F-element optical and NMR
spectroscopy-insights from experiment
and theory
30th Helsinki Winter School in
Theoretical Chemistry
17/12/2014
Optical Spectroscopy 1:
Lanthanides
Louise Natrajan
louise.natrajan@manchester.ac.uk
Research Themes
Pure oxidation state
actinide compounds
and fundamental
chemistry
Transition metal polypyridyls:
two photon spectroscopy
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)
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)
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. 1- 43.
J. Yuan et al., Journal of Fluorescence, 2005, 15, pg. 559 - 568.
Near Infra-red Excitation/Emission
> 500 microns depth
penetration of near infrared light
Yb3+, Nd3+
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
Vertical emission (radiative decay)
state
Kasha’s rule: emission from
lowest vibrational (relaxed) state
E
S1 *
ground
state
Internuclear separation
S0
Fluorescence-radiative decay between
states of the same multiplicity (2S+1)
Photochemistry Re-cap:
excited singlet
state (S1*)
Hund’s Rule: for every singlet
excited state, there is a triplet state
which lies lower in energy
ISC-intersystem crossing (S1- T1):
horizonal transition
E
excited triplet
state (T1*)
ground
State (S0)
Internuclear separation
radiationless process between 2
states of different multiplicity
T1*
S0
Phosphorescence-radiative decay
between states of different multiplicity
More Rules!
El Sayed’s Rule
• 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
0.012
E / 103 cm-1
-2
2
Pr(Tf2N)3 10 M in BumimTf2N
25
3P
J
20
2
1
0
absorbance
0.01
0.008
1
0.006
1I
0
6
1D
2
0.004
500
550
600
650
•Line-like absorption and emission spectra
•Transitions between states derived from
Russell-Saunders spin orbit coupling (J)
•Pale colours (formally forbidden transitions)
6
15
10
1G
5
450
1I
1D
2
0.002
0
400
3P
0
4
4
3
2 3F
6
5 3H
4
4
Pr3+, 4f2,3H4
f-f Electronic Transition Intensities
• Judd-Ofelt Theory
• Theoretical oscillator strengths f (J, J’) of the J → J’
transition at the mean frequency ν is given for an
electric dipole transition
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
Label S
P
D
F
G
H
I
J
K
L
1
2
3
4
5
6
7
8
2S+1L
0
J
J = L+S, L+S-1, L+S-2…, |L-S|
Hund’s 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
l =3
3 2
1 0 -1 -2 -3
Lmax = 3 + 2 +1 = 6 = I state
ml
J = |L+S|…|L-S| = 12/2 + 3/2, 12/2 + 3/2 -1…12/2 - 3/2
= 15/2, 13/2, 11/2, 9/2 =
4I
9/2
Russell Saunders Coupling: Nd3+
Higher energy
levels
4F
[Xe] 4f 3
•Transitions from excited state
to ground 4IJ ground state
4F
11/2
4F
7/2
4F
5/2
4F
3/2
•Series of absorption and
emission lines
4I
15/2
4I
Electronic
configuration
4I
13/2
4I
11/2
4I
9/2
Interelectronic
repulsion
Spin-oribit
coupling
Crystal field
splitting
•Crystal field small-can be
observed in some transitions
(noteably Nd3+, Yb3+)
S = 1/2 + 1/2 = 1, 2S+1 = 3
Pr3+ (f2):
L=3+2=5=H
l =3
3 2
1 0 -1 -2 -3
ml
J = 5+1…5-1, = 6, 5, 4
Ground state = 3H4
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+)
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
5
1
D2
1
5
2 00 00
5
5
G4
D4
G 1 1/ 2
(2 D ,2P) 3/ 2
D1
4
3
P
3 1
P0
G 9/ 2
4
G 7/ 2
D0
E / c m- 1
2
3
3
3
F2
4
G 5/ 2
F 3/ 2
F 9/ 2
F3
4
S3 /2
F4
2
F 7/ 2
G 5/ 2
1
S 3 /2
4
4
u =3
F 9/ 2
u= 4
4
•Bigger gap = longer
lifetime
I9/ 2
u =2
F 3/ 2
1 00 00
4
F 11/ 2
4
F 9/ 2
1
4
G4
u= 2
3
7
7
F5
7
F4
7
7
F
7 0
F1
7
F
7 2
F3
7
F4
F6
7
F3
4
H5
I1 5/ 2
6
4
I1 3/ 2
6
6
4
F5
2
F7 /2
0
Yb 3+
Eu 3 +
7
3
F6
Tb3 +
4
H6
T m 3+
3
F4
3
F3
H 1 3/ 2
3
u= 1
I13/ 2
•Non radiative
quenching can reduce
the lifetime e.g. by
vibrations
u =0
F2
3
H6
H 1 1/ 2
4
u= 0
H 9 /2
I1 1/ 2
7
F2
7
F
7 1
F0
u =1
F 7/ 2
4
F 5/ 2
4
F 3/ 2
4
F 1/ 2
H4
I9/ 2
N d3 +
u= 3
•Tb3+ and Eu3+
millisecond lifetimes
I1 1/ 2
4
3
u =4
u= 5
D2
H9 /2
F5 /2
•Energy gap =
wavelength of
emission
u =6
H 11/ 2
4
H 11 /2
4
4
2
4
u =7
u =5
G 7/ 2
4
4
2
I6
4
6
H 7 /2
3
H5
6
H 5 /2
3
H4
Sm 3 +
3
P r3+
H4
Er 3+
O-H
O -D
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
 = 1/kobs (s)
•The lifetime of the excited state is given by:
•During this time, a fraction 1/e of the excited molecules
return to the ground state (e = 2.73)
400000
Lifetimes of Ln3+ (
intensity
1/e I0
Tb3+, Eu3+ milliseconds
200000
Sm3+, Dy3+, Yb3+, Er3+ microseconds
Pr3+, Nd3+ nanoseconds (10 - 100’s)
t=t
0
0
4
8
time/microseconds
12
Lifetime Determination from
Kinetic Traces
1. Stepwise increase of time delay (manual or software driven)
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, 10877−10886.
Lifetime Determination from
Kinetic Traces
2. Time Correlated Single Photon Counting (TCSPC)
10
 = 74 microseconds
Ln (Emission Intensity)
8
6
4
2
0
0.0
5
5.0x10
6
1.0x10
6
1.5x10
6
2.0x10
time / nanoseconds
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
Yb3+ in water, Ge-diode detector, 980 nm, ns/μs timescale;
Reconvolution fit with a scatterer
L.S. Natrajan, P.L. Timmins, M. Lunn, and S.L. Heath, Inorg. Chem., 2007, 46, 10877−10886.
Grow-in Component (Rise Time)
Yb3+ (D2O)
Yb3+ (H2O)
• 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
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 (00 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-often quadrupolar transitions: J = 2
e.g. 5D0 7F2 in Eu3+
Mechanism: mixing of the 4f states with ligand states
Some Examples: Eu3+
Normalised Emission Intensity
1
J = 1
J = 2
•Eu3+ ion in high symmetry
environment
J = 4
0.8
0.6
•J = 1, J = 2 and J = 4
transitions strong
0.4
J = 3
0.2
•J = 0 transition very weak
0
500
600
700
800
Wavelength (nm)
•Eu3+ ion low symmetry
environment
•J = 1 weak
•J = 0 fairly strong
•J = 2 very strong
•Split into two components
Hypersensitive Transitions in Nd3+
Absorption
2H
4
4
9/2, F5/2 I9/2
Nd(BrO3)3.9H2O (s)
CN = 9
4F
[Nd(H2O)9]3+(aq)
3/2
 4I13/2
Emission
CN = 9
NdCl3 6H2O (s)
CN = 9
780
800
820
Faulkner et al., J. Am. Chem. Soc., 2009, 131, 9916 – 9917.
Other Transitions: Absorptions
• f-d transitions: allowed by Laporte’s selection rule
Highly energetic except for Ce3+, Pr3+ and Tb3+ (4f to 5d)
[Ce(H2O)9]3+, D3h symmetry
[Ln(H2O)9]3+
 ~ 800 M-1 cm-1
 / M-1cm-1
Tb
300
200
Pr
100
225
250
300 nm
0
Ce3+ [Xe]5d1 generates two
levels, 2D3/2 and 2D5/2
•4 transitions
49 47 45 43
E / 103 cm-1
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, 10877 – 10866.
Sensitised Emission
• 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’
hv
Light harvesting
hvLn
Energy transfer
400
Light emission
500
600
700
Wavelength (nm)
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
O
O
O
O
Eu
O
O
O
O
hv
[Eu(dbm)4][NEt4]
Dbm = dibenzoylmethane
Eu3+
Courtesy of Helen Aspinall, Liverpool University, UK.
Jablonski Diagram
Sensitised emission: Energy level scheme
S1
•Heavy atom effect
-promotes ISC
T1
kisc
5D
2
5D
1
kbet
5D
0
ket
Competing pathways:
•Ligand fluorescence
•Ligand phosphorescence
Relative
Energy
(cm-1)
kfluorescence
7F
•Back energy transfer
7F
kphosphorescence
Triplet mediated
sensitisation path
6
5
7F
4
7F
3
7F
2
7F
7F
S0
Ligand
Eu3+
1
0
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.
Ar(S0)Ln3+ Ar(S1)Ln3+ Ar(T1)Ln3+ ArLn2+ 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.
Emission (f-f)
Emisison Intensity / Arb. Un.
Excitation (ligand)
But
{5D0  7FJ}
J = 2
O
S
But
Bu
But
Eu OO But
O
t
O
But
But
O
O
K
But
O
J = 0
J = 3
J = 1
550
600
650
J = 4
700
wavelength / nm
300
350
400
Natrajan et al., Dalton Trans., 2006, 4456.
Vibrational Quenching by O-X Oscillators
1
G
4
3Phen
4
G 1 1/ 2
(2 D ,2P ) 3/ 2
20000
3
3
4
F4
2
4
u =4
u=4
S 3 /2
G 5/ 2
4
1
D2
u= 5
G5/2
2
H11/2 4F
F 9/ 2
4
S3 /2
4S
3/2
H9 /2
2H
9/2
4F
3/2
u =3
u=3
9/2
u= 4
F9/2
F 3/ 2
10000
4
F 11/ 2
4
F 9/ 2
1
4
u =2
u=2
2
4
G4
I9 / 2
F5/2
u= 2
3
4
H5
I1 5/ 2
6
H1 3 / 2
7
4
4
I1 3/ 2
F6
67F
H5
1 1/2
67F4
H 9 /2
I1 1/ 2
7F
3
7H
F27 /2
7
6 F1
7H
F05 /2
6
3
4
H6
I9/ 2
0
T m 3+
u =1
u=1
F 7/ 2
4
F 5/ 2
4
F 3/ 2
4
F 1/ 2
H4
N d3 +
Eu
S3+
m3 +
u= 3
I1 1/ 2
4
3
u =7
u =6
4G
2
9/2 H 1 1/ 2
4
G7/2 4
H 11 /2
4
F3
D4
5F
D03/ 2
G 5/ 2
E / cm-1
2
5
G 7/ 2
u =5
u=5
3
P
3 1
4G P 0
11/2 4
(2D,2 P)3/2 F 7 / 2
4
4
4
I6
D1
G 9/ 2
4
G 7/ 2
F2
4
1
D2
5
4
3
5
3
3
F4
F3
7
F 3F
2
7 0
F3
7 F1
H
6
7 F2
3
7F
4
u= 1
I13 / 2
u=0
u =0
4
u= 0
I13/2
4I
11/2
7F
5
3
4
4I
15/2
H5
7F 3
6H4
4
I9/2
Tb3+ P r 3 +
2
3
H4
F7/2
Nd3+ Er 3+
Yb3+
O-H
O-H
O -D
•Ln excited states are quenched by: O-H, N-H and C-H vibrations
•Limited quenching by O-D oscillators:- (Hooke’s law)
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, 5972.
Faulkner, et al., Inorg. Chem. Commun. 2001, 187
.
Example: [Eu(DTPA)]2Q. For the Eu3+ complex determine q:
H2O = 0.63 milliseconds (ms)
D2O = 2.60 milliseconds
q = A(kH2O - kD2O-B)
A = 1.2 ms
B = 0.25
q = 1.2((1/0.63 - 1/2.60) - 0.25)
 q= 1.1 ie. one molecule of water is coordinated to europium
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
•Excite with UV pulse
I em
•Allow background
fluorescence to decay
•Collect Ln3+ emission
(Eu, Tb)
time
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.
N
N N
N
N
N N
N
Ln3+ Luminescence as a Signalling
Method
A. Modification of the Ln3+ inner coordination sphere
Analyte
•Binding of an analyte reduces O-H quenching and q decreases
B. Modulation of the energy transfer processes
Analyte
•Binding of an analyte switches on or off the Ln3+ emission
Courtesy of Jean-Claude Bünzli
Example: pH Sensor
pH 4, q = 1.6
pH 10, q = 0.1
Change in Eu3+ coordination sphere reflected in ratio of
hypersensitive transitions
Parker et al., J. Am. Chem. Soc., 2001, 123, 7601.
Example: Anion sensors
Bicarbonate sensor-ratiometric probe
Lactate and citrate assay
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
H2O = 1.67 ms
•Targetted probes/imaging
agents
D2O = 3.25 ms
q = 1.2
Faulkner et al., Chem. Commun., 2006, 909.
http://www.youtube.com/watch?v=xcE4VXUgZbY
Properties of the lanthanide ions. Note; eff – the calculated values and show good
agreement with experimental values with the exceptions of SmIII (eff exp = 1.4 -1.7) and
EuIII (eff exp = 3.3-3.5) due to their excited states being close to their ground state.