ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTElectron donor/acceptor interaction and reorganization parameters from temperature-dependent intramolecular electron-transfer ratesP. Finckh, H. Heitele, M. Volk, and M. E. Michel-BeyerleCite this: J. Phys. Chem. 1988, 92, 23, 6584–6590Publication Date (Print):November 1, 1988Publication History Published online1 May 2002Published inissue 1 November 1988https://doi.org/10.1021/j100334a022RIGHTS & PERMISSIONSArticle Views257Altmetric-Citations71LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (857 KB) Get e-Alerts
We explore charge migration in DNA, advancing two distinct mechanisms of charge separation in a donor (d)–bridge ({B j })–acceptor (a) system, where {B j } = B 1 ,B 2 , … , B N are the N-specific adjacent bases of B-DNA: ( i ) two-center unistep superexchange induced charge transfer, d*{B j }a → d ∓ {B j }a ± , and ( ii ) multistep charge transport involves charge injection from d* (or d + ) to {B j }, charge hopping within {B j }, and charge trapping by a. For off-resonance coupling, mechanism i prevails with the charge separation rate and yield exhibiting an exponential dependence ∝ exp(−βR) on the d-a distance (R). Resonance coupling results in mechanism ii with the charge separation lifetime τ ∝ N η and yield Y ≃ (1 + δ̄ N η ) −1 exhibiting a weak (algebraic) N and distance dependence. The power parameter η is determined by charge hopping random walk. Energetic control of the charge migration mechanism is exerted by the energetics of the ion pair state d ∓ B 1 ± B 2 … B N a relative to the electronically excited donor doorway state d*B 1 B 2 … B N a. The realization of charge separation via superexchange or hopping is determined by the base sequence within the bridge. Our energetic–dynamic relations, in conjunction with the energetic data for d*/d − and for B/B + , determine the realization of the two distinct mechanisms in different hole donor systems, establishing the conditions for “chemistry at a distance” after charge transport in DNA. The energetic control of the charge migration mechanisms attained by the sequence specificity of the bridge is universal for large molecular-scale systems, for proteins, and for DNA.
Abstract Die für das Leben auf der Erde wohl wichtigste „Erfindung”︁ der Natur ist die Konversion von Sonnenlicht in biochemisch nutzbare Energieformen. 25 Jahre nach der gelungenen Isolation eines bakteriellen Reaktionszentrums, des Protein‐Pigment‐Komplexes, in dem die ersten entscheidenden Schritte der Photosynthese ablaufen, und zehn Jahre nach dessen Strukturaufklärung ist diese Energieumwandlung in den wesentlichen Grundzügen der Dynamik und Energetik verstanden. Zu verdanken ist dies vor allem modernen physikalischen Meßmethoden wie der Ultrakurzzeit‐Spektroskopie in Verbindung mit der theoretischen Beschreibung des Elektrontransfers und der systematischen Veränderung äußerer Parameter wie Temperatur oder magnetische und elektrische Felder.
From previous thermal and photoinduced charge-transfer reactions in duplex DNA there is accumulative evidence for an attenuation parameter β of the distance dependence in the range 0.6−0.8 Å-1, with the exception of one specific system exhibiting β = 1.5 Å-1 which is reinvestigated in this paper. Femtosecond to nanosecond time-resolved pump−probe spectroscopy has been used to follow photoinduced charge-shift dynamics in DNA duplexes containing a covalently appended, protonated 9-alkylamino-6-chloro-2-methoxyacridine chromophore. This acridine derivative (X+) resides in the DNA duplex at a specific abasic site, which is highly defined as reflected in the monoexponentiality of the kinetics. In the presence of only neighboring A:T base pairs, no charge transfer occurs within the excited-state lifetime (18 ns) of the chromophore. However, the presence of a guanine nucleobase as either a nearest neighbor or with one interspersed A:T base pair does result in fluorescence quenching. In the case of nearest neighbors, the intermediate radical state X• is formed within 4 ps and decays on the 30 ps time scale. Placing one A:T base pair between the X+ and guanine slows down the forward transfer rate by 3 orders of magnitude, corresponding to an apparent β value of >2.0 Å-1. This dramatic decrease in the rate is due to a change in charge-transfer mechanism from a (nearly) activationless to a thermally activated regime in which the forward transfer is slower than the back transfer and the X• state is no longer observed. These observations indicate that the distance dependence of charge injection in the X+-labeled DNA duplex is not solely caused by a decrease in electronic couplings but also by a concomitant increase of the activation energy with increasing distance. This increase in activation energy may result from the loss of driving force due to excited-state relaxation competing with charge transfer, or reflect distance-dependent changes in the energetics, predominantly of the low-frequency reorganization energy in this charge-shift reaction, on purely electrostatic grounds. To test the hypothesis of distance-dependent activation energy, guanine has been replaced by 7-deazaguanine, its easier-to-oxidize purine analogue. In these duplexes, a similar change of charge-transfer mechanism is found. However, consistent with an a priori larger driving force this change occurs at a larger donor−acceptor separation than in the X+-guanine systems. Independent of the detailed contributions to the distance-dependent activation energy, this phenomenon illustrates the complex nature of experimental β values.
Abstract Excited state dynamics in two strong organic electron acceptor systems, TCNQ and F 4 TCNQ single crystals, was studied. After absorption of a single photon, dianions are formed in both crystals on ultrashort timescale: TCNQ τ < 50 fs, F 4 TCNQ τ = 4 ps. By use of transient absorption spectroscopy, we demonstrate that the dianion formation in F 4 TCNQ is mediated by the radical anion precursor which is described by a two-step model. Our measurements show the phenomenon that in this quinoid acceptor crystals in the absence of additional donor molecule, it is possible to resolve the two step formation of a doubly charged anion upon absorption of a single low energy photon (2.6 eV).
G-quadruplex nucleic acids represent a unique avenue for the building of electrically conductive wires. These four-stranded structures are formed through the stacking of multiple planar guanine assemblies termed G-tetrads. The diverse folding patterns of G-quadruplexes allow for several geometries to be adopted by stacked guanine bases within the core and at the dimeric interface of these structures. It is currently not clear how different G-tetrad stacking arrangements affect electron hole mobility through a G-quadruplex wire. Using a combined quantum mechanics and molecular dynamics approach, we demonstrate that the electron-hole transfer rates within the G-tetrad stacks vary greatly for different stacking geometries. We identify a distinguished structure that allows for strong electronic coupling and thus enhanced molecular electric conductance. We also demonstrate the importance of sampling a large number of geometries when considering the bulk properties of such systems. Hole hopping within single G-tetrads is slower by at least two orders of magnitude than between stacked guanines; therefore, hole jumping within individual tetrads should not affect the hole mobility in G-quadruplexes. The results of this study suggest engineering G-tetrads with continuous 5/6-ring stacking from an assembly of single guanosine analogs or through modification of the backbone in G-rich DNA sequences.
The two-photon-induced singlet fission was observed in rubrene single crystal and studied by use of femtosecond pump-probe spectroscopy. The location of two-photon excited states was obtained from the nondegenerate two-photon absorption (TPA) spectrum. Time evolution of the two-photon-induced transient absorption spectra reveals the direct singlet fission from the two-photon excited states. The TPA absorption coefficient of rubrene single crystal is 52 cm∕GW at 740 nm, as obtained from Z-scan measurements. Quantum chemical calculations based on time-dependent density functional theory support our experimental data.
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