Electron and energy transfer reactions in covalently connected donor-bridge-acceptor assemblies are strongly dependent, not only on the donor-acceptor distance, but also on the electronic structure of the bridge. In this article we describe some well characterised systems where the bridges are pi-conjugated chromophores, and where, specifically, the interplay between bridge length and energy plays an important role for the donor-acceptor electronic coupling. For any application that relies on the transport of electrons, for example molecule based solar cells or molecular scale electronics, it will be imperative to predict the electron transfer capabilities of different molecular structures. The potential difficulties with making such predictions and the lack of suitable models are also discussed.

The photophysics of a butadiyne-linked porphyrin dimer has been investigated by spectroscopy and quantum mechanical calculations. Primarily, the influence of conformation on the ground and first singlet excited states was studied, and two spectroscopically distinct limiting cases were identified. Experiments show that the twisted and planar conformers are separate spectroscopic species that can be selectively excited and have unique absorption and emission spectra. Calculated ground-state spectra compare well with experimental spectra of the two species. A spectrum of the planar conformer was obtained by the addition of a dipyridyl pyrrole ligand, which forms a 1:1 complex with the dimer and thus forces it to stay planar. The absorption spectrum of the twisted conformer could be deduced from the excitation spectrum of its emission. The interpretation of the ground-state spectrum of the free noncomplexed dimer is that it represents an average of a broad distribution of conformations. Calculations support this conclusion by indicating that the barrier for rotation is relatively small in the ground state (0.7 kcal/mol). Studies of the temperature dependence of the fluorescence spectrum of the dimer indicate a mother-daughter relationship between the twisted and planar conformations in the excited state, where the former has approximately 3.9 kcal/mol higher energy. Furthermore, time-correlated single-photon counting experiments also suggest that the twisted population adopts a planar configuration in the first singlet excited state with a rate constant of k rot ) 8.8 × 10 9 s -1 in 2-MTHF at room temperature. The temperature dependence of the fluorescence lifetimes indicated that an activation energy barrier of approximately 2 kcal/mol, in part related to solvent viscosity, is associated with this rate constant.

A series of donor--bridge--acceptor (D--B--A) systems with varying donor-acceptor distances have been studied with respect to their triplet energy transfer properties. The donor and acceptor moieties, zinc(II), and free-base porphyrin, respectively, were separated by 2-5 oligo-p-phenyleneethynylene units (OPE) giving rise to edge-to-edge separations ranging between 12.7 and 33.4 A. The study was performed in 2-MTHF at 150 K and it was established that triplet excitation energy transfer occurs with high efficiency in all of the studied D--B--A systems. The distance dependence was exponential with an attenuation factor, beta, equal to 0.45 +/- 0.015 A(-1). The experimental study was also supported by quantum mechanical DFT and TD-DFT calculations on a series of closely related model systems. A thorough analysis of the OPE-bridge conformational dynamics led to an equation that quantitatively models the distance dependence of the electronic coupling found in the experimental study.

Long-range electron transfer (ET) and triplet-energy transfer (TET) are governed by the through-bond electronic coupling (V DA ). As this coupling is of the exchange type and is related to orbital overlap, it is generally believed to decay exponentially with distance [Eq. (1)].In Equation (1), R DA is the donor-acceptor (D-A) separation, V 0 is the electronic coupling at contact distance, and b is an attenuation factor characteristic of the intervening medium. This simple expression has been used to analyze the distance dependence for electron and energy transfer in a vast number of donor-bridge-acceptor (DBA) systems for which characteristic b values have been suggested. [1][2][3][4][5] In addition, it is expected that the electronic coupling is proportional to the inverse of the energy gap between the relevant bridge and donor states [Eq. (2)].This relation, first derived by McConnell, [6] gives the through-bond donor-acceptor interaction for a chain of m identical units with nearest neighbor interactions v and energy gaps D between the relevant donor and bridge localized states. For v/D ! 1, it can easily be shown that Equation (2) leads to exponential distance dependence. In DBA systems with p-conjugated bridges ("wires"), it is more difficult to dissect the bridge into well-defined chain elements and the validity of Equations (1) and (2) might be questioned. In general, experimental studies of the distance dependence of ET and TET are analyzed with Equation (1) and the estimated b values regarded as bridge-specific parameters. We will show in this communication that: 1) b is not a bridgespecific parameter for p-conjugated bridges; 2) Equation (1) is not valid in cases for which the energy of the bridge states vary strongly with bridge length, that is, the distance dependence of the exchange interaction might be nonexponential.We have been interested for quite some time in understanding bridge-mediated electronic coupling. Carefully designed DBA systems with porphyrin donors and acceptors and with different bridging structures have been investigated experimentally. The metallation states of the porphyrins were varied to allow selective studies of ET, TET, and singletenergy transfer, SET. In one set of systems with constant D-A separation, the donor-bridge energy-gap dependence was mapped out, [7] and in another set the distance dependence was investigated. [8][9][10] In parallel to the experimental studies, we have developed a DFT-based quantum-mechanical method to calculate the electronic coupling for ET, TET, and SET. In particular, the calculations for TET were shown to give excellent quantitative agreement with the experiments if an appropriate conformational-averaging procedure was employed.[8] Inspired by the success of the calculations, we have expanded our study to include other important pconjugated bridges and to specifically investigate how the distance dependence for the TET electronic coupling varies with the donor-bridge energy gap for a set of oligo(phenyleneethenylene) (OPE) bridges.The electr...