Bimolecular Electron Transfers That Follow a Sandros鈭払oltzmann Dependence on Free Energy

详细信息    查看全文

• 作者：Samir Farid ; Joseph P. Dinnocenzo ; Paul B. Merkel ; Ralph H. Young ; Deepak Shukla
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：April 6, 2011
• 年：2011
• 卷：133
• 期：13
• 页码：4791-4801
• 全文大小：940K
• 年卷期：v.133,no.13(April 6, 2011)
• ISSN：1520-5126

文摘

Rate constants (k) for exergonic and endergonic electron-transfer reactions of equilibrating radical cations (A^鈥? + B A + B^鈥?) in acetonitrile could be fit well by a simple Sandros鈭払oltzmann (SB) function of the reaction free energy (螖G) having a plateau with a limiting rate constant kb>limb> in the exergonic region, followed, near the thermoneutral point, by a steep drop in log k vs 螖G with a slope of 1/RT. Similar behavior was observed for another charge shift reaction, the electron-transfer quenching of excited pyrylium cations (P⁺*) by neutral donors (P⁺* + D 鈫?P^{鈥?/sup> + D鈥?). In this case, SB dependence was observed when the logarithm of the quenching constant (log kb>qb>) was plotted vs 螖G + s, where the shift term, s, equals +0.08 eV and 螖G is the free energy change for the net reaction (Eb>redoxb> 鈭?Eb>excitb>). The shift term is attributed to partial desolvation of the radical cation in the product encounter pair (P鈥?/sup>/D鈥?), which raises its free energy relative to the free species. Remarkably, electron-transfer quenching of neutral reactants (A* + D 鈫?A鈥⑩垝 + D鈥?) using excited cyanoaromatic acceptors and aromatic hydrocarbon donors was also found to follow an SB dependence of log kb>qb> on 螖G, with a positive s, +0.06 eV. This positive shift contrasts with the long-accepted prediction of a negative value, 鈭?.06 eV, for the free energy of an A鈥⑩垝/D鈥? encounter pair relative to the free radical ions. That prediction incorporated only a Coulombic stabilization of the A鈥⑩垝/D鈥? encounter pair relative to the free radical ions. In contrast, the results presented here show that the positive value of s indicates a decrease in solvent stabilization of the A鈥⑩垝/D鈥? encounter pair, which outweighs Coulombic stabilization in acetonitrile. These quenching reactions are proposed to proceed via rapidly interconverting encounter pairs with an exciplex as intermediate, A*/D exciplex A鈥⑩垝/D鈥?. Weak exciplex fluorescence was observed in each case. For several reactions in the endergonic region, rate constants for the reversible formation and decay of the exciplexes were determined using time-correlated single-photon counting. The quenching constants derived from the transient kinetics agreed well with those from the conventional Stern鈭扸olmer plots. For excited-state electron-transfer processes, caution is required in correlating quenching constants vs reaction free energies when 螖G exceeds +0.1 eV. Beyond this point, additional exciplex deactivation pathways鈥攆luorescence, intersystem crossing, and nonradiative decay鈥攁re likely to dominate, resulting in a change in mechanism.}