Molecular Approaches to Solar Energy Conversion. Michael R. Wasielewski

Transcription

1 Molecular Approaches to Solar Energy Conversion Michael R. Wasielewski

2 Photosynthesis: Self-assembly Provides Emergent Function Periplasm B850-B fs LH1-RC 35 ps Membrane edge Car B Side BChl a BChl a 2 (P865) +. A Side 3.5 ps BChl a B800-B ps 0.9 ps BPh a BPh a LH2-LH1 3-5 ps B800-B fs Membrane edge Cytoplasm Q B 200 µs ps Q A

3 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. Molecule A Molecule B T 2 T 2 S 1 S 1 T 1 T 1 hn S 0 S 0 Photon Absorption Charge Separation Charge Separation and Transport: Self-assembly is used to prepare molecular materials in which photogenerated 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. Charge Transport Charge Collection

4 Light Harvesting: Singlet Exciton Fission (SF) Originally observed in anthracene and tetracene by Siebrand, Schneider, Swenberg, Pope, and Geacintov: A B T 2 T 2 S 1 S 1 T 1 T 1 S 0 S 0 E(S 1 ) > 2E(T 1 ) E(T 2 ) > 2E(T 1 ) Optimized Electronic Coupling k(tt sep ) >> k(tt annih ) SF can increase the efficiency of solar cells from 33% to 45% Hanna, M.C.; Nozik, A.J. J. Appl. Phys. 2006, 100,

5 Singlet Fission Mechanisms [ 1 (S 1 S 0 ) «CT «1 (T 1 T 1 ) ] 1 (T 1 T 1 ) T 1 + T 1 3,5 (T 1 T 1 ) 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.

6 Singlet Fission Mechanisms: Recent Examples S + S 1 2T 1 vs. J. Am. Chem. Soc. 135, (2013). J. Phys. Chem. A 119, (2015). Singlet Fission J. Am. Chem. Soc. 139, (2017). Angew. Chem. Int. Ed. 54, (2015). 1 (S 1 S 0 ) 1 (T 1 T 1 ) Charge Transfer Solvent Polarity CT Nat. Chem. 8, (2016). Nat. Comm. 8, (2017). S 1 S 1 T 1 T 1 S 0 S 0 S 1 S 1 T 1 T 1 S 0 S 0 J. Phys. Chem B 120, (2016). J. Am. Chem. Soc. 138, (2016). ChemPhotoChem 2, (2018).

7 Terrylenediimide (TDI) n Good absorption in the solar spectrum λ max = 650nm (93,000 M -1 cm -1 ) n E(S 1 ) - 2E(T 1 ) = 0.33 ev E(S 1 ) = 1.87 ev (optical bandgap) E(T 1 ) = 0.77 ev (phosphorescence) n High stability 6 DA CH 2 Cl 2 t D = 2.1 ± 0.1 ns Wavelength (nm) 1.0 ps 100 ps 500 ps 2.0 ns 3.0 ns 5.0 ns Intensity Wavelength (nm)

8 FsTA of the Slip-stacked TDI Dimer with a Biphenyl Offset 0.04 CH 2 Cl DA ps 1.0 ps 3.0 ps 5.0 ps 20 ps 50 ps 200 ps 500 ps 1.7 ns Absorbance (AU) TDI Wavelength (nm) Absorbance (AU) TDI Wavelength (nm) Wavelength (nm) 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).

9 DA FsTA of Slip-Stacked TDI Dimer with a Biphenyl Offset Anthracene-Sensitized Triplet State 20 ns 150 ns 300 ns 1.0 µs 4.0 µs 15 µs 0.00 (toluene) Wavelength (nm) DA (toluene) t SF = 2.2 ps, t D = 1.2 ns 0.75 ps 1.5 ps 3.0 ps 5.0 ps 10 ps 50 ps 300 ps 1000 ps 4000 ps Wavelength (nm) 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).

10 Population Dynamics and Triplet Yield Population (Normalized) f T = 133% GSB S 1 T 1 Energy 1 (S 1 S 0 ) k CT k SF k TTA1 CT 1 (T 1 T 1 ) Time (ps) Reaction Coordinate (1/3) 1 (S 1 S 0 ) (2/3) 1 (T 1 T 1 ) N TT = e ESS(ETT kt N SS = 18 mev 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).

11 Transient Mid-IR Spectroscopy of a Slip-stacked TDI Dimer Absorbance (norm.) XanTDI XanTDI 2 C 15 TDI Wavelength (nm) θ q = 20 o slip = 8.6 Å p-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, (2018).

12 IR Spectra Normalized Absorption Measured XanTDI * * * * * * XanTDI 2 * C 15 TDI Calculated Intensity Computed Singlet GS Singlet ES Triplet Anion Cation Wavenumber (cm -1 ) 0.4 Wavenumber (cm -1 ) C 15 TDI Anion In CD 2 Cl 2 Absorbance cm cm cm cm Wavenumber (cm -1 )

13 FsTA and FsIR Data for Xan-TDI CH 2 Cl DA t S = 2.7 ± 0.1 ns 1 ps 50 ps 205 ps 750 ps 2 ns 4 ns 7 ns Wavelength (nm) ,4-dioxane DA t S = 3.2 ± 0.1 ns Wavelength (nm) 1 ps 49 ps 204 ps 749 ps 2 ns 4 ns 7 ns

14 FsTA Data for Xan-TDI 2 A (mod) CH 2 Cl 2 1,4-dioxane CH 2 Cl 2 1,4-dioxane 1.0 ps 5.0 ps 10 ps 1.1 ps 3.0 ps 10 ps 49 ps 174 ps 279 ps 52 ps 100 ps 175 ps 499 ps 1.0 ns 250 ps 500 ps 1.0 ns 2.0 ns 7.0 ns 2.0 ns 4.0 ns 7.0 ns Wavelength (nm) A B A B C

15 DA (mod) ps 50 ps 250 ps 2 ps 10 ps 50 ps 1.2 ps 2.0 ps 3.0 ps FsIR Data TDI Monomers and Xan-TDI 2 1,4-dioxane-d ps 1.4 ns 3.0 ns 100 ps 251 ps 501 ps 5.0 ns 7.2 ns iodoethane CD 2 Cl ps 10 ps 20 ps 1.02 ns 2.02 ns 3.02 ns 50 ps 100 ps 250 ps 1,4-dioxane-d ns 5.02 ns 1.0 ns 2.5 ns 6.02 ns 7.02 ns 5.0 ns 7.2 ns ps 5.0 ps 8.0 ps 10 ps 20 ps 51 ps 100 ps 150 ps 200 ps 310 ps 500 ps 1.0 ns 1.5 ns 2.1 ns 3.3 ns Wavenumber (cm -1 )

16 FsIR Data TDI Monomers and Xan-TDI 2 0A (mod) CD 2 Cl 2 1,4-dioxane-d 8 x 1.5 iodoethane x 3 A: (4.2 ± 0.5 ps) B: (489 ± 11 ps) A: 5.4 ± 2.2 ps B: 110 ± 1 ps C: 1.21 ± 0.02 ps Wavenumber (cm -1 ) S 1 : 1.90 ± 0.17 ns T 1 : >> 8 ns

17 Time Evolution of the Mixed State Population 1.0 Normalized : A TDI Triplet Spectrum Xan-TDI 2 in Dioxane Xan-TDI 2 in DCM Wavenumber (cm -1 )

18 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 (S 1 S 0 ) 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.

19 Charge Separation and Transport: Self-Segregating Charge Conduits Photon Absorption Charge Separation Charge Transport Charge Collection J. L. Logsdon, P. E. Hartnett, J. N. Nelson, M. A. Harris, T. J. Marks, MRW, ACS Appl. Mater. Interfaces ,

20 Route to Ordered Films Focus on 1a: Intensity (Arbitrary) Disordered (CH 2 Cl 2 ) 2925 cm -1 Ordered (NMP) 2918 cm cm cm -1 Absorbance (Normalized) Solution Disordered Film (CH 2 Cl 2 ) Ordered Film (NMP) Wavelength (nm) Intensity Wavenumbers (cm -1 ) SAXS of Ordered Film (NMP) -2 Q(A -1 )

21 GIWAXS of Ordered Films: Comparing Tails Lengths 1a: 1b: Intensity (Normalized) q XY (A -1 ) 1a 1b Intensity (Normalized) q Z (A -1 ) 1a 1b

22 Transient Absorption Spectroscopy and Kinetics 1a in toluene, 525 nm exc. 1a ordered film (NMP), 525 nm exc DA ps 160 ps 2 ps 600 ps 15 ps 2000 ps 50 ps 7500 ps Wavelength (nm) DA (Normalized) DA (N ormalized) Tim e (ps) Ordered Disordered DA (Normalized) K 145K 195K 245K 295K DA a = Time (ns) Time (ns) Time (ns)

23 Probing the CT State using TREPR and TR-Microwave Conductivity Intensity (arb. units) a e t = 100 ns Energy Zeeman Effect on e-h pair energy levels S 2J T +1 T 0 T -1 F A e e T 0 F B Relative Microwave Power Absorbed (arb. units) Magnetic Field (mt) Ordered (NMP) Disordered (CH 2 Cl 2 ) Magnetic Field (B) Free carrier yield in ordered films of 1a-b. T -1 Time (µs)

24 A Bio-inspired Approach Functional G-Quadruplexes: Hoogsteen edge Watson-Crick edge

25 Functional G-Quadruplexes t CR = 10 ps t CR = 1 ns Y.-L. Wu, K. E. Brown, and MRW, J. Am. Chem. Soc. 135, (2013). + - Rapid hole hopping in the GQF core Y.-L. Wu, K. E. Brown, D. M. Gardner, S. M. Dyar and MRW, J. Am. Chem. Soc. 137, (2015).

26 G-Quadruplex Frameworks (GQFs) Photon Absorption Charge Separation h + e 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, (2017).

27 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, (2017).

28 Crystalline GQF: Strong PXRD 2.25 (39.2 Å) 3.14 (28.1 Å) d(nh 2 NH 2 ) ~ 39.2 Å Intensity 2.50 (35.0 Å) 3.45 (25.6 Å) d(nh 2 NH 2 ) ~ 34.9 Å 3.90 (22.6 Å) d(nh 2 NH 2 ) ~ 21.3 Å q (deg)

29 Facile Electron Movement in a GQF e Intensity (norm.) G2NDI NDI linewidth 1/ N G2PDI PDI N = Magnetic Field (mt) Magnetic Field (mt)

30 Long-lived and Mobile Charge Carriers in a GQF DA G 2 NDI 520 nm ps Time (ps) Wavelength (nm) DA G 2 PDI 550 nm ps Time (ps) Wavelength (nm) TRMC Intensity (norm.) Time (ns) G2NDI G2PDI NDI

31 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.

32 Photodriven Catalysis: Photosensitizers for Energy Demanding Reactions Solar Fuels Molecular Approach to Photoelectrochemical Cells Photoanode Photocathode H + Dye Catalyst 4 2 H + 2 e - H 2 2H 2 O O 2 4 e - 4 H + Catalyst Dye 2 Image from SOFI e -

33 Overall Strategy Use time-resolved spectroscopy to probe photo-initiated multi-step catalytic mechanisms: One step at a time < 4 ps < 65 ps Vagnini, M. T. et al. Proc. Natl. Acad. Sci. 2012, 109, < 10 ps 17.2 ns Vagnini, M. T. et al. Chem. Sci. 2013, 4, 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.

34 Photoexcited NDI anions are Super-reductants Reduced at 0.48, 0.99 V vs SCE *NDI 1 has 2.08 V reducing power *NDI 2 has 3.07 V reducing power R O N O O N O R t = 141 ps D. Gosztola, M. P. Niemczyk, W. Svec, A. S. Lukas and MRW, J. Phys. Chem. A, 2000, 104,

35 Photoexcited PDI anions are Super-reductants Reduced at 0.43, 0.73 V vs SCE *PDI 1 has 1.73 V reducing power *PDI 2 has 2.45 V reducing power R O N O O N O R t = 137 ps D. Gosztola, M. P. Niemczyk, W. Svec, A. S. Lukas and M. R. Wasielewski, J. Phys. Chem. A, 2000, 104,

36 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. Re(R2-bpy)(CO)3L: R n N N L OC COCO ReI R n n bpy ligand reduced at ( ) V vs SCE and Re center reduced at ( ) V vs SCE (varies depending on R) Binds CO2 very poorly after one electron reduction Binds CO2 very well (and catalysis initiated) after second reduction and loss of L

37 Photoexcited Radical Anions as Super-reductants n n n Use a triad to enhance the charge shift lifetimes. Diffusive encounter with CO 2 should be more facile. Longer charge shift lifetimes allow the study of intermediates, charge accumulation, and catalysis. J. F. Martinez, N. T. La Porte, C. M. Mauck, and MRW, Faraday Discuss. 198, (2017).

38 Electrochemistry of the Re Complex Current (µa) Fc NDI 0/- Bpy 0/- NDI -/2- Re I/0 ANT 0/ Potential (V vs. SCE) NDI: V, V DPA: V Bpy: V Re: V Excited State Reduction Potential of *NDI : V

39 Excitation Window Absorbance (norm.) Ligand Complex Ligand with TDAE Complex with TDAE Wavelength (nm) NDI - can be selectively excited at nm.

40 Femtosecond Transient Absorption in the Vis/NIR and mid-ir t CS1 = 21 ps t CS2 < 4 ps DA NDI 1 NDI Wavelength (nm) 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 DA ps 6.6ps 19.9ps 49.6ps 172ps 7.18ns Re(bpy 0 ) Re(bpy ) Wavenumber (cm -1 ) Long reverse charge shift lifetime t RCS = 43.4 ± 1.2 µs

41 Electrochemistry of the Re Complex Current (µa) Fc NDI ANT 0/- Bpy 0/- Re I/0 0/- NDI-/2- NDI: V, V DPA: V Bpy: V Re: V Excited State Reduction Potential of *NDI : V Potential (V vs SCE)

42 Light-driven Reduction of a Re-based CO 2 Reduction Catalyst NDI Radical Anion: Reduction of the noninnocent bpy ligand: NDI is reversibly reduced at 0.48 V vs SCE The excited NDI radical anion has 2.12 V of reducing potential using near-infrared light at 800 nm. Ground state spectra: Femtosecond IR spectra: Re(bpy œ ) t CS = 5 ps Absorbance (norm.) Ligand Complex Reduced NDI on Ligand Reduced NDI on Complex Re(bpy 0 ) t RCS = 24.4 ns Wavelength (nm)

43 Electrocatalytic CO 2 Reduction DMF, 0.1 M TBAPF6, 0.5mM Triad 30 µa Argon CO Potential (V vs Ag/AgCl)

44 Bulk Photoelectrolysis Intensity CO -0.6V vs. SCE applied potential, 0.1 M TBAPF6, 0.1 M MeOH, 0.1mM Triad. l >520nm Time (minutes)

45 Light-driven Super-Reductants for CO 2 Reduction The next step: 520 nm e nm e - Use two electron transfer steps that take advantage of both visible and near-infrared photons.

46 Summary 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 CO 2 reduction catalysts.

47 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