Donor Bridge Acceptor Systems Research Articles

Molecular-wire behaviour in p -phenylenevinylene oligomers

Electron transfer from electron-donor to electron-acceptor molecules via a molecular ‘bridge’ is a feature of many biological andchemical systems. The electronic structure of the bridge component in donor–bridge–acceptor (DBA) systems is known to play a critical role in determining the ease of electron transfer1,2. In most DBA systems, the rate at which electron transfer occurs scales exponentially with the donor–acceptor distance — effectively the length of the bridge molecule. But theory predicts that regimes exist wherein the distance dependence may be very weak, the bridge molecules essentially acting as incoherent molecular wires3,4,5,6. Here we show how these regimes can be accessed by molecular design. We have synthesized a series of structurally well-defined DBA molecules that incorporate tetracene as the donor and pyromellitimide as the acceptor, linked by p -phenylenevinylene oligomers of various lengths. Photoinduced electron transfer in this series exhibits very weak distance dependence for donor–acceptor separations as large as 40 A, with rate constants of the order of 1011 s−1. These findings demonstrate the importance of energy matching between the donor and bridge components for achieving molecular-wire behaviour.

Wide-band and narrow-band approximations for donor–bridge–acceptor systems

The techniques of wide-band and narrow-band approximations, which have been used in solid state theory, are applied to the theory of donor–bridge–acceptor models which are commonly used to represent molecular switches or wires. The approximations lead to analytical solutions which closely match the numerical calculations. These approximations are based on one of the simplest one-electron models where the interaction is time independent and the bridge band is completely initially empty. The form of the analytical solutions and the stability of the numerical calculations are discussed.

Rigid donor—bridge—acceptor systems: photoinduced charge transfer in solution and in a supersonic jet

In this paper the fluorescent behaviour is studied under jet-cooled conditions as well as in solution of two donor—bridge—acceptor molecules ( 1 and 2) in which the donor (D) and acceptor (A) are held rigidly apart by an extended saturated hydrocarbon bridge with an effective length of three sigma-bonds. It was found that even in the isolated molecules no excess excitation energy at all is needed to induce charge separation and that charge transfer (CT) fluorescence can be observed from both these molecules. Furthermore, broad red-shifted excitation bands are observed indicating that excitation can take place directly from the ground state into the CT state, even at energies considerably lower than that required to reach any of the locally (in D or A) excited states. Comparison of the radiative rate constants for CT fluorescence in media of different polarity indicates important intensity borrowing from local transitions in media of low polarity. Nevertheless, the admixture of locally excited configurations in the CT state is too small to diminish significantly the CT nature of the emissive state even in the gas phase.

Dispersion relations for electron and hole transfer in donor—bridge—acceptor systems

Simple dispersion relations are derived which allow for the separation of the contribution of direct electron transfer and hole transfer to the total matrix element between donor and acceptor states in donor—bridge—acceptor charge transfer systems. The proposed relations are based on the calculation of the total matrix element for complex tunneling energies and do not assume diagonalization of the bridge orbitals. A numerical procedure to perform such calculations for proteins and other large electron transfer systems is described.

Exciplex formation in jet-cooled donor—bridge—acceptor compounds incorporating bridges with three degrees of flexibility

Intramolecular exciplex formation was studied in three types of donor—bridge—acceptor systems under jet-cooled conditions. While each of these contains the same aniline/cyanonaphthalene D/A pair the saturated hydrocarbon bridges differ in flexibility and length. With a flexible trimethylene bridge at least three conformers are concluded to be present in the jet, which display different mechanisms of exciplex formation. The main conformer is probably fully extended and required an excess excitation energy Δ E⩾ 1100 cm −1 for exciplex formation. This is thought to correspond with a mechanism in which IVR is the primary process bringing D and A in closer proximity. A broad long wavelength excitation is furthermore assigned to the presence of a fully folded conformer undergoing direct excitation into the exciplex state. In addition a partly folded conformer appears to be present from which exciplex formation can occur at negligible excess energy. It is argued that in this partly folded conformer D and A are within “harpooning range” even for Δ E = 0, implying that after excitation at the spectral origin of the acceptor chromophore electron transfer can occur followed by electrostatically driven folding to form the emissive exciplex. With two more rigid types of bridges a harpooning mechanism is also concluded to be involved in formation of the exciplex. With these bridges however, the ground-state donor—acceptor distance is much better defined and is furthermore too large to allow electron transfer at Δ E = 0. As a result exciplex formation sets in only at excess energies sufficiently high to extend the “harpooning range” to or beyond the actual ground-state distance.