Time-resolved Fourier Transform Infrared Spectroscopy (FTIR) in Soft Matter research
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Transcript of Time-resolved Fourier Transform Infrared Spectroscopy (FTIR) in Soft Matter research
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Time-resolved Fourier Transform Infrared Spectroscopy (FTIR) in Soft Matter research
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Outline
Physical processes in the IR spectral range IR spectrometry
Fourier Transform Infrared Spectroscopy (FTIR)
Quantitative information from IR spectra Effects of external fields on the molecular level
Time resolved FTIR Chemical reactions Conformational changes ...
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Example: CO2 gas
Rotational – vibrational transitions
IR spectral range
-11[cm ]
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IR spectra of condensed matter
Gases show complex vibrational-rotational spectra In soft matter absorption bands are significantly broader
Martin Chaplin, www.physics.umd.edu
IR spectral range
H2O
CO2
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Oscillations – selection rules
Covalent bonds can be described by Morse or LJ potential curves
Quantum harmonic oscillator is a good approximation Both stretching and bending modes
Single photon is absorbed by interaction with oscillating dipole – transition dipole moment
Absorption coefficient: No absorption normal to the transition dipole moment
IR spectral range
nmp m n d
Δn=±1Others weakly allowed, due to anharmonicity(overtones)
i iq rd
: dipole operator
2
p E
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IR spectroscopy as analytical tool
Widely used as analytical tool Easier preparation than NMR, less quantitative
Underestimated! IR and Raman spectroscopy are very powerful techniques
IR spectral range
1-octanol
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Grating IR spectrometer
Requirements: Well collimated beam Monochromator
Largest part of light intensity is not used Calibration is necessary
IR spectrometry
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Fourier Transform Infrared Spectroscopy
Michelson interferometer Interferogram: intensity vs optical
path difference Intensity at all wavelengths is
measured simultaneously
-0.01 0.00 0.01-0.2
-0.1
0.0
0.1
0.2
Inte
nsity
(ar
b. u
nits
)
Optical retardation (cm)
0 0det
0ig
, cos 42 2
1 cos 42
I II
II d
IR spectrometry
Optical path difference for each wavelength
γ
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FTIR spectroscopy
Spectrum is easily obtained from the Fourier transform of the interferogram
IR spectrometry
ig 0
0
0 : 0I I d
igig 0
0
ig0
ig0 ig
0 1cos 4
2 2
0Re
2 2
02Re
2
II I d
IF I
II F I
4000 3500 3000 2500 2000 1500 1000
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsi
ty (
arb
. un
its)
wavenumber (cm -1)
no sample silk
3500 3000 2500 2000 1500 10000
1
2
3
Abs
orba
nce
wavenumber (cm -1)
-0.01 0.00 0.01-0.2
-0.1
0.0
0.1
0.2
Inte
nsi
ty (
arb
. un
its)
Optical retardation (cm)Fourier transform
Division
solvent solvent
„white light“ position
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Resolution – Apodization
Problem: impossible to integrate interferogram from - to + Equivalent to multiplying “ideal”
interferogram with a “box” function FT of a product is the convolution of
FT‘s
Resolution depends on maximum mirror path ~ Δ-1
Artefacts! Multiplying with other functions
improves quantitative accuracy, but reduces resolution
Apodization=”removing feet”
Apodizationfunction
Fourier transform of Iig(γ)
Shape ofinfinitely thin lines
IR spectrometry
( ) ( )F f g F f F g
Fourier Transform Infrared Spectrometry,P. R. Griffiths, J.A. de Haseth, Wiley
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Advantages of FTIR
Jacquinot advantage FTIR not as sensitive to beam misalignment, allowing for
larger aperture – throughput
Fellget advantage (“multiplex”) All frequencies measured together
Connes advantage Built-in calibration, mirror position determined by He-Ne
laser
FTIR is exclusively used nowadays
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Transmission – reflection modes
Simplified: no interference, etc.
Transmission - absorption Specular reflection
Absorbance
Absorption coefficient αMolar absorption coefficient ε=α/c
Lambert-Beer law:
1
0
logI
AI
1 0 0e el clI I I
ln10 ln10
l clA
Reflectivityref
0
IR
I
Normal incidence in air2
1
1
nR
n
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Complex refractive index
The imaginary part is proportional to the absorption coefficient
Dielectric function Real and imaginary parts are related through Kramers-
Kronig relations
Example:polycarbonate
n n in
0
0
exp 2
exp 4 exp 4
4
t
t
E x E i n x
I I i n x n x
n
2n
Fourier Transform Infrared Spectrometry,P. R. Griffiths, J.A. de Haseth, Wiley
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Polarization dependence
Example: salol crystal All transition dipoles (for a certain transition) are perfectly
aligned Intensity of absorption bands depends greatly on crystal
orientation
Dichroism: difference of absorption coefficient between two axes Biaxiality (all three axes different)
IR spectral range
salol
Vibrational Spectroscopy in Life Science, F. Siebert, P. HildebrandtJ. Hanuza et al. / Vib. Spectrosc. 34 (2004) 253–268
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Order parameter
Non-crystalline solids: molecules (and transition dipole moments) are not (perfectly) aligned Rotational symmetry is common Different absorbance A|| and A Dichroic ratio R= A|| / A
Molecular order parameter
IR spectral range
Reference axis
Molecular segment
Transition dipole
||
2
2
3 cos 1
2molS P
1
0 :2
mol RS
R
1
: 22 2
mol RS
R
“parallel” vibration
“perpendicular” vibration
2
2
1 2cot 2
2 2cot 1mol R
SR
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1050 1000 950
0.1
0.2
0.3
0.4
Pol
y(al
anin
e)
(Ala
Gly
) n
Pol
y(gl
ycin
e) I
Abs
orba
nce
wavenumber (cm-1)
Pol
y(gl
ycin
e) I
I
Pol
y(al
anin
e)
0°
polarization
90°
0,0
0,2
0,4
0,6
0
30
60
90
120
150
180
210
240270
300
330
0,0
0,2
0,4
0,6
Abs
orba
nce
Smol=0.25
0,0
0,2
0,4
0,6
0
30
60
90
120
150
180
210
240270
300
330
0,0
0,2
0,4
0,6
Smol=0.50
Abs
orba
nce
0,0
0,2
0,4
0,6
0
30
60
90
120
150
180
210
240270
300
330
0,0
0,2
0,4
0,6
Smol=0.80A
bso
rba
nce
0
2
4
0
30
60
90
120
150
180
210
240
270
300
330
0
2
4
Smol=0.93
Abs
orba
nce
High order of alanine-rich crystalsLow order of glycine-rich amorphous chains
Order of crystals and amorphous phase in spider silk
Experimental
2
p E
p: transition dipole moment
Papadopoulos et al., Eur. Phys. J. E, 24, 193 (2007)Glisovic et al. Macromolecules 41, 390 (2008)
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Examples of structural changes in soft matter
Phase transitions liquid crystals
Conformational changes Protein secondary structure
In many cases these processes take place very fast (< s) Cannot be probed by X-rays or NMR
Lemieux, R. P. Acc. Chem. Res. 2001, 34, 845-853
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Time-resolved measurements
Two possibilities: Collect interferogram as fast as possible (“rapid scan”) Synchronize spectrometer with external event (“step scan”)
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Rapid scan - kinetics
Interferograms are collected successively
Time resolution down to a few ms (depending on spectral resolution)
Non-repetitive processes Cannot average scans
noise 0 1 2 3 4 5
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsi
ty (
arb
. un
its)
time (min)
trigger
-0.01 0.00 0.01-0.2
-0.1
0.0
0.1
0.2
Inte
nsi
ty (
arb
. un
its)
Optical retardation (cm)-0.01 0.00 0.01
-0.2
-0.1
0.0
0.1
0.2
Inte
nsi
ty (
arb
. un
its)
Optical retardation (cm)
Time-resolved FTIR
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Irreversible processes
Rapid scan is useful for studying chemical reactions and phase transitions
Crystallization of a liquid crystal by T-jump
Synthesis of polyurethane
For faster processes:Static measurements at different spots of a flow cell
1t
2tReaction time
Time-resolved FTIR
de Haseth et al., Appl. Spectrosc., 47, 173 (1993)Takahashi et al. J. Biol. Chem. 270, 8405 (1995)
amorphous
crystal
90°C 36°C
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Step scan
Differences from rapid scan kinetics: Interferograms are not measured successively Triggered event is repeated for every mirror step
Allows study of very fast processes down to ns, ps -> chemical reactions Lower noise than kinetics
Disadvantages: Limited to repetitive processes Sensitive to system instabilities
Time-resolved FTIR
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-0.01 0.00 0.01-0.2
-0.1
0.0
0.1
0.2
Inte
nsi
ty (
arb
. un
its)
Optical retardation (cm)
-0.01 0.00 0.01-0.2
-0.1
0.0
0.1
0.2
Inte
nsi
ty (
arb
. un
its)
Optical retardation (cm)
Step scan
Stroboscopic technique Mirror moves stepwise All measurements after a certain
dt from trigger are assembled to make a single interferogram
All interferograms are collected in a single scan
One scan takes longer than rapid scan, but much higher time resolution
Time-resolved FTIR
0 200 400 600 800 1000
0.0
0.2
0.4
0.6
0.8
1.0
1.2 rapid scan step scan
op
tica
l pa
th d
iffe
ren
ce (
arb
. u
nits
)
time (arb. units)
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4000 3500 3000 2500 2000 1500 1000
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(ar
b. u
nits
)
wavenumber (cm -1)
no sample silk (0 ms) silk (20 ms)
3500 3000 2500 2000 1500 10000
1
2
3
Abs
orba
nce
wavenumber (cm -1)
0 ms 20 ms
1000 9500.3
0.4
0.5
0.6
Abs
orba
nce
wavenumber (cm -1)
0 ms 20 ms
Step scan example: spider silk
Time-resolved FTIR
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Combined IR and mechanical spectroscopy
polarizer
IR beam
Piezo crystals –DC motors
Force sensor
IR detector
sample
Tracing microscopic effects of strain
Possible to extract order parameter dependence on external fields
Dynamic Infrared Linear Dichroism (DIRLD)
Transmission mode using microscope
Experimental
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Preparation of Step Scan measurement
Process studied with Step Scan FTIR should be reproducible
Several cycles should be run before actual measurement
Measurement should start at this point to ensure reproducibility
Time-resolved FTIR
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DIRLD in polymers
Dichroic ratio depends on strain Polymer chains become better oriented Different trend for dipole moments parallel and normal to
the chain
S. Toki et al. / Polymer 41 (2000) 5423–5429I. Noda et al. / Appl. Spectrosc. 42 (1988) 203–216
Natural rubber (polyisoprene)
polystyrene
Time-resolved FTIR
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External – crystal stress comparison: Phase
The step-scan technique allows IR measurements with high time resolution
Crystal stress can be measured as a function of time under sinusoidal external field
Phase shift < 2°
R. Ene et al. / Soft Matter, 2009, 5, 4568–4574
Time-resolved FTIR
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What is the origin of frequency shifts?
Vibrational frequency depends on: Atom mass Bond force constant Number of atoms involved in vibration Perturbations
H-bonding Conformation
Anharmonicity Thermal expansion External fields (in this case)
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C
CH3H
C
O
NH
NH
C
O
-1 -18 cm GPad d
pertV F r -4,68
-4,66
hc
Quantum Perturbation Theory
The shift is ~ 0.3 % QPT is applicable
The bond anharmonicity gives rise to the shift of energy levels
0( ) 20 0 (1 )a r rU U U e
0 1 2 3 4-6
-4
-2
0
2
4
Ene
rgy
(10
-19 J
)
r (Å)
Morse potential
Morse potential+
perturbation
N-C
3 eV
0.12 eV
F r 0.17 eV
disN CE
P. Papadopoulos et al. Eur. Phys. J. E 24, 193 (2007)
Theoretical value
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Microscopic – macroscopic stress in silk
Crystal stress is equal to the externally applied At time scales from µs to
hours Independent of sample
history
Serial connection of crystals
0 ,0 0 ,2 0 ,4 0 ,6 0 ,8 1 ,0 1 ,2 1 ,4
9 6 1
9 6 2
9 6 3
9 6 4
9 6 5
stress (GPa)
wa
ven
um
be
r (c
m-1
)
S tat ic
-2.6 cm - 1 GPa - 1
Kinetics
Step Scan
PP, J. Sölter, F. Kremer Eur. Phys. J. E 24, 193 (2007)
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Photoinduced protein folding
Bacteriorhodopsin structure changes after visible photon absorption IR photons do not have enough energy to change structure,
just probe vibrations!
Pulsed laser is synchronized with spectrometer Retinal conformational changes during the complete
cycle (~ms) are observed
reti
nal
R. Rammelsberg et al. Appl. Spectrosc. 51, 558 (1997)
Time-resolved FTIR
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Folding kinetics of peptides after T-jumps
Alanine-based peptide Secondary structure depends on temperature (coil at higher T) Reaction rate “constants“ can be studied by T-jumps
IR laser pulses synchronized with spectrometer heat the sample by ~ 10°C
The sum ku+kf is determined by kinetics, ratio ku/kf by equilibriumu
f
folded unfoldedk
k
exp u fk k t
T. Wang et al. J. Phys. Chem. B 108, 15301 (2004)
Time-resolved FTIR
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Summary
Fourier Transform IR spectroscopy is an ideal tool to study fast processes High sensitivity Information for different molecular groups High time resolution
Time resolved measurements Rapid scan Step scan
Effects of external perturbations in various systems: Polymers Proteins Liquid crystals, ...
http://www.uni-leipzig.de/~mop/lectures
Thank you for your attention!
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N-term. C-term.
GGXGAAAAAAAA
Repetitive pattern
GGXGGX GGX GGX GGX
n
AAAAAAA GPGXX GPGXX GPGXX GPGXX GPGXX
n
MaSp2
MaSp1
Hydrophobic Slightly hydrophilic
Chemical structure of dragline silk and PA6
Block copolymer Two high-MW proteins (MaSp1 and MaSp2) Semi-crystalline High Ala- and Gly- content
PA6 (Nylon):
Spider silk
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Normal vibrational modes
Simple relations only in diatomic molecules! Vibrations involve more than two atoms
Especially at low frequencies
Example: amide bond C
O
N H
C
k
Amide I Amide II
Amide III
Amide IV
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Absorption spectrum of silk
Typical protein spectrum Amide vibrations
dominate, but ...
They cannot give aminoacid-specific information
The region 1100 – 900 cm-
1 can be used instead1050 1000 950
0.2
0.3
0.4
Po
ly(a
lan
ine
)
(Ala
Gly
) n
Po
ly(g
lyci
ne
) I
Ab
sorb
ance
wavenumber (cm-1)
Po
ly(g
lyci
ne
) II
Experimental
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Poly(alanine) segment
Rotondi, K. S.; Gierasch, L. M. Biopolymers 2005, 84, 13-22. Simmons, A.; Ray, E.; Jelinski, L. W. Macromolecules 1994, 27, 5235-5237.
C-terminus
N-terminus
N-terminus
C-terminus
N-terminus
N-terminus
C-terminus
C-terminus
N C
NC
N C
N C
Antiparallel and parallel -sheet structure
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3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750
0,0
0,3
0,6
0,9
1,2
0,0
0,5
1,0
1,5
-polyalanine
wavenumber (cm -1)
Abs
orb
anc
e
Am
ide
III
Am
ide
II
Am
ide
I
Am
ide
B MA silk ||
Ab
sorp
tion
coe
ffici
ent
(m
-1) A
mid
e A
Polyaminoacid IR spectra
Dragline silk and -polyalanine
A. M. Dwivedi, S. Krimm Macromolecules 15, 186 (1982)
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Similar findings in PA6
Similar to silk, orientation before crystallization induces the high order
Crystal vibration responds linearly to applied stress Both spider silk and PA6 are
glassy at room temperature
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Rotational – vibrational transitions
The fine structure of gas vibrational spectra is due to the vibrational transitions
Selection rules: Δn=±1 ΔJ=±1 (and 0 in certain cases)
Relation between integrated molar absorption coefficient and transition dipole moment:
22
2 202
0 0 0
8
2 3 3t t
med
m c he c
H.C. Haken – H. WolfMolecular Physics and Elements of Quantum ChemistryChapter 15