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30th Helsinki Winter School in
Theoretical Chemistry
F-element optical and NMR spectroscopy-insights from experiment
and theory
17/12/2014
Pure oxidation state actinide compounds and fundamental chemistry
Transition metal polypyridyls: two photon spectroscopy
Geochemistry of Uranyl
Lanthanide upconverting nanoparticle enzyme biosensors and FRET
Research Themes
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 infra-red light
3D optical imaging Biological imaging
Yb3+, Nd3+
Photochemistry Re-cap: Absorption of light: Electronic transitions
• organic molecules, metal complexes (high ε)
E
Internuclear separation
ground state
Vertical absorption
excited state Vertical emission (radiative decay)
Kasha’s rule: emission from lowest vibrational (relaxed) state
Vibrational relaxation (VR) Non-radiative de-activation
S0
S1*
Fluorescence-radiative decay between states of the same multiplicity (2S+1)
Photochemistry Re-cap:
E
Internuclear separation
ground State (S0)
excited singlet state (S1*)
S0
Phosphorescence-radiative decay between states of different multiplicity
T1*
excited triplet state (T1*)
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
radiationless process between 2 states of different multiplicity
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
El Sayed’s Rule • Intersystem crossing is slow unless there is a
change in orbital type
More Rules!
Selection Rules Spin Selection rule: ΔS = 0 allowed transition
• Fluorescence S1* to S0, ΔS = 0, allowed, fast decay (ns)
• Phosphorescence T1* to S0, ΔS = 1, forbidden, slow (s)
Electronic transitions in lanthanides-Absorption • f-f transitions (weak oscillator strengths, ε ~ 1 M-1 cm-1, c.f. d-d transitions ~ 100 and π-π ~ 105) • Laporte selection rules (centrosymmetric cxs (i): Δ l = 1 and g u • f-f Δ 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 Pr(Tf2N)3 10-2 M in BumimTf2N
0
0.002
0.004
0.006
0.008
0.01
0.012
400 450 500 550 600 650
abso
rban
ce
3PJ
1D2 0
1
2
1I6
Pr3+, 4f2,3H4
0
5
10
15
20
25 E / 103 cm-1
1G4
1D2
3H4
3F
1I6
3P
4 5 6 2 3 4
0 1 2
• 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 ν is given for an electric dipole transition
Only really works for doped solids in high symmetry (e.g. cubic)
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 0 1 2 3 4 5 6 7 8
2S+1L 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
3 2 1 0 -1 -2 -3 ml
l = 3 ! Lmax = 3 + 2 +1 = 6 = I state
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 = 4 I9/2
[Xe] 4f 3
4 F
4 I
4 F3/2
4 F5/2
4 F7/2
4 F11/2
4 I9/2
4 I11/2
4 I13/2
4 I15/2
Electronic configuration
Interelectronicrepulsion
Spin-oribit coupling Crystal field
splitting
Higher energylevels
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+)
Pr3+ (f2): S = 1/2 + 1/2 = 1, 2S+1 = 3
L = 3 + 2 = 5 = H
J = 5+1…5-1, = 6, 5, 4
Ground state = 3H4 3 2 1 0 -1 -2 -3 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+)
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
Nd3 +Eu3 + Tb3 +Yb3+ O-H O -DTm3+ Sm3 + P r3+ Er3+
3H 4
4 I13/2
4I1 1/2
3H 4
3H 5
3H 6
3H 6
3H 5
3H 4
0
2 00 00
4 I9/2
4I1 1/2
4I1 3/2
4I1 5/2
4 F3/2
2 H9 /2
4S3 /2
4 F9/2
2H 11 /2
4 G5/2
4 G7/2
4 G9/2
(2 D ,2P)3/2
4G1 1/25D 4
7F 07F 17F 27F 37F 4
7F 5
7F 67F0
7F1
7F2
7F3
7F4
7F5
7F6
5 D0
5D 1
5 D2
2 F5 /2
2 F7 /2
1 00 00
6 H5 /2
6 H7 /2
6 H9 /2
6 H1 1/2
6 H1 3/2
4 F 3/2
4 G5/2
4 F 3/2
4 F 1/2
4 F 5/2
4 F 7/2
4 G7/2
4 F 9/2
4 F 11/2
3 F2
3 F4
3P0
3P 1
3 F3
1I6
1G4
4 I9/2
4F9/2
4S3 /2
4F7/2
3 F4
3 F3
3 F2
1G 4
E /
cm-1
υ=0
υ=1
υ=2
υ=3
υ=4
υ=5
υ=0
υ=1
υ=2
υ=3
υ=4
υ=5
υ=6
υ=7
2H11/2
1D2
• 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
• The intrinsic lifetime is the average time spent in the excited state without non-radiative deactivation processes
• Units of lifetimes is usually ns
• The inverse is ns-1 which is a rate
• The intensity of emission after a pump pulse decays exponentially
• Taking the natural log of this gives a linear relationship between intensity and time
Lifetimes
• 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
• During this time, a fraction 1/e of the excited molecules return to the ground state (e = 2.73)
0
200000
400000
0 4 8 12
time/microseconds
intensity 1/e I0
t = t
Lifetimes of Ln3+ (τ):
Tb3+, Eu3+ milliseconds
Sm3+, Dy3+, Yb3+, Er3+ microseconds
Pr3+, Nd3+ nanoseconds (10 - 100’s)
I(t) = It = 0.e-kt
• The lifetime of the excited state is given by: τ = 1/kobs (s)
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.
2. Time Correlated Single Photon Counting (TCSPC)
0.0 5.0x105 1.0x106 1.5x106 2.0x106
0
2
4
6
8
10
τ = 74 microsecondsLn
(Em
issi
on In
tens
ity)
time / nanosecondsSm3+ 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.
Lifetime Determination from Kinetic Traces
Grow-in Component (Rise Time)
• 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
Yb3+ (D2O) Yb3+ (H2O)
Hypersensitive Transitions
• Some transitions are sensitive to changes in symmetry and/or the inner coordination sphere.
• They display shifts in their maxima, splitting and intensity
• In hypothetical Ln3+, only magnetic dipole transitions are allowed: ΔL = 0, ΔJ = 0, 1
e.g. 5D0→ 7F1 in Eu3+
• In a complex, electric dipole (ED) transitions are induced: ΔL, ΔJ = 0, 2, 4, 6 (0→0 forbidden)
• Some transitions acquire strength by both MD and ED e.g. Tb3+
Hypersensitive transitions-often quadrupolar transitions: ΔJ = 2
e.g. 5D0→ 7F2 in Eu3+
Mechanism: mixing of the 4f states with ligand states
Some Examples: Eu3+
550 600 650 700
Em
isis
onIn
tens
ity /
Arb
. Un.
wavelength / nm
ΔJ = 0
ΔJ = 2
ΔJ = 1 ΔJ = 3ΔJ = 4
550 600 650 700
Em
isis
onIn
tens
ity /
Arb
. Un.
wavelength / nm
FJ}50 →→{ D 7
ΔJ = 0
ΔJ = 2
ΔJ = 1 ΔJ = 3Δ
O
Eu O
OS
O
ButO
But
But
ButO
K
But
But
But
But
O
O
550 600 650 700
Em
isis
onIn
tens
ity /
Arb
. Un.
wavelength / nm
ΔJ = 0
ΔJ = 2
ΔJ = 1 ΔJ = 3ΔJ = 4
550 600 650 700
Em
isis
onIn
tens
ity /
Arb
. Un.
wavelength / nm
FJ}50 →→{ D 7
ΔJ = 0
ΔJ = 2
ΔJ = 1 ΔJ = 3Δ
550 600 650 700
Em
isis
onIn
tens
ity /
Arb
. Un.
wavelength / nm550 600 650 700
Em
isis
onIn
tens
ity /
Arb
. Un.
wavelength / nm
ΔJ = 0
ΔJ = 2
ΔJ = 1 ΔJ = 3ΔJ = 4
550 600 650 700
Em
isis
onIn
tens
ity /
Arb
. Un.
wavelength / nm550 600 650 700
Em
isis
onIn
tens
ity /
Arb
. Un.
wavelength / nm
FJ}50 → FJ}50 →→{ D 7→{ D 7
ΔJ = 0
ΔJ = 2
ΔJ = 1 ΔJ = 3Δ
O
Eu O
OS
O
ButO
But
But
ButO
K
But
But
But
But
O
O
• Eu3+ ion in high symmetry environment
• ΔJ = 1, ΔJ = 2 and ΔJ = 4 transitions strong
• ΔJ = 0 transition very weak
• Eu3+ ion low symmetry environment
• ΔJ = 1 weak
• ΔJ = 0 fairly strong
• ΔJ = 2 very strong
0
0.2
0.4
0.6
0.8
1
500 600 700 800
Wavelength (nm)
No
rm
alised
Em
issio
n I
nte
nsit
y
ΔJ = 1 ΔJ = 2
ΔJ = 3
ΔJ = 4
• Split into two components
Hypersensitive Transitions in Nd3+
780 800 820
2H9/2,4F5/2←4I9/2
Nd(BrO3)3.9H2O (s)
[Nd(H2O)9]3+(aq)
NdCl3.6H2O (s)
CN = 9
CN = 9
CN = 9
Absorption
H2N
N
N
N
N
N N
N
O O
O
O
O
OOO
O
OO O
Nd
Nd
-O
N
NN
N NO
O
O O
O
O
N
N N
O
O
O O
O
O
Nd
Nd
4F3/2 → 4I13/2
Emission
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)
225 250 300 nm
[Ce(H2O)9]3+, D3h symmetry
Ce3+ [Xe]5d1 generates two levels, 2D3/2 and 2D5/2 • 4 transitions
ε ~ 800 M-1 cm-1
300
200
100
0 49 47 45 43
ε / 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
O NN
CO2-
CO2--O2C CO2
-
O NN
CO2-CO2
--O2CCO2
-Eu3+Eu3+
Ar + Eu3+→ Ar• + Eu2+ Transient Ligand radical + Eu(II)
π-π* ε = 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’
Organic chromophore: hv1 + Ar → 1Ar* → 3Ar* → Ln* → Ln + hvLn
hv
Light harvesting Energy transfer
hvLn
Light emission 400 500 600 700 800
Wavelength (nm)
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 β-diketonates
hv
Eu3+
Eu
O
O OO
OO
OO
[Eu(dbm)4][NEt4]
Dbm = dibenzoylmethane
Courtesy of Helen Aspinall, Liverpool University, UK.
Jablonski Diagram Sensitised emission: Energy level scheme
S0
Ligand
Relative Energy(cm-1)
S1
5D0
5D1
5D2
ket
7F0
7F1
7F2
7F3
7F4
7F5
7F6
Eu3+
kfluorescence
kphosphorescence
kbet T1
kisc
• Heavy atom effect -promotes ISC
Competing pathways:
• Ligand fluorescence
• Ligand phosphorescence
• Back energy transfer
Triplet mediated sensitisation path
LMCT Quenching in Eu3+ Complexes
5D0
7FJ
S1
T1
S0
Ligand Eu3+
Relative Energy
kisc
kelectron transferLMCT
Eu2+ + 1Ar*
kelectron transfer
Eu2+
8S7/2
Double electron transfer quenches the Eu3+ emission (NR decay)
• 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.
LMCT Sensitised Yb3+ Emission
2F5/2
2F7/2
S1
T1
S0
Ligand Yb3+
Relative Energy
kisc
kelectron transfer
LMCTYb2+ + 1Ar*
kelectron transfer
• 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.
N
N
ReCl
COOCOC
N
N
N
N N
O
R
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.
300 350 400
550 600 650 700
Emis
ison
Inte
nsity
/Ar
b. U
n.
wavelength / nm
ΔJ = 0
ΔJ = 2
ΔJ = 1 ΔJ = 3ΔJ = 4
550 600 650 700
Emis
ison
Inte
nsity
/Ar
b. U
n.
wavelength / nm
FJ}50 →→{ D 7
ΔJ = 0
ΔJ = 2
ΔJ = 1 ΔJ = 3Δ
O
Eu O
OS
O
ButO
But
But
ButO
K
But
But
But
But
O
O
550 600 650 700
Emis
ison
Inte
nsity
/Ar
b. U
n.
wavelength / nm
ΔJ = 0
ΔJ = 2
ΔJ = 1 ΔJ = 3ΔJ = 4
550 600 650 700
Emis
ison
Inte
nsity
/Ar
b. U
n.
wavelength / nm
FJ}50 →→{ D 7
ΔJ = 0
ΔJ = 2
ΔJ = 1 ΔJ = 3Δ
550 600 650 700
Emis
ison
Inte
nsity
/Ar
b. U
n.
wavelength / nm550 600 650 700
Emis
ison
Inte
nsity
/Ar
b. U
n.
wavelength / nm
ΔJ = 0
ΔJ = 2
ΔJ = 1 ΔJ = 3ΔJ = 4
550 600 650 700
Emis
ison
Inte
nsity
/Ar
b. U
n.
wavelength / nm550 600 650 700
Emis
ison
Inte
nsity
/Ar
b. U
n.
wavelength / nm
FJ}50 → FJ}50 →→{ D 7→{ D 7
ΔJ = 0
ΔJ = 2
ΔJ = 1 ΔJ = 3Δ
O
Eu O
OS
O
ButO
But
But
ButO
K
But
But
But
But
O
O
Natrajan et al., Dalton Trans., 2006, 4456.
Excitation (ligand) Emission (f-f)
Nd3+Eu3+ Tb3+ Yb3+0
20000
4I9/2
4I11/2
4I13/2
4I15/2
4F3/2
2H9/2
4S3/2
4F9/2
2H11/2
4G5/2
4G7/2
4G9/2
(2D,2P)3/2
4G11/25D4
7F07F17F27F37F4
7F5
7F67F0
7F1
7F2
7F3
7F4
7F5
7F6
5D0
5D1
5D2
2F5/2
2F7/2
10000
E / c
m-1
3Phen
O-H
υ=0
υ=1
υ=2
υ=3
υ=4
υ=5
Vibrational Quenching by O-X Oscillators
• Ln excited states are quenched by: O-H, N-H and C-H vibrations Nd3 +Eu3 + Tb3 +Yb3+ O-H O -DTm3+ Sm3 + P r3+ Er3+
3H 4
4 I13/2
4I1 1/2
3H 4
3H 5
3H 6
3H 6
3H 5
3H 4
0
2 00 00
4 I9/2
4I1 1/2
4I1 3/2
4I1 5/2
4 F3/2
2 H9 /2
4S3 /2
4 F9/2
2H 11 /2
4 G5/2
4 G7/2
4 G9/2
(2 D ,2P)3/2
4G1 1/25D 4
7F 07F 17F 27F 37F 4
7F 5
7F 67F0
7F1
7F2
7F3
7F4
7F5
7F6
5 D0
5D 1
5 D2
2 F5 /2
2 F7 /2
1 00 00
6 H5 /2
6 H7 /2
6 H9 /2
6 H1 1/2
6 H1 3/2
4 F 3/2
4 G5/2
4 F 3/2
4 F 1/2
4 F 5/2
4 F 7/2
4 G7/2
4 F 9/2
4 F 11/2
3 F2
3 F4
3P0
3P 1
3 F3
1I6
1G4
4 I9/2
4F9/2
4S3 /2
4F7/2
3 F4
3 F3
3 F2
1G 4
E / c
m-1
υ=0
υ=1
υ=2
υ=3
υ=4
υ=5
υ=0
υ=1
υ=2
υ=3
υ=4
υ=5
υ=6
υ=7
2H11/2
1D2
• Limited quenching by O-D oscillators:- (Hooke’s law)
Calculating the Inner Sphere Hydration Number (q)
• Lanthanide coordination compounds are labile: [Ln(H2O)9]3+ + H2O [Ln(H2O)8(H2O)]3+ + H2O
• 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/τ
A, B = proportionality constants
A corrects for inner sphere quenching
B corrects for outer sphere quenching
O H
H
OH
H
O HH
OH
H
OH
H
OH H
OH
H
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)]2-
Q. For the Eu3+ complex determine q:
τH2O = 0.63 milliseconds (ms)
τD2O = 2.60 milliseconds
NN N
O O
O
O O
O O
O O
O
Eu3+
q = A(kH2O - kD2O-B)
A = 1.2 ms
B = 0.25
q = 1.2((1/0.63 - 1/2.60) - 0.25)
NN N
O O
O
O O
O O
O O
O
Eu3+
OH2
→ 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 (Δλ absorption and emission)
Time-resolved luminescence: allows high signal to noise ratios
Time-gating: removal of background fluorescence
time
I em • Allow background fluorescence to decay
• Excite with UV pulse 2 - 4 ms
• Collect Ln3+ emission (Eu, Tb)
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
NN
NCO2H
CO2H
HO2C
HO2C
N
NN
N
NH2
O
NH2O
H2N
O
H2N O
H4DOTA DOTAM
NN N
OH HO
OH
OH OH
O O
O O
O
H5DTPA
N N
N NN NN N
Ln3+ Luminescence as a Signalling Method
B. Modulation of the energy transfer processes
A. Modification of the Ln3+ inner coordination sphere
• Binding of an analyte switches on or off the Ln3+ emission
• Binding of an analyte reduces O-H quenching and q decreases
Analyte
Analyte
Courtesy of Jean-Claude Bünzli
Example: pH Sensor
N
NN
N
HN
OOO
O
O O
Eu
S
R
O
O
pH 4, q = 1.6
pH 10, q = 0.1
N
NN
N
H2N
OOO
O
O O
Eu
S
R
O
O
OH2
N
NN
NNH
OOO
O
O O
Eu S
R
O
O
H3O+
OH-
Parker et al., J. Am. Chem. Soc., 2001, 123, 7601.
Change in Eu3+ coordination sphere reflected in ratio of hypersensitive transitions
Example: Anion sensors
N N
NNEu3+
O
O
HN
O
O H
H
N
OR
NH
R
NHR
HCO3- N N
NNEu3+
O
O
HN
O
O
N
OR
NH
R
NHR
OHO
- H2O
Bicarbonate sensor-ratiometric probe
Parker et al., Org. Biomol. Chem., 2009, 7,1525.
Lactate and citrate assay
HO O
O OH
O
O
OOO
O
N N
NNH
Eu3+
OHN
O
OH
H
SN
O
NH
CO2-
R
-O2CR
Example: Peptide Conjugates and Targeted Imaging
• Tb3+ complex coupled to a peptidic vector-Tuftsin
• Affinity for macrophage cells
• Targetted probes/imaging agents τH2O = 1.67 ms
τD2O = 3.25 ms
q = 1.2
N N
NN
O
O
O
O
O
O
Tb
HN
O
NH
O
ThrLysProArg
OH2
Faulkner et al., Chem. Commun., 2006, 909.
Electronic configeration Z Element Symbol
Ln LnIII
Ground level
Radius Ln3+ (Å)
eff
cal
58 Cerium Ce [Xe]4f15d16s2 [Xe]4f1 2F5/2 1.061 2.54 59 Praseodymium Pr [Xe]4f36s2 [Xe]4f2 3H4 1.034 3.58 60 Neodymium Nd [Xe]4f46s2 [Xe]4f3 4I9/2 0.995 3.62 61 Promethium Pm [Xe]4f56s2 [Xe]4f4 5I4 0.979 2.68 62 Samarium Sm [Xe]4f66s2 [Xe]4f5 6H5/2 0.964 0.85 63 Europium Eu [Xe]4f76s2 [Xe]4f6 7F0 0.950 0 64 Gadolinium Gd [Xe]4f75d16s2 [Xe]4f7 8S7/2 0.938 7.94 65 Terbium Tb [Xe]4f96s2 [Xe]4f8 7F6 0.923 9.72 66 Dysprosium Dy [Xe]4f106s2 [Xe]4f9 6H15/2 0.908 10.65 67 Holmium Ho [Xe]4f116s2 [Xe]4f10 5I8 0.899 10.60 68 Erbium Er [Xe]4f126s2 [Xe]4f11 4H15/2 0.881 9.58 69 Thulium Tm [Xe]4f136s2 [Xe]4f12 3H6 0.869 7.56 70 Ytterbium Yb [Xe]4f146s2 [Xe]4f13 2F7/2 0.858 4.54 71 Lutetium Lu [Xe]4f145d16s2 [Xe]4f14 1S0 0.848 0
Properties of the lanthanide ions. Note; µeff – the calculated values and show good agreement with experimental values with the exceptions of SmIII (µeff exp = 1.4 -1.7) and EuIII (µeff exp = 3.3-3.5) due to their excited states being close to their ground state.