X-ray Transient Absorption Spectroscopy

X-RAY TRANSIENT ABSORPTION SPECTROSCOPY

Lin X. Chen

Chemical Sciences and Engineering Division, Argonne National Laboratory

Department of Chemistry, Northwestern University

August 7-12, 2017

19th National School on Neutron and X-ray Scattering Summer School

X-ray Transient Absorption Spectroscopy

Taking Snapshots of Photoexcited Molecular Structures in Disordered Media Using Pulsed X-rays, Lin X. Chen, Angew. Chemie. Intl. Ed., 43, 2886-2905 (2004). Ultrafast X-ray absorption spectroscopy, C. Bressler and M. Chergui, Chem Rev. (2004)104,1781-812 Probing Excited State Structures and Dynamics of Metal Complexes Using Time-resolved X-rays, Lin X. Chen, Ann. Rev. Phys. Chem. 56, 221-254 (2005). Molecular Structural Dynamics Probed by Ultrafast X-ray Absorption Spectroscopy, C. Bressler and M. Chergui, Annu. Rev. Phys. Chem. (2010) 61: 263-82. Photochemical Processes Revealed by X-ray Transient Absorption Spectroscopy, Lin X. Chen, Xiaoyi Zhang, J. Phys. Chem. Lett. 4, 4000–4013 (2013).

Two-volume book published 2015 • Combines the theory, instrumentation

and applications of x-ray absorption and emission spectroscopies which offer unique diagnostics to study almost any object in the Universe.

• Is the go-to reference book in the subject for all researchers across multi-disciplines since intense beams from modern sources have revolutionized x-ray science in recent years

• Is relevant to students, postdocurates and researchers working on x-rays and related synchrotron sources and applications in materials, physics, medicine, environment/geology, and biomedical materials.

Today’s lecture is based on Chapter 9 of the book entitled “X-ray transient

absorption spectroscopy” by Lin X. Chen

1. Pump-probe spectroscopy

2. Experimental Considerations

2.1 XTA at a Synchrotron Source

2.2 XTA at an X-ray Free Electron Laser Sources

3. Transient Structural Information Investigated by XTA

3.1 Metal center oxidation state

3.2 Electron configuration and orbital energies of X-ray absorbing atoms

3.3 Transient coordination geometry of the metal center

4. X-ray pump-probe absorption spectroscopy, Examples

4.1 Excited state dynamics of transition metal complexes (TMCs)

4.2. Interfacial charge transfer in hybrid systems

4.3 XTA Studies of Metal Center Active Site Structures in Metalloproteins

4.4 XTA using the X-ray Free Electron Lasers

4.5 Other XTA Application Examples

5. Perspective of Pump-probe X-ray spectroscopy

Outline of today’s lecture


Time scales of fundamental dynamic processes

t=0 t1 t2 t3 … tn t∞

Reaction Coordinates

transient states

product

Energy

time

reactant

Scientists’ dream: “Seeing” molecules along the reaction coordinates, and being able to understand and control reaction paths.

Molecular snapshots and reaction control


7

How can we follow a molecule in an ensemble of NA?

Single molecule (by Muybridge) Ensemble of molecules

Molecular snapshots and reaction control 1. Pump-probe spectroscopy


Molecular snapshots and reaction control 1. Pump-probe spectroscopy


8

t=0 t1 t2 t3 … tn t∞

Reaction Coordinates

laser

pulse

excited

state

transient states

product Probe pulses

Energy

time

reactant

Femtosecond (10-15) laser pulses trigger chemical events synchronously for NA

molecules ensemble, allowing snapshots to be taken.

Photons induce transitions to synchronously generate energetic species, the excited states that then proceed to many different processes. By capturing the optical signatures of transient species, one can follow the reaction kinetics and mechanisms and identify the final products.

S0

S1

S2

SN

T1

Vibration

relaxations

h

exc

itatio

n

fluo

rescen

se

ph

osp

ho

resc

en

se

tf tp

New species

photoinduced processes to be investigated

Discussions: other processes?



Snapshots for molecules: Transient absorption spectroscopy (TA)

460 480 500 520 540 560 580 600 620 640 660

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

D A

Time

Wavelength (nm)

Pump

Probe

t

t<0 t=0 t>0

Probe

At one delay time, many pump-probe cycles are collected for sufficient S/N

level in the data;

Changing the delay time step-by-step to cover the entire kinetics trace;

Time resolution determined by the laser pulse duration (i.e., ~10-100 fs);

Time window determined by the optical delay length (i.e. 0-6 ns).


Pump and probe concept

I0 I1

1

0

I

I

tI

tItOD

onpump

offpump

log),(

),(log),(

t

t


Pump and probe concept

Pump excitation light sources

Direct Ti-Sapphire laser output - 800 nm

Harmonic generation - SHG 400 nm and THG 267 nm

Optical parametric amplification - 250-12000 nm

• White light continuum generation

– Advantage: Easy to use and All wavelengths are generated at once

– Disadvantage: Somewhat limited spectral range 330-1000 nm

• Optical parametric amplification

– Advantage: Broad spectral range 250-12000 nm

– Disadvantage: Requires more sophisticated equipment; Only narrow spectrum range can be used – need to scan wavelength.

Laser probe light sources



S0

S1

S2

T1

A

T2

IC

IC TA

F

IC TA

0 100 200 300 400

OD

Probe Delay (ps)

Probe at 540 nm

0 100 200 300 400

O

D

Probe Delay (ps)

Probe at 525 nm

475 500 525 550 575 600 625

0.4 ps

0.6 ps

0.8 ps1.0 ps

1.5 ps

2.5 ps

5 ps

10 ps

20 ps

100 ps

200 ps

400 ps

O

D

Wavelength (nm)

OD UV-vis

Ground state

depletion and

recovery

Excited state

generation

and decay

“Pump” laser pulse:

Single wavelength at a certain absorption feature of the UV-vis spectrum.

“Probe” laser pulse:

White light pulse with a variable optical path to set delay from the “pump” pulse.

Optical transient absorption spectroscopy


• Light sources for pump and probe pulses;

• Photon energy differences;

• Timing structure (synchronization of laser with X-ray pulses);

• Samples (absorption coefficients, concentration, lifetimes,

stability);

• Detection scheme

• Capability of optical vs. X-ray transient absorption

spectroscopy

Advanced Photon Source,

Argonne, Illinois. USA

LCLS

APS

Lightsources.org

XFEL

X-ray photon flux evolution in sources 2. Experimental Considerations

Pump excitation light sources

Direct Ti-Sapphire laser output - 800 nm

Harmonic generation - SHG 400 nm and THG 267 nm

Optical parametric amplification - 250-12000 nm

• White light continuum generation

– Advantage: Easy to use and All wavelengths are generated at once

– Disadvantage: Somewhat limited spectral range 330-1000 nm

• Optical parametric amplification

– Advantage: Broad spectral range 250-12000 nm

– Disadvantage: Requires more sophisticated equipment; Only narrow spectrum range can be used – need to scan wavelength.

Laser probe light sources


X-ray probe light sources • Third generation synchrotron sources

• Table top laser driven fs pulsed x-ray pulses

• High harmonic generation

• X-ray free electron lasers

Synchronization of laser pump pulse with x-ray probe pulse through synchrotron RF signal

X-ray signal before the laser pulse X-ray signal change resulted from the laser excitation

0 1 2 3 4

154 ns Δt

Laser pump pulse (t1)

Laser/X-ray Probe pulse (t2)

time

S

Transmission signal (Δt = t2-t1)

Fluorescence signal (Δt = t2-t1)


X-ray transient absorption spectroscopy (XTA)

1s

2s, 2p

Valenc

e

Continuum

3s, 3p, 3d, …

EXAFS XANES

4s, 4p, 4d, …

Valence electronic transitions


Photon energy difference: visible vs. hard X-ray

Inner shell electronic transitions

21


Photon energy difference: visible vs. hard X-ray

22

Laser Pump Pulse

X-ray Probe Pulse

Time

ES2 ES1

APS Standard Operating Mode

Time

154 ns

Time

8.33 ms

Excited state population

120 Hz, <50 fs fwhm

6,500,000 Hz, ~80 ps fwhm Digitizing signals from every x-ray pulse to study structural dynamics on different time scales

LCLS

Beamline 11IDD


Timing structures of laser and X-ray pulses

23

Time-resolved x-ray facility at 11IDD

Large scale collaboration with APS

though partner usership and contribute

to general user programs

TOPASNd-YLF Regen Laser w/ SHG & THG

Nd-YLF

Seed Laser

Xe LampMonochromaterPMT

Translation stage

527nm

351nm

470-690 nm

Dref

Dsig

To detection

electronics

sample

CrystalShutter

High Harm.Rej. Mirror

Ion

Chamber

Slits SlitsIon

Chamber

-

Pump

Sample

Laser

Focusing

Lens

Sample circulation

Laser

shutter

Soller slits

Filter ensemble

X-ray from

synchrotron

CrystalShutter


Ion

Chamber

Slits SlitsIon

Chamber

CrystalShutterCrystalShutterCrystalShutter

High Harm.Rej. MirrorHigh Harm.Rej. Mirror

Ion

Chamber

Ion

Chamber

SlitsSlits SlitsSlitsIon

Chamber

Ion

Chamber

-

PumpPump

X-ray Hutch

Q-switch control

Storage RingEvolution272 kHz Shutter

control

Pulse

DelayGenerator

PulseDelayGenerator

PhotodiodeLaser pulseSignal 1kHz

CrystalShutter


Ion

Chamber

Slits SlitsIon

Chamber

-

Pump

Sample

Laser

Focusing

Lens

Sample circulation

Laser

shutter

Soller slits

Filter ensemble

X-ray from

synchrotron

CrystalShutter


Ion

Chamber

Slits SlitsIon

Chamber



Ion

Chamber

Ion

Chamber


Chamber

Ion

Chamber

PumpPump

Sample

Laser

Focusing

Lens

Sample circulation

Laser

shutter

Soller slits

Filter ensemble

X-ray from

synchrotron

Laser Room

X-ray Hutch

Q-switch control


control

Pulse

DelayGenerator

PulseDelayGenerator


Pilatus

X-ray fluo.

detector

Fast

Digitizer

To

computer

-


Translation stage470-

Dref

Dsig

To detection

electronics

RF/4 88MHz

for mode-lock

driver

sample

OPA

RF/4 88MHz

for mode-lock

driver

Amplifier laser

TOPASNd-YLF Regen Laser w/ SHG & THG

Nd-YLF

Seed Laser


Translation stage

527nm

351nm

470-690 nm

Dref

Dsig

To detection

electronics

sample

CrystalShutter


Ion

Chamber

Slits SlitsIon

Chamber



Ion

Chamber

Ion

Chamber


Chamber

Ion

Chamber

-

PumpPump

Sample

Laser

Focusing

Lens

Sample circulation

Laser

shutter

Soller slits

Filter ensemble

X-ray from

synchrotron



Ion

Chamber

Ion

Chamber


Chamber

Ion

Chamber

CrystalShutterCrystalShutterCrystalShutterCrystalShutter


Ion

Chamber

Ion

Chamber


Chamber

Ion

Chamber

-

PumpPump

X-ray Hutch

Q-switch control


control

Pulse

DelayGenerator

PulseDelayGenerator


CrystalShutter


Ion

Chamber

Slits SlitsIon

Chamber



Ion

Chamber

Ion

Chamber


Chamber

Ion

Chamber

-

PumpPump

Sample

Laser

Focusing

Lens

Sample circulation

Laser

shutter

Soller slits

Filter ensemble

X-ray from

synchrotron



Ion

Chamber

Ion

Chamber


Chamber

Ion

Chamber

CrystalShutterCrystalShutterCrystalShutterCrystalShutter


Ion

Chamber

Ion

Chamber


Chamber

Ion

Chamber

PumpPump

Sample

Laser

Focusing

Lens

Sample circulation

Laser

shutter

Soller slits

Filter ensemble

X-ray from

synchrotron

Laser Room

X-ray Hutch

Q-switch control


control

Pulse

DelayGenerator

PulseDelayGenerator


Pilatus

X-ray fluo.

detector

Fast

Digitizer

To

computer

-


Translation stage470-

Dref

Dsig

To detection

electronics

RF/4 88MHz

for mode-lock

driver

sample

OPA

RF/4 88MHz

for mode-lock

driver

Amplifier laser


Synchrotronization of laser pulses with X-ray pulses at the APS

24


Sample requirements

• Lifetime of the transient structure > x-ray pulse duration (e.g. 100 ps) and less the x-ray pulse separation;

• Absorption coefficient or extinction coefficient at the excitation laser wavelength sufficiently high to ensure effective excitation;

• Concentration of the x-ray absorbing atoms sufficiently high for either transmission or fluorescence detection;

• Stability of the sample sufficiently high under illumination of the laser and x-ray pulses;

• Sample quantity sufficiently large to unsure necessary refresh of the sample during the data acquisition period;

• Meaningful samples.


Time

X-ray Pulse Train 3.68 μs

For example, the pump-probe cycle repeats at 1 kHz due to the laser pulse repetition rate resulting in a factor of ~1,700 or 6,500 reduction of the x-ray photon flux! Solutions: Higher x-ray flux, higher repetition rate of laser.

16mA 84mA

(271kHz)

Laser Pulse Train 1 ms

Time

(1kHz)

Time

153 ns

4mA

(6504kHz)

Hybrid mode

24-bunch mode

Sample requirements: X-ray and laser pulse repetition rates

26


Sample requirements: The ground state absorption of laser photons

0.1

0.2

0.3

0.4

0.5

0.60.7

0.80.9

2 4 6 8 10 12 14 16 18 200.0

0.2

0.4

0.6

0.8

1.0

B

=20,000M-1cm

-1

Concentration (mM)

Pu

mp P

uls

e E

nerg

y (

mJ)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.10.2

0.30.4

0.5

0.6

0.7

0.8

0 2000 4000 6000 8000 100000.0

0.2

0.4

0.6

0.8

1.0

C=2mM A

(M-1cm

-1)

Pu

mp

Pu

lse

En

erg

y (

mJ)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

fP e Q

N hex

ktlC f Q eex

kt

[ ]( ) ( )1 10 1

P, laser pulse energy (J); k, rate const., (s-1); t, time, s; ε (λ), absorption coeff. at laser wavelength λ (M-1cm-1); l, thickness (cm); C, concentration (M); N, total number of molecules illuminated by the laser.

350 400 450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

434

414

OD

wavelength (nm)

toluene

25% pyridine (v)

50% pyridine (v)

75% pyridine (v)

pyridine

X10

601

563

527

X-ray

Sample requirements: X-ray and laser photon absorption cross section



Laser



Laser+

X-ray



Laser+

X-ray

0 5 10 15 20

1

10

100

1000

Ab

so

rptio

n (

cm

-1)

[Zn(II)] (mM)

Laser 2.21 eV (560 nm)

X-ray 10,000 eV



fP e Q

N hex

ktlC f Q eex

kt

[ ]( ) ( )1 10 1

Extracting excited state (or transient state) spectrum from XTA

P, laser pulse energy (J);

k, rate const., (s-1);

t, time, s;

ε (λ), absorption coeff. at laser wavelength λ (M-1cm-1);

l, thickness (cm);

C, concentration (M);

N, total number of molecules illuminated by the laser.

)],(),()[(),(

),()(),()]([

),()(),()(),(

tktktftk

tktftktf

tktftktftk

GSESESGS

ESESGSES

ESESGSGS

1

Two unknowns and one equation → need to find fES via other means

A, via calculation


)}()()]()[({(

)(log

)}(),()()]()[,()({(

),(log

)],()(),()([(

),()(log),(

tftf

tfCtfC

C

tCtC

CtOD

ESESESGS

GS

ESGSESESGSGS

GSGS

ESESGSGS

GSGS

1

010

0

0

From laser pump, laser probe transient absorption spectroscopy in a two-state

example, where the total concentration of excited and ground states remain to be

the starting ground state concentration CGS(λ,t=0):

If the extinction coefficient for the excited state at a particular detection

wavelength λ, εES(λ) = 0,

OD

ES

OD

ES

ES

ESESGS

GS

etfetf

tftftf

tOD

11

11

1

1

)()]([

)](log[)]([

log)]()[((

)(log),(

B. via optical transient absorption (e.g. two state system)



C. via calculation (one example by Grigory Smolentsev et al.)

Smolentsev, G.; Soldatov, A. Journal of Synchrotron Radiation 2006, 13, 19-29.

Smolentsev, G.; Soldatov, A. V. Computational Materials Science 2007, 39, 569-574.



Slits

Ion Chamber Detector 1

X-ray Beam

High Harmonic Rejection Mirror Slits

Ion Chamber Detector 1

To fast digitizer

To fast digitizer

S

Fast Digitizer

Electronic Delay

Generator

Q-switch Control

Hybrid Pixel 2D Detector

for XRD

Seed Laser

Amplifier Laser and OPA

271 kHz Ring Evolution Signal

RF/4 88MHz

To Computer Electronic

Delay Generato

r

Photodiode

Detection and sample delivery

Pump laser pulse (on/off) ΔOD(t)

OTA:

• How fast molecule moves; • How to get from R → P; • How energy flows; • Transient species population; • Correlation of electronic transitions; • How molecular movements affect functions.

XTA (New):

• Electronic configuration; • Geometry; • Oxidation states • Molecular orbital energies; • Transient species lifetime; • Correlations of electronic transition with nuclear movements.

Pump: creates changes or triggers reactions Probe: interrogates changes

Sample

Molecular snapshots and reaction control 3. Transient Structural Information Investigated by XTA

Metal center oxidation state in the excited state


Jahn-Teller

Distortion

hν

Franck-Condon

State

MLCT

State—

“Flattened”

kr

Cu(I)

Cu(II)*

Cu(II)*

kr’+knr”

Cu(I)

knr’

Strongly

Interacting

< 2 ns

Weakly

Interacting

> 100 ns

Ground

State

Cu(II)*

MLCT State—

Solvent

Complex<0.1 ps

<20 psJahn-Teller

Distortion

hν

Franck-Condon

State

MLCT

State—

“Flattened”

kr

Cu(I)

Cu(II)*

Cu(II)*

kr’+knr”

Cu(I)

knr’

Strongly

Interacting

< 2 ns

Weakly

Interacting

> 100 ns

Ground

State

Cu(II)*

MLCT State—

Solvent

Complex<0.1 ps

<20 ps

JACS 129, 2147 (2007), 125, 7022 (2003), 124, 10861 (2002).

1s to 4pz

Cu(I)→Cu(II) 1s to 3d

[CuI(dmp)2]+

Electron configuration and orbital energies of X-ray absorbing atoms


7.11 7.12 7.13 7.14 7.15

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Fe(III)Mb

Fe(II)CO-Mb

Fe(II)-Mb after CO

photodissociation

Norm

aliz

ed

(E)

Energy (keV)

7.108 7.112 7.116

MbFe(II)CO MbFe(II)*

3d

hv, -CO

1s

2231yx

ds

231z

ds

Photodissociation of CO from heme in myoglobin

Stickrath et al., J. Phys. Chem. A. 117, 4705 - 4712 (2013)

Transient coordination geometry of the metal center 3. Transient Structural Information Investigated by XTA

Fe(II)CO-Mb CN R(Å) σ2(Å2) ∆E(eV) Fe(II)Mb CN R(Å) σ2(Å2) ∆E(eV)

Fe-C(O) 1 1.76 0.0001 18.10 16.47

Fe-N(His,Heme) 5 1.99 0.0013 5 2.05 0.0042

Fe-O(CO) 1 2.84 0.0002

Fe-C(CNC) 10 3.08 0.0080 10 3.05 0.0034

Fe-C 4 3.38 0.0001 4 3.47 0.0010

Fe-C 10 4.41 0.0001 9 4.21 0.0023

Table 1. Iron heme structural parameters (Residual = 8.11, S2= 0.9)

Photodissociation of CO from heme in myoglobin

Stickrath et al.,

J. Phys. Chem.

A. 117, 4705 -

4712 (2013)

Metal Ligand MLn

MLCT

LMCT

np

ns

(n-1)d

π*

π

σ

E

Cu(dmp)2+

The metal-to-ligand-charge-transfer (MLCT) states

MLCT transitions are often origins for transition metal complexes to be used

in solar energy conversion initiated by electron density shifts between the

metal and the ligands. Examples are DSSC, photocatalysis, etc.

4. XTA spectroscopy, Examples

Excited state structural dynamics of Cu(I) diimine complexes

40

[CuI(dmp)2]+ → [CuII(dmp-)(dmp)]2++ e-

[RuII(bpy)3]2+ → [RuIII(bpy-)(bpy)2]3++ e-

D2d D2

τ1 τ2,τ3

τ4

τ5

τ6

τ1- 1MLCT formation; τ2- J-T distortion, 0.3-0.6 ps; τ3- ISC, 1-20 ps; τ4,

3MLCT “ligation” > 100 ps?;

Τ5,6 3MLCT decay, 1ns to 3µs.

Two key structural factors: • Orientation of two ligand planes: ISC • Solvent accessibility to the Cu* center – electron density transfer

from “ligating” solvent to 3d, 3MLCT lifetime.

McMillin, Sauvage,

Karpishin, Meyer,

Castellano, Chen, Tahara,

Iwamura, Penfold, …

Mara, Fransted, Chen,

Coord. Chem. Rev.,

282, 2-18 (2015).




8.97 8.98 8.99 9.00 9.01 9.02

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

(E

) (n

orm

alize

d)

Energy (keV)

[Cu(I)dmp2]+

Excited State

[Cu(II)dmp2]2+

τ1= 0.6 ps τ1= N/A τ2= 13 ps τ2= 13 ps τ3= 1.6/100 ns τ3= 100/N/A ns

Transient Cu(II) in [CuI(dmp)2]+ MLCT state: 3d hole remains – transient “ligation” and no formal bond forms with the solvent, but only direct access of the solvent to the Cu(II)* center could shorten the MLCT lifetime in coordinating solvent; Cu(II) in [CuII(dmp)2]2+ : 3d hole filled due to the ligation.16

41

8.97 8.98 8.99 9.00 9.01 9.02

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Ab

so

rptio

n (

no

rma

lize

d)

Energy (eV)

Cu(I)(dppS)2

Excited State

Cu(II)(dppS)2

8.976 8.980 8.984-0.010.000.010.020.030.040.050.060.070.080.090.10

(J.-P. Sauvage and coworkers)

8.976 8.978 8.980 8.9820.00

0.05

0.10

0.15

Cu K-edge XANES

1s to 4pz

1s to 3d

Cu(I)→Cu(II)

1s to 3d

1s to 3d

[CuI(dmp)2]+ [CuI(dppS)2]+

[CuI(dmp)2]+ [CuI(dppS)2]+



2 4 6 8 10 12-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5 Ground State

Excited State

(k

)k2

k(Angstrom-1)

(k) N F (k)e e

kr k

krj j

2 k

2r

k j ij

jj

2 2

j

j

( ) sin( ( ))2

2

0 1 2 3 40.00

0.02

0.04

0.06

0.08

0.10

FT

[(k

)k2]

R (before phase correction, Angstrom)

Ground State

Excited State

FT

Cu-N

Cu-C

Removal of

antibonding 3d

electron→bond

shortening

Transient exciplex forms when the space and the MLCT state lifetime allow, but not necessarily forms a normal bond between Cu(II) and solvent.

EXAFS of the MLCT State of [CuI(dmp)2]+



Mara, Coskun, Dimitrijevic, Barin, Kokhan, Stickrath, Ruppert, Tiede, Stoddart, Sauvage, Chen, Angew. Chem. Intl. Ed. 51, 12711–12715 (2012).

232

2IIk

2II OTi](dppS)[CuTiO*(dppS)](dppS)[Cu TE

2s

13.7 ps

100 ns

0.5V

-1.57V

3MLCT

1MLCT


Excited state structural dynamics of Cu(I) diimine complexes 8980 9000 9020

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Energy (eV)

Abso

rptio

n (

no

rma

lize

d)

Energy (eV)

4

4* - 100 ps

8980 9000 9020

4-TiO2

4*-TiO2 - 100 ps

A B

C D

0 2 4 6 8 10 12 14 100 1000

0.0

0.2

0.4

0.6

0.8

1.0

Ab

sorp

tion

(n

orm

aliz

ed

)

Time (ps)

4

4-TiO2

In solution On TiO2

• ~40% injection yield, due to charge recombination; • Linker length independent; • Electron transfer mechanism and pathway: through

bond; • Future design aims at minimize charge recombination.

1s →4pz

MLCT state decay by OTA Mara, Coskun, Dimitrijevic, Barin, Kokhan, Stickrath, Ruppert, Tiede, Stoddart, Sauvage, Chen, Angew. Chem. Intl. Ed. 51, 12711–12715 (2012).

Excited states of metalloporphyrins: electron donors or acceptors, light harvesting pigments, photocatalysis, functional site of heme proteins, photovoltaic materials.

Moore et al.

NN

NNM

Myoglobin

Photosynthetic bacterial reaction center protein

++++

Photosynthetic bacterial light harvesting protein

Metalloporphyrins

FeIII

FeIII

FeII

FeII

O

FeII

FeIV

FeII

FeIII

O∙

1/2O2 hv(λLMCT

)

τspring<200ps

S

(O)S

O

Photocatalyst


Capturing a Photoexcited Molecular Structure Through Time-Domain X-ray

Absorption Fine Structure, L. X. Chen, W. J. H. Jäger, G. Jennings, D. J.

Gosztola, A. Munkholm, J. P. Hessler, Science, 292, 262-264(2001).

• (π,π*)→(d,d,) conversion mechanism; • Multiple electron shift steps but

sequence is unclear; • Influence of excited state structures on

MO energy levels; • Role of the solvent and ligation; • Transient oxidation state of Ni.

3dxz 3dyz

3dxy

3dz2

3dx2-y2

Ni(II) 3d8

Square-planar

π*

π

Porphyrin Ni

8.32 8.34 8.36 8.38 8.40

0.0

0.2

0.4

0.6

0.8

1.0

1.2

[(E

)-

0(E

)]/

0(E

)

Energy (keV)

"t=100 ps" laser-off

"t=100 ps" Excited state

Chen et al., JACS (2007)

1s → 4pz

1s →3d

(LCLS study)

(APS study)

APS results


8328 8329 8330 8331 8332 8333 8334 8335 8336

No laser

t = "0" ps

t = "100" ps

T1

S0

(E

)

Energy (eV)

3dx2-y2

3dz2

• Ni-N and Ni-Cα distances increase in the T1 state due to the reduction of the bond order when the excitation adds electron to antibonding MO.

• The effective radius of Ni(II) is larger and fits porphyrin cavity better, and hence the macrocycle is flatter than in the ground state.


APS: JACS (2007),

Chem. Sci. (2010).

8330 8332 8334

1s -> 3dz2

1s -> 3dx2-y2

Laser 527 nm

No laser

(E

)

Energy (eV)LCLS exp. Shelby et al., J. Am. Chem. Soc. 138, 8752–8764 (2016)


XTA at X-ray free electron laser source

1s → 4pz

1s →3d

• A new transient feature in 1-2 ps detected: Ni(I)?;

• To be confirmed by calculations about the existence of Ni(I) T’ state.

Shelby et al., J. Am. Chem. Soc. 138, 8752–8764 (2016)

• LCLS X-ray pulse: ~50 fs (fwhm), 1012photons/pulse, 103-4 x higher than pump-probe exp. at synchrotrons;

• Self-amplified-spontaneous emission (SASE) with ~50 eV energy range, XANES spectra only.

Lemke et al., J. Phys. Chem. A,

117, 735 (2013)



GS depletion

T1 rise T’* Ni(I)? rise and decay

( GSddTTSk

yxz

kk

3222

21 )3,3(*'*),( 11 (1 ps)-1 (0.3 ps)-1 (>200 ps)-1

Inverted kinetics )()(),(3

0

tPEAtEAi

iitotal



Orbital energies and changes in various excited states (Xiaosong Li)

Exp

Calculation

1s →4pz 1s →3d

3dxz 3dyz

3dxy

3dz2

3dx2-y2

π*

π

Porphyrin Ni

Inputs for excited-state XAS modeling

Wavefunc. S0 S1(π, π*) T’ T(d,d) S0 S0 T(d,d) T(d,d)

Geometry S0 S1(π,π*) T’ T(d,d) T’ T(d,d) S0 S1

Calculated Ni orbital energies (eV)

1s α -8167.47 -0.05 2.02 -1.17 0.37 -0.34 -0.67 -0.61

1s β -8167.47 -0.05 2.03 -1.17 0.37 -0.34 -0.67 -0.61

4pz α 1.39 -0.07 0.18 -0.23 0.06 -0.03 -0.15 -0.16

4pz β 1.39 -0.06 0.05 -0.44 0.06 -0.03 -0.34 -0.34

Energy-specific TDDFT (ES-TDDFT) using the PBE1PBE and Ahlrichs' def2-TZVP basis set with diffuse functions on the nickel atom

S0

T1

T’*



A wide range of transient structures during photochemical reactions have been captured by high quality XTA studies which become routine and sample property dependent;

Capturing transition state in chemical reactions by fs x-ray pulses is still a significant challenge and will bring new insight into matter interactions with which requires X-ray free electron lasers with stable timing and spectral tunability;

Computational and theoretical works are urgently needed to model photochemical pathways through unrelaxed excited states, excited state coherence, initial reaction conditions and core/valence excitations.

Simultaneously obtaining the structural dynamics on the initial and stepwise catalytic reactions remain to be challenging.

5. Perspective of X-ray Transient Absorption Spectroscopy

X-ray Transient Absorption Spectroscopy

Documents

Transcript of X-ray Transient Absorption Spectroscopy