| 1. |
- Wiberg, Joanna, 1980, et al.
(författare)
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Charge recombination versus charge separation in donor-bridge-acceptor systems
- 2007
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 129:1, s. 155-163
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Tidskriftsartikel (refereegranskat)abstract
- Optimizing the ratio of the rates for charge separation (CS) over charge recombination (CR) is crucial to create long-lived charge-separated states. Mastering the factors that govern the electron transfer (ET) rates is essential when trying to achieve molecular-scale electronics, artificial photosynthesis, and also for the further development of solar cells. Much work has been put into the question of how the donor-acceptor distances and donor-bridge energy gaps affect the electronic coupling, V DA , and thus the rates of ET. We present here a unique comparison on how these factors differently influence the rates for CS and CR in a porphyrin-based donor-bridge-acceptor model system. Our system contains three series, each of which focuses on a separate charge-transfer rate-determining factor, the donor-acceptor distance, the donor-bridge energy gap, and last, the influence of the electron acceptor on the rate for charge transfer. In these three series both CS and CR are governed by superexchange interactions which make a CR/CS comparative study ideal. We show here that the exponential distance dependence increases slightly for CR compared to that for CS as a result of the increased tunneling barrier height for this reaction, in accordance with the McConnell superexchange model. We also show that the dependence on the tunneling barrier height is different for CS and CR. This difference is highly dependent on the electron acceptor and thus cannot solely be explained by the differences in the frontier orbitals of the electron donor in these porphyrin systems. © 2007 American Chemical Society.
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| 2. |
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| 3. |
- Albinsson, Bo, 1963, et al.
(författare)
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Excitation energy transfer in donor-bridge-acceptor systems
- 2010
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 12:27, s. 7338-7351
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Tidskriftsartikel (refereegranskat)abstract
- This perspective will focus on the mechanistic aspects of singlet and triplet excitation energy transfer. Well defined donor-bridge-acceptor systems specifically designed for investigating the distance and energy gap dependencies of the energy transfer reactions are discussed along with some recent developments in computational modeling of the electronic coupling.
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| 4. |
- Albinsson, Bo, 1963, et al.
(författare)
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Long-range electron and excitation energy transfer in donor-bridge-acceptor systems
- 2008
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Ingår i: Journal of Photochemistry and Photobiology C: Photochemistry Reviews. - : Elsevier BV. - 1389-5567. ; 9:3, s. 138-155
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Tidskriftsartikel (refereegranskat)abstract
- Donor-bridge-acceptor (D-B-A) systems, either as supermolecules or on surfaces, have been extensively studied with respect to long-range electron (ET) and excitation energy (EET) transfer. In more recent years, the main research objective has been to develop knowledge on how to construct molecular-based devices, with predetermined electron transfer properties, intended for application in electronics and photovoltaics. At present, such construction is in general hampered for several reasons. Most importantly, the property of a D-B-A system is not a simple linear combination of properties of the individual components, but depends on the specific building blocks and how they are assembled. An important example is the ability of the bridge to support the intended transfer process. The mediation of the transfer is characterized by an attenuation factor, beta, often viewed as a bridge specific constant but which also depends on the donor and the acceptor, i.e. the same bridge can either be poorly or strongly conducting depending oil the donor and acceptor. This review gives an account of the experimental exploration of the attenuation factor beta in a series of bis(porphyrin) systems covalently linked by bridges of the oligo(phenyleneethynylene) (OPE) type. Attenuation factors for ET as well as for both singlet and triplet EET are discussed. A report is also given on the dependence of the transfer efficiency on the energy-gap between the donor and bridge states relevant for the specific transfer process. The experimental variation of beta with varying donor and acceptor components is shown fora range of conjugated bridges by representative examples from the literature. The theoretical rationalization for the observed variation is briefly discussed. Based on the Gamow tunneling model, the observed variations in beta-values with varying donors and acceptors for the same bridges is simulated Successfully simultaneously as the observed energy-gap dependence is modelled. (C) 2008 Elsevier B.V. All rights reserved.
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| 5. |
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| 6. |
- Amombo Noa, Francoise Mystere, 1988, et al.
(författare)
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Metal-Organic Frameworks with Hexakis(4-carboxyphenyl)benzene: Extensions to Reticular Chemistry and Introducing Foldable Nets
- 2020
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 142:20, s. 9471-9481
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Tidskriftsartikel (refereegranskat)abstract
- Nine metal-organic frameworks have been prepared with the hexagon-shaped linker 1,2,3,4,5,6-hexakis(4-carboxyphenyl)-benzene (H(6)cpb) by solvothermal reactions in dimethylformamide (dmf) or dimethylacetamide (dmac) with acetic acid or formic acid as modulators: [Bi-2(cpb)(acetato)(2)(dmf)(2)]center dot 2dmf CTH-6 forms a rtl-net; 2(H2NMe2)[Cu-2(cpb)] CTH-7 forms a kgd-net; [Fe-4(cpb)-(acetato)(2)(dmf)(4)] CTH-8 and [Co-4(cpb)(acetato)(2)(dmf)(4)] CTH-9 are isostructural and form yav-nets; 2(HNEt3)[Fe-2(cpb)] CTH-10 and the two polymorphs of 2(H2NMe2)[Zn-2(cpb)]center dot 1.5dmac, Zn-MOF-888 and CTH-11, show kgd-nets; [Cu-2(cpb)-(acetato)(2)(dmf)(2)]center dot 2dmf, CTH-12, forms a mixed coordination and hydrogen-bonded sql-net; and 2(H2NMe2)[Zn-2(cpb)] CTH-13, a similarly mixed yav-net. Surface area values (Brunauer-Emmett-Teller, BET) range from 34 m(2) g(-1) for CTH-12 to 303 m(2) g(-1) for CTH-9 for samples activated at 120 degrees C in dynamic vacuum. All compounds show normal (10-fold higher) molar CO2 versus N-2 uptake at 298 K, except the 19-fold CO2 uptake for CTH-12 containing Cu(II) dinuclear paddle-wheels. We also show how perfect hexagons and triangles can combine to a new 3D topology laf, a model of which gave us the idea of foldable network topologies, as the laf-net can fold into a 2D form while retaining the local geometry around each vertex. Other foldable nets identified are cds, cds-a, ths, sqc163, clh, jem, and tfc covering the basic polygons and their combinations. The impact of this concept on "breathing" MOFs is discussed. I-2 sorption, both from gas phase and from MeOH solution, into CTH-7 were studied by time of flight secondary ion mass spectrometry (ToF-SIMS) on dried crystals. I-2 was shown to have penetrated the crystals, as layers were consecutively peeled off by the ion beam. We suggest ToF-SIMS to be a method for studying sorption depth profiles of MOFs.
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| 7. |
- Andreasson, Joakim, 1973, et al.
(författare)
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Efficient non-radiative deactivation and conformational flexibility of meso-diaryloctaalkylporphyrins in the excited triplet state
- 2000
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Ingår i: Journal of Physical Chemistry A. - 1089-5639 .- 1520-5215. ; 104:41, s. 9307-9314
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Tidskriftsartikel (refereegranskat)abstract
- The excited triplet state deactivation of zinc(II) meso-diaryloctaalkylporphyrins (ZnDAOAP) has been studied over a wide temperature range using transient triplet-triplet absorption spectroscopy together with steady-state and time-resolved phosphorescence techniques, The results from transient absorption measurements show that the depopulation of the initially formed triplet state (T-1A state) is unusually fast at temperatures above 150 K. The efficiency of the deactivation originates from a spin allowed transition to a second tripler state (T-1B state), The transformation process T-1A-->T-1B is therefore the dominating deactivation channel of the T-1A state in this temperature range, and direct intersystem crossing T-1A-->S-0 makes negligible contribution. The subsequent ground-state recovery T-1B-->S-0 is also very efficient in comparison to many other porphyrins. Due to the substantial activation energy found for the transformation process, it most likely involves a conformational distortion of the porphyrin macrocycle. At low temperature, however, the relaxation of the T-1A State occurs by direct intersystem crossing to the ground state.
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| 8. |
- Andreasson, Joakim, 1973, et al.
(författare)
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Photoinduced hole transfer from the triplet state in a porphyrin-based donor-bridge-acceptor system
- 2003
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 107:42, s. 8825-8833
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Tidskriftsartikel (refereegranskat)abstract
- The triplet excited-state deactivation of a gold porphyrin (AuP) in porphyrin-based donor-bridge-acceptor (D-B-A) systems has been studied. The results from room temperature and 80 K measurements are presented. The primary objectives have been to investigate whether electrons/electron holes or excitation energy could be transferred from (AuP)-Au-3 to the appended zinc porphyrin (ZnP) in the dimers. As the bridging chromophores in our D-B-A systems separate the ZnP and AuP moieties by 19 A edge-to-edge, we do not expect a significant contribution to either electron or energy transfer from a direct (through space) exchange mechanism. This gives us the opportunity to scrutinize how the bridging chromophores influence the transfer reactions. The results show that quenching of (AuP)-Au-3 occurs with high efficiency in the dimers that are connected by fully conjugated bridging chromophores, whereas no quenching is observed when the conjugation of the bridge is broken. We also observed that the decay of (AuP)-Au-3 is complex at temperatures below 110 K. In addition to the two previously published lifetimes on the order of some 10-100 mus, we have found a third lifetime on the nanosecond time scale.
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| 9. |
- Andreasson, Joakim, 1973, et al.
(författare)
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Temperature and viscosity dependence of the triplet energy transfer process in porphyrin dimers
- 2002
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Ingår i: Photochemical & Photobiological Sciences. - : Springer Science and Business Media LLC. - 1474-905X .- 1474-9092. ; 1:2, s. 111-119
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Tidskriftsartikel (refereegranskat)abstract
- The temperature and viscosity dependence of the triplet energy transfer (TET) process in porphyrin dimers has been studied. A zinc porphyrin (donor) and a free base porphyrin (acceptor) are covalently linked together by rigid bridging chromophores at a center-center distance of 25 Angstrom. Due to the large donor-acceptor distance and the weakness of the spin forbidden transitions involved, neither direct (through space) electron exchange nor Coulombic mechanisms are expected to contribute to the observed TET process. The results from transient absorption measurements at temperatures between room temperature and 80 K show that TET occurs with unexpectedly high efficiency in the systems connected by fully conjugated bridges and a pronounced temperature dependence of the process is observed. Comparison of the TET efficiencies in dimers connected by different bridging chromophores correlates well with a transfer reaction governed by a through bond exchange (superexchange) interaction. However, in high viscosity media the TET process is dramatically slowed down. This is attributed to a conformational gating of the TET process where the electronic coupling varies strongly with the relative orientation of the donor and the bridging chromophore. Further, the zinc porphyrin donor offers two distinct donor species, T-1A and T-1B. At room temperature, the TET rate constant of the T-1A Species is about two orders of magnitude larger than for the T-1B species. The dimers studied are well suited model systems for materials where the rate of the transfer reactions can be changed by external stimuli.
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| 10. |
- Andreasson, Joakim, 1973, et al.
(författare)
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The gold porphyrin first excited singlet state
- 2002
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Ingår i: Photochemistry and Photobiology. - 0031-8655 .- 1751-1097. ; 76:1, s. 47-50
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Tidskriftsartikel (refereegranskat)abstract
- Gold porphyrins are often used as electron-accepting chromophores in artificial photosynthetic constructs. Because of the heavy atom effect, the gold porphyrin first-excited singlet state undergoes rapid intersystem crossing to form the triplet state. The lowest triplet state can undergo a reduction by electron donation from a nearby porphyrin or another moiety. In addition, it can be involved in triplet-triplet energy transfer interactions with other chromophores. In contrast, little has been known about the short-lived singlet excited state. In this work, ultrafast time-resolved absorption spectroscopy has been used to investigate the singlet excited state of Au(III) 5,15-bis(3,5-di-t-butylphenyl)-2,8,12,18,-tetraethyl-3,7,13,17-tetrameth ylporphyrin in ethanol solution. The excited singlet state is found to form with the laser pulse and decay with a time constant of 240 fs to give the triplet state. The triplet returns to the ground state with a lifetime of 400 ps. The lifetime of the singlet state is comparable with the time constants for energy and photoinduced electron transfer in some model and natural photosynthetic systems. Thus, it is kinetically competent to take part in such processes in suitably designed supermolecular systems.
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