Post on 27-Mar-2020
Molecular Approaches toSolar Energy Conversion
Michael R. Wasielewski
B800-B8501.2 ps
B800-B800500 fsLH2-LH1
3-5 ps
LH1-RC35 ps
B850-B850100-200 fs
B800-B8501.2 ps
B800-B800500 fsLH2-LH1
3-5 ps
LH1-RC35 ps
B850-B850100-200 fs
Membrane edge
Membrane edge
Periplasm
Cytoplasm
3.5 ps
0.9 ps
+.
-. 200 ps
BChl a2(P865)
Car BChl a
BPh a
QAQB
BChl a
BPh a
A SideB Side
200 µs
3.5 ps3.5 ps
0.9 ps0.9 ps
+.
-. 200 ps
+.
-. 200 ps-. 200 ps
BChl a2(P865)
Car BChl a
BPh a
QAQB
BChl a
BPh a
A SideB Side
200 µs
Photosynthesis: Self-assembly Provides Emergent Function
Topics for Discussion• Light Harvesting:
Singlet fission in molecular materials can generate two excited triplet states from one singlet state that can greatly improve use of the solar spectrum to enhance charge generation yields.
• Charge Separation and Transport: Self-assembly is used to prepare molecular materials in which photo-generated charge can be transported long distances.
• Photodriven Catalysis:New photosensitizers that can deliver charge at high potentials to catalysts to carry out energy-demanding reactions.
S1 S1
Molecule A Molecule B
S0S0
T1 T1
T2 T2
hn
Photon Absorption Charge Separation
Charge Transport Charge Collection
• E(S1) > 2E(T1)• E(T2) > 2E(T1) • Optimized Electronic Coupling• k(TTsep) >> k(TTannih) SF can increase the efficiency
of solar cells from 33% to 45%Hanna, M.C.; Nozik, A.J. J. Appl. Phys. 2006, 100, 074510.
Light Harvesting:Singlet Exciton Fission (SF)
Originally observed in anthracene and tetracene by Siebrand, Schneider, Swenberg, Pope, and Geacintov: 1965-1969.
S1 S1
T1 T1
S0 S0
T2 T2
A B
Singlet Fission Mechanisms
Smith et al. Chem. Rev. 2010, 110, 6891; Greyson et al. J. Phys. Chem. B 2010, 114, 14168; Burdett and Bardeen, Acc. Chem. Res. 2013, 46, 1312; Zimmerman et al. J. Am. Chem. Soc. 2011, 133, 19944; Scholes, G. D. J. Phys. Chem. A 2015, 119, 12699; Kolomeisky et al. J. Phys. Chem. C 2014, 118, 5188.
[ 1(S1S0) « CT « 1(T1T1) ] ® 1(T1T1) ® T1 + T1
3,5(T1T1)
vs.
S + S1 ® 2T1
S1 S1T1 T1S0 S0
S1 S1T1 T1S0 S0
J. Am. Chem. Soc. 135, 14701-14712 (2013). J. Phys. Chem. A 119, 4151-4161 (2015).
Angew. Chem. Int. Ed. 54, 8679-8683 (2015).
J. Am. Chem. Soc. 139, 663-671 (2017).
Nat. Comm. 8, 15171 (2017).
J. Phys. Chem B 120, 1357-1366 (2016). J. Am. Chem. Soc. 138, 11749–11761 (2016). ChemPhotoChem 2, 223-233 (2018).
1(S1S0) 1(T1T1)
SingletFission
ChargeTransfer
CTSolven
t
Polarity
Nat. Chem. 8, 1120-1125 (2016).
Singlet Fission Mechanisms:Recent Examples
Terrylenediimide (TDI)
n Good absorption in the solar spectrum• λmax = 650nm (93,000 M-1cm-1)
n E(S1) - 2E(T1) = 0.33 eV• E(S1) = 1.87 eV (optical bandgap)• E(T1) = 0.77 eV (phosphorescence)
n High stability
1100 1200 1300 1400 1500 1600
0
2
4
6
Inte
nsity
Wavelength (nm)500 600 700 900 1000 1100 1200 1300 1400 1500 1600-0.045
-0.030
-0.015
0.000
0.015
0.030
DA
Wavelength (nm)
1.0 ps 100 ps 500 ps 2.0 ns 3.0 ns 5.0 ns
CH2Cl2tD = 2.1 ± 0.1 ns
500 600 700 900 1000 1100 1200 1300 1400 1500
-0.08
-0.04
0.00
0.04
50 ps 200 ps 500 ps 1.7 ns
DA
Wavelength (nm)
0.33 ps 1.0 ps 3.0 ps 5.0 ps 20 ps
FsTA of the Slip-stacked TDI Dimerwith a Biphenyl Offset
600 800 1000 1200 1400 16000.00
0.05
0.10
0.15
0.20
Abso
rban
ce (A
U)
Wavelength (nm)
907
10371325
TDI•-
600 800 1000 1200 1400 1600
0.00
0.05
0.10
0.15
Abso
rban
ce (A
U)
Wavelength (nm)
757
TDI•+
CH2Cl2
E. A. Margulies, C. E. Miller, Y. Wu, L. Ma, G. C. Schatz, R. M. Young and M. R. W., Nat. Chem., 8, 1120 (2016).
500 600 700 900 1000 1100 1200 1300 1400 1500-0.01
0.00
0.01
0.02DA
20 ns 150 ns 300 ns 1.0 µs 4.0 µs 15 µs
Wavelength (nm)
(toluene)
Anthracene-Sensitized Triplet State
FsTA of Slip-Stacked TDI Dimerwith a Biphenyl Offset
500 600 700 900 1000 1100 1200 1300 1400 1500-0.010
-0.005
0.000
0.005
50 ps 300 ps 1000 ps 4000 ps
0.75 ps 1.5 ps 3.0 ps 5.0 ps 10 ps
DA
Wavelength (nm)
(toluene) tSF = 2.2 ps, tD = 1.2 ns
E. A. Margulies, C. E. Miller, Y. Wu, L. Ma, G. C. Schatz, R. M. Young and M. R. W., Nat. Chem., 8, 1120 (2016).
Population Dynamicsand Triplet Yield
1 10 100 1000 10000
0.0
0.5
1.0
1.5
2.0 GSB S1
T1
Popu
latio
n (N
orm
alize
d)
Time (ps)
fT = 133%
𝑵𝑻𝑻𝑵𝑺𝑺
= 𝒆𝑬𝑺𝑺(𝑬𝑻𝑻
𝒌𝑻 = 18 meV
(1/3) 1(S1S0) ⇌ (2/3) 1(T1T1)
1(S1S0)CTkCT
kSF
kTTA1
Reaction Coordinate
Ener
gy
1(T1T1)
E. A. Margulies, C. E. Miller, Y. Wu, L. Ma, G. C. Schatz, R. M. Young and M. R. W., Nat. Chem., 8, 1120 (2016).
θ
400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
Abs
orba
nce
(nor
m.)
Wavelength (nm)
XanTDI XanTDI2 C15TDI
Transient Mid-IR Spectroscopy of a Slip-stacked TDI Dimer
q = 20o
slip = 8.6 Åp-p distance = 3.5 Å
M. Chen, Y. J. Bae, C. M. Mauck, A. Mandal, R. M. Young, and M. R. Wasielewski, J. Am. Chem. Soc. 140, 9184-9192 (2018).
1700 1600 1500 1400 1300 1700 1600 1500 1400 1300
* ** *
XanTDI
* *
Nor
mal
ized
Abs
orpt
ion
*
XanTDI2
Wavenumber (cm-1)
C15TDI
Cal
cula
ted
Inte
nsity
Cation
Anion
Triplet
Singlet ES
Singlet GS
Wavenumber (cm-1)
1700 1600 1500 1400 1300
0.0
0.1
0.2
0.3
0.4
1644 cm-1
1343 cm-1
1532 cm-1
Abso
rban
ce
Wavenumber (cm-1)
1572 cm-1
IR Spectra
C15TDI AnionIn CD2Cl2
Measured Computed
500 600 700 800 1000 1200 1400-0.10-0.08-0.06-0.04-0.020.000.020.040.06
DA
Wavelength (nm)
1 ps 50 ps 205 ps 750 ps 2 ns 4 ns 7 ns
FsTA and FsIR Data for Xan-TDI
tS = 2.7 ± 0.1 ns
500 600 700 800 1000 1200 1400-0.20-0.15-0.10-0.050.000.050.100.15
DA
Wavelength (nm)
1 ps 49 ps 204 ps 749 ps 2 ns 4 ns 7 ns
tS = 3.2 ± 0.1 ns
CH2Cl2
1,4-dioxane
-200
20
-707
-350
35
500 600 700 800 1000 1200 1400-20-10
010
499 ps 1.0 ns
2.0 ns 7.0 ns
49 ps 174 ps 279 ps
1.0 ps 5.0 ps 10 ps
A (m
OD
) 250 ps 500 ps 1.0 ns
2.0 ns 4.0 ns 7.0 ns
52 ps 100 ps 175 ps
1.1 ps 3.0 ps 10 ps
A B
Wavelength (nm)
A B C
FsTA Data for Xan-TDI2
CH2Cl2
CH2Cl2
1,4-dioxane
1,4-dioxane
FsIR Data TDI Monomers and Xan-TDI2
-2024
-4-2024
-4
0
4
1680 1650 1620 1590 1560 1530-0.3
0.0
0.3
600 ps 1.4 ns 3.0 ns
5.0 ns 7.2 ns
1 ps 50 ps 250 ps
6.02 ns 7.02 ns
1.02 ns 2.02 ns 3.02 ns
100 ps 251 ps 501 ps
4.02 ns 5.02 ns
2 ps 10 ps 50 ps
5.0 ns 7.2 ns
1.0 ns 2.5 ns
5.0 ps 10 ps 20 ps
50 ps 100 ps 250 ps
1.2 ps 2.0 ps 3.0 ps
DA
(mO
D)
1.5 ns 2.1 ns 3.3 ns
100 ps 150 ps 200 ps
10 ps 20 ps 51 ps
310 ps 500 ps 1.0 ns
2.2 ps 5.0 ps 8.0 ps
Wavenumber (cm-1)
1,4-dioxane-d8
iodoethane
1,4-dioxane-d8
CD2Cl2
-20246
-0.20.00.20.4
1680 1650 1620 1590 1560 1530-4-2024
A: (4.2 ± 0.5 ps) B: (489 ± 11 ps)
x 1.5
0A
(mO
D)
A: 5.4 ± 2.2 ps B: 110 ± 1 ps C: 1.21 ± 0.02 ps
Wavenumber (cm-1)
S1: 1.90 ± 0.17 ns T1: >> 8 ns
x 3
FsIR Data TDI Monomers and Xan-TDI2
1,4-dioxane-d8
CD2Cl2
iodoethane
Time Evolution of theMixed State Population
1680 1650 1620 1590 1560 1530-1.5
-1.0
-0.5
0.0
0.5
1.0
Norm
aliz
ed :
A
Wavenumber (cm-1)
TDI Triplet Spectrum Xan-TDI2 in Dioxane Xan-TDI2 in DCM
Summary
n FsIR spectra show that the electronic excited states of the TDI dimer have mixed singlet, triplet and CT character.
n At times < 300 fs, the 1(S1S0) state already has significant CT character even in low polarity solvents.
n Nevertheless, the degree of state mixing depends on the solvent polarity, which alters the relative energies of the states and their time evolution.
Photon Absorption Charge Separation
Charge Transport Charge Collection
Charge Separation and Transport:Self-Segregating Charge Conduits
J. L. Logsdon, P. E. Hartnett, J. N. Nelson, M. A. Harris, T. J. Marks, MRW, ACS Appl. Mater. Interfaces 2017 9, 33493.
400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
Abs
orba
nce
(Nor
mal
ized
)
Wavelength (nm)
Solution Disordered Film (CH2Cl2) Ordered Film (NMP)
3000 2950 2900 2850 2800 2750
0.00
0.05
0.10
0.15
0.20
0.25 Disordered (CH2Cl2) Ordered (NMP)
Inte
nsity
(Arb
itrar
y)
Wavenumbers (cm-1)
2849 cm-12854 cm-1
2918 cm-1
2925 cm-1
Route to Ordered Films
Inte
nsity
Q(A-1)
-2
SAXS of Ordered Film (NMP)
Focus on 1a:
0.0 0.5 1.0 1.5 2.0 2.50.001
0.01
0.1
1
Inte
nsity
(Nor
mal
ized
)
qZ (A-1)
1a 1b
0.0 0.5 1.0 1.5 2.0
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(Nor
mal
ized
)
qXY (A-1)
1a 1b
GIWAXS of Ordered Films:Comparing Tails Lengths
1a: 1b:
500 600 700 800
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
DA
Wavelength (nm)
0 ps 160 ps 2 ps 600 ps 15 ps 2000 ps 50 ps 7500 ps
0 2 4 6
0.0
0.2
0.4
0.6
0.8
1.0
-1 0 1 2 3 4
0.0
0.2
0.4
0.6
0.8
1.0
DA
(Nor
mal
ized
)
Tim e (ps)
Ordered Disordered
DA
(Nor
mal
ized
)
Time (ns)
Transient AbsorptionSpectroscopy and Kinetics
100 1000
0.001
0.002
0.003
0.0040.0050.006
a = 0.3D
A
Time (ns)
1a in toluene, 525 nm exc. 1a ordered film (NMP), 525 nm exc.
0 2 4 6
0.0
0.2
0.4
0.6
0.8
1.0
DA
(Nor
mal
ized
)
Time (ns)
95K 145K 195K 245K 295K
e
BFAF
1-T
0T
e2J
S
T-1
T0Ener
gy
Magnetic Field (B)
T+1
Probing the CT State using TREPR and TR-Microwave Conductivity
0 10 20 30 40 50-1.0
0.0
1.0
2.0
3.0
4.0
Ordered (NMP) Disordered (CH2Cl2)
Rela
tive
Mic
row
ave
Pow
erAb
sorb
ed (a
rb. u
nits
)
Time (µs)
Zeeman Effect on e-h pair energy levels
Free carrier yield in ordered films of 1a-b.
340 342 344 346-5
-4
-3
-2
-1
0e
Inte
nsity
(arb
. uni
ts)
Magnetic Field (mT)
a
t = 100 ns
Functional G-Quadruplexes:
Watson-Crickedge
Hoogsteen edge
A Bio-inspired Approach
Y.-L. Wu, K. E. Brown, D. M. Gardner, S. M. Dyar and MRW, J. Am. Chem. Soc. 137, 3981-3990 (2015).
Y.-L. Wu, K. E. Brown, and MRW, J. Am. Chem. Soc. 135, 13322-13325 (2013).
Functional G-Quadruplexes
tCR = 10 ps tCR = 1 ns
Rapid hole hopping in the GQF core
+ -
-
+
+ -
G-Quadruplex Frameworks (GQFs)
e–h+
Photon Absorption Charge Separation
Charge Transport Charge Collection
Y.-L. Wu, N. E. Horwitz, K.-S. Chen, D. A. Gomez-Gualdron, N. S. Luu, L. Ma, T. C. Wang, M. C. Hersam, J. T. Hupp, O. K. Farha, R. Q. Snurr. and M. R. Wasielewski, Nat. Chem. 9, 466-472 (2017).
Synthesis of GQFs
Y.-L. Wu, N. E. Horwitz, K.-S. Chen, D. A. Gomez-Gualdron, N. S. Luu, L. Ma, T. C. Wang, M. C. Hersam, J. T. Hupp,O. K. Farha, R. Q. Snurr. and M. R. Wasielewski, Nat. Chem. 9, 466-472 (2017).
2 4 6 8 10 12 14
3.90° (22.6 Å)
2q (deg)
3.14° (28.1 Å)
2.25° (39.2 Å)
Inte
nsity
3.45° (25.6 Å)
2.50° (35.0 Å)
Crystalline GQF: Strong PXRD
d(NH2–NH2)~39.2Å
d(NH2–NH2)~34.9Å
d(NH2–NH2)~21.3Å
Facile Electron Movement in a GQF
348 350 352 354
Inte
nsity
(nor
m.)
Magnetic Field (mT)
G2NDI NDI
346 348 350 352
Magnetic Field (mT)
G2PDI PDI
linewidth ∝1/ 𝑁�
e–
N = 6-7
Long-lived and Mobile Charge Carriers in a GQF
500 550 600 650 700 750-0.16
-0.14
-0.12
-0.10
-0.08
-0.06
-0.04
-0.02
0.00
0.02
0 1000 2000 3000 4000 5000 6000 7000
-0.14
-0.12
-0.10
-0.08
-0.06
-0.04
-0.02
0.00
100 630 1841 6277
DA
Wavelength (nm)
2.5 ps 10 20 40
DA @
520
nm
Time (ps)
550 600 650 700 750
-0.14
-0.12
-0.10
-0.08
-0.06
-0.04
-0.02
0.00
0.02
0 1000 2000 3000 4000 5000 6000 7000
-0.10
-0.08
-0.06
-0.04
-0.02
0.00
100 397 1988 5814
DA
Wavelength (nm)
0.3 ps 0.9 2.4 20
DA @
550
nm
Time (ps)
0 1500 3000 4500 6000
0.0
0.2
0.4
0.6
0.8
1.0
TRM
C In
tens
ity (n
orm
.)
Time (ns)
G2NDI G2PDI NDI
G2NDI G2PDI
Summary
• Ordered thin solid films of self-segregating ZnP-PDI molecules display charge conduit behavior resulting in independent charge carriers that persist for > 10 µs.
• G-quadruplex frameworks assemble into ordered structures in which ultrafast photo-driven charge separation results in independent charge carriers that also persist for > 10 µs.
Image from SOFIhttp://www.solar-fuels.org/research-applications/
Photodriven Catalysis: Photosensitizersfor Energy Demanding Reactions
Solar FuelsMolecular Approach to
Photoelectrochemical Cells
×2
Dye Catalyst
DyeCatalyst
H+
2H2O O24 e-
4 H+
2 H+
2 e- H2
e-
Photoanode Photocathode
×4
Overall Strategy
< 4 ps
< 65 ps
17.2 ns< 10 ps
Vagnini, M. T. et al. Proc. Natl. Acad. Sci. 2012, 109, 15651-15656.
Vagnini, M. T. et al. Chem. Sci. 2013, 4, 3863-3873.
Lindquist, R. J.; Phelan, B. T.; Reynal, A.; Margulies, E. A.; Shoer, L. E.; Durrant, J. R.; Wasielewski, M. R., J. Mater. Chem. A 2016, 4, 2880.
Use time-resolved spectroscopy to probe photo-initiated multi-step catalytic mechanisms: One step at a time
Photoexcited NDI anions are Super-reductants
O
OO
O
NNR RReduced at –0.48, –0.99 V vs SCE*NDI1– has –2.08 V reducing power*NDI2– has –3.07 V reducing power
t = 141 ps
D. Gosztola, M. P. Niemczyk, W. Svec, A. S. Lukas and MRW, J. Phys. Chem. A, 2000, 104, 6545-6551.
O
O
O
O
NNR RReduced at –0.43, –0.73 V vs SCE*PDI1– has –1.73 V reducing power*PDI2– has –2.45 V reducing power
D. Gosztola, M. P. Niemczyk, W. Svec, A. S. Lukas and M. R. Wasielewski, J. Phys. Chem. A, 2000, 104, 6545-6551.
Photoexcited PDI anions are Super-reductants
t = 137 ps
Photoexcited Radical Anions as Super-reductants
Strategy:
• Couple photoexcited radical anions with hard to reduce catalysts.
• Use multi-step electron transfer to increase the lifetime of the reduced catalyst intermediates.
NNReI
COOC COL
R
R Re(R2-bpy)(CO)3L:n bpy ligand reduced at –(1.2-1.35) V vs SCE
and Re center reduced at –(1.5-1.65) V vs SCE (varies depending on R)
n Binds CO2 very poorly after one electron reduction
n Binds CO2 very well (and catalysis initiated) after second reduction and loss of L
n Use a triad to enhance the charge shift lifetimes.n Diffusive encounter with CO2 should be more facile.n Longer charge shift lifetimes allow the study of
intermediates, charge accumulation, and catalysis.
Photoexcited Radical Anions as Super-reductants
J. F. Martinez, N. T. La Porte, C. M. Mauck, and MRW, Faraday Discuss. 198, 235-249 (2017).
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0-20
-15
-10
-5
0
5
10
15Fc
ANT0/-ReI/0Bpy0/-
NDI-/2-NDI0/-
Cur
rent
(µA)
Potential (V vs. SCE)
Electrochemistry of the Re Complex
NDI: -0.51 V, -1.01 VDPA: -1.85 VBpy: -1.20 VRe: -1.55 VExcited State ReductionPotential of *NDI•– : -2.11 V
Excitation Window
300 400 500 600 700 8000.0
0.5
1.0
Abs
orba
nce
(nor
m.)
Wavelength (nm)
Ligand Complex Ligand with TDAE Complex with TDAE
NDI•- can be selectively excited at 450-850 nm.
2150 2100 2050 2000 1950 1900 1850
-0.002
-0.001
0.000
0.001
0.002
DA
Wavenumber (cm-1)
-9.22ps 6.6ps 19.9ps 49.6ps 172ps 7.18ns
Re(bpy•–)
Re(bpy0)
400 500 700 800-0.012
-0.010-0.008
-0.006
-0.004
-0.002
0.0000.002
0.004
DA
Wavelength (nm)
-3.89 ns 9.25 µs 139 ps 15.1 µs 510 ps 23.1 µs 4.41 ns 35.5 µs 86.5 ns 57.8 µs 1.34 µs 100 µs 4.44 µs 173 µs
NDI0
NDI1–
Long reverse charge shift lifetime tRCS = 43.4 ± 1.2 µs
Femtosecond Transient Absorption in the Vis/NIR and mid-IR
tCS1 = 21 ps tCS2 < 4 ps
0.5 0.0 -0.5 -1.0 -1.5 -2.0
-40
-30
-20
-10
0
10
20
30Fc
ReI/0
NDI0/- NDI-/2-
ANT0/-
Bpy0/-
Cur
rent
(µA)
Potential (V vs SCE)
Electrochemistry of the Re Complex
NDI: -0.52 V, -1.04 VDPA: -1.85 VBpy: -1.30 VRe: -1.50 VExcited State ReductionPotential of *NDI•– : -2.12 V
Re(bpy0)
Re(bpy•œ)
tRCS = 24.4 ns
300 400 500 600 700 8000.0
0.5
1.0
Abs
orba
nce
(nor
m.)
Wavelength (nm)
Ligand Complex Reduced NDI on Ligand Reduced NDI on Complex
NDI is reversibly reduced at –0.48 V vs SCEThe excited NDI radical anion has –2.12 V of reducing potential using near-infrared light at 800 nm.
NDI RadicalAnion:
Ground statespectra:
Femtosecond IRspectra:
tCS = 5 ps
Reduction of the non-innocent bpy ligand:
Light-driven Reduction of aRe-based CO2 Reduction Catalyst
Electrocatalytic CO2 Reduction
0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5-10
0
10
20
30
µA
Potential (V vs Ag/AgCl)
Argon CO2
DMF, 0.1 M TBAPF6, 0.5mM Triad
Bulk Photoelectrolysis
-0.6V vs. SCE applied potential, 0.1 M TBAPF6,0.1 M MeOH, 0.1mM Triad. l >520nm
0 1 2 3 4 50
20
40
60
80
100
Inte
nsity
Time (minutes)
CO
Light-driven Super-Reductants forCO2 Reduction
Use two electron transfer steps that take advantage of both visible and near-infrared photons.
520 nme-
950 nme-
The next step:
• Arylene diimide radical anions can be reversibly reduced at mild potentials to radical anions.
• Arylene diimide radical anions absorb in the visible and near-IR spectral regions.
• Excited states of the radical anions are powerful reductants that can drive energy-demanding reactions such as CO2 reduction catalysts.
Summary
Acknowledgements
Collaboration: Tobin Marks, Joseph Hupp, George Schatz, Omar Farha, Randall Snurr, Mark Hersam, all at Northwestern
Support: Chemical Sciences, Geosciences,and Biosciences Division,Office of Basic Energy Sciences, DOE