Magnetic field effects in chemical kinetics and related phenomena

Steiner, Ulrich; Ulrich, Thomas

doi:10.1021/cr00091a003

Cited by 1,557 publications

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“…Since the viscosity of TMPA TFSI (72.6 cP) is much larger than that of 2-methyl-1-propanol (3.3 cP), the escaping process of the triplet pairs in TMPA TFSI should be slow. Thus a part of the triplet pairs can convert to the singlet pairs, even at 0 T, and disappear by the recombination process, as shown BP + hν (355 nm) f 1 BP* f 3 BP* (1) in Figure 3(a). The escaped radical (Y esc ) at 0 T in TMPA TFSI was estimated to be 0.84, which was discussed later.…”

Section: Resultsmentioning

confidence: 90%

“…The decrease of R(B) at 0 T < B e 2 T and the saturation of MFEs at 2 T < B e 10 T can safely be explained by the ∆gM, which originates from the difference between the isotropic g factors of two radicals in a pair, 1,2,16,18 because the present radical pair of BPH• (g ) 2.0030) and •SPh (g ) 2.0082) has a large ∆g value of 0.0052. 18 Moreover, the plots of R(B) vs B 1/2 have a good linear relationship, as shown in Figure 2 (inset).…”

Section: Resultsmentioning

confidence: 96%

“…If ILs have the local structure like a micellar solution, it can work as a cage for the photochemical reactions and cause the saturation of the MFEs at the low field region. In fact, the local structure or some domains in ILs were recently reported by Hamaguchi et al, 21 Wang et al, 22 Nishikawa et al, 23 and Wishart et al 24 The secondary decrease of R(B) at 10 T < B e 28 T may be due to the relaxation mechanism (RM), which originates from the spin relaxation of T (1 -T 0 and T (1 -S. According to the RM by Hayashi and Nagakura, 25 the spin relaxation rates of k R and k R′ for a radical pair consisting of radical A and radical B can be represented as follows: 1,2,25 Here, k R is the spin relaxation rate between T (1 and T 0 , and k R′ is that between T (1 and S. k dd is the rate constant for the inter-radical relaxation induced by the electron spin-spin interaction. k j (j ) A, B) is the rate constant for the intra-radical relaxation of radical j. k j can be given by Here, k j δHFC and k j δg are the rate constants of spin-lattice relaxation by the anisotropic hyperfine coupling and the anisotropic Zeeman interaction, respectively.…”

Section: Resultsmentioning

confidence: 99%

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The Magnetic Field Effects on Photochemical Reactions in Ionic Liquids

Wakasa

2007

J. Phys. Chem. B

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The magnetic field effects (MFEs) on photoinduced hydrogen abstraction reactions between benzophenone and thiophenol in an ionic liquid, N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl) imide (TMPA TFSI), were studied by a nanosecond laser flash photolysis technique under ultrahigh fields of up to 28 T. Extremely large and anomalous stepwise MFEs were observed for the first time. The escape yield of benzophenone ketyl radical decreased with increasing magnetic field strength (B) at 0 T < B e 2 T. The decrease was almost saturated at 2 T < B e 10 T. At much higher fields (10 T < B e 28 T), the yield decreased again with increasing B, producing a 25% decrease at 28 T.

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Section: Resultsmentioning

confidence: 90%

Section: Resultsmentioning

confidence: 96%

Section: Resultsmentioning

confidence: 99%

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The Magnetic Field Effects on Photochemical Reactions in Ionic Liquids

Wakasa

2007

J. Phys. Chem. B

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“…This coupling drives the so-called "spin-flip" transition, mediating population transfer specifically between the singlet and the T 0 triplet state [22]. If we silence this coupling, then the relatively weak hyperfine field (∼ 1mT) is easily overcome by an externally applied field, leading to MFEs that saturate at very small fields.…”

Section: Apr 2016mentioning

confidence: 99%

A Model of Charge-Transfer Excitons: Diffusion, Spin Dynamics, and Magnetic Field Effects

Lee

Shi

Willard

2016

J. Phys. Chem. Lett.

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In this letter we explore how the microscopic dynamics of charge transfer (CT) excitons are influenced by the presence of an external magnetic field in disordered molecular semiconductors. This influence is driven by the dynamic interplay between the spin and spatial degrees of freedom of the electron-hole pair. To account for this interplay we have developed a numerical framework that combines a traditional model of quantum spin dynamics with a stochastic coarse-grained model of charge transport. This combination provides a general and efficient methodology for simulating the effects of magnetic field on CT state dynamics, therefore providing a basis for revealing the microscopic origin of experimentally observed magnetic field effects. We demonstrate that simulations carried out on our model are capable of reproducing experimental results as well as generating theoretical predictions related to the efficiency of organic electronic materials.1 arXiv:1603.08160v2 [physics.chem-ph] Apr 2016Charge transfer (CT) states play a fundamental role in mediating interconversion between bound electronic excitations and free charge carriers in organic electronic materials.For processes that require this interconversion, such as electroluminescence in organic light emitting diodes (OLEDs) and photocurrent generation in organic photovoltaics (OPVs), low-energy (thermalized) CT states are often implicated as a precursor to efficiency loss pathways [1][2][3][4][5][6][7][8][9][10][11][12]. Despite this, much remains to be understood about the properties of CT states and how they contribute to various energy loss mechanisms. Due to their short lifetime and low optical activity, attempts to interrogate CT states directly have brought limited success. Notably, however, recent experiments that probe CT states indirectly via their response to an applied magnetic field have demonstrated the potential to reveal new information about this elusive class of excited states [13][14][15][16][17][18][19][20][21]. Unfortunately, extracting this information is challenging because it is encoded by a complex interplay of electronic and nuclear spin dynamics [15,22,23]. This interplay is further complicated when the dynamics of the electron-hole spin state (or the specific experimental observable) is coupled to a source of fluctuating microscopic disorder such as charge transport or molecular conformational dynamics [21]. In this letter we focus on disentangling this interplay.The dependence of an experimental observable on an applied magnetic field is generically referred to as the magnetic field effect (MFE). For CT-mediated processes, MFEs require that the observed physical property depends either directly or indirectly on the spin state of the electron-hole pair. For instance, spin selection rules for radiative electron-hole recombination can give rise to a magnetic field-dependent electroluminescence yield [20,[24][25][26]. To understand specifically how CT state properties are influenced by the presence of a magnetic field it is na...

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“…[6][7][8][9] Transient absorption studies 10 have shown that coherent interconversion of the electronic singlet and triplet states of the radical pair gives rise to long lived states of the protein whose quantum yields can be modified by applied magnetic fields via the well-established radical pair mechanism. 11,12 Studies of the magnetic sensitivity of cryptochromes are currently constrained by the relatively high protein concentrations and large sample volumes required for absorption-based spectroscopy and by the photo-degradation caused by the intense laser pulses often needed for such measurements. 13,14 Cryptochromes -in particular the vertebrate proteins -are difficult to express recombinantly in large quantities and tend to aggregate in concentrated solution.…”

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confidence: 99%

Fluorescence-detected magnetic field effects on radical pair reactions from femtolitre volumes

et al. 2015

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We show that the effects of applied magnetic fields on radical pair reactions can be sensitively measured from sample volumes as low as ~100 femtolitres using total internal reflection fluorescence microscopy. Development of a fluorescence-based microscope method is likely to be a key step in further miniaturisation that will allow detection of magnetic field effects on single molecules.Flavins -tricyclic aza-aromatic compounds -occur widely in Nature and perform a variety of biological functions, many of which involve electron transfer. 1 Attention has recently focussed on the flavoprotein cryptochrome which has been proposed as the primary sensory molecule by which migratory birds detect the direction of the Earth's magnetic field. 2-5 Blue-light excitation of the flavin adenine dinucleotide (FAD) chromophore in cryptochrome is thought to trigger intra-protein electron transfers along a triad of tryptophan (Trp) residues to give a stabilised, charge-separated FAD-Trp radical pair. 6-9 Transient absorption studies 10 have shown that coherent interconversion of the electronic singlet and triplet states of the radical pair gives rise to long lived states of the protein whose quantum yields can be modified by applied magnetic fields via the well-established radical pair mechanism. 11,12 Studies of the magnetic sensitivity of cryptochromes are currently constrained by the relatively high protein concentrations and large sample volumes required for absorption-based spectroscopy and by the photo-degradation caused by the intense laser pulses often needed for such measurements. 13,14 Cryptochromes -in particular the vertebrate proteins -are difficult to express recombinantly in large quantities and tend to aggregate in concentrated solution. Further in vitro studies of their magnetic responses will require a detection method compatible with much smaller quantities of protein, the obvious choice being fluorescence.Fluorescence has been used extensively to monitor magnetic field effects (MFEs) on the photochemistry of small organic radicals. In appropriate, usually non-aqueous, solvents singlet but not triplet radical pairs can often recombine to form an exciplex whose fluorescence provides a convenient and sensitive probe of the magnetic response. [15][16][17][18] Effective though such an approach has proved, it is manifestly inapplicable when the radical pair cannot recombine to form a luminescent state. Many radical pairs, including flavin-Trp, do not form excited molecular complexes and cryptochromes have no other electronically excited states that can be populated from the FAD-Trp radical pair.However, flavins do have fluorescent excited states that can be exploited as MFE probes. For cyclic photochemical reactions under conditions of continuous illumination, the fluorescence intensity reflects the steady state concentration of ground state flavin, which in turn depends on the concentrations of long-lived radical intermediates in the photocycle. If all other species are short lived and thus present in low concentra...

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Magnetic field effects in chemical kinetics and related phenomena

Cited by 1,557 publications

References 31 publications

The Magnetic Field Effects on Photochemical Reactions in Ionic Liquids

The Magnetic Field Effects on Photochemical Reactions in Ionic Liquids

A Model of Charge-Transfer Excitons: Diffusion, Spin Dynamics, and Magnetic Field Effects

Fluorescence-detected magnetic field effects on radical pair reactions from femtolitre volumes

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