## Abstract

In this contribution we advance and explore the thermally induced hopping (TIH) mechanism for long-range charge transport (CT) in DNA and in large-scale chemical systems. TIH occurs in donor-bridge-acceptor systems, which are characterized by off-resonance donor-bridge interactions (energy gap δΕ > 0), involving thermally activated donor-bridge charge injection followed by intrabridge charge hopping. We observe a "transition" from superexchange to TIH with increasing the bridge length (i.e., the number N of the bridge constituents), which is manifested by crossing from the exponential N-dependent donor-acceptor CT rate at low N (< N^{x}) to a weakly (algebraic) N-dependent CT rate at high N (>N_{x}). The "critical" bridge size N_{x} is determined by the energy gap, the nearest-neighbor electronic couplings, and the temperature. Experimental evidence for the TIH mechanism was inferred from our analysis of the chemical yields for the distal/proximal guanine (G) triplets in the (GGG)^{+}TTXTT(GGG) duplex (X = G, azadine (^{z}A), and adenine (A)) studied by Nakatani, Dohno and Saito [J. Am. Chem. Soc. 2000, 122, 5893]. The TIH sequential model, which involves hole hopping between (GGG) and X, is analyzed in terms of a sequential process in conjunction with parallel reactions of (GGG)^{+} with water, and provides a scale of (free) energy gaps (relative to (GGG)^{+}) of δ = 0.21-0.24 eV for X = A, δ = 0.10-0.14 eV for X = ^{z}A, and δ = 0.05-0.10 eV for X = G. We further investigated the chemical yields for long-range TIH in (G)_{l}^{+}X_{n}(G)_{l} (l = 1-3) duplexes, establishing the energetic constraints (i.e., the donor (G)_{l}^{+} - bridge base (X) energy gap δ), the bridge structural constraints (i.e., the intrabridge X-X hopping rates k_{m}), and the kinetic constraints (i.e., the rate k_{d} for the reaction of (G)_{l}^{+} with water). Effective TIH is expected to prevail for δ ≲ 0.20 eV with a "fast" water reaction (k_{d}/k_{m} ≃ 10^{-3}) and for δ < 0.30 eV with a "slow" water reaction (k_{d}/k_{m} ≃ 10^{-5}). We conclude that (T)_{n} bridges (for which δ ≃ 0.6 eV) cannot act in TIH of holes. From an analysis based on the energetics of the electronic coupling matrix elements in G^{+}(T-A)_{n}(GGG) duplexes we conclude that the superexchange mechanism is expected to dominate for n = 1-4. For long (A)_{n} bridges (n ≳ 4) the TIH prevails, provided that the water side reaction is slow, raising the issue of chemical control of TIH through long (A)_{n} bridges in DNA attained by changing the solution composition.

Original language | English |
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Pages (from-to) | 12556-12567 |

Number of pages | 12 |

Journal | Journal of the American Chemical Society |

Volume | 123 |

Issue number | 50 |

DOIs | |

State | Published - 19 Dec 2001 |