| 1. |
- Lindberg, David, 1986, et al.
(författare)
-
Steady-state and time-resolved Thioflavin-T fluorescence can report on morphological differences in amyloid fibrils formed by A beta(1-40) and A beta(1-42)
- 2015
-
Ingår i: Biochemical and Biophysical Research Communications. - : Elsevier BV. - 1090-2104 .- 0006-291X. ; 458:2, s. 418-423
-
Tidskriftsartikel (refereegranskat)abstract
- Thioflavin-T (ThT) is one of the most commonly used dyes for amyloid detection, but the origin of its fluorescence enhancement is not fully understood. Herein we have characterised the ThT fluorescence response upon binding to the A beta(1-40) and A beta(1-42) variants of the Alzheimer's-related peptide amyloid-beta, in order to explore how the photophysical properties of this dye relates to structural and morphological properties of two amyloid fibril types formed by peptides with a high degree of sequence homology. We show that the steady-state ThT fluorescence is 1.7 times more intense with A beta(1-40) compared to A beta(1-42) fibrils in concentration matched samples prepared under quiescent conditions. By measuring the excited state lifetime of bound ThT, we also demonstrate a distinct difference between the two fibril isoforms, with A beta(1-42) fibrils producing a longer ThT fluorescence lifetime compared to A beta(140). The substantial steady-state intensity difference is therefore not explained by differences in fluorescence quantum yield. Further, we find that the ThT fluorescence intensity, but not the fluorescence lifetime, is dependent on the fibril preparation method (quiescent versus agitated conditions). We therefore propose that the fluorescence lifetime is inherent to each isoform and sensitively reports on fibril microstructure in the protofilament whereas the total fluorescence intensity relates to the amount of exposed beta-sheet in the mature A beta fibrils and hence to differences in their morphology. Our results highlight the complexity of ThT fluorescence, and demonstrate its extended use in amyloid fibril characterisation.
|
|
| 2. |
- Börjesson, Karl, 1982, et al.
(författare)
-
Conjugated anthracene dendrimers with monomer-like fluorescence
- 2014
-
Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 4:38, s. 19846-19850
-
Tidskriftsartikel (refereegranskat)abstract
- Two generations of highly emissive conjugated anthracene dendrimers containing up to 9 anthracene units are presented. In these dendrimers, anthracene-like absorption and emission properties are preserved due to the relatively weak electronic coupling between the anthracene units, while evidence of fast crosstalk within the molecular framework is still observed.
|
|
| 3. |
- Gilbert Gatty, Melina, 1986, et al.
(författare)
-
Conformational Gating of Charge Separation in Porphyrin Oligomer-Fullerene Systems
- 2013
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:50, s. 26482-26492
-
Tidskriftsartikel (refereegranskat)abstract
- The rate of the photoinduced charge-separation in C-60-terminated butadiyne-linked porphyrin oligomers P-n (n = 4, 6) is strongly influenced by their molecular conformation. In these systems, the presence of the butadiyne linkers gives rise to a broad distribution of conformations in the ground state, due to an almost barrierless rotation of individual porphyrin units in the oligomer chain. The conformational states of these oligomers, either twisted or planar, could be selected by varying the excitation wavelength, thereby providing different initial excited states for charge separation. Charge separation in the different conformers was followed using both steady-state and 2D time-resolved emission using a streak camera system. Singular value decomposition (SVD) analysis applied on streak camera data provides here a powerful tool to study the conformational dependence of the charge separation in long PnC60 systems. Both the kinetics and spectral changes accompanying charge separation could be analyzed for different populations of conformation. From this analysis we show that, for both systems studied, twisted conformations undergo faster charge separation than planar conformations. This disparity in charge separation rates was ascribed mainly to the difference in driving force for charge separation between twisted and planar conformations. Charge separation was also studied in oligomers PnC60 coordinated to an octadentate ligand T8 that hinders the rotation of porphyrin subunits. The semicircular complexes PnC60-T8 show dramatic changes in their spectral properties, as well as slow excitation wavelength independent rate of charge separation and corresponding low efficiency compared to their linear counterparts. This slow charge separation rate was attributed to fast relaxation to the lowest excited vibronic state and lack of driving force for charge separation in these close to planar semicircular systems; i.e., the template systems behave like "normal" donor acceptor systems without slow conformational relaxation. This work illustrates how control of conformation can be used to tune the rate of charge separation.
|
|
| 4. |
- Gilbert Gatty, Melina, 1986, et al.
(författare)
-
Hopping versus Tunneling Mechanism for Long-Range Electron Transfer in Porphyrin Oligomer Bridged Donor-Acceptor Systems
- 2015
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 119:24, s. 7598-7611
-
Tidskriftsartikel (refereegranskat)abstract
- Achieving long-range charge transport in molecular systems is interesting to foresee applications of molecules in practical devices. However, designing molecular systems with pre-defined wire-like properties remains difficult due to the lack of understanding of the mechanism for charge transfer. Here we investigate a series of porphyrin oligomer-bridged donor–acceptor systems Fc–Pn–C60 (n = 1–4, 6). In these triads, excitation of the porphyrin-based bridge generates the fully charge-separated state, Fc•+–Pn–C60•-, through a sequence of electron transfer steps. Temperature dependence of both charge separation (Fc–Pn*–C60 → Fc–Pn•+–C60•-) and recombination (Fc•+–Pn–C60•– → Fc–Pn–C60) processes was probed by time-resolved fluorescence and femtosecond transient absorption. In the long triads, two mechanisms contribute to recombination of Fc•+–Pn–C60•– to the ground state. At high temperatures (≥280 K), recombination via tunneling dominates for the entire series. At low temperatures (
|
|
| 5. |
- Gilbert Gatty, Melina, 1986, et al.
(författare)
-
Photoinduced charge and energy transfer in molecular wires
- 2015
-
Ingår i: Chemical Society Reviews. - : Royal Society of Chemistry (RSC). - 1460-4744 .- 0306-0012. ; 44:4, s. 845-862
-
Forskningsöversikt (refereegranskat)abstract
- Exploring charge and energy transport in donor-bridge-acceptor systems is an important research field which is essential for the fundamental knowledge necessary to develop future applications. These studies help creating valuable knowledge to respond to today's challenges to develop functionalized molecular systems for artificial photosynthesis, photovoltaics or molecular scale electronics. This tutorial review focuses on photo-induced charge/energy transfer in covalently linked donor-bridge-acceptor (D-B-A) systems. Of utmost importance in such systems is to understand how to control signal transmission, i.e. how fast electrons or excitation energy could be transferred between the donor and acceptor and the role played by the bridge (the "molecular wire"). After a brief description of the electron and energy transfer theory, we aim to give a simple yet accurate picture of the complex role played by the bridge to sustain donor-acceptor electronic communication. Special emphasis is put on understanding bridge energetics and conformational dynamics effects on the distance dependence of the donor-acceptor electronic coupling and transfer rates. Several examples of donor-bridge-acceptor systems from the literature are described as a support to the discussion. Finally, porphyrin-based molecular wires are introduced, and the relationship between their electronic structure and photophysical properties is outlined. In strongly conjugated porphyrin systems, limitations of the existing electron transfer theory to interpret the distance dependence of the transfer rates are also discussed.
|
|
| 6. |
- Gilbert Gatty, Melina, 1986
(författare)
-
Photoinduced Electron and Energy Transfer in Π-Conjugated Systems
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Photosynthesis is a fascinating process that provides food and oxygen. In this thesis, focus is on understanding and controlling the early processes of photosynthesis, namely photoinduced energy and electron transfer. Indeed, mimicking these processes in synthetic systems could open to a wide range of applications going from renewable energy production to molecular devices for information exchange. In this work, covalently linked π‒conjugated systems provide a means to get a better understanding of the mechanisms and factors that govern transfer of charge and energy in molecular systems.Photoinduced electron transfer (ET) is investigated in series of donor‒acceptor dyads Pn‒C60 and triads Fc‒Pn‒C60 employing butadiyne‒linked zinc porphyrin oligomers denoted Pn (n = 1‒8) as photoactive electron donor and π‒conjugated bridging structures (wires). In the triads, temperature dependence study of the recombination of the long‒range charge separated state Fc•+‒Pn‒C60•- provides new insights into the mechanistic nature of the charge transport linked to its wire‒like behavior. At high temperatures (> 280 K), the weakly distance‒dependent charge transport arises from coherent electron tunneling for the entire series. At low temperatures, crossover to incoherent hole hopping accompanied by a stronger distance dependence of the charge transport is observed in long Fc‒Pn‒C60 (n = 2‒4). Being able to tune the ET rate in donor‒acceptor systems is another property which is highly desirable for applications of molecules in actual devices. Here, in the long Pn‒C60 (n = 4, 6) dyads, we demonstrate the possibility of varying the rate of electron transfer between the photoexcited porphyrin oligomer (1Pn*) and the fullerene (C60) by either optical or chemical control of the conformation of the porphyrin chain. Additionally, we show that temperature could potentially be used to control the conformation of the porphyrin oligomer. Below 170 K, all studied systems Pn, Pn‒C60 and Fc‒Pn‒C60 spontaneously form highly‒ordered planar aggregates.The thesis also discusses photoinduced excitation energy transfer (EET) in the Pn systems and two anthracene dendrimers. In the latter, despite weak through‒bond electronic coupling, signs of an ultrafast EET between the anthracene dendrons are observed by time‒resolved fluorescence anisotropy. For the Pn systems, pump intensity‒dependent transient absorption measurements reveal at early times (
|
|
| 7. |
- Gilbert Gatty, Melina, 1986
(författare)
-
Probing the Effects of Conformation on the Photophysics of Conjugated Porphyrin Oligomers
- 2013
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Controlling the conformational heterogeneity of molecular systems is fundamental to envisage future applications of these systems, such as in solar cells or molecular devices to name a few. The work presented in this Thesis investigates the effects of conformational flexibility on the photophysical properties of a series of conjugated porphyrin oligomers Pn (n = 1-4, 6, 8). First the influence of temperature on the conformational distribution displayed by these systems has been explored. At low temperature, these porphyrins spontaneously form highly-ordered planar aggregates resulting in characteristic changes in their spectral properties. The observed spectral changes could be explained by a planarization of the oligomers accompanying the aggregate formation. Further the influence of conformation on the electron transfer reactions in long fullerene-appended porphyrin oligomers PnC60 (n = 4, 6) was studied thoroughly. The rate of charge separation could be tuned by selective excitation of different populations of conformations. Twisted conformations showed faster charge separation rates and higher efficiencies than planar conformations. The disparity in charge separation rates was ascribed to the differences in driving force for charge separation between twisted and planar conformations. The conformational distribution of PnC60 systems could also be restrained and controlled by means of coordination to an octadentate ligand T8, thus providing simpler model systems to investigate the influence of conformation on the charge separation. The semi-circular complexes PnC60-T8 showed significant differences in their spectral properties compared to their linear counterparts that were attributed to their planarity and higher rigidity. Charge separation was also investigated in these semi-circular complexes. These complexes displayed an extremely slow rate of charge separation and low efficiency of charge separation. The reason for this slow charge separation was the lack of driving force for charge separation to occur, as expected for more planar conformations.
|
|
| 8. |
- Kärnbratt, Joakim, 1982, et al.
(författare)
-
Self-Assembly of Linear Porphyrin Oligomers into Well-Defined Aggregates
- 2012
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:37, s. 19630-19635
-
Tidskriftsartikel (refereegranskat)abstract
- Conjugated zinc porphyrin oligomers of various lengths are shown to form well-defined planar aggregates at low temperatures. The aggregation occurs over a narrow temperature interval (170-150 K) and is accompanied by dramatic changes in the electronic absorption and emission spectra. Similar changes are found in J-aggregates in which the transition dipole moments of aggregated chromophores couple to form a new and intense transition in the absorption spectrum, red shifted from the monomeric chromophore band. For the present porphyrin oligomers, the dramatic absorption changes are not associated with the formation of large aggregates, but rather with the dimerization accompanied by planarization of the oligomers. Free oligomers have a broad distribution of porphyrin porphyrin dihedral angles and show a broad and unstructured absorption spectrum. As the oligomers stack to form aggregates, they planarize and the width of the conformational distribution is reduced to include virtually only the planar conformers, resulting in the observed change of the absorption spectrum. No experimental evidence for the formation of large aggregates was found, while a small aggregate, probably only dimer, is supported by the minor changes of the fluorescence rate constant upon aggregation and the fact that pyridine has no significant effect on the formation of this aggregate, which otherwise is very effective for inhibiting aggregation of zinc porphyrin oligomers. Compared to most porphyrin aggregates, which show broad absorption spectra and quenched fluorescence, these aggregates give sharp absorption and emission spectra with little change in the fluorescence quantum yield. Similar aggregates were also observed for oligomers substituted with both a fullerene electron acceptor and a ferrocene donor. The results presented here will be potentially useful as tools to understand how electron transfer and delocalization processes are influenced by molecular order/disorder transitions.
|
|
| 9. |
- Mann, J. K., et al.
(författare)
-
Opportunities in nanometer sized Si wires for PV applications
- 2013
-
Ingår i: Progress in Materials Science. - : Elsevier BV. - 0079-6425. ; 58:8, s. 1361-1387
-
Forskningsöversikt (refereegranskat)abstract
- Quantum-confined silicon material has been a very active field of research in the years 1990-2000 with the rapid development of opto-electronics. The main application targeted by this research was a light-emitting device (either LED, or laser). In the years 2000-2010, with the emerging need for efficient and cheap photovoltaic devices, new materials, and in particular new silicon-based materials trigger again a special interest. In particular, all-crystalline-Si tandem solar cells where the high-bandgap material is provided by the 2D confinement of excitons in nm-sized nanowires could provide the high-efficiency potential of a tandem device, while taking benefit of the decade-long buildup of knowhow of crystalline silicon material technology (both science and processing). In a first part of this review, we summarize the features described in the relevant literature for the functioning of a photovoltaic device based on Si NWs. This literature shows that from the conceptual point of view such an all-crystalline-Si-tandem solar cell using quantum confined nanowires should be feasible to produce in order to achieve the goal of inexpensive high efficiency (>30%) Si-based solar cells. Keeping the fabrication of efficient photovoltaic devices as driving theme, we review the dense literature of Si nanowires. The literature on the fabrication of nanometer-sized Si nanowires is reviewed in the second part.
|
|
