| 1. |
- Börjesson, Karl, 1982, et al.
(författare)
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Conjugated anthracene dendrimers with monomer-like fluorescence
- 2014
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Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 4:38, s. 19846-19850
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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.
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| 2. |
- Gilbert Gatty, Melina, 1986, et al.
(författare)
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Conformational Gating of Charge Separation in Porphyrin Oligomer-Fullerene Systems
- 2013
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:50, s. 26482-26492
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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.
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| 3. |
- Gilbert Gatty, Mélina, et al.
(författare)
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Direct evidence of catalyst reduction on dye and catalyst co-sensitized NiO photocathodes by mid-infrared transient absorption spectroscopy
- 2018
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Ingår i: Chemical Science. - : ROYAL SOC CHEMISTRY. - 2041-6520 .- 2041-6539. ; 9:22, s. 4983-4991
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Tidskriftsartikel (refereegranskat)abstract
- Co-sensitization of molecular dyes and catalysts on semiconductor surfaces is a promising strategy to build photoelectrodes for solar fuel production. In such a photoelectrode, understanding the charge transfer reactions between the molecular dye, catalyst and semiconductor material is key to guide further improvement of their photocatalytic performance. Herein, femtosecond mid-infrared transient absorption spectroscopy is used, for the first time, to probe charge transfer reactions leading to catalyst reduction on co-sensitized nickel oxide (NiO) photocathodes. The NiO films were co-sensitized with a molecular dye and a proton reducing catalyst from the family of [FeFe](bdt)(CO)(6) (bdt = benzene-1,2-dithiolate) complexes. Two dyes were used: an organic push-pull dye denoted E2 with a triarylamine-oligothiophene-dicyanovinyl structure and a coumarin 343 dye. Upon photo-excitation of the dye, a clear spectroscopic signature of the reduced catalyst is observed a few picoseconds after excitation in all co-sensitized NiO films. However, kinetic analysis of the transient absorption signals of the dye and reduced catalyst reveal important mechanistic differences in the first reduction of the catalyst depending on the co-sensitized molecular dye (E2 or C343). While catalyst reduction is preceded by hole injection in NiO in C343-sensitized NiO films, the singly reduced catalyst is formed by direct electron transfer from the excited dye E2* to the catalyst in E2-sensitized NiO films. This change in mechanism also impacts the lifetime of the reduced catalyst, which is only ca. 50 ps in E2-sensitized NiO films but is >5 ns in C343-sensitized NiO films. Finally, the implication of this mechanistic study for the development of better co-sensitized photocathodes is discussed.
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| 4. |
- Gilbert Gatty, Melina, 1986, et al.
(författare)
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Hopping versus Tunneling Mechanism for Long-Range Electron Transfer in Porphyrin Oligomer Bridged Donor-Acceptor Systems
- 2015
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 119:24, s. 7598-7611
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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 (
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| 5. |
- Gilbert Gatty, Melina, 1986, et al.
(författare)
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Photoinduced charge and energy transfer in molecular wires
- 2015
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Ingår i: Chemical Society Reviews. - : Royal Society of Chemistry (RSC). - 1460-4744 .- 0306-0012. ; 44:4, s. 845-862
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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.
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| 6. |
- Gilbert Gatty, Melina, 1986
(författare)
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Photoinduced Electron and Energy Transfer in Π-Conjugated Systems
- 2015
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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 (
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| 7. |
- Gilbert Gatty, Melina, 1986
(författare)
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Probing the Effects of Conformation on the Photophysics of Conjugated Porphyrin Oligomers
- 2013
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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.
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| 8. |
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| 9. |
- Huang, Jing, et al.
(författare)
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Covalently linking CuInS2 quantum dots with a Re catalyst by click reaction for photocatalytic CO2 reduction
- 2018
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Ingår i: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 47:31, s. 10775-10783
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Tidskriftsartikel (refereegranskat)abstract
- Covalently linking photosensitizers and catalysts in an inorganic-organic hybrid photocatalytic system is beneficial for efficient electron transfer between these components. However, general and straightforward methods to covalently attach molecular catalysts on the surface of inorganic semiconductors are rare. In this work, a classic rhenium bipyridine complex (Re catalyst) has been successfully covalently linked to the low toxicity CuInS2 quantum dots (QDs) by click reaction for photocatalytic CO2 reduction. Covalent bonding between the CuInS2 QDs and the Re catalyst in the QD-Re hybrid system is confirmed by UV-visible absorption spectroscopy, Fourier-transform infrared spectroscopy and energy-dispersive X-ray measurements. Time-correlated single photon counting and ultrafast time-resolved infrared spectroscopy provide evidence for rapid photo-induced electron transfer from the QDs to the Re catalyst. Upon photo-excitation of the QDs, the singly reduced Re catalyst is formed within 300 fs. Notably, the amount of reduced Re in the linked hybrid system is more than that in a sample where the QDs and the Re catalyst are simply mixed, suggesting that the covalent linkage between the CuInS2 QDs and the Re catalyst indeed facilitates electron transfer from the QDs to the Re catalyst. Such an ultrafast electron transfer in the covalently linked CuInS2 QD-Re hybrid system leads to enhanced photocatalytic activity for CO2 reduction, as compared to the conventional mixture of the QDs and the Re catalyst.
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| 10. |
- Kärnbratt, Joakim, 1982, et al.
(författare)
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Self-Assembly of Linear Porphyrin Oligomers into Well-Defined Aggregates
- 2012
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:37, s. 19630-19635
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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.
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