| 1. |
- Dahlstrand, Christian, et al.
(författare)
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Exploration of the π-Electronic Structure of Singlet, Triplet, and Quintet States of Fulvenes and Fulvalenes Using the Electron Localization Function
- 2012
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 116:20, s. 5008-5017
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Tidskriftsartikel (refereegranskat)abstract
- The singlet ground states and lowest triplet states of penta- and heptafulvene, their benzannulated derivatives, as well as the lowest quintet states of pentaheptafulvalenes, either the parent compound or compounds in which the two rings are intercepted by either an alkynyl or a phenyl segment, were investigated at the (U)OLYP/6-311G(d,p) density functional theory level. The influence of (anti)-aromaticity was analyzed by the structure-based aromaticity index HOMA, the harmonic oscillator model of aromaticity. The extent of (anti)aromatic character was also evaluated in terms of the pi-electron (de)localization as measured by the pi component of the electron localization function (ELF pi). The natural atomic orbital (NAO) occupancies were calculated in order to evaluate the degree of pi-electron shift caused by the opposing electron-counting rules for aromaticity in the electronic ground state (S-0; Hiickel's rule) and the first pi pi* excited triplet state (T-1; Baird's rule). Pentaheptafulvalene (5) shows a shift of 0.5 pi electrons from the 5-ring to the 7-ring when going from the S-0 state to the lowest quintet state (Qu(1)). The pentaheptafulvalene 5 and [5.6.7]quinarene 7 were also investigated in their 90 degrees twisted conformations. From our study it is apparent that excitation localization in fulvalenes, but not in fulvenes, to a substantial degree is determined by aromaticity localization to triplet biradical 4n pi-electron cycles. Isolated benzene rings in these compounds tend to remain as closed-shell 6 pi-electron cycles.
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| 2. |
- Dahlstrand, Christian, et al.
(författare)
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Substituent Effects on the Electron Affinities and Ionization Energies of Tria-, Penta-, and Heptafulvenes : A Computational Investigation
- 2010
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Ingår i: Journal of Organic Chemistry. - : American Chemical Society (ACS). - 0022-3263 .- 1520-6904. ; 75:23, s. 8060-8068
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Tidskriftsartikel (refereegranskat)abstract
- The extent of substituent influence on the vertical electron affinities (EAs) and ionization energies (IEs) of 43 substituted tria-, penta-, and heptafulvenes was examined computationally at the OVGF/6-311G(d)//B3LYP/6-311G(d) level of theory and compared with those of tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (TTF) as representing strong electron-acceptor and -donor compounds, respectively. The substituents X at the exocyclic positions of the fulvenes were either NH2, H, or CN, while the substituents Y at the ring positions were H, CI, F, CN, or NH2. The variations of the EAs and lEs were rationalized by qualitative arguments based on frontier orbital symmetries for the different fulvene classes with either X or Y being constant. The minimum and maximum values found for the calculated EAs of the tria-, penta-, and heptafulvenes were 0.51-2.05, 0.24-3.63, and 0.53-3.14 eV, respectively, and for the IEs 5.27-9.96, 7.07-10.31, and 6.35-10.59 eV, respectively. Two of the investigated fulvenes outperform TCNQ (calcd EA = 2.63 eV) and one outperforms TTF (calcd IE = 6.25 eV) with regard to acceptor and donor abilities, respectively. We also evaluated the properties of bis(fulvene)s, i.e., compounds composed of a donor-type heptafulvene fused with an acceptor-type pentafulvene, and it was revealed that these bis(fulvene)s can be designed so that the IE and EA of the two separate fulvene segments are retained, potentially allowing for the design of compact donor-acceptor dyads.
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| 3. |
- Kilså-Jensen, Kristine, 1972, et al.
(författare)
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Interlayer energy transfer between carbazole and two 9-anthroyloxy derivatives in Langmuir-Blodgett films
- 1999
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Ingår i: Journal of Physical Chemistry B Materials. - 1089-5647. ; 103:40, s. 8514-8523
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Tidskriftsartikel (refereegranskat)abstract
- The interlayer excitation energy transfer between 11-(9-carbazole)undecanoic acid (II-CU) and two 9-anthroyloxy derivatives, 9-(9-anthroyloxy)stearic acid (9-AS) and 2-(9-anthroyloxy)stearic acid (2-AS), in alternating multilayer Langmuir-Blodgett films has been studied. The 11-CU fluorescence is quenched by energy transfer to 9-AS or 2-AS as judged by steady-state and picosecond time-resolved fluorescence measurements. The fluorescence decay curves of 11-CU in the films were analyzed in the framework of several models: (1) a general model for interlayer energy transfer, (2) a two-exponential decay, (3) a Forster model for energy transfer in a two-dimensional system, and (4) a stretched-exponential decay, characteristic of Forster energy transfer in self-similar fractal-like structures. The recovered decay parameters suggest an inhomogeneous mixing of the acceptor molecules in LB films leading to a two-phase system. The phase separation during compression of the acceptor monolayers forms regions of acceptor concentration about 3 times that of the intended and regions with very low acceptor concentration.
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| 4. |
- Kilså-Jensen, Kristine, 1972, et al.
(författare)
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Kinetics for hybridization of peptide nucleic acids (PNA) with DNA and RNA studied with the BIAcore technique
- 1997
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 1520-4995 .- 0006-2960. ; 36:16, s. 5072-5077
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Tidskriftsartikel (refereegranskat)abstract
- The binding of a mixed-sequence pentadecamer PNA (peptide nucleic acid) containing all four nucleobases to the fully complementary as well as various singly mismatched RNA and DNA oligonucleotides has been systematically investigated using thermal denaturation and BIAcore surface-interaction techniques. The rate constants for association (k(a)) and dissociation (k(d)) of the duplex formation as well as the thermal stability (melting temperature, T-m) of the duplexes have been determined. Upon binding to PNA tethered via a biotin-linker to streptavidin at the dextran/gold surface, DNA and RNA sequences containing single mismatches at various positions in the center resulted in increased dissociation and decreased association rate constants. T-m values for PNA.RNA duplexes are on average 4 degrees C higher than for PNA.DNA duplexes and follow quantitatively the same variation with mismatches as do the PNA.RNA duplexes. Also a faster k(a) and a slower k(d) are found for PNA.RNA duplexes compared to the PNA.DNA duplexes. An overall fair correlation between T-m, k(a), and k(d) is found for a series of PNA.DNA and PNA.RNA duplexes although the determination of k(a) seemed to be prone to artifacts of the method and was not considered capable of providing absolute values representing the association rate constant in bulk solution.
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| 5. |
- Kilså-Jensen, Kristine, 1972, et al.
(författare)
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Mediated energy transfer in covalently linked porphyrin dimers
- 1997
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Ingår i: Journal of Physical Chemistry A. - 1089-5639 .- 1520-5215. ; 101:12, s. 2218-2220
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Tidskriftsartikel (refereegranskat)abstract
- Donor-acceptor systems where the rate of radiationless electronic energy transfer is influenced by the electronic structure of the intervening medium are studied. In this paper we report the observation of mediated energy transfer in a series of geometrically well-defined porphyrin dimers. The dimers consist of a free base porphyrin (5,15-diphenyl-alpha,beta-octaalkylporphyrin, H2P) and the corresponding zinc porphyrin (ZnP) as acceptor and donor, respectively, connected by aryl chromophores with varying energies of the lowest excited states.
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| 6. |
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| 7. |
- Kilså, Kristine, 1972, et al.
(författare)
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Bridge-dependent electron transfer in porphyrin-based donor-bridge-acceptor systems
- 2001
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 123:13, s. 3069-3080
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Tidskriftsartikel (refereegranskat)abstract
- Photoinduced electron transfer in donor-bridge-acceptor systems with zinc porphyrin (or its pyridine complex) as the donor and gold(III) porphyrin as the acceptor has been studied. The porphyrin moieties were covalently linked with geometrically similar bridging chromophores which vary only in electronic structure. Three of the bridges are fully conjugated pi -systems and in a fourth, the conjugation is broken. For systems with this bridge, the quenching rate of the singlet excited state of the donor was independent of solvent and corresponded to the rate of singlet energy transfer expected for a Forster mechanism. In contrast, systems with a rr-conjugated bridging chromophore show a solvent-dependent quenching rate that suggests electron transfer in the Marcus normal region. This is supported by picosecond transient absorption measurements, which showed formation of the zinc porphyrin radical cation only in systems with pi -conjugated bridging chromophores. On the basis of the Marcus and Rehm-Weller equations, an electronic coupling of 5-20 cm(-1) between the donor and acceptor is estimated for these systems. The largest coupling is found for the systems with the smallest energy gap between the donor and bridge singlet excited states. This is in good agreement with the coupling calculated with quantum mechanical methods, as is the prediction of an almost zero coupling in the systems with a nonconjugated bridging chromophore.
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| 8. |
- Kilså, Kristine, 1972, et al.
(författare)
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Control of electron transfer in supramolecular systems
- 2001
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Ingår i: Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy. - 1386-1425. ; 57:11, s. 2213-2227
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Tidskriftsartikel (refereegranskat)abstract
- The fluorescence quantum yield of zinc porphyrin (ZnP) covalently linked to 9,10-bis(phenylethynyl)anthracene (AB) is strongly dependent upon the solvent properties. The bichromophoric system ZnP-AB exhibits 'normal' zinc porphyrin fluorescence in solvents that cannot coordinate to the central zinc atom. In contrast, if a Lewis base, such as pyridine, is added to a sufficiently polar solvent, the fluorescence is significantly quenched. Picosecond transient absorption measurements, in conjunction with fluorescence quenching and cyclic voltammetric measurements, suggest that the quenching mechanism is intramolecular electron transfer from ZnP to AB. The charge separated state, ZnP.+-AB(.-), has a lifetime of not more than 220 ps before recombining. If a secondary electron acceptor, iron(III) porphyrin (FeP), is covalently connected to the AB unit, a second electron transfer from AB(.-) to FeP occurs and the charge separated state, ZnP.+-AB-FeP.-, has a lifetime of at least 5 ns. This demonstrates that electron transfer might be sensitively tuned (switched on) by specific solvent effects. (C) 2001 Elsevier Science BN. All rights reserved.
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| 9. |
- Kilså, Kristine, 1972
(författare)
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Energy and Electron Transfer in Porphyrin-Based Donor--Bridge--Acceptor Systems
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The photophysical properties of supramolecular donor--bridge--acceptor systems have been investigated by spectroscopic and quantum mechanical methods. The systems consist of porphyrin donor and acceptor moieties linked by bridging chromophores of varying excitation energy, and are shown to be geometrically well-defined in terms of length and rotational conformation. In order to focus on one photophysical process at a time, several series, each consisting of a specific donor/acceptor pair and four different bridges, have been studied. Special emphasis is put on how the bridges affect singlet energy and electron transfer. In the series where only excitation energy transfer is possible, it was shown that the bridging chromophore can mediate energy transfer, and that this effect is separable from the Förster mechanism. The thermally activated mediation is specific to each bridge, and is inversely proportional to the energy gap between the lowest singlet excited states of donor and bridge. Theoretical modeling of the mediation describes the difference between the bridging chromophores correctly. The series containing a paramagnetic acceptor porphyrin show significantly enhanced S1 -> T1 intersystem crossing in the donor moiety. This, together with bridge-mediated energy transfer, accounts for the observed quenching of donor fluorescence, and no electron transfer could be observed. However, for one of the systems, stepwise electron transfer occurred in highly polar solvents. This process is controllable by selecting specific combinations of the donor auxiliary ligand and solvent polarity. In the series designed to study direct electron transfer, the .pi.-conjugated bridging chromophores were found to promote electron transfer, whereas systems with a non-conjugated bridge showed only energy transfer. The solvent dependent electron transfer rate showed the electronic coupling to be inversely proportional to the energy gap between donor and bridge lowest singlet excited state, just as was found for the energy transfer mediation. Furthermore, quantum mechanically calculated electronic couplings were found to be highly sensitive to molecular conformation, and agree qualitatively with the measured couplings.
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| 10. |
- Kilså, Kristine, et al.
(författare)
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Enhanced Intersystem Crossing in Donor/Acceptor Systems Based on Zinc/Iron or Free-Base/Iron Porphyrins
- 2001
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Ingår i: Chemistry, A European Journal. - 1521-3765 .- 0947-6539. ; 7:10, s. 2122-33
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Tidskriftsartikel (refereegranskat)abstract
- The deactivation pathways of the singlet excited state of a series of zinc or free-base donor porphyrins covalently linked by a bridge to a paramagnetic iron(iii) chloride porphyrin acceptor have been studied. These donor-bridge-acceptor systems all share a similar geometry (25 Å donor-acceptor center-to-center distance), but the bridges vary in electronic structure. In previously reported investigations of zinc/iron porphyrin systems, the fluorescence quenching of the donor has predominantly been assigned to electron transfer. However, for the porphyrin systems studied in this paper, we show that the dominant deactivation channels are enhanced intersystem crossing and singlet energy transfer. In both series, the intersystem crossing rate (S1T1) of the donor moiety is almost doubled in the presence of a paramagnetic high-spin metal-porphyrin acceptor. The significant spectral overlap of the donor fluorescence and acceptor absorption in both series allows for efficient singlet energy transfer (Förster mechanism). Furthermore, the bridging chromophores mediate energy transfer and the enhancement is inversely dependent upon the energy gap between the donor and bridge excited states. Although Marcus theory predicts thermodynamically favorable electron transfer to occur in the systems investigated, the quenching rate constants were found to be independent of solvent polarity, and no charge-separated state could be detected, indicating very small electronic coupling for electron transfer.Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2111/2001/f2779_s.pdf or from the author. 1H NMR spectra of FeP, FeP-OB, FeP-BB, FeP-NB, FeP-AB, ZnP-OB-FeP, ZnP-BB-FeP, ZnP-NB-FeP, and ZnP-AB-FeP.
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| 11. |
- Kilså, Kristine, 1972, et al.
(författare)
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Mediated electronic coupling: Singlet energy transfer in porphyrin dimers enhanced by the bridging chromophore
- 1999
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Ingår i: Journal of Physical Chemistry B Materials. - 1089-5647. ; 103:34, s. 7329-7339
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Tidskriftsartikel (refereegranskat)abstract
- We have studied singlet electronic energy transfer (EET) in two donor-bridge-acceptor series (D-B -A), in which the donor (zinc porphyrin or its pyridine complex) and the acceptor (free base porphyrin) were covalently connected by a geometrically well-defined bridging chromophore. We have investigated how the medium between a donor and an acceptor influences EET by separating the influence of the electronic structure of the bridging chromophore from other effects known to influence the energy transfer. The electronic structure of the bridging chromophore was varied by changing the central unit (bicyclo[2.2.2]octane, benzene, naphthalene, or anthracene) in the bridging chromophore. In all systems the excited state energy separation donor-bridge and bridge-acceptor is large enough to prevent stepwise singlet energy transfer. In addition, the systems were designed to minimize conjugation to preserve the identity of the separate chromophores (donor, bridge, acceptor). Compared with the rate constant expected from the Forster theory, the bridging chromophore with bicyclo[2.2.2]octane as the central unit did not significantly enhance the energy transfer rate constant. However, the bridging chromophores with benzene and naphthalene as the central unit showed a moderate increase, whereas the bridging chromophore with anthracene as the central unit showed the largest increase in energy transfer rate constant. This increase is ascribed to a mediating effect of the bridging chromophore and it is proposed to be strongly correlated to the energy splitting between the singlet excited states of donor and bridging chromophores.
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| 12. |
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| 13. |
- Mikaelsson, Therese, 1980-
(författare)
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Electronic Energy Migration/Transfer as a Tool to Explore Biomacromolecular Structures
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fluorescence-based techniques are widely used in bioscience, offering a high sensitivity and versatility. In this work, fluorescence electronic energy migration/ transfer is applied to measure intramolecular distances in two types of systems and under various conditions.The main part of the thesis utilizes the process of donor-acceptor energy transfer to probe distances within the ribosomal protein S16. Proteins are essential to all organisms. Therefore, it is of great interest to study protein structure and function in order to understand and prevent protein malfunction. Moreover, it is also important to try to study the proteins in an environment which resembles its natural habitat. Here two protein homologs were investigated; S16Thermo and S16Meso, isolated from a hyperthemophilic bacterium and a mesophilic bacterium, respectively. It was concluded that the chemically induced unfolded state ensemble of S16Thermo is more compact than the corresponding ensemble of S16Meso. This unfolded state compaction may be one reason for the increased thermal stability of S16Thermo as compared to S16Meso.The unfolded state of S16 was also studied under highly crowded conditions, mimicking the environment found in cells. It appears that a high degree of crowding, induced by 200 mg/mL dextran 20, forces the unfolded state ensemble of S16Thermo to become even more compact. Further, intramolecular distances in the folded state of five S16 mutants were investigated upon increasing amounts of dextran 20. We found that the probed distances in S16Thermo are unaffected by increasing degree of crowding. However, S16Meso shows decreasing intramolecular distances for all three studied variants, up to 100 mg/mL dextran. At higher concentrations, the change in distance becomes anisotropic. This suggests that marginally stable proteins like s16Meso may respond to macromolecular crowding by fine-tuning its structure. More stable proteins like S16Thermo however, show no structural change upon increasing degree of crowding.We also investigated the possibility of local specific interactions between the protein and crowding agent, by means of fluorescence quenching experiments. Upon increasing amounts of a tyrosine labelled dextran, a diverse pattern of fluorescence quantum yield and lifetime suggests that specific, local protein-crowder interactions may occur.In a second studied system, electronic energy migration between two donor-groups, separated by a rigid steroid, was studied by two-photon excitation depolarization experiments. Data were analysed by using recent advances, based on the extended Förster theory, which yield a reasonable value of the distance between the two interacting donor-groups. To the best of our knowledge, this is the first quantitative analysis of energy migration data, obtained from two-photon excited fluorescence.
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| 14. |
- Ottosson, Henrik, et al.
(författare)
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Scope and Limitations of Baird's Theory on Triplet State Aromaticity : Application to the tuning of singlet-triplet energy gaps in fulvenes
- 2007
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 13:24, s. 6998-7005
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Tidskriftsartikel (refereegranskat)abstract
- Utilizing Baird's theory on triplet state aromaticity, we show that the singlet-triplet energy gaps (ΔEST) of pentafulvenes are easily varied through substitution by as much as 36 kcal mol-1. This exploits the fact that fulvenes act as aromatic chameleons in which the dipoles reverse on going from the singlet ground state (S0) to the lowest ππ* triplet state (T1): thus, their electron distributions are adapted so as to achieve some aromaticity in both states. The results are based on quantum chemical calculations with the OLYP density functional theory method and the CASPT2 ab initio method, as well as spectroscopic determination of ΔEST by triplet sensitization. The findings can also be generalized to fulvenes other than the pentafulvenes, even though the effect is attenuated as the size of the fulvene increases. Our studies thus reveal that triplet-state aromaticity can greatly influence the properties of conjugated compounds in the T1 state.
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| 15. |
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| 16. |
- Rosenberg, Martin, et al.
(författare)
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Influence of excited state aromaticity in the lowest excited singlet states of fulvene derivatives
- 2011
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 13:28, s. 12912-12919
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Tidskriftsartikel (refereegranskat)abstract
- The absorption spectra and excited state dipole moments of four differently substituted fulvenes have been investigated both experimentally and computationally. The results reveal that the excited state dipole moment of fulvenes reverses in the first excited singlet state when compared to the ground state. The oppositely polarized electron density distributions, which dominate the ground state and the first excited singlet state of fulvenes, respectively, reflect the reversed π-electron counting rules for aromaticity in the two states (4n + 2 vs. 4n, respectively). The results show that substituents indeed influence the polarity of fulvenes in the two states, however, cooperative interactions between the substituents and the fulvene moiety are most pronounced in the ground state.
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| 17. |
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| 18. |
- Rosenberg, Martin, et al.
(författare)
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Proton and Hydride Affinities in Excited States : Magnitude Reversals in Proton and Hydride Affinities between the Lowest Singlet and Triplet States of Annulenyl and Benzannulenyl Anions and Cations
- 2010
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Ingår i: Journal of Organic Chemistry. - : American Chemical Society (ACS). - 0022-3263 .- 1520-6904. ; 75:7, s. 2189-2196
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Tidskriftsartikel (refereegranskat)abstract
- Aromaticity has importance for proton and hydride affinities in the singlet ground state (S-0) of annulenyl anions and cations so that, e.g., cyclopentadiene is an acidic hydrocarbon. For the lowest pi pi* excited triplet state (T-1), Baird's rule concludes that annulenes with 4n pi-electrons are aromatic and those with 4n+2 pi-electrons are antiaromatic, opposite to Huckel's rule for aromaticity in S-0. Our hypothesis is now that the relative magnitudes of proton and hydride affinities of annulenyl anions and cations reverts systematically as One goes front S-0 to T-1 as a result of the opposite electron counting rules for aromaticity in the two states. Using quantum Chemical calculations at the G3(MP2)//(U)B3LYP/6-311+G(d,p) level We have examined the validity of this hypothesis for eight proton and eight hydride addition reactions of anions and cations, respectively, of annulenyl and benzannulenyl type. We categorize the (4n+2)pi-electron systems in S-0 and the 4n pi-electron systems in T, to be of A-character and 4n pi-electron systems in S-0 and (4n+2)pi-electron systems in T-1 to be of AA-character (A, aromatic, AA, anti/nonaromatic). The average proton affinities of anions of A- and AA-characters in S-0 are 1447 and 1521 kJ/mol, respectively, and in T-1 they are 1365 and 1493 kJ/mol. The average hydride affinities of A- and AA-character cations in S-0 are 826 and 996 kJ/mol, and in T-1 they are 790 and 879 kJ/mol, respectively. Thus, the calculated proton and hydride affinities are in general lower For anions and cations of A-character than for those of AA-character, in good Support Of Our hypothesis. The findings could likely be applied in synthetic organic photochemistry and other areas where excited state acid-base chemistry plays a role.
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