Zinc-porphyrin/fullerene donor–acceptor assemblies joined by six-fold Hamilton-receptor/cyanuric-acid hydrogen bonding behave as supramolecular 'molecular wires' for photoinduced electron transfer, and the founding 2010 study of such wires (Wessendorf, Grimm, Guldi, Hirsch) reported the first attenuation factor for hydrogen-bond-mediated electron transfer, beta = 0.11 per angstrom. That value, however, is an effective parameter: it was averaged over four different conjugated spacer chemistries, extracted from kinetics recorded under conditions of incomplete association in selected solvents, and assigned without separating through-bond from through-space coupling. The open problem is to replace this effective value with a chemically and mechanistically resolved account of hydrogen-bond-mediated electron transfer in these wires: spacer-chemistry-resolved attenuation factors for charge separation and charge recombination, an explicit through-bond/through-space assignment for the hydrogen-bond junction, association-clean charge-separation quantum yields and electron-/energy-transfer branching ratios, and the solvent and concentration dependence of the association constant, the charge-separation and charge-recombination rate constants, and beta.

Background

In a donor–bridge–acceptor molecule, photoinduced charge separation (CS) creates a radical ion pair — here a one-electron-oxidized zinc porphyrin (ZnP, the electron donor) and a one-electron-reduced C60 fullerene (the electron acceptor, favored for its small reorganization energy, i.e. the small geometrical relaxation energy accompanying electron transfer) — which subsequently decays by charge recombination (CR). In Marcus-type theory the rates of these electron-transfer (ET) reactions are governed by the electronic coupling between donor and acceptor, the reaction driving force, and the reorganization energy; for a bridge of repeating structure the coupling falls off exponentially with donor–acceptor distance, kET∝exp⁡(−βr)k_{ET}\propto\exp(-\beta r), and the distance-decay or attenuation factor β\beta (in Å−1^{-1}) is the standard figure of merit for a molecular wire: fully conjugated wires reach β≈0.04\beta \approx 0.04–0.20.2 Å−1^{-1}, whereas saturated or empty-space gaps give much larger values, so a small β\beta marks an electronically transparent bridge. Energy transfer (EnT), which transfers excitation energy rather than an electron, can compete with ET and must be distinguished when quenching of the donor excited state is analyzed. The Wessendorf et al. 2010 study (J. Am. Chem. Soc. 132, 10786) assembled ZnP–fullerene dyads noncovalently through a Hamilton receptor, a V-shaped ditopic receptor that binds barbiturate or cyanurate guests via six directional hydrogen bonds; association constants KassK_{\mathrm{ass}} of 104^4–105^5 M−1^{-1} were measured by 1^1H NMR (millimolar) and steady-state fluorescence (micromolar), and transient absorption spectroscopy (a pump–probe technique resolving short-lived intermediates) showed ET to be the dominant quenching mechanism, with radical ion pair lifetimes reaching tens of nanoseconds. By varying both the length and the chemistry of the conjugated spacer (p-phenylene-ethynylene, p-phenylene-vinylene, p-ethynylene, and fluorene) connecting chromophore to hydrogen-bonding unit, the authors derived the first β\beta (0.11 Å−1^{-1}) for hydrogen-bonding-mediated ET. This built on earlier hydrogen-bonded dyads (Sessler's Watson–Crick cytidine/guanosine porphyrin–fullerene dyad, 2005; Wessendorf's Hamilton-receptor prototype, 2007) and on D'Souza's 2009 demonstration of through-bond ET — coupling transmitted through the covalent-plus-hydrogen-bond framework, as opposed to through-space coupling by direct interchromophore contact or folded conformations — in a rigid two-point hydrogen-bonded porphyrin–fullerene dyad. Because the 2010 β\beta combines four spacer chemistries with different intrinsic electron-transfer activity, was obtained at micromolar concentrations where a sizeable fraction of porphyrin and fullerene remain uncomplexed (bound fraction well below unity for Kass≤105K_{\mathrm{ass}} \le 10^5 M−1^{-1}), and involved no decomposition of the coupling into through-bond and through-space parts, it leaves open what the hydrogen-bond junction itself contributes to the wire's electronic transparency — the question formulated here.

Problem Statement

For hydrogen-bond-bridged zinc-porphyrin/fullerene donor–acceptor wires of the type assembled by Wessendorf et al. (2010) — chromophores joined through a six-hydrogen-bond Hamilton-receptor/cyanuric-acid junction with conjugated spacers — establish a chemically and mechanistically resolved description of the distance dependence of photoinduced electron transfer, replacing the single effective value β=0.11\beta = 0.11 Å−1^{-1}. Specifically: (1) determine attenuation factors β\beta for charge separation and for charge recombination for each individual spacer chemistry of the Wessendorf et al. (2010) series (p-phenylene-ethynylene, p-phenylene-vinylene, p-ethynylene, and fluorene) rather than averaged over mixed spacers, with driving force controlled or explicitly modeled, and with electronic-structure effects (spacer orbital energies, bridge-mediated orbital alignment) separated from purely geometric distance effects; (2) partition the measured electronic coupling across the hydrogen-bond junction into through-bond (through the hydrogen-bond array and covalent framework) and through-space (direct chromophore contact or folded conformations) contributions, with the assignment supported by structural control or validated computation; (3) quantify the absolute charge-separation quantum yield and the electron-transfer versus energy-transfer branching for the associated state, at conditions where the associated-state population is established or quantitatively accounted for; and (4) characterize how KassK_{\mathrm{ass}}, kCSk_{\mathrm{CS}}, kCRk_{\mathrm{CR}}, and β\beta vary with solvent (polarity and hydrogen-bond competitiveness) and concentration. The objective is a quantitative, mechanistically assigned account of how electronically transparent a hydrogen-bond junction is and why.

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