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Photoinduced electron transfer reactions investigated by ultrafast spectroscopy Eric Vauthey Département de chimie-physique Université de Genève Femto08
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Page 1: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Photoinduced electron transfer reactions investigated by ultrafast spectroscopy

Eric VautheyDépartement de chimie-physique

Université de Genève

Femto08

Page 2: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Outline

Introduction. Photoinduced intermolecular electron transfer reactions, remaining questions

Picosecond optical calorimetric studies: looking at the energetics

Distinguishing different types of ion pairs:- pump-pump-probe spectroscopy- transient IR absorption

Artificial photosynthesis

Excited-state dynamics of radical ions

Femto08

Page 3: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

D* + A D.+ + A.-ET

Photoinduced electron transfer (ET)

ET operative if (Weller equation):

D + A

D*+ A

D .A-˙

D + A-˙

+˙ ΔGET

E*

e(Eox- Ered)

C

ΔGET = - E* + e[Eox(D)- Ered(A)]+C < 0

Femto08

Page 4: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Classical Marcus theory

1.0

0.9

0.8

0.7

0.6

0.5

0.4

k ET

/km

ax

2.52.01.51.00.50.0-0.5ΔGET/λ

normal region

barrierless region

inverted region

AD A-D+

ΔGET

λ

−ΔGET= λ

−ΔGET> λ

−ΔGET< λ

kET ∝ exp −ΔGET + λ( )2

4λkBT

Femto08

Page 5: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

normal region

barrierless region

inverted regionln

kET

2.01.51.00.5 −ΔGET/λ

classical

v=1

v=2

AD A-D+

v=0

solvent coordinate

v=0

semi-clas.

Semi-classical Marcus theory

kET = kET0→v

v=0

∑ ΔGET≠ 0→ v( ) =

ΔGET0→0 + vhυvib + λs4λskBT

Femto08

Page 6: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

- 1984, intramolecular charge shift(Miller, Closs et al.)

- 1987, ‘intermolecular’ charge recombination(Gould, Farid et al.)

- 1985, intramolecular charge recombination(Wasielewski et al.)

- 2001, intramolecular charge separation(Mataga et al.)

- 1997, intermolecular charge shift(Guldi et al.)

Observation of the inverted region

Miller et al., J. Am. Chem. Soc. 106 (1984) 3047

Femto08

Page 7: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Photoinduced bimolecular CS (polar solvents)

A

A* + D (A *. D) (A .- .D.+) A.- + D.+

(A .D)

+ D

DIF CS SEP

CR

D. Rehm, A. Weller, Isr. J. Chem. 8 (1970) 259

Femto08

Page 8: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Hypothesis:

- CS distance increases with driving force (Sutin et al., 1984)

- the product is formed in an electronic excited state (Weller)

- ...

Femto08

Page 9: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

E

A D

A* D

A.- D.+

ΔGCS

A.- D.+*

ΔGCS*

ΔGCS ΔGCS*

1) Formation of the product in an electronic excited state ?

Femto08

Page 10: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

2) ΔGCS dependence of the CS distance

kCS =2πh

V 2

4πλkBT( )1/2exp − ΔGCS +λ( )2

4λkBT

λs =e2

4πε012aA

+22aD

−1r

1n2−1εs

1.8

1.6

1.4

1.2

λs (

eV)

3025201510r (Å)

B. S. Brunschwig, S. Ehrenson, N. Sutin, J. Am. Chem. Soc. 106 (1984) 6858

The barrier of highly exergonic ET is predicted to decrease with distance

Femto08

Page 11: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

S. Murata, M. Tachiya, J. Phys. Chem. 100 (1996) 4064

2) ΔGCS dependence of the CS distanceFemto08

Page 12: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Usual scheme ofCS quenching in polar solvents

(Gould et al., ACR 1996)

CIP (TIP): contact (tight) ion pairs, small ΔGCS

or CT excitation of DA complex

SSIP (LIP): solvent-separated (loose) ion pairs, large ΔGCS

Femto08

Page 13: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Other unanswered questions:

- absence of normal region in the CR of excited donor/acceptor complexes

- structure and geometry of the reactions intermediates

- ....

Femto08

Page 14: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Two types of energy gap law for CR

From T. Asahi and N. Mataga, J. Phys. Chem. 95 (1991) 1956

CT excitation (CIP, TIP)

ET quenching (SSIP, LIP)

Femto08

Page 15: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Picosecond optical calorimetric studies: looking at the energetics

Femto08

Page 16: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

C = −e2

4πε0εsd

Energetics

D + A

D*+ A

D .A-˙

D + A-˙

+˙ ΔGCS

E*

e(Eox- Ered)

C

d d

Femto08

Page 17: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

sample

grating formation

(four-wave-mixing, real-time holography)

2 formalisms: 1) holography 2) nonlinear optics

θ

Picosecond optical calorimetry using transient gratingFemto08

Page 18: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

modulation axis

light intensity

product concentration

reactant concentration

ΛFemto08

Page 19: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Spatial modulation of the optical properties of the sample

˜ n x( ) = ˜ n 0 +Δ ˜ n cos 2πΛ

x

modulation amplitude

˜ n average value

Transient gratingFemto08

Page 20: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

amplitude grating

Δn

ΔAΔnphase grating

population changes

Δnppopulation changesoptical Kerr effect

ΔnKdensity changes

Δnd

changesthermal

ΔndvΔnd

TΔnds

volumeexpansion electrostriction

ΔnKeΔnK

n

nuclear electronic

non resonant interactions

Femto08

Page 21: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Probing the grating

θB

probe pulse

sinθB =λs2Λ

Bragg angle:

Femto08

Page 22: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

diffracted intensity

η =Idif

Is

∝Δ ˜ n 2

≈ c1Δn2 + c2ΔA2

Applications of the transient grating technique:

- population dynamics

- ultrafast calorimetry

- transient dichroism

- …

Femto08

Page 23: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Idif∝Δnd2

23

ΔA = Δnp = 0

Picosecond calorimetry

Measurement of Δnd only: non-resonant probing

Δnd t( ) = Ci R t − t'( )−∞

t

∫ ⋅fi t'( )dt'i∑

R t( ) =1− cos 2πτ ac

t

exp −αacvst( )

Ci α Qi : amount of heat deposited upon process i

R(t) : response of the sample to instantaneous heat deposition

f t( ) = exp −t τ r( )fi(t) : time evolution of the temperature change

Femto08

Page 24: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

I dif (

a.u.

)

1086420time delay/τac

24

- shape of the time profile: dynamics of the heat releasing process

- amplitude of the signal: amount of heat deposited€

τ ac =Λvs

=λpu

2 sin(θ / 2)vs

τr = τac/4 τr = τac τr =4τac

Picosecond calorimetryFemto08

Page 25: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

ON

N

25

Picosecond calorimetry

Photoinduced CS between benzophenone (BP) and DABCO

VR

ISCultrafast heat release (0.5 eV)

CSslow heat release (? eV)

very slow heat release (µs)

A DE(eV)

0

1

2

3

A + D

1A* + D3A* + D

A·¯ + D·+

J. Phys. Chem. 99 (1995) 8652

Femto08

Page 26: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

translation stage

frequencyconverter

LASER

detectorsample

Experimental setup

Femto08

Page 27: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

27

Picosecond calorimetry

VR

ISC CS

E(eV)

0

1

2

3

A + D

1A* + D3A* + D

A·¯ + D·+

600

500

400

300

200

100

0

I dif (

a.u.

)

3000200010000

time delay (ps)

300

200

100

I dif (

a.u.

)

0.05 M

0.3 M

0.05 M0.3 M

[D] Is/If1.901.93

ΔGCS=-0.86 eVC=-0.28 eV

The ions are in close contactJ. Phys. Chem. 99 (1995) 8652

Femto08

Page 28: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Excited-state dynamics of radical ions

Femto08

Page 29: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

E

A D

A* D

A.- D.+

ΔGCS

A.- D.+*

ΔGCS*

ΔGCS ΔGCS*

Formation of the product in an electronic excited state ?

Femto08

Page 30: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Solution: detection of excited radical ions

Problems:

- Essentially nothing is known on the excited-state properties of radical ions

- Only a few fluorescing radical ions known (many artefacts)

- Absorption spectrum of excited radical ions unknown

O

O

O

O

F

F

F

F

F

F

N

N

SS

SSOCH3

H3CO OCH3

Femto08

Page 31: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

N

N

N

NH2

Photophysics of radical ions in liquid

Formation of radical ions:

- Photochemical (bimolecular electron transfer): perylene cation and anion

- Chemical (salts of radical ions): Wurster’s Blue and Wurster’s Red

- Electrochemical: perylene cation and anion

WB WR Pe·+ Pe·¯

For all ions investigated so far: very short excited state lifetime (≤ 5 ps)

Femto08

Page 32: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Photophysics of radical ions in liquid

Formation of radical ions:

- Photochemical (bimolecular electron transfer): perylene cation and anion

- Chemical (salts of radical ions): Wurster’s Blue and Wurster’s Red

- Electrochemical: perylene cation and anion

For all ions investigated so far: very short excited state lifetime (≤ 5 ps)

1.0

0.5

0.0

-0.5

-1.0

Δ A (

a.u.

)

806040200

time delay (ps)

500 nm 580 nm 594 nm

τ1 ~ 4 ps (IC) τ2 ~ 15 ps (cooling)

-15

-10

-5

0

ΔA x

103

750700650600550500450

wavelength (nm)

3 ps 13 ps 5 ps 20 ps 7ps 30 ps

50 ps -steady state

electrochemically produced Pe·¯ (ACN + 0.1 M Bu4NCLO4)

Femto08

Page 33: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Photophysics of Wurster’s Blue

-30

-20

-10

0

10

20

ΔA x

103

151050time delay (ps)

τ1= 0.3 psτ2= 0.5 psτ3= 4.2 ps

-30

-20

-10

0

10

20

ΔA x

103

700650600550500wavelength (nm)

0.0 ps 3.0 ps 0.1 ps 4.0 ps 0.2 ps 10.0 ps 0.4 ps 20.0 ps 2.0 ps

NN

A

700600500400300

wavelength (nm)

Femto08

Page 34: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Photophysics of Wurster’s Blue

Fast decay components independent of solvent (polarity, viscosity)Slow decay component slower in non H-bonding solvent

D0,v=0

D1,v>0

D0,v>0

fast

slow

Why is internal conversion so fast ?- a very small D1-D0 gap cannot be invoked (ΔE (D1-D0)=1.95 eV)- presence of a conical intersection ?

Femto08

Page 35: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Photophysics of Wurster’s Blue

A.C. Albrecht, JACS 1955

A clue: How can a molecule with a 500 fs excited-state lifetime exhibit significant fluorescence?

fluor

esce

nce

(a.u

.)

800750700650

wavelength (nm)

MeOH:EtOH λexc=600nm 140K 95K 110K 92K 105K 90K 100K 88K 98K 85K

rct coord.

hv

barrierin

tens

ity (a

.u.)

800700600500

wavelength (nm)

MeOH:EtOH (1:1) at 85K

Femto08

Page 36: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

At the present stage, the observation of excited radical ions upon highly exergonic CS is highly difficult.

The photophysics of radical ions has to be better understood.

Interesting for the astrochemists

Nature 391 (1998) 259

Femto08

Page 37: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

At the present stage, the observation of excited radical ions upon highly exergonic CS is highly difficult.

The photophysics of radical ions has to be better understood.

Interesting for the astrochemists

Can we obtain new information on ion pair structure from the excited state dynamics of ions?

Femto08

Page 38: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Distinguishing different types of ion pairs

Femto08

Page 39: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Usual scheme of ET quenching in polar solvents

CIP: contact ion pairs, small ΔGCS

or CT excitation of DA complex

SSIP: solvent-separated ion pairs, large ΔGCS

Gould et al., Acc. Chem. Res. 29 (1996) 522

Femto08

Page 40: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

How to distinguish these ion pairs?

1) ion pairs and free ions have essentially the same UV-vis absorption spectrum

3) Look at other properties, which might be sensitive to ion pairing J. Phys. Chem. 110 (2006) 7547

2) Time-resolved resonance Raman spectroscopy:

- no information on singulet systems

- apart from very few exceptions, triplet geminate ion pairs and free ions have essentially the same resonance Raman spectrum (above 500 cm-1).

J. Phys. Chem. 96 (1992) 7356

J. Am. Chem. Soc. 116 (1994) 9182

Femto08

Page 41: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

A + D

A+ D*

A·¯ D·+ A·¯ + D·+pump 1

A·¯···D·+

Pump-pump-probe experiment

A·¯ D·+*

pump 2 probe

D·+*

early time long time

J. Phys. Chem. A 110 (2006) 7547

Femto08

Page 42: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Q :CN

NC

Pe DCB DCE

CN

CN

Pump-pump-probe experiment

Femto08

Page 43: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Pump-pump-probe experiment

Femto08

Page 44: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Pump-pump-probe experiments (GSR of Pe.+) -Δ

A (a

.u.)

121086420Δt23 (ps)

Δt12= 60 ps Δt12= 1 ns

Pe/DCB

- ΔA

(a.u

.)

20151050Δt23 (ps)

Δt12= 60 psΔt12= 1 ns

Pe/DCE

1.1 ps (0.5) + 6.2 ps (0.5)

3 ps

3 ps (0.5) + 10 ps (0.5)

J. Phys. Chem. A 110 (2006) 7547

Femto08

Page 45: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

GSR dynamics as a function of the ‘age’ of the ion

monophasicGSR dynamics

biphasic GSRdynamics

J. Phys. Chem. A 110 (2006) 7547

long timeearly time

Transition from biphasic to monophasic GSR on the 400 ps timescale

3 ps3 ps

6 ps

2 ps

Femto08

Page 46: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

How to distinguish these ion pairs?

1) ion pairs and free ions have essentially the same UV-vis absorption spectrum

4) Time-resolved IR spectroscopy J. Phys. Chem. A 110 (2006) 13676

Ang. Chem. in print

3) Look at other properties, which might be sensitive to ion pairing J. Phys. Chem. 110 (2006) 7547

2) Time-resolved resonance Raman spectroscopy:

- no information on singulet systems

- apart from very few exceptions, triplet geminate ion pairs and free ions have essentially the same resonance Raman spectrum (above 500 cm-1).

J. Phys. Chem. 96 (1992) 7356

J. Am. Chem. Soc. 116 (1994) 9182

Femto08

Page 47: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Time-resolved IR spectroscopy(with E. Nibbering, MBI, Berlin)

J. Phys. Chem. A 110 (2006) 13676

Pe DCB

CN

CN

In acetonitrile (ACN): dissociation into free ions Φion= 30%

In dichloromethane (DCM): no free ions (CR only)

Femto08

Page 48: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

J. Phys. Chem. A 110 (2006) 13676

Pe DCB

CN

CN

1.0

0.5

0.0

ΔA

(nor

m.,

a.u.

)

2130212021102100209020802070Wavenumber (cm -1)

ACN DCM

Pe + DCB (700 ps)

CN stretch region

In acetonitrile (ACN): dissociation into free ions Φion= 30%

In dichloromethane (DCM): no free ions (CR only)

Time-resolved IR spectroscopy

Femto08

Page 49: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Pe TCNE

CNNC

NC CN

τCRΔGCRΔGCS

-0.74 eV-2.17 eV 1.6 ns1)

1) N. Mataga et al., J. Phys. Chem. 90 (1986) 3380, Chem. Phys. 127 (1988) 239

D A

D A

D* A

D·+A·-

CS

CR

The Pe-TCNE system

D A

D·+A·-

CRhν

Femto08

Page 50: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Time-resolved IR spectroscopy

MPe TCNE

CNNC

NC CN

Can one use the sensitivity of C-N stretch to differentiate the stronglycoupled and weakly coupled ion pairs found with Pe/TCNE ?

0.6

0.4

0.2

0.0

ΔA

(mO

D)2200218021602140

Wavenumber (cm -1)

-100 ps 3 ps 7 ps 10 ps 20 ps 30 ps 50 ps 70 ps 100 ps

0.2 M TCNE

Ang. Chem. in print

Femto08

Page 51: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Concentration dependence

MPe TCNE

CNNC

NC CN

0.6

0.4

0.2

0.0

ΔA

(mO

D)

2200218021602140

Wavenumber (cm -1)

-100 ps 3 ps 7 ps 10 ps 20 ps 30 ps 50 ps 70 ps 100 ps

0.2 M TCNE

2.0

1.5

1.0

0.5

0.0

ΔA

(mO

D)

2200218021602140

Wavenumber (cm -1)

-100 ps 1 ps 3 ps 5 ps 7 ps 10 ps 20 ps 30 ps 70 ps

0.9 M TCNE

Femto08

Page 52: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Concentration dependence

MPe TCNE

CNNC

NC CN

1.0

0.9

0.8

0.7

0.6

0.5

0.4

ΔA

(mO

D)

2155215021452140Wavenumber (cm-1)

1 ps 5 ps 10 ps 30 ps 50 ps 70 ps 100 ps 120 ps 150 ps

0.05 M TCNE

0.6

0.4

0.2

0.0

ΔA

(mO

D)

2200218021602140

Wavenumber (cm -1)

-100 ps 3 ps 7 ps 10 ps 20 ps 30 ps 50 ps 70 ps 100 ps

0.2 M TCNE

2.0

1.5

1.0

0.5

0.0

ΔA

(mO

D)

2200218021602140

Wavenumber (cm -1)

-100 ps 1 ps 3 ps 5 ps 7 ps 10 ps 20 ps 30 ps 70 ps

0.9 M TCNE

Femto08

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Two IR spectral components

1) Broad and short lived (dominant at high [TCNE])

2) Narrow and long lived (dominant at low [TCNE])

Strongly coupled ion pairs

Weakly coupled ion pairs

1.0

0.8

0.6

0.4

0.2

0.0

Δ A

(a.u

.)

22102200219021802170216021502140Wavenumber (cm-1)

1 2

Femto08

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Comparing LE and CT excitations

500400300 1000900800700600500

0.0 M 0.2 M 0.6 M 1.0 M

Abso

rban

ce

Wavelength (nm)

x40

LE

2.0

1.5

1.0

0.5

0.0

ΔA

(mO

D)

2200218021602140

Wavenumber (cm -1)

-100 ps 1 ps 3 ps 5 ps 7 ps 10 ps 20 ps 30 ps 70 ps

0.9 M TCNE

Strongly+weakly coupled IP

CT

1.5

1.0

0.5

0.0

ΔA

(mO

D)

2200218021602140

Wavenumber (cm -1)

-100 ps 2 ps 3 ps 5 ps 7 ps 10 ps 12 ps 15 ps 30 ps

CT excitation

Stronglycoupled IP

Femto08

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Structure of the ion pairs ?

Polarisation anisotropy measurements upon 400 nm excitation

0.10

0.05

0.00Pola

rizat

ion

Aniso

tropy

0.1 1 10 100Time Delay (ps)

0.9 M TCNE in ACN

raw smoothed best fit

2150 cm-1

r = 0.1±0.03 on both IR transitions

N

N N

N

ΔA

(mO

D)

2200218021602140

Wavenumber (cm -1)

0.9 M TCNE

Femto08

Page 56: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Structure of the ion pairs ?

Strongly coupled IP: anisotropy r = 0.1 on both CN bands

=> probably sandwich-like coplanar geometry

Femto08

Page 57: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Structure of the weakly coupled ion pairs ?

Solvent-separated ?

All our measurements suggest two ions in contact but poorly oriented.

In bimolecular ET theories, the reactants are always considered as spheres

Distance is the only coordinate modulating the coupling.

Mutual orientation should be considered.

Tight and loose ion pairs is more judicious than

contact and solvent-separated ion pairs

Femto08

Page 58: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Artificial photosynthesis

with Stefan Matile (Geneva)

Femto08

Page 59: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

NN

+H3N

H2NO O

O

O

O

R2

R1

CF3COO-

O

OO

OO

OO

O

NDI

O

NDI

O

NDI

O

NDI

O

NDI

O

NDI

O

NDI

O

NDI

O

Octachromophoric systems

NDI

Femto08

Page 60: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

A

600500400

wavelength (nm)

fluor

esce

nce

800700600500400

wavelength (nm)

NDI chromophores

NN

+H3N

H2NO O

O

O

O

HN

NH

NN

+H3N

H2NO O

O

O

O

HN

Cl

NN

+H3N

H2NO O

O

O

O

O

O

RY B

F. Würthner et al., Chem. Eur. J. 8 (2002) 4742

Femto08

Page 61: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

O

NN

O

H

H

H

H

NN N

O

O

O

ON

O

H

H

OO

H

H

+H3N

+H3N

N NO

O

O

O

NH

R

R

R

R

R =

Self-assembly in lipid bilayers

BB

BB

BB

BB

4

EYPC-LUVegg yolk phosphatidylcholinelarge unilamellar vesicle

Femto08

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Photoinduced transmembrane pH gradient

EDTA

HPTS

O

O

SO3-

Q HPTS

OH-O3S

-O3S SO3-

0

2 105

4 105

6 105

8 105

1 106

350 400 450Wavelength [nm]

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

6.8 7 7.2 7.4 7.6pH

I 462 /

I 405

A B

cps

a)

d)c)b)

e)

irradiation at 635 nm a) 0 s, e) 600 s

Science 313 (2006) 84

Q

EDTA

Femto08

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Photoinduced transmembrane pH gradient

Schulten et al. (1998) PNAS 95, 5935

photosynthetic apparatus of purple bacteria

Femto08

Page 64: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

EDTA

0

2 105

4 105

6 105

8 105

1 106

350 400 450Wavelength [nm]

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

6.8 7 7.2 7.4 7.6pH

I 462 /

I 405

A B

cps

a)

d)c)b)

e)

irradiation at 635 nm a) 0 s, e) 600 s

Science 313 (2006) 84

*CS

-+

Q

EDTA

EDTA+

Q.-QH.*

+

EDTA+

QH-

CS

QH2

O

O

SO3-

Q HPTS

OH-O3S

-O3S SO3-

Photoinduced transmembrane pH gradientFemto08

Page 65: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Blue systems

Time-resolved fluorescence

NN

O

O

O

O

HN

NH

NH

H2N

+H3N

B1

B2

B8

8.1 ns

5.7 ps63 ps av.: 2 ns

7.1 ps

51 ps … ps

av.: 0.2 nsinte

nsity

(a.u

.)

6050403020100

time delay (ps)

B1

B2

B8

Φfl

0.32

0.05

0.01

Efficient self-quenching!

J. Phys. Chem. B, 112 (2008) 8912

Femto08

Page 66: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

-40

-30

-20

-10

0

10

20

ΔA x

103

700650600550500450

wavelength (nm)

0.5 ps 5 ps 20 ps 70 ps 700 ps 1.4 ns

Blue systems

NN

O

O

O

O

HN

NH

NH

H2N

+H3N

B1

B2

B8

Transient absorption

-60

-40

-20

0

20

40

60

Δ A

x 10

3

700650600550500450

wavelength (nm)

0.3 ps 1.2 ps 4 ps 10 ps 20 ps 40 ps

B1·-

B1* + D B1.- + D.+B1*

-15

-10

-5

0

5

10

15

Δ A x

103

700650600550500450

wavelength (nm)

0.3ps 1ps 3ps 6ps 14ps 30ps 50ps 700ps B8

CSS

B8

LES

hν FLIC

CSS

LESCSS

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Page 67: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

CS state dynamics

CS state longer-lived in B8 !

Charge hopping?

Blue systems

ΔA

(a.u

.)

250200150100500

time delay (ps)

D (25 ps) O (60 ps)B2 (25 ps)B8 (60 ps)

J. Phys. Chem. B, 112 (2008) 8912

NN

O

O

O

O

HN

NH

NH

H2N

+H3N

B1

B2

B8B8

LES

hν FLIC

CSS

Transient absorptionFemto08

Page 68: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

r =ΔA|| −ΔA⊥ΔA|| + 2ΔA⊥

polarisation anisotropy(ground state bleach)

evidence of charge hopping Blue systems

NN

O

O

O

O

HN

NH

NH

H2N

+H3N

B1

B2

B8reorientation of the S0-LES transition dipole in 8 ps - too fast for rotational diffusion (200 ps)- charge hopping

0.4

0.3

0.2

0.1

0.0

-0.1

anis

otro

py

140120100806040200

time delay (ps)

~8 ps

200 ps

~50 ps

B1 B8

Transient absorptionFemto08

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15

10

5

0

-5

-10Δ

A·10

3750700650600550500450

wavelength (nm)

2.2ps 2.5ps 3ps 5ps 10ps 20ps 30ps 50ps 80ps 100ps 150ps 200ps 300ps 500ps 1000ps 1900ps

B84

B84 in vesicles (transient absorption)

16

14

12

10

86

4

2

0

ΔA·

103

1 10 100 1000time delay (ps)

tau1 = 5 ps (49%) tau2 = 25 ps (40%) tau3 = 400 ps (11%)

CSS

Long-lived component (400 ps) in vesicles!

Femto08

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Possible improvements ?

NN

O

O

O

ONH

O

OH2N

+H3N

O

O

Y

Y

Y

Y

Y

Y

Y

Y

Y

Increase E-gap for CR

1) Slowing down charge recombination

2) Increasing charge mobility

R

R

R

R

R

R

R

R

R

R

R

NN

O

O

O

ONH

O

OH2N

+H3N

HN

Cl

Change the ‘rod’Increase E-gap for CR

Femto08

Page 71: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

- Investigate the photophysics on gold surface

Perspectives

- Other chromophores with OPE

- Other NDI chromophores

- Effect of excitation wavelength

- Excitation energy migration

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Page 72: Photoinduced electron transfer reactions investigated by ...reseau-femto.cnrs.fr/IMG/pdf/FEMTO2008_Vauthey.pdf · Introduction. Photoinduced intermolecular electron transfer reactions,

Acknowledgements

Pierre-Alain MullerAna MorandeiraOlivier NicoletStéphane PagèsAlexandre FürstenbergOmar MohammedBernhard LangNatalie BanerjiJakob GriljGuillaume DuvanelOksana Kel

Collaboration:Stefan Matile (Geneva)Erik Nibbering (Berlin)Anatoly Burshtein (Weizmann)Anatoly Ivanov (Volgograd)

Femto08


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