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Comparative PCET study of a donor-acceptor pair linked by ionized and nonionized asymmetric hydrogen-bonded interfaces.

ABSTRACT: A Zn(II) amidinium porphyrin is the excited-state electron donor (D) to a naphthalene diimide acceptor (A) appended with either a carboxylate or sulfonate functionality. The two-point hydrogen bond (...[H(+)]...) formed between the amidinium and carboxylate or sulfonate functionalities establishes a proton-coupled electron transfer (PCET) pathway for charge transfer. The two D...[H(+)]...A assemblies differ only by the proton configuration within the hydrogen-bonding interface. Specifically, the amidinium ion transfers a proton to the carboxylate to form a nonionized amidine-carboxylic acid two-point hydrogen network, whereas the amidinium retains both protons when bound to the sulfonate functionality, forming an ionized amidinium-sulfonate two-point hydrogen bond network. These two interface configurations within the dyads thus allow for a direct comparison of the PCET kinetics for the same donor and acceptor juxtaposed by ionized and nonionized hydrogen-bonded interfaces. Analysis of the PCET kinetics ascertained from transient absorption and transient emission spectroscopy reveals that the ionized interface is more strongly impacted by the local solvent environment, thus establishing that the initial static configuration of the proton interface is a critical determinant in the kinetics of PCET.

SUBMITTER: Young ER

PROVIDER: S-EPMC3278395 | biostudies-literature | 2009 Jun

REPOSITORIES: biostudies-literature

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Comparative PCET study of a donor-acceptor pair linked by ionized and nonionized asymmetric hydrogen-bonded interfaces.

Young Elizabeth R ER Rosenthal Joel J Hodgkiss Justin M JM Nocera Daniel G DG

Journal of the American Chemical Society 20090601 22

A Zn(II) amidinium porphyrin is the excited-state electron donor (D) to a naphthalene diimide acceptor (A) appended with either a carboxylate or sulfonate functionality. The two-point hydrogen bond (...[H(+)]...) formed between the amidinium and carboxylate or sulfonate functionalities establishes a proton-coupled electron transfer (PCET) pathway for charge transfer. The two D...[H(+)]...A assemblies differ only by the proton configuration within the hydrogen-bonding interface. Specifically, the ...[more]

PMID: 19489645

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Project description:Donor-acceptor systems forming exciplexes are versatile models for the study of magnetic field effects (MFEs) on charge recombination reactions. The MFEs originate from singlet-triplet interconversion within transient radical ion pairs (RIPs), which exist in a dynamic equilibrium with the exciplexes. Here, we describe the synthesis and MFEs of the chain-linked N,N-dimethylaniline (DMA)/9-methylanthracene (MAnt) donor-acceptor system MAnt-(CH2) n -O-CH2-CH2-DMA for n = 6, 8, 10, and 16. The MFEs are found to increase with increasing chain length. Effects as large as 37.5% have been observed for the long-chain compound with n = 16. The solvent dependence of the MFEs at magnetic field intensity 75 mT is reported. For the range of solvent static dielectric constants ?s = 6.0-36.0, the MFEs go through a maximum for intermediate polarities, for which the direct formation of RIPs prevails and their dissociation and reencounter are balanced. Field-resolved measurements (MARY spectra) are reported for solutions in butyronitrile. The MARY spectra reveal that for n = 8, 10, 16, the average exchange interaction is negligible during the coherent lifetime of the radical pair. However, singlet-triplet dephasing broadens the lineshape; the shorter the linker, the more pronounced this effect is. For n = 6, a dip in the fluorescence intensity reveals a nonzero average exchange coupling of the order of ±5 mT. We discuss the field-dependence in the framework of the semiclassical theory taking spin-selective recombination, singlet-triplet dephasing, and exchange coupling into account. Singlet recombination rates of the order of 0.1 ns-1 and various degrees of singlet-triplet dephasing govern the spin dynamics. In addition, because of a small free energy gap between the exciplex and the locally excited fluorophore quencher pair, a fully reversible interconversion between the RIP, exciplex, and locally excited fluorophore is revealed by spectrally resolved MFE measurements for the long-chain systems (n = 10, 16).

Dataset Information

Comparative PCET study of a donor-acceptor pair linked by ionized and nonionized asymmetric hydrogen-bonded interfaces.

Publications

Comparative PCET study of a donor-acceptor pair linked by ionized and nonionized asymmetric hydrogen-bonded interfaces.

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