Molecular Approaches to Solar Energy Conversion · 2018-10-04 · Topics for Discussion • Light Harvesting: Singlet fission in molecular materials can generate two excited triplet

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


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)


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.



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+



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

Documents

Molecular Approaches to Solar Energy Conversion · 2018-10-04 · Topics for Discussion • Light Harvesting: Singlet fission in molecular materials can generate two excited triplet