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  • Photoinduced electron transfer reactions investigated by ultrafast spectroscopy

    Eric VautheyDépartement de chimie-physique

    Université de Genève

    Femto08

  • 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

  • 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

  • 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

  • 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( ) =ΔGET

    0→0 + vhυvib + λs4λskBT

    Femto08

  • - 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

  • 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

  • Hypothesis:

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

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

    - ...

    Femto08

  • E

    A D

    A* D

    A.- D.+

    ΔGCS

    A.- D.+*

    ΔGCS*

    ΔGCS ΔGCS*

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

    Femto08

  • 2) ΔGCS dependence of the CS distance

    kCS =2πh

    V 2

    4πλkBT( )1/2 exp −

    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

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

    2) ΔGCS dependence of the CS distanceFemto08

  • 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

  • Other unanswered questions:

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

    - structure and geometry of the reactions intermediates

    - ....

    Femto08

  • 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

  • Picosecond optical calorimetric studies: looking at the energetics

    Femto08

  • C = − e2

    4πε0εsd

    Energetics

    D + A

    D*+ A

    D .A-˙+˙

    D + A-˙+˙ ΔGCS

    E*

    e(Eox- Ered)

    C

    d d

    Femto08

  • sample

    grating formation

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

    2 formalisms: 1) holography 2) nonlinear optics

    θ

    Picosecond optical calorimetry using transient gratingFemto08

  • modulation axis

    light intensity

    product concentration

    reactant concentration

    ΛFemto08

  • Spatial modulation of the optical properties of the sample

    ˜ n x( ) = ˜ n 0 +Δ ˜ n cos2πΛ

    x

    modulation amplitude

    ˜ n average value

    Transient gratingFemto08

  • amplitude grating

    Δñ

    ΔAΔnphase grating

    population changes

    Δnppopulation changesoptical Kerr effect

    ΔnKdensity changes

    Δnd

    changesthermal

    ΔndvΔnd

    TΔnds

    volumeexpansion electrostriction

    ΔnKeΔnK

    n

    nuclear electronic

    non resonant interactions

    Femto08

  • Probing the grating

    θB

    probe pulse

    sinθB =λs2Λ

    Bragg angle:

    Femto08

  • diffracted intensity

    η =IdifIs∝Δ ˜ n 2

    ≈ c1Δn2 + c2ΔA

    2

    Applications of the transient grating technique:

    - population dynamics

    - ultrafast calorimetry

    - transient dichroism

    - …

    Femto08

  • 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

  • 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

  • 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

  • translation stage

    frequencyconverter

    LASER

    detectorsample

    Experimental setup

    Femto08

  • 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

  • Excited-state dynamics of radical ions

    Femto08

  • E

    A D

    A* D

    A.- D.+

    ΔGCS

    A.- D.+*

    ΔGCS*

    ΔGCS ΔGCS*

    Formation of the product in an electronic excited state ?

    Femto08

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • Distinguishing different types of ion pairs

    Femto08

  • 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

  • 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

  • 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

  • Q :CN

    NC

    Pe DCB DCE

    CN

    CN

    Pump-pump-probe experiment

    Femto08

  • Pump-pump-probe experiment

    Femto08

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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·-CR

    Femto08

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • Structure of the ion pairs ?

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

    => probably sandwich-like coplanar geometry

    Femto08

  • 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

  • Artificial photosynthesis

    with Stefan Matile (Geneva)

    Femto08

  • 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

  • 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

  • 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

  • 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

  • Photoinduced transmembrane pH gradient

    Schulten et al. (1998) PNAS 95, 5935

    photosynthetic apparatus of purple bacteria

    Femto08

  • 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

  • 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

  • -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

    Femto08

  • 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

  • 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

  • 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

  • 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

  • - Investigate the photophysics on gold surface

    Perspectives

    - Other chromophores with OPE

    - Other NDI chromophores

    - Effect of excitation wavelength

    - Excitation energy migration

    Femto08

  • 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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Photoinduced electron transfer reactions investigated by ultrafast spectroscopy Eric Vauthey Département de chimie-physique Université de Genève Femto08
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