical emission (radiative decay) Kashas rule: emission from lowest vibrational (relaxed) state E S0 S1* Fluorescence-radiative decay between states of the same multiplicity (2S+1) ground state Internuclear separation Photochemistry Re-cap:
excited singlet state (S1*) excited triplet state (T1*) Hunds Rule: for every singlet excited state, there is a triplet state which lies lower in energy ISC-intersystem crossing (S1- T1): horizonal transition radiationless process between 2 states of different multiplicity E S0 Phosphorescence-radiative decay between states of different multiplicity T1* ground State (S0) Internuclear separation More Rules! El Sayeds Rule Spin-Orbit Coupling
Intersystem crossing is slow unless there is a change in orbital type Spin-Orbit Coupling Crossing points occur on the S1 and Tn potential energy curves Application of a magnetic torque to an electron spin can result in a change in spin and a change in angular momentum Most common mechanism of ISC Improved with the addition of heavy atoms Selection Rules Spin Selection rule: S = 0 allowed transition
Fluorescence S1* to S0, S = 0, allowed, fast decay (ns) Phosphorescence T1* to S0, S = 1, forbidden, slow (s) Electronic transitions in lanthanides-Absorption f-f transitions (weak oscillator strengths, ~ 1 M-1 cm-1, c.f. d-d transitions ~ 100 and - ~ 105) Laporte selection rules (centrosymmetric cxs (i): l = 1 and g u f-f l = 0, slow decay results in long lived emission Relaxation of the Laporte selection rule occurs by spin orbit coupling (l and s) Described by Russell-Saunders coupling scheme Example Pr3+, 4f2,3H4 5 10 15 20 25 E / 103 cm-1 1G4 1D2 3H4 3F 1I6 3P 4 6 2 3 1 2 3PJ 1 1I6 1D2 Line-like absorption and emission spectra Transitions between states derived from Russell-Saunders spin orbit coupling (J) Pale colours (formally forbidden transitions) f-f Electronic Transition Intensities
Judd-Ofelt Theory Theoretical oscillator strengths f (J, J) of the J J transition at the mean frequency is given for an electric dipole transition he theoretical oscillator strengths fcal are derived by using the Judd-Ofelt theory. The magnetic dipole transitions, which give only negligible contribution to the transition bands of Nd3+, are not considered. Theoretical oscillator strengths f(J, J) of the J J transition at the mean frequency is given for an electric dipole transition by Eq.(2) Only really works for doped solids in high symmetry (e.g. cubic) Judd-Ofelt Celebration! Russell Saunders Coupling
Electronic structure of the 4f elements Each state will have: S overall spin and L overall orbital angular momentum: 2S+1L (d-orbitals) spin and orbital angular momentums can couple: 2S+1L J J takes values from |L+S||L-S| Spin multiplicity 2S+1, where S = overall spin LabelSPDFGHIJK L 2S+1L J J = L+S, L+S-1, L+S-2, |L-S| Hunds Rules for Ground State
Spin multiplicity must be the highest possible (Smax) If more than one term have the highest multiplicity, the term with the highest value of L is the ground state (Lmax) The ground level has Jmin if the subshell is less than half full The ground level has Jmax if the subshell is greater than half full Example: Nd3+, f3 Smax = 3/2, 2S+1 = 4 The ground state has the maximum possible value for S (Hunds rule) nad maximum possible values for L For less than half full shells, J takes the minimum value For greater than half full shells, J takes the maximum value NB J = total angular momentum ml l= 3 Lmax = = 6 = I state J = |L+S||L-S| = 12/2 + 3/2, 12/2 + 3/2 -112/2 - 3/2 = 15/2, 13/2, 11/2, 9/2 =4 I9/2 Russell Saunders Coupling: Nd3+
Transitions from excited state to ground 4IJ ground state Series of absorption and emission lines Crystal field small-can be observed in some transitions (noteably Nd3+, Yb3+) S = 1/2 + 1/2 = 1, 2S+1 = 3 L = = 5 = H J = 5+15-1, = 6, 5, 4 Ground state = 3H4 Pr3+ (f2): ml l= 3 4f Emission Spectra Line-like emission spectra
Long lived emission (milliseconds - microseconds) Range from UV (Gd3+), to visible (Tm3+, Sm3+,Eu3+, Tb3+, Dy3+) to near infra-red (Nd3+, Pr3+, Er3+, Yb3+) Glow stick time!! L.S. Natrajan, A.N. Swinburne, unpublished results, The University of Manchester, 2013. Defining the Emissive State
Energy gap law: emissive state is the one with the biggest energy gap to the next lowest state Energy gap = wavelength of emission Bigger gap = longer lifetime Tb3+ and Eu3+ millisecond lifetimes Non radiative quenching can reduce the lifetime e.g. by vibrations Lifetimes It is the average time spent in an excited state before radiative decay Given by the equation Units of lifetimes is usually ns The inverse is ns-1 which is a rate The intrinsic lifetime is the average time spent in the excited state without non-radiative deactivation processes Lifetimes The intensity of emission after a pump pulse decays exponentially Taking the natural log of this gives a linear relationship between intensity and time Methods used to measure lifetimes are a statistical methods based on the bulk solution Lifetimes are fitted using least squares regression Exchange processes which occur on timescales faster than the lifetime are averaged out. Exchange processes slower than the lifetime are resolved a separate lifetimes Time Dependence of Emitted Light
I(t) = It = 0.e-kt The lifetime of the excited state is given by: = 1/kobs (s) During this time, a fraction 1/e of the excited molecules return to the ground state (e = 2.73) 1/e I0 t = t Lifetimes of Ln3+ ( Tb3+, Eu3+ milliseconds Sm3+, Dy3+, Yb3+, Er3+ microseconds Pr3+, Nd3+ nanoseconds ( s) Lifetime Determination from Kinetic Traces
1. Stepwise increase of time delay (manual or software driven) The growth and decay of the luminescence at selected wavelengths was detected using a germanium photodiode (Edin- burgh Instruments, EI-P) and recorded using a digital oscilloscope Tb3+ complex in water: 0.1 ms steps (0.1 10 ms) at 545 nm L.S. Natrajan, P.L. Timmins, M. Lunn, and S.L. Heath, Inorg. Chem., 2007, 46, 1087710886. Lifetime Determination from Kinetic Traces
2. Time Correlated Single Photon Counting (TCSPC) The growth and decay of the luminescence at selected wavelengths was detected using a germanium photodiode (Edin- burgh Instruments, EI-P) and recorded using a digital oscilloscope Sm3+ complex 200 microsecond range at 650 nm: Tail fit L.S. Natrajan, J.A. Weinstein, C. Wilson, P.L. Arnold, Dalton Trans., 2004, 28, 3748. Lifetime Determination from Kinetic Traces
3. Photodiode detection with an oscilloscope The growth and decay of the luminescence at selected wavelengths was detected using a germanium photodiode (Edin- burgh Instruments, EI-P) and recorded using a digital oscilloscope Yb3+ in water, Ge-diode detector, 980 nm, ns/s timescale; Reconvolution fit with a scatterer L.S. Natrajan, P.L. Timmins, M. Lunn, and S.L. Heath, Inorg. Chem., 2007, 46, 1087710886. Grow-in Component (Rise Time)
Yb3+ (D2O) Yb3+ (H2O) Grow in longer than the laser pulse (not convoluted with the envelope of the laser pulse/detector response) Grow in exponential fitted before decay-indicates slow energy transfer from sensitiser triplet state to Yb3 emissive excited state (corresponds to emission decay of the organic sensitiser) If different in D2O than H2O, indicates phonon assisted energy transfer is the rate determining step Quantum Yields A measure of the efficiency on fluorescence
Absolute QYs Integrating sphere Hypersensitive Transitions
In hypothetical Ln3+, only magnetic dipole transitions are allowed: L = 0, J = 0, 1 e.g. 5D0 7F1 in Eu3+ In a complex, electric dipole (ED) transitions are induced: L, J = 0, 2, 4, 6 (00 forbidden) Some transitions acquire strength by both MD and ED e.g. Tb3+ Some transitions are sensitive to changes in symmetry and/or the inner coordination sphere. They display shifts in their maxima, splitting and intensity Hypersensitive transitions-some transitions are more likely in certain circumstances-quadripolar or octapolar The electronic spectra of lanthanide(III) complexes have been studied to give information regarding structure and bonding in these complexes. Most of the sharp lines like 4f-4f transitions originating within the 4f configuration of the lanthanide(III) ions are little affected by the environment about the lanthanide ions. A few, however, are very sensitive to the environment and are more intense when the ion is complexed than they are in the corresponding aqua-ions. The oscillator strength and band shape of these transitions in lanthanide(III) complexes are especially sensitive to the structural details and the chemical nature of the ligand environment. Such transitions have been called hypersensitive transitions and the phenomenon is generally referred to as hypersensitivity. The oscillator strength and band shapes of the hypersensitive transitions in the absorption spectra of the lanthanides have been correlated to the complex formation, coordination numbers, coordination geometry, ligand structure, chelate solvent interactions and symmetry of the field around lanthanide ion. These are termed the selection rules for electric dipole transitions. It is clear, for instance, that the electric dipole approximation allows a transition from astate to astate, but disallows a transition from ato astate. The latter transition is called a forbidden transition. Forbidden transitions are not strictly forbidden. Instead, they take placeat a far lower rate than transitions which are allowed according to theelectric dipole approximation. After electric dipole transitions, the nextmost likely type of transition is a magnetic dipole transition, which is dueto the interaction between the electron spin and the oscillating magneticfield of the incident electromagnetic radiation. Magnetic dipoletransitions are typically abouttimes more unlikely than similar electricdipole transitions. The first-order term in Eq.(845) yields so-calledelectric quadrupole transitions. These are typically abouttimes moreunlikely than electric dipole transitions. Magnetic dipole and electricquadrupole transitions satisfy different selection rules than electricdipole transitions: for instance, the selection rules for electricquadrupole transitions are . Thus, transitions which are forbidden aselectric dipole transitions may well be allowed as magnetic dipole orelectric quadrupole transitions. In the hypothetical free lanthanide ion, only magnetic dipole (MD) transitions are allowed. These are selected by the [DELTA]J = 0, [+ or -] 1 (but J = [right arrow] J = is forbidden) rule. Their probability is relativily easily calculated (40) and practically independent of the surrounding matrix. One example of a purely MD transition is the [sup.5][D.sup.0] [right arrow] [sup.7][F.sup.1] emission line of [Eu.sup.3+] (see Figure 2). In a coordinating environment, electric dipole (ED) transitions are induced as the ligand field mixes odd-parity configurations slightly into the [Xe] [4f.sup.n] [5d.sup.0] configuration. Most of the absorption and emission lines are such induced ED transitions. Some transitions acquire strength both by MD and ED schemes: the emission spectrum of [Tb.sup.3+] is dominated by mixed ED/MD transitions. Since ED transitions in lanthanide ions are induced by the ligand field their strengths (or: probabilities) are quite sensitive to it. Strongly asymmetric or strongly interacting ligand fields lead to relatively intense ED transitions. The intensities of some ED transitions are extremely sensitive to coordinating environment, which means that they can be either completely absent or very intense, depending on the ligand field. An example of such a hypersensitive transition is the [sup.5][D.sup.0] [right arrow] [sup.7][F.sub.2] emission line of [Eu.sup.3+] (Figure 2). Hypersensitive transitions-often quadrupolar transitions: J = 2 e.g. 5D0 7F2 in Eu3+ Mechanism: mixing of the 4f states with ligand states Some Examples: Eu3+ Eu3+ ion in high symmetry environment
J = 1 J = 2 J = 3 J = 4 Eu3+ ion in high symmetry environment J = 1, J = 2 and J = 4 transitions strong J = 0 transition very weak Eu3+ ion low symmetry environment J = 1 weak J = 0 fairly strong J = 2 very strong Split into two components Hypersensitive Transitions in Nd3+
4F3/2 4I13/2 Emission 780 800 820 2H9/2,4F5/24I9/2 Nd(BrO3)3.9H2O (s) [Nd(H2O)9]3+(aq) NdCl36H2O (s) CN = 9 CN = 9 Absorption Faulkner et al., J. Am. Chem. Soc., 2009, 131, 9916 9917. Other Transitions: Absorptions
f-d transitions: allowed by Laportes selection rule Highly energetic except for Ce3+, Pr3+ and Tb3+ (4f to 5d) 225 250 300 nm [Ce(H2O)9]3+, D3h symmetry Ce3+ [Xe]5d1 generates two levels, 2D3/2 and 2D5/2 4 transitions ~ 800 M-1 cm-1 300 200 100 / M-1cm-1 E / 103 cm-1 Tb Pr [Ln(H2O)9]3+ LMCT Transitions Occurs in complexes with ligands that can be oxidised (redox reaction)-Laporte allowed Therefore the metal must have an accessible +2 oxidation state Ar + Eu3+ Ar + Eu2+ Transient Ligand radical + Eu(II) -* = 260 M-1 cm-1 LMCT Natrajan et al., Inorg. Chem., 2007, 46, Sensitised Emission hvLn
The special case of 4f elements: indirect excitation In view of weak f-f oscillator strengths, direct excitation (f-f absorption) is inefficient Laporte selection rule overcome by the antenna effect Energy transfer hv Light harvesting hvLn Light emission Organic chromophore:hv1 + Ar 1Ar* 3Ar* Ln* Ln + hvLn S. Faulkner, L. S. Natrajan, W. S. Perry, D. Sykes, Dalton Trans., Perspective Article, 2009, 3890; L.S. Natrajan, Current Inorganic Chemistry, 2011, 1, 61. Eu -diketonates [Eu(dbm)4][NEt4] Dbm = dibenzoylmethane hv Eu3+
Courtesy of Helen Aspinall, Liverpool University, UK. Jablonski Diagram Sensitised emission: Energy level scheme
Heavy atom effect -promotes ISC kbet kfluorescence kphosphorescence Competing pathways: Ligand fluorescence Ligand phosphorescence Back energy transfer Represent energy migration pathways-pass an example around/show them at end of lecture NB vibrational energy levels of S0 not shown for clarity Optimising energy transfer from ligand to metal heavy atom effect The enhancement of the rate of a spin-forbidden process by the presence of an atom of high atomic number, which is either part of, or external to, the excited molecular entity. Mechanistically, it responds to a spin-orbit coupling enhancement produced by a heavy atom. ENERGY MIGRATION PATHWAYS Competing pathways, each has a rate constant, overall lots of rate constants Triplet mediated sensitisation path LMCT Quenching in Eu3+ Complexes
Competitive radiative decays (Eu emission + phosphorescence) Non radiative LMCT decay to ground state S. Faulkner, L. S. Natrajan, W. S. Perry, D. Sykes, Dalton Trans., Perspective Article, 2009, 3890; L.S. Natrajan, Current Inorganic Chemistry, 2011, 1, 61. Double electron transfer quenches the Eu3+ emission (NR decay) LMCT Sensitised Yb3+ Emission
Alternative mechanism of energy transfer Fast energy transfer-mediated via an orbitally allowed transition For NIR emitting Ln3+, a wider range of chromophores can be used: e.g. Here, the LMCT state doesnt directly overlap with the lanthanide emissive state, which precludes back energy transfer, so that the LMCT state can actually sensitise Yb emission-cant happen for Nd as the +2 state is harder to access-need the right redox potential from the ligands-so LMCT rather than triplet feeds the Ln manifold DOWNHILL ENERGY PROCESSES Ar(S0)Ln3+ Ar(S1)Ln3+ Ar(T1)Ln3+ ArLn2+ Ar(S0)(Ln3+)* Ar(S0)Ln3+ S. Faulkner, L. S. Natrajan, W. S. Perry, D. Sykes, Dalton Trans., Perspective Article, 2009, 3890; L.S. Natrajan, Current Inorganic Chemistry, 2011, 1, 61. Excitation Spectra Excitation spectrum: Quanta of light emitted at a given wavelength as a function of excitation wavelength; the excitation spectrum follows the absorption spectrum, as the number of photons emitted is directly proportional to the amount of light absorbed. Excitation (ligand)Emission (f-f) Confirming which electronic transitions are responsible for the emission bands Natrajan et al., Dalton Trans., 2006, 4456. Vibrational Quenching by O-X Oscillators
Limited quenching by O-D oscillators:-(Hookes law) NB. Vibrational overtones-overcome quenching by perfluorination or perdeuteration-but quenching can be useful. Ln excited states are quenched by: O-H, N-H and C-H vibrations Calculating the Inner Sphere Hydration Number (q)
Lanthanide coordination compounds are labile: Use quenching of O-H vs. O-D oscillators to determine q Horrocks Equation: q = A(kH2O - kD2O-B) k = rate constant of decay = 1/ A, B = proportionality constants A corrects for inner sphere quenching B corrects for outer sphere quenching A and B values have been determined for Eu3+, Tb3+, Yb3+ (Nd3+ and Sm3+) Horrocks et al., . J. Am. Chem. Soc. 1997, 119, Faulkner, et al., Inorg. Chem. Commun. 2001, 187. Example: [Eu(DTPA)]2- q = A(kH2O - kD2O-B)
Q. For the Eu3+ complex determine q: H2O = 0.63 milliseconds (ms) D2O = 2.60 milliseconds q= 1.1 ie. one molecule of water is coordinated to europium q = A(kH2O - kD2O-B) A = 1.2 ms B = 0.25 q = 1.2((1/ /2.60) ) 7 coordinate ligand-1 molecule of H2O- Luminescent Chemical Sensors
The spectroscopic properties of the lanthanides make them ideal luminescent probes: Line like emission spectra: easily identifiable spectral fingerprint Long luminescent lifetimes Large Stokes shift ( absorption and emission) Time-gating: removal of background fluorescence Time-resolved luminescence: allows high signal to noise ratios 2 - 4 ms Collect Ln3+ emission (Eu, Tb) Allow background fluorescence to decay time I em Excite with UV pulse Macrocyclic Complexes
Free Ln3+ ions are toxic to living organisms For biological applications e.g. imaging agents, multidentate or macrocyclic ligands are often employed Using the chelate effect to create kinetically stable complexes Coordinatively saturated complexes- to exclude water molecules from the inner coordination sphere e.g. Ln3+ Luminescence as a Signalling Method
A. Modification of the Ln3+ inner coordination sphere Binding of an analyte reduces O-H quenching and q decreases Analyte B. Modulation of the energy transfer processes Analyte Binding of an analyte switches on or off the Ln3+ emission Courtesy of Jean-Claude Bnzli Parker et al., J. Am. Chem. Soc., 2001, 123, 7601.
Example: pH Sensor pH 4,q = 1.6 pH 10,q = 0.1 Parker et al., J. Am. Chem. Soc., 2001, 123, 7601. Change in Eu3+ coordination sphere reflected in ratio of hypersensitive transitions R modulates electron density at N atom and alters the pKa of the sulphonamide N atom-R = CF3, Me, OMe Example: Anion sensors
Bicarbonate sensor-ratiometric probe Lactate and citrate assay Citrate levels are reduced in prostate cancer-usually enzymatic assays for dtermining concentration of analytes Lactate and citrate are key metabolites in the intermediary metabolism of the cell-Citrate key in the Krebs cycle (citric acid cycle). The synthesis and oxidation of citrate provides a major energy supply in cells (70%). -diminished citrate levels linked with varoius aspects of kidney disfunction. Lactate anaylsis of serum is important in sports medicine and clinincal medicine. E.g. lactate is elevated in liver damage and liver disease. Work by cahnges in intensity of the Eu luminescence and by changes in local coordination environment I.e. cahnges in hypersensitive bands Parker et al., Org. Biomol. Chem., 2009, 7,1525. Example: Peptide Conjugates and Targeted Imaging
Tb3+ complex coupled to a peptidic vector-Tuftsin Affinity for macrophage cells Targetted probes/imaging agents This complex used as a luminescent probe containing a Tuftsin targeting vector coupled to a Tb complex that is internalised by macrophage cells-useful probes of the immune response with involvement of macrophage-monocyte cells in a variety of disease states Temporal discrimination from background autofluorescence ns timescale, large stokes shift etc etc H2O = 1.67 ms D2O = 3.25 ms q = 1.2 Faulkner et al., Chem. Commun., 2006, 909. Properties of the lanthanide ions
Properties of the lanthanide ions. Note;eff the calculated values and show good agreement with experimental values with the exceptions of SmIII (eff exp = ) and EuIII (eff exp = ) due to their excited states being close to their ground state.