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| 2. |
- Sprafke, Johannes K., et al.
(författare)
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Belt-Shaped π-Systems: Relating Geometry to Electronic Structure in a Six-Porphyrin Nanoring
- 2011
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 133:43, s. 17262-17273
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Tidskriftsartikel (refereegranskat)abstract
- Linear pi-conjugated oligomers have been widely investigated, but the behavior of the corresponding cyclic oligomers is poorly understood, despite the recent synthesis of pi-conjugated macrocycles such as [n]cycloparaphenylenes and cyclo[n]thiophenes. Here we present an efficient template-directed synthesis of a pi-conjugated butadiyne-linked cyclic porphyrin hexamer directly from the monomer. Small-angle X-ray scattering data show that this nanoring is shape-persistent in solution, even without its template, whereas the linear porphyrin hexamer is relatively flexible. The crystal structure of the nanoring-template complex shows that most of the strain is localized in the acetylenes; the porphyrin units are slightly curved, but the zinc coordination sphere is undistorted. The electrochemistry, absorption, and fluorescence spectra indicate that the HOMO-LUMO gap of the nanoring is less than that of the linear hexamer and less than that of the corresponding polymer. The nanoring exhibits six one-electron reductions and six one-electron oxidations, most of which are well resolved. Ultrafast fluorescence anisotropy measurements show that absorption of light generates an excited state that is delocalized over the whole pi-system within a time of less than 0.5 ps. The fluorescence spectrum is amazingly structured and red-shifted. A similar, but less dramatic, red-shift has been reported in the fluorescence spectra of cycloparaphenylenes and was attributed to a high exciton binding energy; however the exciton binding energy of the porphyrin nanoring is similar to those of linear oligomers. Quantum-chemical excited state calculations show that the fluorescence spectrum of the nanoring can be fully explained in terms of vibronic Herzberg-Teller (HT) intensity borrowing.
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| 4. |
- Göransson, Erik
(författare)
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Bridge Mediated Electron Transfer in Conjugated and Cross-Conjugated Donor-Acceptor Compounds
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Detailed understanding of electron transfer reactions is important in many aspects of chemistry, biology and solar energy conversion.The main aim of this thesis is to provide further insight into electron transfer through highly conjugated bridge structures. Towards this end, three series of donor-acceptor dyads have been studied, all using an oligo(1,4-phenylene-ethynylene) moiety as the bridge. A common theme in these series is that they explore the effects of having either an ethynylene or phenylene as the attachment group between the bridge and the donor or acceptor. Photophysical characterization of these dyads was carried out by means of time resolved laser spectroscopy. The results show that having an ethynylene as attachment group results in higher rates for bridge mediated electron and energy transfer compared to similar systems, where a phenylene was used.It was also found that most of the investigated systems show a fast back electron transfer. A notable exception is a zinc(II) phthalocyanine- gold(III) porphyrin dyad, where very fast photoinduced electron transfer (kPET = 1.0×1012 s-1) was followed by relatively slow back electron transfer (kBET = 1.0×109 s-1). A complementary DFT investigation indicated that the charge shifted state involves a reduction of the gold ion, rather than the porphyrin ring. This results in lower electronic coupling between the reduced gold porphyrin and the bridge and thus slower back electron transfer.A series of zinc porphyrin platinum acetylide dyads was used to explore the effects on electronic coupling of different attachments points on the porphyrin ring. For the investigated system it was found that linking at the meso-position results in an eight-fold increase in electron transfer rate compared to the β-position.In addition, a series of mixed valence compounds was used to investigate electronic coupling mediated by cross-hyperconjugated or cross-π-conjugated bridges. The results indicate coupling elements of 100-400 cm-1, with the cross-π-conjugated bridge having the largest coupling. A complementary TD-DFT study indicates that both through bond and through space coupling can be active in these systems. The relative contribution of these two mechanisms to the electronic coupling is highly conformer dependent.
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| 5. |
- Hedström, Svante, et al.
(författare)
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Thousandfold Enhancement of Photoreduction Lifetime in Re(bpy)(CO)(3) via Spin-Dependent Electron Transfer from a Perylenediimide Radical Anion Donor
- 2017
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 139:46, s. 16466-16469
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Tidskriftsartikel (refereegranskat)abstract
- Spin-dependent intramolecular electron transfer is revealed in the Re-I(CO)(3)(py)(bpy-Ph)perylenediimide radical anion (Re-I-bpy-PDI-(.)) dyad, a prototype model system for artificial photosynthesis. Quantum chemical calculations and ultrafast transient absorption spectroscopy experiments demonstrate that selective photoexcitation of Re-I-bpy results in electron transfer from PD-(.) to Re-I-bpy, forming two distinct charge-shifted states. One is an overall doublet whose return to the ground state is spin-allowed. The other, high spin quartet state, persists for 67 ns due to spin-forbidden back-electron transfer, constituting a more than thousandfold lifetime improvement compared to the low-spin state. Exploiting this spin dependency holds promise for artificial photosynthetic systems requiring long-lived reduced states to perform multi-electron chemistry.
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| 6. |
- La Porte, Nathan T., et al.
(författare)
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Photoexcited radical anion super-reductants for solar fuels catalysis
- 2018
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Ingår i: Coordination chemistry reviews. - : Elsevier BV. - 0010-8545 .- 1873-3840. ; 361, s. 98-119
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Forskningsöversikt (refereegranskat)abstract
- The catalytic transformation of carbon dioxide into fuels is one of the most important reactions for creating a sustainable, carbon-neutral energy economy. Given that the sun is the only plausible energy source that can accommodate the increased global energy demand without contributing to catastrophic climate change, it makes sense to use solar energy to drive this reaction, ideally using the largest possible portion of the solar spectrum. Over the past several years, we have explored the use of reduced rylenediimide chromophores, which absorb wavelengths ranging into the near-infrared, as strongly reducing photosensitizers capable of photosensitizing Re(diimine)(CO)(3)L metal centers towards the binding and reduction of CO2. We have explored the effects of varying the binding geometry, donor-acceptor redox potentials, and excitation wavelength on the kinetics of electron transfer from the reduced rylenediimide to the metal center. So far, we have achieved charge-separated lifetimes in electrocatalytically active complexes of 25 ns when illuminated with near-infrared light, and >250 ns when illuminated with blue light.
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| 7. |
- Pettersson-Rimgard, Belinda, et al.
(författare)
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Ultrafast Interligand Electron Transfer in cis-[Ru(4,4’-dicarboxylate-2,2’-bipyridine)2(NCS)2]4- and Implications for Electron Injection Limitations in Dye Sensitized Solar Cells
- 2018
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Ingår i: Chemical Science. - : RSC Publishing. - 2041-6520 .- 2041-6539. ; 9:41, s. 7958-7967
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Tidskriftsartikel (refereegranskat)abstract
- Interligand electron transfer (ILET) of the lowest metal-to-ligand charge transfer (MLCT) state of N712 (cis-[Ru(dcb)2(NCS)2]4−, where dcb = 4,4′-dicarboxylate-2,2′-bipyridine) in a deuterated acetonitrile solution has been studied by means of femtosecond transient absorption anisotropy in the mid-IR. Time-independent B3LYP density functional calculations were performed to assign vibrational bands and determine their respective transition dipole moments. The transient absorption spectral band at 1327 cm−1, assigned to a symmetric carboxylate stretch, showed significant anisotropy. A rapid anisotropy increase (τ1 ≈ 2 ps) was tentatively assigned to vibrational and solvent relaxation, considering the excess energy available after the excited singlet–triplet conversion. Thereafter, the anisotropy decayed to zero with a time constant τ2 ≈ 240 ps, which was assigned to the rotational correlation time of the complex in deuterated acetonitrile. No other distinctive changes to the anisotropy were observed and the amplitude of the slow component at time zero agrees well with that predicted for a random mixture of MLCT localization on either of the two dcb ligands. The results therefore suggest that MLCT randomization over the two dcb ligands occurs on the sub-ps time scale. This is much faster than proposed by previous reports on the related N3 complex [Benkö et al., J. Phys. Chem. B, 2004, 108, 2862, and Waterland et al., J. Phys. Chem. A, 2001, 105, 4019], but in agreement with that found by Wallin and co-workers [J. Phys. Chem. A, 2005, 109, 4697] for the [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) complex. This suggests that electron injection from the excited dye into TiO2 in dye-sensitized solar cells is not limited by ILET.
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| 8. |
- Trippodo, Elisa, et al.
(författare)
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Air-stable ternary organic solar cells achieved by using fullerene additives in non-fullerene acceptor-polymer donor blends
- 2023
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Ingår i: Journal of Materials Chemistry C. - : ROYAL SOC CHEMISTRY. - 2050-7526 .- 2050-7534. ; 11:24, s. 8074-8083
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Tidskriftsartikel (refereegranskat)abstract
- Organic solar cells (OSCs) based on donor-acceptor blends have shown a rapid improvement in power conversion efficiency (PCE) now approaching, for small cells, those of the state-of-the art commercial solar modules. However, performance degradation remains one of the most critical impediments for OSC technology commercialization. Ternary solar cells where a third component, for instance an acceptor, is added to a non-fullerene acceptor-polymer donor blend are an effective approach for improving both OSC efficiency and long-term stability. Here, we study the role of two fullerene acceptors, ET18 and PCBM, as the third component in P-D:Y6 blends. These fullerene derivatives significantly enhance the cell stability, which retained > 90% of their initial PCEs (13-14%) even after storage in air for 6 months, compared to only similar to 20% retention for the binary devices. GIWAXS, AFM, in situ impedance spectroscopy and femtosecond transient absorption spectroscopy measurements reveal that the enhanced stability of the ternary devices results from a more robust blend morphology reducing charge recombination in the ternary devices during aging.
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| 9. |
- Wang, Gang, et al.
(författare)
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Photovoltaic Blend Microstructure for High Efficiency Post-Fullerene Solar Cells. To Tilt or Not To Tilt?
- 2019
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 141:34, s. 13410-13420
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Tidskriftsartikel (refereegranskat)abstract
- Achieving efficient polymer solar cells (PSCs) requires a structurally optimal donor-acceptor heterojunction morphology. Here we report the combined experimental and theoretical characterization of a benzodithiophene-benzo-thiadiazole donor polymer series (PBTZF4-R; R = alkyl substituent) blended with the non-fullerene acceptor ITIC-Th and analyze the effects of substituent dimensions on blend morphology, charge transport, carrier dynamics, and PSC metrics. Varying substituent dimensions has a pronounced effect on the blend morphology with a direct link between domain purity, to some extent domain dimensions, and charge generation and collection. The polymer with the smallest alkyl substituent yields the highest PSC power conversion efficiency (PCE, 11%), reflecting relatively small, high-purity domains and possibly benefiting from "matched" donor polymer-small molecule acceptor orientations. The distinctive morphologies arising from the substituents are investigated using molecular dynamics (MD) simulations which reveal that substituent dimensions dictate a well-defined set of polymer conformations, in turn driving chain aggregation and, ultimately, the various film morphologies and mixing with acceptor small molecules. A straightforward energetic parameter explains the experimental polymer domain morphological trends, hence PCE, and suggests strategies for substituent selection to optimize PSC materials morphologies.
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