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Donor/Acceptor Electronic Coupling Is a Dynamical Variable Edward W Claremont Colleges Scholarship @ Claremont All HMC Faculty Publications and Research HMC Faculty Scholarship 1-1-2000 Solvent as Electron Donor: Donor/Acceptor Electronic Coupling Is a Dynamical Variable Edward W. Castner Jr. Rutgers University Darcy Kennedy '00 Claremont McKenna College Robert J. Cave Harvey Mudd College Recommended Citation Castner, E.W. Jr.; Kennedy, D.; Cave, R.J. “Solvent as Electron Donor: Donor/Acceptor Coupling is a Dynamical Variable,” J. Phys. Chem. A 2000, 104, 2869. DOI: 10.1021/jp9936852 This Article is brought to you for free and open access by the HMC Faculty Scholarship at Scholarship @ Claremont. It has been accepted for inclusion in All HMC Faculty Publications and Research by an authorized administrator of Scholarship @ Claremont. For more information, please contact [email protected]. J. Phys. Chem. A 2000, 104, 2869-2885 2869 ARTICLES Solvent as Electron Donor: Donor/Acceptor Electronic Coupling Is a Dynamical Variable Edward W. Castner, Jr.*,† BrookhaVen National Laboratory, Chemistry Department, Building 555A, Upton, New York 11973-5000 Darcy Kennedy and Robert J. Cave*,‡ HarVey Mudd College, Department of Chemistry, Claremont, California 91711 ReceiVed: October 14, 1999; In Final Form: December 24, 1999 We combine analysis of measurements by femtosecond optical spectroscopy, computer simulations, and the generalized Mulliken-Hush (GMH) theory in the study of electron-transfer reactions and electron donor- acceptor interactions. Our focus is on ultrafast photoinduced electron-transfer reactions from aromatic amine solvent donors to excited-state acceptors. The experimental results from femtosecond dynamical measurements fall into three categories: six coumarin acceptors reductively quenched by N,N-dimethylaniline (DMA), eight electron-donating amine solvents reductively quenching coumarin 152 (7-(dimethylamino)-4-(trifluoromethyl)- coumarin), and reductive quenching dynamics of two coumarins by DMA as a function of dilution in the nonreactive solvents toluene and chlorobenzene. Applying a combination of molecular dynamics trajectories, semiempirical quantum mechanical calculations (of the relevant adiabatic electronic states), and GMH theory to the C152/DMA photoreaction, we calculate the electron donor/acceptor interaction parameter HDA at various time frames. HDA is strongly modulated by both inner-sphere and outer-sphere nuclear dynamics, leading us to conclude that HDA must be considered as a dynamical variable. Introduction reorganization is complete and that the rate is faster than reorientational or translational diffusion. Combining methods Bimolecular electron-transfer reactions in solution frequently of femtosecond optical spectroscopy, computer simulations, and have rates limited by diffusion of donor and acceptor molecules, the generalized Mulliken-Hush theory54,55 for evaluating as one or both of the reactant species is usually in a low concentration relative to the solvent. To obtain a detailed electron donor/acceptor electronic coupling, we have arrived mechanistic and kinetic understanding of electron-transfer at a substantially new picture for describing the earliest events reactions in solution, chemists have devised ingenious schemes in solution phase electron transfer. in which the two reactants, the donor and acceptor, are held at Applied sensibly, there is no obvious reason the Marcus a fixed distance and orientation, so that diffusion will not theory for bimolecular electron-transfer reactions56 should not complicate the study of the intrinsic electron-transfer rates.1-3 apply to interpretation of our experimental results. Zusman’s Examples of these studies include tethering the donor and extensions of the Marcus theory indicated that the reaction rate acceptor together via covalent bonds,4-16 charge-transfer could be dominated by a nuclear-factor preexponential term complexes,17-20 peptides,21-23 proteins,24-31 or nucleic acid proportional to the inverse solvation time for adiabatic electron- - spacers.32-35 transfer reactions.57 59 The Barbara and Yoshihara groups have In our work, we follow the lead of Yoshihara and co- made use of two-dimensional reaction-coordinate models in workers,36-53 where we do not fix the orientations of donor and analyzing their ultrafast photoinduced electron-transfer rates for - - acceptor, but guarantee that no translational diffusion need take intra- and intermolecular reactions, respectively.38,43 47,50,52,60 67 place by choosing the solvent to be the electron donor. Because They have combined the models of Sumi and Marcus,68 and of the acceptor will then have its first solvent coordination sphere Bixon and Jortner,69,70 so that the electron-transfer reaction composed of nothing but donor molecules, an electron-transfer proceeds along or between two reaction coordinates: a solvation reaction photoinitiated by an ultrashort laser pulse can then coordinate, and a vibrational coordinate. An important point is provide direct information on the intrinsic reaction rate (and to decide whether the inertial components of solvation dynamics hopefully mechanism) by eliminating the complexities intro- are included along the solvation coordinate or along the duced by diffusion. The very rapid rates of electron-transfer vibrational coordinate. The latter is actually sensible, as the observed in a number of systems for which the solvent is the inertial dynamics often can be described in terms of specific donor imply that the electron is transferred before solvent intermolecular normal modes, such as librations or H-bond vibrations, at least for polar liquids. An excellent review of these † E-mail: [email protected]. Present address: Rutgers Uni- theories and relevant ultrafast electron-transfer reactions in versity, Department of Chemistry, 610 Taylor Road, Piscataway, NJ 08854- solution was published in 1993 by Heitele.71 8087. ‡ E-mail: [email protected]. To study this class of ultrafast photoinduced bimolecular 10.1021/jp9936852 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/03/2000 2870 J. Phys. Chem. A, Vol. 104, No. 13, 2000 Castner et al. SCHEME 1 SCHEME 2 electron-transfer reactions, we have used the excited states of substituted coumarins as electron acceptors, and a variety of electron-rich solvents as donors. The photochemical and pho- tophysical properties of substituted coumarins molecules have been widely studied, because of their broad applications as laser dyes. Jones and co-workers first discovered many of the photoinduced electron-transfer reactions studied here. They demonstrated that the excited singlet states of the coumarins can be both reductively and oxidatively quenched by solvents SCHEME 3 such as N,N-dimethylaniline and nitrobenzene, respectively.72,73 Substituted coumarins have been often used as probes of solvation dynamics, where the time-dependent fluorescence Stokes shift (TDFSS) of the coumarin charge-transfer excited state is used as a probe of solvent reorganization dynamics.74-76 Reductive quenching of coumarin excited states by electron donors (such as DMA) has been studied as a probe of diffusion- limited reactions at high donor concentrations of ∼0.5 M, which is at or beyond the so-called “static quenching” concentration limit.77 Yoshihara and co-workers have used several classes of organic laser dye molecules to study the ultrafast bimolecular electron-transfer between solvent and excited-state chromo- phore.37-53 In the experimental parts of this paper, we present studies on the photoinduced bimolecular electron-transfer rates from oromethyl)coumarin), 120 (7-amino-4-methylcoumarin), and solvent donors to substituted coumarin excited-state acceptors. 343 were Kodak Laser Grade and were used as received. The experiments may be categorized using three paradigms. In Coumarins 500 (7-(ethylamino)-4-(trifluoromethyl)coumarin) the first group of experiments, we characterized a series of and 503 (aka 307, 7-(ethylamino)-6-methyl-4-(trifluoromethyl)- molecules that are likely to be good electron donors by coumarin) were Exciton Laser Grade and were also used as measuring the reductive fluorescence quenching of coumarin received. Structures of the coumarins are shown in Scheme 2. 152 in these solvents. In the second series of experiments, we Solvent purity grades were ACS reagent, HPLC, or spectro- used DMA as the electron donor to reductively quench the grade, obtained from Aldrich. N,N-Dimethylaniline was frac- excited states of a series of substituted coumarin molecules. The tionally distilled at reduced pressure (15 Torr) under an argon third group of experiments involves the use of DMA to atmosphere (∼85 °C). Triethylamine (TEA), toluene, chlo- reductively quench the excited state of coumarin 151 or 152, robenzene, and benzene were distilled over calcium hydride, but the focus is on measuring the bimolecular electron-transfer under an atmosphere of argon. Triphenylamine (TPA) was rate dependence as a function of dilution of the donor solvent obtained from Aldrich (98%) and used as received. All solvents with a suitable cosolvent, such as toluene or chlorobenzene. were transferred by gastight syringes to the sample flow A diagram of the photophysical and photochemical reactions reservoir, which was vigorously purged with argon. N,N- we have studied is presented in Scheme 1. On excitation by Dimethyl-o-, -m-, and -p-toluidines (DMOT, DMMT, and the near-ultraviolet laser pulse (hν), the excited state of the DMPT, respectively), dimethyl phthalate (DMP), o-anisidine
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